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Polymer Nanocomposites with Cellulose Nanocrystals Featuring Adaptive Surface Groups Jens Christoph Natterodt, Janak Sapkota, E. Johan Foster, and Christoph Weder Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b01639 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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Polymer Nanocomposites with Cellulose Nanocrystals Featuring Adaptive Surface Groups Jens C. Natterodt,a Janak Sapkota,a E. Johan Foster,b Christoph Wedera* a

Adolphe Merkle Institute, University of Fribourg, Chemin des Verdiers 4, 1700 Fribourg, Switzerland

b

Virginia Polytechnic Institute and State University (Virginia Tech), Macromolecules Innovation Institute (MII), Department of Materials Science and Engineering, 203 Holden Hall, 445 Old Turner Street, Blacksburg, Virginia 24061, USA

Email: [email protected]

ABSTRACT: Cellulose nanocrystals (CNCs) are mechanically rigid, toxicologically benign, fiber-like nanoparticles. They can easily be extracted from renewable bio-sources and have attracted significant interest as reinforcing fillers in polymers. We here report the modification of CNCs with the 2-ureido-4[1H]pyrimidinone (UPy) motif as an adaptive compatibilizer, which permits the dispersion of UPy-modified CNCs in nonpolar as well as polar media. In toluene, the UPy motifs appear to form intra-CNC dimers, so that the particles are somewhat hydrophobized and well-dispersible in this nonpolar solvent. By contrast, the UPy motifs dissociate in DMF and promote dispersibility through interactions with this polar solvent. We have exploited this adaptiveness and integrated UPy-modified CNCs into non-polar and polar host polymers, which include different poly(ethylene)s, a polystyrene-block-polybutadieneblock-polystyrene elastomer, and poly(ethylene oxide-co-epichlorohydrin). All nanocomposites display an increase of stiffness and strength in comparison to the neat polymer, and some compositions retain a high elongation at break, even at a filler content of 15% w/w. Keywords:

adaptive

behavior,

cellulose

nanocrystals,

nanocomposite, 2-ureido-4[1H]pyrimidinone 1 ACS Paragon Plus Environment

CNCs,

hydrogen

bonding,

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INTRODUCTION The modification of polymer properties through the addition of a mechanically reinforcing or toughening nanofiller is a design approach that continues to attract significant interest in academic research and finds increasing technological use in a broad range of materials systems.1-6 Reinforcing fillers are typically characterized by a high Young’s modulus, high strength, and a high specific surface area. Examples include inorganic particles or platelets such as metal oxides and silicates,7 carbonaceous nanofillers such as carbon nanotubes and graphene,8 or polymeric nanofillers.9 Cellulose nanocrystals (CNCs) represent another filler type that is attracting much interest due to the biocompatibility and low toxicity,10 low density, and the abundance and renewable nature of cellulose.11 CNCs can be extracted from several sources, including wood,12 cotton,13 banana plants,14 tunicates,15 bacterial sources16 and other raw materials,17 and the commercial production of CNCs has recently been launched.18 CNCs have been used to reinforce a wide variety of polymers that cover a broad property range.19-21 Several processing schemes have been explored in order to optimize the dispersion of CNCs in the polymer matrix, which is essential for maximizing the reinforcing effect,22 including solvent-casting23 or melt-mixing.24 However, on account of the polar nature of cellulose, these methods are typically only applicable to polar matrix polymers, whereas nanocomposites with hydrophobic polymers, notably polyolefins, require specific processing protocols that may not readily be scalable, such as filling CNC scaffolds with monomer or polymer (solutions).24-29 Good dispersion of CNCs in less polar polymers can also be achieved by modifying the CNC surface with chemical motifs that serve to enhance the compatibility either by chemical modification of the nanocrystals’ surface groups or physical adsorption of compatibilizers. For example, Heux et al. reported the improved dispersion of CNCs in nonpolar solvents using a poly(ethylene oxide) (PEO) based compatibilizer, which is adsorbed on the surface of the CNCs and functions as a surfactant.30 This approach was used

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to create CNC composites with atactic poly(propylene) and low-density poly(ethylene) (LDPE).31-33 The dispersion of the CNCs in the nanocomposites thus made was indeed enhanced, as suggested by the transparent nature of the nanocomposites, but the PEO coating prevented interactions between the nanoparticles and thus almost no mechanical reinforcement was achieved. Ljungberg et al. also investigated CNCs grafted with maleated polypropylene and reported an enhanced dispersion of these modified CNCs, but no mechanical reinforcement was observed, also on account of reduced CNC-CNC interactions.32 Menezes et al. reported on the functionalization of CNCs with aliphatic chains (C6 – C18), but also in this case only an enhanced dispersion but no significant improvement of mechanical properties was observed.28 Recently, Volk et al. used poly(ethylene glycol)-bpolyethylene as a compatibilizer for melt compounding linear low-density poly(ethylene) (LLDPE) with microfibrillated cellulose, which resulted in nanocomposites with almost no visible aggregation.34 However, only a moderate improvement of the mechanical properties relative to a model nanocomposite without the compatibilizer was observed, probably due to the same reason as for the composites mentioned above. In summary, it appears that compatibilization schemes can lead to an enhancement of the CNC dispersion in nonpolar matrices, but at the same time the CNC-CNC interactions are weakened so that the reinforcement displayed by the modified CNCs is often comparable (or even inferior) than that of non-modified CNCs. We here report a new compatibilization strategy that seeks to overcome this problem and which involves the modification of CNCs with ureidopyrimidinone (UPy) groups,35 based on the idea that this motif can be employed as a “switchable surface modifier”. UPy dimerizes via quadruple hydrogen bonding with a high association constant Ka (6·108 M-1 in toluene) and is widely used as a binding motif in supramolecular polymers.36-40 We previously functionalized CNCs with this motif, and utilized this filler type, which is in the following referred to 3 ACS Paragon Plus Environment

