Discontinuous Swelling Research Articles

Charge Regulation of Polyelectrolyte Gels: Swelling Transition

We study the effects of charge-regulated acid/base equilibrium on the swelling of polyelectrolyte gels by considering a combination of the Poisson–Boltzmann theory and a two-site charge-regulation model based on the Langmuir adsorption isotherm. By exploring the volume change as a function of salt concentration for both nanogels and microgels, we identify conditions where the gel volume exhibits a discontinuous swelling transition. This transition is driven exclusively by the charge-regulation mechanism and is characterized by a closed-loop phase diagram. Our predictions can be tested experimentally for polypeptide gels.

On the nature of volume-phase transitions in photo-cross-linked poly(cyclopropylacrylamide) and poly(N-vinylisobutyramide) coatings

Ellipsometry in an inverted configuration was used to characterize the temperature-dependent swelling of thin (100 nm) coatings of photo-cross-linked poly(cyclopropylacrylamide), or poly(CPAAm), and poly(N-vinylisobutyramide), or poly(NVIBAm). Both polymers contained 3 mol% of methacroylaminobenzophenone (MnBP) as the photo-cross-linking unit. Poly(CPAAm-co-MnBP) showed a continuous, 2nd order deswelling transition between 10 and 70 °C with no hysteresis. Poly(NVIBAm-co-MnBP), on the other hand, showed a discontinuous, 1st order deswelling transition at 45 °C with a hysteresis in the swelling curve. The two types of swelling transitions are consistent with the cloud-point measurements of the uncross-linked polymers, which both showed lower-critical solution temperature (LCST) behavior. The LCST for poly(CPAAm-co-MnBP) is located at almost zero polymer concentration, and hence cross-linked poly(CPAAm) cannot cross its cloud point curve during swelling or deswelling. The LCST for poly(NVIBAm-co-MnBP), in contrast, is located at a significant off-zero polymer concentration (>10 wt% polymer), meaning cross-linked poly(NVIBAm) can cross its cloud point curve and experience a discontinuous jump in swelling. Concurrent measurements of the infrared vibrations of the amide groups in both systems further revealed that the amide I band in poly(CPAAm-co-MnBP) showed a single sub-band throughout the swelling transition. Poly(NVIBAm-co-MnBP), on the other hand, showed two distinct sub-bands, one of which grew at the expense of the other throughout the swelling transition, which may explain the off-zero concentration in the cloud point curve and the discontinuous swelling transition.

Discontinuous swelling behaviors of lipophilic polyelectrolyte gels in non-polar media.

Discontinuous volume changes of lipophilic polyelectrolyte gels in non-polar organic solvent mixtures are demonstrated towards the construction of stimuli-sensitive materials in media other than water.

Mechanism of the pH-Induced Discontinuous Swelling/Deswelling Transitions of Poly(allylamine hydrochloride)-Containing Polyelectrolyte Multilayer Films

The mechanism of the discontinuous swelling/deswelling transitions exhibited by polyelectrolyte multilayers containing poly(allylamine hydrochloride) (PAH) was examined by FT-IR spectroscopy, in-situ atomic force microscopy (AFM), and in-situ ellipsometry. Assembly pH was found to play a critical role in determining the postassembly pH-dependent swelling behavior of multilayers containing PAH. Multilayer films assembled at pH < 8.5 were found to exhibit pH-independent swelling behavior over the pH range of 2.0−10.5, whereas dramatic discontinuous swelling transitions were observed when the assembly pH was greater than 8.5. FT-IR spectroscopy was used to demonstrate unequivocally that the pH-triggered, discontinuous swelling/deswelling transitions of PAH/sulfonated polystyrene (SPS) multilayers assembled at high pH (>8.5) are driven by changes in the degree of ionization of free amine groups of PAH that are established during multilayer assembly. The pH trigger points of these swelling/deswelling transitions are determined by the elimination/reestablishment of hydrophobically associated PAH chain segments. Such hydrophobic interactions are responsible for shifting the pKa of the free amine groups of PAH to unusually low values (ca. pH 4.0). The kinetics of deswelling were found to be strongly influenced by the type of polyanion assembled with PAH.

