Another Unusual Property of Water: It Increases the Glass Transition Temperature of a Glassy Polymer

摘要

We report on the effect of water on the glass transition temperature (T_g) of three amorphous systems: an acrylic-based polymer (Eudragit E100), a pharmaceutically relevant, low molecular weight compound (indomethacin), and a 50 : 50 mixture of the two. The Tg of the polymer increased with water content up to RH ~0.06, followed by a decrease at higher water levels. Consistent with the Tg results, the permeability of water in the polymer decreased with water content, up to RH ~0.06, and began to increase with higher water concentration. Conversely, indomethacin showed exclu-sively a decrease in T_g when water was present. For the drug-polymer mixture, the T_g exhibited a plateau in the RH range where the pure polymer T_g increased, followed by a sharp decrease at higher water content, where the two pure components exhibited decreasing T_g. These results are consistent with previous observations of antiplasticization in two ways: (a) low concentrations of plasticizer have been found to increase, not decrease, the rigidity of polymeric materials, and (b) in the case of low molecular weight materials, plasticizers exclusively produce softening effects. However, there has been no previous report of a plasticizer producing an increase in the T_g of any material. The results presented here demonstrate that water can increase the T_g of a polymer. These results provide a new perspective on antiplasticization and negate any theoretical argument precluding the increase in Tg by addition of a small amount of plasticizer. Results on the T_g of the drug-polymer mixture show that the antiplasticizing effect of water is significant and can have important implications when polymers are used to stabilize amorphous systems.