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as CNC-UPy, in connection with a UPy-modified telechelic building block for the design of light-healable supramolecular nanocomposites.41,42 We show here that CNC-UPy can be dispersed in solvents of vastly different polarity, including toluene, dimethylformamide, and water. This is possible because the extent to which the UPy groups dimerize, and consequently how the CNC-UPy presents itself to the environment, changes with the nature of the solvent. We show here that this adaptive behavior permits the fabrication of nanocomposites with LDPE, LLDPE, polystyrene-block-polybutadiene-block-polystyrene (SBR), and poly(ethylene oxide-co-epichlorohydrin) (EO-EPI) via solution casting. In all cases, optical analysis suggests that the CNC-UPy are well dispersed in the polymer matrix and the mechanical properties are significantly improved vis à vis nanocomposites made with unmodified CNCs.

EXPERIMENTAL SECTION Materials. All reagents and solvents were purchased from Sigma-Aldrich and were used as received,

unless

otherwise

mentioned.

LDPE

(grade

1965Z300

nature,

density

δ = 0.924 g·cm-3, average particle size of < 600 µm) was obtained from Sabic Polymers. LLDPE (Dowlex NG 5056 E, δ = 0.919 g·cm-3) was obtained from Dow Polymers. EO–EPI was obtained from Daiso Co. Ltd, Osaka, Japan (Epichlomer®, co-monomer ratio = 1:1, δ = 1.39 g·cm-3). SBR (δ = 0.965 g cm-3) was purchased from Sigma Aldrich. Cellulose nanocrystals derived from cotton (CNC) were isolated according to a procedure that was described previously.43 The CNC suspensions were frozen in liquid nitrogen, and lyophilized using a VirTis BenchTop 2K XL lyophilizer with a condenser temperature of -78 °C to prepare dry CNCs. The dimensions of the CNCs were determined from TEM images, resulting in a length of 250 ± 90 nm, a width of 23 ± 5 nm and an aspect ratio (length/width) of 11 ± 2, as determined by analyzing 5 TEM images, in which the length and width of 20 – 30 CNCs per image were measured.


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Synthesis

of

2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone

(UPy-NCO). This compound was synthesized as reported before44 with the following modifications: 2-amino-4-hydroxy-6-methylpyrimidine (1.0 eq, 80 mmol, 10 g) was reacted with 1,6-diisocyanatohexane (7.0 eq, 560 mmol, 94 g) at 100 °C for 16 h. Hexane was added (100 mL), and the precipitate was filtered off, washed with hexane and dried at 50 °C under reduced pressure to afford UPy-NCO as a white powder (22 g, 95%). 1H-NMR (400 MHz, CDCl3): δ 13.1 (s, 1H, CH3CNH), 11.9 (s, 1H, CH2NH(C=O)NH), 10.1 (s, 1H, CH2NH(C=O)NH), 5.8 (s, 1H, CH=CCH3), 3.3 (m, 4H, NH(C=O)NHCH2 + CH2NCO), 2.2 (s,

3H,

CH3C=CH),

1.6

(m,

4H,

NCH2CH2

+

NHCH2CH2),

1.4

(m,

4H,

CH2CH2CH2CH2CH2CH2). Synthesis of 2(2-ethylhexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone (EHAUPy). This model compound was synthesized according to the procedure described above, but the following modifications were made: 2-amino-4-hydroxy-6-methylpyrimidine (1.0 eq, 21 mmol, 2.7 g) was reacted with 2-ethylhexyl isocyanate (1.5 eq, 32 mmol, 5 g) in pyridine (65 mL) at 130 °C for 2 h. Acetone was added (70 mL), and the precipitate was filtered off and washed with acetone. The white solid was recrystallized from EtOH/CHCl3 (ratio 7/1, 135 mL) at 5 °C overnight, and the precipitate was filtered off and dried at 50 °C under reduced pressure to afford EHA-UPy as a white powder (5.2 g, 86%). 1H-NMR (400 MHz, DMSO-d6): δ 11.4 (s, 1H, CH3CNH), 9.5 (s, 1H, CH2NH(C=O)NH), 7.5 (s, 1H, CH2NH(C=O)NH), 5.8 (s, 1H, CH=CCH3), 3.1 (m, 2H, NH(C=O)NHCH2), 2.1 (s, 3H, CH3C=CH), 1.4 (m, 1H, NHCH2CH), 1.4 (m, 8H, CH3CH2CH2CH2CH(CH2CH3)CH2). Preparation of CNC-UPy by Surface Modification of CNCs with UPy-NCO. CNC-UPy was prepared by the reaction of CNCs with UPy-NCO using the protocol described elsewhere for the modification of CNCs isolated from tunicates.41 The CNC-UPy were not dried and kept suspended in DMF at a concentration of 3 mg/mL, as determined gravimetrically by 5 ACS Paragon Plus Environment

