|M.Sc Student||Pavlovsky Svet|
|Subject||Chemical Sensing Materials: Electrically Conductive|
SEBS Copolymers Systems
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Arnon Siegmann (Deceased)|
|Full Thesis text|
Chemical sensors are of interest due to their ability to detect leakage of organic solvents including gasoline. The objective of this research was to develop and investigate liquid/vapor chemical sensing materials based on conductive carbon black (CB) filled [styrene - ethylene butylene - styrene] triblock-copolymers (SEBS - a thermoplastic, multiphase rubber). Several types of SEBS copolymers were studied, differing in composition, molecular weight and melt viscosity.
Composites of SEBS containing various amounts of CB were prepared by blending at elevated temperatures and then compression molding. Their electrical conductivity was measured and samples containing CB at a level near the corresponding percolation threshold were used for the sensing experiments. In the latter, the conductivity was measured during several exposure/drying cycles. Structure characterization included gel content and M.W between cross-links, scanning electron microscopy, dynamic mechanical analysis (DMA) and calorimetry (DSC). The two phase SEBS polymers (S being the minor phase) exhibit two distinct glass transition temperatures corresponding to poly(ethylene/butylene) and polystyrene. The CB particles were preferentially located in the continuous EB phase.
The SEBS composites exhibit large reversible changes in conductivity upon exposure to a limited number of solvents, e.g., acetone, n-heptane, and air drying cycles. To expand the use of the SEBS composites to additional solvents, e.g., cyclohexane, chloroform and 1, 2-dichloroethane, the polymer matrix was chemically crosslinked, preventing solubility in those solvents. Crosslinked samples exposed to chloroform exhibited the highest resistance increase, i.e., highest sensitivity. This behavior was related to the sorption kinetics, affected by the different characteristics of the solvents (solubility parameter, polarity and molecular volume). The samples’ resistance tended to return to its initial value upon short drying in acetone, and longer drying in other studied solvents.
The observed reversible electrical conductivity changes upon solvent sorption/desorption is based on the polymer reversible swelling, changing the distance between the CB particles and resulting in the breakdown and reconstruction of conductive CB networks, respectively. The natures of the SEBS, the CB content and mixing temperature are all significant parameters, determining the sample’s structure and the resultant sensing property.