Thermotropic Liquid Crystalline Polyimides Research Articles

Synthesis and characterization of thermotropic liquid crystalline polyimides

Non-symmetrical and linear dyad-based mesogens were synthesised containing imine or ester bridging group. In the present work, due to the absence of branching in diamine-based mesogen, the structure has—rigidity inversely imine/ester bridging groups between two benzene rings imparts—flexible property to the mesogen and consequently rigid–flexible property has been balanced. The synthesised mesogens were characterized by different techniques including nuclear magnetic resonance and Fourier transform infrared spectroscopy. Liquid crystalline polymers (LCPs) were synthesised using pyromellitic dianhydride and 4-[(4-aminobenzylidene)amino]aniline or 4-aminophenyl-4-aminobenzoate. Subsequently, thermotropic liquid crystalline polymers (TLCPs) have also been evaluated to obtain optical microscopy textures at different temperatures which demonstrated interesting and notable changes. It is worth noting that marble-like textures were observed upto 200°C.

Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages: Synthesis, Characterization, and Liquid Crystalline Behavior

Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages: Synthesis, Characterization, and Liquid Crystalline Behavior

Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages: Synthesis, Characterization, and Liquid Crystalline Behavior

Thermotropic liquid crystalline (LC) semialiphatic polyimides have been developed from diamines containing siloxane spacer units with pyromellitic dianhydride (PMDA) or 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA). Monomer diamines with siloxane units were prepared by three steps, which include Williamson’s synthesis, hydrosilylation, and reduction. The polymerization was conducted by the conventional method, solution polymerization, followed by thermal imidization. A highly viscose poly(amic acid) solution was obtained and its inherent viscosities were 0.65−1.73 dL/g. These novel polyimides exhibited high thermal stability and a lower crystal-LC transition temperature compared to reported polyimides containing alkylene or oxyethylene spacer units. X-ray studies were performed for investigating the oriented smectic mesophases of each polyimide. In the polyimides derived from PMDA, a smectic A (SmA) phase was formed on gradual cooling from the isotropic liquid phase. On the other hand, the polyimides ...

Synthesis of Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages

Abstract Thermotropic liquid crystalline (LC) semi-aliphatic polyimides have been developed from 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA) and diamines containing siloxane spacer units. These polyimides exhibited high thermal stability and lower transition temperature compared to the corresponding polyimides containing alkyl or oxyethylene spacer units.

The double melting behavior of a liquid crystalline polyimide derived from PMDA and 1,3-bis[4-(4′-aminophenoxyl) cumyl] benzene

The double melting behavior of a thermotropic liquid crystalline polyimide was studied by means of differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). This liquid crystalline polyimide exhibited a normal melting peak around 278 °C and transformed into a smectic A phase. The smectic A phase changed to nematic phase upon heating to 298 °C, then became isotropic melt around 345 °C. The samples annealed or isothermally crystallized at lower temperature showed double melting endotherms during heating scan. The annealing-induced melting endotherm was highly dependent on annealing conditions, whereas the normal melting endotherm was almost not influenced by annealing when the annealing temperature was low. Various possibilities for the lower melting endotherm are discussed. The equilibrium melting points of both melting peaks were extrapolated to be 283.2 °C. Combined analytical results showed that the double melting peaks were from the melting of the two types of crystallites generated from two crystallization processes: a slow and a fast one. Fast crystallization may start from the well-aligned liquid crystal domains, whereas the slow one may be from the fringed or amorphous regions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3018–3031, 2000

Fluorescence study on intermolecular complex formation between mesogenic terphenyldiimide moieties of a thermotropic liquid-crystalline polyimide

Intermolecular complex formation between mesogenic terphenyldiimide moieties of a thermotropic liquid-crystalline (LC) polyimide (P-11TPE) was studied by steady-state fluorescence and fluorescence lifetime measurements together with differential scanning calorimetry (d.s.c.) and wide-angle X-ray diffraction (WAXD). The P-11TPE shows various highly-ordered crystalline phases and a smectic phase during heating, according to the d.s.c. and WAXD results. By comparing the fluorescence spectra and fluorescence lifetimes between P-11TPE and its model compound ( N, N′-diundecylter-phenyldiimide), the fluorescing complex at 438 nm of P-11TPE is confirmed to be an intermolecular ground-state complex between two terphenyldiimide moieties. The fluorescence peak wavelength of this intermolecular complex shifts slightly to shorter wavelength below the glass transition temperature and shifts abruptly to longer wavelength around crystallization, then shifts gradually to shorter wavelength again from various crystalline phases until the isotropic phase. The changes in fluorescence intensity and fluorescence lifetime around phase transition temperatures are also observed. These apparent changes in fluorescence behaviour indicate the differences in the nature and number of intermolecular complexes and radiationless deactivation processes in various phases. The characteristics of fluorescence behaviour for P-11TPE are also compared to those for various LC polyesters studied previously, and it is concluded that the red shift of fluorescnece is due to the apparent change in molecular arrangement between mesogenic moieties and the blue shift of fluorescence is due to the almost identical packing pattern with increasing thermal fluctuation during phase transition for liquid crystal polymers. By using fluorescence technique, molecular interaction between mesogenic moieties of liquid crystal polymer has been proved to show various patterns of changes during phase transition processes.

