Langmuir 1988, 4, 407-410
407
Ellipsometric Study of Polymer Monolayers Spread at the Air-Water Interface. 1. Thickness of Monolayers Masami Kawaguchi,* Masahiro Tohyama, Yuhji Mutoh, and Akira Takahashi Department of Industrial Chemistry, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu, Mie, Japan 514 Received May 21,1987. I n Final Form: September 25, 1987 Polymer monolayers of PEO, PTHF, PVAc, PMMA, and PMLG spread at the air-water interface have been investigated by ellipsometry and surface pressure measurements. From their surface pressure ( T ) vs surface area (A)curves, PEO, PTHF, and PVAc were classified as expanded monolayers,while the other polymers belonged to the condensed monolayers. Ellipsometry gave two parameters of A (the phase difference between the light components perpendicular and parallel to the plane of incidence) and IC. (tan tc, is the ratio of the reflection coefficients of the perpendicular and parallel components). The A values showed a very sensitive change as a function of surface concentration I’ of polymer monolayer spread at the air-water interface. Change in A, 6A, increased linearly with r for PTHF and PVAc monolayers, while 6A values of PEO, PMMA, and PMLG monolayers deviated from a linear relation above a given r at which T reaches a plateau value. Below such surface concentrationsthicknesses calculated from ellipsometry were less than 5 nm, independent of polymer species. With further increase in r the thicknesses of PTHF and PVAc monolayers increased to more than 10 nm.
Introduction Many surface pressure measurements have been performed on polymer monolayers spread a t the air-water interface.’ In some studies it has been assumed that all polymer segments lie on the water surface, according to Singer’s idea? However, there is a possibility that polymer chains at the interface would take the following conformation: some segments anchor a t the interface, while remaining segments extend far into the water phase as loops and tails, as suggested by Frisch and Simhaa3 The extension of polymer chains at the air-water interface would depend on the chain flexibility and on the hydrophilic and/or hydrophobic properties of polymer chains. Thus it is necessary to measure the extension (thickness) of a polymer monolayer normal to the water surface. Ellipsometry is based on the principle that light undergoes a change in polarization when it is reflected at a surface. The refractive index and the absorption coefficient of the surface can be calculated from the phase retardation A and the azimuth $ of the amplitude ratio for light polarized parallel and normal to the plane of incident.*+ If polymers are adsorbed or deposited onto a surface, additional changes in A and $.are observed. From these changes it is possible to determine the thickness and refractive index of the polymer layer adsorbed on the surface. The thickness and refractive index so determined concern the thickness and refractive index of a hypothetical homogeneous layer,’ respectively. The advantage of ellipsometry is that it allows in situ measurement; its main application, however, has been to study the adsorbed layer at solid-liquid as well as solidsas Since the use of the ellipsometry on a gasliquid interface is known to be very difficult, only a few ellipsometric studies have been reported on the interac(1)Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience; New York, 1966. (2)Singer, S. J. J. Chem. Phys. 1948, 16, 872. (3)Frisch, H. L.;Simha,R. J . Chem. Phys. 1956,24,652;1957,27,702. (4)Passaglia, E.; Stromberg, R. R.; Kruger, J., Ellipsometry in the Measurement of Surfaces and Thin Film; NBS Miscellaneous Publication 256, Superintendent of Documents, U.S.Government Printing Office: Washington, D.C., 1964. (5)Bashara, N.M;Azzam, R. M. A. Ellipsometry; North-Holland: Amsterdam, 1976. (6) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977. (7)Takahashi, A,; Kawaguchi, M.; Adu. Polym. Sci. 1982,46,1.
0743-7463/88/2404-0407$01.50/0
tions of lipid8y9and fatty acidlo monolayers spread at the air-water interface. At present these is no substantial application of ellipsometry, to polymer monolayers spread at the air-water interface except for adsorption studies11-14 of surface-active biological and synthetic polymers at the air-solution interface from their aqueous solutions. In this paper the applicability of ellipsometry for investigating the behavior of polymer monolayers spread at the air-water interface is explored. Polymer monolayers used in this study are poly(ethy1ene oxide) (PEO), poly(tetrahydrofuran) (PTHF), poly(viny1 acetate) (PVAc), poly(methy1 methacrylate) (PMMA), and poly(ymethyl-L-glutamate) (PMLG). Their interfacial properties will be discussed as a function of surface concentration and surface pressure. Moreover, the calculation of the thickness and refractive index of the polymer monolayers is performed in the same manner as the polymer adsorption onto the metal surface,’ and the conformation of polymer chains a t the air-water interface also will be discussed in terms of the hydrophilicity or hydrophobicity of polymers.
