Phase Transitions in Spread Monolayers of Cellulose Ethers

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Phase Transitions in Spread Monolayers of Cellulose Ethers Yufang Xiao and Anna M. Ritcey* Department of Chemistry and CERSIM, Laval University, Que´ bec, Canada, G1K 7P4 Received June 2, 1999. In Final Form: October 5, 1999 Cellulose ethers containing alkyl side chains of varying length were prepared from (2-hydroxypropyl)cellulose and alkyl bromides. Two different synthetic methods were employed. The average degree of alkyl substitution of the product polymers ranges from 1.8 to 2.8, as determined from 1H NMR spectroscopy. Surface pressure-area isotherms recorded for spread monolayers depend on temperature, side-chain length, and degree of substitution. Most striking is the temperature dependence. For certain samples, a constant pressure transition is clearly evident in isotherms recorded at higher temperatures. Upon modest cooling, the plateau vanishes, although changes in slope indicate that a phase transition persists. Results are interpreted in terms of partial crystallization in interdigitated side chains. For samples that exhibit a constant pressure transition, the plateau pressure increases with decreasing the subphase temperature. Thermodynamic analysis yields positive entropy and enthalpy changes for the transition, consistent with a transition from a monolayer to a less ordered bilayer. An energy change associated with the transition is calculated as ∆E ) 60 ( 15 kJ mol-1. LB films transferred to solid substrates were characterized by polarized infrared spectroscopy. These results suggest that the alkyl side chains exist in a predominately all-trans conformation in the LB films.

1. Introduction The concept of using rigid-rod-like polymers, decorated with flexible alkyl side chains, to construct stable and homogeneous ultrathin films by the Langmuir-Blodgett (LB) technique has been applied to several types of macromolecules, including cellulosics1,2,3,4 and polyglutamates.5,6 In the case of cellulose derivatives, both ether and ester linkages have been employed to attach the long alkyl chains to the rigid polymer backbone. The pressure-area isotherms recorded for spread monolayers of these cellulose derivatives exhibit a characteristic plateau, which has been attributed both to a phase transition from an expanded to a condensed state7 and to the formation of bi- or multilayers.3,4 Interestingly, this constant pressure plateau is absent in the pressure-area isotherm of cellulose hexadecanoate,1 cellulose octadecanoate, and (trioctadecyl)cellulose.8 The absence of this transition for derivatives containing side chains of 16 or 18 carbon atoms has not been addressed. In the present paper, we examine the monolayer properties of a series of new cellulose ethers containing side chains of this critical length. Because of difficulties related to the solubility of the starting materials, long alkyl chains are grafted onto a soluble cellulose derivative, (2-hydroxypropyl)cellulose, rather than onto cellulose itself. * To whom correspondence should be addressed. (1) Kawaguchi, T.; Nakahara, H.; Fukuda, K. J. Colloid Interface Sci. 1985, 104, 290. (2) Schaub, M.; Fakirov, C.; Schmidt, A.; Lieser, G.; Wenz, G.; Wegner, G.; Albouy, P.-A.; Wu, H.; Foster, M. D.; Majrkzak, C.; Satija, A. Macromolecules 1995, 28, 1221. (3) Basque, P.; de Gunzbourg, A.; Rondeau, P.; Ritcey, A. M. Langmuir 1996, 12, 5614. (4) Itoh, T.; Tsujii, Y.; Suzuki, H.; Fukuda, T.; Miyamoto, T. Polym. J. 1992, 24, 641. (5) Duda, G.; Wegner, G. Makromol. Chem., Rapid Commun. 1989, 9, 495. (6) Duda, G.; Schouten, A. J.; Arndt, T.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988, 159, 221. (7) Kawaguchi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 29. (8) Itoh, T.; Suzuki, H.; Matsumoto, M.; Miyamoto, T. In Cellulose Structural and Functional Aspects; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Ellis Horwood: New York, 1990; p 409.

