Nano-Organization of Amylose-b-Polystyrene Block Copolymer Films

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Nano-Organization of Amylose-b-Polystyrene Block Copolymer Films Doped with Bipyridine Karim Aissou, Issei Otsuka, Cyrille Rochas, Sebastien Fort, Sami Halila, and Redouane Borsali* Centre de Recherche sur les Macromolecules Vegetales - CERMAV, CNRS UPR 5301-ICMG and Joseph Fourier University, BP53, 38041 Grenoble Cedex 9, France ABSTRACT: This paper discusses the self-assembly of rodcoil amylose-b-polystyrene (Mal-b-PS) block copolymer thick and thin films. The nano-organization falls in an interdomain spacing d of about 10 nm, much smaller than flexible-flexible petrol block copolymer systems. Additionally, hydrogen-bonding interactions between carbohydrate rods (amylose) and 40 ,4bipyridine (bipy) molecules induces phase transitions. Indeed, adding bipy in maltooctadecaose-block-polystyrene (Mal18-bPS) copolymers results, at room temperature, in the formation of a lamellar phase having Mal18 bipy-rich nanodomains instead of hexagonal close-packed (HCP) of cylinders made of Mal18, whereas a coexistence of Mal7bipy-rich cylindrical and spherical nanodomains are formed from maltoheptaose-b-polystyrene (Mal7-b-PS) copolymers instead of a poorly organized array of Mal7-based cylinders. On heating, the Mal7bipy-b-PS system shows more rich phase behavior as compared to the Mal7-b-PS one due to weakening of hydrogen bonding with temperature. Such a system is of great interest in developing active layers in light-emitting diodes (LEDs) or in photovoltaic cells to realize devices with an optimal structure, that is, having large interface area and domain size with similar exciton diffusion length (10 nm).

’ INTRODUCTION Self-assembly of block copolymers provides an elegant route to fabricate nanolithographic mask or functional films in which a variety of thermodynamically stable phases are formed with nanodomain size on 10 nm length scales. For example, using rod-coil block copolymers as nanolithographic masks offers a promising way to sensibly increase the integration density of inorganic nano-objects (dots, holes, and pillars) as compared to those already well-documented nanolithographic masks using coil-coil block copolymer systems.1-9 Another important application using rod-coil block copolymer film consists in developing active layers in light-emitting diodes (LEDs)10-16 or in photovoltaic cells17-23 to realize devices with an optimal structure, that is, having large interface area and domain size similar than exciton diffusion length (10 nm). To fabricate thin films having a high nanodomain density required for nanolithographic, proton transport,24 or organic electronic applications, we propose to exploit the strong mutual repulsion between the hydrophilic natural rods and hydrophobic synthetic coils in rod-coil amylose-b-polystyrene (Mal-b-PS) block systems. Additionally, the use of amylose as rod offers several advantages. Indeed, one of them is to realize a nanoporous lithographic template by placing the sample in an aqueous acidic medium bath (HCl or H2SO4) since a scission of glycosidic linkages (acid hydrolysis) occurs in this condition and so permits selective removal of the carbohydrate rods from the synthetic matrix.25 Another advantage resides in the fact that numerous hydroxyl groups on amylose rods allow one to prepare supramolecules in which hydrogen-bonded low-cost active small molecules offer the possibility to create thin films having r 2011 American Chemical Society

photonic properties. However, incorporation of small molecules in the system induces a variation of essential parameters used to build the block copolymer phase diagram26-29 such as (i) the rod volume fraction, φrod, (ii) the intermolecular interaction nature between rods, expressed by the Maier-Saupe interactions, ω, and (iii) the repulsion between rods and coils, expressed by the Flory-Huggins parameter, χ. Using amylose-block-polystyrene copolymer systems, we report in this work that the films obtained self-assemble into different nano-organizations and that the incorporation of 40 ,4bipyridine (bipy) by H-bonding in the systems induces phase transitions due to the simultaneous variation of φrod and to the relative importance of the two driving forces mentioned above which could be quantified by the ω/χ ratio.30,28 Moreover, one observes, as it will be described below, that the equilibrium between spherical and cylindrical domain formation strongly depends on the temperature.

