Bioderived Rubber–Cellulose Nanocrystal Composites with Tunable

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Bioderived Rubber−Cellulose Nanocrystal Composites with Tunable Water-Responsive Adaptive Mechanical Behavior Ming Tian,*,†,‡,§ Xiuchun Zhen,§ Zhifei Wang,§ Hua Zou,†,§ Liqun Zhang,†,‡,§ and Nanying Ning*,†,§ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029P. R. China § Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ‡

ABSTRACT: Adaptive mechanical behaviors in nature have inspired the development of synthetic adaptive composites, with those responsive to water particularly relevant for biomedical applications. Polymer nanocomposites containing cellulose nanocrystals (CNCs) are prime examples of water-responsive mechanically adaptive materials. Although CNCs are biobased, the matrixes of these composites are exclusively petroleum-based synthetic elastomers, in sharp contrast to their biological counterparts. In this work, we attempted to probe the possibility of using bioderived rubber(s) as the matrix to fabricate CNC-nanocomposite with waterresponsive adaptive mechanical behaviors. Specifically, natural rubber (NR) and epoxidized natural rubber (ENR) were used as the composite matrixes. Our results show that the water-responsive sensitivity and reversibility of ENR composites is much more drastic than that of NR composites. This is attributed to the strong CNC−polymer interaction (hydrogen bonding) for ENR, which leads to better filler dispersion and the formation of an extra CNC−polymer network in addition to the CNC−CNC filler network present in the NR composite. The synergistic effect of the dual networks plays a key role in tuning the mechanical properties and water-responsive sensitivity for various potential biomedical applications. Our study further provides guidance to make use of renewable resources to produce high value added water-responsive nanocomposites. KEYWORDS: cellulose nanocrystals (CNCs), epoxidized natural rubber (ENR), dual network, water-responsive, nanocomposites

1. INTRODUCTION Stimuli-responsive polymers that change properties under external stimuli, such as exposure to light,1,2 heat, chemicals,3 moisture,4 magnetic fields,5 etc., have gained significant attention from researchers in both academia and industry.6 Among them, polymers mimicking the stiffness-changing capability of sea cucumbers are particularly intriguing.7 This type of stimuli-responsive mechanically adaptive materials are promising for a variety of applications, including biomedical devices,8 aerospace structures,9 dry adhesives,10 optical devices, and smart materials. Biomimetic mechanically adaptive materials composites were typically designed via a composite approach by introducing cellulose nanocrystals (CNCs) into a rubbery polymer including ethylene oxide-epichlorohydrin copolymer,11 polybutadiene,12 styrene−butadiene rubber (SBR),13 polycaprolactone-co-polyetheneglycol,14 polyurethane rubbers,7 and poly(vinyl-acetate).11,15 These composites were fabricated by various methods including solution casting, compressionmolding, emulsion polymerization, etc.16 The processing methods have significant influence on the morphologies, which in turn affects the stimuli-responsive properties of the composites.1 In particular, structures of composites control the © 2017 American Chemical Society

internal stress transfer by changing the degree of interaction between adjacent nanofibers, which is critical for the composite modulus. In the dry state, the strong filler−filler interactions arising from the hydrogen bonding between CNC fibers leads to high modulus. This is typically considered as the “on” state. Upon exposure to an aqueous environment, water diffuses into the nanocomposites and weakens the filler−filler interactions, resulting in disintegration of the CNC filler network and softening of the nanocomposites (modulus drop).17 This wet and low modulus state is called the “off” state. Although CNCs are biobased, the matrixes of reported biomimetic waterresponsive composites are exclusively petroleum-based synthetic elastomers,18 in sharp contrast to their biological counterparts. The natural question is if it is possible to use bioderived rubber(s) as the matrix to fabricate CNC nanocomposites with water-responsive adaptive mechanical behaviors. In addition, it is highly desirable to fabricate composites with highly tunable water-responsive behaviors in order to meet the demands of potentially diverse applications. To this latter Received: December 19, 2016 Accepted: January 24, 2017 Published: January 24, 2017 6482

