Direct Measurement of Polymer-Chain-End-to-End Distances by Using RAFT Chain Transfer Agent as the FRET Acceptor.

Förster 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.

Synthesis and Morphology of High-Molecular-Weight Polyisobutylene-Polystyrene Block Copolymers Containing Dynamic Covalent Bonds.

Incorporating dynamic covalent bonds into block copolymers provides useful molecular level information during mechanical testing, but it is currently unknown how the incorporation of these units affects the resultant polymer morphology. High-molecular-weight polyisobutylene-b-polystyrene block copolymers containing an anthracene/maleimide dynamic covalent bond are synthesized through a combination of postpolymerization modification, reversible addition-fragmentation chain-transfer polymerization, and Diels-Alder coupling. The bulk morphologies with and without dynamic covalent bond are characterized by atomic force microscopy  and small-angle X-ray scattering, which reveal a strong dependence on annealing time and casting solvent. Morphology is largely unaffected by the inclusion of the mechanophore. The high-molecular-weight polymers synthesized allow interrogation of a large range of polymer domain sizes.

Diketoenamine-Based Vitrimers via Thiol-ene Photopolymerization.

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 ß-triketones, whereby the low energy barrier for exchange facilitates reprocessing and enables rapid depolymerization. Accordingly, an alkene-functionalized ß-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 (Tg = 5.66 °C) by differential scanning calorimetry. Using a Maxwell stress-relaxation fit, the topology-freezing temperature (Tv ) 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 (Tv,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.

Shape-transformation of polymersomes from glassy and crosslinkable ABA triblock copolymers.

Recent developments in the field of polymer vesicles, i.e. polymersomes, have demonstrated that disrupting the equilibrium conditions of the milieu could lead to shape transformation into stable non-spherical morphologies, bringing on-demand shape control to reality and bearing great promise for cell mimicry and a variety of biomedical applications. Here, we studied the self-assembly behavior of glassy amphiphilic triblock copolymers, poly(ethylene glycol)-block-polystyrene-stat-poly(coumarin methacrylate)-block-poly(ethylene glycol) (PEG-b-P(S-stat-CMA)-b-PEG), and their response to various stimuli. By changing the respective molecular weights of both the hydrophobic P(S-stat-CMA) and the hydrophilic PEG blocks, we varied the hydrophobic volume fraction thereby accessing a range of morphologies from spherical and worm-like micelles, as well as polymersomes. For the latter, we observed that slow osmotic pressure changes induced by dialysis led to a decrease in size while rapid osmotic pressure changes by addition of a PEG fusogen led to morphological transformations into rod-like and tubular polymersomes. We also found out that chemically crosslinking the vesicles before inducing osmotic pressure changes led to the vesicles exhibiting hypotonic shock, atypical for glassy polymersomes. We believe that this approach combining the robustness of triblock copolymers and light-based transformations will help expand the toolbox to design ever more complex biomimetic constructs.

Functional Polymers Through Mechanochemistry.

While coupling mechanical and chemical processes is widespread in living organisms, the idea to harness the mechanically induced dissociation of weak covalent and non-covalent bonds to create artificial materials that respond to mechanical stimulation has only recently gained attention. Here we summarize our activities that mainly revolve around the exploitation of non-covalent interactions in (supramolecular) polymeric materials with the goal to translate mechanical stresses into useful, pre-defined events. Focusing on mechano- chromic polymers that alter their optical absorption or fluorescence properties, several new operating principles, mechanosensitive entities, and materials systems were developed. Such materials are expected to be useful for technical applications that range from the detection of very small forces in biological systems to the monitoring of degradation processes and damage in coatings and structural objects.

Mechanoresponsive, Luminescent Polymer Blends Based on an Excimer-Forming Telechelic Macromolecule.

A well-known approach toward mechanochromic polymers relies on the incorporation of excimer-forming fluorophores into a matrix polymer and the disruption of aggregated chromophores when such materials undergo macroscopic mechanical deformation. However, the required aggregates and stress-transfer processes have so far only been realized with select dye/polymer combinations. As demonstrated here, the utility of this approach can be extended by tethering an excimer-forming cyano-substituted oligo(p-phenylene vinylene) fluorophore to the two ends of a telechelic poly(ethylene-co-butylene) and blending small amounts (0.1-2 wt%) of the resulting aggregachromic macromolecule into polymer matrices such as poly(ε-caprolactone), poly(isoprene), or poly(styrene-b-butadiene-b-styrene). All blends display mechanofluorochromic responses, and the ratio between the monomer and excimer emission intensities can be used to correlate the luminescence signal to the extent of deformation and to follow subsequent relaxation processes. The developed approach significantly expands the scope of blend-based mechanoresponsive luminescent materials.

Enhancement of triplet-sensitized upconversion in rigid polymers via singlet exciton sink approach.

