Russell K. W. Spencer

Simulation of nucleation dynamics at the cylinder-to-lamellar transition in a diblock copolymer melt

We examine the dynamical evolution of a stable lamellar phase nucleating from a metastable cylinder phase in a diblock copolymer melt, through large-scale simulations of the time-dependent Landau–Brazovskii model. Ellipsoidal nuclei form, whose minor axis is parallel to the cylinder axis. We use our observation of both shrinking and growing droplets to determine the critical nucleus size as a function of undercooling, and find that the critical size grows as we approach coexistence. The nucleus shape and critical size agree, near coexistence, with the predictions of an approximate theory. This supports the idea that the underlying microstructure produces an anisotropic droplet interfacial tension, and that the interplay between this interfacial tension and a reduction in bulk free-energy is central to the nucleation process. The nucleus interface moves with a time-independent velocity that depends on the interface orientation in a manner that preserves the ellipsoidal droplet shape into the late stages of growth. Near coexistence, the magnitude of the interfacial velocity varies linearly with undercooling, consistent with theoretical predictions and experimental observations.

Manipulating the ABCs of self-assembly via low-χ block polymer design

Significance Molecular sequence and interactions dictate the mesoscale structure of all self-assembling soft materials. Block polymers harness this relationship to access a rich variety of nanostructured materials but typically require energetically unfavorable (high-χ) interactions between blocks. Contrary to this convention, we demonstrate that ABC triblock terpolymers featuring low-χ interactions between end blocks can self-assemble into a unique mixed morphology that subverts the demands of chain connectivity. As a consequence of block–block mixing, the characteristic length scales of these self-assembled structures exhibit an unusual trend: As the total polymer size increases, the domain spacing decreases. These developments expand the vocabulary of block polymer design and open additional avenues for manipulating the self-assembly of synthetic macromolecules. Block polymer self-assembly typically translates molecular chain connectivity into mesoscale structure by exploiting incompatible blocks with large interaction parameters (χij). In this article, we demonstrate that the converse approach, encoding low-χ interactions in ABC bottlebrush triblock terpolymers (χAC ≲ 0), promotes organization into a unique mixed-domain lamellar morphology, which we designate LAMP. Transmission electron microscopy indicates that LAMP exhibits ACBC domain connectivity, in contrast to conventional three-domain lamellae (LAM3) with ABCB periods. Complementary small-angle X-ray scattering experiments reveal a strongly decreasing domain spacing with increasing total molar mass. Self-consistent field theory reinforces these observations and predicts that LAMP is thermodynamically stable below a critical χAC, above which LAM3 emerges. Both experiments and theory expose close analogies to ABA′ triblock copolymer phase behavior, collectively suggesting that low-χ interactions between chemically similar or distinct blocks intimately influence self-assembly. These conclusions provide fresh opportunities for block polymer design with potential consequences spanning all self-assembling soft materials.

Nucleation of the BCC phase from disorder in a diblock copolymer melt: Testing approximate theories through simulation

We examine nucleation of the stable body-centred-cubic (BCC) phase from the metastable uniform disordered phase in an asymmetric diblock copolymer melt. Our comprehensive, large-scale simulations of the time-dependent, mean-field Landau-Brazovskii model find that spherical droplets of the BCC phase nucleate directly from disorder. Near the order-disorder transition, the critical nucleus is large and has a classical profile, attaining the bulk BCC phase in an interior that is separated from disorder by a sharp interface. At greater undercooling, the amplitude of BCC order in the interior decreases and the nucleus interface broadens, leading to a diffuse critical nucleus. This diffuse nucleus becomes large as the simulation approaches the disordered phase spinodal. We show that our simulation follows the same nucleation pathway that Cahn and Hilliard found for an incompressible two-component fluid, across the entire metastable region. In contrast, a classical nucleation theory calculation based on the free energy of a planar interface between coexisting BCC and disordered phases agrees with simulation only in the limit of very small undercooling; we can expand this region of validity somewhat by accounting for the curvature of the droplet interface. A nucleation pathway involving a classical droplet persists, however, to deep undercooling in our simulation, but this pathway is energetically unfavourable. As a droplet grows in the simulation, its interface moves with a constant speed, and this speed is approximately proportional to the undercooling.

Confinement effects on the miscibility of block copolymer blends

Abstract.Thin films of long and short symmetric AB diblock copolymers are examined using self-consistent field theory (SCFT). We focus on hard confining walls with a preference for the A component, such that the lamellar domains orient parallel to the film with an even number

Fluctuation effects in blends of A + B homopolymers with AB diblock copolymer

\nu

Critical Point of Symmetric Binary Homopolymer Blends

of monolayers. For neat melts, confinement causes the lamellar period, D , to deviate from its bulk value, Db, in order to be commensurate with the film thickness, i.e.,

Domain Bridging in Thermoplastic Elastomers of Star Block Copolymer

L=\nu D/2

Boundary Tension Between Coexisting Phases of a Block Copolymer Blend

. For blends, however, the melt also has the option of macrophase separating into

Continuous thermodynamic integration in field-theoretic simulations of structured polymers

\nu^{(\ell)}

Correction to Critical Point of Symmetric Binary Homopolymer Blends

large and