Hydrolytic degradation and crystallization behavior of linear 2-armed and star-shaped 4-armed poly(L-lactide)s: Effects of branching architecture and crystallinity

Linear aliphatic polyesters composed of two poly(l-lactide) arms attached to 1,3-propanediol and star-shaped ones composed of four poly(l-lactide) arms attached to pentaerythritol (2-L and 4-L polymers, respectively) with number-average molecular weight (M-n)=1.4-8.4 x 10(4)g/mol were hydrolytically degraded at 37 degrees C and pH=7.4. The effects of the branching architecture and crystallinity on the hydrolytic degradation and crystalline morphology change were investigated. The degradation mechanism of initially amorphous and crystallized 2-L polymers changed from bulk degradation to surface degradation with decreasing initial M-n; in contrast, initially crystallized higher molecular weight 4-L polymer degraded via bulk degradation, while the degradation mechanism of other 4-L polymers could not be determined. The hydrolytic-degradation rates monitored by molecular-weight decreases decreased significantly with increasing branch architecture and/or higher number of hydroxyl groups per unit mass. The hydrolytic degradation rate determined from the molecular weight decrease was higher for initially crystallized samples than for initially amorphous samples; however, that of 2-L polymers monitored by weight loss was larger for initially amorphous samples than for initially crystallized samples. Initially amorphous 2-L polymers with an M-n below 3.5 x 10(4)g/mol crystallized during hydrolytic degradation. In contrast, the branching architecture disturbed crystallization of initially amorphous 4-L polymers during hydrolytic degradation. All initially crystallized 2-L and 4-L polymers had -form crystallites before hydrolytic degradation, which did not change during hydrolytic degradation. During hydrolytic degradation, the glass transition temperatures of initially amorphous and crystallized 2-L and 4-L polymers and the cold crystallization temperatures of initially amorphous 2-L and 4-L polymers showed similar changes to those reported for 1-armed poly(l-lactide). (c) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41983.