Lu Wanga,
Linjia Hub,
Shangbing Gaoa,
Detao Zhaob,
Liqun Zhang*a and
Wencai Wang*b
aKey Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: zhanglq@mail.buct.edu.cn; Fax: +86-10-64433964; Tel: +86-10-64434860
bState Key Laboratory of Organic-Inorganic Composites, Beijing 100029, China. E-mail: wangw@mail.buct.edu.cn
First published on 18th December 2014
Polydopamine (PDA) is labeled as one category of synthetic melanin because it mimics the intriguing radical-scavenging behaviors of its natural counterpart. In this study, PDA modified montmorillonite (PDA-MMT) is utilized as a thermo-oxidative stabilizer for styrene butadiene rubber (SBR). PDA-MMT is fabricated by an aqueous dip-coating, based on the spontaneous alkaline auto-oxidative polymerization of dopamine hydrochloride in an air atmosphere, and is then associated with the SBR matrix via latex compounding. The PDA coating plays an excellent role as a radical-scavenger, and the uniformly dispersed PDA-MMT significantly functions as a physical barrier, which collaboratively work to reduce thermo-induced radical production during the decomposition process of SBR. This mechanism was attested by an in situ thermo-oxidative ageing test along with electron spin resonance (ESR) analysis. Thermal kinetics calculations showing that the apparent activation energy (Ea) of the SBR compounds is augmented by a large margin in the presence of PDA-MMT also corroborate this trend. Above all, the bio-inspired PDA-coating combined with the homogeneous dispersion of MMT exerts a synergistic effect on the thermo-oxidative stabilization of SBR matrix.
Polydopamine constitutes a fascinating class of bio-mimetic polymer inspired by marine mussel adhesion proteins (MAPs)10,11 and has been demonstrated in literature to possess a versatile adhesion to universal surfaces, owing to the strong hydrogen bonding from its abundant catechol groups.9–13
PDA has been the subject of tremendous research effort associated with surface modification14,15 and functionalization.16,17 For instance, our group has manipulated the electroless plating of a silver layer onto various substrates (containing polymeric nanoparticles,18 electrospun-fibers,19 and magnetic metal nanoparticles20) assisted by PDA pre-deposition, as well as by its metal-ion chelating ability.
PDA is also a popular modifier for PLS nanocomposites. Yang and Phua et al. introduced PDA-MMT into epoxy resin and polyether polyurethane, respectively, and thereby obtained the expectant promoted mechanical properties.9,21–23
It is worth noting that PDA is a somewhat melanin-like substance.10 Natural melanin existing in skin, hair, eyes, and other tissues has a photo-protective function.22,24,25 Melanin is able to dissipate absorbed UV radiation into harmless heat and to scavenge some detrimental species, including free radicals, reactive redox metal-ions, and oxidizing species, which can disrupt the normal metabolism in the body.25,26 Although the pathway by which such dissipation functions is still unknown,24 these protective mechanisms are mainly relevant due to their pronounced inhibition capability against harmful radicals.22,27 Since natural melanin is difficult to systematically characterize, due to its structural heterogeneity and chemical complexity,24–26 a synthetic analog originating from dopamine (3,4-dihydroxy-phenylethylamine)28 or L-DOPA (3,4-dihydroxy-phenylalanine)25,29 has so far been used instead as a common substitute in research. In contrast to natural melanin, the synthetic substitute exhibits a comparable or even superior radical-scavenging ability.25,28
It is widely recognized that the ageing mechanisms of polymeric materials seem to be relevant to either thermo-degradation or photolysis, in terms of molecular chains scission as well as cross-linking initiated by reactive free radicals. Therefore some researchers have employed melanin to serve as a novel anti-ageing additive for polymers. Shanmuganathan et al. applied natural melanin extracted from Sepia Officinalis Ink and synthetic melanin derived from L-DOPA to the PMMA matrix, and achieved a notably increased onset of the decomposition temperature of the composite.25
Furthermore, PDA-modified MMT can also be used to develop the anti-ageing ability of PLS nanocomposites. Phua et al. acquired a greatly enhanced UV-resistance of PP/clay composites with the addition of PDA-MMT.22 However, to the best of the authors' knowledge, the performance related to elastomeric materials has yet not been studied.
