Patterning Dewetting and Self-Healing of Polymer Nanofilms on a

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Patterning Dewetting and Self-Healing of Polymer Nanofilms on a Brush Layer Yu-Hsuan Weng, Heng-Kwong Tsao, and Yu-Jane Sheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10889 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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The Journal of Physical Chemistry

Patterning Dewetting and Self-healing of Polymer Nanofilms on a Brush Layer

Yu-Hsuan Weng,a Heng-Kwong Tsao,b,c,* Yu-Jane Shenga,*

aDepartment

of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617

bDepartment

of Chemical and Materials Engineering, National Central University, Jhongli, Taiwan 32001

cDepartment

of Physics, National Central University, Jhongli, Taiwan 32001

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The Journal of Physical Chemistry 1 2 3 4 5 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 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 59 60

ABSTRACT: Patterning dewetting of a nanoscale polymer film on a brush layer is investigated by many-body dissipative particle dynamics. Four types of autophobic dewetting phenomena are clearly identified: (i) spinodal decomposition, (ii) nucleation and growth, (iii) stable hole, and (iv) metastable self-healing. The outcome depends on the film thickness, grafting density of polymer brush (ρg), brush length, and nucleus size. Two morphological phase diagrams of the dewetting mechanism are acquired for film on the short and long brush layers, respectively. On the short brush, the stable hole appears in the middle range of ρg for the film with the intermediate thickness. On the long brush, the stable hole prevails at low ρg as the film is imposed with a large nucleus. On the basis of the stable hole mechanism, the casted film can dewet on the brush layer and evolve to exhibit a pattern similar to the underlying brush.


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I. INTRODUCTION A polymer thin film on a solid surface has various applications but suffers film rupture (dewetting) during thermal and chemical processing. The suppression of dewetting and stabilization of ultrathin films are generally achieved by surface modification.1,2 The properties of a surface such as wettability and biocompatibility are often tailored by grafting polymers with reactive end groups onto the surface, referred to as polymer brushes. Compared to other methods of surface modification, polymer brushes possess better mechanical and chemical robustness.3-7 In addition, a variety of functional groups can be introduced for polymer brushes to have a high degree of synthetic versatility. The conformation of the surface-tethered polymer chain depends significantly on the grafting density.8,9 At low grafting densities, the grafted chain tends to adopt the conformation like mushroom or pancake. In contrast, at high grafting densities, the distances between neighboring grafting points are short, and thus chain stretching is resulted due to strong steric repulsion. Consequently, the grafted chains exhibit a brushtype conformation.10-12 The common strategy against spontaneous rupture and dewetting is to graft compatibility-enhancing chains on the substrate surface. The wettability of the substrate is modified so that ultrathin films can be stabilized by lowering the interfacial energy.13,14 However, dewetting of a liquid film can still be observed from a layer of identical molecules attached to the underlying substrate. For example, even on the chemically identical polymer brushes, the drop of polymer melt may exhibit partial wetting or the rupture of the film takes place spontaneously.15 For polystyrene (PS) droplets on the PS brushes, it was found that the contact angle (𝜃𝑌) and interfacial tension between melts and brushes increase with decreasing the chain length of PS 3 ACS Paragon Plus Environment


brushes. This decrease in the brush length leads to the increase in the grafting density and the decline in the brush thickness.16-18 Such a phenomenon of the increment in the contact angle of PS droplets on the PS brushes is referred to as ‘autophobic dewetting’ resulted from the balance between the two competing effects. The intermixing at interfaces between the film and brush of the same chemical identity is entropically favorable. On the contrary, the stretching of the brushes to accommodate penetrated polymer melt raises the elastic free energy, which is entropically unfavorable.19 For a polymer thin film on a smooth surface under the condition of partial wetting (finite contact angle), it is either unstable (dewetting) or metastable. The rupture mechanism of thin films is either spinodal decomposition or nucleation and growth.20,21 While the free energy barrier is absent in the former, it exists in the latter to hinder dewetting. Spinodal decomposition amplifies surface disturbances on the free surface due to thermal fluctuations.22,23 The mechanism of nucleation and growth has to start with nucleation sites (e.g., impurities or defects), resulting in the formation of dry patches. The holes grow continuously and they coalesce. Eventually, complete dewetting takes place and spherical droplets are formed.24,25 Dewetting of the polymer nanofilm occurs spontaneously if it is thin enough. On the contrary, if the film is thick, it can sustain thermal fluctuations, and capillary waves get damped. As a dry patch is enforced deliberately in such a metastable film, this nucleation site keeps on growing. Nonetheless, if the initial size of the dry patch is small enough, it will shrink and be filled with the polymer eventually. In other words, the ruptured film can self-heal to develop a metastable layer when the nucleation site is not large enough. The fabrication of micro- and nano-pattern in polymer thin films has been achieved by morphology evolution of the dewetting process on heterogeneous substrates.26,27 On homogeneous surfaces, randomly distributed holes are formed upon 4 ACS Paragon Plus Environment

