Flow birefringence and conformational characteristics of polyamide hydrazide molecules in dimethylsulphoxide

Flow birefringence (FB) and intrinsic viscosity of 19 samples of aromatic polyamide hydrazide (PAH) in dimethylsulphoxide (DMS), previously characterized by their weight-average molecular weights by the light scattering method, have been investigated. The molecular-weight dependence of reduced birefringence according to theory [12] was used to determine the optical anisotropy of a monomer unit Δa = (200 ± 20) 10^?25cm³ and the length of the Kuhn segment $\text{A = (250 ± 30)}\text{A}\text{?}$ of PAH molecules. The second independent evaluation of rigidity of the PAH chain $\text{A = (240 ± 30)}\text{A}\text{?}$ was obtained according to the theory of rotational friction of rigid wormlike chains by using the coefficients of rotational diffusion of PAH molecules determined from the characteristic values of orientation angles of FB. The value of rigidity of the PAH chain obtained by this method is in good agreement with the data on molecular dimensions obtained by light scattering.     

Flow birefringence and optical anisotropy of poly(N-vinylpyrrolidone) molecules

Concentration dependences of flow birefringence and viscosity of poly(N-vinylpyrrolidone) solutions in water and benzyl alcohol are investigated. The intrinsic anisotropy for a poly(N-vinylpyrrolidone) macromolecular segment, (α₁ ? α₂) = ?(82 ± 8) × 10^?25 cm³, is determined from the results of birefringence measurements in benzyl alcohol. For aqueous solutions, a strong concentration dependence of the specific anisotropy of solution is obtained, a result that may be explained by the heterogeneity of coils. A model allowing for this heterogeneity is suggested. It makes it possible to fit the concentration dependence to a hyperbolic function, to separate contributions of heterogeneity anisotropy and form anisotropy to the birefringence of a solution, and to estimate the segment asymmetry parameter as p = 3.0 ± 0.5.     

Capillary condensation of short-chain molecules

A density-functional study of capillary condensation of fluids of short-chain molecules confined to slitlike pores is presented. The molecules are modeled as freely jointed tangent spherical segments with a hard core and with short-range attractive interaction between all the segments. We investigate how the critical parameters of capillary condensation of the fluid change when the pore width decreases and eventually becomes smaller than the nominal linear dimension of the single-chain molecule. We find that the dependence of critical parameters for a fluid of dimers and of tetramers on pore width is similar to that of the monomer fluid. On the other hand, for a fluid of chains consisting of a larger number of segments we observe an inversion effect. Namely, the critical temperature of capillary condensation decreases with increasing pore width for a certain interval of values of the pore width. This anomalous behavior is also influenced by the interaction between molecules and pore walls. We attribute this behavior to the effect of conformational changes of molecules upon confinement.     

Flow birefringence and conformational characteristics of molecules of para-and meta-isomers of polyoxyphenyl-benzoxazoleterephthalamides

Flow birefringence (FB) has been used to study sulphuric solutions of two homologous series of polyoxyphenylbenzoxazoleterephthalamides (POPhBT) differing in the position (para- or meta-) of phenyl ring in the chain. In the framework of the FB method alone by using the theory of flow birefringence for kinetically rigid wormlike chains, it was possible to determine quantitatively the optical anisotropy of the monomer unit Δa = (330 ± 30) 10^?25 cm^?3 and the length of the Kuhn segment $\text{A = (330 ± 30)}\text{A}\text{?}$ and $\text{A = (115 ± 20)}\text{A}\text{?}$ for para- and meta-isomers, respectively. Analysis of possible mechanisms of flexibility in the chains of both polymers gives theoretical values of the rigidity parameter A in good agreement with experimental values of A. confirming the validity of the molecular models used.     

Flow birefringence of temporary polymer networks

The statistical theory of temporary polymer networks is an effort of describing the macroscopic behavior of such networks based on molecular behavior analysis. The calculation of the stress tensor of a network and the satisfactory comparison with experimental viscosity results was shown before [Rheol. Acta 28 (1989) 193]. Here we present the foresights of the theory as regards flow birefringence. The polarizability tensor is calculated first and then the birefringence of a four-functional temporary polymer network is estimated for a stationary simple shear flow. The dependence of the calculated quantities on shear rate is in line with existing experimental evidence.     

