Shear-induced formation of crystal nuclei in polyamide 11 (PA 11) was studied using a conventional parallel-plate rheometer. Crystallization of PA 11 after shearing the melt at different rates for 60 s was followed by the evolution of the complex viscosity. The sheared samples showed in an optical microscope a gradient structure along the radius, due to the increasing shear rate from the center to the edge. The critical shear rate for shear-induced formation of nuclei was identified at the position where a distinct change of the semicrystalline superstructure is observed, being at around 1 to 2 s⁻¹. Below this threshold, a space-filled spherulitic superstructure developed as in quiescent-melt crystallization. Above this value, after shearing at rates between 1 and 5 s⁻¹, an increased number of point-like nuclei was detected, connected with formation of randomly oriented crystals. Shearing the melt at even higher rates led to a further increase of the nuclei number and growth of crystals oriented such that the chain axis is in parallel to the direction of flow. In addition, optical microscopy confirmed formation of long fibrillar structures after shearing at such condition. The critical specific work of flow of PA 11 was calculated to allow a comparison with that of polyamide 66 (PA 66). This comparison showed that in the case of PA 11 more work for shear-induced formation of nuclei is needed than in the case of PA 66, discussed in terms of the chemical structure of the repeat unit in the chains.

Graphical abstract

中文翻译：

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剪切诱导的聚酰胺11结晶

使用常规的平行板流变仪研究了聚酰胺11（PA 11）中剪切诱导的晶核形成。在以不同速率剪切熔体60 s后，PA 11结晶，随后形成复数粘度。由于从中心到边缘的剪切速率增加，剪切的样品在光学显微镜中显示出沿半径的梯度结构。在观察到半结晶超结构的明显变化的位置，在大约1至2 s ^-1处，确定了剪切诱导的核形成的临界剪切速率。低于此阈值，就形成了一个静止的球状超晶格结构，就像在静态熔体结晶中一样。高于此值后，以1到5 s ^-1的速率剪切后，检测到数量增加的点状核，与无规取向晶体的形成有关。以更高的速率剪切熔体导致晶核数进一步增加，并取向为使得链轴平行于流动方向的晶体生长。另外，光学显微镜证实在这种条件下剪切后形成了长的纤维状结构。计算出PA 11的临界比功，可以与聚酰胺66（PA 66）进行比较。该比较表明，在PA 11的情况下，与在剪切链中重复单元的化学结构方面讨论的PA 66相比，需要更多的工作来进行剪切诱导的核形成。

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