nanoclay ternary nanocomposites

Materials and Design 31 (2010) 4693–4703

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Materials and Design journal homepage: www.elsevier.com/locate/matdes

Morphological interpretations and micromechanical properties of polyamide-6/polypropylene-grafted-maleic anhydride/nanoclay ternary nanocomposites Naresh Dayma, Bhabani K. Satapathy * Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e

i n f o

Article history: Received 12 February 2010 Accepted 12 May 2010 Available online 16 May 2010 Keywords: Polymer matrix composites Mechanical Microstructure

a b s t r a c t Ternary nanocomposites were fabricated based on an optimized impact modified polyamide-6 (PA-6)/ polypropylene grafted maleic anhydride (PP-g-MA) blend composition with varied concentrations (0– 6 wt.% at a step of 2 wt.%) of organoclay, Cloisite 30B™. The morphological attributes such as state of intercalation/exfoliation/crystalline organization and fractured surface topography of the nanocomposites were characterized by transmission electron microscopy (TEM), wide angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM) while the thermal characterizations were done by conducting differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The WAXD/DSC studies have revealed that the crystallinity of the nanocomposites remained unaffected. DMA revealed an increase in glass transition temperature (Tg) of the nanocomposites by 14–19 °C relative to the soft polypropylene (PP)-phase, by 7–12 °C relative to the neat matrix PA-6 and by 4–9 °C relative to the optimized impact toughened PA-6 matrix while simultaneously being accompanied by the appearance of a second phase Tg peak progressively at higher temperatures as a function of nanoclay content, indicating the reinforcement effects/restrictions imposed by the nanoclay layers to the polymer chain mobility. The bulk mechanical response of the nanocomposites such as tensile, flexural and impact properties were studied and its related micromechanics aspects have been investigated using composite theories such as Halpin-Tsai, Hui-Shia, Takayanagi and Pukanszky models to analyze the interfacial effects and its role on the stress transfer efficiency. SEM analysis of fractured surface indicated that the failure mode of the nanocomposites undergoes a switch-over from interfacial-effects assisted fibrillation controlled ductile deformation to nanoclay induced soft PP-phase stiffened semi-ductile response via shear-lips formation. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Designing nanocomposite materials with enhanced mechanical performance has always been a problem of toughness-to-stiffness optimization which is functionally manipulated by several material processing, material selection and material modification routes. Polymer composites for engineering applications are mainly limited by several factors pertaining to the resin such as low modulus of the matrix, extreme notch sensitivity and poor thermal resistance. In such a scenario two stepped property modification approach following increasing the room temperature toughness via addition of a soft/rubbery phase as the primary step and subsequently the incorporation of a nanofiller to compensate any loss in the stiffness as a consequence of the step undertaken

* Corresponding author. E-mail address: [email protected] (B.K. Satapathy). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.05.024

previously giving rise to ternary nanocomposites may be adopted [1,2]. It is well known that stiffness can readily be improved by adding micro- and nano-particles since these rigid inorganic particles have a much higher stiffness than the polymer matrices. On the other hand, strength is primarily dependent on the stress-transfer mechanism in which nature of the interface plays a dominant role. Interestingly, enhancements in the energy dissipation efficiency which is related to impact toughness/fracture toughness has also been reported to get enhanced in many cases such as for pseudoductile and semi-ductile polymers via the incorporation of rigid inorganic particles like calcium carbonate and talc [3]. Such mechanical property enhancements may conceptually be related to domain size reduction, inter-particle distance of the nanoparticles or other parameters attached to morphological dimensions [4–6]. For example, the incorporation of layered silicates/organomodified nanoclays into a toughened bi-component blend matrix has been reported to aid particle/domain size reduction of the

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matrix polymer dispersed in the base matrix of the blend. Remarkably enhanced mechanical properties may be realized when high aspect ratio high modulus clay layers are dispersed via exfoliation at the nanoscale promoted by molecular level entanglements post polymer chain infiltration into the clay inter-gallery space [7–9]. In such a case not only the mechanical but also the functional performance such as flame retardance of nanocomposites may get enhanced [10]. Ternary nanocomposites based on binary blends in combination with nanoclay, such as in case of PA-6/PP/nanoclay, have been observed to increase the mechanical properties both in the plastic and elastic regimes. In ternary nanocomposite system with PA-6 as the dispersed phase in a PP matrix combined with the secondary incorporation of nanoclay, the clay tactoids get preferentially embedded in the PA-6 dispersed phase rather than in the PP matrix phase, an aspect which may have a correlation to the large gap in their melt viscosities. These eventualities are construed to be the consequences of two independently competing effects. Firstly, when the nanoclay gets preferentially localized in the minor phase the dispersed phase viscosity shoots up causing the stiffness of the same (soft) phase increased and thereby reducing the modulus-mismatch between the major and minor phases leading to enhanced stress transfer efficiency. Secondly, the alignment of the clay layers along the interface cause a reduction in the interfacial adhesion between the two phases and thereby limits the property enhancement [11]. Complex morphological dynamics with regard to the precise location of nanoparticles, interaction between the components and the preferential mobility of the rigid fillers into one of the phase makes the ternary nanocomposites very interesting systems to investigate the nano-morphological consequences onto mechanical properties [12]. Since neither the crystalline content nor the crystalline phase is modified upon blending, enhanced mechanical properties in the ternary nanocomposites were attributed to the additive effects of reinforcement and selective fibrillation of one of the phases of the blend matrix. The relative localization of the clay tactoids in a ternary nanocomposite system has been observed to play a critical role in controlling the morphology and the mechanical properties. For example, in case of ternary nanocompoites based on poly-butylene-terepthalate/ethylene-vinyl-acetate-grafted-maleic-anhydride/nanoclay (PBT/EVA-g-MA/ organoclay) [13], Nylon6,6/styrene-ethylene-butylene-styrenegrafted-maleic anhydride (SEBS-g-MA)/organoclay [14,15] and polypropylene (PP)/ethylene–vinyl-acetate (EVA) grafted maleic anhydride/organoclay [16] the addition of clay has lead to higher stiffness and lower toughness, which was attributed to the localization of organoclay in the continuous phase instead of the dispersed phase. Conversely, increase in toughness in combination with stiffness has also been reported in case of PP/ethylene-octene-copolymer/organoclay [17] and poly trimethylene terepthalate (PTT)/linear low density polyethylene (LLDPE)/nanoclay [6] based ternary nanocomposites. The toughness and stiffness enhancements are attributed to the decrease in the size of the dispersed phase domains or the formation of a mixed nanomorphology. Interestingly, a few research papers also highlight on the possibility of toughness enhancement when clay tactoids are either interface-localized or dispersed phase localized. The typical examples of such findings include PA-6/ethylene propylene rubber (EPR)/organoclay and PP/styrene-co-butadiene-co-styrene (SBS)/ organoclay based ternary nanocomposite systems reported by Kelnar et al. [18,19] and Li et al. [20] respectively. Such preferentially uneven distribution/localization of these nanoparticles giving rise to selective composition specific morphologies in case of ternary nanocomposites has been intensively investigated by Fenouillot et al. [21] where it was explained that the competition between thermodynamic wetting of the solid by the polymeric phase (of two dissimilar complex polymeric fluids) and the kinetically con-

