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Green & Efficient Synthesis of Dispersible Cellulose Nanocrystals in Biobased Polyesters for Engineering Applications Stephen Spinella, Cedric Samuel, Jean-Marie Raquez, Scott A McCallum, Richard A Gross, and Philippe Dubois ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01611 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 3, 2016

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ACS Sustainable Chemistry & Engineering

Green & Efficient Synthesis of Dispersible Cellulose Nanocrystals in Biobased Polyesters for Engineering Applications ⊥

Stephen Spinella†,‡,§, Cédric Samuel , Jean-Marie Raquez†, Scott A. McCallum§, Richard Gross§, Philippe Dubois†* †

Centre d’Innovation et de Recherche en MAtériaux Polymères CIRMAP, Service des

Matériaux Polymères et Composites, Université de Mons, Place du Parc 23, B-7000 Mons, Belgium ‡

Department of Chemical and Biomolecular Engineering NYU Polytechnic School of

Engineering, Six Metrotech Center, Brooklyn, New York 11201, USA. §

Department of Chemistry and Biology, Rensselaer Polytechnic Institute (RPI), 4005B

BioTechnology Bldg., 110 8th Street, Troy, N.Y. 12180, USA. ⊥

Mines Douai, Department of Polymers and Composites Technology and Mechanical

Engineering, 941 rue Charles Bourseul, CS 10838, F-59508 Douai, France. E-mail: [email protected]

ACS Paragon Plus Environment

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Keywords Nanocomposites, Cellulose nanocrystals, Surface modification, Poly(L-lactide), Poly(methyl methacrylate), Grafting reactions Abstract Despite attractive properties of cellulose nanocrystals (CNCs) such as high natural abundance, inherent biodegradability and high modulus, CNCs tend to degrade and aggregate when exposed to high temperatures during melt processing. In the present work, the surface of CNCs was modified with PMMA to take advantage of the miscibility with various biobased polymers including PLLA when melt-blended. A particular attention was paid to grafting techniques in water medium using two different redox initiators: Fe2+/H2O2 (Fenton’s reagent) and ceric ammonium nitrate (CAN). The successful synthesis of CNC-g-PMMA was verified by gravimetric analysis, FTIR, CP-MAS 13C-NMR and suspension tests. A high grafting efficiency of 77% was achieved using CAN as the redox initiator. Increasing the PMMA content on CNC surfaces led to higher CNC thermal stability. As a consequence of PMMA grafting in water, modified CNCs were found to be pre-dispersed in a PMMA network. PLLA/CNC nanocomposites were then prepared by melt-blending, i.e. in the absence of solvent, and the quality of the dispersion was confirmed by dynamic rheology, TEM and DMA. The presence of a high amount of PMMA grafts on CNC surfaces reduced CNC aggregation and favours the percolation of CNCs with the development of a weak long-range 3D network. Miscibility between PMMA grafts and PLLA as well as the pre-dispersion of CNCs was found to play a key role in the dispersion of CNCs in PLLA. Thermomechanical analysis revealed that PMMA grafts on CNC surfaces significantly enhanced elastic moduli in the glassy and rubbery state. The high dispersion state (related to high PMMA grafting) also showed a positive effect on O2


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permeability of PLLA and a strong beneficial effect on heat deflection temperature (HDT) reaching outstanding temperatures higher than 130 °C. Thus, free-radical grafting of PMMA in water provides an efficient and green route to dispersible (bio)nanofillers by solvent-free extrusion techniques with PMMA-miscible matrices such as PLLA for high-performance applications.


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Introduction Cellulose nanocrystals (CNC) represent a class of very attractive nanofillers given their high abundance, their renewability, biodegradability, high surface area (greater than 100 m2/g)1 decorated with reactive hydroxyl moieties and impressive tensile modulus (close to 150 GPa).2 Cellulose consists of highly-crystalline domains and early work in the 1950’s3 demonstrated the preparation of CNCs by controlled acid hydrolysis of amorphous domains.2 Depending on the cellulose source, rod-like CNC nanoparticles with lengths and aspect ratios from 100 to 1000 nm and 25 to 70, respectively, have been reported.4 During the past few decades, an increasing amount of research has focused on the combination of CNCs with thermoplastic or thermoset matrices to produce high-performances nanocomposites. Currently Poly(L-Lactide) (PLLA) represents one of the promising biobased polymer produced on an industrial scale.5,6 It has many desirable attributes including mechanical properties similar to polystyrene (PS),7 biocompatibility,8,9 degradability under industrial composting conditions,10 melt processability and high transparency. Despite these advantages, PLLA suffers from shortcomings such as poor oxygen barrier properties and a low heat distortion temperature (HDT).11,12 During the past few decades, advances in PLLA-based nanocomposites using nanofillers have led to significant improvements in PLLA thermomechanical properties.13,14 Reinforcing PLLA with CNCs represents a promising route to fully biobased materials with enhanced performances including HDT. Introduction of CNCs into (bio)degradable polyesters such as poly(ε-caprolactone) (PCL)15–17, poly(butylene succinate)18, poly(3-hydroxybutyrate-co3-hydroxyvalerate) (PHBV)19,20 and PLLA21–25 was reported. However, CNC aggregation and degradation during high-temperature extrusion have limited their use. Furthermore, a vast majority of nanocomposite preparations thus far reported were processed by solvent-exchange


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and solvent-casting.24,26 The use of large solvent quantities during CNC preparation and modification reduces their ecological benefits and, consequently, their commercialization potential. While melt processing by twin-screw extrusion is attractive for nancomposite manufacturing, dispersion of hydrophilic CNCs into hydrophobic matrices such as PLLA during melt processing remains challenging. Due to poor CNC dispersion, high CNC loadings in composites (e.g. from 5 to 10%-by-weight) are required15,21,22,27,28 to significantly improve PLLA thermomechanical properties. The observed CNC aggregation in PLLA is a direct consequence of the fact that CNC surfaces are covered with hydrophilic hydroxyl groups.29 Consequently, surface-modification of CNCs is essential to reduce CNC aggregation and enhance their dispersion during meltprocessing into hydrophobic matrices.30 Common CNC modification strategies include silylation and polymer grafting reactions.30 CNCs have been modified by silylation in citrate buffer to introduce acrylic, amino or alkyl groups on CNC surfaces. Subsequently, silylated CNCs were melt blended with PLLA leading to improved mechanical properties.25 Efficient CNC modifications may also be performed by grafting polymer chains on CNC surfaces. Polymer grafting is classically performed by either (i) “grafting onto” processes involving a direct reaction between CNC and a functional polymer such as isocyanate terminated PCL31 or amine-terminated poly(ethylene glycol) (PEG),32 or (ii) “grafting from” processes in which hydroxyl groups on CNC surfaces act as (co-)initiators for polymerization reactions such as ring-opening polymerization (ROP).33 For instance, PLLA and PCL have been grafted from CNC surfaces by ROP resulting in CNC-g-PLLA22 or CNC-g-PCL nanoparticles.15,17 Subsequent blending of CNC-g-polyester where the polyester structure is the same as that of the matrix have shown significant improvements in material thermo-mechanical properties due to


