Interchain Contributions Research Articles

A Tunable, Particle-Based Model for the Diverse Conformations Exhibited by Chiral Homopolymers

Chiral block copolymers capable of hierarchical self-assembly can also exhibit chirality transfer─the transfer of chirality at the monomer or conformational scale to the self-assembly. Prior studies focused on experimental and theoretical methods that are unable to fully decouple the thermodynamic origins of chirality transfer and necessitate the development of particle-based models that can be used to quantify intrachain, interchain, and entropic contributions. With this goal in mind, in this work, we developed a parametrized coarse-grained model of a chiral homopolymer and extensively characterized the resulting conformations. Specifically, the energetic parameter in the angular and dihedral potentials, the angular set point, and the dihedral set point are systematically varied to produce a wide range of conformations from a random coil to a nearly ideal helix. The average helicity, pitch, persistence length, and end-to-end distance are measured, and correlations between the model parameters and resulting conformations are obtained. Using available experimental data on model polypeptoid-based chiral polymers, we back out the required parameters that produce similar pitch and persistence length ratios reported in the experiments. The conformations for the experimentally matched chains appear to be somewhat flexible, exhibiting some helical turns. Our model is versatile and can be used to perform molecular dynamics simulations of chiral block copolymers and even sequence-specific polypeptides to study their self-assembly and to gain thermodynamic insights.

Atomistic Description of Ionic Diffusion in PEO–LiTFSI: Effect of Temperature, Molecular Weight, and Ionic Concentration

Understanding the ionic diffusion mechanism in polymer electrolytes is critical to the development of advanced lithium-ion batteries. We report here molecular dynamics-based characterization of structures and diffusion in poly(ethylene oxide) (PEO) with lithium and bis(trifluoromethysulfonyl)imide (TFSI) ions imbedded into the PEO structure. We consider a range of temperatures (360–480 K), molecular weights (43, 22, 10, and 2 chains with 23, 45, 100, and 450 EO monomers, respectively), and ion concentrations (r = 0.02, 0.04, 0.06, and 0.08 Li:EO) for which there is experimental data. The found dependence of the diffusion coefficients on these variables is in good agreement with experimental measurements. We then analyze how the diffusion performance depends on details of the atomistic diffusion mechanism, the motion of the Li and TFSI along the polymer chains and hopping between them, the role of polymer motion, the temperature dependence of the intrachain and interchain diffusion contributions to the tot...

Structural Analyses of Alanine Trimer and Tetramer Crystals with Antiparallel and Parallel β-Sheet Structures Using Solid-State 1H Spin-Diffusion 2D Correlation NMR Spectroscopy.

Poly-l-alanine (PLA) sequences are key elements of the crystalline domains of spider dragline and wild silkworm silks. In the present work, 1H spin-diffusion two-dimensional (2D) correlation NMR spectra were observed for selectively deuterated (Ala)3 and (Ala)4 crystals to develop the analytical method for the structure of PLA sequences. The build-up curves of the cross peaks for three kinds of 1H pairs in selectively deuterated (Ala)3 and (Ala)4 crystals were observed to obtain spin-diffusion rate constant k j, k from relaxation master equations P i, j(τm). The k j, k values subsequently lead to effective interproton distance r j, keff (obs) values for individual proton-proton pairs, which include intra- and intermolecular contributions. The r j, keff (obs) values were compared to r j, keff (calc) values obtained from the experimentally determined atomic coordinates of antiparallel (AP) β-sheet (Ala)3 and (Ala)4 and parallel (P) β-sheet of (Ala)3 and (Ala)4 crystals. The agreement between the r j, keff (obs) and r j, keff (calc) values was good for AP β-sheet (Ala)3 and (Ala)4 crystals but poor for P β-sheet (Ala)3 and (Ala)4 crystals. These deviations were obtained from the interproton distances of the interchain contributions due to different packing arrangements. The packing arrangements of the PLA region are important when considering the relevant structure and the mechanical properties of silks.

