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Solvent Vapor Mediated Polymer Adsorption in Thin Films F. Thomas Kiff, Randal W. Richards,† and Richard L. Thompson* Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K. Received September 30, 2003. In Final Form: March 8, 2004 The effectiveness of a “solvent annealing” process was investigated for thin (∼150 nm) polystyrene films, in which the diffusion and reorganization of polymer chains were mediated by the controlled absorption of cyclohexane vapor. Results were compared with conventional “thermal annealing” of films under vacuum above the glass transition temperature. Elastic recoil detection analysis (ERDA) was used to determine the surface excesses of fluorocarbon end-capped polystyrene (hPSF) and poly(styrene-b-dimethylsiloxane) (hPS-PDMS) in deuterated polystyrene (dPS) films. Both annealing methods enabled diffusion of the surface-active polymers; however, only thermal annealing gave rise to a surface excess in hPSF/dPS films. The inhibition of hPSF adsorption under solvent annealing was due to the low surface tension of cyclohexane. In contrast, hPS-PDMS, having a larger surface-active group than that of hPSF, was found in excess at the air surface under solvent annealing, and surface excesses were consistent with the formation of saturated monolayers in blended films. The mixing of hPS-PDMS with dPS was inhibited by the unfavorable interaction between the PDMS block of the copolymer and the homopolymer. The slow interdiffusion of hPS-PDMS in dPS is consistent with the formation of micelles, and the formation of an excess layer at the air surface may be kinetically inhibited by the rate of dissociation of hPS-PDMS micelles.
Introduction The use of end-functional and block copolymers is established as a convenient means to achieve spontaneous organization of materials, with potential applications in fields as diverse as controlled drug delivery1 and photonics.2 The interest in and application for organic thin film devices have generated a need to control the organization of molecules over small length scales of the order of the film thickness and below. In multilayer devices, chemical vapor deposition (a “dry process”) is often used to deliver small molecules to a surface,3 whereas large involatile polymer molecules may require a “wet process”, such as spin-coating4 from solution to deposit layers of material. However, with wet processing, there is a risk that the solvent could corrupt pre-existing layers of a multilayer device. One solution to this problem is to render polymer layers insoluble via cross-linking4,5 after deposition; however, this places some restrictions on the chemistry of the materials that are used. In instances where the chemistry of polymers is not amenable to cross-linking and dry processing is not possible, there is a clear need to understand the impact of solvent exposure on the possible reorganization of polymer layers. Furthermore, there are cases where solvent mediated diffusion and organization in polymer films may be useful and it is equally important to explore the limits of these applications. While this technique is known to be effective in organizing ultrathin block copolymer films,6 and to facilitate the diffusion of small dye molecules7 into polymer * To whom correspondence should be addressed. E-mail:
[email protected]. † Current address: EPSRC, Polaris House, North Star Avenue, Swindon SN2 1ET, U.K. (1) Allen, C.; Maysinger, D.; Eisenberg, A. Colloids Surf., B 1999, 16, 3-27. (2) Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers: Synthetic Stategies, Physical Properties, and Applications; Wiley: Hoboken, New Jersey, 2003. (3) Hung, L. S.; Chen, C. H. Mater. Sci. Eng., R 2002, 39, 143-222. (4) Domercq, B.; Hreha, R. D.; Zhang, Y.-D.; Larribeau, N.; Haddock, J. N.; Schultz, C.; Marder, S. R.; Kippelen, B. Chem. Mater. 2003, 15, 1491-1496. (5) Hreha, R. D.; Zhang, Y.-D.; Domercq, B.; Larribeau, N.; Haddock, J. N.; Kippelen, B.; Marder, S. R. Synthesis 2002, 1201-1212.
