Phase separation ionomers

The two matrices in these cements are of a different nature an ionomer salt hydrogel and polyHEMA. For thermodynamic reasons, they do not interpenetrate but phase-separate as they are formed. In order to prevent phase separation, another version of resin glass polyalkenoate cement has been formulated by Mitra (1989). This is marketed as VitraBond, which we term a class II material. In these materials poly(acrylic acid), PAA, is replaced by modified PAAs. In these modified PAAs a small fraction of the pendant -COOH groups are converted to unsaturated groups by condensation reaction with a methacrylate containing a reactive terminal group. These methacrylates can be represented by the formula ... 

Yang, J. C. and Kyu, T. 1990. Kinetics of phase separation of Nafion perfluorinated ionomer and poly(vinylidene fluoride) blends. Macromolecules 23 182-186. 

Landis, F A. and Moore, R. B. 2000. Blends of a perfluorosulfonate ionomer with poly(vinylidene fluoride) Effect of counterion type on phase separation and crystal morphology. Macromolecules 33 6031-6041. 

This condition has been recently used in a vaporization-exchange model for water sorption and flux in phase-separated ionomer membranes. The model allows determining interfacial water exchange rates and water permeabilities from measurements involving membranes in contact with flowing gases. It affords a definition of an effective resistance to water flux through the membrane that is proportional to... 

Figure 3 shows the effects of composition on the Tg s for both ionomer and nonionomer pseudo-IPN. They exhibit good linearity for both pseudo-IPNs. Nonionomer pseudo-IPN reveals two lines due to the phase separation. The bottom most one is PU and the top most one is VMCC, but they shifted inward due to the formation of pseudo-IPN. 

In contrast, the micrographs of the ionomer pseudo-IPN coatings with opposite charge groups. Figure U- (A-2 to C-2), did not reveal any phase separation. No white particles of the VMCC phase were visible in the dark matrix of the PU phase. Presumably the ionic bonds between the carboxyl and tertiary amine groups provided the best opportunity for interpenetration between the linear chains of VMCC and the networks of PU to prevent any possible phase separation from the ionomer pseudo-IPN microphase. The physical properties of ionomer... 

Glass-ionomers have been used in various areas of restorative dentistry since the mid 1970s. They were invented and originally described by Wilson and Kent [208], and consist of a basic glass powder and a water-soluble acidic polymer. The most widely used polymer is poly(acrylic acid), but acrylic/maleic acid copolymer is also widely used [209]. The glass powder is a complex calcium (or strontium) aluminofluorosilicate [210] that is typically at least partially phase separated. 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

Furthermore, in 2001, Ballard entered an alliance with Victrex to produce two new membrane alternatives. One membrane is based on sulfonated poly(arylether) ketone (a variant of PEEK) supplied by Victrex, which may be better suited to PEMFC fabrication applications. In March 2002, U.S. Patent 6,359,019 was issued to Ballard Power for a graft-polymeric membrane in which one or more trifluorovinylaromatic monomers are radiation graft polymerized to a preformed polymeric base. The strucmres of BAM membranes have been studied by way of small-angle neutron scattering (SANS) [97]. The study of the ionomer peak position suggests the existence of relatively small ionic domains compared to Nalion, despite large water content. Phase separation in the polymer matrix is possibly crucial for the membrane s mechanical and transport properties. 

Fig. 33. Intradiffusion coefficients of water and of protons in concentrated aqueous acid solution, homogeneously sulfonated polyaromatic ionomer, and phase-separated PFSA ionomer [95]. (Reprinted by permission of the Electrochemical Society).

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

The product isolated at dilute concentration exhibits other properties different from that of the material isolated at higher concentration. For example, conventional S-PS (1.7%) will form a homogeneous gel in xylene at concentration >3%, but will phase separate to form a gel phase in more dilute solutions, especially <1%. This behavior has been observed with a number of sulfonate ionomers. 

The goal of the work reported here was to devise a theory that predicts the polar/nonpolar phase separation as a favorable thermodynamic process. In addition, the effects of each of the physicochemical forms on the thermodynamic and structural characteristics of the biphasic material were sought. The current molecular model was not developed to predict mechanical or thermal behavior of ionomers. Hence, properties like glass transition and melt temperatures, storage, and shear moduli cannot be determined from the current model. We must also stress that this modeling work is still in its infancy. As such, it has used several tenuous assumptions that must be tested. The formalism is a start, but not the end, to devising a comprehensive treatment of ionomeric structure. 

E.J. Roche, M. Pineri and R. Duplessix, Phase separation in perfluorosulfonic ionomer membrane, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 107-116 C. Heitner-Wirguin, Recent advances in perfluorinated ionomer membranes structure, properties and application, J. Membr. Sci., 1996, 120, 1-33 G. Gebel and J. Lambard, Small-angle scattering study of water-swollen perfluorinated ionomer membranes, Macromolecules,... 

The melt behavior of sulfonated polystyrene ionomers was studied by Lundberg and coworkers41. As was shown in the case of a sulfonated elastomer30, sulfonation of polystyrene leads to an increase in the melt viscosity (measured at 250 °C) of the SPS upon neutralization, indicating increased association of the sodium poly-(salt). A sudden jump of the melt viscosity occurs at the point of complete neutralization, where a critical concentration of Na polystyrenesulfonate is reached, apparently resulting in a sharp phase separation between the ionic and hydrophobic domains (Figure 5). 

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