Crystallization and Rheology of Poly(ethylene oxide) in Imidazolium

Aug 2, 2016 - Lee and Lodge. 2010 1 (13), pp 1962–1966. Abstract: We describe the LCST-type phase behavior of poly(ethylene oxide) (PEO) (M = 5000 a...
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Crystallization and Rheology of Poly(ethylene oxide) in Imidazolium Ionic Liquids Fuyong Liu,†,‡,§ Yuxia Lv,†,‡ Jiajian Liu,†,‡ Zhi-Chao Yan,†,‡ Baoqing Zhang,†,‡ Jun Zhang,*,†,‡ Jiasong He,† and Chen-Yang Liu*,†,‡ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Institute of Environmental Science, Shanxi University, Taiyuan 030006, China S Supporting Information *

ABSTRACT: Polymer and ionic liquid (IL) mixtures have attracted an increasing amount of attention due to their unique properties and potential applications. The interactions between poly(ethylene oxide) (PEO) and imidazolium ILs of different cation alkyl lengths and anion structures have been investigated by measuring melting points (Tm), contact angles, and rheological properties. Tm of crystalline PEO dramatically decreased when it was blended with ILs. Similarly, the contact angles of different ILs on a PEO surface proportionally decreased. The interaction energy, as calculated from melting point depression using the Flory equation, increased with the length of imidazolium alkyl cations and the size of anions. The different anionic structures had a more significant influence on the interaction energy than the alkyl chain lengths of cations. These trends accorded with the solubility obtained by high-energy X-ray diffraction and swelling ratio measurements of PEO in different ILs [Asai et al. Macromolecules 2013, 46, 2369−2375] and the solubility of poly(methyl methacrylate) in different ILs [Ueno et al. Langmuir 2014, 30, 3228−3235]. The rheological behavior of PEO in three different anionic ILs has also been studied to determine the effect of the anions on PEO conformations. The molecular weight dependence of the intrinsic viscosity of PEO in ILs revealed that the solvent quality of ILs (from poor solvents to good solvents) is highly influenced by anionic structures, which was consistent with the results of the melting point depression and contact angle.

1. INTRODUCTION Ionic liquids (ILs) have been widely promoted as “green solvents” and are increasingly popular in chemistry and in the industry due to their low melting point, low volatility, large liquid range, nonflammability, thermal and chemical stability, tunable solvation, and ionic conductivity.1 With respect to polymers, ILs have been widely used as solvents for polymerization,2 plasticizers,3 dissolving and processing celluloses, 4 and polymer electrolytes. 5 There are complex interactions in ILs, including Coulombic interactions, hydrogen bonds, π−π interactions, and van der Waals forces.6 These interactions are highly influenced by the structure of cations and anions in ILs and thereby determine the solvation structure and solubility of polymers in ILs. The solubility of certain polymers in a few ILs has been reviewed.7,8 Poly(ethylene oxide) (PEO) is one of the most important synthetic polymers due to its biocompatibility, low toxicity, and peculiar amphoteric character.9 Many unique properties can be obtained by combining PEO with ILs, which are used, for example, as electrolytes of lithium ion batteries,10 in solar cells,11 as functional solvents in extraction systems,12 as fuel cells, and in thin-film transistors.13,14 Interactions between PEO © 2016 American Chemical Society

and ILs and the conformation of PEO in ILs have been studied by small-angle neutron scattering (SANS), rheology, and dynamics simulations. Triolo et al. revealed that 1-butyl-3methylimidazolium tetrafluoroborate (C4mimBF4) was a good solvent for PEO in a SANS study.15 Using SANS, Atkin and coworkers16,17 found that ethylammonium nitrate (EAN) and lithium tetraglyme bis(trifluoromethanesulfonyl)amide behaved as weak good solvents for PEO. Steady shear viscosity measurements were also performed on PEO dissolved in EAN and propylammonium nitrate (PAN) by Atkin and coworkers.18 This study revealed three distinct solvent qualities: good (water), good-theta (EAN), and theta (PAN). The PEGIL interactions were investigated by molecular dynamics (MD) simulations by Costa and co-workers.19 They found that calculated distributions of end-to-end distances and radiuses of gyration (Rg) were nearly Gaussian and that PEO chains were more extended in 1-butyl-3-methylimidazolium hexafluorophosphate (C4mimPF6) than in 1,3-dimethylimidazolium Received: June 1, 2016 Revised: July 24, 2016 Published: August 2, 2016 6106

