Metastability of polymer crystallites formed at low temperature studied by ultra fast calorimetry: Polyamide 6 confined in sub-micrometer droplets vs. bulk PA6

It is shown that the application of a chip calorimeter, allowing very fast cooling and heating rates up to 10,000 K/s, can be successfully applied to study reorganization phenomena in crystallizable polymers. In this research both bulk Polyamide 6 (PA6) as well as an immiscible (polystyrene/styrene-maleic anhydride copolymer)/Polyamide 6 blend with dispersed Polyamide 6 droplets of sub-micrometer size have been studied. The blends with sub-micrometer PA6 droplets have been shown to crystallize at low temperature via a homogeneous nucleation mechanism, due to a lack of heterogeneities in the small droplets.Upon fast cooling with more than 500 K/s crystallization of PA6 could be totally prevented. No cold crystallization takes place upon subsequent heating with 500 K/s. Upon fast heating of 2000 K/s after isothermal crystallization, the 'real', initial melting of the crystallites formed at very low temperature could be obtained, which was not possible with standard DSC apparatus or HPer DSC. However, even heating with 5000 K/s was not fast enough to completely avoid reorganization. Annealing experiments in the melt-range clearly show the fast reorganization of PA6 crystallites, resulting in improved stability within a timescale in the order of 0.01-0.1 s. Most probably reorganization takes place mainly in the crystalline state, and only to a lesser extent via a melting-recrystallization-remelting process. The reorganization process is not hindered by the confined dimensions of the Polyamide 6 droplets, because both bulk PA6 as well as the confined (PS/SMA2)/PA6 blend give rise to identical reorganization phenomena.