Diffusion in Ionic Liquid–Cellulose Solutions during Coagulation in

Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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Diffusion in Ionic Liquid−Cellulose Solutions during Coagulation in Water: Mass Transport and Coagulation Rate Measurements Artur Hedlund,*,† Tobias Köhnke,† and Hans Theliander‡ †

Bio-based fibers, Swerea IVF, P.O. Box 104, SE-431 22 Mölndal, Sweden Forest Products and Chemical Engineering, Chalmers University of Engineering, SE-412 96 Göteborg, Sweden

‡

S Supporting Information *

ABSTRACT: This article describes central features of the mass transport during the coagulation in water of cellulose−1-ethyl-3methylimidazoium acetate ([C2mim][OAc]) solutions, namely, that the diffusivities are mainly affected by the relative concentrations of water and [C2mim][OAc], that the concentration of cellulose does not affect diffusivities and coagulation rates, that the diffusivities of low-Mw compounds are similar to those in aqueous [C2mim][OAc] solutions without macromolecules, that the polymer concentration is diluted by the large influx of coagulant causing a positive net mass gain, NMG, from diffusive fluxes, and that such NMG, although observed only as a function in time, is also a function in space that has local peaks significantly higher than the mean NMG value. The conclusion from the first three findings was that the diffusion advances through a liquid phase which possesses a continuous pore network and most of the volume. The precipitated cellulose is concentrated into fibrils whose inhibitive effect on the diffusion of small molecules through the surrounding phase is marginal. This key understanding about mass transport during coagulation also simplifies numerical modeling significantly.

1. INTRODUCTION

measures, such as close control of metal trace elements and temperatures, to avoid uncontrolled degradation of NMMO. Therefore, a range of alternative solvents have been investigated over the years.1,4 Among the most promising of these are several ionic liquids (ILs), which are organic salts with melting temperatures below 100 °C.5 1-Ethyl-3-methylimidazoium acetate ([C2mim][OAc]) and 1-butyl-3-methylimidazoium chloride ([C4mim][Cl])6−9 have been the most frequently studied ILs, while new ones are continuously being developed.10−17 Common nonsolvents, NSs, in which IL− cellulose solutions can be coagulated, are water or other protic polar solvents, i.e., the same as for the NMMO−cellulose solutions in the production of lyocell. Protic NSs coagulate IL− cellulose solutions by competing with cellulose hydroxyl groups for the strongly basic IL anion with which both cellulose and NS interact through H bonds. Typically, the required amounts of nonsolvent to cause coagulation are small; generally, such coagulation values (CVs) are in the order of 10%.10,18,19 The polar character and large molecular weights (150−250 Da) of ILs are the reasons for their high viscosities and low diffusivities relative to other common solvents, such as the protic solvents used to coagulate them. Their slow dynamics in solutions may cause several problems, such as limiting cellulose solubility or causing problematic resistance to flow in various process steps. Therefore, an interesting variation on pure IL solvents is the

Mass transport during the coagulation of cellulose solutions is an issue of major industrial importance since it is an intrinsic phenomenon of any fiber wet spinning process. Such processes are the only proven way by which cellulose, isolated from biomass by pulping, can be reshaped into continuous cellulose fibers applicable in textiles.1 In wet spinning processes, a polymer is dissolved, extruded as filaments, and precipitated by immersion in a nonsolvent (NS). Diffusive mass transport determines the rates of coagulation and washout of the solvent. Data on such rates are crucial, e.g., with regard to residence times when designing production plants. However, diffusive rates of solvent and nonsolvent during coagulation also influence the nano- and microstructures formed during coagulation and, consequently, affect the properties of fibers from wet spinning processes.2 In addition, the shaping of cellulose into films and membranes involves these same fundamental process steps. In 2016, forest-based dissolving pulp was converted into 5.3 Mton/a of viscose fibers and 0.2 Mton/a of lyocell fibers, which are the two most important wet spinning processes for manmade cellulose fiber production.3 Although well established, they are not without drawbacks, such as costs for controlling and reducing pollution from viscose production and the thermodynamic instability of the solvent N-methylmorpholine N-oxide (NMMO), used in the lyocell process. Because of this thermodynamic instability, the production of lyocell consumes considerable amounts of antioxidants and requires several safety © XXXX American Chemical Society

