Poly(L-lysine) containing azobenzene units in the side chains: influence of the degree of substitution on liquid crystalline structure and thermotropic behaviour

The structure of poly(L-lysine)s containing between 20% and 100% of azobenzene units in the side chains has been studied by X-ray diffraction, between room temperature and 250 C. Except for samples having very low contents of azobenzene, the polymers are found to exhibit mesomorphic structures of the smectic A1 type deriving from the beta -structure of polypeptides. For polymers in which all lysine residues were substituted, the polypeptide main chains are arranged in layers corresponding to the sheets of a polypeptide 'antiparallel' beta -structure, and the side chains are perpendicular to the smectic layers. For polymers containing both substituted and free lysine side chains, each smectic layer results from the superposition of two layers: one layer contains the free lysine side chains; the other contains the azobenzenemodified lysine side chains and the polypeptide main chains that are arranged in 'antiparallel' beta -structures. All polymers exhibit only one smectic A mesophase as a function of temperature. The thickness of the smectic layers increases with increasing temperature until a thickness is reached that corresponds to the maximum interaction between the azobenzene mesogens in their trans -configuration.     

Structural correlation between the columnar mesophase and the melt in poly(di-n-alkylsiloxane)s

Phase transitions of poly(dialkylsiloxane)s substituted with propyl and butyl groups are reported based on X-ray diffractometry. The structure in the mesophase and in the melt of polymers with 2 to 10 carbon atoms in each side chain is compared. Both phases display a linear dependence of the molecular cross-section on the length of the alkyl group demonstrating their structural similarity. By means of differential scanning calorimetry it is shown that the temperature range of the mesophase remains practically unchanged in width as well as in absolute temperature for dibutyl up to dihexyl substituted polysiloxanes.     

Investigations on liquid crystalline partially fluorinated alkyl and succinimidyl benzoates

The synthesis and mesophase properties of partially fluorinated alkoxy‐substituted benzoic alkyl and succinimidyl (NHS) esters with one, two and three perfluoroalkyl alkoxy chains are reported. The mesophases were studied using differential scanning calorimetry (DSC), polarizing optical microscopy and X‐ray diffraction of non‐oriented samples. The SmA phases of the one‐chain methyl esters are monotropic, while those of the one‐chain NHS esters are enantiotropic. The more wedge‐shaped two‐ and three‐chain alkyl esters do not form mesophases, whereas the succinimidyl analogues exhibit hexagonal columnar phases. Their enhanced mesophase stability is caused by the higher polarity of the succinimidyl ring with respect to the alkyl ester groups. Aggregation of the dipolar succinimidyl groups, together with the microsegregation of the lipophilic and fluorophilic segments of the partially fluorinated alkoxy chains, is assumed to lead to a threefold structured morphology in both the SmA and the Col_h phases. This threefold structuring can be regarded as analogous to known morphologies of ABC triblock copolymers.     

Nanoscale segregation,molecular recognition and fluorophobic effect – from molecular design towards complex mesophase morphologies

Mesomorphic structure formation of hydrogen-bonded complexes of amino substituted 1,3,5-triazines with complementary (semiperfluoro)alkoxybenzoic acids is presented. The substitution pattern of both components was modified systematically in order to elucidate the influence of molecular parameters on the mesophase morphologies of the binary mixed systems. The phase sequence of the triazines, and of their associates with the acids, spans the range from smectic layer structures to discontinuous cubic phases composed of closed inverted micelles. Columnar phases with various two-dimensional lattice symmetries and bicontinuous cubic phases were found as intermediates. The mesophase morphologies are discussed in terms of the microsegregation of rigid polar, lipophilic and fluorinated molecular blocks in different sub-spaces along with tailoring the shape of (curved) aggregates by the space requirement of incompatible molecular fragments.     

Mesomorphic flexible chain polymers based on silicon

Poly(dialkylsiloxane)s and poly(dialkylsilane)s form a similar type of columnar mesophase. Although, the polysilanes are stiffer than polysiloxanes, both classes of polymers may be considered to be flexible due to the ability to form chain-folded crystals. Chain flexibility rather than the presence of chain stiffness determines whether the columnar mesophase is formed. A certain amphiphilic character does not appear to be required, as polysiloxanes with short side groups, e.g. polydiethylsiloxane display the same mesophase behaviour as polydialkylsilanes with long side chains and other nonpolar flexible chain molecules. The importance of the entropy gain upon conformational disordering is reflected in the increase in temperature stability with increasing alkyl side group length and the absence of mesophase behaviour in the case of the dimethyl substituted polymers     

Structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy) phosphazene]

The structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x-ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis-trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.     

