Janus Nanosheets Synchronously Strengthen and Toughen Polymer

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Janus Nanosheets Synchronously Strengthen and Toughen Polymer Blends Yu Hou,†,‡ Guolin Zhang,† Xiuping Tang,† Yan Si,‡ Ximing Song,† Fuxin Liang,*,†,‡ and Zhenzhong Yang*,‡,§

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Liaoning Provincial Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, Liaoning University, Shenyang 110036, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Institute of Polymer Science and Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Strength is usually compromised with toughness of polymer blends. We report a new way to synchronously strengthen and toughen a typical blend of epoxy resin/nitrile-butadiene rubber (EP/LNBR) with Janus nanosheets (JNs). The silica-based JNs contain a reactive epoxide group and nitrile-butadiene rubber on the opposite sides. They are robust of high strength and toughness. The JNs are covalently bound at the EP/LNBR interface upon blending and crosslinking, which can effectively transfer stress in between the two phases. At the saturation coverage of the interface with the JNs, mechanical properties of the blends reach the maxima. Excess JNs will stack into multiple layers at the interface, which will lead to interfacial slippage and thus to worse properties. This report may open a new avenue to achieve higher performance polymer materials by using Janus materials.

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INTRODUCTION Janus materials (JMs) are asymmetric and composed of different compositions distinctly compartmentalized onto different regions of the same object.1,2 In consideration of wettability difference, they are amphiphilic in analogy to molecular surfactants.3−5 JMs are usually used to reduce interfacial tension and stabilize interfaces.6 They are more effective arisen from the synergetic amphiphilic performance and Pickering effect.7,8 As compatibilizers, JMs can efficiently stabilize the interfaces of polymer blends.9−12 All of the reports are focused on controlling the domain size and morphology with the JMs.13−15 It is important to correlate mechanical properties with the microstructure of the JM-modified polymer blends. Epoxy resin (EP) has been extensively used as a typical thermoset resin.16 However, strength of the cured EP is inevitably compromised with toughness while blending with polymers especially rubbers.17 It is challenging to synchronously enhance strength and toughness of polymer blends. Herein, we propose a new way to synchronously strengthen and toughen polymer blends with reactive silica-based Janus nanosheets (JNs). Epoxide group and butadiene acrylonitrile rubber (NBR) are distinctly located onto opposite sides of the JNs (denoted as EP/NBR JN). For the EP/LNBR (liquid butadiene acrylonitrile rubber) blends, the JNs are covalently bound at the interface thus effectively transfer stress in between the two phases. As shown in Scheme 1, the starting © XXXX American Chemical Society

Scheme 1. Illustrative Synthesis of the EP/NBR JN: (1) Selective Grafting NBR onto the Amine Side; (2) Conjugation of Epoxy Group onto the Other Side with the Silane of GPTS

silica JN contains amine and silanol groups onto the opposite sides. NBR is selectively grafted onto the amine groupterminated side to achieve the NBR JN. In the second step, epoxide group is grafted onto the other side of the NBR JN with 3-glycidyloxypropyltrimethoxysilane (GPTS). The EP/ NBR JN is achieved. Received: March 25, 2019 Revised: April 25, 2019

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DOI: 10.1021/acs.macromol.9b00598 Macromolecules XXXX, XXX, XXX−XXX

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mapping mode. Fourier transform infrared (FT-IR) spectra of the JNs were measured with KBr pressed pellets using a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) was performed in air at a heating rate of 10 °C/min on a PerkinElmer Pyris 1 TGA. A Leica EMUC6 ultramicrotome was used to cut 70 nm thick samples at −90 °C. Three-point bending tests of the composites were performed on a universal testing machine (Instron 3365, USA), following ASTM D790 with a support span of 64 mm at a cross-head rate of 2 mm/ min. Tensile tests were performed on an Instron 3365 universal testing machine, following ASTM D638 at a cross-head speed of 2 mm/min. Notched Izod impact tests were performed at room temperature according to ASTM D256 using a XJC-25D impact testing machine with a pendulum energy of 5.4 J. Five random samples were measured for the average values.

