The Design, Preparation And Sensing Characteristics Of Humidity And Gas Sensitive Materials Based On Polymer And Polymer Composites

Four types of humidity sensitive materials based on polymer and polymer composite were designed and prepared,including an interpenetrating polymer network (IPN)composed of poly(4-vinylpyridine)and poly(glycidyl methacrylate) (P4VP/PGMA),an IPN composed of poly(N,N'-dimethylaminoethyl methacrylate)and poly(glycidyl methacrylate)(PDMAEM/PGMA)and its composites with polyaniline (PANI)or polypyrrole(PPy),a composite of sodium polystyrenesulfonate(NaPSS)and multi-walled carbon nanotube(MWNT)and a composite of poly(diallydimethylammonium chloride)(PDDA)and MWNT,and a hyperbranched polyester undergoing terminal group modification.Humidity sensors were prepared by dip-coating these sensitive materials on the interdigital gold electrodes.In particular,the sensors were heated to initiate the simultaneous quaternization and crosslinking reactions to form the IPN structure.Furthermore,gas sensitive materials and gas sensors based on PPy and its composites with MWNT or polymers were prepared by the method of vapor phase polymerization,in-situ solution polymerization and electrospinning.The compositions and morphologies of the sensitive materials were characterized by means of FT-IR,UV-vis,XRD,TGA,SEM,TEM,AFM,Raman spectroscopy,etc.The humidity sensitive properties of the polymer and polymer composites were investigated at room temperature,together with the gas sensitive properties of PPy and its composites towards NH₃,methanol vapor,etc.The effects of the composition,structure and morphology of the sensitive films,preparation techniques on the sensing characteristics of the humidity and gas sensors were discussed.The humidity and gas sensitive mechanisms were explored.The effect of the concentration of dip-coating solution,ratio of the two polymer components,the concentration of crosslinking agents,the preparation techniques (electrode structure,temperature and time of quaternization reaction,aging time, thickness of the sensitive film,protection film,etc.)and the measuring conditions on the humidity sensitive properties of P4VP/PGMA IPN was investigated.It was found that the polymer network exhibited an impedance change of 10³ï½ž10⁶Ωin the range of 95～25%RH,indicating a high sensitivity.In addition,it showed a fast response (response time of 3 s and 21 s for adsorption and desorption,respectively),small hysteresis(～1%RH)and good linearity(R²=0.996).Besides,it was also featured with good durability to high temperature and high humidity,and high resistance to water and solvents.The investigations on the humidity sensitive mechanism by complex impedance spectra revealed that the polymer composite showed sensing characteristics typical of polyelectrolytes,and ion conductivity played a major role in its conduction behavior.The ratio of hydrophobic to hydrophilic components in the polymer network determined whether it exhibited a resistive-type or capacitive-type response.The effect of the composition and preparation conditions on the humidity sensitive properties of PDMAEM/PGMA IPN was investigated.It was found that the polymer network exhibited an impedance change of 10³ï½ž10⁷Ωin the range of 98～18%RH.In addition,it was also featured with a fast response(response time of 4 s and 20 s for adsorption and desorption,respectively),small hysteresis(～2%RH)and high resistance to water.The polymer network was covered by a layer of vapor phase polymerized PPy or soluble PANI to prepare composite humidity sensors.It was found that the composite not only maintained the good humidity sensitive properties of the PDMAEM/PGMA IPN,but also exhibited a much smaller resistance at low humidity,which facilitated the measurement of low humidities(15～30%RH).The effect of introduction of MWNT on the humidity sensitive properties of the composite of NaPSS and PDDA with MWNT was investigated.And the exploration of its sensing mechanism revealed that its conduction behaviors were related to both ionic and electronic conduction.The humidity sensitive properties of hyprebranched polyester(H20)whose terminal hydroxyl group was converted to carboxylic group were investigated.It was found that the properties and numbers of terminal groups of modified H20 affected its humidity sensitive properties.The effect of the properties and concentration of oxidants and dopants,and the introduction of MWNT on the gas sensitive properties of PPy and its nanocomposites prepared by vapor phase polymerization towards NH₃ vapor was investigated.It was found that the inclusion of MWNT improved the sensitivity of the nanocomposite, reaching 10%towards NH₃ vapor of 50 ppm.In addition,a reversible response was observed in the sensing of high concentration NH₃ vapor(12500 ppm),which indicates its great potentials for practical applications.The effect of the preparation conditions on the NH₃ sensitive properties of PPy prepared by in-situ solution polymerization was investigated,and a reversible and linear response towards NH₃ vapor of very low concentrations(32～550 ppm)was observed.The gas sensitive properties of fibers of PPy and its composite obtained by electrospinning towards NH₃ vapor was investigated, and the fibers exhibited a higher sensitivity and better reversibility than the thin films of PPy and the composite.The exploration on their gas sensitive mechanism revealed that the properties of oxidants and dopants,the introduced MWNT all have great effect on their sensitivity and reversibility in the detection of NH₃ vapor.