Synthesis and mesomorphic structures of comb liquid-crystalline polymers with lipopeptidic side chains

Lipopeptidic macromonomers were synthesized from bi-functional lipids by two methods and transformed into comb copolymers with lipopeptidic side chains by radical polymerization of their acrylate, methacrylate, acrylamide or methacrylamide terminal group. X-ray diffraction and differential scanning calorimetry studies showed that comb copolymers exhibit both a thermotropic and a lyotropic behaviour and present smectic and nematic mesophases. The influence of the solvent concentration, the degree of polymerization of the peptidic chains and the nature of the main chains on the structural parameters of the smectic mesophases were established in the case of copolymers with liposarcosine side chains.     

Coexistence of two different side chain packing structures in a smectic phase of liquid-crystalline side chain polymers

Ten varieties of liquid-crystalline side chain polymers, poly(cholesteryl-ω-(methacryloyloxy)alkanoates) (pChMO-n, n = 1, 2, 3, 4, 5, 7, 9, 10, 11 and 15; the carbon number of the alkyl chain), were studied by differential scanning calorimetry and small angle X-ray scattering. On and after the first cooling run from the isotropic state, these polymethacrylates gave the same smectic phase. X-ray investigations showed that pChMO-n with short spacers (n = 1-7) has a two layer (bilayer) S_A packing structure, and pChMO-n with a longer spacer (n= 15) has a single layer (monolayer) S_A packing structure. However, these two types of packing structure appear simultaneously in pChMO-n (n = 9s-11) below their phase transition temperature. To clarify the manner of the coexistence of the two different structures the smectic layer spacing and X-ray diffraction patterns were examined by small angle X-ray scattering at various temperatures.     

X-ray investigation of combined main-chain/side-chain liquid-crystalline polymers

Structural investigations on six combined liquid-crystalline polymers using the X-ray technique are described. The measurements have allowed a characterization of the liquid-crystalline phases and have led to our first ideas about the phase structures. All mesogenic groups are arranged parallel to each other, the shortest segment in the main-chain or in the side-chain determines the smectic layer spacing. A modified designation for liquid-crystalline phases of high polymers is proposed.     

Controlling side-chain density of electron donating polymers for improving their packing structure and photovoltaic performance

A simple and efficient approach of controlling the side-chain density in the electron donating polymers has been demonstrated to tune their 3-D packing structure and HOMO level, which increases the hole mobility and V(oc) values, thus improving the solar cell performance.     

Thermotropic liquid-crystalline polymers. XXVII. Reentrant nematic phase of side chain liquid-crystalline polymers and their optical properties

The phase diagrams of side chain liquid-crystalline acrylic copolymers with cyanobiphenyl mesogenic groups are described. These copolymers are shown to form a reentrant nematic phase. The main regularities of the reentrant behaviour of polymer systems are studied. Certain peculiarities of the electric field induced orientation phenomena are discussed.     

Characteristics of liquid-crystalline cholesteryl and cyanobiphenyl side chains pendent to trans-polypentenamer

trans-Polypentenamers with thermotropic liquid-crystalline side chains cholesteryl and cyanobiphenyl were prepared by ring-opening polymerization of vinylcyclopropane monomers with proper substituents. Molecular weights of the polymers were in the range of 25000 to 80000 and the ratios of weight- to number-average molecular weights M _w/M _n were between 3.3 and 3.8. The glass transition temperature values of the polymers were 35°C ( 4a ) and 39°C ( 4b ). Monomers 3a and 3b present cholesteric and smectic mesomorphism, respectively. On the other hand, polymers 4a and 4b present only a smectic mesophase.     

