Thermal Stability and Rheological Behaviors of High-Density Polyethylene/Fullerene Nanocomposites

Abstract

High-density polyethylene/fullerene (HDPE/C₆₀) nanocomposites with the C₆₀ loading that varied from 0.5 to 5.0% by weight were prepared via melt compounding. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results showed that the presence of C₆₀ could remarkably enhance the thermal properties of HDPE. A very low C₆₀ loading (0.5 wt%) increased the onset degradation temperature from 389^∘C to 459^∘C and decreased the heat release from 3176 J/g to 1490 J/g. The larger the loading level of C₆₀, the better the thermal stability of HDPE/C₆₀ nanocomposites. Rheological investigation results showed that the free radical trapping effect of C₆₀ was responsible for the improved thermal stability of HDPE.

1. Introduction

Soon after the discovery of buckminsterfullerene, C₆₀, it was able to be produced in bulk quantities, which inspired scientists worldwide to explore its fascinating chemistry, and it has now become the most intensely researched single molecule in modern science [1–3]. A significant aspect of the C₆₀ chemistry is its high reactivity towards free radicals, and C₆₀ has 30 carbon-carbon double bonds which can trap more than 34 free radicals; thus it is known as a radical sponge [4]. The free radical reaction of C₆₀ with various compounds has been extensively studied [4–7].

Since the thermal degradation of polymers is a free radical chain reaction, the presence of C₆₀ in polymer may trap the free radicals produced during the degradation process. These suggest the possible use of fullerene as an effective radical scavenger in chain reactions during thermal degradation. It has been reported that 2 wt% fullerene (C₆₀) could increase the onset degradation temperature (𝑇_onset) and the maximum degradation temperature (𝑇_max) of PP by 20 and 62°C, respectively [8, 9]. The presence of C₆₀ could delay the thermal oxidation and improve the thermal stability of PP, which could be attributed to the high reactivity of C₆₀ towards free radicals.

The present paper mainly focused on studying the influence of C₆₀ on the thermal degradation behavior and rheological properties of high-density polyethylene. Concerning that the thermal degradation of polyethylene is done also via a free radical chain scission process as PP, C₆₀ is expected to have positive effect on improving the thermal stability of PE.

2. Materials and Methods

2.1. Materials

High-density Polyethylene (HDPE, 5000 S, MFR = 0.9 g/10 min) was purchased from Yangzi Petrochemical Co., Ltd, and C₆₀ (purity: >99%) was bought from Henan Puyang Co., Ltd.

2.2. Preparation of HDPE/C₆₀ Nanocomposites

HDPE/C₆₀ nanocomposites were prepared via melt compounding at 180°C in a ThermoHaake rheomixer with a rotor speed of 60 rpm for 8 min. Nanocomposites containing 0.5 wt%, 1.0 wt%, 2.5 wt%, and 5.0 wt% C₆₀ were designated as C₆₀-0.5%, C₆₀-1.0%, C₆₀-2.5%, and C₆₀-5.0%.

2.3. Measurements and Characterization

Thermogravimetric analysis (TGA) was performed on a STA 409 PC thermal analyzer at a heating rate of 20°C/min in air atmosphere and N₂ atmosphere, with a scanning range from 30 to 700°C, and each specimen was examined in triplicate.

The rheological properties of HDPE and its nanocomposites were conducted on a controlled strain rheometer (Haake MARS) in air environment. The samples were pressed at 180°C under 15 MPa to get the disklike specimens with 25 mm in diameter and 1.2 mm in thickness. The isothermal dynamic frequency sweeps were performed under the condition of the frequency range, strain amplitude, and temperature being 100–0.01 rads⁻¹ with the strain of 1% at 180°C and 300°C, respectively. Temperature scanning test was performed in the range from 180 to 300°C with the 1% strain and a fixed frequency at 1 rads⁻¹.

The dispersion of C₆₀ in the HDPE matrix was observed by transmission electron microscopy (TEM, JEM-1200EX).

3. Results and Discussion

3.1. Dispersion of C₆₀ in HDPE Matrix

The diameter of the spherical C₆₀ molecule is 0.71 nm, and its crystal size differs with different methods of fabrication [9]. Figure 1 shows the TEM images for pure C₆₀ and C₆₀-5%. The size of C₆₀ crystals used in this work from Figure 1(a) is from 70 to 200 nm. From Figure 1(c), it is shown that many of C₆₀ crystallites in the HDPE matrix were shaped in ellipsoidal, rodlike. This phenomenon may be caused by the strong shear force during blending. The shear force could destroy the primary stack of C₆₀ crystallites and these crystallites rearrange to different type. In the HDPE matrix, C₆₀ crystallites aggregates and does not disperse well through melt blending, as observed from Figure 1(b). The size of some C₆₀ domains in nanocomposites is about 500 nm or even large.

