Hee-Woo Rhee

Humidity sensor using epoxy resin containing quaternary ammonium salts

Humidity-sensitive epoxy monomer, glycidyl trimethyl ammonium chloride (GTMAC) was selected as the humidity-sensing resin. Polypropylene glycol diglycidyl ether (PPGDGE) and methyl tetrahydrophthalic anhydride (MTPHA) were used as a comonomer and a curing agent, respectively. The humidity-sensitive membranes were composed of GTMAC, PPGDGE and MTPHA. When impedance characteristics of the epoxy resins containing quaternary ammonium salts were measured, the impedance decreased linearly with an increase in the content of GTMAC in its semi-logarithmic graph. The impedance changed from 10 7 to 10 3 O between 30 and 90%RH, which was required for a common humidity sensor. Temperature dependence, frequency dependence and response time were also measured. The humidity-sensitive characteristics of the sensor did not change even after soaking in water. # 2001 Elsevier Science B.V. All rights reserved.

Synthetic control of molecular weight and microstructure of processible poly(methylsilsesquioxane)s for low-dielectric thin film applications

Processible poly(methylsilsesquioxane)s (PMSSQs) were prepared in refluxing THF solutions under nitrogen atmosphere in the presence of HCl catalyst. It was found that various reaction parameters such as concentration, temperature, reaction time, relative amount of water, and relative amount of acid catalyst could affect the molecular weight, microstructure, and the amount of functional end-groups of synthesized PMSSQs. PMSSQ thin films prepared with high molecular weight PMSSQ samples synthesized in solutions exhibited a much improved crack resistance over commercially available samples, probably due to the effects of different microstructures of polymers. The dielectric constants of the fully cured thin films prepared in this study were found to be ca. 2.7, which is nearly the same as those for commercially available samples.

Electrochemical and physical properties of composite polymer electrolyte of poly(methyl methacrylate) and poly(ethylene glycol diacrylate)

Abstract The electrochemical and physical properties of composite polymer electrolytes based on a blend of poly(methyl methacrylate) (PMMA) and poly(ethylene glycol diacrylate) (PEGDA) are investigated. The addition of nanometre-size TiO2 filler decreases the crystallinity of the polymer electrolytes and also improves the ionic conductivity and the interfacial resistance between the electrode and the electrolyte. In addition, this ceramic filler enhances the mechanical strength of the electrolytes. A prototype battery, which consists of a graphite anode, a LiCoO2 cathode, and the composite polymer electrolyte coated on a poly(propylene) (PP) separator, shows good cycling performance at high rate.

Present status of solid state photoelectrochemical solar cells and dye sensitized solar cells using PEO-based polymer electrolytes

Due to energy crises in the future, much effort is being directed towards alternate sources. Solar energy is accepted as a novel substitute for conventional sources of energy. Out of the long list of various types of solar cells available on the market, solid state photoelectrochemical solar cells (SSPECs) and dye sensitized solar cells (DSSCs) are proposed as an alternative to costly crystalline solar cell. This review provides a common platform for SSPECs and DSSCs using polymer electrolyte, particularly on polyethylene oxide (PEO)-based polymer electrolytes. Due to numerous advantageous properties of PEO, it is frequently used as an electrolyte in both SSPECs as well as DSSCs. In DSSCs, so far high efficiency (more than 11%) has been obtained only by using volatile liquid electrolyte, which suffers many disadvantages, such as corrosion, leakage and evaporation. The PEO-based solid polymer proves its importance and could be used to solve the problems stated above. The recent developments in SSPECs and DSSCs using modified PEO electrolytes by adding nano size inorganic fillers, blending with low molecular weight polymers and ionic liquid (IL) are discussed in detail. The role of ionic liquid in modifying the electrical, structural and photoelectrochemical properties of PEO polymer electrolytes is also described.

Composite polymer electrolytes reinforced by non-woven fabrics

Composite electrolytes composed of a blend of polyethylene glycol diacrylate (PEGDA), poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) together with a non-woven fabric have been prepared by means of ultra-violet cross-linking. As the non-woven fabric serves as a mechanical support medium, the composite electrolyte has good integrity up to an initial liquid electrolyte uptake of 1000% (ethylene carbonate (EC)–dimethyl carbonate (DMC)–ethylmethyl carbonate (EMC)–LiPF6). The ionic conductivity of the composite electrolytes reaches 4.5 mS cm −1 at an ambient temperature of around 18 ◦ C and are electrochemically stable up to about 4.8 V versus Li/Li + . The conductivity and interfacial resistance remain almost constant even at 80 ◦ C. Scanning electron micrographs show that the high-temperature behavior is associated with structural stability that is induced by chain entanglement between PVdF, PMMA and PEGDA network. A MCMB|LiCoO 2 cell using the composite electrolytes retains >97% of its initial discharge capacity after 100 cycles at the C/2 rate (150 mA), and delivers more than 80% of full capacity with an average load voltage of 3.6 V at the 2C rate. The cell also shows much better cycle-life than one with a PVdF-coated composite electrolyte at high temperatures because of the better liquid electrolyte retention capability.

