Search

Diffusion of deuterated polystyrene(dPS) is probed in PS matricescontaining stringlike chained nanoparticles (cNP) grafted with PS.This investigation connects prior diffusion studies in model sphericaland cylindrical NP systems and provides insight for technologicalapplications, which typically involve irregularly shaped NPs suchas carbon black. We report that the presence of chained NPs in PSmatrices induces a minimum in the diffusion coefficient (D) with increasing cNP concentration when the key length scale, 2Rg/L≤ 1.5, where Rgis the gyration radius of dPS and Lis the mean length of the impenetrable core of the chainedNPs. When 2Rg/L>1.5, Ddecreases monotonically as the NP concentrationincreases.Note that in all cases 2Rgis larger thanthe diameter of these short-stringy NPs. The diffusion minimum isattributed to anisotropic diffusion in the vicinity of the chainedNPs and requires that the long dimension of the cNP be comparableto or longer than the tracer molecule. Two normalizations are exploredto provide insight about the diffusion mechanism: D/D0where D0is the diffusion coefficient in a pure homopolymer matrix and D/Dewhere Deis an effective diffusion coefficient that accountsfor the distinct dynamics in the PS matrix and PS brush regions. For D/D0, a sharp transition froma diffusion minimum to a monotonic decrease is observed as dPS molecularweight increases, while for D/Dethe transition is more gradual. These studies show not onlythat the NPs act as impenetrable obstacles for polymer diffusion butthat the polymer brush grafted to the cNP provides an alternativepathway to control polymer dynamics. [ABSTRACT FROM AUTHOR]

Diffusion of deuterated poly(methylmethacrylate) (dPMMA) is sloweddown in a PMMA matrix filled with hydroxyl-capped spherical silicananoparticles, from 13 to 50 nm in diameter and at loadings up to40 vol %. At constant T– Tg= 75 K, the normalized diffusion coefficients (D/D0) collapse onto a mastercurve, when plotted against the confinement parameter, ID/2Rg, where IDis interparticledistance and 2Rgis probe size. This resultsuggests that the confinement parameter captures the effect of nanoparticlesize, size polydispersity, and volume fraction on polymer dynamicsfor the PMMA composite. For ID< 2Rg, the master curve exhibits a strongly confined regionwhere D/D0decreasesby up to 80%, whereas for ID> 2Rg, the curve falls in a weakly confined region where D/D0decreasesonly moderately by up to 15%. Surprisingly, D/D0is reduced even when IDis8 times larger than 2Rg. A comparisonbetween the master curves for PMMA and polystyrene nanocompositesindicates that attractive interactions in the PMMA system do not significantlyalter the center-of-mass diffusion of macromolecules in polymer nanocomposites. [ABSTRACT FROM AUTHOR]

A stacked oxide semiconductor of n-type ZnO/p-type NiO with diode behavior was proposed as the novel charge-trapping layer to enable low-voltage flash memory for green electronics. The memory performance outperforms that of other devices with high κ and a nanocrystal-based charge-trapping layer in terms of a large hysteresis memory window of 2.02 V with ±3 V program/erase voltage, a high operation speed of 1.88 V threshold voltage shift by erasing at -4 V for 1 ms, negligible memory window degradation up to 10(5) operation cycles, and 16.2% charge loss after 10 years of operation at 85 °C. The promising electrical characteristics can be explained by the negative conduction band offset with respect to Si of ZnO that is beneficial to electron injection and storage, the large number of trapping sites of NiO that act as other good storage media, and most importantly the built-in electric field between n-type ZnO and p-type NiO that provides a favorable electric field for program and erase operation. The process of diode-based flash memory is fully compatible with incumbent VLSI technology, and utilization of the built-in electric field ushers in a new avenue of accomplishing green flash memory.

An analytic model for the slowing down of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chains diffuse. The reduction of polymer diffusion in each cylinder is equated with the reduction of diffusion for a sphere through a cylinder. The distribution of cylinder diameters within the ensemble is determined from statistical mechanical theories based on the packing of spherical particles. For low loadings of spherical particles in nanocomposites, this model results in a master curve for the reduced diffusion coefficient. With no adjustable parameters, the model agrees with recent data for tracer diffusion measurements in polymer nanocomposites at low loading.

High-performance liquid chromatography with tandem mass spectrometry (HPLC/MS/MS) was used to determine melamine oral bioavailability (BA) and urinary excretion. Organ distribution after a 14-day consecutive oral melamine administration (100 mg/kg/day, once a day) was also evaluated. A noncompartmental model was utilized to obtain pharmacokinetic parameters. According to the results, the BA of melamine was estimated to be 98.1%. Approximately 63% of administered melamine was recovered in urine within 96 h after a single oral administration (100 mg/kg). The bladder had the highest melamine concentration of all the organs after a 14-day consecutive oral administration of melamine, and almost no melamine was found in the rat brain. This result indicated that the oral absorption of melamine was almost complete and urinary excretion was the major route for its elimination. Repeated exposure to high-dose melamine may result in only slight accumulation in organs.