Dipole moments and statistical conformations of cyclic and linear dimethylsiloxane oligomers

Abstract

Abstract The statistical conformations of cyclic dimethylsiloxane oligomers [(CH3)2SiO]x with x in the range 4–10 have been studied by using rotational isomeric state models to describe the ‘cyclic’ conformations of the open chain molecules. Stringent criteria were used to select ‘cyclic’ conformations of the equivalent open chains based on the relative orientations and rotational states of their terminal skeletal bonds. The root-mean-square electric dipole moments 〈μ 2 〉 0 1 2 and the temperature coefficients d ln 〈μ 2 〉 0 dT were calculated using a modified form of the rotational isomeric state model of Flory, Crescenzi and Mark. The calculated dipole moments are compared with some experimental values measured for cyclic oligomers in the undiluted state at 298 K. For the ‘cyclic’ conformations of the open chains the magnitude of the calculated temperature coefficients for x = 5–9 were found to be very nearly inversely proportional to the fraction of rotatable skeletal bonds occupying trans rotational states; a functional dependence opposite to that found for the open chains.