- Email: [email protected]
Porous polyurethane films having biomimetic ordered open pores: Indentation properties
G Model
JIEC 2692 1–4 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5
Porous polyurethane films having biomimetic ordered open pores: Indentation properties Q1 Suyeong
An, Byoung Soo Kim, Jonghwi Lee *
Department of Chemical Engineering and Materials Science, Chung-Ang University, 221, Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 September 2015 Received in revised form 18 October 2015 Accepted 19 October 2015 Available online xxx
The dramatically improved indentation hardness and modulus of ordered porous films prepared by directional crystallization were reported herein, which to the best of our knowledge, have not been reported. The directional crystallization process of porous materials seems to have the unique advantage of strengthening along the pore direction. This processing technique provides freedom in the design of porous materials, as the pore direction can be engineered in an arbitrary direction. Anisotropic structures in such materials can be engineered to any direction by simply changing the temperature gradient of the surrounding polymer solutions. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Ordered pore Foam Polyurethane Ice templating Directional melt crystallization Q2 Indentation
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Cellular materials from nature, such as porcupine quills, bird bones, and toucan beaks, have been adapted for use as high strength, lightweight materials with insulating properties [1–5]. Various foaming agents produce artificial cellular plastics, which have been used as the most common materials for protection and insulation [6]. The modern electrical industry also adapts the same methodology to protect integrated elements from external shock or deformation. Thin porous films are often inserted in between electrical elements that require excellent mechanical performance and thin film thickness. Unfortunately, common foaming agents for cellular plastics are not quite appropriate for this application, as they produce relatively large pores compared to the required film thickness, preferably less than 50 mm [7]. Freeze drying has been used as a common drying method for water soluble compounds and ceramic slurries for many decades [8–10]. Recently, this drying method was combined with the directional freezing (crystallization) technique, and as a result, biomimetic ordered cellular structures have been reported [11– 13]. This technique has provided 3D control of pores and walls in graphene, composites, and polymeric materials [14–16]. If organic solvents are used that are crystallizable, this technique can be applied to most polymers [17]. The crystallization of solvents can be engineered to produce well-ordered biomimetic porous structures by carefully controlling the temperature gradients
* Corresponding author. Tel.: +82 28205269. E-mail address: [email protected] (J. Lee).
[18]. Through-thickness porous membranes have been developed from various polymers, which could be used as ideal thin films for protection [17,19]. However, the mechanical properties of thin polymer films having these ordered biomimetic pores have rarely been investigated. This study explores the indentation properties of ordered porous thin films, which are critical for the protection and shockabsorbing performance, for the first time. Tensile or bending tests cannot reliably produce the basic mechanical parameters for protective thin film applications, as their stress conditions are different. Furthermore, compression tests are not quite feasible due to their thin nature. Thus, a micro-indentation test is a reliable solution to probe the basic mechanical properties useful for protection performances [20,21]. Because of the micro-porous nature, the size of the indenter should be sufficiently large compared to the pore size, and therefore nano-indentation is not adequate. Because of the high porosity and the micron size of the pores of these foams, indentation requires a relatively large indenter and accurate measurement of a small indentation force. These quite unusual indentation conditions required us to build our own micro-indentation system. We performed micro-indentation tests on these materials using a relatively large 2-mm spherical indenter and a sensitive load measurement system (readability 9.8 mN). Polyurethane (PU) was selected as a typical material for thin and light foams [22,23]. Ordered pores were first produced by directional crystallization of a solvent and subsequent solvent removal by sublimation. The PU solution was introduced to the top of a liquid nitrogen reservoir
http://dx.doi.org/10.1016/j.jiec.2015.10.023 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
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Fig. 1. Preparation of porous PU films by directional crystallization of a solvent.
