New Approach to Fabricate Novel Fluorosilicone ... - ACS Publications

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New Approach to Fabricate Novel Fluorosilicone Thermoplastic Vulcanizate with Bicrosslinked Silicone Rubber-Core/FluororubberShell Particles Dispersed in Poly(vinylidene Fluoride): Structure and Property Yukun Chen,*,† Youhong Wang,† Chuanhui Xu,‡ Yanpeng Wang,† and Changyun Jiang§ †

The Key Laboratory of Polymer Processing Engineering, Ministry of Education, South China University of Technology, Guangzhou, 510640, China ‡ School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China § Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore 117602 ABSTRACT: Herein, we report a new method, core−shell dynamic vulcanization, to prepare a poly(vinylidene fluoride) (PVDF)-based thermoplastic vulcanizate (TPV) with cross-linking-controlled silicone rubber (SR)/fluororubber (FKM) core− shell particles. The bicrosslinked SR-core/FKM-shell structure effectively stabilized the blending morphology of TPV, avoiding the direct contact of PVDF and SR. Results of transmission electrom microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and Fourier transform-infrared (FT-IR) confirmed the formation of core−shell structure. In the PVDF/FKM/SR (40/30/30) TPV, the size of core−shell particle was ∼2 μm, and the thickness of the FKM shell was ∼400 nm. The cross-link density of SR-core and FKM-shell could be controlled to tailor the properties of the TPV. It was found that the tan δ value of core−shell bicrosslinked TPV could be maintained at around 0.3 in range of 1−1000 cpm. The TPV exhibited good mechanical properties in which its tear strength was as high as 58 kN m−1. The new PVDF/SR/FKM TPV can be an idea potential alternative for expensive fluorosilicone rubbers in some applications.

1. INTRODUCTION Core−shell structures assembled from various molecular building blocks, in which a core structural domain is covered by a shell domain,1−4 have attracted the attention of many researchers. Core−shell structures own the special ability to encapsulate guest molecules to act as molecular flasks to confine chemical reactions,5−7 centers to stabilize reactive molecules,8−10 drug/gene carriers,11 and catalysts.12 Core−shell emulsion polymerization13 is known as a typical way to synthesize core−shell particles. As shown in Figure 1a, it is usually a two-stage process that starts with the preparation of a seed latex first (monomer A) and then is mixed with another monomer B to grow a shell onto and around core A. The core−shell structures can also be formed in multiphase polymer blending to solve the interfacial incompatibility and stabilize blending morphology.14,15 Typically, compatibilization is achieved by using block or graft copolymers with segments which concentrate at the interface between blend components and act as emulsifiers to reduce interfacial tension and inhibit coalescence during melt processing.16 Similarly, the formation of core−shell structure in polymer blending was related to the interfacial tensions between different polymer pairs to minimize the surface free energy of polymer blends.17,18 There has been considerable interest in the formation of core−shell structures in polymer blending.17−24 Favis et al.21 have studied the control of the microstructure in HDPE/PS/PMMA ternary blends. Ke et al.20 have shown a facile method to obtain the core−shell structure in PA6/PB-g-MAH/LDPE blends. Li et al.23 successfully predicted the core−shell morphology of PP/ EPDM/HDPE ternary blends in thermodynamic equilibrium © 2016 American Chemical Society

by a minimum free energy model and controlled the morphology of ternary blends from core−shell structure to separately dispersed structure. Valera et al.18 predicted the morphology of PMMA/PP/PS ternary blends by spreading coefficient, minimum free energy and dynamic interfacial energy phenomenological models. However, no reports has been involved the formation of cross-linked core−shell structures in the field of dynamic vulcanization. Fluorosilicone rubber, a kind of special and promising elastomers, plays many essential roles in various commercial and civil applications, especially for military applications. However, the strait technology barrier, high-cost synthesis, and the consequently high price of fluorosilicone rubbers limited their commercial use. Additionally, fluorosilicone rubbers must be vulcanized before practical use. After vulcanization, the thermoset fluorosilicone rubber lost its processability and recyclability,8,9 this is a waste of the expensive fluorosilicone raw materials. From the view of sustainable development, to design and prepare a new fluorosilicone thermoplastic vulcanizate (TPV)25 is a meaningful and charming topic. A typical TPV usually consists of high content of cross-linked rubber as dispersion phase and low content of thermoplastics as a continuous phase, and the rubber/thermoplastic must have good interfacial compatibility,26 so that it combines the resilience of conventional Received: Revised: Accepted: Published: 1701

December 7, 2015 January 9, 2016 February 2, 2016 February 2, 2016 DOI: 10.1021/acs.iecr.5b04676 Ind. Eng. Chem. Res. 2016, 55, 1701−1709

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) Schematic of core−shell emulsion polymerization and (b) core−shell dynamical vulcanization of PVDF/SR/FKM TPV.

