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1. Polyacrylamide hydrogels. VI. Synthesis-property relation

   https://doi.org/10.1016/j.jmps.2022.105099  

   Wang, Yecheng; Nian, Guodong; Kim, Junsoo; Suo, Zhigang (January 2023, Journal of the Mechanics and Physics of Solids)

   ynthesis-property relation is fundamental to materials science, but many aspects of the relation are not well understood for many materials. Impetus for this paper comes from our recent appreciation for the distinct roles of entanglements and crosslinks in a polymer network. Here we study the synthesis-property relation of polyacrylamide hydrogels prepared by free radical polymerization. Some of the as-prepared hydrogels are further submerged in water to swell either to equilibrium or to a certain polymer content. The synthesis parameters include the composition of a precursor, as well as the polymer content of a hydrogel. Series of hydrogels are prepared along several paths in the space of synthesis parameters. For each hydrogel, the stress-stretch curve is measured, giving four properties: modulus, strength, stretchability, and work of fracture. We interpret the experimentally measured synthesis-property relation in terms of entropic polymer networks of covalent bonds. When the precursor has a low crosslinker-to-monomer molar ratio, the resulting polymer network has on average long polymer segments. When the precursor has a low water-to-monomer molar ratio, the resulting polymer network has on average many entanglements per polymer segment. We show that crosslinks lower strength, but entanglements do not. By contrast, both crosslinks and entanglements increase modulus. A network of highly entangled long polymer segments exhibits high swell resistance, modulus, and strength. 

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2. Quantum supergroups VI: roots of 1

   https://doi.org/10.1007/s11005-019-01209-4  

   Chung, Christopher; Sale, Thomas; Wang, Weiqiang (December 2019, Letters in Mathematical Physics)
3. Pure Pairs VI: Excluding an Ordered Tree

   https://doi.org/10.1137/20M1368331  

   Scott, Alex; Seymour, Paul; Spirkl, Sophie (March 2022, SIAM Journal on Discrete Mathematics)
4. Molybdenum( vi ) tris(amidophenoxide) complexes

   https://doi.org/10.1039/C8DT03392G  

   Erickson, Alexander N.; Brown, Seth N. (November 2018, Dalton Transactions)

   Tris(2-(arylamido)-4,6-di- tert -butylphenoxo)molybdenum( vi ) complexes ( R ap) 3 Mo can be prepared either from (cycloheptatriene)Mo(CO) 3 and the N -aryliminoquinone, or from MoO 2 (acac) 2 and the aminophenol. In contrast to all other reported unconstrained transition metal tris(amidophenoxide) complexes, the molybdenum complexes show a facial geometry in the solid state. In solution, the fac isomer predominates, though a small amount of mer isomer is detectable at room temperature. At elevated temperature the two species interconvert through Rây-Dutt trigonal twists, which are faster than Bailar twists in this system, presumably because of steric effects of the N -aryl groups. Substituents on the N -aryl ring shift the fac / mer equilibrium of the complex, with more electron-withdrawing substituents generally increasing the proportion of the mer isomer. The preference for fac over mer geometry is thus suggested to be due to enhanced π bonding in the fac isomer. In contrast to analogous catecholate complexes, the tris(amidophenoxide) complexes are not Lewis acidic and are inert to nucleophilic oxidants such as amine- N -oxides. 

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5. Synthesis of Molybdenum(VI) Tritylimido Alkylidene Complexes

   https://doi.org/10.1021/acs.organomet.2c00189  

   Paul, Bhaskar; Schrock, Richard R.; Carta, Veronica (November 2022, Organometallics)