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Distribution of Nanodiamond Inside the Nanomatrix in Natural Rubber Asangi Gannoruwa, and Seiichi Kawahara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00761 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Distribution of Nanodiamond inside the Nanomatrix in Natural Rubber Asangi Gannoruwa, Seiichi Kawahara* Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 9401-2188, Japan

ABSTRACT:

The distribution of nanodiamond inside a nanomatrix, which is related to the mechanical and viscoelastic properties, is investigated for a natural rubber-nanodiamond composite. The composite is prepared by reacting nanodiamond with deproteinized natural rubber (NR-ND) in the presence of a tert-butylhydroperoxide (TBHPO) / tetraethylenepentamine (TEPA) radical initiator at 30 °C in the latex stage and subsequent drying. The morphology of the composite is observed by 3D transmission electron microscopy (TEM). NR-ND prepared with an initiator exhibits a nanomatrix structure, whereas NR-ND prepared without an initiator displays an island matrix structure. The nanomatrix is densely loaded with 15 nm or smaller-sized nanodiamond. Both the mechanical and viscoelastic properties of NR-ND depend upon the morphology. The stress at break and plateau modulus are 12 MPa and 1.19 × 106 Pa when NR-ND is prepared with a TBHPO/TEPA initiator and contains 25 w/w% nanodiamond, which are four and eight times higher than those of DPNR, respectively.

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INTRODUCTION Nanomatrix structure is a novel nanostructure, in which the minor component forms a thin matrix whereas the major component or the polymeric particles are dispersed in the matrix. This nano structure shall be distinguished from the conventional island matrix structures, in which the minor component forms isolated aggregates as islands randomly dispersed in the matrix of the major component1. In addition uniform distribution of nanoparticles inside the nanomatrix is very important to improve the mechanical properties. A nanodiamond (ND) nanomatrix is a promising and emerging structure to meet the demands of modern and future applications of polymeric materials. Additionally, ND displays one of the advantageous properties of bulk diamonds; its surface chemistry is highly tunable2. The ND core can be modified to introduce novel properties such as outstanding optical and electrical properties in combination with the superior mechanical properties of the rubbery material2-4. Nevertheless, the main obstacle to form a fine nanomatrix structure with ND is its high tendency to re-aggregate and form larger agglutinates in an aqueous medium5, 6. In a nanomatrix structure, natural rubber particles are covered by a layer of ND with a nanometer thickness. A fine and stable nanomatrix structure is inevitable by forming chemical linkages between ND and natural rubber in the latex stage followed by co-coagulation. These chemical linkages give rise to an increased adhesion of filler and rubber. Consequently, they reduce the energy dissipation and provide better reinforcement7. In our previous work, a ND nanomatrix structure was formed in the presence of a radical initiator8. The morphology of nanomatrices is usually observed by microscopic techniques and scattering techniques. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and


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atomic force microscopy (AFM) are common microscopic techniques as they provide insight into real-space representations of the morphologies9,

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. Among these, TEM is a powerful

technique that provides high-resolution images on the nanometer scale, but the resulting twodimensional (2D) images are merely projections of the nanoparticles arranged in threedimensional (3D) space in the z-direction. In these images, several filler aggregates may be projected on top of each other, appearing as one aggregate. Furthermore, TEM cannot distinguish between complex filler shapes and aggregates on the nanometer scale. In this regard, 3D models developed by the transmission electron microtomography (3D-TEM) are promising11-13. In this method, the specimen is tilted with respect to the electron beam and a series of 2D TEM images of the nanomatrix structure are collected at various angles in regular intervals. These images are then aligned and reconstructed to obtain the 3D structure11. In the present work, we focus on relating the mechanical and viscoelastic properties with the distribution of the ND nanomatrix structure. In order to obtain a fine distribution of ND particles inside the nanomatrix a chemical reaction with an organic redox initiator (TBHPO/TEPA) was used in latex stage at 30°C. The Effect of phospholipids on the reaction is negligibly small14 hence to eliminate side reactions with proteins1, 15, deproteinized natural rubber (DPNR) was used for the study. To relate the morphology with the mechanical properties, two ND concentrations were selected. The one at 25 w/w% is in the threshold concentration region of the nanomatrix formation, whereas the one at 15w/w% lies below the threshold concentration8. The morphology of ND nanomatrix formed in natural rubber (NR-ND), is observed through 2D and 3D TEM. EXPERIMENTAL SECTION


