Phenomenological analysis of elongational flow birefringence in semidilute solutions of hydroxypropylcellulose

The relaxation time of a polymer chain in an elongational flow field was investigated for hydroxypropylcellulose (HPC) semidilute solution systems by two methods: phenomenological analysis of elongational flow-induced birefringence, and dynamic light scattering (DLS) and rheological measurements. To understand the relaxation time of an entangled semiflexible polymer solution in an elongational flow field, scaling analysis of the elongational flow-induced birefringence curve was performed. The results of both temperature and concentration scaling analyses showed that birefringence curves at different temperatures and at several HPC concentrations were described well by a universal birefringence–strain rate curve. This scaling behavior was compared with the "fuzzy cylinder" model. The critical strain rate corresponded to the correlation time of the slow relaxation mode determined by DLS measurement and the relaxation spectrum obtained by dynamic viscoelasticity measurement. The elongational flow-induced birefringence observed in an HPC semidilute solution was concluded to be attributed to the orientation of the HPC segment in the entangled molecular system, because the dominant relaxation mode is found to be the concentration fluctuation of an entangled molecular cluster in a quiescent state.     

Zero-Birefringence Optical Polymer by Birefringent Crystal and Analysis of the Compensation Mechanism

Summary: We reported a method for compensating the birefringence of optical polymers by doping them with inorganic birefringent crystals. In this method, an inorganic birefringent material is chosen that has the opposite birefringence to the polymer and needle-like shape crystals which are oriented when the polymer chains are oriented. The birefringence of the polymer is thus compensated by the opposing birefringence of the crystal. Orientation behavior of the needle-like crystals and polymers was investigated. The orientation function of the needle-like crystal was increased with an increase in the aspect ratio of the needle-like crystal.     

Localized flow birefringence of polyethylene oxide solutions in a four roll mill

We report flow birefringence observations of polyethylene oxide solutions in a four roll mill where the flow field in the central region of the mill approximates well to that of pure shearing flow. When flow birefringence is observed it is seen to be highly localized within a region close to the “outgoing” asymptotic plane of flow. The phenomenon can be explained in terms of the flow birefringence corresponding to high extension of some polymer chains where the localization is caused by the chains requiring sufficient time in the flow field to become extended. This explanation has important consequences in all “persistently extensional flows” and can explain the origin of previously published results of localized flow birefringence observed for polyethylene solutions in axial compression and axial extensional flows.     

Electron microscopic and light scattering observation on a system with two iridescent phases

Electron microscopic observations and classical light-scattering measurements have been carried out for dodecyldimethylaminoxide/hexanol/water mixtures in the concentration range where iridescent colors occur. This system has two different iridescent phases. The iridescent phase with more hexanol forms quickly, and the phase with less hexanol forms very slowly. Three different isotropic phases which show strong flow birefringence are found near both iridescent phases. The electron microscopic pictures show clearly that only one of these isotropic phases with strong flow birefringence is a bicontinuous sponge phase (L_3h -phase). This is the phase which comes out by adding some alkanol to the upper lamellar phase. The flow birefringent phase below the lower lamellar phase forms unilamellar vesicles. The flow birefringent phase which occurs between both iridescent phases contains multilamellar vesicles and is shown to be a precursor of a lamellar phase.     

Birefringence of polyethylene melt in transient elongational flow at constant strain rate

Simultaneous birefringence and elongational viscosity measurements were carried out on molten commercial grade, low-density polyethylenes during simple elongational flow at constant strain rate and constant temperature. The birefringence increased with time during constant strain rate elongation. The increase in birefringence was a linear function of elongational stress throughout whole elongation, but the elongational viscosity increased in two stages. The increase in elongational viscosity can be divided into linear viscoelastic and nonlnear viscoelastic regions. The linear region appeared at small strain and the nonlinear region appeared at strain greater than 0.7. The elongational viscosity in the nonlinear region increased much more rapidly than that in the linear region. The Gaussian approximation, which is commonly used in molecular models, could be used for the transient elongational flow. A constant stress-optical coefficient C = 1.3 × 10^?10 cm²/dyn was obtained for all the elongational experiments, independent of strain rate (0.002-0.2s^?1). The stress-optical coefficients were weakly dependent on temperature, as predicted by the theory of rubber elasticity.     

