A liquid crystal cocktail of rods and plates yields strong nanocomposites

Published in Chemistry
A liquid crystal cocktail of rods and plates yields strong nanocomposites
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The holy grail of nanocomposite manufacturing involves a simple mixing of polymer + nanofiller in a benign solvent, followed by a processing and drying step. The findings reported in our paper [https://doi.org/10.1038/s41467-020-14618-0] offer new design rules for fabricating reinforced polymer-GO nanocomposites. Our results unambiguously demonstrate that the molecular structure of the polymer is paramount; the macromolecular structure must be just right to form a strong interface with nanoparticles [1,2]. Obtaining well-dispersed nanoparticles is a whole different challenge in and by itself [1]. Sonication techniques for dispersing nanoparticles such as carbon nanotubes (CNTs) [1] are energy intensive methods and challenging to scale-up.

Strong nanocomposites came one step closer to reality when the first results on GO-polymer nanocomposites were reported [3]. And, the ability of these oxidized graphene plates to form nematic discotic liquid crystalline phases in water opened up new processing possibilities [4,5]. We conjectured that the water-soluble, sulfonated rigid rod polyamide, poly(2,2’-disulfonyl-4,4’-benzidine terepthalamide) or PBDT has potential as a matrix polymer for preparing strong GO-based nanocomposites. PBDT forms double helices in water with an unusually high aspect ratio (L/D ~ 260) and forms nematic aqueous solutions at concentrations as low as 2.0 wt.% [6]. If GO plates could be intercalated between rigid-rod polymers by simply mixing nematic phases of PBDT rods and GO might strong nanophases result?

 Conventional wisdom suggested that homogeneous nematic mixtures of rods and plates are not compatible and tend to phase separate [7]. Much to our surprise, the two nematics form a stable mesophase when mixed together using a magnetic stirrer only. The result is the first example of a stable rod+plate hybrid biaxial nematic; locally the directors of the rods and plates are mutually orthogonal. Drying the biaxial rod+plate cocktail yields nanocomposite films which exhibit mechanical properties that far surpass results published in literature[3]. The biaxial nematic phase appears to be prerequisite for making strong nanocomposites; composite films from isotropic or biphasic mixtures did not yield strong nanocomposites. Stable, homogenous dispersions of graphene oxide were only obtained when the mixture was.a  hybrid biaxial nematic. Nanocompositie films derived from biaxial nematic mixtures exhibit exceptional mechanical properties. 



References: 

[1] Hegde et al, Macromolecules46, 1492-1503 (2013).

[2] Hegde et al, Polymer55, 3746-3757 (2014).

[3] Papageorgiou et al, Prog. Mater. Sci20, 75-127 (2017).

[4] Dan et al, Soft Matter7, 11154-11159 (2011).

[5] Kim et al, Angew. Chem. Int. Ed.50, 3043-3047 (2011).

[6] Wang et al, Nat. Commun.10, 801:1-8 (2019).

[7] van der Kooij et al, Langmuir 16, 10144-10149 (2000).  

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