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Materials Deproteinization of natural rubber was done by incubating high ammonia natural rubber latex at a DRC of 30 w/w% (HANR, Golden Hope Malaysia, DRC- 62.6 w/w%.) dispersed in 0.1 w/w% sodium dodecyl sulfate (Kishida Chemical, Japan, 99 %) with 0.1 w/w% CO(NH2)2 (Nacalai Tesque, Inc., 99 %) at room temperature for 1 hour. Then the resultant deproteinized latex was centrifuged at 10,000 g for 30 minutes. Cream fraction was dispersed in 0.5 w/w% sodium dodecyl sulfate and washed twice by centrifugation and re-dispersion in alternate. The DPNR latex (DRC- 30 w/w%) used for the study was prepared by dispersing the washed cream fraction in 0.1 w/w% sodium dodecyl sulfate . ND slurry was prepared by stirring ND solid (particle size 5-10 nm, Sumiseki Materials, Japan) with 18 g of ammonia solution (Nacalai Tesque, Inc., 28%). The slurry was homogenized using a Cho-onpa Kogyo Usonic U-D6354 ultrasonic generator at 21 kHz (10 W) for 60 minutes and mixed with DPNR latex (ND concentration: 25w/w%, 15 w/w% to dry rubber) under N2. DPNR and ND were reacted for 4 hours in the presence of tert-butylhydroperoxide (Kishida Chemical, Japan, 68 %) and tetraethylenepentamine (Kishida Chemical, Japan, 95 %) initiator under a N2 atmosphere at 30 °C [The initiator concentration was 6.6 × 10-5 mol g-1 (dry rubber) unless otherwise specified]. The film specimens of NR-ND (~ 1 mm thick) were prepared by casting the reacted latex in to glass Petri dish and subsequently dried at 50 °C. Samples without the nanomatrix structure were prepared without the initiator in a similar procedure without an initiator. Morphology


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2D and 3D TEM was used to observe the morphology of NR-ND. Ultra-thin NR-ND specimen for TEM imaging was prepared with Richert-Nissei FC-S Ultracut microtome at -90 ºC and placed on Cu grids. The TEM images were observed by JEOL JEM-2100 at an accelerating voltage of 200 kV. For each specimen, 3D observations were acquired using ultrathin sections with tilt angles from -60º to +60º with in 1º increments. The 3D TEM image in Figure 5(c) was obtained at a magnification of 15,000×, whereas the others were obtained at a 10000× magnification. The alignment and the reconstruction were performed according to the protocol described in the ref 16. Mechanical Properties The tensile properties were measured with a Toyo Seiki Seisaku-sho Universal Testing Machine at a cross-head speed of 200 mm/min at room temperature according to JIS K6251-7 using a Dumbbell-7 shaped test pieces. The dynamic mechanical properties were measured using an Anton Paar Physica MCR 301 with (12-mm diameter) parallel plate geometry. The angular frequency range was from 0.1 - 100 rad s-1. The measurements were obtained between -70 °C and 130 °C. For all measurements strain amplitude was in the linear viscoelasticity region. The measurements for rubbery plateau were obtained at 25°C. RESULTS AND DISCUSSION 1. Morphology of NR-ND TEM images of NR-ND prepared with and without a TBHPO/TEPA initiator are shown in Figure 1. The dark and light domains represent ND and natural rubber particles, respectively17.


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Figure 1(a) clearly shows that the natural rubber particles are constrained in the ND matrix with a nanometer thickness. On the other hand, the nanomatrix is not visible without using an initiator. Instead, the ND particles are randomly arranged with the natural rubber forming an island matrix structure. Furthermore, no significant amount of ND aggregation is visible when an initiator is used (Figure 1a). Figure 1(c) shows the TEM image of NR-ND at a 15 w/w% ND concentration prepared with the initiator. Although the nanomatrix structure is formed, some spaces on the rubber particles do not contain ND particles. Both ND concentrations have nanomatrices with similar thicknesses.

