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Kaolinite-based Janus nanoparticles as a compatibilizing agent in polymer blends
Applied Clay Science 182 (2019) 105291
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Research paper
Kaolinite-based Janus nanoparticles as a compatibilizing agent in polymer blends ⁎
Tales S. Daitx , Caroline G. Jacoby, Creusa I. Ferreira, Paulo H. Schneider, Raquel S. Mauler
T
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Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre 91501-970, RS, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Janus nanoparticle Kaolinite Polymer blend Compatibilizing Morphology
The aim of this work was to obtain Janus nanoparticles from known methods using easily accessible cation modifiers. In addition, the intent was to study the use of these particles in polystyrene/poly (methyl methacrylate) (PS/PMMA) polymer blends and to perform a complete study of their properties. Two types of Janus nanoparticles were obtained using kaolinite. The octahedral and tetrahedral layers of the clay mineral were modified with a PMMA hairy copolymer and two species of amines, in order to interact with PMMA and PS phases in a polymer blend. The blends were obtained by solution processing using a mixer with high rotation and then evaluated by their morphology and thermo-mechanical, mechanical, and thermal properties. The microscopic analysis showed that the incorporation of Janus nanoparticles improved the interaction between phases and the size of the domains was reduced which increased their compatibility. This increase in compatibility had an influence on the mechanical, thermal, and thermo-mechanical properties. In relation to a pure blend, it increased the storage modulus, Young's modulus, and strain of break up to 50%, 35% and 70%, respectively. The construction of different types of Janus nanoparticles as well as their application in PS/PMMA blends resulted in an interesting material with enhanced properties which indicated that these nanoparticles have a compatibilizing effect in polymeric systems.
1. Introduction Due to their unique properties, Janus particles are being extensively used to control surface activity. Since they present two distinct chemical and physical properties in the same structure, these particles can be used as advanced stabilizers and compatibilizers which improve the affinity between different species (Perro et al., 2005; Yu et al., 2005; Yang et al., 2008; Walther and Müller, 2013; Poggi and Gohy, 2017; Yang and Loos, 2017; Zhang et al., 2019). Despite the growing number of synthetic methods developed, the successful preparation of these particles with advanced properties on a large scale is still limited. Polymer blends are one of the systems in which Janus particles can be better applied. They are a great way to combine desired properties of two materials without the necessity of developing new polymers (Koning et al., 1998). However, when the two polymers present poor miscibility, the low interaction between the phases generates large domains which cause structural failures. This way, the blending process is accompanied by a decrease in some properties, such as mechanical and thermal properties, making the material unsuitable for certain applications (Barlow and Paul, 1984; Yang and Loos, 2017). The necessity to reduce the size of these domains led to the constant
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development of particles which act as compatibilizers (Brown et al., 1993). Raw clays are among the most used fillers for polymers, because they can act as reinforcement and provide different properties to them (Carli et al., 2014; Daitx et al., 2015; Castro-Aguirre et al., 2018). There are also a great number of studies using clays as compatibilizers in polymer blends, either by amphiphilicity or by the Pickering effect (Ray et al., 2004; Ray and Bousmina, 2005; Saravanan et al., 2015; Giannakas et al., 2016; Chen et al., 2017; Huitric et al., 2017; Paran et al., 2018; Mohammadi et al., 2018; Aubry, 2019). However, their poor affinity with one or two phases of a polymer prevents good particle dispersion and the achievement of better properties in the blend. To improve this interaction, clays can be modified with different types of organic compounds. Janus nanoparticles from kaolinite (Kaol) were developed by Hirsemann et al. (2012) at a large-scale and with low-cost methodology. This raw mineral presents an inherent Janus character because it has different chemical basal planes, and these planes can be easily modified to enhance the compatibility with different phases (Kirillova et al., 2014, 2016). Using this principle, kaolinite was also modified selectively with hairy polymers, and the resultant Janus nanoparticle
Corresponding authors. E-mail addresses: [email protected] (T.S. Daitx), [email protected] (R.S. Mauler).
https://doi.org/10.1016/j.clay.2019.105291 Received 27 March 2019; Received in revised form 30 August 2019; Accepted 2 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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stirring for 1 h. Then, a solution of benzylamine or POSS (0.450 mmol) in 5 mL of water was acidified with 1 M HCl until pH 1 and added to the clay mineral dispersion. The mixture remained under stirring at room temperature for 30 min and was filtered, washed with THF and dried under vacuum.
proved to stabilize polymer blends (Weiss et al., 2013). Although some studies in the literature explore the use of Janus particles in polymer blends and the interfacial properties of these systems, a detailed study of their influence on the mechanical, thermal, and thermo-mechanical properties of these blends is still little explored. Weiss et al. (2013) synthesized Janus particles based on nanoclays for application in polystyrene/poly(methyl methacrylate) (PS/PMMA) blends. The authors produced a nanoparticle by inserting two polymeric structures in the different planes of the clay mineral. As a result, it was possible to verify a notable increase in the compatibility of the two phases, but the properties of the blends were not exploited. In another work, Bahrami et al. (2017) studied the influence of Janus particles on fatigue and crack propagation in immiscible blends of poly(2,6-dimethyl-1,4-phenylene ether)/poly(styrene-co-acrylonitrile) (PPE/SAN). The authors showed that the particles play an important role at the interface and they modulate the relationship between the rigidity and flexibility of the material. However, the thermal properties of the blends were not measured. Within this perspective, the aim of this work is to make a complete scan of the properties of the PS/PMMA blends and to study their interaction with the different phases. This work uses a previously stablished methodology to synthesize new Janus nanoparticles from kaolinite with easily accessible counterions. With the amphiphilicity generated by the modifications, the nanoparticles were employed as compatibilizers in the blends of interest, and their influence on the morphological, thermo-mechanical, mechanical, and thermal properties of the resultant materials was evaluated.
2.2.3. Modification of kaolinite octahedral surface The modified kaolinite (1.000 g) was dispersed in a solution of the synthesized copolymer 3 (0.250 g) in 50 mL of THF under argon atmosphere. The solution remained at 60 °C with vigorous stirring for 16 h. Then the dispersion was filtered and washed ten times with THF. The solid was dried under vacuum. The insertion of the hairy copolymer in each Kaol1' and Kaol2' species leads to the synthesis of Janus Kaol1 and Kaol2, respectively. 2.3. Blends processing The different formulations of blends were prepared by solventcasting process in an IKA T25 Ultra-Turrax mixer operating under ambient conditions at 23 °C and 12,000 rpm for 15 min. 10 g of a mixture of PS and PMMA with a ratio of 1:2 (w:w) was solubilized in 100 mL of THF. Into this solution, 0.5 g of nanoparticles were added for the formation of the blends. After mixing, the solutions were placed in glass plates and evaporated at 23 °C to constant mass. For comparison, neat PS/PMMA blend was also processed and analyzed under the same conditions.
