Structure of the B4 Liquid Crystal Phase near a Glass Surface

Abstract

The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness.

Helical nanofilaments of bent-core liquid crystals with a second twist

“…Therefore, we conclude that even the first layer is made of helical filaments. This is very different from prior FFTEM studies on the n ¼ 7 homologue near a flat and rigid glass substrate that revealed focal conic domains formed under the influence of the surface forces near the substrate 17,22 . Our observations of only slightly distorted nanofilaments suggest that the carbon substrate preserves the structural features pertinent to the bulk structure of the HNF phase, despite the relatively small thickness of the samples (40-120 nm).…”

“…Interestingly, Chen et al 17 have already observed that at the surfaces, where parabolic domain walls form, the HNFs axes are tilted by gB38°with respect to a periodic one-dimensional modulation. However, this rotation was not explained, and no subsequent rotations of the filaments with respect to each other was assumed 22 .…”

“…Solar cells require thin films in the range of hundred nanometres, so it is desirable to know the structure of the HNF phase in such thin films. Recent FFTEM studies, for example, showed the formation of focal conic structures instead of the helical nanofilaments near a glass substrate 17 , which indicates that the structure near a surface can be different from the bulk structure. While FFTEM monitors the profile of a broken surface of a bulk (over micrometre thick) material, cryo-transmission electron microscopy (TEM) gives information about the inside of a thin (LB100 nm film) material; therefore, it can provide complementary information of the structures.…”

Charge Generation Measured for Fullerene–Helical Nanofilament Liquid Crystal Heterojunctions

“…This is similar to the structures found at a glass interface from a broken planchet sample in work by Chen et al 10 Because helical nanofilaments cannot make full contact with the surface, instead of forming helical nanofilaments, P-9-OPIMB form some typical lamellar surface structure. These regions are usually large.…”

“…Specifically the repeating, twisted texture seen in Figure 2A has previously been identified as characteristic of the HNF phase. 1,10,25 The topology of the as-spun sample on the surface of the quartz is shown in Figure 2A. The P-9-OPIMB is dissolved in chloroform and spin-coated on the surface of the quartz.…”