Localized flow birefringence of polyethylene oxide solutions in a two roll mill

Experimental observations are reported of the behavior of solutions between two rotating parallel rollers where the form of the flow field between corotating rollers corresponds to that of a col which has nonorthogonal asymptotes. The obliquity of the asymptotes of the flow gives a direct measure of the relative magnitude of the constituent pure shear and rotational components of the flow field. Flow birefringence observations of polyethylene oxide solutions between the corotating rollers show that molecular orientation is limited to a highly localized region near the “outgoing” asymptotic sheet of the flow field in accordance with the predictions of a “persistent extensional flow” analysis presented. Counterrotating rollers show no flow birefringence also as expected. The behavior of polymer solutions and Newtonian fluids are compared. With increasing strain rate using corotating rollers the obliquity of the asymptotes of the flow remains constant for the Newtonian fluid but changes significantly for the polymer solution.     

Refractometry of lyotropic mesophase systems: Ethyl cellulose/m-cresol and ethyl cellulose/acetic acid

An Abbé refractometer with a rotatable polarizer mounted on the eyepiece is used for determining the two principal refractive indices of birefringent concentrated solutions of ethyl cellulose in m-cresol and in acetic acid. At a certain concentration, birefringence appears for both systems, and the concentration agrees with that for the onset of iridescence (here defined as the critical concentration C_a for formation of the liquid-crystalline phase). The birefringence of concentrated solutions depends strongly on concentration and temperature and can be generalized by a master curve. The refractive index increments for both systems are almost independent of temperature at high concentrations.     

Some optical and mechanical properties of poly(vinyl chloride)

The optical and mechanical properties of poly(vinyl chloride) film were examined by observing both the stress and birefringence during stretching at constant rate, during relaxation at constant length and during a dynamic birefringence experiment. Experiments were also done by varying the temperature at constant length. The changes in birefringence are interpreted in terms of changes in negative distortional birefringence, changes in positive orientation birefringence, and possible reversible changes in birefringence with temperature arising from conformational changes in the polymer chain and changes in the contribution of birefringent crystals.     

Polyethylene fiber formation in tubular flow. I. Observations of growth pattern and fiber properties

The process of seeded growth of fibrillar polyethylene crystals has been studied in a tubular flow geometry for 0.01-wt % solutions of a high-molecular-weight polyethylene in xylene. The transformation sequence has been followed visually by using polarized-light illumination in conjunction with a video camera. Data are presented to show that transformation is initiated by the formation of a concentrated, unoriented, amorphous precursor fiber within which oriented birefringent crystals subsequently grow in consequence of the stresses transmitted by the flowing solution. Time constants for the precursor formation, birefringence initiation, and completion of birefringence were measured as functions of temperature and flow rate over a range of growth conditions. Wide-angle x-ray diffraction, overall birefringence, and optical hot-stage melting data were also obtained on the grown fibers. The net result of these observations is to conclude that fibrillar crystal growth during flow is always preceded by the formation of a liquidlike phase transformation which produces the concentrated, unoriented precursor. Subsequent orientation is in consequence of stress-induced crystallization with overall fiber orientation showing an increase with solution flow rate at a fixed temperature and a decrease with temperature at a fixed flow rate. At higher temperatures and lower flow rates, birefringence develops in an oscillatory fashion, indicating a remelting process possibly due to slippage of trapped chain entanglements formed by flow. A discussion is given of the implications of these observations for the understanding of flow-induced structure development, phase transformation, and oriented crystallization; this is expanded upon in a companion paper, Part II.     

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.     

