NANO LETTERS
Solvent-Assisted Formation of Nanostrand Networks from Supramolecular Diblock Copolymer/ Surfactant Complexes at the Air/Water Interface
2005 Vol. 5, No. 7 1309-1314
Qing Lu and C. Geraldine Bazuin* De´ partement de chimie, UniVersite´ de Montre´ al, C.P. 6128, Succursale Centre-Ville, Montre´ al, Que´ bec, Canada H3C 3J7 Received March 18, 2005; Revised Manuscript Received May 13, 2005
ABSTRACT We describe a simple manipulation of an asymmetric diblock copolymer, polystyrene-b-poly(4-vinylpyridine), mixed with 3-pentadecylphenol that generates a dense network of interconnected nanostrands at the air/water interface. This morphology is obtained by compression of the solution immediately after spreading, contrasting with the dot-and-planar morphology obtained by the conventional method, and is attributed to the presence of the solvent that confers sufficient mobility to the system to enable reorganization in response to increased surface pressure. This procedure thus adds a facile yet effective tool for controlling pattern formation at the air/water interface.
Functional materials based on the self-assembly of block copolymers have attracted considerable interest both in fundamental research and for potential nanotechnological applications.1-5 Complexation of surfactant-type molecules with block copolymers to form hierarchical supramolecular assemblies, which combine the meso- and nanoscales and which may also be made responsive to external stimuli, is a particularly appealing and versatile approach. The different length scales and structures in these systems can be manipulated separately by the choice of molecular parameters in either component and by the relative amounts of the components. Self-assembling materials of block copolymer/surfactant complexes that give rise to various nanostructures have been investigated in the bulk5-9 and in solution.10 LangmuirBlodgett (LB) monolayers of such materials transferred from the air/water interface may also be expected to lead to interesting nanostructures. A variety of studies on LB monolayers of amphiphilic block copolymers without surfactant (although some of these systems are composed of blocks with covalently attached alkyl side-chains, which may be considered to be analogous to those with noncovalently attached side-chains) have been published.11-19 They give rise to surface aggregates that range from small circular objects (“nanodots”) to cylindrical (ribbon-, rod-, or spaghettilike) structures to large planar aggregates (of up to hundreds of nm in diameter). 10.1021/nl050530z CCC: $30.25 Published on Web 05/28/2005
© 2005 American Chemical Society
The air/water interface in a Langmuir trough provides an ideal environment for fabricating surface aggregates from amphiphilic block copolymers, given the precise control of various experimental conditions including the continuous variation of the surface density. The surface structures or patterns obtained are determined by a number of parameters, which may include material parameters (such as the nature of the block copolymer, the relative block sizes, and the amount of material spread on the interface) and experimental parameters (such as temperature, compression speed, film deposition speed, and solution concentration). The usual procedure that is followed in the LangmuirBlodgett technique comprises a step where the spreading of the solution on the water surface is followed by a waiting period (typically 20 min) before any barrier compression is effected. This step allows time for the spreading solvent to evaporate and, in principle, for the system to reach equilibrium before varying the surface pressure. In this paper, we demonstrate that this step, too, can be an important factor in controlling the type of surface aggregate obtained. For the system investigated, the conventional application of this step leads to one type of predominant surface morphology (nanodots and planar aggregates), whereas eliminating this step consistently produces a completely different surface morphology. The latter is essentially cylindrical, but because of the extensive interconnectivity and flexibility of the cylindrical segments or strands, we have adopted the term
Figure 1. π-A isotherms of PS-b-P4VP/PDP, PS-b-P4VP and, in the inset, PDP.