We recently examined the rheological properties of monolayers spread from cellulose ethers.9 Interestingly, changes in isotherm shape could be correlated with changes in monolayer mechanical properties. We interpreted these results as an indication of side-chain interdigitation in the surface bilayer, permitting partial sidechain crystallization at lower temperatures. The resulting cross-links between neighboring layers are responsible for an increase in film elasticity that prevents the monolayer-to-bilayer transition from occurring under equilibrium conditions. Appropriately substituted rigid-rod polymers have been shown to be interesting materials for the fabrication of novel supramolecular architectures.10 LB films of cellulose ethers have received considerable attention for a variety of practical applications, including the production of ultrathin membranes11 and the preparation of films possessing nonlinear optical properties.12 The control of desired properties clearly requires a fundamental understanding of the relationship between molecular organization, in both spread monolayers and transferred LB films, and the structure of the constituent macromolecules. 2. Experimental Section 2.1 Materials. Cellulose ethers of the general structure sketched in Figure 1 were prepared from commercially available (2-hydroxypropyl)cellulose (HPC) (Aldrich, molecular weight 100,000, MS ) 4, DS ) 3.0). Because of the sequential addition of propylene oxide moieties during the synthesis of HPC, this polymer must be characterized by both a degree of substitution (DS) and a molar substitution (MS). The DS corresponds to the average number of hydroxyl groups substituted per anhydroglucose ring and cannot exceed a value of 3. The MS is the average number of oxyethyene moieties per repeat unit. Two synthetic methods were employed for the introduction of long alkyl chains onto HPC: (9) Fischer, P.; Brooks, C. F.; Fuller, G. G.; Ritcey, A. M.; Xiao, Y.; Rahem, T. Langmuir 2000, 16, 726. (10) Wegner, G. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1326. (11) Seufert, M.; Fakirov, C.; Wegner, G. Adv. Mat. 1995, 7, 52. (12) Basque, P.; Ritcey, A. M. PMSE Polym. Preprints 1994, 71, 488.

10.1021/la990691g CCC: $19.00 © 2000 American Chemical Society Published on Web 03/24/2000

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Langmuir, Vol. 16, No. 9, 2000 4253 2.3 Sample Characterization. Product polymers were characterized with a Nicolet 550 FTIR and a Bruker 300 MHz NMR spectrometer. IR measurements were made on cast films, and NMR spectra were recorded in CDCl3 solution. Transmission IR spectra of deposited LB layers were recorded on ZnSe substrates. Spectra shown are the sum of 1000 scans, accumulated at a resolution of 2 cm-1.

