A Host–Guest Interaction Assisted Approach for Fabrication of

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

A Host−Guest Interaction Assisted Approach for Fabrication of Polybutadiene Nanocomposites Reinforced with Well-Dispersed Cellulose Nanocrystals Chuang Peng,†,‡ Bo Dong,† Chunyu Zhang,† Yanming Hu,*,† Li Liu,§ and Xuequan Zhang*,† †

CAS Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Science and Technology of China, Hefei 230026, China § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Fabrication of homogeneous hydrophobic polymer/cellulose nanocrystals (CNCs) nanocomposites with high mechanical reinforcement is a tough issue owing to the poor interfacial interactions between the two components and the aggregation of CNCs in hydrophobic matrices. In this study, the polybutadiene (PB) nanocomposites reinforced with cotton-derived CNCs were prepared through a host−guest inclusion complex assisted approach. In the system the pendent guest adamantane moieties of PB reacted with β-cyclodextrins (βCDs) to form inclusion complexes, which acted as competitive binding sites to the CNCs via hydrogen bonds, allowing the CNCs to disperse homogeneously in the matrix. Dynamic mechanical analysis and tensile testing studies of the nanocomposites revealed that the incorporation of CNCs into the polymer matrix significantly improved the mechanical properties. For the nanocomposites with 10 wt % βCDs and 15 wt % CNCs, the storage modulus, Young’s modulus, and tensile strength were enhanced by about an order of magnitude compared to neat PB. The increase in the βCD and/or CNC contents led to a shifting of the tan δ peak of the nanocomposite to higher temperatures, suggesting the presence of filler−polymer interactions. Moreover, scanning electron and polarized optical microscopy analyses revealed that in the matrix the individual CNCs aligned in parallel and longitudinal directions to form long and entangled assemblies. The reinforcement could be attributed to the filler−filler and filler−matrix interactions which consequently led to the stress transfer between polymer and CNCs in the system.

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INTRODUCTION Cellulose nanocrystals (CNCs), a novel form of cellulose, have attracted considerable current interest in material research because of their unique physical and chemical properties, inherent renewability and sustainability, and abundance in nature.1−6 Such materials have find numerous applications in pharmaceutical industry,7 coating materials,6 fiber optics,8 nanocomposites,3,9,10 and other fields.2,11−14 In particular, the impressive mechanical properties,15,16 nanoscale dimensions,5 low density,17 and high aspect ratio3 of CNCs make them ideal candidates for fabrication of polymer nanocomposites. Since Favier and co-workers reported the first example of poly(styrene-co-butyl acrylate)/CNC nanocomposite,18 CNCs have been exploited immensely as model fillers for various polymers. Several CNC-reinforced nanocomposites especially for hydrophilic polymers including poly(vinyl alcohol), poly(vinyl acetate), polyurethane, poly(styrene-co-butyl acrylate), and hydroxypropylcellulose have been successfully prepared, and in some cases significant improvement in mechanical properties was realized even at a low CNC loading.19−23 However, the © XXXX American Chemical Society

aggregation of CNCs in nanocomposites especially for hydrophobic matrices limits the potential of mechanical reinforcement. To address this issue, the strategies including chemical modification and physical adsorption have been developed to control the dispersion state and network structures of CNCs within hydrophobic polymer matrices. The presence of reactive surface hydroxyl groups enables various chemical modifications to render CNCs compatible with nonpolar polymers, allowing the improvement of the CNC dispersion in the matrices. Physical adsorption involving the addition of binders into polymer/CNC nanocomposites is another effective approach to improve the CNC dispersion and compatibility with polymer matrix. On the basis of these two strategies, several CNC-reinforced nanocomposites were successfully fabricated and exhibited significant improvement in mechanical properties.24−27 Received: March 21, 2018 Revised: May 29, 2018

