Short-range orientational order in polymer-solvent systems

This review presents the results on studying short-range order in polymer solutions by polarization optical, electrooptical, and spectroscopic methods. The effect of short-range orientational order on dynamic birefringence, photoelasticity, electric-field-induced birefringence, IR absorption spectra, and fluorescence polarization in polymer-solvent systems has been studied. The problem concerning the account for internal field in optical and electrooptical experiments is considered. We present the results on studying the effect of characteristics of both a polymer and a solvent on the parameters characterizing short-range orientational order in polymer solutions.     

Short-range order in some polymer melts from X-ray diffraction

Wide-angle X-ray diffraction measurements were performed on polymer melts of isotactic and syndiotactic polypropylene (IPP and SPP), poly(ethylenepropylene) (PEP), polystyrene (PS), polyisobutylene (PIB), and polyethylene (PE), to study the dependence of the short-range structure of polymer liquids on chain architecture. Total structure functions, which comprise intra- and intermolecular contributions, were derived from the scattering data. The trivial Fourier components of the intramolecular structure (C(SINGLE BOND)>C ≃ 1.54 Å and C(SINGLE BOND)C(SINGLE BOND)C ≃ 2.55 Å) were subtracted from the total structure functions. The remaining functions contain only those intramolecular contributions dependent on the chain's conformational degrees of freedom, plus the intramolecular contributions. The structural differences between the polymers in momentum space are discerned only when the trivial components are subtracted. This subtraction also reduces the effects of truncation errors on Fourier transformation to real space. The short-range structure of PIB appears very different compared to all the others, which correlates with anomalies in a number of physical properties for this polymer. © 1996 John Wiley & Sons, Inc.     

Prediction of thermodynamic properties of polymer solutions by a group-contribution method

The Flory–Huggins lattice-theory expression for solvent activity in a polymer-solution is commonly used to calculate the thermodynamic interaction parameter χ with the aid of experimental data from vapor pressure osmometry. This expression assumes that χ is independent of composition. However, experimental data for a variety of polymer-solvent mixtures indicate that χ exhibits an appreciable concentration dependence. A group contribution method, UNIFAC (UNIQUAC Functional-Group Activity Coefficients) incorporating the free-volume correction of Oishi and Prausnitz is used to predict the dependence of χ on solvent concentration. Agreement with previously reported experimental data is within 15%. Calculated values of χ obtained from the Flory–Huggins expression for solvent activity and from the corresponding Gibbs free energy of mixing (which does not assume that χ is independent of composition) are compared. Calculations based on the Gibbs free energy of mixing predict a somewhat larger value of χ relative to those based on solvent activity. The specific Gibbs free energy of mixing for polystyrene-solvent mixtures is calculated using the UNIFAC model, and is found to represent qualitatively the phase equilibrium behavior. Quantitative discrepancies are observed, however, for the polystyrene-acetone system in light of the actual experimental solubility reported by Suh and Clark (20). Most of the thermodynamic predictions for polymer-solvent systems investigated herein are correlated qualitatively with the relative mismatch between solubility parameters of both components.     

Dynamics of photoinduced orientational order of azo-dyes in polymer films

Dynamical and spectral behavior of photoassisted electrical poling and photoinduced anisotropy are studied in films of DR1-doped PMMA, DR1-PMMA copolymer and new DR1-Polyimide copolymer. The orientational mobility of DR1 is characterized both in ordering processes (angularly selective trans-cis photoisomerization) and in disordering relaxation processes. Mobility is increased by the photoisomerization and decreases slowly with time, after the end of optical pumping. The transient dichroism recorded simultaneously on six probe wavelengths shows a spectral inhomogeneity of the absorption band of DR1 and of the photoinduced anisotropy (particularly in polyimide films).     

Short-range order in miscible polymer blends: A monte carlo study

Local ordering in miscible binary polymer blends with strong attractive interactions between the two types of chains was investigated by computer simulation of allowable conformations on an incompressible cubic lattice. A tendency toward maximum ordering (in which chains pack in alternate parallel rows) is shown by the calculation of pair correlations and in “snapshots” of the mixtures. A specific directional interaction is not necessarily required for ordering. A comparison of these results with those recently obtained using mean-field theory is presented. The heat capacity of mixing was also calculated and found to be positive, with a value close to that reported experimentally.     

