2. “There currently exists several techniques which make it possible to prepare thin organic films on substrates, each being based on an appropriate family or class of molecules.
The process for forming a coating by centrifuging, or "spin coating" does not require a particular affinity between the molecules deposited and the substrate of interest, which is also the case for the related techniques for forming coatings by immersion (dip coating) or by deposition by spraying (spray coating). This is because the cohesion of the deposited film is based essentially on the interactions between the constituents of the film, which can, for example, be crosslinked after deposition in order to improve the stability thereof. These techniques are highly versatile, can be applied to all types of surfaces to be covered and are highly reproducible. However, they do not make possible any effective grafting between the film and the substrate (simple physisorption is involved). The thicknesses produced are difficult to control, especially for the finest depositions (less than 20 nanometres) and rarely uniform over the entire surface.
Other techniques for forming an organic coating at the surface of a support, such as plasma deposition or else photochemical activation, are based on the same principle: generating unstable forms of a precursor close to the surface to be covered, which unstable forms change with the formation of a film on the substrate. While plasma deposition does not require any specific property of its precursors, photoactivation for its part requires the use of photosensitive precursors, the structure of which changes under irradiation with light. These techniques generally give rise to the formation of adherent films, although it is generally impossible to discern whether this adhesion is due to crosslinking of a film topologically closed around the object or to true formation of bonds at the interface.
The self-assembling of monolayers (SAM for "self-assembled monolayers") is a technique which is very simple to employ which requires, however, the use of generally molecular precursors having a sufficient affinity for the surface of interest to be coated. The term used will then be "precursor-surface pair", such as sulphur compounds having affinity for gold or silver, trihalosilanes for oxides, such as silica or alumina, or polyaromatics for graphite or carbon nanotubes. In all cases, the formation of the film is based on a specific chemical reaction between a part of the precursor molecule (the sulphur atom in the case of the thiols, for example) and certain "receptor" sites of the surface. A chemisorption reaction provides the attachment. However, while the pairs involving oxide surfaces may give rise to the formation of very firmly grafted films (the Si--O bond involved in the chemisorption of trihalosilanes on silica is among the most stable in chemistry), this is not at all the case when oxide-free metals or semiconductors are involved.
Anodic electropolymerization consists of the polymerization of monomer species in the presence of electrons close to an electrically conductive or semiconductive surface. The polymerization results in the formation of a film by precipitation in the vicinity of the surface. However, no bond of covalent nature is created between the surface and the polymer, which results in the films obtained not having an optimal resistance to attacks. Among the monomers that can be used according to this technique, mention may especially be made of pyrrole. The electrografting of polymers is a technique based on the initiation and then the polymerization, by chain propagation, which is electroinduced of electroactive monomers on the surface of interest, which acts both as electrode and as polymerization initiator. It requires the use of precursors suited to its mechanism of initiation by reduction and of propagation, generally anionic, as preference is often given to cathodically initiated electrografting, which can be applied to noble and non-noble metals. Electrografting by anodic polarization can, for its part, be applied only to noble or carbon-based substrates: graphite, glassy carbon, boron-doped diamond. International application WO 03/018212 describes, in particular, a process for grafting and growing a conductive organic film on an electrically conductive surface, the grafting and growing being carried out simultaneously by electro-reduction of a diazonium salt that is a precursor of said organic film.
Electrografting is the only technique which makes it possible to produce grafted films with specific control of the interfacial bonding.”
[Coating Methods, US Patent 8,466,072 (6/18/2013)]
1. Coatings can be applied by many process from the simple wiping, using brushes to spraying and even chemical vapor deposition. (RDC 4/15/2013)
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Roger D. Corneliussen
Maro Polymer Links
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Copyright 2013 by Roger D. Corneliussen.
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* Date of latest addition; date of first entry is 4/15/2013.