Rod Molecules Research Articles

Interactions and mechanical properties of rod–coil ionomer blend

Polymer–polymer blends of rigid-rod and random-coil macromolecules are obtained by ionomer blend formation. The molecular dispersion of the rod molecules in the matrix polymer is achieved by acid–base interactions. The reinforcement effect in terms of the Young's modulus is discussed as a function of the degree of polymerization of the rods. By treating the reinforcer molecule as a fuzzy cylinder the reinforcement effect can be correlated to the reinforcers' size and shape.

Phase and orientational ordering of A–B–A tri-block co-polymers guest in a quenched host of low molecular weight rod molecules

We investigate the effects of thermodynamical variables, intermolecular interactions and block lengths on phase and orientational ordering of guest tri-block co-polymers in a host glassy matrix of short molecular rods. The A and B blocks can align to the short rod molecules. Using a field theoretic formulation we demonstrate the occurrence of a nematic–nematic (N/N) first order transition from a guest stabilized to a guest–host stabilized region, a reentrant transition from a guest stabilized nematic region to a host only stabilized regime via an isotropic phase and the possibility to selectively stabilize the orientation of the A or/and B blocks.

Morphology and Deformation Behaviour of a Liquid Crystalline Polymer Containing Laterally Attached Pentaphenyl Rods

AbstractSummary: The morphology and deformation behaviour of a liquid crystalline polymer consisting of both laterally attached transverse rods and terminally attached rods were studied. A plaque specimen of the polymer was prepared using the melt‐compression method, which exhibited highly anisotropic Poisson's ratio behaviour. In its through‐thickness direction, its Poisson's ratio was approximately 0.54, and in the in‐plane it was approximately 0.43. This anisotropic behaviour is explained by the anisotropic morphology of the polymer plaque resulting from its compression preparation. The high Poisson's ratio in the through‐thickness direction is due to the unusual chain structure of the polymer. A simple analytical model is presented to describe the non‐linear strain dependent Poisson's ratio behaviour of the material.Representation of the arrangement of the main chain and transverse rod molecules, prior to, during, and at complete extension.magnified imageRepresentation of the arrangement of the main chain and transverse rod molecules, prior to, during, and at complete extension.

Observation of very large chiral domains in a liquid crystal phase formed by mixtures of achiral bent-core and rod molecules

Experimental observations on the B4 phase of liquid crystal mixtures formed from bent-core (P-8-O-PIMB) and rod (5CB) achiral molecules reveal the stabilization of the B4 phase with an unusually large size of chiral domains. The phenomenon is interpreted as an intrinsic property of the B4 phase when arising below the isotropic liquid (Iso); actually direct transition between Iso and B4 phases occurs except for only very limited regions of pure P-8-O-PIMB and 5CB molecules. The phase diagram of the mixture shows the existence of a new phase, closely related to the B4 phase.

Analysis of dielectric polarization of organic monolayer at the air–water interface by MDC and SHG spectroscopy

With Maxwell displacement current (MDC) measurement coupled with optical second harmonic generation (SHG) measurement, the dielectric polarization phenomena of monolayers can be examined. We have been developing a novel MDC–SHG experimental system, and analyzing the dielectric monolayers at the air–water interface. From theoretical side, based on a rod molecule model, formula to express the linear and nonlinear polarization of monolayers with C ∞-symmetry have been derived using orientational order parameters. On the other hand, using the developed experimental system, the orientational order parameter has been determined with surface pressure–molecular area isotherm ( π– A isotherm) for liquid crystalline monolayers. Further, phase transitions of monolayers that accompanies the monolayer structural change has been successfully detected.

Phase and orientational ordering of low molecular weight rod molecules in a quenched liquid crystalline polymer matrix with mobile side chains.

We study the phase diagram and orientational ordering of guest liquid crystalline (LC) rods immersed in a quenched host made of a liquid crystalline polymer (LCP) matrix with mobile side chains. The LCP matrix lies below the glass transition of the polymer backbone. The side chains are mobile and can align to the guest rod molecules in a plane normal to the local LCP chain contour. A field theoretic formulation for this system is proposed and the effects of the LCP matrix on LC ordering are determined numerically. We obtain simple analytical equations for the nematic/isotropic phase diagram boundaries. Our calculation show a nematic-nematic (N/N) first order transition from a guest stabilized to a guest-host stabilized region and the possibility of a reentrant transition from a guest stabilized nematic region to a host only stabilized regime separated by an isotropic phase. A detailed study of thermodynamic variables and interactions on orientational ordering and phases is carried out and the relevance of our predictions to experiments and computer simulations is presented.

