Non-isocyanate Synthesis of Covalent Adaptable Networks Based on Dynamic Hindered Urea Bonds: Sequential Polymerization and Chemical Recycling.

The development of environmentally sustainable processes for polymer recycling is of paramount importance in the polymer industry. In particular, the implementation of chemical recycling for thermoset polymers via covalent adaptable networks (CANs), particularly those based on the dynamic hindered urea bond (HUB), has garnered intensive attention from both the academic and industrial sectors. This interest stems from its straightforward chemical structure and reaction mechanism, which are well-suited for commercial polyurethane and polyurea applications. However, a substantial drawback of these CANs is the requisite use of toxic isocyanate curing agents for their synthesis. Herein, we propose a new HUB synthesis pathway involving thiazolidin-2-one and a hindered amine. This ring-opening reaction facilitates the isocyanate-free formation of a HUB and enables sequential reactions with acrylate and epoxide monomers via thiol-Michael and thiol-epoxy click chemistry. The CANs synthesized using this methodology exhibit superior reprocessability, chemical recyclability, and reutilizability, facilitated by specific catalytic and solvent conditions, through the reversible HUB, thiol-Michael addition, and transesterification processes.

Reusable, Ultrasensitive, Patterned Conjugated Polyelectrolyte-Surfactant Complex Film with a Wide Detection Range for Copper Ion Detection.

Conjugated polyelectrolytes (CPEs) are emerging as promising materials in the sensor field because they enable high-sensitivity detection of various substances in aqueous media. However, most CPE-based sensors have serious problems in real-world application because the sensor system is operated only when the CPE is dissolved in aqueous media. Here, the fabrication and performance of a water-swellable (WS) CPE-based sensor driven in the solid state are demonstrated. The WS CPE films are prepared by immersing a water-soluble CPE film in cationic surfactants of different alkyl chain lengths in a chloroform solution. The prepared film exhibits rapid, limited water swellability despite the absence of chemical crosslinking. The water swellability of the film enables the highly sensitive and selective detection of Cu2+ in water. The fluorescence quenching constant and the detection limit of the film are 7.24 × 106 L mol-1 and 4.38 nM (0.278 ppb), respectively. Moreover, the film is reusable via a facile treatment. Furthermore, various fluorescent patterns introduced by different surfactants are successfully fabricated by a simple stamping method. By integrating the patterns, Cu2+ detection in a wide concentration range (nM-mM) can be achieved.

Spontaneously sp2-Carbonized Fluorescent Polyamides as a Probe Material for Bioimaging.

Spontaneously sp2-carbonized polyamides (PA1, PA2) were prepared via Knoevenagel-type side reactions of malonyl moieties under mild conditions in the polycondensation of dicarbonyl chloride and diamine. Both polymers were soluble in water and emissive in the visible region, and the fluorescence (FL) intensity and the maximum wavelength were highly dependent on the excitation wavelength and the pH. Their chemical structures and FL origin were clarified by performing various spectroscopic analyses. π*-π transition was assumed to be allowed in an enol form based on the conjugated structure formed by the side reaction; this was responsible for its pH dependency and high FL quantum efficiency. In particular, PA2, which comprises the tertiary amide linkage, showed quick endocytosis, low cytotoxicity, excellent biocompatibility, and exclusively stained lysosomes with the lowest intracellular pH. These results will help in understanding the origin of the FL emission of carbonized nanomaterials and exploring more advanced functions in the field of bioimaging.

Optical-Dissymmetry Phase Transitions in an Achiral Helical-Spring Polymer through Controlled Noncovalent Interactions.

Noncovalent chemistry may offer diversity in the functions and applications for artificial polymers by allowing various ordered-disordered phase transitions in a precisely controlled manner. To verify this notion from a fundamental perspective, we examined an achiral poly(phenylacetylene) derivative with an α-helical structure as a helical-spring polymer for revealing phase changes through control of intramolecular hydrogen bonding with the chiral solvent and temperature. When an amine capable of hydrogen bonding was used as the chiral solvent, either an irreversible helix-helix or a reversible helix-coil phase change occurred in an optically dissymmetric manner according to the amount of the chiral solvent added and ambient temperature. Considering the hydrogen-bonding strength values of the solvent mixture and the thermodynamic parameters, we could predict if the optical-dissymmetry phase changes would occur and, if so, how they occur. Our results were similar to those see for the denaturation of proteins, induced by solvent and temperature, based on helix-coil phase transition.

Unusual piezochromic fluorescence of a distyrylpyrazine derivative crystals: phase transition through [2 + 2] photocycloaddition under UV irradiation.

