Melting behavior and nonisothermal crystallization kinetics of metallocene polyethylene

Gao Jungang, Li Shurun, Li Zhiting
(Department of Polymer Science, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Received July 29, 2003; Supported by the National Natural Science Foundation of China (No.201068) and Educational Science Foundation of Hebei (No.2000105)

Abstract Melting behavior and nonisothermal crystallization kinetics of Metallocene Polyethylene (mPE-D65) was studied using differential scanning calorimetry (DSC) at various scanning rates. The Avrami equation modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of mPE. The theory of Ozawa was also used to analyze the mPE DSC data. Kinetic parameters such as the Avrami exponent (n), the crystallization rate (Z_c), the peak temperatures (T_p) and the half-time of crystallization (t_1/2) etc. could obtain at various scanning rates. The results showed that the Ozawa analysis failed to provide an adequate description of the nonisothermal crystallization of mPE. The Avrami equation modified by Jeziorny and a method developed by Mo were fit for describing the nonisothermal crystallization process of mPE. The value of n showed that the nonsiothermal crystallization of mPE corresponds to a tridimensional growth with heterogenous nucleation, and the value of Z_c increase with increasing of the cooling rates for mPE.
Keywords Metallocene Polyethylene; Melting behavior; Nonisothermal crystallization kinetics

1 INTRODUCTION            
Polyethylene (PE) ^[1] is the most widely used commercial polymer in the world today. Because of its specific properties, such as high chemical and mechanical resisitance, low specific gravity and cost, the industrial PE market is still growing. Metallocene Polyethylene (mPE) is a linear polyethylene which catalyzed by metallocene. mPE offers the following significant performance advantages compared with conventional polyethylene: (1) tensile, puncture and impact performance improvements, providing superior toughness; (2) sealing performance improvements; (3) easy blending with other polyolefins.
    Investigations of the kinetics of polymer crystallization are significant both theoretically and practically. Most frequently, the investigations are conducted under isothermal conditions because of the convenience of the theoretical treatment of the data. Practically mPE usually undergoes a nonisothermal crystallization environment in the processing. So a study on the nonisothermal crystallization process of mPE is meaningful, but it is so less about this investigation until now.
    In this article, the nonisothermal crystallization process of mPE were investigated by modified Avrami equation and a method developed by Mo Z.S. The theory of Ozawa was also used to analyze the mPE DSC data at different scanning rates ^[2].

2 EXPERIMENTAL
2.1 Materials
The mPE was supplied from Exxon Chemical Company (USA) as an exact grade D65. (MI=0.10g/min; denstiy=0.918g/cm³; melting point=119ºC)
2.2 Thermal Measurements 
The weight of all samples was kept in approximately 10mg in DSC measurement by using a CDR-4P calorimeter under a nitrogen atmosphere. The temperature and melting enthalpy were calibrated with standard indium. In order to erase the previous thermal history, the samples were encapsulated into aluminum pans and were heated to 180ºC and kept at this temperature for 5 minutes. They were then cooled to room temperature at different constant cooling rates (D) (namely D=5,8,10,12,15ºC/min, respectively) and the heat flow during crystallization was recorded as a function of time or temperature. For heating process, the samples that had been erased thermal history were heated at different constant rates (namely D=5,8,10,12,15ºC/min, respectively) from room temperature to 180ºC again, and the heat flow was recorded.

