Electrooptical properties of liquid crystalline side chain polymers with laterally attached mesogenic units

The electrooptical properties of side-chain liquid crystalline polymers were investigated for the case that the mesogenic units were attached laterally rather than longitudinally to a flexible chain backbone via flexible spacer units. The experimental finding is that these polymers display unusual electrooptical properties within the isotropic phase in the neighborhood of the transition into the nematic phase. The polymers are characterized by the occurrence of a fast and a slow electrooptical response both of which show a critical divergence of the Kerr constant and the Kerr relaxation time. In addition, they show deviations between the rise and the decay values of the Kerr constants and in certain cases also of the Kerr relaxation times. Finally an overshoot of the induced birefringence following a sudden stepwise increase of the applied electric field has been found for one of these polymers. All these features can be accounted for on the basis of a newly developed theoretical approach that considers the particular dipolar and optical polarization configurations of these polymers.     

Dipoles-dipole association of mesogenic molecules in solution

The dielectric polarization has been used to study dipolar association of 4-n-pentyl-4'-cyanobiphenyl in benzene solution. The results have been interpreted with the assumption of a monomer-dimer equilibrium. To explain the relatively high effective dipole moment of the dimers, a new structure has been proposed for them.     

Lattice model theory of chain stiffening and relaxation properties of macromolecules in the nematic LC state

The relaxation properties of polymer chains In the nematic LC-state or in the external quadrupole field may depend both on the variation of the conformation in the ordered state and on the activation barrier of the molecular (or external) field. This barrier should be surmounted during reorientation of chain elements. The lattice model theory of chain stiffening of macromolecules in the LC-state is proposed. The calculation and comparison of the longitudinal and transversal relaxation spectra for the continuous and discrete rotameric mechanism of the mobility are performed. For the simplest model of a heterogeneous polymer chain the possibility of the more complex relaxational behavior i.e. the existence of two longitudinal and two transversal relaxation spectra was shown.     

Excited state properties of sizable molecules in solution: from structure to reactivity

We review some recent advances in quantum mechanical methods devised specifically for the study of excited electronic state of large size molecules in solution. The adopted theoretical/computational framework is rooted in the density functional theory (DFT) and its time-dependent extension (TD-DFT) for the characterization of ground and excited states, in the polarizable continuum model (PCM) for the treatment of bulk solvent effects, and in time-dependent quantum mechanical methods for chemical dynamics. Selected applications to the simulation of absorption spectra, to the interpretation of time-resolved experiments, and to the computation of dissociative electron transfer rates are presented and discussed.     

Conformation and conformation-dependent properties of chain molecules

This paper is dedicated to the memory of Professor N.A. Platé for his outstanding contributions to polymer science and his extraordinary leadership in the world scientific community. The long-lasting personal and professional relationship with Nikolai came to the end rather untimely, but his enthusiasm for science and people will always be remembered. The rotational isomeric state treatment has been extended to elucidate conformation and conformation-dependent properties of chain molecules occurring under the condition for which some special care is required. Examples include the odd-even effect of n-alkane chains, dipole moment of polymethylmathacrylate (PMMA), enhancement of optical rotation of vinyl polymers, screw-sense reversal of polyaspartates, and the anisotropy of a flexible spacer involved in the main-chain liquid crystal. Major conclusions are as follows: (1) The distinct odd-even effect is a peculiar character of the methylene sequence -(CH₂) _n -attributable to its bond angle (tetrahedral) and rotational preference (trans). In the other chains including carbonate linkages or oxyethylene units, the correlation along the chain unnecessarily accompany the odd-even phenomenon. (2) When the meso dyad of the PMMA chain takes the trans-trans conformation, the two adjacent ester groups are situated at a separation of only ca. 0.27 nm. The strong dipole-dipole interaction between the two polar ester groups may be alleviated by taking a staggered arrangement. The importance of such neighbor-dependent correlation in treating dielectric properties has been pointed out. (3) The amplification of conformational asymmetry has been discussed for two seemingly dissimilar examples: i.e., flexible vinyl polymers having asymmetric side chains and tightly hydrogen-bonded α-helical polypeptides. The conformation of the side chain has been elucidated for each of the enantiomeric forms of the backbone. The small free-energy difference between the two forms may cooperatively trigger a drastic effect in these polymeric systems. (4) A long oxyethylene spacer carrying mesogenic units on both terminals can be rather easily accommodated in the nematic LC phase. The nematic conformation of the spacer was found to play a crucially important role in determining the orientational order and the packing entropy of the nematic state.     

