MOLECULAR-WEIGHT CHARACTERIZATION OF POLYACRYLAMIDE-CO-SODIUM ACRYLATE .2. LIGHT-SCATTERING

The first large-scale fractionation method for polyacrylamides is reported. This has been optimized to prepare a series of narrowly distributed polymer standards with molecular weights between 10,000 and 1,000,000. These were subsequently hydrolyzed to between 5 and 35% sodium acrylate content through a saponification reaction. It has been found that the classical light-scattering theory, derived for nonionic scattering bodies, can be applied to polyelectrolytes provided the specific refractive index increment (partial derivative n/partial derivative c) is evaluated at constant electrochemical potential (mu). This is obtained by dialyzing the polymer solution against the dialysate (0.2 M Na2SO4 for polyacrylamide-co-sodium acrylate) for a predetermined equilibrium period, specific to the pore size of the membrane used. From such measurements at various copolymer compositions (F), the following empirical relationship is established: [GRAPHICS] where a was found to be -0.076. The validity of the specific refractive index increment at constant chemical potential is established through light-scattering measurements of the weight-average chain length of the hydrolyzed polyacrylamides. These were then compared with the chain lengths of the parent nonionic polymers from which they were derived. Excellent agreement is observed, implying the polyelectrolyte characterization method developed, and specifically the (partial derivative n/partial derivative c)mu's are accurate and reliable.