Unraveling the blend composition influence on crystallization,melting,thermal stability and rheological behavior of PMMA/EVA/MWCNTs nanocomposites

Abstract

In this research, the thermal behavior and stability as well as the rheological properties of the nanocomposites containing 5 wt% multi-walled carbon nanotubes (MWCNTs) filled poly (ethylene-co-vinyl acetate) (EVA)/poly (methyl methacrylate) (PMMA) blends of different blend compositions were investigated. It was confirmed by employing the Winter-Chambon model that the physical gels were formed for the nanocomposites containing 70 wt% EVA and beyond. Hence, they behaved as shear thinning fluids throughout the angular frequency range examined. Moreover, the relaxation exponent, gel strength and fractal dimension of the critical gel were estimated to be about 0.309, 41,304 Pa.sⁿ and 2.21, respectively. The calculation of the wetting coefficient revealed that the MWCNTs thermodynamically tended to migrate from EVA and locate inside the PMMA phase. The results obtained from the crystallization and melting rate of the materials indicated that the crystallization rate was always higher than that of the melting rate and both progressively decreased with the increase in PMMA loading for the nanocomposites. Thermogravimetric analyzer (TGA) was employed to study the thermal stability of the nanocomposites under air and nitrogen atmosphere. The highest thermal stability for air-scanned materials were found for the nanocomposites containing 85 wt% followed by 70 wt% EVA as if their thermal stabilities were even superior to that of the pristine EVA. For instance, the temperature at 50 and 70 wt% weight loss for the nanocomposite containing 85 wt% EVA was found to be 20 and 23°C higher than those of the EVA alone, respectively.

Introduction

The investigation and attempting to develop the innovative composite materials with desirable and tailor-made properties is still an exciting and vibrant topic in polymer science and technology research area. ¹ In recent decades, the nanoparticles such as single- (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), graphene (GN), organo-modified montmorillonite (OMMT) and polyhedral oligomeric silsesquioxane (POSS) has become a part of plastics formulations in academia and industries. The polymers incorporated with these materials could provide much better mechanical, optical, electrical, rheological, biomedical, thermal and barrier properties than those of their unfilled counterparts.^2–7 Thermoplastic materials are preferred to the thermoset ones mainly because of their capabilities for high production, less hazardous chemicals, easy and swift processing, recyclability and higher environmental safety. ⁸ Carbon-based nanoparticles have found their place in polymer nanocomposite productions because of their excellent physico-mechanical properties. ⁹ The finished articles made of these nanocomposites are used in applications such as biomedical, ¹⁰ energy storage, ¹¹ packaging, ¹² tissue engineering, ¹³ and sensors. ¹⁴

Poly (methyl methacrylate) (PMMA) is an atactic, amorphous thermoplastic material with a variety of applications in packaging, optical and biomedical industries. This is due to its desirable optical transparency and gloss, good dimensional stability, low water absorption, excellent weathering resistance and biocompatibility as well as elevated mechanical strength. However, the material posses relatively poor thermal stability, flame resistance and electrical conductivity.^15,16 Poly (ethylene-co-vinyl acetate) (EVA) with totally saturated backbone presents an outstanding ozone, weather, and heat resistance as well as great flexibility and resilience, superb processing and biological inertness property. It may also be a good candidate for substituting PVC as a halogen-free polymer in many applications.^17,18 Thus, EVA could be considered as a good nominee for the modification of the PMMA shortcomings. ¹⁹

It is worth mentioning that only limited number of published works is available about the PMMA/EVA blends and their nanocomposites in literature. Bernini et al. ²⁰ worked on the opto-thermal behavior of PMMA/EVA blend. They reported that the blend underwent a phase transition where its optical properties varied considerably. The authors suggested that the especial optical and thermal behavior of the blend made it attractive for potential application in temperature-controlled optical tools. Tewo et al. ²¹ studied several compositions of paraffin wax (wax)/EVA blend filled with PMMA micro-beads for possible application as investment casting pattern substance. They proposed that 40/20/40 wt% wax/EVA/PMMA blend revealed an optimum balance of properties in which the flexural strength, strain at break and needle penetration hardness were highly improved, at the cost of the augmented melt viscosity, in comparison with those of the paraffin wax itself. Inoue ²² reported that the impact strength of the PMMA/EVA blend fabricated by means of cast-polymerization is much superior to that of the melt-mixed PMMA/EVA blend. The triangle phase diagram drawn from the polymerization of 80/20/0.2 methyl methacrylate (MMA)/EVA/2,2′-azobis (isobutyronitrile) showed that the system forced into a two-phase arrangement when the polymerization continued to the level of about 2 wt% conversion at 80°C. Chen et al.^23,24 studied the phase separation phenomenon during the polymerization of MMA in the presence of EVA copolymer using optical microscopy and light scattering techniques. From the cloud point curves, they found out that the ternary MMA/PMMA/EVA blend created an upper critical solution temperature (UCST), which amplified with the reduction in MMA content. The trigonal phase diagram, which was drawn based on the cloud point curves, showed that the homogeneous zone in the ternary blend was very tiny. Cheng and Chen ²⁵ found out that the PMMA/EVA in situ blends improved the toughness of PMMA and following the addition of EVA, the fracture mechanism of the matrix was transformed from crazing to yielding. Since the EVA domains were placed near to one another, the interconnection yielding stress volumes resulted in the lengthening of elongation and toughening of PMMA. It was also reported that an abundant number of fibrils and voids were created on the fracture surfaces of the PMMA/EVA in situ blends. Therefore, the deformations that were occurred at the boundary of PMMA phase and EVA dispersed domains, avoided the particles to pull out from the PMMA at fracture and hence absorbed high amount of energy. Ying et al. ²⁶ studied the influence of selective localization of silica (SiO₂) on the mechanical properties of PMMA/EVA/SiO₂ composites. They pre-dispersed the filler into either EVA or PMMA material and the other polymer was then blended. It was reported that higher SiO₂ content and more spherical-shaped EVA domains brought about the optimum mechanical properties for the composite prepared by PMMA/SiO₂ pre-dispersed compound. Si et al. ²⁷ evaluated the influence of organoclay on the microstructure of polycarbonate (PC)/poly (styrene-co-acrylonitrile) (SAN), polystyrene (PS)/PMMA and PMMA/EVA blends. They reported that the domain size was appreciably decreased for the blends. The compatibilization effect was suggested to achieve via the interfacial localization of organoclay particles.

