Langmuir 2002, 18, 4471-4477
4471
Synthesis of Chemically Grafted Polystyrene “Brushes” and Their Influence on the Dewetting in Thin Polystyrene Films A. Voronov* and O. Shafranska† Department of Physical Chemistry II, University of Bayreuth, Universitatstrasse 30, D-95440 Bayreuth, Germany Received September 27, 2001. In Final Form: April 2, 2002 End-grafted polystyrene “brushes” of various thickness and grafting density on silica surfaces were synthesized by surface-initiated polymerization. The “brushes” were studied by contact angle measurements, ellipsometry, and scanning force microscopy (SFM). Wetting behavior of thin polystyrene film on the top of the grafted layers was investigated using optical microscopy and SFM. The film stability was strongly dependent on the grafting amount. We identified three grafting density regimes with respect to stability of the top polystyrene films: at small and very large grafting density the top film dewets “brushes”, while at moderate grafting density the polystyrene film was stable after 80 h of annealing time at 155 °C.
Introduction Synthesis of polymer brushes on a solid substrate is an effective way for the control of surface properties that are very important for various technologies, e.g., microelectronics, biomaterials, chromatography.1-3 There are several techniques for polymer brushes preparation including physical adsorption of block copolymers,4 attachment of end-functionalized polymers (grafting to),5 and polymerization initiated from the solid surface (grafting from).6-10 The “grafting from” method is performed by the polymerization initiated by the initiators that are immobilized on the substrate. Self-assembled monolayer approach was described as an effective way to immobilize initiator groups on the surface in one step.11,12 This technique resulted in the synthesis of reproducible grafted polymer layers possessing a high grafting density. However, the presence * To whom correspondence may be addressed. † Permanent address: National University “Lvivska Polytechnika”, Bandera Str. 12, 79053 Lviv, Ukraine. (1) (a) Mansky, P.; Liu, Y.; Huang, E.; Russell, T.; Hawker, C. Science 1997, 275, 1458-1460. (b) Minko, S.; Muller, M.; Usov, D.; Scholl, A.; Froeck, C.; Stamm, M. Phys. Rev. Lett. 2002, 88 (3), 35502-35505. (2) Quali, L.; Franc¸ ois, J.; Pefferkorn, E. J. Colloid. Interface Sci. 1999, 215, 36-42. (3) (a) Brown, H. R.; Deline, V. R.; Green, P. F. Nature 1989, 341, 221-222. (b) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K.-J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289-296. (4) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (5) (a) Tsubokawa, N.; Kuroda, A.; Sone, Y. J. Polym. Sci. Polym. Chem. 1989, 27, 1701-1712. (b) Jones, R. A. L.; Lehnert, R. J.; Schonerr, H.; Vancso, J. Polymer 1999, 40, 525-530. (c) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33, 1043-1048. (6) Boven, G.; Oosterling, M. L. C. M.; Challa, G.; Schouten, A. J. Polymer 1990, 31, 2377-2383. (7) (a) Luzinov, I.; Voronov, A.; Minko, S.; Kraus, R.; Wilke, W.; Zhuk, A. J. Appl. Polym. Sci. 1996, 61, 1101-1110. (b) Luzinov, I.; Minko, S.; Senkovsky, V.; Voronov, A.; Hild, S.; Marti, O.; Wilke, W. Macromolecules 1998, 31, 3945-3952. (8) Minko, S.; Sidorenko, A.; Stamm, M,; Gafijchuk, G.; Senkovsky, V.; Voronov, S. Macromolecules 1999, 32, 4532-4538. (9) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592-601. (10) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813-8820. (11) Tsubokawa, N.; Ishida, H.; Hashimoto, K. J. Macromol. Sci. Pure Appl. Chem. 1995, A32, 525-535. (12) Laible R.; Hamann K. Angew. Macromol. Chem. 1975, 48, 97133.
