Mechanical Properties and Thermal Behaviour of LLDPE / MWNTs Nanocomposites

Multi-walled carbon nanotubes (MWNTs) were incorporated into a linear low-density polyethylene (LLDPE) matrix through using screw extrusion and injection technique. The effect of different weight percent loadings of MWNTs on the morphology, mechanical, and thermal of LLDPE/MWNTs nanocomposite had been investigated. It was found that, at low concentration of MWNTs, it could uniformly disperse into a linear low-density polyethylene matrix and provide LLDPE/MWNTs nanocomposites much improved mechanical properties. Thermal analysis showed that a clear improvement of thermal stability for LLDPE/MWNTs nanocomposites increased with increasing MWNTs content.

It is well known that CNTs can exist in the form of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs).They can both be synthesized by three techniques: laser ablation, arc discharge and chemical vapour decomposition (CVD) 20 .In general, CVD is the most commonly used method for mass production of different types of CNTs.Although SWNTs offer the opportunity for much lower loading versus MWNTs to achieve the same electrical properties, enhancing mechanical properties even further 21 , MWNTs are extensively used, due to lower unit cost, greater availability and fewer dispersion challenges compared with SWNTs.
Linear low density polyethylene (LLDPE) is a commodity polymer, cheap, versatile, with widely uses in a variety of forms.Enhancing of LLDPE properties should be of interest in order to expand its application fields.In this study, LLDPE/MWNTs nanocomposites are fabricated by using traditional twin-screw extruder and injection machine; to improve LLDPE mechanical properties and thermal stability, and the effect of MWNTs loading on mechanical properties is investigated.

Material
MWNTs were supplied by Shenzhen Nano-Technologies Port Co. Ltd., China, with a purity of above 94%, average length of microns, and surface area of 4.26 m 2 /g.Linear low-density polyethylene (LLDPE, with number-average molecular weight of 20000 g.mol -1 ) was supplied by Daqing Petroleum & Chemical Co., and used without any further treatment.Polyethylene glycol (PEG, Aldrich, M n = 20000) was dried by heating at 70 °C for 7 hours under vacuum, used as an organic dispersant, and also played a role of a plasticizer.3-(trimethoxysilyl) propyl methacrylate (KH-570) was used as a coupling agent and paraffin wax (chemical grade) was used as a lubricant.
Adding the gained MWNTs, PEG and KH-570 into ethanol, the mixtures were sonicated using a bath sonicator for 1 hour.Then they were dried in a vacuum oven at 65 °C for 24 hours to remove ethanol, which were mixed with LLDPE and 0.1 wt.(%) of paraffin wax.
The above mixtures were extruded with a TE-35 co-rotating twin-screw extruder (Coperion Machinery Co., Ltd., China).The test specimens were molded using an injection machine (TTI-80, Tonghua machinery Ltd., Canton, China) with a screw of 33.0 mm in diameter, and L/D ratio of 20.

Characterizations
MWNTs size analysis was performed on a Microtrac S3000 analyzer (Microtrac Software Co., USA).Thermogravimetric analysis (TGA) measurement was performed using TGA-7 (Perkin-Elmer).The thermal analyses were carried out with a differential scanning calorimeter (DSC, DuPont 9900) over a temperature range from 70 °C to 155 °C at a heating rate of 10 °C/min, purged with nitrogen gas, and quenched with liquid nitrogen.The cell was calibrated using an indium standard; the weight of the sample was 5-10 mg.The tensile and flexural properties of the nanocomposites were measured on an Instron testing machine (model 4302).The notched Izod impact tests were performed with XJ-40A pendulum apparatus (Wuzhong Material Testing Machine Factory, Jiangsu, China).SEM observation was carried out with a JSM-5900LV scanning electron microscopy.

