Collapse Of Gels Research Articles

Linking single particle rearrangements to delayed collapse times in transient depletiongels

Weak depletion gels with particle radii of∼200–500 nm have been reported to display a time-dependent settling behaviour where aninitially space spanning gel displays a catastrophic collapse after a characteristic period oftime, defined as the delay time. Several experiments suggest that thermally activatedparticle rearrangements promote macroscopic gel coarsening, which ultimately triggers therapid collapse. The delay time is found to be a sensitive function of the colloid volumefraction and polymer concentration. We have performed systematic experiments on thesilica–decalin–polystyrene depletion system to explore how colloid volume fraction, polymerconcentration, particle radius and ratio of polymer radius of gyration to particle radiusinfluence the delayed collapse time of transient gels. We employ a recently developedactivated barrier-hopping theory to make predictions of the timescales over which colloidscan escape localized states as a function of system parameters. Our study shows that,within experimental uncertainty, delay times follow an exponential dependence on acomposite variable that is a function of the three controllable system variables, which isvery similar to that predicted for the thermally activated local rearrangementtimescales. This provides support for the hypothesis that thermally activated particlemotions are a rate-limiting step in determining the timescale for the initiation ofcatastrophic collapse of transient gels. In addition, the model explains why transientgel formation is found to be absent in weak depletion gels as particle sizes arereduced.

Small angle x-ray scattering study of local structure and collapse transition of (1,3)-β-D-glucan-chitosan gels

Scleroglucan is a (1,3)-beta-D-glucan polysaccharide produced by the fungus Sclerotium. Dissolved in water at room temperature it adopts a linear, rigid, triple helical structure. Gelation of scleroglucan can be obtained by Schiff-base formation between partly periodate oxidized scleroglucan and the primary amine groups of chitosan. The scleraldehyde-chitosan gels exhibit a collapse transition when exposed to volume fractions of isopropanol, Wp, larger than 65%. The aim of the present study is to provide structural information concerning the local polymer distribution and the collapse transition in (1,3)-beta-D-glucan-chitosan gels. Small angle x-ray scattering was used to investigate solutions and gels of scleroglucan in water, as well as in an aqueous mixture containing 65% isopropanol. The results reveal that in aqueous solution, the polysaccharide scleroglucan chains have an approximately cylindrical cross section of external diameter close to 17 A. The gels display the same local structure, but form clusters on a longer distance scale. For the collapsed gels in the water-isopropanol mixture, partial phase separation occurs in which ordered domains of approximate size of 110 A develop. This study indicates that local ordering in liquid-crystalline-type domains is a possible molecular mechanism contributing to the collapse of gels composed of semiflexible polymers. The triple helical structure of the molecule appears not to be conserved in the majority phase in this solvent, but it is conserved in the liquid crystalline domains.

Polyelectrolyte networks as highly sensitive polymers

This review presents the results of theoretical and experimental studies concerning collapse of polyelectrolyte gels. Along with classical investigations, a number of new effects are described; they are associated with the formation of ion pairs and with nonuniform distribution of low-molecular-mass counterions over the volume of the polyelectrolyte gel.

Gelatin vs polysaccharide in mixture with sugar.

The structural behavior of a well-characterized gelatin sample has been revisited to investigate the morphology of its network in the presence of sugar. This was then contrasted with the corresponding properties of the gelling polysaccharides agarose, kappa-carrageenan, and deacylated gellan. Small deformation dynamic oscillation, differential scanning calorimetry in plain and modulated mode, visual observations, and transmission electron microscopy were used to identify the structural characteristics of the biopolymers from the rubbery plateau through the transition region to the glassy state. In contrast to the collapse of the polysaccharide gels at intermediate levels of co-solute, gelatin forms reinforced networks. The drop in polysaccharide network strength is accompanied by a decline in the enthalpy of the coil-to-helix transition, whereas the transition enthalpy is more pronounced in gelatin gels in accordance with their strengthening. Tangible evidence of the molecular transformations was obtained using microscopy, with polysaccharides disaggregating and dissolving in the saturated sugar environment. Gelatin, on the other hand, is visualized in an aggregated form thus producing a phase-separated topology with sugar.

Rapid settling of a colloidal gel

We study the rapid collapse of gels formed from strongly aggregating colloidal suspensions. This gravity-driven collapse is associated with the apparition of fractures in the bulk of the gels that provide an easy route to the gel-supernatant interface for the solvent and are the cause for the strong increase of the settling velocity. We propose a model that connects the apparition of a fracture in the gel to the settling velocity of the interface. This description takes into account the microscopic structure of the gel and is consistent with the experimental results.

