Icephobic Durability of Branched PDMS Slippage Coatings Co-Cross

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Surfaces, Interfaces, and Applications

Icephobic Durability of Branched PDMS Slippage Coatings Co-crosslinked by Functionalized POSS Shuhui Gao, Bo Liu, Jie Peng, Kongying Zhu, Yunhui Zhao, Xiaohui Li, and Xiaoyan Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19666 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Applied Materials & Interfaces

Icephobic Durability of Branched PDMS Slippage Coatings Co-crosslinked by Functionalized POSS Shuhui Gao,† Bo Liu,† Jie Peng,† Kongying Zhu,‡ Yunhui Zhao,† Xiaohui Li,† and Xiaoyan Yuan*,† † School

of Materials Science and Engineering, Tianjin Key Laboratory of Composite and

Functional Materials, Tianjin University, Tianjin 300350, China ‡ Analysis

and Measurement Center, Tianjin University, Tianjin 300072, China

KEYWORDS: branched PDMS, POSS, icephobic coating, durability, interfacial slippage

ABSTRACT:

Ice accretion poses a severe impact on diverse aspects of human life. Although

great efforts have been dedicated to prevent or alleviate ice adhesion to the surface of substrates by developing various icephobic coatings, it is still needed to improve the integrated performance. Herein, we present a novel strategy to prepare polydimethylsiloxane (PDMS) slippage coatings by combining a soft architecture-driven branched PDMS with partial short PDMS-functionalized polyhedral oligomeric silsesquioxane (POSS) as a co-crosslinker, in which silicone oil with certain viscosity was added as a lubricant. The chemical structure, surface morphology and icephobic durability of the prepared coatings were investigated with concerns for the potential anti-icing uses. The PDMS slippage coating shed light on extraordinary icephobic durability with the ice shear strength at approximate 11.2 kPa and maintained low

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values below 14 kPa even after 50 icing/deicing cycles. Due to the elaborate control of the cross-link density, the side chains of the branched PDMS provided rich storage space for entrapped silicone oil to the formation of the interfacial slippage. Moreover, the introduction of the functionalized POSS brought about significantly improved mechanical resistance in abrasion and elastic modulus. It is suggested that the branched PDMS slippage coating is promising as a candidate in practical anti-icing applications.

INTRODUCTION

Ice build-up and its subsequent removal on exposed surfaces of infrastructures, such as ship hulls, aircrafts, wind turbines and power lines, may cause severe safety challenges.1-3 Consequently, extraordinary efforts have been dedicated to prevent or alleviate ice adhesion to such facilities surface.4-9 Typically, passive strategies by polymeric anti-icing coatings are favorable because conventional methods are somewhat energy-intensive, environmentally unfriendly and costly.1-3 Previously, ice adhesion strength (τice) has been considered as a key performance indicator of the icephobic coatings by evaluating the shear stress of ice detaching from the coating surfaces it adhered to.10 Generally, icephobic coatings are defined as the coatings with τice less than 100 kPa.11 To date, low values of ice adhesion strength have been reported by using hydrophobic polydimethylsiloxane (PDMS) and integrating a self-healing interpenetrating polymer network elastomer with τice = 6.0±0.9 kPa,12 slippery PDMS surfaces with τice = 3.8±1.8 kPa,13 organogel PDMS coatings containing liquid paraffin with τice = 1.7±1.2 kPa,14 self-lubricating PDMS organogel coatings with τice = ca. 0.4 kPa15 and interfacial slippage on hydrophobic materials with τice = 0.15±0.05 kPa,10 respectively. All of the measurement approaches were conducted on a conventional anti-icing apparatus by horizontally pushing the ice cuvette to slide off the 2


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coating surfaces. Dou et al. discovered that a strong breeze could detach ice from coating surfaces under τice ≤ 27±6 kPa.16 It was noteworthy that the ice adhesion strength lower than 12 kPa was preferred, in which ice could be shed off from such icephobic surfaces under natural forces like wind shear and its intrinsic gravity.17 Rigid hydrophobic materials were conventionally adopted to decrease the ice adhesion strength.18 On the other hand, the distinguished icephobicity on slippery liquid-infused porous surfaces can be obtained through a layer of liquid lubricant, either aqueous lubricating liquids19,20 or low surface energy free silicone oil.4,21 However, the short-lived icephobicity could be mitigated and removed by the depletion of the free-oil layer via evaporation and consumption due to the intrinsic sacrificial nature.21 In general, the adhesion of ice on the smooth hydrophobic coatings is lower than that on hydrophilic ones.22,23 Polysiloxane, a kind of hydrophobic polymer with low-modulus, was universally employed in icephobic surfaces.10,12-15,17,18,22,24-26 Except for environmentally benign, widely available with ease and non-corrosive, PDMS-involved materials are likely to be tuned with the modulus by tailoring the cross-link density.18 Interestingly, soft solvent-free PDMS elastomers in one-step with chain entanglement-free design exhibited their excellent softness and tailored stiffness, and large deformation easily occurred under only slight external forces.27,28 In addition, as an organic-inorganic hybrid nano-scaled molecule, polyhedral oligomeric silsesquioxane (POSS) is of great interests.5,19 According to our previous studies, POSS with eight-cornered groups and low surface energy was able to migrate to the submicron/nano-scaled icephobic surfaces and form rough wavy structures,13,25 or to bring about microphase separation of POSS-containing fluorosilicone acrylate copolymers, endowing excellent hydrophobicity and water-repellency for anti-icing.29-32 In this work, we developed a novel icephobic coating with matrix enhancement and abrasion

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resistance, which integrated branched PDMS, vinyl-functional POSS partially modified with short PDMS chains, and deliberate lubricant. The functionalized POSS was tailored and intended for compatibility and miscibility with the bulk PDMS matrix as well as fulfillment of co-crosslinker. According to our previous study on slippery surfaces formed from polysiloxane and fluorinated POSS,13 ultra-low ice shear strength of the slippery wavy coatings could go upwards after 15 icing/deicing cycles, demonstrating a relatively short-lived icephobicity. Herein, the icephobic durability and abrasion resistance of the as-prepared coatings were explored. It was expected to be collaboratively improved with a fine continuous modulation of PDMS lubricant secretion and POSS introduction with concerns for the potential ice-repellency use.

