TRANSLATIONAL DIFFUSION-COEFFICIENT OF OLIGOMETHYL AND POLY(METHYL METHACRYLATE)S IN DILUTE-SOLUTIONS

The translational diffusion coefficient D was determined from dynamic light scattering (DLS) measurements for 14 samples of oligo- and poly(methyl methacrylate)s (a-PMMA), each with the fraction of racemic diads f(r) = 0.79, in the range of weight-average molecular weight M(w) from 5.02 x 10(2) to 2.18 X 10(5) in acetonitrile at 44.0-degrees-C (THETA). The determination was also made for the same samples except for the four with the smallest M(w) in n-butyl chloride at 40.8-degrees-C (0). A modification of the DLS data acquisition procedure was made in order to remove the effect of some residual aggregates of solute a-PMMA molecules in acetonitrile. As was expected from the dependence on M(w) of the intrinsic viscosity [eta] reported previously, a double-logarithmic plot of DM(w) against M(w) in each solvent deviates downward from its asymptotic straight line of slope 1/2, following an S-shaped curve. A comparison is made of the present results (along with those for two a-PMMA samples with larger M(w) reported previously) with the corresponding theory based on the helical wormlike (HW) touched-bead model. The values of the model parameters determined are found to be consistent with those from the previous analyses of [eta] and the mean-square radius of gyration, confirming that the local and global conformations of the a-PMMA chain in dilute solution may be described typically by the HW chain model.