用正电子束研究分离膜的微纳结构及其选择渗透机理

Because of their high permeability and selectivity for samll molecules and ions, polymeric membranes are very promising to be applied in industries such as water treatment and green energy etc. The role of nanostructure characteristics in separation performance of membranes is of importance and attract great interests. Positron annihilation techniques based on a slow positron beam are novel tools to study the nanostructures (for instance, free volume element) of membranes and thin polymer films. Thus, in this proposal a developed positron microbeam in our lab will be functionalized and it will be applied to study free volume properties of Nafion nanocomposite proton exchange membrane and reverse osmosis (RO) membranes. The main research content includes: 1) to prepare various Nafion membranes containing different inorganic nanoparticles and reverse osmosis (RO) membranes composed of aromatic polyamide thin films, to study the free volumes of the membranes using positron annihilation spectroscopies based on a slow positron beam. 2) to fabricate and setup a new sample chamber with a small window sealed by a strong solid thin film (30-200 nm in thickness）, through which focused positrons are injected into a target sample in an atmospheric environment; and such that nanostructures of membranes in an atmospheric environment will be studied; 3) to design a simple pulsing system, guide cold positrons with small energy dispersion into the buncher and accelerate, focus the short positron pulses on the target; 4) to setup a positron annihilation lifetime spectroscopy based on the pulsed slow positron beam and investigate the role of free volume elements in ion, gas permeability and selectivity of the membranes. The functionalized positron microbeam has an advantage in bunching positrons, and positron annihilation spectra for thin film samples can be measured both in a vacuum and various atmospheric environments. It can be used as a unique platform for studies of material surfaces, underneath layers and thin films as well as for fundamental physics with many positrons and positroniums.

高分子分离膜具有小分子/离子选择性渗透功能，在水处理、清洁能源等领域有广泛应用前景，其微结构与性能关系是迫切的研究课题之一。慢正电子束是研究高分子薄膜非常有用的灵敏工具。本项目拟完善正电子微束功能并开展高分子分离膜的正电子湮没研究：1)利用正电子束研究高分子膜微结构，确立其微结构与功能的关系；2)加工气氛样品靶室，使正电子微束透过SiN薄膜窗入射到气氛下薄膜样品中，开展气氛下分离膜的微结构研究；3）加工简易脉冲集束器，利用捕获阱中正电子能量分散小的特点，将其斩波引出并进行脉冲集束、加速聚焦于样品靶上，获得能量可调的脉冲正电子微束；4)组装基于脉冲束的正电子寿命谱仪，开展真空和气氛下高分子分离膜的正电子湮没寿命测量，研究分离膜自由体积特性影响其选择性渗透功能的机理。功能完善的正电子束有易集束、束斑小、可扫描、可气氛下测量等特点，是研究薄膜、微样的独特平台，也可开展多正电子物理等前沿研究。

高分子分离膜具有小分子/离子选择性渗透功能，在水处理、清洁能源等领域有广泛应用前景，其微结构与性能关系是迫切的研究课题之一。正电子湮没寿命谱仪是研究高分子薄膜非常有用的灵敏工具。本项目加工气氛样品室，利用正电子湮没寿命谱、电化学工作站等研究了多种高分子分离膜微结构表征，并初步确立其微结构与功能的关系。结果表明：质子分离膜的自由体积变化与质子水通道的形成密切相关；正电子湮没参数可以有效表征质子分离膜的微结构在不同湿度下的变化，从而解明影响质子导电率微观机理；同时研究了影响不同掺杂膜的质子导电微观机理。而反渗透膜的水、盐选择性渗透功能不仅仅与平均自由体积大小有关，自由体积分布更是决定盐渗透机理的关键因素。相关分离膜研究有待进一步深入开展。