Composites: Part B 60 (2014) 561–567

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Composites: Part B

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Investigation of dynamic characteristics of nano-size calcium carbonateadded in natural rubber vulcanizate

1359-8368/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2014.01.010

⇑ Corresponding authors. Addresses: School of Materials Science and Engineering,Shenyang University of Chemical Technology, Shenyang Economical and Techno-logical Development Zone, Street No. 11, Shenyang 110142, China. Tel.: +86 24893881 53 (Q. Fang). Department of Chemical Engineering, Faculty of Engineering andScience, Universiti Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak,Kuala Lumpur, Malaysia. Tel.: +60 3 4107 9802 (L.T. Sin).

E-mail addresses: fqh80@126.com (Q. Fang), leets@utar.edu.my (L.T. Sin).

Qinghong Fang a,b,⇑, Bo Song a, Tiam-Ting Tee c, Lee Tin Sin c,⇑, David Hui d, Soo-Tueen Bee c

a School of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang Economical and Technological Development Zone, Street No. 11,Shenyang 110142, Chinab State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, Chinac Department of Chemical Engineering, Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Genting Kelang, 53300 Setapak, Kuala Lumpur, Malaysiad Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 October 2013Received in revised form 17 December 2013Accepted 3 January 2014Available online 9 January 2014

Keywords:A. HybridB. Mechanical propertiesCure: nano-size calcium carbonate

The nano-calcium carbonates (NCC) with spherical and chain polymorphs and 30 nm, 50 nm, and 80 nmsizes of cube shape particle have been used to prepare nano-calcium carbonate (nano-CaCO3)/naturalrubber (NR) nano-composite. The influence of NCC on the properties of rubber vulcanizates such as Mul-lins effect, Payne-effect, loss factor and the dynamic compressed heat generation on the structure ofnano-composite were investigated. The results showed that the Mullins effect of rubber composite filledchain shape NCC was high and it was comparable to the large particle size (80 nm) of cubic NCC. For theanalysis of Payne effect, the value of DG0 of rubber composite filled with spherical shape has the lowestvalue due to weaker filler network resulted largest inter-aggregate distance occurred in the rubbermatrix. Meanwhile, the chain and large particle size cubic NCC have more significant DG0 with theincreasing of strain. The value of damping factor corresponds to energy loss showed that large particlesize NCC has more pronounced values. Both chain and 80 nm cubic NCC have highest rising of tempera-ture compared to spherical NCC added rubber composites.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Calcium carbonate (CaCO3) has been widely used as filler inplastics and rubber industry. It is produced from chalk, limestone,or marble found in upper layers of the earth’s crust. CaCO3 sourcefrom natural ground is the most common and cheapest used in theplastics and rubber industry. There is also exist of chemicallyproduced form of CaCO3 known as precipitated CaCO3 which is fi-ner and high purity, yet also more costly than the natural type. Themost widely reason of blending CaCO3 with polymer is to reducecost without scarifying the tensile strength significantly. Inaddition, CaCO3 can act as processing aids, toughener, improvedproductivity from a combination of high thermal conductivityand lower specific heat in comparison to the polymer materials rel-atively. According to Khanna and Xanthos [1] that all these benefitscan further be optimized with the selection of appropriate particle

size distribution and surface treatments with hydrophobic agentsuch as stearic acid, silane.

While nano-size CaCO3 (NCC) has been produced for 25 yearsago [2], the applications of NCC have gained great attention ofthe researchers in recent decade because of NCC particles can pro-duce higher modulus as well as increasing the impact strength inthe acrylonitrile–butadiene–styrene (ABS) system as compared tomicro-scale CaCO3 [3]. Manroshan and Baharin [4] observed thatacrylic dispersed NCC added in vulcanized latex showed modulusat 100% elongation and modulus at 300% elongation increased withNCC loading. At the mean time, tensile strength and elongation atbreak increased up to 10 phr of filler loading and then decreasedagain. Recently study conducted by He et al. [5] on the compres-sion properties of NCC/epoxy and its fiber composites revealed aremarkable improvement of 13.5%, 6.1%, 42.5% and 106.3% in com-pressive strength, elastic modulus, displacement and the totalfracture work of epoxy resin cast filled with 4 wt.% NCC contrastedto neat epoxy casts. It showed that the modified nano-CaCO3

particles had a strengthening and toughening effect. Also, Kumaret al. [6] conducted morphological analysis on nanocompositesfractured surfaces found that that the NCC stearic acid modifica-tion induced homogeneous and fine dispersion of nanoparticlesinto polymer as well as strong interfacial adhesion between thetwo phases. An increment in the Tg and storage modulus of the

Fig. 1. SEM images of NCC with (A) spherical shape, (B) chain shape and (C) cubeshape.

