Reinforcement of Natural Rubber: The Use of in Situ Regenerated

Article pubs.acs.org/Macromolecules

Reinforcement of Natural Rubber: The Use of in Situ Regenerated Cellulose from Alkaline−Urea−Aqueous System Peng Yu,†,‡ Hui He,*,† Yuanfang Luo,† Demin Jia,† and Alain Dufresne*,‡ †

Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ‡ Univ. Grenoble Alpes, CNRS, LGP2, Grenoble INP, F-38000 Grenoble, France ABSTRACT: Cellulose, especially cellulose nanocrystal and cellulose nanofibril, has captured much attention as a bio-based candidate for the reinforcement of elastomers, but data in the use of regenerated cellulose (RC) from alkaline−urea− aqueous system as reinforcing agent remain scarce. In this contribution, fully bio-based hybrids were produced by flocculating a mixture of natural rubber (NR) latex and cellulose alkaline−urea−aqueous solution, during which NR latex particles were demulsificated to form the polymeric matrix while the porous honeycomb-like RC phase was also generated due to the coagulant-induced gelation, indicating that both the matrix and the filler were synchronously in situ formed by the bottom-up route. RC possesses a unique honeycomb-like structure which apparently could induce extensive physical entanglements, making the filler tightly interlocked with the matrix and therefore favoring the stress transfer. Predictably, RC, without any chemical modification, could endow NR with a pronounced reinforcement. Overall, this work provides (i) a unique inspiration for reinforcing NR with honeycomb-like cellulose and (ii) a better understanding of how could RC truly reinforce NR composites.

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caprolactone,20 methyl methacrylate/butyl methacrylate,21 Llactide/ε-caprolactone22) in the three-dimensional porous RC, suggesting that the formation of RC and polymeric matrix was sequential, not simultaneous. Another method to prepare thermoplastic/RC composite consists in using a “dissolution− gelation−isolation−melt extrusion” methodology.23 In parallel, natural rubber (NR), almost exclusively extracted from the Pará rubber tree in a milky latex form, is a unique biopolymer with strategic importance, and the use of reinforcing filler is often required for NR to obtain improved strength or controlled cost. However, the attempt to use the RC as reinforcing agent for elastomer has been reported rarely. In a recent work,24 we first proposed an environmentally friendly method to prepare NR/RC hybrids by simply precipitating the mixture of NR latex and cellulose alkaline− urea−aqueous solution. During this process the cellulose inclusion complex associated with sodium ion, water, and urea molecule was broken by the coagulant; thus, the cellulose chains contact, entangle, and interact through hydrogen bonding to form the cellulose-rich phase. Simultaneously, a demulsification process of the rubber latex particles was induced by the same coagulant. It means that both polymers were in situ and bottom-up formed synchronously. It was found

INTRODUCTION There is an increasing demand for products made from renewable and sustainable non-petroleum-based resources. Cellulose, the most abundant polymer on earth, is renewable, biodegradable, and nontoxic.1 Since the first report of using nanocellulose as a reinforcing agent of a polymeric matrix,2 cellulose or its derivatives, especially cellulose nanocrystals (CNC) or cellulose nanofibrils (CNF), have been used extensively as model filler in various polymeric matrices,3−12 since it could enhance the mechanical13 or functional14 performances of ensuing composite as well as its “green” attribute that is favoring for a sustainable world. In recent years, a novel solvent for cellulose, the alkaline/ urea aqueous system, has attracted significant interest,15−18 and cellulose could regenerate from this solvent by heat, ethanol, salts, and other chemicals.19 This kind of cellulose is a relatively novel reinforcing agent for polymers compared to CNC or CNF. Moreover, different from rod-like CNC and fiber-shaped CNF, regenerated cellulose (RC) could show a honeycomb-like morphology with plenty of micropores/nanopores,19 which facilitates the interaction between the filler and the matrix through physical interlock/entanglement effect. Therefore, composites involving RC might display pronounced mechanical properties. Recently, several interesting processing methods regarding the incorporation of RC into polymeric matrix have been reported.20−23 Usually, RC was initially prepared, followed by in situ polymerization of various monomers (such as ε© XXXX American Chemical Society