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measuring the solid content after evaporating the solvent from three 1 mL aliquots in an vacuum oven at 50 °C. Prior to the preparation of nanocomposites, the solvent was exchanged from DMF to toluene, THF or DMSO by three cycles of centrifugation at 7000 rpm and replacement of the supernatant with the respective solvent. After the final centrifugation step, toluene, THF or DMSO was added until the concentration of CNC-UPy reached 3 mg/mL, which was confirmed by measuring the solid content as described above. The dispersions of CNC-UPy were sonicated (Bandelin Sonorex Technik RL 70 UH sonicator at 40 kHz) for 1 h before they were used for the preparation of nanocomposites. Preparation of CNC and CNC-UPy dispersions in water, DMSO, THF, DCM and toluene for optical and electron microscope imaging. Prior to imaging, freeze-dried unmodified CNCs were dispersed in DMF (3 mg/mL) by stirring overnight and sonicating for 3 h. For the CNCs and the never-dried CNC-UPy dispersed in DMF the solvent was exchanged from DMF to water, DMSO, THF, DCM or toluene as described above. After the final centrifugation step, the respective solvent was added until the concentration reached 1 mg/mL (optical imaging) or 0.01 mg/mL (electron microscope imaging), which was confirmed by measuring the solid content as mentioned above. The dispersions in the new solvents were sonicated for another 3 h before use. Fabrication of Nanocomposites. LDPE/CNC-UPy and LLDPE/CNC-UPy nanocomposite films were prepared by solution casting from toluene. The polymer (0.85 - 1 g) was dissolved in toluene (50 mL) by stirring the mixture at 100 °C for 30 min using a reflux condenser, before the required amount of a CNC-UPy dispersion in toluene was added to the polymer solution to create a 1 g composite with a CNC-UPy content of 2.5, 5, 10 or 15% w/w (relative to the overall weight). For example, to prepare the 15% w/w nanocomposite, 50 mL of a toluene dispersion containing 3 mg/mL CNC-UPy were added to 0.85 g of the polymer dissolved in toluene. After stirring the mixture for 10 min at 100 °C, the dispersion was cast 6 ACS Paragon Plus Environment

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into a Teflon® Petri dish with a diameter of 12 cm and the solvent was allowed to evaporate at room temperature in a well-ventilated hood overnight. The residual solvent was removed under reduced pressure at 70 °C for 24 h. The nanocomposites thus obtained were compression molded into films with a thickness of 250 µm. This was done in a Carver press between Teflon® sheets and using spacers to control the thickness at 170 °C, first for 3 min without pressure and for another 3 min with a pressure of 5 metric tons, before the samples were removed from the press, and allowed to cool to room temperature between the Teflon® sheets under ambient conditions. Characterization of the samples was performed at least 3 and at most 10 days after the samples had been prepared. SBR/CNC-UPy and EO-EPI/CNC-UPy nanocomposite films were prepared in a similar manner, but the following changes were made: The solvent was changed to THF (SBR) or DMSO (EO-EPI) and the polymer solutions were stirred for 2 h 60 °C before the CNC-UPy dispersion in THF (SBR) or DMSO (EO-EPI) was added. After stirring the mixtures for 10 min at 60 °C, the dispersions were cast into Teflon® Petri dishes and the solvent was allowed to evaporate in a ventilated oven at 60 °C overnight (SBR) or at 80 °C for two days (EO-EPI). The residual solvent was removed under reduced pressure at 80 °C for 24 h. In both cases, compression molding was done at 130 °C. SBR and EO-EPI nanocomposites with non-functionalized CNCs were fabricated in the same way, but the solvent exchange step was omitted and CNCs were directly dispersed in the respective solvent. Materials Characterization. 1H-NMR (400 MHz, 32 scans) and 13C-NMR (400 MHz, 1024 scans) spectra were recorded on a Bruker Advanced III HD in DMSO-d6 or CDCl3. Infrared spectra were recorded on a Perkin Elmer Spectrum 65 spectrometer in ATR mode. Ultraviolet-visible (UV/VIS) spectra were taken on a Jasco V-670 spectrophotometer in DMF, if not otherwise noted. Thermogravimetric analyses (TGA) were made under nitrogen using a Mettler-Toledo STAR thermogravimetric analyzer with a heating rate of 10 °C/min. 7 ACS Paragon Plus Environment

