Molecular thermodynamics for swelling of a mesoscopic ionomer gel in 1 : 1 salt solutions

For a microphase-separated diblock copolymer ionic gel swollen in salt solution, a molecular-thermodynamic model is based on the self-consistent field theory in the limit of strongly segregated copolymer subchains. The geometry of microdomains is described using the Milner generic wedge construction neglecting the packing frustration. A geometry-dependent generalized analytical solution for the linearized Poisson-Boltzmann equation is obtained. This generalized solution not only reduces to those known previously for planar, cylindrical and spherical geometries, but is also applicable to saddle-like structures. Thermodynamic functions are expressed analytically for gels of lamellar, bicontinuous, cylindrical and spherical morphologies. Molecules are characterized by chain composition, length, rigidity, degree of ionization, and by effective polymer-polymer and polymer-solvent interaction parameters. The model predicts equilibrium solvent uptakes and the equilibrium microdomain spacing for gels swollen in salt solutions. Results are given for details of the gel structure: distribution of mobile ions and polymer segments, and the electric potential across microdomains. Apart from effects obtained by coupling the classical Flory-Rehner theory with Donnan equilibria, viz. increased swelling with polyelectrolyte charge and shrinking of gel upon addition of salt, the model predicts the effects of microphase morphology on swelling.     

Plasticized microporous poly(vinylidene fluoride) separators for lithium-ion batteries. III. Gel properties and irreversible modifications of poly(vinylidene fluoride) membranes under swelling in liquid electrolytes

Microporous poly(vinylidene fluoride) (PVdF) separators for lithium-ion batteries, used in liquid organic electrolytes, have been characterized with respect to the swelling phenomena on dense PVdF membranes (obtained through hot pressing). In the first and second parts of this study, we have described the swelling equilibria and swelling kinetics of dense PVdF. Here the thermal properties of PVdF gels and their irreversible modifications induced by swelling are characterized. Particular attention is paid to crystallinity modifications, polymer plasticization, and membrane degradation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2308–2317, 2004     

Relationship of swelling and swelling pressure on silica-water interactions in montmorillonite

In this work, we have used the previously designed controlled uniaxial swelling (CUS) cell to obtain predetermined extents of swelling in montmorillonite. Using the CUS cell, a simultaneous measurement of swelling pressure is done with controlled swelling. Undisturbed clay samples at well-defined swelling (0%-75%) were removed from the CUS cell and analyzed using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. In addition, orientation-dependent microattenuated total reflectance (micro-ATR) spectroscopic investigations are also conducted on the controlled swelled samples. Significant changes in the silicate (Si-O) stretching region (1150-950 cm(-1)) have been observed with changes in swelling and orientation. The band at 1005 cm(-1) (attributed in the literature to arising from Si-O vibrations when montmorillonite platelets are normal to incident radiation) is most pronounced for the 0%-swelled sample and diminishes with swelling. The band associated with perpendicular vibration (at 1078 cm(-1)) increases with swelling. Thus, the intensity of this band increases with misorientation of clay particles. Our results indicate that the reduced particle size, as ascertained from SEM cryoimaging, with increased swelling is related to increased misorientation of the clay platelets. At 0% swelling, the clay platelets are most oriented and have largest particle size. The rearrangement of clay platelets as seen in the orientation-dependent spectra is a direct result of the breakdown of the clay particles with increased hydration resulting from increased swelling.     

Group contribution method for the swelling behavior of thermo‐responsive hydrogels

A group contribution method is introduced to describe the swelling behavior of thermo‐sensitive hydrogel systems. The accuracy of group contribution calculations is strongly dependent on the choice of thermodynamic model. Therefore, we revise the modified double lattice (MDL) model and develop a new expression for the interaction energy parameter using the association theory of Sanchez to take into account complex polymer/solvent mixing. The net Helmholtz energy for a hydrogel is established by combining the revised MDL model and modified Flory–Rehner elastic model. Group parameters are generated by fitting to experimental swelling data from both homopolymer and copolymer gel systems. The effect of salt on the volume phase transition is modeled by introducing an additional salt‐specific parameter to investigate various stimuli‐response swelling behavior. Calculated swelling equilibria using the new group contribution method shows excellent agreement with experimental data and various stimuli‐response volume phase transitions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 455–463     

Closed-packed lattice model for polymer solution systems considering the chain length dependence effect: Correlating LLE,VLE, and gel swelling behaviors using identical interaction energy parameters

We propose a new Helmholtz energy of mixing equation following the original Flory–Huggins (F–H) closed-packed lattice model. Also, to overcome F–H mean-field approximation, we introduce new universal constants to consider chain length dependence of polymer in solvent and consider specific interactions to describe strongly interacting polymer systems. Our proposed model successfully describes liquid–liquid equilibria (LLE) for binary polymer–solvent systems using identical interaction parameters which do not depend on the polymer molecular weight. We also describe vapor–liquid equilibria (VLE) for polymer/solvent systems and swelling equilibria of thermosensitive hydrogel systems using the same energy parameters obtained from LLE calculations.     

