Molecular characterization and crystallization behavior of peroxide-induced slightly crosslinked poly(L-lactide) during extrusion

Reactive extrusion of poly(L-lactide) (PLLA) was carried out in the presence of a small amount of various peroxides with relatively slow decomposition rates. The resulting crosslinked, four-armed randomly branched PLLA (chi-PLLA) was characterized by size exclusion chromatography equipped with multiangle laser light scattering (SEC-MALS), and the results were interpreted according to the type of peroxide used. A new component with a higher molecular weight than the original PLLA was observed in the SEC-MALS chromatograms of the chi-PLLA. The weight-averaged molecular weight (M(w)) of the chi-PLLA was found to increase with increasing effective radical number per PLLA precursor (n), where n is the overall hydrogen abstraction ability of peroxide times the mole ratio of radical to PLLA precursor molecule. This implies that the hydrogen abstraction ability is a good index for the crosslinking efficiency of PLLA. The extent of branching of chi-PLLA was estimated by the shrinking factor, g=< R(g)(2)>(b)/< R(g)(2)>(l), and rationalized with n, where < R(g)(2)>(b) and < R(g)(2)>(l) are the mean square radii of gyration of branched and linear polymers with the same molecular weight, respectively. The nucleation and overall crystallization rate of chi-PLLA in the nonisothermal crystallization from the melt was discussed from the viewpoints of branching and entanglement density. Polymer Journal (2010) 42, 600-608; doi:10.1038/pj.2010.42; published online 26 May 2010