Published papers

This study aims to understand the fundamental mechanical relationship between polypropy-lene (PP)-gels and solid PPs without solvent through mechanical and thermal analyses, by which the mechanical similarities between molten PPs and PP gels were found, leading to the reliable estimate of the mechanical properties of semi-crystalline gels. The gelation of syndiotactic and isotactic polypropylenes (sPP and iPP) was found when PPs were dissolved in 1,2,3,4-tetrahydronaphthalene (tetralin). Interestingly, it was found that the storage modulus of sPP-gel became higher than that of iPP-gel at low PP concentration (<~40 wt%). The result was distinctly different from the result of neat solid PPs (without solvent), where the modulus of solid sPP is generally significantly lower than that of solid iPP. Such inversion behavior in the mechanical property of semi-crystalline gels had not been reported and discussed before. By further investigation of the storage moduli of neat sPP and iPP, it was found that the storage modulus of sPP became higher than that of iPP above the melting points of PP, which was similar to the behavior of the storage moduli observed in the diluted PP-gels. Such similarity between PP-gels and PP melts was also observed within iPP samples with different molecular weights.

Controlling the adhesion and proliferation of endothelial cells is essential for the development of functional vascular materials. In this study, we show that wrinkled diamond-like carbon (DLC) films formed on grooved PDMS substrates regulate endothelial cell alignment and proliferation through combined micro- and nanoscale topographical cues. Human umbilical vein endothelial cells cultured on these hierarchical patterns exhibited directional elongation along the grooves, while their lamellipodia and filopodia preferentially adhered to the wrinkle crests. Enzymatic assays confirmed sustained viability and proliferation across different wrinkle dimensions. These results demonstrate that hierarchical micro-/nanoscale topographies on DLC-coated PDMS substrates can effectively modulate endothelial cell behavior, offering promise for vascular tissue engineering.

Ionic semiclathrate hydrate, which is formed under atmospheric pressure with sustainably safe properties, has a potential for solid-liquid phase change materials(PCMs). In this study, the thermophysical property of tetrabutylphosphonium glycine(TBPGly) hydrate containing no halogen guest compound, glycine (aminoacetic acid), was investigated to evaluate the availability as an eco-efficient thermal energy storage medium. The phase equilibrium temperature was obtained through visual observation method. The heat flow rate curve behavior and dissociation heat were measured via differential scanning calorimetry(DSC). In the dissociation process, a single hydrate dissociation peak emerged, indicating TBPGly hydrate has one type of crystallographic structure. The concentration dependency of the TBPGly aqueous solution on the equilibrium temperature and the dissociation heat was confirmed. The highest equilibrium temperature of 5.8 °C was confirmed at the fraction range wTBPGly =0.368 or xTBPGly =0.0306, and the largest dissociation heat was 177.7 kJ·kg−1 at the fraction range wTBPGly =0.377or xTBPGly =0.0318. The estimated hydration number in the congruent composition was found to be in 31.2 to 32.5. In addition, based on the measured physical property, the optimum operating temperature and volume fraction of TBPGly hydrate was evaluated to determine the concentration for the practical cooling system. These data revealed TBPGly hydrate is available as a solid or slurry PCM under the operating temperature range 2 °C to 5.8 °C. The experimental data as well as the illustrative calculations contribute to accelerate the expansion in the application range, and provide the prospect in selecting the suitable composition toward the actual hydrate-based cooling system.

