Comparison of theory with experiment overall crystallization

4 Comparison of theory with experiment overall crystallization 

Despite these concerns, when crystallization kinetics experiments are conducted so that prior to crystallization the sample is completely molten, the chains in random conformation and degradative processes avoided, the resulting isotherms are extremely reproducible. They are independent of the initial temperature of the melt at which the sample is held.(12,35,36) An important factor is that complete melting be ensured. There are some reports where the expected reproducibility is not achieved. In these cases, the aforementioned conditions may not be operative, or the rate of cooling of the sample is such that isothermal crystallization does not occur at the predetermined temperature. 

The literature concerning the experimental results of the overall crystallization kinetics of homopolymers is quite vast. It encompasses virtually all types of repeating units of crystallizable polymers. From this abundance, a representative set of examples has been chosen that illustrates the basic experimental results. These data allow for a quantitative assessment of the theoretical developments that have been presented so far. 

To compare experimental results with theory we start with the G61er-Sachs free-growth expressions, approximated by Eqs. (9.12) to (9.14). Accordingly, a plot 

The next step in analyzing experiment is to compare the results with one of the derived Avrami expressions. We recall that Avrami introduced a specific mode for termination of the transformation. There are essentially two ways in which the comparison can be made. One reliable, and informative, way is to superpose the experimental isotherms, based on either absolute or relative crystallinity, onto a master isotherm of a 1 - X(t) against log t plot. This master experimental isotherm can then be compared with the theoretical derived Avrami type plots as illustrated in Fig. 9.10. In this way, both the best fit and the extent of the transformation at which deviahon from theory occurs can be clearly and definitively established. Alternatively, taking advantage of Eq. (9.32), a plot of ln(- In X) against In t should give a straight line with slope equal to . It is informative to examine both of these methods. 

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