Polypropylene spherulite

FIGURE 8-62 An isotactic polypropylene spherulite grown from a mixture of 10% isotactic polypropylene and 90% atactic polypropylene [redrawn from a micrograph of H. D. Keith and F. J. Padden, J. Appl. Phys35.1270 (1964)]. 

Figure 3. Additive distributions for polypropylene spherulites containing 0.5 Uvitex OB crystallizing at 125, 130, and 135 °0. Waveform monitor trace of UV image.

The most likely cause of the crystallinity variations in polypropylene spherulites is the accumulation of rejected atactic and low molecular weight impurities. This view is supported by the observation that adding increasing amounts of atactic material to polypropylene purified by octane extraction, leads to changes in the crystallinity distribution (Figure U). 

Samuels, R. J. Yee, R.Y. (1972). Characterization of structure and organization of beta-form crystals in type-iii and type-iv beta-isotactic polypropylene spherulites. Polym. Sci. PartA-2 Polymer Phys., Vol.lO, Issue 3, pp. 385-432. 

Figure 3 Polypropylene spherulites viewed through a tint plate. (Reprinted with permission from Mr R Musgrove, Pira International.)...

Galeski A. Pracella M. Martuscelh E. Polypropylene spherulite morphology and growth rate changes in blends with low density polyethylene. Journal of Polymer Science. PartB. Polymer Physics. 1984, 22, 739-747. 

Fig. 2.4 DSC melting endotherms of high crystallinity polypropylene (7) crystallized frtnn the melt for 3 h at 145 °C, (2) partially melting scan (3) fully remelted after the partial melting From figure 3, Weng J, Olley RH, Bassett DC and Jaaskelainen P, Changes in the Melting Behavior with the Radial Distance in Isotactic Polypropylene Spherulites. J. Polym. Sci. Polym. Phys. 41 2342-2354. Copyright 2003 by John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc...

One of the earliest and most commonly used microscopical methods of examining polymers is between crossed polarizers. Some of the earliest work was determining the birefringence of fibres, then came the study of spherulites in semi-crystalline polymers. Often the spherulites show a simple Maltese Cross pattern where the dark areas show zero-amplitude birefringence, which simply arises from the orientation of the crossed polars. In the polypropylene spherulites (Fig. 2.7a), radial growth has occurred along the a -axis which is the fastest crystal growth direction, while the and c-axis are effectively randomly oriented. 

Spherulites of aliphatic hydrocarbon polymers such as polyethylene generally have their molecular chains with the highest refractive index oriented perpendicular to the radial growth direction. This is the case with the polypropylene spherulites shown in Fig. 2.9a, which were crystallized at 140 °C and consequently contain very few cross-hatched lamellae, so in this respect they are like polyethylene spherulites. The lower refractive index in this view is radial in direction, giving a yellow colour where the radius is parallel to the slow direction of the tint plate these are therefore termed negative spherulites. 

Fig. 2.10 Use of circularly polarized light (top) and phase contrast (bottom) to show development of polypropylene spherulites at 150 °C (Reprinted from Polymer 30 399-409, Olley RH and Bassett DC. On the Development of Polypropylene Spherulites, Copyright (1989), with permission from Elsevier)...

Auriemma F, De Rosa C (2006) Stretching isotactic polypropylene from cross-p to cross-hatches, from a form to y form. Macromolecules 39(22) 7635-7647 Baird DG (1998) Polymer processing principles and design. Wiley-Interscience, New York Bassett DC, Olley RH (1984) On the lamellar morphology of isotactic polypropylene spherulites. Polymer 25(7) 935-943... 

Aboulfaraj M, G Sell C, Ulrich B and Dahoun A (1995)/n situ observation of the plastic deformation of polypropylene spherulites under uniaxial tension and simple shear in the scanning electron microscope. Polymer 36 731-742. 

Figure 35 Interior morphology of a polypropylene spherulite crystallized at 130 °C showing the additional textural element intermediate in scale between the spherulite and the lamella...

Crystalline polymers exhibit the following basic properties They are opaque as long as the size of the crystallites or spherulites, respectively, lies above the wavelength of light. Their solubility is restricted to few organic solvents at elevated temperature. The following crystalline polymers have attained technical importance as thermoplastic materials polyethylene, polypropylene, aliphatic polyamides, aliphatic/aromatic polyamides, aliphatic/aromatic polyesters, poly-oxymethylene, polytetrafluoroethylene, poly(phenylene sulfide), poly(arylene ether ketone)s. 

Addition of small amounts of nucleating agents influence the spherulitic crystallization of polypropylene and improve transparency (Sect. 1.3.3.3 and Example 3-20). 

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. 

The structure of crystalline polymers may be significantly modified by the introduction of fillers. All aspects of the structure change on filling, crystallite and spherulite size, as well as crystallinity, are altered as an effect of nucleation [9]. A typical example is the extremely strong nucleation effect of talc in polypropylene [10,11], which is demonstrated also in Fig. 2. Nucleating effect is characterized by the peak temperature of crystallization, which increases significantly on the addition of the filler. Elastomer modified PP blends are shown as a comparison crystallization temperature decreases in this case. Talc also nucleates polyamides. Increasing crystallization temperature leads to an increase in lamella thickness and crystallinity, while the size of the spherulites decreases on... 

Our findings in regard to the effects of deformation on PTFE accord in some respects with observations on the deformation of polyethylene " and polypropylene, although PE and PP usually have a spherulitic morphology before drawing. 

Figure 1.15 Polarized microscopic image of the spherulitic structure in polypropylene.

Spherulites. As a polymer melt solidifies, several folded chain lamellae spherulites form which are up to 0.1 mm in diameter. A typical example of a spherulitic structure is shown in Fig. 1.15. The spherulitic growth in a polypropylene melt is shown in Fig. 1.16. 

Inoue, M. Spherulitic crystallization and cracking during heat ageing of polypropylene. J. Polymer Sci. 55, 443—450 (1961). 

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