Study on Thermal Stability and Non-isothermal Crystallization Behaviour of Polyethylene/clay Nanocomposites

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1 Study on Thermal Stability and Non-isothermal Crystallization Behaviour of Polyethylene/clay Nanocomposites Study on Thermal Stability and Non-isothermal Crystallization Behaviour of Polyethylene/clay Nanocomposites Fu-An He, a,b and Li-Ming Zhang a* a PCFM Lab and GDHPPC Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat- Sen University, Guangzhou , China b College of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming , China Received: 8 August 2014, Accepted: 11 December 2014 SUMMARY In this work, thermal stability and non-isothermal crystallization behaviour of polyethylene/clay nanocomposites prepared by in-situ polymerization were studied. The thermal degradation kinetics of polyethylene/clay nanocomposites in nitrogen was investigated by a pseudo-first order method indicating that the addition of clay filler in polyethylene matrix could improve the thermal stability of nanocomposites. The Ozawa, Avrami, and modified Avrami-Ozawa methods were applied to study the non-isothermal crystallization behaviour of polyethylene/clay nanocomposite. It was found that the clay filler not only acted as a heterogeneous nucleation for the crystallization of polyethylene but also hindered the transport of the polyethylene molecule chains at the same time, resulting in a decrease of the crystallization growth rate of polyethylene with clay loading. In addition, the nucleation activity and the activation energy for the crystallization of polyethylene/clay nanocomposites were also estimated. Keywords: Polyethylene/clay nanocomposites; Thermal stability; Non-isothermal crystallization kinetics 1. INTRODUCTION Polyethylene (PE) is an important semicrystalline thermoplastic that has wide range domestic and industrial applications. It has been reported that the incorporation of inorganic fillers into PE matrix is an important approach to optimize the properties and the processability of the final materials. In recent years, nanocomposites composed of polyolefin and clay have received considerable attention because of their unique advantages including increased thermal stability, enhanced mechanical property, reduced flammability, and improved gas barrier Moreover, the addition of clay into the PE matrix may also lead to some transformations of the physical structure of PE macromolecule chains, such as changes in a crystallinity degree, melting temperature and crystallization temperature. * Corresponding author: ceszhlm@mail.sysu.edu.cn Smithers Information Ltd., 2015 Many methods, such as solution blending, melt blending, and in-situ polymerization, have been employed to prepare PE/clay nanocomposites. Compared to the other two preparation methods, in-situ polymerization has several advantages: (1) It can produce PE/clay nanocomposites in one step by catalyze ethylene using clay-supported catalyst; (2) It can obtain ultrahigh molecular weight PE/clay nanocomposites and PE/clay nanocomposites with high loading amount of clay; (3) The interfacial interaction between PE and clay are strong, which can improve the mechanical properties of PE/clay nanocomposites and adjust the crystallization behaviour of PE macromolecular chains; (4) The claysupported catalyst can be used in the industrial polymerization equipment directly. In our previous work, we have prepared PE/clay nanocomposites by an in-situ polymerization method using Ziegler- Natta catalyst supported on organoclay/ MgCl 2 support with alkylaluminium compounds as cocatalysts 20. It can be expected that the clay filler may play an important role in determining the properties and the processability of PE/clay nanocomposites due to its influences on the thermal stability, crystallinity, crystallization rate, and crystallization temperature of PE. The isothermal crystallization behaviour of PE/clay nanocomposites at the specified crystallization temperature has been studied by Xu et al. and Kuo et al. 13,14. However, the analysis of the isothermal crystallization kinetics of polymer and its composites is relatively simple because some important factors including the cooling rate and the thermal gradient are neglected. On the other hand, nonisothermal crystallization is carried out under continuous cooling conditions. The study of the thermal stability and the non-isothermal crystallization of PE/clay nanocomposites are of great Polymers & Polymer Composites, Vol. 23, No. 8,

