Thermo‐responsive fluorine‐free foam stabilized by PEO–PPO–PEO triblock copolymer (EO)100(PO)65(EO)100 for pool fire suppression

Research and development of novel fluorine‐free materials to replace fluorinated aqueous film‐forming foam (AFFF) are crucial for improving pool fire suppression performance and protecting the environment. In this study, we report the thermo‐responsive fluorine‐free foam stabilized by triblock PEO–PPO–PEO copolymers (EO)100(PO)65(EO)100 for pool fire suppression. Small‐angle X‐ray scattering (SAXS) and reflected light interferometric techniques are conducted to study the molecular self‐assembly in bulk and film thinning behavior, and the foaming kinetics of copolymer solution and thermophysical properties of the liquid foam are studied by dynamic surface tension and oscillatory rheology analysis. At room temperature, the amphipathic structure of PEO–PPO–PEO makes it possible to absorb at the air–liquid interface forming large‐scale liquid foams containing the mobile films with a detergent state. Upon heating to the surface cooling temperature of burning oil, the mobile films can be actively switched into mechanically strong films with rigid surfaces. The in situ switching of the two interfacial states leads to the significant enhancement of the foam stability, especially under the dual defoaming effects of heat and oil. What's more, it is observed that the confinement of organized copolymer micelles in the Plateau borders and micellar self‐layering in film confinement induce drainage delay of foam and film's stepwise thinning phenomenon, further increasing film thickness and enhancing the thermal stability of the foam. In standard fire‐fighting tests, it is proved that the burnback performance exhibited by thermo‐responsive copolymer foams is three times better than that for classical fluorine‐free foams and almost 1.5 times higher than that for commercial AFFF.

