Enzymatic degradation of poly(L-lactide) and poly (epsilon-caprolactone) electrospun fibers

Poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) ultrafine fibers were prepared by electrospinning. The influence of cationic and anionic surfactants on their enzymatic degradation behavior was investigated by measuring weight loss, molecular weight, crystallinity, and melting temperature of the fibers as a function of degradation time. Under the catalysis of proteinase K, the PLLA fibers containing the anionic surfactant sodium docecyl sulfate (SDS) exhibited a faster degradation rate than those containing cationic surfactant triethylbenzylammonium chloride (TEBAC), indicating that surface electric charge on the fibers is a critical factor for an enzymatic degradation. Similarly, TEBAC-containing PCL fibers exhibited a 47% weight loss within 8.5 h whereas SDS-containing PCL fibers showed little degradation in the presence of lipase PS. By analyzing the charge status of proteinase K and lipase PS under the experimental conditions, the importance of the surface charges of the fibers and their interactions with the charges on the enzymes were revealed. Consequently, a "two-step" degradation mechanism was proposed: (1) the enzyme approaches the fiber surface; (2) the enzyme initiates hydrolysis of the polymer.

Valence stability and change of Eu(II)in oxides

Valence stability and change of Eu(II) in oxides have been studied by luminescence spect a. The results show that the valence stability and change of Eu(II)in oxides is closely related to the radius and electric charge of positive ions substituted by Eu(II) and crystal structure of the host such as Al2O3 which can form alpha-Al2O3 single phase and alpha-Al2O3 and gamma-Al2O3 mixed phases under different reaction temperatures. A, fairly good explanation is made by the proposed relation between energy coefficient and crystal structure for the first time to the observed experiment results. if the energy coefficients of substitution ions is more than that of Eu(II), the lattice substitution of Eu(II)for these ions is not occured generally and valence stare of Eu(II)is not stable and be easily changed into Eu(III). The lattice of gamma-Al2O3 can stablize the valence state of Eu(II)within certain coped concentration and in alpha-Al2O3 crystal lattice Eu(II)can be easily changed into Eu(III).

THE CHARGE-DISTRIBUTION AND THE ELECTRIC-FIELD GRADIENT IN THE HIGH-TEMPERATURE SUPERCONDUCTOR TLBA2CAN-1CUNO2N+3 (N = 1 TO 3)

The net charges at atoms in the high-temperature superconductor TlBa2Can-1CunO2n+3 (n = 1 to 3) are calculated by means of the tight-binding approximation based on the EHMO method. The results indicate that the charge distribution in this kind of compounds possesses a specially layered arrangement. An insulating Ba-Ba layer is inserted between the Cu-O layer and the Tl-O layer. There may exist a weak coupling between the Cu-O layer and the Tl-O layer through the interaction of the same O(2) atom with both the Cu atom and the Tl atom. The existence of the Ca in the compounds can cause the valence fluctuation at the Cu atom. The calculated electric field gradients at atoms implies that the conducting electron or hole may move in the Cu-O layer, which is closest to the Tl-O layer, along the a-b plane.

Theoretical analysis and experimental study of the space-charge-screening effect in uni-traveling-carrier photodiode

A new optimized structure of an UTC (uni-traveling-carrier) photodiode is developed and epitaxied by metal-organic chemical vapor deposition. We fabricated a UTC photodiode of 30 mu m in diameter. Theoretical simulation based on drift-diffusion model was used to analyze the space-charge-screening effect in UTC photodiode primarily in two aspects: the carrier concentrations and the space electric field. The simulation results were generally in agreement with the experimental data.

Quantum mechanical simulation of nanosized metal-oxide-semiconductor field-effect transistor using empirical pseudopotentials: A comparison for charge density occupation methods

The atomistic pseudopotential quantum mechanical calculations are used to study the transport in million atom nanosized metal-oxide-semiconductor field-effect transistors. In the charge self-consistent calculation, the quantum mechanical eigenstates of closed systems instead of scattering states of open systems are calculated. The question of how to use these eigenstates to simulate a nonequilibrium system, and how to calculate the electric currents, is addressed. Two methods to occupy the electron eigenstates to yield the charge density in a nonequilibrium condition are tested and compared. One is a partition method and another is a quasi-Fermi level method. Two methods are also used to evaluate the current: one uses the ballistic and tunneling current approximation, another uses the drift-diffusion method. (C) 2009 American Institute of Physics. [doi:10.1063/1.3248262]

Effects of doped dye on the charge carrier injection, transport, and electroluminescent performance in polymeric light-emitting diodes

The effects of doped fluorescent dye 4-(dicyanomethylene)-2-i-propyl-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTI) on the charge carrier injection, transport and electroluminescence (EL) performance in polyfluorene (PFO)-based polymer light-emitting diodes (PLEDs) were investigated by steady-state current-voltage (I-V) characteristics and transient EL measurements. A red EL from DCJTI was observed and the EL performance depended strongly on the DCJTI concentration. The analysis of the steady-state I-V characteristics at different DCJTI concentrations found that three regions was shown in the I-V characteristics, and each region was controlled by different processes depending on the applied electric field. The effect of the dopant concentration on the potential-barrier height of the interface is estimated using the Fowler-Nordheim model. The dopant concentration dependence of the current-voltage relationship indicated clearly the carrier trapping by the DCJTI molecules. The mobility in DCJTI: PFO changed significantly with the DCJTI concentration, and showed a nontrivial dependence on the doping level. The behavior may be understood in terms of the formation of an additional energy disorder due to potential fluctuation caused by the Coulomb interaction of the randomly distributed doping molecules.

Investigation of charge-carrier injection characteristics in NPB/Alq3 heterojunction devices

The effect of copper phthalocyanine (CuPc) and LiF interfacial layers on the charge-carrier injection in NN'-di(naphthalene-l-yl)N,N'-diphenyl-benzidine (NPB)/tris(8-hydroxyquinoline) aluminium (Alq(3)) organic heterojunction devices have been studied through the analysis of current-voltage characteristics. The investigation clearly demonstrated that the hole injection into NPB from anode is Fowler-Nordheim (FN) tunneling and the electron injection into Alq3 from cathode is Richardson-Schottky (RS) thermionic emission. The barrier heights obtained from the FN and RS models proved that the band alignments for charge-carrier injection are greatly improved by the CuPc and LiF interfacial layers, which should fully clarify the role of the interfacial layer on the improvement of device performance.

Analysis of strip electric saturation model of crack problem in piezoelectric materials

This paper presents a fully anisotropic analysis of strip electric saturation model proposed by Gao et al. (1997) (Gao, H.J., Zhang, T.Y., Tong, P., 1997. Local and global energy release rates for an electrically yielded crack in a piezoelectric ceramic. J. Mech. Phys. Solids, 45, 491-510) for piezoelectric materials. The relationship between the size of the strip saturation zone ahead of a crack tip and the applied electric displacement field is established. It is revealed that the critical fracture stresses for a crack perpendicular to the poling axis is linearly decreased with the increase of the positive applied electric field and increases linearly with the increase of the negative applied electric field. For a crack parallel to the poring axis, the failure stress is not effected by the parallel applied electric field. In order to analyse the existed experimental results, the stress fields ahead of the tip of an elliptic notch in an infinite piezoelectric solid are calculated. The critical maximum stress criterion is adopted for determining the fracture stresses under different remote electric displacement fields. The present analysis indicates that the crack initiation and propagation from the tip of a sharp elliptic notch could be aided or impeded by an electric displacement field depending on the field direction. The fracture stress predicted by the present analysis is consistent with the experimental data given by Park and Sun (1995) (Park, S., Sun, C.T., 1995. Fracture criteria for piezoelectric materials. J. Am. Ceram. Soc 78, 1475-1480).