trolled filler localization as a consequence of mixing dynamics determines the fine morphology and hence its mechanical properties. Kusmono et al. [22] have reported the compatibilizing effects of SEBS-g-MA in terms of mechanical property enhancement with reference to ternary nanocomposite system based on PA6/PP/ organoclay (organically modified montmorillonite). They have found that the longer the alkyl chain of the organic moiety attached to the clay layers the higher the crystallographic basal spacing and the better the exfoliation causing remarkable increase in mechanical property. Similarly, the effects of processing sequence on the critical inter-particle distance which fundamentally controls the brittle-to-ductile transition has also been investigated in case of ternary nanocomposite systems based on PA6/clay/SEBS-g-MA [23]. It was reported pertaining to the system PA6/clay/SEBS-gMA that with the increase in the matrix stiffness due to incorporation of nanoclay the inter-particle distance becomes smaller. In the light of the above literature-findings the present study focuses on the possibility of maximizing the toughness–stiffness combination of a ternary nanocomposite system based on PA 6/ PP-g-MA/nanoclay with a two stepped material engineering approach conforming to toughness maximization criteria, via dispersed soft-phase domain size reduction, which is manipulated by optimization of a blend composition followed by compensating the loss in stiffness by incorporating the nanoclay as a reinforcement into the optimized PA6/PP-g-MA blend system. The domain size reduction of the dispersed phase, i.e., PP-g-MA is expected because of the reduction in the interfacial tension between the two polymers due to polar-polar interactions via the amide (of PA6) and maleic anhydride (of PP-g-MA) linkages. 2. Experimental 2.1. Preparation of PA-6/PP-g-MA blends and nanoclay filled ternary nanocomposites The details of the materials selected and the processing conditions are given in Tables 1 and 2. The PA-6, PA-6/PP-g-MA blends was pre-dried to make them free of absorbed moisture content by keeping them in a vacuum oven at 80 °C for 8 h (overnight). The mixing of the two ingredients for blends and mixing of the optimized blend composition with Cloisite 30B organically modified nanoclay was carried out in a co-rotating type (Prism Euro Lab 16/Thermo Scientific) twin-screw extruder with L/D = 40, and at a screw speed of 300 rpm. The details of the temperature profile of the twin-screw extruder are given in Table 2. The continuous strands thus obtained were later chopped in a granulator and subsequently kept again for drying in a vacuum oven at 80 °C for 3 h prior to being injection molded on an L&T Demag injection molding machine (model PFY 40-LNC4P) to obtain test specimens conforming to ASTM standards. The detailed temperature profiles in the injection molding machine and the selected process parameters are given in Tables 3 and 4, respectively. The optimization of the PA-6/PP-g-MA blend compositions for the selection of the impact modified PA-6 grade to be taken as the matrix for the nanoclay reinforcement for the fabrication of the nanocomposites was done on the basis of the best combination of a set of mechanical properties. 2.2. Morphology characterization 2.2.1. Transmission electron microscopy (TEM) For the TEM investigations, cryocuts with a thickness of about 100 nm were cut from a middle position of the nanocomposite, showing the best combination of toughness-stiffness combination, (i.e., impact modified PA-6 with 4 wt.% of nanoclay), injection

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Table 1 Data sheet of raw materials and their characteristics. Raw material

Grade

Supplier

Characteristics

Polyamide-6 (PA-6)

GUJLON M28RC

GSFC Ltd., India

Polypropylene-grafted-maleic anhydride (PP-g-MA) Nanoclay

OPTIM P-405

PLUSS™ India

Density (q) = 1.14 g/ml; Tm (°C) = 220; MFI (g/10 min) = 28 @230 °C, 2.16 kg Density (q) = 0.91 g/ml; Tm (°C) = 163; MFI (g/10 min) = 15 @190 °C, 2.16 kg; MAH content (%) = medium (0.5–0.8) Density (q) = 1.98 g/cc, Off white powder, doo1 = 18.5 Å

