The influence of short chain branch on formation of shear induced crystals in bimodal polyethylene at high shear temperatures

Molecular dynamics simulation of extension-induced crystallization of branched bimodal HDPE: Unraveling the effects of short-chain branches

Bimodal HDPE models were designed for extension-induced crystallization imitating the architecture of industrial bimodal HDPE copolymerized with ethylene and 1-butene, 1-hexene, or 1-octene. Crystallites of bimodal HDPE experienced the emergence of precursors, shish nuclei, and lamellae. The compact conformation of branched polymers impeded the rolling-over, deposition, and folding of chains on the substrate, and thus the formation of nuclei and lamella. Moreover, this retardation was intensified with the rising branch density and length, causing a depression of crystallinity and an increment of tie-chains concentration. Besides, when branches were all located on long chains, the compact conformation enlarged the resistance to the disentanglement of main chains, thus relatively fewer branched long chains were involved in the precursors or nuclei, resulting in the attenuation of lamella formation. Furthermore, for ethyl branched polymers, the coexistent orthorhombic and monoclinic crystallites were built up, and a few expanded monoclinic cells occurred for butyl branches because of the larger butyl reeling into lamella, while hexagonal crystals were created for ethyl/1-hexyl copolymers because of cocrystallization. Additionally, relative to ethyl, larger butyl and hexyl were preferential to be repelled outside crystals to form tie-chains, and hexyl branched polymers acquired relatively fewer tie-chains because of hexagonal eutectoid.

Electric Response of CuS Nanoparticle Lubricant Additives: The Effect of Crystalline and Amorphous Octadecylamine Surfactant Capping Layers

Octadecylamine-coated CuS nanoparticles were designed and confirmed to play an important role in their electric response and boundary lubrication in the ester lubricant. For the case of CuS nanoparticles coated with crystalline surfactant, the surface potential is 18.47 ± 0.99 mV higher than with amorphous surfactant, owing to the random chain conformations of the octadecylamine molecules. When used as a lubricant additive, CuS nanoparticles (in the form of nanoplates or nanoarrays) with a crystalline surfactant were positively charged due to the presence of the amino headgroup in octadecylamine. The observed friction coefficient decreased from 0.18 to 0.09 and 0.05, respectively, when negative potential (for the copper lower pair) was applied across untreated CuS nanoparticles. However, thermally treated CuS nanoparticles showed good lubricating effect, but almost no effect of potential control since the amino groups were obscured by the disordered carbon chains, hindering electron transfer and weakening the response to externally applied electric field.