Energetics of phase separation in aqueous solutions of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide)

The energetics of phase separation in dilute aqueous solutions of poly(N-isopropylacrylamide) is studied by high-sensitivity differential scanning calorimetry. The temperature dependences of the partial heat capacity of the polymer are obtained. The effect of the heating rate on their shape is examined. The concentration dependences of thermodynamic parameters of phase transition are determined. After phase separation of the system, the partial heat capacity of the polymer is much smaller than its partial heat capacity in the state of a swollen random coil. This finding indicates the occurrence of the polymer hydrophobic structure in the concentrated phase of the system probably in the form of clusters of monomer units. The profile of transition is described by the Schröder-van Laar equation with the van’t Hoff enthalpy independent of polymer concentration. The size of the cooperative unit of the ordered hydrate structure of the polymer is estimated and found to be coincident with the size of the Kuhn segment. High-velocity sedimentation measurements of the polymer are conducted at various temperatures both below and above the binodal curve of the system. It is shown that the sizes of poly(N-isopropylacrylamide) macromolecules before the phase transition temperature and in the diluted phase after phase separation of the system coincide. Thus, in the diluted phase, macromolecules retain the coil conformation.     

Influence of hydrophobe content and salt concentration on dilute solution behaviour of hydrophobically modified ionic polymers from diallylammonium salts/sulfur dioxide cyclocopolymerization: Light scattering and fluorescence spectroscopy

Novel hydrophobically modified polyelectrolytes were synthesized using the cyclocopolymerization of sulfur dioxide, N,N-diallyl-N-carboethoxymethylammonium chloride and the hydrophobic monomer N,N-diallyl-N-octadecylammonium chloride. Aggregation of these polymers in aqueous solutions was characterized in the dilute regime by static light scattering and fluorescence spectroscopy as a function of hydrophobe content and NaCl concentration. Copolymers were observed to associate at very low concentrations (0.005 wt%). The copolymer is capable of associating at this very low polymer concentration because of the extended length of the hydrophobic monomer (C₁₈) that can reach far enough from the backbone to avoid electrostatic repulsion. Aggregation of the polymers increased with increasing hydrophobe content. Upon addition of salt, the apparent molecular weight of polymer aggregates decreased as a result of neutralization of the charges. At high salt concentrations, the size of the polymer aggregates was observed to increase again as a result of increased polarity of the solvent that resulted in more hydrophobic association.     

Polyampholyte solutions in the presence of salt ions having different valences: Computer simulation

The properties of dilute solutions of regular polyampholytes in the presence of a low-molecularmass salt were studied by means of stochastic dynamics simulation. The number-average concentration of polymer units in solution did not exceed 0.038, a value that is below the coil overlap concentration at N= 1024. The effect of charge (valence) of ions of an added salt on the macromolecule size and distribution of the salt inside the polymer coil at different temperatures was considered. It was found that the condensation of salt ions over the temperature range examined (1 ≤T≤2ε/k _B) takes place in the case of multiply charged systems of the (+4?4) or (+4?2) type. The maximal penetration of salt ions into the volume of the macromolecule was observed in the case of a (+2?2) salt.     

Dielectric polarization of a partially neutralized poly(methacrylic acid) in dilute aqueous solutions: Conformational transition

Dielectric permittivity ε′ (at a frequency of 1 MHz) of aqueous solutions of a partially neutralized poly(methacrylic acid) Na-PMAA was studied with varying the degree of ionization α_i from 0 to 0.78 and concentrations w ₂ (g/g) from 1.25 × 10^?4 to 6 × 10^?3 at 25°C. It was shown that the concentration dependence of ε′ for Na-PMAA aqueous solutions at all α_i values contains two regions corresponding to different changes in ε′, and the above regions are separated by the crossover point w*₂. In the first region, ε′ increases with the increasing concentration Δε′/Δw ₂ > 0; in the second region, ε′ decreases with the increasing concentration Δε′/Δw ₂ < 0. The sign inversion of Δε′/Δw ₂ is explained by different structures of water and mechanisms of hydration in regions I and II. In the region corresponding to the ascending branch of the ε′-w ₂ curve, the dipole moments of macromolecules per repeating unit, μ = (〈M ²〉/N)^1/2, were calculated according to the Buckingham theory. The μ value is high; as a macromolecule is saturated with ionized units, this value nonmonotonically changes from ～10 D at α_i = 0 to ～18 D at α_i ～ 0.24. This dependence peaks at ～30 D at α_i ～ 0.1. As is assumed, (〈M ²〉/N)^1/2 depends on the vector sum of partial dipole moments of hydrated nonionized monomer units (μ ～10 D) and fluctuation-induced dipole moments of ionized monomer units. The profile of μ as a function of the degree of ionization of Na-PMAA indicates the occurrence of the conformational transition in an individual macromolecule. This transition takes place at the degree of ionization α_i ～ 0.1. An analysis of molecular interactions in the (ionized coil)-solvent system explans the conformational transition in Na-PMAA.     

