Orthogonal crystal orientation in double-crystalline block copolymer

In this study, we explore the orientation of crystals formed within the lamellar domains of a diblock copolymer composed of two crystallizable blocks, that is, poly(l-lactide)-block-polyethylene (PLLA-b-PE). The orientation of both PLLA and PE crystals with respect to the lamellar interface was examined under two types of crystallization condition with a broad range of crystallization temperatures (T_c). The first type was the "two-stage crystallization", where the PLLA block was allowed to crystallize before PE. The second was the "one-stage crystallization", where PLLA and PE blocks competed to crystallize. A homeotropic crystal orientation was always observed for the PLLA crystals, with the crystalline stems lying parallel to the lamellar normal regardless of the crystallization condition, except when T _c approached the glass-transition temperature of PLLA, where the orientation became random. A homogeneous crystal orientation with the PE crystalline stems oriented perpendicular to the lamellar normal was always identified at low-to-intermediate degree of undercooling, whereas at large undercooling, the crystals showed random orientation. The "orthogonal orientation" disclosed here was preserved over a broad range of undercooling. Our results further demonstrated that the orientation of both PLLA and PE crystals depended mainly on T_c but was independent of the competitiveness of the two crystallization processes. This was a consequence of the strong segregation that made the two blocks crystallize independently within their respective microdomains.