Humidity sensitive properties of copolymers containing phosphonium salts

Abstract (Vinylbenzyl)triphenylphosphonium chloride (VTPC) was prepared for a humidity-sensitive polyelectrolyte. The humidity-sensitive membranes were composed of cross-linked copolymers with different contents of VTPC, n-butyl acrylate (n-BA) and 2-(N,N-dimethylaminoethyl)methacrylate (VTPC/n-BA/DMAMA=10/35/0, 10/45/0, 10/35/4.5 and 10/35/9). When impedance characteristics of the copolymers were measured, the impedance was decreased with an increase in the content of VTPC. The impedance ranged from 107 Ω to 103 Ω between 50% RH and 95% RH, which was required for a humidity sensor operating at high humidity or a dew point. Temperature dependence, hysteresis and a response time were also measured. The humidity-sensitive characteristics of the cross-linked copolymer of VTPC, n-BA and DAEMA changed little even after soaking in water for 120 min.

Thermally Stable Gel Polymer Electrolytes

To prepare miscible polyethylene glycol diacrylate/polyvinylidene fluoride (PEGDA/PVdF) blend gel polymer electrolytes, low molecular weight (M = 742) liquid PEGDA oligomer was mixed with PVdF-HFP dissolved in ethylene carbonate/dimethyl carbonate/LiPF 6 liquid electrolytes, and then cured under ultraviolet irradiation. Room temperature conductivity of PEGDA/PVdF blend films was found to be comparable to that of PVdF-HFP gel polymer electrolytes, and they were electrochemically stable up to 4.6 V vs. Li/Li + Scanning electron micrographs revealed that PEGDA/PVdF blend electrolytes have pore size intermediate between dense PEGDA and highly porous PVdF-HFP. It was confirmed by weight change measurement that liquid electrolyte was likely to evaporate through large pores in PVdF-HFP at 80°C, while PEGDA/PVdF blend showed better liquid electrolyte retention ability. This result was in good agreement with more stable interfacial properties of PEGDA/PVdF blend at 80°C in ac impedance analysis. Consequently, both PVdF-HFP and PEGDA/PVdF gel polymer electrolytes delivered similar discharge capacity at room temperature, but PEGDA/PVdF blend gel polymer electrolyte showed much better cycle performance than pure PVdF-HFP at 80°C.

Preparation, characterization and application of ionic liquid doped solid polymer electrolyte membranes

Solid polymer electrolyte membranes of polyethylene oxide, KI and I2 doped with low viscosity ionic liquid (IL) are developed and used in dye sensitized solar cells. Free charge carriers and reduced crystallinity of polymer electrolyte matrix provided by IL assist in ionic conductivity (σ) enhancement, and the maximum σ is 4.72 × 10−4 S cm−1 at 40 wt% IL concentration. XRD and DSC results affirm the reduction of crystallinity while AFM suggests the good incorporation of IL. The fabricated solar cells with IL-doped polymer electrolytes show improved efficiency (2%) in comparison with solar cells without IL. This improvement is due to the high σ by IL doping.

Fiber-optic biosensor for the detection of organophosphorus compounds using AChE-immobilized viologen LB films

Abstract Acetylcholinesterase (AChE)-immobilized viologen Langmuir–Blodgett (LB) film was fabricated for the development of a fiber-optic biosensor to detect organophosphorus compounds in contaminated water. AChE was adsorbed onto viologen monolayer by electrostatic force. The optimal amount of the dissolved enzyme for the LB film formation was determined based on the enzyme adsorption isotherm and relative enzyme amount adsorbed. The surface characteristics of AChE-immobilized viologen LB film were analyzed at various conditions of surface pressure with atomic force microscopy. The optimal number of AChE-immobilized viologen LB film was determined based on the enzyme activity. The signal output of the sensor composed of AChE–viologen hetero LB film was obtained for the various concentrations of organophosphorus compounds. The response time to the steady signal and the detection range of the sensor were 5 min and 0–2.0 ppm, respectively.

Incorporation of Zirconium Hydrogen Phosphate into Porous Ionomer Membranes

Nation membrane was cast from 5 wt % Nafion/dimethly acetamide/2-propanol solution containing a hydrophobic plasticizer, and the phase-separated low molecular weight component was leached out from the Nafion matrix by solvent extraction using diethyl ether and methanol. The resultant porous Nafion membrane was doped in situ with zirconium hydrogen phosphate (ZHP) by zirconium chloride and phosphoric acid treatment. The maximum loading of ZHP nanoparticles increased to 40 wt % because of additional precipitation in the pore structure. Field-emission scanning electron micrographs showed that the pores were completely filled with in situ doped ZHP particles. Fourier transform infrared study confirmed that more ZHP fillers improved water retention and proton conductivity of the composite ionomer membrane at high-temperature regions above 100°C. As a result, the power density of Nafion/ZHP membranes was much higher than that of an as-received Nafion 115 membrane at elevated temperature.