Admicellar Polymerization Coating of CNF Enhances Integration in

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Admicellar Polymerization Coating of CNF enhances Integration in Degradable Nanocomposites Ulrica M Edlund, Tove Lagerberg, and Eva Ålander Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b01318 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Admicellar Polymerization Coating of CNF enhances Integration in Degradable Nanocomposites Ulrica Edlund, Tove Lagerberg, Eva Ålander Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE100 44 Stockholm, Sweden RISE Bioeconomy, Drottning Kristinas väg 61, SE-114 28 Stockholm, Sweden

KEYWORDS cellulose nanofibrils, composite, admicellar polymerization, surface modification, PBAT

ABSTRACT

A water-based one-pot synthesis strategy for converting cellulose nanofibrils (CNF) into a hydrophobic and processable biopolymer grade is devised. CNF was chemically modified through admicellar polymerization, producing fibrils coated with fatty acrylate polymers. The proposed modification targets a change in the inter-fibrillar interactions and

ACS Paragon Plus Environment

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improved CNF compatibility with a degradable plastic composite matrix, poly(butylene adipate-co-terephthalate), PBAT in composites prepared by melt extrusion. CNF had a clear reinforcing effect on PBAT, increasing Young’s modulus by at least 35% and 169% at 5 and 20% (w/w) CNF content, respectively. However, unmodified CNF showed aggregation, poor adhesion in the matrix, and severely impaired the ductility of PBAT. CNF modified by admicellar polymerization was homogeneously dispersed in the PBT matrix and showed significantly better preservation of the elongation properties compared to unmodified CNF, especially at 5% (w/w) addition level. 1. INTRODUCTION The annual global production of plastics has passed above 300 million tonnes. Most of these plastics are non-biodegradable and today’s plastic waste constitutes a huge environmental problem. A large amount of plastic waste ends up in the oceans stemming from uncontrolled pollution from sewer overflows, storm water discharges, leak from industrial and tourism activities. The plastic debris will be worn down into smaller pieces and will then enter the food chain. The estimated amount of plastics contained in the Pacific patch are of the order of 100 million tonnes.1 Replacing at least a fraction of the conventional plastics used worldwide by biodegradable alternatives could significantly contribute to reducing the constantly increasing build-up of plastic waste and future plastic debris pollution. A number of biodegradable commodity plastics are commercially available and used in applications such as disposable items. Poly(butylene adipate-co-terephthalate), PBAT, is a biodegradable aliphatic-aromatic co-polyester with high elongation capacity and


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documented biodegradability. PBAT is easy to process with conventional methods such as injection molding, in fact PBAT is designed to be processed like low density polyethylene2 with a melting temperature around 119-124 °C. PBAT is typically used in packaging and agricultural mulch films. It is also used in blends with other biodegradable plastics, like polylactide (PLA), to improve the flexibility and processability of the matrix. However, PBAT has some significant drawbacks that hinder a more widespread use: a relatively high production cost, limited heat stability and low strength. Several attempts have been made to improve the mechanical and thermal properties of the material. Composites with PBAT and nanocellulose have been studied by several research groups including Morelli et al.3 and Zhang et al.4 An increase in Young’s modulus as well as an increased degradation onset temperature compared to neat PBAT was reported. In general, and not only for PBAT, the reinforcement of polymers with fillers based on nanocellulose has attracted a great deal of attention and is considered a way to design composites with superior mechanical and thermal properties. Nanocellulose attracts ever-increasing attention and, not in the least, the prospects of replacing petrochemicals as resources in future production of bio-based bulk and advanced materials. Nanocellulose is, as the name implies, cellulose fibrils with their widths in the magnitude of nanometers. Use of cellulose nanofibrils (CNFs) provides access to several attractive features including high stiffness and remarkable strength in combination with high specific surface area and high aspect ratio. Reported intrinsic properties of CNF films, such a Young’s modulus at 10-17 GPa and tensile strengths


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between 130-250 MPa, makes CNF very promising as additive in future plastic composites for improve mechanical properties and reduced material need.5,6,7 Nanocomposites with CNF fillers provide the opportunity to manufacture light-weight materials at affordable costs. However, there are many scientific and technological challenges in the field that should be addressed before these materials could reach their full potential. The mechanical properties exhibited by a nanocomposite are largely determined by the compatibility at the interface of the composite constituents. Materials with similar wettability will have a better compatibility at the interface, and therefore the nanocomposite will attain better mechanical performance. In order to get a good compatibility between the phases in a composite, some sort of modification of at least one the components is often necessary. For composites with nanocellulose and a hydrophobic matrix, two of the major issues are to increase the dispersion of the fibrils in the matrix and to reduce their high tendency to absorb moisture.8,9 An effective adhesion between the fibrils and the matrix is necessary. Chemical surface modification by hydroxyl group derivatization and polymer grafting strategies have been well studied and is commonly used to render the hydrophilic surface of nanocellulose hydrophobic.10,11,12,13,14,15,16 A major challenge to the modification of cellulose is its insolubility in most organic solvents and water. Thus, cellulose modification reactions typically occur heterogeneously or in harsh solvent systems, at the risk of limited yield or concomitant degradation. Not in the least from a sustainability perspective, synthesis tools and modification strategies running under more green and water-based conditions than the conventional methods could offer are needed. Some strategies to


