Herein, the direct morphological evidence of the extension‐induced phase‐separated structures in the electrospinning jet observed by high‐speed video imaging and by light scattering technique is reported. Model solutions of poly(vinyl alcohol) (PVA)/water are electrospun. Two types of internal structures, that is, long strings and short ellipsoids, are found. A light scattering model is derived for the Vv scattering configuration to account for the scattered intensities contributed from the liquid jet itself and those from the internal structures. For the severely stretching jet of PVA/water, the Vv intensity profile is dominant by the internal structures to mask the scattering contribution from the jet itself. Moreover, the Hv intensity profile reflects the anisotropy of the oriented chains parallel to the jet axis. For the 7 wt% solution, the derived extension rate in the vicinity of the Taylor cone apex is about 3420 s−1, which is higher than the Rouse relaxation rate measured by rheometer. It is concluded that extension‐induced phase separation of the single‐phase PVA solution is likely to occur in Taylor‐cone apex to trigger the self‐assembly process for producing strings (and/or bulges) in the flowing jet, which eventually transform to become the nanofibers, after solvent removal, to be collected on the grounded collector.
Adsorption and reactions of 3-bromopyridine and 2-bromopyridine on Cu(100) and O/Cu(100) have been investigated, attempting to explore the chemical processes by identifying a variety of possible reaction intermediates and products, such as bipyridine, pyridyne, pyridine, pyridine oxide, hydroxypyridine, pyridone, and so forth. They can be generated from the Ullmann reaction, dehydrogenation, hydrogenation, hydroxylation, and oxidation of pyridyl groups on the surfaces. Temperature-programmed reaction/desorption, reflection–absorption infrared spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations have been employed for this research. At a monolayer coverage of 3-bromopyridine on Cu(100), a perpendicular 3-pyridyl is generated on the surface below 300 K because of the C–Br bond scission. This surface species undergoes loss of hydrogen, hydrogenation, and ring rupture at ∼450 K. The first two processes result in the pyridine formation. The dissociated fragments continue to react to form H2, HCN, and (CN)2 at higher temperatures. The relative amounts of the products of pyridine, H2, HCN, and (CN)2 are dependent on the coverage of 3-pyridyl. In the presence of surface oxygen atoms (O(ad)), dissociation of 3-bromopyridine also generates 3-pyridyl first. However, the pyridine formation from this intermediate is terminated, with decreased H2, HCN, and (CN)2. Additional products of H2O, CO, CO2, and HNCO are generated. For 2-bromopyridine (1.0 L) on Cu(100), at a coverage slightly higher than a monolayer, a perpendicular 2-pyridyl is generated below 300 K, and its decomposition generates pyridine (∼525 K), together with H2, HCN, and (CN)2 at higher temperatures. At a large exposure of 10.0 L of 2-bromopyridine, additional pyridine is generated at 650 K, which is suggested to be originated from an electronically/structurally strongly perturbed C5NH4 intermediate. On O/Cu(100), no 2-pyridyl intermediate is measured from the primary 2-bromopyridine decomposition; however, 2-oxypyridine is formed instead. This intermediate can be produced from nucleophilic attack on the 2C atom of 2-bromopyridine by O(ad) and/or from recombination of 2-pyridyl and O(ad). The 2-oxypyridine further reacts to form H2O, HCN, CO, CO2, HCNO, and (CN)2.
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