On the Fluorescence Depolarization and the Calculation of Rotational Relaxation Times of Rigid Rod Molecules

The rotational relaxation times of rod like molecules like poly[arylene-ethynylene]s and their low molecular weight model compounds calculated from a simple model agree well with the experimental ones as long as the axial ratio of the corresponding rotational ellipsoid is less than 8. For the polymer (axial ratio > 10) the fluorescence depolarization cannot be described by rotational motion perpendicularly to the long molecular axis. One has to take into consideration bending motions in connection with energy transfer along the bent backbone.

Stretching type II collagen with optical tweezers

Collagens are the most abundant proteins of vertebrates and they provide mechanical and supportive functions in a wide range of connective tissues. Knowledge of the mechanical properties of single collagen molecules is essential in studying the self-assembly of collagen, the interaction between cells and extracellular matrix, the etiology of tissue degeneration and mechanism of regeneration, and the relationship between the structures and mechanical properties of tissues. Here we stretched single type II collagen molecules in neutral pH solution using optical tweezers. The molecular parameters of collagen were obtained by fitting force-extension curves into worm-like chain elasticity model. The molecule length of type II collagen monomer was 295.8 nm. The persistence length of type II collagen monomer was 11.2 nm. These observations indicate that collagen molecules are flexible rather than rigid rod molecules at neutral pH solution.

Calculated phase diagrams for cellulose/ammonia/ammonium thiocyanate solutions in comparison to experimental results

Flory's lattice theory for rigid rod molecules and Kuhn chains is used to calculate phase diagrams for cellulose/ammonia/ammonium thiocyanate solutions. Persistence length values measured by light scattering and reported previously are used as rod length values. Variations in the phase diagram based on varying rod length/solvent composition and cellulose molecular weight are explored. Spinodal curves are calculated in a region of the phase diagram predicting phase separation between two anisotropic solutions. Finally, calculated phase diagrams are compared to published data for the system. Discrepancies between theory and data may be accounted for by soft interactions between cellulose molecules and solvent which are not incorporated into the lattice theory.

Phase behavior and structure formation of hairy-rod supramolecules.

Phase behavior and microstructure formation of rod and coil molecules, which can associate to form hairy-rod polymeric supramolecules, are addressed theoretically. Association induces considerable compatibility enhancement between the rod and coil molecules and various microscopically ordered structures can appear in the compatibility region. The equilibria between microphase-separated states, the coil-rich isotropic liquid and the rod-rich nematic are discussed in detail. In the regime where hairy-rod supramolecules with a high grafting density appear as a result of the association, three phase diagram types are possible depending on the value of the association energy. In the low grafting density regime only the lamellar microstructure is proven to be stable.

Birefringence due to free electrophoresis

We calculate the steady-state birefringence and dichroism for the case of a rigid bent rod molecule due to the free electrophoresis. In order to find the orientational distribution function, we use the Fokker–Planck equation and solve it by the perturbation method (for the steady state) at low electric field. We find that, using convenient parameters to characterize the DNA solution, both, the calculated electric birefringence and the dichroism, for low electric fields, have positive values in concordance with the experimental results.

Phase behavior of symmetric rod–plate mixtures revisited: Biaxiality versus demixing

The phase behavior of symmetric binary rod–plate mixtures has been investigated by numerical minimization of a free energy functional derived by Parsons [J. D. Parsons, Phys. Rev. A 19, 1225 (1979)] and Lee [S. D. Lee, J. Chem. Phys. 87, 4972 (1987)]. Both rod and plate molecules are represented by hard cylinders, with aspect ratios chosen so that the molecular and pair excluded volumes are equal; in this way symmetric phase diagrams in composition are found. The subtle competition between the packing entropy and the entropy of mixing rules out the possibility of a uniaxial nematic–biaxial nematic phase transition and instead favors a demixing phase transition between a rod-rich and a plate-rich nematic phase. It is shown that the biaxial nematic phase is unstable relative to demixing even for symmetric mixtures of very long rod and very flat plates, where the Parsons–Lee theory becomes identical with the Onsager theory. The contradictory predictions obtained in recent studies regarding the stability of the biaxial nematic phase have been resolved by examining the lowest aspect ratio of the rods (κ2) where the Parsons–Lee and Onsager theories become equivalent. It turns out that neglecting the lower order terms in the excluded volumes (so-called end effects) leads to a favoring of the biaxial nematic phases. Only two types of phase transitions are observed in this work: isotropic–nematic phase coexistence and demixing transitions involving either two isotropic or two nematic phases. The stability of the nematic region on mixing is found to be very sensitive to the aspect ratios of the molecules: for moderate aspect ratios of the rods (5<κ2<10), a destabilization of the nematic phase is observed relative to the isotropic phase, while for κ2>10 the opposite tendency is found. A demixing transition between rod-rich and plate-rich nematic phases takes place for all of the systems studied with a widening coexistence region for increasing aspect ratio. Isotropic–isotropic demixing is also observed for aspect ratios κ2>65.5. For the larger values of κ2 the regions of isotropic–isotropic and nematic–nematic demixing broaden, while the isotropic–nematic coexistence is progressively suppressed.

