Lamellar orientation and interlamellar cracks in co-crystallized poly(ethylene oxide)/poly(L-lactic acid) blend

Abstract

The correlation between the lamellar orientation in ring-banded spherulites and crack patterns in blends of two crystalline polymers crystallized in two steps was investigated. The two polymers were poly(ethylene oxide) (PEO) and poly(L-lactic acid) (PLLA) with a low molecular weight. The techniques used for this investigation included polarizing light optical microscopy, scanning electron microscopy and atomic force microscopy. Crack formation was influenced by the ring band patterns in PLLA and overlapping or impingement between PEO spherulites upon cooling. By extracting the water-soluble PEO from the PEO/PLLA blend, the inner morphology of the lamellar textures was further revealed to have a complex pattern. It consisted of the outer macro-lamellae with a flat-on width of 20 μm and twisting micro-lamellae with a 1–2 μm edge-on width. As the excess PEO was etched off from the valley and the interlamellar crevices of the PLLA lamellae, the outer macro-lamellae plates (∼20 μm flat-on width) exhibited up and down periodicity with flat-on orientation. However, the micro-lamellae (with 1–2 μm edge-on width) with twisting were visible inside the cracks. Similar analysis of a PEO-rich composition, PEO/PLLA (80/20) blend with irregular ring bands and no cracks, revealed that the outer macro-lamellae were not present. However, all micro-lamellae twists in the ridge regions irregularly protrude out to create zig-zag banding. Atomic force microscopy (AFM) phase images for the ridge and valley regions of the water-etched poly(ethylene oxide)/poly(L-lactic acid) (50/50) blend crystallized at crystallization temperature (Tc)=110 °C. The AFM graph (5 μm×5 μm) of the ridge region b shows a texture different from that of the valley region c. The surface patterns of the lamellae in the valley and ridge in these two regions appeared as flat-on and edge-on lamellae, respectively. The anisotropicity of the platelet crystals in these two regions was also apparently different. The orientation of the crystal lamellae in the valley (region c) was circumferential, whereas that of the lamellae in the ridge (region b) was radial. The difference in orientation in the ridge and valley regions may account for the cracks in the ridge regions but not in the valley region.