Lignocellulose Nanofiber-Reinforced Polystyrene Produced from

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Lignocellulose nanofiber-reinforced polystyrene produced from composite microspheres obtained in suspension polymerization shows superior mechanical performance Daniel Ballner, Sabine Herzele, Jozef Keckes, Matthias Edler, Thomas Griesser, Bodo Saake, Falk Liebner, Antje Potthast, Christian Paulik, and Wolfgang Gindl-Altmutter ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01992 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 15, 2016

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ACS Applied Materials & Interfaces

Lignocellulose nanofiber-reinforced polystyrene produced from composite microspheres obtained in suspension polymerization shows superior mechanical performance Daniel Ballner1, ‡, Sabine Herzele2, ‡, Jozef Keckes3, Matthias Edler4, Thomas Griesser4, Bodo Saake5, Falk Liebner6, Antje Potthast6, Christian Paulik7, and Wolfgang Gindl-Altmutter1,* 1

Department of Materials Science and Process Engineering, BOKU-University of Natural

Resources and Life Science, Vienna, Konrad Lorenz Strasse 24, 3430 Tulln, Austria 2

Kompetenzzentrum Holz GmbH, Altenbergerstrasse 69, 4040 Linz, Austria

3

Department of Materials Physics, University of Leoben, Jahnstrasse 12, 8700 Leoben,

Austria 4

Department of Polymer Technology, University of Leoben, Otto Glöckel Strasse 2, 8700

Leoben, 5

Zentrum Holzwirtschaft, University of Hamburg, Leuschnerstrasse 91, 21031 Hamburg-

Bergedorf, Germany 6

Department of Chemistry, BOKU-University of Natural Resources and Life Science, Vienna,

Konrad Lorenz Strasse 24, 3430 Tulln, Austria

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Institute for Chemical Technology of Organic Materials, Johannes Kepler University,

Altenbergerstrasse 69, 4040 Linz, Austria KEYWORDS: Cellulose, Lignin, Nanocomposite, Polystyrene, Suspension polymerization

ABSTRACT: A facile approach to obtaining cellulose nanofiber-reinforced polystyrene with greatly improved mechanical performance compared to unreinforced polystyrene is presented. Cellulose nanofibers were obtained by mechanical fibrillation of partially de-lignified wood (MFLC) and compared to nanofibers obtained from bleached pulp (MFC). Residual hemicellulose and lignin imparted amphiphilic surface chemical character to MFLC, which enabled the stabilization of emulsions of styrene in water. Upon suspension polymerization of styrene from the emulsion, polystyrene microspheres coated in MFLC were obtained. When processed into polymer sheets by hot-pressing, improved bending strength and superior impact toughness was observed for the polystyrene – MFLC composite compared to the unreinforced polystyrene.

INTRODUCTION Cellulosic fibers are a key resource for the partial or full replacement of fossil resources by bio-based alternatives. Apart from a well-established position in non-structural interior automotive parts, a real break-through for natural fiber-reinforced composites involving e.g. flax, hemp, or kenaf fibers has not been achieved yet. This is mainly due to the fact that even though natural fibers offer advantages in terms of low density and renewability compared to e.g. glass fiber, true competitiveness in terms of mechanical performance is not given.1 Here, nanocellulose offers a true alternative. This novel bio-based reinforcement material is of 2 ACS Paragon Plus Environment

