Rheological properties of anisotropic poly(para-benzamide) solutions

A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C^*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C^*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C^* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM?)^* ≈ 1.3 × 10⁵ at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C^* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.     

Rheological study of an aromatic polyamide-hydrazide

Rheological and rheo-optical data are reported for the poly(terephthalamide of p-aminobenzhydrazide), X-500, in dimethyl sulfoxide solutions in the concentration range 0.2 to 6.0 g/100 ml. The objective of these measurements is to seek evidence of shear-induced isotropic → nematic phase transition. Three types of measurement, Couette, cone and plate, and capillary rheometer, all indicate that this system exhibits flow instabilities at high shear rates and concentrations. In this region both the viscosity and the primary normal stress difference decrease with time under shear. In the capillary rheometer the apparent viscosity is smaller for shorter capillaries and, for short capillaries, there is a range of shear rates in which plug flow and a coiled extrudate are observed. These instabilities may arise from the existence of a mixture of isotropic and nematic phases. At lower shear rates, where the flow behavior appears normal, the concentration dependence of the flow birefringence increases at a critical polymer concentration C This value is in reasonable accord with the concentration corresponding to the change of slope of logη (measured at low shear rate) vs. logC_p. Both the latter measurements appear to be sensitive to the onset of the phenomenon, which may be due to a shear-induced transition or the rheological effect of chain entanglements.     

Rheology of a mixture of liquid-crystal polymers in solution

The rheological behavior of a mixture of two liquid-crystal polymers, hydroxypropyl cellulose and ethyl cellulose, in acetone solution is studied. The total polymer concentration in the solvent is held constant (40%) as the ratio of the two polymers is varied. The mixtures are anisotropic, isotropic, or biphasic (isotropic/anisotropic), depending on the concentration. Curves of viscosity vs shear rate for all the mixtures studied show three regions of viscosity, as described by Onogi and Asada for liquid-crystal polymers. The viscosity as a function of the weight ratio of the two polymers at constant shear rate exhibits deviations from additivity of viscosities of the two components at all concentrations. In mixtures of two polymers in the melt, deviations are also observed; the negative ones are attributed to phase separation and the positive ones to homogenous mixing (comparison with the phase diagram). All the mixtures studied (anisotropic, isotropic, or biphasic), show ranges of shear rates where the first normal-stress difference is negative, as is generally observed for anisotropic liquid-crystal polymers. It is concluded that the isotropic solutions become anisotropic under shear, as they are not far from the critical concentration. © 1994 John Wiley & Sons, Inc.     

Rheology of concentrated solutions of poly(γ-benzyl-glutamate)

Steady shear viscosity, dynamic viscosity, dynamic modulus, and normal force were measured via rotational rheometry for concentrated solutions of racemic mixtures of poly(benzyl-glutamate) and poly(benzyl-D-glutamate) in m-cresol. A transition from the isotropic state to liquid-crystalline order with increase in concentration was indicated by optical anisotropy and maxima in all four material functions. This occurred at a critical concentration higher than the Flory prediction. Over a well-defined range of concentrations and shear stresses, some of the liquid-crystalline solutions exhibited negative first normal-stress differences that were not due to inertial effects.     

A novel orientation technique for semi-rigid polymers. 1. Preparation of cross-linked cellulose acetate and hydroxypropylcellulose films having permanent anisotropy in the swollen state