Application of nickel‑titanium (NiTi) alloys to medical implant devices is increasing due to their unique characteristics. However, many reports show adverse effects of corrosion of the alloys due to implantation in human body. To ensure good biocompatibility in the human body, fluorine-doped amorphous carbon (a-C:H:F) coating is a promising candidate. Generally, a-C:H:F coating shows poor adhesion on metallic alloys, so that silicon-incorporated interlayer is introduced between a-C:H:F and metallic alloys. However, this membrane design has a risk of delamination at the outermost interface (a-C:H:F//interlayer), and also there is a practical problem that coating time becomes long because the deposition process in multiple stages is required. Here we develop silicon and fluorine doped amorphous carbon (a-C:H:Si:F) film which exhibits high adhesion and excellent biocompatibility. One of the a-C:H:Si:F films “C2H2-doped”, which was deposited using a mixture of TMS, C2H2 and C3F8, showed higher adhesive strength than a-C:H:F with Si-interlayer and the same number of adhesive platelets and leukocytes as a-C:H:F. This result demonstrated that a single film with both adhesion of Si-interlayer and biocompatibility of a-C:H:F was fabricated. Furthermore, this a-C:H:Si:F coating can be anticipated as an effective film coating method in a practical point of view, because the film deposition is completed in one process.

Thermoresponsive gelation of the nanocomposites consisting of poly(lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(lactic acid-co-glycolic acid) (PLGA-b-PEG-b-PLGA) triblock copolymers and LAPONITE (LAPONITE/PLGA-b-PEG-b-PLGA nanocomposites) is investigated by decreasing the molecular weights of PLGA as 1130, 900, and 470 g mol−1. As for the pure triblock copolymers, it is confirmed by UV–vis and SANS that a micellar structure with a hydrophobic core is formed even in the case of PLGA-b-PEG-b-PLGA with the lowest PLGA molecular weight of 470 g mol−1 (Triblock0.5k). As for the nanocomposites with LAPONITE, it is found that the gelation temperature (Tgel) becomes less dependent on the PLGA-b-PEG-b-PLGA concentration and more stable as the PLGA molecular weight decreases, and that Tgel of the nanocomposites almost linearly decreases as the LAPONITE concentration increases. Eventually, the nanocomposite with Triblock0.5k reaches Tgel between room temperature and physiological temperature. In addition, gradual degradation behavior is observed for the nanocomposite with Triblock0.5k. Considering the monomeric molecular weights of LA and GA at the LA/GA ratio of ≈2.0, it is concluded that the PLGA-b-PEG-b-PLGA with the lower PLGA molecular weight (i.e., Triblock0.5k) can be effectively used for thermoresponsive and degradable hydrogels targeting biomedical applications.

A solid poly(2-methoxyethyl acrylate) (PMEA)-based polyurethane (PU) has been synthesized through reversible addition−fragmentation chain transfer (RAFT) polymerization followed by polyaddition. PMEA with four hydroxyl-groups (PMEA-(OH)4) synthesized using RAFT reagents with 4 hydroxyl groups works as an effective prepolymer and crosslinker. The thermoset PMEA-based PU can be obtained by mixing PMEA-(OH)4 and diisocyanate without additional agents for the synthesis. The temperature-independent mechanical property of the thermoset PMEA-based PU due to the chemical crosslinking points is confirmed by dynamic mechanical analysis (DMA) at temperatures ranging from 25 °C to 110 °C. The mechanical property at room temperature is also evaluated by tensile testing: The Young's modulus of PMEA-based PU increased from 454 ± 31 kPa to 574 ± 54 kPa by decreasing Mn of the PMEA prepolymer from 14,000 to 7,600. It is concluded that our synthesized PMEA-based PU exhibits sufficiently elastomeric characteristics as compared with conventional crosslinked PMEA, which may be used as a durable and more stretchable biomedical material.

Electric vehicles (EVs) play key roles in realizing a sustainable society. Lithium-ion batteries (LIBs) implemented in EVs deteriorate due to significant temperature changes. To maintain LIBs in an appropriate temperature range, a competitive thermal energy storage (TES) medium is needed. Phase change materials (PCMs) that can utilize their latent heat have great potential. In this study, we focused on piperazine hexahydrate as a promising candidate for a PCM and investigated its thermodynamic properties that are essential for practical design. The equilibrium temperature and dissociation enthalpy were determined in the mass fraction range of 0.250 to 0.550. The results showed that piperazine hexahydrate had a maximum equilibrium temperature of 42.9 °C and a maximum dissociation enthalpy of 250.0 ± 3.7 kJ kg−1 at a mass fraction of 0.443, which is the congruent point of piperazine hexahydrate. Piperazine hexahydrate can be utilized as a PCM in the temperature range of 34.4 °C to 42.9 °C. By utilizing piperazine hexahydrate instead of a conventional water-cooling system, the volume of the TES tank may be reduced by 31% approximately. The crystal system of the piperazine hexahydrate prepared in the present study was determined to be pseudomonoclinic by powder X-ray diffraction (PXRD) measurements, which is the same as that reported in the literature.