Toward this end, herein, we try to establish a structure–property–function relationship in SBR-based compounds, and to ascertain the thermo-oxidative stabilization mechanism of PDA-MMT by means of various characterizations. The fact that the PDA coating forms strong interfacial interactions and facilitates the uniform-dispersion of MMT is verified in terms of our optical observations (SEM and TEM). The excellent radical-scavenging ability of the PDA coating alone is characterized by a DPPH Assay (UV-vis). The eminent inhibition effect of thermo-induced carbon-centered radicals is then predicated based on the ESR results. Ultimately, the superior thermo-oxidative stability of SBR/PDA-MMT is evidenced by its accelerated activation energy and the decomposition temperature (TGA), as well as the decrease of unsaturated carbonyl compounds in heat treatment (as assessed by TG-IR).
Ingredient | Loadinga (phr) |
---|---|
a Weight parts per 100 weight parts of rubber.b N-Isopropyl-N′-methylphenyl-p-phenylene diamine.c Dibenzothiazole disulfide.d Diphenyl guanidine.e Tetramethyl thiuram disulfide. | |
SBR | 100 |
PDA-MMT | 0/5/10/15 |
Zinc oxide | 5 |
Stearic acid | 5 |
Anti-oxidant 4010NAb | 1 |
Accelerator DMc | 0.5 |
Accelerator Dd | 0.5 |
Accelerator TTe | 0.2 |
Sulfur | 2 |
The SBR/PDA-MMT flocculate rubber was put into a 6 inch double-roll open mill and masticated for 1 minute. Then, zinc oxide, stearic acid, and the anti-oxidant were mixed in turn. Afterwards, the accelerators and sulfur were mingled with the rubber mix. After resting for 24 hours to assure a good diffusion of these ingredients in the matrix, the gross rubber was vulcanized at 143 °C for its corresponding T90 (the optimum cure time). In addition, the control samples of SBR and SBR/MMT were prepared under the same conditions. The final rubber sheets had a thickness ranging from 1.95 mm to 2.05 mm.
The smooth rubber sheets (with sizes of about 10 mm × 10 mm × 2 mm) were scanned from 0.5° to 10° at a scanning rate of 1° min−1 using D/Max2500 VB2+/PC X-ray Diffraction (Rigaku, Japan) with Cu Kα radiation. The TEM observations were conducted on an H-800 Transmission Electron Microscope (HITACHI, Japan) at 200 kV. The ultrathin sections of SBR/clay nanocompounds for TEM were cut using a microtome (LEICA EM FC7). The Scanning Electron Microscope images were captured by a S-4800 SEM (HITACHI, Japan) at an accelerating voltage of 20 kV. The rubber samples were broken off in liquid nitrogen, and the flat worn fractures were selected for SEM. In addition, a thin layer of platinum was sputtered on the sample surfaces prior to the SEM observations.
The mechanical properties were performed by means of a universe electronic testing machine (SANS CMT 4104, China) according to the National Standards of China or ASTM with a tensile speed of 500 mm min−1 at ambient temperature. The rubber sheets were die-cut into dumbbell-shaped samples with a working-district width of 6 mm. For each group of data reported, at least five sample measurements were taken and averaged.
The radical scavenging property of PDA was determined by a DPPH Spectrophotometry Assay22,30 with a slight alteration along with a UV-3600 UV-vis spectrometer (Shimadzu, Japan). The 0.05 mg mL−1 solution of DPPH in ethanol was freshly prepared, and 500 μL of pristine PDA aqueous solution (dopamine 1.5 g L−1, pH = 8.5, 4 h) was diluted with 14 mL of DI water prior to usage. The scavenging activity was characterized by monitoring the decrease in the UV-vis absorption of DPPH at 517 nm at different dose levels of PDA (the addition amounts of diluted PDA solution varied from 100 μL to 800 μL). To enable an impartial comparison, ascorbic acid was chosen as a positive control. The calculation equation for the radical-inhibition capability is as follows:
Inhibition = [1 − (Ai − Aj)/A0]. |
The ESR apparatus used here was a JES-FA 200 X-band Electron Spin Resonance spectrometer (JEOL, Japan) with a temperature accessory (JEOL, CVT Controller). The rod-shaped sample was stored in a spectrosil tube (3.8 mm i.d.) in air atmosphere and measured at a 100 kHz modulation frequency with a microwave power of 0.1 mW. The center magnetic field was 3234.91 Gauss with a sweep width of (±) 75 Gauss. The standard sample of manganese (Mn2+) inserted beside the specimen was applied to collimate the center field and the magnetic field strength. The SBR/clay samples were aged using the temperature accessory at 150 °C for various times, at which point the resonance signals were recorded accurately by ESR.
