Thin Films Prepared by Vacuum Deposition - American Chemical


Received January 13, 1998. In Final Form: September 6, 1998 ... polymer, on the morphology and wettability of the deposited films were investigated...
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Langmuir 1998, 14, 6980-6986

Surface Morphology and Wetting Behavior of Poly(r-methylstyrene) Thin Films Prepared by Vacuum Deposition Yuh-Lang Lee,*,† Chi-Hau Chen,‡ and Yu-Min Yang‡ Department of Applied Chemistry, Chia Nan College of Pharmacy and Science, Tainan, Taiwan 717, Republic of China, and Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 701, Republic of China Received January 13, 1998. In Final Form: September 6, 1998 Poly(R-methylstyrene) of various molecular weights was vacuum-deposited on different substrates including glass, gold, and silicon. The effects of substrate type, as well as the molecular weight of the polymer, on the morphology and wettability of the deposited films were investigated. The advancing and receding contact angles of water on the surfaces were measured by the method of Wilhelmy plate. Besides, the film surfaces were characterized with scanning electron microscopy and electron spectroscopy for chemical analysis.. The results show that the growth of the polymer on gold tends to be of the StranskiKrastanov mode, in which the growth leads to a smoother geometry. On the other hand, the growth on glass and silicon shows the island mode and results in a rougher surface. The differences in diffusion coefficients and viscosities of the polymers of various molecular weights cause distinct island population and surface structure. The contact angles of water on heterogeneous films were examined, and the influence of fractional coverage was interpreted using a theoretical model.

1. Introduction The vapor deposition process is a well-known technique which has been widely used for preparing inorganic and metallic compounds. Successful techniques such as molecular beam epitaxy, atomic layer epitaxy, and chemical vapor deposition have been developed to control the structures and orientations of inorganic thin films. However, the equivalent technique has not yet been achieved for the preparation of organic thin films. Recently, organic thin films have received more attention because of their specifically notable electrical and chemical functions. In the preparation of the thin organic films, two mainly applied methods are the LangmuirBlodgett (L-B) deposition1,2 and the physical-vapor depositions (PVD).3-6 The L-B deposition is known to be capable of preparing thin molecular films with ordered and highly packed structures. However, the PVD method has also drawn much attention because the fabrications of electronic devices have always been designed in a dry system. In the previous research on vapor-deposited organic films, the main interests were concentrated on linearchain molecules.3-5 Few of those studies were done on the vapor-deposited polymer films.6-8 For the polymer * To whom correspondence should be addressed. † Chia Nan College of Pharmacy and Science. ‡ National Cheng Kung University. (1) Ulman, A. Ultrathin Organic Films; Academic Press: New York, 1991. (2) Roberts, G. Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Baran, J.; Marchewka, M. K.; Ratajczak, H.; Borovikov, A. Y.; Byckov, V. N.; Naumovets, A. G.; Podzelinsky, A. V.; Puchkovskaya, G. A.; Styopkin, V. I. Thin Solid Films 1995, 254, 229. (4) Jones, R.; Tredgold, R. H.; Ali-Adib, Z.; Dawes, A. P. L.; Hodge, P. Thin Solid Films 1991, 200, 375. (5) Ishizaki, K.; Horiuchi, T.; Matsushige, K. Thin Solid Films 1992, 214, 99. (6) Mitsuya, M. Langmuir 1991, 7, 814. (7) Asakura, T.; Toshima, N. Jpn. J. Appl. Phys. 1994, 33, 3558. (8) Miyashita, K.; Kaneko, M. Macromol. Rapid Commun. 1994, 15, 511.

