Review of polystyrene diffusion studies in latex particles by small‐angle neutron scattering

This paper reviews small-angle neutron scattering (SANS) and some results from direct nonradiative energy transfer (DET), for the observation of the diffusion coefficients of polystyrene chains at latex interfaces. To compare SANS with DET, doubly labeled polystyrene with deuterium and fluorescence groups were synthesized, showing that while SANS and DET produce comparable data in terms of diffusion coefficients, both results differ in detail, each having their own advantages. Chain confinement, ionic end groups, and short branch effects on interdiffusion were studied. Large polymer chains confined in small particles have non-Gaussian shapes that store rubber elastic energy. Rapid, non-diffusion relaxation is inhibited because the density would be required to become less than normal. Hence confinement effects on the diffusion rate are not significant. Using the DET method, ionic end-groups were found to increase the early-time apparent interdiffusion coefficients during film formation. The early-time apparent diffusion coefficients of polystyrene with varying end-groups were found to increase as follows: The higher apparent diffusion coefficients of the chains with ionic groups are presumably due to a surface segregation of the end-groups caused by the polar, aqueous environment during latex synthesis. The interdiffusion behavior of sulfite-ended polystyrene (Mn ≅ 300 000 g/mol) with H-ends, one sulfite end, and two sulfite ends were compared via SANS and DET. The diffusion coefficients of polystyrene with one or two sulfite end groups were five times and ten times lower than that of polystyrene, respectively. The ionic end group effects on the reduced diffusion coefficients are interpreted as the competition between enhancement by the surface segregation of end groups and reduction by end group aggregation. Noting that sulfate end groups diffused faster, while sulfite end groups diffused slower, the effect is complex, and not yet fully resolved. Diffusion coefficients of polystyrene with branches were studied by DET. Short branches work to decrease the Tg and hence increase the diffusion coefficients. However, after the experimental temperature, T, is converted to a normalized temperature, T-Tg, the diffusion coefficients are found to be almost independent upon the number of branches and the length of branches. The branch length ranged from one-carbon to 40 carbons. Side chains of entanglement molecular weight or longer may be required to significantly reduce the diffusion coefficient. Copyright © 2002 John Wiley & Sons, Ltd.