Energy-sustained reversible nanoscale order and conductivity increase in polymer thin films

When treated with 30 kHz ultrasounds in isothermal conditions, poly-3-hexylthiophene (P3HT) thin films show a marked increase of their nanoscale order. This structural transition is fully reversible, as the system returns to its initial structure when ultrasounds are turned off. The increase of nanoscale order is shown to consists of a more efficient packing and/or alignment of amorphous fragments along pre-existing nano-crystals. Additionally, owing to the reduced structural disorder, the ultrasound treatment leads to the marked increase (up to 100%) of the film conductivity. The effect, being dependent on the thickness and degree of crystallinity of the polymer films, is discussed in terms of ultrasound-induced packing of P3HT chain segments and the subsequent strain-driven relaxation to restore the initial order when sonication is switched off. These results open the way to a new field of study dealing with energy-sustained ordering processes of randomly oriented soft molecular moieties, as a new path to sample quasi-equilibrium states in partially crystalline systems. This novel approach promises new paths to reversible modification and control of polymeric materials and devices.