Tuning self-assembly of hybrid PLA-P(MA-POSS) block copolymers in solution via stereocomplexation

Abstract

We demonstrate the formation of stable hybrid nanoparticles in solution through self-assembly driven by the stereocomplexation between enantiomeric poly(lactide) (PLA) chains in organic–inorganic hybrid diblock copolymers. The well-defined hybrid diblock copolymers (PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS)) are synthesized via atom transfer radical polymerization of methacrylisobutyl POSS (MA-POSS) using either PLLA or PDLA as a macroinitiator. The structure of the block copolymers is confirmed by 1H NMR and GPC. The mixture of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) in THF solution resulted in the formation of self-assembled nanoparticles as confirmed by the light scattering data. It is further verified that the only driving force for self-assembly in THF solution is the ‘stereocomplexation’ between the PLLA and PDLA blocks as no aggregation could be observed in THF solutions of homochiral polylactides at low concentration (∼1 mg mL−1). A solution mixture of 50:50 weight% of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) for all the samples yields the best stereocomplexation results in terms of particles size and aggregation numbers. For a given composition, the size of the stereocomplexed hybrid nanoparticles, however, decreases with the increasing length of the (MA-POSS) block in the copolymer. As an example, for a 50:50 weight% mixture, the mean hydrodynamic radius Rh and apparent aggregation number Nagg of the particles decreased from 220 nm and 890, respectively, to 72 nm and 290, when the degree of polymerization of P(MA-POSS) increased from ∼2 to 11–12. It is assumed that the bulky POSS nanocages of P(MA-POSS) in the PLA-b-P(MA-POSS) block copolymer sterically hinder the formation of larger nanoparticles by block copolymers with longer P(MA-POSS) blocks. The stereocomplexed nanoparticles remain stable over 30 days and are not sensitive to dilution, suggesting the formation of stable hybrid nanoparticles dispersion. In contrast, the homopolymer mixture of PLLA and PDLA turned cloudy and was no longer stable after 12 days due to the formation of larger macroscopic aggregates. This interesting finding opens up new opportunities to tune the size and stability of the stereocomplexed nanoparticles in solution by manipulating the block length of the inorganic P(MA-POSS) segment.