Thermal Degradation Kinetics and Morphology of

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by incorporating latex compounding with self-assembly techniques. The SiO2 nanoparticles are ... jiang, P. R. China) and was pre-vulcanized. Fumed sil-.

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Journal of Nanoscience and Nanotechnology Vol. 6, 1–6, 2006 Vol. 6, 541–546,

Thermal Degradation Kinetics and Morphology of Natural Rubber/Silica Nanocomposites Si-Dong Li,1 2 Zheng Peng,2 3 ∗ Ling Xue Kong,3 and Jie-Ping Zhong1 1

College of Science, Zhanjiang Ocean University, Zhanjiang 524088, P. R. China Agriculture Ministry Key Laboratory of Natural Rubber Processing, South China Tropical Agricultural Product Processing Research Institute, P. O. Box 318, Zhanjiang 524001, P. R. China 3 Centre for Advanced Manufacturing Research, University of South Australia, Mawson Lakes, SA 5095, Australia 2

A novel natural rubber/silica (NR/SiO2 ) nanocomposite with a SiO2 loading of 4 wt% is developed by incorporating latex compounding with self-assembly techniques. The SiO2 nanoparticles are homogenously distributed throughout the NR matrix as spherical nano-clusters with an average size of 75 nm. In comparison with the host NR, the thermal resistance of the nanocomposite is significantly improved. The degradation temperatures (T ), reaction activation energy (E), and reaction order (n) of the nanocomposite are markedly higher than those of the pure NR, due to significant retardant effect of the SiO2 nanoparticles.

Keywords: Nanocomposite, Natural Rubber, Silica, Self-Assembly, Thermal Degradation Kinetics, Morphology.

Natural rubber (NR) has been widely used to manufacture medical products, due to its excellent properties such as good anti virus permeation, excellent human body compatibility, and good formability.1 2 However, as an unsaturated polymer, the NR will degrade gradually and then generate some undesired substances at a high temperature or when exposed to oxygen. This has a significant negative effect on the potential for medical applications, where strict safety and hygiene are required. Although some thermal stabilizers have been successfully developed to improve the thermal stability of NR for various applications,3 4 no effective thermal stabilizers have been identified for medical products so far. Therefore, it is crucial to find new methods to enhance the thermal stability of NRL medical products. Thermal properties of polymer materials can be improved through the introduction of inorganic nanoparticles into the host matrix. Uniformly distributing inorganic nanoparticles into polymer matrices without aggregation is one of the most important criteria in preparing polymeric/inorganic nanocomposites (PINs)5 as only evenly distributed nanoparticles will improve the thermal and mechanical properties of the materials. Decher6 introduced an alternative approach—fabrication of multilayers ∗

Author to whom correspondence should be addressed.

J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 2

by consecutive adsorption of polyanions and polycations— to the Langmuir-Blodgett technique and chemisorption from solution which is far more general and has been extended to other materials such as proteins or colloids. Layer-by-layer assembly by adsorption from solution is a general approach for the fabrication of multicomponent films on solid supports. Materials can be selected from a pool of small organic molecules, polymers, natural proteins, inorganic clusters, clay particles, and colloids. In addition to the buildup of multilayers on macroscopically flat substrates, the production of core–shell materials of given size, topology, and composition can also be achieved.7 In a recent work,8–10 we also developed a state of the art process that incorporates the solution compounding and self-assembly techniques to prepare a bulk polyvinyl alcohol/silica (PVA/SiO2 ) nanocomposites. It is observed that the thermal stability of nanocomposite is improved significantly due to the strong interactions between SiO2 nanoparticles and PVA molecular chains. The thermal degradation temperatures at maximum weight loss rate of side and main chains of nanocomposite increase 11.2  C and 15.6  C over those of the PVA, respectively. The enhancement in the thermal resistance of polymers after the introduction of inorganic particles is largely due to the barrier effect. As observed by Gilman et al.11 and Vyazovkin et al.,12 a polymeric-inorganic char builds up on the surface of the polymer and provides the mass and heat transfer barrier which improves the thermal resistance.

