Structural control in radical polymerization with 1,1 diphenylethylene: 2. Behavior of MMA-DPE copolymer in radical polymerization

Copolymers of 1,1-diphenylethylene (DPE) behave in a very special way in radical polymerization. Particularly, the behavior of MMA-DPE copolymers in radical polymerization is investigated. The results reveal that the semiquinoid structure of the precursor polymer identified in a previous contribution is activated by the attack of free radicals and thus, in a second stage polymerization with a second monomer, block copolymers are formed. The block copolymer yield depends strongly on the ratio between the amount of DPE-containing precursor polymer and the initiator and monomer concentration used in the second stage. The mechanism proposed is able to explain at least qualitatively all experimental results including the restriction of this mode of control of radical polymerization to the formation of diblock copolymers only.     

Comparing emulsion polymerization of methacrylate-monomers with different hydrophilicity

A comprehensive experimental study concerning the influence of various types of initiator-emulsifier systems on emulsion polymerization of methacrylate monomers (2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA) and butyl methacrylate (BMA)) reveals interesting relations between initiator and surfactant hydrophilicity on the one hand and the hydrophilicity of the monomers on the other hand. For the water-soluble HEMA stable latexes are only obtained if hydrophobic initiators such as 2,2′-azobisisobutyronitrile or dibenzoyl peroxide in combination with alkyl sulfate surfactants with carbon chain lengths greater than 10 or surface active initiators of the 2,2′-azobis(N-2′-methylpropanoyl-2-amino-alkyl-1)-sulfonate type with alkyl chain lengths greater than 8 are employed. Stable nano size range poly(2-hydroxyethyl methacrylate) (PHEMA) particles have been prepared also by batch emulsion polymerization using ionic surface active initiators (inisurfs). The results clearly show that the formation of stable latex particles requires a proper choice of the initiator-emulsifier system regarding its hydrophilic-hydrophobic balance. The PHEMA particles prepared with surface-active initiators keep their identity and spherical shape even in the dried state whereas in the case of the other initiator-emulsifier systems complete coagulation and coalescence occurs during drying.     

Kinetics of Thermal Dehydroxylation and Carbonation of Magnesium Hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH)₂ were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X-ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting-sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting-sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH)₂ underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO₃ precipitated on the surface of disrupted Mg(OH)₂ crystals acting as a kinetic barrier to both the outward diffusion of H₂O and the inward diffusion of CO₂.     

Applying a formal analysis technique to the CCITT X.509 strong two-way authentication protocol

In the quest for open systems, standardization of security mechanisms, framework, and protocols are becoming increasingly important. This puts high demands on the correctness of the standards. In this paper we use a formal logic-based approach to protocol analysis introduced by Burrows et al. [1]. We extend this logic to deal with protocols using public key cryptography, and with the notion of duration to capture some time-related aspects. The extended logic is used to analyse an important CCITT standard, the X.509 Authentication Framework. We conclude that protocol analysis can benefit from the use of the notation and that it highlights important aspects of the protocol analysed. Some aspects of the formalism need further study.This research was sponsored by the Royal Norwegian Council for Scientific and Industrial Research under Grant IT 0333.22222, and was performed while K. Gaarder was at Alcatel STK Research Centre.     

Man vs machine: examining the three themes of service robotics in tourism and hospitality

Electronic Markets - There is a growing need in the tourism and hospitality literature to harmonise service robots and artificial intelligence’s (AI) meaning and foundations, while also...     

Energietransport über weite Strecken

If the regenerative energy that can be produced in a country is not sufficient for the successful implementation of the energy transition, one possible solution could be the purchase of regeneratively produced electricity from countries particularly favored by meteorology. This article examines five different technologies for energy transport. The electricity is generated once in a photovoltaic system and once in a wind turbine park. The investments in the transmission technologies sometimes exceed the investments in the power generation technologies many times over. According to the available results, without considering storage, the wind turbine system, although it is specifically more expensive than the photovoltaic system, always leads to lower electricity generation costs after the energy transport. The reason is the higher number of full load hours of the wind turbine.     

Upcycling by grafting onto semi-crystalline polymers using supercritical CO2

The creation of graft copolymers by selectively grafting a second polymer to the amorphous fraction of a semi-crystalline polymer in supercritical CO₂ is demonstrated herein. The graft copolymer is synthesized by free radical polymerization of a vinyl monomer within the semi-crystalline polymer below its melt temperature. Such conditions afford selective grafting on the amorphous regions (block “B”) while leaving the crystalline domains (block “A”) unmodified. Accordingly, unique A-B, A-B-A, A-B-A-B-A, and so forth. block structures are formed. In this work, styrene is polymerized within polyamide 6, polyethylene terephthalate, and isotactic polypropylene. Purification of these material is performed to remove the un-grafted homopolymer, allowing for determination of the graft yield, the portion of polymer which covalently bonds to the semi-crystalline matrix. Grafting yields achieved in polyamide 6, polyethylene terephthalate, and isotactic polypropylene were 98%, 59%, and 15%, respectively. Property enhancements were observed upon further characterization of polystyrene-polyamide 6 copolymers, including high glass transition temperatures, the ability to be remelted, and tunable grafting molecular weight. Additionally, hydrophobicity is controlled by varying polystyrene composition. The remarkable range of accessed properties demonstrates this as a potential route to upcycling plastics.     

Upcycling of polypropylene with various concentrations of peroxydicarbonate and dilauroyl peroxide and two processing steps

The influence of two peroxides (peroxydicarbonate/dilauroyl peroxide) with various concentrations (10–200 mmol/kg PP) and their effective opportunity to introduce long chain branched (LCB) were investigated. The dependence of a single and double extrusion step and the changes of the properties were studied. Experiments were carried out in a single screw extruder at 180°C for the first extrusion step (modification) and at 240°C for the second extrusion step (processing simulation). Melt flow rate and dynamic rheological properties were studied at a measuring temperature of 230°C. For the definitive determination of long chain branched polypropylene (LCB-PP) served the extensional rheology measurements. The mechanical properties were examined via tensile test and impact tensile test. Summarized, LCB (melt strength) could be observed via extensional rheology for all modified specimens and the mechanical properties were maintained or even improved for the modified samples. Particularly, samples containing dilauroyl peroxide display excellent mechanical properties in this study.