Investigations Regarding the Determination of Glass Transition Temperatures from Moisture Sorption Isotherms

Abstract

Moisture sorption isotherm and glass transition temperature (T_g data on polyvinylpyrrolidone (PVP) of molecular weights less than or equal to 40,000 as well as PVP containing added plasticizers were collected to examine the possibility of extracting T, values from moisture sorption data. Moisture sorption isotherms generated for PVP of various molecular weights were similar to those previously reported. The moisture sorption isotherm for PVP-K15 containing added vinylpyrrolidone (VP) and methylpyrrolidone (MP) were predicted reasonably well by simple addition of their respective isotherms with that of PVP-K15. The T_g values for PVP as a function of moisture was similar to that found previously. Decreasing polymer molecular weight or the addition of plasticizers (VP or MP) reduced the T_g of the polymer system, displacing the T_g to lower temperatures. When the moisture content to give a T_g of 25°C (W_g) was indicated on the isotherm, W_g shifted downward with decreasing molecular weight or increasing plasticization by VP or MP while the shape of the isotherm was not distinguishably altered. Consequently, W_g, and thus T_g, would be difficult to extract from the moisture sorption isotherm.