Structural investigations on polymer latices by small-angle x-ray scattering

An overview is given on recent studies of latices by small-angle x-ray scattering (SAXS). It is demonstrated that SAXS-studies performed at different contrast (contrast variation) allows the full elucidation of the fine structure of the particles down to a size-scale of a few nanometers. Examples are given including studies on core-shell latices composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA) as well as of carboxylated latices.     

Real time small-angle x-ray scattering during annealing of polymer single crystals

Experiments at the Cornell high energy synchrotron source (CHESS) have shown that it is possible to obtain 2-D small-angle x-ray scattering patterns from single crystal mats of high density polyethylene with a time resolution of 0.3 s. However, it took up to 5 s to heat the 0.1-mm-thick specimen to the annealing temperature. A logarithmic increase of long spacing was quickly established, after an induction time of <2 s at the higher annealing temperatures. At lower temperatures the original reflection remains, weakening, while a continuous scatter to smaller angles builds up. At 10–20 s annealing time a new maximum becomes clear, and then the logarithmic increase of long spacing begins. The intensity of this reflection is intially low and increases with annealing time. On cooling it decreases again. It seems that here we can directly observe two mechanisms of lamellar thickening, melting then recrystallization at short times and diffusional thickening at long times.     

Analysis of small-angle x-ray diffraction from polymers

The Fourier transform method of Vonk and Kortleve for the analysis of small-angle x-ray diffraction (SAXRD) from semicrystalline polymers has been compared with a modified direct method, originally due to Tsvankin. SAXRD data for three melt-crystallized polyethylene samples have been analyzed in terms of the mean true periodicity, mean crystal length, and mean length of amorphous segments. The two methods of analysis yield substantially equivalent results for all three samples. Calibration curves for the Tsvankin analysis are tabulated, and the relative merits of the two methods of analysis are discussed. With either method, information about the diffracting structure may be deduced that is not available from a simple measurement of the position of the SAXRD maximum. In fact, direct application of the Bragg law to any but the sharpest maximum yields a spacing (the long period) that lacks direct physical significance.     

Morphological characterization during deformation of a poly(ether ester) thermoplastic elastomer by small-angle x-ray scattering

The scattering behavior of pre-drawn and annealed bristles of a highly deformable poly(ether ester) themoplastic elastomer based on poly(butylene terephthalate) as hard segments and poly(ethylene glycol) as soft segments in a ratio of 57/43 wt.-% is studied. Small-angle x-ray seattering measurements with an area detector are carried out on bristles with and without application of stress up to 195% relative deformation. Two-dimensional scattering patterns are used for morphological characterization of the sample.At small deformations one morphology peak is found, corresponding to a periodicity that changes affinely with deformation. The morphology of the smaple represents assemblies of mutually parallel crystalline lamellae, positioned perpendicular to the stretching direction both under and without stress. When macrodeformation increases a second peak appears, and a four-point pattern is observed in the relaxed state. In this intermediate deformation range coexisting morphologies contribute to the scattering. Additional contributions arise from lamellae, which are inclined to the stretching direction, as well as from lamellae, which are again perpendicular to the stretching direction, as a result of microfibril relaxation and loss of interfibrillar contacts. At large deformations the latter morphology dominates and the 2D-scattering pattern again shows a two-point character. A morphological model for this behaviour is discussed, where the break of interfibrillar contacts during deformation and the inhomogeneous stress field in the sample play an important role.Dedicated to the 65th birthday of Prof. E.W. Fischer Prof. Fischer was always a quide and patient teacher to us, he inspired our work with his intense interest and many valuable suggestions. We want to congratulate and thank him sincerely and extend our best wishes for the future.     

Analysis of the conformation of wormlike chains by combination of small-angle x-ray and small-angle neutron scattering

An analysis of the conformation of a wormlike polymer by small-angle scattering is presented. By a combined investigation of small-angle X-ray and of small-angle neutron scattering the effect of the finite size of the repeating units can be eliminated. The procedure suggested herein therefore allows to obtain the scattering function for a respective infinitely thin chain. The latter quantity is compared to current models of the scattering function of wormlike chains.     

Rapid small-angle and wide-angle x-ray studies of crystallization behavior in polymers

Small-angle and wide-angle x-ray scattering measurements, using a position-sensitive detector, were made during melt-crystallization of linear polyethylenes and PEO–PS–PEO triblock copolymer. The scattering measurements indicated that the triblock copolymer grew by the enlargement of regions in which lamellae are regularly stacked. During primary crystallization at higher temperatures similar behavior is observed in two linear polyethylenes. At lower temperatures, changes in the shape of small-angle scattering curves during the primary stage of crystallization indicate that amorphous gaps within the lamellar stacking become filled in. During secondary crystallization at higher temperatures new crystallites appear to grow between those formed in the primary stage. Concurrent decrease of the overall scattered intensity leads to the conclusion that secondary crystallization has two components: crystallization of new lamellae behind the spherulite growth front and the thickening of existing lamellae.     

