Isotactic polypropylene crystallization rate

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. 

A new technique for following rapid rates of crystallization. II. Isotactic polypropylene. Polymer 3, 35—42 (1962). 

The steric configuration is extremely important in the polymer. Only isotactic polypropylene (iPP) has the properties necessary for forming fibers. The molecules are cross-linked only by Van der Waals forces, so it is important that they pack as closely as possible. The isotactic molecules form a 3, helix, as shown in Fig. 12.21,16 and exhibit a high crystallization rate. The atactic molecules, shown in the figure, do not pack well, and although the syndiotactic molecules can pack better and crystallize, this configuration is not a normal product of commonly used catalyst systems. 

PP isotactic polypropylene (aPP — atactic sPP — syndiotactic) T max maximum crystallization rate temperature... 

The purpose of this chapter is to provide a general experimental route for studying the crystallization behaviour of isotactic polypropylene under high cooling rates and pressure. 

C02-induced crystallization has also been observed for bisphenol A polycarbonate [33], tert-butyl poly(ether ether ketone) [34], and methyl substituted poly (aryl ether ether ketone) [35]. This C02-induced enhanced crystallization rate with increasing pressure has been explained in terms of the depression of Tg being far greater than the depression of the T. However, in contrast to the aforementioned polymers, it has been shown that exposure of poly(L-lactide) [36] and isotactic polypropylene [37] to CO2 suppresses the crystallization rate. This observation was suggested to be due to the being depressed to the same extent as the Tg, which prevents the formation of critical size nuclei. It has recently been shown by Hu et al. [38] that the crystallization of polycarbonate may be initiated under scCOz conditions using nano-scale days. 

Zafeiropoulos et al. [16] used four types of flax fiber when studying the crystallization of isotactic polypropylene in the presence of fibers unmodified, dew-retted (pectin removed), flax modified by esterificatirMi with stearic acid and flax modified by high temperature in autoclave. It was interesting to see that transcrystaUizatirHi occurred in both chemically and hydrothermafly modified fibers. Zafeiropoulos also fotmd that the rate of cooling may have an effect on the morphology of TCL. 

Isotactic polypropylene composites with tetra-needle-shaped zinc oxide whisker were prepared via melt blending in a twin-screw extruder. The whisker is a p-nucleating agent, which offers higher crystallization temperature, smaller sphemlites, and faster crystalhzation rate than can be obtained in neat iPP. The addition of whisker also increases heat deflection temperature and thermal degradation temperature of iPP composite. ... 

This low-isotactic polypropylene sample shows a low crystallization rate at room temperature, which allows one to follow the crystallization in situ (Figure 9.21). The initially formed lamellar... 

The crystallization of isotactic polypropylene (iPP) is affected by many factors, such as macromolecular characteristics [82], crystallization conditions (temperature, cooling rate), and stress (shear, elongational fields) [83-85]. In fiber reinforced iPP matrix composites, the crystalline morphology of the polymer is influenced by the fibers that can act as nucleating agents affecting the crystallization process [86-88]. TranscrystaUinity is a well-known structural feature in pol rmers, which occurs as the result of dense nucleation of the semi-... 

A particular case in point is polypropylene. When isotactic polypropylene is crystallized at 145 °C and above, aU the material is sufficiently stable not to undergo reorganization at usual scan rates. Typically an endotherm such as in Fig. 2.4 is seen, curve 1 (Weng et al. 2003). 

Somani RH, Yang L, Zhu L, Hsiao BS (2005) Flow-induced shish-kebab precursor structures in entangled polymer melts. Polymer 46 8587-8623 Sorrentino A, De Santis F, Titomanlio G (2007) Polymer crystallization under high cooling rate and pressure a step towards polymer processing conditions. Lect Notes Phys 714 329-344 Stocker W, Magonov SN, Cantow HI, Wittmann JC, Lotz B (1993) Contact faces of epitaxially crystallized a- and y-phase isotactic polypropylene observed by atomic force microscopy. Macromolecules 26 5915-5923... 

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