Study On Gas Sensing Of Carbon Black/Poly(Lactic Acid) Series Conductive Composites

Carbon black(CB)/poly(lactic acid)(PLA)conductive biopolymer composite were fabricated by casting.Scanning electron microscopy(SEM)pictures show the morphology of CB/PLA,it is found a uniform distribution of CB particles in PLA is achieved.A low percolation value(1.39 vol%)is achieved due to the formation of a two-dimensional conductive network in the CB/PLA composite.Once the filler loading achieves 1.39 vol%,a sharp resistivity decreases about 10 orders of magnitude,indicating that CB networks through the CPC are formed.Conductive polymer composites were prepared with carbon black(CB)as conductive filler filled in immiscible blends of PLA and polystyrene(PS)by melt mixing.SEM pictures show that PLA and PS matrixes form two phases co-continuous structure,and CB is selectively distributed in the PS phase.The percolation threshold of the blend is 5wt%,and resistivity of CB/PLA/PS reduces about 6~7 orders of magnitude.Gas sensing behaviors of the CB/PLA composite against different organic vapors were evaluated detailedly by monitoring the variations in the resistance when exposed to organic vapors like ethyl acetate,carbon tetrachloride,methanol,ethanol,xylene and n-hexane.It is found the vapor sensing responsivity of the composite drastically increased by 10~2-10~4 times of the initial responsivity in vapors of good solvents such as xylene,ethyl acetate and carbon tetrachloride after 60 s exposure;however,in the poor solvent vapors such as methanol,ethanol and n-hexane,the electric resistance only increased several times,i.e.,the composite film exhibits a good selective sensitivity.The variations of the sensing behaviors during exposure-drying processes were associated with the evolvement of CB conductive networks.The results indicate that the Flory-Huggins interactions parameter x12,related to the solubility parameter(thermodynamic factor),molar volume(kinetic factor)and the measurement temperature,affects the vapor sensing behaviors of CB/PLA composite significantly.A comparison of solubility parameter between polymer and solvent investigates gas sensing behaviors CB/PLA/PS samples to dichloromethane?acetone?ethyl acetate and cyclohexane.The two solubility parameters are very close;responsivity of the composites again organic gas is the next highest.Solubility parameters of solvent and macromolecular chain movement are usually influenced by measurement temperature.Solvent evaporation rate speed up due to test temperature increasing.Organic gas concentration effects on the responsivity of CB/PLA/PS composites.Especially,responsivity of the composites to dichloromethane gas changes obviously to text temperature.It is found the vapor sensing behaviors during exposure-drying are related to the evolutions of the conductive paths in CB/PLA/PS system;the vapor sensing behaviors show a nice reversibility and a good reproducibility.These results suggest CB/PLA and CB/PLA/PS composites can be applied as a nice vapor sensor candidate.