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Ionic liquids as solvents for polyaramids

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 copolymercopoly(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 theirrespective 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 polyaramidstheir 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 moderatetemperatures. 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.

Date:1 Jan 2016 →  8 Feb 2021
Keywords:polyaramids, Ionic liquids
Disciplines:Analytical chemistry, Physical chemistry, Organic chemistry, Condensed matter physics and nanophysics, Pharmaceutical analysis and quality assurance, Inorganic chemistry
Project type:PhD project