Light and neutron scattering studies of excess low-angle scattering in moderately concentrated polystyrene solutions

The light scattering (LS) and small-angle neutron scattering (SANS) behaviour of semi-dilute solutions of polystyrene has been determined in both ‘good’ and ‘theta’ solvents. Above a critical concentration related to chain overlap, an excess small-angle scattering component is in evidence for scattering vectors, q, such that qR_g < 1. Application of a number of recent solution scattering theories fails to account for the small-angle scattering observed. The inter- and intramolecular scattering functions are measured experimentally through characterization of the SANS behaviour of solutions containing mixtures of polystyrene and perdeuteropolystyrene. The resultant intermolecular scattering functions depend on the fraction of labelled chains, indicating clearly that the solutions contain large scale fluctuations. LS studies support this hypothesis and further show that the presence of these fluctuations is reproducible, yet dependent on the solution preparation procedure. Similar behaviour is observed in screening length measurements. The excess low-angle scattering is well characterized by the Debye—Bueche random two-phase model, which is subsequently used to estimate the characteristic dimensions of the long-range fluctuations.     

Response of matrix chains to nanoscale filler particles

A new generation of on-lattice MC simulations for dense melts of coarse-grained POE chains has been developed so that the MC simulations include nanoscale filler particles. In this novel approach, filler particles and polymer chains are built from the same chemical structural building unit. Early simulations pointed an ambiguous behavior for the response of the matrix chains to the introduction of the nanoparticles. When the size of nanoparticles is comparable to that of matrix chains, or even smaller, a significant amount of chain expansion (for matrix chains) is always the case, which challenges the current theoretical considerations in this field. Our recent simulations show that the chain expansion behavior is not triggered by an artificially created “extra volume” effect. Furthermore, the relation between chain expansion behavior and a potential enhancement in the free volume is found to be irrelevant based on our analysis on the pair correlation and scattering functions.     

Edge effect on electronic transport properties of graphene nanoribbons and presence of perfectly conducting channel

Numerical calculations have been performed to elucidate unconventional electronic transport properties in disordered nanographene ribbons with zigzag edges (zigzag ribbons). The energy band structure of zigzag ribbons has two valleys that are well separated in momentum space, related to the two Dirac points of the graphene spectrum. The partial flat bands due to edge states make the imbalance between left- and right-going modes in each valley, i.e. appearance of a single chiral mode. This feature gives rise to a perfectly conducting channel in the disordered system, i.e. the average of conductance 〈g〉 converges exponentially to 1 conductance quantum per spin with increasing system length, provided impurity scattering does not connect the two valleys, as is the case for long-range impurity potentials. Ribbons with short-range impurity potentials, however, through inter-valley scattering, display ordinary localization behavior. Symmetry considerations lead to the classification of disordered zigzag ribbons into the unitary class for long-range impurities, and the orthogonal class for short-range impurities. The electronic states of graphene nanoribbons with general edge structures are also discussed, and it is demonstrated that chiral channels due to the edge states are realized even in more general edge structures except for armchair edges.     

The electrical properties of graphene modified by bromophenyl groups derived from a diazonium compound

Graphene field-effect transistors were fabricated with mechanically exfoliated single-layer graphene (SLG) and bilayer graphene (BLG) sheets and the functionalization effects of bromophenyl groups derived from a diazonium compound on its transfer properties were explored. Spectroscopic and electrical studies reveal that the bromophenyl grafting imposes p-doping to both SLG and BLG. The modification of SLG by bromophenyl groups significantly reduces the hole carrier mobility and the saturation current in SLG transistors, suggesting an increase in both long-range impurity and short-range defect scattering. Unexpectedly, the bromophenyl group functionalization on BLG does not obviously increase both types of scattering, indicating that the BLG is relatively more resistant to charge- or defect-induced scattering. The results indicate that chemical modification is a simple approach to tailor the electrical properties of graphene sheets with different numbers of layers.     