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increased modified CNC-matrix adhesion.15,21,22 Covalently-grafted polymer chains from CNC not only modified the interfacial tension with polyesters but also efficiently increase stress transfer due to co-entanglements with the matrix.22,19 However, PLLA grafting “from” reactions require strict water-free conditions and are conducted in a dry solvent to avoid the preparation of CNC-g-PLLA with large fractions of water-initiated non-grafted PLLA chains (e.g. low graft efficiency).21,22,33,34 To achieve such conditions, solvent exchange processes have been used.4,30 Our laboratory has demonstrated that melt-extrusion of PLLA/PMMA blends formed a unique miscible phase in all blend proportions.12,35,36 This discovery prompted the current work on the modification of the CNC surfaces with PMMA grafts in order to improve interfacial interactions between PLLA and the corresponding modified CNCs. If PMMA grafting from CNCs could be conducted in bulk or by aqueous processes that are scalable, and such grafts result in large improvements in interfacial adhesion between CNC-g-PMMA and PLLA, this work could have important practical implications.12,35,36 Also, since progress has been made on developing PMMA from readily renewable carbon,37 the potential future use bio-based PMMA would lead to 100% bio-based PLLA nanocomposites. Efficient redox-initiated free radical polymerization/grafting of (meth)acrylic monomers on polysaccharides (granular starch,38 microfibrillated cellulose, chitin39) could be easily performed in water.40 Common initiators, including Fenton’s reagent (Fe2+/hydrogen peroxide) and ceric ammonium nitrate (CAN), were used to graft poly(glycidyl methacrylate)41–43 and poly(2hydroxyethyl methacrylate) on cellulose fibers.43 In addition, PMMA was successfully grafted using CAN on nanofibrillated cellulose.43 Corresponding molecular weights of grafted acrylates prepared by the above free-radical methods can reach 200,000 g/mol.40 Despite these interesting results in green conditions, little is known about the processability of PMMA-modified CNCs at


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high temperature. Furthermore, the dispersion state of CNC-g-PMMA in PLLA after meltcompounding and the influence of grafting degree on the final physicochemical properties of PLLA/CNC-g-PLLA nanocomposites have not been studied. In the present work, PMMA was grafted on CNCs in water using two different grafting initiators: Fenton’s reagent and CAN. Grafting efficiencies and morphologies of PMMA-modified CNC were evaluated by FTIR, gravimetric analysis, solid-state CP-MAS 13C-NMR, WAXS, TGA and TEM. PMMA-modified CNCs were directly melt-blended with PLLA by melt extrusion to prepare nanocomposites. The dispersion state of 5%-by-wt. (based on cellulose content) unmodified and PMMA-modified CNCs in PLLA was investigated by dynamic rheology and TEM experiments. Subsequently, thermo-mechanical and thermal properties were also examined for the PLLA/CNC-g-PMMA nanocomposites. The influence of PMMA content of CNC-gPMMA on its dispersion in the PLLA matrix is also discussed. Materials and Methods Materials PLA grade 4032D was purchased from Natureworks with 1.5% D-lactide content, Mn of 135.000 ± 5.000 and dispersity (Mw/Mn, Đ) of 2. Methyl methacrylate (98%) was purchased from Sigma, filtered over aluminum oxide to remove inhibitors and stored at -4°C until use. Nitric acid, ceric ammonium nitrate (CAN, (NH₄)₂Ce(NO₃)₆), ferrous ammonium sulfate (FAS) and hydrogen peroxide (H2O2, 30% wt.) were purchased from Sigma, stored at 0°C and used as received. Preparation of cellulose nanocrystals The preparation of CNC from cotton linter follows a previously-developed literature method.44 Briefly, pressed cotton linter pieces were soaked in 2.5 M HCl overnight. Hydrolysis reactions were then conducted in a convention oven with an external temperature set at 150°C for 3 hours


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in air-tight containers (Ball® 8oz Regular Mouth Half Pint Jars) capable of withstanding the pressure buildup during the course of the reaction. The internal temperature of the reaction was measured by an internal thermocouple and was 105°C. The cellulose concentration in HCl for hydrolysis reactions was 4% (w/v). The reaction mixture was mechanically agitated in a Waring laboratory blender for 20 min. The resulting cellulose suspensions were diluted with an equal amount of deionized (DI) water and were repeatedly centrifuged at 8600 g for 3 minutes, with replacement of DI water to remove excess acid. When the pH reached approximately 5-6, the supernatant remained turbid. The resulting turbid solutions were collected and freeze-dried to obtain unmodified cellulose nanocrystal powders that are used for graft polymerizations described below. Graft polymerization of MMA on CNC with Fenton’s reagent The method for MMA graft polymerizations on CNCs using Fenton’s Reagent (H2O2 and Fe2+) was adapted from that developed by Brockway et al. for PMMA grafting on granular starch.45,46 Briefly, CNC (5.0 g) was dispersed in deionized water (200 mL) by sonication at 25 °C for 10 minutes and the solution was degassed by nitrogen bubbling at 70 °C for 30 minutes under continuous stirring. Subsequently, FAS (25 mg) was added, followed by MMA (25 g) and H2O2 (100 mg). The reaction was conducted for 120 minutes at 70 °C under nitrogen atmosphere. Graft polymerization of MMA on CNC with CAN Graft polymerizations of MMA on CNCs using CAN were performed following a literature procedure.47 In summary, CNC (2 g) was dispersed in deionized water (200 mL) by sonication at 25°C for 10 minutes and the solution was degassed by nitrogen bubbling at 35° C for 30 minutes under continuous stirring. Then, the pH of the resulting suspension was adjusted to 1 with nitric


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acid and CAN (1 g) was added followed by MMA (4 g). The reaction was conducted for 120 minutes at 35 °C under nitrogen atmosphere. Purification and gravimetric assessment of PMMA grafting Immediately after graft polymerization reactions, initiators and monomers were removed by five successive centrifugation steps at 10 000 rpm for 30 minutes using deionized water until the pH of CNC suspensions was 6-to-7. The resulting CNC-g-PMMA nanoparticles were separated from “free” PMMA chains (i.e., non-grafted PMMA) by Soxhlet extraction with acetone for 2 days. The recovered CNC-g-PMMA fraction and the “free” PMMA fraction were subsequently dried under vacuum at 60°C until constant weight. The grafting efficiency (Ge), weight fraction of grafted PMMA in modified-CNC nanoparticles after purification (WPMMAgrafted) and MMA conversion (C) for both Fenton’s reagent and CAN methods were evaluated gravimetrically according to previous reports47 and using the following equations: ( )

( !)