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particular interest for future, greener materials. Its high specific surface area and excellent mechanical properties make it an ideal material for polymer reinforcement.2-5 Nanocellulose is usually produced by mechanical fibrillation of dilute aqueous suspensions of delignified wood pulp into typically 10-50 nm thick and several µm long fibrils. At this stage, nanocellulose is either termed nanofibrillated cellulose (NFC), when fibril dimeters around 10 nm or less are obtained. Alternatively, the term microfibrillated cellulose (MFC) is used when also elements with diameters up to 100 nm may occur.6 Acid hydrolysis of non-crystalline domains in cellulose results in even smaller, highly crystalline nanocellulose termed cellulose nanocrystals (CNC).5 The modulus of elasticity of crystalline cellulose representative of CNC has been measured to >100 GPa7 and the modulus of single bacterial cellulose fibrils, which may be more representative of NFC and MFC, has been determined to be 78 GPa.8 Since also the tensile strength of nanocellulose is high, with values in the order of 2-6 GPa,9 stiff and strong bio-based materials could potentially be produced using this material. However, challenges to be overcome with regard to scaling-up the production of nanocellulosereinforced polymers lie in the need for storage and processing in wet condition, be it in water or organic solvent.5 Cellulose is essentially hydrophilic, even though the cellulose crystal also possesses small hydrophobic face parallel to the cellulose I (200) plane.10 Particularly when working with CNC, this amphiphilicity may be exploited e.g. in terms of stabilizing oil in water emulsions.11 However, when referring to MFC or macroscopic cellulose fibers, cellulose may be considered effectively hydrophilic. This is why it shows poor compatibility with non-polar solvents and important technical polymers such as polypropylene, polyethylene, or polystyrene, where it fails to disperse homogeneously without prior hydrophobization by chemical surface modification.12 It was shown for selected non-polar organic solvents and hydrophobic polymers that the lack of natural compatibility with nanocellulose can be overcome when, in addition to 3 ACS Paragon Plus Environment


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cellulose, also certain amounts of residual lignin and/or hemicellulose, the two other main plant cell wall polymers, are present. Hemicellulose is more chemically diverse than cellulose, and is assumed to act similar to a coupling agent between cellulose and lignin in the wood cell wall.13 Compared to hemicellulose-free nanocellulose, hemicellulose-containing nanocellulose termed fibrillated holocellulose showed excellent miscibility with DMF and chloroform.14 Similarly it was shown that residual lignin in nanocellulose, which was termed microfibrillated lignocellulose (MFLC), provides excellent dispersion and reinforcement in the polymers polycaprolactone,15-16 poly(lactic acid),17 and polystyrene.15 Thus the problem of lacking interfacial compatibility between nanocellulose and hydrophobic polymers was solved here, but processing was still done in a wet organic solvent route. While processing in organic solvent is straightforward on a laboratory scale, potential scaleup calls for simple, cost-efficient procedures. Different drying methods18 and subsequent compounding may be applied. Unfortunately, nano-scale morphology of the cellulose fibrils is partly lost due to agglomeration during drying, which limits reinforcement efficiency. In view of the limitations of drying methods, in situ polymerization directly in water19-20 may be an elegant solution to the problem of compounding nanocellulose with polymers. Chemically hydrophobized nanocellulose has been repeatedly shown to be capable of stabilizing oil-inwater emulsions.21 Emulsions of hydrophobic monomer e.g. methyl methacrylate22 or styrene23 stabilized by chemically modified nanocellulose were successfully polymerized, resulting in spherical polymer microparticles covered in nanofibers. Considering the evaluation of the feasibility of in-situ polymerization for nanocellulosic fillers by several authors22, 24-25 we propose that by relying on the amphiphilic properties of lignin outlined above, an extremely simple suspension polymerization procedure can be realized. In the present study, we present such a facile route to potentially widespread utilization of