EXPERIMENTAL SECTION

Materials. All reactive PDMS polymers were purchased from Gelest Inc. (USA) and used as received.

Backbone:

methylhydrosiloxane-dimethylsiloxane

copolymers,

trimethylsiloxy

terminated, ca. 44 silicone hydrogen for per molecule (PDMS-PMHS, HMS-064). Side chain: monovinyl terminated polydimethylsiloxanes-asymmetric (PDMS-V, MCR-V21). Cross-linking chain: dualvinyl terminated polydimethylsiloxanes (V-PDMS-V, DMS-V21). Monohydride terminated polydimethylsiloxanes-asymmetric (PDMS-H, MCR-H11). Dimethylsilicone oil (viscosity ca. 100 mPa·s) was provided by Shanghai Meryer Chemical Technology Co., Ltd., China. Octavinyl polyhedral oligomeric silsesquioxane (OVPOSS) was purchased from Hybrid Plastics Inc. (USA). Sylgard 184 (Part A and Part B) was provided by Dow Corning (USA). Platinum (0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene (Karstedt’s catalyst, Pt ~ 2%) was purchased from Sigma-Aldrich (USA). 3-(Trimethoxysilyl)propyl 4


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methacrylate (γ-MPS, 97%) was provided by Beijing HWRK Chem. Co., Ltd., China. Toluene and tetrahydrofuran (THF) were provided by Tianjin Jiangtian Chemical Technology Co., Ltd., China. All chemicals were used as received without further purification. Synthesis. Branched PDMS was prepared by mixing a multiple silicon hydrogen-functional linear PDMS copolymer PDMS-PMHS with certain mono-vinyl terminated linear PDMS polymer PDMS-V through hydrosilylation reaction. As shown in Scheme 1a, a given amount of PDMS-PMHS (2.0 g) was vigorously mixed with the PDMS-V (2.0 g) in toluene in a three-neck flask to prepare a 15 wt% polymer solution. Then Karstedt’s catalyst (0.1 mol% of Si-H bond) was added dropwise. The solution was allowed to stir at 80 °C for 19 h with a condenser under argon atmosphere, followed by reduced pressure distillation and vacuum desiccation to remove extra toluene via being poured into a round-bottom flask.27 PDMS-functionalized POSS (V3-POSS-PDMS5) was synthesized as a co-crosslinker via hydrosilylation of OVPOSS and PDMS-H, as depicted in Scheme 1b.13 Briefly, 0.2 g of OVPOSS was dissolved in a certain amount of toluene in a three-neck flask, which was equipped with a reflux condenser and under a stream of dry N2. Then, PDMS-H (1.6 g) and Karstedt's catalyst (0.1 mol% of Si-H bond) were added. After reaction at 80 °C for 48 h, the crude product was post-treated in the same manner as mentioned above. Scheme 1. Schematic Illustration of Preparation of Branched PDMS (a), Functionalized POSS V3-POSS-PDMS5 (b), and the PDMS Slippage Coating (c) through Hydrosilylation Reaction

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Preparation of PDMS Slippage Coatings. As illustrated in Scheme 1c, a given amount of the prepared branched PDMS was mixed with V-PDMS-V and the functionalized POSS co-crosslinker in THF. Additionally, the silicone oil was also added into above mixture in a 1/2 mass of the branched PDMS to prepare a 20 wt% polymer solution eventually. Afterwards, the mixed solution was violently stirred on a turbine mixer for 1 h after adding certain Karstedt’s catalyst. The icephobic coating was prepared by casting the solution onto a stainless steel sheet (20 mm × 20 mm), left at ambient temperature for 1.5 h. Prior to use, the stainless steel substance was treated with oxygen plasma (PDC-32G-2, Harrick plasma, USA) at 18 W and a pressure of 800~1000 mTorr for 3 min, and followed by incubation in a silanization solution γ-MPS at room 6


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temperature overnight before thorough rinse with ethanol and desiccation with air stream.33 The desired coating thickness was obtained by adjusting the volume of the coated solution. The drop coated mixture was subsequently cross-linked in an oven at 100 °C for 4 h, resulting in the desired PDMS slippage coatings.13 For designation, bPDMSO refers to the branched PDMS-based coating containing silicone oil, while for bPDMSOx, the number x represents the integral mass percentage of the functionalized POSS in the as-prepared icephobic coating. The coating samples of Sylgard 184 and bPDMS act as controls. Compositions of the reactants used in preparing the icephobic coatings were collected in Table 1. Specially, the viscous solution was scraped using a knife coater for the sake of controlling the final thickness. The thickness of the coatings was measured by using a coating thickness gauge (NT220, Beijing Timesun Science & Trade Ltd., China). Characterizations. Chemical structures of the branched PDMS and V3-POSS-PDMS5 were examined by 1H nuclear magnetic resonance (NMR, AVANCE Ⅲ TM HD 400 MHz NanoBAY, Bruker, Germany) via dissolving 5 mg of the specimen in deuterochloroform and Fourier transform infrared spectroscopy (FTIR, Tensor 27 spectrometer, Bruker, Germany), which was carried out in the range from 4000 cm-1 to 400 cm-1 using KBr pellet technique. Standard adhesion tests were carried out using a cross-cut test for evaluating the adhesion of the as-prepared PDMS slippage coatings on the stainless steel substrate according to GB/T 9286-1998 (eqv. ISO 2409: 1992).34,35 Typically, the coated sample was fixed tightly onto the horizontal platform. An elongated “+” pattern was incised at an angle of 90°, with a cutter equipped with 6 blades and a spacing of 1 mm applied evenly, accompanied by sweeping gently along the diagonal using a banister brush. The coatings were evaluated according to the degree of damage to the cutting edge.