of various functional groups in the attached initiator could lead to side chemical reactions between the functional groups of the initiator. A new approach was developed for the attachment of the initiator to the surface by adsorption (physical and chemical) of polyperoxide macroinitiator.13-15 Authors reported on the kinetics of the grafting process and have found that the grafting amount reaches saturation and strongly depends on the polymerization rate. The immobilized initiators can be also attached to the surface by chemical methods16,17 via several consequent steps to fix initiator supplied by appropriate functional groups on the substrate. On the first stage, anchoring molecules are linked to the surface and then they react with the functional initiator.18-19 If polymer chains are linked to the substrate by one end, the macromolecules may be stretched from the surface because of excluded volume effect in the dense brush. This stretching may induce the formation of an interface region with a certain nonzero interfacial tension between grafted polymer brushes and coating polymer deposited on the top of the brush. Therefore, the use of the end-functionalized chemically grafted polymer chains can be considered as an alternative approach for regulation of the solid surface wettability and thin polymer film stability. When spin-coating technique is used, thin polymer films can be prepared even on a nonwettable surface. However, the films are intrinsically unstable and dewet the substrate, thereby resulting in droplets.20,21 Great attention has been focused on the wetting/dewetting processes of thin polymer (13) Voronov, S.; Tokarev, V.; Petrovska, G. Heterofunctional Peroxides. Theoretical Basis of the Synthesis and Application; Lviv Polytechnic State University; Lviv, Ukraine, 1994. (14) Minko, S.; Luzinov, I.; Evchuk, I.; Voronov, S. Polymer 1996, 37, 177-181. (15) Voronov, S.; Tokarev, V.; Datsyuk, V.; Seredyuk, V.; Bednarska, O.; Oduola, K.; Adler, H.; Puschke, C.; Pich, A.; Wagenknecht, U. J. Appl. Polym. Sci. 2000, 76, 1228-1239. (16) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 602-613. (17) Boven, G.; Folkersma, R.; Challa, G.; Schouten, A. J. Polym. Commun. 1991, 32, 50-53. (18) Sidorenko, A.; Minko, S.; Schenk-Meuser, K.; Duschner, H.; Stamm, M. Langmuir 1999, 15, 8349-8355. (19) Ulman, A. An Introduction to Ultrathin Organic FilmssFrom Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (20) Srolowitz, D. J.; Safran, S. A. J. Appl. Phys. 1986, 60, 247-254. (21) Sekimoto, K.; Oguma, R.; Kawasaki, K. Ann. Phys. 1987, 176, 359-392.
10.1021/la011489s CCC: $22.00 © 2002 American Chemical Society Published on Web 05/01/2002
4472
Langmuir, Vol. 18, No. 11, 2002
films.22-27 Rupture and dewetting processes of unstable thin polymer films have been studied experimentally. There are different processes in the films that are annealed above the glass temperature. Initially, smooth films become rough and then break up by creating circular holes.28,29 Reiter studied the stability of thin (less than 100 nm thick) polystyrene films on silicon surfaces30 and found that a characteristic distance between the holes increased with the thickness of the film. The growth velocity was found to be constant with time. Results were in a good agreement with theoretical predictions. Silberzan and Leger31 investigated the spreading of poly(dimethylsiloxane) PDMS on silicon surfaces by the optical microscopy and ellipsometry. They found that for polymeric liquids with a molecular weight higher than the critical molecular weight of entanglement, the film density is not uniform. At the edge of the film the density was only about 60% of the bulk density. The profile of the drops was found to be sensitive to the molecular weight. The results were interpreted by Bruinsma32 taking into account slippage of the chains near the solid surface. Redon et al.33 have monitored the growth of the holes of PDMS on silanized silicon. Films of PDMS were found to dewet by nucleation and growth of dry patches. In films with a thickness larger than 10 µm, holes opened with a constant velocity, i.e., the radius of the dry patch grows linearly with time. In thin films the radius depended on time with a