The Hexagonal Mesophase in Atactic Polyacrylonitrile: A New Interpretation of the Phase Transitions in the Polymer

When unoriented polyacrylonitrile (PAN) is heated from room temperature, two glass transitions are observed at ∼100°C and ∼150°C. Further heating at moderate heating rates, such as 10°C/min, a degradation exotherm is seen at ∼340°C. But if a very fast heating rate (100°C/min) is used, a melting endotherm can be seen at ∼320°C before the degradation exotherm. When highly oriented PAN is heated, only a single glass transition is seen at ∼100°C, whereas the melting/degradation still occurs at ∼340°C. The X-ray fiber pattern of oriented PAN at room temperature shows just two equatorial peaks (d1 = 0.525 nm and d2 = 0.304 nm); any meridional or off-axis scattering is very weak and diffuse. This diffraction pattern remains essentially unchanged until more than 250°C, in the sense that different peaks do not appear or existing peaks disappear. Another surprising and important feature is the existence of a thermoplastic state well below the melting point, which allows the material to be molded into a transparent sheet. Here, a new phase scheme is proposed to interpret the experimental observations on the thermal behaviour of PAN. There are two key ideas: first, there is a thermodynamically stable hexagonal mesophase in PAN; second, unoriented PAN has a two-phase morphology, whereas oriented PAN tends toward a single-phase material. In unoriented samples, there are mesophase and amorphous domains that are both in a glassy state at room temperature. At ∼100°C, the mesophase glass transforms to the mesophase melt, whereas the amorphous domains become rubbery at about ∼150°C. In PAN with a single-phase morphology (highly oriented and/or thermally annealed polymer), the amorphous phase is a minor component and hence only a single glass transition at ∼100°C is observed from the mesophase glass to mesophase melt transformation. The differential scanning calorimeter endotherm at ∼320°C must be interpreted as the mesophase isotropization temperature. On cooling from the mesophase, PAN does not crystallize because of the lack of stereoregularity, and only a mesophase glass is formed. The mesophase has not been detected in the past because of its extreme viscosity, which means the material does not flow easily.