Main-chain Liquid Crystal Polymers Research Articles

Collective chain motions and the glass transition in thermotropic liquid crystal polymers. A transverse deuteron spin relaxation study

Multipulse dynamic nuclear magnetic resonance techniques have been employed to investigate molecular dynamics of a thermotropic main chain liquid crystal polymer. Measurements are reported for the nematic and smectic A phases, at the glass transition and in the glass phase itself. Three different processes have been identified on the basis of their characteristic transverse spin relaxation dispersions and anisotropies. In the nematic phase and at higher temperatures in the smectic A phase, director fluctuations appear to play a significant role in the low frequency spin relaxation mechanism. As the temperature is lowered and the glass transition is approached, rotational motion of individual molecules slows and begins to dominate the relaxation behaviour. The correlation times for this process display a strongly non-Arrhenius temperature dependence which can be accounted for in terms of the Williams-Landel-Ferry equation. A third motion which had not been identified previously appears in the glass phase and is probably related to slow low amplitude collective motion of the polymer chains.

Temperature and Molecular Weight Dependence of the Viscometric Properties of Main-Chain Liquid Crystal Polymers in Nematic Solvents

The twist and bend viscosities of dilute nematic solutions in 4'-(pentyloxy)-4-cyanobiphenyl (5OCB) and in 4'-pentyl-4-cyanobiphenyl (5CB) of the main-chain liquid crystal polymers, TPB10 and TPB13, which have a mesogenic group, 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)butane, separated, respectively, by flexible decamethylene and tridecamethylene spacers, were determined via dynamic light scattering analysis. A theoretical expression for the increment of twist viscosity, γ 1 , derived by Brochard can be applied to interpret the experimental results in terms of changes in the mean square end-to-end distances parallel (R∥2) and perpendicular (R⊥2) to the nematic director

Spectroscopic characterization of trans-gauche isomerization in liquid crystal polymers with two nematic states

Nuclear magnetic resonance (n.m.r.) spectroscopy and Fourier transform infra-red ( FTi.r.) spectroscopy are used to characterize the trans-gauche isomerization of the methylene groups in a new class of main-chain liquid crystal polymers. This new series of liquid crystalline copolymers is based on the 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane mesogen, where R is F, Cl or CH 3 and flexible spacers containing an odd number of menthylene units. These copolymers are particularly interesting because they show two nematic states. Trans-gauche isomerization is characterized by FTi.r. spectroscopy through measurements of the absorbances of characteristically trans bands, and characteristically gauche bands. Trans-gauche isomerization is characterized by 13C n.m.r. spectroscopy through measurements of the 13C chemical shifts in the solid state. FTi.r. spectroscopy shows that an increase in temperature results in an increase in the percentage of gauche isomers for the methylene units in both the spacer and the mesogen. N.m.r. spectroscopy shows 13% more gauche isomers in the high-temperature nematic state, relative to the low-temperature nematic state, for rotation about the C 28C 29 bond axis in the spacer.

Molecular Weight-Dependent Behavior of the Twist Distortion in a Nematic Monodomain Containing a Main-Chain Liquid Crystal Polymer

The twist visoelastic coefficients of dilute nematic solutions in 4'-(pentyloxy)-4-cyanobiphenyl (5OCB) of a main-chain liquid crystal polymer, TPB-10 [OPhPhCH 2 CH(Et)PhO(CH 2 ) 10 ] n were determined over a wide range of molecular weight using electric field-dependent dynamic light scattering. A Mark-Houwink-Sakurada relationship [η]= KM v α was found for the intrinsic twist viscosity, with different exponents for oligomeric and polymeric chains

Field-Cycling NMR Relaxation Spectroscopy of Poly(di-n-alkylsiloxanes) in Solid, Mesomorphic Liquid, and Isotropic Liquid Phases

The frequency dependence of the proton spin-lattice relaxation times T1 and T1., in the laboratory and rotating frames, respectively, is reported for solid and liquid phases of poly(diethylsiloxane) (PDES) and in melts of poly(dimethylsiloxane) (PDMS). The total frequency range is 5 X 102 -3 X 108 Hz and is mainly covered by field-cycling NMR relaxation spectroscopy. The relaxation behavior of PDES in the liquid but ordered mesophase is compared to that of isotropic melts of PDES and PDMS and also to that of nematic main-chain liquid-crystal polymers. The frequency dependences of PDES and PDMS liquids can be represented at low and high frequencies by power laws, section by section. The relaxation behavior in the isotropic melts is entirely equivalent to that previously reported for other polymer species. In the PDES mesophase, the exponents of the power laws are significantly larger and the crossover frequency between the two regimes is reduced. The dynamics in this phase are discussed with respect to the influence of chain modes and order director fluctuations. The main conclusion is, on the whole, that data of the liquid phases are determined by chain modes rather than by local segment fluctuations. The chain dynamics in the PDES mesophase resemble the chain modes in isotropic melts modified for a microstructure with reduced randomness, whereas the influence of order director fluctuations can neither be confirmed nor ruled out.

