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Non-Newtonian Rheology of Liquid Crystalline Polymer Monolayers Kang Sub Yim, Carlton F. Brooks, and Gerald G. Fuller* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025
Dominik Winter and Claus D. Eisenbach Universitat Stuttgart, Stuttgart, Germany Received October 13, 1999. In Final Form: February 3, 2000 A rheological study on a two-dimensional Langmuir monolayer has been conducted by measuring the optical anisotropy and the mechanical surface properties. Experiments were conducted on monolayer mixtures of a liquid crystalline polymer, poly(p-phenylene)sulfonic acid (PPPSH), with an aliphatic fatty acid, stearic acid (C18). It is found that these two molecules mix ideally, as supported by studies of average molecular area, collapse pressure, and Brewster angle microscopy. Both surface moduli and surface viscosity were obtained with an interfacial stress rheometer and showed that PPPSH/stearic acid monolayer mixtures are non-Newtonian. The orientational dynamics of polymers under well-defined planar extensional flow was considered using UV dichroism, which revealed a surface pressure-induced isotropic-nematic transition. This transition is difficult to observe using traditional isotherm analysis; however, it is readily evident in measurements of the rheology of the monolayers.
Introduction Thin films of liquid crystalline polymers have been studied extensively recently because of their high mechanical and thermal stability compared to that of lowmolecular-weight materials. It has been known that liquid crystalline polymers can be used in applications involving optical storage,1 nonlinear optical devices,2,3 and ferroelectric switching.4,5 Using the Langmuir-Blodgett (LB) technique, it is possible to fabricate films with an enhanced alignment of the polymer backbone by transferring polymeric monolayers from the water surface to solid substrates.6 It is important to fabricate films with a high degree of orientational order because the degree of alignment in the film influences device performance in applications such as displays.7,8 Several researchers9,10 have reported that the resulting orientation of the transferred monolayers could be controlled by the flow associated with the LB process, suggesting that knowledge of the flow behavior of these films can provide insight into molecular orientations and interactions at the air-water interface. Many industrial processes involve interfacial flow, such as coating and spraying technologies. Moreover, because of the simplified two-dimensional geometry and the large degree of thermodynamic and conformational control that (1) Eich, M.; Wendorff, J. J. Opt. Soc. Am. B 1990, 7, 1428. (2) Assanto, G.; Neher, D.; Stegeman, G. I.; Torruellas, W. E.; Marques, M. B.; Horsthuis, W. H. G.; Mohlmann, G. R. Mol. Cryst. Liq. Cryst. 1992, 222, 33. (3) Ozaki, M.; Sakuta, M.; Yoshino, K.; Helgee, B.; Svensson, M.; Skarp, K. Appl. Phys. B 1994, 59, 601. (4) Nakamura, T.; Ueno, T.; Tani, C. Mol. Cryst. Liq. Cryst. 1989, 169, 167. (5) Sasaki, A. Mol. Cryst. Liq. Cryst. 1986, 139, 103. (6) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (7) Tieke, B. Adv. Mater. 1991, 3, 532. (8) Seo, D. S.; Matsuda, H.; Ohide, T.; Kobayashi, S. Mol. Cryst. Liq. Cryst. 1993, 224, 13. (9) Nakahara, H.; Mobius, D. J. Colloid Interface Sci. 1986, 114, 363. (10) Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357.
can be exerted on the constituent molecules, monolayers of insoluble amphiphiles (Langmuir monolayers) stand to serve as important model systems for the basic understanding of complex rheological behavior. In the present work, we examine the non-Newtonian viscoelasticity of the liquid crystalline polymers in two-dimensional monolayers. Poly(p-phenylene) derivatives can yield a rigid rod structure and high preferential alignment of the polymer backbone with dichroic ratios of 3.511 during vertical dipping. In this study, alkyl-substituted poly(p-phenylene)sulfonic acid (PPPSH) was mixed with stearic acid (C18) to study the surface rheology. A system with sulfonate groups situated on the backbone was chosen for several reasons, such as a higher thermal stability, a smaller ratio of diameter to rod length, and an enhanced rigidity.12 In our previous paper,13 the surface pressure-induced isotropic-nematic transition was studied in the PPPSH polymer solution. These findings are interesting because such a liquid crystalline transition is not observable in the pressure-area isotherm. It was shown that this transition was affected by comixed small molecules, referred to as a solvent, which are added to improve the mobility of monolayers and make stable floating layers. The isotropic-nematic transition of PPPSH/arachidyl alcohol mixtures was found to correspond to the phase transition observed in the isotherm, while PPPSH/DMPC (1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine) mixtures show the transition around 20 mN/m, which is unnoticeable in surface pressure measurements. Different viscoelastic responses are observed in the isotropic and nematic phases, respectively, suggesting the isotropicnematic transition causes the change of the surface rheology properties. In this work, stearic acid (SA) is used (11) Cimvora, V.; Remmers, M.; Neher, D.; Wegner, G. Adv. Mater. 1996, 8, 146. (12) Rulkens, R.; Schulze, M.; Wegner, G. Macromol. Rapid Commun. 1994, 15, 669. (13) Yim, K. S.; Brooks, C. F.; Fuller, G. G.; Datko, A.; Eisenbach, C. D. Langmuir2000, 16, 4319-4324.
10.1021/la991403j CCC: $19.00 © 2000 American Chemical Society Published on Web 03/29/2000
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Figure 1. Chemical structure of poly(p-phenylene)sulfonic acid (PPPSH).
as a solvent molecule mixed with PPPSH because its lattice structure and rheology are well-known. The surface moduli and surface viscosity were measured with an interfacial stress rheometer to relate the mechanical shear properties to the two-dimensional liquid crystalline transition. Dichroism refers to polarization-dependent attenuation of light and is defined as ∆n′′ ) n1′′ - n2′′, where n1′′and n2′′ are the principal values of the imaginary part of the refractive index tensor. Because dichroism is strongly wavelength dependent, different wavelengths of light can be selected for specific samples. In our group,14 the anisotropy in monolayers of the hairy-rod polymer phthalocyaninatopolysiloxane (PcPS) was studied using visible light of wavelength 543.6 nm. However, most liquid crystalline polymers do not absorb in the visible region of the spectrum but have a greater sensitivity to polarized absorption in the UV region. Therefore, in this investigation, the UV dichroism was measured dynamically by rapidly modulating the polarization of the light to probe the orientational dynamics of the PPPSH monolayers, which are known to absorb deep UV light strongly, in response to an applied flow. Here we compare surface rheological data on twodimensional monolayers of a rigid rod polymer and a fatty acid. The coupling of hydrodynamics and molecular orientation for monolayers containing polymers is then investigated using optical anisotropy under extensional flow at the air-water interface. These studies provide additional information about the viscoelastic properties with our previous findings of an isotropic-nematic transition.
Figure 2. Top view of the four-roll mill to produce extensional flow. The direction of roller rotation and the resulting flow field are shown. A stagnation point of the flow field exists at the center of the device. For the roller rotation in the figure, the compression axis is the vertical line of symmetry, as the extension axis is the horizontal line of symmetry.
Materials and Methods Samples of poly(p-phenylene)sulfonic acid (PPPSH) of Mn ) 5080 synthesized by Dr. Eisenbach’s group are shown schematically in Figure 1. Stearic acid (SA: C18H36O2) was purchased from Sigma. Mixtures of the rigid rod polymer PPPSH with stearic acid were made in chloroform with concentrations varying from 0 to 100 mol % PPPSH monomer. A 25.0 × 7.5 cm2 Langmuir trough made of Teflon was used for all measurements (KSV Instruments, Finland). The surface pressure was monitored using a Wilhelmy balance. All experiments were performed at 22 ( 0.1 °C. The rollers of the four-roll mill were positioned to generate an extensional flow for studying flow-induced molecular orientation (Figure 2). The diameter of the rollers was 1.98 cm, and the distance between adjacent rollers was 3.18 cm. This ratio of roller diameter to roller spacing, 0.625, was known to provide the closest approximation to homogeneous extension for bulk flows.15 The compression and extension axes in this flow can be interchanged by simply reversing the direction of roller rotation. All measurements were performed in the stagnation region located at the geometric center of the rollers, where material has a long residence time. The flow of monolayer films in response to an applied extensional flow field was visualized by a particle tracking method. Monolayers were seeded with sulfur particles whose motion was monitored with a CCD camera (Sony XC 771). This experiment provided the relationship between the angular (14) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Macromolecules 1996, 29, 705. (15) Higdon, J. J. L. Phys. Fluids A 1993, 5, 274.
