Viscoelastic behavior of polymers

We have relied heavily on the use of models in discussing the viscoelastic behavior of polymers in the transient and dynamic experiments of the last few sections. The models were mechanical, however, and while they provide a way for understanding the phenomena involved, they do not explicitly relate these phenomena to molecular characteristics. To establish this connection is the objective of this section. 

In Section 4.2.2 the central role of atomic diffusion in many aspects of materials science was underlined. This is equally true for polymers, but the nature of diffusion is quite different in these materials, because polymer chains get mutually entangled and one chain cannot cross another. An important aspect of viscoelastic behavior of polymer melts is memory such a material can be deformed by hundreds of per cent and still recover its original shape almost completely if the stress is removed after a short time (Ferry 1980). This underlies the use of shrink-fit cling-film in supermarkets. On the other hand, because of diffusion, if the original stress is maintained for a long time, the memory of the original shape fades. 

Shiga, T., Deformation and viscoelastic behavior of polymer gels in an electric fields, Adv. Polym. Set, 134, 131, 1997. 

Shiga, T. Deformation and Viscoelastic Behavior of Polymer Gels in Electric Fields. Vol. 134, pp. 131-164. 

Deformation and Viscoelastic Behavior of Polymer Gels in Electric Fields... 

Dynamic mechanical analysis (DMA) or dynamic mechanical thermal analysis (DMTA) provides a method for determining elastic and loss moduli of polymers as a function of temperature, frequency or time, or both [1-13]. Viscoelasticity describes the time-dependent mechanical properties of polymers, which in limiting cases can behave as either elastic solids or viscous liquids (Fig. 23.2). Knowledge of the viscoelastic behavior of polymers and its relation to molecular structure is essential in the understanding of both processing and end-use properties. 

Forsman,W.C., Grand, H.S. Theory of entanglement effects in linear viscoelastic behavior of polymer solutions and melts. I. Symmetry considerations. Macromolecules 5,289-293 (1972). 

Technological Advances. TMA and DMA are both widely employed in the characterization of viscoelastic behavior of polymers, composites, and other materials. Notably, TMA and DMA are particularly useful in identifying glass transitions and other low energy-associated sub-glass transitions, which may not be easily... 

In a stress relaxation test, a polymer test specimen is deformed by a fixed amount, eo, and the stress required to hold that amount of deformation is recorded over time. This test is very cumbersome to perform, so the design engineer and the material scientist have tended to ignore it. In fact, several years ago, the standard relaxation test ASTM D2991 was dropped by ASTM. Rheologists and scientists, however, have been consistently using the stress relaxation test to interpret the viscoelastic behavior of polymers. 

Differential Viscoelastic Models. Differential models have traditionally been the choice for describing the viscoelastic behavior of polymers when simulating complex flow systems. Many differential viscoelastic models can be described by the general form... 

Integral viscoelastic models. Integral models with a memory function have been widely used to describe the viscoelastic behavior of polymers and to interpret their rheological measurements [37, 41, 43], In general one can write the single integral model as... 

This is an important parameter to analyze the viscoelastic behavior of different materials mainly in the case of polymeric materials where the dependence of tan 8 with the chemical structure of the polymeric materials give important information about the relaxation processess that take place is these systems. tan8 is commonly used as a first experimental approach to obtain information about the viscoelastic behavior of polymers as function of the frequency, where it is possible to reach experimental information about the effect of the side chain structure of the polymers on conformational and relaxational responses. 

T. A. Tervoort, E. T. J. Klompen, and L. E. Govaert, A Multi-mode Approach to Finite, Three-dimensional Nonlinear Viscoelastic Behavior of Polymer Glasses, J. Rheol., 40, 779 (1996). 

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