3. Results and Discussion Figure 1. General structure of the cellulose ethers investigated. The exact substitution pattern of the parent HPC varies statistically along the chain. The representative repeat unit shown here respects the average molar substitution of 4 and a degree of substitution equal to 3. Samples with n ) 15, 17, and 21 are considered and denoted HPC-C16, HPC-C18, and HPC-C22, respectively. Method 1. One gram of HPC was dissolved in 100 mL of anhydrous ethanol. A 12.5-fold molar excess, relative to the number of hydroxyl groups to be substituted, of finely powdered sodium hydroxide was introduced, and the mixture refluxed with stirring for 1 h under nitrogen before the dropwise addition of 1-bromohexadecane (12.5-fold molar excess). The reaction mixture was refluxed for 24 h under nitrogen. After cooling to a room temperature, the mixture was poured into 200 mL of water. The cellulose ether was isolated by extraction with dichloromethane and purified by repeated dissolution in dichloromethane followed by precipitation in methanol. Method 2. One gram of HPC was dissolved in a mixture of 40 mL of tetrahydrofuran (THF) and 13.60 mL of a 1 M solution of potassium tert-butoxide in THF. A 4-fold molar excess, relative to the number of hydroxyl groups to be substituted, of alkyl bromide dissolved in 30 mL THF was added. The reaction mixture was stirred at room temperature for 7 days under nitrogen and then poured into the hot methanol. The precipitated polymer was filtered and dried. The dried material was extracted with chloroform and any insoluble material removed by filtration. The chloroform solution was again poured into hot methanol to precipitate the desired product. Cellulose ethers were prepared in this way from 1-bromohexadecane (C16), 1-bromooctadecane (C18), and 1-bromodocosane (C22). All samples were purified by preparative gel permeation chromatography on Lipophilic Sephadex (Sigma) in CHCl3. The preparative column was 95 cm in length and had a diameter of 4 cm. A Waters Differential Refractometer R401 served as the detector. Sample concentrations of about 20 mg/mL were employed, and each injection was 2 mL. The flow rate was kept constant at 2 mL/min. Under these conditions, the polymeric product appears as the first peak after about 30-50 min. Additional peaks appearing at longer elution times correspond to small molecule reagents present in excess, notably 1-bromoalkanes. 2.2 Langmuir-Boldgett Film Deposition. Cellulose ethers were dissolved in chloroform at a concentration of 0.5 mg mL-1. Approximately 100 µL of the solution was carefully spread on freshly filtered water (Nanopure II, Barnstead) in a KSV model 3000 model film balance. After evaporation of the solvent, the floating film at the air-water interface was compressed at a constant barrier speed of 30 mm min-1. This corresponds to compression rates of between 27 and 29 Å2 repeat unit-1 min-1, depending on the number of molecules spread. Slower barrier speeds result in isotherms identical to those reported here. The surface pressure was monitored by a platinum Wilhelmy plate. Monolayers were transferred to hydrophobic glass, silicon, ZnSe, and aluminum coated substrates by the vertical dipping method at the constant surface pressure of 15 mN m-1 for HPC-C16b and 25 mN m-1 for the other cellulose ethers. Substrates were passed through the interface at a speed of 10 mm min-1, and all transfers were carried out at a subphase temperature of 15 °C. Glass substrates were cleaned with an ultrasonic bath, first in chloroform and 2-propanol for 10 min each, then in a 10% aqueous detergent solution (Decon 75, BDH) for 30 min. Substrates were rinsed repeatedly with Nanopure water before being treated with hexamethyldisilazane in chloroform (5%) in the ultrasonic bath for 15 min.

3.1 Synthesis of HPC Ethers. Procedures for the synthesis of cellulose alkyl ethers of intermediate sidechain length, starting from cellulose acetate, have been previously reported.13 Several nonaqueous solvents for cellulose have also been employed14-19 for the direct modification of cellulose under homogeneous conditions. Unfortunately, these methods are not appropriate for the introduction of alkyl side chains with lengths exceeding 12 carbon atoms because of the limited solubility of both the starting alkylating agents and the resulting partially substituted cellulosics in strongly polar environments. An alternative approach to the preparation of long-chain ethers consists of starting from a soluble cellulose derivative containing functional groups accessible for further substitution. The solubility of the commercially available (2-hydroxypropyl)cellulose (HPC) in a large range of common organic solvents makes it a very attractive candidate. This synthetic route does, however, lead to cellulose ethers of a more complex structure than the simple tri-ethers previously investigated.3 Because of the possible sequential addition of propylene oxide moieties during the synthesis of HPC, not all side chains of the parent polymer are necessarily of the same length. This is illustrated in Figure 1, where a second oxyethylene moiety is placed arbitrarily at position 2 of the anhydroglucose ring. Alkyl ethers of HPC, with side chains containing from 16 to 22 carbon atoms, were prepared according to the procedures described in the Experimental Section. The structure of the resulting polymers, denoted as HPC-C16, HPC-C18 and HPC-C22, is shown schematically in Figure 1. The product HPC ethers were characterized by 1H NMR and FTIR spectroscopy. The successful introduction of long alkyl chains is apparent from the NMR spectrum of HPCC22 presented in Figure 2. The average number of hydrocarbon chains per repeat unit can be evaluated by comparing the integrated intensity of the signal arising from the terminal methyl group of the long alkyl chains at 0.85-0.89 ppm with that of the secondary methyl group at 1.13-1.2 ppm. The ratio of the intensities of these two methyl signals, multiplied by the molar substitution of the parent HPC, yields the degree of alkyl substitution. The results for the various samples are given in Table 1 and are found to vary from 1.8 to 2.8. Infrared spectra of all samples exhibit the characteristic hydroxyl vibration near 3400 cm-1 (see Figure 10, for example), and are thus consistent with degrees of substitution inferior to 3. Two different HPC-C16 samples were synthesized from the two methods described in the Experimental Section. The first method results in a significantly lower degree of substitution, presumably because the product polymer becomes insoluble and precipitates from the reaction medium before etherification can proceed to completion. The first method proved inappropriate for the preparation of longer chain ethers because of the insolubility of higher alkyl bromides in ethanol. 3.2 Surface Pressure-Area Isotherms. Surface pressure-area isotherms recorded for spread monolayers (13) Kondo, T.; Isogai, A.; Ishizu, A.; Nakano, J. J. Appl. Polym. Sci. 1987, 34, 55.