Figure 3. (a) Sulfur particle trajectories and best fit hyperbolic streamlines for 40 mol % PPPSH/stearic acid monolayers at 5 mN/m subject to flow in the four-roll mill. (b) Relationship between roller velocity and the observed strain rate. velocity of the rollers and the strain rate of the extensional flow field. Figure 3a shows the sulfur particle trajectories created using different roller speeds for 40 mol % PPPSH/SA mixtures. The symbols represent the center of mass of the sulfur particles, and the solid curves correspond to the best fit hyperbolic
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Figure 4. Isotherms for PPPSH/stearic acid monolayers at 22 °C. The isotherms are plotted in terms of area per repeat unit, where one repeat unit represents one monomeric unit of PPPSH polymer or one molecule of stearic acid. streamlines through these coordinates. The good agreement between the experimental and theoretical streamlines indicates that the four-roll mill produces a good approximation to planar extensional flow in monolayers of this composition. In Figure 3b, the mean strain rate is plotted against roller velocity. Mixtures of 40 mol % PPPSH/SA produced a linear relationship at low roller velocities, as expected. The proportionality constant relating strain rate to roller velocity is 2.2. Brewster angle microscopy (BAM) was used to visualize the morphology of the monolayer mixtures. P-polarized light from a 10 mW argon laser was incident at the Brewster angle of 53.1°. The reflected beam was magnified with a 50 mm focal length lens, sent through an analyzer to enhance contrast, and recorded with a CCD camera. A description and analysis of the optical train for the dichroism measurements have been discussed previously.13 The instrument operates by modulating the polarization of 275 nm UV light generated by an argon laser (Spectra-Physics 2085-20RS) with a photoelastic modulator (PEM). Following the monolayer, the UV light is measured with a photomultiplier tube. The dichroism, ∆n′′, is recovered from the extinction, δ′′ ) (2π∆n′′d)/λ, where d is the thickness of the monolayer and λ is the wavelength of the incident light. An interfacial stress rheometer was recently constructed to study the rheology of the Langmuir monolayer in our group.16 A magnetized rod oscillates at the air-water interface by applying a sinusoidal magnetic field gradient. The dynamic surface moduli and surface viscosity can be obtained by measuring the amplitude and phase of the resulting rod motion relative to the applied force.
Results and Discussion Isotherms and Ideal Mixing. The surface pressurearea isotherms of mixed monolayers of PPPSH and stearic acid are shown in Figure 4. The isotherm of pure stearic acid exhibits a kink at 24.5 mN/m, which is associated with a phase transition from the L2 phase to the LS phase17 named by Harkins and Copeland.18 According to X-ray diffraction studies,19 the alkyl chain’s tails in the L2 phase tilt toward their nearest neighbor (NN), while in the LS phase the tilt angle tends to zero and the alkyl chains are perfectly upright. Therefore, the L2-LS transition is called a “tilting transition”, as evidenced by observations using BAM that a polydomain structure in the L2 phase is lost when the system reverts to the isotropic state of the LS phase (data not shown). The mean area per molecule (16) Brooks, C. F.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1999, 15, 2450. (17) Bibo, A. M.; Peterson, I. R. Adv. Mater. 1990, 2, 309. (18) Harkins, W. D.; Copeland, L. E. J. Chem. Phys. 1942, 10, 272. (19) Kenn, R. M.; Bohm, C.; Bibo, A. M.; Peterson, I. R.; Mohwald, H.; Alsnielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092.
