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Applications of Polymer, Composite, and Coating Materials

A Novel Method for Preparing Auxetic Foam from Closed-cell Polymer Foam Based on Steam Penetration and Condensation (SPC) Process Donglei Fan, Minggang Li, Jian Qiu, Haiping Xing, Zhiwei Jiang, and Tao Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02332 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

A Novel Method for Preparing Auxetic Foam from Closed-cell Polymer Foam Based on Steam Penetration and Condensation (SPC) Process Donglei Fana,b, Minggang Li*a, Jian Qiua, Haiping Xinga, Zhiwei Jianga, Tao Tang*a a

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b University of Science and Technology of China, Anhui 230026, China ABSTRACT: Auxetic materials are a class of materials possessing negative Poisson’s ratio. Here we established a novel method for preparing auxetic foam from closed-cell polymer foam based on steam penetration and condensation (SPC) process. Using polyethylene (PE) closed-cell foam as an example, the resultant foams treated by SPC process presented negative Poisson’s ratio during stretching and compression testing. The effect of steam-treated temperature and time on the conversion efficiency of negative Poisson’s ratio foam was investigated, and the mechanism of SPC method for forming re-entrant structure was discussed. The results indicated that the presence of enough steam within the cells was a critical factor for the negative Poisson’s ratio conversion in the SPC process. The pressure difference caused by steam condensation was the driving force for the conversion from conventional closed-cell foam to the negative Poisson's ratio foam. Furthermore, the applicability of SPC process for fabricating auxetic foam was studied by replacing PE foam by polyvinyl chloride (PVC) foam with closed-cell structure or replacing water steam by ethanol steam. The results verified the universality of SPC process for fabricating auxetic foams from conventional foams with closed-cell structure. In addition, we explored potential application of the obtained auxetic foams by SPC process in the fabrication of shape memory polymer materials. Keywords: Negative Poisson's ratio, steam treatment, polymer foam, polyethylene, polyvinyl chloride. 1. INTRODUCTION Metamaterial concept offers a promising route to develop the materials with unusual or unprecedented properties. Auxetic materials refer to a class of materials possessing negative Poisson’s ratio. They expand in the transverse direction when being stretched, while shrink under compression.1 Since the publication of the first report on artificial auxetic foam materials in 1987,2 it has been paid much attention because of its novel properties and numerous potential applications. As auxetic materials possess excellent properties, such as high shear modulus and yield strength, large indentation resistance, good resilience, good anti-notch performance and excellent fracture toughness, they have a broad application prospects in plenty of fields, such as sound barrier, insulation, buffer, cushions, especially in the artificial disc, artificial blood vessels, sandwich panels, manufacturing fasteners or seat belts, internal combustion engine catalyst converter carrier materials.3-9 In recent years, a variety of fabricating methods for negative Poisson’s ratio foam (auxetic foam) have been reported. For example, the auxetic foams from the foams with open-cell or partially open-cell structure are generally prepared by bi-axial compression10 or tri-axial compression11 or mechanic-chemic-thermal process12, and the auxetic foams from closed-cell foams are prepared by high pressure or vacuum.13 Lakes has firstly employed the tri-axial compression method in preparing the negative Poisson’s ratio foam, and since then significant effort has been made to prepare auxetic foams.14-16 Although the tri-axial compression equipment has been improved, the negative Poisson’s ratio

value of the obtained foam by this method is still small. Lisiecki et al.12 put forward mechanic-chemic-thermal process to prepare open-cell polyurethane (PU) auxetic foam. The foams were exposed to chemical solvent (acetone) after tri-axial compression to soften the foam, then were stabilizing at 180 oC, and were set at 120 oC finally. This method can prepare foam specimens with more homogeneous cell structure. However, although the prepared auxetic foam has good performance, the use of a large amount of volatile organic solvents brought environmental pollution and hidden trouble for production safety. What’s more, solvent removal, drying processes substantially increased the processing complexity as well as processing time. In addition, the obtained foams have a typical strain-dependent Poisson's ratio. Very recently, Li et al.16 have used environmentally friendly and easily removed CO2 as processing agents to prepare auxetic open-cell foams with constant Poisson's ratio over large deformation at room temperature. They found that the introducing of styrene-acrylonitrile copolymer (SAN) into the PU matrix enhances the loading bearing capability of the foam, which generates a constant negative Poisson's ratio over large deformation for the prepared foam.16 So far the research about the fabrication of auxetic foams from the conventional closed-cell foams is few. Comparing the structures of open-cell and closed-cell foams, it is more difficult to realize the conversion from conventional closed-cell foam to the negative Poisson's ratio foam, because the cell wall (not rib in open-cell foams) in the closed-cell foams needs to be transformed into re-entrant structure, different from the case of open-cell foam. Martz et al.13

