Page 210 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GLM Final Pages
Encyclopedia of Physical Science and Technology EN012c-598 July 26, 2001 15:59

Polymers, Mechanical Behavior 719

the particles. An example of this is illustrated in Fig. 27a,
where poor wetting of the particle existed between the
elastomer and the glass sphere. In Fig. 27b, a coupling
agent was used to bond the particle with the matrix to help
minimize the cavitation process when elongation was un-
dertaken. The particle that was bonded showed much less
cavitation at equivalent or higher elongations than did the
system without the bonding component.
To summarize the two types of particulates discussed
above, it is clear that whether the matrix is hard relative to
the particle, or vice versa, the effect of the dispersed phase
FIGURE 26 Transmission electron micrograph showing crazes is to modify the stress in the area of that inclusion. The
induced in the equatorial regions of rubber particles that are em- exact behavior, however, is influenced by the difference in
bedded in the glassy polystyrene matrix. The arrow labeled with modulus characteristics of the dispersed phase relative to
a σ represents the direction of tensile strain. [Reprinted with per-
mission from Agarwal, B. D., and Broutman, L. J. (1980). “Analysis that of the matrix.
and Performance of Fiber Composites,” Wiley, New York. Copy- The effect of fibers versus particulates will not be dis-
right 1980 John Wiley and Sons.] cussed here. We shall simply point out that because a fiber
has a much higher aspect ratio than more spherical-like
slab. If attention is now placed on the rubber slab compo- particles, there is an effect on the stress transfer along
nent,thereisadesireforthissoftermaterialtoelongateand the fibrous component. This will allow a greater stiffness
to “thin down” around the hard inclusion, thereby placing to be gained and explains why high-strength composites
a hoop stress on the equatorial region of the particle, which often contain fibrous fillers in contrast to particulates. A
is very different from the system discussed above. In ad- final point regarding geometrically anisotropic filler par-
dition, the matrix tends to elongate and pull away from ticles is that their state of physical orientation will influ-
the “north” and “south” poles of the particulate, leading ence the local mechanics of the stress field and hence this
to a possible dewetting or cavitation phenomenon around feature must be considered in any analysis of such filler-
containing materials.

X. MOLECULAR ORIENTATION
AND ANISOTROPIC SYSTEMS

The use of polymers for many mechanical applications
depends on the fact that previous molecular orientation
of molecules has occurred for purposes of influencing the
properties along specific directions. The simplest exam-
ple is that of a drawn fiber in which the strength properties
are important along the fiber axis but generally of little
importance perpendicular to the same axis. Hence, by ori-
entation of the molecules along the stretch direction or
draw axis, the modulus is typically enhanced, as is tensile
strength. Furthermore, the yield point can be eliminated
since the system will have already gone through an orien-
tation step. Generally, maximum or perfect orientation is
not obtained through the drawing operations of bulk sys-
tems due to entanglement effects. This can lead to points of
FIGURE 27 (a) Photomicrographs showing a glass bead em-
stress concentration and failure long before perfect align-
bedded in a silicone elastomer that is undergoing elongation as
indicated by the percentages. The blackish regions at the “poles” ment of all chains is attained.
indicate where dewetting or cavitation is occurring, causing a scat- There is much interest today in trying to achieve max-
tering of light and a black region to be observed. (b) Surface of the imum orientation of polymer chains for purposes of ob-
particle has been treated such that it bonds well with the silicone
elastomer matrix. Less dewetting or cavitation is observed, indi- taining ultrahigh-modulus/high-strength systems. Linear
cating a stronger interface. [Reprinted with permission from Gert, polyethylene has the characteristics to provide an ex-
A. N., and Park, B. (1984). J. Mat. Sci. 19, 1950 and 1951.] tremely high modulus material of the order of 300 GPa, in