Reducing Environmental Impact: Solvent and PEO Reclamation

Oct 14, 2015 - An improved subtractive manufacturing process for fabrication of rectangular, ... Drawing in poly(ε-caprolactone) fibers: tuning mecha...
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Research Article pubs.acs.org/journal/ascecg

Reducing Environmental Impact: Solvent and PEO Reclamation During Production of Melt-Extruded PCL Nanofibers Alex M. Jordan, Tyler Marotta, and LaShanda T. J. Korley* Center for Layered Polymeric Systems, Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States S Supporting Information *

ABSTRACT: An improved subtractive manufacturing process for fabrication of rectangular, high-surface-area poly(ε-caprolactone) (PCL) fibers is presented. PCL fibers were derived from continuous coextruded tapes of poly(ethylene oxide) (PEO)/PCL with 75% reduction in washing time, while still achieving >99 wt % PCL purity with a quantitative yield of PCL fibers. The fabricated PCL fiber mat had a measured surface area of 3.27 ± 0.53 m2/g. A two-stage distillation process was used to recover methanol and water used in composite solvation to remove PEO. Both methanol and water were recovered at ∼100% purity with a fractional recovery of 87 ± 2% and 95 ± 2%, respectively. Solvated PEO was also recovered at a fractional recovery of 94 ± 4% at ∼100% purity. Gel permeation chromatography and thermal analysis revealed no chain scission, thermal degradation, or cross-linking within the recovered PEO, which suggested the possibility of reincorporating recovered PEO to the multilayer coextrusion process for future composite coextrusion. These waste reduction figures represent recovery on the laboratory-scale process with substantial room for improvement in a fully automated, large-scale industrial process. By reducing overall waste generation >90%, fibers derived from multilayer coextrusion may become an industrially viable alternative for nanofiber manufacturing. KEYWORDS: Coextrusion, Reclamation, Reincorporation, Fibers, Distillation

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INTRODUCTION Owing to their high surface area-to-volume ratio, nonwoven mats composed of submicron-scale fibers have generated significant interest in the textile,1 filtration,2 and biomedical fields.3 Particularly within the biomedical community, nonwoven fiber mats have been used in applications as wideranging as tissue engineering scaffolds, 4−6 soft tissue prosthetics,7,8 and controlled drug delivery.9,10 Nonwoven fiber mats have also seen considerable use as a reinforcing material in composite materials,11−14 as well as a separating material in lithium-ion batteries.15,16 There are a myriad of techniques currently employed to process synthetic polymers to form micro/nano-scale fibers, including electrospinning,17,18 rotary jet spinning,19 gas jet processing,20 melt spinning,21 and melt blowing,22 each with its own advantages and drawbacks. As a solvent-based process, electrospinning relies on repulsive electrical forces to overcome a solution’s capillary surface tension to form a charged polymeric jet. Electrostatic whipping forces then elongate the charged jet as it passes through air and solidifies before being deposited on a grounded collector.23 Although electrospinning easily produces submicrometer diameter fibers, individual fiber quality with defects in the form of beads and welded fiber joints, is highly susceptible to processing parameters, such as solution conductivity24,25 and concentration,26−30 as well as electric field strength and flow rate.31−38 Rotary jet spinning, which is a relatively new fiber manufacturing technique, places a polymer solution in a © XXXX American Chemical Society

rotating central drum. The rotating drum expels a polymeric jet that solidifies during centrifugal elongation before depositing aligned, continuous fibers on a cylindrical collector.19 Like electrospinning, processing parameters have been shown to adversely affect fiber quality.39 Insufficient rotational velocity of the central drum has been shown to result in bead formation, as opposed to continuous fibers, and solvent volatility has been shown to alter fiber size.40 Gas jet processing, a third solventbased technique, relies on pressurized air flow to elongate and solidify a polymer jet and fiber quality is highly dependent upon capillary number.20 Each of these solvent-based processes suffers from low throughput and inherently difficult solvent sequestration, resulting in evaporation and loss of solvent to the atmosphere. Melt-based approaches, such as melt spinning and melt blowing, have high throughputs and do not rely on harmful organic solvents, making them industrially relevant.41 Both melt spinning and melt blowing are initiated by using a series of conveying and heating elements to force solid resin into a viscous molten state. Melt spinning forces the molten polymer through a die with many small orifices, termed a spinneret. The molten fibers are spooled and drawn to impart crystalline orientation and enhance the thermo-mechanical properties of Received: September 4, 2015 Revised: September 30, 2015