Figure 1. TEM images of NR-ND prepared (a) with and (b) without initiator at a 25 w/w% ND concentration, and (c) at a 15 w/w% ND concentration with initiator. Figure 2 shows TEM images of two NR-NDs with 25 w/w% ND prepared with6.6 × 10-5mol g-1 and 3.3 × 10-4 mol g-1 TBHPO/TEPA initiators, respectively. Both TEM images show the nanomatrix structure, in which natural rubber particles were dispersed in the ND-nanomatrix. The nanomatrix structure was independent of the initiator concentration. This may be explained due to the same nanodiamond concentration in spite of the difference in the initiator


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concentration. Hereafter, we used NR-ND prepared with 6.6 × 10-5mol g-1 TBHPO/TEPA initiator.

Figure 2. TEM images of NR-ND at 25 w/w% ND concentration prepared with (a) 6.6 × 10-5mol g-1, (b) 3.3 × 10-4mol g-1 TBHPO/TEPA initiator.

Figure 3. TEM images of NR-ND nanomatrix at a 25 w/w% ND concentration with increasing magnification. Figure 3 shows TEM images of the ND nanomatrix structure at higher magnifications. Figures 3 (b & c) show that the ND particles with a few-nm diameters are closely packed forming nanomatrix structures that are about 40-nm thick. However, a small amount of ND particles is


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visible as they come inside the rubber particle few nm away from the nanomatrix (Figure 3c). Because a heat treatment is not used, it is unclear whether ND penetrates into the natural rubber particles. Thus, the exact arrangement of the particles in the nanomatrix cannot be accurately determined via normal TEM imaging, even under a higher magnification. To elucidate the 3D structures of NR-ND, 3D-TEM techniques were employed11.

Figure 4. 3D-TEM images of NR-ND prepared (a) with and (b) without initiator at a 25 w/w% ND concentration. Figure 4 depicts the 3D structures of NR-ND prepared with and without a TBHPO/TEPA initiator at a 25 w/w% ND concentration, where the white particles represent the ND and the black voids (inside the box) are natural rubber particles respectively. These images confirm that the nanomatrix structure is formed when a TBHPO/TEPA initiator is used. However, without an initiator, the ND particles are randomly distributed and form an island matrix structure. During the latex stage reaction in the presence of an initiator, chemical bonds between the ND particles and the natural rubber should be formed. These chemical bonds prevent re-aggregation of ND


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particle hence formation of larger aggregates. Thus after drying, the nanomatrix formed will consist of much smaller and fine ND particles leading to a uniform nanoparticle distribution in the matrix. On the other hand, without an initiator, the ND particles do not strongly interact with the rubber particles. Consequently, the particles are randomly distributed inside the latex. During the drying process, an island matrix structure is formed due to the strong filler–filler attraction. In other words, the ND tends to re-aggregate6, 18. Figures 5 (a & c) show the nanomatrix structure over a wider area. The ND particles are arranged in the nanomatrix. To visualize the morphology of the nanomatrix around a single natural rubber particle more clearly, 3D-TEM images (Figure 5b) are given in relation to their respective 2DTEM images (Figure 5a). The surfaces of the rubber particles are closely stacked and the interface between the natural rubber particles is completely covered by a thin layer of ND particles, similar to Figure 5(a). The tomography images indicate that the nanoparticles, which are appeared to come inside the natural rubber particles in the 2D-TEM image, do not actually exist inside the rubber particles. The ND nanoparticles exist outside in the nanomatrix. This may be explained due to the nature of the 2D image which is obtained by transmission of electron beam through the film.


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Figure 5. (a) TEM image and (b & c) 3D-TEM images of the NR-ND nanomatrix structure at a 25 w/w% ND concentration. Figure 6, which shows the 2D and 3D TEM images at high magnifications, depicts the arrangement of the ND particles in the nanomatrix. The ND nanomatrix is about 40 nm thick. Tomography image (Figure 6d) and 2-D TEM image (Figure 6b) reveal that ND Particles less than 10 nm in size are closely arranged and form the nanomatrix. Thus, the nanomatrix may consist of the ND particles and bound rubber19, 20 to form filler network, proposed by Isono and co-workers (2013) as bridged filler netork 21and Wang and co-workers (1998, 2000) as the ruber shell around the filler particles

22, 23

. This is assured by formation of chemical linkages between

the ND particles and natural rubber due to the reaction with the TBHPO/TEPA initiator.


