Rubbery phases

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. 

Figure 8.16. Schematic diagram of modulus versus temperature for two materials A and B to be shaped in the rubbery phase in the temperature range T]-T2. In this range the modulus of A is above a critical figure C above which atmospheric pressure is insufficient to shape sheet of a given thickness. Such material could therefore not be vacuum formed. The type B material would, however, present no problem on this score...

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. 

Following the success in blending rubbery materials into polystyrene, styrene-acrylonitrile and PVC materials to produce tough thermoplastics the concept has been used to produce high-impact PMMA-type moulding compounds. These are two-phase materials in which the glassy phase consists of poly(methyl methacrylate) and the rubbery phase an acrylate polymer, usually poly(butyl acrylate Commercial materials of the type include Diakon MX (ICI), Oroglas... 

Blends have also been produced containing neither acrylonitrile and styrene in the glassy phase nor polybutadiene in the rubbery phase. 

The mechanical properties of two-phase polymeric systems, such as block and graft polymers and polyblends, are discussed in detail in Chapter 7. However, the creep and stress-relaxation behavior of these materials will be examined at this point. Most of the systems of practical interest consist of a combination of a rubbery phase and a rigid phase. In many cases the rigid phase is polystyrene since such materials are tough, yet low in price. 

It is another well-known problem that classical triblock TEP s, i.e. those of the Kraton-type, are often confined to applications under rather mild conditions, due to the relatively low Tg of the glassy phase, and/or the medium thermal stability of the rubbery phase In principle at least, good answers to that challenge can be offered by the type of synthesis discussed in this section. 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

Between Tg and T the polymer contains a crystalline and an amorphous, rubbery phase. The rubbery phase is responsible for the high impact strength. 

With calendering and also with vacuum forming the polymer must be in the rubbery condition in the first case the sheet must be taken off from the last roll under a certain stress, which it can withstand in the rubbery phase only. With vacuum forming the heated sheet is placed on the mould this is only possible when it possesses enough coherence and handability. The word molten is therefore not relevant in these cases. 

Other papers reported the phase separation behavior for the composition showing dual phase morphology [7,20,21,43-45], Delides et al. [43] proposed that the viscosity at the point of phase separation is sufficiently large enough to inhibit diffusion of the epoxy through the rubber (CTBN) and result in the generation of the occluded phase, which is the inclusion of epoxy domains within the rubbery phase. 

The formation of voids in the rubbery phase in HIPS influences its mechanical properties. The formation of voids is believed to facilitate the energy dissipating deformation processes, i.e., crazing and shearing. Crazing and shearing are facilitated under conditions in that the rubber particles can easily cavitate. 

In the mechanical mixture, the dispersion of the rubbery phase is coarse in the graft copolymer, by contrast, it is so fine that it is difficult to detect the elastomer by this optical technique of limited resolving power. However, measurements of the temperature dependence of the mechanical loss show that the elastomer is present as a distinct phase. 

Monomer diffusion in the rubbery phase of PVC-rich reaction products is difficult, and this was demonstrated by polymerizing vinyl chloride (200 grams) at 70°C in the presence of a crude polymerizate (360 grams) containing 9% total rubber suspended in an aqueous solution of poly-(vinyl alcohol) (1.2% with respect to the polymer + monomer weight), so as to obtain a ratio of water/crude + monomer = 2.4 and by using benzoyl peroxide (0.38% with respect to the reacting monomer). 

As discussed in the previous section, the toughening effect depends both on the matrix, where the shear bands are propagating, and the rubbery phase, which induces cavitation and crack bridging. 

The effect produced by a rubbery phase is based on the following mechanism ... 

Chain orientation influences the melting point, which is understandable from the equation Tm - AH/AS. When the polymer is stretched in the rubbery phase, its entropy is already so much decreased (see Figure 4.3 and, later on, in 5.1), that AS has decreased. For natural rubber (cis-1.4-polyisoprene) a relation between Tm and the strain has been established experimentally as shown schematically in Figure 4.4. 

Strongly cross-linked systems such as thermosets, however, show in the rubbery phase a much higher modulus than a rubber, but still considerably lower than in the glassy state. Figure 5.5 schematically presents how E changes with T for systems... 

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