Stress-Strain Behaviors and Crosslinked Networks Studies of Natural Rubber-Zinc Dimethacrylate Composites

This article was downloaded by: [Yukun Chen]On: 16 May 2012, At: 08:14Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

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Stress-Strain Behaviors and CrosslinkedNetworks Studies of Natural Rubber-ZincDimethacrylate CompositesYukun Chen a & Chuanhui Xu ba School of Mechanical and Automotive Engineering, South ChinaUniversity of Technology, Guangzhou, Chinab College of Material Science and Engineering, South ChinaUniversity of Technology, Guangzhou, China

Available online: 18 Oct 2011

To cite this article: Yukun Chen & Chuanhui Xu (2012): Stress-Strain Behaviors and CrosslinkedNetworks Studies of Natural Rubber-Zinc Dimethacrylate Composites, Journal of MacromolecularScience, Part B: Physics, 51:7, 1384-1400

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Journal of Macromolecular Science R©, Part B: Physics, 51:1384–1400, 2012Copyright © Taylor & Francis Group, LLCISSN: 0022-2348 print / 1525-609X onlineDOI: 10.1080/00222348.2011.629904

Stress-Strain Behaviors and CrosslinkedNetworks Studies of Natural Rubber-Zinc

Dimethacrylate Composites

YUKUN CHEN1 AND CHUANHUI XU2

1School of Mechanical and Automotive Engineering, South China Universityof Technology, Guangzhou, China2College of Material Science and Engineering, South China Universityof Technology, Guangzhou, China

The stress-strain behaviors of natural rubber (NR)-zinc methacrylate (ZDMA) compositehave been studied by uniaxial tension. The results indicated that there was a largereinforcement by ZDMA and the NR/ZDMA composites exhibited a high stress-softeningeffect. Meanwhile, the recovery stretch curve was close to the second stretch curve; thusa weak stress recovery of the composites was shown. The analysis of crosslink densityindicated that the damage to the crosslink network was mainly due to the breakage ofionic crosslinks at low strain (100%). A more developed ionic crosslink network wasformed at a higher content of ZDMA. When the vulcanizate is subjected to loading intension, the ionic crosslink network will suffer the force first. Next, the slippage of ionicbonds will take place under the stress. A new ionic crosslink network might be formedrapidly after the ionic bonds were broken during the stretching. Therefore, it could notreturn to the initial state. The analysis of crosslink density and stress recovery indicatedthat the rubber chains could be adsorbed to the ZDMA aggregates due to the formationof poly-zinc methacrylate (PZDMA). A molecular analysis of NR/ZDMA composites isproposed in the last part of this article.

Keywords crosslink density, natural rubber, networks, stress-softening, stress-strainbehavior, zinc methacrylate

Introduction

Metal salts of unsaturated carboxylic acids have usually been used as coagents in peroxide-cured rubbers due to the enhancement of both the crosslinking reaction in the vulcanizationprocess and the crosslink density.[1] Recently, some of these metal salts of unsaturatedcarboxylic acids, such as zinc methacrylate (ZDMA), have been used as new reinforcingagents for rubbers. Rubbers reinforced with this kind of reinforcing agent have high tensilestrength, tear strength, and modulus, while still retaining a high elongation at break. Metalsalts of unsaturated carboxylic acids can be added into rubbers directly or prepared in situthrough the neutralization of metal oxides and acids.[2,3] It has been reported that manykinds of rubbers can be reinforced by metal salts of methacrylic acids (MAA), such as

Received 16 June 2011; accepted 8 September 2011.Address correspondence to Yukun Chen, School of Mechanical and Automotive Engineering,

South China University of Technology, Guangzhou, 510640, China. E-mail: [email protected]

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Stress-Strain and Networks Studies of NR/ZDMA 1385

natural rubber (NR), butadiene rubber (BR), ethylene-propylene-diene rubber (EPDM),nitrile-butadiene rubber (NBR), and poly (α-octylene-co-ethylene) elastomer (POE).[4–11]

When a peroxide is used as a curing agent, the metal salts of unsaturated carboxylicacids will be polymerized simultaneously during rubber vulcanization no matter whetherin homo-polymerization or graft-polymerization. Ionic bond crosslinks are formed dueto the graft-polymerization of metal salts onto the rubber chains, while the poly-(metalsalt) chains separate from the rubber matrix and aggregate into nano-scaled fine particles,forming salt crosslinks.[12–14] The fine particles, with diameters of 20–40 nm, were foundto disperse in the rubber matrix.[2,11,14] This structure, originating from the polymerizationof metal salts in rubber, is the reason for the reinforcement.[12] However, there has not beena clear and well-accepted explanation of the mechanism and microstructure model for thereinforcement of rubbers filled with metal salts of unsaturated carboxylic acids to date.

