Activity coefficients of solvents in elastomers measured with a quartz crystal microbalance

Abstract

A piezoelectric quartz crystal microbalance (QCM) has been used to measure the sorption of solvents into elastomers. The equipment can operate from ambient temperature and pressure up to 180°C and 60 bar. The polymers are deposited on the quartz crystal from solution to form films on the order of 0.1 to 1 μm. Because the time to reach equilibrium scales with the square of the film thickness, this technique can generate data much more rapidly than alternative gravimetric techniques, such as electronic or quartz spring microbalances. However, at high mass loading the frequency–mass response of the QCM becomes non-linear. For the systems studied this typically occurred at solvent vapor sorption levels of 20 to 30 wt.%. From the sorption data, we have generated the activity coefficient on a weight fraction basis for solvents in these polymers from about 0.01 to 0.3 weight fraction. To test this method on coatings of polymers with heterogeneous morphology, we measured the sorption of solvents into a series of triblock copolymers comprising styrene endblocks and midblocks of butadiene, isoprene, or hydrogenated butadiene. The microphase domain sizes in these polymers are about an order-of-magnitude smaller than the typical film thickness used in these experiments. The results have been correlated with elastomer structure. Where comparisons are possible, these activity coefficients agree reasonably with those measured by conventional methods at finite concentration using bulk polymer samples, but sometimes appear low relative to activity coefficients at infinite-dilution measured by inverse gas chromatography (IGC). Advantages and limitations of the QCM are summarized.