Surface-Initiated Controlled Radical Polymerization Approach To

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A surface-initiated controlled radical polymerization approach to enhance nanocomposite integration of cellulose nanofibrils Julien R.G. Navarro, and Ulrica M Edlund Biomacromolecules, Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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A surface-initiated controlled radical polymerization approach to enhance nanocomposite integration of cellulose nanofibrils Julien R. G. Navarro and Ulrica Edlund*

Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE100 44, Stockholm, Sweden ABSTRACT A strategy is devised for the conversion of hydrophilic cellulose nanofibrils (CNF) into hydrophobic CNF that form a stable nanocomposite dispersion for functional reinforcement of a polypropylene matrix. For that purpose, CNF was converted to a CNF–based microinitiator through an esterification reaction on the nanofibril surfaces, which efficiently initiated the controlled radical grafting polymerization of stearyl acrylate. The grafting-from modification was performed with and without a sacrificial initiator and verified with solid-state 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy. CNF based nanocomposites were prepared using the combination of a twin screw mini extruder and melt pressing. Scanning electron microscopy reveal a homogeneous dispersion of the hydrophobic CNF in composite matrix with no signs of aggregation. Hydrophobic CNF showed a strong compatibility with the PP matrix. 1 ACS Paragon Plus Environment

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KEYWORDS: Cellulose nanofibrils, controlled radical polymerization, grafting-from, composite

INTRODUCTION

The use of non-edible renewable and biodegradable material resources and sound methods for converting them to functional materials that can replace oil-based products are integral parts of the global strive to meet the grand challenge of a future sustainable society. Benign, inexpensive, and easily managed strategies for doing so are keys for any realistic future implementation. In this context, cellulose nanofibrils (CNF) offer many intrinsic property advantages including high specific surface area and aspect ratio, high Young’s modulus, tailorable barrier properties, and remarkable strength that render them highly promising in future materials, not in the least as reinforcing additives in plastic and fibre composites.1–3 Depending on the preparation and post-treatment protocol, CNFs can come in different grades that vary quite significantly in terms of surface charge and pendant groups4, e.g. cationic5, anionic6, phosphorylated7 and so forth. Still, a generic remaining challenge is the high hydrophilicity and low miscibility with polymers and solvents of low or no polarity, leading to fibril agglomeration, moisture absorption, poor interfacial compatibility with conventional composite matrices and phase separation from organic solvent suspension.

Surface grafting, or labelling entities onto the nanocellulose backbone, may produce chemically modified nanocellulose with concomitant changes in property profiles8–15 and with careful choice of modification chemistry, this presents a viable route to overcome deficiencies in the CNF intrinsic properties. Cellulose modification and grafting is mediated by functionalization of hydroxyl groups on the cellulose sugar rings to serve as initiator groups in a subsequent polymerization reaction with appropriate monomers (grafting-from) or serve as

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immobilization sites for pre-fabricated polymer chains (grafting-to).16,17 Surface initiated controlled radical polymerization (CRP) offer versatile and facile pathways for modification with superior control of graft length and end group chemistry. In addition, CRP methods nitroxide-mediated

polymerization

(NMP),18–20

atom-transfer

radical

polymerization

(ATRP),21,22 and Single Electron Transfer Living Radical Polymerization (SET-LRP)23–29 proceed efficiently with preserved control in polar media and at ambient temperature30 which render these methods highly interesting for CNF modification and functionalization. In addition, Cu(0)-mediated CRP has been shown viable for acrylate monomers with hydrophobic pendant groups.31–33 A handful reports have to date been presented regarding CRP for CNF modification. For instance, CNF was grafted with methyl and ethyl acrylates in a radical polymerization process redox-initiated by cerium ammonium nitrate,34 and surface initiated ATRP was employed to graft TEMPO-oxidized CNF with styrene.35 Surface initiated CRP was shown to produce a CNF that stays stable in both aqueous and organic suspension.36 We recently demonstrated the successful conversion of CNF into a macroinitiator followed by SET-LRP

copolymerization

producing

a

methyl

acrylate

and

acrylic

acid

N-

hydroxysuccinimide ester grafted CNF that was further functionalized with a fluorescent end group and shown to work as a viable biomarker sensor.37

Pushing the limits of graft chain hydrophobic character and molecular weight will widen the scope of CNF applicability and extend the avenues of handling and processing. Our aim was to devise a facile and efficient synthetic route to produce a hydrophobic CNF grade that is highly dispersible and show an improved compatibility with, hydrophobic plastic composite matrices, herein represented by polypropylene.