“nanostrand network”. A similar pattern was reported recently in LB monolayers of a blend of two block copolymers and attributed to the low glass transition (Tg) of one of the components,18 as will be discussed later. This pattern was also noted as one of several features (but not a predominant one) in LB monolayers of a highly asymmetric poly(styrene)-b-poly(ethylene oxide) (PS-b-PEO, 7 wt % PEO) copolymer,17 and along edges and contact lines of LB monolayers of an asymmetric heteroarm PS-PEO star polymer.20 We investigated LB monolayers of the system composed of an asymmetric poly(styrene)-b-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer [Mn(PS) ) 40 000; Mn(P4VP) ) 5600; Mw/Mn ) 1.09] mixed with a stoichiometric amount (relative to the 4VP units) of 3-n-pentadecylphenol (PDP). In the bulk, hydrogen bonding between phenol and pyridine leads to supramolecules that selfassemble to give a hierarchical structure of nanoscale lamellae within block-copolymer length-scale lamellae below an order-disorder transition of the P4VP/PDP block at about 60 °C.21 For our experiments, the copolymer (Polymer Source) was used as received, and PDP (Aldrich) was recrystallized twice from petroleum ether and dried in vacuum at 40 °C. Unless specified otherwise, the copolymer and surfactant in equimolar 4VP/PDP proportion were dissolved separately in chloroform and then mixed together; about 200 µL of the resulting solution (0.35 mg/mL of the copolymer) were spread on 20 °C Milli-Q water (18.2 MΩ.cm) in the standard trough (777 cm2) of the KSV 3000 Langmuir Blodgett system, symmetric compression was effected at 15 cm2/min, and LB films were deposited at a speed of 10 mm/min onto 1 cm2 freshly cleaved mica. The films were dried for a day at room temperature and were imaged by atomic force microscopy using a multimode AFM and a Nanoscope IIIa controller (Digital Instruments) with a silicon cantilever (resonance frequency 300-400 kHz). Surface pressure-area (π-A) isotherms of PS-b-P4VP/ PDP, PS-b-P4VP, and PDP on pure water are shown in Figure 1. They were obtained by symmetric compression at 1310
a rate of 15 cm2/min and recorded to a precision of 0.1 mN/m using a Wilhelmy balance and a platinum plate after a 20-min waiting period following deposition of the solution. The PDP isotherm shows a transition between 0.30 and 0.35 nm2/molecule at a surface pressure of about 6 mN/m. The limiting area of 0.27 nm2/molecule, obtained by extrapolating the linear portion of the isotherm in the pressure region between 10 and 50 mN/m to π ) 0 mN/m, is indicative of a transition from an expanded to a condensed monolayer of PDP, involving its reorientation from prone or tilted to nearly vertical relative to the water surface.22 The PS-b-P4VP isotherm shows a monotonic increase in surface pressure, with a limiting area of 19 nm2/molecule. The isotherm of the PS-b-P4VP/PDP system is more expanded compared to that of PS-b-P4VP, especially at lower pressures, and there appears to be a weak transition near 7 mN/m (30-35 nm2/molecule). These isotherms resemble those for other amphiphilic diblock copolymers where the hydrophilic block is much shorter than the hydrophobic block, in particular for analogous PS-P4VP+Cn diblock copolymers where P4VP+Cn indicates P4VP that is fully quaternized by linear alkyl chains of n carbons.12,13b,15 In the course of preliminary investigations involving AFM imaging of transferred monolayers from the lower pressure region of the PS-b-P4VP/PDP isotherm, it was discovered that, if moderate compression of the spread solution is effected without the conVentional wait for solVent eVaporation, the morphology of the resulting monolayer is drastically different from that obtained after the usual wait. This is shown in Figure 2 for monolayers transferred at a surface pressure of 10 mN/m. In both cases, the PS-b-P4VP/PDP solution was spread to give a surface pressure of less than 5 mN/m (usually ca. 2 mN/m). Then, for Figure 2a, there was a 20-min wait at the initial low pressure, after which the barriers were compressed to 10 mN/m, followed by monolayer transfer onto mica (“conventional procedure”). For Figures 2b and 2c, in contrast, the compression of the barriers to 10 mN/m was initiated as