Improved Cellulose Nanofibril Dispersion in Melt-Processed

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Improved cellulose nanofibril dispersion in meltprocessed polycaprolactone nanocomposites by a latexmediated interphase and wet feeding as LDPE alternative Giada Lo Re, Joakim Engstrom, Qiong Wu, Eva Malmström, Ulf W. Gedde, Richard T. Olsson, and Lars A. Berglund ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00376 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Applied Nano Materials

Improved cellulose nanofibril dispersion in meltprocessed polycaprolactone nanocomposites by a latex-mediated interphase and wet feeding as LDPE alternative Giada Lo Re1*, Joakim Engström2,3, Qiong Wu1, Eva Malmström3, Ulf W. Gedde4, Richard T. Olsson4 and Lars Berglund1,2 1

Division of Biocomposites, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden 2

Division of Coating Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden 3

WWSC Wallenberg Wood Science Center, Teknikringen 56, Stockholm, SE-100 44, Sweden

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Division of Polymeric Materials, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden *Corresponding Author: * Giada Lo Re; Email: [email protected]; Ph: +46-8-790 8037; Fax: +46 8 207865 KEYWORDS: wet feeding, melt processing, enzymatic cellulose nanofibrils, latex, nanocomposite, nanocomposite rheology, interface compatibilization

ACS Paragon Plus Environment

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ABSTRACT. This work reports the development of a sustainable and green one-step wet feeding method to prepare tougher and stronger nanocomposites from biodegradable cellulose nanofibrils (CNF)/polycaprolactone (PCL) constituents, compatibilized with RAFT-mediated surfactant-free PMMA latex nanoparticles. When a PMMA latex is used, a favorable electrostatic interaction between CNF and the latex is obtained, which facilitates mixing of the constituents and hinders CNF agglomeration. The improved dispersion is manifested in significant improvement of mechanical properties compared with the reference material. The tensile tests show much higher modulus (620 MPa) and strength (23 MPa) at 10 wt.% CNF content (compared to the neat PCL reference modulus of 240 MPa and 16 MPa strength), while maintaining high level of work to fracture of the matrix; seven time higher than the reference nanocomposite without the latex compatibilizer. Rheological analysis showed a strongly increased viscosity as the PMMA latex was added, i.e. from a well-dispersed and strongly interacting cellulose nanofibrils network in the polycaprolactone.

Introduction Plastic waste is a major global challenge and from the 8.3 billion tons of plastic produced it is estimated that as much as 6.3 billion tons are accumulated in landfill or in nature, including the marine environment.1 Biodegradable plastics and nanocomposites are therefore replacement candidates, but their performance needs to be competitive with established non-biodegradable polymer systems. Cellulose nanocomposites are of interest in this context but successful dispersion of cellulose nanocrystals (CNC) or cellulose nanofibrils (CNF) in biodegradable biopolymers is still challanging.2-5 Among the major biopolymer thermoplastics, poly(εcaprolactone) (PCL) shows facile melt-processing and high deformability similar to that of the


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low-density polyethylene (LDPE).6, 7 However, even with significant progress in producing PCLbased materials, the higher cost of PCL compared to LDPE and these polymers are also limited by low modulus (PCL: 250-420 MPa, LDPE: 200-400 MPa)7 and low tensile strength (PCL: 1627 MPa, LDPE: 8-23 MPa).7 In fact, even a partial replacement of LDPE with more sustainable nanocomposites could have a large scale impact, since the LDPE worldwide market has reached a volume of about US$33 billion in 2013.8 Efforts to improve mechanical properties and biobased content are therefore of interest. Melt blending of PCL and nanocellulose has been used to improve the mechanical performance of PCL-based materials while maintaining its biodegradable characteristic.9-12 Polymer composites based on hydrophobic PCL and dry nanocellulose by melt compounding show cellulose agglomeration and most likely poor adhesion between nanocellulose and the PCL matrix.13-14 A feasible strategy for melt compounding is to perform wet feeding of the cellulose component. Wet feeding was evaluated for pulp/PCL based composites for cellulose contents ranging from 10-20 wt.% during feeding.15 Attempts to transfer this approach to nanostructured cellulose failed and resulted in composites with inferior properties compared to the pulp-based ones. The reason was the low content of nanocellulose (0.5-1,5 wt.%) in the water dispersions, inefficient mixing, and thus nanocellulose ended up in agglomerated state. In contrast, Yano et al.16 prepared well-dispersed composites based on 10 wt.% cellulose nanofibril (CNF) surfacemodified by ester group, in high-density polyethylene (HDPE). The mechanical performance of the composites was much improved compared with the reference material. Spinella et al.17 demonstrated that surface modification of CNC is critically important in engineering CNCpolymer nanocomposites with optimal performance properties.


