Study an biodegradable polymer materials based on poly(lactic acid). I. Chain extending of low molecular weight poly(lactic acid) with methylenediphenyl diisocyanate

Methylenediphenyl diisocyanate (MDI) was used as the chain extender for low molecular weight poly(lactic acid) (PLA) to produce high molecular weight biodegradable polymer material with a better heat resistance. PLA prepolymer with a number-average molecular weight (M-n) of 5800 and a weight-average molecular weight (M-w) of 9800 was produced by direct polycondensation using stannous octoate as the catalyst. After 40 min of chain extension at 175 degrees C, the resulting polymer had a M-n of 15,000 and a M-w of 57,000. The glass transition temperature (T-g) of the low molecular weight PLA prepolymer was 48.6 degrees C. After chain extension, the T-g of the resulting polymer was raised to 67.9 degrees C, as determined by DSC. DMA results also indicate that the heat resistance was improved by the chain extension. The DSC spectrum and X-ray diffraction pattern of annealed samples showed that both the crystallinity and rate of crystallization of PLA were lowered by chain-extension reaction due to the formation of branched molecular structure. (C) 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2546-2551, 1999.