Hydrocarbon polymers, ionic groups

Ionic polymers are a special class of polymeric materials having a hydrocarbon backbone containing pendant acid groups. These are then neutralized partially or fully to form salts. lonomeric TPEs are a class of ionic polymers in which properties of vulcanized rubber are combined with the ease of processing of thermoplastics. These polymers contain up to 10 mol% of ionic group. These ionomeric TPEs are typically prepared by copolymerization of a functionalized monomer with an olefinic unsamrated monomer or direct functionalization of a preformed polymer [68-71]. The methods of preparation of various ionomeric TPEs are discussed below. 

Introduction of small amounts of ionic groups in hydrocarbon polymers exerts a profound effect on their mechanical properties. These ionic groups neutralized with suitable metal ions act as physical cross-hnks within the polymer matrix. The ionic associations can be thermally relaxed to permit sufficient melt flow at the processing temperature. The ion containing polymer thus behaves as a thermoplastic elastomer having the unique ability to act as cross-linked elastomer at ambient temperature, and to melt and flow at elevated temperatures like thermoplastics [50-53]. 

To diminish methanol crossover, PEMs have been researched for decreasing three stages of methanol crossover (adsorption, diffusion, and desorption). Rigidity and lower swelling of hydrocarbon-based polymers can induce minimization of adsorption and diffusion of methanol. Hydrocarbon-based PEM is needed for optimization in mobility and stiffness of ionic groups. Control of ionic and non-ionic phases also can be developed for satisfactory ionic conductivity and methanol crossover at the same time. A smaller number of ionic groups reduce methanol adsorption to PEM. Distribution of ionic sites in PEM affects the morphology... 

Fig. 123a. Simplified structure of the polyion complex LB films showing ionically bound mono-layers of conjugated polymer sandwiched between stearylamine spacer groups with interdigitated hydrocarbon tails, b Simplified organization of the heterostructure LB films showing alternating layers of 1 1 PTAA-StNH2 and SPAn-StNH2 [767]...

The difference in ionic conductivity can be attributed to the difference in freedom of the imidazoUum cation. This effect has already been confirmed empirically in our laboratory [11, 12]. For a simple system such as l-ethyl-3-vinylimidazolium TFSl, the ionic conductivity decreased by about four orders upon polymerization. The ionic conductivity of IL polymer brushes having PEO or hydrocarbon chains as spacers between the vinyl group and imidazolium salt are almost the same before and after polymerization. In spite of their rubberlike physical properties, IL polymer brushes had excellent ionic conductivity, around 10 " Scm , at room temperature. Details of the IL polymer brushes are given in Chapter 31. The ionic conductivity of P3 and P4, in which the imidazolium cation is fixed to the main chain, was very low even after salts were added to the matrix. P5 and P6 having the counter anion on the main chain displayed an ionic conductivity fom orders higher than P3 or P4 (about 10 S cm at 50°C). The distance between the vinyl polymer and the imidazolium cation is important for the high ionic conductivity in IL polymers. 

Oligomeric sulfates are surfactive and will be incorporated in micelles or adsorbed by polymer particles, the ionic end group remaining in the aqueous phase. Thus, the radical activity at the other end of the molecule is transported into the hydrocarbon phase of micelles or particles where relatively high monomer concentrations prevail and polymerization proceeds rapidly. 

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