59 60 61 62 63 64 65 66 67 68 69 70 71 72
at a speed of 200 mm/s (Fig. 1) [24]. As a result, crystal nucleation started from the region closest to the liquid nitrogen reservoir and propagated into the rest of the sample. The direction of the crystal growth lay between the direction of the sample movement and the tangential direction to the liquid nitrogen reservoir, resulting in a slanted direction. A temperature gradient in the polymer solution also developed in the same direction. Therefore, crystallization propagated along the slanted direction, from the bottom to the top, and from the one side close to the reservoir to the other side. As we previously reported, this condition greatly reduced the strain mismatch from cryostress and desiccation stress [24]. As a result, defect-free thin porous films could be prepared. The crystallization of solvent molecules by cooling along a temperature gradient separates a PU solution into crystal phases
and cryoconcentrated phases [25,26]. Since the solubility of polymer chains in the crystal phases of the solvent is extremely poor, the majority of polymer chains are expelled into the cryoconcentrate phases [18]. The crystal phases become pores after sublimation. Therefore, the content of the solvent is approximately the porosity of the final materials, which is 0.9 (0.89, if the densities of PU and dioxane are considered to be 0.9 and 1.033 g/cm3) [19]. The crystal phases of the solvent should be highly connected due to the low possibility of homogeneous nucleation, compared to that of heterogeneous nucleation, common in crystallization [27]. Thus, connected open pores generally form [18]. The crosssectional images of Fig. 2 show the ordered structures of pores and their connectivity. The major line structures are slanted, which
Fig. 2. SEM images of cross-sections (left) and surfaces (right) of porous PU films with different hard segment contents: (a) 50%, (b) 37%, and (c) 25% (scale bar = 100 mm).
Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
73 74 75 76 77 78 79 80 81 82 83 84 85 86
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0.20
Load (N)
stiffness of porous films, wherein increases in the hard segment content result in greater indentation hardness and moduli. Although these values are obtained from the initial linear regimes, they can represent the general stiffness trend of materials beyond the linear regime, as shown in the tangential slopes of Fig. 3. The values are not small, and considering the porosity of the materials, they exhibit outstanding stiffness. The dependence of the modulus on porosity has been investigated and various empirical equations have been established [1,29]. A popular equation proposed by Phani and Niyogi is
Hard segment 50% Hard segment 37% Hard segment 25%
0.15
0.10
0.05
0.00 0.00
123 124 125 126 127 128 129 130 131 132 133
p f E ¼ E0 1 ; pc 0.01
0.02
0.03
0.04
0.05
0.06
Displacement (mm) Fig. 3. Typical load versus displacement curves of porous PU films with different hard segment contents.
87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122
3
reflects the slanted temperature gradients and related crystal growth direction as discussed above. Although one concentration was used for all samples in Fig. 2, the pore sizes became significantly different. Generally, a smaller pore size resulted with more hard segments. Various influences need to be considered to explain this result, such as the viscosity, diffusion coefficient, solvent affinity, and crystallization behavior of the hard segments, but the chain conformation of PU in 1,4dioxane might be a cause. An increase in the hard segment will increase the radius of the gyration of PU chains, which slows their diffusion rate. When they are expelled by solvent crystal fronts during directional crystallization, their slow diffusion characteristics restricts the crystal growth more and creates smaller crystals. An increase in the content of hard segments also leads to an increase in viscosity [28]. The high viscosity sustains crystal growth, since the moving of crystal growth front has difficulty in expelling viscous cryoconcentrate phases. As a result, crystal growth front becomes more branched (pinned), and smaller crystals (thinner spacing between pore walls) results. Upon uniaxial compression, cellular materials first undergo an initial linear region (elastic cell wall deformation) [1]. Due to the buckling and fracture of the cell walls, the materials then pass through a yield point and reach a plateau region after sufficient densification. The indentation results show a slightly different series of deformation (Fig. 3). The initial linear elastic regime is followed by a slightly curved plastic regime. Yield points are difficult to define, and plastic deformation is dominant with a distinct hysteresis between loading and unloading curves. With an increase in the hard segment content, the hysteresis of the energy becomes more significant. Therefore, relatively brittle microdeformations, such as wall buckling, might be the main reason for the hysteresis. The increased hysteresis of PU with an increase in the hard segment content indicates significant energy dissipation, which could be related to the excellent shock adsorbing properties. The averages of the indentation hardnesses and moduli are shown in Table 1. The hard segments distinctly contribute to the
