Yoan C. Simon

Impedance mismatch block copolymers, combining rubbery polyisobutylene (PIB) with glassy polystyrene (PS) segments, show promise for the design of high-strain-rate material applications. However, the molecular-level processes of energy absorption and dissipation under high strain rates in the solution state are poorly understood. Using a grafting-from approach, we synthesized a library of well-defined PS–PIB–PS triblock copolymers with and without mechanophores at the block interfaces to evaluate the effects of molecular weight, block composition, and dispersity on chain scission induced by high-strain-rate elongational and shear flows under solution-state sonication. Synthesis of block copolymers without the lowest reaction enthalpy bonds identified by DFT analysis allowed interrogation of the site of chain cleavage. GPC and fluorescence spectroscopy analysis revealed that chain scission did not occur preferentially at the block interface, at the lowest enthalpy, or at the chain center in the triblock copolymers but rather was dictated by chain conformation, plasticity, and physical properties of the sonication medium and that stress concentrated at the interface can be tuned by the length and composition of the blocks. Our findings establish an experimental baseline for understanding the effects of factors such as chain architecture, conformation, and sites of stress concentration on the solution-state response of block copolymers under shear and elongational forces.

Organic excitonic polymeric materials underpin a wide range of optoelectronic, photonic, and biointerfacing technologies, from organic photovoltaics and light-emitting devices to near-infrared imaging and organic bioelectronics. With increased use in biological, wearable, and environmentally transient contexts, the need and interest for chemically degradable and depolymerizable organic excitonic systems is growing more apparent. Traditionally, high excitonic and electronic performance has been associated with uninterrupted -conjugation along polymer backbones. However, emerging evidence demonstrates that efficient excitonic function can persist, and in some cases be enhanced, in architectures that intentionally break or segment conjugation. In this Perspective, we examine recent advances in degradable organic excitonic conjugated polymers through an architecture-driven lens, spanning fully conjugated backbones, partially conjugated and pseudo-conjugated polymers, and systems incorporating dynamic covalent linkages. By summarizing developments across organic electronics, photonic nanomaterials, and bioelectronics, we highlight common excitonic principles that unify these seemingly disparate approaches. Emerging evidence suggests that efficient excitonic and charge-transport behavior can persist even in partially conjugated or dynamically bonded architectures when intermolecular packing and energetic homogeneity are preserved. At the same time, most current degradable strategies still exhibit important trade-offs between device performance, operational stability, and chemically accessible degradation triggers. This Perspective outlines key opportunities for future research toward organic excitonic materials that balance functional performance with controlled degradation and circular lifecycle considerations.

Vitrimers are an emerging class of covalent adaptable networks, which can be reprocessed at elevated temperatures while preserving crosslinking density. In these systems, the onset temperature of bond exchange is often dubbed "topology freezing transition temperature (T_v)" and is characterized by a sharp reduction in material viscosity. Here, we provide a universal and external stress-free method to determine T_v in submicrometer (<μm) supported films, by measuring their thickness change as a function of temperature by ellipsometry. This study investigated a range of vitrimer systems, including catalyst-free, externally catalyzed, and internally catalyzed networks, to confirm the general applicability of our approach. We demonstrate the high sensitivity of ellipsometry in detecting changes in the apparent thermal expansion behaviors of vitrimer films, specifically linked to the onset of bond exchange in vitrimers, which is distinguished from most other methods that primarily capture macroscopic thermomechanical behaviors. Our results also suggest that the mechanism by which ellipsometry reveals the T_v in vitrimers is governed by their change in relaxation dynamics, which are fundamentally distinct from the thermodynamically driven glass transition observed in conventional polymers. We believe the ellipsometric method can not only streamline the characterization of T_v in vitrimers but also provide deeper insights into their dynamic exchange mechanisms by distinguishing between their microscopic and macroscopic properties.

Understanding the physical and chemical response of materials to impulsive deformation is crucial for applications ranging from soft robotic locomotion to space exploration to seismology. However, investigating material properties at extreme strain rates remains challenging due to temporal and spatial resolution limitations. Combining high-strain-rate testing with mechanochemistry encodes the molecular-level deformation within the material itself, thus enabling the direct quantification of the material response. Here, we demonstrate a mechanophore-functionalized block copolymer that self-reports energy dissipation mechanisms, such as bond rupture and acoustic wave dissipation, in response to high-strain-rate impacts. A microprojectile accelerated towards the polymer permanently deforms the material at a shallow depth. At intersonic velocities, the polymer reports significant subsurface energy absorption due to shockwave attenuation, a mechanism traditionally considered negligible compared to plasticity and not well explored in polymers. The acoustic wave velocity of the material is directly recovered from the mechanochemically-activated subsurface volume recorded in the material, which is validated by simulations, theory, and acoustic measurements. This integration of mechanochemistry with microballistic testing enables characterization of high-strain-rate mechanical properties and elucidates important insights applicable to nanomaterials, particle-reinforced composites, and biocompatible polymers.