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DOI: 10.1021/acssuschemeng.5b01019 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Post-processing removal of PEO matrix from the PCL/PEO composite through solvation and delamination/hydroentanglement resulting in a PCL fiber mat.

the spun fibers.42,43 However, the fibers typically fabricated via melt spinning exceed 10 μm in diameter.21 Melt blowing overcomes the fiber size limitation of melt spinning, producing fibers with typical fiber sizes 1−2 μm. After passing through the exit die, molten polymer jets are solidified and attenuated by a pressurized air stream before deposition on a rotating drum.22 Because of the variability of the hot air attenuation process, the fibers resulting from melt blowing are often fragmented and randomly aligned within the nonwoven mat and are not easily oriented on the molecular scale.41 Traditional multilayer coextrusion uses a series of layer multiplication elements to fabricate continuous polymeric layered films with individual layer thicknesses ranging from 5 nm to 10s of μm with tunable dielectric,44,45 barrier,46,47 and mechanical properties.48−53 Recently, we demonstrated an alternative multilayer coextrusion scheme to fabricate a continuous rectangular poly(ε-caprolactone) (PCL) fiber− poly(ethylene oxide) (PEO) matrix composite. The PEO matrix was removed using a 24 h water solvation followed by 24 h of methanol solvation and 0.5 h of a hydroentanglement procedure.54,55 By using this subtractive manufacturing process, we demonstrated removal of the PEO matrix, producing continuous PCL fibers with lateral dimensions of 1.6 ± 0.4 μm by 2.6 ± 0.6 μm. The fibers were uniaxially drawn to impart chain orientation on the molecular scale, providing a pathway to tailored thermo-mechanical properties and fibers with tunable lateral dimensions down to 130 ± 20 nm by 310 ± 50 nm. This fiber manufacturing technique provided access to the nanoscale regime utilizing a melt processing technique, while also leveraging traditional melt processing benefits, such as fiber drawing, to impart molecular orientation. However, during matrix removal, a considerable amount of methanol, water, and PEO are utilized over a lengthy 48.5 h procedure.55 By using low volatility solvents, such as water and methanol, in bulk solvation vessels, it is possible to sequester any solvent used in this process on an industrial scale. Solvent sequestration provides a unique avenue to reincorporate any solvents utilized during the postmanufacturing process, which is in stark contrast to solvent-based fiber processing that relies on the immediate evaporation of volatile organic solvents for fiber formation. Herein, we report an optimized matrix removal procedure, reducing removal time from 48.5 to 12.25 h along with a solvent requestration, refinement, and reintroduction procedure to drastically reduce the environmental impact of our fiber manufacturing process, and to overcome the functional limitations of solvent-based fiber manufacturing processes.

