Spherulites optical properties

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. 

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. 

Transparency of PP products can be efficiently improved both with traditional nucleating agents and clarifiers, which are various sorbitol derivatives. The improvement in the optical properties depends on the efficiency in destroying the spherulitic structure of PP or at least decreasing the size of the spherulites below a critical value. Strong nucleation... 

Occasionally, the positive and negative spherulites can coexist in one sample. For example, syndiotactic polystyrene and its blends with atactic polystyrene show the coexistence of the spheruhtes of the opposite optical properties when cooled from the melt [25]. 

Methods used to compare and contrast the crystallization behavior of water soluble crystalline polymers with dispersed silicates may include cross-polarization optical microscopy (CPOM) or atomic force microscopy (AFM), depending upon physical properties of the materials such as spherulite size and optical properties. Other methods used to study crystallization behavior of such materials include differential scanning calorimetry (DSC) and x-ray diffraction (XRD). 

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). 

Copolymerization also affects morphology under other crystallization conditions. Copolymers in the form of cast or molded sheets are much more transparent because of the small spherulite size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallization, films are soft and rubbery, with low modulus and high elongation. After crystallization, they are leathery and tough, with higher modulus and lower elongation. 

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