3D Morphological Characterization of Complex Soft Matter Assemblies at the Sub-Unit Cell Level

The double gyroid (DG) microdomain structure is a complex 3D tubular network structure found in block copolymers as well as in butterfly wings and amphiphilic phases.   We employ a slice and view electron microscopy technique to directly generate a 3D tomogram of the DG nanoscale structure.  The material studied is a polystyrene (PS) – polydimethylsiloxane (PDMS) diblock copolymer that exhibits the DG network morphology with a lattice parameter of ~ 130nm, with feature sizes on the 20 nm scale, typical of many soft matter assemblies.   By alternating between a thin ion beam slice and a secondary electron image using a low voltage incident electron beam, the voxel size is approximately 3 x 3 x 3 nm³ allowing analysis of a comprehensive set of sub-unit cell morphological descriptors and enabling critical comparison to theoretical models of the structure.  We find that the PS-PDMS material does not exhibit cubic symmetry, but rather a range of triclinic shapes, most likely due to distortions of the structure from solvent induced shrinkage during film preparation.  Analysis of the triclinic unit cell determines the magnitudes and directions of the shear and tensile deformations that can be re-expressed as an eigen-matrix of principal compressive/tensile strains (average compressive strain of ~ -20% and tensile strain about + 20%).  Morphological characteristics are analyzed including direct measures of the distributions of the distances between the interface between the two blocks and the skeletal graph and the distance between the interface and the triply periodic gyroid minimal surface, the mean and Gaussian curvatures of the interface, the dihedral angle between adjacent nodes, as well as the node-node strut lengths and directions. These very detailed experimental measures are compared with self consistent field theory calculations of a PS-PDMS melt undergoing the ODT under boundary conditions that are matched to the deformed cubic (i.e. triclinic) unit cell.
 
References:
 
Prasad, I., Jinnai, H., Ho, R-M., Thomas, E. L. and Grason, G. “Anatomy of triply-periodic network assemblies:  Characterizing skeletal and inter-domain surface geometry of block copolymer gyroids,” Soft Matter, 14, 3612-3623 (2018).
 
X. Feng, H. Guo and E. L. Thomas, “Topological Defects in Tubular Network Block Copolymers,” Polymer, (2019) https://doi.org/10.1016/j.polymer.2019.01.085

Bio: Edwin L. “Ned” Thomas served as William and Stephanie Sick Dean of the George R. Brown School of Engineering at Rice from 2011 to 2017. He holds joint appointments in the Departments of Materials Science and NanoEngineering and Chemical and Biomolecular Engineering and collaborates with scientists and engineers in the Richard E. Smalley Institute for Nanoscale Science and Technology at Rice.
 
Thomas is a materials scientist and mechanical engineer and is passionate about promoting engineering leadership and student design competitions. His research is currently focused on using 2D and 3D lithography, direct-write and self-assembly techniques for creating metamaterials with unprecedented mechanical and thermal properties.
 
Thomas is the former head of the Department of Materials Science and Engineering at the Massachusetts Institute of Technology, a position he held from 2006 until his appointment at Rice in July 2011. He was named Morris Cohen Professor of Materials Science and Engineering in 1989 and is the founder and former director of the MIT Institute for Soldier Nanotechnology (2002-2006).
 
Before joining MIT in 1988, Thomas founded and served as co-director of the Institute for Interface Science and was head of the Department of Polymer Science and Engineering at the University of Massachusetts. He is a recipient of the 1991 High Polymer Physics Prize of the American Physical Society and the 1985 American Chemical Society Creative Polymer Chemist award. He was elected to the National Academy of Engineering and the American Academy of Arts and Sciences in 2009, Inaugural Fellow of the Materials Society in 2008, Fellow of the American Association for the Advancement of Science in 2003 and Fellow of the American Physical Society in 1986. He wrote the undergraduate textbook, The Structure of Materials, and has coauthored more than 450 papers and holds 20 patents.
 
Thomas received a B.S. in mechanical engineering from the University of Massachusetts and his Ph.D. in materials science and engineering from Cornell University.