Figure 5. (a) Average molecular area at zero surface pressure against the concentration of monomer. The data show a good correlation with the additivity rule of eq 1. (b) Collapse pressure against the concentration of monomer. It is shown that the collapse pressure decreases continuously as PPPSH concentration increases.
increases monotonically as polymer is added, and the L2LS phase transition disappears in isotherms for mixtures containing >40 mol % PPPSH. The miscibility of the two components of a monolayer is an important factor when studying the thermodynamics of such mixtures. One method of studying miscibility behavior is the excess area criterion.20,21 If the components are mixed ideally or completely immiscible, the average area per molecule in the mixed film will follow the simple additivity rule,
A12 ) x1A1 + x2A2
(1)
where A12 is the average molecular area in the mixed monolayer at zero surface pressure, xi are the mole fractions of the components, and Ai are the molecular areas in the pure films at the same surface pressure. In Figure 5a, the average area per molecule (extrapolated to zero surface pressure) of the mixed film is plotted against PPPSH mole fraction. The solid line represents eq 1, and the data are in good agreement with the additivity rule. From this analysis, the average area for stearic acid is 25.9 Å2/molecule, and it is 64.2 Å2/monomer for the PPPSH monomer. To determine whether the two components are miscible, the collapse pressure data were examined (Figure 5b). If two immiscible components have different collapse pressures, the resultant monolayer should start to collapse at the lower value, and the surface pressure should rise again until the higher collapse pressure is reached. In contrast, ideally mixed monolayers will only have a single collapse at a pressure different from that of either component. (20) Gaines, J. G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (21) Dorfler, H. D. Adv. Colloid Interface Sci. 1990, 31, 1.
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Figure 7. Surface moduli at 15 mN/m of a stearic acid monolayer as a function of the strain amplitude.
Figure 6. Brewster angle microscopy (BAM) in (a) pure stearic acid, (b) pure PPPSH, and (c) a mixture of 65 mol % PPPSH/ stearic acid.
Figure 5b shows the collapse pressure decreases continuously between 47.9 and 28.8 mN/m, the collapse pressures of each component, as PPPSH is added, which indicates complete miscibility for the PPPSH/SA mixture system. The addition of a small amount of stearic acid (20 mol %) results in a large increase in the collapse pressure, verifying that stearic acid incorporates into the PPPSH monolayer and stabilizes it. It is known that the collapse pressure depends on the rate of compression and the number of cycles.22 For consistent results, all measurements were performed during the first compression with a sufficiently slow compression rate. In addition, Brewster angle microscopy (BAM) was used to image monolayer mixtures. Both pure stearic acid (Figure 6a) and PPPSH (Figure 6b) monolayers form distinct crystalline phases. It is known that the molecules of long-chain fatty acids including stearic acid at low (22) Sims, B.; Zografi, G. J. Colloid Interface Sci. 1972, 41, 35.