Ionic semiclathrate hydrates mainly consist of water typically together with tetrabutylammonium and tetrabutylphosphonium salts. Since ionic semiclathrate hydrates have the large dissociation heat under ambient pressure and temperature conditions, various ionic semiclathrate hydrates have been studied as safety and eco-friendly phase change materials. In this study, tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were proposed as thermal energy storage media for air conditioning and cooling lithium-ion batteries. The dissociation heat, which was a significant thermophysical property to design thermal energy storage systems, were measured at various mass fractions. The largest dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were 212.9 ± 0.9 kJ⋅kg−1 and 200.4 ± 2.2 kJ⋅kg−1. As a result of the comparison of the dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates with those of other ionic semiclathrate hydrates, it was found that tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates had the promising thermophysical properties as thermal energy storage media for air conditioning and cooling lithium-ion batteries, respectively.

A tetrabutylammonium bromide (TBAB) hydrate is the only ionic semiclathrate hydrate that has been commercialized as a medium for efficient thermal energy storage targeting general air conditioning. However, TBAB is an environmental pollutant, while liquid water and ice that have been widely used as conventional media, also have difficulty in increasing the thermal energy storage density as well as the coefficient of performance (COP) of a refrigerator to form solid. In this study, a tetrabutylphosphonium phosphate ((TBP)3PO4) hydrate was proposed as an alternative green medium. The dissociation heat and the phase equilibrium temperature, which were both crucial factors to evaluate the thermal energy storage density, and COP was experimentally determined. It was found that the maximum dissociation heat was 154.1 ± 2.0 kJ・kg−1 at the mass fraction of 0.309. The highest phase equilibrium temperature was 10.0 °C at the mass fractions of 0.290, 0.299, and 0.309. The thermal energy storage density of (TBP)3PO4 hydrates was 3.7 times larger than that of water. (TBP)3PO4 hydrates increased the COP of a refrigerator to form solid by 42% as compared with ice, indicating the potential size-reduction of conventional devices with the decrease in the power consumption of a refrigerator. The effective balance between environmental protection and the system performance through (TBP)3PO4 hydrates was also discussed.

Ionic semiclathrate hydrates, the water-based phase change materials (PCMs) have potential for thermal energy storage media. There are a number of hydrates, reported in the literature, which are suitable as PCMs for air conditioning system of the required temperature range 10–15 °C. In this study, Tetrabutylammonium picolinate hydrate (TBAPi hydrate) was newly synthesized as a PCM for the cold chain, which has a lower phase equilibrium temperature than the previously reported hydrates. Thermophysical properties of TBAPi hydrate were experimentally measured. The hydrate dissociation heat was determined using a differential scanning calorimeter (DSC). The phase equilibrium temperature was measured by visually observing system with a CMOS camera and microscope. Also, the cycling test was conducted to confirm the long-term stability of thermophysical properties. The largest dissociation heat was 157.7 ± 4.9 kJ・kg−1 at a mass fraction of TBAPi aqueous solution, wTBAPi of 0.37. The highest phase equilibrium temperature of TBAPi hydrate was 5.5 °C at 0.34 ≤ wTBAPi ≤ 0.36. These thermophysical properties did not change even after 200 cycles of hydrate formation and dissociation. Based on the experimental results, TBAPi hydrate can be applied to the transport and storage of vaccines, blood packs, and perishable foods.

