Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering

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known as plasticizers. When PVC contains these compounds, the glass temperature is

lowered. This makes PVC more ductile and workable and is known as vinyl (not to be

confused with the vinyl group mentioned here and in other places). PVC is used to

make three-ring binders, pipes, tiles, and clear Tygon

tubing.

In polytetraﬂuoroethylene (PTFE or Teﬂon

), all four hydrogen atoms in the

polyethylene structure are replaced by ﬂuorine. The monomer again is symmetrical, and

the strength of the polymer is not much greater than that of polyethylene. However, the

C-F bond permits PTFE to have a high melting point with the added beneﬁt of low

friction, nonstick characteristics that make the polymer useful for bearings and cook-

ware. Teﬂon

was invented by accident by Roy Plunkett, who was working with

tetraﬂuoroethylene gas. He found a tetraﬂuoroethylene gas cylinder that had no pres-

sure (and, thus, seemed empty) but was still heavy. The gas inside had polymerized into

solid Teﬂon

Branching prevents dense packing of the chains, thereby reducing the density, sti¤-

ness, and strength of the polymer. Low-density (LD) polyethylene, which has many

branches, is weaker than high-density (HD) polyethylene, which has virtually no

branching (Table 16-2).

Crystallization and Deformation Crystallinity is important in polymers since it a¤ects

mechanical and optical properties. Crystallinity evolves in the processing of polymers

as a result of temperature changes and applied stress (e.g., formation of PET bottles). If

crystalline regions become too large, they begin to scatter light and make the plastic

translucent. Of course, in certain special polymers, localized regions crystallize in re-

sponse to an applied electric ﬁeld and this is the princi ple by which the liquid crystal

displays work. As we have discussed previously, encouraging crystallization of the pol-

ymer also helps to increase density, resistance to chemical attack, and mechanical

properties—even at higher temperatures—because of the stronger bonding between the

chains. In additi on, deformation straightens and aligns the chains, producing a pre-

ferred orientation. Deformation of a polymer is often used in producing ﬁbers having

mechanical properties in the direction of the ﬁber that exceed those of many metals and

ceramics. This texture strengthening (Chapter 8), in fact, played a key role in the dis-

covery of nylon ﬁbers. During their processing, PET bottles develop a biaxial texture

and strength along the radial and length direction.

Tacticity When a polymer is formed from nonsymmetrical repeat units, the structure

and properties are determined by the location of the nonsymmetrical atoms or atom

groups. This condition is called tacticity, or stereoisomerism. In the syndiotactic ar-

rangement, the atoms or atom groups alternatively occupy positions on opposite sides

of the linear chain. The atoms are all on the same side of the chain in isotactic polymers,

whereas the arrangement of the atoms is random in atactic polymers (Figure 16-7).

The atactic structure, which is the least regular and least predictable , tends to give

poor packing, low density, low strength and sti¤ness, and poor resistance to heat or

chemical attack. Atactic polymers are more likely to have an amorphous structure with

a relatively high glass temperature. An important example of the importance of tactic-

ity occurs in polypropylene. Atactic polypropylene is an amorphous wax-like polymer

with poor mechanical properties, whereas isotactic polypropylene may crystallize and is

one of the most widely used commercial polymers.

Copolymers Similar to the concept of solid solutions or the idea of composites, linear

addition chains composed of two or more types of molecules can be arranged to form

copolymers. This is a very powerful way to blend properties of di¤erent polymers. The

arrangement of the monomers in a copolymer may take several forms (Figure 16-8).

CHAPTER 16 Polymers510

Figure 16-7

Three possible arrangements of

nonsymmetrical monomers: (a) isotactic,

(b) syndiotactic, and (c) atactic.

Figure 16-8 Four types of copolymers: (a) alternating monomers, (b) random monomers,

different monomers.

