King-Fu Lin

Core-Shell Particles Designed for Toughening the Epoxy Resins. II. Core-Shell-Particle-Toughened Epoxy Resins

The diglycidyl ether of bisphenol A–m-phenylene diamine (DGEBA–MPDA) epoxy resin was toughened with various sizes and amounts of reactive core-shell particles (CSP) with butyl acrylate (BA) as a core and methyl methacrylate (MMA) copolymerized with various concentration of glycidyl methacrylate (GMA) as a shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either core or shell. Among the variables of incorporated CSP indicated above, the optimal design was to obtain the maximum plastic flow of epoxy matrix surrounding the cavitated CSP during the fracture test. It could be achieved by maximizing the content of GMA in a shell-crosslinked CSP, the particle size, and the content of CSP in the epoxy resin without causing the large-scale coagulations. The incorporation of reactive CSP could also accelerate the curing reaction of epoxy resins. Besides, it was able to increase the glass transition temperature of epoxy resins if the particle size ≤0.25 μm and the dispersion was globally uniform.

Kinetic model of thermal degradation of polymers for nonisothermal process

A new kinetic model was developed to describe the thermal degradation behavior of polymers. The model was applied to predict the degradation of poly(methyl methacrylate) (PMMA) blended with propyl ester phosphazene (FR). The results showed that the thermal degradation mechanism of pure PMMA was dominated by zero- and first-order reactions. For PMMA blended with FR, the thermal degradation mechanism was dominated by first- and second-order reactions due to the formation of anhydride from the ester groups of PMMA. In addition, the major thermal degradation temperature of blends was greater than pure PMMA. By using our model, the activation energy of the thermal degradation PMMA was calculated to be 180 kJ/mol; this activa- tion energy increased as FR was added to PMMA. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1855-1868, 1997

Kinetics study of imidazole-cured epoxy-phenol resins

The reaction kinetics of diglycidyl ether of bisphenol A (DGEBA) cured with different concentrations of imidazole and bisphenol A (BPA) were investigated by using differential scanning calorimetry. Both dynamic and isothermal DSC were stud- ied. Two initiation mechanisms were found to play roles in the curing reactions. One was based on adduct formation of epoxy groups with pyridine-type nitrogen and the other was based on ionic complexes of imidazole and BPA. The subsequent propagation was composed of three main reactions, viz. the epoxide/phenol reaction, the acid/base reaction, and the epoxide/R-O 2 reaction. A generalized kinetics model was developed and used to predict the conversion of epoxide groups using a wide range of imidazole and BPA concentrations, and cure temperature.

Enhancing photovoltaic performance of all-solid-state dye-sensitized solar cells by incorporating ionic liquid-physisorbed MWCNT

Multi-walled carbon nanotubes (MWCNT) coated with a thin layer of 1-(2-acryloyloxy-ethyl)-3-methyl-benzoimidazol-1-ium iodide were successfully fabricated by physical adsorption. They were then incorporated into poly(1-(2-acryloyloxy-ethyl)-3-methyl-imidazol-1-ium iodide (poly(AMImI))-based electrolytes to create an all-solid state dye-sensitized solar cell (DSSC). The power conversion efficiency (PCE) of the resulting DSSC was significantly increased, higher than that using poly(AMImI)-grafted MWCNT incorporated in the poly(AMImI)-based electrolytes. With gradual increases in the 1-(2-acryloyloxy-ethyl)-3-methyl-benzoimidazol-1-ium iodide-physisorbed MWCNT content from 0 to 0.5 wt%, the PCE increased from 1.156 to 3.551% at full sun and the short-circuit current density (JSC) increased from 2.367 to 8.505 mA cm−2. According to the linear sweep voltammetry measurements, the presence of 1-(2-acryloyloxy-ethyl)-3-methyl-benzoimidazol-1-ium iodide-physisorbed MWCNT in the solid-state electrolytes significantly increased the limiting current and the diffusion coefficient of I3−. Notably, the relationship between the JSC and the increase of limiting current with the content of 1-(2-acryloyloxy-ethyl)-3-methyl-benzoimidazol-1-ium iodide-physisorbed MWCNT is linear.

High temperature resins based on allylamine/bismaleimides

Abstract Synthesis, curing, and physical/mechanical properties of 4,4′-bismaleimidodiphenylmethane (BDM), 4,4′-bismaleimidodiphenylether (BDE), 3,3′-bismaleimidodiphenylsulfone (3-BDS), and 4,4′-bismaleimidodiphenylsulfone (4-BDS) adducted with various amount of allylamines were investigated and compared with each other. BDM was reacted with allylamines exclusively through the Michael addition reaction, whereas 3-BDS and 4-BDS were reacted with allylamines by amidation along with the cleavage of an imide-ring CN bond. Only BDE underwent both reactions to yield BDE/allylamine adducts. Three types of curing reactions might occur depending on the amount of adducted allylamines: (1) thermal homopolymerization through the maleimide double bonds; (2) accelerated homopolymerization by allylamines; and (3) polymerization of the cleaved allylamines by themselves or with the maleimido groups. Of all the BMIs/allylamines adducts under study, the cured BDM/50 mol% allylamine and BDE/50 mol% allylamine adducts have superior mechanical properties. In addition, the former resin has a glass transition temperature ( T g ) of 335°C and a degradation temperature ( T d ) of 471°C, whereas the latter has a T g of 349°C and a T d of 436°C.

