Laboratoire Sciences et Ingénierie de la Matière Molle, CNRS UMR7615, ESPCI Paris, PSL Research University, 10 rue Vauquelin, F-75231 Paris, France.
Soft Matter. 2016 Oct 4;12(39):8143-8154. doi: 10.1039/c6sm01206j.
When a dry soluble polymer is put in contact with a large quantity of solvent, it swells and forms a transient gel, and eventually, yields a dilute solution of polymers. Everyday lab experience shows that when the molar mass is large, namely tens of times larger than entanglement mass, this dissolution process is slow and difficult and may require stirring. Here, in agreement with previous results, we found that the time needed to turn a dry grain into a dilute solution is not limited by water diffusion in the glassy or semi-crystalline dry polymer, but rather by the life-time of the transient gel made of entangled chains. In addition, we shed new light on the dissolution process by demonstrating that, in contrast to theoretical predictions, the gel life-time is not governed by reptation. We show instead that swelling is simply controlled by the osmotic pressure and the gel permeability until the overlap concentration is reached within the gel. At this stage, the gel turns into a dilute solution in which polymers are dispersed by natural convection. The observed dependence of the dissolution process on the molar mass therefore originates from the molar mass dependent overlap concentration. Under stirring, or forced convection, the polymer gel disappears at a higher critical concentration that depends on the shear rate. We suggest a description of the experimental data which uses the rheological flow curves of the solutions of the considered polymer. Inversely, dissolution times of polymer powders under stirring can be inferred from classical rheological measurements of the polymer solutions at varied concentrations.
当一种干燥的可溶聚合物与大量溶剂接触时，它会膨胀并形成一种瞬态凝胶，最终形成聚合物的稀溶液。日常实验室经验表明，当摩尔质量较大，即数十倍于缠结质量时，这种溶解过程缓慢且困难，可能需要搅拌。在这里，我们与之前的结果一致，发现将干燥颗粒转变为稀溶液所需的时间不受玻璃态或半晶态干燥聚合物中水分子扩散的限制，而是由缠结链形成的瞬态凝胶的寿命决定。此外，我们通过证明，与理论预测相反，凝胶寿命不受蠕动控制，而是由溶胀控制，从而为溶解过程提供了新的认识。我们表明，溶胀仅受渗透压和凝胶渗透性控制，直到凝胶内达到重叠浓度。在这个阶段，凝胶会变成一种稀溶液，其中聚合物通过自然对流分散。因此，观察到的溶解过程对分子量的依赖性源自分子量依赖性的重叠浓度。在搅拌或强制对流下，聚合物凝胶会在更高的临界浓度下消失，该浓度取决于剪切速率。我们提出了一种描述实验数据的方法，该方法使用了所研究聚合物溶液的流变流动曲线。相反，通过在不同浓度下对聚合物溶液进行经典流变测量，可以推断出聚合物粉末在搅拌下的溶解时间。
