A Mechanistic Investigation of the Effect of Stress on the Photochemical Degradation of Polymers

研究表明，应力会加速许多聚合物的光化学降解，但降解速率增加的原因尚不清楚。本研究的主要目的是确定应力诱导速率增加的机制起源，并找到与应力和光降解速率定量相关的一般表达。为了避免与光化机制复杂性相关的问题，并促进快速的实验进展，将使用三个关键的实验策略。首先，为了简化典型聚合物复杂的机械降解途径，提出了研究含有金属-金属键沿其骨干周期性放置的聚合物。这些聚合物通过一种直接的机制光降解，没有不必要的副反应。其次，为了消除由限制速率的氧扩散引起的复杂性，自由基陷阱将沿着聚合物主干嵌入。第三，计算机控制装置将用于收集光降解量子产率作为施加在聚合物薄膜上的应力的函数。这些数据将与三个降解假设的预测进行比较：(1)Plotnikov假设，(2)“自由基重组效率降低”假设（一种将降解率与自由基在压力影响下重组能力下降联系起来的理论），以及(3)Zhurkov方程的光化学模拟。这项工作可能具有相当大的实际意义，因为聚合物越来越多地用于结构应用，它们受到应力和光的影响。因此，正确理解光和应力之间的协同作用对于准确估计聚合物寿命和开发稳定体系至关重要。这项研究也将揭示固态中自由基重组的重要基本原理。

Studies have shown that stress will accelerate the photochemical degradation of many polymers, but reasons for the increased degradation rates are not well understood. The primary objectives of this study are to determine the mechanistic origins of the stress-induced rate increases and to find a general expression for quantitatively relating stress to photodegradation rates. In order to circumvent the problems associated with the mechanistic complexities of photogradation and to facilitate rapid experimental progress, three key experimental strategies will be used. First, to simplify the mechanistically complicated degradation pathways of typical polymers, is proposed to study polymers containing metal-metal bonds placed periodically along their backbones. These polymers photodegrade by a straightforward mechanism with no unwanted side-reactions. Second, to eliminate the complexities caused by rate-limiting oxygen diffusion, radical traps will be built-in along the polymer backbones. Third, a computer controlled apparatus will be used to collect photdegradation quantum yields as a function of stress applied to polymer films. The data will be compared to the predictions of three degradation hypotheses: (1) the Plotnikov hypothesis, (2) the "Decreased Radical Recombination Efficiency" hypothesis (a theory that relates degradation rates the decreased ability of radical to recombine under the influence of stress), and (3) a photochemical analog of the Zhurkov equation.%%%This work may be of considerable practical significance because polymers are increasingly being used in structural applications, by which they are subjected to stress and light. A proper understanding of the synergy between light and stress is therefore essential for the accurate estimation of polymer lifetimes and for the development of stabilizing systems. Important fundamental principles about radical recombination in the solid state should also emerge from this study.