Tensile stress-strain measurements have been one of the useful methods for charac-terizing rubber vulcanizates.[15] The tensile stress at a certain strain is usually used forcharacterizing the properties of the vulcanizates and interpreting the network structureand reinforcement mechanisms.[16] Numerous constitutive models have been developed todescribe the mechanical behavior of stretched rubber. Possible sources of damage understress included debonding and recreation of crosslinks, chain residual stretch, and chainbreakage.[17–29] However, these descriptions cannot determine the network deformation ofrubber reinforced by metal salts of unsaturated carboxylic acids when it is stretched, andthis indetermination limits further research on the reinforcing mechanism and the rela-tionship between structure and properties of the vulcanizates. It is necessary to study thenetwork deformation of rubber reinforced by metal salts of unsaturated carboxylic acidsduring uniaxial tension on the microstructure level.

In this article, we focus on an NR/ZDMA system and analyze the deformation andbreakage of networks (containing covalent crosslinks and ionic crosslinks) in differentstretching cycles at different strains by using equilibrium swelling. In this system, the exis-tence of the two types of crosslinks results in a complex microstructure of the composite. Itis considered that the reinforcement of the ZDMA–rubber is affected by various structuralfactors, that is, unsaturation, polarity, formation of a nano-dispersion, ionic crosslinkednetwork, etc. One of the most important factors among them is the crosslinked network.Thus, this article mainly deals with the crosslinked network of the vulcanizate during uni-axial stretching at room temperature. Investigations of the evolution of the network duringvulcanization of the NR/ZDMA system and other influencing factors for reinforcement arein progress.

Experiments and Methods

Materials and Sample Preparation

NR (Malaysia 1#) was provided by Guangzhou Rubber Industry Research Institute (China).MAA, purchased from Guangzhou Xin’gang Chemical Factory (China), was purified bydistillation under nitrogen at reduced pressure. Zinc oxide (ZnO) was purchased fromTianji Yaohua Chem. Co., Ltd. (Guangzhou, China). Dicumyl peroxide (DCP), purchasedfrom Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), was purified by anhydrousalcohol recrystallization before use.

In this article, NR was reinforced by in-situ prepared ZDMA. Rubber compounds wereprepared in a two-roller mill. ZnO and MAA were added into the NR and mixed for severalminutes, then DCP was added. Theoretically 1 mol ZnO and 2 moles MAA will react to

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1386 Y. Chen and C. Xu

form ZDMA and water completely. Equivalent amounts of ZnO and MAA (ZnO/MAAmolar ratio 0.5) were used to react in the NR matrix during mixing. A high degree ofconversion to ZDMA from the in-situ reaction of ZnO and MAA had been verified bymany articles (e.g., refs. 11, 13). Here, the neutralization was assumed to have occurredcompletely to form ZDMA. The compounds containing 100 weight parts of NR, 1.5 phr(parts per 100 parts of rubber) DCP, and 0–40 phr ZDMA were sheeted on a two-roll milland then press cured to 2-mm-thick sheets at 155◦C.

Tensile Stress-Strain Characteristics

Stress-strain characteristics were obtained by uniaxial tension. Samples for the tensiletests were cut from the pressed films with a thickness of about 2 mm in a dumbbellshape (6-mm-wide cross section). Tensile tests were carried out at room temperature usinga Computerized Tensile Strength Tester (UT-2080) produced by U-CAN Dynatex Inc.(Nantou, Taiwan) with a crosshead speed of 500 mm/min.

The stretching force (F) value was converted to engineering stress (σ ) using the relationσ = F(λ)/A0. A0 is the cross section of the sample before stretching, λ defines the principalstretch ratio by λ = L/L0 where L0 and L are the sample lengths before and after stretching,respectively. Therefore, the strain ε is defined as ε = (L − L0)/L0.

The specimen was stretched to a fixed strain and then the stress was retraced to zero,then the operation was repeated again. Because the third and fourth stretch curve almostoverlapped each other, the third stretch curve is defined as the equilibrium stretch curve.The recovery stretch curve is defined as the stress-strain curve obtained after the specimenhad rested after the equilibrium stretch for 24 h at room temperature to ensure full recovery.

Crosslink Density Measurement

The apparent crosslink density was determined by equilibrium swelling experiments. Todetermine the crosslink density at different elongations, the dumbbell-shaped specimenswere stretched to 100%, 200%, 300%, and failure elongation. Then, the middle section ofthe stretched samples was chosen for the equilibrium swelling experiment. To determine thecrosslink density for different stretch cycles, the dumbbell-shaped specimens were stretchedseveral times to a fixed strain in the same way as described before. Then, again, the middlesection of the stretched samples was chosen for the equilibrium swelling experiments.