EXPERIMENTAL SECTION Materials. 3 ACS Paragon Plus Environment

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1,1′-Carbonyldiimidazole (CDI), 2-bromo-2-methylpropionic acid 98%, imidazole ≥99%, stearyl acrylate (SA) 97%, Ethyl 2-bromo-2-methylpropionate, isopropanol, acetonitrile, ethanol, acetone, chloroform Toluene were purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO, ≥99%) was purchased from Merck. Copper wire (diameter 0.812 mm) was purchased from Fisher. For the production of cellulose nanofibrils a bleached never-dried softwood sulphite dissolving pulp (Domsjö mill, Domsjö Fabriker AB, Sweden) was used. The pulp has a hemicellulose content of 4% as determined by the ISO 699:2015 standard method.38 A monocomponent endoglucanase (FiberCare R, Novozymes, Denmark) was used as received. Milli-Q water was used for the solvent exchange procedure. Polypropylene (PP) produced by A. Schulman GMBH (degree of crystallinity 50%) was a kind gift from Innventia AB (Now: RISE Bioeconomy).

Extraction of cellulose nanofibrils from wood pulp. The CNF was prepared by a combined refining and enzymatic pre-treatment procedure as previously reported.39,40 The process resulted in a 2.1 % (w/w) CNF aqueous gel.

Solvent exchange procedure. 10 g of the CNF aqueous gel (2.1 % w/w) was suspended in 80 mL of water and subsequently stirred for 5 h. DMSO (80 mL) was slowly added to the aqueous suspension under stirring. The suspension was finally centrifuged (4000 RPM, 20 min) and the supernatants were discard and replaced with fresh DMSO. The centrifugation operation was repeated 4 times.

Synthesis of the CNF-based macroinitiator. 4 ACS Paragon Plus Environment

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The CNF gel (10 g, 1 % w/w) was suspended in DMSO (100 mL) and the temperature was raised to 55 °C. Imidazole (3 g, 44 mmol) was added to the suspension. Separately, 2-bromo2-methylpropionic acid (4 g, 24 mmol) was dissolved in 60 mL DMSO and CDI (4 g, 24 mmol) was slowly added, under stirring, at room temperature for 1 h. Finally, the solution was slowly added to the CNF suspension. The reaction proceeded for 16 h. The modified CNF was purified by centrifugation (4 000 RPM / 20 min). The supernatants were discarded and replaced with DMSO. The purification steps were repeated 8 times.

Synthesis of Tris[2-(dimethylamino)ethyl]amine (Me6-TREN). A solution composed of formaldehyde (160 mL) and formic acid (160 mL) was cooled to 0°C and tris(2-aminoethyl)amine (15 g, 0.1 mol) was added dropwise. After 1 h stirring, the solution was warm up to room temperature and finally refluxed for 16 h. The yellow-orange solution was concentrated by rotary evaporation. A saturated solution of NaOH was then added to the solution, and the addition was continued until the product did form an oily layer. The product was extracted with dichloromethane. The combined organic layers were dried over MgSO4, and the dichloromethane was removed by rotary evaporation, yielding a yelloworange oil. The oil was purified by vacuum distillation.

General procedure for SET-LRP grafting of CNF. A copper wire (diameter=0.812 mm, length=6.25 cm) was immersed in a concentrated HCl solution for 10 min, then rinsed with acetone and dried prior to use. The CNF-based macroinitiator (4 g, 1 % w/w) was suspended in DMSO (30 mL), and SA (in 15 mL toluene) was added. The HCl-treated copper wire was added and the suspension was degassed via nitrogen purging for 10 min. The temperature was then raised to 40°C. The Me6TREN ligand was added and the reaction was allowed to proceed, under a nitrogen atmosphere. The SA-


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grafted CNF (CNF-g-SA) was precipitated with isopropanol and collected through centrifugation (4 000 RPM / 20 min). CNF-g-SA was repeatedly washed (6 times) with a toluene:isopropanol (1:4) mixture (100 mL). All polymerizations were performed at a [M]0/[I]0/[L]0 ratio of 100/1/0.2 All CNF-g-SA products were characterized by ATR-FTIR spectroscopy and solid-state MAS 13C NMR.

