Electronic states of 1,4-bis(phenylethynyl)benzene: A synchrotron radiation linear dichroism investigation

The electronic transitions of 1,4-bis(phenylethynyl)benzene (BPEB) were investigated by UV synchrotron radiation linear dichroism (SRLD) spectroscopy in the range 25,000-58,000 cm⁻¹ (400-170 nm) on molecular samples aligned in stretched polyethylene. The investigation was supported by variable temperature spectroscopy and by quantum chemical calculations in the LCOAO and TD-DFT models. The molecular alignment of BPEB in stretched polyethylene was found to be remarkably efficient, leading to an orientation factor equal to 0.95 for the long molecular axis. The observed band shape depended strongly on the degree of stretching and on temperature. The combined experimental and theoretical evidence leads to the characterization of several previously unobserved transitions and supports the assumption that BPEB adopts a nearly planar conformation in stretched polyethylene.     

Stretched versus compressed exponential kinetics in α-helix folding

In a recent paper (J. Bredenbeck, J. Helbing, J.R. Kumita, G.A. Woolley, P. Hamm, α-helix formation in a photoswitchable peptide tracked from picoseconds to microseconds by time resolved IR spectroscopy, Proc. Natl. Acad. Sci USA 102 (2005) 2379), we have investigated the folding of a photo-switchable α-helix with a kinetics that could be fit by a stretched exponential function exp(−(t/τ)^β). The stretching factor β became smaller as the temperature was lowered, a result which has been interpreted in terms of activated diffusion on a rugged energy surface. In the present paper, we discuss under which conditions diffusion problems occur with stretched exponential kinetics (β < 1) and under which compressed exponential kinetics is obtained (β > 1). We show that diffusion problems do have a strong tendency to yield stretched exponential kinetics, yet, that there are conditions (strong perturbation from equilibrium, performing the experiment in the folding direction) under which compressed exponential kinetics would be expected instead. We discuss the kinetics on free energy surfaces predicted by simple initiation-propagation models (zipper models) of α-helix folding, as well as by folding funnel models. We show that our recent experiment has been performed under condition for which models with strong downhill driving force, such as the zipper model, would predict compressed, rather than stretched exponential kinetics, in disagreement with the experimental observation. We therefore propose that the free energy surface along a reaction coordinate that governs the folding kinetics must be relatively flat and has a shape similar to a 1D golf course. We discuss how this conclusion can be unified with the thermodynamically well established zipper model by introducing an additional kinetic reaction coordinate.     

Mechanical properties and enzymatic degradation of poly[(R)-3-hydroxybutyrate] fibers stretched after isothermal crystallization near Tg

Poly[(R)-3-hydroxybutyrate] (P(3HB)) fibers with high tensile strength were prepared by stretching the fibers after isothermal crystallization near the glass transition temperature. Two samples with different molecular weights (M_w = 0.7 × 10⁶ and 4.3 × 10⁶) were used to investigate the effect on tensile strength. Increasing the time for isothermal crystallization of P(3HB) fibers resulted in a decrease in the maximum draw ratio. But, the tensile strength of P(3HB) fibers increased remarkably when the isothermal crystallization time was prolonged to more than 24 h. The tensile strength of low-molecular-weight drawn fibers was higher than that of high-molecular-weight fibers. Therefore, it can be concluded that this procedure does not increase the tensile strength of the high-molecular-weight drawn fibers. This is because, in this drawing method, small crystal nuclei grow initially during the isothermal crystallization process. Then, the molecular chains between the small crystal nuclei that acted as the entanglement points are oriented by stretching. In the case of the high-molecular-weight fibers, because the molecular length between the entanglement points of the small crystal nuclei is too long, the molecular chains are not sufficiently oriented by the stretching process. However, in the case of the low-molecular-weight fibers, the molecular length is suitable for generating the extended chains. Based on the result of X-ray analysis of P(3HB) fibers stretched after isothermal crystallization, fibers have the oriented α-form crystal with 2₁ helix conformation and β-form with planar zigzag conformation. The enzymatic degradation of the stretched P(3HB) fibers was performed by using an extracellular PHB depolymerase purified from Ralstonia pickettii T1. The enzymatic erosion rate of β-form was faster than that of α-form in the P(3HB) fibers stretched after isothermal crystallization.     

