I. Study of Macromolecular Brownian Motion by Laser Light Scattering. II. Light Scattering Investigation of Amorphous Polymethylmethacrylate

Part I We have studied the light scattered from dilute solutions of three different molecular weight (MW = 2 x 106, 4.1 x 106, 7.1 x 106) polystyrene molecules in three solvents (cyclohexane, diethylmalonate, 1-chloroundecane) at their respective theta temperatures. The autocorrelation functions collected at small x = k2RG (k is the wave vector, RG is the radius of gyration) were fitted to a single exponential function decay time which yielded the diffusion coefficients D. For concentrations below 1000 μg/cc, D's were found to be independent of concentration and to be well represented by a Stokes-Einstein relation D = kBT/6TπƞsξG where ƞs is the solvent viscosity. We determined ξ to be 0.70 ± .02. At higher x, the autocorrelation functions have contributions from the first mode of internal motion and analysis of data in terms of two exponentials yield the relaxation time of this mode, τ1. τ1's were measured for four polystyrene-cyclohexane solutions with varying concentrations for both the four and seven million molecular weight samples. The relaxation times were found to be independent of concentration. The same experiment was repeated for both molecular weight samples for one solution each in diethylmalonate and in 1-chloroundecane. The relaxation times obtained agreed well with the theoretical predictions based on Rouse-Zimm model. The results of this research program confirm the accuracy of laser light scattering technique to measure diffusion coefficients of macromolecules. They also establish laser light scattering as a new technique for quantitative determination of terminal relaxation time of linear flexible polymers. The accuracy of these measurements are comparable to those obtained by viscoelastic methods. Part II We have studied the light scattered from amorphous polymethylmethacrylate (PMMA) using the correlation technique for a range of temperatures from 6°c to 165°c encompassing the glass transition temperature (Tg ~ 120°C). The data were analyzed in terms of two exponential decays and the angular dependence of each of the corresponding relaxation frequencies was examined. The results for the high frequency relaxation mode are angular independent and fall reasonably well on two straight lines of different activation energy(~ 8 Kcal/mole at high temperatures and ~1 Kcal/mole at low temperatures), indicating the presence of two coupled relaxation mechanisms. The low frequency relaxation results are quite sensitive to the inhomogeneities of unannealed samples and have in this case irregular angular dependences. The angular dependence disappears for samples close to Tg and for annealed samples. Below Tg this relaxation process has a fairly constant frequency of about 3 Hz independent of temperature. Above Tg its frequency increases very rapidly with temperature to reach 130 Hz at 165°C and follows the backbone main-chain relaxation frequency measured by other techniques. The results of this research program show that laser light scattering is a valuable tool to study the various relaxation mechanisms occuring in solids and supercooled liquids.