2.2. Synthesis and structural investigation of HBPSi-R
The synthesis of hyperbranched polysiloxane (HBPSi-R) bearing Si-O-C molecular backbone is straightforward using A2+B3 ester-exchange polycondensation, where R represent their terminal groups of amino (HBPSi-NH2), epoxy (HBPSi-EP) and vinyl group (HBPSi-V). Synthetic details are described inS1. 2 . Briefly, a simple diol (1,3-propanediol, PDO) was employed as A2monomer to react with (3-aminopropyl) triethoxysilane, (3-glycidyloxypropyl) triethoxysilane and triethoxyvinylsilane, respectively, yielding liquid products (Figure 1 ). The concentration of hydroxyl group (-OH) was quantified by titration experiments, with details given in S2.3 , yielding approximate -OH concentration of 1.69 × 10-2 mol g-1, 1.33 × 10-2 mol g-1 and 1.45 × 10-2 mol g-1 for HBPSi-NH2, HBPSi-EP and HBPSi-V, respectively. Since the -OH concentration of HBPSi-R is much lower than that of epoxy resin, thus the incorporation of HBPSi-R does not change the equivalence dosage of epoxy formulation.
The chemical structure of HBPSi-NH2, HBPSi-EP and HBPSi-V are evidenced by fourier transform infrared (FT-IR),1H nuclear magnetic resonance (1H-NMR), respectively. To prove the proceeded polycondensation, the distillates from reaction systems were collected for IR detection, showing the same IR profiles compared with that of standard ethanol (See Figure S1a ), demonstrating the as-expected polycondensation.1H-NMR was performed to assign the chemical shifts of protons in spectra of HBPSi-R (Figure 1b ) and their monomers (Figure S2 ). It’s noted that the protons from H1 and H2 in PDO are separated into three different modes in each spectrum of HBPSi-R, which is attributed to various linked modes of protons in dendritic, linear and terminal sites upon hyperbranched structure.[44] Besides, the protons from amino, epoxy and vinyl groups can be recognized as marking a-e involving in the synthetic polymers (Figure 1b ), indicating their characteristic groups. The FT-IR information of HBPSi-R and their monomers are presented in Figure 1c and S2.1 , respectively. In the spectrum of HBPSi-NH2, two peaks around 3300 cm-1 are attributed to the stretching vibration of primary amine group, which can also be founded in the spectrum of its monomers (Figure S1b ). In the spectrum of HBPSi-EP, the peak at 880 cm-1 corresponds to the C-O-C absorption in epoxy group. In the spectrum of HBPSi-V, the peak at 1600 cm-1 is attributed to the stretching vibration of carbon-carbon double bond.[45] These results prove the hyperbranched structure information with the presence of characteristic terminals.
2.3. Fabrication ofHBPSi-R/EP co-polymer system
The HBPSi-R/EP co-polymer system was fabricated using a thermoset workflow (S1.3 ). The resin matrix consisted of an epoxy resin (DGEBA, E51) and an anhydride curing agent (methyl tetrahydrophthalic anhydride, MTHPA). The fabrication of xHBPSi-R/EP composites involved a thermocuring process by casting method, where ’x’ represents the mass fraction of HBPSi-R in the whole resin matrix. In brief, different mass fractions (3%, 6%, 9%) of HBPSi-R were mixed with 70 g of DGEBA epoxy resin and stirred at 80 °C for 15 minuntil a clear yellowish solution was obtained. Subsequently, 56 g of MTHPA and two drops of tris(dimethylaminomethyl)phenol (DMP-30, curing accelerator) were added and stirred for another 15 min. The mixture was then poured into a preheated mold, and then follows thoroughly degassing in an 80 °C vacuum for 30 min. The curing process involved a heating procedure of 120 °C for 2 h, followed by 150 °C for 3 h and 180 °C for 2 hours. After curing, the samples were demolded and prepared for testing.
Figure 2 illustrates a schematic model describing the distinct reaction modes of HBPSi-R with epoxy substrates. These HBPSi-R structures possess unique terminals and abundant -OH groups upon their molecular backbone (Figure 2a ). The -OH groups can react with both DGEBA epoxy resin and anhydride curing agent, resulting in clear and uniform resin solutions after brief pre-polymerization. The characteristic terminal groups of HBPSi-R control over their crosslinking behavior and aggregate state of HBPSi-R. Specifically, the amino group in HBPSi-NH2 react with both epoxy group and anhydride group, while the epoxy group in HBPSi-EP participates in the curing reaction likewise another epoxy component, and the carbon-carbon double bond (vinyl) in HBPSi-V does not react with epoxy resin and anhydride-type curing agent, which plays a supportable role in interpenetrating thermoset network with hyperbranched structure.