Control of Morphological Features in Micropatterned ...The films were investigated by AFM or by SEM at low voltages (1 kV) in order to obtain high contrast within the ultrathin films

Reprint : Solidification and Crystallization, Ed. Herlach, Dieter, Wiley-VCH, Sept. 2004, ISBN 3-527-31011-8

Control of Morphological Features in Micropatterned Ultrathin Films

E. Meyer, H.-G. Braun Max Bergmann Center of Biomaterials Dresden (TUD/IPF), D-01069 Dresden, Hohe Str.6

Abstract

Crystallization in ultrathin films obeys diffusion controlled growth processes (DLA) which result in branched growth morphologies. For the first time different morphological features related to diffusion limited aggregation could be created on request by controlled initiation of crystallization processes in ultrathin polymer films (polyethylene oxide). Prerequisite for the controlled initiation is the generation of amorphous- with respect to crystallization –metastable films which can be realized by film formation on micrometer-sized isles surrounded from non-wetting barriers. Inside of this isolated amorphous microareas heterogeneous nucleation can be initiated by contact with an AFM-tip. Morphological features of the DLA- structures such as tip-radius, branch widths and correlation lengths are varied in dependence of the growth kinetic which is influenced by temperature, surface properties or other parameters. In addition influences of limited material reservoirs in confined areas on film morphology are discussed.

1. Introduction

Nature offers a wide range of examples with highly branched morphological features. Typical dimensions of the branches may range from the atomic (1) to the macroscopic scale. The branching structure generally originates from non-equilibrium growth processes which may appear during crystal growth- such as electrocrystallization (2)- as well as during bacteria colony growth (3). Theoretically the processes are commonly described as diffusion limited aggregation (DLA) (16). Recent studies demonstrated the presence of highly branched structures also for the crystallization of polymers in ultrathin films of homopolymers (4,5) and in blends (6,7). A theoretical approach that takes into account the peculiarities of macromolecular crystallization has recently been published by Reiter and Sommer (8,9,10). Whereas previous work focused on the crystallization of polymers on homogeneous surfaces our studies deal with crystallization processes of ultrathin films in constrained microstructures (15). The microstructured films are prepared by controlled dewetting of polymer solutions on chemical heterogeneous surfaces (11,12,15). Surface heterogenization can be achieved by microcontact printing (13) or by electron beam lithography of self-assembled monolayers (14) of appropriate organic molecules. In our experiments a gold surface is hydrophilized by chemisorption of an organic molecule which contains a polar group (-COOH) introducing hydrophilicity and a sulphur end-group binding to the gold surface. Irradiation with an electron beam changes the molecular structure causing a change from hydrophilic to hydrophobic properties. Dip-coating of patterned surfaces in


chloroformeous PEO solution causes dewetting of the hydrophilic polymer polyethylene oxide (PEO) from the hydrophobic areas (15) leading to microstructured films.

2. Experimental

Plasma cleaned (Harrick Plasma Cleaner) silicon substrates were coated with a 3 nm Chromium layer as adhesion promoter and with a 50 nm gold layer. The gold surface is hydrophilized by adsorption (30 minutes) of 11-Mercaptoundecanoic acid (11-MAU) dissolved in ethanol (10-3 molar). Hydrophobization of the 11-MAU treated surface within predefined micropatterns is achieved by electron beam lithography with a critical dose of 1000 µAs/cm2 . The E-beam lithography is carried out in a Zeiss Gemini DSM 982 low voltage scanning electron microscope equipped with a Raith Quantum Plus lithography system. Film formation on the microstructured substrates is done by dip-coating in chloroformeous PEO-solutions (~ 0.15 % by weight, Mw = 10000) with a lift-off velocity of the substrate of 2 mm/sec. Crystallization at various temperatures was initiated on a peltier heating stage under light microscopic observation (ZEISS Axiotech). The light microscope is equipped with an AFM head (Ultraobjective SIS® ). The films were investigated by AFM or by SEM at low voltages (1 kV) in order to obtain high contrast within the ultrathin films.

3. Results & Discussion

Ultrathin film formation on microheterogeneous substrates allows to generate metastable amorphous areas which are created on micrometer-sized isles surrounded from non-wettable barriers (Figure1). The existence of these long time stable amorphous domains is a prerequisite for study of crystallisation processes under controlled thermal and geometrical conditions. Immediately after dip coating the films are amorphous all over the surface. In presence of

Figure 2. Crystallized PEO-layer grown from a spherical nucleus.

Figure 1. Amorphous PEO-isle (C) separated by a non wettable hydrophobic barrier (B) in a crystallized film (A).


heterogenous nuclei crystallization process is initiated and the growth of highly branched lamellae structures of PEO proceeds (A in Figure 1). But inside the isolated film fragments the probability to find a nucleus is rather low and therefore the domains on micrometer-sized isles (C in Figure 1) which are separated by non-wettable barriers (B in Figure 1) remain amorphous.

