Control of Gelation and Network Properties of Cationically Copolymerized Mono- and Diglycidyl Ethers

International audienceThe development of low temperature curing systems has become a major objective in thermoset technologies for both environmental and economic reasons. The use of protic and chelating additives have recently been underlined for the control of the cationic ring opening polymerization of epoxies, a curing mode that is very efficient at temperatures close from the ambient but that can easily runaway. In this paper, we propose to use this strategy to control the kinetics of the cationic copolymerization of a diepoxy monomer(diglycidyl ether of bisphenol A, DGEBA) with a monoepoxy monomer(phenyl glycidyl ether, PGE). The purpose of the study is to tune the crosslink density (ν e) in order to control the mechanical properties of the materials. The sol-gel transition was first investigated in details at several frequencies by using the Fourier transform mechanical spectroscopy method (FTMS). We found that the gel time (t gel) and the critical conversion (α gel) can be controlled to a great extent by promoting transfers and complexing cationic species involved in the polymerization mechanism. The FTMS method also gives some insight into the structure of the polymer clusters at the sol-gel transition. The results indicate that the various additives used to control the transition have mostly no influence on the clusters' structure. The properties of the fully-cured networks were then investigated via swelling and dynamic mechanical measurements. Both methods indicate that ν e is strongly influenced by the crosslinker content (DGEBA) bu

Control of reactions and network structures of epoxy thermosets

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Ester-Containing Imidazolium-Type Ionic Liquid Crystals Derived from Bio-based Fatty Alcohols

The need to take into account the life cycle of ionic liquids (ILs), from the sourcing of the raw materials involved in their synthesis to their disposal and degradation, has become paramount in the design of new IL-type molecular structures. In the case of 1-alkyl-3-methylimidazolium salts, one of the prominent IL families, there is an increasing demand for synthetic methods involving (i) substitution of the petro-based alkyl derivatives by readily available bio-sourced surrogates and (ii) functionalization of the alkyl tail with heterofunctional groups enabling the (bio)degradation of ILs after use. Herein, a straightforward and industrially viable synthesis of lipidic imidazolium salts is reported, starting from different bio-sourced fatty alcohols, including oleic, stearyl, and lauryl alcohols. This procedure is based on the acrylation of fatty alcohols, followed by the aza-Michael addition of the imidazole group onto the acrylate moiety. Subsequent quaternization, using either methyl iodide or methyl tosylate, provides a library of 1-alkylpropionate-3-methylimidazolium salts with various alkyl chain lengths (C18, C12, and C11) and incorporating different types of counteranions (iodide, tosylate, and tetrafluoroborate). These ester-containing analogues of classical 1-alkyl-3-methylimidazolium salts are all ILs, that is, with a melting point below 100 °C. In addition, most of them exhibit a liquid-crystal behavior and can be referred to as IL crystals (ILCs). The thermal stability, as well as the phase transitions of these ILs, has been investigated by thermogravimetric analysis, as well as differential scanning calorimetry, respectively, while the molecular structure into the crystalline phase and the mesophase is studied by X-ray scattering. Interestingly, ILCs featuring unsaturated alkyl tails exhibit a low melting point, close to room temperature, and the presence of the ester function is shown to provide an enhanced stabilization of the mesophase

Pathogenesis of adolescent idiopathic scoliosis in girls - a double neuro-osseous theory involving disharmony between two nervous systems, somatic and autonomic expressed in the spine and trunk: possible dependency on sympathetic nervous system and hormones with implications for medical therapy