Received: August 3, 2017 Revised: September 3, 2017

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DOI: 10.1021/acs.macromol.7b01663 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Photos of the swelling behavior for NR/RC compounds in toluene at various times: (A) NR, (B) NR/5RC, (C) NR/10RC, (D) NR/ 15RC, (E) NR/20RC, (F) NR/25RC, and (G) NR/30RC. NR/20RC, 0.625 g NR/25RC, and 0.65 g NR/30RC; the rubber content in all samples was 0.5 g) were divided into scraps and dipped in toluene at room temperature. The photos of the samples during swelling during the first 3 days were recorded with a camera. After 3 days, the samples were filtered to remove the dissolved fraction, and fresh toluene was added to ensure sufficient swelling. These procedures were repeated every 3 days for 5 times. After 15 days, the swollen sample was dried in a vacuum oven and then weighted again to determine the bound rubber content according to the equation

that the honeycomb-like RC dispersed homogeneously in the matrix, and RC and NR phases interlaced/interpenetrated each other to form a semi-IPN/fully IPN (interpenetrating polymer network) structure. The goal of the present study consists in evaluating the interaction between RC and NR as well as RC’s reinforcing capability toward NR. Properties such as swelling, mechanical, and dynamic mechanical for NR/RC hybrids are presented to enable a better understanding of the structure−properties relationship in NR/RC hybrid.

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bound rubber content =

EXPERIMENTAL SECTION

W1 − (W2 − W3) × 100% W1

(1)

where W1 is the NR mass in the NR/RC composite before swelling, W2 is the initial mass of NR/RC composite, and W3 is the mass of NR/RC composite after swelling and drying. The swelling for the NR/RC films in water was done as follows: NR/RC samples (circular sheet, the diameter and thickness were 20 mm and around 0.6 mm, respectively) were first dried and then submerged in DI water at room temperature. The films were removed from the water at specific intervals and weighted (the adhering water on the sample’s surface was wiped off with a dry cloth before weighting) and then submerged again in water. The water uptake (Wu) was determined from the relative gain in weight of the specimen according to the equation

Materials. Microcrystalline cellulose powder was purchased from Sigma-Aldrich (USA). Urea (purity ≥99.5%, BioScience-Grade) and NaOH were obtained from Carl Roth (Karlsruhe, Germany). Natural rubber latex (type: CENTEX FA, full ammonia, pH 10−11, dry rubber content 59−61%) was kindly provided by Centrotrade Deutschland GmbH (Eschborn, Germany). Ethanol and other reagents were provided from local sources and used as received. Preparation of NR/RC Composites. First, a 4 wt % transparent cellulose solution was prepared following the protocol developed by Qi et al.18 The resulting cellulose solution was centrifuged to exclude bubbles and slightly insoluble fraction before mixing with the diluted NR latex (the solid content of NR latex was diluted to 30%). Subsequently, the well-mixed mixture was cocoagulated by sufficient ethanol overnight, and the coagulum was first washed with deionized (DI) water and then immersed in DI water at room temperature for 1 week with renewing water every day to remove NaOH and urea. Afterward, the sediment was dried in a oven at 50 °C until a constant weight was reached. To prepare NR/RC films, the NR/RC sediments were first preheated for 3 min in a hydraulic press (Mécanique Outillage, Model Saint-Eloi, France), and they were compressionmolded under 42 bar at 145 °C for another 3 min to obtain films. Subsequently, the films were immediately cold-pressed at room temperature for 7 min to avoid shrinkage. The as-prepared films were conditioned at 23.5 °C and 50% relative humidity (RH) for the following mechanical testing. The compounds were denoted as NR/ χRC, where χ represents the χ phr of RC in the compounds (phr refers to parts per hundred of rubber). Characterization. The swelling for the NR/RC sediments in toluene was done as follows: the sediments of NR and NR/RC (0.5 g neat NR, 0.525 g NR/5RC, 0.55 g NR/10RC, 0.575 g NR/15RC, 0.6 g

