On the "Tertiary Structure" of Poly‐Carbenes; Self‐Assembly of sp3‐Carbon‐Based Polymers into Liquid‐Crystalline Aggregates. Issue 35 (12th July 2013)

• Record Type: Journal Article
• 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
• Is Part Of: Chemistry. Volume 19:Issue 35(2013)
• Page Start: 11577
• Page End: 11589
• Publication Date: 2013-07-12
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201301403
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The self‐assembly of poly(ethylidene acetate) (<italic>st</italic>‐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<sup>3</sup>‐carbon backbone polymers. Although the dense packing of polar ester functionalities along the carbon backbone of <italic>st</italic>‐PEA could perhaps be expected to lead directly to rigid‐rod behavior, molecular modeling reveals that individual <italic>st</italic>‐PEA chains are actually highly flexible and should not reveal rigid‐rod induced LC behavior. Nonetheless, <italic>st</italic>‐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 <italic>st</italic>‐PEA into higher‐order aggregates. According to MM/MD modeling <italic>st</italic>‐PEA single helices adopt a flexible helical structure with a preferred <italic>trans</italic>‐<italic>gauche syn</italic>‐<italic>syn</italic>‐<italic>anti</italic>‐<italic>anti</italic> 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 <italic>st</italic>‐PEA into higher‐order structures, and suggest the formation of rod‐like particles. The dimensions derived from these light‐scattering experiments correspond with <italic>st</italic>‐PEA triple‐helix formation. Langmuir–Blodgett surface pressure–area isotherms also point to the formation of rod‐like <italic>st‐</italic>PEA aggregates with similar dimensions as <italic>st‐</italic>PEA triple helixes. Upon increasing the <italic>st‐</italic>PEA concentration, the viscosity of the polymer solution increases strongly, and at concentrations above 20 wt % <italic>st‐</italic>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 <italic>st</italic>‐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 <italic>st‐</italic>PEA triple helices are also present in the solid state, up to temperatures well above the melting point of <italic>st‐</italic>PEA. Formation of higher‐order aggregates explains the observed LC behavior of <italic>st‐</italic>PEA, emphasizing the importance of the "tertiary structure" of synthetic polymers on their material properties.</p> </abstract>