(a)

(b)

(c)

Figure 1 

TEM microphotographs for pristine C60 (a) and C60-5.0% (b) and (c).

3.2. Thermal Properties of HDPE/C₆₀ Nanocomposites

The thermal stability of HDPE and HDPE/C₆₀ nanocomposites was tested by TGA, and their TG and DTG curves in nitrogen atmosphere are shown in Figure 2, with detailed data listed in Table 1. At the initial stage of degradation (before 400°C), C₆₀ reacts easily with low-molecular-weight alkyl radicals with formation of remarkable persistent products RnC₆₀    (where 𝑛=1,2,3…) [10, 11], which caused that the decomposition of HDPE/C₆₀ nanocomposites is slower than pure HDPE. The onset thermal decomposition temperature (𝑇_onset) of HDPE is noticeably increased with the addition of C₆₀. For example, the 𝑇_onset of C₆₀-0.5% is 459°C, about 70°C higher than pure HDPE. With increasing C₆₀ content, the onset temperature of HDPE/C₆₀ nanocomposites changes slightly. From DTG curves, the maximum decomposition temperature (𝑇_max) is obtained, with 𝑇_max 497°C for C₆₀-0.5%, about 14°C higher than pure HDPE.

Table 1 

Detailed data obtained from TGA tests for HDPE and HDPE/C₆₀ nanocomposites in nitrogen.

(a)

(b)

Figure 2 

TG (a) and DTG (b) curves for pristine HDPE and HDPE/C60 nanocomposites in nitrogen.

Figures 3 and 4 present the TG and DSC curves for HDPE and its composites in air atmosphere, with detailed data given in Table 2. In air atmosphere, the presence of oxygen could enhance the thermal oxidation decomposition of HDPE nanocomposites remarkably. HDPE experiences a rapid thermal oxidation decomposition accompanied by hydrogen abstraction [12]. The (𝑇_onset) of HDPE is about 323°C, and two-step decomposition processes are observed at around 398°C and 456°C (see Table 2). The first step decomposition is the oxidation of HDPE [13]. The second step may be the decomposition of oxidation products formed by the oxidation of HDPE. C₆₀, as the radical sponge, could capture the free radicals produced during the degradation process. Obviously, the presence of C₆₀ delays the oxidation degradation of parent polymer and the first-step 𝑇_max disappears, and the 𝑇_onset of the nanocomposites noticeably increase with the increase of C₆₀ content. As for C₆₀-2.5%, its 𝑇_onset is around 431°C, 108°C higher than pure HDPE, which indicates that the presence of C₆₀ slows down the thermal oxidation decomposition of HDPE remarkably. However, when the contents of C₆₀ in the matrix exceed 2.5 wt%, the 𝑇_onset of the nanocomposites decreases with the content of C₆₀. At high loading (as for C₆₀-5.0%), C₆₀ may tend to attract each other, which causes the agglomeration (as shown in Figure 1) instead of trapping the alkyl fragments radicals bringing down the thermal stability of HDPE/C₆₀ nanocomposites.

Table 2 

Detailed data obtained from TGA and DSC tests for HDPE and HDPE/C₆₀ nanocomposites in air.

(a)

(b)

Figure 3 

TG (a) and DTG (b) curves for pristine HDPE and nanocomposites in air.

Figure 4 

DSC curves for HDPE and HDPE/C60 nanocomposites.

The difference in thermal behavior between air and nitrogen atmospheres indicates that the presence of oxygen could speed up the thermal oxidation decomposition of HDPE nanocomposites remarkably. The oxygen and alkyl fragment radicals trapped by C₆₀ in nanocomposites during the thermal decomposition are competitive.

DSC measurements could provide the heat enthalpy of thermal degradation during the decomposition process of materials. Figure 4 presents the DSC curves of HDPE/C₆₀ nanocomposites in air atmosphere. In air atmosphere, HDPE/C₆₀ nanocomposites experiences rapid exothermal oxidation decomposition at high temperature. The enthalpy (ΔH_𝑑) is an important parameter since it could quantify the heat evolution produce in the process of oxidation decomposition. The enthalpies of the nanocomposites are greatly reduced from 3176 J/g for HDPE to 1490 J/g for C₆₀-0.5%, indicating that the nanocomposites release much less heat in the process of oxidation dehydrogenization and this phenomenon is favorable to flame retarded polymer materials.