Polarization and orientational order in polymer ferroelectric films based on vinylidene fluoride

Phase transitions and the development of orientational order are studied for three-dimensional polymer systems with the anisotropy of local intra- and interchain orientational-deformation interactions of chains with the dipole-type potential. In the proposed model of chains composed of elastically deformed segments with a fixed mean-square length (in the modified model of Gaussian subchains), there is a certain critical temperature at which the second-order phase transition from the isotropic state to the orientationally ordered state occurs. The temperature dependences of the parameter of the dipole order for thick films are calculated, and these dependences are compared with the corresponding dependences within the mean-field approximation according to the Ising model for ferromagnetics and within the Langevin continuum model for ferroelectric materials as well as with the experimental data on the thermal depolarization in the films based on the vinylidene fluoride-trifluoroethylene copolymer. The order parameter is calculated as a function of the film thickness (the length of chains) under certain boundary conditions imposed on film ends, and the calculated values are compared with the values predicted by the phenomenological theory and with the experimental data on the polarization distribution in the ferroelectric films based on vinylidene fluoride.     

Vapor-pressure osmometry and inverse gas chromatography in the analysis of thermodynamic properties of polymer solutions

The primary objective of this work is to check the utility of vapor-pressure osmometry as an experimental technique in obtaining thermodynamic data on polymer/solvent systems. The second objective is to obtain experimental values for the exchange energy parameters ΔP, X, and ζ of the theory of Sanchez and Lacombe. With respect to the first objective, solvent activities of polyiso-butylene/benzene mixtures have been measured at several concentrations up to 600 g/kg of the solvent and at two temperatures. From the activities, it is possible to determine free energies, entropies, and enthalpies and their concentration dependence. Satisfactory agreement has been found between the results obtained and data provided by other techniques such as membrane osmometry and the Cahn electrobalance. These and other data obtained by inverse gas chromatography for the same mixture in the range 35–200°C have been used in calculating the exchange energy parameter of the lattice fluid theory at different concentrations and temperatures.     

Influence of the chain composition on the thermodynamic properties of binary and ternary polymer solutions

Thermodynamic properties of binary and ternary polymer solutions (one or two uncharged polymers in one solvent) were studied. Poly(vinyl pyrrolidone) (PVP), fully hydrolyzed poly(vinyl alcohol) (PVA) homopolymers, and water-soluble poly(vinyl alcohol-co-vinyl acetal, -vinyl propional, and -vinyl butiral) copolymers with various acetal content and chain structure, respectively, were used in the experiments. The hydrophilic/hydrophobic character of the PVA-based macromolecules and their compatibility with the PVP homopolymer were systematically regulated by changing the chemical structure of the copolymers (acetal content and/or length of side chains). The water activities in binary and ternary solutions of the chemically different polymers were determined by a gel-deswelling method developed here for ternary solutions. On the basis of the Flory-Huggins theory, the relevant solvent-segment and segment-segment pair interaction parameters (chi) have been calculated. The chi12 segment-solvent interaction parameters proved to be sensitive indicators for changes in the chemical structure of the copolymers. With increase of either the acetal content or the length of side chains in the copolymer, chi12 approached the value characteristic of a theta condition. No significant differences could be revealed in the segment-segment interaction parameters obtained for the PVP-copolymer mixtures with various acetal derivatives, when the chi12 and chi13 interaction parameters determined in binary solutions were used in the calculations for chi23. Determination of the parameters chi1,23 as suggested by Panayiotou, however, showed that increasing the acetal content or the length of the hydrophobic side chains in the copolymer resulted in a reduction in the interaction between the PVA "acetals" and PVP molecules.     

Optical properties and orientational order of deoxyribonucleic acid molecules at interfaces

A high degree of planar orientational order of DNA molecules in thin films has been discovered by the oblique-polarized-beam method. The coefficients of surface birefringence of films with different thicknesses are measured. The thickness of optically anisotropic surface layers, which are characterized by macroscopic values higher than the diameter of DNA molecules by three orders of magnitude in terms of thickness, is determined. A difference in the main polarizabilities of the monomer unit of DNA, which is estimated for the films, is compared with the corresponding value for the solution measured by flow birefringence. The effect of temperature on the coefficient of birefringence of the films is examined. A comparison of the data on the native and denatured DNA shows that the degree of orientational order for the single-helix structure is smaller than that for the double helix.     

Unified analysis of thermodynamic and rheological properties of high polymer solutions. II. Ternary systems

We extend to ternary solutions our previous study of conformational, thermodynamic, and rheological properties of semidilute polymer solutions in good solvent. Osmotic pressure and viscosity measurements were performed in several mixtures of two compatible polymers in a common solvent. Renormalization group results were used to analyze the data, using de Gennes's blobs model to connect dynamic and conformational properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1071–1079, 2002     

Unified analysis of thermodynamic and rheological properties of high polymer solutions. I. Binary systems

We present a comprehensive experimental study of thermodynamic and rheological properties of semidilute polymer solutions in good solvent. Osmotic pressure and viscosity measurements have been done in several polymer‐solvent systems at different temperatures. A renormalization group technique was applied to analyze the data using de Gennes's blobs model to connect dynamic and conformational quantities. The behavior of polymer systems in the whole range from dilute to semidilute solutions can be satisfactorily described using only a few nonuniversal quantities experimentally determined. An adequate agreement between experiments and theory was found, showing universal behavior with a system‐dependent constant β that does not depend on molecular weight or concentration. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 290–301, 2002     