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The use of a toluene borne surfactant in a multi-step approach was also demonstrated as an approach to improve the dispersion of nanocellulose in polypropylene, where the material was solvent-cast prior to melt-processing.18 Low molar mass surfactants, however, often induce a plasticization of the polymer during high temperature processing.19 Migration of these substances can also cause aging effects lowering mechanical properties.20 To some extent, the drawbacks of surfactant-mediated dispersion can be overcome by using higher molar mass polymeric surfactants.21 Amphiphilic design of diblock copolymers is often an efficient compatibilization strategy for immiscible polymer blends,20 where a substantially higher molecular weight is targeted than for traditional surfactants. In the composite context, amphiphilic copolymer can be designed with a hydrophilic block, which interacts with the cellulose by forming an electrostatic or noncovalent “anchor” bonding.22,

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The hydrophobic tails could provide compatibility with the polymer

matrix, assisting in the dispersion. Recently, Yano and co-workers reported surface engineering of cellulose nanofibers by adsorption of diblock copolymer dispersant for nanocomposites.24, 25 Their approach at 10 wt.% CNF, resulted in substantial mechanical reinforcement showing a doubling of the Young’s modulus and the tensile strength compared with neat HDPE. However, the adsorption of different diblock copolymers on the CNF surface required the use of a normal pyrrolidone (NMP – solvent carrier)/water emulsion stabilized by a diblock copolymer as poly(lauryl methacrylate)-block-poly(2-hydroxyethyl methacrylate) (PLMA-b-PHEMA).24, 25 An alternative approach, avoiding the solvent carriers for the diblock copolymers, is to use block copolymers with hydrocarbon tails such as polymethyl methacrylate (PMMA) connected to a poly(dimethylaminoethyl methacrylate) (PDMAEMA) prepared by RAFT-mediated surfactant-free emulsion polymerization.22, 23 The chain extension of the PDMAEMA during the


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polymerization of MMA results in stable spherical latex nanoparticles in water. This latex had a hydrophobic core of PMMA and a positively charged corona of PDMAEMA for favorable electrostatic interactions with the weakly electronegative cellulose surface.22, 23 This study reports a one-step wet feeding method to prepare cellulosic CNF/PCL nanocomposites compatibilized by an amphiphilic latex. The effect of the wet feeding and the compatibilization on the composite morphology was assessed, and this influenced the work of fracture of the composites. The content of CNF in the composites was varied between 0 and 20 wt.% and the resulting thermal and mechanical properties of the composites were assessed by DSC, TGA, tensile testing and DMA. Moreover, the melt rheology of the composites was characterized. Oscillatory rheology is a sensitive method to study the structure of complex fluids like polymer carbon nanotube nanocomposites.26, 27 Dynamic moduli showed that the viscosity increased strongly as interconnected structures of the anisometric fillers formed a network in the polymer melt. The present rheological measurements show strongly increased viscosity in CNF/latex compositions with better dispersed CNF. The relationships between the synergistic effect of the CNF and latex in the nanocomposite melt and the rheological percolation threshold, together with the changes in the frequency dependent rheological properties are discussed. Finally, it is demonstrated that the latex as amphiphilic diblock copolymers improved the mechanical properties and caused increased viscosity, which was due to improved dispersion of the nanofibrils in the PCL. In combination with wet-feeding of CNF, this doubled the modulus and resulted in an almost ten-fold increase in work to fracture with respect to that of the CNF/PCL composite without copolymer latex.


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Experimental Materials and methods PCL grade Capa™ 6506 in powder form (Mw 50000 g/mol, melt flow index = 11.3-5.2 g/10 min with 2.16 kg, 1 inch PVC die at 160°C) was kindly provided by ©Perstorp Holding AB, Sweden. N,N-Dimethylaminoethyl methacrylate (DMAEMA, Aldrich, 98%), methyl methacrylate (MMA, Acros, 99%), 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA, Aldrich, 97%), hydrochloric acid (HCl, VWR Prolabo, 35 wt.%, technical grade) and 1,3,5-trioxane (Aldrich, ≥99%) were used as received. Water was deionized prior to use (MilliQ Water purification system, Millipore). The RAFT agent, 4-cyano-4-thiothiopropylsulfanyl pentanoic acid (CTPPA), was prepared according to Boursier et al.28 by reacting 4,4-azobis(4cyanopentanoic acid) (ACPA) with bis(propylsulfanylthiocarbonyl) disulphide.

Preparation of enzymatic CNF CNF was prepared by enzymatic pretreatment of never-dried pulp (supplied by Nordic Paper, Sweden) containing 13.8 wt.% hemicelluloses and 0.7 wt.% lignin according to Henriksson et al.29 Briefly, the process included enzymatic (Novozym 476) pretreatment of the pulp followed by eight passes through a microfluidizer (Microfluidics Inc., USA). After disintegration, CNF was obtained as a viscous gel with ∼1.6 wt.% dry content. The morphology of CNF was assessed by TEM analysis and the diameter distribution of the fibrils had an average of 11.6 nm and a standard deviation of 4.1 nm, as shown in Figure S1.

