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Abstract

In this study, the effect of nanoclay on the rheological and morphological properties of polyamide 6 (PA6)/acrylonitrile–butadiene–styrene (ABS) blends was investigated. The scanning electron microscopy micrographs showed that with increment in the nanoclay content, the dispersed phase droplets size and their polydispersity index decreased, and the finer and more uniform dispersed phase was obtained. The transmission electron microscopy micrographs of nanocomposites indicated well-dispersed nanoclay tactoids in the polymer matrix produced by exfoliation of the nanoclay in the polymeric blends. Dynamic strain sweep experiments showed that the extent of the linear viscoelastic region is sensitive to the nanoclay content and compatibilizer. With increasing nanoclay content in the blend, the extent of the linear viscoelastic region decreased. On the other hand, the rheological measurements revealed that the nanoclay content has a significant effect on the moduli and complex viscosity of the blends. These results have indicated that with increasing nanoclay content the storage modulus (G′), loss modulus (G′′) and complex viscosity (η*) increased. In addition, the results of creep experiments revealed that with the addition of compatibilizer (polyethylene octene elastomer grafted with maleic anhydride) and nanoclay to PA6/ABS blends, creep and recovery strain, over time, decreased remarkably and the recovery percentage increased. It was concluded that there is a good conformity between the results obtained from morphological and rheological investigations.

Keywords

PA6 ABS nanocomposites rheological morphological

Introduction

In recent years, polymer blends have obtained a high popularity in the industrial and academic centers due to their ability to merge the advantages of polymer blend components for specific applications. ¹ However, most polymer blends are immiscible due to undesirable interactions and hence form two-phase blends.^2,3 The unfavorable interactions between the components of polymer blends resulted in weak interfacial adhesion among phases, coarse and unstable morphology, and poor final performance of blends. So, for immiscible polymer blends it is necessary to improve interfacial interactions using appropriate compatibilizer. ^4,5 Due to some interesting properties, such as high tensile and impact strength, high thermal, chemical and weathering resistance, polyamide 6 (PA6)/acrylonitrile–butadiene–styrene (ABS) blends have found already some important commercial applications, e.g. automotive, airspace industries and electronic devices. ^6,7 These applications can still be extended by improving thermal stability and mechanical properties of the blend using layered nanoclay as a reinforcing agent.⁸

Many published results show that few percentage of nanoscale dispersed in a polymeric material can lead to significant increase in physical mechanical properties of the matrix, which is due to high-specific surface area and aspect ratio of nanoclay.⁹ It is clear that the morphology and the content of the dispersed phase play a key role in the performance of polymeric nanocomposites. Yongjin et al.⁸ investigated cocontinuous PA6/ABS nanocomposites and indicated that greatly exfoliated nanoclay platelets were mainly located in the PA6 phase and rubber particles were only dispersed in the styrene–acrylonitrile (SAN) phase. They also found that the heat resistance of the nanocomposite blends increased with an increase in the nanoclay content in the nanocomposites due to the cocontinuous structure of the blends and the selective location of the nanoclay. Sailer et al.¹⁰ studied the effects of reactive compatibilization on the rheological properties and morphology of PA6/SAN blends and revealed that reactive compatibilization considerably increases the complex modulus of PA6/SAN blends at low frequencies. They also reported that the results of the PA6/SAN (50/50) and the PA6/SAN (30/70) blends displayed that an elastic network between PA6 domains was formed.

Lele et al.¹¹ studied creep behavior of uncompatibilized and compatibilized polypropylene/nanoclay using maleated polypropylene as a compatibilizer agent and their results indicated that creep resistance increases in the presence of compatibilizer. Yang et al.^12
–14 investigated the rheological properties of PA 66 nanocomposites and reported a high increase in the creep resistance of polymers using different kinds of nanofillers. They also studied in another work, the effect of nanoparticles on creep behavior and they found that the nanoparticles may avoid slippage of polymer chains and therefore reinforced long-term characteristics of polymers. Jia et al.¹⁵ studied the creep and recovery behavior of polypropylene (PP)/multi-walled carbon nanotube nanocomposites. They found that the creep strain increased with temperature and decreased with the amount of carbon nanotubes.

