Sensitive thermal transitions of nanoscale polymer samples using the bimetallic effect: Application to ultra-thin polythiophene

Abstract

A sensitive nanocalorimetric technology based on microcantilever sensors is presented. The tech- nology, which combines very short response times with very small sample consumption, uses the bimetallic effect to detect thermal transitions. Specifically, abrupt variations in the Young’s modu- lus and the thermal expansion coefficient produced by temperature changes have been employed to detect thermodynamic transitions. The technology has been used to determine the glass transition of poly(3-thiophene methyl acetate), a soluble semiconducting polymer with different nanotechno- logical applications. The glass transition temperature determined using microcantilevers coated with ultra-thin films of mass = 10 − 13 gis5.2 ◦ C higher than that obtained using a conventional differential scanning calorimeter for bulk powder samples of mass = 5 × 10 − 3 g. Atomistic molecular dynamics simulations on models that represent the bulk powder and the ultra-thin films have been carried out to provide understanding and rationalization of this feature. Simulations indicate that the film-air in- terface plays a crucial role in films with very small thickness, affecting both the organization of the molecular chains and the response of the molecules against the temperature.