Molecular and Microstructural Factors Affecting Mechanical Properties of Polymeric Cover Plate Materials
Elizabeth J. Clark
DEPARTMENT OF COMMERCE
National Bureau of Standards
Center for Building Technology Gaithersburg, 20899
U.S. Department of Energy Office of Solar Heat Technologies ating and Cooling Division QC
NATIONAL BUREAU OF STANDARDS LIBRARY
MOLECULAR AND MICROSTRUCTURAL FACTORS AFFECTING MECHANICAL PROPERTIES OF POLYMERIC COVER PLATE MATERIALS
Elizabeth J. Clark
DEPARTMENT OF COMMERCE
National Bureau of Standards Center for Building Technology Gaithersburg, MD 20899
U.S. Department of Energy Office of Solar Heat Technologies
DEPARTMENT OF COMMERCE, Malcolm
NATIONAL BUREAU OF STANDARDS,
Ernest Ambler. Director
Table of Contents Page
Introduction Background Aspects of Polymer Physics and Chemistry 3. Polymer Molecules 3.1 Bonding 3.2 Solidification 3.3 Glass Transition 3.4 Polymeric Cover Plate Materials 4. Deformation and Mechanical Properties of Polymers 5. Polymer Deformation and Fracture 5.1 Mechanical Properties 5.2 Effect of Microstructural and Molecular Factors on Mechanical 6. Properties Molecular Weight 6.1 General 6.1.1 Effect of Molecular Weight on Mechanical Properties 6.1.2 Crystallinity 6.2 Crosslinking and Branching 6.3 Copolymerization 6.4 Plasticizers 6.5 Orientation 6.6 Residual Stresses 6.7 Degradation of Polymeric Cover Plates 7. Discussion 8. General 8.1 Polymer Microstructure 8.2 Methods to Study Polymer Degradation and Deformation 8.3 Summary 9. 10. References 1
3 4 5 7
16 16 23
27 28 28 30 35 40 41 41
42 45 46 52 52 52 55 57 58
This paper reviews the dependence of mechanical properties of polymer on various microstructural factors. The microstructural and molecular
factors considered are:
molecular weight, crystallinity, crosslinking,
branching, copolymerization, plasticization, orientation, and residual stresses. The types of mechanical properties considered are: loading,
fatigue, creep, wear and abrasion, and environmental stress
cracking. The effects of polymer deformation and fraction at the
molecular level are discussed. Cracking, crazing, and shear yielding are described. Polymeric cover plate materials are discussed and their
degradation reviewed. Methods to measure microlevel changes in polymers are identified.
Introduction In solar collectors the cover plate simultaneously serves as a window
for the sun and a structural component of the collector,
prime importance on the optical and mechanical properties of the material.
Polymers have long been used in applications requiring transparency, strength, and durability, jet plane canopies,
so it is natural to consider them in collector glazing
applications. Polymeric cover plate materials may play an important role in the future development of solar energy collectors if they can be shown
Use as solar collector cover plates
to have adequately long service life.
imposes more severe demands on polymeric materials than most past
large-scale uses because degradation may occur when polymers are used as
collector glazings. Possible advantages of polymeric, rather than inorganic glass cover plates are:
higher transmittance, better impact
resistance (from some materials), lower cost, easier processing, lighter weight,
and greater design flexibility. On the other hand,
not as inert to their environment as the inorganic glasses they might
The most significant difference between solar collector cover plates and other polymer applications requiring transparency,
is that cover plates are exposed
simultaneously to elevated
temperatures and natural outdoor weather. Optical and mechanical
properties of polymers can be reduced by exposure to elevated temperatures or to outdoor weathering,
and simultaneous exposure to both can lead to
loss of solar transmittance and embrittlement of the polymer. Service life prediction of polymeric cover materials is difficult because
knowledge of the relationships between polymer use, composition, morphology,
and long term behavior are not well enough developed. As
knowledge in polymer structure-property relationships is gained, and as
correlations are established between weather factors and polymer degradation,
service life prediction will improve. Past research on cover
plate materials has emphasized transmittance changes as a measure of
polymer degradation. This paper reviews the dependence of polymer
mechanical properties on
their composition and microstructure and shows
the relationship between cover plate polymer degradation and
microstructural changes. 2.
Background Mechanical properties are associated with resistance of a material to
deformation under load. The mechanical properties of polymers are complex due to their viscoelastic nature which is demonstrated by a strong
dependence of the mechanical properties on time and temperature . This is a
critical consideration for cover plate materials which encounter
extended use at temperatures above ambient. Failure,
in the sense of
mechanical properties, occurs when the material breaks, tears, cracks, flows,
The mechanical properties of polymers are dependent on many factors
including microstructure, chemical composition, residual stresses, flaws,
processing methods and history. Mechanical properties are properties of the bulk material, but they reflect phenomena occurring at the microscopic
scale down to the molecular level. Polymer degradation begins on a
molecular level with alteration of the polymer molecules and generally is
accompanied or followed by changes in morphological properties (e.g.,
crystallinity or orientation) at the microscopic level. As continues,
the molecular and microscopic changes manifest themselves as
macroscopic changes, e.g., changes in strength, elongation, and hardness.
Measures of the microscopic level changes which occur earlier in the
degradative process than macroscopic types, may be Important as early
degradation indicators. For service life prediction,
it is desirable to
have diagnostic tools able to detect and measure early changes which are
clearly precursors in degradation processes. Since mechanical properties are so dependent on structural properties, monitoring microstructural
changes should give an indication of mechanical property deterioration.
Understanding the relationships between structure and properties of polymers has received increased emphasis in recent years but is not as
well-developed as for metals and ceramics. There remains a major technical challenge to develop
a basic understanding of the relationships
between structure and end use performance and the scientific and technological framework needed for improving the durability of polymeric materials. As insight develops into the influence of microstructure on
it permits better choice of materials,
processing, and an improved ability to predict if a material will meet the
requirements of the application. It has been recognized that additional polymer research is needed to determine accurately the service life of
commercial polymers used as glazing materials and to develop new polymers with long term durability under the exposure conditions of solar
collectors [2,3]. 3.
Aspects of Polymer Physics and Chemistry To understand the mechanical properties of polymers,
to appreciate the nature of polymers. The two important considerations
are the types of molecules and bonds which in turn influence the molecular
morphology of polymers. These factors, along with the chemical
composition control the solidification, flexibility, strength, and stability of polymers.
Polymer Molecules Polymer molecules consist of very long chains of covalently bonded
atoms containing multiple sequences of a chemical structure,
repeating unit being unique for each polymer. Polymer properties are dependent on chemical composition and physical arrangement of the long
molecular chains in space. The simplest polymer chain, polyethylene,
made up primarily of covalently bonded carbon atoms where the side
structure on these carbon atoms is a hydrogen atom,
Polymers vary by having other atoms or molecules substitute for C in the main chain backbone or H as the side attachment. The flexibility of
polymer chains depends on the atoms in the chain backbone, the size of the side groups, bond angles,
the ease of rotation about the bonds,
energies. Bulky side groups make rotation and packing more difficult and may reduce structural regularity and interchain bonding. Ring structures in the main
chain increase rigidity. Oxygen in the main chain provides
greater ease of rotation and increases flexibility . The flexibility of the bulk polymer is a direct result of the flexibility of the polymer
chain on a microscopic level. The structural repeat unit of several common cover plate polymers is shown in table
Poly(vinyl fluoride) (PVF) and poly( vinylidene
fluoride) (PVDF) have the most simple repeat unit with two C atoms in the
backbone and F substituted for some of the side H atoms. Fluorinated
ethylene propylene (FEP) is a random copolymer consisting of 80-90 mol# of the -C
- Fij- units
and 10-20 mol# of the -CF 2 -CF-CF -units. 3
methacrylate) (PMMA) has two C atoms in the main chain but bulky side
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groups. The repeat units of polycarbonate (PC) and poly(ethylene
terephthalate) (PET) are large and oontaln a number of atoms in the main
chain including both benzene rings and oxygen atoms. 3.2 Bonding
The strength and mechanical properties of polymers are influenced by the bonding between the molecules and the three-dimensional nature of the
bulk material. Several types of bonding exist within polymers:
bonds between atoms within an individual polymer chain; secondary bonding (
intermolecular forces) between adjacent molecules (e.g., London
induced polar-dipolar forces, and dipolar forces
including hydrogen bonding); and physical entanglements of the long chains with adjacent chains , 3.3 Solidification
Most polymeric materials used as cover plates are fabricated by
casting or extrusion of the molten material. The microstructure obtained on solidification is dependent on the polymer composition,
and stresses or orientation resulting from processing. The
solidification of a linear polymer from the liquid gives either an amorphous solid or a partly crystalline one. In amorphous polymers the
polymer chains are disordered. Semicrystalline polymers contain regions
having both crystalline and amorphous material. The crystallization behavior of polymers is unusual and results in a
complex morphology due to the chain-like structure of the polymer molecules. Polymers crystallize by their constituent chains folding back and packing parallel to one another into thin platelets, as shown in
As a crystal grows,
the platelet thickness remains fixed while
Semicrystalline polymer illustrating chain folded crystals, tie molecules, and the amorphous regions.
the other two dimensions grow very large in comparison. The thickness of
polymer crystals is on the order of 50-200 A . This plate-like crystal
structure is a major difference between crystals from polymers and
nonpolymeric materials. Most polymers crystallize only in randomly
distributed crystallites of microscopic size. Polymer crystals are surrounded by amorphous material but are
connected by polymer chains that loop out of the crystal through the amorphous regions and participate in other crystallites. These tie chains give strength to the material by providing physical connections between the crystallites which hold the structure together.
