Angular correlations in linear copolymers from the compositional dependence of their dipole moments. V. Solvent–polymer interactions

Abstract

Mean square effective dipole moments μ2e per structural unit have been determined at 298 K from a set of poly(4-chlorostyrene, 4-methylstyrene) copolymers, P (4CS,4MS), in p-xylene solutions containing 10 mole % copolymer. Coefficients which are related to local chain conformations have been evaluated from an expansion of the dipole moment ratio R (x) =μ2e(x)/μ2e(1) into a series of linearly independent functions of the mole fraction x of 4 CS units in the copolymer. The coefficient of the quadratic term, F2=1.01±0.06, obtained from these solutions is compared with the value 1.36±0.09, reported previously for the same set of copolymers in the undiluted state at 428 K. Extrapolations based on smoothed temperature coefficients which were derived from measurements in the range 360 to 440 K, yield a value F2=1.15±0.04 for the undiluted copolymers at 298 K. Uncertainties are standard deviations. It is argued that the copolymer method is relatively immune to the usual difficulties of determining dipole moments of molecules in condensed matter because it is framed in terms of ratios of experimental values of effective dipole moments. It is therefore suggested that the observed difference in values of F2 at 298 K reflects a difference in the local chain conformations resulting from the difference in the ’’solvent’’–polymer interactions in the two cases. Approximate values of the Kirkwood correlation factor g=μ2e/μ21 of P4CS, where μ1 is the moment of an equivalent freely orienting 4CS unit, are calculated from our results and compared with values obtained by others. This comparison supports the finding of Burshtein and Stepanova that g of P4CS increases toward unity as the thermodynamic quality of the solvents increase. We suggest that the copolymer method may provide a reliable means of separating a local component from the long range (excluded volume) dimensional changes that occur with changes of solvents.