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Characterizing the naturally occurring sacrificial bond within natural rubber
Accepted Manuscript Characterizing the naturally occurring sacrificial bond within natural rubber Xuan Fu, Cheng Huang, Yong Zhu, Guangsu Huang, Jinrong Wu PII:
S0032-3861(18)31110-8
DOI:
https://doi.org/10.1016/j.polymer.2018.12.005
Reference:
JPOL 21092
To appear in:
Polymer
Received Date: 18 September 2018 Revised Date:
27 November 2018
Accepted Date: 2 December 2018
Please cite this article as: Fu X, Huang C, Zhu Y, Huang G, Wu J, Characterizing the naturally occurring sacrificial bond within natural rubber, Polymer (2019), doi: https://doi.org/10.1016/j.polymer.2018.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Natural rubber (NR) shows better mechanical properties than its synthetic counterpart, polyisoprene (PI). One widely accepted mechanism to interpret this phenomenon is the existence of naturally occurring network in NR, which is formed by linking the terminal groups of PI chains with the aggregrates of proteins and phospholipids through hydrogen or ionic bonds. However, how this naturally occurring network works to improve the mechanical properties remains largely unknown. Herein we consider that the naturally occurring network can dissociate and re-associate upon deformation, thus playing a role of sacrificial bonds to dissipate energy. To prove this speculation, we remove the free and bonded proteins from NR, and find that both these two types of proteins influence the performances of NR. The proteins has huge influence on the mechanical properties of un-vulcanized NR, while has little impact on that of vulcanized NR. However, the existence of bonded proteins provides extra energy dispassion in the vulcanized NR (S-NR) matrices, as well as prolongs the fatigue life of S-NR. The results reveal that the superior properties of NR are due to the combine effect of the vulcanized network and the pseudo network constructed by the linked protein and other non-rubber components. The breaking down of the weak pseudo network could provide effective energy dissipation to avoid material failure. The regeneration of the pseudo network helps preserving the sacrificial bonds, which ultimately leads to the remarkable fatigue resistance of NR.
ACCEPTED MANUSCRIPT Characterizing the naturally occurring sacrificial bond within natural rubber Xuan Fu, Cheng Huang, Yong Zhu, Guangsu Huang*, Jinrong Wu College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China E-mail addresses: [email protected]
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Abstract: The influence of protein on the superior properties of NR was analyzed in term of sacrificial bonds. It was found that the presence of the naturally occurring sacrificial bond provided by protein increases the tensile strengthen and toughness, furthermore prolongs the fatigue life of NR. The superior properties of NR are due to the combine effect of the vulcanized network and the pseudo network constructed by the linked protein and other non-rubber components. The breaking down of the weak pseudo network could provide effective energy dissipation to avoid material failure. The regeneration of the pseudo network helps to preserve the sacrificial bonds, which ultimately leads to the remarkable fatigue resistance of NR. The impact of protein of the network structure is essential to its effect on the properties of NR. Keywords: energy dissipation; sacrificial bonds; protein
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1. Introduction: Natural rubber (NR) is one of the bio-synthetic materials that are proved to be indispensable in industrial applications. The superior mechanical properties of NR to that of its synthetic counterpart, poly (cis-1,4,-isoprene) rubber (IR), has attracted much interest for half a century. Abundant researches have been devoted to distinguish the difference between NR and IR to unravel the basic mechanism of the superior performances of NR, while a concrete conclusion remains to be reached. At the dawn of this century, a new hypothesis was brought out, suggesting that the existence of “none rubber components” is the reason for the superior properties of NR. The “none rubber components” includes proteins (~2.2 wt%), lipids (~3.4 wt%), carbohydrates (~0.4 wt%), metal salts and oxides (~0.2 wt%) and others (~0.1 wt%)(1, 2). Among them, proteins and phospholipids are considered as the crucial components in determining the properties of NR. By applying selective enzymatic and transesterification treatments, Sakdapipanich et al(3-5) proved that each NR macromolecule is composed of a linear poly-isoprene chain with two active terminal groups, which interact with protein and phospholipid aggregates through hydrogen or ionic bonds. As a result, they proposed a network structure in the gel phase of NR, a branched structure in the sol phase of NR and a linear structure in the transesterified NR. Based on their research, Toki(6) studied the influence of the natural impurities on the strain-induced crystallization (SIC) behavior of un-vulcanized NR. The results showed that the SIC of un-vulcanized NR is largely controlled by the existence of the natural impurities. The removal of proteins deteriorates the SIC of NR, while further removing phospholipids results in complete suppression of SIC, similar with the case in the un-vulcanized IR. Taking the large molecular weight of NR macromolecules into consideration, Toki(7, 8) complemented Jitladda’s hypothesis with a pseudo end-linked network model. The large agglomerates of none rubber components at both ends of NR molecules can act as joint point to form a temporary physically-linked network. Meanwhile, this end-linked network can
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also make entanglements as fixed points to induce SIC. This multi-scaled microstructure that was constructed by proteins and phospholipids was further conformed by SAXS/WAXD and optical microscopy(9). Despite intense researches on the structure and constitute of raw NR(10-12), these researches were mostly confined in un-vulcanized NR. However, the vulcanization of rubber is crucial for the industrial applications of NR. To date, there are few researches regarding the impact of none rubber components on the mechanical performances of vulcanized NR. Recent studies reveal that the toughening mechanism of natural materials with superior strength and toughness, such as bone, muscle, spider silk and mussel byssus often involves sacrificial bonds(13-18). The sacrificial bonds are often recognized as the weak bonds that rapture before the failure of strong covalent bonds during deformation. The rapture of sacrificial bonds dissipates a huge amount of energy, which helps maintaining the overall integrity of the natural material(19). Inspired by nature, introducing sacrificial bonds has become an emerging strategy to fabricate high performance materials(20-34). In these materials, there is usually a weak network that preferentially breaks and dissipates energy upon deformation, and thus acting as sacrificial bonds; while the strong network distributes stress and avoids stress concentration, and thus maintaining the integrity of the materials despite the internal break of sacrificial bonds. Along this line, we can envision that similar toughening mechanism exists in vulcanized NR, as the physically-linked network formed by proteins, phospholipids and entanglements can firstly dissociate to serve as sacrificial bonds, while the strong network formed by vulcanization and SIC can confer NR with high mechanical strength. Therefore, the investigation of none rubber components as sacrificial bonds is necessary. However, previous studies on the impact of none rubber components on NR were mostly focused on their influences on the SIC behavior(6-8). The potential role of none rubber components as sacrificial bonds has not been reported. In this article, we examined the influence of proteins on the various properties of raw NR and vulcanized NR. The changes in the tensile strength, toughness, crystallization and the specific nonlinear viscoelastic behavior of NR that are caused by deproteinization are discussed in terms of the microstructure and entanglements in the rubber matrices. Furthermore, the impact of proteins on the mechanical properties of NR is analyzed in analogue to sacrificial bonds. The results proved the existence of the naturally occurring sacrificial bonds within rubber matrices and its huge influence on the dynamic mechanical properties of vulcanized NR. The naturally occurring sacrificial bonds are originated from the pseudo network that is constructed by protein and other non-rubber components. The pseudo network generates extra energy dissipation through preferential dissociation upon deformation, providing NR with good toughness and superior fatigue resistance. 2. Experimental section 2.1 Materials Natural rubber latex used in the present work was commercial high ammonia natural rubber (HANR) latex provided by the Thai Rubber Latex Co, Thailand. Total solid content and dry rubber content of the HANR latex, determined according to ASTM D1076, were 61.3% and 60.8 w/w%, respectively. Triton X-100 and protease (P-5380) was provided by Sigma-Aldrich. Curing reagents including sulfur, zinc oxide (ZnO), N-cyclohexyl-2-benzothiazolesulfenamide(CZ), stearic acid (SA), and antioxidant N-isopropyl-N0-phenyl-p-phenylenediamine (4010NA) were directly purchased from Sichuan Haida Rubber Group Co. Ltd.
