Biomimetic strain hardening in interpenetrating polymer network hydrogels

In this paper, we present the systematic development of mechanically enhanced interpenetrating polymer network (IPN) hydrogels with Young's moduli rivaling those of natural load-bearing tissues. The IPNs were formed by synthesis of a crosslinked poly(acrylic acid) (PAA) network within an end-linked poly(ethylene glycol) (PEG) macromonomer network. The strain-hardening behavior of these PEG/PAA IPNs was studied through uniaxial tensile testing and swelling measurements. The interaction between the independently crosslinked networks within the IPN was varied by (1) changing the molecular weight of the PEG macromonomer, (2) controlling the degree of PAA ionization by changing pH, and (3) increasing the polymer content in the PAA network. Young's moduli and the maximum stress-at-break of the swollen hydrogels were normalized on the basis of their polymer content. Strain hardening in the IPNs exhibited a strong dependence on the molecular weight of the first network macromonomer, the pH of the swelling buffer, as well as the polymer content of the second network. The results indicate that the mechanical enhancement of these IPNs is mediated by the strain-induced intensity of physical entanglements between the two networks. The strain can be applied either by mechanical deformation or by changing the pH to modulate the swelling of the PAA network. At pHs below the pK_a of PAA (4.7), entanglements between PEG and PAA are reinforced by interpolymer hydrogen bonds, yielding IPNs with high fracture strength. At pHs above 4.7, a “pre-stressed” IPN with dramatically enhanced modulus is formed due to ionization-induced swelling of the PAA network within a static PEG network. The modulus enhancement ranged from two-fold to over 10-fold depending on the synthesis conditions used. Variation of the network parameters and swelling conditions enabled “tuning” of the hydrogels' physical properties, yielding materials with water content between 58% and 90% water, tensile strength between 2.0 MPa and 12.0 MPa, and initial Young's modulus between 1.0 MPa and 19.0 MPa. Under physiologic pH and salt concentration, these materials attain “biomimetic” values for initial Young's modulus in addition to high tensile strength and water content. As such, they are promising new candidates for artificial replacement of natural tissues such as the cornea, cartilage, and other load-bearing structures.     

Crystallization behavior of carbon nanofiber/linear low density polyethylene nanocomposites

Crystallization behavior of LLDPE nanocomposites is reported in the presence of three types of carbon nanofibers (CNFs) (MJ, PR‐19, and PR‐24). During nonisothermal crystallization studies, all three crystalline melting peaks for LLDPE matrix were observed in the presence of PR‐19 nanofibers (up to 15 wt % content), but only the high‐ and low‐temperature peaks were observed in the presence MJ nanofibers. The broad melting peak at low‐temperature became bigger, suggesting an increase in the relative content of thinner lamellae in the presence of MJ nanofibers. TEM results of nanocomposites revealed transcrystallinity of LLDPE on the surface of CNFs, and a slightly broader distribution of lamellar thickness. STEM studies revealed a rougher surface morphology of the MJ nanofibers relative to that of PR nanofibers. Also, BET studies confirmed a larger specific surface area of MJ nanofibers relative to that of PR nanofibers, suggesting that the larger and the rougher surface of MJ nanofibers contributes toward the different crystallization behavior of MJ/LLDPE nanocomposites. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Coil design optimization for an induction evaporation process: Simulation and experiment

For the purpose of utilizing induction heating in the evaporation process, the effects of induction coil design and droplet size on induction heating efficiency are investigated. Electro-magnetic simulations with various induction coil designs were conducted to predict the electro-magnetic field distribution. The induction coils were fabricated in order to verify the simulation results under atmospheric evaporation test conditions. The electro-magnetic simulation results indicated that the magnetic field became widened around the Zn droplet when the size of the Zn droplet increased. This in turn attributed to the increase in induction heating energy efficiency. The energy efficiency of the induction coil with 4-windings was the highest among the 3-, 4-, and 5-windings induction coils. Energy efficiency tendencies derived by the atmospheric evaporation tests corresponded well to the simulation results, and maximum energy efficiency was measured to be 42% under the atmospheric evaporation tests.     

Fractal analysis of pressure fluctuations in a three phase bubble column reactor operating at low pressure

Pressure fluctuations in a three phase bubble column reactor operating at relatively low pressure (92 KPa) have been analyzed by adopting the spectral and fractal analyses to get the engineering informations for the on-line control and fault diagnosis of the reactors. The mean value, standard deviation, skewness and kurtosis of the pressure fluctuations have been obtained. The local fractal dimension has been determined from the Pox diagram obtained by means of the rescaled range analysis of the pressure fluctuations based on the fractional Brownian motion. The local fractal dimension of pressure fluctuations has increased and thus the pressure fluctuation signals have become less persistent and irregular, with increases in the gas flow rate reaction temperature, particle size and solid content in the slurry phase. The local fractal dimension has been well correlated in terms of the operating variables.     

Low-bandgap copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges

Two donor–acceptor-type alternating copolymers consisting of 2,1,3-benzoselenadiazole and carbazole derivatives with thiophene or selenophene π-bridges were synthesized by Suzuki cross-coupling polymerization, and their optical, electrochemical, and photovoltaic properties were compared. The selenophene π-bridged copolymer (PCz-DSeBSe) exhibited a smaller band-gap (1.82 eV) than the thiophene-bridged polymer (PCz-DTBSe; 1.89 eV). PCz-DSeBSe also showed a deeper highest occupied molecular orbital energy level (−5.36 eV) than PCz-DTBSe (−5.20 eV). Moreover, the PCz-DSeBSe thin film showed higher crystallinity and hole mobility than the PCz-DTBSe thin film. Organic photovoltaic devices were fabricated using the polymers as the donors and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor. The device using PCz-DSeBSe showed a higher open circuit voltage (V_oc), short circuit current density (J_sc), and power conversion efficiency (PCE) than that using PCz-DTBSe. The fabricated indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/PCz-DSeBSe:PC₇₁BM/LiF/Al device showed the maximum PCE of 2.88% with a J_sc of 7.87 mA/cm², an V_oc of 0.80 V, and a fill factor of 0.50 under AM 1.5G irradiation (100 mW/cm²).     

