Chemistry Reference
In-Depth Information
CHAPTER 5
General Properties
5.1 SOME GENERAL INFORMATION
Many of the properties of the polysiloxanes have been tabulated in hand-
books of polymer science and engineering.
1-3
Recent work has included
the stretching of polydimethylsiloxane (PDMS) chains,
4
in some cases to
their rupture points.
5
The nature of the bonding in siloxane molecules has been of long-
standing interest.
6 -9
Force fields for calculations of PDMS properties have
been revised over the years and are now at an advanced state of develop-
ment.
10-13
Some of the simplest approaches employ the methods of mo-
lecular mechanics.
14
Most of the experimental results have been obtained
on solutions of polysiloxanes in thermodynamically good solvents.
15
5.2 CONFORMATIONS AND SPATIAL CONFIGURATIONS
5.2.1 Symmetrically Substituted Polysiloxanes
The first member of this series, poly(dimethylsiloxane) (PDMS),
[-Si(CH
3
)
2
O-]
x
, has been studied extensively with regard to its
configuration- dependent properties.
16-21
PDMS (figure 2.1) is very similar
in structure to the polyphosphate chain in that the successive bond angles
are not equal. The Si-O bond length in polysiloxanes is 1.64 Å, and bond
angles at the Si and O atoms are 110 and 143°, respectively. This inequality
of bond angles causes the all-
trans
form of the molecule (with rotational
angles
φ
= 0°) to form a closed structure after approximately eleven repeat
units. The torsional barrier for rotations about the skeletal bonds is very
low, which accounts for the high dynamic flexibility and low glass
