A-spherulites

Figure 4.13 Schematic illustration of the leading edge of a lathlike crystal within a spherulite.

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

Figure 7 Scheme of the internal structure of a spherulite. Reproduced from Mercier, Zambelli and Kurz [1], Elsevier (2002) with permission of Elsevier. Copyright Elsevier 2002. 

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.)...

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... 

In addition to the main types described in Table 2, crystals may form treelike patterns known as dendrites, with such aggregates being termed dendritic or arboraceous. If an aggregate is composed of tiny crystals radiating from a center, it is termed a spherulite or rosette. 

The self supporting wafers of zeolites (3 to 4 mg.cm ) presented an i.r. absorption continuum depending on the particle size. The most intense absorptions were obtained for the samples 5 and 6 with particle size of ca. 6 pm and a spherulitic shape. 

Fig. 8 a Spherulitic growth rates for PPDX and the PPDX block within D7732C2310 diblock copolymer. Solid lines are fits to Lauritzen and Hoffman theory, b Lauritzen and Hoffman kinetics theory plot for PPDX (K = 17.2 x 104 K2) and the PPDX block within D7732C2310 diblock copolymer (K = 46 x 104 K2). (From [103]. Reproduced with permission of the Royal Society of Chemistry)... 

Figure 4.9 Illustration of a spherulite growing into a melt...

The linewidth-temperature relation of the polyethylene oxide samples are given in Fig. 5. Despite the large differences in molecular weight, these samples have about the same linewidth, 300-350 Hz, in the crystalline state at 25°C. They all also possess a spherulitic type of morphology. The influence on the linewidth of the different types of supermolecular structures... 

Figure 3. Plot of linewidth, against temperature at 67.9 MHz for two linear PE samples having a spherulitic morphology but differing levels of crystallinity degree of cryHallinity 0.78, (O) degree of crystallinity 0.50, (9).

FIGURE 2.15 Structure of a spherulite from the bulk, (b) A slice of a simple spherulite. As further growth occurs, filling in, branch points, etc. occur as shown in (a). The contour lines are simply the hairpin turning points for the folded chains. 

Fig. 1-8 Structural organization within a spherulite in melt-crystallized polymer.

A methoxylated polyamide 78 analogous to Nylon 6 was obtained in several steps fi om D-glucose [61, 63] through the preparation of a dimeric active ester of 6-amino-6-deoxy-2,3,4,5-tetra-(9-methyl-D-gluconic acid (49). This polyamide was highly crystalline, and gave resistant films with a spherulitic texture. 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

Fig. 9. Drawing of a 2D projection of a spherulite formed from radially grown lamellae with a small degree of branching...

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. 

Our findings in regard to the effects of deformation on PTFE accord in some respects with observations on the deformation of polyethylene " and polypropylene, although PE and PP usually have a spherulitic morphology before drawing. 

Price and Wendorff31 > and Jabarin and Stein 32) analyzed the solidification of cholesteryl myristate. Under equilibrium conditions it changes at 357.2 K from the isotropic to the cholesteric mesophase and at 352.9 K to the smectic mesophase (see Sect. 5.1.1). At 346.8 K the smectic liquid crystal crystallized to the fully ordered crystal. Dilatometry resulted in Avrami exponents of 2, 2, and 4 for the respective transitions. The cholesteric liquid crystal has a second transition right after the relatively quick formation of a turbid homeotropic state from the isotropic melt. It aggregates without volume change to a spherulitic texture. This process was studied by microscopy32) between 343 and 355.2 K and revealed another nucleation controlled process with an Avrami exponent of 3. 

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. 

Blends of polyphenylenevinylene with water-soluble polymers have been prepared by mixing solutions of the sulfonium precursor with polyethyleneoxide, hydroxy-propylcellulose and polyvinylmethylether317). Polyethyleneoxide forms spherulites which impose a spherulitic texture to the polyphenylenevinylene that is retained after transformation. As a result of this open network, high conductivities are reached at only 10% conducting polymer. 

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