’ EXPERIMENTAL METHODS Sample Preparation. Natural-synthetic rod-coil amylose-blockpolystyrene (Mal-b-PS) copolymers were synthesized by clicking azidoterminated polystyrene blocks, prepared from a hydroxylethyl-polystyrene following the procedure of Fallais et al.,31 with alkynyl-terminated amylose ones, obtained from the method described previously,32 using CuBr/PMDETA as the catalyst system in a dimethylformamide (DMF) solution maintained at 40 °C for 3 days (see Scheme 1). Synthetic Received: December 12, 2010 Revised: January 30, 2011 Published: March 11, 2011 4098

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Scheme 1. Preparation of Natural-Synthetic Amylose-block-Polystyrene Copolymer from Alkyne and Azide Functionalised Building Blocks

Table 1. Molecular Parameters for Amylose-block-Polystyrene Block Copolymers no. of rod units

no. of PS units

Na

φPSb

Mal7-b-PS37

7

36

43

0.82

Mal18-b-PS37

18

36

56

0.64

block copolymer

a

N is the number of volumetric repeat units of each block copolymer with a PS unit as reference volume. b φPS is the coil volume fraction using dPS = 1.05 g.cm-3 and damylose = 1.50 g.cm-3. The Mal18 sample was purchased from Hayashibara Biochemical Lab (Japan).

Figure 2. Synchrotron SAXS profiles of Mal7-b-PS37 obtained at selected temperature. Red and black arrows show the peak positions corresponding to the HCP and BCC phases, respectively.

Figure 1. Synchrotron SAXS profile of Mal18-b-PS37 obtained at room temperature. polymer chains used in this work have a constant coil size or degree of polymerization Dp = 36 (Mn,PS = 3700 g 3 mol-1), and the rod volume fractions, φrod, are 0.18 and 0.36 where maltoheptaose (Mal7) and maltooctadecaose (Mal18) were used as rigid blocks. Thick films made of Mal-b-PS37 and Malbipy-b-PS37 were prepared by slowly evaporating a diluted tetrahydrofuran (THF) solution (5% w/w) stirred vigorously during 24 h when bipy was added and then were annealed under vacuum at 170 °C during 24 h to promote their nano-organizations. Thin films were prepared by spin coating technique using more diluted solutions (0.5% w/w) onto Si(100) substrates with a native oxide surface and then were annealed under vacuum at 170 °C during 24 h. Small Angle X-ray Scattering (SAXS). SAXS experiments were performed on the BM02 beamline of the European Synchrotron Radiation Facility (Grenoble, France). Films were disposed in sample

Figure 3. Synchrotron SAXS profile of Mal18bipy-b-PS37 obtained at room temperature. holders equipped with an integrated heating system. The experiments were performed using 14 keV (λ = 0.089 nm) X-rays. Nanoscale features, averaged over an X-ray spot size on the order of 100  150 μm2, were recorded during 5-20 s exposures on a CCD detector placed 4099

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Figure 5. Synchrotron SAXS profile of Mal7bipy-b-PS37 obtained at room temperature. Coexistence of two phases is observed, since we observe an undefined supplementary peak noted q** plus two peaks relative to a bcc phase.

Figure 4. (a) AFM phase image obtained from MAL18bipy-b-PS37 thin film. (b) Phase cross section profile corresponding to the continuous blue line on the AFM phase image. at 51 cm behind the sample. Iq2 vs q representation of the scattered intensity was systematically used to increase higher-order peak magnitude. Atomic Force Microscopy (AFM). AFM phase images were realized using a Picoplus Molecular Imaging atomic force microscope operating in tapping mode, with a silicon cantilever of resonant frequency 145-230 kHz.

