Durable Anti-Superbug Polymers: Covalent Bonding of Ionic Liquid

4-styrenesulfonate-co-glycidyl methacrylate) (P(SSNa-co-GMAx)) through a free ... The unsaturated ionic liquids, 1-vinyl-3-butylimidazolium chlori...
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Durable Anti-Superbug Polymers: Covalent Bonding of Ionic Liquid onto the Polymer Chains Jipeng Guan,†,‡,§ Yanyuan Wang,† Shilu Wu,† Yongjin Li,*,† and Jingye Li‡ †

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College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People’s Republic of China ‡ Shanghai Institute of Applied Physics, Chinese Academy of Sciences, No.2019, Jialuo Road, Jiading District, Shanghai 201800, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Here, we fabricated the ionic liquid (IL) grafted poly(vinylidene fluoride) (PVDF) (PVDF-g-IL) via electron-beam irradiation to fight common bacteria and multidrug-resistant “superbugs”. Two types of ILs, 1-vinyl-3-butylimmidazolium chloride (IL (Cl)) and 1-vinyl-3-ethylimidazolium tetrafluoroborate (IL (BF4)), were used. It was found that the PVDF-gIL exhibited superior antibacterial performance, with almost the same mechanical and thermal performance as unmodified PVDF. Nonwovens and films made from PVDF-g-IL materials exhibited broad-spectrum antimicrobial activity against common bacteria and “superbugs” with the strong electrostatic interactions between ILs and microbial cell membranes. With extremely low IL loading (0.05 wt %), the cell reduction of PVDF-g-IL (Cl) nonwovens improved from 0.2 to 4.4 against S. aureus. Moreover, the antibacterial activity of PVDF-g-IL nonwovens was permanent for the covalent bonds between ILs and polymer chains. The work provides a simple strategy to immobilize ionic antibacterial agents onto polymer substrates, which may have great potential applications in healthcare and household applications.

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INTRODUCTION With the overuse of antibiotics and rapid rise of antibioticresistant pathogenic bacteria, antimicrobial materials have attracted great attention for both modern healthcare and industrial applications.1−4 Antimicrobial resistance may become catastrophic if not dealt with in a timely manner, and is one of greatest threats to modern medicine.5 Currently, antibacterial agents, such as peptides,6 bacteriophages,7 silver,8−11 carbon materials,12 and ionic compounds/polymers,13−17 have been widely studied for their broad-spectrum antibacterial activities. The antibacterial mechanisms primarily relate to the disturbance of bacterial cell membranes, inhibiting DNA or RNA synthesis, or acting on enzymes. Various types of antibacterial agents can be incorporated into a matrix to fabricate antibacterial composites.13−15 However, antibacterial agents are usually detrimental to the performance of the matrix, including mechanical, thermal, or processability aspects. An ideal antibacterial material should possess many features, including high antibacterial efficiency, a simple synthesis process, low cost, broad-spectrum antimicrobial activity, durable activity, excellent mechanical properties, and good biocompatibility. Increasing attention has been paid to cationic antimicrobial polymers substituted with quaternary ammonium,14 phosphonium,18 pyridinium,19 pyrrolidinium,20 or imidazolium,21−24 which exhibit antibacterial activity and high selectivity for © 2017 American Chemical Society

bacterial over mammalian cells. In these antimicrobial materials, the primary antibacterial mechanism has two aspects: the electrostatic interactions of the cationic moieties with the microbial cell membranes and the hydrophobic effect of the alkyl chains of the polymers, which can insert themselves into the hydrophobic regions of the lipid membranes and cause leakage of electrolytes and the death of bacteria.2,25 These materials can act on the bacteria without concern for antimicrobial resistance.18−24 Meanwhile, antibacterial agents, such as imidazolium, pyrrolidinium, and pyridinium cations, show good compatibility with blood and low cytotoxicity and can satisfy the requirements for antimicrobial materials.20,22−24,26 For instance, Yan and co-workers synthesized pyrrolidinium-type polyionic liquids (PIL) membranes via reversible addition− fragmentation chain-transfer polymerization (RAFT) and photocross-linking reactions, which exhibited excellent antimicrobial activity against both Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli).20 In addition, Kallitsis et al. successfully prepared the cross-linked membranes of poly(4-vinylbenzyl chloride-co-acrylic acid) (P(VBC-co-AAx)), and poly(sodium 4-styrenesulfonate-co-glycidyl methacrylate) (P(SSNa-co-GMAx)) Received: September 30, 2017 Revised: November 4, 2017 Published: November 7, 2017 4364

