Langmuir 1990,6, 492-496
492
cluster cations A q+ ( n = 5-13) on silver zeolite-Y substrates, Ag55-Y,4%a * in the visible region (400-650 nm) shows that the spectra are identical, displaying identical band positions. The spectra also bear somewhat similar features to our difference absorption spectra from pulse radiolysis (Figure 1) and laser photolysis (Figures 3-5). Clearly, any conclusion as to the precise nature of the transients in the present work is made difficult by these spectral similarities: silver atom clusters versus silver cluster cations, not to mention the nuclearity of the various cluster species. In the time frame of the picosecond laser experiments (30 ps to 10 ns), formation of silver atom clusters and/or silver cluster cations with n > 2 a t some surface defect site is not precluded by the present study, their formation being dependent on the rate-determining surface migration of Ag+ and Ago. Thus, while spectra of Figures 1 and 2 must be ascribed to silver atoms, Ago (and/or dimeric Ag, molecules) under the conditions of the experiment, the spectra of Figures 3-5 may reflect those of higher silver clusters. Recent picosecond studies on Ag+/crown ether micellar solutions show transient absorption spectra nearly identical with those of Figures 3-5; the transients can only be Ago or Ag, clusters.50
Concluding Remarks Both pulse radiolysis and laser flash photolysis techniques have been employed to examine the behavior of AgI colloids of small to very small dimensions (100, 35, and 25 A) in acetonitrile media under conditions where Ag+ ions are reduced to metallic silver by acetonitrile (50) Serpone, N.; Pelizzetti, E., to be published.
reducing radicals ((CH,CN),-, CH,CN-, and e,,-) as well as by conduction band electrons under a 355-nm photolyzing laser pulse. The transient absorptions resulting from these redox conditions are those of silver atoms or dimeric Ag, silver molecules (pulse radiolysis) and probably higher Ag, clusters ( n > 2) under photolysis conditions. The absorption spectra of the AgI colloids exhibit bands that shift to higher energy (by about 0.7 and about 1 eV) as the particle size decreases from -100 to -25 A, consistent with the notion of increased effective band gap from the confinement of charge carriers in very small AgI semiconductor particles. The rates of formation of these silver clusters correlate inversely with size. An understanding of the particle size dependence and intensity dependence of the rates of Ag cluster formation must await computer modeling (in progress51)followingthe suggestion of possible multinucleation sites by Marquardt and c o - w o r k e r ~ . ~ ~
Acknowledgment. Financial support from the SIZ for Science of SR Serbia and from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. We have also greatly appreciated the very useful discussions and constructive comments from Dr. Me1 Sahyun of 3M Corporate Research (St. Paul, Minnesota) and Dr. Dani Meisel of Argonne National Laboratories (Illinois). Registry No. TEA, 102-71-6; AgI, 7783-96-2; Ag, 7440-224; CH,CN, 75-05-8. (51) Sahyun,M. R. V.; Serpone, N. et al., work in progress. (52) Marquardt, C. L.; Gingerich, M. E.; Williams, J. W. In Growth and Properties o f Metal Clusters; Bourdon, J., Ed.; Elsevier: Amsterdam, 1980; pp 345-353.
Compatibility of Polymer Chains at the Air/Water Interface Masami Kawaguchi* and Ryuji Nishida Department of Industrial Chemistry, Faculty of Engineering, Mie University, 1515 Kamihama-cho, Tsu,Mie 514 Japan Received July 20, 1989. I n Final Form: September 12, 1989 Surface pressure measurements of binary mixtures among poly(ethy1ene oxide) (PEO), poly(methy1 acrylate) (PMA), poly(viny1 acetate) (PVAc), and poly(methy1 methacrylate) (PMMA) have been performed at the air/water interface using the Wilhelmy plate method as a function of mole fraction of one component in the binary mixture. From the plots of the experimental mean area of the mixed polymer films at a constant surface pressure as a function of the mole fraction of one component in the films, we estimated the compatibility or incompatibility of both polymers at the air/water interface. PEO/ PMMA mixtures and PMA/PVAc mixtures are compatible in their entire composition. In PVAc/ PMMA mixtures, the repulsive interaction is dominant, and the mixtures are thermodynamically unstable and should not be compatible. For PVAc/PEO mixtures, the attractive interaction is observed below the collapse surface pressure of PEO, i.e., 10 mN/m. Above this value, the surface pressures of the mixtures are almost independent of the composition. These results correlate well with the compatibility of the corresponding mixtures in the bulk state.