In our previous work, ²⁸ we investigated the effect of MWCNTs content (i.e. 0, 2.5, 5 and 10 wt%) on the physical and mechanical properties of PMMA/EVA based nanocomposites containing 30 wt% EVA. The results revealed that the maximum tensile properties were obtained for the material containing 5 wt% MWCNTs. The main objective of this research was to scrutinize the influence of PMMA/EVA weight ratio in the nanocomposites having 5 wt% MWCNTs on the possibility of critical gel formation, the rheological behavior and thermal stability of the materials. Hence, the structure-property relationship was established for this far less studied system in order to tailor the properties of PMMA/EVA/MWCNTs-based nanocomposites by adjusting their compositions. It is worth mentioning that any similar report has yet to be found in the literature, to the best of the authors’ knowledge, on this subject.

Materials and methods

Materials

EVA with the trade name of EVA VS430 was procured from Hyundai Séetec (South Korea). The density, melt flow index (190°C, 2.160 kg) and vinyl acetate content of the copolymer was reported as 0.94 g/cm³, 1.7 g/10 min and 19%, respectively. PMMA with the commercial name of Acryrex CM-205 was purchased from Chei Mei Corporation (Taiwan) with the melt flow index (230°C/3.8 kg) and density of 1.8 g/10 min and 1.19 g/cm³, respectively. Its weight average molecular weight (M_w) and polydispersity index were also reported to be 79,400 g/mol and 1.73, respectively. ²⁹ MWCNTs with the trade name of Nanocyl^TM NC 7000 was supplied from Nanocyl S.A (Belgium). It’s mean diameter and length as well as purity were reported to be of 9.5 nm, 1.5 μm and about 90%, respectively. ³⁰

Fabrication of the nanocomposites

The nanocomposites were fabricated through the use of a Brabender-type laboratory mixer (Germany) after the materials were dried in an oven at 70°C for 12 h. EVA and MWCNTs were first melt mixed at 110°C and 80 r/min for 15 min with the intention of preparing the EVA/MWCNTs master-batches required. The nanocomposites with appropriate compositions were then fabricated via amalgamation of the proper amounts of the master-batch and the neat EVA and PMMA. The master-batch and pristine EVA were loaded into the same mixer and the process accomplished at 180°C and 80 r/min for 7 min followed by the addition of PMMA into the chamber where mixing was continued for another 9 min. The mixtures were compressed by a hydraulic press in a square sheet mold of about 1 mm thick at 210°C, 25 MPa and 5 min. Table 1 presents the selected codes and formulations of the materials studied.

Table 1.

Codes and formulations of the compounds prepared.

Sample	EVA (wt.%)	PMMA (wt.%)	MWCNTs (wt.%)
PMMA	0	100	0
80P15E5C	15	80	5
65P30E5C	30	65	5
45P50E5C	50	45	5
25P70E5C	70	25	5
10P85E5C	85	10	5
EVA	100	0	0

Characterization

The static contact angle measurements were carried out on the neat surface of compression molded sheets of PMMA and EVA. The data acquisition was accomplished at about 20°C, using the probe liquids of double distilled water (H₂O) and diiodomethane (CH₂I₂) by means of a contact angle measuring device Krűss G10 (Germany). In each case, the measurements were carried out on minimum seven individual droplets of 6 μL volume located at different places and the average value was then calculated. DSC1 Mettler Toledo (Switzerland) was used at a cooling/heating rate of 10°C/min under a nitrogen atmosphere to evaluate the crystallization and melting behavior of the materials prepared. Each sample with the weight of 10 ± 0.2 mg was encapsulated in an aluminium closed pan and heated from −50 to 160°C at the rate mentioned above. The samples were then remained at 160°C for 5 min, then cooled to −50°C and after 5 min heated again to 160°C. The degree of crystallinity ( $X_C$ ) of EVA in composite samples was estimated according to equation (1)^31,32;