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2.2 Preparation of deproteinized raw NR samples All samples were prepared through centrifugal process and enzymolysis approach. Untreated raw NR latex was directly casted on the glass plate to form a thin film and chosen as standard sample NR. Centrifugation of NR latex (10ml) at the speed of 10000r/min for one time, the obtained upper layer was re-dispersed with distilled water and cast on the glass plate to produce sample DPNR1. Sample DPNR2 was prepared with a similar method as sample DPNR1, but by centrifugation for three times. After the centrifugal process of NR latex, sample DPNR3 and DPNR4 were prepared by enzymatic reaction at 60℃ for 24h with different amount of proteinase (0.39mg and 0.78mg) in the presence of 0.15% v/v Triton X-100 and 1 wt% aqua ammonia.
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2.3 Vulcanization of the deproteinized NR samples All samples were dry at 40℃ for 48h in vacuum oven before vulcanization. The NR and curing agents were directly mixed on a two-roller mill at room temperature. The vulcanizations were
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carried out in a standard hot press at 143℃ for 13 min. The cure time of NR was determined using a torque rheometer. The formulation of all samples is as followed: NR 100phr, stearic acid 2phr, ZnO 5phr, sulfur 2phr, CZ 1phr and 4010NA 1phr. (SNRX represents vulcanized NRX)
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2.4 Characterizations The molecular weight characteristics of raw NR and deproteinized NR were determined by gel-permeation chromatography (HLC-8320GPC, TOSOH, Japan) with two columns inseries, packed with cross-linked polystyrene gel and equipped with a differential refractive index detector. Tetrahydrofuran (THF, LabScan, HPLC grade) was used as an eluent with a flow rate of 0.6mL/min at 40±0.1℃. The rubber samples were dissolved in THF at a concentration of0.05% w/v in THF and filtered through a pre-filter and 0.45μm membrane filter. The N content and P content of raw NR and deproteinized NR was characterized by an elemental analyzer (CARLO ERBA 1106, Italian). The results were listed in Table 2. Mechanical properties was tested with an Instron 5567 material testing machine at room temperature with tensile rate of 500mm•min-1 according to the GB/T1040-92 standard. The specimen was a dumbbell-shaped thin strip (25×4×1 mm) and five parallel measurements were carried out and the average value was taken for each sample. The network chain density (v) of all the samples were estimated according to the following equation based on the classical theory of rubber elasticity: ) Equation 1 σ= ( − where σ is the force per unit area, k is the Boltzmann constant, T is the absolute temperature, and α is the elongation ratio. Mc=ρ/v is the average mass of network chains, and ρ (0.92g/mL) is the density of the rubber and the nanocomposites. Dynamic measurements were performed on DMA Q800 (TA instruments) in the range of temperature from -70℃to 50℃(2℃/min) at 1Hz and deformation from 0.01 to 100% at room temperature. The rectangle samples were shaped in a dimension of 10×10×1 mm. Fatigue resistance abilities of the vulcanized samples were carried on a Mechanical Testing & Simulation (MTS) machine. The testing specimen was in the shape of a cylinder (10*52 mm). The high-cycle fatigue experiments were performed with amplitude of 1.5 mm at 5Hz. The sample temperature during the fatigue testing was recorded by a FLIR T610 thermal imager.
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=
∗ 100%
Equation 2
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In situ WAXD experiments were carried out at room temperature on the beamline BL16B1 in shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the X-rays used is 0.124 nm. Two-dimensional WAXD patterns were recorded by a MAR-CCD detector (MAR USA). The image acquisition time for each frame was 20 s. The sample-to-detector distance in WAXD experiments was 172mm. A tensile stretching device, allowing the symmetric deformation of the sample, was used in this study. The device allowed the illumination of X-ray to monitor the structure change on the same sample position during deformation. This instrument is a modified tabletop stretching machine (Linkam 353) provided by the Linkam Inc. The chosen deformation rate was 5 mm/min. The original clamp-to-clamp distance is 10 mm. The initial rate of deformation was 0.007 s−1.All measured images were corrected for beam fluctuations and sample absorption. The WAXD patterns were integrated and the scattering profiles were de-convoluted into individual indexed peaks and amorphous halos using the Levenberg–Marquardt method(35, 36). The crystallinity Index (CI) was calculated by the following equation(37).
Mn(*105)
NR
7.75
DPNR1
7.70
DPNR2
7.68
DPNR3
7.55
DPNR4
7.45
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Table 1. Molecular Weight Characteristics of NR, DPNR1, DPNR2, DPNR3 and DPNR4 Samples tested by GPC Mw(*105)
Mw /Mn
12.9
1.67
12.8
1.66
12.5
1.62
11.7
1.55
11.1
1.49
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3. Results and discussion 3.1 Preparation of the two kinds of deproteinized NR It is well known that NR latex contains various proteins. Generally, they can be classified into two categories: one category is dispersed in the latex with rubber particles and can be easily remove by physical approach, thus we call it free proteins; the other one is directly linked to the macromolecular chains through functional groups(3, 4), so we refer it as bonded proteins. Here we use centrifugation to remove free proteins, while decomposing bonded proteins by proteinase. In this way, we prepared 4 different samples of deproteinized NR (DPNR) with different kind of protein and deproteinization degree. The N content of raw NR and DPNR are listed in Table 2. DPNR1 and DPNR2 are fabricated by centrifugation for different times, and DPNR2 is considered to contain only linked proteins as further centrifugation will not decrease the N content. It is clear that, centrifugation decreases the N content of NR from 0.6wt.% to 0.1wt.%, which suggests that the majority of the protein in NR exists in the dissociated state. Moreover, treating with protease is very efficient in removing linked proteins after centrifugation, as the resulting DPNR3 and DPNR4 show N contents of 0.06wt.% N and <0.03wt%, respectively. Meanwhile, the influence of deproteinization on the molecule weight of NR is analyzed. The GPC results in Table 1 shows the similar MW characteristics between NR and deproteinized NR, suggesting that the degradation of the NR Chains during the enzymolysis reaction is highly impossible.
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DPNR2 (S- DPNR2)
DPNR3 (S- DPNR3)
DPNR4 (S- DPNR4)
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0.2%
0.1%
0.06%
<0.03%
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3.2 The impact of protein on the properties and structure of NR
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NR (SNR)
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Fig.1 Various properties of the raw NR and the deproteinized NR samples: (a) Representative stress–strain curves, (b) tensile strength and maximum elongation, (c) crystallization capacity during deformation and (d) the specific nonlinear viscoelastic behavior. (e) Presumed structure evolution after the deproteinization of raw NR.
