Molecular coils and their deformation

The study of stress-effects on chain conformation has always been a fruitful - and sometimes highly controversial - issue in macromolecular science. In this paper chain scission in transient elongational flow is investigated. Particular attention is given to the possible nature of stress transfer from the deforming highly dilute polymer solution to the embedded molecular (PS) chain. The dependencies of the measured degradation efficiency on molecular weight, solvent viscosity, nozzle geometry and flow rate exclude the classical mechanism of chain loading through strain-rate dependent viscous friction. On the basis of their findings the authors propose a model where the loading of hairpin-like segments in almost static friction works against the internal relaxation of these structures.     

Preparation and use of diacetylene-containing polyesters for studying deformation micromechanics in model polyester-polyolefin blends

A range of diacetylene-containing polyesters have been prepared by polycondensation of terephthaloyl chloride with diacetylene diols of structure HO—(CH₂)_n—C≡C—C≡C—(CH₂)_n—OH, where n = 1, 2, 3, 4 and 9. The results from characterization of the polyesters by DSC and Raman spectroscopy are presented. The polyesters from the diols with n = 3 and n = 9 have melting points (T_m) of 186 and 88 °C, respectively, and cross-polymerise under irradiation, but not thermally. These polyesters were blended with isotactic polypropylene (T_m = 167 °C) and the blends compression-moulded into dumbbell specimens, which then were exposed to ⁶⁰Co γ-radiation to effect cross-polymerization of the polyester phases. The dumbbell specimens were subjected to simultaneous tensile testing and resonance Raman spectroscopy, and the shift in wavenumber of the C≡C stretching band of the polydiacetylene crosslinks used to determine the local stress in the polyester phases independent of the overall applied stress, thereby facilitating determination of the efficiency of stress transfer in the blends.     

Elastic deformation of membrane bilayers probed by deuterium NMR relaxation

In deuterium ((2)H) NMR spectroscopy of fluid lipid bilayers, the average structure is manifested in the segmental order parameters (S(CD)) of the flexible molecules. The corresponding spin-lattice relaxation rates (R(1Z) depend on both the amplitudes and the rates of the segmental fluctuations, and indicate the types of lipid motions. By combining (2)H NMR order parameter measurements with relaxation studies, we have obtained a more comprehensive picture of lipids in the liquid-crystalline (L(alpha)) state than formerly possible. Our data suggest that a lipid bilayer constitutes an ordered fluid, in which the phospholipids are grafted to the aqueous interface via their polar headgroups, whereas the fatty acyl chains are in effect liquid hydrocarbon. Studies of (2)H-labeled saturated lipids indicate their R(1Z) rates and S(CD) order parameters are correlated by a model-free, square-law functional dependence, signifying the presence of relatively slow bilayer fluctuations. A new composite membrane deformation model explains simultaneously the frequency (magnetic field) dependence and the angular anisotropy of the relaxation. The results imply the R(1Z) rates are due to a broad spectrum of 3-D collective bilayer excitations, together with effective axial rotations of the lipids. For the first time, NMR relaxation studies show that the viscoelastic properties of membrane lipids at megahertz frequencies are modulated by the lipid acyl length (bilayer thickness), polar headgroups (bilayer interfacial area), inclusion of a nonionic detergent (C(12)E(8)), and the presence of cholesterol, leading to a range of bilayer softness. Our findings imply the concept of elastic deformation is relevant on lengths approaching the bilayer thickness and less (the mesoscopic scale), and suggest that application of combined R(1Z) and S(CD) studies of phospholipids can be used as a simple membrane elastometer. Heuristic estimates of the bilayer bending rigidity kappa and the area elastic modulus K(a) enable comparison to other biophysical studies, involving macroscopic deformation of thin membrane lipid films. Finally, the bilayer softness may be correlated with the lipid diversity of biomembranes, for example, with regard to membrane curvature, repulsive interactions between bilayers, and lipid-protein interactions.     

