Toward understanding the cross-linking from molecular chains to aggregates by engineering terminals of supramolecular hyperbranched polysiloxane

Crosslinking thermosets with hyperbranched polymers confers them superior comprehensive performance. However, it still remains a further understanding of polymer crosslinking from the molecular chains to the role of aggregates. In this study

level, however, in case of a multi-components system, [23] although many efforts have been made toward concepting supramolecular interpenetration or other related ideas among hydrogels and elastomers, [24][25][26][27][28] The crosslinking modes should be included but not limited in covalent-bonds and weakly supramolecular interactions.Especially, in case of a thermoset polymer system, over-agglomeration of additives has always been regarded as an undesired matter that dampens material properties, [29,30] but the aggregates at nanoor micro-scales could be the positive one reasoned from their nano-phase separation and forming a strong interface between double-phase polymer. [31]s a representative crosslinked example, epoxy resins (EPs) feature excellent thermal and mechanical properties due to their highly-crosslinked network-forming architecture, [32][33][34] which are widely involved in human society for aerospace, navigation, wind power.To meet the ever-growing demands for advanced application, the integrals on realizing high-performance and multi-function purposes still require an incisive understanding from structure-toproperty relations toward in-principle building and tailoring polymers.The state-of-the-art work focuses their insights overlapping supramolecular chemistry, [35][36][37][38] dynamic covalent and/or no-covalent chemistry [39][40][41][42] to engineer the polymer network and to regulate the chemical crosslinks and physical entanglement, which inspires us insightful viewpoints to further understand the cross-linking, and the real formation of polymer. [43]n this study, three kinds of hyperbranched polysiloxane (HBPSi-R) were designed and synthesized, respectively, each featuring with different terminals but similar molecular backbone (Si-O-C), and they were copolymerized with epoxy resin/anhydride system to construct supramolecular HBPSi-R/epoxy interpenetrating polymer networks.The thermal performance, curing behaviors and mechanical properties of resulting materials were studied in a fresh viewpoint of aggregation and dispersion to co-polymer crosslinking.To contrast the terminal effects, the aggregation states and nanointerface were revealed, as the double-crosslinking modes and their aggregate mechanism were proposed in combining their mechanical properties and different terminal groups.Highlighting the importance for understanding the polymer crosslinking from the concept of aggregate science, this work provides theoretical guidance toward in-principle tailoring material properties from a more refined molecular structure in polymeric science.

Reagent materials and methods
The reagent materials and method information are supplied in S1.
The chemical structures of HBPSi-NH 2 , HBPSi-EP and HBPSi-V are evidenced by Fourier transform infrared (FT-IR), 1 H nuclear magnetic resonance ( 1 H-NMR), and 29 Si nuclear magnetic resonance ( 29 Si-NMR, Figure S4), respectively.The distillates from reaction systems were collected and detected as standard ethanol in IR profiles (See Figure S2a), demonstrating the as-expected polycondensation. 1 H-NMR was performed to assign the chemical shifts of protons in spectra of HBPSi-R (Figure 1B) and their monomers (Figure S3).It is 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-NH 2 (Figure S3b), 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.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.

Fabrication of HBPSi-R/EP co-polymer system
The HBPSi-R/EP co-polymer system was fabricated using a thermoset workflow (Supporting information 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 wt%, 6 wt%, 9 wt%) 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, The synthesis route of HBPSi-R featuring with varying terminals, (B) 1 H-nuclear magnetic resonance (NMR) spectra and (C) Fourier transform infrared (FT-IR) information of the synthetic polymers.and then follows thoroughly degassing in an 80 • C vacuum for 30 min.The curing process subjected to a heating procedure of 120 • C for 2 h, 150 • C for 3 h and 180 • C for 2 h.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, with typical reactions described in Figure S1.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-NH 2 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.

