Polymeric Materials: Science & Engineering 2000, 83,448 Environment Friendly and Switchable Polymer Brushes Sergiy Minko 1 , Alexander Sidorenko 2 , Evgeniy Goreshnik 1 , Denys Usov 1 , and Manfred Stamm 1 (1) Department of Polymer Interfaces, Institut für Polymerforschung Dresden e.V., Hohe straße 6, Dresden, 01069, Germany, fax: 49-351-4658- 281, minko@ipfdd.de, (2) Department of Materials Science and Engineering, Iowa State University, Gilman Hall 0315, Ames, IA 50011 Introduction The behavior of a solid surface with respect to surrounding medium is essential for many applications in industry, biology, medicine and environmental science, where wetting or in other cases non-wetting situations are envisaged. In this respect the concept of the hydrophobic or hydrophilic nature of a surface is widely used describing the interaction of water with materials. It was shown that wetting behavior of solid surfaces can be tuned by composition of random copolymers grafted to the solid substrates 1 . We have addressed the question whether one can prepare a surface, which can change its behavior depending on the environment, and whether this change is even reversible. Thus for instance an initially hydrophobic surface will gradually change to hydrophilic in water, in that way responding to the changing environment and optimizing its wetting properties, while it will become hydrophobic again in contact with a nonpolar organic solvent. It thus can be reversibly switched between those two states and can also adopt any intermediate state in between. This behavior has some resemblance to an artificial chameleon, which can adopt its appearance to changing surroundings. Our approach for the fabrication of a surface with the chameleon behavior is synthesis of a binary polymer brush composed of two incompatible polymers of a very different solubility in water. We assume that the interaction of the binary polymer brush with a selective solvent causes a change of surface properties of the brush when one of the two polymers preferentially occupies a top layer. With this mechanism one can approach unique opportunities to change the surface properties of a polymer film as a response to the change of the surrounding media. Experimental Materials. Monomers - styrene (Merck) and 2-vinylpyridine (VP, Aldrich) were purified with ALOX B chromatographic column and distilled under reduced pressure under argon. Solvents. Toluene, tetrahydrofurane (THF), 1,4-dioxane and hexane were distilled after drying with sodium, dimethylsulfoxide (DMSO) was distilled under reduced pressure after drying with calcium hydride, methanol and ethanol were used as received. Dichloromethane was dried on molecular sieves. Initiators - 4,4’-azobis(4-cyanopentanoic acid) (ACP) from Aldrich and 4,4’- azobis(isobutyronitrile) (AIBN) from Fluka were purified by recrystallization from methanol. All reagents were used immediately after purification. Silicon wafers obtained from Wacker-Chemitronics GmbH (Burghausen, Germany) were cleaned with dichloromethane in an ultrasonic bath, hot piranha solution and rinsed several times in Millipore water. 3- glycidoxypropyltrimethoxysilane (GPS) (Aldrich), p- Aminophenyltrimethoxysilane (APTS) from ABCR GmbH & Co. (Karlsruhe, Germany) and phosphorus pentachloride (Merck) were used as received. Triethylamine was dried on calcium hydride. Attachment of the Initiator. We used two different methods of the introduction of the azo-initiator onto surface of the Si-wafer described by Tsubokawa et al. 2 and Schouten et al. 3 Due to the first method the Si- wafers were treated by GPS from 5% solution in toluene for 8h. Then the Si-wafers were washed by methanol. On the next step ACP was introduced on the surface of the Si-wafers from 2% solution in DMSO with catalytic amount of α-picoline (Aldrich) at 50 0 C for 5h. The resulting samples of Si- wafers with chemically attached initiating groups were rinsed 6 times with freshly distilled THF. Due to the second method the Si-wafers were treated by APTS from 2% solution in toluene for 12 h. Then the Si-wafers were washed by toluene and ethanol in ultrasonic bath. Separately the acid chloride derivative of ACP was prepared by adding of the slurry