Formation of Stacked Three-Dimensional Polymer "Single Crystals"

Stacks of crystalline lamellae all having a uniquely oriented hexagonal shape, referred to as "3D (three-dimensional) single crystals", were obtained via isothermal crystallization of isotactic polystyrene at a temperature close to the melting point. The height of the stacks of lamellae reached several micrometers corresponding to hundreds of superposed lamellar crystals. We propose that the mechanism of self-induced nucleation allowed propagating the orientation of the basal lamellar crystal to all other lamellae in the stack. The unique orientation of all lamellae was reflected by the preference of cracks to form along the diagonals of the hexagonal stack. Cracks were caused by a mismatch in the coefficients of thermal expansion of the substrate and "3D single crystals" during quenching from the crystallization temperature to room temperature. We believe that the presented growth mechanisms leading to "3D single crystals" can be observed for all crystallizable polymers including block copolymers with a noncrystallizable block.