Fabrication and characterization of non-fluoro based transparent easy-clean coating formulations optimized from molecular dynamics simulation

Progress in Organic Coatings 136 (2019) 105306

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Fabrication and characterization of non-fluoro based transparent easy-clean coating formulations optimized from molecular dynamics simulation Sushanta K. Sethi, Uday Shankar, Gaurav Manik

T

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Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Easy-cleaning Contact angle Interaction energy Molecular dynamics simulation Transparent

Polymer industries are increasingly adopting molecular dynamics simulations for the prediction of targeted properties before going for any material formulation and product development. In this investigation, the impact of co-polymerizing poly(vinyl acetate) (PVAc) with poly(dimethyl siloxane) (PDMS) in developing a suitable easy-cleaning polymeric coating material through both simulation and experimental validation has been reported. PDMS-g-PVAc with different wt.% of PVAc have been molecularly simulated and easy-cleaning ability has been assessed through estimation of properties such as surface energy, substrate adhesion and transparency. Subsequently, the pre-optimized 20 wt.% PDMS-g-PVAc was synthesized experimentally, coated on a substrate and characterized to validate the simulation estimates based on its thermal, optical and surface behavior. Interestingly, properties such as glass transition temperature (Tg), transparency and water contact angle (WCA) and transparency of the optimized PDMS-g-PVAc were found in good accordance with the experimental findings. Additionally, the surface coverage of the methyl (−CH3) groups and acetate (−OCOCH3) groups in PDMS-gPVAc were found to strongly dictate the surface and interfacial properties as confirmed through surface and Al substrate-interaction energy findings. The simulation protocols suggest reliability by displaying close agreements of simulated density (ρ), solubility parameter (δ ), refractive index and WCA of pristine PDMS and PVAc with the reported experimental values. When compared with poly(tetrafluoroethylene) (PTFE), the optimal non-fluoro based graft copolymer was found to exhibit similar or improved thermal degradation, transparency, surface energy and WCA. The significance of this work is believed to broaden the usage of the graft copolymer with 20 wt.% PVAc in PDMS (PDMS-g-PVAc) as an alternative material to fluoropolymers for applied as easy-cleaning coatings.

1. Introduction Over the past many decades, researchers have developed numerous coating materials [1–5] to protect various structural, automobile and aircraft from dirt, oil grease, etc. Among them, some coating materials have drawbacks of poor sustainability at high/low-temperature applications, weaker substrate adhesion, poor durability, etc. In general, low surface energy fluorine-based polymers such as PTFE [6] and PFO [7] are widely used for a broad variety of applications like anti-fouling, anti-reflection, anti-corrosion, anti-wetting, etc [3]. PTFE, a special hydrophobic polymer, exhibits WCA of ∼100° [8], transparency of ∼94% [9], and surface energy of 21.77 mN/m [10]. But such polymers have some major drawbacks such as they produce toxic products when subjected to overheating during application. Additionally, throughout their processing, they also require extra precautions in order to remove the off-gases produced due to the degradation of the product [11].

⁎

Consequently, there is a tremendous need to fabricate non-hazardous easy-cleaning coatings. Hence, in this study, we attempt to fabricate and simulate an easy-cleaning coating material using non-hazardous bio-compatible and bio-degradable polymers. Among several hydrophobic polymers, PDMS is one of the most popular bio-compatible and low surface energy polymer [12], that exhibits excellent easy-cleaning nature but has a drawback of poor substrate adhesion. PVAc, on the other hand, is bio-compatible and biodegradable [13], which has potential applications in coating and gluing industries [14] due to its better film forming ability. The introduction of multifaceted nature by incorporating PVAc in PDMS to form a graft copolymer could enhance the substrate interaction and thereby its durability alongside easy-cleaning potential. Such coating materials would ensure a clean surface when water drops make a reasonable kinetic impact on the dirty surface. The formulated coating material may additionally exemplify excellent properties like high thermal stability

Corresponding author. E-mail address: [email protected] (G. Manik).

https://doi.org/10.1016/j.porgcoat.2019.105306 Received 25 June 2019; Received in revised form 29 August 2019; Accepted 2 September 2019 Available online 10 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Progress in Organic Coatings 136 (2019) 105306

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forcefield has broad coverage and specifically been designed for the major material science applications [20]. Additionally, in our previous study [21] the same forcefield have shown good agreements with experimental reported results for pristine PDMS and PVAc. For amorphous cell construction, chains of PDMS-g-PVAc of different chain lengths (n = 5, 10, 15, 20, 25 and 30) were generated and subsequently subjected for geometry optimization in order to relax the structures and to eliminate the chain biasedness. Ewald Summation method [22] was considered for electrostatic interactions with an accuracy level of 0.001 kcal/mol/Å and an Atom-based summation method adopted for van der Waals’ interactions. Energy minimized configurations were chosen for each composition to construct amorphous cells for performing MD simulations. The procedure for cell equilibration and estimation of critical chain length (nc) was similar to our previously reported [17,21] methods. The sample nomenclature chosen for ease of description was 20PV80PD, 40PV60PD, 60PV40PD and 80PV20PD for 20, 40, 60 and 80 wt.% of PVAc in PDMS.

