Parallel-stripe structures in PLLA/POM blend films

Abstract

Poly(l-lactic acid)/poly(oxymethylene) bulky blend is a typical system exhibiting ring-banded spherulites. In this work, however, the parallel ridges and valleys were fabricated for the first time in the blend film by the precise control of film thickness. They are named as “parallel-stripe structures” to distinguish from general “ring-banded structures”. Our results indicate that the cracks on thinner films prevent the development of ridge/valley rings along the film, accounting for the formation of these parallel structures. Consequently, they can reduce the influence of the fractal and resultant branching lamellae which makes the investigation of ring-banded structures more complicated. On the other hand, a novel in-situ etching method has been established to investigate the composition distribution of PLLA and POM in film thickness direction. As a result, the tri-layer structures including top-PLLA layer, blend layer and bottom-POM layer were reconstructed and validated by melting contact angles. Based on the atomic force microscope and transmission electron microscope results, it is concluded that the twisting model dominates the formation of parallel banded structures in PLLA/POM blend film. Furthermore, the crystallization temperature and composition dependence of lamellae twisting has been investigated in detail. On one hand, the increase of crystallization temperature leads to the well-developed POM crystal lamellae, which is the reason for the slower twisting. On the other hand, blend films with higher PLLA weight fraction produce bigger period and smaller radius during POM lamellae twisting because of the depression of the top PLLA wetting layer.