Novel phosphorus-containing halogen-free ionic liquids: effect of sulfonate anion size on physical properties, biocompatibility, and flame retardancy

Abstract

Two novel phosphorus-containing ionic liquids with different size sulfonate anions ([Pmim]CH₃SO₃, [Pmim]Ts) were synthesized and their physicochemical properties including thermal stability, solubility, viscosity, and melting points were comparably investigated. [Pmim]Ts, with a tosylate anion, shows a lower melting point, higher viscosity, and higher thermal stability due to the increasing anion size and uniform electron delocalization. The cytotoxicity of [Pmim]CH₃SO₃ and [Pmim]Ts against HeLa cells was also evaluated in vitro by MTT assay. It was found that [Pmim]CH₃SO₃ and [Pmim]Ts show lower toxicity than the commercial ILs with sulfate anions. Meanwhile, [Pmim]CH₃SO₃ and [Pmim]Ts were used as flame retardants for PA6, and it was found that they present different flame retardant effects. The limiting oxygen index (LOI) values of PA6/[Pmim]CH₃SO₃ and PA6/[Pmim]Ts are 26.2, and 24.4%, respectively, when their addition is 30 wt%. In the cone calorimeter test, the total heat released (THR) values of PA6 are decreased with the addition of [Pmim]CH₃SO₃ and [Pmim]Ts, and the burning residues from barren to abundant. Thermogravimetric analysis and thermal degradation kinetics were further investigated and it was found that both [Pmim]CH₃SO₃ and [Pmim]Ts can accelerate the decomposition of PA6. Because of the acidity difference of CH₃SO₃ and the Ts anion, [Pmim]CH₃SO₃, whose anion has strong acidity, shows high catalysis decomposition ability.

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Introduction

Ionic liquids (ILs) are low-temperature organic salts in general composed entirely of ions, and have shown great potential in electrochemistry,^1–3 catalysis,^4–6 liquid–liquid extraction processes,⁷ organic synthesis,^4,8 spectroscopy,⁹ carbon dioxide capture,¹⁰ etc. due to their unique properties, such as good electrical conductivity, a relatively wide electrochemically stable window, negligible vapor pressure, non-flammability, great chemical and thermal stabilities as well as excellent designability. In recent years, ILs have attracted the attention of many polymer materials scientists, and they have been extensively investigated as solvents for indissolvable macromolecules,^11–14 plasticizers,^15,16 antistatic agents^17–19 and compatibilizers for enhancing their interfacial compatibility with carbon nanotubes (CNTs)^20,21 and montmorillonite (MMT).^22,23

Research on using ILs as flame retardants for polymers is still in the primary stage. Ding et al. synthesized a phosphorus-containing ionic liquid ([PCMIM]Cl) and used it as an acid source as well as carbonization agent for polypropylene (PP). It is found that when the weight ratio of ammonium polyphosphate (APP) and [PCMIM]Cl is 5 : 1 and the total amount of intumescent flame retardant (IFR) is kept at 30 wt%, the LOI value of PP/APP/[PCMIM]Cl composite reaches 31.8%, and UL-94 testing rating is V-0.²⁴ Their further research reveals that the phosphorus-containing ILs [PCMIM]PF₆ has a synergistic effect with graphene, carbon nanotubes, which enhances the flame retardance of polylactide (PLA).^25,26 The research²⁷ reported by Li et al. found that polyoxometalates-based ionic liquid [BMIm]₃PMo is a good catalyst to enhance the efficiency of intumescent flame retardants (IFR). And her further investigation²⁸ demonstrates that the anions of PIL play a great role in the flame retardant properties of PP/IFR. Thereinto, the IL consisted of phosphotungstic acid (PWA) anions shows better synergistic effect than that containing silicotungstic acid (SiWA), or phosphomolybdic acid (PMoA). Although these results illustrate that ILs can be used as flame retardants or flame retardant synergists for some polymers, the new kind of ILs still need to be designed and explored for other polymers.

It is well known that the different kind of ILs can be designed with specific properties by changing the structure of the component ions. For rational design, the basic physicochemical properties of ionic liquids can be tuned.^29–34 In general, the thermal stability of ILs is critically dependent on the nature of both ions: imidazolium salts are more stable than tetraalkyl ammonium salts, and the stabilities of common anion are BF₄⁻ and bis(triflate)amide⁻ > tris(triflate)methyl⁻, PF₆⁻ and AsF₆⁻ > I⁻, Br⁻ and Cl⁻.^31,33 Greaves et al.³⁴ found that longer alkyl substitution on the cation can significantly increases the viscosity of ILs, which is owed to the increase of ion–ion interactions, hydrogen bonding and asymmetry of the molecules. Besides, the change to the anion structure has greater influence on the viscosity than the same changes to the cation. As for as the melting points are concerned, Ngo et al.³¹ found that the melting points of ILs decrease with the more asymmetrical substituted cations. However, the anion effect on the melting points is more complex, there is not an overall correlation between the kind or composition of anion and the melting points.^32,35

In recent years, there is growing realization that some ILs maybe not as green as expected, and they can be just as toxic as any other class of chemicals.^36,37 Therefore, it is crucial to test the toxicity for the new ILs before they used in industry. According to the regulations and laws with regard to human security and ecological (such as RoHS, WEEE and REACH), some halogen-containing flame retardants have been restricted due to the highly toxic and potentially carcinogenic dioxins formed during combustion.³⁸ Just like these halogen-containing flame retardants, the halogenated ILs also have hazardous and toxic effect to environment and human body, and they will evolve toxic and corrosive products during decomposition.^39,40 On this account, some more secure ILs with halogen-free anion should be synthesized and used as flame retardants for polymer materials.