The melting point, T m> of a semicrystalline polymer is one of the most
important characteristics governing its usefulness. T m is influenced by a
number of characteristics of the repeating unit, which are: (3)
the most important of
close-packing ability, and (A) interchain attraction. High melting
points are generally associated with either highly regular structures, rigid molecular structures, or a combination of these , For example,
PC has a high melting temperature, which is related to the molecular
stiffness of the backbone which has two benzene rings. A feature in the structure of amorphous polymers which affects
mechanical properties is entanglements. The coiling motion of the polymer chains in the melt causes them to become entangled. However, a critical molecular length is necessary before the entanglements affect flow. Once chains are long enough to become entangled, flow becomes much more difficult because forces applied to one polymer chain become transmitted to and distributed among many other chains. Below the critical molecular
weight, Mw> polymer viscosity is roughly proportionately to the weight
average molecular weight, M w
Above the critical molecular weight,
polymer viscosity is related to M w raised to about the 3.4th power . The relationship between flow viscosity and 1^ is illustrated in figure 2. For most polymers,
the critical molecular weight is between
40,000 . Since the viscosity increases as the molecular weight increases, more energy is required to process high molecular weight
polymers. Thus an optimum range based on processability is usually
selected for commercial polymers.
Flow viscosity as a function of molecular weight.
3.4 Glass Transition
Most polymers have a characteristic glass transition temperature, T gf above which the thermal motion of the polymer molecules is increased. This is important in cover plate applications since mechanical properties of
polymers show profound changes in the region of the glass transition temperature. For this reason, knowledge of the T_ can be vital in ©
selecting a polymer for applications in which mechanical properties are important . Above T g| the continuous vibrational motion of segments of
amorphous chains results in a myriad of different shapes of the polymer
if the chain is long enough,
the motion enables it to form
physical entanglements with itself and its neighbors. Below T gt the thermal energy is insufficient to activate movement of chain segments . The T
is related to chain stiffness and the geometry of the polymer
chain. The practical uses of various polymers depend greatly on their T
value. The main requirement for a polymer to serve as a useful plastic at
room temperature is that T
for amorphous polymers and T ra
in the case of
crystalline polymers, occur well above room temperature. The T less than T ra
An approximate empirical relationship of 1/2
(with temperature in
has been developed .
The segmental motion above T
is temperature dependent and it
basis for polymer viscoelasticity  which implies behavior similar to
both viscous liquids and to purely elastic solids. In viscous systems all work done on the system is dissipated as heat whereas in elastic systems all work is stored as potential energy as in a stretched spring ,
The relative importance of the various deformation modes changes with the temperature, and it is convenient to distinguish four physical states of polymers,
although in practice the divisions are not always clear cut.
Starting at the low temperature end, the states are:
retarded highly elastic (leathery),
instantaneous highly elastic
and (4) viscous . In the glassy state ideally there is
only normal elasticity, although in reality some creep will be found.
Above Tg the glassy state gives way to the leathery state where thermal
agitation is sufficient to promote retarded high elasticity and viscous flow.
About 30 degrees above Tg the leathery state merges into the
rubbery state where the motion of molecules as a whole becomes dominant and where viscous flow rate occurs at an increased rate. Finally,
still higher temperatures the viscous flow rate increases to a point at
which it dominates the other deformation modes. Below T a hard,
the material is
rigid glass. As the temperature is increased for high molecular
weight polymers, the material becomes leathery, then rubbery, and finally liquid. Only the Tg is well marked,
the other divisions being somewhat
intermediate due to overlapping of the deformation modes . Polymers are normally used at temperatures which put them into the glassy or
leathery state. The load versus deformation curves for glassy,
Load-elongation curve for glassy, semicrystalline, and rubbery polymers.
semicrystalline, and rubbery polymers are illustrated in figure
can observe the change in elongation as the temperature is increased from
below to above T g# 4.
Polymeric Cover Plate Materials The polymers used most frequently in cover plate applications are
listed in table
along with their chemical repeat unit, glass transition
temperature, and melting temperature. A brief description of each
material follows. Poly( vinylidene fluoride) is a semicrystalline polymer with a degree of crystallinity ranging from 35~65 percent,
depending on thermal history
. The symmetric arrangement of the H and F atoms attached to the C
backbone contributes to the ease of crystallization. T m of commercial films is about 170 °C [11,12], although it depends on the method of
polymerization and/or crystallization history. The T
is -45 °C.
extruded into films which can be oriented to increase mechanical strength. Cover plate film is about 0.13 mm thick. PVDF has high levels of
mechanical and impact strength, abrasion resistance, resistance to most
chemicals and solvents, and reportedly has good resistance to sunlight. Poly(vinyl fluoride) is a tough, flexible semicrystalline polymer
having one fluorine atom on every other carbon atom in the backbone. Since F has a
Van der Waals radius near that of
the regularity of the main
chain is retained and PVF readily crystallizes. The degree of
crystallinity ranges from 20-60 percent, depending on the polymerization method . The melting temperature is about 200 °C  and the maximum
continuous use temperature is 107 °C . PVF polymer is converted to film by melt extrusion. As a cover plate it is used in thicknesses of
about 0.1 mm. The extruded film can be biaxially oriented to varying
degrees to improve tensile strength,
resistance. Because of the high temperatures needed for processing, heat
stabilizers are added to PVF; however no plasticizers are used . PVF has exceptional resistance to water adsorption, chemical attack and
unusual solvent resistance. PVF has low solubility in all solvents at
temperatures below 100 °C and is reported to have excellent resistance to damage from sunlight. However,
the early PVF cover plate films discolored
and embrittled . Apparently the combination of elevated temperature
and sunlight caused degradation. Inhibitor systems in PVF have since been altered.
Fluorinated ethylene propylene (FEP) is a semicrystalline copolymer produced by copolymerization of tetrafluoroethylene and hexaf luoropropylene. FEP has a crystalline melting temperature of 290 °C and a T
between -5° and 29 °C [12,16]. The continuous use temperature of
FEP is 204 °C . FEP cover plates are made from extruded film having a
thickness of about 0.025 mm. The crystallinity of FEP after processing depends on the cooling rate of the molten polymer but can reach 70 percent .
FEP is a relatively soft material with lower tensile strength,
resistance, and creep resistance than many polymers. Its strength
decreases with increasing temperature. FEP is chemically inert to
essentially all industrial solvents and chemicals and is virtually unaffected by sunlight.
Poly(methyl methacrylate) used in cover plates is an amorphous polymer
with a Tg -105
The bulky side groups off the main chain raise the T
and decrease the regularity, preventing the polymer from crystallizing.
The maximum recommended use temperature is about 80-85 °C  which can
present a problem for certain cover plate applications. PMMA cover plate
materials are made by a casting process. Since it is a glassy material
which is fairly brittle at ambient temperature,
the thickness of PMMA
cover plates is generally 1-1.5 mm or greater to provide sufficient
strength to span across the collector. Prolonged subjection to even quite low stresses can produce surface crazing . Differences in temperature
and/or humidity on the inner and outer surfaces in glazing may bow the sheet in the direction of the higher value. Temperature differences act rapidly, within a few hours, but recovery is also fast when the gradient is removed.
Differences in humidity act slowly over weeks . Although
PMMA has a history of good resistance to outdoor weathering, UV
stabilizers are added to improve resistance to sunlight . When
cleaning a cover material the chemical resistance must be considered. PMMA is chemically resistant to many liquids,
however aromatic and chlorinated
and ketones attack it.
Poly( ethylene terephthalate)
is a thermoplastic polyester which has
varying degrees of crystallinity, depending on its thermal treatment . The T
is reported as 69
°C and T m as high as 280°,
although the T m of
most commercial products ranges from 255~265° due to reduced crystallinity
resulting from chemical impurities . In semicrystalline PET both crystalline and amorphous regions contribute to the overall properties and a range of degrees of order exist.
In the absence of nucleating agents
and plasticizers, PET crystallizes slowly and performs best in
applications where strength and crystallinity can be increased through orientation
which is accomplished by stretching the film and inducing a
high level of molecular chain alignment and extension. Orientation
significantly improves creep resistance, tensile strength, and modulus. If an oriented film is heated above its T
additional crystallization may
thus PET cover plates may increase crystallinity if the
temperatures exceed 70 °C. Oriented PET has good chemical resistance being
resistant to most organic solvents and mineral acids . Unaged PET is
strong film, however its properties degrade substantially on
weathering. PET used as a cover plate has a thickness of 0.13 mm and is made by an extrusion process. PET is particularly susceptible to
hydrolysis which can result in decreases in molecular weight and associated reductions in mechanical properties . Upon exposure to sunlight,
PET loses elasticity, breaks easily and develops a crazed
surface. It is common for both UV and heat stabilizers to be added to PET or for UV absorber to be applied on the surface.