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nanocellulose for polymer reinforcement and illustrate this novel approach exemplarily for polystyrene. EXPERIMENTAL DETAILS Production of microfibrillated cellulose. A batch of 6 kg bark-free beech wood chips was partly delignified in an autoclave by means of a water/ethanol (50/50 weight) mixture containing 0.75 w% H2SO4 as a catalyst at a mass ratio of 1:4 (wood:solvent). For the treatment time of 90 min the temperature was 170 °C, which resulted in a pressure of 15 MPa. After delignification the reactor was cooled to 40 °C and the material was washed with the same 50/50 water/ethanol mixture used for treatment in order to remove dissolved substances, in particular lignin degradation products. Thereafter, the wood chips were disintegrated in a disc refiner. Fibrillation was carried out at a consistency of 1 w% in deionized water using first a Masuko Supermasscollider at 1500 rpm and a gap clearance of -50 µm, and subsequently in an APV high-pressure homogenizer (20 cycles, 80 MPa). Due to its high residual lignin content this batch of fibrillated material was termed microfibrillated lignocellulose (MFLC) Conventional microfibrillated cellulose (MFC) was obtained from the University of Maine (http://umaine.edu/pdc/process-and-product-development/selectedprojects/nanocellulose-facility/) and was homogenized before use (15 cycles, 80 MPa). Suspension polymerization and molding. The stabilizer was removed from styrene (Alfa Aesar, 99.5%, stabilized with 4-tert-butylcatechol) by distillation. Emulsions of 50 g styrene containing 1570 mg (3.14%) Azobis(isobutyronitril) (obtained from MOLEKULA, MW:164.21) were produced in 450 g deionised water containing a) 2.25 g MFC, and b) 2.25 g MFLC using an Ultra Turrax high shear mixer operated at 10.000 rpm. After transfer to a 1000 ml round bottom flask, polymerization was carried out under reflux and constant stirring at a temperature of 100 °C for 12 h. After cooling, the reaction product was removed from the



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flask and dried in an oven at 100 °C for 24 h. Pure PS was polymerized in bulk using the same amount of radical starter and the same temperature and duration. For mechanical characterization production the dry reaction product was molded into sheets in a hot press. For this purpose, the press was pre-heated to 150°C. 10 g of pure PS or PS-MFLC, respectively was placed onto the lower steel platen using parchment paper as an intermediate layer to avoid sticking of the PS. After covering the sample with a second layer of parchment paper, the press was closed to a gap width of 1.5 mm and the heating was turned off, leaving the press and molded sample to cool to room temperature within 1h. Microscopy. Emulsions were characterized with a Zeiss Axioplan fluorescence microscope using Nile Red, Nile Blue and Calcoflour white stain. SEM of MFLC-polystyrene microspheres was done in high-vacuum mode using a Quanta™ 250 scanning electron microscope with a Shottky field emission gun. AFM was done in tapping mode using a Dimension Icon AFM (Bruker, Karlsruhe, Germany) and standard OTESPA probes by the same manufacturer. Sample preparation for AFM was accomplished by placing a drop of the respective 0.001 w% aqueous fibril suspension onto freshly cleaved mica, and evaporating the water at ambient conditions. Chemical characterization. The lignin content of MFC and MFLC was evaluated gravimetrically from the dry residue after total acid hydrolysis of carbohydrates in concentrated sulfuric acid (72 w%, Merck, Darmstadt) at 30°C for 60 min. The content of glucose and xylose was determined by means of acid methanolysis followed by gas chromatography for MFC, and by Borat-AEC of sulfuric acid hydrolysates for MFLC, respectively.26 Crystallinity of cellulose was evaluated according to ref.27 from X-ray powder diffractograms obtained with a Rigaku SmartLab 5-Axis X-ray diffractometer using glass capillaries. As a note of caution it is mentioned that this method delivers results that should not be interpreted as absolute values of crystallinity, whereas it is deemed well-suited for 6 ACS Paragon Plus Environment

