Polymer Gel Network Research Articles

The first example of thermochromism of dyes embedded in transparent polymer gel networks

Novel dye containing transparent hydrogels responding to changes in temperature with reversible color changes (thermochromism) are presented. The betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate (DTPP), and the well known indicator dye Cresol Red ( Fig. 1 ) exhibit an outstanding thermochromism, if they are embedded in an aqueous polyvinyl alcohol–borax–surfactant gel network. In case of DTPP at pH = 8.5, the color changes gradually from colorless at 10 °C to a deep violet at 80 °C. Similarly, for Cresol Red a change from yellow to wine-red was observed. An explanation for the observed reversible color changes in the closed hydrogel systems is given by a temperature-induced shift of the proton-transfer equilibrium between the phenolate and the phenol form of the dye molecules in the micro-environment of the gel network. The new thermochromic hydrogels can be useful in a wide range of applications, such as temperature sensors, large-area displays, and so-called smart or intelligent windows.

Thermotrope Materialien für den Einsatz in „intelligenten Fenstern" / Thermotropic materials for application in "intelligent windows"

Abstract Thermotropic materials as integrated part of „intelligent windows" based on polymer gel networks are developed. Systems change the transparency in dependence on temperature. These windows are able to protect buildings against superheating by sunshine. The aim of this work is the formation of a thermotropic material which operates like a heat filter. The effect is based on reproducible reversible phase transitions in the material. A decisive aspect is the adjustment of these transitions between the optical clear or opaque state of the windows, controled by the solar energy, to the perception of the human eye.

The optical behavior of lyotropic liquid crystalline polymer gel networks: Dependence on temperature

Gel networks with interesting optical properties have been prepared on the basis of a highly ordered aqueous polymer lyotropic liquid crystalline phase, embedded in a thermally stable polymer network. The optical transparency changes reversibly with an increasing or decreasing temperature of a few degrees up to a contrast ratio of 85:1. Different lyotropic phases exist in close proximity in the system, for example a schlieren texture (see Figure) and a radial droplet structure. magnified image

Micro-Electro-Mechanics of Ionic Polymeric Gels As Electrically Controllable Artificial Muscles

Ionic polymeric gels are three-dimensional networks of cross-linked macromlecular polyelectrolytes that swell or shrink in aqueous solutions on addition of alkali or acids, respectively. Reversible dilation and contraction of the order of more than 1000 percent have been observed in our laboratory for polyacrylonitrile (PAN) fibers. Furthermore, it has been experimentally observed that swelling and shrinking of ionic gels can also be induced electrically. Thus, direct computer control of large expansions and contractions of ionic polymeric gels by means of a voltage gradient appears to be possible. These gels possess an ionic structure in the sense that they are generally composed of a number of fixed ions (polyions) pertaining to sites of various polymer cross-links and segments and mobile ions (counter ions or unbound ions) due to the presence of a solvent which is electrolytic. Electrically-induced dynamic deformation of ionic polymeric gels such as polyacrylic acid plus sodium acrylate cross-linked with bisacrylamide (PAAM), or poly(2-acrylamido-2-methylpropanesulfonic acid) or PAMPS or various combinations of chemically-doped polyacrylic acid plus polyvinyl alcohol (PAA-PVA) can be easily observed in our laboratory. Such deformations give rise to an internal molecular network structure with bound ions (polyions) and unbound or mobile ions (counterions) when submerged in an electrolytic liquid phase. In the presence of an electric field, these ionic polymeric networks undergo substantial contraction accompanied by exudation of the liquid phase contained within the network. Under these circumstances, there are generally four competing forces acting on such ionic networks: the rubber elasticity, the polymer-liquid viscous interactions due to the motion of the liquid phase, inertial effects due to the motion of the liquid through the ionic network, and the electrophoretic interactions. These forces collectively give rise to dynamic osmotic pressure and network deformation and subsequently determine the dynamic equilibrium of such charged networks. On the other hand there are situations in which a strip of such ionic polymeric gels undergoes bending in the presence of a transverse electric field with hardly any water exudation. Under these circumstances there are generally three competing forces acting on the gel polymer network: the rubber elasticity, the polymer-polymer affinity and the ion pressure. These forces collectively create the osmotic pressure which determines the equilibrium state of the gel. The competition between these forces changes the osmotic pressure and produces the volume change or deformation. Rubber elasticity tends to shrink the gel under tension and expand it under compression. Polymer-polymer affinity depends on the electrical attraction between the polymer and the solvent. Ion pressure is the force exerted by the motion of the cations or anions within the gel network. Ions enter the gel attracted by the opposite charges on the polymer chain while their random motions tend to expand the gel like an ionic (Fermionic) gas. Two mechanisms are presented for the reversible nonhomogeneous large deformations and in particular contraction/expansion with water exudation as well as bending of strips of ionic polymeric gels in the presence of an electric field. An analytical model is first presented for the dynamics of contraction of ionic polymeric gels with liquid exudation in the presence of an electrical field. The proposed model considers the dynamic balance between the internal forces during the contraction. These forces are assumed to be due to the viscous effects caused by the motion of the liquid, the inertial forces due to the motion of the liquid in and out of the network, and the electrophoretic forces due to the motion of the charged ions in the solvent as it exudes from the ionic polymeric gel network. The effects of rubber elasticity of the network as well as ion-ion interactions have been assumed negligible in this case compared with the inertial, viscous and electrophoretic effects. The governing equations, thus obtained are then solved exactly for the velocity of liquid exudation from within the network as a function of time and radial distance in cylindrical samples. The relative weight of the gel sample is then related to this velocity by an integral equation. This integral equation is then numerically solved to obtain a relationship between the amount of contraction as a function of time, electric field strength and other pertinent material and geometrical parameters. The results of the numerical simulations are compared with some experiment results on PAMPS contractile fibers and satisfactory agreements are observed. Next, the case of electrically-induced bending of strips of ionic polymeric gels is considered. Exact expressions are given relating the deformation characteristics of the gel to the electric field strength or voltage gradient, gel dimensions and other physical parameters such as the resistance and the capacitance of the gel strip. It is concluded that direct voltage control of such nonhomogeneous large deformations in ionic polymeric gels is possible.