Rigid rod polymers with flexible side chains. X. Thermotropic mesophases from aromatic stiff-chain polyamides bearing n-alkoxy side chains

The synthesis of stiff-chain poly(1,4-phenylene terephthalamide)s substituted by two as well as by four flexible side chains per repeating unit is described. The solubility of the materials bearing only two side chains is still very low. Appending of four side chains leads to polyamides which dissolve in common organic solvents. All polyamides reported herein form layered structures in the solid state as well as in the mesophase. Polyamides with two side chains have a very weak tendency for crystallization and do not exhibit a transition to the isotropic state even for the longest side chains. Polyamides with four side chains show three reversible thermal transitions: a disordering transition of the side chains, a transition to a layered, smectic-like mesophase, and finally the transition to an isotropic melt. It is shown that the phase behavior of these materials is mainly governed by the strong segregation of main- and side-chains which can be compared best to the microphase separation in block copolymers. © 1993 John Wiley & Sons, Inc.     

Synthesis and characterization of liquid crystalline elastomers bearing fluorinated mesogenic units and crosslinking mesogens

Several new side‐chain liquid crystalline (LC) polysiloxanes and elastomers ( IP ‐ VIP ) bearing fluorinated mesogenic units and crosslinking mesogens were synthesized by a one‐step hydrosilylation reaction with poly(methylhydrogeno)siloxane, a fluorine‐containing LC monomer 4′‐undec‐10‐enoyloxy‐biphenyl‐4‐yl 4‐fluoro‐benzoate and a crosslinking LC monomer 4′‐(4‐allyloxy‐benzoxy)‐biphenyl‐4‐yl 4‐allyloxy‐benzoate. The chemical structures and LC properties of the monomers and polymers were characterized by use of various experimental techniques such as FTIR, ¹H‐NMR, EA, TGA, DSC, POM and XRD. The effect of crosslinking mesogens on mesomorphic properties of the fluorinated LC polymers was studied as well. The obtained polymers and elastomers were soluble in many solvents such as toluene, tetrahydrofuran, chloroform, and so forth. The temperatures at which 5% weight loss occurred (T_d) were greater than 250°C for all the polymers, and the weight of residue near 600°C increased slightly with increase of the crosslinking mesogens in the fluorinated polymer systems. The samples IP , IIP , IIIP and IVP showed both smectic A and nematic phases when they were heated and cooled, but VP and VIP exhibited only a nematic mesophase. The glass transition temperature (T_g) of polymers increased slightly with increase of crosslinking mesogens in the polymer systems, but the mesophase–isotropic phase transition temperature (T_i) and smectic A–nematic mesophase transition temperature (T_S‐N) decreased slightly. It suggests that the temperature range of the mesophase became narrow with the increase of crosslinking mesogens for all the fluorinated polymers and elastomers. In XRD curves, the intensity of sharp reflections at low angle decreased with increase of crosslinking mesogens in the fluorinated polymers systems, indicating that the smectic order derived from fluorinated mesogenic units should be destroyed by introduction of more crosslinking mesogens. Copyright © 2008 John Wiley & Sons, Ltd.     

Mesophase structure of flexible-chain polymers

A structure of polyethylene (PE) and poly[bis(2, 2,2-trifluoroethoxy)phosphazene] (PPh) has been studied by means of the X-ray diffraction technique, scanning electron microscopy and differential scanning calorimetry. The polymers under investigation are examples of the organic and inorganic mesophase flexible-chain polymers not containing the mesogenic groups. When heated above the melting point both polymers form the condis crystal mesophase state which is stable in a wide temperature range. The PE mesophase reveals the pseudohexagonal structure, while the structure of mesophase PPh has some peculiarities arising from the sharp increase in the chain stiffness within a certain temperature range. The PPh mesophase structure is characterized by the perfect one-dimensional long-range order of the monomolecular layers and by the short-range order only along the two other directions within the layer. This structure may be due to the difference between the interactions of the inorganic backbones and regularly alternating side trifluoro-ethoxy groups.     

Sheet-shaped mesogens based on 1,3,5-triazines: variation of columnar mesophases through intermolecular hydrogen bonding

The mesomorphic behaviour of a liquid crystalline sheet-shaped 2,4,6-triarylaminothe 1,3,5-triazine bearing six peripheral decyloxy chains has been investigated in binary mixtures with two-fold alkoxy substituted non-mesomorphic benzoic acids. Each investigated equimolar mixture exhibits an enantiotropic columnar phase with either a hexagonal or a rectangular lattice. The two-dimensional lattice parameters depend on length and positions of the flexible alkoxy chains of the aromatic carboxylic acids. The structure formation of the mixed systems giving rise to variations of the mesophase structure of the pure triarylaminotriazine results from intermolecular hydrogen bonding between the complementary melamine and acid components.     

