Chemistry Reference
In-Depth Information
0.18
43%
0.16
0.14
0.12
0.10
25.9%
0.08
20.2%
17.5%
0.06
0.04
0.02
9.3%
0.2
0.4
0.6
0.8
1.0
α
-1
Figure 9.9:
Stress-strain isotherms for PDMS-polystyrene (PS) composites.
129
Each curve is labeled
with the wt % PS present in the composite. The dashed line locates the linear portion of
the curve useful for quantitative interpretation.
384
the matrix. The R' groups on the particle surfaces participate in the po-
lymerization, thereby bonding the elastomer chains to the reinforcing
particles. Alternatively, the R'Si(OC
2
H
5
)
3
can be used as one of the end-
linking agents, to place unsaturated groups at the cross links. In any case,
good reinforcement is observed for PS domains in the roughly spherical
state. (figure 9.9)
129
Ex situ techniques can also be used. The effects of particle dispersion on
the melt state have been documented using nanocomposites containing
PS homopolymers or PS-grafted silica.
131
PS has also been grafted onto the
cross links in a siloxane polymer
132
and onto filler particles used to rein-
force an elastomer.
133
The PS domains have the disadvantage of having a relatively low
glass transition temperature (T
g
≈10 0
°
C)
134-136
and in being totally amor-
phous. Above T
g
they soften and presumably lose their reinforcing ability.
For this reason, studies have been carried out using crystalline
poly(diphenylsiloxane) as the reinforcing phase.
137
Measurements on co-
polymers containing diphenylsiloxane blocks indicate it has a melting
point (and thus a softening temperature) as high as 550
°
C.
138
It is possible to convert the essentially spherical PS particles just de-
scribed into ellipsoids.
42,
139-143
First, the PS-PDMS composite is raised to a
temperature well above the T
g
of PS; it is then deformed and cooled while in
the stretched state. The particles are thereby deformed into ellipsoids, and
