Evaluations of forcefields for aromatic polysiloxanes, and some applications to poly(diphenylsiloxane)

Two forcefields were evaluated for possible use in polysiloxanes containing aromatic phenyl or phenylene groups, either as side chains or as part of the chain backbone. The criterion was reproducing results on the crystal structure of the cyclic diphenylsiloxane trimer, and both forcefields were satisfactory. The better of the two was used to obtain conformational energies for estimating some of the statistical properties of the corresponding polymer poly(diphenylsiloxane) (PDPS) [Si(C₆H₅)₂–O–]. The calculations were based almost entirely on the rotational isomeric state (RIS) theory, but the possibility of using a Metropolis Monte Carlo method was also considered. Since the stiffness of the chains was of primary interest, the quantities calculated were the unperturbed dimensions and its temperature coefficient and the radii of gyration. Degrees of polymerization ranged from 40 to 400, and temperatures from 300 to 1000 K. The “characteristic ratio” (of the mean-square unperturbed dimensions to those of the corresponding freely-jointed chain) was found to be approximately 65 in the limit of very long chains. This is an order of magnitude larger than that of the much studied and very flexible poly(dimethylsiloxane), and the associated chain stiffness this indicates for PDPS seems to be consistent with its high transition temperatures and other properties.