The SBR/clay samples were heated from 40 °C to 600 °C at a heating rate of 10 °C min−1 in air atmosphere with a purge rate of 50 mL min−1 by means of Thermo-Gravimetric Analysis (METTLER-TOLEDO, Switzerland). To ascertain the apparent activation energy (Ea), the samples were heated at different heating rates of 5 °C min−1, 10 °C min−1, 20 °C min−1 and 30 °C min−1, respectively. Ea was calculated according to the classical Flynn–Wall–Ozawa method.31,32 Additionally, Thermo-Gravimetry coupled to Fourier Transform Infrared spectroscopy was conducted to investigate the gaseous products of the specimens evolved during the thermal treatment. TG measurements were performed as described above (heating from 40 °C to ∼700 °C). Infrared spectra were recorded using a Nicolet 6700 FT-IR (Thermo, USA) equipped with a DLaTGS detector in the 400–4000 cm−1 range with 4 cm−1 spectral resolution over 32 scans. The temperature of the transfer line linking TG and FT-IR was 215 °C.
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Scheme 1 Proposed mechanism for the polymerization of dopamine (a-oxidation, b-cyclization, c-rearrangement, and d-dismutation). |
XPS is used to probe the chemical composition and corresponding bonding status of MMT and PDA-MMT. After being coated by PDA, the signal of the characteristic nitrogen is detectable at binding energy (BE) of 399.75 eV from the wide scan spectra of XPS (see Fig. 1(a) and (b)). In Fig. 1(c), the C 1s core-level spectrum of unmodified MMT can be curved-fitted with two peak components, with BE at 282.5 eV for the C–Si impure species, and at 284.6 eV for the C–H species (carbon dioxide contamination). The C 1s core-level spectrum of PDA-MMT can be curved-fitted with three peak components in Fig. 1(d), with BE at 284.6 eV for the C–H species, 285.5 eV for the C–N species, and 287.5 for the CO species. In Fig. 1(f), the N 1s core-level spectrum of PDA-MMT comprises two peak components at 388.5 eV for –N
, and 389.5 eV for –N–H. The C
O species from the quinone derivatives and the –N
species from the indole compounds illustrate the formation of polydopamine deposited onto MMT, which consolidates the proposed polymerization mechanism of dopamine as mentioned above.
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Fig. 1 XPS wide-scan spectra, C 1s core-level spectra and N 1s core-level spectra of (a, c and e) MMT and (b, d and f) PDA-MMT. |
The TEM images are exploited here to produce direct evidence of the morphological alterations of MMT before and after the PDA surface modifications (see Fig. 2(a) and (b)). As PDA exerts an influence of adhesion rather than intercalation to some degree,9,21 ultrasonic treatment is carried preceding the deposition of PDA, to decrease the aggregation of MMT. In Fig. 2(a) and (b), after ultrasonic treatment, most MMT is in the form of a mono-layer distribution, with a width ranging from 200 nm to 1000 nm, whereas PDA-MMT is still dispersed arbitrarily on the TEM grids in similar dimensions.
Since XRD is not a quantitative analysis in which PDA-MMT might be easily diluted by the SBR matrix, conclusions regarding the spatial distribution based on XRD alone are not adequate. Related TEM images are captured in Fig. 2(c)–(f). A much rougher and thicker agglomeration of MMT (500–1000 nm) is observed in SBR/MMT, indicating an inferior dispersion, namely an intercalated and flocculated dispersion.4 Paradoxically, uniformly-intercalated PDA-MMT layers can be scarcely discerned from the low contrast background (as shown in Fig. 2(e), in which the black dots should relate to some ingredients in the SBR/clay composite). In amplifying the magnification in Fig. 2(f), PDA-MMT is in the form of a slim laminate (200–500 nm), which assumes a light-fog appearance.
