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Polypropylene–clay nanocomposites prepared by in situ grafting-intercalating in melt
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 64 (2004) 1383–1389 www.elsevier.com/locate/compscitech
Polypropylene–clay nanocomposites prepared by in situ grafting-intercalating in melt Yu-Qing Zhang a, Joong-Hee Lee
b,*
, John M. Rhee b, Kyung Y. Rhee
c
a
b
Department of Chemical Engineering, Henan University of Science and Technology, Luoyang 471039, China Department of Polymer Science and Technology, Chonbuk National University, Chonju, Chonbuk 561-756, Republic of Korea c School of Mechanical and Industrial Systems Engineering, Kyunghee University, Yongin 449-701, Republic of Korea Received 11 April 2003; received in revised form 30 October 2003; accepted 31 October 2003 Available online 19 December 2003
Abstract Polypropylene–clay nanocomposites (PPCN) were prepared by in situ grafting-intercalating in melt. The organoclay was first modified with maleic anhydride (MA) in solution with a small quantity of a co-swelling agent and an initiator. It was then blended with PP in melt to obtain PP/clay grafting-intercalating composites (GIC). Finally, the GIC were blended with PP in melt to obtain PPCN. The nanostructure materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). The XRD patterns show that the basal distance in the MA-modified organoclay was 30 nm, which was larger than that of the original organoclay (19.6 nm). The XRD and TEM results for GIC and PPCN demonstrate that the layers of clay were partially exfoliated in the GIC and fully exfoliated in the PPCN. PPCN showed good thermal stability in the TGA analysis. The increased storage modulus indicates that the clay has a reinforcing effect in the PP matrix. The increased Tg of the PP/clay nanocomposites in dynamic mechanical analysis (DMA) implies that the PP macromolecules are intercalated or exfoliated between the interlayers of the silicate. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructure; PP–clay nanocomposites
1. Introduction Polymer/inorganic nanocomposites are a class of hybrid materials composed of an organic matrix and an inorganic reinforcement in which inorganic particles of nanoscale dimension are imbedded [1–6]. Due to the nanometer effect, the physical and mechanical properties of the polymer are improved dramatically by using a small quantity of a nanofiller, especially a nanofiller with a large aspect ratio. Polymer nanocomposites have unexpected hybrid properties synergistically derived from the two components, such as a higher heat distortion temperature, improved flame resistance, increased modulus, bet-
* Corresponding author. Present address: Center for the Composite Materials, University of Delaware, Newark, DE 19716-3144, USA. Tel.: +1-302-831-0378; fax: +1-302-831-8525. E-mail address: [email protected] (J.-H. Lee).
0266-3538/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2003.10.014
ter gas barrier properties, and better dimension stability [7,8]. Due to these advantages, many studies have examined these compounds. However, there are still some difficulties in making them commercially available and in getting good dispersion due to the strong interparticle interaction of the high-surface-area nanoparticles. Since there are also handling problems relating to inhalation health hazards, their application has been limited [9]. A technique to produce anisotropic nanoparticles in situ during the process of nanocomposite formation has been developed. This method is especially suitable for layered silicates. The exfoliation of a layered silicate modified with an organic amine can be realized using three approaches: (1) the insertion of a suitable monomer into the layered silicate and its subsequent polymerization, (2) the direct insertion of polymer chains into layered silicates in solution or in melt, and (3) the insertion of a polymer via polymerization with a catalyst supported on the silicate [10,11].
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Y.-Q. Zhang et al. / Composites Science and Technology 64 (2004) 1383–1389
The current preferred anisotropic nanofiller is montmorillonite (MMt), which has a layered structure. Since it is hydrophilic, MMt is not compatible with most polymers, so it must be chemically modified to render its surface more hydrophobic. The most popular surface treatment is ion exchange using organic ammonium cations with long nonpolar alkyl groups. This not only renders the surface more hydrophobic, but also expands the spaces between the silicate layers. Polypropylene (PP) is one of the most widely used polyolefin polymers. Since PP does not have any polar groups in its backbone, it is not compatible with clay and homogeneous dispersion of the silicate layer in PP matrix cannot be realized using modified hydrophobic MMt [12–15]. To make the clay disperse well in the PP matrix, two major methods for preparing PP–clay nanocomposites have been developed. The first method is melt intercalation using a polar functional oligomer as a compatibilizer [13,14]. In this method, the organoclay is first blended with the compatibilizer in melt, and then the resulting pre-intercalated clay is melt-blended with PP. When the miscibility of the oligomers with PP is sufficient, exfoliation of the pre-intercalated clay takes place. The second method is intercalating polymerization with a catalyst (TiCl3 or a metallocene) supported on the modified clay [10,16,17]. Although the second method is very promising, the most versatile and environmentally friendly method is based on direct polymer melt blending with modified clay. In this study, we attempted to use grafting-intercalating in situ to synthesize nanocomposites with a low compatibilizer content. Polypropylene, an organoclay treated with maleic anhydride (MA), a distending agent, and an initiator were blended together in melt. The graft reaction and high shearing forces simultaneously led to good dispersion of the silicate layer in the grafted polypropylene matrix. The composites resulting from grafting-intercalating in situ were used as a master batch and blended with PP to give the final nanocomposites. The thermal properties and dynamic behavior of the nanocomposites were measured to characterize the composites.
