Triazole End-Grafting on Cellulose Nanocrystals for Water

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Triazole End-Grafting on Cellulose Nanocrystals for Water-Redispersion Improvement and Reactive Enhancement to Nanocomposites Le Li, Han Tao, Bolang Wu, Ge Zhu, Ke Li, and Ning Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03407 • Publication Date (Web): 16 Sep 2018 Downloaded from http://pubs.acs.org on September 16, 2018

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ACS Sustainable Chemistry & Engineering

Triazole End-Grafting on Cellulose Nanocrystals for Water-Redispersion Improvement and Reactive Enhancement to Nanocomposites Le Li, † Han Tao, † Bolang Wu, † Ge Zhu, † Ke Li, ‡ and Ning Lin † * †

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, Luoshi Road #122, Wuhan 430070, P. R. China ‡

Intelligent Transport Systems Research Center, Wuhan University of Technology, Heping

Road #1178, Wuhan, 430063, P. R. China

KEYWORDS. Cellulose nanocrystals; end modification; redispersion; steric stability; composite.

Corresponding Author: * Dr. Ning Lin, Email: [email protected]

ABSTRACT Different from the conventional surface modification strategy, the end reaction based on active aldehyde groups of cellulose nanocrystal (CNC) provides a targeted modification under the protection of its surface chemistry. With the purpose of promoting its redispersibility in water, the strategy of triazole end-grafting performed on CNC was proposed in this study, exhibiting the significant improvement on the redispersion and stability of nanocrystals in the aqueous

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suspension attributed to synergistic effect of steric stabilization and electrostatic repulsion. The end-modified CNC was then introduced into natural rubber (NR) matrix to fabricate the composites with reactive compatibility from thiol-ene click reaction. Ascribed to the formation of covalent linkage between nanofillers and matrix together with the architecture of rigid percolating network, the mechanical properties of obtained composites were remarkably advanced. With the introduction of 10 wt% end-modified CNC, the tensile strength, Young’s modulus and storage modulus of the prepared composite increased by 160%, 468% and 1041% in contrast with those of neat NR material. More importantly, this composite retained a high level of elongation at break (1575%) similar as the raw rubber material attributed to the designed covalent linkage and resultant reactive enhancement of end-modified CNCs to NR matrix. INTRODUCTION Typically involved a process of chemical acid hydrolysis to release the crystal domains, the rigid cellulose nanocrystal (CNC) can be prepared from the native cellulose with diverse sources and regarded as a fascinating building block of novel nanoscale material in future.1 Attracted by its impressive physicochemical properties, e.g. sustainability, biodegradability, biocompatibility, low toxicity, high specific modulus, reactive surface chemistry, liquid crystalline, special rodlike morphology and high crystallinity, CNC has received tremendous attention from both academia and industry, which is widely used in the field of composites,2 paper-making,3 packaging,4 catalysis,5 energy and optical materials,6,7 electronics,8 sensor,9 environmental and biomedical applications.10,11 Most studies concerning the presence of CNCs require a good redispersion from the dried powders into the suspensions and effective prevention of the nanoparticles’ aggregates for the subsequent reaction, modification or application. The introduction of rigid CNCs into polymeric composites is the most extensive application, which is


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mainly attributed to its superior specific modulus (potentially stronger than steel)

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as the

nanoreinforcing filler. To synthesize CNCs-reinforced composites, the redispersibility and stability of CNCs play a critical role in the reinforcing effect to improve the mechanical properties of the obtained composites, while the inhomogeneous dispersion of CNCs may induce the inferior interfacial compatibility and even microphase separation of the composites.13 As discussed before, the reliable dispersion and redispersion of nanoparticles in the composites is one of the important factors to affect the resulting performance of the obtained materials, which commonly serves as the prerequisite in the development of novel nanomaterials. In the case of CNC, the formed hydrogen bonding and van der Waals attraction during the dehydration process14 will inevitably induce the aggregation of these rigid nanoparticles, and therefore result in a weak redispersion and uniformity in the suspension. When utilizing the common ovendrying treatment, the self-aggregation of redispersed CNCs is even more serious with the rapid sedimentation and visible agglomeration in the suspension, which can’t be eliminated by the conventional treatments such as the vigorous stirring and intense sonication. For the purpose of improving redispersion, steric stabilization and electrostatic repulsion from the surface grafted molecules on CNCs, the two strategies are effective. In the case of electrostatic repulsion, the typical sulfuric acid (H2SO4) hydrolysis can convert partial surface hydroxyl groups of CNCs as anionic sulfate groups (−ܱܱܵଷି ), covering the negatively-charged surface of nanocrystals and promoting their dispersion in solvents.15 However, this original dispersion effect from the H2SO4 hydrolysis is insufficient to support the redispersion of CNC powder into the suspension after drying treatments. Regarding the strategy of steric stabilization, an early study reported the efficient improvement of CNC dispersion by poly(ethylene glycol) grafting in an aqueous suspension.16 The limitation of this approach is the consumption of


