The adsorption of the strong polyelectrolyte polystyrene sulfonate (PSS) to the air surface of dilute aqueous solutions was investigated as a function of molecular weight and salt concentration. Detailed segment profiles of the deuterated polymer were determined by neutron reflection.Surface activity was also examined through surface tension measurements.In general, the segment profiles are composed of a thin layer (10-20t hick) of high concentration at the air surface, followed by a distinct second layer of much lower segment concentration that extends to larger depths into the liquid. The high segment density at the air sutiace is due to a strong surface attraction, arising from the low surface tension of the PSS backbone relative to the surface tension of water. At low salt concentration, the profiles tend toward a single dense layer, suggesting that the chains lie nearly flat at the interface in that limit. The adsorbed amount increases with salt concentration, with a stronger dependence for higher molecular weight chains. The adsorbed amounts at the air/water interface are higher than reported previously for PSS adsorbed onto neutral solid surfaces, consistent with the fact that the air-liquid interface provides a stronger surface attraction. While the trends of adsorbed amount and layer thickness with salt concentration and molecular weight are in good agreement with numerical self-consistent field lattice calculations, the measured bilayer profiles are rather different from the smoothly decaying theoretical profiles. The surface tensions of the PSS solutions are significantly lowered relative to those of pure salt solutions. Combining the reflectivity and surface tension measurements, an approximately linear relationship is established between the surface pressure and the PSS adsorbed amount.

The adsorption of the strong polyelectrolyte sodium poly(styrenesulfonate) (NaPSS) to the air surface of water was investigated as a function of polymer concentration from the dilute regime to the beginning of the semidilute regime. Detailed segment profiles of the deuterated polymer were determined by neutron reflection (NR). Data were obtained for 0.67 and 2.50 M KCl. For two samples differing widely in molecular weight (1150 and 56.1 kg/mol), we find that with increasing polymer concentration the adsorbed amount first increases, reaches a maximum, and then decreases strongly. The PSS concentration at which the maximum is reached is dependent on both the molecular weight and the salt concentration in a manner that correlates with the chain overlap concentration. Regarding the segment profiles, at low polymer concentration the profiles are composed of a thin layer of high concentration at the air surface (trains), followed by a distinct second layer of much lower segment concentration that extends to larger depths into the liquid (loops and tails). Complementary X-ray reflection (XR) revealed a localization of ions about 10 Å below the surface for dilute PSS concentration. This layer becomes more diffuse at higher PSS concentration, in conjunction with the decrease in PSS adsorbed amount measured by NR. This surprising behavior of the adsorbed amount with polymer concentration is not explained by current SCF theory treating the adsorption of strong polyelectrolytes to neutral surfaces. We discuss a few possible explanations for this desorption transition. Finally, we observe that the surface tension decreases monotonically with increasing concentration of PSS in bulk solution but is not correlated with the adsorbed amount of PSS at the surface.

coupled double quantum wells show distinctive chblet structures related to the tunnel-split ground w.xb-level st.atee, The magnetic field behavior uf the upper transition from the antisymmetric state strongly dqmnds on sample mobilty, In a lower mobility sample, the transition energy displays an N-type kinkwith fidci (namely a nmxir.num followed bY a minimum], whereas higher mobility samplea have a hear dependence, The fcrmer is titrihuted to a coupl@ mechanism due to hornogenxxmsbroadening of the electron and hcJe states. The results are in good qrcent with recent theoretical catcdati"m.

The effect of the density and in-plane distribution of interfacial interactions on crack initiation in an epoxy-silicon joint was studied in nominally pure shear loading. Well-defined combinations of strong (specific) and weak (nonspecific) interactions were created using self-assembling monolayers. The in-plane distribution of strong and weak interactions was varied by employing two deposition methods: depositing mixtures of molecules with different terminal groups resulting in a nominally random distribution, and depositing methyl-terminated molecules in domains defined lithographically with the remaining area interacting through strong acid-base interactions. The two distributions lead to very different fracture behavior. For the case of the methyl-terminated domains (50 m on a side) fabricated lithographically, the joint shear strength varies almost linearly with the area fraction of strongly interacting sites. From this we infer that cracks nucleate on or near the interface over nearly the entire range of bonded area fraction and do so at nearly the same value of local stress (load/bonded area). We postulate that the imposed heterogeneity in interfacial interactions results in heterogeneous stress and strain fields within the epoxy in close proximity to the interface. Simply, the bonded areas carry load while the methyl terminated domains carry negligible load. Stress is amplified adjacent to the well-bonded regions (and reduced adjacent to the poorly bonded regions), and this leads to crack initiation by plastic deformation and chain scission within the epoxy near the interface. For the case of mixed monolayers, the dependence is entirely different. At low areal density of strongly interacting sites, the joint shear strength is below the detection limit of our transducer for a significant range of mixed monolayer composition. With increasing density of strongly interacting sites, a sharp increase in joint shear strength occurs at a methyl terminated area fraction of roughly 0.90. We postulate that this coincides with the onset of yielding in the epoxy. For methyl-terminated area fractions less than 0.85, the joint shear strength becomes independent of the interfacial interactions. This indicates that fracture no longer initiates on the interface but away from the interface by a competing mechanism, likely plastic deformation and chain scission within the bulk epoxy. The data demonstrate that the in-plane distribution of interaction sites alone can affect the location of crack nucleation and the far-field stress required.