Thermotropic Liquid Crystalline Polyurethanes Research Articles

Investigation of bifurcated hydrogen bonds within the thermotropic liquid crystalline polyurethanes

The polyurethanes are synthesized from the biphenyl-4,4′-diol (mesogenic biphenol) and 1,3-Bis(isocyanatomethyl) cyclohexane, using (CH2) of 2, 6 and 11 units as flexible alkylene spacer, respectively. FTIR detects the hydrogen bond in the thermotropic liquid crystalline polyurethane. FTIR spectra show a new CO absorption with lower wavenumber at around 1658 cm−1 is assigned to “bifurcated” hydrogen bonded CO group—a CO with higher strength hydrogen bonds. The distributions of “bifurcated” hydrogen bonded CO are increased substantially along with increasing the flexible spacer length in polymer backbone. The “bifurcated” hydrogen bond existed not only at the temperature below Tg, but also existed at the temperature far higher than Tm and Ti. It almost is independent of temperature and exhibits a stable interaction (or strucuture) throughout a wide temperature range, differences from the normal liquid crystalline polyurethanes. It is worthy of predicting the thermotropic liquid crystalline polyurethane with “bifurcated” hydrogen bond would enhance its performances.

Synthesis and Properties of Liquid Crystalline Polyurethanes from 4,4-bis (6-hydroxyhexoxy) Biphenyl and 3,3'-Dimethyl-4, 4’-Biphenylene Diisocyanate

A thermotropic liquid crystalline polyurethane (LCPU) was synthesized by the polyaddition reaction of 3,3'-dimethyl-4,4'-biphenylene diisocyanate (TODI) with 4,4-bis(6-hydroxyhexoxy)biphenyl (BHHBP). The liquid crystalline behavior of the polymer was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide angle X-ray diffraction (WAXD). The LCPU exhibited a nematic phase with a threaded texture and had a wide mesophase temperature range. And thermogravimetric analysis (TGA) indicated the decomposition temperature of the LCPU was >300°C. The observation of POM showed that the LCPU was a thermotropic nematic liquid crystalline polymer at certain temperature range.

Synthesis and characterization of thermotropic liquid crystalline polyurethanes from 4,4′‐bis(6‐hydroxyhexoxy) biphenyl and aliphatic diols

AbstractA series of thermotropic liquid crystalline polyurethanes (LCPUs) were synthesized by the polyaddition reactions of 2,4‐toluene diisocyanate (2,4‐TDI) with 4,4′‐bis(6‐hydroxyhexoxy)biphenyl (BHHBP) and aliphatic diol. The intrinsic viscosities of the polymers were measured by Ubbelohde viscometer, and the chemical structure was confirmed by Fourier transform infrared spectroscopy (FT‐IR). The LCPUs were examined by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide angle X‐ray diffraction (WAXD), and thermogravimetric analysis (TGA). The intrinsic viscosities were 0.56–0.83 dl/g. According to the melting point (Tm) and the isotropic temperature (Ti) of the LCPUs, the temperature range of the liquid crystalline phase became wider with increased number of methylene spacers in the polyurethane. The LCPUs exhibited a nematic phase with a threaded texture and had a wide mesophase temperature range. The decomposition temperature of the LCPUs was >300°C. On WAXD, the LCPUs give a dispersing peak at 2θ ≈ 20°, and a strong diffraction peak at 2θ ≈ 25°. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis and Properties of Thermotropic Main-Chain Type Liquid Crystalline Polyurethanes Containing Biphenyl Mesogen

A new type of thermotropic main-chain liquid crystalline polyurethanes containing biphenyl units was synthesized by polyaddition reaction of diisocyanates such as 2,6-tolylene diisocyanate, 2,5-tolylene diisocyanate, 2,4-tolylene diisocyanate, and 1,4-phenylene diisocyanate, with 4,4′-Bis(8-hydroxyoctoxy)biphenyl (BP8). The structure of the monomer and the corresponding polymers were confirmed using FT-IR and 1H-NMR spectroscopic methods. BP8 exhibited a smectic type mesophase, however, nematic phases were found for all synthesized liquid crystalline polyurethanes except for 1,4-phenylene diisocyanate/BP8 based polyurethane. For example, polyurethane 2,5-TDI/BP8 exhibited monotropic liquid crystallinity in the temperature ranging from 172 to 160 °C on the cooling stage. Properties of these polyurethanes were studied by differential scanning calorimetry (DSC), and optical polarizing microscopy. The FT-IR study indicated that the hydrogen bonding among urethane linkages attributed to the mesomorphism.

