Interchain Contacts Research Articles

A model for the plateau in the relaxation modulus for linear chain polymer melts

A simple model is considered for the free energy associated with the relaxation of a linear chain polymer melt. The relaxation of the chain configurations results in an adjustment of the locations of the interchain contacts. The change in the distribution of the positions of the contacts between a pair of relaxing chains is hindered by the presence of other nearby chains in the melt. There is less configuration space available to the relaxing chains, due to the noncrossability of the chain backbones, than would be the case for phantom chains. This results in an increase in the free energy of the relaxing system. The analysis presented indicates that, given the free energy models considered, the extent of relaxation decreases as the length scale for relaxation increases. This results in a plateau in the relaxation modulus. This qualitative prediction of a plateau does not rely on the assumption of a specific mechanism for the chain dynamics, and is relatively insensitive to the form chosen for the terms in the free energy. If reasonable assumptions are made concerning the form of the free energy, then it is shown that the plateau which results depends on monomer length, Kuhn length, the monomer density for the melt, and, for solutions, the polymer concentration in a manner consistent with experimental data.

Sensor System for Alcohol and Chlorinated Hydrocarbon Vapours on the Basis of Semiconducting Polymers

A novel polymer-chemosensor microsystem which can be operated without additional energy consuming heating is presented. Production steps were reduced to metal coating of ceramic plates and structuring of an interdigital array by CAD-supported pulsed laser ablation (PLA) instead of conventional lithographic techniques which yield low production costs only for high production numbers. The active sensor film consists of a semiconducting polymer which can be deposited as a polymer precurser fluid onto the transducer, and can-be tempered in-situ to yield the required polymeric species and chemical functionality. A precurser, poly-2,5-furylene-hydroxy-ethylene, is coated on the pulse-laser processed interdigital array structures on an Al2O3 substrate and converted to various poly-2,5-furylene-vinylene (PFV) derivatives by special heat treatments under inert atmosphere. Costly capacitive modulation and lock-in data acquisition techniques were replaced by simple direct current measurements. Certain sensor types exhibited remarkable sensitivity e.g. for 1,1,2-trichlo-roethylene and propanol, but moderate cross sensitivity for humidity. The response time is of the order of few minutes, and the current signal is linear in a wide analyte pressure range. Sensitivities for alcohols and chlorinated hydrocarbons are most probably related to hydrophobic van der Waals interactions whereas Brönstedt acidity or Lewis basicity seem to play minor roles. Electronic or van der Waals interaction leads to deviation of the idealy flat and rigid structure of the conjugated polymer thus allowing for more flexibility of the chains, more intimate inter-chain contacts and higher conductivity.

Crystallographic Evidence for C α–H···O=C Hydrogen Bonds in a Collagen Triple Helix

The crystal structure of the collagen triple-helical peptide (Pro-Hyp-Gly) 4- Pro-Hyp-Ala-(Pro-Hyp-Gly) 5shows evidence for the existence of interchain contacts between α-carbon hydrogens from Gly and Hyp residues, and carbonyl groups from Gly and Pro residues on neighboring chains. The geometrical disposition of these contacts makes it reasonable to describe them as C α–H···O=C hydrogen bonds. Two repetitive patterns can be identified, and one of them is identical to a similar type of interaction reported recently for β-sheets in globular proteins, which suggests a more universal character for C–H···O hydrogen bonds in building protein secondary structure elements. They are presumably much weaker in energy than the interchain N–H···O=C hydrogen bonds responsible for the alignment of the three chains in the collagen triple helix, and therefore their contribution will be a small but cooperative decrease on the total interchain hydrogen bonding energy.

Clusters of charged residues in protein three-dimensional structures.