Ò

Cloisite 30B

Southern clay

Table 2 Extrusion temperature profile. Zones

Z-1

Z-2

Z-3

Z-4

Z-5

Z-6

Z-7

Z-8

Z-9

Z-10

Temp. (°C)

220

225

225

230

230

235

235

240

240

245

Table 3 Temperature profile set in injection molding machine. Feed (°C)

Z-I (°C)

Z-II (°C)

Z-III (°C)

Nozzle (°C)

40

222

230

245

260

2.3.2. Dynamical mechanical analysis (DMA) Dynamic mechanical analysis (DMA) measurements have been carried out on the nanocomposite test specimens with dimensions of 20  8  1 mm3 in tensile mode on an Q800 (TA Instruments, USA) to characterize the storage modulus, loss modulus and Tan d for qualitatively investigating the reinforcement effects and quantitatively ascertaining shift (if any) in glass transition temperature of the nanocomposites in the temperature range of 30 and 140 °C at a frequency of 1.0 Hz (6.28 rad/s) and heating rate of 5 °C/ min. 2.4. Mechanical property investigations

Table 4 Processing conditions used in injection molding machine. Process parameter

Value

Injection pressure Injection time Oil temperature Cooling time

80 bar 4s 50 °C 25 s

moulded bars in flow direction using a Leica Ultracut EM UC6/EM FC6 ultramicrotome (Leica Microsystem GmbH, Wien, Austria). The ultramicrotome was equipped with a diamond knife with a cut angle of 35° (Diatome, Biel, Switzerland). The TEM used is a Philips CM 12 model operated at 100 kV and the micrographs were taken using an energy filter in zero loss imaging mode for an optimal contrast of the various phases at a lower magnification of 19,500 and at a higher magnification of 66,000.

Tensile tests were performed according to ASTM D 638 norms on a Zwick Z010 testing machine at room temperature with a test speed of 50 mm/min. Three injection molded tensile bars were tested for each composition. Flexural tests were conducted according to the ASTM D 790 method-I and notched Izod impact tests were conducted on specimen bars conforming to ASTM D 256 norms. 2.5. Fractured surface morphology The cryo-fractured surface morphologies of virgin PA-6, optimized impact modified PA-6/PP-g-MA blend and the nanocomposites have been investigated using scanning electron microscopy (SEM) on an EVO 50 apparatus operating at 20 KV to analyze the associated failure-mechanisms and structural integrity of such multiphase nanostructured materials. The surfaces of the specimens were gold sputter coated prior to examination to make the surfaces conductive. 3. Results and discussion

2.2.2. 2D Wide angle X-ray diffraction (WAXD) Wide angle X-ray diffraction (WAXD) was carried out on the extruded granules (quasi-isotropic sample) to characterise crystallinity and orientation in the samples, apart from investigating the changes in the peaks corresponding to different crystal planes as a function of nanoclay content. The measurements were done with a X-ray diffractometer Philips X’PertPRO, PANalytical diffractometer using Cu Ka radiation (k = 1.54 Å) at 40 kV and 40 mA with a sample-to-detector distance of 120 mm and a radial scattering range of 2–40°. The crystallinity was evaluated by applying the peak-area method while integrating in the range of 2h = 5–35° (as typical for PA-6) with applying an amorphous scattering curve which was realized by experimental and theoretical experiences. 2.3. Thermal charactrization 2.3.1. Differential Scanning Calorimetry (DSC) DSC measurements, to obtain information about the influence of the nanoclay on the crystallisation behaviour of the impact modified PA-6, were conducted with a DSC 7 instrument (Perkin– Elmer) at a scan rate of ±10 °C/min in a temperature range of 40–250 °C.

3.1. Optimization of PA-6/PP-g-MA blends (impact modified PA-6) for fabrication of nanocomposites The mechanical properties of the binary PA-6/PP-g-MA blends are shown in Fig. 1. It can be observed from Fig. 1a that the tensile strength increased by 30% with the increase in PP-g-MA content up to 10 wt.% followed by a consistent decrease by 21% till a PPg-MA content of 30 wt.%. This may be attributed to the fact that above 10 wt.% of PP-g-MA content in the blend the crystallinity has been observed to decrease substantially by 25–35% (based on WAXD measurements)/50–70% (based on DSC measurements) and will be exclusively discussed in a future communication [24]. In another independent study pertaining to PA6/PP/PP-g-MA based blend system Sathe et al. [25] have explained that a decrease in the yield strength may possibly be due to enhanced plasticization effect of the low molecular weight PP-g-MA, when the content of PP-g-MA exceeds certain saturation level to act as only compatibilizer. However, the relevance of such findings to the present investigation may need further detailed investigations. The tensile modulus, however, remained inappreciably affected in the entire composition range of the blends indicating that the modulus is more matrix-controlled, i.e., PA-6 determined. On the other hand

4696

70

Tensile Strength [MPa]

60

50

500 600 500

450

400 300 200

400

100 0

5

10

15

20

25

30

350

PP-g-MA content [wt.%]

300

40 Tensile Strength Tensile Modulus

250

Tensile Modulus [MPa]

Elongation at break [%]

a

N. Dayma, B.K. Satapathy / Materials and Design 31 (2010) 4693–4703

30 200 0

b

5

10 15 20 25 PP-g-MA content [wt.%]

30

35

50

1050

Flexural Strength Flexural Modulus

45 950

900

40

Flexural Modulus [MPa]

Flexural Strength [MPa]

