Crosslinked poly(L-lactide) and poly(epsilon-caprolactone)

Several methods of introducing crosslinks into poly(L-lactide) (PLLA) were studied. Electron beam irradiation was not effective in crosslinking PLLA, and, moreover, caused severe polymer degradation and embrittlement of the material. Peroxide crosslinking of PLLA with dicumyl peroxide (DCP), however, resulted in gelation. At high peroxide concentrations (13-25 wt%) and high curing temperatures (192 degrees C), a gel fraction equal to 1 could be determined gravimetrically. Peroxide crosslinking with DCP caused chemical modification of the PLLA polymer chain, with the decomposition products having a plasticizing effect on the resulting network. For the in situ crosslinking, a tetrafunctional monomer, 5,5'-bis(oxepane-2-one) (5,5'-BO), was synthesized and used in the copolymerization with epsilon-caprolactone and L-lactide. Differential scanning calorimetry (d.s.c.) and swelling experiments showed that network formation did take place. A pronounced difference in reactivity between the L-lactide and the 5,5'-BO crosslinker was observed, with this difference in reactivity being found to be comparable to the difference between L-lactide and epsilon-caprolactone. Swelling experiments carried out with crosslinked PLLA showed that incorporation of this crosslinker into the polymer network increased at higher polymerization temperatures and longer polymerization times. When 1 mol% of crosslinker was used in the copolymerization with L-lactide, a significant increase in the impact and tensile strength was obtained when compared to uncrosslinked PLLA. Copyright (C) 1996 Elsevier Science Ltd.