Interpolymer radical coupling reactions during sonication of polymer solutions

Productive chemistry induced by mechanochemically generated macroradicals

Large or repeated mechanical loads degrade polymeric materials by accelerating chain fragmentation. This mechanochemical backbone fracture usually occurs by homolysis of otherwise inert C-C, C-O and C-S bonds, generating highly reactive macroradicals. Because backbone fracture is detrimental on its own and the resulting macroradicals can initiate damaging reaction cascades, a major thrust in contemporary polymer mechanochemistry is to suppress it, usually by mechanochemical release of "hidden length" that dissipates local molecular strain. Here we summarize an emerging complementary strategy of channelling mechanochemically generated macroradicals in reaction cascades to form new load-bearing chemical bonds, which enables local self-healing or self-strengthening, and/or to generate mechanofluorescence, which could yield detailed quantitative molecular understanding of how material-failure-inducing macroscopic mechanical loads distribute across the network. We aim to identify generalizable lessons derivable from the reported implementations of this strategy and outline the key challenges in adapting it to diverse polymeric materials and loading scenarios.

Does power ultrasound affect hydrocarbon Ionomers?

The effect of low-frequency high-power ultrasound on hydrocarbon-based ionomers, cation exchange sulfonated phenylated polyphenylene (sPPB-H+) and anion exchange hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), was studied. Ionomer solutions were subjected to ultrasonication at fixed ultrasonic frequencies (f = 26 and 42 kHz) and acoustic power (Pacous = 2.1 - 10.6 W) in a laboratory-grade ultrasonication bath, and a probe ultrasonicator; both commonly employed in catalyst ink preparation in research laboratory scale. Power ultrasound reduced the polymer solution viscosity of both hydrocarbon-based ionomers. The molecular weight of sPPB-H+ decreased with irradiation time. Changes in viscosity and molecular weight were exacerbated when ultrasonicated in an ice bath; but reduced when the solutions contained carbon black, as typically used in Pt/C-based catalyst inks. Spectroscopic analyses revealed no measurable changes in polymer structure upon ultrasonication, except for very high doses, where evidence for free-radical induced degradation was observed. Ionomers subjected to ultrasound were used to prepare catalyst layers and membrane electrode assemblies (MEA)s. Despite the changes in the ionomer described above, no significant differences in electrochemical performance were found between MEAs prepared with ionomers pre-subjected to ultrasound and those that were not, suggesting that fuel cell performance is tolerant to ionomers subjected to ultrasound.

Synthesis and application of core-shell liquid metal particles: a perspective of surface engineering

Liquid metal micro/nano particles (LMPs) from gallium and its alloys have attracted tremendous attention in the last decade due to the unique combination of their metallic and fluidic properties at relatively low temperatures. Unfortunately, there is limited success so far in realizing the highly controllable fabrication and functionalization of this emerging material, posing great obstacles to further promoting its fundamental and applied studies. This review aims to explore solutions for the on-demand design and manipulation of LMPs through physicochemically engineering their surface microenvironment, including compositions, structures, and properties, which are featured by the encapsulation of LMPs inside a variety of synthetic shell architectures. These heterophase, core-shell liquid metal composites display adjustable size and structure-property relationships, rendering improved performances in several attractive scenarios including but not limited to soft electronics, nano/biomedicine, catalysis, and energy storage/conversion. Challenges and opportunities regarding this burgeoning field are also disclosed at the end of this review.

Water-processable liquid metal nanoparticles by single-step polymer encapsulation

Stabilizing metastable colloids such as gallium-based liquid metal nanoparticles (LM NPs) and using them in complex machining processes is a great challenge due to their simple and sensitive surface chemistry against ligand modification and solution processing. Water, a green solvent, is unfortunately not favored by LM NPs in the entire workflow from synthesis to functionalization and application. This dilemma is relieved herein by presenting a sonochemical polymer deposition technique that can produce a water-dispersible LM@polymer hybrid nanomaterial in one step. Water-soluble polymers such as poly(vinyl pyrrolidone) (PVP) can be directly deposited onto eutectic gallium indium (EGaIn) as a coating shell (up to 20 nm) and prevent their rapid and continuous oxidation in aqueous solutions. EGaIn@PVP NPs are stable after preservation for 30 days in water and 60 days in ethanol, and the polymer coating shows solvent-responsive swelling/shrinking behaviors. The LM@polymer NPs can be stably composited with other materials through complex processing and perform desired functions after composition. For demonstration, EGaIn@PVP NPs are employed as active electrode materials for lithium-ion batteries by compositing with water-soluble binders, and exhibit excellent cycling performances with no obvious decay (capacity retention >95%) in more than 700 cycles at 4 A g-1. This strategy enriches the toolbox for surface engineering of LMs especially under harsh conditions, which can benefit the applications of LM NPs in various scenarios.

Does power ultrasound affect Nafion® dispersions?

The effect of low frequency power ultrasound on Nafion® ionomer used for fabricating proton exchange membrane fuel cell (PEMFC) and water electrolyzer (PEMWE) catalyst inks was investigated. In this study, a series of Nafion® dispersions having three concentrations (10, 5, and 2.5% w/v) were studied under various irradiation durations (t_us), at fixed ultrasonic frequency (f = 42 kHz) and ultrasonic power (P > 2 W), under either controlled or unregulated bulk solution temperature conditions using a laboratory ultrasonic cleaning bath. Viscosity (η), thermal degradation, and glass transition temperature (T_g) for all Nafion® dispersion samples was measured and compared to untreated Nafion® samples. In our conditions, it was found that power ultrasound lowered the viscosity of all tested Nafion® dispersion samples; whilst thermogravimetric and differential scanning calorimetry analyses showed that for all ultrasonically irradiated samples, a negligible overall polymer degradation and no obvious change in T_g was observed under controlled and unregulated bulk temperature conditions. It was found that it is possible that acoustic cavitation causes depolymerisation followed by a polymerisation initiation step during ultrasonication. By comparing the ultrasonically treated and high-shear mixed samples, it was also observed that acoustic and hydrodynamic cavitation played an important role in the reduction of dispersion viscosity.

