Thermal degradation kinetics of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate)

The degradation kinetics of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate), a member of the Nodax family of polymers, were investigated using transient constant shear rate and dynamic time sweep rheological tests. The rate of chain scission at several times and temperatures was correlated with viscosity data and verified using molecular weight determination of the degraded samples. The experimental results show that the molecular weight and the viscosity of Nodax decrease with time over the range of temperatures that were studied (155–175°C). The degradation kinetics, which exhibited first‐order behavior, were determined as a function of the flow history and thermal history. An apparent activation energy of 189 ± 5 kJ/mol for thermal degradation was found by modeling variations in the rate with temperature using an Arrhenius law model. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 66–74, 2005     

Nonaqueous poly(methyl methacrylate) dispersions: radical dispersion polymerization in the presence of AB block copolymers of polystyrene and poly(dimethyl siloxane)

Well defined AB block copolymers of polystyrene (PS) and poly(dimethyl siloxane) (PDMS) have been prepared with PS molecular weights in the range 8 800 to 43 600 and PDMS molecular weights in the range 2 400 to 48 000. Provided the PS and PDMS molecular weights have a ratio within the range 0.5 to 4.0, these block copolymers stabilize particles of poly(methyl methacrylate) in n-alkanes. The particle size over the range 0.1 to 0.5 μm may be varied by performing dispersion polymerizations of methyl methacrylate as a function of monomer content of the seed stage and as a function of the concentration, molecular weight and composition of the block copolymer. From silicon analyses of the poly(methyl methacrylate) particles, values of the surface area stabilized per PDMS chain were established. The results indicate complete surface coverage of the particles.     

Comments on the use of buckminsterfullerene encapsulated in zeolite Y as a potential catalyst

Buckminsterfullerene encapsulated within zeolite Y is found not to be a reactive radical initiator for the formation of dimethoxyethane from dimethyl ether below 200° C. Above this temperature the expected acid catalysed conversion of dimethyl ether to hydrocarbons is observed. Dimethoxyethane formation is observed when bibenzoyl peroxide is used as a radical initiator. These studies indicate that any radical formed by the encapsulated buckminsterfullerene is relatively unreactive.     

The electrochemical reduction of VO2 in acidic solution at high overpotentials

An investigation has been carried out to attempt to understand the unusually low apparent symmetry factor observed during the reduction of V(5) at higher overpotentials at carbon electrodes (typically <0.13, or >460 mV decade⁻¹). This reaction is of interest because it occurs in vanadium redox-flow batteries (VRBs) during discharge. Polarisation curves were measured using a rotating disk electrode (RDE). The reaction was not solution mass transport controlled, was pH independent (ca from 0 to 1), and the observed Tafel slope was unaffected by V(5) concentration over a range from 0.031 to 280 mM. Electrode double layer capacitance measurements were also carried out in sulphuric acid with and without vanadium. These tests showed that the presence of V(5) caused a suppression of the normal carbon surface quinone pseudocapacitance, as well as the appearance of two new pseudocapacitance peaks, one around 0.175-0.2 V and the other around 0.675-0.725 V versus SCE. The observed results do not appear consistent with a precipitated film causing diffusion limitations or causing IR drop. A model is developed to try to explain the data, which involves electron transfer through an adsorbed layer of vanadium.     

Solvent free liquid phase oxidation of benzyl alcohol using Au supported catalysts prepared using a sol immobilization technique

Solvent free oxidation of benzyl alcohol was investigated in the absence of a base using Au catalysts prepared by sol immobilization on titania and carbon supports. Comparison between the Au supported catalysts revealed that activity and distribution of products was dependent on the nature of support and heat treatment. Specifically, heat pre-treatment of the Au catalysts has a beneficial effect in terms of activity, but is detrimental in terms of selectivity to the benzaldehyde. We conclude that sol immobilization is a suitable technique for preparing gold catalysts with small particle size and narrow particle size distributions and very high activity and selectivity for benzyl alcohol oxidation.     

