Polymer crystallization in thin films at high cooling rates

Crystallization kinetics in PCL films was studied in a wide range of scanning rates. Cooling rates equal or above 400 K/s results in amorphous PCL. At even higher cooling rates the influence on nucleation was studied. Heating curves after different thermal histories were recorded to identify differences in nucleation density. The heating after cooling faster than 400 K/s reveals decreasing cold crystallization and melting peaks. That is indication that on previous cooling we not only avoid crystallization, but also create fewer nuclei. Above 10,000 K/s cooling rate the cold crystallization peak reduction saturates. This can be explained by the absence of homogeneously formed nuclei at cooling. The absence of homogeneous nuclei and the possibility to jump fast enough to any temperature, again avoiding nuclei formation, allow us to measure time and temperature dependency of nucleation. Nucleation at 185 K (15 K below Tg) yields a time constant of about 100,000 s. At temperatures above Tg, e.g. 220K, nucleation saturates and eventually superimposes with isothermal crystallization. Comparing the relaxation times of nucleation with previously derived dielectric relaxation data places the nuclei formation process intermediate between $\alpha $- and $\beta $- process, following the temperature dependence of the $\alpha $-process.