Sacrificial initiator.

SET-LRP grafting of CNF was carried out exactly according to the procedure described above except that ethyl 2-bromo-2-methylpropionate at a concentration of 1.5 mM or 3 mM was added to the CNF suspension as a sacrificial initiator.

CNF-based nanocomposite preparation. Composites were prepared from PP and 5% (w/w) CNF-g-SA (added in its gel form, 10% w/w) using a twin-screw mini extruder (DSMXplore 5 cm3 Micro-Compounder). Composite samples from PP and non-modified CNF were prepared as a reference. The screw speed was set to 100 rpm with a temperature of 190 °C during 5 min in counter-rotating mode. The extruded material (5.5 g) was melt-pressed at a temperature of 210 °C in a dogbone shape mold with the following dimensions: width = 1 cm, length = 7 cm and thickness = 2.88 mm. The materials were pressed for 13 min within a force of 10 kN, followed by a 5 min pressing with a force of 100 kN. Dogbones from pure PP were prepared as reference samples using the same procedure. Characterization. All 13C NMR spectra were acquired with a Bruker 500 Avance III HD spectrometer at Larmor frequencies of 125 MHz and 500 MHz for 13C and 1H, respectively. The samples were packed 6 ACS Paragon Plus Environment

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in 4 mm zirconia rotors and spun at 8 kHz. Ramped cross-polarization (CP) 13C MAS NMR spectra were recorded with a 13C nutation frequency of 50 kHz and a contact time of 1.5 ms. High-power 1H decoupling was achieved by the TPPM technique using a nutation frequency of 80 kHz. 4096 signal transients were accumulated with relaxation delays from 3 to 15 s, depending on relaxation time estimated for each sample. Signal apodization by a 30 Hz Lorentzian broadening was applied before Fourier transformation and 13C chemical shifts are quoted relative to neat tetramethylsilane (TMS). 1

H-NMR was performed on a Bruker Avance DMX-400 NMR operating at 400 MHz at room

temperature and the spectra were analyzed using MestReNova software version 9.0.0. Samples were dissolved in CDCl3 (Larodan Fine Chemicals) and transferred to NMR tubes with 5 mm outer diameter. ATR-FTIR (Attenuated Total Reflection Fourier Transform Infrared Spectroscopy) was performed using a PerkinElmer Spectrum 2000 spectrometer. All spectra were obtained as means of 64 scans in the spectral region of 4000−500 cm−1, with a spectral resolution of 4 cm−1. Static water contact angle measurements were made using a Contact Angle Meter CAM-200 from KSV Instruments LTD. The sessile drop method was used with an automated dispenser applying 5 µl droplets of Milli-Q water on the substrates and recording the static contact angle with a Basler A6021-2 camera. Each reported value is the calculated mean of 10 individual measurements. Thin, flat substrates of CNF and grafted CNF were prepared by suction filtration of CNF suspensions followed by air drying. The morphology of the different CNF was observed using a JSM-7500F (JEOL, Japan) at an acceleration voltage of 5 kV. The dried CNF were attached to the sample supports using carbon tape, and coated with a 5 µm layer of Pd/Pt.


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The thermal behavior of CNF, CNF-g-SA and composites were monitored using a Differential Scanning Calorimeter (DSC) equipment Mettler Toledo DSC 820 module. Each sample (approximately 5 mg) was encapsulated in a 40 µL aluminium pan. The temperature program was (I) heat from 25 to 300 °C, (II) cool to 0 °C and (III) heat for a second time to 300 °C. The heating and cooling rate was 10 °C/ min under a nitrogen atmosphere (nitrogen flow rate 50 mL/min). The approximate degree of crystallinity of the materials (Xc) was calculated according to equation (1): Xc= ∆Hf / 209 fp (1) Where ∆Hf is the fusion enthalpy (J/g), 209 is the estimated enthalpy of fusion in J/g of 100% crystalline PP41,42, and fp is the weight fraction of PP. Thermogravimetric analysis of the samples was carried out using a Mettler Toledo TGA/DSC1. The samples (approximately 5 mg) were placed in 70 µL aluminium oxide crucibles and heated to 800 °C at a rate of 10 °C/ min under a nitrogen atmosphere (nitrogen flow rate 50 mL/min). Tensile tests on the melt-pressed dogbones were performed using an INSTRON 5566 module. Five specimens were tested for each material. The measurements were performed with a 10 kN load cell at a strain rate of 5 mm/min. The samples were preconditioned at 23 °C and 50% RH for 40 h according to the standard ASTM D618-08.