Microscopic theory of orientational order, structure and thermodynamics in strained polymer liquids and networks

A microscopic integral equation theory of the segmental orientational order parameter, structural correlations and thermodynamics of strained polymer solutions, melts and networks has been developed. The nonclassical problem of the consequences of intermolecular excluded volume repulsions and chain connectivity is addressed. The theory makes several novel predictions, including effective power law dependences of the orientational order parameter on monomer concentration and chain degree of polymerization, and strain hardening of the bulk modulus. The predictions of a nearly classical strain dependence, and supralinear scaling with segment concentration, of the strain-induced nematic order parameter is in agreement with nuclear magnetic resonance experiments. The absolute magnitudes of the a priori calculated orientational order parameter agree with simulations and experiments to within a factor of 2. The possible complicating influence of "trapped entanglements" in crosslinked networks is discussed. Extensions of the theory are possible to treat the mechanical response of flexible polymer liquids and rubbers, and the structure, thermodynamics, and mechanical properties of strained liquid crystal forming polymers.     

聚电解质溶液热力学性质的研究

本文采用柱形胞腔模型以及Vink的近似方法, 求解聚电解质溶液的Poisson-Boltzmann方程, 得到不同条件下聚离子周围静电势的分布。进而得到了不同条件下聚离子、反离子和同电荷离子的活度系数及溶剂的渗透系数。所得结果与实验值能较好地吻合。     

The molecular-level relationship between the properties of liquid water molecules and orientational order

The relationship between the orientational (tetrahedral) order (q) of an individual liquid water molecule and its various properties such as Voronoi volume, potential energy, kinetic energy, and nearest neighbors was thoroughly examined using molecular dynamics simulations of TIP5P model at 278, 298, and 318 K. By constructing Voronoi polyhedra (VP), we found that the average volume of water molecules classified according to q decreased monotonically as q increased, while the surface of VP increased in the range of high q. Kinetic energy was almost invariant but potential energy decreased monotonously as q increased. The volumes of molecules having a very large q increased as temperature decreased, implying a possible density maximum phenomena. Using time correlation functions, it was shown that total energy rather than potential energy was a more significant factor in the determination of the orientational order. With varying temperature, the relation between the properties of central molecules and those of nearest neighbor molecules were investigated. It required a very systematic cooperative motion to obtain LDL (low-density liquid) formed by ordering. It was supposed that density maximum phenomena should be accomplished by the growth of LDL and HDL (high-density liquid) of low quality with a consistent population of HDL and a drop of LDL of high quality as temperature lowered.     

Calorimetric effects of short-range orientational order in solutions of benzene or n-alkylbenzenes in n-alkanes

A Picker flow microcalorimeter was used to determine molar excess heat capacities, C^E_p, at 298.15 K, as function of concentration, for the eleven liquid mixtures: benzene+n-tetradecane; toluene+n-heptane, and +n-tetradecane; ethylbenzene+n-heptane, +n-decane, +n-dodecane; and +n-tetradecane; n-propylbenzene +n-heptane, and +n-tetradecane; n-butylbenzene+n-heptane, and +n-tetradecane. In addition, molar excess volumes, V^E, at 298.15 K, were obtained for each of these systems (except benzene+n-tetradecane) and for toluene+n-hexane. The excess volumes which are generally negative with a short alkane, increase and become positive with increasing chain length of the alkane. The excess heat capacities are negative in all cases. The absolute ¦C^E_p¦ increased with increasing chain length of the n-alkane. A formal interchange parameter, C_p12, is calculated and its dependence on n-alkane chain length is discussed in terms of molecular orientations.     

Evaluation of unperturbed and thermodynamic parameters of polymer solutions from viscometric data

The existing approximate theories of polymer solutions are generalized by a simple equation. It is combined with the Kirkwood- Riseman perturbation treatment of viscosity together with the Kuhn model of random coil dimensions to produce two expressions for the unperturbed and thermodynamic parameters of polymers in solvents. A test of these equations on a large variety of polymer systems yields results which are comparable with the experimental values.     

Comparison of thermodynamic and rheological properties of binary polymer mixtures

The dependence of viscosity on concentration for a series of binary mixtures of polymers differing in their chemical nature and for mixtures of polymer homologues was compared with the peculiarities of their phase diagrams as well as with the concentration dependence of their thermodynamic interaction parameters. A definite correlation was found between the composition dependence of the viscosity and the thermodynamic state of the system in the melt at a given temperature. The universal character of this correlation was shown both for the mixtures of various polymers and for those of the polymer homologues.