Generating Triple Crystalline Superstructures in Melt Miscible PEO-b-PCL-b-PLLA Triblock Terpolymers by Controlling Thermal History and Sequential Crystallization

The morphology, crystallization behavior, and properties of multi-crystalline polymer systems based on triple crystalline biodegradable PEO-b-PCL-b-PLLA triblock terpolymers are reviewed. The triblock terpolymers, with increasing poly(l-lactide) (PLLA) content, exhibit a triple crystalline nature. Upon cooling from melt, the PLLA block crystallizes first and templates the spherulitic morphology of the terpolymer. Then, the poly(ε-caprolactone) (PCL) block crystalizes and, finally, the poly(ethylene oxide) (PEO) block. These triblock terpolymers are melt miscible according to small angle X-ray scattering (SAXS) results. Thus, the crystallization of PCL and PEO blocks takes place within the interlamellar zones of the PLLA spherulites that are formed previously. Therefore, the lamellae of PLLA, PCL, and PEO exist side-by-side within a unique spherulite, constituting a novel triple crystalline superstructure. The theoretical analysis of SAXS curves implies that only one lamella of either PCL or PEO can occupy the interlamellar space in between two contiguous lamellae of PLLA. Several complex competitive effects such as plasticizing, nucleation, anti-plasticizing, and confinement take place during the isothermal crystallization of each block in the terpolymers. New results on how successive self-nucleation and annealing thermal treatment can be used as an additional suitable technique to properly separate the three crystalline phases in these triple crystalline triblock terpolymers are also included.