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dewetting. The hole expands with annealing time, and the underlying material within the hole is exposed. By cooling the substrate below the glass transition temperature of the polymer, dewetting can be arrested and holes with tuneable diameters can be acquired.28 For example, patterned substrates were produced from dewetting of bilayers of two immiscible polymers, polystyrene (attracting proteins) and poly(methyl methacrylate) (lower affinity for proteins).29 Thermally annealing the system leads to spontaneous dewetting of the latter. The distribution of the islands has to be tuned by varying a few parameters, including film thickness, annealing time, and molecular weight of the employed polymers. To control the spatial layout of polymer structures accurately, deliberately tailored heterogeneous surfaces have to be used by conﬁning polymer to a defined geometry. The modern experimental techniques such as microcontact printing and photolithography allow us to create structured substrates.30-32 The dewetting dynamics of a polymer nanofilm on a smooth substrate has been investigated by many-body dissipative particle dynamics (MDPD) approach. The simulation results are consistent with the experimental observations.33-35 In this work, on the basis of the same method, the instability mechanism and dewetting phenomena of a nanoscale polymer film on a brush layer grafting on smooth, partial wetting substrate were explored. The dewetting mechanisms were identified based on the evolution of the polymer nanofilm morphology on the polymer brush layer. The influences of the brush length, grafting density of brush, polymer film thickness, and nucleus size were investigated. Two morphological phase diagrams on the short and long brush layers were acquired. The interesting phenomena of stable hole and selfhealing were observed, and used to develop a patterned polymer film on the patterned brush layer, referred to as patterning dewetting.



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II. SIMULATION METHODS MDPD is a modified version of the dissipative particle dynamics (DPD) which is a coarse-grained particle-based approach. Because the momentum conservation is obeyed, MDPD has been employed to study the hydrodynamics and contact line movement. Moreover, MDPD can be used to explore vapor-liquid systems in comparison with DPD.36-39 Three interparticle forces in the DPD scheme are still adopted. The random force and dissipation force are the same as those in DPD. Nonetheless, the conservative force (𝐹𝐶𝑖𝑗) includes attraction, in addition to repulsion. Both of them are soft and linear within the interparticle distance (𝑟𝑖𝑗), 𝐹𝐶𝑖𝑗 = 𝑎𝑖𝑗(1 ― 𝑟𝑖𝑗/𝑟𝑎)𝐞𝑖𝑗 + 𝑏𝑖𝑗(𝜌𝑖 + 𝜌𝑗)(1 ― 𝑟𝑖𝑗/𝑟𝑏)𝐞𝑖𝑗,

(1)

where 𝐞𝑖𝑗 = 𝒓𝑖𝑗/𝑟𝑖𝑗 denotes a unit vector along the force direction. The forces decrease with 𝑟𝑖𝑗 and become 0 as 𝑟𝑖𝑗 ≥ 𝑟𝑎 for attraction and as 𝑟𝑖𝑗 ≥ 𝑟𝑏 for repulsion. The cut-off distances are choosen as 𝑟𝑎 = 1.00 and 𝑟𝑏 = 0.75 to assure a stable liquid– vapor interface.40-43 The strengths of attraction and repulsion are described by the interaction parameters 𝑎𝑖𝑗 < 0 and 𝑏𝑖𝑗 > 0, respectively. Note that the many-body force law with 𝑏𝑖𝑗 in eq (1) is non-integrable unless 𝑏𝑖𝑗 is constant. 44 The magnitude of repulsion depends on the average local density 𝜌𝑖 at the position of the bead 𝑖, which is calculated by 𝜌𝑖 = ∑

15

𝑟𝑖𝑘 2

(1 ― ) , if 𝑟

k ≠ 𝑖 2𝜋𝑟3𝑏

𝑖𝑘

𝑟𝑏

< 𝑟𝑏; 0, if 𝑟𝑖𝑘 > 𝑟𝑏.

