Figure 3: Textures of HNF domains of an 8S5/NOBOW HNF network mixture.

In a c=30% 8S5 (an achiral, rod-like LC)/NOBOW mixture, NOBOW phase-separates first on cooling from the isotropic blend, forming low-birefringence B4 domains (a) in which the helical nanofilaments grow radially outwards from a central nucleus and have very low birefringence (Δn=n_z–n_⊥∼0.001). The domains are chiral and cause optical rotation, revealed by decrossing the analyser (b,c). The 8S5 is still isotropic at this temperature (the guest phase is indicated in parentheses in all images). (d) Molecular and layer organization in a single HNF with refractive indices n_z and n_⊥ defined for the polarization directions indicated. The very low observed HNF birefringence is consistent with an estimate obtained by averaging values of the principal components of the optical dielectric tensor typical for bent-core mesogens, ɛ_a≈2.20, ɛ_b≈2.38 and ɛ_c≈2.89 (ref. 69), over the continuous reorientation around the filament axis (Supplementary Discussion 2). When the 8S5 transitions from Iso to N (e–f), the birefringence of both chiral domains becomes positive (Δn∼0.012), due to the additional contribution by the nanoconfined 8S5 nematic. However, this Δn is much smaller than that which would be obtained if the nematic 8S5 were uniformly aligned (n_e–n_o∼0.12, (Supplementary Fig. 10)). Such a reduction in Δn requires a guest nematic director that is twisted in the pores, with a small net average orientation parallel to the local HNF axis. There is no observable difference between the left- and right-handed HNF domains between crossed polarizers at any temperature, indicating that they have the same Δn. The scale bar in (a) is 100 μm. (g) Transmitted white light intensity through a c=30% 8S5/NOBOW cell between crossed polarizer and analyser. Incident light passes through a 30 μm diameter spot selected to be within a spherulite and to have the HNF axes making an average angle of 45° with respect to the polarizer axis. (h) Resulting measured birefringence, Δn. Moving to other positions in either L or R domains with polarizer and analyser crossed produces minor variations in the intensity curves due to the textures being slightly different. When 8S5, which is achiral, transitions from Iso to N, the birefringence of both chiral domains changes from negative to positive, giving a minimum in the transmitted intensity at around 78 °C. This change of sign shows that the growth of nematic order can produce ordering of sign different from that due to the surface ordering in the isotropic phase. The magnitude of the birefringence decreases as the 8S5 transitions from the nematic to the SmA phase, consistent with quasi-cylindrical guest smectic layers growing around the surfaces of the HNFs (as in Fig. 7f). The bulk phase transitions of the 8S5 in the network are indicated as vertical dashed lines on the plots.