Article pubs.acs.org/JPCB
Observing Nucleation Transition in Stretched Natural Rubber through Self-Seeding Han Liu, Guangsu Huang,* Jian Zeng, Lili Xu, Xuan Fu, Siduo Wu, Jing Zheng, and Jinrong Wu College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: Potential competition between fringed-micelle nucleation (N1) and folded-chain nucleation (N2) widely exists in strain-induced crystallization (SIC). However, during uniaxial deformation, no in situ observational evidence of nucleation transition from N2 to N1 in SIC of natural rubber (NR) has been reported yet. In this work, self-seeding provides an effective way for this observation. By the introduction of residual TIC (temperature-induced crystallization)-melting crystallites into pure NR system, in situ synchronic WAXD revealed the formation of low-oriented crystal in the initial deformation stage, which gradually evolves into highly oriented crystal at last. The low-oriented crystal is related to secondary folded-chain nucleation (N2) on the surface of residual TIC-melting crystallites (self-seeding), while newly formed highly oriented crystal is associated with N1. For the first time, the concept of “selfseeding” is innovatively applied to SIC process so that NR exhibits clear nucleation transition phenomenon. Further, theoretical computation of nucleation barrier in the special NR system well reflects that self-seeding has the role of both increasing critical strain of nucleation transition and decreasing onset strain of SIC, thus providing conditions for the observation.
1. INTRODUCTION Strain-induced crystallization (SIC) has been widely researched in natural rubber (NR) during uniaxial deformation,1−5 which generally follows the mechanism proposed by by S. Toki and M. Tosaka:6,7 the short chain between network junctions is first fully stretched to act as oriented nucleus, then the surrounding coiled chains partially align parallel to the nucleus to form the crystallite. In this mechanism, the oriented nucleus nearly parallel to stretching direction leads to highly oriented crystal. Contrast to SIC, temperature-induced crystallization (TIC)8 induces unoriented folded-chain nuclei and further randomly oriented crystals. According to mainstream nucleation theory,9,10 SIC of NR is thought to be the imperfect intermolecular fringed-micelle nucleation (N1) with some intramolecular folded-chain components, while TIC conforms to the intramolecular folded-chain nucleation (N2) with some intermolecular chain component. N1 leads to highly oriented crystals with less chainfolding fraction, while N2 induces unoriented crystals nearly made up of folded-chain. Previous research11,12 combined SIC and TIC by observing crystal morphology of static stretched NR under low temperature, and suggested a transition of crystal morphology from spherulites to fibrous crystallites with increasing strain. That is, as strain increases, the nucleation type N2 gives way gradually to N1. Because of its much lower end surface free energy, N2 is a preferred habit of TIC.13 But upon stretching, N1 could gradually get over the unfavorable surface free energy due to its higher stability.9,10,13 Further, molecular simulation of SIC by Y. Nie and W. Hu10 has validated the potential competition between coexisting N1 and N2 with strain and has revealed a critical strain for the © XXXX American Chemical Society
nucleation transition from N2 to N1. Nevertheless, no in situ observational evidence of nucleation transition in SIC of NR during uniaxial deformation has been reported yet, even at low temperature.2 Self-seeding may provide a way for this observation. Generally, self-seeding14−16 is defined as nucleation on the surface of residual crystallites of polymer during the melting− recrystallization process of TIC. With residual crystallites providing surfaces for newly formed nuclei, nucleation barrier would significantly decrease to readily benefit crystallization, and orientation of newly formed crystal is mainly determined by these residual crystallites’ orientation.16 In this work, the concept of “self-seeding” would be innovatively applied to SIC in NR during uniaxial deformation by introducing residual TICmelting crystallites into NR system, and SIC behavior of the special NR system would be studied to reveal the nucleation transition phenomenon.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. NR latex used in this study was fresh field latex provided by the Thai Rubber Latex Co, Thailand. The fresh NR latex was preserved with 0.6 (v/vol %) NH3 (NR latex 100 mL/NH3 solution 0.6 mL) at room temperature for 3 months. Due to the presence of naturally occurring network,2,17,18 unvulcanized NR is able to crystallize at room temperature during uniaxial deformation. Here, two Received: May 28, 2015 Revised: August 7, 2015
