1478
Langmuir 1991, 7, 1478-1482
Ellipsometric Study of Mixture Films of Poly(methy1 acrylate) and Poly(viny1 acetate) at the Air/Water Interface Masami Kawaguchi' and Katsutoshi Nagata Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu, Mie 514, Japan Received October 12, 1990. In Final Form: January 16,1991 Binary mixed films of poly(methy1acrylate) (PMA) and poly(viny1acetate) (PVAc) and the individual polymer films at the air/water interface have been investigated by surface pressure measurements and ellipsometry. Both films of PMA and PVAc showed surface pressurearea (T-A) isotherms of the expanded type film type. However, the limiting area for PMA was smaller than that of PVAc and the collapse surface pressure for PVAc was higher than that for PMA. These results were in agreement with those of Crisp. On the other hand, ellipsometry showed there was no difference between PMA and PVAc films. For the mixed films of PMA and PVAc, the thicknesses and the refractive indices depended on the mixture ratio at the same total surface concentration due to repulsive forces between PMA and PVAc. An increase in PMA in the mixture induced an increase of the thickness as well as a decrease in the refractive index of the mixed films. Immiscibility of PMA and PVAc was confirmed by further experiments on films prepared by using a separate addition of the individual polymers. Therefore, it is concluded that PMA and PVAc mixtures were incompatible at the air/water interface.
Introduction Investigation of polymer films spread a t the air/water interface leads to the understanding of the static and dynamic properties of polymer chains in two-dimensional spaces. For this, several techniques have been applied to the surface films of polymers. Among them, surface pressure-area (T-A) isotherms were measured and were very useful in exploring the thermodynamic properties of polymer chains in two dimensions.' However, from the T-A isotherm alone, the conformational properties of polymer chains adsorbed on the water surface could not be directly deduced. Traditionally, since the limiting area obtained by the extrapolation of the straight, steepest portion of the T-A isotherm to zero surface pressure is in agreement with the data of X-ray spacing of the corresponding polymers as well as the two-dimensional spatial area estimated from using the molecular model, it has been accepted that polymer chains spread a t the air/water interface take a flat conformation and their thicknesses are 1-2 nm.2 Recently, ellipsometry,3-6 X-ray fluorescence,6 and neutron reflection' were used to determine the structural properties of adsorbed polymer layers at the air/water interface. Neutron reflection showed the surface layers have a thickness that is comparable to the radius of gyration in s ~ l u t i o n .Such ~ a high thickness was observed for PEO at the plateau surface pressure. From this result, we may discard the idea that every polymer chain spread a t the air/water interface should take a flat conformation with a thinner layer thickness. Stereostructural difference in a polymer, regardless of identical chemical structure, induced the variation in the (1) Gaines, G. L., Jr. Insoluble Monolayers a t Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Crisp, D. J. J. Colloid Sci. 1949, 1 , 49. (3) Kawaguchi, M.; Tohyama, M.; Mutoh, Y.; Takahashi, A. Lungmuir 1988, 4, 407. (4) Kawaguchi, M.; Tohyama, M.; Takahashi, A. Langmuir 1988,4,
All.
( 5 ) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G.; Kim, M. W. Macromolecules 1989,22, 2332. (6) Sansone, M.; Rondelez, F.; Peiffer, D. G.; Pincus, P.; Kim, M. W.; Eisenberger, P. M. Phys. Rev. Lett. 1985,54, 1039. (7) Rennie, A. R.; Crawford, R. J.;L+ee, E. M.;Thomas, R. K.; Crowley, T. L.; Roberta, S.; Qureshi, M. S.;Richards, R. W. Macromolecules 1989, 22, 3466.
