Page 212 - 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 721

are quenched from either the melt or from above T g into
the glassy state. The viscosity rises greatly as the glass
transition temperature is approached during the cooling
process causing the material not to be able to maintain its
true equilibrium structure as it passes into the glassy state,
thereby placing it into a nonequilibrium state. Specifically,
the material will posses a greater amount of “free volume”
due to the fact that equilibrium thermal contraction could
not occur. As a result, this excess free volume, small as it
may be, slightly lowers the density of the quenched glassy
material and, as a result of possessing this excess free vol-
ume, its mechanical properties are also altered relative to
what its true equilibrium state would provide. Generally,
the result is that during aging below T g , commonly called
physical aging, the system undergoes slow densification of
FIGURE 28 Plot of specific modulus versus specific tensile
strength for several materials including steel, glass-oriented the material if local-scale molecular motion can occur as
®
polyethylene (Spectra ), liquid crystalline aromatic polyamides we have discussed is common for polymers in the glassy
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(Kevlar ), etc.
state. The degree and rate of physical aging is dependent
on a number of factors such as how far below the glass
surfaces are highly important, but it is not always easy transition temperature one stores the material and how
to measure them by a simple test and apply the results rapid the cooling process was. (How far from equilibrium
to a specific application. Other important parameters are was the structure when placed into the glassy state?) Of
the tear characteristics and fatigue and cyclic loading be- much importance is that this slow densification or physical
havior of materials. Picture, for example, an application aging process has drastic effects on a number of parame-
of a polymeric hinge where folding back and forth oc- ters, one of these being yield stress. In particular, as shown
curs over its life-time. How many cycles of folding can in Fig. 29, amorphous polyethylene terephthalate (PET)
occur at a given rate and temperature before failure is shows that the yield stress distinctly grows with time upon
induced? Another important area is environmental stress aging a quenched sample at room temperature, which is
crack resistance where certain agents, such as detergents approximately 40 C below its glass transition tempera-
◦
or oil components, may promote the cracking of polymeric ture (1 Hz). A number of other mechanical parameters
systems if they are under stress. are also altered with time but the author will not focus
As the reader is aware, there are several materials that on additional detail in this basic paper. However, those
are used structurally but which are of a cellular nature, working with materials that are polymeric glasses or con-
i.e., open-cell and closed-cell foams along with other ma- tain glassy phases should become more aquatinted with
terials such as even wood itself. Often the closed-cell
foams are for insulation purposes or to help lighten the
weight of a system. Polymeric structural foams are often
utilized in a diversity of applications, including as auto-
mobile bumpers. In contrast, the open-cell foams allow air
to pass through when the system is loaded as is common
in the cushioning applications of polyurethane foams. Of
importance is to recognize that the nature of the cellular
structure does influence the mechanical properties of the
materials and as a result can play an important role, when
analyzing such materials. Although not discussed in this
paper, the interested reader should consult the reference
by Gibson and Ashby (1988) for more information on cel-
lular materials and their mechanical behavior.
On a different note, if a polymeric system is below its
glass transition temperature, one tends to often think that
FIGURE 29 Engineering stress–engineering strain plots for
the system is stable mechanically since there is limited
quenched glassy poly(ethylene terephthalate) determined at dif-
backbone motion. However, this can be very far from the ferent times following physical aging at 23 C. [From Tant, M. R.,
◦
truth. Typically, when glassy materials are prepared, they and Wilkes, G. L. (1981). J. Appl. Polym. Sci. 26, 2813.]