A deep insight into the spontaneous fragmentation of two-dimensional crystalline platelets

As typical polymer lamellar crystals, two-dimensional (2D) platelets obtained from seeded growth of crystallizable block copolymers enable the regulation of their metastability through rational manipulation of crystallization pathways. Herein, the metastability of 2D platelets with crystalline poly(ε-caprolactone) (PCL) core is regulated by changing of crystallization temperatures, molecular weight of PCL and compositions of crystalline seeds. Experimental results have shown that colloidally stable PCL platelets would be created at a high crystallization temperature (4 °C), and these platelets are sufficiently stable to resist the structural change during aging process. In contrast, less stable PCL platelets are formed via seeded growth at a low crystallization temperature (-25 °C). Spontaneous fragmentation of less stable PCL platelet would occur when low-molecular-weight PCL composition is incorporated into PCL platelet particles, resulting in the formation of 2D hollow platelet structure. Moreover, it is further observed that the platelet fragmentation rate would accelerate when compositionally distinct crystalline seeds are used for seeded growth, which is due to the larger lattice mismatching degree between PCL and crystalline seeds. Mechanistic study has shown that the PCL platelets formed from low-molecular-weight PCL has a low melting temperature in ethanol solution, making them more susceptible to lamellar thickening via melt-recrystallization during aging/annealing process.