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modify nanocellulose heterogeneously in aqueous solution include controlled radical polymerization and click chemistry.17 Admicellar polymerization offers another strategy to circumvent the solubility challenges. Admicellar polymerization resembles emulsion polymerization but occurs at surface of a substrate rather than in bulk.18 The process is governed by surfactants that form a bilayer, so called admicelles, at the substrate surface. The result is a thin polymeric film, a coating, inside the bilayer of absorbed surfactants. Admicellar polymerization stand out as a potentially green chemical pathway since it occur at moderate temperature, the demand of chemicals is low, and organic solvents are not needed at all.19,20 Admicellar polymerization has been studied for textile coating17 and has also been applied to bacterial cellulose and cellulose nanocrystals using styrene as a model monomer.21 This work strives to identify sustainable and relevant routes for future industrial largescale production of degradable nanocomposites by devising and investigating a waterbased admicellar polymerization coating process of CNF. A route to produce hydrophobic CNF with improved compatibility and good dispersion in plastic composite matrices processed by extrusion is presented, and herein applied on the PBAT matrix.

2. EXPERIMENTAL

2.1 Materials


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For the production of CNF a bleached never-dried softwood sulfite dissolving pulp (Domsjö Fabriker AB, Sweden) with a hemicellulose content of 4% was used.

A

monocomponent endoglucanase FibreCare R was used as received from Novozymes. Hexadecylpyridinium chloride monohydrate (CTP) and ammonium persulfate (ACS reagent, ≥98%) were used as received from Sigma Aldrich. Sodium pyrosulfite (≥98%) was used as received from Fluka Chemie AG. Ethanol (96 %(v/v)) was purchased from VWR chemicals. Hexyl acrylate (HA) (98 %, containing 100 ppm hydroquinone as an inhibitor) was purchased from Sigma Aldrich and destabilized with silica gel 60 (particle size: 0.040-0.063 mm) (Merck) before use. Poly(butylene adipate-co-terephthalate) (PBAT) was supplied under the trade name Origo-Bi® ES01G.

2.2 Preparation of CNF The manufacturing procedure of CNF used includes mild enzymatic hydrolysis (0.17 µL enzyme per gram fiber (dry matter), 2 h incubation at 50°C) in between two mechanical refining steps. Nanofibrils were then liberated by passing the fiber suspension at 2% (w/w) concentration

through

a

high-pressure

fluidizer

(Microfluidizer

M-110EH

from

Microfluidics Corporation, USA) repeatedly. The detailed procedure is described by Päkkö et al.22 No biocide was added.

2.3 General procedure for admicellar polymerization coating of CNF CNF was modified by admicellar polymerization in a one-pot four-step process:


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Step 1. Admicelle formation. A 0.5% (w/w) suspension of CNF in water (300 g) was mixed with CTP (0.6 mM) and stirred for 72 h at room temperature. A reference experiment was performed in which the time for admicelle formation was 24 h. Step 2. Monomer adsolubilization. HA (3% (v/v) with respect to the aqueous medium) was added and the reaction mixture was left for 48 h at room temperature. Step 3. Polymerization. The polymerization was initiated by adding ammonium persulfate (1% (w/w) with respect to the added HA) and half of the molar amount of sodium pyrosulfite. The temperature was increased to 80 °C and the reaction was allowed to proceed for 2 h. The reaction mixture was then cooled to room temperature before centrifugation and washing. Step 4. Removal of the upper surfactant layer. The reaction mixture was centrifuged at 5000 rpm for 10 min. The supernatant was removed and the pellet was washed with a mixture of water and ethanol at a volume ratio of 70:30 and again centrifuged. The supernatant was filtered through a PTFE syringe filter with a pore size of 0.45 μm and the absorbance at 258 nm was recorded using a Shimadzu UV-2550 UV-vis Spectrophotometer. The washing procedure was repeated at least five times with deionized water as the washing liquid, or until residual CTP could no longer be detected in the supernatant (Supporting information, Figure S1). The final product, herein denoted CNF-HA, was stored cold.