Structure of gyroid mesophase formed by monodendrons with fluorinated alkyl tails

The structure of a three-dimensional mesophase has been studied by the methods of small-angle X-ray diffraction and reconstruction of electron-density maps of the cubic lattice. In the oriented samples based on monodendrons with partially fluorinated alkyl tails, this mesophase has been shown to coexist with a two-dimensional columnar phase through a wide temperature interval. Epitaxial relationships between (10) planes of the hexagonal lattice and (211) planes of the cubic lattices lead to the ten-point pattern of azimuthal intensity distribution for the first X-ray 211 reflection and to the six-point intensity distribution for the second 220 reflection. The observed 12-point pattern of the 220 reflection is due to the presence of twin “crystallites” of the three-dimensional phase, and their [110] axis is parallel to the axis of cylinders in the columnar phase. The reconstructed electron-density maps show that the regions with increased electron density, which are composed of fluorinated aliphatic tails, form a bicontinuous gyroid structure.     

Mesomorphic complexes through hydrogen bonding between alkoxy-substituted triarylmelamines and semiperfluorinated benzoic acids

The thermal behaviour of two 2,4,6-triarylamino-1,3,5-triazines carrying either three or six peripheral alkoxy chains have been investigated in binary mixtures with two-chain and three- chain partially fluorinated benzoic acids by means of polarizing microscopy, differential scanning calorimetry and X-ray scattering. The melamines form hydrogen-bonded aggregates with the complementary carboxylic acids. Each investigated equimolar mixture exhibits a hexagonal columnar mesophase. In the case of the six-fold alkoxy-modified triazine the individual columns are built up by descrete hetero-dimers with a circular cross-sectional shape. The segregation of fluorinated from lipophilic side chain regions leads to a superstructure within the hexagonal lattice. The hexagonal columnar phases of 1:1 compositions of the triazine, incorporating just three aliphatic chains with the semiperfluorinated benzoic acids, are formed by pairs of H-bonded dimeric supermolecules.     

Thermotropic mesophases in linear polysiloxanes

Flexible linear polysiloxanes with inorganic backbones such as poly(diethylsiloxane) and poly-(dipropylsiloxane) contain no traditional mesogenic groups neither in the main chain nor in side chains and, nevertheless, they are able to form thermotropic mesophases. Recent developments in the study of thermodynamics, kinetics, structure and morphology of these mesophase polymers are considered. The temperature interval of existence of the mesophase is strongly dependent on the length of side radicals and molecular weight. These dependences are examined in detail. X-ray structure and the optical textures seen in the polarizing microscope are discussed. It is shown that large mesophase lamellae represent a very characteristic feature of the morphology of poly(diethylsiloxane). The linear growth rate of the lamellae and the overall calorimetric kinetics of the mesophase formation have been studied at various temperatures and the results obtained have been analyzed using the Avrami equation and the kinetic theory of the linear growth. As a result of the kinetic analysis two-dimensional growth has been suggested in accordance with the microscopic observation. The formation of the mesophase in slightly crosslinked poly(diethylsiloxane) samples can be initiated by stretching. The stress-strain and thermomechanic (deformation calorimetry data) behaviour of such samples is briefly discussed.     

Influence of Perfluoroarene–Arene Interactions on the Phase Behavior of Liquid Crystalline and Polymeric Materials

A stabilization of the liquid-crystalline mesophase and thus an enlarged temperature range of the mesogenic phase is achieved by adding perfluorotriphenylene to a chiral liquid-crystalline triphenylene. This mesophase is based on 1:1 perfluoroarene–arene interactions (see picture). In a polymer with triphenylenes as mesogens in the side chains, the addition of perfluorotriphenylene led to crystallization.     

Glasses of new 2,3,9,10,16,17,23,24-octasubstituted phthalocyanines forming thermotropic and lyotropic discotic mesophases

The synthesis and mesogenic properties of a new class of liquid crystalline metal-free and metalated phthalocyanines with different numbers of alkylthio side chains are presented. The effect of the substitution pattern in a series of differently octasubstituted phthalocyanines on the width and order of the thermotropic mesophase is investigated. The exchange of the alkylthio groups from chlorine atoms alters the mesophase width and the melting behaviour. Glassy phthalocyanines, without any tendency to crystallize, can be synthesized in this way. Furthermore, mixtures of differently 4,5-disubstituted benzenedicarbonitriles are tetramerized statistically to the corresponding mixtures of substituted phthalocyanines. The effect of decreasing symmetry at the molecular level on the properties of the liquid crystalline materials has been investigated. Highly oriented columnar systems can be obtained with these mixtures. For all compounds containing less than eight alkylthio groups, lyotropic mesomorphism in hexadecane solution is also found.     