Molecular Dynamics Simulations of Ionic Diffusion in PEO-LiTFSI Polymer Electrolyte: Effect of Temperature, Molecular Weight, and Ionic Concentration

Polymer electrolytes are promising materials for development of the next generation of energy storage devices, including lithium-ion polymer batteries, due to their exceptional chemical and mechanical stability, high energy density and long lifetimes. Understanding the ionic diffusion mechanism in polymer electrolytes is critical to the development of advanced lithium-ion batteries. A molecular dynamics-based characterization of structures and diffusion in poly(ethylene oxide) (PEO) with lithium and bis(trifluoromethy-sulfonyl-imide) (TFSI) ions imbedded into the PEO structure have been performed across a range of temperatures, molecular weights and ion concentrations, with relative ionic diffusion coefficients shown to be in good agreement with experimental measurements. To determine details of the atomistic diffusion mechanism, the chain coordination of Li atoms, polymer motion, and temperature dependences of the intrachain and interchain diffusion contributions into the total ionic diffusion coefficients on ionic concentration and molecular weight were analyzed. We find that the most diffusive Li atoms exhibit frequent interchain hopping, whereas the least diffusive Li atoms rather oscillate or “shift” coordination between two or more polymer chains. These shifts may actually reduce the segmental motion of the PEO-LiTFSI polymer, which is important for the fast lithium-ion diffusion. Good agreement between experiment and theory validates the approach and methodology used in this study and its further applications for predicting the structure and ionic conductivity of new advanced polymer materials for a new generation of electrochemical devices. Acknowledgement This work was supported by Bosch Energy Research Network Grant No. 13.01.CC11.

Single-Chain Conformation for Interacting Poly(N-isopropylacrylamide) in Aqueous Solution

The demixing phase behavior of Poly(N-isopropylacrylamide) (PNIPAM) aqueous solution is investigated using small-angle neutron scattering. This polymer phase separates upon heating and demixes around 32 ∘C. The pre-transition temperature range is characterized by two scattering modes; a low-Q (large-scale) signal and a high-Q dissolved chains signal. In order to get insight into this pre-transition region, especially the origin of the low-Q (large-scale) structure, the zero average contrast method is used in order to isolate single-chain conformations even in the demixing polymers transition region. This method consists of measuring deuterated and non-deuterated polymers dissolved in mixtures of deuterated and non-deuterated water for which the polymer scattering length density matches the solvent scattering length density. A fixed 4% polymer mass fraction is used in a contrast variation series where the d-water/h-water fraction is varied in order to determine the match point. The zero average contrast (match point) sample displays pure single-chain scattering with no interchain contributions. Our measurements prove that the large scale structure in this polymer solution is due to a transient polymer network formed through hydrophobic segment-segment interactions. Scattering intensity increases when the temperature gets close to the phase boundary. While the apparent radius of gyration increases substantially at the Lower Critical Solution Temperature (LCST) transition due to strong interchain correlation, the single-chain true radius of gyration has been found to decrease slightly with temperature when approaching the transition.

Entangled polymer chain melts: Orientation and deformation dependent tube confinement and interchain entanglement elasticity

The phenomenological reptation-tube model is based on a single chain perspective and was originally proposed to explain the remarkable viscoelastic properties of dense entangled polymer liquids. However, simulations over the last two decades have revealed a fundamental tension in the model: it assumes that bonded, single-chain backbone stresses are the sole polymer contribution to the slowly relaxing component of stress storage and elasticity, but mounting evidence suggests that at the local level of forces it is interchain contributions that dominate, as in simple liquids. Here we show that based on a chain model constructed at the level of self-consistently determined primitive paths, an explicit force-level treatment of the correlated intermolecular contributions to stress that arise from chain uncrossability can essentially quantitatively predict the entanglement plateau modulus associated with the soft rubbery response of polymer liquids. Analogies to transient localization and elasticity in glass-forming liquids are identified. Predictions for the effect of macroscopic deformation and anisotropic orientational order on the tube diameter are also made. Based on the interchain stress perspective the theory reproduces some aspects of the rheological response to shear and extensional deformations associated with the single chain tube model.

Interchain and intrachain emission branching in polymer light-emitting diode doped by organic molecules

A blend of the polymer poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) and the electron-transport molecule tris-(8-hydroxyquinoline) aluminum (Alq3) has been investigated by means of electroluminescence and fluorescence spectroscopy, upon variation of the Alq3 content in the blend. A decreased interchain emission is observed upon increasing Alq3 content, due to lower packing of the MEH-PPV chains which leads to a reduction in the interchain interaction, excimer formation, and emission probability. A branching of MEH-PPV interchain and intrachain emissive contributions is clearly time resolved and analyzed as a function of the Alq3 content. At high doping concentration, direct emission from Alq3 molecules is observed.

