Showing papers by "Jacky Wing Yip Lam published in 2001"

Tuning the Chain Helicity and Organizational Morphology of an l-Valine-Containing Polyacetylene by pH Change

Abstract: The helical chirality and self-assembling structures of an unnatural polymer, poly(p-ethynylbenzoyl-l-valine) (1), are readily manipulated by a simple environmental perturbation of pH change. The amino acid appendages of l-valine create an asymmetric force field, inducing the polyacetylene backbones to helically rotate, and form intra- and interchain hydrogen bonds, stabilizing the screw-sense conformation of the polymer chains. The polymer exhibits a large Cotton effect in methanol, which decreases with an increase in pH upon addition of KOH into the polymer solution. The change in the chain helicity is reversible: the unfolded polymer chains refold back to their original helical conformations when the solutions are neutralized. Natural evaporation of the methanol solutions of 1 on mica gives long, bundled nanofibers of macromolecular assemblies; in contrast, evaporation of the methanol/KOH solutions yields short, unraveled nanofibers with sizes of roughly single macromolecular chains. The ionization of...

Influence of electric field on the photoluminescence of a liquid crystalline monosubstituted polyacetylene

Abstract: It has been shown recently that strong photoluminescence (PL) can occur in monosubstituted polyacetylenes (PAs). In this letter, we investigate the PL of tetrahydrofuran solutions of a highly luminescent liquid crystalline PA, poly(11-{[4′-heptoxy-4-biphenylyl)carbonyl]oxy}-1-undecyne) (PBU), under the influence of direct current electric fields. The applied electric fields can dramatically tune the PL structures of a moderately concentrated PBU solution (∼11.3 mM) at and above an electric field threshold of about 3×105 V/m. Combined with the director simulations, our results indicate that the electrically induced molecular reorientation and intermolecular interaction are responsible for the observed spectral changes.

Synthesis of fullerene-containing sol-gel glasses

Abstract: Macroscopically homogeneous and visually transparent fullerene-containing glasses are fabricated from sol-gel mixtures of aminated C60 derivatives and tetraethyl orthosilicate (TEOS) through physical blending and chemical reaction. The aminated fullerenes are synthesized by the amination reactions of C60 with 6-amino-1-hexanol, cyclohexylamine, 2-(2-aminoethoxy)ethanol, and 3-aminopropyltriethoxysilane at 100°C under nitrogen. The amination products are purified by filtration, precipitation, and column chromatography and are isolated in good to excellent yields (32–82%). Characterization by NMR, MS, and TGA analyses reveals that the aminated fullerenes possess molecular structures H x C60[NH(CH2)6OH] x $$(1;{\bar x = 7)}$$ , H x C60(NH-cyclo-C6H11) x $$(2;{\bar x = 11)}$$ , H x C60[NH(CH2CH2O)2H] x $$(3;{\bar x = 11)}$$ , and H x C60[NH(CH2)3Si(OCH2CH3)3] x $$(4;{\bar x = 4)}$$ . All the aminated fullerenes except 2 are completely soluble in aqueous alcoholic solutions of TEOS and can be incorporated into silica gel networks by sol-gel process in the absence or presence of drying-control chemical additives, giving crack-free monoliths of large sizes (up to ∼60 mm). Electronic absorption spectrum of the fullerene glass continuously red shifts with an increase in the C60 content, suggesting the formation of fullerene nanoclusters in the sol-gel process. The fullerene glasses are thermally and optically stable, resisting continuous attack of strong laser pulses of 532 nm for a prolonged period of time without losing their optical limiting power.

Rapid fabrication of three-dimensional porous films with biomimetic patterns by natural evaporation of amphiphilic polyacetylene solutions under ambient conditions.

Abstract: A simple process for fast fabrication of thin films with biomimetic morphological structures from a group of linear homopolymers is developed. Natural evaporation of tetrahydrofuran, chloroform, and hexane-dichloromethane solutions of poly(phenylacetylene)s that contain amino acid and ethylene glycol moieties under ambient conditions instantly produces three-dimensional porous films with structural patterns reminiscent of honeycombs and radiolarian shells. Morphological analysis by optical and electronic microscopy suggests that vesicles of the amphiphilic polymers serve as building blocks in the self-organization to the biomimetic structures.