SEM observation
The diameter distribution of as-received MWNTs is measured, as shown in Figure 1.The diameter of 93% MWNTs is smaller than 100 nm with average diameters = 68 nm, and about 0.8% large than 200 nm.
It is well known that the dispersion of CNTs in the polymer matrix and the interfacial interactions between the polymer matrix and nanotubes are the key factors to ultimately determine many properties of polymers/CNTs nanocomposites.A uniform dispersion of CNTs and strong interfacial interactions between them can effectively improve the thermal, solid mechanical, adhesive, and electrical performances of the nanocomposite.To reveal dispersion of MWNTs in the LLDPE matrix, SEM observations were performed for cryo-fractured surfaces of LLDPE/MWNTs nanocomposites, as shown in Figures 2a-e.A fibrous fractured surface is observed due to the elongation at break of LLDPE/MWNTs nanocomposites; the random dispersed bright dots because of MWNTs high conductivity are the ends of the broken carbon nanotubes 22 .In addition, it is found that some MWNTs are broken apart, and, as a result of poor interfacial adhesion, some MWNTs are pulled out of the matrix before the breakage, forming caves on the fractured surface; moreover, other MWNTs are observed with their one end still strongly embedded in the LLDPE matrix as an inset.Such interesting and typical breakage phenomenon of the MWNTs indicates that a strong interfacial adhesion exists between MWNTs and LLDPE matrix and that the load transfer takes place efficiently from the matrix to the nanotubes.The strong interfacial adhesion is usually responsible for the significant enhancement of the mechanical propertie 23 .
At low concentration of MWNTs, from Figures 2a-c, it can be seen that the bright dots (i.e., MWNTs) embedded in the LLDPE matrix and the caves (i.e., MWNTs) pulled out from the LLDPE matrix are well dispersed; however, with an increase in MWNTs concentration, from Figures 2d, e), a nonuniform dispersion of MWNTs is observed in the nanocomposites, and a large aggregate of MWNTs having a diameter of over 500 nm is presented; while in the case of pristine MWNTs, its diameter is about 30-40 nm, as shown in Figure 1.Those indicate that the MWNTs were dispersed as nanotubes aggregates due to the imperfect mixing of the masterbatch.
When the content of MWNTs is no more than 1.0 wt.(%), owing to producing carboxylic or hydroxylic groups on the surface of the MWNT by HNO 3 treatment 24,25 , which ensures the high dispersion quality of the MWNTs and prevents the severe aggregation of MWNTs 26,27 , reasonably uniform distribution of the MWNTs is observed in Figures 2a-c.On the other hand, at compositions containing greater amounts of MWNTs, a small amount of aggregates is shown in Figures 2d, e).These results are in good agreement with the results of Wu et al. 28 and Bikiaris et al. 29 who reported that increasing the content of SiO 2 leads to larger agglomerates; this is also the case in LLDPE/MWNTs nanocomposites.In fact, there are more or less agglomerates of MWNTs formed in Figure 2. It is worth nothing, due to the strong interaction among the nanoparticles, the limited shear force provided by the mixing device and the high melt viscosity of polymer melts, especially the high viscosity of the LLDPE melt (MFI about 1 g/10 min) 30 , might be impossible to pursue an efficient nanoscale dispersion of MWNTs.

Mechanical property of LLDPE/MWNTs nanocomposites
There are many researches on the effectiveness of nanoparticles to bear impact toughness.The amounts of MWNTs have great effects on the impact strength of LLDPE/MWNTs nanocomposites in Figure 3.The Izod impact strength of notched specimen initially increases with the increasing of MWNTS, and reaches the maximum value (37.15 kJ.m -2 ) at MWNT content of 1.0 wt.(%), a 239% increase compared with that of pure LLDPE.The measured mechanical properties truly reflect the homogeneous dispersion of MWNTs in polymer matrix leads to improving impact strength, as shown in Figures 2a-c.This is because the size of the intercalated MWNTs is in the similar order as that of macromolecular segments, and the increased free volume in the composites makes it possible for the segments to move when composites are subjected to an impact force 31 .But at high MWNTs contents exceeding 1.0 wt.(%), they tend to agglomerate, as shown in Figures 2d, e, and act as stress concentrators, which may decrease the impact strength.
In order to evaluate the reinforcing effect of MWNTs into the LLDPE matrix, mechanical properties during extension were measured.In Figure 4, the variation of tensile strength for LLDPE/MWNTs nanocomposites is shown as a function of MWNTs content.The increase of tensile strength is observed for MWNTs contents up to 1.0 wt.(%), reaching 22% in regard to the initial value; the reduction in tensile strength observed for MWNTs concentrations higher than 1wt.(%) should be rather associated with MWNTs aggregation, which increased with increasing MWNTs amounts, as was observed in SEM micrographs discussed above.It is obvious that MWNTs content plays an important role as far as their efficiency in acting as reinforcement agents is concerned.
In the case of Young's modulus, its variation for LLDPE nanocomposites at different MWNTs concentrations is shown in Figure 5.An increase in modulus with filler loading is observed for LLDPE/MWNTs nanocomposites, which reaches up to 54% compared to neat LLDPE and indicates increased stiffness due to the rising filler content.A change of deformation behavior from ductile to more brittle occurs at higher contents of MWNTs.
The results of elongation at the break point measurements are presented in Figure 6.It is clear that incorporation of MWNTs initially has a negative effect on the elongation at the break point and a greater decrease reaches up to 53% compared to neat LLDPE; however, as MWNTs concentrations is more than 1 wt.(%), the elongation at the break increases with increasing MWNTs amounts.It seems that, at MWNTs content of no more than 1.0 wt.(%), its dispersion is uniform; meanwhile, MWNTs, which are rigid, almost no elongation, and included in LLDPE matrix, behave like physical cross-linking points and restrict the movement of polymer chains, and have this property in the final composite material.As a result they inhibit the elongation of the nanocomposite, making it less ductile.On the other hand, when the concentration is higher than 1.0 wt.(%), the elongation at the break increases; this is because MWNTs agglomerate are formed in the matrix (Figures 2d, e), resulting in a profoundly deleterious effect on the properties of LLDPE/MWNTs nanocomposites, for they induces premature fracture and frequently becomes sources of the secondary crack 32 .Sternstein et al. 33 , have primarily attributed the reinforcement mechanism of nano-filled polymer, to trapped entanglements that existed in the temporary bonding between polymer chains and the filler surface 33 , which has both near -and far-field effects on matrix chain motion, and plays the role of physical cross-linking network, thus leading to greatly enhanced modulus of matrix 34 ; when load is transferred to the physical cross-linking network, the debonding (disentanglement) of chain segments from the filler surface facilitates the relaxation of the matrix entanglement structure, leading to the higher impact toughness.Therefore, following above analysis, we have gained not only a higher modulus but also higher impact toughness in LLDPE/MWNTs nanocomposites.On the other hand, the choice of suitable filler, as well as a suitable quantity, grain size and high compatibility with the polymer matrix, is a key factor for efficient toughening, which can accomplish improvement of polymer's mechanical properties, except elongation at break and toughness.On the addition of filler, the interface conditions for the brittle-ductile transition of matrix should be in accordance with the following requirements: (1) a strong adhesion should exist between the matrix and filler so that the stress is transferred through the interfacial area; and (2) the interfacial area should be a lower stiffness than the matrix and deform, thereby yielding prior to the matrix and inducing matrix yielding 35 .In our experiment, due to the poor interfacial bonding between MWNTs and LLDPE macromolecules, the interface stiffness is lower and the tensile strength reduces.

