Session T32: Advanced Mechanical Characterization Including Mechanoresponsive Molecules for Stress Sensing

Modeling the rheology of gelation using recovery rheology

Gels are materials that continue to be heavily used in the food industry, biological applications, numerous commercial products, as well as other applications. The process of forming polymer gels often involves crosslinking polymers so that the bulk undergoes a phase transition from liquid to solid at a critical extent of reaction. Typical mechanisms that initiate gelation include chemical reactions induced by exposure of UV or elevated temperature, physical reactions, ionic complexation, and addition of particles. Given the wide usage of gelation for product design, there are limited studies in developing rheological models to understand the material responses during gelation under different external stimuli. Moreover, the current understanding of gelation fails to explain interesting experimental observations such as the existence of an overshoot in the loss modulus. In the present work, we construct a phenomenological model of the rheology of gelation based on recovery rheology and demonstrate its generality on systems that gel via a variety of mechanisms. The model accounts for many qualitative features and is quantitatively accurate for some materials. We show valuable insights in understanding gelation from decomposing traditional dynamic moduli into elastic (recoverable) and viscous (unrecoverable) contributions. We believe the new understanding can be further used to design and understand manufacturing processes and engineer novel formulations.   

A comparison of phase separation in gels using different elastic models

Phase separation in gels can occur in several different geometries, which can be roughly categorized as one, two, or three dimensional. Here we calculate candidate phase diagrams for five elastic models in the three geometries and show that finite extensibility can lead to anomalous-looking phase diagrams because of extreme stretching induced at high solvent fraction. We also show that neo-Hookean elasticity cannot support phase separation in certain cases. We apply our finding to recent experiments as well as gels with two solvents. 

    

Hierarchical, Porous Hydrogels Demonstrating Structurally Dependent Mechanical Properties

The unique properties of natural tissues are derived from highly complex arrangements of biomolecules spanning several orders of magnitude. Although replicating tissue properties is of great interest, little consideration has been given to mimicking their natural structures. Here, we demonstrate a process by which porous microstructures are incorporated into hydrogels via rapid spinodal decomposition of amphiphilic ABA triblock copolymer solutions when injected into water. At the nanoscale, the hydrophobic chain-ends of the block copolymer self-assemble into a network of micelle cores bridged by the hydrophilic mid-blocks. Simultaneously, the diffusion of the solvent triggers rapid spinodal decomposition, leaving micron-sized voids within the micelle network. In this work, we show how the pores introduce an additional deformation mode in the hydrogels, which strongly influences the mechanical properties of the hydrogels. The gels exhibit both high elasticity and extreme softness yet can reversibly deform to multiple times their initial length with no hysteresis. We investigate factors influencing the porous microstructure and mechanical properties, such as relative block composition and initial solution conditions. This research presents an exciting class of new materials, with diverse applications and an opportunity to further explore the complex physics of block-copolymer self-assembly.   

A Molecular Theory for Liquid Crystal Elastomers: Nematic Ordering, Shape Deformation and Mechanical Response

Modeling liquid crystal elastomers (LCEs) at the molecular level is crucial for the predictable design of energy-conversion and stimuli-responsive materials. Here, we develop a self-consistent field theory for LCEs which captures the coupling between nematic ordering, backbone alignment and network deformation. Molecular features such as density of elastic strands, strength and architecture of local chemical hinge, and LC grafting density are systematically included. Crosslinking suppresses nematic ordering as a result of the elastic energy stored during network deformation. Higher work capacity can be achieved by less crosslinked LCEs. The spontaneous shape change of end-on side-chain LCEs can be either elongation or contraction depending on the competition between the local and global couplings. Adjusting LC grafting density is found to be an effective way to fine-tune the deformation mode. We elucidate a universal scaling relationship between the transition temperature and the shear modulus as T_NI,0 - T_NI ~ μ^4/5. Furthermore, we predict that the first-order nematic phase transition can be degraded into a continuous manner upon applied stress. Coupled with nematic ordering, the mechanical response of LCEs significantly deviates from classical rubber elasticity. A plateau in the stress-deformation curve appears accompanied by the nematic phase transition. Our theoretical predictions are in good agreement with the experimental results reported in the literature.   