Global fluid phase behavior in binary mixtures of rodlike and platelike molecules

The phase behavior of a liquid-crystal forming model colloidal system containing hard rodlike and platelike particles is studied using the Parsons–Lee scaling [J. D. Parsons, Phys. Rev. A 19, 1225 (1979); S. D. Lee, J. Chem. Phys. 87, 4972 (1987)] of the Onsager theory. The rod and plate molecules are both modeled as hard cylinders. All of the mixtures considered correspond to cases in which the volume of the plate is orders of magnitude larger that the volume of the rod, so that an equivalence can be made where the plates are colloidal particles while the rods play the role of a depleting agent. A combined analysis of the isotropic–nematic bifurcation transition and spinodal demixing is carried out to determine the geometrical requirements for the stabilization of a demixing transition involving two isotropic phases. Global phase diagrams are presented in which the boundaries of isotropic phase demixing are indicated as functions of the molecular parameters. Using a parameter z which corresponds to the product of the rod and plate aspect ratios, it is shown that the isotropic phase is unstable relative to a demixed state for a wide range of molecular parameters of the constituting particles due to the large excluded volume associated with the mixing of the unlike particles. However, the stability analysis indicates that for certain aspect ratios, the isotropic–nematic phase equilibria always preempts the demixing of the isotropic phase, irrespective of the diameters of the particles. When isotropic–isotropic demixing is found, there is an upper bound at large size ratios (Asakura and Oosawa limit), and a lower bound at small size ratios (Onsager limit) beyond which the system exhibits a miscible isotropic phase. It is very gratifying to find both of these limits within a single theoretical framework. We test the validity of the stability analysis proposed by calculating a number of phase diagrams of the mixture for selected molecular parameters. As the hard rod particles promote an effective attractive interaction between the hard-plate colloidal particles, the isotropic–isotropic demixing usually takes place between two rod-rich fluids. As far as the isotropic–nematic transition is concerned, a stabilization as well as a destabilization of the nematic phase relative to the isotropic phase is seen for varying rod–plate size ratios. Moreover, isotropic–nematic azeotropes and re-entrant phenomena are also observed in most of the mixtures studied. We draw comparisons between the predicted regions of stability for isotropic demixing and recent experimental observations.

Constrained dynamics in supramolecular structures of poly(p-phenylenes) with ethylene oxide side chains: A combined dielectric and nuclear magnetic resonance investigation

The complex dynamics in supramolecular structures of poly(p-phenylenes) with short ethylene oxide (EO) side chains (known as “hairy rods”) have been studied by combining temperature and pressure dependent dielectric spectroscopy (DS) with different nuclear magnetic resonance (NMR) techniques. The system forms a periodic lattice of layers of the main chains separated by layers of the amorphous EO chains. Since the dipole moment is located on the EO side chain, DS is probing mainly the side-chain motions and only indirectly the backbone dynamics. On the other hand, the different NMR techniques are also sensitive to the backbone mobility and provide the molecular origin of the dynamic processes. Two dielectrically active processes exist, a “fast” β (which differs from the usual Johari–Goldstein β process found in homopolymers and low molecular weight glass-forming liquids) and a “slow” α process with the latter carrying most of the intensity. Comparison of the NMR and dielectric results identifies the outer EO units as solely responsible for the dielectric β process whereas both the short EO segments and the unfreezing of the internal backbone dynamics including bending motions contribute to the α process. The unfreezing of the backbone dynamics does not lead to the dissolution of the structure that only happens at even higher temperatures. Hairy rod molecules thus can serve as model systems which provide the means of studying the effect of confinement on the dynamics. In this respect, the effect of confinement is mainly on the temperature dependence of the α-process relaxation times.

Isomerization and Oxidation of Vitamin A in Cone-Dominant Retinas: A Novel Pathway for Visual-Pigment Regeneration in Daylight

The first step toward light perception is 11- cis to all- trans photoisomerization of the retinaldehyde chromophore in a rod or cone opsin-pigment molecule. Light sensitivity of the opsin pigment is restored through a multistep pathway called the visual cycle, which effects all- trans to 11- cis re-isomerization of the retinoid chromophore. The maximum throughput of the known visual cycle, however, is too slow to explain sustained photosensitivity in bright light. Here, we demonstrate three novel enzymatic activities in cone-dominant ground-squirrel and chicken retinas: an all- trans-retinol isomerase, an 11- cis-retinyl-ester synthase, and an 11- cis-retinol dehydrogenase. Together these activities comprise a novel pathway that regenerates opsin photopigments at a rate 20-fold faster than the known visual cycle. We suggest that this pathway is responsible for sustained daylight vision in vertebrates.

