The Laboratory for Research on the Structure of Matter (LRSM) at the University of Pennsylvania is a center of excellence for materials research and education. It facilitates collaboration between researchers from different disciplines – physics, chemistry, engineering, biology, and medicine – to advance transformative scientific projects and solve societal challenges.
   The 2017-23 MRSEC supported IRG & Seed senior investigators, who published more than 525 research articles. This research depended on the support of personnel and in-house scientific experimental facilities. The critical outcomes of IRG research are described below.
   IRG-1 Rearrangements & Softness in Disordered Solids: IRG1 participants developed a microscopic understanding of the mechanical response of disordered solids composed of particulates with a wide range of length scales above yielding, from atoms to grains. To this end, they introduced a novel structural parameter, softness, which is a weighted average of many local structure functions where the weights are determined by machine learning to correlate strongly with the susceptibility of a particle to rearrange. They demonstrated that softness predicts particle rearrangements and sample dynamics in different disordered solids. (Figure 1) A structuro-elastoplastic model that explicitly incorporates local microstructural information via softness was developed and tested, and it was shown that it successfully describes the interplay of local structure, plasticity, and elasticity on material response. The concept of excess entropy was established: this is an average order parameter that can be computed from the sample’s pair correlation function and provides a measure of particle caging. Both particle dynamics and material rheological behavior correlate with excess entropy. IRG participants showed that particle aspect ratio can be manipulated to significantly enhance the fracture toughness of nanoparticle pillars infiltrated with polymers.
   IRG-2 Structural Chemo-Mechanics of Fibrous Networks: IRG2 members and their MRSEC collaborators developed new fibrous materials with unique nonlinear mechanical responses to shear and uniaxial deformation, in which reactions are controlled by local stress or deformation. They defined the molecular mechanisms by which either soft or rigid inclusions within fibrous networks radically alter their response to uniaxial deformation, resulting in the ability to produce tissue-like materials from a minimal set of synthetic constituents. Innovations in organic chemistry and peptide synthesis led to the production of artificial fibrous networks with mechanical properties similar to those of biological fibrous polymer networks, and a combination of organic synthesis and molecular biology enabled specific coupling of enzymes and other ligands to the fibrous elements both in vitro and within cells. Controlled fracture of resisting or load-bearing fibers within a network was explored theoretically and experimentally to determine how macroscopic responses depend on local rearrangements. (Figure 2) It was shown how a 2D network within its membrane protects the cell nucleus and how similar concepts might be applied to the densely packed DNA fibers within the nucleus. IRG participants combined electrospinning, 3D printing, and other synthetic strategies to produce new scaffolds that can function in bioengineering and tissue engineering applications and suggest new material strategies for wound healing. They have also made new materials in which elastic and dissipative properties can be independently altered and have shown how viscous dissipation impacts cell-matrix constructs.
   IRG-3 Pluperfect Nanocrystal Architectures: IRG3 participants exploited surface chemistry and geometric cues in ‘hard’ fabricated and ‘soft’ liquid-crystal templates and created new materials, new probes, and new models of the response of nanoparticles in ordered or partially ordered arrangements. Experimentalists exploited self- and programmed- -assembly and templating to create nanocomposites, quantum sensors, and anisotropic organization of colloids and polymers. These new platforms were created to develop new technologies and devices, including metamaterial-based computing, microsensing, and charge probes. Significant advances in the fundamental science of liquid crystals were made. New approaches to the simulation and modeling of liquid crystal anchoring suggested new experimental routes to create and control nano- and micro-particle interactions with a liquid-crystalline solvent. Theoretical work motivated studies of the response of liquid crystals to inclusion, leading to understanding and controlling giant director fluctuations. These studies motivated the creation of biological analogs using liquid-crystalline polymers.
The LRSM offered programs and activities for students at all levels, from elementary school to graduate school, and provided professional training at the post-doctoral level. These activities included summer camps and workshops that introduced students to the exciting world of materials science through hands-on experiments and demonstrations, research opportunities and mentorship that allowed students to participate in scientific projects and learn from experts in the field, and outreach events and online resources that highlighted the diversity and impact of materials science to the broader public. A Partnership for Research & Education in Materials grant (DMR 1523463, DMR 2122102) with the University of Puerto Rico helped identify, attract, and support minority students. The LRSM also provided access to state-of-the-art facilities and equipment for materials research, allowing researchers at Penn, universities, government laboratories, and industries to advance their research activities.
 

Last Modified: 02/11/2024
Modified by: Eric A Stach