TY - JOUR
T1 - On the "tertiary structure" of poly-carbenes; Self-assembly of sp3-carbon-based polymers into liquid-crystalline aggregates
AU - Franssen, Nicole M.G.
AU - Ensing, Bernd
AU - Hegde, Maruti
AU - Dingemans, Theo J.
AU - Norder, Ben
AU - Picken, Stephen J.
AU - Alberda Van Ekenstein, Gert O.R.
AU - Van Eck, Ernst R.H.
AU - Elemans, Johannes A.A.W.
AU - Vis, Mark
AU - Reek, Joost N.H.
AU - de Bruin, Bas
PY - 2013/8/26
Y1 - 2013/8/26
N2 - 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. Coming around again: The self-assembly of "polycarbenes" into van der Waals stabilized liquid-crystalline (LC) aggregates is described. The LC behavior of these materials is unexpected for flexible sp3-carbon backbone polymers. The experimental measurements, supported by molecular mechanics-based molecular dynamic studies, suggest that the LC behavior is due to self-aggregation of st-PEA into triple-helix aggregates (st-PEA=syndiotactic poly(ethylidene acetate).
AB - 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. Coming around again: The self-assembly of "polycarbenes" into van der Waals stabilized liquid-crystalline (LC) aggregates is described. The LC behavior of these materials is unexpected for flexible sp3-carbon backbone polymers. The experimental measurements, supported by molecular mechanics-based molecular dynamic studies, suggest that the LC behavior is due to self-aggregation of st-PEA into triple-helix aggregates (st-PEA=syndiotactic poly(ethylidene acetate).
KW - liquid crystals
KW - polymers
KW - sp backbone
KW - supramolecular aggregates
KW - triple helix
UR - http://www.scopus.com/inward/record.url?scp=84882904602&partnerID=8YFLogxK
U2 - 10.1002/chem.201301403
DO - 10.1002/chem.201301403
M3 - Article
C2 - 23852805
AN - SCOPUS:84882904602
VL - 19
SP - 11577
EP - 11589
JO - Chemistry : A European Journal
JF - Chemistry : A European Journal
SN - 0947-6539
IS - 35
ER -