where E is the Young’s modulus of a porous material with porosity p, E0 is the modulus of the solid material, pc is the porosity at which the effective modulus becomes zero, and f is a parameter dependent on the grain morphology and pore geometry [1,29]. Often, one of these assumptions are used: pc = 1 or f = 1. With a series of porous films from different solution concentrations of 5, 10, and 15 wt%, the pc obtained was near unity at 0.964. From the typical percolation theory based on common near spherical or elliptical pore morphology, f was calculated to be 2.1. Table 1 shows the results of the E calculation with f = 1 or 2.1. The measured values are all much larger than the calculated values of E, as much as one or two orders of magnitude larger. The other simple equations, such as Gibson and Ashby, produce the same conclusion, regardless of their adjusted parameters [1,29]. This discrepancy, huge increases in modulus, seems to result from the assumption of either the spherical or elliptical pore morphology. Indeed, the pore morphology of our films with ordered open pores with high connectivity is quite different from that of ordinary foams. The aligned and ordered pores prepared in PU films distinctly improved the hardness and modulus of porous films. Furthermore, the directional crystallization process could align polymer chains in the cryoconcentrate region, resulting in strengthening along the pore direction. The chain alignment effect by directional crystallization was previously reported to a certain degree [14]. The dramatically improved hardness and modulus of ordered porous films prepared by directional crystallization could be useful for various applications of light-weight materials, which to the best of our knowledge, has not been reported. The directional crystallization process of porous materials seems to have the unique advantage of strengthening along the pore direction. This processing technique provides freedom in the design of porous materials, as the pore direction can be engineered in an arbitrary direction. Anisotropic structures in such materials can be engineered to any direction by simply changing the temperature gradient of the surrounding polymer solutions. Specifically, strengthening perpendicular to the film surfaces is possible, which has been quite difficult to achieve because the direction is typically perpendicular to the processing (flow) direction.
134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172
Experimental
173
Songstomer P-7195A, P-7185A, and P-3170A (polyester-based thermoplastic PU, hard segment contents = 50%, 37%, and 25%,
174 175
Table 1 Indentation modulus and hardness of porous PU films with different hard segment contents. Hard segment (%)
Hardness (MPa)
Indentation modulus (MPa)
Bulk indentation modulus (MPa) (p = 0)
E (MPa) calculated with f = 1
E (MPa) calculated with f = 2.1
50 37 25
0.65 0.11 0.50 0.29 0.16 0.02
8.26 0.45 3.96 0.44 2.32 0.24
22.65 0.46 16.14 0.60 10.40 0.59
1.50 1.07 0.69
0.076 0.054 0.035
Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
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176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206
respectively) polyurethanes (PUs) from Songwon Industry (Ulsan, Republic of Korea) were used. Sulfuric acid (Duksan, Ansan, Republic of Korea), hydrogen peroxide (Duksan, Ansan, Republic of Korea), 1,4-dioxane (Wako Pure Chem., Osaka, Japan), and silicon wafers with a 4-inch diameter (Sehyoung Wafertec, Seoul, Republic of Korea) were used. Silicon wafers were cleaned in a Piranha solution (sulfuric acid:hydrogen peroxide = 1:1) for 2 h, followed by washing with double distilled water. A homogeneous PU/1,4-dioxane solution (10 wt%) was prepared at 60 8C, and cast on a silicon wafer at 25 8C into a 100-mm thick film by a doctor blade (HaeChang, Seoul, Republic of Korea). The cast solution was directionally crystallized by a liquid nitrogen reservoir, which used melt crystallization equipment that we made and previously reported in detail [19]. Briefly, the solution was moved at 200 mm/s over an 1156 mL liquid nitrogen reservoir (17 cm 17 cm 4 cm). The distance between the bottom surface of the wafer substrate and the top surface of the reservoir was 4 cm. The crystallized solvent was removed by a freeze dryer (Eyela FDU-2200, Tokyo, Japan) for 24 h to produce the ordered porous PU. A scanning electron microscope (S-3400N, Hitachi, city, Japan) was used for morphological investigations (5 kV) and samples were coated with Pt by a coater (E-1010, Hitachi, Tokyo, Japan) at 15 kV for 120 s. Cross-sections were prepared by cryofracturing in liquid nitrogen. An aluminum spherical indenter with a 2-mm diameter was indented into the sample surfaces at a displacement speed of 1.667 mm/s, which was driven by a stage motor (VT-80, PI MiCos, Freiburger, Germany) and the force was measured using a microbalance (FX-200i, A&D company, Tokyo, Japan). Forcedisplacement data of at least five samples were converted into indentation hardness and modulus using the Oliver–Pharr method [30].
207
Acknowledgements
208 Q3 This work was supported by a National Research Foundation 209 grant funded by the Korean Ministry of Science, ICT and Future
Planning (NRF-2013R1A1A2021573 & Engineering Research Center of Excellence Program No. 2014R1A5A1009799). SA thanks the Chung-Ang University Research Scholarship Grants in 2014.
