75 research outputs found
Measurement of Dynamic Light Scattering Intensity in Gels
In the scientific literature little attention has been given to the use of
dynamic light scattering (DLS) as a tool for extracting the thermodynamic
information contained in the absolute intensity of light scattered by gels. In
this article we show that DLS yields reliable measurements of the intensity of
light scattered by the thermodynamic fluctuations, not only in aqueous polymer
solutions, but also in hydrogels. In hydrogels, light scattered by osmotic
fluctuations is heterodyned by that from static or slowly varying
inhomogeneities. The two components are separable owing to their different time
scales, giving good experimental agreement with macroscopic measurements of the
osmotic pressure. DLS measurements in gels are, however, tributary to
depolarised light scattering from the network as well as to multiple light
scattering. The paper examines these effects, as well as the instrumental
corrections required to determine the osmotic modulus. For guest polymers
trapped in a hydrogel the measured intensity, extrapolated to zero
concentration, is identical to that found by static light scattering from the
same polymers in solution. The gel environment modifies the second and third
virial coefficients, providing a means of evaluating the interaction between
the polymers and the gel
Structure of Polyelectrolytes with Mixed of Monovalent and Divalent Counterions: Poisson-Boltzmann Analysis and SAXS Measurements
International audienceWe have studied by Small Angle X Ray Scattering (SAXS) the structure of salt free polyelectrolytes solutions containing monovalent and divalent counterions. We have considered mixtures of sulfonated polystyrene with monovalent (Na+) and divalent (Ca2+) counterions and measured the position of the scattering peak, q*, as a function of the monomer concentration cp and the monovalent / divalent content. The aim is to understand the variations observed in q* position when the valence of the counterions is gradually increased. This work is a continuation of a previous study in which first measurements were performed on a rather small number of sodium-PSS / calcium-PSS mixtures. In the present work, we used synchrotron radiation improved the quality of the data and varied the monovalent / divalent ratio with a much finer step. Indeed this gives new interesting results in the ranges of low and large divalent content. We analyzed SAXS results through the isotropic model and scaling approach description introduced by de Gennes et al. and developed by Dobrynin et al.. In this model, one key parameter is the chemical charge and / or the effective charge fraction feff of the polyions. Although the chemical charge fraction f of sodium-PSS and calcium-PSS polyelectrolyte is fixed by the synthesis, the effective charge fraction in mixtures varies with the monovalent / divalent ratio. This quantity has been calculated using the resolution of the Poisson-Boltzmann (PB) equation in the frame of the cell model for various monovalent / divalent contents and different concentrations. Severe deviations can be found in the effective charge values of mixtures at finite concentrations compared to the classical Manning-Oosawa prediction (infinite dilution limiting law). We demonstrate that the evolution of q* is still compatible with the isotropic model and the scaling approach in the low concentration range provided that the divalent content is not too high. In particular, a power law relation q * ~ f eff~ 0.3 can be found which looks very close to the one observed for weakly charged polyelectrolytes ( q*~ f 2 / 7 in good solvent or q*~ f 1/ 3 in theta solvent). Mixtures finally provide a way to adjust the effective charge fraction without changing the chemical nature of the polyions. However this procedure gives improvement of data prediction only in a limited range; it is still not able to fully explain the high concentration range, as well as the high divalent content mixtures. This is certainly due to the fact that the PB equations are not able to take into account the local interactions between monomers and divalent counterions, which goes beyond the mean field approach
Hybrid Nanocomposites with Tunable Alignment of the Magnetic Nanorod Filler
For many important applications, the performance of polymer-anisotropic
particle nanocomposite materials strongly depends on the orientation of the
nanoparticles. Using the very peculiar magnetic properties of goethite
({\alpha}-FeOOH) nanorods, we produced goethite-poly(hydroxyethyl methacrylate)
nanocomposites in which the alignment direction and the level of orientation of
the nanorods could easily be tuned by simply adjusting the intensity of a
magnetic field applied during polymerization. Because the particle volume
fraction was kept low (1-5.5 vol \%), we used the orientational order induced
by the field in the isotropic phase rather than the spontaneous orientational
order of the nematic phase. At the strongest field values (up to 1.5 T), the
particles exhibit almost perfect antinematic alignment, as measured by optical
birefringence and small-angle X-ray scattering. The results of these two
techniques are in remarkably good agreement, validating the use of
birefringence measurements for quantifying the degree of orientational order.
