Binary Mixed Homopolymer Brushes Tethered to Cellulose Nanocrystals

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Binary mixed homopolymer brushes tethered to cellulose nanocrystals: a step towards compatibilized polyester blends Rosica Mincheva, Latifah Jasmani, Thomas Josse, Yoann Paint, Jean-Marie Raquez, Pascal Gerbaux, Samuel Eyley, Wim Thielemans, and Philippe Dubois Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b00932 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 24, 2016

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Binary mixed homopolymer brushes tethered to cellulose nanocrystals: a step towards compatibilized polyester blends Rosica Mincheva,*a Latifah Jasmani,b Thomas Josse,a,c Yoann Paint,e Jena-Marie Raquez,a Pascal Gerbaux,c Samuel Eyley,d Wim Thielemans,d and Philippe Duboisa,f a

Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in

Materials and Polymers (CIRMAP), University of Mons (UMONS), Place du Parc 23, 7000 Mons, Belgium. b

School of Chemistry and Process and Environmental Engineering Research Division,

University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom. c

Interdisciplinary Center for Mass Spectrometry, Organic Synthesis and Mass Spectrometry

Laboratory, University of Mons (UMONS), Place du Parc 23, 7000 Mons, Belgium. d

Renewable Materials and Nanotechnology Research Group, Department of Chemical

Engineering, KU Leuven, Campus Kulak Kortrijk, Etienne Sabbelaan 53, 8500 Kortrijk, Belgium. e

Analysis and Characterization Unit, Materia Nova, Avenue Copernic 1, 7000 Mons,

Belgium.

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f

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Materials Research and Technology Department (MRT), Luxembourg Institute of Science

and Technology (LIST), Rue du Brill 41, 4422 Belvaux, Luxembourg. KEY WORDS. Polymers brushes, nanoparticles, poly(lactide), poly(butylene succinate), cellulose nanocrystals, grafting “onto”, heterogeneous click reaction

ABSTRACT. This article reports on the successful preparation and characterization of cellulose nanocrystals (CNCs) surface-modified with polylactide (PLA) and poly(butylene succinate) (PBS) binary mixed homopolymer brushes. Their synthesis was designed as a three-step procedure combining polyester synthesis and surface-modification of CNCs with simultaneous polyester grafting via a heterogeneous copper(I)-catalyzed azide-alkyne cycloaddition reaction. For comparison, single homopolymer brushes tethered to CNCs (PLLA-g-CNC and PBSBDEMPAM-g-CNC) were obtained applying the same procedure. The hairy nanoparticles were characterized in terms of chemical composition and thermal properties. Spectroscopic analyses suggested “rippled” microphase separation of both immiscible homopolyesters in the mixed brushes, while other showed impeded homopolyester crystallization after surface-grafting. Synergistic relationship between the polyesters and CNCs was also suggested, i.e. the polyester grafting increases the CNC thermal resistance, while CNC presence imparts char formation. The as-obtained binary homopolymer brushes tethered to nanoparticles makes these surface-modified cellulosic nanomaterials attractive as compatibilization/reinforcement agents for PLA/PBS blends.

INTRODUCTION. Binary mixed homopolymer brushes tethered to nanoparticles (“hairy” nanoparticles) are composed of a core (organic or inorganic, spherical, cylindrical or rod-like) and a layer of two chemically distinct homopolymer chains tethered/grafted to the core.1 Their design allows the combination of mechanical, electrical, and optical properties of the core with the environmental compatibility/responsiveness of the polymer brushes.2,3 Thus, the 2 ACS Paragon Plus Environment

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hybrid materials can find applications from self-cleaning materials4 to sensing5 and in biotechnology.6 Generally, they can be obtained by applying either a “grafting from”, or a “grafting onto” approach, but only the second involves polymers of known molecular characteristics.7 Amongst all available nanoparticles, an interesting and vastly developing class is cellulose nanocrystals (CNCs).8 These typically rod-like particles are derived from annually renewable resources and are known for their high strength.9 Particular interest has been drawn to their incorporation into the bio-based polyester poly(lactic acid) (PLA) matrix.10,11 Indeed, although PLA is obtained from annually renewable resources and is considered as an alternative to commodity plastics,12 it suffers from significant drawbacks such as low crystallinity/crystallization rate and poor shock resistance. This is where CNCs are expected to intervene as reinforcing agents.10,11 However, their hydrophilic character, strong aggregation upon drying, and low thermal stability impede the preparation of high performance CNC/PLA nanocomposites. Surface-modification by grafting of PLA chains represents a promising technique for overcoming these drawbacks.10 The “grafting from” technique was used and showed satisfactory results.10 Another possibility to improve the mechanical properties of PLA is polymer blending with the poly(butylene succinate) (PBS) – another bio-based polyester displaying higher flexibility.13-18 Initially, PLA and PBS were reported to be miscible in the amorphous regions,13 but further studies concluded that phase-separation occurs if the PBS content exceeds 20 wt%.14 Therefore, multiple efforts have been undertaken on the compatibilization of the two polyesters via reactive blending.15-17 However, physical blending in presence of an interfacial compatibilizer still represents a more convenient, lower cost and compositioncontrolled method.18 This blending often uses carefully engineered block or graft copolymers;