Experimental Section Materials. The polymers used here were obtained from various sources. PEO with a narrow molecular weight distribution was purchased from Toyo Soda Co., and its molecular weight was determined to be 145 X lo3 by light scattering. PTHF was prepared by cationic bulk polymerization of tetrahydrofuran (THF) using trifluoroboron-THF-ethylene oxide catalyst by
Fujimoto, Kawahashi, Nagasawa, and Takahashi,lsand its molecular weight was determined t~ be 33 X ld by intrinsic viscosity measurement. This sample has also a narrow molecular weight distribution. PVAc was synthesized by free radical solution polymerization and fractionated. We used one fractionated PVAc sample whose molecular weight was determined to be 240 X lo3 by intrinsic viscosity measurement. PMMA was a commercially (8) den Engelsen, D.; de Koning, B. J.Chem. SOC.,Faraday Trans. 1 1974,70, 1603. (9) den Enaelsen, D.; de Koning, B. J. Chem. SOC., Faraday Trans. 1 1974,70,2106. (10)Minc, S.Electroanal. Chem. Interfac. Electrochem. 1975,62,291. (11)Benjamins, J.; de Feijter, J. A.; Evans, M. T. A.; Graham, D. E.; Phillips, M. C. Discuss. Faraday SOC. 1976,59,218. (12)de Feijter, J. A,; Benjamins, J.; Veer, F. A. Biopolymers 1978,17, 1759. (13)Graham, D. E.; Phillips, M. C. J. Colloid Interface Sci. 1979,70, 415. (14)Kawaguchi, M.; Oohira, M.; Tajima, M.; Takahashi, A. Polym. J . 1980,12,849. (15)Fujimoto, T.; Kawahashi, M.; Nagasawa, M.; Takahashi, A. Polym. J. 1979,ll, 193.
0 1988 American Chemical Society
408 Langmuir, Vol. 4, No. 2, 1988
Kawaguchi et al.
Light Source
Photometer
Figure 1. Schematic representation of ellipsometric arrangement. available sample of Scientific Polymer Products and was fractionated. We chose one fractionated PMMA sample. Its molecular weight was determined to be 280 X l@by intrinsic viscosity measurement. PMLG was received as a gift from Dr. Kinoshita at Nagoya Institute of Technology. It was prepared by the NCA method, and its molecular weight was determined to be 4.4 X lo3 by intrinsic viscosity measurement. Spectrograde benzene was used as a spreading solvent for PEO, PTHF, PVAc, and PMMA, and spectrograde chloroform was used as a spreading solvent for PMLG. Surface Pressure Measurement. A Teflon trough with a diameter of 15 cm was filled with deionized water supplied from a Millipore Q-TM system, and its temperature was controlled within 25 f 0.1 "C by circulating thermostated water. The trough was covered by a box of Plexiglass in order t o minimize air turbulence as well as the fallout of dust particles and to maintain a high humidity. Contaminations of the water surface were removed by sweeping a Teflon bar over the trough. Monolayers were applied t o the water surface in the trough by delivering the polymer solution from a Hamilton microspinge. At least 30 min was allowed for evaporation of the spreading solvent. The surface pressure A of polymer monolayers was measured by means of a glass plate or a platinum plate attached to a force transducer (Shinko Denki Co. Type 1301) fed into a home-built phase-sensitive amplifier. A was determined with precision of *0.05 dyn/cm. The surface concentration I' of polymer spread on the water surface was varied either by stepwise addition of polymer solution or by spreading an appropriate amount of the solution on a clean water surface to give the desired surface concentration (one-shot spreading). Duplicate runs were made to check the reproducibility of surface pressure measurements. Ellipsometry. Ellipsometry of polymer monolayer spread a t the air-water interface was performed by using the same trough as for the surface pressure measurements. The polymer monolayers were formed by stepwise addition or one-shot spreading. As shown in Figure 1we used a Shimadzu horizontal ellipsometer with a monochromic light source (A = 546 nm), which is obtained from a Toshiba SHL-100UV high-pressure mercury lamp. The polarizer and analyzer are made of