Qi-Feng Zhou

Liquid crystalline polymers

Liquid Crystal States of Matter Theories of Liquid Crystalline Polymers Molecular Engineering of Liquid Crystalline Polymers Characterization of Liquid Crystalline Polymers Liquid Crystalline Polymers as High Performance Fiber and Structural Materials Physical Properties of Liquid Crystalline Polymers.

Reverse atom transfer radical polymerization of methyl methacrylate in imidazolium ionic liquids

Abstract Reverse atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) employing 2,2-azobisisobutyronitrile (AIBN)/CuCl2/bipyridine(bipy) as the initiating system was approached at 80 °C in two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) and 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12mim][BF4]), respectively. The polymerization in [C12mim][BF4] proceeded in a well-controlled manner as evidenced by kinetic studies, end group analysis, chain extension, and block copolymerization results, but not in [C4mim][BF4] presumably due to poor solubility of PMMA in it. The kinetic study of reverse ATRP of MMA in recycled [C12mim][BF4] suggested that this ionic liquid could be re-used as reaction solvent after simple purification, without affecting the living nature of polymerization.

Water soluble multi-walled carbon nanotubes prepared via nitroxide-mediated radical polymerization

Nitroxide-mediated radical polymerizations of 4-vinylpyridine and sodium 4-styrenesulfonate were carried out on the surfaces of MWNT to afford poly(4-vinylpyridine)-grafted MWNT (MWNT-P4VP) and poly(sodium 4-styrenesulfonate)-grafted MWNT (MWNT-PSS). The covalent bonding of polymers to the MWNT surface was confirmed by IR and TGA investigations. SEM, TEM and AFM investigations also showed a polymer-wrapped MWNT structure. Due to the presence of basic pyridine units in the structures, MWNT-P4VP exhibited good solubility in acidic aqueous solutions, but tended to aggregate in neutral or basic solutions. In comparison, MWNT-PSS could form stable dispersion solutions in aqueous solutions throughout the pH range from 1 to 14.

Morphological study of oriented films obtained from side-chain liquid crystalline polymers

Abstract The morphological features of side-on fixed liquid crystalline polymers in an oriented state are investigated using polarizing microscopy, small angle light scattering (SALS) and infra-red dichroism techniques. A conventional side-chain polymer with mesogenic units attached longitudinally to the backbone is also studied for comparison. Banded texture usually only reported for main-chain liquid crystalline polymers is observed for these side-on fixed polymers under the crossed polarizing microscope, and SALS measurements also show typical grating diffraction patterns. This implies that side-on fixed rigid mesogens may significantly affect the conformation of the main chain of molecules, and the molecule as a whole may become rather rigid, exhibiting behaviour similar to that of the main chain. Infra-red dichroism measurements indicate that the main-chain axes of the molecules tend to orient in directions close to the shear direction. The side-on fixed mesogens are packed around the backbone, but their axes are not parallel to the latter.

Restudy of the Old Poly-2,5-di(benzoyloxy)styrene as a New Liquid Crystal Polymer

Abstract The 40 year old polymer, poly-2,5-di(benzoyloxy)styrene, has been restudied from the view point of the “mesogen-jacketed liquid crystal polymer.” This old polymer was synthesized in 1952 as a precursor of an electron exchange polymer and is now found to be thermotropically liquid crystalline, from which the characteristic banded texture of rigid and semi-rigid liquid crystal polymers may be obtained. The monomer of this polymer, 2,5-di(benzoyloxy)styrene, was then also not realized as liquid crystalline, is now found to be liquid crystalline forming too. This finding offers probably the first example of liquid crystal rigid rod-like molecules with lateral but without terminal substitutions. The synthesis and primary characterization of a series of the closely related polymers poly-2,5-bis[(4-substituted-benzoyl)oxy styrenes are also reported.

Synthesis of a New Class of Side Chain Liquid Crystal Polymers—Polymers with Mesogens Laterally Attached Via Short Linkages to Polymer Backbones

Abstract New monomers, 2,5-(4-alkoxybenzoyl-oxy)benzyl acrylates were synthesized. Upon radical polymerizations of these monomers thermotropic liquid crystalline polyacrylates with mesogens laterally attached to polymer backbones via short linkages were obtained. The polymers were found non-crystalline by WAXS measurements. Above glass transition all the polymers had a stable mesophase as revealed by DSC and a polarizing microscope. The glass transition and the isotropization temperatures were about 100°C and about 165°C respectively, both varying with the size of the R substituents in the mesogens. The term “mesogen-jacketed polymer” is proposed to describe this type of polymers of which the main chains are jacketed with laterally substituted mesogens and to distinguish this new class of side chain liquid crystal polymers from the conventional ones.