$X_C(\%)=\frac{\mathrm{\Delta }{H}_f}{\phi .\mathrm{\Delta }{H}_f^{*}}\times 100$

(1)where $\mathrm{\Delta }{H}_f$ is the enthalpy of melting which is found from the area under the peak for each sample, $\mathrm{\Delta }{H}_f^{*}$ is the melting enthalpy for 100% crystalline PE with a value of 293 J/g ³³ and φ is the EVA weight fraction. An oscillatory stress-controlled rheometer MCR501 (Anton Paar, Austria) with a 25 mm diameter parallel-plate flow geometry and one mm gap was employed to clarify the rheological behavior of the materials at 180°C. The experiments were carried out at the angular frequency (ω) in the range 0.01 to 600 rad/s at a strain amplitude of 0.25% under a nitrogen atmosphere. The strain amplitude was chosen from the linear viscoelastic region of the samples based on the analysis worked out on strain and time sweep experiments. Thermal stability of the compositions was investigated through thermogravimetric analysis using the Thermal Analyzer TGA1 (Mettler Toledo, Switzerland). The weight of each sample was about 15 ± 0.3 mg and they were tested from ambient temperature to 600°C at the heating rate of 10°C/min in both nitrogen and air gases. The balance chamber flow rate was taken as 20 mL/min and that of the furnace was set to 50 mL/min. The wide angle x-ray diffraction (WAXD) analysis was performed using Philips X’pert X-ray diffractometer (The Netherlands) with CuK_α radiation (λ = 0.154 nm) at a generator voltage and current of 40 kV and 40 mA, respectively. The samples were scanned at room temperature over the 2θ range of 5 to 50° and at the rate of 0.02°/s.

Results and discussion

Localization of MWCNTs

In order to realize how a material such as a (nano)filler is preferentially distributed in an immiscible blend, the particle preferred location can theoretically be predicted by calculating the wetting coefficient (ω_a) value. ³⁴ The ω_a value is obtained using equation (2). This relationship, which has been suggested by Sumita, ³⁵ only considers the thermodynamic aspects but not the kinetic features of the process. ³⁶ For our system, the equation has been adopted as^37,38;

${\omega }_a=\frac{{\gamma }_{PMMA-MWCNTs}-{\gamma }_{EVA-MWCNTs}}{{\gamma }_{PMMA-EVA}}$

(2)where ${\gamma }_{PMMA-MWCNTs}$ , ${\gamma }_{EVA-MWCNTs}$ and ${\gamma }_{PMMA-EVA}$ represent the interfacial surface free energies or interfacial surface tensions between the PMMA and MWCNTs, between the EVA and MWCNTs and between the PMMA and EVA, respectively. If ${\omega }_a$ >1, the nanoparticles are selectively located in the EVA phase, if ${\omega }_a$ <−1, the MWCNTs will be accommodated in the PMMA domain and when −1< ${\omega }_a$ <1, they are located at the interface of the PMMA and EVA phases. ³⁹

Several methods are now available for obtaining the interfacial surface free energy of the materials ( ${\gamma }_{1-2}$ ) theoretically and experimentally. ⁴⁰ However, the geometric- and harmonic-mean are the most frequently used equations that have been suggested by Owens and Wendt ⁴¹ and Wu, ⁴² respectively. The geometric- (3) and harmonic-mean (4) equations, by which the ${\gamma }_{1-2}$ of a pair of components is estimated, are as follows^43,44;

${\gamma }_{1-2}={\gamma }_1+{\gamma }_2-2\left(\sqrt{{\gamma }_1^d{\gamma }_2^d}+\sqrt{{\gamma }_1^p{\gamma }_2^p}\right)$

(3)

${\gamma }_{1-2}={\gamma }_1+{\gamma }_2-4\left(\frac{{\gamma }_1^d{\gamma }_2^d}{{\gamma }_1^d+{\gamma }_2^d}+\frac{{\gamma }_1^p{\gamma }_2^p}{{\gamma }_1^p+{\gamma }_2^P}\right)$

(4)where ${\gamma }_1$ and ${\gamma }_2$ are the surface free energies of the components 1 and 2 (e.g. EVA, PMMA and MWCNTs) and also ${\gamma }_1^d$ , ${\gamma }_2^d$ and ${\gamma }_1^p$ , ${\gamma }_2^p$ are denoted as the dispersive and polar parts of the surface free energy of the ingredients one and two, respectively.

