RheologyâStructure Interrelationships of Hydroxypropylcellulose

Article pubs.acs.org/Macromolecules

Rheology−Structure Interrelationships of Hydroxypropylcellulose Liquid Crystal Solutions and Their Nanocomposites under Flow Veronica V. Makarova,† Maria Yu. Tolstykh,† Stephen J. Picken,‡ Eduardo Mendes,‡ and Valery G. Kulichikhin*,§ †

A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Pr. 29, 119991 Moscow, Russia Section NanoStructured Materials, Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628BL Delft, The Netherlands § Chemical Department, Moscow State University, Leninskiye Gory, 11991 Moscow, Russia ‡

ABSTRACT: A new approach to study the flow-induced orientation in LC polymer-based nanocomposites has been developed. This approach is a combination of optical, mechanical, and X-ray methods that allowed us to “visualize” physical processes taking place in these systems under flow. The HPC/water solution (matrices) concentrations were selected based on the accurate phase equilibrium lines determined with the optical interferometry method. Flow curves and the concentration dependence of the viscosity provide additional information on the phase state and structure of the samples. Rheo-X-ray data were obtained using a specially developed micro-Couette geometry consisting of a coaxial arrangement of two standard X-ray capillaries (the inner one rotates at different speeds). The orientation development of HPC in two-phase and full anisotropic solutions, as well as of the clay and HPC in the nanocomposites were measured (separately) as a function of shear rate. In addition, the decay of the orientation parameter with time after cessation of flow was analyzed to reveal several stages related to the disorientation and reformation of the cholesteric helix in HPC solutions. Crystalline nanoclay in flowing LC solutions also formed a mesophase structure. Specifically, the columnar mesophase of clay formed under certain conditions could be completely or partially converted to a discotic phase. Comparison of these results with optical images of the regular (circle-like) morphology observed under strong flow suggested that the elasticity is the general driving force for development of the regular instability in length scales of optical and X-ray scattering measurements.

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INTRODUCTION

allowing relatively easy study of them with a wide variety of techniques. After the first experimental observation of the aramides LC solutions behavior under shear flow6,7,10,11 and pioneering theoretical work,12,13 it became evident that the rheological response of LC polymers under shear flow is strongly affected by phase state and structural transformations of the director field. Therefore, it is very useful and informative to combine rheological and structural measurements for flowing LC systems. There are already many examples in literature combining radiation scattering under shear and rheological studies, using light (optical),14,15 neutron scattering,16 or X-ray scattering.17−20 Introduction of nanoparticles to LC-solutions leads to understanding of the role of LC-ordering in preparation of nanocomposites with anisotropic matrices. The most suitable for this aim are aqueous solutions of HPC and, as nanoparticles, aluminsilicate, e.g., hydrophilic Na−montmorillonite, because

Rheological properties of LC solutions and melts are interesting from both applied and fundamental viewpoints. One of the reasons to investigate main-chain lyotropic LC systems is that they orient spontaneously on a molecular scale and can be aligned easily under flow. This orientation leads to development of ultrahigh strong fibers (i.e., Kevlar, Twaron, Terlon, Bocell). During the spinning process of such fibers, the main driving forces are shear and elongation forces that convert the disoriented LC multidomain structure at rest into an uniaxially oriented fiber structure. Hydroxypropylcellulose (HPC) is one of a few polymers capable to form both thermotropic,1 and lyotropic LC phases.2,3 Early studies of lyotropic LC were concentrated mainly on the polyglutamates4,5 and aramide solutions.6−9 Difficulties to handle these systems restricted the scientific research to a few groups. Fundamental investigations became more accessible with the observation of cholesteric and nematic phases in cellulose derivatives, thereby activating renewed scientific interest in the main-chain LCPs. Some of these polymers are water-soluble and available in large quantities, © 2013 American Chemical Society

Received: May 30, 2012 Revised: January 22, 2013 Published: February 1, 2013 1144

dx.doi.org/10.1021/ma301095t | Macromolecules 2013, 46, 1144−1157

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The above brief overview for the nanocomposite rheology suggests that the rheology-morphology interrelationships for heterophase systems has been elucidated to some extent but not fully especially under strong flows. For deeper understanding, direct observation of the structure evolution under the flow combined with rheological measurements is needed. The most appropriate polymer matrices for this aim are lyotropic LC polymers, because their order is easily detected by the X-ray methods and they exhibit complex phase equilibria and specific rheological behavior. Thus, we studied the rheological properties of aqueous solutions of hydroxypropylcellulose (HPC) without and with clay, by conducting wideangle X-ray scattering (WAXS) measurements with a specially designed capillary Couette cell. Interrelationships of rheology and structure of such nanocomposites, elucidated from those rheo-WAX data as well as optically detected clay morphology under strong flow, are summarized and discussed in this paper.