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Macromolecules hexafluorophosphate (C1mimPF6). Yethiraj and co-workers20 investigated the Rg of PEO as a function of molecular weight (MW) from 400 to 1800 g/mol in C4mimBF4 and water solutions by atomistic simulation methods. They determined power law relationships Rg ∼ N0.9 for PEO in C4mimBF4 and Rg ∼ N0.5 for PEO in water. This indicated that PEO chains were in a more stretched conformation in C4mimBF4 than in water. In a more recent study,21 the authors investigated the behavior of PEO in C4mimBF4 using a coarse-grained method and found that the exponent for N ranged from 0.28 to 0.34, which was similar to the 0.33 value for collapsed chains. The solvating mechanism and hydrogen bond interactions of PEO in ILs have also attracted attention. Asai et al.22 studied the solvation structures of PEO in imidazolium-based ILs using high-energy X-ray diffraction and molecular dynamics simulation methods to determine the relationships between the microscopic structures and the macroscopic properties of PEGbased gels (e.g., swelling ratio and χ parameters). The hydrogen bonds between PEO (or PEO derivatives) and imidazoliumbased ILs were also investigated by NMR.23,24 Oxygen atoms of PEO were found to form hydrogen bonds with protons at the imidazolium cation. Although a few solubility parameters and polarity scales have been associated with the solubility of certain polymers,7,8,25 no universal theory has been developed to assess the solubility of polymers in ILs. While the interaction parameter (χ) is used to determine the miscibility of polymers and molecular solvents in the Flory theory, there exists more complex intramolecular (between the cations and anions in ILs) and intermolecular interactions. Moreover, the conformation of PEO in IL solutions remains unresolved.15,16,19,20 To obtain universal and representative results, two groups of imidazolium ILs were employed in this study (Scheme 1). Group I ILs have the same

anions, the molecular weight of PEO, and the test temperatures were determined.

2. EXPERIMENTAL SECTION Materials. Different molecular weight (ranging from 20 to around 4000 kg/mol) poly(ethylene oxide)s and one poly(methyl methacrylate) (PMMA) were purchased from Sigma-Aldrich. The molecular weight and distribution (Mn and Mw/Mn) were determined by gel permeation chromatography (GPC). GPC test for PEO samples was performed in 0.1 M NaNO3 aqueous solution at 35 °C on a Waters 1515 chromatograph equipped with Ultrahydrogel 2000 (7.8 × 300 mm) column and refractive index detector at a flow rate of 0.5 mL/min. PEO standards from Polymer Standard Service were used for calibration. GPC test for PMMA sample was performed in HPLCgrade tetrahydrofuran at 25 °C on the same chromatograph equipped with Ultrastyragel (19 × 300 mm) column and refractive index detector at a flow rate of 0.5 mL/min. Poly(methyl methacrylate) standards from Polymer Standard Service were used for calibration. The results are shown in Table 1. All ionic liquids used in this study

Table 1. Molecular Characterization of Polymer Samples sample

Mw (kg/mol)

Mn (kg/mol)

Mw/Mn

PEO-20K PEO-100K PEO-400K PEO-1M PEO-4M PMMA-1M

21.2 95 396 982 3850 995

19.3 79 330 720 2450 700

1.10 1.20 1.20 1.36 1.57 1.42

were purchased from the Center for Green Chemistry and Catalysis of Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. The residual water in the ILs was removed by rotary evaporation at 90 °C, followed by vacuum drying under P2O5 at 80 °C for 48 h to yield colorless liquids with undetectable water content by Karl Fischer titration. Preparation of Polymer/IL Solutions. The polymers and ILs were combined in ampules and heated to approximately 100 °C with stirring to melt the polymer and homogenize the polymer/IL solutions. The solutions were then dried under vacuum with P2O5 for at least 48 h to remove any residual moisture. All solutions were prepared on a weight basis, and the polymer volume fraction ϕ was calculated by assuming no volume change on dissolution. Differential Scanning Calorimetry. A 10 mg sample was hermetically sealed within an aluminum pan and placed in a differential scanning calorimetry (DSC) instrument (TA Instruments Q20), with an initial heating to 80 °C for 10 min to erase the thermal history. After a quench to the appointed temperature, the sample was held for enough time to obtain a well-crystallized sample before recording a heating scan at 5 °C/min. DSC software (TA Universal Analysis 2000) was employed to analyze thermograms. The melting temperature was identified as the position of the endothermic minimum. Wide-Angle X-ray Diffraction (WAXD) Measurements. The diffraction pattern of a sample was obtained with a Ragaku D/max2500 diffractometer using Ni-filtered Cu Kα radiation (λ = 0.1542 nm, 40 kV, 200 mA). Measurements were performed at 2θ = 5° to 40° at a scanning rate of 5°/min. Extracted PEO and Remixed PEO/IL Mixture. To test whether ILs combined with PEO crystals to form new eutectic crystals, a pure PEO sample was extracted from each crystallized PEO/IL mixture for further DSC and WAXD tests. A crystallized PEO/C4mimBF4 mixture (10 or 30 wt % PEO-20K) was soaked in acetone and stirred at room temperature followed by suction filtration. The resultant filter cake was washed three times in acetone to remove residual C4mimBF4. Finally, the filter cake was dried under vacuum for at least 24 h at room temperature to obtain extracted PEO. One of the extracted PEO samples (obtained from a 10 wt % PEO/C4mimBF4 mixture) was remixed with different ILs for additional DSC tests. The extracted

