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Polymerized ionic liquids (PILs) with anions of bis(trifluoromethylsulfonyl)imide (TFSI−), hexafluorophosphate (PF6−), and tetrafluoroborate (BF4−); and cations of poly[1-(4-vinylbenzyl)-3-alkyl-imidazolium] P[VBCnIM]+ with alkyl lengths C1, C2, C4 were successfully synthesized and characterized. X-ray scattering showed an increase of backbone-to-backbone spacing (db) by 0.8 Å per CH2 added to the alkyl side chain. Rheological and dielectric measurements were used to measure rates of chain relaxation and ion dissociation/association. The glass transition temperatures Tg follow the trend: PC4-TFSI < PC2-TFSI < PC1-TFSI< PC1-BF4 < PC1-PF6, which correlates well with their dielectric behaviors. However, the fragility mDR from dielectric relaxation increases with decreasing Tg, which is the opposite of the dependence of the fragility mRheo from rheology for both our PILs and of neutral polymers. The dielectric and rheological relaxations of our PIL’s are expected to be influenced by both their anion-cation binding energies and their relative free volumes, while for neutral polymers, only free volume influences relaxation. The increase of fragility of mDR with decreasing Tg, therefore suggests that dielectric relaxation is influenced more by anion-cation binding energy than by free volume, while the reverse is true for mRheo. The plateau modulus GN and entanglement molecular weight Me estimated from rheological measurements agree with predictions of the packing model, using only a small modification of the Flory characteristic ratio C∞ from that of a neutral polymer. Packing lengths of p= 6.0 ~ 9.3 Å and tube diameters dt= 11 ~ 17 nm are found, depending on specific cation and anion structures. 

A high-salt phase-separation re-entry is observed in mixtures of poly (diallyldimethyl ammonium chloride) (PDADMAC), a strong polycation, and poly (acrylic acid) (PAA), a partially charged polyanion, within the pH range 4.7 to 5.3. This intriguing phenomenon exclusively occurs at salt concentrations exceeding the critical salt concentration required for dissolving the coacervate formed at low salt concentrations, here named the “Upper Critical Salt Concentration” (UCSaC), and at monomer concentrations exceeding 0.1M for each polymer. The transition from associative phase separation at low salt concentration, to a single solution, and ultimately to segregative separation at high salt concentration called the Lower Critical Salt Concentration (LCSaC), arises from the interplay between electrostatic interactions and the hydrophobicity of neutral PAA monomers in a high-salt solvent. To explain this transition, we use a theory combining short-range ion pairing and counterion condensation with long-range electrostatics using the random phase approximation (RPA), and with hydrophobic interactions between PAA neutral monomers and water. The latter is modeled through a Flory-Huggins χ parameter of around 0.6. Literature observations of a continuous transition from associative to segregative phase transition with increasing salt concentration, without a homogeneous single-phase solution at intermediate salt concentration, are also predicted and discussed. 

A thorough study is made of the dependences on salt concentration and polymer chain lengths of the low-frequency plateau of coacervates of poly (diallyl dimethyl ammonium chloride), PDADMAC, and poly (sodium 4-styrenesulfonate), PSS. The reliability and reproducibility of these measurements are carefully checked by determining the frequency-dependent stress limits of the rheometer through the use of reference fluids and by repeat experiments with coacervates. Long-time frequency sweeps show that coacervates with less salt are more repeatable than those with higher salt. A low-frequency plateau reliably appears only below a critical salt concentration, and the magnitude of the plateau depends strongly on salt concentration and chain lengths of both polycation and polyanion. It is only present for the molecular weight of the polycation, PDADMAC, higher than 100 kDa, but the magnitude of the plateau is more strongly influenced by the chain length of the polyanion, PSS. Possible causes of the low-frequency plateau are discussed.