EXPERIMENTAL SECTION

Materials. 3-Aminopropyl triethoxysilane (APTES), tetraethyl orthosilicate (TEOS), styrene, maleic anhydride, 2,2-azobisisobutyronitrile, thionyl chloride, methylamine, dicyclohexylcarbodiimide (DCC), N-hydroxysuccinimide (NHS), GPTS, and boron trifluoride ethylamine were purchased from Sigma-Aldrich. Bisphenol A EP E-51 (epoxide equivalent value 0.51 mol/100 g resin) with an average molecular weight of 400 was purchased from Nantong Synthetic Material (China). A liquid carboxyl group-terminated butadiene acrylonitrile (CTBN) copolymer (1300 × 13 CTBN) (carboxyl-group content: 0.057 and molecular weight: 3150) was purchased from CVC Thermoset Species. Nitrile-butadiene rubber (LNBR-40) (molecular weight 1000−2000) was purchased from Lanzhou Petrochemical Company (China). Fluorescein isothiocyanate (FITC) was purchased from J&K Chemical Co. The other reagents were purchased from Sinopharm Chemical Reagent Company. Synthesis of the Amine/Silanol Silica JN.18 The amine/silanol silica Janus hollow sphere was synthesized by the self-organized sol− gel process at an emulsion interface. After 15.0 mL of hydrolyzed styrene/maleic anhydride (10.0 wt %) was dissolved in 75.0 mL of water, the hydrochloric acid solution (2.0 mol/L) was added to adjust pH ∼2.5. APTES (1.0 mL), toluene (25.0 mL), and TEOS (5.6 g) were mixed as the oil phase. After the oil phase was added in the aqueous phase, an oil/water emulsion was formed by shearing at a speed of 13 000 rpm for 5 min. The emulsion was heated and stood at 70 °C for 12 h, forming a Janus silica hollow sphere. After the system was cooled to room temperature, the Janus hollow sphere was centrifugated and washed with water. The amine/silanol silica JN was achieved by crushing the Janus hollow sphere with a colloid mill. Synthesis of the NBR JN. CTBN was previously treated by drying at 120 °C for 12 h. CTBN (2.0 g) and dimethylformamide (0.5 mL) were mixed in 20.0 mL of toluene. Thionyl chloride (65 μL) was added for the chloroformylation at 70 °C for 24 h. The solvents were removed by reduced pressure distillation. The chloroformylated CTBN was achieved. The amine/silanol silica JN (500 mg) was dispersed in 30.0 mL of dichloromethane. In an ice-water bath, the above-modified CTBN solution was added in the silica JN dispersion under stirring for the reaction at 50 °C for 24 h in nitrogen. Afterward, the JN was centrifugated and washed with dichloromethane. The modified JN (500 mg), methylamine (2.0 g), DCC (0.805 g), and NHS (0.403 g) were added in 30.0 mL of dichloromethane for the reaction at room temperature for 24 h. After washing with dichloromethane and ethanol, the methylamineterminated CTBN (NBR) JN was synthesized. Synthesis of the EP/NBR JN and Dying. GPTS (6.0 mL) was added in 50.0 mL of toluene containing 500 mg of the CTBN JN under refluxing for 24 h; the epoxy group was conjugated onto the silanol side of the BNR JN. After centrifugation and washing with dichloromethane and ethanol, the EP/NBR JN was achieved. FITC labeling of the EP/NBR JN was performed by stirring 25 mg of the EP/NBR JN and 5 mg of FITC in 10.0 mL of tetrahydrofuran at room temperature for 2 h. Preparation of the EP/JN/LNBR Blends. After 26.6 g of E-51 and 10 phr LNBR were mixed at 150 °C for 10 min, a given amount of the EP/NBR JN was added under stirring for 10 min. Upon cooling down at 65 °C, 0.798 g of boron trifluoride ethylamine was added under stirring. After degassing in vacuum for 15 min, the mixture was poured in a Teflon mold for cross-linking at 120 °C/3 h, 200 °C/1 h, and 175 °C/12 h. Characterization. The morphology of the JNs and fractured surfaces of the blends were observed with scanning electron microscopy (SEM S-4800 at 15 kV) equipped with an energydispersive X-ray analyzer. The samples were ambient-dried and vacuum-sputtered with Pt. A transmission electron microscope (TEM, JEOL2200FS at 200 kV) was used to observe the samples. An atomic force microscope (AFM) was used to measure thickness of the JNs using a Digital Instrument Multimode Nanoscope IIIA. Derjaguin− Müller−Toporov (DMT) modulus imaging was recorded under ambient conditions in the peak force quantitative nanomechanical