The surface free energy of the two polymers and their dispersive and polar values of the surface free energy can be found from the contact angle experiments using the following equations^45,46;

${\gamma }_l\left(1+\cos \hspace{0.17em}\theta \right)=2\left(\sqrt{{\gamma }_p^d{\gamma }_l^d}+\sqrt{{\gamma }_p^p{\gamma }_l^p}\right)$

(5)

${\gamma }_p={\gamma }_p^d+{\gamma }_p^p$

(6)

${\gamma }_l={\gamma }_l^d+{\gamma }_l^p$

(7)where ${\gamma }_p$ , ${\gamma }_p^d$ and ${\gamma }_p^p$ are the surface free energy and the dispersive and polar fractions of the surface free energy of solid polymer materials (i.e. PMMA and EVA), respectively, $\theta$ is the contact angle value and ${\gamma }_l$ , ${\gamma }_l^d$ and ${\gamma }_l^p$ are the surface tension and the dispersive and polar fractions of the surface tension of probe liquids, respectively. According to the above equations, two probe liquids of different polarities are usually needed to figure out the surface free energy of different materials in the solid state.

Table 2 tabulates the surface tension values of H₂O and CH₂I₂^47,48 as well as their contact angles on the surfaces of EVA and PMMA at 20°C, however, those related to the nanofiller were taken from two separate published studies.^49,50 In addition, all quantities were regulated for the temperature of 180°C at which our melt mixing process was carried out. This was ensued by means of using the temperature coefficient ( $\frac{d\gamma }{dT}$ ) proposed for different substances in literature.^42,51 Table 3 presents the data obtained at 20 and 180°C for the three substances used in this work.

Table 2.

Surface free energies of the probe liquids and their contact angles on the EVA and PMMA surfaces at 20°C.

Liquids	${\gamma }_l^d$ (mJ/m2)	${\gamma }_l^p$ (mJ/m2)	${\gamma }_l$ (mJ/m2)	Contact angle on EVA (o)	Contact angle on PMMA (o)
H2O	21.80	51.00	72.8047,48	91.0	68.7
CH2I2	50.80	0.00	50.8047,48	38.5	27.3

Table 3.

Surface free energies of the two pristine polymers and the nanofiller.

Material	${\gamma }_s$ at 20°C (mJ/m2)	Polarity	${\gamma }_s$ at 180°C (mJ/m2)	${\gamma }_s^d$ at 180°C (mJ/m2)	${\gamma }_s^p$ at 180°C (mJ/m2)	dγ/dT (mJ/m2K)
EVA	41.10	0.018	32.46	31.88	0.58	−0.054 42
PMMA	51.78	0.125	39.62	34.67	4.95	−0.076 42
MWCNTs	27.80 49	0.367	27.80	17.60	10.20	0 51
MWCNTs	45.30 50	0.594	45.30	18.40	26.90	0 51

The interfacial surface free energy between different couples of ingredients and the ${\omega }_a$ values estimated through the harmonic- and geometric- mean approach, are shown in Tables 4 and 5, respectively. The results revealed that apart from the surface free energy value assumed for MWCNTs, the nanofiller demonstrated superior interactions with the PMMA phase. Consequently, they might be eager to transfer from the EVA domains and reside in the PMMA phase. However, it should be noted that in contrast to the ${\omega }_a$ predictions, the results attained from various experimental techniques indicated that the localization of nanofillers was not always dictated simply by thermodynamic considerations. ⁵² Different kinetic factors such as viscosity ratio of the polymers, convective forces, mixing sequence, and processing conditions are still among the factors that can control the kinetic of migration and specify the final localization of MWCNTs. Furthermore, based on the results obtained from the rheological studies, which will be discussed later, the values of storage modulus (G′) and complex viscosity (η*) of EVA was significantly lower than those for PMMA throughout the ω range tested. However, physical gel was formed for the nanocomposite in which EVA was its major component. This could be for the benefit of lingering of MWCNTs in EVA or at least for remaining the major part of the nanofiller in EVA phase.^53–55 It has also been suggested that the substantial amounts of contact locations are established between the polymeric chains and the surfaces of the nanofillers after the nanoparticles are being mixed with the first polymer. Hence, the activation energy for the molecular chains desorption can be very large and the procedure may not completely take place. ⁵⁶ In the same vein, if thermodynamic evaluations indicate that the carbon nanotubes are preferred to be accommodated within the PMMA phase, only the nanofillers that are not wholly covered or wrapped with EVA and/or the ones that are able to totally desorb the EVA chains can be transferred and located in PMMA phase. Selective extraction of the nanocomposites was carried out by acetone to assess the localization of MWCNTs within the polymers. The PMMA phase was completely dissolved by the solvent at room temperature. The liquid part and the solid residue (EVA) were both become visible as black after the dissolution, in agreement with the notion that a part of MWCNTs was transferred from the EVA phase and located in the PMMA domains. Our results were similar to those reported for NBR/EVA/MWCNTs ⁵⁷ and PLA/EVA/MWCNTs ⁵⁸ materials in which the authors found out that the carbon nanotubes were positioned in both phases. Nevertheless, a high amount of the nanofiller particles yet lingered in their original phase.

Table 4.

Interfacial surface free energies based on the harmonic- and geometric-mean equations at 180°C.

