Stimuli-responsive materials have exhibited a wide range of applications, because the necessity of designing novel “smart” systems usually requires materials that can change their chemical and/or physical properties upon exposure to changed environmental conditions. Two representative applications are stimuli-responsive porous membranes and polymer actuators which are subjects of this thesis. On one hand, stimuli-responsive porous membranes are a class of membranes that can adjust their mass transfer and interfacial properties to manipulate permeability and selectivity in response to changing environmental conditions, such as pH, light, temperature, redox, and so on. There has been a rapid development in this field over the past decades or so. However, there are some limitations for these stimuli-responsive porous membranes. For example, for pH-responsive porous membranes, addition of acids and bases into the solution results in salt accumulation that can contaminate the system and diminish the switchability, while
temperature or light responsive porous membranes may be damaged by the stimuli to a certain extent. Therefore, it is still urgent to develop environmentally friendly and cost-effective stimulation modes for stimuli-responsive porous membranes. On the other hand, polymer actuators have aroused scientists’ extensive research interest because of their potential applications in biomedicine, artificial muscles and soft robots. Instead of utilizing mechanical force to achieve motion or deformation, polymer actuators can
move or deform under thermal, optical or electrical stimulation. Among them, photo-thermal-responsive polymer actuators based on liquid crystal polymer networks (LCNs) have emerged as a particularly promising materials system. For this kind of actuator, the photothermal agents are needed to convert optical energy into thermal energy to induce the LC-to-isotropic phase transition that drives the actuator to deform macroscopically. However, the photothermal reagents often have poor compatibility with organic polymeric matrices, resulting in a dilemma that reducing the doping percentages of photothermal reagents would weaken the photo-actuation speeds of the LCN actuators, whereas increasing the doping percentages would lead to serious phase segregations, and then sacrifice the mechanical properties of the LCN actuators. Moreover,fabricating photothermal-responsive LCN actuators that can perform light-driven caterpillar-type motion on untreated surfaces is also challenging. The main topic of this thesis is to learn from nature to design and fabricate the stimuli-responsive porous membranes for the size separation and nanofiltration and the stimuli-responsive polymer actuators based on LCN for remotely controlled motion. We introduce the CO2-responsive polymers into membrane separation, in a bid to develop a new external stimulus that can reversibly “open” and “close” the membrane pore sizes and further impact the
size selectivity of the membrane. Compared with other stimuli, using CO2 as trigger has several merits, such as environmental friendliness, easy operation, excellent repeatabil-ity without any damage and contamination and good depth inside the membranes. Moreover, we designed a photothermal-responsive LCN-based trilayer actuator that can perform near-infrared (NIR) light -guided bending, moving waves and motion on un-treated, either horizontal or inclined surface. The research works in this thesis mainly contain above-mentioned two topics, presented in three chapters.
In the first work, we proposed a novel concept that gas-tunable pore sizes as well as gas-controlled separation of species can be successfully realized by using CO2/Ar as trigger. In this work, a CO2-responsvie polymer, poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA), was grafted to the polydopamine (PDA) modified polyvinylidene fluoride (PVDF) membrane to undergo reversible contraction/extension in response to CO2/Ar, resulting in the corresponding opening and closure of membrane pore. The reversible rejection of gold nanoparticles (AuNPs) with the diameter of 50 nm can be
realized by alternating CO2/Ar bubbling time. This novel modality that integrates CO2-responsive polymers, tunable membrane pore size and membrane separation shows the great potential of developing smart membranes for applications requiring or involving tunable, size-selective separation of molecular species or nanoparticles.
On the basis of the first work, in order to improve the separation capability of CO2-responsive polymer membranes, we developed a CO2-responsive nanofiltration mem-brane based on pyrene modified PDEAEMA (Py-PDEAEMA) and graphene oxide (GO) for water purification. This composite nanofiltration membrane had a number of attrib-utes potentially appealing for water treatment, such as the reversible, gas-tunable water permeability, both high water permeability and rejection of organic dye molecules and
gas-tunable changes of the charge sign. To our knowledge, this is the first report about CO2-responsive nanofiltration membrane. This work combines the advantages of a CO2-responsive polymers and GO-based nanofiltration membrane, demonstrating new perspectives in developing smart stimuli-responsive nanofiltration membranes for water purification.
In the third work, we designed a LCN-based actuator that performed NIR light-guided locomotion. The actuator had a trilayer structure, including a thin reduced graphene oxide (RGO) top layer, an inactive polymer middle layer and an active LCN bottom layer. When exposing the RGO side to a moving NIR laser, a moving wave along the strip actuator is generated, which makes the strip an effective caterpillar walker that could move on untreated, either horizontal or inclined surface under the guide of NIR
laser. Moreover, while known actuators using photothermal effect are usually fabricated by mixing the nanofiller as NIR light heater with the polymer, which may weak the reversible deformation degree and raise the compatibility concern, the easy trilayer fabrication method laminates directly a “sheet” of RGO on thick polymer layers, which circumvents the potential problems