562 Q. Fang et al. / Composites: Part B 60 (2014) 561–567

resulting nanocomposites was observed with the increasing ofCaCO3 ratio. Moreover, thermogravimetric results showed a lowerdegradation temperature with the increase of CaCO3 ratio in thepolymer matrix.

In the rubber industry, NCC is also commonly used as the fillerfor acrylonitrile–butadiene rubber [8], styrene–butadiene rubber(SBR) [9], chloroprene rubber [10] and etc. Addition of NCC canproduce outstanding stiffness, toughness, and dimensional stabil-ity rubber compound. Nevertheless, the outstanding performanceof rubber compound by addition of NCC is still greatly dependingon the dispersion of its nano-particles in rubber matrix [7]. Hence,the depth understanding on the relationship between microstruc-ture and mechanical properties of NCC are essential to improve theend-use properties of rubber composite. Most of the elastomericcomponents in practical applications are deformed statically and

dynamically where specific dynamic properties characterizationsare crucially required. Commonly, the durability of elastomericcompounds was analyzed in accordance to the effect of strainamplitude on the dynamic modulus. The modulus of filled rubbersdecreases with increasing of applied dynamic strain up to interme-diate amplitudes. After adding the filler, the low strain modulus Go

rises more than the high strain modulus G1, resulting in a non-lin-ear viscoelastic behavior, which is known as Payne-effect Go–G1[11,12]. The Payne effect happens in rubber vulcanizates due tothe diminishing of filler–filler interactions or separation of polymerchains from filler surface when subjected to strain. Ramier et al.[13] reported the Payne effect of the styrene–butadiene-rubbervulcanizates can be reduced by silane treatment of the nano-sizesilica. On the other hand, the improvement of mechanical proper-ties, however, is always limited because NCC with high surfaceenergy tends to agglomerate. Such condition was observed by Quet al. [14] who compared the mechanical properties of bulk NCCand co-precipitated NCC in SBR vulcanizates. They found thatwhen the amount of co-precipitated NCC and bulk NCC is identical,the mechanical properties of the former can achieve tensilestrength of 13.38 MPa which was superior over the later. Thiswas due to the NCC in the former had better dispersion and inter-face bonding force than that in the later, which led to the bettermechanical properties. Zhang et al. [15] showed that the surfacemodified NCC also exhibited better processing capability than thatof carbon black. Subsequently, they suggested that the processabil-ity of carbon black filled rubber could be improved by thecombination of NCC.

This study is aiming to analyze the mechanical properties ofrubber nano-composites filled with the NCC in the context ofMullins effect, Payne effect, the loss factor tand and dynamic heatgeneration. In particular, the influence of specific surface area,polymorph, structure and different of particles size of NCC on theMullins effect and Payne-effect of natural rubber (NR) compositewere investigated. The strength of the filler network and the fil-ler–polymer interaction in the green compound and vulcanizatewere studied using a wide range of shear amplitudes performanceto correlate with the fracture mechanism [16].

2. Experimental

2.1. Materials

Natural rubber (NR) grade SCR 20 was supplied by Xi ShuangBan Na Tian Zheng Trade Co., Ltd., China. Nano-size calcium car-bonate (NCC) with cube shape with particle size 30 nm, 50 nm,80 nm, spherical shape, and chain shape were purchased fromHenankeli New Material Co., Ltd., China. Zinc oxide, stearic acid,sulfur, N-isopropyl-n0-phenyl-p-phenylenediamine (IPPD 4010),N-oxyoliechylene benzothiazole-2-sulfenamid (NOBS) were ob-tained from Rhein Chemie Rheinau GmbH, Germany. IPPD 4010is used as antioxidant and NOBS is used as curing accelerator. Allwere used as received.

