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- Title
On the 'Tertiary Structure' of Poly-Carbenes; Self-Assembly of sp<sup>3</sup>-Carbon-Based Polymers into Liquid-Crystalline Aggregates.
- Authors
Franssen, Nicole M. G.; Ensing, Bernd; Hegde, Maruti; Dingemans, Theo J.; Norder, Ben; Picken, Stephen J.; Alberda van Ekenstein, Gert O. R.; van Eck, Ernst R. H.; Elemans, Johannes A. A. W.; Vis, Mark; Reek, Joost N. H.; de Bruin, Bas
- 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 sp3-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.
- Subjects
TRIPLE helix structure (Molecules); MOLECULAR self-assembly; POLYMERS; LIQUID crystals; MOLECULAR models; NUCLEAR magnetic resonance spectroscopy
- Publication
Chemistry - A European Journal, 2013, Vol 19, Issue 35, p11577
- ISSN
0947-6539
- Publication type
Article
- DOI
10.1002/chem.201301403