(2)

Note that long-range van der Waals forces which would not be captured in typical MDPD models may play a role in film dewetting.45

For a polymer comprising N beads, the two adjacent beads are joined by the spring force depicted by the finitely extensible nonlinear elastic (FENE) model, 6 ACS Paragon Plus Environment

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= 𝐹𝐹𝐸𝑁𝐸 𝑖𝑗

C(𝑟𝑖𝑗 ― 𝑟𝑒𝑞)

,

1 ― [(𝑟𝑖𝑗 ― 𝑟𝑒𝑞)/(𝑟𝑚𝑎𝑥 ― 𝑟𝑒𝑞)]2

(3)

where the spring constant is set as C = 100, the equilibrium bond length 𝑟𝑒𝑞 = 0.7, and the maximum bond length 𝑟𝑚𝑎𝑥 = 1.1.46-49 For simplicity, the length of polymers containing N beads is (N ― 1)𝑟𝑒𝑞 + 𝑟𝑎, designated as NL. In this work, the length of polymers to form a thin film is NL = 20. For short and long brush layers, the lengths of grafted polymers are NB = 3 and NB = 10, respectively. In our simulations, all the quantities are scaled by the cut-off distance of attraction 𝑟𝑎, bead mass 𝑚, and thermal energy 𝑘𝐵𝑇. Therefore, time is nondimensionalized by (𝑚𝑟2𝑎/𝑘B𝑇)

1/2

. The simulation system to obtain the phase diagram

is a rectangular box (𝐿𝑥, 𝐿𝑦, 𝐿𝑧) = (90, 90, 60). The number densities of the polymer beads and solid beads in the substrate are 6 and 8, respectively. At least 2 × 105 steps (𝑡 = 0.01) are run for each simulation. The repulsive parameter between any two beads is always set as 𝑏𝑖𝑗 = 25. The attractive parameters 𝑎𝑖𝑗 for the polymer film (𝑝) and brush layer (𝑏) beads are 𝑎𝑝𝑝 = 𝑎𝑏𝑏 = ―40.40 If polymer and brush have the same chemical composition, one has 𝑎𝑝𝑏= ―40; otherwise, 𝑎𝑝𝑏 > ―40. The attractive parameter between polymer and solid beads (𝑎𝑝𝑠 = 𝑎𝑏𝑠) is choosen as ― 35 to exhibit the equilibrium contact angle 𝜃𝑌  73°.33 Initially, a stable nanofilm is developed by imposing the total wetting condition (𝑎𝑝𝑠 = 𝑎𝑏𝑠 = ―50). At 𝑡 = 0, 𝑎𝑝𝑠 and 𝑎𝑏𝑠 are switched to ― 35, and the film starts to evolve toward a metastable film or spinodal decomposition. A dry hole with radius 𝑅0 may be imposed on the metastable film to create a nucleus site.



III. RESULTS AND DISCUSSION Consider a polymeric liquid (NL = 20) which is partially wetting on a smooth surface. Because the gravity effect is negligible for nanoscale phenomena, the equilibrium shape of this polymeric liquid corresponding to minimum surface energy is a spherical cap with an equilibrium contact angle (𝜃𝑌  73°). The liquid drop may be scraped into a metastable film on the substrate when the border of the film is pinned intentionally. In our simulations, the pinning effect is accomplished by either pinning edges (e.g., surface roughness) or periodic boundary condition. When the film thickness is small enough, it becomes unstable and ruptures. To enhance surface wettability, grafting polymers on the surface to form a brush layer is employed. Two lengths of brush polymers are considered, NB = 3 and 10. Figure 1 shows the equilibrium morphology of the brush layer on a smooth surface at various grafting densities (g) defined as the number of brush polymers per unit area. The quantitative characterization of the brush layer is given in Figure S1 of the supporting information, including the brush density profiles, mean height, and root mean square height. When the brush polymer length is short (NB = 3), the surface is fully covered with polymer only as g  0.33. In contrast, when the brush polymer length is long (NB = 10), the full coverage can be achieved at g = 0.20. The thickness of the brush layer is 0.9 for NB = 3 at g = 0.33 and 1.58 for NB = 10 at g = 0.20. Note that the bicontinuous structure appears at low grafting density g = 0.1.


