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Abstract:In order to reduce the effects of hydrolytic degradation and to maintain sufficient viscosity during processing of biomass based poly(L-lactic acid) (PLLA), various epoxy functional reactive oligomers have been characterized and incorporated into the degraded fragments as chain extenders. The molecular weight of PLLA increased with the increase in functionality of the reactive oligomers. No further increase in molecular weight was observed for oligomers with functionality of greater than five. Under our experimental conditions, no gelation was found even when the highest functionality reactive oligomers were used. This is attributed to the preferential reaction of the carboxylic acid versus the negligible reactivity of the hydroxyl groups, present at the two ends of the degraded PLLA chains, with the epoxy groups. The study provides a clear understanding of the degradation and chain extension reaction of poly(lactic acid) (PLA) with epoxy functional reactive oligomers. It is also shown that a higher functionality and concentration of the reactive oligomers is needed, to bring about a sufficient increase in the molecular weight and hence the hydrolytic stability in circumstances when PLA chains suffer significant degradation during processing.

Based on a series of well-defined oligomers of lactic acid with different end-groups, calorimetric measurements and vibrational spectroscopic studies have been carried out to examine chain conformation and chain packing. These studies yielded data that are directly related to the models used for conformational analysis of both crystalline and amorphous PLA chains. The previously proposed rotational isomeric model dominated by the trans−gauche′−trans (tg′t) sequences has been confirmed. The linear relationship between enthalpy of fusion and reciprocal of lamellar thickness for chain-extended crystals was used to calculate the equilibrium enthalpy of melting of the α crystalline form of PLA and the value was found to be in close agreement with the previously proposed value of 93 J/g reported by Fischer. Using the same technique, the enthalpy of melting for the α′ phase has been determined to be 60 J/g. This confirms the value of 57 J/g reported by Kalish.

A systematic comparison between graft poly( L-lactide) copolymers with different topologies and their ability to form stereocomplexes with poly( D -lactide) (PDLA) is performed. Comb and hyperbranched copolymers based on functional poly(ethylene glycol) and poly( L-lactide) with molecular weights in the range of 2000-90 000 g mol − 1 and moderate molecular weight distributions ( M w / M n = 1.08-1.37) are prepared via the combination of anionic and ring-opening polymerization. Two "topological isomers," a linear poly(ethylene oxide)/poly(glycerol) (PEG/PG) copolymer and a branched PEG/PG copolymer are used as backbone polymers. Furthermore, the stereocomplex formation between PDLA and the hyperbranched and comb copolymers containing poly( L-lactide) arms is studied. Stereocomplex formation is confi rmed by DSC as well as by Fourier transform IR (FTIR) and Raman spectroscopy.