Neutron scattering in polymer gels at the theta temperature

Small angle elastic neutron scattering was performed on three polystyrene networks swollen to equilibrium in cyclohexane at the theta temperature. Comparison of the SANS spectra with similar measurements in uncross-linked solutions reveals that in the intermediate Q region the gels behave like solutions of lower concentration than the equivalent solutions. We assume that the gel spectra are composed of a static component plus a dynamic, or solution-like part. By a suitable fitting procedure, the former can be estimated, yielding the mean square static concentration fluctuation <Δφ²> generated by cross-linking contraints in the gel. Independent measurement of the swelling pressure of these gels permits an estimate to be made of the scattering intensity I(Q=0) of the dynamic part of the spectrum. Plausible agreement is found between the macroscopic and microscopic estimations of the osmotic compressibility if allowance is made for the concentration polydispersity in the gels.     

THE STRUCTURAL CHANGES IN BOVINE ROD OUTER SEGMENTS IN THE PRESENCE OF ATP

Abstract— We have, previously, described a light-induced near infrared (700–850 nm) light scattering transient obtained in the presence of ATP from bovine rod outer segments suspensions in which the plasma but not the disk membranes were perforated (Uhl et al ., 1979a). This transient was termed the 'A' signal. To elucidate its possible origin, we have analyzed their angular and wavelength dependencies. These data have been compared with osmotically controlled (non-light) induced light scattering changes from identical control rod outer segments suspensions. It has been found that A_D (the dark light scattering signal obtained in the presence of ATP) and A_LS (the slow component of the actinic flash induced light scattering signal, A_L) can be assigned to the swelling of the disk membranes while A_Lf (the fast component of this latter signal) can be attributed to the change in refractive index of the ROS caused by the hypsochromic spectral shift of photolyzed rhodopsin. The collective disk swelling associated with A, and A_LS is consistent with the pumping of ions into the disk lumen by the action of a disk membrane bound ATPase.     

High-resolution diffusion-ordered spectroscopy to probe the microenvironment of JandaJel and Merrifield resins

The success of organic reactions performed on a gel-phase resin is highly dependent on the accessibility of solvents, catalysts, and reagents to the interior of the resin. A variety of techniques including EPR, fluorescence, and Hildebrand solubility parameters (delta) have been used to probe reaction capabilities and in particular the microenvironment of a gel-phase resin. To provide a more detailed picture of the matrix in question, researchers have turned to NMR for the determination of the diffusion coefficients of solvents and small molecules in swollen beads to provide a means to compare the microenvironment of swollen beads. Since Merrifield and JandaJel resins display different swelling properties and have significantly different kinetic behavior, we undertook a comparative study of the diffusion coefficients of solvents and small molecules in both resins by high-resolution (1)H DOSY NMR. Our results show the following: (1) diffusion values for all studied solvents and small molecules are 20-30% higher in JandaJel compared to Merrifield resins, (2) in the absence of interactions between the resin and a given molecule, the diffusion values mirror the swelling properties of the resin, and (3) in the presence of strong intermolecular interactions between the gel and the considered molecule, the diffusion behavior in the gel is primarily influenced by the strength of the interactions and secondarily by the swelling properties of the resin. These results clearly show that the microenvironment of JandaJels is more "solution-like" than that of Merrifield resins, presumably due to the higher swelling capacity.     

Optical studies of the stress-induced crystallization of rubber

The low-angle light scattering by films of stretched natural and synthetic rubbers was investigated. Intense V_v scattering is found under conditions when crystallization occurs which is characteristic of the scattering from aggregates of dimensions comparable with the wavelength of visible light. These were identified with the γ fibrils described by Andrews. The dependence of scattering was studied as a function of light polarization direction, orientation direction, elongation, temperature, degree of swelling, type of swelling liquid, and degree of crosslinking. It was concluded that the scattering unit consists of an assembly of crystals with their chain axes parallel to the stretching direction, but (in the case of natural rubber at high elongations) with the fibril axis at a slight angle to the stretching direction. The scattering is not affected much by swelling but is decreased upon increasing the temperature. Upon recooling the scattering returns, but does so over several hours, indicating that much of the scattering arises from secondary crystallization.     

Small-angle neutron and x-ray scattering study of swollen nylon-6

Results of swelling and small-angle scattering experiments on samples of nylon-6 swollen with heavy water are discussed on the basis of the lamellar and switchboard models. The small-angle neutron scattering (SANS) intensity is very sensitive to the distribution of water in swollen samples, while the small-angle x-ray scattering (SAXS) data characterize the dry samples. The observed values of the mean-square fluctuation of scattering-density can be explained by a model with assumed inhomogeneous swelling of the amorphous phase.     