Introduction There are many studies of mixed polymer films spread at the air/water interface.'-' In particular, Gabrielli and (1) Ries, H. E., Jr.; Walker, D. C. J. Colloid. Sci. 1961, 16, 361. (2) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interface; Interscience: New York, 1966. (3) Wu, S.; Huntsberger, J. R. J. Colloid. Interface Sci. 1969, 29, 138. (4) Gabrielli, G.; Puggelli, M.; Faccioli, R. J. Colloid Interface Sci. 1971, 37, 213.
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c o - w o r k e r ~have ~ - ~ significantly contributed to the investigation of various mixed polymer films in order to obtain information concerning the general reasons for compatibility and incompatibility in the two-dimensional state. (5) Gabrielli, G.; Puggelli, M.; Faccioli, R. J. Colloid Interface Sci.
1973, 44, 177.
(6) Gabrielli, G.; Puggelli, M.; Ferroni, E. J. Colloid Interface Sci.
1974, 47, 145.
(7) Gabrielli, G.; Baglioni, P. J. Colloid Interface Sci. 1980, 73,582. (8) Gabrielli, G.; Puggelli, M.; Baglioni, P. J. Colloid Interface Sci. 1982, 86, 485.
0 1990 American Chemical Society
Polymer Chains a t the AirlWater Interface One of their important conclusions is that the compatibility of mixed polymers spread at the air-water interface strongly depends on the interfacial orientation of the polymer chains, such as a predominant horizontal orientation, with the hydrophobic chains parallel to the interface, and a predominant vertical orientation, with the hydrophobic chains perpendicular to the interface. The components that show compatibility have the same interfacial orientation, and the incompatible ones have a different orientation. In general, the compatibility of the mixed films (monolayers) is determined from the plot of the mean areas at a constant surface pressure versus molar fraction of one component in the binary mixture. If the plot is a linear relationship, i.e., the surface areas are additive, the mixed films can be regarded as an ideal mixture or as a completely immiscible mixture in the entire mixture range. Since the additivity of the surface areas is in itself not sufficient evidence to demonstrate either the complete incompatibility or the ideal miscibility of the two components, we should rely on a plot of the collapse surface pressure as a function of mole fraction of one component in the binary mixtures: the collapse surface pressure of the ideal mixture films depends on the composition, whereas for the completely immiscible mixture it is independent of the components. The deviation from the linear relation stems from the contribution of intermolecular interaction between both substances: a negative deviation means that the mixtures are considered to be stable and compatible, whereas a positive deviation indicates that the mixtures are less stable than the components alone at the interface. On the other hand, investigating the compatibility of two polymers in the bulk state (polymer blends) is one of the central problems in polymer science and polymer processing. Many pairs of polymers were found to be compatible.' However, there are only a few studies that reveal whether the compatibility of polymer mixtures in the bulk state correlates well with that in the twodimensional state or not.3 All polymers do not form stable films a t the air/water interface therefore, and then it is not easy to determine good pairs to compare the compatibility in the bulk state with that in a film spread a t the airlwater interface. Among them, blends of poly(ethylene oxide) (PEO) and poly(methy1 methacrylate) (PMMA)1° as well as poly(methy1 acrylate) (PMA) and poly(viny1 acetate) (PVAc) are well-known to be compatible in the bulk state.' These four polymers do form stable films a t the air/water interface,' while, PMMA and PVAc blends are incompatible in the bulk state.' The aim of this paper is to compare the compatibility of polymers in the two-dimensional state with that of polymers in the bulk state. We measured the surface pressure of the binary mixture films spread a t the air/ water interface among PEO, PMA, PVAc, and PMMA. According to Gabrielli et al., the interfacial orientations of all four polymer chains a t the air/water interface are considered to possess a horizontal orientation a t the air/ water interface, and all mixtures among them should be compatible a t the air/water interface.
