Enthalpy-Enhanced Janus Nanosheets for Trapping Nonequilibrium

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

Enthalpy-Enhanced Janus Nanosheets for Trapping Nonequilibrium Morphology of Immiscible Polymer Blends Huarong Nie, Xincheng Liang, and Aihua He* Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China S Supporting Information *

ABSTRACT: Janus particles are promising for self-assembly at the liquid/liquid interface due to coexistence of the Pickering effect and the different wettability of both sides. In this study, we synthesize a Janus nanosheet and evaluate its ability to trap nonequilibrium morphology by interfacial selfassembly. The Janus nanosheet was synthesized by selectively grafting polymer chains, polystyrene (PS) or polyisoprene (PI), on each side of a silica nanosheet. The selective grafting method endow the nanosheet with two different wettabilities, which make it ideal for self-assembly and further jamming at the PS/PI interface. The interfacial jamming of the nanosheet trapped the intermediate, nonequilibrium morphology during phase separation of the polymer blends. Compared to other Janus materials, the Janus nanosheet has higher interfacial activity and reduces the free energy of the system more effectively due to its 2D structure. Only 2 wt % of Janus nanosheet is required to break the threshold and reach the jammed state. The Janus nanosheet is easily scaled up and has potential as a compatibilizer in polymer materials.

1. INTRODUCTION

of the nanoparticles is more stringent but absolutely necessary for trapping the phase morphology at nonequilibrium.2,8 Janus particles (JPs) are entities with two distinct components and different physics−chemistry properties in the same objects but on two opposite sides.13−20 The welldefined structure of Janus particles, especially in biphasic polymer brushes, is promising as the next generation of compatibilizers in polymer blends due to their combined intrinsic properties, the amphiphilic performance of two compartment brushes, and the “Pickering effect” of the particulate character.21−24 Compared to the interfacial activity of different types of compatibilizers, such as block copolymers, polymer grafted nanoparticles, and Janus particles with two native polymer brushes, the interactions between Janus particles and the two phases are most energetically favorable, allowing the largest desorption energy from the interface.25 Incorporation of such JPs into polymer blends significantly slows domain growth, reflected as smaller phase domains during later stages of the phase separation process.26−29 While significant progress has been made in the control and application of spherical JPs in polymer blends, research interest has been limited to isotropic nanoparticles.22,30,31 However, the percolated network of anisotropic particles at the liquid−liquid interface allows a more stable emulsion because their rotation at the interface is highly restricted.32,33 Recently, the sheet-shaped

Polymer blends have been widely utilized in many fields; however, phase separation caused by environmental disturbance leads to the instability and failure of these materials. Suppression of the phase-separated system is therefore desirable and often used to fulfill the design requirements of advanced materials, yet remains a significant challenge.1,2 Solid colloids enable the generation of stable biphasic systems by selfassembly at the liquid−liquid interface to minimize the free energy of the system, which is subjected to the intricate balance of entropy and enthalpy contributions.3−10 Aside from the square of the particle radius and the interfacial tension, the energetic gain for placing particles at the liquid−liquid interface predominantly scales with the neutralization of particles by both phases.11,12 Preferential wetting of these particles by one phase can lead to their detachment from the interface to the phase that normally exhibits strong interactions with particles. Thus, phase coarsening leads to instability of the phase morphology until a thermodynamic state is reached. Considering the enthalpy, tethering small molecules or polymer brushes, which preserve similar chemical properties with one phase, onto the nanoparticles somewhat offsets the preference of particles for another phase. Finding the balance between particle−liquid interactions encourages a significant increase in the energy holding of nanoparticles at the interface. Particularly in polymer blends, where the interfacial tension between the demixing liquids is quite low, precise control over the surface chemistry © XXXX American Chemical Society

Received: January 8, 2018 Revised: March 20, 2018

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DOI: 10.1021/acs.macromol.8b00039 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules kaolinite Janus particles have shown compatibilization to polymer blends, although a high concentration is required.34 JP concentration remains a critical parameter governing the properties of polymer blends; JPs at high loading of above 3 wt % are required for the slow coarsening of phase morphology, ultimately negating the economic benefits of using Janus particles.19,31,35 As the percolation threshold of particles decreases inversely with particle aspect ratio, it is more practical and interesting to develop anisotropic, neutrally wettable Janus nanosheets to mediate the phase morphology of polymer blends. In this work, silica nanosheets that can be produced scale up24 and used as the common fillers in polymer materials are functionalized by grafting two phasic polymer brushes onto their respective compartment faces to mediate interactions between the particles and two fluids. We hypothesize that the complex interplay between entropic and enthalpic contributions resulting from the combination of silica nanosheets and compartmentalized polymer brushes could increase the energetic holding of particles at the interface, slow the coarsening of phase morphology in polymer blends, and ultimately trap the nonequilibrium phase structure.