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Table 1. Compositions of the PDMS Slippage Coatings Branched

V-PDMS-V

Silicone oil

Si-H:C=C

PDMS (g)

(g)

(g)

(wt%)

(g)

(mol:mol)

bPDMSO

0.30

0.30

0

0

0.15

1:1.03

bPDMSO1

0.30

0.28

0.010

1.4

0.15

1:1.03

bPDMSO3

0.30

0.26

0.021

2.9

0.15

1:1.03

bPDMSO4

0.30

0.25

0.031

4.2

0.15

1:1.03

bPDMSO7

0.30

0.21

0.052

7.3

0.15

1:1.03

bPDMSO10

0.30

0.18

0.072

10.3

0.15

1:1.03

bPDMSO15

0.30

0.13

0.103

15.1

0.15

1:1.03

bPDMS

0.30

0.30

0

0

0

1:1.03

Sample

V3-POSS-PDMS5

a)

a) bPDMSOx

stands for the branched PDMS-based coating containing silicone oil, and the

number x represents the integral mass percentage of the functionalized POSS in the as-prepared icephobic coating. The surface topography was viewed by atomic force microscope (AFM, CSPM5500A, Guangzhou Being Nano-Instruments Ltd., China) at room temperature in a tapping mode. Moreover, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were conducted in the field emission scanning electron microscope (S-4800, Hitachi, Japan) equipped with an EDS detector (Genesis XM2, EDAX, USA) to analyze the morphology and the elements distribution in the cross-section of the coatings. The sample bPDMSO4 on an aluminum sheet tailored with a cutter was glued onto the sample stage with conductive adhesive and sputtered with gold before SEM and EDS observation. In addition, X-ray photoelectron spectroscopy (XPS, PHI 5000C ECSA, Perkin-Elmer, USA) was performed to characterize the elemental 8


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compositions on the coating surface under a pressure of 510-8 Torr with a tilted platform at 45°. POSS in the coatings was observed under a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan) by directly dropping of a drop of 1 wt% polymer solution in THF on the carbon-coated copper grids. Water contact angle (WCA), advancing contact angle (θadv) and receding contact angle (θrec) were measured by a contact angle meter (JC2000D, Shanghai Zhongchen Equipment Co., Ltd., China) with 5 µL water droplets. Additionally, contact angle hysteresis (CAH) was obtained by calculating the difference between θadv and θrec. The value was recorded by the multiple 3 independent tests on each of three parallel samples. Equilibrium Swelling Behavior. The equilibrium swelling test was carried out at room temperature with toluene (molar volume V = 105.9 mL/mol, density ρs = 0.87 g/cm3) as the probe solvent. The coating samples were weighed as-prepared (m0) and swollen for 48 h to reach the equilibrium. Afterwards, the swollen coating was wiped off lightly with a filter paper to remove excess toluene and weighed immediately as mswell. Eventually, the swollen coating was exposed to the air to reach the semi-dry state, after which being transferred to the oven at 80 °C for 24 h and exactly weighed as mass of dried coating (mdry).36-38 For each sample, three pieces of the prepared coatings were measured. The gel fraction (GF) was estimated corresponding to the following equation (1): GF (%) 

mdry m0

100

(1)

The cross-link density (ρCL) was calculated from equilibrium swelling data by means of Flory-Rehner equation (2) defined as the number of cross-link sites per unit volume.36,39-41

 CL (mol/m3 )  

ln(1  Vr )  Vr  Vr2 106 2V (Vr1/3 - 0.5Vr )

(2) 9



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where χ is the Flory-Huggins interaction parameter (χ = 0.465) between the PDMS polymer (density ρp = 0.970 g/cm3) and the solvent toluene. Vr referred to the volume fraction of polymer in the swollen state,36,39-41 and was depicted as following equation (3):

Vr 

m0 / p m0 / p  (mswell  m0 ) / s

(3)

Quasi-Static Nanoindentation Tests. Quasi-static nanoindentation tests were commanded in nanoindenter (Piuma, Optics11, Amsterdam, the Netherlands) by using a cylindrical diamond flat punch between the sphere probe and the cantilever. In particular, probes were used with a tip diameter of 39.5 μm to stiffness of 3.56 N/m and a tip diameter of 20 μm to stiffness of 41.5 N/m for measuring the reduced Young’s modulus of the coatings in different depths, respectively. The reduced Young’s modulus was obtained based on the load-indentation curve as the references described.12,42-44 Each value was recorded by the average of 8 independent measurements. Anti-Icing Tests. The freezing delay time (Td) of the as-prepared coatings was measured, with the specimen commanded onto the Peltier plate kept at -15 C for 10 min under a stream of N2. Once a certain amount of deionized water (5 μL) was syringed onto the coating surface, its total droplet freezing process on the coating surface was recorded by regular digital-camera shooting. And the time difference, ranging from dropping the drip onto the coating surface immediately to initial freeze of water droplets appeared on the coating surface, was regarded as Td.32,45-47 Every value was recorded by the multiple average of 3 independent tests on each of three parallel samples. Ice adhesion strength, which was denominated as ice shear strength in this study to evaluate the horizontal shear strength of ice detaching from the coating surfaces it adhered to, was measured using a custom-built apparatus according to our previous reports.29-31,48,49 Typically, a

