Understanding the Cohesive and Adhesive Failures in Novel Multilayered SWCNT-Reinforced PDMS Composites using Nano Scratch Technique

This study focuses on investigating the nanoscale failure of composites based on a synthetic elastomer-polydimethylsiloxane (PDMS). Nano scratch test is used as a nanoprobing technique on multilayered thin films made of neat PDMS and single-walled carbon nanotubes (SWCNT) reinforced PDMS composites to study the cohesive and adhesive failure in soft materials. Two types of novel layered composites are comparatively considered in this study: (i) $\boldsymbol{L B L}$ PNCs: Multilayered films with alternating layers of SWCNT and PDMS, and (ii) Bulk PNCs: Multilayered films with SWCNT reinforced in the bulk of PDMS matrix. Composites with four incremental concentrations of SWCNT at $0.05 \mathrm{wt} \%$, $0.2 \mathrm{wt} \%, 0.5 \mathrm{wt} \%$, and $1 \mathrm{wt} \%$ are comparatively studied. Nano scratch tests (unidirectional single passes) are conducted to understand the nanoscale interfacial damages in detail: (i) the normal force used to initiate the scratch on soft films (referred to as cohesive damage) and (ii) the interfacial delamination in the composites which is revealed as a breakpoint in the normal force vs lateral force plots (referred to as adhesive damage). In LBL PNCs, the interfacial delamination is used as an indicator of the adhesive damage between the SWCNT and PDMS. From this study, it is understood that the novel stacked architecture in LBL PNCs promotes the surface-level interfacial mechanics between SWCNT and PDMS even at ultra-low concentrations of SWCNT.