Flow birefringence and flexibility of the molecules of copolymers of para- and meta-aromatic polyamides

Flow birefringence (FB) of solutions of poly-p-phenyleneterephthalamide (PpPhTPhA), poly-m-phenyleneisophthalamide (PmPhIPhA) and 13 of their copolymers, with various compositions, has been investigated. The number S of monomer units in a Kuhn segment was determined for all samples from the experimental values of the shear optical coefficient Δn/Δτ. Molecular models for homo- and copolymers were considered. On the basis of these models, an equation expressing the dependence of the chain flexibility parameter 1/S on copolymer composition Z was derived. Good agreement between the experimental dependence of 1/S on Z and the theoretical value confirms the additivity of the structural and deformation mechanisms of flexibility in polymer chains.     

Flow and electric birefringence in solutions of ladder polychlorophenylsilsesquioxane

Flow birefringence (FBR) and electric birefringence (EBR) have been investigated for dilute solutions of a number of fractions of cyclolinear (ladder) polychlorophenylsilsesquioxane (CLChPhS). FBR and EBR are negative in sign and their characteristic values increase with molecular weight in accordance with properties characteristic of kinetically and equilibrium—rigid chain polymers. In sinusoidal fields, dispersion of EBR characteristic of kinetically rigid chains was found: its dependence on molecular weight quantitatively agrees with the theory of relaxation and rotatory friction of rigid molecules. Analysis of experimental data confirms the validity of the molecular model of CIChPhS as a double-chain ladder cis-structure and permits us to obtain quantitative information concerning important molecular parameters such as the character of rotation of phenyl side groups the values of valence angles in chains and in oxygen bridges and dipole moments of the monomer unit of the chain and of the SiO bond.     

Flow birefringence of concentrated polymer solutions in two-dimensional flows

The results of flow birefringence measurements are reported for polymer solutions of moderate concentration subjected to a wide range of two-dimensional flows. These flows were generated in a four-roll mill which enables one to systematically vary the ratio of the vorticity to the rate of strain in the flow while holding the velocity gradient constant. It is shown that steady-state birefringence data collected over a wide range of flow types can be correlated against the eigenvalue of the velocity gradient tensor, in agreement with criterion for strong and weak flows from model calculations. Transient birefringence measurements in which purely extensional flows were started from rest are also reported. It was observed that the birefringence went through a pronounced overshoot in time for two different polymer/solvent systems. Flow induced increases in the solution turbidity were also observed and the increased turbidity remained constant over a period of many hours after extensional flows were arrested. The birefringence, on the other hand, decayed to zero almost immediately after the flows were stopped. These changes in the turbidity suggest that crystallization of the polymer was occurring. The qualitative results of experiments are compared to recent network model calculations using the theory of Yamamoto for concentrated polymer systems. It is found that this model can predict qualitatively many of the experimental observations if the function describing the breakage of polymer chain entanglements is allowed to depend on the conformation of the polymer segments bridging the entanglements. In particular, this dependency of the entanglement breakage on the conformation of the network segments leads to a predicted overshoot of birefringence when purely extensional flows are started from rest. It is also demonstrated through this model that birefringence data taken over a wide range of flow types can be used to estimate the degree to which the network deforms affinely with the flow field.     

Dynamic birefringence and conformation of molecules of aromatic polyamide-hydrazide in solutions

Flow birefringence (FB) in solutions has been investigated for a number of samples of polyamide-hydrazide (PPAH) in dimethylsulphoxide; intrinsic viscosities [η] of the same solutions have been measured. Characteristic values of FB, [η], and orientation angles [χ/g] of the solutions were determined. Molecular weights of the PPAH samples were calculated from values of [χ/g] and [η]. The use of characteristic values of [n] and [η] and the theory of optical anisotropy of persistent chains gives quantitative data on the equilibrium rigidity of PPAH molecules. It was shown that the length of the statistical segment of the PPAH chain is 800 Å. The possible reasons for the fact that the equilibrium flexibility of PPAH molecules is greater than that of poly-p-phenylene terephthalamide molecules have been discussed.     

Dynamic birefringence and rigidity of poly-p-benzamide molecules in solutions

Flow birefringence (FB) in solutions of a number of samples of poly-p-benzamide (PPBA) in sulphuric acid has been investigated; intrinsic viscosities [η] of the same solutions have been measured.Characteristic values of FB, [n], and orientation angles [χ/g] of the solutions were determined. Molecular weights of all the PPBA samples were calculated by using the values of [χ/g] and [η]. The dependence [η] = 1.6 × 10^?5 M^1.7 shows that the conformation of PPBA molecules in solution is close to that of a straight rod.The use of characteristic values of [n] and [χ/g] and the theory of optical anisotropy of persistent chains gives quantitative data on the equilibrium rigidity of PPBA molecules. It was shown that the number of molecular units in a statistical segment of the PPBA chain is 320 and the corresponding length of the segment is 2000 Å.     