Morphological modification of UHMPE fibers

UHMPE fibers can display several morphological features, including skin-core, microfibrillar and shish-kebab structures. In this work, an attempt was made to eliminate these morphologies by treating the UHMPE in a two-roll mill, at temperatures ranging from ambient to 145°C. Up to 130°C the tensile strength and modulus of the fiber were retained, and thermal analysis and crystallographic data showed that ordering in the fiber remained intact. The shish-kebab and microfibrillar morphologies were greatly reduced and the fiber cross-section was altered from circular to a flat ribbon.     

Theory of tagged polymer method in small-angle X-ray scattering

Summary It was shown that the conformation of a single polymer chain in concentrated solutions and in undiluted state can be estimated from the small-angle X-ray scattering of randomly tagged polymer mixed in the systems. The randomly tagged polymer means the polymer containing heavy atoms at randomly selected positions along the molecule. The excess scattering of the tagged polymer is obtained by subtracting the scattering of the untagged polymer solution (or bulk) from that of the mixture solution (or bulk) containing the tagged and untagged polymers. The excess scattering obtained in this way contains the contributions of some undesirable cross terms of the scattering amplitude. The method for the experimental evaluation of these terms and the conditions under which these terms are negligible are discussed. It was found that the cross terms can be neglected when the contrast in scattering power between the tagged and untagged scattering units is sufficiently large, and the number of the tags per tagged molecule is not so small.The distribution of the tags in the tagged molecules was also taken into consideration. By averaging the theoretical scattered intensity over all possible positions of the tags, it was found that the excess scattering is practically proportional to the scattering function of the untagged polymer when the number of the tags per tagged molecule is not so small. Experimental verification of this theory was shown.It was also shown experimentally that the conformational change, which may be caused by the effect of the tags, can be eliminated by the extrapolation of the results obtained for a series of tagged polymers with various tag contents to zero tag content.With 2 figures     

A small-angle x-ray scattering study of blends of isotactic and atactic polystyrene

Small-angle x-ray scattering (SAXS) from blends of isotactic and atactic polystyrene has been studied. Results have been interpreted and compared using the Tsvankin, Vonk, and Hosemann techniques. The studies suggest that segregation of the atactic component occurs during crystallization within the growing spherulite of the isotactic component. However, since the interlamellar distance does not increase with atactic content, segregation is believed to occur with the formation of domains larger than interlamellar but smaller than spherulite size.     

Assessment of branching in polymers from small-angle x-ray scattering (SAXS)

Small-angle x-ray scattering from solutions was applied in assessing branching using well-characterized dextran hydrolyzates. It was found that the longitudinal and cross-sectional properties of the polysaccharides vary in a manner predictable by the molecular topology of the chains. A hydrodynamically equivalent linear model for dextran is proposed and it has been shown that this model correctly predicts values of Φ and K, Flory's universal hydrodynamic constant and the Flory–Fox–Schaefgen parameter, respectively.     

The influence of crystallinity distribution on small-angle x-ray scattering from semicrystalline polymers

Theoretical x-ray scattering curves have been developed for the lamellar structure in semicrystalline polymers in which there are present distributions of lamellar thickness and crystallinity. The models have been tested against samples of linear low-density, low-density and crosslinked polyethylenes. When variation in crystallinity is present in a material, a major effect is an increase in the magnitude of near-zero angle scattering. The Bragg maximum can appear as an ill-defined hump on an apparently high level of background scattering. The shape of the Bragg peak is influenced more by crystallinity distribution than by lamellar thickness distribution. Of the polymers we have studied so far only linear low density polyethylene shows significant crystallinity distribution effects. A “rule-of-thumb” method for rapid estimation of crystallinity distribution effects has been developed, obviating the need for lengthy simulation.     

Small-angle x-ray scattering by crystalline polymer fibers 2. Investigation of the linear paracrystalline model using various materials

The linear position-sensitive detector is well-suited to measure quantitatively the distribution parallel to the fibre axis of the intensity of small-angle x-ray scattering (SAXS) by polymer fibres, except that in the case of four-point patterns their width is greater than that of the detector window. A method is described which overcomes this problem, and which has high angular resolution. Using this method, the variation of scattered intensity with angles from 0.3° to 2.5° has been measured for fibres of poly(ethylene terephthalate), nylon, and low density poly(ethylene) (LDPE), and compared with that predicted by the linear paracrystalline model. In all cases except LDPE, when the distribution of phase lengths was given by the Reinhold function, there was no significant disagreement between the measured and predicted scattering except for a very small range of angles on the low angle side of the peak intensity. With LDPE small but significant discrepancies were found at other angles as well, and these were worse if the symmetrical Gaussian distribution function was used. The method enabled quantitative parameters describing the morphology to be obtained. It is concluded that the morphology of the linear paracrystalline stack is consistent with the SAXS intensity distribution, and that the Reinhold function is a reasonable approximation to the distribution of phase lengths. A small modification so that this decays more rapidly at long lengths might be necessary to explain the scattering for all materials over the entire angular range and other small changes might be needed with LDPE, although the asymmetrical nature of the distribution must be retained.On leave from Department of Physics, University of Technology Malaysia, 81300 Sekudai, Malaysia.     