Interpretation of quasi-elastic light scattering data for flexible chains: model dependence

The autocorrelation functions and corresponding relaxation times obtained from the forward depolarized quasi-elastic light scattering experiment are exhibited for two quite similar models of flexible polymer chains in solution. A very small change in the chain dynamics is found to be sufficient to change the relaxation time from a relatively short time independent of chain length, with an autocorrelation function suggestive of an unweighted sum of contributions from all the relaxation times in the spectrum of chain motion, to a long time with an autocorrelation function identical with that for the end-to-end vector, strongly dependent upon chain length and dominated by the longest relaxation time in the spectrum. These results raise the question whether widely-used models in which information about short-range chain structure and motion is deliberately omitted can be expected to be appropriate for the interpretation of depolarized scattering experiments.     

Thermodynamics of polyolefin blends: small-angle neutron scattering studies with partially deuterated chains

Small-angle neutron scattering is used to evaluate the binary interaction parameter χ in molten blends of polyethylenes having different levels of ethyl branching along the backbone. The labelled chains were partially deuterated to minimize the isotope effect on χ. The present results for equal-volume-fraction binary blends at 150°C can be summarized as χ=0.4×10⁻⁴+0.014ΔX²_br. The first term is from the hydrogen-deuterium isotope effect for chains labelled with about 33% deuterium. The second term is due to chemical composition differences, expressed here as the difference in X_br, the fraction of four-carbon repeat units having an ethyl branch. The observed contributions to χ are in reasonable accord with calculated estimates.     

Mechanical properties of nylon-6 after treatment with metal halides

Conditioning of nylon-6 by immersion in metal chloride solutions modifies the mode of deformation under an applied tensile stress. Infra-red spectroscopy of salt-treated films shows significant modification of the spectrum, indicating changes in the intermolecular bonding. Modulus measurements support the hypothesis that modification of the intermolecular bonding in nylon results in some chain stiffening. This stiffening of the network of chains reduces the mobility that is required for shear deformation. This in turn leads to the onset of scission crazing. NaCl, however, spectroscopically shows no evidence for nylon-salt interactions having occurred. The deformation behaviour of both thin-film and bulk samples of NaCl-treated nylon-6 reflect the absence of any salt-amide interaction.     

微波合成(Au)_核·(Ag)_壳纳米粒子及其共振散射光谱研究

以柠檬酸化学还原法制备的金纳米粒子作晶种,采用微波高压液相合成技术,制备出分散性较好、规则球形的(Au)核·(Ag)壳复合纳米粒子。研究了(Au)核·(Ag)壳复合纳米粒子的紫外可见吸收光谱和共振散射光谱特性,在470nm处有最强共振散射峰,在404nm处产生一个吸收峰。结果表明,(Au)核·(Ag)壳复合纳米粒子的形成是导致470nm共振散射的根本原因。     

Reactivity of polymeric carbon chains reduced from poly(tetrafluoroethylene)

Electrochemical reduction of poly(tetrafluoroethylene) (Teflon), which contains very long linear carbon atom chains, yields in the presence of Li⁺ a solid mixture of lithium fluoride and polymeric carbon. The latter is probably in the sp state and is very chemically reactive. Its chains begin to bind into a solid carbon skeleton only after separation from the salt sheaths, so that chemical bonds are formed between certain carbon atoms of neighbouring chains. The form of the skeleton is much influenced by physico-chemical conditions during the separation process. The porous carbon layer consisting from sporadically bound carbon chains was found to be plastic as long as the skeleton does not become more rigid by the formation of additional chemical bonds. Three different carbonaceous materials were prepared from the same mixture (C, LiF) by different technologies. Differences between them were studied by elementary chemical analysis, nitrogen adsorption, wide-angle and small-angle X-ray scattering, and electron microscopy. The obtained results enable to discuss the course of formation of the carbon skeleton.     

The contribution of neutron scattering to polymer science

The contribution of neutron scattering to polymer science is reviewed. A summary is given of the theory of neutron scattering followed by a review of the literature. Particular emphasis is placed on the use of small angle neutron scattering to observe the conformation of polymer chains in bulk polymers and concentrated polymer solutions and on quasi elastic neutron scattering to measure the dynamics of polymer chains.     

Dynamics of polymer molecules using neutron spin—echo

The dynamics of single polymer chains have been investigated in dilute solution and in the melt using the neutron spin—echo technique. In dilute solution the intramolecular motion is described to a first approximation by that for a Rouse chain incorporating hydrodynamic interactions (Zimm). It is characterised by an inverse correlation time which may be normalised by temperature, solvent viscosity and segment size, and which for long chains varies as Q³ at small scattering angles (Q is the change in wavevector on scattering). At very low Q vectors the predicted ‘universal’ regime is observed, but over most of the accessible range chemical structure also becomes important. For short chains, deviations from this Q³ behaviour are associated with overall molecular diffusion. In the melt, chain entanglements come into play and modify the simple Rouse-type motion. The correlation functions are described by a combination of Rouse motion over short distances and times and a long-time slow-motion predicted by the reptation model of melt dynamics.     

