Fabrication of inorganic-organic hybrid nanocomposites (IOHN) using well-defined nanoclusters has attracted much attention over the last decade due to their potential use in varied applications such as smart coatings, insulation layers in electronics, host materials in rare-earth complexes and organic dye.$^{1-4}$ $ The IOHN prepared from siloxane-based nanoclusters such as polyhedral oligomeric silsesquioxanes (POSS) or metal-oxo-clusters containing siloxanes possess more precisely controlled nanostructures and more reproducible properties compared to conventional IOHN made from inorganic and organic components mixed at the nano -scale level. In particular, POSS derived from the hydrolytic condensation of trifunctional organosilicon monomers offer many advantages for the construction of IOHN. However, their synthesis and functionalization via the introduction of functional organic groups, such as methacrylates, epoxides and amines requires complicated chemical modifications.$^5$ A simple technique for the synthesis of well-defined, siloxane-based nanoclusters is sought for a straightforward and reproducible method to fabricate IOHN.
In particular, aminopropyl and aminophenyl have very promising properties for roles as binding sites for DNA, microcrystals, diamond surfaces and photo-functional organic molecules.$^{1,6}$ Also, they can be easily changed into amide or imide bonds, resulting in IOHN with high thermal stability and mechanical strength.
Amine modified oligosiloxanes nanoclusters (AONC) are useful and effective nanoclusters with amine functionality to fabricate IOHN. The AONC have many advantages over POSS, because novel properties can be easily tailored. Their sizes and physical properties can be controlled readily by altering the composition of the reaction mixture. They have attractive optical properties such as a high optical transparency of over 90% in the visible range and a refractive index that may be modulated easily. The additional property of AONC can be easily obtained by simple chemical modification.
We report here the synthesis of amine modified oligosiloxane nanoclusters (AONC) and methacryl modified AONC. Additionally, we report the fabrication of photo-functional hybrid nanocomposites derived from AONC, methacryl modified AONC and vinyl cinnamate modified AONC.
Three different photo-functional hybrid nanocomposites were fabricated by using AONC, derivatives of AONC and dianhydrides. (1) colorless Imide hybrid nanocomposites, (2) photo-patternable imide hybrid nanocomposites and (3) photo-reactive hybrid nanocomposites for photo-alignment of liquid crystals (LCs).
AONC can be easily prepared in a single step by using the barium hydroxide monohydrate-catalyzed condensation reactions between APTS and DPSD. Barium hydroxide monohydrate is an effective catalyst that can activate a condensation reaction between APTS and DPSD, and it restricts self-condensation of the DPSD. The evidence for restricted self-condensation of DPSD is very few water contents, released from DPSD self-condensation, in the final solution. FT-IR results of AONC show a strong and broad vibrational band $(1000\sim1200cm^{-1})$ related to siloxane backbone and small vibration band of NH2 ($\sim3350cm^{-1}$:N-H stretch and 1617: N-H bending).
The $^{29}Si$ NMR data confirmed that the siloxane backbones in AONC were well established by condensation reactions. The size of AONC as determined by SANS is below 2nm. These results represent that AONC were successfully synthesized.
Methacryl modified AONC (AMO) was also prepared by same procedure. The formation of siloxane frames and restricted self-condensation of DPSD were confirmed by $^{29}Si$ NMR spectroscopy. Each oligosiloxane did not agglomerated by further condensation reactions. The sizes of AMO determined by small angle neutron scatterings (SANS) were below 2 nm. These results represent that AMO has uniform and confined sizes.
To fabricate imide hybrid nanocomposites, we used a general method of organic polyimide synthesis. The AONC reacted with the DOCDA to form an AONC amic acid solution as an intermediate state before forming the imide hybrid nanocomposite. In this step, each AONC was connected to short amide chains. The amic acid groups were then changed into imide groups after thermal imidization. Finally, we fabricated the imide hybrid nanocomposites in which each AONC was cross-linked with short diimide chains.
Thermal properties of imide hybrid nanocomposites were characterized by TGA. Their 5% weight loss temperatures under $N_2$ were approximately 430℃. The incorporation of AONC can retard the decomposition at lower temperatures due to the low molecular weight of the short imide segments in the nanocomposites.