SEM observations of the worn fractures of the SBR/clay nanocompounds were performed to illuminate the interfacial interactions between the filler and matrix. The sparkling spots are thought to be some conductive additive (e.g., zinc oxide) added during the mix process. Defects or holes can be barely detected in pristine SBR, while the vicinity of MMT brings in many defects in to the matrix (see Fig. 4(a) and (b)). As shown in Fig. 4(c), with the addition of PDA-MMT, the shortcomings vanish, and the boundaries between clay and SBR blur. In fine detail, the strong hydrogen bonding from the abundant catechol groups of PDA and the π–π conjugation of the pendant aromatic structures give rise to impressive interfacial interactions.
The tensile properties of SBR/clay nanocompounds are shown in Table 2. The tensile strength of SBR is only 1.8 MPa, while it is increased by more than 150% through the addition of only 5 phr PDA-MMT, much higher than that of MMT at the same loading level. As the stress gradually improves, the interfacial interaction associated with the cross-linking network begins to take power.34 The modulus at 300% is increased to 2.6 MPa in SBR/PDA-MMT at a very low loading content (5 phr), implying that PDA-MMT possesses robust interfacial interactions with SBR, which thus function as efficient physical cross-linkers.9,34 Namely, the PDA coating is able to lubricate the interface during the tensile process, which diminishes the slippage and fracture of SBR chains anchored onto the MMT galleries, which favors the stress-average, and thus contributes to the enhancement of tensile strength, as well as to the break elongation.34 However, with the increase in the clay loading, a sustainable improvement of the tensile properties could hardly be found. Moreover, this situation was also found in associated literature.21,22 Besides, as layered silicate is not an optimal reinforcing filler for rubber compared with sphere fillers (e.g., carbon black and silica), the reduction in tensile properties could be partially attributed to the aggregation dispersion of PDA-MMT at higher loading levels.
Sample | Modulus at 100% (MPa) | Modulus at 200% (MPa) | Modulus at 300% (MPa) | Break elongation (%) | Tensile strength (MPa) |
---|---|---|---|---|---|
SBR | 0.8 | 1.1 | 1.3 | 426 | 1.8 |
SBR/PDA-MMT-5 | 0.9 | 1.5 | 2.6 | 470 | 5.0 |
SBR/MMT-5 | 1.1 | 1.4 | 1.8 | 438 | 2.4 |
SBR/PDA-MMT-10 | 1.0 | 1.4 | 1.8 | 669 | 4.3 |
SBR/MMT-10 | 0.9 | 1.0 | 1.2 | 577 | 2.4 |
SBR/PDA-MMT-15 | 1.0 | 1.5 | 2.0 | 647 | 4.5 |
SBR/MMT-15 | 1.0 | 1.4 | 1.7 | 640 | 3.1 |
Nevertheless, the possibility of covalent bonds could not be excluded absolutely,9 as the remaining aminogen (–NH2) from PDA might react with the residual double bonds of the SBR chains.
It is rather interesting to note that even dopamine itself has a radical-scavenging activity through a one-step hydrogen atom transfer (HAT) strategy or metal-ion assisted HAT from its catechol groups to reactive oxygen species (ROS).38 In that case, dopamine itself (as one kind of neurotransmitter) could quench these ROS in neurotransmission systems. However, it is difficult to see how the operation of such a mechanism could come about in PDA, due to the absolutely different chemical structure between the monomer and the corresponding macromolecule.
Since radical-induced chain scission is a common thermo-oxidative mechanism for most polymers,25 it is of great significance in our present work to scrutinize whether PDA-MMT could be employed as an efficient anti-aging additive for SBR.
After the addition of PDA aqueous solution, the dark purple solution of DPPH converted to a yellow-colored one, because of reduction of the stable DPPH radical. Fig. 5(a) shows the decay of the UV-vis absorbance spectra at 517 nm upon the addition of PDA. Fig. 5(b) illustrates the corresponding scavenging activity of PDA compared with the reference ascorbic acid (an effective radical-scavenging anti-oxidant) at the same aliquot level. Both reveal a similar tendency of scavenging activity. The scavenging activity dose-dependently ascends rapidly during the early stage and shows a quasi-linear trend. Eventually, the scavenging effect of PDA, as well as ascorbic acid, reaches the maximum value of 85.1% and 95.6%, respectively. These results signify that the PDA coating has an impressive scavenging capability, which is comparable with ascorbic acid.