Experimental Section Materials. PEO with the narrow molecular weight distribution, having M, = 180 X lo3,was purchased from the Tohso Co. PMA and PVAc were synthesized by radical solution poly(9) Paul, D. R.; Newman, S.Polymer Blends; Academic Press: New
York,1978.
(10) Martuscelli, E.; Pracella, M.; Wang, P. Y . Polymer 1984, 25, 1097.
Langmuir, Vol. 6, No. 2, 1990 493 merization and fractionated. We employed fractionated PMA and PVAc samples. Their molecular weights were determined to be 589 X lo3 and 300 X lo3 by intrinsic viscosity, respectively. PMMA was a commercially available sample from Scientific Polymer Products that was fractionated. We used one fractionated PMMA sample. Its molecular weight was determined to be 280 X lo3 based on its intrinsic viscosity. Ita stereoregularity was determined in chloroform-d at 55 O C with a Bruker MSL-400 lH NMR spectrometer. Its content was 9.5% isotactic, 39.0% heterotactic, and 51.5% syndiotactic. Spectrograde quality benzene was used as the spreading solvent for the polymer films. Surface Pressure Measurements. A Teflon trough with a diameter of 15 cm was filled with deionized water supplied from a Millipore Q-TM system. Its temperature was controlled within 25 f 0.1 "C by circulating thermostated water. The trough was placed on a stage that can be moved up and down to adjust the position of the trough. This apparatus was placed in a Plexiglas box. The water surface was cleaned by aspiration, and the height of the trough was adjusted to touch the edge of a sandblasted platinum plate (24 X 10 X 0.1 mm3) connected to a Cahn electrobalance with the stage. The surface tension of pure water and the surface pressures of the spread polymer films were determined from the electrobalance connected to a digital voltmeter. After confirmation of the surface tension of water being equal to that published in the literature, polymer films were applied to the water surface in the trough by delivering the polymer solutions from a Hamilton microsyringe. At least 10 min was allowed for evaporation of the spreading solvent. For PMA, PVAc, PMMA, and their mixtures, the surface pressure at their higher concentrations showed a strong time dependence, and we regarded it as important for attaining a constant surface pressure, namely, an equilibrium value unless it does not remain constant over 10 min. The accuracy of the surface pressure is *0.03 mN/m. The surface concentration of the polymer spread on the water surface was varied by stepwise addition of the polymer solutions. Duplicate runs were made to check the reproducibility of the surface pressure measurements. The experimental errors in the surface pressure were less 0.1 mN/m.
Results and Discussion Single Polymer Film. In general, the surface pressure is plotted versus the surface area, and we call the plot an a-A isotherm. From the *-A isotherm, we can deduce some important conclusions. The shape of the a-A isotherm is related to the hydrophilicity or hydrophobicity of substances, and the limiting area can be determined by the extrapolation of the steepest portion of the a-A isotherm curve to the zero surface pressure. PEO, PMA, and PVAc films show their surface pressure a t a larger surface area than PMMA film. The surface pressure of a PMMA film more steeply increases with a decrease in surface area than the other three films. The classificationof the shape of the a-A isotherm, as advanced by Crisp," allows that PEO, PMA, and PVAc belong to the expanded film, while PMMA falls in the condensed film category. The measured a-A isotherms are not different from previous data.'"14 PEO and PMMA Mixture. Figure 1 shows the *-A isotherms of the two pure polymers and their typical binary mixtures. We noticed some features in the isotherms of the mixtures. The isotherms at the higher PEO compositions have plateau regions around a surface pressure of 10 mN/m, which corresponds to the collapse surface pressure of PEO film. The width of the plateau regions (11) Crisp, D. J. J . Colloid Sci. 1946, I , 49. (12) Kawaguchi, M.; Komatsu, S.;Matsuzumi, M.; Takahashi, A. J. Colloid Interface Sci. 1984,102, 356. (13) Kawaguchi, M.; Tohyama, M.; Mutoh, Y.; Takahashi, A. Langmuir 1988, 4, 407, 411. (14) Kawaguchi, M.; Sauer, B. B.; Yu, H. Macromolecules 1989,22, 1735.