Figure 2. (a) Effect of DCP and AF/BPP curing agents on the SR and (b) effect of DCP and AF-BPP curing agents on the FKM.

has, as far as we know, not yet been employed for TPVs. The new TPV based on PVDF/SR/FKM with cross-linkingcontrolled core−shell structure is reported for the first time in the field of dynamic vulcanization and shows promising for sustainable development of various functional materials for different applications.

vulcanized rubber and the processability/recyclability of thermoplastic.27 Unfortunately, when the interfacial tension distinct between two blend components is too large, for example, of PVDF and SR, directly blending PVDF with high content of SR is difficult to fabricate the desired PVDF/SR TPV. The formation of core−shell structures in emulsion polymerization offers routes to the design of new fluorosilicone TPV based on PVDF and SR via specific designed dynamic vulcanization. In this study, we report an ingenious method, core−shell dynamic vulcanization, to prepare PVDF/SR TPVs. The crosslinked SR as core is encapsulated by a third component, FKM as the shell, forming cross-linked core−shell particles dispersed in the PVDF phase. The concept is illustrated in Figure 1b. In addition, we can even control the cross-linking of the core and shell, respectively, which is quite different from the existing reports about the formation of linear polymer core−shell particles in melt blending.17,20,21 Such a SR/FKM system can also meet the criterion of good availability of raw materials and

2. EXPERIMENTS AND METHODS 2.1. Materials. PVDF (502) was purchased from Guangzhou Li Chang Fluoroplastics. Co. Ltd. FKM (2463), based on vinylidene fluoride (40%) and hexafluoropropylene (25%) and tetrafluoroethylene (35%), was purchased from Zhonghao Chenguang Research Institute of Chemical Induatry (Zigong, China). SR was a commercial rubber. Dicumyl peroxide (DCP) was purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and was purified by anhydrous alcohol recrystallization before use. Bisphenol AF (AF) and benzyltriphenylphosphonium chloride (BPP) were 1702

DOI: 10.1021/acs.iecr.5b04676 Ind. Eng. Chem. Res. 2016, 55, 1701−1709

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Figure 3. Schematic representation for preparation of cross-linking-controlled core−shell structures.

Figure 4. (a) Torque change with time during dynamic vulcanization of PVDF/FKM/SR TPV, photographs of (b) core cross-linked PVDF/FKM/ SR TPV, (c) core/shell bicrosslinked PVDF/FKM/SR TPV, and (d) dynamically vulcanized PVDF/SR without FKM.

Then the lump-shaped samples were chopped into small granules to make it easy for compression molding. In this paper, the weight ratio of PVDF/FKM/SR was fixed at 40/30/30. The dosage of DCP was 2 wt % of the SR component and AF-BPP (2:1) was 2 wt % of the FKM component. For brevity, we defined PVDF/FKM/SR as the simple blend, PVDF/FKM(AF-BPP)/SR was the shell-crosslinked TPV, PVDF/FKM/SR(DCP) was the core-cross-linked TPV, and PVDF/FKM(AF-BPP)/SR(DCP) was the core/shell bicrosslinked TPV. 2.3. Scanning Electron Microscopy (SEM) and X-ray Spectra (EDX). DMF-etched surface morphology of the sample was observed by a S3700 microscopy (Japan). Energy dispersive X-ray spectra (EDX) were collected using an INCA 350 spectrometer (Oxford, England). 2.4. Contact Angle Measurement. The surface tension of all the components was deduced by the contact angle measurement. Contact angles were measured by the sessile drop technique using an apparatus model OCA 15 PLUS, DATAPHYSICS. Measurement of a given contact angle was carried out for at least 5 times. 2.5. Fourier Transform-Infrared Spectroscopy (FT-IR). The FT-IR absorption spectra were recorded using a Tensor 27 spectrometer (Bruker, Germany) with a resolution of 4 cm−1 and 32 scans. The residual of DMF-extracted samples were oven-dried under a vacuum to eliminate the effects of residual solvent and moisture prior to testing. 2.6. Transmission Electron Microscopy (TEM). TEM observation was carried out on a JEM-100CX II transmission

