Phase Behavior of Salt-Doped A/B/AB Ternary Polymer Blends: The Role of Homopolymer Distribution

Abstract

Salt-doped A/B/AB ternary polymer blends demonstrate a plethora of nanostructured morphologies tunable by the composition of homopolymers (A and B) and the corresponding diblock copolymer (AB). Here, we report a complete phase diagram of lithium bis­(trifluoromethane) sulfonimide (LiTFSI)-doped low-molar-mass polystyrene (PS)/poly­(ethylene oxide) (PEO)/symmetric PS-b-PEO block copolymer (SO) blends and evaluate the spatial distribution of homopolymers in the resulting microstructures. In the isothermal phase triangle at 120 °C and r = [Li+]/[EO] = 0.06, a wide region of lamellae (LAM) is bracketed by small zones of double gyroid (GYR) and wide regions of hexagonally packed cylinders (HEX); adjacent to HEX is a significant region of the C15 Laves phase. At a high total homopolymer composition ϕH = ϕPS,homo + ϕPEO,homo+LiTFSI, the copolymer brush becomes saturated and begins to exclude homopolymers, resulting in a rapid domain size increase and inducing the formation of higher curvature phases, as suggested by small-angle X-ray scattering (SAXS). This phenomenon is distinct from charge-neutral ternary blends. Moreover, small-angle neutron scattering (SANS) profiles of selectively deuterated lamellar and bicontinuous ternary blends with contrast variation confirm the existence of a pure PS homopolymer layer in the middle of the PS microdomain.

Self-Assembly of Partially Charged Diblock Copolymer-Homopolymer Ternary Blends

“…However, a dramatic increase in domain size is observed upon further addition of homopolymers, where the scaling is much closer to a dry-brush limit. Such swelling behavior has been reported before for ion-containing ternary systems with small α values . Meanwhile, in the previous work, this was attributed to the presence of free ions, which caused an increase in segregation strength, and direct comparison among the three systems in our work shows that the poor solubility of the homopolymer toward the corresponding block has little relation with the charge fraction.…”

“…Such swelling behavior has been reported before for ion-containing ternary systems with small α values. 52 Meanwhile, in the previous work, this was attributed to the presence of free ions, which caused an increase in segregation strength, and direct comparison among the three systems in our work shows that the poor solubility of the homopolymer toward the corresponding block has little relation with the charge fraction. In Figure 5, all three blends show a similar trend of A c with ϕ H , regardless of the difference in charge fraction and homopolymer size, increasing almost linearly at low ϕ H before deviating at ϕ H > 0.6.…”

“…More surprisingly, a C15 Laves phase was found for a range of compositions in blends with higher salt loading ([Li + ]/[EO] = 0.06). The emergence of this large curvature change was attributed to the high effective χ N induced by the salt, coupled with the differential wet-brush to dry-brush transition for PS vs PEO . As anions are confined to the polymer chains in the current system, the phase behavior may differ from salt-blended systems.…”

“…Charged ternary blends with either added salt ,,− or attached charged moieties on one of the blocks have been explored along the volumetrically symmetric isopleth. , In both cases, the systems showed a marked resemblance to their neutral counterparts. Typically, a LAM phase persists at low homopolymer volume fractions (ϕ H ) until the Lifshitz composition is approached, which mean-field theory predicts to be equal to 1/(1 + 2α 2 ) for a symmetric ternary blend along the isopleth .…”

“…The emergence of this large curvature change was attributed to the high effective χN induced by the salt, coupled with the differential wet-brush to dry-brush transition for PS vs PEO. 52 As anions are confined to the polymer chains in the current system, the phase behavior may differ from saltblended systems. As noted above, the prior work of Shim et al focused on the volumetrically symmetric isopleth, where the composition window over which the BμE was observed apparently increased with charge fraction, for reasons that remain to be elucidated.…”