Thermal Properties of a Liquid-Crystalline Polyurethanes Containing Biphenyl Mesogen

A new type of thermotropic main-chain liquid crystalline polyurethanes containing biphenyl units was synthesized by polyaddition reaction of diisocyanates such as 2,6-tolylene diisocyanate, 2,5-tolylene diisocyanate, 2,4-tolylene diisocyanate, and 1,4-phenylene diisocyanate, with 4,4′-Bis(11-hydroxyundeyloxy)biphenyl (BP11). The structure of the monomer and the corresponding polymers were confirmed FT-IR and 1H NMR spectroscopic methods. BP11 exhibited a smectic type mesophase, however, nematic phase was found for all synthesized liquid crystalline polyurethanes except for 1,4-phenylene diisocyanate/BP11 based polyurethane. For example, polyurethane 2,5-TDI/BP11 exhibited monotropic liquid crystallinity in the temperature ranges from 173 to 156 °C on the cooling stage.

Main chain thermotropic liquid crystalline polyurethanes containing biphenyl mesogens based on novel AB‐type self‐polycondensation route: FT‐IR and XRD studies

AbstractThe detailed mesophasic characterization of main chain liquid crystalline polyurethanes containing biphenyl mesogen, which were synthesized by the novel AB‐type self‐polycondensation approach, was carried out by using Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM), variable temperature X‐ray Diffraction (XRD), and Fourier Transform Infrared (FT‐IR) spectroscopic studies. The type of mesophase present in these polymers was identified to be the smectic A phase by POM and XRD studies. The smectic layer thickness was found to increase as the length of the spacer increased. The effect of temperature on the hydrogen bonding was analyzed by FT‐IR studies. The curve‐fitting analysis of the NH stretching and CO stretching modes of vibrations indicated a gradual decrease in hydrogen bonding during the transition from the crystalline state to the mesophase. The mesophase to isotropic liquid transition was then accompanied by the complete disappearance of the hydrogen bonding. The biphenyl bands also showed changes during phase transitions due to the coupling of biphenyl vibration modes with the urethane linkage attached to it. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1903–1912, 2005

Synthesis of new thermotropic liquid crystalline polyurethanes containing biphenyl mesogens using a novel AB-type self-polycondensation.

A series of thermotropic main chain liquid crystalline polyurethanes containing biphenyl mesogens and flexible methylene spacers were synthesized using the novel AB-type self-polycondensation approach for the first time.

Studies on thermotropic main chain liquid crystalline segmented polyurethanes I. Synthesis and properties of polyurethanes from high aspect ratio mesogenic diol as chain extender

Thermotropic main chain liquid crystalline polyurethanes were prepared from 4-{[4-(6-hydroxyhexyloxy)phenylimino]methyl}benzoic acid 4-{[4-(6-hydroxyhexyloxy)phenylimino]methyl}phenyl ester (mesogenic diol) and 1,6-hexamethylene di-isocyanate. The effects of partial replacement of the mesogenic diol by 20-50 mol% of poly(tetramethylene oxide)glycol (PTMG) of varying molecular mass (M n =650, 1000, 2000) on the liquid crystalline properties were studied. Structural characterization was carried out by FTIR spectroscopy and the molecular mass distribution was determined by GPC. Differential scanning calorimetry and hot stage polarizing optical microscopy were used to study the mesomorphic properties. It was observed that the partial replacement of the mesogenic diol by PTMG of varying molecular masses influenced the phase transitions and the occurrence of mesophase textures. When the molecular mass of PTMG was enhanced, a higher content of mesogenic agent was needed to obtain liquid crystalline properties.

Synthesis and Characterization of Thermotropic Liquid Crystalline Main-Chain Polyurethanes from High-Aspect Ratio Mesogenic Diols

A series of novel tetrad high aspect ratio mesogenic diol monomers 4-{[4-(n-hydroxyalkoxy)phenylimino]methyl}benzoic acid 4-{[4-(n-hydroxyalkoxy)phenylimino]methyl}phenyl ester (3 a–e) were prepared with varying alkoxy spacer length (n = 2, 4, 6, 8, 10) by reacting 4-formylbenzoic acid 4-formylphenyl ester and 4-(n-hydroxyalkoxy)anilines. Two series of thermotropic main-chain liquid crystalline polyurethanes (LCPUs) 4 a–e and 5 a–e were obtained by the polyaddition of 3 a–e with hexamethylene diisocyanate (HMDI) and 4,4′-methylene bis(cyclohexylisocyanate) (H12MDI) in DMF respectively. Structural elucidation was carried out by elemental analysis, FT-IR, 1H NMR and 13C NMR spectroscopy. Inherent viscosity of the polymers measured in methanesulfonic acid at 26°C was in the range of 0.13–0.65 dL/g. Mesomorphic properties were studied by differential scanning calorimetry and hot stage polarizing optical microscopy and the thermal stability was determined by thermogravimetric analysis. The monomeric diols and the polyurethanes exhibited nematic texture.

