Flocculation mechanism induced by cationic polymers investigated by light scattering

Three cationic polymers with molecular weights and charge densities of 3.0 x 10(5) g/mol and 10%, 1.1 x 10(5) g/mol and 40%, and 1.2 x 10(5) g/mol and 100% were chosen as flocculants to aggregate silica particles (90 nm), under various conditions, including change in polymer dosage, particle concentration, background electrolyte concentration, and shear rate. The size and structure of flocs produced were determined using the static light scattering technique. On the basis of measurements of polymer adsorption and its effect on the zeta potential and floc properties, it has been found that the polymer charge density plays an important role in determining the flocculation mechanism. Polymers with a 10% charge density facilitate bridging, 40% charged polymers bring about either a combination of charge neutralization and bridging or bridging, depending on the polymer dosage, and polymers with the charge density of 100% induce electrostatic patch flocculation mechanism at the optimum polymer dosage and below but bring about bridging mechanism at the polymer dosage approaching the adsorption plateau value. Bridging aggregation can readily be affected by the particle concentration, and an increase in particle concentration results in the formation of larger but looser aggregates, whereas electrostatic patch aggregation is independent of particle concentration. The addition of a background electrolyte aids in bridging aggregation while it is detrimental to electrostatic patch aggregation. It has also been found that the effect of shear rate on the mass fractal dimension depends on polymer charge density.     

Theory of dynamic light scattering by polymers and gels

It is shown under very general conditions that the intermediate scattering function for the generalized Rouse—Zimm model always takes the simple form G(K, t) α exp[?K²(k_BT/f)t], when the scattering vector K becomes sufficiently large. (Here k_B is Boltzmann's constant, T is the absolute temperature and f is the individual bead friction factor.) A microscopic formulation for the bulk modulus and friction factor density of a gel network is incorporated into the viscoelastic continuum model of Tanaka et al. The resulting expression for the apparent long-wavelength diffusion coefficient of the gel is D_G = (k_BT/f)2(1 - 2/Φ), where Φ is the network functionality.     

On the scattering by polymers and copolymers at high angles

In this paper we discuss the interpretation of the large angle scattering for chains, either being rod like, rigid or presenting excluded volume effect. It is shown that on highly branched polymers one can obtain a parameter which depends on the functionality of the crosslinks and on the average length of the blocks in the chain. One shows also how this type of analysis allows the interpretation of neutron results during transesterification.     

Scattering by cyclic polymers and copolymers at large scattering vectors

General formulae allowing the evaluation of the form factors of cyclic block copolymers are established and graphs for cyclic copolymers of the form (A-B)_N are shown. WhenN is large, the linear and the cyclic copolymer have the same behaviour. It is possible to extend at large angle an analytical expression given for linear polymers. This allows to show that, in the study of transesterification by small-angle neutron scattering, one can neglect the effect of cycles.Dedicated to Prof. Fischer on the occasion of his 65th birthday     

Brillouin scattering from polymers and gels

We report results from Brillouin scattering on highly swollen and crosslinked polymeric gels, i.e. methyl methacrylate (MMA) gels crosslinked with ethylene dimethacrylate (EDMA). The study is performed varying the crosslink amount of the gels from 0 to 6%. The Brillouin spectra are measured at different scattering wavevectors in the angular ranges 90°–150°. Ve also measured the spectra at constant wavevector (90°) changing the scattering volume, in particular we select the size and the positions of the sample scattering volume. The wavevector dependence of the measured quantities confirms that in our system a micro—phase separation phenomenon takes place. In particular, for samples with crosslink contents higher than 3%, Brillouin data (studied in terms of the group velocity) give information about the existence of well defined solid- and liquid-like islands (heterogeneities in the gel structure) with an extent of several hundred Angstroms (as confirmed by elastic scattering data). Furthermore, selecting different spatial positions of the scattering volume (of micrometric sizes) and the overall sizes of the same, and performing the measurements at different times, we are able to observe (for the first time with Brillouin scattering) the non–ergodic behavior typical of this system. We report here preliminary results of such a study.     

Librational Raman scattering from solid polymers

Broad, low frequency Raman bands (below 100 cm^?1 were observed in several solid polymers (polymethylmethacrylate) (PMMA), polystyrene, poly n-butyl methacrylate and poly 4-methyl pentene-1) and attributed to the librational motions of the side groups. A structural model for the librating side group is proposed which exhibits the observed frequency dependence on mass.     