(1) = ( ) × 100 = (

!)"#

( )

× 100

(2) $ = (% &'&)"( ) = (3) ( =

( ) (% )

=

( !)"# )

( !)

× 100

× 100

with m1 the mass of recovered CNC nanoparticles after grafting reactions and purification by Soxhlet extraction (g), m2 the initial mass (g) of neat CNC, m3 the mass (g) of non-grafted PMMA recovered after Soxhlet extraction with acetone and m4 the initial mass (g) of MMA monomer. Isolation of grafts and size exclusion chromatography (SEC) Grafts were first isolated from CNCs by acid hydrolysis using the method described by Littunen and coworkers.47 Briefly, modified CNCs (500 mg), acetone (40 mL), tetrahydrofuran (40 mL)


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and sulfuric acid (92%, 4 mL) were stirred and refluxed overnight. The resulting slurry was precipitated into methanol, filtered and dissolved in chloroform. The chloroform-soluble fraction was recovered by filtration and dried for 2 days at ambient temperature. Analysis of the grafted polymer molecular weight was by size-exclusion chromatography (SEC) in THF (concentration 1 mg/mL, calibration PMMA standards) at 35°C using an Agilent liquid chromatograph equipped with a refractive index detector. Melt Extrusion of PLLA/CNC nanocomposites PLLA pellets and CNC nanoparticles were first dried in a vacuum oven at 60°C for 12 hours. The cellulose content of modified CNCs was fixed at 5%-by-wt. for all formulations. CNCs were dispersed directly into the PLLA matrix by melt-compounding with a DSM micro-compounder (total volume 15 cc) at 190°C. The screw speed and residence time were set to 30 rpm during the introduction step (approx. 3 min) and 60 rpm for 7 min during melt-compounding. Rectangular specimens for DMA tests (or bars) (35 mm x 12 mm x 2 mm) were prepared using a DSM micro-injection system as follows: drying overnight at 60°C under vacuum, preheating at 190°C for 3 minutes, injection molding at 190°C with a mold temperature at 60°C. Cylindrical specimens for rheology/permeability tests (diameter 35 – 50 mm, thickness 250 – 400 µm) were prepared by compression-molding at 195°C as follows: melting time for 4 min, low-pressure cycle for 3.5 min at 5 bar, high-pressure cycle for 1 min at 10 bar and cold-water cooling. FTIR analysis of CNC Fourier Transform Infrared spectroscopy (FTIR) was carried out on unmodified CNC and PMMA-modified CNC using a BIO-RAD Excalibur spectrometer with an ATR Harrick Split pea. The spectra were recorded using a spectral width of 700 to 4000 cm-1 with 4 cm-1 resolution over 16 scans.


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CP-MAS 13C-NMR of CNC Unmodified and PMMA-modified CNCs were analyzed by Cross-Polarization Magic-Angle Spinning solid-state 13C-NMR (CP-MAS 13C-NMR), according to the method published by R.E. Taylor.48 All CP-MAS

13

C-NMR experiments were performed on the 600 MHz 89 mm wide-

bore Bruker Advance III spectrometer equipped with a 4 mm HXY solid-state MAS probe (experimental conditions: 8000 scans, spinning rate 11.3 KHz, acquisition time 0.02 s and temperature 278 K). Thermogravimetric analysis of CNC Unmodified and PMMA-modified CNCs were analyzed by thermogravimetric analysis (TGA) using a TA Instruments Q500 under nitrogen flow from ambient temperature to 800 °C at a heating rate of 20 °C·min-1. WAXS analysis of CNC Wide angle X-Ray Spectroscopy patterns were obtained for CNC powders using a Panalytical X’Pert Pro X-Ray diffractometer using Cu Kα radiation (0.154 nm) at 45 kV and 40 mA. The scan rate and diffraction angle was 10°/min and 5° to 60°, respectively. The crystallinity index (CI) was calculated according to the following equation IC = 1 – (I1/I2) with I1 the intensity at 2θ = 18.8° and I2 the intensity at 2θ = 22.8°.49 TEM analysis of CNC and PLLA/CNC nanocomposites Morphology of unmodified and modified CNCs was analyzed by transmission electron microscopy (TEM). A drop of a highly diluted CNC suspension (0.001~0.003 mg/mL) was placed on a 400 square mesh copper grid with Formvoar carbon film and allowed to dry. The grid was floated in a 2% uranyl acetate solution for 3 min and then washed with DI water. Electron micrographs were then recorded with a FEI LIBRA 200FE transmission electron


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microscope at an accelerating voltage of 80 kV. Concerning PLLA/CNC nanocomposites, injection-molding and annealed specimens were ultramicrotomed at -50°C using a Leica XL microtoming instrument equipped with a Diatome Cry 35 diamond knife. The microtomed samples were then transferred to a holey-carbon coated copper grid and stained with 2% uranyl acetate. CNC dispersion in the PLLA nanocomposites was observed using a JEOL High Resolution Transmission Electron Microscope (HR-TEM). Dynamic rheology of PLLA/CNC nanocomposites Rheological experiments were performed using a parallel plate rheometer Haake Mars III from Thermo Scientific in small amplitude oscillation mode (plate diameter 35 mm, gap 300 µm). All experiments were conducted within the linear viscoelastic range in controlled strain mode (strain approx. 0.1). Samples were allowed to equilibrate for 5 min prior to measurement and frequency sweeps were performed with an angular velocity from 100 to 0.01 rad.s-1. Thermo-mechanical properties of PLLA/CNC nanocomposites Thermo-mechanical properties of nanocomposites were evaluated by dynamic mechanical analysis (DMA) carried out on a Q800 DMA from TA instruments operating in dual cantilever mode on injection-molded and annealed samples (frequency 1 Hz, amplitude 20 µm, heating rate 2°C.min-1 from 25°C to 130°C). All materials were annealed for 2 hours at 130 °C to reach constant crystallinity index in all samples and therefore discard crystallinity effects. Storage modulus values at 30°C (E’30°C) and at 100°C (E’100°C) were extracted and the α-relaxation temperature (Tα) was taken at the tanδ peak (average of 3 measurements). The HDT was evaluated according to the correlation by Takemori.50 Thermal Properties of PLLA/CNC nanocomposites


