Small-angle x-ray scattering experiments conducted with compositionally asymmetric low molar mass poly(isoprene)-b-poly(lactide) diblock copolymers reveal an extraordinary thermal history dependence. The development of distinct periodic crystalline or aperiodic quasicrystalline states depends on how specimens are cooled from the disordered state to temperatures below the order-disorder transition temperature. Whereas direct cooling leads to the formation of documented morphologies, rapidly quenched samples that are then heated from low temperature form the hexagonal C14 and cubic C15 Laves phases commonly found in metal alloys. Self-consistent mean-field theory calculations show that these, and other associated Frank-Kasper phases, have nearly degenerate free energies, suggesting that processing history drives the material into long-lived metastable states defined by self-assembled particles with discrete populations of volumes and polyhedral shapes.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation under grants DMR-1104368 and DMR-1333669. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, under contract DE-AC02-06CH11357. Portions were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the APS, supported by E. I. DuPont de Nemours and Co., The Dow Chemical Company, and Northwestern University. K.K. and F.S.B. designed the research on PI-PLA. K.K. synthesized the PI-PLA samples and acquired and analyzed SAXS data. M.W.S., R.M.L., and M.A.H. designed the research on poly(ethylethylene)-b-poly(?)-lactide (PEE-PLA). M.W.S. and R.M.L. synthesized the PEE-PLA samples and performed SAXS experiments. M.W.S. conceived of and demonstrated the liquid nitrogen thermal processing technique with the PEE-PLA and initially identified the C14 phase. A.A. and K.D.D. designed, executed, and interpreted SCFT calculations. F.S.B. supervised the overall project and drafted the initial manuscript. All authors reviewed and edited the manuscript and supplementary materials.
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