A solvent-free processed low-temperature tolerant adhesive

Ultra-low temperature resistant adhesive is highly desired yet scarce for material adhesion for the potential usage in Arctic/Antarctic or outer space exploration. Here we develop a solvent-free processed low-temperature tolerant adhesive with excellent adhesion strength and organic solvent stability, wide tolerable temperature range (i.e. −196 to 55 °C), long-lasting adhesion effect ( > 60 days, −196 °C) that exceeds the classic commercial hot melt adhesives. Furthermore, combine experimental results with theoretical calculations, the strong interaction energy between polyoxometalate and polymer is the main factor for the low-temperature tolerant adhesive, possessing enhanced cohesion strength, suppressed polymer crystallization and volumetric contraction. Notably, manufacturing at scale can be easily achieved by the facile scale-up solvent-free processing, showing much potential towards practical application in Arctic/Antarctic or planetary exploration.

Up to now, a majority of traditional adhesives are based on polymer as the main component [9][10][11] , for example, commercially available hot melt adhesives include ethylene-vinyl acetate copolymer (EVA), polyamide (PA) and polyether sulfone (PES), etc.Despite their widespread use in daily life, they still have some bottlenecks [12][13][14][15][16][17] , especially at low temperature: 1) high crosslinking density and low surface energy lead to di cult bonding and easy debonding between substrate surface and adhesive; 2) poor interfacial in ltration effect with easily formed thicker adhering layer, resulting in undesired residual stress; 3) the traditional polymer molecules tend to be frozen at low temperature, leading to volumetric contraction, enhanced fragileness, weakened mechanical force transmission across the substrates, and reduced resistance to crack propagation and 4) the long-term stability in low temperature is generally unmet, and the adhesion mechanism especially that under low temperature has been less investigated.Although some strategies, such as adding plasticizer/crosslinking agents or non-polar substituents, can elevate the temperature tolerance range of adhesives to some extent 15,17,18 , the lowest temperature resistance for most of commercial hot melt adhesives is above − 50°C.Therefore, the novel functional polymer-based adhesives that can be used at ultra-low temperature are still demanded yet largely unmet for speci c scenarios like Arctic/Antarctic or outer space exploration.
Polyoxometalates (POMs) have triggered a urry of attentions to the elds of chemistry and materials science owing to their unique physic-chemical properties [19][20][21][22] .Actually, POMs are frequently deemed as desired building blocks for constructing adhesive due to the following advantages [23][24][25][26] : 1) favorable interface adhesion might be promoted by regulating the crosslink density of polymer using POMs to reduce the residual stress; 2) cohesion strength of adhesive would be markedly enhanced by the interaction with POMs carrying multiple hydrogen protons and oxygen-rich surface, resulting in the strong interaction energy and large energy dissipation [27][28][29] and 3) POMs with well-de ned structures and compositions can act ideal templates for the theoretical calculations 30,31 .Nevertheless, the currently reported POMs-based adhesives usually rely on solvents (e.g., organic or aqueous agents) [32][33][34][35][36][37] , since it can break intermolecular forces of POMs and other counterparts, and subsequently assemble into viscous materials 38,39 .However, low temperature would lead to phase transition of solvents and media, which makes adhesive become brittle or deformed to seriously weaken their functionality or applicability 40 .Up to now, it has been reported that POMs based adhesive can act as low temperature adhesion by removing the solvent 41 , yet the presence of solvents would result in many inconveniences in practical applications including storage, transportation, or processing processes, as well as the possible performance suppression caused by the residue of solvents.Thus, based on the above considerations and inspired by the pioneering works, the exploration of solvent-free method to facilely prepare lowtemperature tolerant adhesive would be an intriguing target for practical usage like Arctic/Antarctic or outer space exploration, yet related research works have been rarely reported as far we know.
