Comparative performance of linear, cross-linked, and plasma-deposited PMMA capacitive humidity sensors

Abstract

The role of cross-linking density in the performance of poly(methyl methacrylate) (PPMA) capacitive relative-humidity (RH) sensors is investigated. Four materials are studied: linear PMMA (no cross-linking), PMMA cross-linked with 5% ethylene glycol dimethacrylate (EGDM), ‘low-density’ plasma-deposited methyl methacrylate (PPMMA), and ‘high-density’ PPMMA. We find that the capacitance of the linear and cross-linked PMMA sensors increases by about 20% from 0 to 100% RH, and the maximum sensitivity of the plasma-deposited materials to moisture is about 13%. The hysteresis of the capacitance versus RH characteristic is found to be signifiant only for the cross-linked PMMA sensor below 1 kHz excitation frequency. A dielectric loss peak is present at approximately 50 Hz in the linear and cross-linked PMMA materials, but is absent in the PPMMA materials. All materials investigated exhibit evidence of a loss peak above 100 kHz. The linearity at 100 kHz of the response to RH of the devices using PPMMA as the dielectric is higher than that of the devices using linear and cross-linked PMMA. PPMMA films are promising materials for moisture-sensing applications.