’ RESULTS AND DISCUSSION The molecular parameters of amylose-b-PS37 copolymers are summarized in Table 1. The SAXS profile of Mal18-b-PS37 shows the existence of one main peak, q*, at 0.29 nm-1 and higher-order peaks at 31/2q*, 41/2q*, 71/2q*, and 131/2q* (see Figure 1). This result is consistent with a hexagonal close-packed cylinder (HCP) array having an interdomain spacing, d = 25.0 nm (d = (2π/q)(4/3)1/2). For the Mal7-b-PS37 sample, the corresponding SAXS profile shows, in addition to a distinct broad first-order peak, q*, at 0.63 nm-1, one observes two shoulders at q/q* = 1.66 and 2.56, corresponding closely to hexagonal symmetry having an interdomain spacing, d = 11.7 nm (see Figure 2). As compared to the Mal18-b-PS37 system, the broadness of the observed first peak suggests that the assembled

Figure 6. (a) AFM phase image obtained from Mal7bipy-b-PS thin film. (b) Phase cross section profile corresponding to the continuous blue line on the AFM phase image inset. 4100

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Langmuir cylinders do not present a long-range order. This phenomenon combined with the presence of a cylinder form factor33,34 could explain the absence of the peak located at 41/2q*. Indeed, for this system, a cylinder having a √ radius Rcyl, calculated to be about 2.6 nm (RCyl = d[((1 - φPS) 3)/2π]1/2) with φPS being the PS volume fraction) does show a minimum value of the calculated form factor close to the 41/2q* value. Increasing the temperature (heating ramp 1 °C/min) for the Mal7-b-PS37 sample results to a phase transition: indeed, the peak located at q/q* = 1.66 becomes gradually difficult to observe and completely disappeared at 250 °C and is replaced by two peaks located at 21/2q* and 31/2q* appearing roughly at 245 °C (Figure 2). These two higher-order peaks indicate a well-developed cubic phase. Due to the presence of an additional scattering peak at 71/2q*, one concludes that the cubic structure is arranged into a bcc lattice in which the interdomain spacing is about 12.3 nm with d = (2π/ q*)(3/2)1/2.

Figure 7. Schematic of the left-hand part of the block copolymer microphase separation. Spherical, cylindrical, lamellar, and disordered phases are indicated by S, C, L, and dis, respectively. χN and φrod are the incompatibility product and the rod volume fraction, respectively. Forming supramolecules from amylose-b-PS systems induces (i) an increase of φrod due to the presence of bipy in amylose nanodomains and (ii) a decrease of the χN product caused by a moderation of the hydrophilic character of amylose bipy blocks. These phenomena imply a shift in positions on the phase diagram of different systems which are indicated by arrows.

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In the presence of 1 equiv of 40 ,4-bipyridine (bipy) molecule per maltoheptaose hydroxyl group, the rod-coil block copolymer chains form supramolecular architectures driven by hydrogen-bonding interactions between carbohydrate rods and the pyridine groups. Figure 3 shows a SAXS profile of Mal18bipy-bPS37 at room temperature. The binding of bipy molecules by hydrogen-bonding to Mal18-b-PS37 induces the formation of a lamellar phase confirmed by the presence of a main peak, q*, localized at 0.24 nm-1, where d = 2π/q* = 26.2 nm, and higherorder peaks at q/q* = 2, 3, and 4. AFM phase image (tapping mode), shown in Figure 4a, confirms the lamellar phase nanoorganization in thin films. The different viscoelastic properties of the Mal18bipy-rich (light) and PS-rich (dark) domains account for the high contrast that allows for the easy observation of the well-aligned parallel oriented lamella. Figure 4b shows the phase cross-sectional profile corresponding to the continuous blue line in Figure 4a. This profile reveals that the mean interdomain spacing of about 27.5 nm is in good agreement with SAXS results. Figure 5 shows the SAXS profile of Mal7bipy-b-PS37 at room temperature. The data reveal a combination of two different morphologies in the film. The more reflective structure presents a first order peak at q* = 0.69 nm-1 and higher-order peaks at 21/2q*, 31/2q*, 61/2q*, and 71/2q* which are ascribed to spherical domains arranged in a cubic lattice, with a domain spacing of d = 11.2 nm, whereas the less reflective one exhibits only a firstorder peak located at q** = 0.59 nm-1. Figure 6a illustrates AFM phase image (tapping mode) at different magnifications obtained for the Mal7bipy-b-PS37system. The isotropic Mal7bipy-rich (light) domains are less organized than strip patterns made from Mal18bipy-b-PS37. Figure 6b shows the phase cross-section profile corresponding to the continuous blue line of the Figure 6a inset. This profile reveals that the mean interdomain spacing about 11.6 nm is in agreement with the sphere period obtained by SAXS measurement (see Figure 5). All these results show that the binding of bipyridine molecules on amylose-based block copolymer systems influences their phase behavior since one observes the formation of different phases when these small molecules are present. For the Mal18-b-PS37 system, formation

