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PEO blend
Industrial Crops & Products 143 (2020) 111883
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Impact of PEO structure and formulation on the properties of a Lignin/PEO blend
T
L.-S. Ebersa,b,*, R. Auvergnec, B. Boutevinc, M.-P. Laboriea,b a
Chair of Forest Biomaterials, University of Freiburg, Werthmannstr. 6, 79085 Freiburg im Breisgau, Germany Freiburg Materials Research Center, Stefan-Meier-Str. 21, 79104 Freiburg im Breisgau, Germany c Institut Charles Gerhardt UMR 5253 – CNRS, UM, ENSCM – 240 Av Prof Emile Jeanbrau, 34296 Montpellier, France b
ARTICLE INFO
ABSTRACT
Keywords: Lignin Carbon fiber precursors Coupling agent Crystallinity Mechanical properties
To improve the performance of lignin/PEO fibers as precursors of carbon fibers, blend formulations were designed with an epoxidized PEO and with carbon nanotubes. Model studies demonstrated that the epoxidized PEO could covalently link to lignin. With incorporation of epoxidized PEO, the precursor fibers exhibited enhanced degree of crystallinity of 70 % (52 % without epoxidized PEO) and a doubling of their young modulus (from 200 to 450 MPa) and tensile strength (from 6 to 12 MPa). In contrast, the addition of carbon nanotubes did not impact the morphology nor the mechanical performance of the resulting fibers. It is argued that with its ability to couple lignin to the PEO matrix, epoxidized PEO plays the role of a stress transfer agent between the rigid lignin regions and the flexible PEO regions, thereby improving the blend performance.
1. Introduction Due to the increasing demand for carbon fibers and the high price of its precursor polyacrylonitrile (PAN), much research effort has been recently devoted to using lignin for the production of carbon fibers (Aslanzadeh et al., 2017; Baker et al., 2012; Baker and Rials, 2013; Bengtsson et al., 2019a, 2018; Byrne et al., 2018; Dai et al., 2018; Fang et al., 2017; García-Mateos et al., 2018; Hosseinaei et al., 2017; Jiang et al., 2018; Jin et al., 2018; Jin and Ogale, 2018; Kubo and Kadla, 2005; Lin et al., 2014a; Lin and Zhao, 2016; Liu et al., 2018b, a; Meek et al., 2016; Nar et al., 2016; Norberg et al., 2013; Nordström et al., 2013; Oroumei et al., 2018; Qu et al., 2018, 2016; Schlee et al., 2019; Shi et al., 2018; Wang et al., 2016) and carbon fiber precursors (Awal and Sain, 2013; Bengtsson et al., 2019c, b; Culebras et al., 2018; Dong et al., 2015; Imel et al., 2016; Kubo and Kadla, 2003; Seydibeyoğlu, 2012). Indeed, as a major constituent of lignocellulosic biomass, lignin is largely available and comes from renewable resource, all important factors in times of climate change and decreasing fossil resources (Sain and Faruk, 2016). To improve lignin spinnability for the manufacture of carbon fiber precursors, lignin has either been modified or blended with a spinnable polymer (Fang et al., 2017). In the case of polymer blends, polymers of high spinning ability and good miscibility with lignin such as polyvinyl alcohol, polyethylene terephthalate (PET) or polyethylene oxide (PEO) have been commonly used (Cui et al., 2018; Kadla and Kubo, 2003; Kubo and Kadla, 2006, 2004; Lin et al., 2014b; Yu et al., ⁎
2015). It was shown for example that blending lignin with PEO resulted in improved spinnability and mechanical performance of the resulting fibers (Kubo and Kadla, 2004; Yu et al., 2015). To address the remaining challenge of the low mechanical properties of precursor fibers from PEO and lignin (Baker et al., 2012), the reinforcement of precursor with carbon nanotubes has been proposed. Indeed, the addition of carbon nanotubes (CNT) to carbon fiber precursors has been shown to increase strength and modulus in ligninbased carbon fibers (Baker et al., 2011; Wang et al., 2016) . In general, it is well established that the addition of CNT enhances the mechanical properties of polymer composites (Bikiaris, 2010; Coleman et al., 2006; Grady, 2012; Liu and Kumar, 2014) and that they can initiate a nucleating effect (Grady, 2012; Jiang et al., 2017; Kim et al., 2012; Liu and Kumar, 2014; Schawe et al., 2017; Zhang et al., 2014). In addition, it was hypothesized that a better stress transfer at the lignin/PEO interphase could result in further improvements in mechanical properties. While PEO and lignin are well established to be miscible (Kadla and Kubo, 2003; Kubo and Kadla, 2006, 2004), the use of a coupling agent that would covalently link lignin to the PEO could further result in improved stress transfer and mechanical performance of the precursor fiber. An ideal coupling agent would consist of a functionalized PEO whose functionality would enable covalent linking with lignin and the remaining PEO chain would still be able to cocrystallize or entangle with the matrix PEO polymer. In this contribution, we evaluate a series of lignin/PEO precursor
Corresponding author at: Chair of Forest Biomaterials, University of Freiburg, Werthmannstr. 6, 79085 Freiburg im Breisgau, Germany. E-mail addresses: [email protected] (L.-S. Ebers), [email protected] (M.-P. Laborie).
https://doi.org/10.1016/j.indcrop.2019.111883 Received 15 May 2019; Received in revised form 8 September 2019; Accepted 17 October 2019 Available online 12 November 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis route of monofunctional epoxidized PEO.
fibers in which carbon nanotubes are added as reinforcing and structuring agents and an epoxidized PEO is utilized as possible coupling agent between the PEO matrix and lignin. We thus examine in detail the impact of formulation design on the thermal, morphological and mechanical performance of the precursor fibers with a view to shedding light on the possible strengthening role of CNTs and epoxidized PEO. Alike other authors who examined lignin-based precursor fibers in a first place (Awal and Sain, 2013; Bengtsson et al., 2019c,b; Culebras et al., 2018; Dong et al., 2015; Imel et al., 2016; Kubo and Kadla, 2003; Seydibeyoğlu, 2012), in this paper we focus exclusively on the attributes and properties of the precursor fibers.
pyridine and deuterated chloroform (1.6/1 v/v) was prepared together with an internal standard cholesterol solution (43 mg/ml) (SafouTchiama et al., 2017). 30 mg OSL was weighed in a volumetric flask to which 0.5 ml of the solvent mixture, 100 μl of the chromium(III)acetylacetonate relaxation agent solution (5 mg/ml) and 100 μl of the internal standard solution was added. The mixture stirred over night at room temperature. 15 min before 31P-NMR measurement, 50 μl of the phosphitylation reagent 2-Chloro-4,4,5,5-Tetramethyl-1,3,2-Dioxaphospholane (TMDP) was added. NMR spectra were retrieved from a Bruker Avance 400 MHz spectrometer. 128 scans were retrieved with a relaxation time of 15 s.
