Molecular Structure
The primary structural unit of polyacrylonitrile (PAN) fiber is acrylonitrile. Acrylonitrile copolymers are linear carbon-chain polymers with side groups, where the distribution of comonomers along the molecular chain is random. The acrylonitrile units are primarily linked in a head-to-tail fashion, with a methylene group separating the carbon atoms attached to the nitrile groups. The PAN macromolecule features a carbon-chain structure with good chemical stability; it exhibits high structural regularity and a compact molecular configuration. The cyano group (–C≡N) within the chain is a strongly polar group; the highly uneven distribution of electron cloud density between the carbon and nitrogen atoms facilitates the formation of hydrogen-bond-like interactions and dipole-dipole bonds between macromolecules.
PAN macromolecules are held together by van der Waals forces, hydrogen-bond-like interactions, and dipole interactions, resulting in strong intermolecular forces. Cyano groups on the same macromolecule repel each other due to identical polarity, while those on adjacent macromolecules attract each other due to opposing polarity; this causes the molecular chain to adopt a helical conformation. The incorporation of second and third monomers further disrupts this conformation, making it difficult for the PAN backbone to form a highly regular structure.
Supramolecular Structure
Due to interactions between the strongly polar cyano groups, the PAN molecular chain adopts a helical conformation characterized by irregular kinks and twists. The packing of these irregular helical chains within the fiber results in a structure that is laterally two-dimensionally ordered but longitudinally disordered-a state known as a paracrystalline structure. Based on the degree of macromolecular alignment regularity, the supramolecular structure is categorized into regions of high, medium, and low lateral order; the high-order regions are referred to as paracrystalline or mesomorphic zones.
