1,721,743 research outputs found
Debondable epoxy-acrylate adhesives using β-amino ester chemistry
Full text linkThe reuse of multilayered materials, which are held together by structural epoxy adhesives, is a major challenge since the bonded substrates cannot be easily separated for recycling. In this research, we explore a one-pot strategy based on beta-amino ester chemistry for the development of modified epoxy adhesives with on-demand debonding potential. For this, a formulation of commercially available acrylate, epoxy and amine compounds is used. The research starts with a systematic study, demonstrating the influence of the different compounds on the thermal and adhesive properties of the materials. Subsequently, the potential for debonding is demonstrated using rheological measurements and tensile tests. The fast, catalyst-free Aza-Michael reaction enables the straightforward preparation of such epoxy-based adhesives, while the reverse reaction allows for debonding at 120 degrees C. In general, a chemical design is demonstrated for producing an industrially attractive generation of debondable epoxy-based adhesives
Conformational influence of fluorinated building blocks on the physical properties of polyesters
No full textThe interplay of the relative configuration of diastereomeric vicinal difluoride groups on the conformational properties of polyesters has been investigated and compared to their non-fluorinated and per-fluorinated counterparts. The incorporation of syn and meso vicinal difluoride units in the polymer backbone was expected to influence the rigidity and stacking behaviour of the polymers in a significant way, and therefore to result in different thermal properties, such as melting point and melting enthalpy. Both syn and meso-2,3-difluoro butanediol have been reacted with the diester dimethyl succinate, leading to the formation of polyesters that have been characterized with H-1, C-13 and F-19 NMR. The polyesters showed molar masses up to 20 kg/mol (SEC). Surprisingly, the syn and meso-polymers displayed identical crystallization and melting behaviour. In contrast, differences were observed in the crystallization kinetics and melting points of syn and meso oligomers. Relying on time-resolved synchrotron SAXS and WAXD experiments, the complex multiple melting behaviour of these oligomers was explained in terms of crystal size and surface effects. The slower crystallization kinetics for the meso oligomers was tentatively associated with a stronger tendency to adopt gauche configurations. Apparently, such effects no longer affect the crystallization kinetics when larger polymers crystallize. It was also found that the syn and meso-polymers have identical equilibrium melting points and melting enthalpies notwithstanding molecular and crystallographic differences
Vinylogous urethane vitrimers
Full text linkVitrimers are a new class of polymeric materials with very attractive properties, since they can be reworked to any shape while being at the same time permanently cross-linked. As an alternative to the use of transesterification chemistry, we explore catalyst-free transamination of vinylogous urethanes as an exchange reaction for vitrimers. First, a kinetic study on model compounds reveals the occurrence of transamination of vinylogous urethanes in a good temperature window without side reactions. Next, poly(vinylogous urethane) networks with a storage modulus of ≈2.4 GPa and a glass transition temperature above 80 °C are prepared by bulk polymerization of cyclohexane dimethanol bisacetoacetate, m-xylylene diamine, and tris(2-aminoethyl)amine. The vitrimer nature of these networks is examined by solubility, stress-relaxation, and creep experiments. Relaxation times as short as 85 s at 170 °C are observed without making use of any catalyst. In addition, the networks are recyclable up to four times by consecutive grinding/compression molding cycles without significant mechanical or chemical degradation. Catalyst-free vitrimers based on the transamination of vinylogous urethanes are prepared from readily accessible chemicals. These high Tg, cross-linked materials exhibit excellent mechanical properties, while the exchangeable bonds enable full stress-relaxation on short time scales and recycling over many cycles.Fil: Denissen, Wim. University of Ghent; BélgicaFil: Rivero, Guadalupe. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Nicolay, Renaud. University of Ghent; BélgicaFil: Leibler, Ludwik. University of Ghent; BélgicaFil: Winne, Johan. University of Ghent; BélgicaFil: Du Prez, Filip E.. University of Ghent; Bélgic
One-pot thermo-remendable shape memory polyurethanes
Full text linkFuran-based thermoset polyurethanes have been prepared in a one-pot fashion with the ability to self-mend under mild temperature conditions, by making use of a Diels-Alder shape-memory assisted self-mending (DASMASH) approach. For this, thermoreversible covalent bonds, obtained by Diels-Alder chemistry, are introduced as crosslinkers into a polycaprolactone (PCL) containing polyurethane material. It is demonstrated that, after introduction of a crack into the PU-thermoset, Diels-Alder bonds preferentially break, regenerating free furan/maleimide functional groups, while the shape memory effect favors the crack closure at temperatures above the melting point of PCL, simultaneously resulting in a reformation of the reversible crosslinks. The reversibility and shape memory ability of the materials were optimized and studied by FTIR, 1H NMR and tensile measurements. Different compositions were used to properly understand the role and influence of each component. The polyurethane materials healed at 50°C after mechanical damage induced by either the application of a large tensile deformation or by performing controlled macro/micro scratches with a depth sensing indenter. On-line FT-IR monitoring provided a kinetic description of the system reversibility for numerous cycles. Furthermore, mechanical recovery with complete disappearance of the microscratches was accomplished after multiple cycles of large tensile deformation. The results were not only confirmed by an optical inspection and scanning electron microscopy, but also with confocal microscopic mapping, by comparison of the cross-section profiles of the microscratches before and after healing.Fil: Rivero, Guadalupe. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación En Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingeniería; ArgentinaFil: Nguyen, Le-thu T.. University Of Ghent. Faculty Of Sciences. Department Of Organic Chemistry. Polymer Chemistry Research Group; BélgicaFil: Hillewaere, Xander K. D.. University Of Ghent. Faculty Of Sciences. Department Of Organic Chemistry. Polymer Chemistry Research Group; BélgicaFil: Du Prez, Filip E.. University Of Ghent. Faculty Of Sciences. Department Of Organic Chemistry. Polymer Chemistry Research Group; Bélgic
