Mimicking Neuromuscular Junctions Using Controlled Crystallization

Article pubs.acs.org/crystal

Mimicking Neuromuscular Junctions Using Controlled Crystallization of Solvents: A Surface and Interface Engineering Technique for Polymers Published as part of a Crystal Growth and Design virtual special issue of selected papers presented at the 12th International Workshop on the Crystal Growth of Organic Materials (CGOM12) Leeds, UK Hyun Jin Kim, Suyeong An, and Jonghwi Lee* Department of Chemical Engineering and Materials Science, Chung-Ang University, 221, Heukseok-dong, Dongjak-gu, Seoul, 156-756, Republic of Korea S Supporting Information *

ABSTRACT: Recently, crystallization engineering has become a novel processing technique for various materials, including ceramics, polymers, and composites. Herein, a novel processing technology of polymers based on controlled directional crystallization was developed for biomimetic surfaces and interfaces. Solvent was allowed to come into contact with a polymer surface for a limited time, followed by controlled crystallization of the solvent along a temperature gradient perpendicular to the surface. As a result, perpendicular pores of well-defined patterns were successfully prepared, as well as adhesive-free strong interfaces mimicking neuromuscular junctions. By increasing the temperature of the polymer or solvent contact time, the pore depth and contact angle increased. Highly hydrophobic surfaces of polycarbonate were efficiently prepared, and interfacial adhesion with polydimethylsiloxane was improved by more than 4-fold. This novel processing technique based on crystal engineering could open completely new application possibilities, particularly for biomedical devices, soft lithography, microfabrication, soft sensors, and flexible and stretchable electronics.

1. INTRODUCTION

cell membrane of a muscle cell); this is referred to as a neuromuscular junction (NJ). 7 The sarcolemma has a postjunctional folds structure, which increases the surface area of the membrane to facilitate the transport of acetylcholine molecules and prevent any junction failure, and the presynaptic axons have a structure that protrudes from the postjunctional folds, which are called terminal boutons (or presynaptic terminals).7 The folds have wide longitudinal crests and narrow lower interfold parts.8 An important reason why engineering the interface structure has seldom been the major approach to improve adhesion is the absence of a convenient preparation method. For example, the regular structure of NJ has never been utilized in the interfaces of artificial materials. In this study, a novel, simple, and efficient engineering tool for the interface structures, which mimic the NJ structures and maximize microdeformations, has been developed based on a directional crystallization technique. Directional crystallization has first been adapted as a processing technique for ceramic materials and their

The properties of surfaces and interfaces have been critical to materials research and development.1 Nature presents numerous examples of elegantly architectured surfaces and interfaces, which determine the adhesion strength between two different materials.2−4 Adhesion is generally proportional to the surface free energy, but other parameters such as surface roughness, slippage, contaminants, and viscoelastic properties matter as well.5 The majority of the adhesion energy measured is usually dissipated into heat, and only a small portion of energy is used for breaking intermolecular forces.5 Therefore, the viscoelastic properties of materials are critical, and particularly, surface microdeformations have been reported as the major phenomenon that absorbs significant energy.5,6 Engineering interface structures to maximize microdeformations is the best choice for improving adhesion, but for a long time, this has been overwhelmed by the engineering strategy of intermolecular bonding and interactions. A million years of evolution have enabled animals and plants to use optimized interface structures for proper adhesion between different tissues and cells. For example, when neurons are in contact with muscle cells, an ideal junction structure has been developed between the motor axons and sarcolemma (the © XXXX American Chemical Society

Received: January 25, 2017 Revised: March 18, 2017 Published: March 30, 2017 A

DOI: 10.1021/acs.cgd.7b00130 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Fabrication of porous surfaces by directional melt crystallization of solvent after the precooling step (a, precooling method) and the predissolving step of PC (b, predissolving method). The nucleation and directional growth of solvent crystals determine pore morphology (c).

