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Using a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), we document the composition-dependent morphologies of 39 new poly(lactide-block-1,4-butadiene-block-lactide) (LBL) block polymers, comprising a broad dispersity B segment (Mn = 4.5-17.7 kg/mol; D= Mw/Mn = 1.72-1.88) and narrow dispersity L end blocks (Mn = 0.6-15.3 kg/mol; D= 1.10-1.21) with volume fractions 0.26 ≤ fB ≤ 0.95. A subset of these samples undergo melt self-assembly into cylindrical, lamellar, and apparently bicontinuous morphologies. By assessing the states of order and disorder in these triblock polymer melts using temperature-dependent SAXS, we find that broad B segment dispersity increases the minimum segregation strength xN ≳ 27 required for LBL triblock self-assembly relative to the self-consistent mean-field theory prediction xN ≥ 17.9 for narrow dispersity analogues. While B segment dispersity has previously been shown to thermodynamically stabilize the self-assembled morphologies of low x/high N ABA triblocks, the present study indicates that broad B block dispersity in related high x/low N systems destabilizes the microphase-separated melt. These observations are rationalized in terms of recent theories that suggest that broad segmental dispersity substantially enhances fluctuation effects at low N, thus disfavoring melt segregation.
Bibliographical noteFunding Information:
We gratefully acknowledge financial support from the National Science Foundation (DMR-1307606 and DMR-1631598). This work also relied on critical core instrumentation facilities funded in part by NSF grants (CHE-9974839) and the University of Wisconsin NSEC (DMR-0832760) and CEMRI (DMR-1121288), which are part of the NSF-funded Materials Research Facilities Network. Synchrotron SAXS analyses were conducted at the DuPont−Northwestern−Dow Collaborative Access Team (DND-CAT) beamline located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.
© 2017 American Chemical Society.
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