Self-organization of amphiphilic macromolecules with local helix structure in concentrated solutions

Concentrated solutions of amphiphilic macromolecules with local helical structure were studied by means of molecular dynamic simulations. It is shown that in poor solvent the macromolecules are assembled into wire-like aggregates having complex core-shell structure. The core consists of a hydrophobic backbone of the chains which intertwine around each other. It is protected by the shell of hydrophilic side groups. In racemic mixture of right-hand and left-hand helix macromolecules the wire-like complex is a chain of braid bundles of macromolecules with the same chirality stacking at their ends. The average number of macromolecules in the wire cross-section is close to that of separate bundles observed in dilute solutions of such macromolecules. The effects described here could serve as a simple model of self-organization in solutions of macromolecules with local helical structure.     

Compactization of rigid-chain amphiphilic macromolecules with local helical structure

Conformational characteristics of amphiphilic macromolecules with secondary local helical structuring are studied by the method of molecular dynamics for different properties of a helix (bending angles between neighboring vectors of the bond and internal rotation angle) and different rigidities of its fixation. Extended helices with high distances between helical turns and dense helices in which neighboring turns directly adjoin each other are studied. As the quality of a solvent deteriorates, extended helices experience a well-pronounced coil-globule transition, whose amplitude increases with an increase in chain rigidity, while the dimensions of dense helices gradually change. In a poor solvent, extended helices formed “collagen-like” structures, flexible chains of dense helices produce hairpin structures, and rigid macromolecules of dense helices form rodlike globules with an almost ideal local helical order. Independently of helix parameters, a deterioration in solvent quality leads to stabilization of the local secondary structure.     

Association of Chitosan in Aqueous-Alcohol Solutions

The association of chitosan in aqueous-alcohol solutions is studied via probe fluorescence spectroscopy and dynamic light scattering. It is shown that, if a certain critical concentration is reached, the associates of macromolecules 12–38 nm in size are formed in chitosan solutions. The addition of 5–60 vol % ethanol to the solvent leads to reduction in the critical association concentration. The tendency of chitosan macromolecules toward association is the most pronounced at 34 vol % alcohol.     

Temperature dependence of the structure of a carbosilane dendrimer with terminal cyanobiphenyl groups: Molecular-dynamics simulation

The molecular-dynamics simulation of the structure and molecular mobility of an individual macromolecule of a fourth-generation carbosilane dendrimer with terminal cyanobiphenyl groups in a highly diluted chloroform solution in the range 213–323 K is performed. Upon a change in temperature, the dendrimer undergoes structural rearrangement that depends on the ability of terminal segments to penetrate into the dendrimer. At temperatures close to the boiling point of the solvent, aliphatic spacers of terminal segments can penetrate deep into the dendrimer. As temperature decreases, the terminal segments are grouped only on the surface of the molecule; this leads to a 45% increase in the number of solvent molecules in the treelike part of the macromolecule. These results make it possible to give a new interpretation of temperature effects previously observed in NMR experiments for dilute solutions of these macromolecules.     

Simulation of the electrical double layer of a spherical micelle of an anionic substance with regard to the solvent structure

Computer simulation methods are employed to consider the structure of the electrical double layer of a spherical micelle in aqueous surfactant solutions with allowance for the contribution of the solvent. Three micelle models were used in the calculations, namely, a macroion with discretely distributed charges and a continual solvent, a spherical model micelle with a coarse-grained representation of the solvent, and a spherical model micelle in an aqueous phase with an explicit account for water molecules. Based on these three models, the radial profiles of the local densities and electric potentials in the electrical double layer, as well as the degrees of binding single-, double-, and triple-charged counterions by the macroion in aqueous surfactant solutions, are calculated with regard to the Lennard-Jones and electrostatic interactions. The allowance for the molecular structure of the solvent leads to qualitatively different local dependences of the electric potential as compared to both the continual and coarse-grained representation of the solvent.     

Solution behavior of methyl cellulose mixtures with poly(<Emphasis Type="Italic">N</Emphasis>-vinylformamide) in water and dimethyl sulfoxide

The rheological properties of diluted and moderately concentrated solutions of methyl cellulose mixtures with poly(N-vinylformamide) in water and dimethyl sulfoxide are investigated. Negative deviations of the viscosities of these solutions from the additive values are observed and explained by different mechanisms of solvation of macromolecules with water and dimethyl sulfoxide. Incompatibility of the polymers in a common solvent is discovered. This circumstance leads to the formation of a new system of hydrogen bonds and rearrangement of the structural organization of solutions.     