2.2. Preparation of rubber nano-composites

The blends of rubber were prepared in accordance with the ba-sic combination of natural rubber (100 phr), zinc oxide (5 phr),stearic acid (2 phr), IPPD 4010 (2 phr), NOBS (0.75 phr) and sulfur(2.5 phr). Meanwhile, the amount of NCC was varied accordingly.All these ingredients were compounded using a two rolls mill ma-chine with the cooling water heat removal function. The preparedcompounds were moulded into sheets using a hydraulic press at150 �C and 10 MPa. All specimens were then cut into form oftesting sheets.

05

1015202530

Amount of CaCO3 (phr)

Tens

ile s

tren

gth

(MPa

)

Spherical shape Chain Cube 30 nm Cube 50 nm Cube 80 nm

400500600700800900

20 40 60 80 100

20 40 60 80 100

Amount of CaCO3 (phr)

Elon

gatio

n at

bre

ak

(%)

Spherical shape Chain Cube 30 nm Cube 50 nm Cube 80 nm

Fig. 2. Tensile strength and elongation at break of NR vulcanizates added with different polymorph and particle sizes of NCC.

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2.3. Dynamic mechanical analysis

Dynamic performance and Payne effects of NCC rubber vulca-nizates were analyzed using rubber process analyzer (RPA 2000,Alpha Technologies Akron, Ohio, United States) under temperature60 �C, frequency 1 Hz, strain range: 1–100%. Tensile and Mullinseffect of the nanocomposite were measured using an Instrontensometer with strain range: 1–300%. In cyclic strain tests, theshear modulus can be simply expressed as a complex modulus,G� ¼ G0 þ iG00 where G0 is the store energy modulus, G00 is the lossenergy modulus and i is the imaginary unit. The loss angle tangentis given by tan d ¼ G00=G0 [17]. Meanwhile, heat generation of thesample was tested by heat compression testing machine GT-RH-2000 (Gotech Testing Machines, Inc., Taiwan) at 1.0 MPa prestress,55 �C and 5.71 mm stroke. The morphologies of the NCC were in-spected on a field emission scanning electron microscope (ModelS-4800, Hitachi).

3. Results and discussion

3.1. Mechanical and morphologies analyses

Fig. 1 shows the SEM images of the spherical, chain, and cubeshapes of NCC. It can be noticed that the spherical shape of NCChas low aspect ratio compare to the others. As shown in Fig. 2regardless type of NCC, when amounts of NCC added in rubber vul-canizates increased, the tensile strength initially improved up to anoptimum level at 60–80 phr and followed by exhibiting inferiorityeffects. The poor dispersion of NCC caused inferior effect to thepolymer matrix. In other words, the inverse relationship is due tothe non-homogeneous distribution of NCC agglomerates thatcauses stress concentration on the NR matrix when subjected toextension [8]. The phase separation destroyed the continuity ofrubber matrix while further reducing the interfacial interactionof rubber matrix with NCC [18] which weakened the mechanicalperformance of the composite [19]. On the other hand, it was alsofound that the tensile strength of the spherical NCC added NRvulcanizates exhibited the highest tensile strength. This can beexplained that the spherical shape of NCC has larger surface areawhich can interact well with the polymer matrix. Although thestearic acid is added to react with zinc oxide to promote the cross-linking of natural rubber, the addition of excess stearic acid canalso act as the surface modifier to improve the hydrophobic