we can expect the components to be compatible at least partially. Polymer−clay nanocomposites have become rather popular due to significant reinforcing of commodity thermoplastics and introduction of some functional properties at small filler content.21,22 In order to produce such systems, extensive studies have been conducted for characterization of nanoparticles, compatibility of polymers and particles via reactive polymerization,23 melt- or solution mixing,24−26 morphology of clay particles distribution in polymer matrices,27−29 and properties of the final nanocomposites (mostly mechanical properties).30−32 At the same time, processing conditions have not been discussed so intensively, although some attempts have been made in relation to intercalation or exfoliation of clay crystalline structure.33−37 Nevertheless, the rheology is of fundamental importance since any filled polymer system should be most likely processed in a liquid state and under flow for conversion to a solid composite material. So, it is desired to elucidate not only the basic rheological properties but also structure transformation which a heterophase viscoelastic system undergoes during processing. Processing is the stage where structure and final properties of nanocomposites are developed. In the case of the combination of liquid crystal polymers and nanoparticles in solution, we deal with systems composed of two phases, which are able to be ordered separately under flow, since most of the nanoparticles (clays, CNT’s) have highly anisotropic shape. Clay particles38−41 can form discotic nematic, or columnar mesophases in dispersions (due to the flat shape of elementary platelets). If such anisotropic nanoparticles are combined with isotropic polymer melt, a LC phase can be formed by themselves.43,44 On the other hand, an intrinsically anisotropic LC matrix can become isotropic due to a disturbing effect of the nanoparticles.45 However, studies of such particle/LC composites are still very limited, and a brief review of those studies is given below. In an attempt to understand the influence of flow on the rheology-morphology interrelationship, aqueous isotropic and LC solutions of hydroxypropylcellulose (HPC) filled with model spherical polystyrene (PS) particles have been examined to elucidate two significant effects, suppression of the negative normal stresses of a LC solution in presence of the PS particles and formation of long strings of those particles in both isotropic and anisotropic solutions. For clay/polymer composites,48 closed regular circles of clay particles were found to be formed in a rotational sphere-plate unit. This formation took place under strong flows at rather high shear stresses close to the “spurt” or an unstable flow regime.49 A similar phenomenon, i.e., formation of optically detectable alternating rings, was noted for the neat polymer melts as well. The main driving force of this texturing of the polymer matrix itself was related to its specific elastic ordering under strong flow.50,51 Small particles added to this matrix arrange themselves along interfaces (streamlines) separating neighboring rings of the matrix and act as tracers. For LC systems, it is known that a hydrodynamic field affects the position of the equilibrium lines in the phase diagrams.46 Moreover, there are examples of transitions of isotropic solutions to the LC-state due to a mechanical field.47 We have not come across similar transitions for clay platelets but can still expect that the orientation caused by the mechanical field (in shear or extension flows) leads to ordering. To confirm this expectation, we need to perform structural measurements in situ, i.e., directly under the flow.

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EXPERIMENTAL SECTION

Materials. Hydroxypropylcellulose (Klucel EF, Aqualon Co) with a molecular weight, Mw = 80 000, was used for the preparation of solutions in distilled water. The HPC powder and the appropriate amount of water were mixed by hand. Then, as-prepared solutions were kept at 40 and 20 °C alternatively for 3−10 days to ensure uniform dissolution.52 In this way, solutions with the HPC concentration in the range of 30−80% (with 5% increment) were prepared. The Na−montmorillonite Cloisite Na+ (Southern Clay Products, USA) was dried at 100 °C overnight before mixing. A 6% suspension of clay in distilled water was prepared by hand. For preparation of clay/HPC dispersions, powders of HPC and clay were preliminary grinded in an agate mortar. Subsequently, the calculated amount of distilled water was added to the mixture and the paste-like presolutions were treated at 40 and 20 °C in a way mentioned above to prepare the dispersions of various HPC concentrations containing 5% of clay. For understanding the influence of clay on the equilibrium phase diagram of HPC−water system and estimation of shear stresses developed in the Couette X-ray cell, rheological tests were carried out for the neat solutions and also the dispersions of clay (5%) in the same solutions at ambient temperature. Detailed knowledge of the phase diagram of the HPC−water system is required to set the appropriate experimental conditions. Therefore, it is important to recall the phase behavior of the HPC solutions under study here. Several versions of phase diagrams have been published and some of them are shown in Figure 1.

Figure 1. “Combined” phase diagram for HPC in water: ▲ −;53 ● −;54 ■ −.55 The upper curve was taken from the paper.19 1145


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(between relaxed specimen at rest and specimen at flow) were obtained. The orientation attained under flow can be expressed quantitatively in terms of the orientation parameter P2 defined below. The intensity distribution can be described as

From the thermodynamic viewpoint, similar diagrams exhibiting transitions from the isotropic I to the LC state via the biphasic corridor (I + LC) have been reported. In addition, the crystal solvate phase (CS) at high concentrations of HPC was observed in some studies. Nevertheless, the exact location of the equilibrium lines varies significantly from study to study. This means that the phase diagram of HPC−water system needs to be determined accurately. Here, we report results from microinterference, polarizing microscopy and viscometry which gave a reliable phase diagram with precisely located equilibrium boundary lines for the HPC−water system. In isothermal conditions, the isotropic, biphasic, and fully LC phases emerged on change of the HPC concentration, and boundaries between these phases were accurately determined. Methods. Two optical methods were used to construct the phase diagram: microinterference and polarizing microscopy. For the optical microinterference measurements,56 a HPC film was placed in a wedgeshaped gap between two semitransparent optical windows. To an open end of the cell, water was added to make contact with the edge of the HPC film. Interdiffusion between water and HPC was monitored over time by a video camera connected to a PC. This provided information on the dissolution rate and the position of the phase boundaries. In some cases, the interdiffusion zone was examined using the polarizing microscopy method to verify the anisotropy of the system. In this way, it was possible to determine the equilibrium lines along the diffusion profile, i.e., the position of the phase boundaries under isothermal conditions. Performing the same experiment at different temperatures, the phase diagram as a whole was constructed. Two rheometers, Rheotest-2 (Haake, former DDR) and Rheostress 600 (Thermo Electron, Germany) having the cylinder-cylinder and cone and plate geometries, respectively, were used for rheological characterization of the systems under study. Most of experiments were carried out at ambient temperature in the shear rate range 10−2−103 s−1 (steady-state flow) and 10−1−103 s−1 (low-amplitude oscillatory regime). For rheo-X-ray measurements,17,18 the following equipment was used. Two glass coaxial capillaries were combined in a Couette cell (Figure 2). The outer capillary of radius Rout was fixed, and the inner