210 211 212
References
213
[1] S.E. Naleway, M.M. Porter, J. McKittrick, M.A. Meyers, Adv. Mater. (2015), http:// dx.doi.org/10.1002/adma.201502403. [2] M. El-Shahawi, H. Alwael, A. Arafat, A. Al-Sibaai, A. Bashammakh, E. Al-Harbi, J. Ind. Eng. Chem. 28 (2015) 147. [3] M. El-Shahawi, A. Al-Sibaai, A. Bashammakh, H. Alwael, H. Al-Saidi, J. Ind. Eng. Chem. 28 (2015) 377. [4] M. El-Shahawi, A. Bashammakh, M. Orief, A. Alsibaai, E. Al-Harbi, J. Ind. Eng. Chem. 20 (2014) 308. [5] J.-H. Kim, Y.-S. Lee, J. Ind. Eng. Chem. 30 (2015) 127. [6] Y. Yang, M.C. Gupta, K.L. Dudley, R.W. Lawrence, Nano Lett. 5 (2005) 2131. [7] X. Cao, L.J. Lee, T. Widya, C. Macosko, Polymer 46 (2005) 775. [8] J. Lee, Y. Cheng, J. Control. Release 111 (2006) 185. [9] M.Y. Kim, J. Lee, Carbohydr. Polym. 84 (2011) 1329. [10] H. Lee, J. Lee, J. Ind. Eng. Chem. 21 (2015) 1183. [11] Q. Shi, Z. An, C.K. Tsung, H. Liang, N. Zheng, C.J. Hawker, G.D. Stucky, Adv. Mater. 19 (2007) 4539. [12] H. Zhang, I. Hussain, M. Brust, M.F. Butler, S.P. Rannard, A.I. Cooper, Nat. Mater. 4 (2005) 787. [13] M.K. Lee, J. Lee, Nanoscale 6 (2014) 8642. [14] M.K. Lee, J. Lee, Polymer 74 (2015) 176. [15] M.K. Lee, M.H. Rich, A. Shkumatov, J.H. Jeong, M.D. Boppart, R. Bashir, M.U. Gillette, J. Lee, H. Kong, Adv. Healthc. Mater. 4 (2015) 195. [16] M.K. Lee, M.H. Rich, J. Lee, H. Kong, Biomaterials 58 (2015) 26. [17] M.K. Lee, N.-O. Chung, J. Lee, Polymer 51 (2010) 6258. [18] H. Zhang, A.I. Cooper, Adv. Mater. 19 (2007) 1529. [19] B.S. Kim, M.K. Lee, J. Lee, Macromol. Res. 21 (2013) 194. [20] M.-K. Chang, S.-S. Hwang, S.-P. Liu, J. Ind. Eng. Chem. 20 (2014) 1596. [21] K. Nayana, T. Venkatesha, J. Ind. Eng. Chem. 26 (2014) 107. [22] I. Bica, E. Anitas, L. Averis, J. Ind. Eng. Chem. 28 (2015) 86. [23] U. Baig, A.A. Khan, J. Ind. Eng. Chem. 29 (2015) 392. [24] B.S. Kim, J. Lee, J. Cryst. Growth 373 (2013) 45. [25] N.-O. Chung, M.K. Lee, J. Lee, Int. J. Pharm. 437 (2012) 42. [26] B.S. Kim, J. Lee, J. Nanosci. Nanotechnol. 13 (2013) 2276. [27] B.C. Knott, V. Molinero, M.F. Doherty, B. Peters, J. Am. Chem. Soc. 134 (2012) 19544. [28] Y. Li, Z. Ren, M. Zhao, H. Yang, B. Chu, Macromolecules 26 (1993) 612. [29] J. Kova´cˇik, J. Mater. Sci. Lett. 18 (1999) 1007. [30] E.G. Herbert, G.M. Pharr, W.C. Oliver, B.N. Lucas, J.L. Hay, Thin Solid Films 398 (2001) 331.
