| In the current practice, the conductive polymer composites as gas sensor are usuallyfabricated as films with the thickness from decades to hundreds of microns through hotpressing or casting, which is beneficial to vapor permeating, and resulting in high vaporsensitivity. However, when being used to monitor the flowing vapors in the pipes, thefilmy polymer sensors are seriously limited by easily getting ruptured, less operationarea, instability, and so on. Thus, it is still a tough challenge to establish alternativetechniques to fabricate polymer based gas sensor for flowing vapors in practicalapplication.Structurally, the conductive polymer composites with an aligned porous structurewill be an ideal sensor when the diameter of the pores and thickness of the wall is tomicrons or to the nanometer level. Compared with the ordinary film-like polymer gassensors, this new aligned micro-porous composite sensor is much more suitable formonitoring flowing organic vapors based on the following reasons:(1)The micro-pores greatly increase the surface area of the materials and intensifythe contact between the gas molecules and materials.(2)The even thinner pore walls in the new aligned micro-porous sensor can result inhigher vapor sensitivity.(3)One-dimensional micro-porous structure can facilitate the high speedtransportation of flowing vapors and benefit detection.(4)This kind of sensor has stronger pressure resistance than the film sensors and ismore applicable under the condition of pressure difference.However, there are two basic questions must be solved to achieve the alignedporous conductive composites. One is the construct of the aligned porous structure inpolymer matrix. The other is the formation of the conductive network in an alignedporous structure. For the first question concerned, there are a variety of methods can beused to realize the construction of an aligned porous structure in a polymer matrix, suchas micro processing method, photolithography, soft lithography, etc. But these methodshave shortcomings such as high cost, low efficiency and polymer selectivity andhindered the application in polymer processing. In contrast, the newly developeddirectional freeze-drying process has been paid much attention by the people because ofthe advantages such as simple preparation device, efficient and does not exist chemical reaction.Herein, a conductive polymer composite with an aligned micro-porous structure isobtained through directional freeze-drying process. And, the relationship between thepreparation parameters and structure, the conductivity property and structure,conductivity mechanism, vapor-sensitive response behavior and mechanism are studiedin this paper.The main results are listed below:①Preparation of an aligned porous poly (vinyl alcohol)When poly (vinyl alcohol)(PVA) solution was immersed in liquid nitrogen with thevelocity of2mm/min, after freeze-drying process, the fiber structure was observed inthe specimen’s cross section prepared from1%PVA solution. Regular aligned porousstructure was obtained from the specimen prepared from5%PVA solution. With theincrease of PVA solution concentration, the specimen prepared from10%PVA solutionis fish-bone like structure. And the micro-porous structure of the specimen preparedfrom15%PVA solution is only a trend to orientation.The results show that the structure will change with the immersing velocity. WhenPVA solution was immersed in liquid nitrogen with the velocity of10mm/min, thespecimen prepared from1%PVA solution no longer is fiber network structures.Regular aligned porous structure can also be obtained from5%PVA solution, but thereare some bridges appeared between the pores. The specimen prepared from10%PVAsolution presents special fish-bone like structure. When the concentration of PVAsolution increased to15%, compared with the specimen prepared from2mm/min, theorientation trend of the specimen decreased.When PVA solution was immersed into-70℃ethanol with the velocity of4mm/min, the structure of the specimen prepared from1%PVA solution is floe in group,that is different from the fiber structure. When the concentration of PVA solutionincreased to5%and7.5%, the structure of the specimen presents fish-bone likestructure and the fish-bone has the trend to been thicker. The branches of the fish-bonesare formed by the edge of the large ice crystals. When the concentration of the solutionincreased to10%or above, the morphology changed gradually to a layer structure andthe orientation trend decreased.②Preparation of an aligned porous polyurethanea. Bayer Impranil1380water-borne polyurethane dispersion was used as the rawmaterials for directional freeze-drying process in this section. When polyurethane dispersion was immersed in liquid nitrogen with the velocity of4mm/min, thespecimen prepared from5%polyurethane dispersion by directional freeze-dryingprocess is fiber network structures and the diameter of the fibers is about2-3μm. That issimilar to the structure of the specimen prepared from1%PVA solution. The specimenprepared from20%polyurethane dispersion presents an unobvious aligned porousstructure. And, when the concentration of polyurethane dispersion increased to40%,there are a few holes and only a trend to orientation in the micro-structure of thespecimen. When polyurethane dispersion was immersed in-50℃ethanol with thevelocity of4mm/min, the specimen presents a cleavage morphology and there is noaligned pores in it.b. Bayer UH240water-borne polyurethane dispersion was used as the raw materialsfor directional freeze-drying process. When polyurethane dispersion was immersed in-70℃ethanol with the velocity of4mm/min, there are only a trend to orientation in themicro-structure of the specimen prepared from10%and40%polyurethane dispersionby directional freeze-drying process. However, the specimen prepared from20%polyurethane dispersion by directional freeze-drying process presents regular alignedporous structure, and the diameter of the pores is about25μm and pore wall thickness isabout4μm. When polyurethane dispersion was immersed in-70℃ethanol with thevelocity of1and10mm/min respectively, the result showed that the diameter of poresand pore wall thickness prepared by10mm/min were smaller than1mm/min. Whenpolyurethane dispersion was immersed in-50℃,-70℃,-90℃and-110℃respectively,the specimen prepared by directional freeze-drying process presents regular alignedporous structure. And the temperature of cryogenic lower, the smaller of diameter ofpores and pore wall thickness.③Preparation and conductivity of aligned porous conductive poly (vinyl alcohol)and polyurethane compositeAfter carbon black or carbon nanotubes were added into5%PVA solution orcarbon nanotubes were added into20%polyurethane dispersion, the solutions wereimmersed in-70℃ethanol with the velocity of4mm/min. In this way, aligned porousconductive carbon black/poly (vinyl alcohol), carbon nanotube/poly (vinyl alcohol) andcarbon nanotube/polyurethane composites can be obtained by directional freeze-dryingprocess. Electrical conductivity of composites such as black/poly (vinyl alcohol),carbon nanotube/poly (vinyl alcohol) and carbon nanotube/polyurethane composites asa function of carbon black or carbon nanotubes content is obtained in this paper. These curves showed that the resistivity of the composite decrease significantly when theconductive filler increased to certain content. When the resistivity decreased to a certaindegree, as the conductive filler continue to increase, the resistivity change is not large.When these curves are fit using the percolation concept, the volume percolationconcentration of these composites were obtained as12.5%,6.2%and2.0%respectively.And, the resistivity critical exponents are2.8,6.1and2.7respectively and not in therange of percolation theory described.④Aligned porous conductive poly (vinyl alcohol) or polyurethane composite usedas a sensor for flowing vapors.When aligned porous conductive black/poly (vinyl alcohol), carbon nanotube/poly(vinyl alcohol) and carbon nanotube/polyurethane composites were obtained bydirectional freeze-drying process, the gas sensitive property of these composites toflowing vapors were studied in this paper. The electrical resistance responses of thesesaligned porous conductive composites against different flowing organic vapors weremeasured. The maximum response of these materials against benzene, tetrahydrofuranand acetone in turn reduced. The maximum response increased as the flowing vapourtemperature increase. |
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