Material	Interfacial free energy based on harmonic-mean equation (mJ/m2)	Interfacial free energy based on geometric-mean equation (mJ/m2)
PMMA/EVA	3.56	2.20
EVA/MWCNTs 49	12.70	8.02
PMMA/MWCNTs 49	10.23	7.39
EVA/MWCNTs 50	28.81	21.42
PMMA/MWCNTs 50	20.12	11.32

Table 5.

The wetting coefficients attained by harmonic- and geometric-mean equations at 180°C.

Material	The value of ωa based on harmonic-mean equation	The value of ωa based on geometric-mean equation	Prediction
PMMA/EVA/MWCNTs 49	−1.49	−1.92	In PMMA
PMMA/EVA/MWCNTs 50	−2.44	−4.59	In PMMA

X-ray diffraction analysis

The XRD spectra of the parent polymers and prepared materials are shown in Figure 1. On the one hand, double important reflections were unveiled by the semi-crystalline copolymer, which could be observed at about 2θ of 21.3 and 23.4°C. ⁵⁹ The former is assigned to the plane (110) and the latter to that of (200), which are both related to the polyethylene orthorhombic unit cell. ⁶⁰ On the other hand, PMMA revealed no distinct reflections but only a broad halo diffraction, reflecting that PMMA was an amorphous polymeric material. ⁶¹ The results showed that the reflection peaks become visible at almost the same locations, similar to those of the unfilled EVA, for the nanocomposites prepared. The appearance of the reflections at similar places and the lack of the formation of new peaks revealed that MWCNTs was unable to modify the crystalline structure of the semi-crystalline component of the nanocomposites. In addition, no visible reflections could be seen at the 2θ of around 26 (stronger peak) and 44° (weaker peak), which were assigned to the graphite (002) and (100) planes of the well-ordered concentric tubes of the nanofiller. ⁶² This might be attributed to the good dispersion of the nanofiller particles throughout the polymers.

Figure 1.

XRD diffractions of the parent polymers and the nanocomposites.

Crystallization and melting behavior

The crystallization and melting response for the crystallizable materials, obtained by DSC, are shown in Figure 2(a) and (b), respectively. All the thermograms were scrutinized to determine the crystallization onset temperature (T_con), crystallization peak temperature (T_cpt), melting peak temperature (T_mpt), melting end temperature (T_met) and X_C as well as the rate of crystallization (S_cry) and rate of melting (S_mel) of the materials. The data obtained are tabulated in Table 6. Both T_con and T_met were found using two different methods. In one method, the desired temperature was determined by drawing two tangents at the high-temperature zone of the crystallization and/or melting thermogram and the baseline at which these lines intersected each other at one point (i.e. T_con1 and T_met1). The other method was derived from drawing a line along to the baseline until it intersected the thermogram at one point. The intersection point showed the temperature at which the crystallization curve was deviated from the baseline because the crystallization process just initiated (T_con2) and/or the melting curve reached the baseline (T_met2), revealing the temperature at which the last trace of crystals (i.e. the thickest lamella) were disappeared. Moreover, S_cry and S_mel were obtained by calculating the slope of the linear section of the crystallization and melting thermograms at their high-temperature zones, respectively. ⁶³ The results indicated that with the increase in PMMA content, at the constant amount of MWCNTs (i.e. 5 wt%), T_cpt, T_con1, T_mpt remained nearly untouched except for the highest PMMA loading at which the values reduced a few degree of Celsius. The values of T_met1 and T_met2 are also relatively constant. It was observed that T_con2 is much higher for the nanocomposites than that of EVA. This evidently indicated that the addition of PMMA and MWCNTs increased the heterogeneous nucleation sites and thus encouraged the crystallization process to be started at higher temperatures than that of EVA itself. This could be continued until reaching the nanocomposites containing up to 65 wt% PMMA beyond which T_con2 reduced but the value still remained about 4°C higher than that of EVA. The crystallization ability of the EVA domains is decreased with the increase in PMMA loading because the crystallizable domains are being more separated from each other, and as the PMMA continuous phase has already become solidified before starting the EVA crystallization process, the movement of EVA chains to the crystallization front is highly prevented. Therefore, the X_c value is gradually reduced with the increase in PMMA content. The values of S_cry and S_mel are steadily reduced with the addition of PMMA in the nanocomposites, implying that the rate of crystallization and melting both decreased with the increase in PMMA loading. This could be attributed to the dilution effect and imposing restrictions on EVA molecular mobility by PMMA. However, the rate of crystallization was larger than that for the melting process in each case, reflecting the initial part of crystallization process was taken place faster than the final part of melting process.

Figure 2.

(a) Crystallization and (b) melting thermograms of EVA and the nanocomposites.

Table 6.

Crystallization and melting data for EVA and the other materials.