Liquid crystal polymers: Studies of labelled parts by Neutron scattering

The studies of the conformation of comb-like liquid crystal polymers have been made by Small Angle Neutron Scattering (SANS) with samples containing mixtures of totally protonated polymers and homologous polymers with deuterated backbones. For a polymethacrylate the conformation of a chain is determined in the isotropic nematic and smectic phases. Theoretical predictions of Renz and Warner (similar to the hairpin model of de Gennes) are verified from the temperature dependence of the radius, parallel to the director, of one chain in the smectic phase. When the terminal groups of the pendant part of the monomer are deuterated a segregation seems to occur. The problem of the generalization of results is raised.     

Characterization of star polymers in solution by small angle neutron scattering

In this paper we present the small angle neutron scattering characterization of two polyisoprene and one poly(ethylenepropylene) star polymers in solution. In the limit of low scattering vector, Q, and low concentration, φ, the molecular characteristics were determined. The molecular weights obtained are in good agreement with light scattering data. Deviations in second virial coefficients are due to a difference in solvent quality. Radii of gyration of the stars are well determined by the use of Kratky plots as is shown by the good agreement with average values of Zimm and Guinier analysis. Additionally, some thermodynamic properties were investigated in dilute and semidilute solutions. We have found that the concentration dependence of the osmotic compressibility is influenced by an expected step at the overlap concentration in both concentration regions.     

Dynamic small-angle x-ray scattering from crystalline polymers

The variation in small-angle x-ray scattering (SAXS) for samples of crystalline polymers subjected to oscillatory strains has been observed employing the 10-m SAXS apparatus with a two-dimensional position-sensitive detector at the National Center for Small-Angle Scattering Research at Oak Ridge National Laboratory. Signal averaging has been carried out in accordance with the π/2-sector technique by gating the detection of scattered intensity in synchronism with the sample strain. In-phase and out-of-phase intensity changes have been observed for a high-density polyethylene at various frequencies (0.05–3.2 Hz) and temperatures (0–70°C). Changes are greatest at small scattering angles. Frequency dependence is greatest at lower temperatures, indicating a time-dependent response of the structural changes leading to scattering at these temperatures. The behavior is not pronounced at elevated temperatures. The similarity of the scattering at both azimuthal angles of 0° and 90° suggests the existence of an additional mechanism, perhaps microvoid formation. To evaluate void contribution, the dynamic invariant function is determined as a function of frequency and temperature. It appears that the microvoid formation lags behind the applied strain. The static and swelling SAXS studies present further evidence of void generation during elongation.     

Laser-Raman spectra of vinyl chloride-vinylidene chloride copolymers

Several copolymers as well as homopolymers of vinylidene chloride and vinyl chloride have been examined by Raman spectroscopy. Probabilities of concentrations of monomer sequences have been calculated from known reactivity ratios. Surface scattering intensities at some frequencies were found to be linearly proportional to specific microstructure concentrations. Normalization of scattering intensities was accomplished using the intensity of the CH₂ asymmetric stretching mode at 2926 cm^-1, which is common to all samples examined. Good correlation was found for the concentrations of comonomer sequences {B}, {BB}, {BBB}, {AA} and {AAAA} in which A denotes vinylidene chloride and B vinyl chloride.     

Small angle scattering of partially oriented polymers

Summary A general method for the calculation of the small angle scattering pattern of partially oriented polymers with rotational symmetry about the fiber axis is explained. The method can be used with widely varying models for the basic scattering unit.

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Zusammenfassung Es wird eine allgemeine Methode zur Berechnung des Kleinwinkelstreudiagramms von partiell orientierten Polymeren mit Rotationssymmetrie bezüglich der Faserachse angegeben. Als Modell für die streuende Grundeinheit können die verschiedensten Strukturen verwendet werden.

     

Coherent anti-Stokes Raman scattering microscopy for polymers

Coherent anti-stokes Raman scattering (CARS) microscopy is a label-free chemical imaging modality capable of interrogating local molecular composition, concentration, and even orientation. In comparison to traditional Raman spectroscopy/imaging, CARS generates signals that are typically orders-of-magnitude stronger, enabling high-throughput and large-area imaging with superior spectroscopic fidelity. In this review, we present an overview of CARS microscopy as applied to polymer science, covering such timely and important topics as drug release and reaction kinetics to 3D molecular structures and orientation. We also discuss outstanding opportunities and challenges to using CARS microscopy as a quantitative measurement method.     