We also demonstrate that the ordering induced by the field in the isotropic
suspension is preserved in the final material after field removal. This work
illustrates the interest, for such problems, of considering the field-induced
alignment of anisotropic nanoparticles in the isotropic phase, an approach that
is effective at low filler content, that avoids the need of controlling the
nematic texture, and that allows tuning of the orientation level of the
particles at will simply by adjusting the field intensity
Characteristic-time of strain induced crystallization of crosslinked natural rubber
International audienceReal time Wide-Angle X-ray Scattering (WAXS) measurements during cyclic tensile tests at high strain rates (from 8 sâ1â280 sâ1) and at room temperature on crosslinked Natural Rubber (NR) are performed thanks to a specific homemade device. From the observed influence of the frequency on the crystallization index at the maximum sample elongation, a characteristic crystallization time is deduced. This is done taking into account the material self-heating during such unusually high strain rates. Two regimes for the dynamic process of strain induced crystallization are evidenced. For the NR tested, the obtained characteristic time is around 20 ms when the material average elongation during the cyclic test is above a critical elongation value λc. λc is the minimum elongation needed to induce crystallization during low strain rate tensile tests. Moreover, a rapid increase of this characteristic time is found when the average elongation decreases below this critical value
Water vapour adsorption and contrast-modified SAXS in microporous polymer-based carbons of different surface chemistry
International audienceThe adsorption of water vapour on highly microporous activated carbons with different surface chemistry is investigated by small angle X-ray scattering (SAXS) as well as by adsorption isotherms. The water changes the intensity of the SAXS in a way that depends on how the pores are filled. With wetting liquids such as hexane, a pseudo binary model can be assumed in which pore-filling in reciprocal space q is described by a density function p(q). For water, clusters develop, even in the most oxidized carbon, creating a fully ternary system. In the Porod scattering region, however, the final slope is insensitive to the liquid-vapour interfaces. In this region, for the less oxidized samples, p(q) shows reasonable agreement with the adsorption isotherms. At low relative pressure P/P0, however, the SAXS results indicate a small degree of filling (about 10%) that is not reflected in the isotherms. The highly oxidized sample attains a degree of filling of about 70% that, unlike the corresponding isotherm, is constant for P/P0 > 0. These differences may be due to kinetic effects and/or ageing, involving either redistribution of the water molecules or modifications of the surface groups
Birefringence Modulated Scattering in Gels
We describe a novel effect that arises when a monochromatic light beam passes through a deswollen rigid-rod gel. When viewed perpendicular to the direction of propagation, under suitable conditions of polarisation, a pattern of fringes is observed. The phenomenon is a simple geometrical optical effect caused by the birefringence of the sample. For a given wavelength of incident light, observation of the spatial frequency of the fringes, , offers an inexpensive means of measuring the magnitude of the birefringence . With white incident light, the fringes transform into an iridescent pattern
Shear induced crystallization of MXD6 with and without nucleating additives
International audienceThe shear induced crystallization of the poly(m-xylylene adipamide) (MXD6) which is a semi-aromatic polyamide, was studied for a virgin (PA1) and a nucleated (PA2) grades using a shearing hot stage coupled with a microscope. Half crystallization times were measured according to the crystallization conditions (crystallization temperature, shear rate and shearing time). The effect of shear on the crystallization kinetics was shown by a strong decrease of the crystallization times for both materials. PA2 sensitivity to shear was much lower than that of PA1. This was attributed to the presence of nucleating agents which increased the primary nucleation density in the unsheared quiescent melt, leading to a higher necessary shear rate to overcome the quiescent nucleation. Kinetic models were proposed to predict the crystallization process as a function of the crystallization conditions. They were based on both Avrami and Hoffman-Lauritzen theories and modified to take into account the effect of shear. In the model the nucleation rate of the crystalline entities was related to the shear rate by a power function. Besides, crystalline morphology and orientation were studied by wide and small angle X-ray scattering to confirm the orientation effect of the shear in the crystalline part of the material
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