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In addition to the above described protocol, a reference experiment was performed without any added surfactant. Another reference experiment was performed in which no polymerization initiators were added.

2.4 Homopolymerization of HA HA was polymerized in emulsion to prepare a reference homopolymer sample, poly(HA) for analysis. HA (4.5 mL), water (150 mL) and CTP (1 mM) was kept on magnetic stirring overnight to reach a stable emulsion. Ammonium persulfate (1% (w/w) with respect to the added HA) and half of the molar amount of sodium pyrosulfite were then added and the temperature was raised to 80 °C. After 2 h, the reaction mixture was cooled and a saturated solution of NaCl in water was added. The precipitated polymer was recovered be centrifugation, washed and again centrifuged several times. The recovered polymer was then freeze-dried. The structures of the monomer and the polymerization product were verified by ATR- FTIR.

2.5 Preparation of composites Composites were prepared from PBAT (Figure 1) with 5 and 20% (w/w) CNF-HA. Composites from PBAT and unmodified CNF (5 and 20% (w/w), respectively) were prepared as references.


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O

O

O O

O

O O

Figure 1. Repeating unit building blocks of PBAT. The PBAT pellets (~4 mm) were first dried in an oven at 110°C to remove moisture. The pellets were then stored in a freezer at -80°C for some days before milling at 18000 rpm using a Retsch ZM 200 system cooled by dry ice. The average diameter of milled PBAT particles was 500 µm as measured with laser diffractometry (Supporting information, Figure S2). Before extrusion, milled PBAT particles was pre-mixed with CNF-HA or unmodified CNF in a viscous water suspension. The mixtures were lyophilized and then extruded at 130 °C and 50 turns/min in a Thermo Scientific HAAKE MiniLab Rheomex CTW5. PBAT without any addition of CNF or CNF-HA was also processed according to the same protocol as the composites to serve a reference sample.

2.6 Fourier transform infrared spectroscopy with attenuated total reflection (ATR-FTIR) Lyophilized samples of CNF, poly(HA), CNF-HA and products from reference experiments with CNF were analyzed with a PerkinElmer Spectrum 2000 spectrometer. The measurements were performed in the spectral range 4000-600 cm-1 with a resolution of 4 cm-1. Each spectrum was calculated as an average of 16 individual scans with


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corrections made for atmospheric water and carbon dioxide. Perkin-Elmer Spectrum software was used for data acquisition.

2.7 Contact angle measurements Films for contact angle measurements were prepared from unmodified CNF and replicates of CNF-HA from five different batches. Each sample was diluted to a 250 mL aqueous dispersion at a concentration of 0.1% (w/w) by stirring overnight and air bubbles were removed by vacuum suction for 1.5 h. Degassed samples were then filtered and dried under vacuum for 2 h to form films using a 0.65 μm Omnipore membrane filter. The formed films were air dried for 2.5 h at ambient temperature and at 55 °C overnight. The dried films were carefully removed from the filters and preconditioned at 50% relative humidity and 23 °C for 3 days. Static contact angles of water droplets (3.3-3.9 µL in volume) were then measured at different times (0.1, 0.5 and 1 s) on both sides of the films using a Fibro DAT 1100. Four to six droplets were analyzed on each side of the films.

2.8 Thermogravimetric analysis (TGA) Lyophilized samples were analyzed with a Mettler-Toledo TGA/DSC STAR system that was previously calibrated using the melting points of standards In, Zn and Al. Each sample (3-5 mg) was put in a 70 µL aluminium crucible and heated from 25 °C to 800


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°C at a heating rate of 10 °C/min under a nitrogen atmosphere (50 mL/min). STARe Evaluation software as used for data acquisition.

2.9 Differential scanning calorimetry (DSC) Lyophilized samples were taken directly from the freeze-dryer and analyzed with a Mettler Toledo DSC 820 instrument. Each sample (5-10 mg) was put in a 40 µL aluminium crucible and heated from 25 °C to 180 °C, then cooled to 25 °C and finally heated again to 180 °C under a nitrogen atmosphere (50 mL/min). The heating/cooling rate was 5 °C/min. STARe Evaluation software as used for data acquisition.

2.10 Field-emission scanning electron microscopy (FE-SEM) All composites were analyzed with a Hitachi S-4800 field emission scanning electron microscope operating at 5 kV. Samples were mounted on carbon tape-coated stubs and sputter coated with a 5 nm thick layer of gold/palladium under inert atmosphere using a Cressington 20HR Au/Pd sputter coater. In addition, the residual ash from TGA analyses were analyzed to determine the elemental composition by the same FE-SEM connected to an X-MaxN energy-dispersive spectrum (EDS) from Oxford instruments. AZtec 3.0 software was used for imaging SEM-EDS data.

