Self-assembly Behavior Of Thermo And Photo-responsive Diblock Copolymers In Ionic Liquids

In this paper,photo-and thermo-responsive smart materials in ionic liquids were prepared based on butyl acrylate（BA）and photo-induced unit 4-phenylazophenyl methacrylate（Azo MA）.The P（Azo MA-r-BA）homopolymer,diblock copolymer PDMA-b-P（Azo MA-r-BA）and ionic gel were studied in three parts.The photo-and thermo-responsive behavior in ionic liquids is listed as follows:1.Preparation of P（Azo MA-r-BA）copolymers containing different azobenzene contents and investigation of phase transition behavior of the copolymer in ionic liquids.A random copolymer with uniform molecular weight was prepared by reversible cleavage-adaptive chain transfer（RAFT）polymerization.The phase transition temperature of the polymer in the ionic liquid was measured by a transmittance meter.The results showed that:（1）The lower critical solution temperature（LCST）of the P（Azo MA-r-BA）in ionic liquids decreased with the increase of azobenzene content,and the it has big impact on the value of LCST.（2）The LCST of the P（Azo MA-r-BA）dissolved in single ionic liquid[C₂mim][NTf₂]is lower than room temperature,which is difficult to measure.When dissolved in[C₄mim][NTf₂],the transition temperature of the system is usually too high（>80℃）.But,when[C₂mim][NTf₂]and[C₄mim][NTf₂]were mixed together,the LCST can be regulated within a reasonable range.（3）Due to the higher polarity of the azo groups under ultraviolet light,the transition temperature of the polymer in the cis-state in the ionic liquid is increased by 22°C.（4）Based on the principle of（3）,the ultraviolet light and the visible light can adjust the phase transition process of the solution when the temperature is set at an intermediate value.2.Preparation of diblock copolymer PDMA-b-P（Azo MA-r-BA）and Study on Aggregation Behavior in ionic liquids.The polymer PDMA with RAFT agent at the end was prepared by RAFT polymerization method,and then another P（Azo MA-r-BA）was reacted at the end.The prepared product was characterized by ¹H NMR,and the molecular weight was characterized by gel permeation chromatography.The thermo and photo-induced assembly of PDMA-b-P（Azo MA-r-BA）in[C₂mim][NTf₂]and[C₄mim][NTf₂]ionic liquid mixtures was determined by dynamic light scattering（DLS）.The experimental results are as follows:（1）The diblock copolymer PDMA-b-P（Azo MA-r-BA）has a thermal response behavior in an ionic liquid,and can form a double-layer micelle having a particle diameter of about 200 nm when the temperature is raised to a certain temperature.The transfer temperature is the critical micelle concentration（LCMT）,in which the content of azobenzene can significantly reduce the LCMT.（2）Similar to the interaction of the radom copolymer in ionic liquids,[C₂mim][NTf₂]and[C₄mim][NTf₂]can also tune the value of LCMT in a reasonable range.The value of LCMT decreased with the[C₂mim][NTf₂]content elevated,while increased as the content of[C₄mim][NTf₂]increases.Based on this mechanism,the LCMT can be designed within the room temperature range.（3）When the dilock copolymer is irradiated with UV light,the temperature required to convert into micelles is increased.Compared to the visible light irradiation,the ultraviolet light can increase the LCMT by about 8°C.By placing the system at an intermediate temperature,the reversible micellization and demicellization can be realized.3.Preparation of Thermo-and Photo-responsive actuators.The ionic gel was prepared based on using Azo MA-BA system using[C₄mim][NTf₂]as solvent and EDGMA as cross-linking agent.The structure of the gel was determined by ATR-FTIR.The performance test of the gel is mainly carried out in the following two ways:（1）The gel is cut into tiny gels with a certain width and thickness,and its volume change is observed under a microscope.By changing the temperature of the hot stage,the volume of the gel shrinks at a certain value of temperature,and the ultraviolet light can elevate the temperature of volume change significantly.Ultraviolet light and visible light can tune the volume change if the gel is set at an intermediate temperature.（2）.The gel is cut into strip shape and immersed into the ionic liquids,then the temperature is set an intermediate value.The strip gel can be bent under the irradiation of ultraviolet light,and the shape returns to the initial state when the visible light is irradiated.