To calculate the crosslink density of the rubber, five weighed test pieces of rubber wereimmersed in toluene at about 23◦C for a period of 72 h. Then, the samples were blotted withtissue paper to remove excess solvent and immediately weighed on an analytical balance.Finally, the samples were dried in a vacuum oven for 48 h at 60◦C until constant weight.The volume fraction of swollen rubber in the gel, V r, which was used to represent thecrosslink density of the samples, was determined by the following equation:

Vr = m0ϕ(1 − α)ρ−1r

m0ϕ(1 − α)ρ−1r + (m1 − m2)ρ−1

s(1)

where m0 is the sample mass before swelling; m1 and m2 are the swelled sample massesbefore and after drying, respectively; ϕ is the mass fraction of NR rubber in the unswollenvulcanizates; α is the mass loss of the vulcanizates after swelling; and ρr and ρs are therubber and toluene density (ρs = 0.865 g/cm3), respectively.

To distinguish ionic crosslinks from covalent crosslinks, the above samples wereswollen in a mixture of toluene and chloroacetic acid once again for 120 h to destroy the

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ionic crosslinks, followed by swelling in toluene for 72 h and weighed, then vacuum driedand reweighed. Finally, V r1 was calculated from Eq. (1), which represents the covalentcrosslink density. V r2, which is calculated by subtracting V r1 from V r, was used to representthe ionic crosslink density.[14,20–23] In the discussion, the crosslink density of a rubber witha given ZDMA content was the average over five test pieces from one sample.

Results and Discussion

Stress-Strain Behavior

Stress-strain curves of NR reinforced by different amounts of ZDMA stretched to rupturewere obtained with standard dumbbell-shaped samples. Maximum elongation and strengthat rupture can be evaluated from these curves. In addition, the samples had sufficient piecesafter stretching to be studied by the equilibrium swelling experiments. NR gum processedsimilarly but without the ZDMA, which was used as a reference sample. In Fig. 1, the stress-strain curves with low ZDMA concentration (see 10 and 20 phr) display low tensile stressat small deformations of the rubbers, followed by an abrupt increase at high deformations.The tensile strength of the NR gum was surprisingly high, with the stress at rupture, σ max,of 7–8 MPa. In the presence of ZDMA, the magnitude of the stress increased rapidly withZDMA concentration, which we attribute to a great reinforcement by the in-situ ZDMA.The maximum failure stress occurred for the highest loading of ZDMA (40 phr) used,about 26 MPa. However, the strain at break decreased as the loading of ZDMA increased.Regardless, the elongations were maintained at relatively large values.

Stress-Softening Behavior

Figure 2 shows the various stress-strain curves to 100% strain of the ZDMA-reinforcedNR. The most important information which can be obtained from Fig. 2 is the presence ofthree main regimes for 100% strain:

Figure 1. Stress-strain curves to the breaking strain of the ZDMA-reinforced NR.

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Figure 2. Stress-strain curves at 100% strain of the ZDMA-reinforced NR (color figure availableonline).

• With ZDMA content in the range from 0 to 10 phr, the samples showed very weakstress-softening behavior in which all four curves essentially superimposed. Thestress-strain behavior of the 10 phr ZDMA sample was similar to the NR gumsample.

• With 20 phr ZDMA, stress-softening behavior was able to be distinguished. However,the second stretch, the equilibrium stretch, and the recovery stretch curves seemedto overlap each other.

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• With 30 and 40 phr ZDMA, the different curves can be distinguished clearly. Therecovery stretch curve is close to the second stretch curve rather than the initialstretch curve, showing a weak recovery.

From Fig. 3, two main regimes can be seen with 200% strain imposed:

• For the NR gum, the stress-softening behavior was very weak. The stress-straincurves seem to overlap with each other.

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• With ZDMA content from 10 to 40 phr, the different stretch curves can be distin-guished clearly. The recovery stretch curve was again much close to the secondstretch curve than the first, showing a weak recovery.

At 300% strain imposed, additional important information can be obtained, as shownin Fig. 4:

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• For the NR gum, the second stretch and equilibrium stretch curves can be distin-guished to a small degree, but the recovery stretch curve and the initial stretch curvewere superimposed, showing a good recovery.

• From 10 to 40 phr ZDMA, the different curves can again be distinguished clearly.In these cases, the second stretch, equilibrium stretch, and recovery stretch curveswere very close to each other when the strain did not exceed 200%. However, theyseparated from each other when the strain was between 200% and 300%, and thestress of the recovery stretch curve showed a more abrupt increase in this strainrange than the second and equilibrium stretch curves.