materials, because of their complicated structures and physical properties, the polymer thin films are always prepared by the solution method despite the urgent desire to be compatible with the vacuum process for the production of electronic devices. One of the applications of polymer films is in the wettability modification of the solid surface, which has been used to improve the wettability of silicone lenses9 and to enhance the dropwise condensation.10 Various hydrophobic or hydrophilic surfaces are required for specific demands. To have better control on the wettability, one should resort to the controlling of macroscopic and microscopic structures. Many studies have been devoted to the epitaxial growth of organic films on substrates.3-5 However, geometrical homogeneity of films is also important, especially for practical application in the wettability. In this work, poly(R-methylstyrene) (PMS) was vacuumdeposited onto the surfaces of glass, silicone, and vacuumdeposited gold. The effects of film thickness and substrate type on the morphology of the films were investigated by a scanning electron microscope (SEM). The wettabilities of the surfaces were determined by measuring the advancing and receding contact angels (θa and θr, respectively). The relation between the contact angle and the fractional coverage of the polymer on the surfaces was also discussed. 2. Experimental Methods The optically flat glass plates with dimensions of 24 × 32 × 0.2 mm3 were used as substrates. The glass plates were cleaned ultrasonically in succession with detergent, trichloroethylene, a hydrofluoric acid buffer solution, methanol, and pure water. After this treatment, the measured advancing contact angles of water on the glass surfaces were nearly 0°, revealing the high cleanness and (9) Ho, C. P.; Yasuda, H. J. Biomed. Mater. Res. 1988, 22, 919. (10) Marto, P. J.; Looney, D. J.; Rose, J. W.; Wanniarachchi, A. S. Int. J. Heat Mass Transfer 1986, 29, 1109.

10.1021/la9800607 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/06/1998

Poly(R-methylstyrene) Thin Films

uniformity of the surfaces. The gold substrates were prepared by vacuum deposition of Ti (20 nm) and Au (80 nm) in succession on the pretreated glass plates. The titanium was used as an intermediate layer to enhance the adhesion of gold to the glass surface. After gold deposition, the titanium surface was found to be completely covered by the gold, as indicated by an electron spectroscopy for chemical analysis (ESCA). An n-type, mirror-polished silicone wafer with an orientation of (100) was used as the substrate and was cleaned using the same procedure as that for glass plates. Poly(R-methylstyrene) with different number-average molecular weights (Mn), 685 and 1300, were obtained from Aldrich Chemical Co. and were used to study the effect of chain length. The deposition was proceeded in a small coater of model ULVAC VPC-260 made by Sinku-Kiko Co. The base pressure of the vacuum chamber was controlled at 1 × 10-5 Torr, a measurement made by an ionization gauge. An electrically heated tungsten boat was used as the evaporator. The substrates were kept at room temperature (25 °C), and the deposition rate was controlled at 0.3 nm/s by the temperature-controlled evaporation boat. The deposition rate and the film thickness were monitored by the frequency shifts of a quartz oscillator. The wettability of the surface was expressed in terms of the contact angles of water on that surface. Doubledistilled water, which has a measured surface tension of 72 mN/m, was used in this analysis. For the substrates of glass and gold, deposition was proceeded on both sides so that the contact angles could be measured by the Wilhelmy plate technique. For each experimental condition, three specimens were prepared for the Wilhelmy plate analysis and the mean value of the data was taken as the final result. The chemical elements of the prepared films were analyzed by X-ray photoelectron spectroscopy (XPS). The XPS spectrometer is supplied by VG Scientific Ltd. (mode ESCA-210) with monochromatized Mg KR (1253.6 eV) radiation. Because the PMS film and glass substrate are insulators, an electron flood gun is used to eliminate the accumulated electric charge on the surface. Besides, for the purpose of visualization with a SEM, a gold film of about 10 nm in thickness was deposited on the solid surface. 3. Contact Angle and Hysteresis on Heterogeneous Surfaces For an ideally smooth and homogeneous surface, the contact angle of a liquid upon it should have only one equilibrium value. However, the results are rarely obtained for real solid surfaces. The advancing and receding contact angles are not always identical because of the heterogeneity and roughness of the solid surface.11 A measurement of the equilibrium contact angle, θ, of a drop placed on a horizontal plane is always found to have a value between the advancing and receding contact angles. The difference between θa and θr is called contact angle hysteresis. Because of the hysteresis of the contact angle, a single value of equilibrium contact angle hardly exists, and the advancing and receding contact angles should be measured instead. At the early stage of thin film growth, the substrate was partially covered by the deposited materials and thus the surface was heterogeneous in composition. While (11) Chappuis, J. In Multiphase Science and Technology; Hewitt, G. F., Delgaye, J. M., Zuber, N., Eds.; Hemisphere: New York, 1982; p 387.

