Asymmetric Growth of Co-Crystallized Nano- and Micrometer-Sized Lamellae to Janus-Faced Spherulites in Poly(l-lactic acid) with Amorphous Poly(methyl methacrylate)

Surface topology and interior crystals of highly anisotropic Janus-faced spherulites from crystallization of semicrystalline poly(l-lactic acid) (PLLA) with amorphous poly(methyl methacrylate) (PMMA) have been characterized using polarized optical, atomic-force, and scanning electron microscopes. The morphologies in the two faces differ dramatically in optical birefringence, growth rates, as well as crystal dimensions and assembly. Dissection into the interior lamellar arrangement of the optically asymmetric spherulites is conducted via fracturing and selective etching with a proper solvent. Large α′-form platelet lamellae, along the direction of the sheaf-nuclei, grow faster, periodically bend or branch out, appearing as densely fan-like dendritic region(s) with strong birefringence. As opposed to this, the α-form nanosize rod-like lamellae grow more slowly in the perpendicular direction, with each rod slanting at an angle to the substrate, appearing as a bivalve-shaped region of weak birefringence. Both types of lamellae concomitantly grow into a half-and-half Janus-faced PLLA spherulite.