Crystallization Behavior of Soft, Attractive Microgels

Abstract

The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N'-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH = 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.

Prevention of Collagen-Induced Platelet Binding and Activation by Thermosensitive Nanoparticles

“…Previous studies have shown that the addition of hydrophilic co-monomers to poly(NIPAm) results in decreased temperature sensitivity of the resulting nanoparticles as the charged groups serve to increase overall solubility and resist the temperature-induced hydrophobic collapse (39,52,53). The addition of multiple hydrophilic groups to the backbone of our particles presents a possibility that their temperature sensitivity could be reduced to the extent that they are no longer a viable mechanism of controlled drug loading and release.…”

“…Depending on the application, this collapse can trigger rapid burst release of the loaded drug or can result in decreased porosity and a more controlled drug release profile (35)(36)(37). Additionally, poly(NIPAm) is readily copolymerized with acrylic acid (AAc), which adds easily modified carboxylic acid functional groups to the backbone of the particle, allowing for the addition of targeting ligands while simultaneously increasing the colloidal stability of the nanoparticle (38)(39)(40)(41)(42). In addition to carboxylic acid, the charged sulfated comonomer 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) has been added to poly(NIPAm) nanoparticles, resulting in colloidally stable, hemocompatible sulfated poly(NIPAm-AMPS) nanoparticles (29,43,44).…”

Steric hindrance effect on thermo-responsive behaviors of well-defined water-soluble semi-rigid polymers

“…More remarkable molecular weightdependence and concentration-dependence on thermo-responsive behaviors were demonstrated in the P(2-HPPVTA) compared with the P(1-M-2-HEPVTA). The cloud points of P(2-HPPVTA) and P(1-M-2-HEPVTA) solution elevated by 35 ) were 9.9 and 13.3 C lower in D 2 O than in H 2 O, respectively. These results suggest that the steric hindrance of polymers on thermo-responsive behaviors have a significant impact.…”

“…It may influence solubility of polymers, 29 polymerization rate, [30][31][32] anomeric effect, 33 molecular recognition, 34 crystallization behavior, 35 and so forth. In addition, Okada and Tanaka suggested that the cooperativity between the neighboring water molecules hydrogen-bonded onto the polymer chain is responsible for the cloud point behavior of the PNIPAm, where the steric hindrance by the hydrophobic isopropyl group is the main origin of the cooperativity.…”

Color-Tunable Etalons Assembled from Poly (N-Isopropylacrylamide) Based Microgels

“…Enlightened by Kawaguchi's work, other researchers used capillary forces [25,26], the environment [27] and nature of the particle [28,29], and the surface chemistry [30] to control the spacing and color of the resultant films. Subsequent work has further expanded this knowledge to include an understanding of the effect of temperature [31,32], charge or magnetic field [33,34], pH [35], and hydrophobic/hydrophilic interactions [36] on the formation of microgel films and their optical properties.…”