Liquid fuel is widely used in the transportation, aerospace, chemical industry, electric power, and other fields and is the "blood" of these industrial fields.A pool of liquid fuel catching fire, which is termed a "pool fire," is among the most common accidents in the industry.Once a such fire occurs, it will soon lose control and lead to severe fire or explosion accidents, [1][2][3] which seriously harm people's lives and property as well as the ecological environment.Therefore, developing efficient fire extinguishing materials for pool fire suppression plays an important role in fire safety in fuel utilization.
5][6][7][8] It was the fluorocarbon surfactants contained in AFFF that were essential to enhance the pool fire suppression performance. 9,10owever, the perfluorooctyl-based surfactant was shown to be bio-persistent, bioaccumulative, and toxic because of the extremely high stability of fluorine-carbon bonds in hydrophobic tails. 11,12The US Environmental Protection Agency (EPA) restricted the production of perfluoroctnyl surfactants in 2003, and countries around the world are phasing out perfluoroalkyl surfactant products.To reduce toxicity and enhance the biodegradability of fluorocarbon surfactants in foaming agents, surfactant manufacturers have made the following modifications in the past decade: (i) reducing the number of carbon atoms in perfluorinated chains from C8-C10 to C4-C6; (ii) incorporating branching structures into perfluorinated chains; and (iii) introducing hydrocarbon fragments (-CH 2 -) n into perfluorinated surfactants.However, these modifications cannot provide sufficient biocompatibility for fluorocarbon surfactants and do not allow us to classify them as safe substances. 13Besides, the long-term environmental fate and routes into the environment of new fluorocarbon surfactants remain unclear. 14Replacing the fluorocarbon surfactants with environmentally friendly fluorine-free surfactants in the foaming agent will lead to an effective reduction of its environmental impact.
Recently, much research on fluorine-free fire-fighting foam that entirely eliminated the fluorocarbon surfactants in foaming agents was carried out, 3,[15][16][17][18][19][20][21][22][23][24][25][26][27][28] including silicone surfactant foam, protein foam, and nanoparticle stabilized foam.However, it has not yet been successfully developed a fluorine-free fire-fighting foam with fireextinguishing and burnback performance comparable to the current fluorinated AFFF, especially for liquid fuel with low flash points such as n-heptane and gasoline.Perfluorinated surfactant tails are unique in that they are both oleophobic and hydrophobic, and Yu et al. 22 indicated the fluorinated foam could resist the defoaming effect of oil and described that oil droplets were "expelled" from the liquid film containing fluorocarbon surfactants.To replace fluorinated surfactants with fluorine-free surfactants in firefighting foams, oleophobic (or nonlipophilic) compounds must be used.The commonly used hydrocarbon surfactants (SDS, CTAB, or APG) are no longer applicable in the foaming agent for liquid fuel fire suppression because fuel molecules such as n-heptane can interact with alkane chains of these hydrocarbon surfactants through weakly induced dipoles based on the rule of "like dissolves like." 23Research from US NRL pointed out that compared with fluorinated foam, n-heptane vapor was more likely to penetrate and destroy SDS foam than fluorinated foam, allowing fuel to maintain steady combustion on the surface of the foam blanket. 6he current strategy for selecting or designing fluorine-free surfactants for pool fire suppression is guided by the physical and chemical interactions between surfactant molecules and liquid fuels.Besides, the hydrophobicity of the tail and hydrophilicity of the head must be in balance to ensure the optimum amphiphilicity for fluorine-free surfactants to generate large-scale liquid foams.What's more, in foaming liquids, the generation of organized structures for example micelles or bilayers is critical because the confinement of these structures in the foam films induces an additional disjoining pressure and helps to enhance the foam stability.Taking into account these characteristics, we focus on PEO-PPO-PEO triblock copolymers as the foaming agent that have less methyl and methylene groups than the classical siloxane or hydrocarbon surfactants, which weakens the attraction to alkane fuels.In addition, at high temperatures, the close packing and ordering of PEO-PPO-PEO micelles promote the formation of gelled foam films.Therefore, the foam prepared from PEO-PPO-PEO triblock copolymers is "smart" that can respond to heat based on self-assembly in bulk or at air-liquid interfaces.In other words, the foam appears as flowing fluid at room temperature, and once the foam contacts with burning oil, the flowing foam changes into gel foam in situ rapidly which significantly promotes the fire-fighting efficiency due to the increased thermal stability, adhesion, durability, and other physical properties.
In our study, small-angle X-ray scattering (SAXS) tests were conducted to obtain the structural information of the molecular aggregates in PEO-PPO-PEO solutions.The foaming kinetics of PEO-PPO-PEO solution and the thermophysical properties of foam were studied by both dynamic surface tension and oscillatory rheology analysis.
Thinning behavior and thickness of the liquid PEO-PPO-PEO film containing the ordered micelles were visualized and measured quantitatively by using the reflected light interferometric technique.Finally, we evaluated and compared the fire extinction and burnback performance of our thermo-responsive PEO-PPO-PEO foam, classical fluoride-free foam, and the commercial fluorinated AFFF by standard fire-fighting tests.Our study provides a better understanding of thermo-responsive smart foam for reducing the damage caused by pool fires and offering environmental advantages compared to fluorocarbon surfactants.