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achieved the transformation from conventional closed-cell polyethylene (PE) foam to negative Poisson’s ratio foam under high pressure or vacuum condition. In their work, the processing temperature should be higher than softening temperature of PE, at which the relaxation of PE and cell wall bending of PE foam could be realized. Finally, the permanent deformation of the cell structure was achieved by decreasing the temperature and letting the specimen cool before the load was removed. This method is relatively simple but can only obtain negative Poisson's ratio in small deformation (32% strain). Thus more exploration is urgently required to find the novel method to prepare the negative Poisson’s ratio foam material from closed-cell polymer foams. In this work, we demonstrate a novel method for fabricating negative Poisson’s ratio foam from closed-cell PE foams. This method is based on steam treatment under atmospheric pressure through water steam penetration and condensation (SPC) process, so it is simple and green. As we know, the cell wall of closed-cell polymer foam is composed of polymeric film with various thicknesses, which shows permeability to gas and deformability under some conditions. In fact, the permeation rate (P) of small penetrants is determined by P=D·S, where D is the diffusion coefficient and S is the solubility of penetrant.17 As the thickness and crystalline state of polymer films strongly influence the diffusion coefficient and solubility of penetrant, the permeability of gas or liquid through polymer films will be determined by these factors. Normally polymer films have “double-edged sword” behaviour for the penetration or barrier of gas, depending on the testing time. So the effect of steam-treated temperature and time on the negative Poisson’s ratio conversion efficiency was studied. The morphologies of the original foam and the auxetic foam were compared. The mechanism of SPC process for PE conventional foam transforming into negative Poisson’s ratio material was discussed. Finally the applicability of SPC process for fabricating auxetic foam was studied by replacing PE foam by polyvinyl chloride (PVC) foam with closed-cell structure or replacing water steam by ethanol steam. 2. EXPERIMENTAL SECTION 2.1. Materials. Commercial closed-cell PE foams with 20, 30, and 45 expansion ratio were purchased from Shijiazhuang Qihong Rubber Product Limited Company. The closed-cell PVC foam with the density of 52 Kg/m3 was fabricated in our laboratory. 2.2. Preparation of negative Poisson’s ratio foam by SPC process. As shown in Fig. 1, steam penetration and condensation (SPC) process was established to fabricate auxetic foams from conventional closed-cell polymer foams. In particular, first the PE foam was cut to approximately 10 cm x 10 cm x 10 cm in length, width and thickness, respectively. And then the sample was put into the environment of water steam stayed for a certain time at specified temperature. Finally, it was taken out and let it cool to obtain the PE auxetic foam. The auxetic foams prepared in this method are uniform except the edge, which is very important for the accuracy of Poisson's ratio value. The PVC foam was treated with the same method. In some cases, ethanol steam was used to replace water steam for treating PE foam. In the following context, the OF-X represented the original foam with an expansion ratio of X, STF-X represented

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the steam-treated foam with an expansion ratio of X, OHF-X represented the oven-high-temperature-treated foam with an expansion ratio of X, and ETF-X represented the ethanol-treated foam with an expansion ratio of X. The OHF-X-Y represented the oven-high-temperature-treated foam with an expansion ratio of X at Y °C, and the STF-X-Y represented the steam-treated foam with an expansion ratio of X at Y °C. For example, the OF-20 meant the original foam with an expansion ratio of 20, and STF-45-100 represented the steam-treated foam with an expansion ratio of 45 at 100 °C. As control, oven high temperature treatment was used to study the mechanism of SPC process. In detail, the mass of the foam specimen with different expansion ratio was weighed before placing into the oven, and then heated at 100 °C for 6 h. The treated sample (OHF-X) was weighed after the oven treatment. The mass change of the specimens from steam treatment and oven treatment was compared to determine whether water steam entered the cell during the steam treatment. 2.3. Characterization. For mechanical testing, the middle part of the foam sample after SPC was selected and cut into rectangular sample with approximately 5 cm x 1.5 cm x 0.6 cm in length, width and thickness, respectively. Firstly, a line which is vertical to the direction of stretch was drawn on the rectangular sample. And the original length (l0) and thickness (t0) were measured with a vernier caliper. Then, the rectangular sample was tested with a tensile rate of 5 mm/min by the electronic universal tensile tester (Changchun Intelligent Instrument and Equipment Limited Company). During the test, the length (l) and thickness (t) of the drawn line were measured under different engineering strains. The longitudinal strain (εl) and the thickness direction strain (εt) were calculated by the following formula:

εl=

l - l0 l0

(1)

εt=

t − t0 t0

(2)

Poisson's ratio (the thickness direction Poisson's ratioν t) was calculated by the classical definition:

ν t=-

εt εl



(3)

