Second osmotic virial coefficient revisited: Variation with molecular weight and temperature from endothermal to exothermal conditions

Light scattering measurements for six samples of polystyrene (M = 20 400 to 2.5×106) were performed in tert-butylacetate from 0–140 °C. Within this T interval, the second virial coefficients A2 change from negative to positive, pass through a maximum at intermediate temperatures, and get negative again. The M dependence vanishes when A2 = 0 at ϑ+ = 15.5 °C for endothermal, and at ϑ− = 109.5 °C for exothermal conditions. The temperature of athermal behavior increases with M from 42 °C (M = 20 400) to 62 °C (M = 2.5×106). For sufficiently high temperatures A2 grows larger as M is raised, in contrast to the normal situation. There exist strong indications that limM→∞ A2 is generally different from zero, except for theta conditions. The enthalpy part of the second virial coefficient A2,H becomes independent of M at Ξ+ = 31 °C for endothermal solutions and at Ξ− = 90 °C for exothermal. Between these temperatures A2,H decreases as M rises, else it increases. The unperturbed dimensions are the same at ϑ+ and ϑ−. The observations concerning A2(T;M) can be explained theoretically on the basis of the corresponding states principle. For a better understanding of the influence of M, a modified excluded volume treatment is presented; it accounts for the differences in intra- and intermolecular contacts between segments. The interrelation of the chain expansion coefficient and A2 is in agreement with the Zimm–Stockmayer–Fixman equation.