power function. Stamm et al.34 studied dewetting of thin films of end-functionalized mono- and difunctional polystyrenes on silicon substrates as a function of the initial film thickness, molecular weight, and functionality of the chains. The authors pointed out that high molecular endfunctionalized chains retarded the dewetting of thin polymer films. As described in the literature, thin films on nonwettable substrates are mostly unstable and metastable. By coating a substrate with an appropriate monolayer, it is possible to change the short-range interactions. However, the coating of a surface with a monolayer may also lead to so-called “wetting autophobicity”; i.e., the liquid does not spread even on a monolayer of the same chemical nature.35 Reiter et al.36 reported about the dewetting of polystyrene films from polymer “brushes” of end-functionalized polystyrene chains. Using optical microscopy, they followed the growth of the holes depending on the grafting density. The thickness was determined by ellipsometry and X-ray reflectometry. The authors suggested that quite long grafted polymer molecules could stabilize the melted polymer flms. Klein et al.37 studied wetting of a polymeric (22) Martin, P.; Buguin, A.; Brochard-Wyart, F. Europhys. Lett. 1994, 28, 421-426. (23) Qu, S.; Clarke, C. J.; Liu, Y.; Rafailovich, M. H.; Sokolov, J.; Phelan, K. C.; Krausch, G. Macromolecules 1997, 30, 3640-3645. (24) Jones, R. A. L. Polymer 1994, 35, 2160-2166. (25) Lambooy, P.; Phelan, K. C.; Krausch, G. Phys. Rev. Lett. 1996, 76, 1110-1113. (26) Stange, T. G.; Evans, D.; Hendrickson, W. A. Langmuir 1997, 13, 4459-4465. (27) Reiter, G. Macromolecules 1994, 27, 3046-3052. (28) Yerushalmi-Rozen, R.; Klein, J.; Fetters, L. W. Science 1994, 263, 793-795. (29) Shull, K. R.; Karis, T. E. Langmuir 1994, 10, 334-339. (30) Reiter, G. Langmuir 1993, 9, 1344-1351. (31) Silberzan, P.; Leger, L. Macromolecules 1992, 25, 1267-1271. (32) Bruinsma, R. Macromolecules 1990, 23, 276-280. (33) Redon, C.; Brzoska, J. B.; Brochard-Wyart. F. Macromolecules 1994, 27, 468-471. (34) Henn, G.; Buckhall, D. G.; Stamm, M.; Vanhoorne, P.; Jerome, R. Macromolecules 1996, 29, 4305-4313. (35) Hare, E. F.; Zisman, W. A. J. Phys. Chem. 1955, 59, 335-340. (36) Reiter, G.; Auroy, P.; Auvray, L. Macromolecules 1996, 29, 21502157.
Voronov and Shafranska
network by a thin film of the melt of the same polymer. The network was formed by cross-linking the polymer layer by ion-beam radiation. The permeability as a function of the cross-linking degree was studied by atomic force microscopy (AFM), light microscopy, and nuclear reaction analysis. The authors found changes from complete wetting through partial wetting and again to complete wetting with an increasing of the cross-linking density. They explained the partial wetting by the autophobicity phenomenon and reentrant wetting by the increased film roughness. In this paper we report about the stability of thin polymer films on the top of poly(styrene) brushes chemically attached to the surface of silica. In contrast to the work reported by Reiter,36 we study dewetting on the brushlike polymer layers of much larger range of grafting densities. The brushes were formed by radical polymerization initiated by the monolayer of an azoinitiator chemically attached to the surface of the silicon wafer. We grafted a large variety of the polymer layers to test the possibility to control the surface wettability. The main goals of the present work were the synthesis of chemically grafted polystyrene brushes of various grafting density and systematic study of the wetting behavior of thin polymer films on the top of the synthesized brushes. Experimental Section Materials. Monomer. Styrene (Merck) was purified with ALOX B chromatographic column and distilled under reduced pressure under nitrogen. Solvents. Toluene and hexane were distilled after drying. Dichloromethane was dried on molecular sieves. Methanol and ethanol were used as received. Initiator. 