films, we are unaware of any application to promote polymer interdiffusion and organization in thicker (>100 nm) films. Interdiffusion over this range is particularly relevant to the function of organic light-emitting devices (OLEDs), since these are typically up to 200 nm in total thickness.3 The adsorption and diffusion of block copolymers and polymers with surface-active end groups in homopolymer matrixes have been the focus of numerous experimental8-15 and theoretical16,17 studies. The surface activity of polymers end-functionalized with fluorocarbon groups is wellknown8-10,12. The equilibrium grafting density of these polymers tends to be quite low, and only a relatively small proportion of the free surface is covered with the end groups. Consequently, the surface modification, as determined by the change in contact angle,10 achieved by the adsorption of end-functionalized polymers in these films is negligible. Many examples of surface-active block copolymers are found in the literature,18-21 and siloxane polymers are (6) Konrad, M.; Knoll, A.; Krausch, G.; Magerle, R. Macromolecules 2000, 33, 5518-5523. (7) Pschenitzka, F.; Sturm, J. C. Appl. Phys. Lett. 2001, 78, 25842586. (8) Hopkinson, I.; Kiff, F. T.; Richards, R. W.; Bucknall, D. G.; Clough, A. S. Polymer 1997, 38, 87-98. (9) Hutchings, L. R.; Richards, R. W.; Thompson, R. L.; Bucknall, D. G.; Clough, A. S. Eur. Phys. J. E 2001, 5, 451-464. (10) Hutchings, L. R.; Richards, R. W.; Thompson, R. L.; Bucknall, D. G. Eur. Phys. J. E 2002, 8, 121-128. (11) Mansfield, T. L.; Iyengar, D. R.; Beaucage, G.; McCarthy, T. J.; Stein, R. S.; Composto, R. J. Macromolecules 1995, 28, 492-499. (12) Elman, J. F.; Johs, B. D.; Long, T. E.; Koberstein, J. T. Macromolecules 1994, 27, 5341-5349. (13) Clarke, C. J.; Jones, R. A. L.; Clough, A. S. Polymer 1996, 37, 3813-3817. (14) Clarke, C. J. Polymer 1996, 37, 4747-4752. (15) Clarke, C. J.; Jones, R. A. L.; Edwards, J. L.; Shull, K. R.; Penfold, J. Macromolecules 1995, 28, 2042-2049. (16) Shull, K. R. J. Chem. Phys. 1991, 94, 5723-5737. (17) Shull, K. R. Macromolecules 1996, 29, 2659-2666. (18) Volkov, I. O.; Gorelova, M. M.; Pertsin, A. J.; Filimonova, L. V.; Torres, M.; Oliveira, C. M. F. J. Appl. Polym. Sci. 1998, 68, 517-522. (19) Schaub, T. F.; Kellogg, G. J.; Mayes, A. M.; Kulasekere, R.; Ankner, J. F.; Kaiser, H. Macromolecules 1996, 29, 3982-3990. (20) Hu, W. C.; Koberstein, J. T.; Lingelser, J. P.; Gallot, Y. Macromolecules 1995, 28, 5209-5214.