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The stress-strain curves of the raw NR and deproteinized NR (DPNR) with different deproteinization degree and N content are shown in Fig 1(a). It is clear that the tensile strength is relatively high for NR, since it shows an earlier and steeper upturn in the stress-strain curve at large deformations; while the tensile strength decreases with decreasing protein content in the DPNR, but at the same time the elongation increases. Such a phenomenon is similar to that found in the filler-filled soft polymer matrices(38-41). To explore this phenomenon, in situ WAXD experiments are carried out and the calculated crystallinity index (CI) as a function of strain is shown in Fig 1(c). We can see that NR shows the smallest onset strain of SIC and the highest CI; while with the decrease of protein content, DPNR shows largely delayed SIC and smaller CI, and finally the SIC becomes very weak in DPNR4. According to the widely accepted crystallization theory proposed by Toki and Tosaka(42-46), the crystal nuclei in natural rubber are first induced from the short chains between cross-linking during deformation. Thus, the presence of SIC behavior in raw NR indicates the existence of a network structure in rubber matrix, and the weak SIC behavior after deproteinization of raw NR indicates the destruction of such network structure. To examine the change in the network upon deproteinization, strain sweep experiments are
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performed and the results of storage modulus as a function of strain are shown in Fig. 1d. The specific nonlinear viscoelastic behavior of crosslinked materials, known as Payne effect, is usually used to characterize the destruction of network structure. As we can see in Fig. 1d, un-vulcanized NR shows the highest storage modulus; while decreasing the protein content decease the storage modulus. Considering that there is no other additional additive in our system, the lower storage modulus of DPNR samples than that of NR1 is the evidence for the lower network density in DPNR matrices. Since it has been revealed that the two terminals of each NR macromolecule link with proteins and phospholipids to form a temporary physically end-linked network, it is rational that the remove of protein leads to the partial destruction of such pseudo network (Fig. 1e).
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Fig.2 Various properties of the SNR and the deproteinized SNR samples: (a) Representative stress–strain curves, (b) tensile strength and maximum elongation, (c) crystallization capacity during deformation and (d) the specific nonlinear viscoelastic behavior. (e) Presumed structure evolution after the deproteinization of SNR. Table 3. network chain density, v (×10-4mol/cm3) and average mass of network chains, Mc (g/cm3) of vulcanized NR and vulcanized DPNR. S-DPNR1
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S-DPNR2
S-DPNR3
S-DPNR4
ν
1.39
1.36
1.32
1.31
1.23
Mc
6620
6765
6970
7020
7480
Compared to the raw NR without vulcanization, the vulcanized NR (SNR) possesses a much more robust network, which endowed NR with sustainable and reliable properties for industrial applications. Since the vulcanization network is so robust, it may smear out the influence of the deproteinization on the mechanical properties of SNR. As such, the stress-strain curve of vulcanized NR is compared with that of DPNR, as shown in Fig 2a. The S-DPNR samples with different N contents possess the same maximum elongation, while the deproteinization only leads to a slightly decrease in tensile strength. Fig 2c shows the crystallization capacities for vulcanized NR and vulcanized DPNR samples. The removal of proteins results in a slight delay in the crystallization process, but the similar SIC behavior is observed in all the S-NR and S-DPNR
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samples. The strain sweep test is also applied on vulcanized NR and DPNR to characterize the network change in the vulcanized system. The specific nonlinear viscoelastic behaviors of S-NR and S-DPNR are showed in Fig. 2d. S-NR shows obvious Payne effect, which indicates good integrity of network structure in pristine vulcanized rubber. The deproteinzation of S-NR leads to a slight decrease in storage modulus and S-DPNR4 with the least N content shows the lowest storage modulus. The result reveals that even though the vulcanization helps to sustain the network integrity of NR, the removal of proteins still causes some change to network structure. Furthermore, the network chain density (v) of vulcanized NR and vulcanized DPNR are calculated to support above assumption. Seen in Table 3, the network chain density of S-NR slightly decreases with the amount of protein. Thus, the deproteinzation indeed cause a destruction of the network structure in S-NR at some level. Compared to un-vulcanized NR, the influence of protein on the mechanical properties of S-NR is much smaller. However, it is prior to point out that the above properties of SNR are mostly characterized under static conditions. While in industrial applications, NR is usually subjected to dynamic loadings. Thus, here we cannot conclude that proteins have no impact on the mechanical properties of vulcanized NR.
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3.3 Characterization of the naturally occurring sacrificial bond
Fig.3 Energy dissipation during the tensile cycle: (a) cyclic stress-strain curves. (b) Hysteresis loss of the first tensile cycle and (c) the second tensile cycle of SNR, S-DPNR1, S-DPNR2, S-DPNR3 and S-DPNR4. It has been revealed that soft materials containing both weak and strong networks can be toughened through the sacrificial break of the weak network. For vulcanized raw NR (SNR), the existence of vulcanized network and pseudo end linked network matches the requirement for the formation of sacrificial bonds. To quantify the influence of protein contents on the energy dissipation of the weak network, cyclic loading-unloading tests are carried out for vulcanized SNR samples and the corresponding hysteresis losses are shown in Fig. 3. An interesting phenomenon
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is observed in Fig. 4b and Fig. 4c. The hysteresis loss during the first tensile cycle is the same for SNR, S-DPNR1 and S-DPNR2 at different strains, while S-DPNR3 and S-DPNR4 show a smaller hysteresis loss compared to them. The same tendency could also be seen during the second tensile cycle. It should be noted that all curves in Fig. 3a show negligible instantaneous residual strain. Moreover, taking the similar SIC behavior for all vulcanized samples during deformation into consideration, the difference in hysteresis loss during tensile cycle should represent the different dissipated energy. According to Fig. 3 SNR, S-DPNR1 and S-DPNR2 provide higher energy dissipation than S-DPNR3 and S-DPNR4 during cyclic tension. One must wonder, what cause the difference between all these samples? As we mentioned in the first paragraph, S-DPNR1 and S-DPNR2 were prepared through centrifugation, while S-DPNR3 and S-DPNR4 underwent both centrifugation and enzymolysis processes. The centrifugation process erases the free proteins which are previously dispersed in rubber matrix, while the protease treatment removes the bonded proteins from the rubber macromolecules. Thus, the existence of the linked proteins is the essential difference between SNR, S-DPNR1, S-DPNR2 and S-DPNR3, S-DPNR4. Sakdapipanich et al proposed the presence of peptide groups at the initiating terminal of a rubber chain(4). The peptide groups are likely the linked proteins we refer in this article. If the linked proteins can form a network structure with other none-rubber components, the association and dissociation of this network could provide extra energy dissipation for the vulcanized rubber matrix during deformation. This can explain the phenomenon we observe in Fig. 3b and Fig. 3c.
