Surfactant‐Assisted Growth of Novel PbS Dendritic Nanostructures via Facile Hydrothermal Process

Abstract

lated and exfoliated PMMA and PS±clay nanocomposites foams were prepared using supercritical CO 2 as the foaming agent. Presence of a small amount of clay nanoparticles greatly reduces foam cell size and increases the cell density. Exfoliated nanocomposites yield foams with the smallest cell size and the highest cell density. Cell morphology can be furthered manipulated by adjustment of polymer±clay surface±CO 2 interaction and foaming conditions to achieve microcellular and submicrocellular foams. The high nucleation efficiency can produce microcellular nanocomposite foams at less stringent processing conditions, leading to cost savings and processing flexibility. ExperimentalMaterials: Methylmethacrylate (MMA), Styrene (St) and 2,2¢-azobisisobutyronitrile (AIBN) were purchased from Aldrich. PS (CX 5197) is from AtoFina Petrochemicals, while PMMA (PL25) is from Plaskolite. Two types of organically modified MMT clays were used. Cloisite 20A (20A) is an MMT modified by dimethyl dihydrogenated tallowalkylammoniumonium cations (Southern Clay Products). Na + ±MMT (cation exchange capacity 92.4 milliequivalent/100 g, from Southern Clay Products) was modified using a reactive cationic surfactant 2-methacryloyloxyethylhexadecyldimethylammonium bromide (MHAB) via ion-exchange reaction [16]. The resulting organoclay is denoted as MHABS. Polymer±20A nanocomposites were prepared using a Leistritz ZSE-27 intermesh twin-screw extruder (L/D = 40, d = 27 mm) in co-rotating mode between 200±220 C and 200 rpm (revolutions per minute) screw speed. In-situ polymerization was carried out to prepare PMMA and PS±MHABS nanocomposites. The monomer, MHABS, and AIBN (0.5 wt.-%) were mixed using a high shear mixer. The mixture was reacted isothermally (60 C for styrene, 50 C for MMA) for 20 h, after which the temperature was raised to 105 C for another 30 min. A two-stage method was also used to prepare PS±MHABS nanocomposites. First a 20 wt.-% nanocomposite masterbatch was prepared by in-situ polymerization. It was then blended with neat PS to prepare nanocomposites with desired clay concentration using a DACA microcompounder at 200 C and 250 rpm. Soxhlet extraction was used to extract non-bonded PMMA from PMMA±MHABS nanocomposites, using dichloromethane as the solvent. The unextractable portion consists of MHABS and a substantial amount of PMMA (64 % of the total weight of the unextractable nanocomposite). The resulting material was blended with neat PS to produce PS±(MHABS±PMMA). (PS±MHABS)±PMMA was prepared via blending a PS±MHABS nanocomposite with PMMA. These two materials have the same weight composition (PS/ PMMA/MHABS = 86:9:5).Foaming of Nanocomposites: The foaming agent, bone-dry grade carbon dioxide, was provided by Praxair. Samples were placed in a stainless steel vessel and CO 2 was delivered via a syringe pump. The system was allowed to equilibrate at the foaming temperature and pressure for sufficient time to ensure equilibrium. The pressure was then rapidly released and the foam cells were fixed by cooling with an ice and ...

Hydrothermal synthesis and thermoelectric properties of PbS

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Size, shape-controlled Synthesis of BaSiF6 nanostructures in a hydrothermal process

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