Seung Man Noh

Thiol-responsive block copolymer nanocarriers exhibiting tunable release with morphology changes

New thiol-responsive nanocarriers of amphiphilic block copolymers consisting of a pendent disulfide-labeled methacrylate polymer block (PHMssEt) and a hydrophilic poly(ethylene oxide) (PEO) block were reported. These well-controlled block copolymers were synthesized by atom transfer radical polymerization (ATRP) of a new pendent disulfide-functionalized methacrylate (HMssEt) in the presence of the PEO-Br macroinitiator. Due to its amphiphilic nature, the PEO-b-PHMssEt with narrow molecular weight distribution self-assembled in aqueous solution to form monomodal micellar aggregates with PHMssEt cores surrounded with hydrophilic PEO coronas. In response to thiols, the disulfide linkages were cleaved, and thus self-assembled micelles were either converted to core-crosslinked micelles or destabilized to further disintegrate, depending on the amount of added thiols. Such change in morphology led to tunable release of encapsulated model drugs in aqueous solutions.

Dual Sulfide-Disulfide Crosslinked Networks with Rapid and Room Temperature Self-Healability.

Polymer-based crosslinked networks with intrinsic self-repairing ability have emerged due to their built-in ability to repair physical damages. Here, novel dual sulfide-disulfide crosslinked networks (s-ssPxNs) are reported exhibiting rapid and room temperature self-healability within seconds to minutes, with no extra healing agents and no change under any environmental conditions. The method to synthesize these self-healable networks utilizes a combination of well-known crosslinking chemistry: photoinduced thiol-ene click-type radical addition, generating lightly sulfide-crosslinked polysulfide-based networks with excess thiols, and their oxidation, creating dynamic disulfide crosslinkages to yield the dual s-ssPxNs. The resulting s-ssPxN networks show rapid self-healing within 30 s to 30 min at room temperature, as well as self-healing elasticity with reversible viscoelastic properties. These results, combined with tunable self-healing kinetics, demonstrate the versatility of the method as a new means to synthesize smart multifunctional polymeric materials.

Fluorescence Detection of Microcapsule-Type Self-Healing, Based on Aggregation-Induced Emission

An extrinsic self-healing coating system containing tetraphenylethylene (TPE) in microcapsules was monitored by measuring aggregation-induced emission (AIE). The core healing agent comprised of methacryloxypropyl-terminated polydimethylsiloxane, styrene, benzoin isobutyl ether, and TPE was encapsulated in a urea-formaldehyde shell. The photoluminescence of the healing agent in the microcapsules was measured that the blue emission intensity dramatically increased and the storage modulus also increased up to 105 Pa after the photocuring. These results suggested that this formulation might be useful as a self-healing material and as an indicator of the self-healing process due to the dramatic change in fluorescence during photocuring. To examine the ability of the healing agent to repair damage to a coating, a self-healing coating containing embedded microcapsules was scribed with a razor. As the healing process proceeded, blue light fluorescence emission was observed at the scribed regions. This observation suggested that self-healing could be monitored using the AIE fluorescence.

Dual temperature and thiol-responsive POEOMA-multisegmented polydisulfides: Synthesis and thermoresponsive properties

New thermoresponsive polydisulfides of POEOMA multiblocks linked with disulfide bonds having redox-responsive properties are reported. These POEOMA-multisegmented polydisulfides were synthesized by a new method employing a combined RAFT/aminolysis and reversible thiol-disulfide redox reaction that centers on the synthesis of new disulfide-labeled difunctional RAFT agent. RAFT polymerization proceeded in living fashion, yielding well-defined POEOMA copolymers with middle disulfides and terminal RAFT species. They were then used as precursors for thiol-disulfide polyexchange induced by aminolysis and reductive reaction followed by oxidation: these polydisulfides with different molecular weights and end groups exhibited tunable thermoresponsive properties and thiol-responsive degradation.