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DOI: 10.1021/acs.jpcb.5b05113 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry B kinds of unvulcanized NR films were prepared, where NR-1 is the special NR system with residual TIC-melting crystallites and NR-2 is the common NR system. They were prepared as follows: First, the latex was diluted to 30% dry content and subjected to centrifugation at a speed of 10 000 rpm for 40 min in order to remove sludge and water-soluble impurities, such as amino acids, sugars, and metal ions. The cream fraction was collected and dried in an oven at 40 °C for 2 days. Second, this NR sample was used to prepare unvulcanized NR film by hot press containing three procedures: first, to preheat the NR sample at 150 °C for 3 min, and then, to press the sample with the pressure of 15 MPa at the same temperature for 5 min; finally, to cool the pressed film with the same pressure at room temperature for 5 min. Although hot press would undermine naturally occurring network, numerous such networks still remain.2,17 Third, for NR-1, in order to introduce residual TIC-melting crystallites, the prepared NR film was subjected to TIC-melting process:19 the film was stored in a fridge at 2−6 °C for 6 weeks, and then taken out and preserved under room temperature (25 °C) for about 48 h just before sample characterization. For NR2, the film was preserved under room temperature without suffering TIC-melting process. 2.2. Sample Characterization. Measurements of differential scanning calorimentry (DSC) for NR-1 and NR-2 were performed on Q200 (TA Instruments) from 298 to 373 K with a heating rate of 10 K/min. The weights of the samples were in the range of 7−8 mg, and the DSC measurements were performed under nitrogen atmosphere. In situ synchrotron WAXD experiments for NR-1 and NR-2 during uniaxial deformation were carried out under room temperature at the beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. A wavelength of 0.124 nm was used, and the WAXD patterns were recorded every 5 s. Rectangular films for this characterization were about 1 mm thick, 8 mm wide, and 30 mm long. A homemade mechanical stretching machine was used for symmetric deformation at a drawing speed of 20 mm/min. The strain ε = (l − l0)/l0 was determined from the distance between the clamps during deformation, in which l0 is the initial length (10 mm) of the sample and l is the length of the elongated one. Similarly, the extension ratio (α) is defined as l/l0. The WAXD patterns were background corrected20 and processed using Fit2D software for further analysis. To determine the crystallinity,21 these corrected WAXD patterns were integrated along the azimuthal direction from 0° to 360°. Then, the resultant profiles were deconvoluted into corresponding crystal diffraction (the 200, 120, and 201 reflections) and amorphous peaks. All crystalline and amorphous peaks were described by the Guassian functions. In this way, the crystallinity (Xc) can be calculated from the diffraction intensity data by using the equation Xc = ∑Ac/(∑Ac + ∑Aa), where ∑Ac and ∑Aa represent the integrated intensities of the crystalline and amorphous regions, respectively.
Figure 1. Sequential WAXD patterns of NR-1 with strain.
1.75, which were gradually oriented to stretching direction from ε = 1.75 to ε = 3.05 and finally replaced by highly oriented crystals at ε = 3.47. Figure 2 shows the sequential azimuthal intensity distribution curves of 200 plane with strain to further reflect the above crystal orientation trend clearly.
Figure 2. Sequential azimuthal intensity distribution curves of 200 plane with strain in NR-1.