The stress-softening effect, often discussed in connection with filler reinforcement andreflecting a kind of viscous loss, can be used to characterize the rubber–filler and filler–fillerinteractions.[24] Figures 2–4 show stress-softening behaviors of the vulcanizates at 100%strain, 200% strain, and 300% strain, respectively. It can be seen from these figures that theNR gum exhibits a weak stress-softening effect, even at 300% strain. The stretch curves ofNR gum almost overlap for both 100% strain (see Fig. 2[a]) and 200% strain (see Fig. 3[a]),and the stretch curves for 300% strain were also very close to each other (see Fig. 4[a]).These results indicate that the uniaxial tension had little influence on the crosslink networkof the NR gum. The stress increased quickly with increasing content of ZDMA for anyfixed strain, and the composites showed a high stress-softening effect with high content ofZDMA. From the stress-strain characteristics of the filled samples for 100%, 200%, and300% maximum strain, similar behaviors were observed in that the stress for any fixed strainin the second stretch decreased in comparison to the first stretch, and the equilibrium stretchwas slightly lower than the second stretch. Here, a special behavior should be pointed out,that is, the recovery stretch curves (recovery for 24 h at room temperature) were close tothe second stretch curves rather than the first, which shows a poor recovery of the filledsamples. This behavior is different from that of the NR filled with conventional reinforcingfillers, such as carbon black, silica, or clay.[30] A general stress-softening recovery behavior,which can be observed in rubber/conventional-filler systems, is that the recovery stretchcurve is close to the first stretch curve, showing very good stress recovery.[26,27]

The stress-softening effect is related to the destruction and reconstruction of crosslink-ing bonds, local orientation of rubber molecular chains, and interactions between filler andrubber chains.[24] In the case of a conventional reinforcing filler, such as carbon black, aset of NR chains between two aggregates will be bound to their active sites through theadsorption effect. These NR chains generally represent parts of longer NR chains. In thecourse of stretching, the NR chains slide on or debond from the aggregates’ surface. Thisdebonding starts with the shortest interconnecting chain and gradually involves longer andlonger chains. The strength of monomer bonds within the NR chains is far higher thanthat of NR–filler bonds. Thus, at a fixed strain, the NR chains would not break but ratherdebond from their bonding sites on the aggregate surface.[28,29] Such interactions could berebuilt after a rest period at room temperature. The recovery of stress-softening has alsobeen observed through the recovery of the permanent set, the return to initial values of thestress at a fixed strain, or of the complete stress-strain response at a high temperature. Forinstance, Mullins[30] studied the stress recovery of a filled NR. He measured the stress atan elongation of 200% after recovery at 100◦C and compared it to the stress measured onthe material stretched to the same elongation for the first time. The material recovered 80%of the softening after only 2 days. Similarly, Laraba-Abbes et al.[31] showed a completerecovery of the softening of a carbon-black filled NR exposed to 95◦C for 48 h. Thesestudies indicate that the physical adsorption of rubber chains on reinforcing filler particles

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is dynamic and can be rebuilt after a long time of relaxation or upon heat treatment. Fol-lowing unloading, the debonded chains reattached back to the aggregates surface to rebuilda new network similar to the original. Therefore, in the recovery stretch experiments, thetension stress increased and was close to the first tension stress, showing a good recovery.

However, in the situation of ZDMA as fillers, as indicated above, the peroxides ini-tiated the polymerization of ZDMA (both homo-polymerization and graft-polymerizationon the NR).[12,13] As a result, there were three types of ZDMA components in the vulcan-izate: homo-PZDMA (poly-zinc methacrylate), P(NR-g-ZDMA), and residual monomericZDMA. Thus, the vulcanizates contained both covalent crosslinks and ionic crosslinks,and the slippage of ionic bonds occurred under the effect of stress. As a result, the stress-softening mechanism of ZDMA differs from that of conventional reinforcing filler such ascarbon black. The deformation mechanism of the crosslinked network of NR vulcanizatereinforced by ZDMA in the uniaxial tension will be discussed in the crosslink densitysection below.

Deformation of Crosslinked Network During Stretching

The stress-strain behavior indicates a remarkable reinforcing effect of in-situ formed ZDMAin NR and a particular stress-softening behavior. The reinforcing mechanism of ZDMA isalso different from that of conventional fillers. It is thus essential to analyze the deformationof the crosslinked network of the samples in the range of elongation investigated.

Figure 5 shows the crosslink density of the vulcanizates with different amounts ofZDMA. The gross crosslink density (V r) and the ionic crosslink density (V r2) increasewith increasing ZDMA content. On the contrary, the covalent crosslink density (V r1) showsa decrease. As shown in Figs. 1–3, the stress for any given strain increased rapidly withZDMA concentration, which is attributed to the increasing ionic crosslink density. The ioniccrosslinks are also considered as a main reason for the reinforcement of the composites.[32]

As shown in Fig. 6, the V r, V r1, and V r2 curves decreased with increasing strain.All three types of crosslink densities reached a minimum at the failure elongation. Thesignificant variation in the crosslink density indicates that damage of the micro-structure

Figure 5. Effect of ZDMA content on crosslink density of NR/ZDMA vulcanizates.

