Filippidi Emmanouela, Dhiman Anuj K, Li Benke, Athanasiou Thanasis, Vlassopoulos Dimitris, Fytas George
Department of Materials Science and Engineering, University of Crete, Heraklion 70013, Greece.
Institute of Electronic Structure and Laser, FORTH, Heraklion 70013, Greece.
J Phys Chem B. 2024 Dec 19;128(50):12628-12637. doi: 10.1021/acs.jpcb.4c06492. Epub 2024 Dec 4.
The response of soft materials to an imposed oscillatory stress is typically frequency dependent, with the most utilized frequency range falling in the range of 10-10 rad/s. In contrast to most conventional contact techniques for measuring material elasticity, like tensile or shear rheology and atomic force microscopy, or invasive techniques using probes, such as microrheology, Brillouin light spectroscopy (BLS) offers an optical, noncontact, label-free, submicron resolution and three-dimensional (3D) mapping approach to access the mechanical moduli at GHz frequencies. Currently, the correlation between the experimental viscoelastic (at lower frequencies) and elastic (at higher frequencies) moduli has fundamental and practical relevance, but remains unclear. We utilize a series of solvent-free epoxy polymer networks with variable cross-link density as models to compare the storage modulus, ', (in the MPa range) obtained from shear rheology and the longitudinal modulus, ', (in the GPa range) extracted from BLS. Our results show that ' exhibits a much stronger increase with increasing cross-link density than ' (by a factor of about 3.5). This finding is discussed in the context of the phantom network model for ' and Wood's inverse rule of mixtures for '. The epoxy polymer network displays an unexpectedly fast hypersonic dispersion compared to its uncross-linked precursor. These results testify the importance of obtaining reliable information about the elasticity of networks and will hopefully trigger further investigations in the direction of bridging the elasticity of soft materials at different scales.
软材料对施加的振荡应力的响应通常与频率有关，最常用的频率范围在10 - 10 rad/s之间。与大多数用于测量材料弹性的传统接触技术（如拉伸或剪切流变学以及原子力显微镜）或使用探针的侵入性技术（如微观流变学）不同，布里渊光散射（BLS）提供了一种光学、非接触、无标记、亚微米分辨率和三维（3D）映射方法，用于在GHz频率下获取机械模量。目前，实验粘弹性（在较低频率下）和弹性（在较高频率下）模量之间的相关性具有重要的理论和实际意义，但仍不清楚。我们使用一系列具有可变交联密度的无溶剂环氧聚合物网络作为模型，比较从剪切流变学获得的储能模量G'（在MPa范围内）和从BLS提取的纵向模量M'（在GPa范围内）。我们的结果表明，随着交联密度的增加，G'的增加比M'强得多（约为3.5倍）。这一发现是在关于G'的幻影网络模型和关于M'的伍德混合反比规则的背景下进行讨论的。与未交联的前体相比，环氧聚合物网络显示出意想不到的快速超声波色散。这些结果证明了获得关于网络弹性的可靠信息的重要性，并有望引发在弥合不同尺度软材料弹性方面的进一步研究。