6/541/006 1533-4880/2006/6/001/006

doi:10.1166/jnn.2006.114

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1. INTRODUCTION

Si-Dong Li et al.

Thermal Degradation Kinetics and Morphology of Natural Rubber/Silica Nanocomposites

However, the mechanism of such an effect, in particular the kinetic aspect of the thermal degradation, has not yet been well understood. The present work will therefore introduce a novel selfassembly synthetic method by evenly dispersing SiO2 nanoparticles into the NR matix in an attempt to significantly improve the thermal stability of the NR materials. The morphology of the nanocomposite and structure of the SiO2 nano-clusters in NR matrix are analysed. To fundamentally understand the degradation mechanism and investigate the effect of SiO2 on the degradation of the resulting nanocomposite, we also study the thermal degradation kinetics with the Coats-Redfern model.13

2. EXPERIMENTAL DETAILS 2.1. Materials

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Natural rubber latex (NRL), with a total solid content of 62%, was provided by Shenli Rubber Plantation (Zhanjiang, P. R. China) and was pre-vulcanized. Fumed silica nanoparticles (average diameter: 14 nm; surface area: 200 m2 /g ± 25 m2 /g) and poly (diallyldimethylammonium chloride) (PDDA) (mol wt ca. 100000 ∼ 200000; 20 wt% in water) were purchased from Sigma-Aldrich (SigmaAldrich, Louis, MO). All experimental materials were used as received. 2.2. Preparation of NR/SiO2 Nanocomposite The process of self-assembling NR/SiO2 nanocomposites involves two assembly steps (Fig. 1). At the first stage, SiO2 particles are negatively charged at a pH of 10,14 and act as templates to assembly the PDDA molecular chains that are positively charged at the same pH value of 10.14 The driving force is the electrostatic adsorptive interaction. At the second step, the NRL particles are negatively charged. As the protein particles adsorbed on the surface of the NRL particles contain carboxyl and amino functional groups, acidic ionization of the proteins can be generated

to obtain negative charges. These NRL particles with negative charges are then assembled onto the surface of the SiO2 particles that are covered with PDDA as treated in the first stage. Finally, the SiO2 nanoparticles are uniformly dispersed in NR matrix to form nanocomposite which is then dried in a vacuum oven at 50  C for 1 week to obtain NR/SiO2 films, which contain 4 wt% SiO2 . The self-assembly in this work is based on the electrostatic attractive interaction of oppositely charged polyelectrolytes. The electrostatic adsorptive interactions between NR latex particles, PDDA molecular chains, and silica nanoparticles are used as driving forces to ensure an effective inter-assembly at latex state. Different from other conventional methods such as sol–gel, intercalation and blending process, this novel process combines the selfassembly and latex compounding technique and offers a new approach to develop bulk NR/SiO2 nanocomposites. The main advantage of the approach is the constraint of free movement of nano SiO2 particles in MR matrix during the synthesis which reduces the probability of particle aggregation. 2.3. Characterization Scanning electron microscopy (SEM) micrographs were taken with a Philips XL30-EDAX instrument (Philips, Eindhoven, Netherlands). The fracture surface was obtained by splitting bulk samples being quenched in liquid nitrogen. A sputter coater was used to pre-coat conductive gold onto the fracture surface before observing the microstructure. The accelerating voltage of the observation is 18 KV. Thin films for transmission electron microscopy (TEM) were prepared by cutting bulk samples being quenched in liquid nitrogen. TEM observation was done on a JEM100CXII instrument (JEOL, Peabody, MA) with an accelerating voltage of 100 KV. A Perkin Elmer TGA-7 thermogravimetric analyser (TGA) (Perkin-Elmer, Fremont, CA) was used for the thermal degradation measurement. In nitrogen, the measurement of the films (ca. 10 mg) was carried out from 100  C to 600  C at the heating rate of 5  C/min, 10  C/min, 15  C/min, 20  C/min, and 25  C/min, respectively. The flowing rate of the carrying gas is 80 ml/min.

3. RESULTS AND DISCUSSION 3.1. Morphology of NR/SiO2 Nanocomposite

Fig. 1.

2 542

The schematic of the self-assembly process.