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Figure 2. 300 MHz 1H NMR of HPC-C22 recorded in CDCl3. The intensity ratio of the signals arising from the two methyl groups labeled a and b permits the evaluation of the average number of alkyl side chains per repeat unit.

Figure 4. Surface pressure-area isotherms of HPC-C16b as a function of subphase temperature: (a) 10 °C, (b) 15 °C, (c) 20 °C, (d) 25 °C, (e) 30 °C, and (f) 35 °C.

Table 1. Degree of Alkyl Substitution of Cellulose Ethers as Evaluated by 1H NMRa sample name

degree of alkyl substitution

HPC-C16a HPC-C16b HPC-C18 HPC-C22

1.7 2.8 2.5 2.7

a HPC-C16a and HPC-C16b were obtained from the first and second synthetic methods, respectively.

Figure 5. Surface pressure-area isotherms of HPC-C18 as a function of subphase temperature: (a) 10 °C, (b) 15 °C, (c) 20 °C, (d) 25 °C, (e) 30 °C, and (f) 35 °C.

Figure 3. Surface pressure-area isotherms of HPC-C16a as a function of subphase temperature: (a) 10 °C, (b) 15 °C, (c) 20 °C, (d) 25 °C, (e) 30 °C, and (f) 35 °C.

of the four samples, at various temperatures between 10 and 35 °C, are presented in Figures 3-6. Isotherm shape is found to depend on temperature, side-chain length and degree of substitution. Most striking is the temperature dependence observed for HPC-C16b and HPC-C18. For

these samples, a constant pressure transition is clearly evident in isotherms recorded at higher temperatures. Upon modest cooling, the plateau vanishes, although changes in slope indicate that a phase transition persists. The isotherms of HPC-C22 and HPC-C16a are much less sensitive to temperature changes in the limited range investigated. Constant surface pressure plateaus have been observed in monolayers spread from a number of cellulose esters7,4 and ethers.8,3 In general, the plateau is present for intermediate side-chain lengths (8-12 carbon atoms) but vanishes for longer chain derivatives. Although originally attributed to side-chain ordering,7 the transition is now considered to correspond to the formation of bi- or multilayers.3,4 The isotherms of Figures 3-6 also show that the limiting molecular area, evaluated by extrapolation of the first pressure rise in the compression isotherm to zero surface

Phase Transitions in Cellulose Ether Monolayers

Figure 6. Surface pressure-area isotherms of HPC-C22 as a function of subphase temperature: (a) 10 °C, (b) 15 °C, (c) 20 °C, (d) 25 °C, (e) 30 °C, and (f) 35 °C.

Figure 7. Compressional modulus and surface pressure, evaluated at the transition midpoint, as a function of subphase temperature for spread monolayers of HPC-C16b (diamonds) and HPC-C18 (circles). Inflection points are indicated by the arrows.

pressure, varies both with temperature and side-chain length. The changes, however, do not appear to be systematic. In general, mean molecular areas between 100 and 170 Å2 per repeat unit are found. These areas are larger than the 75 Å2 predicted for the close packing of three hydrocarbon chains perpendicular to the water surface. Recent Brewster angle microscopy images of HPCC18 monolayers9 show, however, that, at the limiting area, the surface film is not composed of a single liquid phase. As expected, above the limiting area, a condensed liquid phase is observed to coexist with a low-density gaseous phase. Surprisingly, both phases persist during compres-

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Figure 8. Temperature dependence of mean molecular areas delimiting the phase transition observed in surface pressurearea isotherms of HPC-C16b. The areas corresponding to the beginning and the end of the transition are plotted as square and circular symbols, respectively.

