Copyright 2008 American Chemical Society
JULY 1, 2008 VOLUME 24, NUMBER 13
Letters Tunable Hierarchical Assembly on Polymer Surfaces: Combining Microphase and Macrophase Separation in Copolymer/Homopolymer Blends E. Ibarboure,† A. Bousquet,† G. Toquer,‡ E. Papon,† and J. Rodrı´guez-Herna´ndez*,† Laboratoire de Chimie des Polyme`res Organiques (LCPO-UMR5629), CNRS, ENSCPB, UniVersity Bordeaux 1, 16 AVenue Pey Berland, 33607 Pessac-Cedex, France, and Centre de Recherche Paul Pascal (CRPP-UPR8641), CNRS, 115 AVenue Albert Schweitzer, 33600 Pessac-Cedex, France ReceiVed February 5, 2008. ReVised Manuscript ReceiVed May 15, 2008 Nanopatterned surfaces with feature spacing on the nanometer scale have attracted considerable attention during the last few decades. Structured surfaces find interest among others in the fabrication of devices applied, in turn, as biosensors and masks and in biomedicine to tailor cell-surface interactions or for tissue engineering purposes. Several techniques have been reported, including photolithography,1 polymer imprinting,2 and selfassembly3 to produce nano-organized surfaces. The self-assembly of block copolymers, which is well understood nowadays, has potential uses in numerous nanotechnologies because the size, shape, and arrangement of the nanostructures can be finely tuned through the synthesis of the building blocks. In addition, from a technological point of view, self-assembly is fast, inexpensive, and easily scalable.4 A relatively easy way to control the nanostructuration and create unprecedented structures consists of the use of homopolymers and block copolymer blends. In these mixtures, there is an interplay between two phase-transition phenomena, i.e., the coexistence of homopolymer domains as a result of macrophase separation with microstructured domains rich in diblock co* Corresponding author. E-mail:
[email protected] † University Bordeaux 1. ‡ Centre de Recherche Paul Pascal (CRPP-UPR8641).
(1) Sorribas, H.; Padeste, C.; Tiefenauer, L. Biomaterials 2004, 25, 3707. (2) Lee, W.; Jin, M.-K.; Yoo, W.-C.; Lee, J. K. Langmuir 2004, 20, 7665. (3) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, U.K., 1998. (4) Darling, S. B. Prog. Polym. Sci. 2007, 32, 1152.
polymers.5 Such structures with dual periodicities (micro and macro) are known as mesoscopic superlattice structures.6 Several examples have been reported in which double hydrophobic block copolymers were blended with a high-molecular-weight homopolymer. These studies concern mixtures such as PS-b-PI/ PS,7 PMMA-b-PS/PMMA or PS,8 and PS-b-PB-b-PS/PS.9 This letter presents the elaboration of multiscale patterned surfaces combining two phase-separation phenomena: microphase separation and macrophase separation to produce droplets of nano-organized domains of diblock copolymers in a hompolymer matrix. More interestingly, the microdomain morphology is formed by an amphiphilic diblock copolymer having a hydrophilic polypeptide block (i.e., poly(L-glutamic acid)), which may induce morphological changes depending on the environmental conditions. Synthetic polypeptides are attractive candidates for the preparation of functional and stimuli-responsive nanostructured materials.10 This interest relies on the ability of polypeptides to (5) (a) Ito, A. Phys. ReV. E 1998, 58, 6158. (b) Hong, K. M.; Noolandi, J. Macromolecules 1983, 16, 1083. (c) Matsen, M. W. Macromolecules 1995, 28, 5765. (6) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1994, 27, 6532. (7) Tanaka, H.; Hashimoto, T. Macromolecules 1991, 24, 5713. (8) Lo¨wenhaupt, B.; Steurer, A.; Hellmann, G. P.; Gallot, Y. Macromolecules 1994, 27, 908. (9) Han, C. D.; Baek, D. M.; Kim, J.; Kimishima, K.; Hashimoto, T. Macromolecules 1992, 25, 3052. (10) (a) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 1619. (b) Jaworek, T.; Neher, D.; Wegner, G.; Wieringa, R. H.; Schouten, A. J. Science 1998, 279, 57. (c) Hartmann, L.; Kratsmu¨ller, T.; Braun, H.-G.; Kremer, F. Macromol. Rapid Commun. 2000, 21, 814.