Studies on Macromolecular Complexing Agents. II. Effects of Polypropylene Oxide on the Copolymerization Reaction between Butadiene and Styrene

ABSTRACT

Ultra-high-molecular-weight poly[(R)-3-hydroxybutyrate](P(3HB)) was biosynthesized from glucose by a recombinant Escherichia coli XL-1 Blue (pSYL105) harboring Alcaligenes eutrophus PHB biosynthesis phbCAB genes. Six kinds of P(3HB) samples with differ-ent weight-average molecular weight (M_w ) from 1.1 × 10⁶ to 11 × 10⁶ measured by multi-angle laser light scattering were respectively produced at pH values of 7.0 to 6.5 in culture media. Solvent-cast P(3HB) films of high-molecular-weights over M_w of 3.3 × 10⁶ were stretched easily and reproducibly at 160°C to a draw ratio of 400-650%. Mechanical properties of the stretched P(3HB) films were markedly improved relative to those of solvent-cast film. The elongation to break, Young's modulus, and tensile- strength of stretched film (M_w = 11 × 10⁶) were 58%, 1.1 GPa, and 62 MPa, respectively. X-ray diffraction patterns indicated that the stretched film was highly oriented and had a high crystallinity over 80%. When the stretched film was annealed at 160°C for 2 hours, the mechanical properties were further improved (elongation to break = 67%, Young's modulus = 1.8 GPa and tensile strength = 77 MPa). The mechanical properties of the stretched-annealed film remained almost unchanged for 6 months at room temperature, suggesting that a high crystallinity of the stretched-annealed film avoids a progress of secondary crystallization.

     

Morphology and mechanical properties of ultrahigh molecular weight polypropylene prepared by gelation/crystallization at various temperatures

Films of ultrahigh molecular weight (5.4×10⁶) polypropylene were produced by gelation/crystallization at various temperatures from dilute decalin solutions according to the method of Smith and Lemstra. The temperatures chosen were 20°, 30°, 50°, and 60°C. With increasing the temperature, the long period and crystallinity of the resultant gel film increased. By contrast, when the films were stretched up to 50 } 60 times, the increases in Young's modulus and crystallinity become more significant, as the temperature of the gelation/crystallization became lower. This interesting phenomenon is thought to be due to the dependence of the number of entanglements on the temperatures concerning gelation/crystallization and evaporation of solvent from the gel to form a film.     

‘Tail’ phenomenon and fatigue crack propagation of PC/ABS alloy

The fatigue crack propagation (FCP) behavior of the alloy of polycarbonate (PC) and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length and optical and scanning electron microscopes (SEM) are used to observe the features of crack tip deformation in situ. ‘Tail’ phenomenon has been observed at the initial stage of fatigue for each specimen, which is regarded as a reflection of the transition process of accumulation of damage and plastic deformation during FCP. The law of FCP from low to high crack growth rate (10⁻⁶-10⁻³ mm/cycle) is obtained and described with Paris law. Porous or dimple features govern the fatigue crack surfaces and coarse features have been seen on the crack surfaces with higher crack growth rate, while smooth features have been observed on the crack surfaces with lower crack growth rate. A stretched band appears when the crack growth transforms from lower to higher region of FCP rate.     