RESULTS AND DISCUSSION

Chemical modification of the cellulose nanofibrils. To produce a hydrophobic grade of cellulose nanofibrils (CNF), we chemically modified the nanofibrils through an esterification reaction followed by a SET-LRP grafting-from polymerization. Pristine CNF carries an abundance of surface hydroxyl groups, which enhance the inter-fibril interaction and thus 8 ACS Paragon Plus Environment

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limit their stability in dispersion in various media due to the fibril aggregation. The growth of surface immobilized polymer grafts will aim to suppress those inter-fibril interactions and enhance the stability of CNF suspension thereby increasing the applicability of CNF as a strengthening additive in composites. The choice of monomer will play a major role in the stability of the suspension. First, we converted pristine CNF to a CNF-based macroinitiator through an esterification reaction between 2-bromo-2-methylpropionic acid and the hydroxyl groups of CNF (Fig. 1A). In a second step, the CNF-based macroinitiator initiated the controlled radical polymerization of stearyl acrylate (SA) in the presence of Cu(0) and a tetradendate tertiary amine ligand (Me6TREN) in a DMSO:toluene mixture (Fig. 1B). Interestingly, the CNF-based macroinitiator was only stable in DMSO, while the solubility of the monomer was poor in DMSO. SA is soluble in toluene and the addition of toluene to the CNF-based macroinitiator DMSO suspension did not destabilize the nanocellulose suspension (Fig. 1C). As the controlled radical polymerization proceeded and yielded the poly(SA)grafted CNF (CNF-g-SA) a phase separation occurred (Fig. 1D).


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Fig.1 (A) Chemical pathway for the CNF-based macroinitiator synthesis, (B) Polymer grafting onto the CNF-based macroinitiators with stearyl acrylate (SA) via CRP. Pictures of the CNF suspension in a DMSO:toluene mixture at t=0 min (C) and t=18 h (D). At t=18h (D), the CNF-g-SA produced a biphasic system in the DMSO:toluene mixture.

The upper phase was mainly composed of the CNF-g-SA, while the lower phase contained the monomer, catalyst and Me6-TREN. The phase separation was observed after 30 min reaction and did seemingly not disturb the CRP since the polymer grafts continue to propagate onto the CNF. Similar results were reported by Haddleton et al.32,33 while performing SET-LRP with 10 ACS Paragon Plus Environment

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lauryl acrylate as the monomer. After several purification steps, the CNF-g-SA was collected, suspended in toluene and centrifuged to obtain a transparent gel (Fig. 2A) with a concentration of 10% (w/w) and with no macroscopic signs of inter-fibril interaction and agglomeration in the organic medium. The UV-Vis spectrum of the CNF-g-SA in toluene (Fig. 2B) shows a high transmittance value over the visible wavelength range demonstrating the high stability of the CNF-g-SA in toluene.

Fig.2 (A) Photograph of a CNF-g-SA gel in toluene. For a better comparison, the initial and unmodified CNF gel, in water, was also added to the picture. (B) UV-Vis spectra of the CNFg-SA suspension in toluene.

The successful chemical modification of CNF with the SA monomer was monitored with ATR-FTIR (Fig. 3). The unmodified CNF spectrum (Fig. 2A) shows the characteristic bands of nanocellulose with bands localized at 3320 cm-1 (O-H), 2950 and 2895 cm-1 (C-H), 1430 cm-1 (C-H) and 1161 cm-1 (C-O-C). The spectrum of CNF-g-SA (Fig. 2B), in addition to the characteristic bands of the nanocellulose, shows strong additional absorption bands localized at 2917 and 2850 cm-1 (C-H), 1733 cm-1 (C=O), 1467 cm-1 (C-H), 1162 cm-1 (C-O-C) and 721 cm-1 (-CH2-, rocking) verifying the presence of ester groups and an abundance of methylene


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groups in the main and side chains of each graft repeating unit and hence the successful chemical modification of the CNF.

Fig. 3 ATR-FTIR spectra of unmodified CNF (A) and CNF-g-SA (B).

The successful grafting of SA onto CNF was also investigated through CPMAS

13

C NMR.

