Lattice-patterned LC-polymer composites containing various nanoparticles as additives
© Sim et al; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
In this study, we show the effect of various nanoparticle additives on phase separation behavior of a lattice-patterned liquid crystal [LC]-polymer composite system and on interfacial properties between the LC and polymer. Lattice-patterned LC-polymer composites were fabricated by exposing to UV light a mixture of a prepolymer, an LC, and SiO2 nanoparticles positioned under a patterned photomask. This resulted in the formation of an LC and prepolymer region through phase separation. We found that the incorporation of SiO2 nanoparticles significantly affected the electro-optical properties of the lattice-patterned LC-polymer composites. This effect is a fundamental characteristic of flexible displays. The electro-optical properties depend on the size and surface functional groups of the SiO2 nanoparticles. Compared with untreated pristine SiO2 nanoparticles, which adversely affect the performance of LC molecules surrounded by polymer walls, SiO2 nanoparticles with surface functional groups were found to improve the electro-optical properties of the lattice-patterned LC-polymer composites by increasing the quantity of SiO2 nanoparticles. The surface functional groups of the SiO2 nanoparticles were closely related to the distribution of SiO2 nanoparticles in the LC-polymer composites, and they influenced the electro-optical properties of the LC molecules. It is clear from our work that the introduction of nanoparticles into a lattice-patterned LC-polymer composite provides a method for controlling and improving the composite's electro-optical properties. This technique can be used to produce flexible substrates for various flexible electronic devices.
Owing to its impact on device performance, the phase separation behavior of materials and its effect on the device morphology have attracted considerable attention as one of the powerful methods for fabricating flexible electronic devices, such as organic photovoltaics, organic field effect transistors, organic nonvolatile memory devices, and liquid crystal displays [LCDs] [1–6]. The phase separation of a mixture is attributed to the difference in surface free energy among the components and their interactions with each other. Lattice-patterned liquid crystal [LC]-polymer composites, which are characterized by phase separation of the mixture of LC and the miscible photoreactive monomers upon UV light irradiation under a patterned mask, are one of the most important fabrication materials for flexible substrates that can be used in flexible electronics, owing to their sophisticated and controllable non-contact characteristics [7, 8].
As the region of the mixture that is irradiated by UV light undergoes a photoreaction to form polymerized polymer walls that act as a supporting structure, the monomer and LC simultaneously diffuse into polymer-rich and polymer-poor regions, respectively, through dynamic phase separation. This is the cause of the difference in the surface free energy and the low miscibility between the LC molecules and the UV-cured polymers. The phase separation can be used to determine the features of cells containing the LC surrounded by polymer walls. These structures are resistant to bending stress, satisfying a fundamental requirement of flexible electronic substrates.
However, as in all organic material systems, the control of physical and electro-optical properties of LC-polymer composites is limited due to the restricted properties of the organic materials. Nowadays, in order to overcome the limitations of all organic material systems, many research groups have become interested in enhancing phase separation using hybrid materials, which involves introducing inorganic materials into the system. To minimize the deterioration of the display properties, such as the transparency, it is preferable to use inorganic materials in the form of nanoparticles as additives [9–13].
In this study, we show the effects of introducing inorganic nanoparticles into lattice-patterned LC-polymer composites on the phase separation behavior and electro-optical properties of the composites. Prepolymers containing nanoparticles were prepared by mixing UV-curable monomers and SiO2 nanoparticles of varying sizes and with various surface functional groups. Photoinduced phase separation was caused by exposing the LC-prepolymer mixtures to UV light by using a lattice-patterned photomask. The phase separation structures of the lattice-patterned LC-polymer composites were then studied using polarized optical microscope imaging, and the electro-optical properties of the LC were investigated by measuring the contrast ratio and the driving voltage of the lattice-patterned LC-polymer composites.
Lattice-patterned LC-polymer composites were prepared by exposing the mixture containing the prepolymer solution (50 wt.%) and LC (E7, Merck KGaA, Darmstadt, Germany; 50 wt.%) to UV light. The LC cell was prepared using two ITO glass slides coated with rubbed polyimide, between which the LC-prepolymer mixture was injected by capillary effect at 100°C, which was above the clearing temperature. To form the polymer walls, UV light was radiated for 400 s through a photomask comprising 300 × 300 μm2 of dark square patterns with a 30-μm spacing. The sample temperature was maintained at 100°C. Polarized optical microscope images of the lattice-patterned LC-polymer composites were obtained, and the polymer wall thickness was determined by using a microscope system (Eclipse LV100D, Nikon Co., Shinjuku, Tokyo, Japan). In order to confirm the effect of the various SiO2 nanoparticles on the electro-optical properties of the LC, the contrast ratio and the driving voltage of the LC-polymer composites were measured.
Results and discussion
In the case of MPS-SNP, the positive effect of SiO2 nanoparticles on the electro-optical properties was more evident than in the case of BTMA-SNP. The contrast ratio of the LC increased from 32 to 43, and the driving voltage decreased to about 9.5 V as the quantity of MPS-SNP increased to 5 wt.%. MPS-SNP nanoparticles were modified by using an MPS functional group, which had a long alkyl chain and a methacryloyl group, for the photopolymerization reaction. Similar to BTMA-SNP, MPS-SNP also shows hydrophobic properties because of its long alkyl chains and is thus expected to remain in the polymeric region during photoinduced phase separation. However, compared with BTMA-SNP, MPS-SNP has a photoreactive methacryloyl functional group and can also participate in the photopolymerization of prepolymers. Therefore, MPS-SNP can be uniformly distributed in the region around the polymer walls after photoinduced phase separation and can prevent the aggregation of MPS-SNP in the polymer walls. Owing to the uniform distribution of MPS-SNP in the polymer walls, the surface region of the walls might have been well covered by MPS-SNP, and thus, the improvement in the electro-optical properties of the LC-polymer composites was more evident than it was for BTMA-SNP.
We found that phase separation behavior and interfacial properties of lattice-patterned LC-polymer composites were significantly affected by the inclusion of various SiO2 nanoparticles in the prepolymers. The distribution of SiO2 nanoparticles in the LC-polymer composites was affected by the surface functional groups of the nanoparticles. Untreated SiO2 nanoparticles were mainly located in the LC region, and thus, the behavior of the LC molecules was disturbed by these nanoparticles. Surface functionalized SiO2 nanoparticles remained in the polymeric region after photoinduced phase separation due to the hydrophobic alkyl chains of the surface functional groups. The SiO2 nanoparticles in the polymer walls were closely related to the interfacial properties between the LC and the polymer walls. In particular, SiO2 nanoparticles with a photoreactive methacryloyl group could participate in the photopolymerization of the prepolymers. Owing to the uniform distribution of the SiO2 nanoparticles with a methacryloyl group, the electro-optical properties of the LC-polymer composites were effectively improved by the inclusion of the nanoparticles. By using customized nanoparticles with various sizes and surface functional groups as additives, it might be possible to control the phase separation behavior and the interfacial properties of lattice-patterned LC-polymer composite systems. Such control can facilitate the use of the LC-polymer composite system for the fabrication of various flexible electronic devices.
This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (11-BD-03).
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