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Organic Electrolyte Solutions as Solvents for Polyaramids. Towards more benign methods of polyaramid synthesis and processing
Boek - Dissertatie
Polyaramids are a class of high-performance polymers that are renowned for
their chemical/thermal stability, non-flammability and exceptional mechanical
strength. The three most commonly produced polyaramids are: (1)
the para-aramid poly-p-phenylene terephthalamide (PPTA), (2) the metaaramid
poly-m-phenylene isophthalamide (PMIA) and (3) the copolymer
copoly(p-phenylene/3,4-diphenylether terephthalamide) (ODA/PPTA). These polyaramids can be processed into fibers which can be used in many demanding applications, such as heat/fire protection, cut-resistant clothing, ropes and cables, material reinforcement, ballistic vests, military/aerospace applications and many more. The source of the superior properties of polyaramids is in their chemical and supramolecular structure. Polyaramids consist almost entirely out of aromatic repeating units that are connected via amide bonds. The aromatic backbone, along with the partial double bond character of the amide groups, provides a great deal of rigidity to the polymer chains. Furthermore, the amide groups of neighbouring polymer chains can undergo hydrogen bonding between their
respective amine hydrogens and carbonyl oxygens. The rigid polymer backbone reduces the mobility of the amide groups which enables more effective hydrogen bonding. This leads to the formation of a vast network of intermolecular hydrogen bonds between adjacent polymer chains, which is further supplemented by additional pi-stacking interactions. The end result is a highly crystalline polymer with potentially high degrees of orientation, granting the polyaramids
their mechanical strength and high melting points. However, this supramolecular structure also makes polyaramids notoriously difficult to dissolve, as the stable hydrogen bond network needs to be broken up. The melting points of polyaramids lie well above their decomposition temperatures (>500 °C) which precludes them from being synthesized and processed from the melt. The only way to produce polyaramid products is from solution, which is problematic due to their low solubility. The currentsynthetic procedures generally involve the use of amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA) and N,Ndimethylformamide (DMF). To enhance the solubility, an inorganic salt such as CaCl2 or LiCl can be added. The purpose of these inorganic salts is to provide free Cl- anions which can interact with the amine hydrogens of the polymers, effectively interrupting the intermolecular hydrogen bonds. However, suspected of being teratogenic, the above mentioned amide solvents have been placed on a 'Substance of Very High Concern' (SVHC)-list by the European Commission. These solvents are now subject to REACH restrictions, and strict REACH legislation is likely to follow in the future, creating the need for alternative solvent systems for polyaramid synthesis and processing. Our research group has previously explored the potential of Ionic Liquids (ILs) to serve as more benign alternative solvents for the p-aramid PPTA. ILs are solvents that consist entirely out of ions, but are still liquid at ambient to moderate
temperatures. They generally exhibit a negligible vapour pressure which limits their exhaust to the atmosphere, giving them the title of 'green solvents'. Furthermore, the charged species that make up ILs can potentially undergo strong hydrogen bonding interactions with solutes. These solvating properties have already been widely reported on and employed successfully in the field of cellulose dissolution. In the case of polyaramids, ILs with strongly coordinating anions such as chlorides, acetates and dialkylphosphates were found to be good solvents for PPTA oligomers. The IL 1-methyl-3-octylimidazolium chloride [C8MIm][Cl] could be used to synthesize PPTA polymers with a mediocre molar mass, whereas using a mixture of this IL and NMP resulted in high molar mass PPTA that was on par with the industrial standard. dissolution and synthesis of PPTA in ILs by also considering the other polyaramids PMIA and ODA/PPTA. Furthermore, these polyaramids are generally easier to dissolve than PPTA, allowing for a more convenient and more in-depth study of the dissolution mechanism. Considerable attention is given to IL/co-solvent mixtures, otherwise known as Organic Electrolyte Solutions (OESs), as previous work indicated that IL/NMP mixtures