The piezochromic fluorescence (FL) of a distyrylpyrazine derivative, 2,3-diisocyano-5,6-distyrylpyrazine (DSP), was investigated in this study. Depending on the recrystallization method, DSP afforded two different crystals with green and orange FL emission. The orange color FL emission crystal (O-form) was easily converted to the green color FL emission one (G-form) by manual grinding. The G-form was also converted to a slightly different orange color FL emission crystal (RO-form) by a weak UV irradiation. When the RO-form was ground again, the G-form was regenerated. The FL colors changed between the G- and RO-forms over several ten times by repeated mechanical grinding and UV irradiation. The FL, UV-visible, 1H-NMR and XRD results showed that the O (or RO)-to-G transformation induced by mechanical stress results from the change of degree of molecular stacking from dense molecular stacking structure to relatively loose molecular stacking structure, whereas the G-to-RO reconversion by UV irradiation results from return to dense molecular stacking structure again due to lattice movement (lattice slipping) allowed by photocycloaddition in solid-state.

QCM-Based HCl Gas Sensors Using Spin-Coated Aminated Polystyrene Colloids.

Hydrogen chloride (HCl) gas is highly toxic to the human body. Therefore, HCl gas detection sensors should be installed at workplaces where trace HCl gas is continuously generated. Even though various polymer-based HCl-gas-sensing films have been developed, simpler and novel sensing platforms should be developed to ensure the cost effectiveness and reusability of the sensing platforms. Therefore, we present a simple strategy to fabricate reusable HCl-gas-sensing platforms using aminated polystyrene (a-PS) colloids and investigate their sensitivity, reusability, and selectivity using a quartz crystal microbalance (QCM). The reusable a-PS(1.0) colloidal sensor with a high degree of amination (DA) exhibited the highest binding capacity (102 µg/mg) based on the frequency change (Δf) during the HCl gas adsorption process. Further, its sensitivity and limit of detection (LOD) were 3.88 Hz/ppm and 5.002 ppm, respectively, at a low HCl gas concentration (<10 ppm). In addition, the sensitivity coefficient (k*) of the a-PS(1.0) colloid sensor with respect to HCHO was higher than that in the case of HF because of the lower binding affinity of the former with the a-PS(1.0) colloids. Based on these results, highly sensitive and reproducible a-PS colloids could be reused as an HCl-gas-sensing platform and used as an HCl sorbent in a gas column filter.

Control of Intramolecular Hydrogen Bonding in a Conformation-Switchable Helical-Spring Polymer by Solvent and Temperature.

A substituted poly(phenylacetylene) derivative (PPAHB ) with two hydroxymethyl groups at the meta position of the side phenyl ring was examined as a conformation-switchable helical spring polymer that responds to solvent and heat stimuli in a precisely controlled manner. Intramolecular hydrogen bonds, which cause the helical structure of the polymer, were broken and re-formed by adjusting the hydrogen-bonding strength values (pKHB ) of various combinations of solvents or by varying the temperature. In this process, a reversible conformational change from cis-cisoid to cis-transoid, accompanied by a phase transition in the form of a helix-coil transformation occurred, with the polymer exhibiting critical changes of color fading and recovery in specific environments. These results demonstrate that PPAHB can be used as either a pKHB indicator or a thermometer. The color changes of the polymer solution are described in detail based on spectroscopic analyses and thermodynamic considerations.

Visualization of Sweat Fingerprints on Various Surfaces Using a Conjugated Polyelectrolyte.

A conformation-variable conjugated polyelectrolyte responding to oppositely charged biomolecules was examined as an imaging agent for the detection of latent fingerprints (LFPs). Sulfonated poly(diphenylacetylene) (SPDPA) produces high-resolution fluorescence (FL) LFP images by simple wetting of the target objects with the polymer solution without any additional treatment. SPDPA readily interacts with LFP sweat components (especially amino acids) via electrostatic interactions, leading to significantly enhanced FL images in a "turn-on" mode. The FL emission enhancement was examined in a model reaction between SPDPA and an amino acid standard. Visualization with SPDPA is effective on various surfaces, including both rough (paper) and smooth (glass and plastic) ones. Moreover, SPDPA readily interacts with extremely thin sweat LFPs, especially on smooth glass surfaces.

Highly emissive 'frozen-in' conjugated polymer nanofibers.

Conjugated-polymer nanofibers with a thermodynamically stable, coarsened, disordered structure in an amorphous glassy state were fabricated via a freeze-drying method using a poly(diphenylacetylene) derivative. The nanofibers were extremely emissive, with a fluorescence (FL) quantum yield of approximately 0.34, which was much higher than that of both the cast film (0.02) and the solution (0.21). Similarly, the amplitude-weighted average FL lifetime of the nanofibers was 0.74 ns, which was much longer than that of the film (0.29 ns) and the solution (0.57 ns). This unusual and enhanced FL-emission behavior was attributed to the abruptly quenched chain structure that was created by the freeze-drying process. The polymer chains in the nanofibers remained frozen-in and the side phenyl rings were retained in a relaxed state. The metastable chains did not undergo vibrational relaxation and collisional quenching to generate the radiative emission decay effectively.

