软物质本构理论的多尺度模型

A theoretical framework for bottom-up multiscale modeling is studied aiming at the soft matter dominated with solid components. The issues involving invariants of geometric structures and thermodynamical consistencies at every level of scales, along with the basic principles of macroscopic continuum mechanics, are discussed. In an atempt to overcome the intrinsic limitations in the classical continuum mechanics applied to soft matters like oligomers, materials with large deformations, and deformations in micro-regions, new structural variables are introduced by employing approaches of direct reduction descriptions or coarse-graining of microstructures of the materials. Comparing with the 'internal variables' normally introduced in the classical continuum mechanics models for describing the large deformations and deformation in micro-regions, these new structures possesses the advantage in physics such as the deeper and direct characterization of microstructures of the soft matters. The theories of soft matter mechanics are developed, which do not addopt the local balance hypothesis, a fundamental assuption in the classical continuum mechanics. GENERIC formalism involving a generalized irreversible process is considered, which includes both disppation and disppationless processes, and the relavent new geometric structures are discussed. Accordingly, on the basis of multiscale models for soft matter, the methods of closure approximations are investigated. Based upon these methods, mechanics models are developed, which are applicable to macroscopic continuum soft matter. Finally, the relationship between the macroscopic mechanic behaviors of silicon-based oligomers/silica (composite) aerogels at the levels of macroscopic scale and the microscopic dynamics of their constituent atoms and molecules is examined. The validities of meodels are tested with a series of experimental data obtained with polycarbosilane and silica aerogels.

针对以固体形式描述的软物质，开展以从下而上的形式构建软物质体多尺度力学模型的理论框架的研究。探讨在各个尺度下数学几何结构的不变性和热力学自洽以及宏观连续介质力学的基本准则。通过直接对微结构进行约化描述或粗化开始，针对至上而下形式的经典连续介质力学模型在应用于低聚物等软物质、固体大变形和微区变形等条件下的局限性，构建比传统定义的"内变量"有更深刻更直接的微结构背景的结构状态变量。发展连续介质力学基本假设之一的局部平衡假设失效条件下的软物质力学理论，建立考虑具有更普遍意义的不可逆过程（包括耗散和非耗散）的耗散势形式的GENERIC热力学框架和新的几何结构。探索对多尺度力学模型的封闭近似方法，构建基于多尺度模型的软物质宏观连续介质力学模型。通过开展对典型含硅低聚物和（复合）气凝胶的实验研究，关联微结构与宏观力学行为，探索软物质多尺度力学模型在具有复杂分子结构的含硅低聚物和（复合）气凝胶中的应用。

针对经典连续介质力学在应用于低聚物等软物质、固体大变形和微区变形下的局限性，研究构建比传统“内变量”有更深刻更直接微结构背景的结构状态变量，发展考虑具有更普遍意义的不可逆过程（包括耗散和非耗散）耗散势的热力学框架和连续介质的多尺度模型，并利用实验手段研究典型含硅低聚物和气凝胶的多尺度模型及其微结构与宏观力学行为的关联，探索软物质多尺度模型在具有复杂分子结构的含硅低聚物和气凝胶中的应用。项目按计划执行，基本达到项目目标，共发表相关SCI和EI检索学术论文15篇，获授权国家发明专利1项，完成博士学术论文3篇，硕士学术论文7篇。重要研究成果有1)二氧化硅气凝胶的多尺度模型。本成果发展了二氧化硅气凝胶的常压干燥制备方法并成功制备了满足标准的块体力学试样，获国家发明专利1项；提出了多孔颗粒概念及连续介质意义下的多尺度微结构模型和结构特征参数，给出了该特征参数的计算方法；探讨了特征参数与二氧化硅气凝胶试样宏观力学性能的关联 (Journal of Materials Science 53(2):994 (2018)，Materials Letters 204:93 (2017), Materials Chemistry and Physics 162:345 (2015), Progress in Chemistry 26(8):1329 (2014)）。2)低聚物流变性能与微结构。发展了聚碳硅烷低聚物熔体稳态实验方法，获得了重复性良好的聚碳硅烷低聚物熔体的流变测试数据，建立了聚碳硅烷低聚物熔体零剪切粘度方法（Key Engineering Materials 726:90 (2017)，Key Engineering Materials 726:95 (2017) )；提出了根据应变速率频率方法构建动态流变响应主曲线的新判据（Journal of Central South University 23(8 ):1873 (2016)）。