Based on the literature,³⁷ we found that ILs with sulfate anions are relatively non-toxic than other ionic liquids. In addition, sulfur is a common-used flame retardant element. So, in this paper, two novel phosphorus-containing ionic liquids, whose anion is methanesulfonate [CH₃SO₃]⁻ or tosylate [TS]⁻, respectively, were synthesized and applied as a flame retardant for polyamide 6 (PA6). To the best of our knowledge, there are a few researches focused on the discussion of the effect of anion size on the physical–chemical properties of ILs, not to mention the roles in the flame retardant performance. In order to make the above issue clear, the thermal stability, solubility, viscosity in water and melting points of synthesized ILs were comparably investigated. Their cytotoxicity against the human cervical cancer cells (HeLa) was evaluated by MTT assay. Furthermore, the effect of anion size on the flame retardance, thermal decomposition behaviors of PA6 was also discussed.

Experimental

Materials

Pentaerythritol (PER), phosphorus oxychloride (POCl₃), methylsufonyl chloride, and para-toluenesulfonyl chloride (TsCl) were provided by Tianjin BoDi Chemical Ind. Co. Ltd. (Tianjin, China). N-Methylimidazole, dioxane, triethylamine, dichloromethane (CH₂Cl₂), n-hexane, acetone, and ethanol were all purchased from Kelong Chemical Reagent Co (Chengdu, China) and used as received. PA6 pellets (YH-800, M_n ≈ 30k) was purchased from Hunan Yueyang Baling Petrochemical Co, Ltd., China. 3-(4,5-Dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) were obtained from Beyotime Institute of Biotechnology (Shanghai, China). For the purpose of comparison, 1-butyl-3-methylimidazolium methanesulfonate ([Bmim]CH₃SO₃), and 1-butyl-3-methylimidazolium tosylate ([Bmim]Ts) were purchased from Shanghai Cheng Jie Chemical Co. Ltd., China.

Synthesis of pentaerythritol octahydrogen tetraphosphate (PEPA)

The phosphorus-containing ionic liquid precursor, 1-oxo-4-hydroxymethyl-2,6,7-trioxa-1-phosphabi-cyclo[2.2.2] octane (PEPA), was prepared according to the literature.^41,42 The total yield of PEPA was 87%. ¹H NMR (DMSO-d₆, δ, ppm): 5.12 (1H, –OH), 4.60 (6H, O–CH₂), 3.29 (2H, CH₂–OH).

Synthesis of 1-cage phosphate-3-methylimidazolium methanesulfonate ([Pmim]CH₃SO₃)

[Pmim]CH₃SO₃ was synthesized by two steps. The synthesis procedure for [Pmim]CH₃SO₃ was described as follows (Scheme 1): 90.0 g (0.5 mol) PEPA, 250 mL dichloromethane, 50.6 g (0.5 mol) triethylamine, were charged into a 500 mL four-neck flask equipped with mechanical stirrer, reflux condenser and addition funnel. After the mixture was stirred for 15 min at room temperature, 57.3 g (0.5 mol) methylsufonyl chloride was added slowly. The reaction was kept at room temperature for 5–6 h, and then the white power, (1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octan-4-yl)methyl methanesulfonate (PS) was filtered and washed with distilled water. The PS product (yield 82.5%) was dried at 80 °C in vacuum until a constant weight was obtained. Then PS was dissolved in N-methylimidazole at 70 °C and maintained for 72 h. After the extra N-methylimidazole was removed by vacuum distillation, [Pmim]CH₃SO₃, was obtained. Finally the target product was washed with acetone, and dried in a vacuum oven at 70 °C to a constant weight. The total yield of [Pmim]CH₃SO₃ was 84.6%.

Scheme 1 Synthesis route for [Pmim]CH₃SO₃ and [Pmim]Ts.

¹H NMR, (400 MHz, δ_H, (CD₃)₂SO, ppm): 9.1 (1H, s, H^a), 7.7 (1H, s, H^b), 7.6 (1H, s, H^c), 4.7 (6H, d, H^d), 4.3 (2H, s, H^e), 3.8 (3H, s, H^f), 2.3 (3H, s, H^g). ¹³C NMR, (400 MHz, δ_c, (CD₃)₂SO, ppm): 138.5 (C^a), 124.4 (C^b), 123.7 (C^c), 75.9 (C^d), 45.9 (C^e), 40.3 (C^g), 37.3 (C^h), 36.4 (C^f). ³¹P NMR, (400 MHz, δ_H, (CD₃)₂SO, ppm): −7.77 (s). Elemental analysis, calcd for C₁₀H₁₇O₇N₂PS: C 35.3, H 5.0, N 8.3, S 9.4; found: C 35.9, H 5.2, N 8.6, S 9.4%. ESI-MS, calcd for [Pmim]⁺: m/z = 245.19, found: 245.07; calcd for CH₃SO₃⁻: m/z = 95.09, found: 95.07.

