Green sonochemical synthesis, kinetics and functionalization of nanoscale anion exchange resins and their performance as water purification membranes

This paper reports on sonochemically catalyzed atom transfer radical polymerization (SONO-ATRP) polyelectrolyte synthesis and chain-end functionalization to single-walled carbon nanotubes (SWCNT). This all aqueous process is kinetically facile without use of initiator, or reducing agents and with very low concentrations of catalyst. The process achieves high functionalization density of polymer onto the SWCNTs. These functionalized nanoscale resins (NanoResins) exhibit high performance as fast and sustainable water purification materials. SONO-ATRP of vinyl benzyl trimethyl ammonium chloride (vbTMAC) was performed in aqueous medium resulting in short polyelectrolyte strands with high atom economy and high monomer conversions (93%) at room temperature using a thin probe sonicator (144Wcm^-2, 20 kHz, for 4 h). Kinetics analysis showed first order kinetics with respect to monomer concentration in presence of or absence of sonication power. Low temperature SONO-ATRP functionalization of SWCNTs is achieved within two hours without added reducing agent while similar functionalization density using reducing agents without sonochemistry required 12 h under reflux conditions. Functionalized NanoResin membranes were tested against surrogate analyte and demonstrated high performance Thomas Model breakthrough curves with a maximum adsorption capacity of 139 ± 1 mgg^-1 and water flux of 692 Lm^-2h^-1bar^-1 at one atmosphere pressure. Moreover, these materials are easily regenerated and reused without loss of performance or degradation.

Synthesis of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) with low polydispersity using ultrasonic irradiation

Poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) having low polydispersity was synthesized in mixed solvent of ethanol and water using ultrasonic irradiation without any chemical polymerization initiator. The effects of the volume fraction of ethanol in the solvent, the molar ratio of two monomers, the monomer concentration and the ultrasonic power intensity on the time courses of the conversion to the polymer, the number average molecular weight, and the polydispersity of synthesized polymer were investigated in order to determine the optimal conditions to synthesize the copolymers with a narrow molecular weight distribution (i.e. low polydispersity). The optimum volume fraction of ethanol in the solvent was 60 vol% to synthesize the copolymers with a low polydispersity. A higher ultrasonic power intensity resulted in a faster polymerization rate and a lower number average molecular weight. The polydispersity was less than 1.5 for all ultrasonic power intensities up to 450 W/dm³ applied in this work. A higher monomer concentration gave a faster polymerization rate and a higher number average molecular weight. The polydispersity was less than 1.5 when the monomer concentration was lower than 0.4 mol/dm³. A higher molar ratio of N-isopropylacrylamide resulted in a higher polymerization rate and a lower number average molecular weight. The copolymers with polydispersity less than 1.5 can be obtained regardless of the molar ratio of N-isopropylacrylamide. The copolymers synthesized by the ultrasonic polymerization method had a high temperature responsibility.

Control of molecular weight distribution in synthesis of poly(2-hydroxyethyl methacrylate) using ultrasonic irradiation

Poly(2-hydroxyethyl methacrylate) (PHEMA) was synthesized using ultrasonic irradiation without any chemical initiator. The effect of the ultrasonic power intensity on the time course of the conversion to polymer, the number average molecular weight, and the polydispersity were investigated in order to synthesize a polymer with a low molecular weight distribution (i.e., low polydispersity). The conversion to polymer increased with time. A higher ultrasonic power intensity resulted in a faster reaction rate. The number average molecular weight increased during the early stage of the reaction and then gradually decreased with time. A higher ultrasonic intensity resulted in a faster degradation rate of the polymer. The polydispersity decreased with time. This was because the degradation rate of a polymer with a higher molecular weight was faster than that of a polymer with a lower molecular weight. A polydispersity below 1.3 was obtained under ultrasonic irradiation. By changing the ultrasonic power intensity during the reaction, the number average molecular weight can be controlled while maintaining low polydispersity. When the ultrasonic irradiation was halted, the reactions stopped and the number average molecular weight and polydispersity did not change. On the basis of the experimental results, a kinetic model for synthesis of PHEMA under ultrasonic irradiation was constructed considering both polymerization and polymer degradation. The kinetic model was in good agreement with the experimental results for the time courses of the conversion to polymer, the number average molecular weight, and the polydispersity for various ultrasonic power intensities.

Preparation of hydrogels via ultrasonic polymerization

Several acrylic hydrogels were prepared via ultrasonic polymerization of water soluble monomers and macromonomers. Ultrasound was used to create initiating radicals in viscous aqueous monomer solutions using the additives glycerol, sorbitol or glucose in an open system at 37 degrees C. The water soluble additives were essential for the hydrogel production, glycerol being the most effective. Hydrogels were prepared from the monomers 2-hydroxyethyl methacrylate, poly(ethylene glycol) dimethacrylate, dextran methacrylate, acrylic acid/ethylene glycol dimethacrylate and acrylamide/bis-acrylamide. For example a 5% w/w solution of dextran methacrylate formed a hydrogel in 6.5min in a 70% w/w solution of glycerol in water at 37 degrees C with 20kHz ultrasound, 56Wcm(-2). The ultrasonic polymerization method described here has a wide range of applications such a biomaterial synthesis where initiators are not desired.

Bubbles in an acoustic field: an overview

Acoustic cavitation is the fundamental process responsible for the initiation of most of the sonochemical reactions in liquids. Acoustic cavitation originates from the interaction between sound waves and bubbles. In an acoustic field, bubbles can undergo growth by rectified diffusion, bubble-bubble coalescence, bubble dissolution or bubble collapse leading to the generation of primary radicals and other secondary chemical reactions. Surface active solutes have been used in association with a number of experimental techniques in order to isolate and understand these activities. A strobe technique has been used for monitoring the growth of a single bubble by rectified diffusion. Multibubble sonoluminescence has been used for monitoring the growth of the bubbles as well as coalescence between bubbles. The extent of bubble coalescence has also been monitored using a newly developed capillary technique. An overview of the various experimental results has been presented in order to highlight the complexities involved in acoustic cavitation processes, which on the other hand arise from a simple, mechanical interaction between sound waves and bubbles.

Acoustic Emission Spectra from 515 kHz Cavitation in Aqueous Solutions Containing Surface-Active Solutes

The effect of adding surface-active solutes to water being insonated at 515 kHz has been investigated by monitoring the acoustic emission from the solutions. At low concentrations (<3 mM), sodium dodecyl sulfate causes marked changes to the acoustic emission spectrum which can be interpreted in terms of preventing bubble coalescence and declustering of bubbles within a cavitating bubble cloud. By conducting experiments in the presence of background electrolytes and also using non-ionic surfactants, the importance of electrostatic effects has been revealed. The results provide further mechanistic evidence for the interpretation of the effect of surface-active solutes on acoustic cavitation and hence on the mechanism of sonochemistry. The work will be valuable to many researchers in allowing them to optimize reaction and process conditions in sonochemical systems.