The physical and structural properties of acrylamide gels have been characterized by osmotic deswelling, mechanical compression, and x‐ray scattering. These properties vary considerably with the concentration of the crosslinking agent bisacrylamide, at fixed total monomers concentration (10% wt/wt water). In particular, changes in the properties appear more prominent at a crosslinking level of about 5‐6% (wt bisacrylamide/wt monomers). The compression modulus of as‐prepared and swollen gels passes through a maximum at this level of crosslinking. The swelling pressure curves can be separated into osmotic and elastic contributions of the gel network. The elastic part exhibits similar behavior to the compression modulus. The scaling of the osmotic part with the gel concentration varies with the degree of crosslinking, changing from 2.33 to 3.09. This indicates that the solvent power of water decreases with increasing crosslinking level, towards Φ conditions. The scattering patterns from the gels have been analyzed as arising from additive contributions from a homogeneous gel matrix, and embedded heterogeneities having a higher crosslinking density. These heterogeneities become much more prominent at the same level of crosslinking about 5‐6%. Hysteresis observed in the sorption/desorption behavior of polyacrylamide gel suggests that further irreversible structural changes may occur at water activities lower than probed by osmotic deswelling. © 1992 John Wiley & Sons, Inc.

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The rheology of wormlike micelles ("worms") formed by surfactants in water often follows nonmonotonic trends as functions of composition. For example, a study by Raghavan et al. (Langmuir 2002, 18, 3797) on mixtures of the anionic surfactant sodium oleate (NaOA) and the cationic surfactant octyl trimethylammonium bromide (OTAB) reported a pronounced peak in the zero-shear viscosity eta0 as a function of NaOA/OTAB ratio at a constant surfactant concentration (3 wt %). In this work, we study the origins of rheological changes in the NaOA/OTAB system and the relations between the composition and structural characteristics using cryo-transmission electron microscopy (cryo-TEM). When either surfactant is in large excess, the dominating morphology is that of spherical micelles. As oppositely charged surfactant is added to the mixture, the spheres grow into linear worms and these continue to elongate as the viscosity peak (which occurs at a 70/30 NaOA/OTAB ratio) is approached from either end. At the viscosity peak, the sample shows numerous long worms as well as a small number of branched worms. Taken together, NaOA/OTAB rheology can be primarily understood on the basis of micellar growth, which is explained primarily by packing arguments. While the size of the hydrophobic micellar core continuously decreases as the short amphiphile OTAB is added at the expense of NaOA, screening of charges goes through a maximum, which contributes to the asymmetry of the viscosity curve. With regard to micellar branching, there is no significant difference in the density of branched worms on either side of the viscosity peak. Therefore, it appears that in contrast to the behavior of some surfactant/salt systems, branching does not have a significant influence on the rheology of this mixed catanionic surfactant system. Instead, our data clearly indicate that the origin of the viscosity peak is linked with micellar growth and micellar shortening.

The LCST phase-transition of aqueous PNIPA solutions in rising concentrations of the strong chaotropic salt KSCN was studied microcalorimetrically by DSC and apparently for the first time by ITC. An endothermic (entropy driven) binding of KSCN onto PNIPA was observed, explained by electrostatic perturbation of hydrophobic hydration by adsorbed ions. A good fit was found for the one-typeof-sites binding model, and the binding affinity increased with rising temperature from 15 to 20 °C but decreased at 25 °C. DSC measurements emphasized the lowering and broadening of the endothermic peak of PNIPA phase-transition with rising KSCN concentration, explained by reduced cooperativity of coil-toglobule collapse with increased heterogeneity along the polymer chain, caused by salt adsorption. A hysteresis was observed between heating and cooling DSC peaks, which decreased asymptotically with rising KSCN concentration, further supporting that binding occurs. This work provides new insights into the mechanisms of chaotropic salt effects on polymers and biopolymers in aqueous solutions.

The association behavior, critical micellization concentration (CMC), and enthalpy of demicellization (DeltaHdemic) of bovine beta-casein were studied, for the first time by isothermal titration calorimetry, in a pH 7.0 phosphate buffer with 0.1 ionic strength and in pure water. In the buffer solutions, the CMC decreased asymptotically from 0.15 to 0.006 mM as the temperature was raised from 16 to 45 degrees C. DeltaHdemic decreased with increasing temperature between 16 and 28 degrees C but increased from 28 to 45 degrees C. Thermodynamic analysis below 30 degrees C is consistent with the Kegeles shell model, which suggests a stepwise association process. At higher temperatures, this model exhibits limitations, and the micellization becomes much more cooperative. The CMC values in water, measured between 17 and 28 degrees C, decreased with increasing temperature and, expectedly, were higher than those found in the buffer solutions. beta-Casein micelles were visualized and characterized, for the first time in their hydrated state, using advanced digital-imaging cryogenic transmission electron microscopy. The images revealed small, oblate micelles, about approximately 13 nm in diameter. The micelles shape and dimensions remained nearly constant in the temperature range of 24-35 degrees C.

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The swelling of poly(acrylamide) (PAAm) gels and the osmotic pressure of linear PAAm in aqueous solutions were predominantly affected by anion type and increased according to the lyotropic series ranking of sodium halide anions: F− < (H2O) < Cl− < Br− < I−. The osmotic pressure of PAAm in all examined salt solutions followed the scaling theory, with an exponent of 2.3 ± 0.1. In solutions of a sodium halide series, the value of the pre‐exponential factor seemed to depend on salt concentration, anion radius, and the apparent “anionic‐portion radius” of the water molecule. This radius, extracted from the literature data, marks a transition point of the anion radius effect. Larger anions increase the osmotic pressure of PAAm more significantly as their concentration increases and vice versa. The effects of the anions on the osmotic pressure of PAAm are related to their preferential interactions with the polymer. Iodide, which increased the osmotic pressure of PAAm with respect to its value in pure water, seemed to preferentially adsorb onto the polymer with a binding constant of Kb = 9.7 ± 2.0 M−1 determined by isothermal titration microcalorimetry. However, fluoride, which decreased the osmotic pressure, was preferentially repulsed. The mechanisms of attraction and repulsion were attributed to ion‐water‐polymer interactions and the solvent quality of the hydrated ions. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 508–519, 2003

The "Efficiency of Energy Absorption" (or "Efficiency") and "Ideality" parameters were evaluated for several plastic foams and were found to be very useful in choosing a n appropriate cushion for the protection of a specific product. The maxima in these parameters were found to be in the same range of stresses, when derived from dynamic measurements or from predicted stress-strain curves based on previously proposed "Reference" and "Modified Boltzman Superposition" Models. For the rate independent foams the maxima in these parameters do not change with strain rate and can therefore be derived from slow, constant rate ("static") experiments. For rate dependent foams however, the maxima from "static" measurements were found to be in a lower range of stresses than those derived from dynamic ones. As a result, slow rate compressive measurements do not predict well the behavior of the foams during impact and the use of the "Reference" and "Modified Boltzman Superposition" Models is required for good predictions. It was found that the stiffer PS and PE foams attain maximum Efficiency and Ideality at higher stresses than the more flexible PUR foams.