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Figure 1. The top view of the brush layer formed by polymers with (a) NB = 3 and (b) NB = 10 at various grafting densities (g).

A.

Polymer Film On A Brush Layer.

Consider a polymeric liquid film

deposited on a brush layer grafted on a smooth substrate. Note that polymer and brush have the same chemical composition (𝑎𝑝𝑏 = 𝑎𝑝𝑝 = 𝑎𝑏𝑏 = ―40). The initial film thickness is characterized by ℎ0 corresponding to the equilibrium thickness of this polymer film on a smooth, total wetting substrate. For a smooth, partial wetting surface with 𝜃𝑌  73° (𝑎𝑝𝑠 = ―35), spinodal decomposition occurs as ℎ0 ≤ 1.62. That is, the film with ℎ0 > 1.62 is metastable.33 However, a polymer film with ℎ0 = 1.90 on a short brush layer (NB = 3) with the grafting density g = 0.4 exhibits the dewetting behavior, as demonstrated in Figure 2a. The film rupture occurs spontaneously, and the holes with fuzzy boundaries grow with time. In fact, such spontaneous dewetting takes place as long as ℎ0 < 2.4. On the basis of the critical thickness, this outcome reveals that the polymer nanofilm on a brush layer becomes more unstable than that on a 9 ACS Paragon Plus Environment


smooth substrate. It may be attributed to the thermal disturbance associated with the rough brush layer. Moreover, unlike the morphological evolution on a smooth substrate,33 coalescence of the holes is hindered and thus drops with the spherical cap shapes cannot be developed. Eventually, the randomly ruptured pattern of the film seems to be kinetically frozen on the brush layer, as shown in Figure 2b for ℎ0 = 1.25. According to the critical nanofilm thickness and the evolution of morphology, the dewetting behavior on the brush layer (autophobic dewetting) differs distinctly from that on the smooth substrate. The arrest of the spinodal pattern may be correlated with the quenched disorder in the underlying film.

Figure 2. The top view of dewetting dynamics for a polymer (NL = 20) thin film (a) (ℎ0 = 1.90) and (b) (ℎ0 = 1.25) on a brush (NB = 3) layer grafting (g = 0.4) on substrate with 𝜃𝑌  73°.

As the nanofilm on a brush layer is thick enough (ℎ0  2.4), it is able to sustain thermal fluctuations, and the capillary wave gets damped. When a dry hole is created on purpose in such a metastable nanofilm, however, this nucleation site will grow 10 ACS Paragon Plus Environment

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continuously. This process is corresponding to the mechanism of nucleation and growth. Figure 3a demonstrates the nucleation and growth process (top view) of a polymer nanofilm with ℎ0 = 2.6 on a brush layer, which does not suffer spinodal decomposition. After imposing a small hole with the radius 𝑅0 = 8, the dry zone broadens with time, and its shape is maintained as a circle. However, when the polymer nanofilm is thicker (ℎ0 ≥ 2.8), the enforced dry zone tends to shrink first, but form a stable hole after a long time, as illustrated in Figure 3b. This result implies the mechanism associated with pattern formation. As the film thickness is thick enough (ℎ0 ≥ 3.1), the dry zone diminishes with time, and it will be filled with polymer finally, as shown in Figure 3c. This process corresponds to the self-healing mechanism. The time evolution of the dry zone in terms of its radius 𝑅(𝑡) on the brush layer (NB = 3 and g = 0.4) for different polymer film thicknesses (ℎ0) is shown in Figure 4. At 𝑡 = 0, one has 𝑅(𝑡) = 𝑅0 = 8. The three different outcomes can be clearly depicted from 𝑅

(𝑡  ). That is, 𝑅()  represents nucleation, 𝑅() constant denotes stable hole, and 𝑅() 0 characterizes self-heal. Obviously, the self-healing rate is increased with ℎ0.