Figure 8. Synchrotron SAXS profiles of Mal7bipy-b-PS37 obtained at selected temperature. Red and black arrows show the peak positions corresponding to the HCP and BCC phases, respectively. The intensity difference between the main peak structures is noted Δq. 4101

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Langmuir of the lamellar phase in the presence of bipy instead of a HCP array of cylinders could be easily understood, since Mal18bipy blocks are more voluminous than Mal18 ones and therefore a more symmetrical phase could be expected. However, this simple consideration could not explain the formation of spheres instead of cylinders when adding bipy. So, one supposes that the binding of bipy molecules on amylose rods renders less incompatible the hydrophilic carbohydrate blocks with the hydrophobic PS ones (i.e., χMal7/PS37 > χMal7bipy/PS37) and so permits to the system to rich a less segregated phase as illustrated in Figure 7. Because the increase of temperature gradually decreases the hydrogen bonding between the hydroxyl groups of the amylose rods and bipy, we then studied, using SAXS, the effect of temperature (heating ramp 1 °C/min) on the phase evolution in the Mal7bipy-b-PS37system. Figure 8 shows SAXS profiles of Mal7bipy-b-PS37 at selected temperatures. Between room temperature and 205 °C, increasing the temperature tends to favor the formation of a less reflective structure, since (i) the intensity difference between the main peak structures, Δq = q* - q**, progressively diminishes and (ii) higher-order peaks located at 31/2q** and 41/2q** are formed. This last result is consistent with the formation of a hexagonal phase having an interdomain spacing of d = 12.3 nm. Note that the cylinders formed in Mal7bipy-b-PS37 films are about 5% larger than those observed in Mal7-b-PS37 one. From 205 to 215 °C, the difference in the intensity between the main peak structures does not vary and describes a perfect equilibrium between the formation of cylinders, driven by the gradual breaking of the hydrogen bonds between the hydroxyl groups of the rods and bipy, and the formation of spheres to decrease the stretching free energy of PS blocks due to a thermal effect on the χ ∼1/T parameter value. Moreover, one observes that the first-order peak corresponding to the spherical structure is located at q* = 0.66 nm-1 which is slightly lower than the spherical one observed at room temperature. Finally, one can observe at 225 °C that the intensity difference between the main peak structures, Δq, starts to increase which traduces a predominant mechanism of sphere formation.

’ CONCLUSION We report in this work the study of the phase behavior in rodcoil amylose(40 ,4-bipyridine)1.0-block-polystyrene copolymer systems with two different amylose volume fractions. The results show at room temperature that (1) the system self-assembles into nano-organized phases that depend on the volume fraction as expected; and (2) the hydrogen-bonding interactions between 40 ,4-bipyridine and amylose rods leads to different phase formation as compared to the corresponding amylose-blockpolystyrene copolymer system alone. Thus, for Mal18-b-PS37 system, one observes that the cylinder phase evolves toward a lamellar phase when supramolecular Mal18bipy-b-PS37 chains are formed by addition of bipyridine molecules in the block copolymer systems, whereas a mixture of cylindrical and spherical nanodomains is obtained with Mal7bipy-b-PS37 block copolymer chains instead of a poorly organized cylindrical phase formed with Mal7-b-PS37. One also observes that increasing the temperature when investigating the Mal7bipy-b-PS37 system results in more rich phase behavior as compared to the Mal7-bPS37 one due to weakening of hydrogen bonding with temperature.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: 00 33 4 76 03 76 40. Fax: 00 33 4 76 03 76 29.

