Relaxation curves ionomers

Undervacuum Stress Relaxation Studies. The stress relaxation behavior of the Nafion system presents some unusual characteristics. The relaxation master curves of the precursor, as well as of Nafion in its acid and salt forms, are very broad and are characterized by a wide distribution of relaxation times. The individual stress relaxation curves and the master curves for the precursor (45), Nafion acid and Nafion-K (46), are shown in Figures 14, 15 and 16 with the reference temperatures Indicated in the captions. Time-temperature superposition of stress relaxation data appears to be valid in the precursor and in the dry Nafion acid, at least over the time scale of the experiments. In the case of Nafion-K, time-temperature superposition is not valid, because it leads to a breakdown at low temperatures, which is reestablished at high temperatures (above ISO C). Similar behavior was also observed for a low molecular weight (5x10 ) styrene ionomer. The addition of small amounts of water to the Nafion acid can lead to a breakdown in the time-temperature superposition. The Influence of crystallinity and of strong ionic interaction will be discussed in the section on underwater stress relaxation studies. 

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. 

Figure 3. The individual stress relaxation curves and master curve reduced to the a glass transition temperature for Nafion acid, compared with those of polystyrene and two styrene ionomers reduced to their glass transition temperatures (31).

The apparatus is described and details given of its use with PETP homopolymer, PS/poly(vinyl methyl ether) miscible blend and styrene-styrenesulphonic acid copolymer/ethyl acrylate-4-vinylpyridine copolymer ionomer blend with ionic interactions. Orientation and relaxation curves were obtained for all three samples. It is concluded that the technique is very efficient for obtaining curves with high precision. For these three systems, the relaxation rate increases with temperature. 

Fig. 9. Stress relaxation curves for an annealed polsKethylene-co-methaciylate) ionomer containing 8 mol% of acid groups, 47% of which were neutralized with NaOH (modified from Ref 38).

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... 

Stress relaxation experiments performed in the dry state on the aromatic ionomer films were complicated by the hydrophilic nature of the materials. The curves obtained to date are characteristic of the water plasticized system rather than the dry material. More reliably, experiments were... 

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