2.6. Mechanical performance of HBPSi-R/EP
The mechanical strength of HBPSi-R/EP was tested to contrast the terminal effects among the three hyperbranched structures modifying epoxy resin (Figure 4a-c ), following the test results in Table S4 . The impact fracture microsurfaces are presented in Figure 4d-g . Generally, the impact failure of thermosets often owns to their high crosslinking density and rigid polymer chains that are difficult to dissipate the impact energy.[48] As the natural EP displayed typical brittleness “river-like” fracture covering a large microscopic area (Figure 4d ), with impact toughness of only 12.9 kJ/m2. In contrast, the incorporation of the hyperbranched structures, benefiting the merit of hyperbranched architecture and flexible Si-O-C chains, all demonstrate excellent strengthening and toughening effects, with a gradual decrease as the content is further increased. As shown in Figure 4a-c, HBPSi-V performs superior reinforcement effects regarding toughness, strength and modulus with respect to the other two hyperbranched structures, with an increasingly tough feature from native EP to HBPSi-NH2, HBPSi-EP and HBPSi-V in SEM photographs (Figure 4d-g) . The optimal impact strength of 28.9 kJ/m² was achieved with 3% incorporation of HBPSi-V, nearly three times higher than that of the pristine EP. Based on the aforementioned analyses, these hyperbranched structures possess similar molecular backbones with approximate hydroxyl concentration, as their terminals give rise to such significant differences in mechanical properties, which might be closely related to their unique crosslinking and aggregate behavior.[49, 50]
Among the three structures, HBPSi-V bears a moderate aggregation and dispersion behavior due to its non-reactive vinyl group, which could forms an appropriate nano-silicone aggregates to toughen the polymer network in case of elastomer mechanism being dominant.[51, 52] Meanwhile, their abundant -OH terminals can interact covalently and/or non-covalently within the epoxy network, acting as a supramolecular “nano-rivets” that tightly entangle within the network for strengthening effort.[53] However, it should be note that excessive aggregation of HBPSi-R does a negative effect on mechanical performance, where the bulk agglomeration of excessive hyperbranched polymer becomes a stress concentration area, leading to the deterioration in impact strength. It is worth mentioning that the deterioration in impact strength is less significant in HBPSi-V/EP compared to HBPSi-NH2/EP at the same filler load. Furthermore, the impact toughness of HBPSi-NH2/EP is even lower than that of the pristine EP, indicating that HBPSi-NH2 has a lower tendency to form bulk agglomerations than HBPSi-V at high filler additions. Regarding strength and modulus (Figure 4b, c) , the flexural strength increases by 36.4% from 106.1 MPa to 144.7 MPa with the incorporation of 3% HBPSi-EP. Both HBPSi-V and HBPSi-EP exhibit better strengthening effects compared to HBPSi-NH2, and HBPSi-V performs better at high filler loads (9%). Therefore, it is evident that the nano-reinforced and supramolecular ”nano-rivets” mechanisms both depend on appropriate aggregate and uniform dispersion of supramolecular HBPSi-V.
The thermomechanical behaviors were evaluated using dynamic thermomechanical analysis (DMA), as shown inFigure 5 . Overall, the temperature-dependent storage modulus (E’) curves in Figure 5d-f exhibit a consistent trend in all the three systems with increasing addition of hyperbranched polysiloxane. The co-crosslinking of HBPSi-R significantly improves the E’ value from the room temperature region to the glass transition of the materials (Figure 5a ), indicating the excellent reinforcement effect of HBPSi-R. The overall decreasing in crosslink density (dcrosslink ) is presented in Figure 5baccording to the rubber elasticity theory (S3.1 ).[54, 55] As a results, the decrease in dcrosslink is lower for HBPSi-NH2 compared to HBPSi-EP and HBPSi-V, which further confirms the tightly aggregated nature of HBPSi-NH2 that is unfavorable for forming the free volume in crosslinking network. As a whole, these hyperbranched structures can reduce the crosslinking density of thermoset but do not deteriorate their mechanical strength, which is attributed to their highly branched chains and abundant reactive terminals.[44, 56] The glass transition temperatures (Tg ) of EP, 6HBPSi-NH2/EP, 6HBPSi-EP/EP, 6HBPSi-V/EP are 131oC, 118 oC, 111 oC and 110 oC, respectively, indicating they are glassy polymer at room temperature.[57] The decrease inTg is foreseeable due to the amorphous structure, as well as the flexible molecular chains of HBPSi-R. Therefore, the co-crosslinking of HBPSi-R encourages a weakly-crosslinked but high-strength thermoset network, and its terminal nature comprehensively affects the aggregation, dispersion, even to crosslinking and the final material properties.