Scheme 1. Structures of Different ILs

anion (tetrafluoroborate, BF4−) but different cation alkyl lengths (Cnmim+, n = 2, 4, 6, and 8). Group II ILs have the same cation (C4mim+) but three different anions (BF4−, PF6−, and Tf2N−). Therefore, the interactions between PEO and the imidazolium ILs and their effects on thermal properties and rheological behavior can be systematically investigated. Effects of the alkyl length of imidazolium cations, the different types of 6107

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Macromolecules PEO crystal was dipped into different ILs and then treated in a vacuum oven for 12 h at room temperature. This removed air bubbles and ensures sufficient PEO crystal surface contact with the ILs to yield remixed PEO/ILs. Contact Angle Measurement. Contact angles of different ILs on a PEO sheet surface were measured on a KRUSS DSA100 instrument. Under ambient conditions, IL droplets were deposited on the PEO sheet and monitored with a CCD camera. The flat PEO sheet was prepared by hot press molding. The reported data are averages of five measurements taken at different positions on the sample. Each measurement was determined at a constant contact time, i.e., 2 min after the IL droplet was deposited on the PEO sheet. Rheology. Steady shear viscosity measurements were carried out with a stress-controlled rheometer by TA Instruments (AR-2000ex). The lower fixture incorporated a Peltier plate, providing temperature control, while the upper fixture was equipped with a parallel plate fixture of a diameter of 40 and 20 mm for measuring low viscosity (100 Pa·s) samples, respectively. Sample edges between the two fixtures were sealed using liquid paraffin to prevent moisture buildup. Comparison tests showed that sealing with liquid paraffin did not influence the results.

3. RESULTS AND DISCUSSION 3.1. Interactions between PEO and ILs Based on Melting Point Depression and Surface Contact Angle. The solubility, crystallization, and melting of semicrystalline PEO in different types of imidazolium ILs were studied. The melting temperatures of PEO crystals in ILs were lower than that of pure PEO. The melting point depression values were mainly attributed to the interactions between the polymer and the ILs. As a representative example, the thermal properties of the PEO/C4mimBF4 solution are presented. To eliminate kinetic influences (which are severe for high MW PEOs with high viscosity), a low-MW PEO-20K polymer was used in this section. PEO-20K can completely dissolve in C4mimBF4 when mixed at 80−110 °C for approximately 30 min. When the solution was quenched to room temperature for approximately 30 min, PEO crystallized from the solvent and formed a uniform, white, pastelike sample. The pastelike sample became transparent upon heating to a certain temperature, dependent on the PEO concentration. Differential scanning calorimetry (DSC) results for fully crystallized PEO/C4mimBF4 samples at 0 °C are shown in Figure 1a. The melting temperature (Tm) of PEO in C4mimBF4 decreased with decreasing PEO concentration (Figure 1b). PEO in other ILs exhibited similar trends; however, the melting point depression values were dependent on the structure of the IL (Figure S1). Many small molecules, such as HgCl2,26 ZnCl2,27 and urea,28 can associate with PEO chains to form eutectic crystals, which alters the crystal structure of PEO and the Tm. To determine whether C4mimBF4 combined with PEO crystals to form new eutectic crystals, extracted PEO samples were prepared for DSC and WAXD analyses. The extracted PEOs (from 10 and 30 wt % PEO/C4mimBF4 mixtures) had the same melting curve profiles as that of pure PEO (Figure S2). The results demonstrated that C4mimBF4 can be completely removed from the PEO/C4mimBF4 mixtures and that PEO and ILs do not cocrystallize. The diffraction patterns for pure PEO, PEO/ C4mimBF4 (30 wt %), and the corresponding extracted PEO were compared in Figure S3. The three samples had identical diffraction peaks, indicating that the crystal forms were the same. The melting point depression without changing crystalline structure is primarily attributed to the interactions between solvent and solute. According to Flory−Huggins theory,29 the

Figure 1. (a) DSC melting curves of PEO-20K/C4mimBF4 blends with different PEO weight fraction after fully crystallized at 0 °C. The heating rate was 5 °C/min. (b) Melting points as a function of PEO weight fraction in C4mimBF4.

melting point depression of a crystal polymer−solvent system can be described by30 (1/Tm 0 ′ − 1/Tm 0)/ϕ1 = (RVu/V1ΔHu)(1 − BV1ϕ1/RTm 0 ′) (1)