As a proof-of-concept, a type of H 4 SiW 12 O 40 (SiW 12 ) based solvent-free polymer (SSFP) adhesive has been successfully designed and prepared (Scheme 1).The SSFP adhesive exhibits high adhesion strength, favorable interfacial adhesion ability, excellent organic solvent stability and ultra-low temperature tolerance, which is superior to commercially available hot melt adhesives.Moreover, theoretical calculations prove that the strong interaction energy between SiW 12 and polyethylene glycol (PEG) though abundant hydrogen bonds endows SSFP adhesive with favorable adhesion performance under a wide temperature range.This work may promote the development of solvent-free adhesives for potential applications of Arctic/Antarctic or planetary exploration.

Synthesis and characterizations of SSFP adhesive
A white solvent-free SSFP adhesive is facilely prepared on a kilogram-scale through a heat-assisted process (Supplementary Fig. S1, detail see Methods).Fourier-transform infrared (FT-IR) spectra (Fig. 1a and Supplementary Fig. S2) show that four typical characteristic peaks of SiW cm − 1 is slightly shifted to 1108 cm − 1 after forming SSFP adhesive 30 , which is ascribed to the possible hydrogen-bonding interaction.Besides, the powder X-ray diffraction (PXRD) certi es the amorphous nature of SSFP adhesive (Fig. 1b), which is different from PEG and SiW 12 , or even their physical mixture (Supplementary Fig. S3).These results suggest that SiW 12 is dispersed in SSFP adhesive matrix, and the crystallization of PEG is obviously inhibited after hybridizing with SiW 12 .Additionally, their structures, chemical compositions and states of PEG and SiW 12 in adhesive have been con rmed with X-ray photoelectron spectroscopy (XPS) (Supplementary Figs.S4 and S5), 32 Si NMR (Fig. 1c) spectra 42 , 1 H NMR spectra (Fig. 1d), solid-state and liquid-state 13 C NMR tests (Figs. 1, e and f, and Supplementary Fig. S6).The low eld nuclear magnetic resonance (LF-1 H NMR) reveals that a ~ 45 times decrease of crosslink density (from 69.93 × 10 − 4 to 1.55 × 10 − 4 mol mL − 1 ) can be detected in SSFP adhesive when compared with PEG (Fig. 1h), indicating that SiW 12 would be hybridized with PEG and occupies a certain space to decrease the number of cross-linked bonds in PEG.Moreover, the results are further supported by the decaying proton transverse relaxation curves (Fig. 1g) 43 .Besides, the scanning electron microscopy (SEM) tests show that SSFP adhesive has a denser and atter surface than that of PEG treated under similar heating process (Fig. 2a and Supplementary Fig. S7), proving the vital role of SiW 12 in decreasing the residual stress.Furthermore, energy-dispersive X-ray spectroscopy (EDS) element mapping analyses indicate the uniform dispersion of SiW 12 in SSFP adhesive (Fig. 2a).
The adhesion effect plays a vital role in the application of adhesive materials 44 .Satisfyingly, this obtained SSFP adhesive (Fig. 2b) exerts favorable adhesion capability on different types of arti cial and natural materials (Supplementary Fig. S8).Subsequently, the quantitative tests of adhesion strength of SSFP adhesive (Fig. 2e) are measured.Strong adhesion forces are presented on high surface energy substrates, such as stainless steel (SS, 3.7 MPa), glass (3.1 MPa), and copper (Cu, 2.7 MPa), owing to the existence of strong chemical bonds and mechanical interlocking 45 .Meanwhile, relatively weaker adhesion forces are shown on low surface energy substrates, such as polycarbonate (PC), polypropylene (PP) and polytetra uoroethylene (PTFE) (Fig. 2e).Notably, this SSFP adhesive with excellent adhesion performance is superior to the almost all of the POMs based adhesives that have been reported so far (Supplementary Fig. S9 and Table 1) 30, 33-36, 38, 42 .Besides, the SS substrates joined with SSFP adhesive can easily tolerate a weight of ~ 50 kg (Fig. 2c), indicating the remarkable adhesion capabilities of SSFP adhesive.Furthermore, it can be observed that the adhesion strength of SSFP adhesive on SS still maintains at about 3.7 MPa after multi-recyclable adhesion and deadhesion recycling (Supplementary Fig. S10).The distribution of SSFP adhesive on SS after detachment implies that the adhesion failure mainly occurs in interfacial adhesion between adhesive and substrates, indicating the high cohesion interaction of SSFP adhesive (Supplementary Fig. S11) 46,47 .
