Chemistry Reference
In-Depth Information
for this improvement is increased capability of the in situ-produced silica
to tie up hydroxyl chain ends that participate in the degradation reaction.
Attempts at further improvements of silica fillers are continuing.
122
A variety of techniques have been used to further characterize
in situ
-
filled elastomers.
36,
42
Density measurements, for example, yield informa-
tion on the nature of the particles. Specifically, the densities of the
ceramic-type particles are significantly less than that of silica itself, which
implies the particles contain some unhydrolyzed alkoxy groups or voids,
or both.
The low-temperature properties of some in situ-filled materials have
been studied by the calorimetry techniques mentioned in chapter 1. Of
particular interest is the way in which reinforcing particles affect the crys-
tallization of a polysiloxane, both in the undeformed state and at high
elongations.
123
Electron microscopy (both transmission and scanning) has been used
to reveal (i) the nature of the precipitated phase (particulate or nonpar-
ticulate), (ii) the average particle size, if particulate, (iii) the distribution
of particle sizes, (iv) the integrity of the particles, and (v) the degree of
agglomeration of the particles.
42
A number of studies using x-ray and neutron scattering
124 -127
have
also been carried out on filled PDMS elastomers.
36,
37
Although the re-
sults are generally consistent with those obtained by electron micros-
copy, there are some intriguing differences. Some fillers that appear to
be particulate in electron microscopy appear to consist of a continuously
interpenetrating phase by scattering measurements. Additional experi-
ments will certainly be forthcoming. Determining whether particle-
induced network formation occurs is obviously a question of consider-
able importance.
128
9.2.2 Glassy Particles Deformable into Ellipsoidal Shapes
It is possible to obtain reinforcement of a PDMS elastomer by polymeriz-
ing a monomer such as styrene to yield hard glassy domains within the
elastomer.
129,
130
Low concentrations of styrene give a low molecular
weight polymer that acts more like a plasticizer than a reinforcing filler.
At higher styrene concentrations, however, roughly spherical polystyrene
(PS) particles are formed, leading to good reinforcement. The particles are
relatively easy to extract from the elastomeric matrix, which implies min-
imal bonding exists between the components. A trifunctional R'Si(OC
2
H
5
)
3
where R' is an unsaturated group can be used to couple the particles to
