Polystyrene Freeze Dried from Dilute Solution:Tg Depression and Residual Solvent Effects

摘要

The glass transition temperature, Tg, of freeze-dried polystyrene from dilute solution has beenreported to be depressed by 2 to 45 K depending on the sample, solvent, solvent concentration, andfreezing conditions. Moreover, the reduced Tg can also recover back to the bulk value by simple annealingat moderate temperatures near or above Tg. Although such measurements have been reported by manyresearch groups, the impact of residual solvent has not been fully clarified. One issue involves the validityof using weight loss measurements by thermogravimetric analysis (TGA) to ensure the absence of residualsolvent since these are normally performed on compacted samples above the bulk Tg, where diffusionlength scales may be long. In this work, the calorimetric glass temperature was measured as a function ofthe fraction of the residual solvent. In addition, the structure of the freeze-dried polystyrene was examinedusing Fourier transform infrared spectroscopy. We find that the residual solvent has a significant effect onthe Tg depression observed for polymers freeze-dried from dilute solution; whether there is a depressionobserved in the absence of residual solvent will also be discussed.