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3.3 The influence of deproteinization on the fatigue resistance of NR In practical applications, NR is usually subjected to dynamic loadings. Thus, in addition to strength and toughness, the ability to maintain its mechanical properties under cyclic forces is highly demanded. NR shows much better fatigue resistance than its counterpart IR and the reason for that has been remaining a mystery for decades. Here we investigated the influence of the sacrificial bonds on the fatigue resistance of deproteinized NR. The vulcanized cylindrical NR sample was first compressed for 2.5 mm and then subjected to a dynamic force at a displacement of 1.5 mm for 3*106 cycles. The loading process is illustrated at Fig. 4a and the stress-strain curves at selected cycles are shown in Fig. 4b. Fig. 4c presents the variation of dynamic stiffness as the function of cycle numbers for S-NR and S-DPNR. It is clear that S-DPNR3 and S-DPNR4 show smaller dynamic stiffness than S-NR, S-DPNR1 and S-DPNR2 at the initial of the fatigue test and much smaller dynamic stiffness after 3*106 cycles of dynamic force. If the dynamic stiffness of each sample during the cyclic test is dived by its initial value, the fatigue life of SNR can be deduced. Fig. 4d shows that S-NR, S-DPNR1 and S-DPNR2 can maintains its initial dynamic stiffness even after millions of cycles of dynamic force, while the dynamic stiffness of S-DPNR3 and S-DPNR4 drops quickly. When the dynamic stiffness of the vulcanized rubber drops under 80% of its initial value, the vulcanized rubber is considered to be fatigue failure, the corresponding cycle for this event is recognized as the fatigue life of the rubber. According to Fig. 4d, the dynamic stiffness of S-NR, S-DPNR1 and S-DPNR2 maintain above 80% of its initial value after 3*106 cycles of dynamic force, which means that S-NR, S-DPNR1 and S-DPNR2 possess a fatigue life > 3*106.On the other hand, the fatigue life is calculated to be around 1*106 for S-DPNR3 and even smaller than 1*105 for S-DPNR4. These dramatic differences between S-NR, S-DPNR1, S-DPNR2 and S-DPNR3, S-DPNR4 indicate that proteins play a crucial part in determining the fatigue resistance of NR.
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On the base of above results, we know the fact that the linked proteins are the only difference between S-NR, S-DPNR1, S-DPNR2 and S-DPNR3, S-DPNR4. The break of the pseudo network formed by the linked protein provides extra energy dissipation for the vulcanized NR during MTS test. Meanwhile, the pseudo network is a temporary network which linked by physical or other reversible chemical bonds(6-8). Since the pseudo network in NR is reversible and can regenerate after the break, therefore, the sacrificial bonds provided by the pseudo network exist not only in the first cycle, but also after millions of cycles. The massive energy dissipation created by the linked protein all along with the MTS experiment will inhibit the crack initiation, slow down the crack growth and ultimately prolong the fatigue life of NR. To prove the above assumptions, we calculate the hysteresis loss of each cycle during the MTS tests, the results are presented in Fig. 5e. According to the consideration of the similar network structure within the pristine S-NR and S-DPNR, the decrease of the hysteresis loss can be referred as the decrease of sacrificial bonds. Seen in Fig. 5e, all SNR samples show similar hysteresis loss at the initial stage of the MTS test. For S-DPNR1 and S-DPNR2, the hysteresis loss decreases slowly with the increase of the cyclic number, while the hysteresis loss of S-NR maintains high after millions of cycles. On the other hand, a drastic drop in the hysteresis loss happens for S-DPNR3 and S-DPNR4 at 1*104 th cycle. These results suggest that the existence of proteins helps preserving the sacrificial bonds provided by the pseudo network. Furthermore, we calculated the amount of remaining sacrificial bonds of each sample after the MTS tests. At the end of 3*106 cycles, SNR could maintain 94% of its initial sacrificial bonds. The remaining sacrificial bonds for S-DPNR1 and S-DPNR2 are 56% and 50%, while the remaining sacrificial bonds for S-DPNR3 and S-DPNR4 are only 25% and 19%. The ability to maintain the sacrificial bonds sustained cyclic forces could help to preserve the network integrity and mechanical properties of the vulcanized rubber, which explains the superior fatigue resistance of NR. The above results support our hypothesis of the regeneration of the pseudo network after breaking down.
Fig. 4 MTS test results for neat SNR and the deproteinized SNR samples: (a) presentation for the MTS test experiments; (b) stress-strain curves at selected cycles; (c) the variation of dynamic
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stiffness as the function of cycle numbers; (d) the variation of fatigue resistence at the function of cycle numbers; (e) hysteresis loss for each cycle during the MTS test; (f) network integrities at 3*106 fatigue.
Fig. 5 Dynamic heat build -up during the MTS test: (a) Infrared thermal images at selected cycles for SNR1; (b) Infrared thermal images at selected cycles for SNR5; (c) Temperature rising of neat SNR and deproteinized SNR.
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The dynamic heat build-up is also a critical aspect that could determine the fatigue life of NR. Here we monitor the temperature variation during the first 105 cycles of the MTS experiment for the neat SNR and the deproteinized SNR. The representative infrared thermal images at selected cycles for S-NR and S-DPNR4 are presented in Fig5a and b. The pictures in Fig. 5 show clear temperature rising in both S-NR and S-DPNR4 samples after 105 cycles of dynamic force. Furthermore, we calculated the exact temperature rising during the MTS test for all SNR samples and the results are compared in Fig. 5c. The deproteinization of SNR leads to a decrease in temperature rising. As a result, the temperature rising in the vulcanized deproteinized rubber is consistent with the protein amount in the rubber matrices. As we discussed before, the amount of protein determine the amount of sacrificial bonds in NR. The more the sacrificial bonds provided by protein, the more energy dissipation they can generate, hence, higher the temperature rising. It should be point out that the difference in the temperature rising between the SNR samples is smaller than 2℃, thus the aging of rubber caused by the high temperature can be ignored. 4. Conclusion
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In this work, we prepared raw NR and vulcanized NR samples with different N content. The influence of protein on both the macroscopical properties and the micro-network structure of NR matrices were investigated. Raw NR contains only the pseudo network constructed by protein and other non-rubber components. The remove of protein would lead to massive destruction of the temporary network and alteration of mechanical properties of NR matrices. In opposite, vulcanized NR possesses a rigid network which was linked by sulfur through chemical bonds. The destruction of the pseudo network has little effect on the network structure of the vulcanized rubber in the static environment. However, for the vulcanized NR containing both the vulcanized network and the pseudo network, a double-network structure was formed within the NR matrix. Upon external stress, the pseudo network could break down to provide energy dissipation to avoid the material failure, giving NR extra toughness. The existence of the naturally occurring sacrificial bond provided by protein was confirmed by step-cycle loading-unloading experiments. Meanwhile, the sacrificial bonds were likely provided by the bonded protein which participated in the construction of the pseudo network, rather than the free protein that was immersed in the NR latex. The MTS tests reveal that the naturally occurring sacrificial bond has huge influence on the dynamic properties of NR. Considering that the pseudo network is linked though physical or other reversible bonds, the regeneration of the pseudo network is likely possible. Thus, the energy dissipation effect by the naturally occurring sacrificial bond could sustain much longer than the traditional sacrificial bonds, providing NR with good fatigue resistance. Our work was the first to link the performance of NR in the dynamic environment with the existence of sacrificial bonds. The result provided new insight into the difference between NR and synthetic IR and new idea of synthesizing rubber with superior properties for industrial applications.
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Acknowledgment: The authors acknowledge the financial support of National Natural Science Foundation of China (Grant No. 51333003), Foundation (Grant No. 51673120), National Natural Science Foundation of China (Grant No. 51790501) and the Special Fund for Agro-scientific Research in the Public Interest (Grant No. 201403066).