Rheological characterization of director dynamics in a nematic monodomain containing mesogenic polymers of differing architectures

The rheological behavior of dilute nematic mixtures containing liquid crystal polymers (LCP) [CH 2 C(CH 3 )(COOR)] n (R= (CH 2 ) 6 OPhC(Me)=C(Me)PhOMe) and [OPhPhCH 2 CH(Et)PhO(CH 2 ) 15 ] n dissolved in low molar mass nematogens is investigated. Addition of the side-chain LCP to a flow-aligning nematic solvent, pentylcyanobiphenyl (5CB), alters the rheology to director-tumbling character. Conversely, dissolution of the main-chain LCP in a director-tumbling nematic, octylcyanobiphenyl (8CB), suppresses the tumbling behavior and produces a flow-aligning response.

A new type of main-chain liquid-crystal polymer derived from 4′-hydroxybiphenyl-4-carboxylic acid and its smectic mesophase behaviour

A homologous series of polymers (PHBC-n) has been prepared from 4′-hydroxybiphenyl-4-carboxylic acid and ω-bromoalkan-1-ols containing five to nine methylene units which can serve as mesogen and flexible spacer groups, respectively. The PHBC-n polymers are characterized by having two different linkage groups, ether and ester groups, to connect the biphenyl mesogen to the methylene spacer, and are classified on a new type of main-chain liquid-crystal polymer. All polymers exhibit the enantiotropic smectic phase. The smectic behaviour and structure were examined by differential scanning calorimetry, optical microscopic and X-ray diffraction methods, and are discussed in comparison with the corresponding date of the BB-n polyesters, which include only the ester linkage group.

Thermoplastic and thermoset main chain liquid crystal polymers prepared from biphenyl mesogen

AbstractMain chain liquid crystal thermoplastic polyesters and thermosetting epoxy resins were prepared using biphenyl mesogens. The melting points of the polyesters were effectively decreased by incorporating flexible methylene spacers into the polymer main chain. The liquid crystal epoxy resins exhibit high glass transition temperature, low thermal expansion coefficients, high dielectric strengths, and low dielectric loss. They are suitable for the preparation of self‐reinforcing molecular composites. © 1993 John Wiley & Sons, Inc.

Odd-even effect in the viscoelastic properties of main-chain liquid crystal polymer-low-molar-mass nematogen mixtures

The viscoelastic properties of dilute nematic mixtures containing 4'-(pentyloxy)-4-cyanobiphenyl (5OCB) and the main-chain liquid crystal polymer [OPhPhCH 2 CH(CH 3 )PhO(CH 2 ) m ] n (TBP-X) were determined using dynamic light scattering and Freedericksz transition measurements. An odd-even variation in viscoelastic properties with spacer length of TPB-X was found. Specifically, TPB-2n/5OCB mixtures have substantially lower splay and twist decay rates and slightly higher bend decay rate than those of TPB-2n+1/5OCB mixtures. These differences are primarily due to changes in the associated viscosity coefficients

Collective Motions and Viscoelastic Properties in Main Chain Liquid Crystal Polymers. A Deuteron Spin Relaxation Study

AbstractMultipulse dynamic NMR techniques have been employed to investigate collective motions and viscoelastic properties in a main chain liquid crystal polymer and low molar mass analogues in the nematic phase. Transverse deuteron spin relaxation times measured as a function of pulse spacing using a modified Carr‐Purcell‐Meiboom‐Gill pulse sequence are analysed in terms of a simple hydrodynamic model for nematic director fluctuations. Combining these results with those of NMR slow sample spinning experiments allows determination of the effective viscosity and the average elastic constant of the material. Results are compared for compounds with different molecular weights and with the results of analogous proton field‐cycling experiments. The viscosity is found to increase approximately as the square of the molecular weight, Mn, whilst the elastic constant shows a much shallower Mn dependence apparently tending to a plateau value at moderate molecular weights.