Short-range order and collective dynamics of poly(vinyl acetate): A combined study by neutron scattering and molecular dynamics simulations

The structure and collective dynamics at local length scales of poly(vinyl acetate) (PVAc) have been investigated by combining neutron scattering on a deuterated sample and fully atomistic molecular dynamics simulations. The experimental data have provided microscopic information at molecular level and a critical check to validate the simulated cell. In order to gain deep insight on the structure factor, we have grouped the simulation results in terms of three molecular substructures: the main chain, the carboxyl group, and the methyl group. The study of the resulting static partial structure functions has revealed the origin of the diffraction peaks, and a real-space evaluation of the characteristic radial distribution functions has allowed separating intra- and interchain contributions to the total correlation functions. The extension of this study to the dynamic structure factor has revealed a coupling between the side-group motions and the main-chain dynamics at intermolecular distances. Moreover, a decrease in coherency of the main-chain motions at intermolecular level is observed with increasing temperature, leading to a rather decoupled behavior between the main chains at 200 K above the glass transition.

Deuteron and proton spin-lattice relaxation dispersion of polymer melts: Intrasegment, intrachain, and interchain contributions

Proton and deuteron field-cycling NMR relaxometry was applied to deuterated and undeuterated bulk polyethyleneoxide and polybutadiene melts and mixtures thereof with molecular weights above the critical value. Spin-lattice relaxation data due to intrasegment (quadrupolar) couplings and intra- and interchain (dipolar) interactions were evaluated. Diverse dynamic limits are identified both with the proton and deuteron frequency dispersion data. The comparison between the intrachain and the interchain contributions leads to the conclusion that only model theories based on largely isotropic chain dynamics can account for the experimental findings. The extremely anisotropic character of the well-known tube/reptation model is too restrictive in this respect.

Elastic interaction among transition metals in one-dimensional spin-crossover solids

We present an exact examination of a one-dimensional (1D) spin-phonon model describing the thermodynamical properties of spin-crossover (SC) solids. This model has the advantage of giving a physical mechanism for the interaction between the SC units. The origin of the interaction comes from the fact that the elastic constant of the spring linking two atoms depends on their electronic states. This leads to local variation of the elastic constant. Up to now, all the statistical studies of this model have been performed in the frame of the mean-field (MF) approach, which is not adequate to describe 1D systems with short-range interactions. An alternative method, based on the variational approach and taking into account the short-range correlations between neighboring molecules, was also suggested, but it consists in an extension of the previous MF approximation. Here, we solve exactly this Hamiltonian in the frame of classical statistical mechanics using the transfer-matrix technique. The temperature dependence of the high spin fraction and that of the total energy are obtained analytically. Our results clearly show that there is a clear tendency to a sharp transition when we tune the elastic constants adequately, which indicates that first-order phase transition takes place at higher dimensions. In addition, we demonstrate the existence of an interesting isomorphism between the present model and Ising model under effective interaction and effective ligand field energy, in which both depend linearly on temperature and both come from the phonon contribution. We have also studied the effect of the pressure (the tension) on the thermodynamical properties of the high spin (HS) fraction and have found a nontrivial pressure effect that while for weak tension values, the low spin state is stabilized for the pressure above a threshold value, it enhances the interaction between the HS states. Finally, we have also introduced elastic interactions between the chains. Treating exactly (in mean field) the intrachain (interchain) contributions, we found that our model leads us to obtain first-order spin transitions when both short- and long-range interactions are ferroelastic. We show also that competing (antiferroelastic short-range and ferroelastic long-range) interactions between spin-state ions reproduce qualitatively the two-step-like spin-crossover transitions.

Local Structure of Syndiotactic Poly(methyl methacrylate). A Combined Study by Neutron Diffraction with Polarization Analysis and Atomistic Molecular Dynamics Simulations

The local structure of syndiotactic poly(methyl methacrylate) (PMMA) has been investigated by combining neutron scattering and fully atomistic molecular dynamics simulations. Selectively deuterating parts of the PMMA monomer, we have accessed five different partial structure factors in the glassy state by neutrons, and polarization analysis has allowed isolating the coherent contribution to the total scattering. In addition, the temperature dependence of the static structure factor has been determined on the fully deuterated sample. The different measured partial structure factors show qualitatively different features with respect to peak positions and heights and provide a very critical check to validate the simulated structure. To gain deep insight into the structure, we have grouped the simulation results in terms of three molecular substructures: the main chain, the α-methyl group, and the ester side group. The study of the resulting partial structure functions has revealed the origin of the diffraction peaks, including those in the X-rays pattern reported in the literature. In addition, a real-space evaluation of the characteristic radial distribution functions has allowed separating intra- and interchain contributions to the total correlation functions. We have found that (i) PMMA exhibits a strong local order with an average main-chain distance of ≈8.6 Å, (ii) this is the only average interchain distance and thus no precursor effect of a layered structure is found, (iii) the main chain shows a persisting all-trans structure, and (iv) a strong anticorrelation between the main chain and the ester side groups, together with an interdigitation of the side groups, suggests a marked separation between the backbone and the side-group spatial arrangements.

Optical investigation of intra and interchain charge dynamics in conducting polymers

Reflectance measurements on tensile-drawn films of polypyrrole (PPy) doped with PF 6 are analyzed in terms of intra and interchain contributions to the optical conductivity σ( ω). The intrachain σ( ω) is typical of a disordered Drude metal with σ( ω→0)≈2×10 3 S/cm, while the interchain σ( ω) is dominated by spectral features of strongly localized conductors, implying the interchain charge dynamics is incoherent.

Sulfonic Acid Doped Thermoreversible Polyaniline Gels. 2. Influence of Sulfonic Acid Content on Morphological, Thermodynamical, and Conductivity Properties

Thermoreversible gelation behavior of polyaniline (PANI) in the presence of varying concentrations of sulfonic acids are studied. Four different sulfonic acids are used, e.g., dinonylnaphthalene sulfonic acid (DNNSA), dinonylnaphthalene disulfonic acid (DNNDSA), (±)-camphor-10-sulfonic acid (CSA), and n-dodecyloxo sulfonic acid (DOSA). The composition range studied here is W PANI = 0.05-0.80 (W PANI is the weight fraction of PANI in the gel). It has been found from the SEM study that for all the sulfonic acids in the composition range W PANI = 0.05-0.40, fibrillar network is present. The TEM study also supports the above viewpoint. The thermodynamic study of the gels has been done by DSC-7, and for all the systems broad peaks consisting of two fused gel melting/gel formation peaks are observed. After proper deconvolution of the two peaks the gel melting/gel formation temperatures are measured. When they are plotted with W SO3H (weight fraction of sulfonic acid), typical phase diagrams consisting of two almost parallel curves are found. They are explained by considering the lamellar model where the bilayer and monolayer portions of the surfactant form different crystalline domains. The higher melting point curve is due to the former and the lower one is due to the later in each phase diagram. The lowering of gel melting/gel formation points with increase in PANI concentration of each curve has been attributed to the dilution effect of PANI in the gel. The corresponding enthalpy values (ΔH) also show similar decrease with increasing PANI concentration and are explained also from the dilution effect of PANI. The conductance of these gels varies with weight fraction of PANI (W PANI ), showing a maximum, and it has been explained by considering that conductivity in the gel is due to both intrachain and interchain contributions. Attempt is made to discuss the above results by using a molecular model of the PANI-DOSA system with the help of MMX program.

Thermoreversible polyaniline gels

Polyaniline (PANI) forms thermoreversible gels in different sulphonic acids e.g. dinonyl napthalene sulphonic acid (DNNSA), dinonylnapthalene disulphonic acid (DNNDSA), ± camphor - 10 - sulphonic acid (CSA) and dodecyl sulphonic acid (DSA) when processed from the formic acid medium. The surfactant concentration has been varied from weight fraction of PANI (W PANI ) = 0.05 - 0.60. In most cases at W PANI = 0.05 - 0.40 compositions fibrillar network structures are observed from SEM study. They also exhibit reversible first order phase transition during both heating and cooling in DSC. The melting temperature and the gelation temperature increases with increase in surfactant concentration of the gel. From the WAXS pattern it is concluded that crystallization of the surfactant tails anchored from the nitrogen atom of PANI through its SO 3 H head group is responsible for gelation. The conductivity of all the gels with increase in PANI concentration showed a maximum with composition. The maximum conductivity is ∼0.01 S/cm, for W PANI = 0.22. The conductivity variation has been explained by considering it as a function of both interchain and intrachain contributions.

Conformational effects on electrical and spectroscopic properties of bi-, ter-, and poly-thiophenes

Using conformatcnally constrained bi- and ter-thiophenes, the effects of inter-ring torsional angles on spectroscopic properties of the corresponding polymers are demonstrated; effects on conductivity are complicated by inter-chain contributions.