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measuring the contact angles, the assignment of the contact line is controlled by the distribution of the two different materials. Thus, the sticking, stretching, and jumping effects occur in the contact line from position to position across the surface to minimize the surface free energy. As a result, various contact angles will be obtained on surfaces of various coverage ratios and composition distributions. This phenomenon has been studied theoretically in the literature.11-14 However, few of the experimental results were found to match those of the theoretical model. In this paper, the Chappuis model11 was used to interpret the dependence of the contact angle on the coverage ratio of surfaces and was amended to give a good simulation of our experimental results. A surface of high surface energy (such as glass, which will be labeled 1 hereafter) was considered to be covered with small patches of low surface energy material (such as polymer, which will be labeled 2). The material 2 is assumed to disperse in very small patches (less than 0.01 mm2) and cover only a small fraction of the substrate. The advancing and receding contact angles were proposed to have the following relations with fractional coverage:

cos θa ) xR cos θ2 + (1 - xR) cos θ1

(1)

θ r = θ1

(2)

where R is the fractional coverage of material 2; θ1 and θ2 are the equilibrium contact angles of liquid on the pure homogeneous surfaces of 1 and 2, respectively. Equation 2 reflects the fact that the receding contact angle is governed by the material of high surface energy and is approximately equal to θ1when the fractional coverage, R, is small. On the other hand, when the fractional coverage of material 2 is high, it corresponds to disperse small patches of high surface energy material (glass) over the material of low surface energy (polymer). The advancing contact angle is then governed by the material of low surface energy, i.e.

θ a = θ2

(3)

and the receding contact angle is expressed as

cos θr ) x1 - R cos θ1 + (1 - x1 - R) cos θ2 (4) Equations 1 and 2 are used when the fractional coverage is small, and on the contrary, eqs 3 and 4 should be used while the fractional coverage is high. These equations will be verified with the experimental data of this work. 4. Results and Discussion To verify that the PMS molecules did not crack during the thermal evaporation process, the deposited polymer on the glass was dissolved with n-hexane and analyzed by a mass spectroscope. The spectra show that the molecular weight distribution of the deposited PMS is nearly identical with that prior to deposition. Effect of Molecular Weight on Surface Morphology. Both PMS molecules of molecular weights 685 and 1300 were deposited on glass substrates, and the surface morphologies of the thin films with different thicknesses are shown in Figures 1 and 2. The sequence of pictures (12) Schwartz, L. W.; Garoff, S. Langmuir 1985, 1, 219. (13) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (14) Schwartz, L. W.; Garoff, S. J. Colloid Interface Sci. 1985, 106, 422.

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Figure 1. Series of SEM micrographs showing the variation of surface morphology of poly(R-methylstyrene) (Mn: 685) grown on glass substrates with various film thicknesses of 10 (a), 100 (b), 150 (c), and 300 nm (d).

Figure 2. A series of SEM micrographs showing the variation of surface morphology of poly(R-methylstyrene) (Mn: 1300) grown on glass substrates with various film thicknesses of 10 (a), 100 (b), 150 (c), and 300 nm (d).

shows the variation of surface morphology with the stage of film formation. Similar results were found for both PMS molecules of different molecular weights. The island growth mode was obtained in the early stage of the film growth. During this stage, the substrate surface was partially covered by polymer islands and the continuous films were formed at a thickness of about 150 nm. However, when more attention was devoted to these pictures, it was found that the PMS with 685 in molecular weight has a smaller island size and higher island density. A polymer with a smaller molecular weight is known to have a smaller viscosity and a higher diffusion coefficient. Thus, the mobility of the adsorbed molecule on the substrate surface is higher for the polymer of molecular weight 685. It has been recognized that the vapor molecules are first adsorbed on the substrate surface in the initial stage of the film formation.15 There are more chances for the ad-molecules of higher surface mobility to join the critical nuclei and thus a higher nucleation rate and island density result. These results are consistent with that obtained by the simulation model proposed by Lee and Maa.16 At the later stage of film formation, the ripplelike morphology shows the fold structure of the film surface for both PMS molecules with different molecular weights. (15) Bassett, G. A.; Montor, J. W.; Pashley, D. W. In Structure and Properties of Thin Films; Neugebauer, C. A., Newkirk, J. B., Vermilyea, D. A., Eds.; John Wiley: New York, 1959; p 11. (16) Lee, Y. L.; Maa, J. R. Int. Commun. Heat Mass Transfer 1991, 18, 479.