Synthesis of 1-cage phosphate-3-methylimidazolium tosylate ([Pmim]Ts)

The synthesis procedure for [Pmim]Ts is similar to that of [Pmim]CH₃SO₃, and the detailed process is presented as follow: 250 mL acetone, 50.6 g (0.5 mol) triethylamine, 90.0 g (0.5 mol) of PEPA were added into a 500 mL four-neck flask equipped with mechanical stirrer, reflux condenser, addition funnel and dry nitrogen inlet. The mixture was stirred for 15 min at room temperature, and then the acetone solution contained 95.3 g (0.5 mol) para-toluenesulfonyl chloride was dropped into the reaction mixture slowly. The reaction was carried out at 80 °C for 5–6 h, and the white powder, (1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octan-4-yl)methyl 4-methylbenzenesulfonate (PTS), was obtained by filtration. Finally, the product was washed with distilled water and ethanol, respectively, and dried at 80 °C in vacuum. The total yield of PTS was 80.6%. PTS was dissolved in N-methylimidazole at 80 °C for 72 h, and the extra N-methylimidazole was removed by vacuum distillation. The product ([Pmim]Ts, yield 86.3%) was washed with acetone, and dried to a constant weight in a vacuum oven at 70 °C.

¹H NMR, (400 MHz, δ_H, (CD₃)₂SO, ppm): 9.1 (1H, s, H^a), 7.7 (1H, s, H^b), 7.6 (1H, s, H^c), 7.5, 7.1 (4H, d, Hⁱ), 4.7 (6H, d, H^d), 4.3 (2H, s, H^e), 3.8 (3H, s, H^f), 2.3 (3H, s, H^g). ¹³C NMR, (400 MHz, δ_c, (CD₃)₂SO, ppm): 145.8, 128.7, 125.9 (Cⁱ), 138.4 (C^a), 124.5 (C^b), 123.7 (C^c), 76.0 (C^d), 46.0 (C^e), 37.3 (C^h), 36.5 (C^f), 21.2 (C^g). ³¹P NMR, (400 MHz, δ_H, (CD₃)₂SO, ppm): −7.91 (s). Elemental analysis, calcd for C₁₆H₂₁O₇N₂PS: C 46.1, H 5.1, N 6.7, S 7.7; found: C 45.7, H 4.9, N 7.0, S 7.2%. ESI-MS, calcd for [Pmim]⁺: m/z = 245.19, found: 245.07; calcd for Ts⁻: m/z = 171.19, found: 171.07.

Preparation of flame retardant PA6 samples

Polyamide 6 pellets, [Pmim]CH₃SO₃, and [Pmim]Ts were all dried at 80 °C in vacuum for 8 h before processing. According to the formulation, PA6 with different amount of [Pmim]CH₃SO₃ or [Pmim]Ts were melt-blended via a co-rotating two-screw extruder (CTE20, Coperion Keya Machinery Company, Ltd, China) with a screw speed of 50 rpm at the temperature range of 210–245 °C. The extrudates were compression molded at 245 °C to obtain the standard specimens for testing.

Cell viability assays

The cytotoxicity of [Pmim]CH₃SO₃, [Pmim]Ts, [Bmim]CH₃SO₃, and [Bmim]Ts against the human cervical cancer cells (HeLa) was evaluated in vitro by MTT assay. The HeLa cells were obtained from American Type Culture Collection. The cells were seeded at a density of 4000 cells per well in 96-well plates in 200 μL complete DMEM containing 10% fetal bovine serum (FBS) and incubated in a humidified atmosphere at 37 °C under 5% CO₂ for 24 h and 48 h. Then the culture media were removed, and [Pmim]CH₃SO₃, [Pmim]Ts, [Bmim]CH₃SO₃, and [Bmim]Ts solutions with different concentrations (10⁻⁷ to 10² mmol L⁻¹) were added. After 24, and 48 h incubation, 20 μL MTT solution (5 mg mL⁻¹ PBS) was added to each well and the cells was incubated for another 1 h. After that, the medium was removed and 100 μL DMSO were added in each well to dissolve formazan crystals. The absorbance was measured by Model 680 Microplate Reader (Bio-Rad Laboratories, Inc, USA) at a test wavelength of 570 nm and a reference wavelength of 630 nm. The cell viability (%) was calculated based on the following eqn (1):where A_sample and A_control represent the absorbance of the sample and control wells, respectively.

Characterization

Nuclear magnetic resonance (NMR) measurement were performed with a Bruker Avance III400 spectrometer (Rheinstetten, Germany) operating at 400 MHz for ¹H, 100 MHz for ¹³C and 161.9 MHz for ³¹P, using d₆-dimethyl sulphoxide (DMSO) as a solvent and TMS as an internal reference. Elemental analyses (EA) were performed with a CARLO ERBA1106 instrument (Carlo Erba, Italy). ESI mass spectrometry were recorded and analyzed with a LCMS-IT-TOF spectrometer (Shimadzu, Japan).