DOI: 10.1021/acsami.6b16308 ACS Appl. Mater. Interfaces 2017, 9, 6482−6487

Research Article

ACS Applied Materials & Interfaces

ments, Rmetric Scientific, United States). The samples were ovendried prior to the measurement. Tests were conducted in a tensile mode using a temperature sweep method (−100 to 100 °C; strain, 0.1%; frequency, 1 Hz; heating rate, 5 °C/min). The aqueous swelling behavior of nanocomposite films was monitored in deionized water at 37 °C. The samples were first dried under vacuum at 60 °C for 24 h and weighed. They were then immersed in deionized water. The samples were removed every 24 h, gently blotted using filter paper, weighed, and immediately immersed in deionized water. Swelling measurements were conducted for 4 days for the samples and were carried out in triplicate. The water uptake (WU) was determined from the relative gain in weight as follows:

point, we note that the water-responsive properties of composites are affected by the filler, the matrix, or a combined effect of the filler and matrix, yet the reported examples of water-responsive composites focused mainly on the effect of the filler−filler network. In this study, we designed and prepared two kinds of biomimetic water-responsive elastomer CNC nanocomposites utilizing bioderived natural rubber (NR) and epoxidized natural rubber (ENR) as the matrix. Because of the lack of strong polymer−filler interaction in CNC/NR nanocomposites, only a single CNC−CNC filler network is expected. In contrast, we hypothesize that two sets of networks (filler−filler and filler− polymer) can be formed in CNC/ENR nanocomposites due to the additional strong hydrogen bonding between hydroxyl groups of CNCs and epoxy groups of ENR. The dual network in the CNC/ENR nanocomposites could have a synergistic effect that can be explored as an additional freedom to tune the water-responsive behavior and mechanical properties of the composites. Efforts along these lines are reported hereafter.

WU =

Mt − M0 × 100% M0

where Mt and M0 are the masses of the sample after water immersion for a certain period of time t and before immersion in deionized water, respectively. The water-responsive mechanical behavior was evaluated with DMA Q800 instrument (TA Instruments), using a submersion clamp where the samples were submerged in deionized water during an isothermal run at 37 °C under tensile loading for 40 min (with a strain of 0.2% and static force of 0.05 N, 1 Hz, and scanning rate of 1 °C/min). To determine the kinetics of softening and stiffening of the nanocomposites during wetting and drying, selected measurements were also carried out under isothermal conditions at 35 ± 2 °C upon adding and removing water. Swollen nanocomposites were dried in an oven at 45 °C for 24 h to remove water for selected repeat measurement.

2. EXPERIMENTAL SECTION 2.1. Materials. The cellulose filter papers used as raw material of CNCs were supplied by Hangzhou Special Paper Industry Co., Ltd. (China). Sulfuric acid (H2SO4, 98%), toluene, and cyclohexane were purchased from Beijing Chemical Reagents Co., Ltd. (China). NR latex (solid content of 60%, Mw = 8.3 × 105 Da) and ENR latex (epoxy degree of 50%, solid content of 32%, Mw = 2.1 × 105 Da) were obtained from Chinese Academy of Tropical Agricultural Sciences. 2.2. Preparation of CNCs. CNCs were prepared by acid hydrolysis of cotton from cellulose filter paper (10 g) using 64 wt % sulfuric acid (87.5 mL) at 45 °C for 2 h. The obtained suspension was diluted with 500 mL of deionized water to terminate the reaction and centrifuged at 1000 rpm for 10 min to remove the cellulose with low degree of hydrolysis and/or large size. Then the suspension was centrifuged again at 5000 rpm for 10 min to concentrate the cellulose and remove excess acid. The resultant precipitate was rinsed, recentrifuged, and dialyzed against deionized water for 7 days until constant neutral pH was achieved in the effluent. The suspension was then dispersed by ultrasonic treatment for 1 h to prepare CNCs by freeze-drying. 2.3. Preparation of ENR and NR Nanocomposite Films. ENR and NR nanocomposite films with different contents of CNCs (1− 15%, all volume percentages unless otherwise noted) were prepared by emulsion blending followed by in situ drying and filming. Specifically, CNCs were mixed in deionized water, stirred, and ultrasonic treated for 2 h to make a CNC water suspension. Next, 10 g of ENR and NR latex were dissolved in the as-prepared CNC water suspension, and the solution was stirred at room temperature for 2 h until the rubber was dissolved completely. Finally, ENR and NR nanocomposite films (thickness about 0.5 mm) were formed after casting and drying under vacuum for 48 h at 50 °C. The obtained NR and ENR nanocomposites are denoted as NR-X or ENR-X, with X representing the volume percentage of CNCs. For reference, two samples containing no CNCs were similarly obtained and are denoted as NR and ENR. 2.4. Characterization. The morphologies of CNCs and the microstructures of nanocomposites were characterized by a scanning electron microscopy instrument (SEM, S-4800, Hitachi Co. Japan). For the observation of the microstructures of CNCs, the nanocomposites were first soaked in a mixed solution of toluene and cyclohexane (1/1) to partially remove NR and ENR and better expose the CNCs; they were then coated with platinum before SEM characterization. The change of hydrogen bonds of ENR and NR nanocomposite films was characterized by Fourier transform infrared (FTIR) spectroscopy (Tensor 27, Bruker Optik GmBH) with the resolution of 2 cm−1 and 32 scans. The dynamic mechanical properties of the nanocomposites were measured with a dynamic mechanical analyzer (DMTA,TA Instru-