Deformational,swelling, and potentiometric behavior of ionized poly(methacrylic acid) gels. I. Theory

By using the model of a randomly coiled chain, a relation is derived describing the equilibrium stress–strain behavior of variously ionized polyelectrolyte gels swollen in solutions of a uni–univalent salt. The effect of the concentration of bound counterions calculated on the basis of the cylindrical model and the effect of the change of length of the statistical chain segment with the change in ionization of the gel on stress–strain, swelling, and potentiometric equilibria is discussed.     

Deformational,swelling, and potentiometric behavior of ionized poly(methacrylic acid) gels. II. Experimental results

The deformational, swelling, and potentiometric behavior of poly(methacrylic acid) gels was measured as a function of the degree of crosslinking, ionic strength, and degree of ionization. The comparison of the stress–strain behavior with theoretical relations derived in the preceding part has shown that the relations are valid only if an increase is assumed in the number of monomeric units in the statistical chain segment with increasing degree of neutralization of the gel. This dependence is affected by the salt content in the swelling solution and is also dependent on the activity coefficient of counterions. The pK₀ values for an undissociated gel approach pK₀ = 5.0 and increase somewhat with degree of neutralization. The swelling equilibria are in qualitative agreement with theoretical assumptions; their quantitative agreement depends on the activity coefficient of counterions.     

Sorption and swelling in ultra-thin polymer films

Different polymers were investigated with respect to the sorption of solvents and gases. Depending on the chemical nature of the polymers this sorption leads to polymer swelling. The degree of swelling D/D₀ was measured utilizing Small angle X-ray scattering (SAXS) as well as Surface plasmon resonance (SPR). From the change in film thickness after swelling in different solvents Hildebrand parameters of the polymers were determined. By crosslinking of the polymer films the degree of swelling can be controlled. In the case of ultra-thin polyimide films a higher degree of crosslinking led to a decreased selectivity of the transport of gases through the membrane. Reptation of macromolecules was also investigated and the influence of polymer swelling in different solvents has a great influence on the selectivity and diffusion coefficient.     

Hysteresis in clay swelling induced by hydrogen bonding: accurate prediction of swelling states

We perform grand-canonical molecular simulations to study the molecular mechanism of clay swelling hysteresis as a function of the relative humidity. In particular, we focus on the transition from the one- to the two-layer hydrate and the influence of three types of counterions (Li+, Na+, and K+). Our results cover the experimental relative humidity region where swelling and shrinking usually occur. We show that the thermodynamic origin of swelling hysteresis is a free-energy barrier separating the layered hydrates. This free-energy barrier is dominated by breaking and formation of hydrogen bonds between and within water layers. This network of water molecules is similar for all counterions, but the positions of these counterions depend upon their size. The relatively large K+ counterions show more affinity for clay surface adsorption, which increases the free-energy barrier and inhibits swelling. On the other hand, the relatively small Li+ counterions are quite well-accommodated in the water network, and thereby, they can form a new swelling state with a basal spacing of approximately 13.5 A. This new swelling state is an alternative explanation for the widely accepted simultaneous occurrence of two or more swelling phases.     

Guest-responsive reversible swelling and enhanced fluorescence in a super-absorbent, dynamic microporous polymer

A swell idea! The guest-responsive reversible swelling and fluorescence enhancement of a dynamic, microporous polymer network is presented. Guest-induced breathing of hydrophobic pores imparts multi-functional properties, such as super-absorbency, phase-selective swelling of oil from water and encapsulation of C(60) (see figure), to this soft micro-porous organic polymer.     

Elastomer swelling and Hansen solubility parameters

The prediction of how much an elastomer swells in contact with a given solvent is important in many industrial processes. In this article, Hansen solubility parameters (HSP) of two EPDM elastomers and one FKM elastomer were determined before vulcanisation by correlating solubility in those solvents that fully dissolved the polymers. These values were then used to generate plots of solvent swelling after the same elastomers were vulcanised. HSP correlations were also established for the same vulcanised elastomers at different levels of swelling. This makes its possible to predict the swelling of the elastomers studied in contact with many untested solvents.     

Dynamic and equilibrium swelling of a sulfonic acid superabsorbent copolymer in salt solutions

Superabsorbent copolymers were synthesized by free radical reverse emulsion from the monomers acrylamide and 2‐acrylamido‐2‐methyl‐propanosulfonic acid with a divinyl crosslinker (methylene‐bisacrylamide). The copolymer synthesized was then identified by Fourier transform infrared spectroscopy, its porosity was examined by electronic microscopy, and its specific gravity was determined by the conventional procedure. The dynamic and equilibrium swelling behavior of the copolymer was studied, in water and in salt solutions, at 25 °C (chlorides and sulfates of sodium and magnesium). As to the equilibrium swelling, the influence of the pH as well as of the salt type and its concentration was observed. As to the dynamic swelling, it was interpreted in terms of both a simple power of time equation, and a more detailed model based on a coupled diffusion‐relaxation mechanism. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 505–514, 2004     