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surface active hydroxyl groups and therefore may restrict the reactivity of subsequent chemical modification or reaction. Recent studies proposed the introduction of counterion of sodium salt onto the surface of CNCs to enhance re-dispersion and stability in the suspension,17 whereas the sensitive ionic environment made against the change of processing condition may affect the utilization of this method in the practical application. The idea inspired by the combination of different dispersing effects should be efficient to meet the demands of good redispersion and stability of CNCs in the suspensions, requiring the simple treatment, mild reaction and preservation of original structure of nanocrystals (surface chemistry). Recent studies point on the modification of CNCs active aldehyde groups, they oxidized aldehyde group to carboxyl group and the reacted with two amino groups compound,18,19 inspired by this, we have explored the end grafting of triazole (cyclic molecule) based on the reaction with active aldehyde groups of H2SO4-hydrolyzed CNCs in two different methods, which provided the synergistic effect of steric stabilization from end-grafted molecules and electrostatic repulsion from surface sulfate groups and therefore achieved the purpose of redispersion improvement for CNCs in aqueous suspension. The centrifugation/sedimentation experiment together with the birefringence observation of suspensions demonstrated the promoted redispersion and stability of end-modified CNCs after the conventional freeze-drying and oven-drying treatments. The end-modified CNC having the reactive thiol groups was then introduced into the natural rubber-based composites, constructing the covalent linkage from thiol-ene click reaction via the UV irradiation. The reactive enhancement of end-modified CNCs to the elastomeric matrix effectively improved the mechanical properties of the obtained composites, particularly with the simultaneous enhancement of strength and toughness. Under mild conditions, the proposed strategy of end modification on CNCs can preserve its surface


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chemistry with negative sulfate groups and active hydroxyl groups, which significantly promoted the redispersion in aqueous suspension meanwhile the possibility of further modification on the surface of the nanocrystals can be retained. EXPERIMENTAL SECTION Materials. 3-Amino-1,2,4-triazole-5-thiol (ATT, 98%) as the end-modified reagent and 2hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, >98%) as the watersoluble photoinitiator were purchased from the Aladdin corporation (Shanghai, China). N-(3Dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC, 98%), N-hydroxysuccinimide (NHS, 98%), sodium triacetoxyborohydride (STAB, 90%) were supplied by the Macklin corporation (Shanghai, China). Sodium chlorite (NaClO2, 80%), potassium chloride (KCl, 99.5%), sodium carbonate anhydrous (Na2CO3, 99.5%), acetic acid (>99.7%), natural rubber latex (60% solid content), sulfuric acid (H2SO4, 98%) and hydrochloric acid (HCl, 37%) were purchased from the Aladdin corporation and directly used without any pre-treatment. Preparation of cellulose nanocrystals (CNCs) from cotton linter. Cellulose nanocrystals were prepared from the cotton linter by strong acid hydrolysis.20 Briefly, the cotton fibers were purified with the diluted alkaline solution of NaOH (2 wt%) for 12 h, and then hydrolyzed in H2SO4 solution (64 wt%) for 1 h at 45 °C to remove the amorphous regions. Washing treatment of the suspension was performed thrice through centrifugation at 8000 rpm to remove the free acid and further purified by dialysis against distilled water for several days. To investigate the redispersion behavior, the CNC suspension was freeze-dried (FD) at -58 °C for three days or oven-dried (OD) at 45 °C for one day to release the powder, and then re-suspended into the water.


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End modification of CNCs via aldimine condensation. Two strategies on the end modification to CNCs were compared on the basis of the reaction with the active aldehyde groups, namely the direct aldimine condensation and indirect carboxyamine condensation. According to the previous report,21 the pH condition of CNCs aqueous suspension (10 mg mL-1) was adjusted as 9.2 by Na2CO3 buffer solution. A total of 3 mmol ATT and 45 mmol STAB as the reducing agent was added in the CNC suspension under the protection of nitrogen at 70 °C. In order to achieve a high end-grafting efficiency, the modifying agent ATT was divided into three portions then added to the CNCs suspension in three consecutive days (1 mmol each day). The small amount of 3 M HCl solution (20 mL) was then added to stop the reaction, and the resulting suspension was dialyzed against the distilled water for three days to remove the unreacted reagents. The incubation treatment to the suspension was performed by the addition of KCl solution, ultrasonicated for 10 min, followed by 24 h magnetic stirring of the suspension. Finally, the suspension containing end-modified CNC-ATT-1 was obtained after the purification by dialysis against the distilled water to remove the excess electrolytes. End modification of CNCs via carboxyamine condensation. Regarding the indirect carboxyamine condensation for the end modification, the end aldehyde groups of CNCs were firstly converted as the carboxyl groups (-COOH), and then reacted with the amino groups of ATT under the catalysis of EDC and NHS.22 Briefly, under the pH regulation by acetic acid as 3.5, the NaClO2 solution (250 mM) was added into the CNCs aqueous suspension (10 mg mL-1) for 20 h at room temperature to achieve the oxidization. After the purification by repeated centrifugation/washing and dialysis treatments, nitrogen gas was bubbled through the oxidized CNCs suspension (10 mg ml-1) for 30 min to drive out air and the pH was adjusted to 7.0, followed by addition of the co-catalyst NHS (0.3 mmol) and EDC (3 mmol). The pH of the