Mononuclear rearrangements of heterocycles in water/β-CD: information on the real site of reaction from structural modifications of substrates and from proton concentration dependence of the reactivity

The effect of β-cyclodextrin (β-CD) on the reactivity in the base-catalyzed pathway for the rearrangement in water of some (Z)-hydrazones of 3-benzoyl-1,2,4-oxadiazoles (1b-f) into the relevant triazoles (2b-f) was investigated, finding different behavior as a function of the proton concentration. ESIMS and ¹H NMR data evidence the formation of host-guest complexes. The whole of the experimental and calculated (MM2) data enabled us to draw some intriguing conclusions concerning the influence of the structures of the substrates and the nature of the formed host-guest complexes on the real site of the reaction.     

ATR IR spectra and structure of boundary layers of atactic polystyrene

The ATR IR spectra of solutions of atactic polystyrene in chloroform at the boundary with KRS-5 glass in the ranges 2800?C3100 and 1550?C1650cm^?1 are studied. It is shown that the parameters of orientation ?? and polarizability anisotropy ???? _p ? ???? _s of macromolecules estimated from the absorption and refractive indexes and the average complex polarizability of polystyrene boundary layers on KRS-5 vary in an extremum manner with the concentration of polystyrene in a solution. For dilute polystyrene solutions, a high degree of orientation of chains adsorbed on the glass surface is found. The inversion of the sign of polarizability anisotropy ???? _p ? ???? _s and an increase in orientation parameter ?? of macromolecules in boundary layers on KRS-5 are associated with the compression and interpenetration of coils during the transition from semidilute to concentrated solutions of polystyrene.     

Dipole moment and conformational properties of poly(methacrylic acid) in solvents with hydrogen bonding

Dielectric polarization of solutions of un-ionized linear poly(methacrylic acid) in polar associated liquids is studied in the temperature range 20–50°C. The solutions are in methanol, with the molar fraction of polymer units x ₂ = (3 × 10^?3)?(1.5 × 10^?2), and in water, with x ₂ = (4 × 10^?5)?(4 × 10^?3). The permittivity ε₁₂ of the polyacid solutions in methanol is shown to be lower than the permittivity of the pure solvent ε₁; the permittivity of the polyacid solutions in water exceeds ε₁ of water in the concentration range x ₂ = (4 × 10^?5)?(2.13 × 10^?4) and becomes lower than ε₁ as the polymer concentration in the solution increases further. A procedure for estimating the dipole moment μ₂ per monomer unit of the polymer macromolecule in solution is proposed. The estimation is based on Buckingham’s statistical polarization theory for a two-component mixture of polar molecules under the conditions of infinite dilution. The μ₂ values amount to 2.76–2.14 D (x ₂ < 1.5 × 10^?2) in methanol at 20–50°C and to 11.4?3.8 D (x ₂ < 2.13 × 10^?4) in water at 20–40°C. The difference in the dipole moments of the polyacid and in the patterns of their temperature dependences in methanol and in water is due to the effects of the polyacid-solvent hydrogen bonding, to intramacromolecular hydrogen bonds, and to specificity of the local structure of the solvent. It is shown that the μ₂ value corresponds to the dipole moment of the solvates and decreases with temperature owing to changes in the stoichiometry of the solvates, to the formation of cyclic associates in the macromolecule, and to conformational changes in the chain.     

Three new olanzapine structures: the acetic acid monosolvate,and the propan‐2‐ol and propan‐2‐one hemisolvate monohydrates

