Anisotropic Subsurface Inter- and Intramolecular Properties of Heterogeneous Polymers Revealed by Torsional Force Spectroscopy

Abstract

Atomic force microscopy (AFM) is widely recognized as an essential technique for the nanomechanical surface characterization of polymers. However, probing subsurface inter- and intramolecular properties of polymers remains a significant challenge with conventional AFM techniques, such as tapping mode. In this study, we utilized torsional force spectroscopy on annealed polystyrene-block-polybutadiene (SB) diblock copolymer films, which consist of rigid polystyrene (PS) cylinders embedded in a softer polybutadiene (PB) matrix, to analyze their inter- and intramolecular interactions as a function of the AFM tip indentation depth. The slow cantilever dynamics in torsional force spectroscopy along the z-direction allows for greater indentation depths compared to tapping mode, facilitating deeper tip penetration into the SB polymer films. This approach enables a precise analysis of the molecular interactions between the AFM tip and the PS and PB polymer blocks. By converting the available observables, such as torsional frequency shift and torsional excitation amplitude, into mechanical quantities, specifically in-plane tip-sample force, shear stress, and the dissipated energy between the lateral tip motion and the polymer blocks, we determined their dependence on the block alignment relative to the tip trajectory. We found that PS cylinders exhibit considerable rigidity when subjected to shear along their length axis with an AFM tip. In contrast, when shear stress is applied perpendicular to the length axis, the PS cylinders can wobble in the softer PB matrix. Additionally, we observed that the molecular interactions of individual molecules within the PS cylinders are approximately 0.22 pN. Our findings highlight the capability of torsional force spectroscopy to visualize local nanomechanical properties and to reveal the differences in inter- and intramolecular interactions within polymeric films. This insight can significantly contribute to the design of functional nanomaterials by providing a deeper understanding of molecular interactions on the nanometer scale.