Research ArticleNANOMATERIALS

Polymer ligand–induced autonomous sorting and reversible phase separation in binary particle blends

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Science Advances  23 Dec 2016:
Vol. 2, no. 12, e1601484
DOI: 10.1126/sciadv.1601484
  • Fig. 1 Illustration of ligand-induced phase separation in particle brush blends.

    (A) UCST phase behavior. (B) LCST phase behavior. LCST blends allow for reversible cycling of blend through homogeneous one-phase (1 P) and phase-separated two-phase (2 P) states.

  • Fig. 2 Phase separation in 8SiO2-MMA194/8SiO2-S205.

    AFM phase images of film morphologies at varying composition and annealing temperature after t = 48 hours of thermal annealing. Dark phase corresponds to PMMA. (A) PMMA/PS = 25:75; T = 200°C. (B) PMMA/PS = 25:75; T = 140°C. (C) PMMA/PS = 50:50; T = 200°C. (D) PMMA/PS = 50:50; T = 160°C. (E) PMMA/PS = 75:25; T = 200°C. (F) PMMA/PS = 75:25; T = 140°C. (G) UCST phase diagram for a reference PS/PMMA homopolymer blend with NPS = 205 and NPS = 194 calculated using Flory-Huggins theory (see text for more details). Solid line, binodal; dotted line, spinodal. The region enclosed by the spinodal marks the “unstable regime” where separation by SD is expected. Region highlighted in gray is not accessible to experiments due to thermal degradation of PMMA. Scale bars in (A) to (F), 1 micron.

  • Fig. 3 Structure characterization of films before and after thermal annealing.

    Images show structure evolution in a symmetric blend of 8SiO2-MMA194/8SiO2-S205 (50:50). (A) Bright-field TEM image after thermal annealing at T = 140°C for t = 0. (B) Bright-field TEM image after thermal annealing at T = 140°C for t = 24 hours. (C) AFM phase image after thermal annealing at T = 140°C for t = 0. (D) AFM phase image after thermal annealing at T = 140°C for t = 24 hours. Insets in (B) and (D) show magnified view of the phase-separated structure. PS domain appears dark in (A) and (B) (RuO4 stain); PS domain appears bright in (C) and (D). Electron micrographs further reveal uniform distribution of particle centers (small dark dots) thus confirming uniform film structure before and after thermal annealing. Scale bars, 500 nm (main figures) and 250 nm (insets).

  • Fig. 4 Structure and topology of phase-separated films.

    (A) AFM height image of 8SiO2-MMA194/8SiO2-S205 (50:50) after 24 hours of thermal annealing at T = 140°C. (B) AFM phase image of same film. Z scale in (A) is in nanometers, z scale in (B) is in degrees. Image size in (A) and (B) is 5 μm; black lines indicate position used for line scans in (C) to (E). (C) Line scan of height image depicting long wavelength corrugations with amplitude ≈15 nm that occur on the scale of 1 to 2 μm. Red box (C and D) depicts short wavelength corrugations with amplitude ≈3 nm that coincide with domain positions. (E and F) Enlarged scans. Maxima in height images correspond to PS and minima to PMMA constituent; for phase images, the PS domain corresponds to minima positions (indicated by green arrows).

  • Fig. 5 Analysis of domain growth kinetics.

    (A) Characteristic distance d of 8SiO2-MMA194/8SiO2-S205 (50:50) blend system during thermal annealing at T = 140°C as a function of annealing time, t, revealing the scaling relation d ~ t0.22. The areal fraction of PMMA is fa = 0.45 and independent of annealing time (error bars are within the symbol size). Inset of main figure shows the dependence of d on annealing temperature for a constant annealing time of t = 24 hours. (B) AFM phase image of film at t = 12 hours. (C) AFM phase image of film at t = 166 hours. Scale bars, 1 μm.

  • Fig. 6 Structure formation depends on degree of polymerization of tethered chains.

    AFM phase images of symmetric blends 8SiO2-MMA68/8SiO2-S62 (A), 8SiO2-MMA194/8SiO2-S205 (B), and 8SiO2-MMA350/8SiO2-S360 (C) after 1 week of thermal annealing at T = 140°C. Insets show AFM phase images of respective samples before thermal annealing. Scale bars, 1 μm. Short-chain brush system (A) is found to remain uniform after thermal annealing, whereas phase separation is observed in long-chain brush particle systems (B and C). Nonuniformity of preannealed state is observed for long-chain ligands [inset of (C)] indicating that phase separation commences during casting of films. The scheme illustrates the transition of interaction between brush particles with increasing degree of polymerization of tethered chains: hard sphere–type in case of short polymer chains [concentrated particle brush (CPB) regime] and polymer-like for long-chain systems [semidilute particle brush (SDPB) regime], see Discussion.

  • Fig. 7 Ligand-induced phase separation in large particle blend system.

    Bright-field TEM images of monolayer films of 60SiO2-MMA1470/60SiO2-S1300 (50:50) blends in the presence of 15% DMP revealing SD-type phase separation. (A) As-cast film. (B) After t = 24 hours of thermal annealing at T = 140°C. Dark phase corresponds to PS (RuO4 stain). Insets show magnified image. Scale bars, 500 nm (main figure) and 200 nm (insets).

  • Fig. 8 Reversible structure evolution in 8SiO2-MMA257/8SiO2-SAN262 (50:50) LCST system.

    Ten percent of DMP was added to accelerate kinetics. (A) Illustration of thermal processing steps: annealing at 403 K (130°C) for 24 hours; 443 K (170°C) for 24 hours; and 383 K/403 K (110°C/130°C) for 48 hours along with corresponding domain size d (see text for more detail). The star symbol corresponds to preannealed state, filled black circles represent annealing states of which microstructures are presented. The dotted blue line at T ~ 351 K (75°C) indicates the Tg of the blend in the presence of DMP. TEM images depict film microstructures after subsequent annealing steps (RuO4 stain of PSAN component). (B) Twenty-four hours of annealing at 403K (130°C). (C) Twenty-four hours of annealing at 443 K (170°C). (D) Twenty-four hours of annealing at 383 K (110°C). (E) Forty-eight hours of annealing at 383 K (110°C). Scale bars, 200 nm. The micrographs reveal a transition from uniformly mixed structure (B) to bicontinuous phase-separated structure (C) after annealing at T > LCST. Subsequent annealing at T < LCST results in progressive reduction of domain size (D and E).

  • Table 1 Summary of molecular characteristics of particle brush systems.

    Sample IDs reflect sample characteristics where the first number indicates the approximate particle radius in nanometers. S, styrene; MMA, methyl methacrylate; SAN, styrene-r-acrylonitrile with (S/AN) = 3:1 graft composition. N is the degree of polymerization of surface grafted chains, Mn, GPC is the number average molecular weight as determined by gel permeation chromatography (GPC), Mw/Mn is the molecular weight dispersity, and θs is the surface grafting density. wt %, weight percent.

    Sample IDNMn, GPCMw/Mnθs (nm−2)SiO2 wt %
    8SiO2-S62626,5001.220.7656
    8SiO2-S20520521,4001.360.4922
    8SiO2-S36036037,9001.140.6414
    60SiO2-S13001300135,2001.290.5925
    8SiO2-MMA68686,8001.300.4158
    8SiO2-MMA19419419,4001.170.6525
    8SiO2-MMA25725725,7001.320.4921
    8SiO2-MMA35035035,0001.120.5613
    60SiO2-MMA14701470147,0001.180.9316
    8SiO2-SAN26226222,3001.150.6419

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