The crystal structures of three new solvates of olanzapine [systematic name: 2‐methyl‐4‐(4‐methylpiperazin‐1‐yl)‐10H‐thieno[2,3‐b][1,5]benzodiazepine], namely olanzapine acetic acid monosolvate, C₁₇H₂₀N₄S·C₂H₄O₂, (I), olanzapine propan‐2‐ol hemisolvate monohydrate, C₁₇H₂₀N₄S·0.5C₃H₈O·H₂O, (II), and olanzapine propan‐2‐one hemisolvate monohydrate, C₁₇H₂₀N₄S·0.5C₃H₆O·H₂O, (III), are presented and compared with other known olanzapine forms. There is a fairly close resemblance of the molecular conformation for all studied analogues. The crystal structures are built up through olanzapine dimers, which are characterized via C—H...π interactions between the aliphatic fragment (1‐methylpiperazin‐4‐yl) and the aromatic fragment (benzene system). All solvent (guest) molecules participate in hydrogen‐bonding networks. The crystal packing is sustained via intermolecular N_host—H...O_guest, O_guest—H...N_host, O_guest—H...O_guest and C_host—H...O_guest hydrogen bonds. It should be noted that the solvent propan‐2‐ol in (II) and propan‐2‐one in (III) show orientational disorder. The propan‐2‐ol molecule lies close to a twofold axis, while the propan‐2‐one molecule resides strictly on a twofold axis through the carbonyl C atom. In both cases, the water molecules present positional disorder of the H atoms.     

Self-organization of sulfonated polystyrene ionomers with various contents of ionogenic groups in toluene: Neutron scattering study

The structural organization of sulfonated polystyrene-based ionomers containing different amounts of sodium sulfonate groups (SO₃Na) in the concentration range from 0.5 to 5.8 mol % in deuterotoluene has been studied by cold neutron scattering. An analysis of molecular correlations shows that ionomer chains retain the Gaussian coil conformation even at a degree of sulfonation of 5.8 mol %, when the distance between ionogenic groups along a chain is comparable with the Kuhn segment length. At a content of SO₃Na groups of 1.35 mol %, the mutual penetration of polymer chains predominates and is accompanied by the formation of associates including up to seven macromolecules (effective chains) and functioning as main structural elements of the system. At a higher content of SO₃Na groups, the mutual penetration of macromolecules becomes weaker owing to the enhanced intramolecular association of ionogenic groups. As a result, the effective chains with sizes smaller than the size of the PS coil arise. These chains are composed of 1–2 ionomer macromolecules and are compacted structures in which the concentration of PS units is a factor of 4 higher than the concentration of units in the nonsulfonated PS coil. The self-organization of such particles in toluene yields secondary structures in the form of clusters composed of four to five effective chains.     

Hydrodynamic, electrooptical, and conformational properties of fullerene-containing poly(2-vinylpyridines) in solutions

For C₆₀ fullerene-containing poly(2-vinylpyridines) synthesized by anionic polymerization, the molecular mass and hydrodynamic size of macromolecules in solutions have been determined by molecular hydrodynamics (translational diffusion and viscometry) and electrooptics in dilute benzene and THF solutions. Under the same conditions in the molecular mass range (9.8–123) × 10³, the hydrodynamic behavior of linear poly(2-vinylpyridines) and their molecular-mass dependences have been examined and the conformational characteristics of macromolecules have been established. The branching of macromolecules has been characterized by comparing the properties of star-shaped fullerene-containing and linear poly(2-vinylpyridines). With consideration of the hydrodynamic data interpreted within the framework of regular star model, it is inferred that on average three to four linear polymer chains with a molecular mass of (8 ± 3) × 10³ for each chain are attached to a fullerene core of C₆₀ in molecules of fullerene-containing poly(2-vinylpyridines). The specific Kerr constant of fullerene-containing poly(2-vinylpyridines) in dilute benzene solution is ?(14 ± 1) × 10^?12 cm⁵/[g (300 V)²]. As evidenced by the electrooptical data, the incorporation of fullerene into the polymer weakens self-association of macromolecules in solution.     

Sizes and conformations of hydrophilic and hydrophobic polyelectrolytes in solutions of various ionic strengths

The hydrodynamic characteristics of macromolecules of a random copolymer of N-methyl-N-vinylacetamide and N-methyl-N-vinylamine hydrochloride containing 43.6% charged units in the molecular-mass range of 27 × 10³ to 355 × 10³ are studied. For solutions in 0.2 M NaCl, sedimentation and translational-diffusion coefficients are determined. For salt-free solutions and for solutions in 0.2 and 5.0 M NaCl, the intrinsic viscosities of the fractions are found. The lengths of the statistical segments of the chains are estimated in terms of the Gray-Bloomfield-Hearst theory. The behavior of the polycation, whose uncharged counterpart is a hydrophilic polymer, is compared to the behavior of poly(sodium 4-styrenesulfonate), whose uncharged counterpart is a hydrophobic polymer. The comparison is based on normalized scaling relations. It is shown that the level of compaction of macromolecules of strong polyelectrolytes at a high ionic strength is determined by the degree of hydrophobicity of their polymer chains. Polyelectrolytes based on hydrophilic polymers cannot be compacted into a preglobular state; their chains preserve a swollen coil conformation up to maximally high values of ionic strength.     

