Preparation of narrowly dispersed stereocomplex nanocrystals: a step towards all-poly(lactic acid) nanocomposites
Stereocomplexed polylactide-based nanocrystals were designed through a two-stage procedure comprising (i) the stereocomplexation of enantiomeric polylactide-based triblock copolymers and (ii) their selective recovery after acid-hydrolysis of the amorphous blocks. A “one-pot” synthetic process to prepare two enantiomeric P(D,D-LA)-b-P(rac-LA)-b-P(D,D-LA) and P(L,L-LA)-b-P(rac-LA)-b-P(L,L-LA) triblock copolymers of n = 10 100 g mol−1 and Đ = 1.10–1.11 was developed via a two-step ring-opening polymerization. The triblock copolymers were then subjected to stereocomplexation of the enantiomeric blocks followed by acidic hydrolysis of the amorphous racemic blocks to obtain uniform in size stereocomplex nanocrystals (scNCs) consisting solely of near-perfect stereocomplex. Hydrolysis conditions were optimized based on DSC analyses and the scNC crystal structure was confirmed by XRD. The morphology of sc-b-P(rac-LA)-b-sc and scNCs was studied by peak-force tapping atomic force microscopy, allowing simultaneous topography and adhesion mapping image analyses. The results clearly evidence the recovery of narrowly dispersed low-adhesive spherical nanoparticles with an average diameter of 15 ± 4 nm with concentration-controlled shape and size. Such unprecedented all-PLA stereocomplexed nanocrystals might find applications as renewable nanofillers in all-PLA nanocomposites.