Preparation of epoxy/polyelectrolyte IPNs for flexible polyimide-based humidity sensors and their properties

Resistive humidity sensors based on an epoxy interpenetrating polymer network (IPN) polyelectrolyte films, which had strong adhesion to polyimide substrate, were developed for flexible polyimide electrode. Humidity sensitive epoxy IPN films were prepared using two stage cross-linking between the diglycidyl ether of bisphenol-A (DEGEBA) and a phosphate ester curing agent (EPEs), followed by photocuring of copolymer of 2-(cinnamoyloxy)ethyl methacryate (CEMA) and [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (METAC). Under optimal conditions ([METAC/CEMA]/[DGEBA/EPEs] = 2/1), a semi-log graph of film resistance was plotted linearly by four orders of magnitude (from 103 to 107 Ω) on increasing humidity over a humidity range of 20–95%RH and exhibited good linearity (Y = −0.0529X + 7.8048, R2 = −0.9944). Furthermore, reliability testing (in terms of water resistance and high temperature, humidity, and long-term stabilities) of the epoxy IPN/gold electrode/polyimide substrate showed that they are satisfactory for use as flexible humidity sensors.