Due to the strong hydrogen bonding interaction and high surface free energy, the fumed SiO2 nanoparticles usually exhibit a strong tendency to form large aggregates,15 which lead to separation in the composites. Some micro disfigurements such as micro voids and micro fissures at the interface of two phases can be clearly seen under the SEM, J. Nanosci. Nanotechnol. 6, 541–546, J. Nanosci. Nanotechnol. 6, 1–6, 2006

Si-Dong Li et al.

Thermal Degradation Kinetics and Morphology of Natural Rubber/Silica Nanocomposites

200 nm Fig. 4.

Fig. 2. SEM micrograph of NR/SiO2 nanocomposite.

nano-clusters in the first step assembly. Therefore, the SiO2 is not distributed in NR matrix as individual nanoparticles, but as nano-clusters. Similar assumption was found in polyethylene oxide (PEO)/silica nanocomposites. Gunb’ko et al.16 reported that each PEO molecules could interact with many primary fumed SiO2 particles to form small aggregates. We also observed this with the PVA/SiO2 nano composites where primary particle aggregation was found at a lower level of SiO2 particle content while both primary and secondary aggregations were observed at a SiO2 content higher than 5 wt%.8 SiO2 clusters appear to be in a spherical structure as observed in SEM micrograph of higher magnification (Fig. 4), where the SiO2 clusters mainly present as hemispherical spheres while another half is encapsulated by the NR. Some primary SiO2 nanoparticles can even be seen in the clusters, though there is little significant difference in the phase contrast. Another illustrative evidence is given by the TEM micrograph, where the spherical nanoclusters with a diameter of around 50 ∼ 110 nm (presented as dark circle pies) are uniformly distributed in NR matrix (Fig. 5).

Percentage (%)

40 30 20 10 0

40

50

60

70

80

90

100

110

200 nm

Diameter (nm) Fig. 3. Size distribution of the SiO2 clusters in NR matrixes.

J. Nanosci. Nanotechnol. 6, 541–546, J. Nanosci. Nanotechnol. 6, 1–6, 2006 2006

Fig. 5.

TEM micrograph of the SiO2 nano-clusters in NR matrix.

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even though the phase separation cannot be observed from its macrostructure. However, there are no any defects of microstructure in the SEM micrograph of NR/SiO2 nanocomposite (Fig. 2). The good miscibility between SiO2 and NR in this work may be resulted from the self-assembling treatment of SiO2 nanoparticles. After two steps of the assembly, the SiO2 particles are completely covered with PDDA and NR layers. The strong particle–particle interaction of SiO2 particles which leads to particle aggregation is therefore prevented. Thus, heavy aggregation of SiO2 that may cause phase separation is suppressed and is not observed. The dark phase in the SEM micrograph represents the NR matrix and the bright phase corresponds to the SiO2 particles, which are strongly embedded by NR matrix. From the statistic information of nano-clusters’ size distribution (Fig. 3), the SiO2 clusters are dispersed in NR matrix with a very narrow normal distribution. This suggests that the dispersion of SiO2 should be very homogenous. The size of nano-clusters ranges from 40 to 110 nm, among which 70 ∼ 80 nm accounts for the maximum percentage (29.8%). Primary agglomerates are generated as the average size of original nanoparticles is 14 nm. These primary aggregations are unlikely caused by the strong particle–particle interaction, but the adsorption between polymer molecular chain and SiO2 nanoparticles, i.e., PDDA molecular chains adsorb quite a few separate nanoparticles to form

SEM micrograph of the SiO2 nano-clusters in NR matrix.

Si-Dong Li et al.

Thermal Degradation Kinetics and Morphology of Natural Rubber/Silica Nanocomposites

Weight % (%)

90 NR (TG)

70

–30

NR /SiO2 (TG) 50

NR (DTG) NR /SiO2 (DTG)

30

–10

10 –10 200

300

400

Derivative Weight % (%/ min)

–50

10 600

500

Temperature (˚C) Fig. 6. in N2 .