Anthropometric data from three groups of adolescent girls - preoperative adolescent idiopathic scoliosis (AIS), screened for scoliosis and normals were analysed by comparing skeletal data between higher and lower body mass index subsets. Unexpected findings for each of skeletal maturation, asymmetries and overgrowth are not explained by prevailing theories of AIS pathogenesis. A speculative pathogenetic theory for girls is formulated after surveying evidence including: (1) the thoracospinal concept for right thoracic AIS in girls; (2) the new neuroskeletal biology relating the sympathetic nervous system to bone formation/resorption and bone growth; (3) white adipose tissue storing triglycerides and the adiposity hormone leptin which functions as satiety hormone and sentinel of energy balance to the hypothalamus for long-term adiposity; and (4) central leptin resistance in obesity and possibly in healthy females. The new theory states that AIS in girls results from developmental disharmony expressed in spine and trunk between autonomic and somatic nervous systems. The autonomic component of this double neuro-osseous theory for AIS pathogenesis in girls involves selectively increased sensitivity of the hypothalamus to circulating leptin (genetically-determined up-regulation possibly involving inhibitory or sensitizing intracellular molecules, such as SOC3, PTP-1B and SH2B1 respectively), with asymmetry as an adverse response (hormesis); this asymmetry is routed bilaterally via the sympathetic nervous system to the growing axial skeleton where it may initiate the scoliosis deformity (leptin-hypothalamic-sympathetic nervous system concept = LHS concept). In some younger preoperative AIS girls, the hypothalamic up-regulation to circulating leptin also involves the somatotropic (growth hormone/IGF) axis which exaggerates the sympathetically-induced asymmetric skeletal effects and contributes to curve progression, a concept with therapeutic implications. In the somatic nervous system, dysfunction of a postural mechanism involving the CNS body schema fails to control, or may induce, the spinal deformity of AIS in girls (escalator concept). Biomechanical factors affecting ribs and/or vertebrae and spinal cord during growth may localize AIS to the thoracic spine and contribute to sagittal spinal shape alterations. The developmental disharmony in spine and trunk is compounded by any osteopenia, biomechanical spinal growth modulation, disc degeneration and platelet calmodulin dysfunction. Methods for testing the theory are outlined. Implications are discussed for neuroendocrine dysfunctions, osteopontin, sympathoactivation, medical therapy, Rett and Prader-Willi syndromes, infantile idiopathic scoliosis, and human evolution. AIS pathogenesis in girls is predicated on two putative normal mechanisms involved in trunk growth, each acquired in evolution and unique to humans

Control of the curing process and of the properties of an epoxy material via supramolecular chemistry

Contrôler la gélification des résines époxy est un enjeu majeur de la chimie des matériaux thermodurcissables. Dans cette étude, nous décrivons une méthode de contrôle du temps de gel (tgel) et de la conversion au point de gel (xgel) dans le cas de la polymérisation cationique de résines époxy commerciales. Pour ce faire, des additifs hydroxylés sont utilisés comme agents de transfert afin de contrôler xgel. Parallèlement, des additifs de type oligo-oxyéthylène permettent de contrôler tgel en complexant les amorceurs de la polymérisation (cations aniliniums). La combinaison de ces deux leviers de contrôle permet d'explorer une large gamme de valeurs de tgel et de xgel. Lorsqu'un oligo-oxyéthylène cyclique (18-crown-6) est utilisé, les cations aniliniums sont stabilisés sur de très longues périodes grâce à de fortes interactions de type " clé-serrure ". En l'absence d'agent de transfert, une élévation modérée de la température est suffisante pour observer la dissociation du complexe aniliniumo18-crown-6 suivie d'un rapide amorçage de la polymérisation. Ce complexe est isolable et peut être utilisé comme un amorceur thermodéclenchable. En présence d'agent de transfert, une élévation de la température s'accompagne d'un long retard à la polymérisation (temps d'induction) suivi d'une rapide réticulation du matériel. La composition chimique du mélange initial permet de contrôler le temps d'induction. Ainsi, la réticulation du matériel devient programmable dans le temps, à la manière d'une minuterie. Cette chimie est ensuite appliquée à la copolymérisation de résines mono- et di-époxy afin de contrôler la densité de réticulation du matériel et donc ses propriétés mécaniques.Controlling the pot life and the gelation of epoxy resins is a crucial issue in thermosets processing. In this study, we report about the control of the gel time (tgel) and the gel conversion (xgel) for the cationic polymerization of commercial epoxy resins. To this end, hydroxyl additives are used as chain transfer agent to control xgel. Concurrently, oligo(ethylene oxide) additives are used to vary tgel as a result of the supramolecular complexation of the anilinium cations responsible for the polymerization’s initiation. We show that the combination of these two control levers enables the exploration of a wide range of tgel and xgel values. When a cyclic oligo(ethylene oxide) (18-crown-6) is used, anilinium cations are stabilized on very long period as a result of strong host-guest interactions. In absence of transfer agent, a slight increase of temperature is enough to observe the dissociation of the anilinium•18-crown-6 complex and the fast initiation of polymerization. The anilinium•18-crown-6 complex can be isolated and used as a thermoresponsive initiator presenting a high apparent activation energy. When used in combination with hydroxyl additives, a temperature increase results in a long delay of polymerization followed by the rapid completion of the material’s reticulation. By varying the composition of the initial mixture it is possible to tailor the duration of the polymerization’s delay. Thus, the resulting system is programmable and exhibits a clock like behavior. This chemistry is then extended to the copolymerization of diepoxy and monoepoxy resins to control the crosslink’s density of the material and thus, the properties of the final networks

