Differential scanning calorimetry and fast scanning chip calorimetry heating experiments were carried out in a wide range of rates of temperature change from 0.2 to 60,000 K s(-1) for isothermally crystallized polyamide 6. Multiple melting peaks were observed. With increasing heating rate, the highest-temperature endotherm shifts toward lower temperatures and finally disappears due to suppression of the reorganization. The critical heating rate to suppress reorganization was 15-50 times higher than the critical cooling rate to cause complete vitrification. On heating at rates higher than the critical heating rate to suppress reorganization, there were observed two melting processes of different kinetics. Four possible assignments were considered regarding the two crystal populations. These are (i) crystals grown during primary and secondary crystallization, (ii) crystals grown in the bulk and nucleated at the surface/substrate, (iii) crystals, which are subjected to different local stress originating from heterogeneities in interlamellar regions, and (iv) the crystal/mesophase polymorphism. (C) 2016 Wiley Periodicals, Inc.