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stainless steel sheet with the as-prepared coating was placed onto the Peltier plate, on which a hollow glass cylinder (inner diameter of 9.84 mm) hydrophobically treated with 1H,1H,2H,2H-perfluorooctyl trichlorosilane was placed. Then, 450 μL of deionized water was pipetted in the cylinder, left in a chamber with a stream of dry N2 at a temperature of -15 °C for more than 3 h to ensure its total freezing that adhered to the coating surface. Subsequently, the linear transition stage with a digital pull & push force gage mounted was moved at a shear rate of 0.125 mm/s. The probe applied a shear force on the very bottom of the cylinder (the distance from the coating surface < 1mm) to minimize the torque on the iced cylinder and gain the pure shear force. The maximum shear force of the coating and the loading curve were recorded. The ice shear strength was obtained by dividing the peak value of the shear force by contact area between the iced cylinder and the coating surface. Each value was recorded by the average of 5 independent measurements. In addition, the icephobic durability of the coating was examined by measuring the ice shear strength during 50 ice/deicing cycles. Abrasion tests were conducted by using a weight with a diameter of 3.15 cm and a mass of 200 g, under which a piece of 400-grit sandpaper (20 mm × 20 mm) was attached. For abrasion, the coating sample (75 mm × 40 mm) was clamped down and abraded at 25 cycles/min under a pressure of 4.9 kPa and a stroke length of 2.54 cm.10,12,18 The weight was refaced before each set of abrasion cycles by replacing a brand-new sandpaper. Repeated icing/deicing cycles were carried out to evaluate ice shear strength after corresponding continuous abrasion cycles. Each value was recorded by the average of 3 independent measurements.

RESULTS AND DISCUSSION

A novel icephobic slippery coating was developed in this study via hydrosilylation reaction by 11



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integrating functional POSS and branched PDMS with a certain amount of silicone oil introduced. To ensure the branched structure and lower entanglement design in the bulk matrix of the PDMS network, the icephobic coatings were prepared by a two-step procedure. On one hand, a branched PDMS was synthesized by mixing a multiple silicon hydrogen-functional linear PDMS copolymer with certain mono-vinyl terminated linear PDMS polymers through hydrosilylation. On the other hand, the icephobic PDMS slippage materials were prepared by integrating dual-vinyl terminated PDMS polymer as cross linkage into the material matrix to form a cross-linked network, accompanied by the introduction of functionalized POSS as a co-crosslinker and dimethyl silicone as a lubricant, which was immiscible with water, into the network structure. The icephobic durability and abrasion resistance could be improved with a fine continuous modulation of PDMS oil secretion and POSS incorporation. Preparation of the PDMS Slippage Coatings. A sequence of PDMS slippage coatings were prepared with compositions shown in Table 1, of which the syntheses of precursors branched PDMS and V3-POSS-PDMS5 were illustrated in Scheme 1. The multiple-functional PDMS copolymer (PDMS-PMHS) abundant in silicone hydrogen groups (ca. 44 silicone hydrogen for each) was selected as the backbone of the branched molecule. PDMS-V, each carrying one terminal vinyl group, was grafted onto PDMS-PMHS through hydrosilylation, which could covalently conjugate the branched PDMS via the addition of silicone hydrogen to unsaturated vinyl groups. The 1H NMR and FTIR spectra of the branched PDMS confirmed the chemical structure as shown detailedly in Figure S1 and S2 (Supporting Information). Averaged 9 molar amounts of PDMS-V were successfully grafted onto a PDMS-PMHS backbone, resulting in 100% grafting efficiency and 20% grafting degree as expected. Coincidentally, 4.63 calculated molar amounts of Si-H were covalently bound with C=C double bonds of OVPOSS, drawing the

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conversion of 92.6% for Si-H in the synthesis of the functionalized POSS on the whole. More details were given in the Figure S3 and S4. Furthermore, an appreciably excellent adhesion of the coating bPDMSO4 to the substrate which is of a major concern was conducted and depicted in detail in Figure S5, exerting an effective cross-linking between the C=C bonds exposed to the underlying substrate treated with γ-MPS and the residual Si-H bonds on the branched PDMS. Morphology. The surface topography of the coating samples characterized by AFM was shown in Figure 1a. Relatively nano-scaled successive wrinkle structures were observed, and all the samples showed root-mean-squared roughness below 2 nm with the exception of bPDMS (29.5±4.4 nm) (Table 2), suggesting that the prepared coatings surface was relatively smooth. Additionally, the higher the content of POSS co-crosslinker was added, the greater the root-mean-squared roughness of the coating, accompanied by the more obvious protuberances and grooves were formed (Figure 1a). This indicated that the functionalized POSS introduced into the coatings was proposed as a co-crosslinking agent, which could also improve the roughness of the coatings as well. To observe the morphology and elements distribution along the depth of the coatings, SEM equipped with EDS characterization was conducted and shown in Figure 1b. The cross-section of the sample bPDMSO4 was homogeneous with neither aggregation nor phase separation, as depicted in concert with the surface morphology by AFM. Elements of C, O and Si along the white line in Figure 1b manifested uniform distribution by EDS line scan, which could also echo the results of the EDS area line in Figure S6. In addition, XPS measurements were carried out and the relevant spectra were presented in Figure S7. It could be seen that the representative binding energy peaks of O1s, C1s, Si2s and Si2p appeared at 534.4, 286.4, 155.2 and 104.0 eV, respectively. Relatively elemental percentages of the as-prepared PDMS slippage coatings were

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then listed in Table S1. The almost similar elemental percentages were detected by XPS of the coating surfaces with different POSS co-crosslinker contents, demonstrating favorable dispersion of C, O and Si elements during the formation of the PDMS slippage coatings built on multiple Si-O-Si bonds in PDMS-based polymers.

