Study on the stretch induced phase transition of α trans-1,4-polyisoprene by in-situ SAXS and WAXS measurements

Trans-1,4-polyisoprene (trans-PI) is an important industrial polymer, which exhibits pronounced polymorphisms. However, understanding of the crystal form transition of trans-PI is rather limited. In the present work, the stretch-induced crystal form transition of α trans-PI at room temperature was investigated in detail. It was found that the β crystals with high molecular chain orientation appeared at strain = 0.,42 and the total crystallinity increased at the same time. Meanwhile, the average long spacing (lac), thickness of lamellae (lc), and amorphous layers (la) along the stretching direction had undergone multiple evolution processes during stretching. Results also showed that part of the amorphous phase and the recrystallization process of α crystals both contributed to the formation of the β form. Based on the combination analysis of the in-situ SAXS (small angle X-ray scattering), WAXS (wide angle X-ray scattering), differential scanning electron microscopy (DSC), and Fourier transform infrared spectroscopy (FTIR), it was proved that the β mesophase with layered structure emerged before the formation of β crystals. Finally, a possible multistage evolution mechanism was proposed to interpret the stretch-induced crystal form transition of α trans-PI.