The CNF-g-SA spectrum, along with peak assignments, is shown in Fig. 4. The characteristic bands of nanocellulose are located at 84 ppm (C4) and 62 ppm (C6) for the surface carbon sites, and at 89 (C4) and 66 ppm (C6) for the carbon inside the crystalline region. The spectrum also exhibits a peak localized at 175 ppm attributed to the C7 and C12 sites (carbonyl bond) and intense peaks in the region 40-20 ppm for the long alkyl chain of the SA polymer grafts (C14 to C20 sites).


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Fig. 4 Solid state

13

C NMR spectra at 500 MHz of CNF-g-SA. The spinning side bands are

marked (*).

The stability of the CNF-g-SA was investigated in various solvents (Fig. 5, top). From all the used solvents, acetonitrile, ethanol, acetone, chloroform, isopropanol and toluene, the poly(SA)-grafted CNF was so far only stable in toluene. The suspension stability of the CNFg-SA in toluene was also investigated at various graft polymerization times. At t=0 min, the CNF-based macroinitiator suspension is not stable in toluene but early on in the reaction, the CNF-g-SA suspension becomes stable in toluene.


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Fig. 5 (Top) The CNF-g-SA never dried toluene gel (t=18 h) was re-suspended in various solvents: (A) Acetonitrile, (B) Ethanol, (C) Acetone, (D) Chloroform, (E) Isopropanol, (F) Toluene. (Bottom) CNF-g-SA suspended in toluene, after different polymerization times.


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Fig. 6 1H NMR spectra (in CDCl3) of the progressive controlled radical polymerization of the isolated homopolymers SA, at different reaction times: 1, 2, 4, 6 and 18 h.

The nature of the grafted polymer onto the cellulose nanofibrils still remained difficult to fully characterize. Hence, two approaches were investigated: (1) after the synthesis of the CNF-gSA suspension, and further purification, a minute amount of unbound poly(SA) homopolymers was isolated, purified and characterized. (2) The use of a sacrificial initiator, free initiator in solution, which generated free homopolymers of SA in solution. The isolated unbound SA homopolymer fraction (approach 1) was assessed with solution state 1

H-NMR to provide information about the kinetic progression of the grafting reaction (Fig. 6).

The peaks localized at 5.9 ppm were attributed to the vinyl bond of the SA monomer. As the monomer was used in excess and only very small amounts of homopolymers are formed without any sacrificial initiator, there was no significant gradual disappearance of this peak as 15 ACS Paragon Plus Environment

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the CRP propagates. But it was still possible to clearly see the signal of the growing SA homopolymers with peaks localized at 4.0, 2.27 and 1.6 ppm, respectively attributed to the main and side chain methylene hydrogens as the polymerization proceed. However, the –CH2proton (main polymer chain) peak is difficult to resolve from the overlaid proton signal of the co-solvent DMSO (2.63 ppm). SA homopolymer formation was therefore detectable after 1 h of reaction as evident from the NMR traces (Fig. 6) and reaches its maximum conversion within 6 h (Fig. 6). Moreover, by having a closer look on the CNF-g-SA suspension picture (Fig.5 bottom), the hydrophobic behavior of the purified modified CNF suspension was observed. A kinetic plot is shown in Fig. 7. The relationship between the conversion and the ln [M]0/[M] appears to be linearly dependent on reaction time. This suggests a first order rate of propagation and a controlled radical polymerization (CRP) for the homopolymerization of SA which further indicates that the CNF-g-SA also had the characteristic of a CRP.

Fig. 7 Kinetics plots for the Cu(0) mediated CRP formation of SA homopolymers. Each data point represents an individual experiment.


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To further compare this experiment with the self-initiated unbound SA homopolymer, we performed the grafting polymerization under the same conditions as before in combination with an added sacrificial initiator (approach 2). The free unbound generated polymer was then isolated and characterized. The homopolymers produced with a sacrificial initiator provide a good indication of the average graft length if determined by SEC.43,44 The SA homopolymer had a number average molecular weight (Mn) of 16700 g mol-1 and a very low dispersity (Đ=1.06) at a sacrificial initiator concentration of 1.5 mM and close to twice as high molecular weight was recorded when the initiator concentration was doubled (Mn=31800 g mol-1, Đ=1.08).

The high hydrophobicity of the CNF-g-SA was further manifested in a clear increase in water contact angles upon grafting (from 43o to 107o, Fig. 8).