Received: July 27, 2017 Revised: September 19, 2017

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DOI: 10.1021/acs.macromol.7b01594 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

diffusive processes and has not been proven for cellulose coagulation. In fact, tetrabutylammonium fluoride−DMSO− cellulose solution coagulated in water is the only process for which the distribution curves of nonsolvent inside the coagulating material have been studied, and the resulting concentrations were continuous and smooth.33 However, the nonsolvent distribution curves remain unknown for other cellulose solvent−nonsolvent combinations. The actual nonsolvent distribution curves and their relations to the coagulation front are thus open and important questions concerning nonsolvent diffusion, which apparently needs further elucidation. In addition to measurements of apparent Ds during coagulation, there is growing literature on self-diffusivity in various mixtures of IL with either a nonsolvent or cellulose, based on diffusion-NMR experiments.34−37 However, neither the magnitude nor the mechanism of cellulose’s effect on diffusion, or on the apparent Ds during coagulation, has been quantified and clarified. For instance, some authors entertain the idea that the mobility of even very small molecules is strongly inhibited when cellulose chains link into a rigid network.27,38 Their basis for this idea is the Stokes−Einstein equation, which implies that D is ∼ηmacro−1, i.e., that D decreases radically, even infinitely, in coagulated material. However, the validity of the Stokes−Einstein equation does not cover small molecules in polymer solutions.39 Gavillon and Budtova25 have compared models for the influence of cellulose concentration on the diffusion in hetero- and homogeneous gels to their measurements of solvent diffusion during the coagulation of NaOH and NMMO cellulose solutions in water. Because the Ds measured were significantly below literature data for Dself , their conclusion leaned toward homogeneous gel diffusion models. However, as stated above, their measured Ds should be considered 4 times larger. In that case, the data for NMMO diffusion, in particular, could be interpreted differently in favor of the heterogeneous gel. In this work, we challenge the ideas about a rigid network’s inhibitive effects on diffusion during coagulation by constructing a simple numerical model for diffusion. This model is built on suppositions in contrast to those of our predecessors. The deciding question is if it can emulate experiments. The experimental methods used in the present study were inspired by the work by Liu et al.40,41 They have investigated mass transport and coagulation in cellulose−ammonia− ammonium thiocyanate solutions, coagulated in various alcohols. An adapted version of the coagulation front propagation method used in their first paper40 was developed to accurately observe and measure the coagulation rate. Their latter work41 has treated the “mass transfer rate difference”, which describes the asymmetry of the inflow of nonsolvent and outflow of solvent. However, the term does not isolate the relative rates of nonsolvent versus solvent from the general rates of both. Consequently, deeper interpretations of the results in terms of simple rate coefficients are difficult and ambiguous as the results are composed of two underlying variables. In the present article, the distinction is made between the “rate” and the “mass transfer difference” or, as we have chosen to term it, the net mass gain or NMG. Our newly developed method also allows the quantification of the crucial inflow of nonsolvent, which was achieved through the complement of solvent outflow measurements to the mass transfer rate difference measurements. With this method, all species exchanged between an extruded precipitate and a