PA6 and ABS, two of the most commercially available polymers, were selected as the blend components. Processability and physical mechanical properties of the polymer blend nanocomposites highly depend on their rheological and morphological properties. So, this work has been devoted to study the effects of the nanoclay and a maleated POE compatibilizer on the melt rheological and morphological properties of the PA6/ABS nanocomposites. The rheological behavior of the nanocomposite samples was investigated using the following three experiments: (1) dynamic strain sweep; (2) frequency sweep in the linear viscoelastic region; (3) creep and recovery responses. The morphology was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Experimental

Materials

PA6 (Ultramid B3S HP) a commercial product from BASF (Berlin, Germany) with a melt flow index (MFI) of 175 g/10 min at 275°C and 5 kg test weight, density of 1.13 g/cm³ and ABS (SD-0150 grade) with an MFI of 1.7 g/10 min at 200°C, a density of 1.04 g/cm³ prepared from Tabriz Petrochemical Company, Iran, have been used as received in their granular forms. Polyethylene octene elastomer grafted with maleic anhydride (POE-g-MA) (Fusabond MN493D) with a density of 0.87 g/cm³, a product of DuPont Dow elastomers also received in the granular form and used without any further treatment. Cloisite^® C30B from Southern Clay products Inc. (Texas, USA), which is methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium chloride modified montmorillonite with a basal spacing of 18.5 Å, has been used after drying in a vacuum oven at a temperature of 80°C.

Blends preparation

All the blend samples were prepared by melt blending in a corotating twin-screw extruder having L/D = 80/2 (Brabender, Germany) at a temperature range of 230, 231, 232, 233, 234 and 235°C and a screw speed of 100 rpm. Before melt blending, all components were dried in an oven at a temperature of 80°C for 24 h and before feeding to the extruder hopper all components are mixed in solid form. A small amount of Irganox 1010 and Irgafos 168 was added to avoid thermal degradation of the blend components during the blending process. The processing conditions were the same for all the samples. The extrudates after granulation were dried and used for preparation of the rheological and morphological samples. The formulation samples are presented in Table 1. For polymer blends, the weight ratios of two components have always taken constant values. Therefore, the weight percentage of clay subtracted from both blend components is relative to their weight in the blends.

Table 1.

Formulation and composition of the prepared samples.

Sample code	PA6 (wt%)	ABS (wt%)	POE-g-MA (wt%)	Nanoclay (wt%)
P	100	—	—	—
A	—	100	—	—
PA	75	25	—	—
PAP	70	25	5	—
PAPN1	69.26	24.74	5	1
PAPN3	67.80	24.20	5	3
PAPN5	66.30	23.70	5	5

PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride; PAP:PA6/ABS/POE-g-MA.

Morphological characterization

The morphology of the samples was examined by SEM using a VEGA TESCAN model. At first, the samples were fractured in liquid nitrogen and for improving image contrast, the samples were etched using tetrahydrofuran to dissolve the ABS phase at room temperature for 48 h. Then the samples were dried in a vacuum oven at a temperature of 80°C for 6 h and coated by a thin layer of gold. After coating, the morphology was examined by SEM at an accelerating voltage of 20 kV.

To determine the droplet size, the SEM images were analyzed using the Image Processing Software and the number-average diameter (D _n), volume-average diameter (D _v) and polydispersity index (PDI) of the droplets in the blends were calculated using the following equations:

D_{n} = \frac{\sum N_{i} D_{i}}{\sum N_{i}}

D_{v} = \frac{\sum N_{i} D_{i}^{4}}{\sum N_{i} D_{i}^{3}}

P D I = \frac{D_{v}}{D_{n}}

where N_i denotes the number of droplets with diameter D_i .^16,17 To measure the above parameters, at least 200 droplets were analyzed per sample.

