Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2014)
    Thin Films Wednesday Sessions
       Session TF-WeP

Paper TF-WeP36
Negative Thermal Expansion of Polystyrene Ultrathin Films Supported on Si Substrates revealed by X-Ray Reflectivity: Quench Rate Dependence

Wednesday, December 10, 2014, 4:00 pm, Room Mauka

Session: Thin Films Poster Session
Presenter: Kazuki Nishimori, Kwansei Gakuin University, Japan
Authors: K. Nishimori, Kwansei Gakuin University, Japan
S. Nakahara, Kwansei Gakuin University, Japan
K. Sekiya, Kwansei Gakuin University, Japan
Y. Chunming, Shanghai Institute of Applied Physics, China
I. Takahashi, Kwansei Gakuin University, Japan
Correspondent: Click to Email
Understanding of ultrathin functional polymers is important for various fields of nanotechnology. However, physical property of polymeric thin films with thicknesses comparable to the size of molecules is significantly different from that of bulk due to the confinement effect and heterogeneity peculiar to thin films sandwiched between air and solid substrate. As one of the deepest and most interesting unsolved problems, origin of negative thermal expansion (NTE) often observed in ultrathin glassy polymer films has attracted considerable attention. In the present study, we find key factors for the emergence of NTE with polystyrene (PS) ultrathin films quenched from the rubbery state with different cooling rates. Samples are mono-dispersive atactic PS with molecular weight Mw = 955,000 and glass transition temperature of 375 K. Thin films around 6 nm thick were formed by spin-coating technique on Si (100) substrates. Prior to the rapid cooling, thin PS films were annealed at 405 K for 12 hours in a low vacuum. From 405 K, the films were cooled to the liquid nitrogen temperature with different cooling rates and heated to room temperature in vacuum. Thermal expansivity was evaluated with X-ray reflectivity (XR) measured isothermally from room temperature to 405 K, which affords us precise temperature dependence of thickness, average electron density and root mean square of surface roughness. Relaxation in glassy state is also obtained through temporal variation in thickness at a certain temperature that can be monitored by XR. A strong correlation between cooling-rate-dependent-NTE and relaxation behavior at room temperature is revealed in this study.