Phase Behavior and Flow Properties of “Hairy-Rod” Monolayers

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Phase Behavior and Flow Properties of “Hairy-Rod” Monolayers Peter Fischer,* Carlton F. Brooks, and Gerald G. Fuller Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025

Anna M. Ritcey, Yufang Xiao, and Tarik Rahem Department of Chemistry, Laval University, Que´ bec G1K 7P4, Canada Received June 17, 1999. In Final Form: September 17, 1999 Brewster angle microscopy, interfacial stress rheometry, and Π-A isotherm measurements are employed to characterize spread monolayers of two new cellulose derivatives, one containing octadecyl side chains, the other p-nitroaniline substituents. Similar monolayer phase behavior is found in both cases. At high surface area, a liquid phase is observed to coexist with a dilute gaseous phase. Monolayer compression eventually leads to complete coalescence to yield a uniform liquid phase. Surprisingly, a homogeneous phase is observed only at surface pressures significantly higher than that characterizing the gas-liquid phase equilibrium. This observation is attributed to dipolar repulsion between neighboring liquid domains. Compression beyond surface areas at which a continuous monolayer is observed provokes a phase transition, appearing as either a constant pressure plateau or as changes in slope in the surface pressure area isotherms. BAM images reveal only uniform surface films at molecular areas within the transition region. Surface rheological measurements indicate significant changes in monolayer mechanical properties during compression. Importantly, these changes can be correlated with isotherm shape. In all cases, monolayers exhibit a predominantly viscous behavior at high surface areas where two phases coexist. Homogeneous films achieved upon compression exhibit rheological properties that depend on both temperature and the nature of the side chain substituents. In the case of long alkyl side chains (HPC-C18), the uniform film is found to be predominantly viscous at high temperatures, and more elastic at low temperatures. This observation is interpreted as an indication of the partial crystallization of interdigitated side chains, made possible by bilayer formation. Highly dipolar chromophores incorporated as side chain substituents increase interlayer interactions and interdomain repulsion.

Introduction The spontaneous adsorption of surfactants at the airwater interface is well-known. Amphiphilic molecules significantly reduce the surface free energy by adopting orientations in which the hydrophilic part is submerged in the aqueous phase while the hydrophobic tail typically points toward the air. When amphiphiles are studied as Langmuir monolayers, the reduction in surface energy is expressed as the surface pressure, Π ) γ0 - γ, where γ0 and γ are the surface tensions of the clean and monolayer covered surfaces, respectively. As the accessible surface area is reduced, the monolayer can undergo several phase transitions. In the case of aliphatic fatty acids, one observes gaseous, liquid, and solid phases, analogous to the familiar phase behavior of three-dimensional systems. An extensive description of the statistical thermodynamics of liquid surfaces is given by several authors.1-4 Rigid polymers substituted with flexible side chains, so-called hairy rods, have been shown to form stable surface films when spread at the air-water interface.5 Typically, these polymers do not contain highly polar * To whom correspondence should be addressed. E-mail: peter. [email protected]. Permanent address: ETH Zu¨rich, Universita¨tsstrasse 2, 8092 Zu¨rich, Switzerland. (1) Birdi, K. S. Lipide and Biopolymere Monolayer at Liquid Interfaces; Plenum Press: New York, 1989. (2) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; VCH Publishers: New York, 1994. (4) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1990. (5) Wegner, G. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1326.

functional groups and therefore direct comparison with the phase behavior of small molecule amphiphiles is not possible. Upon compression, a characteristic constant pressure plateau is observed in the monolayer surface pressure-area isotherms of a number of rigid polymers. In the case of spread monolayers of cellulose esters, Matsumoto et al. attribute the corresponding transition to the formation of bilayers, partly on the basis of film thickness measurements by electron microscopy.6 Kawaguchi et al., however, surmised that the transition region corresponds to an ordering of the alkyl side chains.7 A similar transition region is also evident in the isotherms of synthetic polypeptides. In this case, ellipsometry measurements by Motschmann et al. indicate that the plateau region corresponds to the piling up of undefined films near the barriers rather than the formation of a discrete bilayer.8 The exact nature of the transition still remains unclear. The Langmuir-Blodgett (LB) technique allows for the fabrication of ultrathin films through the deposition of monolayers from the air-water interface to solid substrates. LB films typically exhibit a high degree of orientational order that is induced at the water surface by the geometrical restriction in two dimensions. The control of molecular orientation offered by this technique is responsible for several possible applications of LB films, (6) Matsumoto, M.; Itoh, T.; Miyamoto, T. In Cellulosics Utilization; H. Inagaki, H., Phillips, G. O., Eds.; Elsevier Applied Science: London, 1989; p 151. (7) Kawaguchi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 29. (8) Motschmann, H.; Reiter, R.; Lawall, R.; Duda, G.; Stamm, M.; Wegner, G.; Knoll, W. Langmuir 1991, 7, 2743.