Voltammetric properties of electrodes modified by poly-[N-(9,10-anthraquinone-2-carbonyl)ethylenimine] and used in aqueous solution

Three differently loaded anthraquinone polymers have been prepared by condensation of 2-anthraquinonecarbonylchloride and poly(ethylenimine). They have been adsorbed on vitreous carbon or mercury electrodes by dipping the electrode in a solution of pyridine or methylene chloride containing 0.01–0.05% polymer. The influence of adsorption parameters and of electrochemical variables on the voltammetric behavior of the polymer-coated electrodes in aqueous buffer solutions is described. Changing the dip time in pyridine from 30 s to 5 min and changing the polymer concentration have very little effect on the apparent coverage (=1.3-1.8×10^?9 mol cm^?2) and peak shape. In contrast, an increase of the loading (the number of monomer units loaded with quinone) or the use of methylene chloride as a dip-coating solvent instead of pyridine affected the shape of the cathodic and anodic peaks which broaden and tend to separate. The peak shape is characterized by a tailing which tends to disappear at slow scan rate. The modifications of the peak shape and position when the scan rate is changed, have been shown to fit qualitatively with a proposed polylayer model in which electron (proton) transfer to the sublayer nearest the underlying conductor is slow. It has been observed that the weak acid dissociation constants for the dihydroanthraquinone and hydroanthraquinone anion are both different for the corresponding monomer in solution (pK_A=8.65 and 11.6) and the adsorbed polymer (pK_A=9 and 13).     

Dependence of Polymer Chain Entanglements on the Solution Concentration

For the viscometric determination of molecular weights of polymers, sufficiently dilute solutions have to be used so that entanglements of the polymer chain are absent. The concentration of the polymer should be such that the relative viscosity (η_r) lies in the range 1.1–1.5 [1]. Similarly, for molecular weight determination by light scattering, the suggested concentration for polymer with weight-average molecular weight ( M _w ) > 10⁵ is 0.5 wt%; for those with M _w < 10⁵, up to 1% may be used [2].

The limits of polymer concentration for such measurements are not clearly known. On dissolution, the polymer molecules adopt a more or less extended configuration whose shape depends on the structure and molecular weight of the polymer, the properties of the solvent, and the temperature

[3]. The molecules of flexible linear polymers acquire a coiled configuration due to free rotation about the C-C bonds. When a dilute solution satisfies theta conditions, the polymer molecules are free from all kinds of interaction and move freely. Then their solution properties could possibly be related to their end-to-end distance. Based on this concept, our attempt to establish the permissible limits of polymer concentration for dilute solutions of several polymers of different molecular weights is reported here.     

Conformational characteristics of polycaproamide in dimethylacetamide containing lithium chloride

The viscosity behavior, index of refraction, and density of dilute polycaproamide solutions in DMAc containing lithium chloride have been studied. On the basis of temperature dependences of the index of refraction and the density of solutions, the θ-condition of polycaproamide solutions in DMAc containing 2.5% lithium chloride is estimated as 21.3°C. From intrinsic viscosity [η] measurements, the coefficients of swelling of a polymer coil, the mean-square distance between chain ends, and other parameters of macromolecules in solution are calculated.     

Influence of the Molecular Mass of Poly(<Emphasis Type="Italic">N</Emphasis>-vinylpyrrolidone) on Formation of Cu<Subscript>2</Subscript>O Nanoparticles During Reduction of Divalent Copper Ions with <Emphasis Type="Italic">tert</Emphasis>-Butylamine Borane in Polymer Solution

The process of reduction of divalent copper ions with tert-butylamine borane in dilute aqueous solutions of poly(N-vinylpyrrolidone) is investigated. The influence of polymer molecular mass on properties of the resultant Cu₂O sols is studied. It is shown that Cu₂O nanoparticles with an average diameter of 6–8 nm independent of polymer molecular mass and a relatively narrow size distribution of particles are formed in the systems under study. The contour length of macromolecules and the hydrodynamic diameter of a poly(N-vinylpyrrolidone) macromolecular coil are compared with the diameter of Cu₂O particles. Poly(N-vinylpyrrolidone) with M ≥ 1 × 10⁴ can be used to produce Cu₂O nanoparticles. Poly(N-vinylpyrrolidone) with M > 4 × 10⁴ should be used for the formation of long-living Cu₂O sols.     