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sacrificial bonds; protein; pseudo network; energy dissipation
S0032-3861(18)31110-8
DOI:
https://doi.org/10.1016/j.polymer.2018.12.005
Reference:
JPOL 21092
To appear in:
Polymer
Received Date: 18 September 2018 Revised Date:
27 November 2018
Accepted Date: 2 December 2018
Please cite this article as: Fu X, Huang C, Zhu Y, Huang G, Wu J, Characterizing the naturally occurring sacrificial bond within natural rubber, Polymer (2019), doi: https://doi.org/10.1016/j.polymer.2018.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Natural rubber (NR) shows better mechanical properties than its synthetic counterpart, polyisoprene (PI). One widely accepted mechanism to interpret this phenomenon is the existence of naturally occurring network in NR, which is formed by linking the terminal groups of PI chains with the aggregrates of proteins and phospholipids through hydrogen or ionic bonds. However, how this naturally occurring network works to improve the mechanical properties remains largely unknown. Herein we consider that the naturally occurring network can dissociate and re-associate upon deformation, thus playing a role of sacrificial bonds to dissipate energy. To prove this speculation, we remove the free and bonded proteins from NR, and find that both these two types of proteins influence the performances of NR. The proteins has huge influence on the mechanical properties of un-vulcanized NR, while has little impact on that of vulcanized NR. However, the existence of bonded proteins provides extra energy dispassion in the vulcanized NR (S-NR) matrices, as well as prolongs the fatigue life of S-NR. The results reveal that the superior properties of NR are due to the combine effect of the vulcanized network and the pseudo network constructed by the linked protein and other non-rubber components. The breaking down of the weak pseudo network could provide effective energy dissipation to avoid material failure. The regeneration of the pseudo network helps preserving the sacrificial bonds, which ultimately leads to the remarkable fatigue resistance of NR.
ACCEPTED MANUSCRIPT Characterizing the naturally occurring sacrificial bond within natural rubber Xuan Fu, Cheng Huang, Yong Zhu, Guangsu Huang*, Jinrong Wu College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China E-mail addresses: [email protected]
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Abstract: The influence of protein on the superior properties of NR was analyzed in term of sacrificial bonds. It was found that the presence of the naturally occurring sacrificial bond provided by protein increases the tensile strengthen and toughness, furthermore prolongs the fatigue life of NR. The superior properties of NR are due to the combine effect of the vulcanized network and the pseudo network constructed by the linked protein and other non-rubber components. The breaking down of the weak pseudo network could provide effective energy dissipation to avoid material failure. The regeneration of the pseudo network helps to preserve the sacrificial bonds, which ultimately leads to the remarkable fatigue resistance of NR. The impact of protein of the network structure is essential to its effect on the properties of NR. Keywords: energy dissipation; sacrificial bonds; protein
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1. Introduction: Natural rubber (NR) is one of the bio-synthetic materials that are proved to be indispensable in industrial applications. The superior mechanical properties of NR to that of its synthetic counterpart, poly (cis-1,4,-isoprene) rubber (IR), has attracted much interest for half a century. Abundant researches have been devoted to distinguish the difference between NR and IR to unravel the basic mechanism of the superior performances of NR, while a concrete conclusion remains to be reached. At the dawn of this century, a new hypothesis was brought out, suggesting that the existence of “none rubber components” is the reason for the superior properties of NR. The “none rubber components” includes proteins (~2.2 wt%), lipids (~3.4 wt%), carbohydrates (~0.4 wt%), metal salts and oxides (~0.2 wt%) and others (~0.1 wt%)(1, 2). Among them, proteins and phospholipids are considered as the crucial components in determining the properties of NR. By applying selective enzymatic and transesterification treatments, Sakdapipanich et al(3-5) proved that each NR macromolecule is composed of a linear poly-isoprene chain with two active terminal groups, which interact with protein and phospholipid aggregates through hydrogen or ionic bonds. As a result, they proposed a network structure in the gel phase of NR, a branched structure in the sol phase of NR and a linear structure in the transesterified NR. Based on their research, Toki(6) studied the influence of the natural impurities on the strain-induced crystallization (SIC) behavior of un-vulcanized NR. The results showed that the SIC of un-vulcanized NR is largely controlled by the existence of the natural impurities. The removal of proteins deteriorates the SIC of NR, while further removing phospholipids results in complete suppression of SIC, similar with the case in the un-vulcanized IR. Taking the large molecular weight of NR macromolecules into consideration, Toki(7, 8) complemented Jitladda’s hypothesis with a pseudo end-linked network model. The large agglomerates of none rubber components at both ends of NR molecules can act as joint point to form a temporary physically-linked network. Meanwhile, this end-linked network can
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also make entanglements as fixed points to induce SIC. This multi-scaled microstructure that was constructed by proteins and phospholipids was further conformed by SAXS/WAXD and optical microscopy(9). Despite intense researches on the structure and constitute of raw NR(10-12), these researches were mostly confined in un-vulcanized NR. However, the vulcanization of rubber is crucial for the industrial applications of NR. To date, there are few researches regarding the impact of none rubber components on the mechanical performances of vulcanized NR. Recent studies reveal that the toughening mechanism of natural materials with superior strength and toughness, such as bone, muscle, spider silk and mussel byssus often involves sacrificial bonds(13-18). The sacrificial bonds are often recognized as the weak bonds that rapture before the failure of strong covalent bonds during deformation. The rapture of sacrificial bonds dissipates a huge amount of energy, which helps maintaining the overall integrity of the natural material(19). Inspired by nature, introducing sacrificial bonds has become an emerging strategy to fabricate high performance materials(20-34). In these materials, there is usually a weak network that preferentially breaks and dissipates energy upon deformation, and thus acting as sacrificial bonds; while the strong network distributes stress and avoids stress concentration, and thus maintaining the integrity of the materials despite the internal break of sacrificial bonds. Along this line, we can envision that similar toughening mechanism exists in vulcanized NR, as the physically-linked network formed by proteins, phospholipids and entanglements can firstly dissociate to serve as sacrificial bonds, while the strong network formed by vulcanization and SIC can confer NR with high mechanical strength. Therefore, the investigation of none rubber components as sacrificial bonds is necessary. However, previous studies on the impact of none rubber components on NR were mostly focused on their influences on the SIC behavior(6-8). The potential role of none rubber components as sacrificial bonds has not been reported. In this article, we examined the influence of proteins on the various properties of raw NR and vulcanized NR. The changes in the tensile strength, toughness, crystallization and the specific nonlinear viscoelastic behavior of NR that are caused by deproteinization are discussed in terms of the microstructure and entanglements in the rubber matrices. Furthermore, the impact of proteins on the mechanical properties of NR is analyzed in analogue to sacrificial bonds. The results proved the existence of the naturally occurring sacrificial bonds within rubber matrices and its huge influence on the dynamic mechanical properties of vulcanized NR. The naturally occurring sacrificial bonds are originated from the pseudo network that is constructed by protein and other non-rubber components. The pseudo network generates extra energy dissipation through preferential dissociation upon deformation, providing NR with good toughness and superior fatigue resistance. 2. Experimental section 2.1 Materials Natural rubber latex used in the present work was commercial high ammonia natural rubber (HANR) latex provided by the Thai Rubber Latex Co, Thailand. Total solid content and dry rubber content of the HANR latex, determined according to ASTM D1076, were 61.3% and 60.8 w/w%, respectively. Triton X-100 and protease (P-5380) was provided by Sigma-Aldrich. Curing reagents including sulfur, zinc oxide (ZnO), N-cyclohexyl-2-benzothiazolesulfenamide(CZ), stearic acid (SA), and antioxidant N-isopropyl-N0-phenyl-p-phenylenediamine (4010NA) were directly purchased from Sichuan Haida Rubber Group Co. Ltd.
