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Morphological behavior of model poly(ethylene-alt-propylene)-b-polylactide diblock copolymers



A set of well-defined poly(ethylene-alt-propylene)-b-polylactide (PEP-PLA) diblock copolymers containing volume fractions of PLA (fPLA) ranging between 0.08 and 0.91 were synthesized by a combination of living anionic polymerization, catalytic hydrogenation, and controlled coordination-insertion ring-opening polymerization. The morphological behavior of these relatively low-molecular-weight PEP-PLA diblock copolymers was investigated with a combination of rheology, small-angle X-ray scattering, and differential scanning calorimetry. The ordered microstructures observed were lamellae (L), hexagonally packed cylinders (C), spheres (S), and gyroid (G), a bicontinous cubic morphology having Ia3d space group symmetry. The G morphology existed in only a small region between the L-C morphologies in close proximity to the order–disorder transition (ODT). Transformations from L to G were observed upon heating in several samples. The efficacy of the reverse G to L transition in one sample was cooling rate dependent. The PEP-PLA Flory–Huggins interaction parameter as a function of temperature χPEP-PLA(T) was estimated from TODT's by mean-field theory and subsequently used in the construction of the experimental PEP-PLA morphology diagram (χN versus fPLA). The resultant morphology diagram was symmetric there were the well-defined L-C morphology boundaries. The low molecular weight of the materials imparted no significant deviation from previously documented diblock systems. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2364–2376, 2002