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comparing the relative crystallinity of two samples. ATR-IR spectra were recorded with a Perkin Elmer FT-IR Spectrometer equipped with a Universal ATR Sampling Accessory. Each sample was scanned in quadruplicate from 650 cm-1 to 4000 cm-1 with a resolution of 4 cm-1 for 32 times. XPS was done with a K-Alpha spectrometer (Thermo Fischer Scientific). Survey scans were done with a pass energy of 200 eV and a step size of 1.0 eV. High resolution scans were done with a pass energy of 50 eV and a step size of 0.1 eV. For the determination of molecular weight, pure PS and PS-MFLC composites were dissolved in DCM, whereby the fibrous fraction of PS-MFLC was removed by filtration. After precipitation in MeOH, lyophilization, and crushing, the samples were suspended in DMAc/LiCl (0.9%) and subjected to GPC analysis. DSC was performed with milled PS and PS-MFLC samples on a Netzsch DSC 200F3 at a heating rate of 10 k min-1 using aluminum crucibles. Tg was evaluated from the DSC curves using the half peak height method Mechanical performance. Three-point bending tests were done with a Zwick-Roell 20 kN universal testing machine equipped with a 500 N load cell. Testing was done at a free sample length of 20 mm and a cross-head speed of 10 mm min-1. Impact bending was performed on a Zwick-Roell instrumented Charpy 5N impact pendulum at a support distance of 22 mm. For both tests, 10 replicate samples with a length of 60 mm and a width of 10 mm were used. The fracture surfaces of specimens tested in bending were sputter-coated with gold and examined in a Zeiss-LEO SEM in high vacuum mode at an acceleration voltage of 10 kV. RESULTS AND DISCUSSION In order to assess the potential suitability of MFLC for stabilizing emulsions of styrene in water in comparison to MFC derived from bleached pulp, its basic chemical characteristics were determined. As shown in results compiled in Figure 1, MFLC is clearly different both in terms of bulk- and surface chemistry. For MFLC, ATR-FTIR reveals peaks characteristic of



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aromatic lignin (aromatic skeletal vibrations at 1595 cm-1 and 1505 cm-1,28 and xylan, the most important hemicellulose (unconjugated C=O at 1733 cm-1 28 as well as syringyl ring breathing and C-O stretching in lignin and xylan at 1242 cm-1 29), whereas MFC only shows characteristic features of cellulose (Figure 1a). This agrees well with wet-chemical analysis of MFLC and MFC (Figure 1b) showing only glucose as monomer constituent of MFC, which is thus essentially pure cellulose, compared to MFLC, which also contains significant amounts of xylose indicative of the hemicellulose xylan, and lignin.

Figure 1. Comparison of important chemical characteristics of MFLC and MFC by means of ATR FT-IR spectroscopy (a) and wet-chemistry as well as XRD and XPS (b) (arithmetic mean of two determinations is shown, COV was < 10 % for wet chemical methods and < 2% for XRD and XPS). While the overall crystallinity of both materials determined from XRD was the same, XPS showed clearly different surface chemistry, with a significantly higher C/O ratio and more abundant C-C and C-H moieties in MFLC compared to MFC, hinting at a less polar surface character. In addition to these differences in bulk- and surface chemistry, also the morphology of MFLC and MFC as revealed by AFM is different (Figure 2). MFC (Figure 2a, b) tended to agglomerate during sample preparation, which was much less the case for MFLC (Figure 2 c, d). Also, globular particles, presumably lignin, appear to be present on individual fibrils on 8 ACS Paragon Plus Environment

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several instances. Disregarding occasional larger agglomerations of individual fibrils, average fibril diameters derived from AFM height images were 10.5 +/- 2.8 nm for MFC and 6.8 +/1.5 nm for MFLC. In spite of repeated homogenization, MFLC proved recalcitrant to fibrillation and few fiber fragments with diameters of 100 nm to several 100 nm remained, which is why the material is termed microfibrillated lignocellulose instead of nanolignocellulose.

Figure 2. AFM images of MFC (a, b) and MFLC (c, d). Topography (a, c) and phase images (b, d) are shown. Phase images show a comparably uniform appearance for MFC, whereas MFLC is highly heterogeneous, with numerous bright spots indicating the presence of non-cellulosic substances, presumably lignin. In summary, these characterization results confirm chemical and surface chemical heterogeneity and a certain amphiphilic character for MFLC as expected from literature.15, 30 9 ACS Paragon Plus Environment


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Emulsions of styrene in water containing MFC or MFLC for stabilization were prepared with apparent success (Figure 3). Even after 4 h, no separation of styrene from the emulsion was visible macroscopically. Notably, the volume taken up by MFC remained constant during this time, whereas the volume taken up by MFLC in the suspension shrunk to 60 % of its initial value (Figure 3a). The remaining 40 % volume was taken up by water containing very small amounts of MFLC.