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2.2 Preparation of deproteinized raw NR samples All samples were prepared through centrifugal process and enzymolysis approach. Untreated raw NR latex was directly casted on the glass plate to form a thin film and chosen as standard sample NR. Centrifugation of NR latex (10ml) at the speed of 10000r/min for one time, the obtained upper layer was re-dispersed with distilled water and cast on the glass plate to produce sample DPNR1. Sample DPNR2 was prepared with a similar method as sample DPNR1, but by centrifugation for three times. After the centrifugal process of NR latex, sample DPNR3 and DPNR4 were prepared by enzymatic reaction at 60℃ for 24h with different amount of proteinase (0.39mg and 0.78mg) in the presence of 0.15% v/v Triton X-100 and 1 wt% aqua ammonia.
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2.3 Vulcanization of the deproteinized NR samples All samples were dry at 40℃ for 48h in vacuum oven before vulcanization. The NR and curing agents were directly mixed on a two-roller mill at room temperature. The vulcanizations were
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carried out in a standard hot press at 143℃ for 13 min. The cure time of NR was determined using a torque rheometer. The formulation of all samples is as followed: NR 100phr, stearic acid 2phr, ZnO 5phr, sulfur 2phr, CZ 1phr and 4010NA 1phr. (SNRX represents vulcanized NRX)
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2.4 Characterizations The molecular weight characteristics of raw NR and deproteinized NR were determined by gel-permeation chromatography (HLC-8320GPC, TOSOH, Japan) with two columns inseries, packed with cross-linked polystyrene gel and equipped with a differential refractive index detector. Tetrahydrofuran (THF, LabScan, HPLC grade) was used as an eluent with a flow rate of 0.6mL/min at 40±0.1℃. The rubber samples were dissolved in THF at a concentration of0.05% w/v in THF and filtered through a pre-filter and 0.45μm membrane filter. The N content and P content of raw NR and deproteinized NR was characterized by an elemental analyzer (CARLO ERBA 1106, Italian). The results were listed in Table 2. Mechanical properties was tested with an Instron 5567 material testing machine at room temperature with tensile rate of 500mm•min-1 according to the GB/T1040-92 standard. The specimen was a dumbbell-shaped thin strip (25×4×1 mm) and five parallel measurements were carried out and the average value was taken for each sample. The network chain density (v) of all the samples were estimated according to the following equation based on the classical theory of rubber elasticity: ) Equation 1 σ= ( − where σ is the force per unit area, k is the Boltzmann constant, T is the absolute temperature, and α is the elongation ratio. Mc=ρ/v is the average mass of network chains, and ρ (0.92g/mL) is the density of the rubber and the nanocomposites. Dynamic measurements were performed on DMA Q800 (TA instruments) in the range of temperature from -70℃to 50℃(2℃/min) at 1Hz and deformation from 0.01 to 100% at room temperature. The rectangle samples were shaped in a dimension of 10×10×1 mm. Fatigue resistance abilities of the vulcanized samples were carried on a Mechanical Testing & Simulation (MTS) machine. The testing specimen was in the shape of a cylinder (10*52 mm). The high-cycle fatigue experiments were performed with amplitude of 1.5 mm at 5Hz. The sample temperature during the fatigue testing was recorded by a FLIR T610 thermal imager.
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=
∗ 100%
Equation 2
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In situ WAXD experiments were carried out at room temperature on the beamline BL16B1 in shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the X-rays used is 0.124 nm. Two-dimensional WAXD patterns were recorded by a MAR-CCD detector (MAR USA). The image acquisition time for each frame was 20 s. The sample-to-detector distance in WAXD experiments was 172mm. A tensile stretching device, allowing the symmetric deformation of the sample, was used in this study. The device allowed the illumination of X-ray to monitor the structure change on the same sample position during deformation. This instrument is a modified tabletop stretching machine (Linkam 353) provided by the Linkam Inc. The chosen deformation rate was 5 mm/min. The original clamp-to-clamp distance is 10 mm. The initial rate of deformation was 0.007 s−1.All measured images were corrected for beam fluctuations and sample absorption. The WAXD patterns were integrated and the scattering profiles were de-convoluted into individual indexed peaks and amorphous halos using the Levenberg–Marquardt method(35, 36). The crystallinity Index (CI) was calculated by the following equation(37).
Mn(*105)
NR
7.75
DPNR1
7.70
DPNR2
7.68
DPNR3
7.55
DPNR4
7.45
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Table 1. Molecular Weight Characteristics of NR, DPNR1, DPNR2, DPNR3 and DPNR4 Samples tested by GPC Mw(*105)
Mw /Mn
12.9
1.67
12.8
1.66
12.5
1.62
11.7
1.55
11.1
1.49
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3. Results and discussion 3.1 Preparation of the two kinds of deproteinized NR It is well known that NR latex contains various proteins. Generally, they can be classified into two categories: one category is dispersed in the latex with rubber particles and can be easily remove by physical approach, thus we call it free proteins; the other one is directly linked to the macromolecular chains through functional groups(3, 4), so we refer it as bonded proteins. Here we use centrifugation to remove free proteins, while decomposing bonded proteins by proteinase. In this way, we prepared 4 different samples of deproteinized NR (DPNR) with different kind of protein and deproteinization degree. The N content of raw NR and DPNR are listed in Table 2. DPNR1 and DPNR2 are fabricated by centrifugation for different times, and DPNR2 is considered to contain only linked proteins as further centrifugation will not decrease the N content. It is clear that, centrifugation decreases the N content of NR from 0.6wt.% to 0.1wt.%, which suggests that the majority of the protein in NR exists in the dissociated state. Moreover, treating with protease is very efficient in removing linked proteins after centrifugation, as the resulting DPNR3 and DPNR4 show N contents of 0.06wt.% N and <0.03wt%, respectively. Meanwhile, the influence of deproteinization on the molecule weight of NR is analyzed. The GPC results in Table 1 shows the similar MW characteristics between NR and deproteinized NR, suggesting that the degradation of the NR Chains during the enzymolysis reaction is highly impossible.
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DPNR2 (S- DPNR2)
DPNR3 (S- DPNR3)
DPNR4 (S- DPNR4)
0.6%
0.2%
0.1%
0.06%
<0.03%
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3.2 The impact of protein on the properties and structure of NR
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N content(wt.%)
NR (SNR)
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Fig.1 Various properties of the raw NR and the deproteinized NR samples: (a) Representative stress–strain curves, (b) tensile strength and maximum elongation, (c) crystallization capacity during deformation and (d) the specific nonlinear viscoelastic behavior. (e) Presumed structure evolution after the deproteinization of raw NR.
