Crystallization kinetics study on N-(p-n-alkyloxybenzylidene)-p-n-alkylanilines (nO.m compounds)

A comparative systematic kinetic study of crystallization among various smectogens of higher homologues of the benzylidene aniline nO.m series (9O.2, 13O.2, 14O.2, 15O.2, 16O.2, 2O.16, 11O.16, 16O.16 and 18O.16) has been carried out by thermal microscopy and differential scanning calorimetry (DSC). The DSC thermograms were run from crystallization temperature to the isotropic melt for different time interval. The liquid crystalline behaviour together with rate of crystallization of smectic ordering in newly synthesized nO.m compounds (16O.2, 2O.16, 11O.16, 16O.16, 18O.16) with respect to their lower homologues are discussed in relation to the kinetophase (which occurs prior to the crystallization), end chain lengths, the odd-even effect and chain length ratio. The molecular mechanism and dimensionality in the crystal growth were computed from the Avrami equation. The characteristic crystallization time (t*) at each crystallization temperature was deduced from the individual plots of log t vs. DeltaH. Further, a qualitative approach was made to the crystallization process in smectic layers.     

Orientational order of p-n-alkoxybenzylidene-p-fluoroanilines. X-ray diffraction and electro-optic response of the chiral (S)-p-2-methylbutyloxy derivative

This work continues our study of the mesomorphic properties of the p-n-alkoxybenzylidenep-fluoroanilines. Previous studies have shown that the straight chain homologues, with the number of carbon atoms in the chain ranging from 4 to 8, form smectic phases. In this paper we describe H NMR studies of the homologues with n 4, 5 and 8 and the synthesis and properties of a chiral branched chain derivative, (S)-p-2-methylbutyloxybenzylidene-p-fluoroaniline. This compound is not mesomorphic, but does show solid state polymorphism. From an optical, electro-optical, DSC and X-ray diffraction study we infer that the higher temperature crystalline form is a CrE phase.     

Orientational order of p-n-alkoxybenzylidene-p-fluoroanilines. X-ray diffraction and electro-optic response of the chiral (S)-p-2-methylbutyloxy derivative

This work continues our study of the mesomorphic properties of the p-n-alkoxybenzylidenep-fluoroanilines. Previous studies have shown that the straight chain homologues, with the number of carbon atoms in the chain ranging from 4 to 8, form smectic phases. In this paper we describe H NMR studies of the homologues with n 4, 5 and 8 and the synthesis and properties of a chiral branched chain derivative, (S)-p-2-methylbutyloxybenzylidene-p-fluoroaniline. This compound is not mesomorphic, but does show solid state polymorphism. From an optical, electro-optical, DSC and X-ray diffraction study we infer that the higher temperature crystalline form is a CrE phase.     

聚｛11-[(4′-正庚氧基-4-联苯基)羰基]氧-1-十一炔｝的相结构与相转变

采用示差扫描量热法(DSC)、一维(1D)、二维(2D)广角X-射线衍射(WAXD)和偏光显微镜(PLM)等研究手段对聚{11-[(4′-正庚氧基-4-联苯基)羰基]氧-1-十一炔}(PA-9,7)的本体相转变和相结构进行研究,并采用分子动力学方法对相结构进行模拟.结果表明,样品的相转变为近晶B相(SmB)近晶A相(SmA)各向同性态(Iso).在近晶B相中,侧链在层状结构中排列成具有六次对称性的准长程有序结构。     

Thermotropic phase behavior of cationic gemini surfactants and their equicharge mixtures with sodium dodecyl sulfate

The lyotropic phase behavior for the neat cationic gemini surfactants alkanediyl-alpha,omega-bis(alkyldimethylammonium bromide), designated here as m-s-m, has been investigated previously in several works, but the thermotropic behavior has not been well characterized. Only for 15-s-15 and 14-s-12 have thermotropic liquid crystals (Lc) been reported. In this work, for the first time and in contrast to previous reports, we observe thermotropic Lc formation for m-2-m geminis with m = 12, 14, 16, and 18, by means of polarizing microscopy and differential scanning calorimetry (DSC). Furthermore, we investigate mixtures of m-2-m and SDS, m-2-m Br2.2SDS, which exhibit crystal-to-crystal phase transitions at lower temperature and, at high temperature, smectic Lc phases. The transition temperatures and enthalpies for Lc phases, obtained by DSC, present clear trends upon increase of the chain lengths. Combining Langmuir film experiments, possible lamellar arrangements for the different phases are tentatively discussed.     