Electrically induced reversible structural change of a highly swollen polymer gel network

By using high-intensity X-rays from a synchrotron radiation source the structural response of a non-ionic polymer gel of poly(vinyl alcohol)(PVA) in dimethyl sulfoxide to the application of an electric field has been investigated. The gel used as the electroactive gel actuator showed an excellent response: There was an increase of 8% in relative strain within ca. 200 ms at an electric field strength of 2.5 × 105 V m–1. To clarify the initial stages of this electrically induced actuating phenomenon, scattering experiments were carried out in the range of the electrically induced relative strain below 0.8%. As shown by the scattering curves, the application of the electric field barely affected the long-range structure of the gel, but induced a reversible change in the scattering curve corresponding to a dimension of period ca. 60 A. Both the analysis of the distribution of distances and the estimate of correlation lengths imply local structural change over the above length scale when an electric field is applied. The present results suggest that the application of an electric field to a non-ionic gel can induce a reversible rearrangement of polymer chains around chemical crosslinking points to create a local scattering density fluctuation which provides a macroscopic effect.

MASS-PRODUCTION OF BIOCATALYST-ENTRAPPING ALGINATE GEL PARTICLES BY A FORCED OSCILLATION METHOD

Abstract A forced oscillation method applying reverse-piezo electric effect has been proposed for mass-production of biocatalyst-entrapping polymer gel particles. A jet of aqueous alginate solution flowing out through an orifice was disintegrated into droplets by an oscillating pressure which was exerted by a ceramic plate vibrated by the reverse-piezo electric effect. The droplets formed were immediately solidified into gel particles in calcium chloride solution. The particle production rate by the method was up to 270 particles s−1 or 1 × 10−6m3.s−1 The size of the particles produced were down to I mm depending on the applied frequency. The smallest matrix size of the gel's polymer network was estimated to be around 5 nm. The method was applied for immobilization of yeast cells. The diameter of immobilized particles was 2.5 mm. The immobilized yeast grew well in the particle. The damage of yeast cells caused by the immobilization was not remarkable

Transport Phenomena in Gels.

When water passes through a polymer gel, a frictional resistance arises between the water and the polymer network of gel. If the network is permanently crosslinked, it is natural to assume that the friction, normalized by the viscosity of water, shouid be independent of temperature. We found, however, that the friction decreases reversibly by three orders of magnitude and appears to diminish as the gel approaches a certain temperature, This phenomenon occurs in spite of the fact that the network structure and the total volume of the gel, thus the average density of the network, are unchanged.

Heat-induced reversible gelation of arachin: kinetics, thermodynamics and protein species involved in the process

Arachin forms a heat-reversible gel under certain experimental conditions. The minimal gelling concentration for this sytem is 7.25%. Above minimal gelling concentration calculation of thermodynamic parameters for gelation of arachin revealed a constant ΔH bonding (—1220 cal.mol −1 where ΔS bonding values varied with an increase in protein concentration (ranging from −4.01 e.u. at 7.5% to −3.48 e.u. at 10.0%). The main steps involved in the gelation phenomenon include thermal denaturation of arachin, partial aggregation of heat-denatured protein molecules, setting of protein solution and maturation of the gel formed. Gel maturation process follows first order kinetics and is characterized by a large positive ΔG + + (22030 cal.mol −1). Determination of ΔH + + and ΔS + + for this process reveaed that mostly ΔS + + (—62.9 e.u.) contributes to the large positive ΔG + +, thus decreasing the overall rate of gel maturation process. This large negative ΔS + + value probably arises from a loss of entropy of protein molecules because of their increased involvement in gel network formation. The polymer gel network seems to be primarily contributed by a part of both arachin dodecameric and hexameric species.