| Interfacial and bulk properties of the foaming solution
The structural parameters of the copolymer micelle can be obtained by dealing with the SAXS data (Figure 1A).In this study, the scattering pattern of block copolymer micelle and Gaussian size distribution in SASfit are interpreted to fit the SAXS data.The fitted pattern mainly drives from the model assumed by Pedersen and   Gerstenberg (PG model), [29][30][31] in which the core is homogeneous and the chains are noninteracting and obey Gaussian statistics.And the form factor of a block copolymer micelle can be described as Equations (S1-S2) in Supporting Information.
The structural parameters are listed in Table 1.The average radius of micelle R ¯and aggregation number N agg present an increasing change by increasing PF127 concentrations.PF127 gel can be formed only at a high temperature when the micellar radius and the aggregation number are enough.The increase in aggregation number can be observed when the PF127 concentration rises from 10 wt% to 20 wt%.Considering the critical gel concentration measured by the tube inversion method, 32 the critical gel temperature of 30 wt% PF127 is around room temperature and as the concentration of PF127 decreases, the critical gel temperature of PF127 solution increases.It should be noted that the values of the gyration radius of chain R g seem to be independent of the PF127 concentration, and the fitting value is slightly lower than the expected value.It may be caused by the folding or entanglement of the Gaussian chains in space. 30igure 1B shows the equilibrium surface tension and the interfacial tension with n-heptane as a function of PF127 concentration.The equilibrium surface tension slightly decreases with the increasing concentration of PF127 and then remains unchanged.The surface tension of PF127 in the flat region in the surface tension curve is about 35-37 mN/m.Interfacial tension isotherm shows a changing trend similar to the equilibrium surface tension curve and the value is in the range of 3-15 mN/m.For comparison, the interfacial tension of the micellar solution containing low-molecular-weight hydrocarbon surfactant APG is very low, less than 1.5 mN/m. 33The high interfacial tension value indicates that PF127 molecules have a weak attraction to alkanes and do not easily adsorb at the oil-water interface.
Dynamic surface tension plays an important role in foam generation. 34In this study, Rosen's approach 35 is used to fit the dynamic surface tension curves in Figure 1C.
where γ 0 is the equilibrium surface tension of water (~75 mN/m); γ m is the meso-equilibrium surface tension; t* is a constant, with the dimensions of time, and represents the diffusion potential energy of surfactant molecules from the bulk solution to the subsurface; n is a dimensionless constant, representing the adsorption potential energy from subsurface to the surface.The smaller value of the t*, the smaller encumbrance of molecules diffusion, and the greater value of the n, the less difficult for the surfactant to adsorb on the surface. 36,37The n value first significantly falls in the low concentration region and slightly rises with the increase of PF127 concentration (Figure 1D).On the contrary, the t* value slightly increases at first and then greatly increases by increasing PF127 concentration.Before the PF127 concentration reaches 1 wt%, the diffusion characteristic time t* conforms to the common concentration gradient principle of diffusion like lowmolecular-weight surfactants. 36As shown in Figure 1E, one sees the foaming height of the solution increases with increasing PF127 concentration.Similar to the results of low-molecular-weight surfactants, 36 the increase of foaming ability at low PF127 concentration (<1 wt%) can be explained by the decrease of diffusion characteristic time t*.However, a slight decrease in foaming ability is observed when the PF127 concentration is greater than 10 wt%.The formation of large, stable, and rigid micelles in bulk results in a slower demicellization rate and higher diffusion characteristic time t*.Moreover, SAXS data in Table 1 shows that when the concentration of PF127 increases from 5 wt% to 20 wt%, the aggregation number of PF127 increases by about three times.Besides, the aggregation of PF127 molecules leads to the increase of micelle size and solution viscosity (Figure 1F), thus slightly reducing the foaming ability.

