With the miniaturization of technology, ultra-thin polymer films have received close scientific attention from the point of view of both physics and technology. It is important to study the ultra-thin films in presence of substrates to understand fundamental phenomena. However, these ultra-thin polymer films exhibit kinetically nonuniform layered structure in which chain mobility at the surface region is high whereas molecular chains at the interface region are strongly bound. As a result, the glassy state of polymer thin films exhibits a strong deviation from the bulk. Therefore, clarification of the mechanism of polymer thin films is necessary scientifically and industrially. In this study, the time-dependent relaxation process of annealed polystyrene (PS) ( Mw= 955,000 g mol-1, Mw/Mn=1.09, Rg=30 nm) ultra-thin films (∼6nm) supported on SiO2 were investigated in the temperature range of 30oC (Tg bulk -80°C) to 140oC (Tg bulk +30°C) to understand the relaxation mechanism below, near and above Tg bulk. X-ray reflectivity (XRR) was used to measure the precise change in film thickness, and the data obtained were fitted by the Kohlausch-Williams-Watts function. Differential scanning calorimetry and XRR were utilized to measure Tg bulk (~110°C) and Tg apparent (~80°C) of 6 nm PS film respectively. An interesting and unusual thickness relaxation behavior in PS ultra-thin films was observed (Fig 1). The ultra-thin films showed positive thickness relaxation (expansion in thickness with time) at temperatures ≤60°C and suddenly reversed its behavior by depicting negative thickness relaxation (contraction in thickness with time) at temperatures ≥70°C up until 110°C and again reversed the behavior for temperatures &gt;110°C. We are currently investigating the thickness relaxation in the temperature range of 60°C to 70°C and 110°C to 120°C to find the temperature where the nature of relaxation is reversed.

Unique Thickness Relaxation Behavior Characterized in ultra-thin polystyrene films using X-Ray reflectivity: Effect of temperature on physical aging

ACS Spring Meeting 2021

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