Optical Imaging of Large Gyroid Grains in Block Copolymer Templates by Confined Crystallization

Block copolymer (BCP) self-assembly is a promising route to manufacture functional nanomaterials for applications from nanolithography to optical metamaterials. Self-assembled cubic morphologies cannot, however, be conveniently optically characterized in the lab due to their structural isotropy. Here, the aligned crystallization behavior of a semicrystalline-amorphous polyisoprene-b-polystyrene-b-poly(ethylene oxide) (ISO) triblock terpolymer was utilized to visualize the grain structure of the cubic microphase-separated morphology. Upon quenching from a solvent swollen state, ISO first self-assembles into an alternating gyroid morphology, in the confinement of which the PEO crystallizes preferentially along the least tortuous pathways of the single gyroid morphology with grain sizes of hundreds of micrometers. Strikingly, the resulting anisotropic alignment of PEO crystallites gives rise to a unique optical birefringence of the alternating gyroid domains, which allows imaging of the self-assembled grain structure by optical microscopy alone. This study provides insight into polymer crystallization within a tortuous three-dimensional network and establishes a useful method for the optical visualization of cubic BCP morphologies that serve as functional nanomaterial templates.

: Variation of birefringent textures of solvent-annealed ISO thin films with temperature. (a) At room temperature, after solvent vapor annealing and before heating, extended domains of uniform birefringence are visible. (b) At 55 • C, the birefringent texture disappears. The residual non-uniformity in the image is caused by the surface roughness of the terpolymer film. (c) The birefringence gradually reappears upon cooling at ≈ 40 • C. Birefringent features are visible and increase in density upon further cooling to 36 • C. (d) The initial birefringent texture reappears at 26 • C with a pattern nearly identical to the one shown in (a). The domains exhibit an increased defect density, which is likely due to incomplete crystallization. The crossed arrows indicate the orientation of the polarizers. Scale bars: 500 µm.
To confirm the crystalline nature of the extended domains of uniform birefringence (Figure 1b) and to confirm that the birefringence is the result of the semicrystalline PEO and not any underlying structural anisotropy, heating experiments were carried out ( Figure S1). A heating chamber with transparent windows allowed the in situ observation of the birefringent textures by polarizing optical microscopy (see Experimental Section for details). Samples exhibiting extended domains of uniform birefringence ( Figure S1a) were heated at a rate of is consistent with the melting temperature of PEO as determined by DSC (T PEO m ≈ 55 • C; Figure S2). The samples were kept at 55 • C for 1 min, after which they were allowed to cool.
At a temperature of ≈ 40 • C, the birefringence began to reappear, becoming increasingly clear as the temperature decreased ( Figure S1c). Below ≈ 26 • C, no further changes were observed ( Figure S1d). Figure panels S1a and S1d show nearly identical birefringent textures and patterns. , respectively), differential scanning calorimetry (DSC) was performed to determine these key parameters ( Figure S2).
The peaks at 55 • C during heating and 30 • C during cooling are consistent with values previously reported for the melting and crystallization temperatures of PEO ( Figure S2(a)). 1 A shallow T PS g can be seen at ≈87 • C ( Figure S2(b)), determined by the 50% heat capacity change during vitrification. The corresponding T PS g during heating is ≈90 • C and thus slightly higher than during cooling. The higher T PS g during heating is probably related to its proximity to the PEO melting peak in the heating curves. The "true" liquid base line is expected to be less steep and any error in its identification shifts both the onset and half-step temperatures to higher values (i.e. away from the T PS g measured during cooling).

Grazing-Incidence Wide-Angle X-Ray Scattering
Intensity (a.u.) Figure S3: Grazing-incidence wide-angle X-ray scattering (GIWAXS) of as-spun and solvent-annealed terpolymer thin films. Azimuthally-averaged GIWAXS data for as-spun (black dots) and solvent-annealed (red dots) samples. The blue line is the curve fitted to the as-spun data using two peaks (red lines). The gray line indicates the expected (120) crystallite peak for uniaxially-oriented PEO (homopolymers) after stretching and melt quenching. 2