10.1021/la990779u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/12/1999

Phase Behavior and Flow Properties of “Hairy-Rod” Monolayers

Figure 1. Chemical structure for HPC-C18 (a) and Cell-C6NA (b).

including those in the field of nonlinear optics. Order in the transferred LB layers is clearly a function of the molecular orientation at the air-water interface. Other monolayer properties, including phase behavior, domain structures and viscoelasticity, are also influenced by the molecular organization of the films. Furthermore, during layer deposition, the monolayer experiences shear and extensional flow that can imprint additional orientational properties on the sample. Anisotropy in deposited films, due to velocity gradients in the floating monolayer during vertical deposition, has been demonstrated.9,10 Knowledge of the rheological properties as a function of strain, frequency, surface pressure, and temperature can provide insight into molecular interactions and organization at the air-water interface. In this study, surface rheometric measurements, in conjunction with morphology characterization by Brewster angle microscopy (BAM), are used to study monolayers formed from two new cellulose derivatives. The first sample is an alkyl ether, containing a flexible 18-carbon side chain. The second polymer contains aromatic chromophores tethered to the cellulose backbone via a hydrocarbon spacer. Experimental Section 1. Samples. All samples are cellulose derivatives modified by the addition of flexible side chains in order to be suitable for monolayer formation.11,12 Samples without attached chromophores are alkyl ethers prepared from (2-hydroxypropyl) cellulose. The chemical structure of HPC-C18 is shown in Figure 1a. The polymer has a degree of polymerization of about 250 and (9) Schwiegk, S.; Vahlenkamp, T.; Yu, X.; Wegner, G. Macromolecules 1992, 25, 2513. (10) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1996, 12, 1594. (11) Basque, P.; Gunzbourg, A. D.; Rondeau, P.; Ritcey, A. M. Langmuir 1996, 12, 5614. (12) Mao, L.; Ritcey, A. M.; Desbat, B. Langmuir 1996, 12, 4754.