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RESULTS AND DISCUSSION The representative silica JN was synthesized by crushing the Janus hollow sphere.18 Both sides of the silica JN are smooth (Figure 1a). The cross-sectional TEM image (inset Figure 1a)

Figure 1. SEM and inset cross-sectional TEM images of the three representative JNs: (a) silica; (b) NBR; and (c) EP/NBR.

shows that the silica JN is homogeneous. The JN is thick 35 nm measured by AFM (Figure S1). One side of the JN is covered with the amine group from APTES, whereas the other side is covered with the silanol (Si−OH) group from TEOS.18 The carboxylic-terminated butadiene acrylonitrile rubber (CTBN) was chlorinated with SOCl2 and grafted onto the amine side of the silica JN via nucleophilic substitution. Termination of the residual acyl chlorine group on the JN will ensure selective grafting of another material onto the opposite side. The NBR JN was achieved. The side becomes coarsening, whereas the silanol group-terminated side remains smooth (Figure 1b). The cross-sectional TEM image shows that the NBR JN displays a bilayered structure (inset Figure 1b). The gray NBR layer is thick 23 nm, and the dark silica layer keeps the same thickness 35 nm. The NBR JN becomes thicker 58 nm from 35 nm (Figure S2). The NBR JN contains 44.4 wt % of rubber measured by TGA (Figure S3). After the silanolterminated side was modified with GPTS, the epoxide group was grafted thereby. The EP/NBR JN was achieved. The corresponding side becomes slightly rough (Figure 1c). The epoxide group layer is too thin to recognize under TEM (inset Figure 1c). The AFM image result reveals that the EP/NBR JN becomes slightly thicker 60 nm (Figure S4). The EP/NBR JN contains 1.6 wt % of the epoxide group. B


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cm −1 (−CN), 1730 cm −1 (−COOH), 1443 cm −1 (−CH2−), and 965 cm−1 (−C−H) appear, confirming the presence of CTBN (Figure 2a-2).19 Meanwhile, the peaks assigned to the amine group disappear. After termination with methylamine, a new peak at 1380 cm−1 assigned to the −CH3 group appears whilst the peak 1730 cm−1 (−COOH) becomes dramatically weaker (Figure 2a-3). In the EP/NBR JN, the new peak at 910 cm−1 appears, which is assigned to the epoxide group (Figure 2a-4).20 DMT modulus imaging under ambient conditions was used to distinguish the difference between two sides of the EP/NBR JN. Two moduli were detected (Figure 2b). The dark side is corresponded to the low modulus of 0.9 GPa thus the NBR-terminated side. The light side is corresponded to the high modulus of 2.0 GPa thus the epoxide group-terminated side. In comparison, only one modulus of 2.2 GPa was detected from the silica JN (Figure 2c). The EP/NBR JN can be used to enhance compatibility of the EP/LNBR blends. High-performance blends are expected after cross-linking. The JN was dyed with FITC displaying green under a fluorescence microscope (FM). In the example blend of EP/10-LNBR containing 0.20 phr of the JN, green circles are found under FM (Figure 3a). This implies that the JNs are exclusively located at the EP/LNBR interface. The enveloped spherical domains should be the minor LNBR phase. The blend was ultramicrotomed and etched with a good solvent CH2Cl2. The cross-sectional TEM image shows that

The composition of the Janus NPs after medications was characterized with FT-IR spectroscopy. The presence of the amine group onto the silica JN is confirmed by the peaks at 1640 and 1530 cm−1 (Figure 2a-1). The new peaks at 2238

Figure 2. (a) FT-IR spectra of the silica JN (1); (2,3) the CTBN JN and after termination with the methyl-group; (4) the EP/NBR JN; (b) DMT modulus image of the EP/NBR JN; (c) DMT modulus image of the silica JN.