surface pressure are packed into a distorted hexagonal lattice and are tilted at some angle with respect to the surface normal.23 Therefore, the domain contrast can be inverted by rotating an analyzer used in the BAM, indicating the presence of anisotropy in the optical properties of the monolayer. On the other hand, the image of the PPPSH monolayer does not show a range of contrasts, suggesting PPPSH, a rigid-rod polymer, is not tilted with respect to the surface but is positioned parallel to the surface. Figure 6c shows that the 65 mol % PPPSH/ SA mixture is composed of nearly homogenized domains, unlike the pure components. On the basis of the above results (the average molecular area, the collapse pressure, and the BAM images), it is concluded that PPPSH and stearic acid are ideally miscible at low surface pressures over a wide range of compositions. Rheology of Pure Stearic Acid Monolayers. Figure 7 shows the surface storage Gs′ and the loss modulus Gs′′ as functions of the strain amplitude in the L2 phase of the pure stearic acid monolayers. Over this broad range of strain amplitudes, both moduli remain nearly constant, suggesting that the stearic acid monolayer is in the linear viscoelastic regime. The surface rheology parameters in the L2 phase are plotted against the frequency of the imposed shear in Figure 8. The surface loss modulus Gs′′ is larger than the storage modulus Gs′, and at higher frequencies Gs′ becomes undetectable as the phase difference between the stress and the strain approaches 90°. Gs′′ is approximately linear on a log-log scale, with a power law dependence on frequency of 1.1, which is close to the relation of G′′ ∝ ω for an ideal Newtonian-like material. This is in agreement with our previous observations on fatty alcohol monolayers.16 The surface viscosity is 0.017 mN‚s/m and is independent of frequency. This frequency dependence demonstrates that stearic acid monolayers behave as Newtonian interfaces under these conditions. Coperland et al.24 reported that the logarithm of the surface viscosity (ηs) is a linear function of the surface pressure (Π), on the basis of the two-dimensional theory of reaction rates by Moore and Eyring25
ln ηs ) ln ηs° + kΠ
(2)
where ηs° and k are constants for a particular monolayer system. It was reported that various monolayer systems such as aliphatic acids,26 phospholipids,27,28 and proteins29 obey eq 2. (23) Kaganer, V. M.; Mohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (24) Copeland, L. E.; Harkins, W. D.; Boyd, G. E. J. Chem. Phys. 1942, 10, 357. (25) Moore, W. J. J.; Eyring, H. J. Chem. Phys. 1938, 6, 391. (26) Boyd, E.; Harkins, W. D. J. Am. Chem. Soc. 1939, 61, 1188. (27) Vilallonga, F. Biochim. Biophys. Acta 1968, 163, 290.
Liquid Crystalline Polymer Monolayers
Figure 8. Frequency dependence of (a) the dynamic moduli and (b) the surface viscosity at 15 mN/m for stearic acid. Gs′′ is nearly proportional to ω, and the magnitude of the surface viscosity is independent of ω, which indicates a Newtonian fluid.
Figure 9 shows the surface moduli (9a) and viscosity (9b) as functions of pressure. The isotherm of stearic acid is shown in a box. The surface storage modulus is negligible because stearic acid is essentially purely viscous. The logarithm of the surface viscosity has a good linear correlation with the surface pressure. Interestingly, the thermodynamic phase transition at 24.5 mN/m is clearly marked by a change in the slope of eq 2, which verifies the fact that the L2-LS transition is second order. Previous results in our laboratory on docosanoic acid and azobenzene-containing fatty acid suggest that the surface rheological properties are sensitive to the occurrence of phase transitions in Langmuir monolayers.30 From these results, we conclude that stearic acid forms a Newtonian interface in the L2 phase with a surface viscosity independent shear rate, which is in agreement with previous studies using a canal viscometer31,32 and damped oscillation of a torsion pendulum.26 For this reason, stearic acid is a reasonable choice as a solvent in this study. It is also found that the thermodynamic (tilting) transition of the monolayers is correlated to the rheological shear properties, which provides evidence of the coupling between the microstructural molecular transition and mechanical shear properties. Non-Newtonian Rheology of PPPSH Monolayers. The dependence of the surface moduli on strain amplitude is shown in Figure 10 for a 40 mol % PPPSH/SA mixture. There is a linear regime where the shear moduli remain constant, but at higher amplitudes, Gs′ and Gs′′ drop exponentially, unlike those for pure stearic acid. All (28) Relini, A.; Ciuchi, F.; Rolandi, R. J. Phys. II 1995, 5, 1209. (29) MacRitchie, F. J. Macromol. Sci., Chem. 1970, A4, 1169. (30) Yim, K. S.; Fuller, G. G. In preparation. (31) Joly, M. J. Colloid Sci. 1956, 11, 519. (32) Jarvis, N. L. J. Phys. Chem. 1965, 69, 1789.
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Figure 9. (a) Dynamic surface moduli and (b) surface viscosity for stearic acid as a function of surface pressure. Both the surface loss moduli Gs′′ and the surface viscosity ηs result in a big increase at 24.5 mN/m, where the phase transition from L2 to LS occurs. And the quantitative relationship, eq 2, of surface viscosity relative to surface pressure shows different constant values beyond this transition.