Volume compression factor was the value of the final density divided by the original density. And the density was measured in buoyancy method. For STF, it was the value of the density of steam-treated foam divided by the original foam density. For OHF, it was the value of the density of oven-high-temperature-treated foam divided by the original foam density. The tensile and compression testing was carried out with a tensile rate of 5 mm/min by the electronic universal tensile


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tester (Changchun Intelligent Instrument and Equipment Limited Company). The Poisson's ratio value given in the

Figure 1. The schematic drawing of SPC process for the preparation of negative Poisson’s ratio foam. manuscript were the average values of the treated foams under the same conditions after five times measurements. The standard deviation is between 0.028 and 0.179. We presented the standard deviation of the Poisson's ratio of the STF-45-100 as an example in Fig.S1 in SI. Morphologies of the foam specimen were observed by Phenom ProX desktop scanning electron microscopy (SEM) of Fenna Scientific Instruments Limited Company. The cross sections of all specimens were exposed by cryo-fracturing under liquid nitrogen. The melting point of the foam before and after steam treatment was measured by METTLER TOLEDO's DSC1 differential scanning calorimeter. The 5-10 mg foam specimen was loaded in aluminum pans and heated under a nitrogen atmosphere. A first heat scanning was carried out from 25 °C to 180 °C at a rate of 10 °C/min, and the specimen was held for 5 min in the molten state to eliminate thermal history. Then it was cooled to 25 °C at a rate of 10 °C/min and held for 5 min. Finally, a second heat scanning was executed with heating to 180 °C at a rate of 10 °C/min. The liquid water content was measured by SDTQ600 thermal gravimetric analyzer made in TA Instruments. A heat scanning was carried out from 25 °C to 900 °C at a rate of 10 °C /min under air atmosphere. 3. RESULTS AND DISCUSSION Fig. 2 shows the relationship between the Poisson's ratio and engineering strain of the original foam (OF-X, X represents expansion ratio of foam) and steam-treated foam (STF-X specimens) by SPC process (treating conditions: at 100 °C for 6 h; see the detailed process in the experimental section). It can be seen that the Poisson's ratio value of the original foam is positive, while the Poisson's ratio of the steam-treated foam becomes small and even negative value. The minimum Poisson's ratios of the steam-treated foam specimens with 20, 30 and 45 expansion ratio in the stretch testing are -0.278, -0.439 and -0.487, respectively. When being compressed, the minimum Poisson's ratios of the steam-treated foam specimens with 20, 30 and 45 expansion ratio are -0.026, -0.090 and -0.118, respectively. The Poisson's ratio increases with engineering strain in both stretch testing and compression testing. The negative Poisson’s ratio behavior vanishes when engineering strain in the stretch testing increases to 0.201, 0.303, and 0.604 for foam specimens with 20, 30 and 45 expansion ratio, respectively. This phenomenon has been observed in the previous reports for negative Poisson’s ratio foam,12,13,18 in which the auxetic foam can be obtained only within 0.322 engineering strain.13 In contrast, the negative Poisson's ratio behaviour can maintain to 0.604 engineering strain in this work. The above results indicate that the steam treatment realized the

conversion from conventional closed-cell foams to negative Poisson's ratio foams, and the greater the expansion ratio of the foams is, the easier this conversion is. Fig. 3 shows the SEM images of the foam specimens before and after steam treatment. It can be seen that the untreated foams have a full cell structure, and the cell walls are straight. The cell wall thicknesses of three PE foams with 20, 30 and 45 expansion ratio are 6.1 µm, 4.4 µm and 2.2 µm, respectively. The particulate matter on the cell walls are inorganic particles added by the manufacturers as foaming nucleating agents, which was verified by TGA measurements. After the steam treatment at 100 °C for 6 h, the full cell of the foams shrink inward, and the cell walls are obviously bent, as a result, the re-entrant structure appears. The transformation degree of re-entrant structure from normal cell structure is more obvious in the foams with higher expansion ratio, which is consistent with the results of Fig. 2. Actually, we observed the following phenomenon during the fabrication of auxetic foams: After the PE foam was put into the environment of water steam at specified temperature, the volume of the foam expanded quickly. When the foam was taken out, the foam quickly shrunk in several seconds at room temperature, and then the shape of auxetic foams did not change. Now there are two questions about the fabrication process of auxetic foam by SPC process: What is the driving force for strong shrinking of PE foam after fabrication process of auxetic foam by SPC process? Why can the shape of shrunk PE foam be kept? Fig. 4 presents a schematic drawing for possible mechanism of preparing the negative poisson's ratio foam by SPC process. The key factors should include penetrability of water steam through the cell wall, and deformation ability for shrinking stage of the foam and shaping ability for keeping re-entrant structure of foam cell in the final cooling stage. In the cases of semi-crystalline polymer foams, the above factors are closely related to crystalline state and crystallization behaviour of PE foams.

Figure 2. The relationship between Poisson's ratio and engineering strain of PE foam specimens with different expansion ratio before and after steam treatment (The steam-treated time and temperature were 6 h and 100 °C, respectively).



