Depth-dependent inhomogeneous characteristics in supported glassy polystyrene films revealed by ultra-low X-ray reflectivity measurements

Abstract

This study reports X-ray reflectivity measurements of the glass transition of polystyrene thin films supported on Si substrates and heated at low heating rates that ranged from 0.14 to 0.01 1C min À1 . At a heating rate of 0.14 1C min À1 , the glass transition temperature T g was independent of the film thickness down to a thickness of 6 nm. However, at a heating rate of 0.04 1C min À1 , the value of T g decreased with decreased thickness. The reduction in T g was most significant at the ultra-low heating rate of 0.01 1C min À1 . Furthermore, with decreased film thickness, the linear thermal expansivity in the glassy state a glass slightly decreased at a heating rate of 0.14 1C min À1 , whereas a glass exhibited a significant increase at the ultra-low heating rate of 0.01 1C min À1 . Reconstructed depth profiles of thermal expansivity, which were obtained by fitting the a glass values using an integral model, indicated a decrease in the thickness of the interfacial dead layer with a decrease in the heating rate, whereas the volume fraction of the free surface region increased under this condition. The observed reduction in T g can be attributed to surface and interface effects perturbing the glass transition dynamics of the thin films under slower probing conditions.

Thickness and Substrate Dependences of the Relaxation of Spin-Coated Polymethyl Methacrylate Ultrathin Films Supported on SiO2 and SiOH Substrates

“…Advancing literature on the confinement effect in polymer systems has confirmed some unusual behaviors, such as (i) the strong dependence of the glass transition temperature, T g , on the thickness and interface conditions; − (ii) the acceleration and deacceleration of the aging dynamics in thin and ultrathin films, respectively; − (iii) the negative thermal expansion (NTE) of some glassy polymers even after prolonged, multiple annealing; − and (iv) the memory and rejuvenation effects on the aging dynamics. − …”

Combined X-ray Reflectivity and Infrared Study of the Effect of Hydrogen Bonding of the OH Group on the Relaxation Behavior in Ultrathin Polyvinylphenol Films on SiO2

“…As technology progresses, the miniaturization of electronic, mechanical, and optical devices has become more widespread, and the use of ultrathin films has become increasingly popular. Ultrathin polymer films in their confined state display several anomalous behaviors, − such as deviation of the glass-transition temperature ( T g ) − from its bulk counterpart, acceleration of physical aging with decreasing thickness, − and negative thermal expansion (NTE). − The most aberrant behavior found in thin films is currently thought to be related to the dynamic heterogeneity of the system, which emerges due to differences in the characteristics of the surface or interface region. Keddie et al , used ellipsometry to conduct the first systematic study of T g in thin supported polymer films.…”