Molecular Layer Deposition of Ultrathin Polyester Films

Molecular Layer Deposition (MLD) is an attractive vapour-phase technique to deposit organic thin films with thickness control at the subnanometer level. Thus far, a large deal of research has been dedicated to study MLD on flat substrates. However, MLD on particle substrates is becoming increasingly relevant since depositing organic films is highly sought for several applications, such as controlled release of pharmaceutical and food products and protection of pigments. This project focuses on the MLD growth of polyester films on both inorganic, TiO2, and organic particles, proteins. The deposition is executed in atmospheric-pressure fluidized bed reactors for a wide range of number of cycles, from 5 to 50, using either malonyl chloride or diethyl succinate as reactant, and either 1,2-butanediol or 1,4-butanediol as coreactant. To verify the success of the process, the photocatalytic activity of MLD-coated TiO2 and the dissolution behaviour of MLD-coated protein are tested. FTIR and TGA show that the MLD growth on TiO2 using malonyl chloride and 1,2- butanediol evolves linearly with the number of cycles, thus confirming one of the main characteristics of MLD. However, the film deposited with these precursors is not effective in suppressing the photocatalytic activity as the model pollutant is adsorbed by the coating. The MLD growth using diethyl succinate, independently of the diol, stops after a few cycles. Nonetheless, the photocatalytic activity of TiO2 is reduced by one-half. Regarding proteins, the MLD growth using malonyl chloride potentially leads to denaturation and thus to structural changes of the proteins, which affect the inherent dissolution behaviour. Therefore, no clear conclusion can be drawn by the comparison of the dissolution profiles of uncoated and MLD-coated proteins. The results show that the only successfulMLD approach for the deposition of polyesters involves the use of an acid dichloride (e.g. malonyl chloride) and a diol. However, it can only be applied to chemically stable particles as TiO2. Nonetheless, malonyl chloride changes the colour and thus the intrinsic photocatalytic properties of TiO2.