Figure 1. AFM images of the PDMS slippage coatings (a), SEM images and EDS line scans of C, Si and O elements on the cross-section of the sample bPDMSO4 (b), as well as the variation of water contact angle values with time on the PDMS slippage coatings (c). The bulk dispersion of the POSS co-crosslinker was observed by TEM in Figure S8. POSS is a kind of organic-inorganic hybrid nanoparticles with a diameter of 1~3 nm in terms of a single molecule.50,51 The interaction between the POSS molecules and the solvent was relatively weak owing to its superior low surface free energy, making it difficult to achieve mono-dispersion 14


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even in the excellent solvent. It was noteworthy that the aggregation of the functionalized POSS nanoparticle appeared in the as-prepared PDMS slippage coatings to some extent. As shown in Figure S8, the POSS aggregate occurred in the sample bPDMSO4 with a size of about 18 nm, whereas an aggregate with a size of approximate 50 nm was detected in the sample bPDMSO15 due to the higher content of the POSS co-crosslinker with 15.1 wt%, suggesting a good dispersion into the coatings on the whole. It was incident to self-assemble for the POSS co-crosslinker when dispersed into the PDMS slippage coatings, of which the Si-H bond in the backbone reacted intensely with 3 molar equivalent residual C=C double bonds in POSS. Actually, the POSS co-crosslinker with short PDMS segments tended to interpenetrate into the coating ingredients, leading to minor aggregate sizes. Simultaneously, the aggregate size measured by TEM corresponded to the results of the root-mean-squared roughness characterized by AFM in Table 2 and the reduced Young’s modulus depicted in Figure 2a and Figure 2b delightedly. Wettability of the Icephobic Coatings. Hydrophobic silicone polymers endowed themselves with inherent hygroscopicity. As a consequence, variations of WCA with time on the coatings were recorded in 6 min under ambient conditions and presented in Figure 1c and Figure S9. The samples Sylgard 184, bPDMS and bPDMSO were also tested as controls. It could be seen that WCA of the commercial product Sylgard 184 declined gradually, ranging from original 111.2° to 102.6° in 6 min, exhibiting better hydrophobicity. However, the WCA value of the sample bPDMS

decreased rapidly from 107.5° to 102.1° within 20 s due to quick water absorption. The

successive slower decrease was similar to the tendency observed for the sample Sylgard 184, reaching the lowest value of all the coatings at 6 min eventually, with a trend to a relatively fast decline of the hydrophobicity. This finding of variations coincided with the results of the

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prepared PDMS slippage coatings shown in Figure S9, with an analogical but merely minor decline of the hydrophobicity. The results indicated the molecular channels inside the coatings that the organic silicon polymer itself has certain moisture absorption, resulting in a slight drop of the hydrophobicity. Simultaneously, WCA values at initial moments and 6 min were collected in Table 2. It is universally acknowledged that increasing WCA and reducing CAH are convincing approaches to pursue ice repellent capability.52 Higher WCA values can be attained only by texturing the hydrophobic surfaces.53-55 Naturally, the higher content of POSS co-crosslinker was expected to generate increased WCA of icephobic coatings at the beginning and 6 min, together with the more obvious hydrophobicity. It was exactly the added POSS co-crosslinker that alleviated the hygroscopicity of the coatings. The CAH values on the coatings surface were also listed in Table 2, which was derived from the difference between θadv and θrec. The sample Sylgard 184 had the highest CAH among the coatings, exhibiting its intrinsically weak repellency to water as a traditional commercial product. Particularly, CAH showed an increasing tendency with the POSS co-crosslinker content to a certain extent, whereas the sample bPDMSO4 had a relatively higher value by virtue of adsorption of water in the coating. Swelling Properties. The thickness of the as-prepared coatings listed in Table 2 was measured by using a coating thickness gauge before swelling. It was noteworthy that the thickness of all the samples was controlled below 50 μm via adjusting the volume and concentration of the solutions except for the sample bPDMS which was absent of the oil lubricant. Subsequently, both of the gel fraction (GF) and the cross-link density (ρCL) calculated by the above mentioned equations (1) and (2) were presented in Table 2. It should be noted that the sample Sylgard 184 owned a higher ρCL of 723±7 mol/m3 and a GF over 90%, suggesting a

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highly cross-linked network with few unreacted dangling polymer chains dissolved. On the contrary, the minimum ρCL of 39±4 mol/m3 was detected for the sample bPDMS. On account of the addition of the oil lubricant in the PDMS slippage coatings, that was, the mass of the lubricant accounted for 1/3 of the total solution, the proportion of the insoluble polymeric chains came up to around 2/3, leading to a GF of 69.6±1.1% for the sample bPDMS. Subsequently, both of the gel fraction (GF) and the cross-link density (ρCL) calculated by the above mentioned equations (1) and (2) were presented in Table 2. It should be noted that the sample Sylgard 184 owned a higher ρCL of 723±7 mol/m3 and a GF over 90%, suggesting a highly cross-linked network with few unreacted dangling polymer chains dissolved. On the contrary, the minimum ρCL of 39±4 mol/m3 was detected for the sample bPDMS. On account of the addition of the oil lubricant in the PDMS slippage coatings, that was, the mass of the lubricant accounted for 1/3 of the total solution, the proportion of the insoluble polymeric chains came up to around 2/3, leading to a GF of 69.6±1.1% for the sample bPDMS. More significantly, the cross-link density of the as-prepared PDMS slippage coatings monotonically enlarged with the increased POSS co-crosslinker contents. The gel fraction of all the PDMS slippage coatings maintained stable between 55~60% as expected, which was evidenced in Table 2. Considering the total constant molar ratio of Si-H bond and C=C double bond basically, we just substituted the functionalized POSS for original V-PDMS-V. While insufficient cross-link was obtained due to certain steric hinerance of the functionalized POSS, with a lower GF than the sample bPDMS, it was within the acceptable range. In other words, this was indicative that the introduction of the POSS co-crosslinker facilitated the cross-link density of the as-prepared PDMS slippage coatings considering the total constant molar ratio of Si-H bond and C=C double bond basically.