Tm0

where is the equilibrium melting temperature of the pure polymer, Tm0′ is the equilibrium melting (or dissolution) temperature of the polymer in solution, ϕ1 is the volume fraction of the diluent, Vu and V1 are the molar volumes of the repeating unit and diluent, R is universal gas constant, ΔHu is the heat of fusion of the perfectly crystallized polymer per mole of repeat unit, and B is a polymer−diluent interaction energy parameter. According to this equation, a plot of the quantity (1/Tm0′ − 1/Tm0)/ϕ1 against ϕ1/Tm0′ should be linear. Therefore, ΔHu and B can be calculated from the interception (RVu/V1ΔHu) and the slope (−BV1/R) once the molar volumes of the repeating unit and solvent are known. First, the data from the isothermally crystallized PEO and PEO/C4mimBF4 blends were analyzed by the Hoffman and Weeks method to obtain Tm0′ (Figure S4a). Values of Tm0′ were plotted as a function of ILs volume fractions (Figure S4b). Figure 2a shows the plots of (1/Tm0′ − 1/Tm0)/ϕ1 as a function of ϕ1/Tm0′ and the straight line fitting according to eq 1 for PEO in ILs with different cation alkyl lengths. Figure 2b shows the results for different anions. The heat of fusion ΔHu of the PEO repeating unit and the interaction energy parameter B of PEO in different ILs were estimated from eq 1, where Vu = 38.1 cm3/mol,31 and the V1 values of the different ILs were obtained from ref 32. First, the value of ΔHu was 1910 (±10%) cal/mol for six different systems in the present work, which agreed with literature data 6108

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Macromolecules Table 2. Summary of Parameters Charactering the Interaction between PEO and ILs

ILs group I

group II

C2mimBF4 C4mimBF4 C6mimBF4 C8mimBF4 C4mimBF4 C4mimPF6 C4mimTf2N

interaction parametera (B, J/cm3)

Tmb of remixed PEO/ILs (°C)

contact angle of ILs on PEO surface (deg)

−9.2 −12.1 −16.6 −21.5 −12.1 −23.6 −28.3

48 44 41 38 44 30 25

42.5 29.6 18.8 13.4 29.6 18.3 10.6

Interaction parameter (B) was calculated from the slope of (1/Tm0′ − 1/Tm0) × 103/ϕ1 versus ϕ1 × 103/Tm0′ shown in Figures 2a and 2b according to eq 1. bThe extracted PEO (obtained from 10 wt % PEO/ C4mimBF4 mixture) was remixed with different ILs to evaluate the depression of Tm of PEO in different ILs. a

PEO crystals (obtained from the 10 wt % PEO/C4mimBF4 mixture) were dipped into different ILs to prepare remixed PEO/ILs. The melting curves of remixed PEO/ILs (Figures S5) reveal that the Tm of the extracted PEO crystals in different ILs decreased with increasing imidazolium cation alkyl lengths and decreased with different anions in the order of BF4− > PF6− > Tf2N−. When the Tm values of the remixed PEO/ILs were plotted in Figure 2c as a function of the interaction energy parameter (B), the Tm of the remixed PEO/ILs was proportional to B. Furthermore, when the Tm reached that of pure PEO (65 °C), the extrapolated value of B was close to zero. This consistency between two sets of experimental data indicates that both methods can be used to investigate the interaction between PEO and ILs. The contact angle reflects the strength of the interactions of liquids on a solid surface. The contact angles for different ILs on PEO surfaces were measured (Figure 3a). In general, the contact angle decreased with increasing imidazolium cation alkyl lengths and decreased with different anions in the order of BF4− > PF6− > Tf2N−, as shown in Figure 3b. These trends were consistent with the melting point depression trends. Therefore, the B values, contact angles, and Tm of extracted PEO crystals in different ILs (listed in Table 2) are indicative of the interactions between PEO and ILs and are dependent on the ionic structures of the ILs. First, the interactions increased with the imidazolium cation alkyl length (group I) and with the size of the imidazolium anions (group II). Second, the anionic structures of the ILs have a stronger effect than the alkyl chain length of the cationic structure, as shown by the red line and the blue line in Figure 3b. The effects of the alkyl chain length on the solubility and miscibility of PEO in ILs have been reported in the literature. PEO chains in ILs with longer alkyl chain lengths have more extended conformations,21 better miscibility,12,24 and better solubility.34 Watanabe’s group25 has studied the solubility of PMMA in 1-alkyl-3-methylimidazolium ILs with different anionic structures by using nearly monodisperse PMMAgrafted silica nanoparticles (PMMA-g-NPs) as a measurement probe. The hydrodynamic radius (Rh) of PMMA-g-NPs in the ILs as measured by dynamic light scattering (DLS) reflected the PMMA-solubility change in the ILs. They reported that the predominant factor for determining the PMMA solubility was the nonpolar properties of the anion; the nonpolarity of the imidazolium cation was a secondary effect. For example, Rh

Figure 2. (a) (1/Tm0′ − 1/Tm0) × 103/ϕ1 versus ϕ1 × 103/Tm0′ for PEO-20K in ILs with different alkyl lengths. The solid line is a leastsquares fitted line; the fitted equations are labeled. (b) (1/Tm0′ − 1/ Tm0) × 103/ϕ1 versus ϕ1 × 103/Tm0′ for PEO-20K in ILs with different anions. The solid line is a least-squares fitted line; the fitted equations are labeled. Interaction parameter (B) was calculated from the slope. (c) Tm of extracted PEO-20K in different ILs (10 wt %) versus interaction parameter (B).