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Figure 6. Crosslink density of vulcanizates at different strains (color figure available online).

occurred when the samples were stretched. As shown in Fig. 6(a), the V r values for 100%strain were close to the un-stretched one when the ZDMA content did not exceeded 20phr, whereas the V r values for larger strains were lower than that of the un-stretched one(10 and 20 phr). This indicated that the effects of low strain (100%) on the damage of thenetworks of low content ZDMA composites (10 and 20 phr) were limited, but the higherstrains (>100%) did destroy the networks of the composites. As seen from Fig. 6(b), theV r1 values for 100% strain were also close to the un-stretched one when the ZDMA contentexceeded 20 phr, whereas the V r curve for 100% strain was lower than the un-stretchedcurve in Fig. 6(a). Thus the changes of V r (30 and 40 phr) for 100% strain were attributedto the decreases of the V r2 which was shown in Fig. 6(c). This indicates that the damage ofthe crosslink network of high content ZDMA composites at low strain (100%) was mainlydue to the breakage of the ionic crosslink network. This could be explained that the highcontent of ZDMA could form a more developed ionic network which was more sensitiveto the strains.

As shown in Fig. 6(c), the values of V r2 at a low concentration of ZDMA (10 phr) werealmost superimposed. Thus the changes of V r (see Fig. 6[a], strain>100%) were attributedto the V r1 (see Fig. 6[b]). The no changes of V r2 could be explained that the low content ofZDMA (10 phr) could only form a small quantity of ionic crosslinks (see Fig. 5) rather thana developed ionic crosslinked network. Thus, the strains had hardly any influence on theV r2 of 10 phr ZDMA/NR composite. We have mentioned in the introduction that the in-situpolymerization of ZDMA results in the formation of a nano-dispersion during peroxidecuring.[7] It is also well known that the strength of the covalent bonds of NR chains is farhigher than that of ionic bonds. Thus, we suggest that an adsorption effect, as with carbon

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black, existed in the peroxide-cured ZDMA/NR composite. In Figs. 2(b) and 3(b), thestress-strain behaviors for 10 phr ZDMA were similar to the NR gum, which is evidenceof the adsorption effect to some extent. Therefore, the observed V r1 is not a pure covalentcrosslink density but a multiple type crosslink density containing covalent crosslinks andphysical adsorption crosslinks (there might also be some other type of physical crosslinksthan adsorption on the PZDMA which will be further studied, e.g., rubber entanglement).

Crosslink Density of Different Stretching Stages

To investigate the dependence of the changes of crosslink densities on the strain amplitude,and to see whether the ionic crosslink densities recovered after 24 h, crosslink densities fordifferent stretching stages were evaluated, with the initial crosslink densities being used asa standard reference.

Figure 7 shows the (a) gross crosslink density, (b) covalent crosslink density, and(c) ionic crosslink density of different stretching cycles at 100% strains depending on theZDMA content. The decreases of V r after different stretching cycles (see Fig. 7[a]) aremainly attributed to the damage of V r1 at low content of ZDMA (see Fig. 7[b], such as10 and 20 phr). As long as the content of ZDMA was in excess of 20 phr, V r1 remainednearly constant and was not affected much by different numbers of stretching cycles. Forhigh ZDMA content, V r2 played the main role in the network breakage (see Fig. 7[c]).Considering Fig. 5, V r2 grew quickly, while the V r1 maintained a slow decline with increas-ing ZDMA. And as seen in Figs. 2(d) and (e), the second stretch curve was far below thefirst curve when the content of ZDMA was in excess of 20 phr. All these results indicate that

Figure 7. Crosslink density for different stretch cycles at 100% strain (color figure available online).

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a more developed ionic crosslink network may be formed at a higher content of ZDMA.When the vulcanizate is subjected to a loading in tension, the ionic crosslink network willsuffer the force first, resulting in slippage of the ionic bonds under the stress.

A similar trend as in Fig. 7 can be found in Fig. 8 with 200% strains. Compared with100% strain in Fig. 7, the crosslink network was stretched to a larger degree, to 200% strainhere. In this case, ionic crosslinks were also deformed even at low content of ZDMA (seeFig. 8[c]). The covalent crosslinks were also inevitably destroyed. But at higher contents ofZDMA, the effects on the covalent crosslinks became smaller and smaller (see Fig. 8[b]).What should be mentioned here is that, after recovery for 24 h at room temperature, V r

showed a recovery to a certain degree which was mainly attributed to the recovery of V r1

(V r2 recovered to a very limited extent, see Figs. 8[b] and [c]). This result is consistentwith the tensile stress-strain behavior. As seen in Fig. 3, different stretch curves can bedistinguished clearly even at 10 phr ZDMA, and the recovery stretch showed a weakrecovery, being close to the second stretch.

