(552i) Crystallization and Thickness Dependence on Fictive and Glass Transition Temperature in Thin Poly(D,L-lactic) Copolymer Films

The glass transition temperature (Tg), fictive temperature (Tf) and melt temperature (Tm) were investigated in crystallizable thin films of poly(D,L-lactic-co-glycolic acid) supported on silicon substrates by ellipsometry and compared to the bulk behavior. Coatings were spin-cast between 10nm and 200nm and annealed above the glass transition temperature but below the melt temperature for up to five days. The bulk Tg was 55°C and the bulk Tm was 136°C. It was found that the crystallization kinetics in the thin films were an order of magnitude 9 times faster than the bulk, whereas the melt temperatures were up to 15°C lower than the bulk. The fictive temperatures, measured upon heating, of the thin films were also consistently less than the bulk, by up to 5 ËšC. These findings stand in contrast to the Tg, measured upon cooling from the melt state, which were approximately the same (within 1 ËšC), although a slight depression in Tg was observed for films less than 50 nm. Interestingly, the fictive temperature in thin films increased up to 2°C due to crystallization, and there was a slight enhancement in Tf for films less than 50nm. Atomic force microscope images revealed that surface crystallization in thin films occurred upon cooling, yet this crystallization was found undetectable using ellipsometry. The present findings expand the understanding of the complex role that crystallization and thickness have in tuning the behavior of Tg, Tf and Tm in thin film, crystallizable polymers.