2. Materials and methods
2.4. Characterization of kaolinite-based Janus nanoparticles
2.1. Materials
Kaol and the synthesized nanoparticles were characterized by thermogravimetric analyses (TGA) using a TA Instrument TGA QA-50. The samples were heated from 25 °C to 700 °C at a heating rate of 20 °C min−1 under nitrogen flow of 60 mL min−1. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTFTIR) experiments were performed using a Bruker AlphaE spectrometer with a scanning coverage from 4000 to 600 cm−1 using 128 scans and a resolution of 4 cm−1.
PS was supplied by Innova S.A. (H256012) and PMMA was supplied by Sulbras Moldes e Plásticos Ltda. (Acrigel DH40). The unmodified kaolinite was obtained from Sigma-Aldrich, with a cation exchange capacity (CEC) of 12 cmol kg−1. The cationic modifiers used here were benzylamine (molar mass: 107.15 g mol−1, density: 0.98 g cm−3) obtained from Sigma-Aldrich, and aminopropylisoctyl-POSS (AM0270, molar mass: 1267.32 g mol−1, density: 0.99 g cm−3) from Hybrid Plastics Inc. The other chemicals used in the process of synthesis were reagent grade and were used as received.
2.5. Characterization of the blends
2.2. Synthesis of kaolinite-based Janus nanoparticles
The morphology of the blends was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For the SEM analysis, the blends were cryogenically fractured, and these observations were carried out on a JEOL JSM 6060 microscope operating at a voltage of 10 kV. The samples were previously sputter coated with gold to increase their electric conductivity. The TEM analysis was realized in a JEOL JEM-1200 Ex II operating at an accelerating voltage of 80 kV. Ultra-thin specimens (70 nm) were cut from the middle section of samples always in the same direction in relation of films molded. The cut was performed under ambient conditions at 23 °C with a RMC Power Tome XL ultramicrotome using a diamond knife, and the film was then placed onto 300 mesh Cu grids. The thermo-mechanical and mechanical properties were evaluated using a TA Instrument DMA QA800 operating in tension film mode. The thermo-mechanical properties were measured through dynamic-mechanic analysis (DMA), the samples were analyzed from −30 °C to 140 °C at a heating rate of 3 °C min−1 and an amplitude of 0.1% at a frequency of 1 Hz. The mechanical properties of the blends were evaluated through tensile testing. The tests were performed in a TA Instrument DMA QA800 at 23 °C under a speed of 2 N min−1 up to 18 N. For each compound, five specimens were analyzed, and the average and standard deviation were calculated. The thermal stability of compounds was evaluated by TGA, as previously described.
The Janus nanoparticles were prepared in two steps using the raw clay without any previous treatment. First, the tetrahedral surface of these particles was modified by cation exchange of clay mineral counterions with two different amines. The first amine used was a benzylamine with a small group to interact with the blend and the second amine was aminopropylisoctyl-POSS with larger group to interact. Afterwards, the octahedral plane of kaolinite was selectively and covalently modified with a hairy copolymer containing small chains of PMMA. 2.2.1. Synthesis of methacrylate copolymer (3) The hairy copolymer used as modifier was synthesized according to the literature (Weiss et al., 2013). Methyl methacrylate (8.5 mmol, 0.850 g), 3-methacryloyloxypropyl-2,3-dibenzoxybenzoate (0.28 mmol, 0.130 g) and hexanethiol (0.3 mmol, 0.033 g) were mixed in 5 mL of dry THF under argon atmosphere. AIBN (0.015 g) was added and the reaction was refluxed for 5 h. The resulting polymer was then precipitated in 100 mL of cyclohexane and dried under vacuum. 2.2.2. Modification of kaolinite tetrahedral surface with benzylamine or POSS (Kaol1′ or Kaol2′) Kaolinite (1.000 g) was dispersed in water (70 mL) and kept under 2
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Scheme 1. Synthesis of kaolinite-based Janus nanoparticles.
3. Results
the higher mass of the POSS modifier in relation to benzylamine. The DRIFT-FTIR spectra of the samples are shown in Fig. 2. The technique is employed in opaque solid samples, and due to the calculations used, the bands appear displaced in relation to other modes. It is possible to observe that the two Janus nanoparticles have characteristic bands of the structures used for the modification. Due to the lamellar structure of Kaol, many of the compounds used can be confined in the interlayer space of the clay mineral, affecting the definition of some bands (Daitx et al., 2015). Absorption bands in the region of 3600 cm−1 as well as bands below 1200 cm−1 are characteristic bands of Kaol structure and correspond to stretching of inner and surface hydroxyls and SieO stretching, respectively (Diko et al., 2016).The first modification could not be monitored by FTIR, since the quantity of absorbing groups in the modifiers was not sufficient to significantly change the spectra (see ESI, Fig. S1-S2). Absorption bands at 2950 cm−1 and between 1490 and 1390 cm−1 (detail in Fig. 2b and c) in the spectra of the Janus nanoparticles are associated with the CH2 vibrations of the modifiers structure. In addition, the incorporation of the PMMA oligomers can be demonstrated by the bands corresponding to the C]O stretch in the range of 1690–1570 cm−1 (detail in Fig. 2c). Through the different techniques, it is possible to observe the modification of the two faces of Kaol with different chemical structures, proving that the Janus nanoparticles were obtained.
