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M W Matsen

Publications and source records attributed to M W Matsen.

10 recordsLinked to original sources
PubMed2006

Converting the nanodomains of a diblock-copolymer thin film from spheres to cylinders with an external electric field.

We investigate the ability of an applied electric field to convert the morphology of a diblock-copolymer thin film from a monolayer of spherical domains embedded in the matrix to cylindrical domains that penetrate through the matrix. As expected, the applied field increases the relative stability of cylindrical domains, while simultaneously reducing the energy barrier that impedes the transition to cylinders. The effectiveness of the field is enhanced by a large dielectric contrast between the two block-copolymer components, particularly when the low-dielectric contrast component forms the matrix. Furthermore, the energy barrier is minimized by selecting sphere-forming diblock copolymers that are as compositionally symmetric as possible. Our calculations, which are the most quantitatively reliable to date, are performed using a numerically precise spectral algorithm based on self-consistent-field theory supplemented with an exact treatment for linear dielectric materials.

Journal Article↗
PubMed2006

Monte Carlo phase diagram for diblock copolymer melts.

A partial phase diagram is constructed for diblock copolymer melts using lattice-based Monte Carlo simulations. This is done by locating the order-disorder transition (ODT) with the aid of a recently proposed order parameter and identifying the ordered phase over a wide range of copolymer compositions (0.2<or=f<or=0.8). Consistent with experiments, the disordered phase is found to exhibit direct first-order transitions to each of the ordered morphologies. This includes the spontaneous formation of a perforated-lamellar phase, which presumably forms in place of the gyroid morphology due to finite-size and/or nonequilibrium effects. Also included in our study is a detailed examination of disordered cylinder-forming (f=0.3) diblock copolymers, revealing a substantial degree of pretransitional chain stretching and short-range order that set in well before the ODT, as observed previously in analogous studies on lamellar-forming (f=0.5) molecules.

Journal Article↗
PubMed2006

Effect of large degrees of polydispersity on strongly segregated block copolymers.

We investigate the effect of polydispersity on the lamellar phase of a diblock copolymer melt using self-consistent field theory (SCFT). A previous SCFT calculation predicted that polydispersity increases the domain spacing consistent with experiment, but it also suggested that the effect vanishes with increasing segregation contrary to experiment. We attribute this disagreement to a problem of slow convergence of the Gaussian-quadrature technique used to integrate over the molecular-weight distribution when either the segregation or polydispersity index is large. Here the problem is overcome by a new efficient algorithm that allows high-order quadratures for relatively little computational cost. When implemented, we find that the elevated domain spacing does indeed persist into the strong-segregation regime consistent with experiment. This conclusion is also substantiated by the analytical strong-segregation theory (SST).

Algorithms↗
PubMed2005

Stability of a block-copolymer lamella in a strong electric field.

Using self-consistent field theory, we examine the stability of a lamellar layer of diblock copolymer subject to strong orthogonal electric fields. Two competing instabilities are identified; one is a peristaltic mode that leads to perpendicular lamellae, and the other is an undulatory mode that results in the formation of an undesirable grain boundary. The former kinetic pathway is favored when the central domain is relatively thin and composed of the low-dielectric material.

Computer Simulation↗
PubMed2005

Scaling behavior of a brush-homopolymer interface in the limit of high grafting density.

The interface between a polymer brush and a chemically equivalent homopolymer is examined using self-consistent field theory (SCFT). Focusing on ultrahigh grafting densities, we extract how the properties scale with the brush thickness, L, and compare with predictions based on strong-stretching theory (SST). Although the scaling exponents are consistent, the overall agreement is poor. We attribute this to the inaccurate way the SST-based calculation treats chain fluctuations at the extremity of the brush. This accounts for a previous disagreement between SCFT and SST in regards to autophobic dewetting, and brings into question a number of other SST predictions. Our conclusion is that SST requires a more sophisticated treatment of finite-stretching corrections, along the lines of that proposed by Likhtman and Semenov [Europhys. Lett. 51, 307 (2000)].

Journal Article↗
PubMed2004

Investigating the dominant corrections to the strong-stretching theory for dry polymeric brushes.

The accuracy of strong-stretching theory (SST) is examined against a detailed comparison to self-consistent field theory (SCFT) on dry polymeric brushes with thicknesses of up to approximately 17 times the natural chain extension. The comparison provides the strongest evidence to date that SST represents the exact thick-brush limit of SCFT. More importantly, it allows us to assess the effectiveness of proposed finite-stretching corrections to SST. Including the entropy of the free ends is shown to rectify the most severe inaccuracies in SST. The proximal layer proposed by Likhtman and Semenov provides another significant improvement, and we identify one further effect of similar importance for which there is not yet an accurate treatment. Furthermore, our study provides a valuable means of rejecting mistaken refinements to SST, and indeed one such example is revealed. A proper treatment of finite-stretching corrections is vital to a wide range of phenomena that depend on a small excess free energy, such as autophobic dewetting and the interaction between opposing brushes.

Journal Article↗
PubMed2003

Comment on "Cylindrical phase of block copolymers: stability of circular configuration to elliptical distortions and thin film morphologies".

Pereira [Phys. Rev. E 63, 061809 (2001)] has recently predicted that the hexagonal symmetry of the cylindrical phase in AB diblock copolymer melts is highly unstable. This is in stark disagreement with experiment, and can be attributed to the fact that the connectivity of the A and B blocks is not enforced in his implementation of the strong-segregation theory. Here, the stability of the hexagonal symmetry is supported by alternative calculations based on a more advanced strong-segregation theory that enforces the connectivity as well as the more rigorous self-consistent field theory.

Comment↗
PubMed2001

Predicting the mesophases of copolymer-nanoparticle composites.

The interactions between mesophase-forming copolymers and nanoscopic particles can lead to highly organized hybrid materials. The morphology of such composites depends not only on the characteristics of the copolymers, but also on the features of the nanoparticles. To explore this vast parameter space and predict the mesophases of the hybrids, we have developed a mean field theory for mixtures of soft, flexible chains and hard spheres. Applied to diblock-nanoparticle mixtures, the theory predicts ordered phases where particles and diblocks self-assemble into spatially periodic structures. The method can be applied to other copolymer-particle mixtures and can be used to design novel composite architectures.

Journal Article↗
PubMed2000

Improving polymeric microemulsions with block copolymer polydispersity.

Recent experiments have demonstrated that block copolymers are capable of stabilizing immiscible homopolymer blends producing bicontinuous microemulsion. The stability of these polymeric alloys requires the copolymer to form flexible, nonattractive monolayers along the homopolymer interfaces. We predict that copolymer polydispersity can substantially and simultaneously improve the monolayers in both of these respects. Furthermore, polydispersity should provide similar improvements in systems, such as colloidal suspensions and polymer/clay composites, that utilize polymer brushes to suppress attractive interactions.

Journal Article↗