Heterogeneous nucleation of isotactic

Heterogeneous Nucleation of Isotactic Poly(Propylene Oxide) Crystallization... 

The microscopic data on primary nucleation of isotactic pol)rpropylene (iPP) for melt samples annealed at 180°C prior to crystallization and then crystallized isothermally in the temperature range from 122 to 145°C are presented in Table 1. The nucleation in iPP in these experiments was found to be heterogeneous and instantaneous. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

In contrast to polyethylenes the crystallization of polypropylene is initiated by heterogeneous nucleation. The order in polypropylene is a result of the structural regularity of the polymer chains it s isotacticity. Normal polypropylenes are isotactic with the methyl groups lined up on the same side of the polymer chain and they crystallize in a helical configuration. TREE separation is expected to be dependent on the perfection of the stereoregularity along the chain which may be influenced by the nature of the catalyst, the polymerization conditions and also the use of comonomers, such as ethylene. The effect of comonomer is to disrupt the isotactic runs in the polypropylene chain. 

Figure 1 illustrates the nucleation activity of isotactic polypropylene in thin films determined in isothermal experiments. The data are differentiated to represent the contribution of new nuclei activated by the temperature decrease by 1°C. Samples in the form of thin films (20-30 g,m) were crystallized isothermally on a microscopic hot stage. It is seen that the number of nuclei increases initially as the temperature of crystallization decreases. The number of new nuclei at the temperature below 115°C levels off. Apparently all heterogeneous nuclei present in... 

Numerous attesnpts have been made to predict the effectiveness of potential nxacleating agents with particular attention having been paid to the crystallisation of isotactic polypropylene, isotactic polystyrene (6), polyethylene (2 9) ard poly(ethylene terephthalate) (10,11). However, little can be concluded about the fundamental mechanism of heterogeneous nucleation, or the propeirties of effective nucleating agents. 

The crystallization of isotactic polypropylene from melt could be enhanced in the region of the temperature where heterogeneous nucleation is observed by adding some extra heterogeneous nuclei. The interest... 

Martuscelli and coworkers studied the effects of isotactic fractions, and catalyst type on the overall crystallization of iPP [154, 155). For a given crystallization temperature and catalyst type, ti/2 increased with increasing/jso- However, the effect of the choice of the catalyst was strong enough to contravene the effect of even a large change in/jsQ. The dependence of Ti/2 on catalyst type was probably due to a combination of two effects the different impurity levels left in the resin with the different catalysts, that acted as heterogeneous nucleation sites, and the different distribution... 

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