shape of the u-A isotherms. A typical and well-known example is poly(methy1methacrylate) (PMMA). The P A isotherm of isotactic PMMA was more expanded than that of syndiotactic PMMA.&*O The limiting area of the isotactic PMMA was twice as large as that of the syndiotactic PMMA. On the other hand, poly(methy1 acrylate) (PMA) differs from poly(viny1 acetate) (PVAc) only in the reversed position of the ester group. From surface pressure measurements, the PMA film had less expansion a t the wide surface area than PVAcm2This discrepancy was interpreted by the difference in the surface orientations, which are from the molecular model, a t the air/ water interface under the assumption of a flat conformation. However,there has been no attempt to determine directly the differences in the thicknesses between the spread polymer films with similar chemical structure. In this paper, in order to explore the difference in the interfacial properties between the films of PMA and PVAc spread a t the air/water interface, we used both techniques of surface pressure measurements and ellipsometry for the individual polymers as well as the binary mixtures of PMA and PVAc. We expect that ellipsometry would give clear experimental evidence for the difference in the interfacial structures of PMA and PVAc films. An application of ellipsometry to the binary mixtures of PMA and PVAc should allow us to discuss their interfacial properties, such as compatibility and conformation a t the airlwater interface.
Experimental Section Materials. We used a fractionated PMA with M , = 589 X lo3and a fractionated PVAc sample with M , = 300 X 1Oa. Their molecular weights were determined by intrinsic viscosity in benzene for PMA at 25 O C and in acetone for PVAc at 30 O C . Spectrograde quality benzene was used as the spreading solvent for polymer films.
Surface Pressure Measurements. Surface pressuresof the individual polymer films and the binary mixture films spread at the air/water interface were determined by using the same (8) Beredjick, N.; Ahlbeck, R. A.; Kwei,T. K.;Ries, H. E., Jr. J. Polym. Sci. 1960, 46, 268. (9) Beredjick, N.; Kwei, R i a , H. E., Jr. J. Polym. Sci. 1962,62, S64. (10) Takahashi,A.;Ohwaki, S.; Kagawa, I. Bull. Chem. SOC. Jpn. 1970, 43, 1262.
0743-7463/91/2407-1478$02.50/0 0 1991 American Chemical Society
Mixture Films of PMA and PVAc instruments as reported previously.3 A Teflon trough with a diameter of 15cm was filled with deionized water supplied from a Millipore Q-TM system. The temperature of the water in the trough was controlled to 25 f 0.1 "C by circulating thermostated water. The trough was placed on a stage. The trough and stage were covered by a Plexiglas box to prevent dust particles. After the water surface was cleaned by aspiration, the sandblasted platinum plate (24 X 10 X 0.1 mma) was attached to a Cahn 2000 electrobalance, and the trough was slowly moved up using the stage until the edge of the plate touched the water surface. The electrobalance was connected to a digital voltmeter capable of 0.001-mV readings (AdvantestTR 6846). The sensitivity of the surface pressure was 0.02 mN/m.ll Polymer films were spread on the water surface in the trough by delivering the polymer benzene solution with a Hamilton microsyringe. Binary mixtures in benzene were prepared by mixing stock solutions of the individual polymers. Binary films are prepared by spreading the benzene solution of mixtures, which is designated as the simultaneous spreading, or by a separate addition of benzene solutions of each polymer. Surface concentrations of polymer were adjusted by changing the amount of the spread solution. A t least 10min was allowed for evaporation of the spreading solvent. At the higher surface concentrations, the surface pressure showed a strong time dependence, and we regarded it as important for attaining a constant surface pressure, namely, an equilibrium value, unless it did not remain constant over 10 min. It takes at least 1 h to attain equilibrium surface pressure. Duplicate runs were at least made to check the reproducibility of the surface pressure measurements. The experimental errors in the surface pressure were less than 0.1 mN/m. Ellipsometry. An instrument for ellipsometry was used as in the previous papers.3J1 Settings and readings of goniometers on which a polarizer, an analyzer, and a quarter-wave plate were placed were 30 s. Ellipsometry gives the phase difference (A) and azimuth ($) of the amplitude ratio for light polarizedparallel and normal to the plane of incident. The values of A and $ were obtained from the readings of the polarizer and analyzer, respectively. The changes in A and $ between the clean water and the water surface covertd with polymer films_wereexpressed as 6A = (A - A) and 6$ = ($ - $), respectively: A and $ for the pure water and A and $ for the water surface covered with the polymers. Incident angle was 70°. In particular, we started the measurements of ellipsometry at least 1h after spreading for the higher surfaceconcentrations. The 1h elapsed time shouldensure attaining equilibrium and is longer than that for the previous case.3 We calculated the thickness and refractive index of the polymer films spread at the air/water interface from the experimental data of A and $using a FACOM M760/6 computer system with a modified version of McCrackin's program.'* The calculation method was based on the iteration procedure where the thickness and refractive index values are sought, which reproduce the measured ellipsometric parameters of A and $ by taking account of the same three terms as those in the previous papers? (1)The maximum differences in both A and $ between the measured and calculated values are 0.01O. (2) The refractive index of the environment corresponding to air is regarded as unity. (3) The calculated refractive index of the polymer films must be lower than the refractive index of polymer in the bulk state. To check the reproducibility of ellipsometric measurements, we have repeated at least two runs at the same surface concentration. The experimental errors in the values of A and $ were less than 0.03 and O.O2O, respectively.