Preparation of PMMA latex with RAFT-mediated surfactant-free emulsion polymerization A surfactant-free latex was designed, having a hydrophilic cationic corona and a hydrophobic


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core, where the cationic part could interact strongly with the anionic surface of the CNF and a to provide a favorable dispersion in the polymeric matrix, especially in the in the extruder.22,

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Moreover, a high glass transition temperature (Tg) for the diblock was targeted as critical to control the viscoelastic property of the system at the interface, as recently demonstrated by Sakakibara et al.25 The molecular mobility of the adsorbed polymeric dispersant on the CNF surfaces is affected by both its Tg and the magnitude of the stress transfer at the interface between the dispersant and the polymer matrix. The PMMA latex was obtained using a preformed PDMAEMA-based macroRAFT, synthesized in water according to a previously reported procedure.22, 23 In a typical procedure, the macroRAFT agent (0.150 g, 38 µmol for DP=25) was added to a 25 mL round bottom flask equipped with a magnetic stir bar followed by the addition of deionized water (13.2 mL). The monomer, MMA (2.56 g, 25.6 mmol) was added. An aqueous solution of the initiator 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA) (3.4 g L-1) was added to the reaction mixture (1.19 mg, 4.4 µmol in 1:8.25 molar ratio to the macroRAFT). The flask was placed in a water/ice bath and the reaction mixture was degassed with argon for 30 minutes and thereafter immersed into an oil bath which was pre-heated to 70 °C. The reaction was conducted for 2 h. The conversion of monomer to polymer was monitored by gravimetric analysis of the dry content by withdrawing samples during the reaction. Latex were analyzed by nuclear magnetic resonance (1H-NMR) spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC). The latex produced from the PDMAEMA (DP=25) macroRAFT, with a targeted DP of 705, showed a conversion above the 90% (with a dry content 16.7 wt.%). The average molar mass Mn of 214 000 g/mol (polydispersity index of 1.79) was assessed by DMF-SEC. The average diameter measured on crude latexes (diluted in MilliQ


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water at around 3 g/L) with dynamic light scattering (DLS) showed 103 nm (polydispersity index of 0.051), while the average diameter of the dried latex assessed from 100 particles by FESEM micrography was 63 (± 10) nm. The onset and the melting temperatures were assessed by DSC on freeze-dried latexes as 115 and 125 °C, respectively. Figure 1 shows the latex made with RAFT-mediated surfactant-free emulsion polymerization of MMA using a DMAEMA-based macroRAFT.

Figure 1. The latex formation schematic (a) with SEM image of the latexes (b), after drying on gold surface in ambient conditions at low concentration.

Fabrication of CNF/PCL based nanocomposites Figure 2 shows a sketch/overview of the method used to prepare the CNF/PCL based composites. Briefly, prior to extrusion, PCL in micrometric powder form using different amounts (70-100 wt.% in relation to CNF) was added to a water dispersion of enzymatic CNF or a premixed water dispersion of enzymatic CNF with crude PMMA latex under magnetic stirring.


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The initial dry contents of the CNF water dispersion and latex water dispersion were 1.5 wt.% and 15 wt.%, respectively. The final amount of latex was matched to reach 10 wt.% dry weight content of the total composite, after water evaporation during the processing. The final amount of CNF was targeted ranging between 0 and 20 wt.% of the total composite mass. The fraction of water was reduced to 50 wt.% by evaporating under a fume hood. The 50% water dispersions were melt-blended at 120 °C using a DSM twin-screw micro-compounder (DSM, Holland, Explore, 15 cc). The feeding was carried out at 30 rpm for 5 min and then at 100 rpm for 10 min, to make sure that the screw force, recorded during the processing, reached a constant value for all the materials. This was assumed to represent complete evaporation of water during the processing. After compounding, according to the standard ISO 527-2, dumbbell shaped (1BA) specimens (60x10x1 mm) and disks (25 mm in diameter, 2 mm thickness) were prepared by injection molding using a HAAKE™ MiniJet-Pro (Thermo Fisher Scientific) with the injection pressure of 1000 bar and an oven temperature of 120 °C and mould temperature of 40 °C. The composition of different samples, their initial and final water content, and acronym are displayed in Table S1.

Scanning and transmission electron microscopy High resolution scanning electron micrographs were acquired by using a Hitachi SEM S-4800 (Japan) with an accelerating voltage of 1 kV. For the enzymatic CNF used in these composites and mixtures of CNF-latex, SEM samples were prepared from a water mixture (CNF:latex=1:1 wt.%) before the processing. Composite samples were also analyzed after cryo-fracture as well as cryo-microsection of injected dog bone shaped specimens. The samples were conditioned in dry nitrogen at -100°C, cryo-fractured or cryo-microsectioned using RMC MT-XL


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ultramicrotome, and finally Pt/Pd (60/40) sputtered with for 20 s at a current of 80 µA using a Cressington 208HR sputter coater prior to imaging. Transmission electron microscopy (TEM) imaging were performed using a Hitachi HT7700 TEM at 100 kV accelerating voltage. Mixtures of CNF-latex aqueous dispersion were deposited onto hollow carbon-coated 400 mesh copper grids (TED PELLA, USA) and examined in the microscope after drying. The same sample was finally analyzed after annealing at 120 °C for around 20 min to mimic the thermal conditions prevailing during processing.