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although another category of interfacial agents has been approached, i.e. (nano)fillers (mostly inorganic) which proved interesting alternative with a small size and high surface-to-volume ratio. Interestingly these (nano)fillers become even more efficient for polymer blend compatibilization when they are organically modified19 as they can alter the morphology of the blend, tune viscosity ratios and migrate to the interface to be compatibilized. The latter reduces interfacial tension, suppresses coalescence, and strengthens adhesion between polymer phases. Based on this knowledge, the combination of PLA and PBS with CNCs in the form of binary mixed homopolymer brushes tethered to nanoparticles appears to be a promising solution to the PLA/PBS compatibilization issue. With this in mind, the present study aims to prepare binary mixed homo-PLA/homo-PBS brushes tethered to CNCs via the “grafting onto” method. homoPLA- and homoPBS-grafted CNCs were also prepared and characterized for sake of comparison. Propargyl alcohol (Pr-OH) initiated Ring-Opening Polymerization (ROP) and conventional two-step melt polycondensation in the presence of diethyl(methyl-2propargyl)malonate (DEMPAM) were used in the synthesis of Pr-PLA and PBSBDEMPAM containing C≡C triple bonds, respectively. CNCs were surface modified with azide functionalities and heterogeneous “click”-chemistry was applied for grafting both types of functionalized polyester chains. Surface grafting was confirmed by different methods and thermal stability of the modified CNCs was evaluated.

EXPERIMENTAL PART Materials: Dimethyl succinate (DMSu, > 98 %, Kosher), 1,4-butanediol (BDO, > 98 %, Kosher), diethyl methylmalonate (DEMMA, 99 %, Acros), hydroquinone (99 %, Fluka), propargyl bromide (80 wt.% stabilized in toluene, Aldrich), propargyl alcohol (Pr-OH, 99 %, 4 ACS Paragon Plus Environment

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Aldrich), tin(II) 2-ethylhexanoate (Sn(Oct)2, ≈ 95 %, Aldrich), titanium(IV) n-butoxide (TBT, 99 %, Acros), cotton wool (BP grade, 100 %, Fisher Scientific UK), sulfuric acid (> 95 %, Fisher Scientific UK), ethanol (95 %, Fisher Scientific UK), N,N-dimethylformamide (99 %, Fisher Scientific UK), dichloromethane (99 %, Fisher Scientific UK), sodium azide (> 99 %, Fisher Scientific UK), potassium bromide for IR spectroscopy (> 99 %, Fisher Scientific UK), toluene (>99.8 %, Fisher Scientific UK), pyridine (AnalaR NORMAPUR® , 99.7 %, BDH Prolabo), Amberlite® MB6113 mixed bed ion exchange resin, H+/OH- with indicator (Merck), Spectrum Laboratories Spectra/Por® 4 regenerated cellulose dialysis tubing (MWCO, 12 - 14 kDa) and thionyl chloride (> 99 %, Sigma-Aldrich) were used as received. L- and D-lactide (LLA, Purasorb® L and DLA, Purasorb® D) with optical purity > 99.5 % (free acid < 1 meq kg−1, water content < 0.02 %) were supplied by Purac Biochem BV (the Netherlands) and stored in a glove box prior to use. Triphenylphosphine (Ph3P, ≥ 99 %, Merck) was recrystallized from diethylether, dried at 25 °C under reduced pressure overnight and then by three consecutive azeotropic distillations with dry toluene prior to use. Copper(I) bromide (CuBr, 98 %, Aldrich) was purified by stirring overnight with acetic acid, then by washing with ethanol and diethylether and drying under nitrogen and reduced pressure at r.t. to constant weight. N,N,N’,N’,N’’-pentamethyldiethylenetriamine (PMDETA, > 99 %, Acros) was distilled prior to use. Toluene and THF were dried using an MBraun solvent purification system under N2 flow, and all other reagents were analytical reagent (AR) grade and used as received. Preparation of binary mixed homopolymer brushes tethered to cellulose nanocrystals: Synthesis of DEMPAM: DEMPAM was prepared according to the literature.20


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Synthesis of PBSBDEMPAM: The synthesis of PBSBDEMPAM was performed as described by Mincheva et al.21 The ratio [DMSu] / [DEMPAM] was fixed to 99:1 (mol/mol) and the ratio [BDO] / [DMSu + DEMPAM] = 1.5:1.0 (mol/mol). Synthesis of Pr-PLLA and PDLA: Pr-OH initiated ROP of LLA and BDO initiated ROP of DLA were performed in bulk, using autoclave reactors at the conditions described before.22 Synthesis of CNC-N3: Azidated cellulose nanocrystals were synthesized based upon a previously published procedure.23 In short, cotton wool (80 g) was added to sulfuric acid (64 %, 700 mL) and heated at 45 °C under mechanical stirring. The acid-hydrolysis was left for 40 minutes. Following hydrolysis, the suspension was diluted with an equal volume of cold deionized water and subjected to centrifugation. Three successive centrifugations were carried out at 10,000 rpm for 40, 60 and 20 min with intermittent water washes. After centrifugation, the suspension was dialyzed using cellulose membrane tubing (MWCO 12-14 K diameter 48 mm) against running tap water. The suspension was then sonicated using a sonication horn to homogenize the suspension and separate aggregates prior to filtration through a fritted‐glass filter (porosity 2). After sonication, the filtrate obtained was mixed with Amberlite MB6113 mixed bed ion exchange resin and filtered again before freeze‐ drying. The resulting cellulose nanocrystals were Soxhlet extracted with ethanol for 72 hours before drying in vacuo. Dry cellulose nanocrystals (5 g) were then suspended in a solution of pyridine (40 mL, 500 mmol) and dry toluene (400 mL) under argon. A solution of thionyl chloride (20 mL, 110 mmol) in dry toluene (20 mL) was added dropwise before subsequent heating at 65 °C for 16 hours. The resulting light brown suspension was filtered and the product washed with dichloromethane (20 mL), deionized water (50 mL) and acetone (20 mL) before Soxhlet