Polycarbonate used in cover plates is an extruded polyester based on bisphenol
is about 150 °C,
however, due to the size and
flexibility of the repeat unit and vibrational motion of parts of the chain,
PC does not become brittle until -100 °C. Polycarbonate
crystallizes only after lengthy annealing at 190 °C in the absence of oxygen and moisture . Thus it is unlikely that cover plates contain
crystalline PC. However, PC can crystallize from amorphous material by treatment with swelling agents or their vapors. Polycarbonate is soluble in or swelled by a number of aromatic and chlorinated hydrocarbons and
other chemicals. With solar collectors it may be necessary to assure that heat transfer fluids and cleaners are compatible with the polymer. PC
also exhibits some hydrolytic sensitivity , The heat distortion
temperature is 135-1
in order and in stiffness .
Annealing below T
leads to an increase
Stresses can be removed by annealing
above 100 °C. PC is a tough, stiff, is not scratch resistant.
impact resistant material, however it
Scratch resistant coatings have been developed
Strong sunshine combined with humidity causes superficial yellowing
and embrittlement to fracture. Thus thermal and UV stabilizers are added to PC [2*1].
Fiber reinforced plastics
FRP) are generally thermosetting polyester
resins with glass fiber added for strength. Due to the highly crosslinked nature of these thermosets,
they have no T
g or Tm<
mpac t resistance
and strength are aided by the presence of chopped glass fibers. As with
most polyesters, FRP materials are generally susceptible to moisture
damage and they are discolored by exposure to heat.
Deformation and Mechanical Properties of Polymers
Mechanical properties of a polymer are those properties involving loading or deformation of the material under the influence of applied forces. Polymer mechanical properties are complex because of their
viscoelastic nature. Viscoelastic behavior is demonstrated by a strong
dependence of the mechanical properties on time and temperature . A brief overview of the deformation of polymers and their mechanical
properties is provided here. The reader is directed elsewhere for more detailed discussions of polymer deformation and mechanical properties [7,10,24-40]. 5.
Polymer Deformation and Fracture When a polymer system is subjected to an applied stress, a number of
chemical and rheological events (often having opposing effects) may occur at a given temperature. The net effect of the several competing processes
depends on many factors,
including the temperature,
the time scale of the
experiment with reference to the time scale of the process, the environment,
the basic molecular properties of the polymer,
composition . At sufficiently low stresses and strains,
the mechanical response is
reversible while at higher stresses and strains an irreversible component of deformation becomes evident.
variety of events takes place,
during deformation and fracture, a
including homogeneous shearing,
heterogeneous shearing (shear banding), dilatational processes, and bond rupture . Polymers exhibit a wide range of elastic properties
depending upon their structure and the testing conditions. Figure the variation of Young's modulus,
with temperature for an amorphous
polymer. At low temperatures the polymer is glassy with a relatively high
modulus. The modulus falls rapidly through the region of the T
O where the
polymer is viscoelastic and the modulus is very rate- and
At a sufficiently high temperature the polymer
becomes rubbery. If the polymer is crosslinked the modulus remains
approximately constant with increasing temperature, otherwise the polymer flows like a viscous liquid .
Semicrystalline polymers behave like amorphous polymers below the T of the non-crystalline phase. However,
properties between T
they have interesting and useful
and T m where the structure consists of crystals in a
rubbery matrix. The modulus of the material depends upon the degree of
crystallinity and degree of orientation. The modulus values for different types of polymer are summarized in table 2 .
Typical variation of Young's modulus, E, with temperture for a polymer showing the effect of crosslinking upon E in the rubbery state .
Typical Values of Young's Modulus, E, and Fracture Strength, o it for Different Types of Polymers Measured at Room Temperature and Moderate Testing Rates 
Type of Polymer
Rubber Semicrystalline polymer (above T Semicrystalline polymer (below Tg) Glassy polymer
Fracture Strength Oi(MPa)
- 50 - 70
Two major deformation micromechanisms of importance in polymers are shear yielding and crazing. They represent the main source of energy
absorption in the material. Both involve localized, or inhomogeneous, plastic deformation of the material. The difference between the
mechanisms is that shear yielding occurs essentially at constant volume whereas crazing occurs with an increase in volume. Thus, unlike shear yielding, crazing is a cavitation process.
Ductile plastics tend to have a fairly well-defined yield point with the value of yield strain typically of the order of 5~1 0 percent
higher than the value for metals or ceramics. Some semicrystalline
polymers can be cold-drawn to extension ratios in excess of 20 during
which a stable neck extends along the specimen. Most amorphous glassy polymers are brittle in tension but yield and flow to high strains in
compression or pure shear
Shear deformation consists of a change of
shape without significant change in volume. Molecules translate past each other during shear yielding. The shear yielding of polymers is highly
relevant to the mechanics and mechanisms of polymer fracture. Shear
yielding limits the strength of the polymer if brittle fracture can be suppressed. A polymer must have a high yield stress in order to be strong and if bulk, homogeneous yielding does occur the polymer is likely to be tough. Recent evidence suggests that shear yielding,
in the form of
microshear bands, plays a key role in the initiation of cracks.
crack propagation in polymers usually involves localized viscoelastic and plastic energy dissipative processes occurring in the vicinity of the
crack tip A
craze is initiated when an applied tensile stress causes microvoids
to nucleate at points of high stress concentrations in the polymer created
flaws, cracks, dust particles, molecular heterogeneities,
etc. The microvoids do not coalesce to form a true crack since they
become stabilized by fibrils of plastically deformed, oriented polymeric
material spanning the craze. The resulting localized yielded region therefore consists of an interpenetrating system of voids and polymer fibrils.
Molecular entanglements play an important role in controlling craze geometry. Unlike a crack, a craze is capable of transmitting loads
between its faces. However, when cracks do initiate and grow, they do so by means of the breakdown of the fibrillar structure in a craze. This
typically results in slow cavity expansion followed by crack extension through the craze, and finally crack propagation during which a craze or crazes precedes the crack tip through the remainder of the material .
Crazes may initiate, grow and break down to give cracks at stresses below those necessary to cause bulk shear yielding and so lead to brittle
fracture requiring only a relatively low value of fracture energy, Gj c> 0 r
stress-intensity factor, Kj c . Crazing therefore plays an important
role in determining the strength of a polymer. Temperature is important in determining the balance between crazing and shear response.
relationship between yield stress, crazing stress, and microvoid
nucleation stress for polystyrene is shown as a function of temperature in figure
Both the crazing stress and yield stress decrease as
temperature rises. More detailed discussion of the mechanisms of crazing and shear yielding is given in references
33 ] and .
Whatever the detailed yielding response may be, attainment of the critical energy for crack propagation depends on the balance between energy stored and energy dissipated and, hence, on the relative importance of the various modes of deformation and their consequences in terms of
strength of the molecular aggregations that oppose the crack's progress . Many possible combinations of deformations and defects may exist
thus making mechanistic correlations difficult. While fracture mechanics
may serve to characterize a material, a given level of behavior may arise from a multiplicity of different competing molecular events . The
Relationship between yield stress, crazing stress, and microvoid nucleation stress in polystyrene .
General deformation for a polymer: stress-strain curve as observed in one material as a function of temperature of rate of strain, or for a wide variety of materials. (I) Hookean elasticity; also linear viscoelasticity; (II) nonlinear viscoelasticity; (III) yield point; (Y) yield stress; (H) hysteresis of return curves; (N) necking region; (IV) beginning of strain hardening; (S) set, permanent deformation, incipient failure; (R) rupture; (T) tearing; (V) plastic flow; (ER) partial elastic recovery, elasticoviscous, or plasticoelastic (VI) strain hardening and rupture; (DR) ductile rupture (VII) ductile failure . ;
general deformation of a polymer in simple tension is illustrated in figure
and as shown,
the response may be quite complex. The events
shown have been described by Hertzberg and Manson  as follows. "At first (Region I) bonds are stretched and energy is stored in a
quite reversible manner (Hookean elasticity, linear viscoelasticity). As the stress rises further,
the response (Region II) becomes nonlinear;
energy is still recoverable on release of the stress, but slowly (anelastic response). Eventually, yielding occurs (Region III),
accompanied by the onset of components of irreversible deformation. (This is not to say that no reversible component exists in this region;
some point, even crazes may exhibit a high degree of reversibil ity
up to .
