Depression of T_g in polystyrene by freeze-drying

The calorimetric glass temperature of polystyrenes with molecular weights ranging from 3.0 × 10³ to 43.7 × 10⁶ g/mol are measured as a function of cooling rate for both bulk material and for samples freeze-dried from dilute solution. We find that T_g is depressed approximately 5 K for samples which can fully entangle and also the same amount for ultrahigh molecular weight samples which cannot achieve full entanglement. The lowest molecular weight samples show only 2 K depression on freeze-drying. Annealing eliminates the depression in T_g. The results indicate that the reduction of the glass temperature due to freeze-drying cannot be due to the reduced entanglement concentration induced by freeze-drying.