⎦

I = I0 + Aeα[cos (φ − φ0)⎤ 2

where φ is the azimuthal angle, α is a parameter characterizing the width of the intensity distribution. From inherent α values for for HPC and clay, we can calculate P2 for both components separately as 1

⟨P2⟩ =

∫−1 P2(cos φ)eα[cos 1

∫−1 eα[cos

2

φ]

2

φ]

d[cos φ]

d[cos φ]

The rheo-X-ray Couette cell did not allow to measure a torque and, consequently, shear stress. For estimation of the shear stress developed in the cell, the data of independent rheological measurements (flow curves) were used to find the shear stress corresponding to the shear rate realized in the two cells (having two different gaps).

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RESULTS AND DISCUSSION The Phase Diagram of the HPC−Water System. From the optical interference measurements (Figure 3a), four

Figure 2. Scheme of the experimental device for “rheo-X-ray” measurements.

one of radius Rin rotated. Two pairs of capillaries of radii Rout = 0.5, Rin = 0.25 and Rout = 0.7, Rin = 0.5 and ∼10 cm in length were used. The first pair had a large gap, but it was more convenient for loading high viscous solutions and used as the basic fixture. To prove that such a large gap still gave reasonable results, the second pair with a narrower gap was used in some cases. Nevertheless, keeping in mind that shear rate varies across the wide gap, the average shear rate γ was calculated as γ = r dω/dr = ωR̅ /Rout − Rin = 2πn(Rout − Rin)/2/60(Rout − Rin), where ω is the angular velocity and n is the rotation speed (rpm). The Couette cell was correctly aligned with respect to the collimator and 2D detector with the aid of a x,y,z-stage of the goniometer, an alignment laser beam, and a video camera. The collimated X-ray beam (with a diameter of 0.5 mm) passed through the center axis of the Couette geometry (via two slits along the gradient axis). In all cases, the differential X-ray diffractograms

Figure 3. Microinterference patterns of interdiffusion zone at two temperatures (a) and the phase diagram of HPC−water systems based on these data (b).

different sections are observed along the diffusion zone at temperatures lower than 37 °C. These sections correspond to the traditional phase sequence for solutions of stiff-chain polymers (from left to right): isotropic (I), biphasic (II), LC (III), and CS (IV). The exact position of boundaries between them is also shown. With increasing temperature above 37 °C, the interference pattern changes dramatically. The sections I and II disappear and the sharp border identified as an amorphous phase separation emerges. The sections III and 1146


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to solutions leads to the increase in the viscosity in all cases. The magnitude of the viscosity increase due to clay is larger at low shear rates than at high shear rates. It means that the viscosity anomaly is enhanced by the clay particles to narrow the Newtonian flow range. At the same time, the clay particles hardly affect the qualitative feature of the flow curves. As typical for LC polymers, the 50 and 60% solutions exhibit three regions in the flow curve (see, for example,8,10) whereas the boundaries between these regions are smeared for less concentrated solutions. The first decrease of the viscosity with increasing shear rate is attributed to the defect network that disturbs transition of the polydomain structure at rest to the monodomain structure at flow. The second plateau-like region corresponds to the flow of the monodomain, and the third region where the viscosity decreases again, to fragmentation of the monodomain. The viscosities of unfilled and filled solutions (matrix and dispersion) at the shear rate of 0.25 s−1 are compared in Figure 6. Clearly, the maxima in the viscosity coincide completely for the unfilled and filled solutions, which indicates again that the clay does not influence the phase transition of HPC bulk solutions from the isotropic to the LC state. Change of HPC concentration due to adsorption or intercalation of HPC molecules into clay is localized in the close vicinity of the clay surface and does not significantly affect the rheological behavior of the dispersions. Comparing these rheological features with the phase diagram, we see that the maximum viscosity for both dispersion and HPC matrix corresponds to the appearance of the LC phase (∼40%) that leads to a decrease in the hydrodynamic resistance due to the alignment in the LC state. The minimum is located in the middle of a biphasic region (∼50%). Above 54%, the LC phase becomes dominant and the viscosity increases with increasing concentration. On the basis of these results, the 30% isotropic solution, 50% biphasic solution with the lowest viscosity in the anisotropic region, and fully anisotropic 60% solution were chosen for further experiments using the rheo-X-ray setup. Rheo-X-ray Data. Flow Alignment of HPC Solutions. For the 50% HPC solution, plots of the intensity against scattering angle 2θ (diffractogram) at different average shear rates are shown in Figure 7. The strongest diffraction emerges at 2θ = 6.1° and its position does not change with the shear rate. Thus, this diffraction is later utilized as the intrinsic diffraction of HPC (that characterizes interchain order of the oriented HPC macromolecules). Some of the diffraction data showing the evolution of HPC alignment at different shear rates are presented in Figure 8 for the 50 and 60% HPC solutions. On the sample loading into the Couette cell, the HPC macromolecules were shear-oriented to some extent in the direction of the capillary (cell) axis so that the HPC diffraction is located predominantly on the equator. The Couette flow induces the HPC reorientation along the shear direction to turn the reflection from the equator to the meridian direction. In the case of a biphasic solution, the intensity of this diffraction increased with increasing shear rate and became constant at rates larger than γ =226.1 s−1. In the case of the full LC solution, the intensity of this reflection was the strongest at moderate rates, but decreased at high rates (compare the patterns for the rates 339.1 and 471.0 s−1). The distribution of the scattering intensity in the azimuthal angles (Figure 9, parts a and b) confirms the weak initial orientation on the sample loading explained above and the