composites.9−13 Outstanding toughness has been reported in nacre-mimetic ceramic materials and composites prepared by directional water crystallization of ceramic slurries, which is called ice-templating.11 Later, the same technique was utilized to prepare polymers and their composites, where water or organic solvents were directionally crystallized.14,15 After crystals were removed, lamellar or cylindrical pores were

obtained, and unique porous materials such as foams and membranes could be prepared efficiently. We hypothesized that if this technique is combined with confined crystallization on polymer surfaces,16 lamellar or cylindrical porous surfaces can be prepared and used for engineering interface enhancing microdeformations. As a proof of concept, we chose polycarbonate (PC) as a substrate B



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Figure 2. Temperature profiles measured on the surface of PC plates during cooling by a liquid nitrogen reservoir: (a) D25, (b) D80, (c) D450, (d) D1000. Polymer surfaces start to contact with solvent at point i, followed by dissolving of surfaces and crystallization of solvent, which stop at point iii. DaeJung Chemicals, Gyeonggi-do, Republic of Korea) for 15 min, following a procedure from previous publications.22,23 Silicone plates (5.1 mm thick, Daihan Chemicals, Seoul, Republic of Korea) were used for the preparation of molds. 2.2. Controlled Crystallization for the Preparation of Porous Surfaces. A PC plate with a silicone mold (10 × 60 × 5 mm3) was placed on top of a liquid nitrogen reservoir, and a temperature wire probe (Center 306 datalogger thermometer, Center, Taiwan) was attached to the top surface of the PC plate (Figure 1a, Precooling Method). The temperature was monitored at every 1 s by a computer (Center, Taiwan). After the PC plate was precooled to 5 or 15 °C, 1.5 mL of 1,4-dioxane was poured into the mold on the top surface of the plate. 1,4-Dioxane became cooling-crystallized by the unidirectional temperature gradient imposed by liquid nitrogen. The average cooling rate was −14 °C/min when measured without using 1,4-dioxane. After the crystallization finished, the PC plate was immersed in 2 L of isopropyl alcohol under stirring at 100 rpm and −17 °C for 24 h to remove 1,4-dioxane and dried in a convection oven for more than 3 h. A set of samples was prepared with longer solvent contact times. 1,4-Dioxane (1.5 mL) was poured onto the surface of the PC plate at 24−25 °C, and the contact between PC and 1,4-dioxane remained for 5 or 15 min (Figure 1b, Predissolving Method). Then, the PC plate with 1,4-dioxane was transferred to the top surface of the same liquid nitrogen reservoir for controlled directional crystallization, followed by the same procedure as described above. As a control, the PC plate was polished by sandpaper (No. 1000, 3M, St. Paul, MN, USA) for 10 s by hand. After sequential washing by 100 mL of distilled water and 100 mL of isopropyl alcohol, the sample was dried in a convection oven for more than 3 h (S-PC). 2.3. Characterization. A scanning electron microscope (S-3400N, Hitachi, Tokyo, Japan) was used for the investigation of the internal material structures at 5 kV or 15 kV. Samples were coated with Pt using a coater (Hitachi, Tokyo, Japan) at 15 kV for 120 s. Cryofractured cross sections were prepared by fracturing a sample in liquid nitrogen after immersion in liquid nitrogen (500 mL) for more

polymer for surface engineering and investigated its adhesion with polydimethylsiloxane (PDMS). The surface energies of the two polymers are quite different (the surface energies of PDMS and PC = 19.8 and 34.2 mN/m, respectively, at 20 °C).17 Since the interfacial adhesion becomes poor as the difference between the surface energies of polymers becomes larger, the suitable adhesion between PC and PDMS is extremely tough to achieve.18 Due to the poor miscibility, chain interpenetration and entanglement between the two polymers are difficult. Recently, PDMS has been widely investigated as a substrate for biomedical devices, soft lithography, microfabrication, soft sensors, and flexible and stretchable electronics, but none of the bonding techniques, such as thermal bonding, solvent bonding, adhesive bonding or lamination, have been satisfactory so far.19 The surface oxidation of PDMS has been the most popular method of adhesion improvement, but in many applications, poor interfacial adhesion has limited the pressure window and the durability of devices. Despite this disadvantage, PDMS has become more and more popular nowadays due to its useful properties, such as low cost, high transparency, chemical resistance, low modulus, low surface energy, and high strength.20 PC is also a popular transparent polymer with high toughness, low moisture absorption, and a high glass transition temperature.21

