Viscoelastic properties of polystyrene using dynamic rheometry

Abstract

Dynamic mechanical thermal analysis (DMTA) of polystyrene samples of different molecular weights has been investigated using a shear-sandwich unit. Information on the viscoelastic properties of such materials at melt temperatures is of interest as this can lead to an improved understanding of polymer behaviour in processing and fabrication technologies. Samples were subjected to a strain which varied in a sinusoidal manner with time and the resultant response of the sample in terms of storage shear modulus ( G′) and loss shear modulus ( G″) was investigated (at a number of different temperatures) with increasing frequency of the applied strain. Results were assessed using the time–temperature superposition principal. Master curves show four characteristic features: a terminal zone at low frequencies, a fluid–elastic transition zone, a rubbery zone and a glassy zone at high frequencies. These features are indicative of different stress relaxation processes. The average molecular weight between entanglements ( M e) was determined from the storage modulus associated with the rubbery zone. This was found to be approximately half the value of M c (the critical average molecular weight for entanglement). The effects of molecular weight were clearly seen in the terminal zone where the polymer behaves analogous to a Newtonian fluid. The non-Newtonian behaviour of the material was reflected by a sharp increase in G′ and a decrease in the dynamic viscosity ( Y′) with frequency. Acceleration factors derived from time–temperature superposition showed good adherence to the Arrhenius relationship with an activation energy for viscous flow found to be essentially independent of polymer molecular weight suggesting the unit of flow is a collection of chain segments rather than the whole chain itself.