Notwithstanding this, PDA-MMT is also endowed with a higher specific surface area, which means more available reactive sites,22,28 and thus it exhibits an inferior scavenging ability on DPPH radicals by this method. The reason for this could be that PDA-MMT is a sort of suspension rather than a solution, whereas a homogeneous solution is fundamental when applying spectroscopic measurements. Nevertheless, based on the pronounced scavenging activity on DPPH of PDA, it is a reasonable presumption to make that the high-dispersion clay platelets deposited by PDA would impart an excellent inhibition effect to thermo-induced radicals in SBR matrix. Further research related to this matter of PDA-MMT is outlined below.
Elastomeric material is also a major subject for ESR measurement.44–46 ESR measurements allow reasonable mechanism analyses, with most of these measurements achieved below the glass transition temperature of polymer41–44 under which the motion of polymer chains is blocked to a large extent. Herein, it is doubtful whether the practical anti-ageing property of polymeric materials could be determined under such rigid conditions. To circumvent this issue, we established a sort of in situ thermo-oxidative ageing test along with ESR. The evolution of the spectral shape and the relative spin generation efficiency were determined to ascertain the expectant excellent scavenging activity of PDA-MMT.
As shown in Fig. 6(a) and (b), thermo-induced radicals generated in SBR/clay compounds give a broad and para-symmetrical singlet eventually between the two standard Mn2+ peaks. It is known that alkyl radicals are implicated in the thermal degradation of polymer, therefore the authors tentatively assigned these spectra to alkyl radicals (maybe some alkoxyl radicals concerned47). The interpretation that the broad singlet originates from alkyl radicals could be verified by using pristine SBR for ESR testing, which is known to produce such radicals. As shown in Fig. 6(c), the spectrum gives a similar singlet that is even sharper.
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Fig. 6 ESR spectra measured at 150 °C at various ageing times of (a) SBR/MMT-10, (b) SBR/PDA-MMT-10, and (c) SBR. (d) The evolution of ESR spectra after ageing for 30 minutes at 170 °C. |
The spectral differences between SBR and SBR/clay indeed reveal the outstanding thermo-oxidative stabilization of PDA-MMT. At the early stage of degradation, SBR/PDA-MMT and SBR/MMT exhibit similar spectra shapes. The resonance signals are rather weak, and the features of the central part are masked by a drift of the baseline, which is caused by the high executive temperature (Fig. 6(a) and (b)). The height of the singlet of SBR/MMT increases rapidly as the ageing-times prolong, while the emergence of such a sharp singlet is delayed substantially in the pattern of SBR/PDA-MMT.
It could be inferred that, firstly, high-dispersion clay platelets prevent the intrusion of heating, as well as the diffusion of contagious species, and then the PDA coating handicaps the generation of thermo-induced radicals. In contrast to those without PDA in Fig. 6(d), the characteristic singlet shifts forward. Both the spectral transformation and the aforementioned alterations of the peak shape confirm the occurrence of chemical scavenging on the carbon-centered radicals of the PDA coating.43,47
Moreover, the relative spin generation was calculated by the intensity ratio of the SBR/clay and Mn2+, in which the standard sample Mn2+ was utilized to calibrate for the errors arising from the slight fluctuation of the measuring parameters. The relative spin generation is plotted in Fig. 7 as a function of aging time. The relative spin generation efficiency is obtained from the slope of the plot. SBR/PDA-MMT gives an efficiency value of 0.021 a.u. min−1, which is lower than that of SBR/MMT (0.036 a.u. min−1), indicating a superior inhibition effect on the reactive species of PDA-MMT.