3 RESULTS AND DISCUSSION
3.1 Theories of Kinetic Parameter Determinations        
The relative crystallization (X_t), ^[3] as a function of temperature is defined as:
    (1)
where and are the onset and end of crystallization temperature, respectively. is the heat flow at temperature T.
    The time of the fastest crystallization (t_max) is the time of crystallization starts to the appearance of crystallization peak. As a function of temperature (T) and cooling rate (D), t_max is defined as:
(2)
where T_p is the temperature of crystallization peak.
    The half-time of crystallization (t_1/2) ^[4] is the time required for 50% crystallization.
    Sample crystallinity (X_c) ^[5] is defined as:
(3)
where and are the melting enthalpy of PE sample and 100% crystallization PE, respectively, where =273J/g ^[6]. is acquired by the integral area under the DSC heating curve.
    The kinetic parameters of nonisothermal crystallization were determined, based on the simplified assumption that crystallization occurs under constant temperature. In this case, the Avrami equation can be used ^[7-8]:
(4)
or (5)
where X_t is the relative crystallinity at time t, t can be obtained by the equation (6).
(6)
where T is the temperature at crystallization time t, and D is the cooling rate. From Eqs.(5) and (6), we have equation (7):
(7)
where n is the Avrami exponent, which depends on the type of nucleation and growth dimension, and the parameter Z_t is a composite rate constant that involves both nucleation and growth rate parameters. The Avrami exponent (n) and crystallization rate (Z_t) can be obtained from the slope and intercept of the line in the plot of versus. Considering the effect of the cooling rate, Z_t is corrected by cooling rate ^[9]:
(8)
where Z_c is the kinetic crystallization rate.
    Ozawa ^[10] had extended the Avrami equation to the nonisothermal condition. Assuming that the nonisothermal crystallization process may be composed of infinitesimally small isothermal crystallization steps, the following equation was derived:
(9)
or (10)
where K (T) is the function of cooling rate, m is the Ozawa exponent, which is depended on the dimension of the crystal growth.
    A method developed by Mo ^[11] was employed to describe the nonisothermal crystallization for comparison. For the process, physical variables relating to the process are the relative crystallization X_t, cooling rate D, and crystallization temperature T. Both the Ozawa and Avrami equation can relate to these variables as follows:
(11)
and by rearrangement:
(12)
where refers to the cooling rate value, which must be chosen within unit crystallization time when the measured system amounts to a certain relative crystallization; a is the ratio of the Avrami exponent n to the Ozawa exponent m (a=n/m). According to eq. (12), at a given relative crystallization, by plotting log D versus log t it yields a linear relationship between log D and log t. The data of kinetic parameter F (T) and a can be estimated from the intercept and slope.
3.2 Melting and Crystallization Behavior of mPE       
Figure 1 shows DSC curves of mPE at different heating rates. From these curves, some useful data for describing their nonisothermal melting behavior can be obtained, such as the melting curve shape and peak temperature (T_p) at which mPE has the fastest melting rate, melting enthalpy () and crystallinity (X_c). The values of the parameters determined are given in Table 1. From these melting curves we can see that there are two melting peaks during different heating rates, it showed that this mPE (D65) molecules used has not homogenizing tacticity and grafting chain distribution in the molecules, some molecules on chains segment have not good tacticity that will be melting earlier and have a higher degree of graft, and the other later, which have a good tacticity and a less degree of graft.

Fig.1 DSC curves of mPE at different heating rates. 1-5ºC/min; 2-8ºC/min; 3-10ºC/min; 4-12ºC/min; 5-15ºC/min

    This fact can be also showed from the crystallization exotherms process of this mPE at different cooling rates which are showed in Figure 2. As seen from the figure 2, we can also see that there are two peaks during the nonisothermal crystallization of mPE.From these curves, some useful data can be obtained for describing their nonisothermal crystallization behavior, such as the peak temperature (T_p), relative crystallinity (X_t), the half-time of crystallization (t_1/2), and the crystallization rate (Z_c) etc. And the values of the parameters determined are given in Table 2. From Figure 2, we can see that T_p moves to a lower time scale that allows the polymer to crystallize as the cooling rate increasing, therefore requiring a higher power to initiate crystallization. It is clearly seen from Table 2 and Figure 2; the changement of parameters for this mPE at different cooling rates is corresponding to that at different heating rates. Most of molecules which has a higher tacticity and less degree of graft begin to crystallize at higher temperature whereas the others at lower temperature.

Fig. 2 DSC curves of mPE at different cooling rates. 1-5ºC/min; 2-8ºC/min; 3-10ºC/min; 4-12ºC/min; 5-15ºC/min

Table 1 Parameters of mPE Sample during Nonisothermal heating Process

Table 2 Parameters of mPE Sample during Nonisothermal Crystallization Process

3.3 Nonisothermal Crystallization Kinetic of mPE       
Figure 3 shows a good linear relationship when log [-ln(1-X_t)] is plotted versus log[(T₀-T)/D] for mPE at each cooling rate. Two adjustable parameters, Z_t and n, can be obtained by a linear regression. The Z_t and n parameters are not the same physical meaning as in the isothermal crystallization, because the temperature changes constantly in nonisothermal crystallization. This affects the rates of both nuclei formation and spherulite growth ascribed to their temperature dependence. Therefore Z_t must be calibrated by Jeziorny method. The results are listed in Table 2. It is clearly seen from Table 2, the value of Z_c increase with increasing cooling rate. The Avrami exponent (n), depends on the type of nucleation and growth dimension. If the crystallization corresponds to a tridimensional growth with homogenous that n=3+1=4, then nucleation rate increases with time, and if with heterogeneous n=3+0=3, the nucleation rate have nothing to do with time ^[12]. From Table 2, the value of n is not changing almost with the changement of the cooling rates, it suggested that the nonisothermal crystallization of mPE corresponds to a tridimensional growth with heterogeneous nucleation. It showed that this mPE has some molecules which have higher molecular weight, have an action of nucleation.

