Crystals polypropylene

Fig. 17. Loss tangent of a crystallizing polypropylene at different times after cooling from T = 100 °C to T = 40 °C (below Tmelt). Data from Lin et al. [68].

A review article by Bicerano [64] delves into the phenomenology of polymer crystallization in far greater detail including the trends for Tm and for the other crystallization parameters of homopolymers and copolymers, as well as the trends and some of the models for the kinetics of crystallization. Polypropylene, polyethylene terephthalate), and copolymers of propylene and of ethylene terephthalate, are all given detailed consideration as case studies. 

As shown by Price (6), rejected noncrystallizable impurities will be pushed ahead of the growing spherulite as a wave, leaving a lower concentration within the spherulite than in the original melt. Frank and Lehner (8) and Curson (9) have used UV transmission microscopy to observe additive distributions in crystalline polyolefins. This method can be used to observe the redistribution process in action by quenching partially crystallized polypropylene to freeze the additive concentrations... 

All isotactic vinyl polymers can be arranged in four ways in the usual crystals. Polypropylene has as stable conformation a 2 3/1 helix that consists of successive tg(—) or tg rotational isomeric states (left-handed or right-handed helices). In addition, it was possible for the helices to have the —CHj groups point up or down , relative to the crystallographic c-axis. In the ideal, monoclinic crystal form I (P2j /c) all four types of helices occur only at specific, symmetry-related positions The equilibrium melting temperature and entropy are 460.7 K and 15.1 J/(K mol), respectively. 

Kardos, J. L., Christiansen, A. W., and Baer, E. 1966. Structure of pressure crystallized polypropylene. Journal cf Polymer Science, Part B Polymer Physics 4 777-778. 

Obtained results showed clearly that the presence of the flax fiber itself, in the crystallizing polypropylene, causes no formation of hexagonal phase (Fig. 10.1). For samples where fiber was moved, the X-ray patterns revealed the diffraction maximum at 20 16.2°, which proves the presence of p form. The amount of this form was calculated firom the diffractograms according to the well-known Turner-Jones formula [1]. It was shown that the formation and the amount of this form in composite matrix depend on the temperature at which the fiber is moved in... 

Industrial application of fiber pulling inside the crystallizing polypropylene matrix can be a method for manufacturing products characterized by a higher impact resistance and elasticity however, the advantages of the products are the result of the presence of hexagonal form [9]. 

Figure 3.7. The volume of unit cell of crystallizing polypropylene vs. concentration of nucleating agent (sodium benzoate). [Adapted, by permission, from Xu, T Lei, H Xie, C S, Mater. Design, 24, 227-30,2003.]...

However, as pointed out by Tanner (2003), the stress law of (4.24) over simplifies the real picture of structure. The additive rule envisages parallel components of amorphous and crystalline phase at each point. Photographs of crystallizing polypropylene such as those shown by Koscher and Fulchiron (2002) and many others do not support this assumption. 

Fig. 4.1 Flat-plate X-ray diffraction pattern obtained from an isotropic sample of melt-crystallized polypropylene.

Kardos, J.L. Christiansen, A.W. Baer, E. (1966), Structure of pressure-crystallized polypropylene.. Journal of Polymer Science, Polymer Physics Edition 4, 777-8. 

Snetivy D and Vancso G J 1994 Atomic force microscopy of polymer crystals 7. Chain packing, disorder and imaging of methyl groups in oriented isotactic polypropylene Po/yme/ 35 461... 

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... 

Aqueous solutions of 50% acrylamide should be kept between 15.5 and 38°C with a maximum of 49°C. Below 14.5°C acrylamide crystallizes from solution and separates from the inhibitor. Above 50°C the rate of polymer buildup becomes significant. Suitable materials of constmction for containers include stainless steel (304 and 316) and steel lined with plastic resin (polypropylene, phenoHc, or epoxy). Avoid contact with copper, aluminum, their alloys, or ordinary iron and steel. 

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

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