2. Materials and methods
2.3. Synthesis of functionalized PEO
2.1. Materials
An allylation was first performed and followed by an epoxidation (Scheme 1). PEO was added to a two-neck round-bottom flask and diluted in THF. An azeotropic distillation was performed to remove water from PEO and THF. After the distillation, potassium tert-butoxide (PTB) was added little by little under stirring and bubbling of argon. The solution stirred for 2 h prior to adding allyl bromide with a syringe and letting to react overnight. To neutralize PTB, the same amount of water as PTB was added to the PEO solution. For the purification of PEO, the solution was firstly evaporated to remove THF. Afterwards, PEO was dissolved in dichloromethane (DCM) and two liquid-liquid extractions were performed with water. The solution was dried with sodium sulfate (Na2SO4), filtered and DCM evaporated. The allylated and purified PEO was given to a round-bottom flask and solubilized in DCM. In an Erlenmeyer flask, 3-Chloroperbenzoic acid (MCPBA) was dissolved in DCM. By using a burette, the MCPBA solution was added to PEO drop by drop at room temperature. After the addition was finished, the flask was heated to 60 °C in a water bath. The mixture was left to stir for one night. After that time, potassium carbonate was diluted in as least distilled water as possible and this solution was added to the reaction flask. Under a high degree of agitation, the mixture reacted for 1 h. For purification of the functionalized PEO, a liquid-liquid extraction with water was performed, followed by drying over Na2SO4, filtration and evaporation of DCM using a rotary evaporator.
Beech organosolv lignin (OSL) was kindly provided by the Fraunhofer Center for Chemical-Biotechnological Processes (Leuna, Germany). Alkox E30 PEO (Mw: 400,000 g/mol) and PEO methyl ether with a molecular mass of 2000 g/mol were purchased from SigmaAldrich. Poly(ethylene glycol) diglycidyl ether (difunctional PEO) with a molecular mass of 500 g/mol was used in model studies to assess the reactivity of functionalized PEO and lignin (Sigma-Aldrich). Graphistrength® CM12-30 were used as multi-walled carbon nanotubes (MWCNT) and were kindly provided from Arkema (Région de Pau, France). Acetone (≥ 99.5 %), 1-butanol (≥ 99.5 %), dichloromethane (DCM) (≥ 99.8 %), ethanol (≥ 99.8 %), ethyl acetate (≥ 99.5 %), tetrahydrofuran (THF) (≥ 99 %) and toluene (≥ 99.5 %) were purchased from VWR. Anisole (≥ 99 %) and benzaldehyde (≥ 99.5 %) were purchased from Carl Roth. Allyl bromide (97 %), 2-Chloro-4,4,5,5Tetramethyl-1,3,2-Dioxaphospholane (TMDP) (95 %), 3Chloroperbenzoic acid (MCPBA) (≤ 77 %), cholesterol (≥ 99 %), chromium(III)acetylacetonate (99.99 %), cyclohexanol (99 %), diacetone alcohol (99 %), o-dichlorobenzene (99 %), diethyl ether (≥ 99.5 %), dimethyl formamide (≥ 99.8 %), 2-ethyl-1-butanol (98 %), ethylene glycol (≥ 99 %), potassium carbonate (≥ 99 %), potassium tertbutoxide (PTB) (≥ 98 %), propylene carbonate (99 %), propylene glycol (FG), pyridine (99.8 %), sodium sulfate (≥ 99 %) and triethylene glycol (99 %) were purchased from Sigma-Aldrich. Methanol (≥ 99.8 %) was purchased from Th. Geyer. Dimethyl sulfoxide (99.98 %) was purchased from Fisher Chemical. Deuterated chloroform (99.8 %) was purchased from Eurisotop (Saint-Aubin, France).
2.4. Compatibility studies To assess the compatibility between OSL, PEO, epoxidized PEO and CNT, Hansen solubility parameters were determined as: 2
=
2 d
+
2 p
+
2 h
(1)
where d , p and h are the Hansen solubility parameters due to the nonpolar dispersive interactions, polar interactions and hydrogen bonding, respectively (Hansen, 2007). The Hansen’s relative energy distance (RED) allows to draw conclusions on the combability of two components.
2.2. Lignin characterization For gel permeation chromatography (GPC), ca. 10 mg of OSL was diluted in 1 ml of DMF and filtered prior to injection in a Polymer Laboratories PL-GPC 50 system. The system used two PL1113-6300 ResiPore (300 × 7.5 mm) columns with DMF as eluent at a flow rate of 0.8 ml/min. Temperature was set to 70 °C and calibration of the GPC system was done using narrow poly(methyl methacrylate) (PMMA) standards. Results were obtained with the software Cirrus. DSC was performed on 11.7 mg of OSL sample placed in steel capsules on a NETZSCH DSC200 calorimeter from -100 to 100 °C at a heating rate of 20 °C/min. The preparation of OSL for 31P-NMR followed the literature procedure (Granata and Argyropoulos, 1995) except that cholesterol rather than cyclohexanol was used as internal standard. A solvent mixture of
RED =
Ra = R0
4(
D2
D1)
2
+(
P2
R0
P1)
2
+(
H2
H 1)
2
(2)
where Ra is the distance between the spheres to be compared and R 0 represents the radius of the sphere. RED values smaller than 1 indicate compatibility between the components (Hansen, 2007). The Hansen solubility parameter was assessed on 38 solvents by solving 5 mg of sample in 1 ml of solvent and observing the solutions after 24 h. The selected solvents included acetone, anisole, benzaldehyde, 1-butanol, cyclohexanol, diacetone alcohol, o-dichlorobenzene, diethyl ether, 2
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dimethyl formamide, ethanol, ethyl acetate, 2-ethyl-1-butanol, ethylene glycol, methanol, propylene carbonate, propylene glycol, toluene, triethylene glycol and mixtures thereof (table S4). Data were analyzed with the software HSPiP, 5th edition.
2.7. Characterization of processed formulations 2.7.1. Thermal analysis TGA was conducted on a Perkin Elmer Pyris 1 TGA from 30 to 900 °C at 10 °C/min. in N2 atmosphere. DSC scans were performed on a Perkin Elmer DSC 8500 in the range of -80 to 100 °C at a heating or cooling rate of 15 °C/min. with isotherms of 5 min. between cycles. The melting temperature (Tm) and degree of crystallinity (Xc) values were computed from the second heating cycle.
2.5. Assessment of reactivity between lignin and the functionalized PEO A model study of OSL and difunctional PEO was conducted to assess whether cross-linking between these two components could take place. Namely, OSL was mixed with a commercial difunctional epoxidized PEO and the solution was placed in an oven for 12 h at 120 °C. The obtained film was placed in THF for 24 h to dissolve unreacted species and the gel content was computed as:
Gel content (%)
(m2 ) x 100 m1
2.7.2. Microscopic analysis Freshly processed blends were sampled in their cross-section in 1000 and 2500 nm thin slices with a Microtome Leica Ultracut after cooling with liquid N2. The slices were placed with glycerin on cover glasses and observed under polarized optical microscopy (POM) on a Axio Scope.A1 equipped with the camera Axiocam 105 color (Zeiss, Germany) and using the software ZEN. Spherulites radii were recorded as the length from the distribution of chord intercepts with the spherulite boundaries (Karger-Kocsis, 1995) and averaged from 20 measurements. Environmental scanning electron microscopy (ESEM) was performed on a Quanta 250 FEG (FEI) with EDX (Oxford). The samples were placed in a sample holder and a plane surface was cut in the flow direction with the help of a microtome. Low-vacuum and a VCD detector were used.
(3)
where, m2 represents the dried residue mass and m1 the mass of the original specimen. 2.6. Blending procedure and sample composition 2.6.1. Preparation of formulation OSL, epoxidized PEO (Mw: 2000 g/mol), unmodified PEO (Mw: 400,000 g/mol) and CNT were weighed and pre-mixed in a beaker using a spatula. The mixture was compounded at 120 °C and 240 rpm for 2 min. in a DSM micro-compounder (Xplore, Netherlands) in selected compositions. The compounded material was immediately shredded into small pellets, and thereafter fed into a capillary rheometer (Göttfert) operating at 120 °C and at four different shear rates (10, 100, 1000 and 10,000 s−1). Samples were stored in a desiccator at 25% RH after processing.
