Thermal diffusivity measurements on three polymer melts were made using the Forced Rayleigh Light Scattering technique. The polymers, which were tested at room temperature where they are in the molten state, included a polydimethylsiloxane and two polyisobutylenes. The optical setup and procedures developed in this study to conduct thermal Forced Rayleigh Light Scattering experiments are shown to be capable of producing thermal diffusivity data with a high degree of accuracy and precision. From measurements on a reference fluid (ethanol), experimental error was estimated to be no greater than 2%, and could be reduced to less than 1% by appropriate design of a series of experiments. Discrepancies of 4 and 14% in thermal diffusivity data on the polymer samples between measured values and those found in the literature were observed. It is suggested that these deviations are attributable to either sample variations or to errors in the techniques used in previous investigations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1069–1078, 1999

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The effect of diffusion on linear, step-growth polymerizations has been quantified via "chainby-chain" simulations of the polymerization process and a diffusion model consistent with reptation theories. The results show that diffusion limitations can narrow the molecular weight distribution and alter the way it evolves during polymerization; e.g., when diffusion is the limiting step, the polydispersity index (PDI1 ) Mw/Mn) converges to a value of ∼1.3 at complete conversion. Nevertheless, diffusion limitations have little effect on the time dependence of conversion. Consequently (for this type of polymerization), modeling of diffusion phenomena is important only when an accurate calculation of the MWD is needed. We also show that the numerical value of the Hatta number, at the point of entanglement (φ), can be used to determine whether diffusion limitations are likely to be important in a given system. Guidelines for the estimation of φ are given.

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