| Popis: |
The incorporation of a small amount of ionic groups on a hydrocarbon polymer backbone profoundly alters its thermal, rheological and electrical properties. During this study, the effects of ionic content, neutralization level and processing history on ionic group mobility have been clarified. Randomly sulfonated polystyrenes (SPS) along with their sodium and zinc salts were prepared and carefully characterized via element analysis, differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA). These techniques all indicate a reduction in polymer chain mobility by enhanced glass transition temperatures (T$\sb{\rm g}$) and rubbery plateau moduli and decreased dielectric relaxation strength with increasing sulfonation and neutralization levels. In addition, the importance of processing history was observed through increased rubbery plateau moduli and ionic transition temperatures with increasing molding temperature. A secondary relaxation mechanism associated with ionic domains was revealed by the failure of the Williams-Landel-Ferry (WLF) equation to describe frequency plane shifts of DMTA and DETA data. These results are consistent with a model of ionomer morphology in which polar groups phase separate into thermally labile ionic crosslinks of high functionality with trapped hydrocarbon inclusions. For comparison with recent theories on relaxations of semi-crystalline polymers, a series of ethylene-methyacrylic acid(E-MAA) copolymers and their sodium salts were prepared and characterized by infra-red(IR) spectroscopy, DSC and DMTA. All E-MAA samples exhibited mechanical relaxations similar to low density polyethylene (LDPE). In the acid form, typical copolymer behavior was observed where the alpha transition temperature (T$\sb{\alpha}$) associated with the crystalline phase decreased and the beta transition temperature (T$\sb{\beta}$) or T$\sb{\rm g}$ increased with increasing MAA content. The gamma transition (T$\sb{\gamma}$) associated with local methylene segment motions remained independent of MAA content. The sodium neutralized samples displayed characteristics of a phase separated system in which T$\sb{\beta}$ was independent of MAA content, but about 5$\sp\circ$C higher than LDPE,and T$\sb{\gamma}$ decreased slightly with increasing MAA. The ionic transition temperature of E-MAA materials was found to be about 100$\sp\circ$C lower than the equivalent SPS material, indicating the weaker clustering behavior of carboxylate compared to sulfonate ionomers and the greater flexibility of ethylene- than styrene-based copolymers. Preliminary experiments probing ionic group and cation mobility in E-MAA samples on a molecular scale were undertaken using $\sp{23}$Na nuclear magnetic resonance (NMR) spectroscopy and DETA. The results indicated a broad step increase in the spin-spin relaxation time (T$\sb2$) with increasing temperature in ethylene glycol and water saturated materials. This increase was correlated with a broad dielectric "water relaxation" peak. |