4,4-Azobis(4-cyanpentanoic acid) (ACP) from Aldrich was used as received. Silicon wafers obtained from the CrysTec GmbH were cleaned with a snow-jet technique. (p-Aminophenyl)trimethoxysilane from ABCR GmbH & Co. (Karlsruhe, Germany) and phosphorus pentachloride (Merck) were used as received. Triethylamine was dried over calcium hydride. The Introduction of the Azo-Initiator onto the Surface of the Si Wafer. Si wafers were treated by (aminophenyl)trimethoxysilane from a 2% solution in toluene overnight.38 Then, the wafers were washed by toluene and ethanol in an ultrasonic bath. Separately, the acid chloride derivative of 4,4′-azobis(4cyanopentanoic acid) was prepared by adding the slurry of phosphorus pentachloride to a suspension of ACP in dichloromethane at 0 °C. After crystallization from hexane-dichloromethane mixture at 0 °C, the product (ACPC) was washed and dried in vacuo. At the next step ACPC was deposited on the surface of the modified Si wafers from a 5% solution in dichloromethane with a catalytic amount of triethylamine at room temperature overnight. The resulting samples of Si wafers with chemically attached initiating groups were rinsed in ethanol in an ultrasonic bath. Every step of the modification of Si wafers was controlled by ellipsometric measurement of the layer thickness. Synthesis of Polystyrene Brushes. Samples of the Si wafers with the chemically attached initiator were placed in a glass flask filled with toluene solution of monomer (styrene) under argon atmosphere. The flasks were immersed in a water bath (55, 60, 67, and 75 °C, (0.1 °C thermostating accuracy) for different periods of time (1-50 h). After the polymerization, the Si wafers were rinsed six times with THF. The ungrafted polymer was removed by Soxhlet extraction with THF for 8 h. For every specimen the molecular weight of polymer extracted from solution was determined using a gel permeation chromatography tech(37) Kerle, T.; Yerushalmi-Rosen, R.; Klein, J. Macromolecules 1998, 31, 422-429. (38) Minko, S.; Usov, D.; Goreshnik, E.; Stamm, M. Macromol. Rapid Commun. 2001, 22, 206-211.
Chemically Grafted Polystyrene Brushes
Langmuir, Vol. 18, No. 11, 2002 4473 Table 1. Characteristics of the Grated Polystyrene Layers distance contact between fraction of angle grafting surface a grafting Si covered by water, time, coverage, density, with PS deg min mg/m2 chain/nm2 point, nm
Figure 1. Time dependence of ellipsometric thickness (dry film) for chemically attached polystyrene brushes (temperature of polymerization: 1, 55 °C; 2, 60 °C; 3, 67 °C; 4, 75 °C).
Figure 2. Time dependence of the calculated fraction of silica surface covered with a polymer for chemically attached polystyrene brushes (temperature of polymerization: 1, 55 °C; 2, 60 °C; 3, 67 °C; 4, 75 °C). nique. All polystyrene coverings after drying in nitrogen were annealed for 5 h at 150 °C. Contact Angle Measurements. Static contact angles of water (Millipore) were determined with a sessile drop method. The measurements were carried out within 1 min after the equilibration of the drop shape. For each sample three to five successive measurements were made. In most cases the deviation did not exceed 2°. Fraction of Si wafers covered by grafted polymer was evaluated by the Cassie-Baxter equation:39,40 cos(θ) ) φ1 cos(θ1) + φ2 cos(θ2), where θ is the experimental contact angle on the surface of grafted polystyrene brush, φ1, θ 1 and φ2, θ2 are parts of area and contact angles corresponding to the bare silica covered with initiator and unporous polystyrene layer, respectively (θ1 ) 58°, θ2 ) 90°). Ellipsometric Measurements. Ellipsometry was used to measure the amount of the initiator attached to the surface as well as the amount of polystyrene grafted to the silicon wafers. The measurements were carried out with a computer-controlled ellipsometer (Sentech) with an angle of incidence fixed at 70°. For the data interpretation, a multilayer model of the coating was assumed. It considers a coating with the following layers: Si wafer with a silica layer on the top, attached initiator and grafted polystyrene layer. Thickness of the SiO2 layer as well as the thickness of the layer of the attached initiator was measured for each sample. Thickness of the grafted polystyrene layers was measured for the dry layers on two to four various places of the (39) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (40) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11-16.