Thermal properties
It is of great interest to study the crystallization and melting behavior of neat LLDPE and its nanocomposites because crystal structures and crystallinity play significant roles in the mechanical and other properties of crystalline polymers.DSC analysis is a generally convenient method for analyzing first order transitions like melting and crystallization.Figure 7 shows the thermal diagram measured during both heating and cooling.The LLDPE/MWNTs nanocomposites possess a melting endothermic peak (T m ) during heating and a crystallization exothermal peak (T c ) during cooling; it can be observed that T m and T c increase very slightly in the LLDPE/MWNTs nanocomposite compared with those in neat LLDPE, and increase with increase of MWNTs concentration, which indicates that the MWNTs presence does not prevent LLDPE crystallization and melting, and act as an nucleation reagent for the LLDPE crystallization; meanwhile, the crystallization enthalpy (∆H c ) and melting enthalpy (∆H m ) which are determined from the area under the exotherm and endotherm, are increased in the nanocomposites as compared with that of neat LLDPE.The incorporation of MWNTs enhances the crystallization of LLDPE in the nanocomposites compared with that of neat LLDPE, which should be attributed to the strong heterogeneous nucleation of MWNTs 27 .
Figure 8 shows TGA thermograms of neat LLDPE, MWNTs and LDPE/MWNTs nanocomposites.The decomposition temperature (onset of inflection) for neat LLDPE is lower than those of its nanocomposites, indicating that the thermal stability of nanocomposites has been   improved because of addition of MWNTs.Besides that, the residual weight of LLDPE/MWNTs nanocomposites left increases steadily with the increase of MWNTs loading.As shown in Figure 8, the weight loss at 432 °C for neat LLDPE is about 85%, whereas LLDPE/MWNTs nanocomposites are only around 58-72%.This also indicates that the thermal stability of LLDPE is significantly improved on incorporation of MWNTs.
Figure 9 shows the dependence of the decomposition temperature (T d , 5% weight loss temperature) on MWNTs content.Neat LLDPE and pure MWNTs start to loss weight at 371 °C and 467 °C, respectively.It can be seen that the overall thermal stability of LLDPE/MWNTs nanocomposites, compared with neat LLDPE, is clearly improved; in Figure 9, the addition of 0.5 wt.(%) MWNTs causes the decomposition temperature of LLDPE increase more than 10 °C.The thermal stability of LLDPE/MWNTs nanocomposites may be closely related to following factors: the dispersion state and the loading content of MWNTs.The decomposition temperature of LLDPE/MWNTs nanocomposites increases slightly with increasing MWNTs loading content, probably due to the ease of compact char formation for the nanocomposites during the thermal degradation.On the other hand, high concentration MWNTs would definitely prevent its fine dispersion, and more aggregation or bundles could often be formed because of van der Waals force among the MWNTs, thus deteriorating the thermal stability of the nanocomposites.It is necessary to research the competing effect between the dispersion state and the loading content of MWNTs in depth.
Morphological analysis shows that, at low concentration of MWNTs, it is well dispersed; however, with an increase in MWNTs concentration, it is a nonuniform dispersion, and a large aggregate of MWNTs having a diameter of over 500 nm is formed.Mechanical properties showed that the Izod impact strength of notched specimen and the tensile strength initially increased with the increasing MWNTs content, and subsequently decrease with increasing MWNTs content.DSC analysis showed that T m and T c increased very slightly in the LLDPE/MWNTs nanocomposite compared with those in neat LLDPE, and increased with increase of MWNTs concentration.The analysis of thermal degradation in airflow showed a clear improvement of thermal stability for LLDPE/MWNTs nanocomposites, proportionally to MWNTs content.

Figure 3 .
Figure 3. Izod impact strength of the LLDPE nanocomposites with different MWNTs content.

Figure 4 .
Figure 4. Tensile strength of LLDPE nanocomposites as a function of MWNTs content.

Figure 9 .
Figure 9. Decomposition temperature (T d , 5% weight loss temperature) as a function of MWNTs loading.