Statistically significant charge clusters (basic, acidic, or of mixed charge) in tertiary protein structures are identified by new methods from a large representative collection of protein structures. About 10% of protein structures show at least one charge cluster, mostly of mixed type involving about equally anionic and cationic residues. Positive charge clusters are very rare. Negative (or histidine-acidic) charge clusters often coordinate calcium, or magnesium or zinc ions [e.g., thermolysin (PDB code: 3tln), mannose-binding protein (2msb), aminopeptidase (1amp)]. Mixed-charge clusters are prominent at interchain contacts where they stabilize quaternary protein formation [e.g., glutathione S-transferase (2gst), catalase (8act), and fructose-1,6-bisphosphate aldolase (1fba)]. They are also involved in protein-protein interaction and in substrate binding. For example, the mixed-charge cluster of aspartate carbamoyl-transferase (8atc) envelops the aspartate carbonyl substrate in a flexible manner (alternating tense and relaxed states) where charge associations can vary from weak to strong. Other proteins with charge clusters include the P450 cytochrome family (BM-3, Terp, Cam), several flavocytochromes, neuraminidase, hemagglutinin, the photosynthetic reaction center, and annexin. In each case in Table 2 we discuss the possible role of the charge clusters with respect to protein structure and function.

On the High Viscosity of Aqueous Solution of Lysozyme Induced by Some Organic Solvents

A remarkably high viscosity has been induced in protein aqueous solutions by the addition of certain structurally related organic solvents. The effect has been observed for lysozyme aqueous solutions containing tetramethylurea (TMU), dimethylsulfoxide (DMSO), dimethylformamide, and hexamethylphosphortriamide. The effect has also been induced in ferrocytochrome c aqueous solutions by TMU. Critical concentrations for both the protein and organic solvent were verified for the onset of the viscosity increase. A common feature of the solvents which were able to induce the effect is a dipolar moiety (C=O, S → O and P → O) and a nonpolar region represented by the methyl groups. The resulting fluids show an extremely restricted flow and a typical non-Newtonian, pseudoplastic behavior. Use was made of1H nuclear magnetic resonance (NMR) and Raman spectroscopy to characterize protein structural modifications and of13C NMR to investigate changes in relaxation times and chemical shifts in the solvent/water solutions. A systematic rheological characterization of the systems was undertaken for some of the solvents, and unusual patterns of viscous effects were identified for the solvent/water systems both with and without protein. The process was found to be at least partially reversible, as concluded from the recovered original solution rheological characteristics and the original protein1H NMR spectrum, after eliminating the organic solvent by ultrafiltration. The whole process was characterized as consisting of two mutually independent stages. The first involves an extensive conformational transition of the polypeptide backbone, from a predominantly α-helical to increased random coiled and β-sheet structures, with the occurrence of nonorthodox protein secondary structures at regions above the solvent critical point. The second stage consists of short-lived interchain contacts leading to an entanglement of the macromolecular system as a whole. A microphase reversion in the organic solvent/water mixture, supported by13C NMR and rheological results, is proposed as the driving force causing the observed behavior.

Structural aspects of the neutral-to-ionic transition in mixed-stack charge-transfer complexes

Original and intriguing features associated with the so-called neutral-to-ionic (N-I) transition have been examined in some organic mixed-stack charge-transfer (CT) complexes, involving simultaneously electronic, structural and magnetic aspects. The origin of the N-I transition and the essential parameters governing it are still controversial. Structural aspects of the N-I transition in the prototype compound tetrathiafulvalene- p-chloranil (TTF-CA) are reviewed. In the ionic phase, a ferroelectric arrangement is apparent between the dimerized chains. However, these long-range dipolar interactions are too weak to drive the phase transition. At atmospheric pressure, a significant volume variation occurs at T N-I with an abrupt change in the cell parameters in directions perpendicular to the stacking axis. This clear three-dimensional aspect of the compound is also evident from several close interchain contacts. Intramolecular deformations observed at the transition have been directly related to the ionicity change and the symmetry breaking, in agreement with the evolution of the 35Cl NQR spectrum. The phase diagram has been determined and a clear change in the nature of the transition has been observed with a tendency towards second-order character at high pressures.

