pubs.acs.org/Langmuir © 2010 American Chemical Society
Nonsolvent Annealing Polymer Films with Ionic Liquids Lian R. Hutchings,† Craig J. R. Douglas,† Catherine L. Rhodes,† W. Douglas Carswell,† Maximilian W. A. Skoda,‡ John R. P. Webster,‡ and Richard L. Thompson*,† †
Department of Chemistry, Durham University, Science Site, Durham DH1 3LE, U.K., and STFC ISIS facility, Rutherford Appleton Laboratories, Chilton, Didcot OX11 0QX, U.K.
‡
Received February 8, 2010. Revised Manuscript Received August 27, 2010 Neutron reflectometry has been used to determine the interface structure and swelling of thin polymer films, when annealed in contact with a series of 1-alkyl-3-methylimidazolium ionic liquids (ILs). By choosing immiscible polymer/IL combinations, we have established that thin polymer films can be annealed for several hours in contact with ILs at temperatures well above the glass transition temperature and that this nonsolvent annealing environment can be exploited to direct self-assembly in polymer films. The ingress of IL into polymer films was quantified in terms of the swelling up to 10%. The polymer/IL interfacial width generally also increased from 0.9 nm up to ∼3 nm, but there was remarkably little correlation between interfacial width and swelling. For one combination of polymer and IL (deuterated PMMA and Bmim-BF4) the interfacial width decreased slightly with increasing temperature, consistent with LCST behavior for this system. All of the ILs tested had a profound influence the distribution of carboxy-endfunctionalized deuterated polystyrene, “dPS-COOH”, in blended films with polystyrene homopolymers. The ILs promoted dPS-COOH adsorption at the film/IL interface and the simultaneous rapid desorption at the film silicon-oxide interface. The rate of desorption was found to correlate with the swelling behavior of the polymer with respect to the IL anion species: PF6- < Br- < Cl- < BF4-, suggesting that the polymer films are plasticized by the IL as it penetrates the film.
Introduction Over the past decades, there has been an ongoing effort to replace volatile organic compounds (VOCs) from formulations and industrial processes.1 VOCs are problematic in that the vapor pressure is not sufficiently high to enable rapid extraction, as is the case for supercritical fluids,2 but it is high enough to ensure that significant quantities of often harmful vapor are released over prolonged periods. While supercritical fluids have shown considerable promise as solvents for many small molecule systems, they are relatively poor solvents for most polymers with a few notable exceptions such as highly fluorinated polymers, silicones, and some polyethers, for which the cohesive energy density is low.3 At the opposite extreme, ionic liquids (ILs) circumvent the problem of evaporation due to their extremely low vapor pressure4 and are often excellent solvents for polymers. Some ILs can even be used as solvents for notoriously insoluble materials such as cellulose.5 Despite showing many promising and quite unique properties, ILs have yet to be accepted as mainstream replacements for VOCs, partly due to their cost, but also because of their toxicity.6 Therefore, it is most likely that ILs will largely be utilized in relatively small volume applications for the foreseeable future. It is also the case that while many ILs are solvents for many polymers, they are certainly not universally good solvents for all polymers. Attempts to rationalize polymer/IL *Corresponding author: tel þ44 191 3342139; fax þ44 191 3844737; e-mail
[email protected]. (1) Cody, R. D. Prog. Org. Coat. 1993, 22, 107. (2) Woods, H. M.; Silva, M.; Nouvel, C.; Shakesheff, K. M.; Howdle, S. M. Eng. Films 2004, 14, 1663. (3) O’Neill, M. L.; Cao, Q.; Fang, R.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37, 3067. (4) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831. (5) Zhang, H.; Wu, J.; Zhang, J.; He, J. S. Macromolecules 2005, 38, 8272. (6) Latala, A.; Nedzi, M.; Stepnowski, P. Green Chem. 2010, 12, 60.
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behavior in terms of polarity, dielectric constant, and more recently by Lewis acidity/basicity have been moderately successful, but while it is possible to rationalize experimental results for series of polymer/IL mixtures, understanding of these complex mixtures is still not sufficient make accurate a priori predictions of the miscibility of polymer IL mixtures.7-9 Although there have been some very encouraging studies on polymer/IL mixtures in which the IL is the minor component (e.g., as plasticizers10,11 or high-temperature fuel cell membranes12), the literature on polymer/IL mixtures remains dominated by synthetic studies where the IL has been used as a solvent for monomers and catalyst for polymerization.9,13-15 For this reason, the main focus of attention has been single phase systems in which polymers are the minor component in IL solutions. While insoluble polymer/IL mixtures are commonly observed and the tendency to form gels is now well established,7-9 the nature of the phase boundary and particularly the polymer-rich side of the phase boundary have received comparatively little attention. The possibility of exploiting ILs to direct the self-assembly of block copolymers has recently been demonstrated by Virgili et al.,16 who observed that ILs can act as selective solvents for polystyrene-b-vinylpyridine copolymers and that temperature (7) Lu, J. M.; Yan, F.; Texter, J. Prog. Polym. Sci. 2009, 34, 431. (8) Ueki, T.; Watanabe, M. Macromolecules 2008, 41, 3739. (9) Winterton, N. J. Mater. Chem. 2006, 16, 4281. (10) Scott, M. P.; Rahman, M.; Brazel, C. S. Eur. Polym. J. 2003, 39, 1947. (11) Scott, M. P.; Brazel, C. S.; Benton, M. G.; Mays, J. W.; Holbrey, J. D.; Rogers, R. D. Chem. Commun. 2002, 1370. (12) Sekhon, S. S.; Park, J. S.; Cho, E.; Yoon, Y. G.; Kim, C. S.; Lee, W. Y. Macromolecules 2009, 42, 2054. (13) Kobryanskii, V. M.; Arnautov, S. A. Makromol. Chem., Macromol. Chem. Phys. 1992, 193, 455. (14) Kubisa, P. Prog. Polym. Sci. 2004, 29, 3. (15) Zhang, H.; Hong, K.; Mays, J. W. In Ionic Liquids in Polymer Systems; Brazel, C. S., Rogers, R. D., Eds.; American Chemical Society: Washington, DC, 2005. (16) Virgili, J. M.; Hexemer, A.; Pople, J. A.; Balsara, N. P.; Segalman, R. A. Macromolecules 2009, 42, 4604.
