Peter E. Mallon

Hydrophobicity recovery of corona-modified superhydrophobic surfaces produced by the electrospinning of poly(methyl methacrylate)- graft-poly(dimethylsiloxane) hybrid copolymers*

Superhydrophobicity is dependent on both the surface energy and the texture of the surface. These factors are discussed in terms of a series of electrospun poly(methyl methacrylate)-graft-poly(dimethylsiloxane) (PMMA-g-PDMS) copolymers with different poly(dimethylsiloxane) (PDMS) content. These copolymers are synthesized via conventional free radical copolymerization of methyl methacrylate (MMA) and monomethacryloxypropyl-terminated PDMS macromonomers. It is shown how these copolymers can be electrospun to produce copolymer fibers with diameters in the 100-1000 nm range. The effect of the copolymer composition (and hence the surface energy) and the electrospinning tip-to-collector distance (TCD) on the fiber morphology is discussed. The surfaces produced by the electrospinning process show superhydrophobic properties where the preferential surface segregation of the PDMS component is combined with the roughness of the fiber surface. The surface energy of the fibers is varied by variation of the PDMS content in the copolymers as well as by post-spinning modification with corona discharge. The hydrophobicity of the surfaces shows a greater dependence on the PDMS content than on the average fiber diameter. After exposure of these fiber surfaces to corona discharge, the initial superhydrophobic surfaces become easily wettable despite the fact that much of the surface roughness is maintained after exposure. The samples show the phenomena of hydrophobocity recovery after corona exposure. The rate and extent of this recovery depends on the PDMS content and the corona exposure time. Despite the recovery, scanning electron microscopy (SEM), swelling measurements, and confocal Raman spectroscopy show that permanent surface changes have taken place. The surfaces do not recover to their original superhydrophobic state.

Positron annihilation spectroscopy study of high-voltage polydimethylsiloxane (PDMS) insulators

Abstract Polydimethylsiloxane (PDMS) compounds are used in outdoor high-voltage applications. Pure PDMS and compounds containing silica and aluminum trihydrate are studied using a mono-energetic positron beam. Large changes in the S parameter profiles as a function of depth from the surface are observed after samples are exposed to corona discharge. In addition, there are significant changes in the profiles on increasing relaxation time after corona treatment. Evidence is found for a silica-like layer at the surface after corona treatment. It is shown that the positron beam technique can provide important information on the mechanism of hydrophobicity loss and recovery that the samples show after corona treatment.

Positron studies of polymeric coatings

Abstract In complicated coating systems, positrons have shown sensitivity in detecting the early stage of deterioration due to weathering, specially, in probing a specific location or depth of coatings from the surface through interfaces and the bulk. Existing extensive experimental positron data show that positron annihilation signals respond quantitatively to the deterioration process due to weathering. Now it is possible to detect the very early stage of coating deterioration at the atomic and molecular scale by using positrons, typically in days as compared to years by conventional methods. This paper summarizes recent positron studies in polymeric coatings. Correlations between positron data and a variety of chemical, physical and engineering data from ESR, AFM, cross-link density, gloss, and cyclic loading are presented.

Micromechanics of ultra-toughened electrospun PMMA/PEO fibres as revealed by in-situ tensile testing in an electron microscope

A missing cornerstone in the development of tough micro/nano fibre systems is an understanding of the fibre failure mechanisms, which stems from the limitation in observing the fracture of objects with dimensions one hundredth of the width of a hair strand. Tensile testing in the electron microscope is herein adopted to reveal the fracture behaviour of a novel type of toughened electrospun poly(methyl methacrylate)/poly(ethylene oxide) fibre mats for biomedical applications. These fibres showed a toughness more than two orders of magnitude greater than that of pristine PMMA fibres. The in-situ microscopy revealed that the toughness were not only dependent on the initial molecular alignment after spinning, but also on the polymer formulation that could promote further molecular orientation during the formation of micro/nano-necking. The true fibre strength was greater than 150 MPa, which was considerably higher than that of the unmodified PMMA (17 MPa). This necking phenomenon was prohibited by high aspect ratio cellulose nanocrystal fillers in the ultra–tough fibres, leading to a decrease in toughness by more than one order of magnitude. The reported necking mechanism may have broad implications also within more traditional melt–spinning research.

Antibacterial Properties of Tough and Strong Electrospun PMMA/PEO Fiber Mats Filled with Lanasol—A Naturally Occurring Brominated Substance

A new type of antimicrobial, biocompatible and toughness enhanced ultra-thin fiber mats for biomedical applications is presented. The tough and porous fiber mats were obtained by electrospinning solution-blended poly (methyl methacrylate) (PMMA) and polyethylene oxide (PEO), filled with up to 25 wt % of Lanasol—a naturally occurring brominated cyclic compound that can be extracted from red sea algae. Antibacterial effectiveness was tested following the industrial Standard JIS L 1902 and under agitated medium (ASTM E2149). Even at the lowest concentrations of Lanasol, 4 wt %, a significant bactericidal effect was seen with a 4-log (99.99%) reduction in bacterial viability against S. aureus, which is one of the leading causes of hospital-acquired (nosocomial) infections in the world. The mechanical fiber toughness was insignificantly altered up to the maximum Lanasol concentration tested, and was for all fiber mats orders of magnitudes higher than electrospun fibers based on solely PMMA. This antimicrobial fiber system, relying on a dissolved antimicrobial agent (demonstrated by X-ray diffraction and Infrared (IR)-spectroscopy) rather than a dispersed and “mixed-in” solid antibacterial particle phase, presents a new concept which opens the door to tougher, stronger and more ductile antimicrobial fibers.