Aqueous-Based Fabrication of Low-VOC Nanostructured Block

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Aqueous-Based Fabrication of Low-VOC Nanostructured Block Copolymer Films as Potential Marine Antifouling Coatings Kris S Kim, Nikhil Gunari, Drew MacNeil, John A. Finlay, Maureen E. Callow, James A. Callow, and Gilbert C Walker ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04629 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 24, 2016

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ACS Applied Materials & Interfaces

Aqueous-Based Fabrication of Low-VOC Nanostructured Block Copolymer Films as Potential Marine Antifouling Coatings Kris S. Kim±, Nikhil Gunari±, Drew MacNeil±, John Finlayǂ, Maureen Callowǂ, James Callowǂ, Gilbert C. Walker±,* ±

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H4, Canada

ǂ

School of Biosciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT UK

KEYWORDS: marine fouling, dissolution, phase transfer, adhesion forces, zoospore settlement Abstract The ability to fabricate nanostructured films by exploiting the phenomenon of microphase separation has made block copolymers an invaluable tool for a wide array of coating applications. Standard approaches to engineering nano-domains commonly involve the application of organic solvents, either through dissolution or annealing protocols, resulting in the release of volatile organic compounds (VOCs). In this paper, an aqueous-based method of fabricating low-VOC nanostructured block copolymer films is presented. The reported procedure allows for the phase transfer of water insoluble triblock copolymer, poly (styrene-block-2 vinyl pyridine-block-ethylene oxide) (PS-b-P2VP-b-PEO), from a 1

ACS Paragon Plus Environment


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water immiscible phase to an aqueous environment with the assistance of a diblock copolymeric phase transfer agent, poly (styrene-block-ethylene oxide) (PS-b-PEO). Phase transfer into the aqueous phase results in self-assembly of PS-b-P2VP-b-PEO into core-shell-corona micelles, which are characterized by dynamic light scattering techniques. The films that result from coating the micellar solution onto Si/SiO2 surfaces exhibit nanoscale features which disrupt the ability of a model foulant, a zoospore of Ulva linza, to settle. The multilayered architecture consists of a pH-responsive P2VP-“shell” which can be stimulated to control the size of these features. The ability of these nanostructured thin films to resist protein adsorption and serve as potential marine antifouling coatings is supported through atomic force microscopy (AFM) and analysis of the settlement of Ulva linza zoospore. Field trials of the surfaces in a natural environment show the inhibition of macrofoulants for 1 month.

Introduction The development of various coating materials for antifouling applications has been of high interest in a wide variety of fields; ranging from coating biomedical implants and biosensors to aquaculture nets and ship hulls.1–3 In particular, marine biofouling has long plagued aquaculture and shipping industries.4 The accumulation of marine organisms on ship hulls has led to increased roughness, which results in significant hydrodynamic drag. As of 2012, marine fouling has been estimated to cost shipping industries over $200 billion annually.5 Fouling has resulted in a number of negative environmental effects as well. Some impacts of fouling on the environment include a significant increase in greenhouse gas emissions, over 250 million additional tons of CO2,6 as well as the transportation of non-native species across various geographical regions which has resulted in devastating effects on native populations.7–9 Although numerous antifouling materials have proven to be effective, stricter regulatory regimes have outlawed several antifouling paints, most notably tributyltin oxide (TBT) due to its endocrine disrupting effects in several gastropod species at low environmental concentrations.10–12 2


