Using Plasma Etching to Access the Polymer Density Distribution and Diffusivity of Gel Particles.

In this paper we examine the polymer density distribution of gel particles and its effect on solvent diffusivity through the polymer network. In order to access the inner particle regions, external polymer layers were removed by plasma etching, thus reducing them from the outside. Higher polymer densities after erosion showed internal heterogeneity, with the density increasing towards the center of the particles. An exponential decay polymer density model is proposed, and the spatial relaxation length measured. The diffusion of solvent through the particles, before and after the plasma oxidation, revealed a correlation between the diffusion coefficient and the internal density.

Gels and microgels for nanotechnological applications.

In recent years, "smart" materials have been the focus of considerable interest, from both fundamental and applied perspectives. Polymer gels are within this category; they respond to specific environmental stimuli by changing their size. Thus, the internal structure, the refractive index, and the mechanical properties of the polymer network change. They are considered super absorbent materials, as they can absorb solvent up to several hundred times their own weight. They respond rapidly to local environmental variations, an important fact in device miniaturization and microsensor developments. As size changes are accompanied by changes in internal dimensions, microgels have found application as carriers of therapeutic drugs and as diagnostic agents. They have also been used as microreactors, optically active materials, for template synthesis of nanoparticles or fabrication of artificial muscle. In this paper we review a set of application based on the special features associated to this systems. Basic concepts on the physical-chemistry of gel swelling is first described, followed by different applications covering drug delivery, composite materials using polymer gels to modulate optical or magnetic and electrical properties, molecular imprinting, gel-based biosensors and polymer sensors and actuators used in the field of artificial muscles.

Ion-specific and reversible wetting of imidazolium-based minigels.

Cross-linked imidazolium-based [poly(ViEtIm +Br -)] microparticles were synthesized, and their wetting properties were studied by optical microscopy, after addition of aqueous solutions of sodium halides. Particle wetting showed ion specificity due to counterion binding, described by Desnoyer's model. The interaction between anions and the microparticles allowed exchanging halogenides between them in a reversible way. A salt-independent characteristic wetting time was found as well as a decreasing power law with salt concentration, for the network diffusion coefficient. It modified the polymer network elasticity as ion concentration increased, making the network softer.

Swelling kinetics of poly(N-isopropylacrylamide) minigels.

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.

Diffusion coefficients in swelling polypyrrole: ESCR and Cottrell models.

Reliable diffusion coefficients, D, for the diffusion of perchlorate anions into polypyrrole films during polymeric oxidation were obtained from chronoamperometric results. Two different models were used to calculate D: the Cottrell equation and the electrochemically stimulated conformational relaxation (ESCR) model. As expected, the initial Cottrell hypothesis was far from swelling/shrinking polymeric electrodes and the obtained D range was from 10(-10) to 10(-6) cm(2) s(-1). The ESCR model, based on the internal diffusion that takes place from regions where the steady state of oxidation has already been reached to regions where the oxidation is only just beginning, provided values of D ranging from 0.4 x 10(-9) to 2.2 x 10(-9) cm(2) s(-1), which is close to the values expected for a gel. When a constant amplitude is kept for the potential step, D increases with increasing initial anodic potentials, i.e., from increasingly swollen films. When it is stepped to the same oxidation potential, D decreases when starting from more cathodic potentials, i.e., from a more compact structure. These changes in D can be attributed to (i) swelling processes during oxidation, giving a gel-like structure; (ii) compacting processes at increasing cathodic potentials; (iii) the increasing thickness of the film during oxidation; and (iv) a decrease in film viscosity during the swelling process.