3.3. Mechanism for the Thermal Stability of HDPE/C₆₀ Nanocomposites

In order to clarify the mechanism for C₆₀ improving the thermal stability of HDPE, the rheological measurements were introduced to investigate the viscoelastic behaviors of the nanocomposites. For investigating the viscoelastic behavior of the nanocomposites in the heating process, temperature scanning measurements were performed. Figure 5 presents the curves of temperature dependence of complex viscosity (𝜂^∗) for HDPE and its nanocomposites. Clearly, with the increase of temperature, the complex viscosity (𝜂^∗) of nanocomposites decreases, and then increases sharply. The critical temperature is defined as 𝑇_c, at which 𝜂^∗ starts to increase or crosslink reaction occurs. The easier movement of polymer chains during heating, or the occurrence of degradation of polymer can cause the decrease of 𝜂^∗. The increase of 𝜂^∗ is due to the occurrence of an oxidation crosslink or the oxidation crosslink reaction overwhelming the polymer decomposition. In the case of pure HDPE, it gives a 𝑇_c of 285°C. For HDPE/C₆₀ nanocomposites, their 𝑇_c is about 45°C lower than that of HDPE, implying that another kind of crosslink reaction occurred in the process of heating for HDPE/C₆₀ systems, which speeds up the crosslink reaction whether before or after oxidation crosslinking takes place.

Figure 5 

Dependence of complex viscosity (𝜂∗) on temperature for HDPE and HDPE/C60 nanocomposites.

The viscoelastic behaviors of the nanocomposites at 180°C were studied and those of parent polymer, HDPE, were also used as a comparison. Figures 6 and 7 show the storage moduli (𝐺^′) and complex viscosity (𝜂^∗) as a function of frequency (𝜔) of HDPE and its nanocomposites. Some researchers have found that both 𝐺^ and 𝜂^∗ have much larger values than those of parent polymers in the low 𝜔 regime for the nanocomposites containing CNTs [14–17] or clay [18, 19], suggesting that the presence of CNTs or clay affects the relaxation and motion of polymer chains due to their spatial geometry. Most authors attribute these viscoelastic behaviors to the formation of CNT or clay networks in the polymer matrix. However, in the high 𝜔 region, the addition of CNTs and clay does not significantly affect the 𝐺^ or 𝜂^∗ of polymers. Unlike CNTs and clay, whether in the low 𝜔 region or high 𝜔 region, not much obvious difference in 𝐺^ and 𝜂^∗ is observed for the HDPE/C₆₀ nanocomposites with various C₆₀ contents. The oxidation degradation of HDPE does not occur at 180°C (as shown in Figure 3 and Table 2). And not enough free radicals react with C₆₀. Therefore, the incorporation of C₆₀ does not affect the movement and relaxation of polymer chain segments remarkably.

Figure 6 

Plots of storage moduli (𝐺) versus shifted frequency for consecutive small amplitude oscillatory shear sweeps performed at 180°C.

Figure 7 

Plots of complex viscosity versus shifted frequency for consecutive small amplitude oscillatory shear sweeps performed at 180°C.

The viscoelastic behaviors of the nanocomposites at 300°C after dynamic temperature scanning measurements were also studied. Figures 8 and 9 show the storage moduli (𝐺^) and complex viscosity (𝜂^∗) as a function of frequency (𝜔) of HDPE and its nanocomposites at 300°C. The increase of complex viscosity with C₆₀ content is concomitant with the increase of the storage moduli. The storage moduli for the nanocomposites show a monotonic increase at all frequencies with increasing C₆₀ content. The 𝐺^ versus frequency curve for the nanocomposites appears to be approaching a plateau at low frequencies. It has been proposed that this “plateau” effect is derived from interconnected structures of isometric fillers that result in an apparent yield stress which is manifest by a plateau in either 𝐺^ or 𝜂^∗ versus frequency plots [20, 21]. Exposed at 300°C for a long time, many alkyl radicals appear, coupled with the interfacial interactions between free radicals and C₆₀. The restraint for the move and relaxation of polymer chains increased 𝐺^ and 𝜂^∗, which is the evidence for the presence of a chemical reaction between the HDPE matrix and C₆₀.

Figure 8 

Plots of storage modulus versus shifted frequency for consecutive small amplitude oscillatory shear sweeps performed at 300°C.

Figure 9 

Plots of complex viscosity versus shifted frequency for consecutive small amplitude oscillatory shear sweeps performed at 300°C.

4. Conclusion

The presence of C₆₀ could enhance the thermal stability of HDPE. A very low C₆₀ loading (0.5 wt%) increases the 𝑇_onset from 389°C to 459°C and decreases the heat release from 3176 J/g to 1490 J/g. The free radical trapping effect of C₆₀ is responsible for the improved thermal stability of HDPE. Thus, C₆₀ is expected to be an efficient thermal stabilizer to the polymer materials that degraded with a free radical chain scission process.

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (no. 51073140).