However, the folding ripples are thinner and are distributed more densely for the PMS of molecular weight 685. As a consequence, a porous and fiberlike structure was formed on the surface because of the alignment of the folding ripples. This structure was not found for the PMS of molecular weight 1300, which has a rougher ripple. It was also interesting to find small hollows formed on the island of the polymer with molecular weight 685 at a thickness of 100 nm, while the island’s surfaces were only slightly concave and the hollow was not found for the molecular weight 1300. This phenomenon is caused by the liquidlike behavior of the polymer island.17 However, the formation of concave and hollow surfaces is not a spontaneous process, since from the viewpoint of thermodynamics, surfaces tend to minimize the surface energy by reducing the surface area. The surface re-forming is probably induced by the bombardment of molecular beams from the evaporation boat during the deposition process. The surface re-formation inside the islands proceeded more easily for the PMS of smaller molecular weight which has a smaller viscosity. However, for the PMS of higher molecular weight, the higher viscosity causes only slight deformation on the island surfaces. On the basis of the above discussion, it can be expected that polymers with different molecular weights would have different morphologies at the later growing stage. (17) Bassett, G. A. In Condensation and Evaporation of Solid; Gordon and Breach: New York, 1964; p 599.

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Figure 3. Series of SEM micrographs showing the variation of surface morphology of poly(R-methylstyrene) (Mn: 1300) grown on silicon substrates with various film thicknesses of 20 (a), 100 (b), and 150 nm (c). Table 1. ESCA Analysis of Atomic Ratios of Poly(r-methylstyrene) Films for Various Thicknesses and Substratesa PMS (Mn: 1300) on glass

on gold

on silicon

thickness (nm)

C (%)

Si (%)

O (%)

C (%)

O (%)

Au (%)

C (%)

Si (%)

O (%)

0 2 5 10 20 50 100 150 200 300

15

27

58

32

39 6 2 1 0 0

72 33 25 11 0 0 0 0 0 0

53

19 3 1 1 0 0

0 0 0 0 0 1 0 0 0 0

15

42 91 97 98 100 100

28 67 75 89 100 99 100 100 100 100

42 55 100 100 100

38 25 0 0 0

20 20 0 0 0

a

Sensitivity factor: C1s (1), Si2p (0.87), O1s (2.85), Au4f (9.79).

Figure 4. Series of SEM micrographs showing the variation of surface morphology of poly(R-methylstyrene) (Mn: 1300) grown on gold substrates with various film thicknesses of 100 (a), 150 (b), 200 (c), and 300 nm (d).

Effects of Substrates on Surface Morphology. Figure 3 shows the morphologies of PMS (Mn: 1300) films grown on the silicon surfaces. These morphologies are similar to those on the glass substrate. By inspection of the atomic ratios of the surfaces obtained from the analysis of ESCA shown in Table 1, it is found that the carbon compositions on bare silicon and glass surfaces are also similar. The advancing and receding contact angles of water on the silicon were 30° and 18°, respectively, as measured by the method of sessile drop. The small contact angles reveal that the silicon surface has a high surface energy, which is also a property of the glass surface. Nevertheless, the small difference in the contact angles between silicon and glass still leads to evident discrepancy in morphology. At the film thickness of 100 nm, the

polymer islands on silicon do not have the complete shape of the spherical cap found on the glass. The lengthened islands resulting from the coalescence and the nonspherical oblate caps both reflect the higher affinity of the PMS to the silicon than to the glass. The scanning electron micrographs of the PMS (Mn: 1300) deposited on the gold surfaces are shown in Figure 4. The morphologies are quite different from those on the glass and silicon surfaces. Under the resolution of SEM, we were not able to find any polymer islands at a magnification of 10 000. This indicates that either the clusters are too small to be visualized by SEM or the continuous film has been quickly formed. This situation suggests that the growth of PMS on the gold tends to be the mode of layer by layer in the early stage of film growth.