especially the large surface area of spherical nano-size particlesof NCC [20–22]. In addition, the mimic homogenous dispersion ofspherical NCC in rubber vulcanizates matrix as observed inFig. 3(A) also attributes to the superior tensile strength of sphericalNCC added NR vulcanizates. By referring to Fig. 3(A), the surfacemorphologies of spherical NCC added NR vulcanizates was ob-served to be smooth without the occurrence of NCC particlesagglomeration and voids. This also indicates the excellent interfa-cial adhesion between spherical NCC particles and NR matrixwhich could effectively transfer the stress from rubber matrix tospherical NCC particles during stretching, thus providing thereinforcing effect to NR matrix. Unlike the chain type NCC whichinitially believed to act like a fiber which can provide reinforce-ment effect to the rubber vulcanizates. The results showed thatchain type NCC has worse effect and possessed similar outcomeswith the 80 nm cube type NCC when added at high amount100 phr. This is due to the chains and cube NCC particles tend toagglomerate together into larger aggregates particles in rubbervulcanizates matrix as observed in Fig. 3(B and C). The agglomera-tion of NCC could reduce the effective interfacial adhesion betweenNCC particles and rubber vulcanizates matrix and cause the occur-rence of voids between the NCC aggregates and rubber matrix. Thiscould further cause the NCC aggregates to act as stress concentra-tor point when subjected to stretching and thus lowering thetensile strength of chain and cube NCC added NR vulcanizates. Thisindicates that NCC tends to work as filler in rubber vulcanizateseven though it has high aspect ratio. This can be also evidencedby analyzing different particles sizes of cube shape NCC. The80 nm cube NCC possessed generally lowest tensile strengthamong the three particles of cube shape NCC. The small size cubeNCC not only promotes dispersion in the rubber vulcanizates ma-trix, while the nano-sized can embedded well into the entangledchains of polymer matrix. This would produce superior externalforces transfer over the entire rubber composite results outstand-ing mechanical performance.

Meanwhile, analysis on the elongation shows that low loadinglevel of NCC (<60 phr) has better elongation at break comparedhigh loading level. This is because low quantities of NCC wouldnot cause disruption to the entangled polymer chains to slidefreely when subjected to extension. Besides, the phenomenoncan also due to the inter-aggregate distance becomes smaller withincreasing surface area of NCC and thus the probability of forminga network raises. Meanwhile, it can also be observed that the bothspherical and chain shape NCC possessed high elongation at break

Fig. 3. SEM images of rubber vulcanizate nanocomposite with NCC (A) sphericalshape, (B) chain shape and (C) cube shape.

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when added 60 phr below. This may be due to the low cross-sec-tional area of the spherical and chain shape NCC would induce alubricating like effect which promoted the flowability of the poly-mer chains [23]. Moreover, this can be further justified wherebythe large 80 nm cube shape NCC has lowest elongation at break.Such larger particle size of NCC would cause phase discontinuitysubsequently ruin the elongation of rubber vulcanizates. Impor-tantly, this effect becomes pronounced due to the hydrophilicsurface of NCC which reduces the interfacial interaction with rub-ber vulcanizate matrix. Nevertheless, such incompatible conditionhas been minimized with the blending of stearic acid transform theNCC surface with hydrophobic characteristic. Also, it is expectedthat sharp edge of cube NCC would exhibited low elongation atbreak compared to other shapes because shape edge tends to cause

more inter-molecular abrasion of rubber chains when extensionoccurs. This will cause unfavorable stress concentration withinthe polymer matrix which leads to earlier failure of sample whensubjected to external forces.

3.2. Mullins effect

When a rubber vulcanizate specimen is subjected to static cyc-lic loading, it will demonstrate non-linear elastic behavior as wellas damage-induced stress-softening phenomena which is knownas Mullins effect. The Mullins effect is the irreversible softeningof the stress–strain curve that occurs whenever the load of the rub-ber vulcanizate increases beyond its all-time maximum value. Assuch, Fig. 4 shows the Mullins effect rubber vulcanizates compos-ites with 60 phr NCC. It was found that the spherical shape NCCpossessed the smallest Mullins effect of rubber vulcanizate wherethe different between 1st time maximum loading (4 MPa) and 2ndtime maximum loading (3 MPa) is 1 MPa. This is mainly attributedto that the spherical NCC can easily slip between macromolecularchains in NR matrix. Therefore, it can decrease the stress softness.Besides, the good dispersion and low agglomeration of nanoparti-cle also contributed to the low Mullins effect of spherical NCC.Low agglomeration of filler is favorable in order to avoid disruptionof the polymer matrix continuity while enabling better interfacialinteraction of filler and polymer. Meanwhile, the rubber vulcaniz-ates filled with 80 nm cubic NCC experienced the most pronounceMullins effect with the different reaches 2 MPa. Indeed, such pro-nounce effect was also found with the chain shape NCC whichhas the Mullins effect almost comparable to the large particle size(80 nm) of cubic NCC. This can be explained where large particlesize NCC can hinder the mobility of macromolecular chains whenpulling. This will cause higher stress concentration at the localizedspot to cause chains breakage to occur easily [24]. Further investi-gation by comparing the 2nd and 3rd loading found that only cubicNCC with 80 nm particle size has most pronounce different(0.5 MPa) among the specimens. This is due to the large particleNCC lack of mobility requires subsequent re-orientation withinthe entangled macromolecule chains. Such substantial re-orienta-tion would still lead to formation of internal stress whereby thebreakage of chains expected to occur in minor causing loss ofmechanical strength. For specimens containing spherical andchains NCC still undergoing insignificant changes of stress after2nd loading. This is mainly due to viscoelastic behavior of disen-tanglement of macromolecules as well as minor debonding of poly-mer chains which caused insignificant loss of mechanicalperformance.

