Probing Polymer-Chain Flexibility and Defect Mechanics in Block Copolymer Films by Torsional Force Spectroscopy

Torsional force spectroscopy is employed to probe the in-plane nanomechanical properties of solvent-annealed polystyrene-block-polybutadiene (SB) diblock copolymer films, which feature stiff polystyrene (PS) cylinders embedded in a softer polybutadiene (PB) matrix. By converting the torsional frequency shift and excitation amplitude into in-plane shear stress, storage shear modulus, and torsional dissipated energy, we map nanoscale variations in polymer-chain flexibility within three different defect structures. (1) An isolated ring defect reveals distinct direction-dependent shear behavior: when shear is applied perpendicular to the PS cylinder length axis, lower in-plane shear stress compared to parallel shearing indicates enhanced “wobbling” of the cylinders due to limited intramolecular resistance. (2) A previously unreported, sphere-like protrusion exhibits highly inhomogeneous in-plane shear stress yet nearly uniform dissipated energy, suggesting a swollen, mushroom-like PS cap structure connected to concealed PS cylinders beneath the surface. (3) Matrix dislocations and grain boundary defects show elevated in-plane shear stress and reduced dissipated energy at PS cylinder branching points, particularly at dislocation cores and grain boundary nodes, caused by chain compression and restricted flexibility. In contrast, bridging segments between the nodes exhibit lower in-plane shear stress and higher dissipated energy due to increased polymer-chain flexibility. These findings demonstrate that defect structures originating from local swelling, chain compression, and orientation mismatch introduce strong anisotropy into in-plane nanomechanical properties, offering pathways to tailor stiffness and flexibility in block copolymer-based nanotechnologies.