2 Fu-An He and Li-Ming Zhang significance in practical application because industrial processes are usually related to continuously changing thermal conditions. Therefore, it is very necessary to better understand the thermal stability and the nonisothermal crystallization behaviour of PE/clay nanocomposites for practical processing. In this work, we use the Chan-Balke method to evaluate the thermal stability of PE/clay nanocomposites, and study the nonisothermal crystallization kinetics of PE/ clay nanocomposites in details by the Ozawa method, the Avrami method, the modified Avrami-Ozawa method, and the calculation of nucleation activity and activation energy. 2. MATERIALS AND EXPERIMENTAL METHODS The details of the preparation and characterization of the PE/clay nanocomposites prepared by the insitu method have been described in our previous research 20. The loading amounts of clay in the PE matrix are listed in Table 1. The thermal stability was investigated by thermogravimetric (TG) analysis using NetzschTG-209 in nitrogen at a purge rate of 50 ml/min. The heating rate was set as 20 o C/min and the scanning temperature was in the range from 50 to 700 o C. Non-isothermal crystallization measurements were carried out with a Perkin-Elmer DSC-7 differential scanning calorimeter (DSC) calibrated with indium under a nitrogen atmosphere. Sample was initially heated to 160 o C at a rate of 100 o C /min. It was held for 5 min at this temperature to eliminate previous thermal histories before cooling at a specified cooling rate. The cooling rates employed were 5, 10, 15, and 20 o C /min. From the DSC curves of melting crystallization, the values of relative crystallinity at different temperature, X T, can be calculated according to the following equation: (1) where T 0 and T are the temperatures at which crystallization starts and ends, and dh c /dt is the heat flow rate. Figure 4 shows the relative degree of crystallinity (X T ) vs. the temperature (T) for PE and its nanocomposites at various cooling rates. In nonisothermal crystallization process, the relationship of the time t and the temperature T can be defined as follow: (2) where T is the temperature at crystallization time t, T 0 is the temperature at which crystallization begins (t=0) and β is the cooling rate. The activation energy DE for the transport of the polymeric segments to the growing crystal surface can be determined using Kissinger approach, Augis-Bennett approach and Takhor approach by calculating the variation of T p with the cooling rate β as follows : (1) Kissinger approach, (3) where R is the gas constant and other parameters have the same meaning as earlier indicated. (2) Augis-Bennett approach, (4) where T 0 is the initial crystallization temperature, (3) Takhor approach, 3. RESULTS AND DISCUSSION (5) 3.1 Thermal Behaviour of PE and PE/Clay Nanocomposites Figure 1 shows TG and DTG curves from the thermal degradation of PE and its nanocomposites under a nitrogen atmosphere at a scan rate of 20 o C /min. At the initial stage of the thermal degradation, the PE/clay nanocomposites degraded faster than pristine PE. Acceleration of the degradation in this stage is due to a catalytic effect of acidic sites originating from the Hoffman elimination of the organic treatment 2,13. As show in Table 1, the maximum decomposition temperature of all the PE/clay nanocomposites obtained from the tip of the DTG curve peak was higher than that of PE. Moreover, the maximum decomposition temperature of the PE/clay nanocomposites increased with the increment of clay contents. It suggests that the nanoscale dispersion of the clay filler in PE matrix can improve the thermal stability of nanocomposites due to its barrier effect on diffusion of degradation products from the bulk PE to the gas phase, which slows down thermal volatilization of the degradation PE. Table 1. T onset and T max of PE and its nanocomposites Sample PE PEC1 PEC2 PEC3 Clay loading (wt%) T onset ( ) T max ( ) Chan and Balke proposed a pseudofirst order method for calculating the thermal degradation activation energy of a sample from the thermal gravimetric data based on a single constant heating rate experiment Polymers & Polymer Composites, Vol. 23, No. 8, 2015

