Chemistry Reference
In-Depth Information
The characteristic ratio of PDMS is known to increase with increasing
temperature. Numerous computations have been carried out to interpret
this result.
16, 20, 21, 30
Thermal expansion is expected since the low energy
conformation is the closed polygon discussed in section 1.1, figure 1.1,
16,
20,
21
and an increase in temperature provides thermal energy for switch-
ing from these compact, low-energy conformations to higher-energy
states that are less compact.
Comparisons between the experimental and theoretical values of the
characteristic ratio and its temperature coefficient give values of the
chain conformational energies, which are then used to predict a number
of other configuration-dependent properties. Dipole moments calcu-
lated in this way are in excellent agreement with experiment for small
chain length; the agreement at longer chain length is less satisfactory,
possibly because of the large specific solvent effect31
31
already mentioned.
Additional information on the dipole moments of siloxanes is given in
section 5.5
Stress-optical coefficients have been determined using PDMS networks
both unswollen and swollen with a variety of solvents. Only qualitative
agreement was obtained, presumably because of the vanishingly small opti-
cal anisotropy of the PDMS chain.
17
Similar studies have been carried out on
other polysiloxanes
16, 20, 21
—for example, on poly(methylphenylsiloxane)
32
and poly(tetramethyl-
p
-silphenylene-siloxane).
33
Molecular mechanics and more sophisticated computational techniques
are being applied to siloxane conformational problems to study both the
chain backbone and the side chains,
34
and polysiloxane molecular motions
in general.
35
It is interesting to note that the PDMS chain and polyphosphate chain
have approximately the same characteristic ratio. Isolated
gauche
states,
of relatively high spatial extension, are more prevalent in the polyphos-
phate chain, but
pairs
of
gauche
states (g
±
g
±
) of the same sign are less prev-
alent so the two effects largely offset one another.
Much experimental and theoretical work has been reported on the cy-
clization of dimethylsiloxane chains, and on the properties of these cy-
clics.
36-41
Cyclization has been investigated for a wide range of polymer
chain lengths, but in this chapter we discuss only the results for the be-
havior of long chains. The interpretation of the results for shorter chains
may be complicated by failure of the Gaussian distribution function em-
ployed for the end-to-distances, directional correlations between termi-
nal bonds prior to cyclization, ring-strain contributions to the heat of
the reaction, and the need to revise some of the statistical weight factors
for the chains. In the limit of large chain length, the agreement between