inclusion of a polar aprotic cosolvent, such as methylimidazole,20,21 dimethylacetamide,20,22 dimethylformamide22 or dimethyl sulfoxide (DMSO),20,22,23 which reduces the solution viscosity and increases diffusivity. Research into mass transfer during the coagulation of cellulose has focused on the outward diffusion of solvent by measuring its concentration in fibers, in membranes, in other coagulated shapes, or in the coagulation liquids as a function of residence time in the coagulation liquid. Such studies are available for both NMMO (Liu and Hu;24 Gavillon and Budtova;25 Biganska and Navard;26 Hauru et al.27) and ILs (Jiang et al.;28 Sescousse et al.,29 Hauru et al.27). However, in many cases, the values for diffusion coefficients DIL and DNMMO reported in these studies are not consistent, which probably is due to the variations in mathematical models used to calculate D from data. For example, Liu and Hu24 and Hauru et al.27 have found initial (higher) and final (lower) apparent Ds, as the result of improper mathematical models. In the case of Hauru et al.,27 it is necessary to distinguish between their numerically modeled data, mentioned above, and their experimental data. The latter is rather consistent with the data of Biganska and Navard,26 Gavillon and Budtova,25 and Sescousse and coworkers.29,30 These latter publications are consistent with each other, but a closer look at their calculations reveals that they use eq 1, which is the equation for a membrane exposed from both sides. In eq 1, d* is the full thickness, 2d, of the membrane exposed from both sides, but the text declares that eq 1 was applied with d* = d. In that case, the “4” in the denominator should be a “2” as in eq 6, later in section 3.1 (cf. Crank.31). ⎛ M (t ) d * ⎞ 2 π D=⎜ ⎟ ⎝ M tot 4 ⎠ t

(1)

The corresponding authors of the latter four studies have been contacted in the matter, and the resulting discussions have concluded that all four studies did indeed contain this same mistake. Another example is the work by Jiang et al.,28 which declares significantly lower Ds for [C4mim][Cl] than in the work by Sescousse et al.29 A more rigorous description of these issues is included in the Supporting Information. The papers described above have not measured the mass transport of NS. Neither has there been any other satisfactory study of NS mass transport into coagulating IL−cellulose solutions, whereas it is in fact the rate-determining parameter of the coagulation process. The need to include the aspect of nonsolvent diffusion was made particularly clear for the case of NMMO by Laity et al.,32 who used NMR imaging to show that the diffusion of NMMO through a cellulose solution gradually increased as a result of the initial infusion of water and the resulting phase separation. Considering the CVs in the range of 5−30 wt %18 and the higher diffusivities of the nonsolvents compared to ILs, nonsolvent transport is obviously the main contributor to phase separation. This constitutes another reason to focus on nonsolvent transport. Biganska and Navard26 have observed coagulation front propagation to measure nonsolvent uptake in NMMO solutions. The same method was recently picked up by Hauru et al.27 to measure the diffusion of water in a number of IL−cellulose solutions. However, this technique is based on the assumption that the whole coagulated volume is composed of nonsolvent; i.e., it is distributed as a step function with 100% NS in the coagulated part and 0% elsewhere, or “shrinking core” behavior. Such discontinuous behavior is uncommon in B

DOI: 10.1021/acs.macromol.7b01594 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Compositions of Solutions Used in the Study Solution designation (wt[C2mim][OAc]:wtDMSO - wt %MCC)

wt % MCC

wt % [C2mim][OAc]

wt % DMSO

99:1-5%MCC 99:1-14.3%MCC 99:1-25%MCC 50:50-14.3%MCC

5 14.3 25 14.3

94.05 84.85 74.25 42.85

0.95 0.85 0.75 42.85

coagulation liquid are followed to generate a holistic picture of these processes.