Swelling behavior of radiation-polymerized polyampholytic two-component gels: Dynamic and equilibrium swelling kinetics

Polyampholytic hydrogels, with varying degrees of crosslinking and ionic content, were prepared by radiation polymerization of p-sodium styrene sulfonate (SSS) and vinyl benzyl trimethylammoniumchloride (VBT). These gels were investigated for their dynamic and equilibrium swelling kinetics. Dynamic swelling of these gels established that the gels containing equal amounts of SSS and VBT strictly follow Fickian diffusion. The hydrogels containing excess of SSS followed the case II type of diffusion, whereas those containing excess of VBT followed anomalous diffusion. Equilibrium swelling kinetics of these gels in aqueous system, ethanol–water mixture, at different pHs, and in the presence of solutions of biological interest was studied. It was seen that gels containing equal amounts of SSS and VBT show the lowest equilibrium swelling. Swelling of the polyampholytic gel decreased with an increase in the radiation dose imparted and the amount of crosslinking agent incorporated in the gel. The gels having an excess of VBT showed higher equilibrium swelling in comparison to those having an excess of SSS. Differential scanning calorimetry (DSC) studies showed that crosslinking of the gels decreases equilibrium swelling but increases the bonded nonfreezable water content of the gels. The organic solvents like ethanol cause abrupt collapse of the polyampholyte gels containing excess of SSS and those containing equal amounts of both the monomers at some critical ratio of water and ethanol in swelling medium. However, the deswelling in the water–ethanol mixture was gradual for gels containing an excess of VBT and the extent of deswelling was also low for these gels in comparison to other gels. The swelled gels of all compositions deswelled when they were transferred to solutions at pH in the range 2–12. Biologically important solutes like urea, glucose, and surfactants like Triton-X tend to further swell the polymer matrices, whereas NaCl causes their deswelling. The additive effect is more prominent for polyampholyte gels containing excess of either of the monomers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 730–742, 2003

Swelling and Collapse of Swiss-Cheese Polyelectrolyte Gels in Salt Solutions

A simple theory of swelling of microporous Swiss-cheese polyelectrolyte gels (i. e., polyelectrolyte gels containing water voids) in the solution of 1–1 low-molecular-weight salt is developed. Due to the Donnan effect the overall concentration of salt within porous Swiss-cheese gel can be significantly higher than that within the corresponding homogeneous gel due to the effective absorption of salt by the embedded voids. The degree of this absorption increases with the increase of average size of the voids and of their concentration. In the case of relatively large water voids with radii of about few μm the salt concentration within the water voids is approximately equal to that in the external solution, while the salt concentration in the polymer matrix can be much lower. It is thought that polyelectrolyte Swiss-cheese gels are promising for use as suitable media for microreactors for nanotechnology.

Formation and collapse of gels ofsterically stabilized colloidal particles

Colloidal silica spheres (diameter 88 nm) with a thick stericstabilization layer of polystyrene (PS; Mw = 26 600 g mol-1)were prepared. In cyclohexane, a marginal solvent for PS, particleaggregation and gelation were observed on lowering the temperature.Near the gelation temperature and at particle concentrations of afew per cent by weight, the gels were sufficiently weak to slowlycompact under gravity. On quenching to slightly lower temperatures,the gels still settled, but the top of the sediment did not becomeflat, as is usually the case. This seems to be related to anunusual mechanism for gel compaction, which starts by forming a moredense structure at the top of the sample. It is proposed that thisis related to the entangled polymer chains on neighbouring particlesresisting substantial rearrangement of the local structure. Thetransient gelation phenomenon, observed previously for mixtures ofcolloid and non-adsorbing polymer, has so far not been observed forour system.

SAXS Study of ι-Carrageenan−Surfactant Complexes

Small-angle X-ray scattering (SAXS) was used to study the structure of ι-carrageenan gels formed in the presence of cationic surfactant cetylpyridinium chloride (CPC) at various concentrations of the polysaccharide and as a function of the surfactant concentration. A physical gel of low ι-carrageenan concentration displays no characteristic peak at small angles. After addition of CPC to the ι-carrageenan solution central scattering and equidistant characteristic peaks appear due to ordering in the gel structure. This ordering is the result of complex hydrophobic and electrostatic interactions between the polymer chains and the charged surfactants. The ι-carrageenan gelation in the presence of CPC leads to self-assembly of surfactant molecules inside the physical gel similarly to micellization in a solution. In this case the shrinking or collapse of polyelectrolyte gels takes place. The additional central scattering emerges due to the process of surfactant micellization. The relative positions of the main ...

