Rubbery type ionomer

As the author pointed out in the first edition of this book, the likelihood of discovering new important general purpose materials was remote but special purpose materials could be expected to continue to be introduced. To date this prediction has proved correct and the 1960s saw the introduction of the polysulphones, the PPO-type materials, aromatic polyesters and polyamides, the ionomers and so on. In the 1970s the new plastics were even more specialised in their uses. On the other hand in the related fields of rubbers and fibres important new materials appeared, such as the aramid fibres and the various thermoplastic rubbers. Indeed the division between rubbers and plastics became more difficult to draw, with rubbery materials being handled on standard thermoplastics-processing equipment. 

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. 

In this work we used polystyrene-based ionomers.-Since there is no crystallinity in this type of ionomer, only the effect of ionic interactions has been observed. Eisenberg et al. reported that for styrene-methacrylic acid ionomers, the position of the high inflection point in the stress relaxation master curve could be approximately predicted from the classical theory of rubber elasticity, assuming that each ion pah-acts as a crosslink up to ca. 6 mol %. Above 6 mol %, the deviation of data points from the calculated curve is very large. For sulfonated polystyrene ionomers, the inflection point in stress relaxation master curves and the rubbery plateau region in dynamic mechanical data seemed to follow the classical rubber theory at low ion content. Therefore, it is generally concluded that polystyrene-based ionomers with low ion content show a crosslinking effect due to multiplet formation. More... 

While ionomers of many types have been made and characterized [1,2,3], there is little work on the overall relaxation mechanisms. For polymers with low ionic concentrations, there is general agreement on the fundamental relaxation step. The stress relaxes by detachment of an ion pair from one cluster and reattachment to another. For the styrene/methacrylic acid Na salt (ST/-MAA-Na) system, there is a secondary plateau in the relaxation modulus which depends on the ionic content and can be described as a rubbery modulus [4], While a rubbery modulus with stress relaxation due to ionic interchange has been invoked earlier, it does not adequately describe the relaxation curves. A different approach is taken here. 

Block Ionomers. The block ionomers to be discussed are of AB or ABA type, in which one of the blocks is ionic (eg, sodium methacrylate) and the other consists of nonionic units (eg, polystyrene). While ionic block copolymers in a micelle form in both aqueous and nonaqueous solutions have been studied extensively (99-101,130,131), the viscoelastic properties of block ionomers in bulk have not received much attention (132-137). If the short ionic blocks formed micelle-like aggregates, which were surrounded by nonionic blocks, the viscoelastic properties of the diblock ionomers would be very similar to those of stars or polymers of low molecular weight (136). Thus, above the Tg of the nonionic blocks, as the temperature increased the modulus dropped rapidly with a very short rubbery plateau. For example, in a dynamic mechanical study, it was found that a homopolymer containing 490 styrene units showed a Tg at ca 115°C, and started to flow at ca 150°C. However, in the case of a diblock ionomer containing 490 styrene units and 40 sodium methacrylate ionic units showed a Tg at ca 116°C, and flow behavior was observed above ca 165°C, which was only 15°C higher than that of nonionic polystyrene (135). 

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