and low surface energy due to the presence of PDMS. With the growing efficiency and availability of high-speed computers, molecular dynamics (MD) simulations at atomistic and coarsegrain scale provide the user an option in microscopic modeling of the next generation polymers. Now, researchers extensively use such computational tools [15–17] to make simulation-based cost-effective predictions through allowing individual molecules for movement, based on their nature and interaction with the surrounding atoms, and thereby, estimate the essential performance properties. Thus, it might be said that the simulation relies on basic theoretical foundations and replaces the approximations normally associated with the theory by elaborate calculations effectively and efficiently. This paper explores and presents two studies: (1) optimization of PVAc fraction in the PDMS-g-PVAc based on the water and oil contact angles (WCA/OCA), substrate adhesion, transparency and surface energy using MD simulations, followed by, (2) synthesis and characterization of the optimized PDMS-g-PVAc and validation with the simulated estimates. In addition, a comparative analysis of simulation outcomes with the previously available literature has also been undertaken.

3.1. Density (ρ) and solubility parameter (δ ) estimation We prepared initial random structures of pristine PVAc and PDMS, and their grafted copolymers, at different PVAc concentrations using the amorphous cell construction function of the software. Subsequently, the geometry optimization has been accomplished in order to eliminate the possible chain overlapping during the formation of initial simulation cells. The ρ and δ have been estimated using Forcite module for structures with different chain lengths, and thereby, used them for predicting nc based on the saturations obtained for performing further simulations.

2. Material selection and synthesis procedure 2.1. Materials Vinyl Acetate monomer (VAc, 99% purity) and Sylgard 184 were obtained from TCI Chemicals India Pvt. Ltd. and Dow Corning, respectively. Both the monomers contained hydroquinone inhibitor required to be purified before use for synthesis as per the previously reported method [18]. Aluminum oxide (activated, basic, Brockmann) and 2,2’-azobisisobutyronitrile (AIBN) were supplied from Sigma-Aldrich and purified by recrystallization method before use. Dimethylformamide (DMF, 99.9% purity) was obtained from Alfa Aesar. The other chemicals sourced were analytical grade and used as received.

3.2. Glass transition temperature (Tg) estimation The paints and coatings generally need a higher surface hardness and lower tackiness at the process temperatures which acquires by higher Tg. Thereby, the obtained coating shall provide adequate hardness and sufficient dirt pick up resistance. For the estimation of Tg, the earlier obtained equilibrated simulation cells were subjected a 50 ps MD in NPT ensemble to obtain specific volume at different temperatures in a temperature range of 90–460 K separated by 20 K. Later on, the inflection points were noted in the obtained plots of specific volume versus temperature curves and considered as Tg. The details of the method have been described in our previous article [21].

2.2. Synthesis of grafted copolymer PDMS-g-PVAc The solution polymerization technique was used to synthesize grafted copolymer (PDMS-coPVAc)) of DMS and VAc monomer. The reaction was executed in a round-bottomed flask containing 200 ml of DMF solution with DMS and VAc monomers (80:20 wt.%) and AIBN as an initiator (1 wt.% of the monomers content). The reaction was carried out at 80 °C for 6 h as shown in Scheme 1. Subsequently, the synthesized product has been precipitated by the addition of an excess of butanol and then dried at 60 °C in a vacuum oven.

3.3. Contact angle analysis with water and oil (decane) as testing fluids To quantify the phobicity/philicity of the coating material, the wellknown SPC/E water model originally developed by Berendsen et al. [23] was used for this study. Around 1000 water molecules were considered and Shake procedure [24] employed to maintain the internal geometry of SPC/E water molecules. In order to ascertain the oleophobic nature of the coating material 300 oil-representative decane molecules were simulated separately. In this line, Cansoy et al. [25] studied the contact angles of graft copolymer surfaces against low surface energy (LSE) liquids of different chain lengths such as octane, decane, tetradecane and hexadecane. The oleophobicity of the surfaces was found to be dependent on the liquid surface tension. They found that the liquid chain length controls the cohesion among its molecules, and hence, a larger chain length liquid would be expected to have high cohesion, and hence, increased contact angles when applied on a surface. Hence, in our case if the coating surface sufficiently repels decane then it may be assumed that it would show even better repellency against oils with higher chain length. In a similar way, 300 oil-representative decane molecules were simulated separately in order to ascertain the oleophobic nature of the coating material. After density equilibration, these water and oil droplets were kept on the coating

