Article pubs.acs.org/IECR
Self-Organizing Preparation and Marine Fouling Bioassays of a Honeycomb Microstructure Surface with Controllable Dimensions Based on Silicone−Acrylate Copolymers Li Wang, Cunguo Lin,* Haiping Gao, Jiyong Zheng, Jinwei Zhang, Fengling Xu, and Yongqiang Sui State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266101, China S Supporting Information *
ABSTRACT: This work describes a large-scale preparation method of a honeycomb microstructure surface (HMS) based on silicone−acrylate copolymers (PSiA) with abundant Si−OH and organic amine. The mean diameter and depth of the honeycomb microstructures are controllable via changing the species and concentration of the organic amine as well as the silicone−acrylate mass ratio of copolymer. The formation mechanism of HMS was investigated, and the results showed that the tiny liquid droplets of organic amine acted as liquid state cores for the formation of honeycomb microstructures. The zoospore settlement and diatom attachment assay demonstrated that the cell density of organisms is lower on the microstructure surface than that on the smooth surface, confirming its preferable antifouling properties. Therefore, this work demonstrates a feasible approach of preparing HMS with controllable pore sizes via changing its composition, which can be used as good candidates for environmental-friendly antifouling surfaces.
1. INTRODUCTION Marine biofouling does enormous harm to ships and other sea facilities. Usually, it can be reduced or resisted by the use of antifouling coatings. However, with the ban of environmentally harmful tributyltin (TBT)-based paint products,1−3 it is quite urgent to develop environmentally friendly antifouling coatings. The current nontoxic biofouling control is based mainly on the surface engineering, turning the substrate into low or nonsticking4−6 material. The previous studies have shown that microscale topography of material surface influences the bioadhesion significantly.7−10 When organisms settle onto a surface, the surface microstructures may be lead to form a bridge structure between biofilm and material surface, and this will increase tension along the unsupported regions of the biofilm and reduce the area of contact between the organisms and surface, which would reduce the overall adhesion strength.11 This will be very useful in limiting marine fouling. Brennan’s research group prepared many engineered pillars, ridges and biomimetic topography replicated in polydimethylsiloxane elastomer and investigated the antifouling performance of engineered antifouling microtopographies. Zoospore settlement was reduced by ∼85% on the finer and more complex Sharklet AFTM topographies.12 Berntsson’s group designed five longitudinal grooves, and their effects were investigated on barnacle fouling. All microtextured surfaces reduced barnacle Balanus improvisus recruitment, and the most efficient texture reduced recruitment by 98%.13,14 The fouling behavior of smooth and roughened superhydrophobic coatings was investigated in detail by Zhang et al.15 The analysis indicated rough surfaces (roughness ratio >2.7) exhibited high resistance to fouling of cypris larvae over a 6 month period. These results confirmed that the designed microscale topography surfaces may be an effective tool and predictive model for the design of unique nontoxic, nonfouling surfaces for marine applications.16,17 © 2014 American Chemical Society
The honeycomb microstructure surface (HMS) is a porous surface with two-dimensionally arranged honeycomb nano/ micropores, and its unique surface properties have made HMS a potential material for applications in antifouling materials.9,18 It could be prepared by the technology of the Template method,19 hot embossing, etching and molecular selforganizing. Molecular self-organizing is a key concept in supramolecular chemistry20,21 and which means that the organizing of molecules is directed through noncovalent interactions (e.g., hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π−π interactions and/or electrostatic) as well as electromagnetic interactions. Because this process adopts a defined arrangement without guidance or management from an outside source, this method has a big advantage in constructing a variety of different shapes and sizes of nano/microstructure surfaces. Molecular selforganizing of polymers gelling around condensates in volatile organic solvents22 is usually used as the HMS preparation method, named the “breath figure” formation technique.23 For example, A nanostructured surface was constructed by wetting poly(lactic-co-glycolic acid) (PLGA) films in ethyl acetate followed by dewetting under wet conditions.24 The resulting films had nanostructured surfaces with pores in the nanoscale range between 200 and 500 nm, and the observations revealed that pore sizes affected the bacteria adhesion significantly. So far, many studies have suggested a variety of polymers could support the “breath figure” technique for formation of regular honeycomb porous structures in thin films, including of star polymers,25 branched polymers,26,27 block copolymers28,29 and so on. The synthetic methods of these well-architected Received: Revised: Accepted: Published: 17636
