mz0c00124_si_001.pdf (5.04 MB)
Emergence of a C15 Laves Phase in Diblock Polymer/Homopolymer Blends
journal contribution
posted on 2020-03-27, 19:45 authored by Andreas J. Mueller, Aaron P. Lindsay, Ashish Jayaraman, Timothy P. Lodge, Mahesh K. Mahanthappa, Frank S. BatesThe observation of complex, Frank-Kasper
(FK) particle packings in diblock polymer melts has until recently
been limited to low molecular weight, conformationally asymmetric
polymers. We report temperature-dependent small-angle X-ray scattering
(SAXS) studies of blends of a sphere-forming poly(styrene-block-1,4-butadiene) (SB) diblock polymer (Mn = 33.3 kg/mol, Đ = Mw/Mn = 1.08, fB = 0.18) with two different poly(1,4-butadiene) (B) homopolymer
additives. When the B additive Mn is the
same as that of the diblock core-forming B segment, these blends remarkably
form tetrahedrally close-packed FK σ and Laves C14 and C15 phases
with increasing B content. However, binary blends in which the B additive Mn is 60% of that of the diblock B segment form
only the canonical body-centered cubic (BCC) particle packing and
hexagonally-packed cylinders (HEXc). The observed phase behavior is
rationalized in terms of “wet” and “dry”
brush blending, whereby higher B Mn drives
stronger localization of the homopolymer in the particle cores while
preserving the interfacial area per SB diblock chain. The consequent
packing constraints in these blends destabilize the BCC packing, and
FK phases emerge as optimal minimal surface solutions to filling space
at constant density while maximizing local particle sphericity.