Toward understanding the cross-linking from molecular chains to aggregates by engineering terminals of supramolecular hyperbranched polysiloxane
Yuanbo Zhang, Junshan Yuan, Jingzhi Hu, Zhixuan Tian, Weixu Feng, Hongxia Yan*
Y. Zhang, J. Yuan, J Hu, Z. Tian, Prof. W. Feng, Prof. H. Yan
Shaanxi Key Laboratory of Macromolecular Science and Technology, Xi’an Key Laboratory of Hybrid Lumines-cent Materials and Photonic Device, School of Chemistry and Chemical engineering, Northwestern Polytechnical University, Xi’an 710129, China
E-mail: hongxiayan@nwpu.edu.cn
Keywords: hyperbranched polymers, aggregates, crosslinking
Abstract:Crosslinking thermosets with hyperbranched polymers confers them superior comprehensive performance. However, it still remains a further understanding of polymer crosslinking from the molecular chains to the role of aggregates. In this study, three hyperbranched polysiloxane structures (HBPSi-R) are synthesized as model macromolecules, each featuring distinct terminal groups (R denotes amino, epoxy, and vinyl groups) while similar molecular backbone (Si-O-C). These structures were subsequently copolymerized with epoxy monomers to construct interpenetrating HBPSi-R/epoxy/anhydride co-polymer systems. The spatial molecular configuration and flexible Si-O-C branches of HBPSi-R endow them with remarkable reinforcement and toughening effects. Notably, an optimum impact strength of 28.9 kJ mol-1 is achieved with a mere 3% loading of HBPSi-V, nearly three times that of the native epoxy (12.9 kJ mol-1). By contrasting the terminal effects, the aggregation states and crosslinking modes were proposed, thus clarifying the supramolecular-dominant aggregation mechanism and covalent-dominant dispersion mechanism, which influences the resulting material properties. This work underscores the significance of aggregate science in comprehending polymer crosslinking and provides theoretical insights for tailoring material properties at a refined molecular level in the field of polymer science.
1. Introduction
Aggregation and dispersion have long been regarded as mutually exclusive concepts, but this dichotomy is in fact a fundamental feature of the natural world. From the gravitational coalescence of dispersed celestial bodies in the universe, to the islands in the ocean, and even to the intricate network-formation of polymer cross-linking, this trend seems like ubiquitous across all scales of the natural world. This universal significance underscores the importance of understanding the mechanisms that govern aggregation and dispersion phenomena in diverse fields of science.[1- 5]
Conventionally, the cross-linking of polymer is known as a network-formation process from linear monomer to three-dimensional structures, namely the crosslinked network.[6- 8] But if substitute the linear monomer with a spatial topological macromolecule, such as dendrimer,[9]star polymer,[10]hyperbranched polymer.[11] These topological macromonomers will occupy all or part of the cross-linking site to form a topologically-extended network with respect to their three-dimensional one, we coined it topological crosslinking networks (TCNs) that link through all or part of topological units. In such system, it could be recognized that the macromonomers should undergo both aggregation and dispersion behaviors during polymer crosslinking, which relies on their interface nature (for example, functional groups, surface energy, system temperature).[12- 14] Therefore, the aggregate state and nano-interface control over the cross-linked structure, and attributes the final properties of the resulting polymer.[15- 19]
Aggregate science contributes a fresh enlightenment to incline the view of the polymer crosslinking into a more refined and precise direction.[15- 19] People customarily consider the crosslinking of chemical bonds at the molecular level, however, in case of a multi-components system,[23]although many efforts have been made toward concepting supramolecular interpenetration or other related ideas among hydrogels and elastomers,[24- 28] The crosslinking modes should be included but not limited in covalent-bonds and weakly supramolecular interactions. Especially, in case of a thermoset polymer system, over-agglomeration of additives has always been regarded as an undesired matter that dampens material properties,[29, 30] but the aggregates at nano- or micro- scales could be the positive one reasoned from their nano-phase separation and forming a strong interface between double-phase polymer.[31]
As a representative crosslinked example, epoxy resins (EPs) feature excellent thermal and mechanical properties due to their highly-crosslinked network-forming architecture,[32- 34] which are widely involved in human society for aerospace, navigation, wind power. To meet the ever-growing demands for advanced application, the integrals on realizing high-performance and multi-function purposes still require an incisive understanding from structure-to-property relations toward in-principle building and tailoring polymers. The state-of-the-art work focuses their insights overlapping supramolecular chemistry,[35- 38]dynamic covalent and/or no-covalent chemistry[39- 42] to engineer the polymer network and to regulate the chemical crosslinks and physical entanglement, which inspires us insightful viewpoints to further understand the cross-linking, and the real formation of polymer.[43]
In this study, three kinds of hyperbranched polysiloxane (HBPSi-R) were designed and synthesized, respectively, each featuring with different terminals but similar molecular backbone (Si-O-C), and they were copolymerized with epoxy resin/anhydride system to construct supramolecular HBPSi-R/epoxy interpenetrating polymer networks. The thermal performance, curing behaviors and mechanical properties of resulting materials were studied in a fresh viewpoint of aggregation and dispersion to co-polymer crosslinking. To contrast the terminal effects, the aggregation states and nano-interface were revealed, as the double-crosslinking modes and their aggregate mechanism were proposed in combining their mechanical properties and different terminal groups. Highlighting the importance for understanding the polymer crosslinking from the concept of aggregate science, this work provides theoretical guidance toward in-principle tailoring material properties from a more refined molecular structure in polymeric science.