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The stress-strain curves of the raw NR and deproteinized NR (DPNR) with different deproteinization degree and N content are shown in Fig 1(a). It is clear that the tensile strength is relatively high for NR, since it shows an earlier and steeper upturn in the stress-strain curve at large deformations; while the tensile strength decreases with decreasing protein content in the DPNR, but at the same time the elongation increases. Such a phenomenon is similar to that found in the filler-filled soft polymer matrices(38-41). To explore this phenomenon, in situ WAXD experiments are carried out and the calculated crystallinity index (CI) as a function of strain is shown in Fig 1(c). We can see that NR shows the smallest onset strain of SIC and the highest CI; while with the decrease of protein content, DPNR shows largely delayed SIC and smaller CI, and finally the SIC becomes very weak in DPNR4. According to the widely accepted crystallization theory proposed by Toki and Tosaka(42-46), the crystal nuclei in natural rubber are first induced from the short chains between cross-linking during deformation. Thus, the presence of SIC behavior in raw NR indicates the existence of a network structure in rubber matrix, and the weak SIC behavior after deproteinization of raw NR indicates the destruction of such network structure. To examine the change in the network upon deproteinization, strain sweep experiments are
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performed and the results of storage modulus as a function of strain are shown in Fig. 1d. The specific nonlinear viscoelastic behavior of crosslinked materials, known as Payne effect, is usually used to characterize the destruction of network structure. As we can see in Fig. 1d, un-vulcanized NR shows the highest storage modulus; while decreasing the protein content decease the storage modulus. Considering that there is no other additional additive in our system, the lower storage modulus of DPNR samples than that of NR1 is the evidence for the lower network density in DPNR matrices. Since it has been revealed that the two terminals of each NR macromolecule link with proteins and phospholipids to form a temporary physically end-linked network, it is rational that the remove of protein leads to the partial destruction of such pseudo network (Fig. 1e).
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Fig.2 Various properties of the SNR and the deproteinized SNR samples: (a) Representative stress–strain curves, (b) tensile strength and maximum elongation, (c) crystallization capacity during deformation and (d) the specific nonlinear viscoelastic behavior. (e) Presumed structure evolution after the deproteinization of SNR. Table 3. network chain density, v (×10-4mol/cm3) and average mass of network chains, Mc (g/cm3) of vulcanized NR and vulcanized DPNR. S-DPNR1
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S-DPNR2
S-DPNR3
S-DPNR4
ν
1.39
1.36
1.32
1.31
1.23
Mc
6620
6765
6970
7020
7480
Compared to the raw NR without vulcanization, the vulcanized NR (SNR) possesses a much more robust network, which endowed NR with sustainable and reliable properties for industrial applications. Since the vulcanization network is so robust, it may smear out the influence of the deproteinization on the mechanical properties of SNR. As such, the stress-strain curve of vulcanized NR is compared with that of DPNR, as shown in Fig 2a. The S-DPNR samples with different N contents possess the same maximum elongation, while the deproteinization only leads to a slightly decrease in tensile strength. Fig 2c shows the crystallization capacities for vulcanized NR and vulcanized DPNR samples. The removal of proteins results in a slight delay in the crystallization process, but the similar SIC behavior is observed in all the S-NR and S-DPNR
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samples. The strain sweep test is also applied on vulcanized NR and DPNR to characterize the network change in the vulcanized system. The specific nonlinear viscoelastic behaviors of S-NR and S-DPNR are showed in Fig. 2d. S-NR shows obvious Payne effect, which indicates good integrity of network structure in pristine vulcanized rubber. The deproteinzation of S-NR leads to a slight decrease in storage modulus and S-DPNR4 with the least N content shows the lowest storage modulus. The result reveals that even though the vulcanization helps to sustain the network integrity of NR, the removal of proteins still causes some change to network structure. Furthermore, the network chain density (v) of vulcanized NR and vulcanized DPNR are calculated to support above assumption. Seen in Table 3, the network chain density of S-NR slightly decreases with the amount of protein. Thus, the deproteinzation indeed cause a destruction of the network structure in S-NR at some level. Compared to un-vulcanized NR, the influence of protein on the mechanical properties of S-NR is much smaller. However, it is prior to point out that the above properties of SNR are mostly characterized under static conditions. While in industrial applications, NR is usually subjected to dynamic loadings. Thus, here we cannot conclude that proteins have no impact on the mechanical properties of vulcanized NR.
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3.3 Characterization of the naturally occurring sacrificial bond
Fig.3 Energy dissipation during the tensile cycle: (a) cyclic stress-strain curves. (b) Hysteresis loss of the first tensile cycle and (c) the second tensile cycle of SNR, S-DPNR1, S-DPNR2, S-DPNR3 and S-DPNR4. It has been revealed that soft materials containing both weak and strong networks can be toughened through the sacrificial break of the weak network. For vulcanized raw NR (SNR), the existence of vulcanized network and pseudo end linked network matches the requirement for the formation of sacrificial bonds. To quantify the influence of protein contents on the energy dissipation of the weak network, cyclic loading-unloading tests are carried out for vulcanized SNR samples and the corresponding hysteresis losses are shown in Fig. 3. An interesting phenomenon
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is observed in Fig. 4b and Fig. 4c. The hysteresis loss during the first tensile cycle is the same for SNR, S-DPNR1 and S-DPNR2 at different strains, while S-DPNR3 and S-DPNR4 show a smaller hysteresis loss compared to them. The same tendency could also be seen during the second tensile cycle. It should be noted that all curves in Fig. 3a show negligible instantaneous residual strain. Moreover, taking the similar SIC behavior for all vulcanized samples during deformation into consideration, the difference in hysteresis loss during tensile cycle should represent the different dissipated energy. According to Fig. 3 SNR, S-DPNR1 and S-DPNR2 provide higher energy dissipation than S-DPNR3 and S-DPNR4 during cyclic tension. One must wonder, what cause the difference between all these samples? As we mentioned in the first paragraph, S-DPNR1 and S-DPNR2 were prepared through centrifugation, while S-DPNR3 and S-DPNR4 underwent both centrifugation and enzymolysis processes. The centrifugation process erases the free proteins which are previously dispersed in rubber matrix, while the protease treatment removes the bonded proteins from the rubber macromolecules. Thus, the existence of the linked proteins is the essential difference between SNR, S-DPNR1, S-DPNR2 and S-DPNR3, S-DPNR4. Sakdapipanich et al proposed the presence of peptide groups at the initiating terminal of a rubber chain(4). The peptide groups are likely the linked proteins we refer in this article. If the linked proteins can form a network structure with other none-rubber components, the association and dissociation of this network could provide extra energy dissipation for the vulcanized rubber matrix during deformation. This can explain the phenomenon we observe in Fig. 3b and Fig. 3c.