Figure 3. SEM images of PE foam specimen before and after steam treatment ((a), (b) and (c) original foams with the expansion ratio of 20, 30 and 45, respectively; (d), (e) and (f) steam-treated foams with the expansion ratio of 20, 30 and 45, respectively. The steam treatment time and temperature were 6 h and 100 °C, respectively).

Figure 4. Schematic drawing for possible formation mechanism of re-entrant structure from closed-cell foams by SPC process. Generally a part of crystalline fraction of cell wall in the PE foams will melt during steam treatment if the steam temperature is high, strongly depending on the specific temperature and time for steam treatment, which lead to softening of the cell wall (Fig. 5a). So the cells expand due to expansion of the gas inside cells. Meanwhile the penetration of water steam into the inside of cells will occur during steam treatment. In order to confirm the penetration of water steam during steam treatment, the mass change of the steam-treated foam and oven-treated foam at the same temperature (100 °C) for the same treatment time (6 h) was compared to explore the mechanism for the formation of auxetic foam with negative Poisson’s ratio (Table 1). From Table 1, it can be seen that the mass change of the foam specimen before and after oven treatment at 100 °C is very small. There is also no significant change in the volume of foam specimen before and after oven-high-temperature treatment, and the volume compression factor of OHF is approximately 1. Importantly, no negative Poisson’s ratio behaviour appears in the foams after oven treatment under the mentioned conditions. In contrast, the mass of foam specimens after water steam treatment increases, especially the foams with 45 expansion ratio. Meanwhile these steam-treated foams present significant volume shrinkage after

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cooling. The higher the increased weight of the steam-treated foam is, the greater the volume compression factor after cooling. These results illustrate that water steam has penetrated into the foam inside during the steam treatment. After some time, the internal and external water steam concentration reaches equilibrium. When the sample is taken out from the steam environment, the outside temperature drops quickly. At the same time, the water steam within the cell is rapidly condensed, which result in rapid pressure drop inside the foam cells. As a result, the pressure outside the cell (1 atm) is significantly greater than the pressure inside the cell, therefore the cells rapidly shrink and invaginate because of the pressure difference, and the re-entrant structure is formed. Therefore, the driving force for the formation of re-entrant cell structure results from the pressure difference between the outside of cell and the inside of cell during SPC process. Obviously both the treatment time and treatment temperature during SPC process will affect the formation of pressure difference between the inside of cell and the outside of cell. The above results come from “double-edged sword” behaviour of polymer films for the penetration or barrier of gas, depending on the observation time. A gas can penetrate through polymer film in a long observation time, but the gas will be obstructed in a short observation time. In the SPC process, the transformed foam structure is fixed after the cooling of the foam, probably due to the partial recrystallization of cell wall, and finally the conversion from positive Poisson's ratio to negative Poisson's ratio is realized ultimately. As we know, the key for preparing auxetic foam with negative Poisson’s ratio is that the cell structure is transformed into re-entrant structure.2 It can be seen that both the stage for steam penetration through cell wall and the stage for steam condensation within the cell are two key processes for the formation of re-entrant structure in this method. Under the same steam conditions, the transformation efficiency of re-entrant structure should be determined by steam penetrability, and deformation ability and shaping ability of cell wall. These properties mainly depend on the conditions of steam treatments (temperature and time). The temperature for steam treatment strongly influences the crystalline state and deformation ability of the foam matrix during SPC process. Table 1 The relationship of volume compression factor with mass change fraction of PE foams with different expansion ratio after steam treatment and oven treatment. Expansion ratio

20

30

45

Mass change after oven treatment at 100 °C for 6 h (wt %)

1.6

2.0

2.1

1.0

1.0

1.0

19.2

42.6

63.1

1.3

5.1

5.9

Volume compression factor Mass change after steam treatment at 100 °C for 6 h (wt %) Volume compression factor


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Figure 7. The relationship between Poisson's ratio and engineering strain of PE foam specimens at different treatment time (The steam treatment temperature was 100 °C and the expansion ratio was 45 times).

Figure 5. (a) DSC heating curves of PE foam specimen before (OF-x) and after (STF-x) steam treatment (Treatment conditions: 100 °C for 6 h); (b) DSC first heating curves of OF-45 (OF-45-1UP) and the heat-treated foam specimens at 100 °C (OHF-45-100-1UP), 80 °C (OHF-45-80-1UP) and 60 °C (OHF-45-60-1UP); (c) The test program of (d) and (e); (d) DSC curves of OF-45 annealed at 100 °C (OF-1-45-100: black line-the first heating curve from 25 to 100 °C, blue line-cooling curve from 100 to 25 °C after annealing at 100 °C for 1 h, red line-the second heating curve from 25 to 180 °C); (e) DSC curves of OF-45 annealed at 60 °C (OF-1-45-60: black line-the first heating curve from 25 to 60 °C, blue line-cooling curve from 60 to 25 °C after annealing at 60 °C for 1 h, red line-the second heating curve from 25 to 180 °C). The heating or cooling rate was 10 °C/min in all the measurements.

