Mechanophores are molecular probes that can report on mechanical stresses and strains in polymers via changes in their optical signal. Their design and development have attracted significant attention because they allow us to address both fundamental and technologically important questions about how polymeric materials fail under mechanical load.

In this talk, I will present two generalizable mechano-mapping platforms for the visualization of heterogeneous mechanical deformation in polymeric materials over a wide applied strain range. For the first platform, we developed a hierarchically structured microparticle additive, or “mechano-pigment”, that contains non-close-packed silica nanoparticles in an elastic matrix cross-linked with a spiropyran mechanophore (1). Small strains lead to shifts in the photonic reflection of the pigment, and large strains cause the mechanochemical transformation of spiropyran to fluorescent spiropyran. We successfully deployed the mechano-pigments to visualize localized deformation phenomena, such as necking and indentation, in poly(ethylene) and poly(dimethylsiloxane). In the second approach, we used supramolecular mechanophores, which change their fluorescence emission at relatively small molecular-level forces (2). These probes enabled the in situ detection of small molecular-level forces involved in heterogeneous strain distributions around deliberately introduced defects, such as holes and inclusions, in polyurethane matrices, using conventional widefield and confocal fluorescence microscopy. Both platforms allowed us to create quantitative strain maps, which showed how local strains close to defects can greatly deviate from the externally applied strain. We are currently developing strategies to implement fluorescent mechanophores for nanoscopic imaging, to enable force mapping of the smallest defects.

Elastomeric thin films are widely used in microelectronics, healthcare, and optics, often confined between rigid surfaces and subjected to external loads. This confinement induces higher stresses than in bulk systems, making it crucial to understand stress distribution in compressed thin films to mitigate failure risks. This study explores the confinement effect and stress distribution in polydimethylsiloxane (PDMS) films in contact with a flat, rigid body. By incorporating a mechano-responsive material, the spiropyran mechanophore (MP), into the elastomer, we analyze stress behavior under compression. Using a custom-built indentation device, we indent the MP-incorporated elastomer with a rigid flat punch under controlled pre-load conditions and observe the MP's fluorescence activation during elastomer deformation via confocal microscopy. The study varies the punch ratio (a) to the elastic layer thickness (h) ratio to examine the confinement effect, especially when the assumption of negligible confinement (a/h<<1) no longer applies. Fluorescence activation of the MP reveals stress distribution, showing the highest intensity at the punch perimeter, consistent with classical mechanics predictions of stress concentration at contact edges. Additionally, by adjusting the a/h values, we visualize confinement's effect on stress distribution. As the thickness of the film decreases, the normalized MP fluorescence response increases, indicating that higher confinement yields a higher stress state. This study enhances our understanding of confinement's impact on elastomer stress and ultimately aids in early detection of damage and failure.

Stimuli-responsive polymers, capable of undergoing considerable property changes upon subtle stimuli, are useful for drug delivery and sensing applications. Among various stimuli, temperature is the most widely used, as many polymers feature lower critical solution temperature (LCST) behavior, characterized by a coil-to-globule transition. To expand the functional versatility, light was introduced as an additional stimulus, generating dual-responsive polymers that allow rapid, precise, and non-invasive polymer solubility alteration. Here, we present a thermoresponsive poly(N,N-dimethylacrylamide) (PDMA) copolymer incorporating varying contents of photoswitchable azobenzene (AzBn) side group. Through UV-induced trans-cis isomerization and blue light-driven back-isomerization, the copolymer hydrophilicity, and consequently its LCST, can be modulated. However, since the LCST shift upon irradiation is typically small,[1] we introduced alanine and valine as spacers between PDMA and AzBn to enhance the photoresponse. Dynamic light scattering reveals a counterintuitive trend: Illumination decreases the LCST, despite the more polar cis-isomer being expected to increase it. Meanwhile, the maximum LCST shift is 10 °C. Small-angle X-ray scattering indicates that all copolymers aggregate into cylinders rather than molecularly dissolving, with trans-isomers forming longer cylinders than the cis-isomers below the LCST. Upon heating, the trans-cylinders contracts while the cis-cylinders extends longitudinally, suggesting distinct LCST transition mechanisms. Moreover, the isomerization remains quantitatively reversible for at least 10 cycles, demonstrating the robustness of the light responsiveness.

Amphiphilic polymer co-networks (APCNs) are soft solids comprising hydrophilic and hydrophobic components, enabling them to swell in water and organic solvents. This unique property results in environmentally responsive viscoelasticity and selective permeability for hydrophilic and hydrophobic substances, making them widely applicable in soft contact lenses(1), membranes, drug delivery systems, and tissue engineering(2). Understanding the relationship between these networks' mesoscopic structure and macroscopic properties is essential to enabling targeted material design for advanced applications.
We studied a model APCN synthesized via a hetero-complementary coupling reaction between 2-(4-nitrophenyl)-benzoxazinone-terminated tetra-poly(ε-caprolactone) (t-PCL) and amino-terminated tetra-poly(ethylene glycol) (t-PEG), as described by Bunk et al.(3). To gain valuable insights into the structural length scales within the network, isorefractive dynamic light scattering (DLS) was used to investigate the diffusion of spherical silver nanoparticles (AgNPs) and esterified dextrans of varying molecular weights in the APCN swollen in toluene.
Diffusion data was analyzed using hydrodynamic and obstruction models, with the hydrodynamic model proving more suitable for such networks. Our results revealed scaling laws for the correlation length as a function of the polymer volume fraction, alongside the determination of the hydrodynamic screening length, marking the transition from the Rouse to the Zimm regime. Additionally, we demonstrated how structural length scales evolve with swelling, offering more profound insights into the structure-property relationships of APCNs. Furthermore, comparative diffusion measurements on non-crosslinked t-PEG/t-PCL solutions highlighted differences due to network crosslinks on both the length scales and the scaling.