Fig. 3 Plots of log[-ln(1-Xt)] versus log[(T0-T)/D] for crystallization of mPE

Fig. 4 Ozawa plots of log[-ln(1-Xt)] versus logD for crystallization of mPE at different crystallization temperatures

Fig. 5 Plots of log D versus log t for mPE at each given relative degree of crystallization

    Figure 3 shows the results for mPE according to Ozawa's method. The curvature in Figure 4 presents an accurate analysis of nonisothermal crystallization data. When crystallization temperature is lower than 114ºC, this can be explained that, at a given temperature, the crystallization process at different cooling rates are different stages, that is, the lower cooling rate processes is toward the end of the crystallization process, whereas at the higher cooling rate, the crystallization process is an early stage. Although Ozawa's approach can be used to describe the nonisothermal crystallization behavior of mPE, the changement in the slope with temperature [Fig.4] means that the parameter m is not a constant during crystallization, and indicated that Ozawa's approach is not fit for describing the nonisothermal crystallization process of mPE.
    The method developed by Mo.Z.S was also employed to describe the nonisothermal crystallization for comparison. According to eq. (12), at a given degree of crystallization, plotting logD versus logt (Fig.5) yields a linear relationship. The data of kinetic parameter F (T) and an estimated from the intercept and slope for mPE are listed in Table 3. It can be seen from Table 3 that F (T) systematically increases with the increase in the relative degeree of crystallization. It is clear that this approach is successful in describing the nonisothermal process of mPE.

Table 3 Nonisothermal Crystallization Parameters of mPE Sample

4 CONCLUSIONS
The mPE (D65) molecular chain has not a homogenizing tacticiy, and has two melting peaks and two crystallization peaks. The Ozawa analysis failed to provide an adequate description of the nonisothermal crystallization of mPE through of the comparisons of different stages of crystallization at different cooling rates. The Avrami analysis modified by Jeziorny and a method developed by Mo.Z.S were successful in describing the nonistothermal crystallization process of mPE. That the value of n is hardly changing with the changement of the cooling rates suggested that the nonisothermal crystallization of mPE corresponds to a tridimensional growth with heterogeneous nucleation, and the value of Z_c increases with increasing cooling rates for mPE.

REFERENCES
[1] Mori. H, Ohnishi. K, Terano. M. Macromol. Chem. Phys, 1998, 199: 393.
[2] Liu Z H. Introduction to Thermal Analysis. Beijing: Chemical Industry Publishing Co., 1991: 2.
[3] Kong X H, Yang X N. Euro. Polym. J., 2001, 29(3): 2.
[4] Zhang Z Y, Wu S Z, Du Y H, et al. Chin. J. Polym. Sci., 1991, 9(4): 319.
[5] Dong Y M. The Practical Analyze Technology of Polymer Journal 2001, 37: 1855.
[6] Dong Y M. The Practical Analysis Technology of Polymer Meterials(Gaofenzi Cailiao Shiyong Poxi Jishu). Beijing: Petrochemical Industry Press, 1979: 293.
[7] Xu R M, Jun T X, Lin S, et al. J. of Appl. Polym. Sci., 2001, 80: 124.
[8] Xu W, Ce M, He P. J. of Appl. Polym. Sci., 2001, 82: 2281.
[9] Jeziorny. A. Polym., 1978, 19: 1142.
[10] Ozawa. T. Polym., 1971, 12: 150.
[11] Liu, T X, Mo Z S, Wang S E, et al. Polym. Eng. Sci., 2001, 82: 2281
[12] He M J, Chen W X, Dong X X, Polymer Physics (Gaofenzi Wuli). Shang Hai: Fudan Publisher, 1990: 71.

　

[ Back ] [ Home ] [ Up ] [ Next ]Mirror Site in USA  Europe  China  GBNet