Chapter 10
Soft Lithography on Block Copolymer Films: Generating Functionalized Patterns on Block Copolymer Films as a Basis to Further Surface Modification 1
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Martin Brehmer , Lars Conrad , Lutz Funk , Dirk Allard , Patrick Théato , and Anke Helfer 1
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Institute of Organic Chemistry,Johannes Gutenberg University, D-55099 Mainz, Germany (email:
[email protected]) Department of Chemistry, Bergische Universität Wuppertal, D-42097 Wuppertal, Germany 2
Functionalized patterns on the surfaces of amphophilic diblock copolymer films were generated using polar/apolar interactions applied by soft lithographic techniques. Further modification of the patterned surfaces included e.g. the deposition of conducting and semiconducting material, which offers the opportunity to build sensor structures ranging from the micron to the submicron size.
Introduction Block copolymers consist of two or more homopolymer chains that are linked by covalent bonds. One characteristic feature of these polymers is the phenomenon called micro-phase separation, which is due to the immiscibility of the different homopolymers on the one hand and their covalent connection on the
© 2004 American Chemical Society
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130 other. The immiscibility arises from the reduced contribution of the polymer chains to the entropie term in the free energy term of the system. As a rule, the enthalpic term of the equation has a positive value, as long as there are not any attractive interactions between the different polymer chains. If the entropie term cannot compensate the value of the enthalpic term, which means that the free energy becomes negative, a miscibility gap will occur. The connectivity of the immiscible polymer phases leads to microscopic structures in the polymer sample with morphologies determined by the volumefractionof the phases . The most well known morphologies are spheres or cylindrical morphologies, gyroid structures, perforated layers and lamellae. If one of the two phases is removed the remaining polymer framework can be used as an etch mask for the preparation of semiconducting capacitors . The morphologies described are characteristic for the internal structure of a block copolymer sample so far. However things are different at the surface. Here the adjacent medium leads to an enrichment of the polymer phase that has the lowest surface energy towards this medium . Consequently the surface of a block copolymer sample consist only of one sort of segments if enough material is available. As the enrichment is a dynamic process, it is necessary to heat the sample above the glass transition temperature of the polymer . We use this property of enrichment of one polymer phase at the surface to generate functionalized patterns on thin spin-casted films of amphiphilic block copolymers. By using amphiphilic block copolymers we are able to switch from polar segments at the surface to nonpolar just by replacing the adjacent apolar by a polar medium or vice versa . This process was studied by contact angle and A F M measurements. To get a patterned surface, we applied polar and apolar surface interactions with the help of soft lithographic techniques that were introduced by G. M . Whitesides et al. . We think that a combination of these concepts of surface reconstruction on amphiphilic block copolymers and soft lithography is of special interest. By doing so we should also be able to functionalize the pattern generated on the spin-casted block copolymer film. Therefore we synthesized diblock copolymers via nitroxide mediated controlled radical polymerization containing phenolic (1) or hydroxy (2) groups in the hydrophilic segments, whereas the hydrophobic segments remained unfiinctionalized. These functional groups show up where the hydrophilic segments come to the surface allowing various surface modifications. A few of these modifications like "grafting from" polymerization, electroless deposition of copper, polyelectrolyte multilayers or deposition of mesoporous transition metal oxides are shown later in this article. The deposition of conducting and semiconducting material on the patterned surface, with pattern size ranging in the micron and submicron scale, opens up prospects to construct various sensor structures. 1
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Experimental Section
Reagents 4-Acetoxystyrene (3) was bought from Aldrich. 4-Octylstyrene (4) was synthesized according to ref. (7.), the initiator (5) for the nitroxide mediated free radical polymerization according to ref. (8.). The monomer acetic acid 2-[2-(4vinyl~phenoxy)-ethoxy]-ethyl ester (6) was prepared as described below and outlined in Figure 1. All solvents were freshly distilled and all reactions were carried out under nitrogen. Diglyme was freshly distilled from potassium hydroxide to remove peroxides. Potassium carbonate was ground and dried overnight in vacuo at 150°C.
Figure 1: Synthesis of Monomer (6)
Synthesis of Monomer (6)
Synthesis of acetic acid 2-(2-chloro-ethoxy)-ethyl ester (7)
To a mixture of 114.5 g 2-chloro-ethoxy-ethanol (8) (0.92 mol) and 93.9 g acetic anhydride (0.92 mole) was added a few drops of sulfuric acid under stirring whereupon an exothermic reaction started. When the heat emission decreased, the mixture was stirred at 100°C for two more hours and poured on ice water afterwards. The aqueous phase was extracted three times with ethyl acetate and the combined organic phases were neutralized with sodium carbonate. The ethyl acetate was evaporated and the residue distilled in vacuo (112°C at 2*10" bar). The product, a colorless liquid, was given in 80% yield. 2
132 Synthesis of acetic acid 2-[2-(4-formyl-phenoxy)-ethoxy]-ethyl ester (9)
15.12 g 4-Hydroxybenzaldehyde (0.12 mol), 41.25 g acetic acid 2-(2chloro-ethoxy)-ethyl ester (7) (0.25 mol) and 34.5 g potassium carbonate (0.25 mol) were suspended in 200 ml dimethylformamide and refluxed for 2 days. Afterwards the reaction mixture was poured in 200ml water and the aqueous phase was extracted three times with ethyl acetate. The combined organic phases were washed with 2 η potassium hydroxide in order to remove the remaining 4hydroxybenzaldehyde. The organic solution was dried with potassium carbonate and the solvent was evaporated. The crude product was purified by column chromatography (petrolethenethyl acetate 2:1). The product was obtained as white crystals. Yield 33%.
