Conjugated polymers are promising materials for flexible electronics and energy-relevant applications due to their combination of semiconducting properties, mechanical flexibility and ease of melt or solution processing. Molecular weight can be a key factor in influencing the nanoscale and macroscale properties of polymer chains including crystal architecture, solubility, phase transition temperatures, rheological response, and charge transport efficacy. These, in turn, have been shown to affect device performance in various devices, including transistors, photovoltaics and light emitting diodes. Nevertheless, accurate characterization of the molecular weight of conjugated polymers remains a challenge. Previous work has demonstrated gel permeation chromatography (GPC), when calibrated with polystyrene standards, underrepresents the molecular weight of polymers such as poly(3-hexylthiophene-2,5-diyl). Thus, absolute molecular weight characterization often relies on nuclear magnetic resonance by quantifying the number of chain ends. This approach is limited to modest molecular weights, due to difficulties in measuring chain ends at high molecular weight; furthermore, chain ends are not always well defined for materials synthesized through polycondensations that are prevalent for the synthesis of leading donor-acceptor alternating conjugated polymers. We show that GPC with an in-line viscometer (universal calibration) can accurately characterize the molecular weight of conjugated polymers. We find that a crucial component of this approach is to measure the differential refractive index, such that we can confirm the solution concentration, and thereby determine minimal loss of material in the system. Molecular weights obtained from universal-calibration GPC are validated using static light scattering and measurements of the intrinsic viscosity in an Ubbelohde viscometer.

Characterizing the molecular weight of conjugated polymers using gel permeation chromatography and static light scattering

2019 Spring National Meeting

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