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contains, on average, 2.3 octadecyl chains per repeat unit. Details of synthesis and characterization are given elsewhere.13 Cellulose derivatives containing N-methyl-p-nitroaniline were prepared from cellulose acetate by a previously reported method.14 The structure of the sample studied here, denoted hereafter as Cell-C6-NA, is given in Figure 1b. This polymer was prepared from cellulose acetate with a molecular weight of 100 000 and is completely substituted, containing three chromophores per anhydroglucose unit. Monolayers were spread from dilute chloroform solutions with a Hamilton microsyringe (50 mL) on deionized water (Millipore Corp.) in two different KSV (Helsinki, Finland) troughs. A 5000 System III symmetric compression film balance, with a maximum allowable interface of 110 mm × 700 mm and located in a class 1000 clean room, was used to obtain the BAM images. Monolayer rheology measurements were carried out on a smaller symmetric compression Minitrough system (maximum allowable interface of 75 mm × 330 mm) under normal laboratory conditions. Subphase temperatures were varied from 15 to 35 °C in increments of 5 °C and a Wilhelmy plate was used to monitor the surface pressure. The compression speed for all samples investigated was 0.25-1 mm/s. 2. Brewster Angle Microscopy (BAM). Brewster angle microscopy was carried out with a laboratory-built microscope that has been described in detail elsewhere.15-20 The setup consists of a laser light source-polarizer unit and a lens-analyzercamera unit. The laser light is p-polarized (electric field perpendicular to the surface) before hitting the air-water interface. Under Brewster angle conditions, p-polarized light is fully transmitted and no reflection is observed from a pure airwater interface. In the presence of a surface film, sufficiently dense to alter the refractive index, the Brewster angle condition is no longer met and reflection occurs. The resulting contrast permits the observation of monolayer structures and domains. The reflected light passes through the lens-analyzer unit and is detected by a CCD camera. Maximum contrast was obtained with the analyzer set at 60° with respect to the direction of p-polarization. BAM images of the flowing monolayers under compression were recorded on videotape and later digitized with Apple Macintosh VideoPlayer software. 3. Interfacial Rheology. An interfacial stress rheometer (ISR) was used to investigate the rheological properties of monolayers at the air-water interface. The rheometer is equipped with a Minitrough, enabling the variation of surface concentration and the simultaneous monitoring of surface pressure.21 This is an advantageous feature since the thermodynamic state of the film can be altered without having to spread a new film. A schematic of the setup is shown in Figure 2. A magnitized rod, situated in a channel built out of two glass slides, is used to probe the interface’s resistance to deformation. The meniscus created between the glass slides stabilizes the rod along the channel centerline. The channel is open at both ends so that the monolayer spreads uniformly over the surface and the equilibrium surface pressure is maintained at the rod during compression and expansion. Lateral stabilization of the needle is accomplished by two positioning coils (not shown in Figure 2). The coils are designed to create a uniform magnetic field at the location of the rod. The current through these coils is constant. A second set of two driving coils are used to create a constant gradient in the magnetic field at the location of the rod. It is this magnetic field (13) Xiao, Y.; Ritcey, A. M. to be submitted to Langmuir. (14) Basque, P.; Ritcey, A. M. Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering; American Chemical Society: Washington, 1994; Vol. 71, p 488. (15) Ho¨nig, D.; Overbeck, G. A.; Mo¨bius, D. Adv. Mater. 1992, 4, 419. (16) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (17) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Macromolecules 1996, 29, 705. (18) La¨uger, J.; Robertson, C. R.; Frank, C. W.; Fuller, G. G. Langmuir 1996, 12, 5630. (19) Maruyama, T.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Science 1996, 274, 233. (20) Maffetone, P. L.; Grosso, M.; Friedenberg, M. C.; Fuller, G. G. Macromolecules 1996, 29, 8473. (21) Brooks, C. F.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1999, 15, 2450.

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Figure 2. Top view of the interfacial stress rheometer showing glass barriers, magnetic needle, Helmholtz coils, flow field, and detection of needle position. gradient that generates a force to induce the motion of the rod. In all experiments, the needle is moved back and forth around a fixed position and the displacement is monitored. The current through one of the coils is controlled by a function generator to create an oscillatory magnetic field. The field is constant over a 6 cm region, and therefore the force exerted on the rod is constant. The rod’s position is determined by monitoring the motion of the rod’s edge at the interface. The shadow is imaged with an inverted microscope and detected with a photodiode array. From the rod’s position (strain) in relation to the applied current (stress), one is able to detect both the delay in phase between the strain and the stress and the ratio of their amplitudes. With these values, the complex surface modulus, Gs*, can be determined. This setup has several advantages in comparison with steady-state surface viscometers such as channel viscometers or oscillatory shear surface rheometers. It achieves high sensitivity (applied force: 10-9 F < 10-6 N; complex surface viscosity: |ηs*| > 0.01 mPas) in oscillatory shear.22,23 The magnetized rod is first placed between the glass slides on a clean air-water interface. The edge of the rod’s shadow is positioned onto the photodiode array by changing the current setpoint in the magnetic coils (large corrections) or by simply shifting the position of the array (small corrections). The position of the needle then is observed by the microscope position sensor unit and monitored with a computer via a data acquisition board (National Instruments, Austin, TX). The spreading solution is then deposited at the interface. Once the solvent has completely evaporated and the temperature of the trough has equilibrated, the monolayer is compressed to the desired surface pressure. During compression, the needle may move out of the aperture of the microscope. This is because the channel and the needle are not sitting directly in the middle of the trough. A correction of the needle’s position is achieved by altering the direct current of one of the positioning coils. This adjustment of the magnetic field gradient must be applied very carefully since fracture or major distortions of the monolayer can occur and significantly change the rheological properties. Rheological properties were investigated as a function of both strain and frequency. Within the theory of linear viscoelasticity, the complex surface modulus, Gs*, the complex surface viscosity, |ηs*|, the storage modulus, Gs′, and the loss modulus, Gs′′, can be derived. The storage modulus, Gs′, represents the elastic properties while the loss modulus, Gs′′, represents the viscous properties of the sample. An introduction to linear viscoelasticisty is given elsewhere.24 The oscillatory movement of the needle (22) Fischer, P.; Rehage, H. Rheology of Interfaces; Internal Report, Universita¨t Essen, 1994. (23) Mannheimer, R. J.; Schechter, R. R. J. Colloid Interface Sci. 1968, 27, 2. (24) Tschoegl, N. W. The Phenomenological Theory of Linear Viscoelastic Behavior; Springer-Verlag: Berlin, 1989.