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Differential scanning calorimetry (DSC) was conducted under nitrogen using a MettlerToledo STAR system differential scanning calorimeter with a heating and cooling rate of 10 °C/min. Dynamic mechanical analysis (DMA) measurements were conducted on a TA Instruments Model Q800 dynamic mechanical analyzer (tensile mode at 1 Hz frequency and 15 µm amplitude, temperature ramp between -100 and 130 °C) using rectangular shaped samples (10 mm x 5.3 mm x 0.25 mm). Stress-strain measurements were performed on the same instrument at 25 °C and with a strain rate of 10 %/min on samples that had been cut into a dog-bone shape with a width of 2.1 mm and measurement length of 5.5 – 6.5 mm. Electron micrographs were taken on a FEI Tecnai spirit after drop-casting 0.1 mg/mL dispersions of CNC or CNC-UPy onto carbon-coated copper grids and drying the samples prior to analysis at 70 °C for 1 h.

RESULTS AND DISCUSSION The CNCs used in this study were isolated from cotton pulp by hydrolysis with sulfuric acid according to a previously reported procedure.43 Their dimensions were determined by analysis of transmission electron microscopy (TEM) images, which resulted in an average length of 250 ± 90 nm and a width of 23 ± 5 nm. Conductometric titration (Figure S1) shows that the CNCs had a charge density of 40 mmol/kg, associated with sulfate ester groups that are introduced during hydrolysis.


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A

Figure 1. (A) Synthesis of UPy-functionalized cellulose nanocrystals (CNC-UPy). (B) TEM images of non-functionalized CNCs and CNC-UPy deposited from dispersions (0.01 mg/mL) in selected solvents onto carbon-coated copper grids. Scale bar = 1 µm.

The CNCs were decorated with the hydrogen-bonding UPy motif by reaction of a CNC dispersion in DMF with 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone (UPy-NCO) using the conditions that were previously successfully employed for the modification of CNCs isolated from tunicates (Figure 1a).41 The modified CNCs (CNC-UPy) were separated from the reaction mixture by centrifugation, washed with DMF, and kept dispersed in this solvent. Fourier transform infrared (FT-IR) spectra (Figure S2), elemental analysis (Table S1), and UV-Vis absorption spectra (Figure S3), confirm the success of the


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reaction, while TEM images (Figure 1a) reveal that the morphology of the nanocrystals remains unchanged. The IR spectra of the CNC-UPy and the model compound 2(2ethylhexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone (EHA-UPy, Figure S2) both show the occurrence of additional bands between 1400 and 1800 cm-1, which were reported to be associated with the CO vibrations in UPy groups.45 The UV/Vis spectra of CNC-UPy dispersed in DMF (Figure S3) reveal an absorption peak around 290 nm, which is also apparent in a DMF solution of EHA-UPy and caused by the UPy motif. A quantitative analysis of the absorption spectra of CNC-UPy against the spectra of a series of EHA-UPy solutions with known concentrations indicates that the UPy units account for 26% w/w of CNC-UPy. This corresponds to a degree of substitution (DS) of 0.16, defined as the fraction of all anhydroglucose units present in the CNCs that have reacted. Two factors have to be taken into account when interpreting this DS value. A considerable fraction of the CNC hydroxyl groups resides inside the crystalline nanoparticles and is thus unavailable for reaction; as a result, the DS of CNCs derived from cotton cannot exceed a value of 0.57.46 In addition, the significantly reduced reactivity of the secondary OH groups vis-à-vis the primary OH groups makes the former inaccessible in many reactions, and the maximum DS is therefore reduced to 0.19. Since only about 0.6% of all primary hydroxy groups are substituted by sulfate groups, the maximum DS is not significantly reduced by the presence of sulfate groups. Thus, almost all primary hydroxyl groups available for reaction were derivatized with UPy. This value is corroborated by elemental analysis data (Table S1), which shows an increase in the nitrogen content and points to a composition that is consistent with a DS of 0.17. Gratifyingly, TEM images of CNC-UPy deposited from DMF or other solvents (Figure 1b) reveal no difference in shape vis-à-vis the neat CNCs and the dimensions are also unchanged.


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The dispersibility of the CNC-UPy and the unmodified CNCs in different solvents was qualitatively probed in a series of solvents with decreasing polarity (H2O > DMSO > DMF > DCM > THF > toluene) by comparing the appearance as a function of time (Figure 2). Except for DMF (the solvent in which CNC-UPy was originally made and kept), the CNCUPy dispersions were accessed by solvent exchange, had a concentration of 1 mg/mL, and they were ultrasonicated before the initial inspection. Non-functionalized CNCs were dispersed in DMF and transferred to other solvents by the same procedure. In the more polar solvents (water, DMSO, DMF) the non-functionalized CNCs disperse very well and remain well-dispersed after 24 h, which is consistent with previously reported findings.47 The CNCUPy also disperse well in these solvents, but tend to settle a bit after 24 h. In less polar solvents (THF, DCM) both the non-functionalized CNCs and CNC-UPy disperse well, but show some sedimentation over the course of 24 h. Not surprisingly, the non-functionalized CNCs cannot be dispersed in toluene, whereas CNC-UPy are dispersible in this solvent and only slowly settle over 24 h. TEM images (Figure 1b) acquired after depositing CNC-UPy and CNCs from dispersions in the various solvents corroborate these trends. The deposition of freshly prepared CNC-UPy dispersion affords well-separated nanoparticles from all solvents, indicating good dispersibility. Non-functionalized CNCs are well separated when deposited from highly polar solvents, aggregate formation increases in solvents with decreasing polarity, and they could not be processed at all from toluene.


