Design,synthesis, and characterization of a combined main-chain/side-chain liquid-crystalline polymer based on ethyl cellulose

A novel combined main-chain/side-chain liquid-crystalline polymer based on an ethyl cellulose main chain containing azobenzene mesogens (AzoEC) was successfully synthesized. Molecular characterization of the resulting polymers with different degrees of substitution (DS) was performed with proton nuclear magnetic resonance (¹H NMR), Fourier-transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). Thermal stability was investigated by thermogravimetric analysis (TGA). The phase transitions and liquid-crystalline behavior of these polymers were investigated by differential-scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). The results indicate that DS has substantial effect on the liquid-crystalline behavior of these polymers. AzoEC with low DS only shows the cholesteric phase similar to ethyl cellulose (EC). However, when DS increases to a specific value, AzoEC begins to show fascinating supramolecular structures. The supramolecular structure of AzoEC with maximum DS consisted of a large-scale ordered lamellar structure formed by EC main chains and a small-scale ordered structure formed by azobenzene mesogens.     

Well-defined side-chain liquid-crystalline polysiloxanes

A route to well-defined side-chain liquid-crystalline polysiloxanes (ratio of weight-to number-average molar masses M̄_w/M̄_n < 1.2 is reported. Anionic ring-opening polymerization of pentamethylvinylcyclotrisiloxane yielded a poly(dimethylsiloxane-co-methylvinylsiloxane) backbone. A flexible disiloxane spacer was used to connect 4-(ω-alkenyloxy)-4′-cyanobiphenyl mesogenic molecules to the vinyl groups which belong to the backbone, leading to a side-chain liquid-crystalline polysiloxane (SCLCP) which has its mesogens distributed regularly along the main chain. Preliminary measurements indicate an electro-optic switching time τ_s = 1 min at 20°C and 7 s at 32°C (dc, 5 V/μm)).     

Semi-rigid thermotropic liquid-crystalline copolycarbonates comprising main-chain hexafluoropentane segments

Novel semi-rigid fluorocarbon-containing coplycarbonates composed of biphenyl moiety and rigid rod-like hexafluoropentane chain were prepared by melt polycondensation of 6,6′-(biphenyl-4,4′-diyldioxy)dihexanol ( 1 ) and 2,2,3,3,4,4-hexafluoro-1,5-pentanediol ( 2 ) with alkylene diphenyl dicarbonates 3 of various aliphatic lengths (m = 6, 8, 10 and 12). The structures of the copolymers 4 were confirmed by FT-IR, ¹H and ¹³C NMR spectra and elemental analyses. The thermal and liquid-crystalline (LC) properties were examined by differential scanning calorimetry (DSC), polarizing microscopy and temperature-changeable X-ray analyses. These measurements indicated that the copolymers show well-defined thermotropic nematic textures and have block-like sequences. It is suggested that the introduction of the hexafluoropentane chain into the main chain gives more stable LC phases.     

DSC and X-ray studies on side-chain crystallization of comb-like polymers

Thermal properties of acrylate and methacrylate monomers containing long-fluorocarbon chains (H(CF₂)_nCH₂OCOCH=CH₂, (F_nA) and H(CF₂)_nCH₂OCOC(CH₃)=CH₂, (F_nMA), wheren=6, 8, 10) and their comb-like polymers have been investigated by differential scanning calorimeter (DSC) and X-ray diffraction. The comb-like polymers (PF₁₀A and PF₁₀MA) with sufficiently long-fluorocarbon chains showed a simple melting and crystallizing behavior. For the fusion of PF₁₀A in 1st heating, enthalpy change H _f was 18 kJ mol^–1 and entropy change S _f was 45 J K^–1 mol^–1, while for PF₁₀MA the values H _f and S_f were 5.3 kJ mol^–1 and 14 J K^–1 mol^–1, respectively. Melted PF₈A crystallized slowly, whereas PF₈MA with same fluorocarbon chain and also both of PF₆A and PF₆MA with shorter fluorocarbon chains can hardly crystallize by the aggregation of side-chains. Effects of the length of side-chain and the flexibility of main chain on the side-chain crystallization of comb-like polymers are clear. Crystallization process of the methacrylate monomers was sensitively affected by the scanning rate of DSC measurement and the length of fluorocarbon chains.     

The packing structure of side chains of liquid-crystalline side-chain polymers before and during heat treatment

Six varieties of liquid-crystalline side-chain polymers, poly (cholesteryl ω-(methacryloyloxy)alkanoates) (p-ChMO-n, n = 1,2,3,4,5,7, the carbon number of the alkyl chain), were studied by differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS). On and after the first cooling run from the isotropic state, these polymethacrylates gave the same smectic phase. However, on the first heating run for virgin samples, unique phenomena were observed, similar to those frequently observed on the first heating run for thermotropic liquidcrystalline polymers. The thermal properties of the first heating run for these six polymers showed different tendencies between pChMO-n (n-1-3) and pChMO-n (4-7). Although the DSC curve of pChMO-n (n = 4, 5 and 7) exhibited the melting and clearing points on the first heating run, the DSC curve of pChMO-n (n = 1,2 and 3) exhibited an exothermal peak between the glass transition and the clearing point on the first heating run. This difference, probably due to the difference in the spacer length of these polymers, was investigated quantitatively by small-angle X-ray scattering by which the micro-change of the packing structure of the side chains of the polymers was traced.     