TEM was applied to examine the distribution and dispersion of the nanoclay platelets in the nanocomposite blends. The sample was prepared and ultramicrotomed under cryogenic conditions. The TEM images were examined using a LEO 912AB model at an accelerating voltage of 120 kV.

Rheological measurements

The rheological behavior of the blends was investigated using a stress constant rheometer (Physica MCR 301 model) made by Anton paar (Osterreich, Austria). A 25 mm parallel plate geometry with a gap of 0.8 mm was used for all rheological measurements. Dynamic strain sweep experiments were conducted for neat blends and their nanocomposites to determine the linear viscoelastic region at a constant frequency of 10 rad/s. Dynamic frequency sweep tests were carried out for all the samples at a constant strain of 1% and the frequency range applied was 0.1–600 rad/s. Before rheological measurements, all samples were dried in a vacuum oven at a temperature of 150°C for 20 h.

The creep and recovery behavior as a function of time for the PA6/ABS nanocomposite blends was also studied. In these tests, the applied constant stress was 50 Pa, which was removed after t = 600s. All rheological measurements were performed at a temperature of 245°C.

Results and discussion

Morphology

Figure 1 depicts SEM micrographs of the PA6/ABS blend with and without compatibilizer agent and nanocomposite samples containing different levels of nanoclay. Considering concentration and viscosities of two components of blend, one can conclude that PA6 should form the continuous phase of the blend and the gross droplets should be the ABS component. Due to incompatibility, melt blending of PA6 with ABS in the absence of a suitable compatibilizer could not provide appropriate dispersion of the dispersed phase in the PA6 matrix, which can be verified in the SEM image of the uncompatibilized blend sample presented in Figure 1(a). Figure 1(b) shows an SEM image of the blend sample including 5 wt% of POE-g-MA. As seen, introducing this compatibilizer reduced dispersed phase particle size and increased interfacial area. These results must be due to the strong interfacial interaction of the compatibilizer with two blend components and formation of covalent bonds between the POE-g-MA groups and the amino end group of PA6.^18
–20 Similar results were reported by Holsti-Miettinen et al.^4,21

Figure 1.

SEM images with the number-average diameter of droplets (D _n) of PA6/ABS/POE-g-MA/nanoclay blends: (a) PA, (b) PAP (PA6/ABS/POE-g-MA), (c) PAPN1, (d) PAPN3 and (e) PAPN5. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Figures 1(c) to (e) show SEM micrographs of PA6/ABS nanocomposites containing different concentrations of nanoclay. By comparison of these figures, it can be concluded that addition of clay to the blend reduces considerably the size of droplets and increases their uniformity.

The calculated characteristics of droplets including the average diameters and PDI are presented in Table 2. These results show that the size of dispersed phase droplets decreases significantly by the addition of compatibilizer and increase in nanoclay concentration. It can be seen that with an increase in the clay content from 1% to 5%, PDI of the droplets decreased from 1.33 to 1.09 (Table 2). These results reveal that clay acts as a compatibilization agent and reduces size and PDI of the droplets and that with increasing clay concentration, the effects of the presence of clay on the blend characteristics increase. It should be mentioned that average droplet size reduction could be due to considerable increase in interfacial surface energy, viscosity and normal forces produced by the presence of clay in the matrix phase.

Table 2.

Effects of compatibilizer and clay content on the droplet average diameter and PDI in morphology of PA6/ABS blends.

Sample code	D _n (μm)	D _v (μm)	PDI
PA	4.4551	6.5338	1.4665
PAP	2.3672	3.1598	1.3348
PAPN1	1.8867	2.2714	1.2039
PAPN3	1.4596	1.7213	1.1793
PAPN5	1.2318	1.3479	1.0940

PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; PDI: polydispersity index.

