Reactive processing of syndiotactic polystyrene with an epoxy/amine solvent system

Syndiotactic polystyrene (sPS) is a new semi-crystalline thermoplastic which is believed to fill the price-performance gap between engineering and commodity plastics. In order to reduce the high processing temperature of sPS (>290°C), an epoxy-amine model system was used as a reactive solvent. Such a processing aid can be used to achieve a 50 to 500 fold lowering of the melt viscosity. When initially homogeneous solutions of sPS in a stoechiometric epoxy-amine mixture are thermally cured, Reaction Induced Phase Separation (RIPS) takes place, leading to phase separated thermoplastic-thermoset polymer blends. We focus our study on low (wt% sPS < 20%) and high concentration blends (wt% sPS > 60%) prepared by two processing techniques (mechanical stirring in a laboratory reactor or internal mixer/ reactive extrusion respectively). These blends have different potential interests. Low concentration blends (sPS domains in an epoxy-amine matrix) are prepared to create new, tunable blend morphologies by choosing the nature of the phase separation process, i.e. either crystallisation followed by polymerization or polymerization followed crystallisation. High concentration blends (sPS matrix containing dispersed epoxy-amine particles after RIPS) are prepared to facilitate the extrusion of sPS. In this case, the epoxy amine model system served as a reactive solvent. The time to the onset of RIPS is in the order of 7-9 min for low concentration blends, while it increases to 20-45 min for high concentration samples, as the reaction rates are substantially slowed down due to lower epoxy and amine concentrations. During the curing reaction the melting temperature of sPS in the reactive solvent mixture evolves back from a depressed value to the level of pure sPS. This indicates a change in the composition of the sPS phase, caused by (complete) phase separation upon reaction. We conclude that our epoxy amine system is suited for reactive processing of sPS, where final properties depend strongly on composition and processing conditions.     

Macromonomers and coordination polymerization

The present work discusses the synthesis of well-defined comb-shaped polymers or graft copolymer structures based on coordination (co)polymerization of macromonomers. Polystyrene macromonomers with various polymerizable entities were synthesized first by induced deactivation reactions. The homopolymerization of these macromonomers in the presence of selected early or late transition metal catalysts was examined. Comb-shaped polymers could be obtained over a large range of DP values. The results were compared to those obtained by anionic homopolymerization. Some results on the copolymerization of these PS macromonomers with ethylene in the presence of VERSIPOL^TM type catalysts were presented.     

Imaging the Footprint of Nanoscale Electrochemical Reactions for Assessing Synergistic Hydrogen Evolution

This work proposes a novel method for measuring the intrinsic activity of single metal-based nanoparticles towards water reduction in neutral media at industrially relevant current densities. Instead of using gas nanobubbles as proxy, the method uses optical microscopy to track the local footprint of the reaction through the precipitation of metal hydroxide, which is associated to the local pH increase during electrocatalysis. The results show the electrocatalytic activities of different types of metal nanoparticles and bifunctionnal core-shell nanostructures made of Ni and Pt, and demonstrate the importance of metal hydroxide nano-shells in enhancing electrocatalysis. This method should be generalizable to any electrocatalytic reaction involving pH changes such as nitrate or CO₂ reduction.     

Macromonomers as well-defined building blocks in macromolecular engineering

The present work compares the efficiency of different polymerization methods to design well-defined comb-shaped structures based on macro monomers. Anionic polymerization remains the method of choice and allows the control of polymerization degree of the main chain and the length of the grafts. The presence of an active chain end on the backbone enabled the synthesis of a new type of hyperbranched polymers by reaction with appropriate low molar multifunctional compounds. Free radical polymerization is less efficient for the controlled homopolymerization of macromonomers but less sensitive to the presence of impurities. It requires in most cases long fractionation procedures to access well defined comb-shaped fractions characterized by high molar masses. The controlled free radical polymerization constitutes an interesting alternative. The homopolymerization of macromonomers with late transition metal catalysts was also examined and comb-shaped polymers characterized by a syndiotactic backbone and atactic grafts could be obtained.     

Aggregation phenomena in polyelectrolyte multilayers made from polyelectrolytes bearing bulky functional,hydrophobic fragments

The functionalization of polyelectrolyte multilayers often implies the use of bulky functional fragments, attached to a standard polyelectrolyte matrix. Despite of the high density of non-charged, often hydrophobic substituents, regular film growth by sequential adsorption proceeds easily when an appropriate polyelectrolyte counter ion is chosen. However, the functional fragments may cluster or aggregate. This complication is particularly evident when using chromophores and fluorophores as bulky pendant groups. Attention has to be paid to this phenomenon for the design of functional polyelectrolyte films, as aggregation may modify crucially the properties. The use of charged spacer groups does not necessarily suppress the aggregation of functional side groups. Still, clustering and aggregation depend on the detailed system employed, and are not obligatory. In the case of cationic poly(acrylamide)s labeled with naphthalene and pyrene fluorophores, for instance, the polymers form intramolecular hydrophobic associates in solution, as indicated by strong excimer formation. But the polymers can undergo a conformational rearrangement upon adsorption so that they are decoiled in the adsorbed films. Analogous observations are made for polyanions bearing mesogenic biphenyls fragments. In contrast, polycations functionalized with the dye coumarin 343 show little aggregation in solution, but a marked aggregation in the ESA films.     

DGEBA monomer as a solvent for syndiotactic polystyrene

Syndiotactic polystyrene (sPS) has to be processed at high temperatures (i.e. >290°C due to its melting point of 270°C), which approaches its degradation temperature. We aim to facilitate the processing of sPS by lowering its melt temperature and viscosity with a curable epoxy/amine model system as reactive solvent, which will result in a thermoplastic-thermoset polymer blend. As a first step we therefore investigated the melting behaviour of sPS in epoxy monomer, established its phase diagram, and investigated the crystalline form of sPS in these mixtures. DGEBA epoxy monomer is found to be a solvent for syndiotactic polystyrene at temperatures above 220°C. The DGEBA-sPS phase diagram was established by means of DSC, on the basis of crystallization and melting peaks. The form of the curve in the phase diagram indicates that DGEBA is a poor solvent for sPS. In WAXS studies of blends only the β crystalline form was detected, not the δ form, thus no sPS-DGEBA polymer-solvent compounds (clathrates) were detected. However, DGEBA can still serve as a monomer for improved processing as it depresses the crystallization temperature by 20 to 60 K upon addition of 20 to 90 wt% DGEBA respectively, while a 16 to 45 K melting peak depression can be observed by adding 20 to 90 wt% DGEBA.     

Si-Tethered Bis- and Tris-Malonates for the Regioselective Preparation of Fullerene Multi-Adducts

Bis- and tris-malonates constructed around a silicon atom have been prepared by reaction of malonate derivatives bearing an alcohol function with di-tert-butylsilyl bis(trifluoromethanesulfonate) and tert-butyl(trichloro)silane, respectively. These compounds have been used for the regioselective bis- and tris-functionalization of C₆₀ under Bingel conditions. By changing the nature of the linker between the central Si atom and the reactive malonate groups, the malonate precursors have been optimized to produce specific bis- and tris-adducts with excellent regioselectivity. A complete understanding of the electronic and stereochemical factors governing the regioselectivity has been obtained by combining computational studies with a complete analysis of the by-products formed during the reactions of the Si-tethered tris-malonates with C₆₀. Finally, desilylation reactions of the resulting fullerene bis- and tris-adducts have been carried out to generate the corresponding acyclic fullerene bis- and tris-adducts bearing alcohol functions.