Polycyanoacrylates are promising materials for numerous applications because they are highly sensitive to the electron beam effect and reveal strong adhesion both to hydrophobic and hydrophilic surfaces. Superthin uniform and defectless films of polycyanoacrylates can be deposited by Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) techniques. In particular, LB films of polycyanoacrylates have been successfully used to produce electron beam resist coatings [1] and to immobilize various proteins, such as enzymes and antibodies, for the preparation of biologically active media [2].
In our studies, we try to realize in practice the idea of "molecular architecture" [3] using LB, LS, and related techniques in the application to biotransformation processes, chemical sensors, biosensors, and various bioanalitical operations (see, for example, [4]). One of the reasons to develop molecular organized films of the specially designed alternate-monolayer structure is the requirement of high stability of sensing elements and biocatalytic media in terms of long-term storage, elevated temperature, and environmental conditions. Alternation of biological compounds (as a rule poorly stable) with chemically and physically stabilizing materials can result just in the preservation of their functional activity.
In particular, the incorporation of polycyanoacrylate monolayers, which possess the enhanced mechanical and specific chemical properties, into such film results in better structural stability of the latter. Moreover, even very small exposure of the deposited film to the electron beam often causes a cross-linking of neighbouring monolayers due to mentioned above features. However, it is not evident that the desired layered structure will be realized after the film preparation. In addition, polymer LB films are mostly poorly ordered and thus the possibility to achieve molecular resolution in the normal direction to the layer plane in the superlattices based on polycyanoacrylate monolayers should be proved.
In present work, an attempt is made to solve this structural problem by using X-ray and neutron diffraction techniques. The combination of X-ray and neutron scattering as the method of investigation enables to detect very delicate processes of thin film structure rearrangement, which may take place without any visible film morphology deterioration. LB superlattices with the structure (bilayer of Ba stearate - bilayer of cyanoacrylate copolymer - bilayer of deuterated Ba stearate - bilayer of cyanoacrylate copolymer)10 were deposited and studied before and after exposure to the electron beam. The exposure dose was chosen in such a way to make polycyanoacrylate bilayers insoluble in the organic solvents, but not to destroy the structure of Ba stearate bilayers. Cyanoacrylate copolymer was a copolymer of heptylcyanoacrylate (HCA) and trichlorobutadiene (TCB) with the composition of (HCA)4(TBC)1. Chemical formula of cyanoacrylate copolymer and schematic film structure are shown in Fig.1. Film morphology was controlled by using optical microscopy. Average thickness of cyanoacrylate copolymer monolayer was estimated by measuring a total thickness of monocomponent films composed of 100 monolayers by interference technique.
Fig.1. Structure of superlattice period (a) and copolymer formula (b).
On the basis of these experimental data conclusions are made on the quality of superlattice deposition and on the realization of the required order of monolayer alternation. Values of the superlattice period measured experimentally and calculated from the thicknesses of the monolayers coincide with each other with a satisfactory accuracy. Although a major part of the substrate surface was coated by the alternate-monolayer film some inclusions of separate Ba stearate phase can be detected as well. Probable models of film structure are discussed which are in the agreement with the measured curves of X-ray and neutron scattering.
References
1. V.I. Troitsky, N.K. Matveeva. Thin Solid Films, 327-329 (1998) 659.
2. T.S. Berzina, L. Piras, V.I.Troitsky. Thin Solid Films, 327-329 (1998) 621.
3. H. Kuhn. Thin Solid Films, 99 (1983) 1.
4. C. Nicolini. Trends in Biotechnology, 15 (1997) 395.