Understanding and controlling the structure of polypropylene/layered silicate nanocomposites

Authors

  • K. Chrissopoulou,

    Corresponding author
    1. Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, P. O. Box 1527, Heraklion Crete 711 10, Greece
    • Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, P. O. Box 1527, Heraklion Crete 711 10, Greece
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  • I. Altintzi,

    1. Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, P. O. Box 1527, Heraklion Crete 711 10, Greece
    2. Department of Physics, University of Crete, Heraklion Crete 710 03, Greece
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  • I. Andrianaki,

    1. Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, P. O. Box 1527, Heraklion Crete 711 10, Greece
    2. Department of Physics, University of Crete, Heraklion Crete 710 03, Greece
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  • R. Shemesh,

    1. Carmel Olefins Ltd, R & D, P.O. Box 1468, Haifa 31014, Israel
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  • H. Retsos,

    1. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853
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  • E.P. Giannelis,

    1. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853
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  • S. H. Anastasiadis

    Corresponding author
    1. Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, P. O. Box 1527, Heraklion Crete 711 10, Greece
    2. Department of Physics, University of Crete, Heraklion Crete 710 03, Greece
    3. Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 541 24, Greece
    • Institute of Electronic Structure and Laser, Foundation for Research and Technology–Hellas, P. O. Box 1527, Heraklion Crete 711 10, Greece
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Abstract

The miscibility and structure in polypropylene/layered silicate nanocomposites is systematically investigated utilizing a maleic-anhydride grafted polypropylene with a low degree of functionalization acting as the compatibilizer. The morphology of the hybrids can be modified from phase separated to almost completely exfoliated in a controlled way by varying the ratio α of the compatibilizer to the organophilized clay; this ratio α is found to be the most important parameter in determining the final structure whereas exfoliated structures can be obtained for α values of 9 or higher. Furthermore, utilization of a “masterbatch” procedure can enhance the degree of exfoliation even for smaller values of α; in that case, polypropylene is essentially mixed with the already dispersed “hairy” platelets. Investigation of the thermal stability of the micro- and nanocomposites shows that high degree of exfoliation is vital in increasing the temperature that the polymer starts to degrade. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2683–2695, 2008

INTRODUCTION

Polymer composites have been widely used in numerous applications, because they offer an appealing improvement of the properties of the individual components.1 Nevertheless, important compromises are often required in material design because, for example, an increase in strength is often accompanied by a loss in toughness and an increase in brittleness or a loss of optical clarity; it is anticipated, that these problems can be overcome if the inorganic additive can exist in the form of a fine dispersion within the polymeric matrix producing a nanocomposite.2 In these cases, the final properties of the hybrid are determined mainly by the existence of many interfaces.3 Among nanocomposites, the ones comprising polymer and inorganic layered silicates (PLSN) have been considered as a new generation of composite materials because of their anticipated unique properties.4–10

Mixing polymers with layered silicates results in three different types of structure depending on the interactions between the chains and the inorganic surfaces:11 the phase separated, where the polymer and the inorganic are mutually immiscible, the intercalated, where the polymer chains reside between the layers of the inorganic material resulting in a well-ordered multilayer with a repeat distance of a few nanometers, and the exfoliated, where the periodicity of the inorganic material is destroyed and the inorganic platelets are dispersed within the polymeric matrix. It has been reported that to demonstrate all the advantages of polymer/layered silicate nanocomposites, large surface area, high aspect ratio, and good interfacial interactions are essential, and so the silicate must be exfoliated and the platelets should strongly adhere to the polymer.12, 13 Thus the optimal nanocomposite structure consists of disordered, exfoliated clay platelets dispersed in the polymer matrix whereas theoretical models predict that incomplete exfoliation decreases the reinforcing efficiency of the clay.14 Nevertheless, exfoliation is practically never complete and a mixed intercalated/partially exfoliated structure is usually reported.15–18

Natural layered silicates are usually hydrophilic and their interactions are favored only with polar polymers.19–23 In the case of hydrophobic polymers, intercalation24–27 or exfoliation28 can be achieved only with organophilized clays, that is, with materials where the hydrated cations within the galleries are replaced by proper surfactants (e.g., alkylammonium) via a cation exchange reaction. Among the various nonpolar polymers, polyolefins are the most widely used in many applications, so that efforts to optimize their properties via the fine dispersion of “nanoobjects” to develop a nanohybrid have been a subject of scientific and industrial interest. Many attempts to synthesize polyethylene, PE,29–33 or polypropylene, PP,13, 15, 18, 34–46 nanocomposites have been presented in the past with a high or low degree of success. As far as polypropylene is concerned, it was early recognized that, because of its quasi apolar nature, the organophilization of the silicate was not sufficient to lead to intercalated or exfoliated structures. Thus, in the majority of the studies, a compatibilizer containing polar groups, such as polypropylene grafted maleic anhydride (PP-g-MA),13, 18, 35–46 is generally introduced as a third component to compensate the polarity difference between PP and the filler. Important factors for achieving exfoliation and homogeneous dispersion of the nanolayers in the hybrids are both the strong interaction of the compatibilizer with the nanolayers and its miscibility with the PP polymer. At the same time, PP-g-MA exhibits inferior mechanical properties than PP; therefore, its addition may result in a deterioration of the final properties of the composites despite the exfoliated structure. Hence, one has to investigate the effect of PP-g-MA on the degree of dispersion to optimize its concentration.39 Besides PP-g-MA, hydroxyl-functional,47 chlorosulfonated,48 or diethyl maleate grafted40 polypropylenes have been utilized as compatibilizers; however, it is reported that PP-g-MA shows the best results concerning compatibilization of PP with modified clay.40 Alternatively, PP nanocomposite formation through polymer functionalization or through fluorination of the surfactants used has been reported.15, 34

The behavior cannot be fully explained by considering only the state of dispersion of the clay; other factors, like the preparation and processing methods appear to be involved in the complex structure-properties relation.49 Understanding of the complex mechanism of dispersion and exfoliation of the clay tactoids may allow a better control of the final morphology and of the homogeneity of clay nanocomposites and, thus, of their macroscopic properties. Additionally, polypropylene, like many semicrystalline polymers, can crystallize in different crystalline forms, which exhibit vastly different properties. The influence of the addition and the state of dispersion of the nanofillers on the crystallization of the semicrystalline polymers has been largely overlooked. Therefore, determination of how PP crystallizes in the presence of nanofillers is of utmost importance. Thus, despite the numerous publications in this field, it is clear that there are still contradictions about the factors influencing the degree of exfoliation and/or the final structure as well as the interactions that define the properties of the hybrids. In a previous work, we have successfully synthesized polyethylene nanocomposites utilizing various compatibilizers and we have demonstrated that the most important factor controlling the structure is the ratio of the compatibilizer to the inorganic.29

In this article, we present a systematic effort to understand and control the structure in polypropylene/layered silicate nanocomposites by utilizing a polypropylene grafted maleic anhydride compatibilizer. We show how one can control the structure, from a phase separated microcomposite to a completely exfoliated nanocomposite, by varying the ratio α of compatibilizer to organoclay. The degree of exfoliation is maintained or even further enhanced if a masterbatch procedure is followed: the compatibilizer is first mixed with the silicate to exfoliate the platelets, which can, then, be further mixed with the matrix PP behaving like hairy particles.

EXPERIMENTAL

Materials

The polypropylene used is an impact copolymer, Capilene® TT 75 AV, with a melt flow index (MFI) of 25 that was provided by Carmel Olefins. A maleic-anhydride-grafted polypropylene (Aldrich) with MFI 115 and a maleic anhydride content wMA ∼ 0.6% was utilized as the compatibilizer. Two organoclays were utilized: Cloisite 20A (C20A) was obtained from Southern Clay, whereas Dellite 72T (D72T) was provided by Laviosa Chimica Mineraria. They are both organophilized via a cation exchange reaction using dimethyl dihydrogenated tallow quaternary ammonium chloride as the organic modifier (hydrogenated tallow is a product consisting of a distribution of hydrocarbon chains with approximate composition 65% C18; 30% C16; and 5% C14). The two producers quote that the weight loss of the organoclays on ignition is 38 and 37–41 wt %, respectively. X-ray diffraction experiments to be discussed in section “Results and Discussion”, however, show different interlayer distances of 25.9 Å for C20A and 30.2 Å for D72T. This indicates the presence of excess surfactants in D72T that may complicate and/or disturb the mixing with PP and PP-g-MA as well as cause a deterioration of the mechanical and/or flammability properties of the final composites.43 Thus, the organoclays were used after the excess surfactants were washed away. The organoclays were washed three times with ethanol following by the centrifugation of the suspension, the removal of the supernatant solution ant the subsequent drying of the material. After washing, the interlayer distance of D72T was found as 26 Å, similar to that of C20A that did not change.

Experimental Techniques

Microextruder

A DSM 5 cm3 twin screw micromixer and microextruder was utilized to facilitate the melt mixing of the polymer with the inorganic material as well as with the compatibilizer. The polymer(s) was (were) initially placed into the mixer and was (were) left to melt and homogenize at 180 °C and 100 rpm for ∼5 min under nitrogen flow. The inorganic material was subsequently introduced in the appropriate amount to obtain the desired concentration. Following homogenization of the mixture for ∼20 min, the specimens were obtained in the form of cylindrical extrudates of 1–3 mm in diameter. In the case of samples prepared utilizing the masterbatch process, a nanocomposite consisting only of compatibilizer and organoclay was synthesized first, according to the aforementioned procedure, followed by its subsequent mixing with the polypropylene. Thermogravimetric (TGA) experiments verify that at the temperature of sample preparation, no degradation of either the polymer or the surfactants has occurred.

X-ray Diffraction

Structural characterization of the nanocomposites was performed with X-ray diffraction, using a RINT-2000 Rigaku Diffractometer. The X-rays are produced by a 12 kW rotating anode generator with a Cu anode equipped with a secondary pyrolytic graphite monochromator. The Cu Kα radiation was used with wavelength λ = λCu Kα = 1.54 Å. Measurements were performed for 2θ from 1.5° to 30° with step of 0.02°. The organoclays were measured in a powder form, whereas the pure polymer and the nanocomposites in sample holders suitable for the cylindrical extrudates. Materials with periodic structure like the layered silicate clays show characteristic (00l) diffraction peaks which are related to the spacing of the layers according to Bragg law, = 2d00lsin θ, where λ is the wavelength of the radiation, d001 is the interlayer distance, and is the diffraction angle. It is expected that when the polymer is intercalated within the layers of the inorganic material, there will be an increase in the interlayer distance, resulting in a shift of the diffraction angles toward lower values. In the case of exfoliated nanocomposites, the layered structure will be destroyed and the diffraction peaks will disappear.

Transmission Electron Microscopy

Bright field TEM images of the nanocomposites were obtained at 120 kV under low-dose conditions, with a Philips 400T microscope. The nanocomposite samples were cryomicrotomed with a diamond knife at −110 °C to give sections with a nominal thickness of 70 nm. The contrast between the silicon-containing phase (shown as dark lines) and the polymer (bright region) was sufficient for imaging, and no further staining was required.

Scanning Electron Microscopy

Scanning Electron Microscopy (SEM) of fractured surfaces was performed using a Phillips XL 20. Samples were gold sputtered prior to observation. The preparation of the samples involved creation of brittle fracture surfaces after immersion of the sample in liquid nitrogen for 30 min.

Differential Scanning Calorimetry

The thermal properties of the micro- and nanocomposites as well as of the initial components were measured with a PL-DSC (Polymer Laboratories). The temperature range covered was between −50 and 220 °C with a heating/cooling rate of 10 °C/min, whereas two heating/cooling cycles were performed in all cases. The melting, Tm, and crystallization, Tc, temperatures were always obtained from the second cycle to ensure the elimination of the samples thermal history. All the measurements were performed under nitrogen flow to prevent the decomposition of the samples, and the cooling was performed using liquid nitrogen.

Thermogravimetric Analysis

The thermal stability of the various parent materials and the micro- or nanocomposites were investigated with thermogravimetric analysis (TGA) on a SDT600 TGA/DTA apparatus (TA instruments). Heating scans were performed from room temperature to 500 °C at a heating rate of 10 °C/min under argon atmosphere.

RESULTS AND DISCUSSION

Figure 1 shows the X-ray diffractograms of the pure materials and their composite. The polypropylene polymer shows a number of diffraction peaks, the position of which as well as their relative intensities indicate that the corresponding structure is that of the isotactic alpha form of polypropylene.50 Table 1 summarizes the results for the observed diffraction angles together with crystallographic data for PP known from the literature. Figure 1(b) shows data for the organoclay, Cloisite20A which shows a main diffraction peak at 2θ = 3.4° which corresponds to d001 = 25.9 Å. The fact that the first and second maxima are not exactly equidistant in the 2θ scale may be attributed to a possible random interstratification of the organoclay platelets.26 The d001 spacing of C20A is in accordance with the material datasheet. The peak observed in the data of the organoclay at 2θ ∼ 20° corresponds to a different crystallographic plane,51 whereas the ones at angles higher than 2θ ∼ 25° are due to remaining minute impurities even after the purification process; for example, the peak at 2θ ∼ 26.5° indicates the presence of quartz. It is noted that the peaks corresponding to the nanoparticles appear in a different angular range than those corresponding to the polymer; thus, any possible intercalation can be positively identified. Figure 1(b) includes data for another organoclay, Dellite 72T which has been organophilized with the same surfactant chains with C20A. Indeed, its diffraction curve shows that the main diffraction is at exactly the same angle with C20A. Both organoclays have been utilized for the synthesis of polypropylene hybrids.

Figure 1.

X-ray diffractograms of (a) polypropylene PP, (b) organoclays Cloisite 20A and Dellite72T, and (c) PP:C20A 90:10 composite.

Table 1. Experimentally Measured Diffraction Angles and Crystallographic Data for Isotactic Alpha Form Polypropylene (λ = 1.54 Å)
hklexp (°)2θ (°) [50]Relative Intensity
11014.014.14very strong
04016.816.92very strong
13018.518.55strong
11121.121.31strong
131, 04121.721.86strong
150, 06025.525.35strong
20027.227.18medium
22028.528.51medium

Figure 1(c) shows the X-ray diffractograms for a composite prepared by melt mixing PP with C20A containing 10 wt % C20A. The data do not provide any indication for either intercalation of the polymer chains or exfoliation of the silicate platelets because the main diffraction peak appears at the same angle with that of the pure inorganic material. Thus, it is clear that the organophilization is not sufficient for PP to intercalate and that the composites are phase separated systems as expected from the nonpolar and very hydrophobic character of PP. Additionally, the peaks corresponding to PP appear very similar with those of the pure polymer, except from two small peaks that appear at 16.0° and 19.6°. These values are close to the ones corresponding to the (300) reflection of the β-phase (Table 2) of polypropylene47 and the (130) plane of the γ-phase.35b, 50 This may indicate that the presence of the inorganic material can modify the crystalline structure of PP at least in the vicinity of the surfaces; such a modification should be more evident in an exfoliated nanocomposite. Nevertheless, it is clear that for the synthesis of PP/layered silicate nanocomposites one has to modify the interactions between the polymer and the inorganic surfaces. Consequently, polypropylene grafted with maleic anhydride groups, PP-g-MA, was selected as the compatibilizer. It is anticipated that the polarity of the maleic anhydride groups will lead to the intercalation of the chains between the layers via hydrogen bonding with the oxygen atoms of the surfaces.

Table 2. Crystallographic Data for Beta Form Polypropylene (λ = 1.54 Å)
hkl2θ (°) [47]
30016.2
30121.2

To examine the effectiveness of the specific PP-g-MA as a compatibilizer, binary hybrids of just the “compatibilizer” and the silicate were first prepared. Figure 2(a) shows the X-ray diffraction data of these composite materials containing PP-g-MA and C20A to examine the compatibilizer's ability either to intercalate between the layers of the inorganic material or to exfoliate the structure. The diffractograms of the pure materials are shown as well whereas the curves have been shifted vertically for clarity. The data of PP-g-MA shows a series of peaks that resemble the crystalline pattern of PP. In the composite with 60 wt % polymer (as well as those with less polymer), the diffraction peak corresponding to the periodic structure of C20A is observed at exactly the same angle as in the inorganic clay signifying mainly a phase separated structure; it is only the increase of the scattered intensity at low angles that indicates some percentage of platelet exfoliation. As the ratio of PP-g-MA to organoclay increases, the characteristic peak decreases in intensity but remains at the same position and it is only for PP-g-MA to C20A ratios 9:1 by weight and higher that it vanishes. It is noted that in a similar case of polyethylene nanocomposites in which polyethylene grafted with maleic anhydride with similar amount of functional groups was utilized as the compatibilizer, the ratio of compatibilizer to organoclay, α, was identified as the crucial parameter for the exfoliation of the structure.29 Nevertheless, in that case, full exfoliation was achieved at much lower α > 4, whereas in this case for the same values of α a clear diffraction peak can be still seen and higher αs are necessary to delaminate the structure.

Figure 2.

X-Ray diffractograms of (a) binary composites of PP-g-MA and C20A in different concentrations and (b) ternary composites of PP, PP-g-MA and C20A at similar ratio α as in (a).

It should be kept in mind that layer distances deduced from XRD reflections are average values, which do not necessarily reflect the complexity of the structure. Additionally, it is well accepted that as far as exfoliation is concerned, XRD is not the most suitable technique since the results are based on the absence of the peaks that may be due to alternative reasons as well.51 In this case the support of a complementary technique, like Transmission Electron Microscopy, is necessary. Actually, it has been illustrated52 that a clear picture of the overall nanoscale dispersion of the clays in the polymer can be best obtained when the XRD data are properly interpreted and combined with TEM results. Figure 3 shows representative TEM images of two hybrids with ratios of PP-g-MA to C20A, 60:40 [Fig. 3(a)] and 90:10 [Fig. 3(b)] by weight. The dark lines represent the edges of the silicate layers and the white region the polymeric matrix. Clear differences are observed between the two systems. It is evident that a coexistence is observed in Figure 3(a) with clay particles retaining the layered structure and clay platelets dispersed within the polymer matrix. For the higher concentration of PP-g-MA in Figure 2(b), a uniform dispersion of exfoliated platelets is observed. These results are in excellent agreement with the X-ray diffraction data. It is noted that multiple TEM images were taken for every hybrid to get a complete picture of the structure of the sample.

Figure 3.

TEM images of (a) PP-g-MA:C20A 60:40 and (b) PP-g-MA:C20A 90:10.

It was previously observed that, for polyethylene, the important parameter that defines the structure of the hybrids is the relative amount of compatibilizer (or functional groups) to the organoclay.29 In the present case, that would be the ratio α = wt % PP-g-MA/wt % organoclay. For the hybrids of Figure 2(a), α ranges from 1.5:1 (for the PP-g-MA:C20A 60:40) to 19:1 (for the PP-g-MA:C20A 95:5). To examine whether the above finding is correct for the present system, three-component hybrids were synthesized, utilizing PP, PP-g-MA, and C20A in such a way that the ratio α is similar to the one in the binary systems of Figure 2(a). These hybrids were synthesized by placing the three materials together at the microextruder under the same conditions that were used for the two-component systems. The X-ray diffractograms of these hybrids with α = 1.35:1, 4.5:1, and 7.7:1 are shown in Figure 2(b). Comparison between Figures 2(a) and 2(b) shows that each pair of specimens with similar α exhibits exactly the same behavior, with the diffraction curves having exactly the same shape. In particular, for the composite with α = 1.35, the main diffraction peak is at 2θ = 3.3°, at the same position with that of the corresponding binary mixture and that of the organoclay, leading to the conclusion that the lamellar structure is preserved and that the system is phase separated. An increase of the ratio α leads to an increase of the degree of exfoliation of the system, manifested by the decrease of the intensity of the main peak and its concurrent increase at low angles. In conclusion, it seems that indeed the structure is determined by the value of the ratio of PP-g-MA to C20A, that is, by α, irrespectively of the presence or absence of PP.

The development of the proper processing route in view of potential applications is sometimes almost equally important as the understanding of the interactions in the systems thus another preparation method was subsequently followed. Hybrids that comprised PP-g-MA and C20A organoclay with a specific α were utilized as a masterbatch and were further mixed with polypropylene in different compositions. Figure 4 shows the X-ray diffractograms of such composites with α = 4 [Fig. 4(a)] and α = 9 [Fig. 4(b)]. It is obvious that in both cases mixing the masterbatch with the polymer results in even higher degrees of exfoliation evidenced by the disappearance of even small peaks present in the diffractogram of the masterbatch; this is reproducible and holds for every composition of the masterbatch. A possible explanation is that achieving a certain degree of exfoliation utilizing the compatibilizer leads on one hand to the weakening of the interactions between the layers of the inorganic material and on the other hand to the creation of a friendly environment for the PP polymer. Thus, a “hairy particle” is formed with PP-g-MA being the “hair” chains, which can be homogeneously mixed with PP dispersing the silicate layers even more. Equally important with the exfoliation of the silicates is the change observed in the crystalline structure of the polymer via the development of a peak at 2θ = 16.0° and an increase of the peak at 2θ = 21.1° (note the change in the relative intensities at 2θ = 21.1° and 2θ = 21.7° between the PP and the exfoliated ternary nanocomposites), which indicates a higher amount of PP crystallized in the beta form. This was observed in the phase separated system, as well, but it is strongly increased with exfoliation and the presence of more surfaces. It is noted, though, that this can be seen only for PP and not for PP-g-MA either in a two or a three component system. The later can be attributed to the stronger interactions and the hydrogen bonding that can develop in the maleated polypropylene.

Figure 4.

X-Ray diffractograms of binary hybrids with (a) PP-g-MA:C20A 80:20 and (b) PP-g-MA:C20A 90:10 together with their mixtures with PP.

As discussed earlier, the support of TEM is very useful to discuss the quality of dispersion and the degree of exfoliation of the nanocomposites. Consequently, TEM images were recorded for a binary mixture, PP-g-MA:C20A 80:20, and its resulting ternary composites [of Fig. 4(a)] and are shown in Figure 5. The PP-g-MA:C20A 80:20 specimen in Figure 5(a) shows a degree of dispersion of the individual layers intermediate between those of the two hybrids of Figure 3, as expected; there are no big particles within the polymer matrix but only individual platelets. It is interesting to note that the platelets apparently exhibit a more-or-less parallel orientation, which, however, can be attributed to the extrusion process;53 the distances between the layers are larger than the larger distance that can be measured with X-ray diffraction and, thus, the measurement indicates a “pseudoexfoliated” structure. Furthermore, it is clear that the composites prepared with the masterbatch procedure utilizing the PP-g-MA:C20A 80:20 as the additive to PP show a higher degree of dispersion than the binary masterbatch. The degree of exfoliation apparently increases with PP concentration as one can see by comparing Figures 5(b) and 5(c) (the fact that the mixture in Figure 5(c) contains a lower amount of inorganic material should be kept in mind). It is, thus, evident that the TEM data support fully the picture that emerged from the discussion of the X-ray diffraction data.

Figure 5.

TEM images of (a) binary PP-g-MA:C20A 80:20 (“mastebatch”), (b) 50% (PP-g-MA:C20A 80:20) + 50% PP, and (c) 25% (PP-g-MA:C20A 80:20) + 75% PP.

The morphology of all the samples was studied utilizing scanning electron microscopy, SEM, as well. Images of fractured surfaces are shown in Figure 6. It is clear that the surface of the pure polymer is very smooth in contrast to the one of the hybrid containing PP and 10 wt % C20A, which is a microcomposite. The latter possesses a rough surface with large aggregates of 1–2 μm size, which are attributed to the clay particles. Despite, the mixing during the sample preparation, it seems that these aggregates are not uniformly dispersed in the polymeric matrix. Additionally, the adhesion between the polymer and the silicate is apparently very poor. The SEM data from two binary hybrids with different values of α are also shown in Figure 6. As α increases from 1.5 (PP-g-MA:Dellite 72T 60:40) to 9 (PP-g-MA:Dellite 72T 90:10), the smoothness of the fractured surface changes with the surface morphology resembling, at high α, that of the pure polymer. It is noted that the PP-g-MA polymer has been examined and its morphology was very similar to that of PP. Additionally, the dispersion of either the clay particles or the silicate nanolayers in the polymer matrix is much better than in the phase separated composite, proving the enhanced interactions between the polymer and the inorganic surfaces. The SEM images of ternary hybrids prepared using the masterbatch procedure are along these lines; for example the specimen containing 25 wt % (PP-g-MA:Dellite 72T 90:10) +75% PP exhibits a fractured surface that resembles very much that of PP (it is of course recognized that it contains only 2.5 wt % clay). It is relatively homogeneous and there is no indication of aggregate formation but a high degree of dispersion which proves that the “masterbatch” preparation method, indeed leads to hybrids with enhanced exfoliation in accordance with the X-ray diffraction and TEM results.

Figure 6.

SEM images of (a) PP, (b) PP:C20A 90:10, (c) PP-g-MA:D72T 60:40, (d) PP-g-MA:D72T 90:10, and (e) 25 wt % (PP-g-MA:D72T 90:10) +75% PP. The scale bar in all pictures corresponds to 2 μm.

The thermal stability of the parent materials as well as of all the micro- and nanocomposites was examined utilizing thermogravimetric analysis, TGA. All the materials were heated to 500 °C and their weight loss curves are shown in Figure 7. The weight of Cloisite 20A is constant up to ∼200 °C and shows a very broad decrease between 215 and 425 °C that corresponds to the decomposition of the surfactant chains utilized for the organophilization of the inorganic surfaces. This weight loss is 32.3%, in good agreement with the amount of organic part of C20A (∼38 wt %), whereas the decomposition temperature of the surfactant chains is 312 °C (decomposition temperature is considered the temperature of the maximum slope). The polypropylene as expected is completely decomposed at a temperature 456 °C while the curve of the weight loss is much narrower than the one of the C20A. The microcomposite with 90 wt % PP and 10 wt % C20A shows a very small initial weight loss of ∼3.2 wt % at temperature ∼312 °C followed by a large drop at higher temperatures that starts at 395 °C and corresponds to the decomposition of the polymer matrix at 439 °C, that is, the decomposition temperature of PP in the presence of the inorganic material in a phase separated composite is lower than that of the pure polymer. Nevertheless, a more careful examination of the weight loss curves shows that the decomposition temperatures at various intermediate weight losses are T10% = 419 and 426 °C, T20% = 433 and 432 °C, T50% = 452 and 439 °C, and T80% = 464 and 447 °C for the PP polymer and PP/C20A composite, respectively. This means that, in the microcomposite, the polymer decomposition is complete at lower temperatures but it begins at much higher temperatures than in the pure polymer. The solid remaining at the end of the process is ∼6 wt % that is in good agreement with the composition of the hybrid and the true inorganic content of C20A.

Figure 7.

TGA curves of (a) silicate, (b) PP and a microcomposite with PP:C20A 90:10, (c) nanocomposites PP-g-MA:C20A 90:10 and PP-g-MA:C20A 80:20 with different degree of exfoliation together with PP-g-MA, and (d) nanocomposites with 50% (PP-g-MA:C20A 90:10) + 50% PP and 50% (PP-g-MA:C20A 80:20) + 50% PP.

The thermal stability of samples with varying degree of exfoliation was examined as well. Figure 7(c) shows the weight loss curves of specimens with PP-g-MA:C20A 90:10 and 80:20 (ratios α = 9 and α = 4, respectively) together with that of pure PP-g-MA. It is noted that the decomposition temperature of PP-g-MA is measured to be the same with that of PP. The results are very similar with the ones of the PP:C20A microcomposite. The decomposition temperature decreases but the onset of decomposition is delayed and is much sharper than the respective of the pure polymer. Specimens prepared with the masterbatch process were also examined and are shown in Figure 7(d). The samples shown are the ones of Figure 7(c) further processed with 50% PP so that the degree of exfoliation and, consequently, the dispersion of the inorganic platelets is even further increased. In both cases, a very small decrease of the weight is observed around 312 °C that corresponds to the loss of the surfactants as mentioned above. At higher temperatures, the polymer decomposition occurs; both nanohybrids show the sharp transition that characterizes all the composites and moreover the decomposition temperature resembles the one of the pure polymer. The solids remaining are found 7.1 and 3.4 wt %, in accordance with the hybrids composition. Thus, it can be concluded that the presence of the inorganic material leads to a decrease in the decomposition temperature of the polymer, a process that nevertheless starts at higher temperatures irrespectively of the hybrid structure whereas a high degree of exfoliation accompanied with a fine dispersion is necessary to increase the overall thermal stability of the material as well. These results are in agreement with previous measurements mentioned in the literature54 although there are reports that show improvement,55 worsening,56 or no effect57 on the degradation temperature in polyolefin nanocomposites.

The effect of the inorganic additive and the micro-or nanocomposite structure on the melting and crystallization of the semicrystalline PP was investigated as well. Figure 8 shows the DSC curves (during heating) of PP-g-MA as well as of the binary PP-g-MA:C20A composites with varying degree of exfoliation. In all cases, two cycles were recorded and the thermal characteristics were evaluated always from the second one to erase any previous thermal history of the samples. It is clear that the melting of PP-g-MA, either in the bulk or in the composites, is always a broad transition which shows an asymmetric wing toward lower temperatures. It is mentioned that the crystallization of PP (not shown) is much narrower and symmetric. It is the broadness and the asymmetry of the melting peak that makes the evaluation of the heat of fusion difficult. Nevertheless, the melting and crystallization temperatures have been estimated and are reported in Table 3 as a function of the composition of the binary hybrid (which means also degree of exfoliation). It seems that, in general, there is not any important effect of the presence of the inorganic material on the crystallization and melting characteristics. One may also point out that it seems that the simple addition of the inorganic material results in a small decrease of the crystallization temperature, Tc, and an increase of the melting temperature, Tm, whereas on the contrary, high degree of exfoliation leads to an increase of Tc and a decrease of Tm. We estimate that it is probably the combination of these two opposing effects, which may cancel each other, together with the fact that the melting transition is very broad due to the polydispersity of the commercial polymer that lead to the lack of any obvious dependence and to the various contradictory results that are reported in the literature. It is the difference between the thermograms of PP and PP-g-MA that prevented the estimation of the thermal characteristics in the ternary mixtures as well. Further measurements, combining DSC, optical microscopy, and small-angle X-ray scattering are in progress to evaluate the effect of the inorganic material and of the different structure on the crystallinity and crystalline characteristics of the hybrids investigated in this article as well as in hybrids that the polymer is more monodisperse and of higher crystallinity.58

Figure 8.

DSC melting curves of the PP-g-MA and composites with varying degree of exfoliation. The curves have been vertically shifted for clarity.

Table 3. Melting (Tm) and Crystallization (Tc) Temperatures of the Binary Mixtures
SampleTm (°C)Tc (°C)
PP-g-MA164 ± 2115 ± 1
PP-g-MA:C20A 95:5160 ± 1117 ± 1
PP-g-MA:C20A 90:10165 ± 2115 ± 1
PP-g-MA:C20A 80:20166 ± 1111 ± 1
PP-g-MA:C20A 60:40164 ± 2113 ± 1

CONCLUDING REMARKS

The morphology of polypropylene layered silicate nanocomposites was investigated by X-ray diffraction, TEM, and SEM, and the factors that allow a systematic variation of the structure were identified. The structure of the hybrids was varied in a controlled way utilizing maleic anhydride functionalized polypropylene as the compatibilizer. It was found that the important parameter, which determines the final structure, is the ratio α of the weight of compatibilizer to that of the organoclay; it was found that for complete exfoliation this ratio α should be 9 or higher. Moreover, high degrees of exfoliation can be obtained, even for smaller values of the ratio α, when an alternative masterbatch process is utilized. In that case, the appropriate binary mixture of compatibilizer and organoclay (“masterbatch”), which acts as “hairy inorganic particles,” is further mixed with the polymer leading to even enhanced dispersion. Investigation of the thermal properties and thermal stability of the micro- and nanocomposites shows that the presence of the inorganic material does not affect or even deteriorates the behavior in comparison with that of the pure polymer whereas high degrees of exfoliation is very important in significantly increasing the stability.

Acknowledgements

The authors acknowledge that part of this research was sponsored by the European Union in the framework of GROWTH Programme (NANOPROP Project No. G5RD-CT-2002-00834), by the Greek General Secretariat for Research and Technology (PENED Programme 03ED581), and by NATO Scientific Affairs Division (Science for Stability Programme).