Chemistry Reference
In-Depth Information
they retain this shape when cooled. Uniaxial deformation of the compos-
ite give prolate (needle-shaped) ellipsoids, and biaxial deformations give
oblate (disc-shaped) ellipsoids.
140,
144
Prolate particles can be thought of
as a conceptual bridge between the roughly spherical particles used to
reinforce elastomers and the long fibers frequently used for reinforce-
ment in thermoplastics and thermosets. Similarly, oblate particles can
be considered analogues of the clay platelets used to reinforce a variety
of polymers,
145 -150
but with dimensions and compositions that are
controllable.
Ellipsoidal particles have been characterized using both scanning and
transmission electron microscopy to determine their axial ratios and
measure of the extent to which their axes were aligned in the stretch di-
rection. In these anisotropic materials, elongation moduli in the direction
of the stretching are significantly larger than those of the untreated PS-
PDMS elastomer, whereas in the perpendicular direction they are signifi-
cantly lower (figure 9.10).
140
Such differences are expected from the
anisotropic nature of the filler particles.
In the case of nonspherical particles degree of orientations is also of
considerable importance. The effects of orientation can be removed by dis-
solving away the host polymer and redispersing the particles isotropically
in another, elastomer matrix. There have been simulations to better un-
derstand the mechanical properties of such composites.
151-154
0.20
0.18
2.2
0.16
1.7
0.14
0.12
1.0
0.10
1.7
0.08
2.2
0.06
0.4
0.6
0.8
1.0
α
-1
Figure 9.10:
Stress-strain isotherms of the PDMS-PS composites.
140
Values of the draw ratio and test-
ing directions are indicated on each curve. The PS introduced during the in situ polymer-
izations was 35.0 wt %.