2.11 Tensile testing


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To prepare specimens for tensile testing from the composites, each sample was first hot pressed at 150 °C and 100 kN in a Fontijne laboratory platen press. The samples were first pressed for 40 s to remove air bubbles, then slightly released and pressed again for 3 min. The resulting thickness of the hot pressed samples was approximately 0.95 mm. Dogbone-shaped specimens with the dimensions of 18 x 4 mm (length x width of the narrow section) were then cut from the hot pressed sheets. Specimens were conditioned at 50% relative humidity and 23 °C for 3 days before tensile testing. Tensile testing was performed with an Instron 5566 instrument and Bluehill software was used for test control and collection of data. Five specimens were tested for each material. The measurements were performed with a 500 N load cell at a strain rate of 3 mm/min.

3. RESULTS AND DISCUSSION

3.1 Coated CNF by admicellar polymerization We have devised a strategy for achieving hydrophobically coated CNF involving one-pot admicellar polymerization of adsolubilized HA monomer in aqueous phase. The first step of the admicellar polymerization involved the addition of a surfactant to a dilute aqueous suspension of CNF (0.5 %w/w), The surfactant is supposed to assemble at the fibril surfaces and form micelles, so called admicelles, and create bilayers. Hexadecylpyridinium chloride monohydrate (CTP), was chosen as a surfactant because


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of its cationic character, originating from deprotonated primary hydroxyl groups and anticipated to provide good surface absorption to the inherently negatively charged CNF. Successful admicellar polymerization relies on a balanced concentration of surfactants in the reaction suspension. Aggregation will be insufficient below the critical admicelle concentration (CAC). Free micelles will form in the bulk above the critical micelle concentration (CMC) while electrostatic interactions will energetically favor surfactants adherence to the substrate surface below the CMC rather than forming free micelles. The concentration of CTP was hence kept below its CMC, 0.9 mM in water at 25 °C,23 to avoid competition from emulsion homopolymerization in bulk free micelles. Extensive stirring for several days of the dispersion containing CNF and CTP was necessary for complete admicelle formation, given the high surface area of CNF. A control experiment in which the admicelle formation step was shortened to 24 h was performed and demonstrated that the subsequent polymerization step resulted in poor fibril coverage. Another control experiment was performed in which no surfactant was added at all resulting in incomplete bulk homopolymerization of the monomer and no coated fibrils.


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Figure 2. A schematic illustration of the admicellar polymerization of HA onto CNF. Step 1) admicelle formation with CTP in H20 at ambient temperature, Step 2) monomer adsolubilization with HA in H20 at ambient temperature, Step 3) polymerization of HA in H20 at 80 °C, Step 4) washing in H20 and ethanol at ambient temperature. In the second step, monomer is added to the aqueous reaction mixture resulting in adsolubilization, incorporation of the monomer within the micelles, driven by the hydrophobicity of the monomer. Hexyl acrylate (HA) was chosen as a monomer because of its fatty acid character and a realistic future possibility to derive acrylate monomers


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from renewable carbon resources.24,25

In addition, HA is in liquid state at room

temperature which facilitates monomer dispersion in the aqueous reaction medium without agglomeration. In the third step, the adsolubilized monomer is polymerized by adding heat activated water soluble radical initiators to the aqueous suspension. The free radicals travel from the bulk to the admicelles where homopolymerization of the monomer generates a coated layer around the fibrils. In a control experiment, performed under the exact same conditions but with no added initiators, no polymerization took place leaving the reaction mixture with unreacted monomers and uncoated fibrils. Finally, the upper surfactant layer is removed by washing with ethanol and water so that the newly formed polymer coating is exposed. The lower surfactant layer remains as a compatibilizer between the substrate and the polymer. The removal of the upper CTP layer was monitored with UV-Vis spectroscopy (Supporting information, Figure S1) and the washing procedure was repeated until residual CTP could no longer be detected in the washing liquid.


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Figure 3. ATR-FTIR spectra of a) CNF before modification, b and c) CNF-HA analyzed at different surface spots, d) PolyHA, a reference homopolymer of HA, and e) the monomer HA.

