Possibilities and limits of photon correlation spectroscopy in determiningpolymer molecular weight distributions

The effect of concentration and polydispersity on the collective diffusion coefficient D-c, evaluated using Photon Correlation Spectroscopy (PCS), has been investigated on poly(methyl methacrylate) (PMMA) in acetone solutions . The concentration dependence of the collective diffusion coefficient foll ows a linear regression law, the slope being fairly independent of polydisp ersity, molecular weight and temperature. The diffusion coefficient at infi nite dilution D-0 obeys the scaling law D-0 = A (M) over bar(w)(-v) in the range from (M) over bar(w)= 10000 to (M) over bar(w)= 800000; the value of the scaling exponent, v = 0.57, proves the good solvent quality of acetone. The inversion of the scattered intensity autocorrelation data by the regul arization method CONTIN allowed the evaluation of the molecular weight dist ribution function of the polymeric samples. Although this algorithm gives v aluable information on average quantities or on the width of the distributi on, it has limited resolution power; therefore a comparison with the result s obtained by Size Exclusion Chromatography (SEC) was carried out for a set of samples having monomodal and bimodal distribution functions.