Formation and morphology of “shish-like” fibril crystals of aliphatic polyesters from the sheared melt

We found the formation of “shish-like” fibril crystals of aliphatic polyesters such as poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), poly(12-hydroxydodecanoic acid) (PHDA) and poly(16-hydroxyhexadecanoic acid) (PHHA) from the sheared melt with shear rate γ˙ = 5 s−1 observed by polarizing optical microscope (POM). The melting temperature Tms of obtained fibril crystals of PLLA and PCL were higher than those of spherulites and were close to the equilibrium melting temperature Tm0. The small angle X-ray scattering (SAXS) patterns from the bulk sample including fibril crystals, small amount of unoriented small crystals and amorphous showed no peaks arose from the existence of long periods in fibril crystals. These are the evidence that the observed fibril crystals consist of assemblies of a lot of extended chain crystals (ECCs). We observed the morphology of moderately extracted single strand of fibril crystals at the magnification of POM by means of scanning electron microscope. We found that macroscopic fibril crystals of PLLA with diameter d = 10 μm consist of the bundle structure of microscopic fibril crystals with d = 2 μm. From POM observation of the formation of fibril crystals of PLLA and PCL, we showed phase diagrams of molecular weight M and crystallization temperature Tc for the formation of fibril crystals. From these phase diagrams, we evaluated a critical M and Tc for the formation of fibril crystals. Moreover, from the sequential melting and crystallization experiments, it was implied that the entanglement and transesterification play an important role on the formation of fibril crystals of aliphatic polyesters.

Graphical abstractLeft: Typical example of the formation of shish-like fibril crystal observed by polarizing optical microscope. Right:M–Tc phase diagrams of the formation of shish-like fibril crystal.Figure optionsDownload full-size imageDownload as PowerPoint slide