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extraction with dichloromethane (24 hours) and ethanol (48 hours). The light brown chlorinated nanocrystals were isolated by drying in vacuo. Chlorinated cellulose nanocrystals (4 g) were suspended in N,N-dimethylformamide (200 mL) with sodium azide (5.42 g, 310 mmol) and heated at 100 °C for 2 days. The solid product was separated by filtration and washed with deionized water (4 x 50 mL) and ethanol (2 x 20 mL) before dispersing in deionized water and dialysis against deionized water for 2 days. The product was subsequently isolated by freeze-drying. Found: C, 40.6; H, 5.6; N, 4.3%. Calc. for DS = 0.17 (C6H9.83N0.51O4.83)n: C, 43.3; H, 6.0; N, 4.3%. ̅ (KBr) /cm-1 3345 ν(O-H), 2901 ν(C-H), 2112 ν(N3), 1635 δ(H2O), 1430 δ(C-O-H), 1337 δ(C-O-H), 1317 δ(C-O-H), 1205 ν(C-O-C, glycosidic), 1163 ν(C-O-C, glycosidic, asym.), 1113 ν(C2-OH), 1059 ν(C3-OH), 1035 ν(C6-OH), 704 ω(C-OH), 667 ω(C-OH). Grafting onto: The “click” reaction between PBSBDEMPAM, Pr-PLLA or PBSBDEMPAM/Pr-PLLA (1:1, mol/mol) and CNC-N3 was performed via copper(I)catalyzed azide-alkyne cycloaddition as described before.21 Characterization: Matrix-assisted laser desorption ionization time-of-flight (MALDI-ToF) mass spectra were obtained using a Waters Q-ToF Premier mass spectrometer equipped with a nitrogen laser, operating at 337 nm with a maximum output of 500 J m−2 delivered to the sample in 4 ns pulses at 20 Hz repeat rate. Time-of-flight mass analyses were performed in reflection mode at a resolution of approximately 10,000. All samples were analyzed using trans-2-[3-(4-tertbutylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) matrix.24 This matrix was prepared as a 20 mg mL−1 solution in chloroform. The matrix solution (1 µL) was applied to a stainless steel target and air-dried. Polymer samples were dissolved in dichloromethane to obtain 1 mg mL−1 solutions. 1 µL aliquots of these solutions were applied onto the target area already bearing the matrix crystals, and air dried. For the recording of the 7 ACS Paragon Plus Environment

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single-stage MS spectra, the quadrupole (RF-only mode) was set to pass ions from 500 to 10,000 m/z, and all ions were transmitted into the pusher region of the time-of-flight analyzer where they were mass-analyzed with 1 s integration time. 1

H NMR spectra were collected in CDCl3 solution on a Bruker AMX-500 NMR

spectrometer. FTIR spectra were recorded in ATR mode with a Bruker Tensor 27 infrared spectrometer using dry powder samples. Size-exclusion chromatography (SEC) was performed in chloroform at 30 °C using an Agilent liquid chromatograph equipped with an Agilent degasser, an isocratic HPLC pump (flow rate = 1 mL min−1), an Agilent autosampler (loop volume = 200 µL, solution conc. = 2.0 mg mL−1), an Agilent-DRI refractive index detector and three columns: a PL gel 10 µm guard column and two PL gel Mixed-D 10 µm columns (linear columns for separation of Mw PS ranging from 500 to 106 g mol−1). Polystyrene standards were used as calibration standards. X-ray photoelectron spectroscopy (XPS) was performed using a VG ESCALAB 220iXL spectrometer. Data were collected using monochromatic Al Kα radiation at 1486.6 eV. Photoelectrons were collected from a 1 mm2 sample area at a take-off angle of 90°, giving an estimated probe depth of about 10 nm. For each sample, a survey spectrum was recorded with 117.4 eV pass energy and 43 W beam power, followed by high-resolution spectra for the C 1s, O 1s and N 1s peaks with pass energy of 23.5 eV. Atomic compositions were derived from peak areas using photoionization cross-sections calculated by Scofield, corrected for the dependence of the escape depth on the kinetic energy of the electrons and corrected for the analyzer transmission function of the spectrometer. High-resolution spectra were deconvoluted in CasaXPS software using peak shapes based on Voigt functions with 8 ACS Paragon Plus Environment

