Structural and morphological characterization of model elastomeric ion containing polymers

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Virginia Polytechnic Institute and State University

Over the last several years, there has been widespread interest in the structure property behavior of ion containing polymers. Of particular interest are those materials termed ionomers which contain ionic groups that are typically less than 15 mole percent. These ionic interactions provides an additional means of controlling polymer structure and properties. In the conventional ionomers, the ionic groups are placed randomly along the polymeric backbone resulting in a network structure that is rather poorly defined. An alternate approach is to prepare ionomers where the ionic groups are placed at well defined locations along the polymer backbone such as the telechelic ionomers, block ionomers, or the segmented ionomers where the ionene polymers serve as an example. The structure-property behavior of all these three types of ionomers have been investigated in this study.

The telechelic ionomers were based on either a polyisoprene (PIP) or polybutadiene (PBD) backbone. The PIP ionomers have been neutralized with various cations and neutralized to different levels. The bulk properties were found to be highly dependent on the nature of the cation, its valency, and its ionic radius. A series of sulfonated and carboxylated ionomers of similar molecular weight have been studied in order to compare the performance of sulfonated ionomers to that of the carboxylated counterpart. As expected, the nature of ionic association in the sulfonated ionomer was found to be much stronger than in the carboxylate ionomer. The extent of neutralization as well as variations in the non-ionic segment molecular was studied in the PBD telechelic ionomers. For the first time, a bimodal type of SAXS peak was observed for 2000 M̅n, PBD ionomer.

In the segmented ionene polymers, the effect of the non-ionic segment (PTMO) molecular weight, the nature of the counterion (Cl vs. Br vs. I), and the architecture of the ionene segment were investigated. From SAXS analysis, a change in the morphology was indicated when the PTMO segment molecular weight exceeded 3400 M̅n. It was estimated that the interfacial region was very sharp in all the materials investigated. One of the main limitations of the ionene polymers discussed herein is that the material undergoes irreversible degradation at elevated temperature. It has been shown that with the addition of an ionic plasticizer such as zinc stearate, the softening temperature of the ionene polymer is lowered which allows thermal processing without a major loss of properties.

A detailed morphological investigation of methacrylate based block ionomers was conducted. The effect of ionic block length, the architecture of the segments, and variations in the polarity of the glassy segments were investigated. Though the diblock materials exhibited poor tensile properties, the morphological features of these materials were very interesting. Multiple scattering peaks were observed in the diblock materials using SAXS. Both highly ordered as well as disordered regions were observed from TEM measurements. The spacing observed from TEM measurements correlates quite well with that observed from SAXS. Surprisingly, long range ordering was absent in the triblock ionomer with similar ion content. Finally, the spacing between the ionic domains were found to be a strong function of the ionic block length.