Understanding isothermal crystallization and subsequent melting behavior of syndiotactic polypropylene

Various issues related to isothermal quiescent crystallization and subsequent melting behavior of syndiotactic polypropylene (sPP) were investigated in this dissertation. On the study of isothermal melt- and cold-crystallization kinetics and subsequent melting behavior of sPP, the overall crystallization rate parameters for melt-crystallization process, when plotted as a function of crystallization temperature, exhibited an unmistakable double bell-shaped curve; whereas, those for cold-crystallization process showed the typical bell-shaped curve. Comparison of the overall crystallization rate parameters obtained for both melt- and cold-crystallization processes indicate that crystallization from the glassy state proceeds at a much greater rate than from the melt state. The multiple-melting behavior observed in subsequent melting endotherms is attributed to the contributions from: 1) melting of the secondary crystallites and their re-crystallization, 2) partial melting of the less stable fraction of the primary crystallites and their re-crystallization, 3) melting of the primary crystallites, and lastly 4) re-melting of the re-crystallized crystallites formed during the heating scan.

Analysis of the linear growth rate data of sPP#1 and other data sets taken from the literature in the context of the Lauritzen-Hoffman secondary nucleation theory suggested an unmistakable regime II-III transition at the crystallization temperature of 110°C. Regardless of the crystal structure, if the growth is assumed to occur on the bc plane, the lateral surface free energy σ = 11.3 erg-cm^-2 and the fold surface free energy σ_e = 63.7 ± 7.1 erg-cm^-2 were found. On the other hand, if the growth is assumed to occur on the ac plane, the fold surface free energy is found to be σ_e= 82.4 ±9.1 erg-cm^-2, while the lateral surface free energy is the same as previously noted. The sensitivity of the crystal growth parameters on changes in the values of the input parameters was also investigated.

Isothermal crystallization behavior after partial or complete melting of syndiotactic polypropylene was also investigated. On subsequent crystallization after partial melting, the total concentration of predetermined nuclei was found to decrease with increasing fusion temperature and increasing time period the sample spent at a specific fusion temperature. On subsequent crystallization after complete melting, the total concentration of predetermined nuclei was found to approach a constant value, which is the concentration of infusible heterogeneous nuclei (e.g., impurities, catalyst residues, etc.) present originally in the sample. At a specific fusion temperature, the concentration of predetermined athermal nuclei was found to decrease exponentially with the time period spent in the melt.

Applicability of four macrokinetic models; namely the Avrami, Tobin, Malkin, and simultaneous Avrami models; in describing the time-dependent relative crystallinity data, using sPP as the model system, was tested using a non-linear multivariable regression program. Based on the quality of the fit, only the Avrami, Malkin, and the simultaneous Avrami models were found to describe the experimental data well, resulting in the rejection of the Tobin model in describing isothermal crystallization data of sPP. Comparison of the Avrami kinetics parameters obtained from the program with those obtained from the traditional analytical procedure suggested that use of non linear multi-variable regression program in data analysis is satisfactorily reliable.

Lastly, a technique of using differential scanning calorimeter (DSC) to study crystallization behavior and the kinetics of the process at high crystallization temperatures or low degrees of undercooling was proposed. The technique was carried out based on the observations of, and the measurements of the enthalpy of fusion from, the subsequent melting endotherms after isothermal crystallization for various time intervals. Comparison of the overall crystallization data obtained from this proposed technique with those obtained from the traditional technique evidently indicated that the proposed technique of using information acquired from subsequent melting endotherms in studying crystallization kinetics is at least reliable and applicable to describe isothermal crystallization of sPP at the conditions studied. Apparent advantages and disadvantages of the proposed technique were also given.