To increase the practical usefulness of solid-state sensitized upconversion (UC) materials as components of solar energy harvesting systems, it is important to identify and suppress loss mechanisms, and increase the UC quantum yield (Φ UC). Here we focus on a benchmark UC system consisting of the emitter 9,10-diphenylanthracene (DPA) and the sensitizer platinum octaethylporphyrin (PtOEP) in a rigid poly(methyl methacrylate) (PMMA) matrix, and show that one of the major losses originates from Förster resonant energy transfer (FRET) from DPA back to PtOEP. Even though DPA emission lies within the PtOEP transparency window, the quantitative assessment of singlet exciton diffusion for samples with a high DPA content evidences that long-range FRET results in effective exciton trapping by PtOEP. A dramatic factor-of-6 reduction of the DPA emission quantum yield occurs even at PtOEP concentrations as low as 0.05 wt%. To alleviate this problem, we demonstrate a new concept based on the introduction of highly emissive sink sites to trap the singlet excitons produced upon annihilation prior to their quenching by the sensitizer. For DPA/PtOEP blends in PMMA, 1,6-bis-[2,5-di(dodecyloxyphenyl)ethynyl]pyrene is shown to be a useful sink, which results in 1.5-fold increase of the Φ UC. A maximum Φ UC of 2.7% was achieved, which is among the highest reported values for rigid sensitized UC polymers.

Polymersomes: Breaking the Glass Ceiling?

Polymer vesicles, also known as polymersomes, have garnered a lot of interest even before the first report of their fabrication in the mid-1990s. These capsules have found applications in areas such as drug delivery, diagnostics and cellular models, and are made via the self-assembly of amphiphilic block copolymers, predominantly with soft, rubbery hydrophobic segments. Comparatively, and despite their remarkable impermeability, glassy polymersomes (GPs) have been less pervasive due to their rigidity, lack of biodegradability and more restricted fabrication strategies. GPs are now becoming more prominent, thanks to their ability to undergo stable shape-change (e.g., into non-spherical morphologies) as a response to a predetermined trigger (e.g., light, solvent). The basics of block copolymer self-assembly with an emphasis on polymersomes and GPs in particular are reviewed here. The principles and advantages of shape transformation of GPs as well as their general usefulness are also discussed, together with some of the challenges and opportunities currently facing this area.

Fabrication of single-chain nanoparticles through the dimerization of pendant anthracene groups via photochemical upconversion.

We report on the use of visible light as the driving force for the intramolecular dimerization of pendant anthracene groups on a methacrylic polymer to induce the formation of single-chain nanoparticles (SCNPs). Using a 532 nm green laser light source and platinum octaethylporphyrin as a sensitizer, we first demonstrated the use of TTA-UC to dimerize monomeric anthracene, and subsequently applied this concept to dilute poly((methyl methacrylate)-stat-(anthracenyl methacrylate)) samples. A combination of triple-detection size-exclusion chromatography, atomic force microscopy, and UV-visible spectroscopy confirmed the formation of the SCNPs. This report pioneers the use of TTA-UC to drive photochemical reactions in polymeric systems, and showcases the potential for TTA-UC in the development of nanoobjects.

Mechanochemical Fluorescence Switching in Polymers Containing Dithiomaleimide Moieties.

Polymers that display useful mechanochemical responses, such as changes of their fluorescence characteristics, are attracting great interest. Here, we introduce the fluorescent dithiomaleimide (DTM) motif as a mechanofluorophore and report the mechanoresponse of two polymer types containing this motif. Poly(methyl acrylate) (PMA) and poly(ε-caprolactone)s (PCL) featuring one DTM moiety in the center of each chain (PMA-DTM and PCL-DTM) were synthesized by controlled radical and coordination-insertion ring-opening polymerizations using bifunctional DTM-containing initiators. Upon ultrasonic treatment of PMA-DTM or PCL-DTM of sufficiently high initial molecular weight, both the molecular weight and the fluorescence intensity decreased with similar kinetics, while no significant fluorescence changes were observed for DTM-free reference polymers. The results show that the DTM motif can serve as a mechanophore that displays a mechanically induced fluorescence turn-off.

Nanodroplet-Containing Polymers for Efficient Low-Power Light Upconversion.

Sensitized triplet-triplet-annihilation-based photon upconversion (TTA-UC) permits the conversion of light into radiation of higher energy and involves a sequence of photophysical processes between two dyes. In contrast to other upconversion schemes, TTA-UC allows the frequency shifting of low-intensity light, which makes it particularly suitable for solar-energy harvesting technologies. High upconversion yields can be observed for low viscosity solutions of dyes; but, in solid materials, which are better suited for integration in devices, the process is usually less efficient. Here, it is shown that this problem can be solved by using transparent nanodroplet-containing polymers that consist of a continuous polymer matrix and a dispersed liquid phase containing the upconverting dyes. These materials can be accessed by a simple one-step procedure that involves the free-radical polymerization of a microemulsion of hydrophilic monomers, a lipophilic solvent, the upconverting dyes, and a surfactant. Several glassy and rubbery materials are explored and a range of dyes that enable TTA-UC in different spectral regions are utilized. The materials display upconversion efficiencies of up to ≈15%, approaching the performance of optimized oxygen-free reference solutions. The data suggest that the matrix not only serves as mechanically coherent carrier for the upconverting liquid phase, but also provides good protection from atmospheric oxygen.

