Subject rubbery polymers

This review does not aim for an encyclopedic coverage of the rapidly expanding field of membrane science. The topics treated do. however, present a reasonable sampling of the current literature on membrane science. The subjects treated reflect directly the interests of the authors and many of the topics are fundamentally related to the fact that the membrane is a high polymer. Indeed, the transport and sorption theories presented provide parameters whose physical interpretation often is a useful complement to other methods of physical analysis of the polymeric state. The study of gas sorption and transport in rubbery polymers is peritaps the simplest example of such a case and will be treated first. 

The electrical properties of polymers have been widely studied and the results subjected to a number of excellent reviews (e.g. Seanor (1972), Link (1972) and Parker (1972)). The writer has discussed, in qualitative terms, the effect of molecular structure on some electrical properties such as dielectric constant and power factor (Brydson, 1975). The rubbers show no particular distinctive features in respect of electrical properties over other polymers so this topic will be dealt with very briefly here. Perhaps the main point to be made is that the electrical properties of rubber compounds are often more dependent on the additives than on the rubbery polymer itself. 

Supercritical carbon dioxide tends to lower the glass transition temperature of glassy polymers, by as much as 60" C in some cases. This makes the polymer more rubbery and flexible and allows carbon dioxide to diffuse into these polymers. Shieh et al. (95) reported that amorphous polymers absorb carbon dioxide to a greater extent than glassy polymers and are therefore subject to increased plasticization. Carbon dioxide diffusivities have been measured to be on the order of cm /s in rubbery polymers (97), which is of... 

Whether or not a polymer is rubbery or glass-like depends on the relative values of t and v. If t is much less than v, the orientation time, then in the time available little deformation occurs and the rubber behaves like a solid. This is the case in tests normally carried out with a material such as polystyrene at room temperature where the orientation time has a large value, much greater than the usual time scale of an experiment. On the other hand if t is much greater than there will be time for deformation and the material will be rubbery, as is normally the case with tests carried out on natural rubber at room temperature. It is, however, vital to note the dependence on the time scale of the experiment. Thus a material which shows rubbery behaviour in normal tensile tests could appear to be quite stiff if it were subjected to very high frequency vibrational stresses. 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

The viscosity of the polymer increases rapidly as the temperature is lowered toward Tg, and the polymer chains exist in many conformations as compact coils. The polymer undergoes reversible stretching when subjected to instantaneous stress in the rubbery plateau as it goes from a low to a high entropy value. 

The amorphous or disordered phase of a polymer normally shows a transition from a rubbery to a glassy condition at some characteristic transition temperature Tg. Crystallisation can only proceed above the glass transition temperature where sufficient molecular mobility is present for reorganisation to occur. The glass transition is a difficult phenomenon to explain in detail and the subject is developed further in Section 3.3. 

The measurement method described in this article is an embodiment of the non-resonance, direct-force-excitation approach that subjects a double-lap shear sample of damping polymer to force from a vibration shaker. In concept this approach can be applied irrespective of whether the material is in a rubbery, glassy, or intermediate state. Each material specimen is small in size and behaves as a damped spring over the entire frequency range. The small specimen size is in contrast with some alternate approaches in which the specimens have sufficiently large dimensions to be wave-bearing. 

We have approached the subject in such a way that the book will meet the requirements of the beginner in the study of viscoelastic properties of polymers as well as those of the experienced worker in other type of materials. With this in mind. Chapters 1 and 2 are introductory and discuss aspects related to chemical diversity, topology, molecular heterodispersity, and states of aggregation of polymers (glassy, crystalline, and rubbery states) to familiarize those who are not acquainted with polymers with molecular parameters that condition the marked viscoelastic behavior of these materials. Chapters 1 and 2 also discuss melting processes and glass transition, and factors affecting them. 

It is well known that volume changes on mixing are likely to occur in polymer-solvent systems, and both theoretical and experimental studies have been devoted to this subject (26,27,28). When the volumes of the components are not strictly additive, both theoretical and phenomenological approaches include an excess volume term in the thermodynamic equations for the system. These treatments, however, concern polymeric solutions in equilibrium, i.e. the mixture is liquid or rubbery, not glassy. For solutions in the glassy state, no theoretical descriptions of the excess volume of mixing seems to be available up to date. 

Other measurable features in SMPs relate to the ability of the polymer to fix the imposed strain when subjected to deformation after cooling and offloading, otherwise known as shape fixity (Tobushi et al., 1998). Another relates to the ability of the polymer to recover from the collated strain during deformation after reheating to its former rubbery state. This is often referred to as shape recovery (Hu, 2007 Tobushi et al., 1998). Further elaborations on these important parameters are beyond the scope of this chapter and readers are referred to the targeted literature (Liu et al., 2007 Kang and Nho, 2001 Adler et al., 1991 Tobushi et al., 1998). 

---