Results and Discussion Individual Polymers. Figure 1 shows surface pressure area (*-A) isotherms of PVAc and PMA. The shape of both isotherms belongs to a typical expanded-type isotherm in which changes in the surface pressure are detected at large surface area, i.e. low surface concentration. The surface pressure gradually increases with a decrease in surface area and attains a plateau value. We noticed some (11)Nagata, K.; Kawaguchi, M. Macromolecules 1990,23, 3967. (12) McCrackin, F. L. NBS Tech. Note (U.S.) 1969,479.
Langmuir, Vol. 7, No. 7, 1991 1479
A
0
0
5 A
10
/m2.mg-'
Figure 1. Surface pressure (+area (A) isotherms of PVAc ( 0 ) and PMA (0) films at the air/water interface. The inset shows the enhanced PA isotherms of both polymers at the low surface areas. differences between the isotherms: First, from the PA isotherms, we can estimate the limiting area from the traditional extrapolation method.' They were 1.5 m2/mg for PMA and 1.75 m2/mg for PVAc, respectively. The limiting area of PMA is clearly smaller than that of PVAc. Second, the apparent collapse surface pressure of PVAc (25.7 mN/m) is larger than that of PMA (19.3 mN/m). These findings are in agreement with the data of Crisp.2 From these experimental results, we believe that PVAc behaves more like expanded-type film than PMA at the air/water interface in spite of the similar chemical structure. Next, we studied the polymer film layers spread at the air/water interface using ellipsometry in order to prove that the less expanded-type PMA makes a more compact film layer on the water surface than PVAc. Ellipsometry is a suitable technique for in situ measurements, yielding simultaneous estimates of both the thickness and refractive index of adsorbed layers at various interfaces. Experimentally, however one is not able to determine both thickness and refractive index for films on water when 6$ is too small.3 Figure 2 shows the 6A and 6$ values for PVAc and PMA films as a function of surface concentration (I?) together with some 6A values cited from the previous papers3The 6A values for both polymers almost linearly increase with an increase of I' in the entire concentration range. Since the 6A value is a measure of mass density, which stems from the spread amount on the water surface, the continuous linear increase in 6A with surface concentration means that both polymers are adsorbed at the air/water interface without any desorption of the whole polymer chain from the interface or any dissolution into the water phase. By further addition of polymer chains, however some segments must desorb from the interface. This is different from water-soluble PEO and condensed films of PMMA, whose 6A values reach a plateau value above the surface concentrations where their surface pressures become ~ o n s t a n t .The ~ 6A values for PVAc film are always located slightly below from a straight line fitting on the data points of PMA films in the entire surface concentration range. On the other hand, since the changes in 6$ due to the presence of polymer chains at the air/water interface are less sensitive to those of 6A, the 6$ values are much smaller than the 6A values over entire surface concentrations. However, above r = 2.8 mg/m2 for PMA and 3.5 mg/m2 for PVAc, respectively, in which both polymers are above in the surface concentration of the limiting area, the values
Kawaguchi and Nagata
1480 Langmuir, Vol. 7,No. 7, 1991 3
1 ' 1 ' 1 . 1 ' 1 . 1 0.1
Q,
E
0 0,
0.05
0.5 Lc
5
a 2
E0, Q,
U
\
0
3
2
1
5
4
a - 1
/mg"
/-
Figure 2. Plots of 6A and 6+ as a function of surface concentration r for PMA (6A,0; a$, e)and PVAc @A, m; 6$,E).The open squares indicate the previous data of 6A for PVAc.S Solid straight lines are drawn as fitting on the 6A values of PMA and PVAc, respectively. I
I
I
I
0
1
2
I r P v ac
3
/mg.