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All thermal characterizations were performed using a Mettler Toledo differential scanning calorimeter. PLLA melting temperature (Tm) and crystallinity index (χ) of injected and annealed nanocomposites were evaluated using the first heating scan from 25°C to 190°C at 20°C·min-1, by using 93 J·g-1 as the melting enthalpy for 100% crystalline PLLA.51 Then, an isothermal crystallization step at 130°C was performed to extract the PLLA crystallization half-time (t1/2). Finally, the glass transition temperature (Tg) was determined at the inflection point on the second heating scan from -40°C to 190°C at 20°C.min-1. Oxygen permeability of PLLA/CNC nanocomposites O2 permeability measurements were performed on thin films by using a LABTHINK VAC-VBS permeability tester at 22 °C (clean time 60 s, lower chamber vacuum time 60 s, high & lower chamber vacuum time 8 h, 10% proportional mode). All materials were annealed for 2 hours at 130 °C to reach constant crystallinity index in all samples and therefore discard crystallinity effects. Samples were then conditioned for at least 48 h at 20 °C under relative humidity of 50%. Measurements were averaged out of 2 samples for each formulation. .


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Results and Discussion This study was motivated by the discovery that PMMA and PLLA are miscible in the melt state.12 Melt blending of PMMA with PLLA resulted in significant modifications of PLLA’s glass transition temperature, crystallization rate and melting temperature. Consequently, this paper describes the preparation of CNC-g-PMMA and, subsequently, discusses the extent that these grafts enhance the compatibility of the corresponding modified CNCs with the PLLA matrix. CNCs were then prepared by controlled acid hydrolysis of cellulose according to a previously-reported method.44,52 Subsequently, MMA was specifically grafted on as-produced CNCs in water to prepare CNC-g-PMMA (Scheme 1). Grafting of PMMA was performed using the following different and well-known initiating species: i) Fenton’s reagent and ii) ceric ammonium nitrate (CAN). Detailed mechanisms on the grafting of (meth)acrylic monomers with these redox initiators are described elsewhere.40,43,47

Scheme 1: Free-radical grafting of MMA in water to produce CNC-g-PMMA nanoparticles using a) Fenton’s reagent and b) CAN as initiators.


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The extent of PMMA grafting on as-produced unmodified CNCs was first analyzed by gravimetric measurements. Non-grafted PMMA chains were removed by Soxhlet extraction with acetone. MMA conversion, grafting efficiency and the weight fraction of grafted PMMA on modified CNCs were calculated using Equations 1–3 and the results are listed in Table 1. Grafting initiated by Fenton’s reagent and CAN lead to MMA conversions of 60 and 45%, respectively. These results are consistent with previous reports regarding redox-initiated MMA polymerization/grafting from polysaccharides.40,43,47 Soxhlet extraction experiments revealed both the production of significant fractions of grafted PMMA on CNC nanoparticles and nongrafted PMMA chains. However, large differences in grafting efficiency were observed between the two initiators (Table 1). Fenton’s reagent gave a relatively low grafting efficiency (12%) with a corresponding high amount of non-grafted PMMA chains. In comparison, PMMA grafting using CAN gave a much higher grafting efficiency (77%). These results agree with previous studies of vinyl monomer grafting on starch,53 cellulose nanofibers47 and cellulose nanocrystals.42,54 Observed differences in grafting efficiencies for polymerizations conducted with Fenton’s reagent and CAN are attributed to corresponding different reaction mechanisms.40 Fenton’s reagent only produces hydroxyl radicals in the vicinity of cellulose followed by radical transfer to cellulose via hydrogen abstraction. In contrast, CAN results in the direct formation of radicals on the cellulose backbone via oxidation of cellulose with cleavage of anhydroglucose unit (AGU) C2-C3 bonds (Scheme 1).55 Following purification, a significant amount of PMMA is grafted on the surface of CNCs (Table 1). Graftings conducted with Fenton’s reagent and CAN result in CNC-g-PMMA containing 15% and 50% PMMA by weight, respectively. CNC modified by Fenton’s reagent is referred to as CNC-g-PMMA15 and, likewise, CNC modified


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using CAN is designated as CNC-g-PMMA50. Further confirmation of surface modification by grafting is discussed below. Table 1: Results of grafting MMA on unmodified CNC using Fenton’s reagent and CAN. Initiator

CMMA(%)a,b

Ge (%)a,c

WPMMAgrafteda,d

Name

Fe2+/H2O2

60

12

15

CNC-g-PMMA15

CAN

45

77

50

CNC-g-PMMA50

a

Calculated using Equations 1-3 shown in the experimental section. MMA monomer conversion c Grafting efficiency d Percent-by-weight PMMA grafted b

ATR-FTIR spectra were recorded for unmodified CNCs, CNC-g-PMMA15 and CNC-g-PMMA50 (Figure S1). Compared to unmodified CNCs, new vibrational bands are observed for modified CNCs, especially at 1724, 1152, 950 and 750 cm-1. These peaks are attributed to grafted PMMA and corresponding ester carbonyl stretching (C=O), CH3 twisting, C-O-C single bond stretching, C-C stretching and C=O out of plane bending, respectively.56 Furthermore, consistent with the gravimetric results and corresponding values of Ge and WPMMAgrafted (Table 1), the intensity of the carbonyl stretching band is significantly enhanced for CNC-g-PMMA50 obtained by CAN initiation relative to CNC-g-PMMA15 prepared using Fenton’s reagent. To gain further insights into the structure of CNC-g-PMMA synthesized herein, CP-MAS

13

C-

NMR spectra were recorded. Indeed, previous work demonstrated that CP-MAS 13C-NMR is an effective tool for characterization of modified CNCs.52,57 Figure 1 shows CP-MAS

13

C-NMR

spectra of unmodified CNCs, CNC-g-PMMA15 and CNC-g-PMMA50. The assignments of

13

C-

NMR resonances for AGU and PMMA chains are based on previous literature.58–61 CP-MAS


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13

C-NMR spectra of CNC-g-PMMA15 and CNC-g-PMMA50 reveal

13

C-NMR resonances at 20,

45, 53, 56 and 178 ppm assigned to grafted PMMA carbons 9, 7, 11, 8 and 10, respectively.60,62 Comparison of relative intensities for PMMA