Rheological studies on the phase separation of hydroxypropylcellulose solution systems

Phase behavior of hydroxypropylcellulose (HPC) in a mixed solvent of glycerol and water was investigated by two different rheological methods: rheooptical birefringence measurement in an elongational flow field and viscometric measurement in a shear flow field. The association process of the HPC chain during phase separation observed by the elongational flow birefringence method was also investigated by the shear viscometric method. The temperature dependence of chain rigidity was determined by measuring the intrinsic viscosity, and change in the conformation was investigated by observing elongational flow birefringence over the temperature range from the one‐phase to inside a phase boundary. The results focus on the molecular process of phase separation. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1976–1986, 2001     

Optical, dynamic, and electro-optical properties of poly(N-acryloyl-11-aminoundecanoic acid) in solutions

The optical, electro-optical, and dynamic characteristics of poly(N-acryloyl-11-aminoundecanoic acid) in organic solvents and of the sodium salt of its monomer in water were studied via the methods of flow birefringence, equilibrium and nonequilibrium electric birefringence, and dynamic light scattering. It is shown that, in aqueous solutions, the monomer forms coarse particles of both symmetric and asymmetric shapes. The linear dimensions of these particles are estimated from the data of translational and rotational diffusion. Polymer macromolecules in organic solvents feature negative anisotropy of optical polarizability. Contributions of optical microform and macroform effects to the observed flow birefringence are analyzed in detail. The intrinsic optical anisotropy of the monomer unit of the polymer, which correlates well with the corresponding values for comb-shaped polymers of a similar structure, is estimated. It is shown that polymer molecules lack marked intrinsic permanent macromolecular dipoles and that electric birefringence in their solutions is associated with macroscopic induced dipole moments that appear during orientation of the dipole moments of polar groups in side chains of the polymer under application of an electric field.     

DIGITAL ANALYSIS TECHNOLOGY FOR MORPHOLOGY OF POLYMER CHAIN COILS IN FLOW FIELDS

Polymer chain coils with entanglement is a crucial scale of structures in polymer materials since their relaxation times are matching practical processing times. Based on the phenomenological model of polymer chain coils and a new finite element approach, we have designed a computer software including solver, pre- and post-processing modules, and developed a digital analysis technology for the morphology of polymer chain coils in flow fields (DAMPC). Using this technology we may simulate the morphology development of chain coils in various flow fields, such as simple shear flow, elongational flow,and any complex flow at transient or steady state. The appli cations made up to now show that the software predictions arecomparable with experimental results.     

Crystallization and thermoreversible gelation of poly(butylene terephthalate)–epoxy mixtures

The thermoreversible gelation of solutions of poly(butylene terephthalate) (PBT) and a liquid diglycidyl ether of bisphenol-A epoxy has been investigated. The morphology of the gels and the conditions under which they form have been characterized by optical microscopy, thermal analysis, and x-ray scattering. Gels were found to form under two different conditions and with different morphologies. Gels formed after a considerable delay when homogenous PBT-epoxy solutions were cooled to slightly below the dissolution temperature of crystalline PBT. These gels contained large, irregular PBT spherulites and smaller birefringent interspherulitic matter. The melting of these gels and the onset of macroscopic flow coincided with the melting of the interspherulitic matter, and occurred before the melting of the large spherulites. Thermoreversible gels formed very quickly when PBT-epoxy solutions were self-nucleated by heating a dispersion of crystalline PBT in epoxy slightly and briefly above the dissolution temperature and then cooling. These gels displayed only a weak background birefringence and were molten when the weak birefringence disappeared. In both cases, gelation occurred by the formation of a three-dimensional PBT network in the epoxy liquid, and the nodes of the network were crystalline PBT particles. $ 1994 John Wiley & Sons, Inc.     