4
5
mZ
Figure 4. Plots of 6A as a function of surface concentration of PVAc in the mixtures of 114 mixture of PMAand PVAc (a)),111 mixture of PMA and PVAc (O),and 411 mixture of PMA and PVAc (e). The continuous line indicates the simple addition model. Dashed and chain (- -1 lines indicate the straight line fitted on data of 6A values for PMA and PVAc, respectively.
-
e 0
0' D
0
I
O
0
I
A
/m2.mg"
Figure 3. Surface pressure (r)-area (A) isotherms of PMA (O), PVAc(u), 114 mixture of PMA and PVAc (e),111 mixture of PMA and PVAc (a)),and 411 mixture of PMA and PVAc ( 0 ) . The inset shows the mean areas at the surface pressures of 5,10, and 15 mN m as afunction of molar fraction of PVAc. Open circles cite are from the previous paper.lS
d
of 6$ undoubtedly exceed the precision of $ for both polymers and they are plotted in Figure 2. The fact that the 6$ values for PVAc do not exceed the precision of $ up to r = 3.0 mg/m2 and are smaller than 0.03O is in agreement with the previous paper.3 Thus, one is able to calculate the thickness ( t )and the refractive index (nf) for both polymer films from A and $ values under the assumption of a homogeneous layer a t higher surface concentration than I' = 2.8 mg/m2 for PMA and 3.5 mg/ m2 for PVAc, respectively. However, it was found that a little change in the S$ often strongly affects on the calculated t and nf values if the G$ value is as large as 0.03'. For example, such a small G$value happens to lead to 1order of magnitude difference in the calculated thickness, and then the large thickness of ca. 30 nm for polymer films reported in the previous papers3v4 may not be reputable. Thus, it has been concluded that the values of t and nfshould be reported only if Gt) exceed 0.05'. Moreover, even if 6$ > 0.05', the experimental errors in A and t)yielded the thickness and the refractive index with errors of ca. 10%. Since all 6$ values for PMA and PVAc films are less than 0.03O, ellipsometry gives no clear difference in PMA and PVAc and this is contrary to our expectation described previously. Binary Mixtures. Figure 3 shows surface pressurearea isotherms of the binary mixtures of PMA and PVAc for three mixtures of 1/4, 1/1, and 4/1, of which these ratios of PMA and PVAc are expressed in monomer molar fraction, respectively. The mean surface areas a t constant surface pressures of 5,10, and 15 mN/m as a function of the molar PVAc in the mixtures are displayed in the inset
of Figure 3. The plot of the mean surface area versus the molar fraction of one component in the mixture is useful for an estimate of compatibility of the corresponding mixtures.' These mean surface areas almost fit on the dashed line, which represents the additive line calculated from the following equation
A = X,A,
+XdA,
(1)
where A is a mean area in the mixed film a t a given surface pressure, X1 and XZ are molar fractions of two pure polymers such as PMA and PVAc films in this study, and A1 and A2 are a surface area of the corresponding pure polymers a t the same pressure, respectively. The fact that data poinb fit on the additive line means that PMA and PVAc mixtures are ideally miscible or completely immiscible a t the air/water interface. Since both polymers are similar for chemical structures, we are led to the conclusion that they were ideally mixed a t the air/water interface in the previous paper,13regardless of invariance of the collapse surface pressure on the compositions in the mixture. Figure 4 shows the GA values for three binary mixtures as a function of PVAc surface concentration rPVAe in the mixture. The values of 6A almost linearly increase with an increase in rPVAc in the entire concentration range. The data points almost fit on the straight solid line calculated from the simple addition of GA for the individual polymers a t a given surface concentration. This simple addition of 6A is interpreted by a theoretical reason, that is, 6A is sensitive to mass density a t the surfaces and should be directly additive in the mixtures since both polymers are adsorbed a t the air/water interface without desorption of the whole polymer chain. A similar agreement between the measured 6A values and the calculated ones was also observed for the binary mixture films of PEO and PMMA with low PEO content," where water-soluble PEO molecules contribute to an increase in the changes in the 6A value due to no desorption of PEO from the water's surface. Thus, the reason why the simple addition of 6A stands up for the PMA and PVAc mixture films may result from their water-insoluble and strongly adsorbable character. (13)Kawaguchi, M.; Nishida, R. Langmuir 1990, 6, 492.
