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1. Thermally Tunable Dynamic and Static Elastic Properties of Hydrogel Due to Volumetric Phase Transition

   https://doi.org/10.3390/polym12071462  

   Jin, Yuqi; Yang, Teng; Ju, Shuai; Zhang, Haifeng; Choi, Tae-Youl; Neogi, Arup (July 2020, Polymers)

   null (Ed.)

   The temperature dependence of the mechanical properties of polyvinyl alcohol-based poly n-isopropyl acrylamide (PVA-PNIPAm) hydrogel was studied from the static and dynamic bulk modulus of the material. The effect of the temperature-induced volumetric phase transition on Young’s Modulus, Poisson’s ratio, and the density of PVA-PNIPAm was experimentally measured and compared with a non-thermo-responsive Alginate hydrogel as a reference. An increase in the temperature from 27.5 to 32 °C results in the conventional temperature-dependent de-swelling of the PVA-PNIPAm hydrogel volume of up to 70% at the lower critical solution temperature (LCST). However, with the increase in temperature, the PVA-PNIPAm hydrogel showed a drastic increase in Young’s Modulus and density of PVA-PNIPAm and a corresponding decrease in the Poisson’s ratio and the static bulk modulus around the LCST temperature. The dynamic bulk modulus of the PVA-PNIPAm hydrogel is highly frequency-dependent before the LCST and highly temperature-sensitive after the LCST. The dynamic elastic properties of the thermo-responsive PVA-PNIPAm hydrogel were compared and observed to be significantly different from the thermally insensitive Alginate hydrogel. 

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2. Unusual Lower Critical Solution Temperature Phase Behavior of Poly(benzyl methacrylate) in a Pyrrolidinium-Based Ionic Liquid

   https://doi.org/10.3390/molecules26164850  

   Carrick, Brian R.; Seitzinger, Claire L.; Lodge, Timothy P. (August 2021, Molecules)

   Polymer/ionic liquid systems are being increasingly explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Poly(benzyl methacrylate) in certain ionic liquids constitute unusual LCST systems, in that the second virial coefficient (A2) in dilute solutions has recently been shown to be positive, indicative of good solvent behavior, even above phase separation temperatures, where A2 < 0 is expected. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide for three different molecular weights (32, 63, and 76 kg/mol) in concentrated solutions (5–40% by weight). Turbidimetry measurements reveal a strong concentration dependence to the phase boundaries, yet the molecular weight is shown to have no influence. The critical compositions of these systems are not accessed, and must therefore lie above 40 wt% polymer, far from the values (ca. 10%) anticipated by Flory-Huggins theory. The proximity of the experimental cloud point to the coexistence curve (binodal) and the thermo-reversibility of the phase transitions, are also confirmed at various heating and cooling rates. 

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3. Retention of poly( N ‐isopropylacrylamide) thin films on polycarbonate via polymer interdiffusion

   https://doi.org/10.1002/pol.20200261  

   Zhang Newby, Bi‐min; Malekzadeh, Elham; Alghunaim, Abdullah (September 2020, Journal of Polymer Science)

   Abstract This study explored the possibility of polymer interdiffusion for retaining thermoresponsive poly(N‐isopropylacrylamide) (pNIPAAm) on polycarbonate (PC). It was hypothesized that interdiffusion could be facilitated either by increasing the annealing temperature or by treating PC using air plasma (AP) and ultra‐violet ozone (UVO). The results showed that increasing annealing temperature only moderately improved pNIPAAm retention. Treating PC with AP led to an increase in surface‐active groups and a greatly enhanced retention of pNIPAAm. UVO treatment, however, severely damaged the PC layer with no noticeable enhancement on pNIPAAm retention. The retained pNIPAAm films on PC exhibited thermoresponsive behavior as evidenced by water contact angle and desired cell attachment/detachment behaviors. These results illustrate the simplicity of using polymer interdiffusion to successfully retain pNIPAAm films on a polymer, and the resulting substrates would be less expensive and more versatile than those retained on brittle supports (e.g., glass) for applications that require resilient thermoresponsive substrates. 

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4. Aggregation and gelation in a tunable aqueous colloid–polymer bridging system

   https://doi.org/10.1063/5.0101697  

   Gallegos, Mariah J.; Soetrisno, Diego D.; Park, Nayoung; Conrad, Jacinta C. (September 2022, The Journal of Chemical Physics)

   We report a colloid–polymer model system with tunable bridging interactions for microscopic studies of structure and dynamics using confocal imaging. The interactions between trifluoroethyl methacrylate-co-tert-butyl methacrylate copolymer particles and poly(acrylic acid) (PAA) polymers were controllable via polymer concentration and pH. The strength of adsorption of PAA on the particles, driven by pH-dependent interactions with polymer brush stabilizers on the particle surfaces, was tuned via solution pH. Particle–polymer suspensions formulated at low pH, where polymers strongly adsorbed to the particles, contained clusters or weak gels at particle volume fractions of ϕ = 0.15 and ϕ = 0.40. At high pH, where the PAA only weakly adsorbed to the particle surface, particles largely remained dispersed, and the suspensions behaved as a dense fluid. The ability to visualize the suspension structure is likely to provide insight into the role of polymer-driven bridging interactions in the behavior of colloidal suspensions. 

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5. Salt-Dependent Phase Re-entry of Weak Polyelectrolyte Complexes: from Associative to Segregative Liquid–Liquid Phase Separation

   https://doi.org/10.1021/acs.macromol.3c01468  

   Li, Huiling; Liu, Ying; Lan, Fujie; Ghasemi, Mohsen; Larson, Ronald G (October 2023, Macromolecules)

   Hillmyer, Marc A (Ed.)

   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. 

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