Effects of nucleating agents on the crystallization and melting behavior isotactic polypropylene (iPP) were studied by differential scanning calorimetry (DSC). Dibenzylidene sorbitol(EC-1), (p-chloro, p-'methyl)-dibenzylidene sorbitol (EC-4), and bis (p-ethylbenzylidene) sorbitol (NC-4) were used as nucleating agents. A nonisothermal crystallization kinetic equation as well as the simple Avrami equation was employed to analyze the crystallization characteristics of iPP with or without the nucleating agents from DSC crystallization thermograms.
In the isothermal crystallization kinetic study, the simple Avrami equation may describe the crystallization behavior of the unnucleated iPP fairly well. However, the Avrami equation has a limit to follow the whole crystallization process of the nucleated samples. Regression to determine the kinetic parameters for the nucleated iPP was made using the DSC data obtained below the relative crystallinity of 30%. With the values of rate constants corrected assuming the three dimensional spherulite growth, the number of effective nuclei in the nucleated iPP was calculated to be $10^2$~$10^5$ time higher than that in the virgin sample. It was observed that the number of effective nuclei decreased with increasing crystallization temperature for both the virgin and the nucleated iPPs. The temperature dependence of the nucleation activity varied with the nucleating agent incorperated and was presented in terms of the deactivation factor defined in this study.
The nonisothermal crystallization kinetic equation in the study took into account the growth processes initiated by either heterogeneous or homogeneous nucleation as well as the growth site impingement. The equilibrium melting temperature of iPP necessary for the nonisothermal kinetic study was obtained by the conventional extrapolation method to be 209℃. The nonisothermal crystallization kinetic analysis for the unnucleated iPP at different cooling rates was possible by assuming the spherulite growth initiated simultaneously by heterogeneous and homogeneous nucleation. On the other hand, the crystallization kinetics of the nucleated iPP was described satisfactorily by the heterogeneous nucleation and growth process alone. The addition of the nucleating agent up to its saturation concentration in iPP increased the crystallization peak temperature by 11-17℃ and the number of effective nuclei by 2-4 orders of magnitude. A high concentration of the nucleating agents caused agglomeration of the agents to lower the number of effective nuclei.
A double melting behavior was observed in the iPP melting study. For the samples crystallized at 110 and 120℃, the double melting behavior resulted from the reordering or recrystallization of the crystalline fractions with lower degrees of perfection was displayed on heating. By a systemetic annealing study on the isothermally crystallized iPP, it was confirmed that the recrystallization could occur within the time scale of DSC scans at 2.0-8.0℃/min. The double melting behavior was also observed for the samples crystallized at 130 and 140℃ but was different in nature from that observed for the sample crystallized at 110 and 120℃. It was found that both the lower and higher melting peaks represented melting of the preexisting different crystal fractions formed during the isothermal crystallization. An intermediate melting behavior was observed for the samples crystallized at 125℃.
The melting behavior of iPP was hardly affected by the presence of nucleating agents as long as the samples had been crystallized at the same temperature. However, when the polymers were crystallized dynamically at a constant cooling rate, the melting behaviors of the nucleated iPPs were observed to be less scan-rate dependent than the virgin sample crystallized at the same condition.
A series of sodium p-substituted benzoates was incorperated with iPP to investigate the nucleation activities of the organic compounds in terms of their structural features. Among the sodium salts of eleven p-substituted benzoic acids prepared, four compounds showed good nucleation activities. Sodium p-tert-butylbenzoate showed the best result, followed by sodium benzoate, then sodium p-methylbenzoate and sodium p-phenylbenzoate. It was observed for the series of sodium p-alkylbezoates that the nucleation activity decreased with increasing alkyl chain length except for sodium p-tert-butylbenzoate. Sodium p-alkoxybezoates gave poor nucleation activities. It was found that melting points of the p-substituted benzoic acids were drastically increased by the salt formation with sodium. However, no obvious correlation existed between nucleation activity and melting point of a nucleation agent. The high nucleating activities of some compounds might result from their crystalline geometry and surface structure which render favorable conditions to accomodate the critical nuclei. Solution mixing of a nucleating agent with iPP powder could result in either a good or a poor dispersion of the agent in the polymer according to its solubility in the solvent. Finally, it is suggested that the nucleation activity of a particular compound appears not to be an exclusive function of any one property or variable, but is probably the effective summation of many conditions operating at the same time.