Mixture Film of PMA and PVAc
0,
m
0.4
1 %
a ' 0.21
-
/e e
,geee+
0
i
e
" Lo
Langmuir, Vol. 7, No.7, 1991 1481
0
,_
0 0 0
0
1
:---b-*3zw--.+3?:,. ,
2
3
4
5
rpvAc /mg m2 Figure 5. Plots of 64 as a function of surface concentration of PVAc in the mixture for the three mixtures of 1/4,1/1,and 411 of PMA and PVAc, respectively. Symbols are the same as in Figure 4. A solid line indicates the straight line fitted on the data of 64 for PMA and PVAc. Dashed, chain, and dotted lines indicate the simple additive lines for the three mixtures of 114, 111,and 411 mixture of PMA and PVAc, respectively. Table I. Ellipsometric Data and Adsorbed Amounts Calculated from Equation 3 as a Function of Surface Concentration r for PMA and PVAc Mixtures surface concn thickness refractive adsorbed amount r, mg/m* t, nm index nf m,, mg/m2 m,/r 3.05 3.56 4.07 4.57 6.11
10.7 12.5 9.9 11.0 10.5
3.93 4.42 4.91 5.40 5.89 6.87 7.36 9.33 9.82 10.70 11.30 11.78 12.77
6.4 12.6 15.9 22.5 23.1 27.7 19.1 25.1 22.3 24.2 26.7 22.6 25.9
1/1 mixture 1.371 1.371 1.384 1.376 1.409 4/1 mixture 1.417 1.384 1.380 1.369 1.371 1.369 1.384 1.382 1.392 1.394 1.388 1.403 1.399
3.46 4.03 4.24 4.53 6.70
1.13 1.13 1.04 1.15 1.10
4.55 5.41 6.28 6.89 7.47 8.48 8.22 10.31 11.13 12.53 12.41 13.29 14.53
1.16 1.21 1.28 1.28 1.27 1.23 1.12 1.11
1.13 1.16 1.10 1.13 1.14
In Figure 5, the 6$ values for the three mixtures are shown as a function of rPVAc and they increase with increasing rpVAc. All 6$ values of 1/4 mixtures of PMA and PVAc are less than 0.05'. At the same rPVAc the 6$ value increases with an increase in total spread amount of PMA and PVAc, and this dependence is similar to the 6A values. However, a simple addition as described above is not applicable for 6$ values since the measured 6$ values deviate above the additive line. The additive lines are drawn by assuming that 61) values for both polymers a t higher surface concentrations increase through the solid line fitted with the values of a$. These large 6$ values should result in large thicknesses. This will come true from calculation results of the thickness and refractive index of the mixture films in the following paragraph. Results of the thickness and refractive index for the two mixtures are illustrated as a function of surface concentration, r of PMA + PVAc in Table I, in which errors in the t and nf values are ca. 10%. If both polymers are ideally compatible, we expect that thickness and refractive index of a mixed polymer film should be independent of the mixture ratio a t the same total amount of the two components as the mixture. However, a comparison of ellipsometric data for the mixtures shows that the thicknesses of the 1/1 mixtures are almost constant up to a surface concentration of ca. 6.0 mg/m2,
whereas in the similar surface concentration ranges the thickness of the 4/1 mixtures increases. In particular, the thicknesses for the 1/1 and 4/1 mixtures are about 11 and 23 nm a t r of ca. 6.0 mg/m2 and the corresponding refractive indices of the mixture films are 1.409 and 1.371, respectively. Regardless of the similar surface concentration, the thickness and refractive index depend on the composition of the mixtures. In other words, an increase in PMA molecules in the mixture further induces the repulsion forces between PVAc and PMA chains and thus it results