1000

850 35 0

Notched Izod Impact strength [J/m]

10

15 20 25 PP-g-MA content [wt.%]

30

35

0.40

140 130

0.35

120 110

0.30

100 0.25

90 80

0.20 70 60

Notched Izod Impact strength Impact strength to Young's modulus ratio

0.15

Impact strength/Young's modulus ratio

c

5

50 0

5

10

15

20

25

30

35

PP-g-MA content [wt.%] Fig. 1. (a) Variation of tensile strength, tensile modulus and elongation at break with PP-g-MA content; (b) Variation of flexural strength and flexural modulus with PP-g-MA content; (c) Variation of notched Izod impact strength and Impact strengthto-Young’s modulus ratio with PP-g-MA content.

the elongation to break increased from 150% in case of PA-6 to more than 500% for the blend composition with 10 wt.% of PP-gMA followed by a decrease by 100% till the blend composition with 20 wt.% of PP-g-MA before eventually dropping down to a

level of strain at break less than that of the virgin PA-6 matrix, i.e., an elongation to break of about 100% for the blend composition with 30 wt.% of PP-g-MA. The cryo-fractured specimens have shown prominent fibrillar surface morphology which is discussed in a subsequent section. Chow et al. [26] have reported extensive necking induced formation of fibrillar fracture surface while investigating on PP-g-MA compatibilized PA6/PP/organoclay based nanocomposites. The flexural properties showed less composition dependence in comparison to the tensile properties as shown in Fig. 1b. For example, the flexural modulus remained unaffected till 10 wt.% of PP-g-MA content in the blend beyond which it consistently increased indicating an enhanced bending resistance/elasticity of the blend compositions. On the other hand the flexural strength decreased though the extent of decrease/variation in the flexural strength data remained <10% irrespective of the blend composition. Interestingly, the impact properties (notched Izod impact strength) attained a maximum at 20 wt.% of PP-g-MA content in the blend i.e., it showed an increase by 224% and 355% when compared to virgin PA-6 and PP-g-MA, respectively, as shown in Fig. 1c. Such a systematic increase in the impact strength may be attributed to the enhanced volume fraction of the soft PP-phase though above 20 wt.% of PP-g-MA the extent of increase compared to the PA-6 matrix was reduced due to enhancement in the PP-domain size and possibly thereby affecting the state of distribution of the PP-phase in the PA-6 matrix. This evidently indicates the much reduced notch sensitivity of the blend composition in contrast to the PA-6 matrix. Further, the stiffness-normalized impact strength data (ratio of impact strength to Young’s modulus) which theoretically/semi-empirically expresses the toughness-to-stiffness combination has also showed a maximum for the blend composition with 20 wt.% of PP-g-MA (Fig. 1c), an observation imperatively suggesting that the blend composition with 20 wt.% of PP-g-MA may be the optimally impact toughened PA-6 matrix, which was taken as the impact modified PA-6 matrix for the fabrication of the ternary nanocomposites with different concentration of nanoclay. The study on the fractured surface morphology of the virgin PA6, and the PP-g-MA blend via SEM also supports the optimized blend composition with 20 wt.% of PP-g-MA through exhibiting characteristics of enhanced plastic deformation prior to failure at the deformed/fractured surface (Fig. 2). 3.2. Morphology of the ternary nanocomposites from TEM and WAXD The details of the investigated ternary nanocomposites based on the optmised blend composition (NPC0) i.e., PA-6 (80 wt.%)/ PP-g-MA (20 wt.%) are given in Table 5. The bulk morphology as seen from the TEM micrographs conducted on microtomed slices (of 120 nm) of the nanocomposite based on PP-g-MA impact modified PA-6 with 4 wt.% of nanoclay content (NPC4) reveal PPdomains dispersed in the PA-6 matrix as regions of maximum grey/white contrast and are shown in Fig. 2a. It is also clearly seen that there are regions of overlapping grey domains appearing white which explaining the coalescence of PP-droplets in the melt stage that are frozen-in as the sample was cooled down to room temperature. The TEM micrographs also indicate that the incorporation of nanoclay has effectively facilitated the domain size reduction of the dispersed PP-phase in the ternary nanocomposite. Such domain size reduction of PP-phase may have occurred via the intersection of PP-domains by nanoclay as indicated from the sharp interfaces amidst some of the PP-domains. The multiple layers of tactoids of thickness 10–20 nm layers are clearly seen to have caused the domain size reduction by penetrating through the dispersed PP-phase. Such domain size reduction and selective impingement (uneven distribution of nanofillers in the two phases of a blend) of nanofillers into soft-phase has also been reported in other ternary nanocomposite systems [6,13–21]. On the other