Figure 6. The relationship between Poisson's ratio and engineering strain of PE foam specimens at different treatment temperatures (The expansion ratio was 45 times and the steam treatment time was 6 h).

Fig. 6 shows the relationship between the Poisson's ratio and the engineering strain of foam specimens by steam treatment at different temperatures. The Poisson's ratio of the foam specimen decreases with the increase of treatment temperature. The negative Poisson’s ratio begins to appear when the treatment temperature reaches 80 °C, and the foams show an obvious negative Poisson's ratio behavior when treated at 100 °C. From the DSC heating curve (Fig. 5a), it can be seen that the temperature has a key effect on the crystalline state of the PE foam, which will influence the deformability of cell wall and the permeability of water steam through cell wall. The fraction of melted crystal is very low at 60 °C, and the PE foam partially melts at 80 °C. A large of the crystalline fraction will melt at 100 °C and completely melt at 112 °C. On the other hand, the saturated vapor pressure of water will increase with the temperatures, for example, 19.9 kPa at 60 °C, 47.4 kPa at 80 °C and 101.3 kPa at 100 °C, respectively. Hence, the steam treatment at 100 °C is more efficient to realize the conversion of negative Poisson’s ratio in the process of steam treatment. As mentioned-above, polymer films show “double-edged sword” behavior for the penetration or barrier of gas, depending on the testing time. The permeation of water steam through the cell wall (PE film) is a slow process, which is controlled by water molecule diffusion and solubility of water in the PE matrix.19,20 Therefore the water steam treatment time should have an important effect on the negative Poisson's ratio conversion. Fig. 7 shows the relation between Poisson's ratio and engineering strain of the foam specimen by steam treatment at 100 °C for different treatment time. It can be seen that Poisson's ratio decreases gradually with the increase of steam treatment time from 2 h to 6 h, and reaches the lowest value after treating for 6 h, then gradually increases with extending treatment time. This phenomenon is ascribed to that the evaporation and condensation of water steam is a reversible process, in which a part of water steam within the cell starts to condense into liquid water due to supersaturation. The longer the treatment time is, the higher the content of liquid water within the cell is (Fig. 8). When the foam is



cooled, the presence of liquid water partly counteracts the volume shrinkage of the foams. Therefore, the efficiency of

Figure 8. The mass change vs treating time for the PE foam with 45 expansion ratio (The steam treatment temperature was 100 °C).

Figure 9. DSC first heating curves of OF-20 and the heat-treated foam specimens at 100 °C (OHF-20-100-1UP), 80 °C (OHF-20-80-1UP) and 60 °C (OHF-20-60-1UP). the negative Poisson's ratio conversion decreases when the steam treatment time surpasses 6 h. In addition, the temperature for steam treatment also influences recrystallization ability after shrinking, which determines the shaping ability of the resultant foam. As mentioned above, the deformation ability for shrinking stage of the foam and shaping ability for keeping re-entrant structure of foam cell are closely related to crystalline state and crystallization behavior of PE foams. Fig. 5b shows DSC heating curves (the first heating run) of the heat-treated foam specimen at 100, 80 and 60 °C for 6 h, simulating the steam treatment process. Compared to the DSC curves of original foams (OF-45), a new broad melting peak appears in the left side of the main melting peak in all the foams treated by heating treatment at three temperatures, which is similar to the cases of steam treatment (Fig. 5a). A similar phenomenon was observed in the PE foams with the expansion ratio of 20 (Fig. 9). This result shows that the broad melting peak at the lower