Optical properties and long-lived carrier generation efficiency in oriented polyacetylene

A simple tight-binding model extended to include the zig-zag structure of the chains and the interchain electronic coupling has been used to account for the dispersion of the real and imaginary part of the dielectric function of polyacetylene as derived from optical measurements. Intrachain and interchain contributions to the perpendicular absorption coefficient have been evaluated. It has been found that while interchain effects have a relatively small impact on the optical properties, they play a major role in determining the photogeneration rate of long-lived carriers in oriented trans-(CH) x.

The local structure of molten poly(tetrafluoroethylene)

We report wide angle neutron diffraction experiments performed on molten poly(tetrafluoroethylene) (PTFE). The experiments have been carried out both on the time of flight LAD diffractometer at ISIS (UK) and on the diffractometer D20 at the ILL (Grenoble). This study is part of a large programme concerned with the development of a detailed understanding of the local structure of non-crystalline polymers. Polytetrafluoroethylene is particularly suited for such an investigation, due both to the simplicity of its chemical structure and to the rigidity of its chain. We have carried out a quantitative analysis of the experimental results by comparing them with a computer built statistical model for the PTFE chain. In particular, a separation between correlations arising from chain segments belonging to the same molecule (intrachain correlations) and from segments of differing chains (interchain contributions) has allowed us to show the high degree of stiffness of the single polymer chain and the considerable level of correlation of segmental orientation between different chains.

Electronic, vibrational and resonance Raman spectra of the layered semiconducting compound NbS3

AbstractThe electronic (900–250 nm), infrared and Raman (600–10 cm−1) spectra of polycrystalline samples of NbS3 were investigated. A complete vibrational assignment is proposed and the band wavenumbers are compared for NbS3 and the related ZrS3 and NbS2Cl2. All the assignments have been checked by a complete valence force field calculation including intrachain, interchain and interlayer contributions. The potential energy distributions and mean square vibrational amplitudes so calculated and the values of force constants are in agreement with the two‐dimensional character of the lattice dynamics in NbS3.In addition, an extensive resonance Raman study has been performed using various exciting radiations in the visible (676.4–457.9 nm) and near‐UV regions (363.8 and 351.1 nm). The experimental unresolved Raman excitation profiles in the contour of two low‐energy electronic transitions (λmax≈︁ 600 and 560 nm) exhibit interesting interference and exact resonance effects depending on the nature of the vibration. Taking account of multi‐state interference phenomena and inhomogeneous broadening effects, these profiles compare well with the computed ones on the assumption of weakly displaced oscillators. This allows conclusions to be drawn about the scattering mechanisms and the nature of the first electronic excited states.

Interchain coulomb interactions and the neutral-ionic interface

Long-range Coulomb interactions −Vα stabilize both the neutral(n) and ionic(i) lattice against charge-transfer (CT) excitations for Madelung constants α > 1 and lead to a finite ionicity jump Δq(x) at the n-i interface of the linear array for x = ⋎ t ⋎ / V(α 1 −1) < x c = 0.33 . Interchain contributions −Vα′ lead in mean field to larger Δq for attractive ( α′ > 0) interactions and to mixed phases for α′ < 0 and x < x c. The ionicities q n and q i at the interface are sensitive to α′ even when α′ ⪡ α 1 = 2 ϵn2, but the CT spectrum at fixed q is independent in mean-field theory of interchain interactions. Interchain interactions contribute to both the n-i and the regular-dimerized (r-d) instability of TTF-Chloranil, whose structure has α′ > 0 ( < 0), respectively, in the ac(ab) planes.

Spectroscopic observations on nonequilibrium glassy poly(vinyl chloride) and polystyrene

AbstractThe relationship of valence‐coordinate deformation to the temperature dependence of some infrared peak‐absorption frequencies in Poly(vinyl chloride) (PVC) and polystyrene (PS) is stated. A skeletal band and a CH2 rocking band in PVC and a ring‐mode band in PS were studied in two kinds of experiments: steady heating and cooling of a quenched (nonequilibrium, glassy) sample through its glass‐transition temperature, Tg, and long‐term annealing of quenched samples below Tg, followed by steady heating and cooling. The results, a slope discontinuity, ΔM, in the v(T) relation at Tg and a frequency shift, Δviso, during isothermal annealing below Tg, are analyzed in two theoretical approaches. Interchain and intrachain contributions to the observed frequency shifts are expected to occur with a differing relative significance in different kinds of molecular vibrations, leading to one possible method of distinguishing valence‐coordinate deformation (chain strain) from other effects.