16-5 Structure–Property Relationships in Thermoplastics 511

These include alternating, random, block, and grafted copolymers. A BS, composed of

acrylonitrile, butadiene (a synthetic elastomer), and styrene, is one of the most common

polymer materials (Figure 16-9). Styrene and acrylonitrile form a linear copolymer

(SAN) that serves as a matrix. Styrene and butadiene also form a linear copolymer, BS

rubber, which acts as the ﬁller material. The combination of the two copolymers gives

ABS an excellent combination of strength, rigidity, and toughness. Another common

copolymer contains repeat units of ethylene and propylene. Whereas polyethylene and

polypropylene are both easily crystallized, the copolymer remains amorphous. When

this copolymer is cross-linked, it behaves as an elastomer. Dylark

is a copolymer of

maleic anhydride and styrene. Styrene provides toughness, while maleic anhydride pro-

vides high-temperature properties. Carbon black (for protection from ultraviolet rays

and enhancing sti¤ness), rubber (for toughness), and glass ﬁbers (for sti¤ness) are added

to the Dylark

copolymer. It is used to make instrument panels for car dashboards.

The Dylark

plastic is then coated with vinyl that provides a smooth and soft ﬁnish.

Blending and Alloying We can improve the mechanical properties of many of the

thermoplastics by blending or alloying. By mixing an immiscible elastomer with the

thermoplastic, we produce a two-phase polymer, as we found in ABS. The elastomer

does not enter the structure as a copolymer but, instead, helps to absorb energy and

improve toughness. Polycarbonates used to produce transparent aircraft canopies are

also toughened by elastomers in this manner.

Liquid Crystalline Polymers Some of the complex thermoplastic chains become so sti¤

that they act as rigid rods, even when heated above the melting point. These materials

are liquid crystalline polymers (LCPs). Some aromatic polyesters and aromati c poly-

amides (or aramids) are examples of liquid crystalline polymers and are used as high-

strength ﬁbers (as to be discussed in Chapter 17). Kevlar

, an aromatic polyamide, is

the most familiar of the LCPs and is used as a reinforcing ﬁber for aerospace applica-

tions and for bulletproof vests. Liquid crystal polymers are, of course , used to make

electronic displays.

16-6 Effect of Temperature on Thermoplastics

Properties of thermoplastics change depending upon temperature. We need to know

how these changes occur because this can help us (a) better design components, and (b)

guide the type of processing techniques that need to be used. Several critical temper-

atures and structures, summarized in Figures 16-10 and 16-11, may be observed.

Thermoplastics can be amorphous or crystalline once they cool below the melting

temperature, (Figure 16-10). Most often, engineered thermoplastics consist of regions

Figure 16-9 Copolymerization produces the polymer ABS, which is really made up of two

copolymers, SAN and BS, grafted together.

CHAPTER 16 Polymers512

that are amorphous and crystalline. The crystallinity in thermoplastics can be in-

troduced by temperature (slow cooling) or by stress-induced crystallization (the appli-

cation of stress that can untangle chains, Chapter 7). Similar to dispersion strengthen-

ing of metallic materials, the formation of crystalline regions in an otherwise

amorphous matrix helps increase the strength of thermoplastics. In typical thermo-

plastics, bonding within the chains is covalent, but the long coiled chains are held to

one another by weak van der Waals bonds and by entanglement. When a tensile stress

is applied to the thermoplastic, the weak bonding between the chains can be overcome

and the chains can rotate and slide relative to one another. The ease with which the

chains slide depends on both temperature and the polymer structure.

Figure 16-10 The effect of temperature on the structure and behavior of thermoplastics.

Figure 16-11

The effect of temperature on the

modulus of elasticity for an

amorphous thermoplastic. Note that

and T

are not fixed.

16-6 Effect of Temperature on Thermoplastics 513

Degradation Temperature At very high temperatures, the covalent bonds between the

atoms in the linear chain may be destroyed, and the polymer may burn or char. In

thermoplastics decompositio n occurs in the liquid state, in thermosets the decom-

position occurs in the solid state. This temperature T

(not shown in Figure 16-11), is

the degradation (or decomposition) temperature. When plastics burn, they create smoke,

and that is dangerous. Some materials (such as limestone, talc, alumina etc.) added to

thermoplastics are thermal or heat stabilizers. They absorb heat and protect the poly-

mer matrix. Fire retardant additives, such as hydrated alumina, antimony compounds,

or halogen compounds (e.g., MgBr, PCl

) are added to retard the ﬂammability of poly-

mers. Some additives retard ﬁre by excluding oxygen but generate dangerous gases and

are not appropriate for certain applications.

Exposure to other forms of chemicals or energy (e.g., oxygen, ultraviolet radiation,

and attack by bacteria) also cause a polymer to degrade or age slowly, even at low

temperatures. Carbon black (up to @3%) is one of the commonly used additives that

helps improve the resistance of plastics to ultraviolet degradation.

