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Nanorheology of Dioctyl Phthalate Confined between Surfaces Coated with Long Alkyl Chains Yoshisada Kayano,† Hiroshi Sakuma, and Kazue Kurihara* Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan ReceiVed May 18, 2007 To shed light on the mechanism related to the high viscosity of a precipitated calcium carbonate (PCC) dispersion in dioctyl phthalate (DOP), the viscosity of DOP in a nanometer space was investigated using the shear resonance measurement. We used mica surfaces modified with dioctadecyldimethylammonium bromide (DODA) as a model of a PCC surface which bears long alkyl chains and bare mica surfaces as a reference. We found that a resonance peak for DOP of high water content (1164 ppm) confined between the DODA-modified surfaces gradually decreased upon approach from a distance of 320.6 nm and disappeared at ca. 57 nm. This indicated that the viscosity of DOP increased with decreasing distance, and a highly ordered state appeared at a large separation of 57 nm. This highly ordered state, however, cannot be observed for DOP of low water content (469 ppm) confined between the DODAmodified surfaces. The resonance peak for DOP of high water content (1164 ppm) between bare mica surfaces gradually decreased upon approach from a distance of 358.4 nm and disappeared at 10.7 nm, which is much shorter than the value for the DODA-modified surfaces. These results could be correlated with the high macroscopic viscosity of a dispersion of PCC coated with long alkyl chains in DOP. We propose the interpenetration of the long alkyl chains and the hydrogen-bonding network of DOP molecules mediated by water molecules as a plausible model for interpreting the high viscosity of coated PCC dispersions.
Introduction Colloidal particles are widely used as materials in industrial products such as paper, rubber, plastics, paints, sealants, and cosmetics1-5 as well as in nanotechnology.6,7 In many applications of colloidal particles, it is important to regulate the dispersive and rheological properties of colloidal particles. Surface modifications with surfactants are known to be efficient for regulating these properties.8,9 Nanocolloidal particles of precipitated calcium carbonate (PCC) coated with stearic acid and other fatty acids are increasingly used as a rheological modifier with the plasticizer of dioctyl phthalate (DOP) for paints and sealants.10,11 For these products, it is essential to stabilize the PCC dispersions of a high viscosity in DOP (DOP sol).12,13 To increase the viscosity of the DOP sol, the PCC surfaces are commonly modified with a fatty acid bearing a long alkyl chain.11 The mechanism for the high viscosity of this type of DOP sol has been proposed as due to the strong attractive interaction between the modified PCC particles.13 However, this proposed mechanism has not yet been * To whom correspondence should be addressed. E-mail: kurihara@ tagen.tohoku.ac.jp. Phone: +81-22-217-5673. Fax: +81-22-217-5674. † Also Shiraishi Kogyo Kaisha, Ltd., 78 4-chome, Motohama-cho, Amagasaki-city, 660-0085, Japan. (1) Mouri, H.; Akutagawa, K. Rubber Chem. Technol. 1999, 72, 960-968. (2) Ambrosi, M.; Dei, L.; Giorgi, R.; Neto, C.; Baglioni, P. Langmuir 2001, 17, 4251-4255. (3) Zhang, H.; Chen, J. F.; Zhou, H. K.; Wang, G. Q.; Yun, J. J. Mater. Sci. Lett. 2002, 21, 1305-1306. (4) Yabe, S.; Sato, T. J. Solid State Chem. 2003, 171, 7-11. (5) Alince, B. J. Appl. Polym. Sci. 2005, 98, 1879-1883. (6) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111-1114. (7) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18-52. (8) Ettlinger, M.; Ladwig, T.; Weise, A. Prog. Org. Coat. 2000, 40, 31-34. (9) Zaman, A. A.; Singh, P.; Moudgil, B. M. J. Colloid Interface Sci. 2002, 251, 381-387. (10) Evans, R. M. Polyurethane Sealants, Technology and Applications; Technomic Publishing Co., Inc.: Lancaster, PA, 1993. (11) Kayano, Y.; Morioka, I.; Hosoi, K. U.S. Patent 2005-0004266-A1, 2005. (12) Cavalier, K.; Larche, F. Colloids Surf., A 2002, 197, 173-181. (13) Lisse, L.; Cavalier, K.; Larche, F. Proceedings of the XIIIth International Congress on Rheology, Cambridge, U.K., 2000; pp 163-165.