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mixture was adjusted to 6.5 and KCl (1 M) solution was carefully added to offer ionic strength. Under the protection of nitrogen, 3 mmol ATT was added in the suspension to start the carboxyamine reaction at room temperature by the pH regulation as 9.2 with 0.1 M Na2CO3 buffer solution. Incubation treatments were done in a similar way (as the aldimine condensation) above. ATT was added to the suspension in portions of 1 mmol per day for three consecutive days and finally the suspension containing the end-modified CNC (CNC-ATT-2) was obtained after the purification by dialysis against the distilled water to remove the unreacted reagents. Preparation of NR/CNC-x and NR/mCNC-x composites. The natural rubber (NR)-based composites were prepared by the liquid compounding and casting evaporation of the mixture of NR latex and pristine CNC or end-modified CNC-ATT-1.23 The water-soluble photoinitiator (Irgacure 2959) was dissolved in water (2 wt% according to the NR weight) and slowly added into the mixture and kept under magnetic stirring for 6 h. The possible click reaction between nanocrystals and NR in the homogeneous mixture was initiated by UV irradiation (365 nm, highintensity ultraviolet lamp, SB-100PC/FC, SPECTROLINE, USA) with the controlled distance, intensity and regulated durations (10 min, 30 min, 60 min, 90 min, 120 min).24 The treated mixture was vacuum-degassed for 30 min at room temperature to remove the bubbles, and finally casted in PTFE molds at 45 °C overnight to obtain the composites. Two series composites were coded as the nomenclatures of NR/CNC-x and NR/mCNC-x containing the pristine CNC and end-modified CNC-ATT-1 (using the mCNC to simplify its name) with various loading levels ranging from 0 wt% to 15 wt%. Characterization. The

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C solid-state nuclear magnetic resonance (ssNMR) was performed by a Bruker Avance

400 NMR spectrometer with the 4 mm CP-MAS probe, and recorded at 100.61 KHz, 12000 Hz


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spinning speed, 6 s relaxation delays. The high resolution X-ray photoelectron spectroscopy (XPS) was recorded by a ESCALAB 250Xi equipment (Thermo Fisher Scientific, USA) at 100 W power with 20 eV pass energy. Elemental analysis (EA) was performed to trace the elemental change of cellulose nanocrystals before and after end modification, which was analyzed by the Vario EL cube elemental analyzer (Elementar Analysensysteme GmbH). The thermogravimetric analyzer (TGA, NETZSCH STA449F3, Germany) and D8 Advance X-ray diffractometer (XRD, Bruker, Germany) were used to investigate the influence of end modification to the thermal stability and crystalline property of nanocrystals. The samples for TGA were heated from 100 to 600 °C at a heating rate of 10 °C/min in nitrogen atmosphere. The XRD was performed using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 60 mA with the diffraction angles (2θ) ranged from 5° to 45° at a speed rate of 0.02 °/sec. The crystallinity index (χୡ ) of nanocrystals were calculated from the XRD data according to the Segal equation.25 All the samples were recorded by a Fourier transform infrared spectroscopy (FTIR) iS5 spectrometer (Nicolet, Madison, USA) in the range of 4000-400 cm-1. The morphological observation of cellulose nanocrystals before and after modification was carried out by transmission electron microscope (TEM) on a Tecnai G2 F30 instrument (FEI, USA) at 300 kV. The suspensions containing pristine or end-modified cellulose nanocrystals were negatively stained with uranyl acetate solution at the controlled concentration before the observation. In addition, the end modification was further proved by the cellulose nanocrystals/silver nanoparticles tagging observation based on the high affinity of grafted thiol groups to the metal particles (Ag). The silver nitrate solution (200 µL, 10 mM) was added into the end-modified CNC suspension (2 mL, 1 mg mL-1) with the magnetic stirring for 30 min. The


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reduced reagent of sodium borohydride solution (2 mL, 5 mM) was introduced in the suspension to transform the silver ions to silver nanoparticles and performed the TEM observation. The redispersion states of end-modified cellulose nanoscrystals were evaluated by the sedimentation test and turbidity measurement. After the treatment of freeze-drying (FD) or ovendrying (OD), the completely dehydrated powders of nanocrystals were redispersed in the distilled water and ultrasonicated for 10 min by a Branson Sonifier equipment. The redispersed suspensions were left standing for 6 days to record the sedimentation states. Furthermore, the end-modified CNC-ATT-1 was suspended in the water (0.5 wt%) and kept in the oven with elevated temperatures of 60 °C, 90 °C, 120 °C for 5 h, 10 h, 24 h, in order to observe the temperature stability of suspensions.26 The dispersing stability of cellulose nanocrystals before and after end modification in suspensions was further analyzed by the turbidity measurements with gradient centrifugation speeds at 1000 rpm, 3000 rpm, 5000 rpm, 7000 rpm, 9000 rpm and 11000 rpm for 5 min. After the centrifugation, 4 mL supernatant suspension was taken for the UV spectroscopy analysis (UV-2600 spectrophotometer, Japan) to determine the normalized turbidity (As/Ai) by the absorbance at the wavelength of 550 nm. The normalized turbidity can be defined as the divisor of the supernatant’s absorbance after centrifugation (As) to that of the initial sample (Ai).27 The birefringence behavior is a typical optical phenomenon of homogeneous suspensions containing individually-dispersed nanoparticles, which can reflect the uniformity of suspensions. The dehydrated powders of cellulose nanocrystals before and after modification were redispersed in the distilled water and treated with the ultrasonication for 10 min. The birefringence based on the macroscopic anisotropy of rodlike CNCs was observed between two crossed polarizers. The particle size distributions and zeta potentials of suspensions containing pristine and end-modified