2.7.3. Tensile tests Tensile tests were carried out on an inspect mini 100 N testing machine (Hegewald & Peschke Meß- und Prüftechnik GmbH) operating at 10 mm/min. in accordance with DIN EN ISO 527-1. Five specimens were tested for each formulation to retrieve Young moduli, ultimate tensile strength and toughness (Gilmore, 2015) from the stress-stain curves. 3. Results and discussion
2.6.2. Sample compositions and factorial design The blends comprised 20 wt% OSL, 80 % PEO in different kinds and 0.1 or 1 wt% CNT with reference to PEO content (Table 1). As for the selection of PEO, the base formulation comprised neat high molecular weight PEO (PEO-HM, 400,000 g/mol). In alternate formulations, PEOHM was partially replaced by either a low molecular weight epoxidized PEO (E-PEO-LM) in 3 or 10 wt % of the total PEO content or by a neat low MW PEO (PEO-LM) in similar amounts as a control formulation (Table 1). This latter control formulation was deemed necessary to deconvolute the impacts of molecular weight changes from that of epoxidation. The complete experimental table followed a L 23 factorial design, in which three factors viz. PEO type, CNT content and shear rates were varied to two levels. The two levels consisted in the presence or absence of E-PEO-LM, CNT loadings of either 0.1 or 1.0 wt%, and shear rates of either 10 s−1 (low) or 10,000 s−1 (high). The effect of each factor was evaluated with the software R and the package RcmdrPlugin.DoE. F tests were conducted with a 95 % confidence interval.
3.1. Synthesis of functionalized PEO The 1H-NMR of pure, allylated and epoxidized PEO in chloroform obtained according to Scheme 1 are shown in Fig. 1. Successful allylation of PEO is evidenced in Fig. 1b with the appearance of the double bond resonances (g and f). Upon epoxidation (Fig. 1c) these resonances disappear, while new chemical shifts appear between 2.5 and 3.5 ppm (j and i), which can be attributed to the epoxy group of epoxidized PEO. Thus, the epoxidation of PEO according to Scheme 1 was successful. 3.2. Compatibility studies Table 2 summarizes the Hansen solubility parameters for OSL, CNT and the various PEOs. Note that for the CNT, values were obtained from literature (Badorrek et al., 2018). The scores for all components and solvents can be seen in table S4. The obtained values for OSL are well in
Table 1 Design of formulations. Sample
0%E-PEO-LM-0.1%CNT 0%E-PEO-LM-1.0%CNT 3%E-PEO-LM-0.1%CNT 3%E-PEO-LM-1.0%CNT 10%E-PEO-LM-0.1%CNT 10%E-PEO-LM-1.0%CNT 10%PEO-LM-0.1%CNT 10%PEO-LM-1.0%CNT
Fixed Components
Variable Components
OSL (%)
PEO-HM (%)
CNT (w%)
E-PEO-LM (%)
PEO-LM (%)
20 20 20 20 20 20 20 20
80 80 77 77 70 70 70 70
0.1 1.0 0.1 1.0 0.1 1.0 0.1 1.0
– – 3 3 10 10 – –
– – – – – – 10 10
3
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Fig. 1. Characterization by 1H-NMR of poly(ethylene oxide) methyl ether (a); poly(ethylene oxide) methyl ether allylated (b) and poly(ethylene oxide) methyl ether epoxidized (c).
lignin, potentially enabling crosslinking in case of difunctional PEO.
Table 2 Hansen solubility parameters of blend components. Sample OSL PEO-HM MWCNT E-PEO-LM
D
[MPa1/2]
18.9 18.2 18.9 21.0
P
[MPa1/2]
16.0 8.6 8.5 12.1
H
[MPa1/2]
11.4 8.4 8.5 4.0
R [MPa1/2]
3.4. Formulation of a precursor fiber
12.2 4.4 6.7 12.8
3.4.1. Thermal analysis and morphology studies Melting temperature and degree of crystallinity (Xc) were computed from the DSC thermograms (figure S7), using an enthalpy of fusion for 100 % crystalline PEO of 213.7 J/g (Shin et al., 2002).
accordance with literature (Badorrek et al., 2018). To assess the compatibility of the components the RED values were calculated. For OSL and CNT a RED value of 1.04 reveals a blend at the limit of compatibility/incompatibility. RED between OSL and PEO-HM and E-PEO-LM equals to 0.67 and 0.78, respectively, demonstrating good compatibility between OSL and the respective PEOs. With a RED value of 0.49, CNT seem to be well dispersible in E-PEO-LM. In contrast, CNT are not compatible with PEO-HM (RED value of 1.5). This suggests that E-PEOLM may act as a compatibilizer between lignin/PEO and CNT.
Xc =
H x 100 H0
(4)
For all formulations, Tm and Xc increase with shear rate (Fig. 2 and figure S8) demonstrating that shear facilitates the formation of crystalline structures, as expected and described for other semicrystalline polymers (Bojda and Piorkowska, 2016; Huang et al., 2011; Liu et al., 2013; Migler et al., 2001; Refaa et al., 2018). It appears that CNT content has only little effect on both Tm and Xc (figure S12). Regardless of CNT content, compared to the base formulation, Tm is lower for samples with PEO-LM and with 10 wt% E-PEO-LM, suggesting that the Tm is influenced by the addition of a lower MW PEO (Fig. 2, figure S8). E-PEO-LM significantly increases the degree of crystallinity (Fig. 2b), an effect that can be clearly ascribed to the presence of the epoxide group, since the opposite trend is observed for non-epoxidized low molecular weight PEO (PEO-LM). Kinetic information can further be obtained by plotting the evolution of crystallinity Xt with time from the crystallization exotherm (Avrami, 1941, 1940, 1939):
3.3. Reactivity of lignin with the functionalized PEO – model study with difunctional PEO A high gel content of 99.3 % was measured from the model reaction of difunctional PEO and OSL. Additionally, proton NMR of the recovered THF did not display residual components (figure S1), demonstrating successful crosslinking of OSL with difunctional epoxidized PEO. That confirms a good reactivity between the epoxy group and the 4
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Fig. 2. Melting temperature (Tm) (a) and degree of crystallinity (Xc) (b) as a function of shear rate for samples containing 0.1 wt% CNT.
Xt =
t 0 0
( ) dt ( ) dt dH dt
dH dt
(5)
where dH/dt denotes the relative heat flow rate and t the respective crystallization time (figure S9). The half-life crystallization time is computed from the crystallization exotherms at the position Xt = 0.5. t1/2 clearly decreases with increasing shear rate, confirming the prior proposition that shearing promotes and accelerates crystallization as observed with other polymers (Feng et al., 2015) (Fig. 3). In comparison to the base formulation, adding PEO-LM increases t1/ 2 while adding E-PEO-LM, decreases it. Hence alike the effect of E-PEOLM on Xc, the addition of functionalized PEO accelerates crystallization and this effect cannot be ascribed to its lower molecular weight. Again, this is clear evidence that the epoxidation of the low molecular weight PEO exerts a distinct effect on the crystallization ability of the high
Fig. 3. Evolution of the half-life crystallization time (t1/2) as a function of shear rate for samples containing 0.1 wt% CNT.
Fig. 4. POM images of sample 0%E-PEO-LM-0.1%CNT (a: 10s−1, c: 10,000 s−1) and 3%E-PEO-LM-0.1%CNT (b: 10s−1, d: 10,000 s−1).
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Fig. 5. Mechanical properties of sample sets: stress-strain curves for 10%E-PEO-LM-1.0%CNT with a shear rate of 10 s−1 (a), sample before and after tensile test (b), comparison of modulus (c), ultimate tensile strength (d), elongation at break (e) and toughness (f) as functions of shear rate. Table 3 Summary of results, indicating significant (according to F-tests) influences of factors on responses.