Scheme 1. Janus Nanosheets (PS-silica-PI) Fabricated by Chemoselective Surface Modification on Silica Nanosheets

analysis, PS chains were etched from silica nanosheets by 5% vol/vol of HF aqueous solution. Before further modification of Cl-PS-silica-NH2 on the opposite side, azido-terminated PS brushes were operated as follows: 200 mg of sodium azide dissolved in 150 mL of anhydrous DMF at 90 °C; then Cl-PS-silica-NH2 suspension in DMF was added and stirred at 90 °C for 24 h. The resulting red-orange solution was centrifuged and washed with DMF and toluene. N3-PS-silica-NH2 sheets were finally collected, as shown in step 3 of Scheme 1. 2.2.3. PS-Silica-Br Nanosheet Synthesis after Br-Terminated Silica Surface. PS-silica-Br particles were synthesized as follows. 2 mL of 2bromoisobutyryl bromide and 0.3 mL of trimethylamine were added to the suspension of azido-terminated PS-silica-NH2 in anhydrous toluene. The mixture was stirred for 24 h at room temperature and appeared as a thick brown solution. After centrifugation and washing with toluene and cyclohexane, the PS-silica-Br sheets were then dispersed in anhydrous cyclohexane, as shown in step 4 of Scheme 1. 2.2.4. PS-Silica-PI Janus Nanosheet Synthesis by Grafting PI onto the SiO2 Surface. After 150 mL of anhydrous cyclohexane and 30 g of dry isoprene were added to a vacuum-dried flask, 0.3 mL (2.5 M) of nbutyllithium was injected. After reaction at 25 °C for 2 h, the residual monomer was pumped out. The above PS-silica-Br sheet dispersion was transferred into the flask under high purity nitrogen for the reaction at room temperature for 24 h. 3 mL of methanol was injected to terminate the active sites. The product was washed with toluene and centrifuged several times until no free PI was detected in the supernatant. In addition, the supernatant was collected and precipitated in ethanol to obtain the free PI fraction. The molecular weight and molecular weight distribution of PI chains were evaluated by GPC (Mn = 160K, PDI = 1.32). The microstructures of PS brushes and PI brushes (72 mol % of cis-1,4; 25 mol % of trans-1,4; 3 mol % of 3,4) were verified by 1H NMR spectra (Figure S1). The PS-silica-PI Janus nanosheets were then dispersed in toluene. 2.3. Characterization. HLC-8320 GPC with a refractive index detector was used to measure the molecular weight of the polymers. Tetrahydrofuran was used as the carrier solvent. Samples for AFM observation were prepared by spin-coating the silica nanosheet suspension onto a clean silicon wafer. AFM was performed by scanning probe microscope (Bruker Multimode 8) in tapping mode. 1 H NMR (500 MHz) spectra were recorded at 25 °C with a Bruker 500 MHz spectrometer with samples in CDCl 3 containing tetramethylsilane as standard. FT-IR spectra of the samples were recorded on a TENSOR27 FTIR spectrometer (Bruker, Germany) using KBr pellets. TEM characterization was performed using a JEM 2100 operated at an acceleration voltage of 200 kV, and samples were stained with ruthenium tetraoxide prior to observation. TGA was performed using the TG 209 F1 Libra (Germany Netzsch) in air at a heating rate of 10 °C/min to determine the grafting density which was calculated by the equation