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Figure 2. Photographs of dispersions of non-functionalized CNCs and 2-ureido-4[1H]pyrimidinone functionalized CNCs (CNC-UPy) in different solvents (concentration: 1 mg/mL). The pictures were taken immediately after ultrasonication for 3 h (top), and after letting the samples settle for 1 h (middle) or 24 h (bottom).

The good dispersibility of CNC-UPy in toluene is initially surprising, since the polar UPy moieties cannot a priori be expected to provide a substantial stabilization in non-polar solvents.48 The high association constant of the UPy motif (KA = 6·108 M-1 in toluene at room temperature)36 suggests that even under dilute conditions a substantial fraction of the UPy groups should be dimerized in this solvent. Indeed, UV/Vis spectra (Figure 3a) reveal that the characteristic absorption band around 290 nm associated with dissociated UPy motifs49 is very weak for solutions of EHA-UPy and dispersions of CNC-UPy in toluene. However, the absorbance increases in both cases by a factor of ca. 10 if the solvent is changed to DMF, which is well known to break UPy-UPy interactions due to competitive hydrogen bonding.50 The absence of substantial aggregation of CNC-UPy in toluene suggests that dimerization occurs preferably between UPy groups attached to the same nanocrystal, and not between different particles. Apparently, this causes the nanoparticles to be hydrophobized and well12 ACS Paragon Plus Environment

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dispersible in a nonpolar environment. By contrast, the UPy motifs dissociate in DMF and promote the dispersibility of the CNCs through interactions with this polar solvent.

A CNC-UPy 0.1 mg/mL DMF CNC-UPy 0.1 mg/mL Toluene

0.6

Absorbance

0.5 0.4 0.3 0.2 0.1 0.0 250

300

350

Wavelength (nm)

B

EHA-UPy 0.01 mg/mL DMF EHA-UPy 0.01 mg/mL Toluene

0.2 Absorbance

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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0.1

0.0 250

300

350

Wavelength (nm)

Figure 3. (A) Ultraviolet–visible (UV/Vis) absorption spectra of dispersions of ureidopyrimidinone-functionalized cellulose nanocrystals (CNC-UPy) in DMF and toluene. The intensity of the peak at 293 nm, which is related to dissociated UPy motifs, reflects that the extent of dimerization is much higher in toluene than in DMF. (B) UV/VIS spectra of solutions of the model compound (2(1-ethylhexylaminocarbonylamino)-6-methyl-4[1H]pyrimidinone) (EHAUPy) in DMF and toluene; the concentrations of the UPy motifs are comparable in all cases.

The adaptive dispersion behavior of CNC-UPy permits processing from different solvents, which in turn enables facile incorporation as reinforcing filler into polymer matrices spanning a broad range of polarity/dissolution behaviors. This was demonstrated through the investigation of four series of nanocomposites with LDPE, LLDPE, SBR, and EO-EPI as matrices. In


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all cases, the CNC-UPy content was varied between 2.5 and 15 % w/w. All nanocomposites were prepared by solution casting and shaped into thin films of a thickness of ca. 250 µm by compression-molding. The solvents employed were selected to match the solubility of the polymers, i.e. toluene in the case of LDPE and LLDPE, THF for SBR, and DMSO for EOEPI. To evaluate the reinforcing capability of CNC-UPy, reference films with unmodified CNCs and EO-EPI or SBR were made. In the case of LDPE and LLDPE reference composites with unmodified CNCs are not accessible by solvent casting, but the data reported by Sapkota et al., who used a mixing scheme for unmodified CNCs and LDPE, were used as benchmark.27 The visual appearance, specifically the absence or presence of visible aggregates and coloration effects, were used to optimize the processing conditions for each system. Films of the neat polymers and the corresponding nanocomposites with 10% w/w of CNC-UPy (Figure 4) made under conditions that were thus established, have the same transparent appearance, which provides a first indication for a homogeneous dispersion of the CNC-UPy in the various matrices. The detailed procedures can be found in the experimental section, but a few specific aspects deserve to be highlighted. In the case of the LDPE and LLDPE nanocomposites that were processed from toluene, homogeneous materials were only obtained if the solvent was evaporated at ambient temperature (Figure S4). As in recent work on composites of LDPE with unmodified CNCs,25 slight yellowing was observed upon compression molding, which can be attributed to the fact that this step was carried out at 170 °C, i.e., at a temperature that overlaps with the onset of thermal degradation of CNCs prepared by sulfuric acid hydrolysis.47 The protonation and deprotonation of UPy motives in strong basic and acid conditions makes conductometric titration of CNC-UPy as a measure of sulfate groups impossible, however generally organosulfate groups are normally attacked by strong nucleophiles such as NaOH,18 and not by electrophiles such as isocyanates; thus the sulfate content is assumed to be not affected by the UPy functionalization and it explains the


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yellowish appearance of composites exposed to elevated temperatures. On the other hand, SBR/CNC-UPy composites only had a homogeneous appearance if the solvent was evaporated at 60 °C, whereas large-scale phase separation was observed upon drying at ambient temperature. The strong influence of the processing temperature on the composites’ properties can be attributed to the differences of the polymer solubility and possibly the dispersibility of the CNC-UPy at different temperatures, although this aspect was not investigated in detail.