purchased from Zigong Tianlong Chemical Co. Ltd. Other chemicals were used as received. 2.2. Preparation of PVDF/FKM/SR TPVs with CrossLinking-Controlled Core−Shell Structures. The PVDF/ FKM/SR TPVs were prepared in a Haake Rheocord 90 at 190 °C with a rotor speed of 90 rpm. We adopted different curing agent to vulcanize FKM and SR, respectively. The vulcanization curves were shown in Figure 2a,b from which we could conclude that DCP is very effective to cross-linking SR but has little effect on curing FKM while AF-BPP has an opposite effect. The high pertinence effects of DCP and AF-BPP on cross-linking of respective SR and FKM provide the possibility to design and control the cross-linking status of the core−shell structures. The schematic representation for preparation of crosslinking-controlled core−shell structures is illustrated in Figure 3. FKM and SR were first mixed at a weight ratio of 1:1 in an internal mixer to prepare the SR/FKM compound. For the simple blend, PVDF was first shear-melted. Then the SR/FKM compound was added and mixed for another 5−6 min. For the core or shell single cross-linked TPV, after SR/FKM compound was well-mixed with melted PVDF, DCP or AF-BPP was added and the mixing was continued 5−6 min to achieve the final equilibrium torque. For the core−shell bicrosslinked TPV, after PVDF matrix and SR/FKM compound were uniformly mixed, DCP was added and 3−4 min later AF/BPP was added and the mixing was continued to reach the final stable torque. Subsequently, the resultants were removed from the cavity of Haake Rheocord 90 and cooled down to the room temperature. 1703


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Industrial & Engineering Chemistry Research electron microscope (JEOL, Japan) with an accelerating voltage of 100 kV. Ultrathin sections (about 90−100 nm in thickness) were sliced using a ultramicrotome (Leica EMUC6, Germany). 2.7. Rubber Process Analyzer (RPA). Rubber Process Analyzer (RPA 2000, Alpha Technologies) was utilized to study the viscoelastic behavior of the samples. Frequency sweep tests were performed at constant strain amplitude of 7% and temperature of 130 °C. The frequency was logarithmically increased from 1 to 1000 cpm. 2.8. Mechanical Properties Measurements. Measurements of tensile and tear properties were conducted on a Computerized Tensile Strength Tester (UT-2080, U-CAN Dynatex Inc., Taiwan) with a crosshead speed of 500 mm/min, according to ISO 37-2005 and ISO 34-2004, respectively. Dumbbell shaped and unnicked 90° angle specimens with 1 mm of thickness were used for tensile and tear test, respectively. Shore A hardness was measured according to ISO 868-2003. At least five specimens were measured for each sample and averaged results were showed.

Figure 5. (a) photographs of dissolving/swelling experiments; (b) FTIR results: a- cross-linked SR, b- DMF-extracted core-cross-linked PVDF/FKM/SR (40/30/30) residual (only cured by DCP), c- crosslinked FKM, d- DMF-extracted core/shell bicrosslinked PVDF/FKM/ SR (40/30/30) residual (cured by DCP and AF-BPP), e-PVDF;.

