Effects of plasticizers on the strain-induced crystallization and mechanical properties of natural rubber and synthetic polyisoprene

Abstract

Effects of liquid isoprene (LIR-50) and naphthenic oil (NPO) on the strain-induced crystallization (SIC) measured by in situ synchrotron wide-angle X-ray diffraction (WAXD) and mechanical properties of vulcanized natural rubber (NR) and synthetic polyisoprene (IR) were studied. The onset strain (α_c) of SIC of NR and IR was 250% and 350%, respectively. NR and IR exhibited stress upturns at strain 383% and 450% in stress–strain curves, and the crystallinities of NR and IR were 7.6% and 8.6%, respectively. After vulcanization, LIR-50 became part of the rubber network, while NPO still existed as free small molecules in rubber networks. After the respective addition of LIR-50 and NPO, the α_c of NR composites rarely changed, while the α_c of IR composites increased. In addition, the crystallinity and tensile strength of NR and IR filled with LIR-50 and NPO, respectively, decreased, and the reduction in IR composites was higher than that in NR composites. The crystallinity and mechanical properties of the NR and IR plasticized by LIR-50, respectively, were higher than those plasticized by NPO. The maintenance of high crystallinity of NR or IR composites may ensure their good mechanical properties. Therefore, LIR-50 can be used as a reactive plasticizer to maintain good mechanical properties for NR and IR.

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1. Introduction

Natural rubber (NR) is an indispensable elastomeric material in the rubber industry, and NR consists of more than 99.9% cis-1,4-polyisoprene. The high stereoregularity of NR allows its molecules to orient along the stretching direction and crystallize when the strain exceeds a critical value (defined as the onset strain (α_c) of crystallization) during stretching.¹ This phenomenon, which is known as strain-induced crystallization (SIC),^2–4 imparts NR with a self-reinforcing characteristic.^5–7 Therefore, NR exhibits excellent physical properties, such as high elasticity, tensile strength, and crack growth resistance. NR is one of the most important natural materials that is widely used in the rubber industry.^8–10

NR comes from the Hevea trees. The production capacity of NR is restricted by the natural conditions of the geographical environment. Research has focused on the synthesis of its synthetic analog (cis-1,4-polyisoprene, IR). Although the backbone of IR is similar to that of NR, there are several differences between NR and IR. NR consists of 94% polyisoprene and 6% natural components including proteins (2.2%), phospholipids and neutral lipids (3.4%), carbohydrates (0.4%), metal salts and oxides (0.2%) and other materials (0.1%).^11,12 Functional groups at both ends of the chains in NR interact with the natural components to create a naturally occurring network that improves the SIC of NR.^13–15 The polyisoprene component in NR is composed of 100% cis-1,4 conformation, and the gel content of NR is larger than 20%. In contrast, IR is synthesized using Li-, Ti, and Nd-based catalyst systems.^16–19 IR contains only polyisoprene molecules (100%). The polyisoprene component in IR contains only 92–98% of the cis-1,4 conformation, and the gel content of IR is less than 10%.^3,13

Due to the self-reinforcing characteristic of SIC for crystallizable rubber during stretching, the SIC characteristics of NR and IR have been extensively studied with the development of in situ synchrotron wide-angle X-ray diffraction (WAXD),^1,20–24 in which the high intensity of WAXD enables the structural development and stress–strain relationship of rubbers to be measured without holding.^25,26 Amnuaypornsri et al.^14,15 and Toki et al.^11,13 concluded that the naturally occurring network and entanglements in unvulcanized NR improved the SIC, modulus and tensile strength of NR during stretching. Tosaka et al.^9,21 revealed that NR exhibited lower α_c and faster crystallinity than IR because the stereoregularity of IR was lower than that of NR.²⁷ Toki et al.¹³ reported that the α_c of NR and IR decreased with an increase in the temperature when the temperature was above −25 °C. Chenal et al.^2,28 explored the effect of crosslink density (γ) on the crystallization rate of NR during stretching. They found that with increasing γ of NR, the crystallization rate of NR increased when γ was lower than 1.2 × 10⁻⁴ mol cm⁻³, which was governed by the nucleation of crystallites, and the crystallization rate of NR decreased when γ was higher than 1.2 × 10⁻⁴ mol cm⁻³, which was governed by the growth of the crystallites uniaxial deformation. It was concluded that the maximum crystallization rate was obtained when γ was approximately 1.2 × 10⁻⁴ mol cm⁻³. Weng et al.²⁹ studied the effect of filler on the SIC of NR. The results showed that NR filled with a small amount of multiwalled carbon nanotubes exhibited a higher crystallinity and lower α_c than NR in the absence of fillers. Ozbas et al.⁷ determined that the α_c of a NR/functionalized graphene sheet composite was lower than that of a NR/carbon black composite because the specific surface area of functionalized graphene sheets was approximately 8 times higher than that of carbon black.

