Polymeric humidity sensor using phosphonium salt-containing polymers

New phosphonium salt-containing monomer, (vinylbenzyl)tributyl-phosphonium chloride (VTBPC) was prepared and polymerized for humidity-sensitive polyelectrolyte. The humidity-sensitive membranes were composed of cross-linked copolymers with different contents of VTBPC, n-butyl acrylate (n-BA) and 4-vinylpyridine (VP): VTBPC/n-BA/VP=5/0/0, 5/1/0, 5/1/5 and 4/1/6. When the resistance dependencies on the relative humidity of the copolymers were measured, it was found that the impedance was decreased with an increase in the content of VTBPC. The impedance ranged from 107 to 103 Ω between 20 and 95% RH, which was required for a humidity sensor operating at ambient humidity. Temperature dependence, hysteresis and a response time were also measured. The humidity-sensitive characteristics of the cross-linked copolymer of VTBPC, n-BA and VP changed little even after soaking in water for 120 min.