For further investigation of PA6/ABS/clay nanocomposites, the morphology of a typical nanocomposite was examined by TEM, which can directly reveal the dispersion quality of clay layers in the polymer matrix.²² TEM micrographs of a compatibilized PA6/ABS/nanoclay nanocomposite on both micro- and nanoscale are presented in Figure 2. The dark lines, which can be seen in Figure 2(a), represent aggregates of intercalated silicate layers of nanoclay. A high-resolution image of the nanocomposite presented in Figure 2(b) shows well-dispersed clay tactoids in the polymer matrix, which suggests almost exfoliated structure for organoclay in the polymer blend.

Figure 2.

TEM images of PA6/ABS/POE-g-MA/nanoclay (PAPN3) blends: (a) 0.4 μm; (b) 60 nm. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Rheology

The dynamic strain sweep tests were performed in order to determine the linear viscoelastic region at 245°C and a constant frequency of 10 rad/s. Strain dependency of storage modulus of the virgin blends and their nanocomposite samples is shown in Figure 3. This figure shows that the extent of the linear viscoelastic limit is sensitive to the content of clay and compatibilizer and with increasing clay content the linear viscoelastic region decreases. It could be concluded that the presence of compatibilizer might increase the interfacial interaction between polymer chain and nanoparticles.²³ So, the strains necessary for partial rupture of the system connectivity and the presentation of nonlinearities decrease. In addition, it was found that introducing nanoclay into polymeric blend systems can reinforce the interfacial interaction of the blend components. On the other hand, the increase in the nanoclay content may be explained as the increase in the strength of the network system. The critical strain values (γ _c), at which nonlinearity behavior of samples starts, are presented in Figure 4. This figure indicates that the critical strain decreases with an increase in the clay content almost linearly. However, the linear relation between logarithm of γ _c and clay volume content seen in the work of Aubry et al.²⁴ cannot be seen here.

Figure 3.

Storage modulus (G′) as a function of strain for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Figure 4.

Critical strain (γ _c%) as a function of clay volume concentration (%).

To avoid the effects induced by morphology changes during the rheological tests, the frequency sweep dynamic experiments were conducted in the oscillatory shear mode at the linear viscoelastic region.²⁵ Figures 5 and 6 show, respectively, storage modulus (G′) and loss modulus (G′′) as a function of frequency for PA6/ABS nanocomposite compatibilized blends and virgin polymeric components. It can be seen that the storage modulus and loss modulus of PA6/ABS blends compatibilized by POE-g-MA are greater than that of the uncompatibilized blends. The higher storage modulus and loss modulus may be attributed to the stronger interfacial interaction due to the formation of covalent bonds between the POE-g-MA groups and the amino end group of PA6.²¹

Figure 5.

Storage modulus (G′) as a function of frequency for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Figure 6.

Loss modulus (G′′) as a function of frequency for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

As seen, with an increase in the clay content, the storage modulus and loss modulus were considerably increased. Increase in storage and loss moduli of PA6/ABS nanocomposite blends could be attributed to the strong interfacial interaction between nanocomposite components in the presence of the clay. Increase in the nanoclay content in the presence of the compatibilizer (POE-g-MA) leads to an increase in the storage and loss modulus. Figure 5 clearly depicts that in the presence of the compatibilizer and the nanoclay, the dependencies of storage modulus on frequency in the low-frequency region decrease and the storage modulus curves display a plateau in the low-frequency region particularly for the sample containing higher nanoclay content, which may be due to the formation of a percolation network in the nanocomposite blends.^26,27 On the other hand, these results infer that due to high interaction between compatibilizer and nanoclay because of their high affinity, which present almost similar polarities.²⁸

Figure 7 shows the complex viscosity (η*) of PA6, ABS and their blends with various amounts of nanoclay. This figure shows that with increasing frequency, the complex viscosity strongly decreased, which reveals high shear-thinning behavior of these polymer blends and their nanocomposites at the melt state.²⁸ It can also be seen that with an increase in the nanoclay content the complex viscosity increases. The significant increase in the complex viscosity, especially in the low-frequency region, should be due to nanoclay exfoliation and induction of high interfacial surface between the polymer components and nanoparticles. Figure 7 indicates that complex viscosity behavior of the uncompatibilized blend is similar to the PA6 matrix phase, whereas that of the samples containing compatibilizer and clay is more similar to that of the dispersed phase. Similar results can also be observed for the dynamic and loss modulus of different blends and nanocomposites.

