Symmetrically Substituted Polysiloxanes

2 CONFORMATIONS AND SPATIAL CONFIGURATIONS 5.2.1 Symmetrically Substituted Polysiloxanes 

Trans states are of lower energy than gauche states ( j) = +120°) in the PDMS chain. This conformational preference may arise from 

There is renewed interest in relating conformational descriptions to crystal structures, which has raised questions about the applicability of this simple rotational isomeric state model to wide-angle scattering results. Conformational rearrangements have been reported for PDMS chains at the air/water interface, and near surfaces of silica or mica.  

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 optical anisotropy of the PDMS chain. Similar studies have been carried out on other polysiloxanes —for example, on poly(methylphenylsiloxane) andpoly(tetramethyl-p-silphenylene-siloxane).  

Molecular mechanics and more sophisticated computational techniques are being applied to siloxane conformational problems to study both the chain backbone and the side chains, and polysiloxane molecular motions in general.  

---

Conformations and Spatial Configurations Symmetrically Substituted Polysiloxanes... 

Table 5 Symmetrically substituted polysiloxane phases, specially designed for high-temperature work...

Finally, melting point depression measurements have been conducted on several symmetrically substituted polysiloxanes, specifically the dimethyl, diethyl, di-n-propyl, and diphenyl polymers. Interpretation of such experimental results yields entropies of fusion. Although it is difib-cult to extract a reliable configurational entropy from this quantity, such results could help elucidate the configurational characteristics of the chains thus investigated. ... 

---