2. EXPERIMENTAL SECTION 2.1. Materials. Isoprene (IP) (polymerization grade) was supplied by Shandong Huaju Polymer Materials Co., Ltd. (China), and styrene (polymerization grade) was supplied by Aladdin (China). Both monomers were distilled over CaH2 under reduced pressure prior to use. 4-Chloromethylphenyltrimethoxysilane (CMPTS) (90%, Alfa Aesa r), tr imethylam ine (99%, Aladdin), pe ntam ethyldiethylenetriamine (PMDETA) (99%, Aladdin), 2-bromoisobutyryl bromide (99%, Aladdin), sodium azide (99.5%, Xiya Chemical reagent Co, China), and n-butyllithium (2.5 M in hexane) (n-BuLi, Aladdin) were all used as received. Copper bromide (CuBr, Aladdin) was purified with acetate before use. Cyclohexane (Fuyu Chemical reagent, China) and toluene (Laiyang Excellence Special Chemicals Research Co., Ltd., China) were treated with Na and diphenyl ketone until the solution became dark blue and then distilled before use. Dimethylformamide (DMF, Sinopharm chemical reagent Co., Ltd., China) was distilled with CaH2 under reduced pressure. Silica nanosheets were prepared by a self-assembled sol−gel process at an emulsion interface to form a shell.23 The two polymer matrixes, polystyrene (PS, Mw = 270K, Aladdin) and polyisoprene (PI, Mw = 610K, cis-1,4 > 95 mol %, Qingdao Yikesi New Material Company, China), were used as received. 2.2. Fabrication of Near Neutrally Wettable Silica Janus Nanosheet (PS-Silica-PI). 2.2.1. Cl-Silica-NH2 Nanosheet Synthesis after Termination of the Chloride Group onto the Silica Surface. After 1 g of silica nanosheets was dispersed in 100 mL of toluene, 2 mL of CMPTS and 0.3 mL of triethylamine were added while stirring. The reaction proceeded under continual stirring at 80 °C for 24 h to allow selective termination of the hydroxyl group on the silica nanosheets. After centrifugation and washing with toluene, the Clsilica-NH2 sheets were dispersed in ethanol (20 mL), as shown in step 1 of Scheme 1. 2.2.2. PS-Silica-NH2 Nanosheet Synthesis after Attaching PS onto the Silica Surface. 700 mg of purified copper bromide and 50 mL of styrene were added to 120 mL of the above-mentioned Cl-silica-NH2 sheet ethanol dispersion in a sealed vial. The mixture was purged with high purity nitrogen prior to the addition of PMDETA. The reaction was carried out under a nitrogen atmosphere for 24 h at 70 °C. After centrifugation and washing with ethanol, THF, and DMF, in that order, the PS-silica-NH2 particles were dispersed in DMF (20 mL), as shown in step 2 of Scheme 1. Here, the chlorine functional groups remained on the end of PS chains, which can be termed Cl-PS-silicaNH2. The molecular weight and molecular weight distribution of PS chains were analyzed by GPC (Mn = 310K, PDI = 2.5). Before GPC B

DOI: 10.1021/acs.macromol.8b00039 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Gr =

W1 W2 − 1 − W1 1 − W2

where Gr is the grafting density and W1 and W2 are the residual weights after silica and PS-silica-PI Janus nanosheets are heated to 900 °C, respectively. The morphology, distribution, and location of JPs in polymer blends were observed using SEM (Jeol 7500F) at an acceleration voltage of 5 kV. In order to study the interfaces of the PS/ PI system, samples were immersed in heptane for 24 h to extract the PI phase before being broken under liquid nitrogen. Optical microscopy images were recorded using an Olympus BX51 microscope. A PS/PI (6/4, wt/wt) mixture was dissolved in dichloromethane (3% wt/vol) with 0.5 wt % of antioxidant butylated hydroxytoluene (BHT) in solid recipe. The suspension of Janus nanosheets in dichloromethane was added into the above polymer solution while stirring. 100 μL of the mixtures was spin-coated at 1000 rpm (Best tools, LLC (SC100)) onto a glass substrate at room temperature, forming thin films. Thermal annealing of some samples was performed at 150 °C under the protection of nitrogen atmosphere.

Figure 2. TEM images of the polymer grafted silica Janus nanosheets (a) PS-silica and (b) PS-silica-PI.

results of AFM. Then, the sandwiched structure appears after the attachment of PI brushes. The polymer thickness of PS and PI on silica nanosheets should be approximately 48 and 45 nm, respectively. To obtain the neutrally wettable particles, the polymers attached onto the silica nanosheets should be strictly compartmentalized; otherwise, the interfacial activity of particles is impeded. As shown in Figure S4, the aminefunctionalized PS nanoparticles are selectively adsorbed on one side where the attached PS brushes were sulfonated. This indicates that the provided grafting route ensures that the polymer chains selectively residing on silica Janus sheets are in compartmented fashion. The grafting density of PS and PI measured by TGA analysis is 33.3 and 50.4 wt %, respectively (Figure S5). Tailoring the morphology of PS/PI blends is beneficial for obtaining advanced high toughness polymer materials. Evaporation of the solvent during spin-coating leads to phase separation of the PS/PI blends, achieving the nonequilibrium cocontinuous microphase-separated structure. After annealing for 1 h at 150 °C, which is above the glass transition temperature of both PS and PI, the morphology continuously coarsens into a sea−island structure with approximately 20 μm of PS-rich phases dispersed in the PI-rich matrix (Figure 3).