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3.3 The influence of deproteinization on the fatigue resistance of NR In practical applications, NR is usually subjected to dynamic loadings. Thus, in addition to strength and toughness, the ability to maintain its mechanical properties under cyclic forces is highly demanded. NR shows much better fatigue resistance than its counterpart IR and the reason for that has been remaining a mystery for decades. Here we investigated the influence of the sacrificial bonds on the fatigue resistance of deproteinized NR. The vulcanized cylindrical NR sample was first compressed for 2.5 mm and then subjected to a dynamic force at a displacement of 1.5 mm for 3*106 cycles. The loading process is illustrated at Fig. 4a and the stress-strain curves at selected cycles are shown in Fig. 4b. Fig. 4c presents the variation of dynamic stiffness as the function of cycle numbers for S-NR and S-DPNR. It is clear that S-DPNR3 and S-DPNR4 show smaller dynamic stiffness than S-NR, S-DPNR1 and S-DPNR2 at the initial of the fatigue test and much smaller dynamic stiffness after 3*106 cycles of dynamic force. If the dynamic stiffness of each sample during the cyclic test is dived by its initial value, the fatigue life of SNR can be deduced. Fig. 4d shows that S-NR, S-DPNR1 and S-DPNR2 can maintains its initial dynamic stiffness even after millions of cycles of dynamic force, while the dynamic stiffness of S-DPNR3 and S-DPNR4 drops quickly. When the dynamic stiffness of the vulcanized rubber drops under 80% of its initial value, the vulcanized rubber is considered to be fatigue failure, the corresponding cycle for this event is recognized as the fatigue life of the rubber. According to Fig. 4d, the dynamic stiffness of S-NR, S-DPNR1 and S-DPNR2 maintain above 80% of its initial value after 3*106 cycles of dynamic force, which means that S-NR, S-DPNR1 and S-DPNR2 possess a fatigue life > 3*106.On the other hand, the fatigue life is calculated to be around 1*106 for S-DPNR3 and even smaller than 1*105 for S-DPNR4. These dramatic differences between S-NR, S-DPNR1, S-DPNR2 and S-DPNR3, S-DPNR4 indicate that proteins play a crucial part in determining the fatigue resistance of NR.
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On the base of above results, we know the fact that the linked proteins are the only difference between S-NR, S-DPNR1, S-DPNR2 and S-DPNR3, S-DPNR4. The break of the pseudo network formed by the linked protein provides extra energy dissipation for the vulcanized NR during MTS test. Meanwhile, the pseudo network is a temporary network which linked by physical or other reversible chemical bonds(6-8). Since the pseudo network in NR is reversible and can regenerate after the break, therefore, the sacrificial bonds provided by the pseudo network exist not only in the first cycle, but also after millions of cycles. The massive energy dissipation created by the linked protein all along with the MTS experiment will inhibit the crack initiation, slow down the crack growth and ultimately prolong the fatigue life of NR. To prove the above assumptions, we calculate the hysteresis loss of each cycle during the MTS tests, the results are presented in Fig. 5e. According to the consideration of the similar network structure within the pristine S-NR and S-DPNR, the decrease of the hysteresis loss can be referred as the decrease of sacrificial bonds. Seen in Fig. 5e, all SNR samples show similar hysteresis loss at the initial stage of the MTS test. For S-DPNR1 and S-DPNR2, the hysteresis loss decreases slowly with the increase of the cyclic number, while the hysteresis loss of S-NR maintains high after millions of cycles. On the other hand, a drastic drop in the hysteresis loss happens for S-DPNR3 and S-DPNR4 at 1*104 th cycle. These results suggest that the existence of proteins helps preserving the sacrificial bonds provided by the pseudo network. Furthermore, we calculated the amount of remaining sacrificial bonds of each sample after the MTS tests. At the end of 3*106 cycles, SNR could maintain 94% of its initial sacrificial bonds. The remaining sacrificial bonds for S-DPNR1 and S-DPNR2 are 56% and 50%, while the remaining sacrificial bonds for S-DPNR3 and S-DPNR4 are only 25% and 19%. The ability to maintain the sacrificial bonds sustained cyclic forces could help to preserve the network integrity and mechanical properties of the vulcanized rubber, which explains the superior fatigue resistance of NR. The above results support our hypothesis of the regeneration of the pseudo network after breaking down.
Fig. 4 MTS test results for neat SNR and the deproteinized SNR samples: (a) presentation for the MTS test experiments; (b) stress-strain curves at selected cycles; (c) the variation of dynamic
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stiffness as the function of cycle numbers; (d) the variation of fatigue resistence at the function of cycle numbers; (e) hysteresis loss for each cycle during the MTS test; (f) network integrities at 3*106 fatigue.
Fig. 5 Dynamic heat build -up during the MTS test: (a) Infrared thermal images at selected cycles for SNR1; (b) Infrared thermal images at selected cycles for SNR5; (c) Temperature rising of neat SNR and deproteinized SNR.
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The dynamic heat build-up is also a critical aspect that could determine the fatigue life of NR. Here we monitor the temperature variation during the first 105 cycles of the MTS experiment for the neat SNR and the deproteinized SNR. The representative infrared thermal images at selected cycles for S-NR and S-DPNR4 are presented in Fig5a and b. The pictures in Fig. 5 show clear temperature rising in both S-NR and S-DPNR4 samples after 105 cycles of dynamic force. Furthermore, we calculated the exact temperature rising during the MTS test for all SNR samples and the results are compared in Fig. 5c. The deproteinization of SNR leads to a decrease in temperature rising. As a result, the temperature rising in the vulcanized deproteinized rubber is consistent with the protein amount in the rubber matrices. As we discussed before, the amount of protein determine the amount of sacrificial bonds in NR. The more the sacrificial bonds provided by protein, the more energy dissipation they can generate, hence, higher the temperature rising. It should be point out that the difference in the temperature rising between the SNR samples is smaller than 2℃, thus the aging of rubber caused by the high temperature can be ignored. 4. Conclusion
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In this work, we prepared raw NR and vulcanized NR samples with different N content. The influence of protein on both the macroscopical properties and the micro-network structure of NR matrices were investigated. Raw NR contains only the pseudo network constructed by protein and other non-rubber components. The remove of protein would lead to massive destruction of the temporary network and alteration of mechanical properties of NR matrices. In opposite, vulcanized NR possesses a rigid network which was linked by sulfur through chemical bonds. The destruction of the pseudo network has little effect on the network structure of the vulcanized rubber in the static environment. However, for the vulcanized NR containing both the vulcanized network and the pseudo network, a double-network structure was formed within the NR matrix. Upon external stress, the pseudo network could break down to provide energy dissipation to avoid the material failure, giving NR extra toughness. The existence of the naturally occurring sacrificial bond provided by protein was confirmed by step-cycle loading-unloading experiments. Meanwhile, the sacrificial bonds were likely provided by the bonded protein which participated in the construction of the pseudo network, rather than the free protein that was immersed in the NR latex. The MTS tests reveal that the naturally occurring sacrificial bond has huge influence on the dynamic properties of NR. Considering that the pseudo network is linked though physical or other reversible bonds, the regeneration of the pseudo network is likely possible. Thus, the energy dissipation effect by the naturally occurring sacrificial bond could sustain much longer than the traditional sacrificial bonds, providing NR with good fatigue resistance. Our work was the first to link the performance of NR in the dynamic environment with the existence of sacrificial bonds. The result provided new insight into the difference between NR and synthetic IR and new idea of synthesizing rubber with superior properties for industrial applications.
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Acknowledgment: The authors acknowledge the financial support of National Natural Science Foundation of China (Grant No. 51333003), Foundation (Grant No. 51673120), National Natural Science Foundation of China (Grant No. 51790501) and the Special Fund for Agro-scientific Research in the Public Interest (Grant No. 201403066).
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sacrificial bonds; protein; pseudo network; energy dissipation