Self-assembly

Block molecules separate

ACS Nano http://doi.org/b5m6 (2017)

Liquid crystals and block co-polymers are characterized by a morphological order that results from interactions at the molecular level. Whereas in block co-polymers the incompatibility between the blocks causes a phase separation that minimizes the interfacial energy, in liquid crystals intermolecular interactions are mediated by molecular orientation. Now, Berrocal et al. have studied an intermediate regime in which molecules composed of incompatible blocks of naphthalenediimides (hard block) and oligodimethylsiloxanes (soft block) assemble in defined morphologies.

The researchers study two different families of compounds. In the first one, either one or two naphthalenediimide groups are end-capped by dimethylsiloxanes of various lengths. In the second one, the naphthalenediimide groups terminate the dimethylsiloxane oligomer. The first family self-assembly behaviour is driven by the π–π stacking of the hard moiety forming columnar structures in which the intermediate volume is occupied by the oligomeric chains. The second family, instead, self-assembles into a lamellar morphology with no evidence of intermolecular interaction between the hard blocks. Remarkably, in both cases, these features are under 10 nm.

Using a number of characterization techniques and theoretical studies, Berrocal et al. show that block-molecule morphologies are not the result of interfacial energy minimization, as in the case of block co-polymers. Rather, they result from the interplay between the tendency of the hard block to crystallize and the thermodynamic incompatibility between the hard and the soft blocks, which, in turn, strongly depends on the orientation of the intermolecular interactions, a feature shared with liquid crystals.

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Moscatelli, A. Block molecules separate. Nature Nanotech (2017). https://doi.org/10.1038/nnano.2017.93

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