Field measurement and estimation of ventilation flow rates by using train-induced flow rate through subway vent shafts

This study measured and estimated the subway vent shaft air flow rate induced by moving trains in the tunnel. This work estimated the flow rate via the tunnel structure and train movement to determine the quantitative effect of vent shafts as air purification systems of natural ventilation to improve the air quality management of a subway. The amount of air suctioned into the tunnel is significantly larger than that vented from the tunnel. Thus, placing vent shafts near subway stations is desirable for natural ventilation systems. Experimental approaches to measure train-induced flow rates have not yet been published. Results of this study provide useful fundamental data to study the natural ventilation in a subway. Therefore, this study suggested the significant design factors required to control indoor air quality in a subway.     

Inter-laboratory correlation exercise on a light-duty diesel passenger vehicle to verify nano-particle emission characteristics by Korea particle measurement program

The Light Duty Inter-Laboratory Correlation Exercise (ILCE) final report, performed with the ‘PMP Golden Vehicle’ at nine laboratories in the EU, Korea and Japan to demonstrate repeatability and reproducibility of the particle number concentration emissions measurement techniques proposed by the Particle Measurement Program (PMP), was released in 2007. The ILCE was conducted by the Korea Particle Measurement Program (KPMP) with a domestic diesel passenger vehicle equipped with a diesel particulate filter (DPF) between three certification laboratories and the research center of an automotive manufacturer to meet future regulations (EURO 5 and EURO 6) of particle number concentration for light-duty vehicles in early 2008. This research focused on measuring the particulate matter emission (particle number and mass) levels of a representative light-duty diesel passenger vehicle during new European driving cycle (NEDC) mode to analyze the repeatability and reproducibility between laboratories in Korea. From the ILCE test results in Korea, the mean total particle number concentration levels ranged from 5.43E+10 #/km to 1.58E+11 #/km and 0.0003 g/km to 0.0036 g/km for particle mass. Repeatability between participating laboratories ranged from 32% to 66% for particle number, 11% to 70% for particle mass; the reproducibility level was 46% for particle number, and 66% for particle mass emission. This paper was recommended for publication in revised form by Associate Editor Kyoung Doug Min Simsoo Park received his B.S. and M.S. degrees from Seoul National University in 1997 and 1979, respectively, and a Ph.D. from the State University of New York at Stony Brook. He served as a Chief Research Engineer at Hyundai Motor Company, a Director for Publication of the KSME, a Technical Advisor of Hyundai-Kia Motor Company, and an Editing Director, Project Director, International Director, Accounting Director, and General Affair Director of KSAE. He is currently Vice President and of Editor-in-Chief of IJAT at KSAE and a professor in school of mechanical engineering at Korea University.     

Damaged layer characteristics in micro-endmilling of copper using an ultrahigh-speed air spindle

This paper presents an investigation on the characteristics of damaged layer in micro-machining by using the ultrahigh-speed air spindle. The damaged layer in metal cutting is derived from plastic deformation and transformation of metal structure. In this study, micro-cutting force, surface roughness, and plastic deformation layer according to the variation of machining conditions were investigated by experiments. The damaged layer was measured using optical microscope for the samples prepared by metallographic techniques. Its scale was dependent on cutting process parameters, especially feed per tooth. According to experimental results, it was verified that the thickness of damaged layer was increased with increasing of feed per tooth and cutting depth, also thickness of damaged layer was reduced in down-milling compared to upmilling during micro-endmilling operation.     

Large eddy simulation of flow characteristics in an unconfined slot impinging jet with various nozzle-to-plate distances

To study the detailed flow structure of the unconfined plane impinging jet, a 3-dimensional flow analysis is carried out focused on the flow mechanism with the non-dimensional distance, L/d, as flow parameter by using the LES turbulent technique. The symbols d and L represent the nozzle width and the nozzle-to-plate distance, respectively. Then, the flow structures along both the stream-wise and spanwise direction are investigated. For these purposes, the plane impinging jet with Reynolds number of 11,000 is analyzed. The nozzle width was fixed at 1.5mm, but the nozzle-to-plate distance was varied between 4mm and 24mm. As a result, the plane impinging jet shows different flow patterns with L/d. In conclusion, the plane impinging jet is classified into three types with the non-dimensional variable, L/d. L/d≤4: Stable impinging jet, 4<L/d<11: periodically-stable impinging jet and L/d≥11: Unstable impinging jet.     

Delay of ignition in early direct-injected PCCI engine combustion using fuel-evaporative cooling and cooled EGR

In early direct-injected PCCI combustion, one of the several approaches to ideal PCCI, the early injected fuel forms a nearly homogeneous air-fuel charge, but most or all of the mixture ignites and burns during the compression stroke, which limits its application to light load operation range with lower-than-expected thermal efficiency. In the present research a unique split fuel injection technique was developed to delay the combustion of the entire fuel for mixing with air before burning. Evaporative cooling effects provided by the fuel in the second injection at precise timing held the self-ignition of the early injected fuel, and thus allowed the entire fuel to burn in PCCI mode at a favorable time in a cycle, namely a little after the top center. The single cylinder test engine run this way demonstrated the diesel-like efficiency along with an-order-of-magnitude lower NOx emissions than in conventional diesel combustion.