Based on the above results, SS substrate is selected as a model substrate to further monitor the adhesion performance and elucidate the interaction mechanism of SSFP adhesive.Interestingly, the adhesion strength of SSFP adhesive on SS is positively proportional to SiW 12 content (mass ratio, 2 : 5 to 2 : 3) (Supplementary Figs.S12 to S16).Nevertheless, the adhesive will transform into a very hard and brittle material with negligible viscosity when adding excessive SiW 12 .Furthermore, the molecular weight of PEG has been screened from ~ 2000 to ~ 20000 based on its viscosity in macroscopic level, and the optimal viscosity can be achieved for PEG (~ 10000).Therefore, the SSFP adhesive with the optimized composition (PEG, ~ 10000; mass ratio = 2 : 5) is selected as a model sample for subsequent in-depth study.It is noteworthy that the adhesion strength of SSFP adhesive is far superior to the SiW 12 based solvent-assisted polymer (SSAP) adhesive 30 (approximately 65 times, Supplementary Figs.S17 and   S18).As expected, a similar H 3 PW 12 O 40 (PW 12 ) based adhesive (PSFP) can also be produced by the same solvent-free method (Supplementary Fig. S1), and has been con rmed by various characterizations (Supplementary Figs.S19 to S21).Whereas, the adhesion strength of PSFP adhesive is weaker than that of SSFP adhesive, which testi es that SiW 12 with more H protons is more conducive to the formation of high-strength adhesives (Supplementary Fig. S16).Additionally, no adhesive is observed by heat-assisted process after replacing PW 12 with Na 3 PW 12 O 40 (Supplementary Figs.S22 to S24), con rming the vital role of H protons for the preparation of solvent-free adhesives.In addition, we further investigate the in uence of polymer on the formation of adhesive by replacing PEG with polycaprolactone (PCL) carrying less hydrogen bond acceptors.The result displays that similar adhesive is still formed, yet its adhesion strength is much weaker than that of SSFP adhesive (Supplementary Figs.S25 and S26).
Besides, no adhesive is formed when replacing PEG with polyethylene (PE) and polyvinylidene di uoride (PVDF) (Supplementary Fig. S25), manifesting the crucial role of hydrogen bond acceptor for the formation of solvent-free adhesives.Beyond that, the PEG analogues, PEGME bearing the methyl group and hydroxyl group at each terminus and PEGdME bearing the methyl groups at both termini, can also crosslink with SiW 12 to form adhesives (Supplementary Fig. S27), and the rheology and lap-shear adhesion test results show that these adhesives possess similar shear strength (Supplementary Fig. S28) and viscosity (Supplementary Fig. S29), suggesting that the formation and behavior of adhesive are primarily attributable to hydrogen bond interaction between H protons of SiW 12 and etheric oxygen groups of PEG rather than the terminal groups.
As is well-known, most of adhesives based on polymers tend to be dissolved or swelled in organic solvents, resulting in markedly weakened adhesion performances and seriously hindered practical applications 48 .Hence, the maintenance of robust adhesion strength in organic solvents is an eyecatching trait for adhesives.Interestingly, the SSFP adhesive is insoluble in some organic solvents, such as mesitylene (TMB), dioxane (Diox), octanoic acid (OA), 1,4-dibromobutane (DiBrb), petroleum ether (PE) and N-hexane (N-hex).For example, no separation or displacement phenomenon has been observed in a long-term adhesion test for at least 14 days (Supplementary Fig. S30).In addition, the adhesion strength remains relatively stable for 14 days of immersion in different organic solvents (Fig. 2f).Moreover, SSFP adhesive is capable of performing rapid adhesion (Supplementary Fig. S31 and video S1) and preventing an emergency leakage for organic solvents (Fig. 2d and video S2).In sharp contrast, ethyl vinyl acetate (EVA), a kind of commercially available hot melt adhesive, shows negligible adhesion strength after soaking in mesitylene for only 7 days when compared to that of SSFP adhesive (Supplementary Fig. S32).
