Solubility of neutral and charged polymers in ionic liquids studied by laser light scattering

doi:10.1016/j.polymer.2010.11.034

Polymer

Volume 52, Issue 2, 21 January 2011, Pages 481-488

https://doi.org/10.1016/j.polymer.2010.11.034 Get rights and content

Abstract

The solubility and chain conformation of different types of homopolymers in low viscosity ionic liquids (ILs), 1-allyl-3-methylimidazolium chloride ([AMIM][Cl]) at 50 °C and 1-butyl-3-methylimidazolium formate ([BMIM][COOH]) at 25 °C, were studied by laser light scattering (LLS). For neutral polymers, such as polyvinyl alcohol and polysulfonamide, aggregation occurred in all the cases except for polyvinyl alcohol in [BMIM][COOH]. For negative polyelectrolytes, such as DNA and polystyrene sulfonate, single chain conformation was observed. However, the hydrodynamic radius of both polymers was much smaller than that in good solvents, suggesting that the chains were condensed. Cellulose was soluble in [AMIM][Cl], and non-diffusive mode was observed by dynamic light scattering. Zeta potential analysis indicated that cellulose exhibited the feature of polyelectrolyte. The solubility of homopolymers could be qualitatively explained by treating polymer/IL as a ternary system: polymer, cation, and anion. It was the mutual interactions determined the solubility and conformation of polymers in ILs.

Graphical abstract

Introduction

Ionic liquids (ILs) are usually composed of large organic cations and either inorganic or organic anions [1]. Compared with the traditional inorganic salts, most ILs are liquid under ambient conditions [2]. When used as solvents, ILs have many distinct advantages [3], such as extremely low volatility and toxicity, excellent thermal and chemical stability, high ionic conductivity and ease of recycling. ILs have found many practical applications in the fields of synthesis and catalysis [4], [5], extraction and separation [6], [7] crystallization [8], electrochemistry [9], and are considered as the potential “green solvents” for chemistry [10], [11].

In the past decade, ILs also attract a great deal of attention in polymer science. On one hand, ILs are used as the media for several types of polymerization processes, [12], [13] such as living radical polymerization, group transfer polymerization, and polycondensation. On the other hand, ILs are served as the solvents into which the polymer transformation or material processing to be carried out. It has been reported that ILs show good solubility to natural polymers and biopolymers, such as cellulose [14], chitin [15], various polysaccharides [16], Bombyx mori silk [17], and wool keratin [18], most of which are insoluble in water or in conventional organic solvents. As for synthetic polymers, some of them, such as poly(ethylene oxide) (PEO) [19], poly(methyl methacrylate) [20], polyacrylonitrile [21], poly(m-phenylene isophthal amide) [22], and polyarylsulfone [23] are soluble in ILs. But most synthetic polymers, such as polyethylene, polyester, polyuethane, and nylon, have not found suitable ILs for dissolution.

ILs are different from water and conventional organic solvent to certain extent. Ueki and Watanabe [24] reported that poly(N-isopropylacrylamide) in 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone) imide [EMIM][NTf₂] exhibited an upper critical solution temperature (UCST) behavior, in contrast to its typical lower critical solution temperature (LCST) behavior in aqueous solutions. He et al. [25] reported the micellization behavior of poly((1,2-butadiene)-block-ethylene oxide) (PB-PEO) in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]), as well as the temperature-induced transportation of PB-PEO between ILs and water [26]. Susan et al. proposed that the compatible IL and polymer binary systems form ion gels, in which the ion transport is decoupled from the segmental motion of the polymer [27].

Understanding the forces governing the solubility and solvation of polymers in ILs are still at early stage of development. Compared with molecular solvents, ILs combine strong Coulomb interactions and many other weak interactions, including hydrogen bonding, cation-π interaction, van der Waals interactions and so on. The general principle of “like dissolves like” is not applied to polymer/IL systems [28]. The slow kinetics of dissolution, as well as the occurrence of phase separation[29] and gel formation [30], made the situation even more complicated. It is not an easy task to predict the factors responsible for polymer solubility in ILs [31].

