Modified graphene oxide (GO) was synthesized by covalently grafted alkylated chains on GO sheets and their compatibilization effects on the morphologies and mechanical properties of immiscible polypropylene/polystyrene (PP/PS) blends were investigated. Alkylated-grafted-GO/PP/PS batches were fabricated by melt-mixing approach and displayed different morphologies with various modified GO loadings. When the content of alkylated-grafted-GO is 0.2 wt%, the tensile strength of obtained composite could reach 17.97 MPa, showing a 36.3% enhancement compared to that of pristine PP/PS, indicating the positive compatibilization of modified GO in polymer blends. Moreover, the mixing order also plays an important role in achieving the desired improvement in properties. Due to the preferential location of modified GO in PP phase, a favorable “transition zone” could be formed at the interfacial region of two polymers when alkylated-grafted-GO was premixed with PS and subsequently mixed with PP, leading to an improvement of compatibilization between two polymers and an enhancement of mechanical properties. However, serious phase separation and declined tensile strength were obtained with a reversed mixing sequence.

1. Introduction

Blending of two or three polymeric materials is an effective way to produce new multiphase materials with excellent ultimate performances. The properties of the multiphase polymeric materials are usually affected by the properties of each component and the formed morphologies, especially near the interfacial regions. However, most of polymers are immiscible with each other because of the positive Gibbs energy of mixing, which results in serious phase separation, poor adhesion in interfaces, and deteriorated ultimate properties. One of the simplest and most effective ways to compatibilize immiscible polymers is to use appropriate compatibilizers such as graft or block copolymers [1, 2] and inorganic nanofillers such as clay or layered silicates [35], carbon nanotubes [6, 7], and graphene oxide (GO) or its derivatives [8, 9].

The high modulus and low cost make inorganic nanofillers very attractive to compatibilize immiscible polymer blends. GO, as a product of the oxidative exfoliation of natural graphite, consists of multilayers of sp2-hybridized carbon atoms with a mixture of carboxyl, hydroxyl, and epoxy functional groups on the basal plane and the edges [10, 11]. They can be easily decorated with strongly bound oxidative debris and have been applied in many applications, such as polymer composites, biosensors, and drug delivery systems [12, 13]. Recently, GO and its derivatives are usually employed to compatibilize immiscible polymer blends and the π-π stacking effects between GO/modified GO and aromatic rings of some polymers could be used to improve the interfacial interactions, resulting in an improved compatibility and better mechanical strength. Chemically coupling of polymethylmethacrylate (PMMA) on GO sheets (PMMA-g-GO) effectively improved the compatibility of immiscible polyvinylidenefluoride/acrylonitrile butadiene styrene (PVDF/ABS) blends, resulting in dramatic increments of 84% in Young’s modulus and 124% in the yield strength for the blends compatibilized by PMMA-g-GO [14]. Modified GO sheets with polyethylene molecules (PE-g-GO) were adopted to reactively compatibilize blends of low density polyethylene and polyethylene oxide (LDPE/PEO), which led to a uniform dispersion of PEO in LDPE matrix and resulted in a significant improvement in the mechanical properties [15]. Polypropylene-grafted-reduced graphene oxide (PP-g-rGO) was also reported as a novel compatibilizer for polypropylene/polystyrene (PP/PS) immiscible polymer blends and the strong interactions of PP-g-rGO between PP and PS chains lead to the enhancements of the tensile strength and elongation at break of the PP/PS blends [16]. Thus, GO/modified GO in polymer blends could not only serve as compatibilizers but also act as reinforcing fillers due to their high modulus, which makes GO and its derivatives preferable to traditional copolymer compatibilizers.

Besides the surface modification of the filler and suitable compatibilizers, the processing technique also plays an important role in achieving the desired improvement in properties. The efficiency of the processing technique in dispersing the fillers becomes critical, especially with nanofillers, which have a very strong tendency to agglomerate due to their high surface energies or selective localization in the multiphase composites. Amani et al. [17] developed a new and effective strategy for compatibilizing immiscible polymer nanocomposites of polyethylene/PS/GO by a combination of solution intercalation and a melt-mixing method. Their studies showed that varying the mixing conditions resulted in different states of dispersion and different localizations of the fillers, leading to large discrepancies for the properties. Therefore, functionalization of appropriate compatibilizers and optimal processing technique are essential to influence the ultimate morphologies and properties of polymer blends [18, 19].

PP/PS blends are typical immiscible polymer mixtures among plastics. Both of them have been widely used in many areas and played a significant role in polymer industry. But PP/PS blends are immiscible due to the semicrystallinity of PP phase and make these polymer pairs chemically and mechanically incompatible. In our previous work, alkylated chains were successfully grafted to the surface of CNTs and the functionalized CNTs greatly improved the thermal stability and mechanical properties of polyimide matrix [20]. Meanwhile, alkylated-grafted-GO has also been confirmed to be good fillers to enhance the mechanical properties of polymers, like poly(lactic acid), polystyrene, and so forth [21, 22]. However, the compatibilization effects of alkylated-grafted-GO on the polymer blends have not been reported (to our knowledge). In this study, alkylated chains (-CONHC18H37) were covalently grafted on GO sheets through the reaction between carboxylic acid groups (-COOH) on GO and CH3(CH2)17NCO. Then the obtained modified GO sheets were applied as compatibilizers for PP/PS blends. The compatibilization effects of alkylated-grafted-GO on the morphologies and properties of PP/PS blends with various modified GO loadings were investigated. Furthermore, the alkylated-grafted-GO was either premixed with PP or PS to study the effects of mixing orders on the morphologies and tensile properties of composites.

2. Experimental Section

2.1. Materials

Graphite oxide (GO) was made by modified Hummer’s method [23]. Octadecyl isocyanate (CH3(CH2)17NCO) with a purity >99% was supplied by Sigma-Aldrich. Polypropylene (T30S, MFI = 3 g/10 min) was purchased from Maoming Petrochemical Chemical and polystyrene (PS144C) was obtained from BASF, China. N,N-Dimethylformamide (DMF), toluene, and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification.

2.2. Preparation of Alkylated-grafted-Graphene Oxide

Dried GO powder (300 mg) and CH3(CH2)17NCO (3.5 g) were put into a 100 mL three-necked flask with 30 mL DMF as a medium and stirred for 24 h at room temperature under N2 atmosphere. The mixture was poured into toluene; then the sediments were centrifuged and washed by acetone several times. Then the final products of alkylated-grafted-GO (noted as aGO) were obtained.

2.3. Fabrication of Alkylated-grafted-GO Compatibilized PP/PS Blends

The alkylated-grafted-GO compatiblized PP/PS (70 : 30, w/w) blends were fabricated by melt-mixing method. A certain amount of modified GO with PP and PS was mixed by using Haake MiniLab microcompounder at 190°C with a screw speed of 60 rpm for 10 min, yielding aGO/PP/PS blends with different modified GO loadings. The various feeding ratios of aGO are 0.2 wt%, 0.5 wt%, 1.0 wt%, and 5.0 wt%, and the samples are designated as aGO/PP/PS-0.2, aGO/PP/PS-0.5, aGO/PP/PS-1.0, and aGO/PP/PS-5.0, respectively. Furthermore, two other blending batches with different mixing orders were fabricated: aGO was firstly premixed with PP in the microcompounder at 190°C for 10 min and subsequently mixed with PS for more 10 min to get products (PP/aGO)/PS; and the other one was fabricated by just inversing the adding orders of PP and PS and the obtained blends were noted as (PS/aGO)/PP. In both (PP/aGO)/PS and (PS/aGO)/PP systems, the ratio of modified GO in PP/PS (70 : 30, w/w) blends was controlled as 0.2 wt% and 0.5 wt%, which are noted as (PP/aGO)/PS-0.2, (PS/aGO)/PP-0.5, (PP/aGO)/PS-0.2, and (PS/aGO)/PP-0.5, respectively.

2.4. Measurements

The morphology and microstructure of the samples were investigated by field emission scanning electron microscope (FESEM, HitachiS-8000) and transmission electron microscope (TEM, JEM-2100). Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR spectrometer and the spectra in the range of 2000–500 cm−1 were collected by averaging 32 scans at a resolution of 4 cm−1. Raman spectra were conducted using a LabRam-1B Raman spectroscope with He-Ne laser excitation at 632.9 nm and scanning for 50 s. For measurement of the mechanical properties of the composites, the sample were cut to sheets with a width of 10 mm, and the measurement was conducted with XQ-1 instrument made in China. The lower grip was fixed and the upper grip rose at a rate of 5 mm/min. The gauge length was 20 mm.

3. Results and Discussion

3.1. Synthesis and Characterization of Alkylated-grafted-GO

It is known that graphene or graphene oxide nanosheets exhibit poor interactions with nonpolar PP chains but strong interactions with PS molecules throughπ-π stacking [24]. Similar to our previous work [20], alkylated-grafted-GO was prepared through the reaction between carboxylic acid groups (-COOH) on GO and CH3(CH2)17NCO as illustrated in Figure 1. The chemical structure and morphologies of obtained alkylated-g-GO were carefully characterized. Fourier transform infrared (FTIR) spectra were first carried out to characterize the chemical structures of modified GO and the spectra of GO, aGO, and CH3(CH2)17NCO are shown in Figure 2(a). It has been reported that GO sheets consist of lots of oxygen groups such as hydroxyl and epoxy groups on the basal planes and carboxyl groups on the edges [13]. From the FTIR spectra, characteristic signals of GO are displayed at ~1720 cm−1, ~1400 cm−1, ~1226 cm−1, and ~1081 cm−1, which could be attributed to C=O stretching mode in -COOH, O-H bending mode, C-OH stretching mode, and C-O stretching mode, respectively. The spectrum of aGO exhibits the new peaks at 3339 cm−1 and 1576 cm−1, corresponding to the N-H stretching and bending mode in amide group [20]. Other peaks of aGO at ~1238 cm−1, ~1468 cm−1, and ~722 cm−1 could be ascribed to the C-N stretching, C-H stretching in alkylated chains, and C-C stretching mode, respectively. Comparing with the line for CH3(CH2)17NCO, no signal of N=C=O peak at ~2190 cm−1 appears for aGO, indicating the chemical reaction between -COOH groups on GO with -NCO . These observations clearly demonstrate that alkylated chains have been covalently bonded onto GO surfaces. Furthermore, Raman spectroscopy is also a powerful tool for characterizing the structure of graphene based materials. As shown in Figure 2(b), both aGO and GO show the D band at ~1350 cm−1 and G band at ~1580 cm−1. The presence of D peak is associated with the defectiveness of the materials and the G peak indicates the graphitized degree of carbon materials [25, 26]. It is clearly seen that aGO shows a larger ratio than that of GO, suggesting the increasing defects on aGO sheets after the reaction with CH3(CH2)17NCO.

TEM was also employed to observe the morphologies of dispersions of GO and aGO. As shown in Figure 3, the sheets of aGO could well remain the rippled nanosheets with wrinkles of GO sheets. The experimental observation shows that GO in dimethylformamide (DMF) easily agglomerates and settles to the bottom of a container within several days. But aGO can be uniformly dispersed in DMF after a couple of weeks, as shown in the inset of Figure 3(a). TEM image in Figure 3(b) also shows the modified graphene nanosheets with crumples due to its thin thickness. Moreover, the elemental analysis from Table 1 demonstrates an increase in content of N atoms in aGO, further proving the existence of isocyanate groups on GO sheets.

3.2. Theoretical Prediction for the Localization of aGO in the PP/PS Blends

The final localization of fillers in polymer blends is often determined by combination of thermodynamic and kinetic effects during the melt blending, and correct interpretations of these states need the knowledge about the thermodynamic driving force and practically processing parameters relating to aGO movements. The wetting coefficient () is often used to predict the thermodynamic location of fillers in polymer blends. Generally, the interfacial affinity between polymer components and inorganic fillers and the preferential localization of the fillers in the composites can be predicted by interfacial tensions between polymers and additives. According to Young’s equation, is proposed to define the location of aGO in an equilibrium state in PS/PP blends according to the following equation [27]: where represents the interfacial tension between aGO and polymer component PP, is the interfacial tension between aGO and PS, and describes the interfacial tension between PP and PS. aGO is preferentially located in PS when , at the interface between the two phases when and in PP component when .

The interfacial tension () can be determined from the surface energy and its disperse/polar components. It can be calculated according to the following two methods including Harmonic-mean and Geometric-mean equations. The former is valid between low-energy materials and the latter is often applied for a low-energy material and a high energy material [28, 29].

Harmonic-mean equation is as follows:and Geometric-mean equation is as follows:where is the surface energy of component and and are the dispersive parts and polar parts of the surface tension of components, respectively.

The method used to determine the surface energy is based on the contact angle measurements between the liquid meniscus and the polymer surface. The geometric-mean method can be used to calculate the surface energy [30]:where is the contact angle, subscripts “LV” and “SV” denote the interfacial liquid-vapor and surface-vapor tensions, respectively, and superscripts “” and “” denote the disperse and polar components, respectively. Then the total surface tension () can be calculated by the equationThen the surface energy can be calculated by the contact angles of some representative liquids on the surface of polymer. Here, the selected representative liquids are distilled water (H2O) and ethylene glycol ((CH2OH)2). The surface energy data of H2O and (CH2OH)2 are  mNm−1 and  mN m−1 for H2O and  mN m−1and  mN m−1 for (CH2OH)2. The average contact angle between aGO and H2O is 107.12° and that of aGO and (CH2OH)2 is 87.94°. Then, according to (4) and (5), the values of the surface energy of polymers are calculated and summarized in Table 2. All the results we used in this work are collected at room temperature.

As shown in Table 2, the surface energy of aGO is derived as 14.75 mN m−1, lower than the value of graphene in reported literatures [31, 32], which might be attributed to the long alkylated chains grafted on graphene oxide surfaces [33, 34]. The interfacial tension between PP and aGO is much smaller than that of PS and aGO, indicating the discrepancy of aGO tendency in PP and PS blends. Then the interfacial tensions between each component can be calculated according to Harmonic-mean and Geometric-mean equations and the results are shown in Table 3. Finally, the wetting coefficient can be obtained and the values are −1.46 and −1.58 as displayed in Table 4, respectively. The low wetting coefficient (<−1) indicates that aGO is preferentially located in the PP phase. However, kinetic migration of aGO plays a decisive role on the localization of aGO in the blends. The migration of aGO from one polymer bulk to another or to the interfacial areas is dependent on the diffusion, collisions, and Brownian motion of aGO in polymer bulk (PS or PP) as well as the viscoelastic properties of each polymer [35, 36].

3.3. Morphologies and Mechanical Properties of aGO/PP/PS Blends

To show the compatibilization effects of aGO on PP/PS blends, PP/PS (70/30, w/w) blends with different aGO contents were firstly fabricated through direct melt blending of all ingredients. The changes in phase morphologies of the samples as a function of aGO contents are depicted in Figure 4. For PP/PS (70 : 30) system, as shown in Figures 4(a) and 4(a′), obvious phase separation could be observed, indicating the incompatibility of two components. After adding aGO with a content of 0.5 wt% in Figures 4(c) and 4(c′), the PS domain size decreases and the large PS agglomerates gradually disappear when the loading of aGO reached 1.0 wt% as displayed in Figures 4(d) and 4(d′). Although the calculated results show a preferential location of aGO in PP, the FESEM observation clearly demonstrates the positive compatibilization effects of aGO nanosheets on PP/PS blends. It is reported that the aGO sheets could absorb some PS chains on their graphene basal planes through π-π interactions [37, 38]. Meanwhile the theoretical calculation demonstrates that these functionalized nanosheets exhibit strong intermolecular interactions with PP phase, which might probably be due to the long alkylated chains grafted on their surfaces. Thus the aGO nanosheets tend to locate at the PP/PS interfacial regions to form a “transition zone” between polymer components, reducing unfavorable contacts and interracial tensions. Consequently, the compatibility between PP and PS is improved and the size of the dispersed phase domains decreases. However, when the content of aGO increases to 5 wt%, aGO/PP/PS-5.0 shows a coarse cross-section surface as displayed in Figures 4(e) and 4(e′), which might be attributed to the increased PS domains and the aggregation of aGO sheets in the blends.

Generally, compatibilization can dramatically enhance the ultimate properties of immiscible polymer blends. The mechanical properties of PP/PS blends compatibilized by aGO with various loadings are given in Figure 5. The average tensile strength and tensile modulus of the PP/PS polymer blends are 13.19 MPa and 2.70 GPa, respectively. The average tensile strength of aGO/PP/PS-0.2, aGO/PP/PS-0.5, aGO/PP/PS-1.0, and aGO/PP/PS-5.0 are 17.97 MPa, 16.52 MPa, 14.88 MPa, and 12.18 MPa, respectively, and their corresponding average tensile moduluses are 2.89 GPa, 2.68 GPa, 2.65 GPa, and 2.64 GPa, respectively. It is obvious that the incorporation of aGO obviously improves the tensile strength and modulus of the composites, and the tensile strength of aGO/PP/PS-0.2 reaches 17.97 MPa, showing a 36.3% enhancement compared to that of PP/PS. The possible reason for the improvement of mechanical properties might be the enhanced interfacial interactions among aGO and the two polymers. Besides, the reduced PS particle size caused by the addition of aGO also can promote the interfacial stress transfer. It should be mentioned that aGO can not only have compatibilizing effect on PP/PS blends but also increase their stiffness because of the high modules of graphene, and then the tensile modulus of aGO/PP/PS increases to 2.89 GPa. However, when the feeding ratio of aGO rises higher than 0.5 wt%, the tensile strength gradually declines and the modulus shows the similar level to that of pristine PP/PS blends, which are consistent with the observation in the FESEM images in Figure 4.

3.4. Effects of Blending Orders on the Compatibilization of aGO in PP/PS Blends

To further explore the effects of blending orders on the localization of modified GO in PP/PS blends, two other different blends were prepared by controlling the mixing orders of aGO, PP, and PS (Experimental Section), which are denoted as (PP/aGO)/PS and (PS/aGO)/PP, respectively (the loading weight of aGO is 0.2 wt% and 0.5 wt%). In Figure 6, compared with aGO/PP/PS-0.2 in (a) and (a′), (PP/aGO)/PS-0.2 in (b) and (b′) displayed an obvious phase separation with a larger diameter of PS domains, which demonstrated obvious PS spheres, indicating the poor compatibilization of PP and PS. While (PS/aGO)/PP-0.2 in (c) and (c′) shows a more continuous morphology than those of (PP/aGO)/PS-0.2. Similar results also could be obtained for blending systems with 0.5 wt% loading of aGO as shown in Figure 7. Therefore, the mixing order has a great effect on the morphologies of final blends: when aGO is premixed with PP ((PP/aGO)/PS system), the preferential location of aGO in PP phase gives an unfavorable compatibilization effect on the PP/PS blends and induces the obvious phase separation; while when aGO is premixed with PS ((PS/aGO)/PP system), the selective location of aGO in PP drives it moving from PS phase into PP, leading it to concentrate on the phase interface to form a “transition zone,” which gives a positive influence on the compatibilization of PP and PS.

Furthermore, the tensile properties of PP/PS and aGO/PP/PS with different mixing orders are also investigated and given in Figure 8. The tensile strength and elongation at break of the PP/PS polymer blends are 13.19 MPa and 0.93%, respectively. The tensile strength for (PP/aGP)/PS-0.2, (PP/aGO)/PS-0.5, (PS/aGO)/PP-0.2, and (PS/aGO)/PP-0.5 is 11.42 MPa, 8.91 MPa, 17.58 MPa, and 15.21 MPa, respectively. After adding alkylated-grafted-GO, (PP/aGP)/PS-0.2 and (PP/aGO)/PS-0.5 show reduced mechanical properties with the tensile strength of 11.42 MPa and 8.91 MPa, respectively, indicating the prior mixing of alkylated-grafted-GO with PP immobilizes the modified GO nanosheets in PP phase, resulting in the serious phase separation with declined mechanical properties. Meanwhile (PS/aGO)/PP-0.2 and (PS/aGO)/PP-0.5 displays an improved tensile strength of 17.58 MPa and 15.32 MPa with the elongation of 5.54% and 1.53%, respectively, suggesting the enhanced interfacial interaction among modified GO loadings and the two polymers. Besides the compatibilizing effects, the modified GO also increases their stiffness because of high Young’s modulus of graphene. As shown in Figure 8(b), with a loading of 0.2 wt% alkylated-grafted-GO, (PS/aGO)/PP displays the largest tensile modulus of 2.95 GPa, further confirming the selective localization of modified GO loadings with different mixing order and their great influence on the properties of PP/PS blends.

4. Conclusions

In conclusion, we successfully fabricated alkylated-grafted-GO sheets by covalently grafting CH3(CH2)17NCO on GO sheets and studied their compatibilization effects on the morphologies and mechanical properties of PP/PS blends. The results showed that an appropriate content of modified GO in PP/PS blends could be used as a favorable compatibilizer to improve the mechanical properties of PP/PS blends. With 0.2 wt% content of alkylated-grafted-GO, the tensile strength of aGO/PP/PS-0.2 could reach 17.97 MPa, showing a 36.3% enhancement compared to that of PP/PS. Besides, the mixing orders also greatly affect the morphologies and properties of polymer blends. When modified GO was premixed with PS and then mixed with PP, the obtained (PP/aGO)/PS batches displayed a more continuous morphology and a better mechanical property compared to those of (PS/aGO)/PP.

Competing Interests

The authors declare that they have no competing interests.