Reinterpretation of the DSC data for atactic polystyrenes by Stadnicki et al.

Reexamination of published DSC traces for first and second heatings of atactic polystyrenes with M̄ n from 2050 to 1.99 × 10 6 , M̄ w /M̄ n ∼ 1.1, is discussed in terms of some newly enunciated principles of calorimetry for the liquid state of atactic polymers. These traces were originally stated to reveal only an endothermic peak above T g , identified with T ll at M̄ n < ca. 10 5 . Three types of behavior are now distinguished: (1) First heating, M̄ n < M c (the entanglement molecular weight), an endothermic slope change attributed to T u , followed by an exotherm at T exo ascribed to melt flow and/or wetting of the DSC pan; (2) first heating for M̄ n above M c , only an endothermic slope change; (3) second heating, all molecular weights, only an endothermic slope change for T ll and, in some cases, T lp . T ll thus defined reaches an asymptotic limit when plotted against log M̄ n , just as T g does. Texo begins to level off, and then, starting at M c , increases without limit. T ll is an isofree volume state as is T g ; T exo is an isoviscous state and hence a pure relaxation process. DSC traces on first heating at M̄ n < 10 5 provide the first known experimental evidence for both the transitional and the relaxational aspects of T ll in the same experiment. The role of zero shear melt viscosity, η 0 , in DSC is noted: T ll depends only on M̄ n ; T exo on η 0 and hence on M̄ w . The T ll temperature is independent of physical form of the specimen: powder, pellet, film. DSC studies on bimodal blends of PS show T ll uniquely dependent on M̄ n , while the T f – ( T exo ) exotherm depends on M̄ w . Such blends can eliminate interference between the T ll endotherm and the T f exotherm. Exotherms using weighted pan lids and exotherms from sintering are also presented. DSC evidence for T lp > T ll is indicated.