Andrei Zvelindovsky

Morphology of symmetric block copolymer in a cylindrical pore

The influence of confinement on morphology formation in copolymer systems is an important area of interest in theoretical research. We apply dynamic density functional theory to investigate the effect of pores on the morphology formation in a symmetric diblock copolymer system. The pore is represented by a perfect cylindrical tube. Porous systems are important in biology and are gaining interest for applications in nanotechnology. We show that for the pore sizes under investigation two equilibrium morphologies are possible depending on the surface interaction: a perpendicular or slab morphology and a parallel or multiwall tube morphology. The latter is referred to in the article as dartboard morphology. In the dynamic pathway towards this morphology an intermediate metastable helical phase is found. An important observation is that, for a wide range of pore radii and variations of polymer chain length, no mixed parallel/perpendicular morphologies were found: All observed morphologies are insensitive to the pore diameter.

Phase behavior in thin films of cylinder-forming ABA block copolymers: mesoscale modeling.

The phase behavior of cylinder-forming ABA block copolymers in thin films is modeled in detail using dynamic density functional theory and compared with recent experiments on polystyrene-block-polybutadiene-block-polystyrene triblock copolymers. Deviations from the bulk structure, such as wetting layer, perforated lamella, and lamella, are identified as surface reconstructions. Their stability regions are determined by an interplay between surface fields and confinement effects. Our results give evidence for a general mechanism governing the phase behavior in thin films of modulated phases.

Block copolymers confined in a nanopore: Pathfinding in a curving and frustrating flatland

We have studied structure formation in a confined block copolymer melt by means of dynamic density functional theory. The confinement is two dimensional, and the confined geometry is that of a cylindrical nanopore. Although the results of this study are general, our coarse-grained molecular model is inspired by an experimental lamella-forming polysterene-polybutadiene diblock copolymer system [K. Shin et al., Science 306, 76 (2004)], in which an exotic toroidal structure was observed upon confinement in alumina nanopores. Our computational study shows that a zoo of exotic structures can be formed, although the majority, including the catenoid, helix, and double helix that were also found in Monte Carlo nanopore studies, are metastable states. We introduce a general classification scheme and consider the role of kinetics and elongational pressure on stability and formation pathway of both equilibrium and metastable structures in detail. We find that helicity and threefold connections mediate structural transitions on a larger scale. Moreover, by matching the remaining parameter in our mesoscopic method, the Flory-Huggins parameter chi, to the experimental system, we obtain a structure that resembles the experimental toroidal structure in great detail. Here, the most important factor seems to be the roughness of the pore, i.e., small variations of the pore radius on a scale that is larger than the characteristic size in the system.

Role of dissimilar interfaces in thin films of cylinder-forming block copolymers.

We study the effect of dissimilar interfaces on the phase behavior of cylinder forming block copolymers in thin films by means of dynamic density-functional theory. In this article, we show that dissimilarity of the interfaces induces hybrid structures. These structures appear when the surface fields at the two interfaces stabilize different surface structures and/or reconstructions. We propose a general classification of hybrid structures and give an unifying description of phase behavior of cylinder forming block copolymer films. Our results are consistent with experimental observations.

Cubic phases of block copolymers under shear and electric fields by cell dynamics simulation. I. Spherical phase

Cell dynamics simulation is used to investigate pathways of sphere-to-cylinder transition in block copolymer melt under applied simple shear flow and electric field. Both fields can induce the transition when their strength is above some critical value. At weak fields the spherical phase is preserved, with spheres being deformed into ellipsoids. Weak shear flow is found to improve order in the spherical phase. Observed sliding of layers of spheres under shear is very similar to the experimental finding by Hamley et al. [J. Chem. Phys. 108, 6929 (1998)]. The kinetic pathways are sensitive to the degree of microphase separation in the system and hence affected by temperature. The details of the pathways are described by means of Minkowski functionals.

Nanostructured Soft Matter

Nanostructured soft matter , Nanostructured soft matter , کتابخانه دیجیتال جندی شاپور اهواز

Scaling behavior of the reorientation kinetics of block copolymers exposed to electric fields

We have followed the reorientation kinetics of various block copolymer solutions exposed to an external electric DC field. The characteristic time constants follow a power law indicating that the reorientation is driven by a decrease in electrostatic energy. Moreover, the observed exponent suggests an activated process in line with the expectations for a nucleation and growth process. When properly scaled, the data collapse onto a single master curve spanning several orders of magnitude both in reduced time and in reduced energy. The power law dependence of the rate of reorientation derived from computer simulations based on dynamic density functional theory agrees well with the experimental observations. First experiments in AC electric fields at sufficiently high frequencies confirm the notion that the reorientation process is dominated by differences in the dielectric constants rather than by mobile ions.

Collective behavior of self-propelling particles with kinematic constraints: The relation between the discrete and the continuous description

In two papers we proposed a continuum model for the dynamics of systems of self propelling particles with kinematic constraints on the velocities and discussed some of its properties. The model aims to be analogous to a discrete algorithm used in works by T. Vicsek et al. In this paper we derive the continuous hydrodynamic model from the discrete description. The similarities and differences between the resulting model and the hydrodynamic model postulated in our previous papers are discussed. The results clarify the assumptions used to obtain a continuous description.

Mechanisms of electric-field-induced alignment of block copolymer lamellae

We demonstrate that two mechanisms of lamellae reorientation observed experimentally under applied electric field [A. Boker H. Elbs, H. Hansel, A. Knoll, S. Ludwigs, H. Zettl, V. Urban, V. Abetz, A. H. E. Muller and G. Krausch, Phys. Rev. Lett., 2002, 89, 135502] which have been previously described within dynamic self consistent field theory [A. V. Zvelindovsky and G. J. A. Sevink, Phys. Rev. Lett., 2003, 90, 049601] can be fully explained within a much more simple model using the Ginzburg–Landau Hamiltonian. A third alignment mechanism has been identified which was not previously reported. A more complete picture of reorientation under electric field emerges that clarifies the crucial role of structural defects.

Mesoscopic Simulations of Lamellar Orientation in Block Copolymers

Mesoscopic simulation techniques are employed to investigate lamellar orientation in block copolymers subjected to oscillatory shear Dynamic mean-field density functional theory (MesoDyn) is able to capture parallel lamellar and perpendicular lamellar states at law and higher shear rates. At higher shear rates a third orientation state is identified from cell dynamics and MesoDyn simulations, and corresponds to predominantly paralled-aligned lamellae. This is explained on the basis of partial shear-melting at higher shear rates. The results are compared to the lamellar alignment diagram obtained experimentally for polystyrene/polyisoprene block copolymers.