494 Langmuir, Vol. 6, No. 2, 1990
Kawaguchi and Nishida
0
A
/m2. mg-’
Figure 1. n-A isotherms for mixtures of PEO and PMMA at the air/water interface: (0)PEO; (0)PEO/PMMA = 0.95/ 0.05; (e) PEO/PMMA = 0.82/0.18; (0) PEO/PMMA = 0.69/ 0.31; ( 8 )PEO/PMMA = 0.53/0.47; ( 0 )PEO/PMMA = 0.22/ 0.78; (A)PMMA.
0
1.0
Mole fraction of PMMA Figure 3. Mean surface areas in the mixtures of PEO and PMMA as a function of mole fraction of PMMA. The numbers in the figure indicate the surface pressure.
.-
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z
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tration in the mixtures of PEO and PMMA ( 0 )PEO/PMMA = 0.90/0.10. The symbols are the same as in Figure 1. The dashed line indicates the concentration dependence of the surface pressure for PEO.
increases with increasing mole fraction of PEO. The plateau surface pressure, which can be regarded as the lower collapse surface pressure, depends on the PEO/PMMA mole ratios. This is clearly found in the plot of the surface pressure as a function of PMMA concentration in Figure 2. Below the plateau surface pressure, the surface pressure at the same area is lower than that of the PEO film and decreases with an increase in the PMMA component. Above the plateau surface pressure, on the other hand, the composition dependence of the surface pressure is quite the reverse; Le., the reduction in area after the plateau pressure has occurred and the higher collapse surface pressure (15-17 mN/m) increases with an increase in PEO content. A relation between the mean areas ( A ) at a given surface pressure and molar ratios of component 1 (X,) and component 2 ( X , ) in the mixture (X, X , = 1) is expressed by eq 1if the two components are ideally miscible or completely immiscible:
+
A = X i A i + X2A2 (1) where A , and A , correspond to the molecular areas of components 1 and 2, respectively. This relation can be applied to the mixed polymer films by using the values of A , A,, and A , as an area per unit mass instead of a molecular area. Plots of the mean areas at the surface pressures of 4 and 8 mN/m versus mole fraction of PMMA are shown in Figure 3. The dashed line represents the additivity line of eq 1. The mean areas deviate from the dashed line, and the negative deviation for the large surface pres-
,
,
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_1 10
/m2.mg-’
Figure 4. P A isotherms for mixtures of PVAc and PMA at the air-water interface. The inset shows the surface pressure at low surface areas in order to enhance the composition dependence: (e) PVAc; ( 0 )PVAc/PMA = 0.89/0.11; ( 0 ) PVAc/ PVAc/PMA = 0.50/0.50; (e)PVAc/ PMA = 0.67/0.33; (0) PMA = 0.20/0.80; ( 0 )PVAc/PMA = 0.11/0.89; (X) PMA.
sure is smaller than that for the small one. The negative deviation means that the intermolecular interaction between PEO and PMMA is attractive and that the mixture of PEO and PMMA at the air/water interface is nonideally miscible and stable. Moreover, both polymers more easily interpenetrate each other. The compatibility of both polymers is also guaranteed by the composition dependences of the plateau surface pressure as well as the higher collapse surface pressure. Though the existence of the plateau region in the a-A isotherms of mixed films as well as the reduction in area is sometimes interpreted by the squeezing of one component, which corresponds to PMMA in the PEO/ PMMA mixture, out of the mixed films, this interpretation does not hold for the PEO and PMMA mixtures. The compatibility of the mixture at the air/water interface correlates well with that of blends of PEO and PMMA. PVAc and PMA Mixture. a - A isotherms of the two pure components and five PVAc-PMA mixtures are displayed in Figure 4. Since both pure components have a similar chemical structure, the shapes of the *-A isotherms are similar, but the collapse surface pressure is much different: 25.7 mN/m for PVAc and 19.3 mN/m for PMA. The a-A isotherms of some mixtures have an apparent plateau region around 20 mN/m, and the magnitude of the plateau surface pressure increases with an increase in the PMA component. Below the PVAc/ PMA molar ratios of 1 / 8 , the width of the plateau region also increases with increasing PMA component. This dependence is more clearly displayed in the plot of the surface pressure as a function of PVAc concentration in Figure 5. Except for the plateau region, the surface pressure for the same area increases with an increase in PVAc component of the mixture.