| Rheology of thermo-responsive gel foam
Figure 2 shows the rheological response of liquid foams at different temperatures.As shown in Figure 2A, with the increase of angular frequency, the curves of PF127 foam (except 20 wt%) exhibit a developing trend that G" is greater than G', and the loss modulus G" in the higher angular frequency range of 6.18-10 rad/s shows a power law growth with the index of 0.5.20 wt% PF127 foam has intertwined modulus curves, and the G" growth index is significantly less than 0.5, which suggests that it has higher surface elasticity and viscosity.When it comes to 45°C (Figure 2B), it is observed that the storage modulus G' of PF127 foam increases by one or two orders of magnitude compared with the cases at 20°C, indicating that the higher temperature induces stronger elastic foam films.At the high-frequency stage of 10-100 rad/s (except 1 wt% PF127 foam), the G" curves grow with an index lower than 0.5 (Figure 2B).With an increase in temperature, the dependence of G' and G" on frequency decreases, which suggests the formation of a stronger structure at 45°C.Heat can accelerate the liquid drainage of foam, expand the gas, expand the surface area of foam films, and weaken the bubble films. 6,38However, the values of G' are always large than those of G" at c PF127 ≥ 10 wt%, and thus these foams behave like an elastic solid that is uneasy to deform under external force.
In the oscillation strain sweep curves (Figure 2C), the loss modulus G" for all cases is greater than the storage modulus G' at 20°C, meaning that foam shows a The stress-strain curves and their corresponding yield points determined by the kink between the linear regime and nonlinear regime are plotted in Figure 2E,F.Note that the yield stress of 20 wt% PF127 foam increases by two orders of magnitude from 20°C to 45°C, up to 100 Pa which is 1000 times greater than the yield stress (<0.1 Pa) of the APG foam. 33Before reaching the yield stress, the oscillation stress at 20°C can be expressed as σ Gγ = n (n ≈ 1).A similar linear relationship was also observed at 45°C.Below the yield strain or stress, the foam acts as a viscoelastic solid, while above the yield, it flows as a non-Newtonian liquid.0][41] All in all, the PF127 foams perform a strong liquid-like fluidity than elasticity at 20°C meaning that the foam can flow easily in the foam system or foam pipe.On the contrary, when the temperature is 45°C (instantaneous cooling temperature of fuel surface in Section 2.4), before reaching the yield stress, the foam shows obvious elastic characteristics, with the mobile gas-liquid interface transforming into the rigid interface.In particular, 20 wt% PF127 foam exhibits the characteristics of the strong gel interface and 10 wt% PF127 foam shows weak gel characteristics.The relaxation index in angular frequency sweep and the yield stress also indicate the generation of the gel interface.gradually, the gel phase transforms into the flowing sol, and the liquid both in the bubble film and Plateau border rapidly drains from the foam under the effect of gravity, resulting in the foam volume decreasing at an extremely fast rate.When the liquid fraction of foam drops to a rather low value, the drainage effect is no longer prominent for foam decay, and the declining trend of foam volume slows down at ambient temperature.It is proved that the foam stability can be tuned with temperature as smart foam, 42 and the falling temperature will make the foam no longer highly stable, so it is helpful to clean the residual agent in the later stage of the fire suppression task.Figures 4, S2 display the foam height of PF127 foam as a function of time in the presence and absence of n-heptane at different temperatures.The presence of heat and n-heptane can significantly accelerate the aging mechanism of fluorine-free foam and decrease foam stability. 6Thermodynamic coefficients E, S, and B were calculated to explain the oil-induced foam degradation according to Equations (S3-S5). 43,44s shown in Table S1, the strong positive values of E, S, and B coefficients indicate that oil droplets tend to emerge and spread on the surface of bubbles and induce foam collapse.Unexpectedly, the rise of temperature causes the improvement of the stability of 20 wt% PF127 foam even under the dual defoaming effect of heat and oil: the half-life time of 20 wt% PF127 at 45°C in the presence of n-heptane is more than 7 h (Figure 4B).The formation of the gelled gas-liquid interface can inhibit foam drainage and suppress bubble coarsening and coalescence.Although the stability of 10 wt% PF127 foam has not been significantly improved, it is distinctly better than others (except 20 wt% PF127) at 45°C.It is perhaps due to the generation of micro gel interfaces in 10 wt% PF127 foam as the temperature rises, which has been confirmed in the rheological results mentioned above.We also compare the foam stability of 20 wt% PF127 foam with other foams prepared by widely studied surfactants (Figures 4D-F, S3), and the results show that PF127 foams have better foam stability than silicone and fluorinated surfactants in the presence of n-heptane at 45°C.The enhanced film thickness and stability of PF127 foam at 45°C lead to a decrease in fuel vapor transport (Figure 4F), which contributes to improving the fire-fighting performance of the foam.accumulates at the base of the container in 1 wt% PF127 foam while liquid drainage cannot be observed in 20 wt% foam after 1 h, which is known as the drainage delay.In Figure 5B, at low PF127 concentration (<5 wt%), PF127 foam has similar liquid drainage behavior to the lowmolecular-surfactant foams: at the moment when the foam is produced, the liquid drainage has already started, and the drainage rate is extremely fast within the first minute.The liquid drainage rate and 50% drainage time (t 50% ) increase with PF127 concentration at 20°C and 45°C (Figure 5C).When PF127 concentration is greater than 10 wt%, drainage delay can be observed and PF127 foam does not drain in the initial stage after generation although the PF127 foam is a viscous fluid at 20°C.The increase of solution viscosity from 1 mPa•s to 5 mPa•s (Figure 1F) can slow down the liquid drainage rate but is unlikely to induce the drainage delay of foam.The drainage delay of PF127 foam is more related to the increased molecular aggregation of PF127 because the larger micelles and supramolecular aggregates can block the Plateau borders to stop the liquid flow within the foam (Figure 5D).Rising temperature will accelerate the drainage rate and reduce the 50% liquid drainage time (Figure 5C) at low PF127 concentration (<5 wt%).However, at high PF127 concentrations, heat performs the opposite effect because one sees the decreased liquid drainage rate of 10 wt% and 20 wt% PF127 foams at 45°C.At 45°C, the slow drainage kinetics of the foam can be attributed to the formation of a gel interface and the confinement of the organized micelles in the foam structure.The close-packed micelles block the liquid drainage channel, the Plateau border, and nodes in the foam.What's more, the formation of a heat-induced gel interface further increases the film thickness and effective diffusion coefficient to slow down the bubble coarsening (see Section 2.3.3), and therefore the drainage delay time becomes longer.