Figure 4. Images of ca. 250 µm thick films of the matrix polymers (left) and the corresponding polymer/CNC-UPy composites (right). (A) LDPE; (B) LDPE/CNC-UPy; (C) LLDPE; (D) LLDPE/CNC-UPy; (E) SBR; (F) SBR/CNC-UPy; (G) EO-EPI; (H) EO-EPI/CNC-UPy. All materials contain 10% w/w CNC-UPy.


Biomacromolecules

A

B

8 7 Stress (MPa)

Storage Modulus (MPa)

104

3

10

102

LDPE/CNC-UPy 15% w/w LDPE/CNC-UPy 10% w/w LDPE/CNC-UPy 5% w/w LDPE/CNC-UPy 2.5% w/w LDPE neat

101 -100

6 5 4 3

LDPE/CNC-UPy 15% w/w LDPE/CNC-UPy 10% w/w LDPE/CNC-UPy 5% w/w LDPE/CNC-UPy 2.5% w/w LDPE neat

2 1 0

-50

0

50

0

100

10

20

30 40 Strain (%)

Temperature (°C)

C

D

2

10

LLDPE/CNC-UPy 15% w/w LLDPE/CNC-UPy 10% w/w LLDPE/CNC-UPy 5% w/w LLDPE/CNC-UPy 2.5% w/w LLDPE neat

10 LLDPE/CNC-UPy 15% w/w LLDPE/CNC-UPy 10% w/w LLDPE/CNC-UPy 5% w/w LLDPE/CNC-UPy 2.5% w/w LLDPE neat

5

0

-50

0

50

0

100

100

Temperature (°C)

F

104

SBR/CNC-UPy 15% w/w SBR/CNC-UPy 10% w/w SBR/CNC-UPy 5% w/w SBR/CNC-UPy 2.5% w/w SBR neat

103

2

10

200 300 Strain (%)

1

SBR/CNC-UPy 15% w/w SBR/CNC-UPy 10% w/w SBR/CNC-UPy 5% w/w SBR/CNC-UPy 2.5% w/w SBR neat

0

-50

0

0

50

100

Temperature (°C)

G

500

2

101

100 -100

400

3

Stress (MPa)


60

15

103

101 -100

E

50

20

Stress (MPa)


104

200 300 Strain (%)

H

104 EO-EPI/CNC-UPy 15% w/w EO-EPI/CNC-UPy 10% w/w EO-EPI/CNC-UPy 5% w/w EO-EPI/CNC-UPy 2.5% w/w EO-EPI neat

103

102

400

500

EO-EPI/CNC-UPy 15% w/w EO-EPI/CNC-UPy 10% w/w EO-EPI/CNC-UPy 5% w/w EO-EPI/CNC-UPy 2.5% w/w EO-EPI neat

0.3 Stress (MPa)


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

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0.2

0.1

101

100

0.0

-60

-40

-20

0

20

40

0

60

50

Temperature (°C)

100

150 200 Strain (%)

250

300

Figure 5. Graphs showing the storage modulus E' (A, C, E, G), and stress-train curves (B, D, F, H) of films of the matrix polymers and the composites with CNC-UPy. (A, B) LDPE; (C, D) LLDPE; (E, F) SBR; (G, H) EO-EPI. Curves are representative of 3-5 experiments.


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Table 1. Mechanical properties of matrix polymers and nanocomposites with CNC-UPy. Storage Modulus at 25 °C (MPa)a 215 ± 50