Laser light scattering (LLS) is a powerful technique to study the solubility of polymers in ILs at low concentrations. However, LLS study on polymer/IL system is limited. On one hand, the viscosities of ILs are generally two or more orders higher than those of molecular solvents under similar conditions. The diffusion of polymer chains in IL is extremely slow. Sometimes it will take unrealistic long time to measure the time-averaged intensity–intensity correlation functions. On the other hand, it is difficult to remove all the impurities, including water, from ILs [1]. The amount of impurity is in the order of ∼1% or more, higher than the polymer concentrations used in LLS. Moreover, the water content varies with time because of the hygroscopicity of ILs. It is difficult to determine the dn/dC values of polymers in ILs by conventional differential refractometer. Without dn/dC value, the weight-averaged molecular weight (M_w) cannot be accurately determined by LLS.

In this work, we attempt to elucidate the solubility of different kinds of homopolymers in ILs by LLS. To bypass the above mentioned difficulties, we chose two ILs with relatively low viscosity, 1-allyl-3-methylimidazolium chloride ([AMIM][Cl]) and 1-butyl-3-methylimidazolium formate ([BMIM][COOH]). Their formulae are shown in Scheme 1. The chosen homopolymers form clear solutions in either or both ILs, and they are divided into three categories: (1) neutral polymers, including hydrophilic polyvinyl alcohol (PVA) and hydrophobic polysulfonamide (PSA); (2) polyelectrolytes, mainly including anionic polymers of DNA and sodium polystyrene sulfonate (PSS); and (3) cellulose. The polymer samples dissolved in [AMIM][Cl] are studied at 50 °C to alleviate the effect of high viscosity, while those samples in [BMIM][COOH] are studied at 25 °C. Our study focuses on the chain conformation of different polymers in ILs. The calculated dn/dC value is applied when the determination of M_w is necessary. Our study indicates that the solubility of polymers in ILs is quite different from that in water or organic solvents.

Section snippets

Materials

PVA (98–99% hydrolyzed, M_w = 1.5–1.9 × 10⁵), salmon testes DNA (∼2000 bps), and PSS (M_w: 1.0 × 10⁶) were purchased from Sigma–Aldrich (USA). PSA (4.0 × 10⁵ determined by GPC) was provided by Shanghai Tanlon Fiber Co. (China). Scheme 1 shows the structure of PSA. All these polymers were used as received. The cotton cellulose with degree of polymerization (DP) about 2400 was received from Shandong Helon Co., Ltd (China) as a gift. It was vacuum-dried at 60 °C for 24 h to remove the moisture

Results and discussion

To clarify the solubility of polymers in ILs, we firstly study the behavior of ILs themselves by LLS, and then conduct a detailed study on each of the polymers following the order of neutral polymer, charged polymer and cellulose. On the basis of the obtained results, we propose a qualitative dissolution mechanism in the end.

Cellulose in ILs

The solubility of cellulose in ILs has been widely investigated by experimental studies and computer simulations [43], [44], [45], [46], [47], [48], [49]. It is generally accepted that the anions of ILs form hydrogen bonds with the carbohydrate hydroxyl protons during the dissolution process [14], [45]. The solubility of cellulose, therefore, is determined by the hydrogen bond basicity of ILs, and strong basicity is necessary to weaken the inter- and intra-molecular hydrogen bonds of the

Conclusions

LLS study yielded essential information on the solubility of polymers in ILs. The polymer/IL should be treated as a ternary system rather than binary system. Therefore, a variety of mutual interactions, some of which were polymer specific, worked together to control the solubility and conformation of polymers in ILs. Even though it was difficult to work out the exact rules governing the dissolution of polymers in ILs, our preliminary study indicated that, to achieve dissolution at molecular

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (20774004, 50873025), Shanghai Science and Technology Commission (09JC1400800), Shanghai Municipal Education Commission (08GG11), and the innovation funds for Ph.D students (Ye Chen) of Donghua University.

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Solubility of neutral and charged polymers in ionic liquids studied by laser light scattering

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Results and discussion

Cellulose in ILs

Conclusions

Acknowledgments

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