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Thermomechanical properties of conjugated polymers (CPs) are greatly influenced by both their microstructures and backbone dynamics. In the present work, to investigate the effect of the side-chain branch position on the backbone's mobility and molecular packing structure, four poly (3-alkylthiophene-2,5-diyl) derivatives (P3ATs) with different side chains, either branched or linear, were synthesized by a quasi-living Kumada catalyst transfer polymerization (KCTP) method. The side-chain branch position greatly influences the glass transition temperature (Tg) of the backbone of P3ATs as well as the melting temperature (Tm), as measured by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) respectively. Placing the branching point closer to the conjugated backbone leads to increased backbone Tg and Tm. Also, according to grazing incidence wide-angle X-ray scattering (GIWAXS …