Obviously, the low-oriented crystal at ε = 1.75 is formed by secondary nucleation, i.e., folded-chain nucleation (N2) on the surface of unoriented residual TIC-melting crystallites (selfseeding), while the highly oriented crystals at ε = 3.47 comprise newly formed oriented crystals induced by N1 as well as previous low-oriented crystals rotating to stretching direction. From ε = 1.75 to ε = 3.47, the phenomenon of nucleation transition from N2 to N1 is completely captured online by recording crystal orientation in SIC process. It is attractive to observe nucleation transition with the help of self-seeding. And in the following section, the role of self-seeding for observing nucleation transition would be clarified. 3.2. Nucleation Barrier and Self-Seeding. In Figure 1, the initial WAXD pattern at ε = 1.18 shows no obvious crystal existing in NR-1. To confirm the presence of residual TICmelting crystallites, Figure 3 gives the DSC curves of NR-1 and NR-2, and a small endothermic peak in the range of 60−70 °C appears in NR-1, which certainly belongs to the residual TICmelting crystallites.22 Moreover, according to WAXD data in
3. RESULTS AND DISCUSSION 3.1. Crystal Orientation and Nucleation Transition. Because of the presence of residual TIC-melting crystallites, NR-1 exhibits unique evolution of crystal orientation. Figure 1 gives the sequential WAXD patterns of NR-1 with strain. Unlike highly oriented crystal in common NR system,2,17,18 low-oriented crystals appear in NR-1 upon onset of SIC at ε = B
DOI: 10.1021/acs.jpcb.5b05113 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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Figure 5. Schematic illustration of four kinds of nucleation: (a) N2; (b) N1; (c) N2/self-seeding; (4) N1/self-seeding.
Figure 3. DSC curves of NR-1 and NR-2: the blue arrow indicates an endothermic peak in NR-1.
extension ratio α. For folded-chain nucleus, Δsm,α approximates to Δsm,1 here by assuming stretching of polymer chains did not significantly affect the probability of forming nuclei of folded chains,10 although it should decrease with increasing strain. Thus, Δg(T,α) = Δg(T,1). And because of Δsm,1 = Δhm/Tm,1, so Δg(T,α) = Δhm(Tm,1 − T)/Tm,1, where Tm,1 is the equilibrium melting point in unstretched state. To reach a critical nucleus, the conditions should be met that
Figure 1, Figure 4 gives the crystallinity evolution of NR-1 and NR-2 with strain. Because of self-seeding by residual TIC-
∂ΔGfolding ∂a
=
∂ΔGfolding ∂b
=
∂ΔGfolding ∂c
=0
(3)
Then, the free energy for critical nucleus is calculated that 32γ 2γe 32γ 2γe ΔG folding = = * [Δg (T , α)]2 [Δhm(Tm,1 − T )/Tm,1]2 (4)
Similarly, the free energy of forming a parallelepiped-like fringed-micelle nucleus (Figure 5b) is ΔGfringed = 2c(a + b)γ + 2abγe′ − abc Δg ′(T , α)
Figure 4. Crystallinity evolution of NR-1 and NR-2 with strain: the green arrows indicate onset of SIC, and the solid line is only a guide for the eye.
where γe′ = mγe and Δg′(T,α) = Δhm ′ − Δsm,α ′ . For fringedmicelle nucleus, Δs′m,α = Δs′m,1 + Δsdef, where Δsdef is the difference of the entropy between the stretched and unstretched states, and according to classical theory of rubber elasticity,24 Δsdef is a function of extension ratio α:
melting crystallites, NR-1 exhibits much easier SIC behavior compared with NR-2. Namely, self-seeding significantly decrease nucleation (N2) barrier of secondary folded-chain nuclei in SIC process. The quantitative calculations are as follows: According to classical nucleation theory,9,23 in common NR system the free energy of forming a parallelepiped-like foldedchain nucleus (Figure 5a) is ΔGfolding = 2c(a + b)γ + 2abγe − abc Δg (T , α)