Young’s Modulus (MPa)b 98 ± 6

LDPE/CNC-UPy 2.5% w/w

420 ± 55

LDPE/CNC-UPy 5% w/w

Sample Neat LDPE

Yield Stress Strain at (MPa)b Break (%)b 5.8 ± 0.9

65 ± 15

140 ± 17

7.0 ± 0.5

26 ± 8

520 ± 50

160 ± 20

7.4 ± 0.6

19 ± 7

LDPE/CNC-UPy 10% w/w

590 ± 50

205 ± 20

7.6 ± 0.5

13 ± 3

LDPE/CNC-UPy 15% w/w

755 ± 65

232 ± 41

7.8 ± 0.5

7.5 ± 1.2

Neat LLDPE

435 ± 30

125 ± 15

8.7 ± 0.9

465 ± 40

LLDPE/CNC-UPy 2.5% w/w

570 ± 35

135 ± 10

9.5 ± 0.2

435 ± 65

LLDPE/CNC-UPy 5% w/w

595 ± 50

145 ± 20

9.9 ± 0.6

445 ± 50


750 ± 65

165 ± 5

10.0 ± 0.5

480 ± 30


900 ± 10

195 ± 10

13.0 ± 0.5

495 ± 15

Neat SBR

2.8 ± 1.2

3.0 ± 1.8

1.7 ± 0.3

425 ± 20

SBR/CNC-UPy 2.5% w/w

15 ± 2

5.6 ± 1.6

2.1 ± 0.2

435 ± 10

SBR/CNC-UPy 5% w/w

16 ± 4

11 ± 5

2.6 ± 0.2

420 ± 25

SBR/CNC-UPy 10% w/w

26 ± 8

16 ± 6.9

2.8 ± 0.2

405 ± 75

SBR/CNC-UPy 15% w/w

54 ± 13

26 ± 11

3.0 ± 0.3

400 ± 85

Neat EO-EPI

2.8 ± 0.6

0.7 ± 0.1

0.23 ± 0.08

265 ± 35

EO-EPI/CNC-UPy 2.5% w/w

6.7 ± 1.2

1.5 ± 0.4

0.24 ± 0.06

135 ± 25

EO-EPI/CNC-UPy 5% w/w

8.8 ± 1.3

3.0 ± 0.8

0.27 ± 0.05

75 ± 22


11 ± 2

4.4 ± 0.9

0.28 ± 0.09

40 ± 13


20 ± 3

5.0 ± 1.2

0.34 ± 0.11

35 ± 8

a

Based on DMA measurements. The data represent averages from at least three samples per composition. b Based on stress-strain measurements at 25 °C. The data represent averages from at least three samples per composition.


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The mechanical properties of the neat LDPE, LLDPE, SBR and EO-EPI and of the corresponding nanocomposites with 2.5 - 15% w/w CNC-UPy were probed by dynamic mechanical analysis (DMA) and tensile testing. The data thus acquired are compiled in Figure 5 and Table 1. Figure 5a shows representative curves of the storage modulus as a function of temperature for the series of LDPE/CNC-UPy nanocomposites and the neat LDPE. The graph clearly shows that the storage modulus increases over the entire temperature range with the CNC-UPy content. At room temperature (25 °C), the storage modulus E’ is increased more than three-fold for the nanocomposite with 15% w/w CNC-UPy (≈ 760 MPa) in comparison to the neat LDPE (E’≈ 235 MPa). A similar trend was observed for the Young’s modulus determined from the tensile tests (Figure 5b). The yield stress only increased slightly with increasing CNC-UPy content, while the strain at break decreased drastically from ≈ 65% for the neat LDPE to ≈ 7% for the composite with 15% w/w CNCUPy. This increase in brittleness is common and has previously been reported for many cellulose nanocomposites.25, 51 In order to probe to what extent the reinforcement imparted by the CNC-UPy might be caused by secondary effects, in particular a nucleation-induced change of the crystallinity, differential scanning calorimetry (DSC) experiments were conducted (Figure S5a, Table S2). The DSC traces reveal an identical melting temperature of ca. 108 °C, independent of the CNC-UPy content, while the crystallinity was between 25 and 28%, without a correlation to the CNC-UPy content. Thus, the primary cause of the reinforcement is not a change of the morphology induced by nucleation effects, but the intrinsic reinforcement imparted by the CNC-UPy. At least at temperatures below 300 °C, the thermogravimetric analysis (TGA) traces (Figure S5b) are identical for all compositions, suggesting that the thermal stability is not negatively influenced by the incorporation of the filler. Non-functionalized CNCs were previously incorporated into the same polymer grade by premixing the components in water and subsequent melt-mixing and compression molding; 18 ACS Paragon Plus Environment

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however, the reinforcement in the materials thus made was significantly lower than the effects reported here.27 For example, LDPE reinforced with 15% w/w CNC-UPy (corresponding to 11% w/w CNCs and 4% w/w UPy) displays a storage modulus of 760 MPa, whereas an LDPE composite with 10% w/w of unmodified CNCs (fabricated via premixing) has a storage modulus of 540 MPa. To evaluate the reinforcing potential of CNC-UPy in other grades of polyethylene, similar composites were prepared with LLDPE as the matrix. The DMA traces and stress-strain curves of the neat LLDPE and LLDPE/CNC-UPy composites are shown in Figure 5c,d. The DMA traces mirror the trend seen for the composites made with LDPE, although the storage moduli of the neat polymer (≈ 430 MPa at 25 C) and the composites (≈900 MPa for the composite containing 15% w/w of CNC-UPy at 25 °C) are slightly higher. The same trend is seen for the Young’s modulus and yield stress, which increased from 125 and 8.7 MPa (neat LLDPE) to 195 and 13 MPa (15% w/w of CNC-UPy). As in the case of the LDPE composites, DSC and TGA characterization showed no effect of the CNC-UPy content on the melting temperature (≈ 120 °C), crystallinity (33 – 36%) and onset of thermal degradation (Figure S6, Table S2). Interestingly, all LLDPE composites with CNC-UPy display a strain at break of >400%. This desirable behavior, i.e., a significant reinforcement/stiffening without embrittlement, is not common for polymer composites with unmodified CNCs,52, 53 including LLDPE/CNC nanocomposites made by a templating process.25 A prominent exception was reported by McKee and coworkers, who investigated a one-component composite consisting of CNCs that had been surface-grafted with poly(methacrylate) carrying UPy groups and observed plastic deformation and a high elongation at break.54 This behavior was explained with the stress-induced dissociation of UP-dimers, which served as sacrificial bonds. While this mechanism might also be at play in the materials investigated here, the formation of intraCNC rather than inter-CNC UPy dimers in the polymer, and as a result the absence of a 19 ACS Paragon Plus Environment