It has been predicted that unusually good mechanical properties can be obtained by drying swollen networks of semi-rigid chains while they are in the deformed state, as described in several theoretical investigations [Macromolecules,23: 5335, 5341 (1990),24: 901 (1991)]. The present investigation involves the preparation of networks of this type from cellulose acetate (CA) and hydroxypropylcellulose (HPC), in order to test these concepts. The cross-linking required to maintain anisotropy during the drying process was obtained using formaldehyde, while the polymers were in either the anisotropic or isotropic state. Control of the cross-linking was obtained by studying the effects of the concentration of formaldehyde, temperature, and reaction time.The liquid-crystalline phase separations in CA and HPC, and in their networks, were studied with cross-polarized optical microscopy. CA and HPC showed anisotropic phases in trifluoroethanol and in methanol, respectively, and under shear the HPC systems exhibited the band textures associated with macroscopic orientation. In the case of the uncross-linked polymers, this band texture disappeared shortly after shearing was discontinued. The networks prepared by cross-linking the HPC in either liquid-crystalline solutions or in isotropic solutions also showed band textures, but these textures now persisted long after removal of the shearing stress. As shown in the following paper, the extensibility required in the proposed processing technique was highest for the networks prepared in the isotropic state, suggesting that these materials should have the greatest potential for dramatic improvements in mechanical properties.     

Equilibria of extended-chain polymers exhibiting crystalline and liquid-crystalline phases

The phase diagram of poly(p-benzamide) (PBA) in N,N-dimethylacetamide/LiCl solutions was determined for two PBA samples having weight-average molecular weights of about 10,000. The various equilibria were studied using analytical, viscometric, and optical microscope measurements. The phase diagram at 25°C, taking as variables the concentration of polymer (C_p) and LiCl (C_s), involves several equilibria which can be summarized as follows: solid ? isotropic solution when 0.75 ≤ C_s ≤ 2 g/dl, solid ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, isotropic solution ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, and C_p > 6 g/dl, and dilute isotropic solution ? gel when C_s > 4 g/dl. In the C_p range in which the isotropic and anisotropic phases coexist, enrichment of the high-molecular-weight component of the polymer in the anisotropic phase becomes more marked as the volume fraction of the latter phase is decreased. The two PBA samples exhibit noticeable differences in solubility, absolute viscosity, and in their viscosity-concentration behavior. The location of the maximum in the latter dependence does not necessarily coincide with the first appearance of the anisotropic phase. In the absence of a flow field, anisotropic solutions exhibit an irreversible increase in viscosity. Inclusion of the equilibria involving the crystalline state furnishes insight into some of the common observations for extended-chain polymers. A diagram illustrates the superposition of the solubility curves for a crystalline polymer and the liquid-crystal regions. This indicates that, for the high melting crystalline polymers, the crystalline phase should be stable relative to the concentrated anisotropic phase of the wide biphasic region.     

Formation of band textures in hydroxypropylcellulose liquid crystals

Upon shearing, some polymeric liquid crystals develop a particular texture, called “band texture,” consisting of fine equidistant black lines as viewed between crossed polarizers. In mesomorphic hydroxypropylcellulose–water solutions, these bands are due to relaxation of the shear. The band texture appears after shearing at low shear rates and seems to exist during shearing at high shear rates (> 200 S^?1). In the latter case, the steady-state first normal-stress difference is negative. The bands relax in a few minutes and parts of the bands form elongated domains. This texture gives a H_v light-scattering pattern which can be interpreted by the theory of scattering by anisotropic rods. These domains lose their elongated shape with time.     

Viscometric behavior of mesomorphic ethyl cellulose solution in chloroform

A simple modification of the viscometer used to determine the viscosity of concentrated solutions is described which permits the ready measurement of viscosity versus shear rate behavior above room temperature. Basic experimental viscometry was performed on the ethyl cellulose–chloroform system using a B-type (Brookfield-like) rotating-cylinder viscometer. Data are compared with theoretical predictions. The flow curves exhibit yield stress at low shear rate. The yield stress is dependent on concentration and temperature. The viscosity versus concentration curves show typical mesomorphic behavior. The concentration dependences of viscosity and yield stress are similar. The critical concentration increases with temperature. In the single-phase region (isotropic or anisotropic state) the viscosity decreases with temperature and the apparent activation energy (E_a) for flow is positive, but in the biphasic range the viscosity increases with temperature and E_a is negative. The experimental results for the critical concentration appear to agree with Flory's theory of rodlike molecules when the ratio of persistence length to diameter of the chain is taken as the aspect ratio.     