Synthesis and characterization of new chiral mesogenic monomers

New mesogenic monomers carrying substituted biphenylyl rings linked to acryloyl or methacryloyl moieties through straight or chiral flexible spacers have been synthesized. All the investigated monomers, independent of the side group chirality, have been found to form ordered smectic phases, most probably of the S₁ type. The asymmetric methyl substitution on the flexible spacer leads to an expansion of the temperature range of mesophase stability. The chiral monomers induce a helical structure when mixed with appropriate nematogens. The fingerprint texture of a contact mixture of the chiral methacrylate prepared in this work, with a nematic methacrylate synthesized previously, has been found not to change after UV-initiated polymerization.     

Influence of peripheral alkyl chain length on the mesomorphic behaviours of hexasubstituted triphenylene 2,3-dicarboxylic esters

A comparative study of our established synthetic approaches to hexasubsituted triphenylenes 2,3-dicarboxylic esters containing four identical β-alkoxy and two adjacent β-alkoxycarbonyl side chains shows that the phase behaviours of small-sized discotic liquid crystals can be tailored over a wide range by simply varying the length of the peripheral alkyl chains. All the prepared esters in two series were observed to form a single hexagonal columnar phase, except for Tp4-1 having four β-butyloxy and two adjacent β-methoxycarbonyl chains which displays two columnar mesophase behaviours with a transition from the columnar plastic phase to hexagonal columnar phase. A significant difference between the two mesophase was observed in the variable temperature X-ray diffraction studies, and the mesophase assignment was also confirmed by polarising optical microscopy and differential scanning calorimetry. Moreover, the prepared esters in each series display the general trend of decreasing clearing temperature upon increasing alkoxy or alkoxycarbonyl chains length. The intermediate triphenylene 2,3-dicarboxylic acids were also found not only to exhibit columnar hexagonal mesophase over a narrower temperature range by maintaining high melting and clearing points but also to form organogel on mixing with toluene or dichloromethane with the assistance of hydrogen bonding.     

Hydrogen-bonded block mesogens derived from semiperfluorinated benzoic acids and the non-mesogenic 1,2-bis(4-pyridyl)ethylene

Thermal properties of benzoic acids carrying one or two semiperfluorinated alkoxy tails on the aromatic core have been investigated in binary mixtures with the non-liquid crystalline bidirectional trans-1,2-bis(4-pyridyl)ethylene. The hydrogen bonded complexes built from the complementary molecular species show a significantly enhanced mesophase stability compared with the fluorinated acids in their pure states. The mesophase morphologies of the complexes are governed mainly by the number of the partially fluorinated chains grafted to the acid component. Mixed systems comprising the one-chain acids exhibit a smectic C phase followed by a smectic A phase at more elevated temperatures. Incorporation of a second semiperfluorinated chain into the acid leads to the formation of columnar mesophases. These columnar phases of the H-bonded complexes should represent ribbon phases resulting from the collapse of the smectic layers.     

SELF-ASSEMBLED MICRO-DOMAINS ON THE UPPERMOST SURFACE OF FLUORINATED POLY（CARBONATE URETHANE）S WITH FLUORINATED SIDE CHAIN ATTACHED ON HARD SEGMENTS

The surface phase separated structure of polyurethanes is always desired due to the advantage of better biocompatibility, compared with the homogeneous one. The key issue is how to control and characterize the surface morphology. In this work, we report the uppermost surface morphology of fluorinated poly(carbonate urethane)s with fluorinated side chains attached to hard segments as studied by AFM, XPS and contact angle measurement. A self-assembled micro-domain with the fluorinated side chain standing up on the uppermost surface has been proposed for polyurethane with higher fluorinated content, based on the result obtained     

Discotic Metallomesogens: Effects of Sidechains Density in Transition Metal Bis(β-Diketonate) Complexes

The preparation and mesogenic properties of a series of discotic β-diketonate metal complexes are reported. The results show that the density of side chains, positions of side chains, and the geometries of the metal centers play important roles in determining the mesomorphic behaviors and thermodynamic stability of these complexes. In the series of copper complexes 3, all these disc-like molecules with eight alkoxy side chains exhibit columnar hexagonal disordered (D_hd) mesophases. In the series of copper complexes 2 with six side chains, only compounds substituted with longer alkoxy chains (n = C₁₄ or C₁₆) exhibit discotic columnar mesophase. However, in the series of complexes 1, only crystal-to-isotropic transitions were observed. The results showed that induction of liquid crystallinity not only depends on the numbers of side chains (i.e. side chain density), but also on the degree of distribution over the central core. Palladium complexes analogs exhibit similar discotic mesophases, and due to their greater core-core organization, they also have higher clearing points and wider temperature range of mesophases than copper complexes.