IV are conserved. On the basis of these data, the phase diagram was constructed (Figure 3b). The phase boundary exhibits a smooth, shallow minimum against temperature, indicating that it is the binodal boundary for two amorphous phases (the liquid−liquid equilibrium). Heating these solutions to temperatures above this boundary leads to the formation of a strong gel. All lyotropic LC transitions (below the binodal) are close to the literature data except the following point. In our case, the right branch of the binodal is located at a lower HPC content compared with the literature data, and it roughly coincides with the boundary between biphasic and LC states. Our attempts to perform the same experiments with clay particles were only partially successful. When distributed in water, clay particles spontaneously coagulated at the interface to form the aqueous dispersion with a porous clay layer being penetrable for water. So, we cannot exclude a partial adsorption of HPC macromolecules on the particle surface in local preinterface layer or even intercalation of HPC into interspaces of the clay structure. However, the position of the equilibrium lines in the dispersion was the same as that in the neat HPC− water system (pure matrix). Thus, clay does not affect significantly the location of the equilibrium boundaries of HPC in the phase diagram. The interaction between HPC and clay is further discussed below for the viscosity and rheo-X-ray data. Viscometry. For HPC−water solutions of various concentrations, dependence of the viscosity on the shear rate is shown in Figure 4. The viscosity does not change with the

Figure 4. Dependence of the viscosity on the shear rate for HPC− water solutions with HPC concentration: 30 (1), 35 (2), 40 (3), 50 (4), and 60% (5) at 25 °C.

concentration monotonically: the lowest curve is for 30% solution while the highest one is for 40% solution. The curves for all other concentrations, including those above 40%, lie in between. The shape of curves changes with increasing concentration from a typical pseudoplastic non-Newtonian fluid to a yielding (viscoplastic) system. The latter shape is intrinsic for LC polymer solutions and melts, and the yield stress reflects a disclination network and a transition from the multidomain to monodomain structure occurring on an increase of the shear rate.57,58 Parts a and b of Figure 5 display the dependence of the viscosity on the shear rate for the neat HPC solutions and the same solutions filled with 5% of clay. As can be seen, the highest viscosity is observed for the 40% solution and the lowest one for the 30% solution. The introduction of 5% of clay 1147


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Figure 5. Dependences of the viscosity on the shear rate for 30 (1, 4), 35 (2, 5) and 40% (3,6) neat (1−3), and filled (4−6) HPC−water solutions (a). The same is for concentration 50 (1, 3) and 60% (2, 4) (3, 4 for the filled solutions) (b).

ratio in the range of 1.4−2.0, the experimental data coincide completely. This result strongly suggests that the data were hardly affected by the shear rate gradient in the gap and are reliable. The corresponding intensity data of the 60% solution are shown in Figure 10b. Differing from the behavior of the less concentrated 50% solution, the diffraction intensity of the 60% solution rapidly increases on the increase of γ to 28.2 s−1 and then decreases on a further increase of γ. These data indicate higher stability of biphasic solution in strong shear flow (at γ >400 s−1, for example) compared with fully anisotropic solution. Presence of interfaces and elongation of the isotropic phase droplets in the LC matrix appear to stabilize the biphasic solution. In the full LC solution these mechanisms do not work well and tumbling or fragmentation of the LC phase possibly destabilizes the orientation of HPC under strong flow. Relaxation. The relaxation of the orientation after cessation of the shear flow can also provide useful insight on the LC structure in these solutions and elastic properties. In the biphasic system, it is expected that the presence of deformable phase boundaries and their elastic recoil will be a significant factor that reduces the orientation induced under the flow. In the full anisotropic solution, the strong flow likely induced some disturbances (tumbling and/or elastic turbulence) that reduced the orientation, as discussed for Figure 10b. Similar effects were observed in ref 42, and the instability in LC solution of HPC was reported to start already at the shear rate of 100 s−1. In addition, this could be a regular elastic instability leading to formation of ordered morphology as observed in refs 48 and 50. On relaxation of HPC solutions the loss of the director orientation should not result in chaotic random texture, but should be transformed to the cholestric texture inherent to solutions of cellulose derivatives at rest. This chiral arrangement is converted easily to the uniaxial nematic texture under the flow, as is indeed observed in the flow experiments reported here. The informative and convenient analysis of the orientational relaxation can be made for ⟨P2⟩ evaluated from the diffractograms in the azimuthal angles. Figure 11 shows “log (⟨P2⟩/ ⟨P2⟩o) − log t” plots, where ⟨P2⟩o is the maximal order parameter during flow (just before its cessation), and ⟨P2⟩ is the order parameter at time t after the cessation. The ⟨P2⟩o value of the 50% solution is the same for all shear rates higher than ∼200 s−1. For the 60% solution, the orientation under the steady flow is maximal at the shear rates lower than 226 s−1, as shown/discussed in Figure 10b.

Figure 6. Concentration dependences of the viscosity for unfilled (1) and filled (2) solutions measured at the shear rate of 0.25 s−1.

Figure 7. Diffractograms of the 50% HPC solution at different shear rates as indicated.

change of the main diffraction direction (90°, rotation) due to the flow-alignment of HPC. For the biphasic solution the monotonous increase in the azimuthal intensity with increasing shear rate witnesses the increase in the magnitude of orientation. For the full LC solution, this magnitude, evaluated as the intensity, passes through a maximum and then decreases. For the 50% solution, the above rheo-X-ray measurements were conducted with two sets of Couette cell having different gaps. The values of the order parameter at various average shear rates calculated from the azimuthal diffractograms are presented in Figure 10a. As seen in the figure, regardless of the Rout/Rin 1148


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Figure 8. Two-dimensional diffraction patterns for 50 (a) and 60% HPC solutions (b) at different average shear rates (indicated in patterns in s−1).