Based on above-mentioned high adhesion strength of SSFP adhesive, its viscosity, energy storage and loss modulus have been further traced using rheology tests.Compared to the PEG, the SSFP adhesive exhibits higher viscosity and lower liquidity in rheological characterization (Supplementary Fig. S33).Rheology measurements verify that the modulus and viscosity are both inversely proportional to the operating temperature (Figs. 3, a and b).The SSFP adhesive remains gel-like or solid-like state in macroscopic behaviors at below ~ 50°C.Speci cally, loss modulus of the SSFP adhesive (Fig. 3a) exceeds storage modulus (G″ > G′) at above ~ 50°C, resulting in a viscosity-dominated viscoelasticity state that can accelerate the interfacial bonding.Moreover, the reversibility of storage modulus (G′), loss modulus (G″), and complex viscosity (η*) can be realized in cycling tests under circulating temperatures, which might be attributed to the solvent-free phase and invertible hydrogen bond interaction that enable reversible temperature-induced rheological behaviors.

Ultra-low temperature-resistant performance
Viscous materials with low-temperature resistance play an important role in exploration under extreme environments especially in a wide temperature-variable range, such as, research at the poles of the Earth (e.g., South Pole, 20.7 to -94.2 ℃) and exploration of the outer planets (e.g., Mars, 20 to -140 ℃).However, traditional polymer based adhesives tend to become brittle, and increase its residual stress as the temperature decreases 49 .Here, the thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) tests display that the glass transition temperature (T g ) of SSFP adhesive is -31.1°C, which is lower than that of PEG (48.3°C) (Supplementary Figs.S34 and S35), implying that the SSFP adhesive has better exibility at relatively low temperature.Impressively, the SSFP adhesive adhered between SS slices can easily tolerate a 2 kg weight under liquid nitrogen conditions, and still maintain the original state after returning to room temperature (Fig. 3c and video S3).In addition, the SSFP adhesive is not observed to have signi cant contraction or rupture after freezing with liquid nitrogen (Supplementary Fig. S36).In contrast, the obvious volumetric shrinkage and rupture for PEG and EVA adhesive occur at 25°C and − 196°C, respectively (Supplementary Figs.S37 and S38).The above results show that volumetric contraction of SSFP adhesive can be signi cantly inhibited at -196°C after crosslinking SiW 12 with PEG.To support it, the adhesion strength of SSFP adhesive has been tested at different temperatures.Particularly, the adhesion strength is negligible for SSFP adhesive at 55°C, then gradually increases as the temperature decreases to 4°C (~ 3.8 MPa) (Fig. 3d and Supplementary Fig. S39).After that, the adhesion strength gradually decreases when the temperature decreases to -196°C.More signi cantly, the SSFP adhesive can still maintain relatively high adhesion strength of 2.96 MPa after immersion in liquid nitrogen for 60 days (Supplementary Fig. S40), while the EVA adhesive is immediately frozen-cracked once it entered liquid nitrogen (Supplementary Fig. S41).These results demonstrate that the SSFP adhesive has admirable ultra-low temperature-resistant adhesion performance than that of commercial hot melt adhesive.Furthermore, the low-temperature rheological measurements (Fig. 3e) showed that SSFP adhesive has a stable storage modulus (G′) and loss modulus (G″) in the temperature range from − 120 to 25°C, which is consistent with its actual performance.Moreover, the temperature-dependent FT-IR spectra (Fig. 3f and Supplementary Fig. S42) indicate negligible change in the four signals of both SiW 12 and PEG, manifesting that the interactions in SSFP adhesive remains almost intact under wide temperature range 50 .
