Langmuir 1988,4, 411-413
411
Ellipsometric Study of Polymer Monolayers Spread at the Air-Water Interface. 2. Adsorbed Amount of Polymers Masami Kawaguchi,* Masahiro Tohyama, and Akira Takahashi Department of Industrial Chemistry, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu, Mie, J a p a n 514 Received June 1,1987. I n Final Form: Septemher 25, 1987 Ellipsometry has been used to calculate the refractive index and thickness of polymer monolayers of poly(ethy1eneoxide) (PEO), poly(tetrahydrofutan) (PTHF), poly(viny1 acetate) (PVAc), and poly(methy1 methacrylate) (PMMA) spread at the &water interface as a function of surface concentration and surface pressure. From the refractive index and thickness of the adsorbed monolayers we attempted to calculate the adsorbed amounts of polymers in the monolayers in terms of the Lorentz-Lorenz relation for an ideal mixed layer of polymer and surrounding media as a function of the refractive index n, of the surrounding media. The adsorbed amount of polymer so calculated for n, = 1 correspondingto air is much larger than that for n, = 1.334correspondingta water. The latter adsorbed amount is in agreement with the real spread amount of polymer in the entire surface concentration range irrespective of polymer species. This agreement indicates that the polymer chains are surrounded by water molecules by taking an extended conformation normal to the water surface for PTHF and PVAc and a flattened one for PEO and PMMA at higher surface concentration.
Introduction In the preceding paper' in this issue we have applied ellipsometry to monolayers of polymers, such as poly(ethylene oxide) (PEO), poly(tetrahydr0furan) (PTHF), poly(viny1 acetate) (PVAc), poly (methyl methacrylate) (PMMA), and poly(methy1-L-glutamate)(PMLG) spread a t the air-water interface, and their surface pressure was monitored. Surface pressure measurements proved that PEO, PTHF, and PVAc are classified as expanded-type polymer monolayers and the other polymers are the condensed type. Here, we particularly have focused on the evaluation of the thickness of the monolayers in order to clarify the conformation of polymer chains adsorbed a t the air-water interface. It was found that the higher hydrophilic polymer of PEO and the hydrophobic polymers of PMMA and PMLG take flattened conformations a t all surface concentrations, while the moderately hydrophilic polymers of PTHF and PVAc adopt flattened conformations a t lower surface concentration and change into an extended conformation normal to the water surface at higher surface concentration. Therefore, we could demonstrate that ellipsometry is a very powerful method to investigate the interfacial properties of polymer monolayers spread a t the air-water interface. Besides the monolayer thickness, ellipsometry gives another important quantity, the refractive index. The refractive index of the adsorbed polymer layer can be replaced by the polymer concentration in the layer. Even for the inhomogenous layers when the refractive index is a linear function of the polymer concentration we can calculate the adsorbed amount of polymer in the layer from both the thickness and the refractive index using the equation
A = [(nt - nl)tl/(dn/ac)
(1)
where A is the adsorbed amount, nf and t are the refractive index and the thickness of the adsorbed layer, respectively, nl is the refractive index of the solvent, and anlac is the refractive index increment of the polymemlvent system. This relationship is useful to estimate the adsorbed amount for the adsorption of polymers from their solution onto the solid surface.2 Also, eq 1 was employed for evaluating ~
(J) f(awaguchi, M.; Tohyama, M.; Mutoh,
muir, in press.
Y.; T a u a s h i , A. Lang-
adsorbed amounts of surface-active b i ~ l o g i c a l and ~ - ~ synthetic polymers6*' a t the air-water interface from their aqueous solutions, and the adsorbed amount so calculated has been confirmed independently by the radio tracer m e t h ~ d .For ~ ~insoluble ~ polymer monolayers spread a t the air-water interface, however, eq 1 is not applicabIe because the refractive index increments in water cannot be determined. In this paper we attempt to calculate the adsorbed amount ip the monolayers of various polymers of PEO, PTHF, PVAc, and PMMA from the refractive index and thickness of the adsorbed monolayers using the LorentzLorenz relation. As a result, we can discuss further the interfacial behavior of polymer chains trapped a t the air-water interface in terms of the hydrophilicity and hydrophobicity of polymers.