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Table 2. Characteristics of the PDMS Slippage Coatings WCA (°)

Thickness

Rq a)

Cross-link density

Gel fraction

(μm)

(nm)

(mol/m3)

(%)

t=0

bPDMSO

45.6±3.0

0.5±0.1

50±3

55.8±0.2

bPDMSO1

43.0±2.1

0.4±0.1

68±1

bPDMSO3

47.5±2.9

1.1±0.1

bPDMSO4

44.3±3.8

bPDMSO7

θadv

θrec

CAH

t = 6 min

(°)

(°)

(°)

106.1±0.6

94.6±0.6

106.3±1.3

102.1±1.2

4.2±1.3

55.9±0.6

105.9±0.6

95.1±1.6

107.6±0.7

102.8±0.8

4.8±1.1

67±2

54.0±0.6

104.6±0.9

96.3±0.4

106.6±1.4

102.4±1.2

4.2±1.4

0.7±0.2

74±3

55.3±0.5

105.8±1.3

95.3±0.9

107.5±0.8

100.8±1.5

6.6±1.0

46.8±3.8

0.7±0.3

98±3

58.6±0.2

105.1±0.3

96.3±0.8

109.1±0.5

104.6±0.6

4.5±0.8

bPDMSO10

45.8±4.9

1.2±0.2

106±13

56.0±0.3

107.9±0.5

97.1±0.3

108.5±0.4

102.9±0.6

5.6±0.7

bPDMSO15

41.0±2.6

1.4±0.2

150±23

57.5±2.0

108.3±0.5

96.9±0.3

108.8±0.5

102.6±0.7

6.2±0.7

bPDMS

53.7±5.4

29.5±4.4

39±4

69.6±1.1

107.5±0.3

91.6±0.8

109.7±1.2

103.2±1.0

6.5±1.0

Sylgard 184 b)

41.4±2.5

0.6±0.1

723±7

91.3±3.0

111.2±0.9

102.6±1.5

117.2±2.0

107.4±1.3

9.8±1.4

Sample

a) Root-mean-squared

b) The

roughness estimated by AFM analysis.

sample Sylgard 184 as a control was conducted by mixing part A and part B at a ratio of 10:1 and cured at 100 °C for 4 h.15

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Elastic Moduli. The critical shear stress required to push off a hard object (eg. ice) from a soft film (eg. coating surface) is given by τice ∝ (WadhE/t)1/2, which was expounded by Chaudhury and Kendall,10,18,56,57 where Wadh is the work of adhesion on the coating surface, E is the elastic modulus of the soft film and t is the thickness of the soft coating, respectively.10,18 It is quite obvious that the elastic modulus is crucial in the soft coatings. As a consequence, the quasi-static nanoindentation test was conducted gradually to evaluate the elastic modulus of the as-prepared icephobic coatings, as illustrated in Figure 2. We selected two divergent probes with tip diameters at 39.5 and 20 μm that applied to stiffness of 3.56 and 41.5 N/m, respectively, detecting the reduced Young’s modulus of the coatings with different depths, respectively. Remarkably, Young’s modulus is proportional to the reduced Young’s modulus,12 which was obtained with ease by linear-fitting the slope of the initial position on the unloading curve in an Oliver model via a corresponding matched software. Building on the existing concepts discussed above, the reduced Young’s moduli were observed to be almost above 600 kPa of the coatings in the indentation of ca. 12 μm (Figure 2b). However, it was extremely different from those of the coatings involving the reduced Young’s modulus less than 150 kPa considering an indentation of about 2 μm (Figure 2a), demonstrating a gradient distribution of the stiffness along with the depth of the as-prepared PDMS slippage coatings. It was exactly because the lubricant was migrated onto the coating surface, on which a layer of silicone oil appeared that the coating surface owned stiffness decline along with the reduced Young’s modulus. On the other hand, due to the pre-treatment of the underlying substrate by means of γ-MPS, the residual Si-H bonds on the branched PDMS might be involved in the reaction with C=C bonds exposed to the substrate, leading to a certain improvement in both of the stiffness and strength of the coatings.

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Figure 2. Reduced Young's moduli (a,b) of the PDMS slippage coatings, and the typical load-indentation curves (c,d) measured on the samples with depth at 2 m and (a,c) and 12 m (b,d), respectively. It is noteworthy that soft materials with low modulus are inclined to obtain low adhesion strength. As depicted in Figure 2a and Figure 2b, the reduced Young’s moduli with depth at 2 m and 12 m of the sample bPDMSO were measured about 59.5±13.2 kPa and 617.2±131.2 kPa, respectively. However, the higher reduced Young’s modulus of the as-prepared POSS-contained PDMS slippage coatings indicated reinforcement that the POSS co-crosslinker endowed the coatings in different depths with significantly improved stiffness. Compared with the sample bPDMSO4, thicker and coarser sample bPDMSO3 tended to obtain higher stiffness. The samples were applied a certain load by means of the cantilever to press it down to the

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maximum load at a constant velocity. Then it was followed by unloading after holding stably at a certain depth range, which was recorded and displayed two kinds of remarkably different load-indentation curves between the coating surface and the internal coating, as shown in Figure 2c and Figure 2d, respectively. The creep behavior expressed as an inflexion form appeared at the top of the load-indentation curve in Figure 2d, which could be illustrated by its intrinsic viscoelasticity using soft PDMS as a matrix. Typically, the absolute value of the ordinate corresponding to the lowest point on the unloading curve referred to the adhesion force between the coating surface and the cantilever. It could be seen that the adhesion between the cantilever and the coating surfaces was almost below 7 μN (Figure 2c), whereas there exhibited the adhesion lower than 75 μN between the cantilever and the internal coating differently (Figure 2d). Indeed, what remained to be explained was the wavy curve in Figure 2c by the coating surface, to which the oil lubricant was added, adhering the cantilever actually. Anti-Icing Properties. Freezing delay times for the PDMS slippage coatings were recorded to evaluate the anti-icing performances at -15 °C, which was selected to match the temperature of the ice shear strength measurement. The freezing delay time of a water droplet on the samples was the time ranging from the startup moment to the onset of water freezing on the coating surface, as presented by the successive pictures captured by a microscope in Figure 3. Four steps involved in the process of water droplets freezing on the coating surfaces referring to water cooling, rapid kinetic freezing and isothermal freezing as well as ice cooling. The sample bPDMS showed a freezing delay time of 257 s, that was, the water droplet began to freeze as it dropped onto the coating surface for a while. By contrast, three PDMS slippage coatings exhibited a relatively longer freezing delay time of 504171, 508145 and 478110 s for the sample bPDMSO1, bPDMSO3 and bPDMSO4, respectively. The longest freezing delay

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time was detected for the sample bPDMSO3 with 2.9 wt% of POSS co-crosslinker, but without significant differences. It was supposed that the increased tendency of the freezing delay time was attributed to the relatively higher root-mean-squared roughness and hydrophobicity for the sample bPDMSO3 accompanied with lower subcooled temperature, giving rise to postpone the transfer of heat between the water droplet and the as-prepared icephobic coatings.