⎞ 1 ⎛ 2 Δsdef = − νk ⎜α 2 + − 3⎟ ⎠ α 2 ⎝
(6)
where ν is network-chain density and k is the Boltzmann constant. Thus, the corresponding free energy for critical nucleus is
(1)
32γ 2γe′ ΔG fringed = * [Δg ′(T , α)]2
where c, a, and b are the sizes of the nucleus in the direction of 002, 200, and 020 plane, respectively. γ and γe are the lateral and chain end surface energies. Δg(T,α) is the melting Gibbs free energy per unit volume at a given extension ratio α and temperature T: Δg (T , α) = Δhm − T Δsm, α
(5)
=
32mγ 2γe ⎡Δh ′ (T′ − T )/T′ + m,1 ⎣ m m,1
νkT 2
(α2 + α2 − 3)⎤⎦
2
(7)
(2)
However, in special NR system with residual TIC-melting crystallites, secondary nucleation on the surfaces of residual crystallites (self-seeding) is much easier than the above primary
where Δhm is the heat of fusion per unit volume independent of α, and Δsm,α is the entropy of fusion per unit volume at the C
DOI: 10.1021/acs.jpcb.5b05113 J. Phys. Chem. B XXXX, XXX, XXX−XXX
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trends of the four kinds of free energy for critical nucleus with strain, where two other critical strains (pink arrows) for nucleation transition from N2 to N1 exist in the special NR system besides the general critical strains (green arrow) of common NR system. On the other hand, calculation of onset strain (ε0) of SIC conforms to the relationships as follows:25
nucleation. Specifically, free energy of forming a parallelepipedlike secondary folded-chain nucleus (Figure 5c) is ′ ΔGfolding = 2bcγ + 2abγe − abc Δg (T , α)
(8)
And the free energy for critical nucleus can be calculated that 4bγγe 4bγγe ′ ΔG folding = = * Δg (T , α) Δhm(Tm,1 − T )/Tm,1
ε0 ∝ τ ∝ 1/Ṅ
(9)
where τ is the induction time needed to observe crystallization and Ṅ is the crystallization rate defined below:
Similarly, the free energy for critical secondary fringedmicelle nucleus (see Figure 5d) is calculated that
Ṅ = N0̇ D1N1
4bγγe′
4mbγγe νkT 2
′ − T )/Tm,1 ′ + Δhm′ (Tm,1
⎛ −ΔG ⎞ *⎟ N1 ∝ exp⎜ ⎝ kT ⎠
(α 2 + α2 − 3) (10)
Table 1. Parameter Values of ΔG*folding, ΔG*fringed, ΔG*′folding, and ΔG*′fringed Existing Research Data10,23,25 γ (J/m ) 0.0033
γe (J/m ) 2
Δhm (J/m3)
0.0066 6.1 × 107 approximate treatment
Δh′m(≈ Δhm) (J/m3) 6.1 × 10
7
m
T′m,1(≈ Tm,1) (K) 308.5
Tm,1 (K)
1.52
k (J/mol/K)
308.5 1.38 × 10−23 selected empirical values
T (K) 298
ν (mol/cm3) −4
2 × 10
(13)
where ΔG* is the free energy for critical nucleus. So the lower ΔG* is, the earlier SIC occurs. Now it is clear that self-seeding in special NR system has the role of both increasing critical strain of nucleation transition and decreasing onset strain of SIC by decreasing nucleation barrier of forming folded-chain nucleus (N2) as well as fringedmicelle nucleus (N1). And in general, onset strain of common NR system is equal or greater than critical strain,10 but in the special NR system, self-seeding can reverse this situation to observe nucleation transition phenomenon. 3.3. Crystallization Trend and Nucleation Competition. Figure 7 shows schematic illustration of preferred crystallization trend related to nucleation competition in NR1 and NR-2 during uniaxial deformation. As strain increases, NR-2 exhibits potential nucleation transition, since in the initial deformation stage folded-chain nucleus has no enough crystallization rate and induction time to cross free energy for critical nucleus.