outperformed pure ILs in the synthesis of PPTA. However, as the goal is to develop an environmentally benign alternative solvent, co-solvents other than NMP were investigated. The first being gamma-valerolactone (GVL), which is a bio-sourced polar solvent that is praised for its renewability, biodegradability and low toxicity. The second co-solvent is N-butyl-2-pyrrolidone (NBP), which structurally resembles NMP and shares many of the same characteristics while being far less reprotoxic. It was found that PMIA and ODA/PPTA are highly soluble in ILs that consist of heteroaromatic cations (imidazolium/pyridinium) and strongly coordinating anions. Using spectroscopic techniques (FTIR, 1H/13C NMR) it was found that hydrogen bonding between the anions of the IL and the hydrogens on the amides of the polymer was very strong, and crucial to the dissolution mechanism. The presumed role of cation is to interact with the aromatic sections of the polymer through dispersion forces, while hydrogen bonding between the cation and anions can increase the stability of the solvation shell. OES consisting of GVL and ILs displayed even higher solubilities than the pure ILs. Even phosphonium based ILs, which are unable to dissolve polyaramids on their own, could make suitable solvents if mixed with GVL. This synergy between GVL and the IL indicated that GVL could in part take over the role of cation and solvate the aromatic parts of the polymer, freeing up more IL to interact with the amide groups. The GVL-based OESs could successfully be used as solvents for the synthesis of high molar mass PMIA. Surprisingly, only a very small amount of IL was needed to keep PMIA in solution. During the synthesis, the organic base -picoline is added to scavenge HCl released during this reaction. This forms a protic pyridinium IL in situ which also partakes in the dissolution process. The reaction mixture could be used directly for the wet-spinning of PMIA fibers, and it was possible to recover the solvent via a sequence of distillation steps. The synthesis of ODA/PPTA in the GVL-based OESs yielded only mediocre results, whereas PPTA could not be synthesized at all. NBP-based OESs proved to be better solvents for this purpose. High molar mass ODA/PPTA could be synthesized and directly spun from this solvent. PPTA with high molar mass could also be synthesized, although the industrial standard was not reached. However the results were nevertheless promising and further optimization can undoubtedly lead to improved results.
copoly(p-phenylene/3,4-diphenylether terephthalamide) (ODA/PPTA). These polyaramids can be processed into fibers which can be used in many demanding applications, such as heat/fire protection, cut-resistant clothing, ropes and cables, material reinforcement, ballistic vests, military/aerospace applications and many more. The source of the superior properties of polyaramids is in their chemical and supramolecular structure. Polyaramids consist almost entirely out of aromatic repeating units that are connected via amide bonds. The aromatic backbone, along with the partial double bond character of the amide groups, provides a great deal of rigidity to the polymer chains. Furthermore, the amide groups of neighbouring polymer chains can undergo hydrogen bonding between their
respective amine hydrogens and carbonyl oxygens. The rigid polymer backbone reduces the mobility of the amide groups which enables more effective hydrogen bonding. This leads to the formation of a vast network of intermolecular hydrogen bonds between adjacent polymer chains, which is further supplemented by additional pi-stacking interactions. The end result is a highly crystalline polymer with potentially high degrees of orientation, granting the polyaramids
their mechanical strength and high melting points. However, this supramolecular structure also makes polyaramids notoriously difficult to dissolve, as the stable hydrogen bond network needs to be broken up. The melting points of polyaramids lie well above their decomposition temperatures (>500 °C) which precludes them from being synthesized and processed from the melt. The only way to produce polyaramid products is from solution, which is problematic due to their low solubility. The currentsynthetic procedures generally involve the use of amide solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA) and N,Ndimethylformamide (DMF). To enhance the solubility, an inorganic salt such as CaCl2 or LiCl can be added. The purpose of these inorganic salts is to provide free Cl- anions