proved. It should be useful if we can understand at the molecular level the role of the hydrocarbon chains in increasing the viscosity. Shear measurements employing the surface forces apparatus (SFA) have been developed to investigate confined liquids in a nanospace.14-19 Various modes of shear measurements are proposed for monitoring shear responses. Although they take advantage of the precise determination of the surface separation using fringes of equal chromatic order (FECO), different measurement modes have certain advantages and disadvantages. The resonance mode18-24 developed by us has a high sensitivity for investigating the viscosity, friction, and lubrication of confined liquids between solid surfaces and is easy to use because we monitor relatively large resonance signals. The shear response of confined liquids was observed around the resonance frequency of the shear unit. The frequency and amplitude of the resonance peak are highly sensitive to the change in the viscosity. This method should be suitable for elucidating the high viscosity of the DOP sol. In this study, we measured the shear responses of DOP confined between dioctadecyldimethylammonium bromide (DODA)modified mica surfaces at various surface separations. The cationic DODA was used to coat the surfaces with long alkyl chains instead of stearic acid, because it strongly binds to the negative mica surfaces in a homogeneous manner. (14) Israelachvili, J. N.; McGuiggan, P. M.; Homola, A. M. Science 1988, 240, 189-191. (15) Van Alsten, J.; Granick, S. Phys. ReV. Lett. 1988, 61, 2570-2573. (16) Klein, J.; Perahia, D.; Warburg, S. Nature 1991, 352, 143-145. (17) Raviv, U.; Tadmor, R.; Klein, J. J. Phys. Chem. B 2001, 105, 8125-8134. (18) Dushkin, C. D.; Kurihara, K. Colloids Surf., A 1997, 130, 131-139. (19) Dushkin, C. D.; Kurihara, K. ReV. Sci. Instrum. 1998, 69, 2095-2104. (20) Dushkin, C. D.; Kurihara, K. Prog. Colloid Polym. Sci. 1997, 106, 262265. (21) Kurihara, K. Prog. Colloid Polym. Sci. 2002, 121, 49-56. (22) Mizukami, M.; Kusakabe, K.; Kurihara, K. Prog. Colloid Polym. Sci. 2004, 128, 105-108. (23) Mori, K.; Kusakabe, K.; Haraszti, T.; Kurihara, K. Trans. Mater. Res. Soc. Jpn. 2001, 26, 909-912. (24) Sakuma, H.; Otsuki, K.; Kurihara, K. Phys. ReV. Lett. 2006, 96, 046104.
10.1021/la701466n CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007
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Kayano et al. further decreases, and if the liquid forms a highly ordered structure, the upper and lower surfaces are connected by ordered liquid molecules, which exhibit a high viscosity and molecular friction, and move together, resulting in the disappearance of the peak at the separation side and the appearance of the resonance peak at frequencies close to the contact position (Ω0,MC). Analysis of Resonance Curves. To perform the quantitative analysis of the viscoelastic properties of DOP, the resonance curves were fitted to mechanical models (Figure 2b).19,23,24 In the case of the surfaces separated in air, the equation of motion is described as follows:
Figure 1. Molecular structures of (a) DOP and (b) DODA.
