Fluorescence study of thermotropic liquid-crystalline polyesters

Photoluminescence behavior (polarization, lifetime) related to liquid-crystal (LC) formation was examined for the thermotropic liquid-crystalline polyesters poly [(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %) and poly [(ethylene 2,6-naphthalene dicarboxylate)-co-(p-oxybenzoate)] (PEN50/OBA50) (OBA:50 mol %). The Growth of liquid-crystalline (LC) phases of PET40/OBA60 proceeded during annealing. even at low temperature (e.g., 138°C) and were promoted by an increase in annealing temperatures T_a in the experimental temperature range 138–260°C. The concentration dependence of fluorescence spectra of PET40/OBA60 in solution suggested that the fluorescences at 325 and 395 nm can be attributed to monomer and ground-state dimer, respectively. The increase in dimer fluorescence intensity and the decrease in the fluorescence anisotropy ratio r from 0.06 to –0.14 were observed with growth of LC phases. These effects are explained by an increase in the ground-state dimer population and a slight change in the dimer configuration, respectively. PEN50/OBA50 showed monomer fluorescence at 395 nm due to naphthalenedicarboxylate segments and excimer fluorescence at 430 nm. The r value for the excimer fluorescence decreased from zero to about ?0.14 with growth of the LC phase. Such an extraordinary phenomena, in comparison with the usual excimer fluorescences which occurs through energy migration, could be interpreted as the result of formation of high-concentration excimer sites induced by chain orientation in LC domains. © 1993 John Wiley & Sons, Inc.     

Dynamic x-ray diffraction studies of liquid-crystalline polyesters

Mesophase transitions in liquid-crystalline (LC) polyesters were studied by dynamic x-ray diffraction using a synchrotron radiation source. Powder and fiber samples were examined by continuous heating from 50°C to 270°C in a hot stage. The polymer systems consisted of two types of thermotropic polyesters with mesogenic cores composed of combinations of substituted terephthalate, oxybenzoate and hydroquinone units combined with aliphatic spacers placed in the main chain. One of these samples was a chemically homogeneous LC polyester (HTH12) while the other LC polyester possessed chemical heterogeneity (BP6). BP6 could also be processed to form fibers which showed thermal transition behavior by x-ray diffraction and no detectable melting or clearing transition by thermal measurements. LC textures were observed using polarized light microscopy. Results of the dynamic x-ray diffraction studies of these two LC polymers are described.     

Fullerene C60 as a stabilizer in the synthesis of liquid-crystalline polyesters

It has been shown that owing to the use of fullerene C₆₀ (～0.01 wt % based on the monomer weight) as a stabilizer in the synthesis of a liquid-crystalline (LC) polyesters via high-temperature acceptorless poly-condensation, the temperature interval of reaction may be widened to 220°C and the crosslinking side reactions may be eliminated.     

Synthesis and phase behaviour of liquid-crystalline side group polyesters

Several series of liquid-crystalline side group polymers with a polyester backbone structure have been synthesized using mesogenic diethylmalonate and 1,3-propanediol derivatives. The structures of the polymer backbone and the mesogenic units have been varied systematically. As shown by differential scanning calorimetry, polarizing microscopy and X-ray diffraction, the side group polyesters exhibit nematic and/or smectic mesophases. Polyesters synthesized from mesogenic diethylmalonate derivatives can tolerate long non-mesogenic segments in the polymer main chain without losing their liquid crystal properties. X-ray studies suggest that some of the smectic polymers exhibit interdigitated bilayer S_A phases.     

Polymerisation in lyotropic liquid-crystalline phases of dioctadecyldimethylammonium bromide

The polymerisation of styrene in lyotropic liquid-crystalline (LC) phases of dioctadecyldimethylammonium bromide (DODAB) in water is explored. Amphiphile concentrations between 20 and 50 wt % are employed. The study is set out as a model study for polymerisation reactions in nonstabilised, nonfunctional bilayer systems. X-ray characterisation was used to assess the phase behaviour of the lyotropic mesophases before, during and after polymerisation. The DODAB/water system forms the lamellar phase within the concentration range considered. Addition of styrene to the lamellar phase of DODAB at an equimolar ratio induces a phase shift to a bicontinuous cubic phase at elevated temperatures near the phase-transition temperature. Upon polymerisation within this cubic phase, the phase structure is maintained if the system is kept at constant temperature; however, if the polymer/amphiphile phase is cooled, the lamellar phase, being typical of the DODAB/water system, is restored. It is concluded that, as a result of phase separation between the polymer and the amphiphile phase, the polymerisation in lyotropic LC phases does not provide a stable copy of the templating amphiphile phase. This is in analogy to the observations for polymerisations in other lyotropic phases. Received: 16 March 2000 Accepted: 1 July 2000     

Conformational structure of liquid-crystalline polyesters with mesogens and spacers in the main chain

The conformations of thermotropic polyesters with alternating mesogens and poly(methylene) spacers in the main chain were investigated by conformational calculations using the data of linear-dichroism IR-spectroscopy, X-ray analysis and differential scanning calorimetry. The conformations of monomeric units and of the macromolecules as a whole in different phase-aggregated states were estimated. It is shown that in the liquid-crystalline (LC) state the polymer chains are strongly extended. The results are compared with the data of various theories of LC ordering.     

