The Effect of the Cooling Rate on the Morphology and Crystallization of Triple Crystalline PE-b-PEO-b-PLLA and PE-b-PCL-b-PLLA Triblock Terpolymers

We study the non-isothermal crystallization and morphology of two triblock terpolymers of practically perfect linear polyethylene (PE), poly(ethylene oxide) (PEO) or poly(ε-caprolactone) (PCL), and poly(l-lactide) (PLLA) with three crystallizable blocks. The two triblock terpolymers PE212.6-b-PEO324.0-b-PLLA475.9 and PE217.1-b-PCL124.2-b-PLLA6723.0 (subscripts indicate the composition in wt %, and superscripts refer to the number average molecular weights in kg/mol) were synthesized by a combination of polyhomologation and ring-opening polymerization techniques, using a “catalyst-switch” strategy. We have applied cooling ramps from the melt at 20 and 1 °C/min while simultaneously performing in situ SAXS/WAXS (small-angle X-ray scattering/wide-angle X-ray scattering) measurements. Parallel experiments performed by differential scanning calorimetry (DSC) and polarized light optical microscopy (PLOM) at identical rates allowed us to unequivocally identify the crystallization sequence of the three blocks from the melt, as well as their superstructural morphology. SAXS indicated that the samples are weakly segregated in the melt as breakout occurs upon crystallization. WAXS and DSC results demonstrated that when the crystallization occurs at 20 °C/min, the blocks crystallize in the following peculiar sequence: PE first, then PLLA, and finally PEO or PCL depending on the triblock terpolymer. The faster crystallization kinetics of the PE block, in comparison with the PLLA block, is responsible for this crystallization sequence. Only when the cooling rate is reduced to 1 °C/min can the PLLA block crystallize first and only for the terpolymer with the highest amount of PLLA. The cooling conditions determine the morphology and properties of these fascinating materials.