’ ACKNOWLEDGMENT We acknowledge financial support from the CNRS and Nanoscience Foundation of Grenoble (RTRA Nanoscience Project # FCSN-2007-13P, Grenoble). ESRF beam time allocations BM2 (CRGD2AM)) are also gratefully acknowledged. We also acknowledge J.-L. Putaux for the gift of Mal18 sample. ’ REFERENCES (1) Shin, K.; Leach, K. A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Nano Lett. 2002, 2, 933. (2) Aissou, K.; Baron, T.; Kogelschatz, M.; Den Hertog, M.; Rouviere, J. L.; Hartmann, J.-M.; Pelissier, B. Chem. Mater. 2008, 20, 6183. (3) Thurn-Albrecht, T.; Schotter, J.; K€astle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (4) Tseng, Y.-T.; Tseng, W.-H.; Lin, C.-H.; Ho, R.-M. Adv. Mater. 2007, 19, 3584. (5) Seifarth, O.; Krenek, R.; Tokarev, I.; Burkov, Y.; Sidorenko, A.; Minko, S.; Stamm, M.; Schmeisser, D. Thin Solid Films 2007, 515, 6552. (6) Crossland, E. J. W.; Ludwigs, S.; Hillmyer, M. A.; Steiner, U. Soft Matter 2007, 3, 94. (7) Lo, K.-H.; Tseng, W.-H.; Ho, R.-M. Macromolecules 2007, 40, 2621. (8) Park, S.; Kim, B.; Wang, J.-Y.; Russell, T. P. Adv. Mater. 2008, 20, 681. (9) Milliron, D. J.; Caldwell, M. A.; Wong, H.-S. P. Nano Lett. 2007, 7, 3504. (10) Olsen, D. B.; Segalman, R. A. Mater. Sci. Eng. R 2008, 62, 37–66. (11) Chochos, C. L.; Kallitsis, J. K.; Gregoriou, V. G. J. Phys. Chem. B 2005, 109, 8755. (12) Chen, P.; Yang, G. Z.; Liu, T. X.; Li, T. C.; Wang, M.; Huang, W. Polym. Int. 2006, 55, 473. (13) Tzanetos, N. P.; Kallitsis, J. K. Chem. Mater. 2004, 16, 2648. (14) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (15) Osaheni, J. A.; Jenekhe, S. A. J. Am. Chem. Soc. 1995, 117, 7389. (16) Malliaras, G.; Friend, R. Phys. Today 2005, 58, 53. (17) Yang, X.; Loos, J. Macromolecules 2007, 40, 1353. (18) de Boer, B.; Stalmach, U.; van Hutten, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou, G. Polymer 2001, 42, 9097. (19) van der Veen, M. H.; de Boer, B.; Stalmach, U.; van de Wetering, K. I.; Hadziioannou, G. Macromolecules 2004, 37, 3673. (20) Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 5464. (21) van de Wetering, K.; Brochon, C.; Ngov, C.; Hadziioannou, G. Macromolecules 2006, 39, 4289. (22) Sivula, K.; Ball, Z. T.; Watanabe, N.; Frechet, J. M. J. Adv. Mater. 2006, 18, 206. (23) Hagberg, E. C.; Goodridge, B.; Ugurlu, O.; Chumbley, S.; Sheares, V. V. Macromolecules 2004, 37, 3642. (24) Rubatat, L.; Li, C.; Dietsch, H.; Nyk€anen, A.; Ruokolainen, J.; Mezzenga, R. Macromolecules 2008, 41, 8130. (25) Hoover, R. Food Rev. Int. 2000, 25, 369. (26) Wiliams, D. R. M.; Fredrickson, G. H. Macromolecules 1992, 25, 3561. (27) Matsen, M. W.; Barrett, C. J. Chem. Phys. 1998, 109, 4108. (28) Reenders, M.; ten Brinke, G. Macromolecules 2002, 35, 3266. 4102

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