Fourier transform infrared (FTIR) spectra were conducted with a Nicolet FTIR 170 SX spectrometer (Nicolet, America) using the KBr powder and the wavenumber range is from 4000 to 400 cm⁻¹.

The viscosity of the obtained ionic liquid in water was conducted by Ubbelohde viscometer, and calculated according to the following eqn (2). And the density of [Pmim]CH₃SO₃ and [Pmim]Ts in water was measured with a 25 mL specific gravity bottle at 25 °C.

where η₁ and η₀ are the viscosity of samples and deionized water, respectively; ρ₁ and ρ₀ are the density of samples and deionized water, respectively; t₁ and t₀ are the flow time of samples and deionized water tested by Ubbelohde viscometer, respectively.

Differential scanning calorimetry (DSC) curves were recorded with a TA Q200 DSC apparatus at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere at a flow rate of 50 mL min⁻¹.

Thermogravimetric analysis (TGA) and thermal degradation kinetics were investigated by NETZSCH TG 209 F1 apparatus. In this work, the mass of each sample was 5.0 ± 0.5 mg and the carrier gas was nitrogen with a flow rate of 50 mL min⁻¹. Each sample was heated in Al₂O₃ pans from 40 to 700 °C at various rates of 5, 10, 20 and 40 °C min⁻¹. The Flynn–Wall–Ozawa method was used to analyze the thermal degradation kinetics parameters.

The limiting oxygen index (LOI) was surveyed on the Oxygen Index Flammability Gauge (HC-2C) according to ASTM D 2863-97. The samples were molded and cut into a standard size of 120 × 6.5 × 3.2 mm³. The vertical burning test (UL-94) for the samples with the size of 130 × 13 × 3.2 mm³ was carried out by a vertical burning instrument (CF-2) according to ASTM D 3801. The cone calorimeter (cone) was performed with a FIT (UK) cone calorimeter according to ISO 5660-1. The samples with the size of 100 × 100 × 100 mm³ were measured under a heat flux of 50 kW m⁻².

Results and discussion

Chemical structure characterization

[Pmim]CH₃SO₃ and [Pmim]Ts were synthesized by two steps, and the detailed synthetic routes are shown in Scheme 1. Firstly, pentaerythritol octahydrogen tetraphosphate (PEPA) was esterified with methylsulfonyl chloride or para-toluenesulfonyl chloride with 1 : 1 molar ratio in dichloromethane or acetone, respectively, using triethylamine as an acid-binding agent. Then [Pmim]CH₃SO₃ or [Pmim]Ts can be obtained with high yield via the quaternization reaction of the corresponding intermediate product with N-methylimidazole under mild conditions. The 3D structure of [Pmim]CH₃SO₃ and [Pmim]Ts is presented in Fig. 1.

Fig. 1 3D structure of [Pmim]CH₃SO₃ and [Pmim]Ts simulated by Chembio 3D MM2 Minimize Energy.

The chemical structure of the obtained ILs is ascertained by ¹H, ¹³C, and ³¹P NMR, which are shown in Fig. 2 and 3. As shown in Fig. 2 and 3, the chemical shifts of H protons in cation of [Pmim]CH₃SO₃ and [Pmim]Ts are similar. The peaks appearing at 9.1, 7.7, and 7.6 ppm correspond to the H^a, H^b, and H^c of imidazole ring. The double peak at 4.7 ppm is assigned to H^d, and the single peak at 4.3 ppm is assigned to methylene H^e protons. The chemical shift of substituent methyl in imidazole ring (H^f) appears at 3.8 ppm. The single peak at 2.3 ppm is assigned to methyl H^g protons in anion, and the double peaks at 7.5, 7.1 ppm are assigned to Ph-Hⁱ of anion. Similarly, the carbon signals in cation of [Pmim]CH₃SO₃ and [Pmim]Ts are basically the same. The carbon signals of imidazole ring appear at 138.4, 124.5, 123.7 ppm, and the carbon signals of caged phosphate show at 76.0 (C^d), 37.3 (C^h) ppm. The carbon signal of the substituent methyl in imidazole ring (C^f) appears at 36.5 ppm, and the carbon signal of methylene between imidazole and caged phosphate (C^e) is found at 46.0. Besides, the carbon signals of benzene ring (Cⁱ) appear at 145–125.9 ppm, and the peak at 21.2 ppm is assigned to methyl in anion (C^g). Simultaneously, there is only a single peak at −7.7, −7.91 ppm in the ³¹P NMR spectra of [Pmim]CH₃SO₃ and [Pmim]Ts, respectively, suggesting that only one type of phosphorus exists in these two ionic liquids. All the results demonstrate that [Pmim]CH₃SO₃ and [Pmim]Ts have been successfully synthesized.

Fig. 2 ¹H, ¹³C and ³¹P NMR spectra of [Pmim]CH₃SO₃.

Fig. 3 ¹H, ¹³C and ³¹P NMR spectra of [Pmim]Ts.