Study of low-frequency molecular motions in polydimethylsiloxane polymers by neutron inelastic scattering

The intrachain and interchain vibrations below 900 cm^?1 of polydimethylsiloxane (PDMS) have been studied by slow neutron inelastic scattering. A composite motion observed at +25°C for the methyl groups corresponds to nearly free rotation about the threefold axis of symmetry together with a large-amplitude rotation of the entire methyl group. At ?123°C, rotation about the threefold axis evolves to a torsional oscillation. The large-amplitude rotation evolves to the skeletal vibrations of a helical conformation. Vestiges of the cooperative skeletal vibrations of the conformation at ?123°C persist into the 25°C spectrum. The results indicate the presence of interrupted helical conformations at 25°C, which result from thermal disordering of the low temperature helices. The effects of crosslinking, low molecular-weight oils, and silica filler on the freedom of the methyl group motions and on skeletal vibrations have been determined. The effects of different crosslinking agents and different relative amounts of filler and oil on both the macroscopic physical properties and the observed molecular motions of PDMS can also be interpreted in terms of an interrupted helix.     

The determination of the lamellar thickness in semi-crystalline polymers by small-angle scattering

Three small-angle scattering methods for determining the thickness of the crystalline lamellae in polymers are described. These are based on 1. observation of the identity period, 2. determination of the specific surface of the phase boundary, and 3. analysis of the shape of the peak at the origin. The conditions for application of these methods and the types of average obtained are discussed, and some results are presented.     

Brillouin scattering and hypersonic relaxation in amorphous polymers

Mechanical relaxation at hypersonic frequencies is measured using Brillouin spectroscopy for polyisobutylene, atactic polypropylene, polydimethyl siloxane, and polyvinyl acetate. The temperatures of maximum loss determined in the gigahertz range are compared to the published transitions maps for the above polymers. It is found that the hypersonic relaxation data fall on an extrapolation of the secondary main chain glass–rubber relaxation line above the region where the primary and secondary lines merge.     

Small-angle x-ray scattering studies of crazed glassy polymers

A small-angle x-ray scattering (SAXS) study of the relaxed craze structure in polystyrene was performed using the Oak Ridge National Laboratory 10-m SAXS facility. Coupled with known results from transmission electron microscopy studies, the SAXS patterns can be interpreted as scattering from an open-cell foam with void spaces interspersed among the fibrils. Results have shown the scattering centers in crazed polystyrene can be modeled as cylinders the axes of symmetry of which are parallel to the tensile axes. Scattering centers are bimodal in their size distribution, with aspect ratios of 1.0 and 2.6. Crazes in lower-molecular-weight polystyrene have more and larger scattering centers than crazes in higher-molecular-weight polystyrene, while variations in strain rate and test temperature during craze formation have no effect on the relaxed craze morphology. A comparison of SAXS patterns from polystyrene and polycarbonate indicates that the morphologies of their respective crazes are significantly different.     

Small angle X-ray scattering pole figures of semicrystalline polymers obtained by synchrotron radiation

A goniometer for measuring small angle X-ray scattering pole figures by using synchrotron radiation has been constructed. In order to obtain the scattering in a large range of scattering angles, position-sensitive detectors (Gabriel counter and vidicon system) are applied. The integrated intensity as well as the position and the half width of the maximum are determined as functions of the angle between the incident beam and the normal to the film surface of the sample. The time necessary to determine a pole figure was approximately 3 hours. Samples of poly(ethylene terephthalate) were drawn under different conditions: uniaxially and biaxially at 92°C under homogeneous deformation, and uniaxially at 40°C by necking. The influence of the drawing conditions on the orientation of the crystal lamellae was determined. The results of the small angle scattering are compared with pole figures obtained by wide angle scattering. In addition some results with polypropylene were obtained. The small angle X-ray scattering is strongly affected by the limited size of the crystals and by the incomplete regular arrangement of the crystals. This gives rise to difficulties in the interpretation of the results. The problem will be discussed.     

Investigation of structural changes in semi-crystalline polymers during deformation by synchrotron X-ray scattering

The mechanical behavior of polymer materials is strongly dependent on polymer structure and morphology of the material. The latter is determined mainly by processing and thermal history. Temperature-dependent on-line X-ray scattering during deformation enables the investigation of deformation processes, fatigue and failure of polymers. As an example, investigations on polypropylene are presented. By on-line X-ray scattering with synchrotron radiation, a time resolution in the order of seconds and a spatial resolution in the order of microns can be achieved. The characterization of the crystalline and amorphous phases as well as the study of cavitation processes were performed by simultaneous SAXS and WAXS. The results of scattering experiments are complemented by DSC measurements and SEM investigations. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1574–1586, 2010