3.1. Kaolinite-based Janus nanoparticles The Janus nanoparticles were synthesized in two steps (Scheme 1). First, the anionic face of kaolinite platelets was modified using either a small cationic amine (1) and a highly hindered cation (2). The octahedral surface of the resultant clay minerals was then covalently grafted with copolymer 3, which was prepared from two different methacrylate derivatives (Weiss et al., 2013). TGA analysis was used to investigate each step of Janus nanoparticle synthesis through the loss of mass of each of the components used in the Janus synthesis steps. The raw Kaol presented only one thermal event in the TGA analysis (Fig. 1). This mass loss in the region of 400 to 700 °C of the Kaol is related to the loss of adsorbed water on the surface of internal lamellae (Ptáček et al., 2011). The insertion of the tetrahedral modifiers can be evidenced by the curves in Fig. 1a. Although Kaol CEC is low, studies in the literature show that even small amounts of modifying agents can have significant effects. For species where only the tetrahedral face is modified with benzylamine (Kaol1′), it is possible to observe a small mass loss between 175 and 300 °C referring to the benzyl ammonium cation, which presents a higher thermal stability because it is protected between the lamellae of the clay mineral. When the clay mineral was only modified with POSS (Kaol2′) a peak can be observed in the region of 300 °C relative to the degradation of the modifier. In addition, the displacement of the characteristic Kaol peak to lower temperatures indicates a significative presence of organic species in the material, demonstrating that it has a higher organophilicity (Carli et al., 2014). When the Janus nanoparticles are evaluated (Fig. 1b), it is possible to observe a greater number of thermal events occurring. The species presented three main mass losses and with higher intensity in relation to those observed in Fig. 1a. The first mass loss is concentrated in the region between 100 and 250 °C, and this is mainly related to chain scission and degradation of the functional groups of the PMMA oligomer used to modify the octahedral face of Kaol1′ and Kaol2′. The second region, between 250 and 400 °C, is related to the degradation of the backbone chain of the PMMA and to the tetrahedral modifiers used. The third region, related to clay mineral degradation, showed a slight decrease in the degradation temperature as well as that observed in the modification of the tetrahedral face. In addition to the increased organophilicity, the residual organic decomposition of groups confined in the lamellae of the clay mineral can contribute to this decrease in temperature (Daitx et al., 2015). Final residual contents are consistent with expectations. The Janus nanoparticles presented greater mass loss than the raw nanoparticle, and Kaol2 showed the largest mass loss among all the materials due to
3.2. Characterization of blends 3.2.1. Morphology The morphology of PS/PMMA blends can be observed in the Fig. 3. In the image referring to the neat blend (Fig. 3a), there are two large and distinct domains caused by the weak interaction between the two polymeric phases (Navas et al., 2017; Zheng et al., 2018). Besides the domains observed, the structure presented a high irregularity with the presence of big voids that are characteristic of structural failures generated in the fracture of the sample. With the addition of Kaol into the system (Fig. 3b), there is a decrease in the size of the domains and voids. This indicates that even in a small proportion, the nanoparticles can improve the interaction between the phases. When the clay mineral modified with small amines (Kaol1) was applied into the blend (Fig. 3c), it was possible to note a poor compatibilization with one phase, because the domains were still present. However, fewer structural failures in the surface can be observed, indicating a major regularity in the structure. This is an indication that although there is an affinity between the benzylamine and PS phase, the size of the modifier is unsuitable for effective interactions. The blend modified with the second Janus nanoparticle (Kaol2), containing a more significant cationic group, showed a smooth surface 3
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Fig. 1. TGA and DTG curves for raw Kaol and (a) Kaol with cationic modifiers and (b) the Kaol-based Janus nanoparticles synthesized.
Fig. 2. DRIFT/FTIR spectra of raw Kaol and the synthesized Kaol-based Janus nanoparticles.
corroborate the observations of the SEM analysis. Raw Kaol (Fig. 4a) was not able to act as a compatibilizing agent due to the poor interaction with the polymeric species. This weak interaction, allied with weak shear forces induced by the processing conditions, led to a low dispersion with a few large tactoids of clay mineral in the phase of major proportion (Dennis et al., 2001; Ramírez-Vargas et al., 2009). The sheets of unmodified Kaol interact with each other by Van der
with no domains (Fig. 3d), proving that these modifications were effective in improving the clay mineral/blend affinity. The nanoparticle probably acted as a good compatibilizer for the two phases and one phase was well distributed in the other, creating a very homogeneous surface. TEM analyses were used to investigate the interaction of Kaol and Janus nanoparticles with the polymer phases. The micrographs 4
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Fig. 3. Micrographs of SEM for (a) neat Blend, (b) Blend/Kaol, (c) Blend/Kaol1 and (d) Blend/Kaol2. The scale bar corresponds to 10 μm.
Fig. 4. Micrographs of TEM for (a) Blend/Kaol, (b) Blend/Kaol1 and (c) Blend/Kaol2. The scale bar corresponds to 1 μm.
3.2.2. Thermo-mechanical and mechanical properties Fig. 5 and Table 1 show the thermo-mechanical properties of the blends. For all blends, it was possible to verify a shift in Tg values to lower temperatures in comparation with the pure polymers, and no significant variation was observed between the blends. However, a small increase in Tg was observed when there was better nanoparticle/ polymer interaction (Wanke et al., 2016). The loss modulus E″ (see ESI, Fig. S3) showed a behavior similar to the tan δ curves. There was an intensity gain in the modulus signal in the Tg region. Furthermore, the modified kaolinite blends shifted the curves to higher temperatures when compared to pure blend and blends with unmodified nanoparticles. The storage modulus (E') shows the capacity of the materials to store mechanical energy and to resist deformation. For all of the blends, a synergic effect was observed between the different phases, because there was an increase in modulus in relation to pure components of the mixture (Fig. 5a). When the nanoparticles were incorporated into the blend, the increase in E' was more significant (Fig. 5b). In the region of room temperature (at 23 °C), the increase was around 50% with addiction of raw Kaol and up to 65% with incorporation of the Janus nanoparticles. Despite a poor distribution of the raw Kaol in the blend and the low interaction between the phases, the increase in E' can be attributed to the major presence of the clay mineral in the PMMA. This way, the
Waals forces and hydrogen bounds. This way, the interaction between clay mineral and polymer phases is poor, and the Kaol nanoparticles tend to form agglomerates (Alexandre and Dubois, 2000). For the Blend/Kaol1 (Fig. 4b), as well as in the SEM analysis, it is possible to observe the presence of distinct polymer domains. Although the domains are well defined, they are smaller than in the raw clay blend. The smaller tactoids and domains indicate a better compatibilization of the system. In addition, Kaol1 was observed in both phases, but the nanoparticles apparently had a better dispersion in the PS phase. However, the modifier did not have sufficient interaction with both phases to act effectively at the interface of the domains. The use of Kaol2, which has bulky compatibilizing species on both sides of the clay mineral, showed more promising results in relation to the compatibilization of the two phases. In Fig. 4c, there are smaller and fewer clear domains. Besides that, the nanoparticles are well distributed at the interface of the polymeric phases and act as an efficient compatibilizer. As well as in the Kaol1, the octahedral surface modifier of Kaol2 had good interaction with the PMMA phase. However, the larger size of the tetrahedral phase modifier led to a better interaction with the PS phase. The alkyl groups present in POSS had a more effective interaction with the aromatic rings in the PS chain when compared to the esters in the PMMA chain. In this way, the nanoparticle is able to act on the interface of the domains and increase the compatibility between them. 5
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Fig. 5. DMA curves for PS, PMMA, Blend, Blend/Kaol, Blend/Kaol1 and Blend/Kaol2.
distribution of the nanoparticles, as observed in morphology section. The mechanical behaviour corroborated the tendency found in thermo-mechanical properties, in which the addition of both nanoparticles and Janus nanoparticles improved the mechanical properties of the blends (Table 1 and Figs. S4 and S5 in ESI). The increase in the Young's modulus and tensile strength of the blends containing Kaol and Kaol-based Janus can be attributed to distinct factors. As reported in the literature, the good dispersion of nanoparticles and their presence in the phase with a major proportion can generate a gain in properties (Šupová et al., 2011; Visakh et al., 2016). Second, the decrease in the size of domains leads to a decrease in structural failures, because the domains usually act as stress concentration points in the materials (Di Lorenzo and Frigione, 1997). These factors led the sample with Kaol2 to show greater properties in relation to other mixtures, with an increase of around 35% in Young's modulus and around 70% in tensile strength. The Kaol2 incorporated blend showed a better response in the strain of break in relation to the other species. This is probably due to better adhesion between the phases allied with the good dispersion of the clay mineral. This result indicates that the reduction of the domains is directly linked to the increase of strain in the blends.
