Polymer brushes––surface immobilized polymers Boyes, Stephen G.; Granville, Anthony M.; Baum, Marina ...
Surface science,
10/2004, Letnik:
570, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
The synthesis of tethered block copolymer brushes via the use of controlled/‘living’ free radical polymerization techniques presents many significant advantages over traditional free radical ...polymerization techniques. In our group, we have found that the most versatile controlled/‘living’ free radical polymerization techniques are atom transfer radical polymerization (ATRP) and reversible addition fragmentation transfer (RAFT) polymerization. Both diblock and ABA type triblock copolymer brushes have been synthesized using either ATRP or RAFT. Of particular interest with block copolymer brushes, are their ability to reversibly rearrange upon treatment with selective solvents. This rearrangement of block copolymer brushes can result in the formation of unusual surface morphologies that have been attributed to the formation of either ‘pinned micelles’ or ‘folded’ structures. We have demonstrated that other external stimuli besides block-selective solvents can be used to induce brush reorganization, namely temperature and treatment with supercritical CO
2.
Diblock copolymer polyelectrolyte brushes of either polystyrene (PS) or poly(methyl acrylate) (PMA) and poly(acrylic acid) (PAA) were synthesized using sequential monomer addition from a tethered ...2-bromoisobutyrate initiator. Si/SiO2//PS-b-poly(tert-butyl acrylate) (P(t-BA)) and Si/SiO2//PMA-b-P(t-BA) were prepared by the “grafting from” method using atom transfer radical polymerization. The PAA block was prepared by hydrolysis of poly(tert-butyl acrylate) (P(t-BA)) and the polyelectrolyte was formed by subsequent treatment with either aqueous silver acetate or sodium tetrapalladate. Conversion of P(t-BA) to PAA and attachment of either silver or palladium ions to the PAA block was confirmed using ATR-FTIR spectroscopy, water contact angle measurements, and X-ray photoelectron spectroscopy. Solvent switching of the diblock copolymer brush using DMF and anisole was incomplete both before and after treatment with silver acetate, although the degree of switching was larger before treatment. The polyelectrolyte diblock copolymer brushes were used for the synthesis of inorganic nanoparticles by reduction of the treated PAA. These samples were characterized using AFM, XPS, and ATR−FTIR.
Sensors based on organic thin-film transistors (OTFTs) are of considerable interest for chemical and biological detection applications, and the development of highly sensitive, chemically specific, ...low-cost sensors operating in aqueous environments will have a profound impact. However, the behavior of the dielectric and semiconducting thin films in OFTF-based sensors during underwater operation is not well understood. Here we investigate OTFT-based sensor materials, specifically a polymer dielectric film of cross-linked poly(4-vinylphenol) (x-PVP), used in OTFTs operating in aqueous environments. We show that immersing x-PVP thin films in a 90:10 water–methanol (model analyte) solution causes swelling of nearly 30% and a corresponding 300% increase in the film dielectric constant. Hence, to quantify the charge-transport behavior of organic molecules within aqueous environments, this drastic change in the capacitance must be accounted for in sensor material design.
We found that interactions of dipalmitoylphosphatidylcholine (DPPC) lipid monolayers with sugars are influenced by addition of NaCl. This work is of general importance in understanding how ...sugar–lipid–salt interactions impact biological systems. Using Langmuir isothermal compressions, fluorescence microscopy, atomic force microscopy, and neutron reflectometry, we examined DPPC monolayers upon addition of sugars/polyols and/or monovalent salts. Sugar–lipid interactions in the presence of NaCl increased with increasing complexity of the sugar/polyol in the order glycerol ≪ glucose < trehalose. When the anion was altered in the series NaF, NaCl, and NaBr, only minor differences were observed. When comparing LiCl, NaCl, and KCl, sodium chloride had the greatest influence on glucose and trehalose interactions with DPPC. We propose that heterogeneity created by cation binding allows for sugars to bind the lipid headgroups. While cation binding increases in the order K+ < Na+ < Li+, lithium ions may also compete with glucose for binding sites. Thus, both cooperative and competitive factors contribute to the overall influence of salts on sugar–lipid interactions.
When downsizing technology, confinement and interface effects become enormously important. Shear imposes additional anisotropy on a liquid. This may induce inhomogeneities, which may have their ...origin close to the solid interface. For advancing the understanding of flow, information on structures on all length scales and in particular close to the solid interface is indispensable. Neutron scattering offers an excellent tool to contribute in this context. In this work, surface sensitive scattering techniques were used to resolve the structure of liquids under flow in the vicinity of a solid interface. Our results are summarized as follows. First, for a Newtonian liquid we report a depletion distance on the order of nanometers which is far too small to explain the amount of surface slip, on the order of micrometers, found by complementary techniques. Second, for a grafted polymer brush we find no entanglement-disentanglement transition under shear but the grafted film gets ripped off the surface. Third, by evaluating the local structure factor of a micellar solution close to the solid interface it turns out that the degree of order and local relaxation depends critically on the surface energy of the solid surface.
The effects of CO sub(2) annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si ...substrates were investigated. By using in situneutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as P= 2.9 MPa and at T= 48 degree C which is below the bulk melting temperature (T sub(m)). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air viasubsequent annealing at given temperatures below T sub(m). Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO sub(2)-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO sub(2) annealing: the limited plasticization effect of CO sub(2) at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.
The phase behavior of a molecular brush-C18 grafted to the surface of both a silicon wafer and SiO2 nanoparticles was investigated as a function of temperature using neutron reflectometry (NR) and ...small-angle neutron scattering (SANS), respectively. The experiments demonstrate a phase change in the brush layer characterized by a straightening of the molecular configuration, increase in shell thickness, and increase in solvent concentration with decreasing temperature that corresponds to gelation in the nanoparticle dispersion.
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