Polymers can be usefully classified in many ways, such as by source of raw mate- rials, method of synthesis, end use, and fabrication processes. Some classifications have already been considered in this chapter. Polymers are grouped by end use in this section, which brings out an important difference between macromolecules and other common materials of construction. This is that the chemical structure and size of a polymeric species may not completely determine the properties of an article made from such a material. The process whereby the article is made may also exert an important influence.
The distinction between elastomers, fibers, and plastics is most easily made in terms of the characteristics of tensile stressstrain curves of representative sam- ples. The parameters of such curves are nominal stress (force on the specimen divided by the original cross-sectional area), the corresponding nominal strain (increase in length divided by original length), and the modulus (slope of the stressstrain curve). We refer below to the initial modulus, which is this slope near zero strain.
Generalized stressstrain curves look like those shown in Fig. 1.2. For our present purposes we can ignore the yield phenomenon and the fact that such curves are functions of testing temperature, speed of elongation, and charac- teristics of the particular polymer sample. The nominal stress values in this figure are given in pounds of force per square inch (psi) of unstrained area (1 psi56.93103N/m2).
Elastomers recover completely and very quickly from great extensions, which can be up to 1000% or more. Their initial moduli in tension are low, typically up to a range of about 1000 psi (7 MN/m2) but they generally stiffen on stretch- ing. Within a limited temperature range, the moduli of elastomers increase as the temperature is raised. (This ideal response may not be observed in the case of samples of real vulcanized rubbers, as discussed inSection 4.5.) If the tempera- ture is lowered sufficiently, elastomers become stiffer and begin to lose their rapid recovery properties. They will be glassy and brittle under extremely cold conditions.
Figure 1.3a illustrates the response of an elastomer sample to the applica- tion and removal of a load at different temperatures. The sample here is assumed to be cross-linked, so that the polymer does not deform permanently under stress.
23 1.8 Elastomers, Fibers, and Plastics
Fibers have high initial moduli which are usually in the range 0.53106 to 23106psi (33103to 143103MN/m2). Their extensibilities at break are often lower than 20%. If a fiber is stretched below its breaking strain and then allowed to relax, part of the deformation will be recovered immediately and some, but not all, of the remainder will be permanent (Fig. 1.3b). Mechanical properties of com- mercial synthetic fibers do not change much in the temperature range between
250C and about 150C (otherwise they would not be used as fibers).
[As an aside, we mention that fiber strength (tenacity) and stiffness are usually expressed in units of grams per denier or grams per tex (i.e., grams force to break a one-denier or one-tex fiber). This is because the cross-sectional area of some fibers, like those made from copolymers of acrylonitrile, is not uniform. Denier and tex are the weights of 9000- and 1000-m fiber, respectively.]
Plastics generally have intermediate tensile moduli, usually 0.53105 to 43105psi (3.53102to 33103MN/m2), and their breaking strain varies from a few percent for brittle materials like polystyrene to about 400% for tough,
120
(a)
(c) (d)
(b) Stress (psi × 10−3) Stress (psi × 10−3)
Strain (%)
Elongation at break
Yield stress Tensile strength
Strain (%) Strain (%) Strain (%) 80
40
0
10 8 6 4 2 0
12
2.0 1.6 1.2 0.8 0.4
0 200 400 600 800 10
8 6 4 2
0 20 40 60 80
10 20 10 20
FIGURE 1.2
Stress-strain curves. (a) Synthetic fiber, like nylon 66. (b) Rigid, brittle plastic, like polystyrene. (c) Tough plastic, like nylon 66. (d) Elastomer, like vulcanized natural rubber.
semicrystalline polyethylene. Their strain recovery behavior is variable, but the elastic component is generally much less significant than in the case of fibers (Fig. 1.3c). Increased temperatures result in lower stiffness and greater elongation at break.
Some chemical species can be used both as fibers and as plastics. The fiber- making process involves alignment of polymer molecules in the fiber direction.
This increases the tensile strength and stiffness and reduces the elongation at break. Thus, typical poly(hexamethylene adipamide) (nylon-66, structure 1-6) fibers have tensile strengths around 100,000 psi (700 MN/m2) and elongate about 25% before breaking. The same polymer yields moldings with tensile strengths around 10,000 psi (70 MN/m2) and breaking elongations near 100%. The macro- molecules in such articles are randomly aligned and much less extended.
Synthetic fibers are generally made from polymers whose chemical composi- tion and geometry enhance intermolecular attractive forces and crystallization.
A certain degree of moisture affinity is also desirable for wearer comfort in textile
ElongationElongationElongation
Load applied
Elastic deformation
Elastic recovery Delayed recovery
Load removed
Permanent deformation Time
Time
Time (a)
(b)
(c)
FIGURE 1.3
Deformation of various polymer types when stress is applied and unloaded. (a) Crosslinked ideal elastomer. (b) Fiber. (c) Amorphous plastic.
25 1.8 Elastomers, Fibers, and Plastics
applications. The same chemical species can be used as a plastic, without fiber- like axial orientation. Thus most fiber-forming polymers can also be used as plas- tics, with adjustment of molecular size if necessary to optimize properties for par- ticular fabrication conditions and end uses. Not all plastics can form practical fibers, however, because the intermolecular forces or crystallization tendency may be too weak to achieve useful stable fibers. Ordinary polystyrene is an example of such a plastic material, while polyamides, polyesters, and polypropylene are prime examples of polymers that can be used in both areas.
Elastomers are necessarily characterized by weak intermolecular forces.
Elastic recovery from high strains requires that polymer molecules be able to assume coiled shapes rapidly when the forces holding them extended are released.
This rules out chemical species in which intermolecular forces are strong at the usage temperature or which crystallize readily. The same polymeric types are thus not so readily interchangeable between rubber applications and uses as fibers or plastics.
The intermolecular forces in polyolefins like polyethylene (1-3) are quite low, but the polymer structure is so symmetrical and regular that the polymer segments in the melt state are not completely random. The vestiges of solid-state crystallites that persist in the molten state serve as nuclei for the very rapid crystallization that occurs as polyethylene cools from the molten state. As a result, solid polyeth- ylene is not capable of high elastic deformation and recovery because the crystal- lites prevent easy uncoiling or coiling of the macromolecules. By contrast, random copolymers of ethylene and propylene in mole ratios between about 1/4 and 4/1 have no long sequences with regular geometry. They are therefore non- crystallizing and elastomeric.