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.
Highlights
Poly(lactic acid) (PLA) is one of the few bio-based polymers currently being produced on a commercial scale with an attractive cost structure [1,2]
Because of the preferential reaction of the epoxy group with the carboxylic acid group, more poly(lactic acid) (PLA) chains will be grafted onto the reactive oligomer chains leading to a larger increase in molecular weight, or second, cross linking may take place with increasing functionality of the chain extender if sufficient hydroxyl groups react with the epoxy groups
In order to determine the degree of polymerization (DP) by MALDI (Matrix-assisted laser desorption/ionization spectroscopy), a low molecular weight homopolymer of E2EA was synthesized using a [Monomer]/[Chain transfer agent] ratio of 10 as MALDI data could not be obtained for the reactive copolymers
Summary
Poly(lactic acid) (PLA) is one of the few bio-based polymers currently being produced on a commercial scale with an attractive cost structure [1,2]. Despite the attractive mechanical properties achieved, it was found that because of the hydrophilic nature of the soft mid-block used in the triblock copolymer, the degradation of PLLA was accelerated in these blends during melt processing leading to a significant drop in the molecular weight of PLLA. Because of the preferential reaction of the epoxy group with the carboxylic acid group, more PLA chains will be grafted onto the reactive oligomer chains leading to a larger increase in molecular weight, or second, cross linking may take place with increasing functionality of the chain extender if sufficient hydroxyl groups react with the epoxy groups. In this study, we have attempted to quantify the functionality of various reactive oligomers and characterize their effects on the molecular weight increase of PLLA, and the stereocomplex crystallization, mechanical properties and hydrolytic stability of triblock copolymer/PLLA/Plasticizer blends. The resultant effects on hydrolytic stability, crystallinity and mechanical properties are reported
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