soon as possible (1-2 min) after spreading of the solution; this was followed by a 20min wait at 10 mN/m and then by monolayer transfer to mica (“solvent-assisted procedure”). When the conventional procedure is followed (Figure 2a), the dominant features observed are essentially circular aggregates ranging in size from small dots (of less than 50 nm in diameter) to large planar aggregates (of more than 500 nm in diameter). The larger aggregates are about 4.5 nm in height, and are surrounded by a rim that is higher by 1-1.5 nm and is 50-60 nm wide (inset of Figure 2a). The smaller aggregates are 6-7 nm in height. This morphology is consistent with what has been observed previously for the analogous PS-P4VP+Cn diblock copolymers with short P4VP+Cn blocks.12 The nanodots are surface micelles (dubbed “starfish” micelles12) composed of PS in the center surrounded by the hydrophilic P4VP+Cn block that is spread equatorially on the surface.12,23 The large planar aggregates were interpreted as being composed of a PS monolayer whose thickness corresponds to the PS random coil dimension and that is adsorbed to the water or substrate surface Nano Lett., Vol. 5, No. 7, 2005
Figure 2. AFM images of PS-b-P4VP/PDP LB monolayers deposited at 10 mN/m: (a) prepared by the conventional procedure; (b, c) prepared by the solvent-assisted procedure (see text for details). The inset in (a) gives the height profile along the white line to the left of the inset.
via an intervening layer of the hydrophilic P4VP+Cn.12 The rim around the planar aggregates and the greater height of the nanodots compared to the inner part of the planar aggregates is attributed to an increase in the random coil thickness of PS as a result of its greater exposure to, and thus more unfavorable interactions with, the water surface when the hydrophilic block is spread equatorially away from the PS instead of located between the PS and the water surface.12 When the solvent-assisted procedure is followed (Figures 2b, 2c, and 3), the surface morphology is dominated by a dense network of interconnected, flexible strands. These strands show smooth contours and are remarkably uniform in width and height (about 80 and 8 nm, respectively, in the height profile shown in Figure 3a). In some places, the strands are strongly aligned (Figure 3c). Other morphologies may also be present to a minor extent (Figure 3d). Immediate compression of the barriers to π ) 5 mN/m is not sufficient to generate dominant strand morphology (although it may be present to a small extent, as is the case also in the conditions of Figure 2a), whereas more mixed morphologies are obtained for immediate compression to π ) 7-8 mN/m (Figure 4). Interestingly, the planar-type aggregates in Figure 4 are, on average, large and tend to have very irregular contours. The rim around the aggregates is always present and of constant width. Immediate compression to surface pressures greater than 10 mN/m (π ) 20, 30 mN/m) results in a similar morphology to that for π ) 10 mN/m (however, still higher final pressures, such as 40 mN/m, give morphologies indicative of collapsed films). The above observations suggest that the equilibrium morphology of the system studied is dot-and-planar at low surface pressures, whereas it is cylinder-like at higher pressures. HoweVer, without the presence of the solVent to facilitate mobility, the reorganization upon changing the pressure does not take place. This is not unlike what was observed by Seo et al.18 for a blend of poly(styrene-bferrocenyl silane) and poly(styrene-b-2-vinyl pyridine) spread at the air/water interface, which also shows a transformation in morphology from predominantly spherical to a dense network of strands upon increasing the surface pressure. The mobility allowing the transformation to take place in that case was attributed to the plasticizing effect of the low-Tg Nano Lett., Vol. 5, No. 7, 2005
Figure 3. AFM images of PS-b-P4VP/PDP LB monolayers prepared by the solvent-assisted procedure: (a, c, d) height images, (b) phase image; (a, b) expanded to and deposited at 5 mN/m (following compression to 10 mN/m), (c, d) deposited at 10 mN/m. The height profile shown above (a) corresponds to that recorded along the white line in (a).
