Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A50: Chirality in Polymers and Soft Matter I: From Molecular to Hierarchical ScalesFocus
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Sponsoring Units: DPOLY GSOFT DBIO Chair: Nicholas Kotov, University of Michigan Room: BCEC 252B |
Monday, March 4, 2019 8:00AM - 8:12AM |
A50.00001: Confinement-induced liquid crystalline transitions in amyloid fibril tactoids Gustav Nystrom, Mario Arcari, Raffaele Mezzenga Chirality is ubiquitous in nature and plays crucial roles in biology, medicine, physics and materials science. Understanding and controlling chirality is therefore an important research challenge with broad implications. Unlike other chiral colloids, such as nanocellulose or filamentous viruses, amyloid fibrils form nematic phases but appear to miss their twisted form, the cholesteric or chiral nematic phases, despite a well-defined chirality at the single fibril level. In this contribution, I will report about our discovery of cholesteric phases in amyloids, using β-lactoglobulin fibrils shortened by shear stresses. The physical behavior of these new cholesteric materials exhibits a large structural complexity, with confinement-driven ordering transitions between at least three types of nematic and cholesteric tactoids. We use energy functional theory to rationalize these results and observe a chirality inversion from the left-handed amyloids to right-handed cholesteric droplets. These findings contribute to our understanding of cholesteric phases as well as their use in soft nanotechnology, nanomaterial templating and self-assembly. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A50.00002: Use of Coiled-Coil Domains to Direct the Self-Assembly of Protein-Polymer Conjugates Justin Paloni, Bradley David Olsen Protein-polymer conjugates have been demonstrated to greatly enhance protein activity and sensitivity in catalytic and biosensing applications. To achieve this enhanced biosensor sensitivity, however, it is critical that the conjugates self-assemble into well-defined nanostructures. In this study, strongly associating coiled-coil domains are added to the protein block of protein-polymer conjugates to promote self-assembly. One of the two constituent alpha helices of a heterodimeric coiled-coil is appended to either the N- or C-terminus of a binding protein previously demonstrated to exhibit weak ordering. Small-angle X-ray scattering (SAXS) measurements indicate that homogeneous solutions of the conjugates containing a single coiled-coil domain generally self-assemble as weakly as the unmodified binding protein. When one of these conjugates is combined in a molar ratio with another conjugate containing the other alpha helix in the coiled-coil heterodimer, though, well-defined nanostructures are formed with SAXS peak full width at half maximum (FWHM) values 2- to 4-fold smaller than those for the bare binding protein. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A50.00003: Phase field collagen fibrils: modelling both axial and radial structure of collagen fibrils Samuel Cameron, Andrew Rutenberg, Laurent Kreplak Collagen fibrils are rope-like biomaterials that provide structural integrity to tendons, skin, and other tissues in the human body. These fibrils self-assemble from aggregates of collagen molecules. Decades of experimental research have uncovered many properties of collagen fibrils, from the ubiquitous "d-band" (periodic modulations of density along the cylindrical axis of the fibril) to the twisted orientation of molecules visible at the fibril surface. While models exist of either the axial d-band or of radial double-twist structure, reconciling the radial and axial structure has only been done qualitatively with a projective coupling hypothesis. In this talk, I present a coarse-grained model of collagen fibrils that combines liquid crystal theory and phase-field-crystal techniques, and uses simple equilibrium arguments to predict both the collagen fibril d-band spacing and the radial double-twist describing the average molecular orientation. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A50.00004: Twisted by fate, or screwing up purpose: self-limiting fibers from achiral vs. chiral filament assembly Gregory Grason Twisted bundles of cohesive filaments or chains are common architectures in a range of supramolecular materials, from protein filaments to self-stacking, organogel fibers. Several theoretical models have shown that assembly twist can lead to a thermodynamic self-limitation of bundles due to the build-up of the costs of geometric frustration with lateral diameter. Common to all these models is the assumption of intrinsic chirality of the constituent filaments as the driving force for collective twist. However, recent experimental and simulation results point to the possibility of self-limitation by the spontaneous twisting of achiral fibers. In this talk, I describe a generic continuum elastic framework for comparing formation of twisted fibers of chiral vs. achiral filament bundles of various states of order (e.g. columnar, solid, polymer nematic). I will contrast predictions for the evolution of pitch with lateral size as well as possible ranges of self-limitation. This analysis argues that self-limited fibers of chiral bundles may reach mesoscopic (multi-filament) dimensions, while self-limitation of spontaneously twisting fibers (achiral filaments) should be expected only up to diameter of a few filaments wide. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A50.00005: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 9:00AM - 9:12AM |
A50.00006: Mechanical basis for the morphology of fibrillar aggregates Thomas Michaels, L Mahadevan The self-assembly of fibrillar aggregates is of importance in biology, biomedicine and materials science, yet understanding the range of possible shapes for these structures remains an open question. We propose a coarse-grained approach that averages over specific molecular details to suggest that the spatial complexity of self-assembling fibrillar structures is due to the competing effects of (the bending and twisting) elasticity of individual filaments and the adhesive interactions between them. We show that a theoretical framework accounting for this allows us to capture a number of diverse fibril morphologies observed in natural and synthetic systems, ranging from Filopodia to multi-walled carbon nanotubes, and leads to a phase diagram of possible fibril shapes. We also show how the extreme sensitivity of these morphologies can lead to spatially chaotic structures. Together, these results suggest a common mechanical basis for the micronscale fibril morphology as a function of the nanoscale mechanical properties. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A50.00007: Phase Behavior of Frustrated ABC2 Miktoarm Star Triblock Copolymer Qi Zhang, Wei-hua Li The phase behavior of ABC triblock copolymer can be classified into “nonfrustrated” and “frustrated” cases according to the relative strength of the three Flory-Huggins interaction parameters. In the frustrated case, χACN is much weaker than χABN and χBCN. Thus, the triblock copolymers prefer to form structures with A/C interfaces. Under the competition of interfacial energy and the entropic energy, the phase behavior of ABC triblock copolymer is more complex, and it can form the phases of the Knitting pattern, double and triple helices on cylinders. However, stable single helical structure has not been reported yet. In order to get the stable single helical structure, we designed the molecule structure of ABC2 miktoarm star triblock copolymer. Our design principle sheds light on the phase behavior of the frustrated case as well as the spontaneous curvature induced by the conformational asymmetry. Through the calculation of Self-Consistent Field Theory (SCFT), the triangular phase diagram shows the region of stable single helical structure as well as the knitting pattern phases. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A50.00008: Amplification of vibrational circular dichroism in chiral block copolymers driven by self-assembly Kai-Chieh Yang, Rong-Ming Ho Herein, various self-assembled phases including double gyroid (DG), double diamond (DD) and helical (H*) phases could be obtained from the self-assembly of chiral block copolymers with equivalent volume fraction of chiral segment, giving building blocks with equal contribution of the optical activities for self-assembly. Interestingly, with comparable chiral segment fraction as evidenced by electronic circular dichroism (ECD), significant enhancement of vibrational circular dichroism (VCD) signals could be found in the chiral H* phase while the VCD signals in the achiral phases of DD and DG remain after microphase separation for ordering, reflecting that the VCD enhancement is attributed to the formation of hierarchical helices with preferential handedness; the observed behaviors are represented as VCD enhancement by self-assembly. This finding provides a new concept to boost optical activities in the applications of chiroptic from self-assembly. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A50.00009: Effects of Polymer Helical Chain Shape on Block Copolymer Self-Assembly Beihang Yu, Scott Danielsen, Anastasia Patterson, Emily C Davidson, Glenn Fredrickson, Rachel Segalman While helical chain shapes in block copolymers have been shown to produce unique morphologies, the details of how chain shape influences the thermodynamics of self-assembly are unclear. Here, we utilize model coil–coil and coil–helix block copolymers based on polypeptoids, for which the chain shape can be tuned from helix to coil via monomer chirality with constant chemistry. This model block copolymer system is used to probe the effects of chain helicity on block copolymer self-assembly. With identical domain spacings in the lamellar morphology, the coil–helix block copolymer has a lower order–disorder transition temperature (TODT) than its coil–coil analogue. There is minimal difference in the enthalpic contribution to mixing. The most significant contribution in lowering the TODT of the coil–helix block copolymer is the helical chain experiencing larger chain stretching penalties in the lamellar morphology, which leads to a larger entropic gain upon disordering. This yields insight into the importance of space filling and chain stretching of polymer chains in block copolymer self-assembly. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A50.00010: Effect of chain flexibility on methylcellulose fibril formation Svetlana Morozova, Peter Schmidt, S. Piril Ertem, Theresa M. Reineke, Frank Bates, Timothy Lodge Methyl cellulose (MC) is a water soluble cellulose derivative, which upon heating spontaneously forms ~ 15 nm diameter fibrils in aqueous solutions. The lateral size scale of these structures has been shown to be independent of the molecular weight of the MC chains, the concentration, and the temperature of formation, but could be related to MC chain flexibility if the chains are twisting together into a bundle. We have investigated the effects of the persistence length (lp) on MC fibril formation by systematically grafting 800 – 2000 g/mol poly(ethylene glycol) to the backbone. Keeping the grafting densities below 30%, the persistence length increases systematically by as much as a factor of 10 from the added excluded volume produced by the grafts. Upon heating, the resulting fibril structure has been investigated with small angle X-ray scattering, small amplitude oscillatory shear, and atomic force microscopy. At grafting densities below 10%, the fibril radius increases as ~ (lpd)1/2, where d is the average space between chains. For higher grafting densities, the fibril formation is suppressed. These results support a recently proposed MC fibril substructure, in which the chains twist together into a bundle and the diameter is set by MC chain flexibility. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A50.00011: Aligning stem orientation: confined chiral and epitaxial growth of the α-phase crystals of isotactic polypropylene Yan Cao, Xingming Zeng, Hiroshi Jinnai, Shuailin Zhang
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Monday, March 4, 2019 10:12AM - 10:24AM |
A50.00012: Chirality Enabled Liquid Crystalline Physical Gels with High Modulus but Low Driving Voltage Huan Ruan, Haiyan Peng, Xiaolin Xie Liquid crystalline physical gels, which stabilize the fluidic liquid crystals by non-covalent three-dimensional fibrous network, are not only able to maintain the unique electro-optical response capability but also capable of affording the self-supporting function. Herein, we report liquid crystalline physical gels with high modulus but low driving voltage. This behavior is enabled by chirality transfer from molecular level to three-dimensional fibrous networks during the self-assembly of 1,4-benzenedicarboxamide phenylalanine derivatives. Interestingly, the critical gel concentration is as low as 0.1 wt%. Our findings open doors to understanding and exploiting the role of chirality in organic gels. |
Monday, March 4, 2019 10:24AM - 11:00AM |
A50.00013: Universal Effects of Chirality on the Self-Assembly of Chiral Block Copolymers and Polymers Invited Speaker: Rong-Ming Ho Here, we aim to investigate the universal effects of chirality on the self-assembly of chiral block copolymers (BCPs*) and polymers. Poly(cyclohexylglycolide) (PCG)-containing BCPs* have been synthesized for self-assembly to give systematic comparisons with polylactide (PLA)-containing BCPs*. Opposite handedness of PCG helical chains in enantiomeric BCPs* were identified by the vibrational circular dichroism (VCD) results of carbonyl group (C=O) stretching due to intramolecular chiral interactions. By taking advantage of intermolecular chiral interactions as evidenced by the VCD results of C-O-C vibration, the self-assembly of the PCG-containing BCP* gave helical phase (H*) with preferential handedness as recognized by electron microscopy tomography, suggesting the chirality effect on BCP self-assembly and the homochiral evolution from molecular to hierarchical scales; the results are in line with theoretical prediction based on chiral orientational self-consistent field theory. For chiral polylactides, twisted lamellae in crystalline banded spherulite with preferred handedness could be formed, reflecting the homochiral evolution in crystallized chiral polylactides. |
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