(298e) Selectively Probing the Glass Transition Temperature in Block Copolymer Films

We have used fluorescent probes to measure the glass transition temperature, Tg, of poly(styrene-b-methyl methacrylate), P(S-b-MMA), symmetric diblock copolymer films. These spin coated films self-assemble into a lamellar morphology upon annealing above Tg with the styrene block preferentially residing at the free surface and the methyl methacrylate block at the substrate interface. Changes in the intrinsic fluorescence spectrum from the phenyl ring in the styrene block have been used to monitor the time scale required for the self-assembly process, and found to be in agreement with similar measurements using neutron reflectivity.

By selective choice of probe dyes and labels we have been able to measure the Tg of the individual blocks as a function of total film thickness. Previous studies of neat PS films on similar substrates have shown reductions in Tg with decreasing film thickness, while neat PMMA films show increases in Tg with decreasing film thickness due to competing free surface and substrate effects. Thus, this block copolymer system is of interest because the different blocks should experience competing thermodynamic effects upon confinement to ultrathin films. We have found that the fluorescent pyrene dye employed segregates to the styrene block and reports that the Tg of the PS block decreases with decreasing film thickness in agreement with neat PS films. We have simulated the lamellar morphology of the block copolymer films using multilayer films of homopolymers of equivalent thickness. Using the homopolymer multilayer films, we can measure the Tg value of individual PS layers within the film. To within experimental error, we obtain good agreement for the Tg values measured in the PS blocks of the block copolymer films and the PS layers of the homopolymer multilayer films of equivalent morphology.