Wu =

Mt − M0 × 100% M0

(2)

where M0 and Mt are the weights of the samples before and after a time t of immersion, respectively. Results were done in duplicate. Differential scanning calorimetry analysis (DSC) was performed on a DSC Q100 (TA Instruments, New Castle, DE). NR/RC films, sealed in aluminum pans, were scanned from −100 to 30 °C at a rate of 10 °C/min under a N2 atmosphere. The specific heat capacity increment ΔCpn and weight fraction of constrained rubber χim for NR/RC composites were calculated as follows:

ΔCpn = ΔCp/(1 − w)

(3)

χim = (ΔCp0 − ΔCpn)/ΔCp0

(4)

where ΔCp is the specific heat capacity increment at glass transition, which was calculated by the TA universal analysis 2000 software, w is B


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Macromolecules the weight fraction of RC in the compound, ΔCpn is normalized to the rubber weight fraction, ΔCp0 is the specific heat capacity increment for neat NR, and χim is the weight fraction of constrained rubber domains. Scanning electron microscopy (SEM) observations were performed for the morphological investigation of NR and NR/RC composites using a Quanta 250 FEI microscope with an acceleration voltage of 10 kV in high vacuum mode. For the NR/RC sediment (before hotpressing), the surface and cross section (cut with a razor blade) were investigated. For the NR/RC film (after hot-pressing), the brittled surface (fractured in liquid nitrogen) was investigated. Tensile tests were conducted on a RSA3 (TA Instruments, USA) apparatus with a 100 N load cell with a cross-head speed of 1 mm s−1 at room temperature. The specimens were cut into 30 mm long strips, the distance between jaws was set at 10 mm, and the width and the thickness of the samples were measured before testing. Five measurements were carried out for each sample. The fracture energy (W) was calculated from the area under the stress−strain curve before rupture by the equation

W=

ε = εmax

∫ε=0

σ dε

(5)

Figure 2. Bound rubber content for NR/RC composites.

where σ is the stress (MPa) and ε is the strain. The tensile fracture surface of the rubber films was observed with a Zeiss AX10 (Carl Ziess AG, Jena, Germany) AX10 optical microscope with a 10× magnification lens. Dynamic mechanical analysis (DMA) was performed on a METRAVIB DMA 50 equipment in tensile mode at a frequency of 1 Hz with 0.08% strain amplitude. The measurements were carried out from −100 to 10 °C at a heating rate of 3 °C min−1. The storage modulus (E′), loss modulus (E″), and tangent of the loss angle (tan δ = E″/E′) were recorded during the isochronal DMA testing.

could attract water molecules through hydrogen bonding26 and therefore facilitate water diffusion. The evolution of water absorption for various samples as a function of time is presented in Figure 3. It is clearly seen that the water

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RESULTS AND DISCUSSION Swelling Analysis. Figure 1 shows photos of the swelling behavior for NR/RC compounds during the first 3 days of immersion in toluene. It is generally known that toluene is an excellent solvent for NR while it is a nonsolvent for RC. As expected, neat NR is dissolved in toluene within 4 h. The NR/ 5RC specimen could almost dissolved in toluene while the ensuing solution was less clear compared to neat NR solution. When the RC content was higher than 15 phr, the NR/RC compounds even could not be fully swollen in toluene. It is obvious that the inclusion of RC into NR could enhance the toluene resistance of the resultant materials. There are two factors correlating with swelling behavior for NR/RC blends: (i) the tortuosity effect imparted upon RC addition, the RC filler is conducive to forming more tortuous solvent paths and hindering toluene diffusion in the NR matrix; (ii) the decreasing free rubber volume upon RC addition, the honeycomb-like RC could entangle/interlace with the NR domains, leading to tightly interlocked components and increasing the constrained rubber volume, which, in turn, results in a lower toluene permeability. Figure 2 reports the evolution of the bound rubber content for NR/RC materials after 15 days of toluene swelling. As expected, the bound rubber content increased with the RC content, demonstrating that more rubber domains are trapped or caged by the honeycomb-like RC phase and hence a stronger filler−matrix interaction. Contrarily to toluene swelling experiments, and as expected, the introduction of RC into NR should increase the water swelling ratio. The transport property of elastomer composites is closely related to their composition and the character of each parent component.25 It is known that NR is a typical nonpolar elastomer with hydrophobic character while RC is a hydrophilic polymer containing large amounts of −OH groups, which