As shown in Fig. 9 with a large strain of 300%, the covalent crosslinks and ioniccrosslinks were both destroyed by stretching, and the breakage of the ionic crosslinks wasvery significant at high content of ZDMA. Comparing Fig. 7 to Fig. 9 and using the curvesbefore stretch as a reference curve, the greater the content of ZDMA or the higher thestrain, the larger the degree of reduction of V r and V r2 that results. The crosslink densitiesof the second stretch and the equilibrium stretch are consistent with the tensile stress-strainbehaviors. The recovery of V r after 24 h is mainly attributed to the recovery of V r1, V r2

maintains at a relatively stable state after being stretched. When a sample was stretched,some bonds (such as physical adsorption) debonded, as in the case of carbon black as

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mentioned before, but lots of ionic bonds slipped and broke, especially at a high content ofZDMA. At the same time, a new ionic crosslink network was formed. Once the stress wasrelieved, the slippage of ionic bonds ceased, and thus the stress recovery was very low.[33]

Crosslink Density at Different Strains

Figure 10 shows the effect of strain on the crosslink density of the NR/ZDMA vulcanizates.This gives more intuitive information of the internal network of the vulcanizate followingstretching. The NR gum vulcanizate only has covalent crosslink density and it is almost ahorizontal line (see Fig. 10[b]). This means that there is almost no damage to the crosslinknetwork of NR gum in the stretching. Note that in Figs. 1(a), 2(a), and 3(a), the stress-strainbehaviors also showed no damage to the crosslink network of NR gum. V r1 decreasedsignificantly with the addition of ZDMA and drops with increasing strain, and reached itslowest value at the breaking strain (see Fig. 10[b]). As shown in Fig. 10(c), V r2 exhibitsa weak declining trend for 10–30 phr ZDMA with increasing strain; however, the 40 phrZDMA, the highest content in this particular experiment, shows an apparent decrease. Thiscan be interpreted that the slippage of the ionic bonds in stretching of the low contentZDMA rubbers led to a rapid destruction–reconstruction of the ionic crosslink networks,resulting in a relatively stable ionic crosslink network. A more developed ionic crosslinknetwork was formed initially at a high loading of ZDMA (40 phr), showing more sensitivityto strain.

Even considering the possible uncertainties in these measurements, it is clear that theformation of the ionic crosslink network in a filled rubber with high content of ZDMA can

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Figure 10. Effect of strain on crosslink density of NR/ZDMA vulcanizates.

be considered as one of the main reasons for the remarkable reinforcement, which is alsosupported by the stress-strain behaviors.

Micro-Structural Model for ZDMA/NR Composites

In our studies, poly-ZDMA was assumed to form the aggregates of diameter 20–40 nm,which have been observed by others in rubber matrices.[2,7,11,34] At the micro-level, multi-plet clusters were formed by the aggregates consisting of ion pairs because of the strongelectrostatic interaction between ion pairs in poly-ZDMA molecules.[35] Therefore, these

Figure 11. Microstructure model of vulcanizates: left, before stretching; right, after stretching.

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Figure 12. Microstructure of the change of ionic crosslink: left, before stretching; right, afterstretching.

multiplet clusters restrict the mobility of adjacent polymer chains (NR and poly-ZDMA),and some of the NR chains near poly-ZDMA (mainly the NR-g-PZDMA) can also berestricted by the adjacent multiplet clusters. The multiplet clusters formed by the graftedpolymer and homopolymer will act as a new kind of crosslink—ionic crosslinks. Simul-taneously, some NR chains are adsorbed to the aggregates to form adsorption bonds byVan Der Waals force, which contribute to the crosslinking. Thus, comparing the crosslinkdensity of the stretched networks and the stress-strain behaviors, stress-softening can beattributed to both the adsorption bonds’ rupture or slippage along the aggregates’ surfaceand ionic bonds’ slippage when the chains reached their limit of extensibility.

On the basis of our study of stress-strain behaviors and crosslink densities, we proposea microstructure model of NR/ZDMA composites and the PZDMA phase. Figure 11 showsschematics of the network of NR/ZDMA vulcanizates containing covalent crosslinks andionic crosslinks (before and after stretching). The adsorption before stretching and de-adsorption (loss of adsorption points) by stretching are used to explain the recovery ofstress-strain behavior and the recovery of V r1. Figure 12 shows the change of ionic crosslinkafter stretching. The ionic crosslinks after stretching does not decrease so much, but it alsodoes not recover to the initial arrangement, and thus does not contribute so much to thestress-strain properties.

