Direct Measurement of the Thermomechanical Properties of Poly(3-hexylthiophene) Thin Films on Ionic Liquid Surfaces

Thermomechanical properties of conjugated polymers (CPs) have emerged as critical factors for the long-term stability of CP-based electronics. However, very little is known about the thermomechanical properties of CP thin films (thickness ≤100 nm) without constraints from a supporting substrate. Herein, we investigate the intrinsic thermomechanical properties of CP thin films by direct measurement of thermal strains of thin films floating on an ionic liquid (IL). The IL surface provides almost frictionless gliding and thermal expansion of the pseudo-free-standing CP thin films for a wide range of temperatures up to 250 °C. We apply the method to examine the thermomechanical properties of poly(3-hexylthiophene)s (P3HTs) with different regioregularities (RRs). With increasing IL temperature, the P3HT thin film exhibits a linear in-plane thermal expansion, followed by a rapid increase in thermal strain above the melting temperature (Tm). When RR decreases from 95 to 60%, we observe a higher coefficient of thermal expansion (CTE) (from 164 to 395 ppm K–1) and a lower Tm (from 189 to 57 °C) due to the reduced crystallinity and crystal size. At the same RR, both CTE and Tm values decrease as the film becomes thinner. When the thickness of 82% RR P3HT film decreases from 550 to 45 nm, CTE and Tm values reduce from 294 to 201 ppm K–1 and from 160 to 128 °C, respectively. The in-plane hardening of the amorphous phase decreases the CTE, while the smaller crystal size from the thickness limitation reduces Tm. Our methods enable the direct measurements of thermomechanical properties of the CP thin films over a wide temperature range.