Synthesis and properties of novel polyurethanes containing the mesogenic moiety of α-methylstilbene derivatives

4,4 ′-Bis(ω-hydroxyalkoxy)-α-methylstilbene with various spacer lengths (HMS n, where “ n” is the carbon number of the hydroxyalkoxy group) and two series of novel thermotropic liquid crystalline polyurethanes containing the mesogenic core of 4,4 ′-bis(ω-hydroxyalkoxy)-α-methylstilbene were synthesized. One series was obtained by polyaddition reaction of 2,4-toluene diisocyanate (2,4-TDI) with HMS n, denoted as TDI n, and the other was obtained by the reaction of 4,4 ′-diphenylmethane diisocyanate (MDI) with HMS n, denoted as MDI n. The polyurethanes were investigated by IR spectroscopy, DSC, wide angle X-ray diffraction, polarizing microscope and thermogravimetric analysis. All the polyurethanes except for TDI11 exhibited enantiotropic smectic phases which are not similar to those of corresponding HMS n. The transition temperatures and the temperature ranges of mesophases changed with the length of alkoxy spacer and the kind of diisocyanate. The thermal degradation behaviours under nitrogen atmosphere depend on the spacer length but not much on the diisocyanate moiety.

Synthesis and mesomorphism of thermotropic liquid crystalline polyurethanes based on meta-diisocyanates with 4,4′-bis( ω-hydroxyalkoxy) biphenyls

There are two series of novel thermotropic liquid crystalline polyurethanes (LCPUs), obtained from the polyaddition reaction of 4,4′-bis( ω-hydroxyalkoxy) biphenyls (HB n, n: the number of methylene units in hydroxyalkoxy group) with 1,3-bis(isocyanatomethyl) benzene (XDI) and 1,3-bis(isocyanatomethyl) cyclohexane (H6XDI), respectively. The Fourier transform infrared spectroscopy, differential scanning calorimetry, wide angle X-ray diffraction, polarizing optical microscope and thermogravimetric analysis were used for characterizing these polyurethanes. All of the liquid crystalline polyurethanes exhibited enantiotropism. Except that XDI/HB11 showed trimorphism, all the other polyurethanes presented monomorphism in this study. The H-bonded urethane domain is stronger when the polyurethanes are composed of the benzene ring urethane moiety and longer alkoxy chain, and they show a higher transition temperature associated with the better stability in mesophase. All the thermal degradation of LCPUs are higher than 330°C. The polyurethanes of H6XDI/HB2 and XDI/HB2 show one-step degradation, while the others show two-step degradation.

Solid-state NMR analyses of the structure and dynamics of polymers in the different states

This paper reviews our recent investigations of the structure and molecular motions of synthetic polymers in the different states, which were mainly carried out by solid-state 13C or 2H NMR spectroscopy. First, the crystalline-noncrystalline structure is characterized in detail for metallocene-catalyzed linear low density polyethylene, which was isothermally crystallized from the melt. Secondly, the characteristic chain conformation, the alternate trans and rapid trans-gauche exchange conformation, is revealed for the spacer methylene sequence in the frozen liquid crystalline region for thermotropic liquid crystalline polyurethane, which was crystallized from the isotropic phase through the nematic liquid crystalline phase. Thirdly, different types of splittings of CH resonance lines, which will be induced mainly by the formation of intramolecular hydrogen bonds in the triad sequence, are shown in the frozen solution states for poly(vinyl alcohol) samples with different tacticities. It is finally pointed out by solid-state 2H NMR analyses that both distributions in flip frequency and in flip angle should be taken into account for the precise characterization of the phenylene flip motion in glassy poly(ethylene terephthalate).

Solid-State 13C NMR Analyses of the Structure and Chain Conformation of Thermotropic Liquid Crystalline Polyurethane Crystallized from the Melt through the Liquid Crystalline State

Solid-state 13C NMR analyses of the structure and chain conformation have been performed for a thermotropic liquid crystalline polyurethane that was polymerized from 3,3‘-dimethyl-4,4‘-biphenyldiyl diisocyanate, 1,10-decanediol, and 1-hexanol with a mole ratio of 25/24/2. This sample was crystallized by cooling from the isotropic melt through the liquid crystalline state at a rate of 1 °C/min. DSC thermograms of the cooling scan exhibit only one endothermic peak, whereas two exothermic peaks corresponding to the melting and isotropic points appear in the heating scan. However, the polarizing optical microscopic observation confirms that the crystallization occurs almost at the same time after the appearance of the liquid crystalline phase, although some part of the liquid crystalline phase is frozen without crystallization. T1C analyses reveal that the sample contains three components with different T1C values, which correspond to the crystalline, medium, and noncrystalline regions. From the line shape an...