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asymmetric correction for vibrational structure in polymers. Each component peak (( )) consists of multiple Voigt functions ( ) separated by the relevant vibrational energy levels (

assigned from FTIR in eV) e.g. ( ) = ( ) + 0.45 ( +

) + 0.2 ( + 2

) + 0.1 ( + 3

) + 0.05 ( + 4

). Differential scanning calorimetry (DSC) measurements were carried out with a DSC Q2000 apparatus from TA Instruments under nitrogen flow. A heat/cool/heat procedure from -80 to 200 °C at a rate of 10 °C min−1 was applied under N2 atmosphere. The thermal transitions were acquired from the second heating run. X-ray diffraction (XRD) morphological analyses were performed on a Siemens D5000 diffractometer using Cu-Kα radiation (λ=1.5406 Å, 40 kV, 30 mA)) at room temperature. The samples were step-scanned from 10° to 25° 2θ with steps of 0.02° with fixed counting time of 4 s. The thermal decomposition behavior was recorded with thermogravimetric analysis (TGA, Q5000, TA Instruments). The samples were heated from room temperature to 800 °C at 10 °C/min under N2 atmosphere. Scanning Electron Microscopy (SEM) was performed using a Hitachi SU8020. Pictures were acquired using the transmission electron detector. The electrons were accelerated using a high voltage of 30 kV. RESULTS AND DISCUSSION. The preparation of binary mixed homopolymer brushes tethered to cellulose nanocrystals (PBSBDEMPAM/PLLA-g-CNC) was designed as a threestep process combining the synthesis of a single triple-bond containing PLA and PBS as a first step, followed by the surface modification of CNCs to azide-containing nanocrystals (CNC-N3), then the simultaneous grafting of the polyesters onto the modified CNCs via a 9 ACS Paragon Plus Environment

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heterogeneous copper(I)-catalyzed azide-alkyne cycloaddition reaction. It accounts for the peculiarities of the syntheses of both polyesters, the ease of CNC-N3 preparation and the possibility for simultaneous grafting of PLA and PBS onto the surface of the CNCs. For the purpose of comparison, the single homopolymer brushes tethered to CNCs (PLLA-g-CNC and PBSBDEMPAM-g-CNC) were prepared using the same procedure. Polyester synthesis and characterization. The polyesters containing a single triple-bond at one chain end or along the polymer backbone – Pr-PLLA and PBSBDEMPAM, respectively, were obtained through a Pr-OH initiated melt ROP of LLA21 and via a classical two-step melt polycondensation of BDO, DMSu and DEMPAM, respectively (Scheme 1). O e'

O + n

O

melt ROP O

OH

f

g

d

O

Pr-OH

m = 2n-1

O x

OMe + m SuA

O

OH + y

HO

O

Pr-PLLA

OMe

MeO DEMPAM

BDO esterification

(a) OH

O

O

MeO

d' m

e

O

LLA

O

MeOH

(b)

polycondensation

O

O O

O H

O

O OH

O

n O

O

PBSBDEMPAM x:y = 99:1 (mol:mol); (x+y):m = 1:1.5 (mol:mol)

Scheme 1. Synthetic pathway to Pr-PLLA (a) and to PBSBDEMPAM (b).


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The triple-bond content per polyester chain was kept low in order to avoid any possible crosslinking reactions in further experiments. The targeted molar masses for both polyesters were also relatively low ( M n ± 5,000 g mol-1) to ensure flexibility and to limit steric hindrance effect upon grafting.

(a)

(b)


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Figure 1. MALDI-ToF of Pr-PLLA (a) and 1H NMR spectra of PBSBDEMPAM (b). MALDI-ToF spectra were used to confirm the polymer structure and the presence of triplebonds for Pr-PLLA and PBSBDEMPAM, respectively. As can be seen, the Pr-PLLA shows good agreement between theoretical predictions (m/ztheo) and experimental data (m/zmeas) (Figure 1a). Concerning PBSBDEMPAM, however, the obtained results were difficult to interpret due to wide polymer polydispersity and trace amounts of side products resulting from polycondensation (data not shown). In this case, 1H NMR was applied to confirm the polyester structure (Figure 1b). The molecular weight characteristics of both polyesters were obtained by SEC (Table 1) showing monomodal elution profiles.

Table 1. Yields and molecular characteristics of PBSBDEMPAM and Pr-PLLA SEC* Polyester

*

Yield, %

Mn ,g mol-1

MW , g mol-1

Mp

PBSBDEMPAM 96

3,900

9,000

9,200

2.29

Pr-PLLA

5,000

9,800

8,600

1.90

98

, g mol-1

Ð

Molecular characteristic features as obtained by SEC in chloroform using PS standards. For

Pr-PLLA the corrected molar masses using K = 1.048 and α = 0.4055 are listed.

