Computational modelling of declination loops under shear flow

Optical observations of sheared lyotropic and thermotropic liquid crystals have shown them to have a rich microstructure dominated by disclination loops and networks. In thermotropic liquid crystalline polymers it has been possible to freeze in the microstructures, which are then sectioned and analysed by optical and electron microscopy, to enable the identification of the predominant types of disclinations. In this work a computational model is presented which simulates the development of texture in liquid crystalline materials. The model has been designed so that it is possible to study large localised distortions which are subjected to a flow field. In the aspect of the work reported here, simulations have been used to study the influence of simple shear on individual disclination loops placed in an otherwise undeformed sample. The subsequent deformation of the loop is shown to be dependent on the angle that the rotation vector of the loop makes with the vorticity direction.     

Elastic constant anisotropy and disclination interaction in nematic polymers II. Effect of disclination interaction

A new tensorial approach introducing unequal elastic constants in a two-dimensional lattice model is applied to simulate the influence of disclination interaction on the structures of half-wedge disclinations, for the understanding of the variation of apparent elastic anisotropy ε_a measured from different disclinations in a texture (see preceding paper, part I [1]). The apparently random variation of ε_a implies that disclination interaction has a strong effect on the structure of disclinations; as elastic anisotropy increases, its impact becomes overpowering. Nevertheless the disclination interaction still acts in the same way as in the case of equal elastic constants. Analysis of the free energy of disclination pairs shows that the structures of both +1/2 and -1/2 disclinations are changed under the influence of a neighbouring disclination, but in different ways. For +1/2 disclination, the splay and bend distortions vary with the relative orientation of its -1/2 neighbour. Meanwhile, for -1/2 disclination, the bend and splay distortions are remarkably insensitive to the relative orientation of its +1/2 disclination neighbour.     

Modelling of microstructure in mesophases

Small-molecule liquid crystals show textures which are readily studied at low magnification in the optical polarizing microscope. In polymeric liquid crystals, however, the textures are often much finer, taxing the microscope's resolution. Nevertheless, studies of microstructure in such polymers have been made and it is apparent that they can differ widely both from small-molecule liquid crystals and, indeed, from polymer to polymer. This paper sets out to account for these variations by exploring the effect on microstructure of the marked differences between the magnitudes of the splay, twist and bend elastic constants which are a characteristic of many liquid crystalline polymers. We report a computer model which simulates the development of microstructure for different ratios of the elastic constants. When these are approximately equal, textures characteristic of small-molecule liquid crystals result, such as those involving escape into the third dimension with the degeneration of line defects into points. When the splay energy is high in comparison with bend and twist, as is the case for many thermotropic liquid crystalline copolyesters, escape does not occur and half integral disclination lines predominate. For simulations involving planar boundary conditions, layered microstructures result, frequently with little matching of the orientation from layer to layer. Within the layers the trajectory of the orienting units is sinuous. This simulated microstructure resembles textures observed in thermotropic copolyesters, studied both in this laboratory and elsewhere. The computer model uses a lattice approach which is similar in some respects to that developed by Lebwohl and Lasher. It should not be thought of as a molecular scale model, however, but rather as one based on assemblies of molecules which share a common director.     

The effect of annealing on the local structure of glassy polycarbonate

Annealing of polycarbonate glasses at temperatures belowTg leads to an increase in yield stress and a drop in the impact strength. Although such behaviour may be related to the corresponding reduction in free volume upon annealing, variations in the wide-angle X-ray scattering curves indicate some modification to the local structure. The area of an intrachain peak at s 0.7 Å^–1 is monitored with respect to annealing temperature and time. It is proposed that the variations may be described by an increasing level of interlocking or (nesting) between neighbouring chain segments, a process which is a natural consequence of the molecular shape of polycarbonate.     

Diffraction from thermotropic copolyester molecules

Analytical expressions for the meridional scattering from highly oriented random copolyesters are presented. These procedures are utilised in developing the relationship of the features in the diffraction pattern to compositions and relative lengths of the constituent monomer units. The difference between the scattering patterns for random and block copolymers are discussed. The theory is applied to an example of a liquid crystal froming random copolyester.Summary of terms B _a (s) the meridional scattering intensity for the chemical unitA - s width or scattering peak measured at half height - f _a number fraction ofA units in chains - f _b number fraction ofB units in chains - ¦f_N number fraction of designated chemical units - [ (N) hydroxynaphthoic residues - B hydroxybenzoic - F _m (s) scattering function associated withm ^th neighbours along chain - G(s) function proportional to scattering intensity - H _m (r) linear disposition function form ^th neighbours along chain - radiation wavelength - L _a average length of sequence ofA units (=n _a r _a) - N _a total number ofA units in chain - n _a average number ofA units in sequences of such unints.n _a= n _a;im _i ² n _a;i;m _i wheren _a;i is the number of sequences containing m; units ofA - P _a probability of a unit being of identityA (P _a=f _a for a random (Bernoullioun) copolymer) - q _aa probability of a unitA following a unitA - [q _ab probability of a unit B following a unitA ] - r _a length of chemical unitA within chain - S scattering vector defined as 4 sin/ - 2 scattering angle - W(r) distribution function of interunit distances - Z(s) function proportional to scattering intensity along chain axis (meridional)     

The effect of aggregation on the electrical conductivity of spin-coated polymer/carbon nanotube composite films

This paper considers the feasibility of replacing indium tin oxide (ITO) with spin-coated, polymer-based composite films that are filled with multiwalled carbon nanotubes (MWNTs). The coating mixture consists of a solvent with low volatility, a dissolved thermoplastic polymer, and MWNTs. The high aspect ratio of MWNTs and their good electrical conductivity enable electrical percolation at very low concentrations, so that films can be prepared that conduct electricity while retaining good optical transparency. Although the MWNTs are driven to aggregate by Van der Waals interactions, the high viscosity of the polymer/solvent solution enables the preparation of metastable, homogeneous dispersions. However, exposing the mixtures to shear leads to aggregation, the magnitude of which depends on the duration of the shear. This effect could be observed directly in spin-coated films using both optical microscopy and conductivity measurements, with aggregation causing a drop in conductivity at high nanotube loading, and more complex non-monotonic behavior at concentrations approaching the percolation threshold.     

Neuropeptide delivery to the brain: a von Willebrand factor signal peptide to direct neuropeptide secretion

Background  

Multiple neuropeptides, sometimes with opposing functions, can be produced from one precursor gene. To study the roles of the different neuropeptides encoded by one large precursor we developed a method to overexpress minigenes and establish local secretion.