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Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
JIEC 2692 1–4 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5
Porous polyurethane films having biomimetic ordered open pores: Indentation properties Q1 Suyeong
An, Byoung Soo Kim, Jonghwi Lee *
Department of Chemical Engineering and Materials Science, Chung-Ang University, 221, Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 September 2015 Received in revised form 18 October 2015 Accepted 19 October 2015 Available online xxx
The dramatically improved indentation hardness and modulus of ordered porous films prepared by directional crystallization were reported herein, which to the best of our knowledge, have not been reported. The directional crystallization process of porous materials seems to have the unique advantage of strengthening along the pore direction. This processing technique provides freedom in the design of porous materials, as the pore direction can be engineered in an arbitrary direction. Anisotropic structures in such materials can be engineered to any direction by simply changing the temperature gradient of the surrounding polymer solutions. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Ordered pore Foam Polyurethane Ice templating Directional melt crystallization Q2 Indentation
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Cellular materials from nature, such as porcupine quills, bird bones, and toucan beaks, have been adapted for use as high strength, lightweight materials with insulating properties [1–5]. Various foaming agents produce artificial cellular plastics, which have been used as the most common materials for protection and insulation [6]. The modern electrical industry also adapts the same methodology to protect integrated elements from external shock or deformation. Thin porous films are often inserted in between electrical elements that require excellent mechanical performance and thin film thickness. Unfortunately, common foaming agents for cellular plastics are not quite appropriate for this application, as they produce relatively large pores compared to the required film thickness, preferably less than 50 mm [7]. Freeze drying has been used as a common drying method for water soluble compounds and ceramic slurries for many decades [8–10]. Recently, this drying method was combined with the directional freezing (crystallization) technique, and as a result, biomimetic ordered cellular structures have been reported [11– 13]. This technique has provided 3D control of pores and walls in graphene, composites, and polymeric materials [14–16]. If organic solvents are used that are crystallizable, this technique can be applied to most polymers [17]. The crystallization of solvents can be engineered to produce well-ordered biomimetic porous structures by carefully controlling the temperature gradients
* Corresponding author. Tel.: +82 28205269. E-mail address: [email protected] (J. Lee).
[18]. Through-thickness porous membranes have been developed from various polymers, which could be used as ideal thin films for protection [17,19]. However, the mechanical properties of thin polymer films having these ordered biomimetic pores have rarely been investigated. This study explores the indentation properties of ordered porous thin films, which are critical for the protection and shockabsorbing performance, for the first time. Tensile or bending tests cannot reliably produce the basic mechanical parameters for protective thin film applications, as their stress conditions are different. Furthermore, compression tests are not quite feasible due to their thin nature. Thus, a micro-indentation test is a reliable solution to probe the basic mechanical properties useful for protection performances [20,21]. Because of the micro-porous nature, the size of the indenter should be sufficiently large compared to the pore size, and therefore nano-indentation is not adequate. Because of the high porosity and the micron size of the pores of these foams, indentation requires a relatively large indenter and accurate measurement of a small indentation force. These quite unusual indentation conditions required us to build our own micro-indentation system. We performed micro-indentation tests on these materials using a relatively large 2-mm spherical indenter and a sensitive load measurement system (readability 9.8 mN). Polyurethane (PU) was selected as a typical material for thin and light foams [22,23]. Ordered pores were first produced by directional crystallization of a solvent and subsequent solvent removal by sublimation. The PU solution was introduced to the top of a liquid nitrogen reservoir
http://dx.doi.org/10.1016/j.jiec.2015.10.023 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
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S. An et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Fig. 1. Preparation of porous PU films by directional crystallization of a solvent.
59 60 61 62 63 64 65 66 67 68 69 70 71 72
at a speed of 200 mm/s (Fig. 1) [24]. As a result, crystal nucleation started from the region closest to the liquid nitrogen reservoir and propagated into the rest of the sample. The direction of the crystal growth lay between the direction of the sample movement and the tangential direction to the liquid nitrogen reservoir, resulting in a slanted direction. A temperature gradient in the polymer solution also developed in the same direction. Therefore, crystallization propagated along the slanted direction, from the bottom to the top, and from the one side close to the reservoir to the other side. As we previously reported, this condition greatly reduced the strain mismatch from cryostress and desiccation stress [24]. As a result, defect-free thin porous films could be prepared. The crystallization of solvent molecules by cooling along a temperature gradient separates a PU solution into crystal phases
and cryoconcentrated phases [25,26]. Since the solubility of polymer chains in the crystal phases of the solvent is extremely poor, the majority of polymer chains are expelled into the cryoconcentrate phases [18]. The crystal phases become pores after sublimation. Therefore, the content of the solvent is approximately the porosity of the final materials, which is 0.9 (0.89, if the densities of PU and dioxane are considered to be 0.9 and 1.033 g/cm3) [19]. The crystal phases of the solvent should be highly connected due to the low possibility of homogeneous nucleation, compared to that of heterogeneous nucleation, common in crystallization [27]. Thus, connected open pores generally form [18]. The crosssectional images of Fig. 2 show the ordered structures of pores and their connectivity. The major line structures are slanted, which
Fig. 2. SEM images of cross-sections (left) and surfaces (right) of porous PU films with different hard segment contents: (a) 50%, (b) 37%, and (c) 25% (scale bar = 100 mm).
Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
73 74 75 76 77 78 79 80 81 82 83 84 85 86
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JIEC 2692 1–4 S. An et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
0.20
Load (N)
stiffness of porous films, wherein increases in the hard segment content result in greater indentation hardness and moduli. Although these values are obtained from the initial linear regimes, they can represent the general stiffness trend of materials beyond the linear regime, as shown in the tangential slopes of Fig. 3. The values are not small, and considering the porosity of the materials, they exhibit outstanding stiffness. The dependence of the modulus on porosity has been investigated and various empirical equations have been established [1,29]. A popular equation proposed by Phani and Niyogi is
Hard segment 50% Hard segment 37% Hard segment 25%
0.15
0.10
0.05
0.00 0.00
123 124 125 126 127 128 129 130 131 132 133
p f E ¼ E0 1 ; pc 0.01
0.02
0.03
0.04
0.05
0.06
Displacement (mm) Fig. 3. Typical load versus displacement curves of porous PU films with different hard segment contents.
87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122
3
reflects the slanted temperature gradients and related crystal growth direction as discussed above. Although one concentration was used for all samples in Fig. 2, the pore sizes became significantly different. Generally, a smaller pore size resulted with more hard segments. Various influences need to be considered to explain this result, such as the viscosity, diffusion coefficient, solvent affinity, and crystallization behavior of the hard segments, but the chain conformation of PU in 1,4dioxane might be a cause. An increase in the hard segment will increase the radius of the gyration of PU chains, which slows their diffusion rate. When they are expelled by solvent crystal fronts during directional crystallization, their slow diffusion characteristics restricts the crystal growth more and creates smaller crystals. An increase in the content of hard segments also leads to an increase in viscosity [28]. The high viscosity sustains crystal growth, since the moving of crystal growth front has difficulty in expelling viscous cryoconcentrate phases. As a result, crystal growth front becomes more branched (pinned), and smaller crystals (thinner spacing between pore walls) results. Upon uniaxial compression, cellular materials first undergo an initial linear region (elastic cell wall deformation) [1]. Due to the buckling and fracture of the cell walls, the materials then pass through a yield point and reach a plateau region after sufficient densification. The indentation results show a slightly different series of deformation (Fig. 3). The initial linear elastic regime is followed by a slightly curved plastic regime. Yield points are difficult to define, and plastic deformation is dominant with a distinct hysteresis between loading and unloading curves. With an increase in the hard segment content, the hysteresis of the energy becomes more significant. Therefore, relatively brittle microdeformations, such as wall buckling, might be the main reason for the hysteresis. The increased hysteresis of PU with an increase in the hard segment content indicates significant energy dissipation, which could be related to the excellent shock adsorbing properties. The averages of the indentation hardnesses and moduli are shown in Table 1. The hard segments distinctly contribute to the
where E is the Young’s modulus of a porous material with porosity p, E0 is the modulus of the solid material, pc is the porosity at which the effective modulus becomes zero, and f is a parameter dependent on the grain morphology and pore geometry [1,29]. Often, one of these assumptions are used: pc = 1 or f = 1. With a series of porous films from different solution concentrations of 5, 10, and 15 wt%, the pc obtained was near unity at 0.964. From the typical percolation theory based on common near spherical or elliptical pore morphology, f was calculated to be 2.1. Table 1 shows the results of the E calculation with f = 1 or 2.1. The measured values are all much larger than the calculated values of E, as much as one or two orders of magnitude larger. The other simple equations, such as Gibson and Ashby, produce the same conclusion, regardless of their adjusted parameters [1,29]. This discrepancy, huge increases in modulus, seems to result from the assumption of either the spherical or elliptical pore morphology. Indeed, the pore morphology of our films with ordered open pores with high connectivity is quite different from that of ordinary foams. The aligned and ordered pores prepared in PU films distinctly improved the hardness and modulus of porous films. Furthermore, the directional crystallization process could align polymer chains in the cryoconcentrate region, resulting in strengthening along the pore direction. The chain alignment effect by directional crystallization was previously reported to a certain degree [14]. The dramatically improved hardness and modulus of ordered porous films prepared by directional crystallization could be useful for various applications of light-weight materials, which to the best of our knowledge, has not been reported. The directional crystallization process of porous materials seems to have the unique advantage of strengthening along the pore direction. This processing technique provides freedom in the design of porous materials, as the pore direction can be engineered in an arbitrary direction. Anisotropic structures in such materials can be engineered to any direction by simply changing the temperature gradient of the surrounding polymer solutions. Specifically, strengthening perpendicular to the film surfaces is possible, which has been quite difficult to achieve because the direction is typically perpendicular to the processing (flow) direction.