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tomed samples. Thus TEM investigation qualitatively indicates that phenomenon of exfoliation/intercalation of the organically modified nanoclay layers as an effect vis-à-vis domain size reduction via decrease in the vigor of domain coalescence. In Fig. 1b the TEM picture displays the increase in clay gallery space, which however, needs to be precisely quantified by conducting TEM on much thinner samples. Further Fig. 1b also amply indicates that the nanoclay is well intercalated in the ternary nanocomposite system with few clay layers apparently showing some features of exfoliation (localized). The WAXD studies conducted on the extruded strands of nanocomposites has also confirmed the complete absence of any peak in I (Intensity) versus 2h plot in the 2h range of 0–5° (Fig. 3a). Such an observation may lead to the understanding that the exfoliation characteristics are much favored in multiphase blend based systems. Theoretically such an observation may directly be inferred as a sign of near perfect exfoliation/dispersion of clay tactoids though the consequences of the same in terms of enhancement in the thermo-mechanical/electrical properties need to be carefully investigated, without which the inferences are prone to be misleading [27]. It must be mentioned here that the peak width substantially decreased on incorporation of organo-clay into the impact modified PA-6 matrix in the 2h range of 20–25 ° indicating that the crystal structure undergoes structural reorganization induced by nanoclay while simultaneously supporting the fact that in the nanocomposites (NPC2, NPC4 and NPC6) the crystalline structures are more defined as compared to its matrix (NPC0). Further the position of the peak in the 2h range of 15° has also been observed to be changed on incorporation of nanoclay into impact modified PA-6 (NPC0). Kulshrestha et al. [28] while studying on the aspects related to enhancement in mechanical properties due to nano addition of raw bentonite to PP have reported similar observations. It was claimed that the peak may shift to lower angles due to the combinatorial effects of intercalation of clay layers and shortening of the period size of the PP causing a nominal drop in crystalline width. However, the estimation of overall crystallinity from WAXD studies indicated that the percentage crystallinity remained nearly unaffected due to incorporation of nano-platelets/nanoclay. Such an observation in terms of its trend has also been reconfirmed by the DSC measurements and is shown in Fig. 3b. However, the quantitative appreciation of the crystallinity data as obtained from WAXD and DSC measurements lack strict equivalence which may be due to the differences in the structural changes of the sample when subjected to different characterization techniques. Fig. 2. Transmission electron microscopy (TEM) images of NPC4 (a) Low magnification: 19,500; (b) High magnification: 66,000.

Table 5 Details of the investigated ternary nanocomposites. Sl. no.

Composite designation

Wt.% of the impact modified optimized blend i.e. (PA-6/PP-g-MA; (80:20)) as the matrix for nanocomposite

Wt.% of nanoclay (CloisiteÒ 30 B)

1 2 3 4

NPC0 NPC2 NPC4 NPC6

100 98 96 94

0 2 4 6

hand the Fig. 2b the nanoclay layers in general are seen to be nicely intercalated and may partly be exfoliated, though the morphological characterization pertaining to the degree of exfoliation needs further investigation by conducting HR-TEM on thinner micro-

3.2.1. Dynamic mechanical properties (DMA) The dynamic mechanical properties of the composites are shown in the plots of storage modulus (E0 ), loss modulus (E00 ) and loss tangent (tan d) as a function of temperature (Fig. 4). From Fig. 4a it was observed that E0 increased from 2600 MPa to 2840 MPa at sub-zero (<0 °C) temperatures with the incorporation of nanoclay into the impact modified PA-6 matrix. However, as the temperature was slowly increased the enhancement in the E0 due to nanolay has been observed to be clearly distinguishable, i.e., the nanocomposite with 2 wt.% of nanoclay (NPC2) has shown a modulus-decay over a lower temperature regime than the other composites, such as the ones with 4 wt.% and 6 wt.% of nanoclay, i.e., NPC4 and NPC6. The modulus-decay slope for PP-g-MA has been observed to be broad indicating higher elastic energy storage ability of this phase. Similarly in the plots with E00 versus temperature (Fig. 4b) and tan d versus temperature (Fig. 4c) plots a shift in the loss-peaks indicating shift in the glass transition temperatures (Tg) towards higher temperatures has been observed in the nanocomposites. The Tg peak corresponding to the PA-6 phase shifts from 31 °C in the impact modified PA-6 matrix to 35 °C,

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2

4

PA-6 PP-g-MA NP20 (NPC0) NPC2 NPC4 NPC6

Cloisite 30B

6

8

10

2 Theta [deg.]

a

3500 PA-6 PP-g-MA NPC0 NPC2 NPC4 NPC6

3000

Storage modulus [MPa]

Intensity [counts]

Intensity [counts]

a

4.83 [ o ]; 18.29[Å]

4698

2500 2000 1500 1000 500 0

5

10

15

20

25

30

-50

35

0

100

b

40

200 PA-6 PP-g-MA NPC0 NPC2 NPC4 NPC6

180

Loss modulus [MPa]

Crystallinity [%]

35

30 Crystallinity WAXD Crystallinity DSC

25

150

o

2 Theta [deg.]

b

50

Temperature [ C]

20

160 140 120 100 80 60 40 20

15

0 -50 0

2

4

0

6

c

0.14

3.2.2. Mechanical properties of the ternary nanocomposites The bulk mechanical properties of the impact modified PA-6/ nanoclay based ternary nanocomposites are shown in Fig. 5. It was seen that the tensile and flexural modulus values of the nanocomposites increased by more than 135% and 60%, respectively,

150

PA-6 PP-g-MA NPC0 NPC2 NPC4 NPC6

0.12 0.10

Tan δ

40 °C, and 39 °C for nanocomposites with nanoclay contents of 2, 4 and 6 wt.%, respectively, i.e., for NPC2, NPC4 and NPC6. Thus the Tg of the nanocomposites increased in the range of 7–12 °C, 15–20 °C and 4–9 °C with respect to neat PA-6, PP-g-MA and optimized impact modified PA-6 (NPC0) blend (PA-6/20 wt.% PPg-MA) respectively. This evidently indicates that incorporation of organically modified nanoclay has not only caused restriction of mobility of the polymer chains since the glass transition shifted to higher temperatures but also has indicated that the nanoclay is dispersed/infiltrated into both the phases, i.e., in PA-phase and PP-phase of the nanocomposites, an aspect which is also in compliance to the TEM (distinct layers of nanoclay platelets) and WAXD (absence of any peak in the 2h  0–5°) observations. However, the extent of increase in Tg of the PP-phase has been observed to be more uniform and consistent with the nanoclay content indicating an ease in the nanoclay localization amidst the PP-phase when compared to the PA-6 phase. This aspect has also been apparently observed from the TEM micrographs (Fig. 2).

