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22 results on '"Toh, Cher Ling"'

5. Hydrogen bond-directed self-assembly of peripherally modified cyclotriphosphazenes with a homeotropic liquid crystalline phase

Author

Toh Cher Ling, Jianwei Xu, and Chaobin He

Subjects
Phase transition, Differential scanning calorimetry, Polymers and Plastics, Hydrogen bond, Chemistry, Liquid crystal, Mesogen, Organic Chemistry, Homeotropic alignment, Polymer chemistry, Materials Chemistry, Supramolecular chemistry, Mesophase
Abstract

The synthesis and characterization of hydrogen-bonded star-shaped complexes consisting of stilbazolyloxy, azopyridyl, and Schiff base-substituted cyclotriphosphazenes (3a, 3b, and 3c, respectively) and monoalkyloxy, bis(dodecyloxy), and tris(dodecyloxy)benzoic acids are reported. The thermal behaviors of complexes are studied by the means of differential scanning calorimetry, polarizing optical microscopy, and X-ray diffractometry. Only 3a and 3b with monoalkyloxybenzoic acids show a homeotropic smectic A mesophase. The effect of azo and ethylene linkage of mesogenic groups in the cyclotriphosphazenes and the length of the flexible chain in monoalkyloxybenzoic acids on mesophase transition behaviors are investigated, revealing that the linkages in mesogenic groups governs the phase transition temperatures, and the length of flexible chain in proton donors plays an important role in controlling the magnitude of enthalpy and entropy of mesophase transitions in this supramolecular liquid crystal system.

Published
2008

6. Poly(ethylene terephthalate)/clay nanocomposites with trisilanolphenyl polyhedral oligomeric silsesquioxane as dispersant : simultaneously enhanced reinforcing and stabilizing effects

Author

Toh, Cher Ling, Yang, Liping, Pramoda, Kumari Pallathadka, Lau, Soo Khim, Lu, Xuehong, and School of Materials Science & Engineering

Subjects
Engineering::Materials::Ceramic materials [DRNTU]
Abstract

This paper presents a new approach for the preparation of poly(ethylene terephthalate) (PET)/clay nanocomposites using surfactant-free clay (sodium montmorillonite, Na-MMT) with trisilanolphenyl polyhedral oligomeric silsesquioxane (Tsp-POSS) as dispersant. The dispersion of clay in the PET/Na-MMT/Tsp-POSS nanocomposites is enhanced over that in PET/Na-MMT by using a very small amount of Tsp-POSS, which acts as functional spacer to keep clay platelets apart and pull monomers in, and, at the same time, acts as a PET chain extender. As a result, thermomechanical properties and thermo-oxidative stability of PET/Na-MMT/Tsp-POSS are improved simultaneously compared with those of PET/organoclay nanocomposites. ASTAR (Agency for Sci., Tech. and Research, S’pore)

Published
2013

20. Temperature and pH Dual-Responsive Behavior of Dendritic Poly( N-isopropylacrylamide) with a Polyoligomeric Silsesquioxane Core and Poly(2-hydroxyethyl methacrylate) Shell.

Author

Bai, Yu, Yang, Liping, Toh, Cher Ling, He, Chaobin, and Lu, Xuehong

Abstract

Novel temperature and pH dual-responsive dendritic polyoligomeric silsesquioxane (POSS)-poly( N-isopropylacrylamide) (PNIPAm)-poly(2-hydroxyethyl methacrylate) (PHEMA) copolymers are prepared via atom transfer radical polymerization and click reactions. The cloud points ( Tc) decrease with decreasing pH from 10.0 to 5.0 due to the weakened inter-molecular interactions and enhanced intra-molecular hydrogen bonding, whereas the Tc exhibits a small increase from pH 5.0 to 4.0 because of the better solvation of PHEMA at highly acidic conditions. The above findings are corroborated by the different sizes of aggregates observed by dynamic light scattering. The encapsulation of a fluorescent dye and stimulated release by temperature and pH changes are also demonstrated. [ABSTRACT FROM AUTHOR]

Published
2013
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21. Effects of clay on polymorphism of polypropylene in polypropylene/clay nanocomposites

Author

Zheng, Wenge, Lu, Xuehong, Toh, Cher Ling, Zheng, Tong Hua, and He, Chaobin

Abstract

The effects of clay on polymorphism of polypropylene (PP) in PP/clay nanocomposites (PPCNs) under various thermomechanical conditions were studied. In extruded PP and PPCN pellet samples, only α-phase crystallites existed, as they were prepared by rapidly cooling the melt extrudates to room temperature. Under compression, β-phase crystallites can develop in neat PP under various thermal conditions, of which isothermal crystallizing at 120 °C gave the highest content of β-phase crystallites. In contrast, no β-phase crystallite was detected in the PPCN samples prepared under the same conditions. This indicated that clay significantly inhibits the formation of β-phase crystallites. The likely reason is that the presence of clay in PPCNs greatly sped up the crystallization process of the α phase, whereas it had an insignificant effect on the crystallization rates of the β phase. The results also showed that clay may slightly promote the formation of γ-phase PP crystallites in PPCNs. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1810–1816, 2004

Published
2004
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22. Packing behaviors of structurally different polyhedral oligomeric silsesquioxane-imidazolium surfactants in clay.

Author

Toh CL, Xi L, Lau SK, Pramoda KP, Chua YC, and Lu X

Abstract

This paper reports the packing structures of two types of polyhedral oligomeric silsesquioxane (POSS)-imidazolium surfactants with different molecular rigidity in the intergalleries of montmorillonite clay. Wide-angle X-ray scattering (WAXS) and molecular modeling studies suggest that the POSS-imidazolium cations have a bilayer packing structure in clay with the long axes of the molecules largely tilted with respect to the basal plane. Direct evidence for the bilayer structure is provided by transmission electron microscopy (TEM). WAXS and differential scanning calorimetry (DSC) results indicate that the relatively flexible POSS-imidazolium cation is able to form a two-dimensionally ordered structure in the clay intergalleries, while the relatively rigid one exhibits a disordered structure in clay. Furthermore, with reducing surfactant loading, the clay modified with the rigid surfactant exhibits increased interlayer d-spacings probably because the surfactant is able to take more extended conformation at lower loadings. With its low organic content and disordered surfactant packing structure, the clay modified with the rigid surfactant also exhibits excellent thermal and thermo-oxidative stabilities.

Published
2010
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