Figure 9. Distribution of azimuthal intensity of the strongest HPC diffraction for 50 (a) and 60% solution (b) at different shear rates (indicated in figure).

Figure 10. Dependence of the order parameter on the shear rate: (a) for the 50% solution in two Couette cells (△ Rout/Rin = 2, ◆ Rout/Rin = 1.4); (b) comparison of 50 and 60% solutions.

For all rates of the flow, Figure 11 shows that the relaxation of both biphasic and full LC solutions dominantly proceeds at least in two, fast and slow stages. In the fast stage, the relaxation is faster for larger preshear rate. In the slow stage, the relaxation rate is insensitive to the preshear rate for the biphasic solution, but for the LC solution the relaxation faster for higher preshear rate. In particular, for the highest preshear rate, ⟨P2⟩/⟨P2⟩o of the 60% solution significantly relaxes rapidly, whereas for low preshear rates the relaxation is very slow and the orientation developed under flow does not seem to relax completely. This can be related to formation of the band-like structures similar to that observed in refs 50 and 51.

The intersection point between the fast and slow relaxation processes may provide us with useful information for the relaxation mechanism. For the 50% solution the time for the intersection decreases (from ∼15 to 6.7 min) with increasing preshear rate, whereas the intersection time (∼10 min) remains insensitive to the preshear rate for the 60% solution. The disperse phase droplets in the biphasic 50% solution, being elongated and oriented (probably up to critical Taylor radius) can break up to accelerate the fast relaxation in biphasic solution compared with the full LC solution, as observed in Figure 11. 1149


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Figure 11. Decay of the relative order parameters in time for 50% (a) and 60% solutions (b) after cessation of flow at various rates (indicated in the figure). The plots are shown in the double-logarithmic scale.

Figure 12. Evolution of the intensity in the azimuthal angles with the increase in the shear rate and at relaxation (a) and the change of the order parameter with increasing the shear rate (b) for the 30% HPC solution containing 5% of clay.

Nanocomposite Precursors. At first, two isotropic dispersions were studied: 6% clay in water and 5% clay in the 30% HPC solution. The dispersion in water showed a weak basal reflection of clay at 6.7° only under fast shear (at ∼500 s−1). The calculated order parameter, 0.24, was rather low. This result coincides with that obtained in60 for clay-water suspensions, but contradicts to the well-defined orientation of hydrophobic clay particles dispersed in oligomeric polybutens.61 In the last case the specific interaction between short matrix chains and dioctadecyl tails of the clay modifier may have provoked the local elasticity that could align the particles. The situation changes drastically when the water is replaced with the polymeric matrix, the 30% HPC solution in water. Since this polymer solution was isotropic, the orientation of the HPC chains in shear flow was not achieved. At the same time, the clay orientation was clearly noted and could be characterized quantitatively. The diffraction intensity of the basal reflection at the azimuthal angles (∼3.2°) inherent for the wet clay grows up prominently with increasing the shear rate, as shown in Figure 12a. This leads to a significant increase in the order parameter, up to 0.85 at γ ∼ 500 s−1 (Figure 12b). The orientational relaxation of clay in the 30% HPC solution proceeded slower compared to that in water and a residual orientation remained after cessation of strong flow for 5 min or more, as noted in Figure 12a. This difference in the behavior of clay in the HPC solution and water can be related to higher viscosity and elasticity of the HPC solution. Large shear and normal stresses (not the shear rate itself) should have resulted in the clay orientation. What

A multistage character of the relaxation curves possibly reflects different processes: the healing of instability disturbance developed under the strong flow, the loss of uniaxial orientation, probably accompanied by polydomain texture formation, and its transformation into the cholesteric phase. Hypothetically, we can assume that the defects should be healed just after the cessation of flow, and the uniaxial orientation developed under the flow relaxes, in a chaotic manner, in the fast stage. Finally, the transformation of this intermediate chaotic phase into the cholesteric phase results in the slow relaxation. The above observation on the orientation relaxation is similar to what has been found for the stress relaxation in the lyotropic PBLG/m-cresol solution,59 in which the stress relaxation occurs faster for higher preshear rates. The evolution of the global orientation parameter for a series of aqueous HPC solutions both during flow and after cessation of flow has been reported in.20 Time-resolved orientation measurements were also made in situ using X-ray rheometer: after cessation of the flow, the orientation was found to decay to zero for low preshear rates, whereas to a small but nonzero value for high preshear rates. Probably, this effect corresponds to the development of the ordered structures under strong flow, which do not relax (disorder) completely. This interpretation was not advanced in reference20 where the acceleration of the relaxation for higher preshear rates was noted. This result is similar to the result seen for the fast process in this paper but may require separate analysis of the fast and slow processes. 1150


