Langmuir 2002, 18, 9985-9989
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Role of End-Group Functionality on the Surface Segregation Properties of HBPs in Blends with Polystyrene: Application of HBPs as Dewetting Inhibitors Joshua A. Orlicki and Jeffrey S. Moore* Department of Chemistry, The Beckman Institute for Advanced Science and Engineering, University of Illinois, Urbana, Illinois 61801
Ibrahim Sendijarevic and Anthony J. McHugh* Department of Chemical Engineering, University of Illinois, Urbana, Illinois 61801 Received July 22, 2002. In Final Form: September 12, 2002 The surface properties of polyetherimide (PEI) hyperbranched polymers (HBPs) are tunable over a broad range of surface energies by proper functionalization of their end-groups. In turn, our data indicate that the surface segregation of PEI HBPs in blends with polystyrene (PS) is primarily determined by the differences in their surface energies. Therefore, HBPs with higher surface energies than that of PS segregate near the substrates, while HBPs with lower surface energies tend to concentrate near the air interface of thin films. The stability (wettability) of low molecular weight PS thin films on a silicon substrate was improved by the addition of high surface energy PEI HBPs.
Introduction Thin polymer films are important in many commercial applications, such as dielectric coatings, electronic packaging, lithographic resist layers, and lubricating surfaces. All these applications require stable and homogeneous films, which are increasingly more difficult to produce as the thickness requirements for films decrease. When they are heated above the glass transition temperature, these films tend to spontaneously dewet the substrates and coalesce into droplets. The dewetting process has recently been studied theoretically1 and experimentally;2 however, the end-goal may be either to develop strategies to suppress it or to use it as a guide in making mesoscale patterns for various applications in opto- and microelectronics. Various approaches have been utilized to stabilize thin polymer films. For example, the use of end-functionalized chains which form a polymer “brush” adsorbed on the substrate stabilizes thin films.3 It has also been reported that surface roughness on the substrate inhibits dewetting, as do various techniques used to create entanglements in films.3,4 In a recent study, Barnes et al.5 used fullerenes (C60 or “buckeyballs”) to inhibit the dewetting of polystyrene on a silicon substrate. They show that fullerenes concentrate near the silicon substrate, creating a diffuse layer which minimizes the interfacial energy between the (1) (a) Sharma, S.; Khanna, K. Phys Rev. Lett. 1998, 81, 3486. (b) Konnur, R.; Kargupta, K.; Sharma, A. Phys. Rev. Lett. 2000, 84, 931. (c) Kargupta, K.; Sharma, A. Phys. Rev. Lett. 2001, 86, 4536. (d) Brochard-Wyart, F.; De Gennes, P. G.; Hervert, H.; Redon, C. Langmuir 1994, 10, 1566. (2) (a) Herminghaus, S.; Jacobs, K.; Mecke, K.; Bischof, J.; Fery, A.; Ibn-Elhaj, M.; Schlagowski, S. Science 1998, 282, 913. (b) Seeman, R.; Heminghaus, S.; Jacobs, K. Phys. Rev. Lett. 2001, 86, 5534. (c) Redon, C.; Brzoska, J. B.; Brochard-Wyart, F. Macromolecules 1994, 27, 468. (3) (a) Henn, G.; Bucknall, D. G.; Stamm, M.; Vanhoorne, P.; Je´roˆme, R. Macromolecules 1996, 29, 4305. (b) Yerushalmi-Rozen, R.; Klein, J.; Fetters, L. J. Science 1994, 263, 793. (4) Tolan, M.; Vacca, G.; Wang, J.; Sinha, S. K.; Li, Z.; Rafailovich, M. H.; Sokolov, J.; Gibaud, A.; Lorenz, H.; Kotthaus, J. P. Phys. B 1996, 221, 53. (5) (a) Barnes, K. A.; Karim, A.; Douglas J. F.; Nakatani, A. I.; Gruell, H.; Amis, E. J. Macromolecules 2000, 33, 4177. (b) Barnes, K. A.; Douglas J. F.; Liu, D.; Karim, A. Adv. Colloid Interface Sci. 2001, 94, 83.