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DOI: 10.1021/acs.macromol.8b00606 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Chemical Modification of Polybutadiene with βCD/Adamantane Inclusion Complexes; (b) Illustration of Polybutadiene/CNC Nanocomposites Based on Inclusion Complexes between βCD and Adamantane Groups on the Polymer Side Chain

also provide the potential for fabrication of CNC-based nanocomposites; the primary and secondary hydroxyl groups on the outer surface of βCD molecules could interact with CNCs via hydrogen bonding, thus rendering CNCs compatible with nonpolar polymers. βCDs can be introduced into nonpolar polymers through appropriate guest molecules linked on nonpolar polymers to form polymeric inclusion complexes. We hypothesized that the βCD−guest inclusion complexes could improve the dispersion of CNCs in nanocomposites and the interaction strength between the CNCs and polymers. On the other hand, the reinforcing performance of CNCs is not only dependent on the dispersion in the matrix and percolation threshold but also closely related to their aspect ratios which depend on the source of the cellulose. Although the highaspect-ratio tunicate-derived CNCs generally have a better reinforcement effect, from a practical viewpoint, the application of CNCs from native cotton is more meaningful because of their much more abundant sources, albeit with the low aspect ratios.5 Herein, we reported a host−guest inclusion complex assisted approach for the preparation of homogeneous polybutadiene/cotton CNC nanocomposites (Scheme 1). The approach was based on the synthesis of the guest polymer and the formation of host−guest inclusion complexes with the pendent adamantane moieties on the polymer and βCDs. Furthermore, the mechanical properties of the CNC-reinforced nanocomposites and the morphology of CNCs in the matrix were investigated to enable a better understanding of the structure−properties relationship in our system, and a plausible mechanical reinforcing mechanism was proposed.

As an important class of hydrophobic polymers, rubbers such as nature rubber, styrene−butadiene rubber, and polybutadiene have been widely used in the automotive industry for tires, seals, shock absorbers, and many other elastic materials. Incorporation of nanofillers is often required to improve the mechanical performance of rubber matrix.28−32 However, the examples of using CNCs as reinforcing agents for rubbers are quite limited due to the incompatibility of these two components. Weder and co-workers successfully prepared homogeneous poly(styrene-co-butadiene)/tunicate-derived CNC nanocompostites by using a preformed three-dimensional nanofiber template.33 The nanocomposite with 17% v/v CNCs exhibited significantly enhanced mechanical properties; the storage modulus increased from ca. 1 MPa for the neat polymer to 236 ± 22 MPa.13 Ikkala and co-workers elegantly fabricated cross-linked polybutadiene/CNC nanocomposites based on thiol−ene click chemistry, in which the formation of noteworthy self-assembled intercalated CNC domains was realized and no macroscopic aggregation was observed even at a high CNC content of 80%, albeit with modest enhancement in mechanical properties compared to the best biomimetic materials.34 Introducing polar groups into rubber by postfunctionalization to improve its compatibility with CNCs is an effective choice. Tian and Ning prepared water-responsive epoxidized natural rubber (ENR) and natural rubber (NR) nanocomposites. The analyses of the resulting materials revealed that CNC reinforcement effect for ENR was much more significant than NR owing to the stronger filler−polymer interaction in the former system. The host−guest chemistry of β-cyclodextrins (βCDs) has been extensively studied, and the formation of noncovalent inclusion complexes through host−guest interactions between βCDs and a variety of guest polymers has led to numerous functional materials.35−39 The βCD−guest inclusion complexes

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EXPERIMENTAL SECTION

Materials. Cellulose nanocrystal (CNC) powder was purchased from Qihong Technology (Guilin, China). Polybutadiene (PB, Mn = 11.7 × 104, Mw/Mn = 2.2, cis-1,4 content of 97.0%) was kindly B

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Macromolecules Scheme 2. Proposed Mechanism Illustrating the Synthesis of ADPB