The synthesis and mesogenic properties of side chain polymethacrylates with terminal branched groups

The synthesis is described of six polymethacrylates attached through oxytetramethylene and oxyhexamethylene spacers to an azobenzene moiety consisting of branched terminal iso -butoxy, iso -amyloxy and n -amyloxy groups. The monomers show a nematic mesophase while their polymers show an additional higher order smectic C phase. The effects of the terminal branched alkoxy group and the spacers on mesomorphic properties are discussed. The phase characteristics, thermal stabilities and molecular masses of the polymers were investigated by DSC, XRD, TGA and GPC. The polymers were also characterized by UV-vis, IR and NMR spectroscopy.     

The influence of vicinal threo-difluorination on electro-optic and mesogenic properties of propyleneoxy-linked nematic liquid crystals

A protocol for the preparation of a series of liquid crystals is presented, where the structures carry a central propyleneoxy motif carrying two vicinal threo-C–F bonds. The negative dielectric anisotropy (−Δ?) of the resultant liquid crystals has been explored. The stereoelectronic relationship between the vicinal C–F bonds was anticipated to orientate the fluorine atoms gauche to each other when the propylene chain is extended. This is shown to be the case. This orientation of the C–F bonds generates a net dipole perpendicular to the molecular axis, a prerequisite for the design of dielectrically negative liquid crystals. However the molecules adopt a conformation where one C–F bond reinforces the net molecular dipole, and the other has a counter effect, thus the introduction of the motif has an almost neutral effect on dielectric anisotropy (Δ?_virt) of these candidate liquid crystals. However, introduction of the difluoro motif raises the melting points of the liquid crystals and increases their conformational rigidity.     

The synthesis and mesogenic properties of side chain polymethacrylates with terminal branched groups

The synthesis is described of six polymethacrylates attached through oxytetramethylene and oxyhexamethylene spacers to an azobenzene moiety consisting of branched terminal iso-butoxy, iso-amyloxy and n-amyloxy groups. The monomers show a nematic mesophase while their polymers show an additional higher order smectic C phase. The effects of the terminal branched alkoxy group and the spacers on mesomorphic properties are discussed. The phase characteristics, thermal stabilities and molecular masses of the polymers were investigated by DSC, XRD, TGA and GPC. The polymers were also characterized by UV-vis, IR and NMR spectroscopy.     

Refractometric studies of mesogenic molecules in isotropic solutions

The Lorenz-Lorentz equation for binary isotropic mixtures, consisting of anisotropic molecules, has been derived using the point-dipole approximation taking into account only pair molecular correlations. This permits the calculation of the effect of molecular correlations on the refractive index. Special attention has been paid to the case of infinite dilution in the solvents consisting of isotropic molecules, where an experimental check of the equations is possible. The specific refraction of some solutes with different molecular polarizability anisotropy and polarity has been studied in various solvents. It has been shown, that the main theoretically predicted features are observed in these experiments, but for a quantitative comparison information on the two-particle distribution function is needed.     

On the equation of state of hard chain molecules

The chain-of-rotators (COR) equation of state developed by Chien, Greenkorn, and Chao makes an assumption on the hard chain partition function based on results for monomers and dimers, i.e., spheres and dumbbells. Here this assumption is checked for trimers by comparison with the results of Boubik's hard convex body equation. It turns out that the COR assumption is, in general, a good approximation. Its quality depends somewhat on the bond length and the bond angle which it does not consider. The agreement is especially good for angles somewhat less than 90°.     

Influence of the alkyl chain length of some mesogenic molecules on their Langmuir film formation ability

Langmuir films of members of two homologous series, the 4‐n‐alkyl‐4′‐cyanobiphenyls (nCB) for n = 2–14 and trans‐4‐n‐alkyl(4′‐cyanophenyl)cyclohexanes (PCHn) for n = 2–12, have been studied by recording surface pressure/area isotherms and by Brewster angle microscopy. It has been found that the compounds with very short chains (n3) and very long chains (n>12 for nCB, n>10 for PCHn) are unable to form compressible monolayers at the air–water interface. Other members of both series can form stable Langmuir films, but both their rigidity and stability as well as the molecular packing vary with the alkyl chain length. The isotherms and BAM images imply that the organization of the liquid crystal molecules in the films is to some extent correlated with their ability to form corresponding mesophase in the bulk: nematogenic compounds tend to form rounded droplet‐like domains, whereas smectogenic compounds tend to form flat domains.     

Model theory of relaxation properties of macromolecules in the nematic LC state

The continuous and lattice dynamic models of the macromolecules in the LC state are considered. The relaxation properties of polymer chain in the nematic LC state may depend on both the variation of the conformation in the ordered state and on the activation barrier of the molecular field. The calculation and comparison of the longitudinal and transversal relaxation spectra for the continuous and discrete rotameric mechanism of the mobility are performed. For the simplest model of the heterogeneous polymer chain for continuous dynamic models the complicated relaxational behavior with two longitudinal and two transversal branches is manifested. The dynamic properties of the lattice dynamic model with spacer consisting of several bonds are considered.     