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Figure 6. Morphology of the nanomatrix observed by TEM (a) at 60,000× magnification, (b) 100,000× magnification and (b & c) 3D TEM with a high magnification.

2. Tensile properties The formation of a nanomatrix structure strongly contributes to mechanical properties of NRND. Our previous studies at 30 and 40 w/w% ND concentration showed a storage modulus that was ten times higher than DPNR. Furthermore, the mechanical and viscoelastic properties for NR-ND dramatically increased above the threshold ND concentration, which was 20–30 w/w%8.


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This study employs two ND concentrations: one in the threshold concentration region (25 w/w%) and one below the critical concentration (15 w/w %). Figure 7 shows the stress–strain curves for NR-NDs prepared with and without the initiator as well as the control using neat DPNR. Tensile properties of DPNR agrees with the literature reported values24, 25. The stress at break for NR-ND prepared without an initiator at a 25 w/w% ND concentration is 6.9 MPa at a strain of 4.9, whereas the stress at break is 3 MPa at 15 w/w%, which is a similar value to DPNR. For both ND concentrations, using an initiator in the preparation, the stress at break becomes exceptionally higher than not using an initiator. This confirms that the formation of a chemical linkage between the ND and natural rubber increases the mechanical strength. For NR-ND at 25 w/w%, the stress at break is 15.5 MPa, which is markedly higher than that of the 15 w/w% ND concentration (6.5 MPa). At 25 w/w% nanodiamond concentration, stress-strain curves for NR-ND with 3.3 × 10-4 mol g-1 and 6.6 ×10-5 mol g-1 TBHPO/TEPA were similar to each other. In fact, to form a nanomatrix structure, which improve the mechanical properties, the nanomatrix should contain a minimum ND concentration. The TEM images of NR-ND at a 15 w/w % ND concentration in Figure 1 support this assertion. These images show that the nanomatrix structure is formed, but some spaces on the rubber particles do not contain ND particles due to an insufficient ND concentration or space defects8.


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Figure 7. Stress–stain curves for NR-ND prepared with (a) 6.6×10-5 mol g-1, (b) with 3.3×10-4 mol g-1 TBHPO/TEPA initiator, (c) without an initiator at a 25 w/w% ND concentration, at a 15 w/w% ND concentration prepared (d) with and (e) without initiator, and (f) DPNR. 3. Viscoelastic properties Figure 8 shows the storage modulus (G′), loss modulus (G″), and loss tangent (tan δ) of NR-ND prepared with and without a TBHPO/TEPA initiator at 25 w/w% and 15 w/w% as well as neat DPNR in the rubbery plateau region. Neat DPNR shows a storage modulus on the order of 105 Pa, which is similar to the value reported in the literature. For a 15 w/w% ND concentration (Figure 8a), the increase in G′ is only about 1.7-times higher than that of DPNR, even with an initiator. This is similar to the island matrix structure, and is attributed to insufficient ND to form the nanomatrix structure (space defects). At a 25 w/w% ND concentration, G′ increases about 3times higher than that of DPNR without an initiator. On the other hand, using a 6.6 × 10-5 mol g-1 initiator concentration increases G′ to more than 8 times that of DPNR. Interestingly, the value increases to about 38 times higher than that of DPNR when the initiator concentration increases to 3.3 ×10-4 mol g-1. This increase in the plateau modulus is an advantage over the conventional