Thermal analysis

The basic thermal properties of ILs are vital for their application in polymers. DSC traces of [Pmim]CH₃SO₃ and [Pmim]Ts at 10 °C min⁻¹ are shown in Fig. 4. [Pmim]CH₃SO₃ exhibits a higher melting point (234 °C) than that of [Pmim]Ts (216 °C). And no glass transition temperatures are found in their curves. It is well known that the symmetry of ILs chemical structure, H-bonding ability, the van der Waals interactions, as well as the size of ions will affect the melting points of ILs.³⁵ Generally, ILs with lower structural symmetry, weaker intermolecular forces, as well as, more uniform distribution of anions and cations on the charge present lower melting points.⁴³ Therefore, [Pmim]Ts has lower melting point than [Pmim]CH₃SO₃ because of large anion size and well electron delocalization.

Fig. 4 DSC thermograms of [Pmim]CH₃SO₃ and [Pmim]Ts for the first heating scans at 10 °C min⁻¹.

For [Bmim]CH₃SO₃ and [Bmim]Ts containing the same anions with [Pmim]CH₃SO₃ and [Pmim]Ts, their melting points are found as low as 72 °C and 57 °C, respectively (shown in Fig. S1†). This illustrates that the high melting points of [Pmim]CH₃SO₃ and [Pmim]Ts are resulted by the different substitution on imidazolium ring. As we know that the melting points will be enhanced with the increase of the length or branching degree of alkyl chain substituted on the cation ring.^29,43 For [Pmim]CH₃SO₃ or [Pmim]Ts, both of them have unsymmetrical, large caged phosphate ester structure in cation, which make them show high melting points than [Bmim]CH₃SO₃ and [Bmim]Ts. Katritzky et al.⁴⁴ also found that the melting points of some imidazolium bromides ILs with larger substituent on imidazole ring are higher than 200 °C.

It has been reported that the thermal stability of ILs is heavily dependent on the structure of ionic liquids.^43,45 The thermal stabilities of [Pmim]CH₃SO₃ and [Pmim]Ts in nitrogen were performed by TGA, and their TG and DTG curves are shown in Fig. 5. The corresponding data such as onset decomposition temperature (T_5%), the temperature at maximum weight loss rate (T_max), and char residue in 700 °C (R₇₀₀) are summarized in Table 1. It is found that the T_5%, T_max and char residue of [Pmim]Ts are 344.2 °C, 371.4 °C and 42.8%, respectively, however, the T_5%, T_max and char residue of [Pmim]CH₃SO₃ are 339.2 °C, 368.3 °C and 36.6%, respectively. In general, for ILs with certain cation, the thermal stability increases with increasing anion size.⁴⁵ The benzene ring in anion of [Pmim]Ts benefits forming more stable carbon layer structure at high temperature, which make it show relatively high decomposition temperature and residue.

Fig. 5 TG (a) and DTG (b) curves of [Pmim]CH₃SO₃ and [Pmim]Ts in nitrogen atmosphere.

Table 1 Thermal stabilities of [Pmim]CH₃SO₃ and [Pmim]Ts in nitrogen atmosphere

Sample	T5% (°C)	Tmax (°C)	R700 (%)
[Pmim]CH3SO3	339.2	368.3	36.6
[Pmim]Ts	344.2	371.4	42.8

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Solubility and viscosity in water

There are few reports concerning the miscibility of ILs with organic solvents. To find the solubility behavior of [Pmim]CH₃SO₃ and [Pmim]Ts in some common solvents, 5 mg IL was dissolved into 1 mL solvent to observe their dissolving process, and the results are presented in Table 2. From the Table 2 it can be seen clearly that both [Pmim]CH₃SO₃ and [Pmim]Ts can easily and rapidly dissolved in proton (H₂O, CH₃OH, AcOH) or non-proton polar solvent (DMSO), and are insoluble in nonpolar solvents or less polar solvents, such as DMF, cyclohexane, THF, CHCl₃, etc. This means the synthesized ILs have larger polarity leading to their excellent solubility in polar solvents. As for as DMF or ethanol is concerned, although both of them have high polarity, [Pmim]CH₃SO₃ and [Pmim]Ts show different solubility behaviour, i.e. [Pmim]Ts is partially soluble in DMF and insoluble in ethanol; [Pmim]CH₃SO₃ is insoluble in DMF and partially soluble in ethanol. This illustrates that besides polarity, the dispersion force or hydrogen-bond forming ability will also affect the solubility of ILs.

Table 2 Solubility of [Pmim]CH₃SO₃ and [Pmim]Ts^a

Sample	[Pmim]CH3SO3	[Pmim]Ts	Sample	[Pmim]CH3SO3	[Pmim]Ts
a +++: dissolved immediately; ++: completely dissolved after stirring; +: partially dissolved; —: insoluble.
H2O	+++	+++	THF	—	—
CH3OH	+++	++	EtOAc	—	—
DMSO	++	++	Acetone	—	—
AcOH	++	++	CH2Cl2	—	—
Ethanol	+	—	CHCl3	—	—
DMF	—	+	Et2O	—	—
Cyclohexane	—	—	Benzene	—	—
1,4-Dioxane	—	—

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In order to investigate their viscosity, we prepared [Pmim]CH₃SO₃ and [Pmim]Ts water solution with different concentration due to their high melting point and solid state at room temperature. The variations in the viscosity of [Pmim]CH₃SO₃ and [Pmim]Ts are comparatively represented in the plots depicted in Fig. 6. It can be seen clearly that the viscosity of [Pmim]Ts is always higher than that of [Pmim]CH₃SO₃. As we know, the viscosity of ILs has great relationship with van der Waals interactions, Coulomb force and H-bonding.⁴⁶ Besides these, the geometry and molar mass of the anions also have strong influence on the viscosity of ILs.⁴³ And our results are in accord with the previous reports. That is to say, the more uniform charge delocalization, and the bigger size of anion make [Pmim]Ts to show high viscosity. Since water has a significant effect on the viscosity of ILs, all the detected values are much lower than that of ILs measured without water.^33,43

Fig. 6 Viscosity of [Pmim]CH₃SO₃ and [Pmim]Ts in water dependence its concentration.