3 Study on Thermal Stability and Non-isothermal Crystallization Behaviour of Polyethylene/clay Nanocomposites They proved that the relationship of the thermal degradation activation energy E a, the weight fraction W, the rate of weight loss r (-dw/dt), and the temperature T could be expressed by the following equation: Figure 1. (a) TG curves for PE and its nanocomposites under a nitrogen atmosphere, and (b) DTG curves for PE and its nanocomposites under a nitrogen atmosphere (6) It means that, for a first order reaction, a plot of ln(r/w) versus -1/RT should give a straight-line slope, and then the thermal degradation activation energy can be obtained. Figure 2 presents a typical plot of ln[-dw/dt]/w] plotted against -1/(RT) for sample PEC3 which is a curved line. PE and all other nanocomposites samples had the same curvilinear trend as sample PEC3. The non-linearity in this plot is caused by a change in thermal degradation mechanism as the thermal degradation temperature increases. It seems that the thermal degradation of sample PEC3 was a two-step reaction with different activation energy. We assumed that the pseudo-first order method was still applicable in our PE/ clay nanocomposites, and attributed the non-linearity to a change in E a. Therefore, the curve was fitted by two straight lines including region I and region II. Region I and region II represented the initial stage and the main stage of the thermal degradation, respectively. A summary of the kinetic parameters obtained using the pseudo-first order method, shown in Table 2, suggests that all samples in initial degradation stage (region I) has lower activation energies and frequency factors lna than those in main degradation stage (region II). The activation energy value of pure PE was higher than that of all PE/clay nanocomposites in region I owing to the Hoffman elimination reaction which decreased the thermal stability of PE matrix at the initial degradation stage. Whereas in region II for main degradation stage the activation energy value of all PE/clay nanocomposites is higher when compared to pure PE. Table 2. The kinetic parameters obtained using the pseudo-first order method Sample Region I Region II lna(min -1 ) Ea(kJmol -1 ) lna(min -1 ) Ea(kJmol -1 ) PE PEC PEC PEC Polymers & Polymer Composites, Vol. 23, No. 8,

4 Fu-An He and Li-Ming Zhang This result should be attributed to the nanoscale dispersion of the clay filler in the nanocomposites and then its barrier effect during the thermal degradation process which can improve the thermal stability of PE matrix. 3.2 Crystallization Behaviour of PE and PE/Clay Nanocomposites Figure 3 shows DSC curves from heat flow as a function of temperature at different cooling rates for PE Figure 2. A typical plot of ln[-dw/dt]/w] plotted against -1/(RT) for sample PEC3 Figure 3. DSC curves of (a) PE, (b) PEC1, (c) PEC2 and (d) PEC3 at various cooling rates and its nanocomposites. The initial crystallization temperature (T 0 ), the peak crystallization temperature (T p ), and the final crystallization temperature (T e ) can be obtained from these curves (see Table 3). It is found that, as the rate of cooling increased, T 0, T p, and T e shift to lower temperatures. Moreover, T 0 and T p of all nanocomposites are higher than those of PE at a given cooling rate. This phenomenon can be explained by the heterogeneous nucleation effect of clay on PE macromolecule segments, which leads to the crystallization of PE at a higher crystallization temperature. According to Eq. (2), the value of T on the X-axis in Figure 4 can be transformed into the crystallization time t as shown in Figure 5. The half crystallization time (t 1/2 ) of PE and its nanocomposites can be determined from Figure 5, and the results are listed in Table 3. It is obvious that the t 1/2 values of PE/clay nanocomposites at various cooling rates are higher than those of neat PE. Moreover, the t 1/2 values increase with the increment of clay loading amount. The result indicates that clay plays two roles in the crystallization progresses of PE. One is a heterogeneous nucleating agent to facilitate crystallization of PE as mentioned above. The other is a hindrance to retard crystallization of PE, which may be attributed to interfacial adhesion between the PE matrix and the clay filler. Such interfacial adhesion can reduce the movement ability of PE chains during crystallization progresses. Similar result has been reported by Tjong et al. in the polyamide 6/clay nanocomposites 22. Non-isothermal Crystallization Kinetics of PE and its Nanocomposites The Ozawa, Avrami, and modified Avrami-Ozawa methods were used to analyze the non-isothermal crystallization kinetics of PE and its nanocomposites. Assuming that the polymeric melt was cooled at a constant 548 Polymers & Polymer Composites, Vol. 23, No. 8, 2015

