Mri measurements of T1, T2 and D for gels undergoing volume phase transition

Gels consist of crosslinked polymer network swollen in solvent. The network of flexible long-chain molecules traps the liquid medium they are immersed in. Some gels undergo abrupt volume change, a phase transition process, by swelling-shrinking in response to external stimuli changes in solvent composition, temperature, pH, electric field, etc. We report that during volume phase transition changes of NMR longitudinal relaxation time T(1), NMR transverse relaxation time T(2), and diffusion coefficient D of the PMMA gel, and D of the NIPA gel. We describe how the gels were synthesized and the reason of using the snapshot FLASH imaging sequence to measure T(1), T(2), and D. Since T(1), T(2) and D maps have identical field of view and data are extracted from identical areas from their respective maps, these values can be correlated quantitatively on a pixel-by-pixel basis. Thus a complete set of NMR parameters is measured in-situ: the gels are in their natural state, immersed in the liquid, during the phase transition. The results of spectroscopic method agree with that of snapshot FLASH imaging method. For the PMMA gel T(1), T(2) and D decrease when gels undergo volume phase transition between deuterated acetone concentration of 30% and 40%. At its contracted state, T(1) is reduced to a little less than one order of magnitude, T(2) over two orders of magnitude, and D over one order of magnitude, smaller from values of PMMA gel at the swollen state. At an elevated temperature of 54 degrees C the thermosensitive NIPA gel is at a contracted state, with its D reduced to almost one order of magnitude smaller from that of the swollen NIPA at room temperature.     

Beating polymer gels coupled with a nonlinear chemical reaction

We report on a beating polymer gel that exhibits periodical volume changes (swelling and deswelling) in a closed solution without external stimuli, like autonomous heartbeat. The mechanical oscillation is driven by the chemical energy of the oscillatory Belousov-Zhabotinsky (BZ) reaction. The gel is a copolymer gel of N-isopropylacrylamide (NIPAAm) in which ruthenium tris(2,2(')-bipyridine) [Ru(bpy)(3)], known as a catalyst of the BZ reaction, is covalently bonded to the polymer chain. The poly[NIPAAm-co-Ru(bpy)(3)] gel provides an open system where the BZ reaction proceeds, when immersed in an aqueous solution containing the reactants of the BZ reaction (with the exception of a catalyst). The chemical oscillation in the BZ reaction generates the periodical changes of the charge of Ru(bpy)(3) in the gel network between reduced [Ru(II)] and oxidized [Ru(III)] states. The gel swells at the oxidized state because the hydrophilicity of the polymer chains increases, while at the reduced state the gel deswells. Thus, the chemical energy is transduced into the mechanical energy to drive the polymer gel oscillation with a period of about 5 min, depending on the composition of the surrounding solution. The oscillation mode of the gel depends on its size scaled by the wavelength of the BZ pattern. Sufficiently small bead-like gels demonstrate isotropic beating. A large rectangular gel shows mechanical oscillation with a peristaltic motion coupled with the propagating chemical waves. The dynamic behavior of the chemical and mechanical oscillations have been analyzed with a model simulation. (c) 1999 American Institute of Physics.     

Gels and glasses in a single system: evidence for an intricate free-energy landscape of glassy materials

In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by "hesitations" between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated.     

Ionic conductivity study on electron beam irradiated polyacrylonitrileben polyethylene oxide gel