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.     

On the origin of Grandjean-Cano lines in liquid-crystalline blue phases

A model is proposed to explain the Grandjean-Cano lines occurring in wedge-shaped samples of liquid-crystalline blue phases I and II. Between stretched and compressed areas of the cubic BP lattice edge dislocations occur which give rise to the observed Cano lines. The results for the BP I are in agreement with a b.c.c. lattice in (110) orientation parallel to the surface whereas for the BP II a s.c. lattice in (100) orientation was derived.     

Blends of an epoxy thermosetting polymer with aromatic liquid-crystalline polyesters

A series of aromatic liquid-crystalline polyesters with different composition have been synthesized to adjust transition temperatures and molecular weight. Miscibility of polyesters with bisphenol-A-diglycidyl ether (DEGBA), 4,4′-methylene-bis(3-chloro-2,6-dimethylaniline) (MCDEA) and the influence on transition temperatures has been studied. Miscibility of binary and ternary mixtures was found over the whole range of composition depending on the temperature. Thermoset formation by curing of LC-polyester / DEGBA / MCDEA mixtures containing different amounts of polyester resulted in reaction-induced phase separation with polyester content from 30 to 50 wt.%. Cloud point techniques, scanning electron microscopy (SEM) and dynamic mechanical thermal analysis (DMTA) have been applied.     

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.     

Characterization of the segmental mobility of liquid-crystalline side-chain polyesters by fourier-transform infrared spectroscopy

Fourier-Transform infrared (FTIR) polarization spectroscopy has been applied to study segmental orientation of a new class of liquid-crystalline side-chain (LCSC) polyesters as a function of laser irradiation. From the experimental results qualitative structural models and quantitative results in terms of an order parameter were derived. Time-dependent investigations yielded a detailed insight into the orientational response of the mesogens, and temperature-dependent studies were applied to monitor the erasure of the induced orientation.     

Synthesis and properties of thermotropic liquid-crystalline polyesters containing 9,10-diphenylanthracene moiety in the main chain

We synthesized thermotropic liquid-crystalline polyesters in which 9,10-diphenylanthracene moieties are incorporated into the main chain type of polyester forming the chiral smectic C (Sm C*). The polymers were prepared by the isopropyltitanate-catalyzed reaction of biphenyldicarboxylic acid and the corresponding diols, with different ratios of diol of 9,10-diphenylanthracene moiety to the alkane diols (1, 5, and 10 mol %) under nitrogen atmosphere. The polymers exhibited thermotropic liquid crystals despite the presence of a bulky diphenylanthracene moiety in the main chain. The circular dichroism spectra revealed that a Sm C* phase was formed in the polymer with 1 mol % of anthracene moiety, although only an Sm A phase was formed in the other polymers. This is the first example of a Sm C* polyester containing a diphenylanthracene moiety in the main chain. Furthermore, we measured the optical properties of the polymers and found that they exhibited very high fluorescent efficiency. The fluorescence spectra of the thin film differed from that of a CH₂Cl₂ solution.     

Equilibria of extended-chain polymers exhibiting crystalline and liquid-crystalline phases

The phase diagram of poly(p-benzamide) (PBA) in N,N-dimethylacetamide/LiCl solutions was determined for two PBA samples having weight-average molecular weights of about 10,000. The various equilibria were studied using analytical, viscometric, and optical microscope measurements. The phase diagram at 25°C, taking as variables the concentration of polymer (C_p) and LiCl (C_s), involves several equilibria which can be summarized as follows: solid ? isotropic solution when 0.75 ≤ C_s ≤ 2 g/dl, solid ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, isotropic solution ? anisotropic solution when 2 ≤ C_s ≤ 4 g/dl, and C_p > 6 g/dl, and dilute isotropic solution ? gel when C_s > 4 g/dl. In the C_p range in which the isotropic and anisotropic phases coexist, enrichment of the high-molecular-weight component of the polymer in the anisotropic phase becomes more marked as the volume fraction of the latter phase is decreased. The two PBA samples exhibit noticeable differences in solubility, absolute viscosity, and in their viscosity-concentration behavior. The location of the maximum in the latter dependence does not necessarily coincide with the first appearance of the anisotropic phase. In the absence of a flow field, anisotropic solutions exhibit an irreversible increase in viscosity. Inclusion of the equilibria involving the crystalline state furnishes insight into some of the common observations for extended-chain polymers. A diagram illustrates the superposition of the solubility curves for a crystalline polymer and the liquid-crystal regions. This indicates that, for the high melting crystalline polymers, the crystalline phase should be stable relative to the concentrated anisotropic phase of the wide biphasic region.     