Experimental Section Materials. Polymers used here were the four species, PEO, PTHF,PVAc, and PMMA. W A C and PMMA were prepared by radical polymerization and fractionated to obtain samples of a relatively narrow molecular weight distributiqn. PEO and PTHF were prepared by cationic polymerization and had a narrow molecular weight distribution. These polymers are the same as used in part 1, and their molecular characteristicsare described in detail in the preceding paper. Spectrograde benzene was used as a spreading solvent for polymer samples. Surface Pressure Measurement and Ellipsometry. Surface pressure mwurements and ellipsometry of polymer monolayers spread at the air-water interface in a Teflon trough with a diameter of 15 cm were performed by using the same instruments as employed in part 1.' The polymer monolayers were formed by stepwise addition of the polymer solution or one-shot spreading (i.e., delivering an appropriate amount of the polymer solution on a dean water surface to give desired surface concentration). The temperature of the water phase in the trough was controlled within 25 & 0.1 OC by circulating thermostated water. The (2) Takahashi, A.; Kawaguchi, M. Adu. Polym. Sci. 1982, 46, 1.
(3) Benjamins, J.; de Feijter, J. A.; Evans, M. T. A.; Graham, D. E.; Phillips, M. C. Discuss. Faraday SOC.1975, 59, 218. (4)de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymer 1978, 17, 1759. (5)Graham, D.E.;Phillips, M. C. J. Colloid Interface Sci. 1979, 70, 415. (6) Kawaguchi, M.; Oohira, M.; Tajima, M.;Takahashi, A. Polym. J. 1980, 12, 849. (7) de Feijter, J. A.; Benjamins, J. J. Colloid. Interface Sci. 1981,81, 91.
0743-746318812404-0411$0l.5O/O 0 1988 American Chemical Society
Kawaguchi et al.
412 Langmuir, Vol. 4 , No. 2, 1988 precision of surface pressure measurement is 10.05 dyn/cm. Ellipsometry gives two basic parameters, such as the phase difference A and the azimuth Q of the amplitude ratio for light components perpendicular and parallel to the plane of incidence. The accuracies of readings for A and Q are 60" and 30" obtained by averaging polarizer and analyzer settings over four zones, respectively.
Background For calculation of adsorbed amount in the monolayers spread at the air-water interface it will be helpful to examine the Lorentz-Lorenz (L-L) relation for a mixture of substances, which form a uniform layer. In general, the L-L relation for the refractive index n of a mixture of substances is given as (n2- l)/(n2 + 2) = R,Nl + RzNz+ R3N3+ ... (2) where Ri is the molar refractivity of substance i and Ni is the number of moles of substance i per unit volume. For a pure substance the L-L relation is written as po
= [(M/R)(n2 l)]/(n2
+ 2) = M N
(3)
where po is the density per unit volume and M is the molecular weight of the pure substance. Now, we consider an adsorbed layer of thickness t with the refractive index nf, which consists of the pure substance, and then the adsorbed amount m of the pure substance is given by m = t p o = [t(M/R)(nf2- 1)]/(nf2 + 2)
(4)
For a mixture of polymer (p) and the surrounding media (s) the L-L relation can be expressed by
(nf2 - l)/(nf2 + 2) = R,Ns + RJV, = (R,/Ms)p, + (Rp/Mp)Pp(5) If it is assumed that the mixture is ideal, the volume fraction of polymer is expressed as uppp, using the partial specific volume of polymer up. For a pure polymer substance the relation upppo = 1 is satisfied. The remaining volume fraction (1- uppp) has the density pso of the surrounding media. Thus, we obtain pB = p s o ( l - uppp). Therefore, the following equation is derived from eq 5:
(nf2- U/(nf2 + 2) = ( R s / M , ) p s 0 (-~uPpp) + (Rp/Mp)Pp= (n,2 - l)/(ns2 + 2)(1 - UPPP) + (Rp/Mp)Pp (6) where n, is the refractive index of the surrounding media. From eq 6 we can calculate the adsorbed amount mp of polymer in an adsorbed layer consisting of polymer and the surrounding media: mp = tp, = [3tF(nf,n,)l/[(R,/Mp) - up{(nS2- l)/(n,2 + 2111 (7) with F(nf,n,) = (n? - n,2)/[(n?+ N n s 2+ 2)1 In eq 7 both t and nf can be determined by ellipsometry, R , values for polymer species can be estimated from the molar refractivity of atoms and atom groups contained in a monomer unit of each polymer, and up can be obtained from the density of pure polymer. Therefore, mp is evaluated as a function of n,.