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CNCs (0.01 wt%) were measured by a Malvern Zetasizer Nano ZS (Malvern Instruments Co., Britain) under room temperature. With the aim of investigating the effect of end modification to the change of surface charges, conductometric titration was performed to measure the contents of sulfate groups on the surface of pristine and end-modified CNCs. To prevent the influence of dissolved CO2 to the conductivity, the nitrogen was bubbled into the suspensions during titration experiments.28 The suspension (100 mL, 0.1 wt%) was titrated by 0.01 M NaOH solution dropwise and recorded the pH and conductivity values until the pH reached to 11.0. The mechanical properties of the NR/CNC-x and NR/mCNC-x composites were measured by the tensile tests (CMT 6503, Shenzhen) with a 500 N load cell at the strain rate of 150 mm/min. The reactive enhancement of end-modified CNC-ATT-1 (mCNC) to the rubber composites was further investigated by dynamic mechanical analysis (DMA, PE DMA8000, USA) in the tensile mode with the temperature range from -100 to 40 °C at a heating rate of 3 °C/min and 1 Hz frequency. The microstructures of the composites were observed by scanning electron microscopy (SEM, JEOL JSM-IT300, Japan) at an accelerating voltage of 1.0 kV and coated with gold using a sputter coater before the observation of cross-sectional morphology. RESULTS AND DISCUSSION Analysis on two strategies of end modification on CNC. The end modification of grafting triazole on CNC was achieved by two strategies including the direct aldimine condensation (product CNC-ATT-1) and indirect carboxyamine condensation (product CNC-ATT-2), as shown in Fig. 1 A. Regarding the chemical mechanism of CNC-ATT1 (Fig. 1 B), the active aldehyde groups on the end of CNCs were susceptible to be attacked by the nucleophilic primary amino of ATT molecules, which induced the aldimine condensation accompanied with the removal of one molecule water. The covalent linkage between CNC and


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ATT was initially achieved by the formation of unstable -C=N- groups, and further reduced as the stable -C-N- groups by the reaction of STAB. The chemical mechanism of CNC-ATT-2 with the indirect carboxyamine condensation was more complicated, generally divided as three steps (Fig. 1 C). The end aldehyde groups of CNCs were firstly converted as the carboxylic groups in the presence of NaClO2 solution. The end-caboxylated CNCs combined with the EDC catalyst to form the O-acylisourea active intermediate, and then transformed as the active NHS-CNC-ester product by the catalysis of NHS. The NHS-CNC-ester was cleaved off during the amide formation, which induced the covalent grafting to obtain the end-modified CNC-ATT-2 under the nucleophilic attack of primary amino of ATT molecules.


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Figure 1. General chemical routes (A) possible reaction mechanisms of two end modification for CNC-ATT-1 (B) and CNC-ATT-2 (C). The end modification was confirmed by the ssNMR, XPS, elemental analysis and comparisons between pristine CNC and mCNCs. As shown in Fig. 2 A, the characteristic peaks of ssNMR spectra can be observed by the resonances located at 105 ppm, 89 ppm, 70-80 ppm and 65 ppm, assigned to the C1, C4, C2, 3, 5 and C6 of anhydroglucose units from cellulose molecules.29 After the end modification, the additional chemical shifts at 172-173 ppm were detected from the spectra of CNC-ATT-1 and CNC-ATT-2, which can be ascribed to the quaternary carbons from grafted triazole molecules on the end of modified nanocrystals (Fig. 2 B). The end grafting efficiency of CNC-ATT-1 seemed to be higher than that of CNC-ATT-2 in comparison with the peaks intensities, which will be discussed in the further analysis.

Figure 2.

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C solid-state NMR spectra of pristine CNC and end-modified CNCs (A) and

enlargement of spectral regions at 190-160 ppm (B). Due to the relatively low sites of end reaction comparing the conventional surface reaction, the end modification on CNCs was further confirmed by the XPS and elemental analysis (Table S1)


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through tracing the change of nitrogen and sulphur elements. The apparent N1s signal (399 eV) appeared in the XPS spectra of end-modified nanocrystals, derived from four nitrogen atoms of grafted triazole molecules in the general XPS spectra (Fig. S1). Consistent with the result of ssNMR, the analysis of additional N1s peaks intensities indicated the higher grafting efficiency of CNC-ATT-1 than that of CNC-ATT-2 (Fig. 3). It was worth noting that the S2p divided as the sulfate groups (−ܱܱܵଷି , 169 eV) and thiol groups (-SH, 164 eV) from the XPS spectra,21 which demonstrated the preservation of negatively-charged sulfate groups on the surface of endmodified nanocrystals. The crystalline properties and thermal degradability of CNCs before and after end modification were investigated by the XRD and TGA, as shown in Fig. S2. The typical cellulose I crystalline features can be observed on the patterns of pristine CNC and end-modified CNC-ATT-1 and CNC-ATT-2,30 possessing the high crystalline index of 77.0%, 75.4% and 80.9% respectively. Furthermore, the end modification provided the small influence on thermal stability of CNCs, exhibiting the slight advancement to the temperatures of thermal degradation for the end-modified CNC-ATT-1 and CNC-ATT-2.31