Figure 3. Evolution of water uptake as a function of immersion time for NR/RC composites.

absorption for all samples increased monotonically until an equilibrium state was achieved. For all composites, and as reported elsewhere,27 two zones can be identified: a first one (zone I) of fast absorption kinetic for t < 20 h and a second plateau-like one (zone II) with slow absorption kinetic. The water uptake at equilibrium increases with the RC content, for instance, the water uptake after 260 h immersion was 16.9% for NR/30RC, which is nearly 3.7 times higher than for neat NR (4.6%). Also, it takes less time to achieve the water absorption equilibrium state for the samples containing higher RC contents. The reduction of the saturation time upon cellulose addition was also observed when using CNF to reinforce NR.11 Moreover, a water absorption jump for the composites is noticed when the RC content was higher than or equal to 10 phr. Apart from the fact that cellulose presents a high affinity toward water28 and cellulose−matrix interface could accommodate additional water,29 adjacent RC domains could interact through physical entanglements or H-bonding to form a percolating network within the NR matrix when high RC C


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that the ΔCpn value decreased monotonously with RC content. The value of ΔCpn at glass transition is proportional to the number of internal degrees of freedom of molecular motion in the composite directly.30 It is worth noting that according to eq 3 the decrease of NR content is accounted for the calculation of ΔCpn. The decrease of ΔCpn is most probably attributed to constrained NR domains interacting with honeycomb-like RC; that is, the constrained NR domains are at least partially dead, behaving like a filler, and insensitive to the glass transition. It is logical that a higher NR fraction is immobilized or trapped when increasing the RC content leading therefore to stronger filler−matrix interaction. The results are also certified by the increase of χim. A slight increase of the Tg,mid value for the rubber blends upon RC addition could further demonstrate that RC could confine the NR macromolecular chain mobility. It is known that immobilized rubber chains can prevent the macrophase separation between rubber and filler,31 which is in favor of the external stress transfer and final mechanical performance of ensuing composites. Morphology Analysis. Generally, the structure and size of the filler are critical for preparing high-performance elastomer composites. However, in the present study the pore size for RC varies from several hundred nanometers to several micrometers. The pore size of RC is affected by the type of precipitate. It is also affected by the NR phase because NR could penetrate the pores of RC during the coprecipitation process, which might broaden the pore of the RC. It is therefore difficult to give a precise size for RC pores. Figure 5 shows the SEM images of the surface and cross section for NR/10RC and NR/30RC

content is considered. That is, a continuous hydrophilic pathway could further promote water infiltration. DSC Analysis. The mobility of NR macromolecules is sensitive to the local environment because of their long-chain characteristic, and the specific heat capacity increment ΔCpn at glass−rubber transition could give valuable information regarding the structural variation. Figure 4 shows the

Figure 4. DSC curves for NR/RC composites.

endothermic glass transition region and parameters (ΔCpn, χim, and Tg,mid) for neat NR and NR/RC blends. It is observed

Figure 5. SEM images of the surface (a, NR/10RC; b, NR/30RC) and cross section (a′, NR/10RC; b′, NR/30RC) for the sediments before hotpressing. The yellow arrows indicate the interpenetration of the NR phase with the RC phase, the green arrows indicate the pores of RC, the red arrow indicates the gap between RC and NR, and the blue circles indicate the interlock/interlacement between NR and RC. D


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Figure 6. SEM images of the cryo-fractured surface for NR and NR/RC composites (after hot-pressing).

Figure 7. Photos for NR and NR/RC samples before and after tensile testing.

