Macro- and micro-lamellar assembly and mechanisms for unusual large-pitch banding in poly(l-lactic acid)

Lamellar patterns and orientations in blends of two crystalline polymers: poly(ethylene oxide) (PEO) and poly(l-lactic acid) (PLLA) were investigated using polarizing-light optical microscopy (POM) and atomic-force microscopy (AFM). Specific etching of PEO off the lamellae was used to reveal the interior complex PLLA lamellae assemblies in correlation to POM birefringence patterns. AFM analysis on the lamellar morphology of the water-etched PEO/PLLA = 50/50 blends demonstrated that the PLLA spherulites with large-pitch banding (50–100 μm) were composed of wavy macro-lamellar plates resulted from the assemblies of lamellae. The lenticular-shaped cavities are associated with only the macro-lamellar plates, and located only on the ridge portion of spherulites. Ring-banded PLLA spherulites of large pitches (∼50 microns) are constructed by synchronized wavy lamellae as the major species from top views. In addition, there might be some sub-lamellae underneath the macro-lamellae, which could be identified only by side views via dissecting across sample thickness. The mechanisms and correlation between the lamellar assembly and ring bands are further exemplified.