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Since its ban, TBT has been widely replaced with copper13 due to its broad spectrum of toxicity towards aquatic organisms.14 Since the increased application of copper in antifouling paints, copper concentrations have become elevated in areas of high boating activity and have surpassed the environmental quality standard (EQS) of 5 µg/L.14 For example, waterborne copper concentrations of 21 µg/L were recorded in the San Diego Bay, which can lead to harmful effects on the development of residing marine species such as Mytilus galloprovincialis.15,16 Due to the harmful effects of copper-based antifouling paints, a number of countries including Sweden, Netherlands, and Denmark have recently banned antifouling copper paints on recreational vessels in certain regions.17 The increasing number of regulations against these paints has sparked a resurgence in the development of a wide range of polymers for antifouling surfaces. In general, antifouling surfaces function by minimizing the intermolecular forces between extracellular molecules and a targeted surface. By considering the Kendall model,18 fouling-release coatings, such as polydimethylsiloxane (PDMS) and polystyrene (PS), offer a facile release of foulants at low shear forces due to their low modulus and hydrophobic surface properties.1 On the other hand, hydrophilic surfaces, such as polyethylene glycol (PEG), have shown to have effective “antifouling” properties due to their low polymer-water interfacial energies, which helps to avoid the initial settlement of fouling organisms.19 Amphiphilic block copolymeric coatings have attracted interest because they show partially low interfacial energies that may prevent strong adhesion due to a single adhesive from a foulant. For example, diblock copolymers possessing perfluoroalkyl-tagged oligoethylene glycol moieties have been shown to form a lamellar morphology that have a combination of foul-releasing properties from the hydrophobic fluoroalkyl groups and antifouling properties from the hydrophilic PEGylated groups.20 Brush coatings utilizing PEG at the polymer/water interface have been commonly applied for antifouling applications due to its ability to resist protein adsorption.21–24 Schilp and colleagues25 have shown that loosely packed PEG surfaces inhibits the settlement of spores of Ulva due to steric repulsion of the 3



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chains forming a diffuse interphase, while well-defined and densely-packed PEG chains form a sharp interface attractive to early stage foulants, such as spores. More recently, PDMS based hierarchical wrinkled coatings have been developed, which combine topographical features, ranging from tens of nanometers to a fraction of a millimeter, with surfaces incorporated with fluorine and ethylene oxide chemistries.26 The study showed that topography can be used to inhibit the settlement of organisms of a particular size by reducing the organism-substrate contact area. Other polystyrene-based diblock copolymers, such as poly(styrene-block-2-vinyl pyridine) (PS-b-P2VP) and poly(styrene-block-methyl methacrylate) (PS-b-PMMA), have shown to self-assemble into cylindrical nano-patterns that reduced settlement of zoospores of the green algae Ulva, relative to unpatterned surfaces.27 More recently, studies have extended the application of triblock copolymers to the development of antifouling coatings. Weinman and coworkers28 have modified a triblock copolymer precursor, poly(styrene-block-(ethylene-ran-butylene)-block-isoprene) (PS-b-P(E/B)-b-PI), to produce amphiphilic triblock surface-active block copolymers (SABCs) for antifouling purposes. The amphiphilic nature of the SABCs were found to reduce both density of settled zoospores of Ulva linza and strength of attached sporelings (young plants), while the low modulus further reduced the attachment strength of sporelings. Although resulting films can be effective, the chemical properties of several polymers only allows for dissolution in organic solvents. Fabricating such films with controlled morphologies and properties commonly requires coating organic solutions of polymers onto surfaces, which further undergo various solvent,29,30 or thermal,31,32 annealing procedures. While acceptable in a laboratory setting, industrialscale applications are not feasible as common methods of coating larger surfaces, such as aquaculture nets and ship hulls, require dip, spray, or brush coating procedures, wherein volatile organic chemicals are less attractive. For this reason, preparing block copolymers in aqueous solutions has been of great interest. 4


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Traditionally, two principal approaches are applied for preparing amphiphilic block copolymers in aqueous environments, namely direct and indirect dissolution. Polymer dissolution has been extensively studied and applied in developing materials in microlithography,33,34 recycling of plastics,35 and drug delivery.36,37 Polymers poorly soluble in water often require indirect dissolution methods, such as dialysis and solvent evaporation. Unfortunately, these procedures suffer from a number of drawbacks that limit scalability. Dialysis is a slow process that is limited to low working volumes, while solvent evaporation is an expensive process that releases volatile organic compounds (VOCs).20,38 Although newly developed polymers offer an effective and environmentally safer alternative to copper antifouling paints, the need for organic solvents, or time and energetically costly indirect dissolution techniques, to fabricate these materials significantly reduces and limits their “green-ness” and largescale applicability. To provide a solution to the previous limitations, we describe an aqueous-based process of fabricating nanostructured block copolymer films which allows the phase transfer of water insoluble block copolymers from a water immiscible organic phase into an aqueous environment. An amphiphilic triblock copolymer, poly(styrene-block-2 vinyl pyridine-block-ethylene oxide) (PS-b-P2VP-b-PEO), was phase transferred from chloroform to water with the assistance of a diblock copolymer, poly(styreneblock-ethylene oxide) (PS-b-PEO), as a phase transfer agent. The rate of phase transfer from chloroform to water was explored at various concentrations of PS-b-PEO phase transfer agent. The transfer resulted in solvent-free solutions of self-assembled PS-b-P2VP-b-PEO, readily applicable to surfaces through dip, spray, or brush coating procedures for antifouling applications, making this technique a more environmentally friendly and scalable process. Experimental Section

