Published papers

Polystyrene (PS) has not been much used as an optical material due to the considerably high birefringence of PS. For the extensive use of PS in the imaging-lens systems, lower birefringence should be attained for the enhancement of the resolution. Previously, we have developed PS with extremely low birefringence (LB-PS) even when the molecular chain orientation of PS occurred. It indicates that PS may now become an effective material with excellent properties for optical lenses. In this paper, we compared the birefringence of our developed LB-PS with ones of commercial optical polymers that are most commonly utilized in imaging-lens systems. It was confirmed that the birefringence of LB-PS was comparable or even better than that of the commercial ones with high resolution. By designing an imaging-lens system, it was also confirmed that employing PS with the Abbe number of 31.5 could be highly effective in compensating for chromatic aberration as well as obtaining high optical resolution. Moreover, the resolution performance of several lens systems was calculated, analyzed, and compared by considering the birefringence of cyclo-olefin polymer (COP), normal PS, and LB-PS. It was found that LB-PS could maintain high resolution, while normal PS with high birefringence exhibited significant decrease in resolution. It was also found that LB-PS generated higher resolution than COP. All these results revealed that LB-PS could be an excellent optical material to compensate for the chromatic aberration of lens systems, leading to the enhancement of the actual resolution performance. It was, therefore, expected that LB-PS could contribute to improving the resolution performance of smartphone cameras and security cameras.

Assessment of the toxicity of insoluble particulate matter (PM) in the atmosphere is crucially important. However, because of challenges associated with extracting insoluble PM from filters, the toxicity of insoluble PM remains largely uncharacterized. For this study, the water-soluble filter made of poly (vinyl alcohol) (PVA) was developed with high particle collection efficiency, durability, and weather resistance, which are necessary for ambient PM sampling. The PVA filter is water-soluble under certain conditions, allowing for the recovery of insoluble PM, which can be exposed directly to cells or animals for toxicological studies. The PVA filter was made from PVA nanofibers using the electrospinning (ES) method. Its durability and weather resistance were comparable to those of polytetrafluoroethylene (PTFE) and quartz fiber (QF) filters, and were superior to those of commercially available gelatin filters. The PVA filter particle collection efficiency was evaluated by comparing concentrations of inorganic elements (Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu and Zn) and water-soluble ionic components (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−) in the PVA filter-collected ambient PM with those in the PTFE filter-collected PM. The concentrations of almost all measured components in the PVA filter-collected PM were comparable to those in the PTFE filter-collected PM. When the PVA filter dissolved in cell culture medium was exposed to cells, no significant difference was found in cell viability compared to a blank solution containing only culture medium, suggesting that the PVA filter has potential use for cell exposure experiments.

Fabrication and structural characterization of syndiotactic propylene-ethylene (sPPEt) copolymer nanofibers were conducted. When using a mixture of methylcyclohexane/DMF/acetone (8:1:1 (wt/wt)) as a solvent, semi-crystalline sPPEt copolymers with ethylene compositions ranging from 1.9 to 11.8 mol% were successfully fabricated into nanofibers via electrospinning. Copolymerization of sPP with ethylene significantly promoted the formation of thinner and smoother nanofibers by suppressing gelation of the spinning solution, thereby enhancing the electrospinnability of the sPP-based copolymer. FTIR and X-ray structural analyses on sPPEt nanofiber membranes revealed that the electrostatic stretching force during the electrospinning process led to a larger proportion of syndiotactic propylene units in the trans-planar conformation and the resulting formation of mesomorphic form crystals, similar to what has been observed during mechanical stretching and subsequent relaxation.

