The phase transition behavior of crystalline region in at-PAN fiber during heat treatment was investigated based on the temperature-dependence measurements of in situ synchrotron wide-angle X-ray diffraction (WAXD)

DSC and FTIR.According to the results of WAXD

the position of diffraction arc for (110) planes was shifted to low q value with the increasing of temperature.However

there was an obviously mutation at 110℃ during this shifting.At this temperature

the linear expansion rate of crystalline region increased from 4.1510-4 nm/K (1) to 1.1310-3nm/K (2)

and the DSC curve also emerged an exothermal peak around 110℃

which results from the phase transition of crystalline region.From the FTIR measurements

the relative intensity of 1230 cm-1and 1250 cm-1 bands

which stands for the 3/1 helix and planar zigzag conformation respectively

was also changed during heat treatment.Through the relationship between C=I1250/I1230 and heat treatment temperature

there was also a dramatical change at 110℃

where C was decreased obviously.It implies that the molecular chains in at-PAN fiber were more likely to take the planar zigzag conformation below 110℃ and transferred to 3/1 helix conformation above 110℃.So it can be concluded that the phase transition of the at-PAN fiber at 110℃ was originated from the transformation of planar zigzag to 3/1 helix in chain conformation.