Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation
Capturing the Transient Microstructure of a Physically Assembled Gel Subjected to Temperature and Large Deformation
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捕获经受温度和大变形的物理组装凝胶的瞬态微观结构
DOI：
10.1021/acs.macromol.1c00895
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发表时间：
2021
期刊：
影响因子：
5.5
通讯作者：
Kundu, Santanu
中科院分区：
文献类型：
--
作者：
Prado, Rosa Maria;Mishra, Satish;Ahmad, Humayun;Burghardt, Wesley R.;Kundu, Santanu
物理组装凝胶的微观结构取决于机械载荷和环境刺激，如温度。在这里，我们报告了由10和20wt%的聚（苯乙烯）-聚（苯乙烯）-聚（苯乙烯-聚（苯乙烯-聚）组成的物理组装的三嵌段共聚物凝胶结构的实时变化。在矿物油中的（异戊二烯）-聚（苯乙烯）[PS-PI-PS]三嵌段共聚物（i）在随温度降低的胶凝过程中，（ii）经受大的振荡变形，以及（iii）在施加阶跃应变之后的应力松弛过程中。在高于流变学确定的凝胶化温度（Tgel）的温度下松散结合的PS聚集体的存在表示跨越宽温度范围的渐进凝胶化过程。然而，该微结构在充分低于Tgel的温度下充分发展。随着施加的应变，微观结构沿拉伸方向取向。在振荡应变循环中，在低应变下观察到这种取向结构。然而，在大应变下，取向结构由于应变局部化而分裂，这表明只有一小部分PI块参与承载。使用拉伸指数模型，在应力松弛过程中的微结构恢复和随时间变化的模量已被捕获后的应用程序的步骤应变。然而，已发现的微观结构恢复时间是2个数量级慢于在室温下的应力松弛时间，表明应力松弛和微观结构恢复过程的复杂性质，涉及中间块松弛，端块拔出，和重新关联。由于其粘弹性性质，这些凝胶的机械响应对应变、温度和变形速率敏感。因此，深入了解这些参数对微结构变化的影响将有助于这些凝胶的实际应用，并设计具有改进性能的新凝胶。
The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 and 20 wt % of poly(styrene)–poly(isoprene)–poly(styrene) [PS–PI–PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step strain. The presence of loosely bounded PS aggregates at temperatures higher than the rheologically determined gelation temperature (Tgel) signifies the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures sufficiently lower thanTgel. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low strain. However, at large strain, the oriented structure splits because of strain localization suggesting that only a fraction of PI blocks participates in load bearing. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step strain have been captured using a stretched-exponential model. However, the microstructure recovery time has been found to be 2 orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of stress relaxation and microstructure recovery processes involving midblock relaxation, endblock pullout, and reassociation. Due to their viscoelastic nature, these gels’ mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural change as a function of these parameters will assist these gels’ real-life applications and design new gels with improved properties.
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