ABSTRACT
When the polymer has a flexible chain structure, this polymer could develop into highly ordered crystal structures. However, for polymer materials, it is very difficult to achieve 100% crystallinity. Thus, semi-crystalline polymer is usually used to name this group of polymer materials. Several factors hinder the formation of crystal phase in polymers, including double/triple bonds, bulky groups (such as phenyl groups), highly polar molecular structures, etc. These structures are able to stiffen the molecular chain structures and create obstacles for polymer chains to fold into crystal lamellae in polymer materials.The effects of crystal structures on material properties are more complicated than that of molecular weight. Factors including crystallinity, crystal phase and morphologies are all contributing factors to the properties of polymer materials. In general, the
increase in crystallinity could improve the strength and modulus of polymer materials, and enhance barrier properties, while ductility is reduced. Meanwhile, different crystal phases also possess different mechanical performances. For example, β-crystal in polypropylene is considered as a tougher phase compared with a-crystal in polypropylene, thus many attempts have been made to grow more β-crystal in polypropylene for better toughness.Regarding dielectric performances, the effects of crystal structures are more significant. Poly(vinylidene fluoride) (PVDF) is the best example. PVDF has several different crystal phases (a, β, g, δ) showing different polarizability in electric field. While it is the most common and stable crystal phase in PVDF, a-phase is less polar compared with other crystal phases. Thus, in terms of dielectric and piezoelectric performances, a-phase is not as desirable as other polar phases. The creation of different crystal phases could be achieved via controlling processing conditions, such as stretching and recrystallization [2-8], as well as using nanomaterials. More discussion on PVDF will be done in Chapter 5.