for 10 min, the immersed parts of PVDF and un-cross-linked FKM were dissolved completely. However, DMF is a poor solvent for SR (δ = 7.3−7.6 (cal/cm3)1/2). It shows that the uncross-linked SR sample was only slightly swollen rather than dissolved after immersed in hot DMF for 10 min. The results of dissolving/swelling experiments revealed that the PVDF and un-cross-linked FKM had already been removed completely after DMF-extraction for 48 h. Figure 5b shows the FT-IR spectra of SR (curve a), the residual of DMF-extracted PVDF/ FKM/SR(DCP) (curve b), FKM (curve c), the residual of DMF-extracted PVDF/FKM(AF-BPP)/SR(DCP) (curve d), and PVDF (curve e). As expected, the FT-IR of the residual of the PVDF/FKM/SR(DCP) (curve-b) is quite similar to that of SR (curve-a), and the FT-IR of PVDF/FKM(AF-BPP)/ SR(DCP) (curve-d) is almost same as that of FKM (curvec). This strongly confirmed that the SR phase was completely encapsulated by the FKM phase in the PVDF matrix, forming core−shell structure. 3.3. TEM Observation. The TEM images of the core/shell bicrosslinked PVDF/FKM/SR (40/30/30) TPV are shown in Figure 6a,b, which provides the evidence for core−shell structure. The annular area in light color in the TEM images represents the FKM-shell and the darker elliptical-shaped area enclosed by the light color annular is the SR-core. The size of the core−shell SR/FKM particles is ∼2 μm, and the thickness of the FKM shell is approximately 380−400 nm. The formation of such core−shell sphere structures reduced the interfacial tension between PVDF and SR and obtained a compatible and intimate interaction between the rubber particles and PVDF continuous phase,36 stabilizing the phase morphology.17 3.4. SEM-EDX Analysis. The morphology structure of the bicrosslinked core/shell particles was observed by SEM. To expose the bicrosslinked core/shell particles, the cryo-fracture surfaces of TPVs were undergone hot N,N-Dimethylformamide (DMF) etching to remove away the PVDF. The results of bicrosslinked PVDF/FKM/SR (40/30/30) and PVDF/FKM (40/60) are shown in Figure 7a,b. The SEM images reveal that the cross-linked rubber phases, namely, the SR/FKM (Figure 7a) and the FKM (Figure 7b), are presented in spherical particles which are uniformly distributed in the PVDF matrix. Comparing parts a and b of Figure 7, the cross-linked FKM particles have a relative smooth surface, while the bicrosslinked core−shell SR/FKM particles exhibit a rough surface, and the average size of the bicross-linked core−shell SR/FKM particles is somewhat larger than the cross-linked FKM particles. The

3. RESULTS AND DISCUSSION 3.1. Core−Shell Dynamic Vulcanization. The torque change in core−shell dynamic vulcanization performed in a Haake rheomix is shown in Figure 4a. The first two sharp peaks were attributed to the melting of the PVDF pellets (40 wt %) and SR/FKM (30 wt %/30 wt %) compound, respectively. Afterward, the torque became very flat, suggesting the complete melting of the components and the full homogenization of the blends.28 A sharp increment in torque after adding DCP demonstrated the cross-linking of the SR phase29 (see Figure 2a). Under the strong shearing, SR with increasing cross-links could not coalesce but transformed into the cross-linked particles.30,31 The cross-linking increased the hardness, modulus, and elasticity of the SR and thus reduced the friction between the SR and the other components, resulting in a slight decline in torque (as shown in Figure 4a at 7−10 min). The appearance of the core-cross-linked TPV at this moment showed an integral elastomeric material (Figure 4b), which was distinct from the powder-like failed PVDF/SR sample as shown in Figure 4d. Without FKM, the inherent characteristics of the low surface energy of components and the poor interfacial compatibility between PVDF and SR resulted in powder-like failed sample which was unable to be compression molded for further processing.32 It is reasonable to deduce that the uncross-linked FKM component encapsulate the cross-linked SR particles at this moment, avoiding phase separation which happened in the failed PVDF/SR blend without FKM. At last, adding AF-BPP led to a further increase in the final torque, which suggested the cross-linking of the FKM component. The final bicrosslinked PVDF/FKM/SR TPV had a deeper color and shows a similar appearance to the SR-cross-linked TPV, as shown in Figure 4c. 3.2. FT-IR Analysis. To further confirm the structure of the rubber phase, we used hot DMF to extract the TPV for 48 h which was then dried at 60 °C for 8 h, and the residuals were characterized by FT-IR. The solubility parameters (δ) of DMF and PVDF are 12.1(cal/cm3 ) 1/2 and 12.2(cal/cm 3 ) 1/2 , respectively. According to the “similar dissolve mutually theory”,33−35 DMF is an excellent solvent for PVDF. The digital photographs of dissolving/swelling experiments are shown in Figure 5a to give a direct proof to support the analysis in FT-IR. It is clearly seen that the PVDF and un-cross-linked FKM were easily dissolved in DMF at 100 °C. After immersed 1704


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Figure 6. (a,b) TEM images of core/shell bicrosslinked PVDF/FKM/SR TPV.

