Complementary use of various modes of deformation for thermomechanical characterisation

Abstract

Dynamic mechanical analysis and rotational or compressional rheometry provide complementary techniques for determining the thermomechanical properties of a sample over a wide temperature range. From the glassy to the melt phase, the modulus of a material may decrease by a factor of 10 6 or more. Such a range is difficult to cover with reasonable accuracy with a single instrument. Those optimised for fluids tend to perform poorly with solid samples, and vice versa. Dynamic mechanical analysers are designed to operate through the glassy and rubbery regimes, in which a large load is required to deform the sample sufficiently to provide displacement signals with a reasonably high signal-to-noise ratio. In the flow regime, better resolution is provided by rheometers, which are optimised to deal with fluids, including polymer melts, and are generally more sensitive. Rotational rheometers (mostly the controlled-stress type) are now complemented by a novel rheological measuring technique, introduced here as dynamic compressional rheology. Dynamic compressional rheology is a novel technique (robust and cost-effective) for which the analytical expressions yielding the shear loss and storage moduli have only recently been developed.