Figure 10. Surface moduli at 10 mN/m of a 40 mol % PPPSH/ stearic acid monolayer as a function of the strain amplitude. Note that the dynamic surface moduli decrease exponentially beyond a certain amplitude limit, which means the existence of a nonlinear regime.
experiments are performed below this strain limit to ensure that the measurements are made in the linear viscoelastic regime. Figure 11 shows the frequency dependence of this PPPSH solution. Both surface moduli, Gs′ and Gs′′, are linear on log-log scales with powers of 0.28 and 0.42, respectively (Figure 11a), whereas the loss modulus for pure stearic acid is proportional to frequency (Figure 8). The surface viscosity decreases linearly as frequency increases, and finally approaches a constant value (Figure 11b). This shear-thinning is typical non-Newtonian behavior, which is not observed in the stearic acid monolayer. Note that both the surface moduli and the surface viscosity of the PPPSH polymeric monolayer are increased more
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Figure 11. Frequency dependence of (a) the dynamic moduli and (b) the surface viscosity at 10 mN/m for 40 mol % PPPSH/ stearic acid. The dynamic surface moduli exhibited a weaker dependence on frequency than that for stearic acid, and the surface viscosity decreases as frequency increases, indicating a non-Newtonian interface.
than 100-fold compared to those of pure stearic acid, indicating strong polymer-polymer interactions. Surface Pressure-Induced Isotropic-Nematic Transition. In our previous work,13 the orientational dynamics of the polymer was examined using UV dichroism, revealing a surface pressure-induced isotropicnematic transition caused by polymer-solvent interactions in the absence of any observable thermodynamic phase transition. In this paper, we compare the orientational dynamics with surface rheology measurements to examine this transition in greater detail. In Figure 12, the optical anisotropy (12a) and orientation angle (12b) are shown as functions of time for a reversal in the flow direction for a 40 mol % PPPSH/SA monolayer. Since the average orientation angle, χ, is restricted to either 0° or 90° in extensional flow, the angular information was incorporated into the sign of the optical anisotropy by multiplying the anisotropy by cos 2χ. Negative values of this product correspond to a backbone orientation of 90°, and positive values indicate alignment at 0°. It is evident that the applied extensional flow couples to the molecular orientation of PPPSH, causing flow alignment along the extension axis. Figure 13 shows a series of orientation-relaxation dynamics at different surface pressures in PPPSH/SA monolayers. First, the monolayer is subjected to an extensional flow with the orientation angle at 90°. At zero time, the rotation direction of the rollers is reversed, and the PPPSH reorients parallel to the new extension axis. After 200 s, the flow is stopped. At low surface pressure, there is an initial relaxation with an exponential decay, approaching the isotropic state. As the surface pressure increases, the relaxation becomes slower. The orientation does not relax completely at the highest surface pressure (8-9 mN/m), suggesting that a nematic ordered phase is
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Figure 12. Orientational response of 40 mol % PPPSH/stearic acid monolayers to an extensional flow reversal at 7 mN/m: (a) optical anisotropy; (b) orientation angle.
