Temperature-Dependence of Persistence Length Affects Phenomenological Descriptions of Aligning Interactions in Nematic Semiconducting Polymers

Electronic and optical properties of conjugated polymers are strongly affected by their solid-state microstructure. In nematic polymers, mesoscopic order and structure can be theoretically understood using Maier-Saupe (MS) models, motivating us to apply them to conjugated macromolecular systems and consider the problem of their material-specific parametrization. MS models represent polymers by worm-like chains (WLC) and can describe collective polymer alignment through anisotropic MS interactions. Their strength is controlled by a phenomenological temperature-dependent parameter, v(T). We undertake the challenging task of estimating material-specific v(T), combining experiments and Self-Consistent Field theory (SCFT). Considering three different materials and a spectrum of molecular weights, we cover the cases of rod-like, semiflexible, and flexible conjugated polymers. The temperature of the isotropic-nematic transition, T_IN, is identified via polarized optical microscopy and spectroscopy. The polymers are mapped on WLC with temperature-dependent persistence length. Fixed persistence lengths are also considered, reproducing situations addressed in earlier studies. We estimate v(T) by matching T_IN in experiments and SCFT treatment of the MS model. An important conclusion is that accounting explicitly for changes of persistence length with temperature has significant qualitative effects on v(T). We moreover correlate our findings with earlier discussions on the thermodynamic nature of phenomenological MS interactions.