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molecular weight PEO. A higher CNT content also leads to a clear decrease of t1/2, suggesting that CNT also accelerate the crystallization kinetics of PEO, thus also possibly acting as nucleating agents. This behavior is consistent with prior literature reports (Grady, 2012; Jiang et al., 2017; Kim et al., 2012; Liu and Kumar, 2014; Schawe et al., 2017; Zhang et al., 2014). In addition to the crystallization kinetics, the Avrami exponent n can be assessed from (Avrami, 1939):
X (t ) = 1
exp( kt n)
and 12 MPa are achieved for formulations containing the epoxidized PEO as coupling agent. This compares well with the values reported in literature. For example, for formulations containing 25/75 Alcell lignin and PEO, Kubo and Kadla (2004) reported tensile modulus and strength of 610 MPa and 10.2. MPa. Other authors achieved a tensile strength of 0.96 MPa and an elastic modulus of 4.26 MPa for samples consisting of 50 % lignin and 50 % PEO (Cui et al., 2018). Overall, the addition of EPEO-LM to lignin/PEO blend appears beneficial for lignin/PEO precursor fibers.
(6)
where t represents the crystallization time, k the crystallization rate constant and n the Avrami exponent. For the series of formulations tested, we found that the Avrami coefficient lies between 3 and 3.4, suggesting spherulitic growth. Further it is the lowest for samples with 10 % E-PEO-LM and the highest for 0 % E-PEO-LM (figure S10). Values around 3 indicate an instantaneous nucleation, whereas higher values suggest a more random nucleation (Sharples, 1966). As a result, the addition of E-PEO-LM seems to lead to a more instantaneous nucleation. To confirm the morphology of the samples, polarized optical microscopy was also performed (Fig. 4). The typical Maltese cross of the spherulite structure is clearly visible as expected from the Avrami analyses. For the base formulations (Fig. 4a and c), spherulites become clearer and less disrupted as shear rate increases; hence shear promotes spherulitic growth. For samples with E-PEO-LM (Fig. 4b and d), the crystalline morphology is already clearly visible for samples with 10 s−1. In addition, the radius of spherulites is smaller in presence of EPEO-LM (figure S11). Together with the thermal analysis, which revealed an increase in Xc and in crystallization rate with the addition of E-PEO-LM, these observations suggest a possible nucleating effect of EPEO-LM. Again, such a nucleating effect cannot be explained by differences in molecular weight since the PEO-LM has an opposite effect to E-PEO-LM. ESEM images (figure S5) revealed that CNT tend to form agglomerates for small shear rates. As shear rate increases, the dispersion of CNT is enhanced.
4. Conclusions The aim of this work was to evaluate the impact of adding carbon nanotubes and an epoxidized poly(ethylene oxide) to lignin/PEO precursor fiber formulations. An epoxidized PEO was successfully synthesized and model studies revealed the ability of the epoxy functionality to covalently bind to lignin. Analysis of Hansen solubility parameters revealed that lignin and PEO are compatible whereas CNT appear to have little dispersibility in lignin. In this case, E-PEO-LM, which is compatible with both CNT and lignin, might act as a compatibilizer between the components. It was shown that the addition of E-PEO-LM in precursor fiber formulations increases the degree of crystallinity, accelerates crystallization and leads to the formation of smaller spherulites. As expected, the higher degree of crystallinity also leads to a significant increase of modulus and ultimate tensile strength of the precursor fiber. Related with other studies, the elongation of samples is high while still having comparatively high values for modulus and ultimate tensile strength. Thus E-PEO-LM appears as a suitable compatibilizer for PEO/lignin blends reinforced with CNT. This effect was ascribed to the ability of the epoxidized PEO to covalently bind to lignin, thereby acting like a nucleating agent for the PEO matrix. Despite their nucleating effect on PEO, CNT did not influence the mechanical performance of the precursor fibers. The impact of thereby improved properties of precursor fiber on the carbon fibers themselves remains to be determined.
3.4.2. Mechanical performance of lignin-based fibers Mechanical testing revealed reproducible stress-strain curves (Fig. 5a), which resulted in highly permanently elongated specimens (Fig. 5b). With increasing shear rate, mechanical properties decrease for all formulations (Fig. 5c to f). Here again, the formulation with the epoxidized PEO stands out among all formulations. Namely, modulus and ultimate tensile strength are clearly higher for samples containing E-PEO-LM than for the base formulation (Fig. 5c and d). For example, at a shear rate of 10 s−1 the partial replacement of PEO-HM with 10 % EPEO-LM results in a ca. 100 % increase in both modulus and strength. Such an effect is not observed with the PEO-LM. Thus E-PEO-LM has a unique reinforcement effect on modulus and strength as a result of the epoxidation. In contrast, elongation at break (Fig. 5e) and toughness (Fig. 5f) were not specifically affected by the presence of the E-PEO-LM. Likewise, CNT content appeared to have a rather small influence on the mechanical performance of the fibers (figure S12). Table 3 summarizes the significant factors found on the various responses tested based on F tests (figure S12, table S3). The significant increase in modulus and strength with the addition of E-PEO-LM (Table 3) can be explained by the reaction of the epoxide with the lignin. The so-immobilized low molecular weight chains might serve as a nucleating surface and can eventually co-crystallize with PEO-HM. In other words, it appears that E-PEO-LM binds OSL with PEO-HM. This interfacial binding between lignin and the PEO matrix could explain the large improvement in modulus and in strength determined for these formulations. In fact, the over 100 % increase in modulus and strength cannot be solely ascribed to the 10 % increase in crystallinity but likely bears some contribution from the improved interfacial adhesion between lignin and the PEO matrix. Overall, precursor fibers with tensile modulus and strength on the order of 450 MPa
Acknowledgements This work was partially funded by CARBOPREC (CARBOPREC: Renewable source nanostructured precursors for carbon fibers, was funded by the European Union's Seventh Framework Program FP7NMP, project number 604215). We gratefully acknowledge Fraunhofer CBP (Leuna, Germany) for providing organosolv lignin and Arkema (Région de Pau, France) for providing carbon nanotubes. The authors would like to thank Elke Stibal and Lukas Walter, who provided technical assistance. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2019.111883. References Aslanzadeh, S., Ahvazi, B., Boluk, Y., Ayranci, C., 2017. Carbon fiber production from electrospun sulfur free softwood lignin precursors. J. Eng. Fibers Fabr. 12 (4), 33–43. Avrami, M., 1939. Kinetics of phase change. I general theory. J. Chem. Phys. 7 (12), 1103–1112. https://doi.org/10.1063/1.1750380. Avrami, M., 1940. Kinetics of phase change. II transformation‐time relations for random distribution of nuclei. J. Chem. Phys. 8 (2), 212–224. https://doi.org/10.1063/1. 1750631. Avrami, M., 1941. Granulation, phase change, and microstructure kinetics of phase change. III. J. Chem. Phys. 9 (2), 177–184. https://doi.org/10.1063/1.1750872. Awal, A., Sain, M., 2013. Characterization of soda hardwood lignin and the formation of lignin fibers by melt spinning. J. Appl. Polym. Sci. 129 (5), 2765–2771. https://doi. org/10.1002/app.38911. Badorrek, J., Walter, M., Laborie, M.-P., 2018. Tuning intermolecular interaction between lignin and carbon nanotubes in fiber composites – a combined experimental and abinitio modeling study. J. Renew. Mater. https://doi.org/10.7569/JRM.2017.634183.