2. MATERIALS AND METHODS 2.1. Materials. PDMS (Dow Corning, Sylgard184 Silicone Elastomer Kit, Midland, MI, USA), dehydrated 1,4-dioxane (Wako Chemicals, Tokyo, Japan), and distilled water (Daeung, CP grade, Siheung, Republic of Korea) were used. PC plates of 550 μm thickness were separated from DVD disks (LG, DVD-R 16x (4.7 G), Seoul, Republic of Korea) and cleaned in isopropyl alcohol (100 mL, 99.5%, C



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Table 1. Designations of Samples with the Average Area, Depth, and Volume of Pores Measured from SEM Micrographs designations

solvent contact time before crystallization (s), Preparation method

precooling temperature or predissolving time

fraction of pore area over surface area

D25 D80 D450 D1000

25, precooling 80, precooling 450, predissolving 1000, predissolving

5 °C 15 °C 5 min 15 min

0.58 0.54 0.59 0.52

than 30 min. Energy-dispersive X-ray spectroscopy (EDS, EX-250, Horiba Energy, Kyoto, Japan) was performed on the cross sections of the interface area at 20 kV. The porous structures were analyzed by microcomputed tomography (Micro CT, SKYSCAN1172, Bruker microCT, Belgium, resolution 1337 × 2000, 30 kV), and the results were converted into 3D images by NReconServer, CTVox, and CTAn (Bruker microCT, Belgium). X-ray diffraction was analyzed using an X-ray diffractometer (D/ MAX-2500/PC, Rigaku, Tokyo, Japan) at 40 kV and 100 mA. For glazing incidence X-ray diffraction (GI-XRD), a 4-cycle diffractometer (SMARTLAB, Rigaku, Tokyo, Japan) was used with a 1.5° incident angle, 0.06° sampling width, and 1°/min scan rate. Contact angles were analyzed using a contact angle analyzer (Phoenix 450, Surface Electro Optics, Gyunggido, Republic of Korea). A total of 10.7 μL of water was dropped on the surfaces and five repeated tests were averaged. Peel tests were performed using sandwich samples of PC-PDMSPC.24 The base and curing agent (10:1 (w:w)) of PDMS were first mixed for 10 min, and the homogeneous mixture was degassed in a vacuum oven for 5 min at room temperature. The mixture was poured onto the surfaces of PC and covered by another PC plate with a porous surface (20 × 70 × 1 mm3). As a result, both porous surfaces faced the PDMS mixture. Curing followed at 30 °C for 24 h. The peel test employed an Instron (model 3344, Canton, MA, USA) at 1 mm/ min. An adhesion test was first tried using asymmetric double cantilever beams.25 We inserted a razor blade into the interface to measure the fracture propagation. However, the unexpectedly strong interface prevented any visible fracture propagation for 24 h.