polymer and the substrate. Similarly, Mackay and coworkers6 showed that perfect dendrimers inhibit dewetting of PS thin films. Herein we report on the study of hyperbranched polymers (HBPs), functionalized with various end-groups, in blends with linear polystyrene (PS). First, the endgroup composition of HBPs is correlated to the changes in surface energy of HBPs, that consequently affect the surface segregation properties of HBP/PS blends. The surface segregation properties of HBP/PS blends are correlated to the stability (wettability) of thin PS films on silicon wafers. Materials The syntheses and surface characterization study of the polyetherimide (PEI) hyperbranched polymers (HBPs), with various end-groups, are reported elsewhere.7,8 The surface properties of PEI HBPs as a function of the end-group compositions are presented in Table 1. Advancing contact angle data were obtained with deionized H2O and CH2I2, and the surface energy was obtained by the method outlined by Owens.9 Polystyrene (PS) and Ultem (poly(bisphenol-A-co-4-nitrophthalicanhydride-co-1,3-phenylenediamine)) were both purchased from Aldrich and used as-received, as were all solvents. PS (Mw ) 280 000 g/mol) was used for the surface segregation studies and contact angle measurements in blends with PEI HBPs. Its surface properties are also listed in Table 1. For the dewetting studies we utilized a low molecular weight PS (Mw ) 3700 g/mol). Ultem used in the dewetting studies has the reported values Mw ) 30 000 g/mol and Mn ) 12 000 g/mol.
Experiments Blend Preparation. Polymer stock solutions were prepared by dissolving the PS, Ultem, and PEI HBPs in chloroform (CHCl3) at a concentration of 13.7 mg/mL. For dewetting experiments (6) Mackay, M. E.; Hong, Y.; Jeong, M.; Hong, S.; Russell, T. P.; Hawker, C. J.; Vestberg, R.; Douglas, J. F. Langmuir 2002, 18, 1877. (7) Orlicki, J. A.; Thompson, J. L.; Markoski, L. J.; Sill, K. N.; Moore, J. S. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 936. (8) Orlicki, J. A.; Viernes, N. O. L.; Moore, J. S.; Sendijarevic, I.; McHugh, A. J. Langmuir 2002, 25, 9990. (9) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
10.1021/la020662x CCC: $22.00 © 2002 American Chemical Society Published on Web 11/09/2002
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Table 1. Surface Properties of PEI HBPs with Indicated End-Groups and PS (280 000 g/mol) polymer
end-group
H2Oa
CH2I2a
γb
I II III IV V VI VII VIII IX X XI XII XIII PS
OH O(CH2CH2O)3Me di-C1 mono- C1 phenyl mono-C8 CF3 di-C8 TBS OCH2(CF2)3CF3 mono-C18 di-C18 OCH2(CF2)7CF3
76 77 84 86 86 92 92 97 97 95 99 110 113 91
38 40 27 28 23 46 55 54 55 65 65 63 89 37
45 44 46 46 47 37 33 32 32 27 27 27 14 42
a Advancing contact angle data. b Surface energies (γ) obtained by the method outlined by Owens.9
Figure 1. Advancing contact angle data with deionized H2O, measured on thin spin-coated films of pure PEI HBPs and blends of PEI HBPs with PS (280 000 g/mol). Films annealed at 240 °C for 24 h. appropriate volumes of the stock solutions were combined to provide solutions containing 99 wt % PS and 1 wt % HBP or Ultem. For contact angle measurements stock solutions were mixed to produce solutions containing 90 wt % PS and 10 wt % HBP. Before spin-coating onto silicon wafers, solutions were filtered through a 0.45-µm syringe filter. Silicon wafers (100) were etched with a piranha solution (70/ 30 volume ratio of concentrated sulfuric acid (70 wt %) and hydrogen peroxide (30 wt %) solutions) for 2 h at 70 °C. The silicon wafers were then washed with water and dried with a nitrogen stream. 70 µL of the appropriate polymer solution was spin-coated onto treated silicon wafers at 4000 rpm for 60 s, followed by an immediate annealing in vacuo at 150 °C. To avoid undesired effects associated with heating and cooling of one sample, different samples were used for each annealing time.6 Optical Microscopy. Thin film topologies were captured with optical microscopy, employing a Zeiss Axiovert 135 inverted optical microscope with incident halogen lamp illumination. Images were captured at 10× zoom with a CCD camera and analyzed with the ImageJ software. Contact Angle Measurements. Advancing contact angle measurements were performed on the surfaces of thin film blends with deionized water (γL ) 72 mN/m, γLD ) 22 mN/m) using a Rame-Hart goniometer model 100-00.