Figure 3. The top view of an initial dry hole 𝑅0 = 8 and final state for a polymer (NL = 20) thick film with different thickness (ℎ0) on a brush (NB = 3) layer grafting (g = 0.4) on substrate with 𝜃𝑌  73°.

10 h0 = 2.59

8 6

R

1 2 3 4 5 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 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 59 60

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h0 = 2.8

4

h0 = 3.1

2 h0 = 3.27 0

0

2000

4000

t

6000

Figure 4. The effect of the film thickness on the dewetting phenomena of a polymer nanofilm on a brush layer (g = 0.4) surface with 𝜃𝑌  73°, manifested by R(𝑡) 12 ACS Paragon Plus Environment

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subject to various film thickness (ℎ0) with an initial dry holes 𝑅0 = 8. The polymer film length is NL = 20 and brush length is NB = 3.

On the short brush layer (NB = 3 and g = 0.4), the final outcome of the polymer film depends on the film thickness ℎ0 subject to the nucleus size 𝑅0 = 8. On the other hand, the fate of the polymer film with a specified thickness is in turn determined by the properties of the brush layer: the length of brush polymer (NB) and grafting density (g). Consider the polymer film with ℎ0 = 2.4 on the long brush layer with NB = 10, the effect of the grafting density on the final morphology is examined. The film is metastable regardless of g. However, subject to an initial nucleus size 𝑅0 = 10, the outcome of the film depends on g. At low grafting density g = 0.33, the dynamics associated with the self-healing mechanism is observed, as depicted in Figure 5a. As the grafting density is increased to g = 0.5, the process for the development of a stable hole is acquired, as shown in Figure 5b. As the grafting density is high g = 0.75, the nucleation and growth phenomenon emerges, as illustrated in Figure 5c.



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Figure 5. The top view of the dynamics of a polymer film NL = 20 on long brush NB = 10 with ℎ0 = 2.4, and 𝑅0 = 10 with different grafting density (g) on

substrate

of 𝜃𝑌  73. B.

The Morphological Phase Diagram. According to our simulation results,

there are four types of wetting phenomena for a polymeric nanofilm on a brush layer grafted on the substrate: (i) spinodal decomposition, (ii) nucleation and growth, (iii) stable hole, and (iv) metastable self-healing. The outcome relies on the grafting density (g), polymer film thickness (ℎ0), and the nucleus size (𝑅0). Note that the final morphology on a smooth substrate (no brush layer) depends on the surface wettability (𝜃𝑌), ℎ0 and 𝑅0. Figure 6 demonstrates the morphological phase diagram of wetting phenomena for the polymer film with NL = 20 on a short brush layer (NB = 3). As the film is thin enough (e.g., ℎ0 < 2.4), spinodal decomposition predominates. For polymer film with


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ℎ02.4, an initial dry hole is imposed and the nucleus size is fixed at 𝑅0 = 8. When the film is thick enough (e.g., ℎ0 = 3.4), the mechanism of nucleation and growth prevails at low grafting densities (g < 0.2), while the mechanism of metastable self-healing dominates at high grafting densities (g ≥ 0.2). When the film thickness is in-between, the morphological outcome depends subtly on g and ℎ0. Nucleation and growth is seen for both low and high g. In the intermediate range (e.g., 0.2 < g < 0.4), the mechanism of the stable hole appears, in addition to metastable self-healing. On the other hand, at a given grafting density (e.g., g = 0.4), the film morphology varies with the thickness, from spinodal decomposition (ℎ0 < 2.4), nucleation and growth (2.4 ≤ ℎ0 ≤ 2.6), stable hole (2.8 ≤ ℎ0 ≤ 2.94), to metastable self-healing (ℎ0 ≥ 3.1). Note that changing the hole size in Fig. 6 shifts the phase boundary, but the main features remain the same qualitatively.