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Figure 3. XPS spectra of end-modified CNC-ATT-1 and CNC-ATT-2 tracing the signals of N1s and S2p. It was reported that the parallel arrangement of cellulose molecules on the structural construction of isolated cellulose nanocrystals provided only one reducing end (containing the reactive aldehyde groups) for the possible reaction.32 In this case, the reduction of silver nitrate to Ag nanoparticles was designated to prove the end modification of nanocrystals, through the adsorption of introduced thiol groups to Ag-metal nanoparticles.22 The negatively-stained concentrations of CNCs suspensions were intentionally reduced to avoid the interference of larger-scale CNCs to the observation of Ag nanoparticles. As shown in Fig. 4, the rodlike morphologies of nanocrystals can be observed from TEM images of CNC, CNC-ATT-1 and CNC-ATT-2 with the length of 100-300 nm and width of 10-20 nm, while the spherical Ag nanoparticles of 1-2 nm in diameter can be found on the end of CNC-ATT-1 and CNC-ATT-2 particles. It was apparent that much more adsorbed Ag nanoparticles can be observed on the TEM image of CNC-ATT-1/Ag than that of CNC-ATT-2/Ag, indicating more thiol groups and therefore higher grafting efficiency for CNC-ATT-1 than CNC-ATT-2.

Figure 4. TEM images of pristine CNC (A), and end-capping silver nanoparticles on CNC-ATT1 (B) and CNC-ATT-2 (C). Effects of end modification to redispersion and physical properties of nanocrystals.


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Based on the previous results, the direct aldimine condensation is favorable to higher efficiency on end modification of CNC than the indirect carboxyamine condensation, which can provide more grafted triazoles for CNC-ATT-1 and therefore subsequent studies on the redispersion and reactive composites focused on this end-modified product only. The redispersion states of pristine CNC and end-modified CNC-ATT-1 were evaluated by the common drying techniques with the freeze-drying (FD) or oven-drying (OD) treatment. As shown in Fig. 5 A, the endmodified CNC-ATT-1 exhibited superior redispersion in water during 6 days regardless of suffering the FD or OD treatment, in contrast to the apparent sedimentation of pristine CNC suspension after the drying treatment. The centrifugation-sedimentation experiment was used to measure the turbidities of aqueous suspensions containing nanoparticles based on the UVabsorbance results. As shown in Fig. 5 B, both pristine CNC and end-modified CNC-ATT-1 suspensions exhibited high levels of turbidity under the low centrifugation speeds, indicating their stable dispersion states at the initial phase (without drying treatment). However, the CNCATT-1 suspension can keep the homogeneous dispersion state even at the high centrifugation speeds, in contrast with there is sharp increase in turbidity and self-aggregation of pristine CNC in suspension at centrifugation speeds higher than 7000 rpm. Regarding the redispersion behaviors, the apparent reduction of turbidities can be observed by pristine CNC suspensions suffering the FD or OD treatment, while the end-modified CNC-ATT-1 suspensions can sustain the homogeneous dispersion even after the drying treatments. The improved redispersion of CNC-ATT-1 suspension can be attributed to synergistic effect of steric stabilization from endgrafted triazoles and electrostatic repulsion from surface negatively-charged sulfate groups. It should be pointed out that the strategy of triazole end-grafting in present study can provide the improved water-redispersion and meanwhile preserve the surface chemistry to CNC


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nanoparticles, which ensures its ensuing reaction and application based on the surface active hydroxyl groups.

Figure 5. (A) Digital photos of dispersion and redipsersion states for 1.0 wt% nanocrystals’ aqueous suspensions (a) CNC, (b) redispersed CNC after freeze-drying treatment (r-CNC/FD), (c) redispersed CNC after oven-drying treatment (r-CNC/OD), (d) CNC-ATT-1, (e) redispersed CNC-ATT-1 after freeze-drying treatment (r-CNC-ATT-1/FD), (f) redispersed CNC-ATT-1 after oven-drying treatment (r-CNC-ATT-1/OD); (B) the calculated turbidities of nanocrystals’ aqueous suspensions after centrifugation treatments at different speeds. Considering its possible reaction and potential application (e.g. as the additive in drilling fluids for oil recovery) 33 under high temperatures, the temperature stability of CNC-ATT-1 suspension was observed at three elevated temperatures. As shown in Fig. S3, the CNC-ATT-1 nanoparticles can homogeneously disperse in the water at 60 °C, 90 °C, 120 °C and also keep the good dispersion states after cooling to the room temperature. The birefringence observation provides a direct indication of well-dispersed and randomlyoriented cellulose nanocrystals in aqueous suspension, through the strong absorption of the


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polarized rays and therefore displaying two refractive indices.34 The birefringence behaviors of pristine CNC and CNC-ATT-1 in suspensions were viewed through the cross polarizers as shown in Fig. 6. Without the drying treatment, the original dispersion of CNC and CNC-ATT-1 in suspensions displayed the beautiful birefringence phenomenon (images a and d). The similar birefringence appeared only in the CNC-ATT-1 redispersed suspensions after the FD or OD process (images e and f), indicating the presence of individual nanoparticles in these suspensions. As a contrast, the absence of birefringence was observed for the pristine CNC redispersed suspensions after the drying process (images b and c) due to the aggregation tendency of nanoparticles.