Figure 7.

Complex viscosity (η*) as a function of frequency for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

It can be expected that the nanoclay content and its dispersion, distribution and morphology could deeply affect the nanocomposite material functions especially elastic modulus of the composite. So for further study, the effects of nanoclay characteristics on the storage modulus of nanocomposites (G′_{nanocomposite}) have been investigated. In general, the contribution of intercalated nanoclay to G′ of the nanocomposites (G′_{nanocomposite}) could be indicated in terms of two effects: the confinement effect (G′_confinement) and the interparticle interactions (G′_interaction), which cause increase in elastic modulus of nanocomposite when compared with the polymer matrix (G′_matrix), that is,²⁹

G_{n a n o c o m p o s i t e}^{'} = G_{m a t r i x}^{'} + G_{i n t e r a c t i o n}^{'} + G_{c o n f i n e m e n t}^{'}

Here G′_interaction comes from frictional interactions between the tactoids of nanoclay. By increasing the nanoclay concentration, these interactions can remarkably increase. G′_confinement results from the confinement of silicate layers with an interlayer distance lower or of the same order of the size of the chain coils.³⁰ G′_{nanocomposite} may be controlled by G′_interaction due to the higher contribution of G′_interaction than those of G′_matrix and G′_confinement, especially at the low-frequency regions. Additionally, if the percolation network structure is ruptured, the viscoelastic response may change remarkably.³⁰ Figure 8 clearly shows that with an increase in the nanoclay content, the G′_interaction + G′_confinement is increased. Figure 9 indicates the effect of preshear on the storage modulus (G′) of the PA6/ABS/POE-g-MA/Nanoclay sample. As seen, the storage modulus of the presheared PAPN3 sample decreases significantly. This behavior could be attributed to the fact that the percolation network structure is destroyed under preshear.

Figure 8.

G′ (nanocomposite)/G′ (matrix) as a function of frequency for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Figure 9.

The effect of preshear on the storage modulus (G′) of the PAPN3 sample.

Creep and recovery behavior

Figure 10 demonstrates the important parameters in this test corresponding to the Burger model schematically.¹⁵ The creep and recovery behavior of PA6/ABS/nanoclay nanocomposites as a function of time was studied and the results are presented in Figures 11 and 12. In these experiments, a constant stress (50 Pa) is applied to the sample and when t = 600 s, the stress was removed. Figure 11 shows that by introducing POE-g-MA to neat blend, creep and consequently recovery strain were decreased remarkably. With increasing clay content, creep and recoverable strain of PA6/ABS compatibilized blends over time were also decreased (Figure 12). These results imply that the creep behavior is improved by the presence of compatibilizer and nanoclay.

Figure 10.

Strain as a function of time for creep and recovery tests corresponding to the Burger model.

Figure 11.

Creep strain as a function of time for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Figure 12.

Recovery percentage (RP) and the permanent creep strain (γ _∞) of PA6/ABS nanocomposite blends have been calculated via equations (5) to (7) and the results are presented in Table 3.

γ_{M a x} = γ_{M S} + γ_{K V} + γ_{\infty}

R P = [\frac{γ_{M a x} - γ_{\infty}}{γ_{M a x}}] \times 100 %

γ_{\infty} (%) = [\frac{γ_{\infty}}{γ_{M a x}}] \times 100 %

where γ _Max, γ _MS, γ _KV and γ_∞ denote maximum or total deformation, immediate elastic deformation related to the Maxwell model, delayed elastic deformation related to the Kelvin–Voigt model and the Newtonian or unrecoverable deformation, respectively (Figure 10).¹⁵

Table 3.

