聚芴共轭聚合物在稀薄溶液中形态变化的静态光散射研究

PF8聚芴共轭高分子材料是应用于制作蓝色光激发光器件的一种有潜力功能材料, 故本文利用静态光散射技术研究了PF8聚芴共轭高分子在稀薄甲苯溶液中的形态结构变化特点。研究结果显示, PF8共轭高分子在甲苯稀薄溶液中单一分子链的持续长度与Kuhn链结长度均明显受到温度的影响。这表明在较高温度下高分子与溶剂间的相互作用力较大, 导致其呈现较伸展的虫状形态结构; 而在较低温度下高分子与溶剂间相互作用力较小, 导致其呈现收缩的结构形态。因此在较高温度状态下PF8共轭高分子应该具有比较刚硬的特征。     

Surface diffusion dynamics of a single polymer chain in dilute solution

Comprehensive three-dimensional dissipative particle dynamics simulations are carried out to elucidate the diffusion mechanism of a strongly adsorbed polymer chain on a solid surface in dilute solutions. We find Rouse and reptation dynamics for polymer chain diffusing on smooth and rough surfaces (with obstacles or sticking points), respectively. Combining with scaling analysis, we find that the interactions between the surface and the fluid screen the hydrodynamic interaction. The different scaling as found for a polymer chain diffusing on a fluid membrane [Phys. Rev. Lett. 82, 1911 (1999)] and on a solid surface [Nature (London) 406, 146 (2000)] may be explained by the solid surface inhomogeneity that induces reptation.     

Electro-optical properties of dilute solutions of flexible molecules ; the wormlike chain model

A theory of propagation and scattering of light in dilute solutions of flexible chain molecules is developed. The solvent is represented as a continuous isotropic dielectric medium. The polymer molecules are modelled as dielectric objects which may have an anisotropic dielectric tensor. The theory, applied to the wormlike chain model, represents the polymer molecules as flexible cylinders of constant cross section and with an isotropic internal dielectric constant. The increment in the refractive index of the solution and the isotropic and anisotropic light scattering intensities due to the presence of the chain molecules are derived. The numerical results of our model for the depolarization ratio as a function of the contour length and the persistence length of the polymer and of the refractive index of the solvent are presented and compared with the corresponding results for the popular bond additive approximation (BAA). It is found that the differences between our model and this approximation are due to the neglect of intramolecular dipole-induced dipole interactions in the BAA.     

A comprehensive molecular dynamics study of a single polystyrene chain in a good solvent

In this study, molecular characteristics of polystyrene (PS) was calculated measuring its dilute-solution properties in toluene at 288.15 K via molecular dynamics (MD) simulations. The solution models consisted of PS chains with different number of repeating units all of which were in a dilute regime. In order to investigate the compatibility between the polymer and the solvent molecules, interaction energy and Flory-Huggins (FH) interaction parameter were estimated. The simulation results indicate that increasing the chain repeating units enhanced the interaction between the solute and the solvent. Additionally, the chain dimensions were evaluated calculating the radius of gyration (R_g) and end-to-end distance, r0. To determine the dynamic behavior of the chains in the solutions, mean square displacement (MSD) and diffusivity coefficient were calculated. The simulation results indicated that the chain rigidity at low molecular weight and chain flexibility with increasing the molecular weight influenced chains dynamic behavior and diffusivity. Moreover, radial distribution function (RDF) illustrated the effect of steric hindrance of the chains in dilute solution on capturing the solvent molecules. In addition, solution viscosity was calculated by performing non-equilibrium molecular dynamics simulation (NEMD). The obtained results of chain characteristics and viscosity showed a good agreement with experimental results published previously. This agreement confirms the accuracy of the applied simulation method to characterize the dilute solutions and the chains characteristics.     

High intrachain hole mobility on molecular wires of ladder-type poly(p-phenylenes)

We have studied the high-frequency (34 GHz) mobility of positive charge carriers on isolated ladder-type polymer chains in dilute solution. We find that the high-frequency mobility is limited by the chain ends on chains as long as 35 monomers. The intrachain motion of charge carriers can be described by one-dimensional diffusion between infinitely high reflecting barriers, representing the chain ends. Our data indicate that the intrachain mobility for ladder-type polymer chains is close to 600 cm(2)/V s. With this high mobility the ladder-type polymer is a promising candidate for future use as an interconnecting wire in molecular electronics.     