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island matrix structure. The variation of the loss modules of the NR-NDs is consistent with the G′ values. In Figure 8(b) the G″ values of the NR-NDs prepared with the TBHPO/TEPA initiator is higher than those prepared without an initiator. At the 25 w/w % ND concentration the G″ value increases as the TBHPO/TEPA initiator concentration increases. The increase in G″ may be explained to be due to rupture and reformation of filler network23. In particular, the bound rubber of the filler network may contribute to the increase in G”, since it is deformable against force. This is consistent with the increase in the G″ values with increasing initiator concentration, since the bound rubber content is dependent on the initiator concentration. The tan δ values for DPNR vary linearly with the frequency (Figure 8c). At higher frequencies, the tan δ values for NR-ND at a 25 w/w% ND concentration are independent of the frequency with the formation of the nanomatrix structure. This has been previously observed in the NR-graft-polystyrene and NR-silica nanomatrix structure10, 26. However at a 15 w/w% ND concentration, this variation in tan δ is not prominent. This may be due to the insufficient ND concentration (space defects) in formation of the nanomatrix structure.


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Figure 8. Logarithmic (a) storage modulus, (b) loss modulus, and (c) the loss tangent of NR-ND prepared (●) with 3.3×10-4 mol g-1, (▲) with 6.6×10-5 mol g-1 TBHPO/TEPA initiator, (♦) without an initiator at a 25 w/w% ND concentration, (∆) with 6.6×10-5 mol g-1 TBHPO/TEPA initiator () without an initiator at a 15 w/w% ND concentration, and () DPNR in the rubbery plateau region. The horizontal shift factor (aT) and vertical shift factor (bT) used for the construction of G′, G″, and tan δ master curves through the time–temperature superposition principle are evidence of the formation of the nanomatrix structure. Figure 9 shows the temperature dependences of the horizontal and vertical shift factors of NR-ND. The respective master curves are given in the supporting information (Figure S2 - S15). Horizontal shift factor aT is expressed by the Williams-Landel-Ferry (WLF) equation (eq1).

1

Where C1 and C2 are experimentally found constants, T is the measured temperature, and Tr is the reference temperature (Figure S1). Thus aT depends only on the reference temperature27. In conventional filler structures, the log aT value depends on the segmental mobility or the


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relaxation of the polymer matrix and does not change by introducing the fillers21, 28, 29. As seen in figure 9(a), log aT values of NR-ND at low temperature agrees well with the WLF equation27, 30. However, at higher temperatures, except for NR-ND prepared at 15w/w% without an initiator, the samples deviate from the WLF plots. The deviation is influenced by the ND concentration and the use of an initiator. At low temperatures the segmental mobility is minimally affected by the fillers29. Hence the nanomatrix or the nanoparticles do not affect the polymer relaxation at low temperatures, but at higher temperatures, polymer relaxation is affected by the nanoparticles and the interaction between the rubber particles and the formed nanomatrix28, 29. The rubber elasticity is known to be entropic elasticity. According to Isono and co-workers (2013) the energetic elasticity arises from the rupture of the filler network at the large shear21. A slope of the log bT Vs. T is positive for entropic elasticity, whereas it is negative for energetic elasticity. Hence the slope of the log bT Vs. T may reflect the effect of the nanomatrix structure and the interaction between ND and NR particles. In Figure 9(b), neat DPNR showed a positive slope, suggesting the entropic elasticity. The slope for NR-NDs at 25 w/w% ND concentration prepared with TBHPO/TEPA was negative, suggesting the energetic elasticity due to strong filler–filler and filler–polymer interactions31. Despite the fact that the nanomatrix structure is absent in NRND at 25 w/w% ND concentration prepared without the initiator, the slope was slightly negative. This may be because ND concentration is sufficiently high to bring the ND aggregates closer giving rise to a strong filler–filler interaction5,

6, 32

.The slope for NR-ND at 15 w/w% ND

concentration prepared without an initiator was positive and coincides with DPNR. It became negative when an initiator was used for NR-ND at the same ND concentration. This change may be explained to be due to the formation of chemical linkages between ND and NR particles, which makes elasticity change to energetic from entropic. The distribution of the ND particles


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and the formation of chemical linkages were found to play a vital role in improving the mechanical and viscoelastic properties of NR-NDs.