100 o

Nanoclay content [wt.%] Fig. 3. 2D wide angle X-ray diffractometry (WAXD), (a) intensity versus 2h diffractograms for the blend components and nanocomposites, (b) variation of crystallinity (%) as function of nanoclay content as measured from WAXD and DSC.

50

Temperature [ C]

0.08 0.06 0.04 0.02 -50

0

50

100

150

o

Temperature [ C] Fig. 4. Dynamic mechanical analysis (DMA) plots (a) storage modulus (E0 ) versus temperature, (b) loss modulus (E00 ) versus temperature and (c) Tan d versus temperature.

as compared to that of the impact modified PA-6 matrix (NPC0) (Fig. 5a and b). Similarly the tensile strength and flexural strengths have also increased with the increase in nanoclay content in the composites. It must be emphasized that the enhancements in the mechanical properties have remained consistent in the entire composition range till the maximum nanoclay loading of 6 wt.% with consistently increasing loss in the elongation to break, i.e., the loss

N. Dayma, B.K. Satapathy / Materials and Design 31 (2010) 4693–4703

a

650

58 Tensile strength Tensile modulus

600

54

550

52 500 Elongation at break [%]

50 48 46 44 42 0

2

450 400 350 300 250 200 150 100 50 0 -50

450 400 350 0

2

4

6

Nanoclay content [wt.%]

4

Tensile modulus [MPa]

Tensile strength [MPa]

56

300

6

Nanoclay content [wt.%]

b

2400 Flexural strength Flexural modulus

2200

80

2000

70

1800 1600

60

1400 50

1200 40

Flexural modulus [MPa]

Flexural strength [MPa]

90

1000

0

2

4

6

Nanoclay content [wt.%]

Notched Izod impact strength Impact strength to Young's modulus ratio

0.35

110

0.30

100

0.25

90

0.20

80

0.15

70

0.10 0

2

4

Impact strength/Young's modulus ratio

Notched Izod impact strength [J/m]

c 120

95% in the case of naocomposite with 6 wt.% nanoclay content (NPC6). This conceptually may demonstrate a change in the mechanism of the deformation mode as a function of nanoclay concentration, which however, may be critically investigated in future. The variations in the notched Izod impact strength and the toughness-to-stiffness ratio are shown in Fig. 5c. It was observed that the impact strength increased by 170% due to the incorporation of nanoclay when compared to virgin PA-6 matrix however, when compared to the impact toughened PA-6 matrix (NPC0) the impact strength suffered a reduction by 25% indicating some possible damping restraining mechanism (nanoclay induced mobility restrictions) to be initiated due to nanoclay. However, the toughness-to-stiffness ratio (impact strength-to-Young’s modulus) has been observed to decrease abruptly with the incorporation of 2 wt.% of nanoclay (NPC2) into the impact toughened PA-6 blend matrix which however substantially suffered a further reduction beyond 4 wt.% of nanoclay content (NPC4) in the ternary nanocomposite system. This inevitably indicates that the composition domain of the nanocomposite with 2–4 wt.% of nanoclay offers maximised overall mechanical performance compared to the PA6, PP-g-MA and impact toughened PA6 grade (NPC0). The enhancements in impact property is close to threefold when compared to the impact modifier PP-g-MA, enhancements in stiffness is close to 170% in contrast to PA-6, without any substantial compromise in the yield strength of and with more than 165% enhancement in the impact strength in contrast to that of PA-6. The optimum mechanical performance enhancement in this composition is also supported by the structural features where apparently the core matrix PA-6-phase and the soft-dispersed PP-phases have been observed to be toughened/reinforced by the entrapment of nm-scale clay tactoids as shown in Fig. 2b. 3.3. Theoretical modeling of mechanical properties

800

30

4699

6

Nanoclay content [wt.%] Fig. 5. (a) Variation of tensile strength, tensile modulus and elongation at break with nanoclay content; (b) variation of flexural strength and flexural modulus with nanoclay content; (c) variation of notched Izod impact strength and impact strengthto-Young’s modulus ratio with nanoclay content.

in the ductility index (Fig. 5). This is revealed from the fact that the elongation to break decreased by 65%, 88% in the nanocomposites with 2 and 4 wt.% of nanoclay, i.e., in NPC 2 and NPC4. Strikingly, the magnitude of decrease of the elongation at break is

3.3.1. Low strain mechanical response; modeling of elastic modulus The stress transfer efficiency of these ternary nanocomposites may be analysed by adopting several successful models for particle/particulate filled polymer systems. The models that have been explored here are Halpin-Tsai model, Hui-Shia model, Takayanagi model and Voigt upper bound and Reuss lower-bound prediction models for elastic moduli of filled polymer systems. The HalpinTsai model [29,30] is most widely used in the fiber composites industry to predict the tensile modulus of unidirectional composites as a function of aspect ratio. The Halpin-Tsai model can deal with a variety of reinforcement geometries, including discontinuous filler reinforcement such as fiber-like or flake-like fillers. The Young’s modulus of a composite material in the Halpin-Tsai is proposed as in Eq. (1),

