Unusual thermal properties of polylactides and polylactide stereocomplexes containing polylactide-functionalized multi-walled carbon nanotubes

摘要

A novel and simple method for the preparation of linear, high-molar-mass, thermally stable polylactide stereocomplexes (sc-PLA) with multiwalled carbon nanotubes (MWCNTs) covalently attached to enantiomeric PLA chains is presented. The MWCNTs modified with organic spacers terminated with -OH groups were used as initiators for the ring-opening polymerization (ROP) of enantiomeric l- and d-lactides. The weight percentage of MWCNTs in the final stereocomplexes was approximately 1 wt %. The nascent (as-polymerized) polymer samples obtained by the ROP in bulk at 130 °C exhibited unusually high melting temperatures (≥190 °C) as well as enthalpies of fusion close to that estimated for polylactides with 100% crystallinity. PLA stereocomplexes were prepared from equimolar mixtures of l-PLA and d-PLA that also contained MWCNT-g-PLA, either by precipitation from solution or, for the first time, as a thin film via the slow evaporation of solvent. Crystallization in the form of stereocomplexes after melting was, unusually, completely reversible, without formation of any homochiral crystallites. Shish-kebab morphologies for the thin-film stereocomplexes containing 1 wt % of MWCNT-g-PLA were observed by atomic force microscopy.