3. Computational strategy All simulation estimations were accomplished using Material Studio 7.0 (MS) [19]. To investigate the condensed phase and easy-clean properties of the coating material, a general all-atom COMPASS forcefield for all atomistic simulations has been considered. The COMPASS

Scheme 1. Synthesis of the graft copolymer (PVAc-g-PDMS) by free radical polymerization. 2

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γ=

EThin film − EAmorphous cell 2A

(3)

Where EThin film is the simulated energy of thin film, EAmorphous cell represents the energy of the previously simulated equilibrated bulk amorphous cells, and 2A refers to the area of two surfaces due to the generation of thin film. Fig. 1. Illustration of the generated initial structure of water droplet on a presimulated PDMS.

3.6. Optical transparency The CASTEP module of the Material studio software was employed for the estimation of refractive indices (RI) of PVAc, PDMS and their grafted copolymers. The density functional theory using a planewave pseudopotential method used in these calculations. The exchange-correlation effects were treated within the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof functional [26]. The geometries for all the systems were optimized, using the conjugate gradient technique in a direct minimization of the Kohn–Sham energy function which employs pseudopotentials to represent core electrons. Plane-wave functions were used as basis [27] and plane-wave cut-off energy of 340 eV has been employed throughout. Reflectance (R) and transmittance (T) of any material can be estimated from the RI values using the following Fresnel equations:

materials individually using a build layer option as shown in Fig. 1. Following this, geometry optimization was carried out to refine the structure until the forces and stresses became lower and saturate. After this, NVT MD simulations were performed COMPASS forcefield at T =298 K for 1000 ps with a time step of 1 fs using a Nosé–Hoover thermostat (with Q ratio of 0.01) to allow the droplets to reasonably interact with the coating materials. Kumar et al. [7] observed that there is no significant change in the contact angles beyond 500 ps time period. Hence, in this investigation, we thought of simulating all the contact angle profiles upto 1000 ps time period to ensure contact angle equilibration. 3.4. Alumina (Al)/ PDMS-g-PVAc interface: Strength/interaction energy assessment

2

R − RA ⎞ R=⎛ m ⎝ Rm + RA ⎠ ⎜

For the durability of the polymer materials applied to chosen substrates, the energy of interaction between polymer and substrate needs to be predicted. The polymer coating-substrate adhesion is primarily due to van der Waals forces present at their interface, specifically when the substrate is metal. The interaction energy has been estimated in the similar way as estimated in our previous study [17]. Here, aluminum (Al) has been chosen as a substrate as it possesses lower surface energy among all metals. If the coating material adheres to it successfully, then it shall show sufficient adhesion to all other metals. For the estimation of the binding energy of the coating material with Al substrate through simulation, the following equation has been used.

EInteraction = ETotal − (EAl + EPolymer )

⎟

(4)

2

4Rm RA ⎞ T=⎛ ⎝ Rm + RA ⎠

(5)

R+T=1

(6)

⎜

⎟

Where, Rm and RA are the RI values of the medium and air (RA = 1) respectively. 4. Experimental strategies 4.1. FTIR

(1)

The Fourier transform infrared spectra of the synthesized graft copolymer were recorded by L1600400 Spectrum Two DTGS, UK spectrophotometer using KBr pellets, in the range of 4000-400 cm−1 in transmittance mode.

Where ETotal is the simulated energy of Al and polymer system taken together, EAl and EPolymer are the potential energy of the separately simulated amorphous cells of Al and polymer. The total potential energy of a system is the combination of different energies [17] given below:

4.2. Thermal characterization

EPotential = ETotal bond + ENon − bond + ECross = Eb + Eθ + Eϕ + EvdW + EElectrostatic + ECross

4.2.1. Differential scanning calorimetry (DSC) study Tg has been investigated by using DSC on a Netzsch DSC 200 differential scanning calorimeter, using an empty aluminum pan as a reference. The sample was first cooled to −85 °C from room temperature and then heated upto 200 °C, at a linear heating rate of 10 °C/min. Tg was considered as the midpoint of the endothermic step transition. The standard aluminum pan was used for testing the sample.

(2)

Here, total bond energy (ETotal bond ) comprises of bond stretching energy (Eb ), angle bending energy (Eθ ), and dihedral torsion energy (Eϕ ). The non-bond energy (ENon − bond ) is composed of van der Waals energy (EvdW ), and electrostatic energy (EElectrostatic ). ECross is the energy of cross terms between any two of the bonded energy, such as the bond-angle cross term and the bond-bond cross term.

4.2.2. Thermal gravimetric analysis (TGA) Thermogravimetric analysis (TGA) was performed on a NETZSCH simultaneous thermal analyzer (STA-429). Samples weighing ∼5.0 mg were heated under a nitrogen atmosphere from 25-800° C at a heating rate of 10 °C/min.