Figure 6. Birefringence observation of nanocrystals’ aqueous suspensions viewed through cross polarizers (a) CNC, (b) r-CNC/FD, (c) r-CNC/OD, (d) CNC-ATT-1, (e) r-CNC-ATT-1/FD, (f) rCNC-ATT-1/OD. The analysis on particle size distribution of CNC and CNC-ATT-1 with or without drying treatment was performed by the Nanosizer analysis. As shown in Fig. 7, the redispersion of the pristine CNCs after the drying treatment induced the aggregation of nanoparticles and the increase of Z-average parameter (Zave) with the peaks shifting to large-size regions or even twodivided peaks (after the OD treatment). On the contrary, the CNC-ATT-1 exhibited the stable


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dispersion and redispersion in suspensions with the Zave particle sizes ranged from 157.2 nm to 173.9 nm, avoiding the presence of serious aggregation and large agglomeration.

Figure 7. Particle size distributions of CNC, r-CNC/FD, r-CNC/OD, and CNC-ATT-1, r-CNCATT-1/FD, r-CNC-ATT-1/OD. The electrostatic repulsion serves as an important effect to promote the dispersion and stability of CNCs in water from negatively-charged sulfate groups induced by sulphuric acid hydrolysis. The contents of sulfate groups on the surface of CNC and CNC-ATT-1 were measured as 0.236 mmol/g and 0.232 mmol/g by conductometric titration (Fig. 8 A and B), which indicated the preservation of surface sulfate groups and negative charges during the end modification. Associated with electrostatic repulsion, the zeta potential of CNC corresponds to the coverage of negative electrostatic layer on nanocrystals and therefore can reflect its dispersion state in the suspension. As shown in Fig. 8 C and D, the CNC-ATT-1 in redispersion retained high levels of zeta potentials as -32.3 mV and -31.6 mV after the FD and OD treatments, while the zeta potentials of pristine CNC in redispersion markedly reduced to -28.0 mV and -16.3 mV after the


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same drying treatments. The decrease of zeta potentials of redispersed CNC in suspensions can be explained by the self-aggregation and shielding effect of surface charges of nanocrystals, in contrast with the good dispersion and high zeta potentials of redispersed CNC-ATT-1 in suspensions.

Figure 8. Conductometric titration curves of (A) CNC suspension, (B) CNC-ATT-1 suspension; zeta potential distributions of (C) CNC, r-CNC/FD, r-CNC/OD, and (D) CNC-ATT-1, r-CNCATT-1/FD, r-CNC-ATT-1/OD suspensions. Thiol-ene click between CNC-ATT-1 and NR. In order to prove the possible click reaction between CNC-ATT-1 (thiol groups) and NR (unsaturated double bonds), the reacted product under UV irradiation was separated by the centrifugation (8000 rpm) and purified with repeated washings against the distilled water for once, anhydrous ethanol for twice, methylbenzene for thrice, anhydrous ethanol again for once, and finally collected the product for the oven-drying at 35 °C overnight. The mixture of pristine


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CNC and NR was also prepared under UV irradiation as the control sample and treated for the same centrifugation/repeated washings as the comparison. Fig. 9 A showed the FTIR spectra for raw materials (NR, NR-i and CNC) and freeze-dried mixtures of NR and CNC or CNC-ATT-1. The NR and NR-i (the neat NR with the irradiation treatment for 30 min) materials exhibited typical features of polyisoprene and completely same spectra, indicated the weak influence of UV irradiation at 30 min to the chemical structure of rubber. This is the necessary proof to exclude the possible influence of UV irradiation to the performance of composites when discussing the enhancement of CNC or end-modified CNC. It was apparent to observe an additional peak located at 835 cm-1 on the FTIR spectrum of freeze-dried mixture of CNC-ATT1 and NR, which was ascribed to the out-of-plane bending vibration of –CH– on NR35 and therefore proved the occurrence of click reaction between two components. The

13

C solid-state

NMR was performed to further confirm this click reaction between CNC-ATT-1 and NR, as shown in Fig. 9 B. The characteristic peaks at the chemical shift range of 32-23 ppm were observed on the spectrum of the resultant product from CNC-ATT-1 and NR, which can be attributed to the features of isoprene on NR structure including C1 (23 ppm), C4 (28 ppm) and C5 (32 ppm).36 The FTIR and ssNMR analysis proved the covalent reaction between CNC-ATT1 and NR induced by UV irradiation, in comparison with the absence of NR features mentioned before in two analyses for pristine CNC/NR mixture and therefore the weak adsorption of hydrophobic NR on the surface of hydrophilic CNC.


