Angle-dependent X-ray photoelectron spectroscopy (ADXPS) is used to measure end group concentration depth profiles for blends of surface active ω-fluorosilane polystyrene with nonfunctional polystyrene. The fluorine signal is in all cases enhanced at the surface, indicating surface segregation of the lower surface tension fluorosilane end groups. End group segregation is enhanced by an increase in the concentration of ω-fluorosilane polystyrene, an increase in the nonfunctional polystyrene molecular weight, or a decrease in the molecular weight of the ω-fluorosilane polystyrene. A self-consistent meanfield lattice theory is developed to model the surface structure and properties of blends containing endfunctional polymers. Lattice model calculations provide estimates of concentration depth profiles as a function of the blend composition, the normalized chain lengths of the blend constituents, and the surface and bulk interaction parameters, χ s and χb, respectively. Two end-functional polystyrene architectures are considered: R-functional polystyrene for which the lattice reference volume is set equal to that of the entire fluorosilane end group and R,β-functional polystyrene where the fluorosilane end group is assumed to occupy two adjacent lattice sites at the chain end. The lattice model for both architectures provides excellent representations of experimental ADXPS data over a wide range of blend compositions and constituent molecular weights. The R,β-functional polymer model is shown to be superior on two accounts: the lattice reference volume and polymer repeat unit volumes are similar, and the optimal values of χ s ) -2.18 and χb ) 1.59, obtained by regression of this model to ADXPS data, are consistent with group contribution estimates of these parameters.
The JKR technique was applied to study the influence of interfacial reactions on the adhesion between functional elastomer gels and functional solid substrates. The gelation chemistry of poly(dimethyl siloxane) (PDMS) gels cured by hydrosilylation reactions was purposely adjusted to produce an excess of either silane or vinyl functionality. Hemispherical lenses of these materials were then contacted under load with a variety of functionalized solid substrates: poly(styrene-b-butadiene) copolymers with vinyl functionality, vinyl-terminated trimethoxysilane self-assembled monolayers, and a,x-functional PDMS brushes terminated with either monomethoxy or hydroxyl groups. To rule out chain interpenetration effects, the molecular weights were kept below the entanglement molecular weight or immiscible polymers were employed on opposite sides of the interface. Significant adhesion enhancement was observed for most systems, indicating that a variety of different interfacial reactions can occur across the interface between PDMS elastomers and solid polymeric substrates. The overall nature of the adhesion enhancement found is consistent with the predictions of the Lake-Thomas theory for failure of elastomers, increasing linearly with the length and areal density of covalent linker chains that span the interface.
Non-member Gary Stevens ** Non-member Two very different kinds of polymer nanocomposites have been prepared, characterized and investigated by dielectric spectroscopy to investigate the effects of polymer-nanofiller matrix difference on the dielectric response of nanodielectric composites. Linear low density polyethylene (LLDPE) is a non-polar thermoplastic which has a high viscosity even in the melt phase and bisphenol-A epoxy resin with an anhydride hardener is a polar low viscosity thermosetting resin. Nanometric sized aluminium oxide filler was chosen as the common inorganic phase for both nanodielectrics. Generally, nanoparticles aggregate easily and are difficult to separate due to strong surface interactions. In this study various mixing methods were employed from ultrasonic liquid processing to controlled shear flow mixing to investigate the dispersion of the nanofillers. The resultant epoxy and polyethylene nanocomposites were characterized with SEM, TEM, and DSC. The dielectric properties and frequency response of the nanocomposites were measured in the frequency domain from 10-2 Hz to 10 6 Hz at different temperatures. In polyethylene nanocomposites, significant interfacial polarization is clearly seen. However, in epoxy nanocomposites, no obvious interfacial polarization is found. The results are discussed in terms of the difference in the electrical characteristics of the interfacial region between the polymers and the nano-alumina.
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