Distinct Cation-Anion Interactions in the UCST and LCST Behavior of Polyelectrolyte Complex Aqueous Solutions.

Polyelectrolyte complexes (PECs) are recently observed to possess diversified thermoresponsive phase transition behaviors in aqueous solutions. Herein, by adjusting the initial polymer concentrations () of poly(styrenesulfonate) (PSS)/poly(diallyldimethylammonium) (PDADMA) PEC in the same saline solution, in addition to previously reported lower critical solution temperature (LCST), we experimentally observed the upper critical solution temperature (UCST)-type phase transition behavior of PSS/PDADMA PECs at a lower polymer concentration. As elucidated by temperature-dependent Raman spectroscopy and two-dimensional correlation analysis, at temperatures lower than UCST, more hydrophobic polyelectrolyte chains lead to a high proportion of contact ion pairs (CIPs), contributing to UCST-type solid-liquid phase transition; however, at higher concentrations of PEC, the less hydrophobic polyelectrolyte chains correspond to a higher proportion of solvent-separated ion pairs (SIPs), which enables the occurrence of LCST-type liquid-liquid phase transition.

Exploration of Doubly Thermal Phase Transition Process of PDEGA-b-PDMA-b-PVCL in Water.

Understanding of phase transition mechanism of thermoresponsive polymers is the basis for the rational design of smart materials with predictable properties. Linear ABC triblock terpolymer poly(di(ethylene glycol)ethyl ether acrylate)-b-poly(N,N-dimethylacrylamide)-b-poly(N-vinylcaprolactam) (PDEGA-b-PDMA-b-PVCL) was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The doubly thermal phase transition of PDEGA-b-PDMA-b-PVCL in aqueous solution was investigated by a combination of nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), turbidimetry, and dynamic light scattering (DLS).