Fischer et al. must be kept relatively small so as not to exceed the linear viscoelasticity regime nor destroy the sample. For this reason, one normally performs an amplitude sweep (shear strain sweep) in which the applied coil current, and hence the amplitude of the needle displacement or, equivalently, the surface stress, is progressively increased as the applied frequency is held constant. In our experiments, the magnetized rod was subjected to an oscillating force at 0.92 rad‚s-1 (0.1465 Hz), the approximate geometric mean of the range of frequencies probed by the present device. Within the linear regime, the properties of the monolayer are not altered by this rheological experiment. With large strains, flow can influence the microstructure of the film, resulting in a nonlinear stress-strain relationship. In the linear regime, Gs* is independent of strain, and, presumably, the imposed flow does not alter the structural properties of the monolayer. Within the linear viscoelastic regime, frequency sweep experiments, in which the frequency of the oscillatory motion of the needle is systematically increased while the applied shear stress (the amplitude) is kept constant, were also performed. The sinusoidal current through the driving coils was modulated by the function generator from 0.1 to 10 rad‚s-1. The strain was typically held constant over the frequencies probed. However, to obtain a constant readout of the needle’s position on the detector array the current through the driving coils might be increased with the frequency. This technique is especially useful for rigid monolayers where a constant current does not result in a constant deformation at all frequencies. The dependence of Gs* on frequency reveals information about the elasticity and viscosity of the monolayer on different time scales. For example, the monolayer may behave as a viscous fluid on long time scales, but appear as an elastic solid at short time scales at which the molecules cannot rearrange rapidly enough to accommodate the imposed deformation.

Results and Discussion 1. Surface Pressure-Area Isotherms. Surface pressure-area isotherms recorded for spread monolayers of HPC-C18 are presented in Figure 3a. The isotherm shape exhibits a striking temperature dependence. A constant pressure transition is clearly evident at 35 °C, extending from 120 to 50 Å2. Upon modest cooling to 25 °C, the plateau vanishes, although changes in slope indicate that a phase transition persists. As indicated in the Introduction, the plateau transition is characteristic of rigid polymer monolayers and has been attributed to both side chain ordering and the formation of bi- or multilayers.7 Significantly, the transition region extends, at the high compression end, to molecular areas below 50 Å2 per residue. This area is smaller than that required to accommodate either three hydrocarbon chains in a closely packed all-trans conformation or an anhydroglucose ring. For this reason, it is certain that the molecules must leave the surface, and at the end of the transition, the film no longer exists as a monolayer. Whether the transition corresponds to the formation of a discreet bilayer or rather a disordered bulk phase is less clear. Previously reported polarized infrared spectra of spread monolayers of a closely related polymer (HPC-C16) indicate that there is no significant change in side chain orientation during compression through the plateau region.25 These results suggest that the transition region does not correspond to side chain ordering, and rather the hypothesis of bilayer formation is retained. The surface pressure-area isotherm recorded for monolayers of the second sample, Cell-C6-NA, is shown in Figure 3b. Although no constant pressure plateau is observed, a highly compressible phase is observed between surface areas from 90 to 40 Å2 per repeat unit. Unlike that of HPC-C18, the isotherm shape is insensitive to temperature changes over the limited range studied. (25) Xiao, Y.; Bourque, H.; Pe´zolet, M.; Ritcey, A. M. Thin Solid Films 1998, 327, 299.

