Superheating in linear polymers studied by ultrafast nanocalorimetry

To study phase transition kinetics on submillisecond time scale a sensitive ultrafast nanocalorimeter was constructed. Controlled ultrafast cooling, as well as heating, up to 10⁶K/s was attained. The method was applied for the measurements of the superheating phenomenon in a set of linear polymers: iPS, PBT, PET, and iPP. A power law relation between the superheating and the heating rate holds in the heating rate range 10^-2-10⁴K/s. A limiting superheating of about 10% of the melting temperature was observed at rates above 10⁴-10⁵K/s. This limit depends on annealing conditions before sample melting. The observed superheating limit, as well as the power law, can be accounted for the internal stresses near the crystalline amorphous interface in semicrystalline polymers induced by heating, which are related to the thermal expansion gradients inherent in a semicrystalline material.