Different mass percent polyacrylonitrile (PAN)—polyethylene oxide (PEO) gels were prepared and irradiated by an electron beam (EB) with energy of 1.0 MeV to the dose ranging from 13 kGy to 260 kGy. The gels were analysed by using Fourier transform infrared spectrum, gel fraction and ionic conductivity (IC) measurement. The results show that the gel is crosslinked by EB irradiation, the crosslinking degree rises with the increasing EB irradiation dose (ID) and the mass percents of both PAN and PEO contribute a lot to the crosslinking; in addition, EB irradiation can promote the IC of PAN—PEO gels. There exists an optimum irradiation dose, at which the IC can increase dramatically. The IC changes of the PAN—PEO gels along with ID are divided into three regions: IC rapidly increasing region, IC decreasing region and IC balanced region. The cause of the change can be ascribed to two aspects, gel capturing electron degree and crosslinking degree. By comparing the IC—ID curves of different mass percents of PAN and PEO in gel, we found that PAN plays a more important role for gel IC promotion than PEO, since addition of PAN in gel causes the IC—ID curve sharper, while addition of PEO in gel causes the curve milder.     

Equilibrium concentration profiles and sedimentation kinetics of colloidal gels under gravitational stress

We study the sedimentation of colloidal gels by using a combination of light scattering, polarimetry and video imaging. The asymptotic concentration profiles (z,t → ∞) exhibit remarkable scaling properties: profiles for gels prepared at different initial volume fractions and particle interactions can be superimposed onto a single master curve by using suitable reduced variables. We show theoretically that this behavior stems from a power law dependence of the compressive elastic modulus versus , which we directly test experimentally. The sedimentation kinetics comprises an initial latency stage, followed by a rapid collapse where the gel height h decreases at constant velocity and a final compaction stage characterized by a stretched exponential relaxation of h toward a plateau. Analogies and differences with previous works are briefly discussed.     

Mechano-chemical oscillations of a gel bead

We present a formalism which describes the spatio-temporal evolution of a gel submitted to an autocatalytic chemical reaction to which it is responsive. This theory is based on an extension of a hydrodynamical multi-diffusional approach of the gel dynamics, which is plunged into a chemically active mixture. Emergent and autonomous volume self-oscillation dynamics of the gel are obtained from the nonlinear coupling of the elastic deformation, the chemical kinetics and the transport phenomena, that take place in the system. We apply this formalism to a spherical bead of gel plunged in a Belouzov-Zhabotinsky oscillatory chemical reaction, for which Yoshida et al. have obtained numerous experimental results. The case of a gel immersed in an autocatalytic bistable chemical reaction is also considered. We show that such formalism describes the autonomous volume self-oscillation dynamics of the gel beads.     

Phase transition in polymer gels due to local heating by illumination of light

The volume phase transition in gels induced by visible light and its related properties are presented, the mechanism of which is based on local heating of a polymer network by illumination of light. The gels consist of a covalently cross-linked copolymer network of thermosensitive N-isopropylacrylamide and a chromophore. Without light illumination, the gel volume changes sharply, but continuously at approximately 34°C when the temperature is varied. At a fixed temperature of an appropriate value, a discontinuous volume transition is observed when the light intensity is gradually changed. The phase transitions can be understood in terms of the temperature increment at the immediate vicinity of polymer chains due to the local heating via light absorption and subsequent thermal dissipation of light by the chromophore. The results can be qualitatively described by the Flory-Huggins mean-field equation of state of gels. In order to make clear the mechanism of the light-induced phase transition in the present system, we measured the light transmitting properties and the swelling as well as shrinking kinetics. These preliminary results are described semi-qualitatively by making use of a simple phenomenological model.     

Gels at interfaces

We discuss the adsorption of polymer gels on flat surfaces. Even in cases of complete wetting where the spreading power S is positive and where an equivalent liquid would spread, the elastic stresses due to the gel deformation upon adsorption oppose the spreading. The competition between elasticity characterized by the bulk shear modulus G and capillarity characterized by the spreading power S defines a typical length scale ℓ = S/G for the deformation in the gel. For loose gels ℓ can be of the order of 1 μm. Macroscopic gels larger than ℓ deform only at their edges over a region of size ℓ. Microscopic gels smaller than ℓ show a finite deformation despite the elastic stresses. The elastic stresses limit the spreading of the polymer, but solvent can be sucked out of a swollen gel by wetting the surface. The thin solvent film can extend rather far from the gel edge and carry solvent. We calculate the kinetics of the solvent film formation and of the solvent transfer from a more swollen gel to a less swollen gel. Received 16 July 2001     

