Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Materials Science and Engineering

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Visualizing fiber end geometry effects on stress distribution in composites using mechanophores
(Soft Matter, 2024-11-14) Haque, Nazmul; Chang, Hao Chun; Chang, Chia-Chih; Davis, Chelsea S.
Localized stress concentrations at fiber ends in short fiber-reinforced polymer composites (SFRCs) significantly affect their mechanical properties. Our research targets these stress concentrations by embedding nitro-spiropyran (SPN) mechanophores into the polymer matrix. SPN mechanophores change color under mechanical stress, allowing us to visualize and quantify stress distributions at the fiber ends. We utilize glass fibers as the reinforcing material and employ confocal fluorescence microscopy to detect color changes in the SPN mechanophores, providing real-time insights into the stress distribution. By combining this mechanophore-based stress sensing with finite element analysis (FEA), we evaluate localized stresses that develop during a single fiber pull-out test near different fiber end geometries—flat, cone, round, and sharp. This method precisely quantifies stress distributions for each fiber end geometry. The mechanophore activation intensity varies with fiber end geometry and pull-out displacement. Our results indicate that round fiber ends exhibit more gradual stress transfer into the matrix, promoting effective stress distribution. Also, different fiber end geometries lead to distinct failure mechanisms. These findings demonstrate that fiber end geometry plays a crucial role in stress distribution management, critical for optimizing composite design and enhancing the reliability of SFRCs in practical applications. By integrating mechanophores for real-time stress visualization, we can accurately map quantified stress distributions that arise during loading and identify failure mechanisms in polymer composites, offering a comprehensive approach to enhancing their durability and performance.
Mechanical Deformation Behavior of Polymer Blend Thin Films
(Macromolecular Rapid Communications, 2024-12-31) Pokhrel, Geeta; Jo, Hyungyung; Christ, Nicholas M.; Son, Hyeyoung; Howarter, John A.; Davis, Chelsea S.
Examining the mechanical properties of polymer thin films is crucial for high-performance applications such as displays, coatings, sensors, and thermal management. It is important to design thin film microstructures that excel in high-demand situations without compromising mechanical integrity. Here, a polymer blend of polystyrene (PS) and polyisoprene (PI) is used as a model to explore microscale deformation behavior under uniaxial mechanical testing. Six thin film compositions ranging from pure PS to a 4.5:5.5 ratio of PS to PI are fabricated. The thin films are deformed under compression, tension, and cyclic loadings, while monitoring the behavior utilizing a micromechanical stage and optical microscopy. To calculate the plane strain modulus, a strain-induced elastic buckling instability technique is employed. The results show that as the PI concentration increases, the plane strain modulus of the films decreases while the fracture strain increases. For the 4.5:5.5 ratio of PS to PI with a continuous rubbery PI phase, the thin films show major recoverable mechanical performance. This behavior is attributed to the mechanical strength of glassy PS combined with the strain energy absorption capability of rubbery PI, enabling elastic recovery. These fundamental observations provide valuable insights for designing mechanically resilient thin films for coatings and flexible devices.
Two-Step Close-Space Vapor Transport of MAPbI3 Solar Cells: Effects of Electron Transport Layers and Residual PbI2
(ACS Applied Energy Materials, 2022-09-11) Kuba, Austin G.; Harding, Alexander J.; Richardson, Raphael J.; McCandless, Brian E.; Das, Ujjwal K.; Dobson, Kevin D.; Shafarman, William N.
The effect of the electron transport layer (ETL) on the growth of methylammonium lead iodide (MAPbI3) thin films by two-step close-space vapor transport (CSVT) is reported. Nanocrystalline CdS, as well as amorphous SnO2 and C60, were selected as ETLs on indium tin oxide-coated glass substrates prior to two-step CSVT. The ETL affected the PbI2 growth, leading to different morphological and crystallographic properties. These differences carried over through the methylammonium iodide reaction to the MAPbI3 phase, but compact films with a reasonable morphology could be made on each ETL. The ETL also affected the PbI2-to-MAPbI3 reaction rate. Solar cells processed on each ETL showed a low level of residual PbI2 was important for good photovoltaic conversion efficiency (PCE). The PCEs were similar on average, but trade-offs in J–V parameters depended on the ETL selection. When films on each ETL were reacted beyond an optimal PbI2 residual content, solar cells had lower performance driven by decreases in different J–V parameters. The ETL also had practical effects on J–V performance, namely, hysteresis and air stability. The hysteresis of solar cells on C60 was much less than on SnO2 and CdS. However, the solar cells with C60 ETLs were not stable in air, exhibiting FF and Jsc losses in as little as 15 min of air exposure, while those made on the other ETLs were stable for hours. Thus, the choice of ETL for two-step CSVT affects the growth of PbI2 and its reaction to MAPbI3, but interfacial chemistry considerations and effects on current and atmospheric stability appear to be more important for device performance and yield.