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because of the orientation along the capillary axis (cell axis) on sample loading. However, under shear flow, those diffractions change their directions: the HPC diffraction moves to the meridian, and the clay diffraction remains in the equator. There exists an essential difference in behavior of the dispersions in biphasic and the full LC solutions at high shear rates. In the 50% solution the preferable direction of the clay diffraction oscillates with increasing shear rate, while in the 60% HPC solution, the clay diffraction splits into the equator and the meridian directions to give 4-spots pattern. The HPC reflection in both solutions remains in the meridian. These features of the clay and HPC orientation in the two matrix solutions are clearly noted also in the diffractograms in the azimuthal angles, as shown in Figure 15. In the two matrix solutions, evolution of the orientation of each component proceeds in a different way. In the 50% solution, the orientation angle of HPC becomes equal to 180° already at low shear rates and the intensity of the characteristic diffraction monotonically increases with increasing shear rate. The maximum orientation of the clay in this matrix solution is observed after loading the sample into the capillary gap, and the flow leads to the decrease in the basal diffraction intensity. With increasing the shear rate its intensity starts to increase monotonously, but at high shear rates a position of this diffraction oscillates between 245 and 270° relatively the equator position. In the 60% solution the HPC diffraction moves to the meridian after diminishing the loading effect, and its intensity increases with increasing shear rate. The clay diffraction in the full anisotropic matrix demonstrates more complicated changes. Its maximal intensity is observed on the sample loading, and under flow it changes its position moderately and loses its intensity with increase in the shear rate. The most drastic perturbation for the clay orientation is noted at the shear rates >350 s−1. The clay diffraction splits into the equator and the meridian directions. The intensity of the meridional diffraction increases, while the intensity of the equatorial diffraction decreases with increasing shear rate. Dependence of the order parameters on the average shear rate suggested that the clay orientation in both solutions exhibits rather abrupt change at a critical shear rate (∼250 s−1), as noted in Figure 16.

will happen if a matrix is not only viscoelastic but also anisotropic, i.e., in the matrix forms mesophase? To answer this question, we conducted experiments for the 5% dispersions of clay in 50 and 60% HPC solutions. First of all, we need to identify separately the inherent diffractions of HPC and clay. Looking at diffractograms for the filled 50% solution (Figure 13), one can see that the basal clay diffraction in an aqueous

Figure 13. Evolution of the diffractograms with increasing shear rate for the 50% HPC solution containing 5% clay.

solution is shifted to ∼3.2° compared with the diffraction typical for the “air-dried” HPC powder emerging at 6−7°, while the reference HPC diffraction remains at 6.6°. Thus, the diffractions of clay and HPC are easily distinguished. In addition, the HPC diffraction at rest is rather weak, which could be related to adsorption/intercalation of the major fraction of HPC molecules on/into the clay particles. With the increase in the shear rate, the redistribution of intensities of the clay and HPC diffractions occurs. This process can be induced either by release of HPC molecules from the clay surface due to the flow (anti-intercalation) or by enhancement of LC-ordering due to the clay surfaces. Further details of the structural change under flow can be examined for the rheo-X-ray data of the clay dispersions in the 50 and 60% HPC solutions, as shown in Figure 14. As noted in Figure 14, in the initial state at rest the clay and HPC diffractions commonly emerge in the equator direction

Figure 14. Two-dimensional diffraction patterns for the filled systems based on the 50 (a) and the 60% HPC solutions (b) at average shear rates indicated in patterns. 1151


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Figure 15. Distribution of the azimuthal intensity of the components of the dispersions in the 50% (a, HPC; b, clay) and the 60% HPC solutions (c, HPC; d, clay) at different average shear rates (indicated in the figure).

Figure 16. Evolution of the order parameters for HPC and clay with the average shear rate observed for the dispersions in the 50 (a) and the 60% HPC solutions (b) filled with 5% clay.

In the 50% solution the HPC order parameter in this region of the shear rates (Figure 16a) demonstrates a broad minimum, and the clay order parameter was dependent on the flow time (or on the total strain). The clay order parameter decreased sharply on start-up of flow (dotted line in Figure 16a), but increased with time to the “normal” stable values. In the 60% solution (Figure 16b), an intensity of the HPC reflection remains more or less stable though its value (∼0.5) is lower than that in 50% solution (>0.6). Interestingly, the presence of clay stabilizes the HPC structure under strong flow. In fact, comparison of Figures 10b and 16b indicates that the HPC order parameter in the 60% solution is stabilized by clay. Therefore, in the full LC solution the high elasticity should have stabilized the flow of the matrix. However, the clay orientation therein is not predictable beforehand. The clay diffraction in this region of the shear rates splits into the

For analysis of this orientational behavior, the 50 and 60% HPC solutions without and with clay were newly prepared and their rheological features were examined. The steady flow curves and the frequency dependence of the modulus are shown in Figure 17. The viscosity and especially the modulus are significantly larger for the 60% solution than for the 50% solution. Both solutions clearly exhibit the yielding region as well as the transition region to the plateau in their flow curves. The storage modulus G′ does not exhibit the terminal behavior (characterized with the power-law behavior G′ ∼ ω2), which is indicative of strong structure formation the solutions and compositions. For the flow curves, we note that the irregular instability (“spurt” behavior) starts at the shear rate of ∼500 s−1 (two last points in Figure 17a). Increase of the elasticity on addition of clay (cf. Figure 17b) influences the orientation of both components in the dispersions, but in a different manner. 1152


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Figure 17. Dependence of the viscosity on the shear rate (a) and the storage modulus on frequency (b) measured for the neat solutions and dispersions.

Figure 18. Distribution of the clay diffraction intensity in the azimuthal angles at different shearing time (indicated in the figure) measured for the dispersions in 50 (a) and 60% HPC solutions (b) and full distribution of the HPC diffraction intensity in the biphasic matrix under flow for 5 min at shear rate of 471 s−1 (c).