Contact-killing antibacterial materials are attracting attention owing to their ability for sustained antibacterial activity. However, contact-killing antibacterial polystyrene (PS) has not been extensively studied because its chemically stable structure impedes chemical modification. In this study, we developed an antibacterial PS sheet with a contact-killing surface using PS synthesized from 2,2′-azobis-[2-(1,3-dimethyl-4,5-dihydro-1H-imi-dazol-3-ium-2-yl)]propane triflate (ADIP) as a radical initiator with cationic moieties. The PS sheet synthesized with ADIP (ADIP-PS) exhibited antibacterial activity in contrast to PS synthesized with other azo radical initiators. Surface ζ-potential measurements revealed that only ADIP-PS had a cationic surface, which contributed to its contact-killing antibacterial activity. The ADIP-PS sheets also exhibited antibacterial activity after washing. In contrast, PS sheets containing silver, a typical leachable antibacterial agent, lost all antibacterial activity after the same washing treatment. The antibacterial ADIP-PS sheet demonstrated strong broad-spectrum activity against both Gram-positive and Gram-negative bacteria, including drug-resistant bacteria. Cytotoxicity tests using L929 cells showed that the ADIP-PS sheets were noncytotoxic. This contact-killing antibacterial PS synthesized with ADIP thus demonstrated good prospects as an easily producible antimicrobial material.

Oligomeric protein nanocages often disassemble into their subunits and reassemble by external stimuli. Thus, using these nanocages as cross-linkers for hydrogel network structures is a promising approach to allow hydrogels to undergo stimuli-responsive gel-sol transitions or self-healing. Here, we report hydrogels that show a reversible gel-sol transition resulting from the heat-induced dissociation and reassociation of protein nanocages. The hydrogel contained the 60-mer artificial protein nanocage, TIP60, as a supramolecular cross-linker for polyethylene glycol network structures. The hydrogel showed a gel-to-sol transition upon heating at a temperature above the melting point of TIP60 and immediately returned to a gel state upon cooling to room temperature. During the heating and cooling treatment of the hydrogel, small-angle X-ray scattering analysis suggested the dissociation and reassociation of TIP60. Furthermore, we demonstrated redox-responsive cargo release from TIP60 in the hydrogel. These results showed the potential of TIP60 as a component of multi-stimuli-responsive hydrogels.

A polymer with high contents of ester bonds and iodine atoms was synthesized, exhibiting sufficient biodegradability and radioactivity for biomedical applications. The iodine moieties of the synthesized polyester can generate halogen bonding between molecules, which may develop additional functional properties through the bonding. In this study, poly(glycerol adipate) (PGA) was selected and synthesized as a polyester, which was then adequately conjugated with three different types of iodine compounds via the hydroxy groups of PGA. It was found that the iodine compounds could effectively work as donors of halogen bonding. The thermal analysis by differential scanning calorimetry (DSC) revealed that the glass transition temperature increased with the increase in the strength of interactions caused by π-π stacking and halogen bonding, eventually reaching 49.6 °C for PGA with triiodobenzoic groups. An elastomeric PGA with monoiodobenzoic groups was also obtained, exhibiting a high self-healing ability at room temperature because of the reconstruction of halogen bonding. Such multifaceted performance of the synthesized polyester with controllable thermal/mechanical properties was realized by halogen bonding, leading to a promising biomaterial with multifunctionality.

The birefringence of optical polymers is a great issue in optical devices, inhibiting major applications of polymers to optical lenses and films. In this study, we have synthesized effective bottlebrush polymers with which we could attain almost zero birefringence when mixed with optical poly(methyl methacrylate) (PMMA). In detail, the PMMA bottlebrush polymers (PMMA-BBP) were synthesized by the ring-opening metathesis polymerization (ROMP) of norbornene-terminated PMMA macromonomers prepared via atom transfer radical polymerization (ATRP). Linear PMMA and PMMA-BBP were mixed to fabricate blend-film samples (PMMA/PMMA-BBP), which were uniaxially drawn to introduce molecular orientations. Linear PMMA possessed a negative value for its orientation birefringence, while the value of PMMA/PMMA-BBP increased as the PMMA-BBP content increased, whose orientation birefringence could reach almost zero when the ratio of the linear PMMA to PMMA-BBP became 73:27, regardless of the magnitude of the strain. The results reveal that the orientation birefringence of PMMA can be effectively controlled and removed by blending the appropriate content of PMMA-BBP.

