Propylene- spherulites

Figure 5-23. An it-poiy(propylene) spherulite as seen under the phase-contrast microscope (left) and the polarization microscope (right) (after R. J. Samuels).

Figure 11-22. The onset of break at the amorphous positions in it-poly(propylene) spherulites, that is, between spherulites and radially within the spherulites. (After H. D. Keith and F. J. Padden, Jr.).

Figure i-24. if-Poly(propylene) spherulites as seen under the polarizing microscope (alter... 

Figure 4.12 Spherulites of poly( 1-propylene oxide) observed through crossed Polaroid filters by optical microscopy. See text for significance of Maltese cross and banding in these images. [From J. H. MaGill, Treatise on Materials Science and Technology, Vol. lOA, J. M. Schultz (Ed.), Academic, New York, 1977, with permission.]...

Figure 11 Left Spherulites of a Ziegler-Natta isotactic poly(propylene) with Mw = 271,500 g/mol and mmmm — 0.95, isothermally crystallized at 148°C. Right Banded spherulites of a linear polyethylene with Mw = 53,600 g/mol slowly cooled from the melt.

Recent developments have allowed atomic force microscopic (AFM) studies to follow the course of spherulite development and the internal lamellar structures as the spherulite evolves [206-209]. The major steps in spherulite formation were followed by AFM for poly(bisphenol) A octane ether [210,211] and more recently, as seen in the example of Figure 12 for a propylene 1-hexene copolymer [212] with 20 mol% comonomer. Accommodation of significant content of 1-hexene in the lattice allows formation and propagation of sheaf-like lamellar structure in this copolymer. The onset of sheave formation is clearly discerned in the micrographs of Figure 12 after crystallization for 10 h. Branching and development of the sheave are shown at later times. The direct observation of sheave and spherulitic formation by AFM supports the major features that have been deduced from transmission electron and optical microscopy. The fibrous internal spherulite structure could be directly observed by AFM. 

Figure 17 Isothermal melting of Ziegler-Natta isotactic poly(propylene). (a) Spherulites with mixed birefringence at Tc = 148°C. The top middle figure displays the melting for the same thermal history, (b) Subsequent to crystallization, the temperature was raised to 171°C spherulites acquire negative birefringence, (c), (d) and (e) Isothermal melting at 171°C for 80, 200 and 300 min, respectively. Reproduced with permission from W.T. Huang, Dissertation, Florida State University, 2005. (See Color Plate Section at the end of this book.)...

Recent advances in catalysis have allowed the production of polyolefins with low crystallinity. Spherulitic structures (see next section) only occur in propylene-ethylene copolymers when the crystallinity exceeds 45% (Fig. 3.22). Sheaf-like structures occur when the crystallinity is between 30 and 45%, whereas axialites and isolated lamellae occur between 15 and 30% crystallinity. Axialites are multi-layer aggregates of lamellar crystals which splay out from a common edge. Embryonic axialites occur for crystallinity from 5 to 15%. Therefore, as the crystallinity is reduced, the microstructures become simpler. 

Copolymers of isotactic propylene (iPP) with a-oleflns also exhibit diffraction peaks due to iPP crystallites. As one would predict, increasing the a-olefln content decreases the percent crystallinity. When the copolymer is blended with iPP homopolymer, the copolymer will cocrystallize with the homopolymer, a phenomenon that is rare in polymers. Cocrystallization is believed to substantially contribute to the improved mechanical properties found in the blend (Starkweather, 1980). Large spherulites are not generally found in these blends as opposed to the homopolymer, and the crystal form is monoclinic rather than smectic (Kresge, 1984). 

Spherulitic growth is a special case in crystallization. Spherulites form only within a specific temperature range for example, with it-poly(propylene) with a melting point of ITO C, they are first formed below IIS C. With spherulites, the rate of advance of the spherulite boundary is followed. This boundary encloses the crystalline portion of the spherulite. Since spherulites also contain noncrystalline material, however, the spherulite growth rate thus corresponds to the linear crystal growth rate. As the molecular weight increases, the rate of crystallization falls, since the rate of diffusion of segments and molecules decreases. 

Semicrystalline polyolefin blends and method of their preparation were described. The blends were reported to show enhanced inter-spherulitic and interlamellar strength. The first polymer should have higher crystallinity and crystallization temperature than the second. Thus, 50-99.9 wt% PP was blended with ethylene-a-olefin copolymers, either a stereo block polypropylene or an ethylene-propylene copolymer, EPR... 

Figure 5.10 Optical micrograph of the spherulites produced by a propylene-ethylene...

Furthermore, the crazes in PP show other similar characteristics to those of amorphous polymers. They grow apparently normal to the direction of major tensile stress which somewhat deviates from the tensile direction because of spherulitic structure. There are similar environmental effects on craze initiation (see also Environmental stress cracking of polypropylene in this book). Crazing is also an important source of toughness in toughened PP alloy systems such as propylene-ethylene block copolymers. 

The 7-modification of iPP (y-iPP) may form in degraded, low molecular weight iPP or in samples crystallized under high pressure [5, 6]. Certain propylene copolymers with low comonomer content (4-10 wt.%) crystallize preferentially in y-form, as well. y-iPP has a face-centred ortho-rombic unit cell with parameters a = 0.85 run, b = 0.993 nm and c = 4.241 nm containing isochiral helices. The cell structure proposed by Bruckner et al. [5] is unique in polymer crystallography the chain axes in adjacent crystal layers are not parallel. The angle between the chain stems is about 80°. y-iPP is not usually observed as an independent phase, but crystallizes with and within the a-spherulites. According to Lotz et al. [7], the positive spherulites observed in samples with mixed polymorphic composition of a- and y-iPP are probably made of a... 

The rate at which these spherulites grow is also affected by tacticity. Measurements were made of the Isothermal rate of spherulitic growth on a hot stage of a microscope. For this purpose, a sequence camera was used to photograph automatically the growing spherulites during crystallization. Dilatometric measurements were also made on these polymers in order to determine the rate of isothermal crystallization from the melt. The fraction of the polymer which had crystallized at any time was calculated from the measured density and the known values of the density of crystalline and amorphous poly(propylene oxide) (8,2,... 

A set of controlled-rheology propylene homopelymers and ethylene-propylene block copelymers have been analyzed. The investigation has been focussed on the influence of the DTBP addition on the supramolecular characteristics as the spherulite size and distributuion... 

Regarding the evolution of the supramolecular properties of the propylenes homopolymers, the average spherulite size seems to enhance and the distribution width seems to reduce, attaing a more unifrom distribution, as the DTBP content increases. The reduction of Mw/ promoted by peroxide addition, leads to the maintenace of the crystalline degree, with the only growth of the spherulite size at the expense of the reduction in the density of tie molecules. However, for very high peroxide content, this trend is not followed because of the presence of pores. 

On unusual feature of the structure-properties relationship is the existence of a number of crystalline forms of PP. This might have remained a scientific curiosity, except that some dyestuffs used for colouring the plastic nucleate the unusual crystalline form, the jS-form, which is associated with type III spherulites, and inferior mechanical properties. Some batches of PP, particularly propylene-ethylene sequential copolymers, have a propensity to form... 

In the study of spherulites formed in poly(ester urethane) multiblock copolymer [51], FTIR imaging was used to reveal valuable information about both the orientation of the polymer chain and the composition of the spherulites. This information was not available from the measurements obtained from atomic force microscopy or polarized light microscopy. A separate study has also demonstrated that linear polarized FTIR imaging is a powerful tool for the investigation of the crystalline and amorphous structures and chain orientation of spherulites of PHB and isotactic poly(propylene oxide) [52]. Recently, a novel multipolarization calculation method has been proposed and applied to obtain FTIR images showing band structure in poly(L-lactic acid) and PHB spherulites with the indication of local molecular chain orientation [53]. 

Merten C, Kowalik T, Asshoff SJ, Hartwig A. FTIR imaging of poly(3-hydroxybutyrate) and isotactic poly(propylene oxide) spherulites. Macromol Chem Phys 2010 211 1627-1631. 

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