°C/min and cooled down at a rate of 10 °C/min. The mechanical properties of nanocomposites were analyzed by dynamic mechanical analysis (DMA Metravib DMA+450) in tensile mode and tensile test (INSTRON-5869). The DMA curves were achieved by scanning at a heating rate of 3 °C/min and a frequency of 1 Hz in a temperature range of −120 to 90 °C. Rectangular slices with length (4 cm) and width (1 cm) were used. Tensile tests were carried out using dumbbell samples with a crosshead speed of 50 mm/min at room temperature. The structure and morphology of CNCs in the nanocomposites were analyzed via an XL-30 ESEM FEG scanning electron microscope (SEM FEI COMPANY) and polarized optical microscopy (POM). POM images were acquired by a Zeiss Imager A1m AX 10 microscope in bright-field mode with ProgRes CapturePro 2.6 software and a ProgRes C5 camera. The film samples for POM and SEM test were prepared through solution-casting method as the same with above statement in nanocomposite preparation part. The initial morphology of CNCs was acquired by atomic force microscopy (AFM) through a multimode nanoscope V scanning probe microscopy system (Bruker, USA). The sample for AFM test was prepared by solution casting of CNCs diluent THF suspension onto a freshly cleaved mica surface until it dried naturally.

provided by Dushanzi Petrochemical Company, PetroChina. Tetrahydrofuran (THF, AR, Tian in Fuyu Fine Chemical CO.) was distilled through sodium/benzophenone prior to use. 1-Adamantanecarboxylic acid (99%, J&K), β-cyclodextrin (βCD, 99%, J&K), N-bromosuccinimide (NBS, 99%, J&K), and other reagents were used as received. Synthesis of PB Partially Modified with Adamantane (ADPB). ADPB was prepared by a procedure altered from the literature.40 A typical step was given as follows: under an inert atmosphere, PB (1.0 g, 18.5 mmol of double bond) and 1adamantanecarboxylic acid (1.0 g, 5.5 mmol) were dissolved in dry THF (60 mL) in a 100 mL flask. The reaction mixture was cooled to 0 °C, and NBS (0.17 g, 0.96 mmol) was added. After stirring for 1 h at 0 °C, the reaction solution was warmed to room temperature and stirred for another 22 h. The resulting polymer was precipitated into a large amount of ethanol and washed repeatedly with ethanol. The purified polymer was acquired after further two times above precipitation procedure. Finally, ADPB was dried under vacuum at 30 °C until invariable quality. The molar ratio of adamantane moieties relative to the double bonds of PB was 4.74% determined by 1H NMR analysis. Host−Guest Interaction Study of βCD and Adamantane in PB. 1H NMR measurement was conducted to identify the formation of βCD/adamantane inclusion complexes. All the speciments were dissolved in CDCl3 (0.5 mL with two drops of CF3OOD). All the samples were put statically overnight before measurement to guarantee enough time for host−guest interaction. Preparation of the Nanocomposites. Polymer/CNC nanocomposites were prepared by solution-casting from THF. First, the transparent ADPB/THF solution (50 mg/mL) with βCD (10 wt % based on ADPB) was configurated using a RCT basic IKA machine (300 rpm) at room temperature for 12 h. The resulting ADPB/βCD solution was then mixed with the preprepared CNC (10 wt % based on ADPB)/THF suspension,24 which was obtained by sonication in a KQ-100E ultrasonic bath for 30 min. Subsequently, the well-mixed mixture was casted into a round Teflon Petri dish with 12 cm diameter. The solvent was mostly volatilized naturally in a place with good ventilation. Afterward, the sediment was dried in a vacuum oven at 30 °C to a constant weight. The dried sediment was then hotpressed in a vulcanizing press (QLB-400*400) at 100 °C for 10 min with 10 MPa pressure using spacers to adjust the slice thickness to about 2 mm. Finally, the slices were cut for varying samples with different requirement in a sheet-punching machine (TY-4025) and immediately stored in a desiccator with some desiccant at an ambient temperature until the mechanical tests to minimize wet absorption. Other nanocomposites with mutative amounts of βCDs (0, 1, 5, and 10 wt %) and CNCs (0, 5, 10, and 15 wt %) were prepared following the same protocol. The volume of mixtures was controlled to achieve the desired concentration in nanocomposites. The compounds were denoted as polymer/xβCD/yCNC, where x and y represent the x or y phr (parts per hundred of rubber) relative to the polymer of βCD or CNC in the compounds. Characterization. 1H NMR spectra were recorded on a Varian Unity 400 MHz spectrometer at room temperature with tetramethylsilane as an internal reference. Fourier transform infrared spectroscopy (FTIR) analysis was performed using Nicolet IS10 on a KBr disc with moderate samples prepared by casting THF solution (ca. 5 mg/mL) of the polymers. Differential scanning calorimetry (DSC) analysis was performed on a Q20 DSC (TA Instruments) under a helium atmosphere. The samples were heated at a rate of 10

