Saide Tang

Self-Stratified Antimicrobial Acrylic Coatings via One-Step UV Curing

We designed and synthesized a novel quaternary ammonium methacrylate compound (QAC-2) bearing a perfluoroalkyl tail on one end and an acrylic moiety on the other. Via one-step UV curing of QAC-2 and methyl methacrylate (MMA) with ethylene glycol dimethacrylate (EGDMA) as the cross-linker, we obtained cross-linked coatings with excellent antimicrobial property, as demonstrated by the total kill against both Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus epidermidis (S. epidermidis) at a QAC-2 concentration as low as ∼0.06 mol % (∼0.4 wt %) relative to MMA, which was substantially lower than the QAC amount needed in the coatings containing QACs with a hydrocarbon tail. A zone of inhibition test confirmed that the antimicrobial effect was on the basis of contact killing and there was no leaching of antimicrobial species from the cross-linked coating. The high antimicrobial potency in QAC-2-containing films was the consequence of strong surface enrichment of the fluorinated QAC, as confirmed by X-ray photoelectron spectroscopy (XPS).

In Situ Characterization of Binary Mixed Polymer Brush-Grafted Silica Nanoparticles in Aqueous and Organic Solvents by Cryo-Electron Tomography

We present an in situ cryo-electron microscopy (cryoEM) study of mixed poly(acrylic acid) (PAA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles in organic and aqueous solvents. These organic-inorganic nanoparticles are predicted to be environmentally responsive and adopt distinct brush layer morphologies in different solvent environments. Although the self-assembled morphology of mixed PAA/PS brush-grafted particles has been studied previously in a dried state, no direct visualization of microphase separation was achieved in the solvent environment. CryoEM allows the sample to be imaged in situ, that is, in a frozen solvated state, at the resolution of a transmission electron microscope. Cryo-electron tomograms (cryoET) were generated for mixed PAA/PS brush-grafted nanoparticles in both N,N-dimethylformamide (DMF, a nonselective good solvent) and water (a selective solvent for PAA). Different nanostructures for the mixed brushes were observed in these two solvents. Overall, the brush layer is more compact in water, with a thickness of 18 nm, as compared with an extended layer of 27 nm in DMF. In DMF, mixed PAA/PS brushes are observed to form laterally separated microdomains with a ripple wavelength of 13.8 nm. Because of its lower grafting density than that of PAA, PS domains form more or less cylindrical or truncated cone-shaped domains in the PAA matrix. In water, PAA chains are found to form a more complete shell around the nanoparticle to maximize their interaction with water, whereas PS chains collapse into the core of surface-tethered micelles near the silica core. The cryoET results presented here confirm the predicted environmentally responsive nature of PAA/PS mixed brush-grafted nanoparticles. This experimental approach may be useful for the design of future mixed brush-grafted nanoparticles for nano- and biotechnology applications.

The Role of Field Electron Emission in Polypropylene/Aluminum Nanodielectrics Under High Electric Fields

Polymer/metallic particle nanocomposites or nanodielectrics can exhibit colossal dielectric constants with a relatively low dissipation factor under low electric fields and thus seem to be promising for high-energy density dielectric capacitors. To study this possibility, this work focused on the dielectric performance and loss mechanisms in polypropylene (PP)/aluminum nanoparticle (nAl NP) composites under high electric fields. Phosphonic acid-terminated poly(ethylene-co-1-butene) was grafted to the Al2O3 surface layer on the nAl NPs in order to achieve reasonable dispersion in the PP matrix. The dielectric breakdown study showed that the breakdown strength decreased to nearly 1/20 that of the neat PP film as the nAl content increased to 25.0 vol %. The leakage current study revealed three electronic conduction mechanisms in the PP/100 nm nAl nanocomposites, namely, ohmic conduction at low fields, hopping conduction at intermediate fields, and Fowler-Nordheim (FN) field electron emission above a critical field, depending on the filler content. Compared to the 100 nm nAl NPs, smaller (e.g., 18 nm) nAl NPs needed a much higher electric field to exhibit FN field electron emission. It was the FN electron tunneling that induced a substantial reduction in breakdown strength for the PP/nAl nanocomposites. Meanwhile, electron-tunneling injected space charges (electrons) from nAl NPs into the PP matrix, and internal electronic conduction led to significant dielectric nonlinearity at high poling fields. Although polymer/metallic NP composites are not suitable for high-field electric applications, they can be good candidates for electrical switches and quantum tunneling composites operated at relatively low electric fields.

Thermally Stable Metallic Nanoparticles Prepared via Core-Cross-linked Block Copolymer Micellar Nanoreactors

Thermally stable metallic nanoparticles (MNPs) are highly desirable for the melt processing of polymer nanocomposites. However, due to the high surface energy penalty and decreased melting temperature, MNPs are easy to agglomerate and lose their unique properties if there is no protection or confinement layer. In this work, we report a facile and efficient way to synthesize thermally stable MNPs using core-cross-linked polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) reverse micelles as nanoreactors. From infrared results, gold, silver, and palladium ions exhibited distinctive coordination to the 4VP groups with varying chelation strengths. Compared to the non-cross-linked micelles, 1,4-dibromobutane (DBB)-cross-linking of the P4VP cores provided several advantages. First, it prevented severe swelling of the P4VP cores caused by the reducing agents and subsequent merger of swollen micelles. Second, the quaternized P4VP with hydrophilicity enhanced the uptake speed of precursor metal ions into the cores. Third, the cross-linked cores greatly stabilized the MNPs against the high-temperature environment (e.g., 110 °C for 40 h in toluene). In addition, the solubility of the reducing agents also played an important role. Anhydrous hydrazine could swell the P4VP cores and concentric core-shell particle morphology was obtained. On the contrary, triethylsilane could not swell the P4VP cores and thus eccentric core-shell particle morphology was observed. Only the concentric core-shell MNPs exhibited good thermal stability, whereas the eccentric core-shell MNPs did not. This work suggested that these thermally stable MNPs could be good candidates for the melt processing of functional polymer nanocomposites.