Feng Cong, Li Yan, Qu Kunnan, Zhang Zhiming, He Pengfei
College of Materials Science and Engineering, Shanghai Key Lab of Metal Functional Materials, Tongji University Shanghai 201804 China.
School of Automotive Studies, Tongji University Shanghai 201804 China.
RSC Adv. 2019 Mar 26;9(17):9594-9603. doi: 10.1039/c9ra00745h. eCollection 2019 Mar 22.
Perfluorosulfonic acid (PFSA) is widely used as the membrane material for proton-exchange membrane fuel cells, and its mechanical properties directly affect the stability and the life of the internal structure of the proton exchange membrane. In the present research, mechanical behaviors of hydrated membranes are investigated through macro-mechanical experiments and nano-simulation. The commercial products of Nafion® 117 and Nafion® 212 are used as tensile testing samples, and the latter manifests a larger swelling ratio, smaller linear-elastic region, and higher Young's modulus at the same humidity. It is found that in comparison to Nafion 117, humidity (especially at less than 9 wt%) yields profound effects on true stress-true strain curves of Nafion 212. Further, two types of PFSA ionomers, representing the nanoscale parts of Nafion and Aquivion membranes, respectively, are studied on the uniaxial tensile deformation through molecular dynamics simulation, and the effects of side chain, water content, backbone length, and strain rate are examined. It is noticed that long side chains decrease the winding degree of polymer chains for the PFSA ionomers with the same backbone, and pinhole failure causes the declining trend of stress-strain curves. For meso- and nano-scale PFSA polymers, the difference between the volume of uptake water and swelling volume can be used to roughly judge the aggregation degree of polymer chains and explain the elastic-plastic deformation mechanism. The results of this work will serve as a baseline for understanding the swelling and mechanical behavior of PFSA membranes.
全氟磺酸（PFSA）被广泛用作质子交换膜燃料电池的膜材料，其机械性能直接影响质子交换膜内部结构的稳定性和寿命。在本研究中，通过宏观力学实验和纳米模拟研究了水合膜的力学行为。使用Nafion® 117和Nafion® 212的商业产品作为拉伸测试样品，后者在相同湿度下表现出更大的溶胀率、更小的线弹性区域和更高的杨氏模量。研究发现，与Nafion 117相比，湿度（尤其是低于9 wt%时）对Nafion 212的真应力-真应变曲线有深远影响。此外，分别代表Nafion膜和Aquivion膜纳米级部分的两种PFSA离聚物通过分子动力学模拟研究了单轴拉伸变形，并考察了侧链、含水量、主链长度和应变率的影响。注意到对于具有相同主链的PFSA离聚物，长侧链会降低聚合物链的缠绕程度，针孔破坏导致应力-应变曲线呈下降趋势。对于中观和纳米尺度的PFSA聚合物，吸水体积和溶胀体积之间的差异可用于粗略判断聚合物链的聚集程度并解释弹塑性变形机制。这项工作的结果将作为理解PFSA膜溶胀和力学行为的基线。