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Impact of PEO structure and formulation on the properties of a Lignin/PEO blend
T
L.-S. Ebersa,b,*, R. Auvergnec, B. Boutevinc, M.-P. Laboriea,b a
Chair of Forest Biomaterials, University of Freiburg, Werthmannstr. 6, 79085 Freiburg im Breisgau, Germany Freiburg Materials Research Center, Stefan-Meier-Str. 21, 79104 Freiburg im Breisgau, Germany c Institut Charles Gerhardt UMR 5253 – CNRS, UM, ENSCM – 240 Av Prof Emile Jeanbrau, 34296 Montpellier, France b
ARTICLE INFO
ABSTRACT
Keywords: Lignin Carbon fiber precursors Coupling agent Crystallinity Mechanical properties
To improve the performance of lignin/PEO fibers as precursors of carbon fibers, blend formulations were designed with an epoxidized PEO and with carbon nanotubes. Model studies demonstrated that the epoxidized PEO could covalently link to lignin. With incorporation of epoxidized PEO, the precursor fibers exhibited enhanced degree of crystallinity of 70 % (52 % without epoxidized PEO) and a doubling of their young modulus (from 200 to 450 MPa) and tensile strength (from 6 to 12 MPa). In contrast, the addition of carbon nanotubes did not impact the morphology nor the mechanical performance of the resulting fibers. It is argued that with its ability to couple lignin to the PEO matrix, epoxidized PEO plays the role of a stress transfer agent between the rigid lignin regions and the flexible PEO regions, thereby improving the blend performance.
1. Introduction Due to the increasing demand for carbon fibers and the high price of its precursor polyacrylonitrile (PAN), much research effort has been recently devoted to using lignin for the production of carbon fibers (Aslanzadeh et al., 2017; Baker et al., 2012; Baker and Rials, 2013; Bengtsson et al., 2019a, 2018; Byrne et al., 2018; Dai et al., 2018; Fang et al., 2017; García-Mateos et al., 2018; Hosseinaei et al., 2017; Jiang et al., 2018; Jin et al., 2018; Jin and Ogale, 2018; Kubo and Kadla, 2005; Lin et al., 2014a; Lin and Zhao, 2016; Liu et al., 2018b, a; Meek et al., 2016; Nar et al., 2016; Norberg et al., 2013; Nordström et al., 2013; Oroumei et al., 2018; Qu et al., 2018, 2016; Schlee et al., 2019; Shi et al., 2018; Wang et al., 2016) and carbon fiber precursors (Awal and Sain, 2013; Bengtsson et al., 2019c, b; Culebras et al., 2018; Dong et al., 2015; Imel et al., 2016; Kubo and Kadla, 2003; Seydibeyoğlu, 2012). Indeed, as a major constituent of lignocellulosic biomass, lignin is largely available and comes from renewable resource, all important factors in times of climate change and decreasing fossil resources (Sain and Faruk, 2016). To improve lignin spinnability for the manufacture of carbon fiber precursors, lignin has either been modified or blended with a spinnable polymer (Fang et al., 2017). In the case of polymer blends, polymers of high spinning ability and good miscibility with lignin such as polyvinyl alcohol, polyethylene terephthalate (PET) or polyethylene oxide (PEO) have been commonly used (Cui et al., 2018; Kadla and Kubo, 2003; Kubo and Kadla, 2006, 2004; Lin et al., 2014b; Yu et al., ⁎
2015). It was shown for example that blending lignin with PEO resulted in improved spinnability and mechanical performance of the resulting fibers (Kubo and Kadla, 2004; Yu et al., 2015). To address the remaining challenge of the low mechanical properties of precursor fibers from PEO and lignin (Baker et al., 2012), the reinforcement of precursor with carbon nanotubes has been proposed. Indeed, the addition of carbon nanotubes (CNT) to carbon fiber precursors has been shown to increase strength and modulus in ligninbased carbon fibers (Baker et al., 2011; Wang et al., 2016) . In general, it is well established that the addition of CNT enhances the mechanical properties of polymer composites (Bikiaris, 2010; Coleman et al., 2006; Grady, 2012; Liu and Kumar, 2014) and that they can initiate a nucleating effect (Grady, 2012; Jiang et al., 2017; Kim et al., 2012; Liu and Kumar, 2014; Schawe et al., 2017; Zhang et al., 2014). In addition, it was hypothesized that a better stress transfer at the lignin/PEO interphase could result in further improvements in mechanical properties. While PEO and lignin are well established to be miscible (Kadla and Kubo, 2003; Kubo and Kadla, 2006, 2004), the use of a coupling agent that would covalently link lignin to the PEO could further result in improved stress transfer and mechanical performance of the precursor fiber. An ideal coupling agent would consist of a functionalized PEO whose functionality would enable covalent linking with lignin and the remaining PEO chain would still be able to cocrystallize or entangle with the matrix PEO polymer. In this contribution, we evaluate a series of lignin/PEO precursor
Corresponding author at: Chair of Forest Biomaterials, University of Freiburg, Werthmannstr. 6, 79085 Freiburg im Breisgau, Germany. E-mail addresses: [email protected] (L.-S. Ebers), [email protected] (M.-P. Laborie).
https://doi.org/10.1016/j.indcrop.2019.111883 Received 15 May 2019; Received in revised form 8 September 2019; Accepted 17 October 2019 Available online 12 November 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis route of monofunctional epoxidized PEO.
fibers in which carbon nanotubes are added as reinforcing and structuring agents and an epoxidized PEO is utilized as possible coupling agent between the PEO matrix and lignin. We thus examine in detail the impact of formulation design on the thermal, morphological and mechanical performance of the precursor fibers with a view to shedding light on the possible strengthening role of CNTs and epoxidized PEO. Alike other authors who examined lignin-based precursor fibers in a first place (Awal and Sain, 2013; Bengtsson et al., 2019c,b; Culebras et al., 2018; Dong et al., 2015; Imel et al., 2016; Kubo and Kadla, 2003; Seydibeyoğlu, 2012), in this paper we focus exclusively on the attributes and properties of the precursor fibers.
pyridine and deuterated chloroform (1.6/1 v/v) was prepared together with an internal standard cholesterol solution (43 mg/ml) (SafouTchiama et al., 2017). 30 mg OSL was weighed in a volumetric flask to which 0.5 ml of the solvent mixture, 100 μl of the chromium(III)acetylacetonate relaxation agent solution (5 mg/ml) and 100 μl of the internal standard solution was added. The mixture stirred over night at room temperature. 15 min before 31P-NMR measurement, 50 μl of the phosphitylation reagent 2-Chloro-4,4,5,5-Tetramethyl-1,3,2-Dioxaphospholane (TMDP) was added. NMR spectra were retrieved from a Bruker Avance 400 MHz spectrometer. 128 scans were retrieved with a relaxation time of 15 s.
2. Materials and methods
2.3. Synthesis of functionalized PEO
2.1. Materials
An allylation was first performed and followed by an epoxidation (Scheme 1). PEO was added to a two-neck round-bottom flask and diluted in THF. An azeotropic distillation was performed to remove water from PEO and THF. After the distillation, potassium tert-butoxide (PTB) was added little by little under stirring and bubbling of argon. The solution stirred for 2 h prior to adding allyl bromide with a syringe and letting to react overnight. To neutralize PTB, the same amount of water as PTB was added to the PEO solution. For the purification of PEO, the solution was firstly evaporated to remove THF. Afterwards, PEO was dissolved in dichloromethane (DCM) and two liquid-liquid extractions were performed with water. The solution was dried with sodium sulfate (Na2SO4), filtered and DCM evaporated. The allylated and purified PEO was given to a round-bottom flask and solubilized in DCM. In an Erlenmeyer flask, 3-Chloroperbenzoic acid (MCPBA) was dissolved in DCM. By using a burette, the MCPBA solution was added to PEO drop by drop at room temperature. After the addition was finished, the flask was heated to 60 °C in a water bath. The mixture was left to stir for one night. After that time, potassium carbonate was diluted in as least distilled water as possible and this solution was added to the reaction flask. Under a high degree of agitation, the mixture reacted for 1 h. For purification of the functionalized PEO, a liquid-liquid extraction with water was performed, followed by drying over Na2SO4, filtration and evaporation of DCM using a rotary evaporator.