Liquid Polymers Thermop lastics usually do not melt at a precise temperature. Instead

there is usually a range of temperatures over which melting occurs. The approximate

melting ranges of typical polymers are included in Table 16-5. At or above the melting

temperature T

, bonding between the twisted and intertwined chains is weak. If a force

is applied, the cha ins slide past one another and the polymer ﬂows with virtually no

TABLE 16-5 9 Melting, glass, and processing temperature ranges (

C) for selected thermoplastics

and elastomers

Polymer

Melting

Temperature

Range

Glass

Temperature

Range (T

)

Processing

Temperature

Range

Addition polymers

Low-density (LD) polyethylene 98–115 90 to 25 149–232

High-density (HD) polyethylene 130–137 110 177–260

Polyvinyl chloride 175–212 87

Polypropylene 160–180 25 to  20 190–288

Polystyrene 240 85–125

Polyacrylonitrile 320 107

Polytetrafluoroethylene (Teflon) 327

Polychlorotrifluoroethylene 220

Polymethyl methacrylate (acrylic) 90–105

Acrylonitrile butadiene styrene (ABS) 110–125 100 177–260

Condensation polymers

Acetal 181 85

6,6-nylon 243–260 49 260–327

Cellulose acetate 230

Polycarbonate 230 149 271–300

Polyester 255 75

Polyethylene terephthalate (PET) 212–265 66–80 227–349

Elastomers

Silicone 123

Polybutadiene 120 90

Polychloroprene 80 50

Polyisoprene 30 73

CHAPTER 16 Polymers514

elastic strain. The strength and modulus of elasticity are nearly zero and the polymer is

suitable for casting and many forming processes. Most thermoplastic melts are shear

thinning (i.e., their apparent viscosity decreases within an increase in the steady-state

shear rate).

Rubbery and Leathery States Below the melting temperature, the polymer chains are

still twisted and intertwined. These polymers hav e an amorphous structure. Just below

the melting temperature, the polymer behaves in a rubbery manner. When stress is

applied, both elastic and plastic deformation of the polymer occurs. When the stress is

removed, the elastic deformation is quickly recover ed, but the polymer is permanently

deformed due to the movement of the chains. Some of this deformation is recovered

over a period of time. Thus, many polymers exhibit a viscoelastic behavior (Chapter 6).

Large permanent elongations can be achieved, permitting the polymer to be formed

into useful shapes by molding and extrusion.

At lower temperatures, bonding between the chains is stronger, the polymer

becomes sti¤er and stronger, and a leathery behavior is observed. Many of the com-

mercial polymers, including polyethylene, have a useable strength in this condition.

Glassy State Below the glass temperature T

, the linear amorphous polymer becomes

hard, brittle, and glass-like. This is again not a ﬁxed temperature but a range of tem-

peratures. When the polymer cools below the glass temperature, certain properties—

such as density or modulus of elasticity—change at a di¤erent rate (Figure 16-12).

Although glassy polymers have poor ductility and formability, they do have good

strength, sti¤ness, and creep resistance. A number of important polymers, including

polystyrene and polyvinyl chloride, have glass temperatures above room temperature

(Table 16-5).

The glass temperature is typically about 0.5 to 0.75 times the absolute melting

temperature T

. Polymers such as polyethylene, which have no complicated side groups

attached to the carbon backbone, have low glass temperatures (even below room tem-

perature) compared with polymers such as polystyrene, which have more complicated

side groups.

As pointed out in Chapter 6, many thermoplasti cs become brittle at lower temper-

atures. The brittleness of the polymer used for some of the O-rings ultimately cau sed

the 1986 Challenger disaster. The lower temperatures that existed during the launch

time caused the embrittlement of the rubber O-rings used for the booster rockets.

Figure 16-12

The relationship between the density

and the temperature of the polymer

shows the melting and glass

temperatures. Note that T

and T

are not fixed; rather, they are ranges

of temperatures.

16-6 Effect of Temperature on Thermoplastics 515

Observing and Measuring Crystallinity in Polymers Many thermoplastics partially

crystallize when cooled below the melting temperature, with the chains becoming

closely aligned over appreciable distances. A sharp increase in the density occurs as the

coiled and intertwined chains in the liquid are rearranged into a more orderly, close-

packed structure (Figure 16-12).