3.5. Surface free energy (γ) analysis The γ of a coating material suggests about the availability of free energy on the surface to adhere to other material. γ is a characteristic factor which affects several surface properties such as adsorption, wetting and adhesion. Hence, γ forms the basis of interest in understanding surface science in the field of adhesive, biomedical and easycleaning coating applications. It can be said that a very high wettability is achieved on substrates with high γ, since, they possess higher available surface free energy to interact with polymers that are applied as coating materials on them. The γ of the coating materials have been estimated from MD simulation using the following equation:

4.3. Contact angle The most widely used method for measuring contact angles is the Sessile drop method [28]. Here, the sessile drop contact angle analyzer tool has been used in order to assess the hydrophobicity of the optimized sample uniformly coated on a glass slide. For the measurement, ∼6 μl of distilled water was dispensed onto the coated surface under investigation and image captured. The same procedure was repeated for 3

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4 samples and the average estimate are reported.

Table 1 Simulated density and solubility parameter values of pristine and their graft copolymers.

4.4. Transparency

Polymer

The UV–vis transmittance spectra were recorded by UV-spectrophotometer (UV1800, Shimadzu Corporation, Japan) for measuring the optical transparency of the synthesized sample.

PDMS 20PV80PD 40PV60PD 60PV40PD 80PV20PD PVAc

5. Results and discussion 5.1. Critical chain length (nc) It is difficult to use the real-time polymer chain lengths in simulations due to the need for enormous computation efforts. Indeed, even for simulations, our longest chain length (n = 50), extremely long runs were required to gather adequate results. Hence, to replicate real-time polymer behavior from simulations and to identify critical chain length (nc) needed for the same, different amorphous cells were constructed with varying initial chain lengths (n = 5, 10, 15, 20, 25 and 30). In designing new polymers and prediction of their bulk properties, ρ is an important parameter. Thus, there is a need to standardize MD simulation protocols to compute ρ of the polymers. In this case, the energy minimization followed by cell equilibrium was executed for all generated amorphous cells using NPT ensemble by employing COMPASS forcefield to compute average ρ and δ for all equilibrated amorphous cells. The saturation observed in ρ and δ with increasing chain lengths is clear evident from Fig. 2. For pristine PDMS and PVAc, nc = 26 has been considered as already obtained in our previous study [21]. It is evident from Fig. 2 that ρ and δ start saturating beyond n = 20, 20, 15 and 10 for 20PV80PD, 40PV60PD, 60PV40PD and 80PV20PD, respectively. Hence, it is apparent that for different wt.% PDMS-co-PVAc a greater chain length of nc = 25 has been safely considered for PDMS-co-PVAc for further simulations. Post-screening of threshold chain length has been estimated followed by ρ and δ reported (at nc = 25) for different wt.% PDMS-gPVAc in the next section.

Density (gcm−3)

Solubility parameter (J/cm3)1/2

Simulated

Reported

Simulated

Reported

1.025 1.059 1.092 1.102 1.103 1.132

0.97 [29] NA NA NA NA 1.22 [31], 1.2 [32]

13.2 ± 0.05 14.808 15.157 15.829 16.829 17.5 ± 0.05

14.1 [30] NA NA NA NA 17.6 [33], 17.7 [32]

± ± ± ± ± ±

0.03 0.011 0.01 0.07 0.011 0.01

the reported values which suggests the accuracy of the simulation protocol adopted in this work. For instance, the computed ρ of PDMS and PVAc are 1.025 and 1.132 gcm−3, which are in good agreement with reported estimates of 0.97 [29] and 1.22 [31] gcm−3 respectively. Similarly, the computed δ of PDMS and PVAc are 13.2 and 17.5 (J/ cm3)1/2 are in good agreement with the reported values 14.1 [30] and 17.6 [33] (J/cm3)1/2 respectively. This suggests the protocols used in the simulation are quite reliable to compute densities and solubility parameters of systems like PDMS-g-PVAc for which these values and other performance properties are unknown. 5.3. Glass transition temperature (Tg) Pristine PDMS possessing a low Tg of −105 [34] when grafted with PVAc of Tg∼ 30 °C [35] would exhibit a higher desired Tg. The Tg of the different wt.% PDMS-g-PVAc systems were identified [21] from the transitions noticed in specific volume versus temperature graphs as shown in Fig. 3, and are tabulated in Table 2. The Tg for pristine polymers: PDMS and PVAc are consistent with reported literature values depicted in Table 2. This proximity of simulated estimates for pure polymers with reported values further validate the simulation protocols in computing Tg for PDMS-g-PVAc. Evidently, Tg found to increase on increasing the PVAc content in PDMS. Any Tg transition ascribed to the PDMS or PVAc content was not found elsewhere for the tested samples.