Photon upconverting films require high overall conversion efficiencies along with a strong absorption of incident light and a low upconversion intensity threshold (I_th) for their practical implementation (e.g. in solar harvesting, sensing, and photocatalysis). Yet, a dichotomy has emerged whereby high quantum yields seemingly come at the detriment of reasonable absorption. To date, the highest efficiency reported in 9,10-diphenylanthracene-based (DPA) amorphous films was demonstrated at low sensitizer concentrations (0.01 wt%) due to increased back-energy transfer at higher sensitizer content. The need for alternatives that circumvent this trade-off is pressing. Herein we report the rational design and application of a novel 9-phenyl-10-(p-tolylethynyl)-anthracene (PTEA) singlet sink (or collector) in conjunction with a benchmark upconverting pair, viz. platinum(II) octaethylporphyrin (PtOEP) and DPA. By using a highly fluorescent singlet exciton sink, parasitic decay to the sensitizer is circumnavigated, thus improving the upconversion quantum yield (Φ_UC) at elevated sensitizer concentrations (0.05 wt% PtOEP) compared to previous reports – expanding the potential for more practical applications for solid-state TTA-UC systems.

Polymer architecture plays an important role in morphology and mechanochemical response. Here, graft copolymers containing mechanophores at the backbone‐arm junction are synthesized through a combination of post‐polymerization modification of butyl rubber and controlled radical polymerization. A series of graft copolymers with 70:30 or 50:50 polyisobutylene:polystyrene (PIB:PS) volume fractions were synthesized by changing the grafting density and arm length to keep the volume fractions constant. The morphologies for these polymers are disordered but show a decreasing domain size with shorter arm lengths, and the samples with a greater number of PS side chains show greater domain mixing. Using pulsed ultrasound experiments, we found greater backbone scission occurs for polymers with a greater number of short grafts per chain than the long grafts with fewer grafting points. Finally, mechanophore activation is greater for samples with arm molecular weights ≥13 kg/mol, attributed to exceeding the cutoff molecular weight for PS.

Fo''rster resonance energy transfer (FRET) is a powerful tool for measuring distances between two molecules (donor and acceptor) in close proximity (1-10 nm), which can be employed for determining polymer end-to-end distances (Ree). However, previous works for labeling FRET pairs on chain-ends often involve relatively complex steps for materials preparation, potentially limiting their broad use in synthetic polymer systems. In this work, we introduce an anthracene-functionalized chain -transfer agent for reversible addition-fragmentation chain-transfer (RAFT) polymerizations, which can directly yield polymers containing FRET donor and acceptor molecules on respective chain -ends. This approach enables the direct use of FRET for characterizing the averaged Ree of polymers. Building on this platform, we investigate the averaged Ree of polystyrene (PS) and poly(methyl methacrylate) (PMMA) in a good solvent as a function of their molecular weight. Notably, the FRET results show good agreement with simulation results obtained from all-atom molecular dynamics, confirming its measurement accuracy. Overall, this work provides a facile and broadly applicable platform to directly determine the Ree of low molecular weight polymers by using FRET-based methods.

Likened to both thermosets and thermoplastics, vitrimers are a unique class of materials that combine remarkable stability, healability, and reprocessability. Herein, this work describes a photopolymerized thiol‐ene‐based vitrimer that undergoes dynamic covalent exchanges through uncatalyzed transamination of enamines derived from cyclic β‐triketones, whereby the low energy barrier for exchange facilitates reprocessing and enables rapid depolymerization. Accordingly, an alkene‐functionalized β‐triketone, 5,5‐dimethyl‐2‐(pent‐4‐enoyl)cyclohexane‐1,3‐dione, is devised which is then reacted with 1,6‐diaminohexane in a stoichiometrically imbalanced fashion (≈1:0.85 primary amine:triketone). The resulting networks exhibit subambient glass transition temperature (T_g = 5.66 °C) by differential scanning calorimetry. Using a Maxwell stress‐relaxation fit, the topology‐freezing temperature (T_v) is calculated to be −32 °C. Small‐amplitude oscillatory shear rheological analysis enables to identify a practical critical temperature above which the vitrimer can be successfully reprocessed (T_v,eff). Via the introduction of excess primary amines, this work can readily degrade the networks into monomeric precursors, which are in turn reacted with diamines to regenerate reprocessable networks. Photopolymerization provides unique spatiotemporal control over the network topology, thereby opening the path for further investigation of vitrimer properties. As such, this work expands the toolbox of chemical upcycling of networks and enables their wider implementation.