Some thoughts concerning rubber elasticity by a rheologist are described. The author raises some rheologically important issues that seem to be largely neglected for the non-rheologists in the field of rubber science and technology. One is that the network strands of rubbers undergo micro-Brownian thermal motion, and the rubbery network is much denser than that conceived generally. The second is that the mechanical data of rubbers involving finite viscoelastic effects are often analyzed by the theories for purely elastic solids. The third is the risk of the analysis relying on the mechanical data of only uniaxial deformation as well as the significance of various types of deformation in mechanical tests.
This article introduces various types of experimental method for rheological characterization for viscoelastic solid materials as well as the analytical methods for stress-relaxation, creep behavior, dynamic mechanical properties, and plastic deformation using a simple phenomenological model. For the purpose, some experimental data of dynamic mechanical spectra, creep compliance and mechanical yielding for a high density polyethylene are exemplified.
Polymer and rubber materials show remarkable nonlinear viscoelasticity. The nonlinearity may be due to change in material structures, i.e. entanglement and filler structures. The way of thinking leads us to make simultaneous measurements of the responses to large stimuli and additional responses to perturbative stimuli, i.e. differential dynamic modulus and volume resistivity. It has been found that nonlinear viscoelastic properties of entanglement and filler dispersed polymeric systems originate with ruptures in entanglement and filler networks. Rheology gives us extremely effective tool to detect such change in various network structures.
Although molecular simulations are useful tool for material design, it is not straightforward to apply the technique to polymeric materials due to the long relaxation time. The widely used strategy to overcome this difficulty is ‘coarse-graining’ where a set of slow variables is chosen and the other state variables are statistically embedded into the time evolution of the chosen variables. However, there has not been any established method in this coarse-graining procedure, and thus, semi-empirical models have been proposed for each specific problem. This review showcases such models to clarify the position of each model in the context of the coarse-graining.
Rheological properties like linear viscoelasiticity, temperature dependence, shear rate dependence of viscosity and extensional viscosity are closely related to processability of polymer processing. In this article, relationship between rheological properties of polymer melt and their flow behavior in polymer processing devices especially for film and sheet extrusion, pipe and profile extrusion and calendaring are discussed.