Synthesis and characterization of CNC-N3. Cellulose nanocrystals were isolated from cotton wool following the well-established sulfuric acid hydrolysis procedure.8 The resulting cellulose nanocrystals were chlorinated using thionyl chloride to activate the CNC surface for 12 ACS Paragon Plus Environment

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subsequent azidation with sodium azide in N,N-dimethylformamide as reported previously.23 The success of the azidation reaction was confirmed by FTIR spectroscopy, which shows a clear azide stretching vibration at 2112 cm-1. This result is corroborated by XPS analysis, which shows the two characteristic nitrogen 1s peaks for azide at 404.5 eV and 400.8 eV, corresponding to two separate nitrogen environments in the azide moiety (Figure 2).25-27 The larger peak at 400.8 eV has been fitted with two components to represent the outer nitrogen atoms in the azide moiety, which both have resonance structures with a negative charge, while the peak at 404.5 eV is assigned to the central nitrogen atom with a positive charge. The level of azide incorporation (expressed as surface degree of substitution, the number of modified hydroxyl groups per anhydroglucose unit on the surface) was determined by elemental analysis using the equation below,28 where DSsurf is the surface degree of substitution, Nmod is the number of moles of the modification per gram of product, Xcel is the mass fraction of cellulose in the product and NOH is the number of moles of surface hydroxyl groups per gram of cellulose nanocrystals.

=

1.5 !" #$

For CNCs with a cross-section dimensions of 7 x 7 nm, NOH is 3 mmol g-1, which results in a surface-degree of substitution of 0.54. This value is slightly higher than expected because the nucleophilic substitution on cellulose nanocrystals is only expected to occur at C6 (primary hydroxyl group, = 0.5) due to unfavorable conditions for Walden inversion at the secondary hydroxyl groups.28 The most probable explanation for this discrepancy is error in the calculation of the size of cellulose nanocrystals (in this case from pattern fitting of the XRD by Rietveld refinement).


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Raw Data Calculated Profile NN N+ Background

220

210

200

CPS

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190

180

170

160

410

408

406

404 402 400 Binding Energy /eV

398

396

Figure 2. XPS Nitrogen 1s high resolution spectrum of CNC-N3. Synthesis and characterization of (binary) homopolymer brushes tethered nanoparticles. The grafting of Pr-PLLA and of PBSBDEMPAM onto the surface of cellulose nanocrystals was performed via a heterogeneous copper(I)-catalyzed azide-alkyne cycloaddition reaction (Scheme 2) due to the tolerance and compatibility of the reaction towards a wide variety of solvents and functional groups.29 Moreover, this reaction proved highly efficient for cellulose modification.30,31

Polyester

OH

OH

HO

Polyester

N3

OH

OH

HO

OH

N3

HO CNC-N3

OH

OH

N

N N

CuBr/PMDETA, THF, 50°C

HO

N3

OH

PBSBDEMPAM-, PLLA- or PBSBDEMPAM/PLLA-g-CNC

Scheme 2. Polyester grafting onto CNC-N3 using heterogeneous click chemistry.


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Three different compositions were targeted: PBSBDEMPAM/PLLA-g-CNC (PBSBDEMPAM/PLLA = 1:1, mol/mol), PLLA-g-CNC and PBSBDEMPAM-g-CNC and their chemistry/properties were studied by ATR-FTIR, XPS, DSC, XRD and TGA. All samples were purified via Soxhlet extraction with chloroform and dried to constant weight prior to analysis. Based on gravimetric analyses, the amount (%) of grafted polyester(s) on the CNC was calculated to be 11, 12 and 9 wt% for PBSBDEMPAM/PLLA-g-CNC, PLLA-

g-CNC and PBSBDEMPAM-g-CNC, respectively. After grafting, the morphology of the CNCs was verified by FEG-SEM (Figure 3a). As can be seen, the initial rod-like structure of the CNC-N3 was preserved, meaning that the reaction conditions were suitable for grafting. The presence of polyester on the CNC surface in the purified samples was confirmed by both ATR-FTIR and XPS. As expected, pristine CNCs (Figure 3b, black line) are characterized with bands in the range 1200-1000 cm-1 assigned to the carbohydrate rings of the cellulose skeleton, a band at 1633 cm-1 designating OH bending vibrations of bound water32 and a band at 2110 cm-1 ascribed to the azide stretching mode.33 Purified samples grafted with polyester chains present an easily distinguishable new absorption(s) at 1714 cm-1, 1753 cm-1 or both, for PBSBDEMPAM-g-CNC, PLLA-g-CNC and PBSBDEMPAM/PLLA-g-CNC (intensity ratio

ν PBSBDEMPAM /ν PLA COO COO

of 51:49), respectively. The presence of carbonyl absorptions was

accompanied by a significant decrease in N3-stretch intensity, thus providing clear evidence for the occurrence of the grafting reaction through copper(I)-catalyzed azide-alkyne cycloaddition reaction. Intriguingly, N3-consumption in PBSBDEMPAM-g-CNC and PLLA-

g-CNC was between 50 and 60 % (residual N3-stretching), while it disappeared almost completely in the case of the purified binary homopolymer brushes tethered to nanoparticles 15 ACS Paragon Plus Environment