pore depth (μm) 16 55 245 418

± ± ± ±

2 3 5 7

pore volume (μm3) 91200 297000 1445500 2173600

± ± ± ±

11400 16200 29500 36400

nitrogen. At 25 and 80 s in Figure 2, panels a and b, respectively, another discontinuity is observed (point (ii); the temperatures at these points are 10 and 5 °C, respectively, which correspond to the initiation of 1,4-dioxane crystallization (Tm = 11.8 °C)). The slope of the temperature profiles becomes lower after the second discontinuity point because of the release of the heat of fusion. When the crystallization is complete, a third discontinuity (point (iii) appears at 150 and 220 s as seen in Figure 2, panels a and b, respectively. The crystallization processes that correspond to the discontinuity points were visually observable. Even after the third discontinuity point, the crystallization or vitrification of the cryo-concentrated phase might occur, but the heat from this process is relatively low compared to that of the solvent bulk crystallization. Furthermore, vitrification generates relatively smooth transition behavior of a pseudo-second order. Thus, it was not detected in our temperature measurement, but still small secondary pores from the crystallization or vitrification of cryo-the concentrated phase could be found in the pore walls. Since the predissolving method does not have the discontinuity related with the pouring step of 1,4-dioxane during cooling, it produces only two discontinuities in the temperature profiles of cooling (Figure 2c,d), which correspond to the starting and finishing points of crystallization. The competition between liquid nitrogen cooling and latent heat release determines the temperature profiles. Since the amount of liquid nitrogen in the reservoir decreases with time, the general temperature profiles show a decrease in slope with time. The dissolution of the PC plates starts upon contact with 1,4-dioxane and stops upon crystallization. In Figure 2c,d, the predissolving steps of 5 and 15 min precede before the temperature measurement. Thus, the solvent contact times of Figure 2a−d before crystallization are systematically varied and have values of 25, 80, 450, and 1000 s, respectively. Therefore, solvent contact times are used for the designations of samples, i.e., D25, D80, D450, and D1000 (Table 1). In Figure 2, the nucleation of solvent crystals starts at 5−10 °C, crystallization finishes at −1 °C to −5 °C, and the duration time of the crystallization sections, which are the sections between the points (ii) and (iii), was 130−140 s. There are only small variations in these values among the four temperature profiles of Figure 2. This is because the volume of solvent on top of the PC plates was controlled to be the same (1.5 mL), although the dissolution depth (the thickness of the solution layer) varies. 3.2. Structures of Porous Surfaces. The pores obtained by the removal of crystals are the replicas of solvent crystals produced by the systematically designed preparation conditions, which produced porous surfaces of different morphologies (Figures 3 and 4). Thus, the morphology of pores reflects the habit of crystals. The nucleation of solvent crystals starts from the bottom of the solution layer and grows to the top surface following the major temperature gradient. Therefore, pores are generally aligned perpendicular to the plate. The crystallization of solvent progresses with expelling

3. RESULTS AND DISCUSSION 3.1. Directional Melt Crystallization. The contact between the PC plate and 1,4-dioxane under directional cooling induced limited dissolution of PC, followed by directional melt crystallization of 1,4-dioxane (Figure 1). In the precooling method shown in Figure 1a, a PC plate was precooled before contact. Solvent crystals nucleate at the region closest to the liquid nitrogen reservoir (the bottom region of the dissolved PC solution layer) and grow to the top surface following the temperature gradient (Figure 1c). As the solvent crystals grow, dissolved PC molecules are expelled from the crystal phases, forming cryo-concentrate phases. The morphology of the cryo-concentrate phases determines the porous surface morphology after the crystals are removed. In the predissolving method shown in Figure 1b, a separate dissolving step was introduced before cooling to increase solvent contact time. In both methods, temperature decreases rapidly by the liquid nitrogen reservoir under the PC plate, which limits the dissolution of PC molecules. By using the two methods, we could control the contact time of PC with 1,4-dioxane before crystallization over a wide range. Figure 2a,b shows the temperature profile of the precooling methods from Figure 1a, which show three points of discontinuity. Right after pouring the 1,4-dioxane stored at room temperature, the temperature abruptly increases from the precooled temperature of 5 or 15 °C to 20 or 23 °C, respectively, in less than 10 s (point i in Figure 2) and then rapidly decreases by directional cooling imposed by liquid D



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Figure 3. SEM micrographs of treated surfaces: (a, b) D25; (c, d) D80; (e, f) D450; (g, h) D1000 [scale bar = 300 mm (left column), 30 mm (right column)].