Polymer collapse in the presence of hydrodynamic interactions

We investigate numerically the dynamical behaviour of a polymer chain collapsing in a dilute solution. The rate of collapse is measured with and without the presence of hydrodynamic interactions. We find that hydrodynamic interactions both accelerate polymer collapse and alter the folding pathway.     

Degree of swelling and extent of crowding of flexible polymer coils in low concentration region

At infinite dilution, a flexible polymer chain is an isolated coil swollen with solvent. The situation may be treated in a manner analogous to the swelling equilibrium of a lightly crosslinked network. From the thermodynamic relation, the degree of swelling of the polymer coil may be estimated (1, 2). This value, the degree of chain extension, was found to be somewhat larger than that estimated from intrinsic viscosity. This finding was interpreted to mean that the chain segments in the periphery of the coil are more extended than the average coil extension (1). In the present work the dilute but finite concentration range was examined. In this case a polymer coil is in equilibrium with solvent containing polymer chains of the same kind. As the concentration is increased, the excess extension of the chain segments in the periphery of the coil diminishes. With further concentration increase, the polymer coil continues to decrease in size to the size defined by the theta condition, when the free solvent disappears. At that concentration adjacent polymer coils begin to interpenetrate. This behavior is described quantitatively. The input data required for this calculation are the intrinsic viscosity of the polymer-solvent system of interest, that of the polymer at the theta condition, and the thermodynamic interaction parameter as a function of temperature, concentration, and molecular weight of the polymer. Thermodynamic equations and their derivations are described.     

Dissipative particle dynamics simulations of polymer chains: scaling laws and shearing response compared to DNA experiments

Dissipative particle dynamics simulations of several bead-spring representations of polymer chains in dilute solution are used to demonstrate the correct static scaling laws for the radius of gyration. Shear flow results for the wormlike chain simulating single DNA molecules compare well with average extensions from experiments, irrespective of the number of beads. However, coarse graining with more than a few beads degrades the agreement of the autocorrelation of the extension.     

Hybrid MC/RISM technique for simulation of polymer solutions: Monte Carlo+ RISM integral equations

A new (hybrid) method is reported for modelling complex macromolecular systems. The approach combines the traditional atomistic Monte Carlo (MC) computer simulation of flexible polymer chains with the numerical solution of the site-site Ornstein-Zernike-like (RISM) integral equations. The method is used for calculating properties of a linear polymer in dilute solution. Since the condensed-phase environment of a flexible macromolecule affects the equilibrium configuration probability distribution of the macromolecule, the site-site intramolecular correlation function and the intramolecular potential field are treated in a self-consistent manner. Briefly, the MC method is applied to generate the configurations of a single chain molecule. Using the coordinates of chain beads, the averaged intrapolymer correlation function is obtained. Then, solving the coupled RISM equations for a given density of solvent particles, we find the polymer-solvent correlation functions. This yields the medium-induced intrapolymer potential and the corresponding effective intramolecular energies, which are used in the standard Metropolis MC procedure. The structural properties of the polymer chain are computed by averaging over the statistically representative set of configurations. As a result of many such iterations, the intramolecular structure is determined self-consistently. Using the hybrid MC/RISM method, extensive studies have been made of static properties of flexible polymer chains surrounded by LJ particles with purely repulsive interactions between the particles and chain beads. Also, direct molecular dynamics simulations have been carried out and have demonstrated that the hybrid MC/RISM approach gives a quite accurate prediction for condensed-phase effects.     

Nonuniform extension of chain segments of a polymer coil in dilute solution

An isolated, flexible polymer chain in dilute solution assumes a random configuration. In reality such a polymer coil is not completely random, because an excluded volume effect requires a modification from randomness. In addition, the coil placed in a better solvent is more expanded than that in a poor solvent. In other words, the polymer chain segments in a better solvent are more extended, compared to those in a neutral solvent. In the present study we have discovered an indication that the chain segments lying on the periphery of the coil are more extended than those in the interior. This discovery has been made in the course of relating intrinsic viscosity to thermodynamic interaction between polymer and solvent. The resulting relationship provides a means of evaluating the thermodynamic interaction parameter from the measured value of intrinsic viscosity, if the molecular weight of the polymer, its intrinsic viscosity in a neutral solvent, and the degree of excess extension of chain segments are known. The last is the parameter discovered in this work. This observation is independent of particular thermodynamic theory, so long as the experimental data are used in a consistent manner. In this work Maron's theory was used, because it is applicable to infinite dilution as well as concentrated solutions.     