DOI: 10.1021/acs.biomac.7b01416 Biomacromolecules 2017, 18, 4364−4372

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Biomacromolecules through a free radical copolymerization and curing reaction at appropriate temperatures.15 The antimicrobial materials displayed strong and durable antimicrobial activity against S. aureus and P. aeruginosa. However, the polymerization or copolymerization of ionic monomers is relatively complicated, and such ionic polymers are usually brittle and weak.15,20,21,27 Therefore, it is important to prepare new antibacterial materials with both superior antibacterial performance and excellent physical performance. In this work, trace amounts of ionic liquids were immobilized onto PVDF polymer chains (PVDF-g-IL) by an irradiationinduced graft reaction in PVDF/IL blends. The unsaturated imidazolium-type ionic liquids (ILs), 1-vinyl-3-butylimmidazolium chloride ([VBIm][Cl]), and 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIm][BF4]) were first melt-blended with a PVDF matrix, followed by electron-beam irradiation at room temperature.28−32 The prepared materials showed almost the same physical properties as pristine PVDF. Taking advantage of the physical electrostatic interactions between grafted ILs and “superbugs”, the nonwoven mats and films prepared from PVDF-g-IL polymers can effectively induce bacterial fracture, the leakage of electrolytes, and cell death and exhibited broad spectrum antimicrobial activity at extremely low IL loading (0.05 wt %). In addition to the formation of covalent bonds between the polymer chains and ILs, the materials showed durable antibacterial activity against common bacteria and “superbugs”.

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Scheme 1. Schematic Illustration of Fabrication of PVDF-g-IL Graft Copolymer

Table 1. Sample Preparation of PVDF-g-IL Copolymers samples

PVDF (g)

IL (g)

absorbed dosage (kGy)

neat PVDF PVDF-g-IL(Cl)-0.05 PVDF-g-IL(BF4)-0.05 PVDF-g-IL(Cl)-0.2 PVDF-g-IL(BF4)-0.2 PVDF-g-IL(Cl)-1.0 PVDF-g-IL(BF4)-1.0

100 100 100 100 100 100 100

0 0.05 0.05 0.2 0.2 1.0 1.0

45 45 45 45 45 45 45

range was 0 to 200 °C. The thermal stability of PVDF-g-IL graft copolymers was carried out by the Thermogravimetric analysis (TGA, TA Q500). The heating rate was 10 °C/min and the temperature range was 40 to 650 °C. Scanning electron microscope (SEM, Hitachi S-4800, Japan) was used to observe the surface morphologies of nanofibers. The accelerating voltage is 2 kV and working distance is 6−8 cm. The contact angle instrument (DSA 100, Data-Physical, Germany) was used to test the contact angle with purified water at ambient temperature. Tensile tests were carried out using the Instron universal materials testing system (Model 5966). The crosshead speed is 2 mm/min. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis Ultra instrument with monochromatic Al Kα X-ray source. Wide scan was investigated in the range of 0−1300 eV, and the narrow scans were measured for the C 1s regions. In order to investigate the grafting process, the irradiated blend films were extracted with methanol for 24 h. The grafting efficiency (GE) of ionic liquids was measured by gravimetric method and calculated by eq 1:

EXPERIMENTAL SECTION

Chemicals. The poly(vinylidene fluoride) (PVDF; KF850), which has Mw ∼ 209000 and Mw/Mn = 2.0 was purchased from Kureha Chemicals in Japan. The unsaturated ionic liquids, 1-vinyl-3-butylimidazolium chloride ([VBIm][Cl]) and 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIm][BF4]), were purchased from Lanzhou Greenchem ILs, LICP, CAS, which were denoted as IL (Cl) and IL (BF4) below. The solvents such as dimethylformamide (DMF), acetone and methanol (CH3OH) were commercially available and used without any further purification. Preparation of PVDF/IL Blend Films. First, PVDF and ionic liquid were added into the Haake Polylab QC mixer, and melt blended at 190 °C to obtain PVDF/IL blends. The PVDF/IL blends with 0.05, 0.1, and 1.0 wt % IL loadings were thus prepared. Then, the corresponding PVDF/IL blend films was prepared with 300 μm thickness by a hot-press (10 MPa, 200 °C, 10 min) process and a subsequent quench process (10 MPa, 3 min). Preparation of Irradiated PVDF/IL Blend Films. The blend films with various IL loadings were put into the polyethylene bags in the atmosphere at room temperature. Then the blend films were irradiated with electron beam at 45 kGy dosage to obtain the irradiated PVDF/IL blend films (Scheme 1). The irradiation experiment was carried out with the electron-beam irradiation instrument in Shanghai Institute of Applied Physics, Chinese Academy of Sciences. Electrospinning. A total of 2 g of irradiated PVDF/IL blend film was dissolved into DMF/acetone mixed solvent (6/3, g/g) to form homogeneous solution with stirring at 80 °C for 2 h. A plastic syringe (5 mL) with a metallic needle was then used to accommodate the transparent polymer solution. A constant flow of 0.05 mL/h and a high voltage of 13−16 kV were applied to fabricate electrospinning fibers. The obtained PVDF-g-IL nonwovens were kept in desiccator after electrospinning for 12 h before the property measurements (Table 1). Characterization. Fourier Transform Infrared Spectroscopy (FTIR, Bruker Tensor) was used to detect the cross-linking reaction in the samples after electron beam irradiation with a resolution of 4 cm−1 and transmittance mode. The differential scanning calorimeter (DSC, TA, Q2000) was used to evaluate the thermal behaviors of samples. The heating/cooling rate was 10 °C/min, and the temperature

GE = (Wg − W0) ÷ WIL × 100%

(1)

where Wg is the weight of grafted copolymer after methanol extraction; W0 is the weight of neat PVDF polymer; WIL is the weight of ionic liquid in the PVDF/IL blend films before methanol extraction. Antibacterial Test. Antibacterial activity of the PVDF-g-IL nonwovens against E. coli (ATCC 8739), S. aureus (ATCC 6538), methicillin-resistant Staphylococcus aureus (MRSA, ATCC 43300), and vancomycin-resistant Enterococci faecium (VREF) were tested by TüV SüD Products Testing Co. Ltd. (Shanghai, China). The procedures of the test were according to the AATCC (American Association of Textile Chemists and Colorists) test method 100−2012 and ISO (International Organization for Standardization) 22196−2011. The reduction of bacteria by the specimen was calculated by the following eq 2:

reduction = (log10 C24h − log10 A 24h )

(2)

where A24h is the number of bacteria recovered from the inoculated test specimen swatches in the jar inoculated after 24 h; C24h is the number of bacteria recovered from the inoculated control sample after 24 h. 4365

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Figure 1. (A) Grafting efficiency of PVDF-g-IL graft copolymers with electron-beam irradiation; (B, C) DSC analysis of neat PVDF and PVDF-g-IL graft copolymers; (D) Tensile test of neat PVDF and PVDF-g-IL graft copolymers.

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RESULTS AND DISCUSSION Synthesis and Characterization of PVDF-g-IL Graft Copolymers. Immobilizing antimicrobial agents into a polymer matrix may provide new strategies for the fabrication of materials with permanent antibacterial properties. Such strategies can also avoid the leaching of antibacterial agents into the surroundings. In this research, an electron beam was applied to initiate the grafting reaction and immobilize the unsaturated ionic liquids, the antimicrobial agent, onto the PVDF polymer chains via covalent bonds (see Scheme 1). A series of PVDF-g-IL graft copolymers were synthesized with two types of ionic liquids. FTIR measurements were performed to analyze the grafting reaction (Figure S1). The characteristic absorption peaks of the imidazole group were observed at 1573 and 1553 cm−1 at high IL loadings.22 When the IL loading was low, the signal from the imidazole group was too weak to be detected in the FTIR spectra, but the grafting could be confirmed by XPS investigation, as will be shown in the following section. The grafting efficiency was measured by extraction with methanol, since methanol can dissolve IL monomers and PIL homopolymers, but not PVDF, as shown in Figures 1A and S2. It is clear that ILs can be easily grafted onto polymer chains, with grafting efficiencies exceeding 99.0%. The high grafting efficiency can be attributed to the miscibility of PVDF in imidazolium-type ILs, which ensures sufficient contact between the functional IL monomers and polymer segments.28−32 When the IL loading was increased, the grafting efficiency decreased slightly as result of the limited reactive sites on the polymer chains during electron-beam irradiation. The initial degradation temperatures of the PVDF-g-IL graft copolymers were all higher than 350.2 °C, indicating that the grafted materials had excellent thermal stability (Figure S3). As shown in Figure 1B,C, similar thermal behaviors were observed with the DSC measurements for unmodified PVDF and PVDF-g-IL. The crystallization and melting temperatures did not change drastically

and were nearly the same for all the samples. These results indicate that electron-beam irradiation at 45 kGy did not cause significant change to the polymer molecular structure. On the other hand, the mechanical properties were measured with tensile tests, as shown in Figure 1D. The PVDF-g-IL samples showed an obvious yielding behavior, with yielding strengths higher than 45 MPa, as neat PVDF. In addition, all the samples were ductile and showed good stretchability with elongation, breaking after a strain greater than 120%. Overall, the PVDF-g-IL samples showed very similar thermal and mechanical properties as unmodified PVDF. The stable chemical structure supported the further applications of IL-grafted PVDF as antimicrobial materials. Moreover, PVDF-g-IL showed almost same processability as unmodified PVDF. The grafted samples were thermoplastic and can be processed via melt or solvent processing. In the following sections, the PVDF-g-IL graft copolymers were processed into nonwoven mats and films and the antibacterial properties were investigated. Preparation and Characterization of PVDF-g-IL Nonwoven Mats. Nanofibers of PVDF-g-IL were prepared by electrospinning. Figure 2 shows the morphologies of nonwoven mats. In the unmodified PVDF nonwoven mats, many largescale beads were observed. This may be ascribed to the poor conductivity of the PVDF solution, which inhibited the formation of fibers.33−35 When a very small amount of IL (0.05 wt %) was grafted onto the PVDF, the beads disappeared completely. This can be attributed to the improvement in conductivity by the IL moieties. Meanwhile, the diameter of nanofibers markedly decreased from 120 ± 10 nm to 80 ± 6.5 nm when the IL (Cl) loading increased from 0.05 to 1.0 wt % (Figure 2B,D). Through tethering organic salt, the ionic liquids, PVDF-g-IL nonwoven mats without beads were obtained successfully, and the diameters of the nanofibers were significantly decreased. The immobilized IL moieties were responsible for the antimicrobial performance. It is necessary to analyze the concentration 4366

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Figure 2. SEM images of nonwoven samples: (A, A′) neat PVDF; (B) PVDF-g-IL(Cl)-0.05; (C) PVDF-g-IL(Cl)-0.2; (D) PVDF-g-IL(Cl)-1.0; (E) PVDF-g-IL(BF4)-0.05; (F) PVDF-g-IL(BF4)-0.2; (G) PVDF-g-IL(BF4)-1.0.

electron-beam irradiation. However, the C 1s core-level spectra of the PVDF-g-IL nonwovens were fitted with five components using the following approach: The four peak components of about equal intensities, with BE at 285.8 eV for the CH2 species, at 286.2 eV for the CO species, at 288.5 eV for the O-CO species, and at 290.5 eV for the CF2 species. The CO and O-CO species can be assigned to the main PVDF chains. The component with a BE at 284.6 eV was attributed to the hydrocarbon backbone of the immobilized ionic liquids.22 The increase in immobilized IL concentration with [IL] to [PVDF] feed ratio is indicated by the steady increase in the CH (IL) peak component intensity in Figure 3. To compare the average concentration of immobilized IL in the bulk materials with the concentration on the surface of the nanofibers (Cs), the molar ratios of [C]IL/[C]PVDF in the bulk materials and on the surface of nanofibers are listed in Table 2. It was found that the concentrations of IL immobilized on the surface of the nanofibers were higher than the average immobilized concentrations in the