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Figure 3. Surface pressure-area isotherms recorded for spread monolayers of (a) HPC-C18 as a function of temperature [(a) 10, (b) 15, (c) 20, (d) 25, (e) 30, and (f) 35 °C] and (b) Cell-C6-NA at 20 °C.

2. Phase Behavior and BAM Images. Spread monolayers of HPC-C18 were examined by BAM and typical images obtained at 15 and 35 °C are presented in Figures 4 and 5, respectively. In all images, the “onion-ring” pattern originating in the upper left-hand corner is due to interference within the optical train and the coating of the camera chip. Little difference is observed between the images recorded at the two temperatures. At both 15 and 35 °C, the monolayers exhibit rich phase behavior at low surface pressure, followed by the eventual formation of a homogeneous phase upon compression. Brewster angle micrographs at a pressures below our detection limit (0.1 mN‚m-1), and thus evaluated as 0 mN‚m-1, are shown in Figures 5a and 6a. In both cases, a concentrated liquid monolayer phase (bright areas) is observed in coexistence with a dilute, presumably gaseous, phase (dark areas). The continuous observation of these two phases during compression from very high initial spreading areas precludes the possibility of their both being liquid. Furthermore, their coexistence at 0 mN‚m-1 corresponds well to the small gas-liquid equilibrium surface pressures reported in the literature for spread monolayers of pentadecanoic acid.26 The bright areas are clearly fluid and are easily deformed. Furthermore, their elongated noncircular shape is indicative of a relatively low line tension between the two phases. Upon further compression, the two phases continue to coexist until a surface pressure of 5-6 mN‚m-1 is reached (Figures 4b and 5b). This result is somewhat surprising. The observation that the transition to a uniform liquid phase occurs only at surface pressures significantly higher (5-6 mN‚m-1) than the surface pressure characterizing the gas-liquid equi(26) Kim, M. W.; Cannell, D. S. Phys. Rev. A 1976, 13, 411.

Figure 4. BAM images recorded for spread monolayers of HPCC18 at 15 °C and at different surface pressures (Π): (a) 0, (b) 5, and (c) 10 mN‚m-1 (vertical length of all images is 1 mm).

librium appears to violate the phase rule. One possible explanation for this additional surface pressure is dipolar repulsion between neighboring liquid phases. This repulsion creates a barrier that must be overcome before coalescence can occur. This idea is further discussed below for samples containing polar chromophores as side chain substituents. It is important to note that the transition to a homogeneous liquid monolayer occurs at the same surface pressure for each of the investigated temperatures (1535 °C). The homogeneous phase obtained is depicted in Figures 4c and 5c, at temperatures equal to 15 and 35 °C, respectively (Π ) 10 mN‚m-1). Identical images are obtained at all pressures between 5 and 6 and 35 mN‚m-1.

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Figure 5. BAM images recorded for spread monolayers of HPC-C18 at 35 °C and at different surface pressures (Π): (a) 0, (b) 5, and (c) 10 mN‚m-1 (vertical length of all images is 1 mm).

This is perhaps unexpected when one considers the isotherms of Figure 3a. The important temperature dependence of the surface pressure area isotherms is not reflected in the BAM images. At 35 °C, a constant pressure plateau is evident at 22-23 mN‚m-1. The BAM study, however, reveals only a uniform liquid phase through this transition region. The featureless BAM images suggest the regular thickening of a liquid film during compression through the plateau region, rather than an equilibrium between the monolayer and discrete bilayer phases. It is, however, difficult to reconcile the picture of an unstructured liquid multilayer with polarized infrared spectroscopy measurements. In the case of two cellulose ethers, (dodecyl)cellulose and HPC-C16, infrared studies indicate

Fischer et al.