Rheology of concentrated solutions of helical polypeptides

Rheological studies were carried out on concentrated m-cresol solutions of two helical synthetic polypeptides; poly-γ-benzyl-L -glutamate (PBLG; molecular weight, 150,000) and poly-?-carbobenzyloxy-L -lysine (PCBZL; molecular weight, 200,000). Steady shear measurements were made over a range of 0.01–16,000 sec^?1 to obtain steady shear viscosity and first normal stress difference. Dynamic viscosity and dynamic storage modulus were measured both by oscillatory shear between cone and plate and also by an eccentric rotating disk device over frequency ranges of 0.1–400 and 0.1–63 rad/sec, respectively. The concentration ranges were such that both liquid crystalline and isotropic solutions were investigated. The previously reported observations of an apparent negative first normal stress difference within a defined range of shear rate for liquid crystalline solutions were confirmed for the PBLG and PCBZL solutions. At high shear rates the peaks in plots of steady shear viscosity against concentration were profoundly suppressed but peaks in first normal stress difference versus concentration were not. The observation of liquid crystalline order in PCBZL/m-cresol solutions at room temperature constitutes evidence that the inverse coil-helix transition temperature is lower in concentrated solutions than in dilute solutions. The critical concentration for formation of the liquid crystalline phase was higher for PCBZL than for PBLG, despite a higher axial ratio, due to helix flexibility.     

Salting-in behavior of isotropic and anisotropic aqueous hydroxypropylcellulose solutions

 Salts with large polarizable ions are capable of salting-in complex aqueous polymer solutions exhibiting microstructure, thereby inducing changes in the phase behavior and properties of the solutions. In this work, the dynamic rheological properties of isotropic and mesomor-phic hydroxypropylcellulose (HPC) in aqueous media have been investigated in the presence of one such salt, guanidine thiocyanate (GuSCN). Addition of this salt to isotropic aqueous HPC solutions is found to induce an increase in the magnitude of the elastic shear modulus (G′). At HPC concentrations above the isotropic→mesophase transition, however, addition of GuSCN results in a substantial reduction in G′ due to microstructural changes in the chiral nematic HPC mesophase. This reduction in G′ indicates that the microstructure of a water-soluble polymer exhibiting supramolecular organization can be tailored through salting-in, and is likewise expected to facilitate the commercial processing of HPC at high solids concentrations. Received: 4 June 1996 Accepted: 3 September 1996     

Rheology of liquid crystalline phases of alkyloxybenzylidene toluidines

A unique viscometer of the CS rheometer viscometer class designed at the Kazan State University of Technology is used to measure viscosities of two p-n-alkyloxybenzylidene-p-toluidines in the entire temperature range of the liquid crystalline state and transition into an isotropic liquid. The measured shear stresses and flow rates are used to calculate shear rates and plot flow and viscosity curves. The liquid crystalline phase and isotropic liquid are demonstrated to possess Newtonian viscosity, whose viscous flow activation parameters are calculated in the temperature range under study. The results are discussed from the standpoint of intermolecular interactions and structural details of the liquid crystalline phase.     

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.     

Flow properties of branched polydisperse polymers

Viscosity and normal-stress difference as functions of shear rate were examined for poly(vinyl acetate) samples with various degrees of random branching in diethyl phthalate solution. At moderate concentration (c = 0.17 g/ml) the viscosities were depressed by branching, in fair accord with the Bueche theory. However, at higher concentrations (c = 0.35 g/ml) the data showed a progressive trend in the direction of viscosity enhancement by branching, a characteristic which has been observed by other workers in undiluted branched polymers. Accompanying viscosity enhancement was an increase in the temperature coefficient of viscosity, an increase in recoverable compliance (estimated from steady-state normal-stress data) and an early onset in the shear rate dependence of viscosity.     

