Smart Materials

Abstract

The nature and properties of “smart” colloidal microgels are reviewed. Detailed descriptions including mechanistic information are reported with respect to the preparation of a number of physically different microgel structures including homopolymers, copolymers, polyelectrolyte, and core–shell type materials. The principal techniques used for the characterization of microgels are described, including dynamic light scattering and differential scanning calorimetery. Microgel properties are reviewed with respect to temperature sensitivity, pH sensitivity, colloid stability, rheological behavior, osmotic deswelling, and the influence of copolymers on microgels. Applications of these types of materials are also discussed and, include drug delivery devices, binding/adsorption agents, and catalytic compounds.

Interpenetrated PNIPAM−Polythiophene Microgels for Nitro Aromatic Compound Detection

“…The interest in polymer microgels has grown rapidly over the last 10 years because their fast response to external stimuli has prompted applications as drug delivery systems, molecular entrapment matrices, oil recovery devices, or catalytic media. − Depending on the environment conditions such as pH, ionic strength, or temperature, the polymer Flory−Huggins parameter can change, leading to a steep variation of the microgel hydrodynamic radius. , In microgels, this response is fast and has been used to create smart materials of which poly( N -isopropyl acrylamide) (PNIPAM) is, by far, the most investigated system. PNIPAM is a water-soluble polymer with a low critical solution temperature (LCST) at 32 °C that shrinks or swells in response to changes in temperature. , Thus, PNIPAM particles swell in water at room temperature, ordering water molecules around the amide group by means of hydrogen bonding.…”

“…When the temperature increases above the LCST, molecular agitation disrupts these H-bonds and leads to a breakdown of local water structure around PNIPAM chains that triggers hydrophobic attraction among isopropyl groups, with the consequent dehydration of polymer chains. Swelling of microgels depends on the type and concentration of monomer (and/or comonomer), its affinity for the solvent, inclusion of nanoparticles within the microgel, and cross-linking . Thus, the combination of PNIPAM with other materials (inorganic particles, conducting polymers) can modify the LCST, and this variation has to be taken into account when dealing with applications.…”

Semi-quantitative analysis of the monomer composition in co-polymer microgels using solid state Raman and NMR spectroscopy

“…[1][2][3] Colloidal poly(N-isopropylacrylamide) [poly(NIPAM)] perhaps the most widely investigated microgel system has been shown to undergo a temperature-induced reversible volume phase transition (VPT) in water at approximately 34 C. 4 The extent of swelling (or de-swelling) is an interesting property of microgel particles. 5 Their properties make them useful in a variety of fields including the paint industry, in ink jet printing, as drug delivery vehicles, [6][7][8] in the synthesis of engineering ceramics, 9 as biosensors, 10,11 and in other areas of biotechnology. 12 The investigation of the incorporation of co-monomer into a polymer microgel to form a co-polymer microgel is of interest, as this incorporation allows the modification of the physicochemical characteristics of the microgel to suit desired applications.…”

Thermodynamic Considerations of Microgel Swelling Behavior

“…The VPT properties of microgel systems can be manipulated and controlled by, e.g., co-polymerization with monomers possessing ionizable functional groups . Consequently the physicochemical properties of colloidal microgels based on poly(NIPAM/BA) and various copolymer derivatives have been widely investigated 1-2,4,11 in relation to the VPT changes that occur in response to a number of external stimuli such as pH, temperature, , and ionic strength. − Poly(NIPAM/BA) microgel particles exhibit a temperature-induced, reversible VPT in H 2 O at 305 K. , Water behaves as a good solvent at room temperature (microgel is in a swollen state) when polymer−solvent interactions are favored; however, water acts as a worse solvent as the temperature is increased (microgel in a shrunken state), leading to the collapse of the microgel when polymer−polymer attractions are favored over solvent−solvent interactions . At temperatures above the volume phase transition temperature (VPTT) of poly(NIPAM/BA), inter- and intrachain hydrogen bonding and hydrophobic interactions are said to be dominant …”