The Construction And Characterization Of Shape Memory Hydrogel With Thermal-induced Plasticity

Shape memory polymers can memorize a temporary shape under stress and environmental stimuli. Upon exposure to the specific external stimulus, such as temperature, light, electric field, magnetic field, chemicals and pH, they can revert to their permanent shape under stress free condition. Shape memory hydrogels have been attractive materials in areas such as sensors and actuators, structural color materials, biomimetic materials, drug delivery and biomedical, mainly for their combination of functionality and hybrid nature of solid and liquid. The main innovative contribution of this thesis can be divided into the following sections:In this section, we synthesize a hydrophobically modified polyampholyte hydrogel via simple ternary copolymerization of acrylamide, acrylic acid and a cationic surfmer in the absence of organic crosslinkers. We investigated effect of FeCl3, NaCl solution on the hydrogel, respectively. Through either ionic/complex binding of carboxyl groups via trivalent cations or salt-dependent hydrophobic association, the hydrogel can be fixed to a temporary shape successfully, which recovers to its permanent form in the presence of a reducing agent or deionized water, correspondingly. We found the thermal-induced plasticity of this physical hydrogel, which is fascinating and has been rarely reported in other shape memory gel systems. Utilizing this property, the present shape memory hydrogel can reshape its permanent shape conveniently upon heating. A possible mechanism has been discussed in terms of the thermal-induced plasticity.In the second section, we designed thermal/sunlight-activated shape memory effect for a hybrid hydrogel composed of agarose/poly (acrylamide-acrylic acid) semi-interpenetrating network, which is from simple photo-polymerization of acrylamide (AM), acrylic acid (AA) and N,N’-methylenebisacrylamide (MBA) within the gel state of agarose. We investigate the effect of agarose and Fe (Ⅲ)-citric acid solution on the hydrogel. Thermal-activated SME is attributed to the reversible coil to helix transformation of agarose, with organic cross-linked poly (acrylamide-acrylic acid) network to set the permanent shape; Light-triggered shape recovery is driven by the photo-reduction of Fe (Ⅲ) to Fe(Ⅱ) ions in the presence of citric acid, leading to disassociation of the complexation mainly between carboxyl groups and Fe (Ⅲ) ions. The present strategy, on the one hand, avoids the requirement of organic medium, surfactant extraction or complicated molecular design compared to existed methods for fabrication of thermal-activated shape memory hydrogels; On the other hand, provides a simple method for light-activated shape memory hydrogels, especially considering the difficulty for designing such hydrogels currently.