Density measurements on liquid-crystalline side-chain polysiloxanes

We have studied the behaviour of the density, ϱ, as a function of temperature for a number of side-chain liquid-crystalline polymers. The polymers have a polysiloxane backbone and a benzoate ester side group with different tails or flexible spacers. From the values of ϱ in the smectic A phase and from the layer thickness obtained from X-ray experiments we have determined a molecular area in the plane of the layers. The evolution of this parameter shows clearly the asymmetrical role of the tail and the spacer.     

Adhesive liquid-crystalline polymers

The synthesis and characterization of liquid-crystalline polymers with possible good adhesive properties is reported. These polymers are prepared by alternating copolymerization of maleic anhydride and mesogenic alkenes. The spacer length m is varied (m = 2, 3, 4, 6, 8 and 9) and methoxybiphenyl is used as the mesogenic group. The glass transition temperature decreases and the isotropization temperature increases with spacer length. Depending on the spacer length and temperature, S_B and S_Ad mesophases can be observed. After annealing, spin-coated films of these polymers show very regular layered structures with a layer spacing similar to that in the bulk.     

Mesomorphic properties of side-chain cholesteric liquid-crystalline elastomers

New monomer cholesteryl 4-(10-undecylen-1-yloxybenzoyloxy)-4′-ethoxybenzoate (M₁), crosslinking agent biphenyl 4,4′-bis(10-undecylen-1-yloxybenzoyloxy-p-ethoxybenzoate) (M₂) and a series of side-chain cholesteric elastomers were prepared. The chemical structures of the monomers and elastomers obtained were confirmed by element analyses, FT-IR, and ¹H NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. M ₁ showed cholesteric phase, and M ₂ displayed nematic phase. The elastomers containing less than 12 mol% of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability.     

Spontaneous deformation of main-chain liquid-crystalline elastomers composed of smectic polyesters

We observe the spontaneous shape change of a uniaxially deformed liquid-crystalline elastomer composed of smectic main-chain liquid-crystalline polyesters in a cyclic heating–cooling process. Although the elastomer contracts by about 115% on heating up to the isotropic phase, the sample length recovers by 55% on cooling to room temperature in the first heating–cooling process, and the elastomer exhibits an almost complete reversible deformation in the second heating–cooling process. By a comparison of the results of sample observation with those of X-ray analysis, we recognise that the strain λ was linearly coupled with the orientational order parameter S. In addition, the results of the X-ray analysis imply that a cybotactic nematic state, in which smectic clusters lie scattered in a nematic-like matrix, emerges after exposure to the isotropic phase.     

Mixtures of liquid-crystalline polymers

A density-functional expansion method is used to derive the free energy of a polymer mixture. The expression obtained includes the entropy of mixing, the entropy of configuration of the chains and the interactions (both isotropic and anisotropic ones). The chains are modelled as interacting elastic lines (bend curvature). The method is very general, and we only focus our attention on binary mixtures. The phase diagram and the order parameters are calculated. We show some results for two types of mixtures: a nematic polymer in a non-mesomorphic particle (polymer or solvent) and in another nematic liquid crystal (small-molecule or polymer). We discuss the influence of the molecular weights, the persistence length and the interactions on the phase separation.     

X-ray diffraction by thermotropic main-chain polymers with side groups. I. rigid polyesters

Thermotropic poly(sulfonyl hydroquinone terephthalate) with either H or CH₃ in the lateral substituent was aligned by drawing from nematic melts. Virtually identical x-ray intensity distributions were recorded consisting of “disks” of intensity periodically spaced along the meridian and concentric rings of intensity lying in the equatorial plane of reciprocal space. They correspond to what is expected for the hexagonal packing of parallel periodic chains that are randomly displaced along the chain axes. Using interactive graphics, appropriate molecular models were deduced in conformity with the observed diffraction intensities.     

Free volumes in liquid-crystalline main-chain polymer probed by positron annihilation

Free volumes in a copolymer consisting of p-hydroxybenzoic acid (HBA) and poly(ethylene terephthalate) (PET) were probed by positron annihilation technique. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured in the temperature range between 30 and 230°C. Above the glass transition temperature (ca. 60°C), the lifetime of ortho-positronium (o-Ps) annihilated in the free volumes and its annihilation probability were found to increase with increasing specimen temperature. These facts were attributed to the increase both in the size of the free volumes and in the concentration of such regions due to rearrangements of molecules. From the observed lifetime of o-Ps, it was found that the size of the free volumes increases from 0.05 nm³ to 0.1 nm³ in the temperature range between 30 and 230°C. At solid-mesophase transition temperature (150–220°C), the size of the free volumes monotonously increases, while the increase in the concentration of such regions saturates at 174°C. The clear relationship between the data obtained by the positron annihilation and those obtained by differential scanning calorimetry was confirmed. ©1995 John Wiley & Sons, Inc.