振荡剪切对聚乳酸熔体状态及结晶行为的影响

发布时间：2018-05-24 22:22

本文选题：聚乳酸 + 熔体状态　；参考：《郑州大学》2017年硕士论文

【摘要】：结晶性聚合物制品的性能与加工过程中所形成晶体的结构与形态密切相关。研究加工过程中外场作用下熔体状态的演变及结晶行为对于调控聚合物制品的聚集态结构、改善制品性能具有重要意义。与稳态剪切相比,大幅度振荡剪切能够诱导聚合物熔体形成更为丰富的结构,并且可以利用小幅度振荡剪切表征聚合物后续的整个结晶过程,有助于研究流动对晶体生长的影响机制,一直是聚合物加工中的研究热点。本文主要研究振荡剪切条件对聚乳酸(PLA)熔体状态的改变以及具有不同熔体结构的PLA的结晶行为,主要工作如下:1.首先研究了实验用原料PLA(4032D,Nature Works)的基本流变性质,计算了不同温度下的零剪切粘度及松弛时间,拟合出了PLA的非牛顿指数n和黏流活化能Ea,计算了PLA的缠结分子量Me。2.通过研究不同大幅度振荡剪切过程中及剪切后熔体模量随时间的变化,分析了振荡剪切条件(温度、应变、周期、频率)对PLA熔体状态的影响,主要结论如下:(1)PLA熔体的线性粘弹区间随温度的升高而增大;(2)剪切过程中储能模量下降先快后慢最后趋于稳定;(3)储能模量的相对变化随剪切温度的升高而减小,且高温下恢复较快;(4)储能模量的相对变化随剪切应变的增加而增大,高应变下熔体在更低的水平下达到平衡状态;(5)剪切前期,储能模量的相对变化随频率的增大而增加,但最终的储能模量水平差别不是很大。3.通过将不同大幅度振荡剪切后的PLA熔体降到结晶温度,表征相对储能模量随时间的变化,研究了不同状态PLA熔体的结晶行为,主要结论如下:(1)振荡剪切对PLA结晶的促进作用随剪切温度的升高而减小;(2)当剪切应变大于临界剪切应变时,振荡剪切才能够对PLA的结晶行为起到明显的促进作用;(3)振荡剪切对PLA结晶动力学的促进作用随剪切周期的增加而增强;(4)在一定剪切周期范围内,频率能够对PLA的结晶行为造成差异;(5)振荡剪切对PLA结晶的促进作用随松弛时间的延长而减弱。
[Abstract]:The properties of crystalline polymer products are closely related to the structure and morphology of crystals formed during processing. It is important to study the evolution of melt state and crystallization behavior in the process of processing in order to regulate the aggregate structure of polymer products and improve the properties of polymer products. Compared with steady state shear, large amplitude oscillating shear can induce polymer melt to form richer structure, and can be used to characterize the whole subsequent crystallization process of polymer by using small amplitude oscillating shear. It is helpful to study the influence mechanism of flow on crystal growth and has been a hot topic in polymer processing. In this paper, the effects of oscillatory shear conditions on the melt state of polylactic acid (PLA) and the crystallization behavior of PLA with different melt structures are studied. The main work is as follows: 1. The basic rheological properties of the experimental raw material PLA 4032D PLA were studied. The zero shear viscosity and relaxation time at different temperatures were calculated. The non-Newtonian exponent n and viscous flow activation energy EA of PLA were fitted, and the entangled molecular weight of PLA was calculated. The effects of oscillatory shear conditions (temperature, strain, period, frequency) on the melt state of PLA were analyzed by studying the variation of melt modulus with time during and after shearing. The main conclusions are as follows: the linear viscoelastic region of the PLA melt increases with the increase of temperature. During the shear process, the energy storage modulus decreases first, then slowly, and then tends to stabilize. The relative variation of the storage modulus decreases with the increase of shear temperature. And the relative variation of storage modulus increases with the increase of shear strain at high temperature, and the relative change of storage modulus increases with the increase of frequency when the melt reaches the equilibrium state at a lower level under high strain. But the difference of storage modulus level is not very big. The crystallization behavior of PLA melts in different states was studied by lowering the crystallization temperature of PLA melts to the crystallization temperature after different amplitude oscillation shear to characterize the change of relative storage modulus with time. The main conclusions are as follows: (1) the effect of oscillatory shear on the crystallization of PLA decreases with the increase of shear temperature) when the shear strain is larger than the critical shear strain, The effect of oscillating shear on the crystallization behavior of PLA is obvious. The effect of oscillating shear on the crystallization kinetics of PLA increases with the increase of shear cycle. The effect of frequency on the crystallization behavior of PLA is different. The effect of oscillatory shear on the crystallization of PLA weakens with the prolongation of relaxation time.
【学位授予单位】：郑州大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O633.14

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