Cytotoxicity

Before wide applications, ILs should be assessed for their impact on human health and environment. One of the useful evaluation methods is in vitro testing on cell lines, in which dedifferentiated cancer cells provide quick and convenient first information about the toxic potential of substances.⁴⁷ Fig. 7 shows the effect of different [Pmim]CH₃SO₃ and [Pmim]Ts concentrations on HeLa cells viability. The obtained data are expressed as the mean ± SD of three independent experiments performed in triplicate. From the Fig. 7, we can see that HeLa cells are not almost affected for 24 or 48 h incubation when the concentration of [Pmim]CH₃SO₃ and [Pmim]Ts is at less than 1 mol L⁻¹. When [Pmim]CH₃SO₃ and [Pmim]Ts concentration is enhanced to 10 mol L⁻¹, they cause a slight cytotoxicity for 24 h incubation, however, further increase the incubation time to 48 h, a significant cytotoxic effect can be observed. This means HeLa cells viability decreases with the incubation time increased at high [Pmim]CH₃SO₃ and [Pmim]Ts concentration. The cytotoxicity of [Bmim]CH₃SO₃ and [Pmim]Ts on HeLa cell viability at same concentration gradient is also investigated and presented in Fig. S1.† It can be seen that their toxicity is higher than [Pmim]CH₃SO₃ and [Pmim]Ts, i.e. when their concentration is 1 mol L⁻¹, a distinct cytotoxicity cannot be neglected. By comparison the results reported by Cvjetko et al.,⁴⁷ the cytotoxicity of [Pmim]CH₃SO₃ and [Pmim]Ts on HeLa cells is substantially lower than [BMIM][Tf₂N], [BMIM][BF₄], [BMIM][PF₆], [MMIM][PF₆] when the concentration of ILs and incubation time is the same. This result is beyond our expectation, that is to say, introduction of the cage phosphate into the imidazole cation improves the biocompatibility of ILs.

Fig. 7 Effect of different [Pmim]CH₃SO₃ and [Pmim]Ts concentrations on HeLa cell viability for (a) 24 h and (b) 48 h.

The half maximal effective concentration (EC₅₀), defined as the concentration of ionic liquids that resulted in 50% growth inhibition, is calculated by extrapolation method from plots of remaining activity vs. inhibitor concentration for the MTT assay. The detailed EC₅₀ data for [Pmim]CH₃SO₃ and [Pmim]Ts are shown in Table 3, in which the data for commercial [Bmim]CH₃SO₃ and [Bmim]Ts are also listed for comparison. As shown in Table 3, our synthesized ILs present higher EC₅₀ than that of [Bmim]CH₃SO₃ and [Bmim]Ts. [Pmim]CH₃SO₃ inhibits the proliferation of HeLa cells with the EC₅₀ of 11.59 mmol L⁻¹ after 48 h incubation, which is lower than that of [Pmim]Ts (12.55 mmol L⁻¹). And this trend is also presented for 24 h incubation. It demonstrates that [Pmim]Ts has lower toxicity than [Pmim]CH₃SO₃ for HeLa cells. Compared to EC₅₀ values of [Bmim]CH₃SO₃ and [Bmim]Ts (3.42, and 1.70 mmol L⁻¹, respectively, for 48 h), [Pmim]CH₃SO₃ and [Pmim]Ts present less toxicity, illustrating again the caged phosphate ester substituent on imidazole ring decreases the cytotoxicity of ILs for HeLa cells.

Table 3 EC₅₀ values of ILs towards HeLa cells

Incubation time (h)	EC50 (mmol L^−1)
	[Pmim]CH3SO3	[Pmim]Ts	[Bmim]CH3SO3	[Bmim]Ts
24	18.63	21.42	6.37	6.94
48	11.59	12.55	3.42	1.70

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Flame retardant effect for PA6

