Minima in the temperature gradient of the crystallization rates of long-spaced polyacetals at the transition between two polymorphs

Under isothermal crystallization (T_c) from the melt, polyacetals spaced by 12, 18, 19, or 23 methylenes develop up to four layered polymorphs. While the crystallization kinetics of the two high T_c polymorphs can be followed by classical DSC, in this presentation it will be demonstrated that the kinetics of the two low-temperature phases can be extracted via Fast Scanning Calorimetry (FSC). The unusual inversion of crystallization kinetics at T_c in the boundary between two polymorphs observed by DSC is also present for the metastable low temperature phases. While the rate minima observed when transitioning between the two highest temperature polymorphs is attributed to competition in nucleation between the two forms coupled with a self-poisoning effect, a possible heterogeneous to homogeneous nucleation change will be discussed for the transition to the lowest temperature disordered structure. Such a change in nucleation type has been posited to explain similar crystallization rate minima in other polymers at the transition between mesomorphic and crystal structures.