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temperature in the steam treated foams should mainly result from recrystallization of PE. Fig. 5d and 5e compare the effect of annealing temperature on the crystallization behavior and melting behavior of PE foams with the expansion ratio of 45. It can be seen that the sample annealed at 100 °C for 1 h shows a strong exothermic peak during cooling process and double endothermic peaks during reheating process, similar to that of OHF-45-100-1UP (Fig. 5d vs Fig. 5b). Compared to the DSC curve of OF-45-1UP in Fig. 5b, the sample annealed at 100 °C for 1 h shows a new melting peak in the left side of the main melting peak (OF-1-45-100-2UP in Fig. 5d). This new melting peak should result from recrystallization of melted fraction of PE foams during cooling process, which provide shaping ability for keeping re-entrant structure of foam cell in the final cooling stage. In contrast, the sample annealed at 60 °C for 1 h shows a very weak exothermic peak during cooling process and a similar endothermic behavior to that of OHF-45-60-1UP (Fig. 5e vs Fig. 5b) during reheating process. This means that the shaping ability for keeping re-entrant structure of foam cell is weak in the samples annealed at 60 °C. In addition, as mentioned above, the deformability of cell wall is also weak at lower temperature, and there is no enough amount of water steam within the cells of PE foam to form a driving force for the conversion of re-entrant structure from conventional foam structure during cooling, so no negative Poisson's ratio behavior appears in the steam-treated foams at 60 °C. The deformation and shaping (or shape fixation) mechanism of auxetic foam in this work is similar to the process for steam-chest molding of expanded polypropylene (EPP) foams. Zhai et al. stated the formation mechanism of interbead bonding during EPP bead processing21 and used DSC to simulate bead foaming process22. The authors considered that the high temperature during steam-chest molding can melt some of the original crystal, and softened polymer matrix is conducive to the expansion and welding of beads. In fact, the high temperature steam treatment causes the beads expand, and the EPP beads impinge with each other, then deform and eventually achieve welding in a limited mold volume. The molded bead samples were then stabilized by cooling to obtain EPP ware. More importantly, the temperature used for the molding of EPP beads is in between the temperature for the lower melting peak and the temperature for the higher melting peak (the primary melting peak). The reason is that the EPP beads can weld together through interfacial diffusion due to partial melting of EPP and the following recrystallization, but the foam cell structure can be kept. If the temperature for steam-chest molding is too high, all original crystals will completely melt, and also the cell structures of the EPP bead foam will be destroyed. For the SPC process in this work, the melting of part crystals softens PE matrix during the steam treatment process, which facilitates the diffusion of water vapor and the deformation of PE matrix. The pressure difference caused by steam condensation makes the foam shrink quickly during cooling process. Meanwhile, the crystals formed after cooling are used to fix the shape. The melting degree of the original crystals and the crystals formed due to cooling increase with steam treatment temperature. Hence, the steam treatment at 100 °C is more efficient to realize the conversion of negative Poisson’s ratio in the process of steam treatment. In the whole process, the steam treatment temperature is lower than Tm


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(108 °C, the main melting peak in DSC curves), meaning that the crystalline structure formed from the recrystallization of melted crystals determines the shaping ability, and the primary crystalline structure associated with the higher temperature in DSC is kept to maintain the cells structure. The cyclic mechanical test was conducted to study the change of Poisson's ratio. It can be seen from Fig. 10a and Fig. 10b that the auxetic foam can recover its original size and shape after stretch testing and compression testing at small engineering strain (0.1) for 4 cycles, and the Poisson's ratio is basically unchanged. As the times of cycles increases, the recovery performance of auxetic foam decreases and the Poisson's ratio increases in some degree. When the engineering strain is increased to 0.2, the recovery performance of the auxetic foam significantly weakens, and the Poisson's ratio greatly increases with the increase of the number of cycles. The larger the engineering strain is, the more obvious this trend is.

Figure 12. The relationship between Poisson's ratio and engineering strain of PE foam specimens of 45 expansion ratio treated under ethanol steam (the steam treatment temperature and time were 78 °C and 6 h, respectively).

Figure 10. The relationship between the minimum of Poisson's ratio and cycle index at different engineering strain in (a) stretch testing and (b) compression testing. Figure 13. Comparison for Poisson's ratio vs engineering strain for PVC and PE foam specimens before and after steam treatment (STF-PVC: The steam treatment temperature and time were 100 °C and 6 h, respectively; STF-PE: The steam treatment temperature and time were 100 °C and 6 h, respectively).

Figure 11. The relationship between the minimum of Poisson's ratio and oven treatment time for foam samples in (a) stretch testing and (b) compression testing at different treatment temperatures. (c) The relationship between the Poisson's ratio and engineering strain for treated auxetic foam samples under different humidity for different treatment time (code name “30-24” represented auxetic foam samples treated in ambient humidity of 30% for 24 h, and so on).

Figure 14. SEM images of PVC foam specimens (a) before and (b) after steam treatment. The thermal stability and moisture stability of the prepared auxetic samples were investigated. The auxetic foam samples were treated in an oven at different temperature for different treatment time. Fig. 11 shows the relationship