Thermo-sensitive N-n-propylacrylamide gels

Thermo-sensitive gels were prepared by irradiating aqueous solutions of poly(N-n-propylacrylamide) (NNPA) and its copolymers with acrylic acid (AA) with γ-rays from a ⁶⁰Co source. The equilibrium swelling volume of the gel in water was determined as a function of temperature. NNPA gel showed a discontinuous and reversible volume phase transition. The transition temperature and the volume change at the transition decreased with irradiation time. The transition temperature was approximately 12°C lower than that of poly(N-isopropylacrylamide) gel. A discontinuous volume transition was also observed in the copolymer gels of NNPA and AA. The dependence of the transition temperature on the concentration of carboxyl groups revealed a marked difference depending on whether they were protonated or dissociated. For gels having side groups of COONa, the transition temperature rose and the volume change at the transition was elevated as the COONa concentration increased. In contrast, an increase in the COOH concentration resulted in a decrease in the transition temperature. These results are discussed in terms of an equation of state constructed based on scaling theory.     

Temperature-responsive poly(N-isopropylacrylamide) gels containing polymeric surfactants

Poly(N-isopropylacrylamide) (PNIPA) gel containing polymer surfactant poly(2-(methacryloyloxyl)decylphosphate) (PMDP) was synthesized and was found to show rapid volume phase transition above its transition temperature. Interestingly, the phase transition temperature of the PNIPA–PMDP gel was equal to that of the PNIPA gels alone. The concentration gradient of PMDP within the PNIPA gel can be obtained by applying an electric field on the gel, similar to the gel electrophoretic technique. The PMDP-gradient PNIPA gel clearly demonstrated the prevention of skin formation and the acceleration of the phase transition rate of the PNIPA gel by PMDP. The rapid volume phase transition allows potential applications of the PNPA–PMDP gel to soft actuators and drug delivery systems. Recently we also succeeded in synthesizing cylindrical microgels (0.8 μm in diameter, 5 μm in length) by a novel strategy where template-guiding synthesis and photochemical polymerization are combined. The obtained microgels can be characterized in individual level by a laser-trapping/Raman spectroscopy. In this article we also briefly described a famous gel system containing ionic surfactant capable of electrically driven actuators although it is not PNIPA gel.     

Pattern formation in the ferrocyanide-iodate-sulfite reaction: the control of space scale separation

We revisit the conditions for the development of reaction-diffusion patterns in the ferrocyanide-iodate-sulfite bistable and oscillatory reaction. This hydrogen ion autoactivated reaction is the only example known to produce sustained stationary lamellar patterns and a wealth of other spatio-temporal phenomena including self-replication and localized oscillatory domain of spots, due to repulsive front interactions and to a parity-breaking front bifurcation (nonequilibrium Ising-Bloch bifurcation). We show experimentally that the space scale separation necessary for the observation of stationary patterns is mediated by the presence of low mobility weak acid functional groups. The presence of such groups was overlooked in the original observations made with hydrolyzable polyacrylamide gels. This missing information made the original observations difficult to reproduce and frustrated further experimental exploitation of the fantastic potentialities of this system. Using one-side-fed spatial reactors filled with agarose gel, we can reproduce all the previous pattern observations, in particular the stationary labyrinthine patterns, by introducing, above a critical concentration, well controlled amounts of polyacrylate chains in the gel network. We use two different geometries of spatial reactors (annular and disk shapes) to provide complementary information on the actual three-dimensional character of spatial patterns. We also reinvestigate the role of other feed parameters and show that the system exhibits both a domain of spatial bistability and of large-amplitude pH oscillations associated in a typical cross-shape diagram. The experimental method presented here can be adapted to produce patterns in the large number of oscillatory and bistable reactions, since the iodate-sulfite-ferrocynide reaction is a prototype of these systems.     