| Draining and thinning mechanisms of foam and foam films
The 50% foam liquid drainage time is related to many characteristic parameters of foam and can be estimated by the following formula 34 : (2 where η is the dynamic viscosity; Z is the initial height of the foam; ρ l is solution density; g is the gravity constant with a value of 9.8 m/s 2 ; R V is the initial average radius of bubbles; φ l0 is the initial liquid fraction of foam; K p represents the permeability coefficient related to the value of Boussinesq number (Bo), which is related to the mobility of the gas-liquid interfaces: the smaller K p , the lower the interfacial mobility.Under the given conditions in Table 2, the calculated K p value of APG is about 1.08, close to 1, the corresponding Bo«1, and the APG film tends to have "mobile surfaces" and the drainage mechanism could be depicted by the vertex-dominated case.45 Similarly, the calculated K p value of 20 wt% PF127 foam is about 0.32.It is known that when Bo»1, K p ≈ 0.02, the corresponding gas-liquid interface is rigid (the gas-liquid interface is like a solid wall) and the drainage mechanism could be depicted by the Plateau borderdominated case.The K p value of PF127 foam is between 0.02 and 1, biased to 0.02.Therefore, it can be seen that the gas-liquid interface of 20 wt% PF127 foam at 20°C is tended to be semi-rigid or partial mobility, and the liquid drainage is controlled by the Plateau border-dominated and vertex-dominated case.
To reveal the interfacial states of the PF127 foam film, the light interference patterns of draining foam films at different temperatures are shown in Figure 6.The interference phenomena occur when the thickness of the film is comparable to the wavelength of monochromatic light.One can distinguish that the liquid films in Figure 6 exhibit two extreme types of draining behavior.Films in Figure 6A-C have "mobile surfaces" and these films show rapid turbulent motion like "peacock feathers" at the film boundary.The convection driven by the Marangoni effect in the film produces the  Figure 7 shows that the mobile films containing 5 wt% PF127 have a wedge-shaped profile.As the PF127 concentration increases, the thickness profile gradually becomes parabolic (Figure 7C).Increasing the temperature from 20°C to 45°C accelerates the film thinning rate for 1 wt% APG (Figure S4) and 5 wt% PF127 films (Figure 7A) because evaporation thins the film by removing the water.Note that the formation of weak gel in 10 wt% PF127 films counteracts the thermal-induced instability and one sees that the increased temperature has little effect on liquid film thickness.
The draining and thinning behavior of the 20 wt% PF127 liquid film is unique compared with other cases.
As shown in Figure 7D, it is found that the film thinning follows a power law behavior.The thickness of 20 wt% PF127 film at 20°C varies as the −0.75 power of time which is close to the value (−0.8 to −1.2) of the SDS liquid films in the previous studies. 25,46Besides, stepwise film thinning that is governed by the "vacancies condensation" 47 mechanisms can be found in the 20 wt% PF127 film and three micellar layers are observed after 45 min (Figure 6D).At 20°C, one sees that the increase in PF127 concentration from 5 wt% to 20 wt% results in a 7-fold increase in initial film thickness (Figure 7E).The confinement of the organized micellar structures in the liquid films induces an additional disjoining pressure which enhances lamella stability and slows down the liquid drainage of foam. 48t 45°C, 20 wt% PF127 liquid film has a rigid surface because when a vertical film is formed, it does not show any visible turbulence on the surface and the film is very thick (Figure 6D).By generating a 20 wt% PF127 liquid film at 20°C and increasing the temperature from 20°C to 45°C gradually, the thinning exponent decreases from −0.75 to −0.25 (Figure 7D).The previous study has correlated the thinning exponent to interfacial slip conditions and indicated that the thinning exponent of the nanoparticle-stabilized aqueous film without interfacial slip is lower than −0.5. 49The formation of rigid surfaces cannot be explained simply by increasing bulk viscosity (~20 mPa•s) because one can hardly make the rigid film from the surfactant-glycerol solution (increasing bulk viscosity does not significantly change the film thinning exponent).The structure of the adsorption layer and interfacial and bulk mass transfer determine the mobility of the film surface and therefore the measured film thinning rate.

