Polymorphism and Stretch-Induced Transformations of Long-Spaced Sustainable Aliphatic Polyesters

Polyethylene-like bioderived, biodegradable, and fully recyclable unbranched aliphatic polyesters, such as PE-2,18, develop hexagonal crystal structures upon quenching from the melt to temperatures < ~50 °C, and orthorhombic-like packing at higher quenching temperatures, or after isothermal crystallization. Both crystal types are layered. While all-trans CH₂ packing characterizes the structure of the orthorhombic-like form, there is significant conformational disorder in the staggered long CH₂ sequences of the hexagonal crystals. On heating, the hexagonal crystals transform to the orthorhombic type at ~ 60°C via melt-recrystallization, but no change is apparent during heating samples with the orthorhombic form up to the melting point (~ 95°C). The hexagonal structure is of interest not only because it develops under very rapid quenching from the melt, but also because under uniaxial tensile deformation it undergoes a stretched-induced transformation to the orthorhombic structure. Compared to deformation of orthorhombic specimens that maintain the same crystal type, such transformation derives in larger strains and enhanced strain hardening, thus representing a desired toughening mechanism for this type of polyethylene-like materials.