NR is the predominating component as well as to the surface energy discrepancy between NR and RC. For the cross section (panels a′ and b′ in Figure 5), we can observe that the RC

sediments (before hot-pressing). For the surface (panels a and b in Figure 5), we can observe that the RC filler is embedded within the NR matrix. This is probably ascribed to the fact that E


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Figure 8. (a) Typical stress−strain curves, (b) fracture energy, and (c) optical images of the tensile fracture surface for NR and NR/RC samples.

network within the matrix. One can imagine that NR/RC hybrids are quasi-thermodynamically equilibrium systems due to the poor interfacial bonding between hydrophobic NR and hydrophilic RC. However, the RC phase and NR phase are synchronously generated during the fast coprecipitation process, and the subsequent hot-pressing treatment could further enhance the physical interaction between both polymers. Therefore, a strong filler−matrix interaction and a state of “forced compatibility” between both polymers were achieved. A simple naked eye examination of the film’s aspect before and after tensile testing is shown in Figure 7. For the samples before stretching, no air bubbles or inhomogeneous dispersion of RC domains could be observed. Despite the gradual reduction of transparency when increasing the RC content, all the films maintained transparent to some extent, suggesting a macroscopically uniform dispersion of RC in NR. For the samples after stretching, a stress-whitening scenario for the NR/RC composites was clearly observed, suggesting that there is an intense rubber molecular chain sliding and reconformation during tensile testing.32 The white color is a result of light diffraction from the numerous crazes that are generated within the material by the tensile stress. Other researchers also observed a similar phenomenon when using CNC or CNF to reinforce poly(ethylene oxide).7 Mechanical Analysis. Typical stress vs strain curves for NR/RC materials are plotted in Figure 8a, and the related mechanical data are collected in Table 1. The reinforcing effect of RC is clearly manifested by the significant increase of the

phase is dipersed at a microscale instead of nanoscale. Interestingly, RC typically exhibits a unique open-core foam structure which resembles a honeycomb-like structure, which is quite different from rod-like CNC or fiber-shaped CNF, and some pores of RC are interpenetrated with the rubbery phase, displaying a semi-IPN structure. It is expected that the RC’s honeycomb-like structure could maximize the contacts between the filler and the matrix, which should favor an intimate interaction between both polymers. Figure 6 displays the morphology of cryogenically fractured cross-section surfaces for various samples (after hot-pressing). The RC’s honeycomb-like morphology and the semi-IPN structure cannot be observed anymore. It is obviously due to the fact that the NR molecular chains could be pressed into the RC pores and the gaps between two neighboring particles since the NR domains could flow during the hot-pressing process, indicating that the hot-pressing process could enhance the interlacements/entanglements between both components. We can also infer that the agglomerate observed in Figure 6 is a complex of honeycomb-like RC and NR domains held together by RC. It is noteworthy that the RC’s honeycomb-like morphology and the semi-IPN structure still exist after hotpressing although they cannot be identified by SEM anymore. Again, the agglomerates are dispersed at the micrometer scale even for the NR/5RC sample (the RC content is only 5 phr). For higher RC contents (NR/20, 25, 30RC), the uniform distribution of RC is more or less preserved, and it seems that the “homogeneously” dispersed irregular aggregates interconnect to some extent, suggesting the formation of a percolating F


