UNPERTURBED DIMENSIONS OF POLY(BETA-HYDROXYBUTYRATE) IN SINGLE AND BINARY SOLVENTS

Intrinsic viscosities [eta], mean-square radii of gyration <S2> and weight-average molar masses MBARw measured in 2,2,2-trifluoroethanol (TFE) at 298 K for several samples of poly(beta-hydroxybutyrate) (PHB) have been used to derive the unperturbed dimensions (<r2>0/M)1/2 via different extrapolation procedures. Theta conditions, established turbidimetrically at 298 K in two binary solvents, viz. TFE/water (52.6/47.4 v/v) and chloroform/n-heptane (50.2/49.8 v/v), were confirmed by viscosity and light scattering, and enabled the unperturbed dimensions to be determined directly. The mean value of (<r2>o/M)1/2 by all procedures was 0.085 nm g-1/2 mol 1/2, which, in conjunction with the corresponding quantity calculated on the basis of free rotation, affords a value of 1.67 for the steric factor sigma. From values of [eta] in chloroform over the interval 280-318 K, a value of -1.5 x 10(-4) K-1 was obtained for d ln <r2>o/dT. Differential refractometry and light scattering on solutions of PHB in the binary theta mixtures yielded high values for the coefficient of selective adsorption of the good solvent (TFE and chloroform) in each case.