1 þ fg/f Ec ¼ Em 1  g/f    Ef Ef 1 þf where; g ¼ Em Em

ð1Þ ð2Þ

where Ec, Ef, and Em are Young’s modulus of composites, inclusions and polymer matrix, respectively. /f is the filler volume fraction and g is the shape parameter dependent on filler geometry loading direction as given Eq. (2). In particular direction, f = 2 (l/d) for fibers or 2 (l/t) for disk-like platelets. l, d, t are the length, diameter and thickness of the dispersed fillers. In the present investigation l/t is assumed to be 100. As a matter of fact, 2D disk-like clay platelets can make less contribution to modulus than 1D fiber-like inclusion. A modulus reduction factor (MRF) [31] is introduced to modify the Halpin-Tsai model as (Eq. (3)):

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N. Dayma, B.K. Satapathy / Materials and Design 31 (2010) 4693–4703

1 þ fðMRFÞg/f E ¼ Em 1  g/f

ð3Þ

ð4Þ

Conversely, when n ? 0, the Halpin-Tsai model reaches at the lower bound elastic modulus value corresponding to the elastic response, when the reinforcement and the matrix phases experience equal stress (i.e., iso-stress approach), the situation so arrived is normally called as Reuss inverse rule of mixtures (IROM) [32,34]. The Reuss model may be stated as in Eq. (5),

1 /f ð1  /f Þ ¼ þ Ec Ef Em

1000

0 0

2

4

6

Nanoclay content [wt.%]

b

55 Experimental Pukanszky model

54

n ¼ /f þ

ð7Þ

  ð1  gÞa2  2g Em þ 3ð1  /f Þ Ef  Em a2  1

a

3ða2 þ 0:25Þg  2a2 K ¼ ð1  /f Þ a2  1



ð1  bÞ þ b EE10

51 50 49 48

46 0

ð9Þ

where a is the inverse aspect ratio of dispersed fillers and a = t/l for disk-like platelets (a 6 0.1). In another approximation where the effects of interface are neglected (t = 0), Ji’s model [37] for tensile modulus prediction reduces to the two phase Takayanagi model [38,39]. Typically, the Takayanagi model prediction underestimates the Young’s modulus since the interfacial contribution which plays a functional role in stress-transfer mechanism is ignored. The model may be stated as in Eq. (10),

b

52

47

ð8Þ 

Yield stress σ y [MPa]

ð6Þ

"

2000

53

Ec 1   ¼ E m 1  /f 1 þ 3 4 n nþK

E ¼ E0 ð1  bÞ þ

3000

ð5Þ

Similarly, the Hui-Shia model [35,36] was developed to predict the tensile modulus of composites including unidirectional aligned platelets with the simple assumption of perfect interfacial bonding between the polymer matrix and platelets, which is given by

2

Elastic Modulus [MPa]

Ec ¼ /f Ef þ ð1  /f ÞEm

p

Experimental Halpin-Tsai Hui-Shia introducing 0.66 in Halpin-Tsai Voigt ROM Reuss IROM Takayanagi

4000

Based on the conventional rule of mixture principle the elastic modulus may also be predicted by using a modified form of Halpin-Tsai model, especially in the cases when n ? 1. In such a scenario the Halpin-Tsai model predictions concerning the elastic modulus reach the upper bound. Such an iso-strain (i.e., when fiber and matrix undergo the same uniform strain under a certain applied stress) approximation based solution is normally referred as Voigt rule of mixtures (ROM) [32,33]. The Voigt rule may be stated as in Eq. (4),

g¼

a

#1 ð10Þ

pffiffiffiffiffi where, b ¼ /f , /f is the volume fraction of the nanoclay platelets. However, by considering that E0 = 343 MPa and E1 = 178 GPa, and platelet thickness as 50 nm, the tensile modulus prediction tends to follow a trend with the closest proximity to the experimentally determined trend. The various theoretical model fit elastic moduli and the experimental values of the elastic moduli for the nanocomposites are shown in Fig. 6a. Among all the model fits the predicted moduli based on Hui-Shia model and Takayanagi model have revealed the closest proximity to the experimentally determined elastic modulus values. It was also observed that the Halpin-Tsai model can predict the elastic modulus at low nanofiller loadings, i.e., within 2 wt.% of nanoclay loading (i.e., up to NPC2) indicating that the agglomeration and structural inhomogenity related issues may not be well encompassed by this model in defining the elastic response of the investigated nanocomposites. In a sim-

2

4

6

Nanoclay content [wt.%] Fig. 6. Theoretical modeling of (a) elastic modulus and (b) yield strength as a function of nanoclay content using various equations/models based on micromechanics and composite theories.

ilar way the modified Halpin-Tsai model with a modulus reduction factor in the expression has also predicted values that were more linear though the predicted elastic moduli using such a model has suffered from much larger deviation from the experimentally determined values. On the other hand the Hui-Shia model could comprehensively justify the elastic response of the composites in terms of the modulus values up to 4 wt.% of nanoclay loading (NPC4) indicating the possibility of a near perfect state of interfacial bonding between nanoclay and the polymer matrix as per the assumptions of the model. Interestingly, the Takayanagi model fit reveals a resembling trend to that of the experimental values of elastic modulus though the quantitatively the predicted values remained lower. This may be attributed to the fact that the role of polymer–nanofiller interface is completely ignored in this model which is in striking contrast to the Hui-Shia model where the polymer–nanofiller adhesion was considered to be of appreciable significance, as revealed from its corresponding model fit. The Voigt upper bound and Reuss lower-bound predictions have also been estimated and the experimental values of the moduli have remained well within the range. However, the magnitudes of elastic moduli have been observed to remain closer to the experimental values indicating the substantial extent of deviation from the theoretically expected state of exfoliation/dispersion of the platelets/tactoids of nanoclay in the polymer matrix.