Results Surface Segregation. The contact angle data with water for thin films of PEI HBPs and their blends with PS (Mw ) 280 000 g/mol) are plotted versus the end-group
composition of PEI HBPs in Figure 1. The data in Figure 1 and Table 1 show a strong dependence of the surface properties of PEI HBPs on the chemical composition of their end-groups. However, the changes in the contact angle values of thin film blends (10 wt % HBPs, 90 wt % PS) do not correlate with the changes in the contact angle values of PEI HBPs. Instead, for the thin film blends containing the HBPs (I-V) with higher surface energies (or equivalently lower water contact angles) than that of PS, the contact angle values remain nearly equal to that of PS, insinuating the absence of HBPs at the air interface. Conversely, in the thin film blends for which the surface energy of PS exceeds that of HBPs (VI-XIII), the contact angle values at the air interface correlate with the changes in the contact angle value of HBPs, suggesting high concentrations of HBPs at the air interface. These data, therefore, indicate that the surface segregation of HBPs in blends with linear polymers is primarily driven by the differences in their surface energies. This observation is corroborated with prior studies, which show that preferential segregation of one component to the surfaces of blends is predominantly driven by an associated reduction in system enthalpy.10 In accordance with our results, lower energy polymers are expected to reside at the air interface, while higher surface energy polymers concentrate near the substrate. Since the surface segregation trends in HBP/PS blends appear to be primarily driven by the enthalpic effects, it can be assumed that the effects of configurational entropy on the surface segregation properties of polymeric blends are negligible. As far as we are aware, these are the first results which show that the surface segregation properties of HBP/linear polymer blends depend on the end-group composition of HBPs. Therefore, by functionalizing with low surface energy end-groups, HBPs can be tailored-made to preferentially segregate in blends with commodity polymers at the air interfaces of thin films. As a result, low energy HBPs could be utilized as surface modifiers for a number of commercial polymers. Conversely, HBPs capped with high surface energy end-groups most likely segregate near the silicon substrate in thin film polymer blends. In the next section, we will show that this surface behavior of HBPs in blends can be utilized to stabilize thin PS films on silicon substrates. Dewetting. Figure 2 shows optical micrograph images of ∼60 nm thick films of pure PS (3700 g/mol) at different dewetting stages. Nonannealed spin-coated films of PS appear uniform and continuous. Annealing at 150 °C (∼50° above Tg) results in a dewetting pattern which starts with nucleation of holes at early stages, followed by hole growth and finally coalescence. At the late stages of dewetting, a familiar Voronoi pattern of polygons is formed, with its edges composed of coalescent droplets.11 This type of dewetting has been characterized for low Mw PS on silicon wafers of various surface energies.12 Barnes et al.5 showed that the addition of small fractions of C60 fullerene nanoparticles (as low as 1 wt %) stabilized thin PS films. Their data suggest that fullerene particles segregate near the substrate and modify the substratepolymer interaction. Similarly, Mackay et al.6 showed that perfect dendrimers stabilize low Mw PS thin films. (10) (a) Hariharan, A.; Kumar, S. K.; Russel, T. P. J. Chem. Phys. 1993, 98, 4163. (b) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M.; Sokolov, J.; Schwarz, S. A. Phys. Rev. Lett. 1989, 62, 280. (c) Anastasiadis, S. H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1990, 92, 5677. (11) Feng, Y.; Karim, A.; Weiss, R. A.; Douglas, J. F.; Han, C. C. Macromolecules 1988, 21, 484. (12) (a) Reiter, G. Langmuir 1993, 9, 1344. (b) Sharma, A.; Reiter, G. J. Colloid Interface Sci. 1996, 178, 383.
