Examining Stretch Marks in Soft Composites Using Field Emission Scanning Electron Microscope Assisted in-situ Nanoindentation Analysis

This study focuses on nanoscale deformation on the surface of single-walled carbon nanotubes (SWCNT) reinforced polydimethylsiloxane (PDMS) composite ($0.5 \mathrm{wt} \%$) in a novel multilayered architecture with alternating layers of PDMS and SWCNT. For the first time, nanoscale plastic deformation through stretch marks generated on the top surface during nanoindentation is reported. By comparing stretch marks and corresponding force-displacement curves from nanoindentation analysis, we document the effect of the density of SWCNT reinforcement on the mechanical behavior of the composite. The findings elucidate the qualitative impact of the density of SWCNT reinforcement on resistance to force flow along the top surface, observed in the resistance offered by the test structure against stretch mark propagation. The resistance to force flow is comparatively investigated in different areas of the sample with low and high densities of the SWCNT network. Moreover, this nanoscale phenomenon is correlated with the previously reported macroscopic delamination analysis to assess bonding strength across the thickness of the composite thin film. The comparative examination of nanoscale and macroscopic deformations demonstrates that incorporating SWCNT in alternate layers enhances the resistance of the test structure to force propagation in both transverse (-xy plane) and longitudinal (-z plane) orientations. This underscores the efficacy of the innovative multilayered design and its influence on enhancing mechanical integrity, particularly at low SWCNT concentrations within PDMS. Such architectural configurations exhibit promising potential for application in lightweight, high-performance optoelectronic and electromechanical devices.