Odd–even effect in miscibility of main-chain liquid crystal polymers with low molar mass nematogens

Abstract Qualitative phase diagrams were constructed using the contact method for binary mixtures of several chemically-distinct low molar mass nematogens (LMMN) with a main chain liquid crystal polymer (TPB-x) which has a mesogenic group, 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxy phenyl) butane, separated by flexible alkyl spacers of variable length, x. Several interesting effects were observed. TPB-x was found to exhibit an odd-even variation in miscibility in the nematic state (2n + 1 = miscible, 2n=immiscible) with 4′-pentyl-4-cyanobiphenyl (5CB), but not with 4′-pentyloxy-4-cyanobiphenyl (5OCB) in which most polymers were completely miscible. On prolonged isothermal annealing in the biphasic region in 5CB, TPB-2n exhibited spherulitic crystallization of the liquid crystal polymer. These observations are shown to be qualitatively consistent with a modification of the Flory-Huggins theory by Brochard et al.

Magnetic orientation and microstructure of main-chain thermotropic copolyesters

The structural response of a molten main-chain liquid crystal polymer to an applied magnetic field has been investigated microscopically, with the microstructure being characterized as a function of field strength, time in the field and molecular weight. The application of the field to a structure in which the director wandered gradually on a scale of about 100 μm resulted in the formation of domains delineated by well defined boundaries. The domains were elongated along the field axis to an aspect ratio of about 8 in the case of a 1.1 T field, although for the higher field strength of 5.6 T the domains were no longer visible. Evidence is presented that the domain boundaries correspond to a director reorientation of π rad. Etched domain microstructure revealed a uniform distribution of crystallites, representing regions of local ordering in the material, on a scale of a few hundred nanometres. The crystallites had the form of platelets normal to the local director and acted as orientational markers across the domain boundaries.

Characterization of thermotropic main-chain liquid–crystal polymers in dilute solutions

Characterization of thermotropic main-chain liquid–crystal polymers in dilute solutions

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer–solvent mixtures

Abstract The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K 22 and viscosity coefficient γ1. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K 22 coupled with large increases in γ1. The decrements in K 22 are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.

Proton spin-lattice relaxation dispersion of liquid crystal polymers: Characterization of local and collective motions

This paper reports on studies of the longitudinal proton spin relaxation dispersion T1Z(ω) of a nematic main chain liquid crystal polymer (M̄n=30 000) over a very broad Larmor frequency range (1 kHz≤ω/2π≤120 MHz). Analysis of the experiments is achieved in terms of a density operator treatment employing the Redfield approximation. The results show that collective motions contribute to the proton spin relaxation process in the kilohertz regime, as found for low molar mass liquid crystals, whereas the conventional megahertz range is dominated by reorientation of individual molecules. The intramolecular motions consist of trans–gauche isomerization and phenyl ring flips. These motions are the fastest in the hierarchy of time with correlation times of 10−10 s in the nematic melt of the polymer at T≊460 K. The intermolecular (whole molecule) motions are interpreted as rotational diffusion in an orienting potential. They exhibit a T1Z(ω)∼ω0.65 dispersion in the megahertz range and have correlation times ranging from 10−9 to 10−7 s at this temperature. The slowest motions affecting longitudinal spin relaxation can be assigned to nematic order director fluctuations characterized by a broad distribution of thermally activated modes. Analysis of the dispersion profiles in the kilohertz regime provides the viscoelastic parameters of the main chain liquid crystal polymer. At T=460 K, an average elastic constant of K=8×10−11 N and an effective viscosity of η=1×103 Pa s have been determined. Using the experimentally accessible value for the short wavelength cutoff of the elastic modes, one obtains the mean-square amplitude of the director fluctuations <θ02≳=0.02, corresponding to a director order parameter of SOF=0.98. Thus, the contributions of the collective chain motions to the measured order parameters are negligible.

Theory of main chain nematic polymers with spacers of varying degree of flexibility

Abstract Main chain liquid crystal polymers are modelled as either worms or jointed rods. In reality they are composed of mesogenic units (rods) linked by spacers with varying degrees of flexibility. We present a molecular model to describe non-homogeneous nematic polymers. The model takes account of molecular parameters, such as the lengths of the mesogenic group and the spacer units, and the interactions between them. The spacers are found to have an order differing from the mesogenic units. If the spacer is not very long and thus in effect is inflexible, one end of the spacer can retain to some extent the orientation of the other end, allowing orientational correlation between spacers mediated by the intermediate mesogenic unit. This is important in giving the chain a global rod-like behaviour as the nematic field becomes strong or the temperature low. The nematic order of the two components (mesogens and spacers), the nematic-isotropic transition as well as the latent entropy are examined. Furthermore...