10.1021/la800404e CCC: $40.75 2008 American Chemical Society Published on Web 06/03/2008
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Figure 1. Dependence of the macrophase separation on the blockcopolymer concentration within the blend: (A) 15 wt %, (B) 30 wt %, and (C) 60 wt % of diblock copolymer within the blend.
adopt different secondary structures depending on temperature, pH, or solvent, among others. Also, these transitions are in general reversible, being thus changed back and forth by tuning the external conditions. We studied the nanostructuration and the influence of the exposure of the nanostructured films to a solvent selective for the hydrophilic polypeptide block on the final morphology. The synthesis of polystyrene-b-poly(L-glutamic acid) diblock copolymers has been carried out in several steps following previously reported procedures.11 In the first step, end-bromofunctionalized polystyrene was obtained by atom-transfer radical polymerization. The end-bromo functional groups were modified into primary amines by reaction with ethylenediamine in the presence of triethylamine. ω-Amino polystyrene was employed as macroinitiator for the ring-opening polymerization of γ-benzyl ester-L-glutamate N-carboxyanhydride. Finally, the benzylester groups were hydrolyzed under basic conditions. For the elaboration of nanostructured surfaces, polystyreneb-poly(L-glutamic acid) (PS27-b-PGA70) was mixed with a highmolecular-weight linear polystyrene homopolymer (PS600). As depicted in Figure 1, after spin coating from THF solutions, the surface exhibits diblock-copolymer-rich circular domains in a homopolymer-rich disordered phase. Such structures, known as “onionlike”, have already been reported using hydrophobic block copolymers/homopolymer blends exclusively.8 In spite of the very asymmetric structure of the diblock copolymer (i.e., PS27b-PGA70), the system appears to form alternating lamellar microdomains of PS and PGA blocks. This experimental fact (11) Babin, J.; Leroy, C.; Lecommandoux, S.; Borsali, R.; Gnanou, Y.; Taton, D. Chem. Commun. 2005, 1993.
Letters
can be explained by the particular chemical structure (i.e., rod-coil) of the diblock copolymer. Effectively, whereas the hydrophobic polystyrene block is amorphous, the polypeptide segment forms a rodlike R-helical structure. The preferential formation of this secondary structure, compared to the β-sheet or random coil, has been evidenced in poly(L-glutamic acid) thin films prepared using organic solvents.18 In addition, the selfassembly of rodlike peptide diblock copolymers in which the peptide segment exhibits an R-helical structure has been reported to form local hexagonal packing that generally leads to the formation of lamellar mesophases independently of the volume fraction of the comonomers.12 The alternation of the microdomains is directed by the interface between the microphaseseparated domain and the homopolymer domains. The PS block in the copolymer is oriented toward the PS homopolymer. However, this preferential orientation is lost in the center of the droplets when the size increases. Thus, for blends containing 60% diblock copolymer several layers of alternating domains close to the homopolymer interface are observed. On the contrary, inside of the droplet the lamellar