Glan-Thomson prisms, mounted in a respective accurate rotatable goniometer. The angle setting and reading of these goniometers were precise to 30". The retardation plate, which is made of quartz, was placed between the polarizer and the reflecting water surface and also mounted in the goniometer, which also has an accuracy of 30" for angle reading. The incident angle of the light was 70" (Figure 1). The polarizer is adjusted in such a way that the ellipticity of the incident beam obtained through the quarter-wave plate is exactly cancelled by the effects produced by the reflection. The reflected linearly polarized beam is extinguished by rotating the analyzer to the correct position. The extinction settings of both the polarizer (p) and analyzer (a)were determined by measuring their respective orientations a t equal intensities on each side of the minimum, by means of a photomultiplier tube. The values of A and $ are obtained if the retardation of the quarter-wave
+
plate is 90": A = 2p 90 and $ = a. We used the four-zone method as described by McCrackin, Passaglia, Stromberg, and Steinberg16t o obtain the absolute values of A and $ by setting the quarter-wave plate a t 45" or 135" t o the incident plane. The accuracies of A and $ are 60" and 30", respectively. The values of 6A (6A = 2 - A) and 6+ (S+ = $ - $) between the water surface covered with polymer monolayers (A, I) and the pure water surface (A, $) were usually reproducible within 60" and 30", respectively. The area of the surface that is observed in a single ellipsometry measurement is about 2.5 cm2. To check the reproducibility of ellipsometric measurements we repeat a t least triple runs at the same surface concentration.
Results and Discussion For the clean water surface, the and $ values were found to be 0.326 f 0.004° and 24.861 f 0.004°, respectively. A non-zero value of A will be attributed to the existence of a transition layer between water and air. Similar results have been obtained by Engelsen and Koninge8 The existence of such a transition layer on the water surface was widely recognized since Rayleigh's pioneer work in 1892.17-21 The measured values of A and $ give a complex refractive index of water such m 1.3340 f 0.0001 - i0.0044 f 0.0001. Except for PEO, the polymers used here are nearly insoluble in water and form stable spread monolayers at the air-water interface. Since PEO is a water-solublepolymer, we paid much attention to PEO monolayers to examine whether it makes a stable monolayer or not. The K of PEO monolayers spread at the different surface areas with a constant volume of subphase water in the trough where the surface concentration is maintained to be equal was measured and found to be the same. Moreover, compression-expansion hysteresis experiments on PEO monolayers showed no hysteresis. Therefore, the PEO monolayer spread from delivering its benzene solution undoubtedly makes a stable monolayer at the air-water interface as previously r e p ~ r t e d . ~ ~ , ~ ~ (16) McCrackin, F. L.;Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J . Res. Natl. Bur. Stand. 1963, 67A, 363.
(17) Rayleigh, Lord Phil. Mag. 1892, 33, 1. (18) Raman, C. V.;Ramdas, L. A. Philos. Mag. 1927, 3, 220. (19) Bouhet, C. C. R. Acad. Sci. (Paris) 1927, 185, 53; Ann. Phys. (Paris) 1931, 15, 3. (20) McBain, J. M.; Bacon, R. C.; Bruce, H. D. J. Chem. Phys. 1939, 7, 818. (21) Kinosita, K.; Yokota, H. J . Phys. SOC.Jpn. 1965, 20, 1086. (22) Schuler, R. L.; Zisman, W. J. Phys. Chem. 1970, 74, 1523. (23) Kawaguchi, M.; Komatsu, S.; Matsuzumi, M.; Takahashi, A. J . Colloid Interface Sci. 1984, 102,356.
Langmuir, Vol. 4 , No. 2, 1988 409
Ellipsometric Study of Polymer Monolayers
30
r-7 -0
20
40
,-xi$
/g.cm'
60
Figure 3. Plots of SA, S$, and r (-) as a function of surface SA), PTHF (0,Skm,S$), and PVAc concentration I? for PEO (0, (8,SA; 0 , W. A mZ.m 9.' Figure 2. Surface pressure (r)-area ( A ) isotherms of PEO, PTHF, PVAc, PMMA, and PMLG.
