Polypropylene fibers syndiotactic

Fig. 12.21. Diagrams showing (a) irregular atactic, (b) stereoregular isotactic, and (c) stereoregular syndiotactic configurations in polypropylene polymer. (Source Ahmed, M., "Polypropylene Fibers-Science andTechnology," Textile Science and Technology, 5, 16, Elsevier Science Publishers SV, Academic Publishing Div., New York, Amsterdam (1982).) Copyright M. Ahmed. By permission.

Polypropylene was not developed until the 1950s when Ziegler and Natta invented coordination catalysts. The structural difference between polyethylene and polypropylene is the methyl group in the propylene unit. Its presence makes a difference because it makes possible three different polymer structures Isotactic, with all methyl groups in the same plane makes the best plastic syndiotactic, in which the methyl groups alternate in the same plane and atactic, with the methyl groups randomly in and out of the plane is soft and rubbery. Polypropylene is used as film and in many structural forms. It is also used as fibers for carpet manufacture and for thermal clothing. 

We examine briefly some specific instances starting with syndiotactic polypropylene (sPP). Aside from the already discussed hexagonal mesophase which can be obtained both drawing fibers and under quiescent conditions, this polymer presents four crystalline forms phases I [73] and II [74-76] where chains adopt the (T2G2)n helical conformation, forms III [30] and... 

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. 

Description of polypropylene in the literature is often accompanied by terms atactic, isotactic, or syndiotactic. The origin of the terms is as follows. Side methyl groups in polypropylene chains can be all on the same side of plane (as it is shown above in a very simplified manner, as in reality carbon atoms in the chain are in zig-zag stereoconfiguration), on alternate sides, or in a random arrangement with respect to plane of carbon atom chain. These forms of PP are called isotactic (fiber-forming), syndiotactic, and atactic, respectively. All these forms are related to homopolymers of polypropylene. PP used for common applications, including WPC, is atactic and will be referred to here as polypropylene. 

There have been numerous formulations of the metalloeene catalysts in the last 20 years. Table 3.4 presents a few examples of metallocene catalyst systems of interest (see all preceding references) [58-66]. It can be seen that earlier efforts were focused on catalyst efficiency and polymer stereospecificity. More recently, some interests seemed to have turned to syndiotactic polypropylene for various polymer and fiber properties. 

Synonyms Atactic polypropylene Isotactic polypropylene Polypropene PP Propathene 1-Propene, homopolymer Propene polymer Propene polymers Propylene polymer Syndiotactic polypropylene Classification Thermoplastic polymer Definition Polymer of propylene monomers three forms isotactic (fiber-forming), syndiotactic, atactic (amorphous)... 

Polypropylene is a versatile polymer used in applications from films to fibers, with a worldwide demand of over 21 million Ib. It is similar to polyethylene in structure except for the substitution of one hydrogen group with a methyl group on every other carbon. On the surface, this change would appear trivial, but this one replacement changes the symmetry of the polymer chain. This allows for the preparation of different stereoisomers, namely, syndiotactic, isotactic, and atactic chains. These configurations are shown in the introduction. 

Syndiotactic polypropylene (sPP) is characterized by the existence of three regular conformers with different fiber repeats. Three crystalline modifications result, all giving fiber patterns as opposed to what is found in iPP. 

Commercially available polypropylene, in the form of pellets, films, and fibers, exists as isotactic polypropylene, and this is produced by well-controlled stereoregular head-to-tail addition polymerization reaction with Ziegler-Natta-type catalysts. Formed in this manner, isotactic polypropylene is a crystalline polymer. Commercial samples typically contain small amount of atactic and/or syndiotactic polypropylene. Furthermore, blocks with different stereoregularities are also observed. The reference spectra (Reference Spectrum 2) in the Appendix provide the IR and Raman spectra of isotactic polypropylene. 

Most polypropylene used in various textile products is produced from isotactic polymers. Isotacticity of polypropylene has significant influence on tape or fiber property. Only iso- and syndiotactic polymer have sufficient molecular order to create polycrystalline structure and hence acceptable physical and tensile properties for oriented tapes. Isotactic polymer chain is characterized by the arrangement of the functional group on one side of the plane. 

Table 17.4 shows the well-known fiber polymers, and a few which seem unsuitable as fiber material, namely polystyrene and poly(vinyl chloride). The latter finds limited fiber application, but a special syndiotactic grade with some crystallinity is then used. Polypropylene has a fairly low melting point but is nevertheless a large fiber product, because the material is cheap and versatile. Polyethylene is even lower melting and is used only as a superstrong fiber at ambient temperature. 

Another man of genius, G. Natta at Milano Polytechnic, soon extended that type of catalysis to the stereoselective polymerization of propylene into isotactic (and also syndiotactic) stereoregular polypropylene chains (highly cristalline fiber-forming material, m.p. close to 170 °C). 

Sura R K, Desai P aud Abhiraman A S (2001), Oriented crystallization in fiber formation Inferences from the structure and properties of melt spun syndiotactic polypropylene filaments . Journal of Applied Polymer Science, 81,2305-2317. 

Different polypropylenes may be described as isotactic, syndiotactic, or atactic. What does this mean Which polypropylene is used to make commercial textile fibers ... 

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