Formation Of Intermolecular Associations Research Articles

Significance of polymer elasticity on drag reduction performance in dispersed oil-in-water pipe flow

Mechanisms proposed for the drag reduction by high molecular weight polymers range from stress anisotropy, elasticity and the formation of intermolecular associations. This experimental study tests the role of elasticity as indicated by the measurement of normal force, from which Weissenberg number can be calculated. Cationic drag reducing polyacrylamides were injected in a dispersed oil-in-water flow through 30.6-mm pipe. Four polymers were used to investigate the effect of polymer molecular weight and charge density. Three polymers with different molecular weights and 10% charge density were investigated along with a polymer with 45% charge density. The flow in the test section was set at 0.3 oil fraction and mixture velocities of 0.8, 1.0, 1.2 and 1.5 m/s (Reynolds number above 34,400) with polymer concentrations from 10 to 40 ppm. Generally, the maximum drag reduction was obtained for polymer concentrations in the test section higher than 20 ppm. It increased with flow velocity and polymer molecular weight. The rheological measurements indicated that the elasticity of the polymers increase with the increase in molecular weight and change density. Similarly, the maximum drag reduction increased with the increase in molecular weight and charge density, which indicates a correlation between the efficiency of the drag reducing polymer and its elasticity. Drag reduction increased also with the increase in mixture velocity (or Reynolds number). On the other hand, drag reduction was generally independent of the hydrodynamic volume of the polymer above intrinsic viscosity of 4700 cm3/g.

Electron polarizability of molecules of carboxylic acids and their dimers and trimers

Components of the tensor of the electron polarizability of molecules of carboxylic acids and their dimers and trimers with conjugated chemical bonds are calculated according the Hartree–Fock method. The dependences of a change in the anisotropy of polarizability on the average polarizability of a molecule and the number of electrons in a conjugated system are determined. An increase in the anisotropy of electron polarizability during the formation of intermolecular associates through hydrogen bonds is observed.

Local Relaxation Behavior and Dynamic Fragility in Hydrogen Bonded Polymer Blends

The dynamics of intermolecularly hydrogen-bonded polymer blends of poly(p-(hexafluoro-2-hydroxyl-2-propyl)styrene) with poly(vinyl acetate), poly(ethylene[30]-co-vinyl acetate[70]) and poly(ethylene[55]-co-vinyl acetate[45]) are investigated by broadband dielectric relaxation spectroscopy and Fourier transform infrared spectroscopy. Each blend component exhibits a glassy state (β) relaxation, and these relaxations are affected by the formation of intermolecular associations. The glassy state behavior of the blends can be modeled using the Painter−Coleman association model. All blends exhibit a single Tg and a single dielectric segmental (α) relaxation, indicative of strong segmental-level coupling. The fragility of the glass-formers depends on the volume fraction of intermolecularly associated segments, and the association model predicts which compositions have the highest fragilities. A relaxation related to the breaking and reforming of hydrogen bonds is observed at temperatures above the α process, and its temperature dependence varies systematically with ethylene content.

Free-radical copolymerization of binary mixtures of vinyl monomers of various compositions: Initiation rate constants

For a number of vinyl monomers (styrene, butyl acrylate, vinyl acetate, and methyl-, butyl-, and lauryl methacrylates) and their mixtures with a divinyl (network-forming) monomer—triethylene glycol dimethacrylate—with the molar ratio of binary mixture components varying from 0 to 100%, initiation rate constants have been measured through the inhibition method with the use of 2,2,6,6-tetramethyl-1-piperidinyloxy as a counter of free radicals. Viscosities at various temperatures and activation energies for the viscous flow of the monomers under study have been estimated. The relationship between the initiation rate constants and the viscosity of the medium has been established. On the basis of this relationship, the mechanism for the exit of radicals from the cage has been interpreted within the framework of the Franck-Rabinowitch model with allowance made for effects associated with formation of intermolecular associates in the systems under study.

Association behavior of poly( N, N-dimethylacrylamide)- graft-poly(methyl methacrylate) in dilute solution

Association behavior of poly( N, N-dimethylacrylamide)- graft-poly(methyl methacrylate) in dilute solution of mixed solvent of methanol/water has been investigated by using light scattering, where association strength of grafted chains is controlled by changing the water content. The following points are demonstrated: (1) the increase in water content in solvent starting from pure methanol increases the association strength, resulting in the formation of inter-molecular associations at water contents higher than 20–30 wt% of water, (2) the association number, N a, depends on the changing rate of water content of solvent. At higher changing rates, associates of small N a (=2–4) are formed by the dominance of intra-molecular association, while the slower change of water content yields larger associates of N a being of the order of 10. The presence of the optimum changing rate of solvent quality for inter-molecular associations is suggested. This process dependence of associate structure is discussed on the basis of association kinetics.

Hydrogel composition based on carboxymethylcellulose and agar-agar

The results are given of a study of the rheological properties of concentrated solutions of CNC and agar-agar and the influence of their quantitative ratio on the anomalous flow behavior of their solutions. A hypothesis is put forward of the formation of intermolecular associates from CMC and agar-agar which leads to the formation of a stronger structural network of solutions of the initial polysaccharides.

Solution rheology of a hydrophobically modified alkali-soluble associative polymer

Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high concentration dependence of the zero shear viscosity, with η0∼c8. Evidence for shear-induced structuring in steady shear, large amplitude oscillatory shear, and parallel superposed steady and dynamic shear is presented. Such structuring is more pronounced at lower polymer concentrations, consistent with the formation of intermolecular associations at the expense of intramolecular. In contrast to the simple linear telechelic associative polymers considered in a number of previous studies, the network dynamics of the polymer are no longer represented by a single characteristic time. This deviation from a classical Maxwellian response in oscillatory shear is interpreted as a broadening of the relaxation spectrum, arising from the coexistence of both hydrophobic associations and topological entanglements. Mechanistically, stress relaxation is better envisaged in terms of “hindered reptation” [Liebler et al. (1991)] of the chains, rather than Rouse-like behavior moderated purely by the hydrophobe disengagement rate [Annable et al. (1993)].

The theory of shear-thickening polymer solutions

A theory that describes the rheological properties of shear-thickening polymer solutions has been developed. This theory comprises an extension of Flory's theory of the expansion parameter to the case where macromolecules contain segments that can associate in an intramolecular fashion. Analysis of experimental results using this theory enabled determination of the association constant. It is then proposed that, when macromolecules with associating groups are placed in an elongational flow field of sufficiently high strain rate, they extend, thereby breaking intramolecular associations and allowing the formation of intermolecular associations, to an extent dictated by the association constant. The quantitative predictions of this theory are then compared with rheological measurements on a well known shear-thickening polymer, ICI's FM9.

Enzymatic Degradation of Protein Mixtures

The rate of protein enzymatic degradation in a mixture is slower than that of single proteins. In a mixture of haemoglobin or casein with protamine, the release of tyrosine by trypsin and chymotrypsin A is slower. A slower rate in the release of arginine from protamine occurs only in mixtures with haemoglobin. The inhibition of enzymatic degradation of proteins in a mixture is due to formation of intermolecular associations and to changes in their spacial structure.