Thermophysical properties such as phase equilibrium temperature and dissociation heat are important to determine the performance of phase change materials targeting thermal energy storage. The phase equilibrium temperature and dissociation heat of tetrabutylphosphoniumglutarate ionic semiclathrate hydrate were measured in this study. The highest phase equilibrium temperature of tetrabutylphosphoniumglutarate hydrate was experimentally determined 287.4 K at the mass fraction 0.347 and 0.357. The largest dissociation heat was 193.3 ± 1.6 kJ/kg at the mass fraction 0.357. The measurement results in this work show that the dissociation heat can be utilized in the suitable temperature range targeting general air conditioning and data center.

The stress–strain characteristics of a clot during loading/unloading mechanical cycles are significant features to assess the underlying mechanisms of thrombectomy, especially when multiple thrombectomy attempts are required. We investigated a damage model to predict loading/unloading response of clots. To study the validity of the model, we tested theoretical models to reproduce the experimentally obtained mechanical characteristics of clots under various conditions. Three types of clot analogs with different red blood cell (RBC) compositions were prepared. Cylindrical clot analogs were formed for the tensile and compression tests. Loading/unloading tests at 80% of strain were conducted, where the material parameters were determined by fitting the results to a theoretical curve combining the damage model and the elasto-plastic constitutive model. Through the computation for theoretical curves, unique characteristics of clots were revealed such that the hysteresis loss rate did not change by varying RBC contents, except for the clot created with 0% RBC composition, under compressive loading. In addition, the plastic strain decreased as the RBC content decreased under tensile loading, whereas it increased as the RBC content decreased under compressive loading. A three-dimensional finite element method (FEM) was employed with the determined parameters. The FEM could accurately reproduce the experimental stress–strain curves for all types of clot analogs and for both loading types up to a strain of 80%. The results indicate that the theoretical model which incorporates and combines the damage model and the elasto-plastic constitutive model is applicable to predict the non-linear stress–strain behavior of clots under loading and unloading.

© 2019 Acta Materialia Inc. The existing first-generation drug-eluting stent (DES) has caused late and very late stent thrombosis related to incomplete stent endothelialization. Hence, biomaterials that possess sufficient anti-thrombogenicity and endothelialization with the controlled drug release system have been highly required. In this work, we have developed a newly designed drug-release platform composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer, a non-thrombogenic polymer, and micropatterned hydrogenated amorphous carbon (a-C:H), a cell-compatible thin film. The platelet adhesion and the endothelial cell adhesion behavior on the micropatterned substrates were investigated in vitro. The results indicated that the micropatterned a-C:H/MPC polymer substrates effectively supported the human umbilical vein endothelial cell (HUVEC) proliferation, while suppressing the platelet adhesion. Interestingly, the HUVEC exhibited different shape and behavior by changing the island size of the micropatterned a-C:H. By introducing both a non-thrombogenic polymer and cell-compatible thin films through a simple patterning method, we demonstrated that the platform had the potential to be utilized as a base material for DES with cell controllability. Statement of significance: The current first-generation drug-eluting stents (DES) would cause late and very late stent thrombosis due to the incomplete endothelialization of the metal stent material. In this work, we have developed a new DES platform composed of a 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer micropatterned by hydrogenated amorphous carbon (a-C:H). Two types of differently micropatterned a-C:H stent surface were made. Our studies revealed that the micropatterned a-C:H/MPC polymer substrates could effectively enhance the endothelial cell (EC) proliferation, simultaneously suppressing the platelet adhesion, becoming a highly biocompatible material especially for indwelling devices including a drug-release device. The new drug-release platform could be utilized as a base material for cell-controllable coating on DES.

