Linear expansion measurements of polymers by means of a capacitance technique

The design, construction, and operation of a unit for measuring linear expansion properties of polymers is described. This unit utilizes a proximity transducer capacitance gauge to measure variations in capacitance caused by expansion of a sample of polymer held in an Invar (low expansion alloy) sample holder. Measurement of displacement and of temperature is continuously recorded on an X–Y recorder. The effect of operating conditions on the measurement of the linear expansion is described, and certain experimental artifacts are shown. In addition, the application of the instrument to polypropylene, styrene–butadiene rubber, butyl rubber, and to ethylenepropylene rubber is described. Distinct, nonliner regions were found, above the normal glass transition temperatures, for butyl rubber and for ethylene–propylene rubber. In the former case, the effect is probably due to crystallinity; in the latter case, there are probably two minor transitions at about −35°C. and at about −5°C. in addition to the major transition at −67°C. These minor transitions may be due to atactic and to stereoblock propylene segments, respectively.