3.3. Payne effect

As seen in Fig. 5 shear modulus (G0) of the NR vulcanizate com-posites corresponding to different types of NCC gradually increasedwith the increasing of strain [12,25]. This phenomenon is causedby the destruction–reformation of filler–filler networks andadsorption–desorption of polymeric chains at the filler interfaceof the rubber vulcanizates [26]. The fact is that upon loading ofNCC, the inter-aggregate distances become smaller with rising offiller content, therefore the probability for the formation of a fillernetwork increases. Previous researchers [26] suggested that suchnetwork structure is due to the breakdown of carbon blacknetwork structure from van der Walls–London attractive forces be-tween carbon black particles. Meanwhile, there is also researcherbelieve that the Payne effect is due to the debonding process thattake place at the interface between the bulk and bound rubber[27]. Under large deformation, the rubber layers that cover the fil-ler will develop micro-voids which lead to lowering the modulus ofthe rubber vulcanizates.

Fig. 4. Mullins effects of rubber vulcanizate composites with 60 phr NCC (A) spherical, (B) chain, (C) cube 30 nm, (D) cube 50 nm and (E) cube 80 nm.

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It can be found that the rubber vulcanizates filled NCC exhibiteddifferent Payne-effect corresponds to the polymorph and particlessize of NCC. The value of DG0 of rubber composite filled with spher-ical shape has the lowest value due to weaker filler networkresulted largest inter-aggregate distance occurred in the rubbermatrix. In addition, the spherical shape NCC with high uniformitystructure also possesses capability to deform smoothly duringstraining. Although both the aggregating size and the inter-aggre-gate distance decreased with reduced primary particle diameter,the compatibility between nanoparticle and rubber are improvedsignificantly. Nanoparticle is covered by NR matrix firstly followedby the formation of aggregate together. Thus, the different of themodulus are insignificant even at high strain. Further analysisfound that the chain NCC has higher DG0 with the increasing ofstrain. This may be due to the initial high aspect ratio structureof chain NCC has experienced breakage under excessive deforma-tion. Hence, this has caused indirectly irreversible loss of rigidity

of the rubber vulcanizate. Besides that, by comparing the differentparticle size of cube shape NCC, the 30 nm cubic NCC exhibits thelowest Payne effect which corresponds to the lower filler networkand highest surface area. It is also interesting to find out that thelarge particle size of cubic NCC experienced more pronounced dropof DG0. Indeed, the 30 nm cubic NCC’s storage modulus G0 when thestrain exceeded 20% was the highest among the three particles sizeof cubic NCC. Such phenomenon can be described whereby thesubstantial strain can cause large inter-chain separation due tothe motion of large particle size in between the entangled polymerchains. Thus, the regularity of the macromolecule vulcanizate hasbeen affected leading to inferiority of rigidity. Moreover, the simi-lar tendency is displayed by the graph tand versus strain (Fig. 5).The smallest the inter-aggregate distance corresponds to the high-er probability for the formation of a filler network. Consequently,the extent of filler networking is more obvious when the surfacearea rises. It should be noted that the high surface activity of the

Fig. 5. Payne effect of rubber vulcanizate with 60 phr NCC.

