Preparation, Charicteration And Vapor Sensing Behavior Of PEG Conductive Composite Materials

Carbon nanotubes (CNT) have emerged as a new nanomaterial of ever-increasing concerns due to their peculiar structure and unique electronic and physicochemical properties. This investigation carried out mainly focuses on contribution of carbon nanotube and devises a vapor sensing film material with polyethylene glycol (PEG) as matrix. Meanwhile electric response behaviour of PEG-g-CB and of the composite polymer electrolyte membranes induced by solvent vapor moleculars was compared. In addition, the effect of the interaction among polymer, conductive carrier and solvent vapor molecules on gas sensing response perforances was discussed. These studies gave some interesting results, which shall pave the way to developing carbon nanotubes sensor for highly sensitivity and specific molecular detection. In this study, we have got three aspects of results on the basis of experimental results:1. A excellent gas sensing material with nano scale dispersion was prepared by resolution mixing process using PEG with different molecular weights as matrice, doped with furnace black grafted PEG (PEG-g-CB) as a conductive carrier. The effect of molecular weights of PEG on grafting percentage and response to various solvent vapor molecules was investigated in this paper. TEM and UV-vis was used to investigate dispersing behaviour, surface-performance difference and their effect on reproducibility and stability. The results show that the PEG/PEG-g-CB conductive polymer composite film presents higher vapor sensitivity for good solvents such as THF, chloroform and acetone whose resistance rises 10⁴～10⁶ times rapidly. The response sensitivity decreases with increasing the PEG molecular weight. Repeated stability was influenced by dispersion behaviour of carbon black. However, no responsivities to nonpolar solvent vapors such as n-hexane and toluene are observed.2. A novel highly-selective gas sensor working at room temperature was fabricated by a chemical modification on multi-wall carbon nanotubes containing carboxyl groups (MWNT-COOH) with PEG in the presence of N, N-dicyclohexylcarbodiimide (DCC). The resistance responsiveness of the film samples against various organic vapors was investigated. The graft percentage and vibration spectra were characterized by a TGA, a FTIR and Raman spectrometer. By compareing PEG-g-CB, MWCNT-g-PEG sensing film, we found that the ability to form films, response time, sensitivity and reproducibility of the PEG-g-MWCNT film were obviously improved. The film displayed high chemical selectivity, fast response and good reproducibility or long stability to chloroform vapor, which is attributed to the properties of MWNTs grafted PEG polymers. In addition, a hydrogen bond interaction and a swelling theory were utilized to illuminate the different resistance responsiveness induced by solvent vapor molecules interacting with polymer, carbon nanotube. However, only very small responsivities to any other solvent vapors are observed, and the film did not respond basically against nonpolar solvent vapors, for example, n-hexane. This is ascribed to peculiar structure, unique physicochemical properties of carbon nanotube as well as the formation of a polymeric nanoscale layer covering onto the tube surface.3. The vapor-sensitive response behavior of the PEG membranes with different molecular weights was investigated upon exposure to various chemical environments. The effect of lithium concentrations on ion conductivity and response was discussed, and the change rule of the surface microporous structure and vapor-sensitive conductivity with the poly(vinylidene fluoride) (PVDF) content changed was compared for PEG (Da=12000)/ PVDF mixed composite membranes. The experimental results indicated that the PEG/lithium composite membranes exhibit preferential response characteristics, and the response intensities enhanced to acetic acid, chloroform and acetone vapors with molecule weights increased. The ion conductivity was increased, and contrarily the response intensity decreased at higher lithium concentrations. The formation of microporous structures on PEG/PVDF mixed composite membranes surface enlarged its specific aero and improved the response performances strikingly.Four views were suggested to explain the observed experimental phenomenon.(1) Swelling theory. According to "like dissolves like", when a compound system absorbed good organic solvent vapors, polymer matrix will be swelled, which will obstruct or cut off conductive pathway, the gap between conductive particles will be induced. In the meanwhile the resistance will be markedly increased. After vapor mocules were reabsorbed from matrices, polymer molecules will contract and build a new conductive road, thus the resistance retured to the original value.(2) Electron transfer throry. According to semiconductor theory, the interaction between gas mocules and carbon atoms on the suface of semicondutive carbon nanotubes will induce charge transfer, which will indirectly lead to the variation on electro-structures. Metallic MWNTs are hardly sensitive to gas mocules. But it is not eliminated that gas molecules can induce a change in conductivity by adsorption on the MWNT surface. This is still needed to demonstrate in detail.(3) Hydrogen bond theory. The PEG chain consisting of the hydrophobic ethylene units and hydrophilic oxygen atoms may be responsible for the unique characteristics of the composite samples. This structure can form hydrogen bond interaction. Because the weak interaction of intramolecular hydrogen bond shows dynamic reversible character, the composite generates special response performances for outside stimulus from environments. This structural trait allows formation of inter- and intramolecular hydrogen bonds. So the formation of a nano-scale polymer layer covering onto the tube surface will influence on the movement way near PEG segments, the swelling and contracting behaviour of the film as well as the compatibility with polymer. On this basis, we suggested a rational model to illuminate the response mechanism. It is worth noting that the swelling behavior of PEG chains wrapped around CNTs is very different from a pure PEG matrix. Some observations suggest that some solvents could form bridge-bonds via two hydrogen bonds between the oxygen atoms of two neighboring PEG chains. The structure could limit the swelling of PEG, leading to weak response for the film. Herein, chloroform vapor molecule, as a good solvent of PEG, can interact with polymer by the hydrogen bond so that could decreases the sorption and desorption ability of the films.(4) Coordination theory. The oxygen atom on -OH groups of PEG molecules could form a strong coordination action with Li⁺ cation of Li(ClO)_4-salt. This structure is in favor of the dissociation of ion pairs to form isolated cations and anions, which therefore increase ionic condutivity. The higher the content of crystal is, the stronger the abity of coordination is, the stronger the dissolvation or solvation ability is between PEG and Li(ClO)₄. Since these solvents exhibit the ability to promte or restrain coordination, and hence could alter the Li⁺ dissolvation or salvation ability in PEG. Furthermore, this has effect on the dissociation of Li⁺ cation in PEG film. Consequently, the film displays different conductive behaviour in various solvent vapors.