Conclusions

The stress-strain behaviors of NR reinforced by ZDMA were studied in uniaxial stretching.Stress-strain curves of the composites stretched to rupture showed a large reinforcementby in-situ formed ZDMA. The composites showed a high stress-softening effect at highcontent of ZDMA and large strain. In addition, after the equilibrium stretch and consequent24-h rest at room temperature, the composites exhibited a poor stress recovery. This uniqueproperty was different from that of conventional reinforcing fillers, such as carbon black.

The crosslink densities indicated that the ionic crosslinks contributed greatly to themechanical properties. The present study showed that the so-called “V r1,” calculated fromthe swollen equilibrium, was not the pure covalent crosslink density but a multiple crosslinkdensity containing NR matrix covalent crosslinks and physical adsorption crosslinks. Afterrest for 24 h at room temperature, the recovery of V r to a certain degree was mainlydue to the recovery of “V r1.” The slippage of the ionic bonds in stretching led to a rapid

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destruction–reconstruction of the ionic crosslink networks. Thus, the ionic crosslinks afterstretching did not decrease so much, but also did not recover to the initial state and did notcontribute so much to the stress-strain properties. It is suggested that the stress-softeningof NR/ZDMA composite is mainly due to the breaking of the adsorption bonds and theslippage of the ionic bonds in those chains reaching their limit of extensibility.

References

1. Costin, R.; Nagel, W.; Ekwall, R. New metallic coagents for curing elastomers. Rubber Chem.Technol. 1991, 64, 152.

2. Yin, D.H.; Zhang, Y.; Peng, Z.L.; Zhang, Y.X. A comparison between the SBR vulcanizatesreinforced by magnesium methacrylate added directly or prepared in situ. Eur. Polym. J. 2003,39, 99.

3. Yuan, X.H.; Peng, Z.L.; Zhang, Y.; Zhang, Y.X. In situ preparation of zinc salts of unsaturatedcarboxylic acids to reinforce NBR. J. Appl. Polym. Sci. 2000, 77, 2740.

4. Sato, K. Ionic crosslinking of carboxylated SBR. Rubber Chem. Technol. 1983, 56, 943.5. Ikeda, T.; Yamada, B. Simulation of the in situ copolymerization of zinc methacrylate and 2-

(N-ethylperfluorooctanesulphonamido) ethyl acrylate in hydrogenated nitrile-butadiene rubber.Polym. Int. 1999, 48, 367.

6. Medalia, A.I.; Alesi, A.L.; Mead, J.L. Pattern abrasion and other mechanism of wear of tanktrack pads. Rubber Chem. Technol. 1992, 65, 154.

7. Lu, Y.L.; Liu, L.; Yang, C.; Tian, M.; Zhang, L.Q. The morphology of zinc dimethacrylatereinforced elastomers investigated by SEM and TEM. Eur. Polym. J. 2005, 41, 577.

8. Lu, Y.L.; Liu, L.; Tian, M.; Geng, H.P.; Zhang, L.Q. Study on mechanical properties of elastomersreinforced by zinc dimethacrylate. Eur. Polym. J. 2005, 41, 589.

9. Peng, Z.L.; Liang, X.; Zhang, Y.X.; Zhang, Y. Reinforcement of EPDM by in situ prepared zincdimethacrylate. J. Appl. Polym. Sci. 2002, 84, 1339.

10. Rossi, E.F.; Martelli, A.F. The vulcanization of rubber with unsaturated salts: A prelimenary,smal-angle X-ray scatering investigation. Eur. Polym. J. 1972, 8, 351.

11. Lu, Y.L.; Liu, L.; Shen, D.Y.; Yang, C.; Zhang, L.Q. Infrared study on in situ polymerization ofzinc dimethacrylate in poly(α-octylene-co-ethylene) elastomer. Polym. Int. 2004, 53, 802.

12. Yuan, X.H.; Peng, Z.L.; Zhang, Y.; Zhang, Y.X. The properties and structure of peroxide-curedNBR containing magnesium methacrylate. Polym. Polym. Comp. 1999, 7, 431.

13. Dontsov, A.; Candia, F.D.; Amelino, L. Elastic properties and structure of polybutadiene vulcan-ized with magnesium methacrylate. J. Appl. Polym. Sci. 1972, 16, 505.

14. Nie, Y.J.; Huang, G.S.; Qu, L.L.; Zhang, P.; Weng, G.S.; Wu, J.R. Cure kinetics and morphologyof natural rubber reinforced by the in situ polymerization of zinc dimethacrylate. J. Appl. Polym.Sci. 2010, 115, 99.