Table 1 Materials used Name
Characterization
Supplier
PP (J-170S)
MFR (230 °C): 28.0 g/10 min 95% 90%
Honam Petrochemical Co. Aldrich Chem. Co Aldrich Chem. Co.
CEC: 115 mmol/ 100 g 97%
Kunimine Industries Co. Aldrich Chem. Co.
Maleic anhydride (MA) 2,5-Dimethy-2,5-di (t-butylperoxy)hexane Montmorillonite Octadecylamine
collected by filtration, washed three times with hot water to remove the chlorine ions, and freeze-dried. The resulting organophilic montmorillonite (OMMt, 20 g) was mixed with MA (16 g), an initiator (diphenylamine-2-carboxylic acid; DPC, 4.8 g), and a distending agent (acetic ester, 2 g) in acetone at room temperature for 2 h, then dried at room temperature to obtain modified organophilic montmorillonite (MOMMt). 2.2. Preparation of PP–clay nanocomposites The dried MOMMt was melt-blended with PP in a Haake internal mixer at 180 °C for 20 min to obtain in situ grafting-intercalating composites (GIC). The OMMt content of the composites was about 30%. The in situ GIC were blended with PP at 190 °C for 15 min as a master batch. A Haake internal mixer was used to obtain final nanocomposites with different clay contents (designated PPCN). The compositions of the PPCNs are listed in Table 2. Film specimens of the PPCNs were prepared for X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurement by hot pressing and specimens (12 3 1.2 mm) for dynamic mechanical analysis (DMA) measurement were prepared by heat molding at 25 MPa and 180 °C for 5 min. 2.3. Characterization 2.3.1. Scanning electron microscopy The particle morphologies of MMt, OMMt, and MOMMt were observed by scanning electron microscopy (SEM) (Hitachi S-4700).
2. Experiment 2.1. Materials The materials used to prepare the PP/MMt nanocomposites are listed in Table 1. The clay was modified as follows: sodium montmorillonite was dispersed into 5000 ml of hot water using a homogenizer. Octadecylammonium chloride neutralized with HCl solution was poured into the hot montmorillonite–water suspension solution with vigorous stirring for 5 min using the homogenizer. This yielded a white precipitate, which was
2.3.2. X-ray diffraction X-ray diffraction analysis was carried out with a Riga-ku model Dmax 2500 instrument with Cu-Ka
Table 2 Composition of the PPCNs Components GIC (wt%) Organoclay (wt%) PPMA (wt%)
PPCN 1 3.33 1 2.33
PPCN 2 6.67 2 4.67
PPCN 3
PPCN 4
10.0 3 7
13.33 4 9.32
Y.-Q. Zhang et al. / Composites Science and Technology 64 (2004) 1383–1389
radiation. The basal spaces of different montmorillonites and composites were estimated from the (0 0 1) peak in the XRD patterns. 2.3.3. Transmission electron microscopy About 70 nm ultrathin film samples were observed by TEM using an EM9110MEGA model instrument at an acceleration voltage of 120 kV. 2.3.4. DSC analysis Differential scanning calorimeter analysis was performed on a DSC-2910 differential scanning calorimeter thermal analyzer (TA Instruments) at temperatures from 60 to 200 °C at a heating rate of 5 °C/min under a nitrogen atmosphere.
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curve of MOMMt at about 2.6° and 4.5°, corresponding to a basal spacing of 3.2 and 1.8 nm, respectively. This can be explained if there are two components in the organoclay. One component facilitates expansion with the agent whereas the other reacts with the agent and extracts the organo-material from the interlayers. The former makes the interlayer space larger and the latter makes it smaller than that of the organoclay. SEM micrographs of MMt, OMMt, and MOMMt are shown in Fig. 2. The silicate flakes formed in ion exchange with the organic amine are clearly seen. However, the flakes re-aggregated when they were treated with the agents; the re-crystallization of MA may cause the organoclay flakes to agglomerate. However, the basal spacing of MOMMt increased, as shown
2.3.5. Dynamic mechanical analysis The dynamic mechanical behavior of the composites was measured on a TA DMA-983. Dynamic temperature spectra of the samples were obtained in shearing mode at a vibration frequency of 1 Hz in a nitrogen atmosphere, at temperatures from )40 to 150 °C at a scan rate of 5 °C/min.