Figure 9. Surface pressure-area isotherms recorded for spread monolayers for two samples of different degrees of alkyl substitution: HPC-C16a (broken line) and HPC-C16b (solid line).

sion significantly beyond the limiting area, and a uniform liquid phase occurs only at surface pressures significantly higher (5-6 mN/m) than the surface pressure characterizing the gas-liquid equilibrium. This observation appears to violate the phase rule and is attributed9 to dipolar repulsions between neighboring liquid phases that create a barrier to coalescence. The observed limiting area thus represents a weighted average of the gaseous and liquidstate molecular areas. Isotherm limiting areas will depend on the relative quantities of the two phases present at the onset of nonzero surface pressure, and it is therefore not unusual that a complex dependence on temperature and side-chain length is observed. 3.3 Thermodynamics of the Transition. Phase transitions in spread monolayers can be analyzed by what

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Figure 10. Transmission FT-IR spectra of 8 LB layers of HPCC16a transferred to silicon, recorded with incident radiation polarized parallel and perpendicular to the dipping direction. The difference spectrum is shown at the bottom of the figure.

is frequently referred to as the two-dimensional Clapeyron equation:20

∆S h afβ dγ )dT ∆A h afβ

( )

dγo ∆S h afβ dΠ ) dT dT ∆A h afβ

∆H ) ∆E + ∆(PV) + ∆(γA)

(3)

At constant pressure, volume, and surface tension,

(1)

where ∆S h Rfβ and ∆A h Rfβ are the changes in molar entropy and molar area, respectively, that accompany the phase transition from R to β. An alternate form of this equation, in which the temperature dependence of the surface pressure, π, appears rather than that of the surface tension, γ, has been employed in the litterature.7,21 As correctly reported by several authors,22,23 the extraction of thermodynamic parameters for the transition from the temperature dependence of the surface pressure requires the removal of the temperature dependence of the surface tension of water, γo, according to eq 2:

( ) ( )

is plotted as a function of temperature in Figure 7 for HPC-C16b. The transition surface pressure is found to decrease with increasing temperature, but not in a linear fashion. Since any thermodynamic consideration is only reasonable for a phase transition that occurs at constant pressure, the slope, (dΠ/dT), is evaluated between 25 and 35 °C and found to be equal to -0.56 ( 0.09 mN m-1 K-1. When combined with the temperature dependence of the surface tension of water,24 (dγo/dT) ) -0.16 mN m-1 K-1, a value of 0.40 ( 0.09 mN m-1 K-1 is obtained for (dγ/dT). At 30 °C, dA ) -60 ( 5 Å2 per mole repeat unit, and the entropy change that accompanies the transition can be calculated from eq 1 as being equal to 140 ( 40 J K-1 mol-1. The positive entropy change is consistent with our previous conclusion that the surface transition does not correspond to side-chain ordering but rather to bilayer formation. Furthermore, the increase in entropy that accompanies the transition indicates that the first surface layer, located at the air-water interface, is more highly ordered than subsequent bi- or multilayers. This is a reasonable conclusion, as polar interactions between the polymer molecules and the water would be expected to have a structuring influence on the surface film. At 30 °C, the enthalpy change, ∆H ) 42 ( 12 kJ mol-1 repeat unit, can be calculated. The appropriate enthalpy includes the surface work term according to

(2)

