Sunwoong Choi

Second-order nonlinear optical properties and relaxation dynamics of aligned cross-linked polyurethanes with hemicyanine-type chromophores

Three types of different cross-linked polyurethane were prepared by the reaction of poly[(phenylisocyanate)-co-formaldehyde] with three different hydroxy-functionalized nonlinear optical chromophores. For poled and cured polymers the χ(2) values were between 16.8 and 37.4 pm/V, measured at a wavelength of 1.064 μm. Thermal stability studies performed on a sample having bonding sites of vertical–parallel structure between the chromophore and the liquid polymer matrix indicated, by second-harmonic-generation activity, a minimum decay of the χ(2) value owing to lattice hardening of the polyurethane matrix with three bonding sites. The relaxation times of aligned dipoles in vertical–parallel polyurethane were measured by variation of poling and cross-linking conditions. Three decay modes were found to exist: the mode that corresponds to partially bonded or free unbonded chromophores, the mode of thermal relaxation of bonded chromophores, and the mode that is due to the relaxation of an elastically stressed main chain induced by the poling elastic field.

Synthesis of metal oxide decorated polycarboxyphenyl polymer-grafted multiwalled carbon nanotube composites by a chemical grafting approach for supercapacitor application

We present grafting of polycarboxyphenyl polymer on the surface of multiwalled carbon nanotube (MWCNT) via a free radical polymerization and subsequent anchoring of the metal oxide nanoparticles for the evaluation of their potential applicability to supercapacitor electrodes. Here, metal oxide nanoparticles, Fe3O4 and Sm2O3, were created after the oxidation of metal precursors Sm(NO3)3 and FeCl2, respectively, and attached on the surface of polycarboxyphenyl-grafted MWCNT (P-CNT) in aqueous medium. This approach shows a potential for enhancing the dispersion of Fe3O4 and Sm2O3 nanoparticles on the wall of P-CNT. The structure and morphological characteristics of the purified MWCNT, P-CNT, and metal oxide-anchored polycarboxyphenyl-grafted MWCNT (MP-CNT) nanocomposites were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The electrochemical performance of the purified MWCNT electrode, P-CNT electrode, and MP-CNT electrodes was tested by cyclic voltammetry (CV) and galvanostatic charge discharge in a 1.0 M H2SO4 aqueous electrolyte. The results showed that the specific capacitance of the purified MWCNT was 45.3 F/g at the scan rate of 5mV/s and increased to 54.1 F/g after the modification with polycarboxyphenyl polymer. Further modification of P-CNT with Sm2O3 and Fe3O4 improved the specific capacitance of 65.84 F/g and 173.38 F/g, respectively, at the same scan rate.

Numerical analysis of the heat transfer and fluid flow in the butt-fusion welding process

Butt-fusion welding is an effective process for welding polymeric pipes. The process can be simplified into two stages. In heat soak stage, the pipe is heated using a hot plate contacted with one end of the pipe. In jointing stage, a pair of heated pipes is compressed against one another so that the melt regions become welded. In previous works, the jointing stage that is highly related to the welding quality was neglected. However, in this study, a finite element simulation is conducted including the jointing stage. The heat and momentum transfer are considered altogether. A new numerical scheme to describe the melt flow and pipe deformation for the butt-fusion welding process is introduced. High density polyethylene (HDPE) is used for the material. Flow via thermal expansion of the heat soak stage, and squeezing and fountain flow of the jointing stage are well reproduced. It is also observed that curling beads are formed and encounter the pipe body. The unique contribution of this study is its capability of directly observing the flow behaviors that occur during the jointing stage and relating them to welding quality.

Microstructure and Mechanical Properties of the Butt Joint in High Density Polyethylene Pipe

The microstructure and mechanical properties of the butt joint in high density polyethylene (HDPE) pipes were evaluated by preparing the joints with increasing the cooling time from 10 s to 70 s before pressure created for fusion of the pipes. Here, cold fusion flaws in HDPE butt joint were created with increasing the cooling time around 70 s caused by the close molecular contact followed by insufficient interdiffusion of chain segments back and forth across the wetted interface. The tensile failure mechanism of the welded pipes at different fusion time was projected based on the tensile test of dog-bone shaped, fully notched bar type as well as round U-notched specimens. The mechanical properties of the joints at different fusion time were correlated with the corresponding fracture surface morphology. The weld seam as well as tensile fracture surfaces were etched using strong oxidizing agents. The crystallinity of surface etched weld zone by potassium permanganate based etchant was found higher than unetched sample due to the higher susceptibility of amorphous phase of polyethylene with oxidizing agent. The U-notched tensile test of butt welded HDPE pipe and surface etching of the weldments provided clear delineation about the joint quality.

Novel method of measuring polymer melt viscosity using a short length of single screw extruder at the closed discharge state

Theory of single screw extruders has been used for analyzing the processing characteristics of various polymeric fabricated such material as plastics, rubber, and food products. Recently this theory extended to measuring the polymer melt viscosity using the closed discharging state of the short single screw extruder. The batch wise operation of the closed discharged state change the complex extrusion characteristic equation into simple calculation form of shear rate and viscosity equation, which related between the geometrical factors and the screw speed and the axial pressure generation, respectively.

Development of Stress Intensity Factor Equation for the Notched Ring Test (NRT) Specimen

Abstract The Notched Ring Test(NRT) has proven to be very useful in determining the slow crack growth behavior ofpolyethylene pressure pipes. In particular, the test is simple and an order of magnitude shorter in experimental times ascompared to the currently used Notched Pipe Test(NPT), which makes this method attractive for use as the accelerated slowcrack growth test. In addition, since the NRT specimen is taken directly from the pipe, having maintained the cross-section,processing induced artifacts that would affect the slow crack growth behavior are not altered. This makes the direct comparisonto the slow crack growth specimen in pipe from more meaningful. In this study, for comparison with other available slow crackgrowth methods, including the NPT, the stress intensity factor equation for NRT specimen was developed and demonstratedof its accuracy within 3 % of that obtained from the finite element analysis. The equation was derived using a flexure formulaof curved beam bending along with numerically determined geometric factors. The accuracy of the equation was successfullytested on 63, 110, 140, 160, 250, and 400 mm nominal pipe diameters, with crack depth ranging from 15 % to 45 % of thepipe wall thickness, and for standard dimensional ratio(SDR) of 9, 11, and 13.6. Using this equation the slow crack results from110SDR11 NRT specimen were compared to that from the NPT specimen, which demonstrated that the NRT specimen wasequivalent to the NPT specimen in creating the slow crack, however in much shorter experimental times.Key wordsnotched ring test(NRT), notched pipe test(NPT), slow crack growth, stress intensity factor, plastics pipesand fittings.