Determination of the adsorbed layer thickness of polystyrene on glass surface by viscosity measurements to the extremely dilute concentration region

A newly developed theory regarding solute adsorption effect in viscometry of a polymer solution is reviewed and extended for determining the adsorbed layer thickness of polystyrene on a glass surface in viscosity measurements. This theory can adequately describe the concentration dependence of the viscosity of a polymer solution measured by glass capillary viscometer to the extremely dilute concentration region. Using this theory, both the free polymer chains in solution and the adsorbed polymer chain on the viscometer inner wall surface can be characterized. Examples are given for the characterization of six polystyrene samples with different molar masses by measuring their solution viscosity in benzene.     

The utility of resonant soft x-ray scattering and reflectivity for the nanoscale characterization of polymers

The utility of resonant soft x-ray scattering (RSoXS) and reflectivity (RSoXR) is extended and exemplified through the characterization of thin films of polymers relevant to organic solar cells and of dilute polymer solutions. RSoXS and RSoXR are methods that utilize anomalous scattering principles at soft x-ray energies. Soft X-rays cover the carbon, nitrogen and oxygen absorption edges, elements very relevant for polymers and colloids. The rapid changes of optical properties near these absorption edges provide selectivity to specific moieties and high contrast. RSoXR is shown to be a powerful tool for the characterization of bilayers of conducting polymers. The RSoXR results point to an interesting strategy that will allow the chemical interdiffusion and physical roughness at a buried polymer/polymer interface to be determined independently. The high scattering cross sections also allows the investigation of thin films of conjugated polymer blends in transmission at thicknesses for which hard X-rays or neutrons would yield relatively little scattering. By scattering at photon energies that provide strong scattering contrast, even very dilute polymeric solutions yield a useable signal.     

Exciton formation with interchain couplings in organic polymers

Based on the framework of the tight binding approach and the nonadiabatic dynamics, the formation of an exciton in inter-coupled polymer chains is studied. Both a localized exciton in one single chain and a spread exciton between chains are obtained. It is found that an excited electron-hole pair is more inclined to evolve into a localized exciton, and the long range Coulomb e-e interaction is favorable to the exciton formation. By calculating the formation time of an exciton, we show that the optical response time is faster in a dilute solution than that in a solid film of polymer molecules.     

A molecular dynamics study of ion migration in a doped electroactive polymer

Simulating the \hbox{BF}_{4}^{-} -doped poly(3-octylthiophene) lattice by molecular dynamics results in a structure in which dopant ions intercalate as a sandwich between thiophene rings on adjacent polymer chains. The ions occupy sites in channels parallel to the polymer main chain, which retains a high degree of planarity in contrast to the pristine (undoped) polymer. Even when lattice imperfections are created by expanding the cell, Coulomb forces ensure that the intercalation features containing the dopant channels are largely retained. On applying electric fields in the principal directions of the 'perfect lattice' it is found that the ions migrate most readily along the ion-channel directions (the lattice c axis), leaving the lattice undisturbed. Although higher electric fields cause dopant migration to occur perpendicular to the channel directions they destroy the intercalated lattice. In the reduced-order lattice regions substantial motion of the ions are predicted at a critical value of the lattice parameter.     

Jamming and crystallization in athermal polymer packings

Dense packings of chains of hard spheres possess characteristic features that do not have a counterpart in corresponding packings of monomeric spheres especially near the maximally random jammed (MRJ) state. From the modelling perspective the additional requirement that spheres keep their connectivity while maximizing the occupied volume fraction imposes severe constraints on generation algorithms of dense chain configurations. The extremely sluggish dynamics imposed by the uncrossability of chains precludes the use of deterministic or stochastic dynamics to generate all but dilute polymer packings. As a viable alternative, especially tailored chain-connectivity-altering Monte Carlo (MC) algorithms have been developed that bypass this kinetic hindrance and have actually been able to produce packings of hard-sphere chains in a volume fraction range spanning from infinite dilution up to the MRJ state. Such very dense athermal polymer packings share a number of structural features with packings of monomeric hard spheres, but also display unique characteristics due to the constraints imposed by connectivity. We give an overview of the most relevant results of our recent modeling work on packings of freely-jointed chains of tangent hard spheres about the MRJ state, local structure, chain dimensions and their scaling with density, topological constraints in the form of entanglements and knots, contact network at jamming, and entropically driven crystallization.     