The electrorheology of rigid rod poly(n-hexyl isocyanate) solutions

The electroviscosities of solutions of a lyotropic liquid crystalline polymer, poly(n-hexyl isocyanate), were investigated by a slit viscometer. The morphologies of the solutions being studied include the isotropic, the biphasic, and the fully liquid crystalline. All three morphologies exhibit viscosity enhancements with imposition of an electric field. However, the electrorheological behavior of the isotropic solution is different from those of other morphologies. The isotropic solution starts with a higher field free viscosity and its electroviscosity increases gradually with the increasing electric field strength. In contrast, the anisotropic solutions begin with lower zero field viscosities and the electroviscosities increase sluggishly until a critical field strength is reached; the viscosities then increase rapidly and finally exceed that of the isotropic solution. For the morphologies of the biphasic and the fully liquid crystalline, the dependence of the viscosity enhancements on field strength and shear rate can be described by a single variable. The variable scales with the square of the electric field strength and the reciprocal of the shear rate. By introducing the effect of the molecular permanent dipole moments into Doi's theory, the electrorheological effects of PHIC solutions can be satisfactorily interpreted. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1217–1224, 1997     

Rheological and microcalorimetric studies of a model alkali‐soluble associative polymer (HASE) in nonionic surfactant solutions

Rheological experiments were carried out on a 1 wt % hydrophobically modified alkali‐soluble emulsion (HASE) solutions at pH ∼ 9 in the presence of nonionic polyoxyethylene ether type surfactant (C₁₂EO₂₃). The low shear viscosity and dynamic moduli increases at c > cmc until they reach a maximum at a critical concentration, c^m of approximately 1 mM (∼17 times the cmc of free surfactant) and then decrease. The dominant mechanism at cmc < c < c^m is an increase in the number of intermolecular hydrophobic junctions and a strengthening of the overall associative network structure. Above c^m, the disruption of the associative network causes a reduction in the number of junctions and strength of the overall network structure. The influence of C₁₂EO₂₃ on HASE before cmc could not be detected macroscopically by the rheological technique. However, isothermal titration calorimetry enables the determination of complex binding of surfactant to the polymer. Isothermal titration of C₁₂EO₂₃ into 0.1 wt % HASE indicates that the C₁₂EO₂₃ aggregation in water and 0.1 wt % HASE polymer solutions is entropically driven. A reduction in the critical aggregation concentration (cac) confirms the existence of polymer–surfactant interactions. The hydrophobic micellar junctions cause a decrease in the ΔH and ΔS of aggregation of the nonionic surfactant. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2019–2032, 2000     

Concentrated solutions of polydisperse rodlike polymers in the isotropic and lyotropic liquid-crystalline phases. I. The effects of molecular length distribution on phase behavior

The earlier calculations of Moscicki and Williams of the phase behavior of rodlike macromolecules in solution incorporating a Gaussian distribution of rod lengths have been extended to include, in addition to a “basic” Gaussian distribution, a small amount of high-molecular-weight species which are characterized by a δ-function or “box-function” distribution. It is shown that C^*_p, the concentration of polymer at which the biphasic material (isotropic plus anisotropic phase) first appears may be substantially lowered in comparison with that for the system comprising only the basic distribution. The volume fraction Φ_A of anisotropic phase, the compositions of the two phases, and the distribution of species between the phases are calculated for a range of polymer concentration which spans the isotropic, biphasic, and anisotropic phase and, among several features obtained, it is shown that the high-molecular-weight species are essentially responsible for the initial formation of anisotropic phase in the biphasic system. The results of the calculations have important implications for the interpretation of published dielectric relaxation data for polyalkylisocyanates in solution at high concentration and for published shear-flow viscosity data for polyalkylisocyanates and poly(p-benzamide) in solution at high concentration and these are discussed in some detail.     