5.2. Density (ρ) and solubility parameter (δ) The so-obtained saturated estimates of ρ and δ of pristine polymers and their graft copolymers at different wt.% of PVAc are tabulated in Table 1. The simulated values of pristine polymers (PVAc and PDMS) have been compared with previously reported literature but for the remaining graft copolymer samples, the ρ and δ were not available in the literature for comparison. The simulated ρ and δ of pristine PDMS and PVAc are in accord with

5.4. Water and oil contact angle The water droplets simulated separately using SPC/E models are placed on the polymeric coating materials in order to quantify surface hydrophobicity. Similarly, to quantify oleophobicity pre-simulated oil droplets (decane) were placed on a simulated polymer surface to obtain

Fig. 2. The saturation observed in (a) density and (b) solubility parameter of PDMS-g-PVAc at different wt.% of PVAc as a function of chain length. 4

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Fig. 3. Illustration of glass transition temperature (Tg) of (a) 20PV80PD (b) 40PV60PD (c) 60PV40PD and (d) 80PV20PD.

surface displayed the highest WCA among all simulated graft copolymers. It is apparent that increasing the PVAc content reduces the hydrophobic methyl groups concentration and increases the hydrophilicity polar groups inducing on the surface which translates into increased hydrophilicity. The higher contribution of acetate groups at surface induces higher interaction with water molecules, and thereby, displays higher WCA’s. An examination of the interaction of simulated polymers and graft copolymers with oil (decane) molecules indicates that the pristine PDMS and PVAc display highest (51°) and lowest (< 20°) OCA respectively. Such estimates are in good accordance with the previously reported results of 58° [39], 52° [40] and 10° [7]. Further, by increasing the PVAc concentration in PDMS-g-PVAc from 20 to 80 wt.%, the OCA reduces from 40° to < 20° which suggests about the higher possible interaction of decane molecules with PVAc than PDMS. Since least interaction with both water and oil is desirable while fabricating an easycleaning coating, hence, PVAc concentration may be kept limited to 20 wt.% in the graft copolymer to achieve a reasonable hydrophobicity and decent oleophobic nature. Further, the obtained WCA for 20PV80PD of 99° is well comparable with the reported WCA of PTFE coating which is ∼100° [8].

Table 2 Estimated glass transition temperature (Tg) peaks (in °C) of different polymers. Polymer

Simulated

Reported Tg

PDMS 20PV80PD 40PV60PD 60PV40PD 80PV20PD PVAc

−105 −63 −43 −10 10 30

−122.8 [36], −105 [34] NA NA NA NA 30 [35], 28 [32]

a contact angle profile. The obtained profiles have been depicted in Fig. 4, and the estimates tabulated in Table 3. The contact angles were estimated using Hauntman and Klein equation [37]. Simulations predicted a WCA of ∼102 ± 1° and 62 ± 2° for pristine PDMS and PVAc respectively, which are in line with the reported values and validate the simulations methodologies employed. The PVAc and PDMS exhibit hydrophilicity and hydrophobicity because of the polar acetate (−OCOCH3) and non-polar methyl (−CH3) groups inherently present in the side chains as depicted in Fig. 5. To validate this, the concentration of such polar and non-polar groups on the coating surface, expressed in terms of surface coverage, has been estimated and depicted in Fig. 6. As the van der Waals’ and electrostatic interaction cut off was limited to 15.5 Å, hence, the surface coverage assessment was considered only till this section in the previously generated super cells. It was found that the concentration of methyl groups reduced significantly from 61.13% to 36.73% as the PVAc content in graft copolymer is increased from 20 to 80 wt.%. Incidentally, the 20PV80PD system consisting of the highest wt.% of methyl groups on

5.5. Strength of alumina (Al)/ PDMS-g-PVAc interface The interaction of coating material with its substrate is influenced mainly by the chemical properties of its constituents as well as the nature of the surface. While the methyl groups present on the PDMS surface may provide hydrophobicity but may undesirably prevent or reduce its interaction with Al substrate, thereby, reduce its adhesion or 5

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Fig. 4. Snapshots of contact angle profiles of water and oil droplets on (a) PDMS (b) 20PV80PD (c) 40PV60PD (d) 60PV40PD (e) 80PV20PD and (f) PVAc.