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featured particularly prominently in this respect due to their unusually low surface energy.22-24 Lee and Archer21 have reported the surface and interfacial adsorption of poly(styrene-b-dimethylsiloxane) (hPS-PDMS) block copolymers in thick films on the basis of Fourier transform infrared (FTIR) and dynamic contact angle (DCA) measurements. DCA measurements indicated that the exposed surfaces of solvent cast films of these materials blended with polystyrene (hPS) became saturated with poly(dimethylsiloxane) (PDMS) with as little as 2% block copolymer. FTIR measurements revealed that the uppermost 500 nm gradually became enriched in hPSPDMS with annealing at 458 K over a period of several hours. Hu et al.20 reported on the influence of hPS-PDMS on hPS/PDMS interfacial tension. Their results suggested that the hPS/PDMS interface became saturated at low block copolymer concentrations. While there are numerous examples of the adsorption of polymers to surfaces of dilute solutions (see, for example, ref 25), there is comparatively little data on the surface adsorption of polymers from concentrated solutions.26 This may in part be due to the difficulty in characterizing the surface without disrupting its composition.27 The fundamental thermodynamic parameter that characterizes the extent of adsorption of one component to the surface of a mixture of materials is the surface excess. The surface excess of component i (z/i ) is the integral of the adsorbed layer
z/i )
∫0∞(φi(x) - φb) dx
(1)
with respect to depth (x), where φi(x) is the concentration of i and φb is its bulk concentration. While there are many methods available for analyzing the composition of surfaces, few are able to determine the variation in composition with depth necessary to evaluate the integral in eq 1. X-ray photoelectron spectroscopy (XPS) is excellent for revealing the surface composition of materials but cannot probe below depths of a few nanometers. Attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopy can be used to probe composition of the order of 500 nm into a sample21 but gives no information on compositional variation within this range. Secondary-ion mass spectrometry (SIMS) can probe composition versus depth and thus calculate surface excess values; however, erroneous results may be obtained if one component is eroded more rapidly than another. Ion beam analysis and neutron reflectometry are the two techniques of preference for determining z*; neutron reflectometry has the advantage of superior spatial resolution, but ion beam data are comparatively straightforward to analyze, providing a direct map of composition versus depth. We report here the effect of polymer film exposure to solvent vapor on the mobility and organization of constituent molecules. Comparisons will be made to the thermal annealing of films above the glass transition temperature in the absence of any solvent. Elastic recoil detection analysis has been used to determine the surface (21) Lee, H.; Archer, L. A. Macromolecules 2001, 34, 4572-4579. (22) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; Wiley: New York, 1999. (23) Cai, Y. H.; Gardner, D.; Caneba, G. T. J. Adhes. Sci. Technol. 1999, 13, 1017-1027. (24) Kim, D. K.; Lee, S. B.; Doh, K. S. J. Colloid Interface Sci. 1998, 205, 417-422. (25) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (26) Bouchaud, E.; Daoud, M. J. Phys. (Paris) 1987, 48, 1991-2000. (27) Joanny, J. F.; Johner, A.; Vilgis, T. A. Eur. Phys. J. E 2001, 6, 201-209.
Table 1. Molecular Weight Data of Polymers polymer
Mw (g/mol)
Mw/Mn
structure
dPS hPS hPSF hPS-PDMS
71 000 47 000 52 300 57 600-b-6500
1.08 1.05 1.04 1.07
(C8D8)x (C8H8)x (C8H8)x-(CH2)2C6F13 (C8H8)x-(OSi(CH3)2)yCH3
excess of surface-active hydrogenous polymers in a perdeuterated polystyrene matrix. We discuss the efficacy of solvent annealing and thermal annealing for mobilizing and promoting the adsorption of surface-active polymers which are (i) end-functionalized, (ii) block copolymers, or (iii) unfunctionalized. Experimental Section Materials. Fluorocarbon end-functionalized hydrogenous polystyrene (hPSF) and unfunctionalized deuteriopolystyrene (dPS) were synthesized in-house using anionic polymerization processes described elsewhere.8 Poly(styrene-b-dimethylsiloxane) (hPS-PDMS) (sample no. P174-F) was purchased from Polymer Source Inc. and used as received. Unfunctionalized polystyrene (hPS) was supplied by Polymer Laboratories, Church Stretton, U.K., and used as received. Molecular weight data and chemical structures for these polymers are given in Table 1. Sample Preparation. A range of mixtures of each polymer with dPS was made by codissolution with toluene followed by spin-casting onto clean silicon wafers. The films were 150 ( 10 nm thick, and the silicon wafers had a 2.5 nm thick native oxide layer. The oxide layer and blended film thickness was determined by ellipsometry, and film thickness was confirmed via X-ray reflectometry. Bilayer films were produced by spin-coating ∼20 nm of film of hydrogenous polymer from solution onto a silicon wafer and ∼120 nm of dPS film onto a large glass microscope slide. The dPS film was transferred onto the film-coated silicon wafer via flotation on deionized water. The roughness of all films measured by X-ray reflectometry was
