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

Abstract

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, υ­(T). We undertake the challenging task of estimating material-specific υ­(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 υ­(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 υ­(T). We moreover correlate our findings with earlier discussions on the thermodynamic nature of phenomenological MS interactions.

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

“…Further confirmation comes from measurements performed at lower scanning rates, using standard DSC and polarized optical microscopy and spectroscopy (see Supplementary Figures 5,[7][8][9] and Supplementary Note 3). 29,32 Optical Characterization of Aligned Films. A strong enhancement of charge mobility in polymer semiconductors can be obtained with uniaxial backbone alignment, inducing transport anisotropy and favoring improved transport properties along the alignment direction.…”

Semiconducting Single‐Walled Carbon Nanotubes or Very Rigid Conjugated Polymers: A Comparison

“…[49] Fully flexible polymers in solution form random coils with a certain hydrodynamic radius that can be correlated to their molecular weight, which is the basic working principle of M n determination by gel-permeation chromatography. [53] For the ladder-type poly(p-phenylene) MeLPPP a persistence length of 6.5 nm was determined by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS), [54] but values up to 9 nm were reported for polyfluorenes. [53] For the ladder-type poly(p-phenylene) MeLPPP a persistence length of 6.5 nm was determined by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS), [54] but values up to 9 nm were reported for polyfluorenes.…”

“…It decreased at higher temperatures and for lower regioregularity. The rigid repeat units of many semiconducting polymers can further lead to liquid crystalline (usually nematic) solutions (lyotropic) or melts (thermotropic) [53] and thus facilitate large scale chain alignment that can be used to enhance charge transport or enable polarized light emission. Polymers with more rigid and planarized backbones such as DPPT-TT and IDT-BT exhibit extinction coefficients that are about twice as high as more flexible semiconducting polymers.…”

“…Fully flexible polymers in solution form random coils with a certain hydrodynamic radius that can be correlated to their molecular weight, which is the basic working principle of M n determination by gel‐permeation chromatography . Due to the higher rigidity of conjugated polymers—especially for conjugated ladder‐type polymers with fused‐rings restricting the torsional motion along the backbone (e.g., MeLPPP, see below)—they are more planarized and can be described by the worm‐like chain model with a certain persistence length that also quantifies the stiffness of the polymer . For the ladder‐type poly( p ‐phenylene) MeLPPP a persistence length of 6.5 nm was determined by small‐angle neutron scattering (SANS) and small‐angle X‐ray scattering (SAXS), but values up to 9 nm were reported for polyfluorenes .…”

“…A number of π‐conjugated polymers also show a substantial degree of aggregation in solution depending on the solvent, e.g., P(NDI2OD‐T2) in toluene or P3HT in anisole, which typically alters their absorption and emission properties significantly and also affects film formation from these solutions during spin‐coating. The rigid repeat units of many semiconducting polymers can further lead to liquid crystalline (usually nematic) solutions (lyotropic) or melts (thermotropic) and thus facilitate large scale chain alignment that can be used to enhance charge transport or enable polarized light emission …”