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However, the textured structure at thicknesses of 200 and 300 nm (Figure 4c,d) indicates that the growth is three-dimensional at the later stage. Therefore, the growth behavior of the PMS on gold is similar to the mode of Stranski-Krastanov, which reflects a higher adhesion force between gold and PMS. As a consequence, the film grows more smoothly on the gold than on the glass. The growth of metal films has been classified into three modes according to the binding energy of the molecules deposited to the substrate.18,19 In the island mode [Volmer-Weber (VW) mode], the molecules (or atoms) of the deposit are bound more strongly to each other than to the substrate. The mode of layer-by-layer growth [Frank-van der Merwe (FM) mode] displays the opposite characteristics. In addition, an intermediate mode, the layer plus island [Stranski-Krastanov (SK) mode], occurs when the strain energy of the first layer is large. Further deposition produces an ad-molecular layer on the strained monolayer (or monolayers), from which three-dimensional islands eventually nucleate and grow. The various growth modes can be identified from the variation of XPS signals of the substrate with the film thickness.18,19 The atomic ratios of Si or O shown in Table 1 can be taken to account for the signal of the glass substrate. Both values of the two ratios decrease slowly when the film thickness is increased. The explanation for this is that in the island growth mode the substrate is not completely covered by the deposited PMS in the early growth stage. The signal of the substrate will decrease with an increase of the fractional coverage when the PMS film becomes thicker and will reach zero when the continuous film is formed. In Table 1, the signals of the glass substrate are insignificant at film thicknesses between 100 and 150 nm. This result is consistent with that observed from the SEM micrographs. On the contrary, the situation is quite different for the gold substrate. The XPS single of the substrate (atomic ratio of Au) decreases rapidly at small thickness (2 nm) and slowly at the later stage (5 and 10 nm) and reaches zero at a thickness as small as 20 nm. The variation tendency of the substrate signal is a typical result for the SK growth mode. The rapid decrease at small thickness is a result of the formation of the monolayer, and the slow rate at the later stage is caused by the formation of islands. From Table 1, the sampling depth of XPS in PMS is about 15 nm for Au. The sampling depth of XPS is related to the mean free path for inelastic scattering, λ, of photoelectrons and is defined by 3λ sin θ, where θ is the takeoff angle of the electrons relative to the surface.20,21 In this work, Mg KR (1253.6 eV) radiation is utilized and the value of θ is 45°. The λ value is about 7.0 nm when the sampling depth is taken to be 15 nm. The values of λ reported in the literature are different for various polymers and organic materials. The similar values of λ determined for an electron energy of 967 eV by Cadman et al. were 3.9 nm in polyimide and 6.2 nm in polystyrene.22 However, to obtain a more reliable value of λ, repeated experiments are required to improve the precision in the measurement of film thickness and XPS. An alternative method for the proof of the SK growth mode is the thickness dependence of contact angles.6 In (18) Venables, J. A.; Spiller, G. D. T.; Hanbucken, M. Rep. Prog. Phys. 1984, 47, 399. (19) Bauer, E.; Poppa, H. Thin Solid Films 1972, 12, 167. (20) Ratner, B. D.; Castner D. G. In Surface AnalysissThe Principal Techniques; John Wiley: New York, 1997; p 43. (21) Briggs, D. In Practical Surface Analysis; John Wiley: Chichester, U.K., 1990; Vol. 1, p 437. (22) Cadman, P.; Gossedge, G.; Scott, J. D. J. Electron Spectrosc. 1978, 13, 1.

Lee et al.

Figure 5. Receding contact angle of water on PMS film as a function of film thickness for substrates of gold and glass.

the SK mode, the substrate will be quickly covered by a continuous monolayer (or monolayers). Therefore, when the film thickness is increased, the contact angle of water on the film surface will rapidly approach that on a homogeneous deposited film. The dependence of the receding contact angle on the thickness is shown in Figure 5 for substrates of glass and gold. As will be discussed in the next section, the value of θr is approximately equal to that of the substrates when the fractional coverage of PMS is small. In Figure 5, the slow increase of the value of θr on the glass substrate shows the typical island growth mode in which the fractional coverage of PMS on glass is small at film thicknesses smaller than 100 nm. On the contrary, for the substrate of gold, the value of θr increases abruptly at a thickness of 2 nm and remains constant when the thickness is further increased to 5 nm. This result also indicates that a monolayer has been formed at a thickness of 2 nm. There are many possible factors that cause the strain of monolayers in the SK growth mode. In this work, the great distinction in lattice parameters and interfacial properties between the metallic substrate and the deposited polymer is expected to be the main factor. In Figure 5, the value of θr on the gold surface decreases for thicknesses in the range of 10-100 nm. A similar result was also found by Mitsuya,6 who studied the hexatriacontane films on polar substrate. This phenomenon can be explained by either the increasing surface area of the film due to the island growth or the disordering of the molecular array resulting in the exposure of various functional groups.6 The increase and variation of θr for thicknesses larger than 150 nm can be attributed to the effect of surface roughness,23 but the true reason is presently not clear. The strong affinity of the gold substrate with the PMS can be attributed to the higher adsorption ability of gold to the carbonaceous compounds. The advancing and receding contact angles of water on the deposited gold substrate are 90° and 48°, respectively, as measured in the atmosphere. The composition of gold detected from the bare gold substrate, as shown in Table 1, is only 72% and the composition of carbon is as high as 28%. Both the contact angle and the composition results reveal that the gold surface has been contaminated with exposure to the atmosphere. In the literature, many efforts have been devoted to clarification of whether the gold surface is (23) Dettre, R. H.; Johnson, R. E. In Contact AnglesWettability and Adhesion; American Chemical Society: Washington, DC, 1964; p 136.