poly(styrene-b-ferrocenyl silane). The implication is that, if the system is not sufficiently mobile during compression to respond to new thermodynamic conditions, the low-pressure morphology is essentially kinetically frozen in (as was pointed out also in ref 16). We have now shown that a simple way to maintain mobility for some time is to eliminate the conventional waiting period for solvent evaporation after spreading and before compression, thus allowing the solvent to play this role. Once the nanostrand network is formed and the solvent evaporated, this morphology, too, becomes frozen in. This 1311
Figure 4. AFM height images of PS-b-P4VP/PDP LB monolayers prepared by the solvent-assisted procedure under the reference conditions (see text) with the exception that they were deposited at 7-8 mN/m.
is indicated by an experiment where the barriers were reexpanded after the procedure described for Figure 2b, to give a surface pressure of 5 mN/m: the monolayer deposited after a 20-min wait at this point, shown in Figure 3a, continues to result predominantly in the network morphology. This figure also shows a less tightly packed network, in accordance with the expansion effected. The accompanying phase image (Figure 3b) clearly indicates that the nanostrands are distinct in their chemical and/or mechanical properties from the background, which is presumably the substrate. The effect of some other experimental parameters was also tested. The reference conditions are those of Figures 2b and 2c; namely, a 0.35 mg/mL solution was spread on the 20 °C Langmuir bath to an initial pressure of about 2 mN/m, the barriers were compressed as soon as possible at a speed of 15 cm2/min to 10 mN/m, and held at the final pressure for 20 min before the transfer to mica. In what follows, only the parameter specified is different from these reference conditions. Compression speeds that are too slow (e.g., 7.5 cm2/min) do not favor nanostrand network formation, which can easily be attributed to significant solvent evaporation having occurred before the conditions for obtaining nanothreads have been achieved. However, too fast compression speeds (above about 40 cm2/min) are also unfavorable, perhaps because of initial lack of homogeneity in the polymer concentration at the air/water interface coupled with changing thermodynamic conditions. This suggests that a balance must be achieved between (a) having enough solvent present for 1312
maintaining flexibility as described above and (b) allowing time for the polymer to spread sufficiently uniformly before new thermodynamic conditions provoke reorganization into another morphology that may, furthermore, be dependent on concentration. Indeed, varying the concentration of the polymer solution appears to influence the morphology obtained. A lower concentration of 0.1 mg/mL resulted in a nanostrand network predominantly, whereas a higher concentration of 1.0 mg/ mL gave, partly, a more fused network-like morphology and partly, densely packed irregularly shaped planar aggregates that appear to be studded with holes. In addition, it seemed necessary to maintain the final pressure (10 mN/m) for at least 15 min before transferring to the solid substrate in order to obtain predominant nanostrand network morphology (10 min gave a mixed morphology and 5 min mainly circular aggregates). In this connection, it was verified that maintaining the final pressure for some time before transfer, when using the conventional procedure, did not have the same effect: even after 1 h at 10 mN/m before transfer, there was no evidence of nanostrand network formation but just more densely packed dots and planar aggregates predominantly. It must be emphasized that, although the nanostrand network morphology appears to result from the “right combination” of several factors, it was repeatedly obtained as the dominant morphology at and near the above-defined reference conditions for the system studied. The dimensions are always similar at 100 ( 20 nm in width and 7 ( 1 nm in height (noting, however, that the less densely packed strands in Figure 3a, whose dimensions are given above, probably provide the most accurate values). It was also verified that neither the block copolymer by itself nor the PDP by itself gives rise to this morphology. It should be added that we presume (based on studies in the bulk21), but have no proof at the present time, that the PDP is hydrogenbonded to the P4VP blocks at the air/water interface. Its presence is, however, essential to obtaining the nanostrand network (since it is not obtained by either the conventional or the solvent-assisted procedures for the copolymer by itself), at least at the copolymer composition investigated. This may be attributed to a combination of two factors, one being its role in providing the appropriate weight fraction of the P4VP block with which it is associated, such that a cylindrical morphology is favored at higher surface pressures, and the second by its contribution to mobility in the system by (internal) plasticization of the P4VP block (although, as will be discussed below, mobility in the PS block might be expected to be more important for the morphology change). Regarding the first factor, it is of interest that the P4VP block proportion, at 12.2 mol %, is within the composition range (6-14 mol %) in which cylindrical aggregates appear in the PS-P4VP+C10 (i.e., decylated 4VP) system using the conventional procedure.12 Nevertheless, in the system we studied, it is clearly the solvent that is critical for the transformation to the nanostrand network, since without it - i.e., by using the conventional procedure with all other factors being identical - this morphology is not observed. On the other hand, as shown in Figure 2a, short cylinders or Nano Lett., Vol. 5, No. 7, 2005
Figure 5. AFM height images of PS-b-P4VP/PDP LB monolayers prepared by the solvent-assisted procedure in undefined conditions.