Copyright © Materials Research Society 2019. Embolic beads for transarterial chemoembolization (TACE) should possess radiopacity and biodegradability at the same time, to be visualized in a body under fluoroscopy and CT scanning to avoid complicating disease. In this study, we fabricated radiopaque and biodegradable beads composed of Lipiodol (LPD) (ethiodized oil) and polycaprolactone (PCL), a biocompatible and biodegradable polymer. LPD/PCL beads were first fabricated with a home-made microfluidic device. By changing the flow-rate ratio in the microfluidic device, the mean diameter of LPD/PCL beads could be well controlled. The radiopacity was evaluated by the fluoroscopic imaging and the CT number measurements. Furthermore, the biodegradability was evaluated by collecting the weight loss data of LPD/PCL immersed in lipase/PBS solution and PBS. The results showed that LPD/PCL beads obtained in this study had sufficient radiopacity and biodegradability, which would be an alternative embolic agent for TACE.

© 2019, Springer Nature B.V. Cellulose-acetate nanofibers (CA-NF) and regenerated cellulose nanofibers (RC-NF) were separately compounded as reinforcement materials for enhancement of the mechanical property of polylactide (PLA). PLA has a great potential as an alternative to widely-used plastics such as polystyrene and poly(ethylene terephthalate), which are made from petroleum. PLA is well known for its biodegradability and renewability, but the mechanical property of PLA is not sufficient for industrial applications. In this study, CA-NF were successfully fabricated by electrospinning using 20 wt% cellulose-acetate solution with acetone/N,N-dimethylacetamide (6/4, wt/wt) mixture solvent, and RC-NF were synthesized by subsequent saponification, as reinforcement materials for PLA composites. The Young’s modulus of CA-NF/PLA composites increased linearly from 1670 to 1840 MPa with the increase in the nanofiber concentration to 15 wt%. In contrast, the Young’s modulus of RC-NF/PLA composites rapidly increased to ~ 1840 MPa at the relatively low nanofiber concentration of 5.0 wt%. Above 5.0 wt%, the Young’s modulus became stable and almost constant. RC-NF exhibited better reinforcing efficiency due to the excellent mechanical property of RC-NF, although the interfacial compatibility of RC-NF for PLA matrix was relatively poor as compared with that of CA-NF.

© The Royal Society of Chemistry. Peptide amphiphiles are one of the most promising materials in the biomedical field, so much effort has been devoted to characterizing the mechanism of their self-assembly and thermosensitive gelation. In this work, vapor pressure osmometry measurements were carried out to parameterize the thermosensitivity of interactions between peptide amphiphiles in an aqueous solution. The osmometry measurement verified that the peptides became more hydrophobic as temperature increased, which was quantitatively described with the Flory-Huggins χ parameter. Thereafter, a coarse-grained molecular model was used to simulate peptide amphiphiles dissolved in an aqueous solution. The temperature sensitive coarse-grained parameter aHW, which is the repulsive force between the hydrophilic head of the peptide amphiphile and water was estimated from the aforementioned experimentally obtained χ. Furthermore, the effects of concentration and temperature on the self-assembly behavior of peptide amphiphiles were quantitatively studied by dissipative particle dynamics. The simulation results revealed that aHW plays an important role in self-assembly characteristics and in the resulting microstructure of the peptide amphiphiles, which coincides with previous experimental and computational findings. The methodology in quantitatively linking the coarse-grained parameter from experiment and theory provides a sensible foundation for bridging future simulation studies with experimental work on macromolecules.

