Zwitterionic sulfobetaine-containing polyelectrolyte for controlling humidity-sensitivity

Three components copolymers poly(MEPAB-co-DMMSA-co-ST) were synthesized to utilize as humidity-sensitive membranes. The major ingredient of a humidity-sensitive membrane is polyelectrolytes obtained from the copolymers [2-[(methacryloyloxy)ethyl]dimethyl]propylammonium bromide (MEPAB)/N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl) ammonium (DMMSA)/styrene (ST) = 6/1/2, 6/2/1, 7/1/2 and 7/2/1. The humidity sensor prepared from the 20% solution of MEPAB/DMMSA/ST = 6/1/2 and 7/1/2 showed average impedances of 754.4–0.92 and 1137.6–0.65 kΩ, respectively, in a 30–90%RH range. The impedance of the sensor so prepared changes linearly for 3.5 orders of magnitude over the range of 30–90%RH, showing a high sensitivity and good linearity in a semi-logarithmic scale. In addition, it exhibits the advantages of quick response and recovery and very small hysteresis (<1%RH). It is proposed that the good humidity-sensitive properties are related to the introduction of the sulfobetaine-type zwitterion to polymer chain in the humidity-sensing film.