There is a need for chemical sensors for monitoring volatile organic compounds (VOCs) in air. Acute and chronic inhalation of toxic VOCs can cause adverse health effects in humans, so monitoring these analytes is important for ensuring that their concentrations are maintained below maximum permissible levels. Chemical sensors using polydimethylsiloxane (PDMS) to extract VOCs with partial selectivity, coupled with label-free optical detection methods based on refractive index, can overcome the limitations of conventional VOC detection methods. A variety of tunable and high refractive index PDMS materials were developed by incorporating a range of titanium and zirconium concentrations (2.5 – 30 mol % and 2.5 – 15 mol %, respectively) using a simple sol-gel synthesis and by incorporating a range of titanium concentrations (2.5 – 10 mol %) into naphthyl-functionalized PDMS. These materials ranged in refractive index from 1.4023 ± 0.0002 to 1.5663 ± 0.0001 at 635 nm and 1.3942 ± 0.0003 to 1.5510 ± 0.0007 at 1550 nm. The ability to use tunable refractive index PDMS films to differentiate between m-xylene and cyclohexane was demonstrated by monitoring changes in refractive index and thickness following absorption of these analytes using a refractometer at 1550 nm. The sensitivity of the refractive index response to an analyte using a particular PDMS film was dependent upon the difference between the refractive index of the analyte and film, as well as the film-air partition coefficient of the analyte. The detection limits for m-xylene and cyclohexane were 81 ppm and 4940 ppm, respectively, using PDMS-titanium-oxo nanocomposites with 5 and 10 mol % Ti, respectively. A simple planar waveguide sensor with an input grating coupler was developed to monitor changes in refractive index of the cladding through shifts in peak resonance wavelength. Using high refractive index PDMS materials as the waveguide core, we monitored changes in refractive index arising from absorption of VOCs into the grating. Here, the sensitivity of the waveguide response was dependent upon the difference in refractive index of the analyte and polymer, as well as the film-air partition coefficient of the analyte. The detection limits for m-xylene and cyclohexane were 1980 ppm and 18000 ppm, respectively. / Thesis (Master, Chemistry) -- Queen's University, 2013-08-24 11:45:57.642

Silicone rubber based on polydimethylsiloxane is used ashigh voltage outdoor insulation, due to its ability to preservethe hydrophobic surface properties during service and evenregain hydrophobicity after exposure to electrical discharges.The underlying processes for the hydrophobic recovery arediffusion of low molar mass siloxanes from the bulk to thesurface and reorientation by conformational changes ofmolecules in the surface region. Only little is known of whichfactors are responsible for the long-term stability of thishydrophobic recovery. It is therefore important to increase theknowledge about the fundamental mechanisms for the loss andrecovery of hydrophobicity of silicone rubbers, exposed toelectrical discharges. Addition-cured polydimethylsiloxanenetworks, with known crosslink densities, were exposed tocorona discharges and air/oxygen-plasma and the loss andrecovery of hydrophobicity was characterised by contact anglemeasurements. The degree of surface oxidation increased withincreasing exposure time with a limiting depth of 100- 150 nm,as assessed by neutron reflectivity measurements. The oxidationrate increased with increasing crosslink density of the polymernetwork, according to X-ray photoelectron spectroscopy. Withinthe oxidised layer, a brittle, silica-like layer was graduallydeveloped with increasing exposure time. The hydrophobicrecovery following the corona or air/oxygen- plasma exposuresoccurred at a slow pace by diffusion of cyclic oligomericdimethylsiloxanes through the micro-porous but uncrackedsilica-like surface layer or at a much higher pace by transportof the oligomers through cracks in the silica-like layer. Theoligomers were present in the bulk, but additional amounts wereformed during exposure to corona discharges. High-temperaturevulcanised silicone rubber specimens were aged in a coastalenvironment under high electrical stress levels (100 V/mm). Thechanges in surface structure and properties were compared tothe data obtained from specimens exposed to coronadischarges/plasma. The dominating degradation mechanism wasthermal depolymerisation, initiated by hot discharges. Thisresulted in the formation of mobile siloxanes, of which the lowmolar mass fraction consisted of cyclic oligomericdimethylsiloxanes. Oxidative crosslinking resulting insilica-like surface layers was not observed during theseconditions. <b>Keywords:</b>silicone rubber, polydimethylsiloxane,hydrophobicity, corona, air-plasma, oxygen-plasma, surfacecharacterisation, degradation products, crosslink density.

<p>Silicone rubber based on polydimethylsiloxane is used ashigh voltage outdoor insulation, due to its ability to preservethe hydrophobic surface properties during service and evenregain hydrophobicity after exposure to electrical discharges.The underlying processes for the hydrophobic recovery arediffusion of low molar mass siloxanes from the bulk to thesurface and reorientation by conformational changes ofmolecules in the surface region. Only little is known of whichfactors are responsible for the long-term stability of thishydrophobic recovery. It is therefore important to increase theknowledge about the fundamental mechanisms for the loss andrecovery of hydrophobicity of silicone rubbers, exposed toelectrical discharges. Addition-cured polydimethylsiloxanenetworks, with known crosslink densities, were exposed tocorona discharges and air/oxygen-plasma and the loss andrecovery of hydrophobicity was characterised by contact anglemeasurements. The degree of surface oxidation increased withincreasing exposure time with a limiting depth of 100- 150 nm,as assessed by neutron reflectivity measurements. The oxidationrate increased with increasing crosslink density of the polymernetwork, according to X-ray photoelectron spectroscopy. Withinthe oxidised layer, a brittle, silica-like layer was graduallydeveloped with increasing exposure time. The hydrophobicrecovery following the corona or air/oxygen- plasma exposuresoccurred at a slow pace by diffusion of cyclic oligomericdimethylsiloxanes through the micro-porous but uncrackedsilica-like surface layer or at a much higher pace by transportof the oligomers through cracks in the silica-like layer. Theoligomers were present in the bulk, but additional amounts wereformed during exposure to corona discharges. High-temperaturevulcanised silicone rubber specimens were aged in a coastalenvironment under high electrical stress levels (100 V/mm). Thechanges in surface structure and properties were compared tothe data obtained from specimens exposed to coronadischarges/plasma. The dominating degradation mechanism wasthermal depolymerisation, initiated by hot discharges. Thisresulted in the formation of mobile siloxanes, of which the lowmolar mass fraction consisted of cyclic oligomericdimethylsiloxanes. Oxidative crosslinking resulting insilica-like surface layers was not observed during theseconditions.</p><p><b>Keywords:</b>silicone rubber, polydimethylsiloxane,hydrophobicity, corona, air-plasma, oxygen-plasma, surfacecharacterisation, degradation products, crosslink density.</p>