Microstructure and swelling behavior of ion-exchange resin

The microstructure and swelling kinetics of ion-exchange resins having sulfonic acid groups were investigated by small-angle neutron scattering (SANS) and swelling experiments as functions of the crosslinking density (CD), pH, and the salt concentration (C_salt). The swelling kinetics was analyzed on the basis of the Tanaka-Fillmore swelling equation for the cooperative diffusion of polymer gels. The swelling behavior was very sensitive to CD, but not to pH and C_salt. The SANS intensity functions, I(q), were independent of CD and well described with a power law function, I(q) ∼ q^−D, where q and D are the magnitude of the scattering vector and the mass-fractal dimension, respectively. D was estimated to be ∼2, indicating that the resin consisted of a rather coarsely interconnected domains irrespective of CD at swelling equilibrium. It was found that CD is the most important parameter determining the swelling power of ion-exchange resin. However, no remarkable variations were found in the microstructure in the order of tens to hundreds of angstrom. © 1996 John Wiley & Sons, Inc.     

Theory of swelling of a crosslinked substance in equilibrium with a solvent in various phases

In this paper the thermodynamic theory of a body in a liquid, crystalline or vaporized solvent is treated. The equilibrium swelling curves are discussed for the different states of the solvent. The slopes of the swelling curves are dependent on the differential enthalpy of dilution of the solvent and, additionally, on the enthalpies of vaporization, crystallization and sublimation of the solvent related to the state of the swelling agent. The slopes of the swelling curves are determined by the differential heat of vaporization, the differential heat of solution of the solvent or the differential heat of fusion according to the state of the swelling agent. Directly below the melting pointT _m,1, or directly above the boiling pointT _b,1 of the solvent the swelling curves change their slopes with a sharp bend. This phenomenon can be used to determine (₁/w ₁) at constant temperature and pressure, which means the change of the chemical potential ₁, with the change of the weight fractionw ₁ of the solvent. Using a simplified statistical thermodynamic relation it is possible to describe the principal courses of the swelling curves in all states of the solvent.     

Diffusion in mesoporous materials and polymers swelling: a transient calorimetric approach

The diffusion of water and benzene has been followed by DSC using the thermoporosimetry (TPM) approach. The diffusion of water has been observed during the drying of a water impregnated mesoporous silica gel at 40 degrees C under dry air. It was found that the confinement affects the evaporation rate of water. The diffusion of benzene has been observed during the drying and the swelling of a cross linked PDMS sample. The mesh size distributions (MSD) of the elastomer, during swelling and drying, have been calculated at various times using the TPM formalism. Extrapolating the mean mesh size of the polymeric network, it was found that the dry polymer has an average mesh of about 2.5 nm.     

Dynamic swelling performance of hydrophobic hydrogels

Conventional gels manifest monotonous swelling or shrinking performance upon immersing in solvents until reaching an equilibrium state. Recently, we discovered that the “hydrophobic hydrogels” prepared from hydrophobic polymer networks demonstrated dynamic swelling performance without equilibrium states. Upon water immersion, the gels expanded tremendously at the first stage until reaching a swelling peak; subsequently, the gels shrunk at an extremely slow rate. While this phenomenon endows the ...     

Starch swelling and its role in modern ceramic shaping technology

A microscopic real-time visualisation of potato starch swelling in aqueous media at 56°C is performed. The viscosity increase of a starch-water mixture during the swelling step is documented by viscometric measurements. The mechanism of ceramic green-body formation in a new ceramic shaping process, called “starch consolidation”, is discussed. In particular, the rheological changes during swelling and their concomitant structural background can explain certain discrepancies between model predictions and experimental findings observed in previous work.     

Solvent swelling of Dictyonema oil shale: Low temperature heat-treatment caused changes in swelling extent

Equilibrium swelling experiments were used to gain insight into the structural changes of Dictyonema oil shale kerogen (baltoscandian basin black shale kerogen) during low-temperature heating. Room temperature volumetric swelling was measured for samples previously thermally pre-treated at different temperatures under an inert gas environment. The swelling capacity for heat-treated Dictyonema oil shale decreases with increasing pre-treatment temperature from about 150 °C to 380 °C, suggesting that cross-linking reactions occur during low-temperature heat treatment in the pre-pyrolysis temperature region. This behavior is consistent with that observed in non-softening low-tar yield coals.     

Modeling of swelling by the fast transient fluorescence technique in a polymeric gel

The time‐resolved, fast transient fluorescence technique, which uses a strobe master system, was employed for studying the swelling of a disc‐shaped poly(methyl methacrylate) gel. The disc‐shaped gel was prepared by free‐radical copolymerization of methyl methacrylate and ethylene glycol dimethacrylate. Pyrene (Py) was introduced as a fluorescence probe during polymerization. After this gel was dried, swelling and slow‐release experiments were performed in chloroform at room temperature. Lifetimes of Py were measured during in situ swelling and slow‐release experiments. An equation was derived for low‐quenching efficiencies to interpret the behavior of lifetimes in and out of the gel during swelling. Py lifetimes in the gel decreased as swelling proceeded, but Py lifetimes out of the gel stayed constant during the slow‐release experiments. The Li–Tanaka equation was used to determine the cooperative and mutual diffusion coefficients, which were around 10⁻⁵ and 10⁻⁷ cm²s⁻¹, respectively. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 739–746, 2000