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Materials: Poly(styrene-block-ethylene oxide) (PS-b-PEO) diblock copolymer (Polymer Source) and poly(styrene-block-2 vinyl pyridine-block-ethylene oxide) (PS-b-P2VP-b-PEO) triblock copolymers (Polymer Source) were used in these experiments. PS-b-PEO polymer has a number average molecular weight for PS of 3600 g/mol and for PEO of 67000 g/mol with a polydispersity index of 1.07. PS-b-P2VPb-PEO polymer has a number average molecular weight of 13000 g/mol for PS, 13000 g/mol for P2VP, and 36000 g/mol for PEO with a polydispersity index of 1.09. Aqueous solutions of PS-b-PEO, ranging from 0.001% to 0.5% (w/v), were prepared in Mili-Q water through successive sonication and vortex procedures. 1% (w/v) organic solutions of PS-b-P2VP-b-PEO, in chloroform (CHCl3, FW 119.38, Aldrich, 99.8%), were prepared through successive vortex procedures. Polydimethylsiloxane (PDMS) was prepared by mixing Sylgard 184 Silicone Elastomer Base and Sylgard Elastomer Curing Agent at a 10:1 ratio (w/w). The mixtures were transferred onto a mold and placed under vacuum until all air bubbles were removed. Finally, the mixtures were cured at 60 oC for 18 hours, then cooled to room temperature. Phase Transfer: A 1% (w/v) organic solution of PS-b-P2VP-b-PEO was transferred to a separatory funnel in addition to an aqueous solution of PS-b-PEO, at concentrations ranging between 0.0001% to 0.5% (w/v), at a 1:1 ratio (v:v). The mixtures were allowed to settle without further agitation for 15 minutes until the organic and aqueous phases were separated. Once separated, the aqueous solutions were air dried overnight and then oven dried at 50 oC for 12 hours and the resulting mass of PS-b-P2VPb-PEO transferred was reported. This procedure was repeated with varying concentrations of PS-b-PEO. The phase transfer was also repeated with agitation. Once organic and aqueous phases were combined in a separatory funnel, the mixture was vigorously agitated every 2 minutes over a span of 10 minutes. The mixture was allowed to settle for the final 5 minutes and the phases were separated. Resulting aqueous solutions were air dried overnight and then oven dried at 50 oC for 12 hours to

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determine the resulting mass of PS-b-P2VP-b-PEO transferred. This procedure was repeated with varying concentrations of PS-b-PEO. Surface Preparation: Silicon wafers were cleaned in a piranha bath (3:1 sulfuric acid: hydrogen peroxide) for 1 hour. Wafers were then rinsed with Milli-Q water and stored in filtered ethanol. Silicon wafers were then air dried and dip-coated into an aqueous solution of PS-b-P2VP-b-PEO. After dipping the surface multiple times, the surface was allowed to dry in air. Surfaces prepared for macrofouling tests were prepared on one sided microscope slides brushed coated with an aqueous solution of PS-bP2VP-b-PEO and dip coated with PDMS. AFM Imaging and Force Spectroscopy: Atomic force microscopy (AFM, Digital Instruments, Dimension 5000, Santa Barbara) operated in Tapping Mode was used to examine the surface topography of thin film coatings. Samples were imaged using NCH rectangular shaped silicon probes (Nanoworld, Switzerland) with resonance frequencies in the range 280-320 kHz and a spring constant of 40 N/m. Adhesion force measurements were obtained using a Molecular Force Probe AFM (MFP-3D, Asylum Research, Santa Barbara) and bovine serum albumin (BSA) coated AFM probes (Novascan Tech Inc., Ames) with a nominal spring constant of 50 pN/nm. Mechanical properties of the thin film were measured by nanoindentation using Molecular Force Probe AFM and V-shaped silicon nitride cantilevers (MLCT, Bruker) with a nominal spring constant of 0.05 N/m. Prior to measurements, the spring constant was determined by thermal noise method.39 Resulting force extension curves were analyzed with custom analysis software (Igor Pro Ver. 6.22A, Wavemetrics). Experiments were carried out in PBS buffer at room temperature. Dynamic Light Scattering (DLS): The average hydrodynamic diameters of the micelles in water were measured using photon correlation spectroscopy (Zetasizer 3000HS, Malvern Instruments Ltd, UK) with a 10 mW laser operating at 633 nm. Experiments were carried out in triplicates at room temperature at

