Stereocomplexed physical hydrogels with high strength and tunable crystallizability

Abstract

Physical hydrogels crosslinked by non-covalent interactions have attained increasing attention due to their good mechanical properties and processability. However, the use of feasible and controllable non-covalent interactions is highly essential for preparing such hydrogels. In this article, we report on stereocomplexed physical hydrogels prepared by simple casting and swelling of amphiphilic graft copolymers bearing a poly(acrylic acid) (PAA) backbone and poly(L-lactic acid) (PLLA) or poly(D-lactic acid) (PDLA) stereocomplexable side chains. The microstructure, swelling behavior, and mechanical and shape memory properties of the obtained hydrogels can be tuned by varying the copolymer composition and stereocomplex (SC) crystallization of PLLA/PDLA enantiomeric chains. The long PLLA or PDLA chains segregate to form hydrophobic, crystallized domains in water, serving as physical crosslinking junctions for hydrogels. SC crystallization between PLLA and PDLA further enhances the number density of physical crosslinkers of enantiomerically mixed hydrogels. The SC content increases as the PLLA/PDLA ratio approaches 1/1 in enantiomerically mixed hydrogels. The average distance between crosslinking junctions declines for the hydrogels with a high PLLA (or PDLA) mass fraction (M_PLA) and SC content, due to the increased number density of physical crosslinkers. Accordingly, the tensile strength and the Young's modulus increase but the swelling ratio and the elongation-at-break of the hydrogels decrease with an increase in M_PLA and SC content. The hydrogels exhibit shape memory behavior; the shape fixing ability is enhanced by the SC crystallization of PLLA/PDLA side chains in the hydrogels.