Chan-Moon Chung

Sunlight-induced self-healing of a microcapsule-type protective coating.

Photopolymerization behavior of a methacryloxypropyl-terminated polydimethylsiloxane (MAT-PDMS) healing agent was investigated in the presence of benzoin isobutyl ether (BIE) photoinitiator by Fourier transform infrared (FT-IR) spectroscopy. MAT-PDMS and BIE were microencapsulated with urea-formaldehyde polymer. The surface and shell morphology of the microcapsules was investigated by scanning electron microscopy (SEM). Mean diameter and size distribution of the microcapsules could be controlled by agitation rate. A coating matrix formulation was prepared by sol-gel reaction of tetraethyl orthosilicate (TEOS) in the presence of a polysiloxane and by subsequent addition of an adhesion promoter. The formulation and microcapsules were mixed to give a self-healing coating formulation, which was then sprayed to surface of cellulose-fiber-reinforced-cement (CRC) board or mortar. Contact angle measurements showed that both the polymerized MAT-PDMS and the prepared coating matrix are hydrophobic, and the coating matrix has good wettability with MAT-PDMS. It was confirmed by optical microscopy and SEM that, when the self-healing coating is damaged, the healing agent is released from ruptured microcapsules and fills the damaged region. The self-healing coating was evaluated as protective coating for mortar, and it was demonstrated by water permeability and chloride ion penetration tests that our system has sunlight-induced self-healing capability. Our self-healing coating is the first example of capsule-type photoinduced self-healing system, and offers the advantages of catalyst-free, environmentally friendly, inexpensive, practical healing.

Fluorescence sensing of microcracks based on cycloreversion of a dimeric anthracene moiety

Novel fluorescent crack sensors have been developed based on dimeric anthracene moiety-containing polymers. Two anthracene derivatives, 9-anthraldehyde (AA) and 9-anthracenecarboxylic acid (AC), were photodimerized to obtain cyclooctane-type dimers. Crack-sensing polymers (Poly-AA and Poly-AC) were prepared by crosslinking of poly(vinyl alcohol) by using the dimers as crosslinkers. The polymers afforded transparent, hard coatings. Upon cracking, the polymers exhibited strong optical absorption and fluorescence emission while the uncracked original polymers did not. This was explained by regeneration of the anthracene moiety by mechanochemical cycloreversion of the cyclooctane dimer structure. It was found that the crack planes emitted fluorescence having emission maxima in the range of 500–600 nm when excited with 330–385 nm UV light. Absolute fluorescence quantum yield measurements indicated that the polymers could have good capability of fluorescence crack sensing. Preliminary evaluation of the crack-sensing ability of Poly-AA and Poly-AC was performed with the polymer films, and fluorescence emission was clearly observed along the crack planes upon excitation with 330–385 nm UV light. Poly-AA and Poly-AC are promising as fluorescent crack sensors because the anthracene moiety regenerated upon cracking has relatively long excitation and emission wavelengths as well as strong fluorescence.

Repeatable self-healing of a microcapsule-type protective coating

Cinnamide moiety-containing polydimethylsiloxane (CA-PDMS) was prepared and used as a healing agent. The photo-cross-linking behavior of CA-PDMS was investigated by UV-vis and Fourier transform infrared (FT-IR) spectroscopy. Upon photo-irradiation, CA-PDMS generates viscoelastic substances which have intrinsic recoating (or self-healing) capability when scribed with a cutter blade. CA-PDMS was microencapsulated with a urea-formaldehyde polymer, and the mean diameter and size distribution of the microcapsules could be controlled by the agitation rate. The prepared microcapsules were integrated into a commercial enamel paint to create a self-healing coating. It was confirmed by optical microscopy and scanning electron microscopy (SEM) that, when the self-healing coating is scribed, the healing agent is released from ruptured microcapsules and fills the scribed region. The scribed self-healing coating was photo-irradiated to induce photo-cross-linking of the released CA-PDMS. SEM imaging provided visual evidence that, when the scribed and healed region is re-scribed, repeated self-healing is accomplished. It was successfully demonstrated by anticorrosion and electrochemical tests that the CA-PDMS-based self-healing coating system has repeatable self-healing capability. Our self-healing coating is the first example of microcapsule-type repeatable self-healing system, and offers the advantages of simple, inexpensive, practical healing.

Organic–inorganic polymer hybrids based on unsaturated polyester

Abstract Organic–inorganic polymer hybrid materials were first prepared using an unsaturated polyester (UPE) as an organic component. Homogeneous UPE/silica gel hybrids formed via π–π interactions between benzene rings of the two constituents. Photocrosslinking of UPE in the hybrids resulted in the formation of interpenetrating polymer network structure.