Multistep reactions based on thiolactones for the synthesis of functionalized polymers
Full text linkThe aim of this research project was to develop a method for the synthesis of advanced polymeric structures without the use of timeconsuming protecting group strategies. The proposed method is based on the reaction between thiols and unsaturated carbon chains, denoted as thiol-ene or thiol-yne chemistry. Because of the disadvantages of working with thiols (i.e. unpleasant smell, limited commercial availability and instability due to oxidation reactions), there has been a continuous interest in the development of new ways to protect thiols, e.g. as disulfide, thiocarbonylthio-group or methanesulfonate. However, most of these methods require a protecting and a deprotecting step, which is unfavorable in terms of atom-efficiency and overall yield. The strategy herein presented, is based on the use of a thiolactone as a precursor for a thiol, allowing for the direct introduction of a thiol, starting from stable amino compounds without the need for a subsequent deprotection step. Using thiolactones in polymer synthesis has a dual advantage: On the one hand, it offers a chemoselective, atom-efficient way of generating thiols, while at the same time it is possible to introduce functionality via the amine compound. This one-pot amine-thiol-ene conjugation reaction was used for the synthesis and modification of diverse polymeric systems
Expanding the realm of thiolactone-based sequence-defined macromolecules : from synthesis towards applications
No full textBlock versus block-like copolymers: comparative study and exploration of a continuous preparation method
No full textIn polymer research there is a huge interest for amphiphilic block copolymers, consisting of a hydrophilic and a hydrophobic segment, as they show a unique surfactant behaviour with a wide variety of applications. In order to tune the properties of these materials, and as a consequence the resulting applications, the use of a controlled radical polymerization, in particular atom transfer radical polymerization (ATRP), is indispensible.
Traditional block copolymers are synthesized via a macroinitiator strategy. This two-step procedure is rather time and solvent consuming because of the intermediate purification step. By consequence, such a synthetic strategy is not industrially attractive.
Therefore, in this research project, a one-step procedure has been developed based on a sequential monomer addition, with the formation of so-called block-like copolymers. By this procedure, no intermediate purification step is required after the synthesis of the first segment. It would be economically much more interesting to use this one-step procedure compared to the macroinitiator strategy, provided that copolymers can be prepared with similar properties. For this reason, a detailed comparative study of both synthetic strategies has been performed to verify whether equivalent copolymer structures can be prepared.
Moreover, via two column reactors in series, it would be possible to produce the block-like copolymers in a continuous way. In order to fully understand this continuous polymerization process and to allow for a better control over the copolymer structure, a kinetic model for the continuous block-like copolymerization has been set up.
Finally, a novel type of solid-support for copper catalyzed reactions has been developed, based on macroporous cryogels. Because of the unhindered flow through the matrix, this support allows for the application in continuous column reactors with in situ catalyst separation. The new catalyst support has been tested for two types of continuous heterogeneous copper catalyzed reactions: the azide-alkyne click coupling and ATRP
Cryogels and polymeric microcapsules with triggered degradation and release
No full textThe developments in polymeric systems containing cleavable functional groups attract much attention nowadays and serve to the design of modern materials for a wide range of applications in different disciplines ranging from medicine and biotechnology to the microelectronic industry and to the protection of the environment from plastic waste. This thesis contributes to these developments with two different polymeric systems. More specifically, both cryogels and polymeric capsules have been investigated with a main attention to their on-demand degradation mechanism.
The presence of cleavable functional groups in the above mentioned systems gives the advantage of either a triggered degradation or release process in accordance with the final application. In these systems, the degradation or release process can be initiated by manipulating the splitting of the cleavable groups in their structure. In the case of cryogels, the triggered degradation of the cleavable groups causes the solubilization of the crosslinked gel system. The presence of such cleavable groups can also be used to have a spatial control in order to achieve a tailored degradation mechanism. In microcapsules, the same process destroys the microcapsule shell, liberating the encapsulated material
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