Figure 4. Controlled nucleation by AFM-tip

Figure 3. Metastable PEO-film on micrometer-sized area.

More sophisticated geometries of amorphous areas are realizable by electron beam lithography of previously chemisorbed MUA (Figure 3). Complicated traces as guidelines for following crystallization processes can be generate by appropriate choice of the geometry of hydrophobic barriers. Crystallization of amorphous films may start by nucleation from surface scratches, from rims resulting from a dewetting process or from small particles as demonstrated in Figure 2 (heterogeneous nucleation). Controlled nucleation can be initiated by contact with an AFM-tip at predefined times and locations (Figure 4). For the first time diffusion limited growth processes of branched PEO lamellae could be observed within prearranged 2d-confinements. As demonstrated in Figure 4 the initial growth of branched lamellae is radial around the nucleation side. Reaching the 2d-channel a single branch is guided by the channel geometry.

Figure 5. Characteristic features of branched morphological structures R tip radius Ζ correlation length perpendicular to stem X correlation width parallel to stem


Initiation of crystallization processes on request allows to control the film morphology. Morphological features of DLA or DBM growth patterns are characterized by the tip-radius, the correlation length and the correlation width (16) of branched structures (Figure 5). In order to understand the molecular setup of the PEO-structures that occur during the diffusion controlled growth processes in ultrathin polymer films relevant molecular parameters are considered in scheme 1.

TbchscdwpiOtlFcPcafTdzuHamar

Scheme1. Basic structural units involved in the crystallization process. Crystalline lamellae are composed of typically 4 unitcells in which PEO segments are arranged in a 72 helical conformation of 1.948 nm basic length (17).

he initial thickness of the amorphous PEO layer is about 3 nm. The typical thickness of the ranched lamellae is measured to 8 nm as for example demonstrated in Figure 5. Within the rystallographic PEO-unit cell seven ethylene oxide monomer segments are arranged in a 72-elix (17 ) with a c-lattice spacing of 1.948 nm. A single branched lamellae of 8 nm thickness hould contain 4 units cells with a polymer chain orientation perpendicular to the surface. The rystallization in the ultrathin PEO film is accompanied by a thickness increase whereby a epletion zone is created in front of the growing lamellae. The width of the depletion zone varies ith crystallization conditions and geometrical confinements and has to be overcome by the olymer chains diffusing from the amorphous layer to the crystallizing lamellae. Crystallization n ultrathin PEO-films can also result in stacked lamellae structures which will be shown later. ne of the key parameter to influence the morphological features is to control the nucleation

emperature at which the diffusional growth process is initiated and at which the branched amellae grow. igure 6 shows a serious of images in which the crystallization is carried out at varying rystallization temperatures inside circular motifs. The equilibrium melting temperature of the EO is 56°C . The heterogeneous nucleation was initiated approximately at the center of each ircle. The growth history can easily be reconstructed from the direction of the branches which re oriented towards the lamellae growth front. In Figure 6 B it is easy to follow the growth pass rom the center through the small gaps and then symmetrical growing to both sides of the gap. he morphologies show a texture similiar to that in spherolites. Branches which grow from ifferent directions towards each other are separated by a gap which originates from the depletion ones. Figure 7 demonstrates much more pronounced gaps for “spherolitic” growth in an ltrathin homogeneous film with a radial growth around a large number of nucleation sites. igher crystallization temperature and consequently decreasing temperature gap between the

ctual sample temperature (nucleation- and crystal growth temperature) and the equilibrium elting temperature causes branch growth with larger tip-radius, increasing correlation length

nd an overall coarser structure (Figure 6 A-D). The coarser branch structure results from eorganisation processes of polymer chain segments. As a consequence of higher segment


mobility at higher temperatures the strong curved tips increase their radius of curvature and reduce the interfacial energy leading to a more favorable thermodynamic state . In ultrathin polymer films not only the branch morphology is changed but also the lamellae thickness can be increased during the reorganisation processes. At high nucleation temperatures crystallization may even result in multilamellae structures which nearly lost their branching completely (Figure 6D, Figure 8).

