Degradation-Influenced/Induced Self-Assembly of Copolymers with the Combinatory Effects of Changed Molecular Weight and Dispersity

Biodegradable polymers constitute an important class of new materials, in particular biomedical materials. While degradation has been studied extensively as a "destroy" factor of a material for many years, it is less investigated when treated as a "construction" factor to influence a material in a dynamic manner. Herein, we examined a hydrolyzable amphiphilic block copolymer and found significant reorganization of the condensed state of copolymers during degradation in water. The reorganization occurs, depending upon polymer composition and experimental condition, both mesoscopically and macroscopically, which we term as "degradation-influenced/induced self-assembly (DISA)". To this end, we developed dynamic Monte Carlo simulations by introduction of hydrolysis probability while keeping the chain microrelaxation modes. A series of dynamic Monte Carlo simulations of amphiphilic triblock copolymers was carried out in a selective solvent, and three types of DISA were revealed: micelle 1 to micelle 2, sol-gel 1 to sol-gel 2, and precipitate to sol-gel-precipitate. Subsequently, amphiphilic block copolymers poly(D,L-lactide)-b-poly(ethylene glycol)-b-poly(D,L-lactide) (PLA-PEG-PLA) were synthesized, and all of the three DISA types were confirmed in experiments of the aqueous systems of three copolymers of different block lengths. We found that the DISA was regulated by both the decrease of molecular weight (MW) and the increase of dispersity. Here the term "dispersity" emphasizes not only the conventional molecular weight distribution (MWD) of all species but also the more diversified components after degradation, which include blends of copolymers and the degradation-generated homopolymers and oligomers. We also employed fluorescence resonance energy transfer to confirm the percolated network of semibald micelles underlying the temperature-induced physical hydrogel in such systems. The present study illustrates that degradation can act as a new strategy to influence the hierarchical self-assembly of amphiphilic block copolymer chains.