Relaxation mechanisms ionomers

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. 

Dielectric TSDC measurements [25] of the blends revealed four relaxation mechanisms. The subglass secondary 7-relaxation (at -120 K) is associated with local motions of parts of the molecular chain. The /i-relaxation (at -160 K) is attributed to the motions of the polar carbonyl groups of the polymer chain. A systematic change of the magnitude (maximum of the current) and position (temperature of the maximum current) of these two relaxations depending on the composition of the ionomer (PUi content) was not observed. 

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. 

Aside from ion content, a wide range of properties is available in ionomers by control of various processing variables, such as degree of conversion (neutralization), type of counterion, plasticizer content and thermal treatment. Various examples illustrating possible effects of these variables on mechanical relaxation behavior and on such mechanical properties as stiffness, strength, and time- or energy-to-fracture have been given. 

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... 

Of the microphase-structure dependent physical properties of ionomers, perhaps the most widely studied are glass transition temperatures, (Tg), and dynamic mechanical response. The contribution of the Coulombic forces acting at the ionic sites to the cohesive forces of a number of ionomeric materials has been treated by Eisenberg and coworkers (7). In cases in which the interionic cohesive force must be overcome in order for the cooperative relaxation to occur at Tg, this temperature varies with the magnitude of the force. For materials in which other relaxations are forced to occur at Tg, the correlation is less direct. 

Y Relaxation. Unlike the other dynamic mechanical relaxations observed in this study, the Y relaxation does not have an analog in the dynamic mechanical behavior of polyethylene, hydrogenated PP s, or other ionomer systems. In addition, it displays no definite trends in changing temperature or magnitude as the level of sulfonation and thermal history are altered. Coupled with the fact that these systems are known to contain water as well as nitrogen, it is not possible to assign this relaxation to any specific phase or mechanism. Additional studies are necessary before this task can be approached adequately. 

In the case of the perfluorocarboxylate ionomers (129), in addition to the three main peaks, a p peak was also identified, but the mechanism for the relaxation was not identified. A y peak between temperatures for the p and y relaxation was assigned to local motions of the polar side groups in the matrix. 

Page, K. A., Cable, K. M., and Moore, R. B. 2005. Molecular origins of the thermal transitions and dynamic mechanical relaxations in perfluorosulfonate ionomers. Macromolecules 38 6472-6484. 

Tachino, H. Kara, E. Hirasawa, K.Tadano and S. Yano (1993), Dynamic mechanical relaxations of ethylene ionomers. Macromolecules 26,752-757. 

Kyu T, Eisenberg A (1982) Mechanical relaxations in perfluorosulfonate-ionomer membranes. ACS Symp Ser 180 79-110... 

Kyu, T., Eisenberg, A. D. I., Mechanical relaxations in perfluorosulfonate ionomer membranes. In Perfluorinated Ionomer Membranes, Adi Eisenberg, Howard L. Yeager American Chemical Society Washington, 1982 Vol. 180, pp. 79-110. 

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