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Table 2. Advancing and Receding Contact Angles of Water on Poly(r-methylstyrene) Films for Various Thicknesses, Substrates, and Molecular Weights PMS (Mn: 1300) on PMS (Mn: 685) on glass thickness (nm) 0 2 5 10 20 50 100 150 200 300

θa 0

78 81 94 97 100 102

θr

glass θa

gold

θr

θa

θr

0

0

0

29 24 17 61 77 72

72 73 74 94 95 98 99 102

11 15 14 18 17 67 77 72

90 100 101 101 100 100 101 99 100 101

48 75 75 68 71 68 63 72 80 79

hydrophilic or hydrophobic.24 The conclusion which has been widely accepted is that a contaminant-free pure gold surface has a hydrophilic nature. However, when the gold surface is exposed to the atmosphere, a thin layer of organic compounds would immediately adsorb on the gold surface and thus the surface becomes hydrophobic. Effects of Film Thickness on Contact Angles. The advancing and receding contact angles of water on the PMS thin films of various thicknesses and substrates are listed in Table 2. Because of the island growth mode of PMS on the glass substrate, the glass surface is partially covered in the early growth stage and the values of θa or θr vary with increase of the film thickness. The advancing contact angle increases steadily with an increase of the fractional coverage when the film thickness is small and approaches to a constant when the fractional coverage becomes significant. This phenomenon is consistent with that described by eqs 1 and 3. From the model proposed by Chappius,11 the receding contact angles are approximately equal to the contact angle on the glass substrate when the fractional coverage of PMS is small. The reason for this was that when the test plate was receded from the liquid during the wettability analysis of the Wilhelmy plate, the three-phase contact line will progress by following the distribution of glass across the plate. As a consequence, the contact angle obtained is almost the same as that on the glass surface. This phenomenon has also been reflected by eq 2. However, in reality the deposition process would not have behaved like that described in the model. In fact, the receding contact angles have slightly higher values (about 18°) than those on the bare glass surface (0°). On the substrate of glass, as the film thickness grows to 150 nm, the values of θa or θr are nearly constant. This indicates that a continuous PMS film has been formed. This inference can be supported both by the investigation on the surface morphologies that are shown in Figures 1 and 2 and the atomic ratios of the surfaces shown in Table 1. For the PMS molecules of different molecular weights, the contact angles are not identical at the small film thickness. This is caused by the differences in island distribution and fractional coverage between the two PMS molecules. However, when the continuous film is approached, their contact angles are nearly identical despite the distinction in the morphologies as shown in Figures 1d and 2d. The limiting values of θa and θr on the continuous PMS films deposited on glass are about 100° and 75°, respectively, for both molecular weights. These values also resemble those obtained on smooth PMS films grown on the gold. This result is suspicious since it is (24) Smith, T. J. Colloid Interface Sci. 1980, 75, 51.

Figure 6. Fractional coverage of poly(R-methylstyrene) (Mn: 1300) on glass surfaces under various thicknesses.