Re-assessment of the potential impact of physiologically relevant pH changes on the hydration properties of the isolated mammalian corneal stroma

The pH sensitivity of the swelling of the mammalian corneal stroma was reinvestigated to assess whether or not there were detectable differences in the hydration properties of this collagen-keratocyte matrix within a physiologically relevant range (as opposed to extremes of acid or alkaline pH) and at a physiologically relevant temperature. From recent post-mortem eyes of adult cows, square (8×8 mm) samples of corneal stroma were prepared and incubated in an isotonic, buffered (HEPES etc.), mixed salts solution with added glucose at 37°C. The time-dependent changes in wet mass were assessed over 24 h. The rate and magnitude of stromal swelling were different within the range of pH 6.5–8.5. The wet mass of stromal samples increased almost 2-fold within 1 h, and then at lesser rates to realise 3.25–3.75-fold and 4–5-fold increases in wet mass by 9 h and 24 h respectively. The maximum increases were observed at pH 7.25–7.5, with most of the effect being the result of differences in the initial rate of swelling. The discontinuous swelling and the pH effect on the rates of swelling were also evident when the data were fitted to a previous kinetic model (Elliott et al., J. Physiol. (Lond.) 298 (1980) 453–470). It is concluded that pH changes in the physiological range can have a small but reproducible impact on the swelling kinetics of the isolated mammalian corneal stroma ex vivo.

Swelling and shrinking of polyacid gels

A theory is presented which permits the calculation of discontinuous swelling of polyacid gels in water. The essential new aspect is that acidic groups shall not only dissociate but also form hydrogen bonds among each other yielding additional crosslinks of the polymer network. It is shown that the consideration of this effect is necessary to achieve agreement with experimental results. Additionally, an equation for the osmotic pressure in polyelectrolyte gels is derived. For highly s wollen gels this equation reduces to the van't Hoff law for dilute solutions.

Polyelectrolyte hydrogel instabilities in ionic solutions

The phase behavior of polyelectrolyte hydrogels has been examined as a function of relative charge composition, bath salt concentration, and solvent quality. Nonlinear swelling instabilities of 2-hydroxyethyl methacrylate (HEMA) and methacrylic acid (MAAc) copolymer hydrogels manifested themselves as discontinuous first order swelling transitions as a function of bath salt concentration. A modified Flory–Huggins model was used to describe the regions of instability when bath salt concentration and solvent quality are considered as control variables. The role of ion dissociation equilibrium in the change from local or smooth transitions to nonlocal or discontinuous swelling transitions is illustrated within the framework of our model.

Reentrant phase transition of N-isopropylacrylamide gels in mixed solvents

Reentrant volume–phase transitions are observed in N-isopropyoacrylamide gels in the methanol–water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed-loop instability phase boundary having both upper and lower critical points. The closed-loop phase boundary depends on temperature and diminishes to a point at approximately 0 °C. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol–water interaction by presence of polymer network. In the case of ethanol–water mixtures, there appear two closed-loop phase boundaries, whose physico-chemical basis are not yet clear.

Diffusion with discontinuous swelling. IV. Type II diffusion into spherical particles

AbstractIn the last few years some new features of the so‐called type II diffusion have been established which confirm the first theoretical description of such a material transport into a semi‐infinite glassy medium which at a certain concentration of the sorbate is transformed into a gel. The boundary between the glass and the gel progresses at a constant velocity into the interior of the sample thus yielding a linear term in the weight gain. The gradual establishment of the concentration profile in front of this boundary yields at the beginning a square root term in the weight gain. A detailed analysis of the extensive measurements of Hopfenberg, et al. of the diffusion of n‐hexane into extremely small polystyrene spheres demonstrates that the weight gain always starts with a square root of time term. In sufficiently large spheres this contribution is soon completely overridden by the term linear in time. The spherical geometry substantially modifies the concentration profile and the weight gain. In particular the weight gain divided by the square root of time vs the square root of time shows a maximum as soon as the geometrical factors prevail over the effect of the constant velocity progression of the boundary between the glass and the gel.