Structure generation in PTFE porous membranes induced by the uniaxial and biaxial stretching operations

The morphology and its formation has been studied for the steady-rate stretching operation of polytetrafluoroethylene (PTFE) porous membranes, which were prepared from PTFE fine powders with a mean diameter of approximately 2×10² nm through extruding and rolling operations prior to the stretching operation. The uniaxially stretched membranes were produced by the unique stretching operation parallel to the rolling operation, and the biaxially stretched membranes by the dual operations consisting of the parallel and the subsequent perpendicular stretches. The inversion of the stretching direction, i.e., the first stretching operation perpendicular to the rolling operation and the second one parallel has been observed to be impossible due to the occurrence of macroscopic fractures on the membrane during the first stretching operation. The uniaxially stretched porous membranes are comprised of fibrils completely oriented in the stretching direction and remaining island-like fractures of the rolled PTFE sheet. The second stretching operation provides a lattice-like porous structure by giving the island-like fractures further division along the second stretching direction and the fibrils slant from the original orientation. The stretching operation is possible within the range where the relative elongation of the whole membrane along the second stretching direction is less than 50% of that along the first one, indicating that the fibrils yielded by the first stretching operation sustain the lattice-like porous structure induced by the second stretching operation. The distribution of the slant angle of the fibrils is independent of the elongation in the second stretching operation, thus, the division of the island-like fractures linked with the fibrils steadily proceeds during the second stretching operation.     

Effect of bending and stretching of polyethylene strips on the adsorption of surface-active substances

The adsorption of tritium-labeled sodium stearate and ³⁵S-labeled sodium dodecylsulfate from aqueous solution on bent and on stretched polyethylene strips was directly measured. The amount of adsorption increased from 2.5 to 4.5 times on surfaces of various polyethylene samples stretched from 5.2 to 5.3 times the original length. For one sample, adsorption increased 8 and 2.5 times, respectively, on the convex and concave surfaces of a bent strip. Autoradiographs of the strips taken after adsorption revealed the appearance of both homogeneous and heterogeneous adsorption on the stretched surface of the polyethylene. The electron micrographs of the strips showed that many small fissures, about 1–5μ in length, formed on their surfaces after the strips were bent or stretched. Enhanced adsorption of the surface-active substance along these fissures was suggested. On the basis of these observations, it is proposed that in stretching or bending, fissures increase adsorption of the detergent, and the adsorption in turn promotes further development of fissures. Thus, bending and adsorption mutually promote the growth of fissures which finally result in failure of the polyethylene.     

Electronic transitions and intramolecular hydrogen bonding in anthralin. UV-VIS linear dichroism spectroscopy and quantum chemical calculations

The electronic and molecular structure of the antipsoriatic drug anthralin (1,8-dihydroxy-9(10H)-anthracenone) is investigated by UV-VIS linear dichroism (LD) spectroscopy in stretched polyethylene and by quantum chemical model calculations. Seven individual electronic transitions are resolved below 47?000 cm⁻¹ and assigned to calculated π–π* transitions. The low-energy region is characterized by a relatively broad band around 28?000 cm⁻¹ that can be assigned to two overlapping, differently polarized electronic transitions involving a considerable degree of intramolecular charge transfer from the phenolic moieties to the carbonyl group. Computed wavenumbers for these transitions depend significantly on the assumed geometrical parameters for the intramolecular H-bonds in anthralin; best agreement with observed data is obtained with a geometry corresponding to strong H-bonding. The calculations also indicate that excited state intramolecular proton transfer (ESIPT) is likely to occur, leading to the prediction of a very large Stoke's shift.     