134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172
Experimental
173
Songstomer P-7195A, P-7185A, and P-3170A (polyester-based thermoplastic PU, hard segment contents = 50%, 37%, and 25%,
174 175
Table 1 Indentation modulus and hardness of porous PU films with different hard segment contents. Hard segment (%)
Hardness (MPa)
Indentation modulus (MPa)
Bulk indentation modulus (MPa) (p = 0)
E (MPa) calculated with f = 1
E (MPa) calculated with f = 2.1
50 37 25
0.65 0.11 0.50 0.29 0.16 0.02
8.26 0.45 3.96 0.44 2.32 0.24
22.65 0.46 16.14 0.60 10.40 0.59
1.50 1.07 0.69
0.076 0.054 0.035
Please cite this article in press as: S. An, et al., J. Ind. Eng. Chem. (2015), http://dx.doi.org/10.1016/j.jiec.2015.10.023
G Model
JIEC 2692 1–4 4
S. An et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
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respectively) polyurethanes (PUs) from Songwon Industry (Ulsan, Republic of Korea) were used. Sulfuric acid (Duksan, Ansan, Republic of Korea), hydrogen peroxide (Duksan, Ansan, Republic of Korea), 1,4-dioxane (Wako Pure Chem., Osaka, Japan), and silicon wafers with a 4-inch diameter (Sehyoung Wafertec, Seoul, Republic of Korea) were used. Silicon wafers were cleaned in a Piranha solution (sulfuric acid:hydrogen peroxide = 1:1) for 2 h, followed by washing with double distilled water. A homogeneous PU/1,4-dioxane solution (10 wt%) was prepared at 60 8C, and cast on a silicon wafer at 25 8C into a 100-mm thick film by a doctor blade (HaeChang, Seoul, Republic of Korea). The cast solution was directionally crystallized by a liquid nitrogen reservoir, which used melt crystallization equipment that we made and previously reported in detail [19]. Briefly, the solution was moved at 200 mm/s over an 1156 mL liquid nitrogen reservoir (17 cm 17 cm 4 cm). The distance between the bottom surface of the wafer substrate and the top surface of the reservoir was 4 cm. The crystallized solvent was removed by a freeze dryer (Eyela FDU-2200, Tokyo, Japan) for 24 h to produce the ordered porous PU. A scanning electron microscope (S-3400N, Hitachi, city, Japan) was used for morphological investigations (5 kV) and samples were coated with Pt by a coater (E-1010, Hitachi, Tokyo, Japan) at 15 kV for 120 s. Cross-sections were prepared by cryofracturing in liquid nitrogen. An aluminum spherical indenter with a 2-mm diameter was indented into the sample surfaces at a displacement speed of 1.667 mm/s, which was driven by a stage motor (VT-80, PI MiCos, Freiburger, Germany) and the force was measured using a microbalance (FX-200i, A&D company, Tokyo, Japan). Forcedisplacement data of at least five samples were converted into indentation hardness and modulus using the Oliver–Pharr method [30].
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Acknowledgements
208 Q3 This work was supported by a National Research Foundation 209 grant funded by the Korean Ministry of Science, ICT and Future
Planning (NRF-2013R1A1A2021573 & Engineering Research Center of Excellence Program No. 2014R1A5A1009799). SA thanks the Chung-Ang University Research Scholarship Grants in 2014.
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References
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