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PBSBDEMPAM/PLLA-g-CNC (Figure 3b, zoom). Taking into account the gravimetric results and the fact that the structures were obtained from a good solvent, the observed results might be explained by a “rippled” microphase separation of the two immiscible homopolyesters in the mixed brushes.34 However, considering the strong incompatibility of PLA and PBS, and the difference in fixation points between both polymer (at one end for PLLA and along the polymer backbone for PBSBDEMPAM), the formation of “dimple”-like phases cannot be excluded but is not supported by the obtained results. XPS data also confirm the proposed hypothesis of “rippled” microphase separation. Looking at the XPS carbon 1s spectra of the polyester-grafted nanocrystals (Figure 4), a change in surface carbon composition can be detected, dependent also on the grafted species. According to the results obtained here, PLLA grafting to CNCs produced noticeable increase in C-C and O-C=O character in the carbon 1s spectrum (Figure 4(b)), when compared with azidated cellulose nanocrystals (Figure 4(a)). Grafting of PBSBDEMPAM (Figure 4(c)) meanwhile produced a much higher increase in C-C character than in O-C=O character when compared with the PLLA-grafted CNCs. This is to be expected considering the polymer structures of PLLA and PBSBDEMPAM where the ratio of the two carbon environments (CC and O-C=O) in the repeat units is 1:1 and 11:4 respectively. The spectrum of the binary PBSBDEMPAM/PLLA-g-CNCs nanoparticles (Figure 4(d)) shows a mixture of both effects. The nitrogen 1s XPS spectra of the products (Figure 5. XPS nitrogen 1s spectra of (a) CNCN3, (b) PLLA-g-CNC, (c) PBSBDEMPAM-g-CNC and (d) PBSBDEMPAM/PLLA-gCNCFigure 5) show the extent of grafting of the polyesters to the CNC-N3 through loss of the positive nitrogen environment in azide at 405 eV. It is further consistent with FTIR spectroscopy where PBSBDEMPAM-g-CNC shows a lower level of modification with the azide peak at 405 eV still clearly visible. The presence of azide stretching vibrations in


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PLLA-g-CNC despite a lack of an azide peak in the XPS data can be explained by decomposition of the azide over time in the XPS due to instability to irradiation27 All three products have a broad peak around 401 eV, which can be explained as the three closely related nitrogen environments in the 1,2,3-triazole produced by the click reaction.35

(a)

(b)


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Figure 3. SEM-FEG image of the binary PBSBDEMPAM/PLLA-g-CNCs nanoparticles (a) and ATR-FTIR spectra of CNC-N3 and the purified polyester(s)-gCNC (b).

Raw Data

C-C

C-O

O-C-O

O-C=O 800

(a)

Background

Sum

(b)

800 600 600 400

400

200

200

292

CPS

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

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1600

290

288

286

284

282

292

800

(c)

290

288

286

284

282

290

288

286

284

282

(d)

1400 600

1200 1000

400

800 600 400

200

200 292

290

288

286

284

282

292

Binding Energy /eV

Figure 4. XPS carbon 1s spectra of (a) CNC-N3, (b) PLLA-g-CNC, (c) PBSBDEMPAM-gCNC and (d) PBSBDEMPAM/PLLA-g-CNC.


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Raw Data Background 220

Triazole 1 Sum

-

Triazole 2

N Azide

+

N Azide

N Azide

(b)

(a) 210

210 200

200

190

190

180

180

170

170

160 160 408

CPS

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

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380

404

400

396

(c)

408

404

400

396

408

404

400

396

(d) 190

360 180 340 170 320 160 300 150 408

404

400

396

Binding Energy /eV

Figure 5. XPS nitrogen 1s spectra of (a) CNC-N3, (b) PLLA-g-CNC, (c) PBSBDEMPAM-gCNC and (d) PBSBDEMPAM/PLLA-g-CNC. Once the successful grafting of polyesters onto CNCs via a “click” reaction was confirmed, the thermal properties of the homopolymer brushes tethered to nanoparticles were evaluated. DSC analyses were performed to give information about the crystallinity of the polyester(s)g-CNCs. The results are compared to the data of neat PBSBDEMPAM, Pr-PLLA and CNCN3 and are available in Table 2 and Figure 6a. No Tg was detected for PBSBDEMPAM (Table 2, Entry 1) due to the high crystallinity and fast crystallization rate of the polyester.36 A peak ascribed to cold crystallization (Tcc) appeared and merged with the typical melting (Tm). Similarly, Tcc and Tm were observed for Pr-PLLA (Table 2, Entry 2). In this case the Tg was also detected as expected from the low crystallinity and slow crystallization rate of this polymer.37 As expected, no thermal transition was detected for CNC-N3 at the applied conditions. 19 ACS Paragon Plus Environment

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Table 2. Thermal characteristics of purified PBSBDEMPAM-g-CNCs, PLLA-g-CNCs and PBSBDEMPAM/PLLA-g-CNCs as obtained by DSC from the second heating run. ∆Hcc, Entry

Sample

Tg, °C

Tcc, °C

∆Hm, Tm, °C

J/g

J/g*

1

PBSBDEMPAM

nv

88

nc

111

71.72

2

PLLA

50

86

21.57

157

49.02

3

CNC-N3

-

4

PBSBDEMPAM-g-CNCs

nv

-

-

115

73.85

5

PLLA-g-CNCs

59

Nv

-

nv

nv

-

-

114

PBSBDEMPAM/PLLA-g6

11.60

CNCs nv = not visible; nc = not calculated; * calculated per gram of polymer and by subtracting the cold crystallization peak where needed.