The synthesis of biodegradable polymers with functional side chains

Trifunctional hydroxy-terminated oligomeric polyesters, M_n 500, 1000, and 2000, were prepared by initiating ring-opening copolymerization of δ-valerolactone and ε-caprolactone with glycerol. The prepolymers were converted to crosslinked polyester-urethanes by their reaction with hexane-1,6-diisocyanate in proportions corresponding to 70, 80, 90, and 100% of the hydroxyl content. The moduli of the resulting elastomers varied between 0.12 MPa and 3.83 MPa, and the elongation at break between 60 and 2000%. The residual hydroxyl groups were derivatized by heterogeneous reaction with chloroacetic anhydride or excess hexane-1,6-diisocyanate, and these and further transformations of the functional groups were verified by infrared spectroscopy and electron probe x-ray microanalysis. A second series of hydroxy-substituted elastomers was synthesized by copolymerization of δ-valerolactone, ε-caprolactone, and 4-(t-butyldimethylsilyloxy)-ε-caprolactone, using different amounts of 2,2-bis(caprolacton-4-yl)propane as the crosslinking agent; removal of the t-butyldimethylsilyl group to liberate pendant hydroxyl groups was achieved with acetic acid but not fluoride ion. The hydroxylated polyester (but not the polyesterurethanes) was shown to undergo enzymatic surface erosion in rabbit. The biodegradation data were compared with results previously obtained with low-modulus elastomeric polyesters.     

Photochromic liquid-crystalline polymers. Main chain and side chain polymers containing azobenzene mesogens

Abstract

Two classes of thermotropic polymers were synthesized containing the trans-azobenzene unit as both a mesogenic and a photochromic group. In the former class (I) the azobenzene unit is incorporated into the main chain of substituted polymalonates, while in the latter class (II) it is appended as a side chain substituent to a polyacrylate backbone. The liquid-crystalline properties of the polymers were studied as a function of the chemical structure. All of the prepared polymers I have smectic phases. Polymers II are nematic and/or smectic, or cholesteric when including a chiral residue R'. Polymers I and II when radiated at 348 nm in chloroform solution undergo trans-to-cis isomerization of the azobenzene moiety. The calculated rate constants are comparable with those of low molar mass model compounds, and indicate that the macromolecular structure does not significantly affect the photoisomerization rate.     

Branched side chains improve molecular packing of non-fullerene acceptors

正As one of the promising emerging photovoltaic technologies(PVs), organic solar cells(OSCs) have received extensive attention from both scientific and industry communities. The advantages of OSCs, such as low toxicity, high power-toweight ratio, and easy manufacturing and upscaling, guarantee their contribution to versatile applications powered by clean and renewable solar energy, for instance,     

Mesophase structure and some properties of liquid-crystalline organoelement polymers

The structure and physico-mechanical properties of polyphosphazene, poly(trimethylvinylsilane) and blends of polyphosphazene with polyethylene were studied. It was shown that these organoelement homopolymers form mesophase structure which can be described as condis-crystal, and mesophase polymers are effective modifiers when mixed as blends with polyethylene, allowing to make processing more effective and obtain composites with improved mechanical properties.     

Two-dimensional NMR study of slow phenyl-flips in liquid-crystalline side group polymers

Abstract

Glass-forming liquid-crystalline group polymers were investigated by ²H 2D exchange NMR. In oriented systems, the exchange pattern of a single 2D spectrum yields the orientational distribution function with at least as high precision as a full rotation pattern of 1D spectra. In addition, important information about the geometry of the phenyl-flip motion is obtained. The 2D lineshapes are particularly sensitive towards possible slow motion of the flip axis which could be envisaged to fluctuate within the orientational distribution of the mesogenic group. However, the experimental spectra for a frozen nematic and a frozen smectic polysiloxane with an orientational distribution of width ±18·5° and ±12·5°, respectively show no indication of the corresponding features, proving that the flip axis is fixed within ± 3° even after a large number of flips.     

Two-dimensional NMR study of slow phenyl-flips in liquid-crystalline side group polymers

Glass-forming liquid-crystalline group polymers were investigated by ²H 2D exchange NMR. In oriented systems, the exchange pattern of a single 2D spectrum yields the orientational distribution function with at least as high precision as a full rotation pattern of 1D spectra. In addition, important information about the geometry of the phenyl-flip motion is obtained. The 2D lineshapes are particularly sensitive towards possible slow motion of the flip axis which could be envisaged to fluctuate within the orientational distribution of the mesogenic group. However, the experimental spectra for a frozen nematic and a frozen smectic polysiloxane with an orientational distribution of width ±18·5° and ±12·5°, respectively show no indication of the corresponding features, proving that the flip axis is fixed within ± 3° even after a large number of flips.     

Synthesis and characterization of liquid-crystalline side group polymers with benzylideneaniline as mesogenic moiety

Two new thermotropic liquid-crystalline side group polymers were synthesized, characterized and compared with the liquid-crystalline monomeric analogues. Some packing features of these polymeric liquid crystals are discussed.

