No abstract

No abstract

A simple model is proposed for the molecular weight dependence of the relaxation time spectrum of linear flexible polymers with molecules of nearly uniform length. A systematic approach for determining the parameters of the relaxation time spectrum from dynamic mechanical data, G'(w) and G"(o), shows the possibilities and the limits of this model. The necessary parameters are ne, G N O , and A , for the entanglement and flow behavior and A, and n, for the crossover to the high-frequency glass behavior. Generic values are proposed for polystyrene and polybutadiene of high molecular weight.

No abstract

In this paper, we report the mechanical relaxation behavior of low molecular weight glassforming side-chain liquid crystalline compounds with cyclic cores and their linear polymeric analogues near the glass transition temperature. We examined two systems: one based on cyclic and linear siloxane backbones and one based on cyclic (cyclohexane) and linear aliphatic backbones. Dynamic mechanical spectroscopy is used to measure the dynamic shear moduli and the complex viscosity near but above Tg. The temperature dependence of the zero-shear viscosity of the cyclic compounds is well described by the Vogel-Tammann-Fulcher (VTF) equation. The strong temperature dependence of the viscosity along with the values of the fitted parameters of the VTF equation shows that the cyclic LC compounds are "fragile" liquids. All cyclic LC compounds, regardless of chemical structure, show identical relaxation behavior when viscosity is plotted versus normalized temperature (Tg/T), where Tg is the temperature obtained calorimetrically at a heating rate of 10 K/min. All cyclic compounds show lower viscosity than their linear analogues when plotted versus Tg/T. The difference in viscosity between the cyclic and linear siloxane compounds is much more pronounced than the difference observed in the aliphatic compounds. For the cyclic compounds, master curves of G′ and G′′ can be described by a single Maxwell mode. The linear compounds exhibit much broader mechanical spectra, suggesting a more complex relaxation phenomenon is taking place. Our results show that, while there is little difference in relaxation behavior among low molecular weight cyclic liquid crystalline compounds, the behavior of the linear polymeric systems is quite different.