m0
d2x dx ) -k0x - b0 + f0 dt dt2
(1)
Experimental Section Sample Preparation for Shear Resonance Measurement. DOP (di-n-octyl phthalate; Wako Chemicals, standard grade, Figure 1a) was used, and its high purity (>99.9%) was confirmed by gas chromatography (Shimadzu GC 14A). Mica surfaces were used as the substrates. The mica sheets (1 × 1 cm2, thickness 1-3 µm) were freshly cleaved, and silver was deposited on one side of them at a thickness of 50 nm using a standard procedure.24 The sheets were then glued onto cylindrical silica disks (radius of curvature 20 mm, diameter of the disk 10 mm) with the silver-coated sides down on the disk and subjected to the experiment. DODA (Sogo Pharmaceutical Co., Figure 1b) was deposited on the mica surface using the Langmuir-Blodgett (LB) method (USI-2-00KC, USI Co., Ltd.).25 Shear Resonance Measurement. The shear resonance measurement system illustrated in Figure 2a was homemade and used in combination with an SFA (Mark4 Anutech Co.). The upper surface was hung by a pair of stiff leaf springs and laterally moved by a four-sectored piezotube, which was driven by a sinusoidal input voltage, Uin, of frequency f. The upper surface vibrated at a constant angular frequency ω ()2πf). The deflection, x, of the leaf spring was detected using a capacitance probe (MicroSense 4830, Japan ADE Ltd.) as an output voltage, Uout. The Uin and Uout signals were recorded using a LabVIEW program (National Instruments) through a digital oscilloscope (GRD 1602, Gould). The amplitude of the oscillation was then defined as the ratio of Uout to Uin and measured as a function of ω. The lower surface was supported by double cantilever springs (kn ) 100-300 mN/m). The distance between the surfaces, D, was determined using FECO with a resolution of 0.1 nm. The control measurements were carried out with the surfaces separated in air and in contact. The resonance frequency and amplitude were obtained by recording Uout/Uin over a wide range of frequencies (ω ) 50-600 s-1). In the former, the peak frequency, Ω0,SP, was characterized by the mass and the spring constant of the upper surface unit. On the other hand, the resonance frequency of the mica surfaces in contact (mica contact, Ω0,MC) in air were determined by the combination of the mass and spring constant of both the upper and lower surface units. Next, a liquid sample (about 10 µL) was injected into the gap between the surfaces in the chamber. The room temperature was controlled by an air conditioner, and the temperature in the chamber was 23.8 ( 0.1 °C. The measurements were started at D ) 200-400 nm, and the resonance curve was recorded at various separations with the lower surface moving toward the upper surface. In general, the resonance curves reflect the viscosity and frictional properties of the confined liquid. When the viscosity of the liquid is low, the liquid does not transmit the movement of the upper surface to the lower surface. Therefore, the resonance frequency is determined only by the resonance property of the upper unit, and the peak appears at the separated position (Ω0,SP). In this case, the high peak amplitude and the narrow width of the peak reflect the low viscosity of the confined liquid. When the surface distance decreases, the confined liquid often changes its packing to a more ordered form, and the viscosity increases. This led to a significant decrease in the resonance amplitude at Ω0,SP. When the distance (25) Kurihara, K.; Ohto, K.; Tanaka, Y.; Aoyama, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 444-450.
where k0 is the total spring constant, b0 accounts for the total energy loss, m0 is the effective mass of the upper unit (i.e., piezotube, leaf springs, and upper silica disk), f0 is the external force, and x is the displacement of the upper unit. Equation 1 can be rewritten for the input and output voltages as Uout C0 1 ) Uin m0 [(Ω 2 - ω2)2 + (β ω)2]1/2 0 0
(2)
where C0 is the apparatus constant, Ω0 ) (k0/m0)1/2 is the resonance frequency, and β0 ) b0/m0 is the damping coefficient. When DOP is injected, the mass, total spring constant, and total energy loss of the vibrated unit are described as m ) m0 + ∆m, k ) k0 + ∆k, and b ) b0 + ∆b, respectively. Here, the parameters of ∆m, ∆k, and ∆b are contributions from both the sample liquid and the lower unit (for details, see refs 19, 23, and 24). In this case, we can obtain the fitting equation for the pure viscous friction as follows: Uout C 1 ) Uin m [(Ω2 - ω2)2 + (βω)2]1/2
(3)
The experimental data (Uout/Uin and ω) were fitted to eq 3 using a least-squares method, and we obtained the viscoelastic parameters Ω and β. Sample Preparation and Measurement of the Macroscopic Viscosity of the DOP Sol. We used commercial materials such as PCC coated with stearic acid (coated PCC, Shiraishi Kogyo Kaisha, Ltd.) and uncoated PCC (Shiraishi Kogyo Kaisha, Ltd.). The surface of the coated PCC was modified with stearic acid (95% pure; the main impurity is palmitic acid, Wako Chemicals) saponified with NaOH. The purities of the coated and uncoated PCC were analyzed by the XRD (Rigaku Denki Co., Ltd.) patterns, resulting in identification of no mineral other than calcite. Coated particles of PCC were observed to be rhombohedral and uniform in size (ca. 80 nm on a side) on the basis of field emission scanning electron microscopy (FE-SEM; JSM-6330F by JEOL, Ltd.) as shown in Figure 3a. An FE-SEM image in Figure 3b, of the uncoated PCC particles, was compared and showed that the particles were also rhombohedral and uniform in size (ca. 80 nm on a side). Industrial DOP (ca. 99% purity, New Japan Chemical Co., Ltd.) was used to study the high viscosity of the dispersion of PCC in DOP (DOP sol). DOP (400 g) and PCC (300 g) were thoroughly mixed by the mixer (5DMV01-r, Dalton Corp.) before the measurements. For the measurement of the viscosity of the DOP sol, a B-type viscometer (Brookfield-type viscometer, Tokimec Inc.) was used. The velocity of rotation was 20 rpm for 3 min, and the temperature was 20 ( 0.1 °C.