TG/DTG curves of the NR and NR/SiO2 nanocomposite

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3.2. Thermal Degradation of NR/SiO2 Nanocomposites Figure 6 presents thermogravimetric and derivative thermogravimetric (TG/DTG) curves of pure NR and NR/ SiO2 nanocomposite in nitrogen with a heating rate of 20  C /min. There is only one turn in the TG curves and one corresponding weight-loss peak in the DTG curves, which indicates that there is only one obvious decomposition step of NR molecular chains primarily initiated by thermal scissions of C–C chain bonds accompanying with a transfer of hydrogen at the site of scission. In comparison with the pure NR, the degradation curve of prepared nanocomposite markedly is shifted to a higher temperature. The initial degradation temperature (T0 ) and final degradation temperature (Tf ) are calculated with a bi-tangent method from TG curves, and the peak degradation temperature, Tp , the temperature at maximum degradation rate is obtained from the DTG curves. Cp and Cf are the degradation rates corresponding to Tp and Tf , respectively. Various degradation temperatures of the NR/SiO2 nanocomposite are significantly higher than those of the pure NR (Table I). Specifically, the T0 , Tp , and Tf of the nanocomposite increase 17.9  C, 17.0  C, and 14.9  C, respectively, over the host NR, indicating that the thermal resistance of the nanocomposite has been greatly enhanced. Due to the retardant effect of SiO2 nanoparticles, the Cp of NR/SiO2 nanocomposite is 5.0% lower than that of the pure NR, which implies that the thermal degradation is delayed. When the thermal degradation is finished, there is 4.9% of residual carbon remained for the pure NR, while there are 10.3% of residual carbon and SiO2 remained for the nanocomposite.

3.3. Thermal Degradation Kinetics of the NR/SiO2 Nanocomposites The kinetic analysis is very important for polymers as it can provide information on the energy barriers of the process and offer clues to understand degradation mechanism. The challenge for studying thermal degradation kinetics is to find a reliable approach. In the two major thermal degradation models in use, the shortcomings of the singleheating-rate method have been reported by Vyazovkin,17 while the multi-heating-rate method has been extensively used to study the thermal degradation kinetics for polymers, due to its reliability.18 Figure 7 shows the TG curves of thermal degradation for both NR and NR/SiO2 nanocomposite at five heating rates. Because of the influence of heat hysteresis, the TG curves shift toward higher temperatures with an increasing heating rate. Using the multi-heating-rate method, the data in Figure 7 can be analysed with the Coats-Redfern model,13 whose significant advantage is that it can offer the average reaction order and reaction energy for any main degradation step. Our previously work19 20 has illustrated that the Coats-Redfern is very suitable for analyzing the thermal degradation of NR-based materials. By integrating the reaction kinetic equation d/dt = k1 − n

(1)

110 NR 90 70 50 30

Weight % (%)

110

10 –10 110 NR/SiO2

90 70 50

5 ˚C/min 10 ˚C/min

30

15 ˚C/min 20 ˚C/min

Table I. Thermal degradation temperatures and rates of NR and NR/SiO2 nanocomposite.

NR NR/SiO2

4 544

T0 ( C)

Tp ( C)

Tf ( C)

Cp (%)

Cf (%)

382.6 400.5

407.2 424.2

461.2 476.1

50.6 44.6

4.9 10.3

10 –10 250

25 ˚C/min

350

450

550

Temperature (˚C) Fig. 7. TG curves of NR and NR/SiO2 nanocomposite at different heating rates.

J. Nanotechnol. 6, 541–546, J.Nanosci. Nanosci. Nanotechnol. 6, 1–6, 2006 2006

Si-Dong Li et al.

Thermal Degradation Kinetics and Morphology of Natural Rubber/Silica Nanocomposites

and Arrhenius equation k = Ae−E/RT

(2)

the following equations can be obtained:   1 − 2RT /EAR 1 − 1 − 1−n ln = ln T 2 1 − n E E n = 1 − RT and

    1 −  1 − 2RT /EAR ln − ln = ln T2 E E − n = 1 RT

(3)

(4)

Table II. Various thermal degradation kinetics parameters of the NR and NR/SiO2 nanocomposite. NR B ( C/min) 5 10 15 20 25

NR/SiO2 nanocomposite

n

E (kJ/mol)