A novel high-temperature non-aqueous dispersion polymerization route to fully aromatic main-chain liquid-crystal polymers

A novel high-temperature non-aqueous dispersion polymerization route to liquid crystal polymers based on poly(4-oxybenzoate- co-2,6-oxynaphthoate) is described. Novel combinations of polymeric and hydrophobic inorganic stabilizers are shown to give stable dispersions at temperatures up to 320°C. A possible mechanism of stabilization is discussed.

Main-chain side-chain liquid crystal polymer blends for improved physical properties

Previous work has demonstrated that compatible blends of main-chain liquid crystal polymers (LCP) with side-chain LCP's can be prepared. The present work was carried out to extend the scope and application of this finding to liquid crystal polyesters, similar to commercially available LCP's and to demonstrate the effect of the blend composition upon the physical and mechanical properties. A series of melt-processable main-chain polyesters was prepared as well as a series of acrylic polymers and copolymers which possess a side-chain mesogenic unit, similar to that of the main-chain LCP's. While the results of the blending studies of these high molecular weight materials were somewhat ambiguous as to compatibility, there are strong indications that the presence of the side-chain mesogen containing polymers does result in an improvement in the overall mechanical properties of the main-chain LCP.

Cholesteric mesophases

In the past few years, there have been important developments concerning the role and the use of chirality in liquid crystals. An example which is now relatively well known is the chiral smectic C* low molecular weight liquid crystal (LMWLC) which lowers the response time of displays by a factor between one hundred and one thousand [1-3]. In chiral liquid crystal polymers (cholesteric or twisted nematic and smectic C*), it is necessary to understand the relation between the chemical structure and the properties of mesophases in liquid and solid polymer. Furthermore, chirality is also found in abundance in biopolymers and may be important for life processes. Applications in industry (high modulus fibers, sensors, non linear optics...) have already been realised or are being pursued.The aim of this paper is to emphasize the polymer character of these liquid crystals. What is the influence of the molecular weight, the degree of substitution, the viscoelasticity and the existence of a glass transition? How to use the polymer character to obtain new informations on the liquid crystal state or to make new devices?This rapid overview roughly covers the following parts:(1) Formation and general properties of cholesteric mesophases: we give a rapid discussion of thermotropics and lyotropics, of the influence of degree of polymerisation, degree of substitution, concentration, temperature..., on the cholesteric-isotropic transition and on the order observed by textures or as described more quantitatively by the order parameter.(2) Optical properties: we give a brief presentation of a theoretical work concerning the pitch prediction taking into account the asymmetric term in the interaction potential and the comparison with experimental results.(3) Textures and defects which are mainly observed in these mesophases.(4) Flow properties and rheological behaviour which are fundamental for processing.(5) Blue phases near the isotropic-cholesteric transition which are discussed with emphasis on the specificity of the polymer state.It is usual to distinguish main chain and side chain liquid crystal polymers [4, 5], which may be modelled as indicated in Fig. 1.Cellulosic derivatives are good examples of main chain cholesteric polymers [6]. Cellulose is a very abundant polymer: each year, biosynthesis produces about 1011 tons of cellulose. It readily accepts grafting of substituents, thus producing derivatives like cellulose acetate of hydroxypropylcellulose.Examples of side chain liquid crystal polymers are those obtained with homo- or copolymers of polyacrilate, polymethacrylate or polysiloxane.

Non-equilibrium excess order in the isotropic state of main-chain liquid-crystal-forming polymers

A new non-equilibrium phenomenon has been recognized in main-chain liquid-crystal polymers. Persistence of non-equilibrium residual short-range nematic-like order can be detected in the isotropic phase up to several minutes after the nematic-to-isotropic transition has occurred. The effect was observed by differential scanning calorimetry and is closely related to the pronounced non-equilibrium effects in the nematic state. As we reported previously, annealing in the nematic phase, on the timescale of hours, is required to bring the temperature and enthalpy of isotropization up to its equilibrium value. Now we find that even in the isotropic state annealing is required in order to achieve thermodynamic equilibrium; only upon holding above T i is the residual excess nematic-type order fully removed, and thus all memory of previous annealings in the nematic state erased. The non-equilibrium effects are particularly pronounced for higher-molecular-weight polymers. In the case of the α-methylstilbene polyether with alkylene spacers, M ̄ n = 31 000 , the time constant of the exponential approach to equilibrium is found to be 2.1 min at T = T i + 10°C.