Detection of anisotropic subsurface inter- and intramolecular interactions: Torsional force spectroscopy on heterogeneous polymers

By guest authors Marvin Hoffer and Christian Dietz.

Marvin Hoffer^a, Felix Petersein^a, Martin Dehnert^b, Tobias A. Lintner^a, and Christian Dietz^a*

*Email: dietz@pos.tu-darmstadt.de

Link to publication: Anisotropic Subsurface Inter- and Intramolecular Properties of Heterogeneous Polymers Revealed by Torsional Force Spectroscopy

#Done with a DriveAFM: Performance without compromise

Block copolymers such as polystyrene-block-polybutadiene (SB) block polymers are used in demanding mechanical applications, including the fabrication of car tire treads. In this context, the unique combination of rigidity and elasticity provided by SB block copolymers is crucial. During driving, the tread is exposed to complex and anisotropic shear forces as it interacts with the road surface. These directional shear stresses play a critical role in determining wear resistance and service lifetime. To better understand these directional nanomechanical properties, we employed in our recent publication the torsional force spectroscopy technique using the DriveAFM (Nanosurf AG, Liestal, Switzerland) on a self-assembling system featuring rigid polystyrene (PS) cylinders embedded in a softer polybutadiene (PB) matrix. This morphology provides a compelling testbed to explore how inter- and intramolecular interactions depend on the direction of applied shear–a key factor in polymer performance under real-world conditions.

Using torsional force spectroscopy on the DriveAFM, we were able to go beyond conventional indentation methods. The slow z-direction dynamics of the torsional mode enabled deeper tip penetration into the polymer matrix. This allowed us to monitor how mechanical properties evolve with increasing depth—something tapping mode cannot easily achieve. Our measurements releaved a clear directional dependence in the mechanical response of the PS cylinders. When the AFM tip applied shear along the PS cylinders’ long axis, distinct rigidity was detected in contrast to shear applied perpendicular to the cylinder long axis. Additionally, we were able to determine that individual interactions within the PS cylinders corresponds to forces on the order of 0.22 pN, offering a highly quantitative view of molecular cohesion.

Torsional force spectroscopy enables visualization of anisotropic shear properties in heterogeneous polymers. In the depth profile image, anisotropy is evident by the more dominant yellow coloration in the upper region (shearing parallel to the polystyrene cylinders) compared to the lower region (shearing perpendicular to the cylinders).

These findings demonstrate the unique capability of the DriveAFM in capturing subsurface, anisotropic nanomechanical properties–information that is inaccessible with conventional AFM modes. Such insights are invaluable for the design and optimization of functional materials, especially in applications where directional stress plays a central role.

By leveraging torsional force spectroscopy, researchers and engineers can gain a deeper understanding of how molecular structure affects macroscopic performance, paving the way for more durable, efficient, and tailored polymer systems.

To read more:

Easy Moiré Pattern Imaging of 2D Materials with Photothermal Torsional Resonance AFM