Based on above excellent adhesion properties of SSFP adhesive, the adhesion mechanism has been further investigated.In general, cohesion and interfacial adhesion are two main factors affecting adhesion performance.Nevertheless, most of the theoretical calculation of adhesives focus on the simulation between adhesives and substrates, and it is still very scarce for the study of interaction contributing to cohesion.Thus, we have applied the density functional theory (DFT) calculations to investigate the SSFP adhesive at molecular level 31,41 .The results reveal that the interaction energy between them is -229.69kJ/mol for one PEG fragment, -507.14 kJ/mol for two PEG fragments, and − 764.81 kJ/mol for three PEG fragments, respectively (Fig. 4a).Obviously, the binding stability between POMs and PEG can be signi cantly enhanced via strong hydrogen bond interaction.Hence, SiW 12 as a cross-linking agent will be interweaved and anchored into PEG networks, resulting in the formation of stable and durable adhesive.In addition, the molecular dynamics (MD) simulation is further performed to evaluate the temperature-dependent interaction energy and hydrogen bonds.At 25°C, the interaction energy and hydrogen bond percentage between PEG and SiW 12 are average − 1168 kJ/mol and 39.13% in this model system at 2 ns, respectively (Fig. 4b, Supplementary Fig. S43 and video S4).At 55°C, the uctuation of interaction energy is more obvious, and signi cantly reduces to -980 kJ/mol (Supplementary Figs.S44 and S45, video S5).Moreover, the percentage of hydrogen bonds (40.0%) at 55°C remains almost the same with that at 25°C (Supplementary Fig. S46).Remarkably, at -196°C, the interaction energy quickly reaches equilibrium and remain stable within 50 ps (Fig. 4c and video S6).In addition, the dramatically increased interaction energy remains at average − 1250 kJ/mol (Fig. 4d), meanwhile the percentage of formative hydrogen bonds (35.71%) is slightly affected by low temperature (Supplementary Fig. S43).High interaction energy at low temperature will elicit large energy dissipation of SSFP when dragging the adhesive, in which the synergistic interaction of them might be the dominating reasons for the SSFP adhesive to exhibit excellent adhesion performance at ultra-low temperature.As a contrast, when replacing SiW 12 with PW 12 , both the interaction energy and the percentage of formative hydrogen bonds are signi cantly reduced to -817 kJ/mol and 29.27% (Supplementary Figs.S47 to S49), owing to the fewer H protons that can be provided by PW 12 to crosslink with PEG under the same conditions.

Conclusion
In summary, we have prepared a kind of POMs based solvent-free polymer adhesive on a kilogram scale through a heat-assisted process.It is worth noting that the achieved SSFP displays excellent interfacial adhesion ability on different substrates, high adhesion strength and organic solvent stability, wide tolerable temperature range (i.e.-196 to 55°C) and long-lasting adhesion effects (> 60 days) at -196°C.The high-performance of SSFP exceeds that of commercial hot melt adhesives.Furthermore, combined experimental results with theoretical calculations, the strong interaction energy between POMs and PEG is the main factor for the high adhesion performance at low temperature, possessing enhanced cohesion strength, suppressed polymer crystallization and volumetric contraction.This work enriches the types of low-temperature resistance adhesives, and would shed light on the development of advanced solventfree adhesives for Arctic/Antarctic or planetary exploration. Declarations 12 deriving from the stretching vibration bands of W = O d , Si-O a , W-O b -W and W-O c -W, respectively, are still clearly discernible in the SSFP adhesive, indicating the retained structure of SiW 12 within the SSFP adhesive matrix.Simultaneously, the stretching vibration of etheric oxygen groups (-C-O-C-) in PEG at 1113

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