Introducing the Critical Cooling Rate estimated by Fast Scanning Calorimetry into the Fourier–Biot framework to estimate the thickness limits for quenching polymer films

This study investigates the cooling-rate dependence of the crystallization kinetics and its correlation with the glass-forming ability of poly(3-hydroxybutyrate) (PHB) using Fast Scanning Calorimetry over a wide range of cooling rates. Under fast cooling rates, the crystallization behavior shows a strong cooling-rate dependence, following Ozawa’s kinetics. The Critical Cooling Rate (CCR) was estimated by integrating for crystallinity the data from the exotherms recorded upon cooling as compared to the subsequent melting endotherms, as well as based on the Continuous Cooling Transformation (CCT) diagrams derived from the Ozawa’s model. The analysis of the structural relaxation observed for the residual amorphous phase, which was performed through the calculation of the fictive temperature and the relaxation enthalpy at the glass transition, revealed a strong correlation with the crystallization behavior, suggesting that these phenomena could be jointly used as kinetic markers for the CCR. This work also establishes a combined Fourier and Biot (Fo-Bi) framework, incorporating the CCR value obtained experimentally into the dimensionless transient heat transfer equation. This approach provides a practical method for the estimation of a thickness threshold beyond which amorphous polymer films are obtained in given cooling conditions, i.e., a Critical Film Thickness (CFT). In the case of PHB, the Fo-Bi framework revealed that the films should be no thicker than 270 μ m to completely suppress crystallization to the core under forced-air cooling. Interestingly, for a harsher water quenching, i.e., under a higher heat transfer coefficient, the CFT is reduced to 80 μ m . By integrating both crystallization and structural relaxation aspects, this approach provides a solid framework for defining a CCR value for any given polymer with specific crystallization and glass-forming abilities, and provides a valuable insight into the processing–structure relationships for quenching processes in industrial applications as well as for polymer characterizations in extreme conditions. • The cooling-rate dependence of maximum crystallinity aligns well with Ozawa model. • The parameters of structural relaxation can be used to estimate the CCR. • Combining CCR with a Fourier–Biot approach provides thickness limits to quench films. • This work bridges experimental CCR with practical industrial cooling constraints. • The developed framework may help optimizing characterizations in extreme conditions.