Flow-Induced Precursor Formation of Poly(L-lactic acid) under Pressure
Flow-Induced Precursor Formation of Poly(L-lactic acid) under Pressure
复制标题
压力下流动诱导聚（L-乳酸）前体的形成
DOI：
10.1021/acsomega.8b02425
复制
发表时间：
2018
期刊：
影响因子：
4.1
通讯作者：
Li Zhong-Ming
中科院分区：
文献类型：
--
作者：
Song Ying-Nan;Ru Jia-Feng;Xu Jia-Zhuang;Lei Jun;Xu Ling;Li Zhong-Ming
首次利用自制的加压剪切装置，探讨了压力场和流场两个不可避免的加工场对半刚性分子链聚合物聚l-乳酸结晶的影响。结果表明，剪切速率有利于前驱体的生成，因为它诱导了定向段的形成。结果表明，在100 MPa下，生成PLLA前驱体的最敏感剪切温度为180℃。当剪切温度较高（如190℃）时，剪切诱导取向段的弛豫过快，无法诱导PLLA前驱体的生成。相反，在较低的剪切温度（170℃）下，定向段在整个剪切速率范围（3.1 ~ 31.4 s-1）内难以松弛。退火处理促进了取向段的弛豫，有利于PLLA前驱体的形成。与剪切和退火不同，压力在PLLA前驱体的形成过程中起着更为复杂的作用。压力降低了PLLA分子链之间的自由体积，同时增加了PLLA熔体的过冷性。此外，为了适应加压状态，PLLA链倾向于形成局部定向的段束，这有利于PLLA前驱体的形成和后续的结晶过程。这两个因素降低了PLLA链的可动性，抑制了链的弛豫，因此剪切诱导取向有利于压力下PLLA前驱体的形成。在这种情况下，压力和剪切流动对PLLA前驱体的生成和随后的结晶过程表现出协同促进作用。这些有意义的结果有助于理解结晶条件与PLLA结晶行为之间的关系，从而为通过控制PLLA的结晶过程来制造最终产品提供指导。
For the first time, the influences of two inevitable processing fields (pressure and flow fields) on the crystallization of a semirigid molecular chain polymer, that is, poly(l-lactic acid) (PLLA), were explored using a homemade pressuring and shearing device. The results reveal that the shear rate facilitated the generation of precursor because it induced oriented segment formation. It was found that the most sensitive shear temperature for the generation of PLLA precursor under 100 MPa was 180 °C. When the shear temperature was higher (e.g., 190 °C), the relaxation of shear-induced oriented segments was too quick to induce the generation of PLLA precursor. Oppositely, at a lower shear temperature (170 °C), the oriented segments were hard to relax within the whole shear rate range (3.1–31.4 s–1). Annealing treatment was infaust to the PLLA precursor formation because it promoted the relaxation of oriented segments. Different from the shear and annealing, pressure played a more complicated role in the formation of PLLA precursor. Pressure decreased the free volume between PLLA molecular chains and meantime increased the supercooling of PLLA melt. In addition, PLLA chains tended to form locally oriented segment bundles to adapt to the pressurized state, which facilitated the formation of PLLA precursor and the following crystallization process. These two factors lowered the movability of PLLA chains and suppressed the relaxation of chain, so shear-induced orientation facilitated PLLA precursor formation under pressure. In that case, pressure and shear flow showed a synergetic promoting effect on the generation of PLLA precursor and the following crystallization process. These meaningful results could be helpful for comprehending the relationship between crystallization conditions and the crystallization behavior of PLLA and thus would provide guidance to fabricating the final products through controlling the crystallization process of PLLA.