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.