dissolved macromolecular chains. The expelled macromolecules formed cryo-concentrated regions, resulting in pore walls. With an increase in the contact time from 25 to 1000 s, deeper dissolution occurs, resulting in deeper pores. With the limited dissolution of the case shown in Figure 3a (the shortest contact time), crystallization occurring within a relatively thin solution layer makes a short cylindrical, stump-like morphology. This seems to be unique, which can be achieved only by the precooling method, since a very short solvent contact time can be achieved. With longer solvent contact times, thicker solution layers with larger amounts of solute result. As a result, significant crystal growth could progress. The crystal morphology becomes the more lamellar type, and pores are more connected into lamellar shapes. In all three cases of D80, D450, and D1000, lamellar structures were found (Figure 3, panels d, f, and h, respectively). Both morphologies confirm the growth of crystals perpendicular to the surfaces, following the imposed temperature gradient. These unique perpendicular structures enable us to construct NJ−mimetic interfaces with other polymers such as PDMS in the future, which has the characteristics of wide longitudinal crests and narrow interfolds. In the low magnification images of Figure 3 (left), in-plane surface patterns exist regardless of contact times. The surface patterns result from a weak temperature gradient along the inplane directions due to the edge effect of the reservoir and

Figure 4. SEM micrographs (a−h) of cross sections of porous PC plates showing the boundary of treated and untreated surfaces (left) and magnified images of treated surfaces (right): (a, b) D25; (c, d) D80; (e, f) D450; (g, h) D1000. Micro-CT 3D image (i) of D1000 showing the porous structures of surfaces.

samples. Since the edges are exposed to the temperature of the environment, there must be minor temperature gradients that develop along the in-plane directions, although the major temperature gradient develops along the out-of-plane (thickness) direction. The depth of the porous layer is clearly identified in Figure 4, where the untreated (bare) surfaces of the PC plate are compared to the treated surfaces to identify the position of boundaries in the porous surface layers. Figure 4i shows the three-dimensional structure of a porous surface (Figure S1). Surface dissolution makes solvent molecules penetrate the plate, and dissolved macromolecular chains diffuse into the bulk E



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solvent phase. Thus, the top surface boundaries of the porous layers are higher than the level of the untreated surfaces, while the bottom boundaries of the porous layers are lower. The magnified images of Figure 4 show many pores smaller than 10 μm. These are much smaller pores than those in Figure 3 that came from the stump-like or lamellar structures of crystals. Thus, these are the secondary pores inside pore walls, which result from the removal of residual 1,4-dioxane in the cryoconcentrated regions. The small secondary pores are not aligned along the temperature gradient of the thickness direction, while the primary pores are well aligned, as can be seen in Figure 3. However, with longer solvent contact times, the crystals or vitrified phases seem to grow into more distinct phases, as can be found in D450 and D1000. The areas and depths of pores obtained from SEM images are given in Table 1. The pore area values were measured by separately identifying the pore and pore wall in surface images like Figure 3. While the area does not significantly change, the depth significantly increases with an increase in solvent contact time. This is because the solvent contact time was systematically changed here, while the temperature gradient and the related crystallization rate, R, were not significantly varied. The binary diffusion coefficient, D, which is related to concentration, could be a critical parameter in determining lamellae (column) spacing, L, a parameter of pore size. The theoretical proposal based on Fick’s law and material balance, whose qualitative relationship has been successfully proven in the previous directional freezing investigations, provides L = (8DΔT )0.5 R−0.5

Figure 5. Contact angles of porous surfaces (D25, D80, D450, and D1000) and controls (PC and S-PC) with typical images of water droplets.