Structural aspects of the neutral-to-ionic transition in mixed-stack charge-transfer complexes

Original and intriguing features associated with the so-called neutral-to-ionic (N-I) transition have been examined in some organic mixed-stack charge-transfer (CT) complexes, involving simultaneously electronic, structural and magnetic aspects. The origin of the N-I transition and the essential parameters governing it are still controversial. Structural aspects of the N-I transition in the prototype compound tetrathiafulvalene— p-chloranil (TTF—CA) are reviewed. In the ionic phase, a ferroelectric arrangement is apparent between the dimerized chains. However, these long-range dipolar interactions are too weak to drive the phase transition. At atmospheric pressure, a significant volume variation occurs at T N-I with an abrupt change in the cell parameters in directions perpendicular to the stacking axis. This clear three-dimensional aspect of the compound is also evident from several close interchain contacts. Intramolecular deformations observed at the transition have been directly related to the ionicity change and the symmetry breaking, in agreement with the evolution of the 35Cl NQR spectrum. The phase diagram has been determined and a clear change in the nature of the transition has been observed with a tendency towards second-order character at high pressures.

Fluorescence Quantum Yield of Poly(p-phenylenevinylene) Prepared via the Paracyclophene Route: Effect of Chain Length and Interchain Contacts

The monomer 9-[(tert-butyldimethylsilyl)oxy][2.2]paracyclophan-1-ene (1) is polymerized by Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)2 (Ar = 2,6-diisopropylphenyl) in a living manner. Block copolymers containing poly(1) and polynorbornene (poly(NBE)) can be prepared and have narrow polydispersities. Treatment of poly(1) with NBu4F produces poly(9-hydroxy[2.2]paracyclophan-1-ene) which dehydrates to poly(p-phenylenevinylene) (PPV) under mild conditions. Block copolymers containing segments of size-specific and defect-free PPV with poly(NBE) of various dimensions can be prepared readily and are soluble in a variety of common organic solvents. These polymers serve as excellent candidates to measure the effect of chain length and frequency of interchain contacts on PPV's fluorescence quantum yield (Φf). Solution and solid-state quantum yield determinations, coupled to fluorescence lifetime measurements, reveal an acute drop in Φf as the degree of polymerization of the emissive species increases and as the average distance between PPV chains decreases. PPVx-block-poly(NBE)y prepared in solution consistently shows larger Φf than material prepared by solid-state dehydration. The poly(NBE) companion block also serves to protect PPV from atmospheric degradation.

Functional Group Accessibility in Hydrogen Bonded Polymer Blends

Accessibility of functional groups is an important concept in the mixing of polymers and involves factors such as chain connectivity, steric shielding, etc., all of which tend to limit the number of interchain contacts formed. Infrared spectroscopy is well suited to study functional group accessibility in carefully selected miscible hydrogen bonded polymer blend systems, because the fraction of interchain hydrogen bonded groups can be measured quantitatively. Such measurements have been made on blends consisting of a wide range of carbonyl containing (co)polymers with a 2,3-dimethylbutadiene-stat-4-vinylphenol containing 24 wt % 4-vinylphenol. Evidence has been obtained for decreasing accessibility of the carbonyl groups of the poly(n-alkyl methacrylate)s due to steric shielding from bulky side groups. Conversely, when these groups are spaced further apart in an ethylene-stat-vinyl acetate or ethylene-stat-methyl methacrylate copolymer chain, they become much more accessible. The effect of functional grou...

BiP Binding Sequences in Antibodies

During the process of folding and assembly of antibody molecules in the endoplasmic reticulum, immunoglobulin heavy and light chains associate transiently with BiP, a resident endoplasmic reticulum protein that is a member of the Hsp70 family of molecular chaperones. BiP is thought to recognize unfolded or unassembled polypeptides by binding extended sequences of approximately seven amino acids that include bulky hydrophobic residues not normally exposed on the surface of native proteins. We used a computer algorithm developed to predict BiP binding sites within protein primary sequences to identify sites within immunoglobulin chains that might mediate their association with BiP. Very few of the sequential heptapeptides in the heavy or light chain sequences were potential BiP binding sites. Analysis of the ability of synthetic heptapeptides corresponding to 24 potential sites in heavy chains to stimulate the ATPase activity of BiP indicated that at least half of them were authentic BiP binding sequences. These sequences were not confined to a single domain of the heavy chain but were distributed within both the VH and CH domains. Interestingly, when the BiP binding sequences were mapped onto the three-dimensional structure of the Fd antibody fragment, the majority involve residues that participate in contact sites between the heavy and light chains. Therefore, we suggest that in vivo BiP chaperones the folding and assembly of antibody molecules by binding to hydrophobic surface regions on the isolated immunoglobulin chains that subsequently participate in interchain contacts.

