Phase Morphology Evolution and Compatibilization of Immiscible Polyamide 6/Polystyrene Blends Using Nano-Montmorillonite.
In recent years, the study of polymer-matrix nanocomposites has expanded greatly in various areas, including the field of flame retardant polymers [1, 2]. Among the nanocomposites, polymer/layered silicate nanocomposites (PLSN) exhibit a lot of unique, useful properties in the presence of a small amount of silicate, such as enhanced mechanical properties, increased heat distortion temperature, improved thermal stability, and reduced flammability [3, 4]. In recent years, the application of inorganic layered silicate materials has aroused great interest due to its natural source and the high modulus of clay platelets. These layered materials exist in the form of aggregates bonded with physical forces, so they can be exfoliated during the processing of polymer nanocomposites [5-8]. The silicate layers can also play the role of a barrier, which can diffuse and impede transfer of heat and oxygen, so that good flame retardant performances are obtained for the polymer/clay nanocomposites.
As is known, the key for the successful preparation of PLSN is to obtain a widely dispersed nanostructure which can lead to optimization of the organ-clay performance in the polymer matrix. Many researchers have been interested in the influences of organic montmorillonite (OMMT) on the structure and properties of PLSN [9-18]. For instance, Wang et al. reported the effect of the addition of organoclay platelets on morphology and mechanical properties of poly(trimethylene terephthalate)/ethylene-propylene-diene copolymer grafted with maleic anhydride/ organoclay (PTT/EPDM-g-MA/organoclay) nanocomposites and showed that almost all of the exfoliated or intercalated clay platelets existed in dispersed domains of the EPDM-g-MA phase and that enhanced mechanical properties were obtained . Gonzalez et al. investigated the effect of clay loading on the morphology and mechanical properties of PA6 and showed the dispersed particle size increased the mechanical properties after the addition of OMMT . The mechanism of compatibilization and compatibility of OMMT in incompatible polymer blends has been summarized as follows [19-21]: (1) the inorganic ions on the clay layers surfaces can reduce the phase interfacial energy of polymer blends; (2) the inorganic particles surrounding the dispersed phase can serve as solid obstructions that stop the dispersed phase from aggregating; (3) the selective distribution of inorganic particles can change the ratio of the viscosity of the two phases; (4) the network structure generated by the inorganic particles can inhibit the movement of the dispersed phase or matrix when the inorganic particle content is over a critical value; (5) the space steric effect can be induced between polymer molecular chain and inorganic particles.
Polyamide 6 (PA6) and polystyrene (PS) have very good complementary effects on the performances of their blends, and more and more researchers have shown interest in the relationship between their morphological structure and properties [22-30]. The most common preparation method has been direct melting blend. The performance of the PLSN was related to either the surface modifiers or processing parameters. The effects of processing temperature [17, 18], extruder type [21, 31], shear rate [31, 32], and residence time  were also investigated. However, to the best of our knowledge, no works have been reported in the literatures thus far using nano-OMMT as a compatibilizer in immiscible PA6/PS blends. In this article, the phase morphology, compatibilization and mechanical properties of immiscible PA6/PS blends with nano-OMMT were investigated through using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and mechanical property tests.
Polyamide 6 (PA6; 1013B, MFR = 15.4 g/10 min) was purchased from Ube Industries Co., Ltd., Japan. Polystyrene (PS, 1050L, MFR = 12.5 g/10 min) was from Total Petrochemicals Co., Ltd., China. Organic nano-montmorillonite (nano-OMMT, DK2), treated with octadecyl trimethyl ammonium salt, was supplied by Fenghong Clay Co., Ltd., China. Dimethylbenzene (DMB) was supplied by Chuandong Chemical Reagent Factory, China. Tetrahydrofuran (THF) was supplied by Tianjing Chemical Reagent Factory, China.
Preparation of Samples
The preparation of the three types of PA6/PS/OMMT nanocomposites was performed according to the following steps:
1. PA6, PS, and nano-OMMT were dried at 80[degrees]C for 12 h, and then nano-OMMT masterbatch 1 (M1) with PA6 and nano-OMMT, and nano-OMMT masterbatch 2 (M2) with PS and nano-OMMT were melt mixed with mass ratios of 2:1, as described below. The extrudates were cut into pellets and dried at 70[degrees]C for 12 h.
2. Suitable quantities of the masterbatch M1, and further PA6 and/or PS were mixed to obtain final mass ratios of PA6/PS/ OMMT of 70/30/1, 70/30/3, 70/30/5, and 70/30/7.
3. Similarly, masterbatch M2, and PA6 and/or PS were mixed to obtain the final mass ratios of PA6/PS/OMMT of 70/30/1, 70/ 30/3, 70/30/5, and 70/30/7.
4. OMMT, PA6, and PS were mixed directly to obtain the final mass ratios of PA6/PS/OMMT of 70/30/0, 70/30/1, 70/30/3, 70/30/5, and 70/30/7.
5. The mixtures in steps (1) to (4) were blended in a twin-screw extruder (TypeCTE35, Coperion Keya Machinery, Co., Ltd., China) at 200[degrees]C to 230[degrees]C and screw speed of 300 rpm. The extrudates were cut into pellets and then injection molded (CJ80MM3V, Chengde Plastics Machinery Co., Ltd., China) with a melt temperature of about 230[degrees]C and a mold temperature of about 60[degrees]C into various specimens for testing and characterization.
Measurements and Characterization
X-ray Diffraction (XRD) Analysis. XRD patterns were recorded in a D/Max-2200 X-ray diffractometer (Rigaku, Co., Ltd., Japan), with 40 kV, 30 mA, and Cu-Ka radiation. The scan speed was 2[degrees]/min.
Transmission Electron Microscopy (TEM). The TEM samples were embedded in an epoxy resin and ultrathin-sectioned at -50[degrees]C using an ultramicrotome. The PS segments in the composites were stained with ruthenium tetroxide (Ru[O.sub.4]). The micrographs were obtained in a JEM200CX apparatus (JEOL Co., Ltd., Japan).
Scanning Electron Microscopy (SEM). The surfaces of cryogenically fractured specimens were etched for 2 days by tetrahydrofuran to remove the dispersed PS phase. The fracture morphology was observed by a SEM (KYKY-2800B, KYKY Technology Development Co., Ltd., China) after gold coating. The number average ([D.sub.n]) and weight-average particles sizes ([D.sub.w]), and the polydispersity index ([P.sub.d]) of the PS domains were calculated by the following formulae:
[mathematical expression not reproducible]
where [N.sub.,] is the number of nearly spherical particles with diameter [d.sub.i].
Mechanical Properties. Tensile and flexural tests were carried out on a tensile tester (WDW-10C, Shanghai Huang Test Instruments Co., Ltd., China) with the crosshead speeds of 50 mm/[min.sup.-1] and 2 mm/[min.sup.-1], respectively. The dimensions of the specimens were 120 x 10 x 4 [mm.sup.3]. Notched Izod impact strength was measured using a pendulum impact testing machine (ZBC-4B, Shenzhen SANS Co., Ltd., China). The notch depth of the specimens was 2 mm and notch tip radius was 0.25 mm. The dimensions of specimens were 60 x 10 x 4 [mm.sup.3]. All the tests were performed at 23[degrees]C [+ or -] 2[degrees]C.
RESULTS AND DISCUSSION
XRD and TEM Characterization of the Nanostructure
The characterization of the nanostructure of the PLSN was carried out by using XRD and TEM. The XRD plots of nano-OMMT using the masterbatches and the PA6/PS/OMMT nanocomposites prepared by two of the processing methods are shown in Fig. 1. The peaks at 2[theta] = 3.65[degrees] and 5.40[degrees] were characteristic of the OMMT; however, it was clearly observed that the XRD plots of both of the PA6/PS/OMMT nanocomposites with OMMT contents of 1 and 3 phr prepared through the two different processing methods did not show any diffraction peaks attributed to the low OMMT concentration. But when the content of OMMT was increased to 5 and 7 phr, the weak peaks at 2[theta] = 2.29[degrees] and 4.56[degrees] appeared with a slight shift compared to that of the OMMT. The XRD results indicated the formation of an exfoliated nanostructure was achieved by the M1 and M2 processing techniques. In order to characterize the nanostructure, TEM micrographs of the PA6/ PS/OMMT (70/30/5) nanocomposites were obtained with the results shown in Fig. 2. The deep gray continuous domains corresponded to the PA6 matrix, the white dispersed domains corresponded to the PS phase and the black lines corresponded to the clay platelets. Figure 2A and C are photos of the 70/30/5 nanocomposites prepared through the M1 method, and Fig. 2B and D show the nanostructure of these composites prepared by the M2 method. An effect of processing method on the morphology of nano-OMMT in the nanocomposites was obviously observed. The possible reason is that the OMMT masterbatchs contain different polymer materials in Fig. 2A and C and Fig. 2B and D. In Fig. 2A and C, OMMT masterbatchs contain PA, however, the OMMT masterbatchs contain PS in Fig. 2B and D. In Fig. 2A and C, many of the OMMT layers appeared exfoliated and homogeneously dispersed in the PA6 phase with layer orientation by the Ml processing method. In addition, the PS phase and clay layer have a clear orientation in the PA6/PS/OMMMT composites prepared by M1. At the same time, it is clear that the majority of the OMMT layers was in the PA6 phase and formed an unoriented exfoliated structure for the M2 processing method. The difference between Fig. 2A and C and Fig. 2B and D is attributed to the different preparing methods.
SEM Characterization of the Nanostructure
Figures 3-6 show the SEM photos of PA6/PS and the PA6/PS nanocomposites with different contents of nano-OMMT prepared through the three different methods. It is clearly seen that the morphologies of all of the nanocomposites were similar to that of the reference PA6/PS blend in Fig. 3. The black domains represent the positions of the etched PS dispersed phase in the PA6 matrix. For the PA6/PS blend, the PS phase was uniformly distributed in PA6 phase as nearly spherically shaped, very large domains, as shown in Fig. 3. As shown in Figs. 4-6, the incorporation of nano-OMMT had a large effect on the size of the still nearly spherically shaped PS phase domains. The domains size clearly decreased with increasing content of nano-OMMT from 1 to 5 phr and then decreased only slightly further if at all for 7 phr; the distribution of PS in the PA6 matrix was also improved greatly. The SEM results indicate a remarkable compatibilization of the immiscible PA6/PS blends by adding nanoOMMT for all three methods of mixing.
In order to further investigate the relationship of nano-OMMT content and the processing methods, the weight-average particle sizes ([D.sub.w]) and polydispersity indices ([P.sub.d]) of the PA6/PS/OMMT nanocomposites were determined; the results are shown in Figs. 7 and 8, respectively. The values of [D.sub.w] of PA6/PS/OMMT nanocomposites decreased dramatically for contents of nano-OMMT increasing from 0 to 3 phr, indicating the dispersed PS phase size was reduced. The values of [D.sub.w] then decreased only slightly for further increasing the content of nano-OMMT. This suggests that the addition of nano-OMMT can enhance the compatibility between PA6 and PS. At the same time, it was noted from Fig. 7 that the [D.sub.w] value for the M2 masterbatch system was the lowest among the three processing methods when the nano-OMMT content was fixed at 1 phr. For example, the Dw value of the PS phase for the PA6/PS blend was 2.06 [micro]m when the nano-OMMT content was 3 phr, but for the direct blending, M1, and M2 masterbatch systems, the values of [D.sub.w] were dramatically reduced to 0.83, 0.63, and 0.58 [micro]m, respectively. The droplet size of PS dispersed phases and distribution width are gradually reduced when the amount of OMMT increases. According to SEM and the statistics of particle size, the OMMT layers have a cutting effect on the PS dispersed phases, and so the composites prepared by the processes of master-batches display smaller dispersed phase size as well as better distribution than the ones prepared by direct blending. Therefore, the compatibilization of OMMT in the samples prepared by master-batch methods is better than that of direct blending method.
As shown in Fig. 8, the Pd values of the PS phase decreased slightly and the variation was small for the PA6/PS/OMMT nanocomposites prepared by the direct blending method. However, for the nanocomposites prepared by the Ml and M2 masterbatch methods, the values of Pd decreased sharply from 1 phr nano-OMMT to 3 phr, indicating the PS phase tended to become uniform size in the PA6 matrix. No matter which method was used, the PS phase size and distribution changed only slightly when the nano-OMMT content exceeded 3 phr. A possible reason is that the nano-OMMT selectively distributed in the PA6, which increased the viscosity of continuous phase and led to an increase of the viscosity ratio of the continuous phase and dispersion phase.
From the above discussion, it was concluded that the masterbatch methods were superior to the direct blending method for obtaining a dispersed, small PS phase size, and morphology in the PA6 matrix. When the nano-OMMT migrated from the dispersed PS phase to the PA6 matrix for the Ml method, the nano-OMMT played a "cutting action," which further decreased the size and improved the dispersion of the PS phase in the PA6 matrix. Thus, the nanocomposites prepared though the masterbatch M2 method led to the most significant decrease in the particle size of PS dispersed phase and a large simultaneous improvement in size distribution. This can be considered as due to the enhanced compatibilization of the immiscible PA6/PS blends by using the nano-OMMT.
Fine control of phase morphology for multiphase polymers is an important approach to imparting excellent mechanical performance to the materials. Enhanced compatibility between the PA6 and PS can be reflected in an improvement of mechanical properties [34-36]. Figures 9-11 show the tensile strength, flexural modulus, and impact strength of the PA6/PS/OMMT nanocomposites with different nano-OMMT contents prepared through the three different methods. As shown in Figs. 9 and 10, both of the nanocomposites prepared by the masterbatch methods showed a simultaneous increase of tensile strength and flexural modulus, compared to essentially no change for the direct blending method when introducing only a small amount of nano-OMMT (1 and 3 phr). This is due to the dispersed PS phase dimensions being smaller and more homogeneously distributed. This can be considered as due to the improved compatibilization. However, the tensile strength decreased when the OMMT content was more than 3 phr. In addition, as shown in Fig. 11, there was a slight decrease of impact strength with the addition of nano-OMMT up to 3 phr for the PA6/ PS nanocomposites prepared by the above three methods, followed, especially for the masterbatch methods, by a larger decrease. This may be due to the stress concentration effect. We suggest that the rigid nano-OMMT acted as stress concentration sites and could not transfer the stress from the dispersed PS phase to the PA6 matrix when subjected to sudden impact energy; however, the stress would not be on the PS but on the matrix.
PA6/PP/OMMT nanocomposites were prepared by melt blending through three different processing methods. The nano-OMMT mainly distributed in the PA6 phase with exfoliated platelets. When the nano-OMMT was first melt blended with PS and then melt blended with PA6/PS, a few of the nano-OMMT layers would migrate from PS phase to the PA6 phase. For the three different processing methods, the compatibility of PA6/PS blends could be greatly enhanced due to the addition of nano-OMMT. With the addition of nano-OMMT, the dispersed phase size and the polydispersity index of PS decreased. The masterbatch method was superior to the direct blending method in terms of reducing dispersed phase size and improving the dispersion. The tensile strength, particularly for 3 phr, and the flexural modulus of PA6/ PS/OMMT nanocomposites were improved due to the enhanced compatibilization after introducing nano-OMMT. The composites prepared by the masterbatch methods had higher tensile strength and flexural modulus compared with direct blending method, with the Ml masterbatch method having the most outstanding improvement of the tensile strength and flexural modulus.
The authors thank Prof. Jun Qin (Key Laboratory of Karst Drainage, Ministry of Education (Guizhou University), China) for conducting the SEM and XRD measurements.
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Jianbing Guo, (1) Yang Xu, (2) Weidi He, (1) Na Wang, (2) Mengqi Tang, (2) Xiaolang Chen (iD),(2) Shijun Hu, (1) Min He, (1) Shuhao Qin (1)
(1) National Engineering Research Center for Compounding and Modification of Polymer Materials, Guiyang 550014, China
(2) Key Laboratory of Advanced Materials Technology Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China
Correspondence to: X.L. Chen; e-mail: email@example.com
Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 51003088 and 20964001; contract grant sponsor: National Key Basic Research Program (973) of China; contract grant number: 2011CB612313; contract grant sponsor: High Level Innovation Talents Cultivation of Guizhou Province of China; contract grant numbers:  4039;  5667.
Caption: FIG. 1. X-ray diffraction patterns for PA6/PS/OMMT nanocomposites. (A) M1 processing sequence; (B) M2 processing sequence.
Caption: FIG. 2. TEM photomicrographs for PA6/PS/OMMT nanocomposites. (A) M1 processing sequence; (B) M2 processing sequence; (C) enlargement of I region of (A); (D) enlargement of II region of (B). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. SEM photo of the PA6/PS (70/30) blend prepared by direct blending. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 4. SEM micrographs of PA6/PS/OMMT nanocomposites with different contents of nano-OMMT prepared by direct blending: (A) 1 phr OMMT; (B) 3 phr OMMT; (C) 5 phr OMMT; (D) 7 phr OMMT. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 5. SEM micrographs of PA6/PS/OMMT nanocomposites with different contents of nano-OMMT prepared from Ml masterbatches: (A) M1, 1 phr OMMT; (B) M1, 3 phr OMMT; (C) M1, 5 phr OMMT; (D) M1, 7 phr OMMT. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 6. SEM micrographs of PA6/PS/OMMT nanocomposites with different contents of nano-OMMT prepared from M2 masterbatches: (A) M2, 1 phr OMMT; (B) M2, 3 phr OMMT; (C) M2, 5 phr OMMT; (D) M2, 7 phr OMMT. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 7. Weight-average particle size ([D.sub.w]) of PA6/PS/OMMT nanocomposites with different contents of nano-OMMT prepared by three processes.
Caption: FIG. 8. Polydispersity index ([P.sub.d]) of PA6/PS/OMMT nanocomposites with different contents of nano-OMMT prepared by three processes.
Caption: FIG. 9. Tensile strength of PA6/PS/0MMT nanocomposites prepared by three processes.
Caption: FIG. 10. Flexural modulus of PA6/PS/OMMT nanocomposites prepared by three processes.
Caption: FIG. 11. Izod impact strength of PA6/PS/OMMT nanocomposites prepared by three processes.