3. RESULTS AND DISCUSSION 3.1. Dispersion and Networks of CNCs in Nanocomposites. SEM was employed to observe the dispersion and network of CNCs in the nanocomposite, and the results are shown in Figure 1. Pristine NR and ENR show a smooth surface (see Figure 1a1,b1), whereas NR and ENR nanocomposites present typical crumpled morphologies due to the presence of CNCs. For NR nanocomposites, CNCs in the NR matrix shows some aggregation even at the CNC content of 1% (Figure 1a2), and the aggregation increases as the CNC content increases (Figure 1a2−a4). This is due to the hydrogen-bonding interaction among surface hydroxyl groups of CNCs and the lack of such interaction between the matrix and filler.19,20 The aggregation of CNCs forms the filler−filler network. For all the ENR nanocomposites, CNCs are distributed uniformly in the matrix without obvious aggregation (Figure 1b2−b4). This is attributed to the hydrogen bonding between the surface hydroxyl groups of CNCs and epoxy groups of the ENR. The filler−filler network of CNCs is not formed at the CNC content of 1% (Figure 1b2) because of the uniform dispersion and the low content of CNCs. The filler−filler network starts to form as the content of CNCs increases to 3% (see Figure 1b3). A stronger filler−filler network is formed as the CNCs are more closely connected with one another at the filler content of 10% (Figure 1b4). Fourier transform infrared spectroscopy (FTIR) was used to characterize the hydrogen bonding between the epoxy groups of ENR and the hydroxyl groups of CNCs in the ENR nanocomposites, and the results are shown in Figure 2. The characteristic peak of CNCs appears at 1059 cm−1, representing the C−O stretching peak, whereas the C−O stretching peak of the epoxy group in ENR occurs at 878 cm−1 (Figure 2a). Figure 2a shows further that the intensity of the C−O peak (1059 cm−1) of CNCs gradually increases while the intensity of the C−O peak (878 cm−1) of the epoxy group shows the opposite trend. This is more quantitatively reflected 6483

DOI: 10.1021/acsami.6b16308 ACS Appl. Mater. Interfaces 2017, 9, 6482−6487

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Figure 2. (a) FTIR spectra for ENR/CNC nanocomposite with different contents of CNCs and (b) absorbance ratio between CH3 and C−O bands in the FTIR spectra for ENR nanocomposites. Figure 1. SEM micrographs of NR nanocomposites filled with (a1) 0%, (a2) 1%, (a3) 3%, and (a4) 10% CNCs and ENR nanocomposites filled with (b1) 0%, (b2) 1%, (b3) 3%, and (b4) 10% CNCs.