Table I. Limiting area A , and Plateau Surface Pressure rP for Various Polymer Monolayers polymer A,,, A*/monomer unit r0,dyn/cm 10.2 32.9 PEO 21.2 34.7 PTHF 24.2 PVAc 25.0 12.6 15.8 PMMA 17.1 18.1 PMLG
Generally, surface pressure (*)-surface area ( A ) ( P A ) isotherms of polymer monolayers were classified mainly into two categories based on the shape of x-A isotherm as advanced by Crisp.24 The a-A isotherm is related to physicochemical properties such as hydrophilicity or hydrophobicity of polymer compounds. One is an expanded-type a-A isotherm: a is detectable at fairly large surface areas, Le., at small r, gradually increases with decreasing area (increasing I'), and finally reaches a plateau value. Its plateau value does not change even if I? increases. This type of a-A isotherm is usually observed for polymers having a relatively strong hydrophilic property. On the other hand, there is a condensed type x-A isotherm: a is first observed at a smaller surface area than the expanded a-A isotherm, shows a steep increase, and then the monolayer shows collapse, i.e., x will suddenly drop or increse at high r. The condensed-type a-A isotherm in general is observed for polymers having small hydrophilicity and bulky side chains. Therefore, PEO, PTHF, and PVAc belong to the expanded monolayer, while PMMA and PMLG fall within the condensed monolayer, from their a-A isotherms (see Figure 2). In Table I the limiting area A . per monomer unit and the plateau surface pressure apare listed. The areas A. are found by linearly extrapolating the steepest portion of an experimental a-A curve to the limit of zero surface pressure. The-value of apfor PMLG monolayer is taken as the surface pressure at the inflection point of the x-A curve. In Figures 3 and 4,r and 6A are plotted against r for expanded and condensed monolayers, respectively. The point noticed is that the A value is very sensitive to the presence of a thin layer on the surface, in comparison with the other ellipsometric parameter In this experiment the changes in +, 6+, are actually extremely small at relatively low r and are regarded as zero, but they undoubtedly exceed experimental error for PTHF and PVAc monolayers at high r. Their values of 6+ are plotted
+.
(24) Crisp, D.J. J. Colloid Sci. 1946, 1, 49.
0.8
0 0 06)
o o
I
I
20
0
p i 0 8
0
40
/g.cmi2
Figure 4. Plots of SA and r (-) as a function of surface concentration I? for PMMA and PMLG.
in Figure 3. On the contrary, 6+ values of PEO, PMMA, and PMLG can be regarded as zero over the entire surface concentration. We notice some differences in 6A vs I' plots in Figure 3. For the PEO monolayer the value of 6A increases linearly with r, but above r = 7 X g/cm2 6A becomes nearly constant and independent of r. At I' = 7 X g/cm2 x attains a constant, Le., plateau, value. In contrast with PEO, the 6A values of PTHF and PVAc monolayers increase almost linearly with I', but at high r the r values are somewhat scattered. The value of 6A is reproducible to within 0.03" above r = 30 X and 25 X 1 P g/cm2 for PTHF and PVAc monolayers, respectively. As seen from Figure 4,x of PMLG monolayer is first detectable at a higher I' than for PMMA monolayer. This means that PMLG has a more hydrophobic character than PMMA. In both monolayers 6A values increase linearly with r until the onset of a. One can see that the change in 6A is more sensitive than the a measurement at a lower r of the condensed polymer monolayer. The plot of 6A against r for PMMA monolayer shows two distinct steps: the first step coincides with the concentration at which ?r is initially detectable; the second step is found at the beginning portion of attaining the plateau T . With further increase of r, 6A attains a constant value. Similar variation of 6A with I? was found for myristic acid monolayeraspread at the air-water interface. For PMLG monolayer such a first distinct step cannot be observed in the 6A vs I' curve. Above I? giving the limiting area, 17 A2/residue (this limiting area corresponds to a-helix or random coil state of PMLG monolayer),= the values of 6A are scattered because the PMLG monolayer (25) Loeb, G. I.; Baier, R. E.J. Colloid Interface Sci. 1968, 27, 38.
410 Langmuir, Vol. 4, No. 2, 1988
Kawaguchi et al.
