Multitechnique surface spectroscopic studies of plasma modified polymers III. H2O and O2/H2O plasma modified poly(methyl methacrylate)s

The surfaces of poly(methyl methacrylate) (PMMA) films modified by O₂H₂O and H₂O radio-frequency glow discharge plasmas were studied using electron spectroscopy for chemical analysis (ESCA or XPS), low energy ion scattering (LEISS or ISS), Fourier transform IR spectroscopy (FTIR) with attenuated total reflectance (ATR) sampling, and critical surface energy from contact angle measurements. The extent and nature of modification with respect to promotion of a hydrophilic surface compared to the hydrophobic surface of the unmodified PMMA has been probed. Results show drastic decreases in C/O ratio at the near surface, which increases to that of the unmodified PMMA as deeper cross sections are analyzed. In addition peak fitting of ESCA data correlated with FTIR functional group information allows for the qualitative and quantitative analysis of the resulting bonding and structure of the modified layer. From these results combined with the polarity and surface energy differences obtained from contact angle measurements, the structural changes are discussed with respect to plasma reaction mechanisms and differences in the structure of the modified polymer films.     

A multitechnique surface analytical study of a segmented block copolymer poly(ether-urethane) modified through an H2O radio frequency glow discharge

Recent work in our laboratories has fully characterized the surface region of a segmented poly(ether-urethane) (PEU) extending from the air/polymer interfacial region through bulk depths in the micron range. This characterization utilized energy and angle dependent Electron Spectroscopy for Chemical Analysis (ESCA), Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR–FTIR), and Comprehensive Wettability Profiling (contact angle using a homologous series of liquids) as defined by Zisman. In this study this same multi-analytical-technique approach is used to elucidate changes in these PEU surfaces induced through an H₂O Radio Frequency Glow Discharge (RFGD) plasma. This investigation reports both qualitative and quantitative changes due to the modification treatments as well as the permanency of the changes effected on these surfaces through the plasma treatment. From our analyses, the amount of surface residing polyurethane (hard segment) is observed to increase due to a proposed plasma etching mechanism. Further, the addition of oxygen containing functionality is detected at the modified surfaces unique with respect to the unmodified PEU. These surface modifications which show large increases in wettability, are finally observed to be semi-permanent over a time period of 6 months.     

Irradiation of aromatic polymers: The effect of irradiation temperature on crosslinking and scission in polystyrene

Polystyrene of narrow molecular weight distribution, M _w/M _n ≈ 1.03, was subjected to γ irradiation at 100 and 150°C. The yields of scission and crosslinking, G(S) and G(X), where determined from the changes in molecular weight distribution using gel permeation chromatography, (GPC) with supporting evidence from osmometry and viscometry. The ratio G(S)/G(X) increased from 0.02 at 30°C, obtained previously, to 2.8 at 150°C. This was mainly due to a tenfold increase in G(S), whereas G(X) apparently decreased slightly. These results are compatible with increased disproportion of chain scission radicals relative to their combination, analogous to the temperature dependence of mutual termination in the free radical polymerization of styrene. There was no obvious discontinuity through the glass transition temperature, although there may be a change in sign of the temperature coefficient of G(X). This system provides an excellent example of the applicability of measurements of molecular weight distributions and averages to determinations of G(S) and G(X) under conditions when gel measurements are inappropriate, either because of the failure of the system to form gel, or, as in the present case, because of the very large radiation doses required for gel formation.     

Salting-in of alcohols in aqueous solutions by tetraalkylammonium bromides at the freezing temperature

The freezing-point depression of the ternary systems tetraalkylammonium bromides-t-butanol-water for the first five homologs of R₄NBr was measured. In the case of Bu₄NBr, the effect of size of the alcohol (methahol ton-butanol) was also investigated. From the corresponding freezing-point data for the binary systems the apparent salting constants were calculated. The true salting constantsk _s were obtained by extrapolation to infinite dilution. These are all very close to zero at the freezing temperature. From the corresponding thermochemical data the temperature dependence ofk _s was calculated, and above 5°C all the R₄NBr salts int-butanol; the salting-in increases with temperature and with the size of the hydrophobic cations. The scaled-particle theory is at present the only one which can account semiquantitatively for the temperature dependence of the salting-in effect. On leave of absence from Chemistry Department, The University, Sheffield S3 7HF, England To whom correspondence should be addressed.     

Copper(II) coordination to ligands immobilized on photoluminescent porous silicon

Increased preorganization can be achieved by immobilizing ligands on solid supports. Photoluminescent porous silicon, which can undergo facile hydrosilylation, was used as a support for open chain neutral N- and O-donor ligands. The abilities of these ligands to bind the divalent metal ions Ni(2+), Cu(2+), Zn(2+), and Pb(2+) are examined. Immobilized ligands selectively complexed Cu(II) over the other metal ions studied. Ligands immobilized on photoluminescent porous silicon also removed a significant amount, up to 98%, of Cu(II) from copper(II)-spiked, organic-rich, seawater samples.     

Influence of structural changes on the ion selectivities of magnesium ionophores based on malonic acid diamides

In an investigation of octamethylene bis(malonic acid diamides) and their selectivities for magnesium, it was found that presence of secondary amides within the particular ionophore played a considerable role in the enhancement of magnesium selectivity. Similar effects in other ionophores, i.e. tris(malonic acid diamides), were thus systematically looked at with the help of selectivity measurements with the hope of optimizing the number of secondary and tertiary amides so as to improve the magnesium selectivity. The syntheses of several investigated tris(malonic acid diamide) isologues are equally reported.Deceased in November 1992     

Correlating axial and equatorial ligand field effects to the single-molecule magnet performances of a family of dysprosium bis-methanediide complexes

Treatment of the new methanediide–methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh₂S)₂}²⁻) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)₂][Dy(SCS)₂(K(DME)₂)₂] (2Dy), [Dy(SCS)₂][Na(DME)₃] (3Dy) and [Dy(SCS)₂][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)₂][K(DB18C6)(THF)(toluene)] (5Dy, NCN = {C(PPh₂NSiMe₃)₂}²⁻, DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy–5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (∼14 Oe s⁻¹). Fitting the alternating current magnetic susceptibility data for 2Dy–5Dy gives energy barriers to magnetic relaxation (U_eff) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy–4Dy join a privileged group of SMMs with U_eff values of ∼1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy–4Dy, the Dy-components can be grouped into 2Dy–cation/4Dy and 2Dy–anion/3Dy, where the former have almost linear C Dy C units with short average Dy C distances, and the latter have more bent C Dy C units with longer average Dy C bonds. Both U_eff and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M–L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between U_eff barrier and axial linearity in structurally comparable systems.

Studies on equatorial donor and C Dy C angle variation effects on energy barriers to the slow relaxation of magnetisation are reported.