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the scattering angle of 90o. A CONTIN statistical method was used to convert the measured correlation data into a particle size distribution. Transmission Electron Microscopy (TEM): Samples were prepared by placing a drop of aqueous PS-bP2VP-b-PEO micelle solution on a holey carbon grid (Ted Pella) and allowed to air dry. Images were collected using a Hitachi H-7000 TEM at 75 kV. Settlement of Zoospores of Ulva linza: Zoospores were obtained from mature plants of U. linza using a procedure reported by Callow et al.40 A suspension of zoospores (10 ml; 1x106 spores ml-1) was added to individual compartments of quadriperm dishes, in the dark. After 45 minutes in darkness at 20 oC, slides were washed by passing 10 times through a beaker of seawater to remove unsettled (i.e. swimming) spores. Slides were fixed using 2.5% glutaraldehyde in seawater. The density of zoospores attached to the surface was counted on each of the 3 replicate slides using an image analysis system attached to a fluorescence microscope. The procedure was repeated with multiple suspensions of zoospores. Macrofouling Tests: Prepared slides (described in Surface Preparation) were submerged 0.5 m below the water surface at the Florida Institute of Technology static immersion test site. All surfaces were facing south and were kept in a caged environment to deter predation and grazing by fish. After 1 month of immersion, surfaces were removed from seawater and the settlement of macrofoulants, specifically barnacles and tubeworms, were analyzed. Results and Discussion Phase Transfer: In this study, a novel procedure for phase transferring a water insoluble triblock copolymer, PS-b-P2VP-b-PEO, into an aqueous phase with the assistance of a diblock copolymer, PS-bPEO, phase transfer agent is reported.

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Figure 1. (A) A water insoluble poly(styrene-block-2 vinyl pyridine-block-ethylene oxide) (PS-b-P2VP-bPEO) triblock copolymer is phase transferred to an aqueous phase from a water immiscible phase with the assistance of (B) poly(styrene-block-ethylene oxide) (PS-b-PEO), an amphiphilic diblock copolymer. When organic and aqueous phases are combined, the amphiphilic nature of PS-b-PEO diblock copolymer allows it to act as a surfactant. PS-b-PEO adsorbed at the interface of the immiscible phases are situated so that hydrophobic-PS moieties are predominantly oriented towards the organic phase while the hydrophilic-PEO groups are preferentially solvated in the aqueous phase. This alignment of PS-b-PEO at the interface of the two phases is believed to reduce the interfacial energy, thus allowing for PS-b-P2VP-b-PEO to transfer into the aqueous phase, while transfer of triblock is not observed in the absence of the phase transfer agent (Scheme 1). This indirect dissolution technique allows for facile phase transfer of water insoluble block copolymers from a water immiscible organic phase to an aqueous phase. As PS-b-P2VP-b-PEO is transferred into the aqueous phase, the remaining solution of chloroform can be recycled for additional phase transfer procedures.