Fig. 8 Photograph of the CNF-g-SA and unmodified CNF films during static contact angle measurements. The CNF-g-SA film displayed a contact angle of 107o while the unmodified film has a contact angle of 43o. 17 ACS Paragon Plus Environment

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The CNF based macroinitiator, and SA-grafted adducts were further characterized with SEMEDS to verify the nanofibrillar morphology and the associated elemental composition. The CNF-based macroinitiator images (Fig. 9A, C) show a dense matrix of aggregated fibrils, while the CNF-g-SA fibrils (Fig. 9B, D) are completely embedded in the grafted polymer matrix. SEM-EDS data show a clear presence on bromine in the CNF macroinitiator while this element was not detectable at all in the pristine CNF or the final CNF-g-SA where only C and O were detectable. (Fig. S1 & S2, Supplementary Information) C and O are light elements and any quantifications with EDS is uncertain. Yet, there is a clear increase in the C/O ratio as a result of grafting which is expected considering the high amount of immobilized methylene groups in each SA repeating unit in the CNF-g-SA compared to the more oxygen rich pristine CNF.

Fig.9 SEM images of CNF-based macroinitiator (A, C) and CNF-g-SA (B, D) at different magnifications. 18 ACS Paragon Plus Environment

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Composite production and characterization. CNF based nanocomposites with a PP matrix were prepared using the combination of a twin screw mini extruder and melt-pressing. The content of chemically modified CNF was fixed to 5% (w/w). It was not possible to produce analogous nanocomposites of unmodified CNF in the PP matrix as they were clearly incompatible. The poor dispersability of the unmodified CNF in the hydrophobic polymer matrix lead to a systematic aggregation of the nanofibrils. The non-adhesion of the unmodified CNF in the PP matrix, evident by macroscopic aggregation, could even be noticed by the naked eye while extruding the material The nanocomposites based on PP and CNF-g-SA on the other hand were macroscopically homogeneous. An ATR-FTIR spectrum of the CNF-g-SA / PP nanocomposite is shown in Figure 10. The PP spectrum (Fig. 10A) shows the characteristic bands of PP with bands localized at 2951, 2918 and 2838 cm-1 (C-H) and 1457 cm-1 (C-H). The spectrum of the PP/CNF-g-SA (Fig. 2B) shows, in addition to the characteristic bands of the PP, an additional absorption band localized at 1738 cm-1 (C=O), verifying the presence of ester groups in the extruded material and hence the successful incorporation of CNF-g-SA into PP.

Fig. 10 FTIR spectra of the PP (A) a composite of 5% (w/w) of CNF-g-SA in PP (B). 19 ACS Paragon Plus Environment

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Thermal characterization of CNF, CNF-g-SA and a composite composed of PP with 5% (w/w) of CNF-g-SA was carried out with DSC and TGA. From the analysis of DSC traces (Figure 11), it is clear that pristine CNF shows no signs of thermal transitions in the entire temperature range of 0-200 °C, neither during heating nor cooling, which is expected considering the restricted segmental mobility of the polysaccharide chains. After SA grafting however, the modified CNF undergoes a clear melting endotherm during heating and a crystallization exotherm during cooling in the range from 35 – 62 °C which is very similar to the behavior of the SA homopolymer (35 – 53 °C) and another indication of a successful grafting. Pristine PP is semicrystalline with a crystalline melting temperature of 164 °C (heating), a crystallization temperature of 130 °C (cooling), and a degree of crystallinity of 50% as calculated from Equation 1. When incorporating 5% (w/w) of CNF-g-SA in the PP matrix, a composite with a strong PP melting endotherm at 164 °C and a small melting peak originating from the grafted CNF is formed. The degree of crystallinity of the PP phase in the composite is 54%, which is somewhat higher than that of pure PP. This increase in Xc may be caused by a nucleating effect of the dispersed nano-sized fibrils.