One model describing the arrangement of the chains in a crystalline polymer is

shown in Figure 16-13. In this folded chain model, the chains loop back on themselves,

with each loop being approximately 100 carbon atoms long. The folded chain extends

in three dimensions, producing thin plates or lamellae. The crystals can take various

forms, with the spherulitic shape shown in Figure 16-14(a) being particularly common.

The crystals have a unit cell that describes the regular packing of the chains. The crystal

structure for polyethylene, shown in Figure 16-14(b), describes one such unit cell.

Crystal structures for several polymers are described in Table 16-6. Some polymers are

polymorphic, having more than one crystal structure.

Even in crystalline polymers, there are always thin regions between the lamellae, as

well as between spherulites, that are amorphous transition zones. The weight percentage

Figure 16-13

The folded chain model for

crystallinity in polymers, shown in

(a) two dimensions and (b) three

dimensions.

Figure 16-14 (a) Photograph of spherulitic crystals in an amorphous matrix of nylon (200).

(Source: From R. Brick, A. Pense and R. Gordon , Structure and Properties of Engineering

Materials, 4th Ed., McGraw-Hill, 1997.) (b) The unit cell of crystalline polyethylene.

TABLE 16-6 9 Crystal structures of several polymers

Polymer Crystal Structure Lattice Parameters (nm)

Polyethylene Orthorhombic a ¼ 0:742 b ¼ 0:495 c ¼ 0:255

Polypropylene Orthorhombic a ¼ 1:450 b ¼ 0:569 c ¼ 0:740

Polyvinyl chloride Orthorhombic a ¼ 1:040 b ¼ 0:530 c ¼ 0:510

Polyisoprene (cis) Orthorhombic a ¼ 1:246 b ¼ 0 :886 c ¼ 0:810

CHAPTER 16 Polymers516

of the structure that is crystalline can be calculated from the density of the polymer:

% Crystalline ¼

ðr  r

ðr

 r

 100 ð16-4Þ

where r is the measured density of the polymer, r

is the density of amorphous poly-

mer, and r

is the density of completely crystalline polymer. Similarly, x-ray di¤raction

(XRD) can be used to measure the level of crystallinity and dete rmination of lattice

constants for single crystal polymers.

As the side groups get more complex, it becomes harder to crystallize thermo-

plastics. For example, polyethylene (H as side group) can be crystallized more easily

than polystyrene (benzene ring as side group). High-density polyethylene (HDPE) has a

higher level of crystallinty and, therefore, a higher density (0.97 g/cc). Low-density

polyethylene (LDPE) has a density of 0.92 g/cc. The crystallinity and, hence, the den-

sity in LDPE is lower, since the polymer is branched. Thus, branched polymers show

lower levels of crystallinity. A completely crystalline polymer would not display a glass

temperature; however, the amorphous regions in semicrystalline polymers do change to

a glassy material below the glass temperature (Figure 16-12). Such polymers as acetal,

nylon, HDPE, and polypropylene are referred to as crystalline even though the level of

crystallinity may be moderate. Sometimes, the trade names can be confusing. For ex-

ample, what is described as a ‘‘crystal polystyrene’’ is actually an amorphous material.

It looks transparent and shiny though, and hence, the trade name crystal polystyrene.

The following examples show how properties of plastics can be accounted for in di¤er-

ent applications.

EXAMPLE 16-6

Design of a Polymer Insulation Material

A storage tank for liquid hydrogen will be made of metal, but we wish to coat

the metal with a 3-mm thickness of a polymer as an intermediate layer between

the metal and additional insulation layers. The temperature of the intermediate

layer may drop to 80



C. Design a material for this layer.

SOLUTION

We want the material to have reasonable ductility. As the temperature of the

tank changes, stresses develop in the coating due to di¤erences in thermal

expansion, and we do not want the polymer to fail due to these stresses.

A material that has good ductility and/or can undergo large elastic strains is

needed. We therefore would prefer either a thermoplastic that has a glass tem-

perature below 80



C or an elastomer, also with a glass temperature below

80



C. Of the polymers listed in Table 16-2, thermoplastics such as poly-

ethylene and acetal are satisfactory. Suitable elastomers include silicone and

polybutadiene.

We might prefer one of the elastomers, for they can accommodate thermal

stress by elastic, rather than plastic, deformation.