A (×10−14 )

n

E (kJ/mol)

A ×10−22 

2.3 2.2 2.3 2.4 2.3

203.2 215.6 225.6 232.8 238.2

0 03 0 33 2 6 21 5

3.2 3.1 3.3 3.2 3.2

286.2 302.3 320.4 333.2 350.3

0 003 0 077 1 00 8 16 18 7

J. Nanotechnol. 6, 541–546, J.Nanosci. Nanosci. Nanotechnol. 6, 1–6, 2006 2006

4. CONCLUSIONS The process combining latex compounding and selfassembly techniques has successfully been used to prepare a natural rubber latex/silica nanocomposite, in which the SiO2 nanoparticles are uniformly distributed as spherical nano-clusters with an average diameter of 75 nm and a size distribution from 40 to 110 nm. The thermal degradation process of the NR/SiO2 nanocomposite is similar to that of the pure NR, and only involves one degradation step. In comparison to the host NR, the thermal stability of the nanocomposite is significantly improved. The initial, peak and final degradation temperatures of the nanocomposite increase 17.9  C, 17.0  C, and 14.9  C, respectively, over the host NR. At a given degradation temperature, the degradation rate and

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where n is the reaction order,  is the reaction degree, T is the absolute temperature,  is the heating rate, E is the reaction activation energy, R is the gas constant, and A is the frequency factor. When n = 1, a line can be obtained from the plot of ln1 − 1 − 1−n /T 2 1 − n versus 1/T , of which the slope is −E/R, and the intercept is ln1 − 2RT /EAR/E. When n = 1, a line can be obtained from the plot of ln− ln1 − a/T 2  versus 1/T , of which the slope is −E/R, and the intercept is ln1 − 2RT /EAR/E. Adopting the least squares fitting method with different n, the n with the maximum correlated coefficient (r) is the apparent reaction order, and the corresponding E is the activation energy. Table II tabulates various kinetic parameters of thermal degradation for both NR and NR/SiO2 nanocomposite. The average value of n for NR/SiO2 nanocomposite is 3.2, 0.9 larger than that of the pure NR, suggesting the thermal degradation of the NR/SiO2 nanocomposite should be more complex than that of the NR. However, without further information, the role of the SiO2 in the thermal degradation of the nanocomposite is not clear. To reveal its thermal degradation mechanism and understand the impact of SiO2 on the enhancement of thermal resistance of host NR, further investigation on thermal degradation of NR/SiO2 nanocomposite using thermogravimetry/Fourier transform infrared spectroscopy (TG/FTIR), and pyrolysisgas chromatography/massspectroscopy (Py–GC/MS) is being conducted.

E increases with the . Using the linear regression leastsquare method, activation energy for the NR and NR/SiO2 nanocomposite can be described as E = 1 744 + 196 9 and E = 3 182 + 270 8, respectively. Eliminating the influence of heating rates, i.e.,  = 0  C/min, the apparent activation energy (E0 ) for the pure NR is 196.9 kJ/mol, and 270.8 kJ/mol for the nanocomposite. This confirms that the thermal stability of NR/SiO2 nanocomposite is better than that of the pure NR, as quite more energy is required during the thermal degradation. Similar to the activation energy, the frequency factor also increases with the temperature. Due to the retardant effect, the frequency factor, A, of the nanocomposite is significantly lower than that of the pure NR during the thermal degradation. The significantly improved thermal stability of the nanocomposite attributes to the introduction of SiO2 nanoparticles into the NR matrix, where the SiO2 and NR molecular chains are strongly interacted through various effects such as the branching effect, nucleation effect, size effect, and surface effect. Therefore, the diffusion of degradation products from the NR matrix to gas phase is slowed down. Consequently, the nanocomposite has a more complex degradation and better thermal stability than those of the pure NR. Furthermore, while being heated, the SiO2 nanoparticles migrate to the surface of composites to form a SiO2 -NR char because of their relatively low surface potential energy. This inorganic-polymeric char on the surface of the composites, therefore, acts as a heating barrier to protect the NR inside. This is similar to Gilman11 and Vyazovkin’s12 work where a clay/polymer char greatly enhances the thermal resistance of host polymers. The degradation kinetics obtained from Coats-Redfern model basically reveals the effects of silica on the thermal degradation of the nanocomposite. However, to get more rational results, the kinetics compensation should be taken into account.21 A less speculative data analysis method that was suggested by Serra, Nomen, and Sempere22 will be presented in our future work.