In the rubber industry, plasticizers are widely used to improve the processing characteristics and reduce the cost of rubber materials.³⁰ Plasticizers have a great influence on the movement of rubber chains. Therefore, the orientation and crystallization of the rubber chains are affected by the addition of plasticizers. Mark and co-workers^31,32 studied the effect of plasticizers on the stress–strain isotherms of cis-1,4-polybutadiene. These studies concluded that the addition of a plasticizer suppressed the SIC of cis-1,4-polybutadiene. However, the investigation of SIC through stress–strain isotherms obtained via elongation of crystallizable networks is an indirect approach. Therefore, the onset formation of crystals cannot be accurately observed, and the crystallinity cannot be quantitatively determined. In recent years, the effects of plasticizers on the SIC of rubber vulcanizates have been rarely studied by WAXD technology, which is used to directly investigate the SIC of crystallizable rubber during uniaxial stretching.

The main topic of the present work is to analyze the SIC characteristics of NR and IR and the effect of plasticizers–naphthenic oil (NPO) and liquid isoprene (LIR-50) on the SIC characteristics of NR and IR, respectively, by WAXD. The mechanical properties of NR, IR and their respective composites filled with NPO and LIR-50 have been measured. This article allows us to gain further insight into the change of α_c, the relationship between the upturn in stress and crystallinity, and the relationship of and the mechanical properties. It is expected that these experimental results will provide a theoretical basis for the application of reactive plasticizers in NR and IR.

2. Experimental section

2.1 Materials

NR (type SCR WF) was manufactured by Yunnan Natural Rubber Industrial Co., Ltd., China. IR (type IR70) was produced by Qingdao Yikesi New Material Co., Ltd., China. LIR-50 was synthesized in our lab by anionic polymerization.³³ NPO was produced by Nanjing Yangtze Petrochemical Co., Ltd., China. The other rubber additives, such as zinc oxide, stearic acid, and sulfur, were of commercial grade. The structural parameters of NR, IR and LIR are listed in Table 1.

Table 1 Structural parameters of NR, IR and LIR-50

	NR	IR	LIR-50
a Percentage of cis-1,4, trans-1,4 and 3,4- units in raw materials.
Mn (g mol^−1)	390 000	333 000	48 000
Mw/Mn	2.47	3.07	1.23
cis-1,4^a (%)	99.90	92.81	71.29
trans-1,4^a (%)	0.10	3.39	21.73
3,4-^a (%)	—	3.79	6.98

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2.2 Formulation for rubber compounds

Other ingredients of the formulations except for rubber and plasticizers are as follows: zinc oxide (4.0 parts-per-hundred rubber, phr), stearic acid (2.0 phr), N-cyclohexyl-2-benzothiazylsulfenamide (1.2 phr), poly(1,2-dihydro-2,2,4-trimethylquinoline) (1.0 phr), N-isopropyl-N′-phenyl-p-phenylenediamine (1.0 phr), sulfur (2.25 phr) (Table 2).

Table 2 Formulations of the NR and IR compounds

Sample no.	NR-0	NR-1	NR-2	IR-0	IR-1	IR-2
NR (phr)	100	100	100
IR (phr)				100	100	100
NPO (phr)		10			10
LIR-50 (phr)			10			10

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2.3 Specimen preparation

2.3.1 Preparation of rubber compounds. NR was first masticated by a Φ360 × 900 mm two-roll mill (Shanghai Rubber Machinery Works no.) for 3 min. Then, zinc oxide and other ingredients were added to these samples and mixed for 3 min to form NR compounds. IR was first masticated for 0.5 min. Other ingredients were added to these samples and mixed for 3 min to form IR compounds.

2.3.2 Preparation of vulcanizates. The vulcanizing properties of the compounds were determined by a P3555B₂ Disc Vulkameter (Beijing Huanfeng Chemical Machinery Trial Plant, Beijing, China). The vulcanizates were prepared in an XLB-D350 × 350 plate vulcanization machine (Huzhou Dongfang Machinery Co., Ltd., Zhejiang, China) at 145 °C for the optimum cure time t₉₀. The hydraulic pressure was 15 MPa, the thickness of the vulcanizate samples for the WAXD test and simultaneous tensile measurements was approximately 0.4 mm, and the thickness of the vulcanizate samples for the mechanical test was 2.0 mm.