Thermal energy storage for air conditioning is increasingly becoming important because the fluctuating output of renewable energy and the growing cooling demand will need to be addressed. In this study, we proposed tetrabutylphosphonium malonate hydrate as an environment friendly thermal energy storage medium for air conditioning. Equilibrium temperature and dissociation heat, which were significant thermophysical properties for commercialization, were experimentally determined at three mass fractions of tetrabutylphosphonium malonate aqueous solution. The maximum value of equilibrium temperature was determined to be 8.9 °C. The largest dissociation heat was measured to be 148.1 kJ/kg. These results indicate that tetrabutylphosphonium malonate hydrate has appropriate thermophysical properties for a thermal energy storage medium.

Ionic semiclathrate hydrate is a crystalline compound mainly composed of water, which can dissociate at ordinary pressure and temperature. Ionic semiclathrate hydrates have been widely studied to develop prospective green and safety phase change materials utilized in a thermal energy storage system to cool batteries, in air conditioning, and in a cold chain system. The thermophysical properties, such as equilibrium temperature and dissociation heat, and the mass fraction dependence of such properties need to be carefully investigated to design such systems. In this study, the equilibrium temperature and the dissociation heat of tetrabutylphosphonium malonate ionic semiclathrate hydrates were experimentally determined at various mass fractions through an established method: the maximum value of the dissociation heat was 179.70 kJ·kg−1 at w = 0.357 and the equilibrium temperature was 8.9 °C at w = 0.357. By comparing tetrabutylphosphonium malonate ionic semiclathrate hydrates with other media in terms of the measured equilibrium temperature and the dissociation heat, the usability of the hydrates as thermal energy storage media for general air conditioning was discussed.

A solid-solid crystalline transition from β to α forms in syndiotactic polystyrene (sPS) was found during the mechanical deformation in the temperature range of 130-218 °C, studied by X-ray and infrared spectroscopy. α and β forms, in contrast to the other two crystalline structures (γ and δ forms) in sPS, are two major crystalline forms manufactured through a concise molding process which is industrially readily accessible. There have been a number of structural transitions found between two crystalline structures among the four different crystalline forms of sPS. In this work, β to α form transition at well below the melting temperature of sPS (~270 °C) was found by mechanical deformation, which became more pronounced as temperature increased. The structural transition was intimately related to the crystallization temperature and the densities of α and β forms.

Linear biodegradable polyesters with a controlled number of side chains of amino groups have been synthesized. Such polyesters with a large number of amino side groups can be conjugated with a wide variety of functional molecules to obtain multi-functional biodegradable polyesters. In this study, poly(1,3-propylene-co-2-amino-1,3-propylene) adipate) (P(P-co-AP)A) was synthesized by enzymatic polymerization of divinyl adipate, 1,3-propanediol, and N-Boc-2-amino-1,3-propanediol (BocAPDO), and subsequent deprotection under strong acidic conditions. The mild reaction conditions and the specificity of the enzymatic polymerization could effectively inhibit side reactions, while the protecting groups of the monomers prevented polymer branching from the amino groups. It was found that the introduction rates of BocAPDO in the polymers could be well-controlled as the feeding rates changed from 0 to 100 mol%. After the deprotection and desalting, the ester groups in the main chain gradually converted into amide groups by intramolecular ester aminolysis at room temperature for a few days due to nucleophilic free amino groups. It was also found that the synthesized functional copolymers were all amorphous, since the random component of amino groups disrupted the crystallinity of poly(1,3-propylene adipate) (PPA). The glass transition temperature gradually increased with the increase in the content of 2-amino-1,3-propylene adipate (APA), eventually reaching −0.4 °C at the APA content of 100 mol% due to the intermolecular hydrogen bonding. Moreover, P(P-co-AP)A shows high hygroscopicity, where 3.9 wt% and 8.1 wt% of water were absorbed in the polymers with the APA contents of 50 and 100 mol%, respectively, when left in air at 60–70 % humidity for 6 h. Thus, the hydrophilicity dramatically improved with the increase in the APA content, eventually resulting in the acquisition of water solubility. The model post-polymerization modification using butyric acid was also conducted, and it was confirmed that carboxylic acids could be conjugated to the polymers by simple condensation reaction. Since polymerization of functional monomers is generally susceptible to the polarity of metal catalysts, the mild and selective reaction employed in this study could realize a higher content of various functional groups in the side chains of polyesters leading to a newly designed multi-functional polyester.

