Crystallizable Thermo-responsive Hydrogels With Reversible Shape Morphing Behavior

Intelligent hydrogel,which is a novel type of hydrogels that can perceive environment changes and make timely changes in volume or gel state,has promising applications for soft actuators,drug delivery,and tissue engineering,etc.At present,there are two important problems in the field of intelligent hydrogel.One is how to improve and control the properties of hydrogels using a simple but effective way.The other is how to realize the controllable and reversible shape morphing in hydrogels similiarly using a simple but effective way.To address these problems,the common poly(N-isopropyl acrylamide)-based(PNIPAM-based)thermosensitive hydrogels were selected and studied in this paper.By introducing hydrophobic and crystallizable polymer chains,the network structure and the transient structure of hydrogels were successfully controlled,which endowed the hydrogels with improved and controllable physical properties as well as excellent reversible shape morphing ability.To address the problem in performance,poly(lactic acid)(PLA)was introduced into PNIPAM to prepare the PNIPAM-g-PLA grafted copolymers and the corresponding thermosensitive physical hydrogels.The hydrogels were obtained through a simple coating and reswelling method.The hydrophobic poly(L-lactic acid)(PLLA)and/or poly(D-lactic acid)(PDLA)side chains aggregated to form crystallized domains in water during the swelling process,serving as the physical crosslinking junctions in the hydrogels.The physical properties of hydrogel could be well controlled by varying the content of PLA in the copolymer and the mixing ratio of enantiomeric copolymers PNIPAM-g-PLLA and PNIPAM-g-PDLA.Crystalline structure of the hydrogels could be facilely tuned by varing the mixing ratio of enantiomeric pair;this enabled the control over the network structure.The enantiomerically mixed hydrogel possessed a stronger network due to the formation of more but smaller nanoscale hydrophobic domains and the intrinsically rigid nature of the stereocomplex crystallization.Therefore,the enantiomerically mixed hydrogel exhibited a lower water content,higher strength,and better solvent resistance in the swollen state.The strong physical network of the hydrogel also restricted the volume shrinkage and water desorption above the transition temperature.This distinct thermo-responsiveness in hydrogels enabled the design of novel reversible shape-morphing materials based on the spatial heterogeneity in crystalline structure.Although the introduction of stereocomplex crystallization offers an effective approach to improve and control the physical properties of hydrogels,the excellent shape morphing ability still cannot be realized.To address the problem in application,the reasonable hydrogel system was designed.It was found that the chemically crosslinked hydrogel based on PNIPAM and crystallizable stearyl acrylate(SA)exhibited the programmable reversible shape morphing behavior.The hydrogels were prepared by conventional free radical polymerization,and the hydrophobic stearyl groups aggregated to form crystallized domains in water,serving as the physical crosslinking junctions in the hydrogels.Physical properties of hydrogels could be controlled by varying the content of stearyl groups.The hydrophobic stearyl groups played a critical role in the shape morphing;only the hydrogels with medium stearyl groups(2%-8%)displayed the good reversible shape morphing performance.The programmability allowed the hydrogels to reversibly shift between various complex shapes.Besides,the reversible shape morphing behavior was time-dependent and could be erased via long-time relaxation in cold water(e.g.,30 h),which established the basis for reprogrammability.Microstructural change of the hydrogel during the shape morphing were analyzed by synchrotron radiation WAXS and SAXS,and the unique role of the transient structure in the shape morphing was demonstrated.Finally,the possible mechanism for the reversible shape morphing was proposed.According to the mechanism,the reversible shape morphing was mainly caused by the thermoresponsive conformation change of PNIPAM chains,with the stearyl domains providing a transient template for programming.