Understanding the confinement dynamics in polyzwitterion-salt complexes using fast scanning calorimetry.

The confinement seen in crosslinked polyzwitterions (PZIs) is analogous to that of the confinement seen in semi-crystalline polymers. As confinement in a semi-crystalline polymer increases, the breadth of transition increases, the glass transition temperature, T_g, increases, and the heat capacity increment at T_g decreases. In our study, complexes of PZI-LiCl were created using the following PZIs: poly(sulfobetaine methacrylate), PSBMA; poly(sulfobetaine acrylate), PSBA; and poly(ethyl sulfobetaine methacrylate), PESBMA. PZIs were blended with LiCl at various molar ratios surpassing a 1:1 ratio of moles of ZI monomeric unit to moles of LiCl. Through the dissolution of LiCl into the PZIs, disruptions of the crosslinking occur due to the association of Li⁺ and Cl^- ions onto the zwitterionic moiety. As the LiCl content increased T_g decreased while the heat capacity increment at T_g increased showing that LiCl acts as a plasticizing agent. PZIs have high glass transition temperatures relative to conventional polymers/ionic glass formers and have low fragility indices. Using fast scanning calorimetry to avoid thermal degradation, the fragility indices for our PZIs can be studied as a function of LiCl, NH₄Cl, and Na₂SO₄ salt content, and compared to the pure homopolymer state.