Effect of excluded volume on the dipole moments of chain molecules

Abstract

Dielectric constants have been determined for dimethylsiloxane chains (CH3)3Si[OSi(CH3)2]x OSi(CH3)3 in the thermodynamically good solvent cyclohexane and in the undiluted state, for degrees of polymerization x + 1 ranging from 102 to 103, at a number of temperatures in the range 10–60 °C. These data indicate that at constant temperature the dipole moment ratio 〈μ2〉/nm2 (where 〈μ2〉 is the mean-square dipole moment of a chain consisting of n = 2x + 2 bond dipoles of magnitude m) is independent of chain length, as has been predicted for chains of such structural symmetry. Unfortunately, comparison of the experimental values of the dipole moment ratio with those predicted from rotational isomeric state theory is complicated by pronounced specific solvent effects and comparison of experimental and theoretical values of d ln 〈μ2〉/dT is also difficult because of the very small magnitude of this coefficient.