Ultrathin Films Of A Polyelectrolyte With Layered Architecture: Well Defined Assemblies For Model Studies Of Polyelectrolytes

May 07, 1998

The Langmuir-Blodgett (LB) technique offers the possibility to fabricate smooth, highly ordered ultrathin films with monolayer by monolayer control of thickness. Examples of pure materials commonly used for multilayer fabrication with this technique include fatty acids, phospholipids, hairy-rod polymers and some polymers with long side chains. Unfortunately, not all molecules, such as water soluble polymers, can be processed into single component multilayers by this technique. In order to overcome this limitation and obtain hydrophilic surfaces with high carboxylic acid densities desirable for models of superabsorbing polymers, bioadsorption studies and as precursors for subsequent chemical modification, Alan Esker, Christoph Mengel and Gerhard Wegner from the Max Planck Institute for Polymer Research, Mainz, Germany, applied a different approach (Esker, et.al., Science, vol. 280, 8 May 1998). Post transfer modification of preformed LB-films of poly(tert-butyl methacrylate) (PtBMA) and poly(tert-butyl acrylate) (PtBA) yields polyelectrolyte films for polymethacrylic acid (PMAA) and polyacrylic acid (PAA), respectively, in the gas phase. By exposing the precursor films to gaseous hydrochloric acid (60 °C, 6 h), isobutene is eliminated. This converts the ester groups to carboxylic acid moieties, and hence generates polyelectrolytes with different surface properties and chemical reactivity.

The first step of this approach is the formation of stable monolayers at the air/water interface, thereby trapping the molecules in two-dimensional (2d) conformations. For PtBMA and PtBA, this behavior was already well known through monolayer studies of their isotherms, surface rheology and dynamics. This is followed by LB-transfer of the materials to hydrophobic silicon wafers, in which one monolayer is transferred on each up and down stroke of the substrate. This has been confirmed by X-ray reflectivity. The most significant difference between the two systems is the presence of a so called Bragg peak for PtBA films indicating a double layer structure. This is never seen for PtBMA which suggests that the methyl groups along the backbone alter the packing characteristics. Thickness per layer data from X-ray reflectivity and the surface concentration during transfer can be used to calculate PtBMA's film density, which is in excellent agreement with the bulk value. In contrast, PtBA's calculated density is significantly lower than the bulk value. This implies that PtBMA has similar packing at both the air/water interface and in the bulk, whereas PtBA possessing greater flexibility can assume a conformation with all of the hydrophobic groups oriented away from the interface.

Subsequent hydrolysis of the PtBMA and PtBA films does not significantly increase their surface roughnesses. The linear relationship between the thickness of PMAA and PAA and the number of layers originally transferred, along with the absence of dewetting, suggest that although the double layer structure is destroyed, the layered structure may remain (i.e. 2d-conformations are retained). In addition to X-ray reflectivity, IR-spectroscopy was used to confirm the transformation of the tert-butyl ester groups to the desired carboxylic acids. Clean shifts of the carbonyl and C-O stretch, along with the growth of a broad hydroxyl stretch are consistent with the existence of PMAA primarily as acid dimer.

For future applications, networks of these materials are desired. To test the feasibility of this approach, isopentylcellulose cinnamate was mixed with bifunctional-vinylbenzyl-terminated PtBMA. Subsequent photo-induced cross-linking left insoluble network structures which could be hydrolyzed as above. As the PtBMA derivative only contained two cross-linking groups per molecule, well defined network films could only be obtained up to 60 mol % PtBMA monomer.

The possibility of retaining 2d-conformations for PMAA and PAA after hydrolysis by first cross-linking the materials, means that composite structures can be assembled which differ by as little as a single monolayer (one monomer thick). In addition to uses as models of superabsorbing polymer networks and bioadsorption studies, these materials should be ideally suited for the flexible hydrophilic spacing layers needed between supported bilayers and solid substrates for biologically relevant studies of the physical properties of membranes.


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