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RESULTS AND DISCUSSION Synthesis of Guest Polymer (ADPB). The guest polymer containing pendent admantane groups, ADPB, was successfully synthesized via a one-pot electrophilic multicomponent reaction (Scheme 2). The treatment of PB with NBS gave the brominating species, and then the resulting intermediates were sequentially attacked by THF and adamantane acid to afford the target product. It is worthy to note that the reaction was conducted at a quite mild condition without fussy procedures and produced ADPB in a good yield. The structure of ADPB was identified by 1H NMR and IR spectroscopies. In the 1H NMR spectrum (Figure 1), the signals 5, 6, 7, and 8 located at 3.3−4.3 ppm are characteristics of the methine and methylene groups of the polymer backbone

Figure 1. 1H NMR spectrum of ADPB. C

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Macromolecules and the linkers between the main chain and the pendent admantane groups, respectively, and the integral ratio of 1:1:2:2 is in agreement with the theoretical value. The successful functionalization of PB was further confirmed by IR spectroscopy. The absorption peak at 1728 cm−1 characteristic of carbonyl CO appears in ADPB (Figure 2), indicating the

Figure 4. 1H NMR spectra of βCD, ADPB/βCD, and ADPB/βCD/ adamantane. The βCD dosage is 1.5 times that of adamantane on ADPB in the ADPB/βCD/adamantane sample.

tensile test along with neat PB and the PB/CNC nanocomposite for comparison. The tensile storage modulus (E′) and tan δ of the samples as a function of temperature are shown in Figure 5, and the mechanical data measured at room temperature are listed in Table 1. The E′ vs temperature plots of neat PB and PB/CNC nanocomposite are quite different from those of the ADPB-based nanocomposites. As shown in Figure 5a, in the temperature ranging from ca. −100 °C (Tg) to ca. −20 °C, the E′ of neat PB and PB/CNC nanocomposite gradually decrease with increasing temperature, and the drop is not obvious in comparison to that in the glassy−rubber transition stage, probably due to the cold crystallization of high cis-1,4 PB which restricts the polymer chain mobility.19 However, such a phenomenon is not observed in the case of ADPB-based nanocomposites (Figures 5c and 5e) due to the absence of cold crystallization (Figure 3b). It is noted that the ADPB-based nanocomposites prepared by solution mixing ADPB/βCD with CNC showed a remarkable mechanical reinforcement. However, the PB/ 10CNC hybrid exhibited a limited improvement of E′. For example, the room temperature E′ of the ADPB nanocomposite with 10 wt % βCDs and 10 wt % CNCs (ADPB/10βCD/ 10CNC) reached 6.32 MPa. With increasing the CNC loading to 15 wt % (Figure 5e), the E′ increased to 8.61 MPa, which is 8 times that of neat PB (1.09 MPa) (Table 1). By contrast, in the case of PB/10CNC, the increase in E′ is also observed but did not display a significant enhancement (Figure 5a). Generally, in the reinforced polymer systems, when a growing crack encounters the CNC particles, energy dissipation occurs. Therefore, these results provide an indirect evidence of the relatively uniform dispersion of CNCs in the matrix through

Figure 2. FTIR spectra of PB and ADPB.