Figure 3. Emulsions of styrene in water stabilized by MFC and MFLC, respectively, immediately after mixing and 4h after mixing (a). Fluorescence microscopy of styrene in 10 ACS Paragon Plus Environment

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water emulsions stabilized by MFC (b) and MFLC (c), respectively. Microscope images were taken immediately after mixing. Light microscopic inspection of the emulsions revealed styrene droplets of heterogeneous diameter between 10 – 100 µm in the MFC variant (Figure 3b) and the MFLC variant showed similar droplet size distribution. After 4h, no significant change in droplet size distribution was observed for the MFLC variant, whereas a significant increase in droplet size to large agglomerations up to 500 µm in diameter was determined for the MFC variant. The surface of the styrene droplets stabilized by MFLC appeared to be covered with fibrous material, which was not the case for the MFC-stabilized variant (Figure 3c). It is presumed that the amphiphilicity of MFLC hinted at by results shown in Figure 1 leads to an agglomeration of MFLC at the styrene-water interface. Polymerization of the emulsions resulted in a complete breakdown of the MFC variant, which yielded discrete regions of polystyrene and cellulose, respectively (Figure 4a). The formation of a composite between polystyrene and MFC clearly failed, which is why this variant was not considered in further investigations. By contrast, the MFLC variant delivered a macroscopically homogeneous, fluffy and apparently fibrous product suitable for further processing.

Figure 4. Optical appearance of polymerized polystyrene (PS), polystyrene from an emulsion stabilized with MFC (PS-MFC), and an emulsion stabilized with MFLC (PS-MFLC), respectively (a). SEM of PS-MFLC (b, c). 11 ACS Paragon Plus Environment


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SEM revealed that this variant consisted of spherical polystyrene particles with diameters ranging from 20 to 100 µm embedded in fibrous material (Figure 4b, c). Thus compounding of PS and MFLC during suspension polymerization was successful. Polystyrene (PS) and PS polymerized from an emulsion stabilized with MFLC were significantly different in terms of degree of polymerization achieved. A small sample of both material variants subjected to GPC analysis revealed clear differences in Mw, which was 415*10^3 for PS polymerized in the absence of MFLC, and 124*10^3 for PS isolated from PS-MFLC (Figure 5a). This difference in Mw is also manifest in DSC curves from both samples (Figure 5b), where Tg is 101.4 °C for PS and clearly lowered to 89.1 °C for PSMFLC. The significantly reduced degree of polymerization of PS observed in the PS-MFLC variant may be due to the well-known radical scavenging properties of lignin interfering with the radical polymerization of PS.31

Figure 5. Results of GPC (a) and DSC (b) analysis of polystyrene (PS) and polystyrene polymerized from an emulsion of styrene stabilized by MFCL (PS-MFLC), respectively Pure polystyrene (PS) and polystyrene-MFLC (PS-MFLC) composite films were hotpressed for mechanical characterization (Figure 6a). Based on the clearly reduced degree of


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polymerization of PS in the PS-MFLC variant (Figure 5) reduced mechanical performance was expected for this variant according to well-known structure-property relationships.32-33

Figure 6. Optical appearance of hot-pressed polystyrene (PS) and polystyrenemicrofibrillated lignocellulose (PS-MFLC) films (a) and results of mechanical characterization by means of static three-point bending (b) as well as Charpy impact bending (c). However, already during sample preparation for mechanical characterization, PS behaved very brittle and was extremely difficult to mill into small beams required for testing, whereas PS-MFLC was much easier to handle and apparently much less brittle. This first impression of different mechanical strength was impressively confirmed by static bending (Figure 6b) and impact testing (Figure 6c). PS-MFLC, with a cellulose fibril content of 5% (w/w) calculated from the ratio of styrene:MFLC in the initial suspension, showed a slight improvement in modulus of elasticity (PS-MFLC: 2.6 +/- 0.20 GPa and PS: 2.1 +/- 0.25 GPa), and clear improvements in bending strength (PS-MFLC: 15.8 +/- 1.52 MPa and PS: 10.6 +/0.88 MPa) and strain to failure (PS-MFLC 0.69 +/- 0.028 % and PS: 0.49 +/- 0.061 %). In impact testing, which is clearly a critical material parameter for polystyrene, an impressive



