We synthesize poly(N-isopropylacrylamide) (PNIPAM) gels with different sizes in the micrometer scale by a slight variation of a recent emulsion polymerization method (ref 1). The procedure is different than that typically used for obtaining macroscopic PNIPAM hydrogels. The resultant minigel suspension is polydisperse thus allowing the swelling kinetics for different gel sizes to be studied; we do so at temperatures below the volume-transition temperature by wetting with water previously dried particles. The resultant swelling is followed by optical video microscopy. We find that the characteristic swelling time scales with the inverse of the particle dimension squared, in agreement with theoretical predictions (ref 2). The proportionality constant is the network diffusion coefficient D, which for the minigels under consideration appears to be in between that of PNIPAM macrogels and the self-diffusion coefficient of water.

Microgels are novel colloidal gels widely used because of the swelling or de-swelling exhibited under specific external conditions. In this work, the swelling and de-swelling of microgel particles is experimentally described. The medium pH, salt concentration and osmotic stress, are the three variables with which the change of phase has been realised. All transitions are found to occur well above the critical point of the system. Additionally, the basic applications in which the given transition is employed are described. Keywords: Microgel, swelling, flory theory Microgeles: Un Nuevo Material ColoidalLos microgeles son geles coloidales que se emplean por el inflado y/o desinflado que manifiestan ante ciertas condiciones externas. En este trabajo, se describe experimentalmente el inflado y el desinflado de partículas de gel. El pH del medio, su concentración iónica y el esfuerzo osmótico externo son las variables externas que se emplean para provocar el cambio de fase en las partículas. Todas las transiciones ocurren por encima del punto crítico del sistema. Adicionalmente, se presentan algunas de las aplicaciones en donde se emplea la transición provocada por la variable externa concreta. THEORETICAL DESCRIPTION OF THE VOLUME PHASE TRANSITIONThe total osmotic equilibrium inside an ionic gel consists, basically of three contributions: mixing (π m ), elastic (π e ) and ionic (π i ) (1,2). Explicitly:where N A is the Avogadro number, N C the number of chains, k the Boltzmann constant, T the absolute temperature, ν s the molar volume of the solvent, V o and φ o the volume and polymer volume fraction of the gel in the totally collapsed state, χ the Flory interaction parameter, φ the polymer volume fraction for a given state and f the number of counterions per chain inside the gel.