Surface modification of coir fibers I: studies on graft copolymerization of methyl methacrylate on to chemically modified coir fibers

Graft copolymerization of methyl methacrylate (MMA) onto chemically modified coir fiber was studied using a CuSO₄–KIO₄ combination as initiator in an aqueous medium in the temperature range of 50–70 °C. Concentrations or [IO₄−] = 0.005 mol/l and [Cu2+] = 0.002 mol/l produce optimum grafting. The effects of time, temperature, amount of coir fiber, some inorganic salts and organic solvents on graft yield have also been investigated. On the basis of experimental findings, a reaction mechanism has been proposed. Evidence of grafting was studied from fourier transform infrared spectroscopy and scanning electron microscopy of chemically modified coir and MMA‐grafted coir. Tensile properties such as maximum stress at break, extension at break and Young's modulus of untreated, defatted, chemically treated and grafted coir fibers were evaluated and compared. Grafted coir fiber showed an increase in tensile properties such as maximum stress at break, extension at break and Young's modulus. Copyright © 1999 John Wiley & Sons, Ltd.     

Shear induced deformation of polystyrene coils in dilute solution from small angle neutron scattering

The deformation of the overall conformation of polystyrene,M _w =280000 g/ mole in dilute solution in a constant shear gradient has been investigated by small angle neutron scattering (SANS). The experiments were performed with a new Couette-type apparatus, consisting of two concentric quartz cylinders in which the outer one rotates and the solution is confined in the gap between both cylinders. The observation of the elastic coherent small angle scatterring intensity at a shear gradient ofG=400 s^?1 has revealed a distinct anisotropy of the scattering pattern on the two dimensional detector, as estimated from equilibrium chain dynamics. After extrapolation to zero angle and zero concentration two different values for the overall size of the single macromolecule parallel and perpendicular to the flow-direction are deduced.     

Furanocoumarins in celery and parsnips: method and multiyear Canadian survey

The natural occurrence of biologically active furanocoumarins in common vegetables is an area of increasing interest with respect to human health. In this study, an efficient, rugged, and sensitive liquid chromatographic method with ultraviolet photodiode array detection was developed for the estimation of 5 biologically active furanocoumarins (psoralen, bergapten, xanthotoxin, trioxsalen, and angelicin) in celery and parsnips. When authentic samples were spiked with a mixture of furanocoumarins at individual levels of 2 to 10 microg/g, the method produced overall recoveries of 77 and 75% of all furanocoumarins from celery and parsnips, respectively. The method was applied in 2 laboratories to a multiyear survey of more than 200 samples. Of 110 parsnips samples, 109 (99%) contained quantitatable levels of furanocoumarins. The mean level of total furanocoumarins in the positive parsnip samples was 15.1 microg/g; the maximum level detected was 145 microg/g. Of 114 celery samples, 88 (77%) contained quantitatable levels of furanocoumarins. The mean level of total furanocoumarins in the positive celery samples was 1.9 microg/g; the maximum level detected was 15.2 microg/g. Xanthotoxin and bergapten were the most commonly detected furanocoumarins in both celery (68 and 63%) and parsnips (97 and 96%). Xanthotoxin had the highest mean level of positives in both celery (1.3 microg/g) and parsnips (8.5 microg/g). Little year-to-year variation in either total furanocoumarin levels or incidence was noted.     

芹菜中黄酮类物质的提取和测定

采用碱溶酸析法，成功地提取了芹菜中黄酮类物质。经HCl-镁粉反应和红外光谱鉴定，提取物为黄酮类物质。另外，利用分光光度法，以芦丁为标准对芹菜的叶和茎中的总黄酮分别进行了测定，其质量分数分别为10．19mg／g，3．064mg／g。同时讨论了时间及色素对测定方法的影响，结果表明，显色时间在40min内稳定，色素引起误差为5％。     

Microfibril diameter in celery collenchyma cellulose: X-ray scattering and NMR evidence