120 420 900 1200 1560 1860 2820
5.1 6.4 7.5 15.8 20.4 24.4 38.9
0.006 0.007 0.008 0.02 0.02 0.03 0.04
t ) 55 °C 15.3 13.5 12.5 8.6 7.6 6.9 5.5
0.04 0.17 0.24 0.26 0.51 1 1
58 64 66 67 75 90 90
45 90 180 300 420 690 900 1020 1260
0.2 0.3 0.8 1.1 1.3 2.6 2.1 8.4 31.5
t ) 60 °C 2.5E-04 70.9 3.8E-04 57.9 0.001 35.5 0.001 31.7 0.002 28.9 0.003 20.1 0.003 22.4 0.01 11.2 0.04 5.8
0.04 0.09 0.15 0.15 0.25 0.49 0.68 0.87 1
59 61 63 63 66 74 80 86 90
240 690 990 1200 1440 1920 2040
3.2 16.8 18.9 29.4 36.8 37.8 59.9
0.004 0.02 0.03 0.04 0.05 0.05 0.08
t ) 67 °C 17.4 7.5 7.1 5.7 5.1 5.0 4.0
0.84 1 1 1 1 1 1
85 90 90 90 90 90 90
120 420 900 1020 1200 1560 1860 2460 2820
10.1 25.5 37.3 37.3 44.3 60.2 63 112.4 119.7
0.02 0.04 0.06 0.06 0.07 0.09 0.1 0.2 0.2
t ) 75 °C 9.1 5.7 4.7 4.7 4.3 3.7 3.6 2.7 2.6
0.97 1 1 1 1 1 1 1 1
85 89 90 90 90 90 90 90 90
a Experimental values of the molecular weights (M ) are as n follows: 394 000 (75 °C), 450 000 (67 °C), 500 000 (60 °C), 556 000 (55 °C).
specimen with an accuracy of about 10%. It was assumed that the density of the polymer layer deposited on the silicon wafers was the same as for the polymer in the bulk, and therefore, we calculated the grafting amount of the polystyrene from the ellipsometric data. Scanning Force Microscopy (SFM) Measurements. The morphology of chemically grafted polystyrene brushes was imaged with SFM operated in Tapping Mode. All experiments were performed on a Digital Instruments Dimension 3100 scanning force microscope using standard silicon cantilevers (NANOSENSORS, Dr. Olaf Wolter, GmbH). SFM data were flattened to improve the image quality. For every sample from two to four different places were imaged. Dewetting Experiments. Polystyrene films (Mw ) 66 000) were spincast from dilute toluene solution onto silicon wafers modified with the polystyrene brushes possessing different thickness. All samples were annealed at 155 °C at the hot stage in nitrogen for a period varying from 30 min to 85 h. Dewetting dynamics was observed using optical microscopy in reflection mode (Zeiss Axio Tech, Germany) fitted with a video camera and a frame grabber. This setup allowed us to follow dewetting processes in real time (growth of holes). Some of the specimens were imaged with scanning force microscopy after the dewetting occurred. Estimation of Surface Coverage Degree. The surface coverage A (mg/m2) was calculated from the ellipsometry and/or scanning probe microscopy thickness of the layer h (nm) by the following equation: A ) hF, where F is the density of polystyrene (1.05 g/cm3). Grafting density δ (number of chains/nm2)
4474
Langmuir, Vol. 18, No. 11, 2002
Voronov and Shafranska
Figure 3. Change of the polymerization rate with time: on the substrate surface (a) and in the volume (b) at 55 °C (1) and 75 °C (2).
Figure 4. Topography images (2 × 2 µm2, gray scale 0-10 nm for panels a and c and 0-20 nm for panel b) of silicon wafers covered with PS grafted at 60 °C showing the characteristic changes for the structure depending on the grafting time (a, 300 min grafting, h ) 0.9 nm; b, 1020 min, h ) 8 nm; c, 1260 min, h ) 30 nm). was determined from the expression: δ ) 10-21hFNa/Mn ) (6.023A*100)/Mn, where Na is Avogadro’s number and Mn is the number average molecular weight of the grafted polystyrene. Finally the distance between grafting sites D (nm) was calculated using the follow equation: D ) (4/δπ)1/2.
