Avrami constants

For temperatures lower than Tc = 95 °C and higher than Tc = 100 °C only one slope, with an Avrami exponent n = 3, is observed. This means that in these cases the nucleation corresponds to a slow crystallization mode. At low temperatures (Tc — 90 °C) the time required to crystallize is very large (tc = 300 min) because the nuclei appear within the sample very slowly. In this case, the fast crystallization process is not observed because the crystallization temperature is too low. On the other hand, for temperatures above 100 °C (Tc = 105 °C) there is also a single region of crystallization (Fig. 4.21) because the temperature of crystallization is higher, and crystallization proceeds so rapidly (mostly by crystal growth) that it seems reasonable to associate it with the fast mode. Finally, the Avrami constant G increases with Tc indicating that the number of nuclei increases with the crystallization temperature (Balta Calleja et al, 1993). 

The Avrami Constant. The calorimetric data were analyzed according to the general Avrami equation ... 

Plotting the left part of this equation against logt should result in a straight line, from which both Avrami parameters, n (slope) and k (intercept), can be obtained. In Table 3.7 some literature data on the Avrami constants and the half time of crystallization are presented. 

The authors suggested that the Avrami constant could be seen as the sum of two processes a contribution due to nucleation and a contribution due to growth ... 

Since both components cocrystallize [Edward, 1986 Hu et al., 1987 Gupta et al., 1994] the crystalline growth can be considered to be identical. As a consequence, the differences seen in the Avrami exponent, n, in the blends must be due to a difference in the nucleation behavior that depends on the blend composition. A value varying from 0 (instantaneous nucleation) to 1 (sporadic nucleation) was attributed to the contribution of the nucleation of LLDPE and HOPE, respectively, to the Avrami constant, n. The remaining part of n (e.g., 1.94 for HOPE... 

X, is the progressive volume fraction transformed into crystal until the time t X is a scaling factor n is the Avrami constant... 

Molecular weight dependence of overall crystallization rate (k) seems to be very complicated as seen in Fig. 19.2 for PE. The Avrami constant of k is a function of nucleation rate of (1) and crystal growth rate of (G) as expressed by following equation,... 

Figure 5.2. Avrami constants K and n vs. concentration of nucleating agent (N,N ,N -tricyclohexyl-l,3,5-ben-zenetricarboxylamide) in PLA crystallization. [Adapted, by permission, from Wang, L Wang, Y-n Huang, Z-g Weng, Y-x, Mater. Design, 66, 7-15, 2015.]...

Ozawa and Kissinger plots are the most commonly used equations to calculate non-isothermal kinetic data, such as Avrami constant, n and crystallization activation energy, Ea, respectively (Celikbilek et al., 2011 Kissinger, 1956 Ozawa, 1971 Prasad Varma, 2005). 

Cebe studied non-isothermal crystallization from 80 to 380°C, with rates from 1 to 50°C /min. The Avrami constant n was found... 

Avrami constants, k/n n=1.5-3 Grady, B P Genetti, W B, Conductive Polymers and Plastics, Rupprecht, L, Ed., WilliamAndrew, Norwich, 1999. 

PARAMETER Lamellae thickness Rapid crystallization temperature Avrami constants, k/n UNIT nm °C VALUE 23 181 n=2.4-2.6 REFERENCES Elzein, T Brogly, M Castelein, G Schultz, J, J. Polym. Sci. B, 40, 1464-76, 2002. 

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