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clay nanocomposites: Effects of the compatibilizer
Composites Science and Technology 72 (2012) 1697–1704
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Morphology, rheology and mechanical properties of polypropylene/ethylene–octene copolymer/clay nanocomposites: Effects of the compatibilizer Sedigheh Bagheri-Kazemabad a, Daniel Fox c, Yanhui Chen c, Luke M. Geever d, Alireza Khavandi a,⇑, Reza Bagheri e, Clement L. Higginbotham d, Hongzhou Zhang c, Biqiong Chen b,⇑ a
School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK School of Physics and CRANN, Trinity College Dublin, College Green, Dublin 2, Ireland d Materials Research Institute, Athlone Institute of Technology, Dublin Road, Athlone, Co. Westmeath, Ireland e Polymeric Materials Research Group, Materials Science and Engineering Department, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran b c
a r t i c l e
i n f o
Article history: Received 11 November 2011 Received in revised form 18 March 2012 Accepted 8 June 2012 Available online 18 June 2012 Keywords: A. Nanoclays A. Nanocomposites B. Mechanical properties
a b s t r a c t The objective of this study was to investigate the effects of two compatibilizers, namely maleated polypropylene (PP-g-MA) and maleic anhydride grafted poly (ethylene-co-octene) (EOC-g-MA), on the morphology and thus properties of ternary nanocomposites of polypropylene (PP)/ethylene–octene copolymer (EOC)/clay nanocomposite. In this regard the nanocomposites and their neat polymer blend counterparts were processed twice using a twin screw extruder. X-ray diffraction, transmission electron microscopy, Energy dispersive X-ray spectroscopy, and scanning electron microscopy were utilized to characterize nanostructure and microstructure besides mechanical and rheological behaviors of the nanocomposites. Clay with intercalated structure was observed in EOC phase of the PP/EOC/clay nanocomposite. Better dispersion state of the intercalated clay in EOC phase was observed by adding EOCg-MA as a compatibilizer. On the other hand, adding PP-g-MA resulted in migration of the intercalated clay from the EOC to the PP and to the interface regions. It was also demonstrated that the elastomer particles became smaller in size where clay was present. The finest and the most uniform morphology was found in the PP/EOC/clay nanocomposite. In addition, the rheological results illustrated a higher complex viscosity and storage modulus for PP/EOC/PP-g-MA/clay nanocomposite in which clay particles were present in the matrix. Mechanical assessments showed improvements in the toughness of the nanocomposites with respect to their neat blends, without significant change in stiffness and tensile strength values. These results highlight a toughening role of clay in the polymer blend nanocomposites studied. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Polymer blend nanocomposites containing an elastomer and a reinforcing filler have attracted much attention due to the incorporation of the properties of both polymer blends and polymer nanocomposites [1–3]. Polypropylene (PP)/ethylene–octene copolymer (EOC)/clay nanocomposites are such high performance materials. PP is a commodity polymer due to its low price, high performance and good processability. However, the application of PP is still limited by its poor impact resistance, especially at low temperatures and high strain rates [4]. Blending PP with a rubbery phase like ethylene–octene copolymer is a successful way to improve its impact resistance [4–6]. In comparison to conventional elastomers such as ethylene–propylene copolymer and ⇑ Corresponding authors. Tel.: +44 114 222 5958; fax: +444 114 222 5943 (B. Chen), tel.: +98 21 77459151; fax: +98 21 77240480 (A.R. Khavandi). E-mail addresses: [email protected] (A. Khavandi), biqiong.chen@sheffield. ac.uk (B. Chen). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.06.007
ethylene–propylene–diene terpolymer, EOC has performed better compatibility with PP because of its long side chains, which leads to reduction of the interfacial tension [5–10]. Among nanofillers employed for nanocomposite preparation, clay is vastly used [11–13] owing to its unique structure with a high aspect ratio and its natural abundance [2]. The properties of polymer blend/clay nanocomposites are strongly related to the location [1,14–17] and dispersion degree of the clay [11,12,14,17,18] as well as the size, shape and dispersion condition of the polymer dispersed phase, i.e. the elastomer in the blend [11,12,14]. By adding clay to a polymer blend, depending on the location of clay in the matrix or dispersed phase, two types of morphology including separated or encapsulated will be formed [1,14,19,20]. The relative extent to which these structures develop is largely dependent on the interfacial energies between the components [1,15,19,21], miscibility of the blend [19], the shear force during compounding [1], the viscosity of the components [1,21] and the order of addition of the components during compounding [1,15,16,21]. Among these factors, interfacial
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interaction between the clay and the polymer phases plays a crucial role in the preferential location of clay [21]. Functionalized polymers are added into PP/elastomer nanocomposites to adjust the interfacial adhesion between the components. So, if the affinity between the clay and PP is higher than the affinity between the clay and elastomer, clay would reside in PP and vice versa [16,20]. In the present study, homogeneous dispersion of clay minerals without a compatibilizer is not possible in PP [11,12] and EOC [8,9] which are apolar. So, two compatibilizers based on the same structure units with PP and EOC, i.e., PP-g-MA and EOC-g-MA, were introduced to PP/EOC/clay nanocomposite to tailor the location and dispersion of the clay and to study their influence on structure, mechanical and rheological behavior of the nanocomposite. Although, Several groups have studied the properties of PP/EOC/ clay by the addition of PP-g-MA as a compatibilizer [5,10– 12,15,22–24], there has been no systematic study comparing the effects of clay location and dispersion, induced by the use of various compatibilizers, on the properties of this nanocomposite. 2. Experimental
(XRD) of the C20A powder and injection molded nanocomposite disc samples (thickness 2 mm). The XRD were recorded with a step size of 0.02° from 2h = 2° to 10° and a scanning time of 2.5 s per step. The dispersion state and location of the organoclay were examined by a FEI Titan 80–300 transmission electron microscope (TEM) at an acceleration voltage of 300 kV. A Carl Zeiss Auriga CrossBeam FIB–SEM workstation was used to extract sections from the core of the injection molded specimens. The samples were then transferred to a C-shaped copper TEM grid. The copper grids containing the sample lamellas were then stained by dropping 2 wt.% aqueous osmium tetroxide solution (Sigma–Aldrich) on the surface and dried at room temperature in a fume cupboard. Energy dispersive X-ray spectroscopy (EDX) was also carried out to get additional evidence on the location of clay in the polymer blends. The microstructure of samples was examined by scanning electron microscopy (SEM) using a Philips XL30 model at 17 kV. Surfaces of injection samples were cryogenically fractured in liquid nitrogen and etched with heptane at 50 °C for 3 h to remove the EOC.