2.4 Characterization

The number-average molecular weight (M_n) and polydispersity index (M_w/M_n) of the raw materials were measured by gel permeation chromatography (Waters 150-C, Waters Co., Ltd. USA). LIR-50 was dissolved in tetrahydrofuran (THF) at a concentration of about 1 mg mL⁻¹ for 1 day. IR was cut into small pieces and dissolved in THF at a concentration of about 1 mg mL⁻¹ for 1 day. Because of its high molecular weight, and high content of branching and microgels in raw NR, NR was cut into pieces and dissolved in THF at a concentration of about 1 mg mL⁻¹ for 7 days. Furthermore, all rubber solutions were filtrated in PTFE membranes with a membrane pore size of 0.45 μm, and then injected into the GPC system using a 1.0 mL min⁻¹ flow rate at 30 °C.

The mechanical properties were measured according to ASTM D412-06 by CMT4104 Electrical Tensile Tester (Shenzhen SANS Test Machine Co., Ltd. China) at a tensile rate of 500 mm min⁻¹. Shore A hardness was determined according to ASTM D2240-05 on a Shore A durometer (Heinrich BAREISS GmbH, Oberdischingen, Germany). The resilience test was performed on a MZ-4065 Resilience Elasticity Tester (Mingzhu Testing Machinery Co., Ltd., China) according to ASTM D2632-01.

The stress relaxation test was carried out according to ASTM D 674 standard. A relaxation test apparatus was designed and assembled for the purpose of testing the relaxation behavior of rubber materials. Relaxation curves for all samples were recorded at strain = 350% at 20 °C for 2000 s. The stress-relaxation characteristics of the vulcanizates were obtained by plotting the normalized relaxation stresses σ(t)/σ(0) against time, σ(t) and σ(0) being the stress at time t and initial stress, respectively.

The crosslink density (γ) of the vulcanized rubber was obtained using swelling method. The vulcanized rubber samples were immersed in n-hexane at 30 °C for 7 days. Then, excess liquid on the surface of the specimens was quickly removed by blotting them with filter paper. The specimens were weighed in a weighing bottle, and then dried in a vacuum oven at 50 °C for 2 days. The weight of the dried samples was then measured. The γ of the vulcanizates was calculated from the swelling ratio by the Flory–Rehner eqn (1):

where v₂ is the volume fraction of polymer in the swollen mass, V₁ (130.7 cm³ mol⁻¹) is the molar volume of the solvent (n-hexane), and x₁ is the Flory–Huggins polymer–solvent dimensionless interaction term (x₁ is equal to 0.5 for the system NR–n-hexane).

The solvent extraction rate of the vulcanizates was determined by a Soxhlet apparatus at 80 °C. N-Hexane was used as the solvent, and the extraction time was 120 h. The extracted samples were dried in vacuum to a constant weight (m₁) at 50 °C. n-Hexane extraction rate was calculated using eqn (2):

Extraction rate (%) = (m₂ − m₁)/m₂ × 100, (2)

where m₂ is the weight of the sample before the extraction test.

In situ synchrotron WAXD measurements were carried out at the 1W2A beamline in the Beijing Synchrotron Radiation Facility (BSRF) at the Institute of High Energy Physics Chinese Academy of Sciences. The wavelength of the X-ray used was 1.54 Å. Two-dimensional WAXD patterns were recorded by a MAR 165 CCD detector for quantitative image analysis. The typical image exposure time was 20 s, and the sample-to-detector distance was 182 mm. The experiments were carried out at room temperature (approximately 22 °C). The specimen was a rectangular sheet 25 × 6 × 0.4 mm. A Linkam tensile tester, which allowed for the symmetric deformation of the sample, was used for the in situ WAXD study. The original distance between the clamps was 15 mm, and the deformation rate was 10 mm min⁻¹. The stress (σ) was measured as σ = F/d₀ω₀, where F is the force measured by a load cell, d₀ is the initial thickness, and ω₀ is the initial width of the sample. In terms of the deformation ratio, the strain (α) was given by α = (l − l₀)/l₀, where l₀ is the original clamp–clamp distance and l is the extended clamp–clamp distance during deformation. Due to the limitation of the Linkam tensile tester, the maximum distance between the clamps was 95 mm. Therefore, for the WAXD test, the maximum strain was about 530% where all samples were not broken up. The time-resolved WAXD patterns and simultaneous stress–strain relationship were continuously recorded without holding the sample in still conditions during stretching.