Polymer Chains at the AirlWater Interface
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Figure 5. Surface pressures as a function of PVAc concentration in mixtures of PVAc and PMA: (e) PVAc PMA = 0.941 0.06; (@) PVAc/PMA = 0.0610.94. The symbo s are the same as in Figure 4. The dashed line indicates the concentration dependence of the surface pressure for PMA.
40
0 1.o Mole fraction of PVAc
Figure 6. Mean surface areas in the mixtures of PVAc and PMA as a function of the mole fraction of PVAc. The numbers in the figure indicate the surface pressure. In Figure 6, the experimental mean areas at the surface pressures of 5, 10, and 15 mN/m are displayed as a function of mole fraction of PVAc, and the mean areas almost lay on the additivity line within experimental error. This additivity of the mean areas and the variation in the surface pressure at the plateau regions, which corresponds to the lower collapse surface pressure, with the molar ratio demonstrate that the two components are ideally mixed in the two-dimensional phase. The ideal mixing between PMA and PVAc may be strongly attributed to the similar chemical structures of both polymers. Thus, the compatibility of the PMA and PVAc mixture at the air/water interface correlates well with that in the bulk state. PVAc and PMMA Mixture. In Figure 7, *-A isotherms of the two pure components and PVAc-PMMA mixtures of five different compositions are displayed. The surface pressure at low PMMA contents continuously increases with a decrease in surface area, while at high PMMA contents the isotherms have a kink around the surface pressure of 16 mN/m, which corresponds to being slightly above the collapse surface pressure of the PMMA film, and then show a steep increase. For the same area, the surface pressure increases with an increase in PVAc components for the entire ranges. In Figure 8, the surface pressures are plotted as a function of PVAc concentration in the mixture. The surface pressure curves indicate clearly the composition dependence of the collapse surface pressure. Except for the higher PMMA contents, ie., below the PMMA/PVAc mole ratio of 0.6310.37, the collapse surface pressure is almost similar to that of PVAc film. Above PMMA/
0
1
2
3
4
rPVAc /mg.m" Figure 8. Surface pressures as a function of PVAc concentration in the mixtures of PVAc and PMMA (e)PVAcIPMMA = 0.8410.16; ( 0 )PVAcIPMMA = 0.10/0.90. The symbols are the same as in Figure 7. The dashed line indicates the concentration dependence of the surface pressure for PMMA.
0 1.o M o l e fraction of PMMA
Figure 9. Mean surface areas in the mixtures of PVAc and PMMA as a function of the mole fraction of PMMA. The numbers in the figure indicate the surface pressure. PVAc mole ratio = 0.81/0.19, on the other hand, there are two collapse surface pressures, and the low collapse point tends to be close to the collapse surface pressure of PMMA film with an increase in PMMA composition. Plots of the mean areas at the surface pressures of 5 , 10, and 13 mN/m as a function of mole fraction of PMMA are shown in Figure 9. Experimental mean areas are located above the additivity line. The positive intermolecular interaction between PVAc and PMMA indicates that they are thermodynamically less stable than PVAc and PMMA films alone at the air/water interface, they
496 Langmuir, Vol. 6, No. 2, 1990
Kawaguchi and Nishida 30
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Figure 10. P A isotherms for mixtures of PEO and PVAc at the air water interface: (m) PEO; (@) PEO PVAc = 0.94/0.06; (e)P O/PVAc = 0.80/0.20; (0)PEO/P Ac = 0.66/0.34; (e) PEO/PVAc = 0.33f0.67; ( 0 )PVAc.