E 10 C
\
1
0
40
20 /-XI&
60
/g.cm.'
Figure 5. Plots of calculated thickness t as a function of surface concentration r for PEO (O),PTHF (O),PVAc (e),PMMA (o), and PMLG ( 0 ) .
undergoes a phase change from monolayer to multiplelayer of helix or random coil. Engelsen and Koning6 made a pioneer study of condensed lipid monolayers spread at the air-water interface by ellipsometry and proposed a model for lipid monolayers based on the variation of optical anisotropy upon compression, but there is not yet an adequate theory relating the ellipsometric parameters of A and $ to the properties of monolayers. I t is very important to clarify the conformation of polymer chains in monolayers spread at the air-water interface. The thicknesses of the polymer monolayers may give insight into the conformation of adsorbed chains in the polymer monolayer spread at the air-water interface. We attempted to calculate the thickness and refractive index of polymer monolayers spread at the air-water interface with the same program as previously used for the polymer adsorption onto the flat metal surface^.^ In the program employed here an iterative procedure has to be used in which thickness and film refractive index values are sought which reproduce the experimental values of A and I). In our calculations we make the following assumptions: (1) The film is nonabsorbing (in other words, the film refractive index is real) because all polymers used here can be considered to be nonabsorbing substances. Further, this standpoint is widely accepted for most ellipsometric experiments on protein monolayers and polymer adsorption. (2) The maximum errors are 0.01' for A and $. (3) The refractive index of the environment corresponding to air is regarded as unity. (4) If the calculated refractive index of the polymer monolayers exceeds the refractive index of the polymer in bulk state, such data are eliminated. The monolayer thicknesses so calculated are semilogarithmically plotted against r in Figure 5 . For the expanded monolayers below I' = 10 X lo4 g/cm2 the thickness is generally less than 2 nm (with the film refractive
index 1.3470-1.3810), which is attributed to the flattened conformation on the water surface. Except for PEO monolayer with increasing I?, the thickness increases to more than 10 nm and tends to level off (with the film refractive index 1.3810-1.4150). At high r ranges, PVAc and PTHF chains adsorbed at the air-water interface adopt a relatively extended conformation normal to the surface. For the condensed monolayers the thickness could be calculated only below the surface concentrations giving the limiting areas. Most of the thicknesses are less than 5 nm (with the film refractive index 1.3555-1.3810). The thickness of PMLG is somewhat larger than that of PMMA due to the larger bulky side group of PMLG. We take another route to estimate a thickness for PEO and PMMA monolayers above r giving the limiting area, where it is expected that the water surface will be fully covered by polymer monolayers. Thus the refractive index of the monolayer can be assumed to be equal to that of bulk polymer, and we calculate the ellipsometric parameters of A and $ as a function of thickness. To compare the measured and calculated values of A and $, we take into account the maximum differences of 0.02' and 0.01' for A and $, respectively. Comparison shows that the thicknesses of PEO and PMMA gradually change from 1.0 to 1.5 nm with increasing I?. This small thickness means that PEO and PMMA chains spread at the air-water interface pile up without looping, with chain entanglement possible provided that the chains are enough long. The lack of looping is attributed to the strong hydrophilicity of PEO and to the condensed character of PMMA at the air-water interface. Conclusions Five stable polymer monolayers spread at the air-water interface have been studied by ellipsometry as a function of r; the surface pressure was also monitored. A linear relationship between 6A and I? holds for PTHF and PVAc monolayers in the entire concentration range. For the other polymer monolayers of PEO, PMMA, and PMLG, 6A values level off above r where ?r becomes constant. The thicknesses are less than 5 nm for each sample until the plateau T. For PTHF and PVAc monolayers the thickness increases with r to be larger than 10 nm. In contrast to these monolayers the thicknesses of PEO and PMMA do not exceed 5 nm in the entire concentration. Thus 6A is very sensitive to changes in the monolayer state and behaves as a probe of the conformation changes of polymer chains adsorbed at the air-water interface. Registry No. PEO, 25322-68-3; PTHF, 24979-97-3; PVAc, 9003-20-7;PMMA, 9011-147;PMLG (homopolymer), 25086-16-2; PMLG (SRU), 25036-43-5.