Sample	Tcpt (±0.5°C)	Tcon1 (±0.5°C)	Tcon2 (±0.5°C)	Scry (±10% W/goC)	Tmpt (±0.5°C)	Tmet1 (±0.5°C)	Tmet2 (±0.5°C)	Smel (±10% W/goC)	XC (±2%)
EVA	68.0	73.8	78.0	14.30	84.8	92.0	95.5	5.67	24.6
10P85E5C	68.3	75.5	92.5	6.31	84.9	91.7	95.2	4.70	20.2
25P70E5C	67.6	74.8	93.0	3.73	85.1	92.7	96.1	3.49	20.6
45P50E5C	67.7	74.7	90.8	2.90	84.1	91.0	95.1	2.14	16.8
65P30E5C	67.8	73.9	93.8	1.73	80.8	90.0	95.2	1.11	17.2
80P15E5C	64.1	70.6	82.3	0.36	80.5	92.0	95.7	0.19	8.2

Rheological measurements

The plots of G′ and η* with ω for PMMA/EVA/MWCNTs nanocomposites of different compositions and of the pristine polymers are shown in Figure 3(a) and (b). The figures showed that the values of both parameters increased with the addition of the nanofiller, in the terminal region, especially for the nanocomposites having 65 wt% PMMA. The growth of the nanofiller network structures within a material could generally suppress the long-range macromolecular chain motions and consequently enhance the two quantity values. ⁶⁴ The parent polymers and the nanocomposites containing up to 50 wt% EVA acted analogous to the Newtonian fluids at low ω region followed by shear thinning at high ω region. However, those of containing 70 and 85 wt% EVA behaved as shear thinning fluids in the whole ω range examined. Therefore, the two materials could possibly form the gel-like structures at these particular compositions.

Figure 3.

The plot of (a) G′ and (b) η* versus ω for the pristine polymers and the nanocomposites at 180°C.

The slopes of the variations of G′ and loss modulus (G″) with ω at the terminal zone and their crossover points are collected in Table 7 for the materials examined. The parent polymers and three nanocomposites containing 15 to 50 wt% EVA showed crossover points, suggesting the samples behaved similar to liquid and solid at low and high ω zone, respectively. However, no crossover point was detected for 25P70E5C and 10P85E5C, consistent with the notion that they both behaved similar to viscoelastic solids because their crossover points could possibly be placed at much lower ω value. ⁶⁵ In addition, the crossover point was gradually moved to lower ω values with the increase in the EVA content in the nanocomposites. The table also indicated that both slopes were progressively reduced with the addition of EVA up to 70 wt% and the lowest values were obtained for 25P70E5C and 10P85E5C nanocomposites. This could be related to the existence of three-dimensional networks of the MWCNTs in the continuous EVA phase, in which the major amount of the nanofiller was yet located, wherein the minor PMMA domains with lower nanofiller content than that of EVA were dispersed. It is worth noting that, if the majority of the nanofiller had not been remained in the EVA phase, the remarkable increase in G′ and η*, especially at terminal region, could not have happened where EVA was the continuous phase, instead the behavior would relatively have been identical to that for the pristine EVA. ⁶⁶

Table 7.

The slopes of G′ and G′′ versus ω and their crossover points for the samples examined.

Sample	Slope of G′ vs. ω (±0.02)	Slope of G′′ vs. ω (±0.02)	Crossover point (ωc)
PMMA	1.53	0.91	0.43
80P15E5C	1.24	0.81	0.10
65P30E5C	0.93	0.72	0.02
45P50E5C	0.85	0.47	0.01
25P70E5C	0.47	0.34	-
10P85E5C	0.54	0.52	-
EVA	1.59	0.94	26.5

The plot of tanδ against ω for the parent polymers and the nanocomposites is shown in Figure 4. At the terminal zone the elasticity of the materials, especially the parent polymers, were lower because more freedom was available for the molecular chains to move and relax than that of high ω region. The elasticity extent of the nanocomposites increased with the increase in EVA content until 70 wt% at low ω region and tanδ value stayed relatively constant for 25P70E5C and 10P85E5C throughout the ω range studied. However, at medium to high ω range, the damping characteristics of the materials were changed relative to one another. This could mainly be attributed to the fact that the nano-sized MWCNTs were played their roles to impede the long-range motions with length scales more than an entanglement of polymeric structure and not the short-range dynamic motions of the macromolecular chains. ⁶⁷

Figure 4.

The plots of tanδ versus ω for the parent polymers and their nanocomposites at 180°C.

The van Gurp-Palmen plot, which is created by plotting the phase angle (δ) against the complex shear modulus (G*(ω)), ⁶⁸ is a very susceptible tool for the identification of the composition, macromolecular structure and relaxation behavior of polymers and their nanocomposites.^69,70 The constructed van Gurp-Palmen plot for the system studied is depicted in Figure 5. The results indicated that δ for the parents polymers was close to 90° in lower G*(ω) values and also it could be seen that δ decreased with increasing in EVA concentration up to 70 wt% at constant nanofiller content. The flow behavior of the materials, except for 25P70E5C and 10P85E5C, was similar to that of a viscoelastic liquid at low ω region. However, the compositions containing 70 and 85 wt% EVA showed a significant reduction in their phase angle to below 45°, reflecting they could both behave as viscoelastic solids.

Figure 5.