Figure 6. The morphological phase diagram of wetting phenomena for the a polymer nanofilm (NL = 20) on a short brush layer (NB = 3) with a fixed nucleus size 𝑅0 = 8 for ℎ0  2.4. 15 ACS Paragon Plus Environment


Figure 7 shows the morphological phase diagram of wetting phenomena for the polymer film with NL = 20 on a long brush layer (NB = 10). The film with fixed thickness ℎ0 = 2.4 is metastable at any g. However, subject to dry holes with various sizes 𝑅0, the final morphology may vary with g. At a specified nucleus size (e.g., 𝑅0 = 8), nucleation and growth occurs only at high grafting densities (g ≥ 0.66). As g decreases, metastable self-healing dominates but the stable hole mechanism emerges at the low grafting density (g = 0.1). However, when the nucleus size is large enough (𝑅0 > 10), the stable hole mechanism always rules for g ≤ 0.5. In contrast, as the nucleus size is small (𝑅0 < 8), the self-healing mechanism dominates in most of the grafting densities. Note that the scenario of stable hole always observed at the low grafting density g = 0.1 is attributed to the bicontinuous structure associated with the long brush layer (see Figure 1b). Since the film is thick enough, spinodal decomposition is absent. Compared to Figure 6 of the short brush layer, the polymer film is more stable on the long brush layer.

Figure 7. The morphological phase diagram of wetting phenomena for the polymer 16 ACS Paragon Plus Environment

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film (fixed thickness ℎ0 = 2.4 and NL = 20) on a long brush layer (NB = 10) with various nucleus size 𝑅0.

On a smooth surface without brush layer, the nanofilm is either unstable (dewetting) or metastable (self-healing). The instability mechanism is either spinodal decomposition or nucleation and growth.33 The nucleation mechanism requires nucleus (dry hole) with radius 𝑅0 exceeding the critical size 𝑅𝑐. While the hole expands for 𝑅0 > 𝑅𝑐, it shrinks and vanishes eventually for 𝑅0 < 𝑅𝑐, corresponding to self-healing. In contrast, on a surface with brush layer, the hole can be neither nucleation and growth nor self-healing, and a stable hole is observed. The phenomenon of autophobic dewetting on the brush layer is owing to the dominance of stretching of brush over polymer-brush intermixing. However, under certain conditions, the stretching-driven instability can be relieved in the vicinity of the dry hole. As a result, the hole with a constant size can be persisted, and it is kinetically stable because the entanglement between brush and polymer at the edge prevents the growth of the hole. Such a mechanism of stable hole can be applied to develop patterning dewetting, in which the pattern on the brush layer can be created by photolithography.50-52 C.

Patterning Dewetting of Polymer Thin Film on Polymer Brush. Well-

defined and nanopatterned polymeric brushes may serve as soft templates for the manipulation of nanoparticles of various sizes in the fields of biomaterial and biotechnology.31,32,53 If the patterned brush layer has low affinity for nanoparticles, nanoparticles tend to be repelled from the surface. However, a polymer film with high affinity with nanoparticles may be used to coat the brush layer with low affinity and develop the pattern of the underlying brush layer for nanoparticles. The casted film is



expected to dewet on the brush layer but will be arrested eventually by the underlying pattern. Three scenarios are studied. First, consider a polymer thin film with ℎ0 = 0.88 cast on a short brush layer with low grafting density g = 0.1. On the uniformly grafted surface, the rupture of the film is initiated at the location where polymers contact the substrate with 𝜃𝑌  73 (𝑡 < 50), as demonstrated in Figure 8a. The holes are developed and coalesce gradually (50 < 𝑡 < 600). Eventually, the dewetting pattern stops propagating (800 < 𝑡 < 1000), and this random pattern of the film is kinetically frozen on the short brush layer. Secondly, the short brush layer with g = 0.2 surrounds two no grafting squares as shown in Figure 8b. The mean grafting density is about g = 0.1. The polymer nanofilm with ℎ0 = 0.88 ruptures from the no grafting squares and continues to retreat toward the brush layer (50 < 𝑡 < 400). Eventually, the patterned polymer film with no grafting squares are formed (500 < 𝑡 < 1000). In the above two cases, the dewetting kinetics is driven by spinodal decomposition. The former is arrested probably by the quenched disorder in the underlying film but the latter is arrested by the entanglement between brush and polymer at the edge of the pattern (similar to the stable hole mechanism). Thirdly, the no grafting pattern becomes a word “NTU” as illustrated in Figure 8c. The short brush layer has g = 0.25. The dewetting dynamics of the film with ℎ0 = 1.71 also starts from the region without brush. The dry zone grows with time (0 < 𝑡 < 600), and gradually evolves to match the pattern (600 < 𝑡 < 3500). Again, the polymer film with the “NTU” pattern is developed ultimately. Figure 8c corresponds to the arrest of the dewetting kinetics in the nucleated dewetting regime, because the final film thickness is ℎ0 = 2.5 (within the stable hole regime in Figure 6). Our simulation results indicate that patterning dewetting can be used to develop a patterned polymer film on the patterned brush layer. If the polymer thin film and brush layer have different chemical 18 ACS Paragon Plus Environment