Figure 7. (a) SEM and EDX of the core/shell bicrosslinked particles in PVDF/FKM/SR TPV; (b) SEM and EDX of cross-linked FKM particles in PVDF/FKM TPV.

Figure 8. (a) Fluorine element (down) and silicon element (up) concentrations from assumed homogeneous distribution and EDX data and (b) scheme of the possible core−shell structure in PVDF/FKM/SR TPV.

EDX spectra2,37 further supports the formation of core−shell structure, as evident from the observation of fluorine atoms in addition to carbon atoms in the spectrum (insert in Figure 7a). However, two small peaks corresponding to silicon and oxygen atoms (−Si−O−) are also presented in the EDX of core−shell SR/FKM particles. Is it some SR-cores was not completely

encapsulated by the FKM-shell? Considering the FT-IR results (Figure 5b, curve d) as well as the weight ratio of FKM/SR (1/ 1), this speculation could not hold water. We here assume that SR and FKM form a homogeneous distribution, and the concentrations of fluorine and silicon elements can be calculated according to the following equation: 1705


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Figure 9. (a) Surface tension for SR, FKM and PVDF and (b) interfacial tension of PVDF/SR, PVDF/FKM, and FKM/SR pairs.

Figure 10. Viscoelastic behavior of PVDF/FKM/SR TPVs with cross-linking-controlled core−shell structures: (a) G′ and (b) tan δ.

(weight % of fluorine element (or silicon element) in repeat chain segment) × (weight % of FKM (or SR) in the rubber phase). Here, the weight % of fluorine element and silicon element in their repeat chain segment are calculated as 71.7% and 37.8%, respectively. Take the PVDF/FKM/SR (40/30/30) as an example, the concentrations of fluorine element and silicon element in assumed homogeneous distribution can be calculated as 71.7% × [30/(30 + 30)] ≈ 35.9% and 37.8 × [30/ (30 + 30)] = 18.9%, respectively. The calculated results for assumed homogeneous distribution and the experimental data obtained from EDX are illustrated in Figure 8a. As expected, the fluorine element data from EDX is much higher than that from the assumed calculation; while the situation of silicon element is reverse. Even in the PVDF/ FKM/SR (40/10/50) system, the EDX data of the fluorine element is as high as to 67%, much higher than the calculated 12% for the assumed homogeneous distribution. When the FKM concentration is increased up to equivalent to the SR in the PVDF/FKM/SR (40/30/30) system, the EDX data of the fluorine element reaches to ∼71%, which is close to the EDX data of the fluorine element in FKM, ∼71.7%. This provides a direct proof that fluorine element was mostly accumulated at the outside (surface) layer of the SR/FKM microspheres, forming a FKM-shell. On the other hand, the silicon mainly accumulated inside the microspheres, forming a SR-core. However, silicon element still can be detected experimentally even at a much lower SR concentration, e.g., the EDX data of silicon element in PVDF/FKM/SR (40/50/10) was 0.21% which was much lower than the assumed data. Please be noted that the EDX signal is not only from the surface of the sample, the depth of field should also be taken into consideration. The depth of field in EDX might penetrate the FKM layer and detect the trace amounts of silicon element which originated

from the core SR molecule interpenetrating into the FKM-shell. There also may be a possible transition region where the concentration of silicon element is reduced from the core edge to the surface of the shell. On the basis of the above TEM results and the SEM-EDX analysis, we proposed a possible core−shell structure in the PVDF/FKM/SR TPV, which is shown in Figure 8b. However, further detailed and deep study about the formation of core−shell structure in TPV still need to be conducted in the future. 3.5. Formation Mechanism of Core−Shell Structure. The formation mechanism of the SR/FKM core−shell structure can also be simply discussed according to the interfacial tension, since the phase morphology is generally determined by the following hierarchy factors: interfacial tension > viscosity ratio > shear stress.21,38 The surface tension in dispersion and polar components are calculated by eqs 1 and 2. ⎛ γd γd γHp Oγ p ⎞ HO 2 ⎟ + (1 + cos θH2O)γH O = 4⎜⎜ d 2 p d 2 γ + γ p ⎟⎠ H 2O ⎝ γH2O + γ