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Fig. 11 DSC thermograms recorded during heating (left) and cooling (right) of (from top to bottom): a composite of 5% (w/w) of CNF-g-SA in PP, pure PP, CNF-g-SA, a homopolymer of SA, and unmodified CNF. The thermal degradation was shifted to higher temperatures for SA-grafted CNF samples as shown in the TGA curves in Figure 12. Unmodified CNF undergo a small weight loss (< 10%) in the range around 100-150 °C due to the evaporation of bound water while this initial weight loss is not observed at all in the hydrophobized CNF-g-SA. Both samples then undergo a major thermal degradation attributed to the cleavage of glucosidic linkages in the cellulose backbone,45 however this degradation range is shifted toward higher temperatures for CNF-g-SA (330 – 450 °C) compared to unmodified CNF (250 – 360 °C). TGA curves of pure PP and a CNF-g-SA / PP nanocomposite are shown in Figure 12B. Both samples show a massive thermal degradation around 375 – 480 °C due to the thermal decomposition of PP. However, the nanocomposite sample also showed a mass loss of 5% around 300 °C due to the CNF-g-SA degradation.

Fig. 12 TGA curves of (A) unmodified CNF (dash line), CNF-g-SA (solid line) and (B) PP (dash line) and PP-CNF5% (solid line).


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The mechanical properties of the composite of 5% (w/w) of CNF-g-SA in PP and pure PP were investigated by tensile testing. The stress-strain curves of both samples are shown in Figure 13 and the mechanical properties are listed in Table 1. When using modified CNF as a filler, the Young’s modulus slightly decreased to 712 MPa (1013 MPa for the pure PP), as did the tensile stress (22.39 MPa as compared to 26.43 MPa for pure PP). However, the tensile strain increased with the presence of the modified CNF indicated that the grafting modification induced a more thermoplastic behavior of the fibrils with resulting improved ductility at the expense of strength. An inhomogeneous dispersion of the CNF in the plastic matrix would have a dramatic effect with a significant decrease of stress, strain and modulus, hence these data indicate that the CNF-g-SA was homogeneously dispersed in the PP matrix. The absence of pores in the morphology of the fracture surface also indicates that interfacial debonding did not cause the failure at break of the composite.

Fig. 13 Stress-strain curves of PP and CNF-g-SA in PP. 22 ACS Paragon Plus Environment

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Table 1. Summary of the tensile properties of the pure PP and a composite of 5% (w/w) of CNF-g-SA in PP. Modified CNF content [%] in PP

Young’s modulus [MPa]

Tensile strain at break [%]

Tensile stress at break [MPa]

0

1014± 18.0

4.5 ± 1.28

26.4 ± 3.07

5

712± 10.3

6.8 ± 2.59

22.4 ± 1.76

A SEM examination of the composite cross-sections reveals that the CNF-g-SA fibrils were indeed well dispersed within the PP matrix with no signs of pores, large aggregates or network formations of fibrils governed by percolation (Figure S4). A comparison of the fracture surfaces of tensile testing dogbone specimens from pristine PP and a composite of PP with 5% (w/w) of CNF-g-SA shows no apparent differences between the samples. The fracture surface is homogeneous implying good adhesion between CNF-g-SA and matrix. The results consistently show that the devised controllable strategy for grafting of CNF indeed imparts a thermoplastic and hydrophobic behavior. The association of the modified CNF with a hydrophobic composite matrix like PP was intended as a proof-of-concept of the hydrophobic potential of the modified CNF. With its combination of high strength, high surface area, and hydrophobicity, the modified CNF may also have many potential applications, other than as a renewable composite additive. For instance, there is a strong interest in using CNFs as membranes and filters for water purification where a hydrophobized grade would be superior to hydrophilic CNFs for the capture of organic pollutants, such as oils. Aerogels from hydrophobized nanocellulose were shown viable as floating oil absorbents.46 Other interesting uses for hydrophobic CNF could be as rheological modifiers in non-polar liquids or encapsulating matrices for transport and release of hydrophobic


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chemicals and pharmaceuticals. Also from an anti-fouling perspective, hydrophobic CNF would be an interesting material candidate.

CONCLUSIONS A strategy for the production of hydrophobic cellulose nanofibrils is devised based on the principle of surface-initiated controlled radical graft polymerization. The cellulose nanofibrils were converted to CNF-based macroinitiators through an esterification reaction. The CNFbased macroinitiators initiated graft polymerization of stearyl acrylate, yielding a poly(SA)grafted CNF. The graft polymerization of the SA monomer proceeded with a linear dependence on reaction time and proved to be an efficient way to modify the properties of CNF and produce a highly hydrophobic grade. The resulting modified CNF suspension was highly stable in toluene, as evidenced with the UV-Vis spectra and the high transmittance value all over the visible wavelength range. Nanocomposites based on poly(SA)-grafted CNF distributed within a PP matrix were prepared using the combination of a twin screw mini extruder and a melt-press. Hydrophobic CNF-g-SA showed a thermoplastic behavior and a strong compatibility with the non-polar PP matrix.

