Glassy Alfa-Relaxation Promotes Surprising Homo-Crystal Nucleation in the Low-Molar-Mass Enantiomeric Poly(lactic acid) Blend
Glassy Alfa-Relaxation Promotes Surprising Homo-Crystal Nucleation in the Low-Molar-Mass Enantiomeric Poly(lactic acid) Blend
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玻璃态阿尔法弛豫促进低摩尔质量对映体聚乳酸混合物中令人惊讶的同晶成核
DOI：
10.1021/acs.macromol.2c00679
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发表时间：
2022-06
期刊：
影响因子：
5.5
通讯作者：
Wenbing Hu
中科院分区：
文献类型：
--
作者：
Yucheng He;Dan Liu;Kefeng Xie;Wenqing Xu;Pengju Pan;Wenbing Hu
低摩尔质量的对映体聚乳酸（PLA）共混物通常收获的立体复合物晶体，持有交替包装的对映体聚合物有利于更多的分子间成核比分子内成核。聚合物玻璃态的结构弛豫往往促进高温结晶的早期成核。通过快速扫描差示扫描量热法测量，我们首先确定在对称对映体PLA共混物（20 kDa，L/D = 7：3）的玻璃态中，焓松弛在低于35 ° C时由β松弛主导，在高于35 °C时由α松弛主导。然后，我们采用两阶段塔曼分析在等温退火300 s期间在30 °C与40 °C相比，晶体成核，随后在各种高温下等温结晶。高温结晶的结果表明，与没有预先退火的平行情况相比，在30 °C下具有β-弛豫的退火促进了立体复合物晶体成核，这可能是因为只有非常小的立体复合物晶核由于其相对高的熔点而可以存活。相比之下，在40 °C下具有α-弛豫的退火令人惊讶地促进了低于120 °C的高温结晶的均质晶体成核，这可能是因为在这些情况下相对大规模但有限的分子合作比立体复合物晶体的分子间边缘胶束成核更有利于均质晶体的分子内链折叠成核。在120 °C及以上的温度下，同质晶核不能存活，因此剩余的立体复合晶核再次占主导地位用于高温结晶。我们的观察表明，在聚合物晶体成核的早期阶段，分子内晶体成核和分子间晶体成核之间存在着强烈的竞争。
Low-molar-mass enantiomeric poly(lactic acid) (PLA) blends commonly harvest stereo-complex crystals that hold alternatingly packed enantiomeric polymers favoring more intermolecular nucleation than intramolecular nucleation. Structural relaxation in polymer glassy states often promotes early-stage crystal nucleation for the high-temperature crystallization. By means of fast-scanning differential scanning calorimetry measurements, we first identified enthalpy relaxation as dominated by beta-relaxation below 35 °C and by alfa-relaxation above 35 °C in the glassy states of the symmetric enantiomeric PLA blends (20 kDa, L/D = 7:3). We then employed the two-stage Tammann analysis on crystal nucleation during isothermal annealing for 300 s at 30 °C in comparison to 40 °C, followed with isothermal crystallization at various high temperatures. The results of high-temperature crystallization showed that in comparison to the parallel cases without prior annealing, the annealing with beta-relaxation at 30 °C promotes stereo-complex crystal nucleation, probably because only very small stereo-complex crystal nuclei could survive due to their relatively high melting points. In contrast, the annealing with alfa-relaxation at 40 °C surprisingly promotes homo-crystal nucleation for the high-temperature crystallization below 120 °C, probably because the relatively large-scale but limited molecular cooperation under these circumstances favors intramolecular chain-folding nucleation of homo-crystals more than intermolecular fringed-micelle nucleation of stereo-complex crystals. The homo-crystal nuclei could not survive at a temperature of 120 °C and above and hence leave the rest stereo-complex crystal nuclei as dominant again for the high-temperature crystallization. Our observations revealed a strong competition between intermolecular crystal nucleation and intramolecular crystal nucleation at the early stage of polymer crystal nucleation.
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