Conformational properties of polyhydroxyethers

The conformation of four types of polyhydroxyethers and one amine-cured triglycidyl isocyanurate resin were studied. Static and dynamic light scattering as well as small angle neutron scattering revealed a scarcely detectable influence of branching on the overall dimensions. The branching density has, however, a very pronounced effect on the intrinsic viscosity. The behavior of the latter results from the inverse proportionality of the intrinsic viscosity to the segment density. Estimation of this apparent density gave swelling ratios of only 2 to 5. The cured epoxy resins showed no swelling up to molecular weights of 10 000; in the higher molecular weight region the swelling ratios increase continuously up to a factor of 9.     

Combining linear polarization spectroscopy and the Representative Layer Theory to measure the Beer–Lambert law absorbance of highly scattering materials

Visible and Near Infrared (Vis–NIR) Spectroscopy is a powerful non destructive analytical method used to analyze major compounds in bulk materials and products and requiring no sample preparation. It is widely used in routine analysis and also in-line in industries, in-vivo with biomedical applications or in-field for agricultural and environmental applications. However, highly scattering samples subvert Beer–Lambert law's linear relationship between spectral absorbance and the concentrations. Instead of spectral pre-processing, which is commonly used by Vis–NIR spectroscopists to mitigate the scattering effect, we put forward an optical method, based on Polarized Light Spectroscopy to improve the absorbance signal measurement on highly scattering samples. This method selects part of the signal which is less impacted by scattering. The resulted signal is combined in the Absorption/Remission function defined in Dahm's Representative Layer Theory to compute an absorbance signal fulfilling Beer–Lambert's law, i.e. being linearly related to concentration of the chemicals composing the sample. The underpinning theories have been experimentally evaluated on scattering samples in liquid form and in powdered form. The method produced more accurate spectra and the Pearson's coefficient assessing the linearity between the absorbance spectra and the concentration of the added dye improved from 0.94 to 0.99 for liquid samples and 0.84–0.97 for powdered samples.     

Mononuclear rearrangement of heterocycles in zwitterionic micelles of amine oxide surfactants

The pH-induced swelling of poly(2-vinylpyridine) microgel particles was studied using dynamic light scattering. The increase in particle diameter with decreasing pH was modeled using a well-established thermodynamic model for microgel swelling. The Maxwell construction was applied to extend the model and yield a prediction for a pH-responsive microgel across the entire pH range. The model predicts a first order phase transition for polymer-solvent combinations with a Flory interaction parameter, χ, greater than a critical value χ(c). The modified theory compared favorably to the dynamic light scattering data for the hydrodynamic diameter of microgel particles based on 2-vinylpyridine at different pH values. In particular, the swelling transition is both predicted theoretically and observed experimentally to occur at a pH lower than the pK(a) of the polymer.     

A theoretical approach of the measurement of osmotic fragility of erythrocytes by optical transmission

The osmotic fragility of the erythrocyte membrane to hypotonic solutions is investigated theoretically. The fragility curves exhibit a strong transmittance rise. This variation is assumed to result from changes in the scattering properties of erythrocytes under dialysis resulting from swelling and hemolysis. The refractive indices of erythrocytes are obtained through the Lorentz-Lorenz relation based on hemoglobin and water contents. The scattering cross sections (needed to calculate the collimated transmittance) and the forward scattered intensity (needed to calculate the incoherent transmittance) are expressed according to the simple algebraic relations of the anomalous diffraction approximation. It is shown that swelling (or shrinking) has no influence on the collimated transmittance. Hemolysis alone causes the abrupt sigmoidal increase of the collimated transmittance with time. The possible transmittance increase (decrease) observed during swelling (shrinking) is due to incoherent transmittance and depends on the detecting solid angle value of the experimental setup.     

The synthesis of temperature-sensitive PMMA-coating PNIPAM particles via a rapid microwave-assisted polymerization

PMMA-coating PNIPAM particles were prepared via a rapid microwave-assisted polymerization. The PMMA shell was found to have a limitation on the movement of PNIPAM chains, which resulted in a reversible swelling and shrinking. The swelling-shrinking processes of particles were traced by laser light scattering, and the swelling factor of particles was found to be 1.63 with an average radius of 83.4 nm in the swelling state and of 70.8 nm in the shrinking state.     

About different types of water in swollen polyelectrolyte multilayers

The review addresses swelling of polyelectrolyte multilayers in water. Different models for the determination of the water content are compared. It is clearly shown that voids under dry conditions present cavities for water which contribute to the water content of the multilayer in the swollen state. This so-called “void water” does not lead to any changes in thickness but in scattering length density during swelling. The “swelling water” leads to both changes in scattering length density and in thickness. Depending on the preparation conditions like the type polymers, polymer charge density, ionic strength and type of salt the ratio of “void water” differs between 1 and 15 vol.% while the amount of “swelling water” is of several ten's of vol.%.     