The van Gurp-Palmen plots for the parent polymers and their nanocomposites at 180°C.

The Winter-Chambon^71,72 method was utilized to elucidate whether the physical gelation was taken place for the nanocomposites or not. They proposed that the stress relaxation modulus (G(t)) of a critical gel could be designated by a power-law relationship which illustrated the self-similar or fractal formation of clusters at the gel point^73,74;

$G\left(t\right)=S_g{t}^{-n}$

(8)where S_g is the gel stiffness or strength and n is the critical relaxation exponent which varies between 0 and 1. Based on the linear viscoelastic theory, G(t) could be connected to G′(ω) and G″(ω) as follow;

$G^{\prime }\left(\omega \right)=\frac{G^{″}\left(\omega \right)}{\tan \hspace{0.17em}\delta }=\frac{G^{″}\left(\omega \right)}{\tan \left(\frac{\pi n}{2}\right)}=S_g\mathrm{\Gamma }\left(1-n\right)\cos \frac{\pi n}{2}{\omega }^n$

(9)where $\mathrm{\Gamma }$ is the gamma function. The variations of tanδ versus EVA content for several ω values on a log-log scale are depicted in Figure 6. As it can be observed, all tanδ curves of different ω values were approached to one peculiar spot, consisting with the formation of a single self-similar fractal structure at the critical gel point (P_g). ⁷⁵ This explicitly discloses the EVA concentration required, for the nanocomposites examined, to form a critical gel in the system. The critical amount of EVA to make a physical gel was found to be 70 wt%. Therefore, the two compositions of 25P70E5C and 10P85E5C are in the form of gel and hence their rheological behaviors are utterly justified.

Figure 6.

The plots of tanδ versus EVA content for several selected ω values. The critical gel point is shown by an arrow.

The quantity of tanδ at the meeting point could be employed to determine the n value that was estimated to be 0.309 ± 0.007. The G′(ω) value at P_g could be figured out if G′(ω) and $G^{″}\left(\omega \right)/\tan \left(\pi n/2\right)$ were plotted as a function of EVA loading for several ω values (Figure 7). The crossover points are observed at the P_g of 70 wt% EVA, the same value reported earlier (Figure 6). The S_g could be calculated by taking into consideration one G′(ω) value from any crossover point and exploiting equation (9). Nonetheless, the average of seven points was mentioned here for the sake of enhancing the trustworthiness of the value of S_g, which was found to be 41,304 ± 1100 Pa.sⁿ. The values of n and P_g indicated that a tough and strong gel was formed at 180°C in the system studied. Pan and Li ⁷⁶ reported that the polypropylene (PP)/MWCNTs system containing the nanofiller with the aspect ratio of 167 provided a physical gel with the lowest MWCNT content at the gel point in comparison with those prepared with the aspect ratio of 84 and 51. Razavi-Nouri ⁷⁷ found a gel point at 3.9 wt% OMMT for the EVA/acrylonitrile-butadiene rubber (NBR)/OMMT nanocomposite system.

Figure 7.

The plots of G′ and G″/tan (πn/2) versus EVA content for several selected ω values. The former and the later values are indicated by the solid and open symbols, respectively and the critical gel point is shown by an arrow.

Excluded volume screening is generally occurred after the development of chemical or physical gels. Near the point at which a gel is formed, as far as to the screening effect is related, the system behaves analogous to a molten state. ⁷⁸ The statistical self-similarity of a polymeric material is generally demonstrated by a fractal dimension (d_f). Presuming that the excluded and hydrodynamic interactions are totally screened at P_g, the value of d_f could be calculated from ⁷⁹ ;

$d_f=\frac{\left(d-2n\right)\left(d+2\right)}{2\left(d-n\right)}$

(10)where d = 3 (the space dimension for the system examined). It was found that d_f is about 2.21. The value was relatively close to d_f = 2.5, reported for a system wherein excluded volume interactions were entirely screened. Therefore, the system could be considered as if the excluded volume interactions within the physical gel were highly screened and also a stiff and dense network structure was formed. ⁸⁰

