The cationic polymerization of electron rich monomers such as vinyl ethers, vinyl furane, and cyclopentadiene on silica surfaces can be initiated by aryl methyl halides. The reactions yield always soluble polymers (by heterogeneous catalysis) and novel polymer/silica hybrid materials. The link between polymer and solid is caused by covalent Si‐O‐C bonds, by network formation of the polymers during the chain growth, or by a combination of both of them. The analysis of the polymer structures on the surface by 1H MAS NMR spectroscopy in suspension and by solid state 13C CP MAS NMR spectroscopy is described. Proof of Si‐O‐C bonds via DRIFT spectroscopy and 13C CP MAS NMR spectroscopy is given. The most effective method of irreversibly linking the polymer to the silica surface is the network formation. Polyvinyl ethers are bound strongly to the surface, as can be shown by FTIR measurements, but the linkage is not stable due to the Si‐O‐C bonds' susceptibility to hydrolysis.
Poly‐cyclopentadienes (PCPD) are linked to the surface by Si‐O‐C bonds, which show an extraordinary high resistance to acids and bases. Si‐O‐C bond formation of poly‐2‐vinyl furane could not yet be detected by 13C CP MAS NMR spectroscopy and DRIFT spectroscopy. In this case the high degree of coating derives from the bifunctionality of 2‐vinyl furane: it may undergo Friedel‐Crafts‐alkylation at the 5‐position of the furane ring as well as chain polymerization via the vinyl group at the 2‐position.
Aluminum sheets were furnished with a water‐repellent composite layer consisting of microroughened alumina, chitosan (CHS), and poly[octadecene‐alt‐(maleic anhydride)] (POMA). CHS was electrochemically deposited or spin‐coated. In a subsequent reaction step, POMA was covalently bonded onto the immobilized CHS layer. The produced composite layers showed superhydrophobic properties, which are indicated by the water contact angles >150° and negligible hysteresis. Cross‐linking reactions of the CHS layer were able to enhance the layer stability. The superhydrophobic surface properties were also maintained after abrasion of the protruding polymer‐coated alumina peaks. This is due to a CHS/POMA reservoir incorporated in the microroughened alumina surface region.magnified image
Hydrophobically modified chitosan microparticles were produced by various syntheses carried out as heterogeneous phase reactions. Perfluorinated carbonyl components such as carbonic acids and acid chlorides were used as hydrophobization agents. To enhance the layer stability preceding cross-linkages between the chitosan macromolecules were introduced. These cross-linkages involve some of the chitosan amino groups. The use of a highly reactive alkylene-maleic anhydride copolymer produces cross-linkages and hydrophobizes the chitosan layer simultaneously in a one-step reaction. The heterogeneous reactions serve as model reactions for the hydrophobization of chitosan films deposited on supports.
Particles of chitosan have a very polar surface, and thus they are easily wetted by water and also dissolve in acidic media. The hydroxyl and amino groups offer a high potential for grafting reactions. Aldehydes and carbonic acid derivatives were covalently grafted, preferably onto the amino groups. The reactions were carried out in solution as homogeneous phase reactions as well as on the particle surfaces as heterogeneous phase reactions. Covalently bonded alkyl chains impart the chitosan molecules with hydrophobic properties. We employed different methods such as NMR, XPS, elemental analysis and FT-IR spectroscopy to study the reactions and to estimate the degree of functionalization. The wetting behavior of the particles was investigated by a modified Wilhelmy technique, where an adhesive tape completely coated with the particles was dipped in water. Some of the samples having a high degree of functionalization showed super-hydrophobic surface properties. The observed super-hydrophobic effect results from a combination of the hydrophobic properties of the modified particles and the roughness of the particle coating.
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