Chemistry Reference
In-Depth Information
These simulated results are in at least qualitative agreement with the ex-
perimental differences in longitudinal and transverse moduli.
140
The silica or carbon black particles used to reinforce commercial mate-
rials are seldom completely dispersed,
57,
88,
89,
391-393
as is assumed in the
simulations described. The primary particles are generally aggregated.
These aggregates are frequently clustered into less stable arrangements
called agglomerates.
91
Simulations are being carried out on more ordered structures.
394
he
shapes of the aggregates include linear, globular, branched, star-shaped,
and fractal. It is well known in the industry that such structures are im-
portant in maximizing the reinforcement, as evidenced by the fact that
being too persistent in removing such aggregates and agglomerates in
blending procedures gives materials with less than optimal mechanical
properties.
88,
89,
391-393
Simulations should provide guidance on optimizing
the degree of particle dispersion.
If hydrolysis in silane precursor-polymer systems is carried out using
relatively large amounts of the silane, the silica generated can become
the continuous phase, with elastomeric polysiloxane dispersed in it.
13-15,
395- 402
The resultant composite is a polymer-modified glass or ceramic,
frequently of very good transparency. Although thermal stability is in-
ferior to that of the neat ceramic component, there are many applica-
tions for ceramic-type materials where reduced thermal stability is not
a concern.
As might be expected, the properties of polymer-toughened ceramic
materials depend greatly on the relative amounts of the two phases.
Properties of particular interest are modulus, impact resistance, ulti-
mate strength, maximum extensibility, viscoelastic responses, and
transparency. The hardness of such a composite, for example, can be
varied by control of the molar ratio of alkyl R groups to Si atoms, as il-
lustrated for PDMS in figure 9.13.
397
Low values of R/Si yield a brittle
ceramic, and high values give a relatively hard elastomer. The most in-
teresting range, R/Si ≅ 1, can yield a tough ceramic of increased impact
resistance.
Figure 9.14 illustrates some improvement in impact strength in such
composites.
403
Specifically, impact strengths of some PDMS-SiO
2
samples
were determined by the Charpy pendulum test and by the falling-weight
test.
403
The samples investigated were PDMS-modified SiO
2
and SiO
2
/TiO
2