Thermal stability

TGA decomposition behavior of PMMA/EVA/MWCNTs nanocomposites along with the parent polymers in air and nitrogen atmospheres are shown in Figure 8 (a) and (b), respectively. The temperature at 5 wt% (T5), 10 wt% (T10), 20 wt% (T20), 50 wt% (T50) and 70 wt% (T70) weight loss were determined for both atmospheres to examine the thermal stability of different compositions. The corresponding TGA results are gathered in Tables 8 and 9 for air and nitrogen, respectively. The degradation of PMMA was taken place in one ⁸¹ and that for EVA in two stages ⁸² in both atmospheres. PMMA has been reported to decompose in nitrogen due to the random C-C bond scissions of the macromolecular backbone. However, the polymer is degraded because of depolymerization as well as C-C bond breakage of PMMA chains in air. ⁸³ The first degradation stage of EVA, is related to the elimination of acetate side groups as acetic acid, which is also known as deacetylation, leading to generate an unsaturated macromolecular chains or polyene in both atmospheres. The following step is attributed to the allylic chain scission of the polyene in an inert atmosphere and its aromatization into a char in an oxidative atmosphere. The char could then be oxidized into carbon dioxide above 500°C and thus the complete decomposition of EVA might eventually take place.^84–86 The two pristine polymers were found to experience total decomposition as if little or no chars could leave behind in both atmospheres. However, the thermal degradation behaviors of the materials under air are significantly different from those of established under nitrogen atmosphere. The samples scanned under air, mostly because of the auto-oxidation reaction, exhibited lower degradation temperatures in comparison with those examined under nitrogen atmosphere. ⁸⁷ It was revealed that the thermal stability of the nanocomposites, both in air and nitrogen, was higher than that of the pristine PMMA as well as their unfilled counterparts (data not shown here). Therefore, the filled materials experienced higher thermal stabilities in regard with all characteristics. This could be assigned to the dispersion state of the nanofiller and its high aspect ratio, which created both the labyrinth effects, strong physical adsorption of the macromolecular chains on the individual surface of the nanofiller particles and perhaps the role of each MWCNT as a radical scavenger function.^87–89 These could postpone the emission of small gaseous substances trapped into the nanocomposites and shift the degradation of the filled materials to higher temperatures. Figure 8(a) reveals that not only the highest thermal stability belongs to 10P85E5C and then 25P70E5C, but also their thermal stability at high temperatures is even better than that of EVA itself. For instance, T50 and T70 of 10P85E5C are 20 and 23°C superior to those of EVA, respectively. This was in agreement with the existence of a substantial amount of MWCNTs in EVA phase of the nanocomposites. However, this was not the case for Figure 8(b) where it was clear that the thermal stability of the nanocomposites was strongly related to the extent of EVA in each composition. Moreover, the nanofiller was found to be more advantageous in increasing the thermal stability of the nanocomposites under oxidative substance such as air than that of nitrogen.

Figure 8.

TGA decomposition curves of different PMMA/EVA/MWCNTs nanocomposites and the parent polymers (a) under air atmosphere and (b) under nitrogen atmosphere.

Table 8.

TGA results of air atmosphere for various PMMA/EVA/MWCNTs compositions and the parent polymers.

Sample	T5 (±1°C)	T10 (±1°C)	T20 (±1°C)	T50 (±1°C)	T70 (±1°C)
PMMA	293	303	318	346	360
80P15E5C	316	337	358	384	396
65P30E5C	306	327	351	385	394
45P50E5C	298	324	348	378	387
25P70E5C	321	343	369	431	460
10P85E5C	324	343	375	459	472
EVA	327	343	378	439	449

Table 9.

TGA results of nitrogen atmosphere for various PMMA/EVA/MWCNTs compositions and the parent polymers.

Sample	T5 (±1°C)	T10 (±1°C)	T20 (±1°C)	T50 (±1°C)	T70 (±1°C)
PMMA	337	346	357	374	383
80P15E5C	352	363	374	394	407
65P30E5C	350	364	377	399	419
45P50E5C	345	358	374	413	465
25P70E5C	346	364	386	457	473
10P85E5C	345	364	408	466	477
EVA	344	362	438	465	474

Conclusions

First, the PMMA/EVA/MWCNTs nanocomposites including 5 wt% MWCNTs and different polymer weight ratios were prepared through the mixing in the molten state. The preferred localization of MWCNTs were estimated by the determination of the surface free energy and then wetting coefficient. The results indicated that the nanofiller would thermodynamically prefer to migrate from EVA phase and accommodate into the PMMA phase. However, it was found that MWCNTs were located in both phases and a major amount of the nanofiller yet remained within the EVA domains. The rheological examinations pointed out that the nanocomposites containing 70 and 85 wt% EVA behaved as shear thinning materials throughout the entire ω range studied and no crossover point was found between the G′ and G″ of the two compositions. Using the Winter-Chambon method, the formation of a physical critical gel was verified in the nanocomposites containing 70 wt% EVA. The calculated fractal dimension value of the physical gel revealed that the system could be regarded as the one in which the excluded volume interactions were greatly screened. The crystallization and melting thermographs analysis indicated that the rate of crystallization and melting decreased with the increase in PMMA, reflecting PMMA imposed restrictions not only on the formation of crystalline domains of EVA phase but also on their melting. The TGA results showed that the thermal stability behaviors of the materials were significantly different in air and nitrogen atmosphere. The thermal stability of the materials were generally higher in nitrogen than that of in air atmosphere, however, the benefit of MWCNTs in putting off the degradation process was more influential in air than for nitrogen atmosphere. Now it is the time to raise the limitations of the present work. Studies on the influence of mixing sequences and mixing time on the nanofiller localization, morphology, thermal stability, flow behavior and other physical and mechanical properties of the nanocomposites should be accomplished for further analysis of the behavior and performance of the system. In this vein, performing additional characterization experiments such as transmission electron microscopy (TEM), atomic force microscopy (AFM) and dynamic mechanical thermal analysis (DMTA) could also provide some other beneficial information about this novel system.