Morphological and Rheological Responses to Shear Start-up and Flow Reversal of Thermotropic Liquid-Crystalline Polymers
Morphological and Rheological Responses to Shear Start-up and Flow Reversal of Thermotropic Liquid-Crystalline Polymers
复制标题
热致液晶聚合物剪切启动和流动反转的形态和流变响应
DOI：
10.1021/ma000765r
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发表时间：
2000
期刊：
影响因子：
5.5
通讯作者：
Sukky Chang
中科院分区：
文献类型：
--
作者：
P. Mather;H. Jeon;C. Han;Sukky Chang
新型流变光学方法已用于直接观察半柔性主链热致液晶聚合物（TLCP）在剪切启动和反向流动过程中的形态演变。使用专门设计的微流变仪，可以同时进行瞬态光学和机械观察，我们观察到流向反转时带的形成。可以看出，这种能带形成导致 HV 小角度光散射模式中的不对称光衍射，表明规则间隔的能带的倾斜排列。还使用锥板流变仪在与流变光学实验相同的热历史和变形历史下，在低于向列向各向同性转变温度（TNI）的相同温差下，对不同间隔基长度的聚合物进行了剪切启动和流动反转实验。据观察，在剪切启动和流动反转过程中，第一法向应力差 N1 + (A o , t) 表现出较大的超调，随后出现振荡衰减，而剪切应力 U + (A o , t) 则表现出较大的超调，随后出现单调衰减。结果发现，施加的剪切速率越高，N1 + (A o , t) 和 U + (A o , t) 的超调量越大，并且 N1 + (A o , t) 振荡的持续时间越长。在本研究中研究的 TLCP 的剪切启动和流动反转期间，比率 N1 + (A o , t)/ U + (A o , t) 和流动双折射 ∨n + (A o , t) 之间存在相似性。此外，我们发现比率 U + (t,A o )/U 与 A o t 成比例，但比率 N1 + (A o , t )/ N1 则不然，其中 U 表示稳态下的剪应力，N1 表示稳态下的第一法向应力差。
Novel rheo-optical methods have been used to directly observe morphology evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical observations, we observed band formation upon reversal of flow direction. It was seen that this band formation causes asymmetric light diffraction in HV small-angle light scattering mode, indicating a tilted arrangement of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature (TNI). It was observed that, during both shear start- up and flow reversal, the first normal stress difference N1 + (A o , t) exhibits a large overshoot followed by an oscillatory decay, while shear stress U + (A o , t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N1 + (A o , t) and U + (A o , t), and the longer the persistence of oscillations in N1 + (A o , t). Similarity was found between the ratio N1 + (A o , t)/ U + (A o , t) and flow birefringence ¢n + (A o , t) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio U + (t,A o )/U scales with A o t but the ratio N1 + (A o , t )/ N1 does not, where U denotes shear stress at steady state and N1 denotes first normal stress difference at steady state.