Thermal characterization The thermal properties of the composites were assessed by using a Mettler Toledo TGA/DSC1 under nitrogen atmosphere. For the DSC run, a heating/cooling/heating procedure to delete the thermal history over a temperature range from room temperature to 140 °C, then to -80 and again to 140 °C, at heating/cooling rate of 10 °C/min. The glass-transition inflection point temperature (Tg) and the starting of the inflection in the region of the glass transition (Tonset), crystallization peak temperature (Tc), melting peak temperature I and melting enthalpy (∆Ηm) were determined from the second heating. The TGA on all composite samples were measured with one heating ramp from 70°C to 550°C at 10 °C/min.

Mechanical characterization Tensile tests of the PCL-based composites were performed on samples conditioned for 100 h at 23 °C and 50% RH using a Single Column Table Top Instron 5944 tensile micro tester with a load force of 2 kN according to ASTM D638-14. Tensile testing was performed with a gauge length of 30 mm and a deformation rate of 30 mm/min. Five replicates were performed for each


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formulation and the data scattering was ca. 10 %. The PCL-based composites were analyzed by dynamical mechanical analysis (DMA) on samples conditioned for 100 h at 23 °C and 50% RH using a Q800 DMA apparatus from TA Instruments, accordingly with the ASTM standard D5023-07. The DMA measurements were carried out in three-point bending mode, at a constant frequency (1 Hz), amplitude of 40 microns, a temperature range from -80 to 50 °C, and with a heating rate of 2 °C/min. Three replicates were performed for each composite formulation and the data scattering was ca. 5 %.

Rheological characterization The viscoelastic behavior of the composites was analyzed by a dynamic oscillatory rheometer in the molten state. A controlled strain rheometer (DHR-2 rheometer, TA Instruments) equipped with a 25 mm diameter parallel plate geometry was employed for the rheological tests. Disks were directly loaded and molten between the plates and rheological tests were carried out at 120 °C with a gap distance of 1.5-2 mm in nitrogen. First, oscillatory amplitude stress and strain sweep test were performed from the initial stress value of 10 to 1200 Pa and strain value of 1x105

to a final strain value of 2 rad, with the frequency of 0.628 rad/s at the processing temperature

(120°C) to determine the linear viscoelastic region of the samples. Complex modulus (G*), storage modulus (G') and loss modulus (G″) were recorded as a function of stress (τ) and shear strain (γ), respectively, and value of τ0 = 200 Pa and γ0 = 0.1 rad were applied in the frequency sweep tests. In frequency sweep test, a small oscillatory amplitude strain, γ = γ0sin(ωt) was applied to the samples. The shear stress was expressed as: = 0 sin + ′′ cos

(1)


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Moduli (G*, G'), complex viscosity (η*) and the phase angle (δ) were measured as a function of angular frequency (ω) in the range of 0.01–100 rad/s at τ0 and γ0, stress and strain values in the linear viscoelastic region.

Solid surface energy of composite components In order to calculate surface energy for the composite materials, contact angle measurements were performed at 50% RH and 23 °C with a KSV instrument CAM 200 equipped with a Basler A602f camera, using 2 or 3 µL droplets of MilliQ water or diiodomethane. A Young-Laplace fitting mode, supplied by KSV was used to process the images. The contact angle values reported were those observed after 20 s of measurement when the drop had reached its equilibrium spreading on the substrates. Seven contact angle measurements were performed for each liquid and each surface. Dispersive (γsd) and polar (γsp) components of surface energy were determined by using the assessed average contact angles, according to the two-component model developed by Owens and Wendt.30 !

"/$

' "/$

/ !

= ½

&

!

"/$

+() / *

(2)

wherein: γL is the overall surface tension of the wetting liquid, (γLd) and polar (γLp) the dispersive and the polar components of the wetting liquid, respectively, γS is the overall surface tension of the solid, (γSd) and polar (γSp) the dispersive and the polar components of the solid, respectively, and θ the contact angle between the liquid and the solid. Relatively smooth surfaces of the components and composite mixtures were prepared same as for compounding procedure, the mixture of latex and CNF (approx. 1:1 wt.% mix) was also premixed and dried to around 50% dry content, but instead placed on Teflon plastic films and


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treated by hot-pressing with 20 kN at 120 °C for 20 minutes. The PCL reference measurement was performed on the sample from injection molding, due to the formed smooth polymer surface.

Structural characterization Size exclusion chromatography (SEC) was performed using Tetrahydrofuran (THF, Fischer Scientific, HPLC grade) as the eluent at a flow rate of 1.0 mL min-1, and the injection volume was 50 mL. THF SEC was performed with a GPCMAX and autosampler from Malvern Instruments equipped with RI detector and ran at 35 °C with a flow rate of 1.0 mL min-1 using guard column TGuard and two linear mixed bead columns LT4000L, conventional calibration with polystyrene standards. Samples of PCL matrix were analyzed before and after the processing sequences. Using dynamic light scattering (DLS), the hydrodynamic diameter (DH), and polydispersity index (PdI) of the latex particles were determined with a Malvern Zetasizer NanoZS at 25 °C. For the particle size measurements (DH and PdI) two concentrations were used (3 g/L) and the particles were diluted in either pure MilliQ water or 5 mM sodium phosphate buﬀer or 1 mM KCl.