| Bubble morphology
Figure 8 shows the photograph of bubble morphology.The gelled air-liquid interface does not generate in the 20 wt% PF127 foam at 20°C as well as the 10 wt% PF127 foam at 45°C, because the thinning exponents of the two films are about −0.75.As shown in Figure 8C, at 45°C one sees that the bubbles in 20 wt% PF127 foam are deformed and elongated in the transverse and longitudinal directions into an elliptical shape because of the asymmetric force in the formation of the gel interface.The appearance of an elliptical bubble shape verifies the generation of the gel interface again which causes the thinning exponent to decrease to less than −0.25 (Figure 7D).Clear and orderly color interference fringes on the surface of gel bubbles can be observed in Figure 8F, rather than the black film or Newtonian black film (h < 100 nm) in Figure 8E indicating the generation of the stable and thick liquid film (h > 1000 nm).The elastic gas-liquid interface in Figure 8F can stabilize the bubble film against the concentration or thickness interfacial fluctuations, through the Marangoni effect, thus decreasing the possibility of bubble film rupture. 34he bubble diameter distribution and average diameter of liquid foam are counted and plotted in Figure S5 in Supporting information.And it is proved that the bubbles with rigid interfaces have slower coarsening and coalescence rates (Figure S5).

| Pool fire suppression application of thermo-responsive gel foam
The temperature variations of foams applied on the surface of the ignited n-heptane pool are shown in Figures 9A,B.When the foam spreads on the surface of the n-heptane pool, the foam temperature (close to the fuel surface) declines quickly from hundreds of degrees Celsius to less than 60°C in 10 s due to the instantaneous cooling effect of foam.In the process of burnback tests (t > 600 s), the foam temperature is also between 40°C and 50°C.All in all, the internal temperature of the foam blanket is around 40°C-60°C, and the properties of foam at 45°C studied above as the hightemperature condition may be related to the fire extinction performance of PF127 foam.
Figure 9C displays the photographs of fire extinguishing tests of PF127 foam and APG foam at different times.It can be seen that PF127 foam can reduce the flame height significantly in 60 s and the fire is successfully extinguished at t = 100 s, while the fluorine-free APG foam cannot suppress the flame due to the instability of APG foam in contact with heat and oil.Figure 9D shows the photographs of PF127 foam and fluorinated AFFF in the burnback tests at different times.One sees that the fluorinated AFFF is burned through by the flame from the burnback pot and the n-heptane beneath the fluorinated AFFF is re-ignited at t = 14 min.Instead, it can be seen that the integrity of the PF127 foam blanket is not damaged until t = 18 min when the flame overflows from the edge of the fire tray.At t = 21 min, the n-heptane pool totally re-ignited.One can observe from the surface morphology of the PF127 foam that the gel foam is formed during the firefighting test (Figure 9D).The outstanding stability of thermo-responsive gel foam at high temperatures is critical to prevent fuel reignition.
Figure 9E compares the fire extinction and burnback performance of 20 wt% PF127 foam, two kinds of commercial AFFFs, silicone-based foam, and a series of fluorinated FC1157/APG foam.The composition of commercial AFFF is proprietary.The research of the U.S. NRL showed that the simple fluorinated formula containing FC1157/APG/solvent mixtures can serve as the reference for the comparative study on the fire extinction performance of fluorine-free foam. 5It can be seen that the fire extinction time of 20 wt% PF127 foam is much shorter than that of APG-based foam, and the flame is successfully extinguished within 120 s.Although the fire extinction time of PF127 foam is longer than that of fluorinated and silicone-based foam due to the limitation of the expansion ratio (ER) of PF127 foam (ER is about 5.1, 750 g/min), the burnback time of PF127 foam is much longer than that of commercial 3% AFFF, silicone-based foam and most of fluorinated FC1157/ APG foam (ER is about 9.0, 750 g/min), equivalent to that of the optimal home-made fluorinated FC1157/APG foam, about 21 min.Therefore, the PF127 foam displays an extraordinary burnback performance at a low expansion ratio.As shown in Table 3, by reducing the PF127 concentration (or adjusting the dilution ratio in application), PF127 foam with a higher expansion ratio can be obtained, thus resulting in a shorter fire-extinguishing time by sacrificing burnback performance.

