Macromolecules, Vol.53, No.22, 10061-10068, 2020
Side-Chain Length Dependence of Young's Modulus and Strength in Crystalline Poly(3-alkylthiophene) Nanofibers
Understanding the side-chain effect on the mechanical properties of conjugated-polymer nanocrystals is critical for the design of high-performance soft electronic devices. However, due to the complexity of the side-chain effect and hierarchical structure in bulk materials, the classical macroscopic characterizations mask the mechanical properties of nanocrystals. Here, we have measured the mechanical properties of crystalline poly(3-alkylthiophene) (P3AT) nanofibers with different side-chain lengths (with 4, 6, 8, 10, and 12 alkyl carbons per side chain) by the combination of thermal shape fluctuation analysis and single-molecule force spectroscopy (SMFS). We found that poly(3-hexylthiophene) (P3HT, with 6 alkyl carbons) nanofibers exhibit a higher Young's modulus and strength than poly(3-butylthiophene) (P3BT, with 4 alkyl carbons), poly(3-octylthiophene) (P3OT, with 8 alkyl carbons), poly(3-decylthiophene) (P3DT, with 10 alkyl carbons), and poly(3-dodecylthiophene) (P3DDT, with 12 alkyl carbons) nanofibers. Furthermore, we illustrated that the more pronounced J-aggregate in P3HT nanofibers increases the probability of the one-step unfolding in SMFS experiments producing high strength and high Young's modulus. The effects of the regioregularity of the polymer chain and the width of the nanofiber on the Young's modulus of P3AT nanofibers were also investigated. Our results explain how the side-chain length affects the nanomechanical properties of P3AT nanofibers and deepen our understanding of the relationship between the chemical composition and nanomechanical properties of conjugated polymers.