recognized that the apparent contact angles are affected by the surface roughness. According to the Wenzel’s relation,25 a rougher surface leads to more area for the contact between the liquid and solid surfaces during the measurement of contact angles. As a result, the apparent contact angle will vary with variation of the roughness. However, the Wenzel’s relation is applied under the assumption that the liquid should penetrate deeply into the troughs of a rough surface and contact completely with the surface. Because of the hydrophobic property of PMS, it is difficult for the water to penetrate into the porous structure, especially when the roughness is significant and the pore is small. In such a situation, only the outer position of the porous film is contacted by the water, which is just like what happens on a smooth film, and so the apparent result resembles that on a smoother surface. This result also indicates that the difference in the surface morphology cannot be reflected completely by the values of the contact angles. For the application of eq 2, the critical value of the fractional coverage was not defined in the Chappuis model except for the limitation of small fractional coverage. According to the data shown in Table 2, the θr increases sharply when the thickness goes above 100 nm on the glass, which has a fractional coverage of 0.6. The fractional coverages of the PMS islands (Mn: 1300) on the glass surfaces, shown in Figure 6, were obtained from the SEM pictures. In contrast to eq 2, eq 4 should be applied when the fractional coverage of PMS is large. In this situation, the advancing contact angle will approach that on PMS. According to the data shown in Table 2, θa approaches a constant value at a film thickness of 50 nm (θa ) 94°), which has a fractional coverage of 0.4. This critical value of the fractional coverage for the application of eq 4 is consistent with that for eq 2. For the purpose of comparing the experimental data of the contact angles with the predicted values obtained from the Chappuis model, both the experimental data and the values obtained from eqs 1 and 4 are listed in Figure 7. The values of θ1 and θ2 were taken from the equilibrium contact angles of water on glass (5°) and PMS (90°), respectively. Obvious discrepancies were found between the theoretical curves and the experimental data. The discrepancies may be attributed to two factors. The first is the use of equilibrium values of θ1 and θ2. Since these (25) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.

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of glass, and they were both taken as 5° for the calculation. The values of θ2a and θ2r are 100° and 75°, respectively, which are obtained from the homogeneous PMS surfaces. By comparison with the experimental data, the results calculated from eqs 5-8 show a better simulation to the experimental data at a β value of 2. These results are also exhibited in Figure 7. 5. Conclusions

Figure 7. Advancing and receding contact angles of water on poly(R-methylstyrene) (Mn: 1300) grown on glass substrates: a comparison between the experimental and theoretical data. A β value of 2 is used in the calculation of the theoretical values.

equations were used to estimate the advancing and receding contact angles on chemically heterogeneous surfaces, it is more reasonable to use the advancing (or receding) contact angles on the two homogeneous surfaces instead of the equilibrium values. The second is the relatively small values of the predicted θa which indicate that the value of xR used in eqs 1 and 4 was underestimated. Therefore, a weighting parameter, β, is needed to correlate the two equations. Equations 1-4 were thus amended in compliance with the two factors. When the fractional coverage is smaller than 0.5, the following equations were applied.

cos θa ) xβR cos θ2a + (1 - xβR) cos θ1a

(5)

θr = θ1r

(6)

The present results have shown that the surface morphologies of vacuum-deposited PMS films are influenced by the nature of the substrate and the molecular weight of the deposited material. The smaller contact angles of water on glass and silicon reveal the hydrophilic nature of the substrates, which leads to the island growth mode in the early growth stage of PMS film and thus a rougher surface. On the gold surface, because of its high affinity to carbonaceous compound, the surface is found to be hydrophobic when it is exposed to the atmosphere. This character of the gold surface causes the growth mode of layer plus island to occur for the deposition of PMS on gold, and thus, a smoother surface of PMS results. As for the effect of molecular weight, the higher diffusion coefficient and lower viscosity of the polymer with a smaller molecular weight result in a higher island density and smaller island size and consequently, a porous and fiberlike structure was formed on the surface. For a chemically heterogeneous surface comprising a deposited material of low surface energy (PMS) and a substrate of high surface energy (glass), the receding contact angles are approximately equal to that on the glass when the fractional coverage of PMS is below 0.6. On the other hand, the advancing contact angles will approach that on a homogeneous PMS surface when the fractional coverage of PMS is over 0.4. The advancing and receding contact angles on the heterogeneous surface can be accurately predicted by the amendment of the existing theoretical model.

If R is greater than 0.5, then

θa = θ2a

(7)

cos θr ) xβ(1 - R) cos θ1r + (1 - xβ(1 - R)) cos θ2r (8) θ1a and θ1r are the advancing and receding contact angles

Acknowledgment. The support of this research by the National Science Council of the Republic of China through Grant No. NSC 86-2214-E-041-005 is gratefully acknowledged. Professor Jer-Ru Maa and Chien-Hsiang Chang are also acknowledged for their helpful discussions. LA9800607