Imide hybrid nanocomposites prepared from DOCDA and CBDA had excellent transmittance over 90% in the visible range and were entirely colorless. They have UV cutoffs at 295 nm and 270 nm, respectively.
To fabricate PINs, AMO was reacted with DOCDA to form AMO/amic acid solution as intermediate state before imide hybrid nanocomposite. In this step, each AMO was connected with short amide chains. Photo-polymerization of methacryl groups in nanocomposites was conducted at this step, because methacryl groups were decomposed during thermal imidization. After thermal imidization, imide hybrid nanocomposites that each AMO was cross-linked by short diimide chains were fabricated.
PIN consisting of AMO and DOCDA in NMP was chosen for photo-patterning. 2,4,6-Trimethylbenzoyldiphenyl phosphine oxide (TPO) was used as photoinitiator. The PIN film containing TPO showed good photo-patternability.
Photoreactive imide hybrid nanocomposites were prepared from vinyl cinnamate modified AONC and 6-FDA. Vinyl cinnamate groups on AONC surface were photo -dimerized after polarized UV exposures. These photochemical reactions induced the anisotropic orientation of main chain and side groups. The decreased absorption of cinnamate groups around 280nm represented that photo-dimerization occurred in the hybrid nanocomposites films.
The direction of the LC alignment on prepared photo-alignment layer was perpendicular to the electric vectors of linear polarized UV. The introduction of imide bonds offered high thermal stability and mechanical strength. Good photoalignment of LCs with high photosensitivity was shown in the fabricated films of imide hybrid nanocomosites containing vinyl cinnamates groups and orientations of LCs were stable in high temperature.
Various imide hybrid nanocomposites were fabricated by reaction of AONC with dianhydrides. Imide hybrid nanocomposites prepared from AONC and alicyclic dianhydirides had excellent optical transparency in the region of visible light and high thermal stability up to 430℃. Photo-patternable imide hybrid nanocomposites prepared from AMO and alicyclic dianhydrides had good patternability. The resulting pattern of PIN exhibited a 10μm resolution, including a high transmittance, above 85% at 400 nm wavelength. Photo-reactive imide hybrid nanocomposites containing vinyl cinnamate groups showed good photoalignment of LCs with high photosensitivity and thermal stability.
잘 정립된 나노클러스터를 이용하여 제조한 무-유기 하이브리드 나노복합체는 다양한 응용분야에서 높은 잠재성을 가진 재료이다. 특히 실록산 나노클러스터는 비교적 나노구조의 조절이 정확하고 기존의 방법으로 제조된 나노복합체 보다 재현성이 높은 물성을 제공한다. 따라서 잘 정립된 실록산 나노클러스터를 쉽게 제조 할 수 있는 방법에 관한 연구가 반드시 필요하다. 특히 아민 올리고실록산은 매우 유용하고 효과적인 나노클러스터로서 수식되어 있는 아민기는 DNA, nanocrystal, 광기능성 분자등과 결합하기가 쉬운 특성을 가진다. 또한 아마이드와 이미드 결합을 쉽게 형성 할 수있어 열안정성과 기계적특성이 우수한 나노복합체를 제조 할 수 있다. 아민-올리고실록산의 Si-O-Si 결합은 우수한 열안정성 이외에 광학적 성질 또한 우수하다.
본 연구에서는 3-aminopropyltrimethoxysilane과 diphenylsilanediol을 출발 물질로 하여 비 가수 솔-젤법을 통하여 매우 간단한 방법으로 아민-올리고 실록산을 제조하였다. 화학적 수식(chemical modification)을 통하여 메타아크릴과 비닐 시나메이트기가 수식된 올리고실록산도 제조하였다. 제조된 올리고실록산의 크기는 2 nm 정도이며 매우 균일하다. 제조된 올리고실록산과 alicyclic dianhydride를 결합시켜 광특성과 광기능성이 우수한 이미드 하이브리드 나노복합체를 제조하였고 그 특성을 분석하였다. 제조된 이미드 하이브리드 나노복합체는 높은 열안정성과 광투광성을 보인다. 메타아크릴과 비닐 시나메이트기가 수식된 올리고실록산과 alicyclic dianhydride를 이용하여 광패터닝이 가능하고 액정 분자의 광배향성이 가능한 나노복합체를 제조하고 그 특성을 분석하였다.
