Effect of molecular weight and glass transition on relaxation and release behaviour of poly(DL-lactic acid) tablets

Different molecular weight grades of poly(DL-lactic acid) were applied as r elease controlling excipients in tablets for oral drug administration. The role of molecular weight and glass transition in the mechanism of water-ind uced volume expansion and drug release of PDLA tablets was investigated. Mo dulated differential scanning calorimetry (MDSC) was used to determine the glass transition temperature of both dry and hydrated PDLA samples. The abs orption rate and total amounts of sorbed water by the polymer were determin ed by dynamic vapour sorption (DVS). Expansion behaviour of PDLA tablets wa s measured using thermal mechanical analysis (TMA). At 95% relative humidit y all molecular weight grades of PDLA sorbed 1.1-1.3% w/w water, as was det ermined with DVS. MDSC showed glass transition temperature reductions of 10 -11 degreesC for all molecular weight grades of PDLA in water. Volume expan sion studies using TMA showed that the molecular relaxation time and equili brium porosity of the tablets increased with molecular weight. The mean rel axation time increased exponentially with the temperature interval T-g - T. The onset temperature of shape recovery of hydrated tablets was approximat ely 8 degreesC lower than for dry samples. Drug release was only slightly a ffected by molecular weight. It is concluded that volume expansion of compr essed PDLA tablets is related to the glass transition behaviour, originates from water-induced and thermally stimulated shape memory behaviour and is therefore highly dependent on the molecular weight of PDLA. (C) 2001 Elsevi er Science B.V. All rights reserved.