Effects of aromatic side-group structure on thermal transitions of polyzwitterions*

Aromatic structures can have a pronounced effect on the glass forming properties of ionic liquids and polymers. Polyzwitterions are polymers with side groups that contain a covalently linked anion and cation. Depending upon side chain structure, this can lead to high glass transition temperatures, occurring near the onset of degradation. In this study we are using fast scanning calorimetry to investigate how aromatic structures in the side-group alters the glass formation of polyzwitterions. Poly(sulfobetaine vinylimidazole) (PSBVI) and poly(sulfobetaine-4-vinylpyridne) (PSB4VP) were synthesized and cast into films. Both polymers contain identical sulfonate anions allowing for a good comparison of the effect of aromatic structure variations in the side-group. The glass transition could not be observed prior to degradation in either polymer using conventional slow scan DSC. Using fast scanning calorimetry, degradation was avoided by heating and cooling at 2000 K/s which allowed the first measurement of the glass transition process in these materials. We found that PSBVI has a T_g of 250 ^oC while PSB4VP has T_g at 215 ^oC. Analysis of the fragility reveals that PSBVI is a stronger glass former than PSB4VP.