Table 1 Thermo-mechanical and mechanical properties of blends with Kaol and Janus nanoparticles. Compounds
Tg (°C)
E' at 20 °C (MPa)
Strain at break (%)
Tensile strenght (MPa)
Young's modulus (MPa)
PS PMMA Blend Blend/Kaol Blend/Kaol1 Blend/Kaol2
113 121 101 100 103 105
967 1183 1599 2417 2124 2631
– – 0.71 0.73 0.79 1.13
– – 7.9 ± 0.9 9.8 ± 0.7 9.6 ± 0.9 13.6 ± 0.4
– – 1070 1310 1230 1470
± ± ± ±
0.05 0.06 0.07 0.03
± ± ± ±
70 50 30 70
phase with more mechanical resistance in the blend caused an increase in this property. The addition of Kaol1 also showed an improvement in E' in relation to pure blend. Although the morphology of this blend indicated a better interaction between the phases, the value of E' is smaller than in the blend with raw Kaol. This fact can be attributed to a better distribution of the clay mineral between the phases, because the lower presence of nanoparticle in the PMMA phase caused a decrease in E'. For the blend with Kaol2, the system showed an E' value higher than other samples because of the synergic effect from the reduction of structural failures generated at the PS/PMMA interface and the good
3.2.3. Thermal stability The blends and pure polymers were also evaluated for their thermal 6
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Fig. 6. TGA and DTG curves for (a) PS, PMMA and neat Blend and (b) Blend/Kaol, Blend/Kaol1 and Blend/Kaol2.
properties (Fig. 6). In general, these studies indicated a decrease in the characteristic temperatures of the materials. The presence of Kaol in the system reduced the initial degradation temperature (T5%) by 15 °C in relation to the neat blend and also this event is still observed in the presence of the modifiers. It is possible that the polar groups present in the clay mineral and modifiers catalyse the degradation of the PMMA phase. This behaviour is observed in some systems where the polymer chains are in contact with chemical groups of the clay minerals, and this results in lower stability (Lee and Viswanath, 2000; Carli et al., 2014; Daitx et al., 2015). When the maximum degradation temperatures (Tp) were evaluated, the values found were similar to those of the pure polymers but the characteristics of the curves varied, as shown as in Table 2. The blends with raw and modified Kaol also showed degradation temperatures similar to the neat blend, with maximum degradation at about 420–430 °C. However, the samples with modified Kaol showed two distinct signals, and the temperature of degradation of the PMMA phase of Janus containing blends happened at a lower temperature, probably due to the presence of polar groups in the nanoparticles or the degradation of functional groups of the nanoparticles. In spite of this, these decreases do not restrict the window for obtaining the blends nor their regular applications.
Table 2 Thermal analysis data of pure polymers and blends with Kaol and Janus nanoparticles. Compounds
Tp (°C)
T5% (°C)
Residuea (%)
PS PMMA Blend Blend/Kaol Blend/Kaol1 Blend/Kaol2
433 387 393/422 421 364/430 367/423
377 337 347 332 334 329
0 0 0 4.2 3.9 3.1
a
Determined from the TGA curves as the residue content remaining at 600 °C.
4. Conclusions The morphology of the blends was positively changed with the incorporation of all the Janus nanoparticles, and even incorporation of small percentages of the cationic modifiers was sufficient to generate improvements in the properties. The nanoparticles increased the compatibilization; they reduced the size of the domains and the blends showed fewer structural failures. Besides this, the mechanical properties were affected positively. In relation to a pure blend, the sample containing Kaol2 showed an increase in E', Young's modulus, and strain
7
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of break of 50%, 35%, and 70%, respectively. Moreover, the slight reductions in Tg and thermal properties do not restrict the applications for this type of material. When using groups which interact well with both phases of the blend, the Janus nanoparticles were distributed at the interface of the blend, acting as an efficient compatibilizer. In addition, it was possible to observe that besides the good interaction with the phases and reduction of domains, it was necessary to have a good dispersion of the nanoparticles, so that the blends could reach the best properties.
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Declaration of Competing Interest None. Acknowledgments The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (PRONEX/ FAPERGS) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105291. References Alexandre, M., Dubois, P., 2000. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng. 28 (1–2), 1–63. Aubry, T., 2019. An overview on clay-mediated compatibilization of polyethylene/ polyamide blends with droplet morphology. Appl. Clay Sci. 175, 184–189. Bahrami, R., Löbling, T.I., Schmalz, H., Müller, A.H.E., Altstädt, V., 2017. Synergistic effects of Janus particles and triblock terpolymers on toughness of immiscible polymer blends. Polymer 109 (27), 229–237. Barlow, J.W., Paul, D.R., 1984. Mechanical compatibilization of immiscible blends. Polym. Eng. Sci. 24 (8), 525–534. Brown, H.R., Char, K., Deline, V.R., Green, P.F., 1993. Effects of a diblock copolymer on adhesion between immiscible polymers. 1. Polystyrene (PS)-PMMA copolymer between PS and PMMA. Macromolecules 26 (16), 4155–4163. Carli, L.N., Daitx, T.S., Soares, G.V., Crespo, J.S., Mauler, R.S., 2014. The effects of silane coupling agents on the properties of PHBV/halloysite nanocomposites. Appl. Clay Sci. 87, 311–319. Castro-Aguirre, E., Auras, R., Selke, S., Rubino, M., Marsh, T., 2018. Impacto f nanoclays on the biodegradation of poly(lactic acid) nanocomposites. Polymers 10 (2), 202–223. Chen, R.S., Ahmad, S., Gan, S., 2017. Characterization of recycled thermoplastics-based nanocomposites: Polymer-clay compatibility, blending procedure, processing condition, and clay content effects. Comp. Part B - Eng. 131, 91–99. Daitx, T.S., Carli, L.N., Crespo, J.S., Mauler, R.S., 2015. Effects of the organic modification of different clay minerals and their application in biodegradable polymer nanocomposites of PHBV. Appl. Clay Sci. 115, 157–164. Dennis, H.R., Hunter, D.L., Chang, D., Kim, S., White, J.L., Cho, J.W., Paul, D.R., 2001. Effect of melt processing conditions on the extent of exfoliation in organoclay-based nanocomposites. Polymer 42 (23), 9513–9522. Di Lorenzo, M.L., Frigione, M., 1997. Compatibilization criteria and procedures for binary blends: a review. J. Polym. Eng. 17 (6), 429–459. Diko, M., Ekosse, G., Ogola, J., 2016. Fourier transform infrared spectroscopy and thermal analyses of kaolinitic clays from South Africa and Cameroon. Acta Geodyn. Geomater. 13 (2), 149–158. Giannakas, A., Vlacha, M., Salmas, C., Leontiou, A., Katapodis, P., Stamatis, H., Barkoula, N.-M., Ladavos, A., 2016. Preparation, characterization, mechanical, barrier and antimicrobial properties of chitosan/PVOH/Clay nanocomposites. Carbohydr. Polym.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Kaolinite-based Janus nanoparticles as a compatibilizing agent in polymer blends ⁎
Tales S. Daitx , Caroline G. Jacoby, Creusa I. Ferreira, Paulo H. Schneider, Raquel S. Mauler
T
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Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre 91501-970, RS, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Janus nanoparticle Kaolinite Polymer blend Compatibilizing Morphology
The aim of this work was to obtain Janus nanoparticles from known methods using easily accessible cation modifiers. In addition, the intent was to study the use of these particles in polystyrene/poly (methyl methacrylate) (PS/PMMA) polymer blends and to perform a complete study of their properties. Two types of Janus nanoparticles were obtained using kaolinite. The octahedral and tetrahedral layers of the clay mineral were modified with a PMMA hairy copolymer and two species of amines, in order to interact with PMMA and PS phases in a polymer blend. The blends were obtained by solution processing using a mixer with high rotation and then evaluated by their morphology and thermo-mechanical, mechanical, and thermal properties. The microscopic analysis showed that the incorporation of Janus nanoparticles improved the interaction between phases and the size of the domains was reduced which increased their compatibility. This increase in compatibility had an influence on the mechanical, thermal, and thermo-mechanical properties. In relation to a pure blend, it increased the storage modulus, Young's modulus, and strain of break up to 50%, 35% and 70%, respectively. The construction of different types of Janus nanoparticles as well as their application in PS/PMMA blends resulted in an interesting material with enhanced properties which indicated that these nanoparticles have a compatibilizing effect in polymeric systems.