© 2019 by the authors. Poly(ethylene glycol)-b-poly(D,L-lactide-co-glycolide) (PEG-b-PLGA) diblock copolymers are widely known as polymeric surfactants for biomedical applications, and exhibit high solubility in water compared to PLGA-b-PEG-b-PLGA triblock copolymers known as gelation agents. In order to overcome the difficulties in the preparation of thermo-responsive hydrogels based on PLGA-b-PEG-b-PLGA due to the low solubility in water, the fabrication of thermo-responsive hydrogels based on PEG-b-PLGA with high solubility in water was attempted by adding laponite to the PEG-b-PLGA solution. In detail, PEG-b-PLGA with high solubility in water (i.e., high PEG/PLGA ratio) were synthesized. Then, the nanocomposite solution based on PEG-b-PLGA and laponite (laponite/PEG-b-PLGA nanocomposite) was fabricated by mixing the PEG-b-PLGA solutions and the laponite suspensions. By using the test tube inversion method and dynamic mechanical analysis (DMA), it was found that thermo-responsive hydrogels could be obtained by using PEG-b-PLGA, generally known as polymeric surfactants, and that the gelation temperature was around the physiological temperature and could be regulated by changing the solution composition. Furthermore, from the structural analysis by small angle neutron scattering (SANS), PEG-b-PLGA was confirmed to be on the surface of the laponite platelets, and the thermosensitive PEG-b-PLGA on the laponite surface could trigger the thermo-responsive connection of the preformed laponite network.

© 2018 Elsevier Ltd Stereocomplex polylactide (sc-PLA) nanofibers with the average diameter of 367 nm were prepared by the conductivity-controlled electrospinning using 7 wt% PLA solution in the mixed solvents of dichloromethane and pyridine. The sc-PLA nanofibers were used as reinforcement materials for poly(L-lactide) (PLLA) to fabricate a self-reinforced PLA composite, while the fibers were compounded into the PLLA by compression molding. Thermomechanical and optical properties of the resulting self-reinforced PLA composites were investigated. It was found that highly transparent (above 75%) self-reinforced PLA composites were obtained up to the sc-PLA fiber concentration of 15 wt%. Also, at higher temperature in the rubbery state, the storage moduli of the composites increased by adding sc-PLA nanofibers. At the fiber concentration of 15 wt%, the storage modulus of the composite became 70 MPa, which was 24.1 times higher than that of the original pure PLLA. It was therefore concluded that the self-reinforced PLA composite with high transparency and high thermomechanical properties was successfully obtained.

© 2019 Elsevier B.V. Management for the temperature increase of lithium-ion batteries in electric vehicles (EV) is required for safety and energy efficiency. As a cooling medium for lithium-ion batteries, one suitable candidate is trimethylolethane (TME) hydrate, which may be utilized as a phase change material (PCM). This hydrate has a large dissociation heat and will form/dissociate at approximately 30 °C under atmospheric pressure conditions. A cooling system which uses TME hydrate can have a high performance with regards to the efficiency and the stability of heat transfer process. However, in previous studies, the equilibrium temperature and the dissociation heat of TME hydrate have been reported only for a TME mass fraction of 0.625. Since TME hydrate forms in an aqueous solution, it is necessary to know how these thermophysical properties change for a range of TME mass fractions. Therefore, in this study, the equilibrium temperatures and the dissociation heats were measured respectively by the visual observation method and differential scanning calorimetry measurements for TME mass fraction range from 0.20 to 0.80. The highest equilibrium temperature was 29.6 °C at the mass fraction of 0.60. For the cooling system, the largest dissociation heat was 190.1 kJ kg−1, also at the mass fraction of 0.60.

© 2018 Elsevier Ltd To develop the “hydrate thermal energy storage technology” it is desired to reveal the mechanism of the thermophysical property manifestation on ionic semiclathrate hydrate. The influence of valence on thermophysical properties has not been investigated so far. In the present study, the phase equilibrium temperatures and the dissociation heats of tetrabutylphosphonium sulfate ((TBP)2SO4) ionic semiclathrate hydrate were experimentally studied. (TBP)2SO4 hydrate consists of the bivalent anion, SO42−. The highest phase equilibrium temperature was 6.5 °C at the mass fraction range from 0.363 to 0.384. The largest dissociation heat was (168.8 ± 2.4) kJ·kg−1 at the mass fraction 0.374. These values of the phase equilibrium temperature and the dissociation heat were respectively lower and smaller than those of the ionic semiclathrate hydrates with the monovalent anion. As a new choice for thermal energy storage medium, (TBP)2SO4 hydrate would be suitable to the automotive air conditioner and the cold chain for blood or perishable foods.