13

C-NMR resonances of CNC-g-PMMA15 and

CNC-g-PMMA50 provides further confirmation of the higher Ge of CAN over Fenton’s reagent. Furthermore, unmodified CNCs, CNC-g-PMMA15 (Figure 1b) and CNC-g-PMMA50 (Figure 1c) display typical 13C-NMR resonances of the cellulose Iβ polymorph including: a doublet centered on 107 and 105 ppm (C1) as well as signals at 90 ppm (C4), 76 ppm (C3), 74 ppm (C5), 73 ppm (C2 and C5) and 62 ppm (C6).63 Also, the position and relative intensities of unmodified CNC 13

C-NMR resonances are very close to that of the pure Iβ polymorph reported by Kono and

coworkers.63 13C-NMR resonances of C4 and C6 at 85 and 62 ppm, respectively (labeled C4* and C6* in Figure 1a),58 correspond to residual amorphous regions of CNCs after HCl hydrolysis.59 PMMA grafting by the redox initiators resulted in only small changes in the relative intensities of

13

C-NMR resonances corresponding amorphous and Iβ crystalline domains. For CNC-g-

PMMA50, a low intensity broad 13C-NMR resonance is noticed close to 95 ppm (marked by an * in Figure 1c). This signal was previously attributed to modifications within amorphous domains that involve C1 carbons.47 However, without confirmation by additional experiments that verifies the assignment of the signal at 95 ppm, we lack direct evidence for the presence of grafted PMMA. In other words, identification of discret signals by CP-MAS 13C-NMR corresponding to α- or ω-PMMA chain ends was not obtained.


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Figure 1: CP-MAS

13

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C-NMR spectra of unmodified CNC (a), CNC-g-PMMA15 (b), CNC-g-

PMMA50 (c) and neat PMMA (d). Suspension tests in chloroform were performed to further assess whether PMMA grafting on CNCs occurred such that they are covalently linked. Inspection of Figure 2a shows that unmodified CNCs (vial on left) led to an unstable chloroform suspension with rapid (within 5 min) nanocrystal sedimentation to the bottom of the vial. In contrast, the suspension of CNC-gPMMA15 (Figure 2b) remains stable for several hours with high turbidity arising from the presence of suspended CNCs. Furthermore, CNC-g-PMMA50 shown in Figure 2c in chloroform has a gel-like behavior without sedimentation and, within a few minutes, a gel-like phase of CNC-g-PMMA50 appears at the top of the vial. Since PMMA is highly soluble in chloroform, the


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fact that CNC sedimentation is delayed or suppressed is consistent with the formation of PMMA grafts from CNCs for CNC-g-PMMA15 and CNC-g-PMMA50. Further studies that provide further insights into differences in CNC-g-PMMA15 and CNC-g-PMMA50 physical behavior are below. The morphologies of modified CNCs were investigated by transmission electron microscopy TEM (Figures 2b-d). Unmodified CNCs display a typical rod-like morphology with large aggregates of individual CNC nanoparticles.64 These aggregates are due to the hydrophilic nature of CNCs and corresponding strong interactions between particles. Length and width of CNCs are approximately 250-300 nm and 10-15 nm, respectively. The TEM image of CNC-g-PMMA15 is similar in appearance to that of unmodified CNCs. That is, CNC-g-PMMA15 shows nanocrystals with a rod-like morphology with large aggregates of similar length, diameter and shape factor as unmodified CNCs. In contrast, the morphology of CNC-g-PMMA50 is dramatically different than those of unmodified CNCs and CNC-g-PMMA15. CNCs produced using HCl tend to aggregate upon drying2 as evidenced for unmodified CNCs and CNC-g-PMMA15 with CNCs in close proximity each other (Figure 2b and 2c). CNC-g-PMMA50 shows a unique image where CNCs are spread apart, individualized and embedded in a PMMA net with a clear reduction of the number of CNC aggregates. This effect could be ascribed to a drastic reduction of the amount of free hydroxyl groups on the surface of CNCs available for interparticle interactions. Interestingly, CAN-initiated PMMA grafting in water consequently favors a remarkable predispersion effect of CNCs. Further insights into the physical behavior of the CNC-g-PMMA’s result from isolation and characterization of grafted PMMA chains. Isolation of PMMA grafts for CNC-g-PMMA15 and CNC-g-PMMA50 was performed by acid hydrolysis of the corresponding CNCs using a method developed elsewhere.47 SEC determined molar-average


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molecular weights (Mn) of PMMA grafts from CNC-g-PMMA15 and CNC-g-PMMA50 are 760,000 and 210,000 g/mol, respectfully. These Mn values agree with previous reports on acrylate redox-initiated free radical grafting on polysaccharides.45,47

a)

b)

c)

d)

Figure 2: a) suspension tests in chloroform (from left to right: unmodified CNC, CNC-gPMMA15, CNC-g-PMMA50, concentration: 5 mg CNC/1 mL CHCl3); TEM images of b) unmodified CNCs, (c) CNC-g-PMMA15 and (d) CNC-g-PMMA50 (scale bar = 100 nm). These large Mn values for PMMA grafts suggest significant differences in grafting densities and the method for calculating CNC substitution degree from PMMA graft chain lengths (SD, representing the amount of substituted hydroxyl group of CNC, see supporting information Table S1) gives SD values of 1.25 x 10-3 and 2.5 x 10-2 for CNC-g-PMMA15 and CNC-g-


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PMMA50, respectively.65 These low values are consistent with the fact that PMMA grafts are primarily confined to the CNC surface. PMMA grafting initiated by CAN results in an approximately 20-fold increase in cellulose SD. Long PMMA grafts are obtained through redoxinitiated free-radical grafting and CAN enhances the density of PMMA grafts at CNC surfaces. These characteristics are consistent with the development of a long-range 3D network between pre-dispersed CNC-g-PMMA50 (see below), the individualization of CNC-g-PMMA50 observed by TEM (Figure 2d) and entanglement of PMMA grafts that lead to gelation of CNC-gPMMA50-chloroform solutions (Figure 2a). Wide angle X-ray Spectroscopy (WAXS) was used to directly determine the crystallinity of CNCs as a function of their modification by PMMA grafting. Recorded WAXS diffractograms are displayed in Figure S2 and values of the crystallinity index are listed in Table S2. The crystallinity index of CNC-g-PMMA15 and CNC-g-PMMA50 are 82% and 60%, respectively, compared to 84% for unmodified CNCs. The decrease in crystallinity of CNC-g-PMMA50 relative to unmodified CNCs shows that PMMA grafting using CAN as the initiator increases the amorphous content of modified CNCs. Furthermore, WAXS shows that free-radical grafting of MMA initiated by Fenton’s reagent does not substantially influence the crystallinity of modified nanocrystals. Nevertheless, the TEM image of CNC-g-PMMA50 displayed in Figure 2d shows the retention of nanofiber dimensions similar to unmodified CNCs but where the CNCs are decorated with a shell of high molecular weight PMMA. The thermal stability of CNCs is also a critically important parameter that determines the melt processability of CNC-based nanocomposites at elevated temperatures. CNCs prepared by sulfuric acid hydrolysis (sulfate modified-CNCs or H2SO4-CNC) are commonly used but have low thermal stability due to the presence on CNC surfaces of residual sulfate esters.66 The sulfate