Grafting of Polymer Brushes. The grafting of the polymer brushes was performed according to the procedure described elsewhere.38-41 Typical kinetic curves of the grafting process are presented in Figure 1. Figure 2 shows the time dependence of the fraction of silica surface covered with grafted polystyrene. The fraction was calculated from the experimental water contact angles according to the Cassie-Baxter equation. Table 1 presents the characteristics of the polystyrene brushes calculated from the experimental data. The analysis of the experimental data revealed that the substrate surface became completely screened by a grafted polystyrene layer when the thickness of the grafted layer approached 20 nm. This phenomenon was observed for each experimental temperature. Figure 3 shows the variation of polymerization rate (Wp) in the polymerizing medium at the substrate surface as a function of time. The data are normalized by current monomer concentration (Wp/Cm). The sharp increase of the normalized polymerization rate with time can be explained by the Trommsdorf effect and leads to the increase of the rate of
growing of the brush thickness. The same effect was reported earlier.6,7 SFM images of grafted polystyrene layers synthesized at 60 and 75 °C are shown in Figure 4 and Figure 5. A low rate of polymerization and surface coverage was observed at 60 °C. Brush thickness increased slowly and reached a limit after about 22 h. Analysis of SFM data showed that after about 15 h of the grafting the surface was already covered with PS clusters (Figure 4a,b). The contact angle measurements revealed that the layer is well permeable for water and, consequently, the substrate is screened by PS clusters only partially. This type of morphology was observed before7 and is similar to the dimpled structure predicted by theory.42 The theory suggests that further increase of the density should create a homogeneous layer that we, indeed, observed experimentally (Figure 4c). In contrast to these results, at the higher temperature (Figure 5) we observed the formation of the bigger clusters. Due to the high rate of the grafting, the clusters grew immediately after the polymerization was initiated. The surface was very quickly screened by the grafted polymer. It is necessary to note that we observed an increase in the film roughness for the very thick brushes (Figure 6). This fact can be explained by the increase of the polydispertsity of grafted chains and an inhomogeneous growth of the brush in conditions of the pronounced Trommsdorf effect. Thus, we were able to synthesize chemically grafted PS brushlike layers of grafting density ranging from 0.0025
(41) Minko, S.; Stamm, M; Goreshnik, E.; Usov, D.; Sidorenko, A. Polym. Mater. Sci. Eng. 2000, 83, 533-534.
(42) Young, C.; Balazs, A. C.; Jasnow, D. Macromolecules 1993, 26, 1914-1921.
Results and Discussion
Chemically Grafted Polystyrene Brushes
Langmuir, Vol. 18, No. 11, 2002 4475
Figure 5. Topography images (5 × 5 µm2, gray scale 0-20 nm for panels a and b and 0-50 nm for panel c) of silicon wafers covered with PS grafted at 75 °C showing the characteristic changes of the structure depending on the grafting time (a, 120 min grafting, 9.6 nm; b, 1020 min, h ) 35.5 nm; c, 2460 min, h ) 107 nm).
Figure 6. Cross sections (height (nm) × length (µm)) obtained from AFM data for the PS brushes synthesized at 60 °C (a, h ) 8 nm, mean roughness 5.5 nm; b, h ) 30 nm, mean roughness 0.2 nm) and 75 °C (c, h ) 35.5 nm, mean roughness 3.5 nm; d h ) 107 nm, mean roughness 16.5 nm).
to 0.2 chain/nm2, which covers all possible surface concentration regimes from diluted and easily penetrable to densily packed and unpenetrable high tethered brushes. Dewetting Experiments. Figure 7 shows optical micrographs of different stages of dewetting of thin polystyrene films (Mw ) 66 000, thickness ∼30 nm) deposited on the top of the grafted polystyrene layers and annealed at 155 °C for different times. The polystyrene film exhibited a typical pattern of dewetting for the case of ultrathin (0.3-10 nm) brush (Figure 7a-d). At the early phase, nucleation of the circular holes began, and then it was followed by the rapid hole growth and further
coalescence all over the sample surface. Nevertheless, when the brush thickness was increased, the optical measurements indicated that the top polystyrene film remained smooth over the long time period of the thermal treatment. For brushes with a thickness of about 20-35 nm, no dewetting process was observed for about 80 h of the annealing (Figure 7e,f). The polystyrene film remained stable. At the next step, we examined the dewetting behavior on the top of thick polystyrene brushes (>60 nm) and found that stability of the top film dramatically dropped and polystyrene was dewetting with a high rate (Figure 7k,l). The character of dewetting in this case was
4476
Langmuir, Vol. 18, No. 11, 2002
Voronov and Shafranska
Figure 7. Optical micrographs for the dewetting stages of polystyrene film (Mw ) 66 000, h ) 30 nm): a and b, on the 0.8 nm polystyrene brush, annealing time 20 h and 40 h; c and d, on the 2 nm PS brush, 2 and 13 h; e, on the 22 nm PS brush, 80 h; f, on the 35 nm PS brush, 80 h; k and l, on the 60 nm PS brush, 4 and 22 h; m, on the pure Si wafer, 2 h.