Curing and thermal performance of HBPSi-R/EP
To investigate the influence of HBPSi-R with varying terminal groups on the curing performance of epoxy resin, we conducted isothermal differential scanning calorimetry (DSC) of the pre-polymer resin compounds, as shown in Figure 2B.The DSC thermograms revealed a single exothermic peak upon the incorporation of HBPSi-R, similar to that of the native EP.This observation indicates a favorable compatibility of the three HBPSi-R compounds within the epoxy/anhydride matrix.Notably, the temperature at peak initiation, peak value, and peak termination exhibited only slight differences, suggesting that the inclusion of HBPSi-R does not significantly affect the curing behavior of the epoxy resin.Consequently, the optimum curing time and temperature of HBPSi-R/EP were set to mirror the curing conditions of EP/MTHPA, thereby facilitating comprehensive comparisons for their other properties.Furthermore, we conducted rheology analysis to determine the viscosity of neat DGEBA and DGEBA with 6% HBPSi-R.The rheological analysis was conducted at a specific peak temperature (150 • C) in DSC curves (Figure 2B).The time-sweep curves (Figure S7) demonstrate a close initial viscosity for all four systems, with a substantial increase in viscosity observed in the case of HBPSi-NH 2 and HBPSi-EP.This substantial increase indicates their reactivity with DGEBA and their active involvement in the crosslinking process.
To determine the real chemical bonding of HBPSi-R within epoxy resin, Fourier-transform infrared (FT-IR) spectra of the cured resin samples were performed (Figure 2D).Obviously, a wider stretching vibration of Si-O at 1080 cm −1 is recognized in spectra of HBPSi-NH 2 /EP, HBPSi-EP/EP and HBPSi-V/EP, meanwhile, the peak intensity of hydroxyl group is significantly lower than that of the native EP, indicating the consummation of residual hydroxyl groups and confirming the participation of HBPSi-R in the copolymerization and crosslinking with the epoxy network.X-ray diffraction (XRD, Figure S5) profiles revealed that the branching structures did not influence the molecular arrangement, thereby maintaining the amorphous crosslinked structure of the epoxy resin.
Thermogravimetric analysis (TGA) was conducted to assess the impact of HBPSi-R on the thermal properties of materials.Given in Figure 2C, overall, the addition of hyperbranched polysiloxanes primarily enhanced the residual char of the materials while exerting minimal influence on the initial decomposition and main decomposition behavior (as indicated by derivative thermogravimetry, DTG, Figure S5b).This improvement can be attributed to the introduction of the inert silicon-containing component.Notably, compared to neat EP, HBPSi-EP/EP exhibited the most substantial increase in char residue, rising from 6.0% to 11.7%, whereas the amino and vinyl variants showed a char residue of 9.5%.This can be attributed to the superior uniform dispersion of HBPSi-EP within the resin matrix compared to the other variants.The detailed curing and thermal parameters are supplied in Table S4.

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 in Figure 3.It is important to note that HBPSi-R not only possesses abundant -OH groups for reactiveness but also characteristic terminals (-NH 2 , -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 The schematic model and transmission electron microscopy (TEM) morphologies describing the terminals effect and nano-crosslinking modes of HBPSi-NH 2 , HBPSi-EP and HBPSi-V within the thermoset matrix.
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-NH 2 shows typical sea-island characteristic, where the HBPSi-NH 2 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 -NH 2 and -OH terminals drive the assembly of HBPSi-NH 2 , 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 (covalentdominance 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-NH 2 .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 terminaldependent crosslinking of HBPSi-R within epoxy network, consequently influencing the overall material properties.