content of CNCs. E′ − E0′ for NR nanocomposites increases when the content of CNCs is above 1%. In contrast, the E′ − E0′ of ENR nanocomposites starts to increase drastically only when the content of CNCs is above 3%. This is analogous to the critical threshold of electrically conductive composites when the conductive filler network starts to form.21 For the current study, the CNC filler network is established in the NR composites even at CNC content as low as 1%, whereas the CNC filler network is formed in ENR composites at CNC content of 3%, consistent with the observations in Figure 1. The higher critical threshold for ENR composites is attributed to the stronger interfacial interactions between CNCs and ENR and the better dispersion of CNCs in ENR. E′ values of the pristine NR and ENR are similar, but the E′ − E0′ for ENR nanocomposites is much higher than that of NR nanocomposites at the same content of CNCs (especially at 6% and 10%). For instance, the E′ − E0′ of ENR-10 nanocomposite is 23.7 MPa, whereas that of NR-10 nanocomposite is 1.25 MPa. Again, this much better reinforcing effect of CNCs for ENR is due to the stronger interfacial interactions between the polymer and filler. 3.3. Water-Responsive Behavior. 3.3.1. Water-Swelling Behavior. The water-swelling kinetics of NR, ENR, and their nanocomposites are shown in Figure 4a,b. The swelling kinetics is similar for both NR and ENR nanocomposites: their water uptake go up rapidly in 1 day and then slowly increase until reaching the equilibrium state in 2 days. The equilibrium water uptake values are summarized in Figure 4c. As expected, the equilibrium water uptake for both NR and ENR nanocomposites increases gradually with the increase in CNC content. At the same filler content, the equilibrium water

in Figure 2b in which the intensity of the peak of methyl (CH3) at 2900 cm−1 for ENR (IENR(CH3)) is used as an internal reference because it remains constant for the different composite samples. The reduction of IENR(C−O)/IENR(CH3) with increasing content of CNCs observed in Figure 2b indicates the hydrogen-bonding interaction between C−O of the epoxy group and the hydroxyl group in CNCs. 3.2. Mechanical Properties of the Nanocomposites. The tensile storage modulus (E′) as a function of temperature of NR composites and ENR composites are shown in panels a and b of Figure 3, respectively. At a given temperature, both the NR and ENR nanocomposites show significant increase in E′ with the increase in the content of CNCs. At the same content of CNCs, the enhancement in E′ of ENR nanocomposites is much more drastic than that of NR nanocomposites. Tg of ENR nanocomposites (Tg = −12 °C) is much higher than that of NR nanocomposites (Tg= −58 °C). For ENR nanocomposites, E′ below Tg of the nanocomposites is slightly higher than that of pristine ENR at the same temperature. In the rubbery plateau, E′ at a given temperature increases significantly with increasing the content of CNCs, especially at high content of CNCs (>3%). This implies the effective reinforcing effect of CNCs. Such a reinforcement for ENR is much stronger than for NR, especially at high content of CNCs (6% and 10%). In addition, E′ values at human body temperature (37 °C) are summarized in Figure 3c,d. E0′ is the storage modulus of pristine NR and ENR, the E′ − E0′ represents the improvement of storage modulus by adding CNCs. We can see that E′ values of both NR and ENR nanocomposites depend strongly on the 6484

DOI: 10.1021/acsami.6b16308 ACS Appl. Mater. Interfaces 2017, 9, 6482−6487

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Figure 3. Dynamic mechanical thermal analysis traces for (a) CNC/NR and (b) CNC/ENR nanocomposites and the storage modulus (E′) as a function of CNC content at 37 °C for (c) CNC/NR and (d) CNC/ENR nanocomposites.

Figure 4. Water uptake of (a) CNC/NR and (b) CNC/ENR nanocomposites as a function of immersion time and (c) water uptake at equilibrium of nanocomposites as a function of the content of CNCs.

corresponding pristine matrix. The reversibility of the waterresponsive behavior is shown in Figure 5c,d. Herein, we take the dry modulus at 37 °C as Emax. We found that the Emax of NR nanocomposites in the second test is almost identical with that in the first test. Interestingly, the Emax of ENR nanocomposites with high content of CNCs in the second test is even higher than that in the first test, mostly likely due to more favorable reorganization of the CNC network structure in the ENR matrix. Figure 5d shows the ΔE (Emax − Emin) of both NR and ENR composites. The value of ΔE represents the extent of

uptake of ENR composites is higher than that of the NR composites. 3.3.2. Water-Responsive Mechanical Behavior. We hereafter focus on the water-responsive mechanical behavior and the kinetics of the water-triggered stiff−soft transition. Both NR and ENR nanocomposites show a sharp decrease in E′ when immersed in deionized water at 37 °C (Figure 5a,b). E′ of both NR and ENR nanocomposites continually reduces until reaching the minimum (Emin) with increasing the immersion time. It is particularly noteworthy that the Emin of some nanocomposites is even lower than that of the 6485