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repel each other at the air/water interface, and they are not easily compatible. This variation in composition is not in agreement with the previous data of Wu and Huntsberger, who reported that the mean areas of the mixtures follow closely the additive line below the surface pressure of 15 mN/m.3 The positive deviation correlates with the experimental finding that blends of PVAc with PMMA are immiscible in the bulk state. However, from the composition dependence of the collapse surface pressure the mixtures of PVAc and PMMA seem to be compatible at the air/water interface in some compositions. In other words, the binary mixture of PVAc and PMMA at the air/water interface for the higher PMMA portion may be miscible since the collapse surface pressure depends on their compositions. PEO and PVAc Mixture. ?r-A isotherms of the two pure components and PEO-PVAc mixtures of four different compositions are displayed in Figure 10. The isotherms of the mixtures have apparent plateau regions around 10 mN/m, which corresponds to the collapse surface pressure of PEO. The plateau surface pressure is almost independent of the composition. Below 10 mN/ m, the surface pressure for the same area increases with an increase in PEO component of the mixture, while above 10 mN/m it increases with decreasing PEO component. The composition dependence of surface pressure is similar to that of the PEO and PMMA mixture. In Figure 11,the surface pressure is plotted versus the concentration of PVAc in the mixture. The surface pressure curves above the surface concentration of 0.8 mg/ m2 PVAc almost fit on the surface pressure-concentration curve of PVAc alone, irrespective of the composition. Therefore, the higher collapse surface pressure of the mixture is independent of the molar ratios. In Figure 12, the experimental mean areas at the surface pressures of 4 and 8 mN/m are displayed as a function of mole fraction of PVAc, and the negative deviations of the mean areas from the additivity line were observed. This indicates that both polymers thermodynamically penetrate each other, and they are miscible at the air/water interface below the surface pressure of 10 mN/m. Above this value, the mixtures should be incompatible since their surface pressures are the same as that for PVAc film. As a result, the existence of the plateau region in the T-A isotherms would support the fact that in this region PVAc film is squeezed out.
/mg.m-'
Figure 11. Surface pressures as a function of PVAc concentration in the mixtures of PEO and PVAc: ( 0 )PEO PVAc = 0.11/0.89. The symbols are the same as in Figure 10. T e dashed line indicates the concentration dependence of the surface pressure for PEO.
b
1.0 0 1.o Mole f r a c t i o n of PVAc Figure 12. Mean surface areas in the mixtures of PEO and
PVAc as a function of the mole fraction of PVAc. The numbers in the figure indicate the surface pressure.
Conclusions Comparison of the compatibility for the four pairs of mixed polymer films among PEO, PVAc, PMA, and PMMA at the air/water interface and that for the corresponding polymer blends in the bulk state leads to the important conclusion that they relatively correlate with each other. Since ellipsometric measurements allow us to conclude that all four polymers examined take a similar orientation a t the air/water i n t e r f a ~ e , ~W " ~A C / PMMA mixtures in their uppermost ranges and PEO/ PVAc mixtures at the higher surface pressure (above 10 mN/m) seem to not always be in agreement with the general idea of Gabrielli et al., that is, the compatibility between polymer chains having the same orientation at the air/water interface. To elucidate the sources of this disagreement, further investigation is required since the compatibility of mixed polymers spread at the air/water interface should be governed by not only the orientation of the polymers at the air/water interface but also by the thermodynamic interaction between the two polymers. Acknowledgment. We are grateful for the encouragement of Professor A. Takahashi of our laboratory. This work was supported in part by the Grant-in-Aid for Scientific Research by the Ministry of Education, Science and Culture (63550664). Registry NO.PEO, 25322-68-3; PMA, 9003-21-8PVAc, 900320-7; PMMA, 9011-14-7.