For the homopolymer brushes tethered to nanoparticles PBSBDEMPAM-g-CNC and PBSBDEMPAM/PLLA-g-CNC, only a Tm at 114-115 °C was present, which was clearly attributed to melting of PBSBDEMPAM crystallites formed at the CNC surface. Interestingly, while Tm was unaffected by the sample composition, the PBSBDEMPAM enthalpy of melting showed a dramatic composition dependence. For the PBSBDEMPAM-gCNC the ∆Hm (calculated per gram of polymer and by subtracting the cold crystallization 20 ACS Paragon Plus Environment

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peak where needed) remained insignificantly influenced by grafting, while it sharply decreased in the case of binary homopolymer brushes tethered to nanoparticles. As CNCs were not influencing PBSBDEMPAM crystallization in PBSBDEMPAM-g-CNC, this observation might be ascribed to impeded PBSBDEMPAM crystallization in the presence of PLLA. This was somewhat confirmed by the DSC data on PLLA in PBSBDEMPAM/PLLAg-CNC and in PLLA-g-CNC samples. With regard to PLLA, no thermal transitions (except a Tg in the case of PLLA-g-CNC) were recorded for this polyester in PBSBDEMPAM/PLLA-g-CNC and in PLLA-g-CNC samples. This result might be ascribed to Pr-PLLA degradation during the “click” reaction or to inhibited crystallization after grafting to CNCs. A simple test consisting of blending PLLA-gCNC with PDLA ( M n = 5,000 g mol-1) was performed and the occurrence of clear melting at 203 °C revealed stereocomplex formation,38 thus excluding the possibility of Pr-PLLA degradation during the “click” conditions (data not shown). Based on the obtained results and in agreement with the literature,39 one can conclude impeded PLA crystallization after grafting onto CNCs. Intriguingly, Tg values were found to shift to higher temperatures, suggesting more restricted mobility of the PLLA chains grafted onto the CNC surface. This restricted chain mobility, together with the relatively low molar mass of the grafted polyester ( M n of 5,000 g mol-1) might be the origin of the impeded PBSBDEMPAM and PLLA crystallization.


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(a)

CNC-N3 PBSBDEMPAM-g-CNC PBSBDEMPAM/PLLA-g-CNC PLLA-g-CNC

* *

*

(b)

* * *

5

10

15

20

25

2θ /°

30

35

40

45

50

Figure 6. DSC traces (a, second heating run) and XRD patterns (b) of PBSBDEMPAM-gCNCs, PLLA-g-CNCs and PBSBDEMPAM/PLLA-g-CNCs. For comparison the DSC of the neat polyesters and DSC and XRD of CNCs are also given. Black and blue asterisks show CNC and PBSBDEMPAM diffraction peaks, respectively. The results were confirmed by powder XRD performed in WAXS mode (Figure 6b). As can be seen, all samples present diffraction peaks at 2θ = 14.7° (11&0), 15.6° (110), 20.8 (102), 22.8° (200) and 34.5° (004) characteristic for the cellulose Iβ crystalline structure.40 In the case of PBSBDEMPAM-g-CNC additional diffractions were recorded at 2θ = 19.7° and 22 ACS Paragon Plus Environment

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29.1°, reasonably ascribed to the PBS α-form (monoclinic system of 21 helices: 2θ = 16.8°, 19.6°, 21.8°, 22.7°, and 29.1°).41 In agreement with the DSC results discussed above, cellulose Iβ diffraction peaks with a small shoulder at 19.7° (indicative of crystalline PBSBDEMPAM) were detected for PBSBDEMPAM/PLLA-g-CNC. Only cellulose Iβ diffraction peaks were detected in PLLA-g-CNC, which also lacks evidence for crystalline PLLA in DSC analysis. Thermal stability of (binary)homopolymer brushes tethered nanoparticles. As the target application of the binary homopolymer brushes tethered to nanoparticles is dispersion in polymer blends, their thermal stability is an important property. Accordingly, thermogravimetric analysis under inert atmosphere was performed (Table 3, Figure 7). For comparison, data on the thermal degradation of the neat polyesters and CNC-N3 are also provided. In all cases, weight loss (WL, %) up to 100 °C does not surpass 3 % and might be ascribed to loss of water. Thermal degradation was considered to start only at 5 % WL. As might be seen, the Pr-PLLA thermal decomposition proceeded in one stage starting at about 212 °C and reaching the maximum at 298 °C. The same one stage degradation profile was recorded for PBSBDEMPAM where the onset point and maximum were both shifted to higher temperatures (273 °C and 399 °C, respectively). In agreement with the literature,42 multiple degradation steps are recorded for CNC-N3, consisting in dehydration of cellulose, depolymerization and decomposition to gases. As expected for surface-modified CNCs,42 the thermal degradation of PBSBDEMPAM-g-CNC, PLLA-g-CNC, and PBSBDEMPAM/PLLA-g-CNC started at 213-226 °C, but