The interaction energies and its different components estimated using Eq. (1) and (2) have been tabulated in Table 4. Higher the absolute interaction energy higher will be the interaction between polymer and substrate. In this line, it is quite apparent from the results that due to a much higher absolute interaction energy pristine PVAc

sticking ability. The polar acetate group present in PVAc may provide sufficient van der Waals’ interaction with a substrate for the formulation. Hence, the aim of enhancing the interaction between coating material and substrate, without compromising much with its oleo/hydrophobicity, may be aided by synthesizing their grafted copolymer. 6

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Table 3 Simulated water and oil contact angles of pristine PDMS and PVAc and their graft copolymers. Polymer

PDMS 20PV80PD 40PV60PD 60PV40PD 80PV20PD PVAc

WCA

OCA

Simulated

Reported

Simulated

Reported

102 ± 1° 99 ± 1° 91 ± 1° 76 ± 2° 71 ± 2° 62 ± 2°

109° [38] NA NA NA NA 58° [7]

51 40 34 27 < <

58° [39],52° [40] NA NA NA NA < 10° [7]

± 1° ± 2° ± 2° ± 2° 20° 20°

would show much higher substrate interaction than PDMS. Hence, with increasing PVAc concentration, the grafted copolymers shall provide increased substrate interaction due to increasing acetate groups coverage at the polymer-substrate interface. These observations provide compelling evidence that the interactions of polymeric coating material with Al can be fine-tuned by varying PVAc concentration. In addition to this, though the interaction with Al increased with PVAc concentration, it was not found to increase significantly even upto 80 wt.% PVAc content due to a possible seize of segmental motion of PVAc acetate groups by the present methyl groups of PDMS. The interaction of the proposed coating with the Al substrate is majorly influenced by the van der Waals’ interaction of the acetate groups present in the PDMS-g-PVAc. In order to validate this, one oxygen atom in each PDMS and PVAc, located at the same distance from Al have been tracked in the obtained trajectories, and their migration towards the substrate with time was noticed. It was observed for all the PDMS-g-PVAc systems that, while starting positions of both the oxygen atoms from the interface are same, the one in PVAc migrates and reaches the Al substrate faster (Fig. 7). This clearly speaks about the possible higher interaction induced by the PVAc addition to PDMS. Since this investigation was performed by only considering a single atom, hence, the surface coverage in the Al-polymer interface has been examined further. The surface coverage in terms of concentration of acetate and methyl groups have been measured within a 15.5 Å distance from the top surface of Al for both initial (at t =0 ps) and final structure (at t =300 ps) and illustrated in Fig. 8. It was observed for all the PDMS-gPVAc systems that, there is a significant increase in the concentration of acetate groups with simulation progress in the intermediate 15.5 Å thickness. For instance, in 20PV80PD sample, the acetate group concentration in the initial structure was found to be 26.96% which increased upto 30.32% in 300 ps time period. These findings clearly elucidate that the PDMS-g-PVAc structure quickly reorients itself by the driving van der Waals’ force, and thereby, acetate groups reach the substrate faster. It is quite evident from the results tabulated in Table 4, that bonded energies do not play significant role in deciding the overall system interaction with Al. however, among non-bonded energies van der Waals’

Fig. 6. Surface coverage of acetate groups of PVAc and methyl groups of PDMS within the top 15.5 Å of simulated PDMS-g-PVAc cells.

energies were much higher than electrostatic component and found to significantly decide the overall interaction of studied systems against chosen substrate. Hence, it is proposed that the wt.% of PVAc in graft copolymers may be limited to 40 wt.% which may provide sufficient substrate interaction, and yet maintains sufficient surface hydrophobicity. Further, the substrate interaction capability of a polymeric material can also be directly correlated with its surface free energy; Higher the surface free energy, higher will be its interaction capability. Hence, the estimation of surface energy for different simulated graft copolymers has been performed and presented in the next section.

5.6. Surface energy (γ) Techniques to fabricate a hydrophobic surface rely on lowering the γ, which explains whether the water droplet will spread over or form a spherical shape above the surface. It controls the coating-fluid adhesive force that inherently measures the polymer coating affinity towards the substrate. The γ estimated using Eq. 3 has been tabulated in Table 5 for the pristine polymers and their co-polymers. The computed γ of 20.35 and 39.3 mJ/m2 for pristine PDMS and PVAc are in proximity to the reported literature values of 19.8 [40] and 36.5 mJ/m2 [40,41]. It is quite apparent that the latter exhibits much higher γ than the former. Therefore, the γ of all simulated graft copolymers is greater than that of pristine PDMS due to increased contribution from PVAc. Additionally, a comparison of results of γ with the interaction energy trends (section 5.5) indicates that such graft copolymers with a higher γ may also show higher interaction ability. From these findings, it may be proposed that the wt.% of PVAc in grafted

Fig. 5. Schematic representation of the chemical structure of (a) PVAc and (b) PDMS elucidating the polar acetate (−OCOCH3) and non-polar (−CH3) groups present on the surface. 7

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Table 4 Different energy parameters (in (mN/m)) between Al and different polymers undertaken. Polymer

