Polypropylene smectic 5 phase

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. 

Shear-induced smectic phase in isotactic polypropylene. 89... 

In isotactic polypropylene [73,76], the smectic phase or the monoclinic a phase can be obtained according to cooling conditions. Furthermore, the a phase may, in certain conditions, exhibit a bimodal crystalline texture, that is, two populations of crystals with their c or their a-Sixis along the fiber axis, respectively (a is the axis of the reciprocal lattice related to a). This is revealed by additional arcs or spots on the 110 and 130 reflections, while the 040 reflections remain unchanged [79] (Fig. 15.21). This can be interpreted in terms of a cylindritic morphology with two types of lamellae classical radial lamellae and tangential ones obtained by epitaxial growth, which is a particular feature of polypropylene crystallization (see Chapter 8). 

For polypropylene (PP), on the other hand, it has not been possible to observe any IR splittings resulting from intermolecular or crystalline packing. In fact, there are many features in the PP spectrum that look like pairs of bands, and indeed they are. However, the band pairs arise from intramolecular helical splitting, not from intermolecular crystalline splitting. Actually, intermolecular packing influences the IR spectrum of PP very little. In Fig. 4.42, the X-ray patterns of the monoclinic a phase and the smectic S phase are shown. The monoclinic a phase consists of well-ordered crystalline 31 helices, while the smectic S phase consists of 31 helices that are out of register with each other. The smectic phase will convert with time to the... 

Typical pictures obtained during the conversion of smectic polypropylene into the a-phase are shown in Fig. 33. 

Fig. 33. Digitized Videoframes showing the transformation of smectic polypropylene into the ot-phase. The temperature was increased in steps up to the pure a-phase. The line corresponds to the intensity contour along one video line and through the peaks...

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... 

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. ... 

Collings PJ (1990) Nature s delicate phase of matter. Adam Hilger lOP, Bristol, p 196 Datta A, Chen HH, Baird DG (1993) The effect of compatibilization on blends of polypropylene with a liquid-crystalline polymer. Polymer 34(4) 759-766 Davidson P (1999) Selected topics in X-ray scattering by liquid-crystalline polymers. In Mingos DMP (ed) Liquid crystals II. Springer, Berlin, pp 1-39 Dowell F (1988) New theories for smectic and nematic liquid-crystal polymers backbone LCPs and their mixtures and side-chain LCPs. Mol Cryst Liq Cryst Inc Nonlinear Opt 157 (l) 203-227... 

Fig. 4.42. A comparison of the IR spectra and the X-ray diffraction patterns of the monoclinic a phase and smectic 5 phase of isotactic polypropylene.

It is evident from wide-angle X-ray scattering that the solidification at 90 °C led to rather higher ciystallinity of the films while the spectra of the films solidified at 30 °C show high amorphous halo. In this case, the structure appeared to be smectic. The films solidified at 60 °C consist of regular crystalline structure. The highest content of P phase was revealed in the film made of B-PP-H and solidified at 90 °C. Random polypropylenes show lower crystallinity and p phase content as compared to homopolymers. For the enhancement of relative amount of P phase within crystalline portion, high chill roll temperature is desirable. 

---