Deleurence Rémi, Saison Tamar, Lequeux François, Monteux Cécile
Laboratoire Sciences et Ingénierie de la Matière Molle, CNRS, ESPCI Paris, PSL Research University, 10 rue Vauquelin, 75005 Paris, France.
Laboratoire Sciences et Ingénierie de la Matière Molle, Université Pierre et Marie Curie, Sorbonne-Universités, 10 rue Vauquelin, 75005 Paris, France.
ACS Omega. 2018 Feb 13;3(2):1864-1870. doi: 10.1021/acsomega.7b01301. eCollection 2018 Feb 28.
Foams made with polymer hydrogels can be used in a variety of applications, such as scaffolds for biomedical applications or decontamination processes. However, from a practical point of view, it is difficult to introduce bubbles into viscous or viscoelastic fluids and to produce large volumes of hydrogel foams. In the present article, we investigate the foaming process of poly(vinyl alcohol) (PVA)/borax transient hydrogels, where PVA chains reversibly bind to borax molecules. In a previous article, we showed that foams obtained with PVA/borax mixtures are highly stable because of both high interfacial and bulk viscosities and can be used to quickly absorb liquids, which make them suitable for detergency or decontamination processes. To produce these foams, we use a two-step foaming process which consists in first shearing a PVA solution to obtain a PVA foam and second adding borax to the PVA foam under continuous shearing. The obtained PVA/borax foams are stable for weeks. In this study, we observe a shear-induced collapse of the foams for formulations containing a low borax/PVA ratio, whereas they remain stable under shear for high PVA/borax ratios. Using scaling arguments, we find that the shear-induced collapse of the foams and bubbles is obtained below a critical ratio, / = 15, of the number of entanglements per chain, , and the number of borax per chain, . Rheology measurements show that the samples present a shear-thickening behavior that increases with the borax concentration. We suggest that during the foaming process when the shearing rate is of the order of 100 s, the viscosity of these samples diverges, leading to a viscous to fragile transition. To mimic the fast stretching of the PVA/borax thin films during the foaming process, we study the stretching of individual PVA/borax catenoid-shaped thin films at high stretching rates. We observe that the films containing low PVA/borax ratios do not minimize their surface area unlike what is theoretically expected for standard surfactant films. Moreover, the films tend to be unstable and fracture because the PVA/borax network does not have time to rearrange and relax stresses for high stretching rates.
由聚合物水凝胶制成的泡沫可用于多种应用，例如生物医学应用的支架或去污过程。然而，从实际角度来看，将气泡引入粘性或粘弹性流体并生产大量水凝胶泡沫是困难的。在本文中，我们研究了聚乙烯醇（PVA）/硼砂瞬态水凝胶的发泡过程，其中PVA链与硼砂分子可逆结合。在之前的一篇文章中，我们表明用PVA/硼砂混合物获得的泡沫由于高界面粘度和本体粘度而高度稳定，并且可用于快速吸收液体，这使其适用于去污或净化过程。为了生产这些泡沫，我们使用两步发泡过程，该过程包括首先剪切PVA溶液以获得PVA泡沫，然后在连续剪切下向PVA泡沫中添加硼砂。所获得的PVA/硼砂泡沫可稳定存在数周。在本研究中，我们观察到对于硼砂/PVA比例低的配方，泡沫会发生剪切诱导的坍塌，而对于高PVA/硼砂比例，它们在剪切下保持稳定。使用标度论证，我们发现当每条链的缠结数(\nu)与每条链的硼砂数(\beta)的临界比(\nu / \beta = 15)以下时，会发生泡沫和气泡的剪切诱导坍塌。流变学测量表明，样品呈现出随硼砂浓度增加的剪切增稠行为。我们认为在发泡过程中，当剪切速率约为(100 s^{-1})时，这些样品的粘度发散，导致从粘性到脆性的转变。为了模拟发泡过程中PVA/硼砂薄膜的快速拉伸，我们研究了在高拉伸速率下单个PVA/硼砂链状薄膜的拉伸。我们观察到，与标准表面活性剂薄膜的理论预期不同，含有低PVA/硼砂比例的薄膜不会使它们的表面积最小化。此外，由于PVA/硼砂网络没有时间重新排列并松弛高拉伸速率下的应力，薄膜往往不稳定并破裂。