Published on Web 09/09/2010
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Article Table 1. Nomenclature, Scattering Length Density, and Molecular Weight for Polymers
Figure 1. Sketch of 1,3-alkylmethylimidazolium IL structures.
and composition changes can dramatically alter the microphase behavior. Unlike most neutral polymer/organic solvent mixtures, increasing temperature in polymer/IL mixtures does not necessarily increase miscibility. LCST behavior of polymer/IL mixtures is now a well-established phenomenon.17-19 The ability to fine-tune polymer/IL interactions via temperature20 and IL structure8 or by employing mixtures of ILs19 opens up some fascinating possibilities for controlling polymer selforganization. There is considerable interest in arranging block copolymers into vertical columns for their potential in nanoscale lithography, and control of surface and interface interactions has been shown to be key to this process. Russell et al.21 first demonstrated this possibility by using random copolymers to create the neutral surfaces and interfaces required for this kind of self-assembly above and below the block copolymer film. More recently, similar self-organization has been achieved by solvent vapor annealing.22 Clearly a facile route to control self-assembly that requires neither volatile solvents nor ion etching to reveal the block copolymer microstructure is highly desirable. Here, we report what is to our knowledge the first direct measurement of interface properties in polymer/IL mixtures. Second, we demonstrate the use of ILs (as a nonsolvents) to manipulate the distribution of polymers within thin blended films. By better understanding the structure-property relationship between ILs and insoluble polymers, we can direct the surface segregation of binary polymer blends, with potential applications for nanoscale patterned surfaces or solution processed conducting polymer devices.
Experimental Section Materials. Benzene (HPLC grade, Aldrich) and perdeuterated styrene (Cambridge Isotopes) were purified by stirring over calcium hydride (Aldrich, Reagent grade 90-95%) and subjected to a number of freeze-pump-thaw cycles. sec-Butyllithium (1.4 M in cyclohexane, Aldrich), tetramethylethyenediamine (TMEDA, >99.5%, Aldrich), and carbon dioxide gas (>99.8%, Aldrich) were all used as received. All polymers were prepared by living anionic polymerization using standard high vacuum. Carboxy endfunctionalized perdeuterated PS was made by the polymerization of styrene-d8, with sec-butyllithium as the initiator and benzene as the polymerization solvent. The resulting polystyryllithium was endfunctionalized by the addition of high-purity carbon dioxide in the presence of (TMEDA) followed by acetic acid, thereby introducing a terminal carboxylic acid functionality.23 The carboxy-functionalized perdeuterated polystyrene was recovered by precipitation into (17) Ueki, T.; Karino, T.; Kobayashi, Y.; Shibayama, M.; Watanabe, M. J. Phys. Chem. B 2007, 111, 4750. (18) Ueki, T.; Watanabe, M. Langmuir 2007, 23, 988. (19) Kodama, K.; Nanashima, H.; Ueki, T.; Kokubo, H.; Watanabe, M. Langmuir 2009, 25, 3820. (20) Ueki, T.; Watanabe, M. Chem. Lett. 2006, 35, 964. (21) Huang, E.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 1998, 31, 7641. (22) Peng, J.; Xuan, Y.; Wang, H. F.; Yang, Y. M.; Li, B. Y.; Han, Y. C. J. Chem. Phys. 2004, 120, 11163. (23) Hseih, H. L.; Quirk, R. P. Anionic Polymerization, Principles and Practical Applications; Marcel Dekker: New York, 1996.
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polymer
structure
dPS-COOH hPS dPS dPMMA
(C8D8)m-COOH (C8H8)n (C8D8)x (C5D8O2)y
F/A˚-2 10-6 Mw/kg mol-1 Mw/Mn 6.47 1.41 6.47 6.98
82.5 102.5 1500 405.0
1.04 1.03 1.5 1.1
Table 2. Nomenclature, Structure, and Scattering Length Data for Dialkylimidazolium Salt ILs IL
structure
purity/%
F/A˚-2 10-6
Bmim-PF6 Bmim-Br Bmim-Cl Bmim-BF4 Hmim-BF4 Omim-BF4 Omim-Cl
C4H9-C4H6N2þPF6C4H9-C4H6N2þBrC4H9-C4H6N2þClC4H9-C4H6N2þBF4C6H13-C4H6N2þBF4C8H17-C4H6N2þBF4C8H17-C4H6N2þCl-
>96.0 >98.5 >98.0 >97.0 >97.0 >97.0 >97.0
1.53 0.74 0.98 1.3-6.4 10-3i 1.12-5.7 10-3i 0.97-5.1 10-3i 0.58
methanol, and the precipitated polymer was filtered and dried in vacuo to constant weight. Perdeuterated polystyrene (dPS) and poly(methyl methacrylate) (dPMMA) homopolymers were supplied by Polymer Source Inc. (Montreal, Canada) and used as received. Hydrogenous (“normal”) polystyrene, hPS, was prepared in-house using standard living anionic polymerization under high vacuum. ILs were purchased from Fluka and used as received. The chemical structures of the ILs are shown in Figure 1. The molecular weight and neutron scattering length data for the polymers are summarized in Table 1, and the structure, quoted purity (by HPLC), and calculated scattering length density data for the ILs are given in Table 2. Because of the ability of boron to absorb neutrons, the scattering length density of tetrafluoroborate ILs incorporates an imaginary component. Using a similar notation to that adopted elsewhere,11,24-29 the ILs are denoted “Rmim-X”, where “R” is either B (butyl), H (hexyl), or O (octyl), “mim” is methylimidazolium, and “X” is the anion. Polymer Film Preparation. Polymer blend films were prepared by codissolving the polymers in the required proportions in a toluene solution, followed by spin-coating at ∼2000 rpm. The spin-cast films were then annealed at 140 °C for 1 h. This step is necessary to