August 14, 2014 October 17, 2014 October 21, 2014 October 21, 2014 dx.doi.org/10.1021/ie5032343 | Ind. Eng. Chem. Res. 2014, 53, 17636−17644
Industrial & Engineering Chemistry Research
Article
Table 1. Molecular Weights and Polydispersity Index of Synthesized of PSiA mass feed fraction
GPC analysis
samples
f EA
f BA
f VSiOH
mSi/mAcry
Mn/g·mol−1
Mw/g·mol−1
PD
PSiA-1 PSiA-2 PSiA-3
20.0 30.0 40.0
13.33 20.0 26.67
66.67 50.0 33.33
2.0 1.0 0.5
5600 5900 5100
11700 12200 8900
2.07 2.08 1.72
the pH value of siloxane prepolymer solution was regulated to 7.0 using hexamethyldisilazane to end the reaction, and the siloxane prepolymer was yielded after the low boiling point compounds was removed under reduced pressure at 60 °C over 12 h. 2.2.2. Synthesis of PSiA. PSiA were prepared via a conventional free radial copolymerization procedure in a 500 mL three-necked round-bottom flask. The reaction temperature was controlled by the same method above. The mixed solution of toluene and n-butyl alcohol (toluene/n-butyl alcohol = 4/1) was used as the solvent. Azobisisobutyronitrile (AIBN, 4% of monomer total mass), tertiary dodecyl mercaptan (TDM, 2% of monomer total mass) were employed respectively as the initiator and chain transfer agent. Acrylic monomers of ethyl acrylate (EA) and butyl acrylate (BA) were of analytical reagent grade and used without further purification. Three kinds of mass feed fractions of EA, BA and VSiOH were employed, as shown in Table1, and respectively named PSiA-1, PSiA-2 and PSiA-3. The mass ratio of monomers to solvent was 2:3. Polymerizations were completed at 80 °C for 8.0 h. The extent of the polymerization reaction was monitored by Fourier transform infrared (FT-IR) spectrum. 2.2.3. Preparation of HMS. PSiA were used as film formers of HMS without purification and coated on a glass substrate (2.5 × 7.5 cm). 3-MPTS was used as its curing agent. Glass sheets were degreased by an ultrasonic cleaner for 10 min in acetone and washed by deionized water for three times before dried. The mixture of 1.0 mL of PSiA, 10 μL of 3-MPTS and 1.0−2.0% of organic amine (volume percent of mixture) was put into a clean test tube and mixed for 5 min via a turbine mixer, then it was coated on a glass sheet in a fume hood. After 24 h, HMS was formed. All the HMS sheets were immersed in the water for 2 weeks to remove organic amine and air-dried under a fume hood before characterization. 2.3. Characterization of HMS. Molecular weights of VSiOH and copolymer compounds were determined by gel permeation chromatography (GPC 50, Polymer Laboratories) analysis in tetrahydrofuran (THF) at 40 °C. A PL-BV 400RT viscometer and a differential refractive index detector were used. The separation columns setup consisted of a PLgel 5.0 μm guard column (50 × 7.5 mm) followed by two (PL) 300 × 7.5 mm linear MIXED-D columns. Calibration was performed with narrow polystyrene standards ranging from 500 up to 2 × 106 g mol−1. Data were acquired using a high resolution (PL) data stream data acquisition system. FT-IR-iTR analysis was conducted on a Nicolet 8700 system with DTGS detector and Smart iTR. The glass transition temperatures (Tg) of HMSs made of PSiA-1, PSiA-2 and PSiA-3 were measured by differential scanning calorimetry (DSC, Netzsch 204 F1). An adequate amount (10−12 mg) of cured polymer was put in aluminum sample pans. Scanning between −70 and +76 °C at a rate of 10 K min−1 was employed to determine the Tg. SEM (XL-30, PHILIPS) with an X-ray energy spectrometer (Phoenix) was used to obtain the topography and characterize
polymers usually are controlled living polymerization techniques, such as living anionic, atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chaintransfer (RAFT) polymerizations. The conditions of these polymerization procedures are often strict, and their precursors required in polymerization are limited. The desired polymers need multiple purification procedures to remove catalysts and other byproducts. Although the advantages and effectiveness of using these well-architected polymers in honeycomb structured porous films formations are well recognized, their synthetic methods are however unconventional, uneconomic, and difficult to exploit industrially. On the other hand, a humidity control device usually is needed as using “breath figure” method. This also increases the cost of material preparation. So, it is necessary to look for a convenient large-scale preparation method of HMS. In this paper, a honeycomb microstructure surface (HMS) based on silicone−acrylate copolymers (PSiA) and organic amine has been prepared, in which the pore size is controllable via changing the species and concentration of the organic amine as well as the silicone−acrylate mass ratio of copolymer. The topography of HMS was characterized by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). In addition, the formation mechanism of HMS was also investigated in detail. The antifouling performance of the HMS was assayed by Ulva zoospore settlement and diatom attachment.
2. MATERIALS AND METHODS 2.1. Materials. 1,2-Ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (DEPA), ethyl acrylate (EA) and butyl acrylate (BA) were obtained from Tianjin Bodi Chemical Ltd., China. Tetraethoxysilane (TEOS), dimethyl diethoxysilane (DMDEOS), methyltriethoxysilane (MTEOS), triethoxyvinylsilane (VTEOS), dodacyl mercaptan (TDM) and 3-methacryloxypropyltrimethoxysilane (3-MPTS) were purchased from Shanghai Reagent Company, China. Deionized water (d-H2O) was produced by a Millipore Direct-Q pure water machine (Millipore, USA). 2.2. Methods. 2.2.1. Synthesis of Siloxane Prepolymer Containing Vinyl Group (VSiOH). Before Synthesis of VSiOH, the raw materials including dimethyldiethoxysilane (DMDEOS), methyltriethoxysilane (MTEOS), tetraethyl orthosilicate (TEOS) and vinyl triethoxysilane (VTEOS) were purified by distillation. 0.5 mol of DMDEOS and 32.4 mL of dilute hydrochloric acid (pH = 4) were put into a 500 mL three-necked round-bottom flask fitted with a water condenser and magnetic force stirrer. The reaction temperature was controlled by a temperature sensor put into the flask under the liquid level of raw materials. Then it was heated at 55 °C for 20 min. After that, the mixture of 0.15 mol of MTEOS, 0.1 mol of TEOS and 0.15 mol of VTEOS was dripped into the flask, and then continuously heated at 55 °C. About 1.5 h later, the temperature was raised up to 85 °C and kept for 3.0 h. Then 17637
dx.doi.org/10.1021/ie5032343 | Ind. Eng. Chem. Res. 2014, 53, 17636−17644
Industrial & Engineering Chemistry Research
Article
due to the use of a large amount of initiator. The copolymers with different silicone−acrylate mass ratios have similar molecular weight and polydispersity index values. This result indicates that the effect of different silicone−acrylate mass ratios on the molecular weights of PSiA is very weak, and the method of conventional free radial copolymerization is feasible for the synthesis of PSiA. 3.2. FT-IR-iTR Analysis of VSiOH, PSiA and HMS. FTIR-iTR was used to characterize the function group details and compositions of VSiOH, PSiA and HMS. The result is shown in Figure 1.
the surface element distribution of the honeycomb microstructure. The microtopography of HMS was investigated using the CLSM system (OLS4000, OLYMPUS) and the SEM system mentioned above. The static contact angles of deionized water on the HMS were measured in ambient air at room temperature using a DSA-100 contact angle measuring system (Krüss GmbH). The reported contact angle represents the average contact angle for five replicates. 2.4. Ulva Zoospore Settlement and Diatom Attachment Assay. HMS samples containing 2.0, 1.0 and 0.5 silicone−acrylate mass ratios were evaluated for Ulva spores settlement and diatom (Navicula subminuscula and Stauroneis constrita) attachment. Three replicates of each HMS were tested for every fouling organism. The glass sheet without a coating was used as control 1, and a uniformly smooth, 3MPTS cured PSiA-1 sample without organic amine was used as control 2. Ulva pertusa and diatoms were all collected from Qingdao, China (latitude 36°03′ N; 120°21′ E). Ulva spores were released and prepared for settlement experiments as documented previously.30 Spores were checked by microscopy, and the zoospores (with four flagella) were pooled and used in the following settlement experiments. For the culture and attachment experiments of diatoms also, refer to the previous document.31,32 Navicula subminuscula and Stauroneis constrita were individually inoculated into a 250 mL Erlenmeyer flask containing autoclaved f/2 nutrient medium and incubated for 2 weeks at 25 °C in the biological incubator. The diatom films of pure species in the culture flasks were peeled off with sterile brushes and dispersed with ultrasonication. After 5 min of precipitation, the homogeneous suspension of diatoms was transferred into a new autoclaved Erlenmeyer flask. The densities of diatom suspensions were calculated through the blood count plate under the microscope. Samples of HMS in this study were immersed in deionized water to ensure that air was totally displaced from the features and that the samples remained fully wetted during the assay on the HMS. Subsequently, the samples were transferred to assay dishes, and a suspension of Ulva spores and diatoms was respectively added and the solution was diluted to 1.5 × 105 mL−1 with sterilized seawater, then the dishes were placed in darkness for 1 h. The samples were washed gently with filtrated seawater to remove unattached spores. The adhered cells and counts were quantified under an epifluorescence microscope. Fifteen images and counts were obtained from each sample, and the fields of view were taken at 2 mm intervals. The density of spores and diatoms was reported as the mean number of settled cells per mm2 from 15 counts on each of three replicate slides ± standard error (n = 3).