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of the elongation at break plays the leading role, giving rise to a decrease of fracture energy. The optical images of the tensile fracture surface for NR and NR/RC composites are shown in Figure 8c. Neat NR showed a striated and smooth fracture surface with little sign of plastic deformation while NR/RC samples exhibited much more coarse fracture surfaces, as a result of a more tortuous pathway for the crack propagation and a higher energy dissipation for NR/RC samples compared to neat NR. This is ascribed to the intense interlock between honeycomb-like RC and NR as previously emphasized, which is in accordance with the mechanical performances. A schematic representation of the RC’s reinforcing mechanism for NR is illustrated in Figure 9 and could probably be attributed to the following origins: (i) the hydrodynamic effect arising from the inclusion of rigid RC into soft NR matrix, which would definitely enhance the global modulus of resultant composites; (ii) the satisfactory dispersion of RC within the NR matrix induced by the coprecipitation technique, which could avoid stress concentration of the cracks and favor the energy dissipation during the tensile process; and (iii) the expected strong interactions between honeycomb-like RC and NR resulting from interlacements/interlocks. The NR chains could be anchored by the filler; that is, the honeycomb-like RC could act as additional physical cross-linking points for the NR macromolecular chains, resulting in external load that can be readily transferred from soft NR into rigid RC. (iv) For high RC contents, the reinforcement is also governed by a percolation effect originating from hydrogen bonding or entanglement between adjacent RC domains. Once a denser and continuous filler network formed, the mobility of NR domains constrained by the filler network will be substantially depressed and therefore the composites show much higher rigidity. In addition, the RC percolating network would further facilitate the stress transfer with the consequence of improved mechanical performance.

Table 1. Mechanical Properties for Rubber Composites (Data in Parentheses Are the Standard Deviations) samples NR NR/5RC NR/10RC NR/15RC NR/20RC NR/25RC NR/30RC

Young’s modulus (MPa) 0.69 1.77 2.63 4.59 11.7 15.7 20.8

(0.19) (0.21) (0.31) (0.45) (1.8) (1.7) (2.9)

tensile strength (MPa) 0.63 1.36 2.38 4.81 5.44 5.36 6.03

(0.14) (0.17) (0.19) (0.28) (0.37) (0.42) (0.61)

elongation at break (%) 747 612 570 505 484 370 225

(38) (45) (32) (29) (24) (18) (11)

Young’s modulus and tensile strength. For example, compared to neat NR, the Young’s modulus and tensile strength for NR/ 30RC are spectacularly increased by 2914% (from 0.69 to 20.8 MPa) and 857% (from 0.63 to 6.03 MPa), respectively. However, owing to the enhanced polymer−filler interaction between NR and RC, the slippage of rubber domains anchored/interlocked by the honeycomb-like RC during stretching is effectively hindered. Therefore, the elongation at break gradually decreased upon RC addition, meaning that the reinforcing effect of RC toward NR was at the expense of the material’s extensibility. A noteworthy scenario was that the tensile curves behave like rigid plastics, exhibiting definite yielding, which is associated with the destruction of the filler network at large deformation. Such a phenomenon is more obvious for the highly RC loaded samples (NR/20, 25, 30RC). The tensile curve could provide some interesting information regarding the percolating RC network within the matrix (will be discussed below). Notably, when increasing the RC content, the fracture energy value for NR/RC composites first increased and then reduced (Figure 8b). It is clear that the elongation at break and modulus/strength regulate the fracture energy value which was derived from integration of the stress−strain curve. When the RC content is lower than 20 phr, the increasing modulus plays the dominating role, resulting in an increase of fracture energy. When the RC content exceeds 20 phr, the drop

Figure 9. Schematic illustration of NR reinforced with RC from alkaline−urea−aqueous system. G


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phenomenon was observed with carbon nanotubes in natural rubber.39 An increase of Tg is usually observed for reinforced composites since the interaction between the matrix and the filler could confine the polymer chain mobility, as shown from DSC experiments. A possible explanation for the inverse trend of Tg in the present system is associated with the decrease of the modulus drop displayed in the concomitant relaxation process (well-known mechanical coupling effect).40,41 Additionally, a reduction in the magnitude of the tan δ peak and half-peak width upon RC addition was also observed, suggesting a continuous reduction in rubber chain mobility which can be ascribed to the strong confinement effect of honeycomb-like rigid RC toward NR. The drop of tan δ peaks is also ascribed to the decrease of polymer matrix fraction in the composite upon filler loading, viz. a decrease in the number of mobile units participating to the relaxation phenomenon.42 The DMA results agreed well with the DSC results and further illustrate the strong interaction between NR and RC in coprecipitated composites. Looking now to the RC percolating network, as noted before, there are some good indications about the RC’s percolation: (i) from the water swelling study, a dramatic jump in water absorption was observed for the composites when the RC content was higher than or equal to 10 phr, (ii) from SEM observations, it seems that the RC filler domains interconnect when the RC content was higher than or equal to 20 phr, and (iii) from tensile study, for composites containing 15 phr RC or more, the tensile curves exhibit obvious yielding. Thus, we can assume that the percolation of RC domains probably occurs within the matrix above a certain RC loading level. The classical series-parallel model43,44 and the modified one45−47 could be adopted to study the percolating effect of cellulose filler within polymeric matrix, and the theoretical modulus of polymer/ cellulose blend predicted by this approach should match well with the experimental data.20−22,48−50 Within the framework of the classical series-parallel model, the elastic modulus E′ is given as follows:

Table 2. Dynamic Mechanical Properties for Rubber Composites samples

E′ at −80 °C (MPa)

E′ at 0 °C (MPa)

tan δ peak (°C)

NR NR/5RC NR/10RC NR/15RC NR/20RC NR/25RC NR/30RC

1087 3480 4731 4937 4425 4302 5294

1.54 1.56 7.93 20.2 79.3 81.6 152.2

−53.3 −54.5 −56.2 −55.4 −57.2 −57.6 −57.3

DMA Analysis. DMA experiments were conducted to evaluate the dynamic performance of NR/RC composites. The evolution of E′ (storage modulus) as a function of temperature is depicted in Figure 10a. In the low temperature range −100 to −70 °C, NR is in the glassy state; the E′ value for all samples remains roughly stable, and it is higher for NR/RC hybrids compared to neat NR. With increasing temperature, the E′ value decreases sharply due to the NR macromolecular glass− rubber transition, which is correlated with an energy dissipation phenomenon involving the bulk segmental relaxation. When the temperature is slightly above Tg (where E′ decreases from a few GPa down to a few hundred MPa), we could also observe that the magnitude of the E′ drop for the NR/RC composites is decreasing when increasing the RC content. This is probably due to the presence of glassy polymer layers33−36 induced by RC that could make a contribution to the material stiffness. After the glass−rubber transition stage, the curves of E′ became flatter, which is associated with the rubbery state of NR. Notably, the E′ value for NR/30RC is 152.2 MPa at 0 °C, which is much higher than for the elastomer filled with silica/ carbon black.33,37 Such a high E′ value above Tg was also observed when using starch nanocrystals to reinforce NR, which is mostly probably due to the formation of a polysaccharide percolating network.38 In this stage, the E′ value for NR/RC compounds increased with the RC content and hence substantiated the prominent reinforcing effect of RC toward NR. The evolution of tan δ versus temperature for the various materials is depicted in Figure 10b. The tan δ peak, which is associated with the glass−rubber transition, slightly shifts to lower temperatures (less than 5 °C) when adding RC. A similar

E′ =

(1 − 2Ψ + ΨX r )E′r E′s + (1 − X r )ΨE′r 2 (1 − X r )E′r + (X r − Ψ)E′s

(6)

Figure 10. Evolution of (a) the storage modulus (E′) and (b) tangent of the loss angle (tan δ) as a function of temperature for NR and NR/RC composites: NR (■), NR/5RC (●), NR/10RC (▲), NR/15RC (▼), NR/20RC (★), NR/25RC (○), NR/30RC (△). H


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0.7 f

precipitant, various chemicals, such as organic solvents (ethanol, etc.), acids (HCl, H2SO4, etc.), and salts (CaCl2, etc.), could precipitate both the cellulose solution19 and the NR latex; the selection of the suitable precipitatant and its concentration is crucial because the precipitant could directly affect the microstructure of RC; (ii) the concentration of NR latex and cellulose solution; and (iii) the precipating temperature. Moreover, it seems that surfactants and other nanofillers (“nucleating agent”) could also be introduced to tune the selfassembly of cellulose chains and aggregate structure. All these factors will be investigated to have a rational design of RC morphology in NR/RC composites in the near future.

