Mogon Patel

The effects of γ-radiation on the thermal, mechanical, and segmental dynamics of a silica filled, room temperature vulcanized polysiloxane rubber

Abstract RTV-5370 is a filled polydimethylsilxoane–polymethylphenylsiloxane copolymer system originally developed by Dow Corning and now supplied by Rhone Poulenc. There is a desire to develop lifetime assessments of this material for certification programs and as a model system for understanding other filled siloxane polymer systems. We have initiated aging studies on these materials by employing accelerated aging tests with exposure to Co-60 γ-radiation. The effects of radiation exposure have been investigated by thermal (differential scanning calorimetry), mechanical (dynamic mechanical analysis), chemical (solvent swelling), and segmental dynamics (nuclear magnetic resonance) methods. The data show that RTV-5370 undergoes predominately radiation-induced crosslinking reactions over the dose range investigated. These reactions reduce the molecular weight between crosslinks, thereby hardening the composite and reducing the motional dynamics of segmental motion. DSC studies show dose dependent crystallization phenomena.

Accelerated thermal ageing studies on nitrile rubber O-rings

Abstract Poly(acrylonitrile-butadiene) rubber (NBR) O-rings have been aged in air whilst under 12.5% compression at temperatures up to 110 °C. The compression set properties together with oxygen uptake of the O-rings under the applied load conditions were measured and results analysed using time-temperature superposition and Arrhenius kinetics. The compression set results suggest a single degradation process with an activation energy of 81.03 kJmol−1. This compares well with the activation energy of 74.3 kJmol−1 obtained from oxygen consumption. Results from solvent swell and oxygen uptake tests show that the main influence on ageing of the rubber is oxidative cross-linking, leading to the material becoming hard and brittle. The approach presented here predicts that the rubber will reach full compression set by 40 (+54/−7) years based on 95% confidence bands.

Impact of thermal ageing on the tin catalyst species in room temperature vulcanised polysiloxane rubbers

Tin 2-ethylhexanoate is used as a catalyst in the synthesis of room temperature cured polysiloxane rubbers. The catalyst has the potential to influence the important load bearing and ageing properties of the rubber. In this study, the structure of the catalyst and the nature of the tin species in both fresh and thermally aged polysiloxane samples (RTV5370) have been investigated. Infra-red and 13C NMR of the catalyst confirms the presence of 2-ethyl hexanoate ligands on the tin. Mossbauer Spectroscopy of the catalyst shows that the tin exists predominantly in the tin(II) oxidation state with some residual tin(IV) impurities. The tin(IV) phase predominates in the as-synthesised rubber, representing some 67% of the total tin signal, with the remainder being unreacted tin(II) species. The tin(IV) phase in the catalyst and that in the rubber are different. Overall, our observations suggest that the catalyst has taken part in the cure reaction and has been chemically modified as a result. The isomer shift (IS) and the quadrupole splitting (QS) from Mossbauer Spectroscopy suggests that the tin (IV) phase in the rubber is most probably SnO2 (stannic oxide). Toluene extraction studies show that this phase is not extractable from the rubber network, whereas the tin(II) species is easily removed. Both high temperature and room temperature ageing increase the proportion of the tin(IV) in the rubber.

Viscoelastic properties of polystyrene using dynamic rheometry

Abstract Dynamic mechanical thermal analysis (DMTA) of polystyrene samples of different molecular weights has been investigated using a shear-sandwich unit. Information on the viscoelastic properties of such materials at melt temperatures is of interest as this can lead to an improved understanding of polymer behaviour in processing and fabrication technologies. Samples were subjected to a strain which varied in a sinusoidal manner with time and the resultant response of the sample in terms of storage shear modulus (G′) and loss shear modulus (G″) was investigated (at a number of different temperatures) with increasing frequency of the applied strain. Results were assessed using the time–temperature superposition principal. Master curves show four characteristic features: a terminal zone at low frequencies, a fluid–elastic transition zone, a rubbery zone and a glassy zone at high frequencies. These features are indicative of different stress relaxation processes. The average molecular weight between entanglements (Me) was determined from the storage modulus associated with the rubbery zone. This was found to be approximately half the value of Mc (the critical average molecular weight for entanglement). The effects of molecular weight were clearly seen in the terminal zone where the polymer behaves analogous to a Newtonian fluid. The non-Newtonian behaviour of the material was reflected by a sharp increase in G′ and a decrease in the dynamic viscosity (Y′) with frequency. Acceleration factors derived from time–temperature superposition showed good adherence to the Arrhenius relationship with an activation energy for viscous flow found to be essentially independent of polymer molecular weight suggesting the unit of flow is a collection of chain segments rather than the whole chain itself.

The stability of polysiloxanes incorporating nano-scale physical property modifiers

Abstract Reported here is the synthesis and subsequent characterization of the physical and chemical properties of novel polysiloxane elastomers modified with a series of polyhedraloligomericsilsequioxane (POSS) molecular silicas. The physical properties of the formulated nanocomposite systems have been characterized with a combination of dynamic mechanical analysis (DMA), broadband dielectric spectroscopy (BDS) and confocal Raman microscopy. The results of the physical property characterization demonstrate that the incorporation of low levels (1–4% by wt.) of POSS particles into the polysiloxane network leads to significant improvements in the mechanical properties of the elastomer and significantly alters the motional chain dynamics of the system as a whole. The results of studies performed to assess the long-term stability of these novel nanocomposite systems have demonstrated that POSS physical property modifiers can significantly alter the thermal stability of polysiloxane elastomers. Physically dispersed POSS has also been shown in some cases to be both mobile and disruptive within the polysiloxane networks, agglomerating into domains on a micron scale and migrating to the surface of the elastomers. This work demonstrates both the potential of POSS nanoparticles as physical property modifiers and describes the effects of POSS on the physical and chemical stability of polysiloxane systems.