Mesomorphic phase transitions of 4,4'-bis(ω-hydroxyalkoxy)-azoxybenzenes. Appearance of isotropic mesophase

Mesomorphic phase transitions of 4,4'-bis(ω-hydroxyalkoxy)-azoxybenzenes (number of carbons in the alkoxy group n = 2,3,6,8,11 and 12) have been studied by differential scanning calorimetry (DSC) and polarized optical microscopy. For the n = 8 compound, an optically isotropic (01) mesophase reminiscent of the smectic D phase was observed in the temperature range of 4 0 M 1 1 K between smectic C (below 400 K) and nematic (above 41 1 K) phases. On the other hand, the 0 1 mesophase was not observed in the other homologues used here; the n = 2,3 and 6 compounds had only a nematic phase and the n= 11 and 12 compounds had only a smectic C phase. This preliminary work points out that the thermal and optical properties of the 01 phase for the n = 8 compound are very similar to those of the smectic D phase.     

Crystallization kinetics study on higher homologues of benzylidene aniline compounds: impact of phase variant on nucleation process

A systematic kinetic study leading to the crystallization process from the kinetophases (which occur prior to crystal phase) smectic B, crystal G and smectic F is performed on representative compounds of the homologous series p-phenylbenzylidene-p'-alkylanilines (PBnA) and p-n-alkoxybenzylidene-p'-alkylanilines (nO.m) these compounds are p-phenylbenzylidene-p'-nonylaniline (PB9A), p-phenylbenzylidene-p'-tetradecylaniline (PB14A), p-n-pentadecyloxybenzylidene p'-tetradecylaniline (15O.14) and p-n-octadecyloxybenzylidene-p'-nonylaniline (18O.9). The molecular mechanism and dimensionality in crystal growth from the kineto phases are computed from the Avrami equation, while the characteristic crystalline time (t*) at each crystallization temperature is deduced from the individual plots of log t vs. ΔH. The low magnitudes of the dimensionality parameter n infers the occurrence of diffusion-controlled transformations leading to the formation of plates or needles of finite size possessing impinged edges. The degree of variation in the value of n at each crystallization temperature also reveals the existence of an independent nucleation mechanism for any individual member of the series. The influence of the terminal alkyl chain lengths on the rate of crystallization is determined from a comparative study with the reported analogous compounds.     

New series of ferroelectric liquid crystals with four ester groups

Three series of ferroelectric liquid crystals have been synthesized, having three ester groups in the mesogenic core and one ester group in the chiral chain. The ester groups introduced into the core decrease the temperature of crystallization, which results in a broad temperature range ferroelectric smectic C phase in homologues with a long non-chiral chain. These phases exhibit a high spontaneous polarization and relatively short pitch of the helical structure.     

Mesomorphic behaviour of praseodymium(III) alkanoates

In this paper, we describe the synthesis, characterization and thermal behaviour of praseodymium(III) alkanoates. The compounds have the stoichiometry [Pr(C_xH_2x+1COO)₃], where x = 5-19, and were characterized by elemental analysis and infrared spectroscopy. The thermal behaviour was investigated by hot-stage polarizing optical microscopy, DSC and high temperature X-ray diffraction. A highly viscous mesophase M and a smectic A phase were observed for the shorter chain compounds (x = 5-8), whereas only a smectic A phase was observed for the longer chain compounds of this type of metallomesogens. The chain length has a pronounced effect on the transition temperatures.     

Synthesis and characterization of new ferroelectric liquid crystals containing a (2S)-2-[6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'- naphthyl] propionate mesogenic group and oligo(oxyethylene) spacers

Three series of ferroelectric liquid crystals containing a (2S)-2-(6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'-naphthyl)propionate mesogenic group and oligo(oxyethylene) spacers were synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC), optical polarized microscopy (POM), and X-ray powder diffraction measurements. Some of these materials containing four phenyl rings of ester cores (i.e. -Ph-Ph-COO-naph-) and chiral heptyl tail exhibited a rich mesomorphic behaviour, a blue phase (BP), a cholesteric phase (Ch), a smectic A (S_A), a twist grain boundary A (TGB_A), and a chiral smectic C (S_C*) phase. Another series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tails revealed only a S_A phase and a S_C* phase. Moreover, a crystal E phase was observed in the short spacer chain (n = 0 or 1) homologues of three series of compounds. Also, the mesomorphism properties were discussed as a function of spacer units, numbers of aromatic rings of core, and different terminal asymmetric moieties.     