Figure 9. Logarithmic (a) horizontal shift factor and (b) vertical shift factor of NR-ND (▲) with and, (♦) without the nanomatrix structure at a 25 w/w% ND concentration, (∆) with and () without nanomatrix structure at a 15 w/w% ND concentration, () DPNR, and (---) WLF curve. The reference temperature (Tr) is -62.5°C. CONCLUSION The ND nanomatrix structure has closely packed ND agglutinates of about 10 nm. Although both 25 w/w% and 15 w/w% do not exhibit a considerable difference in the nanomatrix thickness,


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15w/w% displays a space defect due to insufficient ND content. The formation of the ND nanomatrix structure will greatly contribute to enhancing the mechanical and viscoelastic properties. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website

Log (G″) Vs. temperature of NR-NDs measured at 10 rad s-1, plots of log (tan δ), log (G′), log (G″) Vs. log ω of NR-NDs measured at different temperatures and corresponding master curves prepared by time – temperature superposition principle (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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This work was supported in part by a Grant-in-Aid (16H02291) for Scientific Research (A) from the Japan Society for the Promotion of Science and JST-JICA SATREPS. ABBREVIATIONS ND, nanodiamond, NR, natural rubber, NR-ND, ND nanomatrix structure

formed into

deproteinized natural rubber, TBHPO, tert-butylhydroperoxide, TEPA, tetraethylenepentamine, DRC, dry rubber content, TEM, transmission electron microscopy, G′, storage modulus, G″, loss modulus, tan δ, loss tangent, WLF, Williams-Landel-Ferry equation, aT, horizontal shift factor, bT, Vertical shift factor, T, temperature, Tr, reference temperature

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TABLE OF CONTENTS GRAPHIC


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Figure 1. TEM images of NR-ND prepared (a) with and (b) without initiator at a 25 w/w% ND concentration, and (c) at a 15 w/w% ND concentration with initiator. 51x16mm (300 x 300 DPI)


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Figure 2. TEM images of NR-ND at 25 w/w% ND concentration prepared with (a) 6.6 × 10-5 mol g-1, (b) 3.3 × 10-4 mol g-1 TBHPO/TEPA initiator. 57x26mm (300 x 300 DPI)


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Figure 3. TEM images of DPNR-ND nanomatrix at a 25 w/w% ND concentration with increasing magnification. 47x14mm (300 x 300 DPI)


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Figure 4. 3D-TEM images of NR-ND prepared (a) with and (b) without initiator at a 25 w/w% ND concentration. 86x47mm (300 x 300 DPI)


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Figure 5. (a) TEM image and (b & c) 3D-TEM images of the NR-ND nanomatrix structure at a 25 w/w% ND concentration. 67x28mm (300 x 300 DPI)


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Figure 6. Morphology of the nanomatrix observed by TEM (a) at 60,000× magnification, (b) 100,000× magnification and (b & c) 3D-TEM with a high magnification. 127x127mm (300 x 300 DPI)


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Figure 7. Stress–stain curves for NR-ND prepared with (a) 6.6×10-5 mol g-1, (b) with 3.3×10-4 mol g-1 TBHPO/TEPA initiator, (c) without an initiator at a 25 w/w% ND concentration, at a 15 w/w% ND concentration prepared (d) with and (e) without initiator, and (f) DPNR. 66x52mm (300 x 300 DPI)


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Figure 8. Logarithmic (a) storage modulus, (b) loss modulus, and (c) the loss tangent of NR-ND prepared (●) with 3.3×10-4 mol g-1, (▲) with 6.6×10-5 mol g-1 TBHPO/TEPA initiator, (♦) without an initiator at a 25 w/w% ND concentration, (∆) with 6.6×10-5 mol g-1 TBHPO/TEPA initiator (↓) without an initiator at a 15 w/w% ND concentration, and (■) DPNR in the rubbery plateau region 56x19mm (300 x 300 DPI)


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Figure 9. Logarithmic (a) horizontal shift factor and (b) vertical shift factor of NR-ND (▲) with and, (♦) without the nanomatrix structure at a 25 w/w% ND concentration, (∆) with and (↓) without nanomatrix structure at a 15 w/w% ND concentration, (■) DPNR, and (---) WLF curve. The reference temperature (Tr) is -62.5°C. 131x213mm (300 x 300 DPI)


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45x27mm (600 x 600 DPI)


Distribution of Nanodiamond inside the ... - ACS Publications

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