The coating of CNF with HA polymers was monitored with ATR-FTIR (Figure 3).26 A table with band assignments is available in supporting information, Table S1. The spectrum of


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unmodified CNF (Figure 3a) shows the characteristic bands of cellulose with a broad OH band ranging from 3600 - 3000 cm-1, overlaid bands in the 2950 - 2850 cm-1 region stemming from C-H stretching, 1425 cm-1 (C-H bending), a sharp band at 1105 cm-1 stemming from O-H bending, and sharp bands at 1159, 1055, and 1033 cm-1 stemming from the C-O-C linkages. The band at 891 cm-1 verifies the presence of -glycosidic bonds. In addition, a very weak band at 1620 cm-1 is observed in the pristine CNF spectrum. This may be due to O-H bending in absorbed water in the dry, yet very hygroscopic, CNF. The spectra of two samples of CNF-HA, (Figures 3b and 3c) clearly show that the surface chemistry of CNF change as an effect of admicellar polymerization. CNF-HA spectra show strong absorption bands at 2917 and 2850 cm-1 stemming from the main and side chain methyl groups (C-H) of poly(HA), 1733 cm-1 (acrylate C=O), 1467 cm-1 (C-H), 1162 cm-1 (C-O-C) and 721 cm-1 (-CH2-, rocking). The spectrum of CNF-HA is very similar to the spectrum of poly(HA), a reference homopolymer of HA (Figure 3d), strongly indicating the presence of a polymer coating on the CNF fibrils. Importantly, the bands at 1638 and 1620 cm-1, corresponding to C=C stretch, in the spectrum of the HA monomer is not visible in the CNF-HA spectra indicating that, if any, only traces of the unreacted monomer are still left in the coated samples. In addition, the acrylate C=O band appears at a lower wave number (1722 cm-1) than in Poly(HA) and CNF-HA. The coating layer seems to be somewhat heterogeneous in terms of thickness, in some surface spots, only bands from


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the polymer coating is recorded (Figure 3c), while the characteristic peaks for unmodified CNF are also present in the spectrum recorded at other surface spots (Figure 3b). This indicates that the coating layer was either so thin that the substrate chemistry is also detected in the ATR-FTIR analysis, or that the surfaces were not completely covered. This conclusion is sustained by SEM visualization the topography of CNF before and after admicellar polymerization (Figure 4). The fine fibrils, albeit densely packed, observed in the freeze-dried CNF are to a major extent embedded in a thin matrix after completed admicellar polymerization and few free fibrils can be distinguished. The macroscopic consistency changed accordingly: the freeze-dried unmodified CNF is fluffy and cottonlike while CNF-HA is denser and more plastic-like. In summary, the results sustain that the CNF was significantly but not always fully coated with poly(HA) in the admicellar polymerization.


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Figure 4. SEM images of CNF before and after admicellar polymerization at magnifications x250 and x2000.

The water contact angle, measured on the top side of films prepared by vacuum filtration, reveals an increase from 38° to in average 93°±7 with the admicellar polymerization modification of CNF. More detailed data on measured water contact angles are found in the Table S2, Supporting information. As a comparison, Boufi et.al

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performed

admicellar polymerization with CTP and ethylhexyl acrylate as a monomer on cellulosic fibers and received a contact angle of 90°±4. Trovatti et.al.21 received values between 50-


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70° with styrene and ethyl acrylate as monomers. In a covalent grafting-from synthesis approach, Navarro et. al.10 received a value of 107° using stearyl acrylate as a monomer. The increase in contact angle induced by the specific modification used in this work clearly demonstrates a change towards hydrophobic behavior, that is the contact angle is larger than 90° 28, which is also a comparable result of the previous studies. Measured water contact angles on the bottom side of the films are consistently higher than on the top side, displaying an increase from 51° to in average 96°±8, where the top and bottom sides of the films refer to contact with air and the filter membrane, respectively, during vacuum filtration. The difference in contact angle on the bottom side and the top side of the film is perceived as a difference caused by the different surface conditions at filtration. In one of the produced batches the contact angle only reached ~ 80° probably indicating a poorer coverage of poly(HA) on the fibril surface. Still, the investigated admicellar polymerization coating process of CNF worked repeatedly, at least for the last four batches, which demonstrates that it is a stable and reliable process. The targeted hydrophobization was achieved which was anticipated to be beneficial for the subsequent production of plastic biocomposites.

