Spectroscopy Of Strain-Induced Mesophase Of PLLA Condensed Structure And Phase Transformation Behavior

In this paper, spectroscopic analysis on cold drawing-induced PLLA mesophaseand mechanism of phase transition were investigated by in suit infrared spectroscopy（FTIR）, wide-angle X-ray diffraction （WAXD） and Differential scanning calorimetry（DSC）.In the present work, special attention has been given to understand themesophase of PLLA that is stretched around Tg by combining WAXD data and thepolarized IR spectral analysis. It is found that, the characteristic backbone vibrationmode (located around918-922cm^-1) shifts to higher wavenumber, but WAXD datashows that the interchain packing of ordering helical chains becomes more compact.Therefore, it proves that as increasing the drawing ratio, the conformation andinterchain packing of mesophase become more ordering simultaneously. CombiningWAXD data and the polarized IR spectral analysis, it is found that the characteristicpeak of PLLA around918cm^-1and interchain packing are linear correlation.Meanwhile, the polarized IR spectral reveals that helical chain orientation ofmesophase in the strain induced ordering domain is always kept perfectly in parallelto the drawing direction, whereas that in the amorphous region increases slightlybefore the formation of10/3helical chain.Through DSC experiment, we observe the melt of mesophase that there is anendothermic peak existed around Tg. When drawing ratio λ changes from1to3,there is only mesophose in PLLA, when λ=4, mesophase coexist with crystal phase,when λ=5, there is only crystal phase by strain-induced instead of mesophase. Andthe crystallization temperature and the cold crystallization rate will improve withincreasing the drawn ratio （λ）.By in situ infrared spectroscopy, it is found that the band of918cm^-1thatrelated to mesophase shifts to921cm^-1that related to crystallization in the meltedprocess of mesophase. This observation demonstrates that the melt of mesophaseaccompany by the formation of ordered helical. And in the process phasetransformation, it is found that the decreasing of band of956cm^-1that related to amorphous phase changes prior to918cm^-1that related to mesophase. It suggeststhat the orientation relaxation of amorphous chains is prior to the melt of mesophase.Through the research above, the correlation between the micro structure ofmesophase and spectrum has been established, and it will also help us understand themesophase of PLLA condensed structure.