Synthesis and photopolymerization of dicarboxylic acid dimethacrylates and their application as dental monomers

New six dicarboxylic acid dimethacrylates (DMAs) were synthesized and studied as photocurable dental monomers. The photopolymerization behavior of the monomers was investigated by FTIR spectroscopy using camphorquinone (CQ) and 2-(dimethylamino)ethyl MA (DMAEMA) as a photoinitiating system. Relatively high conversions (>55%) resulted from photopolymerization of the monomers by visible light. The volume shrinkage of the monomers during photopolymerization was significantly influenced by their molecular weight and degree of conversion. Preliminary tests of the composites formulated with a dicarboxylic acid DMA, a diluent, CQ, DMAEMA, and a glass filler were carried out including a three-point bending test and fluoride release.

Radical deoxygenation of alcohols via their trifluoroacetate derivatives with diphenylsilane

Abstract Trifluoroacetate derivatives of alcohols can be used as precursors for the radical deoxygenation of alcohols with diphenylsilane. The reaction afforded high isolated yields of the deoxygenated products and neutral reaction conditions.

Preparation of robust, flexible, transparent films from partially aliphatic copolyimides

Robust, flexible, and transparent films have been prepared from partially aliphatic copolyimides, and their structure-property relationship was systematically studied. Spiro-type rel-(1′R,3S,5′S)-spiro[furan-3(2H),6′-[3]oxabicyclo[3.2.1]octane]-2,2′,4′,5(4H)-tetrone (DAn) and ladder-type 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) were used as cycloaliphatic dianhydride. Each dianhydride and an aromatic dianhydride, benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA), were polymerized with an aromatic diamine, 4,4′-oxydianiline (ODA). Molar feed ratio of the aliphatic and aromatic dianhydrides was varied, and the molar ratios of dianhydride units in the obtained poly(amic acid)s were in good agreement with the molar feed ratios. Highly flexible and transparent films were obtained from the copolyimides. The relationship between structure and properties such as film flexibility, solubility, optical transparency, and thermal stability is explained by intermolecular interactions, degree of intramolecular conjugation, intermolecular charge transfer complex formation and backbone flexibility including the measurement of d-spacing values by wide-angle X-ray diffraction (WAXD) analysis. The polyimide prepared with the molar feed ratio of DAn:BTDA:ODA of 0.9:0.1:1 gave thin films having high flexibility, transparency, and colorlessness coupled with good solubility, thermal stability, and mechanical properties.

Synthesis and properties of silicon‐containing maleimide polymers based on N‐(trimethylsilyl)maleimide

N-(Trimethylsilyl)maleimide (TMSMI) has been polymerized with various styrenic monomers (XSt) in the presence of a radical initiator to give high-molecular-weight, alternating copolymers in high yields. The copolymers P(TMSMI/XSt) have high glass transition temperatures above 200°C and thermal decomposition temperatures in the range 320-360°C. The thermal and acidolytic deprotection of trimethylsilyl (TMS) groups of TMSMI units in the copolymers have been investigated. UV irradiation and subsequent heating of a film of P(TMSMI/t-BOCSt) containing a photoacid generator resulted in deprotection of the protecting groups by the photogenerated acids, thereby causing a significant change in solubility of the polymer. Thus positive image patterns were obtained with P(TMSMI/t-BOCSt) by photolithographic processes.

Photosensitive polyimides having N-sulfonyloxyimide and N-carbonyloxyimide groups in the main chain

Abstract Photodegradation behavior was investigated for positive-type polyimides simultaneously containing N-sulfonyloxyimide (or N-carbonyloxyimide) groups and cyclobutane rings in the main chain. The polyimides underwent photo-induced decomposition through the cleavage of N–O bond as well as cyclobutane ring, affording maleimide as a photoproduct. The polyimides exhibited enhanced photosensitivity compared to conventional cyclobutane-containing polyimides.

Fully Transparent, Non-volatile Bipolar Resistive Memory Based on Flexible Copolyimide Films

Partially aliphatic homopolyimides and copolyimides were prepared from rel-(1′R,3S,5′S)-spiro[furan-3(2H),6′-[3]oxabicyclo[3.2.1]octane]-2,2′,4′,5(4H)-tetrone (DAn), 2,6-diaminoanthracene (AnDA), and 4,4′-oxydianiline (ODA) by varying the molar ratio of AnDA and ODA. We utilized these polyimide films as the resistive switching layer in transparent memory devices. While WORM memory behavior was obtained with the PI-A100-O0-based device (molar feed ratio of DAn : AnDA : ODA = 1 : 1 : 0), the PI-A70-O30-based device (molar feed ratio of DAn : AnDA : ODA = 1 : 0.7 : 0.3) exhibited bipolar resistive switching behavior with stable retention for 104 s. This result implies that the memory properties can be controlled by changing the polyimide composition. The two devices prepared from PI-A100-O0 and PI-A70-O30 showed over 90% transmittance in the visible wavelength range from 400 to 800 nm. The behavior of the memory devices is considered to be governed by trap-controlled, space-charge limited conduction (SCLC) and local filament formation.