The distinction between dΠ/dT and dγ/dT is crucial, and calculations based on the incorrect quantity can, in some cases, lead to the incorrect sign for the entropy and enthalpy changes that accompany the transition. The equilibrium surface pressure, taken at the midpoint between the two changes in slope delimiting the transition, (14) Hudson, S. M.; Cuculo, J. A. J. Macromol. Sci., Rev. Macromol. Chem. 1980, C18, 1. (15) Ishii, T.; Ishizu, A.; Nakano, J. Carbohydr. Res. 1976, 48, 32. (16) Isogai, A.; Ishizu, A.; Nakano, J. Cellul. Chem. Technol. 1983, 179, 123. (17) Isogai, A.; Ishizu, A.; Nakano, J. J. Appl. Polym. Sci. 1984, 29, 2097. (18) Isogai, A.; Ishizu, A.; Nakano, J. J. Appl. Polym. Sci. 1984, 29, 3873. (19) Isogai, A.; Ishizu, A.; Nakano, J. J. Appl. Polym. Sci. 1986, 31, 341. (20) Motomura, K. Adv. Colloid Interface Science 1980, 12, 1. (21) Tamada, K.; Minamikawa, H.; Hato, M. Langmuir 1996, 12, 1666. (22) Biegajski, J. E.; Burzynski, R.; Cadenhead, D. A.; Prasad, P. N. Macromolecules 1990, 23, 816. (23) Grainger, D. W.; Sunamoto, J.; Akiyoshi, K.; Goto, M.; Knutson, K. Langmuir 1992, 8, 2479.

∆H ) ∆E + γ∆A

(4)

At 30 °C, γ∆A ) -18 ( 2 kJ mol-1 and ∆E ) 60 ( 15 kJ mol-1. The positive energy change associated with the transition can be attributed to favorable interactions, such as hydrogen bonds, between the monolayer and the water subphase, which are lost as polymer molecules leave the water surface to form a second layer. Interestingly, the value of the ∆E ) 60 kJ mol-1 repeat unit corresponds to the energy of approximately 3 hydrogen bonds,25 a reasonable number of possible interactions between an anhydroglucose unit and the water subphase. Unfortunately, the limited temperature range in which isotherms can be recorded prohibits a similar analysis for the other samples. A meaningful value of dΠ/dT can only be obtained for samples which exhibit constant pressure transitions plateaus at several temperatures. Another feature of the surface pressure-area isotherms of HPC-C16b that warrants discussion is the striking temperature dependence of the mean molecular areas that delimit the phase transition. These areas are plotted in Figure 8. The magnitude of the change in area that accompanies the transition, |∆A|, clearly decreases with decreasing temperature. From eq 1, one can predict a corresponding decrease in DS within the temperature where dγ/dT remains constant. The transition entropy change will become less important if the entropy of the bilayer decreases with decreasing temperature more rapidly than that of the monolayer. This may indeed be the case if side-chain interdigitation and partial crystallization are possible in the bilayer at lower temperatures, as suggested by the mechanical properties of the films.9 3.4 Temperature Dependence of Isotherm Shape. The above discussion is valid for first order thermodynamic (24) Handbook of Chemistry and Physics, 58th ed.; CRC Press: Boca Raton, FL, 1977; p F-45. (25) Atkins, P. Physical Chemistry, 5th ed.; Freeman:, New York, 1994; p 771.

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transitions at equilibrium. The isotherms of Figures 3-6 illustrate, however, that the surface phase transition does not always occur at constant surface pressure. For a given side-chain length, the transition may appear in the isotherm either as a plateau or as two changes in slope, depending on the temperature. Similar behavior has been observed for phospholipid monolayers and can be predicted by theories that attribute the transition to the formation of surfactant clusters,26 such as hemi-micelles.27 In the absence of aggregation, no transition is predicted. Aggregation to form infinite or very large domains gives rise to a first-order phase transition, characterized by a constant surface pressure plateau. When the process is modeled as the formation of clusters of intermediate size (10-100 molecules) from a molecularly dispersed monolayer, a second order transition, with surface pressure continuously increasing throughout the transition, is predicted. Important differences exist between phospholipid monolayers and the systems studied here, and an analogous explanation of the temperature dependence of isotherm shape is thus not appropriate. In the case of cellulose ethers, the phase transition is not accompanied by an increase in molecular order. The transition entropy change and the temperature dependence of the equilibrium surface pressure are thus of opposite sign from that observed for phospholipids. The molecular architecture is also very different, and surface micelle formation is unlikely. We recently proposed an alternative explanation for the temperature dependence of the compression isotherms of our samples, based primarily on monolayer rheological properties.9 In the case of HPC-C18, the surface film within the transition region is found to be primarily viscous at high temperatures and becomes increasingly elastic upon cooling. We interpreted these results as an indication of side-chain interdigitation in the surface bilayer, permitting partial side-chain crystallization at lower temperatures. Crystallization results in an increase in film elasticity and a loss of fluidity. In the absence of a fluid bilayer, the phase transition can no longer occur under equilibrium conditions. This explanation is consistent with the observed dependence of isotherm shape on side-chain length. The temperature characterizing the change from a constant pressure plateau to a sloped transition can be evaluated from the temperature dependence of the monolayer compressional modulus,28 Cs-1, defined as the reciprocal of the compressibility:

Cs ) -

1 ∂A A ∂Π

( )

T

(5)

The compressional modulus, as well as the surface pressure, at mid-plateau are plotted in Figure 7 for HPCC16b and HPC-C18. Both parameters allow for the evaluation of the temperature at which the change in isotherm shape occurs. Transition temperatures equal to 23 °C and 31 °C are found for HPC-C16b and HPC-C18, respectively. It is pertinent to compare these temperatures with those characteristic of side-chain melting,29 reported as 54 °C and 62 °C for bulk poly(L-glutamates) containing hexadecyl and octadecyl substituents, respectively. Although surface films of cellulose ethers cannot be directly (26) Ruckenstein, E.; Li, B. Langmuir 1996, 12, 2308. (27) Israelachvili, J. Langmuir 1994, 10, 3774. (28) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1963; p 265. (29) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141.

compared with bulk polyglutamates, the similarity of the temperatures, and particularly the change in melting temperature that accompanies the extension of the alkyl chain by two methylene groups, supports the hypothesis that side-chain crystallization is possible in the compressed monolayers studied here. Furthermore, side-chain melting can be expected to occur with modest heating above room temperature. Above the melting temperature, the surface film is a viscous fluid, and the monolayerto-bilayer transition appears in the compression isotherm as a constant pressure plateau. At lower temperatures, partial crystallization involving side chains in neighboring layers diminishes the fluidity of the bilayer and an equilibrium first-order transition is not observed. In the case of HPC-C22, the side-chain melting temperature can be predicted to be above the maximum temperature at which reliable isotherms of spread monolayers can be recorded, and the isotherm shape remains invariant upon heating. 3.5 Effect of Degree of Substitution. Compression isotherms, recorded at 10 and 35 °C, for two samples of HPC-C16 differing in the degree of alkyl substitution, are plotted in Figure 9. At both temperatures, a decrease in degree of substitution results in a shift of the isotherm to smaller molecular areas. This is an expected result, as molecular areas are determined by lateral side-chain packing. The effect of changes in side-chain density on surface pressure and isotherm shape is more complex. At 35 °C, the monolayer-to-bilayer transition occurs at a lower surface pressure for the more highly substituted derivative. This probably reflects the decreased attraction between the monolayer and the water, resulting from the diminished number of polymer hydroxyl groups. At the lower temperature (10 °C), it is the more highly substituted sample that exhibits the higher surface pressures. According to the hypothesis evoked above, side-chain crystallization can occur at this temperature. Side-chain packing can be expected to be favored in the more highly substituted derivative, resulting in greater interlayer interactions in the bilayer and a correspondingly less fluid film. 3.6 Monolayer Transfer. All samples can be transferred, by vertical dipping, to hydrophobic glass, silicon, ZnSe, and aluminum coated substrates with transfer ratios near 1. Attempts to transfer spread monolayers to hydrophilic surfaces were less successful. A single layer can be deposited on hydrophilic glass during a first upstroke, but near total redeposition is observed on subsequent substrate descent. This suggests that a layer of water is retained between the substrate and the first layer, inhibiting adequate adhesion of the layer to the glass surface. This behavior is different from that observed for trisubstituted cellulose ethers3, where the fabrication of multilayered LB films on hydrophilic glass was found to be possible. This difference is probably the result of the incomplete substitution of the HPC ethers studied here. The presence of residual hydroxyl groups along the backbone certainly contribute to water retention. 3.7 Characterization of LB Films. Side-chain order in transferred LB films was characterized by polarized infrared (IR) spectroscopy. Transmission spectra recorded for 8 LB layers of HPC-C16a transferred to ZnSe are shown in Figure 10, with the incident radiation polarized parallel and perpendicular to the dipping direction. No significant difference is observed between the two spectra, as clearly shown by the difference spectrum in the bottom of the figure. The position and assignments of the major bands are summarized in Table 2. Clearly, neither the cellulose backbone nor the alkyl side chains are oriented along the