that the alkyl side chains retain a constant net orientation perpendicular to the water surface during compression through the plateau region.12,25 Furthermore, in the case of HPC-C16, splitting of the methylene scissoring bands at 1472 and 1463 cm-1 indicates a high degree of order and an all-trans conformation for the side chains. When considering the BAM images, one must not neglect the relatively low lateral resolution. Bilayer domains smaller than ∼5 µm cannot be detected, and the possibility of the formation of a discrete bilayer is therefore not eliminated. Additional compression of the HPC-C18 surface films eventually leads to the collapse to three-dimensional aggregates. These structures appear (not shown here) in the BAM images at surface pressures exceeding 40 mN‚m-1. The collapse pressures, as well as the appearance of the collapsed film, do not depend on temperature. Comparison with the isotherms of Figure 3 illustrates that the observed monolayer collapse is not accompanied by a detectable change in surface pressure. For example, the isotherm recorded at 15 °C shows a smooth pressure rise upon compression through the collapse pressure. At the time being we do not have an explanation for continuous pressure rise during and beyond collapse, although this observation does indicate that a thermodynamic equilibrium does not exist between the uniform surface film and the collapsed state. For the cellulose monolayer with attached chromophores, Cell-C6-NA, the situation is, in general, similar. First a rich, more filigreed phase morphology is observed at lower surface pressures. Coexisting liquid and gas phases persist from 0.1 to 8 mN‚m-1, as shown in Figures 6a and b. A homogeneous monolayer is observed, at all investigated temperatures (15-25 °C) only upon compression to a surface pressure of 10 mN/m. The resulting homogeneous film is shown in Figure 6c. It is important to note the relatively high surface pressure required to achieve complete coalescence of the liquid domains. As discussed above, this pressure is attributed to dipolar repulsion between neighboring domains. This hypothesis is supported by the significant increase in this pressure for the cellulose derivatives containing p-nitroaniline. Side chain orientation perpendicular to the water surface forces a parallel alignment of chromophore dipoles, which would indeed result in significant interdomain repulsion. The phase behavior revealed by the BAM investigation has important implications for the preparation of LB films from these polymers. Uniform films can only be prepared at surface pressures exceeding those necessary for complete coalescence. In the case of the polar materials of interest for nonlinear optical applications, this surface pressure can be large. No collapse of the monolayer formed by Cell-C6-NA is observed up to a surface pressure of 15 mN‚m-1. Due to the geometrical restrictions of the Langmuir trough, and since spreading at nonzero surface pressures is not advisable, we were not able to obtain isotherms and BAM image at higher pressures than 15 mN‚m-1. 3. Monolayer Rheology. 3.A. HPC-C18. Representative strain sweeps for HPC-C18 are shown in Figure 7 for measurements performed at 20 °C and at various surface pressures from 2.5 to 30 mN‚m-1. The storage modulus, Gs′, and loss modulus, Gs′′, are both plotted as a function of strain. For each strain, two or three data points are plotted to illustrate the typical reproducibility of the measurement. In general, the linear viscoelastic plateau, where the rheological properties are not a function of the applied strain, is quite pronounced and no nonlinear effects have to be taken into account. Only at low surface pressures (2.5 and 5 mN‚m-1) strain-thinning is observed.

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Figure 7. Dynamic surface moduli as a function of shear deformation, γ, for spread monolayers of HPC-C18 at T ) 20 °C, Π ) 2.5-30 mN‚m-1, and ω ) 1 rad‚s-1.

Figure 8. Dynamic surface moduli (represented by tan δ) as a function of the surface pressure, Π, for spread monolayers of HPC-C18 (T ) 20-35 °C; solid symbols) and Cell-C6-NA (T ) 15-25 °C; open symbols). Divisions between phases are indicated by a vertical solid line for HPC-C18 and a vertical dashed line for Cell-C6-NA. The horizontal phase division is valid for both samples. Labels H and NH denote homogeneous and nonhomogeneous phases, respectively.

Figure 6. BAM images recorded for spread monolayers of CellC6-NA at 20 °C and at different surface pressures (Π): (a) 0, (b) 8, and (c) 15 mN‚m-1 (vertical length of all images is 1 mm).

Here, both properties decrease as a function of strain, implying a structural breakdown of the sample at that particular surface pressure. From the corresponding BAM images (Figures 4a and 5a), one can easily suggest that the inhomogeneous domain structure, characteristic of the biphasic region, is responsible for the nonlinear behavior. One has to take into account the fact that the detected rheological properties are an unknown combination of deformations of the two types of domains. In curves not shown here, several successive strain sweep experiments performed on the same monolayer (i.e., constant temperature T ) 20 °C and surface pressure Π ) 6 mN‚m-1) produce identical results. The sample thus recovers its original state even after the application of