Structural investigations of polymer liquid-crystalline solutions: Aromatic polyamides,hydroxy propyl cellulose,and poly(γ-benzyl-L-glutamate)

A comparative structural investigation of the characteristics of polymer liquid-crystalline solutions including Kevlar® (PPD-T)/sulfuric acid, poly(Cl-p-phenylene terephthalamide) (Cl-PPD-T)/sulfuric acid, poly(γ-benzyl-L-glutamate) (PγBLG)/dioxane, and hydroxypropyl cellulose (HPC)/water was undertaken. Experimental procedures included polarized light microscopy, light scattering, absorption spectra, and x-ray diffraction on solutions at various concentrations and temperatures. Both the two-phase region at the onset of liquid-crystal formation and the wholly anisotropic phase were investigated. Each solution exhibited distinctive characteristics. The PPD-T and Cl-PPD-T solutions were nematic, and the PγBLG and HPC solutions were cholesteric. In the two-phase region the PPD-T, Cl-PPD-T, and PγBLG (but apparently not the HPC) exhibited negatively birefringent spherulites and aggregates of spherulites. The HPC solutions only exhibited spherulitic structures in the single-phase anisotropic system. The structures and orientations in the anisotropic phase for the various polymer solutions is considered. The helicoidal structural characteristics of the PγBLG and HPC solutions are contrasted.     

Novel high birefringent isothiocyanates based on quaterphenyl and phenylethynyltolane molecular cores

We report the synthesis of new laterally alkyl substituted high birefringent quaterphenyl and phenylethynyltolane liquid crystals with isothiocyanate terminal group. The thermal and optical properties such as birefringence are measured and discussed based on their molecular structures. The compounds exhibit birefringence (Δn) in the range of 0.44–0.63, and are expected to be appropriate components of high birefringent nematic mixtures for various applications.     

Formation of ordered phases of (hydroxypropyl)cellulose in miscible and immiscible isobutyric acid-water mixtures

(Hydroxypropyl)cellulose (HPC) is known to form birefringent liquid-crystalline phases at elevated polymer concentrations in either water or isobutyric acid (IBA). The HPC concentration at which the polymeric phase exhibits birefringence decreases as the IBA content in mixed H₂O/IBA solvents decreases, even though the concentration ?ⁱ_c for the formation of an ordered phase of HPC in water is greater than that in IBA. Water is a spectator component and apparently does not participate in the formation of a birefringent phase when IBA is present. A birefringent phase forms once the concentration of HPC in the solution omitting the H₂O equals the ?ⁱ_c of binary HPC/IBA solutions for temperatures from 23 to 95°C. The strong preferential affinity of HPC for IBA is visually evident as an HPC coagulate separates from dilute solution when the solvent mixture contains as little as 5% IBA. The coagulate dissolves to give a monophasic isotropic solution as the IBA content in the solvent is increased. A heterogeneous system in which a clear supernatant fluid covers a pearly white polymeric phase forms when the solvent mixture is immiscible and the HPC content is less than 50%. At high HPC content, the classical appearance associated with concentrated HPC solutions is seen. The optical and rheological properties of the heterogeneous systems are compared with those of homogeneous solutions at several HPC concentrations.     

Design of new super-high birefringent isothiocyanato bistolanes – synthesis and properties

We report the synthesis of new laterally ethyl-substituted high birefringent phenylethynyltolane liquid crystals with alkylsulphanyl chain and isothiocyanate terminal group. The thermal and optical properties such as birefringence are measured and discussed based on their molecular structures. The compounds exhibit birefringence (Δn) in the range of 0.63–0.75 and are expected to be excellent components of high birefringent nematic mixtures for various applications.     

A visualization study of poly(ethylene terephthalate) flow using potential chain-ordering die geometries

Streak-photographic and stress birefringence techniques were used to analyze the flow of poly(ethylene terephthalate) through potential chain-ordering die geometries. The streak photographs were used to determine velocity distributions and streamlines in various convergent dies. The different contours were seen to have a significant effect on the polymer streamlines and velocity distributions. The measured velocities were used to develop empirical equations, specific for each geometry, which relate velocity to position within the die and to throughput rate. Flow birefringence was used to determine the extent of molecular ordering. The optimum chain-ordering die geometry was determined to be one which included a rapid initial decrease in cross-sectional area. Birefringence was also used to monitor polymer flow instability. An unusual mechanism for instability was observed at intermediate throughput rates.