Nonmonotonic Swelling Research Articles

Nonmonotonic swelling and compression dynamics of hydrogels in polymer solutions.

Hydrogels are sponge-like materials that can absorb or expel significant amounts of water. Swelling up from a dried state, they can swell up more than a hundredfold in volume, with the kinetics and the degree of swelling depending sensitively on the physicochemical properties of both the polymer network and the aqueous solvent. In particular, the presence of dissolved macromolecules in the background liquid can have a significant effect, as the macromolecules can exert an additional external osmotic pressure on the hydrogel material, thereby reducing the degree of swelling. In this paper, we have submerged dry hydrogel particles in polymer solutions containing large and small macromolecules. Interestingly, for swelling in the presence of large macromolecules we observe a concentration-dependent overshoot behavior, where the particle volume first continuously increases toward a maximum, after which it decreases again, reaching a lower, equilibrium value. In the presence of smaller macromolecules we do not observe this intriguing overshoot behavior, but instead observe a rapid growth followed by a slowed-down growth. To account for the observed overshoot behavior, we realize that the macromolecules entering the hydrogel network not only lead to a reduction of the osmotic pressure difference, but their presence within the network also affects the swelling behavior through a modification of the solvent-polymer interactions. In this physical picture of the swelling process, the net amount of volume change should thus depend on the magnitudes of both the reduction in osmotic pressure and the change in effective solvent quality associated with the macromolecules entering the pores of the hydrogel network. We develop a phenomenological model that incorporates both of these effects. Using this model we are able to account for both the swelling and compression kinetics of hydrogels within aqueous polymer solutions, as a function of the size of the dissolved macromolecules and of their effect on the effective solvent quality.

A comparative study of semi-flexible linear and ring polymer conformational change in an anisotropic environment.

We adopt a Langevin-dynamics based simulation to systematically study the conformational change of a semi-flexible probed polymer in a rod crowding environment. Two topologically different probed polymer types, linear and ring polymers, are specifically considered. Our results unravel the significance of the interplay of probed polymer's semi-flexibility and crowding anisotropy. Firstly, both ring and linear polymers show a non-trivial dimensional change including nonmonotonicity and collapse-swelling crossover as their stiffness increases. Secondly, we modulate rod crowder length to investigate the anisotropic effect. We reveal that the formation of an ordered parallel arrangement of the environment can effectively lead to a remarkable stretching effect on the probed polymer. The coupling between the crowding anisotropy-induced stretching and the polymer stiffness can account for the unusual swelling behavior. Lastly, nonmonotonic swelling and shape change of the ring polymer are analyzed. We find out that the ring polymer is subject to most pronounced swelling at robust stiffness. Moreover, the maximum prolate shape is also observed at the same robust location.

Nonmonotonic swelling of agarose‐carbopol hybrid hydrogel: Experimental and theoretical analysis

ABSTRACTA hybrid gel is synthesized by physical mixing of agarose and carbopol and an interpenetrating network of Agarose‐Carbopol (AC) hybrid gel is observed by atomic force microscopy. This hybrid gel exhibits pH‐responsiveness and mechanical stability as well as tunable swelling. These hydrogels depict a nonmonotonic swelling behavior as a function of pH. In particular, the equilibrium degree of swelling increases with pH of bath solution until its maximum value around pH = , followed by a decrease at higher pH values. The degree of swelling is increased by the concentration of carbopol when the concentration of agarose is low [ ], or decreased by the concentration of agarose when is high [ ]. A physics‐based model is also adapted to characterize the swelling‐shrinking behavior of different compositions of AC gels. This swelling‐shrinking behavior of AC hydrogels will have potential applications in smart hydrogel‐based devices. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 444–454

Solvothermal Annealing of Block Copolymer Thin Films

A two-stage annealing process for block copolymer films was introduced consisting of a solvent vapor exposure followed by a thermal cycle. By heating the film but not the chamber, changes in the ambient vapor pressure of the solvent were avoided. Films of block copolymers and homopolymers showed transient nonmonotonic swelling behavior immediately after solvent exposure that was dependent on how the thin film was cast before the anneal. Thermal cycling of the solvent-swelled block copolymer films during the solvent vapor anneal (SVA) caused the films to deswell in 1-10 s and produced well-ordered microdomains in templated 45.5 and 51.5 kg/mol polystyrene-block-polydimethylsiloxane films annealed in toluene and n-heptane vapors for total process times of 30 s to 5 min.

