Order and Disorder in High x/Low N, Broad Dispersity ABA Triblock Polymers

Adam K. Schmitt, Mahesh K. Mahanthappa

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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.

Original languageEnglish (US)
Pages (from-to)6779-6787
Number of pages9
Issue number17
StatePublished - Sep 12 2017

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