Air-Induced Conductivity Loss in Fullerene ETLs Can Drive Charge Extraction Losses in Vapor-Deposited Perovskite Solar Cells
(ACS Applied Energy Materials, 2024-12-04) Kuba, Austin G.; Santiwipharat, Chaiwarut; Richardson, Raphael J.; Das, Ujjwal K.; Dobson, Kevin D.; Shafarman, William N.
The effect of air exposure on all-vapor processed perovskite solar cells using C60 fullerene electron transport layers (ETLs) was investigated. C60 is used in lead halide perovskite solar cells as an ETL to decrease hysteresis and improve stabilized power output. However, air exposure to n-i-p solar cells using C60 ETLs without encapsulation or doping can result in performance degradation due to FF loss and the onset of s-shaped J–V curves. This is correlated to orders of magnitude increase in C60 resistivity upon air exposure. Drift-diffusion simulations provide evidence that a change in the C60 carrier concentration or mobility can lead to the FF loss and s-shaped J–V curve. The degradation does not occur when using inorganic ETLs but does occur in p-i-n architecture using C60 ETLs, further confirming that the C60 layer is the source of the degradation. This is an additional pathway for perovskite solar cell degradation upon air exposure beyond the instability of the perovskite itself. The loss of efficiency can be reduced in p-i-n solar cells using a LiF interlayer, and a better combination of hysteresis and air stability can be achieved in n-i-p solar cells using a C60/SnO2 bilayer ETL.
Driving factors for the peculiar bond length dependence and tetragonal distortion of (Ag,Cu)(In,Ga)Se2 and other chalcopyrites
(JPhys Energy, 2024-12-04) Falk, Hans H.; Eckner, Stefanie; Ritter, Konrad; Levcenko, Sergiu; Pfeiffelmann, Timo; Larsen, Jes; Shafarman, William N.; Schnohr, Claudia S.
The chalcopyrite alloy (Ag,Cu)(In,Ga)Se2 is a highly efficient thin film solar cell absorber, reaching record efficiencies above 23%. Recently, a peculiar behavior in the bond length dependence of (Ag,Cu)GaSe2 was experimentally proven. The common cation bond length, namely Ga–Se, decreases with increasing Ag/(Ag + Cu) ratio even though the crystal lattice expands. This is opposite to the behavior observed for Cu(In,Ga)Se2, where all bond lengths increase with increasing lattice size. To better understand this peculiar bond length behavior, element-specific bond lengths of (Ag,Cu)InSe2 and Ag(In,Ga)Se2 alloys are determined using extended x-ray absorption fine structure spectroscopy. They show that the peculiar bond length dependence occurs only for (Ag,Cu) alloys, independent of the species of common cation (In or Ga). The bond lengths are used to determine the anion displacements and to estimate their contribution to the bandgap bowing. Again, both behaviors differ significantly depending on the type of alloyed cation. A valence force field approach, relaxing bond lengths and bond angles, is used to describe the structural distortion energy for a comprehensive set of I–III–VI2 and II–IV–V2 chalcopyrites. The model reveals bond angle distortions as main driving factor for the tetragonal distortion and reproduces the literature values with less than 10% deviation. In contrast, the peculiar bond length dependence is not reproduced, demonstrating that it originates from electronic effects beyond the scope of this structural model. Thus, a fundamental understanding of bond length behavior and tetragonal distortion is achieved for chalcopyrite materials, benefiting their technological applications such as high efficiency thin film photovoltaics.
Formation of bijels stabilized by magnetic ellipsoidal particles in external magnetic fields
(Soft Matter, 2024-10-08) Karthikeyan, Nikhil; Schiller, Ulf D.
