General Properties

Many of the properties of the polysiloxanes have been tabulated in handbooks of polymer science and engineering. Recent work has included the stretching of polydimethylsiloxane (PDMS) chains, in some cases to their rupture points. The nature of the bonding in siloxane molecules has been of long-standing interest. Force fields for calculations of PDMS properties have been revised over the years and are now at an advanced state of development. Some of the simplest approaches employ the methods of molecular mechanics. Most of the experimental results have been obtained on solutions of polysiloxanes in thermodynamically good solvents. The first member of this series, poly(dimethylsiloxane) (PDMS), [–Si(CH3)2O–]x, has been studied extensively with regard to its configuration-dependent properties. 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 glasstransition temperature of the PDMS chain. Not surprisingly, low temperature properties are superb. Trans states are of lower energy than gauche states (ϕ = ±120°) in the PDMS chain. This conformational preference may arise from favorable van der Waals interactions between pairs of CH3 groups separated by four bonds in trans states. This factor is apparently more important 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 experimental and theoretical values of various configuration-dependent properties, however, yield a value for this energy difference that is significantly larger than that obtained from the semi-empirical calculations of interactions between nonbonded atoms.