Synthesis and characterisation of pyrene-labelled polydimethylsiloxane networks: towards the in situ detection of strain in silicone elastomers

Pyrene-substituted polyhydromethylsiloxanes (PHMS-Pyx) were synthesised by the hydrosilylation reaction of prop-3-enyloxymethylpyrene with polyhydromethylsiloxane (Mn = 3700). The ratio of pyrene substituent to Si–H unit was varied to afford a range of pyrene-functionalised polysiloxanes. These copolymers were subsequently incorporated into polydimethylsiloxane (PDMS) elastomers by curingvia either Pt(0) catalysed hydrosilylation with divinyl-terminated PDMS (Mn = 186) and tetrakis(dimethylsiloxy)silane, or Sn(II) catalysed condensation with α,ω-dihydroxyPDMS (Mn = 26 000) and tetraethoxysilane. An alternative method involving the synthesis and integration of [3-(pyren-1-ylmethoxy)propyl]triethoxysilane (Py-TEOS) into PDMS elastomers was also investigated: a mixture of α,ω-dihydroxyPDMS (Mn = 26 000), tetraethoxysilane, and Py-TEOS was cured using an Sn(II) catalyst. Certain of the resulting fluorescent pyrene-labelled elastomers were studied by differential scanning calorimetry and dynamic mechanical analysis. No significant changes were observed in the thermal or mechanical properties of the elastomers containing pyrene when compared to otherwise identical samples not containing pyrene. All of the pyrene-containing elastomers were demonstrated to be fluorescent under suitable excitation in a photoluminescent spectrometer. Two of the elastomers were placed in a photoluminescence spectrometer and subjected to cycles of extension and relaxation (strain = 0–16.7%) while changes in the emission spectra were monitored. The resulting spectra of the elastomer containing the PHMS-Py50 copolymers were variable and inconsistent. However, the emission peaks of elastomers containing Py-TEOS displayed clear and reproducible changes in fluorescence intensity upon stretching and relaxation. The intensity of the monomer and excimer emission peaks was observed to increase with elongation of the sample and decrease upon relaxation. Furthermore, the ratio of the intensities of the excimer : monomer peak decreased with elongation and increased with relaxation. In neither case was there appreciable hysteresis, suggesting that fluorescent labelling of elastomers is a valid approach for the non-invasive in situ monitoring of stress and strain in such materials.

Volatile evolution from room temperature cured polysiloxane rubber induced by irradiation with He2+ ions

Abstract A room temperature cured polysiloxane rubber (Rhodorsil RTV5370) has been irradiated using an accelerated beam of He 2+ ions. Such an irradiation simulates the effects of large alpha radiation doses. A mass spectrometer linked directly to the sample chamber allowed the analysis of volatile species evolved as a result of exposure. The polydimethylsiloxane rubber showed high resistance to radiation damage when exposed to He 2+ ions at doses up to 3.5 MGy. At higher doses, the cyclic hexamethylcyclotrisiloxane was evolved and is indicative of damage to the main chain. The cyclic octamethylcyclotetrasiloxane was only observed at very high doses (10 MGy and above) and is indicative of significant head-to-tail unzipping reactions. Methane, benzene and carbon dioxide were the three main gases evolved. The ratio of phenyl (2%) to methyl groups (93%) within RTV5370 rubber is very small but the amount of benzene evolved was found to be significant. The results suggest that the aromatic groups within the rubber appear to have a much greater susceptibility to radiation induced volatile evolution than other groups making up the polymer structure. In addition, the depletion of phenyl groups (from the reduction in evolution of benzene) from the irradiated zone corresponds to an increase in damage of the siloxane linkages (from the increased evolution of octamethylcyclotetrasiloxane) suggesting the aromatic groups offer a protective action from the incident radiation.

Shelf Life Assessment of Poly(ethylene-co-vinyl acetate) and Polyester Polyol Resins Used as Adhesives

The thermal stability and ageing properties of Vinamul 3161 poly(ethylene-co-vinyl acetate) and AS1160 polyester polyol resins have been investigated in support of shelf life assessment and also to identify storage conditions that may extend product life. These resins are typically used in the production of adhesives for specialised applications either as binders for filler particles or to minimise the relative movement of materials in multi-material assemblies. Our studies confirm that both these resins are susceptible to moisture and hydrolysis chemistry which potentially limits shelf life. The EVA resin readily accumulates acetic acid through hydrolysis of the pendent acetate groups which increases both the acidity (pH) and volatile outgassing characteristics of the material. The temperature sensitivity of pH combined with Arrhenius kinetics was used to identify a useful shelf life for EVA in conditions representative of normal storage conditions. In a separate set of experiments, relatively short-term thermally accelerated ageing studies have been carried out on AS1160 polyester polyol to investigate sensitivity to humidity, temperature and open/close ageing conditions. This material is hygroscopic, readily accumulates moisture and is susceptible to chain scission with molecular weight changes linked to the hydrolysis-esterification equilibrium. These changes do not however adversely impact adhesive bond strength allowing the resin to be potentially used significantly beyond the manufacturer recommended shelf life limit.