Birefringence of polyethylene melt in transient elongational flow at constant strain rate

Simultaneous birefringence and elongational viscosity measurements were carried out on molten commercial grade, low-density polyethylenes during simple elongational flow at constant strain rate and constant temperature. The birefringence increased with time during constant strain rate elongation. The increase in birefringence was a linear function of elongational stress throughout whole elongation, but the elongational viscosity increased in two stages. The increase in elongational viscosity can be divided into linear viscoelastic and nonlnear viscoelastic regions. The linear region appeared at small strain and the nonlinear region appeared at strain greater than 0.7. The elongational viscosity in the nonlinear region increased much more rapidly than that in the linear region. The Gaussian approximation, which is commonly used in molecular models, could be used for the transient elongational flow. A constant stress-optical coefficient C = 1.3 × 10^?10 cm²/dyn was obtained for all the elongational experiments, independent of strain rate (0.002-0.2s^?1). The stress-optical coefficients were weakly dependent on temperature, as predicted by the theory of rubber elasticity.     

Coil‐stretch transition in deformation flows

We present a novel Monte‐Carlo lattice model for the study of the coil‐stretch transition for polymer chains in deformation flows. Our results indicate that elongational flows are much more effective than shear flows in stretching polymer chains, in full agreement with experimental observation. Our model data also show that the ε˙_c∼M^−1.5 powerlaw observed experimentally for the dependence of critical flow rate on polymer molecular weight can be fully explained through a nonuniform stretching of the chain by the flow. A higher powerlaw exponent is predicted in more affine deformation cases. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2422–2428, 2000     

A study of the chain extending effect of elongational flow in polymer solutions

Summary The effect of elongational flow on the orientation of dissolved macromolecules has been studied by observation of localised birefringence within appropriate regions of the flow field, with the objective of linking up experimentally observed chain alignment with basic theory. Benefitting from previous experience the highest available molecular weight fractions were used, i. e. polystyrene ofM _w = 2 · 10⁶, and two opposed suck jets were employed to provide the high strain rates required. Birefringence set in above a critical strain rate and rose rapidly to a maximum value, confirming expectations from theory. Both the maximum birefringence and critical strain rate were independent of concentration indicating that the chains are independent of each other. The value of the birefringence was consistent with complete chain extension, while the critical strain rate yielded a relaxation time of 3 · 10^–5 s in accord with the value calculated fromZimm's non-free draining model. Other observations yielded an estimate of the critical entanglement concentration. The prospects and limitations of the present kind of experimental approach are discussed.With 5 figures and 2 tables     

Fluorescence resonance energy transfer (FRET) in chemistry and biology: Non-F?rster distance dependence of the FRET rate

Fluorescence resonance energy transfer (FRET) is a popular tool to study equilibrium and dynamical properties of polymers and biopolymers in condensed phases and is now widely used in conjunction with single molecule spectroscopy. In the data analysis, one usually employs the F?rster expression which predicts (l/R ⁶) distance dependence of the energy transfer rate. However, critical analysis shows that this e_xpression can be of rather limited validity in many cases. We demonstrate this by explicitly considering a donor-acceptor system, polyfluorene (PF₆)-tetraphenylporphyrin (TPP), where the size of both donor and acceptor is comparable to the distance separating them. In such cases, one may expect much weaker distance (as l/R ² or even weaker) dependence. We have also considered the case of energy transfer from a dye to a nanoparticle. Here we find l/R ⁴ distance dependence at large separations, completely different from F?rster. We also discuss recent application of FRET to study polymer conformational dynamics. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Toughening of polycarbonate by core‐shell latex particles: Influence of particle size and spatial distribution on brittle‐ductile transition

The impact toughness of polycarbonate modified with acrylic core‐shell latex particles was investigated. Addition of impact modifiers with size ranging from 115.7 to 231.4 nm can result in maximum impact strength. Equations for spatial distribution of modified particles were proposed to associate the interparticle distance with particle size and modifier volume fraction in terms of two possible morphologies, given by T = d[0.91/(φ)^1/3 ? 1] or T = d[0.88/(φ)^1/3 ? 1]. The influence of particle size on brittle‐ductile transition was also studied. The results indicated that critical interparticle distance was not a definitive value and had a narrow region. Moreover, there existed a linear relationship between critical interparticle distance and modifier size, that is, critical interparticle distance would enlarge with the increasing of core‐shell particle size. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1970–1977, 2010     

Tailoring of Polarizing Agents in the bTurea Series for Cross‐Effect Dynamic Nuclear Polarization in Aqueous Media

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ? (¹H) in cross‐effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP‐optimized glycerol/water matrix (“DNP juice”) have been studied. We observe that ? (¹H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e–e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e–e distance or too long a T_1e can dramatically limit ? (¹H). Our study also shows that the molecular structure of AMUPol is not optimal and its ? (¹H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better ? (¹H) than AMUPol itself (by a factor of ca. 1.2).     

