Chemistry Reference
In-Depth Information
theory and experiment is excellent and thus supports the proposed
model for PDMS. In addition, cyclization studies have generated useful
information on excluded volume effects (their absence in the undiluted
amorphous state and their magnitudes in solutions, particularly at high
polymer concentrations), the critical chain length at which the Gaussian
distribution becomes inaccurate, the magnitude of specific solvent ef-
fects, and the validity of gel permeation chromatography theories per-
taining to both linear and branched chain molecules.
Physical properties of cyclics in comparison with linear chains of the
same degree of polymerization have been investigated extensively,
37, 38, 42
as described in chapter 3. Example of such comparisons are solution
viscosity-molecular-weight relationships, bulk viscosities, densities, re-
fractive indices, glass transition temperatures,
29
Si NMR (nuclear mag-
netic resonance) chemical shifts, chain dimensions from neutron
scattering, diffusion coefficients and their concentration dependence,
thermal stability, second virial coefficients, radii of gyration, equilibrium
shapes (from Monte Carlo simulations), static dielectric permittivity, par-
ticle scattering functions, monolayer surface pressures, melting points,
theta temperatures (at which the chains are unperturbed by excluded
volume effects),16
16
critical temperatures for phase separations, melt mobil-
ity (by excimer emission), and conformational dynamics (by ultrasonic
relaxation measurements).
Other symmetrically substituted polysiloxanes have been investi-
gated less thoroughly.
17
Poly(diethylsiloxane) [-Si(C
2
H
5
)
2
-O-]
x
has been
reported to have a characteristic ratio of 7.7 ± 0.2, which is essentially
the same as that of PDMS; its dipole moment is difficult to measure be-
cause of the low polarity of the repeat unit, but it too is approximately
the same as that of PDMS. These results suggest that lengthening of the
side chains must generate self-compensating effects. Furthermore,
poly(di-
n-
propylsiloxane) [-Si(C
3
H
7
)
2
-O-]
x
has been reported to have a
characteristic ratio of 13.0 ± 1.0.
43
The high spatial extension in this
chain could result from the fact that an articulated side chain, such as
-CH
2
CH
2
CH
3
, can adopt more conformations in the form of
trans
-
gauche
states along the chain backbone than in the more restrictive
trans
-
trans
states shown in figure 5.1. Although this “entropic destabilization” of
compact
trans
states would increase the chain dimensions, the large in-
crease in the characteristic ratio over that for PDMS would not have been
anticipated. This intuitive conclusion is supported by some rotational
isomeric state calculations that do take into account the conformational
variability of the siloxane side chains. It should be mentioned, however,
that results of cyclization studies carried out on some stereochemically