On the Concentration Dependence of the Cluster Fractal Dimension in Colloidal Aggregation

We have undertaken the task to calculate, by means of extensive numerical simulations and by different procedures, the cluster fractal dimension (d) of colloidal aggregates at different initial colloid concentrations. Our first approach consists in obtaining d from the slope of the log-log plots of the radius of gyration versus size of all the clusters formed during the aggregation time. In this way, for diffusion-limited colloidal aggregation, we have found a square root type of increase of the fractal dimension with concentration, from its zero-concentration value: d = d⁰ _f + a , with d⁰ _f = 1.80 ± 0.01, a = 0.91 ± 0.03 and = 0.51 ± 0.02, and where is the volume fraction of the colloidal particles. In our second procedure, we get the d via the particle-particle correlation function g_cluster(r) and the structure function S_cluster(q) of individual clusters. We first show that the stretched exponential law g_cluster(r) = Ar^{d –3}e^–(r/) gives an excellent fit to the cutoff of the g(r). Here, A, a and are parameters characteristic of each of the clusters. From the corresponding fits we then obtain the cluster fractal dimension. In the case of the structure function S_cluster (q), using its Fourier transform relation with g_cluster(r) and introducing the stretched exponential law, it is exhibited that at high q values it presents a length scale for which it is linear in a log-log plot versus q, and the value of the d extracted from this plot coincides with the d of the stretched exponential law. The concentration dependence of this new estimate of d, using the correlation functions for individual clusters, agrees perfectly well with that from the radius of gyration versus size. It is however shown that the structure factor S(q) of the whole system (related to the normalized scattering intensity) is not the correct function to use when trying to obtain a cluster fractal dimension in concentrated suspensions. The log-log plot of S(q) vs. q proportions a value higher than the true value. Nevertheless, it is also shown that the true value can be obtained from the initial slope of the particle-particle correlation function g(r), of the whole system. A recipe is given on how to obtain approximately this g(r) from a knowledge of the S(q), up to a certain maximum q value.     

Thermodynamic investigation of the state of water adsorbed by carbonaceous adsorbents

The state of water adsorbed on active carbons and canal soot was studied using the “chemical potential—entropy—temperature” diagram. In the range of the relative pressures from 0.174 to 1, the state of adsorbed water is similar to the state of a stretched liquid. The molar volume, heat of evaporation, and surface tension of stretched water were calculated at different relative pressures. Near the spinodal, the molar volume of stretched water is 25% higher, and the surface tension is considerably lower compared to water. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 464–468, March, 1997.     

Melting behavior of highly stretched isotactic polypropylene film

Isotactic polypropylene film was stretched in poly(ethylene glycol) at 140°C and its melting behavior was investigated by using a differential scanning calorimeter (DSC-1B). The shape of the melting curve depends largely on the stretching ratio, v. A sample stretched to moderate extension (1 < v < 3.5–4) has only a single melting peak (163°C) in the thermogram. When the sample is stretched beyond v = 3.5–4, the thermogram becomes more and more complex with increase of v, and some peaks appear when stretched to 10 < v < 13. The lowest peak which is considered to be the melting peak of the intermolecular crystals produced by the unfolding of chain molecules in the lamellae develops gradually with increase of v. In the thermogram for v = 18 the lowest temperature peak is most pronounced, in contrast to the highest temperature peak which decreases markedly in intensity. The phenomenon shows that large amounts of lamellar crystals are converted to intermolecular crystals in this region. On further stretching (v > 20) a very sharp high temperature peak appears, whose half-width is about 1°C. Qualitatively similar results were obtained for the samples stretched in poly(ethylene glycol) at 150°C and in air at 140 and 150°C.     

Electron correlation effects on the shape of the Kohn–Sham molecular orbital

Even systems in which strong electron correlation effects are present, such as the large near-degeneracy correlation in a dissociating electron pair bond exemplified by stretched H₂, are represented in the Kohn–Sham (KS) model of non-interacting electrons by a determinantal wavefunction built from the KS molecular orbitals. As a contribution to the discussion on the status and meaning of the KS orbitals we investigate, for the prototype system of H₂ at large bond distance, and also for a one-dimensional molecular model, how the electron correlation effects show up in the shape of the KS σ _g orbital. KS orbitals φ^HL and φ^FCI obtained from the correlated Heitler-London and full configuration interaction wavefunctions are compared to the orbital φ^LCAO, the traditional linear combination of atomic orbitals (LCAO) form of the (approximate) Hartree-Fock orbital. Electron correlation manifests itself in an essentially non-LCAO structure of the KS orbitals φ^HL and φ^FCI around the bond midpoint, which shows up particularly clearly in the Laplacian of the KS orbital. There are corresponding features in the kinetic energy density t _s of the KS system (a well around the bond midpoint) and in the one-electron KS potential v _s (a peak). The KS features are lacking in the Hartree-Fock orbital, in a minimal LCAO approximation as well as in the exact one. Received: 11 December 1996 / Accepted: 10 January 1997     