GRAPHICAL ABSTRACT


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SUPPORTING INFORMATION. Fig. S1 shows the SEM-EDS data for the CNF-based macroinitiator, Fig. S2 shows SEM-EDS data of the poly(SA)-grafted CNF, Fig. S3 shows UV-Vis spectra of the CNF-g-SA suspension in toluene (black) and the unmodified CNF in water (red), and Fig. S4 shows SEM images of the CNF-g-SA, PP cross-section after tensile fracture.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Ulrica Edlund)

FUNDING SOURCES The authors thank Formas (project number 2014-151) for financial support.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We thank E. Ålander at RISE Bioeconomy (former Innventia AB) for the preparation of CNF.

ABBREVIATIONS CNF,

Cellulose

nanofibrils;

CRP,

Controlled

radical

polymerization;

PP,

polypropylene; CNF-g-SA, SA-grafted CNF


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Fig.1 (A) Chemical pathway for the CNF-based macroinitiator synthesis, (B) Polymer grafting onto the CNFbased macroinitiators with stearyl acrylate (SA) via CRP. Pictures of the CNF suspension in the DMSO:toluene mixture at t=0 min (C) and t=18 h (D). At t=18h (D), the CNF-g-SA produced a biphasic system in the DMSO:toluene mixture. 25x25mm (300 x 300 DPI)

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Fig.2 (A) Photograph of a CNF-g-SA gel in toluene. For a better comparison, the initial and unmodified CNF gel, in water, was also added to the picture. (B) UV-Vis spectra of the CNF-g-SA suspension in toluene. 361x158mm (72 x 72 DPI)


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Fig. 3 ATR-FTIR spectra of unmodified CNF (A) and CNF-g-SA (B). 19x16mm (600 x 600 DPI)


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Fig. 4 Solid state 13C NMR spectra at 500 mHz of CNF-g-SA. The spinning side bands are marked (*). 32x26mm (300 x 300 DPI)


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Fig. 5 (Top) The CNF-g-SA never dried toluene gel (t=18 h) was re-suspended in various solvents: (A) Acetonitrile, (B) Ethanol, (C) Acetone, (D) Chloroform, (E) Isopropanol, (F) Toluene. (Bottom) CNF-g-SA suspended in toluene, after different polymerization times. 173x124mm (150 x 150 DPI)


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Fig. 6. 1H NMR spectra (in CDCl3) of the progressive controlled radical polymerization of the isolated homopolymers SA, at different reaction times: 1, 2, 4, 6 and 18 h. 160x168mm (300 x 300 DPI)


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Fig. 7 Kinetics plots for the Cu(0) mediated CRP formation of SA homopolymers. Each data point represents an individual experiment. 12x8mm (600 x 600 DPI)


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Fig. 8 Photograph of the CNF-g-SA and unmodified CNF films during static contact angle measurements. The poly(SA)-grafted CNF film displayed a contact angle of 107o while the unmodified film has a contact angle of 43o. 317x270mm (72 x 72 DPI)


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Fig.9 SEM images of CNF-based macroinitiator (A, C) and CNF-g-SA (B, D) at different magnifications. 86x59mm (300 x 300 DPI)


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Fig. 10 FTIR spectra of the PP (A) a composite of 5% (w/w) of CNF-g-SA in PP (B). 16x12mm (600 x 600 DPI)


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Fig. 11 DSC thermograms recorded during heating (left) and cooling (right) of (from top to bottom): a composite of 5% (w/w) of CNF-g-SA in PP, pure PP, CNF-g-SA, a homopolymer of SA, and unmodified CNF. 361x162mm (72 x 72 DPI)


Biomacromolecules

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Fig. 12 TGA curves of (A) unmodified CNF (dash line), CNF-g-SA (solid line) and (B) PP (dash line) and PPCNF5% (solid line). 17x7mm (300 x 300 DPI)


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Biomacromolecules

Fig. 13 Stress-strain curves of PP and CNF-g-SA in PP. 361x261mm (72 x 72 DPI)


Biomacromolecules

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Table of Content graphic 13x4mm (300 x 300 DPI)


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Surface-Initiated Controlled Radical Polymerization Approach To

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