close to the process observed for the very thin brushes and finally resulted in the formation of big droplets of polystyrene. With respect to the obtained data several explanations for the observed dewetting behavior may be suggested. A very thin (low grafting density) polymer brush could only slightly modify the influence of nonwettable silica surface. These brushes do not completely cover the surface of silica and allow polymer chains of the top polystyrene to penetrate to the substrate (as annealing proceeds) with further dewetting. The dense brush is nonpermeable for the top polystyrene, and it can dewet from the brush surface similar to the dewetting from the uncovered silicon wafers (Figure 7m). Chains grafted with high density may repel the top polystyrene macromolecules and form a hardly wettable surface. On the basis of theory43 there is a substantial increase in the interfacial tension between the brush and the free polymer as the grafting density increases. It is noteworthy that in both cases the surface consists of areas having different rates of dewetting during annealing, which could be the result of the roughness variation of the grafted polystyrene brush. To explain the stability of polystyrene film on the brushes with a thickness of about 20-35 nm, it should be noted that according to the contact angle and SFM data these thicknesses are close to the brush density when the brush protects (covers) the substrate completely. According (43) Matsen, M.W.; Gardiner, J. M. J. Chem. Phys. 2001, 115, 27942804.
to SFM data these polymer brushes are also very smooth with roughness in the range of 1-2 nm. It seems that in this case the stabilization of the top polystyrene film is caused by the modification of the interfacial tension between the polymeric liquid and substrate by the brushlike structure covering the silica surface. The possible mechanism for the prevention of the dewetting in this case could be the interplay of two separate contributions: the enthalpic one resulting from unfavorable interactions between polystyrene and the silicon substrate and the entropic autophobic dewetting mechanism. The polymer brush in this case has optimum grafting density and is thick enough to shield homopolymer from the substrate but not yet unnecessarily large as to amplify autophobic dewetting. Conclusions Grafting of polystyrene brushes using an initiator chemically attached to the silica surface permitted to attain various grafting densities of the polymer on the surface up to the complete screening of the substrate by the grafted polymer. Investigation of the stability of thin polystyrene films on the chemically grafted polymer layers demonstrated strong influence of the grafting density and brush thickness on the dewetting process. In the case of relatively thick brushes possessing high grafting density, a dewettable interface was created due to the repulsion of the top layer by the densely grafted
Chemically Grafted Polystyrene Brushes
polymer chains. Dewetting also occurred on the brushes with a low thickness because of the penetration of the top polymer material through the layer to the substrate. On the top of the grafted brushes with a thickness corresponding to the polymer monolayer (20-35 nm), no dewetting phenomena were observed during 80 h of annealing. Wetting/dewetting in this case are defined by the interplay of two separate contributions: the enthalpic one resulting from unfavorable interactions between polystyrene and the silicon substrate and the entropic autophobic dewetting mechanism. As the grafting density of the brush is increased, the enthalpic contribution is reduced and the entropic one is amplified. The optimum
Langmuir, Vol. 18, No. 11, 2002 4477
grafting density occurs at an intermediate value sufficiently large that the homopolymer is shielded from the substrate but not so large as to amplify the autophobic dewetting. Acknowledgment. We thank G. Krausch for fruitful discussions. A.V. thanks the Alexander von Humboldt Foundation for the support of this work. The financial support of Deutsche Forschungsgemeinschaft (SP1052) is gratefully acknowledged. LA011489S