Non-monotonic swelling of a macroion due to correlation-induced charge inversion

It is known that a large, charged body immersed in a solution of multivalent counterions may undergo charge inversion as the counterions adsorb to its surface. We use the theory of charge inversion to examine the case of a deformable, porous macroion which may adsorb multivalent ions into its bulk to form a three-dimensional strongly-correlated liquid. This adsorption may lead to non-monotonic changes in the size of the macroion as multivalent ions are added to the solution. The macroion first shrinks as its bare charge is screened and then reswells as the adsorbed ions invert the sign of the net charge. We derive a value for the outward pressure experienced by such a macroion as a function of the ion concentration in solution. We find that for small deviations in the concentration of multivalent ions away from the neutral point (where the net charge of the body is zero), the swollen size grows parabolically with the logarithm of the ratio of multivalent ion concentration to the concentration at the neutral point.

Self-organization of multivalent counterions in polyelectrolyte brushes

We investigated the swelling behavior and density distributions of polyion segments and multivalent counterions in polyelectrolyte brushes within a primitive model that represents both polyion segments and salt ions as charged spheres and the solvent as a continuous dielectric medium. The numerical performance of a previously proposed nonlocal density functional theory (NLDFT) was tested with Monte Carlo simulations for multivalent systems and compared with the Poisson-Boltzmann equation that ignores the ionic size and electrostatic correlations. The NLDFT predicts that in the limit of low salt concentration, trivalent counterions can self-organize into a one-dimensional wavelike structure within the oppositely charged polyelectrolyte brush. Multivalent counterions introduce electrostatic correlations responsible for the nonmonotonic swelling of the brush which is qualitatively different from that in a monovalent solution. While in a monovalent solution the brush thickness increases monotonically with the grafting density, in a trivalent solution the swelling behavior resembles that for a neutral brush in a poor solvent. The NLDFT was also used to investigate the effects of ionic size on the brush swelling and counterion organization.

Ionic Effects in Collapse of Polyelectrolyte Brushes

We investigated the effect of counterion valence on the structure and swelling behavior of polyelectrolyte brushes using a nonlocal density functional theory that accounts for the excluded-volume effects of all ionic species and intrachain and electrostatic correlations. It was shown that charge correlation in the presence of multivalent counterions results in collapse of a polyelectrolyte brush at an intermediate polyion grafting density. At high grafting density, the brush reswells in a way similar to that in a monovalent ionic solution. In the presence of multivalent counterions, the nonmonotonic swelling of a polyelectrolyte brush in response to the increase of the grafting density can be attributed to a competition of the counterion-mediated electrostatic attraction between polyions with the excluded-volume effect of all ionic species. While a polyelectrolyte brush exhibits an "osmotic brush" regime at low salt concentration and a "salted brush" regime at high salt concentration regardless of the counterion valence, we found a smoother transition as the valence of the counterions increases. As observed in recent experiments, a quasi-power-law dependence of the brush thickness on the concentration ratio can be identified when the monovalent counterions are replaced with trivalent counterions at a fixed ionic strength.

Reswelling transition of poly(sodium acrylate) gels due to destruction of hydrogen bonds observed by ATR FTIR spectroscopy

AbstractWe report the reswelling transition of a physically crosslinked polyelectrolyte hydrogel in water, which was prepared using poly(sodium acrylate) as the main constituent and Al ions as the crosslinker. The nonmonotonic swelling behavior was observed at room temperature (25°C) under the continuous exchange of water; an as‐prepared gel swelled at the first stage (the swelling process) and then shrunk very slowly at the second stage (deswelling process). In the deswelling process, the water exchange was stopped and the diameter of the gel in a limited amount of water was measured as a function of the temperature. The gels reswelled at different transition temperatures in the heating processes, which depended on the initial diameter at 25°C (resulting from the different numbers of water exchange). The reswollen state was stable at 25°C and could shrink again as a result of successive water exchanges at 25°C. In the following temperature increase in the closed system, the reswelling phenomenon was observed again. The destruction of the hydrogen bonds introduced by the water exchange was confirmed by the attenuated total refraction Fourier transform infrared measurements. The reversible transition behavior between the deswollen and the reswollen states is discussed in terms of the formation and destruction of the hydrogen bonds. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

The influence of silicon and phosphorus additions on neutron induced microstructural evolution of Fe [sbnd]Cr [sbnd]Ni ternary alloys at 646–703 K

The influence of silicon and phosphorus additions on void swelling in austenitic stainless steels under fast neutron irradiation has been investigated. A series of silicon containing alloys (Fe 15Cr 20Ni Si) and two series of phosphorus containing alloys (Fe 15Cr 25Ni P and Fe 16Cr 17Ni P) were irradiated in the temperature range of 646–703 K and 2 to 36 dpa. Fast neutron irradiation was performed in both FFTF and JOYO. Non-monotonic swelling in response to the solute level was observed in alloys with higher nickel content and at the solute levels where precipitation was not prominent. The enhanced void swelling observed at relatively low solute levels was mirrored by an enhanced void density. The results suggest that the enhancement of swelling by solute addition occurs when void nucleation was initially difficult.