or near super-hydrophobic surfaces. This method is also easily scalable and repeatedly applicable to any polymeric parts that are already processed. Thus, this simple method of dissolution and crystallization is able to conveniently prepare highly hydrophobic surfaces of polymers. The contact angle of Figure 5 generally depends on solvent contact time; the longer the contact time, the higher the contact angle. Therefore, it can be engineered by the precooling temperature or predissolution time. However, the correlation between contact angle and contact time or pore depth is not statistically linear. It seems to be the case that the two samples prepared from the precooling method (D25 and D80) show similar contact angles, and the other two samples (D450 and D1000) do as well. This might originate from the top surface morphologies developed during the different preparation procedures. The top surface morphologies of the pore walls are the most critical factor in determining contact angles according to the Cassie and Baxter mechanism.27 As we discussed above, enough crystal growth occurred in the cases of the predissolving method. As a reference, a rough surface was prepared by sandpaper polishing, S-PC (Figure S2). In SEM observations, its surface was not similar to the porous surfaces prepared by the dissolution and crystallization method developed here; it became distinctly rough by polishing, but no cylindrical or lamellar structures were identified. Only smeared line structures from polishing sand particles were evident, and the pores identified on the surface were rather irregular. There were indented microcracks generated along the line structures. On the cross section images, no distinct pore structures were visible, but significant step features existed on the surface area. Thus, sandpaper polishing produced plastic microdeformations, and less porous surfaces than the dissolution and crystallization method. The increase in roughness by polishing also increased the contact angle from 100° to 104°, which was a significant increase, but was much smaller than those obtained from our porous surfaces. The bare surface of PC showed no features in SEM observation and had a contact angle of 100°, which was consistent with previous publications.30−32 A unique result came from the surface GI-XRD analysis (Figure 6). The PC used here is isotropic amorphous, showing only an amorphous halo at 10−25°, which is typical in

(1)

where ΔT is the supercooling between lamellae. Generally, an increase in solvent contact time, td, will increase concentration and decrease D, resulting in smaller spacing, and this seems to be consistent with Figure 3. However, L was not easy to clearly define in Figure 3, mainly because of the complex structures of pore walls due to the concentration gradient. Additionally, complications should be considered in further analysis. For example, an increase in td does not linearly increase concentration since the solution layer becomes thicker and a concentration gradient develops in the layer accordingly, which are not considered in eq 1. According to our results, the depth of the porous layer, Dp, almost linearly follow td, according to the following equation, regardless of the fabrication methods: 26

Dp (μm) = 0.42td (s) + 16.23 (R2 = 0.9822)

(2)

The volumes of pores calculated from the area and depth data distinctly increase with an increase in contact time as a natural consequence (Table 1). Therefore, in this surface processing, the pore depth and volume can easily be engineered by solvent contact time, which was controlled by the precooling temperature or predissolution time. 3.3. Characteristics of Porous Surfaces. The water contact angle of porous surfaces was much higher than that of pure PC and S-PC (Figure 5), and the maximum contact angle obtained was 150° ± 4°. Water droplets existed in the Cassie and Baxter state, which depends on pore size and morphology.27 In this method, we did not use any high-cost lithographic techniques or delicate solvent etching techniques, as most previous reports have used to prepare lotus-leafmimetic surfaces.27−29 If a crystallizable solvent can be used, any polymer surfaces can be processed into super-hydrophobic F



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peel strength (N/m−1) = 2F /W

(3)

