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.