Mesoscopic pattern formation of nanostructured polymer assemblies

PII: S0968-5677 (98) 00027-3

Supramolecular Science 5 (1998) 331—336  1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00

Mesoscopic pattern formation of nanostructured polymer assemblies Norihiko Maruyama, Olaf Karthaus, Kuniharu Ijiro and Masatsugu Shimomura* Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan

Takeo Koito and Shinnichiro Nishimura Graduate School of Science, Hokkaido University, Sapporo 060, Japan

Tetsuro Sawadaishi, Norio Nishi and Seiichi Tokura Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan

Hierarchical mesoscopic structures of the nanoscopic supramolecular assemblies, which consist of polyelectrolytes and bilayer-forming amphiphiles, are prepared by a simple and new solvent-casting method. Submicron scale 2-D structures, e.g. regular dots, stripes, and networks, are formed when highly diluted organic solutions of polymer assemblies are cast on solid surfaces. Dynamic mesoscopic regular structures, the so-called ‘dissipative structures’, formed in the non-equilibrium processes of solvent-casting are fixed as hierarchically structured polymer assemblies.  1998 Elsevier Science Limited. All rights reserved. (Keywords: polymer-surfactant polyion complexes; dissipative structure; hierarchical structuring; mesoscopic pattern)

INTRODUCTION Hierarchical structuring of molecules from nanoscopic to macroscopic scale in biological systems are structural and functional bases of life. The nanoscale molecular assemblies, e.g. biological membranes which are assembled from lipid molecules, proteins, etc., are structural components of organellas and cells, which are the mesoscopic molecular organizations constructing biological tissues and organs. One of the key words for hierarchical structuring is ‘self-assembly’ or ‘self-organization’ of the molecules and the molecular assemblies. In the last decade, by using the ‘self-assembling’ technique, chemists have succeeded in constructing many sorts of nanoscopic molecular architectures, e.g. molecular-recognitiondirected molecular assemblies, surfactant bilayer assemblies, polymeric Langmuir—Blodgett films, self-assembling monolayers, and alternatively deposited polyelectrolyte multilayers. The driving forces of the molecular ‘self-assembling’ in the supramolecular chemistry of the nanoscopic world are many weak physicochemical intermolecular interactions, e.g. van der Waals force, hydrogen bonding interaction, hydrophobic interaction, electrostatic forces, and so on. Polyion complexes formed by the electrostatic interaction between polyelectrolytes and oppositely charged bilayer-forming amphiphiles are a typical example of the nanoscopic molecular architectures with regularly ordered two-dimensional layered structures at the air—water interface and in the solvent cast films. * To whom correspondence should be addressed

On the other hand, spatiotemporal structures are known to be formed in dissipative processes under chemical or physical conditions far from equilibrium. Several types of regular patterns; spirals in the Belousov— Zhabotinsky reaction systems, honeycomb and stripes of Rayleigh—Be´nard convection, striped Turing patterns in polymer gel reactors, fingering in granular flow and polymer transport, and hexagons in oscillating granular layers, are formed in the dissipative processes with various spatial scales from mesoscopic size of submicrons to macroscopic centimeter size. Since the hierarchical structuring in the mesoscopic range, e.g. micro-phase separation in block copolymers, is considerably indispensable for designing new polymer materials, we attempt to use the dissipative processes for the mesoscopic structuring of the nanostructured polymer assemblies composed of polyelectrolytes and bilayer-forming amphiphiles. Since the dissipative structure is essentially a dynamic structure requiring energy dissipation, we have to freeze the dissipative structures as static stable structures of polyion complexes. We focused on a casting process of the polyion complex solution on a solid surface because the casting processes affected by many physical parameters, e.g. viscosity, temperature, surface tension, etc., are complex enough to form dissipative structures. Also after rapid solvent evaporation, the dissipative structures formed in the casting solution can be immobilized as the regular polymer patterns on the solid surface. Here we report a novel general method of the mesoscopic structuring of polyion complex assemblies based on freezing the spatiotemporal structures formed in a casting process of polymer solutions.