5 Study on Thermal Stability and Non-isothermal Crystallization Behaviour of Polyethylene/clay Nanocomposites Table 3. Values of T 0, T, T, t, n, Z and Z for PE and its nanocomposites at various cooling rates p e 1/2 t c Sample PE PEC1 PEC2 PEC3 Cooling rate (20 C/min) T 0 ( C) T p ( C) T e ( C) t 1/2 (min) n Z t Z c rate and the mathematical derivation of Evans was valid, Ozawa extended the Avrami equation to non-isothermal conditions 23 : Figure 4. The relative degree of crystallinity with temperature for crystallization of (a) PE, (b) PEC1, (c) PEC2 and (d) PEC3 at various cooling rates (7) where X T is the relative degree of crystallinity at temperature T; K is the Ozawa crystallization rate constant; m is the Ozawa exponent and b is the cooling rate. The double logarithm of the Ozawa equation gives the following relation: (8) A plot of ln[-ln(1-x T )] against ln b at a given temperature should result in a straight line if the Ozawa method is valid. Thus, K(T) and m can be obtained from the intercept and the slope of the lines, respectively. The Ozawa plots of ln[-ln(1-x T )] against lnβ for PE and its nanocomposites are show in Figure 6. However, these plots do not show straight lines implying that the Ozawa equation was not successful in describing the non-isothermal crystallization behaviour of our PE/ clay nanocomposites. 24 We also used the Avrami equation which was developed to describe isothermal crystallization kinetics to study non-isothermal processes of PE/clay nanocomposites. The Avrami equation is expressed as follows 25 : (9) The double logarithm of Eq. (9) gives the following relationship: (10) where t is the crystallization time; X t is the relative degree of crystallinity at time t; the exponent n is a mechanism constant, and the parameter Z t is a growth rate constant involving both nucleation and growth of crystalline. Since the rate of non-isothermal crystallization is related with the cooling rate, Jeziorny suggested that the rate parameter Z t should be corrected owing to the influence of Polymers & Polymer Composites, Vol. 23, No. 8,

6 Fu-An He and Li-Ming Zhang Figure 5. The relative degree of crystallinity with time for crystallization of (a) PE, (b) PEC1, (c) PEC2 and (d) PEC3 at various cooling rates Figure 6. Ozawa plots of ln[-ln(1-x T )] against lnß for crystallization of (a) PE, (b) PEC1, (c) PEC2 and (d) PEC3 not integer ranging from 2.7 to for neat PE, which is consistent with the results of Hay 27,28. While for PE/ clay nanocomposites, the values of n range from to 3. The exponent n of PE/clay nanocomposites is larger than that of neat PE at every cooling rate, indicating that that the clay filler acted as a nucleating agent for the PE matrix. The Z c values of PE/clay nanocomposites are lower than that of the neat PE at the same cooling rate, which can be attributed to the hindrance effect of clay filler on the mobility of PE macromolecule chain segments during crystallization progresses. From above analysis, it is clear that neither the Ozawa nor the Avrami methods explain the crystalline behaviour of PE/clay nanocomposites very well. It has been reported that the kinetics equation by combining the Avrami and Ozawa equations, as proposed by Mo et al. is valid in a number of nanocomposites systems Hence, the modified Avrami-Ozawa equation was used to study the nonisothermal crystallization kinetics of PE/clay nanocomposites. Because the degree of crystallinity is related to both the cooling rate b and the crystallization time t (or temperature T), the relation between b and t can be connected for a given degree of crystallinity. Using Eq. (2), Eq. (8) and Eq. (10), the following equation can be obtained under a certain crystallinity degree: β 26. The corrected parameter Z c was defined as follows: (11) The Avrami plots of ln[-ln(1-x t )] vs. lnt for PE and PE/clay nanocomposites are shown in Figure 7. It can be seen that straight lines are obtained only in the initial stages of crystallization with a low degree of crystallinity probably as a result of the secondary crystallinity of polymer in the later stages. The primary parts of the plots were chosen for the determination of Avrami parameters, and the values of Avrami parameters n, Z t and Z c are given in Table 3. The Avrami exponents n are and by rearrangement (12) (13) where F(T) =[K(T)/k] 1/m, which refers to the cooling rate that must be selected within a unit of crystallization time when the measured system reaches a certain crystallinity degree, and α is the ratio of the Avrami exponent to the Ozawa exponent (n/m). According 550 Polymers & Polymer Composites, Vol. 23, No. 8, 2015

