A Novel Gas Sensor from Poly(ethylene glycol)-Grafted Carbon Black. Responsibility of Electric Resistance of Poly(ethylene glycol)- Grafted Carbon Black against Humidity and Solvent Vapor

Poly(ethylene glycol) (PEG) having different molecular weight was successfully grafted onto carbon black surface by direct condensation of terminal hydroxyl groups of PEG with carboxyl groups on the surface using N,N′-dicyclohexylcarbodiimide as a condensing agent. The electric resistance of a composite prepared from PEG and PEG-grafted carbon black drastically increased to 103–104 times of initial resistance in humidity and vapor of methanol, and ethanol, which are good solvent of PEG, and returned immediately to initial resistance when it was transferred in dry air. But the change of electric resistance of the composite was hardly observed in hexane and toluene vapor, which are poor solvent of PEG. The logarithm of electric resistance is linearly proportional to relative humidity. The sensitivity of electric resistance decreased with increasing molecular weight of PEG. In addition, the electric resistance of the composite from PEG-grafted carbon black lost the responsibility against solvent vapor above melting point of PEG. By crosslinking of the composite from PEG-grafted carbon black with triisocyanate, the responsibility against solvent vapor disappeared completely. These results indicated that the crystalline structure of PEG plays an important role on the responsibility of electric resistance of PEG-grafted carbon blacks against humidity and alcohol vapor.