Fully Biobased Vitrimers: Future Direction toward Sustainable Cross‐Linked Polymers

Improving the sustainability of polymer networks is a crucial challenge in polymer science due to their important role in industry. Their traditional syntheses conflict with several principles of green chemistry as the employed monomers are petroleum-based, their production involves the use of toxic reagents, and their permanently cross-linked structures impede their chemical recycling and reshaping. The development of vitrimers represents a unique solution to address the issue of polymer network end-of-life by enabling reprocessability while maintaining good thermomechanical properties and solvent resistance. Although over the last decades biomass has proved to be an excellent feedstock for the production of permanently cross-linked polymers, the field of biobased vitrimers is still in its infancy. In this review, a comprehensive overview of vitrimers synthesized from biobased monomers is presented. The emphasis is set on the compatibility of the biomass structure with the nature of the dynamic covalent chemistry, as well as the sustainability of the synthetic approaches. Implementing renewable feedstocks and recyclability in the production of polymer networks paves the way for the development of the next generation of sustainable materials

Supramolecular Control of Propagation in Cationic Polymerization of Room Temperature Curable Epoxy Compositions

The cationic ring-opening polymerization (ROP) of room temperature curable epoxy compositions was investigated in the presence of protic (alcohols), weakly chelating (linear polyethers), and strongly chelating (crown ethers) species. Epoxide conversion and gelation were monitored through infrared and rheological measurements. We demonstrate that the propensity of hydroxyl moieties to promote the activated monomer (AM) mechanism and the chelating ability of polyether groups toward the cationic species involved in this propagation mode can be combined to control two fundamental parameters of the gelation process of epoxy resins, the gel time (<i>t</i>_gel) and the critical conversion (conversion at the gel point, <i>x</i>_gel), by adequately adapting the amount of these additives. In the case of crown ether, a strong synergy between these two control tools was found and interpreted by the prolonged stabilization of protons involved in chain transfers, in the form of dormant supramolecular host–guest complexes. These results underline the potential of this new approach to control both the kinetic and architecture of epoxy growing network

Sequence-Controlled Polyhydroxyurethanes with Tunable Regioregularity Obtained from Sugar-Based Vicinal Bis-cyclic Carbonates

International audienceThe carbonation of biosourced 1,2-diols is a sustainable avenue for the synthesis bis-five-membered cyclic carbonates (bis-5CCs), the precursors of a valuable class of polymers, for example, polyhydroxyurethanes (PHUs). In this work, we performed the direct carbonation of optically pure sugar-based butadiene tetraols, namely, (i) meso-erythritol and (ii) its (S,S) diastereoisomer, (L)-threitol. The corresponding vicinal bis-5CCs, erythritol dicarbonate (EDC) and threitol dicarbonate (TDC), respectively, retain the stereochemistry of the starting tetraols. The comprehensive study of their aminolysis reaction, in DMSO and at room temperature, indicates that the kinetics and the regio-orientation of the ring opening of the 5CC are very much dependent on their stereochemistry. The total aminolysis of EDC results in the formation of hydroxyurethanes with an excess of secondary hydroxyl groups, (OH)II, of about 85%, against 60% for TDC. Moreover, when considering the two consecutive aminolyses of EDC (and TDC, respectively), the kinetic rate constant of the first aminolysis k1,EDC (and k1,TDC, respectively) is 1 order of magnitude higher than that of the second aminolysis, k2,EDC ∼ k1,EDC/10 (and k2,TDC ∼ k1,TDC/10, respectively). We used this feature to develop a one-pot, two-step polymerization procedure, offering sequence-controlled PHUs. All the experimental results are well supported by density functional theory calculations. In the end, the comparative study of these two diastereoisomers of simple sugar-based vicinal bis-5CCs provides a new family of PHUs with tunable sequence- and regioregularities