which can interact with the amine hydrogens of the polymers, effectively interrupting the intermolecular hydrogen bonds. However, suspected of being teratogenic, the above mentioned amide solvents have been placed on a 'Substance of Very High Concern' (SVHC)-list by the European Commission. These solvents are now subject to REACH restrictions, and strict REACH legislation is likely to follow in the future, creating the need for alternative solvent systems for polyaramid synthesis and processing. Our research group has previously explored the potential of Ionic Liquids (ILs) to serve as more benign alternative solvents for the p-aramid PPTA. ILs are solvents that consist entirely out of ions, but are still liquid at ambient to moderate
temperatures. They generally exhibit a negligible vapour pressure which limits their exhaust to the atmosphere, giving them the title of 'green solvents'. Furthermore, the charged species that make up ILs can potentially undergo strong hydrogen bonding interactions with solutes. These solvating properties have already been widely reported on and employed successfully in the field of cellulose dissolution. In the case of polyaramids, ILs with strongly coordinating anions such as chlorides, acetates and dialkylphosphates were found to be good solvents for PPTA oligomers. The IL 1-methyl-3-octylimidazolium chloride [C8MIm][Cl] could be used to synthesize PPTA polymers with a mediocre molar mass, whereas using a mixture of this IL and NMP resulted in high molar mass PPTA that was on par with the industrial standard. dissolution and synthesis of PPTA in ILs by also considering the other polyaramids PMIA and ODA/PPTA. Furthermore, these polyaramids are generally easier to dissolve than PPTA, allowing for a more convenient and more in-depth study of the dissolution mechanism. Considerable attention is given to IL/co-solvent mixtures, otherwise known as Organic Electrolyte Solutions (OESs), as previous work indicated that IL/NMP mixtures outperformed pure ILs in the synthesis of PPTA. However, as the goal is to develop an environmentally benign alternative solvent, co-solvents other than NMP were investigated. The first being gamma-valerolactone (GVL), which is a bio-sourced polar solvent that is praised for its renewability, biodegradability and low toxicity. The second co-solvent is N-butyl-2-pyrrolidone (NBP), which structurally resembles NMP and shares many of the same characteristics while being far less reprotoxic. It was found that PMIA and ODA/PPTA are highly soluble in ILs that consist of heteroaromatic cations (imidazolium/pyridinium) and strongly coordinating anions. Using spectroscopic techniques (FTIR, 1H/13C NMR) it was found that hydrogen bonding between the anions of the IL and the hydrogens on the amides of the polymer was very strong, and crucial to the dissolution mechanism. The presumed role of cation is to interact with the aromatic sections of the polymer through dispersion forces, while hydrogen bonding between the cation and anions can increase the stability of the solvation shell. OES consisting of GVL and ILs displayed even higher solubilities than the pure ILs. Even phosphonium based ILs, which are unable to dissolve polyaramids on their own, could make suitable solvents if mixed with GVL. This synergy between GVL and the IL indicated that GVL could in part take over the role of cation and solvate the aromatic parts of the polymer, freeing up more IL to interact with the amide groups. The GVL-based OESs could successfully be used as solvents for the synthesis of high molar mass PMIA. Surprisingly, only a very small amount of IL was needed to keep PMIA in solution. During the synthesis, the organic base -picoline is added to scavenge HCl released during this reaction. This forms a protic pyridinium IL in situ which also partakes in the dissolution process. The reaction mixture could be used directly for the wet-spinning of PMIA fibers, and it was possible to recover the solvent via a sequence of distillation steps. The synthesis of ODA/PPTA in the GVL-based OESs yielded only mediocre results, whereas PPTA could not be synthesized at all. NBP-based OESs proved to be better solvents for this purpose. High molar mass ODA/PPTA could be synthesized and directly spun from this solvent. PPTA with high molar mass could also be synthesized, although the industrial standard was not reached. However the results were nevertheless promising and further optimization can undoubtedly lead to improved results.
Jaar van publicatie:2021
Toegankelijkheid:Open