SELF-CONSISTENT-FIELD CONFORMATIONAL ENERGY STUDY AND CONFIGURATIONAL STATISTICS OF POLY(DIMETHYLSILOXANE)

Self-consistent-field molecular orbital ab initio calculations have been performed to obtain the intramolecular rotational potential energy surface for poly(dimethylsiloxane)(PDMS) with simultaneous rotations about successive single bonds. The minimum energy structure is more accurately described as a slightly twisted alternating trans-syn structure as opposed to the more commonly accepted all-trans structure. When an STO-3G basis set was used, it was found that the minimum occurred at φι= 156° and = 0°, (156,0). There is a minimum also at (0,204) because of symmetry. Monte Carlo simulations produced a value for the characteristic ratio of between 3.44 and 3.76 and a value for the temperature coefficient of the mean-square length of 0.511 X 10~3/deg. More recent efforts using a 3-21G* basis set obtained a minimum at about (144,0) and (0,216) and their symmetric analogues. Incorporating these new values into the Monte Carlo simulations produced a slightly raised characteristic ratio of 4.2 at 423 K. The temperature coefficient of the mean-square length, however, did not change appreciably. © 1990, American Chemical Society. All rights reserved.