Studies on thermotropic liquid crystalline polyurethanes. III. Synthesis and properties of polyurethane elastomers by using various mesogenic units as chain extender

Four series of thermotropic polyurethane elastomers (TPUEs) were synthesized in this study. The hard segments were formed by using 4,4′-methylenedicyclohexyl diisocyanate (H12MDI) reacted with various mesogenic units, such as benzene-1,4-di(4-iminophenoxy-n-hexanol), benzene-1,4-di(4-iminophenol), and 3,3′-(4,4′-biphenylene)dipropanol, which also acted as the chain extender. Poly(oxytetramethylene)glycols (PTMEGs), PTMEG-2000 (Mn 2,000) and PTMEG-1000 (Mn 1,000) were used as a soft segment. The structures of all synthesized thermotropic liquid crystalline polyurethanes (TLCPUs) were characterized by FTIR spectroscopy. The effects of mesogenic units on the LC properties and elastic behaviors of LCPUs were studied. It was difficult to show LC behaviors for the PU elastomers derived from the mesogenic units with a lower aspect ratio, such as 3,3′-(4,4′-biphenylene)dipropanol, or the long soft segments, PTMEG-2000. In addition, these PU elastomers show better elastic properties by using a higher aspect ratio mesogenic unit as the chain extender, such as benzene-1,4-di(4-iminophenoxy-n-hexanol and benzene-1,4-di(4-imino-phenol)). The thermal properties were investigated by DSC measurements, thermal optical polarized microscopy, wide angle X-ray diffraction, dynamic mechanical analysis, and thermogravimetric analysis. The mechanical properties were measured by a tensilemeter. © 1996 John Wiley & Sons, Inc.

Studies on thermotropic liquid crystalline polyurethanes. II. Synthesis and properties of liquid crystalline polyurethane elastomers

AbstractThree series of novel thermotropic liquid crystalline polyurethane elastomers (TLCPUEs) were studied. Hard segments were formed by using hexamethylene diisocyanate (HDI) reacted with a mesogenic unit, benzene‐1,4‐di(4‐iminophenoxy‐n‐hexanol), which also acted as a chain extender. Three diols: 1,10‐decanediol,poly(oxytetramethylene) glycol (PTMEG) Mn = 1000 and PTMEG Mn = 2000 were used as the soft segments. The effects of soft segments of polyurethanes on the liquid crystalline behavior were studied. Higher molecular weight TLCPUEs were obtained by adding 30−50 mol % of mesogenic segments to diisocyanates. In contrast to a conventional chain extender such as 1,2‐ethylene glycol or 1,4‐butyl glycol, the synthesized polyurethane elastomers exhibited a mesophase transition by using a mesogenic unit as the chain extender. Mesophase was found for all synthesized LC polyurethanes except of polymers H2‐A‐12 and H2‐A‐7. The structures and the thermal properties of all synthesized TLCPUEs were studied by using FTIR spectroscopy, wide‐angle x‐ray diffraction (WAXD) and DSC measurements, a polarizing microscope equipped with a heating stage, dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). Mechanical properties were also examined by using a tensilemeter. © 1995 John Wiley & Sons, Inc.

Thermotropic Polyurethanes Prepared from 2,5-Tolylene Diisocyanates and 1,4-Bis(.omega.-hydroxyalkoxy)benzenes Containing No Mesogenic Unit

Thermotropic liquid-crystalline polyurethanes were synthesized by the polyaddition reaction of para-substituted 2,5-tolylene diisocyanate (2,5-TDI) with 1,4-bis(ω-hydroxyalkoxy)benzenes (BHBm : HO(CH 2 ) m OC 6 H 4 O(CH 2 ) m OH ; m is the carbon number of the hydroxyalkoxy group). Intrinsic viscosities of the polymers were in the range of 0.22-0.41 dL/g. Polyurethanes 2,5-TDI/BHBm's (m = 3, 5, 6, 8) prepared from BHBm and 2,5-TDI, exhibited monotropic liquid crystallinity, although these polyurethanes contained no mesogenic core unit. For example, polyurethane 2,5-TDI/BHB5 with [η] = 0.33 prepared from 1,4-bis(5-hydroxypentoxy)benzene (BHB5) and 2,5-TDI exhibited an anisotropic phase from 119 to 51 °C. In contrast, for the series of 1,4-PDI/BHBm (m = 2, 3, 5, 6, 8, 11) having no substituent in a phenylene unit, no explicit mesomorphic behavior was observed by DSC measurement and polarized microscopic observation.