ensure the stability of the films with respect to annealing under liquids.30,31 The annealing procedure eliminates residual solvent and stresses that may be present in spin-cast films. In the case of blends containing carboxy-end-functional polymers, annealing allows the functional polymers to adsorb to the silica surface of the substrate and confers an additional degree of stability to the film. Without first annealing polymer films under vacuum, annealing under ILs almost invariably caused the film to dewet or float off the substrate. For ion beam analysis experiments, 15% (w/w) dPS-COOH/ hPS blended films on silicon wafers were annealed in contact with ILs by depositing a small quantity (∼0.05 g) of IL on the film surface and covering with a glass microscope slide. The polarity of (24) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C.; Heenan, R. K. Langmuir 2004, 20, 2191. (25) Goodchild, I.; Collier, L.; Millar, S. L.; Prokes, I.; Lord, J. C. D.; Butts, C. P.; Bowers, J.; Webster, J. R. P.; Heenan, R. K. J. Colloid Interface Sci. 2007, 307, 455. (26) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S.; Brennecke, J. F. J. Chem. Eng. Data 2004, 49, 954. (27) Zhang, H. W.; Hong, K. L.; Jablonsky, M.; Mays, J. W. Chem. Commun. 2003, 1356. (28) De Roche, J.; Gordon, C. M.; Imrie, C. T.; Ingram, M. D.; Kennedy, A. R.; Lo Celso, F.; Triolo, A. Chem. Mater. 2003, 15, 3089. (29) Triolo, A.; Russina, O.; Arrighi, V.; Juranyi, F.; Janssen, S.; Gordon, C. M. J. Chem. Phys. 2003, 119, 8549. (30) Narrainen, A. P.; Clarke, N.; Eggleston, S. M.; Hutchings, L. R.; Thompson, R. L. Soft Matter 2006, 2, 981. (31) Thompson, R. L.; Hardman, S. J.; Hutchings, L. R.; Narrainen, A. P.; Dalgliesh, R. M. Langmuir 2009, 25, 3184.
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Figure 2. Sketch of sample setup for neutron reflectometry experiments. Note that the thickness of the polymer and IL films have been exaggerated for clarity. the carboxy end functional group of dPS-COOH causes this polymer to adsorb to the interface of blends and polar solids32,33 or liquids30 such as silicon oxide or glycerol. This material is therefore a useful probe of the apparent polarity of polymer-IL interfaces and may indicate the ability of different ILs to control self-organization in polymer blends. Using this technique, it was possible to observe that the entire film surface was covered with a very small volume of IL. Following annealing, the films were quenched to 50 °C on a warm hot-plate before the glass slide and residual IL were removed. At this temperature the polymer film is at least 50 °C below its glass transition temperature, while the IL is sufficiently fluid to enable the glass slide to be slid off the polymer film without damaging it. The film was rinsed with high-purity deionized water at 55-60 °C to remove any residual IL from the surface. Samples for neutron reflectometry experiments were prepared in a similar manner to the blended films that were analyzed by ion beams. In this case, the films were of pure perdeuterated homopolymers (dPS, dPMMA), which were spin-cast onto the polished face of 55 mm diameter 5 mm thick silicon substrates. Perdeuterated polymers were necessary to achieve optimal contrast for neutron reflectometry, and high molecular weight polymers were chosen to maximize the stability of the cast films. The coated substrates were annealed under vacuum as before to maximize their stability with respect to annealing in contact with ILs. Neutron Reflection. Neutron reflectometry (NR) is practically the only technique that can determine the in situ swelling and interfacial properties of polymer films in contact with solvent vapor or condensed phases.34-36 NR experiments were carried out on the SURF time-of-flight reflectometer37 at the ISIS pulsed neutron source, Rutherford Appleton Laboratories, Chilton, UK. Specular reflectivity was measured at three grazing angles of incidence (θ = 0.3°, 0.8°, and 1.5°) with an incident neutron beam having a wavelength range of 0.55 < λ/A˚ < 6.8. Data acquisition required ∼90 min per sample. After correction for the wavelength-dependent transmission through the silicon block, the data for each range were combined to obtain complete reflectivity profiles, R(Q), where Q (= 4π/λ sin θ), is the scattering vector. Using three angles of incidence ensured that R(Q) was measured from before the critical edge (Qcritical ∼ 0.012 A˚-1) to the background (Q ∼ 0.15 A˚-1). The combined data were normalized by scaling to unit reflectivity in the critical region, Q < Qcritical. Samples were placed in contact with the ILs using specially designed fluid cells and allowed to equilibrate above the glass transition temperature for at least 1 h (during (32) Clarke, C. J.; Jones, R. A. L.; Clough, A. S. Polymer 1996, 37, 3813. (33) Elman, J. F.; Johs, B. D.; Long, T. E.; Koberstein, J. T. Macromolecules 1994, 27, 5341. (34) Perlich, J.; Korstgens, V.; Metwalli, E.; Schulz, L.; Georgii, R.; MullerBuschbaum, P. Macromolecules 2009, 42, 337. (35) Shin, K.; Rafailovich, M. H.; Sokolov, J.; Gersappe, D.; Kim, M. W.; Satija, S. K.; Nguyen, D.; Xu, D.; Yang, N. L.; Eisenberg, A. Langmuir 2001, 17, 6675. (36) Koga, T.; Seo, Y. S.; Zhang, Y. M.; Shin, K.; Kusano, K.; Nishikawa, K.; Rafailovich, M. H.; Sokolov, J. C.; Chu, B.; Peiffer, D.; Occhiogrosso, R.; Satija, S. K. Phys. Rev. Lett. 2002, 89. (37) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899.
