New Strategy to Construct Mechanically Strong and Tough Phenolic Networks by Considering the Effect of Curing Reactions and Physical States on the Cross-Linking Density and Cross-Linking Inhomogeneity
Zixuan Lei, Zhongzhou Zhang, Jian Wang, Li Xu, Jian Li, Zhichao Zhu, Yuhong Liu
Abstract
How to regulate the conflicting effect of cross-linking density and cross-linking inhomogeneity on the mechanical properties of a thermosetting polymer is of great scientific significance and the key to realizing high mechanical strength and toughness. Herein, a new regulation strategy for the inhomogeneous cross-linking networks is proposed from the viewpoint of curing reactions and physical states. Here, a reactive graphene oxide aerogel (GES) was prepared and employed to modify the hierarchical microstructure of phenolic resin (GESNH). The analysis of curing behavior, DMA, and DQ 1H NMR revealed that all GESNH samples possess similar cross-linking networks with like biomimetic “bricks in mortar” structure of a low cross-linking density, bulk phenolic networks, and an interfacial phase with a high cross-linking density but with diversely tuned cross-linking density and cross-linking inhomogeneity. As for GESNH cured at 130 °C or that reached the vitrification state in the first curing stage, the volume fraction of the interfacial phase was increased and the “bricks in mortar” structure was decreased, thus increasing cross-linking density and decreasing cross-linking inhomogeneity, compared with GESNH cured at 100 °C or that reached the gelation state. As a result, a higher cross-linking density and lower cross-linking inhomogeneity of the former can facilitate load transfer and restrict chain segment mobility, resulting in improved flexural and tensile strength but decreased ε and KIC. Notably, the flexural strength, tensile strength, ε, and KIC of GESNH-3 were increased by 23.16, 45.85, 82.99, and 102.30% compared with those of GNH, respectively.