PDMS 20PV80PD 40PV60PD 60PV40PD 80PV20PD PVAc

Ebond

Enon-bond

Eb

Eθ

Eφ

EvdW

Eelectrostatic

−6.87 −7.24 −3.95 −16.43 −8.53 −15.45

16.11 −2.20 −4.51 −5.78 −6.40 −5.43

−10.27 5.58 4.80 4.81 −5.68 11.33

−362.59 −460.33 −472.72 −474.74 −476.96 −480.43

−3.41 −11.44 −10.32 −9.48 −8.30 −8.11

copolymers should be limited to 40 wt.% in order to maintain a low γ along with good interaction with Al substrate which may provide sufficient adhesion ability and simultaneously lower affinity towards contact fluids water and oil. We can also correlate these findings with WCA results presented in section 5.4, wherein WCA decreased likewise with increasing PVAc content. Additionally, the obtained γ upto 40 wt. % of PVAc in PDMS-g-PVAc has been found to be very close to the reported γ of 21 mJ/m2 for PTFE, [10] thereby, indicating that developed graft copolymers could potentially substitute fluoro based PTFE in such surface application.

Ecross

Interaction energy

−3.36 −0.09 −0.09 −0.09 −0.09 −47.15

−370.34 −475.71 −486.80 −490.27 −502.31 −545.24

for several potential applications such as on the surface of solar cell panels, elastomeric smart windows and safety glasses. A previously transparent object may become opaque when coated with translucent or opaque coating materials. This may impair the usability of the aforementioned products. Hence, optical properties of the graft copolymer coating materials have been assessed through the estimation of RI values which have been tabulated and compared with reported values in Table 6. As depicted in Fig. 9 an optical transmittance of > 95% has been observed for all grafted copolymer compositions of PVAc and PDMS in the visible light wavelength (300–700 nm). Both the polymers used for this study have been reported earlier by Musetti et al. [43] and Wang et al. [44] to provide good transparency levels. Therefore, a combination of such chosen polymers may sufficiently provide a favorable

5.7. Optical properties Transparency in easy-cleaning coatings carries significant interest

Fig. 7. Migration of oxygen atom of PVAc and PDMS towards Al substrate in (a) 20PV80PD (b) 40PV60PD (c) 600PV40PD and (d) 80PV20PD. A visual of oxygen atom migration in 20PV80PD has also been provided in the supplementary video. 8

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Fig. 8. Surface coverage by acetate and methyl groups in different PDMS-g-PVAc simulated systems observed within 15.5 Å from Al surface at (a) t =0 ps and (b) t =300 ps of simulation time. Table 5 Simulated surface energy for different polymers. Polymer

PDMS 20PV80PD 40PV60PD 60PV40PD 80PV20PD PVAc

Surface energy (mJ/m2) Simulated

Reported

20.35 22.08 23.41 25.33 27.81 39.3

19.8 [40], 21.3 [29] NA NA NA NA 36.5 [40,41]

Table 6 Estimated refractive index values of pristine PDMS, PVAc, and their graft copolymers. Polymer

PDMS 20PV80PD 40PV60PD 60PV40PD 80PV20PD PVAc

Refractive index Simulated

Reported

1.4818–1.5569 1.4771–1.5414 1.4894–1.5689 1.4823–1.5713 1.4814–1.5743 1.4812–1.5497

1.403 [42] NA NA NA NA 1.4814 [42]

Fig. 9. Illustration of the transparency curve of pristine PDMS, PVAc and their graft copolymers obtained from MD simulation.

optical transparent easy-cleaning coating which is also well comparable with the reported transparency level of ∼94% for PTFE coating [9]. 6. How trustworthy are such simulation predictions obtained from MD simulation? Based on MD simulation studies discussed previously, the grafted copolymer 20PV80PD was found the best-suited candidate for easycleaning coating based on estimated WCA/OCA, surface energy, interaction energy and transparency. Hence, a need was felt to extensively characterize the screened formulation further and validate the previously obtained simulation results with experimental findings. Fig. 10. FTIR analysis of PVAc, PDMS, and PDMS-g-PVAc (20PV80PD).

6.1. FTIR Fig. 10 shows the overlaid spectra of FTIR of PVAc, PDMS, and PDMS-gPVAc (20PV80PD). The FTIR spectra show absorption peak at 1750 cm−1 which is attributed to the -C = O stretch in the ester group of PVAc which is present both in PVAc and PDMS-g-PVAc. Also, in

FTIR studies of synthesized graft copolymer have been carried out along with monomers in order to validate the presence of several functional groups and structural changes occurring due to grafting. 9

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6.2. Thermal characterization 6.2.1. Glass transition temperature (Tg) Literature studies indicate that PDMS possesses a Tg of −105 °C [34] while for PVAc it is in the range of 28–30 °C [32,35]. Hence, Tg of its grafted copolymer is expected to theoretically lie in between. The obtained DSC plot of 20PV80PD has been depicted in Fig. 11, where the Tg has been captured to be −67 °C. This estimated Tg is in very good coincidence with that from simulations, as described earlier in section 5.3, for the same graft copolymer (20PV80PD). 6.2.2. Thermal degradation behavior Fig. 12 illustrates the results of a thermogravimetric study of pristine PDMS, PVAc and 20PV80PD performed in an inert nitrogen atmosphere. It can be elucidated from the figure that, decomposition starts slowly at T ∼260 °C, while marginal decomposition (5 wt.% loss) is observed at 338 °C which shows a synergistic effect over pristine PVAc (227 °C) and PDMS (306 °C). TGA curve of the sample indicates a broad temperature range (350–600 °C) of decompositions from 100% to 40%. The improved thermal degradation of the proposed grafted polymer (20PV80PD) is an added advantage for a coating that is subjected to high-temperature application.