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esters at high temperatures during processing are converted to sulfuric acid that catalyzes CNC and PLLA degradation.52 To avoid sulfate-mediated thermal degradation during extrusion, CNCs used in this work were prepared by HCl hydrolysis and were analyzed by thermogravimetric analysis (TGA). The corresponding TGA thermographs of CNC-g-PMMA15, CNC-g-PMMA50 and unmodified CNCs are shown in Figure S3. The presence of PMMA shifts the thermal stability of the corresponding CNCs to higher temperatures. The thermal stability of CNCs, as determined from the temperature at the peak of the derivative weight loss, increases from 335°C to 352°C and 368°C for unmodified CNCs, CNC-g-PMMA15 and CNC-g-PMMA50, respectively. Increased CNC thermal stability is crucial to its use for nanocomposite melt extrusion processes. Temperatures corresponding to a 5% weight loss (T5%) are 288°C, 294°C and 255°C for unmodified CNCs, CNC-g-PMMA15 and CNC-g-PMMA50, respectively. The observed T5% reduction for CNC-g-PMMA50 is primarily attributed to residual nitric acid from the initiation step inducing cleavage of C2-C3 bonds (Scheme 1) at high temperature.38,67 It has been shown that different initiators could also play a key role on the PMMA macromolecular architecture. Local defects in the PMMA grafts such as head-to-head and/or tail-to-tail are known to decrease its thermal stability68,69 explaining the observed T5% reduction. PLLA/CNC nanocomposites were subsequently prepared by melt-extrusion at 210°C in the absence of any solvent. The CNC loading in PLLA blends of CNC-g-PMMA15 and CNC-gPMMA50 was fixed at 5%-by-wt. In other words, the quantity of nanocrystals added was normalized such that each blend had equal contents (5%) of CNCs but different quantities of PMMA grafts. It could be here noticed that the measured torque (or melt force) during nanocomposite compounding at high temperature remains stable (and similar to neat PLLA) and indicate the absence of extensive degradation (Figure S4). The nanoscale microstructure of melt-


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extruded PLLA-based nanocomposites was analyzed by TEM (Figure 3). The incorporation by melt extrusion of unmodified CNCs into PLLA shows that, instead of individualized CNCs, large aggregated structures with dimensions up to approximately 500 nm formed (Figure 3a). Aggregation of unmodified CNCs is consistent with previous literature52 and results from strong hydrogen bonding interactions between CNCs. In contrast to unmodified CNCs, the microstructure of nanocomposites with CNC-g-PMMA appears more homogeneous due to better dispersion of the nanocrystals within the PLLA matrix. Indeed, melt-extrusion of these materials resulted in a continuous and fibrillated network of PMMA-modified CNCs within the PLLA matrix (Figures 3b and 3c). TEM images resulting from melt-blending of PLLA with CNC-gPMMA15 and CNC-g-PMMA50 are similar in appearance. However, for CNC-g-PMMA50, a small decrease in the diameter of the fibrillated CNC network could be visually noticed but further verification proved difficult.

Figure 3: TEM analysis of PLLA/CNC (a), PLLA/CNC-g-PMMA15 (b) and PLLA/CNC-gPMMA50 (c) nanocomposites (scale bar = 100 nm, CNC concentration 5% by weight on a cellulose basis, injection-molded specimens). Based on the above-mentioned TEM observations, CNC dispersion and formation of percolated structures were investigated by dynamic rheology in the melt state at 180°C. The evolution of storage modulus with frequency for neat PLLA and all nanocomposites is shown in Figure 4.


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For neat PLLA, the storage modulus is proportional to ω2 at low frequencies in accordance with typical terminal behavior and large-scale relaxations of fully-relaxed PLLA chains.70 Unmodified CNCs and CNC-g-PMMA15 give rise to small increases in G’ without modifying PLLA’s terminal behavior at low frequencies. However, the terminal behavior of the PLLA/CNC-g-PMMA50 nanocomposite is significantly altered with a clear deviation from linearity. Percolation of CNCs resulting from the development of a CNC long-range 3D network formation was assessed by the evolution of tanδ at low frequencies using the Winter-Chambion criterion.71,72 Tan δ sharply decreases for neat PLLA reflecting a pure liquid-like behavior at low frequencies with a phase shift angle δ close to 90° in accordance with previous evolution of G’ (Figure 4). PLLA-based nanocomposites containing 5%-by-wt (normalized by CNC content) unmodified CNC or CNC-g-PMMA15 displayed a similar liquid-like behavior in agreement with the absence of a percolated CNC network. However, the behavior of the PLLA/CNC-g-PMMA50 nanocomposite is drastically different. A finite tan δ value of approximately 15 is reached at low frequencies with minor decreases with frequency. According to Winter-Chambion, viscoelastic properties at the gel point are frequency-independent and this criterion is mainly satisfied at low frequencies for nanocomposites at the so-called percolation threshold.71,72 From a rheological viewpoint, the viscoelastic response observed for PLLA/CNC-g-PMMA50 is in accordance with a physical gelation induced by the presence of a weak 3D CNC network. In this respect, TEM and dynamic rheology show unambiguously that the presence of PMMA grafts on CNC has a strong beneficial effect on CNC dispersion into PLLA by melt extrusion. The formation of a CNC network in as-prepared nanocomposites is clearly favored by the use of pre-dispersed CNCs obtained through CAN-initiated free radical grafting, affording unprecedented percolation threshold down to 5%-by-weight with direct extrusion techniques.