Filled smectic A phases in binary liquid-crystalline systems of terminal-nonpolar compounds

We present five phase diagrams of binary systems in which one component is a bi-swallow tailed compound and the second component is a compound with pronounced nematogenic properties. The common feature of these binary systems is the induction of smectic A phases which have been investigated using X-ray. The smectic A induction seems largely to be the result of specific steric interactions between the components.     

Lyotropic liquid-crystalline phases formed by Pluronic P123 in ethylammonium nitrate

Aggregation of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer, Pluronic P123, is promoted in a room temperature ionic liquid, ethylammonium nitrate (EAN). A series of lyotropic mesophases including normal micellar cubic (I1), normal hexagonal (H1), lamellar (Lalpha), and reverse bicontinuous cubic (V2) are identified at 25 degrees C by using polarized optical microscopy and small-angle X-ray scattering techniques. Such self-assembly behavior of P123 in EAN is similar to those observed in H2O or 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMim(+)][PF6(-)]) systems except for the presence of the V2 phase in EAN and the absence of the I 1 phase in [BMim(+)][PF6(-)]. This suggests that the ionic solvent of EAN plays similar roles as H2O and [BMim(+)][PF6(-)] during the aggregation process and solvates the PEO blocks through hydrogen-bond interaction. Furthermore, the hydrogen bonds are considered to form between the ethylammonium cations and oxygen atoms of the PEO blocks as confirmed by Fourier transform infrared spectra of P123-EAN assemblies. This deduction is also consistent with the results from differential scanning calorimetry and thermogravimetric analysis. The additional V2 phase appearing in the P123-EAN system is attributed to the higher affinity for the relatively hydrophobic PPO blocks to EAN than to water, which might reduce the effective area of the solvophilic headgroup and increase the volume of the solvophobic part. The obtained results may help us to better understand the self-assembly process for amphiphilic block copolymers in protic solvents.     

Induction of thermotropic bicontinuous cubic phases in liquid-crystalline ammonium and phosphonium salts

Two series of wedge-shaped onium salts, one ammonium and the other phosphonium, having 3,4,5-tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontinuous "gyroid" cubic (Cub(bi)) and hexagonal columnar liquid-crystalline (LC) phases by nanosegregation between ionophilic and ionophobic parts. The alkyl chain lengths on the cationic moieties, anion species, and alkyl chain lengths on the benzyl moieties have crucial effects on their thermotropic phase behavior. For example, triethyl-[3,4,5-tris(dodecyloxy)benzyl]ammonium hexafluorophosphate forms the thermotropic Ia3d Cub(bi) LC phase, whereas an analogous compound with trifluoromethanesulfonate anion shows no LC properties. Synchrotron small-angle diffraction intensities from the Ia3d Cub(bi) LC materials provide electron density maps in the bulk state. The resulting maps show convincingly that the Ia3d Cub(bi) structure is composed of three-dimensionally interconnected ion nanochannel networks surrounded by aliphatic domains. A novel differential mapping technique has been applied successfully. The map of triethyl-[3,4,5-tris(decyloxy)benzyl]ammonium tetrafluoroborate has been subtracted from that of the analogous ammonium salt with hexafluorophosphate anion in the Ia3d Cub(bi) phases. The differential map shows that the counteranions are located in the core of the three-dimensionally interconnected nanochannel networks. Changing from trimethyl- via triethyl- to tripropylammonium cation changes the phase from columnar to Cub(bi) to no mesophase, respectively. This sensitivity to the widened shape for the narrow end of the molecule is explained successfully by the previously proposed semiquantitative geometric model based on the radial distribution of volume in wedge-shaped molecules. The LC onium salts dissolve lithium tetrafluoroborate without losing the Ia3d Cub(bi) LC phase. The Cub(bi) LC materials exhibit efficient ion-transporting behavior as a result of their 3D interconnected ion nanochannel networks. The Ia3d Cub(bi) LC material formed by triethyl-[3,4,5-tris(decyloxy)benzyl]phosphonium tetrafluoroborate shows ionic conductivities higher than the analogous Ia3d Cub(bi) material based on ammonium salts. The present study indicates great potential of Cub(bi) LC nanostructures consisting of ionic molecules for development of transportation nanochannel materials.     

One-dimensional energy migration in crystalline and columnar liquid-crystalline phases of 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene

Thermotropic columnar mesophases capable of giving highly oriented fibres are considered as interesting systems for the study of one-dimensional energy migration. Steady-state and time-resolved luminescence experiments carried out with homogeneous solutions, crystalline and liquid crystalline samples of a disc-like compound, 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene, are reported. Normal fluorescence, phosphorescence and P-type delayed fluorescence related to energy migration have been detected in the pure compound phases.