2.5 Processing of the WAXD data

In this research, air scattering was subtracted from all the WAXD patterns for further analysis using the Fit2D software package. The diffraction intensity near the meridian was normalized and azimuthally integrated in a cake from 75° to 105°, as shown in Fig. 1. For example, Fig. 1 represented the cake-integrated intensity as a function of the 2θ for NR at 483% strain. The resulting profiles were deconvoluted considering the diffraction peaks of the 200 and 120 planes and the amorphous halo using the Peak Analyzer software.⁸ The mass fraction crystallinity index (X_c) at room temperature was estimated from diffraction intensity data (2θ = 5–25°) using eqn (3):

X_c (%) = A_c/(A_c + A_a) × 100, (3)

where A_c is the area below the 200 and 120 crystalline peaks, and A_a is the area below the amorphous halo. The crystallinity of WAXD pattern was not calculated when X_c was less than 0.5%.

Fig. 1 Meridional cake-integrated intensity as a function of the 2θ taken from the WAXD pattern of the stretched vulcanized NR (A11) at α = 483%. The inset shows the integration limits from 75° to 105°.

3. Results and discussion

3.1 SIC behaviors of NR and IR

The stress–strain curves and crystallinity of NR and IR during stretching are shown in Fig. 2 and 3.

Fig. 2 Stress–strain curves and selected WAXD patterns collected during stretching of NR and IR.

Fig. 3 Variation of crystallinity of NR and IR with strain.

As shown in Fig. 2, the WAXD patterns of NR and IR at α = 0 exhibit an isotropic amorphous halo with no preferred orientation. As the strain increased, the stress of NR and IR increases, and the stress of NR is higher than that of IR. The α_c of NR and IR is 250% and 350%, respectively. NR has a smaller α_c than IR. NR and IR exhibit a stress upturn at approximately 383% and 450%, respectively, and their corresponding WAXD patterns exhibit an oriented crystal pattern.

The results in Fig. 3 show that with increasing strain the crystallinity of NR and IR rapidly increases when α > α_c, and the crystallinity of NR is higher than that of IR. At the point of the upturn in stress, the crystallinity of NR and IR is approximately 7.6% and 8.6%, respectively. Therefore, when the crystallinity of NR or IR is approximately 8%, the contribution of crystals for the reinforcement of NR or IR vulcanizates increases, and thus the stress upturns indicate a “self-reinforcement” feature. NR has a naturally occurring network and the stereoregularity of NR is higher than that of IR. During stretching the naturally occurring network acts as constraints in order to align molecules to induce crystals during deformation.¹³ Compared with IR, the higher stereoregularity of NR is also of benefit to the SIC of NR. Therefore, NR has a higher SIC characteristic leading to higher crystallinity and stress than IR.

3.2 Effect of plasticizers on the curing characteristic and extraction rate of NR and IR

Table 3 shows the effect of plasticizers on the curing characteristic and extraction rate of NR and IR.

Table 3 Curing characteristic and extraction rate of NR/plasticizer and IR/plasticizer composites

	NR-0	NR-1	NR-2	IR-0	IR-1	IR-2
t10 (m:s)	8:34	8:56	8:48	15:21	17:11	16:12
t90 (m:s)	13:43	14:15	14:07	20:06	23:22	21:06
γ (× 10^4 mol cm^−3)	3.17	2.68	2.55	2.32	1.94	1.97
Extraction rate (%)	5.62	13.20	5.79	3.77	11.50	3.70

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It can be seen from Table 3 that the scorch time (t₁₀) and optimum cure time (t₉₀) for NR and IR increase slightly, and the crosslink density (γ) decreases after the addition of NPO and LIR-50. NR (NR-0–NR-2) exhibits shorter t₁₀ and t₉₀ and higher γ than IR (IR-0–IR-2) because the proteins and stearic acid, which are inherently contained in NR, can accelerate the vulcanization of NR.^34,35

The extraction rate of NR composites is higher than that of IR composites, indicating that NR composites contain a certain amount of inherent removable stearic acid and proteins. After the addition of NPO, the extraction rate of NR composites and IR composites significantly increases, respectively, and approximately 97% of NPO molecules are extracted from the rubber networks, indicating that NPO exists as free small molecules in the rubber networks. There are repeating double bonds in the main chains of LIR-50, NR and IR. After the addition of LIR-50, the extraction rate of NR and IR barely changes, indicating that the molecules of LIR-50 and rubber are linked through the sulfur crosslinks during vulcanization. LIR-50 becomes part of the rubber networks.³⁶

3.3 Effect of plasticizers on the SIC characteristics and stress relaxation of NR and IR

The effect of NPO and LIR-50 on the stress–strain curves of NR and IR, respectively, is shown in Fig. 4.