Diamond-like carbon (DLC) films are widely used in engineering applications owing to their excellent mechanical, optical, and biological properties. However, the simple deposition of a DLC film on a soft polymer, including poly(dimethylsiloxane) (PDMS), results in a coarsened surface composed of instability-induced hierarchical wrinkles with limited ability to adjust the dimensions. Overcoming this issue would help create fabrication opportunities for functional surfaces. Here, we demonstrate the controlled formation of wrinkles with tunable wavelengths and amplitudes on the surface of DLC-coated PDMS substrates. Pre-treatment of the substrates with Ar plasma prior to film deposition allows for the formation of wrinkles without structural hierarchy. The wavelength and amplitude can be controlled by adjusting the duration of the plasma treatment while maintaining the film thickness constant. The static contact angles of water droplets on the wrinkled surfaces remain nearly constant regardless of the wavelength of the wrinkles. These tunable wrinkles in DLC films on PDMS substrates have potential engineering applications, including cell scaffolds.

For the development of blood-contacting biomedical devices, preventing platelet adhesion and avoiding subsequent thrombosis through antithrombogenic surface are vitally important. Poly (2-methoxyethyl acrylate) (PMEA) is a synthetic viscous polymer with an excellent antithrombogenic property, which has already been used as an antithrombogenic coating for biomedical purposes. The PMEA coating, however, can be easily broken due to its liquid-like feature, since the glass transition temperature (Tg) of PMEA is at −25οC. Solidifying PMEA has, therefore, been desperately desired for the improvement of the stability of PMEA coating. Also, once solidified, PMEA could be utilized not only for coating but also for structural materials for medical devices, expanding the application range of PMEA. In this study, PMEA was first solidified by triblock copolymerization with poly (methyl methacrylate) (PMMA) to obtain thermoplastic elastomers with excellent antithrombogenicity. We synthesized PMMA-PMEA-PMMA triblock copolymers with different volume fractions of PMMA (fMMA) by atom transfer radical polymerization (ATRP). The synthesis and the chemical structures of the new triblock copolymers were characterized and confirmed by proton nuclear magnetic resonance (1H NMR) analyses and gel permeation chromatography (GPC). The synthesized elastomeric triblocks were then compression-molded at 180οC to obtain solid transparent films. The tensile property was measured and the drastic change from the soft-elastomer phase to the hard-plastic phase was clearly observed by varying fMMA. It was found that the new triblocks possessed significantly higher tensile strengths than the other solidified PMEA by the previous studies. The platelet adhesion test revealed that the number of adherent platelets on the triblocks with fMMA of 0.12 and 0.40 was almost the same as that on pure PMEA, indicating that the synthesized triblocks possessed excellent antithrombogenicity similar to liquid PMEA. Further investigation on hydrated water and microphase-separated structures eventually revealed that the new triblock copolymers with a sufficient amount of intermediate water or with microphase separation on the surface resulted in excellent antithrombogenicity.

We synthesized new biodegradable shape-memory polymers (SMPs) by blending synthesized poly(D,L-lactide-co-ε-caprolactone) (PLCL) and stereocomplex polylactide (sc-PLA) to be used as biomaterials for medical devices. PLCL copolymers (Mn ∼80,000) with various monomer ratios (LA:CL) were synthesized by the bulk ring-opening polymerization. The glass transition temperature of the resulting PLCL copolymers could be well controlled from −3 °C to 60 °C, agreeing well with the values by the Gordon-Taylor equation, confirming the homogeneous random distribution of LA and CL monomers in the synthesized PLCL. The synthesized PLCL and sc-PLA were eventually blended for the fabrication of biodegradable SMPS. The PLCL/sc-PLA blend samples possessed the sc-PLA crystallinity of ∼13%, which effectively worked as crosslinkings, exhibiting shape-memory behavior with various switching temperatures (30.5°C–62 °C) and shape-recovery ratios realized by adjusting the CL ratio of the PLCL copolymers: a lower CL ratio resulted in a higher switching temperature and a higher shape-recovery ratio.