presence of ester group. Moreover, the absorption bands at 1078 and 1350 cm−1 typical for C−O−C and adamantane moieties, respectively, are detected. As depicted in Figure 3, the glass transition temperatures (Tg) of PB and ADPB are −103.41 and −100.79 °C, respectively, indicating that the incorporation of 5 mol % adamantane moieties relative of the double bonds of the polymer just slightly hinders the polymer chain mobility. A series of 1H NMR measurements were conducted to test the host−guest interaction between βCD molecules and the ADPB-carrying admantane. As shown in Figure 4, the new signals appear at 4.85 and 4.45 ppm in the ADPB/βCD system which are absent in the spectra of βCD or ADPB, and the intensities of these signals increase by the further addition of admantane molecules in the system, indicating that the inclusion complexes formed by the admantane moieties anchoring into the cavities of βCD molecules. Mechanical Properties of ADPB/βCD/CNC Nanocomposites. A series of ADPB/βCD/CNC nanocomposites with various βCD and CNC contents were prepared, and their mechanical properties were studied by DMA analysis and

Figure 3. DSC curves of PB, ADPB, and ADPB/10βCD/10CNC. D

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Figure 5. Representative DMA traces of PB and ADPB nanocomposites.

Table 1. Mechanical Properties of ADPB/βCD/CNC Nanocomposites samples neat PB PB/10CNC ADPB ADPB/10CNC ADPB/1βCD/10CNC ADPB/5βCD/10CNC ADPB/10βCD/10CNC ADPB/10βCD ADPB/10βCD/5CNC ADPB/10βCD/15CNC

storage modulusa (MPa) 1.09 1.85 1.24 2.09 2.75 6.30 6.32 2.86 4.74 8.61

± ± ± ± ± ± ± ± ± ±

Young’s modulusb (MPa)

0.1 0.3 0.2 0.3 0.4 0.6 0.7 0.3 0.6 0.8

0.7 0.9 0.8 1.1 2.1 4.2 4.5 2.2 2.7 6.9

± ± ± ± ± ± ± ± ± ±

0.1 0.2 0.1 0.3 0.2 0.4 0.5 0.2 0.3 0.4

tensile strengthb (MPa) 0.31 0.39 0.24 0.40 1.69 2.20 2.55 1.51 1.91 3.43

± ± ± ± ± ± ± ± ± ±

0.01 0.07 0.04 0.05 0.08 0.03 0.04 0.02 0.01 0.06

elongation at break (%) 515 492 420 390 270 110 108 341 145 103

± ± ± ± ± ± ± ± ± ±

35 27 30 19 13 9 10 24 12 7

a Data were acquired from DMA at 25 °C and represent averages of five individual measurements, ± standard deviation. bData were acquired from tensile tests at 25 °C and represent averages of five individual measurements, ± standard deviation.

chains in the vicinity of the filler particles, which can lead to a percolating interphase network within the composite and consequently has a significant impact on the properties of polymer nanocomposites.41 Thus, in the present system, the

the assistance of the βCD/adamantane host−guest inclusion complexes. The previously experimental and simulation studies have revealed that the filler−matrix interaction affected the nature of the interphase, that is, a special region of polymer E

