Polymorphism in crystalline polypropylene

This is due to a reduction of the local up du chain disorder, a process which is never complete, making aj only a limit situation. 

This form is often referred to as smectic and was first mentioned in 1958 by Slichter and Mandell who observed a peculiar wide angle X-ray diffraction (WAXD) pattern in a sample melted and then rapidly quenched with dry ice. It is characterized by an order intermediate between those found in crystalline and in amorphous phases and is metastable since annealing at temperatures higher than 70°C leads to the crystallization of a-iPP. While density is low (0.88 g/cm ), infrared (IR) spectra indicate that iPP chains adopt the usual 3j helix conformation. Solid state nuclear magnetic resonance (NMR) shows a closer resemblance to p-iPP while WAXD patterns are in favor of a predominance of very local (pairs of chains) arrangements similar to those found in a-iPP. 

This form is obtained by cold-drawing the polymer quenched from the melt. IR spectroscopy and WAXD techniques indicate that in this modification sPP adopts the alTtrans conformation and the crystallographic density is 0.945 g/cm. Upon annealing fibers of this form at about 100°C for a few hours, the more stable helical form results, without losing the preferred chain orientation along the stretching direction. 

This form has been characterized by Chatani et al. from WAXD data from fibers originally in the planar zigzag polymorph and then exposed to benzene, toluene, or p-xylene vapours below 50°C for several days. A triclinic cell was determined containing six monomer units and with a crystallographic density of 0.939 g/cm. The chain conformation is —TTgG2T2G2)— which may be considered as intermediate between the original —(IT)— conformation and the hehcal conformation 

Bruckner, S., Meille, S.V., Petraccone, V. and Pirozzi, B. (1991) Polymorphism in isotactic polypropylene. Prog. Polymer Sci., 16, 361-403. 

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Mesophase reveals intermediate order between amorphous and crystaUine phases. In the first studies it was labelled as smectic (Natta Corradini, 1960) or paracrystalline (Miller, 1960). Further studies revealed that mesophase is made up of bundles of parallel chains, which maintain typical for all polymorphic forms of polypropylene three-fold helical conformation. Bundles are terminated in the direction of the chain axis by helix reversals or other conformational defects (Androsch et al., 2010). In the bundles long range ordering maintains only along the chain axes, whereas in lateral packing a large amount of disorder is present (Natta Corradini, 1960). The mesophase is formed by quenching of the molten polypropylene (Miller, 1960 Wyckoff, 1962) or by deformation of the crystalline structure (Saraf Porter, 1988 Qiu, 2007). As for the fibres, the mesophase was observed in fibres taken at low take-up velocity (Spruiell White, 1975 Jinan et al., 1989, Bond Spruiell, 2001) in fibres intensively cooled in water with addition of ice or in the mixture of dry ice... 

Investigations of the crystalline structure revealed that inside the fibres the crystalline phase is usually built from a crystals. The a form is one of the three known polymorphic forms of polypropylene (Bruckner et al.,1991 Lotz et al., 1996). It can be easily obtained by crystallization of polymer melts or solutions. It is the most stable and the most often encountered form in different polypropylene products. 

This is in accordance with the fact that syndiotac-tic polypropylene is polymorphous. In fact, it can get two different crystalline forms, which differ because of the chain conformation (fig. 5). 

Whereas most of the early work on crystallization, etc., were concerned with predominantly isotactic chains, the recent developments in synthetic methodologies have enabled the preparation of highly syndiotactic polymers [13,14]. Since the high stereoregularity of these syndiotactic polymers facilitates their crystallization, several papers have been published on the x-ray crystal structure and polymorphism of syndiotactic polystyrene [15-18]. The chain conformation in the crystalline state has also been analyzed using NMR [19]. Similarly, the crystal structure of syndiotactic polypropylene has also been studied by a number of authors [20-22]. 

Polymorphism, which means existence of different crystalline forms, is a phenomenon observed in many organic and inorganic compounds. Also, natural (e.g., cellulose) and synthetic polymers show numerous polymorphic forms. Among synthetic semicrystaUine polymers, which can be a matrix in composites with lignocellulosic materials, the isotactic polypropylene is the one to take a close look on this phenomenon. 

