Unveiling Strong Thin Film Confinement Effects on Semirigid Conjugated Polymers

Abstract

Nanoconfinement has been recognized to induce significant changes in the physical properties of polymeric films when their thickness is less than 100 nm. Despite extensive research on the effect of nanoconfinement on nonconjugated polymers, studies focusing on the confinement effects on dynamics and associated electronic and mechanical properties for semiconductive and semirigid conjugated polymers remain limited. In this study, we conducted a comprehensive investigation into the nanoconfinement effects on both p- and n-type conjugated polymers having varying chain rigidity under different degrees of confinement. Using the flash differential scanning calorimetry technique, it was found that the increased molecular mobility with decreasing film thickness, as indicated by the depression of glass transition temperature (Tg) from its bulk values, was directly proportional to chain rigidity. This relationship between chain rigidity and enhanced segmental mobility was further corroborated through molecular dynamics simulations. Thinner films exhibited a higher degree of crystallinity for all conjugated polymers, and a significant reduction of more than 50% in elastic modulus was observed for films with approximately 20 nm thickness compared to those of 105 nm thickness, particularly for highly rigid conjugated polymers. Interestingly, we found that the charge mobility remained independent of film thickness, with all samples demonstrating good charge mobility regardless of the different film thicknesses for devices measured here. Nanoconfined conjugated polymer thin films exhibited a combination of mechanical compliance and good charge carrier mobility properties, making them promising candidates for the next generation of flexible and portable organic electronics. From an engineering standpoint, confinement could be an effective strategy to tailor the dynamics and mechanical properties without significant loss of electronic property.