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Figure 4: Storage modulus (G’) and tan δ as a function of frequency evaluated by dynamic rheology at 180°C, CNC concentration 5%-by-weight on compression-molded specimens for neat PLLA (a), PLLA/CNC (b), PLLA/CNC-g-PMMA15 (c) and PLLA/CNC-g-PMMA50 (d). DSC analysis was performed on semi-crystalline nanocomposites after their preparation by injection-molding and annealing at 130°C for 2 hours (Figure S5). All PLLA/CNC nanocomposites prepared herein display only small shifts in the PLLA melting temperature (Tf, 168.1-169.7°C), %-crystallinity (χc, 42.0-49.0%) and glass transition temperature (Tg, 56.059.1°C) relative to neat PLLA (Table 2). The slight Tg depression observed for nanocomposites with unmodified CNCs could result from PLLA degradation during compounding with unmodified CNCs and this finding is also consistent with CNC aggregation due to poor dispersion of CNCs into PLLA.73 However, relative to neat PLLA, the presence of CNC-gPMMA strongly influenced PLLA’s crystallization rate that was determined by measuring the crystallization half-time (t1/2) at 130°C from the melt state. The largest decrease in t1/2 (e.g. increase in the crystallization rate) was observed for the nanocomposite containing unmodified CNCs. That is, neat PLLA has a t1/2 of 18.1 min whereas the t1/2 for the PLLA/unmodified CNC nanocomposite is almost 7-fold lower (2.7 min). The strong nucleating effect of CNCs could be


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attributed to the presence of numerous free-hydroxyl groups at the CNC surfaces73,74 but a slight degradation of PLLA and/or transcrystallization effects cannot be totally neglected. Half-time crystallization of PLLA increases to 7.9 min with incorporation of CNC-g-PMMA15 but remains lower than neat PLLA. The nucleation effect is no longer observed with CNC-g-PMMA50 (t1/2 25 min) and is attributed to PMMA inhibition of PLA crystallization.12 The distinct difference in nucleation efficiencies of CNC-g-PMMA15 and CNC-g-PMMA50 is attributed to the higher accessibility of hydroxyl groups at the surface of CNC-g-PMMA15. Furthermore, the above results agree with that CNC-g-PMMA15 has a lower SD value than CNC-g-PMMA50. Our group previously reported on the inhibition of PLLA crystallization by miscible PMMA fractions, an effect observed even at very low PMMA concentrations.12 Thus, inhibition of PLLA crystallization by CNC-g-PMMA50 indicates that long PMMA grafts are (partly)-miscible with PLLA and act as a shell covering CNC surfaces. Furthermore, the fine dispersion of modified CNCs into PLLA also arises from the compatibility/affinity between the miscible PMMA shell and the PLLA matrix, in addition to the pre-dispersing effect previously discussed. Table 2: Thermal properties of PLLA/CNC nanocomposites (CNC concentration 5% by weight, see Experimental Section). Sample

Tf (°C)a

χc (%)a

t1/2 – 130°C (min)

Tg (°C)b

Neat PLLA

168.1

46.0

18.1

59.1

PLLA/CNC

169.4

49.0

2.7

56.0

PLLA/CNC-g-PMMA15

169.4

45.1

7.9

56.7


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PLLA/CNC-g-PMMA50

169.7

42.0

25

58.7

a

Determined during the first heating scan on injected samples (10°C·min-1)

b

Determined during the heating scan after isothermal crystallization (10°C·min-1)

Thermomechanical properties of PLLA/CNC nanocomposites were evaluated by dynamic mechanical analysis (DMA) to determine potential mechanical benefits of CNC modification by PMMA grafts related to the dispersion state, especially in terms of storage modulus (E’), αrelaxation temperature (Tα) and the heat deflection temperature (HDT). DSC studies discussed above showed that changes in the content of PMMA grafting on CNC had little effect on corresponding Tg values of the nanocomposites. This agrees with DMA determined Tα values that do not significant vary over the range of CNCs studied. However, other thermomechanical parameters determined by DMA for the series of nanocomposites prepared herein show they are directly a function of the CNC dispersion state in the PLLA matrix. The evolution of the storage modulus (E’) with temperature for injection-molded and annealed nanocomposites is displayed in Figure 5 and related thermomechanical properties are listed in Table 3. Neat PLLA shows typical behavior of a semi-crystalline polymer with a high storage modulus (2.65 GPa) at room temperature and a progressive drop of the storage modulus above Tα at 67.5°C.


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Figure 5: Storage modulus as a function of temperature for neat PLLA (a), PLLA/unmodified CNC (b), PLLA/CNC-g-PMMA15 (c) and PLLA/CNC-g-PMMA50 (d) (evaluated by DMA at 1Hz, CNC concentration 5% by weight on a cellulose basis, see Experimental Section). By adding unmodified and PMMA-modified CNCs, significant increases in the storage modulus was attained over the full temperature range studied and, most notably, in the rubbery state above Tα. Storage modulus (E’) values measured at 23 °C, in the glassy state, were 4.1 GPa, 4.7 GPa and 5.2 GPa for PLLA nanocomposites with 5%-by-weight of unmodified CNCs, CNC-gPMMA15 and CNC-g-PMMA50, respectively (Table 3). The corresponding storage modulus of neat PLA at 23°C is 2.7 GPa for comparison. Similar effects are also observed at 100 °C with E’ values of 440 MPa, 700 MPa and 860 MPa for for PLLA nanocomposites with 5%-by-weight of


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unmodified CNCs, CNC-g-PMMA15 and CNC-g-PMMA50, respectively (Table 3). Thus, the amount of PMMA grafted on CNC surfaces and the related dispersion state of CNCs allowed for dramatic increases in PLA storage modulus both at ambient and elevated temperatures. CNC-gPMMA50 affording a 3D network in PLLA nanocomposites display the best thermomechanical performances and the heat deflection temperature (HDT) of the nanocomposites was consequently determined from DMA analysis using the Takemori method.50 It has been shown previously that neat PLLA with an crystallinity index close to 45% has an HDT of 70 °C, restricting its use in high-temperature applications.21 The addition of unmodified CNCs and CNC-g-PMMA15 slightly shifts HDT to 75°C and 80°C respectively (Table 3). However, PLLAbased nanocomposites with CNC-g-PMMA50 resulted in HDT values greater than 130°C. IN comparison, Spinella et al. reported an increase in the HDT by 20 °C above that of neat PLLA with the incorporation of 20%-by-wt. lactate-modified CNCs into PLA nanocomposites.52 Here, only 5%-by-weight CNC-g-PMMA50 in PLLA is needed to manufacture 100% biobased materials with outstanding thermal resistance for engineering applications requiring HDT higher 130°C. Table 3: Storage modulus at 23°C (E’23°C), at 100°C (E’100°C), α-relaxation temperature (Tα) and HDT of PLLA/CNC nanocomposites (CNC concentration 5% by weight, standard deviation into brackets) Sample

E’23°C (MPa)

E’100°C (MPa)

Tα (°C)a

HDT (°C)b

Neat PLLA

2650 (55)

235 (25)

67.5 (0.2)