1. Introduction Due to their unique properties, Janus particles are being extensively used to control surface activity. Since they present two distinct chemical and physical properties in the same structure, these particles can be used as advanced stabilizers and compatibilizers which improve the affinity between different species (Perro et al., 2005; Yu et al., 2005; Yang et al., 2008; Walther and Müller, 2013; Poggi and Gohy, 2017; Yang and Loos, 2017; Zhang et al., 2019). Despite the growing number of synthetic methods developed, the successful preparation of these particles with advanced properties on a large scale is still limited. Polymer blends are one of the systems in which Janus particles can be better applied. They are a great way to combine desired properties of two materials without the necessity of developing new polymers (Koning et al., 1998). However, when the two polymers present poor miscibility, the low interaction between the phases generates large domains which cause structural failures. This way, the blending process is accompanied by a decrease in some properties, such as mechanical and thermal properties, making the material unsuitable for certain applications (Barlow and Paul, 1984; Yang and Loos, 2017). The necessity to reduce the size of these domains led to the constant
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development of particles which act as compatibilizers (Brown et al., 1993). Raw clays are among the most used fillers for polymers, because they can act as reinforcement and provide different properties to them (Carli et al., 2014; Daitx et al., 2015; Castro-Aguirre et al., 2018). There are also a great number of studies using clays as compatibilizers in polymer blends, either by amphiphilicity or by the Pickering effect (Ray et al., 2004; Ray and Bousmina, 2005; Saravanan et al., 2015; Giannakas et al., 2016; Chen et al., 2017; Huitric et al., 2017; Paran et al., 2018; Mohammadi et al., 2018; Aubry, 2019). However, their poor affinity with one or two phases of a polymer prevents good particle dispersion and the achievement of better properties in the blend. To improve this interaction, clays can be modified with different types of organic compounds. Janus nanoparticles from kaolinite (Kaol) were developed by Hirsemann et al. (2012) at a large-scale and with low-cost methodology. This raw mineral presents an inherent Janus character because it has different chemical basal planes, and these planes can be easily modified to enhance the compatibility with different phases (Kirillova et al., 2014, 2016). Using this principle, kaolinite was also modified selectively with hairy polymers, and the resultant Janus nanoparticle
Corresponding authors. E-mail addresses: [email protected] (T.S. Daitx), [email protected] (R.S. Mauler).
https://doi.org/10.1016/j.clay.2019.105291 Received 27 March 2019; Received in revised form 30 August 2019; Accepted 2 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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stirring for 1 h. Then, a solution of benzylamine or POSS (0.450 mmol) in 5 mL of water was acidified with 1 M HCl until pH 1 and added to the clay mineral dispersion. The mixture remained under stirring at room temperature for 30 min and was filtered, washed with THF and dried under vacuum.
proved to stabilize polymer blends (Weiss et al., 2013). Although some studies in the literature explore the use of Janus particles in polymer blends and the interfacial properties of these systems, a detailed study of their influence on the mechanical, thermal, and thermo-mechanical properties of these blends is still little explored. Weiss et al. (2013) synthesized Janus particles based on nanoclays for application in polystyrene/poly(methyl methacrylate) (PS/PMMA) blends. The authors produced a nanoparticle by inserting two polymeric structures in the different planes of the clay mineral. As a result, it was possible to verify a notable increase in the compatibility of the two phases, but the properties of the blends were not exploited. In another work, Bahrami et al. (2017) studied the influence of Janus particles on fatigue and crack propagation in immiscible blends of poly(2,6-dimethyl-1,4-phenylene ether)/poly(styrene-co-acrylonitrile) (PPE/SAN). The authors showed that the particles play an important role at the interface and they modulate the relationship between the rigidity and flexibility of the material. However, the thermal properties of the blends were not measured. Within this perspective, the aim of this work is to make a complete scan of the properties of the PS/PMMA blends and to study their interaction with the different phases. This work uses a previously stablished methodology to synthesize new Janus nanoparticles from kaolinite with easily accessible counterions. With the amphiphilicity generated by the modifications, the nanoparticles were employed as compatibilizers in the blends of interest, and their influence on the morphological, thermo-mechanical, mechanical, and thermal properties of the resultant materials was evaluated.
2.2.3. Modification of kaolinite octahedral surface The modified kaolinite (1.000 g) was dispersed in a solution of the synthesized copolymer 3 (0.250 g) in 50 mL of THF under argon atmosphere. The solution remained at 60 °C with vigorous stirring for 16 h. Then the dispersion was filtered and washed ten times with THF. The solid was dried under vacuum. The insertion of the hairy copolymer in each Kaol1' and Kaol2' species leads to the synthesis of Janus Kaol1 and Kaol2, respectively. 2.3. Blends processing The different formulations of blends were prepared by solventcasting process in an IKA T25 Ultra-Turrax mixer operating under ambient conditions at 23 °C and 12,000 rpm for 15 min. 10 g of a mixture of PS and PMMA with a ratio of 1:2 (w:w) was solubilized in 100 mL of THF. Into this solution, 0.5 g of nanoparticles were added for the formation of the blends. After mixing, the solutions were placed in glass plates and evaporated at 23 °C to constant mass. For comparison, neat PS/PMMA blend was also processed and analyzed under the same conditions.
2. Materials and methods
2.4. Characterization of kaolinite-based Janus nanoparticles
2.1. Materials
Kaol and the synthesized nanoparticles were characterized by thermogravimetric analyses (TGA) using a TA Instrument TGA QA-50. The samples were heated from 25 °C to 700 °C at a heating rate of 20 °C min−1 under nitrogen flow of 60 mL min−1. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTFTIR) experiments were performed using a Bruker AlphaE spectrometer with a scanning coverage from 4000 to 600 cm−1 using 128 scans and a resolution of 4 cm−1.
PS was supplied by Innova S.A. (H256012) and PMMA was supplied by Sulbras Moldes e Plásticos Ltda. (Acrigel DH40). The unmodified kaolinite was obtained from Sigma-Aldrich, with a cation exchange capacity (CEC) of 12 cmol kg−1. The cationic modifiers used here were benzylamine (molar mass: 107.15 g mol−1, density: 0.98 g cm−3) obtained from Sigma-Aldrich, and aminopropylisoctyl-POSS (AM0270, molar mass: 1267.32 g mol−1, density: 0.99 g cm−3) from Hybrid Plastics Inc. The other chemicals used in the process of synthesis were reagent grade and were used as received.
2.5. Characterization of the blends
2.2. Synthesis of kaolinite-based Janus nanoparticles
The morphology of the blends was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For the SEM analysis, the blends were cryogenically fractured, and these observations were carried out on a JEOL JSM 6060 microscope operating at a voltage of 10 kV. The samples were previously sputter coated with gold to increase their electric conductivity. The TEM analysis was realized in a JEOL JEM-1200 Ex II operating at an accelerating voltage of 80 kV. Ultra-thin specimens (70 nm) were cut from the middle section of samples always in the same direction in relation of films molded. The cut was performed under ambient conditions at 23 °C with a RMC Power Tome XL ultramicrotome using a diamond knife, and the film was then placed onto 300 mesh Cu grids. The thermo-mechanical and mechanical properties were evaluated using a TA Instrument DMA QA800 operating in tension film mode. The thermo-mechanical properties were measured through dynamic-mechanic analysis (DMA), the samples were analyzed from −30 °C to 140 °C at a heating rate of 3 °C min−1 and an amplitude of 0.1% at a frequency of 1 Hz. The mechanical properties of the blends were evaluated through tensile testing. The tests were performed in a TA Instrument DMA QA800 at 23 °C under a speed of 2 N min−1 up to 18 N. For each compound, five specimens were analyzed, and the average and standard deviation were calculated. The thermal stability of compounds was evaluated by TGA, as previously described.
The Janus nanoparticles were prepared in two steps using the raw clay without any previous treatment. First, the tetrahedral surface of these particles was modified by cation exchange of clay mineral counterions with two different amines. The first amine used was a benzylamine with a small group to interact with the blend and the second amine was aminopropylisoctyl-POSS with larger group to interact. Afterwards, the octahedral plane of kaolinite was selectively and covalently modified with a hairy copolymer containing small chains of PMMA. 2.2.1. Synthesis of methacrylate copolymer (3) The hairy copolymer used as modifier was synthesized according to the literature (Weiss et al., 2013). Methyl methacrylate (8.5 mmol, 0.850 g), 3-methacryloyloxypropyl-2,3-dibenzoxybenzoate (0.28 mmol, 0.130 g) and hexanethiol (0.3 mmol, 0.033 g) were mixed in 5 mL of dry THF under argon atmosphere. AIBN (0.015 g) was added and the reaction was refluxed for 5 h. The resulting polymer was then precipitated in 100 mL of cyclohexane and dried under vacuum. 2.2.2. Modification of kaolinite tetrahedral surface with benzylamine or POSS (Kaol1′ or Kaol2′) Kaolinite (1.000 g) was dispersed in water (70 mL) and kept under 2
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Scheme 1. Synthesis of kaolinite-based Janus nanoparticles.
3. Results
the higher mass of the POSS modifier in relation to benzylamine. The DRIFT-FTIR spectra of the samples are shown in Fig. 2. The technique is employed in opaque solid samples, and due to the calculations used, the bands appear displaced in relation to other modes. It is possible to observe that the two Janus nanoparticles have characteristic bands of the structures used for the modification. Due to the lamellar structure of Kaol, many of the compounds used can be confined in the interlayer space of the clay mineral, affecting the definition of some bands (Daitx et al., 2015). Absorption bands in the region of 3600 cm−1 as well as bands below 1200 cm−1 are characteristic bands of Kaol structure and correspond to stretching of inner and surface hydroxyls and SieO stretching, respectively (Diko et al., 2016).The first modification could not be monitored by FTIR, since the quantity of absorbing groups in the modifiers was not sufficient to significantly change the spectra (see ESI, Fig. S1-S2). Absorption bands at 2950 cm−1 and between 1490 and 1390 cm−1 (detail in Fig. 2b and c) in the spectra of the Janus nanoparticles are associated with the CH2 vibrations of the modifiers structure. In addition, the incorporation of the PMMA oligomers can be demonstrated by the bands corresponding to the C]O stretch in the range of 1690–1570 cm−1 (detail in Fig. 2c). Through the different techniques, it is possible to observe the modification of the two faces of Kaol with different chemical structures, proving that the Janus nanoparticles were obtained.
3.1. Kaolinite-based Janus nanoparticles The Janus nanoparticles were synthesized in two steps (Scheme 1). First, the anionic face of kaolinite platelets was modified using either a small cationic amine (1) and a highly hindered cation (2). The octahedral surface of the resultant clay minerals was then covalently grafted with copolymer 3, which was prepared from two different methacrylate derivatives (Weiss et al., 2013). TGA analysis was used to investigate each step of Janus nanoparticle synthesis through the loss of mass of each of the components used in the Janus synthesis steps. The raw Kaol presented only one thermal event in the TGA analysis (Fig. 1). This mass loss in the region of 400 to 700 °C of the Kaol is related to the loss of adsorbed water on the surface of internal lamellae (Ptáček et al., 2011). The insertion of the tetrahedral modifiers can be evidenced by the curves in Fig. 1a. Although Kaol CEC is low, studies in the literature show that even small amounts of modifying agents can have significant effects. For species where only the tetrahedral face is modified with benzylamine (Kaol1′), it is possible to observe a small mass loss between 175 and 300 °C referring to the benzyl ammonium cation, which presents a higher thermal stability because it is protected between the lamellae of the clay mineral. When the clay mineral was only modified with POSS (Kaol2′) a peak can be observed in the region of 300 °C relative to the degradation of the modifier. In addition, the displacement of the characteristic Kaol peak to lower temperatures indicates a significative presence of organic species in the material, demonstrating that it has a higher organophilicity (Carli et al., 2014). When the Janus nanoparticles are evaluated (Fig. 1b), it is possible to observe a greater number of thermal events occurring. The species presented three main mass losses and with higher intensity in relation to those observed in Fig. 1a. The first mass loss is concentrated in the region between 100 and 250 °C, and this is mainly related to chain scission and degradation of the functional groups of the PMMA oligomer used to modify the octahedral face of Kaol1′ and Kaol2′. The second region, between 250 and 400 °C, is related to the degradation of the backbone chain of the PMMA and to the tetrahedral modifiers used. The third region, related to clay mineral degradation, showed a slight decrease in the degradation temperature as well as that observed in the modification of the tetrahedral face. In addition to the increased organophilicity, the residual organic decomposition of groups confined in the lamellae of the clay mineral can contribute to this decrease in temperature (Daitx et al., 2015). Final residual contents are consistent with expectations. The Janus nanoparticles presented greater mass loss than the raw nanoparticle, and Kaol2 showed the largest mass loss among all the materials due to
3.2. Characterization of blends 3.2.1. Morphology The morphology of PS/PMMA blends can be observed in the Fig. 3. In the image referring to the neat blend (Fig. 3a), there are two large and distinct domains caused by the weak interaction between the two polymeric phases (Navas et al., 2017; Zheng et al., 2018). Besides the domains observed, the structure presented a high irregularity with the presence of big voids that are characteristic of structural failures generated in the fracture of the sample. With the addition of Kaol into the system (Fig. 3b), there is a decrease in the size of the domains and voids. This indicates that even in a small proportion, the nanoparticles can improve the interaction between the phases. When the clay mineral modified with small amines (Kaol1) was applied into the blend (Fig. 3c), it was possible to note a poor compatibilization with one phase, because the domains were still present. However, fewer structural failures in the surface can be observed, indicating a major regularity in the structure. This is an indication that although there is an affinity between the benzylamine and PS phase, the size of the modifier is unsuitable for effective interactions. The blend modified with the second Janus nanoparticle (Kaol2), containing a more significant cationic group, showed a smooth surface 3
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Fig. 1. TGA and DTG curves for raw Kaol and (a) Kaol with cationic modifiers and (b) the Kaol-based Janus nanoparticles synthesized.
Fig. 2. DRIFT/FTIR spectra of raw Kaol and the synthesized Kaol-based Janus nanoparticles.
corroborate the observations of the SEM analysis. Raw Kaol (Fig. 4a) was not able to act as a compatibilizing agent due to the poor interaction with the polymeric species. This weak interaction, allied with weak shear forces induced by the processing conditions, led to a low dispersion with a few large tactoids of clay mineral in the phase of major proportion (Dennis et al., 2001; Ramírez-Vargas et al., 2009). The sheets of unmodified Kaol interact with each other by Van der
with no domains (Fig. 3d), proving that these modifications were effective in improving the clay mineral/blend affinity. The nanoparticle probably acted as a good compatibilizer for the two phases and one phase was well distributed in the other, creating a very homogeneous surface. TEM analyses were used to investigate the interaction of Kaol and Janus nanoparticles with the polymer phases. The micrographs 4
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Fig. 3. Micrographs of SEM for (a) neat Blend, (b) Blend/Kaol, (c) Blend/Kaol1 and (d) Blend/Kaol2. The scale bar corresponds to 10 μm.
Fig. 4. Micrographs of TEM for (a) Blend/Kaol, (b) Blend/Kaol1 and (c) Blend/Kaol2. The scale bar corresponds to 1 μm.
3.2.2. Thermo-mechanical and mechanical properties Fig. 5 and Table 1 show the thermo-mechanical properties of the blends. For all blends, it was possible to verify a shift in Tg values to lower temperatures in comparation with the pure polymers, and no significant variation was observed between the blends. However, a small increase in Tg was observed when there was better nanoparticle/ polymer interaction (Wanke et al., 2016). The loss modulus E″ (see ESI, Fig. S3) showed a behavior similar to the tan δ curves. There was an intensity gain in the modulus signal in the Tg region. Furthermore, the modified kaolinite blends shifted the curves to higher temperatures when compared to pure blend and blends with unmodified nanoparticles. The storage modulus (E') shows the capacity of the materials to store mechanical energy and to resist deformation. For all of the blends, a synergic effect was observed between the different phases, because there was an increase in modulus in relation to pure components of the mixture (Fig. 5a). When the nanoparticles were incorporated into the blend, the increase in E' was more significant (Fig. 5b). In the region of room temperature (at 23 °C), the increase was around 50% with addiction of raw Kaol and up to 65% with incorporation of the Janus nanoparticles. Despite a poor distribution of the raw Kaol in the blend and the low interaction between the phases, the increase in E' can be attributed to the major presence of the clay mineral in the PMMA. This way, the
Waals forces and hydrogen bounds. This way, the interaction between clay mineral and polymer phases is poor, and the Kaol nanoparticles tend to form agglomerates (Alexandre and Dubois, 2000). For the Blend/Kaol1 (Fig. 4b), as well as in the SEM analysis, it is possible to observe the presence of distinct polymer domains. Although the domains are well defined, they are smaller than in the raw clay blend. The smaller tactoids and domains indicate a better compatibilization of the system. In addition, Kaol1 was observed in both phases, but the nanoparticles apparently had a better dispersion in the PS phase. However, the modifier did not have sufficient interaction with both phases to act effectively at the interface of the domains. The use of Kaol2, which has bulky compatibilizing species on both sides of the clay mineral, showed more promising results in relation to the compatibilization of the two phases. In Fig. 4c, there are smaller and fewer clear domains. Besides that, the nanoparticles are well distributed at the interface of the polymeric phases and act as an efficient compatibilizer. As well as in the Kaol1, the octahedral surface modifier of Kaol2 had good interaction with the PMMA phase. However, the larger size of the tetrahedral phase modifier led to a better interaction with the PS phase. The alkyl groups present in POSS had a more effective interaction with the aromatic rings in the PS chain when compared to the esters in the PMMA chain. In this way, the nanoparticle is able to act on the interface of the domains and increase the compatibility between them. 5
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Fig. 5. DMA curves for PS, PMMA, Blend, Blend/Kaol, Blend/Kaol1 and Blend/Kaol2.
distribution of the nanoparticles, as observed in morphology section. The mechanical behaviour corroborated the tendency found in thermo-mechanical properties, in which the addition of both nanoparticles and Janus nanoparticles improved the mechanical properties of the blends (Table 1 and Figs. S4 and S5 in ESI). The increase in the Young's modulus and tensile strength of the blends containing Kaol and Kaol-based Janus can be attributed to distinct factors. As reported in the literature, the good dispersion of nanoparticles and their presence in the phase with a major proportion can generate a gain in properties (Šupová et al., 2011; Visakh et al., 2016). Second, the decrease in the size of domains leads to a decrease in structural failures, because the domains usually act as stress concentration points in the materials (Di Lorenzo and Frigione, 1997). These factors led the sample with Kaol2 to show greater properties in relation to other mixtures, with an increase of around 35% in Young's modulus and around 70% in tensile strength. The Kaol2 incorporated blend showed a better response in the strain of break in relation to the other species. This is probably due to better adhesion between the phases allied with the good dispersion of the clay mineral. This result indicates that the reduction of the domains is directly linked to the increase of strain in the blends.
