Processing in the Rubbery State

There are a number of polymers which in fact cannot be melt processed because of their high molecular weights. These include PTFE, very high molecular weight polyethylene and most grades of cast poly(methyl methacrylate). In such cases shaping in the rubbery phase is usually the best alternative. 

In addition to differences in shaping range, polymers also vary in their modulus or resistance to deformation in the rubbery state. If a polymer is to be 

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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... 

Very high molecular weight polyethylenes (A/ in the range 1-6 X 10 ) prepared by the Ziegler process have also become available. As might be expected from consideration of Figure 3.1 these polymers cannot be processed easily in the molten state without decomposition and it is therefore often necessary to process in the rubbery phase. 

In the preparation and processing of ionomers, plasticizers may be added to reduce viscosity at elevated temperatures and to permit easier processing. These plasticizers have an effect, as well, on the mechanical properties, both in the rubbery state and in the glassy state these effects depend on the composition of the ionomer, the polar or nonpolar nature of the plasticizer and on the concentration. Many studies have been carried out on plasticized ionomers and on the influence of plasticizer on viscoelastic and relaxation behavior and a review of this subject has been given 119]. However, there is still relatively little information on effects of plasticizer type and concentration on specific mechanical properties of ionomers in the glassy state or solid state. 

The problem of the reaction kinetics and structure of the resulting polymer has many facets and at present it is far from being solved 2 6,15>6164 65 70>71 74 78 80>87-97)-It should be noted that the epoxy-amine systems turned out to be the most convenient for experimental and theoretical studies of the process of formation of the topological structure of networks. In many cases their topology in the rubbery state agrees with the theoretical predictions 61, M 80,87,88>. 

In addition to knowing the temperature shift factors, it is also necessary to know the actual value of ( t ) at some temperature. Dielectric relaxation studies often have the advantage that a frequency of maximum loss can be determined for both the primary and secondary process at the same temperature because e" can be measured over at least 10 decades. For PEMA there is not enough dielectric relaxation strength associated with the a process and the fi process has a maximum too near in frequency to accurately resolve both processes. Only a very broad peak is observed near Tg. Studies of the frequency dependence of the shear modulus in the rubbery state could be carried out, but there... 

Several polymer properties are important in determining the ability to sorb vapors. The glass transition temperature, Tg, is the temperature at which a polymer changes from glassy to rubbery, as described in Chapter 4. Above Tg, (in the rubbery state), permeability is governed entirely by diffusional forces and sorption proceeds rapidly and reversibly. The sorption process is very much like absorption into a liquid and, as discussed later in the context of sorption mod-... 

Figure 7.28. A state diagram showing stability and time-dependent changes in the rubbery state with typical processes (Reproduced with permission from Roos and Karel, 1991c, Applying state diagrams to food processing and development. Food Technol. 45, pp. 66,68-71, 107, Institute of Food Technologists.)...

Pseudo-affine model, the deformation process of polymers in cold drawing is very different from that in the rubbery state. Elements of the structure, such as crystallites, may retain their identity during deformation. In this case, a rather simple deformation scheme [12] can be used to calculate the orientation distribution function. The material is assumed to consist of transversely isotropic units whose symmetry axes rotate on stretching in the same way as lines joining pairs of points in the bulk material. The model is similar to the affine model but ignores changes in length of the units that would be required. The second moment of the orientation function is simply shown to be ... 

In this immobilized state, proteins are more stable than in the rubbery state above the Tg. Interestingly, enzymatic activity can be retained by processing techniques such as spray-drying, even though it involves heat, because spray-drying is an evaporative cooling process. 

AY [)mr were measured by a Rheovibron (Toyo Seiki) at a heating rate of 2°C/min and a frequency of lOH over the temperature range from 150 C to 200° C. These phenomenological results suggest that the activation energy for the molecular transport term in polymer crystallization is associated with molecular diffusion in the super-cooled melt above T. This indicates that the activation process in molecular transport in polymer crystalUzation could be similar to that in the confined molecular motion in the rubbery state above T... 

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