Polymer gel/organic dye complexes in aqueous salt solutions

AbstractThe present work is devoted to the study of the complex formation of polymer gel with organic dye and their properties in the aqueous salt solutions. Two systems were studied: 1) polyelectrolyte gel based on poly(diallyldimethylammonium chloride) and water soluble oppositely charged organic dyes (alizarin red S and catechol violet) and 2) organogel based on poly(N‐vinylcaprolactam) and dithizone. The collapse of the polyelectrolyte gels in the presence of oppositely charged dyes together with the effective absorption of dyes was observed. The shrinking degree and the dye absorption by the gel depends on the dye concentration. In the case of PVCa gel in organic media the dye absorption takes place.The main attention has been concentrated on the study of the behaviour of gel/dye complex immersed in the salt solution if dye is the chelating ligand for metal ions. It was shown that polyelectrolyte gels generally form stable complexes with oppositely charged dyes.The behaviour of PVCa‐dithizone‐chloroform system was studied in AgNO3 aqueous solution. The release of dithizone to the external aqueous solution of AgNO3 reservoir is completely suppressed.Absorption spectra of gel/dye and gel/dye/metal ion systems were studied. It was shown that metal ions penetrate inside the gel phase and the dye/metal ion complexes form within the gel. The dependence of the optical density for the systems of gel/dye/metal ion on the salt concentration is observed.

Swelling and Collapse of Physical Gels Formed by Associating Telechelic Polyelectrolytes

A theory is proposed for the swelling and collapse of physical gels formed by associating telechelic polyelectrolytes. It is shown that increasing the of degree of ionization makes the formation of an associative gel less favorable and increases the density of the supernatant phase. This leads to the contraction of the associative gel, in comparison with an analogous chemically cross-linked gel. The results of calculation show that the destruction of the associative gel happens monotonically in good solvent, while in poor solvent it can proceed in a jump-like fashion accompanied by the first-order phase decollapse transition of the associative gel. The addition of a low molecular weight salt to the solution facilitates the association of telechelic molecules which produces the associative gel and a further increase of salt concentration leads to the contraction of the associative gel.

Complexes of polyelectrolyte hydrogels with organic dyes: Effect of charge density on the complex stability and intragel dye aggregation

The swelling behavior of polyelectrolyte gels based on poly(diallyldimethylammonium chloride) (copolymers of diallyldimethylammonium chloride and acrylamide with the variable composition) and poly(methacrylic acid, sodium salt) in the presence of organic water soluble dyes (alizarin, naphthol blue black, rhodamine) was studied. The collapse of the polyelectrolyte gels in the presence of oppositely charged dyes together with the effective absorption of dyes was observed. The shrinking degree and the dye absorption by the gel depend on the charges of the polymer network and the dye, and also on the dye concentration. Stability of the gel–dye complexes in a salt solution of NaCl and Al2(SO4)3 was studied. It was shown that the complex stability in the salt solution depends on the charge density of the polymer chains forming the gel. The increase of charge density of polymer generally leads to the enhancement of the complex stability. For the systems with the fraction of charged poly(diallyldimethylammonium chloride) monomer units above 0.5 the release of alizarin to the external solution of Al2(SO4)3 reservoir is practically completely suppressed. The obtained results show that oppositely charged dyes are generally from stable complexes with polyelectrolyte gels. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1209–1217, 1999

Phase Transitions in Swollen Networks. 3. Swelling Behavior of Radiation Cross-Linked Poly(vinyl methyl ether) in Water

It is demonstrated that the collapse of gels of radiation cross-linked poly(vinyl methyl ether) swollen in water is a truly discontinuous process obeying classic thermodynamic principles. The phenomenon can further be shown to be related to the relevant type of limiting critical demixing of solutions of the corresponding non-cross-linked polymer. There are three such types, including “classic” ϑ behavior, and they can also be recognized in the swelling behavior of the cross-linked analogue.

Interactions between covalently crosslinked ethyl(hydroxyethyl)cellulose and SDS

Abstract The equilibrium swelling of chemically crosslinked gels based on ethyl(hydroxyethyl)cellulose (EHEC) in aqueous solutions of sodium dodecyl sulphate (SDS) was studied as a function of the SDS concentration at various temperatures and salt concentrations. Comparisons were made with gels based on poly-N-isopropylacrylamide (p-NIPA). Both polymers are known to form complexes with SDS above a critical association concentration (cac) of the surfactant, and both display a lower critical solution temperature (LCST) in water. For both types of gels, an increase in the equilibrium gel volume was seen with increasing SDS concentration above the cac, up to a maximum value when the SDS concentration in the external solution reached the critical micelle concentration (cmc). Above the cmc, the equilibrium gel volume decreased slowly with increasing SDS concentration. A volume collapse of the EHEC gels was observed in a temperature interval around the LCST of EHEC in solution. Above the cac, the collapse transition moved monotonically towards higher temperatures with added SDS. At lower SDS concentrations, however, the opposite trend was found. The swelling of the gel was less in the presence of salt and SDS, and a pronounced minimum in swelling appeared with added SDS when the salt concentration was sufficiently high (ca. 10 mmoles/l). Under these salt conditions, the LCST of the linear EHEC also passes through a deep minimum (below room temperature) on addition of SDS.