(ΔHu = 1980 cal/mol31). Values of B calculated from the slope of eq 1 varied over a wide range from −9.2 to −28.3 J/cm3 (Table 2). The negative B value indicated the good miscibility of PEO in the ILs. B has been reported to be negative for systems with special interactions, e.g., strong hydrogen bonds.33 The absolute value of B, as the interaction intensity between PEO and ILs, increased with increasing imidazolium cation alkyl lengths (group I) and with anions (group II) according to the order BF4−< PF6− < Tf2N−. Further comparisons of the B values with other estimations and the dissolution mechanisms or solvation structures of PEO in imidazolium-based ILs will be discussed. The melting point depression is another indicator of the interaction between PEO and ILs. Extracted PEO samples have constant crystal sizes and ratios of surface area to weight; therefore, they are good model samples to study to determine the melting point depression of PEO in different ILs. Extracted 6109

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Figure 3. (a) Contact angle of different ILs on PEO crystal surface. (b) Contact angle as a function of melting points of extracted PEO crystal in ILs.

drastically increased with the anion size: BF4− (60 nm) < PF6− (120 nm) < Tf2N− (260 nm) for PMMA-g-NPs in C4mimbased ILs. However, PMMA solubility (Rh as the indicator) remained steady for different cationic structures. These observations were similar to results obtained in the present work (Table 2). Asai et al. recently studied the dissolution mechanism and the solvation structures of PEO in imidazolium-based ILs by using high-energy X-ray diffraction and molecular dynamics simulations.22 They found that the C2mim+ cation preferentially solvated PEO, and the anion was randomly distributed via van der Waals and weak electrostatic interactions. Moreover, the solubility of PEO in ILs with different anions decreased with decreasing anion size, i.e., Tf2N− > bis(fluorosulfonyl) amide (FSA−) > BF4−. Furthermore, Coulombic interactions between cations and anions in bulk ILs were stronger for smaller anion systems. C4mimBF4, with a stronger cation−anion interaction in the bulk, hindered the disruption of the solvent−solvent interaction in the process of solvation and therefore had poor solubility22 and miscibility.25 However, for increasing imidazolium cation alkyl chain length, the cation size slightly increased and thereby decreased the Coulombic interaction between cations and anions, which facilitated the dissociation of the cation−anion pair. Additionally, increased alkyl chain lengths may further enhance van der Waals interactions between the IL cations and the CH2−CH2 unit of PEO,35 which can account for the observed cation dependencies in ref 25 and in the present work. 3.2. Rheological Behaviors of PEO in ILs with Different Anionic Structures. Because the anionic structure showed more significant effects on the interaction between PEO and ILs, the rheological behavior of PEO in ILs with three different anions was explored to illustrate the effect of ILs anions on the PEO conformation. First, PEO/C4mimPF6 solutions were

investigated. Figure 4a shows the apparent viscosity as a function of shear rate for various concentrations of 980 kg/mol PEO (PEO-1M) in C4mimPF6 at 25 °C. Aqueous PEO solutions were compared as a reference system as shown in Figure 4b. The viscosity of both systems markedly increased with PEO concentration. The absolute viscosity of PEO/ C4mimPF6 was up to 2 orders of magnitude greater than that of the PEO/H2O for similar polymer concentrations. This was primarily due to the higher viscosity of pure C4mimPF6. Additional tests had been made to confirm that high-MW PEO solutions at higher concentrations had a good stability during the solution preparation and rheological measurements, as shown in the Supporting Information (Figures S6−S8). Zero-shear viscosity (η) values of the PEO/C4mimPF6 and PEO/H2O solutions were obtained from the apparent viscosity curves. Then, the specific viscosity (i.e., ηsp = (η − ηs)/ηs, where ηs is the viscosity of the pure solvent), by which the solvents viscosity effects can be eliminated, are plotted as a doublelogarithmic function of the PEO concentration in Figure 4c. The ηsp of the PEO/H2O solution was higher than those of the PEO/C4mimPF6 solutions. A factor of 3 was observed in dilute regime, which indicated that the PEO chains in H2O were of larger size than those in the C4mimPF6 solutions. The ηsp for the two solutions were separated into three regimes, i.e., a dilute regime, a semidilute regime, and an entangled regime with slopes of 1, 2, and 4.7, respectively. As described by scaling theory,36−38 these slopes agreed with the theoretical predictions (i.e., 1, 2, and 14/3) for the neutral polymer in θ solvent. Note that the exponents are 1, 1.3, and 3.9 for neutral polymer in good solvent. C4mimPF6 and water may be considered θ solvents for PEO. However, conflicting results were reported for PEO/H2O solutions in the literature. The concentration-dependent scalings of 1, 2, and 14/3 for the three regimes were observed 6110

