Emulsion polymerization: State of the art in kinetics and mechanisms

Over decades of carefully designed kinetic experiments and the development of complementary theory, a more or less complete picture of the mechanisms that govern emulsion polymerization systems has been established. This required means of determining the rate coefficients for the individual processes as functions of controllable variables such as initiator concentration and particle size, means of interpreting the data with a minimum of model-based assumptions, and the need to perform experiments that had the potential to actually refute a given mechanistic hypothesis. Significant advances have been made within the area of understanding interfacial processes such as radical entry and exit into and out of an emulsion polymerization particle, for electrostatic, steric and electrosteric stabilizers (the latter two being poorly understood until recently). The mechanism for radical exit is chain transfer to monomer within the particle interior to form a monomeric radical which can either diffuse into the water phase or propagate to form a more hydrophobic species which cannot exit. Entry is through aqueous-phase propagation of a radical derived directly from initiator, until a critical degree of polymerization z is reached; the value of z is such that the z-meric species is sufficiently surface active so that its only fate is to enter, whereas smaller aqueous-phase radical species can either be terminated in the aqueous phase or undergo further propagation. For both entry and exit, in the presence of (electro)steric stabilizers, two additional events are significant: transfer involving a labile hydrogen atom within the stabilizing layer to form a mid-chain radical which is slow to propagate and quick to terminate, and which may also undergo β-scission to form a water-soluble species. Proper consideration of the fates of the various aqueous-phase radicals is essential for understanding the overall kinetic behaviour. Intra-particle termination is explained in terms of diffusion-controlled chain-length-dependent events. A knowledge of the events controlling entry and exit, including the recent discoveries of the additional mechanisms operating with (electro)steric stabilizers, provides an extension to the micellar and homogeneous nucleation models which enables particle number to be predicted with acceptable reliability, and also quantifies the amount of secondary nucleation occurring during seeded growth. This knowledge provides tools to understand the kinetics of emulsion polymerization, in both conventional and controlled/living polymerization systems, and to optimize reaction conditions to synthesize better polymer products.     

Effect of Microstructure on the Radioluminescence and Transparency of Ce-Doped Strontium Hafnate Ceramics

In this paper we report on the fabrication and characterization of SrHfO(3):Ce ceramics. Powders were prepared by solid-state synthesis using metal oxides and carbonates. X-ray diffraction measurements showed that phase-pure SrHfO(3) is formed at 1200°C. Inductively coupled plasma spectroscopy confirmed the purity and composition of each batch. SrHfO(3) exhibits several phase changes in the solid, but this does not appear to be detrimental to the ceramics. Microprobe experiments showed uniform elemental grain composition, whereas aluminum added as charge compensation for trivalent cerium congregated at grain boundaries and triple points. Radioluminescence spectra revealed that the light yield decreases when the concentration of excess Sr increases. The decrease in the light yield may be related to the change of Ce(3+) into Ce(4+) ions. For stoichiometric SrHfO(3):Ce, the light yield is about four times that of bismuth germanate (BGO), the conventional benchmark, indicating great potential for many scintillator applications.     

Children with recently diagnosed diabetes: Interactions within their families.

Analyzed how families with a diabetic child interact and how these patterns differ from other families, using 57 diabetic and 48 acutely ill early adolescents and preadolescents from 2-parent families. The 2 samples were contrasted in terms of enabling and constraining interactions of each family member. Findings reveal that diabetic Ss and their parents expressed significantly more enabling (e.g., focusing, problem-solving, active understanding) speeches than comparable members of the acute illness group. There were indications of particular constraining interactions (devaluing) occurring between fathers and diabetic Ss. Several alternative interpretations are offered to account for these results. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of attendance, note-taking, and review on memory for a lecture: Encoding vs. external storage functions of notes.