3. RESULTS AND DISCUSSION Silica nanosheets that can scale up production are used as the templates where the different groups (amine groups and hydroxyl groups) in each compartment provide a simple way to functionalize different molecules for the production of neutrally wettable particles.36 As reported, the Janus hollow silica spheres are first synthesized by self-assembled sol−gel process at an emulsion interface. The hydroxyl groups originated from the condensation of tetraethyl orthosilicate face toward the internal oil phase while the amine groups arising from the water-soluble coupler are prone to face toward the external aqueous phase.23,24 In this case, the obtained silica nanosheets smashed from Janus hollow silica spheres are approximately 600 nm long and 20 nm thick (Figure 1 and Figure S2). The polymers

Figure 1. (A) SEM images and (B) AFM images of silica nanosheets.

attached to the silica nanosheets are very representative, involving saturated (PS) and unsaturated (PI) polymers, signifying a large number of polymer pairs, in contrast to the limitation of saturated plastic polymers.19,27,28,30 In the FTIR spectra (Figure S3), peaks at 1641 cm−1 arise from the benzene skeleton vibration with the CMPTS attachment. The characteristic peak for monosubstitution of benzene at 700 cm−1 becomes stronger, signifying the synthesis of PS chains on the silica nanosheet. The appearance of a new peak at 2100 cm−1 assigned to the vibration of the azide group is observed for the azide-terminated PS-silica-NH2 nanosheets. New peaks at 1661 and 1537 cm−1 are assigned to the amide groups after the introduction of functional bromine on the silica-NH2 face. The TEM images of the different silica nanosheets (Figure 2) show the distinct bilayer structure for the incorporation of PS chains on the nanosheets with a thickness of approximately 20 nm, which is consistent with the

Figure 3. Phase morphologies of PS/PI (6/4 wt/wt) blends after annealing at 150 °C for (A) 0 min and (B) 60 min. The dark phases are PS-rich domains.

Obviously, silica nanosheets without modification are thermodynamically immiscible with the PS/PI blend, reflected as aggregation in the fluids owing to the interactions between silica nanosheets being energetically more favorable than the particle−polymer interactions (Figure 4). The existence of polar groups on two surfaces of the silica nanosheets, including the amine groups and hydroxyl groups, hinders their compatibilization to nonpolar polymer blends. Notably, the film thickness is approximately 700 nm (shown in Figure S6), which is sufficiently thick to ensure the nanosheet does not lie C

DOI: 10.1021/acs.macromol.8b00039 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Phase contrast optical microscopic images of the PS/PI (6/4 wt/wt) blends with different concentrations of silica nanosheets (wt %): (A) 1, (B) 2, and (C) 3. (1) As-spun films and (2) films annealed at 150 °C for 60 min. The dark domain is PS-rich phase.

Figure 5. Phase contrast optical microscopic images of the PS/PI (6/4 wt/wt) blends demonstrating the changes in domain size with the addition of PS-silica-PI Janus nanosheets (wt %) at different concentrations: (a) 1, (b) 2, and (c) 3. (1) As-spun films and (2) films annealed at 150 °C for 60 min. The dark droplet is PS-rich domain, and the surrounding light gray phase represents the PI domain.

flat on the substrate to reduce its compatibilization efficiency. Regarding the absence of special interactions between silica nanosheets and PS/PI blends, although solid colloids are reportedly efficient at trapping the nonequilibrium state of biphasic systems by jamming at the interface, the pinned coarsening of PS/PI blends remains absent, even when adding 3 wt % of silica nanosheets (Figure 4). Thus, the entropy penalty with the addition of silica nanosheets generates particle sediment from polymer mixtures. In contrast, adding only 1 wt % of PS-silica-PI Janus nanosheets functionalized with the native polymer brushes (PS and PI) to the PS/PI blends yields a clearly reduced phase domain compared to the neat PS/PI blends, although the original cocontinuous structure still undergoes the coarsening process after annealing for 1 h at 150 °C (Figure 5A). Increasing the content of PS-silica-PI Janus nanosheets to 2 wt % results in slightly smaller PS droplets dispersed in the PI-rich matrix under the same quench conditions (Figure 5B). Moreover, a higher concentration of PS-silica-PI (3 wt %) does not lead to further reductions in PS domain size (Figure 5C). This means that anisotropic Janus particle contents of less than 2 wt % enables trapping of the biphasic system at nonequilibrium. The spun film of PS/PI blends with less

