Polymorphism polypropylene

De Rosa C, Auriemma F, De Lucia G, Resconi L (2005) From stiff plastic to elastic polypropylene polymorphic transformations during plastic deformation of metallocene-made isotactic polypropylene. Polymer 46(22) 9461-9475... 

A case of polymorphism which is strictly analogous to that of Kevlar seems to be present in syndiotactic polypropylene (s-PP). In fact, depending on the... 

Figure 2.12 Maps of conformational energy of various syndiotactic polymers as function of backbone torsion angles 0 and 0227 (a) syndiotactic polystyrene, (b) polypropylene, (c) poly (1-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i 0i 0202 - - -[s(M/N)2 symmetry] has been assumed. Isoenergetic curves are reported every 5 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. Values of energies corresponding to minima (x) are also indicated. Experimental conformations observed for different polymorphic forms of polymers are indicated by triangles. (Reproduced with permission from Ref. 27. Copyright 1992 by the Socicta Chimica Italiana.)...

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

In syndiotactic polypropylene, the methyl groups are well separated and the TT form is favoured, but there are other energy minima among the gauche conformations and TT/G G and TT/G G sequences can generate left- and right-handed helices, respectively, where the repulsions are minimized (Boyd and Phillips, 1993). The chains may crystallize both in the TT and in the TTG G form, so syndiotactic polypropylene is polymorphic. 

Solid-state NMR spectroscopy has been available for the analysis of polymorphs for isotactic polypropylene (i-PP) for approximately 15 years [1-3]. By different sample preparation (e.g., the method of crystallization), i-PP forms a, /3 and smectic forms. Figure 11.1 shows the CP/MAS NMR spectra of these three forms of i-PP at 20°C. 

Synthetic polymers such as polypropylene also exhibit polymorphism and their properties are highly dependent on their mode of preparation. Using a combination of the WAXS technique to identify polymorphs, and HSM for crystallization studies... 

The synthesis of polypropylene brought a new scenario and major efforts at that time concentrated on controlling the stereoregularity, synthesizing the most regular isotactic polypropylene, and understanding its polymorphism these efforts have continued for many years. 

Menyhard et al. [9] reported the generation of polymer blends based on the P-modification of polypropylene. The authors studied the melting and crystallization characteristics as well as the structure and polymorphic composition of the blends by polarized light microscopy (PLM) and differential scaiming calorimetry (DSC). It was observed that the most important factor of the formation of the blend with P-crystalline phase when semicrystalline polymers were added to isotactic polypropylene (iPP) was the a-nucleation... 

Bokobza, L. Burr, A. Garnaud, G. Perrin, M. Pagnotta, S. (2004) Fibre Reinforcement of Elastomers Nanocomposites Based on Sepiolite and Poly(hydroxyethyl acrylate). Polym. Int. Vol.53, N0.8, pp.1060-1065, ISSN 0959-810 Bonduel, D. Mainil, M. Alexandre, M. Monteverde, F. Dubois, P. (2005) Supvported Coordination Polymerisation A Unique Way to Potent Polyolefin Carbon Nanotube Nanocomposites. Chem. Commun. Vol.l4, No.6, pp.781-783 Bruckner, S. Meille, S. Petraccone, V. Pirozzi, B. (1991) Polymorphism in Isotactic Polypropylene. Prog. Polym. Sci. 16, No.2-3, pp.361-404 Bryning, M. Islam, M Kikkawa, J. Yodh, A. (2005) Very Low Conductivity Threshold in Bulk Isotropic Single-Walled Carbon Nanotube-Epoxy Composites. Ado. Mater. Vol.17, N0.9, pp.1186-1191... 

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. 

The transition of the polymorphic forms of polypropylene was observed many times. It was stated that the p—>a transition is not reversible and occurs in the temperature range of 130 to 150°C (Forgacs et al., 1981 Varga, 1995). For years several mechanisms of the transition have been proposed. Garbarczyk suggested that the p—va transition proceeds in a solid state in three intermediate stages based on rotations and translations of polypropylene chains (Garbarczyk, 1985,1989). Due to considerable differences in unit cells of both modifications, Samuels stated that the p—>-a transition must take via the liquid phase (Samuels Yee,... 

Broda, J. (2003b). Polymorphism in Polypropylene Fibres.. Appl.Polym.Sci., Vol. 89, Issue 12, pp. 3364-3370. 

Broda, J. (2004c). Polymorphic composition of colored polypropylene fibers. Crystal Groivth Design, Vol.4, Issue 6, pp. 1277- 1282. 

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