Liquid Silicone Rubber Quality Assurance Library

Orientation and Crystallinity of Liquid Silicone Rubber

As has been explained several times in previous blogs, polymers like silicones are macromolecular structures which are created through polymerization processes such as addition (chain growth) or condensation (radical initiated) polymerization [1]. Those chains are held together through van der Waals forces. The lengths of the different chains are not the same, so it is possible to find very short and very long chains in the material. When the length of a group of chains is short, its arrangement is random; that is, the zone is not organized and it can be said that the zone is amorphous. However, when the chain length is long enough, organized zones can be created called crystalline zones. The process of obtaining those organized structures in polymers is not easy or simple, and it always depends on the time and temperature. First, the chains fold together to create an ordered region called lamellae, which are fine layers of the chains form the structure. When a group of lamellae is big enough, another morphological structure is created: a spherulite. This structure can be seen by optical microscopy and it directly affects the mechanical properties of the material. Also, another special crystalline arrangement is found at the same level as the spherulites. Shish-Kebab structures are formed by crystals in the shape of circular plates and whiskers. These appear when a shear deformation occurs during solidification. With that, the arrangement of the polymer structure, called the morphology, in a conventional polymer can include a highly crystalline structure found just next to an amorphous zone. Figure 1 depicts the possible structures in a polymer.