Sequential Epitaxial Organization of Poly(9,9-di‑<i>n</i>‑octyl-2,7-fluorene) in an Eutectic System

Abstract

The phase diagram of a binary eutectic system of poly­(9,9-di-<i>n</i>-octyl-2,7-fluorene) (PFO) and hexamethylbenzene (HMB) has been established via onset melting temperatures. Upon homogeneous mixing, the growth of PFO nematic phase in binary mixtures exhibits both lyotropic and thermotropic features. During cooling, a smectic phase of PFO developed epitaxially on the surface of prior-developed HMB crystals, establishing layer stacking via the packing of side chains. Upon the annealing at 120 °C, further epitaxial organization of PFO smectic packing on HMB crystalline substrate results in the growth of β crystalline. Both smectic and β phases of PFO yield characteristic ultraviolet (UV) absorption at 434 nm wavelength and photoluminescence at the wavelength of 441 nm. While HMB crystals were sublimated prior to the annealing treatment, α crystalline form developed instead through the transformation of smectic packing. The UV absorption and fluorescence of α form are relatively shorter wavelengths. As attributing the rise of energy bandgaps to the effect of backbone conformation, organization of less coplanar backbones within α crystalline form is proposed as a result of the lack of epitaxial effect. After the growth of smectic phase via the monotropic behavior of PFO in eutectic mixture, the following routes of phase transformation upon annealing hence unveiled the growth mechanisms of both α and β crystalline forms of PFO