between the minimum of Poisson's ratio and oven treatment time for foam samples in stretch testing (Fig. 11a) and compression testing (Fig. 11b) at different treatment temperatures. It is found that auxetic foam can maintain long-term stability when treated at 20 °C. However, the minimum of Poisson's ratio rapidly increased when treated at 40 °C and 60 °C for 1 h, and this phenomenon was more obvious at 60 °C. This is due to the presence of some liquid water within the auxetic foams. The water will become water steam during the heating process, which causes the foam expands and the re-entrant structure shows a certain recovery to normal cell structure. Extending the processing time, the Poisson's ratio tends to be stable and still retains the characteristic of auxetic foam. We will further improve the temperature stability in the future work. In order to investigate moisture stability of the prepared auxetic samples, the STF-45-100 was treated at 30%, 50% and 70% humidity for 24 h and 48 h, respectively. Fig. 11c shows the relationship between the Poisson's ratio and engineering strain for auxetic foam samples under different humidity for different treatment time. It can be seen that the results under different humidity are very close, meaning that the humidity has no significant effect on the Poisson's ratio of the auxetic foams. So the prepared auxetic foam has good humidity stability. We further study the applicability of SPC process for fabricating the auxetic foams from closed-cell polymer foams, for example, water steam is replaced by ethanol steam. Fig. 12 shows the relationship between the Poisson's ratio and engineering strain of ethanol steam-treated foam at 78 °C. It can be seen that the obtained foam shows a lower negative Poisson's ratio than that of STF-45 treated at 80 °C. Considering the influence of treating temperature, the deformation ability of PE foams in the two cases should have a similar ability due to the close treating temperature. However, the saturated vapor pressures of water (47.4 kPa at 80 °C) and ethanol (101.3 kPa at 78 °C) at treating temperature are different. On the other hand, the penetration process of gas through polymer films is determined by solubility and diffusion of gas.23 The solubility of ethanol in PE film is much higher than that of water.23 As a result, more ethanol steam penetrates into the cell inside of PE foam, and a strong driving force for the transformation of re-entrant structure is formed. At the same time, the PE film that includes some ethanol molecules will be easily deformed due to plasticization of ethanol. Thus the ethanol steam treatment is more efficient than water steam treatment to fabricate auxetic PE foam. The SPC process is also applied in closed-cell PVC foam to fabricate the auxetic foams (Fig. 13 and 14), which verifies the universality of this method. As the PVC foam used in the experiment is brittle, and it breaks when stretched in large strain, so Poisson's ratio value is not obtained when the engineering strain is beyond 0.408. From the above results, it is clear that an amount of water is kept in the inside of the obtained PE auxetic foam by SPC method. The water inside the foam will vaporize or condense with the environmental temperature; as a result, the auxetic foam will expand or shrink with the change of environmental temperature. This behavior can be applied in the fabrication of shape memory materials from the auxetic foam. Firstly PE auxetic foam was adhered with polyurethane (PU) film to

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form a bilayer specimen. Based on the principle of SPC method, the layer of PE auxetic foam will dramatically swell

Figure 15. The shape memory property of bilayer specimen of PE auxetic foam/polyurethane film: (a), (b) and (c) The bilayer specimen was put in hot water at 100 °C; (d), (e) and (f) The bilayer specimen was taken out and kept at room temperature. when it is put in hot water or shrink when taken out at room temperature, while the layer of PU changes a little during the above processes. As a result, the shape of bilayer specimen will change with the ambience. Fig. 15 shows the change of the shape of the bilayer specimen with the environmental temperature. When the bilayer specimen was put into the hot water at 100 °C, the sample quickly bended to form loop shape (Fig. 15a-15c). After the sample was taken out from hot water, the sample rapidly deform to the original shape (Fig. 15d-15f). This confirms that the bilayer specimen of PE auxetic foam/PU film possesses the shape memory properties. 4. CONCLUSIONS In summary, we demonstrated a novel method based on SPC process to prepare polymer auxetic foam with the re-entrant structure in this work. The presence of the water steam within the cell was a critical factor for the negative Poisson’s ratio conversion in the process of steam treatment, and the pressure difference caused by steam condensation was the driving force for the transformation from conventional foam to the auxetic foam. Under the same steam conditions, the transformation efficiency of re-entrant structure was determined by steam penetrability, deformation ability and shaping ability of cell wall. These characteristics mainly depended on the properties of polymer matrix and steam properties, such as crystalline state of polymer matrix and penetration behavior of steam in polymer matrix. Therefore, the conditions for steam treatment, including treating temperature and treating time, strongly influenced the conversion from conventional foam to negative Poisson's ratio foam. In a word, the SPC process provided a novel specific route for fabricating negative Poisson's ratio materials from closed-cell polymer foams under atmospheric pressure. In addition, the obtained auxetic foams by SPC process showed some potential application, such as the fabrication of shape memory polymer materials.


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■ ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The relationship between Poisson's ratio and engineering strain of PE foam specimen with 45 expansion ratio after steam treatment. Video showing the shape memory property of the bilayer specimen of PE auxetic foam/PU film. (AVI) ■ AUTHOR INFORMATION Corresponding Authors *E-mail:[email protected]. *E-mail: [email protected].