Because the initial degradation temperature of [Pmim]CH₃SO₃ and [Pmim]Ts are higher than 330 °C, and the processing temperature of PA6 composites is 210–245 °C, so the [Pmim]CH₃SO₃ and [Pmim]Ts can keep thermal stability during melt processing to obtain the flame retardant PA6 composites. The vertical burning tests (UL-94) and the limiting oxygen index (LOI) have been widely used to investigate the flame retardance of polymer. The testing results of virgin PA6 and PA6 composites containing [Pmim]CH₃SO₃ or [Pmim]Ts with different weight ratio are summarized in Table 4. The pure PA6 is a flammable material with a LOI value of 20.5% and fails in the UL-94 test. When [Pmim]CH₃SO₃ is added, the LOI value of PA6 enhances with the increase of [Pmim]CH₃SO₃ content. Thereinto, PA6/[Pmim]CH₃SO₃30 shows good flame retardance, and its LOI and UL-94 test rating is 26.2% and V-2, respectively. When the same amount of [Pmim]Ts (30 wt%) is introduced, however, its LOI value is only 24.4%. It was reported that when 15 wt% aluminium hypophosphite (AP) was added to PA6, the LOI values increased to 25.7%; when 20 and 25 wt% AP was added, the LOI values increased to 26.3 and 27.8%, respectively.⁴⁸ Compared to AP, [Pmim]CH₃SO₃ and [Pmim]Ts show a bit low flame retardant effect for PA6. During UL-94 test, it is found that both [Pmim]CH₃SO₃ and [Pmim]Ts can accelerate the dripping of PA6, which removes heat from the flame zone and contributes to flame extinguishment. Unfortunately, the cotton indicator is ignited by the flaming particles. We deduce that [Pmim]CH₃SO₃ and [Pmim]Ts will promote PA6 decomposition and take the flame retardant effect, which is just like other phosphorus-containing flame retardants.^48–51

Table 4 Flame retardance and thermal stability of PA6/ILs composites

Sample	IL content (wt%)	LOI (%)	UL-94	T5% (°C)	Tmax (°C)	R700 (%)
a No rating.
PA6	—	20.5	NR^a	390.2	450.6	0
PA6/[Pmim]CH3SO310	10	23.6	V-2	337.7	389.0	4
PA6/[Pmim]CH3SO320	20	24.5	V-2	328.3	376.2	9
PA6/[Pmim]CH3SO330	30	26.2	V-2	328.6	366.1	14
PA6/[Pmim]Ts10	10	23.5	V-2	345.7	394.6	5
PA6/[Pmim]Ts20	20	24.2	V-2	335.9	380.4	10
PA6/[Pmim]Ts30	30	24.4	V-2	330.2	367.8	14

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Cone calorimetry is one of the most useful bench-scale fire tests that attempts to predict the fire performance in real-world fires.^52,53 For better understanding the effect of [Pmim]CH₃SO₃ and [Pmim]Ts on flame retardancy of PA6, cone calorimeter tests with a heat flux of 50 kW m⁻² were conducted. Fig. 8 illustrates the total heat release (THR) curves of all testing samples, and Fig. 9 shows the digital photos of partial samples after the cone test. The heat release rate (HRR) curves, total smoke release (TSR) curves are shown in Fig. S3† and the detailed cone calorimetric results were summarized in Table S1.† Although the addition of [Pmim]CH₃SO₃ and [Pmim]Ts makes the PHRR higher, they reduce the total heat release of PA6 composites. The THR value of neat PA6 is 93 MJ m⁻²; however, the THR values of PA6/[Pmim]CH₃SO₃10, PA6/[Pmim]CH₃SO₃20, and PA6/[Pmim]CH₃SO₃30 decrease to 87, 81 and 67 kW m⁻²; the THR values of PA6/[Pmim]Ts10, PA6/[Pmim]Ts20, PA6/[Pmim]Ts30 decrease to 89, 82 and 75 kW m⁻², respectively. In addition, we also can see from Fig. 9 and Table S1,† pure PA6 leaves nothing after burning (Fig. 9a) and its char residue is zero; however, the burning residues of PA6/[Pmim]CH₃SO₃20 and PA6/[Pmim]Ts20 are intumescent and abundant (Fig. 9b and c).

Fig. 8 The total heat release curves of (a) PA6/[Pmim]CH₃SO₃ and (b) PA6/[Pmim]Ts.

Fig. 9 Digital photographs of the burning residues after the cone calorimeter test: (a) PA6; (b) PA6/[Pmim]CH₃SO₃20; (c) PA6/[Pmim]Ts20.

At the same time, we can see that the time to ignite (TTI) of PA6/ILs composites is shortened and the peak heat release rate (PHRR) is elevated compare to virgin PA6 (Fig. S3 and Table S1†). According to our previous results, the [Pmim]CH₃SO₃ and [Pmim]Ts will promote PA6 decomposition. There will be many small molecules over the surface of polymer during the heat radiation before ignite, which will made them easy to ignite and reach a higher PHRR. From the above analyses, it could be conclude that [Pmim]CH₃SO₃ and [Pmim]Ts will promote the char formation of PA6 during combustion, however, the char is to thin and weak to stamp out the fire adequately.

Thermal degradation investigation

In order to make clearly if [Pmim]CH₃SO₃ or [Pmim]Ts can accelerate PA6 degradation, the thermal stability of the flame retardant PA6 composite is investigated by TGA. Fig. 10 shows the TG and DTG curves of PA6/[Pmim]CH₃SO₃ and PA6/[Pmim]Ts under nitrogen atmosphere, and the T_5%, T_max, and char residue in 700 °C for all samples are summarized in Table 4. The pure PA6 presents one major stage of weight loss, occurring in the range of 400–500 °C, which is attributed to the release of water, hydronitrogens, carbon monoxide, carbon dioxide, and hydrocarbon fragments.⁵⁴ And none residue is left at 700 °C. The addition of [Pmim]CH₃SO₃ or [Pmim]Ts decreases both T_5% and T_max of PA6. On the contrary, the residue yields of PA6 composites increase instead. Comparatively, the thermal stability of PA6/[Pmim]Ts is higher than that of PA6/[Pmim]CH₃SO₃, which demonstrates weak accelerating decomposition effect of [Pmim]Ts.