Bicontinuous interfacially-jammed emulsion gels (bijels) are increasingly used as emulsion templates for the fabrication of functional porous materials including membranes, electrodes, and biomaterials. Control over the domain size and structure is highly desirable in these applications. For bijels stabilized by spherical particles, particle size and volume fraction are the main parameters that determine the emulsion structure. Here, we investigate the use of ellipsoidal magnetic particles and study the effect of external magnetic fields on the formation of bijels. Using hybrid Lattice Boltzmann-molecular dynamics simulations, we analyze the effect of the magnetic field on emulsion dynamics and the structural properties of the resulting bijel. We find that the formation of bijels remains robust in the presence of magnetic fields, and that the domain size and tortuosity become anisotropic when ellipsoidal particles are used. We show that the magnetic fields lead to orientational ordering of the particles which in turn leads to alignment of the interfaces. The orientational order facilitates enhanced packing of particles in the interface which leads to different jamming times in the directions parallel and perpendicular to the field. Our results highlight the potential of magnetic particles for fabrication and processing of emulsion systems with tunable properties.
Visualizing separation at composite interfaces via spirolactam mechanophores
(RSC Mechanochemistry, 2024-10-17) Gohl, Jared A.; Roberts, Tyler J.; Freund, Anna C.; Haque, Nazmul; Rueschhoff, Lisa M.; Baldwin, Luke A.; Davis, Chelsea S.
The failure of interfaces between polymers and inorganic substrates often leads to deteriorated performance, as is the case for polymer matrix composites. Interfacial mechanophores (iMPs) have the potential to fluorescently measure interfacial failures. Spirolactam-based mechanophores are of interest due to their readily available synthetic precursors and compatibility with epoxy matrices. In this work, spirolactam is covalently bound at the interface of silica surfaces and epoxy, chosen due to the industrial relevance of glass fiber composites. The iMPs are mechanically activated through uniaxial tension applied to the composite while the resulting fluorescent response is observed in situ with a confocal microscope. Due to their real time sensing capabilities, iMPs are a promising technique to measure interfacial failures in composite materials more easily than with traditional optical microscopy techniques.
Random field reconstruction of three-phase polymer structures with anisotropy from 2D-small-angle scattering data
(Soft Matter, 2024-10-14) Kronenberger, Stephen; Gupta, Nitant; Gould, Benjamin; Peterson, Colin; Jayaraman, Arthi
In this paper we present a computational method to analyze 2-dimensional (2D) small-angle scattering data obtained from phase-separated soft materials and output three-dimensional (3D) real-space structures of the three types of domains/phases. Specifically, we use 2D small-angle X-ray scattering (SAXS) data obtained from hydrated NafionTM membranes and develop a workflow using random fields to build the 3D real-space structure comprised of amorphous hydrophilic domains, amorphous polymer domains, and crystalline polymer domains. We demonstrate the method works well by showing that the reconstructed 3D NafionTM structures have a computed scattering profile that matches the input experimental scattering profile. Though not demonstrated in this work, such reconstructions can be used for further analysis of domain shapes and sizes, as well as prediction of transport properties through the structure. Our method in this work extends capabilities beyond the previously published random field small angle scattering reconstruction method introduced by Berk [Phys. Rev. Lett. 1987, 58 (25), 2718–2721] that had been used to reconstruct structures from 1D small angle scattering data of two-phase systems. The method in this work can be used to generate isotropic, two-phase reconstructions, but can also handle 2D SAXS profiles from three-phase systems that have structural anisotropy resulting from material processing effects.
Machine learning for analyzing atomic force microscopy (AFM) images generated from polymer blends
(Digital Discovery, 2024-10-21) Paruchuri, Aanish; Wang, Yunfei; Gu, Xiaodan; Jayaraman, Arthi
In this paper, we present a new machine learning (ML) workflow with unsupervised learning techniques to identify domains within atomic force microscopy (AFM) images obtained from polymer films. The goal of the workflow is to (i) identify the spatial location of two types of polymer domains with little to no manual intervention (Task 1) and (ii) calculate the domain size distributions, which in turn can help qualify the phase separated state of the material as macrophase or microphase ordered/disordered domains (Task 2). We briefly review existing approaches used in other fields – computer vision and signal processing – that can be applicable to the above tasks frequently encountered in the field of polymer science and engineering. We then test these approaches from computer vision and signal processing on the AFM image dataset to identify the strengths and limitations of each of these approaches for our first task. For our first domain segmentation task, we found that the workflow using discrete Fourier transform (DFT) or discrete cosine transform (DCT) with variance statistics as the feature works the best. The popular ResNet50 deep learning approach from the computer vision field exhibited relatively poorer performance in the domain segmentation task for our AFM images as compared to the DFT and DCT based workflows. For the second task, for each of the 144 input AFM images, we then used an existing Porespy Python package to calculate the domain size distribution from the output of that image from the DFT-based workflow. The information and open-source codes we share in this paper can serve as a guide for researchers in the fields of polymers and soft materials who need ML modeling and workflows for automated analyses of AFM images from polymer samples that may have crystalline/amorphous domains, sharp/rough interfaces between domains, or micro- or macro-phase separated domains.