However, the pivotal reactive peroxyl radicals in thermo-oxidative decomposition could not be detected, due to their short life under high temperature. However, for a full understanding, detailed and more convincing grounds should be acquired in a more rational manner of ESR measurement and combined with other spectroscopic methods.43
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Fig. 8 TGA and corresponding DTG curves of SBR/clay nanocomposites in an air atmosphere at a heating rate of 10 °C min−1. |
Sample | Decomposition temperature (°C) | ||||
---|---|---|---|---|---|
T10% | T20% | T50% | T70% | Tmax | |
SBR | 395.3 | 408.5 | 427.8 | 439.0 | 426.0 |
SBR/MMT | 389.0 | 405.9 | 425.4 | 443.1 | 422.7 |
SBR/PDA-MMT | 385.3 | 406.3 | 439.0 | 457.7 | 449.5 |
In order to quantitate the thermo-stabilization effect of PDA-MMT, the apparent activation energy of degradation (Ea) with respect to different conversions was investigated by the Flynn–Wall–Ozawa method.25,31,32 The activation energy is calculated from the slope of the linear fitting of the data using the following equation:
The TGA curves of SBR-based compounds at different heating rates are shown in Fig. 9(a)–(c). When the heating rate improves, the corresponding TG-DTG curves shift to higher temperature, due to the thermal hysteresis effect, indicating that at the same weight loss percentage, the decomposition temperature rises with the increasing heat rate.
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Fig. 9 TGA and corresponding DTG curves of (a) SBR/MMT-10 and (b) SBR/PDA-MMT-10 (in air atmosphere at different heating rates). |
The Ea of SBR-based compounds varies at different conversions, as described in Table 4. It gradually increases until reaching a maximum, implying the random chain scission and the formation of a peroxyl radical intermediate (the latter necessitates more energy). The subsequent decrease of Ea signifies the swift auto-catalyzed oxidizing of the SBR chains. The Ea of SBR/PDA-MMT exceeds that of SBR/MMT, with the amount of enhancement falling within a range of 10–90 kJ mol−1 across the different degrees of conversion. On the other hand, the lower Ea of SBR/MMT can be anticipated according to its lower decomposition temperature aforementioned. Based on the above, PDA-MMT instead of MMT itself exerts a remarkable stabilization effect upon SBR.
Conversion (%) | Ea (kJ mol−1) | ||
---|---|---|---|
SBR | SBR/MMT | SBR/PDA-MMT | |
10 | 487.0 | 436.6 | 497.9 |
20 | 497.1 | 443.0 | 528.0 |
30 | 431.5 | 411.4 | 488.4 |
50 | 321.6 | 348.2 | 362.2 |
70 | 301.5 | 334.0 | 332.1 |
To gain more evidence, a method to detect the gaseous products of SBR-based compounds evolved during thermal treatment was developed by overlapping a series of temporal profiles of the infrared absorption bands. The main infrared absorption bands of the thermo-oxidation products lie in the range of 1700–1800 cm−1 of unsaturated carbonyl compounds, 2400 cm−1 of carbon dioxide and 3400–3900 cm−1 of water vapor.
Fig. 10 presents the 3D TG-IR images of SBR and SBR/clay nanocomposites. It is observed that the intense infrared absorption mostly occurs in the temperature interval between 390 °C and 600 °C, which is consistent with the TGA results above. As a consequence of the thermo-oxidative decomposition, the characteristic infrared absorption bands of the unsaturated carbonyl compounds are detectable in the range of 1700–1800 cm−1, which consists of carboxyl (1716 cm−1), ketone (1750 cm−1), lactone (1780 cm−1), and so forth. Compared with pristine SBR, the MMT layers alone restrain the SBR matrix from severe decomposition, as is shown in Fig. 10(a) and (b). It is striking to note that the PDA-MMT layers substantially prevent that trend by decreasing the generation of such oxidation products, as seen in Fig. 10(c).
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Fig. 10 3D infrared absorption spectra as a function of temperature for the gaseous products of (a) SBR, (b) SBR/MMT-10 and (c) SBR/PDA-MMT-10 that evolve during thermal treatment. |
In all, PDA-MMT considerably improves the thermo-oxidative stability of the SBR matrix. Such enhancement could be partially attributed to the strengthened interface, as SBR chains need more energy to defy the steric hindrance of PDA-MMT. However, the predominant factor is that PDA acts as a scavenger of carbon-centered radicals, thus screening the SBR matrix from rapid thermal degradation. Additionally, this polymeric pigment (PDA) has a property of chelating metal-ions.10,16 The thermal stability of the SBR matrix could also be improved in the presence of PDA, in that some residual metal complex may aggravate the decomposition process.
In summary, due to the chemical scavenging activity of the PDA coating as well as the physical barrier of the high-dispersion clay layers, PDA-MMT exerts an effective thermo-oxidative stabilization effect on the SBR matrix.
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