Computerized measurement of viscoelastic properties of macromolecular solutions: Frequency dependence over and extended range of solvent viscosity

The modified Birnboim transducer and a computerized data acquisition and processing system (DAPS) for the measurement of viscoelastic properties of macromolecular solutions are described. The apparatus has a continuous frequency range from 0.01 to ca. 700 Hz and a viscosity range from 2 to ca. 30,000 poise (sample volume 1 to 1.5 cm²). Sample temperature is controlled to within 0.002°C from ?30°C to +80°C. Working displacements are 10² to 10⁴ Å. The DAPS is designed for precise determinations of the magnitudes and relative phasing of two sinusoidally time-varying electrical signals (0.02%, 0.02°, respectively, for signals > 2 V peak) from 10^?2 to 10⁵ Hz. Cross-correlation techniques are used for noise rejection. For frequencies below 30 Hz values of the storage (G') and loss (G″) components of the complex shear modulus (G^*) of 1 dyne/cm² are determined to within 10% and 4%, respectively, for liquids of moderately low viscosity. Proportionately higher measurement accuracies for typical values of G' and G″ and the wide frequency and viscosity ranges permit extrapolation to infinite dilution and studies of limited molecular flexibility for many polymer—solvent systems.     

The Viscosity Properties of Sodium Silicate Solutions

Using a NDJ-1 rotational viscometer and an AR500 rheometer, both static and dynamic viscosities of sodium silicate solutions were measured with changes of concentration, temperature, modulus (molar ratio of SiO₂ to Na₂O), shear rate and chemical additives. Static results show that viscosity increases monotonously with concentration varying from 15 to 55%, decreases with temperature rising from 15 to 70 °C, and has a minimum value at a modulus of about 1.8. Measured data can be fitted quantitatively either by the Krieger-Dougherty expression or the Arrhenius equation with good agreement. This fact suggests that the sodium silicate solutions exhibit the properties of a suspension, in which the silicate anions, mainly constructed of Q ¹ and Q ² groups, act as a binder; the colloidal particles mainly constructed of Q ³ and Q ⁴ groups and small cations, act as effective rigid particles. Dynamic results show a shear thickening property in the high shear-rate regime, and a Newtonian property in the low shear-rate regime.     

Chain overlap and entanglements in dilute polymer solutions: Brownian dynamics simulation

We have analyzed chain conformations and the existence — or otherwise — of chain overlaps and entanglements in dilute polymer solutions (at concentrations c < C^*, c^* = critical concentration). The fundamental problem of existence of chain overlaps in dilute solutions is also related to the drag reduction phenomenon (DR). Some experimental results pertinent to DR are explained in terms of entanglements even for solutions at concentrations defined in ppm. We report results of Brownian dynamics simulations of polymer solutions in which the equations of motion of the chains are solved by using the Langevin equation. Chains move according to actions of a systematic frictional force and a randomly fluctuating force w(t), where t is time. In addition, a shear flow field can be introduced into the model. To evaluate the structure of polymer chains in solution we have devised a measure of interchain contacts and two different measures of entanglements. The results for c = 0.3 c^* demonstrate that both chain entanglements and overlaps take place even in dilute solution. They also confirm predictions from an earlier combinatorial model.     

Melt flow behavior of polypropylene composites filled with multi-walled carbon nanotubes during extrusion

Polypropylene (PP) composites filled with multi-walled carbon nanotubes (MWCNTs) were prepared using a twin-screw extruder. The melt flow properties of the composites were measured with a capillary rheometer in a temperature range from 180 to 230 °C and at various apparent shear rates varying from 100 to 4000 s⁻¹. The results showed that the melt shear stress increased almost linearly while the melt shear viscosity decreased almost linearly with increasing shear rates in a bi-logarithmic coordinate system. The melt shear flow followed the power law relationship and the dependence of the melt shear viscosity on temperature obeyed the Arrhenius equation. The relationship between the melt shear viscosity and the MWCNT weight fraction was roughly linear under the investigated range of temperature or shear rate.