Results and Discussion For the calculation of the thickness and refractive index of the polymer monolayer spread at the air-water interface we used an iterative procedure where the thickness and refractive index values are sought which reproduce the measured ellipsometric parameters of A and \k by taking
Table I. Ellipsometric Data and Adsorbed Amount Calculated on Two Bases: Surrounding Medium Water as a Function of (m p,l) and Surrounding Medium Air ( m Surface Concentration r for Various Polymers adsorbed amount surface concn thickness refractive x lo8, g/cm2 r x lo8, g/cm2 t , nm index nf mp,l mp,2
PEO 1.60 2.08 3.19 6.41
0.76 1.1 2.3 5.5
1.347 1.347 1.347 1.347
0.84 1.26 2.56 6.11
6.53 9.79 19.8 47.3
1.381 1.381 1.356 1.347 1.381 1.415
5.99 12.9 23.1 34.9 37.8 50.4
17.2 36.9 135.0 340.0 122.0 91.8
1.347 1.347 1.381 1.381 1.347 1.356 1.347 1.381 1.381 1.381 1.415 1.415
1.51 2.59 5.75 7.09 36.4 42.2 45.6 52.3 59.5 68.1 94.5 107.0
12.0 20.6 14.0 17.2 290.0 209.0 220.0 127.0 145.0 165.0 145.0 164.0
2.24 3.67 4.09 7.81 10.5
19.8 32.4 36.1 68.9 27.5
PTHF 6.80 13.6 20.4 27.2 40.8 54.4
2.1 4.5 17.4 43.4 13.1 10.3
1.41 3.53 7.07 8.83 35.3 38.9 42.4 49.5 56.5 61.9 82.6 110.1
1.3 2.3 1.4 1.8 32.1 22.6 24.4 12.9 14.7 16.8 13.7 15.5
1.31 2.10 2.62 5.25 9.92
2.3 3.8 4.2 8.1 3.0
PVAc
PMMA 1.347 1.347 1.347 1.347 1.381
account of the following three terms: (1)The maximum differences in both A and \k between the measured and calculated values are 0.01' degree. (2) The refractive index of environment corresponding to air is regarded as unity. (3) The calculated refractive index of the polymer monolayers must be lower than the refractive index of polymer in the bulk state. Typical data on the thickness and refractive index of polymer monolayers are listed in Table I. For the expanded monolayers, below the surface concentration r = 10 X lo+' g/cm2, the thicknesses are generally less than 2 nm, indicating that polymer chains take a flattened conformation. Around l? = 35 X lo-* g/cm2 for PVAc and 30 X g/cm2 for PTHF, the thickness shows a maximum and decreases to level off to around 15 nm with increasing ,'I while the refractive index in the adsorbed layer increases. This plateau thickness does not change even if g/cm2 for PVAc. r becomes as large as 100 X The thicknesses of PMMA monolayers are less than 10 nm over the entire measured r range. At similar surface concentrations the thickness of PMMA is somewhat larger than those of the expanded-type polymer monolayers, due to the higher hydrophobicity of PMMA chains than other polymer chains. We need some parameters in eq 7 in order to calculate the adsorbed amount in the polymer monolayers. According to Guypers et al.s the molar refractivities of monomeric units for each polymer can be estimated from the (8) Cuypers, R. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. T.; Hemker, H. C. J. Biol. Chem. 1983,258, 2426.
Ellipsometric Study of Polymer Monolayers
Langmuir, Vol. 4, No. 2, 1988 413
Table 11. Molar Refractivities R, and Molecular Weights M , of a Monomeric Unit for Each Polymer and Pure Polymer Densities ppo Dolvmer PEO PTHF PVAc PMMA
R,'
M"
On0
10.937 20.231 20.297 24.944
44.05 72.10 86.09 100.11
1.13' l.OSb 1.20c 1.18d
1.6
L
(9) Vogel, A. I.; Gresswell, W. T.; Leicester, J. J. Phys. Chem. 1954, 58, 91. (10)Encyclopedia of Polymer Science and Technology; Mark, H. F., Gavlord, N. G., Bikales, N. M., Eds.: Interscience: New York. 1966: Vol. 4, p 476. (11) Encyclopedia of Chemical Technology; Wiley: New York, 1982; Vol. 18. D 647. (12)'Mckinney,J. E.; Goldstein, M. J.Res. Natl. Bur. Stand., Sect. A. 1974, 78, 331. (13) Olabishi, 0.; Simha, R. Macromolecules 1975, 8, 206. (14) Frisch, H. L.;Simha, R. J . Chem. Phys. 1956,24,652; 1957,27, 702. (15) Silberberg, A. J. Phys. Chem. 1962, 66, 1884. (16) Huggins, H. L. Makromol. Chem. 1965,87, 119.
