Infrared Scanning Near-Field Spectroscopic Studies of Self-Assembled Nanostructures Based on Photo-Crosslinkable Block Copolymers

Block copolymers, i.e., chemically distinct polymer blocks which are covalently bond, are known for their ability to self-assemble into a variety of morphologies on the nanometer scale.^[1] The self-assembly of block copolymers has been extensively investigated for 2D films,^[2,3] however, less attention has been paid to 3D bulk morphologies as well as light-based 3D printed microstructures. To achieve long-range ordered samples in thermodynamic equilibrium, in this work, 3D bulk morphologies were prepared via solvent annealing and 3D printed microstructures were fabricated from pre-self-assembled block copolymer inks.<br/><br/>Infrared scanning near-field optical microscopy (IR-SNOM) offers the unique possibility of infrared chemical imaging and spectroscopy with a spatial resolution down to ∼20 nm.^[4] When irradiating a well-defined diblock copolymer consisting of polystyrene and a methacrylate-based copolymer decorated with photo-crosslinkable units at an independently addressable absorption band of a polymer block, chemical imaging of the blocks’ nano-ordered spatial arrangement is possible.<br/><br/>Here, the internal nanostructure of self-assembled 3D bulk morphologies with lamellar nanostructures is studied by means of IR-SNOM. When selectively exciting each polymer block, a switch of the relative optical phase, related to the local absorption, is observed, revealing a strong phase segregation via spectroscopic contrast. A successful correlation with scanning electron microscopy (SEM) measurements, performed after post-staining with ruthenium tetroxide, furthermore demonstrates, that ruthenium tetroxide creates a specific metal stain for polystyrene. These studies are extended to cylindrical nanostructures as well as two-photon laser printed microstructures exhibiting complex geometry and hierarchically nano-ordered structures.<br/><br/>References:<br/>[1] Y. Mai <i>et al.</i>, <i>Chem. Soc. Rev.</i> <b>2012</b>, <i>41</i>, 5969-5985.<br/>[2] C. J. Hawker <i>et al.</i>, <i>MRS Bull.</i> <b>2005</b>, <i>30</i>, 952-966.<br/>[3] J. Kim <i>et al.</i>, <i>Spectrochim. Acta A</i> <b>2022</b>, <i>274</i>, 121095.<br/>[4] A. Centrone, <i>Annu. Rev. Anal. </i><i>Chem.</i> <b>2015</b>, <i>8</i>, 101-126.