Figure 1. FT-IR-iTR spectra of VSiOH, PSiA and HMS.
Figure 1 shows band intensities in the infrared spectra of VSiOH, PSiA and HMS. For VSiOH (curve a), the bands at 3400, 1591 and 1080 cm−1 are Si−OH, vinyl group and Si−O− Si characteristic absorption peaks, respectively. However, the IR spectrum of VSiOH still has the characteristic peak of Si−O− C2H5 at approximately 970 cm−1. This indicated that the ethoxy group of raw materials was not fully hydrolyzed. Combined with the GPC test results, VSiOH was composed of the polycondensate of DMDEOS, MTEOS, TEOS and VTEOS containing a polymerizable double bond, silicon hydroxyl groups and a small amount of ethoxy groups from the raw materials. For PSiA (curve b), the band at 1591 cm−1 disappears, and the characteristic absorption peak of carbonyl group appears at 1733 cm−1. In addition, the bands of the carbon−carbon double bond from acrylic monomer do not appear in the infrared spectra of PSiA. These results indicate that the VSiOH and other acrylic monomers have completely polymerized in the process of free radial copolymerization; there are almost no residual monomers. PSiA are the copolymers of VSiOH, EA and BA with a number-average molecular weight of 5100−5900 g·mol−1. Curves c and d are respectively the IR spectra of HMS before and after washing with deionized water. For the HMS sample without washing (curve c), the amino group characteristic peak is significant at approximately 3400 cm−1. However, for the sample washed with water and dried (curve d), it is hard to observe the existence of an amino group characteristic peak. This suggested that organic amine on the HMS surface was dissolved and washed away by water, and organic amine maybe played an important role in the formation process of the honeycomb microstructure. In addition, the most conspicuous change between PSiA (curve b) and HMS (curve d) is at wavenumber 3400 and 970 cm−1, the band of Si−OH and Si−O−C2H5
3. RESULTS AND DISCUSSION 3.1. Measurement of Molecular Weight for VSiOH and PSiA. The molecular weight of VSiOH and three copolymer compounds was determined by GPC in tetrahydrofuran (THF) at 40 °C. The Mn of prepolymer VSiOH is approximately 1200 g·mol−1, and the Mw is approximately 3000 g·mol−1. In addition, the prepolymer also contains some components with very low molecular weights; their Mn and Mw are all approximately 200−400 g·mol−1. The results of three PSiA samples are shown in Table 1. As shown in Table 1, The Mn of copolymers produced is between 5100 and 5900 g·mol−1, and the Mw is between 8900 and 12200 g·mol−1. The polymer molecular weight is very low 17638
dx.doi.org/10.1021/ie5032343 | Ind. Eng. Chem. Res. 2014, 53, 17636−17644
Industrial & Engineering Chemistry Research
Article
the honeycomb microstructures are 2.8 ± 0.7, 5.9 ± 1.3 and 27.7 ± 7.8 μm, respectively, and the depths are approximately 4.1, 6.8 and 34.5 μm, respectively. Due to the abundant Si−OH in PSiA, the organic amines are more compatible with the silicone segment of PSiA molecules. For PSiA with a higher silicone−acrylate mass ratio, the same amount of organic amine is uniformly dispersed in the silicone to form smaller organic amine droplets, so the decrease in the silicone−acrylate mass ratio is accompanied by an increase of HMS dimension. 3.4. Effect of Organic Amine on the Topography of HMS. 3.4.1. Effect of Organic Amine Species on the Topography of HMS. PSiA-1 was used as the membrane former of HMS. TEPA, TRTA, DETA and EDA were respectively used as the liquid cores, and the HMS samples were named HMS-4, HMS-5, HMS-6 and HMS-7. The concentration of all organic amines was controlled at 1.0%, the CLSM images are shown in Figure 4. For the organic amines TEPA (a), TETA (b), DETA (c) and EDA (d), the mean diameters of the honeycomb microstructures are 0.5 ± 0.1, 0.8 ± 0.2, 2.2 ± 1.8 and 3.1 ± 2.1 μm, respectively, and the depths are approximately 0.4, 0.6, 5.7 and 7.4 μm, respectively. Their dimensions increase gradually with the decrease of organic amine molecular weight. Although the amount of organic amine added into the PSiA is equal, their vapor pressures had tremendous differences. The vapor pressures of EDA, DETA, TRTA and TEPA are respectively 10.4, 0.37,