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pelletized. PCL was purchased from the Perstorp group (CAPA 6800). Methanol was purchased from Sigma-Aldrich, and tap water was provided by the City of Cleveland, OH. Both methanol and water were used as received without purification. Composite Fabrication. Composites consisting of a PEO matrix with embedded, rectangular PCL fiber-like domains were fabricated via multilayer coextrusion.55 PEO and PCL were dried at 40 °C under vacuum for 48 h prior to coextrusion to remove residual moisture. The composite was extruded at 180 °C to achieve a viscosity match between PEO and PCL (Figure S1), producing a structure with 8 distinct PEO horizontal layers alternating with 8 vertically layered structures consisting of 256 alternating PEO and PCL domains. The calculated compositions based on the nominal flow rates of each extruder were 3 wt % PCL and 97 wt % PEO.55 Solvation Efficiency. Four 2.5 cm strips of composite were cut and placed in a vial with 10 mL of solvent (Figure S2). Vials were agitated at 800 rotations per minute (RPM) on a vortex mixer for set time intervals. As a mixed solvent system, 7 mL of methanol and 3 mL of water were used such that the mixture consisted of 70:30 (by volume) methanol:water. The solvated composites were removed from the solvent mixture and prepared for 1H nuclear magnetic resonance (NMR) studies in deuterated chloroform (CDCl3) on a Varian Inova 600 MHz NMR Spectrometer. Each experiment was performed in triplicate to obtain an average and standard deviation of PCL composition for the various washing times and cycles. The conversion method from 1H NMR integration to wt % has been demonstrated in our previous work.55 For subsequent work requiring larger-scale fiber fabrication, composite strips were bound to a beaker to maintain fiber alignment (Figure S3) during agitation in a 70:30 methanol:water solution using a magnetic stirring rod (Figure 1). Fiber Delamination. Solvated strips of composite with a length of 15 cm and an approximate width of 1.25 cm (exposed area of 18.75 cm2) were fixed to a steel plate and held in place with a wire mesh. The wire mesh (60 mesh, Universal Wire Cloth Company) consisted of woven wires with individual diameter of 190 μm and a spacing of 250 μm for an overall coverage of 30.5%. Tap water at a pressure of 3.45 MPa was passed through a 0.25 mm orifice on a custom built system (Antomizing Inc.) for delamination. Solvated composites were subjected to delamination for varying times (3−25 min) to probe the optimal delamination time, while keeping the nominal area of composite exposed to the water jet equal (Figure 1). Again, each delamination experiment was performed in triplicate. Fiber Mat Analysis. Delaminated fiber mats were analyzed under a scanning electron microscope (SEM, JEOL-JSM-6510LV) after sputter coating with 10 nm of gold (Au) to provide a conductive surface on individual fibers. The specific surface area of the fiber mats subjected to varying delamination times was analyzed via multipoint Brunner− Emmett−Teller (BET) analyzer (Micrometrics, TriStar III) after degassing at 40 °C under nitrogen for 24 h.56 Solvent Recovery. Five repetitions of solvent recovery were performed with the amount of input methanol and water carefully measured. For each repetition, two 70:30 mixtures of methanol and water were prepared. Four 2.5 cm strips of composite were placed in one of the mixtures and agitated at 800 rpm on a vortex mixer for 6 h before being removed and the solvated composite transferred to the second prepared mixture, which was again agitated at 800 rpm for 6 h. The 2.5 cm × 2.5 cm solvated composite was then delaminated at a

EXPERIMENTAL SECTION

Materials. Two grades of PEO powder (POLYOX WSR N-10 and POLYOX WSR N-80) were purchased from Dow Chemical and meltblended at a weight ratio of 30:70 (N-10:N-80) at 140 °C before being B

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ACS Sustainable Chemistry & Engineering pressure of 3.45 MPa for 5 min (0.8 min/cm2) based on the optimum time determined through the delamination study, and the water collected in a rigid tray placed under the steel plate (Figure S4). The extracted fiber mat was isolated for purity examination in 1H NMR using CDCl3, while the water recovered from pressure washing was combined with the methanol/water solvation mixtures, which also contained solvated PEO. Each solvent was distilled using a rotary evaporation apparatus. Methanol (boiling point = 64.7 °C, 101.3 kPa) was removed first at a temperature of 40 °C and a pressure of 26.0 kPa and labeled as “1st Distillate”. Water (boiling point = 100.0 °C, 101.3 kPa) was then removed at a temperature of 55 °C and a pressure of 12.0 kPa and labeled as “2nd Distillate”. Each distillate was weighed to measure percent recovery (R) (eq 1) using the recovered weight (Wr) and input weight (Wi) and analyzed using 1H NMR spectroscopy in deuterated-acetone ((CD3)2CO) to assess purity.

R=

Wr × 100% Wi

(1)

Recovered PEO Analysis. PEO was recovered as a yellow solid after methanol and water were distilled off and weighed to assess recovery. Recovered and virgin PEO were analyzed using gel permeation chromatography (GPC, Agilent 1200 series Liquid Chromatography System) in HPLC grade tetrahydrofuran (THF) at an eluent flow rate of 1 μL/min. Retention time was correlated to molecular weight using calibrated PEO standards with molecular weights ranging from 200 to 1 000 000 Da to assess any chain scission or cross-linking during the processing and recovery steps. Thermal degradation behavior of recovered and virgin PEO were assessed using a Thermal Analysis Q500 thermogravimetric analyzer (TGA) at a heating rate of 10 °C/min between 50 and 600 °C. Crystallization and melting of recovered and virgin PEO were analyzed using a Thermal Analysis Q100 differential scanning calorimetry (DSC) instrument using a heat/cool/heat protocol. The first heating was carried out at 3 °C/min between −90 and +180 °C to assess crystalline structures in virgin and recovered PEO. Cooling was performed at 3 °C/min between +180 and −90 °C to assess the ability of recovered and virgin PEO to crystallize. A second heating was performed at 3 °C/min between −90 and +180 °C to assess the recrystallized structure in both recovered and virgin PEO. The melt processability and ability to reintroduce recovered PEO into the manufacturing process was assessed using a Thermal Analysis ARES G2 rheometer at a shear rate of 10 s−1 from 150 to 220 °C to simulate multilayer coextrusion conditions.

Figure 2. 1H NMR spectra of composites solvated for 24 h in methanol, water, and a methanol/water (70/30) mixture.

approximately 3 wt % PCL to roughly 63 wt % PCL. By using the mixed solvent system, the resulting composition was nearly 80 wt % PCL, suggesting a substantial increase in PEO solvation. As a result, a mixed methanol/water (70/30) solvent system provided the highest efficiency for the initial composite washing. With this more efficient solvation system, we explored the impact on washing time with a goal of reducing from the previously reported 48.5 h process. Composite strands were placed in seven individual vials each containing 10 mL of the methanol/water solution and agitated for either 2, 4, 6, 8, 12, 24, or 30 h before examining the composite composition via 1H NMR spectroscopy (Figure 3a). The 1H NMR spectra qualitatively show that PEO was effectively solvated for the first 6 h of the washing process with very little change over the time points between 6 and 30 h. By using the integrated peak values at 4.06 ppm that correspond to the two α-1H of PCL and 3.63 ppm that correspond to the four 1H in PEO, the approximate weight fraction of the composite was calculated for each time point over three repetitions of the experiment (Figure 3b).55 After a period of 6 h, the composition of the composite reached an asymptote at approximately 83 wt % PCL, which was attributed to the solvent becoming saturated with PEO, limiting further absorption. With a significant portion of the PEO in the fabricated composite solvated after 6 h, we attempted to remove residual PEO by using multiple 6 h washing cycles (Figure 4a). It was shown that, by using two 6 h washing cycles (fresh methanol/ water), the composition of the solvated composite increased from 83 ± 7 wt % PCL to 96 ± 3 wt % PCL. Although expanding to a third and fourth 6 h washing cycle did not significantly improve PEO solvation (Figure 4b), it should be noted that the deviation in composition decreased as the number of washing cycles increased, which we attribute to the increased removal of trace PEO. However, we assert that this minor compositional improvement does not justify an additional 12 h of solvation, which effectively doubles solvent usage. Any small traces of PEO past the 96 wt % PCL purity obtained after two washing cycles are easily removed during subsequent delamination. As mentioned, delamination was used to remove trace amounts of PEO while simultaneously inducing hydroentangle-

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RESULTS AND DISCUSSION Composite Washing. In our previous work, initial solvation was achieved using a two-step washing process: 24 h of agitation in water followed by 24 h of agitation in methanol. To reduce the time required for PEO solvation/ removal, a mixed solvent approach was investigated. Using small-angle neutron scattering (SANS), Hammouda suggested that a mixed solvent system of water (30 vol %) and methanol (70 vol %) forms an interoxygen hydrogen bonding distance of roughly 0.47 nm, similar to that of the bond distance between oxygen atoms in the PEO chain backbone.57 The solvation efficiency of this mixed system was evaluated via 1H NMR spectroscopy as a decrease in the PEO 1H signal at 3.63 ppm58 relative to the α-position 1H in the PCL backbone at 4.06 ppm (Figure 2).55,59 Strips of composite were agitated in either methanol, water, or a mixed methanol/water (70/30) system for 24 h before removing the remaining solid composite and measuring its 1H NMR spectra in CDCl3. Methanol alone did not solvate the PEO matrix effectively, which was most likely due to the decreased solubility of the high molecular weight of PEO utilized in the manufacturing process.60 After agitation in water for 24 h, the composition of the composite increased from C

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Figure 4. (a) 1H NMR spectra of PEO/PCL composite after one, two, three, and four 6 h solvation cycles in methanol/water (70/30) solution; (b) average PCL content after each solvation cycle measured via 1H NMR spectroscopy.

Figure 3. (a) 1H NMR spectra of composite before solvation (0 h) and after 2, 4, 6, and 30 h of solvation; (b) approximate composite composition as a function of agitation time in a methanol/water (70/ 30) solution.

of water jetting led to an increase in the measured surface area to 3.27 ± 0.53 m2/g. The measured value of 3.27 ± 0.53 m2/g is within experimental error of our previously reported value (3.48 ± 0.05 m2/g), which indicated no significant loss of available surface area.55 Additional delamination time did not alter the measured surface area outside of the measured experimental error. Thus, a delamination time of 15 min (0.8 min/cm2) was chosen as it provides a balance of good delamination and minimal water consumption during fiber preparation. It should also be noted that the fiber mats washed for 0.8 min/cm2 yield a PCL purity >99 wt %, which will be discussed in more detail.55 After reducing the solvation and delamination time (∼75% reduction in time and labor) with the same level of purity and available surface area, the amount of solvent introduced and wasted during post-processing of the PCL fibers was still quite substantial. Solvent Recovery. As this new reduced time process still required significant amounts of solvent, we investigated methods to recover methanol and water from the PCL/PEO composite washing process, which can potentially be reintroduced into the process stream to reduce environmental impact. To fabricate a 2.5 cm × 2.5 cm PCL fiber mat using the protocol described, 8.5 ± 0.1 mL (6.7 ± 0.1 g) methanol and 94.8 ± 14.9 mL (94.8 ± 14.9 g) water were used to extract 108.7 ± 4 mg of PEO. Examining these numbers on a normalized scale, for each gram of PEO to be removed, 872 ±

ment of individual fibers to yield high surface area PCL fiber mats. In our previous work, we reported a measured surface area of 3.48 ± 0.5 m2/g following the 48 h solvation and 30 min delamination procedure, which resulted in a nearly 6-fold increase over traditional, circular electrospun fibers of comparable lateral dimensions.55,61 Our goal in this study was to determine the minimum amount of pressure washing time required to fabricate a mat with similar surface area, while simultaneously conserving resources. It was not possible to use pressures in excess of 3.45 MPa without destroying individual undrawn fiber integrity. Fiber delamination was observed both qualitatively via SEM and quantitatively via multipoint BET analysis. Using only the initial two 6 h solvation cycles, the initial surface area of the obtained fiber mat was measured to be 0.32 ± 0.04 m2/g. Using a constant mat area of 18.75 cm2, analysis of SEM images of fiber mats delaminated for 3, 5, and 10 min showed noticeable regions containing adhered fibers that had not yet been delaminated (Figure 5) Beyond 10 min of water jet exposure, qualitative delamination of the PCL fibers was observed. This analysis correlated well with the determination of specific surface area as measured via multipoint BET analysis. After 10 min of delamination, the specific surface area measured up to 2.24 ± 0.32 m2/g. Increasing exposure of the PCL fiber mats to 15 min D

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Figure 5. Qualitative SEM analysis of 18.75 cm2 fiber mats delaminated for 3, 5, 10, 15, 20, and 25 min (scale bar = 20 μm) coupled with quantitative multipoint BET surface area measurements.

137 mL of water and 78 ± 1 mL of methanol were used. A twostep distillation was utilized to separate this three component mixture of methanol, water, and PEO. Distillation relies on the volatility difference between two solvents, such as methanol and water, with no stable azeotrope formed.62 The distillation operates in two stages. In the first stage, the rotary evaporator was set to a pressure of 26.0 kPa and a temperature of 40 °C, removing an average of 7.4 ± 0.2 mL (5.9 ± 0.2 g) of clear liquid, labeled as 1st Distillate (Figure 6a). 1H NMR analysis of the first distillate in (CD3)2CO revealed two distinct peaks. The first peak, which is a doublet centered at 3.30 ppm, corresponds

to the three 1H attached to the methyl carbon. The second peak, a quartet centered at 3.41 ppm, corresponds to the one 1 H attached to the oxygen in the alcohol group, confirming that the first distillate was indeed methanol (Figure 6b).63 No water or PEO were observed in the first distillate, suggesting approximate 100% methanol purity with a recovery of 87 ± 2% over the five experimental trials. The second distillation stage operated at a pressure of 12.0 kPa and 55 °C. Throughout the second stage, 90.3 ± 15.9 mL (90.3 ± 15.9 g) of clear liquid was collected, which was labeled 2nd Distillate (Figure 6a). An opaque brown residue at the bottom of the rotary evaporator flask was also obtained. It may appear that water recovery has exceeded water input, violating our mass balance. However, these discrepancies are due to the large spread of values for water recovery and input, resulting in large apparent error bars. It should be noted that in all five experimental trials, less water was recovered than was input, confirming the mass balance (Table S1). Analysis of the 1H NMR spectra of the second distillate revealed a single peak, a triplet centered at 3.86 ppm (Figure 6b). This peak corresponds to the 1H of the water molecule with the 1:1:1 triplet splitting connected to 2JH,D coupling between water and (CD3)2OD, which results in not only an H2O peak, but also a split HOD peak.63 Again, it should be noted that no PEO or methanol impurities were observed in the 1H NMR spectra of the second distillate, which suggests an approximate 100% water purity with 95 ± 2% recovery over the 5 experimental trials. Using the demonstrated recovery rates for methanol and water, the process losses on a gram of PEO basis were reduced to 10 ± 1 mL and 43 ± 2 mL, respectively. Although water recovery exceeded 95%, improvements can be made in the methanol recovery phase. Losses throughout the process were attributed to transfer between vessels during the recovery process. These small losses are magnified in the fractional recovery of methanol due to the smaller input amount when compared with water losses. An industrial process, which offers continuous large-scale recycling, would minimize these losses, substantially improving the fractional recovery of methanol as well as offering an improvement in water recovery. Solids Recovery. From previous studies, the extracted fiber mat was expected to contain PCL.54,55 For confirmation, 1H NMR spectroscopy in CDCl3 of the extracted fiber mat

Figure 6. (a) Visual image of collected 1st and 2nd Distillates; (b) 1H NMR spectra of 1st (methanol) and 2nd (water) Distillate in (CD3)2CO with chemical structure of water and methanol providing 1 H NMR peak assignments. E

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or a fractional recovery of 94 ± 4% of ∼100% pure PEO was obtained, which could potentially be reintroduced to the multilayer coextrusion fiber manufacturing process.

revealed five peaks (Figure 7a). The first peak is a triplet centered at 4.06 ppm corresponding to the two 1H located in

Table 1. Process Inputs, Recovery, and Purity Values for Multilayer Coextrusion PCL Fiber Fabrication Process material

input

recovered

methanol water PEO PCL

8.5 ± 0.1 mL 94.8 ± 14.9 mL 108.7 ± 4.0 mg 3.4 ± 0.1 mg

7.4 ± 0.2 mL 90.3 ± 15.9 mL 102.5 ± 4.0 mg 3.2 ± 0.1 mg

recovery (%)

purity (%)

± ± ± ±

∼100 ∼100 ∼100 >99

87 95 94 93

2 2 4 3

PEO Reincorporation. Having recovered roughly 94 ± 4% of the input PEO at ∼100% purity, we investigated the possibility of reincorporating this recovered PEO into the multilayer coextrusion process. Chain scission and thermal degradation of PEO chains may occur during the composite coextrusion, which would induce changes in the molecular weight of PEO. Using GPC, we compared the molecular weight distribution of unprocessed, virgin PEO with that of the recovered PEO collected as the bottoms product from the twostage distillation. In the virgin state, the weight-average molecular weight (Mw) of PEO was 139 kDa with a dispersity of 2.52. After multilayer coextrusion, solvation, and distillation to remove methanol and water, the Mw of the recovered PEO was slightly reduced to 102 kDa with a dispersity of 2.41. The molecular weight distributions of the virgin and recovered PEO were quite similar, suggesting that no appreciable chain scission or degradation occurred during coextrusion and recovery and indicating the absence of thermally induced cross-linking (Figure 8a). Via this molecular weight analysis, it is proposed that the PEO before and after recovery are structurally similar. We also examined the thermal degradation and crystallization behavior of the recovered PEO. For comparison, the thermal degradation of virgin PEO occurred over the range 306−400 °C with a 99% weight retention temperature of 302 °C (Figure 8b). To reincorporate the recovered PEO into the coextrusion process, thermal stability above 180 °C is also required. Thermal analysis of the recovered PEO revealed a 99% weight retention temperature of 289 °C with a degradation range between 319 and 400 °C, confirming its suitability for coextrusion at 180 °C and that thermally induced cross-linking did not occur (Figure S5). Differential scanning calorimetry was utilized to compare the crystallization behavior of virgin and recovered PEO. During the first heating, virgin PEO melted with an enthalpy of 207 J/g at a temperature of 69 °C. However, the recovered PEO melted with an enthalpy of 175 J/g at a temperature of 58 °C (Figure S6). The difference in enthalpy indicates a difference in the crystalline fraction of the semicrystalline virgin and recovered PEO. The difference in melting temperature suggested a more highly ordered structure within the virgin PEO as the crystal structure was stable to higher temperatures, which is a result of differences in thermal processing history. Virgin PEO was obtained by melt blending of two PolyOx resins at 140 °C and subsequent cooling from the melt using a metal chill roll. In contrast, the recovered PEO was first melt blended at 140 °C, coextruded with PCL at 180 °C, solvated and dissolved in water and methanol, before being recovered as a solid from the solution state under vacuum. It is well-known that crystallization phenomena differ between the melt and solution

Figure 7. (a) 1H NMR spectra of recovered solids from the distillation process (PEO) and the extracted fiber mat (PCL) with repeat monomer structure of PEO and PCL; (b) visualization of the extracted fiber mat and recovered solids from the distillation process.

the α-position of the PCL backbone. The second peak, another triplet centered at 2.30 ppm, corresponds to the two 1H located adjacent to the ketone group of the PCL backbone. The quintet centered at 1.38 ppm represents the two 1H located along the PCL backbone in the γ-position to the ester linkage. A sextet centered at 1.64 ppm represents the final four 1H located along the PCL backbone.59 The final peak is a weak singlet occurring at 3.63 ppm, which corresponds to a small PEO impurity within the PCL fibers.58 Using previously described quantification of the 1H NMR spectra, it was determined that the purity of the extracted PCL fiber mat is >99%. This small residual PEO impurity was likely the result of viscous encapsulation of PEO into the rectangular PCL domains during the coextrusion process and cannot be removed during post-processing. The extracted mat weighed an average of 3.4 ± 0.1 mg over the five experimental trials, yielding a fractional recovery of 93 ± 3%. (Table S2) As can be seen, the extracted fiber mat was roughly 2.5 × 2.5 cm (5.25 cm2) with uniform fiber coverage over the surface of the mat (Figure 7b), which is crucial for applications such as wound dressings or tissue scaffolding. As mentioned, an opaque yellow residue remained after distillation of water and methanol. The yellow residue was visually similar to the virgin PEO before extrusion, and was suspected to be PEO (Figure 7b). A 1H NMR spectra in CDCl3 revealed one singlet peak at 3.63 ppm, representative of the four chemically similar 1H in the PEO backbone.58 Over the course of five experimental trials, approximately 102.5 ± 4.0 mg F

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Figure 8. Virgin (black) and recovered (red) PEO (a) molecular weight distribution functions; (b) thermal degradation profiles; (c) second heating endotherms; (d) viscosity profiles.

that the melt viscosity of the two materials at 180 °C may be more similar, but inherent noise may have lowered measurement sensitivity. It is acknowledged that the true measure of reincorporation would be to utilize the recovered PEO in subsequent multilayer coextrusion runs. However, this laboratory-scale analysis limited recovery to 102.5 ± 4 mg of PEO with each experimental trial, whereas the minimum amount of material to operate the multilayer coextrusion equipment is 50 g. It is asserted that these initial, small-scale studies highlight the potential scale-up of multilayer coextrusion, composite solvation, and waste reclamation processes for the fabrication of PCL fiber mats with minimization of environmental waste. A process flow diagram (PFD) is proposed with this scaledup solvation and reclamation processes (Figure 9). Initially, the composite of 3 wt % PCL is agitated for 6 h in a 70/30 methanol/water solution. The mixed solvent, now containing PEO, is transferred to a holding unit, whereas the solvated composite (83 wt % PCL) is transferred to a second 6 h agitation unit. Following the second 6 h agitation, the mixed solvent containing PEO is charged to the holding unit. The composite, now 98 wt % PCL fibers, and minor residual PEO, may be uniaxially drawn if desired with a high degree of fiber alignment to tune thermo-mechanical properties and lateral fiber dimensions.55 Following the optional uniaxial drawing,

