The fundamental polymer physics of conjugated polymer, including the tie chains, the entanglement molecular weight (Me) and the molecular weight dependences of glass transition temperature (Tg) and the liquid-crystal-to-isotropic transition temperature (Tc), are lacking. These microstructural parameters ultimately connect the charge transport and mechanical flexibility of conjugated polymer. The molecular weight dependence of Tg is studied using regioregular (RR) and regiorandom (RRa) Poly(3-hexylthiophene-2,5-diyl) (P3HT) by oscillatory shear rheometry. We find two Tg’s for both RR and RRa P3HT. The higher Tg, Tα, is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory−Fox model yields Tα = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a very different molecular weight dependence of Tα is seen below Mn = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. In addition, coupled with the absolute molecular weight distribution, Me of P3HT is extracted by fitting the linear viscoelastic data using tube reptation model. Furthermore, this linear viscoelasticity study is expanded to other conjugated polymers, such as poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2’,2”-diyl) (PFTBT), poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14), to locate Tc and identify the type of liquid crystalline phase, which is verified by polarized optical microscopy and X-ray scattering. Therefore, this fundamental polymer physics study provides a pathway towards being able to predict and design the molecular structure of conjugated polymers with desired mechanical and conductive properties as deformable semiconductors in the future.

Understanding polymer physics of conjugated polymers for future deformable semiconductors

255th National Meeting (2018)

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