Chemistry Reference
In-Depth Information
transition temperature of the PDMS chain. Not surprisingly, low tempera-
ture properties are superb.
22
Tra n s
states are of lower energy than
gauche
states (
φ
= ±120°) in the
PDMS chain.
16,
17,
20,
21
This conformational preference may arise from
favorable van der Waals interactions between pairs of CH
3
groups sepa-
rated by four bonds in
trans
states. This factor is apparently more im-
portant than favorable coulombic interactions between oppositely
charged Si and O atoms separated by three bonds, which are larger in
gauche
states because of the reduced distance. Comparisons between ex-
perimental and theoretical values of various configuration-dependent
properties, however, yield a value for this energy difference that is sig-
nificantly larger than that obtained from the semi-empirical calcula-
tions of interactions between nonbonded atoms. Conformations
involving the unlike pairs g
±
g
∓
about O-Si-O skeletal bond pairs give
rise to “pentane-type interferences”
16,
17,
20,
21
between the bulky Si(CH
3
)
2
groups. Such configurations are therefore completely excluded. The
same conformations about Si-O-Si bond pairs cause interferences be-
tween the smaller O atoms; these configurations can occur with low
probability. Conflicting arrangements between groups separated by
four bonds can be visualized by rotations about pairs of consecutive
skeletal bonds in figure 2.1.
There is renewed interest in relating conformational descriptions to
crystal structures,
23
which has raised questions about the applicability of
this simple rotational isomeric state model to wide-angle scattering re-
sults.
24
Conformational rearrangements have been reported for PDMS
chains at the air/water interface,
25
and near surfaces of silica
26
or mica.
27
The equilibrium flexibility of PDMS can be characterized by its unper-
turbed dimensions, as measured by characteristic ratio <
r
2
>
o
/
nl
2
of the
unperturbed dimensions of the chain relative to the product of the number
n
of its skeletal bonds and the square of their length,
l
. Experimental
values of this ratio are in the range 6.2-7.6, the precise value depending on
the nature of the solvent.
28
The origin of this “specific solvent” effect is
obscure but may involve specific interactions between solvent molecules
and polymer segments in a way that changes the conformational prefer-
ences in the chain. The effect is significant only in the case of
polar
poly-
mers. The unperturbed dimensions appearing in the definition of the
characteristic ratio also appear in the equations for the modulus of the
chains when cross linked into an elastomeric network. Not surprisingly,
therefore, the specific solvent interactions can effect the modulus of swol-
len PDMS networks as well as the dimensions of isolated PDMS chains in
solution.
29