DOI: 10.1021/acsami.6b16308 ACS Appl. Mater. Interfaces 2017, 9, 6482−6487

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Figure 5. Evolution of stiff−soft transition of NR and ENR nanocomposites immersed in deionized water at 37 °C (a, b) and the maximum of storage modulus and the D-value of the maximum and the minimum of storage modulus (ΔE = Emax − Emin) as a function of content of CNCs (c, d). In panels c and d, 1 represents the first water-responsive test results; 2 represents the second water-responsive test results after the samples were redried.

mechanical switching and the water-responsive behavior. For both NR and ENR composites, the ΔE increases with the increase in the content of CNCs, indicating higher waterresponsive sensibility. This also confirms that the waterresponsive behavior strongly depends on the content of CNCs. At the same content of CNCs, the ΔE of the ENR nanocomposites is much higher than that of NR nanocomposites, demonstrating higher water-responsive sensibility of ENR nanocomposites. For instance, the ΔE of the NR-15 nanocomposite is about 0.2 MPa, whereas that of ENR-15 nanocomposite reaches 12.5 MPa. In addition, the ΔE of NR nanocomposites in the second test is almost identical with that in the first test, whereas the ΔE of ENR nanocomposites at the high contents of CNCs in the second test is higher than that in the first test, indicating that ENR nanocomposites have better reversibility. 3.4. Mechanism Analysis for the Water-Responsive Sensibility. The mechanism for the enhanced waterresponsive functionality of CNC/ENR nanocomposites over that of CNC/NR nanocomposites is schematically shown in Figure 6. In NR nanocomposites, the hydroxyl groups on CNCs interact with one another by hydrogen bonding, leading to the formation of the CNC−CNC filler network. After immersion in deionized water, it is more competitive for water molecules to interact with the surface hydroxyl groups of the CNCs to form hydrogen bonding, so that the hydrogen bonding of initial CNC−CNC was disrupted. Thus, the CNC− CNC filler network in NR composites was disrupted, showing an obvious water-responsive behavior. Once the water is removed, the hydrogen bonding of initial CNC−CNC can be reformed. This process is reversible and repeatable. In ENR nanocomposites, there exists not only hydrogen bonding between the surface hydroxyl groups of CNCs but also the hydrogen bonding between ENR and CNCs. As a result, there are dual networks in ENR nanocomposites formed by the CNC−CNC filler network and CNC−polymer networks. The dual networks in ENR composites can be disrupted after

Figure 6. Schematic representation of hydrogen bonding (between CNCs and CNCs as well as CNCs and ENR) in nanocomposites upon addition and removal of water.

immersion in deionized water and can be reformed after the water is removed. Compared to NR nanocomposites, the ENR nanocomposites possess better water-responsive sensitivity because of the synergistic effect of dual networks of CNC− CNC and CNC−polymer interactions.

5. CONCLUSION Utilizing bioderived rubber NR and RNR as polymer matrixes, we have designed and fabricated two series of water-responsive mechanically adaptive CNC nanocomposites. The weak interactions between CNCs and NR results in nanocomposites of a single filler−filler network. In contrast, the strong polymer filler interaction in CNC/ENR nanocomposites ensures better filler dispersion and the formation of a dual network (polymer− filler and filler−filler). The formation of either the single network or dual network allows tuning composite properties. Despite their similar water uptake, the latter exhibits much stronger water-responsive stiffness switching due to the synergistic effect of the dual networks. Both series of 6486

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nanocomposites show good reversibility in terms of their watertriggered switchable mechanical behaviors. Their drastically difference in water-responsive sensitivity is appealing to different potential applications. More importantly, the underlying mechanistic difference suggests ways to further tune the water-responsive mechanically adaptive composites. In addition, this study provides guidance for making use of renewable resources such as natural cellulose and natural rubber to produce nonconventional high value added products.

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +8610-64433964. E-mail: [email protected]. *Tel: +8610-64434860. E-mail: [email protected]. ORCID

Ming Tian: 0000-0002-4820-7372 Funding

N.N. received funding from the National Basic Research Program of China (Grant 2015CB654700 (2015CB654704)); M.T. received funding from the National Natural Science Foundation of China (Grants51525301 and 51521062) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Basic Research Program of China (Grant 2015CB654700 (2015CB654704)) and the National Natural Science Foundation of China(Grants 51525301 and 51521062) for financial support.

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REFERENCES

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DOI: 10.1021/acsami.6b16308 ACS Appl. Mater. Interfaces 2017, 9, 6482−6487