in a thick and diluted film a t the air/water interface. The repulsion forces may stem from the difference in the interfacial properties between PMA and PVAc; that is, PMA chains form a more condensed film a t the air/water interface than PVAc chains. Thus, these ellipsometry data lead to a conclusion that both polymers in high surface concentrations seem to be immiscible rather than ideally compatible. This conclusion correlates well with the constant collapse surface pressure of 25.7 mN/ m, which is almost equal to the collapse surface pressure of PVAc film, irrespective of the mixture.13 Compatibility of PMA and PVAc a t the air/water interface will be further discussed below in light of data from different preparation methods of the mixed films. The refractive index of the adsorbed layer is related t o the polymer concentration in the layer. Since both polymers are water-insoluble, we cannot calculate the average polymer concentration, Cf, in the polymer layer by using a relation that is useful for adsorption of polymers a t the solid/liquid interfaces from their s 0 1 u t i o n s ~ ~ J ~
C, = (nf - n,)/(an/ac)
(2) where no is the refractive index of the solvent and anlac is the refractive index increment. Thus, we derived the simple relationship for calculating the adsorbed amounts in polymer films by application of the Lorentz-Lorenz relation to the polymer layers consisting of polymer and a surrounding media. The final equation contains only one unknown parameter, the refractive index (n,) of the surrounding media in the polymer layers. In other words, we can calculate the adsorbed amounts of polymers a t the air/water interface as a function of n,. The quantitative derivation of our model for the polymer layer is addressed in the previous paper." The adsorbed amount (mp)of polymers expressed by the product of the thickness and the polymer density (p,) in the spread films is given by
mp = tp, = [3t F ( n f , n , ) l / [ ( ~ , / ~ -p ) up{(n?- l)/(n,2 + ~)II(3) with F(nf,n,) = ( n t - n,2)/Kn,2 + Nn,2 + 2)) In eq 3 both t and nf can be determined by ellipsometry, the molar refractivity of a repeating monomer unit of polymer chain (Ap) can be estimated from the molar refractivities of atoms and atom groups contained in the monomer unit of each polymer, and the partial specific volume of polymer (up) can be obtained from the density of pure polymer (ppo). In a previous paper: the real spread amounts of polymers were in good agreement with those calculated by assuming n, = 1.334, which corresponds to the refractive index of water, irrespective of polymer species. Thus, in calculation (14)Takahashi, A.; Kawaguchi, M. Adv. Polym. Sci. 1982,46, 1. (15)Cohen Stuart, M. A,; Cosgrove, T.; Vincent, B. Adv. Colloid Interface Scr. 1986,24, 143.
1482 Langmuir, Vol. 7,No. 7, 1991
Kawaguchi and Nagata
Table 11. Effect of Spreading Method on &A firat spreading, mg/m2 PVAc
second spreading, mdm2 PMA
third spreading, mg/m2 PVAc
1.0 1.0 1.0 0.5 0.5
0.25 1.0 4.0 2.0 2.0
0.5
PMA
PVAc
PVAc
6A, deg
6A," deg
0.17 0.18 0.17 1.05 0.17
0.25 0.40 1.05 0.53 0.63b
0.14 0.25 0.25 1.0 0.18 0.40 1.0 1.0 0.18 1.05 4.0 1.0 2.0 0.5 1.05 0.53 2.0 0.5 0.5 0.17 0.63b 0 6A values for the simultaneous spreading. b The 6A value is estimated by assuming a simple addition of 6A for the PMA/PVAc mixed ratio of 2/1.