Electrically tunable photoluminescence of liquid crystalline polyacetylene solutions

Abstract: Light emission from tetrahydrofuran solutions of a liquid crystalline polyacetylene, poly(11-{[(4′-heptoxy-4-biphenylyl)carbonyl]oxy}-1-undecyne), is investigated in the electrical field. The field exerts little effect on the photoluminescence of the polymer solution with a low concentration (1.03×10 −4 M). The photoluminescence of a concentrated solution (113×10 −4 M) is, however, noticeably quenched under an electrical field with a field strength of >3×10 5 V m −1 . When the field strength is increased to ⩾3.67×10 5 V m −1 , the bimodal emission spectrum of the solution changes to a monomodal one. Thus, both the emission intensity and spectral profile of the photoluminescence of the concentrated solution can be tuned by the electrical field, which is probably caused by the aggregate dissociation and mesogen realignment induced by the external stimulus.

Monomer and dimer emissions in the solutions of a monosubstituted polyacetylene

Abstract: Photoluminescence (PL) of tetrahydrofuran solutions of a monosubstituted polyacetylene with liquid crystalline pendants, poly(11-{[4′-heptoxy-4-biphenylyl)carbonyl]oxy}-1-undecyne) ( 1 ), has been investigated. Intense ultraviolet emission (∼370 nm) has been detected in its dilute solution (2.5 μM). As the solution becomes concentrated (up to 39 mM), the emission color changes to deep blue and finally to blue (∼430 nm). In addition to the large-scale red shift in PL peak, its luminescent intensity changes about two orders of magnitude. Based on the extended Huckel tight-binding calculations, the 370 nm peak is assigned to monomer emission, which originates from the biphenyl mesogen in the pendants, and the peak at 430 nm corresponds to dimer emission, which is related to the aggregates formed in concentrated solutions. Lyotropic mesophase was observed as 1 dissolves in tetrahydrofuran.

The role of the phenyl and biphenyl chromophores in the blue luminescent liquid crystalline polyacetylenes

Abstract: We have studied the photoluminescence (PL) of liquid crystalline polyacetylenes {[HCC(CH 2 ) 9 OCOBiphO(CH 2 ) 6 CH 3 ] n  ( 1 ), [HCC(CH 2 ) 3 OC 6 H 4 CO 2 C 6 H 4 O (CH 2 ) 5 CH 3 ] n  ( 2 ), and [HCCC 6 H 4 CO 2 (CH 2 ) 6 OCOBiphO(CH 2 ) 6 CH 3 ] n  ( 3 )}. The textures of the polymers are characterized by polarized optical microscope. In dilute tetrahydrofuran (THF) solution, 1 emits strong deep-blue PL with a single PL peak at ∼380 nm, while 2 emits faint blue PL with a single peak locating at ∼450 nm. Interestingly, the PL of 3 is composed of two peaks, one of which locates at 380 nm and the other at 450 nm. Using extended Huckel tight-binding method, we have calculated their electronic structures and the results show that the electronic states of the polymers are essentially an ensemble of the extended states characteristic of the backbone and the localized states characteristic of the pendant. Our experimental and calculated results prove that both the absorption and blue emissions take place in the phenyl or biphenyl mesogens in the pendants and the types of chromophores determine the emission colors of the polymers.

Stable controller design for linear systems

Abstract: The paper is concerned with the problem of suboptimal stable mixed H/sub 2//H/sub /spl infin// control for linear time-invariant systems. The designed controllers are required to satisfy a prescribed H/sub /spl infin// performance bound or a prescribed degree of stability. By reducing the stable controller synthesis problem to a multiobjective state feedback control problem for two different state models, sufficient conditions for the solvability of the considered problem are given in terms of solutions to algebraic Riccati equations and matrix inequalities. LMI-based iterative algorithms are developed to solve the stable controller synthesis problem. All of the proposed algorithms are shown to be convergent. An example is given to illustrate the proposed methods.

Liquid crystalline light-emitting thermally stable readily processable substituted polyacetylenes

Abstract: Mesomorphic and luminescent poly(propiolates) with different skeleton structures (-{(R)CequalsC[CO 2 (CH 2 ) 6 OCO-Biph- OC 7 H 15 ]} n -; R equals H (1), CH 3 (2), C 6 H 5 (3), Biph equals biphenylyl} are synthesized. The backbone absorption in 2 and 3 are weak but upon photoexcitation, the polymers emit strong violet light of 369 nm, whose intensities are higher than that from poly(l-phenyl-l-octyne), a well-known highly-fluorescent polyacetylene. The main chain of 3 absorbs strongly at 380 nm, and the polymer is completely nonluminescent. All the polymers are thermally stable and form enantiotropic monolayer SmA phase, with 1 adopting well-ordered packing arrangements.