Results and discussion Wet-feeding of the PCL/Latex/CNF blends Figure 2 shows the approach for wet-feeding of CNF /Latex/ PCL composites, which allows for systematic and reliable preparation of samples of different compositions (Table S1). The illustrated 1-step wet-feeding approach (Figure 2) was developed to reduce the number of processing steps before the melt processing. The amphiphilic PMMA-latex, Figure 2, was


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designed to interact with the hydrophilic surface of the CNF in order to favor the premixing with the water dispersion of CNF.22, 23 The slightly anionic surfaces of CNF, with a charge density around 120 ueq g-1,31 could interact with the cationic corona of the latex. Pre-mixing the latex with the never-dried CNF gel, ensured minimal nanofibril aggregation in the water dispersion. The large surface area of CNF promotes the adsorption. Electrostatic interaction between CNF and the positively charged corona of the spherical latex micelles is present in the water phase (Figure 1a). FE-SEM were carried out to analyze the interactions CNF-latex, by drying the mixtures before imaging. The micrographs (Figure S2) confirmed a evenly dispersed latex in the CNF network.

Figure 2. The water dispersed latex nanoparticles were mixed with CNF gel, followed by a partial evaporation of the water phase to a solid content of ca. 50 wt.%, prior to the wet feeding. The residual water required 15 min at 120 °C for evaporation during the compounding, the materials were then injection molded into dumbbell shaped samples.

After mixing the latex suspension with the CNF suspension at 1.6 wt.%, the amount of water was reduced to 50 wt.% by evaporation (≈ 24h at room temperature), which allowed for an


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optimized concentration that was useful for an effective wet-feeding approach (Figure 2). Excessive amounts of water (over 50 wt.%) resulted in inefficient melt processing. High quality specimens were produced after 15 min of melt compounding, followed by about 5 min of injection molding. Dumbbell shaped specimens, and injection molded bars and disks (not shown), were used. No signs of significant thermomechanical degradation were observed, and was confirmed by a structural investigation of the molecular weight and polydispersity of the polymer matrix, in Figure S3 and Table S2, as well as by thermogravimetrical analysis, in Figures S4-S7 and Tables S3, S4. A 10 wt.% content of CNF was considered a minimum amount of nanofibrils to exploit a significant enhancement in mechanical properties.11

Mechanical and rheological properties of the polymer composite blends Figure 3 shows the tensile stress-strain curves, and Table 1 shows a summary of the mechanical property results. The stress as a function of the strain of the neat PCL showed a behavior of a ductile semicrystalline polymer matrix, above its Tg. The tensile tests were carried out at room temperature (23°C), which is significantly over the glass transition temperature of the matrix (Figure S8 and Table S5), resulting in a pronounced yield stress followed by an orientation of the polymeric chains in the stress direction, and finally to a strain hardening region (elongation at break ≈ 1500%, here not shown for the sake of figure effectiveness in the elastic region). An addition of 10 wt.% latex to the PCL matrix (in absence of cellulose) resulted in an increase in modulus and yield stress by 30 % and 6 %, respectively. PMMA was of high molar mass (high degree of polymerization, DP=25) and showed good miscibility with PCL, since only one glass transition could be observed by DSC analysis (Figure S8 and Table S5). The DSC


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analysis also showed that the crystallinity only changed marginally (within one percent), so that effects on PCL crystallinity could not explain the improved mechanical properties.

Figure 3. Representative tensile stress-strain and DMA curves of the composites and the corresponding polymer matrices.

The addition of 10% CNF to the PCL matrix (in absence of latex) resulted in increased strength and modulus compared to the neat PCL polymer, whereas the elongation at break was severely decreased and occurred at ca. 40%. However, the data suggested fairly good dispersion of the CNF in the PCL matrix, only as a result of the wet-feeding. Previously reported nanocellulose


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composites from traditional dry-feeding of the cellulose component typically show strain values at break lower than 10 %.11

Table 1. Tensile properties of the different materials (Young’s modulus, ultimate strength, strain to failure and work to fracture). Sample

Eyoung [MPa]

σ [MPa]

ε [%]

Work to fracture [MJ m-3]

PCL

240 ± 10

16.3 ± 0.3

1470 ± 60

316 ± 10

10%latex/PCL

310 ± 10

17.2 ± 0.3

1150 ± 50

222 ± 6

10%CNF/PCL (dry feed-df)

280 ± 10

15.7 ± 0.6

7.4 ± 0.3

0.8 ± 0.1

10%CNF/PCL (wet feed-wf)