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Figure 9. (A) FTIR spectra of NR (a), NR with the irradiation treatment for 30 min (NR-i) (b), pristine CNC (c), freeze-dried mixture of pristine CNC and NR (1 : 2, w/w) (d), freeze-dried mixture of CNC-ATT-1 and NR (1 : 2, w/w) (e); (B) 13C solid-state NMR spectra of freezedried mixture of pristine CNC and NR (1 : 2, w/w) (f), freeze-dried mixture of CNC-ATT-1 and NR (1 : 2, w/w) (g). Reactive enhancement of reactive CNC-ATT-1 to rubber-based nanocomposites. The strain-stress curves in Fig. 10 A and B and Table S2 depicted the different enhancing effects to mechanical property of NR-based composites containing CNC or CNC-ATT-1 fillers. It should be pointed out that all the composites containing pristine CNC and CNC-ATT-1 were taken the same UV irradiation treatment (30 min at the controlled distance and intensity), which can get rid of the possible influence of irradiation to mechanical properties and make the clarified understanding of enhancement for the composites’ comparison. In theory, the CNCATT-1 can construct the strong covalent linkage with NR matrix via the thiol-ene reaction, while the pristine CNC only mixed with NR matrix on the basis of physical compounding. The appearances of composites filled with various loading levels of pristine CNC and CNC-ATT-1


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(mCNC) were shown in Fig. S4. Typical nanoreinforcement was involved in the rigid CNC to the soft rubber-based composites, exhibiting the gradual increase of Young’s modulus (E) and decrease of elongation at break (εb) for the NR/CNC-x composites. The processed composites through the simple mixture of nanoparticles and matrix commonly exhibit a trade-off between stiffness and extensibility,37 due to the lack of strong interaction and interfacial adhesion between rigid CNCs and matrix. In comparison with those of NR/CNC-x composites, the tensile strength (σb) and Young’s modulus of NR/mCNC-x composites showed general advancement at the same loading levels of CNC-ATT-1 as pristine CNC. Meanwhile, the elongation at break of NR/mCNC-x composites retained a high level of 1500% with similar as that of the neat NR material, which indicated the effectively reactive enhancement of CNC-ATT-1 to rubber-based composites. In particular, at the optimal loading level of CNC-ATT-1 (10 wt%) the NR/mCNC10 composite exhibited superior mechanical performance with simultaneous enhancement of the

σb, E and εb by 160%, 468% and 8% increases than those of the neat NR material (Table S2). The possible effect of UV irradiation to the bulk of rubber was also investigated (Fig. 10 C and Table S3), which demonstrated the weak influence of 30 min irradiation treatment to mechanical property of the rubber component. This result excluded the influence of UV irradiation at the controlled duration to structure and mechanical performance of NR, consistent with the FTIR analysis in Fig. 9 A. In addition, the selection of 30 min duration for UV irradiation was based on the experimental results of the NR/mCNC-10 composite (Fig. 10 D and Table S4). In this case, the short duration of irradiation can’t ensure the sufficient click reaction, while longer durations of irradiation may result in the degradation of materials.


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Figure 10. Stress-strain curves of (A) NR/CNC-x composites, (B) NR/mCNC-x composites, (C) neat NR and NR-i materials, and (D) the NR/mCNC-10 composite treated with different UVirradiation durations. The reactive enhancement of end-modified nanocrystals to the NR composites was further investigated by DMA results, indicated by gradual advancement of storage modulus (E’) for composites with the increase of CNC-ATT-1 loading levels (Fig. 11 A). In comparison with that of neat NR material (1.06 MPa), the E′ values of composites exhibited a remarkable promotion at loading levels of CNC-ATT-1 higher than 7.0 wt% (9.34 MPa for NR/mCNC-7.5, 12.10 MPa for NR/mCNC-10, 14.90 MPa for NR/mCNC-12.5, 18.30 MPa for NR/mCNC-15). The rigid nanocrystals bonded on polymeric chains can serve as the phase of stress distribution and effectively absorb the imposed energy, which facilitated the improvement of thermomechanical property for the composites. According to the analysis and calculation of percolating model


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(Supporting Information), the exceptional enhancement on storage modulus of composites can be attributed to the formation of a rigid percolating network within the matrix, above the critical percolation threshold (ωRc = 7.49 wt%)38 for cotton-derived CNC-ATT-1. The preserved hydroxyl groups on the surface of nanocrystals during the end modification are the basis of hydrogen-bonding interactions and rigid network architecture (Fig. 11 B).24 Moreover, the introduction of CNC-ATT-1 into the composites showed the weak effect to glass transition temperature (Tg) with the slight decreases of about 2-3 °C (Fig. S5), which can be explained by the restriction of chains’ free motion ascribed to the presence of grafted rigid nanocrystals in polymeric composites. Without the covalent linkage (click reaction) between unmodified CNC and NR, the physical compounding strategy for NR/CNC-x composites provided the weakened enhancement to the storage modulus in comparison with those of NR/mCNC-x composites (Fig. S6), particularly at high loading levels (>7.5 wt%) due to the aggregation tendency.