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Figure 3. Influence of Bmim-Br on neutron reflectivity of dPS films at various temperatures. Data have been shifted with respect to the dPS/air data by successive orders of magnitude. The solid curves are fits to the data, and the dotted line is a guide to the eye. which the samples were aligned) before commencing any measurement. The film-coated silicon blocks were inverted and placed onto another silicon block, which had been etched to hold a small (0.1 g) quantity of IL. Drainage holes and an etched channel allow any excess of IL to drain off, thus ensuring that the IL layer is not thick enough to contribute significant uncertainty to the alignment. In this geometry the neutron beam passes through the polymer-coated silicon block and is reflected at the interfaces between the silicon, silicon dioxide, polymer, and IL, without traversing the relatively “neutron opaque” IL. A simple model was used to analyze the reflectivity in which the thickness, t, and scattering length density of the polymer film were varied to determine the extent of swelling of the polymer film by the IL in contact. The only other variable was the interfacial width, σ, between the polymer film and the IL. The volume fraction profile of the polymer, φp, is described by an error function 1 z-t 1 - erf ð1Þ φp ¼ 2 σ where z is the distance from the silicon dioxide/polymer interface. The location of the polymer/IL interface is given by z = t. Data were fit successfully using this model from the critical region to the background, and examples of the fits obtained are shown in Figures 3-5. Nuclear Reaction Analysis. Following annealing in contact with ILs for controlled times and temperatures, the depth distribution of the perdeuterated carboxy-functionalized polystyrene, dPSCOOH, was determined by nuclear reaction analysis. Each measurement therefore provides a “snapshot” of the influence of annealing under an IL as a function of time. The depth distribution of the perdeuterated functional polymers in the blended films was measured directly by nuclear reaction analysis38,39 using a 5SDH Pelletron accelerator to deliver a 2.4 mm diameter beam of 0.7 MeV 3 Heþ to the sample surface at 83° to the sample normal (7° grazing incidence). Protons generated by the D(3He,p)R nuclear reaction were detected at 170° to the incident beam direction using a 1.5 mm thick PIPS detector. The total beam charge was 5 μC, which was sufficient to obtain sufficient statistical quality data without artifacts due to beam damage in polystyrene samples.40 This technique (38) Payne, R. S.; Clough, A. S.; Murphy, P.; Mills, P. J. Nucl. Instrum. Methods Phys. Res., Sect. B 1989, 42, 130. (39) Tesmer, J. R.; Nastasi, M.; Barbour, J. C.; Maggiore, C. J.; Mayer, J. W. Handbook of Modern Ion Beam Materials Analysis; Materials Research Society: Pittsburgh, PA, 1995. (40) Thompson, R. L.; McDonald, M. T.; Lenthall, J. T.; Hutchings, L. R. Macromolecules 2005, 38, 4339.
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Article Table 3. Solubility of Deuterated Homopolymers in ILs polymer
IL
soluble?
dPS
Bmim-PF6 Bmim-BF4 Bmim-Cl Bmim-Br Hmim-BF4 Omim-BF4 Omim-Cl dPMMA Bmim-PF6 Bmim-BF4 Bmim-Cl Bmim-Br Hmim-BF4 Omim-BF4 Omim-Cl a Polymer becomes translucent but does not dissolve.
Figure 4. Influence of Bmim-BF4 on neutron reflectivity of dPMMA films at various temperatures. Data have been shifted relative to dPMMA/air data by successive orders of magnitude. The solid curves are fits to the data, and the dotted line is a guide to the eye. Note that the high background for the 140 °C data set arises due to loss of beam for this sample but does not affect the region of fitting.
Figure 5. Influence of Bmim-Br on neutron reflectivity profiles for dPS and dPMMA. The data for dPMMA have been shifted by 1 order of magnitude for clarity. The solid curves are fits to the data. and the procedure for converting raw data to volume fraction versus depth are reviewed in detail elsewhere.41,42
Results Miscibility of PMMA and PS with ILs. Sealed glass vials were prepared containing ∼15% (w/w) of homopolymer dPMMA or dPS and in each of the ILs. Each mixture remained in two phases at room temperature and upon heating to 70 °C. After heating for 48 h at 155 °C, some of the polymer/IL combinations became miscible. The solubility of each polymer/ IL combination is summarized in Table 3. Although dPMMA was soluble in some of the ILs, the dPS was highly insoluble in all of the ILs used here, consistent with the earlier work of Snedden et al.43 using various alkylimidazolium ILs. After cooling to room temperature, all of the erstwhile soluble dPMMA/IL mixtures formed gels. (41) Geoghegan, M. In Polymer Surfaces and Interfaces III; Richards, R. W., Peace, S. K., Eds.; John Wiley & Sons Ltd.: Chichester, 1999; p 43. (42) Composto, R. J.; Walters, R. M.; Genzer, J. Mater. Sci. Eng. R 2002, R38, 107. (43) Snedden, P.; Cooper, A. I.; Scott, K.; Winterton, N. Macromolecules 2003, 36, 4549.