Fig. 4 Stress–strain curves and selected WAXD patterns of NR and IR plasticized by (a) NPO and (b) LIR-50.

The first WAXD pattern of each sample is the first pattern with oriented crystal reflection during the WAXD test. The second WAXD pattern of each sample (except sample IR-1) is near the upturn in the stress. As shown in Fig. 4(a), after the addition of NPO to NR, the stress decreases, and the α_c barely changes (i.e., approximately 250%). The strain of stress upturn increases to 410%, and the WAXD pattern near the strain of stress upturn exhibits an apparent oriented crystal pattern. However, after the addition of NPO to IR, the stress significantly decreases, the α_c shifts from 350% (IR-0) to 417% (IR-1), and there is no apparent upturn in the stress. The results in Fig. 4(b) indicate that after the addition of LIR-50 to NR, the stress also decreases, and the α_c is 250%. The strain of stress upturn is approximately 405%, and the corresponding WAXD pattern also exhibits an apparent oriented crystal pattern. The effect of LIR-50 on the stress–strain curve of NR is similar to that of NPO. After the addition of LIR-50 to IR, the stress also decreases, and the α_c shifts from 350% to 383%. In addition, IR/LIR-50 composite exhibits an upturn in stress, and the strain of stress upturn increases to 470%.

Fig. 5 shows the effect of NPO and LIR-50 on the crystallinity of NR and IR. As shown in Fig. 5(a), the crystallinity of NR decreases after the addition of NPO or LIR-50, and the reduction in crystallinity of NR plasticized by NPO is more obvious. At the strain of stress upturn, the crystallinity of NR, NR/NPO and NR/LIR-50 is approximately 7.6%, 8.1% and 8.4%, respectively. The results in Fig. 5(b) indicate that after the addition of NPO or LIR-50, the crystallinity of IR significantly decreases, and the reduction of crystallinity of IR/NPO is higher than that of IR/LIR-50. The crystallinity of IR/NPO is only approximately 3.8% at 517% strain. Therefore, the stress does not upturn in Fig. 4(a).

Fig. 5 Effect of NPO and LIR-50 on the crystallinity of NR (a) and IR (b), respectively.

Fig. 6 shows the effect of NPO and LIR-50 on the stress relaxation of NR and IR. Fig. 6 shows that the normalized relaxation stresses, σ(t)/σ(0), of NR and IR decrease after the addition of NPO or LIR-50. Moreover, σ(t)/σ(0) of NR or IR plasticized by NPO is lower than that plasticized by LIR-50, i.e., the stress relaxation of NR or IR plasticized by NPO is faster than that plasticized by LIR-50, illustrating that LIR-50 links to the molecular chains of rubber networks and NPO exists as small molecules. The equilibrium value of σ(t)/σ(0) for NR is slightly lower than that for IR because NR has crystallized and the crystallinity of NR increases with increasing time at 350% strain. IR does not crystallize at 350% strain. The decrease of σ(t)/σ(0) for IR is mainly due to the rearrangement of the molecules of IR caused by plasticizers.

Fig. 6 Normalized relaxation stresses, σ(t)/σ(0), versus time for of NR and IR vulcanizates plasticized by NPO and LIR-50.

The stress relaxation of NR and IR composites is an important factor affecting the formation and growth of crystals during stretching. As shown in Fig. 7, after vulcanization, LIR-50 links to the molecular chains of NR and IR, and the stress relaxation of these vulcanizates increases slightly. However, NPO exists as free small molecules in rubber networks and apparently increases the stress relaxation speed of these vulcanizates. The stress-relaxation time of rubber chains plasticized by NPO becomes shorter and the force subjected to these chains reduces rapidly, which hinders the orientation of these rubber chains under stress and the formation of crystals. Therefore, as shown by the stretching samples in Fig. 6, the suppressing effect of NPO on the SIC of NR and IR is higher than that of LIR-50.

Fig. 7 Schematic models of the effect of NPO (b) and LIR-50 (c) on the SIC of crystallizable rubber. Chemical crosslink (green circle), NPO (black circle), LIR-50 (red curve) and SIC (gray rectangle).