RP and γ_∞ (%) for PA6/ABS/POE-g-MA/nanoclay nanocomposite blends.

Sample code	γ _∞ (%)	RP
PA	0.9916	0.0084
PAP	0.9090	0.0910
PAPN1	0.8304	0.1696
PAPN3	0.7766	0.2234
PAPN5	0.7031	0.2969

RP: recovery percentage; PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Presented results show that, with the introduction of compatibilizer and increase in the nanoclay content, strain has dramatically decreased, whereas RP significantly increased.

The RP for PA6/ABS compatibilized blends increased by 11 and 36 times, respectively, for virgin compatibilized blend and the compatibilized blend containing 5% of nanoclay with respect to the uncompatibilized blend. It is clear that γ_∞ decreases with an increase in the concentration of compatibilizer and nanoclay, which indicates an improvement in recovery performance. It is worth noting that γ _∞ is subject to the relative slippage between molecules chain of systems. The nanoclay could prevent slippage of polymer chain blends, therefore causes an increase in the viscosity, the permanent strain (γ _∞) decreases and recovery increases. On the other hand, viscosity increase is due to increase in interaction between blend phases induced by introducing nanoclay, which could result in an increase in RP of nanocomposites.^15,29,31

Creep compliance versus time curves of PA6/ABS blends with various nanoclay concentrations are shown in Figure 13. It can be seen that with increasing clay content, the creep compliance over the whole time range decreases. It is found that the creep compliance also decreases when POE-g-MA is added to PA6/ABS blends and introduction of organoclay to the compatibilized blend still decreases the creep compliance. These results are in agreement with those of the dynamic frequency sweep test results.

Figure 13.

Creep compliance versus time curve of PA6/ABS/POE-g-MA/nanoclay nanocomposite blends. PA6: polyamide 6; ABS: acrylonitrile–butadiene–styrene; POE-g-MA: polyethylene octene elastomer grafted with maleic anhydride.

Conclusions

PA6/ABS/nanoclay nanocomposites compatibilized by POE-g-MA prepared using a corotating twin-screw extruder at a temperature of 235°C. The SEM micrographs indicated that the average radius of the dispersed droplet phase for the compatibilized blend with POE-g-MA is smaller than that of the dispersed phase for the uncompatibilized blend. In addition, SEM micrographs showed that with an increase in the nanoclay content, the dispersed phase (ABS) droplet size and their PDI had decreased. TEM micrographs of nanocomposites show well-dispersed clay tactoids in the polymer matrix produced by exfoliation of the clay in the polymeric blends. Dynamic strain sweep experiments showed that the extent of the linear viscoelastic region is sensitive to clay content and compatibilizer and that with increasing clay content, the linear viscoelastic region decreases. It could be concluded that introduction of clay can reinforce interfacial interaction between components of blends. The presented results revealed that, with an increase in the nanoclay content, the storage modulus, loss modulus, and the complex viscosity increased. The creep experiment results showed that with the addition of POE-g-MA and clay to PA6/ABS blends, creep and recovery, over time, decreased remarkably. The presented results also displayed that with the addition of compatibilizer and nanoclay, the RP increased. Therefore, the creep experiments results are in accordance with those of the dynamic measurement results. The RP of the compatibilized PA6/ABS blends increases by 11 and 36 times for samples without clay and samples containing 5% of nanoclay, respectively, with respect to uncompatibilized blend.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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Rheological and morphological behaviors of polyamide 6/acrylonitrile–butadiene–styrene/nanoclay nanocomposites

Abstract

Keywords

Introduction

Experimental

Materials

Blends preparation

Morphological characterization

Rheological measurements

Results and discussion

Morphology

Rheology

Creep and recovery behavior

Conclusions

Footnotes

Funding

References