structure is randomly oriented. The size of the diblock-copolymer-rich domains has been found to vary with the quantity of diblock copolymer within the blend. Whereas mixtures having 15 wt % diblock copolymer led to microphase-separated domains with sizes of ∼200-350 nm, mixtures charged with 30 wt % afforded 400-600 nm domains. The areas formed with a major proportion of diblock copolymer (60 wt %) are rather large (several micrometers) and polydisperse. However, independently of the amount of diblock copolymer within the blend, the block copolymer self-assembled within the droplets and forms a lamellar mesophase. Equally, the constant interlamellar spacing for all blend compositions provides evidence of the impossibility to solubilize the high-molecular-weight homopolymer within the diblock polystyrene microdomains.13 This experimental fact clearly indicates the complete segregation between the diblock copolymer and the homopolymer. Whereas the addition in solvent-cast mixtures of diblock and lowmolecular-weight homopolymers led to its localized/uniform solubilization,14 the incorporation of a diblock copolymer in high-molar-mass matrices (NHOMO . Ndiblock) induces macrophase separation.15 The theory behind these processes was first elaborated by De Gennes by the introduction of the concept of wet and dry brushes.16,17 Thus, careful design of the diblock copolymer (molar mass and composition) and homopolymer (molar mass) may allow the preparation of hierarchically structured surfaces having different levels of order. The block copolymer that was designed includes a hydrophilic polypeptide, poly(L-glutamic acid), that was extensively employed because of its unique ability to change the secondary structure from rigid rod-like to disordered random coils and vice versa as a consequence of small variations in pH, temperature, or ion strength. Although PGA preferentially forms R-helical structures (12) (a) Douy, A.; Gallot, B. Polym. Eng. Sci. 1977, 17, 523. (b) Billot, J.-P.; Douy, A.; Gallot, B. Makromol. Chem. 1977, 178, 1641. (c) Douy, A.; Gallot, B. Polymer 1982, 23, 1039. (d) Gallot, B. Prog. Polym. Sci. 1996, 21, 1035. (e) Minich, E. A.; Nowak, A. P.; Deming, T. J.; Pochan, D. J. Polymer 2004, 45, 1951. (f) Babin, J.; Taton, D.; Brinkmann, M.; Lecommandoux, S. Macromolecules 2008, 41, 1384. (13) Tanaka, H.; Hasegawa, H.; Hashimoto, T. Macromolecules 1991, 24, 240. (14) Prahsarn, C.; Jamieson, A. M. Polymer 1997, 38, 1273. (15) (a) Quan, X.; Gancarz, I.; Koberstein, J. T.; Wignall, G. D. Macromolecules 1987, 20, 1431. (b) Hashimoto, T.; Tanaka, H.; Hasegawa, H. Macromolecules 1990, 23, 4378. (c) Ito, A. Phys. ReV. E 1998, 58, 6158. (16) DeGennes, P. G. Macromolecules 1980, 13, 1069. (17) Feast, W. J.; Munro, H. S. Polymer Surfaces and Interfaces; John Wiley & Sons: New York, 1987. (18) (a) Wu, X.; Yang, S.; Njus, J. M.; Nagarajan, R.; Cholli, A. L.; Samuelson, L. A.; Kumar, J. Biomacromolecules 2004, 5, 1214. (b) Yokoi, H.; Kinoshita, T.; Tsujita, Y.; Yoshimizu, H. Chem. Lett. 2000, 1210.
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Figure 2. AFM images (recorded in tapping mode) evidenced the morphological changes on the local nanostructure by annealing the film to water vapor. The lamellar structure (top) is transformed to a cubic phase (bottom).