0
\
0 0
0
m
Reference 10. *Reference 11. Reference 12. Reference 13. 'Molar refractivities of the atoms and atom groups involved in polymers used are as follows:9 C, 2.591; H, 1.028; =0, 2.122; 0, 1.643.
molar refractivities of atoms and specific atom groups involved in the chemical structure of polymers. The data of Vogel, Gressewell, and Leicesterg were used for estimating the molar refractivities. The calculated molar refractivities R, and molecular weight Mpof a monomeric unit for each polymer and the densities ppo of pure polymers are listed in Table 11. In part 1,' we showed that water-soluble PEO also forms a stable monolayer on the water surface. Therefore, every polymer used here should be located close to the air-water interface. For the calculation of adsorbed amounts for polymers we took two routes: (1)using the n, value 1.334, which corresponds to the refractive index of water a t 25 "C (method l ) , and (2) using the n, value 1.00, which corresponds to the refractive index of air (method 2). The adsorbed amounts calculated from methods 1 and 2 are expressed by mp,land mp,2,respectively. Their values are listed in Table I. Both values of mp,land mp2increase with increasing r irrespective of polymers. The value of mp,l is usually 5-6 times larger than mp,2.At the higher surface concentration the difference between mp,2and mp,l becomes smaller. Comparison of the real spread amount and the calculated adsorbed amounts clearly shows that r is closer to mp,l than to mp,2. This result indicates two possibilities for the status of polymer chains adsorbed at the air-water interface: (1)all polymer chains are penetrated into the water phases; (2) water molecules penetrate into the polymer layer located on the water surface. From this result only we cannot conclude which conformation is similar to the real conformation of polymer chains adsorbed at the air-water interface. The former model is in agreement with the conformation model of Frisch and Simha,14Silberberg,15and Huggins,16who proposed that some segments of polymer chains adsorbed at the air-water interface anchor a t the interface, while the remaining segments extend far into the water phase as loops and tails. Careful inspection of Table I shows that the deviation of the mp,lvalue from the true value r at the lower surface concentration is somewhat larger than that a t the higher surface concentration. This fact is emphasized in Figure 1where the ratio of mP,Jr is plotted as a function of I?. The ratios are located around unity a t the higher surface concentration. We notice some features in the figure. For
k ' l l l l l ' l ' l 'I
0
1
1
1
1
1
1
1
1
1
1
1
Figure 1. Plots of the ratio of mp,l/l? as a function of r: 0,PEO; @,
PTHF; 0 , W A C ; 0,PMMA.
PEO, the initially low ratio increases with increasing r and approaches unity at the higher r. For PMMA the initially high ratio decreases with increasing r and levels off a t unity a t higher r. The reason for the large deviation of the ratio from unity can be considered as follows: From the *-A isotherms we can estimate the limiting area, by extrapolating the straight portion of the *-A isotherm back to the P = 0 axis." In this way the limiting areas of polymer monolayers were determined to be 4.52 m2/mg for PEO, 3.0 m2/mg for PTHF, 1.75 m2/mg for PVAc, and 0.95 m2/mg for PMMA, respectively. The limiting area decreases with increasing hydrophobicity, as judged from the chemical structures. At the so-called limiting area the liquid (water) surface is just completely covered by the polymer chains and the adsorbed polymer chains are considered t o take a flattened conformation. Above the limiting area (Le., lower I?) the water surface has some naked parts since it might be expected that the polymer molecules would be widely separated and the water interface covered by isolated two-dimensional chains. Particularly, PMMA chains on the water surface easily form an island-like structure due to the strong cohesive forces exerted by the a-methyl groups in the vinyl chain in comparison with the expanded-type polymer monolayers. Therefore, below the surface concentration at the limiting area the real state of polymer monolayer should not be considered to be a uniform layer. Even for such a state, the appreciable deviation of the ratio from unity at the lower surface concentration is expected, since ellipsometry gives the thickness and the refractive index for a homogeneous layer.
Conclusions Calculations of adsorbed amount of polymer are done by using a Lorentz-Lorenz relation for the refractive index of a mixture layer of polymer and surrounding media, as a function of the refractive index of the surrounding media. The refractive index of the polymer monolayer adsorbed at the air-water interface is between the refractive indexes of water and pure polymer. By taking the refractive index of the surrounding media to be that of water, the calculated adsorbed amount is well correlated with the real spread amount of polymer at the air-water interface, irrespective of polymer species. This good correlation indicates that the polymer chains are mixed with water molecules. Registry No. PEO, 25322-68-3; PTHF, 24979-97-3; PVAc, 9003-20-7; PMMA, 9011-14-7. ~~
(17) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience; New York, 1966.