Cellulose isolated from celery collenchyma is typical of the low-crystallinity celluloses that can be isolated from primary cell-walls of higher plants, except that it is oriented with high uniformity. The diameter of the microfibrils of celery collenchyma cellulose was estimated by three separate approaches: ¹³C NMR measurement of the ratio of surface to interior chains; estimation of the dimensions of the crystalline lattice from wide angle X-ray scattering (WAXS) measurements using the Scherrer equation; and the observation that microfibrils of this form of cellulose have the unusual property of packing into an irregular array from which small angle X-ray scattering (SAXS) shows features of both form and interference functions. The interference function contributing to the SAXS pattern implied a mean microfibril centre-to-centre distance of 3.6 nm, providing an upper limit for the diameter. However modelling of the scattering pattern from an irregular array of microfibrils showed that the observed scattering curve could be matched at a range of diameters down to 2.4 nm, with the intervening space more or less sparsely occupied by hemicellulose chains. The lateral extent of the crystalline lattice normal to the 200 plane was estimated as a minimum of 2.4 nm by WAXS through the Scherrer equation, and a diameter of 2.6 nm was implied by the surface: volume ratio determined by ¹³C NMR. The WAXS and NMR measurements both depended on the assumption that the surface chains were positioned within an extension of the crystalline lattice. The reliability of this assumption is uncertain. If the surface chains deviated from the lattice, both the WAXS and the NMR data would imply larger microfibril diameters within the range consistent with the SAXS pattern. The evidence presented is therefore all consistent with microfibril diameters from about 2.4 to 3.6 nm, larger than has previously been suggested for primary-wall cellulose. Some degree of aggregation may have occurred during the isolation of the cellulose, but the larger microfibril diameters within the range proposed are a consequence of the novel interpretation of the experimental data from WAXS and NMR and are consistent with previously published data if these are similarly interpreted.     

Mechanisms and micromechanics of fatigue crack propagation in glassy thermoplastics

An iterative approach is used to estimate, from interference optics measurements, the variation of refractive index and, hence, extension ratio along the length of a craze at the tip of a fatigue crack. The finite element method is used to compute craze surface stress distributions which are found to be similar to those obtained for static loading. High extension ratios, in the range 6 to 8 for retarded fatigue crack growth in poly(vinylchloride), are attained in the craze fibrils at the crack tip before crack jump occurs. The craze thickens primarily by surface drawing during the early stages of its growth but in the later stages the fibril creep mechanism predominates. The critical fibril extension ratio is not reached in a single cycle, as in normal fatigue crack propagation, and crack jump does not occur until, typically, after several hundreds of cycles during which the fibrils accumulate considerable damage.Presented in part at the 7th Int. Conference Deformation, Yield and Fracture of Polymers, Cambridge, UK, 11–14 April 1988.     

Inelastic deformation of glassy polyaryleneetherketone: Energy accumulation and deformation mechanism

The plastic deformation of glassy non-annealed polyaryleneetherketone (PAEK) was investigated via deformation calorimetry and thermally stimulated recovery of residual strain. Polymer samples were deformed at room temperature under uniaxial compression up to ε_def =–(40?50)% at a rate of 0.04 min^?1. It was found that PAEK behaves in the deformation process similarly to many other glassy polymers: It stores internal energy excess at loading and contains two types of different inelastic strain carriers, namely the delayed elastic (ε_de) and plastic (ε_pl) strain carriers. The maximum level of the accumulated energy in PAEK reaches ≈ 8.3 J/g, which is close to those for glassy polystyrene and polycarbonate. Nearly all the deformation energy stored in PAEK is carried by the delayed-elastic strain. The carriers of plastic strain carry no extra energy or a very small amount of it. The inelastic deformation of glassy PAEK proceeds in two stages. The carriers of ε_de are nucleated at the first stage of the deformation process, and the carriers of ε_pl are nucleated at the second stage. It was shown that, during glassy-polymer loading, the molecular level structures carrying ε_pl never appear by themselves, but appear only as a result of spontaneous reorganization of ε_de. In other words, the plastic deformation appears in PAEK owing to the two-step process. This situation is typical for all glassy polymers.     

Shear-induced deformation of polystyrene coils in dilute solution from small angle neutron scattering 2. Variation of shear gradient,molecular mass and solvent viscosity

The technique of small angle neutron scattering (SANS) has been used to study the conformation of polystyrene chains in dilute solution under a constant shear gradient. The experiments reveal a distinct anisotropy of the molecular dimensions with regard to the directions parallel and perpendicular to the flow direction on the 2D-multidetector. The deformation ratio of the single polymer chain (R ²/R _iso ² )–1 as a function of the reduced shear gradient=([] · · M _w G)/RT shows a transition from the ideal ²-behaviour for dynamic infinitely flexible coils found at small gradients, to a behaviour with smaller increase at larger. These results are qualitatively consistent with the theory of Cerf for a polymer with finite internal viscosity in a shear gradient. At low(<1), a better agreement with the model of a free-draining coil (Rouse behaviour) than with the Zimm model is observed.     