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 S5.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/m 2 .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-NH 2 , HBPSi-EP and HBPSi-V in scanning electron microscope (SEM) photographs (Figure 4D-G).The optimal impact strength of 28.9 kJ/m 2 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]mong 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. [11,51]Meanwhile, their abundant -OH terminals can interact covalently and/or noncovalently within the epoxy network. [52]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-NH 2 /EP at the same filler load.Fur-thermore, the impact toughness of HBPSi-NH 2 /EP is even lower than that of the pristine EP, indicating that HBPSi-NH 2 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-NH 2 , and HBPSi-V performs better at high filler loads (9%).Therefore, it is evident that the strengthen and toughening effects both depend on appropriate aggregate and uniform dispersion of HBPSi-V.
The thermomechanical behaviors were evaluated using dynamic thermomechanical analysis (DMA), as shown in Figure 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 reinforcement effect of HBPSi-R.The overall decreasing in crosslink density (d crosslink ) is presented in Figure 5B according to the rubber elasticity theory (S3.1). [53,54]As a results, the decrease in d crosslink is lower for HBPSi-NH 2 compared to HBPSi-EP and HBPSi-V, which further confirms that the tightly aggregated HBPSi-NH 2 is unfavorable for enhancing free volume of crosslinking network.As a whole, these hyperbranched structures can reduce the crosslinking density of thermoset but do not deteriorate their mechanical strength, owing to their highly branched nature and abundant reactive terminals. [44,55]The glass transition temperatures (T g ) of EP, 6HBPSi-NH 2 /EP, 6HBPSi-EP/EP, 6HBPSi-V/EP are 131, 118, 111, and 110 • C, respectively, indicating they are glassy polymers at room temperature. [56]The decrease in T g is foreseeable due to the amorphous structure, as well as the flexible molecular chains of HBPSi-R.
Regarding the loss modulus (E′′, Figure S6), the E′′ values of HBPSi-R/EP below T g are all higher than that of the natural EP thermoset.In such case, the motion of polymer chains and the crosslinked network are restrained through the enhanced non-covalent and covalent bonded HBPPB within epoxy network.The increased E′′ indicates that energy is dissipated by the friction of polymer chains, suggesting enhanced damping properties.Therefore, the cocrosslinking 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.

CONCLUSION
In summary, we successfully synthesized three hyperbranched structures with a common Si-O-C molecular backbone but differing terminal groups.These structures were co-crosslinked with thermoset epoxy to explore their terminal effects on affecting the overall material performance.The special molecular characteristics of HBPSi-R involves abundant -OH groups for their reactiveness, as well as distinctive terminals (-NH 2 , -EP, vinyl groups) that impart characteristic interface features.All three variants of HBPSi-R exhibit well compatibility within the epoxy matrix, showcasing diverse nano-interface and aggregation behavior.Benefiting their spatial molecular configuration and flexible Si-O-C branches, HBPSi-R manifests exceptional strengthening and toughening effects regardless of their terminal groups.Among them, HBPSi-NH 2 predominantly exhibits the aggregation behav-ior from supramolecular interaction, while HBPSi-EP show predominantly exhibits dispersion behavior from covalent effect.HBPSi-V achieves a balance of the both, surpassing the other two structures in terms of optimal impact toughness (28.9 kJ mol −1 ) and better mechanical performance under high dosage, where the non-reactive vinyl group acts as a supportable role during polymer crosslinking.These findings not only emphasize the pronounced reinforcement effects of HBPSi-R but also provide a fresh perspective from aggregate science in the context of polymer crosslinking.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interests.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available at http://doi.org/

F
I G U R E 2 (A)The schematic illustration for HBPSi-R/EP co-polymer systems with their unique reactive modes, (B) differential scanning calorimetry (DSC) thermograms for pre-polymer mixtures at heating rate of 15 • C⋅min −1 , (C) weight loss curves under nitrogen atmosphere, (D) Fourier transform infrared (FT-IR) spectra of the cured samples, samples are 6HBPSi-NH 2 /EP, 6HBPSi-EP/EP, and 6HBPSi-V/EP, respectively.

F I G U R E 4
The mechanical performance test of (A) impact strength, (B) flexural strength, and (C) modulus at 25 • C; (D-G) SEM photographs indicates the impact tough facture from native EP to HBPSi-NH 2 /EP, HBPSi-EP/EP, HBPSi-V/EP.

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I G U R E 5 (A) The temperature-dependent storage modulus of 6HBPSi-R/EP with distinct terminals, (B) network crosslinking density, (C) Tan δ curves; (D-F) storage modulus curves with different content of HBPSi-R.
This work was sponsored by National Natural Science Foundation of China (grant number: 22175143), Key Research and Development Project of Shaanxi (grant number: 2022GY-353), Science Center for Gas Turbine Project (grant number: P2022-DB-V-001-001), and Fundamental Research Funds for the Central Universities (grant number: D5000230086).The authors thank Analytical and Testing Center of Northwestern Polytechnical University for test assistance.