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Scheme 1. Schematic representation of water insoluble PS-b-P2VP-b-PEO triblock copolymer phase transferring from a water immiscible organic phase to an aqueous phase with the assistance of PS-bPEO. (A) Organic solution of PS-b-P2VP-b-PEO combined with (B) water results in (C) no observable transfer of block copolymer. Addition of an (D) aqueous solution of PS-b-PEO reduces the interfacial energy between the immiscible phases allowing for the (E) phase transfer of PS-b-P2VP-b-PEO into an aqueous environment, in which self-assembly occurs towards core-shell-corona type micelles. The fraction of PS-b-P2VP-b-PEO transferred into an aqueous phase was quantitatively studied at various concentrations of PS-b-PEO. Results showed the highest fraction (mT/mi) of PS-b-P2VP-b-PEO transferred (mT/mi = 0.495) in the presence of 0.5 wt% of PS-b-PEO, while the lowest yield transferred (mT/mi = 0.021) was observed in the presence of 0.001 wt% of PS-b-PEO, over a constant interaction 10


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time of 15 minutes (Figure 2). To compliment these results, pendant drop measurements revealed that the surface tension of water decreased with an increase in concentration of diblock copolymer until the critical micelle concentration of 0.4 wt% was reached, supporting the hypothesis that PS-b-PEO acts as a surfactant which reduces the interfacial energy between the immiscible phases (Fig. S1).

Figure 2. Ratio of PS-b-P2VP-b-PEO mass transferred to an aqueous phase (mT) to the initial mass in the organic phase (mi) was measured at various concentrations of PS-b-PEO. Increase in the concentration of phase transfer agent resulted in an increase in mT/mi. Additional experiments were conducted to study the fraction of PS-b-P2VP-b-PEO transferred with an increase in interfacial area between the immiscible phases. This was effectively achieved by the application of mechanical agitation sequences to the two phase system over 15 minutes. Experiments repeated while agitating the system resulted in higher fractions of triblock copolymer transferred into the aqueous phase, relative to passive procedures. The effective increase in interfacial area became more significant at higher concentrations of PS-b-PEO. When preparing aqueous solutions for coating applications, 0.01 wt% of PS-b-PEO was used and PS-bP2VP-b-PEO was transferred passively over an interaction time of 40 minutes, resulting in a mT/mi of 0.037 ± 0.013. After completing the transfer, the two phases were separated and the organic phase was reintroduced to a fresh aqueous solution of PS-b-PEO for further phase transfer of PS-b-P2VP-b-PEO. 11



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The proposed method of phase transferring PS-b-P2VP-b-PEO triblock copolymer from a water immiscible organic phase to aqueous phase using a diblock copolymer phase transfer agent presents a number of advantages, such as the simplicity of the procedure, recyclability of the organic phase, and potential for large-scale applications. Characterization: Block copolymers can be tuned to provide a wide array of chemical properties due to the arrangement of different polymers in alternating sequences. In the case of sufficiently strong incompatibility, certain blocks may undergo microphase separation to form distinct domains and periodic nanostructures.41,42 Conventional micelles based on amphiphilic diblock copolymers have been widely reported.43,44 However, the remarkable diversity of block copolymers available has given rise to a wide variety of architectures. The unique arrangement of poly (styrene), poly (2-vinyl pyridine), and poly (ethylene oxide) allows for self-assembly of multi-layered micelles, driven by the solubility of each block in surrounding solvents.45–47 As the water solubility of the middle P2VP block is dependent on the degree of ionization, the triblock copolymer was observed to self-assemble into core-shell-corona type micelles, consisting of a hydrophobic PS core, pH-responsive P2VP shell, and hydrophilic PEO corona, under neutral aqueous conditions (Fig. S2 and Fig. S3). These self-assembled micelles were characterized by dynamic light scattering (DLS), resulting in an average diameter of 176 nm at pH 7.0 (Figure 3B).

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Figure 3. (A) Schematic representation of micelle behavior as a function of the pH; upon protonation under acidic conditions, the pH-sensitive P2VP blocks were stretched, as shown by the increase in the average hydrodynamic diameter of the micelles. (B) Size distribution of PS-b-P2VP-b-PEO triblock copolymer micelles in aqueous environments with varying pH. At pH 7.0 the average diameter of the micelles were 176 nm (solid), while at pH 1.0 the average diameter of the micelles were 194 nm (dashed). When exposed to an acidic aqueous environment, pH 1.0, the diameter of the micelles was observed to increase to 194 nm. With a pKa of 4.5, the majority of the 2-vinyl pyridine moieties in the “shell” component of the triblock copolymeric micelle will be protonated in an environment with a pH of 1.0.48 This protonation results in an electrostatic repulsion between the 2-vinyl pyridine moieties and a consequent increase in hydrophilicity of the polymer, resulting in an increase in diameter relative to aqueous conditions at a pH of 7.0. The hydrodynamic diameter can also be tuned by varying the chain lengths of each block. Shorter chain lengths of all blocks resulted in a smaller average hydrodynamic diameter (Fig. S10).

