Page 205 - 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

714 Polymers, Mechanical Behavior

important points. Cross-linking tends to decrease the mag-
nitude of the glass transition, loss behavior and sometimes
causes it to broaden, as might be expected due to an influ-
ence on the spreading of the relaxation times for coopera-
tive backbone motion. Addition of plasticizer tends to de-
crease the glass transition temperature and often broaden
the loss response, although different plasticizers function
in quite different ways in influencing the breadth of the
respective loss behavior. This allows some control of the
thermal range over which damping occurs. Antiplasti-
cizers can, in fact, depress or eliminate one or more of
the sub-T g loss responses due to limiting the mobility of
the corresponding molecular group responsible for that
transition.
As a final example demonstrating the utility of dynamic
mechanical spectra, let us consider the impact behavior
of polymeric systems. It is well known that bisphenol A
polycarbonate is a very high impact glassy polymer under
ambient conditions, whereas atactic polystyrene is a brittle
glass under similar high loading rate or impact conditions.
The glass transition temperature of polycarbonate at low
◦
frequencies is ∼150 C, whereas that of polystyrene is
taken to be ∼100 C; hence, polycarbonate can be viewed
◦
◦
as 50 “deeper” in the glassy state than polystyrene at
room temperature. The latter phrase should be taken
lightly and is meant only to elucidate the difference be-
tween these two polymers in terms of impact properties.
That is, the impact characteristics do not arise from the fact
FIGURE 19 (a) Plot of the real part of dynamic Young’s modulus
that polystyrene has a lower T g ! Specifically, the dynamic
E versus temperature for a lightly cross-linked epoxy (1 C/min).
◦
mechanical spectrum of bisphenol A polycarbonate is
The three intermediate lines represent intermediate frequencies
with respect to the two that are labeled. (b) Plot of log frequency shown in Fig. 20. Bisphenol A polycarbonate displays a
versus 1/T max for the same epoxy material. The T max values were particularly strong sub-T g dissipation mechanism, which
obtained from the tan δ data not shown. [Reprinted with permis- may well assist in dissipating energy when the glass is
sion from Wetton, R. E., and Stone, M. R. (1983). Proc. Trans.
Relax. Polym. Mater., Melbourne, 1983.] rapidly loaded. Polystyrene displays a low-temperature
loss mechanism, but it is not of great magnitude; hence,
this polymer exhibits less mobility in the glassy phase
greatly with a change in frequency. However, this is not to (data not shown). This indicates the potential use of dy-
indicate that there is no change, for in fact the breadth of namic mechanical spectroscopy in looking for differences
the transition may often undergo alteration over a range of in dissipative modes of energy below the glass transition
loading frequencies. This clearly must be correlated with temperature. Another important feature is apparent from
a distribution in the relaxation times associated with the Fig. 21, which illustrates the frequency dependence of the
specific mechanism responsible for a given loss peak, be loss peaks of polycarbonate. In particular, due to the strong
it a side group rotation, cooperative backbone motion, or sensitivity of the sub-T g peak to loading rate, the peak
otherwise. rapidly shifts upward under high loading rate conditions
and may well be operative under ambient conditions as
a prime source for dissipating mechanical energy placed
A. Factors Influencing Dynamic Loss Behavior
into the material. The actual mechanism is more complex,
As might be expected, variables that affect dynamic me- but the point here is to illustrate the worth of dynamic
chanical behavior are such important factors as level of mechanical spectroscopy in light of a very important
cross-linking, the presence of plasticizer, and phase sep- mechanical property, namely, impact strength. Note that
aration as may occur in block or segmented copolymers, the loss peak associated with T g shifts upscale with
polymer blends, and semicrystalline systems. Though we loading rate at only about 3 to 5 C/decade, as discussed
◦
cannot explore all of these effects, we shall make a few earlier.