rods can be a minor component in the morphology obtained by the conventional procedure. Some of the AFM images captured during our investigations suggest at least two possible mechanisms whereby the circular aggregates transform into the nanostrand network pattern during monolayer compression. First, Figure 4 (especially 4a) shows planar-type aggregates with irregular contours and Figure 3d shows a nanostrand network to which elongated planar-type outgrowths are fused. These images suggest that large planar aggregates may be transformed via the development of excrescences that progressively elongate to ultimately become nanostrands (Figure 4a representing a point near the onset of the transformation, and Figure 3d a point near its completion). A second mechanism, involving the transformation of the nanodots into nanostrands, is suggested by the images shown in Figure 5, obtained in our preliminary experiments under undefined conditions of the solvent-assisted procedure. Here, the morphology is dominated by mainly unconnected short nanostrands (linear or branched). There are also runs of unconnected spheres and/or very short strands, as well as some strands that seem to be composed of connected but still distinguishable spheres. All strands are capped at their ends by a sphere that appears slightly larger in diameter and height than the rest of the strands. These observations suggest that nanostrands are also formed by the linear fusion of spheres and ultimately interconnect to form the network. Both transformations would certainly require extensive rearrangement of the polymer chains, especially the relatively long PS block segments, which must undergo considerable Nano Lett., Vol. 5, No. 7, 2005
disentanglement and re-entanglement. In addition, the hydrophilic block must adapt its disposition accordingly, presumably by extension onto the water surface away from the PS block everywhere except at the rims of the planar aggregates in the first mechanism and by moving out of the way of the points of fusion of the spheres in the second mechanism (where runs of apparently connected but distinguishable spheres are observed may indicate that this moving out of the way has not yet happened). It is easy to conceive that there must be sufficient mobility for all of this to take place. It is also consistent with the relatively long time required at the higher surface pressures for the transformation to complete itself. In conclusion, our study has shown that the presence of the spreading solvent can be a useful tool for obtaining specific morphologies. In the system studied, this has allowed the very infrequently observed18 nanostrand network pattern to be generated. Thus, the usual procedure of waiting for the spreading solvent to evaporate before barrier compression is not always appropriate. The solvent-assisted method offers an attractively simple route for manipulating supramolecular self-assembly of block copolymers at the air/water interface, and hence for generating specific surface nanopatterns. In future work, we plan to investigate this system with other block lengths, with nonequimolar proportions of PDP, with other surfactant-like molecules, and with other solvents. It would be particularly pertinent to identify high-boiling and water-insoluble solvents (at least one for each block as well as a nonselective one), which could allow the amount and location of