Dynamooptical and electrooptical properties of poly(methylphenylsiloxane) in solution and bulk

The dynamooptical, electrooptical, and hydrodynamic properties of a low-molecular-mass poly(methylphenylsiloxane) containing 33% phenyl radicals (with respect to the total amount of side groups) in dilute solutions and in bulk are studied. The size of macromolecules, as well as the molecular mass of the polymer, its shear optical coefficients Δn/Δτ = (0.29 ± 0.3) × 10^?10 (in decalin) and (0.43 ± 0.03) × 10^?10 cm s²/g (in bulk), and the specific Kerr constants K = (2.30 ± 0.02) × 10^?12 (in benzene), (2.23 ± 0.02) × 10^?12 (in decalin), and (2.24 ± 0.09) × 10^?12 cm⁵/[g (300 V)²] (in bulk), are estimated and compared with the corresponding characteristics of poly(dimethylsiloxane). The effect of solvents on the intramolecular mobility, optical anisotropy, and dipole structure of polymer macromolecules is considered.     

Electric birefringence and conformation of polychlorohexylisocyanate in solutions

Electric birefringence was investigated for solutions of polychlorohexylisocyanate fractions for molecular weights 30·6 × 10⁴–1·2 × 10⁴ in tetrachloromethane.Experimentally found dispersion of the Kerr effect is used for estimating the coefficients of rotatory diffusion D_r of molecules. A comparison of rotatory diffusion D_r values with molecular weights M and intrinsic viscosities [η] of fractions shows that the value of D_rM[η] decreases with M. This illustrates the change in the conformation of molecules from a random coil to a rod.On the basis of experimental dependences of D_r and the Kerr constants K on M, the main structural parameters of the polymer investigated were determined: the number of monomer units in a segment, the projection of the length of the monomer unit on the axis of the molecule, the value of the dipole moment μ₀ of the monomer unit and the angle formed by μ₀ and the chain direction.     

The thickness of adsorbed polymer layers at a liquid–solid interface as a function of bulk concentration

With a glass apparatus which is capable of measuring the thickness of adsorbed polymer layers with as accuracy of 0.5 nm, determinations on adsorbed polystyrene layers in contact with extremely dilute solutions (ca. 10^?7 g/ml) in toluene were performed. Thermodynamic equilibrium was proved to exist between the adsorbed polymer layer and bulk solution. The thicknesses found suggest that a fluid mechanical effect is operative, as has recently been calculated. It was shown that in the very dilute region the thickness of the adsorbed layer is initially a linearly increasing function of concentration, and that regions of (very low) concentrations exist over which a first plateau arises, where the layer is as thin as might be expected from isolated, adsorbed coil analysis. A second much higher plateau arises at concentrations in excess of 10^?5 g/ml, which is the plateau usually described.     

Free energy and dimensions of macromolecules under confinement in layered assemblies

The changes in the free energy ΔA accompanying penetration of polymer solutions from bulk into slit-like cavities were determined by lattice simulations. In dilute solutions the thermodynamics of penetration is controlled mainly by the parameter ϵ_w specifying interaction between polymer and walls of repulsive or adsorptive cavities. However, the magnitude of |ΔA| is substantially reduced by increasing concentration ∅︁ in bulk solution. Furthermore, compression of chains by concentration in good solvents and adsorptive cavities was found to be larger in the slit then in the bulk. At intermediate confinement, a region of a minimum coil size was observed at all concentrations and attraction strengths, where molecules are squeezed along all three axes.     

Structural and conformational properties of hyperbranched copolymers based on perfluorinated germanium hydrides

Two series of hyperbranched copolymers based on perfluorinated germanium hydrides of various topological structures are studied in dilute chloroform solutions by the methods of molecular hydrodynamics and optics. The first series is composed of copolymers with various molecular masses (from 2.3 × 10⁴ to 31 × 10⁴) that contain rigid linear chains between branching points and various amounts of branching points in cascades of the dendritic fragment, while the second series is comprised of copolymers that, at close branching degrees, on average, are characterized by a looser structure owing to a large amount of linear units at the periphery of macromolecules with M = (2.5 × 10⁴)?(23 × 10⁴). Macromolecules of the studied polymers have compact dimensions and a high density of the polymer substance; their shape asymmetry is low. In terms of these characteristics, they approach dendrimers. At a fixed molecular mass, the copolymers with a loose structure have higher dimensions of macromolecules and higher intrinsic viscosities.