Investigations were carried out by differential calorimetry (D.S.C.), polarization microscopic observation and X-ray diffraction on non-aligned and magnetic field-aligned samples in the wide and small angle region.

The synthesized polymers contain as mesogenic moiety a benzylideneaniline group which is attached in the 4 position via a hexamethylene spacer to a poly-methacrylate backbone. The benzylideneaniline group is substituted in 4′ position with an ethoxy or butoxy group (PEt or PBu). The monomeric analogues are denoted MEt and MBu. The two polymers show a phase sequence crystalline-smectic A-nematic-isotropic. The liquid-crystalline temperature range is observed between 90 and 150°C. The monomeric MEt exhibits only a monotropic nematic, MBu an enantiotropic nematic and a smectic A phase.     

Dielectric relaxations in bilayer structures of comb-like copolymers: Perfluoralkyl and liquid-crystalline side chains

Abstract : A series of comb-like copolymers and homopolymers containing different liquid-crystalline and perfluoralkyl side chains was studied by frequency and temperature dependent dielectric measurements. The structure and the phase behaviour of the systems were characterized by X-ray scattering and differential scanning calorimetry. The dielectric relaxation spectra of these polymers reflect the molecular mobility in bilayer structures formed by the mesogenic or crystalline side chains. By changing temperature it was possible to investigate the molecular motions in the different phases of the copolymers (smectic-crystalline, smectic-isotropic, and isotropic). The homopolymer containing perfluoralkyl side chains and all copolymers show a β-relaxation at low temperatures, which is assigned to local molecular motions. Above the glass transition temperature, all samples exhibit a segmental (α) relaxation with WLF-like temperature dependence in the activation plots. For the polymers forming LC structures only one relaxation process was detected in all phases, i.e. in the smectic, nematic, and isotropic structure or, in case of the copolymers, in the smectic-crystalline and smectic-isotropic double structures. This process was attributed always to the segmental motions, assisted by tumbling motions of the mesogens. The phase transitions are clearly indicated by discontinuities in the dielectric relaxation times and changes in the relaxation strength Δϵ. The dipole reorientations of the mesogens seem to be more restricted by the crystalline layers in the copolymers as by the smectic order of the LC homopolymers.     

Surface tension of polymers with long unbranched side chains

The surface tension of polymers having long unbranched side chains was measured by a contact-angle technique. It ranges between 20–29 dynes/cm and is almost entirely due to dispersion forces. At the lower end of this range the side chains are attached to every other carbon atom of the main chain and the structure is characterized by extended alkyl side chains at right angles to the main chain with methyl groups forming the interface. As the spacing frequency of the side chains increases and the main chain becomes the site of other bulky groups, this structure becomes less ordered and the surface tension increases to the high twenties. The same effect, to a lesser extent, is observed when the length of the side chains is gradually diminished to 12 carbon atoms from 21. A sudden increase of the polar force contribution of the surface tension at 10–12 carbon atoms indicates collapse of the ordered side-chain structure.     

Dynamic mechanical and thermal behavior of liquid-crystalline polybutadiene-diols with mesogenic groups in side chains

Liquid-crystalline polybutadiene-diols (LCPBDs) with the comb-like architecture were synthesized by reaction of a LC thiol with the double bonds of telechelic HO-terminated polybutadiene (PBD). LCPBDs with various initial molar ratios of thiol to double bonds of PBD, R₀, in the range from 0.15 to 1, were prepared by the radical reaction at temperature 60 °C for 48 h. The experimentally obtained degree of modification, R_e, after the reaction and purification, was determined from elemental analysis - from the amount of sulphur bounded in LCPBDs, GPC and from ¹H NMR spectra. The physical properties were investigated by differential scanning calorimetry and dynamic mechanical spectroscopy. With increasing R_e ratio the glass transition temperature of LCPBDs, T_g, increases from ∼ − 45 °C (neat PBD) to ∼20 °C (R_e ∼ 0.5). LC transition starts at R_e ∼ 0.27 (the transition temperature T_m ∼ 27 °C). With increasing R_e temperature T_m increases and for R_e ∼ 0.5 reaches the value T_m ∼ 74 °C; at the same time also the change in enthalpy at LC transition increases. The LC transition could be detected also by the dynamic mechanical spectroscopy; especially shape and position of mechanical functions on frequency and free volume parameters strongly depend on degree of modification.