Beech organosolv lignin (OSL) was kindly provided by the Fraunhofer Center for Chemical-Biotechnological Processes (Leuna, Germany). Alkox E30 PEO (Mw: 400,000 g/mol) and PEO methyl ether with a molecular mass of 2000 g/mol were purchased from SigmaAldrich. Poly(ethylene glycol) diglycidyl ether (difunctional PEO) with a molecular mass of 500 g/mol was used in model studies to assess the reactivity of functionalized PEO and lignin (Sigma-Aldrich). Graphistrength® CM12-30 were used as multi-walled carbon nanotubes (MWCNT) and were kindly provided from Arkema (Région de Pau, France). Acetone (≥ 99.5 %), 1-butanol (≥ 99.5 %), dichloromethane (DCM) (≥ 99.8 %), ethanol (≥ 99.8 %), ethyl acetate (≥ 99.5 %), tetrahydrofuran (THF) (≥ 99 %) and toluene (≥ 99.5 %) were purchased from VWR. Anisole (≥ 99 %) and benzaldehyde (≥ 99.5 %) were purchased from Carl Roth. Allyl bromide (97 %), 2-Chloro-4,4,5,5Tetramethyl-1,3,2-Dioxaphospholane (TMDP) (95 %), 3Chloroperbenzoic acid (MCPBA) (≤ 77 %), cholesterol (≥ 99 %), chromium(III)acetylacetonate (99.99 %), cyclohexanol (99 %), diacetone alcohol (99 %), o-dichlorobenzene (99 %), diethyl ether (≥ 99.5 %), dimethyl formamide (≥ 99.8 %), 2-ethyl-1-butanol (98 %), ethylene glycol (≥ 99 %), potassium carbonate (≥ 99 %), potassium tertbutoxide (PTB) (≥ 98 %), propylene carbonate (99 %), propylene glycol (FG), pyridine (99.8 %), sodium sulfate (≥ 99 %) and triethylene glycol (99 %) were purchased from Sigma-Aldrich. Methanol (≥ 99.8 %) was purchased from Th. Geyer. Dimethyl sulfoxide (99.98 %) was purchased from Fisher Chemical. Deuterated chloroform (99.8 %) was purchased from Eurisotop (Saint-Aubin, France).
2.4. Compatibility studies To assess the compatibility between OSL, PEO, epoxidized PEO and CNT, Hansen solubility parameters were determined as: 2
=
2 d
+
2 p
+
2 h
(1)
where d , p and h are the Hansen solubility parameters due to the nonpolar dispersive interactions, polar interactions and hydrogen bonding, respectively (Hansen, 2007). The Hansen’s relative energy distance (RED) allows to draw conclusions on the combability of two components.
2.2. Lignin characterization For gel permeation chromatography (GPC), ca. 10 mg of OSL was diluted in 1 ml of DMF and filtered prior to injection in a Polymer Laboratories PL-GPC 50 system. The system used two PL1113-6300 ResiPore (300 × 7.5 mm) columns with DMF as eluent at a flow rate of 0.8 ml/min. Temperature was set to 70 °C and calibration of the GPC system was done using narrow poly(methyl methacrylate) (PMMA) standards. Results were obtained with the software Cirrus. DSC was performed on 11.7 mg of OSL sample placed in steel capsules on a NETZSCH DSC200 calorimeter from -100 to 100 °C at a heating rate of 20 °C/min. The preparation of OSL for 31P-NMR followed the literature procedure (Granata and Argyropoulos, 1995) except that cholesterol rather than cyclohexanol was used as internal standard. A solvent mixture of
RED =
Ra = R0
4(
D2
D1)
2
+(
P2
R0
P1)
2
+(
H2
H 1)
2
(2)
where Ra is the distance between the spheres to be compared and R 0 represents the radius of the sphere. RED values smaller than 1 indicate compatibility between the components (Hansen, 2007). The Hansen solubility parameter was assessed on 38 solvents by solving 5 mg of sample in 1 ml of solvent and observing the solutions after 24 h. The selected solvents included acetone, anisole, benzaldehyde, 1-butanol, cyclohexanol, diacetone alcohol, o-dichlorobenzene, diethyl ether, 2
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dimethyl formamide, ethanol, ethyl acetate, 2-ethyl-1-butanol, ethylene glycol, methanol, propylene carbonate, propylene glycol, toluene, triethylene glycol and mixtures thereof (table S4). Data were analyzed with the software HSPiP, 5th edition.
2.7. Characterization of processed formulations 2.7.1. Thermal analysis TGA was conducted on a Perkin Elmer Pyris 1 TGA from 30 to 900 °C at 10 °C/min. in N2 atmosphere. DSC scans were performed on a Perkin Elmer DSC 8500 in the range of -80 to 100 °C at a heating or cooling rate of 15 °C/min. with isotherms of 5 min. between cycles. The melting temperature (Tm) and degree of crystallinity (Xc) values were computed from the second heating cycle.
2.5. Assessment of reactivity between lignin and the functionalized PEO A model study of OSL and difunctional PEO was conducted to assess whether cross-linking between these two components could take place. Namely, OSL was mixed with a commercial difunctional epoxidized PEO and the solution was placed in an oven for 12 h at 120 °C. The obtained film was placed in THF for 24 h to dissolve unreacted species and the gel content was computed as:
Gel content (%)
(m2 ) x 100 m1
2.7.2. Microscopic analysis Freshly processed blends were sampled in their cross-section in 1000 and 2500 nm thin slices with a Microtome Leica Ultracut after cooling with liquid N2. The slices were placed with glycerin on cover glasses and observed under polarized optical microscopy (POM) on a Axio Scope.A1 equipped with the camera Axiocam 105 color (Zeiss, Germany) and using the software ZEN. Spherulites radii were recorded as the length from the distribution of chord intercepts with the spherulite boundaries (Karger-Kocsis, 1995) and averaged from 20 measurements. Environmental scanning electron microscopy (ESEM) was performed on a Quanta 250 FEG (FEI) with EDX (Oxford). The samples were placed in a sample holder and a plane surface was cut in the flow direction with the help of a microtome. Low-vacuum and a VCD detector were used.
(3)
where, m2 represents the dried residue mass and m1 the mass of the original specimen. 2.6. Blending procedure and sample composition 2.6.1. Preparation of formulation OSL, epoxidized PEO (Mw: 2000 g/mol), unmodified PEO (Mw: 400,000 g/mol) and CNT were weighed and pre-mixed in a beaker using a spatula. The mixture was compounded at 120 °C and 240 rpm for 2 min. in a DSM micro-compounder (Xplore, Netherlands) in selected compositions. The compounded material was immediately shredded into small pellets, and thereafter fed into a capillary rheometer (Göttfert) operating at 120 °C and at four different shear rates (10, 100, 1000 and 10,000 s−1). Samples were stored in a desiccator at 25% RH after processing.