Synthesis,structure, and swelling behavior of two-component copolymer networks

A two-component copolymer network comprising polystyrene (PS) and poly(dimethylsiloxane) (PDMS) as copolymer components was prepared by cross-linking PS with telechelic PDMS with a new method. The microphase structure of the dry PS/PDMS network was found to be irregular from DSC and small angle X-ray scattering measurements. The swelling behavior was observed in three kinds of mixed solvents which were chosen so as to contrast with one another in respect of solubility of the component polymers. The observed swelling degree depended on the solvent composition in a manner peculiar to the respective mixed solvent systems. The Flory swelling equation extended to the two-component polymer network qualitatively reproduced the experimental swelling behavior.     

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.     

Physicochemical characterization of chitosan nanoparticles: electrokinetic and stability behavior

Some physical properties of nanogel particles formed by chitosan ionically cross-linked by tripolyphosphate (TPP) have been studied. Electrokinetic properties and colloidal stability were analyzed as a function of pH and ionic strength of the medium. Chitosan particles showed volume phase transitions (swelling/shrinking processes) when the physicochemical conditions of the medium were changed. Experimental data were mainly obtained by electrophoretic mobility measurements and by photon correlation spectroscopy and static light scattering techniques. Chitosan chains possess glucosamine groups that can be deprotonated if the pH increases. Therefore, modification of pH from acid to basic values caused a deswelling process based on a reduction of the intramolecular electric repulsions inside the particle mesh. Electrophoretic mobility data helped to corroborate the above electrical mechanism as responsible for the size changes. Additionally, at those pH values around the isoelectric point of the chitosan-TPP particles, the system became colloidally unstable. Ionic strength variations also induced important structural changes. In this case, the presence of KCl at low and moderate concentrations provoked swelling, which rapidly turned on particle disintegration due to the weakness of chitosan-TPP ionic interactions. These last results were in good agreement with the predictions of gel swelling theory by salt in partially ionized networks.     

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.     

Inelastic light scattering measurements from linear polyelectrolyte aqueous salt solutions

Inelastic light scattering experiments demonstrate the existence of a conformational change upon ionization of a weak acid polyelectrolyte in aqueous salt solutions. The swelling of the polymeric coil occurs at a neutralization degree increasing with molecular weight.     

Characterization of inhomogeneous polyacrylamide hydrogels

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.     

Arsenic(II) sulfide quantum dots prepared by a wet process from its bulk

Arsenic(II) sulfide nanocrystals were fabricated by a wet process from its bulk through cluster-mediate transformation. The bulk arsenic(II) sulfide can be easily dissolved in ethylenediamine to form a solution-like cluster (As4S4-NH2C2H4NH2)). The cluster can be transformed to nanocrystals assisted by consumption of ethylenediamine in protic polar solvents with mild thermal treatment. The nanocrystals show size-dependent fluorescence ranging from UV to the blue, two photon fluorescence as well as cytotoxic effects on cancer cells.     

Surface plasmon spectroscopy of gold-poly-N-isopropylacrylamide core-shell particles

Highly uniform, core-shell microgels consisting of single gold nanoparticle cores and cross-linked poly-N-isopropylacrylamide (PNIPAM) shells were prepared by a novel, versatile protocol. The synthetic pathway allows control over the polymer shell thickness and its swelling behavior. The core-shell structure was investigated by electron microscopy and atomic force microscopy, whereas the swelling behavior of the shell was studied by means of dynamic light scattering and UV-vis spectroscopy. Furthermore, the latter method was used to investigate the optical properties of the hybrid particles. By modeling the scattering contribution from the PNIPAM shells, the absorption spectra of the gold nanoparticle cores could be recovered. This allows the particle concentration to be determined, and this in turn permits the calculation of the molar mass of the hybrid particles as well as the refractive index of the shells.     

Microstructure and rheology of stimuli-responsive microgel systems--effect of cross-linked density

The effect of cross-linked density on the rheological behavior of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with di-allyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counter-ions trapped in the polymeric network against the ions in bulk solution, which is responsible for the swelling and increase in viscosity. The viscosity exhibits a maximum at approximately 1 wt.% DAP and it decreases to a steady value at 4 wt.% DAP, which is independent of pH and particle concentrations. Static light scattering results confirmed this optimum density as the critical point where sufficient cross-link points are present to produce permanent junctions that permit optimal swelling of the microgel particles. In addition, the variation of relative swelling with cross-linked densities of our model microgel systems agrees with the theoretical scaling law, Q alpha (yalphaN(x))(3/2) for cross-linked densities beyond this optimum point (Q is the swelling ratio, y is the acidic MAA content, N(x) is the average number of monomer units between two cross-linked points, and alpha is the degree of neutralization). By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.