Polymer structure and solvent effects on the selective dispersion of single-walled carbon nanotubes

Combinations of different aromatic polymers and organic solvents have been studied as dispersing agents for preparing single-walled carbon nanotubes solutions, using optical absorbance, photoluminescence-excitation mapping, computer modeling, and electron microscopic imaging to characterize the solutions. Both the polymer structure and solvent used strongly influence the dispersion of the nanotubes, leading in some cases to very high selectivity in terms of diameter and chiral angle. The highest selectivities are observed using toluene with the rigid polymers PFO-BT and PFO to suspend isolated nanotubes. The specific nanotube species selected are also dependent on the solvent used and can be adjusted by the use of THF or xylene. Where the structure has more flexible conformations, the polymers are shown to be less selective but show an enhanced overall solubilization of nanotube material. When chloroform is used as the solvent, there is a large increase in the overall solubilization, but the nanotubes are suspended as bundles rather than as isolated tubes which leads to a quenching of their photoluminescence.     

Temperature Dependence of Adsorption of Polymer Mixtures from Solutions

Adsorption from binary (poly(butyl methacrylate)-CCl(4), poly-styrene-CCl(4)) and ternary (poly(butyl methacrylate)-polystyrene-CCl(4)) solutions has been studied at 10, 25, and 60 degrees C. It was found that with increasing temperature the values of adsorption grew due to worsening of the thermodynamic quality of the common solvent. Worsening of the quality of the solvent leads to a decrease in size of the macromolecular coils and to an increase in the critical concentrations of overlapping macromolecules in solution. As a result, the state of macromolecules in solution depends on temperature and determines adsorption values. From the temperature dependence of adsorption using the Clapeyron-Clausius equation, the differential enthalpy of adsorbate (polymer) DeltaH was calculated for each polymer, by adsorption both from binary solution and from the mixture. Determining DeltaH values from the temperature dependence of adsorption allows us to find this value simultaneously for each polymer in the polymer mixture. It was established that transition of a polymer from solution onto the surface leads to an increase in its enthalpy in the case of adsorption from both binary and ternary solutions. DeltaH increases with increasing coverage of the surface. By transition from solution onto the surface, the enthalpy of the adsorbed polymer increases; i.e., the polymer transits in an energetically less favorable state. This effect is more pronounced for adsorption from the mixture, which may be connected with the presence of the second polymer in the adsorption layer. Copyright 2000 Academic Press.     

The problem of bimodal distributions in dynamic light scattering: Theory and experiment

A new method is proposed for analyzing the results of dynamic light scattering measurements of polymer solutions containing macromolecules with substantially different molecular masses. The processing algorithm makes it possible to correctly allow for effects of the quality of solvent and the conformation and polarizability of macromolecules on the contributions of components of a system to the intensity of scattering. The developed method is tested for a model mixture of two polystyrene samples dissolved in a good solvent (tetrahydrofurane) and a θ solvent (cyclohexane) and is applied to study a system containing a linear polymer and an interpolymer complex.     

粘度法研究壳聚糖与两性聚氨酯胶束间的相互作用

通过粘度法考察了壳聚糖(CS)在盐酸水溶液中与表面带有正电荷的两性聚氨酯(APU)胶束的相互作用，并研究了小分子盐浓度、pH及不同混合方式对复合体系粘度的影响．结果表明：在壳聚糖／APU盐酸混合体系中，较高浓度下APU胶束与壳聚糖分子间的静电斥力作用导致壳聚糖大分子链的伸展，使体系粘度提高，同时降低了壳聚糖溶液对电解质、pH的敏感性；不同混合过程导致壳聚糖大分子构像及其与APU组装、复合形态有很大差别，使混合体系流变性表现出明显的不同．     

Specific features of the formation of polymer-dye systems based on nanostructured polymer matrices prepared by solvent crazing

