Collapse-to-Swelling Transitions in pH- and Thermoresponsive Microgels in Aqueous Dispersions: The Thermodynamic Theory

Abstract

We present a theory of a conformational collapse-to-swelling transition that occurs in aqueous dispersions of multiresponsive (pH-and thermoresponsive) microgels upon variation of ionic strength, temperature, or pH. Our theory is based on osmotic balance arguments and explicitly accounts for ionization equilibrium inside microgel partices. The theory predicts complex patterns in the dependence of the microgel particle dimensions on the control parameters: An increase in temperature leads to worsening of the solvent quality for the gel forming LCST-polymers and to concomitant decrease in the dimensions of the gel particles. This collapse of the gel particles provoked by an increase in temperature occurs either smoothly (at high or low ionic strength), or may exhibit a jump-wise character at intermediate ionic strength. The theory further predicts that the degree of swelling of microgel particles varies nonmonotonously and exhibits a maximum as a function of salt concentration at a pH close to the pK. This nonmonotonous variation of the particle dimensions occurs continuously at temperatures below or slightly above LCST (good or marginal poor solvent strength conditions, respectively), whereas at higher temperatures the jump-wise swelling of the gel particles is followed by either continuous or jump-wise collapse induced by progressive increase in the salt concentration. A decrease/increase in pH leads to deswelling of the weak polyacid/polybase gel particles, which occurs smoothly at temperatures below LCST, but may exhibit a discontinuity above LCST. These theoretical predictions can be used for design of smart stimuliresponsive microgels.

Non-monotonic swelling of surface grafted hydrogels induced by pH and/or salt concentration

“…Swellingdeswelling behavior has been theoretically predicted for starbranched weak polyelectrolytes 40 and weak polyelectrolyte microgels. 41 Reentrant behavior in terms of the solvent composition has been reported for a variety of other polymeric systems, including the condensation/precipitation and subsequent redissolution of globular proteins, 48,49 DNA, [50][51][52] and other strong polyelectrolytes [53][54][55] upon varying the concentration of multivalent ions in the solution. Strong polyelectrolyte gels also display reentrant swelling in response to changes in the concentration of monovalent 56 and multivalent 57,58 salts.…”

“…40 Recently, a reentrant swelling-to-collapse transition was theoretically predicted for micrometer-sized (or larger) weak polyelectrolyte gels in good solvent, assuming local electroneutrality. 41 The authors explain that initially, increasing c promotes dissociation until f d saturates once it reaches the value of the dilute solution. Adding more salt to the solution can only reinforce the screening of the electrostatic repulsions, and thus the gel begins to deswell.…”

Structure of Microgels with Debye–Hückel Interactions

“…They are able to undergo reversible volume phase-transitions in response to environmental stimuli, such as pH, temperature, the ionic strength of the surrounding medium, the quality of solvent and the action of the external electromagnetic field [1][2][3][4]. This renders them potential candidates for a broad-range of applications in drug delivery, sensing, the fabrication of photonic crystals, template-based synthesis of inorganic nanoparticles and separation and purification technologies [5][6][7][8][9][10][11][12][13].…”

Volume phase transition in thermo-responsive hydrogels: constitutive modeling and structure–property relations

“…Equation (28) implies that the energy ph has two minima [35] corresponding to the swollen (η > 0) and shrunken (η = 0) states of a hydrogel [30,36]. The swollen state (with a positive concentration of hydrophilic segments) is characterized by a loose distribution of chains in the equivalent network separated by molecules of bound water.…”

“…Abrupt volume changes at T * are described within the Landau theory of phase transformations by introducing an extra term (depending on some order parameter η) into the specific free energy of a hydrogel. A similar approach was suggested in [28] to explain hysteretic phenomena in pH-sensitive gels, in [29] to account for the effect of pressure on volume phase transition in thermo-responsive gels, in [30] to describe excludedvolume interactions between polymer chains, and in [31] to model self-oscillations of gels driven by redox reactions. The novelty of our approach consists in treatment of the order parameter.…”