2.7.3. Tensile tests Tensile tests were carried out on an inspect mini 100 N testing machine (Hegewald & Peschke Meß- und Prüftechnik GmbH) operating at 10 mm/min. in accordance with DIN EN ISO 527-1. Five specimens were tested for each formulation to retrieve Young moduli, ultimate tensile strength and toughness (Gilmore, 2015) from the stress-stain curves. 3. Results and discussion
2.6.2. Sample compositions and factorial design The blends comprised 20 wt% OSL, 80 % PEO in different kinds and 0.1 or 1 wt% CNT with reference to PEO content (Table 1). As for the selection of PEO, the base formulation comprised neat high molecular weight PEO (PEO-HM, 400,000 g/mol). In alternate formulations, PEOHM was partially replaced by either a low molecular weight epoxidized PEO (E-PEO-LM) in 3 or 10 wt % of the total PEO content or by a neat low MW PEO (PEO-LM) in similar amounts as a control formulation (Table 1). This latter control formulation was deemed necessary to deconvolute the impacts of molecular weight changes from that of epoxidation. The complete experimental table followed a L 23 factorial design, in which three factors viz. PEO type, CNT content and shear rates were varied to two levels. The two levels consisted in the presence or absence of E-PEO-LM, CNT loadings of either 0.1 or 1.0 wt%, and shear rates of either 10 s−1 (low) or 10,000 s−1 (high). The effect of each factor was evaluated with the software R and the package RcmdrPlugin.DoE. F tests were conducted with a 95 % confidence interval.
3.1. Synthesis of functionalized PEO The 1H-NMR of pure, allylated and epoxidized PEO in chloroform obtained according to Scheme 1 are shown in Fig. 1. Successful allylation of PEO is evidenced in Fig. 1b with the appearance of the double bond resonances (g and f). Upon epoxidation (Fig. 1c) these resonances disappear, while new chemical shifts appear between 2.5 and 3.5 ppm (j and i), which can be attributed to the epoxy group of epoxidized PEO. Thus, the epoxidation of PEO according to Scheme 1 was successful. 3.2. Compatibility studies Table 2 summarizes the Hansen solubility parameters for OSL, CNT and the various PEOs. Note that for the CNT, values were obtained from literature (Badorrek et al., 2018). The scores for all components and solvents can be seen in table S4. The obtained values for OSL are well in
Table 1 Design of formulations. Sample
0%E-PEO-LM-0.1%CNT 0%E-PEO-LM-1.0%CNT 3%E-PEO-LM-0.1%CNT 3%E-PEO-LM-1.0%CNT 10%E-PEO-LM-0.1%CNT 10%E-PEO-LM-1.0%CNT 10%PEO-LM-0.1%CNT 10%PEO-LM-1.0%CNT
Fixed Components
Variable Components
OSL (%)
PEO-HM (%)
CNT (w%)
E-PEO-LM (%)
PEO-LM (%)
20 20 20 20 20 20 20 20
80 80 77 77 70 70 70 70
0.1 1.0 0.1 1.0 0.1 1.0 0.1 1.0
– – 3 3 10 10 – –
– – – – – – 10 10
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Fig. 1. Characterization by 1H-NMR of poly(ethylene oxide) methyl ether (a); poly(ethylene oxide) methyl ether allylated (b) and poly(ethylene oxide) methyl ether epoxidized (c).
lignin, potentially enabling crosslinking in case of difunctional PEO.
Table 2 Hansen solubility parameters of blend components. Sample OSL PEO-HM MWCNT E-PEO-LM
D
[MPa1/2]
18.9 18.2 18.9 21.0
P
[MPa1/2]
16.0 8.6 8.5 12.1
H
[MPa1/2]
11.4 8.4 8.5 4.0
R [MPa1/2]
3.4. Formulation of a precursor fiber
12.2 4.4 6.7 12.8
3.4.1. Thermal analysis and morphology studies Melting temperature and degree of crystallinity (Xc) were computed from the DSC thermograms (figure S7), using an enthalpy of fusion for 100 % crystalline PEO of 213.7 J/g (Shin et al., 2002).
accordance with literature (Badorrek et al., 2018). To assess the compatibility of the components the RED values were calculated. For OSL and CNT a RED value of 1.04 reveals a blend at the limit of compatibility/incompatibility. RED between OSL and PEO-HM and E-PEO-LM equals to 0.67 and 0.78, respectively, demonstrating good compatibility between OSL and the respective PEOs. With a RED value of 0.49, CNT seem to be well dispersible in E-PEO-LM. In contrast, CNT are not compatible with PEO-HM (RED value of 1.5). This suggests that E-PEOLM may act as a compatibilizer between lignin/PEO and CNT.
Xc =
H x 100 H0
(4)
For all formulations, Tm and Xc increase with shear rate (Fig. 2 and figure S8) demonstrating that shear facilitates the formation of crystalline structures, as expected and described for other semicrystalline polymers (Bojda and Piorkowska, 2016; Huang et al., 2011; Liu et al., 2013; Migler et al., 2001; Refaa et al., 2018). It appears that CNT content has only little effect on both Tm and Xc (figure S12). Regardless of CNT content, compared to the base formulation, Tm is lower for samples with PEO-LM and with 10 wt% E-PEO-LM, suggesting that the Tm is influenced by the addition of a lower MW PEO (Fig. 2, figure S8). E-PEO-LM significantly increases the degree of crystallinity (Fig. 2b), an effect that can be clearly ascribed to the presence of the epoxide group, since the opposite trend is observed for non-epoxidized low molecular weight PEO (PEO-LM). Kinetic information can further be obtained by plotting the evolution of crystallinity Xt with time from the crystallization exotherm (Avrami, 1941, 1940, 1939):
3.3. Reactivity of lignin with the functionalized PEO – model study with difunctional PEO A high gel content of 99.3 % was measured from the model reaction of difunctional PEO and OSL. Additionally, proton NMR of the recovered THF did not display residual components (figure S1), demonstrating successful crosslinking of OSL with difunctional epoxidized PEO. That confirms a good reactivity between the epoxy group and the 4
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Fig. 2. Melting temperature (Tm) (a) and degree of crystallinity (Xc) (b) as a function of shear rate for samples containing 0.1 wt% CNT.
Xt =
t 0 0
( ) dt ( ) dt dH dt
dH dt
(5)
where dH/dt denotes the relative heat flow rate and t the respective crystallization time (figure S9). The half-life crystallization time is computed from the crystallization exotherms at the position Xt = 0.5. t1/2 clearly decreases with increasing shear rate, confirming the prior proposition that shearing promotes and accelerates crystallization as observed with other polymers (Feng et al., 2015) (Fig. 3). In comparison to the base formulation, adding PEO-LM increases t1/ 2 while adding E-PEO-LM, decreases it. Hence alike the effect of E-PEOLM on Xc, the addition of functionalized PEO accelerates crystallization and this effect cannot be ascribed to its lower molecular weight. Again, this is clear evidence that the epoxidation of the low molecular weight PEO exerts a distinct effect on the crystallization ability of the high
Fig. 3. Evolution of the half-life crystallization time (t1/2) as a function of shear rate for samples containing 0.1 wt% CNT.
Fig. 4. POM images of sample 0%E-PEO-LM-0.1%CNT (a: 10s−1, c: 10,000 s−1) and 3%E-PEO-LM-0.1%CNT (b: 10s−1, d: 10,000 s−1).
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Fig. 5. Mechanical properties of sample sets: stress-strain curves for 10%E-PEO-LM-1.0%CNT with a shear rate of 10 s−1 (a), sample before and after tensile test (b), comparison of modulus (c), ultimate tensile strength (d), elongation at break (e) and toughness (f) as functions of shear rate. Table 3 Summary of results, indicating significant (according to F-tests) influences of factors on responses.