© 2018 Elsevier Ltd A method for the fabrication of homogeneous and well-dispersed polymeric nanofiber composites was investigated. Nanofiber fillers can be used to produce polymeric nanocomposites by mixing the fillers to base polymers, eventually enhancing the mechanical property of the matrix polymers. To produce such composites, nanofibers were usually sandwiched by molten matrix polymers at high temperature before molding. The traditional so-called sandwich method, however, was found to produce rather biased and inhomogeneous composites due largely to the solid entanglement of the nanofibers. In this work, unwoven polymer nanofibers were synthesized through electrospinning by controlling the electrostatic repulsion of the nanofibers. We modified the electrospinning apparatus for the direct synthesis of homogenous composites: nanofibers were electrospun and directly ejected from the electrospinning syringe to the matrix polymer solution (not in a solid state), where a regular metal electrode plate was replaced by an optimized metal container containing the base polymer solution. It was found that this new fabrication method could realize homogeneous mixing of the nanofibers that were eventually uniformly dispersed in the polymer solution. Poly(vinyl alcohol) (PVA) was used for nanofibers and polydimethylsiloxane (PDMS) was used for polymer matrix. The field emission scanning electron microscopy (FE-SEM) revealed the homogeneous and well-dispersed PVA nanofibers in the resulting PDMS composites. The composites also presented higher mechanical properties as compared with the composites fabricated by the traditional sandwich method.

© 2019 Wiley Periodicals, Inc. To enhance the reinforcement effects of regenerated cellulose nanofibers (RC-NF) in poly(ε-caprolactone) (PCL), we synthesized RC-NF-3-aminopropyltriethoxysilane (APS), the surface-modified RC-NF by APS. The RC-NF were fabricated by the saponification of electrospun cellulose–acetate nanofibers. The surface modification by APS was confirmed by the X-ray photoelectron spectroscopy (XPS). To enhance the mechanical property of PCL, the RC-NF and the RC-NF-APS were separately compounded into PCL by compression molding. It was found that, when the fiber concentration of RC-NF-APS was 17 wt %, the Young's modulus at room temperature increased from 698.0 to 744.7 MPa, whereas the storage modulus at 55 °C almost increased from 180 to 220 MPa. The micrographs of the fracture surface of the composites revealed that the surface modification prevented the pull-out of RC-NF from PCL. It was concluded that the mechanical properties of the composites were enhanced due to the improvement of the compatibility between RC-NF and PCL by the surface modification with APS. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48599.

© 2020 Wiley Periodicals, Inc. Poly(ε-caprolactone) (PCL) is one of the ecofriendly biodegradable polymers with excellent moldability but with rather low mechanical properties especially for the industrial and biomedical use. In this research, to overcome the problem, the two types of cellulose nanofibers, the cellulose acetate nanofibers (CA-NF) and the cellulose nanofibers (C-NF), were composited into PCL for the enhancement of the mechanical properties of PCL. CA-NF were prepared by electrospinning and converted into C-NF afterward by deacetylation. It was found that the Young's modulus of the CA-NF/PCL composite at the fiber concentration of 35 wt% significantly increased by ~3 times as compared with that of neat PCL, whereas C-NF/PCL of the same fiber concentration also increased by ~4.5 times. It was also found that the Young's moduli of CA-NF/PCL nearly reached the theoretical values calculated by the equation suggested by Tsai, but that the Young's moduli of C-NF/PCL could not reach the theoretical values. It indicates that CA-NF possessed better compatibility with PCL than C-NF, agreeing well with the fracture-surface analyses of the two composites by the scanning electron microscopy.