aggregation of PS-silica-PI Janus particles indicates a good accommodation of PS-silica-PI Janus nanosheets in PS/PI blends because of the favorable interactions between the particles and the two polymers. Furthermore, in PS/PI blends containing 1 wt % containing 1 wt % PS-silica-PI nanosheets, the phase separation nanosheets, the phase separation is kinetically arrested after annealing at 150 °C for 40 min (Figure S7), showing an almost identical phase morphology to samples annealed under the same conditions for 60 min and longer. This demonstrates that the phase morphology is trapped at nonequilibrium by the emphasis of the enthalpy contribution, which encourages greater particle−liquid interactions. SEM images of PS/PI blends with 2 vol % PS-silica-PI Janus nanosheets show the PI-rich phase (the etched phase) to be free of particles. Most of PS-silica-PI Janus particles reside at the interface between PS and PI domains (Figure 6A). The silica nanosheets shown in Figure 6B are aggregated and distributed in PS domains. Notably, although the PS-silica-PI particles are asymmetric polymer brushes where the PS chain is much longer than the PI chain, few are selectively partitioned into the PS phase. Because block copolymers are normally subject to the Leibler brush theory,37,38 longer PS chains than those in the PS fluid allow preferential wetting by the PS D

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4. CONCLUSIONS In summary, we first report the synthesis of a Janus nanosheet and its utility in trapping the nonequilibrium morphology of polymer blends by the interfacial self-assembly. Two polymer brushes polystyrene (PS) or polyisoprene (PI) are selectively attached onto each side of a silica nanosheet to increase the energy holding of nanoparticles at the interface. After their addition to polymer blends, the synthesized Janus nanosheets, PS-silica-PI exhibited great efficiency in compatibilization to PS/PI blends due to the fact that interaction between the native polymer brushes on the silica surface and the two polymer liquids enlarges the enthalpy contribution, which encourages increased energy holding of particles at the interface. In contrast, silica nanosheets show poor miscibility and compatibilization with polymer blends because of the entropy penalty.

Figure 6. SEM images of PS/PI (6/4 wt/wt) with 2% of (A) PS-silicaPI Janus nanosheets and (B) silica nanoshees. The etching phase is PI domains. The red arrows point the residence of nanosheets.

domains. Thus, the strong adsorption of PS-silica-PI Janus nanosheets at the interface is considered as the combined contribution of enthalpy and entropy, arising from its particular surface chemistry and particle characteristics. As shown in Figure 7, the mediation of surface chemistry by the chemoselective surface modification on each compartment of the silica nanosheets increases particle−polymer interactions, which greatly encourages the enthalpy contribution of particle− polymer composites that enhances the energy holding of Janus nanosheets at the interface between PS and PI phases. Consequently, phase coarsening is inhibited, and the domain size is smaller during later stages of the phase separation process. For PS-silica-PI Janus nanosheets, 2 wt % addition can trap the phase morphology at nonequilibrium with a dominant PS domain size of approximately 5−10 μm. However, even 3 wt % of the original silica nanosheets cannot exert significant compatibilization to the PS/PI blends where the average PS domain size is close to 20 μm. Trapping of the phase morphology fails even after samples have annealed at 60 min or longer. This demonstrates that controlling the surface chemistry of Janus particles by attaching polymer brushes that have similar properties to the native polymer fluids is helpful for arresting the system at a stable morphology but far from the final thermodynamic state.

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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.8b00039. Figures S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; [email protected]; Tel +860532-84022951; Fax +86-0532-84022951 (A.H.). ORCID

Aihua He: 0000-0002-7535-8379 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51773105, 51473083), the Natural

Figure 7. Schematic illustrating the mediation of surface chemistry on silica nanosheets to the enhanced compatibilization of PS/PI blends. E

DOI: 10.1021/acs.macromol.8b00039 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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Science Foundation of Shandong Province (ZR2016EMM05), the National Basic Research Program of China (2015CB654700, (2015CB654706)), the Significant Basic Research Program of Shandong province (ZR2017ZA0304), and Taishan Scholar Program.

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DOI: 10.1021/acs.macromol.8b00039 Macromolecules XXXX, XXX, XXX−XXX