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improvement of roughly 500 % was achieved due to cellulose fibril reinforcement (PS-MFLC 11.4 +/- 1.54 kJ m-2 and PS 2.4 +/- 0.57 kJ m-2). SEM images of fracture surfaces taken from bending specimens (Figure 7) show a clear difference in surface roughness for PS (Figure 7a) and PS-MFLC (Figure 7b), with higher roughness in PS-MFLC. Higher magnification of PS-MFLC (Figure 7c) reveals an inhomogeneous distribution of fibrous reinforcement, where fibril-rich regions alternate with regions apparently consisting of PS exclusively. Fiber pull-out is visible both for unfibrillated fiber fragments (Figure 7c) and fibrillary material (Figure 7d). Thus we presume that reinforcement fibrils contribute to improved mechanics by bridging micro-cracks and limiting crack-growth, and by increasing energy consumption during fracture through fiber pull-out. Even though the reinforcement of polymers with MFC is in focus of current research in biobased fiber reinforced polymers,34 comparably few studies deal with the effect of MFC on the mechanics of PS. Improvements in storage modulus were reported for electro-spun PS fibers35 and extruded PS36 reinforced with cellulose nanocrystals compatibilized by means of surfactant or physicochemical modification. Similarly, reinforcement of PS with TEMPOoxidized MFC resulted in clear improvements of stiffness, but only moderate strength improvements were reported.37 In order to correctly interpret the significance of mechanical improvements observed, it is important to note that the strength of brittle materials such as PS may be affected by surface defects or surface roughness. Indeed, Figure 7b clearly shows that the structure of parchment paper used as an intermediate layer during hot pressing is replicated on the specimen surface, thus causing substantial roughness.


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Figure 7. SEM images of fracture surfaces after bending testing of PS (a) and PS-MFLC (bd). The magnified image d) was taken from the region marked in image c). . CONCLUSION It was demonstrated that using residual lignin as an enabler of suspension polymerization of PS in the presence of MFLC, a PS-biobased nanofiber compound can be obtained in an extremely simple procedure without any use of solvent transfer, drying, or chemical compatibilization of cellulose required. Composites produced from this novel type of PS show clearly improved performance in static bending and superior improvement in impact toughness, a critical material characteristic of PS. We propose that the new approach to bio15 ACS Paragon Plus Environment


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based cellulose nanofiber-reinforced PS presented here may significantly benefit the development of future (partly)bio-based materials, e.g. novel MFLC-PS thermal insulation materials. The procedure described here inherently leads to an inhomogeneous distribution of reinforcement material. In view of a potential transfer to industrial application, an adaptation of production processes in terms of e.g. intense melt-compounding may thus be necessary.

AUTHOR INFORMATION Corresponding Author *W. G-A.: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources None of the authors has received funds for the study reported here.

REFERENCES (1) Bledzki, A. K.; Gassan, J., Composites Reinforced with Cellulose Based Fibres. Prog. Polym. Sci. 1999, 24 (2), 221-274. (2) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T., Review: Current International Research into Cellulose Nanofibres and Nanocomposites. J. Mater. Sci. 2010, 45 (1), 1-33. (3) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J., Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chem. Soc. Rev. 2011, 40 (7), 3941-3994. (4) Klemm, D.; Kramer, F.; Moritz, S.; Lindstrom, T.; Ankerfors, M.; Gray, D.; Dorris, A., Nanocelluloses: A New Family of Nature-Based Materials. Angew. Chemie-Int. Ed. 2011, 50 (24), 5438-5466. 16 ACS Paragon Plus Environment

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TOC Graphic


Lignocellulose Nanofiber-Reinforced Polystyrene Produced from

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