Figure 6. Control of morphological features in confined ultrathin PEO-films crystallized at different temperatures

Nucleation and growth temperature: 20 ° C

Nucleation and growth temperature: 31° C

Nucleation and growth temperature: 33 ° C

Nucleation and growth temperature: 38° C

A B

C D

Diffusion controlled growth processes are not only determined by temperature but ,additionally, if the growth happens in constrained areas (in our experiment in isolated rings) the structure formation is influenced by geometry of pattern. In confined areas the concentration gradient which is the driving force for the structure formation remains not constant during the isothermal growth process. Due to the limited material reservoir the depletion zone ahead the propagating solidification front is continuous increased (Schema1)


which causes a decrease of the tip growth velocity respectively a decrease of number of molecules/ time attaching the growth front. Analogous to the temperature effect tip radii are increased, less folded and thicker lamella crystals of the macromolecular stems and finally multilamella stacks are formed. Figure 6C clearly shows the changing of morphological features due to the constrained geometry. Near the nucleation centre (initiation by AFM-tip) the branches are finer than in areas far from the initiation point.

Figure 7. „Spherolitic“ growth of PEO branched lamellae initiated from different nucleation sites.

Figure 8. Increase of tip-radii and multilamellae at the end of the dendrite due to changing of diffusion conditions during isothermal crystallization at limited material supply.

4. Perspectives

The structural organization of PEO-units at surfaces is of great importance for the biocompatibilisation of man-mad materials. PEO is well known to reject proteins from a surface and therefore to influence cellular growth processes in the spaces in between the PEO-branches. From this perspective it becomes obvious that the understanding of and control over morphological features of PEO on surfaces is a goal for the development of biocompatible surfaces. Crystallization in ultrathin PEO-layers is a molecular self-organization process in which a network of highly branched lamellae structures are formed. The PEO lamellae networks with defined and controllable submicrometer-sized features (branch width, distance) may be an approach to gain control over cell adhesion and cell growth processes on surfaces. The detailed knowledge of the structure formation on surfaces in dependence of pattern geometry and the temperatures at which crystallization is initiated allows a morphological surface engineering of PEO lamellae networks. The lamellae PEO-networks can be permanently fixed to the underlying surface by electron-beam irradiation using PEO-layers as electron-beam resist (Figure 9). As a result the irradiated areas are insoluble in water. Beside the control of morphological features of the network (distances or branch widths etc. for growth guide lines on nm-scale) the electron beam lithography can be also used for structuring of homogenous PEO-films in order to influence cellular growth processes on the scale of several micrometers.


Figure 9. Ultrathin Polyethylene oxide film which has been crystallized on a homogeneous (unstructured) surface and afterwards structured and irreversible attached to the surface by electron beam lithography.

5. Literature

1. Z. Zhang, G. Lagally, Science 1997, 276, 377-383 2. M. Castro, R. Cuerno, A. Sanchez, F. Domiguez-Adame, Phys. Rev. E, 2000,61,161 3. I. Golding, Y. Kozlovsky, I. Cohen, E. Ben-Jacob, Physica A, 1998, 260, 510-554 4. G. Reiter, J.-U. Sommer, Phys. Rev. Lett., 1998, 80, 3771-3774 5. M. Wang, H.-G. Braun, E. Meyer, Macromol. Rapid Commun. 2002, 23, 853-858 6. M. Wang,H.-G. Braun, E. Meyer, Polymer 2003, 44, 5015-5021 7. M. Wang,H.-G. Braun,E. Meyer, accept. for publication in Macromolecules 8. G. Reiter, J.-U. Sommer, J. Chem. Phys., 2000,112, 4376-4383 9. J.-U. Sommer, G. Reiter, J.Chem. Phys., 2000, 4384-4393 10. J.U. Sommer, G. Reiter in Polymer Crystallization LNP 606 (Eds. :J.U.

Sommer,G.Reiter), Springer ,Heidelberg, 2003, Chapter 9 11. H.-G. Braun, E. Meyer, Thin Solid Films, 1999, 345,222-228 12. E. Meyer, H.-G. Braun, Macromol. Mater. Eng., 2000, 276/277, 44-50 13. Y. Xia, G.M.Whitesides, Angew. Chemie Int. Ed., 1998, 37, 550-575 14. C.K. Harnett, K.M. Satyalaksmi, H.G. Craighead, Langmuir, 2001, 17, 178-182 15. H.-G. Braun, E. Meyer, M. Wang, in Polymer Crystallization LNP 606 (Eds. :J.U.

Sommer,G.Reiter), Springer ,Heidelberg, 2003, Chapter 13 16. P. Meakin, Fractals, scaling and growth far from equilibrium,Cambridge University Press,

1998 17. Y. Takahashi, H. Tadokoro, Macromolecules, 1973, 6, 672- 675

Acknowledgement

We gratefully acknowledge financial support of the “Deutsche Forschungsgemeinschaft” within the priority program “Wetting and structure formation at interfaces”

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Control of Morphological Features in Micropatterned ...The films were investigated by AFM or by SEM at low voltages (1 kV) in order to obtain high contrast within the ultrathin films