Ulf W. Gedde

Crosslinked polydimethylsiloxane exposed to oxygen plasma studied by neutron reflectometry and other surface specific techniques

Abstract Spin-coated specimens of crosslinked polydimethylsiloxane (PDMS) exposed to radio-frequency (RF) and microwave (MW) oxygen plasma were studied by specular neutron reflectometry, X-ray photoelectron spectroscopy (XPS), Wilhelmy balance, contact angle measurements, scanning electron microscopy and atomic force microscopy. Neutron reflectometry and XPS showed that the oxygen plasma led to the formation of a smooth (

Hydrophobicity recovery of polydimethylsiloxane after exposure to corona discharges

Abstract A high-temperature-vulcanized polydimethylsiloxane (PDMS) elastomer has been subjected to corona discharges for different periods of time in dry air. The loss and recovery of hydrophobicity of the surface have been characterized by contact angle measurements. Immediately after exposure to corona discharges, samples showed a low surface hydrophobicity and, on storage in dry air, a continuous increase in hydrophobicity finally approaching the hydrophobicity of the unexposed material. The activation energy of the hydrophobicity recovery was two to four times greater than the activation energy of the diffusivity of low molar mass PDMS in PDMS elastomers, indicating that the diffusivity properties of the oxidized surface layer were different from that of the bulk. PDMS elastomers quenched in liquid nitrogen or subjected to small mechanical deformation (

Hydrophobicity changes in silicone rubbers

Water repellency, high surface resistivity, vandalism resistance, low density and good processability have made silicone rubbers based on polydimethylsiloxane (PDMS) very attractive materials in housings for outdoor insulation. Their ability to recover hydrophobicity after oxida. tion or contamination is of paramount importance and this is the topic of this review. A critical evaluation of the chemical and physical mechanisms responsible for hydrophobicity loss and recovery is presented.

Diffusion of small-molecule penetrants in polyethylene: free volume and morphology

Based on desorption and permeation measurements, the diffusivity and solubility of n-hexane and oxygen have been obtained for a wide range of linear and branched polyethylenes (PEs) with crystallinities between 40 and 97% and mass-average molar masses between 103 and 106 g mol−1. The morphology and contents of crystal core (CC), crystal-core-like interfacial (ICC), liquid-like interfacial (IL) and liquid (L) components were assessed by transmission electron microscopy, Raman spectroscopy, 13C cross-polarization/magic-angle spinning nuclear magnetic resonance spectroscopy, differential scanning calorimetry, density measurements and small-angle light scattering. The penetrant solubility in the non-crystalline phases increased with increasing concentration of chain ends and chain branches. This effect was masked at certain crystallinities by the constraining effect of the crystallites. The diffusivity selectivity of oxygen over n-hexane increased strongly with increasing crystallinity and decreasing non-crystalline layer thickness, demonstrating that the crystal-induced constraint on the non-crystalline chains more efficiently retards the diffusion of larger molecules. The fractional free volume of the non-crystalline components decreased strongly with increasing crystallinity in the low-crystallinity range (<60%), above which it remained practically constant. The latter is because the constraining effect of the crystals is compensated for by the plasticizing effect of the chain ends, which leads to a constant free volume in this crystallinity range. A model, based on the Cohen-Turnbull-Fujita (CTF) model, considering the polymers to consist of four components, CC, ICC, IL and L, was applied to the diffusivity data. The branched PEs and the majority of the linear PEs could be described by the modified CTF model. However, the lowest-molar-mass linear PEs exhibited a considerably larger interfacial free volume than the other samples. Real-time Raman spectroscopy on CCl4-swollen samples showed that the changes in the CC and ICC contents during sorption were only small.

Hydrophobic recovery of polydimethylsiloxane after exposure to partial discharges as a function of crosslink density

Polydimethylsiloxanes with different crosslink densities were exposed to corona discharges or GHz air plasma and studied by contact angle measurements, X-ray photoelectron spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. The degree of surface oxidation increased with increasing exposure time with a limiting depth of 100-150 nm. Surface oxidation was faster in more highly crosslinked polymers. Within the oxidised layer, a brittle, microporous silica-like layer with a minimum organic silicone content of 40% gradually developed with increasing exposure time. The strain at which the silica-like layer cracked decreased with increasing dose of corona or air plasma. The hydrophobic recovery following the corona/air plasma exposure occurred at a slow rate by diffusion of oligomers through the microporous but uncracked silica-like layer or at a much higher rate by transport of oligomers through cracks in the silica-like layer.