Application of HBPs as Dewetting Inhibitors
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Figure 2. Micrographs of thin PS films on silicon wafers, annealed at 150 °C in vacuo for indicated times.
Figure 3. Micrographs of thin PS films with 1 wt % indicated PEI HBPs on silicon wafers, annealed at 150 °C in vacuo for indicated times.
Consistent with their observation, we find that PEI HBPs suppress dewetting of PS (3700 g/mol) thin films. As micrographs in Figure 3 demonstrate, the addition of 1
wt % PEI HBPs capped with phenyl (V), di-C1 (III), or ethylene glycol (II) end-groups suppresses the dewetting of low molecular weight PS thin films. As previously
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Figure 4. Micrographs of thin PS films with 1 wt % indicated PEI HBPs on silicon wafers, annealed at 150 °C in vacuo for indicated times.
Figure 5. Micrographs of thin PS films with 1 wt % Ultem on silicon wafers, annealed at 150 °C in vacuo for indicated times.
discussed, PEI HBPs with the indicated end-groups have higher surface energies than that of PS and therefore are expected to segregate near the silicon substrate. This results in a diffuse layer of PEI HBPs near the silicon substrate which is expected to lower the interfacial free energy between the substrate and the polymer, improving the overall stability of thin PS films.
On the contrary, micrographs in Figure 4 show PS thin films with 1 wt % PEI HBPs capped with di-C8 (VIII), di-C18 (XII), or perfluoro (X) end-groups, which dewet similarly to pure PS thin films (Figure 2). As the data in Figure 1 show, these HBPs have lower surface energy than that of PS and therefore are expected to preferentially migrate near the air interface in thin film blends with PS.
Application of HBPs as Dewetting Inhibitors
Therefore, for these blends the interfacial free energy at the silicon substrate should remain the same as that in pure PS films, resulting in similar dewetting patterns to those observed for pure PS thin films (Figure 2). Moreover, the addition of low energy PEI HBPs speeds up the dewetting of PS thin films, with fully developed Voroni patterns appearing at less than 1 h of annealing at 150 °C. A possible explanation for enhanced dewetting is that low surface energy PEI HBPs increase the overall surface energy difference between the substrate and polymer and therefore the driving force of thin films to coalesce. Our data indicate a general trend that higher surface energy additives, which preferentially segregate near a substrate, improve the film stability of low molecular weight PS films. Therefore, in addition to nanoparticles, such as fullerene, dendrimers, and HBPs, other high surface energy additives should effectively stabilize thin polymeric films. In Figure 5 we present micrographs of PS thin films with 1 wt % Ultem. Ultem is a linear polymer that is chemically similar to PEI HBP (IV); as a result, their surface energies should be similar (∼47 mN/m).8 Since Ultem has a higher surface energy than PS, it is expected to concentrate near the substrate and stabilize thin PS films. As anticipated, the micrographs in Figure 5 show that the addition of 1 wt % Ultem inhibits the dewetting of PS thin films, annealed at 150 °C for up to 16 h.
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Conclusions Our data indicate that the surface segregation of PEI HBPs in blends with PS correlates with the changes in the surface energies of PEI HBPs, which depend on their end-group composition. As a result, the PEI HBPs with lower surface energy than that of PS concentrate at the air interface of thin film blends and therefore affect the surface properties of PS films. On the other hand, the PEI HBPs with higher surface energies than that of PS concentrate near the silicon substrate. This creates a diffuse layer of high energy HBPs near the substrate, that reduces the interfacial energy at the silicon substrate, stabilizing low molecular weight PS thin films. Our experiments also indicate that linear polymer (Ultem), having a higher surface energy than that of PS, also concentrates near the silicon substrate and inhibits the dewetting of low molecular weight PS thin films. Acknowledgment. This work has been supported under a grant from the U.S. Army Research Office under contract/grant number DAAG55-97-0126. The authors acknowledge L. J. Markoski (U of I) for helpful discussions. We would also like to thank Professor V. K. Gupta and his group (U of I) for the use of their goniometer. LA020662X