Figure 3. Icing process of water droplets on the surface of the PDMS slippage coatings at -15 °C. The ice shear strength was measured on the prepared icephobic coatings at -15 °C according to our previous studies.13,48 The thickness of the coatings was usually measured before ice shear strength tests in view of intimate links between the thickness and the ice shear strength.56-59 Figure 4a showed the ice shear strength of the icephobic coatings at the first icing/deicing cycle at -15 °C. The ice shear strength of the sample Sylgard 184 was 217.4±32.7 kPa owing to the higher ρCL and GF shown in Table 2, close to the reported value previously.60 However, the ice shear strength of the naked stainless steel sheet was 881.8±172.8 kPa at the first icing/deicing

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cycle, more than three times higher than that of the sample Sylgard 184. By contrast, in the absence of the POSS co-crosslinker and the PDMS lubricant, the sample bPDMS had an ice shear strength of 22.2±6.0 kPa, showing an around 90% reduction compared to the sample Sylgard 184. Above all, the as-prepared PDMS slippage coatings all exhibited a comparatively low ice shear strength, stable at approximately 11 kPa, which were preferred because ice could be shed off from such surfaces under natural forces such as wind,16 vibration and its gravity with the ultra-low ice shear strength (< 12 kPa).17 However, the influence of the POSS co-crosslinker contents on the ice shear strength of the as-prepared PDMS slippage coatings was not obvious significantly. Therefore, a series of PDMS slippage coatings with lower ice shear strengths owing to the lubricant introduction were prepared, accompanied by the addition of the functionalized POSS as a co-crosslinker and an enhancer. In addition, residual ice crystals on the surface of the as-prepared PDMS slippage coatings were photographed after the first deicing measurement at -15 °C, as shown in the insets of Figure 4a. Local icicles in the center of the sample Sylgard 184 and the naked stainless steel sheet could be seen after ice shear strength test, demonstrating that fractures occurred from the icicle, with a few of ice blocks remained on the surface, preventing the detachment of ice from the coating. Moreover, this phenomenon coincided with the relatively higher ice shear strength of the naked stainless steel sheet, with more residual icicles observed. On the contrary, there were no residual icicles on the surface of bPDMS and bPDMSO7, which was indicative of an adhesive failure rather than a cohesive failure at the interface of the PDMS slippage elastomers. More significantly, the PDMS slippage coatings exhibited lower ice shear strength than the sample bPDMS

and demonstrated the formation of a layer of oil lubricant, which was immiscible with

water, on the surface of as-prepared POSS-contained coatings.

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Figure 4. Ice shear strength values of the naked stainless steel sheet (uncoated), Sylgard 184 and the PDMS slippage coatings at the 1st icing/deicing test with the inset pictures (a), and ice shear strength variations of bPDMS (b), bPDMSO (c), bPDMSO1 (d), bPDMSO3 (e) and bPDMSO4

(f) during 50 icing/deicing cycles at -15 °C, respectively.

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Even though the ultra-low ice shear strength was essential for icephobic materials, the unlimited icing/deicing cycles followed by probable reduction in ice shear strength were also concerns for the icephobic durability of the coatings. Hydrogen bond, van der Waals forces as well as electrostatic interactions exerted on the adhesion of ice, in which electrostatic interactions was with the greatest effect.61,62 The adhesion of ice to the smooth hydrophobic coatings is lower than that to hydrophilic ones.22,23 As evidenced in Figure 4b-f, the ice shear strength of the sample bPDMS was 22.2±6.0 kPa at the first icing/deicing cycle in Figure 4a, followed by a steadily stable value at around 24 kPa during 50 icing/deicing cycles without remarkable increase in Figure 4b. The ice shear strength of the sample bPDMSO was found increased rapidly from 9.1±1.4 kPa to 15.5±1.4 kPa after 6 icing/deicing cycles, straight after keeping steady for the following icing/deicing cycles in Figure 4c. However, the initial lower ice shear strength value of the sample bPDMSO was mainly due to the sacrificial nature of the additive oil lubricant, which was desirable for its intrinsic poor mechanical property with a lower elastic modulus, as depicted in Figure 2. In contrast, the as-prepared PDMS slippage coatings maintained low ice shear strength values below 14 kPa even after 50 icing/deicing cycles measured at the same site in an identical process. A slight decline in the holistic average ice shear strength happened for the coatings with increased content of POSS co-crosslinker, ranging from 13.5 kPa of bPDMSO1 (Figure 4d) to 13 kPa of bPDMSO3 (Figure 4e) and 12.5 kPa of bPDMSO4 (Figure 4f). Considering Figure 2a and Figure 2b, the higher reduced Young’s modulus of the as-prepared POSS-contained PDMS slippage coatings demonstrated that the addition of the POSS co-crosslinker endowed the coatings in different depths with significantly improved stiffness, laying a negligible impact on the ice shear strengths during virtually unlimited icing/deicing cycles. As a result, the PDMS slippage coatings containing functionalized POSS possessed better