Fig. 6. Damping factor (tand) of rubber vulcanizate with 60 phr NCC.

0

5

10

15

20

25

Sphericalshape

Chain Cube 30 nm Cube 50 nm Cube 80 nm

Nano-sized CaCO3

Tem

pera

ture

(o C)

Fig. 7. Effect of heat generation of rubber vulcanizate with 60 phr NCC.

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nanostructure results in high interaction with the rubber phaseable to prevent NCC network formation and reducing the lowstrain modulus.

The plots of tand or damping factor versus strain of both poly-morph (spherical shape and chain) and particle of different size(30 nm, 50 nm and 80 nm) composites are illustrated in Fig. 6.The damping peak of the composites with NCC continuously in-creases with increasing strain. The tand of cube (30 nm) andspherical shape are lower than the others. With the increasing ofstrain, the filler network develops and stabilizes a hysteresisbehavior of composite at small and intermediate strains. At highstrain, when part of the filler network is broken down, the com-posite is no longer stabilized and the tand rises drastically. Thefilled sample exhibits the highest tand level which is due to thelarge particle size. In fact, the hysteresis results from the break-down of the filler network and straining disruption would dissi-pate energy. All these occurred are because of the mobility ofrubber segments has been restricted by the diameter increasesin NCC composite. For the different of NCC polymorph, the tandof NCC composite filled with nanoparticles are lowered only whenboth the filler structure in the rubber and the surface activity ofthe filler are high. The structure of chain shape is complex andhas large surface area, so it results in high interaction with therubber phase and hindering the flow of the rubber segments. Final-ly, as can be seen from the tested results in Fig. 7, there is indica-tion of relationship between temperature rising and tand. It alsocan be seen that their rise trend are consistent. Energy loss trans-formed as heat generation, which leads to temperature rising ofthe sample during dynamic compression. In other words, a morestabilize NCC filled system would exhibited minimum energy losswith good durability of its performance

4. Conclusions

This work focused on investigating the influence of NCCmorphological parameters (polymorph and different particlesizes) on mechanical and dynamic properties of NR vulcaniz-ates. It concluded that the addition of NCC is not limited to af-fect the mechanical strength; NCC has also exhibited significantinfluence on the dynamic properties of the rubber compositescorresponding to the polymorph and its particle sizes. Hence,based on the results of this study, the following informationcan be concluded:

1. The tensile strength and elongation of rubber compositeswere dependable on the polymorph and particle sizes ofNCC. Spherical NCC has outstanding mechanical strengthresulted from the good dispersion in the NR compositesas found in SEM observation. This finding is identical to30 nm cubic NCC which also has better mechanical proper-ties when compared to 50 nm and 80 nm cubic NCC. The

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large particle size and high aspect ratio of chain shape NCCis expected to cause stress concentration spot in the rubbermatrix which lead to inferior of mechanical performance.

2. The Mullins effect of rubber composite filled chain shapeNCC was large and it is comparable to the large particle size(80 nm) of cubic NCC. This is mainly due to the large parti-cle size NCC would hinder the mobility of macromolecularwhen pulling, subsequently leading to stress concentrationand chains breakage.

3. The value of DG0 of rubber composite filled with sphericalshape has the lowest value due to weaker filler networkresulted largest inter-aggregate distance occurred in therubber matrix. Meanwhile, the chain and large particle sizecubic NCC have more significant DG0 with the increasing ofstrain. Such large structure of NCC tends to experiencedbreakage easily when straining.

4. The value of damping factor corresponds to energy lossshowed that large particle size NCC has more pronouncedvalues. Energy loss from the straining produced heatcaused increasing of temperature. Subsequently, both chainand 80 nm cubic NCC have highest rising of temperaturecompared to spherical NCC added rubber composites.

Acknowledgments

This research is under the Project of State Key Laboratory of Or-ganic–Inorganic Composite (201304). The authors would like tothank for the financial support from National Natural ScienceFoundation of China (51173110 and 51103086), the Liaoning Prov-ince Natural Science Foundation (201102173).

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