Influence of high pressure treatment on rheological and other properties of egg white

Abstract

The paper deals with the influence of high pressure treatment of fresh egg white on its properties and protein composition (individual amino-acids predicted as a function of pressure and time levels). The rheological properties are changed by high pressure from Newtonian to non-Newtonian behaviour, with increasing apparent viscosity as the pressure and time increased. The pH, whipping ability, foam stability, gel strength of heat induced gels after treatment and the whole protein content, were also predicted.

The results showed that the foam stability is increased with increasing pressure and time of processing. The foam volume is also increased with pressure. The pH did not change with pressure or time of processing. Composition of proteins as indicated by individual amino-acids did not exhibit statistically important changes. Gel strength of heat induced gels prepared from previously pressured liquid whites showed no important change of values with pressure or time of treatment. The modulus of elasticity showed a decrease for samples pressured to 400 MPa for 5 up to 15 minutes.     

Vibrational Spectral Characterization of a Lead Lanthanum Zirconate Titanate During Various Stages of Sol-Gel Processing

Lead lanthanum Zirconate Titanate (PLZT 7/65/35) powders were prepared by a sol-gel process, using lead acetate, lanthanum acetate, zirconium n-propoxide and titanium n-propoxide as precursors along with 2-methoxyethanol as the solvent. Nitric acid was used to catalyze the sol-gel reaction. FTIR spectroscopy characterized the starting precursors, monitored the sol to gel transformations involving solutions with different molar ratios of water to alkoxide precursors, and tracked the crystallization behavior of the gels upon heat treatment. Raman spectroscopy was employed at room temperature to follow vibrational spectral changes for the samples that were cooled after various stages of heat treatment and crystallization of the gels. The resulting spectral changes were analyzed in terms of the structural changes that occurred during the sol-gel process.     

Free-radical cross-linking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate in the presence of trimethoxyvinylsilane

Free-radical cross-linking copolymerization of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDM), and trimethoxyvinylsilane (TMVS) has been investigated in toluene at a total monomer concentration of 4 mol L⁻¹ and at 70 °C. Conversion of monomer, volume swelling ratio, weight fraction, and gel point were measured as a function of the reaction time, TMVS concentration, toluene content, temperature, and chain transfer agents up to the onset of macrogelation. Structural characteristics of the gels were examined by using equilibrium swelling in toluene, gel fraction, and Fourier transform infrared (FTIR) analysis. The morphology of the copolymers was also investigated by scanning electron microscopy (SEM).

Based on the obtained results, it was concluded that the FTIR data did not have the capacity to show the presence of the TMVS moiety in these kinds of copolymers. On the other hand, the variation of weight fraction of gel, W_g, and its equilibrium volume swelling ratio in toluene, q_v, exhibited the same behaviors as that of MMA/EGDM copolymers. Also, the dilution of the monomer mixture resulted in an increase in the gel point and swelling degree and a decrease in the percent of conversion and gel fraction. Finally, TMVS is a suitable silicone co-monomer for this system and it has been proved useful.     

Influence of environmental solution pH and microstructural parameters on mechanical behavior of amphoteric pH-sensitive hydrogels