Proton magnetic relaxation study of water orientation around I− and Li+

Proton relaxation time measurements are performed for 6m aqueous solutions of⁷LiI and⁶LiI in D₂O containing small amounts of H₂O. The measurements are done at low temperatures and yield maxima of the relaxation rate plotted against 1/T. From the maxima of the relaxation rates, proteon-I^– and proton-Li⁺ distances in the first coordination sphere of the ions are determined, and from the knowledge of the ion-water oxygen distance it is shown that for iodide a somewhat broadened H-bonded configuration is valid and that for Li⁺ the electric dipole orientation deviates from the radial direction. In order to test the reliability of the method a proton-¹²⁷I interaction study is also performed in KI solution in glycerol. The I-H distance obtained is in satisfactory agreement with that found in the aqueous system.     

Wellenmechanische Absolutrechnungen an Molekülen und Atomsystemen mit der SCF?MO?LC?(LCGO) Methode. X. Das Methylkation (CH3)+

The (CH₃)⁺ has been investigated ab initio, taking all 8 electrons into account, using the Allgemeines Programmsystem/SCF ? MO ? LC (LCGO ) Verfahren. After varying the C? H distance and the position of the C atom, it was found that the (CH₃)⁺ ion is planar with a bond distance of R_CH = 2.05 a.u. The force constants (C? H stretching, angular vibration) were computed to be k₁ = 18.9 mdyn/Å, and the associated frequencies to be ω₁ = 3256 cm^?1 and ω₂ = 1526 cm^?1. The ionization energy was found to be I = 25.75 eV. The electron affinity was estimated to be A = 5.4 eV.     

Loop-erased self-avoiding random walks in four and five dimensions

Monte Carlo simulations of loop-erased self-avoiding random walks in four and five dimensions were performed, using two distinct algorithms. We find consistency between these methods in their estimates of critical exponents. The upper critical dimension for this phenomenon is four, and it has been shown that the mean square end-to-end distance grows as n(log n)^α. It has recently been established that the mean square end-to-end distance is asymptotically bounded by n(log n)^1/3 (see Ref. 21). Our results show that asymptotic convergence to n(log n)^1/3 in fact obtains and does so rather quickly. In five dimensions we examine the rate of asymptotic convergence to the mean-field model. © 1996 by John Wiley & Sons, Inc.     

Dibromo(pyridoxal semicarbazone‐κ3N1,O3,O3′)copper(II)

The title compound, di­bromo(3‐hydroxy‐5‐hydroxy­methyl‐2‐methyl‐4‐pyridine­carbox­aldehyde semicarbazone‐κ³N¹,O³,O^3′)copper(II), [CuBr₂(C₉H₁₂N₄O₃)], consists of discrete complex units with the tridentate pyridoxal semicarbazone ligand as a zwitterion in an almost planar configuration. The Cu^II ions are in a distorted square‐pyramidal coordination, with the equatorial Br atom at a distance of 2.4017 (6) Å and the apical Br atom at a distance of 2.6860 (6) Å.     

The Effect of Fluoro Substitution upon the β‐Hairpin Fold of a β‐Tetrapeptide in Methanol

The importance of β‐peptides lies in their ability to mimic the conformational behavior of α‐peptides, even with a much shorter chain length, and in their resistance to proteases. To investigate the effect of substitution of β‐peptides on their dominant fold, we have carried out a molecular‐dynamics (MD) simulation study of two tetrapeptides, Ac‐(2R,3S)‐β^2,3hVal(αMe)‐(2S)‐β²hPhe‐(R)‐β³hLys‐(2R,3S)‐β^2,3‐Ala(αMe)‐NH₂, differing in the substitution at the C_α of Phe2 (pepF with F, and pepH with H). Three simulations, unrestrained (UNRES), using ³J‐coupling biasing with local elevation in combination with either instantaneous (INS) or time‐averaging (AVE) NOE distance restraining, were carried out for each peptide. In the unrestrained simulations, we find three (pepF) and two (pepH) NOE distance bound violations of maximally 0.22 nm that involve the terminal residues. The restrained simulations match both the NOE distance bounds and ³J‐values derived from experiment. The fluorinated peptide shows a slightly larger conformational variability than the non‐fluorinated one.     