2.1. Materials 2.4. Rheological properties PP (571P) was an isotactic homopolymer supplied by Sabic (Saudi Basic Industries Corporation, Europe). Its density and melt flow index (MFI) (2.16 kg at 230 °C) were 905 kg/m3 and 5.7 g/ 10 min, respectively. EOC (InfuseTM9500) was supplied by Dow Chemical Corporate. Its density and MFI were 877 kg/m3 and 5 g/ 10 min (2.16 kg at 190 °C), respectively. PP-g-MA with an MFI of 140 g/10 min (2.16 kg at 160 °C) was a commercial grade product of Sigma–Aldrich. The grafting level of maleic anhydride was 8– 10 wt.%. EOC-g-maleic anhydride (AmplifyTMGR216) with a melt flow index (2.16 kg at 190 °C) of 1.3 g/10 min was provided by Dow Chemical Corporate and its grafting level was more than 0.5 wt.%. The organoclay CloisiteÒ 20A (abbreviated as C20A) was obtained from Southern Clay Products, Inc. CloisiteÒ 20A is a natural montmorillonite modified with dimethyl, dehydrogenated tallow and quaternary ammonium chloride. 2.2. Sample preparation The nanocomposites of PP, EOC (ratio of PP:EOC = 3:1) and organoclay (4 phr) without a compatibilizer and with two types of compatibilizer, PP-g-MA and EOC-g-MA, were prepared. The ratio of compatibilizer to organoclay was fixed to 1. As a control, the blends of PP/EOC, PP/EOC/PP-g-MA and PP/EOC/EOC-g-MA with 4 phr compatibilizer and the binary nanocomposites of PP/clay, PP/PP-g-MA/clay, EOC/clay and EOC/EOC-g-MA/clay with 4 phr clay and 4 phr compatibilizer were also prepared. The blends and nanocomposites were prepared by simultaneous mixing of components using a bench-top PrismTM twin screw extruder (L/D = 25, D = 16 mm). The barrel temperature profile was 160, 160, 170 and 180 °C from the hopper to die and the screw speed was set at 200 rpm. All compositions were extruded twice in order to attain better dispersion of clay and elastomer. The extruded materials were injection molded into test specimens using a bench-top RR/TSMP injection molding machine (Ray-ran Test equipment LTD, UK) with the barrel and tool temperatures of 195 °C and 55 °C, respectively and the pressure of 110 MPa. 2.3. Characterization A Phillips PW 1050/80 X-ray diffractometer (40 kV, 20 mA) equipped with a graphite monochromator and with CuKa1 radiation (wavelength = 0.1541 nm) was used for X-ray diffraction
Rheological analysis was performed on a TA Instruments AR1000TM rheometer fitted with 25 mm stainless steel plate geometry. Frequency sweep experiments were conducted between 100 and 0.03 rad/s at 180 °C using a strain of 1%, i.e., within the linear viscoelastic region as determined from strain sweep experiments at a frequency of 1 Hz. 2.5. Mechanical properties Tensile testing was carried out with a universal Hounsfield H10KS at 23 °C, according to ASTM D638 and at a crosshead speed of 50 mm/min using a load cell of 10 kN. Modulus was measured using an extensometer. Flexural modulus was measured by means of an Instron-1011 machine according to ASTM D790 using 4-point bending configuration at 2 mm/min deformation rate. Notched Charpy impact strength of samples was measured with a Jinjian impact tester according to BS ISO 179-1. A side-edge notch with an angle of 45° and a 2-mm depth and a notch tip radius of 0.25 mm was machined on specimens. 3. Results and discussion 3.1. XRD analysis The XRD patterns of organoclay (C20A) and binary nanocomposites of PP/clay, PP/PP-g-MA/clay, EOC/clay and EOC/EOC-gMA/clay are shown in Fig. 1. This figure shows that the C20A exhibits an intensive peak at around 2h = 3.7° corresponding to a basal plane spacing (d001) of 2.4 nm. In PP/clay, the diffraction peak remains almost the same position of neat clay with slight broadening which implies clay is not able to disperse in PP effectively without the presence of a compatibilizer. In comparison, the XRD pattern for EOC/clay shows a shoulder at 2h = 2.8° indicating intercalation has taken place in this composite. These results propose that the clay interacts better with the EOC than with the PP in this case. Therefore, it can be predicted that in the PP/EOC blend nanocomposites, clay favorably resides in the EOC phase in the absence of a compatibilizer. In PP/PP-g-MA/clay, the (0 0 1) peak of the clay shifts to a lower angle (2h = 2.6°) and hence a higher d-spacing (3.4 nm) with respect to PP/clay, revealing an intercalated structure in the presence of PP-g-MA as a compatibilizer. However,
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Intensity (a.u.)
1 1: Closite 20A 2: PP/clay 3: EOC/clay 4: PP/PP-g-MA/clay 5: EOC/EOC-g-MA/clay
4
2 3 5
2 (degree) Fig. 1. XRD patterns of Closite 20A and its binary nanocomposites with PP and EOC in the absence and presence of the compatibilizer.
the diffraction peak of clay in EOC/EOC-g-MA/clay disappears which may be due to the exfoliated structure within the nanocomposite, the orientation effect arising from the disc sample. The XRD patterns of clay and ternary nanocomposites of PP/ EOC/clay presented in Fig. 2 show a broad peak with a shoulder at 2h = 3.3° which is equivalent to the basal plane spacing of 2.7 nm. Since the XRD trace of this sample is similar to that of EOC/clay, it can be concluded that a large amount of clay is distributed in the EOC phase. This result is consistent with the previous report that showed the better dispersion of clay in EOC compared with low-density polyethylene (LDPE) [25]. This performance was correlated to the more regular chain branching and narrower molecular weight of EOC compared with LDPE which led to less shear thinning behavior and increased entanglement of chains during extrusion and therefore the improved dispersion of clay layers. Another possible reason for locating clay in EOC phase can be related to the large disparity between the melting points of EOC and PP. Fenouillot et al. [21] demonstrated when the components of polymer/blend nanocomposites are simultaneously added to the extruder, the clay particles will be incorporated into the phase with a lower melting point. As the melting temperature of EOC is lower than PP, clay particles have a tendency to locate in molten EOC before PP melts. A third possible reason for preferential location of clay in one phase of polymer blends is ascribed to the difference in the viscosities of the components in the processing conditions in which clay will concentrate in the phase with a lower viscosity [26]. However, this can be excluded according to the rheological testing results presented subsequently which show the viscosities for both PP and EOC are similar under the processing
1
1: Closite 20A 2: PP/EOC/clay 3: PP/EOC/EOC-g-MA/clay 4: PP/EOC/PP-g-MA/clay
Intensity (a.u.)
4
2
3
2 (degree) Fig. 2. XRD spectra of Closite 20A and PP/EOC blend nanocomposites without and with different compatibilizers.
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condition. As a result, the most probable reason for the localization of clay in the EOC phase would seem to be due to its higher tendency to EOC phase than PP phase, which is subsequently discussed based on thermodynamic equations. It is also seen in Fig. 2 that in PP/EOC/EOC-g-MA/clay, the peak of the neat clay did not change at the 2h position but becomes widened and weakened. This can be attributed to an increase in the exfoliation degree of clay layers and/or the orientation effect arising from the disc sample. In PP/EOC/PP-g-MA/clay there is a strong peak at 2h = 2.6°, corresponding to the d-spacing of 3.4 nm. This peak is exactly the same as the peak observed in PP/PP-g-MA/clay and implies most of the clay is located in the PP phase rather than the EOC phase due to favorable interaction between polar molecular chains of PP-g-MA and clay. 3.2. TEM analysis TEM image of PP/EOC/clay is shown in Fig. 3. Since the EOC phase was stained with OSO4, it appears as a darker phase. It is clear from Fig. 3a and b that a majority of clay is dispersed in the EOC phase as intercalated structure with no signs of large agglomeration. The EDX spectrum on the dark area, where EOC phase is and no clear clay layers are identified (Fig. 3b), clearly shows presence of silicon element which is evidence of clay in that particular region. This is not in agreement with Su’s study [24] which showed very large agglomerates of silicate layers in PP/EOC/clay in the absence of compatibilizer. As both PP and EOC are non-polar and do not have any polar groups in their molecular backbones, the lack of large aggregates in PP/EOC/clay is interesting. This phenomenon can be ascribed to the high shear rate and high residence time used in this work to improve the dispersion of clay by employing twopass extrusion and high screw speed. It is reported that imposing more intensive shear stress during extrusion by raising the screw speed as well as employing longer residence times favors the dispersion of clay and disintegrates the aggregates of clay particles into tactoids with smaller size. However, an exfoliated structure cannot be obtained unless there is an affinity between the clay and the components [27]. According to TEM image and EDX analysis of PP/EOC/EOC-gMA/clay shown in Fig. 4a and b, clay layers are mostly localized inside the EOC phase again as intercalated structures with fewer amounts in PP phase. The EDX spectra within the EOC phase clearly show the presence of silicon element while the spectrum in the PP phase reveals no silicon element. In addition, comparing the TEM images of Figs. 3 and 4 reveal that the interlayer spacing of clay layers in PP/EOC/EOC-g-MA/clay is higher than PP/EOC/clay due to the enhanced polarity of EOC phase with the addition of EOCg-MA. In fact, the appearance of intercalated structure in PP/EOC/ EOC-g-MA/clay is rational, considering that most parts of clay are located in the EOC phase and thus the actual amount of clay in this phase is more than 4 phr being 16 phr. This high concentration of clay in the EOC phase increases the interaction between clay platelets, leading to the formation of intercalated structures instead of exfoliated structures. This result suggests the disappearance of the (0 0 1) peak in the XRD pattern is mainly due to the encapsulation of clay in the dispersed phase which suppresses detection of clay. Similar observation was reported by Dasari et al. [28] who also demonstrated the presence of intercalated clay in the finely dispersed elastomer phase could not be identified by XRD analysis. The TEM micrographs for PP/EOC/PP-g-MA/clay nanocomposite in Fig. 5 shows distribution of the intercalated clay particles mainly in the PP phase, which is in line with the XRD results and with the morphologies observed in the literatures for similar nanocomposites of PP/EOC/PP-g-MA/clay [11,24]. Again no signs of large agglomerates were detected from TEM images, suggesting good dispersion of the clay in the polymer blend.
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(b) Counts
(a)
Energy (keV)
2
Energy (keV)
2
Counts
200
20 nm 0.5 m
Fig. 3. (a) Low magnification TEM micrograph of PP/EOC/clay showing the state of clay distribution and (b) high magnification TEM image along with EDX analysis of PP/EOC/ clay.
300
(b)
Counts
200
100
(a) 5
10 Energy (KeV)
40
Counts
30 20 10
5 100
20 nm
Energy (KeV)
10
80 Counts
0.5 µm
60 40 20
10
5 Energy (keV)
Fig. 4. (a) TEM image and (b) EDX analysis of PP/EOC/EOC-g-MA/clay nanocomposite (top image is a HAADF (high angle annular dark field) image and bottom image is a BF (bright field) image.
3.3. Determination of the clay location based on thermodynamic equations The preferential location of the clay in the polymer blend can also be estimated by the difference between the interfacial tensions of the components, because the system tends to reach a minimum interfacial energy [21]. According to the literature, this prediction is more valuable when kinetic effects such as the viscosities of the polymeric constituents are comparable [21,29]. Since the viscosity of PP and EOC are relatively similar
at a shear rate range of 20–500 s1(cf. Section 3.5) thermodynamic predictions can be successful in this system. The thermodynamic calculations are done by using Young’s equation (Eq. (1)) [1,14,21]:
W¼
cA-clay cB-clay cAB
ð1Þ
in which W is the wetting parameter, cA-clay is the interfacial tension between EOC and clay, cB-clay is the interfacial tension between PP and clay and cAB is the interfacial tension between PP and EOC.
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0.5 µm 50 nm Fig. 5. TEM images of PP/EOC/PP-g-MA/clay without staining process.
Table 1 Surface energies of PP, EOC and clay. Materials
Temperature (°C)
c
cd
cP
Ref.
PP EOC clay
190 190 –
21.7 17.5 29.6
21.2 15.6 20
0.5 1.9 9.6
[14] [14] [30]
The interfacial tension is determined from geometric mean equation of Wu (Eq. (2)) [1,14,21]:
cAB ¼ cA þ cB 2
qffiffiffiffiffiffiffiffiffiffi
cdA cdB þ
qffiffiffiffiffiffiffiffiffiffi
cPA cPB
ð2Þ
where cA and cB represent the surface tensions of EOC and PP, cdA and cdA are the dispersion parts of their surface tensions and cPA and cPA are the polar parts of their surface tensions. When W < 1, clay will be dispersed in the EOC phase. When W > 1, clay will be found in PP phase and clay will localize in the interface provided that 1 < W < 1 [14,21]. The values of the surface tensions for PP, EOC and clay were taken from the literature [14,30] and listed in Table 1. Note that the values for EOC and PP were given for 190 °C [14], which were assumed to be the same as for our processing temperature that is 180 °C. By calculating the interfacial energies according to Eq. (2) and inserting those values into Eq. (1), the value of W is obtained as <1 which indicates clay has tendency to concentrate in EOC phase. This prediction is only valuable for the PP/ EOC blend which contains no compatibilizer, because the incorporation of compatibilizer will alter the interfacial tension of the components with clay. So, on the basis of thermodynamic estimation, XRD and TEM results, it is reasonable to say that in the case of PP/EOC, clay is preferentially accumulated in the EOC phase and for other nanocomposites containing compatibilizers, clay will reside in the phase that is better compatible with the compatibilizer. 3.4. SEM analysis SEM images of the etched cryogenic fracture surfaces of the nanocomposites and their neat blends without clay are illustrated in Fig. 6a–f. The PP/EOC blend visibly demonstrates two-phase morphology in which EOC is a dispersed phase with spherical shape, relatively rough surface and coarse dispersion (Fig. 6a). This kind of morphology suggests EOC is partially miscible with PP which is expectable for polymer blends like PP/EOC including non-
polar components with similar chemical structures [1,25]. The introduction of clay and compatibilizers separately to PP/EOC causes a clear reduction in the size of EOC dispersed phase. However, the influence of clay in dropping the size of EOC is more noticeable. Baghaei et al. [25] also showed the remarkable reduction in the size of EOC phase by adding clay to a partial immiscible blend of LDPE/EOC even in the absence of compatibilizer. Moreover, comparison of the size of EOC in the nanocomposites with their neat blends reveals that the elastomer particles become smaller in size in the presence of clay. In addition, By comparing the SEM images of the nanocomposites in Fig. 6b, d and f, it can be observed that PP/EOC/clay presents the most uniform morphology and that in PP/EOC/PP-g-MA/clay nanocomposite, the EOC particles lose their spherical shape due to the presence of clay in the matrix and likely at the interface. The presence of clay layers at the interface can be related to the migration of clay layers from EOC phase toward the PP/PP-g-MA phase [31].
3.5. Rheological properties The complex viscosity (g) and storage modulus (G0 ) curves of PP, EOC, PP/EOC and the nanocomposites as a function of frequency measured at 180 °C are also shown in Fig. 7a and b. It is observed from Fig. 7a that the complex viscosity of EOC is lower than PP in low frequency regions and their viscosities are close to each other at high frequency regions (e.g. 20–500 s1) where the melt processing takes place. It is also clear that the viscosity curve of PP/EOC falls between the viscosity curves of PP and EOC. This phenomenon again confirms that PP/EOC is a partially miscible blend, because the viscosity of immiscible blends should be lower than both components [32]. Upon the addition of clay to the blend, the complex viscosity of the nanocomposites increases, particularly at low frequency regions. Polymer/clay interactions which limit the molecular motions of the polymers [13] and the formation of clay network structure [24] are effective in raising the viscosity. On the other hand, although the molecular weight and the viscosity of PP-g-MA are lower than the values for EOCg-MA, the viscosity of PP/EOC/PP-g-MA/clay is higher than that of PP/EOC/EOC-g-MA/clay maybe due to the better dispersion of the clay in the matrix and/or at the interface in the PP/EOC/PPg-MA/clay. This indicates both dispersion state and location of the clay are crucial factors in governing the rheological properties of polymer blend/clay nanocomposites. Mishra et al. [13] reported
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(a)
2 µm
(c)
2 µm
(e)
2 µm
(b)
2 µm
(d)
2 µm
(f)
2 µm
Fig. 6. SEM micrographs of: (a) PP/EOC; (b) PP/EOC/clay; (c) PP/EOC/EOC-g-MA; (d) PP/EOC/EOC-g-MA/clay; (e) PP/EOC/PP-g-MA and (f) PP/EOC/PP-g-MA/clay.
PP EOC PP/EOC PP/EOC/clay PP/EOC/PP-g-MA/clay
(a)
(b)
PP EOC PP/EOC PP/EOC/clay PP/EOC/PP-g-MA/clay PP/EOC/EOC-g-MA/clay
G' (Pa)
|η*| (Pa.s)
PP/EOC/EOC-g-MA/clay
Frequency (rad/s)
Frequency (rad/s)
Fig. 7. (a) Complex viscosity curves of PP, EOC, PP/EOC and PP/EOC blend nanocomposites with and without compatibilizer as a function of frequency at 180 °C and (b) storage modulus curves of PP, EOC, PP/EOC and PP/EOC blend nanocomposites with and without compatibilizer as a function of frequency at 180 °C.
a large enhancement in the melt viscosity of PP/EPDM/PP-g-MA/ clay nanocomposite due to the strong interaction between the clay and matrix. Filippi et al. [33] also found a substation enhancement in the viscosity of LDPE/PA blend by adding organoclay owing to the preferential location of the clay in the interface. It is also seen from Fig. 7b that the storage modulus of all nanocomposites increased with respect to the pure blend especially at low frequency regions. In addition, the trend of storage modulus is relatively the same as complex viscosity. That is, the storage moduli of PP/EOC/clay and PP/EOC/EOC-g-MA/clay are slightly lower than that of PP/EOC/PP-g-MA/clay which clay is located in the matrix.
3.6. Mechanical properties The mechanical properties of neat blends and the nanocomposites are presented in Table 2. The results in Table 2 shows the incorporation of clay into the PP/EOC without compatibilizer causes no statistically significant change in modulus (a slight decrease in tensile modulus and a slight increase in flexural modulus) and a more visible drop in tensile strength. This trend is expectable. Actually, there is a close relationship between the location of clay and mechanical properties in which the encapsulated structures show lower stiffness and higher toughness than separated structures [15,16,20]. As mentioned, in PP/EOC/clay, clay is
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Tensile modulus (MPa)
Flexural modulus (MPa)
Tensile strength (MPa)
Elongation at break (%)
Tensile energy (MJ/m3)
Impact strength (kJ/m2)
PP/EOC PP/EOC/clay PP/EOC/PP-g-MA PP/EOC/PP-g-MA/clay PP/EOC/EOC-g-MA PP/EOC/EOC-g-MA/clay
1308.0 ± 64.9 1241.5 ± 130.5 1200.3 ± 5.9 1170.7 ± 48.7 1118.6 ± 47.6 1058.6 ± 20.5
1066.7 ± 20.6 1101.8 ± 13.7 995.2 ± 32.8 1147.1 ± 31.5 931.4 ± 27.1 907.9 ± 18.2
24.8 ± 2.4 21.1 ± 0.4 22.3 ± 0.1 20.1 ± 0.4 20.9 ± 0.3 19.5 ± 0.1
491.0 ± 287.0 650.6 ± 176.0 365.0 ± 224.0 550.0 ± 105.0 516.0 ± 35.0 864.0 ± 72.0
71.2 ± 41.0 79.7 ± 13.9 34.9 ± 35.8 76.7 ± 12.7 72.2 ± 23.3 93.7 ± 15.0
4.7 ± 0.9 12.0 ± 3.3 3.8 ± 0.6 4.3 ± 1.6 22.6 ± 1.4 25.2 ± 8.7
situated in EOC phase. Thus, the most plausible factor for no significant changes in the modulus and a drop in strength is ascribed to the location of clay in the elastomer phase. Besides, the interfacial adhesion between clay and polymers affects tensile strength rather than the modulus. So, the greater decrease in tensile strength with respect to modulus represents the weak interfacial adhesion between the clay and EOC due to their incompatibility. It is interesting to note that the elongation at break, tensile energy at break and impact strength of PP/EOC/clay are considerably improved compared to the blend which provides further confirmation that the amount of clay aggregates in this sample is not high, because the agglomeration of clay should diminish toughness [5]. This result also lessens the probability of the degradation of the polymers or clay during processing, because degradation leads to decrease in both stiffness and impact strength simultaneously [34]. Several factors including the significant decrease in the EOC particle size induced by clay addition, the presence of clay in the EOC phase and the formation of percolated network of clay layers with EOC can be offered to elucidate toughness enhancement in this sample. According to the literature, decreasing the size of dispersed phase leads to the stronger adhesion between two polymeric components, better stress transfer through their interface and improved toughness [32,35]. In addition, as most part of clay is concentrated in the EOC phase, its concentration in this phase is more than 4 phr as previously described and thus the formation of percolated network of clay layers and EOC chains is more probable [36]. Fenouillot et al. [21] described the formation of such a network causes a superior toughening behavior in polymer blend nanocomposites. Moreover, other factors like changes in crystallinity, spherulite size and crystal structure of PP and the type of facture mechanism by adding clay may also be responsible for toughness enhancement of PP/EOC/clay. These aspects are under investigation in our laboratory. According to Table 2, PP/EOC/EOC-g-MA/clay exhibits the greatest reduction in modulus (tensile and flexural) and tensile strength and the maximum enhancement in elongation at break, tensile energy and impact strength compared to PP/EOC. Apart from the reasons described for toughness improvement in PP/EOC/clay, the rubbery nature of EOC-g-MA and better dispersion state of clay can influence the mechanical properties of PP/EOC/EOC-g-MA/clay nanocomposite. The results also show that EOC-g-MA has much higher toughening effect than clay in this nanocomposite in which the addition of EOC-g-MA to PP/EOC blend causes a remarkable increase in impact strength being 22.6 kJ/m2 compared to 4.7 kJ/m2. In fact, it seems EOC-g-MA acts as an efficient compatibilizer for PP/EOC blend, reduces the EOC particle size and enhances the interfacial adhesion between two phases significantly. Table 2 also shows no further improvement in modulus and tensile strength of PP/EOC/PP-g-MA/clay with respect to PP/EOC/ clay, even though XRD and TEM evaluations indicate a fine intercalated structure in the PP. It should be mentioned that the amount of MA in selected PP-g-MA is high (8–10 wt.%) that presumably leads to decreasing the molecular weight, modulus and strength of PP-g-MA compared to PP and subsequent lowering of modulus
and strength of the blend of PP/EOC/PP-g-MA and PP/EOC/PP-gMA/clay [23,37]. On one hand, increasing MA content up to an optimum level that PP-g-MA is miscible with PP is effective for better dispersion of clay in PP/clay composites and possibly promotes exfoliated structures, whereas on the other hand increasing the MA groups to higher than the optimum level causes the immiscibility of PP-g-MA in PP with no signs of exfoliation [37,38]. So, the appearance of intercalated structure in the PP phase may indicate the immiscibility of PP-g-MA in PP which is detrimental for mechanical properties. Moreover, Lopez et al. [39] proposed the less amounts of PP-g-MA with high polar groups are necessary as a compatibilizer for PP/clay, and the excess amount acts as a lubricant and decreases the mechanical properties. As such it would appear that 4 phr of our PP-g-MA is high for PP/EOC/PP-g-MA/clay. Furthermore, the toughness of PP/EOC/PP-g-MA/clay is lower than the values for the other two nanocomposites due to the presence of clay in the matrix, and the high content of MA groups in the PP-gMA. So, employing another PP-g-MA with a lower MA content might be an effective strategy for better balance of mechanical properties of this nanocomposite. In summary, based on the above results, it can be seen that all the tensile energy, elongation at break and impact strength improved in the nanocomposits compared to their neat blends suggesting the toughening effect of clay in these materials. 4. Conclusions The relation between morphology and properties of polypropylene/ethylene–octene copolymer/clay with and without two compatibilizers of maleated polypropylene and maleic anhydride grafted poly (ethylene-co-octene) was studied. The results showed a close correlation between morphology and properties of the studied polymer blend nanocomposites. Clay platelets had better interactions with the EOC than PP and were located in the EOC phase of PP/EOC/clay as intercalated structures. However in the presence of compatibilizers, nanoclay resided selectively in the phase with higher polarity. Rheological analysis suggested the enhancement of complex viscosity and storage moduli of the nanocomposites compared to PP/EOC blend. Although the stiffness and tensile strength of the nanocomposites were not improved compared to PP/EOC, there was a drastic improvement in the toughness especially in PP/EOC/EOC-g-MA/clay due to the presence of fine intercalated structure of clay layers in the EOC phase, significant decrease in the EOC particle size induced by clay addition, the formation of percolated network of clay layers inside the EOC phase and the elastomeric nature of EOC-g-MA. Acknowledgements We thank the Environmental Protection Agency in Ireland (EPA2008-PhD-WRM-4), Science Foundation Ireland (07/SK/I1220a) and the Irish Government’s Program for Research in Third Level Institutions and Irish Research Council for Science, Engineering and Technology for financial support.
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Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Morphology, rheology and mechanical properties of polypropylene/ethylene–octene copolymer/clay nanocomposites: Effects of the compatibilizer Sedigheh Bagheri-Kazemabad a, Daniel Fox c, Yanhui Chen c, Luke M. Geever d, Alireza Khavandi a,⇑, Reza Bagheri e, Clement L. Higginbotham d, Hongzhou Zhang c, Biqiong Chen b,⇑ a
School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK School of Physics and CRANN, Trinity College Dublin, College Green, Dublin 2, Ireland d Materials Research Institute, Athlone Institute of Technology, Dublin Road, Athlone, Co. Westmeath, Ireland e Polymeric Materials Research Group, Materials Science and Engineering Department, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran b c
a r t i c l e
i n f o
Article history: Received 11 November 2011 Received in revised form 18 March 2012 Accepted 8 June 2012 Available online 18 June 2012 Keywords: A. Nanoclays A. Nanocomposites B. Mechanical properties
a b s t r a c t The objective of this study was to investigate the effects of two compatibilizers, namely maleated polypropylene (PP-g-MA) and maleic anhydride grafted poly (ethylene-co-octene) (EOC-g-MA), on the morphology and thus properties of ternary nanocomposites of polypropylene (PP)/ethylene–octene copolymer (EOC)/clay nanocomposite. In this regard the nanocomposites and their neat polymer blend counterparts were processed twice using a twin screw extruder. X-ray diffraction, transmission electron microscopy, Energy dispersive X-ray spectroscopy, and scanning electron microscopy were utilized to characterize nanostructure and microstructure besides mechanical and rheological behaviors of the nanocomposites. Clay with intercalated structure was observed in EOC phase of the PP/EOC/clay nanocomposite. Better dispersion state of the intercalated clay in EOC phase was observed by adding EOCg-MA as a compatibilizer. On the other hand, adding PP-g-MA resulted in migration of the intercalated clay from the EOC to the PP and to the interface regions. It was also demonstrated that the elastomer particles became smaller in size where clay was present. The finest and the most uniform morphology was found in the PP/EOC/clay nanocomposite. In addition, the rheological results illustrated a higher complex viscosity and storage modulus for PP/EOC/PP-g-MA/clay nanocomposite in which clay particles were present in the matrix. Mechanical assessments showed improvements in the toughness of the nanocomposites with respect to their neat blends, without significant change in stiffness and tensile strength values. These results highlight a toughening role of clay in the polymer blend nanocomposites studied. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Polymer blend nanocomposites containing an elastomer and a reinforcing filler have attracted much attention due to the incorporation of the properties of both polymer blends and polymer nanocomposites [1–3]. Polypropylene (PP)/ethylene–octene copolymer (EOC)/clay nanocomposites are such high performance materials. PP is a commodity polymer due to its low price, high performance and good processability. However, the application of PP is still limited by its poor impact resistance, especially at low temperatures and high strain rates [4]. Blending PP with a rubbery phase like ethylene–octene copolymer is a successful way to improve its impact resistance [4–6]. In comparison to conventional elastomers such as ethylene–propylene copolymer and ⇑ Corresponding authors. Tel.: +44 114 222 5958; fax: +444 114 222 5943 (B. Chen), tel.: +98 21 77459151; fax: +98 21 77240480 (A.R. Khavandi). E-mail addresses: [email protected] (A. Khavandi), biqiong.chen@sheffield. ac.uk (B. Chen). 0266-3538/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2012.06.007
ethylene–propylene–diene terpolymer, EOC has performed better compatibility with PP because of its long side chains, which leads to reduction of the interfacial tension [5–10]. Among nanofillers employed for nanocomposite preparation, clay is vastly used [11–13] owing to its unique structure with a high aspect ratio and its natural abundance [2]. The properties of polymer blend/clay nanocomposites are strongly related to the location [1,14–17] and dispersion degree of the clay [11,12,14,17,18] as well as the size, shape and dispersion condition of the polymer dispersed phase, i.e. the elastomer in the blend [11,12,14]. By adding clay to a polymer blend, depending on the location of clay in the matrix or dispersed phase, two types of morphology including separated or encapsulated will be formed [1,14,19,20]. The relative extent to which these structures develop is largely dependent on the interfacial energies between the components [1,15,19,21], miscibility of the blend [19], the shear force during compounding [1], the viscosity of the components [1,21] and the order of addition of the components during compounding [1,15,16,21]. Among these factors, interfacial
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interaction between the clay and the polymer phases plays a crucial role in the preferential location of clay [21]. Functionalized polymers are added into PP/elastomer nanocomposites to adjust the interfacial adhesion between the components. So, if the affinity between the clay and PP is higher than the affinity between the clay and elastomer, clay would reside in PP and vice versa [16,20]. In the present study, homogeneous dispersion of clay minerals without a compatibilizer is not possible in PP [11,12] and EOC [8,9] which are apolar. So, two compatibilizers based on the same structure units with PP and EOC, i.e., PP-g-MA and EOC-g-MA, were introduced to PP/EOC/clay nanocomposite to tailor the location and dispersion of the clay and to study their influence on structure, mechanical and rheological behavior of the nanocomposite. Although, Several groups have studied the properties of PP/EOC/ clay by the addition of PP-g-MA as a compatibilizer [5,10– 12,15,22–24], there has been no systematic study comparing the effects of clay location and dispersion, induced by the use of various compatibilizers, on the properties of this nanocomposite. 2. Experimental
(XRD) of the C20A powder and injection molded nanocomposite disc samples (thickness 2 mm). The XRD were recorded with a step size of 0.02° from 2h = 2° to 10° and a scanning time of 2.5 s per step. The dispersion state and location of the organoclay were examined by a FEI Titan 80–300 transmission electron microscope (TEM) at an acceleration voltage of 300 kV. A Carl Zeiss Auriga CrossBeam FIB–SEM workstation was used to extract sections from the core of the injection molded specimens. The samples were then transferred to a C-shaped copper TEM grid. The copper grids containing the sample lamellas were then stained by dropping 2 wt.% aqueous osmium tetroxide solution (Sigma–Aldrich) on the surface and dried at room temperature in a fume cupboard. Energy dispersive X-ray spectroscopy (EDX) was also carried out to get additional evidence on the location of clay in the polymer blends. The microstructure of samples was examined by scanning electron microscopy (SEM) using a Philips XL30 model at 17 kV. Surfaces of injection samples were cryogenically fractured in liquid nitrogen and etched with heptane at 50 °C for 3 h to remove the EOC.
2.1. Materials 2.4. Rheological properties PP (571P) was an isotactic homopolymer supplied by Sabic (Saudi Basic Industries Corporation, Europe). Its density and melt flow index (MFI) (2.16 kg at 230 °C) were 905 kg/m3 and 5.7 g/ 10 min, respectively. EOC (InfuseTM9500) was supplied by Dow Chemical Corporate. Its density and MFI were 877 kg/m3 and 5 g/ 10 min (2.16 kg at 190 °C), respectively. PP-g-MA with an MFI of 140 g/10 min (2.16 kg at 160 °C) was a commercial grade product of Sigma–Aldrich. The grafting level of maleic anhydride was 8– 10 wt.%. EOC-g-maleic anhydride (AmplifyTMGR216) with a melt flow index (2.16 kg at 190 °C) of 1.3 g/10 min was provided by Dow Chemical Corporate and its grafting level was more than 0.5 wt.%. The organoclay CloisiteÒ 20A (abbreviated as C20A) was obtained from Southern Clay Products, Inc. CloisiteÒ 20A is a natural montmorillonite modified with dimethyl, dehydrogenated tallow and quaternary ammonium chloride. 2.2. Sample preparation The nanocomposites of PP, EOC (ratio of PP:EOC = 3:1) and organoclay (4 phr) without a compatibilizer and with two types of compatibilizer, PP-g-MA and EOC-g-MA, were prepared. The ratio of compatibilizer to organoclay was fixed to 1. As a control, the blends of PP/EOC, PP/EOC/PP-g-MA and PP/EOC/EOC-g-MA with 4 phr compatibilizer and the binary nanocomposites of PP/clay, PP/PP-g-MA/clay, EOC/clay and EOC/EOC-g-MA/clay with 4 phr clay and 4 phr compatibilizer were also prepared. The blends and nanocomposites were prepared by simultaneous mixing of components using a bench-top PrismTM twin screw extruder (L/D = 25, D = 16 mm). The barrel temperature profile was 160, 160, 170 and 180 °C from the hopper to die and the screw speed was set at 200 rpm. All compositions were extruded twice in order to attain better dispersion of clay and elastomer. The extruded materials were injection molded into test specimens using a bench-top RR/TSMP injection molding machine (Ray-ran Test equipment LTD, UK) with the barrel and tool temperatures of 195 °C and 55 °C, respectively and the pressure of 110 MPa. 2.3. Characterization A Phillips PW 1050/80 X-ray diffractometer (40 kV, 20 mA) equipped with a graphite monochromator and with CuKa1 radiation (wavelength = 0.1541 nm) was used for X-ray diffraction
Rheological analysis was performed on a TA Instruments AR1000TM rheometer fitted with 25 mm stainless steel plate geometry. Frequency sweep experiments were conducted between 100 and 0.03 rad/s at 180 °C using a strain of 1%, i.e., within the linear viscoelastic region as determined from strain sweep experiments at a frequency of 1 Hz. 2.5. Mechanical properties Tensile testing was carried out with a universal Hounsfield H10KS at 23 °C, according to ASTM D638 and at a crosshead speed of 50 mm/min using a load cell of 10 kN. Modulus was measured using an extensometer. Flexural modulus was measured by means of an Instron-1011 machine according to ASTM D790 using 4-point bending configuration at 2 mm/min deformation rate. Notched Charpy impact strength of samples was measured with a Jinjian impact tester according to BS ISO 179-1. A side-edge notch with an angle of 45° and a 2-mm depth and a notch tip radius of 0.25 mm was machined on specimens. 3. Results and discussion 3.1. XRD analysis The XRD patterns of organoclay (C20A) and binary nanocomposites of PP/clay, PP/PP-g-MA/clay, EOC/clay and EOC/EOC-gMA/clay are shown in Fig. 1. This figure shows that the C20A exhibits an intensive peak at around 2h = 3.7° corresponding to a basal plane spacing (d001) of 2.4 nm. In PP/clay, the diffraction peak remains almost the same position of neat clay with slight broadening which implies clay is not able to disperse in PP effectively without the presence of a compatibilizer. In comparison, the XRD pattern for EOC/clay shows a shoulder at 2h = 2.8° indicating intercalation has taken place in this composite. These results propose that the clay interacts better with the EOC than with the PP in this case. Therefore, it can be predicted that in the PP/EOC blend nanocomposites, clay favorably resides in the EOC phase in the absence of a compatibilizer. In PP/PP-g-MA/clay, the (0 0 1) peak of the clay shifts to a lower angle (2h = 2.6°) and hence a higher d-spacing (3.4 nm) with respect to PP/clay, revealing an intercalated structure in the presence of PP-g-MA as a compatibilizer. However,
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Intensity (a.u.)
1 1: Closite 20A 2: PP/clay 3: EOC/clay 4: PP/PP-g-MA/clay 5: EOC/EOC-g-MA/clay
4
2 3 5
2 (degree) Fig. 1. XRD patterns of Closite 20A and its binary nanocomposites with PP and EOC in the absence and presence of the compatibilizer.
the diffraction peak of clay in EOC/EOC-g-MA/clay disappears which may be due to the exfoliated structure within the nanocomposite, the orientation effect arising from the disc sample. The XRD patterns of clay and ternary nanocomposites of PP/ EOC/clay presented in Fig. 2 show a broad peak with a shoulder at 2h = 3.3° which is equivalent to the basal plane spacing of 2.7 nm. Since the XRD trace of this sample is similar to that of EOC/clay, it can be concluded that a large amount of clay is distributed in the EOC phase. This result is consistent with the previous report that showed the better dispersion of clay in EOC compared with low-density polyethylene (LDPE) [25]. This performance was correlated to the more regular chain branching and narrower molecular weight of EOC compared with LDPE which led to less shear thinning behavior and increased entanglement of chains during extrusion and therefore the improved dispersion of clay layers. Another possible reason for locating clay in EOC phase can be related to the large disparity between the melting points of EOC and PP. Fenouillot et al. [21] demonstrated when the components of polymer/blend nanocomposites are simultaneously added to the extruder, the clay particles will be incorporated into the phase with a lower melting point. As the melting temperature of EOC is lower than PP, clay particles have a tendency to locate in molten EOC before PP melts. A third possible reason for preferential location of clay in one phase of polymer blends is ascribed to the difference in the viscosities of the components in the processing conditions in which clay will concentrate in the phase with a lower viscosity [26]. However, this can be excluded according to the rheological testing results presented subsequently which show the viscosities for both PP and EOC are similar under the processing
1
1: Closite 20A 2: PP/EOC/clay 3: PP/EOC/EOC-g-MA/clay 4: PP/EOC/PP-g-MA/clay
Intensity (a.u.)
4
2
3
2 (degree) Fig. 2. XRD spectra of Closite 20A and PP/EOC blend nanocomposites without and with different compatibilizers.
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condition. As a result, the most probable reason for the localization of clay in the EOC phase would seem to be due to its higher tendency to EOC phase than PP phase, which is subsequently discussed based on thermodynamic equations. It is also seen in Fig. 2 that in PP/EOC/EOC-g-MA/clay, the peak of the neat clay did not change at the 2h position but becomes widened and weakened. This can be attributed to an increase in the exfoliation degree of clay layers and/or the orientation effect arising from the disc sample. In PP/EOC/PP-g-MA/clay there is a strong peak at 2h = 2.6°, corresponding to the d-spacing of 3.4 nm. This peak is exactly the same as the peak observed in PP/PP-g-MA/clay and implies most of the clay is located in the PP phase rather than the EOC phase due to favorable interaction between polar molecular chains of PP-g-MA and clay. 3.2. TEM analysis TEM image of PP/EOC/clay is shown in Fig. 3. Since the EOC phase was stained with OSO4, it appears as a darker phase. It is clear from Fig. 3a and b that a majority of clay is dispersed in the EOC phase as intercalated structure with no signs of large agglomeration. The EDX spectrum on the dark area, where EOC phase is and no clear clay layers are identified (Fig. 3b), clearly shows presence of silicon element which is evidence of clay in that particular region. This is not in agreement with Su’s study [24] which showed very large agglomerates of silicate layers in PP/EOC/clay in the absence of compatibilizer. As both PP and EOC are non-polar and do not have any polar groups in their molecular backbones, the lack of large aggregates in PP/EOC/clay is interesting. This phenomenon can be ascribed to the high shear rate and high residence time used in this work to improve the dispersion of clay by employing twopass extrusion and high screw speed. It is reported that imposing more intensive shear stress during extrusion by raising the screw speed as well as employing longer residence times favors the dispersion of clay and disintegrates the aggregates of clay particles into tactoids with smaller size. However, an exfoliated structure cannot be obtained unless there is an affinity between the clay and the components [27]. According to TEM image and EDX analysis of PP/EOC/EOC-gMA/clay shown in Fig. 4a and b, clay layers are mostly localized inside the EOC phase again as intercalated structures with fewer amounts in PP phase. The EDX spectra within the EOC phase clearly show the presence of silicon element while the spectrum in the PP phase reveals no silicon element. In addition, comparing the TEM images of Figs. 3 and 4 reveal that the interlayer spacing of clay layers in PP/EOC/EOC-g-MA/clay is higher than PP/EOC/clay due to the enhanced polarity of EOC phase with the addition of EOCg-MA. In fact, the appearance of intercalated structure in PP/EOC/ EOC-g-MA/clay is rational, considering that most parts of clay are located in the EOC phase and thus the actual amount of clay in this phase is more than 4 phr being 16 phr. This high concentration of clay in the EOC phase increases the interaction between clay platelets, leading to the formation of intercalated structures instead of exfoliated structures. This result suggests the disappearance of the (0 0 1) peak in the XRD pattern is mainly due to the encapsulation of clay in the dispersed phase which suppresses detection of clay. Similar observation was reported by Dasari et al. [28] who also demonstrated the presence of intercalated clay in the finely dispersed elastomer phase could not be identified by XRD analysis. The TEM micrographs for PP/EOC/PP-g-MA/clay nanocomposite in Fig. 5 shows distribution of the intercalated clay particles mainly in the PP phase, which is in line with the XRD results and with the morphologies observed in the literatures for similar nanocomposites of PP/EOC/PP-g-MA/clay [11,24]. Again no signs of large agglomerates were detected from TEM images, suggesting good dispersion of the clay in the polymer blend.
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(b) Counts
(a)
Energy (keV)
2
Energy (keV)
2
Counts
200
20 nm 0.5 m
Fig. 3. (a) Low magnification TEM micrograph of PP/EOC/clay showing the state of clay distribution and (b) high magnification TEM image along with EDX analysis of PP/EOC/ clay.
300
(b)
Counts
200
100
(a) 5
10 Energy (KeV)
40
Counts
30 20 10
5 100
20 nm
Energy (KeV)
10
80 Counts
0.5 µm
60 40 20
10
5 Energy (keV)
Fig. 4. (a) TEM image and (b) EDX analysis of PP/EOC/EOC-g-MA/clay nanocomposite (top image is a HAADF (high angle annular dark field) image and bottom image is a BF (bright field) image.
3.3. Determination of the clay location based on thermodynamic equations The preferential location of the clay in the polymer blend can also be estimated by the difference between the interfacial tensions of the components, because the system tends to reach a minimum interfacial energy [21]. According to the literature, this prediction is more valuable when kinetic effects such as the viscosities of the polymeric constituents are comparable [21,29]. Since the viscosity of PP and EOC are relatively similar
at a shear rate range of 20–500 s1(cf. Section 3.5) thermodynamic predictions can be successful in this system. The thermodynamic calculations are done by using Young’s equation (Eq. (1)) [1,14,21]:
W¼
cA-clay cB-clay cAB
ð1Þ
in which W is the wetting parameter, cA-clay is the interfacial tension between EOC and clay, cB-clay is the interfacial tension between PP and clay and cAB is the interfacial tension between PP and EOC.
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0.5 µm 50 nm Fig. 5. TEM images of PP/EOC/PP-g-MA/clay without staining process.
Table 1 Surface energies of PP, EOC and clay. Materials
Temperature (°C)
c
cd
cP
Ref.
PP EOC clay
190 190 –
21.7 17.5 29.6
21.2 15.6 20
0.5 1.9 9.6
[14] [14] [30]
The interfacial tension is determined from geometric mean equation of Wu (Eq. (2)) [1,14,21]:
cAB ¼ cA þ cB 2
qffiffiffiffiffiffiffiffiffiffi
cdA cdB þ
qffiffiffiffiffiffiffiffiffiffi
cPA cPB
ð2Þ
where cA and cB represent the surface tensions of EOC and PP, cdA and cdA are the dispersion parts of their surface tensions and cPA and cPA are the polar parts of their surface tensions. When W < 1, clay will be dispersed in the EOC phase. When W > 1, clay will be found in PP phase and clay will localize in the interface provided that 1 < W < 1 [14,21]. The values of the surface tensions for PP, EOC and clay were taken from the literature [14,30] and listed in Table 1. Note that the values for EOC and PP were given for 190 °C [14], which were assumed to be the same as for our processing temperature that is 180 °C. By calculating the interfacial energies according to Eq. (2) and inserting those values into Eq. (1), the value of W is obtained as <1 which indicates clay has tendency to concentrate in EOC phase. This prediction is only valuable for the PP/ EOC blend which contains no compatibilizer, because the incorporation of compatibilizer will alter the interfacial tension of the components with clay. So, on the basis of thermodynamic estimation, XRD and TEM results, it is reasonable to say that in the case of PP/EOC, clay is preferentially accumulated in the EOC phase and for other nanocomposites containing compatibilizers, clay will reside in the phase that is better compatible with the compatibilizer. 3.4. SEM analysis SEM images of the etched cryogenic fracture surfaces of the nanocomposites and their neat blends without clay are illustrated in Fig. 6a–f. The PP/EOC blend visibly demonstrates two-phase morphology in which EOC is a dispersed phase with spherical shape, relatively rough surface and coarse dispersion (Fig. 6a). This kind of morphology suggests EOC is partially miscible with PP which is expectable for polymer blends like PP/EOC including non-
polar components with similar chemical structures [1,25]. The introduction of clay and compatibilizers separately to PP/EOC causes a clear reduction in the size of EOC dispersed phase. However, the influence of clay in dropping the size of EOC is more noticeable. Baghaei et al. [25] also showed the remarkable reduction in the size of EOC phase by adding clay to a partial immiscible blend of LDPE/EOC even in the absence of compatibilizer. Moreover, comparison of the size of EOC in the nanocomposites with their neat blends reveals that the elastomer particles become smaller in size in the presence of clay. In addition, By comparing the SEM images of the nanocomposites in Fig. 6b, d and f, it can be observed that PP/EOC/clay presents the most uniform morphology and that in PP/EOC/PP-g-MA/clay nanocomposite, the EOC particles lose their spherical shape due to the presence of clay in the matrix and likely at the interface. The presence of clay layers at the interface can be related to the migration of clay layers from EOC phase toward the PP/PP-g-MA phase [31].
3.5. Rheological properties The complex viscosity (g) and storage modulus (G0 ) curves of PP, EOC, PP/EOC and the nanocomposites as a function of frequency measured at 180 °C are also shown in Fig. 7a and b. It is observed from Fig. 7a that the complex viscosity of EOC is lower than PP in low frequency regions and their viscosities are close to each other at high frequency regions (e.g. 20–500 s1) where the melt processing takes place. It is also clear that the viscosity curve of PP/EOC falls between the viscosity curves of PP and EOC. This phenomenon again confirms that PP/EOC is a partially miscible blend, because the viscosity of immiscible blends should be lower than both components [32]. Upon the addition of clay to the blend, the complex viscosity of the nanocomposites increases, particularly at low frequency regions. Polymer/clay interactions which limit the molecular motions of the polymers [13] and the formation of clay network structure [24] are effective in raising the viscosity. On the other hand, although the molecular weight and the viscosity of PP-g-MA are lower than the values for EOCg-MA, the viscosity of PP/EOC/PP-g-MA/clay is higher than that of PP/EOC/EOC-g-MA/clay maybe due to the better dispersion of the clay in the matrix and/or at the interface in the PP/EOC/PPg-MA/clay. This indicates both dispersion state and location of the clay are crucial factors in governing the rheological properties of polymer blend/clay nanocomposites. Mishra et al. [13] reported
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(a)
2 µm
(c)
2 µm
(e)
2 µm
(b)
2 µm
(d)
2 µm
(f)
2 µm
Fig. 6. SEM micrographs of: (a) PP/EOC; (b) PP/EOC/clay; (c) PP/EOC/EOC-g-MA; (d) PP/EOC/EOC-g-MA/clay; (e) PP/EOC/PP-g-MA and (f) PP/EOC/PP-g-MA/clay.
PP EOC PP/EOC PP/EOC/clay PP/EOC/PP-g-MA/clay
(a)
(b)
PP EOC PP/EOC PP/EOC/clay PP/EOC/PP-g-MA/clay PP/EOC/EOC-g-MA/clay
G' (Pa)
|η*| (Pa.s)
PP/EOC/EOC-g-MA/clay
Frequency (rad/s)
Frequency (rad/s)
Fig. 7. (a) Complex viscosity curves of PP, EOC, PP/EOC and PP/EOC blend nanocomposites with and without compatibilizer as a function of frequency at 180 °C and (b) storage modulus curves of PP, EOC, PP/EOC and PP/EOC blend nanocomposites with and without compatibilizer as a function of frequency at 180 °C.
a large enhancement in the melt viscosity of PP/EPDM/PP-g-MA/ clay nanocomposite due to the strong interaction between the clay and matrix. Filippi et al. [33] also found a substation enhancement in the viscosity of LDPE/PA blend by adding organoclay owing to the preferential location of the clay in the interface. It is also seen from Fig. 7b that the storage modulus of all nanocomposites increased with respect to the pure blend especially at low frequency regions. In addition, the trend of storage modulus is relatively the same as complex viscosity. That is, the storage moduli of PP/EOC/clay and PP/EOC/EOC-g-MA/clay are slightly lower than that of PP/EOC/PP-g-MA/clay which clay is located in the matrix.
3.6. Mechanical properties The mechanical properties of neat blends and the nanocomposites are presented in Table 2. The results in Table 2 shows the incorporation of clay into the PP/EOC without compatibilizer causes no statistically significant change in modulus (a slight decrease in tensile modulus and a slight increase in flexural modulus) and a more visible drop in tensile strength. This trend is expectable. Actually, there is a close relationship between the location of clay and mechanical properties in which the encapsulated structures show lower stiffness and higher toughness than separated structures [15,16,20]. As mentioned, in PP/EOC/clay, clay is
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Tensile modulus (MPa)
Flexural modulus (MPa)
Tensile strength (MPa)
Elongation at break (%)
Tensile energy (MJ/m3)
Impact strength (kJ/m2)
PP/EOC PP/EOC/clay PP/EOC/PP-g-MA PP/EOC/PP-g-MA/clay PP/EOC/EOC-g-MA PP/EOC/EOC-g-MA/clay
1308.0 ± 64.9 1241.5 ± 130.5 1200.3 ± 5.9 1170.7 ± 48.7 1118.6 ± 47.6 1058.6 ± 20.5
1066.7 ± 20.6 1101.8 ± 13.7 995.2 ± 32.8 1147.1 ± 31.5 931.4 ± 27.1 907.9 ± 18.2
24.8 ± 2.4 21.1 ± 0.4 22.3 ± 0.1 20.1 ± 0.4 20.9 ± 0.3 19.5 ± 0.1
491.0 ± 287.0 650.6 ± 176.0 365.0 ± 224.0 550.0 ± 105.0 516.0 ± 35.0 864.0 ± 72.0
71.2 ± 41.0 79.7 ± 13.9 34.9 ± 35.8 76.7 ± 12.7 72.2 ± 23.3 93.7 ± 15.0
4.7 ± 0.9 12.0 ± 3.3 3.8 ± 0.6 4.3 ± 1.6 22.6 ± 1.4 25.2 ± 8.7
situated in EOC phase. Thus, the most plausible factor for no significant changes in the modulus and a drop in strength is ascribed to the location of clay in the elastomer phase. Besides, the interfacial adhesion between clay and polymers affects tensile strength rather than the modulus. So, the greater decrease in tensile strength with respect to modulus represents the weak interfacial adhesion between the clay and EOC due to their incompatibility. It is interesting to note that the elongation at break, tensile energy at break and impact strength of PP/EOC/clay are considerably improved compared to the blend which provides further confirmation that the amount of clay aggregates in this sample is not high, because the agglomeration of clay should diminish toughness [5]. This result also lessens the probability of the degradation of the polymers or clay during processing, because degradation leads to decrease in both stiffness and impact strength simultaneously [34]. Several factors including the significant decrease in the EOC particle size induced by clay addition, the presence of clay in the EOC phase and the formation of percolated network of clay layers with EOC can be offered to elucidate toughness enhancement in this sample. According to the literature, decreasing the size of dispersed phase leads to the stronger adhesion between two polymeric components, better stress transfer through their interface and improved toughness [32,35]. In addition, as most part of clay is concentrated in the EOC phase, its concentration in this phase is more than 4 phr as previously described and thus the formation of percolated network of clay layers and EOC chains is more probable [36]. Fenouillot et al. [21] described the formation of such a network causes a superior toughening behavior in polymer blend nanocomposites. Moreover, other factors like changes in crystallinity, spherulite size and crystal structure of PP and the type of facture mechanism by adding clay may also be responsible for toughness enhancement of PP/EOC/clay. These aspects are under investigation in our laboratory. According to Table 2, PP/EOC/EOC-g-MA/clay exhibits the greatest reduction in modulus (tensile and flexural) and tensile strength and the maximum enhancement in elongation at break, tensile energy and impact strength compared to PP/EOC. Apart from the reasons described for toughness improvement in PP/EOC/clay, the rubbery nature of EOC-g-MA and better dispersion state of clay can influence the mechanical properties of PP/EOC/EOC-g-MA/clay nanocomposite. The results also show that EOC-g-MA has much higher toughening effect than clay in this nanocomposite in which the addition of EOC-g-MA to PP/EOC blend causes a remarkable increase in impact strength being 22.6 kJ/m2 compared to 4.7 kJ/m2. In fact, it seems EOC-g-MA acts as an efficient compatibilizer for PP/EOC blend, reduces the EOC particle size and enhances the interfacial adhesion between two phases significantly. Table 2 also shows no further improvement in modulus and tensile strength of PP/EOC/PP-g-MA/clay with respect to PP/EOC/ clay, even though XRD and TEM evaluations indicate a fine intercalated structure in the PP. It should be mentioned that the amount of MA in selected PP-g-MA is high (8–10 wt.%) that presumably leads to decreasing the molecular weight, modulus and strength of PP-g-MA compared to PP and subsequent lowering of modulus
and strength of the blend of PP/EOC/PP-g-MA and PP/EOC/PP-gMA/clay [23,37]. On one hand, increasing MA content up to an optimum level that PP-g-MA is miscible with PP is effective for better dispersion of clay in PP/clay composites and possibly promotes exfoliated structures, whereas on the other hand increasing the MA groups to higher than the optimum level causes the immiscibility of PP-g-MA in PP with no signs of exfoliation [37,38]. So, the appearance of intercalated structure in the PP phase may indicate the immiscibility of PP-g-MA in PP which is detrimental for mechanical properties. Moreover, Lopez et al. [39] proposed the less amounts of PP-g-MA with high polar groups are necessary as a compatibilizer for PP/clay, and the excess amount acts as a lubricant and decreases the mechanical properties. As such it would appear that 4 phr of our PP-g-MA is high for PP/EOC/PP-g-MA/clay. Furthermore, the toughness of PP/EOC/PP-g-MA/clay is lower than the values for the other two nanocomposites due to the presence of clay in the matrix, and the high content of MA groups in the PP-gMA. So, employing another PP-g-MA with a lower MA content might be an effective strategy for better balance of mechanical properties of this nanocomposite. In summary, based on the above results, it can be seen that all the tensile energy, elongation at break and impact strength improved in the nanocomposits compared to their neat blends suggesting the toughening effect of clay in these materials. 4. Conclusions The relation between morphology and properties of polypropylene/ethylene–octene copolymer/clay with and without two compatibilizers of maleated polypropylene and maleic anhydride grafted poly (ethylene-co-octene) was studied. The results showed a close correlation between morphology and properties of the studied polymer blend nanocomposites. Clay platelets had better interactions with the EOC than PP and were located in the EOC phase of PP/EOC/clay as intercalated structures. However in the presence of compatibilizers, nanoclay resided selectively in the phase with higher polarity. Rheological analysis suggested the enhancement of complex viscosity and storage moduli of the nanocomposites compared to PP/EOC blend. Although the stiffness and tensile strength of the nanocomposites were not improved compared to PP/EOC, there was a drastic improvement in the toughness especially in PP/EOC/EOC-g-MA/clay due to the presence of fine intercalated structure of clay layers in the EOC phase, significant decrease in the EOC particle size induced by clay addition, the formation of percolated network of clay layers inside the EOC phase and the elastomeric nature of EOC-g-MA. Acknowledgements We thank the Environmental Protection Agency in Ireland (EPA2008-PhD-WRM-4), Science Foundation Ireland (07/SK/I1220a) and the Irish Government’s Program for Research in Third Level Institutions and Irish Research Council for Science, Engineering and Technology for financial support.
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