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n n n n n n n y n na n na y y
Stability of Polymer Films under ILs. Polystyrene films were stable with respect to annealing under all ILs for at least 30 min at 150 °C. However, while polystyrene films annealed for longer periods at 150 °C in contact with Omim-Cl, Bmim-Cl, Bmim-Br, or Bmim-PF6 remained intact, the ILs with a BF4anion all caused the PS films to dewet. PMMA films could withstand annealing at 150 °C under Bmim-BF4 and Bmim-Br but were unstable with respect to prolonged annealing under any of the other ILs. Swelling of PMMA and PS Films with ILs. Typical neutron reflectivity data for dPS- and dPMMA-coated silicon blocks are shown in Figures 3-5. Since the scattering length density of silicon (2.07 10-6 /A˚-2) and the ILs ((1.0 ( 0.5) 10-6 /A˚-2) are both much lower than the perdeuterated polymers used in this study, this configuration ensures that incident neutrons can be reflected at both the substrate/polymer interface and the polymer/IL interface. The interference of neutrons reflected at these interfaces gives rise to regular oscillations in R(Q) (Kiessig fringes) from which the precise film thickness may be determined. Because the Kiessig fringes are well-resolved for all of the films, this setup is extremely sensitive to swelling in the polymer films arising from ingress of IL. The fitting routine calculates the variation in χ2 with respect to each fitted parameter, and for our reflectivity data the uncertainty in film thickness was typically 0.5%, which corresponds to an uncertainty of (0.5 nm. Sample data and simulations for best fit (0.5 nm are provided in the Supporting Information (SI-1). It is apparent that annealing in contact with an IL tends to reduce the separation of the Kiessig fringes slightly (identified by the dotted line between the fifth and sixth fringes in Figures 3 and 4), from which it is evident that in some cases the films do become slightly swollen by ingress of IL. The percentage film swelling is defined as % swelling ¼ 100
tswollen - tdry tdry
ð2Þ
where tdry and tswollen are the thickness of the polymer film before and after annealing in contact with an IL. Interfacial Width of Polymer Films Annealed in Contact with ILs. As well as measuring film thickness, from which swelling may be determined, NR is uniquely sensitive to the interfacial width of the immersed films. As the interfacial width increases, the Kiessig fringes become more damped with increasing Q, and R(Q) declines more steeply. This behavior is illustrated in Figure 5 for dPMMA and dPS annealed in contact with BmimBr. Initially, both films were found to have an rms interfacial width (with respect to air) of 0.9 ( 0.3 nm. Clearly, the IL has a much greater effect on R(Q) for the dPS film than for the DOI: 10.1021/la102933g
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others have used previously.32,46-48 The model profile comprises a thin surface layer that may be enriched in dPS-COOH, a thicker middle layer in which the concentration of dPS-COOH is fixed and a lower layer which accounts for the excess of dPS-COOH at the substrate interface. This slablike profile was convolved with the instrumental resolution for the NRA measurement, which was 6 nm at the film surface and 14 nm at the substrate interface and was assumed to vary linearly at intermediate depths. The surface (or substrate) excess concentration, z*, is simply given by z ¼ hðφs - φm Þ
ð3Þ
where φs and φm are the volume fraction of dPS-COOH in the surface (or substrate) and middle layers, respectively, and h is the thickness of enriched layer. Figure 6. Composition versus depth profiles for dPS-COOH in PS after annealing under Bmim-PF6 for 30 min at 120 or 150 °C. The fitted curves are for a three-layer model (10 nm enriched surface layer, ∼130 nm bulk layer, and 10 nm enriched substrate layer) convolved with the instrumental resolution.
dPMMA film, for which the interface is virtually unaffected by the IL. From the fits to the data it appears that the statistical uncertainty in the interface width of the swollen films is less than or equal to 10% of σ. Influence of ILs on Segregation in Blended Films. It is well established that COOH-functionalized polystyrenes adsorb at relatively high energy surfaces such as silicon oxide,33 and Clarke et al.32,44 used self-consistent-field equations to estimate the surface energy of adsorption for dPS-COOH on to silicon from the interfacial excess, obtaining a value of 8.6 ( 0.5 kBT. The surface (or interfacial) excess is defined by the product of the volume of the adsorbing species and the number of molecules per unit area at the surface in excess of the bulk concentration. During spin-casting and annealing under vacuum, this leads to a substantial interface excess at the substrate, but no surface excess at the air surface. Previously, it has been well-established that carboxyfunctionalized polymers have no affinity for exterior surfaces of polymer films unless annealing is carried out under a polar nonsolvent.30,45 The depth distribution profile of dPS-COOH in Figure 6 confirms that the polymer films were ∼150 nm thick, consistent with independent ellipsometry results, and that the large excess anticipated at the substrate interface of the blended film is indeed present, even after annealing for 30 min at 120 °C under an IL. Moreover, some excess of dPS-COOH at the film/IL interface is also present. Further annealing at a higher temperature causes the dPS-COOH excess at the IL interface to grow while the excess of at the substrate interface diminishes rapidly. This behavior appears to be quite generic, and similar results were also obtained for carboxy-functionalized perdeuterated PMMA, “dPMMA-COOH”, in PMMA films, although the greater susceptibility of this polymer to ion beam damage restricted the quality of data that could be obtained. Composition versus depth profiles for dPMMA-COOH/PMMA blend films are included as Supporting Information (SI-2). The solid curve fitted to the data in Figure 6 was obtained by assuming a simple slablike composition profile which we and (44) Clarke, C. J. Macromolecular Chemistry: physical aspects, conference 2008. (45) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2942. (46) Ansari, I. A.; Clarke, N.; Hutchings, L. R.; Pillay-Narrainen, A.; Terry, A. E.; Thompson, R. L.; Webster, J. R. P. Langmuir 2007, 23, 4405. (47) Hutchings, L. R.; Richards, R. W.; Thompson, R. L.; Bucknall, D. G.; Clough, A. S. Eur. Phys. J. E 2001, 5, 451. (48) Clarke, C. J. Polymer 1996, 37, 4747.
