TEMPERATURE COEFFICIENTS FOR THE CHAIN DIMENSIONS OF POLYSTYRENE ANDPOLYMETHYLMETHACRYLATE

We have made precise measurements of the unperturbed radius of gyration [S2]0(1/2) of polystyrene and polymethylmethacrylate molecules in the melt over the temperature range 120-degrees-C-240-degrees-C by small-angle neutron scattering. Contrary to most previous reports, based on dilute solution measurements, we find that neither polymer exhibits a significant change in dimensions with temperature, and we deduce temperature coefficients, kappa = d ln[S2]0/dT, of (0.0 +/- 0.1) x 10(-3) K-1 for polystyrene and (0.1 +/- 0.1) x 10(-3) K-1 for polymethylmethacrylate. These kappa values are close to those given by thermoelastic measurements, but are difficult to reconcile with predictions obtained from the rotational isomeric state model.