Specific features of the formation of polymer-dye systems based on various nanostructured polymer matrices prepared by the method of solvent crazing are discussed. In the general case, the formation of polymer-dye composites includes four main stages: sorption of dye molecules by the highly disperse fibrillar material of crazes, shrinkage of the polymer composite due to the removal of the solvent, migration of dye molecules from their localized sites on the surface of fibrils, and healing of the structure of crazes (internal interfacial boundaries) under thermal treatment. Analysis of the migration of dye molecules in the polymer matrix includes the following assumptions: first, a metastable (nonequilibrium) state of the system after solvent crazing and introduction of dye molecules into the fibrillar craze material and, second, the statement according to which both the depth and direction of the above migration processes are controlled by the free energy of mixing of components. For amorphous glassy systems (PVC, PS, PC), healing of the fibrillar craze material (after shrinkage and removal of the solvent) is observed. In the case of semicrystalline polymers (PP, vinylidene fluoride-trifluoroethylene copolymer) and amorphous crystallizable polymer matrices (PET), the intensity of healing upon thermal treatment decreases due to the presence of crystalline regions, which slow down the motion of macromolecules.     

Phase transitions in liquid crystalline solutions of hydroxypropyl cellulose under deformation

Phase transitions and the physical state of the hydroxypropyl cellulose-dimethylacetamide system under static conditions and in a shear field were studied by the cloud-point and polarized optical microscopy techniques with a polarization-photoelectric setup and a modified plasticorder. The deformation of solutions leads to a change in their structure and elevation of liquid-crystalline phase formation temperatures, a result that is due to the additional orientation of macromolecules in the flow direction. The ability of macromolecules to be oriented in a shear field decreases with an increase in the molecular mass of the polymer. The influence of deformation on phase transitions in hydroxypropyl cellulose solutions is nonmonotonic in character.     

Criteria of negative thixotropy of poly(methyl methacrylate) solutions as a function of temperature and of the thermodynamic quality and viscosity of the solvent

A study of the effect of thermodynamic quality and viscosity of the solvent and of temperature on the negative thixotropy of poly(methyl methacrylate) solutions using criteria suggested in the preceding paper showed that the minimal shear stress is a more general criterion of hydrodynamic conditions of the formation of this effect than the minimal velocity gradient. It is assumed that the occurrence of negative thixotropy is due to the activation of macromolecules combined with their critical deformation and orientation caused by the effect of a certain minimal shear stress. A general quantitative relationship between the induction period and conditions of formation of the negative thixotropic structure was derived and experimentally tested.     

Specific features of the formation of poly(vinylchloride)-dye composites based on solvent-crazed nanostructured polymer matrices

Healing and migration of dye molecules in porous nanostructured amorphous-polymer-dye systems prepared by solvent crazing are studied by the method of spectroscopy in the visible spectral region. The conversion of the above processes is controlled by the temperature in the temperature interval above T _g of the bulk polymer. Both processes proceed simultaneously and are independent. After the removal of AALE from the composite and shrinkage of the polymer sample at room temperature, crazes preserve their fibrillar structure, which can be identified by light scattering at 400–600 nm. The main contribution to healing from the fibrillar material of crazes is provided by the reptational mobility of macromolecules, a conclusion that is confirmed by the power dependence of light scattering of the sample on the duration of its thermal treatment with an exponent of 1/4 as well as by the activation energy of this process, which is ∼400 kJ/mol. The kinetic dependences of the intensity of absorption bands of monomer forms of a dye on annealing time have an exponent of 1/2 and show two linear regions. This behavior can be explained by the migration of dye molecules in their monomer form from their absorbed state on the surface of the fibrillar crazed material first into the volume of fibrils and then into the volume of bulk-polymer regions. All the above phenomena are provided by the unique structure of the solvent-crazed polymer material (alternation of the closely spaced regions containing fibrillar material and bulk-polymer regions).     

Physical gels from PVC: molecular structure of pregels and gels by low-angle neutron scattering

In this paper we report on investigations into the short-range molecular structure of pregels (aggregates formed in dilute solutions) and gels as a function of ageing time, temperature and solvent type. In some solvents (diethyl oxalate and bromobenzene) it is found that the structure of the pregels is cylinder-like or ellipsoid-like. The structure is seen to be the same for the gels and the pregels in the investigated range of transfer momentum which leads us to conclude that the gels are probably fiber-like, a statement in agreement with the morphology observed by Yang and Geil. The fiber section is also about the same order of magnitude as that reported by these authors. Also, further evidence is given for the existence of a second type of physical knot, different from those formed by the more syndiotactic sequences. The validity of the molecular structures deduced from these experiments is discussed in the light of macroscopic properties such as compression modulus and swelling.     