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Discussion Thermal Stability of Materials. There is invariably some concern that ILs, especially those containing fluorine (and water, which is commonly present as an impurity9), may decompose at elevated temperatures, although studies in which decomposition has been observed have typically been carried out at much higher temperatures than we have used.4 We have monitored the thermal stability of the fluorine-containing ILs (Bmim-PF6 and BmimBF4) by thermogravimetric analysis and mass spectroscopy. Our results showed negligible mass loss at 155 °C for either of these materials, which was consistent with earlier work by Scott et al. in which the temperature stability of Bmim-PF6 included in PMMA was found to compare favorably with a traditional plasticizer, dioctyl phthalate.10 The mass spectrum indicated traces of volatiles at 18, 28, and 32 amu attributed to water, nitrogen, and oxygen outgassing from the sample, respectively. Crucially, no evidence for generation of HF (20 amu) as a decomposition product was found. The TGA-MS results are available as Supporting Information, SI-3. Miscibility of Polymer/IL Mixtures. For our intended application of controlling self-assembly within polymer films, it is essential that annealing in contact with an IL does not simply dissolve the film. The phase behavior of IL/polymer mixtures is therefore important to establish over the relevant temperature range. In general, the temperature dependence of polymer/IL miscibility reflects the complexity of these liquids, with both LCST17-19 and UCST20 behavior being reported for different mixtures. Watanabe first established UCST behavior for the combination of poly(N-isopropylacrylamide) and the hydrophobic IL ethylMIM bis(trifluoromethane sulfone)imide with the phase boundary increasing with increasing polymer molecular weight and concentration. They also reported that the bis(trifluoromethane sulfone)imide anions (a weak Lewis base) preferentially interacts with PMMA, for which it is a good solvent, suggesting that the imidazolium cations are peripheral to their solvent properties for this polymer. Of the polymer/IL mixtures that we have explored, dPS was highly insoluble in any of the ILs, whereas the solubility dPMMA was relatively high. This is consistent with established behavior for PMMA and polymers bearing aromatic phenyl rings.8,17,18 Within our limited set of data, summarized in Table 3, it is apparent that for both chlorides and tetrafluoroborates there is a systematic increase in PMMA solubility with increasing alkyl chain length of the imidazolium cation. The one exception to this rule was the dPMMA/Bmim-PF6 mixture, which despite having a relatively short (butyl) chain was miscible as has previously been reported by Mays et al.15 We must therefore conclude that while the anions may be the primary factor in determining PMMA Langmuir 2010, 26(19), 15486–15493
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solubility, one cannot ignore the influence of the imidazolium cation structure. Only the polymer/IL combinations that appeared to be completely immiscible were stable as thin films annealed in contact with ILs. While PMMA did not dissolve in Hmim-BF4 or BmimCl, it was observed that the polymer did become translucent upon prolonged heating in these ILs, indicating some uptake of IL into the polymers. Films of dPMMA were not stable when annealed in contact with either of these ILs. The relative instability of spincast films with respect to ILs is not particularly surprising since they have a macroscopic surface area but a very small volume, ∼2 10-4 cm3. When placed in contact with ∼0.1 cm3 of IL in the fluid cell, the effective concentration of polymer is at least an order of magnitude lower than in the bulk solubility measurements. Lu and co-workers49 have recently followed the contraction of freely floating spin-cast polystyrene films annealed on IL surfaces in order to measure surface glass transition effects. Interestingly, they found no evidence for uptake of some ILs (Bmim-CF3SO3) into the polystyrene films by XPS, which suggests that in some cases the degree of swelling should be negligible. However, when ILs are at least partially compatible with polymers, they tend to act as plasticizers,11 which in our experiments is likely to accelerate dewetting of the polymer film from silicon substrate. Film Swelling and Interfacial Width. The polymer/IL combinations that were stable under prolonged annealing were examined by neutron reflectometry. It is interesting to note that while ILs have a negligible vapor pressure, it is possible to extract appreciable quantities ILs from the bulk into an adjacent polymer layer. Furthermore, the variable swelling of polymers with respect to different ILs suggests that this phenomenon could form the basis of a means for separating and purifying ILs. While annealing films in contact with ILs generally leads to swelling of the film and a more diffuse interface, it is not universally the case and the extent of swelling and interface broadening varies significantly from one polymer/IL combination to another. Figure 5 illustrates this point quite clearly: Annealing dPMMA in contact with Bmim-Br has only a modest effect on the film properties relative to annealing in air, whereas annealing dPS in contact with the same IL causes a steeper Q-dependent fall in R(Q) corresponding to an increase in the dPS/Bmim-Br interface width. Swelling and interfacial width parameters extracted from fits to the neutron reflectivity data are shown in Figure 7. The uncertainties in the fitted parameters shown as error bars demonstrate that in several cases the extent of swelling relative to the native film and the increase in interfacial width (from a starting value of 0.9 ( 0.3 nm) are too small to be discernible. From the perspective of exploiting ILs as nonsolvents to control self-assembly in polymer films, minimal swelling and interfacial widths are positive results since the former guarantees that the problem of extracting the IL after contact does not arise, and the latter indicates a high interfacial tension that provides the thermodynamic impetus for surface segregation. The dPS/Bmim-Cl system stands out in that while the interfacial width is not significantly different from several other dPS/ IL mixtures, the swelling is unusually large. This is an intriguing observation that highlights the complexity of polymer IL mixtures. For simple polymer/nonsolvent mixtures one might expect the interfacial width and swelling to be closely correlated since both parameters are primarily a function of the Flory interaction parameter, whether at the interface or in the bulk. While the precise reason for the anomalous swelling of dPS by Bmim-Cl is (49) Lu, H. Y.; Chen, W.; Russell, T. P. Macromolecules 2009, 42, 9111.
