Aging of poly(lactide)/poly(ethylene glycol) blends. Part 1. Poly(lactide) with low stereoregularity

Poly(lactide) (PLA) is rapidly gaining interest as a biodegradable thermoplastic for general usage in degradable disposables. To improve mechanical properties, a PLA with low stereoregularity was blended with polyethylene glycol (PEG). Blends with up to 30 wt% PEG were miscible at ambient temperature. Blending with PEG significantly decreased the Tg, decreased the modulus and increased the fracture strain of PLA. However, the PLA/PEG 70/30 blend became increasingly rigid over time at ambient conditions. The mechanism of aging primarily under ambient conditions of temperature and humidity was studied. Changes in mechanical properties, thermal transitions and solid state morphology were examined over time. Aging was caused by slow crystallization of PEG. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the Tg. As Tg approached the aging temperature, reduced molecular diffusivity slowed the crystallization rate dramatically. Aging essentially ceased when Tg of the amorphous phase reached the aging temperature. The increase in matrix Tg and the reinforcing effect of the crystals produced a change in mechanical properties from elastomer-like to thermoplastic-like.