The biodegradable polylactide (PLA) and polylactide-co-glycolides (PLGAs) are being widely investigated for use as scaffolds in bone and ligament reconstruction. The glass transition temperatures (T.) for these polymers are generally greater than 37 degrees C, causing PLA and PLGA devices to possess brittle characteristics in physiological conditions. To evaluate the possibility of obtaining PLGA polymers with T-g values below 37 degrees C, we evaluated the determinants of T-g in PLA and PLGA copolymers. The T-g, changes in specific heat capacity (Delta C-p), and enthalpic relaxation (Delta H-g) in two consecutive heating cycles were correlated with lactide/glycolide content and intrinsic viscosity [eta] for PLA, PLGAs 90:10, 75:25, 65:35, and 50:50. A linear correlation was observed between T-g and intrinsic viscosity, with 0.1 dL/g increase in viscosity resulting in an increase in T-g by about 3.55 degrees C. The selection of PLA and PLGA copolymers with [eta] values < 0.19 dL/g, corresponding to a viscosity average molecular weight of < 70 kDa, will obtain PLA/PLGA polymers with T-g values below 37 degrees C. The lowest attainable T-g values were found to be 28-30 degrees C. Intrinsic viscosity also correlated with Delta C-p differences between aged and rapidly cooled polymers, and is therefore important in predicting free volume changes within these polymers upon aging. (c) 2006 Wiley Periodicals, Inc.