Extensive Development of Precursory Helical Pairs Prior to Formation of Stereocomplex Crystals in Racemic Polylactide Melt Mixture

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• 作者：Ching-Feng Yang ; Yi-Fang Huang ; Jrjeng Ruan ; An-Chung Su
• 刊名：Macromolecules
• 出版年：2012
• 出版时间：January 24, 2012
• 年：2012
• 卷：45
• 期：2
• 页码：872-878
• 全文大小：504K
• 年卷期：v.45,no.2(January 24, 2012)
• ISSN：1520-5835

文摘

Melt crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 掳C/min from 270 掳C) was studied via a combination of wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (3₂/3₁) helical pairs below 220 掳C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 掳C; the intensity of this mesophase peak started to decrease at 150 掳C, with concomitant emergence of WAXS- or DSC-discernible formation of stereocomplex (尾_c) crystals. Isothermal measurements at 200 vs 170 掳C revealed the presence of low vs high populations of helical pairs; 尾_c crystals were observed to develop only at 170 掳C but not at 200 掳C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of 尾_c nuclei. The clear change in the melt structure well before the formation of incipient 尾_c crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of 尾_c crystals.