Based on a comparison of the results in Fig. 5(a) and (b), the suppressing effect of these plasticizers on the SIC of NR is lower than that of IR. After the incorporation of NPO or LIR-50, the reduction of crystallinity of NR is considerably lower than that of IR. In addition, the α_c of NR barely changes, while the α_c of IR increases. The different behavior of NR and IR may be caused by the higher stereoregularity and naturally occurring network of NR. The entanglements of the naturally occurring network become pivots for aligning the amorphous chains into crystalline order and reducing the effect of plasticizers on the stress relaxation of the molecular chains.^11,13

3.4 Effect of plasticizers on the mechanical properties of NR and IR

The mechanical properties of NR and IR filled with NPO and LIR-50, respectively, are shown in Table 4.

Table 4 Mechanical properties of NR and IR plasticized by NPO and LIR-50, respectively

	NR-0	NR-1	NR-2	IR-0	IR-1	IR-2
Shore A hardness	44.2	40.0	43.3	39.9	36.9	38.6
Rebound resilience (%)	81.0	80.8	79.4	81.0	80.8	79.0
Modulus at 300% (MPa)	3.4	2.8	2.6	2.5	1.9	2.0
TS (MPa)	29.7	24.4	27.0	24.3	15.3	21.7
EB (%)	649	660	715	644	660	710

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After the incorporation of NPO and LIR-50, the Shore A hardness and modulus at 300% of NR and IR decreases, and the elongation at break (EB) increases because of the decrease in the crosslink density. The reduction of hardness of NR and IR plasticized by LIR-50 is lower than that plasticized by NPO. The rebound resilience of NR and IR barely changes. The tensile strength (TS) of NR and IR decreases. The TS and the EB of NR and IR plasticized by NPO are lower than those plasticized by LIR-50. In particular, compared with the TS of IR vulcanizate, the TS of IR plasticized by LIR-50 decreased by 11%, while the TS of IR plasticized by NPO decreased by 37%.

In comparison with the data in Table 4 and Fig. 5, we can conclude that the addition of NPO and LIR-50 suppresses the SIC of NR and IR. Therefore, the TS of these vulcanizates decreases. The reduction in the crystallinity of NR and IR plasticized by NPO is higher than that plasticized by LIR-50. Therefore, NR and IR plasticized by NPO have a lower TS and EB. The SIC characteristics of NR and IR have a substantial influence on their final properties.

4. Conclusions

In situ synchrotron WAXD during stress–strain measurements have been performed on NR, IR and their plasticized samples. First, it was observed that NR had lower α_c and higher crystallinity than IR during stretching. Second, after the addition of plasticizers, the SIC of NR and IR was suppressed, and the suppressing effect of plasticizers on the SIC of NR was lower than that of IR. The existence of a naturally occurring network could play a very important role in the SIC of NR compared with IR before or after the addition of plasticizers. In addition, the suppressing effect of NPO on the SIC of NR and IR was higher than that of LIR-50 due to the different existing states and structures of these plasticizers. During stretching, the stress upturned when the crystallinity was about 8% for NR and IR, even for the plasticized samples, because the contribution of self-reinforcement of the strain-induced crystals was more apparent when the crystallinity attained a critical value.

Through mechanical tests, we found that the tensile strength of NR and IR decreased after the incorporation of the NPO and LIR-50. The tensile strength and the elongation at break of NR and IR plasticized by LIR-50 were higher than those plasticized by NPO. LIR-50 is more suitable for use as a reactive plasticizer in NR and IR. The SIC characteristic is an important factor affecting the final properties of NR and IR.

Nomenclature

WAXD	In synchrotron wide-angle X-ray diffraction
LIR-50	Liquid isoprene with Mn of about 50 000
Mn	Number-average molecular weight
Mw	Weight-average molecular weight
SIC	Strain-induced crystallization
αc	Onset strain of crystallization
IR	Synthetic polyisoprene
Mw/Mn	Polydispersity index
EB	Elongation at break
t90	Optimum cure time
γ	Crosslink density
TS	Tensile strength
m:s	Minute:second
NR	Natural rubber
NPO	Naphthenic oil
t10	Scorch time
α	Strain

Acknowledgements

The authors acknowledge financial support from the Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics Chinese Academy of Sciences. Prof. Guang Mo and Prof. Zhihong Li of BSRF are warmly thanked for their help with the synchrotron WAXD experiments.

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