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Dispersion and morphology of polypropylene nanocomposites: Characterization based on a compact and flexible optical sensor
Polymer Testing 31 (2012) 800–809
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material characterisation
Dispersion and morphology of polypropylene nanocomposites: Characterization based on a compact and flexible optical sensor R. Matadi Boumbimba a, b, M. Bouquey a, R. Muller a, *, L. Jourdainne b, B. Triki a, P. Hébraud c, P. Pfeiffer d a
Laboratoire d’Ingénierie des Polymères pour les Hautes Technologies, ECPM-LIPHT, University of Strasbourg, 25 rue Becquerel, 67087 Strasbourg, Cedex 2, France Institut de Mécanique des Fluides et des Solides, IMFS, University of Strasbourg, 2 rue Boussingault, 67000 Strasbourg, France c Institut de physique et chimie des Matériaux de Strasbourg, CNRS-UDS, University of Strasbourg, 23 rue du Loess, 67034 Strasbourg, France d Laboratoire des systèmes photoniques, University of Strasbourg, Boulevard Sébastien Brant, BP 10413, F-67412 Illkirch, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2012 Accepted 9 May 2012
A new on-line optical sensor based on light scattering, dedicated to real-time monitoring during processing of polymer blends and polymer nanocomposites, has been used to characterize dispersion and morphology of polypropylene based organoclay nanocomposites. Polypropylene matrix was mixed with various concentration of organo modified montmorillonite. The results given by optical measurements were compared to those of XRD, TEM and rheological characterizations. The results showed that the transmitted light intensity increases with increasing extent of exfoliation. At high filler concentration, the optical sensor allows a qualitative characterization of the nanocomposite morphology. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Optical sensor Transmitted light Scattered light Polymers Nanocomposites Melt mixing
1. Introduction In the field of nano-materials, nanocomposites have been intensively studied during recent decades, particularly polymer-based clay nanocomposites [1–6]. This is mainly because the introduction of a low amount of clay may lead to the enhancement of physical and thermal properties, such as higher modulus [7,8], high thermal stability [9–11] and excellent barrier properties against gas and water [12,13]. Improvement of performance largely depends on the spatial distribution of nanoparticles [1,3], the arrangements of intercalating polymer chains [14] and the interfacial interaction between the silicate layers and the polymer [15,16]. The greatest improvement in properties was observed for exfoliated specimens [17,18].
* Corresponding author. E-mail address: [email protected] (R. Muller). 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2012.05.002
One of the common approaches used to synthesize these materials is melt intercalation [17,19]. In this method, the polymer and the layered clays are mixed above the melting temperature of the polymer. The polymer-layered silicate structure formed during polymer melt intercalation depends on the thermodynamic interaction between the polymer and the silicate, as well as the diffusion of polymer chains from the bulk melt into the silicate interlayers. Because natural montmorillonite (MMT) clay is generally incompatible with polymers due to its hydrophilic nature, interactions between the montmorillonite and the polymer are often enhanced by organic modification of MMT via substitution of cations, for instance alkylammonium cations [3,20,21]. Various methods or tools are needed to characterize and identify the different structures of polymer nanocomposites in order to estimate the extent of clay particles exfoliation. Among these methods, one can mention transmission electron microscopy (TEM), X ray diffraction (XRD) [5] and melt rheology [5,22]. The main drawback of
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all these characterisation methods is that they are carried out off-line, which implies long feedback times to adjust the processing parameters of a production line. The longer the feedback time, the greater is the waste of material and energy. This drawback leads to the development of on- or in-line methods based [23,24] on NIR spectroscopy and ultrasonic measurements, as well as on ultrasonic, Raman or near-infrared spectroscopy. When the characterisation of the melt takes place inside the extruder, the method is called in-line. Due to the proximity of the screws, the lack of space and the difficulty of installing a sensor in a pressurized zone of the extruder, in-line methods are not always possible. In addition, the design of an in-line setup strongly depends on the type of extruder head or line. For those reasons on-line methods are often preferred: a small sample of the melt is taken through a by-pass to a zone where it is easier to install a sensor and to analyze the material. Few experiments based on the interaction between the visible light and organoclay nanocomposites have been reported in the literature. Small angle laser light scattering experiments (SALLS) were realized to monitor the effect of clay addition on the critical temperature of phase separation in a polymer/polymer blend. Light scattering patterns obtained for neat polymer blends were compared to those obtained for polymer blends with clay particles under different temperatures [25,26]. The same kind of experiments were used to monitor the effect of organoclay on the phase separation in a polymer/epoxy mixture [27]. The control of the optical quality is representative of the clay particle dispersion. Wang et al. [28] carried out some measurements on transmitted light in a BR/clay nanocomposite, and thus controlled the dispersion of particles. In a previous paper, we characterized the progress of dispersion of montmorillonite in polyLactic acid by comparing rheological behaviour and light attenuation [29]. A comparison between in-line dielectric spectroscopy and in-line optical measurements was published by Bur et al. [30,31]. These two sensors were integrated at the exit of a twin-screw extruder and enabled real-time monitoring of the exfoliation. To the best of our knowledge, these results are the only ones which associate an optical characterization method and an in-line or on- line design of the sensor. In a previous work, we presented a new on line optical sensor based on light scattering dedicated to realtime monitoring during processing of polymer blends and polymer nanocomposites (Light scattering) [32]. Our sensor presents several advantages: it is equipped with two openings for light intensity measurements, thus allowing measurement of both transmitted light (0 with respect to incident light) and light scattered at 90 (with respect to incident light). Additionally, this sensor is very compact and can be easily connected to many polymer processing devices when standard pressure transducer fittings are available, and can be used along production line. In this work we present a comparative study on the measurement of the extent of exfoliation using TEM, XRD, rheological measurements at low frequencies and light scattering measurements. The aim is to show the good ability of light scattering to measure the degree of exfoliation in polymer nanocomposite systems.
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2. Experimental 2.1. Materials Literature on melt mixed polymer/clay nanocomposites, reveals that improving the dispersion and exfoliation strongly depends on the type of mixer, organoclay, melt mixing temperature and rotation speed. For this reason, two series of PP based organoclay nanocomposite samples were used to study the sensitivity of scattering light measurements to the extent of exfoliation. One of them was obtained in our laboratory, and the second, for which a twin screw extruder, different organoclays and different melt mixing temperatures were used, was provided by Basell Polyolefins. A commercial polypropylene supplied by Basell Polyolefins in the form of pellets (PP Moplen HP500N) was used as the matrix. The nanocomposites were prepared by melt mixing of PP and organoclays. The first series of samples are mixtures of pure PP and a masterbach (Nanomax from Nanocor) composed of 50 wt% organoclay, 25 wt% polypropylene and 25 wt% maleic anhydride grafted polypropylene (pp-g-MA). These samples were prepared by melt-mixing using a BUSS Kneader operating at a temperature of 200 C and a rotation speed of 50 rpm. The second group of nanocomposites was prepared by Basell Polyolefins by melt mixing in a twin screw extruder, operating at a rotation speed of 50 rpm. This group is subdivided into two categories. The first category is composed by three materials; all obtained by melt mixing of a polymer (PP) and 7.7 wt% of Cloisite 20 (C20A). The only difference between these materials comes from the preparation temperature. The second category is composed by two materials obtained from Cloisite 15A and Delitte 67G (High modified montmorillonite from Laviosa chemica Mineraria) (D67G). Unlike samples obtained in our laboratory with high filler concentrations, much better exfoliation is observed for Basell samples (TEM results). Therefore, we used theses samples to show that our sensor is very sensitive to the extent of exfoliation, even for high fillers concentrations. The nanocomposite sample designations and the experimental conditions are listed in Table 1.
2.2. Characterization 2.2.1. Scattering light measurements In a previous work [32], we described a new on line optical sensor based on light scattering dedicated to realtime monitoring during processing of polymer blends and polymer nanocomposites. This sensor has been used to monitor the quality of a polymer blend or of a dispersion of nanofillers in a polymeric matrix during extrusion. It is equipped with three windows: two for light sensor setups (one for transmission and the other for 90 -scattering light) and one for the incident laser beam (see Fig. 1). Thus, both transmitted and scattered light can be simultaneously measured. Only a small fraction of the melt is taken through a by-pass for optical analysis. This by-pass is mounted just before the extruder die.
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Table 1 Nomenclature, formulation and preparation protocol of studied samples. Sample Formulation
Synthesis method
Melt temperature ( C)
Rotation speed (rpm)
Sample ID
PPþ0.5 wt% Nanomax PPþ1 wt% Nanomax PPþ3 wt% Nanomax PPþ6 wt% Nanomax PPþ7 wt% Nanomax PPþ8 wt% Nanomax PPþ7.7 wt% Cloisite 20A PPþ7.7 wt% Cloisite 20A PPþ7.7 wt% Cloisite 20A PPþ8.8 wt% Cloisite 15A PPþ8.8 wt% Delitte 67G
Melt Melt Melt Melt Melt Melt Melt Melt Melt Melt Melt
200 200 200 200 200 200 180 195 210 180 180
50 50 50 50 50 50 50 50 50 50 50
PPN-0.5 PPN-1 PPN-3 PPN-6 PPN-7 PPN-8 PPC1-7.7 PPC2-.7.7 PPC3-7.7 PPC4-8.8 PPD-8.8
mixing mixing mixing mixing mixing mixing mixing mixing mixing mixing mixing
(Buss kneader) (Buss kneader) (Buss kneader) (Buss kneader) (Buss kneader) (Buss kneader) (twin screw-extruder) (twin screw-extruder) (twin screw-extruder) (twin screw-extruder) (twin screw-extruder)
(Basell) (Basell) (Basell) (Basell) (Basell)
A single screw extruder (Fairex (Villeparisis, France). 30–24L/D compact C2) was used to carried out the optical characterizations. As mentioned before, the organoclay nanocomposites were prepared in a previous step either in a BUSS Kneader or in a twin-screw extruder, and the single screw extruder was only used to provide a continuous melt flow through the transducer for the optical characterization. The working temperature was 200 C, and the screw speed of the single screw extruder was fixed at 20 rpm. A typical test consists in first extruding pure PP for 200 s and then feeding the nanocomposite pellets into the empty hopper. In the experimental time scale, zero corresponds to the instant where the pure PP arrives at the optical sensor windows. Since the by-pass and the sensor are placed before the extrusion die and, due to the residence time of the material in the extruder, the optical signal detects the presence of the material after an experimental time of about 390 s. For light scattering analyses of the two series of Basell samples, nanocomposite pellets of the first sample was first
extruded, then the second sample pellets were fed into the empty hopper, followed finally by the third sample. 2.2.2. XRD, TEM and rheological characterization The samples used in the XRD, TEM and rheology analyses were recovered at the exit of the optical sensor. Diffraction patterns were obtained on a D5000 diffractometer, with CuKa radiation (l ¼ 1.54 Å). Transmission electron microscopy (TEM) was used to investigate the particle distribution in the nanocomposites. The samples used for TEM observations are obtained by using a Leica ultramicrotome equipped with a diamond knife. Ultra-thin sections of the nanocomposites with a thickness of approximately 50 nm were cut and observed. Dynamic rheological measurements were carried out on ARES-rheometer machine (TA Instrument, USA), over the frequency range from 100 Hz to 0.01 Hz and at a temperature of 200 C. Samples dimensions were 25 mm in diameter, and 1 mm in thickness. 3. Results and discussion
Fig. 1. Cross section of the on-line sensor: (a): Transmitted light; (b): Lens; (c): Scattered light; (d): Fibre collimator connector; (e): Incident light; (f): Quartz window; (g): Optical fibre connector.
TEM images exhibit good dispersion of the organoclay in the PP matrix, as shown in Fig. 2 for PPN-0.5, PPN-1, PPN3 and PPN-6. An almost exfoliated structure is observed for PPN-0.5, whereas partially exfoliated morphologies are found for higher concentrations. This good dispersion can be attributed to the presence of maleic anhydride which increases the compatibility between the PP matrix and the organoclay. The X-ray diffraction patterns in Fig. 3 are in agreement with the TEM observations for the fourth nanocomposite. In fact, the nanocomposite PPN-0.5 presents exfoliated structure, characterized by the disappearance of the XRD peak of the masterbatch. However, partially exfoliated morphology appears for nanocomposites with higher concentrations. On the XRD patterns, the partial exfoliation structure appears with the presence of the broad diffraction peaks. There is an increase in basal spacing from 22.8 Å for the masterbach to 28.2 Å and 25.6 Å for the nanocomposites PPN-3 and PPN-6, respectively. Fig. 4 gives the log G0 vs log u plots at 200 C for pure PP, PPN-0.5, PPN-1, PPN-3 and PPN-6. In Fig. 4 a very clear difference between pristine PP and PP nanocomposite can be observed, and values of G0 in nanocomposites are increased in the low-frequency region.
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Fig. 2. TEM images of PP Nanocor nanocomposites with various organoclay contents (a): PPN-0.5; (b) PPN-1; (c) PPN-3; (d) PPN-6.
Globally, the value of G0 increases with increasing organoclay loading. However, sample PPN-0.5 presents a higher modulus compared to PPN-1 and PPN-3. Only the sample (PPN-6) with 6 wt% presents a modulus higher than
that of PPN-0.5. For nanocomposites PPN-1 and PPN-3, TEM observations (Fig. 2) show the presence of some stacked organoclay platelets. The higher modulus shown in Fig. 4 for PPN-0.5 compared to PPN-1, PPN-3 is due to the fact that PPN-0.5 presents exfoliated morphology, while for PPN-1 and PPN-3 intercalated morphology is shown. In the intercalated structure, the organoclay layers would strongly interact with each other. The combination of effects between high concentration and intercalated morphology could explain why PPN-6 presents a higher modulus compared to that of PPN-0.5. This enhancement can be attributed to the formation of a physical network by the organoclay and the connecting chains. These results are in agreement with those given by both XRD and TEM observations, in which PPN-0.5 presents an exfoliated morphology and a partially exfoliated structure at concentrations above 0.5 wt%.
3.1. Correlation with online optical sensor measurements
Fig. 3. XRD patterns of the masterbach (Nanomax) and the nanocomposite at different organoclay concentrations.
Figs. 5 and 6 show, respectively, the transmitted and scattered light intensity as a function of experimental time for the neat PP and the PP/organoclay nanocomposite with different organoclay concentrations. The first 390 s on the time scale of these curves correspond to the signal for neat
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Fig. 6. Measurements of scattered light intensity of pristine PP and PP/ organoclay nanocomposites with different concentrations. Fig. 4. Frequency dependence of G0 measured at 200 C for PP/organoclay nanocomposite with various organoclay concentrations.
PP before the feeding of the nanocomposite pellets. Then, after a transient regime corresponding to the residence time distribution in the extruder, a permanent regime is reached when pure nanocomposite flows through the transducer. In this regime, the scattered intensity increases and the transmitted intensity decreases with organoclay concentration. For the pristine PP, one can see that both signals are very flat and the repeatability is very good. We can also note that the optical sensor is very sensitive to the organoclay concentration. The curves for transmitted light (Fig. 5) show that the light intensity decreases with increasing organoclay concentration. At a concentration above 3 wt%, the transmitted light intensity completely vanishes. This decrease in light intensity is probably due to the reduction of optical transparency caused by the presence of organoclay. As expected, the signal shows an increase of scattered light intensity (Fig. 6) with increasing organoclay concentration, due to the increasing organoclay particle size and the increasing of number of scattered particles. In Fig. 7, the scattered light intensity measured for high filler concentrations is shown as a function of experimental time. As previously observed, in the long time
Fig. 5. Measurements of transmitted light intensity of pristine PP and PP/ organoclay nanocomposite with different concentrations.
permanent regime the scattered intensity increases with increasing organoclay concentration. The curve representing PPN-7 is above that of PPN-6. Nevertheless, the PPN-8 curve seems to be below that of PPN-7. The very stable signal obtained for the nanocomposites can lead to the conclusion that the organoclay is well dispersed in the PP matrix. The transient regime between the virgin PP matrix signal and the stationary regime at long times presents mixed behaviour: during the first 80 s, a quasi-linear decrease (resp. increase) of the transmitted (resp. scattered) intensity is observed, followed by a slower decrease of both the scattered and the transmitted intensities. This behaviour is observed for all studied concentrations. In order to compare the intensity evolutions during time for all concentrations, we consider the normalized intensities given by the following equation:
Inor ðtÞ ¼
IðtÞ Iðt ¼ 0Þ Iðt/NÞ Iðt ¼ 0Þ
(1)
For low volume fractions (between 0.5 wt% and 3 wt%), the initial evolution of the normalized intensities (Fig. 8) during the first 80 s of the transient regime, perfectly
Fig. 7. Measurements of scattered light intensity of PP organoclay nanocomposites at different concentrations.
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Fig. 8. Evolution of the normalized scattered intensities as a function of time.
superimpose. At high organoclay concentrations (from 6 wt % to 8 wt%), the transient regime presents a large time spectrum, and a stationary regime, for which there is also superposition of curves appearing at long times. In the transient regime, there is both pure PP and nanocomposite present in different proportions. The observed gradients of concentration are thus due to residence times distribution, and not due to diffusion of the charges inside the polymeric matrix. On the contrary, the longer time decrease of both the scattered and the transmitted intensities above 3 wt% do not yield superimposed curves. Light is more scattered and less transmitted than the master curve obtained at the lowest organoclay fractions. Due to the incompleteness of our scattering measurements (we only collect light at 90 and in the forward direction), we cannot assert the origin of the excess of scattering and lack of transmission for all the volume fractions. Nevertheless, we must admit that some transient change in the system occurs at times of the order
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Fig. 10. Evolution of volume fraction of the aggregates of PPN samples at low volume fractions, as a function of the experimental time.
of 150 s after the beginning of the appearance of inhomogeneities in the scattering volume, due to the distribution of residence times. The excess of scattering could, therefore, be due to the existence of concentration gradients. It could also be due to a slow change in the aggregation state of the organoclay particles after mixing. Both phenomena would lead to an increase of the scattered signal. Fig. 9 shows the evolution of scattered intensity (measured at t infinity) as a function of organoclay concentration. For the first three concentrations, one can observe the linear evolution of scattered intensity as a consequence of the presence of single light scattering behaviour. For concentrations above 3 wt%, a non linear behaviour appears. This behaviour is mainly due to multiple scattering. Indeed, a posteriori observation of the mixture reveals that the most concentrated sample is very turbid. In this case, the optical path rapidly increases with the concentration. As a consequence, both scattering and absorption increase with the volume fraction faster than a linear behaviour, as observed for times ranging from 80 to 200 s after the beginning of the mixture. As absorption and single light scattering would lead to affine behaviour, this proves that multiple scattering plays a role for the most concentrated sample. In summary, the results show the presence of two regimes: A simple scattering regime for organoclay concentration less than 3 wt% and a multiple scattering regime for organoclay concentration above 3 wt%. 3.1.1. Simple scattering regime Considering the transmitted light intensity, taken at the simple scattering regime, and the modified Beer’s law [31], one can estimate the extent of exfoliation of the nanocomposites. The modified Beer law is given by the following equation:
I ¼ I0 ebf Fig. 9. Evolution of the scattered intensity in the long time permanent regime as a function of organoclay content. The straight line is an affine fit to the first three concentrations.
(2)
where I is the transmitted light intensity, b an attenuation coefficient proportional to the difference in polarizability between clay particles and PP matrix, and f the volume
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Fig. 11. Measurements of scattered light intensity of Basell samples in the multiple light scattering regime. (a): PPC1-7.7, PPC2-7.7 and PPC3-7.7. (b): PPC4-8.8 and PPD-8.8.
fraction of aggregate clay particles that scatter light [31]. We assume, as in the work of Bur et al. [31], that the pristine PP matrix represents 0% aggregates or 100% exfoliated filler. The other extreme is 100% aggregate with no exfoliation. In our case, it corresponds to PPN-6, the
concentration at which the transmitted light intensity is equal to zero. The attenuation coefficient b is assumed to be a linear function of the difference in polarizability between PP matrix and organoclay, and is given by the following expression:
Fig. 12. TEM images of Basell Samples. (a): PPC1-7.7. (b): PPC2-7.7. (c): PPC3-7.7.
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Fig. 13. TEM images of Basell samples. (a):PPC4-8.8. (b): PPD-8.8.
b ¼ K D n2
where K is a constant of proportionality and D(n2) is the difference in the square of index of refraction between matrix and organoclay. A value of K was obtained from Eq. (2) using I/I0 data for PPN-6 (no exfoliation). To compute the volume fraction of the organoclay as a function of the organoclay weight content in wt% (w), the following relation is used:
fc ¼ wðw þ ð1 wÞrc =rM Þ1 Here, rC and rM are the densities of the organoclay fillers and the PP matrix, respectively. The volume fraction of the organoclay is denoted by fC. Eq. (2) was used to calculate f in terms of transmitted light intensity. The refraction index of PP was taken equals to nPP ¼ 1.53. From the work of Dantal et al. [33,34], the refraction index of nanocomposites was founded to linearly depend on organoclay concentration. We
took a refraction index of n0.5 wt% ¼ 1.67 for PPN-0.5. The other refraction index was estimated from the equation n ¼ 0.28fc þ 1.53 [31]. We also took rM ¼ 1.19 cm3/g and rC ¼ 2.83 cm3/g. Fig. 10 shows the evolution of volume fraction of aggregates as a function of time. The present results show that the volume fraction of aggregate increases from 0.17% for PPN-0.5 to 0.87% and 2.8% for PPN-1 and PPN-3, respectively. These plots are the quantitative indicator of the extent of exfoliation. Obviously, the nanocomposite PPN-0.5 is more exfoliated than both PPN-1 and PPN-3. According to the assumptions made for calculations, we can conclude that the nanocomposites PPN-0.5, PPN-1, and PPN-3 present extent of exfoliation equal to about 94%, 70% and 7%, respectively. 3.1.2. Multiple scattering regime As mentioned above, in the multiple scattering regime, we observed that the transmitted light intensity completely
Fig. 14. Frequency dependence of G0 measured at 200 C for Basell samples. (a): effect of the melt mixing temperature. (b): effect of the type of organoclay.
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vanishes. Our investigations were, therefore, restricted to scattered light intensity. As we have said before, these spectra clearly give us an idea of both organoclay concentration and dispersion in PP matrix. Even if it is not possible to estimate quantitatively the volume of aggregates in this case, the scattered light intensity appears to be sensitive to the nanocomposite morphology. To show this, we used the two groups of PP based organoclay nanocomposites, developed by Basell, as previously indicated. Fig. 11 shows the scattered light results obtained for PPC1-7.7, PPC2-7.7, PPC3-7.7, PPC4-8.8 and PPD-8.8. According to these results, one can observe that the scattered light intensity decreases as the preparation temperature increases. The increasing preparation temperature seems to reduce the size of particles in the matrix. This result is a well known observation in the field of nanocomposite materials. In fact, the increase of temperature can lead to enhance the extent of exfoliation, or contribute to create a higher number of large aggregates. TEM images of PPC1-7.7, PPC2-7.7 and PPC3-7.7 (Fig. 12), show that there is no change in the morphology of these materials. One cannot establish a clear distinction between the sample morphologies. For the second group, the scattering light results (Fig. 11b) depict an increase of scattered light intensity in the presence of D67G, as a consequence of large particles size compared to those obtained in the presence of Cloisite 15A. TEM observations in this case (Fig. 13) do not show any difference in morphology for those materials. Our results are in agreement with rheological characterizations (Fig. 14). In fact, the G0 modulus of PPC4-8.8 is much higher than that of PPD-8.8. The storage modulus G0 increases more significantly, particularly in the low frequency region. This is evidence of the partial exfoliation. 4. Conclusions Dispersion and morphology of polypropylene organoclay nanocomposites have been studied using a new optical sensor, dedicated to real-time monitoring and based on light scattering. Our results have been compared to those of conventional characterization methods. Validation experiments proved the quality of results in terms of signal repeatability and sensitivity to organoclay particle concentration and nanocomposite structure, respectively. XRD, TEM and rheological characterizations showed that the organoclay is well dispersed in the PP matrix, and the nanocomposites mainly present a partially exfoliated morphology. As the organoclay concentration increases, transmitted light intensity decreases but scattered light intensity increases. At concentrations above 3 wt%, the transmitted light intensity completely vanishes. The investigation of transmitted and scattered light intensity leads to separate two regimes. The first, for organoclay concentration from 0.5 wt% to 3 wt%, corresponds to a simple scattering regime. The second regime appears at concentrations up to 3 wt%, and corresponds to a multiple scattering regime. A modified Beer’s law used in the simple scattering regime, allow us to show that the nanocomposite PP/Nanocor 0.5 wt% is more exfoliated than PP/ Nanocor 1 wt%, and PP/Nanocor 3 wt%. In the multiple
scattering regime, our device seems to give an indication of the nanocomposite morphology, where TEM observations show no difference. Our obtained results in this regime are in agreement with rheological characterizations. Acknowledgements This study was carried out in the frame IP European research program “MULTIHYBRIDS”, No. 026685-2, in the 6th Framework Program. We also thank Mr Christophe MELART and Mr Christophe SUTTER for their help and their availability. References [1] E.P. Giannelis, Polymer layered silicate nanocomposites, Advanced Materials 8 (1996) 29–35. [2] I. Hejazi, F. Sharif, H. Garmabi, Effect of material and processing parameters on mechanical properties of polypropylene/ethylenepropylene-diene-monomer/clay nanocomposites, Materials and Design 32 (2011) 3803–3809. [3] R. Matadi, A. Makradi, S. Ahzi, J.G. Sieffert, S. Etienne, D. Rush, R. Vaudemont, R. Muller, M. Bouquey, Preparation, structural characterization, and thermomechanical properties of poly(methyl methacrylate)/organoclay nanocomposites by melt intercalation, Journal of Nanoscience and Nanotechnology 9 (2009) 2923–2930. [4] J.K. Mishra, K.J. Hwang, C.S. Ha, Preparation, mechanical and rheological properties of a thermoplastic polyolefin (TPO)/organoclay nanocomposite with reference to the effect of maleic anhydride modified polypropylene as a compatibilizer, Polymer 46 (2005) 1995–2002. [5] S. Zhu, J. Chen, Y. Zuo, H. Li, Y. Cao, Montmorillonite/polypropylene nanocomposites: mechanical properties, crystallization and rheological behaviours, Applied Clay Science 52 (2011) 171–178. [6] R. Matadi Boumbimba, K. Wang, N. Bahlouli, S. Ahzi, Y. Rémond, F. Addiego, Experimental investigation and micromechanical modeling of high strain rate compressive yield stress of a melt mixing, polypropylene organoclay nanocomposites, Mechanics of Materials (2012). doi:10.1016/j.mechmat.2012.04.006. [7] R. Matadi, O. Gueguen, S. Ahzi, J. Gracio, R. Muller, D. Ruch, Investigation of the stiffness and yield behaviour of melt-intercalated poly(methyl methacrylate)/organoclay nanocomposites: characterisation and modelling, Journal of Nanoscience and Nanotechnology 10 (2010) 2956–2961. [8] K. Wang, R.M. Boumbimba, N. Bahlouli, S. Ahzi, R. Muller, M. Bouquey, Dynamic compressive behavior of a melt mixed polypropylene/organoclay nanocomposites, Journal of Engineering Materials and Technology 134 (issue 1) (2012) 010905. doi:10.1115/ 1.4005420. [9] J.W. Gilman, Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites, Applied Clay Science 15 (1999) 31–49. [10] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, O. Kamigaito, Mechanical properties of nylon 6-clay hybrid, Journal of Materials Research 8 (1993) 1185–1189. [11] S. Kumar, J.P. Jog, U. Natarajan, Preparation and characterization of poly(methyl methacrylate)-clay nanocomposites via melt intercalation: the effect of organoclay on the structure and thermal properties, Journal of Applied Polymer Science 89 (2003) 1186–1194. [12] J.H. Chang, Y.U. An, Nanocomposites of polyurethane with various organoclays: thermomechanical properties, morphology, and gas permeability, Journal of Polymer Science, Part B: Polymer Physics 40 (2002) 670–677. [13] A.R. Manninen, H.E. Naguib, A.V. Nawaby, M. Day, CO2 sorption and diffusion in polymethyl methacrylate-clay nanocomposites, Polymer Engineering and Science 45 (2005) 904–914. [14] J.H. Chang, K.M. Park, D. Cho, H.S. Yang, K.J. Ihn, Preparation and characterization of polyimide nanocomposites with different organo-montmorillonites, Polymer Engineering and Science 41 (2001) 1514–1520. [15] W.S. Chow, A. Abu Bakar, Z.A. Mohd Ishak, J. Karger-Kocsis, U.S. Ishiaku, Effect of maleic anhydride-grafted ethylene-propylene rubber on the mechanical, rheological and morphological properties of organoclay reinforced polyamide 6/polypropylene nanocomposites, European Polymer Journal 41 (2005) 687–696.
R. Matadi Boumbimba et al. / Polymer Testing 31 (2012) 800–809 [16] A. Somwangthanaroj, W. Ubankhlong, W. Tanthapanichakoon, Solid-state mechanical properties of polypropylene/nylon 6/clay nanocomposites, Journal of Applied Polymer Science 118 (2010) 538–546. [17] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Progress in Polymer Science (Oxford) 28 (2003) 1539–1641. [18] L.M. Wang, Preparation and Characterization of Polypropylene/Clay Nanocomposites (2011) pp. 1584–1587. [19] R.M. Boumbimba, S. Ahzi, N. Bahlouli, D. Ruch, J. Gracio, Dynamic mechanical properties of PMMA/organoclay nanocomposite: experiments and modeling, Journal of Engineering Materials and Technology 133 (issue 3) (2011) 030908. doi:10.1115/1.4004052. [20] S.M. Mohomane, V. Djokovic, S. Thomas, A.S. Luyt, Polychloroprene nanocomposites filled with different organically modified clays: morphology, thermal degradation and stress relaxation behaviour, Polymer Testing 30 (2011) 585–593. [21] L. Százdi, A. Pozsgay, B. Pukánszky, Factors and processes influencing the reinforcing effect of layered silicates in polymer nanocomposites, European Polymer Journal 43 (2007) 345–359. [22] R. Wagener, T.J.G. Reisinger, A rheological method to compare the degree of exfoliation of nanocomposites, Polymer 44 (2003) 7513– 7518. [23] D. Fischer, K. Sahre, M. Abdelrhim, B. Voit, V.B. Sadhu, J. Pionteck, H. Komber, J. Hutschenreuter, Process monitoring of polymers by inline ATR-IR, NIR and Raman spectroscopy and ultrasonic measurements, Comptes Rendus Chimie 9 (2006) 1419–1424. [24] D. Fischer, D. Pospiech, U. Scheler, R. Navarro, M. Messori, P. Fabbri, Monitoring of the sol-gel synthesis of organic-inorganic hybrids by FTIR transmission, FTIR/ATR, NIR and Raman spectroscopy, Macromolecular Symposia 265 (2008) 134–143. [25] K. El-Mabrouk, S. Vaudreuil, A. Zeghloul, M. Bousmina, Effects of shear on phase-separation in polystyrene/poly(vinyl methyl ether)/
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
809
organoclay nanocomposite, Journal of Nanoscience and Nanotechnology 8 (2008) 1895–1900. G. Schlatter, C. Serra, M. Bouquey, R. Muller, J. Terrisse, Online light scattering measurements: a method to assess morphology development of polymer blends in a twin-screw extruder, Polymer Engineering & Science 42 (2002) 1965–1975. M. Peng, D. Li, Y. Chen, Q. Zheng, Effect of an organoclay on the reaction-induced phase-separation kinetics and morphology of a poly(ether imide)/epoxy mixture, Journal of Applied Polymer Science 104 (2007) 1205–1214. S. Wang, Y. Zhang, W. Ren, H. Lin, Morphology, mechanical and optical properties of transparent BR/clay nanocomposites, Polymer Testing 24 (2005) 766–774. N.V. Pogodina, C. Cerclé, L. Avérous, R. Thomann, M. Bouquey, R. Muller, Processing and characterization of biodegradable polymer nanocomposites: detection of dispersion state, Rheologica Acta 47 (2008) 543–553. A.J. Bur, S.C. Roth, Y.H. Lee, M. McBrearty, A dielectric slit die for inline monitoring of polymer compounding, Review of Scientific Instruments 75 (2004) 1103–1109. A.J. Bur, Y.H. Lee, S.C. Roth, P.R. Start, Measuring the extent of exfoliation in polymer/clay nanocomposites using real-time process monitoring methods, Polymer 46 (2005) 10908–10918. L. Jourdainne, R. Matadi Boumbimba, B. Triki, M. Bouquey, R. Muller, P. Hébraud, P. Pfeiffer, Compact and flexible optical sensor designed for on-line monitoring, International Journal of Material Forming 5 (2012) 1–9. B.R. Dantal, S. Gupta, A. Saigal, M.A. Zimmerman, I. Georgakoudi, Characterization of a New Titanium Dioxide-polymer Nanocomposite Material for Electronic Packaging Using Light Scattering Spectroscopy, in, Boston, MA (2009) pp. 1147–1150. Corradi Laure, Cervera Régis, Les nanoparticules dans le verre: application aux guides d’ondes, in: Rapport de Master (2007).
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Material characterisation
Dispersion and morphology of polypropylene nanocomposites: Characterization based on a compact and flexible optical sensor R. Matadi Boumbimba a, b, M. Bouquey a, R. Muller a, *, L. Jourdainne b, B. Triki a, P. Hébraud c, P. Pfeiffer d a
Laboratoire d’Ingénierie des Polymères pour les Hautes Technologies, ECPM-LIPHT, University of Strasbourg, 25 rue Becquerel, 67087 Strasbourg, Cedex 2, France Institut de Mécanique des Fluides et des Solides, IMFS, University of Strasbourg, 2 rue Boussingault, 67000 Strasbourg, France c Institut de physique et chimie des Matériaux de Strasbourg, CNRS-UDS, University of Strasbourg, 23 rue du Loess, 67034 Strasbourg, France d Laboratoire des systèmes photoniques, University of Strasbourg, Boulevard Sébastien Brant, BP 10413, F-67412 Illkirch, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2012 Accepted 9 May 2012
A new on-line optical sensor based on light scattering, dedicated to real-time monitoring during processing of polymer blends and polymer nanocomposites, has been used to characterize dispersion and morphology of polypropylene based organoclay nanocomposites. Polypropylene matrix was mixed with various concentration of organo modified montmorillonite. The results given by optical measurements were compared to those of XRD, TEM and rheological characterizations. The results showed that the transmitted light intensity increases with increasing extent of exfoliation. At high filler concentration, the optical sensor allows a qualitative characterization of the nanocomposite morphology. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Optical sensor Transmitted light Scattered light Polymers Nanocomposites Melt mixing
1. Introduction In the field of nano-materials, nanocomposites have been intensively studied during recent decades, particularly polymer-based clay nanocomposites [1–6]. This is mainly because the introduction of a low amount of clay may lead to the enhancement of physical and thermal properties, such as higher modulus [7,8], high thermal stability [9–11] and excellent barrier properties against gas and water [12,13]. Improvement of performance largely depends on the spatial distribution of nanoparticles [1,3], the arrangements of intercalating polymer chains [14] and the interfacial interaction between the silicate layers and the polymer [15,16]. The greatest improvement in properties was observed for exfoliated specimens [17,18].
* Corresponding author. E-mail address: [email protected] (R. Muller). 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2012.05.002
One of the common approaches used to synthesize these materials is melt intercalation [17,19]. In this method, the polymer and the layered clays are mixed above the melting temperature of the polymer. The polymer-layered silicate structure formed during polymer melt intercalation depends on the thermodynamic interaction between the polymer and the silicate, as well as the diffusion of polymer chains from the bulk melt into the silicate interlayers. Because natural montmorillonite (MMT) clay is generally incompatible with polymers due to its hydrophilic nature, interactions between the montmorillonite and the polymer are often enhanced by organic modification of MMT via substitution of cations, for instance alkylammonium cations [3,20,21]. Various methods or tools are needed to characterize and identify the different structures of polymer nanocomposites in order to estimate the extent of clay particles exfoliation. Among these methods, one can mention transmission electron microscopy (TEM), X ray diffraction (XRD) [5] and melt rheology [5,22]. The main drawback of
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all these characterisation methods is that they are carried out off-line, which implies long feedback times to adjust the processing parameters of a production line. The longer the feedback time, the greater is the waste of material and energy. This drawback leads to the development of on- or in-line methods based [23,24] on NIR spectroscopy and ultrasonic measurements, as well as on ultrasonic, Raman or near-infrared spectroscopy. When the characterisation of the melt takes place inside the extruder, the method is called in-line. Due to the proximity of the screws, the lack of space and the difficulty of installing a sensor in a pressurized zone of the extruder, in-line methods are not always possible. In addition, the design of an in-line setup strongly depends on the type of extruder head or line. For those reasons on-line methods are often preferred: a small sample of the melt is taken through a by-pass to a zone where it is easier to install a sensor and to analyze the material. Few experiments based on the interaction between the visible light and organoclay nanocomposites have been reported in the literature. Small angle laser light scattering experiments (SALLS) were realized to monitor the effect of clay addition on the critical temperature of phase separation in a polymer/polymer blend. Light scattering patterns obtained for neat polymer blends were compared to those obtained for polymer blends with clay particles under different temperatures [25,26]. The same kind of experiments were used to monitor the effect of organoclay on the phase separation in a polymer/epoxy mixture [27]. The control of the optical quality is representative of the clay particle dispersion. Wang et al. [28] carried out some measurements on transmitted light in a BR/clay nanocomposite, and thus controlled the dispersion of particles. In a previous paper, we characterized the progress of dispersion of montmorillonite in polyLactic acid by comparing rheological behaviour and light attenuation [29]. A comparison between in-line dielectric spectroscopy and in-line optical measurements was published by Bur et al. [30,31]. These two sensors were integrated at the exit of a twin-screw extruder and enabled real-time monitoring of the exfoliation. To the best of our knowledge, these results are the only ones which associate an optical characterization method and an in-line or on- line design of the sensor. In a previous work, we presented a new on line optical sensor based on light scattering dedicated to realtime monitoring during processing of polymer blends and polymer nanocomposites (Light scattering) [32]. Our sensor presents several advantages: it is equipped with two openings for light intensity measurements, thus allowing measurement of both transmitted light (0 with respect to incident light) and light scattered at 90 (with respect to incident light). Additionally, this sensor is very compact and can be easily connected to many polymer processing devices when standard pressure transducer fittings are available, and can be used along production line. In this work we present a comparative study on the measurement of the extent of exfoliation using TEM, XRD, rheological measurements at low frequencies and light scattering measurements. The aim is to show the good ability of light scattering to measure the degree of exfoliation in polymer nanocomposite systems.
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2. Experimental 2.1. Materials Literature on melt mixed polymer/clay nanocomposites, reveals that improving the dispersion and exfoliation strongly depends on the type of mixer, organoclay, melt mixing temperature and rotation speed. For this reason, two series of PP based organoclay nanocomposite samples were used to study the sensitivity of scattering light measurements to the extent of exfoliation. One of them was obtained in our laboratory, and the second, for which a twin screw extruder, different organoclays and different melt mixing temperatures were used, was provided by Basell Polyolefins. A commercial polypropylene supplied by Basell Polyolefins in the form of pellets (PP Moplen HP500N) was used as the matrix. The nanocomposites were prepared by melt mixing of PP and organoclays. The first series of samples are mixtures of pure PP and a masterbach (Nanomax from Nanocor) composed of 50 wt% organoclay, 25 wt% polypropylene and 25 wt% maleic anhydride grafted polypropylene (pp-g-MA). These samples were prepared by melt-mixing using a BUSS Kneader operating at a temperature of 200 C and a rotation speed of 50 rpm. The second group of nanocomposites was prepared by Basell Polyolefins by melt mixing in a twin screw extruder, operating at a rotation speed of 50 rpm. This group is subdivided into two categories. The first category is composed by three materials; all obtained by melt mixing of a polymer (PP) and 7.7 wt% of Cloisite 20 (C20A). The only difference between these materials comes from the preparation temperature. The second category is composed by two materials obtained from Cloisite 15A and Delitte 67G (High modified montmorillonite from Laviosa chemica Mineraria) (D67G). Unlike samples obtained in our laboratory with high filler concentrations, much better exfoliation is observed for Basell samples (TEM results). Therefore, we used theses samples to show that our sensor is very sensitive to the extent of exfoliation, even for high fillers concentrations. The nanocomposite sample designations and the experimental conditions are listed in Table 1.
2.2. Characterization 2.2.1. Scattering light measurements In a previous work [32], we described a new on line optical sensor based on light scattering dedicated to realtime monitoring during processing of polymer blends and polymer nanocomposites. This sensor has been used to monitor the quality of a polymer blend or of a dispersion of nanofillers in a polymeric matrix during extrusion. It is equipped with three windows: two for light sensor setups (one for transmission and the other for 90 -scattering light) and one for the incident laser beam (see Fig. 1). Thus, both transmitted and scattered light can be simultaneously measured. Only a small fraction of the melt is taken through a by-pass for optical analysis. This by-pass is mounted just before the extruder die.
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Table 1 Nomenclature, formulation and preparation protocol of studied samples. Sample Formulation
Synthesis method
Melt temperature ( C)
Rotation speed (rpm)
Sample ID
PPþ0.5 wt% Nanomax PPþ1 wt% Nanomax PPþ3 wt% Nanomax PPþ6 wt% Nanomax PPþ7 wt% Nanomax PPþ8 wt% Nanomax PPþ7.7 wt% Cloisite 20A PPþ7.7 wt% Cloisite 20A PPþ7.7 wt% Cloisite 20A PPþ8.8 wt% Cloisite 15A PPþ8.8 wt% Delitte 67G
Melt Melt Melt Melt Melt Melt Melt Melt Melt Melt Melt
200 200 200 200 200 200 180 195 210 180 180
50 50 50 50 50 50 50 50 50 50 50
PPN-0.5 PPN-1 PPN-3 PPN-6 PPN-7 PPN-8 PPC1-7.7 PPC2-.7.7 PPC3-7.7 PPC4-8.8 PPD-8.8
mixing mixing mixing mixing mixing mixing mixing mixing mixing mixing mixing
(Buss kneader) (Buss kneader) (Buss kneader) (Buss kneader) (Buss kneader) (Buss kneader) (twin screw-extruder) (twin screw-extruder) (twin screw-extruder) (twin screw-extruder) (twin screw-extruder)
(Basell) (Basell) (Basell) (Basell) (Basell)
A single screw extruder (Fairex (Villeparisis, France). 30–24L/D compact C2) was used to carried out the optical characterizations. As mentioned before, the organoclay nanocomposites were prepared in a previous step either in a BUSS Kneader or in a twin-screw extruder, and the single screw extruder was only used to provide a continuous melt flow through the transducer for the optical characterization. The working temperature was 200 C, and the screw speed of the single screw extruder was fixed at 20 rpm. A typical test consists in first extruding pure PP for 200 s and then feeding the nanocomposite pellets into the empty hopper. In the experimental time scale, zero corresponds to the instant where the pure PP arrives at the optical sensor windows. Since the by-pass and the sensor are placed before the extrusion die and, due to the residence time of the material in the extruder, the optical signal detects the presence of the material after an experimental time of about 390 s. For light scattering analyses of the two series of Basell samples, nanocomposite pellets of the first sample was first
extruded, then the second sample pellets were fed into the empty hopper, followed finally by the third sample. 2.2.2. XRD, TEM and rheological characterization The samples used in the XRD, TEM and rheology analyses were recovered at the exit of the optical sensor. Diffraction patterns were obtained on a D5000 diffractometer, with CuKa radiation (l ¼ 1.54 Å). Transmission electron microscopy (TEM) was used to investigate the particle distribution in the nanocomposites. The samples used for TEM observations are obtained by using a Leica ultramicrotome equipped with a diamond knife. Ultra-thin sections of the nanocomposites with a thickness of approximately 50 nm were cut and observed. Dynamic rheological measurements were carried out on ARES-rheometer machine (TA Instrument, USA), over the frequency range from 100 Hz to 0.01 Hz and at a temperature of 200 C. Samples dimensions were 25 mm in diameter, and 1 mm in thickness. 3. Results and discussion
Fig. 1. Cross section of the on-line sensor: (a): Transmitted light; (b): Lens; (c): Scattered light; (d): Fibre collimator connector; (e): Incident light; (f): Quartz window; (g): Optical fibre connector.
TEM images exhibit good dispersion of the organoclay in the PP matrix, as shown in Fig. 2 for PPN-0.5, PPN-1, PPN3 and PPN-6. An almost exfoliated structure is observed for PPN-0.5, whereas partially exfoliated morphologies are found for higher concentrations. This good dispersion can be attributed to the presence of maleic anhydride which increases the compatibility between the PP matrix and the organoclay. The X-ray diffraction patterns in Fig. 3 are in agreement with the TEM observations for the fourth nanocomposite. In fact, the nanocomposite PPN-0.5 presents exfoliated structure, characterized by the disappearance of the XRD peak of the masterbatch. However, partially exfoliated morphology appears for nanocomposites with higher concentrations. On the XRD patterns, the partial exfoliation structure appears with the presence of the broad diffraction peaks. There is an increase in basal spacing from 22.8 Å for the masterbach to 28.2 Å and 25.6 Å for the nanocomposites PPN-3 and PPN-6, respectively. Fig. 4 gives the log G0 vs log u plots at 200 C for pure PP, PPN-0.5, PPN-1, PPN-3 and PPN-6. In Fig. 4 a very clear difference between pristine PP and PP nanocomposite can be observed, and values of G0 in nanocomposites are increased in the low-frequency region.
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Fig. 2. TEM images of PP Nanocor nanocomposites with various organoclay contents (a): PPN-0.5; (b) PPN-1; (c) PPN-3; (d) PPN-6.
Globally, the value of G0 increases with increasing organoclay loading. However, sample PPN-0.5 presents a higher modulus compared to PPN-1 and PPN-3. Only the sample (PPN-6) with 6 wt% presents a modulus higher than
that of PPN-0.5. For nanocomposites PPN-1 and PPN-3, TEM observations (Fig. 2) show the presence of some stacked organoclay platelets. The higher modulus shown in Fig. 4 for PPN-0.5 compared to PPN-1, PPN-3 is due to the fact that PPN-0.5 presents exfoliated morphology, while for PPN-1 and PPN-3 intercalated morphology is shown. In the intercalated structure, the organoclay layers would strongly interact with each other. The combination of effects between high concentration and intercalated morphology could explain why PPN-6 presents a higher modulus compared to that of PPN-0.5. This enhancement can be attributed to the formation of a physical network by the organoclay and the connecting chains. These results are in agreement with those given by both XRD and TEM observations, in which PPN-0.5 presents an exfoliated morphology and a partially exfoliated structure at concentrations above 0.5 wt%.
3.1. Correlation with online optical sensor measurements
Fig. 3. XRD patterns of the masterbach (Nanomax) and the nanocomposite at different organoclay concentrations.
Figs. 5 and 6 show, respectively, the transmitted and scattered light intensity as a function of experimental time for the neat PP and the PP/organoclay nanocomposite with different organoclay concentrations. The first 390 s on the time scale of these curves correspond to the signal for neat
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Fig. 6. Measurements of scattered light intensity of pristine PP and PP/ organoclay nanocomposites with different concentrations. Fig. 4. Frequency dependence of G0 measured at 200 C for PP/organoclay nanocomposite with various organoclay concentrations.
PP before the feeding of the nanocomposite pellets. Then, after a transient regime corresponding to the residence time distribution in the extruder, a permanent regime is reached when pure nanocomposite flows through the transducer. In this regime, the scattered intensity increases and the transmitted intensity decreases with organoclay concentration. For the pristine PP, one can see that both signals are very flat and the repeatability is very good. We can also note that the optical sensor is very sensitive to the organoclay concentration. The curves for transmitted light (Fig. 5) show that the light intensity decreases with increasing organoclay concentration. At a concentration above 3 wt%, the transmitted light intensity completely vanishes. This decrease in light intensity is probably due to the reduction of optical transparency caused by the presence of organoclay. As expected, the signal shows an increase of scattered light intensity (Fig. 6) with increasing organoclay concentration, due to the increasing organoclay particle size and the increasing of number of scattered particles. In Fig. 7, the scattered light intensity measured for high filler concentrations is shown as a function of experimental time. As previously observed, in the long time
Fig. 5. Measurements of transmitted light intensity of pristine PP and PP/ organoclay nanocomposite with different concentrations.
permanent regime the scattered intensity increases with increasing organoclay concentration. The curve representing PPN-7 is above that of PPN-6. Nevertheless, the PPN-8 curve seems to be below that of PPN-7. The very stable signal obtained for the nanocomposites can lead to the conclusion that the organoclay is well dispersed in the PP matrix. The transient regime between the virgin PP matrix signal and the stationary regime at long times presents mixed behaviour: during the first 80 s, a quasi-linear decrease (resp. increase) of the transmitted (resp. scattered) intensity is observed, followed by a slower decrease of both the scattered and the transmitted intensities. This behaviour is observed for all studied concentrations. In order to compare the intensity evolutions during time for all concentrations, we consider the normalized intensities given by the following equation:
Inor ðtÞ ¼
IðtÞ Iðt ¼ 0Þ Iðt/NÞ Iðt ¼ 0Þ
(1)
For low volume fractions (between 0.5 wt% and 3 wt%), the initial evolution of the normalized intensities (Fig. 8) during the first 80 s of the transient regime, perfectly
Fig. 7. Measurements of scattered light intensity of PP organoclay nanocomposites at different concentrations.
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Fig. 8. Evolution of the normalized scattered intensities as a function of time.
superimpose. At high organoclay concentrations (from 6 wt % to 8 wt%), the transient regime presents a large time spectrum, and a stationary regime, for which there is also superposition of curves appearing at long times. In the transient regime, there is both pure PP and nanocomposite present in different proportions. The observed gradients of concentration are thus due to residence times distribution, and not due to diffusion of the charges inside the polymeric matrix. On the contrary, the longer time decrease of both the scattered and the transmitted intensities above 3 wt% do not yield superimposed curves. Light is more scattered and less transmitted than the master curve obtained at the lowest organoclay fractions. Due to the incompleteness of our scattering measurements (we only collect light at 90 and in the forward direction), we cannot assert the origin of the excess of scattering and lack of transmission for all the volume fractions. Nevertheless, we must admit that some transient change in the system occurs at times of the order
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Fig. 10. Evolution of volume fraction of the aggregates of PPN samples at low volume fractions, as a function of the experimental time.
of 150 s after the beginning of the appearance of inhomogeneities in the scattering volume, due to the distribution of residence times. The excess of scattering could, therefore, be due to the existence of concentration gradients. It could also be due to a slow change in the aggregation state of the organoclay particles after mixing. Both phenomena would lead to an increase of the scattered signal. Fig. 9 shows the evolution of scattered intensity (measured at t infinity) as a function of organoclay concentration. For the first three concentrations, one can observe the linear evolution of scattered intensity as a consequence of the presence of single light scattering behaviour. For concentrations above 3 wt%, a non linear behaviour appears. This behaviour is mainly due to multiple scattering. Indeed, a posteriori observation of the mixture reveals that the most concentrated sample is very turbid. In this case, the optical path rapidly increases with the concentration. As a consequence, both scattering and absorption increase with the volume fraction faster than a linear behaviour, as observed for times ranging from 80 to 200 s after the beginning of the mixture. As absorption and single light scattering would lead to affine behaviour, this proves that multiple scattering plays a role for the most concentrated sample. In summary, the results show the presence of two regimes: A simple scattering regime for organoclay concentration less than 3 wt% and a multiple scattering regime for organoclay concentration above 3 wt%. 3.1.1. Simple scattering regime Considering the transmitted light intensity, taken at the simple scattering regime, and the modified Beer’s law [31], one can estimate the extent of exfoliation of the nanocomposites. The modified Beer law is given by the following equation:
I ¼ I0 ebf Fig. 9. Evolution of the scattered intensity in the long time permanent regime as a function of organoclay content. The straight line is an affine fit to the first three concentrations.
(2)
where I is the transmitted light intensity, b an attenuation coefficient proportional to the difference in polarizability between clay particles and PP matrix, and f the volume
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Fig. 11. Measurements of scattered light intensity of Basell samples in the multiple light scattering regime. (a): PPC1-7.7, PPC2-7.7 and PPC3-7.7. (b): PPC4-8.8 and PPD-8.8.
fraction of aggregate clay particles that scatter light [31]. We assume, as in the work of Bur et al. [31], that the pristine PP matrix represents 0% aggregates or 100% exfoliated filler. The other extreme is 100% aggregate with no exfoliation. In our case, it corresponds to PPN-6, the
concentration at which the transmitted light intensity is equal to zero. The attenuation coefficient b is assumed to be a linear function of the difference in polarizability between PP matrix and organoclay, and is given by the following expression:
Fig. 12. TEM images of Basell Samples. (a): PPC1-7.7. (b): PPC2-7.7. (c): PPC3-7.7.
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Fig. 13. TEM images of Basell samples. (a):PPC4-8.8. (b): PPD-8.8.
b ¼ K D n2
where K is a constant of proportionality and D(n2) is the difference in the square of index of refraction between matrix and organoclay. A value of K was obtained from Eq. (2) using I/I0 data for PPN-6 (no exfoliation). To compute the volume fraction of the organoclay as a function of the organoclay weight content in wt% (w), the following relation is used:
fc ¼ wðw þ ð1 wÞrc =rM Þ1 Here, rC and rM are the densities of the organoclay fillers and the PP matrix, respectively. The volume fraction of the organoclay is denoted by fC. Eq. (2) was used to calculate f in terms of transmitted light intensity. The refraction index of PP was taken equals to nPP ¼ 1.53. From the work of Dantal et al. [33,34], the refraction index of nanocomposites was founded to linearly depend on organoclay concentration. We
took a refraction index of n0.5 wt% ¼ 1.67 for PPN-0.5. The other refraction index was estimated from the equation n ¼ 0.28fc þ 1.53 [31]. We also took rM ¼ 1.19 cm3/g and rC ¼ 2.83 cm3/g. Fig. 10 shows the evolution of volume fraction of aggregates as a function of time. The present results show that the volume fraction of aggregate increases from 0.17% for PPN-0.5 to 0.87% and 2.8% for PPN-1 and PPN-3, respectively. These plots are the quantitative indicator of the extent of exfoliation. Obviously, the nanocomposite PPN-0.5 is more exfoliated than both PPN-1 and PPN-3. According to the assumptions made for calculations, we can conclude that the nanocomposites PPN-0.5, PPN-1, and PPN-3 present extent of exfoliation equal to about 94%, 70% and 7%, respectively. 3.1.2. Multiple scattering regime As mentioned above, in the multiple scattering regime, we observed that the transmitted light intensity completely
Fig. 14. Frequency dependence of G0 measured at 200 C for Basell samples. (a): effect of the melt mixing temperature. (b): effect of the type of organoclay.
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vanishes. Our investigations were, therefore, restricted to scattered light intensity. As we have said before, these spectra clearly give us an idea of both organoclay concentration and dispersion in PP matrix. Even if it is not possible to estimate quantitatively the volume of aggregates in this case, the scattered light intensity appears to be sensitive to the nanocomposite morphology. To show this, we used the two groups of PP based organoclay nanocomposites, developed by Basell, as previously indicated. Fig. 11 shows the scattered light results obtained for PPC1-7.7, PPC2-7.7, PPC3-7.7, PPC4-8.8 and PPD-8.8. According to these results, one can observe that the scattered light intensity decreases as the preparation temperature increases. The increasing preparation temperature seems to reduce the size of particles in the matrix. This result is a well known observation in the field of nanocomposite materials. In fact, the increase of temperature can lead to enhance the extent of exfoliation, or contribute to create a higher number of large aggregates. TEM images of PPC1-7.7, PPC2-7.7 and PPC3-7.7 (Fig. 12), show that there is no change in the morphology of these materials. One cannot establish a clear distinction between the sample morphologies. For the second group, the scattering light results (Fig. 11b) depict an increase of scattered light intensity in the presence of D67G, as a consequence of large particles size compared to those obtained in the presence of Cloisite 15A. TEM observations in this case (Fig. 13) do not show any difference in morphology for those materials. Our results are in agreement with rheological characterizations (Fig. 14). In fact, the G0 modulus of PPC4-8.8 is much higher than that of PPD-8.8. The storage modulus G0 increases more significantly, particularly in the low frequency region. This is evidence of the partial exfoliation. 4. Conclusions Dispersion and morphology of polypropylene organoclay nanocomposites have been studied using a new optical sensor, dedicated to real-time monitoring and based on light scattering. Our results have been compared to those of conventional characterization methods. Validation experiments proved the quality of results in terms of signal repeatability and sensitivity to organoclay particle concentration and nanocomposite structure, respectively. XRD, TEM and rheological characterizations showed that the organoclay is well dispersed in the PP matrix, and the nanocomposites mainly present a partially exfoliated morphology. As the organoclay concentration increases, transmitted light intensity decreases but scattered light intensity increases. At concentrations above 3 wt%, the transmitted light intensity completely vanishes. The investigation of transmitted and scattered light intensity leads to separate two regimes. The first, for organoclay concentration from 0.5 wt% to 3 wt%, corresponds to a simple scattering regime. The second regime appears at concentrations up to 3 wt%, and corresponds to a multiple scattering regime. A modified Beer’s law used in the simple scattering regime, allow us to show that the nanocomposite PP/Nanocor 0.5 wt% is more exfoliated than PP/ Nanocor 1 wt%, and PP/Nanocor 3 wt%. In the multiple
scattering regime, our device seems to give an indication of the nanocomposite morphology, where TEM observations show no difference. Our obtained results in this regime are in agreement with rheological characterizations. Acknowledgements This study was carried out in the frame IP European research program “MULTIHYBRIDS”, No. 026685-2, in the 6th Framework Program. We also thank Mr Christophe MELART and Mr Christophe SUTTER for their help and their availability. References [1] E.P. Giannelis, Polymer layered silicate nanocomposites, Advanced Materials 8 (1996) 29–35. [2] I. Hejazi, F. Sharif, H. Garmabi, Effect of material and processing parameters on mechanical properties of polypropylene/ethylenepropylene-diene-monomer/clay nanocomposites, Materials and Design 32 (2011) 3803–3809. [3] R. Matadi, A. Makradi, S. Ahzi, J.G. Sieffert, S. Etienne, D. Rush, R. Vaudemont, R. Muller, M. Bouquey, Preparation, structural characterization, and thermomechanical properties of poly(methyl methacrylate)/organoclay nanocomposites by melt intercalation, Journal of Nanoscience and Nanotechnology 9 (2009) 2923–2930. [4] J.K. Mishra, K.J. Hwang, C.S. Ha, Preparation, mechanical and rheological properties of a thermoplastic polyolefin (TPO)/organoclay nanocomposite with reference to the effect of maleic anhydride modified polypropylene as a compatibilizer, Polymer 46 (2005) 1995–2002. [5] S. Zhu, J. Chen, Y. Zuo, H. Li, Y. Cao, Montmorillonite/polypropylene nanocomposites: mechanical properties, crystallization and rheological behaviours, Applied Clay Science 52 (2011) 171–178. [6] R. Matadi Boumbimba, K. Wang, N. Bahlouli, S. Ahzi, Y. Rémond, F. Addiego, Experimental investigation and micromechanical modeling of high strain rate compressive yield stress of a melt mixing, polypropylene organoclay nanocomposites, Mechanics of Materials (2012). doi:10.1016/j.mechmat.2012.04.006. [7] R. Matadi, O. Gueguen, S. Ahzi, J. Gracio, R. Muller, D. Ruch, Investigation of the stiffness and yield behaviour of melt-intercalated poly(methyl methacrylate)/organoclay nanocomposites: characterisation and modelling, Journal of Nanoscience and Nanotechnology 10 (2010) 2956–2961. [8] K. Wang, R.M. Boumbimba, N. Bahlouli, S. Ahzi, R. Muller, M. Bouquey, Dynamic compressive behavior of a melt mixed polypropylene/organoclay nanocomposites, Journal of Engineering Materials and Technology 134 (issue 1) (2012) 010905. doi:10.1115/ 1.4005420. [9] J.W. Gilman, Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites, Applied Clay Science 15 (1999) 31–49. [10] Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi, O. Kamigaito, Mechanical properties of nylon 6-clay hybrid, Journal of Materials Research 8 (1993) 1185–1189. [11] S. Kumar, J.P. Jog, U. Natarajan, Preparation and characterization of poly(methyl methacrylate)-clay nanocomposites via melt intercalation: the effect of organoclay on the structure and thermal properties, Journal of Applied Polymer Science 89 (2003) 1186–1194. [12] J.H. Chang, Y.U. An, Nanocomposites of polyurethane with various organoclays: thermomechanical properties, morphology, and gas permeability, Journal of Polymer Science, Part B: Polymer Physics 40 (2002) 670–677. [13] A.R. Manninen, H.E. Naguib, A.V. Nawaby, M. Day, CO2 sorption and diffusion in polymethyl methacrylate-clay nanocomposites, Polymer Engineering and Science 45 (2005) 904–914. [14] J.H. Chang, K.M. Park, D. Cho, H.S. Yang, K.J. Ihn, Preparation and characterization of polyimide nanocomposites with different organo-montmorillonites, Polymer Engineering and Science 41 (2001) 1514–1520. [15] W.S. Chow, A. Abu Bakar, Z.A. Mohd Ishak, J. Karger-Kocsis, U.S. Ishiaku, Effect of maleic anhydride-grafted ethylene-propylene rubber on the mechanical, rheological and morphological properties of organoclay reinforced polyamide 6/polypropylene nanocomposites, European Polymer Journal 41 (2005) 687–696.
R. Matadi Boumbimba et al. / Polymer Testing 31 (2012) 800–809 [16] A. Somwangthanaroj, W. Ubankhlong, W. Tanthapanichakoon, Solid-state mechanical properties of polypropylene/nylon 6/clay nanocomposites, Journal of Applied Polymer Science 118 (2010) 538–546. [17] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Progress in Polymer Science (Oxford) 28 (2003) 1539–1641. [18] L.M. Wang, Preparation and Characterization of Polypropylene/Clay Nanocomposites (2011) pp. 1584–1587. [19] R.M. Boumbimba, S. Ahzi, N. Bahlouli, D. Ruch, J. Gracio, Dynamic mechanical properties of PMMA/organoclay nanocomposite: experiments and modeling, Journal of Engineering Materials and Technology 133 (issue 3) (2011) 030908. doi:10.1115/1.4004052. [20] S.M. Mohomane, V. Djokovic, S. Thomas, A.S. Luyt, Polychloroprene nanocomposites filled with different organically modified clays: morphology, thermal degradation and stress relaxation behaviour, Polymer Testing 30 (2011) 585–593. [21] L. Százdi, A. Pozsgay, B. Pukánszky, Factors and processes influencing the reinforcing effect of layered silicates in polymer nanocomposites, European Polymer Journal 43 (2007) 345–359. [22] R. Wagener, T.J.G. Reisinger, A rheological method to compare the degree of exfoliation of nanocomposites, Polymer 44 (2003) 7513– 7518. [23] D. Fischer, K. Sahre, M. Abdelrhim, B. Voit, V.B. Sadhu, J. Pionteck, H. Komber, J. Hutschenreuter, Process monitoring of polymers by inline ATR-IR, NIR and Raman spectroscopy and ultrasonic measurements, Comptes Rendus Chimie 9 (2006) 1419–1424. [24] D. Fischer, D. Pospiech, U. Scheler, R. Navarro, M. Messori, P. Fabbri, Monitoring of the sol-gel synthesis of organic-inorganic hybrids by FTIR transmission, FTIR/ATR, NIR and Raman spectroscopy, Macromolecular Symposia 265 (2008) 134–143. [25] K. El-Mabrouk, S. Vaudreuil, A. Zeghloul, M. Bousmina, Effects of shear on phase-separation in polystyrene/poly(vinyl methyl ether)/
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
809
organoclay nanocomposite, Journal of Nanoscience and Nanotechnology 8 (2008) 1895–1900. G. Schlatter, C. Serra, M. Bouquey, R. Muller, J. Terrisse, Online light scattering measurements: a method to assess morphology development of polymer blends in a twin-screw extruder, Polymer Engineering & Science 42 (2002) 1965–1975. M. Peng, D. Li, Y. Chen, Q. Zheng, Effect of an organoclay on the reaction-induced phase-separation kinetics and morphology of a poly(ether imide)/epoxy mixture, Journal of Applied Polymer Science 104 (2007) 1205–1214. S. Wang, Y. Zhang, W. Ren, H. Lin, Morphology, mechanical and optical properties of transparent BR/clay nanocomposites, Polymer Testing 24 (2005) 766–774. N.V. Pogodina, C. Cerclé, L. Avérous, R. Thomann, M. Bouquey, R. Muller, Processing and characterization of biodegradable polymer nanocomposites: detection of dispersion state, Rheologica Acta 47 (2008) 543–553. A.J. Bur, S.C. Roth, Y.H. Lee, M. McBrearty, A dielectric slit die for inline monitoring of polymer compounding, Review of Scientific Instruments 75 (2004) 1103–1109. A.J. Bur, Y.H. Lee, S.C. Roth, P.R. Start, Measuring the extent of exfoliation in polymer/clay nanocomposites using real-time process monitoring methods, Polymer 46 (2005) 10908–10918. L. Jourdainne, R. Matadi Boumbimba, B. Triki, M. Bouquey, R. Muller, P. Hébraud, P. Pfeiffer, Compact and flexible optical sensor designed for on-line monitoring, International Journal of Material Forming 5 (2012) 1–9. B.R. Dantal, S. Gupta, A. Saigal, M.A. Zimmerman, I. Georgakoudi, Characterization of a New Titanium Dioxide-polymer Nanocomposite Material for Electronic Packaging Using Light Scattering Spectroscopy, in, Boston, MA (2009) pp. 1147–1150. Corradi Laure, Cervera Régis, Les nanoparticules dans le verre: application aux guides d’ondes, in: Rapport de Master (2007).