Polymer-Plasticizer Coatings for BTEX Detection Using Quartz Crystal Microbalance

Abstract

Sensing films based on polymer-plasticizer coatings have been developed to detect volatile organic compounds (VOCs) in the atmosphere at low concentrations (ppm) using quartz crystal microbalances (QCMs). Of particular interest in this work are the VOCs benzene, ethylbenzene, and toluene which, along with xylene, are collectively referred to as BTEX. The combinations of four glassy polymers with five plasticizers were studied as prospective sensor films for this application, with PEMA-DINCH (5%) and PEMA-DIOA (5%) demonstrating optimal performance. This work shows how the sensitivity and selectivity of a glassy polymer film for BTEX detection can be altered by adding a precise amount and type of plasticizer. To quantify the film saturation dynamics and model the absorption of BTEX analyte molecules into the bulk of the sensing film, a diffusion study was performed in which the frequency-time curve obtained via QCM was correlated with gas-phase analyte composition and the infinite dilution partition coefficients of each constituent. The model was able to quantify the respective concentrations of each analyte from binary and ternary mixtures based on the difference in response time (τ) values using a single polymer-plasticizer film as opposed to the traditional approach of using a sensor array. This work presents a set of polymer-plasticizer coatings that can be used for detecting and quantifying the BTEX in air, and discusses the selection of an optimum film based on τ, infinite dilution partition coefficients, and stability over a period of time.

Keywords: diffusion; plasticization; polymers; quantify; sensitivity.

Figure 1

Comparison between sorption of benzene (a) and toluene (b) in a commonly used rubbery polymer, PIB, and two plasticized PMMA films.

Figure 2

Frequency–time curves for pure PMMA-toluene (a) and a DINCH-plasticized PMMA system (b).

Figure 3

Comparison of experimental data and estimated sensor response for an 80–20 benzene–toluene binary mixture in a PEMA–DIOA (10%) film.

Figure 4

Comparison of experimental data and estimated sensor response from the model for B–T–EB mixture (1–1–1) in the PEMA–DINCH film.

Figure 5

The average error in mole fraction values (ppm) for the four different cases.

Figure 6

The shift in the coated frequency of the PS–BS (15%) film over three months.

Figure 7

Frequency shifts for benzene, toluene and ethylbenzene in PS–BS (15%) film over three months.