350 ± 20

17.2 ± 0.9

40 ± 4

7 ± 0.3

10%CNF/10%latex/PCL (wf)

620 ± 20

23.2 ± 0.7

280 ± 10

51 ± 1.5


840 ± 30

27.2 ± 0.8

13 ± 3

3 ± 0.1

Lower properties of the composites prepared by dry-feeding can be assigned to the aggregation of the individual fibrils. This resulted in reduced CNF reinforcement efficiency. The consequence is reduced work to fracture and elongation at break, even at low cellulose content. This highlights the potential of wet-feeding for CNF compounding. Using the same CNF (as above) without latex during dry feeding resulted in Young’s modulus, maximum tensile strength, strain and work to fracture of 280 MPa, 15.7 MPa, 7.4% and 0.8 MJ m-3, respectively. The effect of the latex on the CNF dispersion was investigated at 10 wt.% CNF content. For the 10 wt.% CNF (10%CNF/10%latex/PCL) a significantly improved work to fracture was observed compared to the 10 wt.% CNF-based composites without latex. A 77% increase in Young’s modulus was measured, while the tensile strength and elongation at break simultaneously


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increased 35% and 655%, respectively. The work to fracture value increased ca. 7 times when using the latex as a compatibilizer between the neat components of CNF and PCL. The increased ductility can be interpreted as a result of improved dispersion. The positively charged amines of the copolymer may induce a dynamic dipole ionic bonding, as recently demonstrated by Odent et al.32 It is suggested that the increase in stiffness originated from better dispersion and more efficient CNF reinforcement. This may be mediated by the locally higher Tg, where the PMMA chains would entangle with PCL macromolecules, as previously demonstrated for CNF-HDPE nanocomposites.25 For the 20 wt.% CNF, these values increased an additional 35% and 17%, respectively. This suggested that although a stiffer and stronger composite was formed, the full potential of the CNF to reinforce at filler content of 20 wt.% was limited by CNF aggregation. Previously, Yano et al. reported an efficient method to compatibilize the cellulose nanofibers with a water insoluble high-density polyethylene (HDPE) matrix.24, 25 A multistep approach to adsorb a non-water soluble block copolymer, as a dispersant, onto the CNF surfaces resulted in a significantly improved modulus (140 %) but a dramatically decreased ductility of the polymer matrix, i.e. a reduced work to fracture for the nanocomposites. The procedure relied on the use of organic solvent, which is a limitation for development of environmentally sustainable processing. To further confirm the improvement of the mechanical performance of the nanocomposite, thermal-dynamical mechanical measurements (DMA) in three-point bending mode were carried out on the injection-molded bars. The results are summarized in Table 2, and Figure 3 shows flexural storage modulus, the loss modulus, and the tan delta evolution as a function of temperature.


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Table 2. Main DMA results for different compositions. a

Sample

G' at -80°C [MPa]

G″ at 20°C [MPa]

Tg [°C]

PCL

4030 ± 40

570 ± 5

-54 ± 1

0.1 ± 0.002

10%latex/PCL (wet feed-wf)

3440 ± 50

560 ± 5

-53 ± 1

0.09 ± 0.001

10%CNF/PCL (wf)

4980 ± 140

840 ± 14

-53 ± 1

0.09 ± 0.004


6490 ± 60

1690 ± 10

-51 ± 1

0.07 ± 0.001


6860 ± 100

1970 ± 22

-52 ± 1

0.06 ± 0.001

b

DF

a

Estimated as the peak of loss modulus according to the standard ASTM D4092-07. bDamping Factor (DF) assessed as the peak of the tan delta according to the standard ASTM D4092-07.

The neat PCL matrix displayed the typical behavior of a semicrystalline polymer. For temperatures below the glass transition temperature (Tg), the modulus remained approx. constant around 4 GPa. A significant drop of the flexural modulus followed the transition from the glassy state (-54°C, consistent with the Tg assessed by DSC analysis, Figure S8 and Table S5). The addition of the 10% of latex or CNFs in the nanocomposite (not together) provided a moderate reinforcement, confirming a relatively good dispersion of the fibrils via the wet feeding approach. When latex and CNF are premixed and then incorporated in the system, the flexural storage modulus increased compared to the CNF/PCL nanocomposite in both the glassy and rubbery regions. In particular, the evolution of G' as a function of temperature displayed a significant change above the glass transition, with a different slope of the curve above Tg compared to the references. As the temperature increased, the flexural storage modulus decreased more slowly. A corresponding increase in the glass transition for the latex compatibilized nanocomposite (Table 2), indicates constrained mobility of the PCL molecular chains. The results suggest that the latex prevented the CNF aggregation and mediated between


19


Page 20 of 33

the surface energies of the hydrophilic CNFs and the hydrophobic PCL matrix. The slight increase in storage modulus registered for the 20%CNF/latex/PCL composition suggests that more addition of nanofibrils would not result in any further confinement of the mobility of PCL matrix. Contact angle measurements against water were carried out for the different systems to assess their surface energies after annealing for 20 min at 120 °C, as shown in Table 3. Moreover, the surface tension of the different components can be an effective way to evaluate the compatibility of the different interfaces.24, 25

Table 3. Average contact angles (with the standard deviation) and the assessed surface energies using either MilliQ water or diiodomethane (CH2I2) for the solid surfaces, determined using the following parameters: γwaterd = 21.8 mN m-1; γwaterp = 51.0 mN m-1; γwater = 72.8 mN m-1; γCH2I2d = 49.5 mN m-1; γCH2I2p = 1.3 mN m-1; γCH2I2 = 50.8 mN m-1.21

+ ,- .

