Degradation of poly(d,l-lactic acid) microspheres: effect of molecular weight

Degradation behaviors of two poly(d,l-lactic acid) (PDLA) microspheres prepared from two different molecular weights (17000 and 41000) were examined. The degree of the degradation was quantitatively estimated by employing an aqueous gel permeation chromatography (GPC) which allowed the determination of the overall amount of water soluble degradation products released out into aqueous medium. At the initial stage of incubation, short chain oligomers, which were produced in an ultrasonication treatment step during the microsphere preparation were immediately released out. Critical weight average molecular weight of the water soluble oligomers ranged from 1050 to 1150. The polymer degradation behaviors of the microspheres during the 53-day incubation were greatly affected depending on the molecular weight of raw polymers as evidenced by significantly different trends of time-dependent molecular weight change. The lower molecular weight polymeric mocrosphere exhibited a significant degradation with reduced glass transition temperature, while the higher molecular weight polymeric microsphere did not show any detectable change in the degradation until 53 days. It was found the water hydration in the low molecular weight PDLA microsphere immediately allowed the polymer morphology to change from a glassy to rubbery state by lowering the glass transition temperature below the incubation temperature. This led to the more susceptible physical state to the degradation. On the other hand, the high molecular weight microsphere was in the glassy state until they started to degrade.