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compositions (𝑎𝑝𝑏  ―40), the patterning dewetting phenomenon still prevails. For example, essentially the same results are obtained as the interaction attractive interaction parameter between them is reduced to 𝑎𝑝𝑏 = ―39 for a nanofilm with ℎ0 = 2.4.

Figure 8a. The polymer film NL = 20 on short brush NB = 3 grafting (g = 0.1) on substrate with 𝜃𝑌  73 (𝑎𝑝𝑠 = 𝑎𝑏𝑠 = ―35).



Figure 8b. The short brush layer with g = 0.2 surrounds two no grafting squares on substrate with 𝜃𝑌  73 (𝑎𝑝𝑠 = 𝑎𝑏𝑠 = ―35).

Figure 8c. The short brush layer with g = 0.25 and no grafting pattern is a word “NTU” on substrate with 𝜃𝑌  73 (𝑎𝑝𝑠 = 𝑎𝑏𝑠 = ―35).


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IV. CONCLUSIONS

The dynamics of patterning dewetting of a nanoscale polymer film on a brush layer grafted on a smooth, partial wetting surface is investigated by MDPD. Four types of autophobic dewetting phenomena are clearly identified: (i) spinodal decomposition, (ii) nucleation and growth, (iii) stable hole, and (iv) metastable self-healing. The outcome depends on the film thickness (ℎ0), grafting density of polymer brush (ρg), brush length (NB), and nucleus size (𝑅0). Two morphological phase diagrams of the dewetting mechanism are obtained for polymer thin film on short (NB = 3) and long (NB = 10) polymer brush layer, respectively. On the short brush layer, spinodal decomposition predominates as the film is thin enough. When the polymer film has the intermediate thickness subject to a nucleus size 𝑅0 = 8, the nucleation and growth mechanism is observed for both low and high ρg. However, in the middle range of ρg, the stable hole mechanism appears. As the film is thick enough, it can self-heal from the imposed dry hole. On the long brush layer, the fate of the polymer film which does not suffer spinodal decomposition is sensitive to the nucleus size. When 𝑅0 is small enough, the film tends to self-heal regardless of ρg. In contrast, for large 𝑅0, the stable hole mechanism dominates at low ρg, while the nucleation and growth mechanism occurs at high ρg.



The mechanism associated with stable hole arises from the entanglement between brush and polymer at the edge of the dry hole, which prevents the growth and shrinkage of the hole. It can be applied to develop patterning dewetting, in which the patterned brush layer can be created by photolithography. The casted film can dewet on the brush layer due to spinodal decomposition or nucleation and evolve to exhibit a pattern similar to the underlying brush. Moreover, the patterning dewetting phenomenon still prevails when the film and brush layer have different chemical composition. Our MDPD approach is also applicable to branched, hyper-branched, or dendritic polymers.


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SUPPORTING INFORMATION The quantitative characterizations of the brush layer are supplied as Supporting Information, including the brush density profiles, mean height, and root mean square height.

Author Information Corresponding Author E-mail: [email protected] (Y.-J.S.); [email protected] (H.-K.T.).

Acknowledgement Y.-J.S and H.-K.T. thank Ministry of Science and Technology of Taiwan for financial support. Computing times, provided by the National Taiwan University Computer and Information Networking Center and National Center for High-performance Computing (NCHC) are gratefully acknowledged.

Conflicts of interest There are no conflicts of interest to declare.



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Patterning Dewetting and Self-Healing of Polymer Nanofilms on a

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