Rapid cooling through the glass transition transiently increases ductility of PGA/PLLA copolymers: a proposed mechanism and implications for devices

Abstract

Heating bioabsorbable plates above T_g allows for temporary softening to facilitate adaptation to bone. This can, however, transiently alter the mechanical properties, a better understanding of which would provide further insight into the use of these polymers. Two types of unoriented l-lactide/glycolide copolymer wafer specimens (82:18 and 95:5 molar ratios) were heated to 90°C, cooled at various rates, and mechanically tested (three-point bend). Long cooling times (∼8 h) did not change mechanical properties compared to unheated controls, whereas faster cooling rates resulted in increased ductility (50–200% increase in energy to break and peak deformation), however, there was gradual recovery. Under simulated physiological incubation conditions (pH 7.4 buffer, 37°C) partial recovery occurred within 48 h. These results fit well into the theoretical framework of free volume considerations. Following rapid cooling to below T_g, the polymer is not initially at equilibrium, containing excess free volume that contributes to increased molecular mobility and ductile behavior. As equilibrium is approached, free volume decreases and the material behaves as a glassy solid. While there is little clinical consequence as regards internal fixation devices, possible transient changes in permeability and other properties could have implications in drug delivery and other applications.