Morphological Behavior of Model Poly(ethylene-alt- propylene)-b-polylactide Diblock Copolymers SCOTT C. SCHMIDT, MARC A. HILLMYER Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455 Received 10 May 2002; revised 15 July 2002; accepted 16 July 2002 Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com).DOI: 10.1002/polb.10291 ABSTRACT: A set of well-defined poly(ethylene-alt-propylene)-b-polylactide (PEP-PLA) diblock copolymers containing volume fractions of PLA (f PLA ) 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 co- polymers was investigated with a combination of rheology, small-angle X-ray scatter- ing, and differential scanning calorimetry. The ordered microstructures observed were lamellae (L), hexagonally packed cylinders (C), spheres (S), and gyroid (G), a bicon- tinous cubic morphology having Ia3 ¯ d 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  PEP-PLA (T) was estimated from T ODT ’s by mean-field theory and subse- quently used in the construction of the experimental PEP-PLA morphology diagram (N versus f PLA ). 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 Keywords: polylactide; block copolymers; polyesters; microstructure; interaction pa- rameter; morphology; renewable resources INTRODUCTION The synthesis and characteristic self-assembly of various model, immiscible (A-B) diblock copoly- mers has been well documented. 1 Self-assembly of these materials is predominantly governed by the strength of the interaction between the com- ponent monomers (i.e., Flory–Huggins interac- tion parameter ), the overall degree of polymer- ization (N), and the block copolymer composition (e.g., the volume fraction of component A, f A ). 2 The product N determines the degree of segre- gation in the system. When the product of N is 10, entropic factors dominate and the material is disordered. As N is increased, the enthalpic interactions are most important, and the system segregates into an ordered morphology containing A- and B-rich domains with compositional heter- ogeneities on the nanometer length scale. The volume fraction of one of the components primar- ily dictates the ordered-state symmetry, 3–7 and the now classical equilibrium morphologies are lamellae (L), a bicontinuous structure with Ia3 ¯ d space group symmetry or gyroid (G), 8 –10 hexago- nally packed cylinders (C), and spheres packed on a body-centered cubic lattice (S). 11 These materi- als remain captivating as toughened plastics, Correspondence to: M. A. Hillmyer (E-mail: hillmyer@ chem.umn.edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 40, 2364 –2376 (2002) © 2002 Wiley Periodicals, Inc. 2364