Large-Scale Molecular Dynamics Study of Entangled Hard-Chain Fluids

Equilibrium molecular dynamics is used to simulate fluids comprised of chains of tangent hard spheres. Reptation theory predictions of segmental motion are compared with simulation results. In addition to the usual tube confinement, a second entanglement effect is observed. As the chain disengages from the tube, persistent interchain contacts cause a plateau in the segment mean-square displacement and subsequent accelerated diffusion. Associated with the plateau in the mean-square displacement is a corresponding delay in relaxation of the end-to-end vector as interior chain segments are extended during disentanglement.

Chain overlap and entanglements in dilute polymer solutions: Brownian dynamics simulation

AbstractWe have analyzed chain conformations and the existence — or otherwise — of chain overlaps and entanglements in dilute polymer solutions (at concentrations c < C*, c* = critical concentration). The fundamental problem of existence of chain overlaps in dilute solutions is also related to the drag reduction phenomenon (DR). Some experimental results pertinent to DR are explained in terms of entanglements even for solutions at concentrations defined in ppm. We report results of Brownian dynamics simulations of polymer solutions in which the equations of motion of the chains are solved by using the Langevin equation. Chains move according to actions of a systematic frictional force and a randomly fluctuating force w(t), where t is time. In addition, a shear flow field can be introduced into the model. To evaluate the structure of polymer chains in solution we have devised a measure of interchain contacts and two different measures of entanglements. The results for c = 0.3 c* demonstrate that both chain entanglements and overlaps take place even in dilute solution. They also confirm predictions from an earlier combinatorial model.

Viscoelastic response of bidisperse melts in the lateral motion model

A model for stress relaxation of linear chain polymer melts in the linear viscoelastic regime is proposed. This model is developed in the context of the lateral motion model of polymer dynamics, which assumes that polymers diffuse in the melt by sliding laterally along the curvilinear contours of neighboring chains. It is assumed that, in the post-plateau region of the time dependent stress relaxation, the stress is supported by interchain contacts. When each contact breaks, a new contact is formed, since the number of interchain contacts is an equilibrium property. It is also assumed in this model that the new contacts, formed after the application of the strain, support partial stress because the polymer system is not isotropic at the time of the formation of these new contacts. The stress relaxation is evaluated within this model for monodisperse and bidisperse melts using a computer simulation procedure. The results are compared with published experimental data. Very good agreement is found with the experimental results.

STM image of polyaniline with large size counter-ions

A periodical STM image of the polyemeraldine salt from protonated with p-toluenesulfornic acid (PANI-PTSA) on IIOPG substrates synthesized electrochemically was obtained. By association with x-ray diffraction measurement, the resulting STM image was assigned, and a possible chain organization was proposed based on assumption of a tight interchain contact with shifted of a quarter of chain repeat unit of PANI. The parameters, such as chain repeat unit, interchain distance between chains, caulculated from proposed chain structure model are in agreement observations from STM image measurements.

Structure-property relationships in poly(o-phenylenediamine) derivatives

Abstract Various poly(o-phenylenediamine) derivatives (e.g. poly(2,3-diaminotoluene) (PDT) and poly(N-n-butyl-o-phenylenediamine) (PBPD)) have been prepared electrochemically and characterized. Cyclic voltammetry measurements in different acidic conditions have revealed a one-step oxidation process in these polymers which implies the formation of deprotonated imines in PDT and of some oxidized (charged) substituted imines in PBPD. However, in-situ conductivity measurements showed that both materials have low electrical conductivities (with a maximum of 10−4−10−3 S cm−1). On the basis of results obtained on model compounds (5,12-dihydro-5,7,12,14-tetraazapentacene and some derivatives), these low electrical conductivities can be explained by a low concentration of charge carriers and poor interchain contacts.