The rheology of Torlon solutions and its role in the formation of ultra-thin defect-free Torlon hollow fiber membranes for gas separation

Fundamental understanding of the material science and rheological engineering to fabricate Torlon^® 4000T-MV and 4000TF hollow fiber membranes with an ultra-thin and defect-free dense-selective layer for gas separation has been revealed. We have firstly investigated the rheology of Torlon^® 4000T-MV and 4000TF dope solutions, and then determined the effect of temperature-correlated shear and elongational viscosities on the formation of Torlon^® fibers for gas separation. Interestingly, Torlon^® 4000T-MV and 4000TF possess different rheological characteristics: the elongational viscosity of Torlon^® 4000T-MV/NMP solution shows strain thinning, while Torlon^® 4000TF/NMP solution shows strain hardening. The balanced viscoelastic properties of dope solutions, which are strongly dependent on the spinning temperature, have been found to be crucial for the formation of a defect-free dense layer. The optimum rheological properties to fabricate Torlon^® 4000T-MV/NMP hollow fibers appear at about 48–50 °C, and the resultant fibers have an O₂/N₂ selectivity of 8.37 and an apparent dense layer thickness of 781 Å. By comparison, the best Torlon^® 4000TF fibers were spun at 24 °C with an O₂/N₂ selectivity of 8.96 and a dense layer of 1116 Å. The CO₂/CH₄ selectivity of the above two Torlon^® variants is 47 and 53.5, respectively.     

On the frequency factor in electron transfer reactions and its role in the highly exothermic regime

Consideration of current information on the dependence of the electron transfer rate on the radial separation distance and on the reactants′ radial distribution function suggests for adiabatic transfers a frequency factor closer to 10¹²M^?1 s^?1 than to 10¹¹M^?1 s^?1. One effect is to raise the λ values estimated from self-exchange rate constants, and to extend thereby the range of ΔG°'s in which the “inverted region′” is masked by a diffusion-controlled reaction rate.     

Interaction of H2@C60 and Nitroxide through Conformationally Constrained Peptide Bridges

We synthesized two molecular systems, in which an endofullerene C₆₀, incarcerating one hydrogen molecule (H₂@C₆₀) and a nitroxide radical are connected by a folded 3₁₀‐helical peptide. The difference between the two molecules is the direction of the peptide orientation. The nuclear spin relaxation rates and the para → ortho conversion rate of the incarcerated hydrogen molecule were determined by ¹H NMR spectroscopy. The experimental results were analyzed using DFT‐optimized molecular models. The relaxation rates and the conversion rates of the two peptides fall in the expected distance range. One of the two peptides is particularly rigid and thus ideal to keep the H₂@C₆₀/nitroxide separation, r, as large and controlled as possible, which results in particularly low relaxation and conversion rates. Despite the very similar optimized distance, however, the rates measured with the other peptide are considerably higher and thus are compatible with a shorter effective distance. The results strengthen the outcome of previous investigations that while the para → ortho conversion rates satisfactorily obey the Wigner's theory, the nuclear spin relaxation rates are in excellent agreement with the Solomon–Bloembergen equation predicting a 1/r⁶ dependence.     

Stereoelectronic Aspects of the Anomeric Effect in Fluoromethylamine

High-level ab initio calculations have been made for fluoromethylamine to study structural and energetic effects of the relative orientation of the N lone pair to the C? F bond. The anti-conformer (N lone pair anti-planar to the C? F bond) corresponds to the global energy minimum. It has the longest C? F distance, the shortest C? N distance, and is 7.5 kcal·mol^?1 more stable than the related perpendicular conformation (lone pair perpendicular to the C? F bond). The syn-conformation also shows hallmarks of the anomeric effect: long C? F bond, short C? N bond, and energetic stability when allowance is made for the two pairs of eclipsed hydrogens. The transition state for N inversion is close to the syn-structure; rotation about the C? N bond is strongly coupled with this inversion process. Small bond distance changes of ca. 0.02 Å between parallel and perpendicular conformations are associated with dissociation energy differences of ca. 30 kcal·mol^?1. Various criteria for assessing the strength of the anomeric effect are discussed.     