Highly Emissive, Optically Active Poly(diphenylacetylene) Having a Bulky Chiral Side Group.

A highly emissive, optically active poly(diphenylacetylene) derivative, NpSi*-PDPA, was synthesized by introducing a bulky chiral pendant group into the polymer chain. NpSi*-PDPA exists in a glassy state having a highly disordered and amorphous structure. Hence, the fractional free volume is quite high, i.e., 0.29. NpSi*-PDPA emits a green light in solution and a yellow light in film. This polymer is quite emissive, as the photoluminescence (PL) quantum yield is 56.1% in solution and 6.2% in film, and the PL lifetime is relatively long, 1.71 ns in solution and 1.05 ns in film. NpSi*-PDPA shows the strongest circular dichroism (CD) signal at 384 nm, with a magnitude of circular polarization (gCD) of 0.90 × 10-3 and an optical rotation of 110° (4.30 × 10-3 g mL-1, in CHCl3). The CD intensity is significantly increased by annealing at 80 °C, reaching an equilibrium at gCD of 7.10 × 10-3 after 72 h.

Surfactant chemistry for fluorescence imaging of latent fingerprints using conjugated polyelectrolyte nanoparticles.

When aqueous conjugated-polyelectrolyte colloidal solutions containing an adequate amount of surfactant with an appropriate hydrophile-lipophile balance were sprayed onto latent fingerprints (LFPs), the polymer nanoparticles were readily transferred to the LFPs to reveal highly distinguishable fluorescent images, while the LFPs themselves remained intact.

Fluorescence Response of Conjugated Polyelectrolyte in an Immiscible Two-Phase System via Nonelectrostatic Interaction with Surfactants.

This paper reports a unique fluorescence (FL) response and diverse applications of conjugated polyelectrolyte (CPE) through nonelectrostatic interaction with appropriate (bio)surfactants in an immiscible two-phase system. A sulfonated microporous conjugated polymer (SMCP) with a conformation-variable intramolecular stacked structure was used as the CPE film. Despite the extremely high hydrophilicity, the SMCP film responded significantly to the hydrophobic circumstances, either physicochemically or electronically, in the presence of water-in-oil (w/o)-type nonionic surfactants with appropriate hydrophile-lipophile balance (HLB) values. The polymer film became fully wet with hydrophobic solvents due to the addition of small amounts of (bio)surfactant to reveal remarkable FL emission enhancement and chromism. Microcontact and inkjet printing using the SMCP film (or SMCP-adsorbed paper) and the surfactant solution as substrate and ink, respectively, provided high-resolution FL images due to the distinctive surfactant-induced FL change (SIFC) characteristic. Moreover, the additional electrostatic interaction of SMCP film with oppositely charged surfactants further enhanced the FL emission. Our findings will help comprehensive understanding of the nonelectrostatic SIFC mechanism of CPEs and development of novel SIFC-active materials.

Fluorescent Molecular Rotor-in-Paraffin Waxes for Thermometry and Biometric Identification.

Novel thermoresponsive sensor systems consisting of a molecular rotor (MR) and paraffin wax (PW) were developed for various thermometric and biometric identification applications. Polydiphenylacetylenes (PDPAs) coupled with long alkyl chains were used as MRs, and PWs of hydrocarbons having 16-20 carbons were utilized as phase-change materials. The PDPAs were successfully dissolved in the molten PWs and did not act as an impurity that prevents phase transition of the PWs. These PDPA-in-PW hybrids had almost the same enthalpies and phase-transition temperatures as the corresponding pure PWs. The hybrids exhibited highly reversible fluorescence (FL) changes at the critical temperatures during phase transition of the PWs. These hybrids were impregnated into common filter paper in the molten state by absorption or were encapsulated into urea resin to enhance their mechanical integrity and cyclic stability during repeated use. The wax papers could be utilized in highly advanced applications including FL image writing/erasing, an array-type thermo-indicator, and fingerprint/palmprint identification. The present findings should facilitate the development of novel fluorescent sensor systems for biometric identification and are potentially applicable for biological and biomedical thermometry.

Fluorescence turn-on response of a conjugated polyelectrolyte with intramolecular stack structure to biomacromolecules.

An anionic conjugated polyelectrolyte based on polydiphenylacetylene showed a significant fluorescence turn-on response to positively-charged proteins through a conformational relaxation of its intramolecular stack structure.