Figure 3. (a) Fluorescence microscopy image of the EP/0.20-JN/10-LNBR blend (the FITC-labeled EP/NBR JN showing green); (b) crosssectional TEM image of the blend; SEM images of the blends at two contents of the JN (phr): (c) 0.02; (d) 0.20; (e) cavity diameter dependence on the EP/NBR JN content in the EP/10-LNBR blend. The diameter was averaged among N = 50 random cavities. C


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Figure 4. (a) Elongation at break; (b) tensile modulus (●); and strength (gray △); (c) impact strength of the EP/10-LNBR blend as a function of the EP/NBR JN content; (d) optimal mass fraction of the JN for the maximum mechanical properties dependence on Φ2/3 (Φ: mass fraction of the dispersed LNBR phase).

the toughness of cross-linked EP. For example, the impact strength is increased from 1.5 to 3.2 kJ/m2 after feeding 10 phr of LNBR. It is interesting that impact strength of the EP/10LNBR blend can be further increased by feeding the JN. A maximum impact strength of 5.3 kJ/m2 is achieved by adding 0.20 phr of the JN (Figure 4c). Although enhancing toughness of the cross-linked EP by adding the rubber, flexural modulus and strength are remarkably sacrificed. The rubber (10 phr) can lower the flexural modulus to 2.05 GPa from 2.54 GPa, and the flexural strength to 79 MPa from 85 MPa. Unexpectedly, the JN can significantly increase both flexural modulus and strength of the EP/LNBR blends (Figure S7). At 0.20 phr of the JN, flexural modulus and strength reach the maxima 4.32 GPa and 139 MPa, even much higher than the cross-linked EP. The JN (0.20 phr) is thus optimal for maximum mechanical properties when 10 phr of the rubber is used. Similarly, in the case of 5 phr rubber, strength and toughness can also synchronously reach the maxima when 0.14 phr of the JN is fed (Figure S8). In the case of 20 phr of the rubber, the optimal content of the JN is 0.30 phr. It is understandable that a higher amount of the JN is required to completely cover the interface when the dispersed phase volume fraction is increased. At the interfacial coverage saturation, the mechanical properties will reach the maxima. Experimentally, the optimal amount of the JN is exactly proportional to the 2/3rd power of the dispersed phase mass fraction (Figure 4d). In another word, the linear plot guides to design high-performance polymer blends at varied rubber content. SEM imaging was performed to observe the fractured surfaces after the samples were broken in the tensile test. EP is a typical brittle thermoset resin, and the fractured surface is smooth (Figure S9a). The EP/10-LNBR blend exhibits a rough fractured surface (Figure S9b). The EP/LNBR interface appears smooth, and major rubber spheres are detached from the domains. In the sample of EP/0.04-JN/10-LNBR, the dispersed LNBR phase becomes smaller and the EP/LNBR interface becomes rough (Figure S9c). Many rubber spheres are incorporated inside the domains. When the JN content is

