Optical Properties of Spherulites

Another manifestation of optical anisotropy is light scattering (Section 1.5.3) the cloudy appearance of most thermoplastic films and molded pieces results from the scattering of incident light by spherulites. Quite generally, light scattering is caused by fluctuations 

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Spherulites show an imperfect crystalline structure, since the melting point of the spherulite usually lies considerably below the thermodynamic melting point (see Chapter 10). Even then, a further increase in X-ray crystallinity can also be observed when the spherulites have filled the volume. Localized orientation of the crystalline region leads to the characteristic optical properties of spherulites. If spherulites are cross-linked by radiation, the identity of the individual spherulite is retained even after they have been heated above the melting point. The birefringence of oriented... 

Figure 3.2 Maltese extinction cross in spherulites and the optical properties of spherulites. (A) Direction of slow vibration is radial (B) direction of fast vibration is radial.

Another side benefit that accompanies with the use of certain nucleants is improved clarity. Since clarity or transparency is evidently related to the crystalline structure of the polymer and the structure is determined by the conditions of crystallization, parameters characterizing crystallization must be also connected with the optical properties of a PP product. The peak temperature of crystallization (Tc) is one of the quantities often used for the characterization of the crystallization process and efficiency of nucleating agents. With increased crystallization temperature, the thickness of the lamellae increases well. Higher efficiency and concentration of nucleating agent lead to an increase of Tc (as determined by DSC) and decrease of the size of the spherulites. 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial application in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of spherulites by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find applications in packaging and barrier resins markets (191). 

Applications. Optical microscopy finds several important applications in filled systems, including observation of crystallization and formation of spherulites and phase morphology of polymer blends. " In the first case, important information can be obtained on the effect of filler on matrix crystallization. In polymer blends, fillers may affect phase separation or may be preferentially located in one phase, affecting many physical properties such as conductivity (both thermal and electrical) and mechanical performance. 

Abstract. Structural properties of rubrene thin films on cleaved mica (001) surfaces were investigated by optical microscopy and x-ray diffraction. Optical microscopy shows, that the crystallization of rubrene results in formation of spherulites. X-ray specular diffraction reveals polycrystalline and polymorphic nature of rubrene. The pole figure measurements of films prepared at low deposition rates reveal orthorhombic structure and indicate fiber textures with crystallographic planes (121), (131) and (141) preferentially oriented parallel to the substrate surface. High deposition rate thin films in addition show polymorphism, corroborating the existence of the orthorhombic and the triclinic phase. 

In crystallization from the melt, polycrystalline regions sometimes occur which are called spherulites because of their spherical form and optical properties. Microtome sections show that their internal structure is radially symmetric. Circular structures of similar internal construction occur in the crystallization of thin films (Figure 5-23). They are therefore likewise termed spherulites, since they can be considered as cross sections of bulk-crystallized spherulites. 

The differences in the speed of the light result from differences in the refractive index. If the highest refractive index is in the radial direction, one talks of positive spherulites. Negative spherulites show the highest refractive index in the tangential direction. Thus, information about the microstructure of the spherulites can be gained from their optical properties. 

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