Poly(cycloalkyl[c]thiophene)s — syntheses, electrical properties and charge transport mechanism

AbstractPoly(cycloalkyl[c]thiophene)s were synthesized by electrochemical or chemical oxidation of bicyclic, α,α′‐unsubstituted thiophenes. NMR measurements on neutral, soluble poly(cyclododeca[c]thiophene) indicate that the polymers have the expected linear structure. The optical spectra and the electrochemical properties of the films are reported. The electrical conductivity σ of salts of the polymers varies from 50 S/cm to 8 · 10−5 S/cm depending on the ring size of the substituent and degree of oxidation of the polymer backbone. A linear correlation between log σ and the diameter of the alkyl substituent demonstrates that interchain contacts play a dominant role in the conductivity of these polymers. The temperature dependence of the conductivity was measured in the range of 300 to 5 K and its frequency dependence in the range of 1–107 Hz at 300 to 120 K. The conductivity data were analyzed in terms of hopping of localized charge carriers and the localization length of the charge carriers was calculated.

Polarons and Their Stability in Poly (P-Phenylene Vinylene)

Abstract We have parametrized a tight-binding description of the n electrons, with [sgrave] bond compressibility, on a poly (p-phenylene vinylene), PPV, chain by fitting the valence band width, the band gap and the chain geometry to better than 1%. With the resulting Hamiltonian we found that for long chains the polaron forms a localized state 0.18 eV from the band edge. Interchain coupling was described by using the results of ab initio electronic structure calculations and the actual separations of the atoms with the closest interchain contacts. It was found that interchain coupling does not destabilize the polaron in PPV.

On the plateau modulus mixing rule of miscible blends

AbstractBased on Graessley and Edwards scaling concepts, a new mixing rule for the entanglement density of miscible blends has been proposed. The arguments here used apply when the interactions in the blend do not affect too much the entanglement probability. The rule was compared with others (linear and geometric mixing rules) based on the entanglement idea of fixed interchain contact points. The predicted plateau moduli were successfully compared with experimental data of polystyrene‐poly(dimethylphenylene oxide) blends.

An organic-inorganic salt containing mixed-valence TTF chains and the molecular metal oxide cluster [Mo8O26]−4. Preliminary spectroscopic, conducting and magnetic properties of the compound (TTF)7Mo8O26

A new organic-inorganic salt formed by mixed-valence TTF chains and [Mo8O26]−4 is obtained by electrochemical oxidation of the donor in acetonitrile solution containing the polyanion. A unusual zigzag stacking of the donor molecules along the b-axis of the triclinic cell is observed, with close intrachain and interchain contacts through the sulpur atoms (the shortest S-S distances are about 3.5 Å). A semiconducting behavior has been observed with a conductivity σ ≈ 10−3 S.cm−1 at room temperature. The magnetic properties indicate that the compound is non magnetic.

A non-reptation model for polymer dynamics in the melt and concentrated solutions

A model for the dynamics of dense multichain polymer systems involving linear chains is presented. The model is based on the assumption that chain motion perpendicular to the direction of the chain backbone dominates the dynamics over motion along the backbone. Many chains are highly intertwind with any given chain. It is assumed that the chain motion at an interchain contact point proceeds by sliding along the contour of each entangling chain. The requirement that a chain moves by sliding along the random-flight contour of another chain at a contact point results in an effective bead-friction coefficient for three-dimensional motion which is an increasing function of time. The chain dynamics are evaluated within a Rouse-like model employing the time-dependent bead-friction coefficient. The asymtotic large- N dependencies of the center-of-mass diffusion coefficient and the terminal relaxation time are found to be N −2 and N 3, respectively. Calcualtions are performed for chains of various lengths, and the results of the model are qualitatively very similar to those reported from the Monte Carlo simulations and experiments. Calculations of the linear viscoelastic response of monodisperse and bidisperse systems are presented. These show excellent agreement with published experimental data.