Investigations of a homologous series of chiral siloxane based dimesogenic compounds

A homologous series of siloxane based dimesogens was synthesized in racemic and enantiomeric form. The flexible part of the core is composed of a central hexamethyltrisiloxane unit and two polymethylene chains of variable length. Increasing the number of methylene units in the core promotes smectic phase behaviour whose polymorphism was studied by polarization optical microscopy. Transitions into metastable phases were detected and investigated by differential scanning calorimetry using different heating rates. Information about the crystallization tendency was obtained from annealing experiments by changing annealing temperature and duration. In contrast to short dimesogenic homologues which predominantly showed monomesogen-like behaviour, polymer-like behaviour was observed for the longer homologues. Small angle X-ray diffraction provided data on the temperature dependence of the smectic layer spacing of mesogens and their corresponding dimesogens linked by the siloxane spacer. No significant differences between the layer spacings of the two classes of compounds were detected which suggests chain folding in the dimesogenic compounds and a similar mesogenic orientation for the mesogens and dimesogens.     

Asymmetric dimeric liquid crystals The preparation and properties of the α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes

Abstract

The first eleven members of the homologous series of α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes have been synthesized. The compounds are all enantiotropic nematogens and, with the exception of the heptyl, octyl and nonyl homologues, exhibit smectic phases. The thermal stability of the smectic A phase initially increases with the length of the terminal alkyl chain, passes through a maximum and then falls dramatically before disappearing. The smectic A phase subsequently reappears with the decyl homologue which has the highest smectic A-nematic transition temperature of the series. In order to understand this unusual behavior we have determined the entropies of transition for the compounds and we have measured the layer spacing of the smectic A phase for three of them.     

Interrelation between side chain crystallization and dynamic glass transitions in higher poly(n-alkyl methacrylates)

Calorimetric and dielectric results for crystallizable poly(n-alkyl methacrylates) (PnAMA) with C=12, 16 and 18 alkyl carbons per side chain are presented. Degree of crystallization D_cal and melting peak temperature T_M are estimated from conventional DSC measurements. For poly(n-hexadecyl methacrylate) (C=16) the influence of isothermal crystallization is studied by DSC as well as TMDSC. Changes in dielectric relaxation strength Δε and α peak shape during crystallization are investigated. Effects of side chain crystallization on the complex dynamics of PnAMA are discussed. The results are related to the relaxation behavior of lower nanophase-separated PnAMA with two co-existing glass transitions, the conventional glass transition (a or α) and the polyethylene-like glass transition (α_PE) within alkyl nanodomains formed by aggregated alkyl rests. It is shown that amorphous as well as semicrystalline PnAMA can be understood as nanophase-separated polymers with alkyl nanodomains having a typical dimension of 1-2 nm. The results are compared with the predictions of simple morphological pictures for side chain polymers. X-ray scattering data for the amorphous and semicrystalline PnAMA are included in the discussion. Common aspects of nanophase-separated systems in both states as well as differences caused by crystallization are discussed. Indications for the existence of rigid amorphous regions are compiled. Different approaches to explain a similar increase of T_g(α_PE)—the glass temperature of the amorphous alkyl nanodomains—and T_M—the melting temperature of crystalline alkyl nanodomains—with side chain length are considered. Pros and cons of both approaches, based on increasing order within the alkyl nanodomains and confinement effects in nanophase-separated systems, are discussed. Main trends concerning crystallization and cooperative dynamics are compared with those in other systems with self-assembled nanometer confinements like microphase-separated blockcopolymers or semicrystalline main chain polymers.     

Synthesis and characterization of new ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (S_A), a chiral smectic C (S_C) and a crystal E (C_r E) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n = 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic S_A and Cr E phases.     

Mesomorphic behaviour of praseodymium(III) alkanoates

In this paper, we describe the synthesis, characterization and thermal behaviour of praseodymium(III) alkanoates. The compounds have the stoichiometry [Pr(C _x H_2x+1COO)₃], where x = 5-19, and were characterized by elemental analysis and infrared spectroscopy. The thermal behaviour was investigated by hot-stage polarizing optical microscopy, DSC and high temperature X-ray diffraction. A highly viscous mesophase M and a smectic A phase were observed for the shorter chain compounds (x = 5-8), whereas only a smectic A phase was observed for the longer chain compounds of this type of metallomesogens. The chain length has a pronounced effect on the transition temperatures.     

Effects of the alkyl side-chain length on the structures of poly[oxy(N-alkylsulfonylmethyl)ethylene]s

The structures of poly[oxy(N-alkylsulfonylmethyl)ethylene]s (ASE-Ns) were examined with X-ray, IR, DSC, and polarized optical microscopy. The structures of ASE-Ns were strongly dependent on the alkyl side-chain length. ASE-2 and ASE-3, the shortest ones, were amorphous materials. ASE-4 and ASE-5 showed nematic characteristics. ASE-6–ASE-12 had smectic A structures. ASE-14 and ASE-16 could be labeled as more ordered structures higher than smectic A (probably smectic B or smectic E). The d-spacings of the first small-angle reflections were double the most extended side-chain length and linearly increased with a slope of 2.50 Å per methylene unit, regardless of the structural phases. The maintenance of the double-layered structure in all ASE-Ns may be due to the strong dipole–dipole interactions at both sides of the layers against the main chain for all amorphous, nematic, and smectic phases of ASE-Ns. The double-layered structure was maintained above the isotropic temperature, indicating that dipole–dipole interactions were not destroyed although the alkyl side chains melted during the isotropic transition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1868–1874, 2004     

