Polypropylene mesophase

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

The case of isotactic polypropylene (iPP) presents some differences with respect to those just discussed. While both sPP and PET adopt in their mesophases disordered, extended, essentially non-helical conformations, iPP is characterized by a unique, relatively well ordered, stable chain structure with three-fold helical symmetry [18,19,36]. More accurately we can state that an iPP chain segment can exist in the mesophase either as a left handed or as the enantiomeric right-handed three-fold helix. The two are isoener-getic and will be able to interconvert only through a rather complex, cooperative process. From a morphological point of view Geil has reported that thin films of mesomorphic iPP quenched from the melt to 0 °C consist of... 

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... 

Baars et al. functionalized polypropylene imine) dendrimers (generation 1, 3, 5) with pentaoxycyanobiphenyl and decyloxycyanobiphenyl mesogens [132], All dendrimers were found to exhibit smectic A mesophases. The SA-layer... 

Figure 16.10 Conformational changes of polypropylene imine) dendrimers in LC materials from spherical to (a) cylindrical, in hexagonal columnar mesophase [130], and (b) ellipsoid, in smectic A mesophase [132]...

The complex with [Li+]/[PO] = 0.10 exhibits a crystalline melting transition followed by a smectic A mesophase. By increasing the salt concentration as in the case of complexes with [Li+]/[PO] = 0.15 0.30, the smectic A phase is suppressed instead, they exhibit a hexagonal columnar mesophase as evidenced by X-ray scattering. The induction of ordered structure in the melt state of the rod—coil molecule by complexation is most probably due to enhanced microphase separation between hydrophobic blocks and polypropylene oxide) block caused by transformation from a dipolar medium to an ionic medium in polypropylene oxide) coil. 

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... 

The rheology of dimolybdenum and dicopper octanoates was studied in their liquid-crystalline state and was found to be similar to that of conventional viscoelastic polymers such as polyethylene or polypropylene. This strongly supports the observation that these metallomesogens form polymeric chains in their columnar mesophases, and thus can be processed in a way similar to that of conventional polymers to form films and fibers. Note, however, that the chains are believed to be dynamic, constantly being formed and re-formed due to the weak nature of the axial, intermolecular M "0 interactions. 

Isotactic polypropylene (iPP), when quenched from the melt in cold water, gives rise to a metastable, solid mesophase, which transforms into the stable a form by annealing at elevated temperatures [25-27]. The X-ray powder diffraction pattern of this disordered form of iPP presents only broad halos at 20 = 14.8 and 21° (Fig. 33A). Oriented fibers of iPP in this mesomorphic form can be obtained by stretching the corresponding unoriented films [25]. The X-ray fiber diffraction patterns present broad peaks on well-defined layer lines as shown in Fig. 33B the chain periodicity corresponds to 6.5 A, as in the crystalline a form. 

Calorimetry detects an exothermic heat effect of about 600 J mol. The glass transition of the CD glass must occur at about 350 K. The mesophase of polypropylene is not stable under any known condition, it appears only as an intermediate on crystallization, similar to the mesophases found in the fibrous poly(ethylene terephthalate) of Fig. 5.72. Often these metastable mesophases determine the mechanical properties of the materials as demonstrated with Figs. 5.113-115. 

Kuhn length 48 letter abbreviation 17 melting, see polypropylene, stereoisomers, melting mesophase glass 561,562,567,... 

The fact that the behavioural patterns of the different composite families tested are not comparable with regard to their glass transition temperatures and activation energy, can be explained by taking into account the different structural order developed onto the polypropylene matrix as a function of the physico-chemical properties of the filler, as well as the filler loading. This structural order is determined by factors related to the appearance of new crystalline structures, by the existence of mesophases surrounding the particles (5) as well as the result of specific interactions between the phases present in the composite. These lines of approach are further being pursued in research in course. 

Young RJ (1974) Dislocation model for yield in polyethylene. Philos Mag 30 85 Zhou J-J, Liu J-G, Yan S-K, Dong J-Y, Li L, Oian C-M, Schultz JM (2005) Atomic force microscopy study of the lamellar growth of isotactic polypropylene. Polymer 46 4077-4087 Zia Q, Radusch HI, Androsch R (2009) Deformation behavior of isotactic polypropylene crystallized via a mesophase. Polym Bull 63 755... 

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