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Mesoscopic pattern formation: N. Maruyama et al. EXPERIMENTAL A water-insoluble polyion complex was precipitated when an aqueous bilayer solution of dihexadecyldimethylammonium bromide (Sogo Pharmaceutical, Japan) was mixed with a water solution of sodium poly-(styrenesulfonate) (Tokuyama, Japan,) or poly(propylsulfonylthiophene) (Showa Denko, Japan). A highly diluted chloroform solution of the polyion complex was dropped on a clean surface of glass, silicon wafer, and freshly cleaved mica in a glove box under controlled atmospheric humidity. To visualize the evaporation process through an epifluorescence microscope, a small amount of an amphiphilic cationic fluorescence probe, octadecyl rhodamin B (Molecular Probe, USA), was added as a counterion of polystyrenesulfonate as well as the bilayer-forming amphiphiles. Fine talc particles (ca.10 lm in diameter) were added to visualize the convection flow in a thin liquid film of the polymer solution.

MESOSCOPIC REGULAR STRUCTURE FORMATION UNDER DRY CONDITIONS Several microliters of the highly diluted polymer solution (a few hundred micrograms per liter) was spread homogeneously as a liquid film over an area of ca. 1 cm on the hydrophilic solid surfaces. Mainly due to solvent evapor-

Scheme 1

ation and the surface tension the solution front is always directed towards the center of the liquid film. At the first stage of evaporation the solution front receded smoothly, then after a short time it receded intermittently with jumping. The jumping instability with a stick and slip movement of the receding front is ascribable to the local gelation effect of polymer at the three-phase line (liquid—substrate—air boundary) where the polymer concentration is assumed to be higher than the bulk liquid film. The local gelation prevents the front from receding. After deposition of the condensed polymer on the solid surface the front can recede again until next gelation. The condensed polymer left behind at the jumping front are

Figure 1 Fluorescence micrographs of polymer patterns fixed on a solid surface after solvent evaporation. The bright part indicates the presence of the polyion complex assemblies containing fluorescence molecules. (a) Regular jumping lines after periodic stick-slip motion of the front on the mica surface. Interline distance was c.a. 20 lm and the linewidth measured by an atomic force microscope was several hundred nanometers. (b) Jumping lines with fingering fixed on the glass surface. (c) Jumping-striping transition on the mica surface. Under this experimental condition, we can prepare long stripes of several mm in length. A minimum value of height, 3—5 nm, corresponds to the monolayer or bilayer thickness of the polyion complex deduced from X-ray diffraction experiments. (d) A regular dot pattern fixed on the mica surface

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Mesoscopic pattern formation: N. Maruyama et al. observed as periodic lines perpendicular to the receding direction (Figure 1a). Condensation of the polymer at the three-phase line can be observed directly by fluorescence microscopy. The solution front was clearly observed as a bright red boundary line between the dark substrate and the bulk solution. A finding of interest was the condensation of a periodic polymer along the three-phase line, which was observed as a typical fingering phenomenon (indicated

with a white arrow) similar to the ‘tear of strong Wine’. The fingering inhomogeneity of polymer condensation at the three-phase line can be attributed to the Marangoni effect which was visualized as a fast migration and coarsening of the fingers (Figure 2a). Figure 1b shows polymer patterns immobilized on a glass surface as jumping lines with a large number of small fingers. On the surface of the freshly cleaved mica, the fingers often started to grow perpendicular to the three-phase

Figure 2 In situ epifluorescence microscopic observations of the solution front (top view) during solvent evaporation. (a) Snap shots of fast migration and fusion of the fingerings at the three-phase line. Two fingers indicated with a white arrow diffuse laterally along the front line and merge into one large finger. Thirty microliters of chloroform solution (170 mg/L) of the complex of dihexadecyl dimethylammonium and poly-(styrenesulfonate) was spread on the mica surface; time interval, 500 ms. (b) Time courses of regular striping from the fingers. Thirty microliters of chloroform solution (680 mg/L) was spread on the mica surface; time interval, 1.5 s

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Mesoscopic pattern formation: N. Maruyama et al. line. Figure 2b shows snap shots of the stripe formation from the periodically generated fingers. The fingers were straightened as regular stripes concomitant with smooth receding of the solution front. The mica surface is more suitable for the finger growth than the glass surface. Figure 1c shows a fluorescence micrograph of the fixed polymer pattern at the jumping-to-striping transition. Dimensions of the stripe patterns are altered by casting conditions. The higher the concentration and molecular weight of polymers, the larger the dimensions of the stripe. A regularly arranged dot pattern (Figure 1d) formed occasionally on the mica surface when the front receding was faster than polymer deposition or at low polymer concentration. As shown in Figure 3 several circular domains that resemble Be´nard-convection cells were formed in the central portion of the polymer solution. Local convection in the circular domains was observed as a very fast movement of the talc particles which were trapped and turning around in the individual cells. At the first stage of casting, the lifetime of the convection cells was very short