Conjugation of Antibodies and siRNA Duplexes to Polymer Nanoparticles via Maleimide–Thiol Chemistry
(ACS Omega, 2024-11-18) Hoover, Elise C.; Chowdhury, Chitran Roy; Ruggiero, Olivia M.; Day, Emily S.
Polymeric nanoparticles (NPs) have shown great promise as highly modifiable platforms that can be applied across many different disease states. They are advantageous because they can encapsulate a range of hydrophobic and hydrophilic cargoes while having customizable surface properties. Depending on the desired biointerfacing capabilities, the surface of polymeric NPs can be modified with moieties, such as antibodies, peptides, nucleic acids, and more. The work presented here is intended to provide mechanistic insight into how different parameters influence the loading of antibodies, small interfering ribonucleic acids (siRNAs), or both on the surface of poly(lactic-co-glycolic acid) (PLGA) NPs via maleimide–thiol chemistry. Some of the conjugation parameters investigated include the buffer concentration, maleimide to protein ratio, and the addition of an excipient such as Tween-20. Through variation in the concentration of FZD7 antibodies added to the reaction mixture, we established tunable conjugation and found the upper limit of their loading density under the conditions tested. We also confirmed antibody conjugation through two different mechanisms: via a thiol-modified antibody or a thiol-modified poly(ethylene glycol) (PEG) linker. Conjugation of thiolated siRNA duplexes targeting β-catenin was also investigated through variations in both Tween-20 concentration and CaCl2 buffer concentration. Finally, the coconjugation of both antibodies and siRNA duplexes was explored. Overall, this work outlines a basis for tunable biomolecule loading on polymer NPs using maleimide–thiol chemistry and reveals the incredible versatility of polymer NP platforms.
Increased hydrophilicity of lignin-derivable vs. bisphenol-based polysulfones for potential water filtration applications
(RSC Sustainability, 2024-09-18) Mahajan, Jignesh S.; Shokrollahzadeh Behbahani, Hoda; Green, Matthew D.; Korley, LaShanda T. J.; Epps, Thomas H., III
The functionality inherent in lignin-derivable aromatics (e.g., polar methoxy groups) can provide a potential opportunity to improve the hydrophilicity of polysulfones (PSfs) without the need for the additional processing steps and harsh reagents/conditions that are typically used in conventional PSf modifications. As determined herein, lignin-derivable PSfs without any post-polymerization modification exhibited higher hydrophilicity than comparable petroleum-based PSfs (commercial/laboratory-synthesized) and also demonstrated similar hydrophilicity to functionalized BPA-PSfs reported in the literature. Importantly, the lignin-derivable PSfs displayed improved thermal properties relative to functionalized BPA-PSfs in the literature, and the thermal properties of these bio-derivable PSfs were close to those of common non-functionalized PSfs. In particular, the glass transition temperature (Tg) and degradation temperature of 5% weight loss (Td5%) of lignin-derivable PSfs (Tg ∼165–170 °C, Td5% ∼400–425 °C) were significantly higher than those of typical functionalized BPA-PSfs in the literature (Tg ∼110–160 °C, Td5% ∼240–260 °C) and close to those of unmodified, commercial/laboratory-synthesized BPA-/bisphenol F-PSfs (Tg ∼180–185 °C, Td5% ∼420–510 °C). Sustainability spotlight Commercial bisphenol A-polysulfones (BPA-PSfs) are hydrophobic in nature, and post-polymerization modification is often required to increase the hydrophilicity of commercial PSfs for water filtration applications. Such PSf modification adds extra processing steps and often requires harsh reaction conditions/reagents, and these extra functionalization steps also have a detrimental effect on application-specific thermal properties. Herein, bio-derivable PSfs were synthesized using potentially safer, lignin-derivable bisguaiacols. Notably, the lignin-derivable PSfs, without any post-polymerization modification, demonstrated similar hydrophilicity to functionalized BPA-PSfs reported in the literature, and they also exhibited improved thermal properties relative to functionalized BPA-PSfs. Moreover, the thermal properties of bio-derivable PSfs remained close to those of commercial BPA-PSfs. Overall, this work is aligned with the UN's Sustainable Development Goal 12 (responsible consumption and production of chemicals).
