Xerogels comprising n-butyl acrylate and jV-vinyl-2-pyrrolidone (30/70 wt/wt) and varying small amounts of a hexafunctional cross-linking agent have been prepared by y-ray-initiated copolymerization to complete conversion and then swollen in water to equilibrium at several different temperatures (T) within the range 284-349 K. Photographic determination of the sample dimensions afforded the volume fractions of polymer ( 2) and water within the resultant hydrogels, and the effective cross-linking densities (i
SynopsisCrosslinked xerogels in the form of rods have been prepared to > 99.9% conversion by the y-ray initiated copolymerization in vacuo of n-butyl acrylate (BA) with N-vinyl-2-pyrrolidone (VP) in the presence and absence of a crosslinking agent. Thin (about 0.5 mm) disks were swollen in water at 2 l T , the final equilibrium swelling being the true water content only for copolymers of high BA content. At low-medium BA content, swelling was accompanied by the release of a small water-soluble fraction, leading to a slightly reduced apparent value for the water content. True water contents could be obtained more rapidly by prior Soxhlet extraction with boiling water. For xerogels containing > about 75% VP by weight, the resultant hydrogels had water contents > about 80%. The influences of dose, dose rate copolymer composition, and concentration of crosslinker on the water content, sol fraction, and extension ratio were investigated. A high level of reproducibility in properties of the hydrogels was obtained by numerous replicate experiments. The feasibility of scaling up the preparation to produce long (60 cm) xerogel rods of uniform properties has been demonstrated. For these copolymers, swelling in several common organic solvents generally exceeds that in water.
We describe a different, more student-friendly approach to writing Lewis structures for covalent molecules and ions based on Lewis theory and Abegg's rule. Several rules for selecting central atoms are provided. Separate sets of rules are provided for diatomic molecules and ions and for polyatomic molecules and ions. For polyatomic species, we distinguish between cases where surrounding atoms are monovalent and cases where there is at least one polyvalent ligand atom. Examples are given to illustrate these rules.
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