SAXS characterization of the Nafion membrane nanostructure modified by radiation cross-linkage

The present study employs small-angle X-ray scattering (SAXS) to investigate the water-swollen structures of two types of Nafion membranes, commercial Nafion 117 membranes and the membranes synthesized from the Nafion precursor, subjected to gamma radiation. The membrane structure can be characterized by two-length scales, comprised the long-range order of lamellar crystalline domains in the matrix and the local order of ionic cluster domains. Both the long-range order lamella and local order cluster in the membranes are significantly affected by the radiation-induced cross-linking. We have extended a local order model to take into account the polydispersity effect, which can more satisfactorily reproduce the ionomer SAXS peak than other existing models. The structural parameters determined from the SAXS model analysis are self-consistent with those obtained from the model-independent Porod analysis. The modified membrane structure by the radiation cross-linking is very helpful for developing a high performance and low cost of direct methanol fuel cells.     

Dilute-solution structure of charged arborescent graft polymer

The solutions of charged G1 arborescent polystyrene-graft-poly(2-vinylpyridine) copolymers in methanol-d4 and D₂O were investigated over a dilute concentration range ?=0.005-0.05 (?: mass fraction) using small-angle neutron scattering (SANS). Upon addition of acid (HCl) arborescent graft polymers became charged and a peak appeared in SANS data. The interparticle distance (d_exp) calculated from a peak position corresponded to the expected value (d_uni) for a uniform particle distribution. This indicates the formation of liquid-like ordering due to long-range Coulombic repulsions. The smaller dielectric constant of methanol-d4 resulted in long-range electrostatic repulsions persisting to lower polymer concentration than in D₂O. The slow mode scattering was observed by dynamic light scattering measurements for the same polymer solutions, indicating the presence of structural inhomogeneity in the solutions. Both the peak and slow mode disappeared by addition of NaCl or excess HCl into the solutions due to the screening of electrostatic interactions. The G1 polymer grafted with longer P2VP chains (M_w∼30,000 versus 5000 g mol) formed a gel on addition of HCl. This result reveals that molecular expansion is more significant for arborescent polymers with longer (M_w∼30,000) linear polyelectrolyte branches, resulting in gelation for ?>0.01. Upon addition of NaCl or excess HCl a gel transformed back to a liquid resulted from the screening of electrostatic interactions.     

Neutron scattering and amorphous polymers

During the past ten years neutron scattering has become a much more widely used technique. The use of neutron scattering to study the conformation and dynamics of polymer chains in the bulk amorphous state and in solution is reviewed here. The basic theory of neutron scattering is introduced. The types of instruments which are currently used and the factors affecting neutron scattering experiments are discussed. The following sections are each concerned with a different type of scattering experiment and the information which has been obtained. At the beginning of each of these sections the theory relating to the particular topic under discussion is introduced. The three topics covered by this review are: conformation studies of polymers; dynamics of polymer chains and studies of side group motion in polymers.     

Transmission Properties of Translucent Polycrystalline Alumina

Effects of residual pores and optical birefringence on transmission through translucent polycrystalline alumina have been studied with the Mie scattering theory. The in-line and total forward transmissions of the translucent polycrystalline alumina were simulated as a function of porosity, pore radius, and grain size. The model revealed that porosity has a significant effect on both the total forward scattering and the in-line transmission, whereas grain size affects only in-line transmission. The calculated transmittance is in good agreement with the experimental values. The in-line transmission gradually increased with a decrease in grain size, and the effect of birefringence on the total forward transmission is negligible in the visible spectrum. The total forward transmission in the visible spectrum is mainly governed by residual pores.     