ORCID Tao Tang: 0000-0002-1887-0579 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The financial supports from National Natural Science Foundation of China (51233005, 51573179 and 21304089) and the Ministry of Science and Technology of China (2015AA033901) are greatly appreciated. ■ REFERENCES (1) Lakes, R. S. Negative-Poisson’s-Ratio Materials: Auxetic Solids Annu. Rev. Mater. Res. 2017, 47, 1-19. (2) Lakes, R. S. Foam Structures with A Negative Poisson's Ratio Science 1987, 27, 1038-1040. (3) Saxena, K. K.; Das R.; Calius, E. P. Three Decades of Auxetics Research-Materials with Negative Poisson's Ratio: A Review Adv. Eng. Mater. 2016, 18, 1847-1870. (4) Liu, Y.; Hu, H. A Review on Auxetic Structures and Polymeric Materials Sci. Res. Essays 2010, 5, 1052-1063. (5) Choi, J. B.; Lakes, R. S. Fracture Toughness of Re-entrant Foam Materials with A Negative Poisson's Ratio: Experiment and Analysis Int. J. Fracture 1996, 80, 73-83. (6) Imbalzano, G.; Tran, P.; Ngo, T. D.; Lee, P. V. Three-Dimensional Modelling of Auxetic Sandwich Panels for Localised Impact Resistance J. Sandw. Struct. Mater. 2017, 19, 291-316. (7) Kolken, H. M. A.; Zadpoor, A. A. Auxetic Mechanical Metamaterials RSC Adv. 2017, 7, 5111-5129. (8) Caddock B. D.; Evans K. E. Negative Poisson Ratios and Strain-Dependent Mechanical Properties in Arterial Prostheses. Biomaterials 1995, 16, 1109-1115. (9) Scarpa, F. Auxetic Materials for Bioprostheses IEEE Signal Proc. Mag. 2008, 25, 124-126. (10) Li, D.; Dong, L.; Yin, J.; Lakes, R. S. Negative Poisson’s Ratio in 2D Voronoi Cellular Solids by Biaxial Compression: A Numerical Study J. Mater. Sci. 2016, 51, 7029-7037. (11) Chan, N.; K. Evans, E. Fabrication Methods for Auxetic Foams J. Mater. Sci. 1997, 32, 5945-5953. (12) Lisiecki, J.; Blazejewicz, T.; Kłysz, S.; Gmurczyk, G.; Reymer, P.; Mikułowski, G. Tests of Polyurethane Foams with Negative Poisson’s Ratio Phys. Status Solidi B 2013, 10, 1988-1995.

(13) Martz, E. O.; Lee, T.; Lakes, R. S.; Goel, V. K.; Park, J. B. Re-entrant Transformation Methods in Closed Cell Foams Cellular Polymers 1996, 15, 229-249. (14) Li, Y.; Zeng, C. On the Successful Fabrication of Auxetic Polyurethane Foams: Materials Requirement, Processing Strategy and Conversion Mechanism Polymer 2016, 87, 98-107. (15) Bezazi, A.; Scarpa, F. Mechanical Behaviour of Conventional and Negative Poisson’s Ratio Thermoplastic Polyurethane Foams Under Compressive Cyclic Loading Int. J. Fatigue 2007, 29, 922-930. (16) Li, Y.; Zeng, C. Room-Temperature, Near-Instantaneous Fabrication of Auxetic Materials with Constant Poisson’s Ratio over Large Deformation Adv. Mater. 2016, 28, 2822-2826. (17) Dimitroulas, G. D.; Badeka, A. B.; Kontominas, M. G. Permeation of Methylethylketone, Oxygen and Water Vapor through PET Films Coated with SiOx: Effect of Temperature and Coating Speed Polym. J. 2004, 36, 198-204. (18) Clausen, A.; Wang, F.; Jensen, J. S.; Sigmund, O.; Lewis, J. A. Topology Optimized Architectures with Programmable Poisson’s Ratio over Large Deformations Adv. Mater. 2015, 27, 5523-5527. (19) Lee, W. M. Water Vapor Permeation in Closed Cell Foams J. Cell. Plast. 1973, 9, 125-129. (20) Waack, R.; Alex, N. H.; Frisch, H. L.; Stannett, V.; Szwarc, M. Permeability of Polymer Films to Gases and Vapors Ind. Eng. Chem. Res. 1955, 47, 2524-2527. (21) Zhai W.; Kim Y. W.; Jung D. W.; Park C. B. Steam-Chest Molding of Expanded Polypropylene Foams. 2. Mechanism of Interbead Bonding Ind. Eng. Chem. Res. 2011, 50, 5523-5531. (22) Zhai W.; Kim Y. W.; Jung D. W.; Park C. B. Steam-Chest Molding of Expanded Polypropylene Foams. 1. DSC Simulation of Bead Foam Processing Ind. Eng. Chem. Res. 2010, 49, 9822-9829. (23) Tamai, Y.; Tanaka, H.; Nakanishi, K. Molecular Simulation of Permeation of Small Penetrants Through Membranes 2. Solubilities Macromolecules 1995, 28, 2544-2554.



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