where W is the specimen width.37 From an adhesion strength of 7 N for the control PC sample having no mechanical locking structure, a dramatic increase to a maximum of 31 N is obtained in the cases of the NJ−mimetic samples, which is more than a 4-fold increase. This is surprising since it is rare to find a publication on more than even a 2-fold increase in adhesion strength by surface treatments.24,37,38 This achievement without using an adhesive can be combined with other chemical bonding strategies of interfacial strengthening for further improvement. The dynamic peel strength is the maximum when the solvent contact time is 450 s (D450). For D1000, it slightly decreases to 26 N, but the decrease is within the statistical error range. The dynamic peel strength seems to reach the maximum above a certain solvent contact time. For comparison, we prepared S-PC (Figure 7b), which is a typical sample with the mechanical locking mechanism. Its surface was rough, but cannot produce NJ−mimetic interfaces. As a result, its dynamic peel strength was only slightly higher than that of untreated PC. The mechanical locking mechanism does not always work effectively: if an interface forms between materials having poor intrinsic adhesion, such as PDMS and PC, the effect of mechanical locking using conventional rough surfaces becomes marginal. A relatively low dynamic peel strength similar to that of S-PC was observed in the case of D25 as well. Although the surface morphology of Figure 3a was porous, which is even rougher than that of S-PC, the pore depth was not enough to mimic the NJ structure. These comparisons proved that the interfacial strengthening mechanism we discovered here is far superior to the traditional mechanisms of mechanical locking. The dynamic peel strength can be analyzed as a function of pore depth (Figure 8). As pore depth increases, peel strength generally increases up to 3000 N/m, and when pore depth reaches 250 μm, the peel strength reaches its maximum state. Therefore, peel strength can be engineered by the precooling temperature or predissolution time. Enough energy dissipation via surface slippage and related microdeformations seems to be activated by the NJ−mimetic interface structures. The maximum peel strength achieved here is relatively high compared to reported values. With a steel surface, the adhesive fracture energy of PDMS was reported to be 1−100 J/m2, and its peel strength was only 1−100 N/m, depending on the crosslink density.39 Between PDMS films, peel strengths of 670−840 N/m were reported, which were relatively high values among the reported peel strengths.24 The peel fracture energy of 1−4 J/m2 was reported for PDMS with a silicon surface.40 Indeed, bonding PDMS to the surfaces of other polymers is not a trivial task, although it is widely required in numerous applications. Although novel breakthrough techniques have been researched for a long time, only a limited number of techniques of adhesion improvement are available, such as oxygen plasma coating41 and the use of sticky layers of low cross-linking or curing agents.42,43 However, even with these techniques, the bond strength has seldom been improved more than double.43 Our NJ−mimetic interface with a 4-fold increase could break the current limitations of adhesion improvement techniques. Figure 9 shows the cross sections of the peel test samples after failure. The pores shown in Figure 3 were filled with PDMS, resulting in NJ−mimetic interfaces similar to the

Figure 6. GI-XRD patterns of PC, D25, D80, D450, and D1000.

bisphenol A-based PC.33,34 Interestingly, the amorphous halo becomes sharper with an increase in solvent contact time. Furthermore, another new broad peak at 25° starts to develop by our surface treatment, and it becomes distinct in both D450 and D1000. It has been known that the peaks of the amorphous halo in PC are dependent on the stretching (drawing) process.34 Partially ordered domains in PC have been investigated by XRD, NMR, IR, neutron scattering, Raman, etc., which showed densely packed chains having orientational order: macromolecular chains prefer to have trans−trans conformations and locally parallel alignment.35 Similarly, the macromolecular chains dissolved in 1,4-dioxane can be partially aligned by mass and heat transfer during directional crystallization. This alignment could affect the surface properties of PC as well. In a previous publication on directional crystallization, similar chain alignment effects were reported.36 3.4. NJ−Mimetic Interfaces. By sandwiching the surfacetreated PC plates with PDMS, the porous surfaces became NJ− mimetic interfaces between PC and PDMS, and the interfacial adhesion was measured using the typical peel test (Figure S3). This test method of mode I fracture was proper for confining the crack propagation.5 The relatively strong interfacial adhesion made the simple double cantilever beam tests impossible, since crack propagation could not be confined at the interfaces. Cracks easily propagated into the PDMS phases, and this cohesive failure resulted in the premature failure of PDMS. In the peel tests of the PC−PDMS−PC sandwich samples, crack propagation was completely confined between the two PC layers, and mostly cohesive failure in the PDMS phase or near the interfaces was found. PDMS has been used in many application areas, but its surface energy is extremely low to be adhesive to or miscible with other polymers. Its notoriously poor adhesion could be dramatically improved by the NJ−mimetic interfaces, as shown in Figure 7. First, all load versus displacement curves show stable crack propagation after the initiation of crack propagation. No stick−slip type instability is observed. The load smoothly increases to a steady state value without any discontinuity, which seems to be the characteristics of PC/ PDMS interfaces. The constant load at a steady state, F, represents the adhesion strength, since dynamic peel strength is directly related to F as follows: G



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Figure 7. Load vs displacement curves of (a) PC, (b) S-PC, (c) D25, (d) D80, (e) D450, and (f) D1000. The values in the graphs are the dynamic peel strength values (constant loads at steady state).

structure of interlocked fingers. After peel tests, the residual PDMS on PC surfaces appear red in the EDS mapping images. In the case of D25, a small amount of PDMS is found, indicating a lower degree of mechanical interlocking between PC and PDMS. Due to the much tighter interlocking structures of D450, a significant amount of PDMS material is left on the fracture surface of PC. Although the fracture appears to be adhesive in macroscopic observation, it is indeed cohesive in the microscopic observation of D450: the crack propagated inside the PDMS phase near the NJ−mimetic interface. In the D450 case, a fracture surface created from the crack propagation following the NJ−mimetic interfaces of PC and PDMS (true adhesive failure) was not found at all. In the case of D450, many voids are found at the bottom of the interphase region of the NJ−mimetic structure (Figure 9). The voids exist near the bulk PC phase, and no such voids are identified in the cross sections of the samples before peel tests (Figure S4). Therefore, the voids must form during fracture

from the strain mismatch between the NJ−mimetic interphase region and the bulk PC region. Significant microshear deformation will accompany the formation of voids. This is similar to the mechanism of cavitation and the accompanying microshear banding after triaxial stress relaxation,44,45 which has proved to be effective for a large amount of energy dissipation. Similarly, the formation of voids and drawing of interlocked structures will lead to significant energy dissipation. Indeed, energy dissipation through micro-deformations has been considered as the major contribution of adhesion in many cases.17 Figure 9 proves the biomimetic structure; the dotted lines indicate the NJ−mimetic interfaces between PC and PDMS, which was also observable before peel tests (Figure S4). The interface of D25 shows a relatively flat line, above which the PDMS phase of high Si concentration exists, and below which the PC phase of negligible Si concentration exists (Figure S4). As the solvent contact time increases, more interpenetrated H



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processing parameters. The contact angle of the surfaces could be increased significantly by increasing solvent contact time. The pore morphologies perpendicular to the surfaces became NJ−mimetic interfaces with PDMS, and peel strength could be increased by more than 4-fold. This increase was incomparable to what we can achieve by the conventional mechanical locking technique. Cracks propagated in the PDMS phases near the NJ−interfaces with significantly enhanced microdeformations such as voiding. This novel engineering technique based on crystallization could open whole new possibilities in developing future materials and devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00130. Micro-CT 3D images, SEM images, and peel tests results (PDF)

Figure 8. Dynamic peel strength vs pore depth showing the influence of pore structure on adhesion.

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area (the NJ−mimetic interphase) of PC and PDMS forms. Successful preparation of NJ−mimetic interfaces resulted in a dramatic improvement of PDMS adhesion, and the same mechanism of adhesion improvement can be applied to other polymers including natural polymers.46 For the further improvement, this mechanism can be combined with the conventional chemical methods such as surface plasma or dihydroxyphenyl function treatments.47

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jonghwi Lee: 0000-0003-2336-8695 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation grant from the Korea Ministry of Science, ICT and Future Planning (Engineering Research Center 2014R1A5A1009799 and 2016R1A2B4011247).

4. CONCLUSIONS A novel processing technique of polymers for engineering porous surfaces and NJ−mimetic interfaces was developed based on the directional crystallization of solvent on a surface after the limited dissolution of the surface. The crystallization was induced by a temperature gradient perpendicular to the surfaces, which resulted in unique regular porous patterns of perpendicular stump-like or lamellar structures depending on

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REFERENCES

(1) Creton, C.; Ciccotti, M. Fracture and adhesion of soft materials: a review. Rep. Prog. Phys. 2016, 79 (4), 046601.

Figure 9. SEM micrograph (left) and EDS mapping image (right) of cross sections of (top) D25 and (bottom) D450 samples after the peel tests. Adhesive failure occurred during the peel tests, and the cross sections were obtained by cryofracture [scale bar = 100 mm (upper) and 500 mm (lower)]. I



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