Ionic liquids for natural polymer dissolution

A range of IL types for natural polymer dissolution have been reported in current literature, while most of them have been developed to dissolve cellulose. Among those many IL types, in this context, only imidazolium based IL types will be reviewed as they have shown to be a common solvent type for both natural cellulose and protein polymers. Generally, ILs consisting of 1-R1-3-R2- imidazolium cation and either the chloride or acetate anion have been used for dissolving natural polymers acting as non-derivatised solvents [18, 21, 35, 40, 69, 85, 87-89]. Figure 2.9 shows the imidazolium cation and the common natural polymer dissolving anions.

Below, Table 2.1 lists the imidazolium based ILs used to dissolve natural polymers namely; cellulose, silk, and wool and duck feather keratin.

Figure 2.9: Structure of typical cations and anions of natural polymer dissolving ILs

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24 ILs used to dissolve cellulose

Ionic Liquid Solubility (%w) DP (Degree of Polymerisation)

Dissolving conditions Ref Temperature

ILs used to dissolve silk

Ionic liquid Solubility (%w) Silk type Dissolving conditions Ref Temperature (˚C) Time (h)

AMIMCl 10 Wild silk fibroin 100 6 This study

BMIMCl 12.2 Wild silk

cocoon

120 - [89]

DMBIMCl 8.3 Mulberry silk

cocoon

100 - [89]

EMIMCl 23.3 Mulberry silk

cocoon 100 - [89]

BMIMOAc 10.14 Wild silk

cocoon 120 - [35]

ILs used to dissolve wool and duck feather keratin Ionic Liquid Keratin

type

Solubility (%w) Dissolving conditions Ref

Temperature (˚C) Time (h)

[AMIM] = 1-allyl-3-methylimidazolium, [BMIM] = 1-butyl-3-methylimidazolium, [DMBIM] = 1-butyl-2,3dimethylimidazolium, [EMIM] = 1-ethyl-3-methylimidazolium, [HMIM] = 1-hexyl-3-methylimidazolium, [OMIM] = 1-octyll-3-methylimidazolium

Anions:

Br = bromide, Cl = chloride, OAc = acetate, SCN = thiocyanate, Me2PO4= dimethylphosphate

Table 2.1: Imidazolium based ionic liquids used in natural polymer dissolution

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25

2.4.1.1 Dissolving cellulose in imidazolium ionic liquids

Many studies on the dissolution of cellulose in ILs can be found in the literature [18, 35, 69, 85, 88, 89, 92]. Most dissolution studies have used micro-crystalline cellulose (MCC), which is a refined wood pulp of DP 300-600. Some important works in cellulose dissolution are reviewed in this section.

In 2002, Swatloski et al. [85] first reported the use of ILs towards the dissolution and regeneration of cellulose. They tried dissolving MCC in seven different imidazolium based ILs. 1-butyl-3-methlyimidazolium chloride (BMIMCl) was found to be the most effective IL to dissolve cellulose since; it could dissolve MCC up to 10 wt% at 100˚C with conventional heating and up to 25 wt% at 80˚C with microwave heating.

Additionally, it was observed that the non-coordinating anions like BF4- and PF6- were not successful at dissolving cellulose. Further, the presence of water in imidazolium based ILs considerably reduced the solubility of cellulose. Even a 1 wt% water impurity in the IL resulted in no cellulose dissolution [85].

In 2005, Zhang et al. [84, 88] reported the use of 1-allyl-3-methylimidazolium chloride (AMIMCl) as a non-derivatised solvent for cellulose. The authors claimed that AMIMCl could dissolve cellulose up to 14.5 wt% at 80˚C without microwave heating.

Figure 2.10 shows the structures of BMIMCl and AMIMCl.

Figure 2.10: Structure of (a) BMIMCl and (b) AMIMCl

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26 Viscosity of the IL is thought to play an important role in cellulose dissolution. Low viscosity of the IL allows a high mobility of the anion, which can interact with the polymer effectively. Kosan et al. [93] reported that the acetate (Ac) containing ILs could dissolve more cellulose due to the lower viscosity of the solution when compared to the Cl^- anion. Fukaya et al. [91, 94] developed new ILs aimed to further reduce the viscosity by using a functionalised phosphate anion see Figure 2.11 [91]. In particular, the lowest viscosity IL; 1-ethyl-3-methylimidazolium dimethylphospate (EMIMMe2PO4) showed the best dissolving capacity which was 10 wt% at 45˚C.

Figure 2.11: EMIM ionic liquids with different phosphonates investigated by Fukaya et al. [91]

2.4.1.2 Dissolving protein fibres in imidazolium ionic liquids

Very limited research is available in the literature regarding silk, wool and duck feather protein fibre dissolution in ILs.

In 2002, Phillips et al. [89] firstly reported the use of ILs to dissolve domestic Bombyx mori silk fibroin. They chose imidazolium based ILs to determine the solubility of silk.

The authors stated that the pre-existing ability to dissolve cellulose in these ILs lead to selection of these ILs. In this work, 1-ethyl-3-methylimidazolium chloride (EMIMCl) dissolved the highest amount of silk 23.3 wt% whilst BMIMCl was found to dissolve 13.2 wt%. The authors also reported that applying microwave heating was not successful, due to thermal decomposition of the silk.

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27 In 2012, Byrne et al. [35] reported the on the dissolution of up to 10 wt% of wild muga silk cocoons in 1-butyl-3-methylimidazolium acetate (BMIMOAc) at 120˚C. Further, they investigated the effect of coagulant type on the structure of regenerated silk using water, methanol, ethanol, and iso-propanol. The results showed that the coagulant type did impact the properties of the regenerated silk.

In 2005, Xie et al. [18] firstly reported on dissolving wool in a series of ILs using combinations of BMIM⁺ and AMIM⁺ cations and Cl^-, Br^-, BF4- and PF6- anions. The results show ILs containing BF4- and PF6- anions are incapable of dissolving wool, whereas only Cl^- and Br^- based ILs were shown to dissolve wool similar to cellulose dissolution behaviour reported by Swatloski et al. [85]. BMIMCl and AMIMCl were the most efficient at dissolving wool, with 11 wt% and 8 wt% dissolved at 130˚C respectively. Investigations of the structure of regenerated wool using the coagulants, water, methanol and ethanol revealed that the regenerated wool structure is different to that of the native wool structure. The regenerated wool exhibited a β-sheet structure with the disappearance of the α-helix structure. A similar study was carried out by Li and Wang [69] to prepare regenerated wool films from ILs. The ability of wool dissolving in ILs eliminates the use of harsh and volatile solvent mixtures.

The dissolution of duck feather keratin in ILs was first reported by Zhao et al. [40] in 2010. The results show that the duck feather was soluble in both BMIMCl and AMIMCl, with the AMIMCl being the most effective solvent. The authors claimed that the secondary structure of the regenerated duck feather was similar to that of the original feather, which indicates that ILs are non-derivatised solvents for duck feather.

In 2013, Idris et al. [21] used the same ILs to dissolve turkey feathers. In particular, it was found that at 130°C, turkey feather keratin was soluble up to 45 %wt. It was also reported that the dissolution occurred without major chemical change of the polypeptide chain confirmation, which agreed with the previous findings of Zhao et al. [40]. However, material properties of regenerated duck feather or the use of duck feather as a blending material have not been reported to date.

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28 It could be anticipated that the ability of dissolving cellulose and protein fibres in a common IL could potentially lead to new bio-based blend materials, which are a prime focus of this project.

The value of IL solvents lies in their design flexibility. ILs can be designed simply by varying the cation and anion to meet such requirements [80, 81]. The above studies on