苯醚甲环唑在芹菜及其土壤中的残留测定和消解动态研究

采用气相色谱-电子捕获检测, 对芹菜及其土壤中的苯醚甲环唑消解动态和最终残留量进行了研究, 评价了苯醚甲环唑在芹菜上使用后的残留行为和环境安全性. 苯醚甲环唑在芹菜及土壤中的残留消解动态均符合一级动力学方程, 苯醚甲环唑在芹菜上消解快; 苯醚甲环唑最终残留量与施药的剂量、施药次数及采样的间隔时间有关; 水解研究表明, 苯醚甲环唑是稳定的农药, 在不同温度和不同pH的研究条件下水解半衰期均大于166 d, 碱性条件更有利于苯醚甲环唑的降解.     

Compatibility of coils and rods bearing flexible side chains

We present a theoretical treatment of nematic-isotropic phase equilibria in mixtures which consist of random coils and comblike polymers, the latter components being composed of a rigid backbone and flexible side chains. The mixing partition function is evaluated by using the Flory lattice model. The comblike component is characterized by the axial ratio x_r of its rigid main chain and the number of flexible side chains z, each containing m segments. The coiled component is described by its number of segments x_c. The net exchange energy of mixing is assumed to be zero; i.e., we consider athermal solutions. It is shown that the flexible side chains attached to the rigid main chains markedly enhance the compatibility in the isotropic phase. If the ratio of the volume fraction of the side chains to the volume fraction of the main chains is high enough, there is even a finite range of concentration where the random coils mix homogeneously with the comblike component. This is in contrast to mixtures of rods and coils, which have been shown by Flory to be incompatible over nearly the full range of composition. These conclusions hold true only when ordered states are involved. For comblike polymers with flexible backbones mixed with random coils in isotropic melts, the resulting free energy of mixing is given by the familiar Flory-Huggins expression.     

Elastic properties of YAG: First-principles calculation and experimental investigation

The hardness and elastic stiffness of Y₃Al₅O₁₂ (YAG) were investigated by first-principles calculations and experiments. The mechanical properties including the second-order elastic coefficients, hardness, bulk modulus, Young's modulus and shear modulus were calculated by density functional theory (DFT). The calculated results were in good agreement with the experimental values. The hardness of YAG is mainly attributed to Al_tet–O bonds. The elastic anisotropy of YAG was discussed. Zener anisotropy parameter of YAG is close to unit and its universal anisotropy index is very close to zero, which indicates the structure of YAG is nearly centrosymmetric. The longitudinal and transverse sound velocities and Debye temperature were also investigated.     

Elastic modulus of a polymer nanodroplet: theory and experiment

We redevelop a theoretical model that, in conjunction with atomic force microscopy (AFM), can be used as a noninvasive method for determination of the elastic modulus of a polymer nanodroplet residing on a flat, rigid substrate. The model is a continuum theory that combines surface and elasticity theories for prediction of the droplet's elastic modulus, given experimental measurement of its adsorbed height. Utilization of AFM-measured heights for relevant droplets reported in the literature and from our own experiments illustrated the following: the significance of both surface and elasticity effects in determining a polymer droplet's spreading behavior; the extent of a continuum theory's validity as one approaches the nanoscale; and a droplet size effect on the elastic modulus.     

Structure and superstructure of carbon fibres

The structure of carbon fibres consists of stacks of carbon layers oriented parallel to the fibre axis. From X-ray wide-angle scattering studies (WAXS) one obtains various structural parameters characterizing the perfection of the stacking and the orientation of the layers. The latter is quantitatively related to the tensile modulus of the fibres. Small-angle scattering studies (SAXS) are used to determine size, shape and orientation of microvoids. For carbon fibres with high heat treatment temperatures (HTT), a correlation between microvoid content and elongation at break is observed which indicates an optimum value for the microvoid content. Electrochemical intercalation is used to obtain information on the accessibility of the carbon layer structure.     

Chemistry and physics of synthetic fibres

Ohne Zusammenfassung

---

Chemistry and physics of synthetic fibres

---

Die Vortr?ge werden im Journal of Applied Chemistry demn?chst im Druck erscheinen.