Self-assembled thin films as alternative surface textures in assistive aids with users who are blind
(Journal of Materials Chemistry B, 2024-09-05) Swain, Zachary; Derkaloustian, Maryanne; Hepler, Kayla A.; Nolin, Abigail; Damani, Vidhika S.; Bhattacharyya, Pushpita; Shrestha, Tulaja; Medina, Jared; Kayser, Laure V.; Dhong, Charles B.
Current tactile graphics primarily render tactile information for blind users through physical features, such as raised bumps or lines. However, the variety of distinctive physical features that can be created is effectively saturated, and alternatives to these physical features are not currently available for static tactile aids. Here, we explored the use of chemical modification through self-assembled thin films to generate distinctive textures in tactile aids. We used two silane precursors, n-butylaminopropyltrimethoxysilane and n-pentyltrichlorosilane, to coat playing card surfaces and investigated their efficacy as a tactile coating. We verified the surface coating process and examined their durability to repeated use by traditional materials characterization and custom mesoscale friction testing. Finally, we asked participants who were both congenitally blind and braille-literate to sort the cards based on touch. We found that participants were able to identify the correct coated card with 82% accuracy, which was significantly above chance, and two participants achieved 100% accuracy. This success with study participants demonstrates that surface coatings and surface modifications might augment or complement physical textures in next-generation tactile aids.
Self-consistent field theory and coarse-grained molecular dynamics simulations of pentablock copolymer melt phase behavior
(Molecular Systems Design & Engineering, 2024-09-24) Park, So Jung; Myers, Tristan; Liao, Vinson; Jayaraman, Arthi
Block copolymer (BCP) self-assembly leads to nanostructured materials with diverse ordered morphologies, some of which are attractive for transport applications. Multiblock AB copolymers are of interest as they offer a larger design parameter space than diblock copolymers allowing researchers to tailor their self-assembly to achieve target morphologies. In this study, we investigate the phase behavior of symmetric AxByAzByAx and BxAyBzAyBx pentablock copolymers (pentaBCPs) where A and B monomers have the same statistical segment length. We use a combination of self-consistent field theory (SCFT) calculations and molecular dynamics (MD) simulations to link the polymer design parameters, namely the fraction of middle block volume to the volume of all blocks of same type, τ, overall volume fraction of A block, fA, and segregation strength, χN, to the equilibrium morphologies and the distributions of chain conformations in these morphologies. In the phase diagrams calculated using SCFT, we observe broader double gyroid windows and the existence of lamellar morphologies even at small values fA in contrast to what has been seen for diblock copolymers. We also see a reentrant phase sequence of double gyroid → cylinder → lamellae → cylinder → double gyroid with increasing τ at fixed fA. The chain conformations adopted in these morphologies are sampled in coarse-grained MD simulations and quantified with distributions of the chain end-to-end distance and fractions of chains whose middle (A or B) and end (A or B) blocks remain within domains of same chemistry (A or B). These analyses show that the pentaBCP chains adopt “looping”, “bridging”, and “hybrid” (both looping and bridging) conformations, with a majority of the chains adopting the hybrid conformation. The spatial distributions for each of the blocks in the pentaBCPs show that blocks of the same type in a chain locally segregate within the same domains, with shorter blocks segregating towards the domain boundaries and longer blocks filling the domain interior. This combined SCFT-MD approach enables us to rapidly screen the extensive pentaBCP design space to identify design rules for transport-favorable morphologies as well as verify the chain conformations and spatial arrangements associated with the theory predicted reentrant phase behavior. Design, System, Application Block copolymers (BCPs) self-assemble into a variety of nanostructures, such as lamellae, hexagonal-packed cylinders, and double gyroid, which in turn enable engineering of materials with desired transport and mechanical properties. The morphology formed by a given BCP is highly dependent on its design in terms of the monomer chemistry, number of blocks (diblock to multiblock), block lengths, and block sequence. As compared to diblock copolymers, multiblock copolymers have been studied to a smaller extent due to their larger design parameter space. However, it is noteworthy that a handful of computational studies of multiBCPs have uncovered novel nanostructures and phase behavior not seen in the well-studied diblock copolymers. In this study, we focus on linking the design of pentablock copolymers (pentaBCPs) to their morphology in the melt state using self-consistent field theory (SCFT) and molecular dynamics (MD) simulations. Our goal is to identify design rules for experimentalists looking to broaden phase windows of transport-friendly morphologies, such as double gyroid. The combination of theory and simulation allows for faster screening of design parameter space using SCFT as compared to MD simulations and quantification of chain conformations using MD simulations especially when getting distribution of chain conformations from SCFT is non-trivial.