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molecular weight PEO. A higher CNT content also leads to a clear decrease of t1/2, suggesting that CNT also accelerate the crystallization kinetics of PEO, thus also possibly acting as nucleating agents. This behavior is consistent with prior literature reports (Grady, 2012; Jiang et al., 2017; Kim et al., 2012; Liu and Kumar, 2014; Schawe et al., 2017; Zhang et al., 2014). In addition to the crystallization kinetics, the Avrami exponent n can be assessed from (Avrami, 1939):
X (t ) = 1
exp( kt n)
and 12 MPa are achieved for formulations containing the epoxidized PEO as coupling agent. This compares well with the values reported in literature. For example, for formulations containing 25/75 Alcell lignin and PEO, Kubo and Kadla (2004) reported tensile modulus and strength of 610 MPa and 10.2. MPa. Other authors achieved a tensile strength of 0.96 MPa and an elastic modulus of 4.26 MPa for samples consisting of 50 % lignin and 50 % PEO (Cui et al., 2018). Overall, the addition of EPEO-LM to lignin/PEO blend appears beneficial for lignin/PEO precursor fibers.
(6)
where t represents the crystallization time, k the crystallization rate constant and n the Avrami exponent. For the series of formulations tested, we found that the Avrami coefficient lies between 3 and 3.4, suggesting spherulitic growth. Further it is the lowest for samples with 10 % E-PEO-LM and the highest for 0 % E-PEO-LM (figure S10). Values around 3 indicate an instantaneous nucleation, whereas higher values suggest a more random nucleation (Sharples, 1966). As a result, the addition of E-PEO-LM seems to lead to a more instantaneous nucleation. To confirm the morphology of the samples, polarized optical microscopy was also performed (Fig. 4). The typical Maltese cross of the spherulite structure is clearly visible as expected from the Avrami analyses. For the base formulations (Fig. 4a and c), spherulites become clearer and less disrupted as shear rate increases; hence shear promotes spherulitic growth. For samples with E-PEO-LM (Fig. 4b and d), the crystalline morphology is already clearly visible for samples with 10 s−1. In addition, the radius of spherulites is smaller in presence of EPEO-LM (figure S11). Together with the thermal analysis, which revealed an increase in Xc and in crystallization rate with the addition of E-PEO-LM, these observations suggest a possible nucleating effect of EPEO-LM. Again, such a nucleating effect cannot be explained by differences in molecular weight since the PEO-LM has an opposite effect to E-PEO-LM. ESEM images (figure S5) revealed that CNT tend to form agglomerates for small shear rates. As shear rate increases, the dispersion of CNT is enhanced.
4. Conclusions The aim of this work was to evaluate the impact of adding carbon nanotubes and an epoxidized poly(ethylene oxide) to lignin/PEO precursor fiber formulations. An epoxidized PEO was successfully synthesized and model studies revealed the ability of the epoxy functionality to covalently bind to lignin. Analysis of Hansen solubility parameters revealed that lignin and PEO are compatible whereas CNT appear to have little dispersibility in lignin. In this case, E-PEO-LM, which is compatible with both CNT and lignin, might act as a compatibilizer between the components. It was shown that the addition of E-PEO-LM in precursor fiber formulations increases the degree of crystallinity, accelerates crystallization and leads to the formation of smaller spherulites. As expected, the higher degree of crystallinity also leads to a significant increase of modulus and ultimate tensile strength of the precursor fiber. Related with other studies, the elongation of samples is high while still having comparatively high values for modulus and ultimate tensile strength. Thus E-PEO-LM appears as a suitable compatibilizer for PEO/lignin blends reinforced with CNT. This effect was ascribed to the ability of the epoxidized PEO to covalently bind to lignin, thereby acting like a nucleating agent for the PEO matrix. Despite their nucleating effect on PEO, CNT did not influence the mechanical performance of the precursor fibers. The impact of thereby improved properties of precursor fiber on the carbon fibers themselves remains to be determined.
3.4.2. Mechanical performance of lignin-based fibers Mechanical testing revealed reproducible stress-strain curves (Fig. 5a), which resulted in highly permanently elongated specimens (Fig. 5b). With increasing shear rate, mechanical properties decrease for all formulations (Fig. 5c to f). Here again, the formulation with the epoxidized PEO stands out among all formulations. Namely, modulus and ultimate tensile strength are clearly higher for samples containing E-PEO-LM than for the base formulation (Fig. 5c and d). For example, at a shear rate of 10 s−1 the partial replacement of PEO-HM with 10 % EPEO-LM results in a ca. 100 % increase in both modulus and strength. Such an effect is not observed with the PEO-LM. Thus E-PEO-LM has a unique reinforcement effect on modulus and strength as a result of the epoxidation. In contrast, elongation at break (Fig. 5e) and toughness (Fig. 5f) were not specifically affected by the presence of the E-PEO-LM. Likewise, CNT content appeared to have a rather small influence on the mechanical performance of the fibers (figure S12). Table 3 summarizes the significant factors found on the various responses tested based on F tests (figure S12, table S3). The significant increase in modulus and strength with the addition of E-PEO-LM (Table 3) can be explained by the reaction of the epoxide with the lignin. The so-immobilized low molecular weight chains might serve as a nucleating surface and can eventually co-crystallize with PEO-HM. In other words, it appears that E-PEO-LM binds OSL with PEO-HM. This interfacial binding between lignin and the PEO matrix could explain the large improvement in modulus and in strength determined for these formulations. In fact, the over 100 % increase in modulus and strength cannot be solely ascribed to the 10 % increase in crystallinity but likely bears some contribution from the improved interfacial adhesion between lignin and the PEO matrix. Overall, precursor fibers with tensile modulus and strength on the order of 450 MPa
Acknowledgements This work was partially funded by CARBOPREC (CARBOPREC: Renewable source nanostructured precursors for carbon fibers, was funded by the European Union's Seventh Framework Program FP7NMP, project number 604215). We gratefully acknowledge Fraunhofer CBP (Leuna, Germany) for providing organosolv lignin and Arkema (Région de Pau, France) for providing carbon nanotubes. The authors would like to thank Elke Stibal and Lukas Walter, who provided technical assistance. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2019.111883. References Aslanzadeh, S., Ahvazi, B., Boluk, Y., Ayranci, C., 2017. Carbon fiber production from electrospun sulfur free softwood lignin precursors. J. Eng. Fibers Fabr. 12 (4), 33–43. Avrami, M., 1939. Kinetics of phase change. I general theory. J. Chem. Phys. 7 (12), 1103–1112. https://doi.org/10.1063/1.1750380. Avrami, M., 1940. Kinetics of phase change. II transformation‐time relations for random distribution of nuclei. J. Chem. Phys. 8 (2), 212–224. https://doi.org/10.1063/1. 1750631. Avrami, M., 1941. Granulation, phase change, and microstructure kinetics of phase change. III. J. Chem. Phys. 9 (2), 177–184. https://doi.org/10.1063/1.1750872. Awal, A., Sain, M., 2013. Characterization of soda hardwood lignin and the formation of lignin fibers by melt spinning. J. Appl. Polym. Sci. 129 (5), 2765–2771. https://doi. org/10.1002/app.38911. Badorrek, J., Walter, M., Laborie, M.-P., 2018. Tuning intermolecular interaction between lignin and carbon nanotubes in fiber composites – a combined experimental and abinitio modeling study. J. Renew. Mater. https://doi.org/10.7569/JRM.2017.634183.
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