(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.
We first focus on demonstrating that our direct-write 3D printing scheme is, in fact, able to induce kinetic trapping in printed films by precisely controlling drying time. Through in situ microscopy of printed films (Fig. 5, A to C), we confirm this by demonstrating (i) that drying time decreases with increasing printing speed and (ii) that assembly time has a corresponding decrease. To accomplish the first task, we captured a series of side-view videos of the printing meniscus in the transmission geometry (movie S5 and section S14) to track wet film thickness over time with good spatial and temporal resolutions (~10 μm and 0.03 s, respectively). Videos were analyzed in MATLAB to yield the meniscus height profiles depicted in Fig. 5A. Starting from the time the nozzle passes (t = 0), the height of the meniscus falls before reaching a baseline value. This time interval is taken to be the drying time and is plotted in the inset for a range of different printing speeds. The results clearly indicate that faster printing speeds did, in fact, reduce the drying time of the films.
Snapshots of the annealing process are shown in Fig. 6A. Upon exposure to solvent vapor, printed thin films showed a gradual redshift in color over a period of ~1 min, before becoming largely colorless (even under very high light exposure). The system was held for a total solvent exposure time of 300 s to allow for molecular rearrangement in the highly mobile solvent-laden state before the removal of solvent vapor, with the film blueshifting to its stable color over ~10s of seconds. Figure 6B (Tprint = 25°C) and Fig. 6C (Tprint = 50°C) show the 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 (lower curves), after annealing at the print temperature (middle curves), and after annealing at higher temperatures (top curves). These spectra were then fit, and the peak reflectance is plotted in Fig. 6D. Comparison of samples printed and annealed at the same temperature (labeled 25/25 and 50/50 in Fig. 6D) clearly shows that after SVA, the d-spacing for samples printed at different speeds converges to a single value. That is, by eliminating variations in processing time, we eliminate the difference in optical properties, supporting our kinetic trapping hypothesis. 2b1af7f3a8