Core–shell particles to toughen epoxy resins. I. Preparation and characterization of core–shell particles

A two-stage, multistep soapless emulsion polymerization was employed to prepare various sizes of reactive core–shell particles (CSPs) with butyl acrylate (BA) as the core and methyl methacrylate (MMA) copolymerizing with various concentrations of glycidyl methacrylate (GMA) as the shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either the core or shell. The number of epoxy groups in a particle of the prepared CSP measured by chemical titration was close to the calculated value based on the assumption that the added GMA participated in the entire polymerization unless it was higher than 29 mol %. Similar results were also found for their solid-state 13C-NMR spectroscopy. The MMA/GMA copolymerized and EGDMA-crosslinked shell of the CSP had a maximum glass transition temperature (Tg) of 140°C, which was decreased with the content of GMA at a rate of −1°C/mol %. However, the shell without crosslinking had a maximum Tg of 127°C, which decreased at a rate of −0.83°C/mol %. The Tg of the interphasial region between the core and shell was 65°C, which was invariant with the design variables. The Tg of the BA core was −43°C, but it could be increased to −35°C by crosslinking with EGDMA. The Tg values of the core and shell were also invariant with the size of the CSP.

Fluorescence monitoring of polarity change and gelation during epoxy cure

Abstract The fluorescence spectrum of 1-(4-dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (DMA-DPH) dissolved in a stoichiometric mixture of diglycidyl ether of bisphenol A and diethylene triamine was measured as a function of cure time at various cure temperatures. The frequency of the fluorescence maximum for DMA-DPH increased during the curing reactions because of the change in the polarity of the epoxy resin. In an isothermal cure, the fluorescence frequency increased linearly with the cure time until the gelation occurred. The total change in fluorescence frequency that occurred from the beginning of the isothermal cure to the gelation time was 1000 cm −1 and was independent of the cure temperature, implying that the chemical structure of the infinite network at the gelation time was independent of the cure temperature. The rate constant, K T , for the polarity change during an isothermal cure of the epoxy resin, defined as the rate constant for the linear increase in fluorescence frequency, was determined. The activation energy of K T was estimated to be 60 kJ mol −1 .

Gelation of ionic liquid with exfoliated montmorillonite nanoplatelets and its application for quasi-solid-state dye-sensitized solar cells.

The exfoliated montmorillonite (exMMT) nanoplatelets that carry negative charges are capable of adsorbing 1-methyl-3-propyl-imidazolium cations to form a gel-type ionic liquid-based electrolyte system for dye-sensitized solar cell (DSSC). Interestingly, it also increases the power conversion efficiency of DSSC from 6.58% to 7.77% at full sun. The increased efficiency is attributed to the decreased resistance of gel electrolyte system and enhanced reduction reaction rate at the counter electrode, both of which are related to the two-dimensional electrolyte nature of exMMTs that repel the I(-)/I(3)(-) redox couples toward their major conduction pathway.

Improved exchange reaction in an ionic liquid electrolyte of a quasi-solid-state dye-sensitized solar cell by using 15-crown-5-functionalized MWCNT

Nanocomposite, 15-crown-5-functionalized multi-wall carbon nanotubes (denoted as MWCNT-15-C-5) were synthesized and used as an additive along with the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) in the electrolyte of a dye-sensitized solar cell (DSSC); the pertinent quasi-solid-state DSSC showed a far superior photovoltaic performance than that of a cell with bare EMIBF4 or with MWCNT-added EMIBF4 (MWCNT/EMIBF4). The heterocyclic structure of the crown ether, 15-C-5, provides its cavities to capture the lithium ions (Li+) in a DSSC, thereby facilitating the dissolution of Li+ and I− in the electrolyte of the cell. This further contributes to an improvement in the exchange reaction of I−/I3− in the electrolyte with EMIBF4. Consequently, the values of short-circuit current density (JSC) and power-conversion efficiency (η) of the DSSC with both EMIBF4 and MWCNT-15-C-5 in its electrolyte showed an increase from 3.23 ± 0.30 to 5.53 ± 0.38 mA cm−2 and from 1.52 ± 0.04 to 2.11 ± 0.10%, respectively, with reference to the values of a DSSC with bare EMIBF4. Moreover, the at-rest durability of this quasi-solid-state DSSC was found to be unfailing for a period of 1200 h at 100 mW cm−2 illumination. Explanations are substantiated with Raman spectra, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS).

Organotransition-metal complexes as additives for epoxy resins: 1. Their effects on toughness and morphology of epoxy resins

Abstract The toughening effects of chromium and cobalt acetylacetonate (Cr(acac)3 and Co(acac)3) additives on diglycidyl ether of bisphenol A/diethylenetriamine (DGEBA/DETA) and tetraglycidyldiaminodiphenyl-methane/diaminodiphenylsulphone (TGDDM/DDS) epoxy resins were investigated and correlated with the changes of their fracture surface morphology. According to fracture tests of their compact tension specimens (CTS), Cr(acac)3 had much higher toughening effects than Co(acac)3. Scanning and transmission electron microscope investigation of their fracture surfaces implied that the epoxy resins incorporated with Cr(acac)3 had more cohesive structure than those incorporated with Co(acac)3 or neat resins. The results have been further supported by gel permeation chromatography and wide-angle X-ray scattering analyses, which revealed that Cr(acac)3 could interact with hydroxy groups in the epoxy networks and reinforced the structure.