A theory of field-induced birefringence applicable to systems of large molecules

We consider a molecule in an external constant field interacting with a linearly polarized photon. When the field is chosen to be magnetic and directed along the wave vector of the photon (longitudinal), we find that the time dependence of the state function implies a rotation of the plane of polarization of the photon characteristic of the Faraday effect. Similarly when the field is transverse, the time evolution of the state corresponds to the Kerr effect in the case of an electric, and the Cotton—Mouton effect in the case of a magnetic field. The theory is applicable to a dilute system of freely oriented molecules of any size: the long-wavelength approximation is not invoked. The general procedure for averaging over all molecular orientations is carried out explicitly in the case of the Faraday effect.     

Flow birefringence experiments showing a shear-banding structure in a CTAB solution

Experiments of flow birefringence have been performed on a concentrated solution of CTAB submitted to shear in a Couette cell. Qualitative observations of the flow in the annular gap of the Couette device let us clearly see that the flow is divided into two layers when the shear rate exceeds a critical value. This shear banding structure is shown on a set of photographs. The extinction angle and the retardation were measured as a function of the shear rate for different temperatures. The results are in agreement with the predictions of the shear banding theory.     

Flow Properties of Moderately Concentrated Solutions of Cellulose Acetate Polymer

Calculations [1] of rheological properties of polymeric solutions have usually been restricted to very dilute o r to very concentrated solutions. In either case, one starts with a model for the micro- structure. Between these two conceptual extremes lies the broad class of moderately concentrated solutions. To cover this region theoretically, two approaches have been used; 1) network theories focusing attention on entanglement junctions, and 2) single-molecule theories focusing attention on single molecules but including inter- molecular interactions. Williams, Wang, and Zimm introduced a double model; one for a particular macromolecule and another for solutions containing like molecules in which the chosen macro- molecule is suspended. This approach has turned out to be quite successful because spheres immersed in a suspension of like spheres behave as i f suspended in a Newtonian fluid, and the resulting ex- pression for viscosity is quite simple in form and qualitatively correct for moderate concentrations.     

Influence of dye molecules on the birefringence of liquid crystal mixtures at near infrared frequencies

The optical properties of nematic liquid crystals have been extensively exploited in the production of devices working in the visible range of the spectrum. These same properties can be employed to make devices that function in the near infrared as required for telecommunications applications. However, it is generally observed that the birefringence of liquid crystal mixtures decreases with increasing wavelength, making it important to identify new materials, optimized for use in the near infrared region. One route to high birefringence is to operate close to an absorption band edge, which in the present context implies choosing highly conjugated materials which are potentially colored and, thus, not suited to traditional display applications. In this paper we explore the usefulness of dye molecules as birefringence enhancers in mixtures with conventional nematic liquid crystals. The optical properties, in particular, the absorption edge, polarizability, and birefringence, of families of known dyes are calculated at optical (589 nm) and infrared (1550 nm) wavelengths, using electronic density functional theory. We demonstrate the expected correlation between the proximity of the absorption edge and the magnitude of the birefringence, and estimate the birefringence enhancement occurring when each dye is incorporated in a guest-host system.     

Induced birefringence of rubbed and stretched polyvinyl alcohol foils as alignment layers for nematic molecules

Some optical properties of poly (vinyl alcohol) (PVA) films are investigated prior and after the rubbing and stretching of the samples. Birefringence of the prepared foils is enhanced as their stretching degree and rubbing become larger. These processing factors are also affecting the values of the ordinary and extraordinary refractive indices. The induced anisotropy in PVA foils is evidenced in the morphological characteristics observed by optical microscopy. The spreading and adhesion behaviors of a nematic liquid crystal on PVA foils are analyzed by means of contact angle measurements. Preliminary testing of the nematic on the PVA films revealed significant variations in transmitted light intensity during sample rotation under crossed polarizers. The high contrast between dark and bright patterns is indicative of uniform and homogenous alignment of nematic on the rubbed and stretched PVA foils.