Diffusion with discontinuous swelling. V. Type II diffusion into sheets and spheres

AbstractType II diffusion into uniform spheres (radius R) and sheets (thickness 2l) is calculated under the assumption that the glass‐gel boundary proceeds at a constant velocity v from the surface towards the interior of the sample, that the diffusion coefficient Dg in the glass is constant and that the diffusion coefficient Dr of the rubbery gel is so much higher than vR or vl that practically no sorbate gradient is needed for the transport through the gel of the sorbate. The diffusion process is completed when this boundary reaches the center of the sample. The concentration profile of the sorbate in the glassy matrix in front of the boundary varies with time and velocity v. It does not, however, influence the boundary propagation velocity. Hence the often observed increase of the rate of the weight gain just at the end of the diffusion process is not considered at all. The relative weight gain of the sample W(t)/W∞ as a function of time is the only quantity usually measured. From the ordinate intercept A and the initial slope B of the plot of W(t)/t1/2W∞ vs. t1/2, one can calculate the characteristic transport properties, i.e., the diffusion coefficient Dg of the glass and the velocity v of the glass–gel boundary.

Diffusion with discontinuous swelling III. Type II diffusion as a particular solution of conventional diffusion equation

Very often a non-solvent diffuses into a glassy polymer with a steep concentration profile proceeding at an almost constant rate v yielding a weight gain proportional to time. Such a diffusion is called type II diffusion in order to distinguish it from the more usual “Fickian” diffusion proceeding without such a constant concentration front and yielding, at least in the beginning, a weight gain proportional to the square root of time. It turns out that the conventional diffusion equation without any special new term but with a diffusion coefficient rapidly increasing with concentration has a series of solutions representing exactly such type II diffusion with v as a completely free parameter which determines the steepness of concentration front. With the usual boundary conditions and infinite medium the diffusion coefficient has to become infinite at the highest penetrant concentration. This case can be considered as an extreme limit which is approached to a high degree in an actual experiment. The finite sample thickness, however, requires only a very large but not an infinite diffusion coefficient. Hence type II diffusion is only a special case of possible diffusion processes compatible with the conventional diffusion equation without any need for new terms if only the diffusion coefficient increases sufficiently fast with penetrant concentration.

Diffusion in a glassy polymer with discontinuous swelling. II. Concentration distribution of diffusant as function of time

AbstractThe time dependence of concentration distribution of diffusant in front of the advancing boundary between the core of the amorphous glass and the swollen surface layer was calculated under the assumption that 1. in the core the diffusion is FICKian and the diffusion constant D is independent of concentration; 2. the transition from the maximum concentration co in the core and the higher concentration c∞, in the swollen polymer occurs in a very thin boundary layer; 3. the boundary layer moves with constant velocity v; 4. the permeability of the swollen polymer is so high and hence requires such a small concentration gradient that it may be neglected (c = c∞ in the whole region). The concentration distribution in the core starts with a very fast drop which gradually decreases and approaches that of the limiting exponential distribution exp (‐yv/D). The time t∞, for practically reaching the steady state distribution equals 4 D/v2.

Diffusion in a network with discontinuous swelling

Journal of Polymer Science Part B: Polymer LettersVolume 3, Issue 12 p. 1083-1087 Article Diffusion in a network with discontinuous swelling A. Peterlin, A. Peterlin Camille Dreyfus Laboratory, Research Triangle Institute, Durham, North CarolinaSearch for more papers by this author A. Peterlin, A. Peterlin Camille Dreyfus Laboratory, Research Triangle Institute, Durham, North CarolinaSearch for more papers by this author First published: December 1965 https://doi.org/10.1002/pol.1965.110031222Citations: 65AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume3, Issue12December 1965Pages 1083-1087 RelatedInformation

The mechanism of sorption of ethylene dibromide on moist soils

AbstractThe influence of moisture on the swelling of soils and clays has been studied at low moisture contents. No detectable swelling was found with soils at low moisture contents while the montmorillonites showed discontinuous swelling corresponding to the lattice changes indicated by X‐ray analysis. From the sorption isotherms at different moisture contents it is shown that ethylene dibromide is displaced from sorption by water vapour, three distinct stages being apparent. The surface energy of ethylene dibromide solutions has been measured and the Gibbs sorption equation applied to moist soils. It is suggested that sorption on soils at field capacity is partly due to solution in the soil water and partly to sorption at the water interfaces. The surface areas obtained by the B.E.T. method using nitrogen at 78° k are much smaller than areas obtained from the water isotherms and other methods, the difference being apparently due to dehydration of organic matter during degassing.