Electron paramagnetic resonance spectra of γ-irradiated polycaprolactam

The nature of the saturation is investigated for radicals trapped in a matrix of polycaprolactam. The radicals were generated with the help of a Co⁶⁰ source of -rays by irradiating evacuated samples consisting of fibers of kapron with two different structural modifications: stretched and unstretched. It is shown that, from its behavior, the broadening is intermediate between the homogeneous and inhomogeneous cases. A study was made of the effect of irradiating dosages in the interval from 1.5 Mrad to 200 Mrad on the form of the EPR spectra; as a result, it was established that with increasing dosage the form of the spectrum (quintet) is preserved, but the intensity of the middle component decreases. The saturation factor here is substantially decreased, but this effect manifests itself to different degrees for specimens of different structural modifications: in the case of the stretched fibers it is enormously stronger than in the case of the unstretched fibers. The study of saturation in the case of specimens of the two investigated modifications, subjected to identical irradiation dosages, has shown that the saturation factor is less for radicals trapped in the more ordered matrix (stretched kapron fibers). Some questions about the chemical nature of the radicals are also considered.     

NMR chemical shifts in the low–pH form of a-chymotrypsin. A QM/MM and ONIOM–NMR study

 The relationship between hydrogen bonding and NMR chemical shifts in the catalytic triad of low-pH α-chymotrypsin is investigated by combined use of the effective fragment potential [(2001) J Phys Chem A 105:293] and ONIOM–NMR [(2000) Chem Phys Lett 317:589] methods. Our study shows that while the His57 N^δ1−H bond is stretched by a relatively modest amount (to about 1.060 ?) this lengthening, combined with the polarization due to the molecular environment, is sufficient to explain the experimentally observed chemical shifts of 18.2 ppm. Furthermore, the unusual down-field shift of H^ɛ1 (9.2 ppm) observed experimentally is reproduced and shown to be induced by interactions with the C=O group of Ser214 as previously postulated. The free-energy cost of moving H^δ1 from His57 to Asp102 is predicted to be 5.5 kcal/mol. Received: 26 September 2001 / Accepted: 6 September 2002 / Published online: 21 January 2003 Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American Chemical Society, 2001 Correspondence to: J. H. Jensen e-mail: jan-jensen@uiowa.edu Acknowledgements. This work was supported by a Research Innovation Award from the Research Corporation and a type G starter grant from the Petroleum Research Fund. The calculations were performed on IBM RS/6000 workstations obtained through a CRIF grant from the NSF (CHE-9974502) and on supercomputers at the National Center for Supercomputer Applications at Urbana-Champaign. The authors are indebted to Visvaldas Kairys for help with the CHARMM program, and to Daniel Quinn for many helpful discussions.     

Dynamic master curves from the stretched exponential relaxation modulus

The stretched exponential relaxation modulus of regular and polymer modified asphalts is studied. It is shown that this relaxation function can generate the dynamic functions of these materials very well on any finite interval of the reduced frequencies (master curves). By continuation one can, in principle, cover the whole region of master curves of G′ and G″. The dispersive defect diffusion mechanism, which leads to the stretched exponential law, points to the stronger three-dimensional structure of modified asphalt at low temperatures. The method of calculating G′ and G″ from the stretched exponential relaxation modulus is proposed and tested on one regular and one modified asphalt. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1225–1232, 1997     