Kinetics of rapid structural changes during heat setting of preoriented PEEK/PEI blend films as followed by spectral birefringence technique

The influence of composition and preorientation on the development of structural hierarchy during heat setting of PEEK/PEI films was investigated using on-line birefringence, and off-line wide-angle and small-angle X-ray scattering, infrared dichroism and thermal analysis techniques. When the PEEK/PEI blends are drawn to deformation levels below the onset of strain hardening, the subsequent heat setting at high temperature starts with a large relaxation process followed by a fast crystallization and a long-term slow structural rearrangement stages. When the films are predrawn beyond a critical structural level (crystallinity and orientation), the initial relaxation stage disappears. This signifies that beyond a critical structural order a long-range physical network, where the nodes consist of crystallized domains and chain—self and cross-entanglements—are formed. This physical network allows the entropy driven shrinkage stresses to be maintained that results in the development of oriented crystalline phase. The addition of non-crystallizable PEI chains was found to retard the formation of this ‘network structure’ resulting in lower orientation levels.     

Double liquid crystalline side-chain type block copolymers for hierarchically ordered nanostructures: Synthesis and morphologies in the bulk and thin film

Well-defined double liquid crystalline side-chain type block copolymers (DLC-BCPs) were synthesized by nitroxide-mediated living free radical polymerization (NMP) of two different types of styrene monomers containing liquid crystalline moieties. The molecular weights and compositions were controlled by changing the polymerization time and/or the feed amount of the monomers. The bulk morphologies were studied using wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). DLC-BCPs were favored to form microphase-separated lamellar structures over a wide range of volume fractions. By controlling the annealing conditions, further higher-ordered hierarchical nanostructures were obtained with a long-range alignment, in which the semi-crystalline side-chains were oriented parallel to the interface of microphase-separated lamellar structure. Thin film morphology was investigated using atomic force micrography (AFM). The formation of lamellar nanostructure with a long-range alignment vertically oriented to the silicon wafer substrate was achieved by simple thermal annealing.     

PH-induced structure change of poly(vinyl alcohol) hydrogel crosslinked with poly(acrylic acid)

The structure of the hydrogel of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was investigated by small angle X-ray scattering (SAXS) of synchrotron radiation. A physically crosslinked blend gel, which was prepared by repetitive freezing and thawing of an aqueous solution of PVA and PAA, could be chemically crosslinked by esterfication of PVA with PAA even in the hydrogel state. The chemical crosslinking induced the destruction of physical crosslinks into a folded structure, indicating that the chemical crosslinking proceeds at the sites around the physical crosslinks that contain PVA and PAA in much higher concentration than other portion of the gel. The pH-induced structure changes of the PVA hydrogels, chemically crosslinked with poly(acrylic acid) (PAA) were investigated by SAXS on the samples of various chemical crosslinking time. The gels were shrunk at pH4, and swollen at pH8. The results of SAXS showed, that the Porod slope changed with chemical crosslinking time from -3.5 to ?2.9 at pH4, and from ?2.9 to ?2.4 at pH8. The results suggest that a folded structure as a structural domain, which is characterized by fractally rough interface, tends to change into the structure that corresponds to percolation cluster, particularly at pH8. The gels immersed in pH8 showed a remarkable structure change accompanying swelling. The results revealed that a conformational change of PAA chains, induced by the pH change, can be explained by the presence of a structural domain in the gel network, where both PVA chains and PAA chains get entangled and partially form a interpenetrating polymer network(IPN).     

13C n.m.r. chemical shielding tensor elements of solid poly(methyl methacrylate)

The solid-state non-spinning ¹³C nuclear magnetic resonance spectrum of poly(methyl methacrylate) (PMMA) is reassigned. Principal elements of the chemical shielding tensors are obtained for all carbon resonances. The observed anisotropies indicate that the polymer, both with respect to the side chains and the backbone, is essentially rigid at room temperature on the time scale of the experiment (in the millisecond range).     

Potentiality of secondary ion mass spectrometry for chemical,micron-size imaging of multiphase polymer materials

Summary Thin deposited films of the diblock copolymer poly{methylmethacrylate-b-(2,perfluorohexyl-ethyl)acrylate} blended into a poly (methylmethacrylate) matrix were studied by Secondary Ion Mass Spectrometry. The molecular information contained in the negative spectrum and peaks ratio measurement identify the top surface as being saturated with fluorinated chains. A multiphase morphology was also revealed by ion microscopy. The potentiality of Secondary Ion Mass Spectrometry to provide chemical images of micron-sized phases in polymer materials is nicely demonstrated.