Moisture Barrier Polymer Nanocomposites for Organic Device Encapsulation
The advancement in smart technologies for organic conducting polymers as flexible substrates in LEDs, PVs and solid state lighting necessitates the development of ultra-high barrier films to protect the devices from moisture and oxygen. The current encapsulation methodology of using layers of plastics and inorganic oxides has several deficiencies. Alternatively, the use of single layer of polymer nanocomposites is a promising substitute for these inorganic based encapsulation layers. The use of polymer materials have the advantage of flexibility, active electrodes printability and easy to make the devices for large area applications. The nano-fillers with high aspect ratio as nanocomposites ingredient in polymers reinforces its mechanical strength and also acts as a scavenging material for moisture and increases the residence time and/or for the penetrating moisture in the film. Chapter 1 gives the basic overview in the field of barrier technology films and coatings from polymers and inorganic oxide as either mono/multi layer hermetic encapsulation methods. The understanding of both chemistry and physics behind the moisture permeation and its interaction with the film material was discussed. The inclusion of functional nano-fillers as moisture trapping agents in the film provide better device protection achieved. The methods and instruments to measure such ultra-low permeation within the films are discussed. Finally, the advantage of polymer based nanocomposites for low-permeable films with existing materials are briefly discussed in this chapter. In this thesis, we employed both thermoplastic and thermoset polymer nanocomposites as encapsulation layer for device sealing. The use of ion-containing polymers (ionomers) as a sealant layer was also studied. Chapter 2 presents the detailed experimental procedures with materials and methods used in this thesis along with the synthesis methodologies to make films from the polymer. In chapter 3, we used cyclic olefin copolymer COC (copolymer of ethylene and norbornene) as an encapsulation layer with silica and layered silicate nano-fillers. The compatibility between hydrophilic silica and hydrophobic COC was achieved by maleic anhydride grafted PE with anchoring on COC as a compatibilizer and then silica filler was added to make the nanocomposite films. FTIR spectroscopy confirms the bond formation of silica with COC/MA-g-PE. The mechanical (tensile and DMA) and thermal studies (DSC) suggested that there is an improvement observed when adding silica/silicate layers in the polymer matrix with increased tensile strength, storage modulus and Tg. The calcium degradation test show enhanced performance towards moisture impermeation in the film. Chapter 4 deals with the synthesis of PVB based nanocomposite film with silica/layered silicate as nanofillers in the base matrix with varying degree of acetalization in the film. The FTIR and NMR spectroscopy show the evidence for acetal link formation in the in-situ synthesized PVB with silica/silicate nanofillers with three different acetyl contents. The tensile and DMA studies show the observed improvement in mechanical strength (increased tensile strength, storage modulus) were due to the intercalation of clay galleries during PVB formation and the interaction of silica particles interactive bond formation with –OH groups of PVA in PVB. The higher clay/silica particles show agglomerated nature and reduction in film strength. Thermal studies (DSC) show that there is an improvement observed in Tg when adding silica/silicate layers in the polymer matrix with moderate to low acetal content. The calcium degradation test show enhanced performance towards moisture impermeation in the film. Chapter 5 describes the inclusion of ionic groups (ionomers) in PVB and its effects on moisture permeation and mechanical properties. PVB ionomer was synthesized using formyl benzene 2-sulfonic acid sodium salt and 2-carboxy benzaldehyde (both sulfonic and carboxylic acid sources) as co-aldehyde with butyraldehyde and PVA. These acid groups were neutralized with potassium, magnesium and zinc ions. The level of acid content in the films was maintained between 6 to 28 mol percent. The sulfonic acid films with zinc and magnesium ions of 14 mol% exhibit good mechanical strength and low moisture permeation. Chapter 6 deals with the epoxy terminated silicone polymer nanocomposites as moisture barrier coatings for device encapsulation. Both silica and clay silicate layers were used to reinforce the silicone matrix. The silica nanoparticles were grafted with amino-silane groups, this would help in better mixing of silica particles in the silicone matrix due to the amine groups interaction in curing with epoxy groups. The calcium degradation test was used to determine the WVTR of the nanocomposites and device encapsulation was employed to estimate the degradation after exposure to ambient environment. Chapter 7 presents the concluding remarks of the results presented. The benefits as well as limitations of the polymer nanocomposite film and the future developmental work to be carried out are discussed in this chapter.
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