Electrospinning of ultra-thin polymer fibers

The electrospinning technique was used to spin ultra-thin fibers from several polymer/solvent systems. The diameter of the electrospun fibers ranged from 16 nm to 2 μm. The morphology of these fibers was investigated with an atomic force microscope (AFM) and an optical microscope. Polyethylene oxide) (PEO) dissolved in water or chloroform was studied in greater detail. PEO fibers spun from aqueous solution show a “beads on a string” morphology. An AFM study showed that the surface of these fibers is highly ordered. The “beads on a string” morphology can be avoided if PEO is spun from solution in chloroform; the resulting fibers show a lamellar morphology. Polyvinylalcohol (PVA) dissolved in water and cellulose acetate dissolved in acetone were additional polymer/solvent systems which were investigated. Furthermore, the electrospinning process was studied: different experimental lay-outs were tested, electrostatic fields were simulated, and voltage - current characteristics of the electrospinning process were recorded.     

A small-angle x-ray scattering study of the nanostructural features of high-ash carbon materials

Nanostructured carbon-silica composite materials are prepared from different compounds in one-pot. They are characterized by BET, transmission electron microscopy, X-ray diffraction, and small-angle X-ray scattering. An effective method of the quantitative analysis of the structural dispersed characteristics of the template phase of silica in these composites is proposed.     

Determination of the persistence length of an aromatic polyamide-hydrazide by small-angle x-ray diffraction

Small-angle x-ray diffraction measurements were performed upon dimethyl acetamide (DMAc) solutions of four samples of X-500, an aromatic polyamide-hydrazide. The solubility improved as the polymer molecular weight increased, the most soluble sample being in the form of a highly oriented, crystalline fiber. Molecular weights determined from the scattering curves indicate that aggregation was present in all solutions. Its extent varied with the solubility of the sample, increased with polymer concentration and, for the lowest concentrations studied, could be reduced by the addition of acetic acid. Radius of gyration values were smaller than those obtained by light scattering, probably because of insufficient resolution of the camera. The mass per unit length along the chain was found to be 18–20 Å, while the radius of gyration of the cross section was 3.5–4.8 Å. Three of the samples gave nearly the same angular intersection for the coil-and rodlike regions of the scattering curve. Two of these were whole polymers and the other a narrow fraction. The fact that essentially the same intercept was found in all three cases indicates the persistence length as measured for linear chain polymers by small-angle x-ray diffraction is independent of the heterogeneity of the sample. The intercepts, after a small correction for excluded volume effects, correspond to persistence lengths in the range 71–88 Å for DMAc solutions, which approximate those determined for dimethyl sulfoxide (DMSO) solutions by light scattering the intrinsic viscosity without correction for excluded volume, but are substantially larger than the DMSO values, corrected for excluded volume of 30–49 Å. We can offer no explanation for this difference.     

Surface heterogeneity of porous polymer sorbents

The distribution function of the energy of adsorption (heterogeneity function, χ) was studied for porous polymers using inverse gas chromatography. The heterogeneity functions to-wards n-hexane, n-butanol, and ethyl acetate probes were obtained for four porous polymers prepared by polymerization of styrene and divinylbenzene. The value of c for the polar surface depends on the choice of the test sorbate. The possibility of comparing surface heterogeneity of various porous polymers using the parameter χ was shown.     

A small-angle x-ray scattering study of starch gelatinization in excess and limiting water

The architecture of the starch granule, and its subsequent disruption due to the application of heat and water, known as gelatinization, is of wide interest. Small-angle x-ray scattering (SAXS) techniques have been used to study gelatinization in limiting and excess water. SAXS allows the absorption of water into the differing regions of the starch granule to be monitored. In excess water, a process of cooperative melting can be seen. In limiting water, the crystalline order melts at a higher temperature. These features have been studied, and observed features of the gelatinization related to those known from other techniques. © 1993 John Wiley & Sons, Inc.     

Fourier analysis of polymer x-ray diffraction patterns

In analyzing the shapes of diffraction peaks from polymers, it has been mistakenly assumed that it is possible to distinguish between models based on “microstrains” and on paracrystallinity. It is shown that either paracrystallinity or distortions due to dislocations give rise to disorder of the second kind and cannot be distinguished by current diffraction methods. Fourier analysis of shapes of peaks from polymers gives quite satisfactory results, despite comments in the literature to the contrary. The analysis can now be carried out with single peaks; multiple orders are not required.