Table 1 Thermo-mechanical and mechanical properties of blends with Kaol and Janus nanoparticles. Compounds
Tg (°C)
E' at 20 °C (MPa)
Strain at break (%)
Tensile strenght (MPa)
Young's modulus (MPa)
PS PMMA Blend Blend/Kaol Blend/Kaol1 Blend/Kaol2
113 121 101 100 103 105
967 1183 1599 2417 2124 2631
– – 0.71 0.73 0.79 1.13
– – 7.9 ± 0.9 9.8 ± 0.7 9.6 ± 0.9 13.6 ± 0.4
– – 1070 1310 1230 1470
± ± ± ±
0.05 0.06 0.07 0.03
± ± ± ±
70 50 30 70
phase with more mechanical resistance in the blend caused an increase in this property. The addition of Kaol1 also showed an improvement in E' in relation to pure blend. Although the morphology of this blend indicated a better interaction between the phases, the value of E' is smaller than in the blend with raw Kaol. This fact can be attributed to a better distribution of the clay mineral between the phases, because the lower presence of nanoparticle in the PMMA phase caused a decrease in E'. For the blend with Kaol2, the system showed an E' value higher than other samples because of the synergic effect from the reduction of structural failures generated at the PS/PMMA interface and the good
3.2.3. Thermal stability The blends and pure polymers were also evaluated for their thermal 6
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Fig. 6. TGA and DTG curves for (a) PS, PMMA and neat Blend and (b) Blend/Kaol, Blend/Kaol1 and Blend/Kaol2.
properties (Fig. 6). In general, these studies indicated a decrease in the characteristic temperatures of the materials. The presence of Kaol in the system reduced the initial degradation temperature (T5%) by 15 °C in relation to the neat blend and also this event is still observed in the presence of the modifiers. It is possible that the polar groups present in the clay mineral and modifiers catalyse the degradation of the PMMA phase. This behaviour is observed in some systems where the polymer chains are in contact with chemical groups of the clay minerals, and this results in lower stability (Lee and Viswanath, 2000; Carli et al., 2014; Daitx et al., 2015). When the maximum degradation temperatures (Tp) were evaluated, the values found were similar to those of the pure polymers but the characteristics of the curves varied, as shown as in Table 2. The blends with raw and modified Kaol also showed degradation temperatures similar to the neat blend, with maximum degradation at about 420–430 °C. However, the samples with modified Kaol showed two distinct signals, and the temperature of degradation of the PMMA phase of Janus containing blends happened at a lower temperature, probably due to the presence of polar groups in the nanoparticles or the degradation of functional groups of the nanoparticles. In spite of this, these decreases do not restrict the window for obtaining the blends nor their regular applications.
Table 2 Thermal analysis data of pure polymers and blends with Kaol and Janus nanoparticles. Compounds
Tp (°C)
T5% (°C)
Residuea (%)
PS PMMA Blend Blend/Kaol Blend/Kaol1 Blend/Kaol2
433 387 393/422 421 364/430 367/423
377 337 347 332 334 329
0 0 0 4.2 3.9 3.1
a
Determined from the TGA curves as the residue content remaining at 600 °C.
4. Conclusions The morphology of the blends was positively changed with the incorporation of all the Janus nanoparticles, and even incorporation of small percentages of the cationic modifiers was sufficient to generate improvements in the properties. The nanoparticles increased the compatibilization; they reduced the size of the domains and the blends showed fewer structural failures. Besides this, the mechanical properties were affected positively. In relation to a pure blend, the sample containing Kaol2 showed an increase in E', Young's modulus, and strain
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of break of 50%, 35%, and 70%, respectively. Moreover, the slight reductions in Tg and thermal properties do not restrict the applications for this type of material. When using groups which interact well with both phases of the blend, the Janus nanoparticles were distributed at the interface of the blend, acting as an efficient compatibilizer. In addition, it was possible to observe that besides the good interaction with the phases and reduction of domains, it was necessary to have a good dispersion of the nanoparticles, so that the blends could reach the best properties.
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Declaration of Competing Interest None. Acknowledgments The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (PRONEX/ FAPERGS) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105291. References Alexandre, M., Dubois, P., 2000. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng. 28 (1–2), 1–63. Aubry, T., 2019. An overview on clay-mediated compatibilization of polyethylene/ polyamide blends with droplet morphology. Appl. Clay Sci. 175, 184–189. Bahrami, R., Löbling, T.I., Schmalz, H., Müller, A.H.E., Altstädt, V., 2017. Synergistic effects of Janus particles and triblock terpolymers on toughness of immiscible polymer blends. Polymer 109 (27), 229–237. Barlow, J.W., Paul, D.R., 1984. Mechanical compatibilization of immiscible blends. Polym. Eng. Sci. 24 (8), 525–534. Brown, H.R., Char, K., Deline, V.R., Green, P.F., 1993. Effects of a diblock copolymer on adhesion between immiscible polymers. 1. Polystyrene (PS)-PMMA copolymer between PS and PMMA. Macromolecules 26 (16), 4155–4163. Carli, L.N., Daitx, T.S., Soares, G.V., Crespo, J.S., Mauler, R.S., 2014. The effects of silane coupling agents on the properties of PHBV/halloysite nanocomposites. Appl. Clay Sci. 87, 311–319. Castro-Aguirre, E., Auras, R., Selke, S., Rubino, M., Marsh, T., 2018. Impacto f nanoclays on the biodegradation of poly(lactic acid) nanocomposites. Polymers 10 (2), 202–223. Chen, R.S., Ahmad, S., Gan, S., 2017. Characterization of recycled thermoplastics-based nanocomposites: Polymer-clay compatibility, blending procedure, processing condition, and clay content effects. Comp. Part B - Eng. 131, 91–99. Daitx, T.S., Carli, L.N., Crespo, J.S., Mauler, R.S., 2015. Effects of the organic modification of different clay minerals and their application in biodegradable polymer nanocomposites of PHBV. Appl. Clay Sci. 115, 157–164. Dennis, H.R., Hunter, D.L., Chang, D., Kim, S., White, J.L., Cho, J.W., Paul, D.R., 2001. Effect of melt processing conditions on the extent of exfoliation in organoclay-based nanocomposites. Polymer 42 (23), 9513–9522. Di Lorenzo, M.L., Frigione, M., 1997. Compatibilization criteria and procedures for binary blends: a review. J. Polym. Eng. 17 (6), 429–459. Diko, M., Ekosse, G., Ogola, J., 2016. Fourier transform infrared spectroscopy and thermal analyses of kaolinitic clays from South Africa and Cameroon. Acta Geodyn. Geomater. 13 (2), 149–158. Giannakas, A., Vlacha, M., Salmas, C., Leontiou, A., Katapodis, P., Stamatis, H., Barkoula, N.-M., Ladavos, A., 2016. Preparation, characterization, mechanical, barrier and antimicrobial properties of chitosan/PVOH/Clay nanocomposites. Carbohydr. Polym.
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