Reprocessable polylactide-based networks containing urethane and disulfide linkages

One of the ways of improving the mechanical properties of polylactide (PLA), which is produced from renewable resources, is crosslinking. In this contribution crosslinked polylactide was obtained by coupling of PLA star polymers using aliphatic diisocyanate, resulting in poly(ester-urethanes). To obtain crosslinked material, able to reprocess, disulfide groups undergoing exchange reactions were introduced into the PLA network structure during the synthesis step. Networks without disulfide groups both, without and with additional low molecular weight diol (diethylene glycol), replacing diol containing disulfide bond (2-hydroxyethyl disulfide) were also prepared for comparative study of mechanical and rheological properties. DSC and FT IR analyses of the networks indicated the presence of hydrogen bonding in obtained materials which influences the ability of disulfide linkages to exchange. The networks with disulfide groups were able to rearrange only at a temperature at which hydrogen bonds were broken (above 100 °C). The ability to rearrange network structure manifested itself in lower storage modulus (G') above glass transition (Tg = 38–48 °C) as well as faster stress relaxation in comparison with the analogous network without disulfide groups. A PLA-based network containing a sufficiently high density of disulfide groups could be reprocessed at a temperature above 170 °C.