EXAMPLE 16-7 Impact-Resistant Polyethy lene

A new grade of ﬂexible, impact-resistant polyethylene for use as a thin ﬁlm

requires a density of 0.88 to 0.915 g/cm

. Design the polyethylene required

to produce these properties. The density of amorphous polyethylene is about

0.87 g/cm

16-6 Effect of Temperature on Thermoplastics 517

SOLUTION

To produce the required properties and density, we must control the percent

crystallinity of the polyethylene. We can use Equation 16-4 to determine the

crystallinity that corresponds to the required density range. To do so, however,

we must know the density of completely crystalline polyethylene. We can use

the data in Table 16-3 to calculate this density if we recognize that there are

two polyethylene repeat units in each unit cell:

ð4CÞð12Þþð8HÞð1Þ

ð7:42Þð4:95Þð2:55Þð10

24

Þð6:02  10

¼ 0:9932 g=cm

We know that r

¼ 0:87 g/cm

and that r varies from 0.88 to 0.915 g/cm

. The

required crystallinity then varies from:

% crystalline ¼

ð0:9932Þð0:88  0:87Þ

ð0:88Þð0:9932  0:87Þ

 100 ¼ 9: 2

% crystalline ¼

ð0:9932Þð0:915  0:87Þ

ð0:915Þð0:9932  0:87Þ

 100 ¼ 39:6

Therefore, we must be able to process the polyethylene to produce a range of

crystallinity between 9.2 and 39.6%.

16-7 Mechanical Properties of Thermoplastics

Most thermopl astics (molten and solid) exhibit a non-Newtonian and viscoelastic

behavior. The behavior is non-Newtonian (i.e., the stress and strain are not linearly re-

lated for most parts of the stress-strain curve). The viscoelastic behavior means when an

external force is applied to a thermoplastic polymer, both elastic and plastic (or viscous)

deformation occurs. The mechanical behavio r is closely tied to the manner in which the

polymer chains move relative to one another under load. Deformation is more compli-

cated in thermoplastics. The deformation process depends on both time and the rate at

which the load is applied. Figure 16-15 shows a stress-strain curve for 6,6-nylon.

Figure 16-15

The stress-strain curve for 6,6-nylon,

a typical thermoplastic polymer.

CHAPTER 16 Polymers518

Elastic Behavior Elastic deformation in thermoplastics is the result of two mecha-

nisms. An applied stress cau ses the covalent bonds within the chain to stretch and dis-

tort, allowing the chains to elongate el astically. When the stress is removed, recovery

from this distortion is almost instantaneous. This behavior is similar to that in metals

and ceramics, which also deform elastically by the stretching of metallic, ionic, or co-

valent bonds. But in addition, entire segments of the polymer chains may be distorted;

when the stress is removed, the segments move back to their original positions only over

a period of time—often hours or even months. This time-dependent, or viscoelastic,

behavior may contribute to some nonlinear elastic behavior.

Plastic Behavior of Amorphous Thermoplastics These polymers deform plastically

when the stress exceeds the yield strength. Unlike deformation in the case of metals,

however, plastic deformation is not a consequence of dislocation movement. Instead,

chains stretch, rotate, slide, and disentangle under load to cause permanent deforma-

tion. The drop in the stress beyond the yield point can be explained by this phenom-

enon. Initially, the chains may be highly tangled and intertwined. When the stress is

su‰ciently high, the chains begin to untangle and straighten. Necking also occurs, per-

mitting continued sliding of the chains at a lesser stress. Eventually, however, the chains

become almost parallel and close together; stronger van der Waals bonding between the

more closely aligned chains requires higher stresses to complete the deformation and

fracture process (Figure 16-16). This type of crystallization due to orientation played an

important role in the discovery of nylon as a material to make strong ﬁbers.

Figure 16-16 Necks are not stable in amorphous polymers, because local alignment

strengthens the necked region and reduces its rate of deformation.

EXAMPLE 16-8

Comparing Mechanical Properties of Thermoplastics

Compare the mechan ical propertie s of LD polyethylene, HD polyethy lene,

polyvinyl chloride, polypropylene, and polystyrene, and explain their di¤er-

ences in terms of their structures.

16-7 Mechanical Properties of Thermoplastics 519

Askeland D.R., Fulay P.P. Essentials of Materials Science &amp; Engineering

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Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering