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390 Int. J. Microstructure and Materials Properties, Vol. 7, No. 5, 2012
Copyright © 2012 Inderscience Enterprises Ltd.
Influence of loading rates on morpholgy and mechanical properties of PLA/clay nanocomposites
Mustapha Kaci* and Lynda Zaidi Laboratory of Organic Materials, Department of Engineering Processes, Faculty of Technology, University Abderrahmane Mira, Route de Targa-Ouzemmour, Bejaia 06000, Algeria Fax: +21334215105 E-mail: [email protected] E-mail: [email protected] *Corresponding author
Stéphane Bruzaud, Alain Bourmaud, Pascal Médéric and Yves Grohens Laboratoire d’Ingénierie des Matériaux de Bretagne, Université de Bretagne Sud, 56321 Lorient Cedex, France E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]
Abstract: This paper reports some experimental results on the influence of loading rates on morphology and mechanical properties of clay nanocomposites based on polylactide (PLA). The dispersed phase was organically modified montmorillonite commercially known as Cloisite 30B and introduced at various loading rates: 1, 3 and 5 wt.%. The nanocomposites were prepared by melt intercalation in a Brabender Plasticorder mixer. Wide angle X-ray scattering showed that the clay was finely distributed in the PLA matrix and the nanocomposite morphology can be considered as intercalated/exfoliated structure, whatever the filler content. The effect of clay loadings on the mechanical properties of PLA/organoclay was investigated by nanoindentation and tensile measurements. Nanoindentation results showed a significant improvement in modulus and hardness with increase of the clay contents. This is consistent with the large increase in Young’s modulus obtained by tensile tests indicating a good correlation of mechanical properties at the macrometric and nanometric scales.
Keywords: polylactide; PLA; clay; nanocomposites; melt intercalation; morphology; mechanical properties; wide angle X-ray scattering; nanoindentation.
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Reference to this paper should be made as follows: Kaci, M., Zaidi, L., Bruzaud, S., Bourmaud, A., Médéric, P. and Grohens, Y. (2012) ‘Influence of loading rates on morpholgy and mechanical properties of PLA/clay nanocomposites’, Int. J. Microstructure and Materials Properties, Vol. 7, No. 5, pp.390–399.
Biographical notes: Mustapha Kaci received his BS Engg. and MS in Plastics Materials from Algerian Institute of Petroleum in Boumerdes (Algiers) and his Doctorate degree from University Ferhat Abbas of Setif (Algeria). Currently, he serves as a Full Professor at the Department of Engineering Processes, Faculty of Technology, University Abderrahmane Mira of Bejaia in Algeria. He is a member of Algerian Group of Polymers of Algerian Society of Chemistry. He has published more than 50 publications on various aspects of polymer degradation and stabilisation. His research activities include also the durability of films based on polymer nanocomposites involving both synthetic and biopolymer matrices filled with organic and inorganic nanoparticles, the additive migration from the plastics films, and the blending and recycling.
Lynda Zaidi is a PhD candidate in Chemical and Materials Engineering at both the University Abderrahmane Mira of Bejaia (Algeria) where she received her Master degree in 2006 and the University of Bretagne Sud (France) in the framework of an international convention of co-direction of thesis. Her research area involves the degradation of polylactides/clay nanocomposites under various environmental conditions. A part of her research work has already been published in several international journals of high impact factor.
Stéphane Bruzaud received his Doctorate degree in Polymer Chemistry at the University of Bordeaux-1 (France) in 1995. Since 1996, he worked as Teacher-Researcher in the Laboratory of Materials Engineering of Bretagne (LIMATB), Lorient (France) and he is experienced in teaching and research in the fields of eco-design of materials, biocomposites or biopolymers. His scientific activity is documented by more than 40 papers concerning essentially the synthesis of polymer nanocomposites by different routes, studies of structure/properties relationships and ageing.
Alain Bourmaud received his Doctorate degree in 2011 at the University of Bretagne Sud, Lorient, France. He presently serves as Laboratory Engineer in the LIMATB. He is the author and co-author of a dozen of publications in the field of material science and technology, especially in biocomposite and nanobiocomposite polymers. His research activities include also processing of thermoplastics and composites materials and modelling.
Pascal Médéric received his Doctorate degree in Fluids Mechanics from the National Institute of Polytechnic of Toulouse in France since 1991. He is member of both French Group of Rheology and French Group of Polymers. His area of interest involves studies on relationships between macroscopic behaviours and multi-scale structured multiphase systems based on thermoplastic matrices including polymer blends, filled polymers, nanocomposites and other innovative nanostructured materials. His research work is currently carried out through the team of Rheology of the LIMATB. He has published many papers in various journals and conference proceedings.
Yves Grohens received his Doctorate degree in Materials Science from the University of Franche-Comté, France in 1991. Since 2001, he is currently a Professor at the University of Bretagne Sud and the Director of the laboratory LIMATB since 2002. His scientific activities involve physico-chemistry of surfaces and interfaces, adsorption of polymers, adhesion, bioadhesion, polymer blends, etc. He has published more than 100 papers in various international journals leaders in the field of polymer science.
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This paper is a revised and expanded version of a paper entitled ‘Influence of loading rateson morphology and mechanical properties of polylactides (PLA)/clay nanocomposites’ presented at 5th International Conference on Advances in Mechanical Engineering and Mechanics (ICAMEM2010), Hammamet (Tunisia), 18–20 December 2010.
1 Introduction
During the last decade, significant attention has been focused on both biodegradable polymers and polymer layered silicate nanocomposites aiming to improve the functional properties and possible uses of these environmental-friendly polymers as reported by Ray and Bousmina (2005) and Bordes et al. (2009). Among all these polymers, polylactide (PLA) is one of the biodegradable polymers that can be used as promising alternative to the petroleum-based commodity materials, because they can be derived from renewable resources, such as corn, potato and other agricultural products. This view has been supported in the works of both Gupta and Kumar (2007) and Lim et al. (2008). PLA has good mechanical properties, thermal plasticity, high degree of transparency and biocompatibility. Subsequently, PLA holds tremendous promise for various end-use applications such as biomedical fields, household, engineering, packaging industries and so on as reported by Auras et al. (2004). However, some of its properties, like flexural properties, gas permeability and heat distortion temperature, are too low for widespread applications. Therefore, many attempts were carried out to reach exfoliation state in corresponding nanobiocomposites. In his review on biodegradable polymer/layered silicate nanocomposites, Okamoto (2005) states that nanoclays such as montmorillonite are classically used to improve biodegradable polymers tensile stress and stiffness, reduce their gas/water vapour barrier properties, increase their thermal stability and modify their biodegradation rate. In this work, the main objective is to obtain PLA-based nanocomposites morphologies using a melt intercalation process for which the dispersion state is verified by coupling the techniques like wide angle X-ray scattering (WAXS) and nanoindentation.
2 Experimental
2.1 Materials
PLA in the form of pellets was supplied by Biomer under the trade name Biomer L9000® and used as received with a D-lactide content of 4.3%. The material has an average molecular weight nM = 220,000 g/mol and a melt flow index of 6.0 g/10 min. The polymer was dried under vacuum at 60°C prior to use.
Cloisite 30B (C30B) is an organically modified montmorillonite commercialized by Southern Clay Products (Texas, USA). C30B was modified with bis-(2-hydroxyethyl) methyl tallowalkyl quaternary ammonium cations with ~65% C18; ~30% C16 and ~5% C14. The exchange capacity of C30B is 90 meq/100g of clay. C30B is an additive for plastics and rubbers to improve various physical properties, such as reinforcement,
Influence of loading rates on morpholgy and mechanical properties 393
synergistic flame retardant and barrier. The clay was dried under vacuum at 60°C for at least 24 hours before use.
2.2 Preparation of PLA-Cloisite 30B nanocomposites
The PLA-C30B nanocomposites were prepared by melt mixing in a Brabender Plasticorder mixing chamber (model W 50 EHT) having the following characteristics: chamber volume = 55 cm3, sample weight = 40–70 g, maximum couple = 200 N.m. and maximum temperature equal to 500°C. Prior mixing, PLA and the nanofiller were dried at 60°C for 24 h. The major processing parameters were processing temperature, screw speed and mixing time; they were set at 190°C, 60 rpm and 8 min, respectively. The resulting material was granulated, and then compressed to produce thin films of an average thickness of ca. 150 μm with the aid of hydraulic press equipped with two heated plates at 190°C with a pressure of 30 bars for 3 min. Different formulations based on PLA were prepared with various clay contents: 1, 3 and 5 wt.%.
In using the nanoindentation method, samples preparation is very important because accurate results are obtained only if the indentations are significantly deeper than the surface topography of the specimen. A meticulous polishing can significantly reduce the uncertainty in determining the surface property when performing nanoindentation experiments. Hence, all surfaces to be indented were polished to a 3 μm particle size polishing solution finish. The average surface roughness was measured with a profilometer at 0.3 μm. The polished samples were mounted on aluminium cylinders using super glue for subsequent indentation tests.
2.3 Analytical techniques
WAXS was used to analyze the structure of the materials and to determine the interlayer spacing between stacked clay platelets. WAXS experiments were performed by using a Philips diffractometer (PW 1050) operating at the CuKα radiation (wavelength, λ = 0.154 nm), 40 kV and 20 mA. The diffraction spectra were recorded in the reflection mode over a 2θ range of 3–12° at room temperature and a scan rate of 0.017°s–1.
The static tensile tests were carried out in a laboratory where the temperature was 23°C and the humidity was 48% according to ISO 527-2 using a MTS Synergie RT1000 testing apparatus and 5A type specimen. The loading speed was 2 mm/min. A HDE extensometer was used with a nominal gauge length of 49.7 mm. The tests were carried out at least five times for each material and the results were averaged arithmetically.
Nanoindentation tests involve the contact of an indenter on a material surface and its penetration to a specified load or depth. Load is measured as a function of penetration depth. In this case, penetration depth is the displacement into the sample starting from its surface. Calculation methods to determine modulus and hardness are based on the work of Oliver and Pharr (1992). Indentation tests were performed with a commercial nanoindentation system (Nanoindenter XP®, MTS Nano Instruments) at room temperature with a continuous stiffness measurement (CSM) technique. In this technique, an oscillating force at controlled frequency and amplitude is superimposed onto a nominal applied force. The material, which is in contact with the oscillating force, responds with a displacement phase and amplitude. A Berkovich diamond indenter was used to perform the test. A SEM image of a Berkovich tip is shown in Figure 1. The area
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function, which is used to calculate contact area from contact depth, was carefully calibrated by using a standard silica sample, prior to the experiments. Strain rate during loading was maintained at 0.05 s–1. The other experimental conditions were as follows: 3 nm amplitude and 70 Hz oscillation. The nanoindenter tests were carried out in the following sequence: firstly, after the indenter made touched the surface, it was driven into the material with constant strain rate, 0.05 s–1, to a depth of 500 nm; secondly the load was held at maximum value for 60 s; and finally, the indenter was withdrawn from the surface with the same rate as loading until 10% of the maximum load was reached. Nanoindentation experiments were performed as indents matrix.
Figure 1 SEM image of a Berkovich tip
3 Results and discussion
3.1 WAXS analysis
Due to its ease and availability, WAXS is the most commonly used to characterize the nanocomposite structure, and the position, shape and intensity of the different peaks enables one to evaluate the dispersion of mineral sheets within the polymer matrix (intercalated and/or exfoliated structures). C30B and PLA-based nanocomposites with varying amounts of filler have been analyzed by WAXS as shown in Figure 2. As expected, no peak is observed in the WAXS pattern of neat PLA in the range 2–8°, whereas C30B exhibits a sharp peak at around 4.8°. The d-spacing values (basal distance between clay layers) were calculated using Bragg’s law (λ = 2dsinθ ; d is the interlayer d-spacing and λ is the wave length). The d001 peak of C30B appears at 2θ = 4.8°, corresponding to an interlayer spacing of 1.8 nm.
The PLA-based nanocomposites containing 1, 3 and 5 wt.% of organoclay exhibit no significant diffraction peak in the region of 2θ = 2–8°, which suggests predominant, if not complete, exfoliation. However, it is noted, for the sample containing 5 wt.% of C30B, a broad peak of less intensity, which is positioned at higher angle at 2θ ~ 5.2°. According to the literature (Bruzaud et al., 2005), this effect can be explained by a more heterogeneous structure of the nanocomposite, probably due to the difficulty to obtain a homogeneous material when the nanofiller content is high. This indicates a good
Influence of loading rates on morpholgy and mechanical properties 395
compatibility between the organophilic clay and PLA and that the nanocomposite morphology can be considered as intercalated and/or partially exfoliated structure, whatever the filler content. Each layer of the mineral is homogeneously dispersed in the polymer matrix although a small amount of non-exfoliated layers probably still remains, in particular for the most filled sample.
Figure 2 WAXS patterns of Cloisite 30B, neat PLA, PLA-C30B (1, 3 and 5 wt.%) nanocomposites (see online version for colours)
3.2 Tensile measurements
In their work on elaboration of poly(ε-caprolactone)-g-TiNbO5 nanocomposites and the measure of tensile properties, Bruzaud et al. (2004) state that in general, the Young’s modulus, expressing the stiffness of a material at the start of a tensile test, has shown to be strongly improved when layered silicate nanofillers are homogeneously dispersed into the polymer matrix. The mechanical properties of neat PLA and PLA nanocomposites at various clay loadings (1, 3 and 5 wt.%) are summarized in Table 1. Table 1 Tensile properties of PLA and PLA-based nanocomposites at various clay contents
Young’s modulus Tensile strength Elongation at break Samples
(MPa) (MPa) (%) PLA 3401 ± 101 60.4 ± 0.8 4.1 ± 0.7 PLA/C30B (1 wt.%) 3914 ± 69 56.2 ± 1.0 3.2 ± 0.5 PLA/C30B (3 wt.%) 4901 ± 41 49.9 ± 1.2 1.4 ± 0.2 PLA/C30B (5 wt.%) 5577 ± 67 28.8 ± 2.1 0.7 ± 0.1
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The addition of the organically modified clay into the polymer matrix leads to a significant increase of the Young’s moduli of nanocomposites at filler contents as low as a few weight percent. As shown in Table 1, it is drastically increased from 3401 MPa for pure PLA to 5577 MPa for the nanocomposite samples containing 5 wt.% of filler. The reinforcement effect R, which corresponds to the ratio of the tensile modulus of the nanocomposite to the tensile modulus of the pure polymer, can be calculated. PLA-based nanocomposites studied here yield R values of 1.15, 1.44 and 1.64 for 1, 3 and 5 wt.%, respectively, which is significantly high. This enhancement in modulus by the incorporation of a small amount of organically modified clay can be attributed to the portion of mineral sheets exfoliated, resulting in a greater mineral-PLA matrix interfacial area. Concerning the evolution of the maximal strength at break which expresses the ultimate strength that the material can bear before break, the differences observed are sufficiently notable to draw some comments. As indicated in Table 1, the tensile strength is decreased from 60.4 MPa for pure PLA to 28.8 MPa for the nanocomposite containing 5 wt.% of C30B. The relationships between tensile strength, filler/matrix adhesion and dispersion are more complex than for modulus, so no attempt is made at this time to justify the result with quantitative models as reported in the literature (Stretz et al., 2005). Ray and Bousmina (2005) have shown that in general, tensile strength of many nanocomposites has been found to increase with increased clay content. However in this case, the reason is not clear but it though that the decrease in the tensile strength is attributed to the decrease in the elongation at break, which is probably related to the delamination of the polymer-silicate interlayer. Elongation at break tends to decrease as expected for such materials when the interaction between the polymer and the filler becomes stronger.
3.3 Nanoindentation results
The modulus and hardness values obtained for the different PLA samples are shown in Table 2. Table 2 Summary of modulus and hardness values obtained for neat PLA and the different
PLA nanocomposite samples by nanoindentation using Berkovitch indenter
Samples Modulus (GPa) Hardness (GPa) PLA 4.36 ± 0.19 0.219 ± 0.018 PLA/C30B (1 wt.%) 4.38 ± 0.22 0.223 ± 0.022 PLA/C30B (3 wt.%) 4.97 ± 0.17 0.288 ± 0.019 PLA/C30B (5 wt.%) 4.99 ± 0.20 0.298 ± 0.022
Figure 3 shows compared results from tests performed on the different nanocomposites using nanoindentation. In this figure, we can observe the modulus evolution according to the indent displacement into the sample surface. A Poisson’s ratio of 0.35 was used in all modulus calculations. The Table 2 values are averaged on indentation depth of 350–450 nm from a 10 × 10 matrix. It can be observed the high standard deviation values (between 3.5 and 10%). However, the results given in Table 2, after a stabilization phase due to the roughness of the samples, are in a good agreement with those reported in the literature. For instance, the value of modulus obtained for neat PLA is 4.36 GPa and this is consistent with the data reported by Wright-Charlesworth et al. (2005) who found 4.60
Influence of loading rates on morpholgy and mechanical properties 397
GPa, while Pillin et al. (2008) indicated 4.49 GPa. However, these values obtained by nanoindentation present an overestimation of 20% compared with those of modulus measured by tensile test on normalized specimens (4.36 compared to 3.40 GPa for neat PLA modulus). According to the literature (Rodriguez and Gutierrez, 2003), these discrepancies can be interpreted as an effect of a scale factor and size effect between nanoindentation and tensile tests. However, another explanation of this phenomenon could be the effect of the high hydrostatic pressure generated beneath the Berkovitch indenter as suggested by Briscoe and Sebastian (1996).
Figure 3 Modulus profiles of nanocomposites using nanoindentation (see online version for colours)
Displacement into sample (μm)
0 100 200 300 400 500
Youn
g M
odul
us (G
Pa)
2
3
4
5
6
7
8PLAPLA-C30B (1 %)PLA-C30B (3 %)PLA-C30B (5 %)
As expected, Table 2 exhibits a slight enhancement (+14%) of the mechanical properties for structured PLA-based nanocomposites filled with silicate mass fraction above 3 wt.%, which is in a good agreement with the data found by Ray and Okamoto (2003). This property could be attributed to the formation of a percolated network structure at these sufficiently high clay loadings, due to the anisometry of clay particles. On the other hand, this effect is not obvious with the 1 wt.% of C30B reinforced PLA.
In the same way, the average modulus and hardness values are higher for the 3 and 5 wt.% of C30B reinforced PLA as shown in Table 2. However, they are also quite similar. This may be explained by a more heterogeneous structure of the nanocomposites, probably due to the difficulty to obtain a homogeneous composite at high loading rates (5 wt.%), as evidenced by WAXS results. This heterogeneous structure for the 5 wt.% clay composite could induce inaccurate nanoindentation results due to the sharp tip of the Berkovitch indenter. The penetration of the thin tip in the sample could occur in a matrix area and induce a displacement of the clay.
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4 Conclusions
PLA/C30B nanocomposites have been successfully prepared by a melt intercalation method. The materials morphology obtained indicates that the dispersion of mineral platelets within the PLA matrix is relatively homogeneous, as revealed by WAXS measurements. Moreover, it was found a significant improvement of the mechanical properties of the different nanocomposites increased with increase of clay loading, due to the addition of stiff clay nanofillers into the PLA matrix. The variation of the elongation at break evidences a decrease with clay loading indicating an alteration of the plastic deformation of the matrix with the incorporation of clay.
This work demonstrates also the good agreement between the mechanical measurements carried out using tensile tests, which can be considered as a macroscopic characterization technique, and those obtained by the nanoindentation technique which allows the determination of the mechanical properties at the nanometric scale. This good correlation of mechanical properties at the macro- and nanometric scales can be attributed to a high degree dispersion of nanoplatelets within the PLA matrix.
Acknowledgements
The research leading to these results has received financial support from EGIDE through TASSILI Program.
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