Daniel Pinkal

The objective of this study was to identify chitosan formulations suitable for sustainable functional integration into composite materials. To this end, the influence of solvent type (acetic acid and lactic acid) and chitosan molecular weight on film formation, rheological behavior, thermal response, thermo-optical properties, chemical structure, and piezoelectric performance was systematically investigated. Optimized casting and drying procedures produced transparent, homogeneous, and mechanically stable films suitable for comprehensive characterization. Rheological and thermo-optical analyses demonstrated that solvent selection strongly influenced polymer network formation and molecular mobility. Films prepared using acetic acid exhibited denser and stiffer polymer networks with improved dimensional stability, whereas lactic acid produced more flexible and elastic films. Thermogravimetric analysis revealed only minor differences in the intrinsic thermal stability of the investigated films, while FTIR confirmed that the solvent systems did not alter the chemical structure of chitosan. Electrical measurements carried out whilst the system was subjected to periodic mechanical excitation revealed weak but equally periodic electrical signals, which demonstrate a sensor functionality. These results demonstrate that chitosan films possess tunable structural, thermal, and potential mechanoelectrical sensor properties and highlight their potential as sustainable, functionally integrated components in composite material systems.

Electrocaloric materials are currently under investigation because of their potential use in active cooling systems. As electrocaloric transducer materials, electroactive polymers such as PVDF copolymers with trifluoroethylene [P(VDF-TrFE)] and PVDF terpolymers with trifluoroethylene and chlorofluoroethylene play an important role. While the relaxor ferroelectric PVDF terpolymers show significant electrocaloric properties even at room temperature, the ferroelectric PVDF copolymers show high electrocaloric properties mainly at temperatures above the Curie transition, which depend on the copolymer composition and is at least 50 °C. Structural modifications are one possibility for converting the ferroelectric PVDF copolymers into a relaxor ferroelectric state and, thus, their possible electrocaloric use at room temperature. Here, we demonstrate the structural changes of P(VDF-TrFE) copolymers (55/45 mol. %) caused by electron irradiation. The use of irradiation doses of around 400 kGy allows a partial conversion from the ferroelectric phase to the relaxor-ferroelectric phase but does not yet lead to a complete degradation or cross-linking process so that a certain polarization can still be used. This is accompanied by the reduction of the Curie temperature and an increase in the permittivity at, e.g., room temperature. Thus, a maximum non-remanent polarization and electrocaloric activity are observed after irradiation of P(VDF-TrFE) copolymers with doses of around 400 kGy. These irradiated copolymers show at room temperature a temperature change of about 1.8 K and a figure of merit of 0.35 at an electric field of 100 V/μm, in comparison to about 0.7 K and 0.13, respectively, for non-irradiated copolymers.