3. Results and discussion 3.1. Effects of MA on intercalation Fig. 1 shows the XRD patterns of MMt, OMMt, and MOMMt. MMt (curve a) shows a single peak at about 7.8°, corresponding to a basal space of 1.2 nm, and OMMt (curve b) shows a peak at about 4° (1.96 nm basal space) in the XRD pattern. Comparing with the pristine clay, the basal spacing of OMMt is increased by 0.76 nm. By contrast, two peaks appear in the XRD
a : Pristine clay b : Organo clay c : Modified organo clay b
Intensity (arbitrary)
c
a
2
4
6 2 theta
Fig. 1. XRD patterns of the clay.
8
10
Fig. 2. SEM images of (a) pristine clay, (b) organoclay, and (c) modified organoclay.
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in Fig. 1. This indicates that MA with a stronger polarity easily intercalates into the galleries of the organoclay and further increases the distance between the interlayers. 3.2. The dispersion of OMMt in GIC and PPCN The grafting reaction takes place when MOMMt and powdered PP are melt mixed in an internal mixer at 170 °C. The reaction increased the temperature of the internal mixer to 180 °C. Due to the previous intercalation of MA into the clay, the grafting reaction takes place in the interlayers of the clay, which generates intercalated PP–g–MA between the interlayers of the clay in situ. This further increases the distance between galleries in the clay. Consequently, good dispersion of clay in the matrix is obtained with the assistance of the simultaneous reaction heat and high shearing force. Direct evidence of the intercalation is provided by the XRD pattern of GIC in Fig. 3(a). This shows that the MOMMT peak near 2.6° is shifted to a smaller angle, which indicates that the layered silicates are further ex-
foliated by the grafting-intercalating. The same results are observed in the TEM image of GIC in Fig. 4(a). Many of the silicate layers were exfoliated and dispersed in the matrix. However, some ordered stacking remained. Then, the GIC was blended with polypropylene in an internal mixer at 190 °C to produce PPCN. Since the interaction between the clay interlayers is already much weakened by grafting-intercalating, the clay layers are easily exfoliated during the subsequent shear blending process, as shown in Figs. 3(b) and 4(b). This proves that fully exfoliated composites of layered silicate can be successfully obtained using this approach. The good dispersion of clay in PP matrix probably results from the synergistic effects of grafting-intercalation. Liu and Wu [15] prepared PP–clay nanocomposites using graft-melt intercalation. However, the clay layers in these nanocomposites were merely intercalated, and not fully exfoliated. 3.3. Thermal analysis Differential scanning calorimeter thermograms of PPCNs with various clay contents are shown in Fig. 5.
250
200
Intensity
150
100
50
0
0
2
4
(a)
6
8
10
6
8
10
2 theta 300 250
Intensity
200 150 100 50 0
(b)
0
2
4 2 theta
Fig. 3. XRD patterns of (a) GIC and (b) PPCN2.
Fig. 4. TEM images of (a) GIC and (b) PPCN2.
Y.-Q. Zhang et al. / Composites Science and Technology 64 (2004) 1383–1389
almost a vertical line, which means that the decomposition is very fast. The enhanced thermal stability of the polymer–clay nanocomposites is attributed to the lower permeability of oxygen and the diffusibility of the degradation products from the bulk of the polymer caused by the exfoliated clay in the composites [10,18–21].
1 2 3 4 5
Heat flow (arbitrary unit)
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6
9.5
9.0
80
100
120
140
160
180
200
220
Temperature oC
Log G'
8.5
60
Fig. 5. DSC heating thermograms of PP, GIC, and PPCNs with different clay contents. 1, GIC; 2, PPCN1; 3, PPCN2; 4, PPCN3; 5, PPCN4; and 6, iPP.
8.0 1% 2% 3% 4% iPP
7.5
7.0
6.5 -50
0
(a)
50 o Temperature C
100
150
50
100
150
100
150
8.0
7.5
Log G''
With increasing clay content, the melting point of the composite decreases slightly, as compared to pure PP. The melting temperature of PPCN is thought to decrease due to the effects of the PPMA on the crystal integrality of PP, but this interference is very weak. However, the introduction of clay into PP enhances the thermal stability of PP greatly, as shown in Fig. 6. Comparing the thermal decomposition temperature of PPCN with that of pure PP, the temperature at the onset of the thermal decomposition of PPCN with 2% clay content is increased by nearly 130 °C. Since GIC contains a large amount of clay and low-molecular-weight PPMA, its temperature at the onset of thermal decomposition is lower than that of PPCN, but the final decomposition temperature is higher than that of PPCN. A surprising phenomenon is seen in Fig. 6. The weight loss curve of the nanocomposites with 2% clay content is
7.0 1% 2% 3% 4% iPP
6.5
6.0
-50
0
o
(b)
Temperature C -0.8
100 1% 2% 3% 4% iPP
-1.0
-1.2
60 2
Log tan d
Weight (%)
80
3
1 1 : iPP 2 : PPCN2 3 : GIC
40
-1.4
-1.6
20
-1.8
0 0
100
200
300
400
500
600
Temperature oC
Fig. 6. TGA curves of PP, GIC, and PP–clay nanocomposite with 2% clay content.
-50
(c)
0
50 o
Temperature C
Fig. 7. Dynamic mechanical spectra of PPCN: (a) storage modulus, (b) loss modulus, and (c) loss factor.
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3.4. Dynamic mechanical properties of PP–clay nanocomposites Dynamic mechanical analysis is a very important tool for studying relaxation in polymers. Analysis of the storage modulus, loss modulus, and tan d curves is very useful in ascertaining the performance of a material under stress and temperature. DMA not only measures the dynamic mechanical properties of a material, but also detects changes in the solid structure of a polymer after compounding with other materials. Fig. 7 depicts the dynamic mechanical spectra (storage modulus G0 , loss modulus G00 , and loss factor tan d as a function of temperature for PP and PP–clay nanocomposites. As Fig. 7(a) clearly shows, the storage modulus of the PP– clay nanocomposite is higher than that of PP. This indicates that the incorporation of clay into the PP matrix remarkably enhances its stiffness and has good reinforcing effects. As seen in Fig. 7(b) and (c), the peak temperatures in the log G00 curves and log tan d curves of the nanocomposites increase with increasing clay content. This indicates that the glass transient temperatures (Tg ) of the PP–clay nanocomposites, which can be derived from the curves of log G00 –T or log tan d–T , are higher than that of pure PP. In general, introducing the lower-molecularweight PP–g–MA into the PP matrix decreases the Tg of the material due to plastification. By contrast, the Tg of PP–clay nanocomposites does not decrease, but increases with the addition of PP–g–Ma. The Tg of nanocomposites with 4% clay content is 30 °C higher than that of PP. It is well known that the magnitude of Tg of a polymer depends on the mobility of the macromolecule chain segment in the polymer matrix. If the molecular chain is restricted, motion or relaxation of the chain segment becomes difficult, so that the glass transition temperature increases. When PP molecules are intercalated into the silicate gallery, or the silicate layers are exfoliated in the PP, geometric constraints alter the chain conformation of the polypropylene, altering the glass transient temperature and storage modulus of the composites.
4. Conclusion In this study, we successfully prepared PP–clay nanocomposites in a novel way, grafting-intercalating in situ. Due to the synergistic effects of graft-intercalation, the clay layers were dispersed well on a nanoscale in the PP matrix. The introduction of clay into the PP matrix improved the thermal stability remarkably and increased the storage modulus (stiffness) of the PP considerably. The narrow space surrounded by the dispersed clay layers and the interaction between the clay layers and macromolecules restricted the mo-
tion or relaxation of the chain segment of the PP, increasing Tg . Acknowledgements This work was supported by grants from the postdoctoral program of Chonbuk National University (2001) and the Basic Research Program of the Korea Science and Engineering Foundation (2000-2-308-002-3). References [1] Messersmith P, Giannelis E. Synthesis and barrier properties of poly(e-caprolactone)-layered silicate nanocomposites. J Polym Sci: Part A: Polym Chem 1995;33:1047–57. [2] Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Synthesis of nylon 6-clay hybrid. J Mater Res 1993;8(5):1179–84. [3] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, et al. Mechanical properties of nylon 6-clay hybrid. J Mater Res 1993;8(5):1185–9. [4] Yano K, Usuki A, Okada A, Kurauchi T, Kamigaito O. Synthesis and properties of polyimide–clay hybrid. J Polym Sci: Part A: Polym Chem 1993;31:2493–8. [5] Vaia R, Jandt K, Kramer E, Giannelis E. Kinetics of polymer melt intercalation. Macromolecules 1995;28:8080–5. [6] Biasci L, Aglietto M, Ruggeri G, Ciardelli F. Functionalization of montmorillonite by methyl methacrylate polymers containing side-chain ammonium cation. Polymer 1994;35(15):3295–304. [7] Giannelis E. Polymer layered silicate nanocomposites. Adv Mater 1996;8(1):29–35. [8] Wang Z, Pinnavaia T. Nanolayer reinforcement of elastomeric polyurethane. Chem Mater 1998;10:3769–71. [9] Kaempfer D, Thomann R, M€ ulhaupt R. Melt compounding of syndiotactic polypropylene nanocomposites containing organophilic layered silicates and in situ formed core/shell nanoparticles. Polymer 2002;43:2909–16. [10] Ma J, Qi