A photoviscoplastic model for photoactivated covalent adaptive networks
A photoviscoplastic model for photoactivated covalent adaptive networks
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DOI：
10.1016/j.jmps.2014.05.008
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发表时间：
2014-10
影响因子：
5.3
通讯作者：
Jing Ma-;Xiaoming Mu;C. Bowman;Youyi Sun;M. Dunn;H. Qi;D. Fang
中科院分区：
文献类型：
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作者：
Jing Ma-;Xiaoming Mu;C. Bowman;Youyi Sun;M. Dunn;H. Qi;D. Fang
光激活聚合物(LAPS)是一类当光照射时会产生机械变形的当代材料。在光驱动的不同分子机制中，我们研究了自由基诱导的键交换反应(BERS)，它通过加成-裂解过程改变大分子链，其中活性端基连接的自由链随后断裂网络链。因此，BER产生了具有共价自适应网络的聚合物。当搭接试件加载时，BERS的宏观后果是应力松弛和塑性变形。此外，如果光穿过样品的不均匀，导致不均匀的应力松弛，样品在卸载后会变形，以达到平衡。在过去，这种光激活机制被模拟为一个相演化过程，其中链的加成-断裂过程被认为是应力相和出生时未变形和无应力的新生相之间的相变。这样的建模方案以合理的保真度描述底层物理，但计算成本很高。在本文中，我们提出了一种新的方法，将BER引起的大分子网络变化模拟为粘塑性变形过程，基于光照射引起的应力松弛是一个与时间相关的过程，类似于在光照射后具有不可恢复变形的粘弹性固体。将这一建模概念进一步转化为有限变形光力学本构模型。该模型的流变表示是一个与标准线性固体粘弹性模型串联放置的光粘塑性单元。建立了两个含时单元的时间积分的两步迭代隐式格式。我们进行了一系列实验来确定模型中的材料参数，并验证了模型在复杂几何和加载结构下的性能。有限元模拟结果与实验结果的对比表明，该模型能够准确地模拟光应力松弛、拉伸蠕变和弯曲等多种光机耦合加载条件下的搭接响应。此外，我们通过模拟一系列显示复杂的三维、依赖于时间的光变形的例子来展示该模型的多功能性，包括光折纸、光成形和光膨胀测试。
Light activated polymers (LAPs) are a class of contemporary materials that when irradiated with light respond with mechanical deformation. Among the different molecular mechanisms of photoactuation, here we study radical induced bond exchange reactions (BERs) that alter macromolecular chains through an addition-fragmentation process where a free chain whose active end group attaches then breaks a network chain. Thus the BER yields a polymer with a covalently adaptable network. When a LAP sample is loaded, the macroscopic consequence of BERs is stress relaxation and plastic deformation. Furthermore, if light penetration through the sample is nonuniform, resulting in nonuniform stress relaxation, the sample will deform after unloading in order to achieve equilibrium. In the past, this light activation mechanism was modeled as a phase evolution process where chain addition-fragmentation process was considered as a phase transformation between stressed phases and newly-born phases that are undeformed and stress free at birth. Such a modeling scheme describes the underlying physics with reasonable fidelity but is computationally expensive. In this paper, we propose a new approach where the BER induced macromolecular network alteration is modeled as a viscoplastic deformation process, based on the observation that stress relaxation due to light irradiation is a time-dependent process similar to that in viscoelastic solids with an irrecoverable deformation after light irradiation. This modeling concept is further translated into a finite deformation photomechanical constitutive model. The rheological representation of this model is a photoviscoplastic element placed in series with a standard linear solid model in viscoelasticity. A two-step iterative implicit scheme is developed for time integration of the two time-dependent elements. We carry out a series of experiments to determine material parameters in our model as well as to validate the performance of the model in complex geometrical and loading configurations. The comparison between the finite element simulations and experiments shows that the model can accurately capture the response of the LAP under a wide range of coupled photo-mechanical loading conditions, such as light induced stress relaxation, creep in tension, and bending. Furthermore, we demonstrate the versatility of the model by simulating a series of examples that exhibit complex three-dimensional, time-dependent photodeformation, including photoorigami, photoforming, and photobulge tests.