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Figure 7. Influence of ILs on swelling and interface width of dPS and dPMMA films.
unclear, it is notable that this is the smallest of the IL molecules studied. The relative ease with which Bmim-Cl may be absorbed into dPS could indicate that net-neutral Bmim-Cl ion pairs can be more easily removed from the bulk IL than larger ion pairs. Some of the recent literature on IL volatility is therefore pertinent to this discussion. All of the ILs studied here are aprotic and are therefore expected to exist as tightly bound net neutral pairs in the gas phase.50 Similarly, one might expect at low concentrations dispersed in a polymer matrix that they exist as neutral pairs, but this will require further experimentation to determine. Lee and Lee explored the relationship between IL structure and cohesive energy density (CED) and Hildebrand solubility parameter δH (=CED1/2).51 They found that for two series of imidazolium ILs δH (and therefore CED) decreased slightly with increasing alkyl chain length (R in Figure1). The molar internal energy, ΔU (= Vm CED), where Vm is the molar volume, however increases slightly with increasing alkyl chain length since the increase in molar volume outweighs the decrease in CED. Santos et al. obtained a similar result both theoretically and experimentally by direct determination at high temperature,52 while Vaughan et al. measured the dependence of ΔU for a variety of Bmim ILs as a function of anion structure.53 On this basis, it is plausible that the greater ingress of smaller IL molecules into our polymer films may be in part due to their greater volatility. However, the solubility of an IL into a polymer matrix will depend upon the balance between IL-polymer interactions and polymer-polymer interactions as well as the IL-IL interactions discussed above. Clearly the thermodynamic studies of pure ILs tell us little about the interactions involving polymers, and we should be cautious in extrapolating the results to other alkylimidazolium ILs. It is notable that the swelling of dPS by Bmim-PF6 is much smaller than by any other IL that we measured, yet ΔU for Bmim-PF6 is not exceptionally high when compared to other imidazolium ILs.53 This suggests that the lack of ingress of BmimPF6 into dPS arises because of relatively unfavorable interactions between the IL and the polymer rather than the ΔU of the IL. (50) Leal, J. P.; Esperanca, J.; da Piedade, M. E. M.; Lopes, J. N. C.; Rebelo, L. P. N.; Seddon, K. R. J. Phys. Chem. A 2007, 111, 6176. (51) Lee, S. H.; Lee, S. B. Chem. Commun. 2005, 3469. (52) Santos, L.; Lopes, J. N. C.; Coutinho, J. A. P.; Esperanca, J.; Gomes, L. R.; Marrucho, I. M.; Rebelo, L. P. N. J. Am. Chem. Soc. 2007, 129, 284. (53) Swiderski, K.; McLean, A.; Gordon, C. M.; Vaughan, D. H. Chem. Commun. 2004, 2178.
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Furthermore, solubility parameters for polystyrene and poly(methyl methacrylate) are very similar, and both54,55 have been estimated to be ∼19 MPa0.5. Given the similarity between solubility parameters for the polymers, their differing solubility in ILs, and the large difference between solubility parameter between ILs (typically53 ∼30 MPa0.5) and the polymers, it is evident that solubility parameter alone is of little value in predicting polymer solubility in ILs. Neither polymer showed a particularly strong temperature dependence on the interaction with the IL for which this was measured. While the interfacial width of PS films appeared to grow slightly with increasing temperature in Bmim-Br, if anything, the PMMA films appeared to become less swollen and have a narrower interfacial width with increasing temperature in Bmim-BF4. It should be stressed that these trends with increasing temperature are only just discernible from the experimental uncertainty, and these systems should ideally be studied over a wider temperature range to clarify these trends. In these experiments only a relatively limited range of temperature could be studied, defined by the glass transition temperature of the dPMMA film (∼120 °C) and the limit of stability. However, with thicker films or higher molecular weight polymers, it should be possible to increase the temperature range sufficiently to resolve the temperature dependence of film swelling. It is interesting to note that in general the dPS films have significantly greater interfacial widths than dPMMA films, despite their low solubility. While the reason for this behavior is not particularly evident (both have similar initial interface widths in air), we speculate that this may be related to the affinity that has been reported to exist between imidazolium cations for aromatic species,56 which for dPS could be fulfilled by the phenyl rings of the styrene repeat units. Interfacial width, σ, relates to interfacial tension γ by57 σ2 ¼ σ0 2 þ
kB T 4πγ
Z
dQ 2 2 As Q þ k
ð4Þ
where σ0 is the intrinsic interfacial width due to the scattering length density variation across surface (∼0.1 nm), kB is Boltzmann’s constant, and k is the gravitational term, which is vanishingly small compared to Q, over the measured range. Since σ0 is small compared to σ, the interfacial tension is approximately proportional to σ-2. From this relationship it is possible to estimate that the growth in interface width from 0.9 to 3 nm seen for many dPS films corresponds to a reduction in interfacial energy from 40 mJ m-2 (PS/air)55 to ∼22 mJ m-2. Concentration Profiles in Blends Annealed in Contact with ILs. i. Surface Segregation versus Time. The relatively weak temperature dependence of the polymer/IL interaction indicated by neutron reflectometry results indicates that time-temperature superposition46,58 can be used to explore the influence of a broad range of effective annealing times on the distribution of dPS-COOH in blends with hPS. Figures 8 and 9 show qualitatively similar behavior for every blended film/IL combination. The large interfacial excess of dPS-COOH adsorbed at the substrate is reduced by annealing the film in contact with (54) Cowie, J. M. G.; Miachon, S. Macromolecules 1992, 25, 3295. (55) Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999. (56) Holbrey, J. D.; Lopez-Martin, I.; Rothenberg, G.; Seddon, K. R.; Silvero, G.; Zheng, X. Green Chem. 2008, 10, 87. (57) Schwartz, D. K.; Schlossman, M. L.; Kawamoto, E. H.; Kellogg, G. J.; Pershan, P. S.; Ocko, B. M. Phys. Rev. A 1990, 41, 5687. (58) Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. Soc. 1955, 77, 3701.
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Figure 8. Anion dependence of surface (solid points) and substrate (hollow points) excess concentration of 15% dPS-COOH/ hPS blends versus annealing time at 155 °C.