Synthesis and properties of highly birefringent liquid crystalline materials: 2,5-bis(5-alkyl-2-butadinylthiophene-yl) styrene monomers

In order to obtain liquid crystals with high birefringence (Δn), a series of new liquid crystals monomers incorporating styrene, a 2,5-disubstituted thiophene ring and diacetylenes, which were highly conjugated along the molecular long axis, were synthesized. Their physical properties were evaluated. As expected, they exhibited high Δn values of 0.4–0.7 and a low melting point and were of potential use for polymer-dispersed liquid crystal displays (PDLCDs), cholesteric displays and laser beam steering applications.

Synthesis and properties of novel second-order NLO chromophores containing pyrrole as an auxiliary electron donor

A novel series of second-order NLO chromophores containing pyrrole as an auxiliary electron donor was synthesized via Knoevenagel reactions between 5-aminated N-methylpyrrole-2-carbaldehydes and different electron-accepting groups, i.e., malononitrile, picolinium tosylate and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF). Their corresponding NLO chromophores containing thiophene in the place of pyrrole were also prepared for comparison. The resulting NLO chromophores showed good solubility in common organic solvents such as CHCl3, THF and DMF, except for TTCF containing thiophene and TCF, which is soluble in polar aprotic solvents but poorly soluble in less polar solvents. NMR studies of these chromophores showed that, in comparison with thiophene rings in the same type of NLO chromophores, pyrrole rings had higher electron density, as evidenced by the up-field chemical shifts of pyrrole protons. TGA investigations showed good thermal stability of these chromophores in nitrogen with the onset weight loss temperatures in the range of 203 to 296 °C. Positive solvatochromism of 10–44 nm from dioxane to chloroform were found for these chromophores, and moderate to very large molecular static hyperpolarizabilities (β0) of 57–1490 × 10−30 esu were revealed by hyper-Rayleigh scattering measurements. For chemical bonding to polymer chains, hydroxyl-containing NLO chromophores were also prepared and characterized for their linear and nonlinear optical properties.

Crystallization kinetics of new copoly(ethylene terephthalate-imide)s

Crystallization kinetics of copoly(ethyleneterephthalate-imide)s (PETIs) of various imide contents were studied by the DSC technique. The results of isothermal and nonisothermal crystallization kinetics studies showed a consistent trend in that the crystallization of all copolyesterimides comprised a primary stage and secondary stage, and that the crystallization mechanism was a three-dimensional growth with athermal nucleation. The crystallization rate of PETIs first increased and then decreased as the content of imide units in the copolyesterimides increased. This indicates that the imide units in the copolymer decreased the chain-packing regularity of poly(ethylene terephthalate) (PET) and the same units also enhanced the copolymer chain rigidity. The copolyesterimide with 0.2 mol% imide unit content showed a dramatic increase of the crystallization rate, and exhibited the highest crystallization rate of all the studied compositions. Studies of isothermal and nonisothermal crystallization kinetics, showed that small amounts of imide units in the the backbone act as nucleating agents during the crystallization process of copolyesterimides.

Atom-Transfer Radical Polymerization to Synthesize Novel Liquid Crystalline Diblock Copolymers with Polystyrene and Mesogen-jacketed Liquid Crystal Polymer Segments

The synthesis of a series of new rod-coil diblock copolymers with different molecular weights and low polydispersity was achieved by atom transfer radical polymerization. The block architecture (coil-conformation of styrene segment and rigid-rod conformation of 2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene segment) of the diblock copolymers was experimentally confirmed by proton nuclear magnetic resonance (1H NMR) and differential scanning calorimetry (DSC). The liquid crystalline behavior of the copolymers was studied using DSC and a polarized optical microscope (POM). It was found that the liquid crystalline behavior was dependent on the molecular-weight of the rigid segment. Only those copolymers with Mn of the rigid block beyond 9,300 g/mol could form liquid crystalline phases above the glass transition temperature of the rigid block.