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hydrogen bonding and consequently block the slippage of rubber domains during stretching. The tensile test results agree well with the DMA results and further illustrate the strong interaction between the polymer and CNC in our system. The morphology of the representative nanocomposite, ADPB/10βCD/10CNC, was studied with SEM. In contrast to the smooth surface of pure ADPB (Figure 7a), the cryogenically fractured cross section of the nanocomposite shows typically crumpled morphology (Figure 7b), suggesting pull-out patterns and local deformations. The same sample after hot pressing also exhibits smooth regions of the polymers and rough regions presumably attributable to the embedded CNCs within the polymer matrix, and the CNC phase is dispersed in the matrix at a micrometer scale instead of nanoscale (Figures 7c and 7d). To gain an insight into the interactions between the CNCs and the polymeric host−guest inclusion complexes, the morphology of CNCs in the nanocomposites from solution casting were also investigated. From the SEM images in Figures 7f and 7g, some bright domains which resemble the rodlike structure of CNC are observed, and it is noticed that both the length of ca. 30 um and the width of ca. 1 um of these domains are much larger than the original CNC (length ∼180−330 nm and width ∼25−35 nm, Figure 7e), suggesting that the original CNCs self-assemble into the rodlike assemblies and some of them links by physical connection of the polymer (Figure 7h). Moreover, the high-magnification SEM images inserted in Figure 7f and Figure S1 show that the rodlike CNC assembly is constituted by the CNCs aligned in parallel and longitudinal directions, which is similar to the intercalated structure observed in the CNC/PB cross-linked nanocomposites.34 Figure 8 shows POM photographs of a series of ADPB/ βCD/CNC nanocomposites along with PB/CNC and CNC for comparison. The POM observations readily distinguished the difference between CNC assembly and aggregate. For the PB/ CNC system, the irregular aggregates of the CNCs are clearly observed owing to the strong interaction of their surface hydroxyl groups. In the case of the ADPB/βCD/CNC nanocomposites with βCD content higher than 5 wt %, the aggregation of CNCs disappears, and the bright regions appear, indicating that the CNC domains become more uniform and their sizes increase with the increment of βCD loading level. These observations further confirmed the existence of CNC assemblies in the ADPB nanocomposites rather than the aggregation of CNCs. Furthermore, it is clear from the TEM images that the CNC assemblies in the system are assembled by the individual CNCs (Figure 9). On the basis of these observations, we proposed a host−guest inclusion complex induced CNC assembling mechanism (Figure 10). The βCDs of the polymeric host−guest inclusion complexes can interact with the CNCs via hydrogen bonding, thus not only hindering the aggregation of CNCs but also enhancing the CNC− polymer interaction. Consequently, the filler−matrix and filler− filler interactions render the adjacent CNCs to form the “CNC aggregates”, namely CNC assemblies. Meanwhile, the enhanced interaction between the CNCs and polymers enables the CNC assemblies to be uniformly dispersed in the matrix. It is noted that although the orientation of CNCs in nanocomposite is generally difficult to realize without external promotion,42−46 in the current study the CNC−CNC and CNC−polymer interactions via hydrogen bonding achieve a local orientation of the CNCs as revealed by the SEM and POM images, even though these assemblies are randomly arranged within the polymer matrix.

mechanical properties of the resulting composites could be affected by the interaction between the CNCs and polymers through the host−guest inclusion complexes, and note that the formation of macroscale CNC assemblies (will discuss below) should also be considered. For the series of ADPB nanocomposites with the βCD content varied from 0 to 10 wt % and the CNC content fixed at 10 wt %, the E′ first increase from 2.09 to 6.30 MPa with increasing the βCD content from 0 to 5 wt % and then practically leveled off at 10 wt % βCDs (Figure 5c). In the case of the ADPB nanocomposites with the fixed 10 wt % βCD content, the increase in CNC content leads to an increase in E′ (Figure 5e). As shown in Figures 5b, 5d, and 5f, for the βCDfree materials, PB/10CNC, and ADPB/10CNC, the maxima of tan δ peaks are about the same as those of their parent polymers, PB and ADPB, indicating that the incorporation of CNCs into the polymer matrix does not affect the mobility of the polymer chains due to the relatively weak interactions of the polar CNCs and the nonpolar polymer. By contrast, the tan δ peak of the ADPB/βCD/CNC nanocomposite gradually shifts to higher temperatures when increasing the βCD and/or CNC contents, and simultaneously a decrease in the magnitude of tan δ peak and continual broadening of the glass−rubber transition zone are observed, which can be ascribed to the strongly attractive effect arising from the interactions between CNCs and βCD moieties of the pendent host−guest inclusion complexes on ADPB via hydrogen bonding and, in turn, lead to a reduction of the volume fraction of the mobile polymer phase. Figure 6 shows the stress−strain curves of the ADPB nanocomposites and the reference sample of neat PB. The

Figure 6. Stress−strain curves of the neat PB and ADPB nanocomposites.

tensile strength and Young’s modulus for ADPB/10βCD/ 15CNC remarkably increased by 980% (from 0.35 to 3.43 MPa) and 1200% (from 0.9 to 10.9 MPa) compared to neat PB, respectively, whereas only a moderate increase in tensile strength (180%, from 0.35 to 0.62 MPa) and Young’s modulus (140%, from 0.9 to 1.3 MPa) was observed for the PB/10βCD nanocomposite. Furthermore, a similar trend was observed as that of E′; both the tensile strength and Young’s modulus increased with increasing the CNC content. However, the elongation at break decreased with increasing the CNC and βCD contents, and this tendency become more apparent for the nanocomposites with higher βCD contents. These results indicate that the mechanical properties of the nanocomposites depend on the number of the formed βCD/admantane inclusion complexes, which can bind with CNC through F

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Figure 7. (a−d) SEM images of cross section for ADPB and ADPB/10βCD/10CNC. (e) AFM height image of CNCs. (f−h) SEM images of ADPB/10βCD/10CNC from solution casting.