Synthesis and mesomorphism of 4,4'-bis(omega-hydroxyalkoxy)-alpha-methylstilbenes

A homologous series of 4,4'-bis(omega-hydroxyalkoxy)-alpha-methylstilbenes, denoted HAMS-n, has been synthesized. Here n is the carbon atom length of the peripheral n-alkyl chain; in this work n = 2 to 8 and 11. The composition, microstructure, thermal properties, and mesomorphism of the homologues were determined using elemental analysis, ¹H NMR, FTIR, DSC, wide angle X-ray diffraction, and polarizing optical microscopy with a heating stage. The influence of the length and the even/odd numbers of methylene units of the peripheral chains on the variants of mesomorphism and the stability of mesophases, as well as on their transition temperatures, are discussed. The homologues with n = 8 and 11 were enantiotropic monomorphic, CrG (or CrH). The odd homologues showed monotropic trimorphism on cooling, while the even homologues exhibited enantiotropic trimorphism. A pronounced even/odd effect on the melting temperatures was observed.     

Liquid-crystalline oxovanadium(IV) complexes accessed from bidentate [N,O] donor salicylaldimine Schiff-base ligands

A series of bis[4-(n-alkoxy)-N-(4′-R-phenyl)salicylideneiminato]oxovanadium(IV) complexes (n?=?6,?10,?14,?16,?18 and R?=?C₃H₇) were prepared and their mesogenic properties were investigated. The mesomorphic behaviors of the compounds were studied by polarized optical microscopy and differential scanning calorimetry. Ligands display SmA/SmC and unexpected nematic mesophases. The complexes bearing longer alkoxy carbon chain (n?=?10,?14,?16, and 18) showed both monotropic or enantiotropic transitions with smectic A and high ordered smectic E phases. However, the complex with shorter carbon chain length (n?=?6) showed monotropic transition with an unprecedented nematic (N) phase. A density functional theory study was carried out using DMol3 at BLYP/DNP level to obtain a stable optimized structure. A square-pyramidal geometry for the vanadyl complexes has been suggested. A ν_V=O stretching value of ～970?cm^?1 corroborated absence of any V?=?O?···?V?=?O interactions. Cyclic voltammetry revealed a quasireversible one-electron response at 0.61?V for the VO(IV)–VO(V) redox couple. Variable temperature magnetic susceptibility measurements of the vanadyl complexes suggested absence of any exchange interactions among the vanadyl spin centers.     

Mesomorphic properties of cyanobiphenyl dimers with a central malonate unit

A series of cyanobiphenyl dimers attached via alkoxy spacers to a central malonate were prepared, and the mesomorphic behaviour was studied by differential scanning calorimetry (DSC), polarised optical microscopy and X-ray diffraction (Wide-angle X-ray scattering and Small-angle X-ray scattering). Depending on spacer lengths and substitution of the malonate, nematic and smectic A mesophases with pronounced odd–even effect were observed. C-2-unsubstituted malonates formed nematic phases for chain lengths C₆–C₁₄, while C₁₂ and C₁₄ homologues displayed additional smectic A phases. In contrast, malonates with fluorinated tails at C-2 displayed exclusively smectic A phases. Remarkably, the X-ray diffraction profile of the smectic A phase of the C-2-unsubstituted C₁₂ malonate showed a fundamental (001) and the corresponding third-order (003) diffraction peak, but no (002) reflection. Using Fourier analysis, the diffraction pattern was converted to an electron density profile, which was in good agreement with the proposed packing model of the SmA mesophase based on a horseshow- or hairpin-like conformation of the malonate.     

Liquid crystalline properties of 4-isopropyl- and 4-isopropoxy- N -(4- n -alkoxysalicylidene)anilines

New homologous series of 4-isopropyl- and 4-isopropoxy- N -(4- n -alkoxysalicylidene)anilines were synthesized. The phase transitions of these homologues were determined by differential scanning calorimetry and polarizing optical microscopy. No enantiotropic liquid crystalline phases were observed for homologues having a terminal isopropyl group; as the terminal alkoxyl chain is lengthened, smectic C and nematic phases occur monotropically. The members with an isopropoxy substituent exhibit an enantiotropic nematic phase as the terminal alkoxyl chain is lengthened; the octyloxy member exhibits a monotropic smectic C phase as well as an enantiotropic nematic state.