Trilayered Morphology of an ABC Triple Crystalline Triblock Terpolymer

Abstract

Triple crystalline triblock terpolymers are materials with remarkable semicrystalline superstructures. In this work, we report for first time the alternating triple lamellar morphology that self-assembles inside spherulites of a triblock terpolymer composed of poly(ethylene oxide) (PEO), poly(ε-caprolactone) (PCL), and poly(L-lactide) (PLLA). The morphology of the PEO-b-PCL-b-PLLA triblock terpolymer is compared to an analogous PCL-b-PLLA diblock copolymer. Both diblock and triblock form a single phase in the melt. Two crystallization protocols were employed to create particular crystalline morphologies. In both cases, the isothermal crystallization of the PLA block is induced first (at 81 °C, a temperature above the melting points of both PCL and PEO blocks) and PLLA spherulites form a template, whereupon cooling the other two blocks can crystallize within the PLLA interlamellar spaces. WAXS analysis demonstrated the double crystalline and triple crystalline nature of the materials. The lamellar structure was evaluated by AFM observations and SAXS measurements. Moreover, theoretical SAXS curves of one-dimensional structural models were calculated. AFM micrographs of the triblock terpolymer evidenced the three different lamellae of PLLA, PCL and PEO that coexist together within the same spherulite. Three different lamellar thickness were determined, and their dimensions suggested that all blocks crystallized in chain-folded conformations. The evolution of the triple lamellar morphology during heating of tricrystalline samples was followed by in situ synchrotron SAXS measurements. The theoretical analysis of the SAXS curves of the triblock terpolymer allowed us to propose a stacking morphological model, in which a particular trilayer structure exists, where one lamella of PCL or one lamella of PEO is inserted randomly between two adjacent PLLA lamellae.

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

“…DSC measurements demonstrated that the PEO block melts at 45 °C. As conclusion, the WAXS experiments demonstrated without doubt the exceptional tricrystalline structure observed by AFM for this ABC type triblock terpolymer …”

“…Beyond 60 °C, the PLLA block is the only one that remained crystalline and its interlamellar regions are composed of mixed amorphous phase that include PLLA, PCL, and PEO chains. Finally, at around 120 °C, the long period values and, accordingly, the PLLA crystalline lamellar thickness increased (see Figure b,c) …”

“…b) Intensity ratio between WAXS signals PLLA 113/203 and PLLA 110/200 of PCL 43 PLLA 57 15.4 , and (PLLA 113/203 +PEO 120 ) and PLLA 110/200 of PEO 23 PCL 34 PLLA 43 19.9 during heating after crystallizing the samples in two steps. Reproduced with permission . Copyright 2017, American Chemical Society.…”

“…SAXS/WAXS measurements were taken of the as crystallized triblock terpolymer at room temperature, and then during a subsequent heating (see Figure a). The evolution of the long period ( d ) value was registered as a function of temperature . It is very difficult to certainly assign the SAXS peaks to a particular population of lamellae in the triblock terpolymer, since the alternated lamellar structure in which the three blocks co‐exist is not that clear.…”

“…c) Evolution of the PLLA lamellar thickness during heating, after isothermally crystallizing the sample in two steps: first at 81 °C and then at 50 °C. Reproduced with permission . Copyright 2017, American Chemical Society.…”