Fig. 10 TG and DTG curves of the samples in N₂: (a) and (c) TG curves and (b) and (d) DTG curves.

To further demonstrate [Pmim]CH₃SO₃ or [Pmim]Ts will accelerate PA6 decomposition, the calculated and experimental TG curves of PA6/[Pmim]CH₃SO₃20 and PA6/[Pmim]T20 under N₂ are illustrated in Fig. 11. The calculated TG curves are derived from the following equations:

M_(cal)PA6/X20 = 80% × M_(exp)PA6 + 20% × M_(exp)X (3)

where X is [Pmim]CH₃SO₃ or [Pmim]Ts, M_(exp) is the experimental mass of corresponding samples, while M_(cal) is the calculated mass of PA6, [Pmim]CH₃SO₃, or [Pmim]Ts, respectively. As shown in Fig. 11, the experimental decomposition temperatures of PA6/[Pmim]CH₃SO₃20 or PA6/[Pmim]Ts20 are much lower than that of calculated data, and the real residue is almost as same as the calculated one. This also manifests that [Pmim]CH₃SO₃ or [Pmim]Ts has the catalytic activity for accelerating decomposition of PA6.

Fig. 11 Calculated and experimental TG curves of (a) PA6/[Pmim]CH₃SO₃20 and (b) PA6/[Pmim]T20 in N₂.

Furthermore, to investigate the degradation behavior of PA6 composites, the Flynn–Wall–Ozawa method (eqn (4)) is used to analysis their thermal degradation kinetics.⁵⁵ This method represents a relatively simple method for determining the degradation activation energy directly from a log β versus 1/T plot.

where A is the pre-exponential factor, R is the gas constant, E is the activation energy, and β is heating rate, T is the Kelvin temperature, and f(α) is the integrated form of the conversion dependence function. The activation energy data of samples calculated by Flynn–Wall–Ozawa method are listed in Table S2.† And the thermal degradation activation energy curves at 5, 10, 15, 20, 35, 50, 60, and 70% conversion of PA6, PA6/[Pmim]CH₃SO₃ 20 as well as PA6/[Pmim]Ts20 are comparably shown in Fig. 12.

Fig. 12 Plots of activation energy vs. conversion of PA6, PA6/[Pmim]CH₃SO₃20 and PA6/[Pmim]Ts20.

It can be seen clearly from Fig. 12 that the activation energy of PA6 composite at any conversion is lower than pure PA6 when [Pmim]CH₃SO₃ or [Pmim]Ts is added. PA6/[Pmim]CH₃SO₃20 and PA6/[Pmim]Ts20 present the same activation energy at lower conversion (10%). However, at relatively higher conversion, the activation energy of PA6/[Pmim]Ts20 is higher than PA6/[Pmim]CH₃SO₃20. As the conversion increased, this trend becomes more obvious. Although both [Pmim]CH₃SO₃ and [Pmim]Ts can accelerate PA6 decomposition, PA6/[Pmim]Ts is relatively more stable and needs more energy to decompose than PA6/[Pmim]CH₃SO₃, which is consistent with previous TGA results. This could be due to the acidity difference of CH₃SO₃ and Ts anions, thereinto [Pmim]CH₃SO₃ whose CH₃SO₃ anion has strong acidity shows high catalysis decomposition ability.

Conclusion

Two novel phosphorus-containing ionic liquids with different size sulfonate anions were successfully synthesized. Because of different anion size and electron delocalization level, they present different thermal stability, melting points, and viscosity. And [Pmim]Ts shows a relatively higher decomposition temperature, lower melting point, and higher viscosity than that of [Pmim]CH₃SO₃. The difference in anion size also leads to different cytotoxicity, in which Pmim]CH₃SO₃ has a higher toxicity than [Pmim]Ts for HeLa cells. When [Pmim]CH₃SO₃ and [Pmim]Ts is used as flame retardant for PA6, both of them can increase the LOI values of PA6, formed expansion layer during the cone test. But the char layer of the PA6/ILs composites is too thin and weak to stamp out the fire of polymer adequately. Thermogravimetric analysis and thermal degradation kinetic results show that the incorporation [Pmim]CH₃SO₃ and [Pmim]Ts into PA6 can accelerate the decomposition of PA6. Because the acidity of CH₃SO₃ anion is strong, [Pmim]CH₃SO₃ presents higher catalysis decomposition ability for PA6 than [Pmim]Ts. Due to the high accelerating decomposition to form char ability, [Pmim]CH₃SO₃ shows better flame retardant effect on PA6 than [Pmim]Ts. Based on the results, we can draw a conclusion by changing anion size, not only physicochemical or cytotoxicity of ILs but also their flame retardant effect can be tuned.

Acknowledgements

This work was supported financially by the National Science Foundation of China (51573104 and 51421061), and the Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026), and SKLPME.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09515a

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