Research ArticleMATERIALS SCIENCE

Tunable structural color of bottlebrush block copolymers through direct-write 3D printing from solution

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Science Advances  10 Jun 2020:
Vol. 6, no. 24, eaaz7202
DOI: 10.1126/sciadv.aaz7202
  • Fig. 1 PDMS-b-PLA bottlebrush synthesis, solution preparation, and dropcast film.

    (A) Seed anionic ring opening polymerization of hexamethylcyclotrisiloxane to produce PDMS macromonomers. (B) 8-diazabicyclo[5.4.0]undec-7-ene (DBU) catalyzed ring opening polymerization of lactide to produce PLA macromonomers. (C) Sequential graft-through ring-opening metathesis polymerization (ROMP) of PDMS and PLA macromonomers. (D) Molecular weight versus time plot for the synthesis of PDMS-b-PLA bottlebrush. PDMS macromonomers are polymerized first (t < 30 min), and then PLA macromonomers are added and polymerized (t > 30 min). (E) Image of dried, as-synthesized bottlebrush stock material. (F) Microscope camera image of a drop-cast film taken at normal incidence under a ring light. Photograph courtesy of Bijal Patel, University of Illinois.

  • Fig. 2 Direct-write 3D printing scheme.

    (A) Render of the printing setup comprising 3D motion axes, pneumatic dispenser, and computer control. (B) Cartoon of molecular assembly during the solution-casting process. Pressure is applied to cause outflow of polymer solution during translation at velocity, v. Microphase separation occurs simultaneously with solvent evaporation to form lamellae. (C) Programmatic variation of optical properties via modulation of printing speed and temperature. Optical microscopy images of printed meanderline patterns on bare silicon are shown in the figure. At each temperature (pair of rows), images at low magnification (inset A) and high magnification (inset B) are shown. (D) Chameleon patterns printed as continuous prints under constant printing conditions (pressure, printing speed, and bed temperature). (E) Complex pattern printed in three layers at three bed temperatures. Print speed was tweaked on the fly to tune line thickness, color, and intensity throughout the print, leading to intended variation seen in the green/blue 25°C lines.

  • Fig. 3 Optical properties of printed lines correlated to printing speed and substrate temperature.

    (A) Diffuse reflection spectra obtained using the integrating sphere geometry, vertically shifted for clarity. Y axis represents reflectivity (% versus spectralon standard). (B) Lorentzian peak fits describing peak position and FWHM as a function of printing speed. Error bars denote SE of the fit. Raw spectra can be found in fig. S18.

  • Fig. 4 Microstructural characterization.

    (A) Representative cross-sectional SEM of dropcast and printed films with 1-μm scale and guidelines. (B) Left: 2D SAXS data for dropcast sample with sample azimuthal line-cut (white). Middle: 1D linecuts plotted for various azimuthal angles (θ). Right: d-spacing determined from the q interval between adjacent peaks. Solid black line reflects azimuthally averaged data. (C) Left: 2D SAXS data for a sample printed at 50°C and 120 mm/min. The black tick mark indicates the region integrated for 1D profiles. Middle: 1D SAXS profiles for samples printed at 50°C. Deviation at low q (orange curve) caused by positioning the integrating region at an offset from qxy = 0 to avoid diffuse background intensity. Right: Domain d-spacing calculated from SAXS (solid points) versus printing speed. Error bars on colored points represent the range of two scans. Error bars for dropcast (DC) sample represent the SD of nine measurements across three samples. The dashed line represents the contour length of the bottlebrush estimated with a fixed backbone contour length of 0.62 nm per norbornene repeat unit. Faded lines connect the domain size estimates obtained by application of the Bragg-Snell law to optical peaks reported in Fig. 3.

  • Fig. 5 In situ optical monitoring of the 3D printing process.

    Full analysis details and code are provided in section S14. Images beside plot correspond to the numbered ticks. All data shown in (A) to (C) were obtained at a substrate temperature of 50°C and applied pressure of 30 kPa. (A) Meniscus height profile versus time for samples 3D printed at various printing speed. Images below are snapshots of transmission-mode video taken for sample printed at 60 mm/min. Inset contains plots of drying time (x intercept) versus printing speed. (B) Intensity plotted against elapsed time for 3D-printed samples. Images below are snapshots from the reflection-mode video corresponding to labeled tick marks for sample (60 mm/min) within plot. (C) Assembly time (peak intensity) plotted against drying time, showing close matching between the two. Dashed line indicates a slope of one. (D) 2D SAXS pattern for solution (100 mg/ml) of BBCP in THF. (E) 1D azimuthally averaged profiles for backbone DP of 400 (top two curves) and 200 (lower curve). Inset depicts fitting of the low-q peak to a lamellar structure factor. (F) Cartoon of bottlebrush conformation in micellar and lamellar assemblies.

  • Fig. 6 SVA experiments.

    (A) Microscopy images during the SVA process for a sample printed at 30 mm/min at 25°C and annealed at 50°C. At time = 0 s, solvent was introduced to the chamber, and the sample was allowed to swell for 300 s before the cover was removed and the sample deswelled. (B and C) Ultraviolet-visible (UV-Vis) reflection spectra for the line patterns printed at 30 mm/min (solid), 90 mm/min (dashed), and 150 mm/min (dashed-dotted) before annealing (ann.) (lower curves), after annealing at the printing temperature (middle curves), and a higher temperature (top curves). For reflectivity measurements, the detector spot size encompasses the entire printed sample. (D) Plot of peak reflected wavelength for the 12 samples before (hollow symbol) and after (filled symbols) annealing for each printing speed: 30 mm/min (triangles), 90 mm/min (circles), and 150 mm/min (squares). Vertical error bars denote the SE from fitting peaks to Lorentzian profiles.

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