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

Raphael Dehmel,James A Dolan,Yibei Gu,Ulrich Wiesner,Timothy D Wilkinson,Jeremy J Baumberg,Ilja Gunkel,Ullrich Steiner,Bodo D Wilts

doi:10.1021/acs.macromol.7b01528

Raphael Dehmel, James A Dolan + Show 7 more

Open Access

https://doi.org/10.1021/acs.macromol.7b01528

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Abstract

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.

Highlights

Melting of the birefringent textures at TmPEO ≈ 55 °C and subsequent recrystallization corroborate that the birefringence results from the semicrystalline nature of the poly(ethylene oxide) (PEO) block (Supporting Information Figure S1)
Our results demonstrate a preferential PEO crystallite alignment along the ⟨111⟩ and ⟨100⟩ directions of the single gyroid network
While observable by Grazingincidence wide-angle X-ray scattering (GIWAXS), the surprising consequence is the optical visibility of self-assembled gyroid grains which, unlike aligned cylinders or lamellae,[19,56] are structurally isotropic

Summary

■ INTRODUCTION

Block copolymer (BCP) self-assembly has recently received renewed attention because of its ability to control the structure of functional materials from the “bottom up” on the 10 nm length scale.[1,2] Possible applications include nanolithography,[3−5] antireflective coatings,[6,7] optical metamaterials,[8,9] photovoltaic[10−13] and battery materials.[14−18] While many applications, e.g., energy materials, only require structural connectivity and high specific surface area of the nanostructured material, others, e.g., nanolithography, require detailed control of the self-assembled morphology. This orientation is a function of Tc: as Tc is increased, the long axis of the polymer chains, the crystallite c-axis, transitions from a random orientation to parallel and perpendicular with respect to the interface of the polymer blocks, thereby mirroring the microphase-separated morphology.[26] Notable by their absence from studies of crystalline alignment are three-dimensional BCP network morphologies, e.g., the gyroid. We identify the physical mechanism underlying this anisotropy as a preferential alignment of confined polymer crystallites within one single gyroid network This enables optical imaging of the gyroid BCP grain structure, thereby allowing rapid optimization of template fabrication protocols, used here to create gold gyroid optical metamaterials with exceptional long-range order.

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES