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首页> 外文期刊>JCT research >Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation
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Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation

机译：探测表面之下的光降解：使用微化学成像和纳米压痕对紫外线降解的聚合物涂层进行深度剖析研究

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Photodegradation of polymer coatings generally involves photooxidation, resulting in the formation of oxidized products, chain scission, and crosslinking. On severe exposure to ultraviolet (UV) light in the presence of air, chemical degradation transforms into substantial changes in the physical and mechanical properties, leading to failures of the coatings. Systematic research by NIST on service life prediction of polymeric coatings indicates that the degradation of polymer coatings starts from the sub-micrometer degradation-susceptible regions at the surface and then grows in width and depth. Additionally, due to the oxygen diffusion effect and the attenuation of the UV light passing through the polymer, the degradation can be spatially heterogeneous. In this study, the changes with depth of the mechanical and chemical properties of a UV-exposed epoxy/polyurethane system were measured by nanoindentation and Fourier transform infrared spectroscopy (FTIR) microscopy using cross-sectioned specimens. Multilayers of epoxy/polyurethane samples were prepared by a draw-down technique. After curing, samples were exposed to the outdoors in Gaithersburg, MD, for four months. Cross-sectioned slices of the exposed and unexposed samples, approximately 500 nm thick as-prepared by microtom-ing, were used for micro-FTIR imaging. Samples for nanoindentation were prepared by embedding the epoxy/polyurethane multilayers (both exposed and unexposed) in a molding compound, followed by microtoming and polishing the embedded films in the thickness direction. Micro-FTIR images clearly show that, for the outdoor exposed samples, substantial amounts of oxidation products are distributed in the 60 (am deep region from the surface to the epoxy bulk, decreasing in the center of epoxy region and increasing again toward the epoxy/urethane interface. Nanoindentation results also show that the modulus significantly increases in the first 60 nm region after UV degradation, and then decreases gradually with depth until a value slightly higher than the modulus of the undegraded epoxy is reached. The modulus rises again in the region near the epoxy/urethane interface. These similarities in the depth profiles of the properties indicate the linkage between the chemical degradation and the mechanical degradation. The study clearly shows that the spatial distribution of chemical species and mechanical properties is heterogeneous in the thickness direction for polymer coatings after UV degradation. It also demonstrates that cross-sectional analysis using nanoindentation and micro-FTIR imaging techniques is a useful method to characterize the mechanical and chemical depth profiles of polymer coating degradation.
机译：聚合物涂层的光降解通常涉及光氧化作用，导致形成氧化产物，断链和交联。在空气中存在紫外线（UV）的情况下，化学降解会转化为物理和机械性能的重大变化，从而导致涂层失效。 NIST对聚合物涂层的使用寿命预测进行的系统研究表明，聚合物涂层的降解始于表面亚微米级的降解敏感区域，然后在宽度和深度上逐渐扩大。另外，由于氧的扩散作用和穿过聚合物的紫外线的衰减，降解在空间上是不均匀的。在这项研究中，通过纳米压痕和傅里叶变换红外光谱（FTIR）显微镜，使用横截面样品，测量了紫外线暴露的环氧/聚氨酯体系的机械和化学性质随深度的变化。环氧/聚氨酯样品的多层是通过拉伸技术制备的。固化后，将样品在马里兰州盖瑟斯堡的户外暴露四个月。显微切割制备的已曝光和未曝光样品的横截面切片（约500 nm厚）用于显微FTIR成像。通过将环氧/聚氨酯多层膜（暴露和未暴露）嵌入模塑料中，然后在厚度方向上进行切片和抛光，制备出纳米压痕样品。显微FTIR图像清楚地表明，对于室外暴露的样品，大量的氧化产物分布在60（从表面到环氧树脂主体的深区域）中，在环氧树脂区域的中心减小，并再次向环氧树脂/纳米压痕结果还表明，紫外线降解后，模头在最初的60 nm区域内模量显着增加，然后随深度而逐渐降低，直至达到略高于未降解的环氧模量的值，模量在该区域内再次升高这些性质在深度分布上的相似性表明了化学降解和机械降解之间的联系，研究清楚地表明，化学种类和力学性能的空间分布在聚合物的厚度方向上是不均匀的紫外线降解后的涂层，这也证明了使用纳米的横截面分析压痕和微型FTIR成像技术是表征聚合物涂层降解的机械和化学深度分布的有用方法。

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