| Summary: | 碩士 === 國立勤益科技大學 === 化工與材料工程系 === 100 === The transparent polyimide (PI) filmswere synthesized from an alicyclic dianhydride (BCDA) and aromatic diamine (3,4'-ODA) in the cosolvent of DMAc and GBL via one-step process. For the application in flexible displays, the transparency and water barrier property of PI films were enhanced by blending with graphene (RG) or graphene oxide (GO) and by deposition of silicon nitride.
Part I: Synthesis and characteristics of transparent polyimide/graphene or graphene oxide nanocomposite films
Graphene oxide (GO) was first synthesized by the oxidation process of natural graphite. Graphene (RG) was then obtained after thermal reduction process and the successful exfoliation was confirmed by XRD and SEM results A series of PI nanocomposite films were prepared by adding different proportions of RG or GO into PI solution. Adding 0.01 wt% of GO or RG, the storage modulus (E') of PI/GO-0.01 and PI/RG-0.01 was 2474 MPa and 3138 MPa, respectively, indicating enhanced mechanical strength. The coefficient of thermal expansion (CTE) reduced to 41.4 ppm/oC for PI/GO-0.001 indicating good thermal dimensional stability. Moreover, all PI nanocomposite films with various contents of GO or RG exhibit excellent thermal stability. PI/RG nanocomposite films show better water barrier property than PI/GO. The water vapor transmission (WVTR) of PI/GO-0.001 was 31.71 g-mil/m2-day and that of PI/RG-0.001 was only 13.09 g-mil/m2-day, which is 92.7 % lower than that of pure PI (181.33 g-mil/m2-day). The UV-Vis spectra show the optical transmittance at 550 nm was 97% and 85%, respectively, for PI nanocomposites containing 0.001 wt% of GO and RG. . Based on the above experimental results, adding a small amount of GO or RG in PI, the resultant PI nanocomposite maintain high optical transmittance and, at the same time, effectively extend the moisture penetration path and thus the water barrier property was significantly improved.
Part II: Deposition of a silicon nitride gas barrier layer on the transparent polyimide/graphene and graphene oxide nanocomposite films via RF magnetron sputtering
Based on the result from Part I, PI nanocomposite films with improved water barrier property and, simultaneously, remaining high optical transmittance were deposited with silicon nitride on the surface to study the WVTR values. The Si2p and N1s XPS analysis showed the binding energy of Si3N4 at 102.6 and 397.5 eV, respectively, indicating the presence of Si3N4 structure in the deposited barrier layer. The presence of Si-C bonding at the interface of PI and deposited barrier layer was confirmed from the corresponded depth profile of Si2p and N1s spectra. FE-SEM and AFM images show that the deposited barrier layer has the denser packing of particles and the lower surface roughness (less than 1 nm) at the working pressure of 4 m Torr, sputtering power of 80W and the deposition thickness of 30 nm. Hence, the PI nanocomposites deposited with the barrier layer under those parameters show the best water barrier property. The WVTR of 30nm-deposited PI/GO-0.001reduced to 0.17 g-mil/m2-day, which is 99.9% reduction of pure PI before deposition (181.33 g-mil/m2-day). Notably, this barrier layer deposited PI/GO-0.001 nanocomposite film (thickness: 21 μm) exhibits high optical transmittance at 550 nm (81%), and improved barrier property thus was potential substrate material for flexible electronics.
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