Figure 3 In situ epifluorescence microscopic observations of the casting solution close to the final stage of solvent evaporation

in the subsecond range and new cells were continuously supplied from the center of the casting solution. With increasing viscosity of the solution due to solvent evaporation the cells were stabilized. The shape and size of the cells were almost identical during the evaporation process. Bright streams from the cell boundary towards the solution front shown in Figure 3 suggest another convectional flow between the three-phase line and the cell boundary. The talc experiment clearly shows radial convection. The particle repeats going to the solution front and back to the cell boundary. The periodic fingering instability at the three-phase line is assumed to be generated by the radial convectional streams originated from the regular arrangement of the Be´nard-type convection cells in the central portion.

MESOSCOPIC REGULAR STRUCTURE FORMATION UNDER WET CONDITIONS At the final stage of the casting, homogeneous and optically transparent polymer films were prepared under the dry casting condition. Under high atmospheric humidity, however, the films were not optically transparent. Figure 4 shows a fluorescence and an atomic force micrograph of the cast film prepared under the wet condition. The cast film has regular honeycomb morphology with a size of ca. 1.5 lm per honeycomb cell. The pattern consists of open honeycomb structures because the dark parts of the fluorescence micrograph are shown to be void by atomic force microscopy. Based on the in-situ observation of the casting process, the formation mechanism of the honeycomb structure is schematically shown in Figure 5. After placing a droplet of chloroform solution on the substrate, the chloroform starts to evaporate. This leads to a cooling of the solution and micron-size water droplets condense onto the chloroform solution of the polyion complex. The droplets are transported to the three-phase line and are hexagonally packed by the convectional flow or the capillary

Figure 4 (a) Fluorescence micrograph of the cast film prepared under high atmospheric humidity. (b) Atomic force microscopic image of the cast film. Thirty microliters of chloroform solution (1.0 g/L) of the complex of dihexadecyl dimethylammonium and poly-(styrenesulfonate) was spread on the mica surface under 50% relative humidity

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Mesoscopic pattern formation: N. Maruyama et al. force generated at the solution front. Since the surface tension between water and chloroform is reduced by the polyion complex of the bilayer forming surfactants, the water droplets are stabilized against fusion. Upon evaporation of chloroform the three-phase line moves over the hexagonal array of water droplets. The water droplets and some of the polymer between them are left behind. Finally, the water evaporates, leading to the observed honeycomb structure. Regularly ordered honeycomb patterns of polymer assemblies formed by a similar mechanism were proposed by Franiois et al. and Imhof et al.. The most important parameters that control the size of the honeycomb cells are the relative humidity of the atmosphere and the concentration of the polymer

solution. Figure 6 shows the effects of the two parameters on the honeycomb size. The honeycomb wall surrounding the hole becomes thinner with decreasing concentration, and higher humidity leads to larger honeycomb holes. The size of the honeycomb holes can be regulated between 0.5 and 5 lm by the two parameters. The X-ray diffraction pattern of the honeycomb film with Bragg peaks of 3.6 nm repeating period strongly suggests that the honeycomb walls are constituted of nanostructured lamella assemblies of the polyion complexes.

CONCLUSION It is concluded that the spatiotemporal structures based on dissipative structures in polymer solutions are frozen on solid surfaces as the regular mesoscopic polymer assemblies with the hierarchical structures of the nanoscopic bilayer assemblies. We believe that our finding is applicable as a novel general method for the formation of a mesoscopic structure without lithographic procedures because the formation of a dissipative structure is essentially a general physical phenomenon for any polymer material. To fabricate the mesoscopic structure with good reproducibility, we are now preparing the regular stripes under the controlled continuous front receding, e.g. generated in the dipping up procedure of solid substrates from a polymer solution in the controlled conditions (dipping speed, temperature, solvent evaporation, etc.). We have already prepared long stripe structures of double-helical DNA complexed with dihexadecyldimethylammonium bromide on mica surface to measure the photoinduced electron transfer through stacked base pairings.

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Figure 5 Schematic formation mechanism of the honeycomb structure: (a) top view, (b) side view

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