Stereocomplexed and Homocrystalline Thermo-Responsive Physical Hydrogels with Tunable Network Structure and Thermo-Responsiveness

Widespread application prospect of thermo-responsive hydrogels requires the materials enabling robust mechanical properties and tunable responsiveness. Herein, we report the robust thermo-responsive physical hydrogels with tunable network structure and responsiveness by controlling the crystallization manner of hydrophobic blocks. The biocompatible, stereocomplexable poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) were introduced into the thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) to obtain the enantiomeric graft copolymers PNIPAM-g-PLLA, PNIPAM-g-PDLA and the corresponding hydrogels. The hydrophobic PLLA/PDLA domains served as the physical crosslinking junctions in the hdyrogels. Crystalline structure of hydrogels can be facilely tuned by varying the ratio of PLLA/PDLA enantiomeric blocks. Stereocomplex (SC) crystallization between PLLA and PDLA facilitates to form the H-bonded hydrophobic domains with denser chain packing, which endows the racemic hydrogels stronger network structure, higher mechanical strength, and better solvent resistance compared to the enantiopure one. The hydrogels exhibit good thermo-sensitivity in water; the stronger network of racemic hydrogel restricts the volume shrinkage and water desorption at high temperatures, enabling the facile control of thermo-responsiveness. Crystallization-tuned thermo-responsiveness of racemic and enantiopure hydrogels also allows for the design of assembled bilayer hydrogels capable of the thermally-triggered reversible shape morphing.