Figure 19. Two-dimensional diffraction patterns and the kinetics of the order parameter changes for HPC and clay in the clay dispersions in the 50 (a) and the 60% HPC solutions (b) at different observation times. The shear rate is 471 s−1.

orientation in the 50% solution leads us to consider the kinetics of orientation for long-term deformation at the highest average shear rate, 471 s−1. Evolution of the azimuthal diffractograms in time is presented in Figure 18, parts a and b. In the 50% HPC matrix, the clay diffraction moves from the equator to the meridian during the shear over a time of 70−75 min (the total

equator and the meridian. The diffraction in both directions decreases the intensity at the shear rates higher 350 s−1. Thus, the clay orientation suffers some unexpected changes depending on the shearing time (in the 50% HPC solution) and/or the shear rate (in the 60% solution), despite the stable orientation of HPC therein. The puzzling behavior of the clay 1153


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strain is approximately 20 × 105 units). This situation does not change for longer observation times. In the 60% HPC matrix, diffractions in the two directions have emerged already after 5 min of shearing, as shown in Figure 19. The structure transformation in the 50% HPC dispersion (moving the clay reflection into meridian) causes a sudden decrease of its order parameter. In the 60% HPC dispersion, the coexistence of two preferable directions of the clay orientation is observed for the entire period of observation. An increase in the shearing time (total strain) leads to the redistribution of intensities in favor of the meridian direction. The diffractions in the two directions reach the maximal stable values after 35−40 min of flow (the total strain is ∼11 × 105 units). The HPC orientation is also not as simple as it may seem. In addition to the main diffraction in the meridian, a part of it passes to the equator, as shown in Figure 18c. This intensity redistribution in the filled 50% solution, emerging at the average shear rate of 226 s−1, was enhanced with increasing the rate and disappeared after prolonged shear at 471 s−1. Its absence corresponds to the transition of the clay diffraction from the equator to the meridian. Such redistribution can be interpreted in relation to partial intercalation of the HPC macromolecules into interspaces of the clay structure that allows the long axis of HPC macromolecules to become oriented perpendicular to the shear direction. The same effect might emerge through the anchoring of HPC domains on the clay surfaces. Change of the clay orientation, reflected in the transition of its main diffraction from the equator to the meridian, may rotate the HPC orientation by 90° and erase the HPC diffraction in the equator. Before discussion of the driving forces of structure perturbations under strong flows, let us focus on relaxation of clay and HPC in the dispersion after cessation of strong flow at the highest shear rate of 471 s−1. As seen from Figure 20, the

the clay orientation, presumably as a result of anchoring of HPC.

Figure 21. Two-dimensional diffractograms of clay dispersion in 60% HPC solution under strong flow for 70 min at shear rate of 471 s−1 (a) and after cessation of flow followed by 5 min relaxation (b).

At first sight, the distribution of clay orientation along the velocity or vorticity axis is similar to the orientation observed for dilute dispersions of nonspherical hematite particles in rather less elastic polymer solutions.62 The existing theories based on behavior of long slender bodies in second order fluids cannot explain this phenomenon even for dispersions of dilute particles. In spite of low absolute values of elasticity of the polymer solutions used in ref 62, a hypothesis has been proposed for an important effect of elasticity on orientation of particles and formation of aligned aggregates (threads) of particles. In our case, clay exhibits uniaxial orientation (in concentrated biphasic HPC solution) or biaxial orientation (in concentrated full LC solution), i.e., in the matrices being much more elastic compared to the solution examined in ref 62. Therefore, in both 50 and 60% solutions, the clay orientation suffers the drastic transformations in certain conditions. The strong diffraction from rather dilute clay particles (5%) observed in all experiments possibly results from macroordering of clay platelets, or their ensembles, or “tactoids”. Similar to polybutene-organoclay dispersion studied in ref 61, there is no exfoliation of the clay structure but only partial intercalation of the HPC macromolecules into the clay structure interspaces. In other words, HPC forms the extended chains or threads under flow. If the clay diffraction is located in the equator, the normal of the clay platelets is oriented in the shear direction. Such a situation is very similar to the columnar mesophase that looks like “stacks of coins”. Interestingly, this columnar structure becomes stable at the shear rates where the ring-like morphology is formed in the clay dispersion in the 50% HPC solution48 (see corresponding pictures accompanied by the flow curve in Figure 22a). As suggested from the shear rate dependence of the viscosity, the arrangement of particles into concentric circles proceeds at shear rates of about 250−300 s−1. In the same range of the shear rates, dramatic changes in the clay orientation would have taken place. We could not observe the morphology of suspensions in the Couette cell in situ, but optical images before and after shearing indicate the thread-like morphology formation in this case as well: the initially chaotic distribution of particles (Figure 22b) converts to the band-like or strip-like morphology (Figure 22c). We should keep in mind that this optical image was obtained for the bulk dispersion recovered from the Couette cell without any special orientation or treatment for microscopic testing.

Figure 20. Change of the order parameters for the HPC and clay in the biphasic matrix with time after cessation of strong flow.

order parameter of clay in the 50% biphasic matrix remains constant, while the HPC order parameter decreases smoothly with time. This behavior suggests that the clay orientation induced by the flow is stable in the 50% matrix even after removal of flow. In contrast, in the full anisotropic 60% matrix, the 4-spots diffractogram of clay under flow relaxes to the quadrant diffraction close to the equator diffraction in 5 min after cessation of flow (Figure 21). This change of the diffraction direction can be caused by reverse transformation of 1154


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Figure 22. Flow curve and morphology of the 50% solution containing 5% of clay obtained in a cone−plate unit (a), optical microscopic photos of the suspension of clay particles in the 50% HPC solution before (b) and after shearing (c) in the Couette cell.