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Table 2. Peak Position and Assignments for the IR Spectrum of 8-Layer LB Films of HPC-C16a Transferred to ZnSe band position cm-1

assignment

3450 2970 2919 2874 2850 1460 1413, 1374, 1325, 1264 1120, 1080, 1059

hydroxyl stretch asymmetric methyl stretch anti-symmetric methylene stretch symmetric methyl stretch symmetric methylene stretch methylene bending methylene wagging progression backbone ether vibrations

dipping direction. Several features of the IR spectra of Figure 10 indicate that the alkyl side chains are conformationally ordered. First, a symmetric methylene stretching frequency of 2850 cm-1 is observed. This value is close to the 2849.5 cm-1 characteristic of an all-trans conformation30 of a hydrocarbon chain. The introduction of gauche isomers shifts this band to higher frequencies, with displacements of the order of 3.5 cm-1 being observed for the well-known gel-to-liquid crystal transition of many lipids.30 The antisymmetric methylene stretching band similarly appears at a position typical of an ordered chain.31 A second important characteristic of the spectra of Figure 10 is the series of four evenly spaced bands appearing in the region between 1400 and 1250 cm-1. These bands can be assigned to the methylene wagging band progression and arise from the coupling of oscillations in adjacent CH2 groups. This coupling is observed only for hydrocarbon chains in an all-trans conformation31 and thus provides further evidence for the high degree of side-chain order in the LB films studied here. One final minor point that must be made concerns the methyl group vibrations. The cellulose ethers studied here contain two types of methyl groups: the terminal groups of the alkyl side chains and the secondary methyl groups of 2-propyl moieties of the parent polymer. For this reason, the methyl vibrations appear in the spectra of Figure 10 (30) Umemura, J.; Cameron, D.; Mantsch, H. Biochim. Biophys. Acta 1980, 602, 32. (31) Cameron, D.; Casal, H.; Mantsch, H. Biochemistry 1980, 19, 3665.

Xiao and Ritcey

with higher relative intensities than those typical of simple hydrocarbon chains. Furthermore, the mixed nature of methyl signals renders them useless for the analysis of side-chain order. Conclusions A procedure has been developed for the preparation of cellulose ethers with long alkyl side chains. Surface pressure-area isotherms recorded for spread monolayers of these cellulose ethers depend on temperature, sidechain length, and degree of alkyl substitution. In particular, a phase transition occurring at constant surface pressure appears as a plateau in isotherms of certain samples. The changes in entropy and internal energy that accompany this phase transition are both found to be positive, with values consistent with the identification of the transition as bilayer formation. Importantly, the phase transition plateau vanishes upon a modest decrease in temperature. The temperature at which the change in isotherm shape occurs increases with increasing side-chain length, with methylene incremental values similar to those reported for side-chain melting in other systems.29 These observations support our previous hypothesis of partial side-chain crystallization involving hydrocarbon chains in adjacent layers.9 Above the melting temperature, the surface film is a viscous fluid, and the monolayer-to-bilayer transition appears in the compression isotherm as a constant pressure plateau. At lower temperatures, partial crystallization involving side chains in neighboring layers diminishes the fluidity of the bilayer, and an equilibrium first-order transition is not observed. Infrared spectroscopy reveals a high degree of conformational order for the alkyl side chains in transferred LB films. Acknowledgment. The financial support of the NSERC (Canada) and FCAR (Que´bec) is gratefully acknowledged. LA990691G