the solvent to be precisely known, to obtain more detailed insight into the mechanism of the morphology transformation. Acknowledgment. The financial support of NSERC (Canada), FCAR/FQRNT (Que´bec) and Universite´ de Montre´al is gratefully acknowledged. Q.L. was partially supported by the CERSIM (Centre de recherche en sciences et inge´nierie des macromole´cules, Universite´ Laval) postdoctoral program. We also acknowledge our membership in the FQRNT-supported, multi-university Centre for SelfAssembled Chemical Structures (CSACS). Finally, the authors are grateful to Prof. Antonella Badia for useful discussions, help with instrumental issues, and critical reading of the manuscript. References (1) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (2) Lazzari, M.; Lo´pez-Quintela, M. A. AdV. Mater. 2003, 15, 1583. (3) Cox, J. K.; Eisenberg, A.; Lennox, R. B. Current Opin. Colloid Interface Sci. 1999, 4, 52. (4) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407. (5) Fo¨rster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688. (6) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557. (7) Kosonen, H.; Valkama, S.; Hartikainen, J.; Eerika¨inen, H.; Torkkeli, M.; Jokela, K.; Serimaa, R.; Sundholm, F.; ten Brinke, G.; Ikkala, O. Macromolecules 2002, 35, 10149. (8) Thu¨nemann, A. F.; General, S. Macromolecules 2001, 34, 6978. (9) Fahmi A. W.; Stamm, M. Langmuir 2005, 21, 1062. (10) (a) Bronich, T. K.; Ming, O. Y.; Kabanov, V. A.; Eisenberg, A.; Szoka, F. C.; Kabanov, A. V. J. Am. Chem. Soc. 2002, 124, 11872. (b) Burke, S. E.; Eisenberg, A. Langmuir 2001, 17, 8341. 1313
(11) (a) Zhu, J.; Eisenberg, A.; Lennox, R. B.; J. Am. Chem. Soc. 1991, 113, 5583. (b) Zhu, J.; Lennox, R. B.; Eisenberg, A.; J. Phys. Chem. 1992, 96, 4727. (12) (a) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (b) Zhu, J.; Eisenberg, A.; Lennox, R. B. Makromol Chem., Macromol. Symp. 1992, 53, 211. (13) (a) Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B.; Langmuir 1993, 9, 2243. (b) Li, S.; Clarke, C. J.; Lennox, R. B.; Eisenberg, A. Colloids Surf. A 1998, 133, 191. (14) Meszaros, M.; Eisenberg, A.; Lennox, R. B. Faraday Discuss. 1994, 98, 283. (15) (a) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B.; Langmuir 1999, 15, 7714. (b) Cox, J. K. Y., K.; Eisenberg, A.; Lennox, R. B. Phys. Chem. Chem. Phys. 1999, 1, 4417. (16) Baker, S. M.; Leach, K. A.; Devereaux, C. E.; Gragson, D. E. Macromolecules 2000, 33, 5432. (17) Devereaux, C. A.; Baker, S. M. Macromolecules 2002, 35, 1921.
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(18) Seo, Y.-S.; Kim, K. S.; Galambos, A.; Lammertink, R. G. H.; Vancso, G. J.; Sokolov, J.; Rafailovich, M. Nano Lett. 2004, 4, 483. (19) (a) Seo, Y.; Im, J.-H.; Lee, J.-S.; Kim, J.-H. Macromolecules 2001, 34, 4842. (b) Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001, 34, 8735. (20) Peleshanko, S.; Jeong, J.; Gunawidjaja, R.; Tsukruk, V. V. Macromolecules 2004, 37, 6511. (21) Ruokolainen, J.; Saariaho, M.; Ikkala, O.; ten Brinke, G.; Thomas, E. L.; Torkkeli, M.; Serimaa, R. Macromolecules 1999, 32, 1152. (22) Gaines, G. L. J. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sons: New York, 1966. (23) Shin, K.; Rafailovich, M. H.; Sokolov, J.; Chang, D. M.; Cox, J. K.; Lennox, R. B.; Eisenberg, A.; Gibaud, A.; Huang, J.; Hsu, S. L.; Satija, S. K. Langmuir 2001, 17, 4955.
NL050530Z
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