Ultrasonic compatibilization of polyelectrolyte complex based on polysaccharides for biomedical applications

In recent years, there has been an increasing interest in the design of biomaterials for cartilage tissue engineering. This type of materials must meet several requirements. In this study, we apply ultrasound to prepare a compatibilized blend of polyelectrolyte complexes (PEC) based on carboxymethyl cellulose (CMC) and chitosan (CHI), in order to improve stability and mechanical properties through the inter-polymer macroradicals coupling produced by sonochemical reaction. We study the kinetic of the sonochemical degradation of each component in order to optimize the experimental conditions for PEC compatibilization. Scaffolds obtained applying this methodology and scaffolds without ultrasound processing were prepared and their morphology (by scanning electron microscopy), polyelectrolyte interactions (by FTIR), stability and mechanical properties were analyzed. The swelling kinetics was studied and interpreted based on the structural differences between the two kinds of scaffolds. In addition we evaluate the possible in vitro cytotoxicity of the scaffolds using macrophage cells in culture. Our results demonstrate that the ultrasound is a very efficient methodology to compatibilize PEC, exhibiting improved properties compared with the simple mixture of the two polysaccharides. The test with murine macrophage RAW 264.7 cells showed no evince of cytotoxicity, suggesting that PEC biomaterials obtained under ultrasound conditions could be useful in the cartilage tissue engineering field.

Characterization of poly(epsilon-caprolactone)/polyfumarate blends as scaffolds for bone tissue engineering

There is considerable interest in the design of polymeric biomaterials that can be used for the repair of bone defects. In this study, we used ultrasound to prepare a compatibilized blend of poly(epsilon-caprolactone) (PCL) and poly(diisopropyl fumarate) (PDIPF). The formation of post-sonication inter-polymer coupling products was verified by SEC analysis of a blend with azo-labeled PDIPF. We also analyzed the physicochemical and mechanical properties of the compatibilized blend. When compared to PCL alone, the PCL/PDIPF blend showed no difference in its resistance as evaluated by the elastic modulus, although it did show a 50% decrease in ultimate tensile stress (P < 0.05) and an 84% decrease in elongation-at-break (P < 0.05). However, the mechanical properties of this blend were comparable to those of trabecular bone. We next evaluated biocompatibility of the PCL/PDIPF blend, and of homo-polymeric PCL and PDIPF films for comparison, with UMR106 and MC3T3E1 osteoblastic cells. Osteoblasts plated on the compatibilized blend adhered and proliferated more than on either homo-polymer, showed a greater number of cellular processes with a better organized actin cytoskeleton and expressed more type-I collagen and mineral, both markers of osteoblast phenotype. These results support the hypothesis that this new compatibilized blend could be useful in future applications for bone regeneration.

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.

Effect of molecular weight on the ultrasonic degradation of poly(vinyl-pyrrolidone)

The ultrasonic degradation of poly(vinyl-pyrrolidone) (PVP) of different initial molecular weights was studied at a fixed temperature. The effect of solution concentration on the rate of degradation was investigated. A method of viscometry was used to study the degradation behavior and kinetic model was developed to estimate the degradation rate constant. The results were indicated that the rate of ultrasonic degradation increased with increasing molecular weight. It was found that rate constant decreases as the concentration increases. The calculated rate constants correlated in terms of inverse concentration and relative viscosity of PVP solutions. This behavior in the rate of degradation was interpreted in terms of viscosity and concentration of polymer solution. With increasing solution concentration, viscosity increases and it causes a reduction in the cavitation efficiency thus, the rate of degradation will be decreased. The experimental results show that the viscosity of polymers decreased with ultrasonic irradiation time and approached a limiting value, below which no further degradation took place. This study confirms the general assumption that the shear forces generated by the rapid motion of the solvent following cavitational collapse are responsible for the breakage of the chemical bonds within the polymer. The effect of polymer concentration can be interpreted in terms of the increase in viscosity with concentration, causing the molecules to become less mobile in solution and the velocity gradients around the collapsing bubbles to, therefore, become smaller.

Xyloglucan degradation using different radiation sources: a comparative study

Tamarind seed xyloglucan was subjected to different radiation sources-ultrasound, gamma-radiation, and microwave heating, and the effects of these energies upon its molecular and structural properties were characterised by gel permeation chromatography, viscometry, sugar analysis, FT-IR and NMR spectroscopic techniques. In dependence on the degradation methods and experimental conditions used, the decrease of the relative molecular mass (RMM) was accompanied with alteration of the primary structure. Depolymerisation by ultrasound at a frequency of 20 kHz yielded after 120 min products with RMM of about 131 x 10(3) without significant alteration of the primary structure of the polysaccharide. Intense degradation of XG started by microwave heating at pH 1.5 yielding polymers with RMM in the range of higher oligosaccharides, however, with changed sugar composition due to cleavage of the glycosyl side chains. At gamma-irradiation doses >40 kGy, next to chain cleavage, very high-molecular mass components exhibiting UV(254)-absorption were formed, and the RMM decreased to about 50 x 10(3) at the highest applied dose (100 kGy). The results of the comparative study suggest that ultrasonication was the most convenient procedure to decrease the RMM of xyloglucan to 130 x 10(3) and preserve the primary structure of the polysaccharide.