Physical Aging and Phase Behavior of Multiresponsive Microgel Colloidal Dispersions

Abstract

Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following approximately 1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pK(a), the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pK(a), repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.

Effective interactions between soft-repulsive colloids: Experiments, theory, and simulations

“…[5][6][7] In particular, cross-linked microgels such as poly(N-isopropylacrylamide) (PNIPAM) that can undergo reversible volume phase transitions upon variations of external stimuli such as temperature, pH, ionic strength or hydrostatic pressure 8,9 have been employed frequently as model systems for soft colloids. [10][11][12][13][14][15][16][17] The physical nature of these particles lies between that of classical hard sphere colloids and ultra-soft polymeric colloids such as star polymers. 5,18 Due to their soft-repulsive character, microgels can compress or interpenetrate to a certain degree, which allows to create states with densities far above close packing, so called squeezed states, with interesting structural and dynamical properties.…”

“…uniroma1.it and temperature dependent phase behavior. [10][11][12][13][14][15][16][17] However, despite all the experimental and theoretical efforts devoted to these systems, the exact nature of the interaction potential between microgels and its dependence on the particle concentration as a vital ingredient for any attempt to theoretically model and understand these systems still needs to be assessed.…”

Electric field driven self-assembly of ionic microgels

“…In particular, crosslinked microgels such as poly(N-isopropylacrylamide) (PNIPAM) have frequently been used as an excellent model system for so colloids. [19][20][21][22][23][24][25][26] As a result of the so potential, microgels can be packed to an effective volume fraction f eff far above closed packing f cp z 0.74 expected for non-deformable hard spheres, with enormous consequences for the resulting structural and dynamic properties.…”

Deformable particles with anisotropic interactions: unusual field-induced structural transitions in ultrasoft ionic microgel colloids

“…[1][2][3][4][5][6] In particular, cross-linked microgels such as poly(N-isopropylacrylamide) (PNIPAm) have been used frequently [7][8][9][10][11][12][13] as excellent model systems for soft colloids. As a result of the soft potential, microgels can be packed to give an effective volume fraction f eff far above closed packing f cp , with enormous consequences for the resulting structural and dynamic properties.…”