A computational method for rapid analysis polymer structure and inverse design strategy (RAPSIDY)
(Soft Matter, 2024-09-30) Liao, Vinson; Myers, Tristan; Jayaraman, Arthi
Tailoring polymers for target applications often involves selecting candidates from a large design parameter space including polymer chemistry, molar mass, sequence, and architecture, and linking each candidate to their assembled structures and in turn their properties. To accelerate this process, there is a critical need for inverse design of polymers and fast exploration of the structures they can form. This need has been particularly challenging to fulfill due to the multiple length scales and time scales of structural arrangements found in polymers that together give rise to the materials’ properties. In this work, we tackle this challenge by introducing a computational framework called RAPSIDY – Rapid Analysis of Polymer Structure and Inverse Design strategY. RAPSIDY enables inverse design of polymers by accelerating the evaluation of stability of multiscale structure for any given polymer design (sequence, composition, length). We use molecular dynamics simulations as the base method and apply a guiding potential to initialize polymers chains of a selected design within target morphologies. After initialization, the guiding potential is turned off, and we allow the chains and structure to relax. By evaluating similarity between the target morphology and the relaxed morphology for that polymer design, we can screen many polymer designs in a highly parallelized manner to rank designs that are likely to remain in that target morphology. We demonstrate how this method works using an example of a symmetric, linear pentablock, AxByAzByAx, copolymer system for which we determine polymer sequences that exhibit stable double gyroid morphology. Rather than trying to identify the global free-energy minimum morphology for a specific polymer design, we aim to identify candidates of polymer design parameter space that are more stable in the desired morphology than others. Our approach reduces computational costs for design parameter exploration by up to two orders-of-magnitude compared to traditional MD methods, thus accelerating design and engineering of novel polymer materials for target applications.
Harnessing multifunctional collagen mimetic peptides to create bioinspired stimuli responsive hydrogels for controlled cell culture
(Journal of Materials Chemistry B, 2024-08-12) Ford, Eden M.; Hilderbrand, Amber M.; Kloxin, April M.
The demand for synthetic soft materials with bioinspired structures continues to grow. Material applications range from in vitro and in vivo tissue mimics to therapeutic delivery systems, where well-defined synthetic building blocks offer precise and reproducible property control. This work examines a synthetic assembling peptide, specifically a multifunctional collagen mimetic peptide (mfCMP) either alone or with reactive macromers, for the creation of responsive hydrogels that capture aspects of soft collagen-rich tissues. We first explored how buffer choice impacts mfCMP hierarchical assembly, in particular, peptide melting temperature, fibril morphology, and ability to form physical hydrogels. Assembly in physiologically relevant buffer resulted in collagen-like fibrillar structures and physically assembled hydrogels with shear-thinning (as indicated through strain-yielding) and self-healing properties. Further, we aimed to create fully synthetic, composite peptide-polymer hydrogels with dynamic responses to various stimuli, inspired by the extracellular matrix (ECM). Specifically, we established mfCMP–poly(ethylene glycol) (PEG) hydrogel compositions that demonstrate increasing non-linear viscoelasticity in response to applied strain as the amount of assembled mfCMP content increases. Furthermore, the thermal responsiveness of mfCMP physical crosslinks was harnessed to manipulate the composite hydrogel mechanical properties in response to changes in temperature. Finally, cells relevant in wound healing, human lung fibroblasts, were encapsulated within these peptide–polymer hydrogels to explore the impact of increased mfCMP, and the resulting changes in viscoelasticity, on cell response. This work establishes mfCMP building blocks as versatile tools for creating hybrid and adaptable systems with applications ranging from injectable shear-thinning materials to responsive interfaces and synthetic ECMs for tissue engineering.
