Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A58: Frustration in Soft Matter AssembliesInvited
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Sponsoring Units: GSOFT DPOLY Chair: Christian Santangelo, University of Massachusetts Amherst Room: LACC Petree Hall C |
Monday, March 5, 2018 8:00AM - 8:36AM |
A58.00001: Frustration in block copolymer assemblies Invited Speaker: Anchang Shi Frustration is a central concept to understand the self-assembly of block copolymers. At the molecular scale, frustration occurs because the different blocks tend to phase separate whereas the chain connectivity prevents a macroscopic phase separation. At the mesoscopic scale, frustration occurs due to the competition between the tendency for the polymeric domains to maintain a uniform shape and the need to fill the space. Furthermore, frustration could occur due to the topology of multiblock copolymers. Novel ordered morpholgies emerge in orded to alleviate these frustrations. We have carried out extensive theoretical studies on the formation of complex ordered phases from block copolymers. The recent theoretical results provide a good understanding of the origin of various frustrations in block copolymer assemblies. Furthermore, the studies have demonstrated that designed block copolymers, either in the form of multiblock copolymers with different architectures or block copolymer blends, could be utilized to tailor frustrations occurring in block copolymer systems. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A58.00002: Molecular Frustration and Formation of Lyotropic Liquid Crystalline Frank-Kasper Phases Invited Speaker: Mahesh Mahanthappa Minimally hydrated amphiphiles self assemble into lyotropic liquid crystals (LLCs), which exhibit periodic aqueous and hydrophobic nanodomains. Commonly observed lyotropic mesophases include 1D lamellae, 2D columnar phases, 3D bicontinuous networks, and high symmetry 3D sphere packings. Based on hard sphere packings, spherical micelles are intuitively expected to form high symmetry body-centered cubic, face-centered cubic, and hexagonally close-packed structures. However, we recently discovered ionic surfactant micelles can spontaneously produce a low symmetry, tetrahedrally close-packed Frank-Kasper (FK) sigma phase with exceptional long-range translational order. The LLC sigma phase comprises micelles of five different and discrete sizes, which are arranged into a large tetragonal unit cell containing 30 particles. This and other new, low symmetry FK phases arise from a frustrated non-covalent force balance that minimizes local variations in amphiphile solvation, while maximizing global micelle cohesion within the ensemble. We will focus on how surfactant structure dictates lyotropic sphere packing symmetry selection and the thermodynamic stabilities of these complex assemblies. This work may inform new methods for harnessing frustration in self-assembled systems to access complex periodic and aperiodic structures derived from various building blocks. |
Monday, March 5, 2018 9:12AM - 9:48AM |
A58.00003: Competing morphologies and escaping to infinite size in geometrically frustrated assemblies Invited Speaker: Gregory Grason In geometrically frustrated assemblies (GFAs) interactions between self-assembling elements favor local packing that is incompatible with uniform global order in the assembly. This classification applies to a broad range of soft matter assemblies, including self-twisting protein bundles and spherical crystals (e.g. particle coated droplets and protein shells). There is a growing understanding that the ability of soft matter GFA to tolerate and build up gradients of imperfect order leads to anomalous behaviors. Most notably, GFA thermodynamics have the ability to "sense" domain sizes on length scales much larger than size individual sub-units or their interactions, opening up the unique possibility of self-limiting equilibria intermediate to dispersed and bulk assembly regimes. In this talk, I describe size-sensitive assembly predicted by two prototypical models of GFAs: frustrated chiral filament bundles and intrinsically-curved 2D crystals. Any given mechanism of assembly frustration (e.g. via chirality, curvature, etc.) poses the basic challenge to understand what are the fundamental limits on breadth of the self-limiting phase, and in turn, the range of self-limiting sizes possible. Underlying these limits is the potential for GFA to escape frustration (in full or in part) via any one of a number competing and structurally distinct morphological responses, including the formation of topological defects in the "bulk" assembly, the reshaping of the free boundary of the assembly (isotropic vs. anisotropic domains), and the elastic "flattening" of shape incompatibility. Each of these mechanisms relaxes the cost of frustration, but exhibit vastly different dependencies on the size, shape, thermodynamic properties of the assembly. I describe our current understanding of the competition between these distinct morphological responses and its implications for the broader phase diagrams of GFAs. |
Monday, March 5, 2018 9:48AM - 10:24AM |
A58.00004: Slimming down through frustration Invited Speaker: Martin Lenz Controlling the self-assembly of supramolecular structures is vital for living cells, and a central challenge for engineering at the nano- and microscales. Nevertheless, even particles without optimized shapes can robustly form well-defined morphologies. This is the case in numerous medical conditions where normally soluble proteins aggregate into fibers. Beyond the diversity of molecular mechanisms involved, we propose that fibers generically arise from the aggregation of irregular particles with short-range interactions. Using minimal models of frustrated aggregating particles, we demonstrate robust fiber formation for a variety of particle shapes and aggregation conditions. Geometrical frustration plays a crucial role in this process, and accounts for the range of parameters in which fibers form as well as for their metastable, yet long-lived character. |
Monday, March 5, 2018 10:24AM - 11:00AM |
A58.00005: Cumulative geometric frustration: From bent-core liquid crystals to spherulites of twisted molecular crystals Invited Speaker: Efi Efrati Unlike Lego bricks that perfectly assemble next to one another, in molecular assemblies some misfit is almost always present. The molecular constituents thus must distort in order to form an aggregate, resulting in a frustrated assembly. The generation of geometric frustration from the intrinsic geometry of the constituents of a material is not only natural and ubiquitous but also leads to a striking variety of morphologies of ground states and exotic response properties. In this talk I will review the notion of cumulative geometric frustration and discuss two distinct examples of geometrically frustrated assemblies: liquid crystals in 2D, and twisted molecular crystals that form banded spherulites. For liquid crystal we will present how to quantify the frustration and give specific examples that exhibit super-extensive elastic energy. Motivated by the twisted crystals observed for a wide variety of organic molecular crystals studied by the Kahr group in NYU, we study a model of frustrated assembly that in particular conveys the nano-metric pitch length of the constituents to the tens of microns pitch length observed for the twisted crystalline assemblies. |
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