Water as a Promoter of the Complete Oxidation of Volatile Organic Compounds over Uranium Oxide Catalysts

The effect of water addition on the complete oxidation of benzene and propane VOCs by uranium oxide catalysts has been investigated. Benzene oxidation was studied using a silica supported U₃O₈ catalyst. Complete oxidation is promoted by the addition of 2.6% water compared with the reactivity when no water is added to the reactant feed. Increasing the water concentration to 12.1% resulted in a suppression of oxidation activity. Investigation of propane oxidation using U₃O₈ shows a dramatic promotion of activity. Propane conversion was ca. 50% at 600 °C without added water, whilst it increased to 100% at 400 °C with the addition of 2.6% water. A comparison of oxidation activity has been made with Mn₂O₃, an oxide recognised for complete oxidation. In contrast to the U₃O₈ catalysts the addition of 2.6% water suppresses the activity of Mn₂O₃. In situ powder X-ray diffraction studies showed that the bulk U₃O₈ structure was stable under all the reaction conditions. The origin of the increased activity is not clear but may be due to modification of the catalyst surface and the contribution from new reaction pathways such as steam reforming.     

Direct synthesis of hydrogen peroxide from H2 and O2 using zeolite-supported Au catalysts

The direct synthesis of hydrogen peroxide from H₂ and O₂ using zeolite-supported Au catalysts is described and their activity is contrasted with silica- and alumina-supported Au catalysts. Two zeolites were investigated, ZSM-5 and zeolite Y. The effect of calcination of these catalysts is studied and it is found that for uncalcined catalysts high rates of hydrogen peroxide formation are observed, but these catalysts are unstable and lose Au during use. Consequently, reuse of these catalysts leads to lower rates of hydrogen peroxide formation. However, catalysts calcined at 400 °C are more stable and can be reused without loss of gold. The use of zeolites as a support for Au gives comparable rates of hydrogen peroxide formation to alumina-supported Au catalysts and higher rates when compared with silica-supported catalysts. prepared using a similar method. Zeolite Y-supported catalysts are more active than ZSM-5-supported catalysts for the stable calcined materials. It is considered that the overall activity of these supported catalysts may be related to the aluminium content as the activity increases with increasing aluminium content.     

Effects of Al:Si and (Al + Na):Si ratios on the properties of the international simple glass,part II: Structure

High-alumina containing high-level waste (HLW) will be vitrified at the Waste Treatment Plant at the Hanford Site. The resulting glasses, high in alumina, will have distinct composition-structure-property (C-S-P) relationships compared to previously studied HLW glasses. These C-S-P relationships determine the processability and product durability of glasses and therefore must be understood. The main purpose of this study is to understand the detailed structural changes caused by Al:Si and (Al + Na):Si substitutions in a simplified nuclear waste model glass (ISG, international simple glass) by combining experimental structural characterizations and molecular dynamics (MD) simulations. The structures of these two series of glasses were characterized by neutron total scattering and ²⁷Al, ²³Na, ²⁹Si, and ¹¹B solid-state nuclear magnetic resonance (NMR) spectroscopy. Additionally, MD simulations were used to generate atomistic structural models of the borosilicate glasses and simulation results were validated by the experimental structural data. Short-range (eg, bond distance, coordination number, etc) and medium-range (eg, oxygen speciation, network connectivity, polyhedral linkages) structural features of the borosilicate glasses were systematically investigated as a function of the degree of substitution. The results show that bond distance and coordination number of the cation-oxygen pairs are relatively insensitive to Al:Si and (Al + Na):Si substitutions with the exception of the B-O pair. Additionally, the Al:Si substitution results in an increase in tri-bridging oxygen species, whereas (Al + Na):Si substitution creates nonbridging oxygen species. Charge compensator preferences were found for Si-[NBO] (Na⁺), ^[3]B-[NBO] (Na⁺), ^[4]B (mostly Ca²⁺), ^[4]Al (nearly equally split Na⁺ and Ca²⁺), and ^[6]Zr (mostly Ca²⁺). The network former-BO-network former linkages preferences were also tabulated; Si-O-Al and Al-O-Al were preferred at the expense of lower Si-O-^[3]B and ^[3]B-O-^[3]B linkages. These results provide insights on the structural origins of property changes such as glass-transition temperature caused by the substitutions, providing a basis for future improvements of theoretical and computer simulation models.     

Impact damage detection in carbon fibre composites using HTS SQUIDs and neural networks

A neural network-based data analysis tool, developed to speed the damage detection process for the NDE of impact damaged carbon fibre composites, is discussed. A feature extraction method utilising a gradient threshold search function and a feed forward neural network for pattern recognition were used to develop the system. Impact damaged carbon composite sample plates were scanned with an eddy current-based NDE setup using HTS SQUID gradiometers and double-D excitation coils. Detection of damage sites in data affected by noise spikes caused by environmental disturbances is demonstrated. Finally, a possible design for a future entirely automated scanning system is also introduced.