(7)

(8)

where E′s corresponds to the storage modulus of neat NR and E′r to the storage modulus of the percolating RC network; Xc is the volume fraction of RC at the percolation threshold; Ψ is the volume fraction of RC contributing to percolation (“effective skeleton”), and f is the aspect ratio of the filler. The filler’s percolation threshold strongly depends on its geometric dimensions51 and dispersion state in the polymeric matrix. However, the exact geometric dimension of the honeycomb-like RC domains, whose pores are interpenetrated or filled by NR domains, is not clear at the moment. Some researchers assumed that the Xc value is zero since RC is intrinsically percolated as long as it is a “gel”,20−22 making eq 8 irrelevant. For simplicity’s sake, we attempt to use Xc as an adjustable parameter (Xc = 0, 0.05, 0.075, 0.1, and 0.15) to try to fit experimental and predicted data. At sufficiently high temperatures (i.e., above the Tg of NR), the modulus of the matrix is much lower than the one of the percolating network; thus, the elastic modulus of the composite is simply the product of the volume fraction and modulus of the rigid percolating network.52,53 E′ = ΨE′r

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CONCLUSIONS The present work demonstrated that the in situ regeneration of cellulose from alkaline−urea−aqueous system resulted in a macroscopic fine cellulose dispersion within the NR matrix by a simple yet green methodology, i.e., coprecipitation of rubber latex and cellulose NaOH/urea aqueous solution. The effects of RC on the structure and properties of NR composites were fully investigated. The swelling study shows that the bound rubber content of the hybrids increases when increasing the RC content. The water uptake for NR/RC blends increases continuously when enriching the material in RC due to its hydrophilic nature. DSC experiments reveal that RC strongly influences the volume fraction of constrained rubber. SEM observations show that RC possesses a unique honeycomb-like structure. A spectacular reinforcing effect of RC toward NR was evidenced from both nonlinear tensile tests and linear dynamic mechanical analysis. The reinforcing mechanism of RC lies in the fact that its unique honeycomb-like structure favors the intense physical entanglements/interlocks with the matrix and promotes, therefore, the polymer−filler interaction. Moreover, the formation of a RC percolating network originating from hydrongen bonding and entanglement between adjacent RC domains could further enhances the mechanical performance of ensuing materials. Overall, this contribution offers a better understanding on how RC could truly reinforce the elastomer. It is expected that the mechanical performances of NR/RC composites will be further enhanced when the interfacial bonding between reinforcing phase and polymeric matrix is improved by introduction of appropriate compatibilizer or chemical modification.

(9)

Taking 1.5 and 1 g cm−3 for the density of the filler and of the matrix, respectively, and 11.8 GPa for the stiffness of the regenerated cellulose as reported elsewhere at −20 °C,20 the modulus of the composite can be estimated from eq 9. The comparison of the experimental and predicted modulus at −20 °C is shown in Figure 11.

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AUTHOR INFORMATION

Corresponding Authors

*(H.H.) E-mail: [email protected]. *(A.D.) E-mail: [email protected]. ORCID

Alain Dufresne: 0000-0001-8181-1849 Notes

The authors declare no competing financial interest.

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Figure 11. Comparison between experimental and predicted modulus from the percolation approach for NR/RC composites.

ACKNOWLEDGMENTS The authors acknowledge financial support from China Scholarship Council (CSC, grant no. 201606150006), Science and Technology Project of Guangdong Province (2015B010122002), and Science and Technology Project of Guangzhou (201508020090). LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir - grant agreement no. ANR-11LABX-0030) and of the PolyNat Carnot Institut (Investissements d’Avenir - grant agreement no. ANR-11-CARN-030-01).

A poor agreement between experimental and predicted data is observed in the present study, the experimental modulus is much lower than the predicted data. How to predict the percolating effect of microscale porous honeycomb-like RC toward NR necessitates more investigation. Also, it has been established that the microstructure and dispersion of RC in the blend strongly depends on the processing technique and conditions. Possible impacting factors are (i) the type of I


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