3.2 Composite production and characterization


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CNF based biocomposites with a PBAT matrix were prepared using melt-extrusion. The content of the filler (CNF or CNF-HA) was 5 or 20% (w/w). It should be noted that the composites thereby contain different amounts of CNF, as 5% (w/w) CNF-HA consists of less cellulose than 5% (w/w) CNF. The poor adhesion of the unmodified CNF in the PBAT matrix, evident by macroscopic aggregation, could even be noticed by the naked eye while extruding the material. The composites based on PBAT and CNF-HA on the other hand were macroscopically homogeneous. The PBAT composites with CNF-HA were soft and glossy. However, the transparency, glossiness and flexibility where reduced with increasing amount of CNF (Supporting information, Figure S3). All PBAT composites, as well as pristine PBAT, has a fait pink color. SEM-EDS elemental analysis of the PBAT ash remaining after TGA analysis confirm an average content of Fe of 1.5% (w/w), most likely a stabilizer residue and responsible for the coloring. The difference in homogeneity between PBAT composites with CNF-HA or unmodified CNF is even more evident on the microscopic scale (Figure 5). SEM images of fracture surfaces of PBAT with unmodified CNF clearly show aggregates of fibrils as well as cavities, the latter indicating that fibrils were pulled out from the matrix during tensile testing, which in turn is a sign of poor filler adhesion to the matrix. Such bundles and cavities are not observed in the fracture surfaces of PBAT with CNF-HA.


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Figure 5. SEM images at x500 magnification of fracture surfaces of PBAT containing 5 or 20% (w/w) of CNF-HA or unmodified CNF. The scale bar represents 100 m.

The melt extrusion was facilitated by the increase in thermoplastic behavior displayed by CNF-HA compared to unmodified CNF. From the analysis of DSC thermograms (Figure 6), it is clear, that unmodified CNF shows no signs of thermal transitions in the entire temperature range of 25-180 °C, neither during heating nor cooling. This is expected given the restricted segmental mobility of CNF due to strong intra- and intermolecular hydrogen bonding. CNF-HA on the other hand displays a detectable glass transition


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around 60 °C, attributed to the coated layer on the CNF fibrils resulting from admicellar polymerization which induced an increase in the thermal flexibility of the matrix. PBAT displays an expected glass transition at 40 °C and a melting endotherm peaking at 128 °C upon heating and a distinct crystallization peak at 101 °C during cooling. The same behavior was found for PBAT containing CNF-HA, having melting endotherms at 128 °C. The glass transition temperature was slightly higher for the composites of PBAT with CNF-HA (43 °C) than for PBAT without any CNF addition (40 °C).


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Figure 6. DSC thermograms recorded during the 1st heating (left), cooling (middle) and 2nd heating (right) of: a) PBAT, b) PBAT containing 20% (w/w) of CNF-HA, c) PBAT containing 5% (w/w) of CNF-HA, d) CNF-HA, and e) unmodified CNF.

Figure 7 shows the thermogravimetric (TG) weight loss and the derivative (DTG) curves of unmodified CNF, CNF-HA, PBAT and composites based on PBAT with CNF-HA, respectively. An initial weight loss of 3.6% (w/w), attributed to loss of bound water is observed for unmodified CNF. The increase in hydrophobicity following admicellar polymerization is reflected in a reduction in the initial water content in CNF-HA (weight loss 2.4% in the temperature range 25-150 ℃). Unmodified CNF undergo major weight loss between 285 ℃ and 400 ℃ after which 10% of char residue remains. This weight loss, with a peak disintegration temperature of 360 ℃, corresponds to the pyrolysis of cellulose via cleavage of glycosidic linkages.29,30 CNF-HA exhibits two major degradation steps with maximum disintegration temperatures at 317 °C and 395 °C for the initial and second pyrolysis processes, respectively. The first degradation step is attributed to the Poly(HA) coating around the CNF fibrils created by admicellar polymerization while the second degradation step is attributed to CNF. The occurrence of two apparent decomposition events further sustains the successful formation of a coating layer around the CNF fibrils with the admicellar polymerization technique. Interestingly, by coating CNF with HA, the thermal degradation temperature of cellulose is shifted towards higher


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temperatures compared to unmodified CNF. A similar result has earlier been reported for grafted CNC by Morelli et al.3 The TG and DTG curves of PBAT composites containing CNF-HA clearly show the eco-existence of two distinct components: PBAT which undergo major thermal decomposition in the 340 – 460 °C range with a peak disintegration temperature of 406 °C, and CNF-HA with a peak disintegration temperature of 305 °C. The peak disintegration temperature of the PBAT component was only very slightly affected by the CNF-HA addition. The peak disintegration temperature of PBAT was 406 °C, 405 °C, and 404 °C in composites with 0, 5, and 20 % (w/w) addition of CNF-HA, respectively. The decomposition onset temperature is key parameter since it determines the temperature limit for processing. In the case of PBAT composites containing CNFHA, the onset temperature of CNF-HA decomposition, 265 °C sets the upper limit of the processing temperature window. Since the composites were melt extruded at 130 °C, the risk of thermal degradation during processing is safely avoided.