(1 + cos θCH2I2)γCH I

2 2

(1)

p ⎛ γd γd γCH γp ⎞ CH 2I 2 2I 2 ⎜ ⎟ = 4⎜ d + p p⎟ d γ + γ γ + γ CH 2I 2 ⎝ CH2I2 ⎠

(2)

⎛ γ dγ d γ pγ p ⎞ γ12 = γ1 + γ2 − 4⎜⎜ d 1 2 d + p 1 2 p ⎟⎟ γ1 + γ2 ⎠ ⎝ γ1 + γ2

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p

γHd 2O

γHp 2O,

(3)

where γ = γ + γ , γH2O = + and γCH2I2 = + γ is surface tension, d is dispersion component, and p is polar component, θH2O and θCH2I2 are contact angles of the polymer d

γdCH2I2

γpCH2I2,


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Figure 11. (a) Stress−strain curves of PVDF/FKM/SR TPV with cross-linking-controlled core−shell structures, (b) possible model of stress line distribution in PVDF/FKM/SR simple blend, and (c) possible model of stress line distribution in the PVDF/FKM/SR core/shell bicrosslinked TPV.

elastomeric material with low energy dissipation potential applied in a harsh environment, e.g., conveyor belt. 3.6. Mechanical Properties. Figure 11a depicted the stress−strain curves of PVDF/FKM/SR TPV with crosslinking-controlled core−shell structures. Similar to the G′, the tensile strength of TPV was largely improved and tailored by cross-linking the core−shell FKM/SR particles. The disappointed elongation of shell-cross-linked TPV was possibly due to the unexpected impurities in the test specimen. The core/ shell bicrosslinked TPV showed the best tensile strength which was nearly 12 MPa, almost double that of the simple blend, ∼6.6 MPa. This could be explained by that cross-linked rubber phase with 3D networks played an important role on stress transfer. Figure 11b,c is the schematic diagrams of the possible stress line distribution model for uncross-linked and crosslinked core−shell structures during stretching, respectively. When the stress was transferred from the PVDF matrix to the rubber phase, the 3D cross-linked network of the rubber phase could effectively transfer the stress more uniformly. Subjecting to a stress, the cross-linked FKM-shell could act as an effectual transition layer while the uncross-linked one might be easily damaged resulting in an early end of stress-transfer. In addition, cross-linked rubber had a higher strength to bear stress and dissipate the energy, which contributed to the mechanical properties of the TPVs. From the view of partial replacement of the costly fluorosilicone rubber, we chose two kinds of commercial fluorosilicone rubber, FE-271-u and LS 4-9080#, to compare their mechanical properties with the PVDF/FKM/SR TPVs. The data of mechanical properties were collected in Table 1. The core/shell bicross-linked TPV exhibited good mechanical properties in which its tear strength was as high as 58 kN m −1 and tensile strength was 12.4 MPa. Obviously, the tensile strength and tear strength of the core/shell bicross-linked TPV were much higher than that of FE-271-u and LS 4-9080. For example, the tensile strength and tear strength of the core/shell bicross-linked TPV showed 157% and 311% higher than that of the LS 4-9080, respectively. However, elongation at the break needs to be further improved to fit the requirement for various applications.

