Carbon dioxide detection using polymer-coated fiber Bragg grating based on volume dilation mechanism and molecular dynamics simulation

Abstract

Here, we report the polyimide-coated fiber Bragg gratings (FBGs) for CO2 detection based on the mechanism of CO2-induced volume dilation in the polyimide matrix, which resulted in a shift of the Bragg wavelength. The detection results indicate that the polyimide-coated FBGs were of good reversibility and repeatability within CO2-N2 cycles. The molecular mechanism was investigated through the analysis of radial distribution function (RDF) and free accessible volume distributions. The RDF analyses of N2, CH4 and CO2 gases reveal that CO2 has stronger intermolecular interactions with polyimide chains. In the adsorption process, the total fractional free volume inside the polyimide matrix increases as the CO2 concentration increases, simultaneously, the small voids in the polyimide matrix gradually turn into the large voids and the opposite trend is found in the desorption process. The maximum volume dilation values of two polyimide systems are 1.04% (Matrimid® 5218) and 1.13% (P84), thus the corresponding coefficients of gas-sensitive expansion are 3.47 × 10-5/vol% CO2 and 3.77 × 10-5/vol% CO2, respectively. Moreover, theoretical formulas of the relationship between the response strain of the sensor and the gas concentration were obtained, which is invaluable and generic and can be applied to detect various gases.