Study On Humidity Sensitive And Photocatalytic Properties Of Polymer Based Micro-and Nano-Structured Materials

The introduction of micro-or nano-structure at the surface and interface of materials is a well-investigated and effective method for improving the performance of materials and corresponding functional devices.Polymer materials show the advantages of low price,easy processing and modification.Therefore,they are frequently used as active components or matrix materials to prepare composite materials or devices with high performance in many fields including gas adsorption and separation,biomedical application,sensing,catalysis,etc.,by means of constructing micro-or nano-structures at the surface and interface.Humidity sensors monitor the change of environmental humidity,and play an important role in production and daily life.Photocatalyst materials effectively convert light energy into chemical energy and catalyze the degradation of pollutants in wastewater,thus promoting the recycling of water resources.In this paper,a variety of micro-or nano-structured polymer based materials has been prepared,and their humidity sensitive properties or photocatalytic degradation properties fully investigated.Details are shown as follows:（1）Preparation of humidity sensitive materials based on nanostructured porous films of sulfonated polystyrene-b-poly（4-vinylpyridine）（PS-b-P4VP）by solvent induced phase separation of block copolymers and investigations on their humidity characteristics.The humidity sensitive films with nano porous structure were obtained by controlling the phase separation process of PS-b-P4VP.Subsequent sulfonation of PS segment in PS-b-P4VP resulted in the humidity sensitive materials of sulfonated PS-b-P4VP films composed of nano-sized pores,whose humidity sensitive properties were investigated at room temperature.It is found that the introduction of porous nanostructures can increase the contact area between moisture and the humidity sensitive films,and promote the adsorption/desorption and diffusion of water molecules on the sensitive film,leading to improved humidity sensitive characteristics.Under optimized conditions,as-prepared nanostructured sulfonated PS-b-P4VP films exhibited high humidity sensitivity,fast response(t_90%of 10 s and 13 s for adsorption and desorption,respectively),and relatively small humidity hysteresis（～5%RH）.（2）Preparation of humidity sensitive materials of composite fibrous films with micro/nano structures by electrospinning and investigations on their humidity sensitive characteristics.Poly（4-vinylpyridine）/polystyrene（P4VP/PS）fibrous films were prepared by electrospinning,and functionalized to obtain humidity sensitive materials based on crosslinked and quaternized P4VP（QC-P4VP）/PS fibrous films with 3D porous fiber structure.The morphology of composite fibrous films can be modulated by changing the content of PS of the electrospinning solution.Humidity sensitive characteristics of QC-P4VP/PS fibrous films were investigated at room temperature.It is found that the composition and morphology of the composite fibrous films directly affected their humidity sensing properties,including the linearity of the response curve,response time,hysteresis,etc.Under an optimum ratio of QC-P4VP/PS,the composite fibrous film showed uniform and regular morphology,and exhibited good sensing linearity on a semi-logarithmic scale,high sensitivity,small hysteresis（～5%RH）and short response time(t_90%of9 s and 13 s for adsorption and desorption,respectively).The uniform in-situ deposition of Ag nanoparticles at the surface of QC-P4VP/PS composite fibers resulted in the humidity sensing material of Ag/QC-P4VP/PS composite fibrous films with micro/nano structures.The introduction of Ag nanoparticles with high intrinsic conductivity into QC-P4VP/PS composite fibers greatly decreased the impedance of the composite humidity sensitive film under low humidity levels,and extended the humidity detection range to～4.6%RH.（3）Preparation of humidity sensitive materials based on flexible micro/nano composite porous membranes by using the template method and investigations on their humidity sensitive characteristics.With flexible microporous filter membrane of poly（vinylidene fluoride）（PVDF）as both the substrate and the template,the surfactant of cetyltrimethylammonium bromide（CTAB）as the structure regulator,polyaniline（PANI）nanoparticles were deposited on both the surface and walls of pores of PVDF membrane.The resulting PVDF/PANI flexible composite porous membrane with unique micro/nano structure were directly employed as flexible humidity sensors,and their sensitive characteristics were studied at room temperature.It is proved that the introduction of micro/nano structure endowed the flexible humidity sensors with small hysteresis（～5%RH）,and very fast response（it can distinguish the humidity changes caused by exhalation and inhalation,and thus monitor the respiratory process）.Furthermore,the flexible humidity sensors revealed high air permeability,and maintained good humidity sensitive response under bending.The universality of the preparation method is proved by the fabrication of a series of flexible humidity sensors with different hydrophobic microporous filter membranes as the templates and substrates.（4）Preparation of photocatalytic materials based on micro/nano structured composite fibers by the combination of electrospinning and hydrothermal treatment and examinations of their photocatalytic properties.Poly（methyl methacrylate）（PMMA）@TiO₂ composite fibrous materials with micro/nano structures were prepared by hydrothermal treatment of electrospun PMMA fibers containing tetrabutyl titanate.The photocatalytic materials of as-prepared PMMA@TiO₂ fibers exhibited high loading content of TiO₂（42%）and large specific surface area(21.4 m²g^-1).In the photocatalytic degradation of methyl orange（MO）,PMMA@TiO₂fibrous materials showed high adsorption capacity of the dye（34.5%of MO was adsorbed onto the photocatalyst from the solution containing 100 m L MO of 10 mg/L at 30℃in 100 min）,and good photocatalytic degradation efficiency（under UV illumination,100 m L MO of 10 mg/L was completely degraded with 0.1 g of the photocatalyst at 30℃in 50 min）.Moreover,the PMMA@TiO₂ fibrous material is easy to be separated from MO solution,and maintains high photocatalytic degradation efficiency during cycling tests,suggesting its advantages for recycling use.