Results and Discussion Behavior of DOP of High Water Content Confined between DODA-Modified Mica Surfaces. Typical resonance curves for the DOP confined between the DODA-modified mica surfaces are shown in Figure 4. DOP easily absorbs water if we leave it in air for 1 day, and the water content significantly altered the
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Figure 2. (a) Schematic drawing of the shear resonance measurement system and (b) mechanical model of the oscillating shear unit.
Figure 4. Resonance curves of the DOP confined between the DODA-modified surfaces at various separation distances, D. The resonance curves for the surfaces separated in air and the DODA contact are also shown for comparison.
Figure 3. FE-SEM images of (a) PCC particles coated with stearic acid and (b) uncoated PCC particles.
results. The water content of DOP described in this section was measured to be 1164 ( 81 ppm by the Karl Fischer titration instrument (Hiranuma Aquacounter AQ-7). The resonance peak at D ) 320.6 nm appeared at an angular frequency of 209 s-1, which was the same as Ω0,SP (209 s-1). The low amplitude at D ) 320.6 nm relative to the peak of separation in air reflects the larger viscosity of the DOP compared to the value for air. The amplitude of the resonance peak gradually decreased with decreasing D from 320.6 to 63.8 nm. This indicated that the
viscosity of the DOP gradually increased with decreasing distance. At D ) 57.1 nm, the resonance peak abruptly disappeared and a broad curve at a resonance frequency of 311 s-1 emerged. This means that the DOP exhibited a high viscosity between the DODA-modified surfaces and transferred the movement of the top surface to the lower one, leading the two surfaces to couple. When the separation was further decreased, the resonance peak shifted toward Ω0,MC (422 s-1), while the intensity remained low. At D ) 9.9 nm, the resonance peak appeared at a frequency close to Ω0,MC, though the amplitude of the resonance peak was much lower than that of the DODA-modified mica contact in air. This curve shows the coupled movement of the upper and lower surfaces mediated by the ordered DOP molecules at a thickness of 9.9 nm. The lower amplitude of the resonance curves indicated that such molecules are somewhat mobile. Behavior of DOP of Low Water Content Confined between DODA-Modified Mica Surfaces. To verify the effect of water on the viscosity of DOP, the fresh DOP just after opening of the bottle was used. The water content of the fresh DOP was low, 469 ( 36 ppm, and was maintained during the shear measurement. The resonance curves at low water content are shown in Figure 5. At D ) 249.6 nm, the resonance peak appeared at angular frequency Ω0,SP (208 s-1). The amplitude of the resonance peaks
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Figure 5. Resonance curves of the DOP of low water content confined between the DODA-modified surfaces at various separation distances, D. The resonance curves for the surfaces separated in air and the DODA contact are also shown for comparison.
Figure 7. Viscoelastic parameters ((a) damping coefficient, β, and (b) resonance frequency, Ω) of the DOP confined between the DODAmodified and bare mica surfaces. The values next to the symbols show the distances (nm).