| CONCLUSIONS
In this study, we have demonstrated the possibility and prospect of an in situ production of thermo-responsive fluorine-free foams for pool fire suppression.The non-ionic triblock PEO-PPO-PEO copolymers, as the building blocks for thermo-responsive foams, could create interfacial and bulk ensembles that could be reversibly switched between mobile and rigid states at gas-liquid interfaces, in response to changes in bulk concentration and fuel temperature.The optimum amphiphilicity of (EO) 100 (PO) 65 (EO) 100 in the PEO-PPO-PEO family made it possible to absorb at the air-liquid interface forming large-scale liquid foams containing the mobile films with a detergent state (thinning exponent ≈ −0.75).Upon heating to the surface cooling temperature of burning oil (45°C), the mobile films could be actively switched into mechanically strong films with rigid surfaces (thinning exponent ≈ −0.25).The active switching of the two interfacial states led to the significant enhancement of the foam stability, especially under the dual defoaming effects of heat and oil, and thermo-responsive fluorine-free foam had stronger stability than fluorinated foam reported in the previous works. 5,6The rheology also supported the thermal behavior of foam with a relaxation index below 1/2 and the yield stress up to 100 Pa at 45°C, which was 1000 times higher than that of low-molecularweight surfactant foams.In addition to the interfacial states, the confinement-induced micellar self-organization in foam structure played a crucial role in the stability of copolymer foams.And it was observed that the confinement of organized copolymer micelles in the Plateau borders and micellar self-layering in film confinement induced drainage delay of foam and film's stepwise phenomenon, further increasing film thickness and enhancing foam lifetime.In standard fire-fighting tests, we proved that the burnback performance exhibited by thermo-responsive copolymer foam was almost 3 times better than that for classical fluorine-free foams and 1.5 times higher than that for state-of-the-art AFFF.The results of this study contributed to a better understanding of thermoresponsive gel foam and provided great potential for developing fluorine-free foam for pool fire suppression.
Small-angle X-ray scattering (SAXS) profiles for solutions in the presence of different PF127 concentrations.(B) Equilibrium surface tension and interfacial tension with n-heptane as a function of PF127 concentration.(C) Dynamic surface tension of PF127 aqueous solutions as a function of time.(D) Values of n and t* at different concentrations of PF127.(E) Foaming ability as a function of PF127 concentration.(F) Viscosity of the solutions as a function of PF127 concentration.All plots were obtained at 20°C.
T A B L E 1 Fitting parameters of PF127 with different concentrations.Concentration (wt%) Average radius of micelle (nm) Aggregation number N aggGyration radius of chain R g (nm) behavior.When the temperature rises to 45°C (Figure2D), the global shapes remain similar in all curves, but foam samples show a completely different performance in viscoelasticity.The modulus values increase with the increase in PF127 concentration, but at 45°C, the foam shows the characteristics of elastomer and has a certain ability to resist deformation.It should be noted that G' values of 20 wt% PF127 foam in Figure2Dincrease by two orders of magnitude compared with those in Figure2C.At higher strain, the G' and G" curves cross each other, after which G" shows greater values than G', the foam is more fluid than elastic, because of a significant dissipation accompanying the deformation at high strain.

2. 3 |
Stability and film thinning of thermo-responsive gel foam 2.3.1 | Foam stability in the presence and absence of oil at different temperatures The variation of 20 wt% PF127 foam volume in the cooling process is recorded and plotted in Figure 3.The foam volume almost remains the same at 45°C as time goes on.As the temperature of the water bath dropsF I G U R E 2 Rheology of thermo-responsive liquid foam at different temperatures.(A and B) Angular frequency sweep at 20°C and 45°C.(C and D) Oscillation strain sweep at 20°C and 45°C.(E and F) Stress-strain curves and yield points at 20°C and 45°C.

Figure
Figure5Adisplays the evolution of the 1 wt% and 20 wt% PF127 foam samples.One can see that the liquid

F I G U R E 4
The normalized foam height in the presence of varying concentrations of PF127 as a function of time (A) at 20°C with n-heptane; (B) at 45°C with n-heptane.(C) Half-life time of PF127 foam.(D-F) Fuel transportation within the foam and the bubble films stabilized by hydrocarbon surfactants, fluorocarbon surfactants, and triblock copolymers, respectively.

F
I G U R E 5 (A) Evolution of the foam samples containing 1 wt% and 20 wt% PF127.(B) Normalized drainage volume of foam in the presence of varying concentrations of PF127 at 20°C and 45°C.(C) Foam 50% drainage time (t 50% ) as a function of PF127 concentration.(D) Schematic diagram of drainage delay of PF127 foam.T A B L E 2 The values of parameters required for the prediction model in different foam types.
elements of the surface area of the film which accelerates the liquid drainage.

F
I G U R E 6 Light interference patterns of draining foam films at different temperatures.(A) 1 wt% APG, (B) 5 wt% PF127, (C) 10 wt% PF127, (D) 20 wt% PF127.F I G U R E 7 The cross-section of draining foam films at different temperatures: (A) 5 wt% PF127, (B) 10 wt% PF127, (C) 20 wt% PF127.(D) Variation of thickness at the center point of the liquid film over time at different temperatures.(E) Initial liquid film thickness determined by the extrapolating curve to t = 0 s.

F
I G U R E 8 Microscopic image of bubbles: (A) 20 wt% PF127 bubbles at t = 60 min at 20°C; (B) 10 wt% PF127 bubbles at t = 60 min at 45°C; (C) 20 wt% PF127 bubbles at t = 60 min at 45°C; (D) 1 wt% APG bubbles at 20°C; (E) Newtonian black film of the 1 wt% APG bubble with a thickness less than 100 nm; (F) 20 wt% PF127 bubbles with a film thickness of several microns showing color stripes.

F G U R E 9
Variation of temperature near the interface between foam and n-heptane fuel with time: (A) 20 wt% PF127 foam; (B) commercial AFFF.(C) Photographs of PF127 foam and APG foam in the fire extinction tests at different times.(D) Photographs of PF127 foam and AFFF in the burnback tests at different times.(E) Comparison of fire extinction and burnback performance for different foams.Thermo-responsive fluorine-free foam contains 20 wt% PF127.For comparison, commercial AFFF #1 and #2 are provided by Suolong and Jiangya Co., Ltd in China, respectively.The silicone-based foam consists of 1 wt% CoatOsil-77 (Momentive company) and 1 wt% BS-12.The foam flow rate was 750 g/min and the foam expansion ratio was around 9.0 ± 0.5 unless otherwise noted.T A B L E 3 Fire extinction and burnback time of liquid foams containing different concentrations of PF127.