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icephobic stability and durability than the sample bPDMSO, which could be capable of inhibiting the wastage of the lubricant owing to the contribution of the POSS co-crosslinker. This was also indicative of interfacial slippage which could occur on surface of the POSS-contained coatings because of the unreactive PDMS polymer chains in the PDMS slippage elastomers for sliding ice and facilitating ice removal. To sum up, it is a reasonable assumption that the ice shear strength values of the as-prepared PDMS slippage coatings would hold steady even over 50 icing/deicing cycles or more, and the icing/deicing cycles could continuously proceed for further detection because there were no signs of wear, tear or damage. Such durable low ice shear strengths were reminiscent of an autonomous stick-slip motion in early indication,18 as illustrated in Figure 5. The icephobic coatings were cross-linked by original vinyl terminated PDMS and functionalized POSS as co-crosslinker, in which silicone oil was entrapped around the side chains of the branched PDMS in a homo-disperse form. When water droplets set in the cuvette turned into solid ice, a shear force was applied horizontally attempting to remove ice from the as-prepared coatings (Figure 5a). Silicone oil migrated towards the coatings surface gradually owing to its intrinsic osmotic pressure, forming a gradient distribution along the depth of the coatings represented by arrows at the bottom of the as-prepared coating in Figure 5b. This consequently led to the formation of a layer of oil lubricant illustrated in yellow onto the surface, with a relatively weak interaction exerted on the interface. At sufficiently high tensile stress, stick-slip motion of ice occurred under indirect contact between ice and the surface, resulting in slip off slowly till absolute detachment from the coatings. Such interfacial slippage was indicative of an adhesive failure as expected, which could be manifested by insets shown in Figure 5b with no residual icicles on the surface of coatings bPDMSO4

after the first and fiftieth icing/deicing cycles, respectively.

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Figure 5.

Schematic illustration of ice removal from the PDMS slippage coatings under a

stick-slip motion. A shear force was applied horizontally attempting to remove solid ice set in the cuvette from the coating (a). Silicone oil migrated towards the coatings surface gradually represented by arrows at the bottom, leading to the formation of a thin layer of oil lubricant illustrated in yellow onto the surface with a relatively weak interaction exerted on the interface. Ice slip off slowly till absolute detachment from the coating (b). Insets show the digital pictures of the sample bPDMSO4 after the 1st and 50th icing/deicing cycles. The relatively mild abrasion measurement (Figure 6a) was executed to mimic severe wear conditions and assess the abrasion resistance durability of the icephobic materials, as illustrated in Figure 6b. It was a remarkable fact that the as-prepared PDMS slippage elastomers retained their abrasion resistance and icephobicity, conducting up to 175 abrasion cycles with no prominent change in values of the ice shear strength, all of which were below 25 kPa. The poor abrasion resistance of the sample bPDMSO was exhibited, in response to the poor reduced Young’s modulus in Figure 2. To be more precise, the number of abrasion resistance of the 27



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as-prepared PDMS slippage coatings increased gradually with the adding content of POSS co-crosslinker, up to 175 cycles which belonged to the sample bPDMSO15.

Figure 6. Schematic of linear abrasion test (a) and changes of the ice shear strength value of the PDMS slippage coatings (b), measured at -15 °C after a series of abrasion cycles. Insets show the digital pictures of the samples after abrasion damage. Moreover, it was suggested that the cross-link density of the coatings was monotonically raised with the increased POSS co-crosslinker contents, together with the significant resistance to abrasion with relatively thin PDMS slippage elastomers less than 50 μm. Simultaneously, the added unreactive silicone oil was migrated onto the coatings surface and a layer of the oil lubricant was formed and spread automatically, leading to declined ice shear strength. Notably, silicone oil was refrained from dwindling fast on account of evaporation, tear and wear as well as capillary action after multiple icing/deicing cycles, so as to result in hardly increase in the ice shear strength, which coincided with the icephobic durability in Figure 4. Changes in morphology and variations of wettability, to be sure, had a significant impact on the resistance of abrasion and icephobic durability. Rougher surfaces were witnessed and modest 28


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increases in WCA and CAH values were measured, as depicted in Figure S10 and Table S2. Simultaneously, the residual coatings were photographed after the last abrasion cycles, as manifested in Figure 6b. It could be seen that the wear almost took place from the edge of the coatings, causing severe damage to the intermediate worn coatings unwillingly.

CONCLUSIONS

In this work, novel PDMS slippage coatings were developed with ultra-low ice shear strength and significantly improved abrasion resistance as well as icephobic durability. The coatings combined a branch-based PDMS elastomer matrix with a functionalized POSS co-crosslinker under the circumstance of the existence of silicone oil lubricant by a two-step procedure. Such low-modulus elastomer networks demonstrated extraordinary icephobic durability and stability with ice shear strength of 11.2±2.7 kPa, maintaining low ice shear strength values below 14 kPa even after 50 icing/deicing cycles. Furthermore, the introduction of the functionalized POSS co-crosslinker was capable of inhibiting the wastage of the oil lubricant simultaneously and posed a significant influence on the abrasion resistance, which could maintain the low ice shear strength even after 175 abrasion cycles. We envision that such coatings with extremely durable icephobicity for longevity may bring in a new design strategy and have a broad range for anti-icing applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge including 1H NMR and FTIR spectra of the branched PDMS and V3-POSS-PDMS5. EDS area scans, XPS spectra, TEM images and

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variation of water contact angle values of the residual PDMS slippage coatings are also included. AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

ORCID Xiaoyan Yuan: 0000-0002-2895-3730 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is financially sponsored by Natural Science Foundation of Tianjin, China (Nos. 18JCQNJC03800 and 14ZCZDGX00008) and National Natural Science Foundation of China (No. 51273146). REFERENCES (1) Parent, O.; Ilinca, A. Anti-Icing and De-Icing Techniques for Wind Turbines: Critical Review. Cold Reg. Sci. Technol. 2011, 65, 88-96. (2) Ryerson, C. C. Ice Protection of Offshore Platforms. Cold Reg. Sci. Technol. 2011, 65, 97-110. (3) Fakorede, O.; Feger, Z.; Ibrahim, H.; Ilinca, A.; Perron, J.; Masson, C. Ice Protection Systems for Wind Turbines in Cold Climate: Characteristics, Comparisons and Analysis. Renew. Sust. Energ. Rev. 2016, 65, 662-675.

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Table of Contents

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Icephobic Durability of Branched PDMS Slippage Coatings Co-Cross

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