As previously stated in Chapter 3, isotaetie polypropylene is a polymorphic material with four basic crystalline forms, namely the monoelinie (a), trigonal (P), orthorhombic (y) and mesomorphic smectic (intermediate state between ordered and amorphous phase) forms. The a-form exhibits excellent modulus and tensile strength but poor fracture toughness. ... 

Trotignon, J.P., Lebrun, J.L. and Verdu, J. (1982) Crystalline polymorphism and orientation in injection moulded polypropylene. Plastics Rubber Processing Applic. 2, 247-251. 

Configurational and conformational properties of the isolated chain of isotactic polypropylene (iPP) are essential for explaining the polymorphic behavior of crystalline polymer. The two main chain bonds astride a tertiary carbon atom can be distinguished from a configurational point of view, and we shall refer to them as (+) or (—) bonds. A (+) bond is always followed by a (-) bond or vice versa, consequently two distinct directions in moving from one end to the other of the chain [1] exist (Scheme 1). 

Isotactic polypropylene can crystallize in different forms (modifications), such as a, P, y, and smectic, which differ by their unit cell type and thus by their packing density." The most common are the a-form and the y-form. The a-modification is the preferred crystalline form of polypropylenes synthesized by conventional Ziegler-Natta catalysts." High molecular weight isotactic polypropylenes prepared by metallocene catalysts preferentially crystallize in the y-form. The different polymorphic behaviors of metallocene and Ziegler-Natta samples can be related to the... 

Syndiotactic polypropylene (s-PP) presents a very complex polymorphic behavior (the first part of this section on s-PP modifications is reported in the publication [203-212]). s-PP chains crystallize to various polymorphic crystalline phases and,... 

Isotactic polypropylene shows polymorphism. X-ray diffraction shows the presence of ot and y crystals in 50/50 proportions in poly(i-propylene-stat-ethylene) (6.6 mol%) crystallized at 393 K. The measured enthalpy change associated with melting was A/121 — A/ia2i = 44 J g and the onset of occurred at 395 K. The heat of fusion at the equilibrium melting point (460.7 K) is 206] g for the ot phase and 165 J g for the y phase. The specific heats of the crystalline and amorphous components were given by Gaur and Wunderlich (1981) as follows ... 

Isotactic polypropylene, iPP, is a polymorphic material with five crystal modifications [32, 33], a, p, y, S and a modification with intermediate crystalline order. For details see the chapter on morphology of polyolefins in this book [4]. 

This polymorphism is different from the case of isotactic polypropylene, which is also polymorphous, but in which, in the different crystalline forms, always a threefold helix is observed or from the case of polybute-ne-1 which is also polymorphous, but the different helices observed correspond, however, to the same region of minimum of the conformational energy map. Instead in the case of syndiotactic polypropylene, as shown before, the two different crystalline forms correspond to chain conformations which are widely separated in the conformational energy nap. 

The methods used for the crystallinity determination can also be used to determine the ratios of the different polymorphs that are often present in some polymers. In the case of PA6 shown in Figure 2.6a, the scan is resolved into the contribution from its two polymorphs, a and y, along with the amorphous halo [47]. Relative areas of the various peaks are used to calculate the relative amounts of the a and y components as well as the total crystallinity. The method can also be extended to determine when more than one polymer is present in the sample, such as polymer blends [48,53], Figure 2.6b shows an example of a mixture of amorphous poly(2,6-dimethyl-p-phenylene ether) (PPE) and PA6. The amorphous templates of PPE and PA6 were obtained from the scans of the homopolymers as discussed in Section 2.5.1. These templates were used as constraints in least squares fitting the data from the blend. Such analyses were useful in demonstrating that crystallinity and crystallite sizes of the PA6 were smaller in an alloy of the two polymers than in a blend [48]. Similar analyses have been carried out in a blend of two crystalline polymers, polyethylene and polypropylene [53]. 

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