During such deformation, extensive cooperative sequential motions take
place, and energy is, of course, dissipated in a viscous manner. The
nominal stress, then, usually drops somewhat (Region IV) and extension frequently proceeds easily, often with the formation of a neck that
continuously undergoes drawing. Fracture (Region V) may occur shortly after necking begins or, especially with crystalline polymers such as polyethylene, fracture may be delayed due to an orientation of the
molecules (analogous to strain hardening in metals) and the stress may
actually increase, as shown in Region VI.” Polymer ductility decreases as the stress field becomes increasingly severe. Ductile/brittle transitions are due to competition between a
brittle fracture mechanism such as crazing and a ductile mechanism such as bulk shear yielding. Fracture originates from local concentrations of flaws,
scratches or notches. All materials contain such inhomogeneities
which can propagate to cause failure. Crack propagation occurs when the material can deform or flow no longer, i.e. when the limit of non-fracture
deformation mechanisms is reached. The fracture stress, related to the crack length,
by using a modified Griffith fracture
equation  where
for plane stress
where G c is the fracture energy and E is Young’s modulus. The fracture energy is the energy required to form a unit area of crack. A material's fracture strength may be characterized as the energy per unit area
required to produce new surfaces. Polymers tend to have lower fracture strengths than metals and ceramics; typical values of fracture strength, for different plastics are given in table 2. As with Young's modulus,
there is a wide variation. The concepts of fracture mechanics provide a
sound basis on which to describe polymer fracture. In some areas, such as
alow crack growth in glassy polymers,
while in other areas, is little.
there is quite a bit of information,
such as environmental crack growth and fatigue there
Ductile failure is the most difficult to characterize .
The experimental methods used to characterize crack growth generally
employ quite large flaws or cracks and it is easy to use the data to predict what will happen when similarly large defects are present. However, when the flaws present are small,
it is more difficult.
Information is scarce and this area will require more attention in the future. The theories of molecular fracture were originally developed
without any direct evidence that molecular fracture actually occurs.
Techniques are now available to study molecular fracture directly. The polymer fracture process can be broken down into three distinct steps: 1.
The excitation of bonds under the action of an applied stress;
The scission of the excited over-stressed bonds by thermal fluctuations; and
The formation of small submicrocracks or crazes which coalesce to
form larger cracks .
Experimental techniques are now available to study all phases of fracture. The most widely employed technique has been electron spin resonance
spectroscopy (ESR) which can be used to follow scission. It allows the direct observation of free radicals produced when covalent bonds are broken.
It is an extremely powerful technique which allows the number and
the chemical nature of the radicals to be determined. Other experimental
techniques have included infra-red and Raman spectroscopy, small-angle x-ray scattering, and the measurement of molar mass variations . 5.2 Mechanical Properties
The mode of fracture is related to the manner in which the load or force is applied to the polymer. There are at least five conditions of
stress and environment that cause fracture. The five conditions, which
correspond to various types of mechanical tests, are: (2)
wear and abrasion, and (5) environmental
stress cracking . Direct loading involves applications of a stress until fracture is produced. Tensile, compression, and Impact tests are
examples of direct loading. Tensile and compression tests determine the
capacity of a material to withstand static load ; they are performed as stress-strain tests which provide information on the modulus
and elongation of the polymer. A sample tensile
stress strain curve is shown in figure
Toughness is an indication of the energy a material can absorb before breaking. Ductile materials that cold draw are very tough due to the
large elongation to break, whereas brittle materials have low toughness.
Tensile properties of brittle materials are greatly influenced by flaws and submicroscopic cracks. Tensile properties of several cover, plate
materials are listed in table
in the order of their tensile strength
Impact resistance is a measure of toughness under shock loading . Impact tests measure the energy necessary to fracture a material by an impulse load and provide an indication of notch toughness or notch
sensitivity resulting from internal stress raisers. Two important factors which enter into impact behavior are the energy needed to initiate a crack and the energy required to propagate a crack.
Fatigue is a measure of lifetime under cyclic loading . It is a
function of stress, consequently as one increases the maximum stress per cycle,
the number of cycles to failure decreases. Fatigue fracture occurs
after many cycles even though the maximum stress applied during a cycle is less than would be required to produce fracture under direct loading.
Tensile Properties of Several Cover Plate Polymers
Tensile Strength MPa
Elongation at Break
Tensile Modulus GPa
Poly( vinylidene fluoride)
Poly( methyl methacrylate)
Below some value of maximum stress, known as the fatigue or endurance limit,
the fatigue curve levels off, and the material can be subjected to
many cycles without failure . Fatigue life generally decreases as the
temperature increases and the frequency of oscillations increases. Fatigue failure is usually due to the gradual growth of cracks. Materials
generally contain flaws which develop into submicroscopic cracks as the stress exceeds some critical value. The cracks grow a small amount during the peak load of a cycle until one or more of them exceeds microscopic
dimensions. Fatigue life of polymers is also related to their mechanical
damping and the resulting heat build-up in the material,
if the heat
produced is greater than the rate of heat dissipation. The strength of polymers generally decreases with temperature and crack growth becomes easier.
The dimensional stability of polymers is measured by creep and stress
relaxation tests . Creep tests measure the continuing deformation of a material with time as the load is held constant. Creep is a function of stress and temperature. Stress relaxation is the time dependent decrease in the stress of a material which is constrained to a constant
deformation. Where viscous flow occurs the stress can decay to zero at
sufficiently long times, however where there is no viscous flow the stress decays to a finite value. Wear is the unwanted progressive loss of substance from the surface of a material brought about by mechanical action from the rubbing of one
surface against another [M3]. Abrasion, wear, and scratch resistance are all similar and are closely related to friction. The ploughing component of frictional force is especially important in abrasion and in the process of scratching a surface.
It is similar on a microscopic scale to tearing
. Small particles of the material are torn or broken off the wearing or abraded surface.
Environmental stress cracking (ESC) is induced by the interaction between a polymer and a nonsolvent in the presence of mechanical stress. ESC occurs when crack or craze growth is promoted by the presence of an
environment and a stress. The major part of the energy required for fracture is provided by chemical or physical-chemical processes, and the
applied stress plays a secondary, though essential role . Internal residual stresses may be sufficient to induce stress cracking in the absence of an external stress. The exact mechanism of ESC is not known, but the most likely mechanism appears to be that the presence of stress
and a crack or flaw results in a local microvoided zone or craze at the
crack tip and it is the interaction within this zone which gives enhanced crack growth. Fatigue and deterioration of polymers are frequently
attributable to ESC.
Effect of Microstructural and Molecular Factors on Mechanical
Properties The deformation of a polymer is dependent on internal material
characteristics as well as external environmental factors. The following microstructural and molecular factors influence the mechanical properties C731
Crystallinity and crystal morphology
Crosslinking and branching
Their effects on mechanical properties are discussed below. In addition to these, in
the following environmental or external variables are important
determining mechanical behavior : 1
Type of deformation (shear,
Stress and strain amplitude
Heat treatment or thermal history
Nature of surrounding atmosphere
frequency, rate of stressing
The effect of temperature was discussed briefly in Section 3.4. The type of deformation will be considered below. The other factors will not be
General The molecular weight and molecular weight distribution of a
polymer have a large effect on mechanical properties. Commercial polymers
contain molecules having a broad distribution of chain lengths. The molecular weight distribution (MWD) of polycarbonate is shown in figure
The curve labeled o month illustrates the MWD for an unweathered
material. The width and shape of the distributions are established during
processing. The molecular weight is typically reported as an average value which obscures any knowledge of the distribution. The width and shape of the distribution are important because some mechanical properties are governed by the number of short molecules while others are dependent on the long chains.
as the molecular weight increases,
span of the molecular weight distribution increases. The narrower the
molecular weight distribution curve, the more rapid is the increase in
Molecular weight distribution of the irradiated surface layer of polycarbonate at various periods of exposure (0 to 42 months) to outdoor weathering .
strength with molecular weight. Polymers of interest have molecular weights that range from 10^ to 10^ and contain lengths (i.e., of the fully
extended molecule) that range from 0.1 to 10
It has been known for some time that the mechanical properties of
linear polymers are dependent on molecular weight in the sense that the
polymer does not develop any significant strength until a certain critical value of molecular weight is reached where entanglements occur. Molecular
entanglement must take place before a polymer can carry a load. Above this critical molecular weight, mechanical properties increase until a
second critical value of molecular weight is reached beyond which the
dependence of mechanical properties on average molecular weights is greatly reduced or nonexistent . This point is illustrated in figure
The exact shape and slope of the curve varies for different
properties, as does the critical minimum molecular weight. The limiting
value at high molecular weights will vary, depending on the specific polymer. Polar polymers and polymers with hydrogen bonding chains reach
their maximum property values at lower molecular weights than do nonpolar
MOLECULAR WEIGHT 8.
Relationship between molecular weight and polymer properties.
polymers, For most thermoplastic materials, moderate changes in molecular
weight do not appreciably influence mechanical properties such as yield stress or modulus. Thus, they must be toward the upper region of figure
On the other hand, properties relating to rupture such as
ultimate elongation, impact strength, and ultimate strength are affected by moderate molecular weight increases. These must be toward the lower
region of figure 6.1.2
Effect of Molecular Weight on Mechanical Properties An overview of the relationships between molecular weight and
mechanical properties is provided here. A more comprehensive review is
contained in references 45 and 46. The molecular weight and molecular weight distribution of the
polymer are probably the most significant microstructural features
influencing deformation. The relationship between molecular weight and tensile or impact properties has been investigated for many polymers. The general conclusions reached are that the tensile and impact strengths and
elongation at break increase to limiting values at high molecular weight.
Elongation at break versus molecular weight for polycarbonate .
Tensile stress at break versus peak molecular weight of polycarbonate subjected to outdoor and artificial weathering conditions .