Novel green light-emitting diode with high efficiency; synthesis and electroluminescent property of poly(2-(2′-ethylhexylthio)-5-methoxy-1,4-phenylenevinylene) (PMEHTPV)

Poly(2-(2′-ethylhexylthio)-5-methoxy-1,4-phenylenevinylene) (PMEHTPV) was synthesized in thin films via a water-soluble precursor polymer and characterized with thermogravimetric analysis (TGA), FT–IR, and elemental analysis, etc. The PMEHTPV film could be stretched up to 10 times and showed conductivity of 0.13 Scm⁻¹ when doped with FeCl₃. The 2-ethylhexylthio group resulted in blue-shifted absorption and emission compared to those of poly(2-2′-ethylhexyloxy-5-methoxy-1,4-phenylenevinylene) (MEH-PPV). The relative quantum efficiency of the device made by PMEHTPV was 20 times higher than that of MEH-PPV. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2253–2258, 1997     

High‐Performance,Stretchable, Wire‐Shaped Supercapacitors

A general approach toward extremely stretchable and highly conductive electrodes was developed. The method involves wrapping a continuous carbon nanotube (CNT) thin film around pre‐stretched elastic wires, from which high‐performance, stretchable wire‐shaped supercapacitors were fabricated. The supercapacitors were made by twisting two such CNT‐wrapped elastic wires, pre‐coated with poly(vinyl alcohol)/H₃PO₄ hydrogel, as the electrolyte and separator. The resultant wire‐shaped supercapacitors exhibited an extremely high elasticity of up to 350 % strain with a high device capacitance up to 30.7 F g^?1, which is two times that of the state‐of‐the‐art stretchable supercapacitor under only 100 % strain. The wire‐shaped structure facilitated the integration of multiple supercapacitors into a single wire device to meet specific energy and power needs for various potential applications. These supercapacitors can be repeatedly stretched from 0 to 200 % strain for hundreds of cycles with no change in performance, thus outperforming all the reported state‐of‐the‐art stretchable electronics.     

Polarization of electronic transitions in some alicyclic saturated ketones

The direction of polarization of the symmetry-allowed short wavelength transition in alicyclic saturated ketones (λ_max ≈ 180 nm) has been estimated. It was found, from measurements of linear dichroism of steroidal polycyclic ketones incorporated in stretched polyethylene films that this band is polarized along the CO bond axis. This direction is in accord with both π-π^* and n-σ_CC^* assignments.     

PHYTOCHROME MODELS: PART 10. CONCENTRATION,SONICATION AND TEMPERATURE AFFECTING THE POPULATION OF THE GROUND-STATE CONFORMERS OF BILIVERDIN DIMETHYL ESTER IN SOLUTION*

UV-vis absorption and fluorescence spectroscopy were used to show that in alcoholic solutions (methanol, ethanol, 2-methylbutan-1-ol, and 2-octanol) concentration, temperature, and sonication affect the relative populations of the more rigid stretched and the more flexible helically coiled conformers of biliverdin dimethyl ester. In aromatic solvents, e.g. toluene, no such effects could be detected. At concentrations larger than 3 10^-5M in the alcoholic solvents aggregates and monomeric coiled species prevail. Upon dilution, the fraction of stretched monomeric conformers increases. Heating and sonication at concentrations smaller than 3 10^-5M further and irreversibly increase the fraction of the stretched conformers. The activation parameters for this change are compatible with the assessment that no equilibrium is attained between the two families of conformers even after several hours at room temperature, since the barriers arc higher than kT at this temperature. The present results and those previously reported on the excited-state processes of biliverdin dimethyl esters are explained on the basis of a scheme in which the eight possible ring B/C isomers of biliverdin dimethyl ester are interconverted through intra-and intermolecular proton transfers, rotations around C-C single bonds, and E-Z photoisomerizations around C = C double bonds.