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Once aqueous solutions of micelles were characterized, PS-b-P2VP-b-PEO micelles were coated onto piranha cleaned Si/SiO2 surfaces. When micelles settle onto an impenetrable substrate from water, reorganization occurs and the poorly soluble PS-core blocks preferentially adsorb on the substrate. This process continues as PS blocks spread and compete for surface area on the substrate, resulting in a stable film of truncated micelles with PEO blocks oriented at the micelle and water interface. Similar reorganization behavior of polystyrene containing block copolymer micelles, poly (styrene-blockisoprene), has been previously studied by Hinestrosa and colleagues,49 wherein they observed a larger spreading coefficient for solvophobic PS-core at a silicon surface resulting in reorganization and adsorption. AFM height profiles (Figure 4A and 4B) show that the micelle height relative to the spread film of PEO, 30.3 ± 7.3 nm, are less than the lateral diameter, 109.1 ± 10.5 nm. Dip-coating piranha cleaned Si/SiO2 surfaces resulted in randomly dispersed truncated micelle settlement along the surface with a root mean square (RMS) roughness of 17.7 ± 1.9 nm. The nanostructures were observed to be stable when submerged in sea water (Fig. S4) and remained stable after 1 month of static immersion (Fig. S5 and Fig. S9).

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Figure 4. (A) AFM height image of PS-b-P2VP-b-PEO coated onto piranha cleaned Si/SiO2 surface (scale bar represents 700 nm) collected in air. (B) Section analysis of PS-b-P2VP-b-PEO micelles. (C) TEM image of PS-b-P2VP-b-PEO micelles (75 kV; scale bar represents 100 nm). We next investigated the mechanical properties of PS-b-P2VP-b-PEO copolymer films using AFM indentation measurements (Fig. S6). The Young’s modulus (E) of the thin film was determined by considering load-indentation dependence for a paraboloidal tip50 shape given by [Eq. 1]. √

[Eq. 1]

Here F is the loading force (nN), E is the Young’s modulus (Pa), R is the radius of curvature of the tip (nm), δ is the indentation (nm), and v is the Poisson ratio (0.5). As a result, the elastic modulus of PS-bP2VP-b-PEO was calculated to be 163.13 58.18 kPa. The surface energy was also determined using a modified Fowkes’ equation [Eq. 2] and by analyzing contact angle data for various solvents (Table S1), including water, formamide, ethylene glycol, and ethyl acetate (Fig. S7).51

!

-

. + 1 " #$%& '()*

/ " / '()* /

[Eq. 2]

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.

Here '/ is the total surface tension of the liquid, '/+ and '/ are the dispersive and polar contributions .

+ and '()* are the dispersive and polar contributions to the surface tension of the liquid (Table S2), '()*

to the surface energy of the PS-b-P2VP-b-PEO films, and & is the contact angle of the respective liquid .

01234 + phase on PS-b-P2VP-b-PEO films. The resulting '()* was 60.6 mJ/m2, with '()* and '()*

contributions of 15.8 mJ/m2 and 44.8 mJ/m2, respectively (Table S3 and S4). Adhesion Tests: To test the potential antifouling properties of the self-assembled PS-b-P2VP-b-PEO coating, adhesion forces between BSA coated tips and the following three surfaces: uncoated Si/SiO2, PDMS, and PS-bP2VP-b-PEO were measured. Figure 5 shows typical force-extension profiles and force histograms when a BSA coated AFM tip interacts with uncoated Si/SiO2, PDMS, and PS-b-P2PVP-b-PEO micelle coated surfaces. Non-zero mean adhesion forces were observed for both uncoated (217 ± 83 pN) and PDMS (1.19 ± 0.42 nN) surfaces. However, 92% of the adhesion forces measured between BSA coated tips and PS-b-P2VP-b-PEO films exhibited no measurable force of adhesion (

Aqueous-Based Fabrication of Low-VOC Nanostructured Block

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