Compact Rheometer for Dynamic Measurements on Polymer Solutions or Gels

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Abstract
A Pochettino‐type rheometer is described which is suitable for polymer solutions or diluted networks, i.e., gels. It directly measures sinusoidally varying stress and strain together with the phase angle between them. The design makes use of the piezoelectric properties of titanate ceramics for both the ``driver'' and the ``pickup.'' Thanks to these, the rheometer is rather compact and temperature control is greatly simplified. The sample is located in a narrow gap (0.3 mm) between two coaxial cylinders. The very small displacements (0.1 μ) are measured with a high‐resolution inductive displacement transducer. An important feature of the design is a ``compensator'' ceramic, between the driver and the pickup, which eliminates secondary modes of motion generated by the driver. The rheometer is calibrated by an absolute method which avoids the use of standard viscosity samples. The instrument can measure moduli from 500 to 6×107 N/m2. The frequency range—not yet fully exploited—is from 10 to 3500 cps. Measurements have been made between −80° and +30°C. The temperature range possible with the present design is −85° to +100°C. With the above rheometer, the relaxation spectrum of a 25.5% solution of polymethyl acrylate (PMA) in toluene was obtained. Below −25°C this solution exhibits a phase separation on a microscopic scale, leading to a very broad distribution of relaxation times and analogous to that observed for some heterogeneous polymer systems.