Mahmad Rasid Irina, Do Changwoo, Holten-Andersen Niels, Olsen Bradley D
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA.
Soft Matter. 2021 Oct 13;17(39):8960-8972. doi: 10.1039/d1sm00392e.
Recent experimental and theoretical work has shown that sticker clustering can be used to enhance properties such as toughness and creep resistance of polymer networks. While it is clear that the changes in properties are related to a change in network topology, the mechanistic relationship is still not well understood. In this work, the effect of sticker clustering was investigated by comparing the dynamics of random copolymers with those where the stickers are clustered at the ends of the chain in the unentangled regime using both linear mechanics and diffusion measurements. Copolymers of ,-dimethyl acrylamide (DMA) and pendant histidine groups were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. The clustered polymers were synthesized using a bifunctional RAFT agent, such that the midblock consisted of PDMA and the two end blocks were random copolymers of DMA and the histidine-functionalized monomer. Upon addition of Ni ions, transient metal-coordinate crosslinks are formed as histidine-Ni complexes. Combined studies of rheology, neutron scattering and self-diffusion measurements using forced Rayleigh scattering revealed changes to the network topology and stress relaxation modes. The network topology is proposed to consist of aggregates of the histidine-Ni complexes bridged by the non-associative midblock. Therefore, stress relaxation requires the cooperative dissociation of multiple bonds, resulting in increased relaxation times. The increased relaxation times, however, were accompanied by faster diffusion. This is attributed to the presence of defects such as elastically inactive chain loops. This study demonstrates that the effects of cooperative sticker dissociation can be observed even in the presence of a significant fraction of loop defects which are known to alter the nonlinear properties of conventional telechelic polymers.
最近的实验和理论研究表明，贴段聚类可用于增强聚合物网络的韧性和抗蠕变性等性能。虽然很明显性能的变化与网络拓扑结构的改变有关，但这种机理关系仍未得到很好的理解。在这项工作中，通过使用线性力学和扩散测量方法，比较无缠结状态下随机共聚物与贴段在链端聚类的共聚物的动力学，研究了贴段聚类的影响。使用可逆加成-断裂链转移（RAFT）聚合反应合成了N,N-二甲基丙烯酰胺（DMA）和侧链组氨酸基团的共聚物。使用双官能团RAFT试剂合成了聚类聚合物，使得中间嵌段由聚DMA组成，两个端嵌段是DMA和组氨酸功能化单体的无规共聚物。加入镍离子后，形成了作为组氨酸-镍络合物的瞬态金属配位交联。结合流变学、中子散射和使用受激瑞利散射的自扩散测量研究，揭示了网络拓扑结构和应力松弛模式的变化。提出网络拓扑结构由非缔合中间嵌段桥接的组氨酸-镍络合物聚集体组成。因此，应力松弛需要多个键的协同解离，导致松弛时间增加。然而，增加的松弛时间伴随着更快的扩散。这归因于存在诸如弹性无活性链环之类的缺陷。这项研究表明，即使存在已知会改变传统遥爪聚合物非线性性能的大量环缺陷，也能观察到协同贴段解离的影响。