Molecular Dynamics as a Lens into Condensed Matter Mechanochemistry

Mechanochemical reactions are increasingly understood to drive the functional characteristics of technological materials, but the fundamentals are difficult to isolate in condensed phases. All-atom molecular dynamics (MD) is a proven method for probing chemistry, yet tailoring MD simulation protocols to study mechanochemical conditions is not straightforward. Strategies to apply MD to probe mechanochemistry are described, including (1) a virtual analog to the rotational diamond anvil cell experiment, and (2) techniques to bridge classical MD sampling of microstructural mechanics with quantum MD simulations of chemical reactions. Case studies are discussed that highlight the role of MD in the discovery of new mechanochemical effects in organic materials, including in the degradation of polymers, initiation of high explosives, induction of reactive phase transitions, and abiotic synthesis of polypeptides. Perspectives are given on using MD as an applied-research tool for identifying molecular-scale mechanochemical effects and routes to upscale those insights to understand macroscopic material properties. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-855661.   

"Quantifying Localized Stresses in the Matrix of a Fiber-Reinforced Composite via Mechanophores"

A profound understanding of the factors triggering damage in fiber-reinforced polymers is pivotal in enhancing their durability. Incorporating mechanoresponsive molecular force probes, known as mechanophores, offers a promising avenue for directly monitoring stress concentration. Our investigation focuses on the utilization of spiropyran (SP) mechanophores (MPs) embedded within a polydimethylsiloxane (PDMS) matrix to unveil stress localization during fracture within a singular fiber-reinforced polymer. The SP mechanophore undergoes a transformative shift from a non-fluorescent state to an active state, referred to as merocyanine, in response to mechanical forces. Our experimental approach involves uniaxial tensile testing of a single fiber-reinforced polymer, effectively replicating the primary failure modes seen in traditional fiber-reinforced composites. By subjecting samples to uniaxial tensile loading along the fiber's orientation, we observe the concentration of stresses through the activation of mechanophores. Evidently, these stresses tend to amass in the matrix, primarily in the vicinity of the fiber, gradually decreasing away from the fiber's surface. To gain deeper insights, we employ confocal microscopy, allowing us to visually track mechanophore activation and conduct a quantitative evaluation of fluorescence intensity. In parallel, our methodology also incorporates finite element modeling to construct a calibration mechanism for quantifying stress levels based on the observed fluorescence intensity. This approach provides a dynamic framework to understand and measure the stresses encountered in real-time, which can significantly contribute to the formulation of high-performance composites.   

Assembly and disassembly of supramolecular polymers with tunable ionic bonds

The substantial energy required to break C-C bonds (ca. 350 kJ/mol) poses challenges in conventional covalent polymer manufacturing, particularly in terms of reprocessability and recyclability. The development of supramolecular chemistry suggested an alternative way to construct multicomponent polymers based on non-covalent association of oligomer “stickers”. We demonstrate that leveraging the electrostatic interactions between chain-end(s) functionalized oligomers, supramolecular polymers can be “synthesized during use”. The chain conformation and self-assembly of these ionic supramolecular block copolymers are governed by two key factors: (1) the effective bond energy between ionic building blocks, ranging from ca. 2.5 to 250 kJ/mol, depending on end-group chemistry and dielectric properties of the polymer matrix, and (2) potential chain repulsion between dissimilar polymer chains. Notably, the effective bond energy can be readily adjusted by temperature, adding external ionic species, and external stimuli. By fine-tuning these ionic interactions, we exert control over the assembly and disassembly of ionic supramolecular block copolymers, facilitating potential phase transitions and unlocking the reprocessability and recyclability of polymeric materials.   

