Department of Mechanical Engineering, University of Colorado Denver, Denver, CO 80217, USA.
Soft Matter. 2017 Oct 11;13(39):7061-7073. doi: 10.1039/c7sm01174a.
Anisotropic polymers with aligned macromolecule chains exhibit directional strengthening of mechanical and physical properties. However, manipulating the orientation of polymer chains in a fully cured thermoset is almost impossible due to its permanently crosslinked nature. In this paper, we demonstrate that rearrangeable networks with bond exchange reactions (BERs) can be utilized to tailor the anisotropic mechanical properties of thermosetting polymers. When a constant force is maintained at BER activated temperatures, the malleable thermoset creeps in the direction of stress, and macromolecule chains align themselves in the same direction. The aligned polymer chains result in an anisotropic network with a stiffer mechanical behavior in the direction of creep, while with a more compliant behavior in the transverse direction. The degree of network anisotropy is proportional to the amount of creep strain. A multi-length scale constitutive model is developed to study the creep-induced anisotropy of thermosetting polymers. The model connects the micro-scale BER kinetics, orientation of polymer chains, and directional mechanical properties of network polymers. Without any fitting parameters, it is able to predict the evolution of creep strain at different temperatures and anisotropic stress-strain behaviors of CANs after creep. Predictions on the chain orientation are verified by molecular dynamics (MD) simulation. Based on parametric studies, it is shown that the influences of creep time and temperature on the network anisotropy can be generalized into a single parameter, and the evolution of directional modulus follows an Arrhenius type time-temperature superposition principle (TTSP). The presented work provides a facile approach to transform isotropic thermosets into anisotropic ones using simple heating, and their directional properties can be readily tailored by the processing conditions.
具有定向大分子链的各向异性聚合物表现出对机械和物理性能的各向增强。然而，由于热固性聚合物的永久交联性质，几乎不可能在完全固化的热固性中操纵聚合物链的取向。在本文中，我们证明了具有键交换反应 (BER) 的可重排网络可用于调整热固性聚合物的各向异性机械性能。当在 BER 激活温度下保持恒定力时，可延展的热固性在应力方向上蠕变，大分子链沿相同方向排列。定向聚合物链导致各向异性网络，在蠕变方向上具有更硬的机械行为，而在横向方向上具有更柔顺的行为。网络各向异性的程度与蠕变应变的量成正比。开发了多尺度本构模型来研究热固性聚合物的蠕变诱导各向异性。该模型将微尺度 BER 动力学、聚合物链的取向和网络聚合物的定向力学性能联系起来。无需任何拟合参数，它就能够预测不同温度下的蠕变应变演变以及蠕变后的 CAN 的各向异性应力-应变行为。通过分子动力学 (MD) 模拟验证了对链取向的预测。基于参数研究，表明蠕变时间和温度对网络各向异性的影响可以概括为单个参数，并且定向模量的演变遵循阿累尼乌斯型时-温叠加原理 (TTSP)。所提出的工作提供了一种简便的方法，可通过简单加热将各向同性热固性转变为各向异性，并且可以通过加工条件轻松调整其各向异性性能。
