College of Science & Engineering
The Mahanthappa research group focuses on molecular self-assembly approaches to spatially periodic, nanostructured soft materials. In one major research thrust, they are examining the thermodynamically driven supramolecular self-assembly of minimally hydrated amphiphilic molecules ("soaps") into lyotropic liquid crystals (LLCs). In order to access materials with larger unit cells (≥ 50 nm), they are also studying the self-assembly of linear multiblock polymers and bottlebrush block polymers. Amongst these classes of materials, the researchers seek universal molecular design rules that enable access to both 3D network phases and low-symmetry discontinuous micellar morphologies. By virtue of their tunable chemistries that stem from the underlying amphiphile structures and their fascinating mesoscopic structures, these soft materials find applications as water filtration and molecular separation membranes, soft templates for mesoporous inorganic materials synthesis for catalysis and optics applications, and as drug delivery vehicles.
In the area of discontinuous micellar phases, the researchers have recently discovered that both LLCs and diblock polymers form myriad low-symmetry micelle packings beyond the canonical, high-symmetry body-centered cubic (BCC), face-centered cubic (FCC), and hexagonally close-packed (HCP) structures. These new, tetrahedrally-close packed Frank-Kasper (FK) phases are micellar mimics of the well-known structures of various heavy elements and intermetallic alloys. These FK phases are characterized by large, low-symmetry unit cells comprising ≥ 7 particles that exhibit unusual X-ray diffraction patterns with high degrees of long-range translational order, and they are periodic approximants to quasicrystals (QCs). This research seeks to elucidate the mechanisms by which this new and other related FK phases form, by systematically examining how amphiphile structure and sample processing history specify sphere packing symmetry selection and the thermodynamic stabilities of these complex supramolecular assemblies.
Over the last five years, the group has discovered molecular designs that enable the formation of at least seven new micelle packings, including the first dodecagonal quasicrystal derived from a simple amphiphile/oil/water mixture. While they have discovered amphiphile designs that furnish unprecedented access to 3D network phases in LLCs, they are currently working to scale up these molecular design rules to self-assembling block polymers to access structures with larger unit cell parameters for broader applications. Using insights gained in the complementary studies of LLC and block polymer sphere packings, the group is working toward the syntheses and characterization of new, bottlebrush shape filling amphiphiles as part of the recently renewed NSF-funded UMN Materials Research Science & Engineering Center.
Since real-spacing imaging of these aqueous LLC phases using electron microscopy is challenging and slow, the researchers typically use variable temperature synchrotron small-angle X-ray scattering (VT-SAXS) analyses to characterize these unusual assemblies. This group was the first to recognize in 2013 that a Rietveld refinement of these data could be performed to obtain the structure factor ("Fourier") amplitudes for the structure in reciprocal space, which can then be used as inputs for the charge-flipping algorithm implemented within the SUPERFLIP software package to reconstruct electron density maps for the structures formed by self-assembled soft materials to understand how they form. These electron density map reconstructions require large amounts of computing power, incompatible with traditional desktop computers and other computing resources.
Calculations that depend on MSI resources have the potential to expand the applications of LLC and block polymer materials, by elucidating the mechanisms by which they enable “bottom up" molecular design of structures of tailored complexity and useful properties. The notion that frustration at two or more competing length-scales can drive complex pattern formation is a relatively new idea. Thus, this work may ultimately inform new methods for harnessing frustration in self-assembled systems, as a means of rationally accessing complex periodic and aperiodic structures derived from myriad constituent building blocks. As one illustration of this group's work, they recently published a paper acknowledging MSI computing resources for the reconstruction of electron density maps for a C15 Laves phase derived from binary blends of a linear diblock polymer and a linear homopolymer. The computations required to conduct these detailed structural analyses would have been impossible without MSI resources.