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G J A Sevink

Publications and source records attributed to G J A Sevink.

10 recordsLinked to original sources
PubMed2006

Dynamics of terrace formation in a nanostructured thin block copolymer film.

We have used dynamic self-consistent field (DSCF) theory to investigate the structural evolution of an ABA block copolymer thin film placed between a solid substrate and a free surface. In line with the few existing theoretical studies for pure homopolymers and mixtures, the free interface is introduced by a void component. In our calculations, the free surface experiences surface roughening and eventually the formation of terraces, as in the experiments. The kinetic pathway of the microstructures was compared to findings of an existing detailed experimental study (Knoll, A.; Lyakhova, K. S.; Horvat, A.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Nat. Mater. 2004, 3, 886) and was found to be equivalent in detail. This corroborates our assumption in this earlier work that the pathway due to changing film thickness is similar to a pathway due to changing surface energetics. Moreover, our calculations show for the first time that microstructural transitions are a driving force of polymer/air interface curving and the formation of terraces.

Journal Article↗
PubMed2005

Influence of initial order on the microscopic mechanism of electric field induced alignment of block copolymer microdomains.

We investigate the mechanism of microdomain orientation in concentrated block copolymer solutions exposed to a dc electric field by in situ synchrotron small-angle X-ray scattering (SAXS). As a model system, we use concentrated solutions of a lamellar polystyrene-b-polyisoprene block copolymer in toluene. We find that both the microscopic mechanism of reorientation and the kinetics of the process strongly depend on the initial degree of order in the system. In a highly ordered lamellar system with the lamellae being aligned perpendicular to the electric field vector, only nucleation and growth of domains is possible as a pathway to reorientation and the process proceeds rather slowly. In less ordered samples, grain rotation becomes possible as an alternative pathway, and the process proceeds considerably faster. The interpretation of our finding is strongly corroborated by dynamic self-consistent field simulations.

Electricity↗
PubMed2005

Design of chimaeric polymersomes.

We discuss the development of hierarchical polymer particles, or variegated polymersome composites, in which at least two different components are phase separated within one polymersome chimaera. We briefly discuss the present status in experimental polymersome research, and then discuss a speculative design strategy, based on mesoscopic simulations with a dynamical variant of polymer self-consistent field theory (Mesodyn). The main conclusion is that the counter-intuitive co-assembly of demixing block copolymers is the key in controlling hierarchical structures on a mesoscopic scale. This is the classical paradox of a chimaera: the constituents live in the same scaffold, but apart. Block copolymers beyond a certain length will always split the assembly, and without further precautions, polymer based chimaerae are intrinsically unstable. To this end, we propose the application of a branched block copolymer as composite compatibilizer, glueing the separate domains together, and thereby stabilizing the chimaeric polymersome.

Journal Article↗
PubMed2004

Direct imaging and mesoscale modelling of phase transitions in a nanostructured fluid.

The kinetics of phase transitions is essential for understanding pattern formation in structured fluids. These fluids play a key role in the morphogenesis of biological cells, and they are very common in pharmaceutical products and plastic materials. Until now, it has not been possible to follow phase transitions in structured fluids experimentally in real time and with high spatial resolution. Previous work has relied on static images and indirect experimental evidence from spatially averaging scattering experiments. Simulating the processes with computer models is a further challenge because of the multiple time and length scales involved. Our movies based on in situ scanning force microscopy show the time sequence of the elementary steps of a phase transition in a fluid film of block copolymer from the cylinder to the perforated lamella phase. The movies validate a versatile simulation model that gives physical insight into the nature of the process. Our approach provides a means of improving the study and understanding of pattern formation processes in nanostructured fluids. We expect a significant impact on nanotechnology where block copolymers serve as self-organized templates for the synthesis of inorganic nanostructured materials.

Computer Simulation↗
PubMed2004

Kinetic pathways of sheared block copolymer systems derived from Minkowski functionals.

We employ Minkowski functionals to analyze the kinetics of pattern formation under an applied external shear flow. The considered pattern formation model describes the dynamics of phase separating block copolymer systems. For our purpose, we have chosen two block copolymer systems (a melt and a solution) that exhibit a hexagonal cylindrical morphology as an equilibrium structure. Our main objective is the determination of efficient choices for the treshold values that are required for the calculation of the Minkowski functionals. We find that a minimal set of two treshold values (one from which should be equal to an average density value and another to a higher density value) is sufficient to unraffle the phase separation kinetics. Given these choices, we focus on the influence of the degree of phase separation, and the instance at which the shear is applied, on the kinetic pathways. We also found a remarkable similarity of the time evolution of Euler characteristic and the segregation parameter for the average density choice.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2002

Phase behavior in thin films of cylinder-forming block copolymers.

We have experimentally determined a phase diagram for cylinder-forming polystyrene-block-polybutadien-block-polystyrene triblock copolymer in thin films. The phase behavior can be modeled in great detail by dynamic density functional theory. 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.

Journal Article↗