2.5. Aggregation mechanism and crosslinking modes of HBPSi-R/EP
Based on the aforementioned discussion, hyperbranched polysiloxanes with similar molecular backbone (Si-O-C) but varying terminals could serve as nice model to explore the terminal effects on polymer crosslinking. The aggregate behaviors and crosslinking modes are proposed and described inFigure 3 . It is important to note that HBPSi-R not only possesses abundant -OH groups for reactiveness but also characteristic terminals (-NH2, -EP, vinyl groups) for adjusting its overall interface features with epoxy resin. The -OH groups can react simultaneously with the DGEBA monomer (ring-opening) and the anhydride monomer (forming ester bonds), thus covalently crosslinking with epoxy network. In addition, it will also supramolecularly crosslinks within the network through hydrogen bond interactions. Therefore, the three systems should exhibit a consistent double-crosslinking behavior involving both covalent and supramolecular modes regardless of their end groups.
To elucidate the characteristic terminal effects, the distribution states of HBPSi-R were visually observed using transmission electron microscopy (TEM) and scanning transmission electron microscopy tools (STEM, the magnification images in Figure 3). A staining method was adopted to enhance the imaging clarity since HBPSi-R is embedded in the cured resin (S1.4 ).[5] It can be founded that the hyperbranched components are uniformly dispersed with varying aggregation degrees at the nanoscale. This fact indicates that HBPSi-R forms a series of supramolecular hyperbranched polysiloxane aggregates during polymer crosslinking, driven by their low surface energy[46] and intermolecular hydrogen bonds[47].
Specifically, HBPSi-NH2 shows typical sea-island characteristic, where the HBPSi-NH2 acts as the dispersed phase and the resin matrix serves as the continuous phase, featuring the tightest aggregation behavior with size of ~ 90 nm than HBPSi-V and HBPSi-EP as seen in Figure 3, coined as aggregately crosslinking whose supramolecular behavior dominates during polymer crosslinking, where the strong intermolecular hydrogen bonds between -NH2 and -OH terminals drive the assembly of HBPSi-NH2, as clearly defined by the silicon-rich scanning surface in element mapping. In contrast, HBPSi-EP shows dominant uniform dispersion behavior in its TEM image, without apparent nano-phase separation. This is referred to as evenly crosslinking, wherein HBPSi-EP behaves likewise to another epoxy component to covalently bond in such system (covalent-dominance in crosslinking). Between the two structure, HBPSi-V demonstrates uniform dispersion with also stable nano-sized aggregates, whose particle sizes is more regular than that of HBPSi-NH2. This behavior, termed bulky crosslinking, arises from the moderate interface in which the non-reactive vinyl groups of HBPSi-V play a supportive role to make sure a balance between covalent and non-covalent supramolecular forms. These finds provide deeper insights from the holism of aggregate science into the terminal-dependent crosslinking of HBPSi-R within epoxy network, consequently influencing the overall material properties.