Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R33: Chirality in Polymers and Soft Matter: From Molecular to Hierarchical ScalesFocus
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Sponsoring Units: DPOLY DSOFT DBIO Chair: Gerd Schroeder-Turk, Murdoch Univ Room: 505 |
Thursday, March 5, 2020 8:00AM - 8:36AM |
R33.00001: Spontaneous Appearance of Chiral Structures from Lyotropic Liquid Crystals in Confinement Invited Speaker: Mohan Srinivasarao Optical activity, a consequence of reflection-symmetry breaking, since the time of its discovery by Biot in the early 1800s, has captivated the imagination of scientists. Since Pasteur’s time, the appearance of macroscopic chirality during crystallization from both chiral and achiral molecules has frequently been observed. Crystallization apart, optical activity resulting from achiral units has been observed in liquid crystals, and polymeric materials, without a molecular chiral center. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R33.00002: Threading the Spindle: A Geometric Study of Chiral Liquid Crystal Polymer Microparticles Helen Ansell, Dae Seok Kim, Randall D Kamien, Eleni Katifori, Teresa Lopez-Leon Polymeric particles are strong candidates for designing artificial materials capable of emulating the complex twisting-based functionality observed in biological systems. In this study we investigate the swelling behavior of chiral twisted spindle-shaped polymer microparticles, which are formed due to the anisotropic contraction of spherical bipolar polymer liquid crystalline microparticles during deswelling. We observe a relationship between the aspect ratio of the spindle shape and the corresponding twist angle of the polymer at the surface. We find that there are two regions in the deswelling process: shrinking without twisting followed by shrinking due to twisting. We propose a geometric model to interpret these two behaviors as well as describing the observed spiral patterns on the surface. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R33.00003: Chiral nematic liquid crystals in cylinders: Layering transition and conservation of layer structure Jonghee Eun, Sung-Jo Kim, Joonwoo Jeong Chiral nematic lyotropic chromonic liquid crystals (LCLCs) confined in a cylindrical cavity exhibit layering transition and intriguing topological defects. With a planar degenerate anchoring at the cavity wall, the chiral LCLCs have the double-twist (DT) director configuration. To our interest, as we increase the chiral dopant concentration, the degree of twist in the DT configuration increases discontinuously. This "step-like" layering transition occurs because there exist local minima in the elastic free energy landscape; depending on the dopant concentration, one of the minima becomes the ground state. Consequently, meta-stable domains with different twist angles can coexist with dislocation-like defects between them. Lastly, we learn that the meta-stable domains are topologically protected because of the layer number conservation. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R33.00004: Truth about the origin of twist - a circular argument Elisabetta Matsumoto, Alireza Dastan, Doug J Cleaver We examine emergent twist by applying both simulation and molecular theory to chromonic amphiphiles. Specifically, we study intrisically achiral particles: amphiphilic discotics in a solvent of spheres. While these systems exhibit a veritable zoo of hierarchical, chiral self-assembled structures, we restrict ourselves here to a set of parameterizations that lead to twisted bilayer structures. Here, we return to the elephant in the room – what causes and controls the supramolecular twist that develops in these systems? In considering this, we demonstrate that, while chirality is manifested at several levels in the twisted bilayers formed by amphiphilic chromonics, it originates from spontaneously-chiral preferred packing at a molecular level. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R33.00005: Thermoresponsive Colloidal Chains Collapse to Form Helices Bipul Biswas, KP Fayis, Suresh Bhat, Guruswamy Kumaraswamy Colloids linked to form linear chains represent model systems for polymers. Chains are prepared using fluorescent micron sized colloidal polystyrene beads that are coated with 100 nm thermoresponsive PNIPAM microgels. Microgel-coated PS beads are lined up in an AC electric field and are crosslinked through amine groups in the microgels, to form thermoresponsive colloidal chains. Here, we demonstrate that these chains exhibit thermally-induced reversible transitions. Interestingly, we show that chains characterized by intermediate flexibility spontaneously organize into helices on heating. In contrast, stiffer chains exhibit a modest decrease in size, without a qualitative change in shape. More flexible chains exhibit a dramatic decrease in size but do not form an ordered helical structure. Thus, in chains where there is no directional specificity to the interactions between monomeric beads, we obtain helical structures when chains with intermediate flexibility collapse. Our work suggests that a purely mechanical instability for semiflexible filaments can drive helix formation, without the need to invoke any directional interactions. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R33.00006: Skyrmion formation and organization on a shell Viviana Palacio-Betancur, Guillaume Durey, Alexander Cohen, Monirosadat Sadati, Teresa López León, Juan P. Hernandez-Ortiz, Juan De Pablo Blue phases (BPs) arise spontaneously in chiral liquid crystals (ChLCs) as the result of minimizing the global free energy, by forming networks of defects with specific cubic symmetry. Confining cuboidal phases to a channel smaller than the molecular pitch results in frustrating the full development of a BP cell, creating skyrmions that pack with hexagonal order. Beyond confinement in a channel, here, we consider the effect of curvature by confining a ChLC in spherical shells. The equilibrium configurations are obtained following a theoretically-informed Monte Carlo relaxation and a Ginzburg-Landau relaxation of the free energy functional, described within the Landau-de Gennes formalism, and solved numerically through finite element discretization. By considering homeotropic and planar anchoring, we stabilize half and full skyrmions. When stabilized in a shell, the hexagonal packing becomes imperfect, to accommodate for the closed surface, resulting in the creation of 5-7 pairs of skyrmions on the surface of the shell. The control over the formation and packing of the skyrmions offers opportunities for pattern design and engineering 2D ordered nanoparticle assemblies. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R33.00007: Peculiar Phase Morphologies from Twisting of Self-Assembled Ribbons in Chiral Block Copolymers Kai-Chieh Yang, Rong-Ming Ho The chirality effects on BCP self-assembly gives rise to the formation of helical phase (H*) from the self-assembly of polylactide-based chiral block copolymers (BCPs*) with achiral segment-rich fraction. The formation of the helical cylinder is attributed to the packing of helical chains with which twisted cylinder microdomain can be formed from twisting and shifting. By contrast, twisted ribbon with a bilayer-like lamellar texture can be formed in self-assembled polylactide-based BCPs* with chiral segment-rich fraction from twisting and bending. Those twisted ribbons can be further developed to give peculiar phase morphologies in bulk through scrolling, giving a roll cake and concentric lamellar textures. We speculate that those helical and twisted textures are built from the chiral interface of chiral and achiral segments as proposed by the tilted chiral lipid bilayer (TCLB) theory and also the mesochiral assembly of the helical chains predicted by the orientational self-consistent field theory (oSCFT). With the homochiral evolution from helical conformation to hierarchical superstructure via inter-chain chiral interactions, it is appealing to control the handedness of the forming morphologies from BCPs*. |
Thursday, March 5, 2020 9:48AM - 10:24AM |
R33.00008: Folding of chiral colloidal membranes Invited Speaker: Prerna Sharma Rod-like viruses self-assemble into one rod-length thick liquid-like monolayers in presence of depletion attraction induced by non-adsorbing polymers. The global constraint of 2-D geometry of these colloidal membranes is incompatible with local chiral interactions of the rods. Consequently, competition between these opposing tendencies leads to a variety of polymorphic transitions. We illustrate this through phenomena that cause flat colloidal membranes to transform into 3-D structures. Crystallization of colloidal membranes proceeds through the usual nucleation and growth pathway. However, the growing crystalline domains spontaneously wrinkle to satisfy local chiral interactions and on completion of the growth lead to macroscopic buckling of the membrane. In contrast, fluid colloidal membranes can be folded by simply doping them with shorter rods of same chirality. These uniformly mixed bi-disperse membranes become unstable and deform into saddle shapes. The origin of this instability lies in enhancement of Gaussian modulus of the monolayer membranes due to doping. At high doping ratio, membranes fold into higher order structures of increasing complexity. These include unduloids, tri-unduloids, unduloid-enneper surfaces as well as system spanning plumber’s nightmare-like phases. Taken together, these results show how chirality may have similar implications for conventional lipid bilayers which are rich in chiral molecules like cholesterol. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R33.00009: Self-assembly of chiral networks in achiral block copolymer systems using coarse-grained simulations Natalie Buchanan, Krysia Browka, Lianna Ketcham, Hillary Le, Poornima Padmanabhan Ordered network materials, such as the double gyroid, are comprised of two interwoven networks embedded in a matrix and can be formed via the self-assembly of block copolymers. By selecting the interaction parameters that promote elongated conformations, an alternating gyroid morphology can be formed, thereby breaking network symmetry and resulting in a structurally chiral morphology. In this talk, we investigate the self-assembly of model achiral block copolymers into alternating gyroid morphologies using coarse-grained molecular simulation. We show that effective chirality can further be tuned by co-assembling with a suitable species to grow one network and shrink the other. Insights into stability are obtained by measuring polymer end-to-end distances and network topology. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R33.00010: Amino Acids as RNA-Folding Chaperones: Single Molecule Experiments Reveal Chiral Sensitivity David Nicholson, David John Nesbitt Early RNA-based life may have relied on amino acids to promote correct folding in RNA molecules, i.e. to act as RNA-folding chaperones. Chiral specificity in this interaction could have driven evolutionary mechanisms to favor a single chirality of amino acid, which may explain the origin of amino acid homochirality. To probe how chirality determines an amino acid’s influence on RNA folding dynamics, we have performed single molecule spectroscopy on a ubiquitous RNA tertiary motif, the tetraloop-tetraloop receptor (TL-TLR). Our kinetic measurements show that TL-TLR folding is sensitive to many amino acids, but only one amino acid, arginine, is observed to have a chirally-specific effect. The kinetic and thermodynamic features of this chiral interaction are presented, and with the help of all-atom molecular dynamics simulations, we propose a mechanistic interpretation of these results based on nonspecific arginine-RNA interactions. |
Thursday, March 5, 2020 10:48AM - 11:00AM |
R33.00011: Insensitivity of Sterically-Defined Helical Chain Conformation to Solvent Quality in Dilute Solution Beihang Yu, Scott Danielsen, Kai-Chieh Yang, Rong-Ming Ho, Lynn Walker, Rachel A Segalman The interplay between polymer–solvent interaction and interactions that impose secondary structures determines polymer chain conformation in dilute solution. Polypeptoids–poly(N-substituted glycines) can form helical secondary structures primarily driven by steric interactions from chiral, bulky side chains, while a racemic mixture of the same side chains results in unstructured coil chains. Small-angle neutron scattering (SANS) reveals that the helical polypeptoids are locally stiffer but overall flexible. However, the radii of gyration (Rgs) of both helical and coil polypeptoids do not increase with improved solvent quality (A2), and the chain expansion deviates from the universal swelling of polymer chains in dilute solution. Potential effects from solvents disrupting the steric interactions of chiral side chains are excluded by comparing to another chemically analogous coil polypeptoid devoid of side chain chirality. The results indicate that chain conformation change of these polypeptoids cannot be simply captured by excluded volume interactions, nor the steric interactions imposing the helical secondary structure. It is likely that the specific interactions between polypeptoid segments dominate the chain shape as opposed to polypeptoid–solvent interactions. |
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