Crystallization morphology of a thermotropic liquid crystalline polymide

AbstractCrystallization morphology of an aromatic thermotropic liquid crystalline polyimide was studied by means of polarized light microscopy. The polyimide was synthesized from 1, 2, 4, 5‐benzenetetracarboxylic dianhydride (PMDA) and 1, 3‐bis[4'(4′ aminophenoxy) cumyl] benzene (BACB), which exhibited three exothermic peaks at onset temperatures of 292, 279, and 250°C in the DSC cooling curve. The results of polarized light microscopy revealed that the polyimide quenched from 350°C in air exhibited fine structure, frozen liquid crystalline texture. The liquid crystalline texture disappeared when the sample was heated to 298°C. However, the polymer melt still exhibited, to some degrees, birefringence until the temperature reached 340°C, where the polyimide was in the truly isotropic state. Isothermal crystallization experiments were carried out at both the isotropic temperature range and the non‐isotropic temperature range. Two types of negative spherulites with lamellar structure and positive needle‐like crystals formed from the isotropic melt have been observed. Interestingly, with a further decrease of the crystallization temperature to the liquid crystalline state temperature, the whole field was covered by the liquid crystalline texture. However, if the sample was kept at 286°C for a long time, and then reheated to 305°C to melt the liquid crystalline phase, negative spherulites with loose structures formed from the liquid crystalline state could be observed. Surprisingly, if the sample was kept at 305°C for a period of time, further crystallization could be observed using the spherulites formed at 286°C as nuclei. Composite spherulites were developed if the low‐high temperature crystallization process was repeated.

Crystallization kinetics of a thermotropic liquid crystalline polyimide derived from 1,3-bis[4-(4?-aminophenoxy) cumyl] benzene

We have studied the nonisothermal and isothermal crystallization kinetics of an aromatic thermotropic liquid crystalline polyimide synthesized from 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA) and 1,3-bis[4-(4'-aminophenoxy) cumyl] benzene ( BACB ) by means of differential scanning calorimetry (DSC). Polarized light microscopy (PLM) and wide-angle X-ray diffraction (WAXD) results confirm that this polyimide exhibits a smectic texture. Nonisothermal crystallization showed two strong and one weak exothermic peaks during cooling. The phase transition from isotropic melt to liquid crystalline state is extremely fast which completes in several seconds. The mesophase transition has a small Avrami parameter, n, of approximate 1. The isothermal crystallization of 253-258°C has been examined. The average value n is about 2.6 and the temperature-dependent rate constant k changes about two orders of magnitude in the crystallization temperature range of 6°C. The slope of In k versus 1/ (T c ΔT) is calculated to be -2.4 X 10 5 , which suggests nucleation control, via primary and/or secondary nucleation for the crystallization process. During the annealing process, a new phase (slow transition) is induced, which grows gradually with annealing time. At lower annealing temperatures (220-230°C), the slow transition process seems not to be influenced by the crystals formed during cooling process and its Avrami parameter n is ca. 0.3-0.4. However, the slow transition was hindered by the crystals formed during cooling process when annealed at higher temperature (230-240°C).

Structure and Thermal Behaviour of a Thermotropic Liquid Crystalline Polyimide (PI-LC)

Anomalous solid structures formed by a thermotropic liquid crystalline polyimide (PI-LC) were investigated by thermal analysis, polarized light microscopy and x-ray analysis. It was revealed that PI-LC should undergo a phase transition from the crystalline to the isotropic phase through the smectic or nematic phase in the temperature range 277–300 °C. The PI-LC filament extruded at 280 °C, at which temperature the polymer was in liquid crystalline phase, was formed to have a structure similar to the smectic C phase, while that extruded and melt-drawn at 310 °C, at which temperature the polymer was in the isotropic phase, had a structure similar to the smectic A phase.

First simple thermotropic liquid crystalline polyimides derived from aliphatic diamines and 4,4''-terphenyltetracarboxylic acid.

New thermotropic liquid crystalline polyimides (ηinh : 0.58-1.20 dLg-1) having aliphatic methylene chains and imide ring were prepared for the first time by the polycondensation under high pressure of the salt monomers derived from aliphatic diamines (number of methylene unit X=7-12) and diethyl 4, 4-terphenyltetracarboxylate. Mesophases of the eight and eleven-membered polyimides were enantiotropic, whereas those of all the other polyimides were monotropic as observed by the DSC thermograms. According to the DSC and X-ray measurements, distinct odd-even effects were observed in which the mesophase properties and structures were significantly affected by the number of cabon atom in the flexible spacer. The smectic CA phase could be confirmed for the odd-membered polyimides from their wide angle X-ray diffraction patterns which exhibited an inner sharp reflection, so-called layer reflection, and an outer broad split reflection. On the other hand, the nematic phase could be confirmed for the even-membered polyimides. The transition temperature from crystal to liquid crystal transition or isotropic temperature upon heating and all transition temperatures upon cooling exhibited an odd-even effect with the number of methylene unit.