Equilibrium and dynamical properties of gaussian stiff chain molecules

We present a novel analytically tractable model for stiff chain molecules. The equilibrium distribution function of the chain is derived using the maximum-entropy principle. For that purpose, we first formulate a discrete chain model, where the connections of the points and the restriction on bending are taken into account via constraints. We then perform the limit to a continuous chain and show that the mean-square end-to-end distance and the radius of gyration of the continuous chain are identical with the same quantities of the Kratky-Porod wormlike chain. The dynamics of our chain is investigated in dilute solution without hydrodynamic interactions. The linear dynamical equation is solved by a normal mode analysis. We discuss the dependence of the relaxation times on the single parameter of the model, the persistence length. For small persistence lengths we obtain the well known relaxation times of the Rouse model. In the stiff-chain limit, we find the pure bending relaxation times and, in addition, the rotational relaxation time.     

Folded chain crystallization of random terpolymer liquid crystal polymers from the nematic state

The morphology of ca. equimolar random terpolymer liquid crystal polymers of an aliphatic segment of 4–7 carbon atoms, oxybenzoate, and dioxyphenyl crystallized from the nematic state in the form of thin films on glycerine by slow cooling and quenching has been characterized by electron microscopy (TEM) and diffraction (ED). In all cases a folded chain, lamellar structure is found. The ED studies suggest adjacent reentry, and despite a large ΔH, indicate no change in lateral molecular packing at the crystal-“liquid crystal” transition with a transformation to the nematic state at a higher temperature “liquid crystal”-liquid crystal transition. The results are interpreted as suggesting similar folded chain, lamellar morphology in the nematic state. © 1992 John Wiley & Sons, Inc.     

Hydrodynamic and conformational properties of cellulose myristate molecules in solution

Cellulose myristate samples with a degree of substitution of 230–250 have been studied by the methods of molecular hydrodynamics (viscometry, analytical ultracentrifugation (flotation), and isothermal translational diffusion) in chloroform in the range M = (56–652) × 10³. The experimental evidence has been interpreted within the framework of the generalized wormlike Yamakawa-Fuji model with the following parameters: the persistence length a = 115 × 10^?8 cm, the chain diameter d = 45 × 10^?8 cm, and the molecular mass per unit chain length M _L = 270 × 10⁸ cm^?1. It has been inferred that the polymer dissolves in chloroform in the form of dimers.     

Side chain polysiloxanes with fluorinated mesogenic groups. The role of the fluorine content on the properties of the smectic A state

A series of liquid crystalline polymers has been synthesized in which fluorinated or semi-fluorinated mesogenic moieties are used as side groups linked to a polysiloxane backbone. Clear-cut differences in the structure of the smectic A phase are evidenced between fluorinated polymers and their classical non-fluorinated homologues. In a comparative analysis, we discuss the relevance of the steric hindrance of perfluorinated sequences and of the microsegregation of the backbone in response to these modifications as related to the fluorine content in the side group and to the proportion of mesogenic groups.     

Theoretical study on mesogenic core structures of nematic liquid crystalline compounds

The structures and intermolecular interaction energies of 10 dimers, included in the mesogenic core structures of typical liquid crystalline (LC) compounds, are obtained at the MP2/6-31G(d) level of theory. It is proved that the dispersion energy significantly contributes to the total interaction energy of these dimers. Even when bulky substituents are introduced into the core part, the interaction energy is still large. It is also revealed that when a long intermolecular distance is provided by a high steric repulsion originating from the linkage of two phenyl groups, the dispersion energy is significantly small. However, in this range of intermolecular distances, the electrostatic energy caused by a strong quadrupole-quadrupole attractive interaction plays a dominant role, and as a result, a rather stable dimer is formed. In all 10 dimers, the dispersion, electrostatic, and exchange-repulsion energies strongly depend on the geometrical orientation of the molecules. The calculated interaction energies of these dimers are also compared with the corresponding experimentally measured viscosities. The results suggest an explicit linear relationship between the interaction energies and viscosities.     

Electrooptical properties and structure of lanthanide 3-trifluoroacetylcamphorates

Kerr effect and optical activity data indicate that Pr³⁺, Dy³⁺,and Yb³⁺ 3-trifluoroacetylcamphorates in CCl₄ exist predominantly in one form with virtually octahedral orientation of the chelate rings.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 486–489, February, 1990.Deceased.