Thus, the clay dispersions are oriented to nearly the same extent irrespective of the flow cell geometry, and this orientation can be affected by HPC given that HPC itself is highly oriented. This orientation of clay and HPC resulted in concentric circle morphology seen in Figure 22a. The band-like morphology formed in the specimen after shearing indicates the ordering involved both clay and HPC matrix. This argument is in harmony with a viewpoint underlying formulation of a “discrete” model of rheological behavior under strong flow.50 According to this model, the regular elastic instability emerges in polymer solutions and melts to develop spiral morphology preceding the irregular “spurt” behavior under conditions where the strain becomes largely recoverable. As an example, this texture in the case of polyisobutylene (PIB) is shown in Figure 23 (left and bottom). Introducing the clay particles in polymer allows visualization of the development of this morphology with increase of the strain, as shown in Figure 23a−f.48,51

Figure 24. Illustration of possible structures in the clay dispersion in biphasic and full LC solutions on sample loading and under strong flow up to different strain. Red curves indicate HPC macromolecules and/or their domains being highly elongated.

discotic transition appears to be incomplete thereby allowing two kinds of clay mesophase to coexist under flow, as depicted also in Figure 24. Thus, the morphological and structure data can be correlated under strong flow associated with large recoverable strain. The texture formation in polymer matrix as well as the ordering of the clay particles can take place in various length scales. Indeed, the texture formation in the matrix in vicinity of the elastic turbulence point seems to be responsible for the significant ordering of nanoparticles in molecular and meso scales. As explained earlier, the clay particles were significantly oriented not in the Newtonian fluid (water), but in the isotropic 30% HPC solution exhibiting moderate elasticity. The elasticity, partly attributable to entanglements of flexible polymers is enough for the clay orientation but not for ordering the HPC macromolecules. In biphasic and full anisotropic solutions allowing significant orientation and mesophase formation of the particles, the main sources of elasticity are: interfaces between phases, isotropic elasticity inside the droplets of isotropic phase (in biphasic solution), disclination network, Frank elasticity, anchoring effects for HPC (in full LC solution). The anisotropic particles are capable to form their own structure and morphology according to such cumulative elasticity. Under strong flow giving instability of anisotropic matrices and disturbance of the particles arrangement inside columns (rotation, tumbling, etc.), the discotic mesophase would become more favorable. The corresponding columnar−discotic mesophase transition is complete in the 50% biphasic solution but not in the full LC 60% solution (where two mesophase

Figure 23. Texture of the PIB melt (left) and the stages of the circlelike morphology formation in the PIB/clay composite (93/7) obtained at the shear rate of 250 s−1 in the operating unit “sphere-plate”. The strain increases from part a to part f.

In relation to transformations of the clay arrangement, we remember that the concurrence of two kinds of mesophases in the biphasic matrix (50% solution) leads to the smooth decrease in the HPC orientation and to the change of the clay orientation with time (pronounced decrease in the orientation in the start-up stage) noted in Figure 16a. The observed rotation of the clay diffraction could correspond to flowinduced destruction of the columnar-like structure and conversion into a discotic-like mesophase34 with the orientation of the axis transversal to platelet planes along the vorticity axis, as depicted in Figure 24. In the 60% solution, the columnar-to1155


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structures coexists), as explained earlier. At this moment, we cannot determine exactly the nature of such transition, but can suggest that the partial intercalation of the HPC macromolecules into the clay interspaces and their transversal orientation to the shear stream (Figure 18c) could initiate the rotation of the clay ensembles by 90°. Additional optical methods such as light-scattering and birefringence would be useful for elucidating further details of the transition.

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SUMMARY AND CONCLUSIONS The flow induced alignment and relaxation after cessation of the shear flow was examined for HPC−water biphasic (isotropic/LC) and full LC solutions as well as clay dispersions in these solutions and in water using a specially designed rheoX-ray capillary Couette cell. The sample concentrations were chosen according to an accurate phase diagram obtained from the microinterference and optical microscopy measurements. In some cases, location of the phase boundaries was confirmed also with a viscometric method. The X-ray data showed that the orientation of the neat solutions is possible only in presence of very slowly relaxing LC phase. Several stages of relaxation including the loss of the uniaxial (nematic) orientation and conversion into cholesteric spiral structure were detected. For the clay dispersion in isotropic solution of HPC, the significant orientation was observed only for the clay particles. In contrast, both clay and HPC were oriented in the dispersions in biphasic and full LC HPC solutions. The orientation was enhanced at higher shear rates. Essentially, the elasticity of the matrix solution allowed the orientation of either clay alone (in the isotropic solution) or for the both clay and HPC (in the anisotropic solutions). Moreover, strong flow appeared to induce partial intercalation of HPC macromolecules into the interplanar spaces of the clay structure. The presence of clay in anisotropic solutions allowed the clay and HPC to mutually interact to exhibit the orientation, in particular when the mesophase formation decreases the order parameter for one but increases the parameter for the other. In the most specific case, the columnar clay mesophase appeared to be transformed into the discotic mesophase due to this mutual interaction. This transition took place in the shear rates range where regular (ring-like) morphology of the clay particles emerged in the HPC solutions because of the regular elastic instability. Thus, structural, morphological and rheological data are interconnected, allowing us to explain the main features of structure transformations in heterophase dispersions under strong shear flow.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank NWO-Russia bilateral cooperation program (Project No. 047.017.033) for partial financial support of this research. The Russian Foundation for Basic Research (Grant Nos. 05-03-08028, 08-03-12035, 09-0312064, 11-03-12042-ofi-m) is also kindly acknowledged. In addition, we thank Drs. Alexander Semakov, Gleb Vasiliev, and an anonymous reviewer for very useful advices and fruitful discussion. 1156


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