THE EFFECT OF Tg REDUCTION ON CHAIN MOBILITY IN CONFINED POLYSTYRENE FILMS
Michael D. Dimitriou, Bülent Akgün, and Sushil K. Satija
A number of studies have shown that the glass transition temperature (Tg) of polystyrene (PS) thin films decreases with decreasing film thickness. For example, recent fluorescence measurements on pyrene doped PS films demonstrate a 32 K reduction in Tg for a 14 nm PS film relative to the bulk Tg value. Using neutron reflectivity (NR), we probe the interdiffusion of PS and deuterated PS (dPS) bilayer and trilayer thin films on silicon, to elucidate the effect of Tg reduction on polymer mobility at the dPS/PS interface. One bilayer and two trilayer films were studied. The bilayer film consisted of a 42 nm thick dPS bottom layer and a 20 nm thick PS top layer (Si/d42/h20), created to probe interdiffusion 20 nm from the polymer surface. One trilayer film was created using the same bottom layer but floating a 10 nm thick PS middle layer and a 10 nm thick dPS top layer (Si/d42/h10/d10) to compare the mobility at 10 nm and 20 nm from the surface. A third trilayer, Si/d10/h10/d40 was generated to determine the mobility at 40 nm and 50 nm from the film surface. Samples were annealed at temperatures between 65 ºC and 120 ºC for various time periods and quenched rapidly to room temperature before each NR measurement. Using all three samples polymer mobility was determined as a function of depth. NR results indicated negligible mixing between the layers up to 95 ºC for all interfaces.
The reflectivity data also revealed no difference in the mobility of PS chains at the interfaces 10 nm and 20 nm from the surface; data that contradicts previously reported enhanced mobility in the top 10 nm of a thin film.