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Figure 4. Steady shear profiles of (a) PEO-1M/C4mimPF6 and (b) PEO-1M/H2O at 25 °C. (c) Concentration dependence of specific viscosity for PEO in different solvent solutions.

for PEO/H2O solutions (Mw of PEO = 5 × 106 g/mol) in ref 36, which indicated that H2O was a θ solvent.36 In contrast, Selser and co-workers studied aqueous solutions of PEO by light scattering39 and intrinsic viscosity measurements40 and found that Rg ∼ Mw0.58, Rh ∼ Mw0.57, and [η] ∼ Mw0.79, which indicated a good solvent system. Therefore, PEO samples ranging from 95 to 3850 kg/mol (Table 1) were used to further investigate and characterize PEO behavior in the dilute and semidilute regimes. The ηsp data of four different PEOs in C4mimPF6 are plotted as functions of concentration in Figure 5a. The exponents of 1 and 2 were found in the dilute and semidilute regimes, respectively, and the overlap concentration, c*, was obtained at the transition point of the two regimes. Figure 5b shows the double-logarithmic plot for c* versus Mw, where c* ∼ Mw−0.48 was obtained. In terms of the molecular weight of the polymers, the overlap concentration is described by c* ∼ Mw1−3ν, where ν = 1/2 for θsolvents and ν = 0.588 for good solvents. Therefore, the ν value of 0.49 for the PEO/C4mimPF6 solutions was very close to 1/2, indicating that C4mimPF6 is close to the θ conditions for PEO at 25 °C. Note that all the data can be superposed together by

Figure 5. (a) Concentration dependence of specific viscosity ηsp for different PEOs (PEO-100K, PEO-400K, PEO-1M, and PEO-4M) in C4mimPF6 solutions. Solid lines are predictions of scaling law for linear polymer solutions in θ conditions. (b) Molecular weight dependence of c* for PEO/C4mimPF6 solutions. The slope −0.54 indicates that PEO/C4mimPF6 solution is close to the θ-condition at 25 °C. (c) Specific viscosity against c/c* of PEO/C4mimPF6 solutions. (d) Molecular weight dependence of intrinsic viscosity ([η]) for PEO in C4mimPF6 and H2O and data of [η] for PEO/H2O from Selser’s work40 (open square) and Kawaguchi’s work41 (open triangle) are also shown in the figure. The black straight line is the fitted results according [η] = 5.3 × 10−4Mw0.67. All experimental data here were obtained at 25 °C. 6111

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Macromolecules reducing the concentration dependencies of ηsp for different PEOs by c* as shown in Figure 5c. In the lowest concentration regimes examined, i.e., below c*, the size of the polymer chain only depended weakly on concentration. An unperturbed (infinite dilution) chain dimension was mainly affected by solvent properties and can be obtained via the intrinsic viscosity [η]. The relationship between [η] and Mw can de described using the Mark− Houwink−Sakurada (MHS) equation:36,42 [η] = KMM α

three different concentration regimes, which correlates with the predictions of θ-solvent conditions. The values of ηsp of the four different solutions for a given PEO concentration increased in the order of BF4− < PF6− < Tf2N− < H2O, which was consistent with the order of the interactions between PEO and ILs reported in section 3.1. When the data of [η] in Table 3 are Table 3. Intrinsic Viscosities ([η], dL/g) of PEO in Different Solvents

(2)

C4mimBF4

where KM is a pre-exponential factor and α is the exponential factor corresponding to the interaction between solvent and solute. The prediction of the Zimm model for the intrinsic viscosity is described by [η] ∼ Mw3ν−1. Therefore, α is 0.5 for the flexible chain in the θ-solvent, and α is (asymptotically) approximately 0.5−0.8 with the asymptotic approximation for flexible chains in the good solvent.36 The Mw dependence of [η] is a power law relation (Figure 5d), and the α exponent can be obtained from the MHS relation (eq 2). Note that the [η] data for PEO/H2O from Selser40 (except for the two lowest molecular weight samples; the authors excluded these two samples in their analysis due to the abnormal values of the Huggins’ coefficients) are plotted in Figure 5d for PEO samples in the molecular weight range of 250−996 kg/mol. The [η] data reported by Kawaguchi et al.41 are also plotted in Figure 5d for PEO samples in the molecular weight range of 86−11 000 kg/mol. All three sets of PEO/H2O data have a good agreement, and the exponent α for PEO in H2O was 0.67. The exponents α for PEO in C4mimPF6 and H2O were 0.50 and 0.67, respectively. This indicated that C4mimPF6 was a near θ-solvent for PEO at 25 °C, which was consistent with ηsp−c scaling (Figure 4c) and c*−Mw scaling (Figure 5b). Our intrinsic viscosity measurements and results in refs 40 and 41 presented similar results; i.e., water was a good solvent for PEO based on [η] ∼ Mw scaling. However, ηsp−c relations for PEO/H2O solutions in the dilute regime, the semidilute regime, and the entangled regime exhibited features of a θ-solvent. This implied that ηsp−c analysis itself was not sufficient to determine the solvent quality. The uncertainty of exponents in these regimes will be discussed later. Because the PEO-1M/C4mimBF4 systems would crystallize at room temperature, 80 °C was selected as the reference temperature to investigate the rheological behavior of PEO-1M in three imidazolium ILs with different anions (C4mimBF4, C4mimPF6, and C4mimTf2N). Temperature can influence the solvent quality and has an effect on the configuration of the polymer chains in solution.36 However, for the PEO-1M/ C4mimPF6 and PEO-1M/C4mimTf2N solutions, the differences between the concentration dependencies of ηsp at 25 and 80 °C as shown in Figures S9a and S9b were barely observed, implying that for IL solvents used here, the solvent quality was insensitive to temperatures within 25−80 °C. The molecular weight dependencies of [η] for these two systems at 25 and 80 °C were compared in Figure S9c, which shows that there is little difference for the α value at the two temperatures. Therefore, we can compare the four groups of data at different temperatures. The concentration dependencies of ηsp for PEO-1M in C4mimBF4 at 80 °C and in C4mimPF6, C4mimTf2N, and H2O at 25 °C are shown in Figure 4c. The data show that the concentration dependence of ηsp for four different systems follows the same scaling laws, ηsp ∼ c, ηsp ∼ c2, and ηsp ∼ c14/3, in

C4mimPF6

C4mimTf2N

H2O

PEO

80 °C

25 °C

80 °C

25 °C

80 °C

25 °C

PEO-100K PEO-400K PEO-1M PEO-4M

0.68 1.54 2.4 3.78

0.8 1.7 2.6 5

0.9 1.8 3.1 5.7

1 2.4 4.8 8.1

1.1 2.75 5.3 8.8

1.2 3.5 7.1 13

Figure 6. Molecular weight dependence of intrinsic viscosity ([η]) for PEO in C4mimBF4, C4mimPF6, C4mimTf2N, and H2O. The data for PEO in H2O was measured at 25 °C, and others were obtained at 80 °C. PEO in H2O solutions cannot be measured at 80 °C because of the high evaporation of water. PEO-1M/C4mimBF4 systems were tested only at 80 °C to avoid crystallizing at room temperature. Furthermore, Figure S9 shows that data of PEO in C4mimPF6 and C4mimTf2N have very weak temperature dependence between 25 and 80 °C.

plotted as functions of Mw in Figure 6, the exponents α for PEO in C4mimBF4, C4mimPF6, C4mimTf2N, and H2O are 0.44, 0.50, 0.59, and 0.67, respectively. These results indicate that for PEO, C4mimBF4 behaved like a poor solvent, C4mimPF6 was near θ condition, and C4mimTf2N and H2O behaved as good solvents for PEO. The value of [η] is proportional to the size of the polymer coils in the given solvent according to the Zimm model. Therefore, the size of PEO increased according to the size of the anion: BF4− < PF6− < Tf2N−. This trend was exactly same as that observed for the Rh for PMMA-g-NPs in the C4mim-based ILs studied by Watanabe and co-workers.25 The sizes of the PEO coils in water were the largest in all systems studied in the present paper (Figures 4c and 6). However, the reasons as to why the concentration dependencies of ηsp of PEO in the four different ILs with different solvent qualities and of PEO in water follow the same θ-solvent scaling laws, i.e., ηsp ∼ c, ηsp ∼ c2, and ηsp ∼ c14/3, in three concentration regimes remain unknown. Additionally, many other polymer/IL systems also show similar scalings. In our previous work,43 celluloses in ionic liquid 1-allyl-3-methylimidazolium chloride showed similar exponents in the three 6112

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Macromolecules regimes (Figure 7a). Chen et al.44 observed the same scalings for cellulose in C4mimCl at 80 °C. Here, the steady shear