Figure 6. Resonance curves of the DOP confined between the bare mica surfaces at various separation distances, D. The resonance curves for the surfaces separated in air and mica contact are also shown for comparison.
gradually decreased with decreasing D from 249.6 to 4.6 nm. This indicated that the viscosity of the DOP gradually increased with decreasing distance. At D ) 1.1 nm, the resonance peak suddenly disappeared and a broad peak emerged at 318 s-1. At this distance, the DOP exhibited a high viscosity, and the lateral oscillation of the upper surface was transmitted to the lower surface mediated by DOP molecules. The distance 1.1 nm was much shorter than the distance (57.1 nm) for DOP of high water content. This indicated that DOP of low water content did not show high viscosity even if it was confined between DODAmodified mica surfaces. When the separation was further decreased, the resonance peak shifted toward Ω0,MC (420 s-1), while the intensity also remained low. At D ) 0.6 nm, the resonance peak was still the broad curve unlike the resonance curve of DODA contact in air. The broad curve showed the large lubricity of DOP confined between the upper and lower surfaces. The distance where the resonance peaks disappeared upon approach was 1.1 nm for DOP of low water content confined between the DODA-modified surfaces. Behavior of DOP Confined between Bare Mica Surfaces. The resonance curves of the DOP confined between the bare mica surfaces are shown in Figure 6. The water content of DOP used for these measurements was 1164 ppm. At D ) 358.4 nm,
the resonance peak appeared at angular frequency Ω0,SP (209 s-1). The amplitude of the resonance peaks gradually decreased with decreasing D from 358.4 to 13.3 nm. This indicated that the viscosity of DOP gradually increased with decreasing distance. At D ) 10.7 nm, the resonance peak suddenly disappeared and a broad curve emerged at 327 s-1. This means that the DOP exhibited a high viscosity at this surface separation, and the two surfaces started to couple. When the separation was further decreased, the resonance peak shifted toward Ω0,MC (433 s-1), while the intensity also remained low. At D ) 3.0 nm, the resonance frequency reached a value close to Ω0,MC (433 s-1) and the amplitude increased, though the amplitude was still much lower than that of the mica contact in air. This curve shows the coupled movement of the upper and lower surfaces mediated by the tightly packed DOP molecules at a thickness of 3.0 nm. The distance where the resonance peaks disappeared upon approach was 57.1 nm for the DODA-modified surfaces. This value was much greater than the value for the bare mica surfaces, indicating that the modification of the surfaces with long alkyl chains indeed contributes to the higher viscosity of the confined DOP molecules. Viscoelastic Properties of Confined DOP. To quantitatively discuss the properties of the confined liquids, the viscoelastic parameters were obtained by fitting the experimental shear resonance curves to the calculated ones on the basis of eq 3. The fitting curves at various surface distances are shown in Figures 4-6 as solid lines. The obtained fitting parameters, damping coefficient (β), and resonance frequency (Ω) are plotted versus the surface separation in Figure 7. In the case of the DOP of high water content confined between the DODA-modified surfaces, the β value gradually increased from 1.3 to 1.8 s-1 with decreasing
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Figure 8. Schematic drawings of the DOP sol made of PCC coated with stearic acid.
D from 320.6 to 69.7 nm. At D e 57.1 nm, the Ω value started to increase while the β value showed a high value. Since a high resonance frequency indicates the coupled movement of oscillation between the upper and lower surfaces, this means that the DOP between the DODA-modified surfaces had a high damping coefficient and exhibited a high enough viscosity to connect the upper and lower surfaces at D e ca. 57 nm. These results were reproducible as shown in Figure 7a by the open circles, although slight differences were observed in the critical distance where the damping coefficient started to increase. On the other hand, the β of DOP of low water content confined between the DODAmodified surfaces and confined between the bare mica surfaces also gradually increased and the Ω values started to increase at D ) 1.1 and 10.7 nm, respectively. This means that the DOP of low water content confined between the DODA-modified surfaces and between the bare mica surfaces had a low damping coefficient and did not exhibit a high enough viscosity to connect the upper and lower surfaces at D > ca. 1 and 11 nm, respectively. Two independent measurements demonstrated similar results as shown in Figure 7a. Viscosity of the DOP Sol Studied by Viscometry. It should be interesting to compare these nanorheological results with the macroscopic rheological properties of the DOP dispersions of PCC. The water content of the DOP was 2121 ppm. The viscosity of the DOP sol (42.9% PCC) was measured using a B-type viscometer at 20 °C. The viscosity of the DOP sol of the coated PCC (226 Pa‚s) was considerably higher than that of the uncoated PCC (12 Pa‚s). This difference should be attributable to the presence of long alkyl chains on the coated PCC surface as shown in Figure 8. Because the long alkyl chain of stearic acid (C17) and that of DODA (C18) are similar, we should be able to compare the data of the bulk viscosities with those from the nanoscopic shear measurement. When a bulk colloid dispersion exhibits a high viscosity, colloid particles should be connected to each other in a network by either attraction or direct bonding. It is difficult to imagine that the van der Waals attraction extended to form the network. If the liquid between the particles were ordered and connecting two particles, the dispersion would show a high viscosity. To examine this concept, we compared the interparticle distance of the DOP dispersion with the distance at which the confined DOP started to exhibit an increased viscosity. When PCC was perfectly dispersed in the DOP sol, the average distance between PCC particles was 51.4 nm calculated on the basis of the weights of the PCC (300 g), DOP (400 g), and mean particle size of the PCC (80 nm). On the other hand, the distance at which the DOP started to exhibit a high viscosity was 57 nm for the DODA-modified surfaces on the basis of the shear resonance measurement. The two values were in good agreement. We think that a liquid DOP connected the surfaces modified with the long alkyl chains, resulting in the macroscopic high viscosity (226 Pa‚s). On the contrary, the DOP sol of uncoated
Figure 9. Schematic drawings of the plausible high viscosity of DOP of high water content on (a) the DODA-modified and (b) bare mica surfaces.