N. Dayma, B.K. Satapathy / Materials and Design 31 (2010) 4693–4703

3.3.2. High strain mechanical response: modeling of yield stress The yield stress (ry) was analysed using the two phase composite model such as the Pukanszky model to estimate the theoretical yield stress. A characteristic of all theoretical approaches is a relationship between volume fraction (/f) and projected area fraction of the particulate inclusions [38–40]. The Pukanszky model [38– 40] may be stated as in Eq. (11), i.e.,

ryt ¼ rym

1  /f expðB/f Þ 1 þ 2:5/f

ð11Þ

where, B is an empirical parameter characterizing the degree of filler-matrix interaction. The value of the parameter B depends on all factors influencing the load bearing capacity, i.e., strength of the interface and size of the interface. The model was developed for composites containing spherical particles however, anisotropic fillers can lead to increase of the load-transfer efficiency and hence to a higher B value. However, the Pukanszky model was used to describe the theoretical yield stress of polypropylene-layered silicates nanocompos-

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ites by Szazdi and co-workers [41]. Different types of treated layered silicates were used to compare the experimental mechanical results with the model. The B-parameter for the different nanocomposites remains in between 2 and 15 for composites with an intercalated or exfoliated morphology. The model was also found to be valid for the investigated ternary nanocomposite based on PA6/PP-g-MA/nanoclay system and the validity is indicated by the linear correlation between natural logarithm of reduced yield stress plotted against nanofiller content (Fig. 6b). The calculated B-parameter of B = 7.36, comprehensively demonstrates not only an efficient load – transfer mechanism but also the theoretical possibility of a mixed state of partially intercalated and partly exfoliated nanomorphology. However, at low nanoclay-contents model fit leads to a much lower ry than the experimental values. Though the anisotropy and orientation of the nanofillers are not taken into account, they have a definitive influence on the extent of reinforcement. In addition, nucleation effects or other morphological changes can change the matrix properties significantly and could alter the prediction of the mechanical properties with this model [41,42].

Fig. 7. Cryofractured surface micrographs showing the topographical/morphological features of (a) PA-6, (b) NPC0, (c) NPC2, (d) NPC4 and (e) NPC6 using scanning electron microscopy (SEM).

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3.4. Fractured surface morphology investigations The cryo-fractured surface morphologies of the nanocomposites and the matrix-blend were analyzed using scanning electron microscopy to characterize the bulk structural attributes. The SEM micrographs are shown in Fig. 7. It may be seen that the fractured surface of virgin PA-6 (Fig. 7a) apparently shows a surface with many topographical irregularities indicating the homogeneous deformation of the matrix phase prior to fracture. However, in Fig. 7b the morphology of the optimized blend matrix (NPC0) reveals evenly distributed fibrillated structures originating from within the bulk of the material. These fibrillate structures indicate the discontinuous soft PP-phase which was added to the PA-6 matrix for toughness enhancement and to reduce the notch sensitivity. Mechanistically such fibrillated phases indicate to have offered resistance to the fracture process, via energy dissipation mechanism, since PP is a softer matrix with much lower Tg as compared to that of PA-6. In fact many of such fibril like structures have apparently formed localized network like crisscross morphology as indicated on the Fig. 7b. Interestingly, on the incorporation of nanoclay of 2 wt.% (NPC2) and 4 wt.% (NPC4) the surface density of such fibril like structures, believed to be of soft PP-phase, had remarkably decreased (Fig. 7c and d). On increasing the extent of incorporation of nanoclay to 6 wt.% (NPC6) the fibril like structures, in Fig. 7e unlike in Fig. 7d, are observed to be apparently inappreciable and instead the nature of the fractured surface appears to be filled with very regular shear-lip kind of homogeneously deformed topography. Thus it may be well comprehended that with the increase in the nanoclay content the mechanism of soft PP-phase deformation undergoes a transition from ductile behaviour dominated fibrillation to semi-ductile (i.e., attributed to nanoclay induced stiffening of PP-phase) dominated shear-lip formation. Such an observation is well complemented by the enhancements in mechanical properties, as indicted from the increase in modulus, strength and impact strength.

4. Conclusions Ternary nanocomposites with nanoclay as reinforcement have been fabricated based on an optimized blend composition of PA6 (80 wt.%)/PP-g-MA (20 wt.%) as the matrix. The morphological studies reveal nanoclay intermediated domain size reduction of the dispersed soft PP-phase via selective impingement of the quaternary ammonium modified clays that are chemically compatible to the MA moiety on the short PP chains. The WAXD studies revealed the complete absence of any peak in the 2h range of 2– 8° implying a good intercalation and partial exfoliation of the clay platelets. The incorporation of nanoclay into the optimized impact modified PA-6 blend matrix has shown to cause a shift in the Tg of the dominant PA-6 phase to higher temperatures indicating nanoclay induced mobility restrictions of the polyamide chains. The mechanical properties such as elastic modulus and flexural modulus have been observed to get enhanced substantially while maintaining the impact strength of the blend matrix without much quantitative changes. The micromechanics aspects have been studied in terms of the application of the various composite theories to analyse the bulk mechanical response of the ternary nanocomposites. While the theoretical predictions from Hui-Shia model was found to be well in congruence with the experimental data the predictions based on Takayanagi model have clearly revealed a considerable amount of deviation from the experimental moduli values. This reaffirms the presence of a good interface between the optimized impact modified PA-6/PP-g-MA blend matrix and nanoclay. Pukanszky model was found to be successful in predicting the yield strength of the nanocomposites to a closer approxi-

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