Manufacture Of Capacitive Humidity Sensor Based On Soluble Polyimides

Polymer capacitive humidity material has become the most potential moisture sensitive materials. Some disadvantages have been found from polyimide such as low sensitivity, high hysteresis and nonlinear output, when it was utilized as capacitive humidity material. In this paper, several improvement measures including introducing strong hydrophobic side groups trifluoromethyl onto the polyimide backbone and preparing polyimide nanofiber humidity materials by means of electrospinning, have been put forward for production of high sensitivity, low hydrophobic of capacitive humidity material. On the other hand, the influence of fluorine content and morphology geometric of the moisture sensitive film on its humidity sensitive characteristics have also been considered.Firstly,1,4-bis(4-amino-3-(trifluoromethyl)phenoxy)benzene and4-chloronitrobenzene were used as raw material to prepare fluorine and non-fluorinated diamine monomers, the diamine monomers were polymerized with ODPA to obtain a series of polyimides with different contents of fluorine by one-step technique. The structures of the polyimides were characterized by using FT-IR, DSC, and TG and their solubility were also measured. The results shows that glass-transition temperature of the polyimide were higher than200℃, and the temperature of10%weight in nitrogen were higher than550℃. They were readily soluble in numerous solvents,such as N-methyl-2-pyrrolidone(NMP), N,N-dimethylacctamide(DMAC), dimethylsulfoxide(DMSO), N,N-dimethylformamide(DMF). We have prepared two films by solution casting method and electro-spinning respectively, contrasted of both the contact angle and to the former than the latter, on the other hand, the contact angle is proportional to the fluorine content. With those polyimides different forms of capacitive humidity sensor were prepared, and their sensitivity, hysteresis, temperature coefficient and output linearity were discussed. Furthermore, the differences of humidity sensitive characteristics between the capacitors with different fluorine contents and geometric morphologies were compared. It was found that the sensitivity of nanofibers (maximum5.27PF/%RH)were higher than that of the solution casting film membrane. However, the hysteresis of naofiber capacitor was high than that of the casting membrane one and the fluorine content had no influence on the sensitivity and the hysteresis.