Figure 9. Cation dependence of surface (solid points) and substrate (hollow points) excess concentration of 15% dPS-COOH/ hPS blends versus annealing time at 155 °C.
any IL, and the surface excess at the polymer/IL interface grows steadily. While the growth of the polymer/IL surface excess has only a subtle dependence on the IL structure, it is clear that the rate at which dPS-COOH desorbs from the substrate interface has a strong dependence of IL structure. Figure 8 shows that the extent of desorption increases in the order PF6 ∼ Br < Cl < BF4. This correlates with the extent of swelling shown by dPS films, where PF6 < Br < Cl, and BF4 could not be measured due to the instability of the film, suggesting that this would have been highly swollen by Bmim-BF4. While a detailed investigation of the dynamics of polymers swollen by ILs is beyond the scope of this present work, it seems likely that (regardless of the innate IL viscosity, which is much lower than that of the polymer) the greater the quantity of IL absorbed into the polymer film, the greater the reduction in the swollen film viscosity with respect to the dry film. The correlation of desorption rate with extent of swelling is therefore expected if the rate of desorption is diffusionlimited. The rate of desorption also increases systematically with decreasing cation size, which is apparent in Figure 9 and may again be related to the extent of swelling of the polymer film by the IL. On this basis, it appears that the mechanism for dPS-COOH desorption is largely governed by the extent of ingress of IL into the film. This suggests that all of the ILs have a sufficient affinity for the silica substrate to displace the dPS-COOH, effectively reducing their sticking energy to this surface, and that the rate at Langmuir 2010, 26(19), 15486–15493
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Figure 10. Equilibrium surface excess concentration of 15% dPSCOOH/hPS blends for all ILs after annealing at 155 °C for 30 min.
which they diffuse from the substrate into the bulk of the film may be enhanced by the plasticizing effect of the IL. ii. Surface Segregation versus Functionality. The surface excess values for blended films annealed under various ILs shown in Figure 10 indicate that surface segregation of dPS-COOH was induced to some extent for every IL structure that could be annealed in contact with the film without causing dewetting. There is almost no dependence of surface excess on anion species, which is surprising since we have established that this parameter has a significant effect on the swelling and interface width of the homopolymers. It is however conceivable that in some cases the films remain plasticized by the absorbed IL, leading to a reduction in surface excess even after the samples have been cooled to room temperature. Within this limited data set, dPS-COOH surface segregation appears to increase with increasing imidazolium cation size, notably for the chloride and tetrafluoroborate salts. This is consistent with the expectation that the surface excess would be larger if the interfacial tension of the pure homopolymer/IL interface was large, since with would give greater thermodynamic impetus for adsorption. However, we cannot exclude the possibility that the cation dependence of the surface segregation is also governed by dissociation of the COOH at the IL surface to COO--Rmimþ and HþX-. Work is ongoing to establish the origins of this dependence.
Conclusions We have quantified for the first time the swelling and interfacial properties of a series of supported polymer films when annealed in contact with ILs. The miscibility is clearly dependent upon the IL and polymer structure and temperature since we found that all combinations of high molecular weight PMMA and IL formed gels upon cooling to room temperature. Only highly immiscible polymer/IL pairs yielded films that were stable with respect to annealing in contact with ILs. In these films, most were found to be swollen by a few percent by the IL in contact. The only
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exception found so far was deuterated polystyrene and BmimPF6, for which the polymer was not swollen by the IL and the polymer/IL interfacial width did not grow either. The interfacial width of polymers annealed in contact with ILs had little temperature dependence, increasing slightly with increasing temperature for dPS/Bmim-Br and decreasing slightly with increasing temperature (dPMMA/Bmim-BF4), suggesting UCST and LCST behavior for these systems, respectively. Both polymers had very similar solubility parameters with each other when compared to values for the ILs; therefore, the complex variation in solubility and swelling behavior could not be resolved in terms of solubility parameter arguments. Although it was generally the case that polymer films swollen by ILs also had a broader polymer/IL interfacial width than the films annealed in air, the correlation between swelling and interfacial width with was quite weak. Generally, the dPS/IL interfaces were broader than the dPMMA/IL interfaces, which we speculate may be due to the affinity of aromatic phenyl groups for the IL. When placed in contact with blended polymer films, all of the ILs for which the films were stable exhibited qualitatively similar behavior, albeit at markedly different rates. Carboxy-functionalized deuteriopolystyrene, dPS-COOH, was displaced from the polymer substrate interface by all of the ILs and adsorbed at the polymer/IL interface. The rate of desorption from this interface was most clearly related to the chemical nature of the anion (PF6- < Br- < Cl- < BF4-), which is in turn related to the swelling of the polystyrene matrix. This sequence does not correspond to a simple fundamental parameter such as the anion size or melting point and serves to underline the difficulty in establishing trends in systematically varying IL systems. However, within this uncertain framework it is possible rationalize the behavior in terms of the more swollen polymer having faster diffusion and therefore accelerated desorption. The ability of ILs to induce surface segregation appears to underline their potential to control polymer self-assembly, and work is ongoing to understand the relationship between IL structure and induced selforganization. Acknowledgment. We thank STFC (UK) for provision of the neutron reflectometry facilities and financial support (RB820013) and Durham University for support of the analytical facilities. R.L.T. is grateful to Matthew Jackson (Durham University) for helpful discussions on IL behavior. Supporting Information Available: Neutron reflectivity data and fits showing sensitivity of fit to variation in total film thickness (SI-1); nuclear reaction analysis results of PMMA blend films showing similar behavior to results reported for dPS-COOH and hPS blends (SI-2); and thermogravimetric analysis and mass spectroscopy confirming stability of ILs over temperature range used (SI-3). This material is available free of charge via the Internet at http://pubs.acs.org.
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