Infoscience

Thesis

Hybrid nanocomposite gas-barrier coatings on polymers

Gas barrier coatings based on single or multiple layers of metals, oxides or nitrides are widely used on polymer substrates in applications ranging from food packaging to organic displays and solar cells. These coatings exhibit a residual permeability resulting from process-induced defects, which moreover control their mechanical integrity. A major challenge is therefore, to produce a high barrier coating, which can withstand high strain values (typically 4-5%) without loosing its barrier properties. The objective of the research work is to develop hybrid organic-inorganic gas-barrier materials based on organosilane-SiOx layered coatings on poly(ethyleneterephthalate) (PET) substrates. Attention is paid to the defect-healing mechanisms resulting from the polymerization of thermally- and UV-curable organosilane molecules on the defective silica coating, and corresponding improvements in barrier and mechanical performance. To achieve accurate analysis of the properties of such nanosized coatings, and in addition to standard analytical methods, two novel techniques are used in the study, namely reactive ion etching (RIE) to analyze the defect population, and permeation tests under tensile loading to determine the critical strain for loss of barrier performance. The oxygen transmission rate (OTR), defect density and critical strain of untreated, 50 nm thick SiOx coating are 1.8 cm3/(m2.day.bar), 560 mm-2 and 1.8%, respectively. The critical strain for 10 nm thick coatings is 4.0%. The interactions and interphase formation between thermally-curable organosilanes and hydrated silica surface is investigated with particularly taking into account the presence of different amine groups and the pH of the silanes. The permeability and defect density of the silane-silica hybrid coating on PET are reduced up to threefold compared to that of the untreated SiOx/PET film, at solution concentrations as low as 1%wt., irrespective of the pH. This concentration level leads to a dense silane monolayer crosslinked to the silica surface, which is hydrothermally stable. Higher concentrations lead to polysilanol layers, which only marginally contribute to the reduction in OTR. The activation energy for oxygen transport of the uncoated PET and SiOx/PET films is found to be equal to ca. 33 kJ/mol for both materials, and increases to 51 kJ/mol and 57 kJ/mol for neutral silane and basic silane treated SiOx/PET films, respectively. The critical strain for loss of barrier performance is improved by a factor of two, only in case of basic pH. All these results demonstrate the defect healing action of SiOx by silane. In addition to siliconalkoxy functional groups, a higher pH (provided by amine function) is the key factor for the combined improvement of barrier and mechanical properties of hybrid silane-silica coatings on PET. Outstanding combination of property improvement is achieved with UV-curable silanes at concentration in solution as low as 2 wt%. In addition to their ultra-fast processing, these molecules enable more than a two-fold improvement in barrier properties, and a remarkable increase of the critical strain, to more than 5% depending on the conversion level. The latter is analyzed using a nth order kinetic model including an Arrhenius dependence of temperature, with activation energy equal to 13 and 32 kJ/mol for the two types of silanes studied, and a power law dependence of UV intensity, with exponent equal to 0.65 for both silanes. Time-intensity-transformation diagrams are compiled to scale up the present results towards high production rate requirements. A model of the permeation behavior of the hybrid coatings is finally developed to understand the influence of the silane on the intrinsic permeability of the oxide network and on the defect population. It is concluded that the defects contribute to 2/3 of the total permeation, the remaining transport occurring through the disordered structure of the oxide. A correlation is moreover identified between the pH of the silane and the permeability of the hybrid silane-silica network. This thesis work identifies the key factors to produce gas barrier coatings on polymers based on silane-silica nanolayers with very high strain to failure and at a very high processing rate of several hundred meters per minute. These hybrid coatings present clear potential beyond food packaging applications, especially for environmental protection of emerging organic-electronic devices including flexible organic light emitting diodes (OLEDs) and solar cells.

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