Synthesis of acetic acid 2-[2-(4-vinyl-phenoxy)-ethoxy]-ethyl ester (6)
22 g Methyltriphenylphosphoniumbromide (0.057 mol) were suspended in 100ml THF and 6.4 g KOtBu (0.057 mol) in 50 ml THF were added dropwise under stirring. The resulting yellow solution was stirred for another hour, before 9 g acetic acid 2-[2-(4-formyl-phenoxy)-ethoxy]-ethyl ester (9) in 100 ml THF was added dropwise keeping the temperature of the reaction mixture below 20°C. The solution was stirred at room temperature overnight. The solvent was then evaporated and ethyl acetate added, resulting the precipitation of triphenylphosphoniumoxide, which was filtered off. The solvent was again evaporated and the crude product purified by column chromatography (petrolethenethyl acetate 2:1). The product was obtained as a white waxlike substance. Yield 46 %.
Synthesis of polymers (1) and (2) All polymerizations were carried out under nitrogen, degassed by three freezepump-thaw cycles using Schlenk type flasks. For the homopolymerizations the monomer to initiator ratio was calculated for a Mw of 70000 (100% conversion). To get the amphiphilic block copolymers a two-fold molar surplus of the second monomer was added to the homopolymer. 4-Octylstyrene (4) and poly(4-octylstyrene) (10) together with 4-acetoxystyrene (3) were polymerized in substance for 20h to 30h at 123°C. The polymerization of poly(4-octylstyrene) (10) with the monomer acetic acid 2-[2-(4-vinylphenoxy)-ethoxy]-ethyl ester (6) was carried out in diglyme over five days at
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Figure 2: Synthesis route to block copolymers (1) and (2)
123°C. All polymers were dissolved in THF and precipitated in methanol after polymerization. This procedure was repeated three times in order to remove remaining monomer. The complete route of synthesis to block copolymer (1) and (2) is shown in Figure 2.
Hydrazinolysis of block copolymer (11) and (12)
Polymer (11) / (12) was dissolved in THF and the fivefold amount of hydrazine hydrate in respect to the polymer was added. The clear reaction mixture was then refluxed for 4h and stirred at room temperature overnight resulting in a blurry solution. The block copolymer was precipitated by pouring the THF solution into methanol, filtered off and dried in vacuo.
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Characterization of block copolymers and block copolymer
films Glass transition temperatures As the generation of a patterned surface goes along with a reorientation of chain segments, their mobility must be ensured. Therefore the restructuring of block copolymer (2) is carried out above the glass transition temperature. In the case of block copolymer (1) the high T of the hydrophilic segments is lowered below room temperature through swelling these segments with water. The glass transition temperatures of all polymers described in this article were determined by DSC measurements and are listed in Table 1. G
Table 1: Glass transition temperatures of block copolymers Polymer
Poly(4-octylstyrene) (4) Poly(4-octylstyrene)-Woc£-(4acetoxystyrene) (12) Poly(4-octylstyrene)-Woc£-(4hydroxystyrene) (1) Poly(4-octylstyrene)-è/ocÂ:-(acetic acid 2[2-(4-vinyl-phenoxy)-ethoxy]-ethyl ester) (11) Poly(4-octylstyrene)-WocA:-(2-[2-(4-vinylphenoxy)-ethoxyl-ethanol) (2)
TQ hydrophobic T hydrophilic phase phase G
-35°C
-
-33°C
90°C
-33°C
135°C
-35°C
-8°C
-35°C
12°C
Contact angle measurements By measuring the contact angle of a drop of water on a surface one receives information about the hydrophilicity of this surface. Surfaces with a contact angle above 100° are called hydrophobic, otherwise they are called hydrophilic. By measuring the contact angle of a surface of a block copolymer film and then comparing it to the contact angle of the respective homopolymers, one gets information about the ratio of the polar and apolar segments at this surface. The contact angle of the hydrophobic phases was measured after annealing the film overnight under air and above glass transition temperature. The contact angle of the hydrophilic segments was determined after exposing the film surface to water for 20h and then removing the water that sticks to the surface through spinning the sample in a spin coater. In case of polymer (2) no reliable value for
135 the contact angle of the hydrophilic phase could be measured. The values varied, but were always below 50°. Table 2: Contact angles of different polymers
m
Contact angle Contact angle hydrophobic hydrophilic phase phase
Polymer
PDI
Mw
Poly(4-octylstyrene) (4) Poly(4-octylstyrene)-2>/oc£(4-hydroxystyrene) (1) Poly-(acetic acid 2-[2-(4vkyl-phenoxy)-ethoxy]-ethyl ester) Poly(4-octylstyrene)-Wocfc(2-[2-(4-vinyl-phenoxy)ethoxyl-ethanol) (2)
1,23
42000
114°
-
1,48
63000
109°
96°
1,36
63000
-
76°
1,33
58000
109°