Tracking Chain Populations and Branching Structure during Polyethylene Deconstruction Processes
(ACS Central Science, 2024-08-21) Balzer, Alex H.; Hinton, Zachary R.; Vance, Brandon C.; Vlachos, Dionisios G.; Korley, LaShanda T. J.; Epps, Thomas H., III
Catalytic deconstruction has emerged as a promising solution to valorize polyethylene (PE) waste into valuable products, such as oils, fuels, surfactants, and lubricants. Unfortunately, commercialization has been hampered by inadequate optimization of PE deconstruction due to an inability to either truly characterize the polymer transformations or adjust catalytic conditions to match the ever-evolving product distribution and associated property changes. To address these challenges, a detailed analysis of molar mass distributions and thermal characterization was developed herein and applied to low-density polyethylene (LDPE) deconstruction to enable more thorough identification of polymer chain characteristics within the solids (e.g., changes in molar mass or branching structure). For example, LDPE hydrocracking exhibited comparable rates of polymer chain isomerization and C–C bond scission, and the solids generated possessed a broadened molar mass distribution with a disappearance of a significant fraction of highly linear segments, indicating polymer-structure-dependent interactions with the catalyst. Solids analysis from pyrolysis yielded starkly different results, as the resulting solids were devoid of unreacted polymer chains and had a narrowed molar mass distribution even at short times (e.g., 0.2 h). By tracking the polymeric deconstruction behavior as a function of reaction type, time, and catalyst design, we mapped critical pathways toward PE valorization. Synopsis A comprehensive approach to track polymer chain evolution during deconstruction was developed and revealed the influence of deconstruction method and polymer architecture on product distributions.
Tuning the thermal response of 3D-printed bilayer hydrogels via architectural control using binary ethanol–water solvent systems
(RSC Applied Polymers, 2024-08-14) Klincewicz, Francis; Kalidindi, Subhash; Korley, LaShanda T. J.
While stimuli-responsive materials can be prepared via many established procedures, digital light processing (DLP) 3D printing offers a simple and robust technique for the fabrication of hydrogels, including spatially-defined bilayer hydrogels. The use of synthesis solvent mixtures has recently gained attention as a facile alternative to more complicated chemical modifications to tune hydrogel morphology by exploiting solvent-monomer interactions and cononsolvency which, by extension, modulates stimuli-response time and magnitude. In this work, we utilized a binary solvent system consisting of ethanol and water to induce morphological changes within a thermally-responsive poly(N-isopropyl acrylamide) (pNIPAAm) hydrogel during polymerization. By varying the ratio of ethanol to water, we demonstrated that hydrogel properties, such as crosslink density, pore morphology, and thermal response, can be tuned and correlated. While mass expulsion was fastest in gels prepared in 100% ethanol, we found that gels prepared in 75%–25% ethanol–water and 50%–50% ethanol–water maintained mechanical integrity at high temperatures, allowing expulsion of water mass without large amounts of contraction. We utilized the experimental findings from the monolayer hydrogel studies and investigated the response of bilayer structures comprised of pNIPAAm hydrogel layer and a non-responsive poly(2-hydroxyethyl acrylate) (pHEA) hydrogel layer and applied a mathematical model to better understand the fundamental kinematics of these bilayer systems in response to temperature. We also demonstrated the utility of these bilayer hydrogels for use in soft robotics applications. Overall, this work highlights that modulation of binary solvent mixture ratios is a strategy that enables control of morphological and mechanical features of stimuli-responsive hydrogels via 3D printing.
Tuning Excitonic Properties of Monochalcogenides via Design of Janus Structures
(Journal of Physical Chemistry C, 2024-07-25) Querne, Mateus B. P.; Dias, Alexandre C.; Janotti, Anderson; Da Silva, Juarez L. F.; Lima, Matheus P.