70

PLLA/unmodified-CNC

4065 (150)

440 (40)

67.6 (0.4)

75


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PLLA/CNC-g-PMMA15

4770 (300)

700 (90)

68.1 (0.6)

80

PLLA/CNC-g-PMMA50

5200 (250)

860 (50)

66.9 (0.2)

> 130

a

: Evaluated at the tan δ peak

b

: Heat deflection temperature evaluated by the correlation established by Takemori.50

Gas permeability of PLLA nanocomposites was also inspected to increase PLAs market penetration, especially in hot temperature food packaging. The influence of the CNC dispersion state on the O2 permeability of the as-produced PLLA/CNC nanocomposites was investigated and the data are shown in Table S3. PLLA nanocomposites with 5% of unmodified CNCs, PLLA/CNC-g-PMMA15 and PLLA/CNC-g-PMMA50 resulted in O2 permeability values of 0.13, 0.10 and 0.08 cm3/m2.day.0.1MPa, respectively. The O2 permeability of neat PLLA is 0.15 for comparison. Increasing the amount of PMMA attached to CNC surfaces results in a 15%, 33% and 44% decrease in the O2 permeability. The decrease in PLLAs O2 permeability can be also attributed to the enhanced dispersion state of CNCs resulting from the PMMA grafting. The tortuous pathway of oxygen is probably increased but as well as a reduction of diffusion rate due to interactions between oxygen and CNCs.72


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Conclusions CNCs were successfully modified with PMMA grafts in water using two different free-radical redox initiators: Fenton’s Reagent (Fe2+, H2O2) and ceric ammonium hydrate (CAN). Gravimetric analysis revealed that Fenton’s reagent and CAN gave PMMA-modified CNCs containing 15 and 50% of grafted PMMA, respectively. Both modification strategies conducted in water resulted in high MMA conversions and very high grafting efficiencies. Successful modification using both initiators was verified by FTIR and CP-MAS

13

C-NMR experiments.

The high crystallinity of PMMA modified-CNCs demonstrates that the grafting occurs primarily at CNC surfaces in amorphous regions. The formation of stable suspensions of PMMA-modified CNC in chloroform provided further evidence that PMMA chains were chemically linked to CNC surfaces. PMMA-modified CNCs prepared by CAN initiation showed a gel-like behavior in chloroform and a distinct microstructure indicating the absence of CNC aggregation. Furthermore, redox-initiated grafting yields long PMMA grafts from CNC surfaces with a low substitution degree. However, the pre-dispersion of CNCs and the gel-like behavior observed with CAN is favored by higher PMMA graft densities. PMMA grafts form a shell around CNCs significantly increases CNC thermal stability that facilitates its blending by melt-extrusion processes. TEM experiments further showed that CNCs modified using CAN were embedded in a PMMA network, resulting in “pre-dispersion” CNCs in PMMA. PLLA-based nanocomposites incorporating 5%-by-weight unmodified- and PMMA-modified CNCs were processed by twin-screw extrusion and injection-molding. Dynamic rheology in the melt state revealed an improved dispersion state of CNCs with the presence of PMMA grafts. In particular, pre-dispersed CNCs by CAN-mediated grafting of PMMA on CNCs favor the development of a percolated CNC network with a percolation threshold ≤ 5%-by-weight.


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Thermal analysis by DSC revealed that, grafting of PMMA on CNC surfaces that reduced the accessibility of PLLA to CNC hydroxyl groups caused a large reduction in PLLA’s crystallization rate. Therefore, the miscibility between the PLLA matrix and PMMA grafts as well as the pre-dispersion effect of CNCs that occurs during PMMA grafting are responsible for the high extents of CNC dispersion achieved by melt-blending. PLLA-based nanocomposites with PMMA modified CNCs possess higher storage moduli in the glassy and rubbery state (both at 23°C and 100°C), in accordance with an enhanced dispersion of CNCs. Indeed, incorporation of well-dispersed 5% CNC-g-PMMA50 in PLLA gave a nanocomposite that retained a storage modulus of up to 850 MPa at 100°C for an outstanding HDT above 130°C. Surface modification of CNCs by PMMA grafting using CAN as the redox initiator provides an efficient and green route to readily dispersible CNCs in biobased PLLA. Pre-dispersion of CNCs during the grafting step in water and the miscibility of PMMA grafts played a key role on the dispersion of CNCs in molten PLLA by solvent-free extrusion techniques. Increasing PLAs HDT while simultaneously decreasing PLAs oxygen permeability will allow for PLA to penetrate various high performance applications. We envision, due to PMMA miscibility with many different polymer matrices, this green grafting method in water could also efficiently increase CNC dispersion in various biobased polymers via melt extrusion. Corresponding authors * e-mail: [email protected] * e-mail: [email protected] * e-mail: [email protected] Acknowledgments


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The authors are grateful for funding received from the National Science Foundation Partnerships for International Research and Education (PIRE) Program (Award #1243313). Financial support from Wallonia and European Commission in the frame of SINOPLISS-POLYEST project and OPTI²MAT program of excellence and from FNRS-FRFC are also gratefully acknowledged. Authors gratefully acknowledge both the International Campus on Safety and Intermodality in Transportation (CISIT, France), the Nord-Pas-de-Calais Region (France) and the European Community (FEDER funds) for their contributions to funding the dynamic rheometer. Supporting information Supporting Information Available: ATR-FTIR spectra of of neat CNCs and PMMA-modified (Figure S1), Molar substitution, polymerization degree of PMMA grafts and substitution degree of PMMA-modified CNC (Table S1), WAXS analysis of neat CNC and PMMA-modified (Figure S2), Crystallinity index of neat CNC and PMMA-modified CNC evaluated by WAXS (Table S2), Thermogravimetric analysis of unmodified and PMMA-modified CNC (Figure S3), Evolution of the torque during extrusion compounding of PLLA/CNC nanocomposites (Figure S4), DSC analysis of PLLA/CNC nanocomposites (Figure S5), O2 permeability of PLLA/CNC nanocomposites (Table S3). This information is available free of charge via the internet at http://pubs.acs.org/.


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For Table of Contents Use Only

Green & Efficient Synthesis of Dispersible Cellulose Nanocrystals in Biobased Polyesters for Engineering Applications Stephen Spinella, Cédric Samuel, Jean-Marie Raquez, Scott A. McCallum, Richard Gross, Philippe Dubois PMMA was grafted to CNC in water and biobased PLLA/CNC-g-PMMA nanocomposites with outstanding HDT > 130°C were produced via a solvent-less extrusion process.


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Green and Efficient Synthesis of Dispersible ... - ACS Publications

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