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.

Stiff and hierarchical chain nature of atactic and stereoregular poly(substituted methylene)s

“…The helical stereocomplex formation of isotactic and syndiotactic PMMA in solution gives gelation . The stereoregular PM also forms triple helices in solvent that induces gelation of the PM solution …”

“…S5). Previously, it was reported that the main chain of stereoregular PPMO shows a helical conformation in lyotropic LC states in which poly(ethylidene acetate) exhibits the same distinctive peak at ca 1.0 nm on its SAXS profile originating from single helices in the multi‐helical structure . From this experimental result, it seems that the observed peak in our scattering profile corresponds to the helical pitch of the stereoregular PPMO main chain.…”

“…From this experimental result, it seems that the observed peak in our scattering profile corresponds to the helical pitch of the stereoregular PPMO main chain. Furthermore, in the lyotropic LC state, the helices of the stereoregular PPMO form a multi‐helical supramolecular assembly, which is stabilized solely by interchain van der Waals interactions in the condensed state . The formation of a supramolecular structure under Θ conditions, such as the lyotropic state, might lead to the emergence of a peak at 4.5 nm −1 (i.e.…”

“…The helical conformation of the polymer backbone of stereoregular PMs, which depends on the side chain structure, leads to the emergence of LC phases in main‐chain LC polymers. Furthermore, these helices can form higher‐order stereo‐complexes, such as multi‐helices, in their lyotropic LC state, which leads to gelation of the PM solution . Since the dense side chains in PMs relative to other polymers impart an abnormal rigidity on the PM backbone, it is expected that PMs will behave as rods, even in an unperturbed state.…”

Columnar Phase-Forming Polymers

“…Copyright (1989 American Chemical Society) unusual observation. C2 shows a nematic phase, and it was suggested that triplehelix aggregations are behaving as rigid-rod in the nematic phase (Franssen et al 2013).…”