state.64,65 Crystallization from the melt occurs along the temperature gradient as the polymer cools and is highly dependent on any shear-induced alignment of chains during the melt extrusion process.64 Conversely, crystallization from solution occurs along the solvent concentration gradient during evaporation65 with both processes occurring at different rates. Thus, examination of the cooling and second heating provides a direct comparison of the virgin and recovered PEO after erasure of the processing history. Upon cooling, the virgin and recovered PEO exhibited similar crystallization temperatures of 43 and 41 °C, respectively. As expected, the second heating curves of virgin and recovered PEO were nearly identical. Virgin PEO displayed a melting endotherm of 163 J/g at a temperature of 62 °C, whereas recovered PEO melted with an enthalpy of 154 J/g at a temperature of 63 °C (Figure 8c). This similarity in material crystallization behavior after heating supports the use recovered PEO for process reincorporation As a final test of reincorporation, the melt viscosity at a shear rate of 10 s−1 was collected to simulate conditions inside the multilayer coextrusion unit. The melt viscosity profiles between virgin and recovered PEO were remarkably similar to a variation of only ∼10% at 180 °C, providing an adequate viscosity match with PCL for reprocessing (Figure 8d). It is also worth highlighting the significant overlap between the virgin and recovered PEO melt viscosity curves, which suggest G

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Figure 9. Process flow diagram of coextruded PEO/PCL composite solvation and waste reclamation process to fabricate a PCL fiber mat.

leaving only an entangled PCL fiber mat of cross-sectional area 5.25 cm2. Although neither methanol nor water is considered a harmful organic solvent, a two-stage distillation process was used to separate methanol, water, and PEO for recovery and reintegration to the fiber manufacturing process. The first distillation stage operated at 26.0 kPa and 40 °C, removing 7.4 ± 0.2 mL of clear liquid, identified through 1H NMR spectroscopy as ∼100% methanol with a fractional recovery of 87 ± 2%. The second distillation stage was operated at 12.0 kPa and 55 °C, recovering 90.3 ± 15.9 mL of a second clear liquid, identified as ∼100% pure water with a fractional recovery of 95 ± 2%. From the distillation process, 102.5 ± 4 mg of an opaque yellow solid resulted, which was identified as ∼100% PEO. Structural and thermal analysis revealed no chain scission, degradation, or cross-linking of PEO during multilayer coextrusion and the subsequent solvation and reclamation processes, suggesting that along with recovered methanol and water, PEO can be reincorporated into multilayer coextrusion to fabricate additional PEO/PCL composites. When demonstrated on the laboratory scale, the process demonstrated here vastly improves the outlook of industrial scale-up for this subtractive fiber manufacturing process, while minimizing waste production and environmental impact through various reclamation and reincorporation procedures.

residual PEO may be removed and individual fibers delaminated at a pressure of 3.45 MPa and a washing time of 0.8 min/cm2 of mat area; any runoff water and PEO collected is charged to the holding unit. The high surface area, PCL fiber mat is extracted as a solid, entangled mass. The holding tank containing a mixture of methanol, water, and PEO is separated via a two-stage distillation process during which ∼100% methanol, water, and PEO are recovered. It should be noted that the cooling water used during distillation may be cycled as a closed loop system with no appreciable water loss, as represented in Figure 9. However, the distillation process does add to the input energy costs, which is the trade-off between waste generation and energy input. The recovered PEO, methanol, and water may all be reincorporated to the multilayer coextrusion process (PEO) and composite washing process (methanol and water) to reduce substantially waste generation and environmental impact.

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CONCLUSIONS Previously, a three-step, 48.5 h washing process was demonstrated to reduce a coextruded PEO/PCL composite to a PCL fiber mat with >99% purity. Here, we demonstrated an alternative three-step approach using two 6 h solvation steps with a mixed methanol/water (70/30) solvent system and a 15 min water delamination process, totaling 12.25 h. The obtained PCL fiber mats had >99% purity with a specific surface area of 3.27 ± 0.53 m2/g similar to previously obtained mats obtained. Although the time required to fabricate these PCL fiber mats was reduced ∼75%, 8.5 ± 0.1 mL of methanol and 94.8 ± 14.9 mL of water were used to remove 108.7 ± 4 mg of PEO, H

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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/acssuschemeng.5b01019. Experimental details of multilayer coextrusion for fiber fabrication, composite washing schematic, distillation recovery results, and thermal analysis (PDF).

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AUTHOR INFORMATION

Corresponding Author

*L. T. J. Korley. Phone: (216) 368-1421. E-mail: lashanda. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Lindsay Matolyak for her assistance and helpful discussions on GPC of PEO. The authors also acknowledge funding from the National Science Foundation (NSF) Science and Technology Center (STC) Center for Layered Polymeric Systems (CLiPS) under Grant DMR0423914 and NSF under Grant CMMI-1335276.

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