obtained. This observation is surprising because an isotropic-nematic transition is not observed in isotherms near 8-9 mN/m. On the other hand, the surface pressure at which the isotropic-nematic transition occurs in PPPSH/SA is not identical with that for PPPSH/DMPC mixtures (20-25 mN/m),13 which indicates that polymersolvent interactions play an important role on the isotropic-nematic transition, as well as polymer-polymer interactions. Mechanical interfacial rheometry measurements have been carried out to obtain an understanding of the relationship between the liquid crystalline transition and the response to shear. In Figure 14, the surface moduli and viscosity are shown as a function of surface pressure. The isotherm of the 40 mol % PPPSH/SA mixture is shown in Figure 4. Two linear regimes are found in the plot of surface pressure and the logarithm of surface viscosity (Figure 14b). Interestingly, a step change in the slope of eq 2 is observed around 8 mN/m, which corresponds to the isotropic-nematic transition as measured using UV dichroism. It has been demonstrated that the slope of eq 2 is proportional to the area of the flow unit.25 The slopes of eq 2 at the isotropic and nematic states are 0.589 and 0.214, respectively. The lower slope found for the nematic phase compared to the isotropic phase corresponds to the lower flow unit, which makes sense in the sense that molecules in the ordered structures can be packed more closely than randomly dispersed structures. It is also found that the PPPSH/DMPC mixture shows a similar change of the slope at the surface pressure where the isotropicnematic transition is found (data not shown). The rigid-rod model of polymer liquid crystals of Hess and Doi has been studied extensively to describe typical features of the rheological behavior of the nematic phase of such materials. By adding a nematic potential function U, defined as cl2 in two dimensions, where c is the number of rods per unit area and l is the rod length, predictions of the liquid crystalline transition in two dimensions have been possible.33,34 However, this model does not include
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Figure 14. (a) Dynamic surface moduli and (b) surface viscosity for 40 mol % PPPSH/stearic acid as a function of surface pressure. A net change in the slope of eq 2 is found around 8 mN/m, where the isotropic-nematic transition is observed.
the influence of the energetic interaction and the thermodynamic phase transition of monolayers, and it must be fitted with experiments performed at the same surface pressure. Therefore, other experimental investigations as well as the nematic potential model should be added to explain the surface pressure-induced isotropic-nematic transition completely. We examined the surface rheology properties to assist the understanding of the liquid crystalline transition, which indicated a significant correlation between this transition and the surface rheology parameters. Our results are in accord with the results of studies by several groups26-28 that changes in the slope of eq 2 can provide information on possible changes in the monolayer structure and phase transitions. It is important to note that the isotropic to nematic transition observed using UV dichroism and rheology is not evident in the pressurearea isotherms. Conclusions The isotropic to liquid crystalline transition and nonlinear viscoelastic response were studied by both optical and mechanical rheometric techniques on polymer monolayers comprised of rigid chains. UV dichroism employing polarization modulation was used to measure the orientational dynamics of polymer monolayers. A 40 mol % PPPSH/SA mixture shows a surface pressure-induced isotropic-nematic transition at 8-9 mN/m, which is not seen using pressure-area isotherm analysis. To examine the effect of this transition on surface rheology parameters, we compared the rheological responses measured using an interfacial stress rheometer. Measurements for both the pure fatty acid and the liquid crystalline polymer are summarized in Table 1. Monolayers of stearic acid in the Figure 13. Surface pressure-induced isotropic-nematic transition in 40 mol % PPPSH/stearic acid monolayers: (a) 0.5 mN/ m; (b) 3 mN/m; (c) 5 mN/m; (d) 7 mN/m; (e) 9 mN/m. It is observed that the isotropic-nematic transition exists at 8-9 mN/m.
(33) Marrucci, G.; Maffettone, P. L. Macromolecules 1989, 22, 4076. (34) Maffettone, P. L.; Grosso, M.; Friedenberg, M. C.; Fuller, G. G. Macromolecules 1996, 29, 8473.
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Table 1. Comparison of Surface Rheology Data in Pure Fatty Acids and Rigid Rod Polymer Solutions temp, °C
surface pressure, mN/m
stearic acid
22
15
constant Gs′ and Gs′′ until strain of 0.1
40 mol % PPPSH/SA
22
10
decrease exponentially more than strain of 0.005
amplitude sweep
L2 phase act as two-dimensional Newtonian liquids, while the addition of a rigid rod polymer, PPPSH, creates shearthinning behavior. Near the isotropic-nematic transition of this system, two distinct linear regions are found in the
freq sweep
surface viscosity
Gs′: NA
constant with shear rate
Newtonian
thermodynamic L2-LS transition at 24.5 mN/m
shear thinning
non-Newtonian
isotropic-nematic transition at 8-9 mN/m
Gs′′ ∝ ω1.1 Gs′ ∝ ω0.28 Gs′′ ∝ ω0.42
flow type
transition
plot of ln ηs (surface viscosity) and surface pressure, which suggests possible coupling between the molecular transition and mechanical shear properties. LA991403J