Monte Carlo Simulations of Self-Assembling Copolymer Brushes

We studied a simplified model of a polymer brush formed by linear chains, which were restricted to vertices of a simple cubic lattice. The macromolecules consisted of a sequence of two different kinds of united atoms arranged in a specific sequence. The chains were grafted to an impenetrable surface, i.e. they were terminally attached to the surface with one end. The model system was studied at different solvent quality from good to poor solvent. The properties of this model system were determined by means of Monte Carlo simulation using a Metropolis-like sampling algorithm based on local changes of chain's conformations. The size and the structure of the brush were determined.     

Study of ordered structures of syndiotactic poly(methyl methacrylate) in solution and in the solid state

From analysis of infrared spectra it was found that in syndiotactic (s) poly(methyl methacrylate) (PMMA) in solution, long s sequences contain an increased population of diads with a skeletal conformation tt (in the staggered approximation). Self-aggregation of s-PMMA in solution leads to a further increase of the fraction of long s sequences in the extended chain conformation, and to an ordering of easter groups. When solid s-PMMA is isolated from a solution in which it exists in the aggregated state, these characteristics are preserved in the solid. The polymer appears partially crystalline by x-ray scattering, and it exhibits fibrillar morphology under the electron microscope. Ordered structures of s-PMMA melt at temperatures about 150°C, while the presence of residual solvent decreases the temperature of melting. Solid s-PMMA obtained from solutions in which aggregation of the polymer does not take place, like s-PMMA which did not come into contact with solvent, contains a higher proportion of syndiotactic diads with a skeletal conformation tg; these samples are amorphous and morphologically structureless. Analogies between the structure of ordered s-PMMA and the structure of the PMMA stereocomplex are also discussed.     

Dissolution of powder celluloses in the dimethylacetamide-LiCl system and physicochemical characteristics of the regenerated samples

Dissolution of powder cellulose samples from flax wastes and wood pulps in the dimethylacetamide-LiCl solvent was studied. The decisive factors of cellulose dissolution are the degree of chemical “purity” of cellulose and the degree of polymerization. The cellulose I supramolecular structure of the initial samples transforms into a mixture of polymorphous modifications after dissolution and subsequent regeneration from solutions. The samples regenerated from solutions consist of highly porous spherical particles. The powder samples degrade at the fibrillar level in solutions.     

Accumulation of Macromolecules in Pores from Dilute Solutions in Poor Solvent

The phenomenon of spontaneous accumulation of macromolecules in pores from the dilute solutions of flexible polymers in poor solvent is studied using the density functional theory. It is shown that the partitioning of macromolecules between the bulk solution and slitlike pore, on whose walls the segments are predominantly adsorbed, depends on the ability of a semidilute polymer solution to wet the wall of a pore and on its size, as well as on the degree of undersaturation of bulk solution and temperature. Partitioning coefficient increases in a jumpwise manner when the surface first-order phase transition or capillary separation take place. It is revealed that the regularities of capillary separation of polymer solutions in a poor solvent are similar to those observed during the capillary condensation of unsaturated vapor. The applicability of the Kelvin equation to describe the conditions of capillary separation of flexible polymer solutions is analyzed.     

The effect of a low-molecular-mass salt on stoichiometric polyelectrolyte complexes composed of oppositely charged macromolecules with different solvent affinities

The effect of a low-molecular-mass salt on the thermodynamic stability of stoichiometric interpolymer complexes composed of oppositely charged macromolecules with different solvent affinities has been theoretically studied. It has been shown that the dissociation of such complexes with an increase in the concentration of the salt proceeds via several stages. At a low concentration of the salt, complexes retain their structure and dimensions. When a certain critical concentration of the salt n _s^cr is achieved, the dimensions of the complex increase abruptly. At this concentration, macromolecules involved in the complex begin to separate, and at concentration n _s^*, they fully move apart but remain soluble owing to the polyelectrolyte effect. Upon a further increase in the concentration of the salt, the polyelectrolyte effect is shielded and the dimensions of macromolecules decrease. The critical concentration of the low-molecular-mass salt, n _s^cr, increases with an increase in the degree of ionization of macromolecules and a decrease in the affinity of the hydrophilic component for water and diminishes with the degree of polymerization of macromolecules and the degree of hydrophobicity of a polycation. Because of the easy formation of soluble complexes from oppositely charged macromolecules differing in solvent affinities and their high stability in solutions of a low-molecularmass salt, such complexes are promising for wide use in medicine and pharmaceutical practice.