Spherulite texture

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. 

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. 

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. 

In situ X-ray examination of crystallizing polyethylene, at high temperature and pressure, then confirmed this proposal in detail, showing that the wide-angle diffraction pattern changed abruptly with the optical texture [ 10]. That corresponding to the spherulitic texture was of the usual orthorhombic form while the new intermediate phase had two-dimensional hexagonal symmetry, with an increased cross-sectional area per chain, but without... 

Fig. 1 The differing optical textures, between crossed polars, of linear polyethylene after crystallization from the melt at pressures close to the triple point, 0.3 GPa (a) the conventional spherulitic texture of the orthorhombic phase (b) the coarse lamellar texture formed as the hexagonal phase then transformed to orthorhombic during return to ambient temperature and pressure from [ 14]...

Segmented thermoplastic elastomers exhibit structural heterogeneity on the molecular, the domain, and in some cases on a larger scale involving periodic or spherulitic texture. Each level of structural organization is studied by specific methods. Molecular sequence distributions can be studied by chemical methods, such as NMR or IR spectroscopy. 

Increasing EPDM content results in irregular spherulitic texture, smaller spherulite size, and loss of sharpness in the spherulite boundaries. 

Fig. 1.9 AFM tapping mode images of a spherulitic texture in isotact polypropylene. The sample was crystallized to completion at 145°C and subjected to permanganic etching prior to examination. Image (a) shows topography while (b) contains phase information. Scale bars 5 iim.

Solid polyethylene obtained directly from the molten state is also made of lamellae. These are set up radially around nucleation centres they form so-called spherulitic textures and the size of a spherulite is of the order of one micron. In this case, the amorphous fraction is larger (from 10 to 20 per cent). 

The molecular weights of the polyamides 45 and 55 were estimated as 25,000 and 67,000, respectively, on the basis of viscoslmetric measurements. Both polyamides displayed high optical activity they were highly hydrophilic and readily soluble in water and In organic solvents, including chloroform. Polyamide 55 was crystalline and yielded resistant films with a spherulitic texture (Scheme 13B). 

Crosspolarized photomicrographs of PEO 1, EO-Is-EO 2, EO-Is-EO 3, and EO-Is-EO 4 films cast from 1% benzene solutions at 30° C are presented in Figure 1. The spherulitic texture with negative birefringence became less perfect and led to a less clear Maltese cross as the fraction of amorphous Is segment increased. When the EO fraction constituted less than 50%, the texture was not clearly resolved by light microscopy. 

When crystallized from the melt, most polymers show a spherulitic texture (Figure 3.2). The spher-ulites then consist of lamellar stacks of alternating crystalline and amorphous layers, radiating from the center (the primary nucleus). 

Figure 3.2. Schematic representation of the spherulitic texture of a semicrystaUine polymer [Hoffman et at., 1976].

When dealing with miscible blends containing two crystalline components, several modes of crystallization are possible separate crystallization, concurrent crystallization, co-crystallization, etc. Only those blends in which both components are miscible in the melt are considered here (Table 3.3). PET/PBT blends were reported to be an example of separate crystallization [Escala and Stein, 1979 Stein et al., 1981]. A spherulitic crystallization was observed for the neat components as well as for blends with small amounts of one component, and the crystals of the minor component were included within the spherulites of the major component, which results in a coarsening of the spherulitic texture. Transesterihcation is, however, the reason for the homogenous amorphous phase. 

The ethylene-a-olefin copolymer chains that are miscible in the molten state also are excluded out from the crystalline region of iPP as described in Section 9.2. Therefore, the applied crystallization condition affects not only the characteristics of crystalline region such as spherulite texture, degree of crystallinity, and the defects in crystals but also the molecular aggregation state of iPP and the copolymers, which will play a central role in controlling the mechanical properties. 

Figure 22 (B) shows a nematic mesophase in poly-61, which was identified in the X-ray analysis as described later. On the other hand, in the case of 62 and poly-63, on cooling scan, 62 exhibits glass transition at —30 °C and isotropization temperature at 43 °C. And, on cooling from the isotropic state to the glass transition state, spherulite textures were ob-sered at 40 °C for 62 and at 32 °C for poly-63, respectively.

Figure 6.3. Spherulitic texture of a thin film of a styrene-ethylene oxide block copolymer (w = 0.40) obtained on quenching to 20°C (Kovacs, 1967). Photomicrograph taken with film between crossed Nicols ( 100 x).

It is well known that PEO spherulites have a Maltese cross extinction pattern and a veiy fine spherulite texture as observed under POM (Ketabi and Lian, 2012 Choi and Kim, 2004 Xi et al., 2005 Choi, 2004) (refer to Figure 9). Meanwhile, under SEM, the image of pure PEO shows waving smooth and uniform stmeture without any phase separation (Johan et al., 2011). 

The advantage of the coarse spherulite texture of melt-cast PHB is that it makes morphological investigations relatively easy. Its disadvantage is that the resulting structure is extremely brittle. Indeed, when pure polymer is crystallized from the melt on a microscope slide, the resulting large spherulites are frequently cracked and little extra... 

The effect of spherulite size on the mechanical properties of PHB is immediately apparent from the comparison of solution-cast and melt-cast films given in Table 5. Both films have spherulitic textures but the comparatively low crystallization temperature of the solution-cast material ensures that its nucleation density is maximized and the spherulite size reduced by two orders of magnitude with respect to the melt-cast sample. Clearly the smaller spherulite size produces major benefits in improved ductility. 

Cast films exhibit a range of morphologies due to the effect of solvents, substrates and orientation. In the case of block copolymers the choice of solvent is quite important to the final structure. Spherulites, a common textural structure observed in crystalline pol5maers, are formed in many industrial processes where the polymer is melted prior to forming the article of interest. Films produced by these industrial processes differ from films used in model studies as the former are usually not thin in the microscopic sense. In true thin films, the spherulitic texture is two dimensional, whereas in these thicker film materials the spherulites are three dimensional. 

Fig. 4.9 Two examples of microtomed sections viewed in the optical microscope are shown (A) a section of a nylon pellet, in polarized light, reveals a coarse spherulitic texture and (B) a section from a black, molded nylon part, in a bright field micrograph, shows the size and distribution of the carbon black filler.

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