15. Diani, J.; Fayolle, B.; Gilormini, P. A review on the Mullins effect. Eur. Polym. J. 2009, 45, 601.16. Nakajima, N.; Yamaguchi, Y. Effect of fillers and rubber structures on tensile behavior of filled,

unvulcanized compounds of cis-1,4-polybutadienes. J. Appl. Polym. Sci. 1997, 66, 1445.17. Arruda, E.M.; Boyce, M.C. A three-dimensional constitutive model for the large stretch behavior

of rubber elastic materials. J. Mech. Phy. Solids. 1993, 41, 389.18. Marckmann, G.; Verron, E.; Gornet, L.; Chagnon, G.; Charrier, P.; Fort, P. A theory of network

alteration for the Mullins effect. J. Mech. Phy. Solids. 2002, 50, 2011.19. Chagnon, G.; Verron, E.; Marckmann, G. Development of new constitutive equations for the

Mullins effect in rubber using the network alteration theory. Int. J .Solids. Struct. 2006, 43, 6817.20. Du, A.H.; Peng, Z.L.; Zhang, Y.; Zhang, Y.X. Effect of magnesium methacrylate on the mechan-

ical properties of EVM vulcanizate. Polym. Test. 2002, 21, 889.21. Gao, G.X.; Zhang, Z.C.; Zheng, Y.S.; Jin, Z.H. Effect of magnesium methacrylate and zinc

methacrylate on bond properties of thermal insulation material based on NBR/EPDM blends. J.Appl. Polym. Sci. 2009, 113, 3901.

Dow

nloa

ded

by [

Yuk

un C

hen]

at 0

8:14

16

May

201

2


22. Yin, D.H.; Zhang, Y.; Zhang, Y.X.; Peng, Z.L.; Fan, Y.Z.; Sun, K. Reinforcement of peroxide-cured styrene-butadiene rubber vulcanizates by mathacrylic acid and magnesium oxide. J. Appl.Polym. Sci. 2002, 85, 2667.

23. Yin, D.H.; Zhang, Y.; Peng, Z.L.; Zhang, Y.X. Effect of fillers and additives on the properties ofSBR vulcanizates. J. Appl. Polym. Sci. 2003, 88, 775.

24. Mullins, L. Softening of rubber by deformation. Rubber Chem. Technol. 1969, 42, 339.25. Wan, C.Y.; Dong, W.; Zhang, Y.X.; Zhang, Y. Intercalation process and rubber–filler interactions

of polybutadiene rubber/organoclay nanocomposites. J. Appl. Polym. Sci. 2008, 107, 650.26. Yan, L.; Dillard, D.A.; West, R.L.; Lower, L.D. Mullins effect recovery of a nanoparticle-filled

polymer. J. Polym. Sci. Part. B: Polym. Phy. 2010, 48, 2207.27. Ciambella, J.; Paolone, A.; Vidoli, S. A comparison of nonlinear integral-based viscoelastic

models through compression tests on filled rubber. Mech. Mater. 2010, 42, 932.28. Roozbeh, D.; Mikhail, I. A network evolution model for the anisotropic Mullins effect in carbon

black filled rubbers. Int. J. Solids. Stru. 2009, 46, 2967.29. Brieu, M.; Diani, J.; Mignot, C.; Moriceau, C. Response of a carbon-black filled SBR under large

strain cyclic uniaxial tension. Int. J. Fatigue. 2010, 32, 1921.30. Mullins, L. Effect of stretching on the properties of rubber. J. Rubber Res. 1948, 16, 275.31. Laraba-Abbes, F.; Ienny, P.; Piques, R. A new “Taylor-made” methodology for the mechan-

ical behaviour analysis of rubber-like materials: II. Application to the hyperelastic behaviourcharacterization of a carbon-black filled natural rubber vulcanizate. Polymer 2003, 44, 821.

32. Yuan, X.H.; Zhang, Y.; Peng, Z.L.; Zhang, Y.X. In situ preparation of magnesium methacrylateto reinforce NBR. J. Appl. Polym. Sci. 2002, 84, 1403.

33. Du, A.H.; Peng, Z.L.; Zhang, Y.; Zhang, Y.X. Properties of EVM vulcanizates reinforced byin-situ prepared sodium methacrylate. J. Appl. Polym. Sci. 2003, 89, 2192.

34. Du, A.H.; Peng, Z.L.; Zhang, Y.; Zhang, Y.X. Fracture morphology and mechanical properties ofethylene/vinyl acetate rubber vulcanizates reinforced by in situ prepared sodium methacrylate.J. Polym. Sci. Part B: Polym. Phy. 2004, 42, 1715.

35. Eisenberg, A.; Hird, B.; Moore, R.B. A new muliplet-cluster model for the morphology of randomionomers. Macromolecules 1990, 23, 4098.

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Documents

Stress-Strain Behaviors and Crosslinked Networks Studies of Natural Rubber-Zinc Dimethacrylate Composites