the JNs remain anchored at the EP/LNBR interface (Figure 3b). The JNs have been covalently bound at the interface after cross-linking of the EP. Alignment of the JNs at the interface was further characterized by SEM imaging. After the EP/JN/ LNBR blends were fractured in liquid nitrogen, LNBR was selectively etched with acetonitrile, leaving cavities within the EP matrix. In the case of EP/10-LNBR blend without the JN, the size of the cavities is highly scattered (Figure S5a). The average diameter is 50 μm. The cavity surface is smooth (Figure S5b). This implies that EP and LNBR are highly immiscible. After feeding 0.02 phr of the JN, the cavity becomes smaller ∼46 μm in diameter (Figure S5c). All of the JNs lay over the cavity surface, whereas the EP surface is partially exposed (Figure 3c). Coverage of the interface is increased with feeding of the JN, and the cavities become smaller and more uniform. In the presence of 0.14 phr of the JN, the cavity becomes smaller ∼28 μm in diameter (Figure S5d). Majority of the cavity surface has been covered with the JN yet single layer (Figure S5e). In the case of 0.20 phr of the JN, the cavity surface is completely covered with the JN (Figure 3d). The cavity remains the size ∼25 μm in diameter (Figure S5f). At high amount of the JN such as 0.40 phr, the cavity size remains (Figure S5g). However, the JNs start to stack into multiple layers at the surface (Figure S5h). Therefore, the cavity becomes smaller with increasing the JN amount until a plateau at 0.20 phr, whereas the cavity becomes more uniform in size (Figure 3e). This implies that coverage of the interface has been saturated at 0.20 phr of the JN. Introduction of the JN can dramatically improve the tensile property of the EP/10-LNBR blend (Figure S6). Elongation at the break is increased with increasing amount of the JN from the original 9.5% to a maximum 16.5% at 0.20 phr of the JN (Figure 4a). Toughness of the blend is greatly enhanced by the JN. The dissipation energy is accordingly increased from 1.79 to 2.71 kJ/cm2. What is more, tensile modulus is increased from the original 2.3 GPa to the maximum 3.3 GPa at 0.20 phr of the JN, whereas the tensile strength increased from 73 to 88 MPa (Figure 4b). Above 0.20 phr of the JN, both modulus and strength decrease progressively. Rubber LNBR can enhance D


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increased to 0.20 phr, the LNBR spheres are deformed leaving major rubber spheres inside the domains (Figure 5a). No JNs

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00598.

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AFM images, TGA curves, SEM images, stress strain curves, and mechanical properties of some representative samples (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.L.). *E-mail: [email protected] (Z.Y.). ORCID

Ximing Song: 0000-0002-4834-2072 Zhenzhong Yang: 0000-0002-4810-7371 Notes

Figure 5. SEM images of the fractured surface of the two blends after broken in the tensile test: (a,b) EP/0.20-JN/10-LNBR and (c,d) EP/ 0.40-JN/10-LNBR.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51233007 and 51622308).

are adhered onto the LNBR sphere surface. Careful observation shows that the JNs are highly wrinkled and partially curled up from the edge (Figure 5b). After selective dissolution of LNBR, the JNs are exposed beneath the LNBR spheres (Figure S9d). The JNs remain wrinkled and curled up from the edge at the interface (Figure S9e). In another example blend of EP/0.40-JN/10-LNBR above the saturated interfacial coverage, many LNBR spheres are detached from the domains (Figure 5c). Some JNs are present on the top surface of the LNBR spheres. Careful observation indicates that the bottom surface of the rubber spheres is also adhered with the JNs (Figure 5d). The JNs lay flat over the interface. No wrinkling or curling up is observed. At low feeding of the JN before saturation interfacial coverage, the JNs form a single layer and covalently bound at the interface. Transferring stress is thus facilitated. At the saturation coverage, all of the interface participates the stress transferring under the aid by the bound JNs. When the JN content is above the saturation, the JNs are stacked forming multiple layers at the interface. The multiple layers become slippery upon exerting stress, lowering the stress transferring efficiency.

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REFERENCES

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CONCLUSIONS

In summary, the EP/NBR JN as a reactive robust compatibilizer can synchronously enhance strength and toughness of the representative EP/rubber blends. The JNs are covalently armored at the interface from the EP side while catalytic cross-linking EP matrix. The JNs bound at the interface can effectively transfer stress in between the two polymers. There exists an optimal JN content at a given rubber content, making the blend the best both in strength and toughness. The optimal JN content is corresponded to the saturation coverage of the interface. It is expected to further magnify the synchronous enhancement by intensifying the stress transferring at the interface when two sides of the JNs are reactive. This report may open a new avenue to achieve higher performance polymer materials by using JMs. E


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Janus Nanosheets Synchronously Strengthen and Toughen Polymer

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