A free-volume hole-filling model for the solubility of liquid molecules in glassy polymers 2: Experimental validation

A model developed in a preceding paper for the solubility of liquids into glassy polymers has been validated with experimental results for a range of liquids in polycarbonate. The model gave good predictions for an average free-volume hole size of 65 cm3/mol and a free volume of 9%, which are consistent with measurements from other techniques. The fit was good apart from with water and small alcohols, and this is believed to be due to these molecules pairing or clustering within the free-volume holes. Infra-red spectroscopy confirmed the evidence of pairing and once this was included in the model, better predictions resulted. In all cases, the model was within an order of magnitude of the experimental value, which is considered very good as all parameters used are fundamental and quite accessible. The hole-size distribution previously used to fit gas sorption was found to give poor predictions, and reasons for this are discussed. The Flory–Huggins and Flory–Rehner theories produced predictions that are far too low as these only really apply to rubbery polymers.