of the value of mp, we assumed n, = 1.334 and employed the same values of R, = 20.297,Mp= 86.09,and ppo = 1.20 for both PMA and PVAC.~The adsorbed amounts calculated are summarized in Table I together with the ratio, mp/I', of the calculated amount to the real spread amount. Most m , / r values are located around unity and the mixture films are also hydrated. However, in particular, the ratios for the 4/1 mixtures a t r of 4.4-7.0mg/ m2are relatively larger than those for the other mixtures. At the present time, we do not have any explanation for these large ratios. Next, we examined the effect of order of addition on the ellipsometric parameters. We used several methods to apply the polymers to the water surface as follows: (I) PVAc is first spread a t r = 1.0 mg/m2 where PVAc is above the limiting area and then PMA is added to adjust the PMA/PVAc mixed ratios to 1/4,1/1, and 4/1 and vice versa. (11) PVAc is first spread a t r = 0.5 mg/m2 where PVAc has a larger surface area than its limiting area and then PMA is added to adjust the PMA/PVAc mixed ratio to 4/1and vice versa. (111) After preparation of a mixed film by using method 11, further addition of PVAc leads to r p V A c = 1.0 mg/m2. Therefore, the value of r p V A c is maintained a t 1.0 mg/m2 in methods I and 111 and 0.5 mg/m2 in method 11, respectively. In Table 11, the values of 6A are summarized together with those for the simultaneous spreading, and their experimental errors are less than 0.03'. The spreading method for two polymers strongly affects 6A and its magnitude for the separate addition is quite different from that for the simultaneous spreading a t the same surface concentration of PMA + PVAc. As mentioned previously, the ellipsometry data in Table I1 were obtained a t least 1 h after the final spreading. The 1 h elapsed time can be regarded to be enough to attain equilibrium, since the values of A and $ were almost the same as 5 h after spreading. Thus, we believe that the structures of the spread films also should be in an equilibrium 1 h after spreading. We notice some interesting features from Table 11. Irrespective of the order, however, the 6A values are
reversible, and a t the PVAc surface concentration of 1.0 mg/m2, the value of 6A is almost equal to that (0.16') for PVAc film. Moreover, the fact that the S$ values are very small and can be regarded as zero resembles that for PVAc film a t the concentration of 1.0 mg/m2. Even if a t the largest spread amount of PMA PVAc = 5.0 mg/m2, the 6$ value is smaller than 0.03' and we cannot extract the reliable values of thickness and refractive index of the separately spread polymer film from the ellipsometry data. For PEO-PMMA mixtures, which are compatible in the entire composition,11J30nthe other hand, it was found that their 6A values are almost independent of the spreading method.l8 This correlates well with their compatibility in the bulk state. The reason why the separations occur between PMA and PVAc is not clear in spite of their closely related structures, but their compatibility in the bulk state is known to be ambiguous.17To understand why PMA-PVAc mixtures are incompatible and why PEO-PMMA mixtures are compatible a t the air/ water interface, will require further work. Since the neutron reflection technique7 is a powerful method to provide the segment density and the thickness of the adsorbed layer, some experiments of mixed polymer films spread a t the air/water interface will be performed in the near future with this technique. These results of the separately spread films seem to be sufficient evidence for the immiscibility of PMA and PVAc a t the air/water interface and also support the conclusion deduced from the simultaneous spreading. However, a t a PVAc surface concentration of 0.5 mg/m2,which is lower than that a t the limiting area, the value of 6A is larger than that for the simultaneous spreading. At the present moment we do not have a good explanation for the larger 6A value.
+
Conclusions We have demonstrated the differences in the interfacial properties between PMA and PVAc films spread a t the air/water interface by use of surface pressure measurements and ellipsometry. From the r A isotherms, the larger differences in the limiting areas and the collapse surface pressures were reconfirmed and their data were in good agreement with previous data. From ellipsometry, the changes in 6A for PVAc as a function of its surface concentration are slightly smaller than those for PMA. On the other hand, the binary mixture films of PMA and PVAc were also conducted by the surface pressure measurements and ellipsometry. For the same amounts spread a t the air/water interface, the thickness increases and the refractive index decreases with an increase in PMA concentration in the mixtures. This fact indicates that PMA and PVAc mixtures incompatible a t the air/water interface and this conclusion is well supported by the separate spreading of each polymer. Thus, ellipsometry is a powerful technique for an estimate of compatibility of two polymers spread a t the air/water interface. Registry No. PMA, 9003-21-8;PVAc, 9003-20-7. (16) Kawaguchi, M.;Nagata, K. Unpublished data. (17) Krause, S. In Polymer Blends; Paul, D. R., Newman, S., Us.; Academic Press: New York, 1978; Vol. 1, p 15.
