Force sensing and stress imaging with donor-acceptor torsional springs

能够响应外界刺激的智能材料正受到广泛的研究，因为它们在生物学、信息技术、自我修复、传感和能源相关主题中的应用几乎是无限的。机械响应性聚合物在暴露于机械应力时会极大地改变性能，机械应力可用于成像应力或应变、切换反应性或偏置原本难以访问的不同反应路径。最常用的机械变色体系之一是螺吡喃/花菁体系(SP/MC)。这一体系以及其他体系依赖于均相或异相的键断裂，因此显示出内在的局限性，这主要与它们的两态性质以及两种形式之间的关联障碍有关。特别是SP衍生物的另一个缺点是温度不稳定，这阻碍了熔体加工，因此具有广泛的适用性。SP衍生品也会出现光漂白，这通常会限制其长期稳定性。对于许多与生物系统、机器人、高性能材料开发和基础研究有关的应用，具有快速、直接响应和完全可逆性的无障碍机械变色系统将具有显著的优势。这项建议涉及发展供体-受体(D-A)扭转弹簧的新概念，用于力传感和应力成像。这种扭转弹簧的中心成分是一个共价连接的施主-受主二元体，在其平衡几何结构中显示出大的二面角。通过适当的共价键将力施加到聚合物链/基质上，然后允许力诱导D-A体系的平坦化，导致电子相互作用的增加，从而导致电荷转移(CT)带的红移。这种力诱导的平坦化是完全可逆的，并且还允许根据所使用的D-A系统和相关的扭转势来调节力/颜色。拟议的D-A扭转弹簧是一个完全可逆的无障碍系统。通过建立这种机械变色的新概念及其所有优点，我们将推进机械响应聚合物的新领域，并打开前所未有的新机遇和新应用。

Smart materials that are able to respond to external stimuli are under extensive investigation because of their almost unlimited range of applications in biology, information technology, self-healing, sensing and energy-related topics. Mechano-responsive polymers drastically change properties upon exposure to mechanical stress, which can be used to image stress or strain, switch reactivity or to bias distinct reaction pathways that are otherwise difficult to access. One of the most prevalent mechanochromic system is the spiropyran/ merocyanine system (SP/MC). This system, and also others, rely on homogeneous or heterogeneous bond scission and hence show intrinsic limitations that are mainly related to their two-state nature and the associate barrier between the two forms. Particularly SP derivatives have the further disadvantages of being temperature-labile which prohibits melt processing and hence broad applicability. SP derivatives also show photo bleaching which generally limits long-term stability. For many applications related to biological systems, robotics, the development of high performance materials and fundamental investigations, a barrierless mechanochromic system with fast and direct response and full reversibility would hold significant advantages. This proposal is concerned with the development of the new concept of donor-acceptor (D-A) torsional springs for force sensing and stress imaging. The central component of such torsional springs is a covalently linked donor-acceptor dyad exhibiting a large dihedral angle in its equilibrium geometry. The application of force by appropriate covalent linkage to a polymer chain/ matrix then allows force-induced planarization of the D-A system, leading to increased electronic interaction and thus a red-shift of the charge transfer (CT) band. Such a force-induced planarization is completely reversible and additionally allows force/color tunability according to the D-A system and related torsion potential used. The proposed D-A torsional spring is a barrierless system that is fully reversible. By using a combined theoretical and experimental approach, we will investigate and use different torsion potentials of various D-A springs, incorporate them into suitable polymer matrices that allow to transduce macroscopic stress to the D/A spring, and finally investigate the resulting force-dependent change of the optical properties. By establishing this new concept of mechanochromism with all its advantages mentioned, we will advance the new field of mechano-responsive polymers and open new opportunities and applications of unprecedented extent.