although it is not the main nucleation to consider in the whole SIC process. For further comparison, all parameters in ΔG*folding, ΔG*fringed, ΔG*′folding, and ΔG*′fringed are taken as their approximated values based on the existing research data,10,23,25 as well as approximate treatment. The results are listed in Table 1, and then Figure 6 show general evolution
2
(12)
where Ṅ 0 and D1 are constant term and diffusion term, respectively. N1 is the nucleation probability that can be estimated with the equation:
′ = ΔG fringed * Δg ′(T , α) =
(11)
b (Å) 40
Figure 6. General evolution trends of the four kinds of free energy for critical nucleus with strain. D
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Figure 7. Schematic illustration of preferred crystallization trend related to nucleation competition in NR-1 and NR-2 during uniaxial deformation: (a) preferred nucleation trend with strain in NR-2; (b) preferred nucleation trend with strain in NR-1; (c) crystal growth and orientation related to growable nucleus. (2) Toki, S.; Che, J.; Rong, L.; Hsiao, S. B.; Amnuaypornsri, S.; Nimpaiboon, A.; Sakdapipanich, J. Entanglements and Networks to Strain-Induced Crystallization and Stress−Strain Relations in Natural Rubber and Synthetic Polyisoprene at Various Temperatures. Macromolecules 2013, 46, 5238−5248. (3) Che, J.; Burger, C.; Toki, S.; Rong, L.; Hsiao, S. B.; Amnuaypornsri, S.; Sakdapipanich, J. Crystal and Crystallites Structure of Natural Rubber and Synthetic cis-1,4-Polyisoprene by a New Two Dimensional Wide Angle X-ray Diffraction Simulation Method. I. Strain-Induced Crystallization. Macromolecules 2013, 46, 4520−4528. (4) Zhou, W.; Li, X.; Lu, J.; Huang, N.; Chen, L.; Qi, Z.; Li, L.; Liang, H. Toughening Mystery of Natural Rubber Deciphered by Double Network Incorporating Hierarchical Structures. Sci. Rep. 2014, 4, 7502. (5) Zhou, W.; Meng, L.; Lu, J.; Wang, Z.; Zhang, W.; Huang, N.; Chen, L.; Li, L. Inducing Uniform Single-crystal Like Orientation in Natural Rubber with Constrained Uniaxial Stretch. Soft Matter 2015, 11, 5044−5052. (6) Tosaka, M.; Murakami, S.; Poompradub, S.; Kohjiya, S.; Ikeda, Y.; Toki, S.; Sics, I.; Hsiao, S. B. Orientation and Crystallization of Natural Rubber Network as Revealed by WAXD Using Synchrotron Radiation. Macromolecules 2004, 37, 3299−3309. (7) Toki, S.; Sics, I.; Ran, S.; Liu, L.; Hisao, S. B.; Murakami, S.; Senoo, K.; Kohjiya, S. New Insights into Structural Development in Natural Rubber during Uniaxial Deformation by In Situ Synchrotron X-ray Diffraction. Macromolecules 2002, 35, 6578−6584. (8) Che, J.; Burger, C.; Toki, S.; Rong, L.; Hsiao, S. B.; Amnuaypornsri, S.; Sakdapipanich, J. Crystal and Crystallites Structure of Natural Rubber and Peroxide-Vulcanized Natural Rubber by A Two-Dimensional Wide-Angle X-ray Diffraction Simulation Method. II. Strain-Induced Crystallization versus Temperature-Induced Crystallization. Macromolecules 2013, 46, 9712−9721. (9) Hu, W. Principles of Polymer Crystallization; Chemical Industry Press: Beijing, 2013. (10) Nie, Y.; Gao, H.; Yu, M.; Hu, Z.; Reiter, G.; Hu, W. Competition of Crystal Nucleation of Fabricate The Oriented SemiCrystalline Polymers. Polymer 2013, 54, 3402−3407. (11) Luch, D.; Yeh, S. G. Morphology of Strain-Induced Crystallization of Natural Rubber. I. Electron Microscopy on Uncrosslinked Thin Film. J. Appl. Phys. 1972, 43, 4326−4238.
But with the help of self-seeding, NR-1 shows clear nucleation transition phenomenon in the initial stage, because secondary folded-chain nucleation on the surface of residual TIC-melting crystallites is preferred and could lead to the formation of observable low-oriented crystal, which is in accordance with the nucleation barrier relationship described in Figure 6.
4. CONCLUSION To summarize, combining experimental observations of loworiented crystal and theoretical computation of decreasing nucleation barrier, we successfully illustrate the occurrence of nucleation transition phenomenon from folded-chain nucleation to fringed-micelle nucleation in SIC process of NR through introducing residual TIC-melting crystallites into pure NR system, which is the old concept of “self-seeding” in TIC innovatively applied to SIC process to reveal potential nucleation competition in SIC of NR during uniaxial deformation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51333003) for financial support of this research.
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