Elastomeric and nanoporous membranes via Langmuir Blodgett transfer of cross-linked monolayers
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Elastomeric and nanoporous membranes via Langmuir Blodgett transfer of cross-linked monolayers Hui Xu1, Robert Heger2, Frank Mallwitz1,2, Marita Blankenhagel2, Claire Peyratout2 and Werner. A. Goedel1,3* 1
Macromolecular and Organic Chemistry, OC3, University of Ulm, Germany Max-Planck-Institut f. Kolloid- & Grenzflächenforschung, Berlin, Germany 3 BASF-AG, Polymer Physics, Ludwigshafen, Germany 2
ABSTRACT Nanometer thin membranes with considerable application potential in micro mechanics and materials science can be prepared by transferring cross-linked monomolecular layers of polyisoprenes or polyisobutenes with ionic head groups from the water surface to solid substrates with holes. Especially if monolayers of low glass transition polymers are cross-linked, elastomeric membranes are obtained, which might find application in micro mechanical devices like membrane valves and pumps. Incorporation of hydrophobised colloids leads to composite membranes, which can be converted into porous membranes via removal of the colloids. INTRODUCTION The Langmuir-Blodgett (LB) technique offers the opportunity to generate suspended membranes by assembling a monolayer at the air-water interface and transferring it to cover a hole in a solid substrate. However, the preparation of suspended membranes via LB-transfer is generally more difficult than LB-transfer of thin organic coatings onto continuous smooth surfaces: Because it is not supported by an underlying substrate, the suspended membrane itself must be tough enough to withstand mechanical stress during fabrication and final use. Monolayers that are made from low molecular weight compounds or from liquid polymers easily rupture during transfer across a hole. Suspended membranes have been fabricated using glassy polymers, often stabilised by crosslinking 1 2. These membranes are usually rigid. For certain applications, like membranes in micro mechanical valves and pumps, it might be advantageous to have elastomeric thin membranes available and to take advantage of the comparatively large reversible deformation of these materials. Here, we show that tough and mechanically stable freely suspended membranes - spanning millimetre-sized holes in solid substrates - can be obtained from cross-linked monolayers of low Tg polymers with ionic head groups. EXPERIMENTAL DETAILS Polyisobutenes with sulfonate head groups have been synthesised via living cationic polymerisation as described in 3-,4,5. Linear polyisoprene with ionic head group and anthracene side groups has been synthesised via living anionic polymerisation followed by platinum catalysed hydrosilylation of the parent polyisoprene as published in 7 and 11. Monolayers
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on a water surface were prepared using a 20 cm x 46 cm rectangular Langmuir trough made of polytetrafluoroethylene, equipped with one compression barrier and a floating barrier for the detection of the surface pressure via the Langmuir method (Lauda FW2, Germany). The polymers were usually spread from chloroform solutions which contained 0.05 w
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