Two-dimensional (2D) Janus structures offer a unique range of properties as a result of their symmetry breaking, resulting from the distinct chemical composition on each side of the monolayers. Here, we report a theoretical investigation of 2D Janus Q′A′AQ P3m1 monochalcogenides from group IV (A and A′ = Ge and Sn; Q, Q′ = S and Se) and 2D non-Janus QAAQ P3̅m1 counterparts. Our theoretical framework is based on density functional theory calculations combined with maximally localized Wannier functions and tight-binding parametrization to evaluate the excitonic properties. The phonon band structures exhibit exclusively real (nonimaginary) branches for all materials. Particularly, SeGeSnS has greater energetic stability than its non-Janus counterparts, representing an outstanding energetic stability among the investigated materials. However, SGeSnS and SGeSnSe have higher formation energies than the already synthesized MoSSe, making them more challenging to grow than the other investigated structures. The electronic structure analysis demonstrates that materials with Janus structures exhibit band gaps wider than those of their non-Janus counterparts, with the absolute value of the band gap predominantly determined by the core rather than the surface composition. Moreover, exciton binding energies range from 0.20 to 0.37 eV, reducing band gap values in the range of 21% to 32%. Thus, excitonic effects influence the optoelectronic properties more than the point-inversion symmetry breaking inherent in the Janus structures; however, both features are necessary to enhance the interaction between the materials and sunlight. We also found anisotropic behavior of the absorption coefficient, which was attributed to the inherent structural asymmetry of the Janus materials.
Engineering lignin-derivable diacrylate networks with tunable architecture and mechanics
(Materials Advances, 2024-07-03) Wong, Yu-Tai; Korley, LaShanda T. J.
Network engineering strategies offer a promising pathway toward tunable thermomechanical properties of bio-derivable, aromatic (meth)acrylate thermosets to expand their application library. In this work, a series of acrylate thermosets, synthesized from lignin-derivable vanillyl alcohol/bisguaiacol F diacrylates (VDA/BGFDA) and a bio-based n-butyl acrylate (BA), were designed as a sustainable platform to explore structure-architecture-property relationships. Using this approach, we examined a series of processable, fully bio-derivable acrylates. Increasing the diacrylate content across all networks improved storage moduli at 25 °C (E′25) by up to 2 GPa (1.1 GPa for VDA/BA-25/75 vs. 3.1 GPa for VDA/BA-75/25, and 1.5 GPa for BGFDA/BA-25/75 vs. 1.7 GPa for BGFDA/BA-50/50), and led to a more inhomogeneous network as evidenced by lower acrylate group conversion and a broader tan δ peak, indicating heterogeneous relaxation modes. Modifying the aromatic content of the starting diacrylate impacted the final inhomogeneity of the network, with increasing inhomogeneity observed for the bis-aromatic BGFDA relative to the mono-aromatic VDA. Similarly, combining the mono- and bis-aromatic diacrylates generated a network with a biphasic-like thermal relaxation mode. By correlating network architecture and material performance, the increasing architectural complexity suggested a more convoluted thermal relaxation mode while the enhancement of thermomechanical properties could still be achieved for potential application as damping materials. Overall, we presented a design strategy utilizing bio-derivable acrylates to expand the suite of renewable material platforms from a network engineering perspective.
Supramolecular Gelation of Cadmium Oleate in the Synthesis of Nanocrystals for Applications in Photonics and Optoelectronics
(ACS Applied Nano Materials, 2024-05-22) Welsch, Tory A.; Cleveland, Jill M.; Thomas, Jessica A.; Schyns, Zoé Odile Georgette; Korley, LaShanda T. J.; Doty, Matthew F.
Cadmium oleate is widely used as a cation precursor in the synthesis of cadmium chalcogenide nanocrystal quantum dots (QDs) for a broad range of photonic and optoelectronic applications. Cd oleate can noncovalently assemble to form a supramolecular coordination gel, or metallogel, in solvents commonly used to disperse oleate-capped QDs. The gelation severely impedes the purification of oleate-capped QDs from excess Cd oleate, resulting in a gelled product that cannot be reliably characterized or used in further synthesis reactions. Here, we investigate the Cd oleate gel to gain insights into its viscoelastic properties and behavior under conditions relevant to QD synthesis, purification, and storage. We then examine how to effectively mitigate gelation by adding oleylamine as an additional ligand to disrupt noncovalent assembly. We synthesize PbS/CdS core/shell QDs via cation exchange as a case study to illustrate gelation of the reaction product and further demonstrate how this issue can be resolved through a better understanding of the supramolecular gel.

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