FLIM-FRAPP: Near-Simultaneous Characterization of Multi-Scale Polymer Dynamics via Fluorescence Microscopy

Characterizing multi-scale dynamics of polymers is crucial to inform property-process relationships, but there is an outstanding gap between accessibility and comprehensive data collection. To this end, we present FLIM-FRAPP, a near-simultaneous and multi-scale hybrid approach between Fluorescence Lifetime Measurement (FLIM) and Fluorescence Recovery After Patterned Photobleaching (FRAPP), to probe the glass transition temperature and self-diffusion coefficient of a melt-state polymer thin film. Properties measured by FLIM-FRAPP within a few hours are in good agreement with the literature, validating the method as a unique exploration of polymer dynamics.   

Quantification of Stress Fields Ahead of a Cutting Blade via Mechanophores

Fracture initiated by contact with a sharp object is a concern in many industries. No matter whether fracture is desired or unwanted, understanding the effects blade and material properties have on the fracture and stress responses of a material is critical to enable better design of products. This research aims to describe fracture and stress fields inside elastomers due to cutting and the effects blade radius has on these properties. The methodology to accomplish this utilizes a Y-shaped cutting apparatus to quantify fracture toughness and mechanophore incorporation coupled with confocal microscopy to visualize stress in situ. Within the steady state cutting regime, mechanophore activation ahead of the propagating crack tip describes the behavior of fracture induced by different radii blades. The fluorescent response of the activated mechanophore is mapped to calculated stress using finite element analysis, quantifying the stress distribution in the material during cutting. These results demonstrate the importance of using cutting implements designed for the task and a way to quantify the interactions of these cutting tools against elastomers.   

Mechano-Optoelectronic Conjugated Polymeric Thin Films with Nano-Structured Lamellae for Self-Sensing of Mechanical Strain

Mechano-optoelectronic (MO) thin films generate electrical energy in direct current (DC) using an external light source. The generated DC was shown to vary with the mechanical strain that the MO thin films are subjected to. The unique strain-sensitive DC generation characteristic of the MO thin films permitted to devise a self-powered strain sensor for self-sensing mechanical strains. The MO thin films were fabricated by depositing p-n semiconducting polymer solution on a flexible substrate using a spin-coating technique. The p-type conjugated poly(3-hexylthiophene) (P3HT) was known to play a pivotal role in the strain-sensitive DC generation due to varying nano-structures of P3HT in the MO thin films subjected to mechanical deformation. 

In this study, we aim to understand how the MO properties of the thin films are affected by the nano-structures of the P3HT through empirical and simulation studies. The MO thin films will be fabricated using P3HTs with different regioregularities on a flexible substrate using a spin-coating technique. The fabricated MO thin films will be tested to measure DC output with mechanical strains applied onto the thin films. Also, using small angle X-ray scattering (SAXS), the nano-structures of the conjugated polymers will be characterized at different strain levels. The SAXS data will be used for modeling the conjugated polymers with different nano-structures using the molecular dynamics (MD) code to help understand how the nano-structures are related to the MO properties.   

Stress Quantification in a Composite Matrix via Mechanophores

Polymer matrix composites (PMCs) combine the toughness of thermoset polymer with a stiff reinforcement phase which produces a material that is both stronger and tougher. Due to non-uniform straining during mechanical deformation, significant stress concentrations develop within the matrix near the interface. To understand failure mechanisms, developing methods to measure this stress concentration is crucial. Finite element analysis (FEA) is a computational modeling technique that can predict the stresses in PMCs for simple systems but becomes computationally expensive as the model becomes complicated. Mechanophores are a type of stimuli responsive molecules which have a labile bond in the structure, allowing for a change in color, fluorescence, or catalytic abilities after breaking. Nitro spiropyran (SPN) is a common mechanophore that is relatively easy to synthesize. By embedding SPN into a polydimethylsiloxane (PDMS) matrix, we obtain force responsive polymers. Using this SPN PDMS, we fabricated model PMCs using silane functionalized spherical silica particles. By correlating fluorescence intensity data from our micrographs to the maximum principal stress values calculated by FEA, we created a calibration curve that allows us to measure the stresses in a PMC based only on fluorescence intensity. This system was validated, and the versatility of this technique was demonstrated by testing PMCs with differing geometries and complexities which are difficult to model in FEA.