Isotactic nucleation

Term N J, Fairclough P A, Towns-Andrews E, Komanshek B U, Young R J and Ryan A J 1998 Density fluctuations the nucleation event in isotactic polypropylene crystallization Polymer 29 2381- 5... 

Fig.l Differential scanning calorimetry (DSC) cooling scans from the melt, at 10°Cmin 1, of the following materials (from top to bottom) Isotactic polypropylene (iPP) iPP after self-nucleation treatment at TS = 162°C 80/20 polystyrene (PS)/iPP melt mixed blend 80/20 PS/iPP melt mixed blend after self nucleation treatment at Ts = 161 °C 80/20 PS/iPP unmixed blend (UB), see text and atactic PS homopolymer. (From [68] with permission)... 

Figure 7. Nucleation of isotactic polypropylene right unnucleated sample.

In the crystallization of isotactic polypropylene from the melt, the number and size of the spherulites (and hence the rate of crystallization) can be influenced by the addition of certain nucleating agents.The smaller the spherulites, the greater is the transparency of the polypropylene film.The mechanical properties can also be affected in some cases. 

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. 

Boon (1966/1968) investigated the kinetics of crystallisation of isotactic polystyrene. This polymer is extremely interesting as a model substance for crystallisation work. Its rate of growth is so low that the crystallisation can be studied in the whole region from Tg to Tm. Due to the low growth rate the fundamental processes of nucleation and growth can be studied almost separately. 

Among the three known crystalline structures (a, j3, y) of isotactic polypropylene (PP), the -modification is certainly the most fascinating one. While the stable a-structure develops under standard process conditions, the occurrence of the /J-form has to be forced (i) by directional crystallization in a temperature gradient field [1-3] (ii) by shear-induced crystallization [4-12] or (iii) by the addition of specific nucleating agents [13-31]. This latter technique is preferred at the industrial scale. 

Dehydrated calcium pimelate is added (between 0.01 and 0.5 wt %) as the nucleating agent to isotactic polypropylene to produce the (3 modification of iPP [63]. Following molding between two glass plates at 220°C thin films of the polymer are isothermally crystallized at 140°C-143°C in a microscopy hot stage. After the hedritic structures develop, the samples are quenched to room temperature. Prior to AFM examination, the specimens are etched with a 1% solution of potassium permanganate in a mixture of sulfuric and orthophosphoric acid [64, 65]. This procedure is described in detail in the literature Caution consult the literature for safety precautions ) and helps to remove preferentially amorphous phase of PP [64]. Thus, unlike in the many other examples discussed in Sect. 3.2, here the interior of a specimen is analyzed after its exposure. 

Figure 3.49. The effect of compatibilizers (SEES) and (EP) on the nucleation of isotactic PP at various temperatures (a) measured as blend volume and (b) calculated as PP volume fraction Px>ng et al.,1995].

It is not only the crystallization rate, however, which is influenced by nucleating agents, but also the morphology. Isotactic poly(propylene) crystallizes monoclinically in the presence of / -/-butyl benzoic acid and pseudohexagonally when the quinacridone dyestuff. Permanent Red EBB, is added. 

Menyhard et al. [9] reported the generation of polymer blends based on the P-modification of polypropylene. The authors studied the melting and crystallization characteristics as well as the structure and polymorphic composition of the blends by polarized light microscopy (PLM) and differential scaiming calorimetry (DSC). It was observed that the most important factor of the formation of the blend with P-crystalline phase when semicrystalline polymers were added to isotactic polypropylene (iPP) was the a-nucleation... 

Polarized light microscopy micrographs of a P-nucleated isotactic polypropylene/polyamide-6 (iPP/PA-6) blend containing 5 wt% PA-6. Isothermal crystallization was carried out at = 135X for (a) t = 0 min, (b) = 21 min, (c) f = 45 min, and (d) = 60 min. (Reproduced from Menyhard, A., Varga, J., Liber, A., and Belina, G. 2005. Polymer blends based on the P-modification of polypropylene. European Polymer Journal 41 669-677 with permission from Elsevier.)... 

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