Page 211 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012c-598 July 26, 2001 15:59

720 Polymers, Mechanical Behavior

TABLE II Comparison of Moduli for Several Different Types of Materials a
Specific properties
Shear Bulk
6
6
Poisson’s Young’s modulus, G modulus, B Density, ρ 10 E/ρ 10 G/ρ
2
2
3
2
2
Material ratio, µ modulus, G (GPa) (Gpa) (g cm ) (m /sec ) (m /sec )
Metals
Cast iron 0.27 90 35 66 7.5 12.0 4.7
Steel (mild) 0.28 220 86 166 7.8 28.0 11.0
Aluminum 0.33 70 26 70 2.7 26.0 9.6
Copper 0.35 120 45 134 8.9 13.5 4.5
Lead 0.43 15 5.3 36 11.0 13.6 4.8
Inorganics
Quartz 0.07 100 47 39 2.65 38.0 17.8
Vitreous silica 0.14 70 31 33 2.20 32.0 14.0
Glass 0.23 60 25 37 2.5 24.0 9.8
Polymers
Polystyrene 0.33 3.2 1.2 3.0 1.05 3.05 1.15
Poly(methyl methacrylate) 0.33 4.15 1.55 4.1 1.17 3.55 1.33
Nylon-6,6 0.33 2.35 0.85 3.3 1.08 2.21 0.79
Polyethylene (low density) 0.45 1.0 0.35 3.33 0.91 1.1 0.385
Ebonite 0.39 2.7 0.97 4.1 1.15 2.35 0.86
Rubber 0.49 0.002 0.0007 0.033 0.91 0.002 0.00075
a
Also included are Poisson’s ratio and density. The last two columns show the specific Young’s and shear moduli.

contrast to 1 to 3 GPa for compression-molded polyethy- that any property that is determined, when normalized on
lene of the same species (i.e., unoriented). The value of the basis of the density of the material, provides what
the modulus just quoted is actually greater than what is is known as a specific property such as specific modu-
presently achieved with high-strength/high-modulus liq- lus. This normalization procedure therefore allows com-
uid crystalline polymeric fibers such as poly-p-aromatic parisons between all types of structural materials and an
amide systems. Again it is apparent that molecular ori- example of this is illustrated in Table II, which provides
entation can strongly influence mechanical parameters. comparisons of polymers with inorganics as well as metal-
In addition, an awareness of any previous orientation in- lic systems. As is noted from Table II, which contains
ducement in a system before its measurement may be the parameters of Poisson’s ratio, Young’s modulus, shear
very important in deciding how to test the final material. modulus, bulk modulus, density, and the specific Young’s
In multiphase or multicomponent systems, the level of modulus and specific shear modulus, the effects of the
orientation is not necessarily equal in each phase (i.e., normalization procedure bring many of these materials
crystalline versus amorphous), and hence if orientation in line with one another relative to their non-normalized
is being taken account of, component or phase orienta- moduli values. When orientation in addition is taken into
tion should be considered rather than necessarily a sys- account for polymeric systems, which can drastically alter
tem average. Indeed, in oriented semicrystalline systems, the modulus as referred to earlier in this section, the spe-
typically the crystal phase is much more easily oriented cific moduli can distinctly exceed that of many metallics
than that of the remaining amorphous phase. In fact, for or inorganics as is illustrated in Fig. 28. Here one notes
®
fully amorphous systems, the orientation levels obtained that the material designated as Spectra , which is a very
are not particularly high by conventional processes. How- highly oriented and highly crystalline linear polyethylene
ever, the induced mechanical anisotropy in these systems fiber material, has a much higher specific tensile strength
is still very considerable and therefore of great practical as well as modulus relative to steel or glass fibers.
importance.
Due to the fact that orientation has such an impact on XI. MISCELLANEOUS CONSIDERATIONS
mechanical properties, it is one of the principal reasons AND FINAL REMARKS
why promoting orientation of the molecular chains is of
highimportanceinachievinghighstiffnessandotherprop- Manyimportantparametershavenotbeendiscussedinthis
erties (along the stretch direction). Of related interest is article. For example, the wear characteristics of polymer