Yamasaki and Blaga  studied polycarbonate glazings and found that
ultimate tensile stress and elongation at break decrease as molecular weight declines. Gardner and Martin  also obtained similar results. Figure
illustrates the relationship between molecular weight
polycarbonate and elongation at break while figure 10 shows the dependence of tensile stress on molecular weight.
Lee and Turner  found the density of entanglements for PMMA
becomes significant only above a MW of above
but eventually levels off
Stress-strain properties of semicrystalline polymers such
as polyethylene and polypropylene also depend on MW;
dependence seems to be less apparent because the crystallites help hold the material together,
in much the same way as chain entanglements.
Margolies  and Perkins et al.  examined high density polyethylene (
HDPE) and found that the tensile strength increased with MW to 1.5 x 10^,
then leveled off. However Margolies found that the ultimate tensile
elongation increased to a maximum at
0^ to 7.5 x
0^ but then
decreased when the MW increased further. He concluded that the lower elongation values resulted when the ultra high molecular weight chain entanglement increased to a point where straightening was substantially inhibited prior to break. He also proposed that an increase in
intermolecular forces between very high MW molecules became significant.
Margolies also studied impact resistance of HDPE and found that Izod impact strength increased to a plateau then decreased gradually above
This is illustrated in figure 11. Apparently,
mentioned above that affect ultimate tensile elongation also affect impact strength.
Matsuoka and Wang  found that the MW of a polymer affects how much stress the material will be able to sustain beyond the yield point.
Experimentally they found that the critical value for MW above which a polymer begins to sustain stress beyond the yield point is about equal to the critical MW for entanglement in melt rheology. In recent years the
importance of the fatigue behavior of polymers has been acknowledged and work in this area is expanding. Little work has been done to explore the
dependence of fatigue life and fatigue crack propagation (FCP) on MW . From what has been done,
seems fatigue life increases as molecular
Izod impact of HDPE versus molecular weight .
weight increases. Pitman and Ward  found a substantial decrease in
resistance of PC to FCP as the MW of the polymer decreased. Kim et al.  found that FCP rates of PMMA were dependent on MW even when fracture
toughness changed little. Typical FCP curves showing the effect of MW of PMMA are shown in figure 12.
Michel et al.  have recently proposed a simple theory to explain the strong influence of MW on rates of FCP in amorphous polymers. They propose that FCP rates be expressed as the product of two functions, one involving the stress intensity factor (AK) and the other
characterizing the relaxation process occurring in the plastic zone. To provide a physical network in the plastic zone that will sustain fatigue loading,
they propose that one needs a sufficient fraction of molecular
fibrils per unit area (W*) whose lengths are greater than M c> the critical value of MW required for entanglement. They have correlated FCP data for PMMA using this approach. The effect of MW on fracture toughness is also of interest. From figure 13 it can be seen that values (converted to
Typical FCP curves showing the effect of MW on fatigue crack propagation in bulk polymerized PMMA .
Effect of molecular weight on fracture energy of PMMA. Band indicates the range of values .
fracture energy) for all types of polymers are reasonably consistent. This also shows that fracture energy is fairly insensitive to MW above M v - 10 5
The FCP response of some crystalline polymers such as PVDF is
superior to that of glassy amorphous polymers such as PMMA. The
substantial molecular reorientation, as evidenced by stress whitening at the fracture surface, may be important to the improved FCP response of
these crystalline materials . For example, both PMMA and PC are
amorphous and have similar elastic moduli, however, PMMA has a FCP growth rate 1300 times faster than found in PC .
Creep and stress relaxation are almost unaffected by changes in
molecular weight below T
because the motions of the large segments in the
polymer chain are frozen in place. Above T
the behavior is strongly
dependent on MW because the mechanical response of such materials is
determined by their viscosity and elasticity resulting from chain entanglements. Bueche  found the creep compliance of PMMA is
insensitive to MW in the glassy region, however at higher temperatures and/or longer times, creep and creep rate Increase as the MW decreases.
Apparently little has been done to study directly the relationship between wear or abrasion resistance and MW. However, since abrasion involves tearing away of small pieces of material, the tensile
strength and fatigue life are probably important factors. Both of them are improved by higher MW.
Environmental stress cracking (ESC) is not well understood and little has been done relating it to MW. ESC behavior is dependent upon the molar mass of the polymer,
with lower molar mass material having the
worst ESC resistance. Herman and Biesenberger  studied the
relationship between molecular weight distribution (MWD) and ESC in annealed PE and found that a narrow MWD shows superior stress crack
resistance to a polymer of similar average molecular weight but with a broader MWD. They conclude that the greater amount of low molecular weight material and a high molecular weight tail give rise to voids and
stresses causing low ESC resistance in broad distribution resins. Devries et al.
 found an 18 percent decrease in the MW of PET exposed to 0^
for four days. 6.2 Crystallinity
The mechanical response of a semicrystalline polymers is related to the crystalline regions,
the amorphous regions,
and the tie chains which
join the lamellar regions into a mechanically integrated structure. Tie
molecules carry a high level of stress and are broken during deformation and fracture. The behavior of tie molecules may be compared with
entanglements in amorphous thermoplastics
are responsible for the
strength of the polymers and without them both types of polymer are
extremely weak. Figure
illustrates the crystalline/ amorphous matrix of
a semicrystalline material.
A given polymer's tendency toward being
crystalline or amorphous is determined by the chemical structure. The morphology of semicrystalline polymers and their degree of crystallinity
have profound effects on their mechanical behavior. Since these factors
can be varied over a wide range,
the mechanical properties of
semicrystalline polymers take on a perplexing array of possibilities . Polymers do not attain complete crystalline order and the crystallization rate varies widely among polymers.
Crystallinity is generally favored by symmetrical chain structures that allow close packing of the polymer units to take advantage of
secondary forces which are distance related. High interchain (secondary forces)
interactions also favor crystallinity. The extent of the
crystallization is limited by irregularities in the molecular structure of the polymer. Examination of the repeat unit of cover plate materials in
illustrates these points. PVF and PVDF, which have fairly
symmetric chain structures, are fairly crystalline, whereas in PMMA and PC the molecular structure is far less regular.
The degree of crystallinity is also affected by the rate at which the
polymer melt is cooled during fabrication. If it is cooled rapidly, the polymer may solidify without reaching maximum crystallization. Consequently,
continue to crystallize at a very slow rate if the
temperature is increased. Such a situation could occur on a solar
collector where the cover plate is exposed to temperatures elevated above ambient. Crystallinity can also be increased in polymers by mechanically
stretching to align and crystallize the polymer chains. Unlike amorphous polymers the application of fracture mechanics to the fracture of crystalline polymers is not very straightforward even for isotropic materials. This is principally because crystalline polymers are not normally brittle and their deformation can be non-linear but it is
also because there is often large-scale yielding in the vicinity of the
crack tip. Hence it is generally impossible to apply linear elastic fracture mechanics unless the polymer is at low temperature or has been
embrittled through environmental action [333* Isotropic semicrystalline polymers deformed at temperatures above the T
g of the
amorphous phase are usually ductile, and they yield and flow
forming a neck with the specimen often drawing up to 30 times its original length. Figure 6 illustrates the stress-strain curve of a typical polymer.
The relative ease of plastic deformation means that semicrystalline
polymers are generally tough above T
of the amorphous phase. Below the
Tg the amorphous phase is much less compliant and the semicrystalline sample has a higher yield stress but flow is unable to take place and so
failure occurs at a lower strain [333-
Semicrystalline polymers are often so ductile that they can be deformed to very high draw ratios before the specimens fail. In these polymers,
the noncrystalline component is normally rubbery;
the crystals to deform plastically and change shape without cleavage.
During the early stages of deformation the original spherulitic structure becomes elongated through slip and other deformation mechanisms. The chain axes of the crystals which are originally oriented randomly become aligned
parallel to the drawing direction. Eventually the original lamellar and
spherulitic structures break up and a new fibrous structure is formed with the molecules aligned approximately parallel to the extension direction [
Semicrystalline polymers can be induced to behave in a brittle
manner by reducing the temperature or using certain hostile environments.
Fracture surfaces of HDPE deformed at liquid-nitrogen temperature appear relatively smooth when examined at low magnification but detailed studies [593 have shown that plastic deformation occurs on a local level even when
the fracture appears to be completely brittle. Spherulite size has a
pronounced effect on the deformation of semicrystalline polymers. The larger the spherulite size,
the more brittle the polymer.
In semicrystalline polymers,
the high molecular weight molecules
crystallize first resulting in a molecular fractionation. Low molecular weight polymer accumulates at crystallite boundaries. Gedde et al. 
recently showed that for polyethylene, fracture preferentially develops along the weakest link path defined by these regions of segregated low
molecular weight material. It is likely that other semicrystalline polymers behave similarly. On a higher level of morphology, there is still not yet any clear idea how factors such as crystal structure or spherulite size affect the behavior. A few studies have shown that,
cause the polymer to be more brittle, but the micromechanisms are not yet fully understood. This may be contrasted with the studies of the behavior of glassy polymers where a great deal is known about the micromechanisms
of deformation and fracture. This is clearly an area where more work
needs to be done. Similarly,
the application of fracture mechanics to
ductile semicrystalline polymers is in its infancy, again in contrast to
amorphous polymers and metals . Impact resistance of semicrystalline polymers is generally better than that of glassy polymers. Semicrystalline materials have better fatigue
resistance than amorphous polymers, probably because crystalline polymers not only dissipate energy when crystallites are deformed, but they also
apparently reform a crystalline structure that is exceedingly strong . In static fatigue, HDPE,
a semicrystalline material,
fails in a ductile
manner after short loading periods whereas after prolonged loading it fails in a brittle manner . For short loading periods the yield
stress is less than the fracture stress and the polymer is ductile whereas for longer periods of loading the fracture stress is less than the yield
stress and so the material is brittle. The premature failure of semicrystalline polymers in certain
aggressive environments is a long standing problem. ESC is characterized by brittle fracture. Williams and co-workers [61,62] have found a unique
dependence of crack velocity upon K Ic f 0r crack propagation in PE in
environments such as methanol, ethanol, and a detergent. This is illustrated in figure 14 for HDPE in hot (60 °C) detergent. Williams and
Marshall  have interpreted the change in slope in the plot between 10"7 and 10~6 ms “1 as being due to a change in the fracture mechanism.
They suggested that in the slow growth region the crack growth rate is
controlled by viscoelastic "relaxation processes" in the material. However,
at higher velocities it is thought that the different slope
corresponds to crack propagation in this region being controlled by transport of the environment to the crack tip. They were able to
Crack growth data for high density polyethylene in a detergent environment at +60 °C. (Different points represent individual tests) [ 62 ].
correlate the velocity at which the transition between the two types of behavior occurred with the viscosity of the fluid, with the transition taking place at the lowest crack velocities for fluids with the highest viscosity. 6.3 Crosslinking and Branching
Crosslinking is a chemical linking established between adjacent polymer chains which results in a three-dimensional network. Crosslinking
provides stability to the physical shape of a polymeric object because when a load is applied,
the crosslinks resist deformation. Rigidity of a
polymer is increased by crosslinking, depending on the degree of crosslinking. The length of the crosslink and the distance between
crosslinks varies. The shorter, the crosslinks, or the smaller the
separation between them, the stronger and more rigid the polymer becomes. Lightly crosslinked polymers are rubbery whereas heavily crosslinked materials are rigid and may be brittle.
Branching occurs when the backbone of the main polymer chain has chains of identical chemical composition branching from it. The length and frequency of branching affects the properties of polymers. The
branches provide increased physical entanglement between adjacent molecules and consequently increase strength and rigidity. However
branching results in decreased regularity of the chain and of the polymer crystallinity. Branching is established at the time of processing and does not change during a polymer's life. Chain branching affects
viscosity above T
The effect is difficult to quantify because the
length and number of branches vary. Above T in moderate amounts improve modulus,
crosslinking and branching
yield strength, flexural properties,
fatigue life, shear, and impact properties while decreasing elongation to yield,
elongation to break, and creep resistance. Environmental stress
cracking and wear abrasion also benefit.
Copolymerization Copolymerization results when two or more monomers are polymerized
together so that the polymer backbone contains alternating sequences of one repeating unit then the other. Copolymerization is performed to
modify polymer properties. Generally, copolymerization decreases
crystallinity by introducing more irregularity. altered, with the T
g of the
copolymer falling between the parent
homopolymers. Copolymerization occurs during polymer processing and does not change during the life of the polymer. FEP is the only cover plate material that is a copolymer.
crystallinity is lower than that of poly( tetrafluorethylene) (PTFE) (i.e. 70 percent vs 98 percent). This is a result in the loss in chain
regularity from replacement of a fluorine atom by a perf luoromethyl group The effect of copolymerization on mechanical properties depends on the
polymers involved. By judicious choice of the type and structure of the monomers,
a copolymer with specific desired properties can be designed.
The addition of a plasticizer to a polymer is made to lower the glass
thus increasing the flexibility and toughness of
the material at ambient temperature. Plasticizers are low vapor pressure
liquids, with molecular weights in the range of 100 to 1000 which form a
highly concentrated solution with the polymer. The polymer chains in the
amorphous regions are forced apart by the plasticizer decreasing the intermolecular forces between chains and allowing them to slip more
readily over one another. The absorption of small molecules such as water or the entrapment of monomers has a similar effect to the addition of a
plasticizer. In general,
the addition of plasticizers decreases
properties requiring strong interchain bonding such as tensile strength, flexural modulus, and yield strength. Properties assisted by chain
flexibility and slickness such as elongation at break and elongation at yield are increased. Conversely, loss of plasticizer is indicated by increase in yield strength and a drop in ultimate elongation reflecting
embrittlement of the polymer. It is believed that plasticization of the polymer by active chemical species plays a role in failure due to
environmental stress cracking. 6.6 Orientation
Orientation can be produced by either hot stretching of a molten polymer followed by rapidly cooling of the melt, or by cold drawing, or by cold-rolling. Most commonly, orientation is accomplished by stretching the polymer at or above T g> Fixing the orientation occurs when the
stretched polymer is cooled below T
before the molecules have had a
chance to return to their random orientation. Biaxial orientation occurs when a polymer is stretched in two directions at right angles to each other. The polymer chain segments tend to line up parallel to the plane of the sheet. Compared with unoriented materials, biaxially oriented
sheets and films have higher tensile,
and impact strengths .
Typical tensile behavior of oriented brittle polymers is shown in figure 15. Parallel to the orientation direction,
the polymer may become
ductile and have a yield point and high elongation, but in the
perpendicular direction the polymer becomes more brittle with low strength and elongation . The tensile stress-strain behavior of ductile
polymers is illustrated in figure 16. When an oriented polymer is tested in the parallel direction,
it has a
higher yield strength, but its
elongation to break may be less than that of an oriented polymer tested in the perpendicular or transverse direction. The reason for this unusually
high transverse elongation is that on stretching the previously oriented
material in the perpendicular direction,
the molecules first deorient and
The stress-strain behavior = the unoriented state. of orientation. J_ = stress uniaxial |
of typical polymers which are brittle in tensile stress parallel to direction perpendicular to the direction of the orientation .
The tensile stress-strain behavior of ductile polymers: unoriented, measured parallel to the direction of uniaxial orientation, and measured perpendicular to the direction of the orientation .
then reorient in the direction of the applied force. Brittle materials
break before the reorientation process gets started . Polymers have
high strengths and moduli in the direction parallel to the orientation because the applied loads are carried largely by the strong covalent bonds of the polymer chains. The properties are poor (at least for brittle
polymers) In the direction perpendicular to the orientation because the loads are carried primarily by the weak Van der Waals’ bonds. Also, there are small cracks or other imperfections in the polymer,
oriented parallel to the orientation direction. These oriented cracks are strong stress concentrators for loads applied perpendicular to the
Broutman and McGarry [6M] looked at biaxially oriented PMMA and uniaxially stretched PMMA. In the case of the biaxially oriented material cracks could be propagated in different directions perpendicular to the plane of the sheet with the interfacial fracture energy Gj c
300 J m~^,
whereas cracks growing in the plane of the sheet propagated with G Ic -
33 J m
showing clearly the relative ease of cleaving between
molecules rather than across the molecules. G Ic for isotropic PMMA was found to be
250 J m
In the uniaxially stretched PMMA,
G Ic for crack propagation parallel
to the orientation direction was found to drop as the degree of
orientation was increased, falling by over an order of magnitude for polymer stretched by over 100 percent , In similar studies by Curtis  on PMMA,
it was found that Gj
for crack propagation parallel to the
orientation direction could be directly correlated to the birefringence of the oriented polymer, Gj
as shown for PMMA in figure 17.
Again the value of
drops by over an order of magnitude for the most highly oriented
Variation of Gj c pmma with orientation (given by birefringence, An) for crack propagation parallel to direction of orientation .
PVF is an oriented semicrystalline film and PET and PVDF may also be
oriented. When PVF film is heated above 100 °C,
it shrinks three to five
percent . Oriented PVF film is sometimes placed on a frame and heat
shrunk to cause the cover plate to appear taut. This practice places the entire film under tension and undoubtedly induces localized stresses.
Since stressed areas are more prone to degradation, heat shrinking cover plate materials onto frames may hasten the polymer degradation. 6. 7
Residual Stresses During fabrication the polymer is subjected to both a definite thermal
cycle and a definite shear cycle which vary considerably between the different methods of fabrication. The level of residual stresses in a
fabricated material depends upon the response of the material to this
combination of shear and thermal histories. They depend upon both the level of orientation induced at a specific point throughout the
fabrication cycle and the opportunity for its relaxation during the
remaining part of the cycle. The rate of relaxation at a particular temperature will also tend to decrease as the molecular weight increases, and,
in a given fabrication process,
both these factors will contribute
towards a higher level of residual stresses. The improvement in
mechanical properties usually observed with increasing molecular weight on isotropic specimens may not,
therefore, be observed in a fabricated
material, owing to this higher overall level of residual stresses .
An increase in molecular weight also results in an earlier onset of
melt flow defects such as melt fracture. The magnitude of these residual
stresses and flow defects can be minimized by increasing the temperature at which the deformation is carried out,
but this inevitably results in a
longer overall cycle which is economically undesirable. There are often
additional restrictions on the upper temperature that can be used owing to side effects such as crosslinking or chain scission,
development of odor,
color changes etc., resulting from volatilization of additives and oxidation, which may be detrimental to the final performance and/or the
appearance of the component. Some improvements can often be achieved by
modifying the width of the molecular weight distribution, as the rate of decrease in viscosity with increasing shear rate is higher for wide
Degradation of Polymeric Cover Plates The weathering which polymeric cover plates receive on solar
collectors frequently causes degradation and a resultant loss of mechanical properties. The weathering and environmental parameters that supply the energy necessary for cover plate deterioration are shown in table
The first four parameters,
and temperature, will be considered here.
Polymer degradation may be considered any type of chemical change of a polymer involving the main-chain backbone and/or side groups. These
require the breaking of primary valence bonds,
leading to lower
molecular weights, crosslinking, and cyclization. Although degradation is
Weather and Other Environmental Parameters Affecting Polymer Deterioration
0.29 to 0.^0 pm wavelength range of sunlight 2 or O 3 humidity, rain, snow, dew, frost, fog absolute value, thermal gradient, and thermal
UV Radiation Oxygen or Ozone Water Temperature
Active Chemical Environment Chemical Pollutants Particle Bombardment Cyclic Loading
a chemical process,
cycling heat transfer fluid contact S0 2 NO 2 and NaCl dust, sand, hail, bird droppings, and insects wind, snow loads ,
not only is the chemical composition of the polymer
modified, but also physical parameters such as MW, MWD, crystallinity,
chain flexibility, crosslinking, and branching which are governed by the chemical composition. The main processes taking place as dominant events during degradation are random chain scission, depolymerization, crosslinking, bond changes, and side group changes. The type of degradation occurring under a given set of circumstances is dependent upon the reaction rates. When multiple
weathering factors are present, several degradative processes can be
happening simultaneously. Chain scission involves bond breakage in the main backbone of the polymer. Each chain scission creates another molecule and lowers the MW.
polyester (e.g., PET or PC) is an example of a random
chain scission process since the susceptibility of the bond does not depend greatly on molecular size. Depolymerization is a chain scission
occurring in the polymer backbone, generally from an end. During
depolymerization molecules are repeatedly split off from the active end of the polymer chain reducing its length until the polymer molecule no longer
Crosslinking occurs when the reactive end of a broken polymer chain reacts with the backbone of another polymer chain forming a chemical link to the second chain.
Crosslinking entails a localized increase in density
which creates internal stresses and the development of microcracks that embrittle a material and increase the surface area open to chemical attack.
Changes in the polymer backbone can be induced without scission or crosslinking. For example, when PVC is heated, a chlorine atom and an
adjacent hydrogen atom react to form HC1. This changes the backbone
structure to form a double bond which is more susceptible to free radical attack than the fully saturated material. A zippering occurs leaving
behind successive conjugated double bonds. The remaining structure is
highly reactive with metal salts and with oxygen. Since the chain cannot rotate around double bonds,
it is stiffened even in the absence of
crosslinking . The atoms within side groups can react with each other to eliminate a molecule similar to the above reaction with PVC.
group changes such as these are least important in contributing to
degradation which affects the mechanical properties of polymers. The three most dominant processes occurring during degradation in a solar collector (i.e., chain scissions, bond changes in the main chain, and depolymerization) result in shortened polymer chains meaning fewer
entanglements per chain. This translates to less deformation resistance. On the other hand, crosslinking, which occurs less frequently,
the connections between chains and provides greater resistance to
deformation. During processing the material is subjected to very high
thermal and mechanical stresses. These high stresses may initiate a
vriety of polymer degradation processes leading to a deterioration of
properties even during processing. On the other hand,
the damaging of the
material may result in the introduction of various defects in the polymer
which will work as degradation sources during its subsequent service life. For example,
in the presence of oxygen traces during processing, carbonyl
groups can be formed. These will function as built in sensitizers for
photodegradation processes . Because of the dual nature of degradation during processing,
deterioration and the introduction of
defects as potential sources of deterioration,
it is very important to add
stabilizers to the polymer before processing. Polymer degradation can occur either at the surface or in the bulk.
Degradation from photooxidation, thermal oxidation, and hydrolysis are more severe at the surfaces. Damage at the surfaces occurs more readily
through the combination of degradation mechanisms. Thus,
cover plate degradation, the surface exposed to the sun will undoubtedly suffer the most degradation because it is exposed to the greatest
combination of factors. 0 II
Cover plate materials having an ester linkage (-C-0-) (i.e., PC, PET, and FRP) are subject to hydrolysis at the exposed surface which results in
lowering of the MW. In semicrystalline materials, such as PET, hydrolysis
occurs preferentially in amorphous regions. This is undoubtedly one of the reasons PET is reported to become brittle so rapidly . Surface
cracking has been reported in PC [44,69,70], PET [66,71], and FRP [66,70-74]. The thickness of PC and FRP as cover plates is greater, whereas PET is about 0.13 mm. Thus,
if surface degradation took
place to an equal depth in all three, the PET would show loss of
mechanical properties more rapidly because of the higher ratio of the volume of the damaged material to the total volume of material. The
number of chains available to resist deformation is drastically reduced and fracture undoubtedly occurs through the amorphous regions where the tie chains linking the crystallites have been severed.
The major cause of degradation in PMMA is chain scission caused by sunlight. This occurs at the outer surface and results in a reduction in MW.
PC degrades through hydrolysis and photooxidation. Weathering studies
of PC have shown a loss of MW and tensile properties [J|4]. Figure 7 shows
the downward shift in molecular weight distribution of PC with longer
periods of weathering. Changes in elongation at break and elongation at
yield of PC exposed to outdoor and natural weathering are illustrated in figure 18. The decline in elongations parallels the reduction in MW seen in figure 7.
PET degrades through both photooxidation and hydrolysis, and loses
strength rapidly when exposed outdoors . PVF and PVDF are fairly resistant to degradation from sunlight. Thermal degradation is probably the most important degradation mechanism. A likely mechanism of thermal
degradation is abstraction of HF molecules in a manner similar to that
Changes in net elongation at break and elongation at yield of on exposure to outdoor and artificial weathering conditions [H4].
described for PVC earlier, the result is a reduction in MW. Loss in tensile strength and elongation at break for PVF have been reported . The most common mechanisms of degradation affecting cover plate
materials result in a reduction of molecular weight. Most frequently this
degradation occurs at the surface. On a molecular scale, this means that the polymer molecules at the surface are less entangled because they are
shorter than those in the bulk polymer, or not entangled at all if their length is short enough.
(Short molecules may simply volatilize).
Consequently the surface region is less capable of resisting deformation. On a macro scale, cover plate polymers can be subjected to small
deformations on a regular schedule, such as daily thermal expansion and contraction, swelling through absorption of moisture followed by
contraction at its loss, or motions due to wind. While these seem small on a macroscopic level,
at the microscopic level these forces are applying
a cyclic stress to the polymer.
When the MW is reduced so that it
contains few entanglements and offers little resistance to deformation, small local regions readily deform and can initiate a microcrack at the surface. With additional degradation, the deforming stress is then
concentrated at the crack tip and spread over a smaller number of chains than previously, resulting in continued crack growth.
Fukushima  has shown that the deterioration of polymer materials by the simultaneous action of UV-light and diffusive oxygen is a function of depth from the surface and increases in proportion to the square root of time. Thus,
monitoring the MWD (i.e., loss of long molecules and gain
of short molecules) as a function from the surface should show a
degradation profile which can be related to a decline in mechanical properties.
Probably the single greatest consideration with respect to mechanical properties of polymeric cover plates is the fact that they decline during use due to degradation caused by weathering. Degradation is a chemical
process affecting the composition and other physical parameters. The
mechanical properties of polymers are dependent on a number of factors, some of which are inherent in the specific polymer while others are
induced at the time of processing. The key factors relating to the
glass transition temperature, molecular weight and molecular
weight distribution, crystallinity, crosslinking, copolymerization, and plasticization. These factors are not independent variables, rather their
effects are frequently interactive and it is difficult to isolate the effects of each individually. Fabrication of the polymer into a product may introduce orientation, anisotropic (unwanted) residual stresses, flaws,
melt flaw defects,
or weak reactive sites in the chain structure.
These factors are difficult to measure and will vary, depending upon the specific process involved, processing temperatures, and the polymer
characteristics. 8.2 Polymer Microstructure
The glass transition temperature is the most important material
characteristic of a polymer as far as mechanical properties are concerned because it determines whether a cover material reacts to an applied force in a brittle or ductile manner.
is related to the flexibility of
the polymer chain structure and the presence of plasticizers. Plasticizer
added during processing may be lost during use by volatilization due to heat or by leaching out by liquids. In some cases moisture or a
chemically active environment may plasticize lower the T
cover plate material and
The glass transition temperature can be measured in a
variety of ways including dilatometry, refractometry, differential thermal
differential scanning calorimetry (DSC),
spectroscopy, nuclear magnetic resonance (NMR), stress birefringence,
dielectric constant and loss, creep and stress relaxation, and dynamic
76— 78 ]
The molecular weight and molecular weight distribution are the most
significant microstructural features influencing deformation. Higher molecular weight provides more strength through more entanglements in the
amorphous material and through tie chains between crystallites in
semicrystalline materials. The molecular weight of cover materials is reduced by polymer degradation,
bond breaking, due to UV, heat,
moisture, or mechanical stress during processing. The average molecular
weight can be determined by many methods, intrinsic viscosity,
the more common of which are
light scattering, osmotic pressure, and gel
permeation chromatography [77,78]. These methods require the polymer to be dissolved in a solvent which is difficult for some polymers. An
alternative method for determining molecular weight, which does not require solvent,
is a melt viscosity measurement .