Aging of silicone rubber under ac or dc voltages in a coastal environment

Material samples of silicone rubber with known differences in their composition, i.e. different filler content and extra silicone oil added, have been aged at the Anneberg field station on the west coast of Sweden. ac or dc voltage supplied to cylindrical samples at stress levels of 50 or 100 V/mm. The work includes laboratory examination of material changes together with on-site, visual observations and leakage current measurements. Material samplings for the laboratory tests were made after 18 months of electrical aging, which went on for one more year in order to gather further information on the long-term electrical performance of the material. The dominant aging factor was the level of the applied stress, independent of ac or dc voltage. The dc stressed samples showed a higher leakage current and exhibited larger surface degradation compared with samples exposed to ac voltage. The material parameter, an addition of extra silicone oil, initially led to an increase in adhesion of pollutants, whereas the overall performance was improved by the higher suppression of the leakage current related to oligomer diffusion. Samples with lower levels of alumina trihydrate (ATH) exhibited a delayed onset of degradation, but once damaged they degraded more rapidly than the specimens with a higher ATH content. Infrared spectroscopy showed that the ATH was completely consumed at the eroded surface regions. The aging of the surfaces was further assessed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The low molar mass siloxanes present in the pollution layer were extracted and analyzed by size exclusion chromatography and gas chromatography-mass spectroscopy. The results indicated that the main degradation factor was thermal depolymerization activated by electrical discharges. Oxidative crosslinking of the silicone rubber, usually attributed to surface close corona discharges, appeared to have played a minor role.

Transport properties of chitosan and whey blended with poly(ε-caprolactone) assessed by standard permeability measurements and microcalorimetry

Blends of poly(p-caprolactone) (PCL) with chitosan and a whey-protein-isolate (WPI) were prepared by solution mixing and film casting. The purpose was to increase the water vapour resistivity of ch ...

Relaxation processes in hyperbranched polyesters: Influence of terminal groups

Abstract Three hyperbranched polyesters with the same backbone structure but with different terminal groups: hydroxyl, benzoate or acetate groups, were studied by dielectric spectroscopy, differential scanning calorimetry and dynamic mechanical analysis. The benzoate- and acetate-terminated polymers exhibited only one subglass process (β), originating from reorientation of the ester groups, distinct from the glass transition (α). The hydroxyl-terminated sample exhibited a low-temperature subglass process (γ), due to motions of the hydroxyl groups, in addition to the β and α processes. The relaxation strengths of the hyperbranched polymers were found to be considerably lower than for those of linear analogues. The activation energies of the β process in the polymers studied increased in the order of hydroxyl, acetate and benzoate, indicating that the benzoate-terminated polymer is the most constrained.

DC breakdown strength of polypropylene films: area dependence and statistical behavior

The DC breakdown strengths of 4-15 mum thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 cm2. Some samples were studied with larger electrodes. Samples were not aged but were measured as received. The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters alpha and beta were fitted to the data. Depending on the electrode area and the film grade, the alpha-parameter values obtained were between 450 and 850 V/mum. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrode areas. The alpha-values for the large sample areas were extrapolated from the small area data by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 m2 by 40-50% whereas the Weibull extrapolation gave an accuracy of plusmn15%. The extrapolated breakdown strengths for the full-scale power capacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples

Parameters affecting the determination of transport kinetics data in highly swelling polymers above Tg

Abstract Sorption and desorption data for n-hexane–natural rubber and n-hexane–low-density polyethylene were analysed to reveal the cause of the s-shaped sorption curves frequently occurring in highly swollen polymers. The model permitted the influence of solute-concentration-dependent diffusivity, sample geometry, boundary concentrations and swelling-induced mechanical stresses on the transport data to be examined. The calculated solute diffusivity varied by several orders of magnitude, depending on the choice of parameters included in the model. The inclusion of direct mechanical stress relaxation parameters only gave a slight improvement of the fit to the experimental data. The inclusion of a time-dependent surface concentration was the only way to fit the s-shaped sorption curves for both natural rubber and low-density polyethylene. Although isotropic three-dimensional swelling of natural rubber occurred over the whole sorption transient period, this condition was unable to explain the swelling (thickness increase) of low-density polyethylene. In the latter system, a model consisting of two stages had to be adopted: stage I where the swelling was mainly one-dimensional, and stage II which occurred later and was characterized by three-dimensional swelling similar to that occurring in natural rubber. During the transient sorption period, the ratio between natural rubber and low-density polyethylene of the ratio of the thickness to cross-sectional area was close to their bulk modulus ratio, which suggests that it is the bulk modulus rather than the Youngs modulus which determines the sorption characteristics of polymers above T g .