Role of diblock copolymers toward controlling the glass transition of thin polymer films

The physical properties of thin polymer films are often thickness, h, dependent, influenced by confinement and by interfacial interactions between the chains and the external interfaces. We show that the magnitude and film thickness dependence of the average glass transition temperature, T-g, of the polystyrene-silicon oxide (PS/SiOx/Si) system are influenced appreciably with the addition of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. The T-g can be tailored to increase, or decrease, with decreasing h or to remain independent of h. T-g shifts of as much of 35 degrees C are obtained for films of h approximate to 20 nm. Additionally, we report that the critical micelle concentration, phi(cmc), of the copolymer in thin films is considerably larger than for the bulk; specifically, micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains in the system. The h dependence of T-g is not influenced by the phi(cmc) or by the number of micelles in this system.