Irreversibility and polymer adsorption

Physisorption or chemisorption from dilute polymer solutions often entails irreversible polymer-surface bonding. We present a theory of the resultant nonequilibrium layers. While the density profile and loop distribution are the same as for equilibrium layers, the final layer comprises a tightly bound inner part plus an outer part whose chains make only fN surface contacts where N is chain length. The contact fractions f follow a broad distribution, P(f) approximately f(-4/5), in rather close agreement with strong physisorption experiments [H. M. Schneider, Langmuir 12, 994 (1996)]].     

Chains at interfaces

Simha, together with Frisch and Eirich, was among the first to realize that in energetic interaction with a surface a flexible polymer chain would be under a major conformational constraint which could be analyzed by solving a random walk (diffusional) problem near a barrier. These results stimulated more work, and today we have a fairly good knowledge of the factors which determine polymer adsorption from very dilute solution. Conformational problems arise also when more concentrated systems are used and it has become apparent that as concentration increases the surface phase will, more and more, be scaled by the root mean square diameter of the undeformed macromolecule. One can view the presence of an energetically attractive interface as creating a sink for macromolecules. The closer a polymer solution is to its Θ point, the more easily the polymer in solution can enter the sink and the stronger will be the extent of adsorption. The presence of an interface can thus be seen as a means to produce a phase separation in a polymer system which would otherwise be stable. Coatings of adsorbed polymers over colloidal particles create “macromolecular” solutions, where the “macromolecule” is the colloid particle with its adsorbed coat. This causes polymer segment/polymer segment contacts to dominate the encounter energetically. Hence, the solution behavior of the coated colloid becomes reminiscent of the solution behavior of the polymer in the coat. Stabilization by adsorbed polymer layers derives from converting the original solubility characteristics of the colloid, from what they were, essentially to those of the polymer being used.     

Semi-flexible interacting self-avoiding trails on the square lattice

Self-avoiding walks self-interacting via nearest neighbours (ISAW) and self-avoiding trails interacting via multiply-visited sites (ISAT) are two models of the polymer collapse transition of a polymer in a dilute solution. On the square lattice it has been established numerically that the collapse transition of each model lies in a different universality class.     

Kinetics of water flow through a polymer gel

The water flow through the poly(acrylamide) gel under a constant water pressure is measured by newly designed apparatus. The Young modulus and Poisson’s ratio of the rod shape gels are measured by the uni-axial elongation experiments, which determine the longitudinal modulus independently from the water flow experiments. The time evolution of the water flow in the dilute gel is calculated based on the collective diffusion model of the polymer network coupled with the friction between the polymer network and the water. The calculated results are compared with the time evolution of the flow experiments, and the values of the longitudinal modulus and the friction coefficient are estimated. The estimated values are consistent with the results of our mechanical-response experiments and the light scattering experiments reported previously. We find that the time evolution of the water flow is well described by a single characteristic relaxation time predicted by our model for dilute gels.     

Fractionation and Crystal Morphology of Rigid Polymer,Poly(p‐Phenylene Benzobisthiazole)

Abstract

Fractionation of the rigid polymer, poly(p‐phenylene benzobisthiazole) (PBZT), was carried out in dilute solution in concentrated methane sulfuric acid (MSA) using silica gels as packing material of a column. Several combinations of the average chain length of the fractionating materials and the average pore diameter of the gels were examined to improve fractionation resolution. The gels with average pore diameter near the average chain length resulted in high fractionation resolution. Single crystals of the fractionated and unfractionated PBZTs were observed by transmission electron microscopy (TEM). Both single crystals were fundamentally composed of rod crystals with the chain orienting normal to the rods. The unfractionated PBZT made a cluster of parallel rod crystals, where longer chains penetrated a few rod crystals leaving their chain ends within the crystalline core. On the contrary, with the fractionated polymer, extended‐chain rod‐like crystals were dispersed, isolated from each other. This morphology enables us to estimate the chain length visibly by TEM, for which a few milligrams of the material is enough for the observation.