Effect of self-seed crystal structure on growth of polymorphs in poly(butylene 2,6-naphthalate) : a cross-nucleation study

Abstract

Cross-nucleation between different crystal polymorphs is a particular, self-seed assisted type of heterogeneous nucleation, where a fast-growing polymorph nucleates at a pre-existing crystal surface of another polymorph. Here, we present a study on cross-nucleation between different crystalline phases of poly(butylene 2,6-naphthalate) (PBN), employing hotstage polarized-light optical microscopy and temperature-resolved wide-angle X-ray scattering as analysis tools. PBN forms α-crystals at relatively low temperature and β′-crystals at rather high temperature, with cross-nucleation experiments designed such to first obtain few α- or β′-seed crystals (mother phase) which then are transferred to higher or lower temperature, respectively, to monitor the continuation of the crystallization process and possible growth of the other polymorph. In case of cooling β′-crystals to lower temperature where typically α-crystals form in the non-seeded isotropic melt, β′-crystals nucleate growth of α-crystals, following many examples of cross-nucleation in the literature. In contrast, if low-temperature-generated α-crystals are heated to a temperature where β′-crystals form in a non-seeded melt, the cross-nucleation efficacy is reduced as, beside growth of cross-nucleated β′-crystals, also growth of the mother phase is observed. This unexpected result demonstrates the importance of the structure of the nucleating substrate and the interplay between kinetic and thermodynamic aspects of crystal growth.