with water and methylene iodide, respectively. The interfacial tensions γ12, which is the interfacial tension between material 1 and 2, are calculated by the equation of Wu (eq 3),39 γ1 and γ2 are the surface tensions of the two contacting components in the blends. The calculated surface tension and interfacial tension are shown in Figure 9a,b. It is clearly seen that the interfacial tension between the pairs of PVDF/SR was 8.8 mN/ m, much higher than that of PVDF/FKM 3.8 mN/m and SR/ FKM 2.4 mN/m. During the melting blending process, the requirement of minimizing the surface free energy40−42drives the FKM chains located between the PVDF and SR phases. After the SR phase was cross-linked by DCP and transformed to particles, the FKM further encapsulate the cross-linked SR particles, forming the shell which functioned as an emulsifier to prevent the contact between PVDF and SR. Further crosslinking of the FKM shell by AF-BPP enhanced the mechanical properties of the final material and stabilized the structure morphology.17,19 3.5. Viscoelastic Behavior of TPVs with Cross-LinkingControlled Core−Shell Structures. Viscoelastic behavior tightly connected with practical application for rubber products. Herein, we show the viscoelastic behavior of the novel PVDF/ FKM/SR TPVs with cross-linking-controlled core−shell structures in Figure 10. The cross-linking-control of core-SR and shell-FKM could be realized through selectively adding the cross-linking agents during dynamic vulcanization as shown in Figure 3. As expected, as shown in Figure 10a, the core/shell bicrosslinked TPV has the highest G′, followed by shell-crosslinked TPV, core-cross-linked TPV, and non-cross-linked simple blend. The shell-cross-linked TPV, owing to its “hard” shell that had higher ability to resist deformation compared to “soft” shell, had higher G′ than core-cross-linked TPV. It should be noted that the G′ showed no evident changes in the range of frequency window. This might be ascribed to the good interaction and entanglement between FKM molecules and PVDF molecules and fine dispersion of the rubber particles.43 The situations of tan δ are shown in Figure 10b. Cross-linking endowed rubber phase with high elasticity and modulus, which led to a lower hysteresis loss. Therefore, un-cross-linked simple blend obtained the highest hysteresis loss, followed by the shellun-cross-linked TPV, shell-cross-linked TPV, and the last core/ shell bicrosslinked TPV. As well-known, a low tan δ value suggests high elasticity and low heat buildup of material. Notably, the tan δ value of core/shell bicrosslinked TPV remained at around 0.3 in the whole frequency window, and this suggested that it could be an ideal fluorosilicone

4. CONCLUSION In this paper, we have developed a new strategy, core−shell dynamic vulcanization for the fabrication of PVDF/FKM/SR TPVs with cross-linking-controlled core−shell structures. To the best of our knowledge, no literature has reported the core− 1707


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Table 1. Mechanical Properties Data of PVDF/FKM/SR TPV with Cross-Linking-Controlled Core-Shell Structures samples simple blend core-cross-linked TPV shell-cross-linked TPV core/shell bicrosslinked TPV FE-271-ua LS 4-9080b

tensile strength (MPa)

tear strength (kN m −1)

elongation at break (%)

shore A hardness

6.6 ± 0.3 9 ± 0.9

25.2 ± 2.1 30.5 ± 4.2

97 ± 12 131 ± 8

91 ± 1 91 ± 1

9.6 ± 0.5

27.4 ± 1.5

43 ± 21

94 ± 2

12.4 ± 0.4

58 ± 2.6

105 ± 18

94 ± 1

9.0 7.9

16 18.62

300 150

73 80

a ShinEstu commercial fluorosilicone rubber. bDow Corning commercial fluorosilicone rubber.

shell structure formed in TPVs and this represents the first example of core−shell structures made from cross-linked SR and FKM. We adopted SEM and EDX to prove the formation of core−shell SR-FKM structure phase in the PVDF matrix. The results clearly showed that fluorine element was mostly accumulated at the outside (surface) layer of the SR/FKM microspheres and the silicon mainly accumulated inside the microspheres. FT-IR results showed that no characteristic peaks of SR were detected for the shell-cross-linked and core−shell cross-linked samples after the PVDF matrix was completely removed, which indicates that SR was encapsulated by FKM. TEM photographs directly showed core−shell rubber particles in the PVDF matrix. Additionally, we deduced interfacial tension of PVDF/SR, PVDF/FKM, and FKM/SR, respectively, and thus verified the formation of the core−shell structure of SR and FKM in the PVDF matrix theoretically. Viscoelastic behavior and mechanical studies manifested that the core−shell bicross-linked TPV has the most comprehensive properties. Our method could also be further extended to produce various core−shell structures with controlled shells with various crosslink degrees, which could lead to new ways for developing novel fluorosilicone elastomeric materials.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 02087110804. Fax: 02085293483. E-mail: cyk@scut. edu.cn. Notes

The authors declare no competing financial interest.

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REFERENCES

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New Approach to Fabricate Novel Fluorosilicone ... - ACS Publications

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