14/3), as summarized by Takahashi and co-workers.47 It can also be found in Colby’s recent review (Figure 13b of ref 37) that data points follow a steeper line than 14/3. Therefore, our systems in the good solvent showed higher exponents than the theoretical prediction (4.7 vs 3.9) in the entangled regime, but the values are still lower than those earlier data in θsolvents.37,47 These results in the entangled regime are consistent with those observations (i.e., higher intrinsic viscosity data than theta condition data) in the dilute regime to some extent. However, because exponents of 2 and 14/3 for a neutral polymer in θ-solvent were observed in all six polymer/IL solutions (i.e., three PEO/ILs, two cellulose/ILs and one PMMA/ILs) in the semidilute regime and the entangled regime, the experimental uncertainty cannot be presumed to account for the same scalings for the different systems. Complex interactions (or solvation structures) between polymer and ILs and between cations and anions, including Coulombic interactions, hydrogen bonds, π−π interactions, and van der Waals forces, exist in polymer/IL systems. The coil− coil interaction of PEO chains will become more prominent with increasing PEO volume fraction and can significantly influence the relaxation of PEO segments and the subsequent ηsp−c scaling.48,49 However, ILs are nano-heterogeneous solvents composed of polar and apolar domains, which may enhance the PEO coil−coil interactions.50,51 The effect of the nano-heterogeneous structures of ILs and the monomer− monomer interactions of polymer/IL solutions on polymer conformations is another interesting issue that requires further study. However, the exponents of the semidilute regime and the entangled regime may not be good indicators of the solvent quality in these complex systems. Therefore, the single-chain behavior of dissolved PEO in ILs should be further investigated by static and dynamic light scattering measurements.

Figure 7. (a) Concentration dependence of specific viscosity for cellulose in 1-allyl-3-methylimidazolium chloride (AmimCl, ref 43). (b) Concentration dependence of specific viscosity (ηsp) for PMMA and PEO in C4mimTf2N; the data were obtained at 25 °C. The two solutions have the same slopes in every concentration region, following the scaling predictions for neutral polymers in a θ-solvent. The straight lines in dilute and semidilute nonentangled regime are plotted according to scaling predictions for neutral polymers in a θsolvent, and the straight line in the semidilute entangled regime is fitted by data points.

4. SUMMARY AND CONCLUSION The interactions between PEO and ILs with different ionic structures were investigated by measuring the melting point depression and contact angle. The results show that the Tm of crystal PEO dramatically decreased when it was blended with ILs and that the contact angle of different ILs on PEO surfaces decreased with the same order. The interaction strength extracted from these experiments increased with the imidazolium cation alkyl chain length (group I) and with the size of the anions (group II), i.e., BF4−< PF6− < Tf2N−. The anionic structures of the ILs had a stronger effect than the alkyl chain length of the cationic structure. These trends agreed with those observed in previous experiments in refs 22 and 25. Thus, the solubility of PEO in ILs was mainly controlled by the dissociation of the cation−anion ion pair in the ILs. The rheological behavior of PEO in ILs with different types of anions was studied at 25 and 80 °C to further determine the effect of the anions on the solvent quality of ILs (or on the PEO conformations). The molecular weight dependence of the intrinsic viscosity revealed that C4mimBF4 behaved as a poor solvent, C4mimPF6 behaved as a near θ solvent, and C4mimTf2N behaved as a good solvent but as a worse solvent than PEO in water solutions. This order (i.e., C4mimBF4 < C4mimPF6 < C4mimTf2N < H2O) was consistent with the melting point depression and contact angle trends. The consistency between crystallization and rheology behaviors in polymer solutions recently was observed in the literature.52 When relatively poor solvents (e.g., vegetable oils), as opposed

viscosity values of PMMA/C4mimTf2 N solutions were determined at 25 °C (Figure S10). A concentration dependence of ηsp is shown in Figure 7b for the PEO/C4mimTf2N system. Apparently, the PMMA/C4mimTf2N system showed the same scaling laws (1, 2, and 4.7) of the concentration dependence of ηsp as that of the PEO/C4mimTf2N system in the three concentration regimes. The theoretical predictions of scalings in the three regimes were 1, 1.3, and 3.9 for a neutral polymer in good solvent. However, the values were 1, 2, and 14/3 for a neutral polymer in θ-solvent. The large differences in the exponents in the semidilute regime and the entangled regime should be easily observed in experiments. However, because the range of the semidilute regime is quite narrow (typically less than 1 decade),45 the uncertainty of the exponent in this regime may be quite large. Additionally, in the entangled regime (close to the concentrated regime), an absence of iso-monomeric friction correction will lead to an overestimation of the exponent.38,46 It is worth noting that in the earlier papers concentration dependence of viscosity in the entangled regime for θ solutions are reported with higher slopes than the scaling concept (i.e., 6113

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Macromolecules

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to good solvents, were used, the gel spinning processing of ultrahigh molecular weight polyethylene to produce ultrahigh modulus and ultrahigh strength fibers, systems represented smaller melting point depressions and smaller viscosities (more compact coils).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01171. Additional experimental details; DSC, WAXD, and rheological data; Hoffman−Weeks plots and equilibrium melting points; and the temperature dependence data of the rheological behaviors of PEO/ILs solutions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Y.L.). *E-mail: [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21174153 and 21374127). We thank Prof. Er-Qiang Chen, Prof. Qi Liao, and Prof. Hiroshi Watanabe for helpful discussions. Authors appreciate reviewer 2 for the constructive comments.

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REFERENCES

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