EFFECTS OF PHYSICAL AGING, CRYSTALLINITY, AND ORIENTATION ON THE ENZYMATIC DEGRADATION OF POLY(LACTIC ACID)

The effects of physical aging, degree of crystallinity, and orientatio n of poly(lactic acid) (PLA) were studied using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The samples of PLA with 96% [L] and 4% [D] contents were prepared by injection mol ding. The physical aging of PLA strongly depended on time and temperat ure. The change of rate of physical aging was very fast initially and slowed down as time increased. The enzymatic degradation of PLA was ca rried out with proteinase K at 37 degrees C at a pH value of 8.6 in a Tris/HCl buffer solution. The enzymatic degradation rate was found to decrease as a function of physical aging (i.e., excess enthalpy relaxa tion). The rate of enzymatic degradation of PLA decreased with the inc rease in crystallinity. A threshold was observed when the heat of fusi on was less than 20 J/g. The weight loss of PLA with a low level of cr ystallinity had no apparent change during any period of testing time. The rates of enzymatic degradation of stretched and injection-molded s pecimens were comparable. (C) 1996 John Wiley & Sons, Inc.