Figure 8. POM images of CNC, PB/CNC, and ADPB/βCD/CNC nanocomposites.

illustrated in Figure 11, and the following points are worth to note as features for this system: (i) The formation of the inclusions of the CNC assemblies into the polymer matrix induced by hydrogen bonding could enhance the modulus of the nanocomposites owing to hydrodynamic effect and external load transfer from the soft polymer to the stiff CNCs. (ii) The well-dispersed CNC assemblies in the PB matrix could dissipate energy and avoid the local concentration of stress during the stretching process. (iii) The specific matrix−filler interaction via hydrogen bonding through βCD/adamantane inclusion complexes or the host−guest interaction between βCDs and adamantane moieties favors the energy dissipation when loading. (iv) For the systems with high adamantane and βCD

Figure 9. TEM images of ADPB/10βCD/10CNC nanocomposite.

On the basis of the above-mentioned results, the proposed reinforcing mechanism in the ADPB/βCD/CNC system is G

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ment of the mechanical properties. Compared to neat PB, the tensile storage modulus, Young’s modulus, and tensile strength of the nanocomposite increased by almost an order of magnitude upon incorporation of 15 wt % CNCs. SEM and POM were used to study the dispersion and morphology of CNCs in the matrix. The observations showed that the individual CNCs aligned to form assemblies which resembled the CNC’s rodlike structure, and some of the assemblies were connected by the polymer, suggesting that the reinforcing mechanism of CNC lies in the intense physical polymer/CNC interlocks and entanglements in the system induced by the strong hydrogen bonding. Furthermore, the sacrificial units including CNC assemblies, CNC and βCD connections, and βCD/adamantane inclusion complexes might also contributed to the enhancement of the mechanical properties.

Figure 10. Schematic representation of the proposed self-assembly mechanism for CNCs in ADPB/βCD/CNC nanocomposites.

contents, the reinforcement is also regulated by the “crosslinked” network formed by hydrogen bonding. (v) For the systems with high CNC contents, the hydrogen bonding or entanglement of CNC assemblies promotes to form percolation network which confines the mobility of PB domains, leading to high rigidity of the nanocomposites. On the contrary, the simple incorporation of CNC into PB matrix resulted in a limited enhancement of the mechanical properties due to the aggregation of CNCs and relatively weak interactions between CNCs and the PB polymers. Therefore, the filler−filler, filler− polymer, and host−guest interactions in the system and stress transfer between polymer and CNCs are crucial for the improved mechanical properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00606. Specific information of CNCs; SEM images of nanocomposite film; aspect ratio distribution of CNC assemblies in nanocomposites from POM images (c−f) (PDF)

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CONCLUSION In summary, the present work showed a host−guest inclusion complex assisted approach for the fabrication of cotton CNCs reinforced polybutadiene nanocomposites. The host−guest inclusion complexes formed with adamantane groups in the polymer side chain and βCDs as binding sites to CNCs disrupted the CNC−CNC interactions, allowing the formation of homogeneous polybutadiene/CNC nanocomposites even at a high amount of CNC (15 wt %). Incorporation of CNCs into the hydrophobic polymer matrix led to a significant enhance-

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.H.). *E-mail [email protected] (X.Z.). ORCID

Xuequan Zhang: 0000-0003-2487-9438 Notes

The authors declare no competing financial interest.

Figure 11. Schematic illustration of PB reinforced with CNCs. H

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from National Basic Research Program of China (2015CB674702) and National Key Research and Development Program of China (2017YFB0307103).

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