Assessed the importance of note taking (an encoding function) and note reviewing (an external storage function) on memory for a lecture. One week following various conditions of film-viewing, note taking, and review of notes, 144 psychology students (aged 17–19 yrs) completed a quiz based on the contents of a videotaped lecture dealing with an unfamiliar but relevant topic. Ss who reviewed a thorough set of lecturer notes prior to the test performed better than those who had no notes to review, irrespective of whether they had taken personal notes, simply viewed and listened, or had attended the film at all. The latter 3 activities had no significant effect on test scores. Ss who reviewed their own notes performed at an intermediate level, scoring generally lower than those who reviewed lecturer notes but higher than those who did not review. Results are interpreted as support for an external storage rather than an encoding function of note taking. (French abstract) (24 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Monte carlo approach to estimating the photodegradation of polymer coatings

The degradation of a polymer coating and predicting the coating lifetime based on physical properties and distribution within the coating of the polymer binder, pigments, and fillers are economically very important. As technologies advance and allow for control of coatings at the nanoscale level, methods such as Monte Carlo can be used not only to predict the behavior of a nanodesigned coating with time but also to design coatings, such as optimizing pigment particle distributions or optimum hard and soft phase distributions of the binders in multiphase systems for maintaining the desired property with time. Erosion of the coating surface was simulated using Monte Carlo techniques where terrestrial solar flux is the initiator for polymer segment cleavage and removal. The impact on the sensitivity of the polymer adjacent to the detached polymer segment can be increased or decreased in the model based on the chemistry and surface energy of the remaining polymer matrix. Multiple phases with varying sensitivity to degradation can be modeled. The Monte Carlo generates a statistically similar surface topography and chemistry of the coating. The results of the Monte Carlo model are compared to measurable properties such as gloss, fracture toughness, and wetting contact angle, using various published correlations of the property to the surface topology. The simulated properties change through the life-time of the coating in ways that are consistent with observed behavior. Apparently, complicated changes in many properties can be described by the repeated application of simple, random processes. Presented at the 79th Annual Meeting of the Federation of Societies for Coatings Technology, October 28–November 1, 2002, in New Orleans, LA.     

The activity and mechanism of uranium oxide catalysts for the oxidative destruction of volatile organic compounds

Uranium oxide based catalysts have been investigated for the oxidative destruction of volatile organic compounds (VOCs) to carbon oxides and water. The catalysts have been tested for the destruction of a range of organic compounds at space velocities up to 70 000 h⁻¹. Destruction efficiencies greater than 99% can be achieved over the appropriate uranium based catalyst in the temperature range 300–450°C. Volatile organic compounds investigated include benzene, butylacetate, cyclohexanone, toluene, methanol, acetylene, butane, chlorobutane and chlorobenzene. The catalysts are thermally stable, destroy low concentrations and mixtures of VOCs and lifetime studies indicate that deactivation during oxidation of chlorinated VOCs did not occur. A temporal analysis of products (TAPs) reactor is used to investigate the mechanism of oxidation of VOCs by uranium oxide catalysts. Studies indicated that VOCs were oxidised directly to carbon oxides on the catalyst surface. A combination of TAP pulse experiments with oxygen present and absent in the gas phase has indicated that the lattice oxygen from the catalyst is responsible for the total oxidation activity. This has been confirmed by studies using isotopically labelled oxygen which indicates that the catalyst operates by a redox mechanism.     

Community,Ethnicity and Neighbourhood

The empirical focus is two small adjacent estates in West Newcastle; one with a mostly Bangladeshi population, the other white. The contrasts between them form the basis for a discussion of low housing demand and high turnover; forms of social exclusion; community and ethnicity. The Bangladeshi population experience exclusion from housing. Their estate has high demand and low turnover. Their housing options are highly constrained by a combination of low income and fear of crime and harassment. On the other hand they are comparatively well integrated into the local economy and a strong community structure. In contrast, the white population has more housing choice as a result of low demand, but exhibit greater exclusion from the labour market and from 'civil society'. Recent proposals to restructure the housing of the West End could increase housing opportunities for the Bangladeshis, but their impact on the white estate is more uncertain.     

Optimising Channel Assignments for Private Mobile Radio Networks in the UHF 2 Band

Private mobile radio (PMR) services are widely used in business and emergency applications. Typically, these services use the UHF 2 band (450–470 MHz). In order to comply with continental practice, the provision of these services in the UK is to be realigned and a new operational assignment will be generated in 2005. However, this can only be achieved with an appropriate model and the ability to make assignments for large regional networks. The purpose of this paper is to bring our experiences of this problem to the wireless arena. We explain the model used when making assignments for UK PMR networks and identify existing techniques and theory which are appropriate for this problem. We demonstrate that improved assignments and estimates of spectral requirements for large operational services can be found.