Block Copolymer-Coated ATR-FTIR Spectroscopic Sensors for Monitoring Hydrocarbons in Aquatic Environments at High Temperature and Pressure

The detection of hydrocarbons at low concentrations in water is relevant to oil and gas resource development, environmental monitoring, and water resource management. The mid-infrared (MIR) sensor based on a polymer-coated waveguide has been extensively developed for use at ambient laboratory conditions, and the analytical performance is well established. However, real-world deployment may encounter conditions that threaten the actual integrity of the polymer coating and sensor platform such as high salinity, temperatures, and pressures. Previous investigators have found that high temperatures can be particularly challenging for the polymer film. In this paper a new range of styrenic ABA block copolymers (i.e., polystyrene–isoprene–styrene, polystyrene–butadiene–styrene, and Kraton G-1650) have been investigated as materials for hydrocarbon sensing in water at high temperatures and pressures. These polymers were selected due to their combination of low (i.e., rubbery) and high (i.e., glassy) glass transition temperature (Tg) domains which may give rise to improved stability at high temperatures. Initially, a screen of several polymers was performed under ambient conditions where it was found that the polymer Kraton G-1650 was the most sensitive. Kraton G-1650 was next tested under high pressures and temperatures where sensitivity was consistently better than poly(isobutylene) (a highly developed material for sensing). Optical microscope measurements revealed that the Kraton G-1650 film was relatively stable with increasing temperature, and only when exposed to high temperatures (i.e., 80 °C) did the film became separated from the waveguide surface. It is believed that film delamination rather than physical melting or chemical decomposition is the main reason governing nonreproducible sensor response and lower sensitivity at extreme temperatures.