Structural characterization and hydrolytic degradation of solid copolymers of D,L-lactide-co-ε-caprolactone by Raman spectroscopy

A series of copolymers of DL-lactide and epsilon-caprolactone (PLA(X)CL(Y): X, % L-lactyl and Y, % E-caproyl units) were synthesized by ring-opening polymerization. Chemical composition, molecular weight, morphology and configuration were studied by Raman spectroscopy. Hydrolytic degradation was monitored comparatively by various techniques. Preferential degradation appeared in D,L-LA units in all the compounds. After 33 days, crystallinity increased in the degradation products of PLA(12.5)CL(75) due to crystallization of the chain fragments enriched in CL-units, whereas PLA(25)CL(50) and PLA(37.5)CL(25) initially amorphous remained amorphous. The initially amorphous PLA(X)CL(Y) copolymers with X greater than or equal to 40% and Y less than or equal to 20%, i.e. having rather long PLA chain segments, yielded crystalline degradation products identified as composed of an oligomeric stereocomplex formed by L-LA, D-LA crystallizable fragments. From the viewpoint of degradation-induced morphology changes, PLA/CL behaved like PLA/GA copolymers but on a much longer time scale, PGA segments in PLA/GA copolymer playing the same role as the PLA ones in PLA/CL copolymers, i.e. being the more sensitive to hydrolysis. (C) 1999 Elsevier Science Ltd. All rights reserved.