Second-order phase transition of high isotactic polypropylene at high temperature

A second-order phase transition of α2 form isotactic polypropylene (iPP) is found at high annealing temperature (T_a = 159.3 °C) by means of X-ray diffraction method. Although the lattice shape and the space group keep the same as those of the α2 form, i.e. monoclinic and P2₁/c, with increase of T_a, it has been revealed that there are discontinuous increases in the slopes of the lattice constants a and b against T_a plots, while the c and the β keep almost constant. As a result, the slope of the unit cell volume V versus T_a plot also shows a discontinuous increase at T_a = 159.3 °C, indicating the occurrence of the second-order phase transition. In order to distinguish the two phases, the phase above the transition temperature is named α2' phase and the transition temperature is denoted T_α2-α2'. These facts suggested that the α2' form is a mobile phase where the molecular chains would become loosely packed and mobile, promoting the better chain sliding diffusion. A fast lamellar thickening process has been confirmed in the higher temperature region than T_α2-α2', which was reported in the precedent paper. General significance is proposed that mobile phases possibly exist at high temperature, close to the melting temperature and accelerate lamellar thickening, which improves physical properties of polymers.