Morphological and Spatial Aspects in Polymer Degradation: From Heterophasic Polymers to Proton Exchange Membranes Used in Fuel Cells

提议的研究。在2001-2004年NSF资助期间进行的研究已经证明了基于电子自旋共振的成像方法ESRI的优势，可以提供多相聚合物中聚合物降解的空间和形态分辨信息。研究的系统是聚（丙烯腈-丁二烯-苯乙烯）（ABS），和丙烯-乙烯共聚物（HPEC），都含有受阻胺稳定剂（HAS）。HAS衍生的氮氧自由基作为自由基清除剂，并在ESRI实验中提供对比。但问题仍然存在，并将在本研究计划中加以解决。主要目标如下：1.了解紫外线降解中照射波长的影响。2.评估HAS结构和分子量的重要性，以及当地环境对HAS衍生的氮氧化物的形成和消耗的重要性。3.绘制氮氧自由基在各个领域的形成和分布图，并评估形态对其分布的影响。4.通过改进图像重建软件来提高ESRI实验的分辨率。5.启动燃料电池质子交换膜退化的现场研究;这是本提案所述长期规划（四年）中的一个风险部分。拟议的研究包括光谱方法（连续波和脉冲ESR，FTIR，UV）和ESR成像。聚合物降解和稳定化是一个具有重大基础和技术重要性的挑战性课题：基础，因为它反映了由于化学现象而导致的聚合物材料性质的变化，这些化学现象可以作为环境条件的函数而变化。技术，因为聚合物材料的使用正在稳步增加，并且因为在特定应用中所需的日益复杂。我们开发的ESRI方法对降解过程中的早期事件异常敏感，因此具有预测价值。此外，该方法是非破坏性的，这一优点对于半结晶体系如聚丙烯基非均相和其它自组装聚合物体系（例如含离子聚合物）和嵌段共聚物胶束特别重要。该研究计划的预期结果是准确预测聚合物材料的寿命，并更好地了解环境因素（例如热、光、湿度和氧气）;这些结果具有重大的根本意义，对工业应用至关重要。研究生和本科生，以及博士后研究员和访问科学家是研究团队的一部分。虽然侧重于具体目标，但每个参与者的贡献将得到分析，并由所有人分享，并在小组研讨会和会议上传播。国际合作（与日本、法国、波兰、俄罗斯、罗马尼亚和德国的科学家）以及与工业实验室（陶氏化学、丰田、通用汽车和Giner）的科学家的联系已经发展，因为我们对降解问题的方法是新奇的，ESRI方法是通用的。该PI计划在UDM，化学601的专题系列中开发一个名为“燃料电池科学”的新单元，该单元将为研究生和高级本科生量身定制。这个主题出现在我们的离聚物的自组装研究（由聚合物计划支持），并受到我们最近对PEM稳定性研究的兴趣。

Proposed Research. The research performed during the 2001-2004 NSF grant period has demonstrated the strength of the imaging method based on electron spin resonance, ESRI, to provide spatially- and morphologically-resolved information on polymer degradation in heterophasic polymers. The systems studied were poly(acrylonitrile-butadiene-styrene) (ABS), and propylene-ethylene copolymers (HPEC), both containing a hindered amine stabilizer (HAS). The HAS-derived nitroxide radicals act as radical scavengers, and provide the contrast in ESRI experiments. But questions remain, and will be addressed in this research plan. The following major objectives are: 1. Understand the effect of irradiation wavelength in UV degradation. 2. Assess the importance of HAS structure and molecular weight, and of the local environment on the formation and consumption of HAS-derived nitroxides. 3. Map the formation and distribution of nitroxides in various domains, and evaluate the effect of the morphology on their distribution. 4. Increase the resolution in ESRI experiments by refining the software for image reconstruction. 5. Initiate an in situ study of degradation of proton exchange membranes (PEMs) in fuel cells; this is a risky part in the long-range planning described in this proposal (four years). The proposed study includes spectroscopic methods (continuous wave and pulsed ESR, FTIR, UV), and ESR imaging.Intellectual Merit. Polymer degradation and stabilization is a challenging topic of great fundamental and technological importance: Fundamental, because it reflects the change in properties of polymeric materials due to chemical phenomena that can vary as a function of environmental conditions. Technological, because the use of polymeric materials is increasing steadily, and because of the increasing sophistication required in specific applications. The ESRI method that we have developed is exceptionally sensitive to early events in the degradation process, and therefore of predictive value. Moreover, the method is nondestructive, an advantage that is particularly important for semicrystalline systems such as polypropylene-based heterogeneous and other self-assembled polymeric systems, for instance ion-containing polymers, and block copolymer micelles. The expected results of this research plan are accurate prediction of lifetimes for polymeric materials and a better understanding of environmental factors (for instance heat, light, humidity, and oxygen); these results are of great fundamental interest and crucial for industrial applications.Broader Impact. Graduate and undergraduate students, as well as postdoctoral fellows and visiting scientists are part of the research team. Although focused on specific goals, the contribution of each participant will be analyzed and shared by all, and disseminated at group seminars and meetings. International collaborations (with scientists from Japan, France, Poland, Russia, Romania, and Germany) and contact with scientists in industrial labs (Dow Chemical, Toyota, General Motors, and Giner) have developed because of the novelty of our approach to the problem of degradation, and the versatility of the ESRI method. This PI plans to develop a new unit entitled "The Science of Fuel Cells", in the Special Topics series at UDM, Chemistry 601, which will be tailored for graduate and advanced undergraduate students. This topic emerged from our studies of self-assembling of ionomers (supported by the Polymers Program), and is motivated by our recent interest in the study of PEM stability.