Rheology and curing characteristics of dual-curable clearcoats with hydroxyl functionalized urethane methacrylate oligomer: Effect of blocked isocyanate thermal crosslinkers

Rheological and mechanical properties of dual-curable automotive clearcoats containing different blocked isocyanate thermal crosslinkers (HDI, IPDI, and SMBI) have been compared by changing dual-curing sequences, i.e., UV-thermal dual curing and thermal-UV dual curing. Real-time elastic modulus data of clearcoats measured during several curing procedures have been correlated with the formation of crosslinked networks inside clearcoats. Clearcoat with SMBI shows increased modulus data due to the more reactive nature of functional groups in SMBI. Difference of rheological data of clearcoats under different curing temperatures during dual-curing sequences has been interpreted with the transition of chemical reactions inside clearcoats with different chemical structured crosslinkers. It turns out that scratch properties of cured clearcoats are also affected by the dual-curing sequence.

Multiblock Copolymer‐Based Dual Dynamic Disulfide and Supramolecular Crosslinked Self‐Healing Networks

A new multiblock copolymer self-healing strategy is reported that centers on the synthesis of block copolymers designed with different self-healing motifs incorporated into individual blocks. As a proof of concept, a novel pentablock copolymer (ABCBA) consisting of a poly(ethylene glycol) middle block and self-healable symmetric blocks of a polymethacrylate with pendant disulfide linkages and carboxylic acids is synthesized by a combination of consecutive controlled radical polymerization with hydrolytic cleavage. Disulfide exchange reactions of pendant disulfide linkages and metal-ligand interactions of pendant carboxylic acids with ferric ions allow for the formation of dual crosslinked networks with dynamic disulfide and supramolecular crosslinkages. The resultant networks possessing self-healing viscoelasticity enable self-healing on macroscale damages through supramolecular metal-ligand interactions and disulfide exchange reactions at room or moderate temperatures. These preliminary results suggest that the strategy can offer the versatility in the development of multifunctional self-healable materials in dual or multiple self-healable mechanisms.

The effect of thermal radical initiator derived from O-imino-isourea on thermal curing characteristics and properties of automotive clearcoats

Curing characteristics of melamine-based automotive clearcoats with or without thermal radical initiator (TRI) derived from an O-imino-isourea have been examined by means of their real-time rheological properties during a thermal curing process. By changing functionalities of reactive sites at hydroxyl groups and unsaturated double bonds in hydroxyl-functionalized urethane methacrylate oligomer as a main binder, the effect of TRI on various properties of thermally cured clearcoats has been compared by means of rheological, rigid-body pendulum, ultra nano-indentation, and nano-scratch tests. It was found that a small portion of TRI significantly enhanced the final elastic modulus of clearcoats with the lower content of hydroxyl groups and considerably shortened curing time, implying the formation of denser crosslinked networks inside clearcoats by the free-radical polymerization at relatively lower curing temperature. These results were directly linked with the improvement of their mechanical properties such as indentation hardness and scratch resistance. TRI plays a key role in desirably controlling the rheological and mechanical properties of clearcoats under insufficient thermal curing conditions.

Characterization of clearcoats containing phosphoric acid-functionalized acrylic polyols for automotive precoated metal sheet coatings