Ordering transitions, biaxiality, and demixing in the symmetric binary mixture of rod and plate molecules described with the Onsager theory.

The phase behavior of a liquid-crystal forming binary mixture of generic hard rodlike and platelike particles is studied with the theory of Onsager [L. Onsager, Ann. N. Y. Acad. Sci. 51, 627 (1949)] for nematic ordering. The mixture is chosen to be symmetric at the level of the second virial theory, so that the phase behavior of the two pure components is identical. A parameter q is used to quantify the effect of the unlike rod-plate excluded volumes on the phase behavior; a value of q>1 indicates that the unlike excluded volume is greater than the like excluded volume between the rods or plates, and a value of q<1 corresponds to a smaller unlike excluded volume. Two methods are used to solve the excluded volume integrals: the approximate L2 model [A. Stroobants and H. N. W. Lekkerkerker, J. Phys. Chem. 88, 3669 (1984)], in which a second-order Legendre polynomial is used; and a numerical method where the integrals are solved exactly. By varying the unlike excluded volume interaction q, the isotropic phase is seen to be stabilized (small q) or destabilized (large q) with respect to the nematic phase for both models. Isotropic-isotropic demixing is also observed for the largest values of q due to the unfavorable contribution of the unlike excluded volume to the entropy of the system. A second-order nematic-biaxial nematic phase transition is observed for small values of q in the L2 approximation, and for all q in the exact calculation; in the latter case the stability of the biaxial phase is enhanced by increasing q, while in the L2 approximation nematic-nematic phase separation is favored. This result is the most striking difference between the two methods, and is in contrast with the results of previous studies. We show that the accuracy of the L2 expansion is particularly poor for parallel and perpendicular particle orientations.

Nanostructure of a Photochromic Polymer/Liquid Crystal Hybrid Monolayer on a Water Surface Observed by in Situ X-ray Reflectometry

The monolayer of a poly(vinyl alcohol) derivative having an azobenzene (Az)-containing side chain (6Az10-PVA) can induce an alignment transition of liquid crystal molecules in the monolayer by the trans−cis conformational change of the azobenzene unit (command effect). As a model system of this dynamic interface, the structural characterization of the mixed monolayer of 6Az10-PVA and a nematic liquid crystal molecule, 4‘-penthyl-4-cyanobiphenyl (5CB), on a water surface was analyzed by in situ X-ray reflectometry. The depth density profile indicated that this hybrid monolayer in the trans-Az state consists of two distinct layers: a dense lower region and a less dense upper region having the same thickness of ca. 1.5 nm. This suggests that the rod molecules are stretched out in a direction perpendicular to the water surface where 5CB molecules are inserted on the lower side of the Az side chains. Light-induced structural modifications were clearly detected in the hybrid monolayer.

Nanoscale Organization of Conjugated Rods in Rod-Coil Molecules

Conjugated rod molecules bearing flexible coil polymer moieties on both ends self-organize into 1D or 2D structures, and eventually into a 3D tetragonal superlattice (see Figure), depending on the number of monomeric units in the coils. The 3D ordered rods exhibit a highly extended fluorescence lifetime over the 1D or 2D arrays, making synthetic control of the rod-to-coil volume fraction a viable strategy to regulate photophysical properties.

Molecular composites of poly( p-phenylene terephthalamide) anion and poly(propylene oxide): mechanical properties

Molecular composites have been prepared by mixing rigid rod molecules of PPTA anion (K-salt) and poly(propylene oxide) in a common solvent and compression molding the precipitated material. As the PPTA anion content of the composites is increased, an upward shift occurs in both the glass transition temperature and the decomposition temperature. The tensile strength, stiffness and resistance to stress relaxation of the composites are significantly increased, even at relatively low concentrations of the rigid rod reinforcement. The enhancement in mechanical properties results from a good dispersion of the rigid PPTA anion molecules and from the presence of ion-dipole interactions between the ionic groups of the PPTA anion and the dipolar units of the poly(propylene oxide) matrix. Additional increases in strength and rigidity may be achieved by changing the counterion from monovalent K + to divalent Ca 2+.

Roto-translational diffusion of biaxial probes in uniaxial liquid crystal phases

We discuss the problem of roto-translational diffusion of a rigid biaxial molecule dissolved in a uniaxial smectic liquid crystal phase. We examine distorted rod and disklike molecules and show how biaxiality and roto-translational coupling can produce significant effects on some of the correlation functions and spectral densities most useful in analyzing experimental observables.