Renewable and environmentally responsive materials are an energy- and resource-efficient approach in terms of civil engineering applications, e.g. as so-called smart building skins. To evaluate the influence of different environmental stimuli, like humidity or solar radiation, on the long-term actuation behavior and mechanical robustness of these materials, it is necessary to precisely characterize the magnitude and range of stimuli that trigger reactions and the resulting kinetics of a material, respectively, with suitable testing equipment and techniques. The overall aim is to correlate actuation potential and mechanical properties with process- or application-oriented parameters in terms of demand-oriented stimuli-responsive element production. In this study, the impact of solar radiation as environmental trigger on the cellulose-based humidity-sensing material Cottonid, which is a promising candidate for adaptive and autonomously moving elements, was investigated. For simulating solar radiation in the lab, specimens were exposed to short-wavelength blue light as well as a standardized artificial solar irradiation (CIE Solar ID65) in long-term aging experiments. Photodegradation behavior was analyzed by Fourier-transform infrared as well as electron paramagnetic resonance spectroscopy measurements to assess changes in Cottonid’s chemical composition. Subsequently, changes in micromechanical properties on the respective specimens’ surface were investigated with roughness measurements and ultra-micro-hardness tests to characterize variations in stiffness distribution in comparison to the initial condition. Also, thermal effects during long-term aging were considered and contrasted to pure radiative effects. In addition, to investigate the influence of process-related parameters on Cottonid’s humidity-driven deformation behavior, actuation tests were performed in an alternating climate chamber using a customized specimen holder, instrumented with digital image correlation (DIC). DIC was used for precise actuation strain measurements to comparatively evaluate different influences on the material’s sorption behavior. The infrared absorbance spectra of different aging states of irradiated Cottonid indicate oxidative stress on the surface compared to unaged samples. These findings differ under pure thermal loads. EPR spectra could corroborate these findings as radicals were detected, which were attributed to oxidation processes. Instrumented actuation experiments revealed the influence of processing-related parameters on the sorption behavior of the tested and structurally optimized Cottonid variant. Experimental data supports the definition of an optimal process window for stimuli-responsive element production. Based on these results, tailor-made functional materials shall be generated in the future where stimuli-responsiveness can be adjusted through the manufacturing process.

中文翻译：

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太阳辐射对纤维素基湿度传感材料棉纺的化学结构和微机械性能的影响

就土木工程应用而言，例如所谓的智能建筑表皮，可再生且对环境敏感的材料是一种能源和资源高效的方法。为了评估不同环境刺激（如湿度或太阳辐射）对这些材料的长期驱动行为和机械强度的影响，有必要精确地表征触发反应的刺激的幅度和范围以及所产生的动力学。分别使用合适的测试设备和技术进行测试。总体目标是根据需求导向的刺激响应元件生产，将促动电势和机械性能与面向过程或应用程序的参数相关联。在这项研究中，研究了作为环境触发因素的太阳辐射对纤维素基湿度传感材料Cottonid的影响，后者是适应性和自主运动元素的有前途的候选者。为了在实验室中模拟太阳辐射，在长期老化实验中，将标本暴露于短波长蓝光以及标准化的人工太阳辐射下（CIE Solar ID65）。通过傅立叶变换红外光谱和电子顺磁共振光谱分析法对光降解行为进行了分析，以评估Cottonid化学成分的变化。随后，通过粗糙度测量和超微硬度测试研究了各个试样表面的微机械性能变化，以表征与初始条件相比刚度分布的变化。此外，考虑了长期老化过程中的热效应，并将其与纯辐射效应进行了对比。此外，为了研究过程相关参数对Cottonid湿度驱动的变形行为的影响，使用定制的标本架在交替的气候室内进行了驱动测试，该标本架装有数字图像相关（DIC）。DIC用于精确的致动应变测量，以比较评估对材料吸附行为的不同影响。与未老化的样品相比，受辐照的棉衣在不同老化状态下的红外吸收光谱表明表面的氧化应力。在纯热负荷下，这些发现有所不同。EPR光谱可以证实这些发现，因为检测到自由基，这归因于氧化过程。仪器化的驱动实验揭示了与加工相关的参数对经过测试和结构优化的卡迪尼德变种的吸附行为的影响。实验数据支持了刺激反应性元素产生的最佳过程窗口的定义。基于这些结果，将来将产生量身定制的功能材料，其中可以通过制造过程调整刺激响应性。在纯热负荷下，这些发现有所不同。EPR光谱可以证实这些发现，因为检测到自由基，这归因于氧化过程。仪器化的驱动实验揭示了与加工相关的参数对经过测试和结构优化的卡迪尼德变种的吸附行为的影响。实验数据支持了刺激反应性元素产生的最佳过程窗口的定义。基于这些结果，将来将产生量身定制的功能材料，其中可以通过制造过程调整刺激响应性。在纯热负荷下，这些发现有所不同。EPR光谱可以证实这些发现，因为检测到自由基，这归因于氧化过程。仪器化的驱动实验揭示了与加工相关的参数对经过测试和结构优化的卡迪尼德变种的吸附行为的影响。实验数据支持了刺激反应性元素产生的最佳过程窗口的定义。基于这些结果，将来将产生量身定制的功能材料，其中可以通过制造过程调整刺激响应性。仪器化的驱动实验揭示了与加工相关的参数对经过测试和结构优化的卡迪尼德变种的吸附行为的影响。实验数据支持了刺激反应性元素产生的最佳过程窗口的定义。基于这些结果，将来将产生量身定制的功能材料，其中可以通过制造过程调整刺激响应性。仪器化的驱动实验揭示了与加工相关的参数对经过测试和结构优化的卡迪尼德变种的吸附行为的影响。实验数据支持了刺激反应性元素产生的最佳过程窗口的定义。基于这些结果，将来将产生量身定制的功能材料，其中可以通过制造过程调整刺激响应性。