high deformations or frequencies. From an experimental point of view, this means that a change of material is not necessary after each experiment. Therefore, several strain sweeps or frequency sweeps can be performed with a single sample. The dynamic moduli, represented by the loss angle, tan δ ) Gs′′/Gs′, are plotted in Figure 8 (filled symbols) as a function of surface pressures for various temperatures between 20 and 35 °C. The rheological properties of the HPC-C18 monolayer exhibit a strong temperature dependence. At 20 °C, the loss modulus initially exceeds the storage module (tan δ > 1) at low surface pressure where domains are freely swimming in a gaseous phase, and thus respond to the imposed stress as a fluid. Under these conditions the monolayer is morphologically nonhomogeneous (as concluded from the BAM images of Figures 4a and 5a) and viscous; therefore we label this phase as NH, viscous. Upon compression, Gs′ increases more rapidly than does Gs′′, with the two curves crossing at a surface pressure of 6 mN‚m-1 (tan δ ) 1), above which the storage modulus is larger than the loss modulus (tan δ < 1). The rheological properties of the monolayer change from a more viscous to a more elastic behavior. We therefore refer to this phase as H, elastic. From the BAM images it is

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Figure 9. Sketch of bilayer formation indicating side chain interdigitation.

known that Π ) 6 mN‚m-1 corresponds to the surface pressure at which a homogeneous liquid phase is achieved. In Figure 8, one can also see that the crossover point of Gs′′/Gs′ (tan δ ) 1) shifts to higher surface pressures with increasing temperatures. At T ) 20 °C, the transition takes place at the same surface pressure as the BAM images indicate for the transition from a nonhomogeneous (NH, viscous) layer to a homogeneous surface film (H, elastic). For temperatures higher than 20 °C, again a homogeneous monolayer in observed for surface pressures exceeding 6 mN‚m-1 (BAM images in Figures 4c and 5c) but the films are not elastic. Here a third phase, called viscous-homogeneous (H, viscous) is observed. Upon further compression the monolayer can, however, form a continuous molecular network with pronounced elastic properties. At 30 °C, the crossover is observed at 18 mN‚m-1 while at 35 °C the crossover point is not detected due to the long plateau in the isotherm (Figure 3a, line f) and to geometrical restrictions of the Minitrough used in the study. However, since a homogeneous phase is observed in the BAM images at all temperatures, the crossover can be expected at surface pressures higher than 20 mN‚m-1. In conclusion, both BAM and rheological results clearly show that a homogeneous film is established at surface pressures of 6 mN‚m-1 and higher. At lower temperatures, this film exhibits immediate elasticity, whereas at higher temperatures a pronounced viscous behavior is observed for surface pressures up to 20 mN‚m-1. This information is summarized in a schematic rheo-morphological phase diagram shown in Figure 8 (filled symbols and solid lines). The phase labeled NH, viscous, corresponds to the fluid biphasic layer observed below a surface pressure of 6 mN‚m-1 and for all probed temperatures. Phases labeled as H, elastic, and H, viscous are the homogeneous/elastic and the homogeneous/viscous phases, respectively. The borderline between H, elastic, and H, viscous, is given by the crossover of Gs′ and Gs′′, i.e., tan δ ) 1, obtained from the rheological measurements. It is interesting to note that although the BAM images are temperature independent, a significant correlation exists between the rheological data and isotherm shape. At higher temperatures (35 °C), the surface pressurearea isotherm exhibits a distinct plateau transition which corresponds to the formation of a continuous viscous phase. At lower temperatures, where the transition is delimited only by less evident changes in the slope in the isotherm, a continuous elastic phase is observed. These results can be interpreted in the following way: At all temperatures investigated, compression beyond the surface pressure at which a continuous monolayer is first achieved (5-6 mN‚m-1 in this case) forces molecules to leave the water surface. Whether these molecules form a discrete bilayer or a less defined films is unclear, although as mentioned above, previously reported polarized infrared studies support the hypothesis of an ordered bilayer. In either case, the presence of polymer molecules on top of the surface monolayer allows for side chain interdigitation, as sketched in Figure 9. At high temperatures, the

Figure 10. Dynamic surface moduli (represented by tan δ) of spread monolayers as a function of the temperature for (a) HPCC18 (Π ) 5-20 mN‚m-1) and (b) Cell-C6-NA (Π ) 5-14 mN‚m-1). The dashed line (tan δ ) 1) indicates the transition from a viscous to an elastic film.

