Second virial coefficient of oligo- and poly(methyl methacrylate)s near the .THETA. temperature

Abstract

The second virial coefficient A 2 was determined from light scattering measurements for atactic oligo- and poly(methyl methacrylate)s (a-PMMA) with the fraction of racemic diads f r = 0.79 in acetonitrile at various temperatures below and above Θ (20.0-55.0°C) in the range of molecular weight M w from 6.13×10 2 to ca. 5.0×10 5 . The results show that A 2 of a-PMMA depends appreciably on M w over the whole range of M w examined even below 0, in contrast to the previous and literature results for atactic polystyrene (a-PS), for which A 2 is independent of M w for M w > 5×10 3 below Θ. It is shown that the present data may be quantitatively explained by the Yamakawa theory that takes account of the effect of chain ends on the basis of the helical wormlike chain, yielding rather reasonable values of the binary-cluster integral β between intermediate identical beads and the effective excess binary-cluster integrals β 1 and β 2 associated with the chain end beads. It is found that, as in the case of a-PS, the results for the part A 2 (HW) of A 2 without the effect of chain ends are consistent with the two-parameter theory prediction below Θ, giving a single-composite curve irrespective of M w and temperature when A 2 (HW) M w 1/2 is plotted against the conventional excluded-volume parameter z. The M w independence of A 2 below Θ as suggested by Fujita is not generally valid