Thermal Degradation Kinetics and Morphology of Natural Rubber/Silica Nanocomposites

frequency factor of the nanocomposite are lower than those of the pure NR, due to the retardant effect of SiO2 . The reaction order of NR thermal degradation is 2.3, while it increases to 3.2 with the addition of SiO2 . The apparent activation energy for the nanocomposite is 270.8 kJ/mol, which is 73.9 kJ/mol higher than that of the pure NR. Acknowledgments: Mr. Peng would like to acknowledge the financial support from UniSA under the UPS scholarship scheme. We are also grateful to Ms. Chen Wang at Agriculture Ministry Key Laboratory of Natural Rubber Processing, P. R. China for doing the TGA measurement.

References and Notes 1. D. L. Walsh, M. R. Schwerin, R. W. Kisielewski, R. M. Kotz, M. P. Chaput, G. W. Varney, and T. M. To, J. Biomed. Mater. Res. B 68, 81 (2004). 2. M. Lundberg, K. Wrangsjo, K. ErikssonWidblom, and S. G. O. Johansson, Allergy 52, 1057 (1997). 3. C. Sirisinha, P. Saeoui, and J. Guaysomboon, Polymer 45, 4909 (2004). 4. P. B. Sulekha, R. Joseph, and K. E. George, Polym. Degrad. Stabil. 63, 225 (1999).

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5. L. X. Kong, Z. Peng, S. D. Li, and P. M. Bartold, Periodontol 2000 (2005). 6. G. Decher, Science 277, 1232 (1997). 7. F. Caruso, R. A. Caruso, and H. Mohwald, Science 282, 1111 (1998). 8. Z. Peng, L. X. Kong, and S. D. Li, Polymer 46, 1949 (2005). 9. Z. Peng, L. X. Kong, and S. D. Li, J. Appl. Polym. Sci. 96, 1436 (2005). 10. Z. Peng, L. X. Kong, and L. S. D, Synthetic Met. (2005), in press. 11. J. W. Gilman, C. L. Jackson, A. B. Morgan, R. Harris, E. Manias, E. P. Giannelis, M. Wuthenow, D. Hilton, and S. H. Phillips, Chem. Mater. 12, 1866 (2000). 12. S. Vyazovkin, I. Dranca, X. W. Fan, and R. Advincula, Macromol. Rapid Commun. 25, 495 (2004). 13. A. W. Coats and J. P. Redfern, Nature 201, 68 (1964). 14. Y. Lvov, K. Ariga, M. Onda, I. Ichinose, and T. Kunitake, Langmuir 13, 6195 (1997). 15. S. H. Kim, S. H. Ahn, and T. Hirai, Polymer 44, 5625 (2003). 16. V. M. Gun’ko, V. I. Zarko, R. Leboda, and E. Chibowski, Adv. Colloid Interf. 91, 1 (2001). 17. S. Vyazovkin and N. Sbirrazzuoli, Macromol. Chem. Phys. 200, 2294 (1999). 18. S. Vyazovkin, J. Comput. Chem. 22, 178 (2001). 19. J.-P. Zhong, S.-D. Li, H.-P. Yu, Y.-C. Wei, Z. Peng, J.-L. Qu, and C.-K. Guo, J. Appl. Polym. Sci. 81, 1305 (2001). 20. Z. Peng, S.-D. Li, M.-F. Huang, K. Xu, C. Wang, P.-W. Li, and X.-G. Chen, J. Appl. Polym. Sci. 85, 2952 (2002). 21. M. E. Brown, J. Therm. Anal. 49, 17 (1997). 22. T. Vlase, G. Vlase, and N. Doca, J. Them. Anal. Calorim. 80, 207 (2005).

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Received: 1 July 2005. Revised/Accepted: 12 November 2005.

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