Liquid Crystalline Polymers In Nematic Solvents

Monte Carlo simulations of self-avoiding random walks surrounded by aligned rods on a square lattice and a simple cubic lattice were performed to address the topological constraints involved for dilute solutions of flexible polymers in a highly oriented nematic solvent. The nematic constraint exerted by the solvent is modelled by requiring that the distance between nearest neighbour chain segments in the direction of the director of the nematic solvent is such that a discrete number of rods, representing the perfectly ordered nematic solvent, fits exactly in between. Rod lengths of one or two lattice spacings are used. In this case interesting scaling behaviour for the radius of gyration and the end-to-end distance is found. Perpendicular to the director the coil behaves as a two- (or one- in the case of a square lattice) dimensional self-avoiding walk, whereas parallel to the director it behaves as a random walk. The conformations have a disk-like shape with the radius of gyration perpendicular to the director being considerably larger than the parallel component. These simulations address only one aspect of flexible polymers in nematic solvents. In reality the presence of the coil also reduces the entropy of the solvent which has exactly the opposite effect on the shape of the polymer coil.

We present the main types of phase diagrams obtained with flexible polymers in nematic solvents. Data deduced from experimental phase diagrams are compared with two theoretical descriptions.

The conformation of a polymer chain in solution is coupled to the local structure of the surrounding solvent and can undergo large changes in response to variations in solvent density and temperature. The many-body effects of solvent on the structure of an n-mer polymer chain can be formally mapped to an exact n-body solvation potential. Here, we use a pair decomposition of this n-body potential to construct a set of two-body potentials for a Lennard-Jones (LJ) polymer chain in explicit LJ solvent. The solvation potentials are built from numerically exact results for 5-mer chains in solvent combined with an approximate asymptotic expression for the solvation potential between sites that are distant along the chain backbone. These potentials map the many-body chain-in-solvent problem to a few-body single-chain problem and can be used to study a chain of arbitrary length, thereby dramatically reducing the computational complexity of the polymer chain-in-solvent problem. We have constructed solvation potentials at a large number of state points across the LJ solvent phase diagram including the vapor, liquid, and super-critical regions. We use these solvation potentials in single-chain Monte Carlo (MC) simulations with n ≤ 800 to determine the size, intramolecular structure, and scaling behavior of chains in solvent. To assess our results, we have carried out full chain-in-solvent MC simulations (with n ≤ 100) and find that our solvation potential approach is quantitatively accurate for a wide range of solvent conditions for these chain lengths.

The average conformation of a flexible chain molecule in solution is coupled to the local solvent structure. In a dense solvent, local chain structure often mirrors the pure solvent structure, whereas, in a dilute solvent, the chain can strongly perturb the solvent structure which, in turn, can lead to either chain expansion or compression. Here we use Monte Carlo (MC) simulation to study such solvent effects for a short Lennard-Lones (LJ) chain in monomeric LJ solvent. For an n-site chain molecule in solution these many-body solvent effects can be formally mapped to an n-body solvation potential. We have previously shown that for hard-sphere and square-well chain-in-solvent systems this n-body potential can be decomposed into a set of two-body potentials. Here, we show that this decomposition is also valid for the LJ system. Starting from high precision MC results for the n = 5 LJ chain-in-solvent system, we use a Boltzmann inversion technique to compute numerically exact sets of two-body solvation potentials which map the many-body chain-in-solvent problem to a few-body single-chain problem. We have carried out this mapping across the full solvent phase diagram including the dilute vapor, dense liquid, and supercritical regions and find that these sets of solvation potentials are able to encode the complete range of solvent effects found in the LJ chain-in-solvent system. We also show that these two-site solvation potentials can be used to obtain accurate multi-site intramolecular distribution functions and we discuss the application of these exact short chain potentials to the study of longer chains in solvent.

A model system of side-group liquid crystalline polymers (SGLCPs) with systematically varied molecular weight (from 78 to 420 kg/mol; PDI ≤ 1.16) and spacer length (8−12 atoms long) was prepared by polymer analogous synthesis. Matching the structure of the mesogenic units to that of the nematic solvent produced excellent solubility, even at molecular weights an order of magnitude greater than in prior literature on SGLCP solutions. Addition of up to 10 wt % polymer did not affect the ordinary and extraordinary refractive indices of the nematic host (4-pentyl-4‘-cyanobiphenyl, 5CB), indicating that the order parameter was not significantly affected by the polymer.

Osmotic coefficients and counterion distribution functions of rodlike and flexible polyelectrolyte chains have been studied using molecular dynamics simulations of multichain systems with explicit counterions in salt-free solutions. Using the counterion density profile, we have verified different regimes in the phase diagram of rodlike and flexible chains predicted by the two-zone model of Deshkovski et al. The agreement between our simulation results and predictions of the two-zone model is reasonably good for weakly charged rodlike chains. However, for flexible chains our results for the distance dependence of the counterion density profile in different regions of the phase diagram are only in qualitative agreement with the predictions of the two-zone model. The osmotic coefficient changes nonmonotonically with polymer concentration, in agreement with predictions of the two-zone model. The osmotic coefficient decreases with increasing polymer concentration in dilute solutions of both rodlike and flexible polyelectrolytes. In semidilute solutions of flexible chains the osmotic coefficient is an increasing function of polymer concentration. We have found that position of the minimum in the osmotic coefficient is close to the overlap concentration.

The use of two liquid crystals as solvents in the determination of molecular structure has been demonstrated for systems which do not provide structural information from studies in a single solvent owing to the fact that the spectra are deceptively simple, with the result that all the spectral parameters cannot be derived with reasonable precision. The specific system studied was 2‐(p‐bromophenyl)‐4,6‐dichloropyrimidine, for which relative inter‐proton discances have been determined from the proton NMR spectra in two nematic solvents.

In our paper, the expression for ht x z 0 t x z 0 zi is incorrect by a factor of 1 2 . Equation (2) should read

We develop a theory of polymers in a nematic solvent by exploiting an analogy with two-dimensional quantum bosons at zero temperature. We argue that the theory should also describe polymers in an isotropic solvent. The dense phase is analyzed in a Bogoliubovlike approximation, which assumes a broken symmetry in the phase of the boson order parameter. We find a stiffening of the longitudinal fluctuations of the nematic field, calculate the density-density correlation function, and extend the analysis to the case of ferro- and electrorheological fluids. The boson formalism is used to derive a simple hydrodynamic theory which is indistinguishable from the corresponding theory of polymer nematics in an isotropic solvent at long wavelengths. We also use hydrodynamics to discuss the physical meaning of the boson order parameter. A renormalization group treatment in the dilute limit shows that logarithmic corrections to polymer wandering, predicted by de Gennes, are unaffected by interpolymer interactions. A continuously variable Flory exponent appears for polymers embedded in a two-dimensional nematic solvent. We include free polymer ends and hairpin configurations in the theory and show that hairpins are described by an Ising-like symmetry-breaking term in the boson field theory.

The thermal racemisation of (–)-3,3-diphenyl-2-(t-butyl)oxaziridine [(–)-(1)] and (–)-1-chloro-2,2-diphenylaziridine [(–)-(R)-(2)] in nematic liquid-crystal (LC) solvents has been followed directly on a thermostatic microscopic stage by measuring the variations in time of the diameters of the Grandjean–Cano disclination circles, which are associated with the induced cholesteric structure. With respect to isotropic solvents, the nematic phases exert relevant influences on the process, affecting both the activation enthalpy and entropy. Although the two variations partially compensate, the first-order rate constants are smaller in the nematic solvents.

Molecules like acenaphthene which have a point group symmetry of C-2v behave as though their symmetry is C-2 when dissolved in chiral nematic liquid crystalline solvents. To quantify this effect a sample of perdeuterated acenaphthene dissolved in the chiral nematic solvent formed by dissolving poly-(gamma -benzyl-L-glutamate), PBLG, in CHCl3 has been studied by deuterium NMR spectroscopy. The quadrupolar splittings obtained were used to determine the orientational order parameters of acenaphthene-d(10) when dissolved in PBLG/CHCl3. To do this it was necessary to also record and analyse the proton and deuterium spectra given by a sample containing both acenaphthene and acenaphthene-d(10) in a non-chiral liquid crystalline solvent. The proton spectrum is very complex, and was analysed only after first recording and analysing the simpler H-1-{H-2} spectrum given by a sample of acenaphthene-d(6). This procedure finally yielded a set of dipolar couplings for the fully protonated molecule, which after correction for vibrational motion, were used to determine both the relative positions of the protons and the orientational order of the molecules. This information was then used to derive the quadrupolar coupling constants from the measured quadrupolar splittings. The lowering of the symmetry of the orientational distribution function is quantified by the angle alpha by which the principal axes of the molecular orientational order matrix of acenaphthene dissolved in PBLG/CHCl3 are rotated out of the plane defined by the aromatic ring. The values of alpha are in the range 1.5 +/- 0.1 degrees to 1.7 +/- 0.1 degrees for the temperature range 295-330 K.

Main-chain liquid-crystalline polymers (LCPs) in nematic media are ellipsoid-like objects. Long LCPs support hairpin defects. The elasticity of the major axis is then significantly weaker than that of the minor axis. Furthermore, the major axis is aligned with the nematic direction. These two traits give rise to novel interfacial behaviour of solutions of LCPs in nematic solvents. A family of second-order tilting phase transitions is expected for the appropriate combination of deformations and anchoring conditions. These occur in a variety of situations involving a solid/solution interface: confinement, grafting and adsorption. A different family of effects are associated with domain boundaries in mesophases of block copolymers incorporating an LCP block. The first-order isotropic–nematic phase transition results in an abrupt change in the elastic behaviour of the LCP block. This, in turn, results in qualitative changes in the phase diagram of the block copolymers.

Self-assembly of a liquid crystal ABA triblock copolymer in a nematic liquid crystal solvent

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAlkyl chains in a nematic field. 2. Temperature and chain length dependence of orientational orderingDemetri J. Photinos, Edward T. Samulski, and Hirokazu. ToriumiCite this: J. Phys. Chem. 1990, 94, 11, 4694–4700Publication Date (Print):May 1, 1990Publication History Published online1 May 2002Published inissue 1 May 1990https://pubs.acs.org/doi/10.1021/j100374a059https://doi.org/10.1021/j100374a059research-articleACS PublicationsRequest reuse permissionsArticle Views95Altmetric-Citations46LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts

Brushes of liquid crystalline polymers swollen by nematic solvents are described by an extended Alexander model. Long-main-chain LCP with hairpins are considered. For the case of homogeneous anchoring a second-order tilting phase transition is expected as the grafting density is increased. When the grafting density is subcritical the transition can be triggered by very weak electrical or magnetic fields.