In-situ investigation on the structural evolution of mesomorphic isotactic polypropylene in a continuous heating process

•Microstructural variation of mesomorphic iPP in the heating process was revealed.•Microstructural change in the temperature range from 20 and 60 °C was detected.•Plausible structural evolution mechanism in the meso-α transformation was proposed.

The microstructural changes of mesomorphic iPP from mesophase to α transition at a molecular level in a continuous heating process have been studied by in-situ Fourier-transform infrared (FT-IR) spectroscopy, in-situ X-ray scattering using synchrotron radiation, differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Microstructural change corresponding to helical conformation variation in the temperature range between 20 and 60 °C was detected by IR spectroscopy, which may originate from the glass transition of rigid amorphous fraction (RAF). The helical sequence with 12 monomer units is found to exist in RAF. The contents of helical sequences with different number of monomers exhibit different variation trends in the course of meso-α transition and the following process of partial melting and perfection of α crystal. A plausible mechanism was proposed that RAF experiences glass transition firstly at low temperature, and then serve as α nuclei to trigger the meso-α transition at higher temperature. This work provides a new insight into the mechanism of microstructural evolution of the meso-α transformation of iPP.

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