In this study, the various physical and mechanical properties of clearcoats prepared through a new crosslinking method were investigated. The method was aimed at developing clearcoat systems to improve the deep-draw processing and formability performance in precoated metal (PCM) sheets for automotive applications. From phosphoric acid-functionalized acrylic polyols (PAFAPs) first synthesized in this study and glycidyl methacrylate-modified acrylic copolymer (GMAMAC), phosphoric acid-GMA modified acrylic polyols (PAGMAPs) were newly prepared as new binders in automotive clearcoats. Several clearcoats were formulated with different molecular weights and hydroxyl contents from PAFAPs and GMAMAC. Using clearcoats themselves, the crosslinking reactions for these clearcoats were compared by evaluating the curing behaviors with a rigid-body pendulum test (RPT) and the changes of chemical structures via attenuated total reflectance FTIR spectroscopy. The mechanical properties of the clearcoats were systematically characterized, using dynamic mechanical analysis (DMA) and universal testing machine analysis (UTM). Also, various tests were carried out using PCM sheets by depositing clearcoats above the same PCM-based primers and basecoat layers on galvanized steel. The fracture and deformation patterns related to surface damages on the clearcoat surface were visualized using a nano-scratch test, in association with atomic force microscopy. In particular, deep-draw processing tests, based on forming process simulations, were employed to scrutinize the effect of clearcoats developed in this study on the forming feature in PCM sheets. From the results of RPT, DMA, and UTM tests, the primary crosslinking networks of PAGMAPs from the synthesized PAFAPs and GMAMAC, and also succeeding secondary crosslinking networks between PAGMAPs and blocked isocyanates, were closely correlated with the degree of crosslinking (Xc), in accordance with the molecular weight between crosslinks (Mc), and glass transition temperature (Tg). As a result, the presented clearcoats with a long pendulum period, a low rubbery modulus, and a large tensile strain value, which are the significant factors for developing automotive PCM sheet technology, have truly demonstrated more superior formability during the deep-draw process. It is confirmed that properties of clearcoats with toughness and flexibility could be optimally controlled by PAGMAPs for automotive coatings.

Water-adaptive and repeatable self-healing polymers bearing bulky urea bonds

Self-healing polymers bearing reversible and bulky urea bonds were prepared by free-radical copolymerization followed by crosslinking with diisocyanates. Linear prepolymers (PtB) were first synthesized from 2-(tert-butylamino)ethyl methacrylate (tBAEMA), methyl methacrylate (MMA), and n-butyl acrylate (BA), in a molar ratio of 1 : 10 : 10, and then crosslinked with 1,6-diisocyanatohexane (HDI) or isophorone diisocyanate (IPDI) to fabricate self-healing polymers. The reversible bonding–debonding between the tBAEMA of the polymer backbone and the isocyanate units of the crosslinker in the self-healing polymer facilitates rapid, repeatable, and water-adaptive self-healing performance, which has been extensively investigated using 1H-NMR, DSC, FT-IR, AFM, optical microscopy, and rheometric analyses. Percentage recovery (%R) and self-healing efficiency (%SHE) were also studied using tensile and single-scratch tests, respectively, and compared with a control sample. It was revealed through extensive analyses that self-healing against a single scratch can be accelerated and repeated up to a certain number of times even in water, although the self-healing polymers are not water-resistant. We also demonstrated that a self-healable tube capable of containing flowing water could be fabricated from a self-healing polymer sheet by using reversible bonding–debonding characteristics of a reversible bulky urea bond.

Ambient temperature induced Diels–Alder crosslinked networks based on controlled methacrylate copolymers for enhanced thermoreversibility and self-healability

The development of covalently-crosslinked dynamic (reversible) networks has been extensively explored due to their built-in ability to self-repair damages, which prevents catastrophic failure of high-performance materials. Here, we report effective thermoreversible crosslinked networks fabricated at room or mild temperature from reactive blends consisting of a controlled methacrylate copolymer having maleimide pendants (CoPMA) and a trifunctional furan (TFu). Well-defined CoPMA is synthesized by controlled radical polymerization, ensuring predetermined chain lengths with narrow molecular weight distribution, and following post-modification through deprotection of pendant furan-protected maleimide groups, ensuring conversion to the corresponding maleimide pendants. The well-defined CoPMA is reactive to different amounts of TFu as a model polyfuran to form dynamic networks at ambient temperature through a click type Diels–Alder (DA) reaction. Comprehensive analyses indicate that the mole ratio of furan/maleimide group is an important parameter that significantly influences network flexibility. The developed dynamic networks crosslinked with thermo-labile DA cycloadduct linkages enable the dissociation and recombination through retro DA (rDA)/DA reactions at elevated temperatures. Such thermoreversibility, combined with balanced flexibility, allows for the development of dynamic network exhibiting effective self-healability at as low as 110 °C with great mechanical property restoration as well as healing elasticity through reversible restoration of viscoelastic properties.