in the bulk, films exposed to solvents, such as water or TFA, disrupt the structure, and a transition is observed to a rather random-coil structure.18 This ability has been employed to fine tune the structuration within the diblock-copolymer-rich droplets. For that purpose, the films were exposed to water vapor in a closed vessel and held at 90 °C for 2 days. At this temperature, whereas water vapor plasticizes the block copolymer and the mobility of the chains is enhanced, the high-molecular-weight homopolystyrene with a Tg of 100 °C is still in a glassy state with reduced mobility. As a consequence, these annealing conditions prevent the coagulation of the block-copolymer-rich domains. After being annealed, the films were rapidly cooled to room temperature to fix the structure and dried under vacuum. As depicted in the AFM images of Figure 2, the structure observed in the microphase-separated domains changes from a lamellar phase to either a cubic or a perpendicularly oriented hexagonal phase. During annealing to water vapor, water (pH 7) is partially condensed at the surface and deprotonates, at least partially, the carboxylic acid functional groups (pKa 4.8) of PGA. Repulsion between negatively charged side-chain groups leads to monomer repulsion, the R-helical structure is disrupted, and the polypeptide hydrophilic domains change from a rodlike R-helical structure obtained directly after spin coating to a more extended randomcoil conformation. Hence, deprotonation by water of the hydrophilic block involves changes in the secondary conformation, but the hydrophilic/hydrophobic volume ratio is also altered, which may in turn be responsible for the variations of the morphology that is formed.19 Other groups have recently demonstrated that changes in the relative volume can lead to similar structural changes.20 The transition between a lamellar phase and a cubic/hexagonal phase has been additionally confirmed by small-angle X-ray scattering experiments (Figure 3). In spite of the low resolution due to the low quantity of diblock copolymer within the blend (30 wt %), the main peak, q*, was found at 0.035 Å-1, which (19) Israelachvili, J. N. Intermolecular and Surface Forces; Harcourt Brace and Company: London, 1992. (20) (a) Bang, J.; Kim, B. J.; Stein, G. E.; Russell, T. P.; Li, X.; Wang, J.; Kramer, E. J.; Hawker, C. J. Macromolecules 2007, 40, 7019. (b) Ludwigs, S.; Krausch, G.; Reiter, G.; Losik, M.; Antonietti, M.; Schlaad, H. Macromolecules 2005, 38, 7532.
corresponds to an average domain distance of 18 nm. This average size is in relatively good agreement with the calculation of the length by assuming a polystyrene coil and PGA in an R-helical conformation (∼14 nm). This distance corresponds to the lamellar-lamellar spacing because a second-order reflection was found at 0.07 Å-1 (1:2). After annealing, whereas the main feature distance remain constant at the same q value, the peak at 2q (0.07 Å-1) is absent, and a peak at q3 (0.06 Å-1) appeared. Because the reflections at relative positions of 1:3 can be found both in cubic phases and hexagonal morphology, the exact microphaseseparated structure is still unidentified.21 However, the low content of polystyrene (less than 0.27 molar fraction) compared to that of the PGA block (0.72) may favor the formation of spherical domains.3 In the first series of experiments, subsequent annealing to water vapor followed by annealing to air led to the partial recovery of the lamellar structure within the macrophase-separated domains. Several aspects, including annealing time and temperature, have to be controlled to optimize the reversibility in the transition. A more detailed study of this concern is in progress. In summary, we demonstrated that blending an amphiphilic diblock copolymer, PS-b-PGA, with homopolystyrene produced spherical structures with a lamellar alternation of polystyrene and PGA blocks. Moreover, annealing under humid conditions perturbed the polypeptide R-helical secondary structure, inducing morphological changes inside the diblock-copolymer-rich domains between lamellar and either a cubic or a hexagonal phase. Several of these aspects are currently being investigated, including the effect of the homopolymer chain length, the amphiphilic diblock copolymer composition of the morphology, and the reversibility of the transition between the lamellar and hexagonal/ cubic phase.
Experimental Section 1H
NMR spectra of the copolymers were recorded at room temperature on a Bruker Avance 400 MHz spectrometer using the residual proton resonance of the deuterated solvent as an internal standard. Average molar masses and molar mass distributions of the (21) Fairclough, J. P. A.; Hamley, I. W.; Terrill, N. J. Radiat. Phys. Chem. 1999, 56, 159.
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Figure 3. Small-angle X-ray scattering profiles for polymer blends having 30 wt % PS27-b-PS70. In samples either unannealed or annealed to air, the presence of a lamellar structure is evidenced by the scattering reflections at q:2q. By annealing to water vapor, the SAXS pattern changes indicate a transition from a lamellar phase to either a hexagonal or a cubic phase because only two reflections, q:3q, have been observed.
samples were determined by size exclusion chromatography (SEC) using a Varian 9001 pump with both a refractive index detector (Varian RI-4) and a UV detector (Spectrum Studies UV 150). Calibration was carried out using narrowly distributed polystyrene standards and THF as the mobile phase at a flow rate of 0.5 mL min-1. Thick films (
