Page 208 - 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

Polymers, Mechanical Behavior 717

shows the strain at break (ultimate property) as a func- goal to discuss the morphological textures of semicrys-
tion of molecular weight at a given temperature. There talline polymers, but it is important to point out that by
is a decrease in the strain at break for this polycarbon- varying the methods of crystallization (quench rates, etc.)
ate system as rate is increased in this range of ˙ε as ex- one can change the morphological textures in a given
pected. A particularly important point is that, at a fixed polymer, thereby affecting the mechanical behavior even
˙ ε, there is a major change in behavior above a critical though the level of crystallinity may not greatly vary. It is
molecular weight, which is of the order of 10,000 g/mol therefore important to be aware of the crystallization pro-
for this material. This critical molecular weight occurs cedures in conjunction with the level of crystallinity if one
where the length of these macromolecules is sufficient hopes to predict the mechanical behavior of a semicrys-
to promote molecular entanglements, which in turn lead talline system.
to a pseudo network—a more useful material for struc-
tural applications. This critical molecular weight varies
for different macromolecular chain chemistry depending VIII. EFFECT OF COVALENT
on the relative chain stiffness and the mass of the repeat CROSS-LINKING
unit.
Placing covalent cross-links into a system influences the
mechanical properties in a fairly predictable manner. Be-
VII. EFFECT OF CRYSTALLINITY low the glass transition temperature, cross-links increase
ON PROPERTIES the modulus, but the effect is somewhat like that of crys-
tallinity in that the increase is not a large one. As the level
Because many of today’s polymers are semicrystalline, of cross-linking increases and places restrictions on the
we shall discuss briefly how crystallinity influences me- thermal Brownian motion of a chain segment, the glass
chanical properties, as well as other important properties transition temperature is generally shifted upward. Often
such as optical transparency. Let us first consider the ef- this transition is broadened and produces behavior simi-
fect of crystallinity on stiffness or modulus behavior. If lar to that of crystallinity (Fig. 24). It is clear, however,
the system is unoriented (oriented systems will be dis- that (as pointed out earlier) when sufficient cross-linking
cussed in Section X) and if one is below the glass transi- exists to promote an infinite network (i.e., the gel point
tion temperature of the amorphous phase, increasing the is reached), the viscous flow region is no longer avail-
crystallinity of a material has relatively little effect on able to the thermal mechanical spectrum. As expected,
9
modulus; that is, modulus will be of the order of 10 to the degree of cross-linking is directly correlated with the
10 10 Pa, regardless of the level of crystallinity. On the modulus behavior in the rubbery region. In fact, if the level
other hand, if one is above the glass transition temper- of cross-linking becomes considerable, rubbery behavior
6
5
ature of the remaining amorphous component, the pres- (modulus of 10 to 10 Pa) is not likely to be found, but
ence of crystallinity will strongly influence modulus and rather the material will be stiffer and have the properties of
cause it to increase accordingly, as long as one is below a leather-like system. In fact, with excessive cross-linking,
the melting point of the crystals. A general sketch of this the glass transition temperature may never be observed
behavior is illustrated in Fig. 24, where the general ther- before thermal degradation! Of course, cross-linking can
mal mechanical behavior is shown with respect to level of be placed into polymeric materials by different means,
crystallinity. such as sulfur vulcanization, peroxide cross-linking, or
If stress–strain measurements were being made above radiation, but the details of these methods will not be
the glass transition temperature of the amorphous phase, discussed.
the modulus would be enhanced in these measurements as
well, but so would other properties such as strain to break
and stress at break. In the presence of crystallinity, a dis- IX. EFFECT OF FILLERS ON
tinct yield point is often displayed (see Fig. 5). Generally, MECHANICAL BEHAVIOR
the presence of crystallinity may also strongly influence
the nature of cold drawing or ductile flow, although the Often polymers are modified by the placement of soft par-
crystallinity does not have to exist for this phenomenon ticulates within them (e.g., rubber particles within a hard
to occur. It certainly must, however, if a yield or ductile glassy matrix). The reverse approach is also undertaken.
character is to be observed above the glass transition for a An example of the latter is the placement of hard particles
nonfilled homopolymeric material. Crystallites also serve in a softer matrix such as calcium carbonate or glass parti-
basically as “physical cross-links” and therefore strongly clesplacedinarubberymatrix.Alsointhegrowingfieldof
dominate the mechanical properties above T g . It is not our composites, polymeric components serve typically as the