interdigitated bilayer behaves as a viscous liquid and the monolayer to bilayer transition occurs at a constant surface pressure. At lower temperatures, however, the bilayer exhibits significant elasticity. This suggests that partial crystallization occurs in the interdigitated side chains, creating cross-links between neighboring layers. In this case, a thermodynamic first-order transition will not be observed, and no plateau appears in the surface pressure area isotherms. Side chain interdigitation in LB films formed from hairy rod molecules has been indicated by X-ray and neutron reflectometry.27 It has also been suggested to explain compression-expansion hysteresis in spread monolayers of poly(L-glutamic acid).28 Watanabe et al. have reported an extensive investigation of a series of poly(L-glutamates) in which side chain interdigitation and crystallization are invoked to explain systematic variations in thermal and mechanical properties as a function of side chain length.29 Interestingly, these authors report a side chain melting temperature of 55 °C for the octadecyl derivative. In the present study, we observe a transition from an elastic to a predominantly viscous continuous phase near 30 °C. These two temperatures are quite similar, especially when one takes into consideration that the former is obtained for bulk poly(L-glutamate) films, whereas the latter corresponds to a spread surface film of a cellulose ether. This similarity supports the hypothesis that the onset of elasticity is the result of side chain crystallization. The temperature dependence of Gs′ and Gs′′ is plotted in Figure 10 for HPC-C18 at three different surface pressures. In all cases, both moduli decrease with increasing temperature. A reduction in monolayer viscosity at higher temperatures is as expected as molecular movements are facilitated. The simultaneous decrease in the elastic modulus, however, warrants further discussion. The statistical thermodynamic treatment of rubber elasticity predicts that, for a fixed concentration of cross-links, the elastic modulus should increase with increasing temperature. This is because the decrease in entropy associated with chain extension renders deformation more difficult at higher temperatures. The decrease in Gs′ observed here at higher temperatures therefore suggests a decrease in the strength and number of interactions (27) Vierheller, T. R.; Foster, M. D.; Schmidt, A.; Mathauer, K.; Knoll, W.; Wegner, G.; Satija, S.; Majkrzak, C. F. Macromolecules 1994, 27, 6893. (28) Reda, T.; Hermel, H.; Ho¨ltje, H.-D. Langmuir 1996, 12, 6452. (29) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141.

Phase Behavior and Flow Properties of “Hairy-Rod” Monolayers

Langmuir, Vol. 16, No. 2, 2000 733

Figure 12. Dynamic surface moduli as a function of shear deformation, γ, for spread monolayers of Cell-C6-NA at T ) 20 °C, Π ) 5-18 mN‚m-1, and ω ) 1 rad‚s-1.

Figure 11. Dynamic surface moduli obtained for spread monolayers of HPC-C18 plotted as a function of angular frequency, ω: (a) T ) 20 °C, Π ) 5, 20 mN‚m-1; (b) T ) 25 °C, Π ) 5, 20 mN‚m-1.

serving as cross-links between the chains. This is consistent with the above discussion of chain interdigitation with partial crystallization. The frequency dependence of the dynamic moduli are plotted in Figure 11, at two different temperatures and surface pressures, all within the domain of homogeneous surface film formation. Predominantly elastic behavior (Gs′ > Gs′′) is observed for both pressures at 20 °C and for the high surface pressure (20 mN‚m-1) at 25 °C. Furthermore, these two plots are relatively independent of frequency, as expected for a rubber-like material. However, for the higher temperature and the lower pressure (25 °C and 10 mN‚m-1), a more viscous behavior is found with Gs′′ exceeding Gs′. The observed frequency dependence of the moduli is not, however, that of a typical fluid. For a fluidlike sample, the models predict slopes of 1 and 2 for the frequency dependence of Gs′ and Gs′′, respectively. Therefore we are not dealing with a pure two-dimensional liquid but with an already interacting two-dimensional network. Since the dynamic moduli are not found to be a function of frequency, the phase diagram presented in Figure 8 is valid not only for the probed frequency of the strain sweeps of 0.92 rad‚s-1 but also for a wider regime of frequencies (0.1 < ω