2. EXPERIMENTAL METHODS 2.1. Materials. [C2mim][OAc] of purity >90%, produced by BASF was purchased from Sigma-Aldrich. The moisture content was determined to be 0.5% by Karl Fischer titration. Anhydrous DMSO with molecular sieve beads was also purchased from Sigma-Aldrich. The cellulose used was microcrystalline cellulose (MCC) PH-101 with Mw = 28.4 kDa and Mw/Mn = 2.6. Deionized water was used as the NS. 2.2. Solution Preparation. The solvents used were mixtures of [C2mim][OAc] and DMSO of either 99:1 or 50:50 composition (wt[C2mim][OAc]:wtDMSO). The objective of the addition of 1 wt % of DMSO to the “pure” [C2mim][OAc] solvent was to use it as a tracer to allow comparisons both between D[C2mim][OAc] and DDMSO in each solution and between DDMSOs in different solutions. The compositions of four different solutions prepared for the experiments are summarized in Table 1, which includes the designations used hereinafter. MCC was added to the solvent and stirred with an overhead mixer turning a dough hook. The stirring speed was set at 800 rpm initially and then lowered to a level that did not cause the solution temperature to rise above 80 °C. A lid sealed the mixing vessel from the surrounding atmosphere so that no moisture could enter during stirring, which continued for 2 h. The solutions were confirmed to be fully dissolved by verifying the absence of undissolved material between cross-polarizing plates in the microscope. Air trapped during stirring was removed from the solution by centrifugation before the solution was transferred to a syringe for moisture-free storage until use (within a week at room temperature). 2.3. Mass Transport Measurements. A 1 mL plastic syringe (inner diameter 4.65 mm, with tip cut off) was filled with 0.3 g of solution. Then a stainless steel rod (3.95 mm in diameter), with a Teflon flange (4.65 mm diameter) 58 mm from one end, was pushed into the syringe in place of the original plunger. Thus, a 0.35 mm thick layer of solution surrounded the stainless steel rod from the plastic flange and down the 58 mm to its tip, as illustrated in Figure 1. By pressing the rod forward, all the solution could be ejected in the shape of a closed-end tube, 4.65 mm in diameter, 0.35 mm thick, and 58 mm long, around the rod from the flange down to expose the solution. In an experiment, the entire device, composed of rod, syringe, and solution, was weighed on a lab scale of four decimals precision. Then the solution content was ejected directly into 33 mL of coagulation liquid, composed of distilled water. The coagulation liquid was vigorously stirred with a small magnet. The temperature was kept within 21−23 °C during the measurement. After a given time, the device was removed from the coagulation liquid, and any excess liquid was quickly, but gently, wiped off before the device was weighed again. The difference between the two measured masses amounted to the “net mass gain” (NMG) incurred by the submersion in nonsolvent. In addition, the coagulation liquid conductivity was measured to quantify the accumulated outward mass transport of [C2mim][OAc]. For a subset of the experiments, a sample of the coagulation liquid was taken for NMR analysis of DMSO concentrations. For each solution−NS combination, 15−25 measurements were performed with residence times varied between 4 and 105 s. The requisite number of measurements was chosen based on coagulation speed and the mechanical robustness of the coagulated material. 2.4. Coagulation Front Propagation Rate Measurement. In this method, a glass tube (inner diameter of 4 mm) was filled from one end with cellulose solution while the other was sealed by inserting a

Figure 1. Schematic image of the device (syringe and rod plunger) used to produce the circular membranes, featuring the steps of the measurements: first weighing, the event of the membrane’s gradual exposure, and the second weighing from left to right. glass rod. The solution was illuminated through the glass rod so that coagulation was observable as opaque regions. The open solution end was covered with a very fine steel mesh, >90% open and 25 μm thick, to hold the material in place and to achieve a relatively flat and distinct end surface. The tube was then placed vertically (open end upward) and observed in a microscope, from the side, as the coagulation liquid level was raised above the open end. As the coagulation proceeded, images such as in Figure 2 were taken intermittently at times ranging from 100 to 5000 s, and the thickness of the coagulated opaque layer

Figure 2. Side view of the coagulated plug from 5 wt % cellulose solution after 1600 s. The very sparse stainless steel wire mesh, which was held taught over the tube’s open end, is visible on the outside of the glass tube. The tube’s inside and outside diameters were 4 and 6 mm, respectively. The plug was lit from below. C

DOI: 10.1021/acs.macromol.7b01594 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules ⎛ M (t ) d ⎞ 2 π π = (kd)2 D=⎜ ⎟ 4 ⎝ M tot 2 ⎠ t

was measured. The data points for the coagulation depth were plotted against t1/2, and their slope was evaluated. 2.5. [C2Mim][OAc] Concentration by Conductivity Measurements. The coagulation liquid conductivity was measured with a standard hand-held device, 470 Cond Meter from Jenway. To obtain reliable measurements, the most concentrated coagulation baths (the longest times) were diluted with a known weight of distilled water to maintain the conductivities measured within a more narrow and reliable range (