PCC showed a macroscopic low viscosity (12 Pa‚s). This was consistent with the observation that the DOP between the bare mica surfaces exhibited a high viscosity only at the shorter distances. The effect of the long alkyl chains is significant. Therefore, it should be interesting to discuss a plausible mechanism for this effect. Plausible Structure of DOP on the Surfaces Modified with Long Alkyl Chains. It is known that the surface modified with long alkyl chains could orient liquid crystal molecules perpendicular to the surface.26 This effect was interpreted by the interpenetration of molecules into the array of long alkyl chains on the surface. We may expect a similar phenomenon for DOP molecules on the DODA-modified surfaces. DOP molecules bear two C8 hydrocarbon chains which can be interpenetrated in an array of hydrocarbon chains on the surface. If only one chain is interpenetrated, other chains can form a second array of hydrocarbons which then can offer sites for the interpenetration for additional DOP layers. It is difficult to interpret the effect of water. In the condition we used for this study, the ratio of water to DOP was small such as DOP:water ) 49:1 for a water content of 1164 ppm. Here we propose two plausible effects of water. The first is that the oriented network of DOP molecules could be enhanced by the hydrogen bonding mediated by water molecules, resulting in the high viscosity of DOP confined between the DODA surfaces. The second is the formation of a reversed-micelle-type network of DOP. In any case, it is interesting that the organized structure of DOP extended to a long distance of 57 nm. We propose a plausible structure for the DOP confined between the DODA-modified surfaces as shown in Figure 9a. On the other hand, the DOP molecules cannot regularly orient on the surface without the long alkyl chains such as on bare mica (Figure 9b).
Conclusions To investigate the mechanism for the high viscosity of the DOP sol, we studied the behavior of the DOP confined between DODA-modified surfaces and between bare mica surfaces using the shear resonance measurement. We observed a viscosity higher than the bulk value for the confined DOP molecules of high water content between DODA surfaces at distances less than 57 nm which was much longer than the corresponding value for bare mica surfaces of 11 nm. The effect of the hydrocarbon chains can be accounted for in terms of the interpenetration of hydrocarbon chains and hydrogen-bonding network of DOP molecules mediated by water molecules. (26) Janik, J.; Tadmor, R.; Klein, J. Langmuir 1997, 13, 4466-4473.
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The high viscosity observed by the shear resonance measurement is in good agreement with the bulk viscosity. To the best of our knowledge, this is the first study of connecting the macroscopic and the nanoscopic rheological data, which revealed a possible mechanism of a highly viscous dispersion of PCC in DOP. For application of PCC particles, solvents which bear a structure similar to that of the DOP molecules bearing two long alkyl chains are commonly used and not those without long alkyl chains. Our study well demonstrated a reason why DOP is important. This study also shows that the nanoscopic measurement
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is a powerful tool for understanding macroscopic behavior, which has a significant bearing on both basic science and engineering. Acknowledgment. This work was supported by a Grantin-Aid for 21st Century COE Research, Giant Molecules and Complex Systems, from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by the CREST program of the Japan Science and Technology Agency (JST). LA701466N