This method is
actually more sensitive in identifying small changes in average molecular weight. The most common method for determining molecular weight
distribution is gel permeation chromatography [77,78] which separates the polymer molecules by size. In some cases it may be very difficult to
determine the molecular weight of cover plate materials if the material is not readily soluble or if the melt viscosity is too high to be measured in the MW of
the temperature range of the melt rheometer. For example,
poly(vinyl fluoride) is estimated by its producers at MW
because of its high MW and the difficulty in dissolving the polymer. The tendency for a polymer to crystallize is influenced by the regularity and
flexibility of the polymer chain. The size of the crystallites and the
extent of crystallization Is generally determined by the rate of cooling,
although orientation can also produce crystallization. The film cover plate materials are semicrystalline i.e,, PVF, PVDF, FEP, and PET.
Crystallinity provides the film with strength and toughness. Crystallites contribute to deformation resistance by dissipating energy during deformation, but they probably also reform into a strong crystalline
structure. Incomplete crystallization at the time of fabrication can lead to problems for cover plate materials later if the temperature is raised
or the Tg lowered to a level where polymer crystallization can occur.
crystallization would result in a small volume change and induce stress into the material. Crystallinity is detected and its amount estimated by a number of methods:
polarized light microscopy, density, calorimetry, differential thermal analysis,
and nuclear magnetic resonance spectroscopy
76 - 78 ].
Crosslinking occurs either at the time of processing or through polymer degradation. With the exception of the fiber reinforced polyester,
the polymeric cover plates contain little,
at the time of processing,
if any crosslinking
although degradation may induce some
crosslinking at the surface. FRP is a thermoset material which is highly crosslinked. The degree of crosslinking dictates the rigidity of a polymer. Heavily crosslinked materials such as FRP are rigid and may be brittle, whereas lightly crosslinked materials are rubbery. Degradation of most polymers is likely to induce only light crosslinking. Because
crosslinking essentially forms a large polymer network, crosslinked materials are insoluble, however they will swell when exposed to an
appropriate solvent. Crosslinking is measured through swelling experiments or by analysis of the stress-strain curve of a polymer network [78,79]. This latter technique is not appropriate to characterize surface
crosslinking as may occur in cover plate degradation.
Fluorinated ethylene propylene is the only cover plate material which is a copolymer.
Copolymerization is generally accomplished to selectively
control some property of the end product. Once a copolymer is formed,
can be characterized in the same manner as other polymers.
Copolymerization offers one possibility of obtaining different materials for cover plates. By judicious choice of the monomers involved,
it may be
possible to design a copolymer with specific desired properties.
Plasticizers are added to polymers to lower Tg an(j to increase the flexibility and toughness of the material. Cover plate materials probably have very little plasticizer added since plasticizers can be driven out by
elevated temperatures. Water,
in the form of humidity or moisture which
collects on cover materials may be absorbed into the surface layer and act as a plasticizer that enhances the yielding processes which occur during
degradation. Blaga  has postulated a theory for the mechanism of
surface degradation of plastic based sheets, whereby surface microcracks are produced by differential dimensional strains induced by humidity and
in conjunction with resin degradation.
Stresses in the cover plate, such as would result from orientation or from anisotropy introduced during processing, can be measured with birefringence,
infrared dichroism, and polarization of fluorescence .
8.3 Methods to Study Polymer Degradation and Deformation
On a molecular level, cover plate degradation results in chain
scission in the polymer backbone, chemical reactions, e.g., oxidation of the polymer, or crosslinking. In general,
the main degradation mechanisms
for each cover plate material are known, although much more specific
knowledge is needed regarding initiation mechanisms, rates of degradation, and synergistic effects of multiple causative factors. Many methods are
available to study various aspects of polymer degradation.
Changes in the polymer stability as a function of temperature can be
monitored using thermogravimetry (TG), DTA, DSC, and thermal volatilization analysis (TVA) including evolved gas detection and analysis . Volatile products from polymer degradation can be studied using gas
chromatography (GC), gas liquid chromatography, mass spectrometry (MS), and a combination of GC and MS .
Chemical techniques capable of detecting chemical changes related to cover plate degradation include Fourier Transform Infrared Analysis (
Electron Spectroscopy for Chemical Analysis (ESCA), X-ray
Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Chemiluminescence (Cl), and GC/MS . The morphological features of surface degradation can also be studied using both optical and electron microscopy. In cover plate materials,
degradation has been exhibited as either
cracking and crazing of the surface toward the sun or embrittlement of the bulk material eventually resulting in fracture. The surface degradation
relates to a reduction in the molecular weight of the surface layers
accompanied by yielding on a molecular level to permit cracking and
crazing to occur. The cracks and crazes gradually grow from a molecular to a microscopic level and finally to a visible level.
Their growth may
well be enhanced by plasticization from moisture or other chemically
active environment, or by fatigue,
for example from thermal expansion and
contraction or wind. The deformation mechanisms of polymers in general have been described in detail in Section 5.1. Polymer deformation involves molecular fracture and experimental techniques are available to
study all phases of fracture. Chain scission can be followed using ESR
which allows direct measurement of the number and nature of free radicals formed when bonds are broken. The reduction of molar mass due to bond
breakage and the increase in concentration of end groups as measured
can also be used to follow molecular fracture. The formation and growth of microvoids from the accumulation of chain ends and the coalescence of
these voids into crazes and microcracks can be followed using small-angle
x-ray scattering * The concepts of fracture mechanics provide a sound basis on which to
describe polymer fracture, however,
little has been done in this area on
polymers used in cover plates. In some areas, such as slow crack growth in glassy polymers, in other areas, is little.
there is quite a bit of information, while
such as environmental crack growth and fatigue life,
Ductile failure is the most difficult to characterize, and
most cover plate materials, other than PMMA,
are ductile. Information is
scarce in the study of the growth of flaws or cracks in the polymers used as cover materials.
Failure of these semicrystalline materials is complex
due to the multiple mechanisms of failure. It is complicated by yielding and reformation of crystals after an initial deformation. Study of the
growth of cracks and flaws in cover plate materials needs more attention. 9
Summary The mechanical properties of cover plate polymeric materials do not
remain constant during exposure on a solar collector, rather they decline as the polymer ages. The mechanical properties are a measure of the
deformation resistance of the polymer under an applied force. Polymer deformation is a complex phenomenon which is influenced by many factors including type of applied stress, polymer,
temperature, characteristics of the
and uniformity of properties within the polymer. The decrease in
cover plate mechanical properties is a result of polymer degradation which occurs at a molecular level. Mechanical property tests are a measure of the bulk material,
and appreciable degradation must occur prior to
measurable change in test results. Since the decline in mechanical properties results from polymer degradation, more direct measurement of
polymer material properties at the microstructural level would provide an indication of whether the material is changing and whether subsequent
changes in mechanical properties might be expected. Monitoring
microstructural properties of cover plate materials from the new material through various stages of degradation would provide degradation rate data
which would aid in the prediction of service life of polymer cover plates. Unfortunately, the state of the art of structure-property knowledge for
polymers in general is not sufficiently well developed to permit this at this time. However, a number of techniques are available to study cover
plate materials at the molecular level to begin to build the body of
knowledge necessary for accurate service life prediction. Application of modern instrumentation to study microstructure, chemical degradation, and polymer deformation and failure mechanisms will aid in developing more exact relationships between the microstructure of cover plate materials and their mechanical properties.
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2 « 80
DEPT. OF COMM.
Performing Organ. Report NoJ
REPORT NO. NBSIR 85-3197
BIBLIOGRAPHIC DATA SHEET (See instructions) TITLE AND SUBTITLE
Molecular and Microstructural Factors Affecting Mechanical Properties of Polymeric Cover Plate Materials
or other than
national bureau of standards DEPARTMENT OF COMMERCE WASHINGTON, D.C. 20234 9.
SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS
& Period Covered
(Street. City . State. ZIP)
Department of Energy Washington, DC 20585 11.
a computer program; SF-185,
FIPS Software Summary,
(A 200-word or less factual summary of most significant information. bibliography or literature survey, mention it here)
This paper reviews the dependence of mechanical properties of polymers The microstructural and molecular on various microstructural factors. molecular weight, crystallinity, crosslinking, factors considered are: residual branching, copolymerization, plasticization, orientation, and direct The types of mechanical properties considered are: stresses. stress environmental and abrasion, and wear creep, loading, fatigue, The effects of polymer deformation and fraction at the cracking. Cracking, crazing, and shear yielding are molecular level are discussed. Polymeric cover plate materials are discussed and their described. degradation reviewed. Methods to measure microlevel changes in polymers are identified.
KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by degradation; polymers; mechanical properties; microstructure; service life
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