Mechanical thrombectomy has become the standard treatment for patients with an acute ischemic stroke. In this approach, to remove blood clots, mechanical force is applied using thrombectomy devices, in which the interaction between the clot and the device could significantly affect the clot retrieval performance. It is expected that the finite element method (FEM) could visualize the mechanical interaction by the visualization of the stress transmission from the device to the clot. This research was aimed at verifying the constitutive theory by implementing FEM based on the visco-hyperelastic theory, using a three-dimensional clot model. We used the visco-hyperelastic FEM to reproduce the mechanical behavior of blood clots, as observed in experiments. This study is focused on the mechanical responses of clots under tensile loading and unloading because in mechanical thrombectomy, elongation is assumed to occur locally on the clots during the retrieval process. Several types of cylindrical clots were created by changing the fibrinogen dose. Tensile testing revealed that the stiffness (E0.45-value) of clots with fibrinogen could be more than three times higher than that of clots without fibrinogen. It was also found that the stiffness was not proportional to the fibrinogen dose. By fitting to the theoretical curve, it was revealed that the Mooney-Rivlin model could reproduce the hyperelastic characteristics of clots well. From the stress-relaxation data, the three-chain Maxwell model could accurately fit the experimental viscoelastic data. FEM, taking the theoretical models into account, was then carried out, and the results matched well with the experimental visco-hyperelastic characteristics of clots under tensile load, reproducing the mechanical hysteresis during unloading, the stress dependence on the strain rate, and the time-dependent stress decrease in the stress-relaxation test.

Bioplastic poly(butylene succinate-co-adipate) (PBSA) has several advantages because of its flexibility and high biodegradability. PBSA, however, possesses poor mechanical properties and dimensional instability. To overcome this, PBSA composites with TEMPO-oxidized cellulose nanofibers (TOCN/PBSA) were fabricated via the Pickering emulsion method. Porous bio-nanocomposite TOCN/PBSA mediums were made by the Pickering emulsion method, followed by the subsequent lyophilization to obtain TOCN/PBSA films by compression molding. It was confirmed that the Pickering-emulsion condition was a key factor to control the TOCN dispersion state in composite films. The Young's modulus of TOCN/PBSA composite films increased to 305 MPa at the TOCN concentration of 5 wt%, which was almost twice as high as that of pure PBSA. The studies on the morphology and tensile testing also revealed that the dispersion state of TOCNs was important to achieve excellent reinforcement effects and to suppress the fracture of composite films caused by stress concentration. The coefficient of thermal expansion at the TOCN concentration of 5 wt% was reduced to ∼25 × 10−5 K−1, half the value of pure PBSA. It was concluded that compounding TOCNs via the Pickering emulsion method was a very effective reinforcement method to improve the mechanical properties and the dimensional stability of PBSA.

For optical applications, including lenses, widely used polystyrene (PS) has not received much attention due to its considerably high birefringence. In this study, an efficient method was introduced to compensate for the orientation birefringence of PS through bottlebrush polymers with well-controlled PS graft chains (PS-BBP). PS-BBP were synthesized by ring-opening metathesis polymerization of norbornene-terminated PS macromonomers produced via atom transfer radical polymerization. Blend-film samples were then prepared by mixing linear PS and PS-BBP. One of the blend samples with a specific ratio of linear PS to PS-BBP (30:70) demonstrated extremely low orientation birefringence of |Δn| ∼3.9 × 10-4 even under 120% strain. In addition, the blend samples preserved almost the same refractive indices as linear PS, possessing the transparency required for optical polymers across the visible-wavelength range. The obtained PS blend could be utilized as an optical material with excellent optical properties.