+/- 0,-

132

14 2

156578 2

(°)

(°)

(mNm-1)

(mNm-1)

(mNm-1)

latex/CNF (1:1)

70 ± 7

44 ± 5

8.0

37.5

45.6

CNF

39 ± 5

40 ± 4

8.0

39.5

47.5

PCL

79 ± 5

39 ± 4

3.7

40.1

43.8

latex

76 ± 8

56 ± 6

7.2

30.7

37.9

Sample

Contact angles measurements of water on the surfaces latex adsorbed on CNF (latex/CNF) showed a significant hydrophobization effect of the latex, with θ ≈ 70° compared to the 40° recorded for the pristine CNF.33 Observation regarding the surface energies of the


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different component in the system can be formulated taking advantage of the two-component model of Owens and Wendt. 30 The PCL sample showed a γs value of 43.8 mN m-1, which was significantly lower than the value assessed for the CNF surfaces (γs = 47.6 mN m-1). The polar component of the PCL matrix, γsp = 3.7 mN m-1 indicated that the nonpolar hydrocarbon groups were accumulated on the surface and generated a hydrophobic surface, whereas the CNF reference surface showed a polar tension of 8.0 mN m-1 that was attributed to the presence of oxygen. The rather high polar character and low dispersive energy of the annealed latex was attributed to the amphiphilic nature of the latex di-block polymer (compared to the PCL matrix) containing the PDMAMEA cationic corona, displayed as 37.9 mN m-1. A favorable interaction between the CNF and the latex was confirmed by the value of the polar component of the mixed latex/CNF system (1:1 wt.%) and showed as a value of 8.0 mN m-1, which was very close to the CNF and the latex as pristine phases, (Table 3). It is suggested that this mediated value stemmed from the formation of two interfaces related to the latex presence in between the PCL and the CNF. On one end, a dynamic ionic bonding between the positively charged cationic corona of the latex and the negatively charged CNF surface and on the other end, the PMMA latex tails facing towards PCL chains.22, 23 This was supported by the fact that γstotal of the CNF/latex showed a value of 45.6 mN m-1, which is close to the value of the PCL (43.1 mN m-1). Figures 4a-c show the complex viscosity and the storage G' modulus as a function of increasing angular frequency for the prepared materials in their molten state. The neat PCL displayed a Newtonian plateau that remained constant within the measured frequency range, Figure 4a. The viscoelastic behaviour of the PCL was however significantly affected by the


21


Page 22 of 33

combined presence of the latex and CNF together, no longer displaying the constant Newtonian plateau over the frequency range, (Figure 4a).

Figure 4. Complex viscosity (a), viscoelastic storage modulus G' (b), viscoelastic loss modulus G″ (c) and the van Gurp-Palmen plot as a function of the angular frequency ω (d), recorded during the frequency sweep tests in the molten state (T = 120°C) for the different composites.

Instead, a liquid-solid transition was apparent, which suggested and the formation of strongly interacting CNF fibrils or a three-dimensional CNF network, also defined as rheological percolation threshold. The percolation threshold, which is assumed to be mainly related to the geometrical percolation of the nanofibrils, relates to the change in rheological properties of a filler network embedded in a viscoelastic liquid is also called ‘liquid-solid transition’.26


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The shear storage moduli values (G') displayed a small reinforcing effect for the PCL containing only CNF. In contrast, a stronger reinforcing effect could be observed for the PCL containing both latex and CNF (Figure 4b). The shear loss moduli (G'') showed a smaller slope for the CNF/latex/PCL systems, also indicating a more rigid network (Figure 4c). This reinforcing effect was more prominent at lower frequencies. This phenomenon, called ‘liquid-solid transition’ is a feature extensively described by Winter et al.34 The addition of the premixed latex/CNF to the PCL lead to a synergistic non-linear increase of viscosity and moduli, also highlighting a pronounced shear thinning behaviour of the complex melts. Similar behaviours have been reported for multiwall carbon nanotube (MWNT) filled polymer composites, as polyamide 6/MWNT and polycarbonate/MWNT composites, and was described as the rheological and electrical percolation threshold. 26, 34-36 Figure 4d shows the Van Gurp-Palmen plot of phase angle δ versus the complex modulus G* for the unfilled PCL and the polymer PCL containing 10wt.%latex, together with the composites with cellulose nanofibrils. The low-frequency δ of the unfilled polymers (in absence of CNF) is close to 90 degrees, which is indicative of a typical flow behavior of a viscoelastic fluid. The addition of the latex with the CNF nanofibrils, for both the 10 and 20% of CNF/latexbased composites, showed a remarkable decrease in the phase angle δ below 45°, indicating a clear rheological liquid-solid transition in these composite systems.37 The rheological percolation threshold for the composites was achieved by use of the copolymer latex.