T dmax

shifted to higher values

(Table 3). Table 3. TGA of PBSBDEMPAM-g-CNC, PLLA-g-CNC, and PBSBDEMPAM/PLLA-gCNC 23 ACS Paragon Plus Environment

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Entry

Sample

WL100, %

Td5%, °C

Tdmax, °C

Residue, %

1

PBSBDEMPAM

0.2

273

399

2

2

PLLA

0.4

212

298

0

3

CNC-N3

2.9

219

288

24

4

PBSBDEMPAM-g-CNC

1.1

214

311

25

6

PLLA-g-CNC

0.8

226

304

15

5

PBSBDEMPAM/PLLA-g-CNC

1.0

213

308

17

Interestingly, their degradation occurred in one or two stages depending on their composition: PBSBDEMPAM-g-CNC and PLLA-g-CNC degraded in one stage, while two overlapping but well visible maxima were observed for the binary homopolymer brushes tethered to the nanoparticles. It might thus be speculated to be phase separation between the PLLA and the PBS chains on the CNC-surface, leading to two independent thermal degradations of both polyesters. However, further studies are needed to validate/reject this hypothesis and no conclusions are proposed at this stage.


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(a)

(b)

Figure 7. Weight loss (a) and DTG (b) thermograms of CNC-N3, PBSBDEMPAM-g-CNC, PLLA-g-CNC, and PBSBDEMPAM/PLLA-g-CNC.

Intriguing data are related to the residue measured at 600 °C (Table 3). While its amount for the neat polyesters is negligible (0-2 wt%), the residue amount increases to 25 wt% for CNCN3 due to char formation.42 Similar results were obtained for PBSBDEMPAM-g-CNC,


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PLLA-g-CNC, and PBSBDEMPAM/PLLA-g-CNC where residues varied between 15 and 25 wt%. The residue amount was found to depend on the amount of grafts, being higher (25 wt%) for PBSBDEMPAM-g-CNC with the higher CNC content. From the as-presented TGA data a synergistic relationship between polyesters and CNCs might be concluded. From one side, polyester grafting increases CNC thermal resistance, while CNC presence imparts char formation. Thus, polyester(s) grafted to CNCs with improved thermal properties were obtained. The presence of PBS and PLA chains on the surface of the binary homopolymer brushes tethered to nanoparticles makes these surface modified cellulosic nanomaterials attractive as compatibilization/reinforcement agents for PLA/PBS blends. CONCLUSIONS. By applying a three-step procedure consisting of 1) the synthesis of monoalkyne PLLA and PBS via ring-opening polymerization and classical melt polycondensation, respectively; 2) surface modification of CNCs to CNC-N3; and 3) grafting PLLA, PBSBDEMPAM or PLLA and PBSBDEMPAM chains onto CNC-N3 using heterogeneous copper(I)-catalyzed azide-alkyne cycloaddition reaction, (binary mixed) homopolymer brushes tethered to CNC were successfully obtained. The surface grafting was confirmed by FTIR and XPS and suggested “rippled” microphase separation of the two immiscible homopolyesters in the mixed brushes. Thermal properties and crystalline structure were evaluated by DSC and XRD and showed suppressed PBSBDEMPAM crystallization in presence of PLLA (binary mixed homopolymer brushes), while PLLA crystallization was impeded by grafting onto CNCs. Thermal stability was also studied by TGA and a synergistic relationship between polyesters and CNCs proposed: polyester grafting increases CNC thermal resistance, while CNC presence imparts char formation. Thus, the as-obtained binary


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homopolymer brushes tethered to nanoparticles makes these surface modified cellulosic nanomaterials attractive as compatibilization/reinforcement agents for PLA/PBS blends. AUTHOR INFORMATION Corresponding Author *R. Mincheva. * Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons (UMONS), Place du Parc 23, 7000 Mons, Belgium. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Financial support from Wallonia and European Commission in the frame of SINOPLISSPOLYEST project, the OPTI2MAT program of excellence, the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 7/05) and the FNRS-FRFC is acknowledged. L.J., and W.T. thank EPSRC (UK) (EP/J015687/1) for funding. L.J. also thanks the Malaysian government for a fellowship from the Ministry of Science, Technology and Innovation (mosti/bmi/taji-3/2 jilid). S.E. and W.T. further thank Research Foundation Flanders (Odysseus G.0C60.13N) and KULeuven (OT/14/072) for financial support. ACKNOWLEDGEMENTS. J.-M.R. is F.R.S.-FNRS Research Associate. REFERENCES


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(41) Ichikawa, Y.; Kondo, H.; Igarashi, Y.; Noguchi, K.; Okuyama, K.; Washiyama, J. Crystal Structures of a and b Forms of Poly(tetramethylene succinate). Polymer 2000, 41, 4719–4727. (42) Peng, Y.; Gardner, D. J.; Han, Y.; Kiziltas, A.; Cai, Z.; Tshabalala, M. A. Influence of Drying Method on the Material Properties of Nanocellulose I: Thermostability and Crystallinity. Cellulose 2013, 20, 2379–2392.


Biomacromolecules

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