Smectic isotactic polypropylene

Brosse et al. [41] modified isotactic polypropylene and other polyolefins by a cold plasma. In isotactic polypropylene, plasma treatment results in a polypropylene crystallization of paracrystalline or smectic form into a a-crystalline form. Further, the active films are susceptible to react with monomers in a postgrafting reaction. 

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 first work reported in this area was an investigation of isotactic polypropylene which was fatigued at a rate of 10 hz with an elongation of one to five percent. The spectra showed no shifts in frequency but reversible intensity changes were observed. The data were interpreted in terms of the presence of a smectic-like structure as a result of the cyclic stress 304i 305>. 

If isotactic polypropylene is quenched from the melt to room temperature a so-called smectic modification is formed which shows only two crystal reflexions. Cabarcos, Bbsecke, and Zachmann investigated the kinetics of the transition from this modification into the a-modification. Fig. 54 shows the change of wide angle scattering during isothermal annealing at 90 °C. One sees that the 040-reflexion of the a-modification appears after about 200 sec and continues to increase in intensity afterwards. The time until the constant temperature is reached is about 150 sec. Therefore we can say that most of the process occurs at constant temperature. If the same is done at 130 °C the process occurs so rapidly that it is almost finished as constant temperature is reached. 

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. 

Isotactic polypropylene crystallizes below 350 K on quick cooling from the melt increasingly as condis glass This conformationally disordered polypropylene had first been described as bdng smectic mesomorphic or paracrystalline ... 

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

Isotactic polypropylene is known to crystallize when cooled quickly from the melt into a crystal form which was called a smectic, mesomorphic form. The X-ray diffraction pattern resembled clearly that of a smectic material. Later, this structure was also called para-crystalline to indicate the poor crystalline order. It was found that this structure is metastable below 335 K. At room temperature it has been reported to persist for over 18 months so that it should be called a CD-glass. The proposed transition mechanism to the stable crystal form involves intramolecular helix... 

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... 

Li L B and De Jeu W H (2005) Shear-induced smectic ordering and crystaUisation of isotactic polypropylene, Faraday Discuss 128 299-319. 

Figure 3-11. Raman spectrum of isotactic polypropylene from smectic (- - -) to crystalline (—) (tridimensional order) obtained after long annealing processes at low temperatures. Raman regularity bands split into doublets (crystallinity bands) due to phonons at F (k, = ky = kz = 0) (from...

Li L, de Jeu WH. Shear-induced smectic ordering as aprecursor of crystallization in isotactic polypropylene. Macromolecnles 2003 36 4862-4867. 

Addition of talc into isotactic polypropylene (PP) induces two types of crystalline forms, i.e., a and forms [89,92,93 ]. Some reported that PP can form 4 different crystalline forms, i.e., a,jS,y,and(5 (smectic) forms [93]).Ferrageetal. [94] showed crystalline onset temperature shift by addition of talc and an a nucleating agent (sodium 2,2 -methyl-bis-(4,6-di-fert-butylphenyl phosphate) in polypropylene. Addition of triphenodithiazine, pimelic acid with calcium stearate, or quinacridone dye permanent red can increase thephase [94 to 96]. 

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). 

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.

Isotactic polypropylene (iPP) exhibits a stable crystalline a form consisting of chains in the 3i helical conformation (tgtgtg), packed in a monoclinic unit cell. Left- and right-handed helices face each other in the a form. The metastable -form crystals contain hexagonally packed 3i helical chains arranged in groups of the same helical handedness (left or right). There is also a smectic, or partially ordered, form... 

Fig. 9.4. The CP-MAS spectra of isotactic polypropylene in the a form (a), the form (b), and the smectic form (c). (Reproduced with permission from Ref. [30]. 1987 Butterworth-Heinemann, Lid.)...

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