Biodegradable Cellulose Diacetate-graft-poly(l-lactide)s: Thermal Treatment Effect on the Development of Supramolecular Structures

Abstract

Thermal transition property of cellulose diacetate-graft-poly(L-lactide) (CDA-g-PLLA) varies depending seriously on the molar substitution (MS) of lactyl unit, as represented by a drastic Tg depression with increasing MS (0<MS< or =8) and a subsequent crystallization of the PLLA side chains (MS> or =14). To make clear the thermally induced development of supramolecular structures for this series of graft copolymers, physical aging and crystallization experiments were conducted under isothermal conditions at temperatures, respectively, comparable to and higher than their Tg's. For aged copolymers with lower MSs of 4.7 and 22, an enthalpy relaxation was followed by differential scanning calorimetry. The analysis of time evolution of the relaxed enthalpy in terms of a Kohlrausch-Williams-Watts relation revealed that the overall relaxation time and the distribution of relaxation times were, respectively, rather longer and much narrower compared with the corresponding data for plain PLLA. For crystallized copolymers of MS=22-77, a spherulite formation was observed by polarized optical microscopy. The growth rate was much lower than that for PLLA per se, and the developed texture usually contained banded extinction rings unlike the homopolymer. The slower growth kinetics was analyzed quantitatively to estimate the interfacial free energy of PLLA crystals constituting the spherulites, by using a folded-chain crystallization formula expanded for a binary mixing system composed of a miscible crystalline/amorphous polymer pair. Discussion of these experimental results took into consideration the effect of the CDA backbone as "anchoring substrate" and "linked diluent" for the PLLA grafts.