Amphoteric hydrogels contain both ionizable acidic and basic groups attached on the polymer chains, which can change their volume in response to the slight alteration of the surrounding environmental p H. In this paper, a theory of equilibrium swelling of amphoteric p H-sensitive hydrogels which is an extension of the formalism proposed by Marcombe et al. and a new hybrid free-energy density function of amphoteric hydrogels composed of the Edwards-Vilgis slip-link model and the Flory-Huggins solution theory as well as the contributions of mixing the mobile ions with the solvent, and dissociating the acidic and basic groups are presented for the prediction of the influence of environmental solution p H, microstructural parameters and geometric constraints on mechanical behavior. The calculations were modeled on chitosan-genipin gels, and the results were compared to experimental data. Numerical calculations show that the model is able to predict the dependence of swelling on p H and crosslinker qualitatively well and quantitatively close to the experimental data. Each gel shows minimal swelling at low p H but an increase in swelling until a maximum was reached; for most of the p H range, a good fit was achieved except for where the maximum swelling occurs; for experimental data, the maximum swelling appears at about pH = 4 , but for modeled data the maximum swelling appears between pH = 4 and pH = 6 ; each gel swell decreasing with increasing crosslinker concentration was also successfully predicted. The calculated results also show that microstructural parameters and geometric constraints have a significant impact on the mechanical behavior of the amphoteric hydrogels; the gel swells less when the network is more densely entangled and the maximum swelling ratio of the gels under biaxial constraint is only about one-third of the maximum when the gels swell freely. The theory developed here is valuable for the design and optimization of a drug delivery system.     

Infrared Spectral/Structural Investigation of the Heat Treatment of Tetramethoxysilane/Diethoxydimethylsilane-Derived Gels

Structural changes occurring during the heat treatment of gels that were prepared from binary tetramethoxysilane (TMOS)/diethoxydimethylsilane (DEDMS) mixtures were investigated using infrared spectra. Analysis of the spectral data indicated that the Si-O networks of the heat-treated gels increased their connectivity with increased dehydration due to increased treatment temperature or time. Four- and three-membered (Si-O)_n rings were noted for appropriate heat-treated gel compositions. At higher heat-treatment temperatures, methyl groups were removed from the gel network.     

Polyelectrolyte gels in poor solvent: Elastic moduli

We study theoretically using scaling arguments the behavior of polyelectrolyte gels in poor solvents. Following the classical picture of Katchalsky, our approach is based on single-chain elasticity but it accounts for the recently proposed pearl necklace structure of polyelectrolytes in poor solvents. The elasticity both of gels at swelling equilibrium and of partially swollen gels is studied when parameters such as the ionic strength or the fraction of charged monomers are varied. Our theory could be useful to interpret recent experiments performed in Strasbourg that show that if identical gel samples are swollen to the same extent at different pH the sample with the highest charge has the lowest shear modulus. Received 7 April 2000     

Spreading of liquid drops on Agar gels

We study the spreading of pure water drops or water drops with surfactine (surfactant produced by bacteria Bacillus Subtilis) on gels (Agar/Water gel). We find that, surprisingly, the drops do not spread indefinitely, but remain in a state of partial wetting. Eventually the liquid diffuses into the gel on a time scale short with respect to evaporation times. The drops containing surfactant show a complex dynamics: at first the spreading velocity decreases, until the front stops and starts receding at about constant velocity. Concurrently, a second front detaches from the rim of the drop if the agar concentration is sufficiently low, and continues to move outwards.     

Morphology and chain dynamics during collapse transition of PNIPAM gels studied by combined imaging, relaxometry and 129Xe spectroscopy techniques

The temperature-induced shape transition of poly(N-isopropylacrylamide) gels of different cross-link densities was investigated by a combination of NMR techniques allowing the characterization of both the macroscopic collapse as well as the changes on a molecular scale related to the expulsion of water from the gel network. The proton-containing gel phase was visualized by swelling in heavy water, and the volume change was monitored by proton imaging for cross-link densities between 0.5% and 2.5%. Above the transition temperature of 35 degrees C, gel collapse led to a volume change of up to a factor of 17 for the gel of smallest cross-link density. Two spectral lines of 129Xe are found in the gel state and are assigned to the hydrophobic and hydrophilic parts of the network. In the collapsed state, the hydrophobic peak shows a strong shift while the hydrophilic peak disappears. A considerable shortening of both T1 and T2 of the gel protons upon collapse was found at a field of 4.7 T. At lower fields, the effect becomes more pronounced and qualitatively different dispersion behaviors between the swollen and the collapsed states are observed.