Z, Hu Y. Synthesis and characterization of polypropylene/clay nanocomposites. J Appl Polym Sci 2001;82:3611–7. [11] SinhaRay S, Biswas M. Preparation and evaluation of composites from montmorillonite and some heterocyclic polymers: 3. A water dispersible nanocomposite from pyrrole–montmorillonite polymerization system. Mater Res Bull 1999;34(8):1187–94. [12] Kawasumi M, Hasegawa N, Kato M, Uski A, Okada A. Preparation and mechanical properties of polypropylene–clay hybrids. Macromolecules 1997;30:6333–8. [13] Kato M, Usuki A, Okada A. Synthesis of polypropylene oligomer–clay intercalation compounds. J Appl Polym Sci 1997; 66:1781–5. [14] Hasegawa N, Kawasumi M, Kato M, Usuki A, Okada A. Preparation and mechanical properties of polypropylene–clay hybrids using a maleic anhydride-modified polypropylene oligomer. J Appl Polym Sci 1998;67:87–92. [15] Liu X, Wu Q. PP/clay nanocomposites prepared by grafting-melt intercalation. Polymer 2001;42:10013–9. [16] Rong J, Li H, Jing Z, Hong X, Sheng M. Novel organic/inorganic nanocomposite of polyethylene. 1. Preparation via in situ polymerization approach. J Appl Polym Sci 2001;82:1829–37. [17] Heinemann J, Reichert P, Thomann R, Mulhaupt R. Polyolefin nanocomposites formed by melt compounding and transition metal catalyzed ethane homo- and copolymerization in the presence of layered silicates. Macromol Rapid Commun 1999; 20:423–30.
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[20] Jimenez G, Ogata N, Kawai H, Ogihara T. Structure and thermal/ mechanical properties of poly (e-caprolactone)–clay blend. J Appl Polym Sci 1997;6:2211–20. [21] Manias E, Chen H, Krishnamoorti R, Genzer J, Kramer EJ, Giannelis EP. Intercalation kinetics of long polymers in 2 nm confinements. Macromolecules 2000;33:7955–66.
Polypropylene–clay nanocomposites prepared by in situ grafting-intercalating in melt Yu-Qing Zhang a, Joong-Hee Lee
b,*
, John M. Rhee b, Kyung Y. Rhee
c
a
b
Department of Chemical Engineering, Henan University of Science and Technology, Luoyang 471039, China Department of Polymer Science and Technology, Chonbuk National University, Chonju, Chonbuk 561-756, Republic of Korea c School of Mechanical and Industrial Systems Engineering, Kyunghee University, Yongin 449-701, Republic of Korea Received 11 April 2003; received in revised form 30 October 2003; accepted 31 October 2003 Available online 19 December 2003
Abstract Polypropylene–clay nanocomposites (PPCN) were prepared by in situ grafting-intercalating in melt. The organoclay was first modified with maleic anhydride (MA) in solution with a small quantity of a co-swelling agent and an initiator. It was then blended with PP in melt to obtain PP/clay grafting-intercalating composites (GIC). Finally, the GIC were blended with PP in melt to obtain PPCN. The nanostructure materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). The XRD patterns show that the basal distance in the MA-modified organoclay was 30 nm, which was larger than that of the original organoclay (19.6 nm). The XRD and TEM results for GIC and PPCN demonstrate that the layers of clay were partially exfoliated in the GIC and fully exfoliated in the PPCN. PPCN showed good thermal stability in the TGA analysis. The increased storage modulus indicates that the clay has a reinforcing effect in the PP matrix. The increased Tg of the PP/clay nanocomposites in dynamic mechanical analysis (DMA) implies that the PP macromolecules are intercalated or exfoliated between the interlayers of the silicate. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructure; PP–clay nanocomposites
1. Introduction Polymer/inorganic nanocomposites are a class of hybrid materials composed of an organic matrix and an inorganic reinforcement in which inorganic particles of nanoscale dimension are imbedded [1–6]. Due to the nanometer effect, the physical and mechanical properties of the polymer are improved dramatically by using a small quantity of a nanofiller, especially a nanofiller with a large aspect ratio. Polymer nanocomposites have unexpected hybrid properties synergistically derived from the two components, such as a higher heat distortion temperature, improved flame resistance, increased modulus, bet-
* Corresponding author. Present address: Center for the Composite Materials, University of Delaware, Newark, DE 19716-3144, USA. Tel.: +1-302-831-0378; fax: +1-302-831-8525. E-mail address: [email protected] (J.-H. Lee).
0266-3538/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2003.10.014
ter gas barrier properties, and better dimension stability [7,8]. Due to these advantages, many studies have examined these compounds. However, there are still some difficulties in making them commercially available and in getting good dispersion due to the strong interparticle interaction of the high-surface-area nanoparticles. Since there are also handling problems relating to inhalation health hazards, their application has been limited [9]. A technique to produce anisotropic nanoparticles in situ during the process of nanocomposite formation has been developed. This method is especially suitable for layered silicates. The exfoliation of a layered silicate modified with an organic amine can be realized using three approaches: (1) the insertion of a suitable monomer into the layered silicate and its subsequent polymerization, (2) the direct insertion of polymer chains into layered silicates in solution or in melt, and (3) the insertion of a polymer via polymerization with a catalyst supported on the silicate [10,11].
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Y.-Q. Zhang et al. / Composites Science and Technology 64 (2004) 1383–1389
The current preferred anisotropic nanofiller is montmorillonite (MMt), which has a layered structure. Since it is hydrophilic, MMt is not compatible with most polymers, so it must be chemically modified to render its surface more hydrophobic. The most popular surface treatment is ion exchange using organic ammonium cations with long nonpolar alkyl groups. This not only renders the surface more hydrophobic, but also expands the spaces between the silicate layers. Polypropylene (PP) is one of the most widely used polyolefin polymers. Since PP does not have any polar groups in its backbone, it is not compatible with clay and homogeneous dispersion of the silicate layer in PP matrix cannot be realized using modified hydrophobic MMt [12–15]. To make the clay disperse well in the PP matrix, two major methods for preparing PP–clay nanocomposites have been developed. The first method is melt intercalation using a polar functional oligomer as a compatibilizer [13,14]. In this method, the organoclay is first blended with the compatibilizer in melt, and then the resulting pre-intercalated clay is melt-blended with PP. When the miscibility of the oligomers with PP is sufficient, exfoliation of the pre-intercalated clay takes place. The second method is intercalating polymerization with a catalyst (TiCl3 or a metallocene) supported on the modified clay [10,16,17]. Although the second method is very promising, the most versatile and environmentally friendly method is based on direct polymer melt blending with modified clay. In this study, we attempted to use grafting-intercalating in situ to synthesize nanocomposites with a low compatibilizer content. Polypropylene, an organoclay treated with maleic anhydride (MA), a distending agent, and an initiator were blended together in melt. The graft reaction and high shearing forces simultaneously led to good dispersion of the silicate layer in the grafted polypropylene matrix. The composites resulting from grafting-intercalating in situ were used as a master batch and blended with PP to give the final nanocomposites. The thermal properties and dynamic behavior of the nanocomposites were measured to characterize the composites.
Table 1 Materials used Name
Characterization
Supplier
PP (J-170S)
MFR (230 °C): 28.0 g/10 min 95% 90%
Honam Petrochemical Co. Aldrich Chem. Co Aldrich Chem. Co.
CEC: 115 mmol/ 100 g 97%
Kunimine Industries Co. Aldrich Chem. Co.
Maleic anhydride (MA) 2,5-Dimethy-2,5-di (t-butylperoxy)hexane Montmorillonite Octadecylamine
collected by filtration, washed three times with hot water to remove the chlorine ions, and freeze-dried. The resulting organophilic montmorillonite (OMMt, 20 g) was mixed with MA (16 g), an initiator (diphenylamine-2-carboxylic acid; DPC, 4.8 g), and a distending agent (acetic ester, 2 g) in acetone at room temperature for 2 h, then dried at room temperature to obtain modified organophilic montmorillonite (MOMMt). 2.2. Preparation of PP–clay nanocomposites The dried MOMMt was melt-blended with PP in a Haake internal mixer at 180 °C for 20 min to obtain in situ grafting-intercalating composites (GIC). The OMMt content of the composites was about 30%. The in situ GIC were blended with PP at 190 °C for 15 min as a master batch. A Haake internal mixer was used to obtain final nanocomposites with different clay contents (designated PPCN). The compositions of the PPCNs are listed in Table 2. Film specimens of the PPCNs were prepared for X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurement by hot pressing and specimens (12 3 1.2 mm) for dynamic mechanical analysis (DMA) measurement were prepared by heat molding at 25 MPa and 180 °C for 5 min. 2.3. Characterization 2.3.1. Scanning electron microscopy The particle morphologies of MMt, OMMt, and MOMMt were observed by scanning electron microscopy (SEM) (Hitachi S-4700).
2. Experiment 2.1. Materials The materials used to prepare the PP/MMt nanocomposites are listed in Table 1. The clay was modified as follows: sodium montmorillonite was dispersed into 5000 ml of hot water using a homogenizer. Octadecylammonium chloride neutralized with HCl solution was poured into the hot montmorillonite–water suspension solution with vigorous stirring for 5 min using the homogenizer. This yielded a white precipitate, which was
2.3.2. X-ray diffraction X-ray diffraction analysis was carried out with a Riga-ku model Dmax 2500 instrument with Cu-Ka
Table 2 Composition of the PPCNs Components GIC (wt%) Organoclay (wt%) PPMA (wt%)
PPCN 1 3.33 1 2.33
PPCN 2 6.67 2 4.67
PPCN 3
PPCN 4
10.0 3 7
13.33 4 9.32
Y.-Q. Zhang et al. / Composites Science and Technology 64 (2004) 1383–1389
radiation. The basal spaces of different montmorillonites and composites were estimated from the (0 0 1) peak in the XRD patterns. 2.3.3. Transmission electron microscopy About 70 nm ultrathin film samples were observed by TEM using an EM9110MEGA model instrument at an acceleration voltage of 120 kV. 2.3.4. DSC analysis Differential scanning calorimeter analysis was performed on a DSC-2910 differential scanning calorimeter thermal analyzer (TA Instruments) at temperatures from 60 to 200 °C at a heating rate of 5 °C/min under a nitrogen atmosphere.
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curve of MOMMt at about 2.6° and 4.5°, corresponding to a basal spacing of 3.2 and 1.8 nm, respectively. This can be explained if there are two components in the organoclay. One component facilitates expansion with the agent whereas the other reacts with the agent and extracts the organo-material from the interlayers. The former makes the interlayer space larger and the latter makes it smaller than that of the organoclay. SEM micrographs of MMt, OMMt, and MOMMt are shown in Fig. 2. The silicate flakes formed in ion exchange with the organic amine are clearly seen. However, the flakes re-aggregated when they were treated with the agents; the re-crystallization of MA may cause the organoclay flakes to agglomerate. However, the basal spacing of MOMMt increased, as shown
2.3.5. Dynamic mechanical analysis The dynamic mechanical behavior of the composites was measured on a TA DMA-983. Dynamic temperature spectra of the samples were obtained in shearing mode at a vibration frequency of 1 Hz in a nitrogen atmosphere, at temperatures from )40 to 150 °C at a scan rate of 5 °C/min.
3. Results and discussion 3.1. Effects of MA on intercalation Fig. 1 shows the XRD patterns of MMt, OMMt, and MOMMt. MMt (curve a) shows a single peak at about 7.8°, corresponding to a basal space of 1.2 nm, and OMMt (curve b) shows a peak at about 4° (1.96 nm basal space) in the XRD pattern. Comparing with the pristine clay, the basal spacing of OMMt is increased by 0.76 nm. By contrast, two peaks appear in the XRD
a : Pristine clay b : Organo clay c : Modified organo clay b
Intensity (arbitrary)
c
a
2
4
6 2 theta
Fig. 1. XRD patterns of the clay.
8
10
Fig. 2. SEM images of (a) pristine clay, (b) organoclay, and (c) modified organoclay.
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in Fig. 1. This indicates that MA with a stronger polarity easily intercalates into the galleries of the organoclay and further increases the distance between the interlayers. 3.2. The dispersion of OMMt in GIC and PPCN The grafting reaction takes place when MOMMt and powdered PP are melt mixed in an internal mixer at 170 °C. The reaction increased the temperature of the internal mixer to 180 °C. Due to the previous intercalation of MA into the clay, the grafting reaction takes place in the interlayers of the clay, which generates intercalated PP–g–MA between the interlayers of the clay in situ. This further increases the distance between galleries in the clay. Consequently, good dispersion of clay in the matrix is obtained with the assistance of the simultaneous reaction heat and high shearing force. Direct evidence of the intercalation is provided by the XRD pattern of GIC in Fig. 3(a). This shows that the MOMMT peak near 2.6° is shifted to a smaller angle, which indicates that the layered silicates are further ex-
foliated by the grafting-intercalating. The same results are observed in the TEM image of GIC in Fig. 4(a). Many of the silicate layers were exfoliated and dispersed in the matrix. However, some ordered stacking remained. Then, the GIC was blended with polypropylene in an internal mixer at 190 °C to produce PPCN. Since the interaction between the clay interlayers is already much weakened by grafting-intercalating, the clay layers are easily exfoliated during the subsequent shear blending process, as shown in Figs. 3(b) and 4(b). This proves that fully exfoliated composites of layered silicate can be successfully obtained using this approach. The good dispersion of clay in PP matrix probably results from the synergistic effects of grafting-intercalation. Liu and Wu [15] prepared PP–clay nanocomposites using graft-melt intercalation. However, the clay layers in these nanocomposites were merely intercalated, and not fully exfoliated. 3.3. Thermal analysis Differential scanning calorimeter thermograms of PPCNs with various clay contents are shown in Fig. 5.
250
200
Intensity
150
100
50
0
0
2
4
(a)
6
8
10
6
8
10
2 theta 300 250
Intensity
200 150 100 50 0
(b)
0
2
4 2 theta
Fig. 3. XRD patterns of (a) GIC and (b) PPCN2.
Fig. 4. TEM images of (a) GIC and (b) PPCN2.
Y.-Q. Zhang et al. / Composites Science and Technology 64 (2004) 1383–1389
almost a vertical line, which means that the decomposition is very fast. The enhanced thermal stability of the polymer–clay nanocomposites is attributed to the lower permeability of oxygen and the diffusibility of the degradation products from the bulk of the polymer caused by the exfoliated clay in the composites [10,18–21].
1 2 3 4 5
Heat flow (arbitrary unit)
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6
9.5
9.0
80
100
120
140
160
180
200
220
Temperature oC
Log G'
8.5
60
Fig. 5. DSC heating thermograms of PP, GIC, and PPCNs with different clay contents. 1, GIC; 2, PPCN1; 3, PPCN2; 4, PPCN3; 5, PPCN4; and 6, iPP.
8.0 1% 2% 3% 4% iPP
7.5
7.0
6.5 -50
0
(a)
50 o Temperature C
100
150
50
100
150
100
150
8.0
7.5
Log G''
With increasing clay content, the melting point of the composite decreases slightly, as compared to pure PP. The melting temperature of PPCN is thought to decrease due to the effects of the PPMA on the crystal integrality of PP, but this interference is very weak. However, the introduction of clay into PP enhances the thermal stability of PP greatly, as shown in Fig. 6. Comparing the thermal decomposition temperature of PPCN with that of pure PP, the temperature at the onset of the thermal decomposition of PPCN with 2% clay content is increased by nearly 130 °C. Since GIC contains a large amount of clay and low-molecular-weight PPMA, its temperature at the onset of thermal decomposition is lower than that of PPCN, but the final decomposition temperature is higher than that of PPCN. A surprising phenomenon is seen in Fig. 6. The weight loss curve of the nanocomposites with 2% clay content is
7.0 1% 2% 3% 4% iPP
6.5
6.0
-50
0
o
(b)
Temperature C -0.8
100 1% 2% 3% 4% iPP
-1.0
-1.2
60 2
Log tan d
Weight (%)
80
3
1 1 : iPP 2 : PPCN2 3 : GIC
40
-1.4
-1.6
20
-1.8
0 0
100
200
300
400
500
600
Temperature oC
Fig. 6. TGA curves of PP, GIC, and PP–clay nanocomposite with 2% clay content.
-50
(c)
0
50 o
Temperature C
Fig. 7. Dynamic mechanical spectra of PPCN: (a) storage modulus, (b) loss modulus, and (c) loss factor.
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3.4. Dynamic mechanical properties of PP–clay nanocomposites Dynamic mechanical analysis is a very important tool for studying relaxation in polymers. Analysis of the storage modulus, loss modulus, and tan d curves is very useful in ascertaining the performance of a material under stress and temperature. DMA not only measures the dynamic mechanical properties of a material, but also detects changes in the solid structure of a polymer after compounding with other materials. Fig. 7 depicts the dynamic mechanical spectra (storage modulus G0 , loss modulus G00 , and loss factor tan d as a function of temperature for PP and PP–clay nanocomposites. As Fig. 7(a) clearly shows, the storage modulus of the PP– clay nanocomposite is higher than that of PP. This indicates that the incorporation of clay into the PP matrix remarkably enhances its stiffness and has good reinforcing effects. As seen in Fig. 7(b) and (c), the peak temperatures in the log G00 curves and log tan d curves of the nanocomposites increase with increasing clay content. This indicates that the glass transient temperatures (Tg ) of the PP–clay nanocomposites, which can be derived from the curves of log G00 –T or log tan d–T , are higher than that of pure PP. In general, introducing the lower-molecularweight PP–g–MA into the PP matrix decreases the Tg of the material due to plastification. By contrast, the Tg of PP–clay nanocomposites does not decrease, but increases with the addition of PP–g–Ma. The Tg of nanocomposites with 4% clay content is 30 °C higher than that of PP. It is well known that the magnitude of Tg of a polymer depends on the mobility of the macromolecule chain segment in the polymer matrix. If the molecular chain is restricted, motion or relaxation of the chain segment becomes difficult, so that the glass transition temperature increases. When PP molecules are intercalated into the silicate gallery, or the silicate layers are exfoliated in the PP, geometric constraints alter the chain conformation of the polypropylene, altering the glass transient temperature and storage modulus of the composites.
4. Conclusion In this study, we successfully prepared PP–clay nanocomposites in a novel way, grafting-intercalating in situ. Due to the synergistic effects of graft-intercalation, the clay layers were dispersed well on a nanoscale in the PP matrix. The introduction of clay into the PP matrix improved the thermal stability remarkably and increased the storage modulus (stiffness) of the PP considerably. The narrow space surrounded by the dispersed clay layers and the interaction between the clay layers and macromolecules restricted the mo-
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