Figure 11. Curves of storage modulus (log E’) vs. temperature (A) and fitted curves of E’ at 25 °C of NR/mCNC-x composites (B) with various loading levels of end-modified CNC-ATT-1. The cross-sectional morphologies of composites containing pristine CNC or CNC-ATT-1 with low (5 wt%), moderate (10 wt%) and high (15 wt%) loading levels were observed by SEM,


24

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indicated the microstructure and dispersion states of nanofillers in composites. In comparison with the neat NR material (Fig. S7), the NR/mCNC-x composites (Fig.12 A, C, E) exhibited uniform microstructures with homogeneous dispersion of rodlike CNC-ATT-1 nanoparticles (indicated by the arrows in images). However, the apparent phase separation was observed in the images of NR/CNC-x composites (indicated by the circles as CNC-rich domains in Fig. 12 B, D, F), particularly for the composites with addition of 10 and 15 wt% pristine CNCs. The endmodified nanocrystals can be covalently bonded on NR chains, provided the good compatibility with the matrix; while the pristine nanocrystals were simply compounded into the NR matrix, lack of necessary interaction between hydrophilic nanofillers and hydrophobic matrix and therefore

exhibiting

different

microstructural

appearances

for

the

composites.

The

microstructures of composites containing CNC-ATT-1 and CNC were further observed as shown in Fig. S8, with the higher resolutions indicated by the regions as in the SEM images shown. This significantly demonstrated good dispersion of CNCs-ATT-1 with apparent agglomeration of CNCs in the composites.


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Figure 12. SEM images of cross-sectional morphologies of the NR/mCNC-5 (A), NR/CNC-5 (B), NR/mCNC-10 (C), NR/CNC-10 (D), NR/mCNC-15 (E), NR/CNC-15 (F) composites. Discussion The general ideas of this study were shown in Fig. 13 regarding the redispersibility improvement of cellulose nanocrystals (CNCs) through the end modification and their reactive enhancement to natural rubber (NR)-based composites. Considering the relatively low reactive sites of end modification in contrast with surface modification for CNCs, the triazole compound was selected as the grafted molecule in order to sensitively detect the end modification by means of tracing the discriminative N element different from the original carbohydrate (cellulose, C/H/O elements). Meanwhile, the end-grafted triazoles as the cyclic molecules provided additional spatial repulsion, promoting the redispersion and stability of end-modified CNCs in aqueous


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suspension from synergistic effect with electrostatic repulsion of the negatively-charged surface chemistry (top images). Attributed to the purposeful design of end modification, the reactive thiol groups were introduced on the end of modified CNCs, which endowed the covalent linkage with unsaturated rubber via click reaction (bottom images). This structural design making the linkage of rigid nanofillers’ end to matrix’s chains (not the simple physical mixture or conventional surface chemical crosslinking) ensured free motion and possible interaction among nanofillers and therefore played the crucial role in reactive enhancement to mechanical properties of ensuing composites (simultaneous increase of strength, modulus and toughness). As a first attempt on the study of end modification to the redispersibility, we didn’t obtain the redispersion improvement of triazole-end-grafted CNCs in organic solvents, such as toluene, acetone (results not shown). Our further study is constructing the end modification of nonpolar cyclic molecules or even hydrophobic macromolecules for CNCs to promote their redispersion in both water and organic solvents. As the prerequisite for the preservation of original surface properties, in here we provided a novel strategy based on the triazole end modification to improve water redispersion of CNCs and their end-bonding with rubbers for the construction of high-performance nanocomposites.


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Figure 13. Proposed mechanisms for the dispersion of end-modified cellulose nanocrystals in water (top images) and reactive compatibility of end-modified nanocrystals and natural rubber (bottom images). CONCLUSIONS The redispersibility of cellulose nanocrystal (CNC) in aqueous suspension is an important property to determine its potential application, further modification, storage or transportation. Here in we proposed the end modification of grafting triazole molecules based on the active aldehyde groups of CNC to promote its redispersion in water after the freeze-drying or ovendrying treatment. The end modification performed at the mild reaction conditions didn’t destroy the surface chemistry of CNC, which provided the electrostatic repulsion from surface sulfate groups and steric stabilization from end-grafted triazoles for the improvement of their redispersion and stability in aqueous suspension. Therefore, a reactively-enhanced composite was constructed via click linkage between the additional thiol groups on the end of modified


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nanocrystals and unsaturated bonds of natural rubber (NR) chains. The formed percolating network among nanofillers together with the covalent linkage between nanofillers and matrix effectively improved mechanical performance of the composites, exhibiting significant advancement on the strength and modulus as well as the sustainability of toughness in comparison with the raw rubber material. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website, including Table S1-S5, Figures S1-S8. Discussion on mechanical mechanisms of percolating model and Halpin-Kardos model for the composites; elemental analysis, data of mechanical properties, general XPS spectra, XRD patterns and TGA thermograms, dispersion states of CNC-ATT-1 aqueous suspensions at different temperatures, DMA thermograms of tanδ vs. temperature, SEM images with high magnifications and appearances of composites. AUTHOR INFORMATION Corresponding Author: * Dr. Ning Lin, Email: [email protected]. Address: #122 Luoshi Road, Wuhan University of Technology, Wuhan 430070, P. R. China. Tel: +86-2787152611; fax: +86-27-87152611. ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (51603159) and Youth Chenguang Program of Science and Technology in Wuhan (2016070204010102). The authors also wish to acknowledge the financial support of Natural Science Foundation of Hubei Province (2017CFB490).


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For Table of Contents Use Only

Synopsis: Triazole end-grafting was performed on CNC to promote its water-redispersion and simultaneously enhanced strength, toughness and modulus of NR-based composites.


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Triazole End-Grafting on Cellulose Nanocrystals for Water

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