Sub‐Micromolar Pulse Dipolar EPR Spectroscopy Reveals Increasing CuII‐labelling of Double‐Histidine Motifs with Lower Temperature

Electron paramagnetic resonance (EPR) distance measurements are making increasingly important contributions to the studies of biomolecules by providing highly accurate geometric constraints. Combining double‐histidine motifs with Cu^II spin labels can further increase the precision of distance measurements. It is also useful for proteins containing essential cysteines that can interfere with thiol‐specific labelling. However, the non‐covalent Cu^II coordination approach is vulnerable to low binding‐affinity. Herein, dissociation constants (K_D) are investigated directly from the modulation depths of relaxation‐induced dipolar modulation enhancement (RIDME) EPR experiments. This reveals low‐ to sub‐μm Cu^II K_Ds under EPR distance measurement conditions at cryogenic temperatures. We show the feasibility of exploiting the double‐histidine motif for EPR applications even at sub‐μm protein concentrations in orthogonally labelled Cu^II–nitroxide systems using a commercial Q‐band EPR instrument.     

Sterically Congested Phosphite Ligands: Synthesis,crystallographic characterization,and observation of unprecedented eight-Bond 31P,31P coupling in the 31P-NMR spectra

The synthesis and characterization of 2-{1-{3,5-bis(1,1-dimethylethyl)-2-{[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl]oxy}phenyl}ethyl}-4,6-bis(1,1-dimethylethyl)phenyl diphenyl phosphite ( 6 ) is described. In the ³¹P-NMR spectrum (¹H-decoupled) of 6 , an unprecedented eight-bond P,P coupling of J = 72.8 Hz is observed. In the X-ray crystal structure of 6 , an intramolecular P–P distance of 3.67 Å is found, which is within the sum of the van-der-Waals radii of the P-atoms. The observed intramolecular P–P distance suggests that a through-space coupling mechanism is operative. The solid-state conformation of 6 is compared to the conformation obtained by semi-empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid-state structure is near a true energy minimum, but that crystal-packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal-packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6 . The proximity of the P-atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable-temperature ³¹P-NMR spectroscopy. Semi-empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O-atoms in a common plane, with the P-atom lying above this plane.     

Synthesis of ReI tricarbonyl complexes with various sulfur‐ and oxygen‐donating ligands: crystal structures of two ReI dinuclear structures bridged by S atoms

The synthesis and characterization of two dinuclear complexes, namely fac‐hexacarbonyl‐1κ³C,2κ³C‐(pyridine‐1κN)[μ‐2,2′‐sulfanediyldi(ethanethiolato)‐1κ²S¹,S³:2κ³S¹,S²,S³]dirhenium(I), [Re₂(C₄H₈S₃)(C₅H₅N)(CO)₆], ( 1 ), and tetraethylammonium fac‐tris(μ‐2‐methoxybenzenethiolato‐κ²S:S)bis[tricarbonylrhenium(I)], (C₈H₂₀N)[Re₂(C₇H₇OS)₃(CO)₆], ( 2 ), together with two mononuclear complexes, namely (2,2′‐bithiophene‐5‐carboxylic acid‐κ²S,S′)bromidotricarbonylrhenium(I), ( 3 ), and bromidotricarbonyl(methyl benzo[b]thiophene‐2‐carboxylate‐κ²O,S)rhenium(I), ( 4 ), are reported. Crystals of ( 1 ) and ( 2 ) were characterized by X‐ray diffraction. The crystal structure of ( 1 ) revealed two Re—S—Re bridges. The thioether S atom only bonds to one of the Re^I metal centres, while the geometry of the second Re^I metal centre is completed by a pyridine ligand. The structure of ( 2 ) is characterized by three S‐atom bridges and an Re…Re nonbonding distance of 3.4879 (5) Å, which is shorter than the distance found for ( 1 ) [3.7996 (6)/3.7963 (6) Å], but still clearly a nonbonding distance. Complex ( 1 ) is stabilized by six intermolecular hydrogen‐bond interactions and an O…O interaction, while ( 2 ) is stabilized by two intermolecular hydrogen‐bond interactions and two O…π interactions.     

Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol‐Ru(2,2′‐bipyridine)32+ Dyads

Proton‐coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)₃²⁺ (bpy=2,2′‐bipyridine) photosensitizer in acetonitrile, intramolecular long‐range electron transfer from a phenolic unit to Ru(bpy)₃²⁺ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)₃²⁺ distance by increasing the number of bridging p‐xylene units. A distance decay constant (β) of 0.67±0.23 Å^?1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long‐range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light‐to‐chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.