Spin-Labeled Polyelectrolyte Gels Based on Poly(N-isopropylacrylamide). Effects of the Network Structure and the Gel Collapse on the EPR Spectra

The swelling and collapse of aqueous copolymer gels of N-isopropylacrylamide and 2-acrylamido-2-methylpropanesulfonic acid have been studied by EPR spectroscopy using spin-labeled polymers. The swelling of the gels is strongly affected by added electrolytes and pH of the aqueous solvent. The EPR spectra from aqueous solutions of the linear polymer are motionally narrowed three-line spectra at temperatures below and above the critical temperature. Spectra from the gels are composites, i.e. they contain also a broad slow motional component, and their shapes are dependent on temperature as well as on the cross-link density. The intensities of the EPR spectra are suggested to be affected by the protonation of the radical label. Changes of the spectral intensity with temperature may be used to detect conformational changes of the polymer.

Evidence for Polyelectrolyte/Ionomer Behavior in the Collapse of Polycationic Gels

The interaction of slightly cross-linked positively charged gels of poly(diallyldimethylammonium chloride) (PDADMACl) with sodium salts of chloride, bromide, iodide, and acetate in aqueous media has been investigated. At the critical concentration of sodium iodide, a phase transition of the network to the collapsed state was observed. A wide hysteresis accompanied this transition. The presence of other salts did not initiate the network collapse in the investigated concentration range. A decrease in the charge density of the cationic network by copolymerization of DADMACl with acrylamide resulted in the disappearance of the phase transition. In this case, the usual contraction of polyelectrolyte gels in the presence of the mentioned salts was observed. The rate of volume change increased sharply with increasing concentration of sodium iodide. For some cases, two well distinct regions in the kinetic behavior were observed: a region of fast polyelectrolyte contraction and a region of slow collapse which, to our knowledge, had not been observed previously. The experimental results can be explained by the formation of ion pairs and multiplets (ionomer effect) for the sufficiently charged networks in the collapsed state. As a result, the collapsed ionomeric state of the network becomes competitive with the swollen polyelectrolyte state at high enough fractions of the ion-containing segments because of the tendency of the ion pairs to associate. A potential barrier which separates the ionomeric and the polyelectrolyte states of the gel can be used to explain the two-step kinetics of the collapse

Slow reversible collapse of gels based on copolymers of acrylamide and acrylic acid

AbstractThe behaviour of highly swelling hydrogels based on a copolymer of acrylic acid and acrylamide has been studied in solutions of low molecular weight salts. An explanation of the mechanism for gel collapse and swelling in solutions of various metals is presented. The dependences of the sorption rate and the amount of sorbed copper ions on the particle size of the dry powder and on the swelling coefficient were established.

Swelling equilibria for ionized temperature-sensitive gels in water and in aqueous salt solutions

Swelling equilibrium data in water and in aqueous NaCl solutions are presented for thermally sensitive N-isopropylacrylamide (NIPA) hydrogels containing 0–4 mol % quaternized amine (positively ionizable) comonomer. We report the effect of gel charge and solution ionic strength on the temperature-induced collapse of NIPA gels. Experimental swelling equilibria are compared with predictions based on a recently proposed oriented-quasichemical model. This model has been shown previously to describe lower critical solution behavior in uncharged aqueous polymer solutions and gels (i.e., aqueous NIPA gel). We apply the model here to ionized NIPA gel. Semiquantative predictions are obtained for the effects of gel charge and solution ionic strength on temperature-dependent swelling behavior.

Concentration-dependent collapse of polymer gels in solution of incompatible polymers

Mesure du changement de volume de gels de PS en solution semidiluee de PMMA/benzene en fonction de la concentration en PMMA et du degre de reticulation du gel

Collapse of polymeric networks in a mixed solvent

The phenomenon of collapse of polymeric networks has been studied theoretically for the case when it occurs in a mixed low molecular weight solvent. It is shown that collapse is accompanied by significant redistribution of the solvent in the collapsed network. In some cases in the mixed solvent reversed collapse of the network is also possible. Experimental study of the collapse of polyacrylamide gels in a mixed solvent confirms the conclusion on the redistribution of solvent in the collapsed phase.