Morphology of the prepared nanocomposites Figure 5a shows a scanning electron micrograph of the synthesized latex spheres mixed with the cellulose nanofibrils in a weight ratio of 1:1, as dried from a water-dispersion.


23


Page 24 of 33

Figure 5. Morphology of the latex-CNF water dispersion used for the wet feeding approach, prepared at and dried at room temperature: SEM image (a), and TEM image (b) of the CNF/latex premix, dried at room temperature prior to observation. A simulation or the thermal treatment during the melt processing (compounding + injection moulding) on the TEM grid have been performed to follow the melting of the PCL and latex in mixture with CNF. A schematic of the simulation is reported in (c), and (d) shows the morphology of CNF/latex/PCL after annealing.

A sample with the same latex/CNF composition is shown in Figure 5b after being deposited on a TEM grid from a more dilute dispersion. The aqueous mixing of the latex with the CNF had no


24

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impact on the spherical morphology of the latex nanoparticles, and they dispersed as an intercalated phase within the CNF fibril network. Figure 5c displays a sketch of the in-situ formation of the latex/CNF nanohybrid on a TEM grid as a result from the storage at 120 °C for 20 min, a condition consistent with the heat exposure of the material in the micro-compounder during the processing (Figure 5d). The evenly embedded CNF phase morphology in the partially melted latex, on the TEM grid, are in line with the latex/CNF nanohybrids surface energy analysis. One may speculate that the PDMAEMA spheres collapsed with their cationic corona oriented towards the CNF surface, whereas the PMMA tails point towards to the exterior after the thermal treatment.23 The latex copolymer particles were beneficial for phase compatibility between the latex and the CNF surface, and prevented aggregation of the CNFs.

Conclusions In this study, polycaprolactone (PCL) cellulose nanocomposite specimens were produced during 20 minutes of melt processing. Wet feeding of a premixed cellulose copolymer latex PCL material was carried out using an aqueous dispersion. The combination of wet feeding and the copolymer compatibilizer in latex form, resulted in well-dispersed CNF fibrils and strongly improved mechanical properties. The wet feeding approach by itself resulted in improved mechanical properties of the CNF/PCL nanocomposite, with an 8% increase in the Young’s modulus, 9.6% increase in maximum strength, while the work to fracture increased 8 times compared with traditional dry feeding. The Young’s modulus, tensile strength and work to fracture of the 10 wt.% CNF/PCL increased ca. 80%, 35%, and 650%, respectively, when an amphiphilic latex compatibilizer was used to mediate the CNF/PCL interface. This improvement


25


Page 26 of 33

in the mechanical performance of the nanocomposites was due to strongly improved dispersion of the CNF, as supported by microscopy data and strong rheology effects from addition of the copolymer. The frequency sweep results showed a liquid-solid transition, corresponding to the formation of strong CNF interaction or a three-dimensional CNF network for the nanocomposites containing the latex. Transmission electron microscopy confirmed the beneficial interaction between the cellulose nanofibrils and the amphiphilic latex. The morphology is in support of in-situ formation of a latex-CNF nanohybrid during the premixing in water. The present approach suggests strong mechanical property potential of PCL/CNF nanocomposites based on well-dispersed CNF fibrils, including high ductility. Furthermore, since the presented material system is based on a biodegradable polymer, combined with native CNF cellulose, the material is of interest in any application where its inherent properties could be found sufficient to replace LDPE.

ASSOCIATED CONTENT Supporting Information. Transmission electron micrograph of CNF and diameter distribution, Compositions of the CNF/latex/PCL systems before/after melt processing, SEC analysis of PCL matrix before and after melt-processing, SEM micrographs of premixed PCL/CNF/latex dispersion prior meltprocessing, TGA thermograms and main results of components and nanocomposites, DSC results of nanocomposites (PDF) AUTHOR INFORMATION Corresponding Author


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* Giada Lo Re; Email: [email protected]; Ph: +46-8-790 8037; Fax: +46 8 207865 Present Addresses †KTH Royal Institute of Technology, School of Chemical Science and Engineering, Fiber and Polymer Technology/WWSC Teknikringen 56, SE-100 44 Stockholm, Sweden. Funding Sources Financial support from VINNOVA through the BiMaC Innovation Excellence Centre and for LB from SSF grant GMT14-0036 is gratefully acknowledged.

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TOC (Table of Contents - image)

Synopsis (Table of Contents - text) A novel green method to produce PCL/CNF nanocomposites, using a water-borne latex dispersant in combination with a one-step wet feeding compounding is presented as a strategy for interface compatibilization. Enhanced nanocomposites mechanical properties are demonstrated.


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Improved Cellulose Nanofibril Dispersion in Melt-Processed

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