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a)

b)

Figure 7. TG (a) and DTG (b) curves of unmodified CNF, CNF-HA, PBAT and composites based on PBAT containing 5 or 20% (w/w) of CNF-HA.


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The mechanical properties of the composites were investigated by tensile testing. The mechanical properties are listed in Table 1 and mean value stress-strain curves for pure PBAT and composites with 5% CNF filler are shown in Figure 8. The measured mechanical properties are clearly changed by the additions of CNF fillers to the PBAT matrix; Young’s modulus increases while both the tensile strain and tensile stress at break decrease. Taken into account the experimental error, the Young’s modulus increase is comparable for composites with modified and unmodified CNF (Table 1). The slightly higher modulus observed for unmodified CNF reinforced composites could probably relate to the possibility of hydrogen bonding between CNFs. Interestingly at 5% (w/w) filler addition the composite with CNF-HA reinforcement exhibits 30% higher tensile strength than the composite with unmodified CNF, which then implies that a stronger interfacial interaction is accomplished by the modification. Still the mean value stress-strain curves for pure PBAT and PBAT with 5% (w/w) filler additions (Figure 8) express a clear decrease in toughness (area below the curve) of the reinforced PBAT. The elongation and strength with modified CNF do not suffer as severely as with pristine CNF. With CNF-HA, the elongation decreases by 38% and the stress-at-break decreases by 20%, while for pristine CNF addition the corresponding decreases are as high as 89% and 38%, respectively. This result suggests that the hydrophobic modification of CNF makes the fibril matrix more ductile and less aggregated. As described earlier, the


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aggregation of unmodified CNF could already be observed by the naked eye while extruding the material. However, the gain observed at 5%(w/w) with modified CNF seems already lost at 20% (w/w) (Table 1). In conclusion, the hydrophobic modified CNF overall appears more suitable to serve as a reinforcing filler in PBAT, but further work is needed, for example by using another more compatible monomer in the admicellar polymerization, to improve the tensile strength of PBAT.

Table 1. Mechanical data from tensile testing. The values are based on an average of 35 specimens per sample. The percentage change compared to pure matrix is also presented. Additive to

Young’s

Strain at

Stress

Change in

Change

Change

PBAT matrix

modulus

break

at break

Young’s

in strain

in stress

(MPa)

(%)

(MPa)

modulus

at break

at break

(%)

(%)

(%)

0% CNF 49±6

829±21

16.1±1.

9

9

-

-

-


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5% CNF

10.0±0.

20% CNF

-89

-38

75±5

93±10

4

+53

159±16

15±3

9.9±1.4

+224

-98

-39

64±5

514±13

12.9±0.

+35

-38

-20

+169

-97

-45

5% CNF-HA

5 20% CNF-HA

132±4

25±5

8.8±0.4

20

Tensile stress [MPa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

10

5

0 0%

200%

400%

600%

800%

1000%

Tensile strain Figure 8. Mean value stress-strain curves of pure PBAT (dotted line) and PBAT composites with 5% (w/w) CNF-HA (dashed line) and 5% (w/w) unmodified CNF (bold line), 3-5 specimens per sample.


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The obtained results are in good agreement with earlier studies exploiting cellulose nanocrystals (CNCs) for reinforcement of PBAT. Morelli et al

3

studied in situ

poly(butylene glutarate) grafting and Zhang et. al.4 examined modification by acetylation of CNC fillers to introduce hydrophobicity and better compatibility with the PBAT matrix. Both articles report on improved Young´s modulus but reduced elongation and strength in tensile tests. However, in both studies the pristine and modified CNC ended up with tensile strength properties being rather equivalent. Differently in our work, at 5% (w/w) filler content, the reinforcement results in 30% higher tensile strength with modified CNF. This could be an indication of that CNCs due to their smaller aspect ratio than CNFs are less prone to entanglement and hence are easier to disperse in the matrix also without surface modification. However, with increased CNF addition level this effect appears to subside. At 20% (w/w) filler content the effect is no longer apparent (Table 1). The outcome of this study implies that to gain full advantage of the intrinsic properties of CNF in plastic materials it is necessary to advance on the control of good dispersion and compatibility with the polymer matrix. Results show that admicellar polymerization indeed produces a polymeric coating around the fibrils which, depending on the choice of monomer, may improve the hydrophobicity and processability of the matrix and hence facilitate the integration of the modified CNF into a plastic composite such as PBAT. PBAT is already used as a disposable packaging material where an addition of a bio-based filler is beneficial not only as a reinforcing additive but also by increasing the share of renewable matter in the material. CNF-HA shows promising characteristics to serve as a bio-based additive and,


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importantly, does not severely compromise important packaging characteristics such as optical transparency (Figure 9). Further work on the CNF reinforcement of PBAT could open and provide opportunity to manufacture biodegradable materials suitable for many packaging applications.