7 Study on Thermal Stability and Non-isothermal Crystallization Behaviour of Polyethylene/clay Nanocomposites to Eq. (13), at a given crystallinity degree, the plot of lnb vs. lnt should yield a linear relationship. The kinetic parameter F(T) and a are determined from the intercept and the slope of the lines, respectively. Plots of lnb vs. lnt at various degree of crystallinity for PE and PE/clay nanocomposite are presented in Figure 8, which shows a good linearity. The values of lnf(t) and a are listed in Table 4. The F(T) values of PE and PE/clay nanocomposites increase with an increase in X t. At the same X t, the F(T) values of PE/clay nanocomposites are higher than that of neat PE, suggesting that the necessary cooling rate of PE/clay nanocomposites is larger than that of neat PE when they arrives at the same crystallinity degree at unit crystallization time. This phenomenon can be ascribed to the reduced mobility of PE chains as crystallization progresses, due to the existence of clay filler. 3.4 Nucleation Activity Dobreva proposed a simple method for calculating the nucleation activity of foreign substrates in polymeric melt 35. For homogeneous nucleation from a melt near the melting temperature, the cooling rates can be expressed as: Figure 7. Avrami plots of ln[-ln(1-x T )] against lnß for crystallization of (a) PE, (b) PEC1, (c) PEC2 and (d) PEC3 Figure 8. Modified Avrami-Ozawa plots of lnß against lnt for crystallization of (a) PE, (b) PEC1, (c) PEC2 and (d) PEC3 (14) while for the heterogeneous nucleation, (15) where B is a parameter that can be calculated from the following equation: (16) where ω is a geometric factor, σ is a specific energy, V m is the molar volume of the crystallizing substance, n is the Avrami exponent, S m is the entropy of melting and T m 0 is the infinite crystal melting temperature. Nucleation activity (ε) is defined as: (17) If the foreign substrate is extremely active, ε approaches 0; if the foreign substrate is very active, ε approaches 1. Plots of logb vs. 1/DT p 2 for PE and PE/clay nanocomposites are shown in Figure 9, and the values of ε are given in Table 4. It can be seen that the ε values of all PE/clay nanocomposites are smaller than 1, indicating that the clay act as nucleation agents in the PE matrix. Polymers & Polymer Composites, Vol. 23, No. 8,

8 Fu-An He and Li-Ming Zhang Table 4. Non-isothermal crystallization kinetics parameters obtained from combination of Avrami Ozawa equation Sample X t a lnf(t) PE PEC1 PEC2 PEC Figure 9. Plots of logß against 1/DT p 2 for crystallization of PE and its nanocomposites resultant DE value can only give a finite relationship between the peak temperature T p and the cooling rate b. 4. Conclusions The thermal degradation of PE/clay nanocomposites in nitrogen was analyzed by a pseudo-first order method. The result showed that the incorporation of clay in the PE matrix could improve the thermal stability of PE/clay nanocomposites. Several models including Avrami, Ozawa, and modified Avrami-Ozawa were employed to study the non-isothermal crystallization kinetics of PE/clay nanocomposites. The kinetic analysis indicated that the Ozawa model failed to explain the non-isothermal crystallization behaviour of PE and its nanocomposites, and the Avrami analysis modified by Jeziorny could only describe the primary nonisothermal crystallization stage of PE and its nanocomposites. By contrast, the modified Avrami-Ozawa method was successful in describing the nonisothermal crystallization process of PE and its nanocomposites. The nucleation activity of PE/clay nanocomposites was determined by Dobreva s method indicating that clay was an effective nucleating agent for PE crystallization. The crystallization activation energy of PE and its nanocomposites were also estimated. It was found that the value of DE for neat PE was higher than that of PE/clay nanocomposites. 3.5 Activation Energy Figure 10 illustrates plots based on Kissinger approach, Augis-Bennett approach and Takhor approach The activation energy can be calculated from the slopes of these plots and the resultant DE values are listed in Table 5. The values of DE or PE/clay nanocomposites are all lower than that of neat PE for those three approaches. It means that that neat PE crystallizes slower than PE/clay nanocomposites. However, this conclusion is not consistent with the result obtained from the above analyses of crystallization kinetics. A similar observation has been reported by Wang et al. in the nanocomposite system based on poly(ethylene terephthalate) and clay 39. They considered that the values of DE evaluated from above methods are just adjustable parameters which do not possess the same physical meaning as the activation energy. Thus, the Acknowledgments This work was supported by the Natural Science Foundation of Guangdong Province in China (Grant No ). References 1. Chen Z.Q., Chen S.J. and Zhang J., Polym. Polym. Compos. 19 (2011) Zhao C., Qin H., Gong F., Feng M., Zhang S. and Yang M., Polym. Degrad. Stab. 87 (2005) Gopakumar T.G., Lee J.A., 552 Polymers & Polymer Composites, Vol. 23, No. 8, 2015

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