Multiamorphous Phases In Diketopyrrololpyrrole-Based Conjugated Polymers: From Bulk to Ultrathin Films

The glass transition temperature (Tg) of conjugated polymers is a crucial physical parameter that governs their mechanical and electrical properties for applications from sensor technology to skin-like electronics. Despite some well-developed methods that exist for detecting Tg of conjugated polymers, there is still a need for the combination of advanced calorimetric methodologies to characterize Tg in a broad range of temperatures and scales from bulk to nanometer-thin films. In this work, alternating current (ac) chip calorimetry and flash differential scanning calorimetry (flash DSC) are applied to successfully identify the Tgs and segmental relaxations of conjugated polymers. The detected relaxations were interpreted, in agreement with existing observations in similar systems, as a side-chain relaxation (low temperature), mobile amorphous fractions (MAF), and rigid amorphous fractions (RAF). Tgs of MAF and RAF increase and decrease with increasing size of fused thiophene units in the backbone, respectively. The Tgs of diketopyrrolopyrrole-based conjugated polymer thin films are also identified by the same method, with both Tgs showing a decreased value compared to those of bulk samples. Furthermore, the weight fractions of MAF and RAF with various film thicknesses are qualitatively analyzed based on the heat capacity step of each amorphous fraction during the glass transition. This facile method gives insights into the physical nature of amorphous phases in conjugated polymers for better revealing their structure–property relationships, which will guide the designs of new materials with optimized electrical and mechanical properties in the future