Spherulitic superstructures

Figure 9.22. Scanning force microscopy images of polyethylene films formed on a model planar chromium polymerization catalyst. The small white stripes are lamellar crystals. These form the well-known spherulite superstructure upon crystallization from the...

Polytetrafluoroethylene (PTFE) is an attractive model substance for understanding the relationships between structure and properties among crystalline polymers. The crystallinity of PTFE (based on X-ray data) can be controlled by solidification and heat treatments. The crystals are large and one is relieved of the complexity of a spherulitic superstructure because, with rare exceptions, spherulites are absent from PTFE. What is present are lamellar crystals (XL) and a noncrystalline phase (NXL) both of which have important effects on mechanical behavior. 

Adamski and Klimczyk analyzed cholesteryl pelargonate36) and caproate 37) liquid crystal to fully-ordered-crystal transitions over a temperature range of about 25 K. Again, the appearance of the fully ordered crystals was that of a spherulitic superstructure. The nucleation was time dependent, and the linear growth rate of the spherulites decreased with decreasing temperature by a factor 1/2 to 1/3, in contrast to the nonanoate and acetate. The Avrami exponent was close to 4 as judged from the measurement of the crystallized volume in the field of view under the microscope. 

In addition to microphase structures, MDI/BDO-based polyurethane systems have exhibited spherulitic superstructure. Characterization of the birefringence of the spherulites was used to determine the orientation of the hard-segment domains (7). However, because of the sensi-... 

Spherulitic superstructures were formed at low shear strains and an oriented, row-nucleated stacked lamellar texture developed with increasing shear strain. At higher recoverable shear strain values, a fine surface roughness developed due to high melt elastic instabilities. 

During elongation of a semicrystalline polymer several different processes, sudi as elastic deformation of the original spherulitic superstructure, tran ormation of a spherulitic into a fibrillar structure, plastic deformation of microfibrils by sliiqiage processes and elongation of molecular chains in the amorphous regions may occur simultaneously, successively or partly superimposed One of the advanta s... 

FIGURE 9.4 Polypropylene crystallized in the monoclinic a-form consisting of parallel-aligned lamellae (a) reveals a characteristic peak at 0 = 9.3° in its WAXS pattern (b) the lamellae will aggregate and form spherulitic superstructures as shown in a Scanning Force Microscopy phase image (c). 

Under certain conditions, the lamellae of the TPEEs organize into a spherulitic structure, which is characteristic structure of the common semi-crystalline polymers. Based on the results of different methods, it was concluded that crystallization occurs by chain folding through which a spherulitic superstructure is formed. A well developed spherulitic crystalline superstructures, with diameters of about 5-20 pm, can be formed depending on the crystallization conditions [33]. Also, the soft, amorphous phase is embedded between radial crystalline fibrils of the hard segment spherulites. Under some conditions, other structures such as dendrites are developed. 

Dried films of PEO and the blends were heated to 100 C (for 3 miiL) in a hot stage to erase previous thermal history, then rapidly transferred to a second hot stage set at the desired crystallization temperature. The development of die spherulitic superstructure was dien viewed with an Olympus BHSP-300 microscope and the crystallization event recorded widi a video camera and VCR. In the case of non-linear growth, spherulite growth rates (G) were determined at the onset of growth. Average growth rates were determined on from I - 5 spherulites. 

An extruded polypropylene film is stretched first in the machine direction, followed by a second stretching in the transverse direction. The resulting material shows highly orientated crystallites and no, or very little, spherulitic superstructure, which results in the film having both a higher Young s modulus and transparency. 

The nucleation, growth, and kinetics of development of the spherulitic superstructural aggregates are of fundamental and practical interest, and it is the main topic of this chapter. 

For immiscible blends with crystaUizable matrix and amorphous dispersed phase, the crystal growth and the final morphology can be significantly influenced by the noncrystallizable component, with large changes in the primary nucleation density (number of nuclei per unit volume of crystallisable polymer), as well as in the size, shape, size distribution, texture, and crystallinity degree of the spherulitic superstructures. 

Figure 11.3 Morphology map for nonisothermally crystallized ethylene copolymers, drawn schematically based on experimental data for fractions of linear low density polyethylene and hydrogenated polybutadienes. Spherulitic superstructure is observed within the domeshaped region defined by the three axes eopolymer molecular weight (M), branch content, and nominal crystallization temperature. Reprinted with permission from Reference [23]. Copyright 1981 American Chemical Society.

In addition to counit concentration, the size of the branches also affects the degree of spherulite organization. It was observed that as the alkyl side chains become longer, they exert a greater influence on the interfacial structure of the crystallites. This in turn lowers the organization of the lamellar stacks and leads to less well-developed spherulitic superstructures [21]. 

Similar crystallization behavior, where the ordered melt structure was destroyed upon crystallization and replaced by alternating lamellae with a spherulitic superstructure was also observed in diblock copolymers of poly(ethylene oxide-fe-(l,2-butylene oxide)) [109], poly(ethylene oxide-6-ethylethylene) [110],poly(ethylene oxide-6-isoprene) [111], poly(ethylene-6-(head-to-head propylene)) [112],poly(ethylene-6-ethylethylene) [113] and poly(ethylene-6-(ethylene-a/f-propylene)) [113], and a triblock copolymer of poly(ethylene oxide-6-styrene-6-ethylene oxide) [114]. In all instances, the solid-state morphology was characterized by a semicrystalline lamellar structure regardless of the initial melt morphology. 

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