Figure 9. A 1 mm thick free-standing film of PBAT with 5%(w/w) CNF-HA.

4. CONCLUSIONS A water-based admicellar polymerization strategy for production of hydrophobic CNF was established and demonstrated with hexyl acrylate as the adsolubilized monomer forming a coating around the fibrils. SEM images and ATR-FTIR analysis revealed that the CNF was significantly but not always fully coated with poly(HA). The targeted hydrophobization was verified by a water contact angle above 90°. The modified CNF displayed a more


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thermoplastic behavior than unmodified CNF and was successfully incorporated to a PBAT matrix by melt extrusion. The addition of poly(HA) coated CNF to PBAT allowed for an increased Young’s modulus, while both the tensile strain and tensile stress at break decreased compared to pristine PBAT. The hydrophobic CNF modification seems to counteract entanglement and aggregation, resulting in a better preservation of the elongation properties compared to pristine CNF, especially at 5 % (w/w) addition level. A full advantage of the intrinsic properties of CNF in reinforcement of plastic materials requires further research work on the control of good dispersion and compatibility with the polymer matrix, herein explored on PBAT.

ASSOCIATED CONTENT Supporting Information. UV-Vis spectra of the washing water after admicellar polymerization. Particle size distribution of grinded PBAT. Photographs of melt-extruded composites. A table of measure water contact angels of CNF, before and after admicellar polymerization. The following file is available free of charge: Supporting information-admicellar pol CNF.pdf


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AUTHOR INFORMATION E-mail: [email protected] ORCID Ulrica Edlund: 0000-0002-1631-1781 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources FORMAS: The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, project number 2014-151. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank FORMAS (project number 2014-151) for their financial support.

ABBREVIATIONS


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CAC, critical admicelle concentration; CMC, critical micelle concentration; CNF, cellulose nanofibrils; CTP, hexadecylpyridinium chloride monohydrate; HA, hexyl acrylate; PBAT, poly(butylene adipate-co-terephthalate);

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PBAT

Admicellar polymerization

Twin-screw extrusion


39


Figure 1. Repeating unit building blocks of PBAT. 274x42mm (120 x 120 DPI)


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Figure 2. A schematic illustration of the admicellar polymerization of HA onto CNF. Step 1) admicelle formation with CTP in H20 at ambient temperature, Step 2) monomer adsolubilization with HA in H20 at ambient temperature, Step 3) polymerization of HA in H20 at 80 °C, Step 4) washing in H20 and ethanol at ambient temperature. 206x143mm (120 x 120 DPI)



Figure 3. ATR-FTIR spectra of a) CNF before modification, b and c) CNF-HA analyzed at different surface spots, d) PolyHA, a reference homopolymer of HA, and e) the monomer HA. 201x288mm (300 x 300 DPI)


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Figure 4. SEM images of CNF before and after admicellar polymerization at magnifications x250 and x2000. 209x144mm (96 x 96 DPI)



Figure 5. SEM images at x500 magnification of fracture surfaces of PBAT containing 5 or 20% (w/w) of CNFHA or unmodified CNF. The scale bar represents 100 m. 167x124mm (120 x 120 DPI)


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Figure 6. DSC thermograms recorded during the 1st heating (left), cooling (middle) and 2nd heating (right) of: a) PBAT, b) PBAT containing 20% (w/w) of CNF-HA, c) PBAT containing 5% (w/w) of CNF-HA, d) CNFHA, and e) unmodified CNF. 623x296mm (300 x 300 DPI)



Figure 7. TG (a) and DTG (b) curves of unmodified CNF, CNF-HA, PBAT and composites based on PBAT containing 5 or 20% (w/w) of CNF-HA. 288x201mm (300 x 300 DPI)


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Figure 7. TG (a) and DTG (b) curves of unmodified CNF, CNF-HA, PBAT and composites based on PBAT containing 5 or 20% (w/w) of CNF-HA. 296x209mm (300 x 300 DPI)



Figure 8. Mean value stress-strain curves of pure PBAT (dotted line) and PBAT composites with 5% (w/w) CNF-HA (dashed line) and 5% (w/w) unmodified CNF (bold line), 3-5 specimens per sample. 139x94mm (120 x 120 DPI)


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Figure 9. A 1 mm thick free-standing film of PBAT with 5%(w/w) CNF-HA. 679x430mm (120 x 120 DPI)



Table of Content graphic 223x86mm (120 x 120 DPI)


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Admicellar Polymerization Coating of CNF Enhances Integration in

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