of phosphorus pentachloride to a suspension of ACP in dichloromethane at 0 o C. The product (ACPC) after crystallization from hexane - dichloromethane mixture at 0 o C was washed and dried in vacuo. On the next step ACPC was introduced on the surface of the Si-wafers from 5% solution in dichloromethane with catalytic amount of triethylamine at room temperature for 10h. The resulting samples of Si-wafers with chemically attached initiating groups were rinsed in ethanol in an ultrasonic bath. Every step of the modification of Si-wafers was controlled by ellipsometric measurement of the layer thickness. Graft polymerization. Oxygen was removed from the solution of a monomer (styrene or 2-VP, 5÷6 mol/l) and AIBN 4 (5÷9)×10 -4 mol/l in 1,4- dioxane using five freeze-pump-thaw-cycles. The samples of the Si-wafers with the chemically attached initiator were placed in a monomer solution under argon atmosphere in a glass flask. The flasks were immersed in a water bath (60±0.1 0 C) for various periods (1 ÷ 40 h). The Si-wafers were rinsed 6 times with THF. In the next step the same procedure was used to graft the second polymer using the Si-wafers with the first grafted polymer. The ungrafted polymers were removed by a Soxhlet extraction for 8 hours using THF. Characterization of the layers. Null ellipsometry was used to measure the amount of the chemisorbed initiator as well as grafted amount of PS and PVP. For the data interpretation, a multilayer model of the grafted film was assumed. In addition the layer composition was determined with FTIR in a transition mode (using Si-wafers polished from both sides). XPS measurements were used to study PVP and PS chain distribution in the grafted layer. Contact angles of water were determined with the sessile drop method. Morphology of the grafted films was studied with AFM. The detail description of the grafting and characterization is published elsewhere. 5 Results and Discussion The synthesis of binary polymer brushes was performed in three stages: (1) introduction of the initiator on the surface of Si-wafers. This stage consists of two procedures. In the beginning reactive (epoxy or amino) groups were introduced on the substrate surface with GPS or ATPS respectively. The layer thickness was of 8.5 Å and 7 Å respectively. Then the azo-initiator was introduced on the surface via the reaction of the epoxy derivative with ACP or the amino derivative with ACPC. The last reaction is well reproducible and the resulting layer thickness is about 21 Å, while the reaction of the epoxy groups with ACP shows poor reproducibility with the layer thickness for different experiments in the range of values between 12 Å and 22 Å. (2) Grafting of PS chains was performed by in situ radical chain polymerization initiated by thermal decomposition of the azo-initiator covalently attached to the surface of Si-wafers. (3) The grafting of PVP was carried out after the first grafting polymerization was finished and ungrafted polymer was washed out. In this stage a residual amount of the azo-initiator is used to carry out the graft polymerization. The grafting density after every stage depends on time of the grafting procedure. We obtained an increase of the layer thickness after every grafting stage typically in the order of 100-300 Å. Finally, we synthesized binary brushes of 0.05 - 0.3 nm -2 grafting density. Molecular weight of PS and PVP chains was varied from 5×10 4 to 3×10 5 g/mol. The composition of grafted layers was also determined using FTIR. The characteristics of several representative samples are presented in Table 1. In this table we demonstrate that many parameters of the binary brush can be varied: grafting density, molecular weight of both polymers and their ratio, that can be concluded from the graft polymerization mechanism. 6 Table 1. Composition of the Binary Brushes Amount of grafted PS, mg/m 2 Amount of grafted PVP, mg/m 2 M n of PS, g/mol M n of PVP, g/mol Total grafted density, nm -2 6.3 3.1 54 200 173 000 0,08 2.0 25.0 215 000 220 000 0,08 21.9 11.0 164 000 120 000 0,14 45.4 44.2 210 000 230 000 0,25 With respect to composition the binary brushes can be subdivided into symmetric (the ratio PS:PVP≈1:1) and asymmetric brushes. AFM