Nanostructured functional materials prepared by atom transfer radical polymerization

Stereoregulation of thermoresponsive polymer brushes by surface-initiated living radical polymerization and the effect of tacticity on surface wettability

In this study, stereocontrolled poly(N-isopropylacrylamide) (PIPAAm) brushes were grafted from surfaces by atom transfer radical polymerization (ATRP) in the presence of a Lewis acid, and the effect of PIPAAm brush tacticity on the thermoresponsive wettabiliy was investigated. PIPAAm grafted by ATRP in the presence of Y(OTf)(3) showed high isotacticity, while the control brush polymerized in the absence of Y(OTf)(3) was clearly atactic. The isotacticity and molecular weight of PIPAAm brushes were controlled by polymerization conditions. The wettability of isotactic PIPAAm-grafted surfaces decreased slightly below 10 °C, although the phase transition temperature of atactic surface was 30 °C, and the bulk isotactic polymer was water-insoluble between 5 and 45 °C.

Pickering emulsions stabilized by nanoparticles with thermally responsive grafted polymer brushes

A study is presented of emulsification by silica nanoparticles with poly(2-(dimethylamino)ethyl methacrylate) brushes grafted from their surfaces (SiO(2)-PDMAEMA) by atom-transfer radical polymerization (ATRP). The grafted nanoparticles were used to stabilize xylene-in-water and cyclohexane-in-water Pickering emulsions. PDMAEMA is a water-soluble weak polyelectrolyte with a pH-dependent lower critical solution temperature (LCST). Accordingly, SiO(2)-PDMAEMA nanoparticles were thermally responsive, as shown by the fact that they displayed a critical flocculation temperature (CFT) when heated. ATRP provides a high degree of control over the brush grafting density and degree of polymerization, two of the principal variables examined in this study. The effects of the solvent quality of the "oil" for the PDMAEMA brush were studied in addition to the effects of aqueous pH, ionic strength, and temperature relative to the CFT. The preferred emulsion type was oil in water in all cases. The lowest grafting density particles (0.077 chains/nm(2)) proved to be the most efficient and robust emulsifiers, producing stable emulsions using as little as 0.05 wt % particles in the aqueous phase and successfully emulsifying over a broader range of solution conditions than for the higher grafting density particles (0.36 and 1.27 chain/nm(2)). Both good (xylene) and poor (cyclohexane) solvents could be emulsified, but the poor solvent could be emulsified over a broader range of conditions than the good solvent. Emulsions have been stable for over 13 months, and some have dispersed as much as 83 vol % oil in the emulsion phase. Thermally responsive emulsions were created with the SiO(2)-PDMAEMA particles such that stable emulsions prepared at low temperature were rapidly broken by increasing the temperature above the CFT.

Superhydrophilic surfaces via polymer-SiO2 nanocomposites

A novel procedure for the preparation of superhydrophilic surfaces is described. The method employs fabricating the surface from a mixture of silica nanoparticles (NPs) and polymers containing reactive trimethoxysilyl (TMOS) groups. Suitable polymers include quaternized poly(2-(dimethylamino)ethyl methacrylate) (PQDMAEMA) and poly(3-(trimethoxysilyl)propyl methacrylate) (PTMOSPMA). Condensation of the TMOS groups in a deposited film occurs under mild conditions and results in formation of a cross-linked polymer-SiO(2) nanocomposite coating covalently anchored onto a glass substrate. When silica nanoparticles, containing micrometer-sized agglomerates, are introduced into the film, a hierarchical micro/nanostructure within the coating is built up. Superhydrophilic behavior is achieved with a high weight ratio of fumed silica NPs or polymer/fumed silica NP bilayer coatings. The superhydrophilic surfaces have high stability and antifogging behavior and display easy cleaning characteristics. Furthermore, the superhydrophilic nanocomposite coatings containing PQDMAEMA exhibit antimicrobial properties against E. coli due to the presence of quaternary ammonium groups.

Size separation of macromolecules during spreading

Spreading of homogeneous mixtures of bottle-brush and linear macromolecules of poly(n-butylacrylate) on a solid substrate has been monitored on the molecular scale by atomic force microscopy. Despite the nearly identical chemical composition and similar molecular weight, brush-like macromolecules move markedly slower than linear chains. Moreover, smaller bottle-brushes have been shown to flow faster than the larger bottle-brushes, resulting in fractionation of the macromolecules along the spreading direction. This behavior was explained by the difference in sliding friction coefficient between the bottle-brush macromolecules and linear chains with the substrate. A theoretical model of molecular size separation is in a good agreement with experimental data.

Thermocurable hyperbranched polystyrenes for ultrathin polymer dielectrics

Thermocurable hyperbranched polystyrenes were successfully synthesized using atom transfer radical polymerization and exhibited superior ultrathin film formation capabilities in comparison with the linear analogues, as assessed by the minimal film thickness attainable by spin-coating without dewetting. They were suitable as ultrathin film organic dielectrics, with parallel plate specific capacitances as high as ∼680 nF/cm2. Similar to high performance inorganic dielectrics, capacitance measurements pointed to the presence of "dead" interfacial capacitance, which could be accounted for by considering the geometric effect of roughness "incommensurability" between metal electrode and polymer film.

Copper-catalyzed remote sp3 C-H chlorination of alkyl hydroperoxides

A copper-catalyzed methodology to functionalize remote sp(3) C-H bonds in alkyl hydroperoxides is presented. The atom-transfer chlorination utilizes simple ammonium chloride salts as the chlorine source, and the internal redox process requires no external redox reagents.

Multifunctional shape-memory polymers

The thermally-induced shape-memory effect (SME) is the capability of a material to change its shape in a predefined way in response to heat. In shape-memory polymers (SMP) this shape change is the entropy-driven recovery of a mechanical deformation, which was obtained before by application of external stress and was temporarily fixed by formation of physical crosslinks. The high technological significance of SMP becomes apparent in many established products (e.g., packaging materials, assembling devices, textiles, and membranes) and the broad SMP development activities in the field of biomedical as well as aerospace applications (e.g., medical devices or morphing structures for aerospace vehicles). Inspired by the complex and diverse requirements of these applications fundamental research is aiming at multifunctional SMP, in which SME is combined with additional functions and is proceeding rapidly. In this review different concepts for the creation of multifunctionality are derived from the various polymer network architectures of thermally-induced SMP. Multimaterial systems, such as nanocomposites, are described as well as one-component polymer systems, in which independent functions are integrated. Future challenges will be to transfer the concept of multifunctionality to other emerging shape-memory technologies like light-sensitive SMP, reversible shape changing effects or triple-shape polymers.