Ionomer thermal studies

Spectroscopic and Thermal Studies of Ionic Interactions in Ionomers... 

Studies of PMMA-based ionomers also demonstrate the influence of thermal treatment on deformation modes (16). For Na salts of PMMA-based ionomers of 6 and 12 mol% that were cast from DMF, only crazes were observed on straining. However, after an additional heat treatment (48 h at 160°C), which also removes any DMF solvent that is present, shear deformation zones are induced. Hence, the ionic cluster phase, which was destroyed by the polar solvent, has been restored by the heat treatment. 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

Stefanithis, I.D. and Mauritz, K.A., Microstructural evolution of a silicon oxide phase in a perfluorosulfonic acid ionomer by an in situ sol-gel reaction. 3. Thermal analysis studies, Macromolecules, 23, 2397, 1990. 

Ionomer membranes based on perfluorocarbon polymers became available In the late 196O s. These materials have excellent chemical resistance, thermal stability, mechanical strength and strong acid strength, A number of functionalities have been studied. Including carboxylate, sulfonate and sulfonamide, but only the first two are available as commercial materials. Ferfluorlnated lonomers have been evaluated as membranes In a variety of applications, such as water electrolysis, fuel cells, air driers, Donnan dialysis In waste metal recovery, and acid catalysts, but the primary interest in these materials is for the permselective membrane In electrochemical processes such as In the production of chlorine and caustic (58). 

It should be emphasized that in the Nafions, as in other polymers, and especially ionomers, the glass transition temperature can be strongly influenced by the thermal history and the moisture content of the polymer. Furthermore, in the present case, some decomposition can be seen at ca. 190°C in the acid samples, which show considerably lower thermal stability than is observed in the salts. These results are consistent with those reported earlier by Yeo and Eisenberg (31), based on weight loss in thermogravimetric studies. This feature appears to be a common phenomenon in sulfonated systems for example, in the sulfonated polysulfones, improved thermal stability is also observed in the neutralized materials (2). 

The glass transition in a material of the type examined in this study is affected by several factors. On the basis of simple copolymer-type arguments (37,38) one would expect the glass temperature to increase as the extent of sulfonation is increased. However, recent studies on ionomer systems demonstrate that the. onset of clustering can cause a much more dramatic rise in Tg than can be explained on the basis of copolymer effects alone (2,39,40). Varying the level of sulfonation and thermal history alters the degree of crystallinity, which also affects the Tg in polymers of this type. The complex interaction of these variables then, must determine the behavior of the ft relaxation if it is assigned correctly. 

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. 

Because of the molecular structural complexity and the diversity of segmental motions in Nafion ionomers, explanations in the limited reports on DSC data for Nafion membranes have been rather vague and inconsistent. In contrast, explanation of TG data on Nafion seems explicit. The thermal stability of Nafion membranes has been widely investigated by several groups (Wilkie et al., 1991 Tiwari et al, 1998 Lage et al., 2004a,b). Perhaps the most cited paper about Nafion thermal durability is of Wilkie et al. (1991). To study the interaction of poly(methyl methacrylate) and Nafion, Wilkie et al. 

Page KA, Jarrett W, Moore RB (2007) Variable temperature F-19 solid-state NMR study of the effect of electrostatic interactions on thermally-stimulated molecular motions in perfiuor-osulfonate Ionomers. J Polym Sci B Polym Phys 45(16) 2177-2186... 

Escoubes M et al (1984) Ion clustering in styrene-based ionomers - calorimetric and gravimetric hydration studies and effect of ion concentration and thermal history. J Appl Polym Sd 29(4) 1249-1266... 

---