Nucleation spherulite formation

The results of Rory and McIntyre (22) for the rate of formation of spherulitic centers in molten poly(decamethylene sebacate) provide a good set of data to examine whether Eq. (9.170) or (9.171) is operative. In these experiments the undercooling ranges from 11 °C to 16 °C. Concomitantly the initiation rate increases by a factor of 10. This result is clearly indicative of nucleation control. The log of the rate of spherulite formation, log N, for this polymer is plotted against (T / T)(l/ AT) and (Tra/T)(l/ AT) in Fig. 9.38. Excellent straight lines are obtained with either plot. Thus, it is not possible to discriminate between the two distinctly different initiation nucleation processes by analysis of the temperature coefficient. 

The properties of a given polymer will very much depend on the way in which crystallisation has taken place. A polymer mass with relatively few large spherulitic structures will be very different in its properties to a polymer with far more, but smaller, spherulites. It is thus useful to consider the factors affecting the formation of the initial nuclei for crystallisation (nucleation) and on those which affect growth. 

The overall rate of crystallization is determined by both the rate of nuclei formation and by the crystal growth rate. The maximum crystal growth rate lies at temperatures of between 170 and 190 °C [71, 72], as does the overall crystallization rate [51, 61, 75], The former is measured using hot stage optical microscopy while the latter is quantified by the half-time of crystallization. Both are influenced by the rate of nucleation on the crystal surface and the rate of diffusion of polymer chains to this surface. It has been shown that the spherulite growth rate decreases with increasing molecular weight due to the decrease in the rate of diffusion of molecules to this surface [46, 50, 55, 71, 74],... 

Both the rate of nuclei formation and the crystal growth rate can also be expected to influence the spherulite size. It has been reported (hat, in the temperature range 130-180 °C, the spherulite size increases with increasing temperature [74], This trend can be expected to extend to higher temperatures as the nucleation rate decreases. On the other hand, the presence of nucleating... 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

With CaC03, the spherulite size is significantly reduced (Ds = 10-15 pm) and the particle surface chemistry induces some specific microstructural characteristics of the PP matrix small size surface treated CaC03 particles promote formation of the p phase. Without surface treatment, CaC03 has a nucleating effect the degree of crystallisation is increased by about 20%(X(. = 65%). 

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