Crystal structure nucleating agents

In addition to the nucleating agents discussed in Section 18.4, many other materials have been found to be effective. Whilst the nylons may be self-nucleating, partieularly if there is some unmelted crystal structure, seeding with higher melting point polymers can be effective. Thus nylon 66 and poly(ethylene terephthalate) are reported to be especially attractive for nylon 6. 

Metal salts of organic acids (for example, sodium benzoate) are used as nucleating agents, while many other materials will function similarly they are added in order to increase the crystallization in certain materials, notably polyamides, and have the effects of reducing the size of crystal structures but inducing the formation of more of them within a shorter period of time. More... 

When a polymer crystallizes from the melt without disturbance, it normally forms spherical structures that are called spherulites [1,2]. The dimensions of spherulites range from micrometers to millimeters, depending on the structure of the polymer chain and the crystallization conditions, such as cooling rate, crystallization temperature, and the content of the nucleating agent. The structure of spherulites is similar regardless of their size they are aggregates of crystallites [1-6]. 

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. 

Fig. 38 Evolution of /u, the J-Integral accounting for the fracture toughness, and of the elongation at break, break> as a function of the long period, LP, (left) and the lateral size of /3-crystals, L300, (right). Tests have been performed at room temperature on injection molded samples at 1.5 m s-1 to determine /Icj and at 50 mm min 1 to measure ebreak-Mechanical and structural responses have been varied by varying the concentration of NU-100, the nucleating agent from 0 to 1300 ppm...

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. 

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