Mechano- and Thermoresponsive Photoluminescent Supramolecular Polymer.

Mechanoresponsive luminescent (MRL) materials change their emission color upon application of external forces. Many dyes with MRL behavior are known, but they normally do not display useful mechanical properties. Here, we introduce a new approach to overcome this problem, which relies on combining MRL compounds with the concept of supramolecular polymerization. As a first embodiment, a cyano-substituted oligo(p-phenylenevinylene), whose MRL behavior is associated with different solid-state assemblies, was derivatized with two ureido-4-pyrimidinone groups, which support the formation of a dynamic supramolecular polymer. The new material displays the thermomechanical characteristics of a supramolecular polymer glass, offers three different emission colors in the solid state, and exhibits both MRL and thermoresponsive luminescent behavior.

The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses.

Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (>25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.

Single-Component Upconverting Polymeric Nanoparticles.

Low-power light upconversion is a highly desirable feature for a broad range of applications and new materials enabling this process are sought in both bulk and particulate form. Here, the preparation of upconverting nanoparticles is reported from a methacrylic terpolymer bearing diphenylanthracene and meso-phenoxytris(heptyl)porphyrin pendant groups by a microemulsion technique. The use of a terpolymer in which the upconvering dye molecules are covalently attached mitigates some of the drawbacks of triplet-triplet annihilation upconverting nanoparticles made by other techniques, in particular dye leakage from the nanoparticles, and limited control of the sensitizer and emitter concentration within each nanoparticle. Size and morphology of the new upconverting nanoparticles are investigated by dynamic light scattering and transmission electron microscopy and elucidated their upconverting properties by luminescence spectroscopy.

A mechano- and thermoresponsive luminescent cyclophane.

The first fluorescent cyclophane with mechano- and thermoresponsive solid-state fluorescence characteristics is reported. The new cyclophane comprises two 9,10-bis(phenylethynyl)anthracene moieties that are bridged by tetraethylene glycol spacers. The stimuli-responsiveness is based on molecular assembly changes.

A Thermo- and Mechanoresponsive Cyano-Substituted Oligo(p-phenylene vinylene) Derivative with Five Emissive States.

Multiresponsive materials that display predefined photoluminescence color changes upon exposure to different stimuli are attractive candidates for advanced sensing schemes. Herein, we report a cyano-substituted oligo(p-phenylene vinylene) (cyano-OPV) derivative that forms five different solvent-free solid-state molecular assemblies, luminescence properties of which change upon thermal and mechanical stimulation. Single-crystal X-ray structural analysis suggested that tolyl groups introduced at the termini of solubilizing side-chains of the cyano-OPV play a pivotal role in its solid-state arrangement. Viewed more broadly, this report shows that the introduction of competing intermolecular interactions into excimer-forming chromophores is a promising design strategy for multicolored thermo- and mechanoresponsive luminescent materials.

Visualization of Polymer Deformation Using Microcapsules Filled with Charge-Transfer Complex Precursors.

The visualization of polymer deformation using microcapsules filled with charge-transfer complex precursors is reported. Poly(urea-formaldehyde) microcapsules filled with toluene and either hexamethylbenzene as donor or chloranil as acceptor were prepared by in situ polymerization of an oil-in-water emulsion. The resulting microcapsules were incorporated in a poly(dimethylsiloxane) matrix by dispersing them in the monomer and subsequent curing. The microcapsules were shown to rupture when the polymer was placed under mechanical forces (i.e., tensile and compression) such that the donor and the acceptor are released into the polymeric matrix, where their combination to a charge-transfer complex results in a pronounced yellow-to-red color change. The factors influencing the extent of the color change (composition and mechanical strain) were investigated and a deformation threshold seems to be present. The new damage-sensing materials may be useful for a range of applications and the general concept applied seems to be readily applicable to other host polymers.

Mechanochemistry in Polymers with Supramolecular Mechanophores.

Mechanochemistry is a burgeoning field of materials science. Inspired by nature, many scientists have looked at different ways to introduce weak bonds into polymeric materials to impart them with function and in particular mechano-responsiveness. In the following sections, the incorporation of some of the weakest bonds, i.e. non-covalent bonds, into polymeric solids is being surveyed. This review covers sequentially π-π interactions, H-bonding and metal-ligand coordination bonds and tries to highlight some of the advantages and limitations of such systems, while providing some key perspective of what may come next in this tantalizing field.