Biomacromolecules

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hydrogen-bonded CNC network, is perhaps more probable on the basis of the solution data reported above. The difference between the LDPE/CNC-UPy and LLDPE/CNC-UPy networks may be related to the mobility of the amorphous phase, which is reported to be higher in polymers with more branches (such as LLDPE compared to LDPE) and thus limits the growth of microcracks from the CNC-UPy during elongation.55 We also investigated the mechanical data acquired for nanocomposites made from CNC-UPy and SBR (Figure 5e,f) or EO-EPI (Figure 5g,h). Both these matrix polymers are elastic and their storage moduli (ca. 3 MPa in both cases) are much lower than those of the two polyethylenes used. As a result, the stiffness increase is much more pronounced; at 25 ºC, E’ of the nanocomposites containing 15% w/w CNC-UPy is increased by a factor of 20 (SBR) and 7 (EO-EPI), respectively, vis-à-vis the neat polymers. Such prominent reinforcement is quite common for the integration of (unmodified) CNCs in low-modulus polymers; in fact, a similar reinforcement has been reported for solution-cast EO-EPI and SBR composites with non-functionalized CNCs processed from the same solvent (Figure S9 and Table S3). As mentioned before, the properties of nanocomposites containing 15% w/w CNC-UPy should be compared with those of a composite containing 11% w/w unmodified CNCs. Thus for the EO-EPI matrix, the storage modulus of a composite with 11% w/w non-functionalized CNCs (29 MPa) was slightly higher than that of the material containing 15% w/w CNC-UPy (20 MPa). In the case of SBR, the storage modulus of a composite with 10% w/w CNCs (38 MPa) was considerably lower than the corresponding material made with 15% w/w CNCUPy (54 MPa). The stress-strain curves of the SBR-based composites show, very much like the materials based on LLDPE, no tendency of a reduction of strain at break upon introduction of the CNC-UPy and the data thus support the conclusions drawn above. However, similar results were observed for SBR composites made with the unmodified CNCs (Table S3), suggesting that the matrix has a significant influence on how integration of CNCs affects the 20 ACS Paragon Plus Environment

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strain at break. This is also reflected by the fact that the stress-strain curves of the nanocomposites based on EO-EPI do show a reduction of the strain at break with increasing CNC-UPy content. In this case, the polymer matrix is much more polar and, perhaps more importantly, processing was done from a hydrogen-bond-breaking solvent (DMSO). Thus, it is possible that at least some inter-CNC UPy interactions and UPy-polymer interactions are formed.

CONCLUSIONS In summary, UPy-modified CNCs can be dispersed in nonpolar as well as polar solvents on account of adaptive interactions with the environment. In nonpolar environment the dispersibility is enabled by the formation of intra-CNC UPy dimers, which appear to cause some hydrophobization of the nanoparticles. By contrast, the UPy motifs dissociate in more polar solvents and promote dispersibility through hydrogen-bonding with the solvent. We have exploited this adaptiveness and integrated UPy-modified CNCs into non-polar and polar host polymers, including different poly(ethylene)s, polystyrene-block-polybutadiene-blockpolystyrene, and poly(ethylene oxide-co-epichlorohydrin). All nanocomposites display an increase in stiffness and strength, while in the case of LLDPE and SBR nanocomposites with CNC-UPy, a high strain at break was maintained. More importantly, the surface modification of CNCs described here is, to the best of our knowledge, the first successful approach that permits the incorporation of one type of CNCs into a broad range of both polar and apolar polymers without any visible aggregates.

SUPPORTING INFORMATION FT-IR and UV/VIS spectra of CNC-UPy; images showing the influence of the solution cast temperature on nanocomposites; DSC and TGA graphs of nanocomposites; NMR spectra; elemental analysis; crystallinity of nanocomposites; additional mechanical data.


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ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support from the Swiss National Science Foundation (NRP66: Resource Wood, Nr. 406640_136911/1) and the Adolphe Merkle Foundation. Received: Month XX, XXXX; Revised: Month XX, XXXX; Published online: DOI:


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TOC Figure Non-polar solvent

Cellulose Nanocrystal

Polar solvent

UPy dimers

Dissociated UPy motif


Polymer Nanocomposites with Cellulose ... - ACS Publications

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