Fig. 11. Differential Scanning Calorimetry plot of the grafted copolymer, PDMS-g-PVAc (20PV80PD).

PDMS-g-PVAc peaks appear at 1261 cm−1 due to the CH3 deformations in Si−CH3, at 792 cm−1 characteristic of Si-C stretching and − CH3 rocking in Si−CH3 linkages. The absorption peak appears at 1020 to 1070 cm−1 due to Si–O stretching. In summary, the FTIR study confirms the formation of PDMS-g-PVAc and validates the synthesis technique adopted.

6.3. Contact angle The surface wettability was investigated by Sessile droplet method using a contact angle analyzer. Fig. 13 depicts the surface profile of a water droplet on grafted system (20PV80PD) surface pre-coated on a

Fig. 12. Thermal degradation profiles of (a) PVAc (b) PDMS and (c) 20PV80PD. 10

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Fig. 13. Snapshot of water contact angle profile of (a) PDMS (b) PVAc and (c) 20PV80PD obtained from sessile droplet analyzer.

7. Conclusion and future recommendations

glass substrate. WCA was found to be 98° at room temperature (25 °C) which is in close agreement with the simulated estimates of 99° as described earlier in section 5.4. This confirmation ensures that graft copolymer retains its higher water repellency nature despite incorporation of 20 wt.% of PVAc in PDMS backbone.

Different wt.% of PVAc (20, 40, 60 and 80 wt.%) in PDMS-g-PVAc were simulated and targeted properties such as water/oil contact angle, surface energy, interaction energy, transparency, and Tg evaluated. Subsequently, 20 wt.% PVAc containing grafted copolymer (20PV80PD) was screened based on molecular simulated data like optimal contact angles (WCA and OCA), surface energy, and substrate interaction energy. Among all co-polymer formulations, 20PV80PD exhibited the lowest surface energy, a WCA of 99° and reasonable substrate adhesion ability, and was considered for experimental validation and further characterization. When analytically explored, this optimized polymeric coating displayed similar performance as PTFE coating with analogous WCA value (99° vs 100°), slightly improved transparency (95% versus 94%), and comparable surface energy (21.77 versus 22.08 mJ/m2). The thermal degradation results revealed the improved thermal stability of optimized grafted copolymer (20PV80PD) with a T5wt.% loss of 338 °C versus pristine polymers (PVAc∼227 °C and PDMS∼306 °C). Furthermore, the experimental results such as WCA, Tg and transparency of optimized PDMS-g-PVAc structure were found to be in close coincidence with the simulation findings validating the simulation protocols adopted. Compared to commercial fluoropolymers (PTFE), PDMS-g-PVAc showed easy-clean nature and similar performance in an addition to offering non-hazardous nature. While this approach extensively explores an optimal PDMS-g-PVAc matrix, the future work further may involve the addition of specific fillers to improve the easy-cleaning nature, especially by reducing the interaction with oil or oil-like materials. The development of the next generation coating may be oriented by employing appropriate fabrication techniques to generate suitable surface roughness with further higher contact angles.

6.4. Transparency Both the polymers PVAc and PDMS used for synthesis are colorless and transparent in nature. Hence, a combination of these two will definitely demonstrate good transparency level when applied as a coating. Different glass slides pre-coated with PVAc, PDMS and 20PV80PD displayed an optical transmittance of > 95% in the visible wavelength as depicted in Fig. 14. This finding is in close agreement with the simulation estimates described earlier in section 5.7.

Declaration of Competing Interest There are no conflicts to declare.

Fig. 14. Illustration of optical transmittance (as a function of wavelength) of pristine PDMS, PVAc and their grafted copolymer PDMS-g-PVAc (20PV80PD) obtained using UV–vis spectrometer. 11

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Acknowledgments [18]

The first and second authors are thankful to the Ministry of Human Resource Development (MHRD), India, for providing financial support in the form of Senior Research Fellowship. The authors would also like to thank Professor Rishi Pal Chauhan, (Head, Department of Physics), National Institute of Technology, Kurukshetra for allowing to run CASTEP module of Material Studio in his lab.

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[21]

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105306.

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