Research Article www.acsami.org
Dual-Functional Antifogging/Antimicrobial Polymer Coating Jie Zhao,† Li Ma,‡ William Millians,† Tiehang Wu,§ and Weihua Ming*,† †
Department of Chemistry, Georgia Southern University, P.O. Box 8064, Statesboro, Georgia 30460, United States Department of Physics, Georgia Southern University, P.O. Box 8031, Statesboro, Georgia 30460, United States § Department of Biology, Georgia Southern University, P.O. Box 8042, Statesboro, Georgia 30460, United States ‡
ABSTRACT: Dual-functional antifogging/antimicrobial polymer coatings were prepared by forming a semi-interpenetrating polymer network (SIPN) of partially quaternized poly(2-(dimethylamino)ethyl methacrylate-co-methyl methacrylate) and polymerized ethylene glycol dimethacrylate network. The excellent antifogging behavior of the smooth coating was mainly attributed to the hydrophilic/hydrophobic balance of the partially quaternized copolymer, while the covalently bonded, hydrophobic quaternary ammonium compound (5 mol % in the copolymer) rendered the coating strongly antimicrobial, as demonstrated by the total kill against both Gram-positive Staphylococcus epidermidis and Gramnegative Escherichia coli. The antimicrobial action of the SIPN coating was based on contact killing, without leaching of bactericidal species, as revealed by a zone-of-inhibition test. This type of dual-functional coating may find unique applications where both antimicrobial and antifogging properties are desired. KEYWORDS: functional coating, antifogging, antimicrobial, quaternary ammonium compound (QAC), semi-interpenetrating polymer network (SIPN)
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INTRODUCTION Fogging can significantly reduce the clarity of a transparent substrate, resulting in not only inconvenience but also potential danger in daily life. Therefore, there has been great demand for effective antifogging surfaces1−5 that can be applied in windshield, eyeglass, camera lens, mirror, goggle, display device in analytical instrument, and so on. For medical procedures, such as laparoscopic6,7 and endoscopic ones,8,9 lens fogging is a common problem and may lead to sudden loss of vision for the operator and interruption of the procedure, potentially provoking complications. An extensively reported strategy to mitigate fogging problems is to apply a superhydrophilic coating, since condensing water vapor would form a very thin water layer on a superhydrophilic surface, leading to much reduced light scattering.10−20 However, complicated procedures were normally used to prepare superhydrophilic surfaces.10−17 It was recently suggested that dry antifogging could be obtained on a superhydrophobic surface,21 which has not yet been materialized. In addition, both superhydrophilic and superhydrophobic surfaces may suffer from mechanical vulnerability due to their microroughened surface. On the other hand, a surface does not have to be superhydrophilic to be effectively antifogging, and smooth antifogging coatings22−27 have been recently prepared by carefully balancing the hydrophilicity and hydrophobicity of a coating, such as an antifogging/self-cleaning coating with both perfluoroalkyl and poly(ethylene glycol) (PEG) segments22 and © XXXX American Chemical Society
zwitter-wettable, antifogging coatings via layer-by-layer assembly involving PEG segments 23 or a chitosan/cellulose complex.24 We recently developed a smooth antifogging coating based on a semi-interpenetrating polymer network (SIPN) comprising either a binary copolymer25 poly(2(dimethylamino)ethyl methacrylate-co-methyl methacrylate) [poly(DMAEMA-co-MMA)] or a terpolymer26 containing DMAEMA segments and a network from polymerized ethylene glycol dimethacrylate (EGDMA). Different from a superhydrophilic antifogging surface, smooth coatings with a proper hydrophilic/hydrophobic balance can rapidly absorb water from the surrounding,23−26 not allowing the formation of discrete water droplets on the surface. For medical devices, a common way to prevent lens fogging is to apply an antifog solution, but it is only temporary and normally requires multiple applications during a medical procedure.6,8,9 Therefore, there is great need for more effective, permanent antifogging coatings for medical devices. Furthermore, it would be advantageous if the antifogging coating was also antimicrobial, which may help reduce, and even eliminate, potential pathogenic infection. There have been very few studies28,29 on coatings that are both antifogging and antimicrobial, one based on a superhydrophilic polymer−SiO2 nanocomposite28 and the other on a UV-cured coating;29 Received: January 19, 2016 Accepted: March 15, 2016
A
DOI: 10.1021/acsami.6b00748 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of Partial Quaternization of Poly(DMAEMA-co-MMA)
synthesized. The molar percentage of three monomer units (QAC, DMAEMA, and MMA units) in the quaternized copolymers was determined by 1H NMR, as listed in Table 1.
however, the combined antifogging and antimicrobial performance still needs major improvement. In this work, we describe a dual-functional antifogging/ antimicrobial coating based on a SIPN between the partially quaternized linear copolymer poly(DMAEMA-co-MMA)25 and polymerized EGDMA network. We envisaged that the incorporation of a proper amount of covalently bonded, hydrophobic quaternary ammonium compound (QAC) would render the SIPN coating highly antimicrobial, while the excellent antifogging property that originated from the binary copolymer poly(DMAEMA-co-MMA) was maintained. QACs, especially those with long hydrophobic tails, have been extensively incorporated into antimicrobial coatings30−42 due to their strong antimicrobial activity. The dual-functional SIPN coating can be obtained in three simple steps: (1) synthesis of poly(DMAEMA-co-MMA);25 (2) partial quaternization of poly(DMAEMA-co-MMA), leading to various QAC amounts in the copolymer; and (3) formation of SIPN coatings25,26 via photopolymerization of EGDMA in the presence of the partially quaternized copolymer.
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Table 1. Copolymer Composition (calculated from 1H NMR) and Glass Transition Temperature (Tg, determined by DSC) for B-75 and Partially Quaternized Copolymers
a
copolymer
molar composition QAC:DMAEMA:MMAa
Tg (°C)
B-75 Q-5 Q-7 Q-10
0:75.0:25.0 4.7:71.9:23.4 6.8:67.9:25.3 10.3:64.6:25.1
39.6 43.5 46.7 56.8
QAC refers to the quaternized DMAEMA unit.
SIPN Coating Preparation. Glass slides (2.2 × 2.2 cm2) were consecutively sonicated in acetone and ethanol for 30 min and followed by blow-drying with air. A copolymer (B-75, Q-5, Q-7, or Q10, 0.2 g), EGDMA (0.5 wt % with respect to the copolymer, which is the optimal amount, as determined previously25), and HHMP (2.0 wt % relative to EGDMA) were codissolved in 2 mL of toluene to obtain homogeneous solutions. The solution was spin-coated on a clean glass slide at 800 rpm for 15 s. After that, the coating was cured under UV irradiation using HHMP as the photoinitiator in a UVP CL-1000 ultraviolet cross-linker apparatus (365 nm, 15 W) for 45 min, and then dried in a vacuum oven at 70 °C for 24 h. The resulting SIPN coatings were labeled as SIPN-B-75, SIPN-Q-5, SIPN-Q-7, and SIPN-Q10, respectively, according to the copolymer used. These coatings were typically about 800 nm thick (determined by atom force microscopy), which was sufficient to ensure excellent antifogging performance.25,26 Fogging Test. A sample was first stored in a freezer at −20 °C for 30 min, and photos were taken 5 s after the sample was exposed to ambient conditions (∼20 °C, 50% relative humidity). An antifogging experiment was also performed by holding the sample 5 cm above a hot water bath (60 °C) for 60 s. In addition, light transmission over the 400−700 nm range was collected on an Agilent 8453 UV−vis spectrophotometer during fogging tests. To help reveal the antifogging mechanism, evolution of water contact angles on all SIPN coatings was monitored on a Ramé-Hart 290 instrument (every 10 s over a 600-s period) under ambient conditions. Antimicrobial Test. Antimicrobial tests were performed according to standard antimicrobial susceptibility test protocols, as described in detail elsewhere.41 Escherichia coli (Carolina #155065A) and Staphylococcus epidermidis (Carolina #155556) were chosen as representative Gram-negative and Gram-positive bacteria, respectively. We also examined possible leaching of bactericidal species from the SIPN coating by using a typical zone of inhibition test.41 Other Measurements. The glass transition temperature (Tg) of copolymers was measured on a TA Instruments DSC Q100 instrument, over the range from −50 to 120 °C at a heating rate of 10 °C/min. 1H NMR spectra were collected on an Agilent 400 MHz instrument with CDCl3 as the solvent, and 1H chemical shifts were internally referenced to the tetramethylsilane (TMS) signal. The number-average molecular weight (Mn) of polymer was determined by gel permeation chromatography (GPC), using a Waters 515 HPLC pump and an OPTILAB DSP interferometric refractometer (Wyatt
EXPERIMENTAL SECTION
Materials. Monomers including 2-(dimethylamino)ethyl methacrylate (DMAEMA, 98%) and ethylene glycol dimethacrylate (EGDMA, 99%), and free radical initiators including azobis(isobutyronitrile) (AIBN, 99%) and 2-hydroxy-4-(2-hydroxyethoxy)2- methylpropiophenone (HHMP, 98%) were purchased from Aldrich. Methyl methacrylate (MMA, 99%) and 1-bromoundecane were purchased from Alfa Aesar. Inhibitor was removed from MMA and EGDMA by passing them through an aluminum oxide column. Solvents including toluene, n-hexane, acetonitrile, chloroform, acetone, and ethanol were purchased from Fisher and used as received. Synthesis of Copolymer Poly(DMAEMA-co-MMA). The copolymer was prepared via a free radical solution polymerization25 as follows. Into a 250 mL flask were added 9.6 g of DMAEMA (0.6 mmol) and 2.0 g of MMA (0.2 mmol), and toluene was added to obtain 10 wt % solution, followed by the addition of 0.058 g of AIBN as the thermal initiator (0.5 wt % with respect to the total monomer mass). After being purged by argon for 20 min, the polymerization was conducted at 70 °C for 24 h, and the final binary copolymer poly(DMAEMA-co-MMA) with 75 mol % of DMAEMA segments, designated as B-75, was purified by repeated dissolution in chloroform and precipitation in hexane. Partial Quaternization of Poly(DMAEMA-co-MMA). The partial quaternization of poly(DMAEMA-co-MMA) was carried out via its reaction with various amounts of 1-bromoundecane (Scheme 1). The binary copolymer B-75 (1.0 g) and 0.063 g of 1-bromoundecane (5 mol % with respect to the DMAEMA units in B-75) were first dissolved in 15 mL of acetonitrile in a 50 mL flask, followed by reaction at 70 °C for 24 h. The product was then purified after dissolution in chloroform and precipitation in hexane and dried in a vacuum oven at 45 °C for 24 h. The partially quaternized copolymer with a 95% yield was labeled as Q-5, according to the molar fraction of the QAC unit in the final copolymer. Similarly, partially quaternized copolymers with higher QAC contents, Q-7 and Q-10, were B
DOI: 10.1021/acsami.6b00748 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. 1H NMR spectra of copolymers: (A) B-75 and (B) Q-7. Technology) detector, with dimethylformamide (DMF) as the elute (flow rate 1 mL/min, at room temperature), and was calibrated with polystyrene standards. Coating thickness was estimated by atom force microscopy (AFM) on an NT-MDT NTEGRA Prima instrument in the semicontact mode with a gold-coated cantilever NSG 10. A razor blade inscribed through the coating on the glass, followed by AFM height analysis to estimate the coating thickness.
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the control glass fogged severely (Figure 2a). In contrast, SIPNB-75 demonstrated excellent antifogging property, showing no
RESULTS AND DISCUSSION
Preparation of Partially Quaternized Poly(DMAEMAco-MMA). We first synthesized the binary copolymer poly(DMAEMA-co-MMA), B-75, with the DMAEMA/MMA molar ratio of 75/25 by conventional free radical polymerization. It was previously determined that a DMAEMA/MMA molar ratio of 70/30 in the binary copolymer led to the optimal hydrophobic/hydrophilic balance for optimal antifogging/ frost-resisting performance.25 In this study, we chose the DMAEMA/MMA molar ratio as 75/25 in anticipation that the subsequent partial quaternization would alter the hydrophobic/ hydrophilic balance. The Mn of the copolymer B-75 was 31 000, with Mw/Mn = 2.1, as determined from GPC. The DMAEMA/ MMA molar ratio in B-75 was confirmed by 1H NMR (Figure 1A) to be 3:1, which was consistent with the feed ratio. Partially quaternized poly(DMAEMA-co-MMA) was prepared by reacting the copolymer with 1-bromoundecane (Scheme 1). A typical NMR spectrum (for Q-7, Figure 1B) clearly shows chemical shifts of various protons due to the partial quaternization, which allowed us to calculate the copolymer composition (Table 1). The partially quaternized copolymers were designated as Q-5, Q-7, and Q-10, according to the QAC molar percentage. A single glass transition temperature (Tg), ranging from 40 to 57 °C (Table 1), was observed for the copolymers. The observed single Tg clearly indicated that different monomer units were randomly distributed in the copolymer, which would help ensure the optical transparency of the copolymer coating.25,26 In addition, the Tg of the partially quaternized copolymer depended on the QAC content: a higher QAC content led to a higher Tg for the copolymer. The partially quaternized copolymers and B-75 were subsequently used to prepare SIPN coatings. Antifogging Performance of the SIPN Coating. We evaluated the antifogging performance of SIPN coatings by first storing them at −20 °C in a freezer for 30 min and then examining their appearance 5 s after exposure to ambient conditions (∼20 °C, 50% relative humidity). Unsurprisingly,
Figure 2. Photos of different samples: (a) control glass and (b) SIPNB-75, (c) SIPN-Q-5, (d) SIPN-Q-7, and (e) SIPN-Q-10 coatings, which were first stored at −20 °C for 30 min and then exposed for 5 s to ambient lab conditions (∼20 °C, 50% relative humidity).
fog or frost on the surface (Figure 2b), which can be attributed to the hydrophilic DMAEMA segments in the copolymer that allowed water molecules from the surroundings to be rapidly absorbed23−26 into the SIPN coating and, possibly, to spread43 along the coating surface. Once inside the coating, water molecules are hydrogen-bonded to the network at a molecular level;23,25,44,45 water is thus nonfreezing, and no large lightscattering water domain would be formed. The SIPN coatings with different QAC contents (SIPN-Q series) also demonstrated excellent antifogging performance (Figure 2c−e), despite that up to 10 mol % of hydrophobic QAC had been incorporated into the coating. To evaluate antifogging property more quantitatively, the light transmission data over the 400−700 nm range were collected (Figure 3). Prior to the test, all SIPN coatings exhibited comparable light transmission (about 92%, Figure 3a) to the control glass, indicating that the effect of the SIPN coating on the light transmission was negligible. During the fogging test, the light transmission through the control glass reduced markedly to ∼15%, while all four SIPN coatings maintained high light transmission (>91%, Figure 3b). It became apparent that the presence of up to 10 mol % of the hydrophobic QAC units did not compromise the affinity of water for the quaternized copolymer, likely because the C
DOI: 10.1021/acsami.6b00748 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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binary copolymer poly(DMAEMA-co-MMA), which led to relatively poor antifogging performance against hot moist air.25 To help reveal the antifogging mechanism, time-dependent evolution of water contact angle (CA) on all SIPN coatings was monitored over a 600-s period under ambient conditions (∼20 °C, 50% relative humidity). Unlike a typical superhydrophilic antifogging surface (CA approaching 0°), all SIPN coatings exhibited initial water CAs well above 60° (Figure 5a), Figure 3. Light transmittance at the normal incident angle for various samples: (a) as prepared and (b) 5 s under ambient condition (∼20 °C, 50% relative humidity) after being stored at −20 °C for 30 min. The spikes in the spectra were due to background noise from the light bulb.
hydrophobic effect of the aliphatic tail was counterbalanced by the hydrophilic quaternary ammonium moiety. The antifogging performance of the SIPN coatings was also evaluated against hot moist air by placing the samples 5 cm above a hot water bath (∼60 °C) for 60 s. Again, the control glass fogged up immediately upon contact with hot water vapor, deteriorating the light transmission (Figure 4). For SIPN
Figure 5. (a) Evolution of water contact angle on various samples as a function of time. (b) Basal diameter change of the water droplet on various samples over the 600-s period, expressed as ΔD/D0, where ΔD = D − D0 and D0 and D (shown in the inset) are the initial diameter (time zero) and the diameter at different times, respectively, of the basal area of the droplet.
reinforcing the recent findings from us25,26 and others22−24,27 that an effective antifogging coating does not have to be superhydrophilic. A higher QAC content in the SIPN coating led to a greater initial water CA (Figure 5a), likely due to surface enrichment of long alkyl chains at the coating surface.41 During the 600-s time interval, the water CAs on all SIPN coatings decreased much more rapidly than on the control glass (due to water evaporation only), indicating that some water had likely diffused into the SIPN coating. We also monitored the change in the basal diameter of a water droplet on the coating surface. While no obvious change in the basal diameter was observed on the control glass over the 600-s period, the droplet basal diameter increased on all SIPN coatings to various extents (Figure 5b), depending on the QAC content in the coating. SIPN-B-75 had the largest expansion in the droplet basal diameter (15%) over the observation period, followed by SIPN-Q-5 (13%), SIPN-Q-7 (11%), and SIPN-Q10 (7%). These results again suggested that water had diffused23−26 into the SIPN coating, leading to the expansion of the droplet basal area on the coating surface. Water spreading along the coating surface may have also contributed to the basal area expansion.43 It should be pointed out that our SIPN coatings were quite smooth, as indicated by a typical root-mean-square (RMS) roughness of 2−3 nm over an area of 2 × 2 μm2 from AFM measurements. In addition, AFM revealed that these coatings were not nanoporous either, so their water-imbibing ability was not nanoporosity-driven10−13,15,17 but originated from the hydrophilic segment in the copolymer. Obviously, the incorporation of a higher amount of hydrophobic QAC led to a decrease in the waterabsorbing capability (less water expansion) of the SIPN coating, which is consistent with the antifogging performance of these SIPN coatings we discussed above. Too high an amount of QAC in the copolymer would definitely compromise the water-absorbing ability of the SIPN coating (as in the case of SIPN-Q-10) and, consequently, its antifogging performance against hot moist air in particular. Antimicrobial Activity of SIPN Coating. We examined antimicrobial activities of SIPN coatings by using S. epidermidis
Figure 4. Light transmission at the normal incident angle for various samples after 60-s exposure to hot moist air (5 cm above a 60 °C water bath) under ambient lab conditions (∼20 °C, 50% relative humidity). The spikes in the spectra were due to background noise from the light bulb.
coatings, different antifogging performance was observed: both SIPN-Q-5 and SIPN-Q-7 demonstrated excellent antifogging property, as indicated by high light transmittance (∼90%), which was slightly higher than that of SIPN-B-75. In contrast, there was major reduction in the light transmission (from 92% to ∼82%) for SIPN-Q-10. A major difference between these two antifogging tests was that the total amount of water to be absorbed by a coating in the test against hot moist air would be greater than the test under ambient conditions (for the sample being taken out of a freezer). Incorporation of a hydrophobic QAC into poly(DMAEMA-co-MMA) might slightly reduce the hydrophilicity, and thus the water-absorbing capability, of the copolymer. This hydrophilicity-reducing effect only became noticeable when the amount of the incorporated hydrophobic QAC was too high (as in SIPN-Q-10, on which slight fogging was observed against hot moist air since there was too much water to be absorbed). For SIPN coatings with lower amounts of QAC (SIPN-Q-5 and SIPN-Q-7), the presence of the hydrophobic QAC appeared to have very little effect on the overall hydrophilicity of the copolymer; thus, the excellent antifogging property of the coating (even against hot moist air) was maintained. It is worth noting that the introduction of a hydrophobic QAC also helped eliminate the lower critical solution temperature (LCST) effect that originated from the D
DOI: 10.1021/acsami.6b00748 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces and E. coli as representative microorganisms. The reduction of the number of viable bacterial cells (log scale) as colony forming units (cfu) within 24-h incubation was recorded. As listed in Table 2, SIPN-B-75 demonstrated some antimicrobial Table 2. Bacterial Log Reduction after 24-h Incubation with Initial Bacterial Concentration of 105 Bacteria/mL Treated with Various Samples (2.2 × 2.2 cm2) bacterial log reduction sample
E. coli
S. epidermidis
control glass SIPN-B-75 SIPN-Q-5 SIPN-Q-7 SIPN-Q-10
0.6 3.6 5 5 5
0.4 2 5 5 5
Figure 6. Zone-of-inhibition test result of (a) SIPN-Q-5 and (b) SIPN-Q-10 in a cultured lawn of E. coli.
activity against E. coli (3.6-log reduction) and S. epidermidis (2log reduction), which may be attributed to the formation of temporary QAC between the tertiary amine in the DMAEMA units and water at neutral pH, which is close to the pKa of PDMAEMA homopolymer.38,46 PDMAEMA homopolymer was found to be able to inhibit bacterial growth.38 More importantly, the introduction of permanent, hydrophobic QAC to the copolymer greatly enhanced the antimicrobial activity of the SIPN coating: all of the SIPN-Q coatings demonstrated superior bacteria-killing efficiency, all reaching 5-log reduction (total kill, Table 2) against both bacteria. Notably, the excellent antimicrobial property of the SIPN coating with the lowest QAC content (SIPN-Q-5) in our study clearly highlights that covalent binding of a small amount of hydrophobic QAC to the copolymer poly(DMAEMA-co-MMA) has allowed us to obtain very effective, dual-functional antifogging/antimicrobial coating. The interplay between the water-absorbing, hydrophilic DMAEMA segments and the bacteria-killing, hydrophobic QAC dictates the design of effective dual-functional coatings. Obviously, a high QAC content would enhance antimicrobial activity but might compromise antifogging behavior if too much QAC is incorporated into the copolymer. Both SIPN-Q-5 and SIPN-Q-7 have demonstrated excellent dual functions in this study. It is highly desirable for an antimicrobial coating not to leach out any bactericidal species; otherwise, released bactericidal species may not only become an environmental hazard but also trigger antibiotic resistance.47 We used the zone-of-inhibition test to examine possible leaching of bactericidal species from our dual-functional coating. As shown in Figure 6, E. coli proliferated right next to the two coatings (SIPN-Q-5 and SIPN-Q-10) examined, indicating that no bacterial inhibition zone existed, and therefore, no bactericidal QAC species had leached out of the SIPN coatings. Lack of a bacterial inhibition zone further suggested that the bactericidal action of our dualfunctional coating was based on contact killing, which can be attributed to the covalent bonding of QAC to the copolymer.
units led to covalently bonded hydrophobic QACs, rendering the coating strongly antimicrobial, as demonstrated by total kill against Gram-positive S. epidermidis and Gram-negative E. coli bacteria. This type of dual-functional coating may find unique applications where both antimicrobial and antifogging properties are desired, such as lenses in medical devices and transparent food-packaging materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support of this research at Georgia Southern University from USDA/NIFA (Award No. 2011-6702230229) is gratefully acknowledged.
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REFERENCES
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CONCLUSIONS In summary, we have designed and prepared a dual-functional polymer coating with excellent antifogging/antimicrobial properties, on the basis of SIPN of partially quaternized poly(DMAEMA-co-MMA) and polymerized EGDMA network. The antifogging behavior originated primarily from the delicate hydrophilic/hydrophobic balance of the partially quaternized copolymer. Meanwhile, partial quaternization of the DMAEMA E
DOI: 10.1021/acsami.6b00748 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.6b00748 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
