title: On the "tertiary structure" of poly-carbenes; self-assembly of sp3-carbon-based polymers into liquid-crystalline aggregates.
abstract: The self-assembly of poly(ethylidene acetate) (st-PEA) into van der Waals-stabilized liquid-crystalline (LC) aggregates is reported. The LC behavior of these materials is unexpected, and unusual for flexible sp(3)-carbon backbone polymers. Although the dense packing of polar ester functionalities along the carbon backbone of st-PEA could perhaps be expected to lead directly to rigid-rod behavior, molecular modeling reveals that individual st-PEA chains are actually highly flexible and should not reveal rigid-rod induced LC behavior. Nonetheless, st-PEA clearly reveals LC behavior, both in solution and in the melt over a broad elevated temperature range. A combined set of experimental measurements, supported by MM/MD studies, suggests that the observed LC behavior is due to self-aggregation of st-PEA into higher-order aggregates. According to MM/MD modeling st-PEA single helices adopt a flexible helical structure with a preferred trans-gauche syn-syn-anti-anti orientation. Unexpectedly, similar modeling experiments suggest that three of these helices can self-assemble into triple-helical aggregates. Higher-order assemblies were not observed in the MM/MD simulations, suggesting that the triple helix is the most stable aggregate configuration. DLS data confirmed the aggregation of st-PEA into higher-order structures, and suggest the formation of rod-like particles. The dimensions derived from these light-scattering experiments correspond with st-PEA triple-helix formation. Langmuir-Blodgett surface pressure-area isotherms also point to the formation of rod-like st-PEA aggregates with similar dimensions as st-PEA triple helixes. Upon increasing the st-PEA concentration, the viscosity of the polymer solution increases strongly, and at concentrations above 20 wt % st-PEA forms an organogel. STM on this gel reveals the formation of helical aggregates on the graphite surface-solution interface with shapes and dimensions matching st-PEA triple helices, in good agreement with the structures proposed by molecular modeling. X-ray diffraction, WAXS, SAXS and solid state NMR spectroscopy studies suggest that st-PEA triple helices are also present in the solid state, up to temperatures well above the melting point of st-PEA. Formation of higher-order aggregates explains the observed LC behavior of st-PEA, emphasizing the importance of the "tertiary structure" of synthetic polymers on their material properties.
authors.affiliation: Van 't Hoff Institute for Molecular Sciences (HIMS), Universiteit van Amsterdam, P. O. Box 94720, 1090 GS Amsterdam, The Netherlands.
authors.forename: Nicole M G
authors.initials: NM
authors.lastname: Franssen
authors.forename: Bernd
authors.initials: B
authors.lastname: Ensing
authors.forename: Maruti
authors.initials: M
authors.lastname: Hegde
authors.forename: Theo J
authors.initials: TJ
authors.lastname: Dingemans
authors.forename: Ben
authors.initials: B
authors.lastname: Norder
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Picken
authors.forename: Gert O R
authors.initials: GO
authors.lastname: Alberda van Ekenstein
authors.forename: Ernst R H
authors.initials: ER
authors.lastname: van Eck
authors.forename: Johannes A A W
authors.initials: JA
authors.lastname: Elemans
authors.forename: Mark
authors.initials: M
authors.lastname: Vis
authors.forename: Joost N H
authors.initials: JN
authors.lastname: Reek
authors.forename: Bas
authors.initials: B
authors.lastname: de Bruin
mesh_terms: D002244:Carbon / Q000737:chemistry*; D050866:Liquid Crystals; D046911:Macromolecular Substances / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D011108:Polymers / Q000737:chemistry*
chemical_list: D046911:Macromolecular Substances; D011108:Polymers; D002244:Carbon
keywords: liquid crystals; polymers; sp3 backbone; supramolecular aggregates; triple helix