and distribution of ILs on the surface of nanofibers. The concentration immobilized on the surface was determined from the XPS-derived carbon (IL) to carbon (PVDF) molar ratios. Considering that the unmodified PVDF main chain has a [C]IL/[C]PVDF molar ratio of zero, the surface immobilized concentration, expressed in terms of the number of ionic liquid units per PVDF repeat unit, and can be obtained from the XPSderived surface [C]IL/[C]PVDF ratio. Figure 3 shows the C 1s core-level spectra of nonwoven mats prepared from unmodified PVDF and PVDF-g-IL graft copolymer with various IL loadings. Note that all the materials were irradiated with the electron beam before electrospinning. In the case of unmodified PVDF nonwovens, the C 1s core-level spectrum can be fitted with four components, with binding energies (BE) at 285.8 eV for the CH2 species, 286.2 eV for the CO species, 288.5 eV for the O-CO species, and 290.5 eV for the CF2 species.36−38 Specially, the CO and O-CO species can be ascribed to the reaction between the PVDF main chains and O2 during 4367

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of ILs on the surface of the nanofibers. Meanwhile, the nonwoven mats immobilized with the hydrophilic IL (Cl) induced a decrease in contact angle with increasing IL loading. The water contact angle decreased from 140.3° to 137.5° when 1.0 wt % IL (Cl) was added to the unmodified PVDF nonwoven mats. In contrast, the contact angle increased gradually with increasing IL loadings for the PVDF-g-IL (BF4) samples. This makes sense because IL (Cl) is hydrophilic, while IL (BF4) is hydrophobic. Antimicrobial Activity of PVDF-g-IL Nonwoven Mats. Figure 4 shows the antimicrobial activity against Gram-positive and Gram-negative bacteria of PVDF-g-IL nonwoven mats with the two types of ionic liquid. The antimicrobial effect was measured in terms of the reduction in bacterial cells after 24 h in contact with each material. As seen in Figure 4A, the unmodified PVDF nonwoven mats and PVDF-g-IL (Cl) nonwoven mats with various IL loadings all exhibited excellent antimicrobial activity against E. coli, which are Gram-negative bacteria. The reduction values were all more than five, which meant that the disinfection rate of bacteria reached 99.99%. However, the unmodified PVDF nonwoven mats exhibited poor antimicrobial activity against the Gram-positive bacteria S. aureus. The antimicrobial activity increased with increasing IL loading. Only 0.05 wt % IL loading greatly improved the antimicrobial activity, with the reduction improving from 1.6 to 4.4. Furthermore, the reduction improved from 1.6 to 5.8 when 1.0 wt % IL (Cl) was added. The highly efficient antimicrobial properties of PVDF-g-IL nonwoven mats can be attributed to the strong electrostatic interactions between the ILs and the microbial cell membranes, which can destroy the lipid bilayer structure of the membranes and cause leakage of electrolytes. This may be fatal to the bacteria, as shown in Scheme 2. However, in the unmodified PVDF nonwoven mats, the difference in antimicrobial activity between E. coli and S. aureus may be ascribed to the different structures of the bacterial cell walls. The cell wall of Gram-negative bacteria is composed of two thin layers (about 9−15 nm), while the cell wall of Gram-positive bacteria is thick, about 20−80 nm.20 Therefore, it is hypothesized that the cell wall E. coli, a Gram-negative bacteria, would deform more easily than in a Gram-positive bacteria. The polarization of the C−F bond on the surface of PVDF nonwoven mats can induce the fracture of bacteria, ultimately leading to death due to electrolyte leakage. In Figure 4B, the PVDF-g-IL (BF4) shows almost the same results as PVDF-g-IL (Cl) against bacteria E. coli. However, PVDF immobilized with IL (Cl) showed higher antimicrobial activity against the Grampositive bacteria S. aureus at comparable grafting ratios. This result can be attributed to the higher concentration of immobilized IL (Cl) on the surface of the nanofibers, which are listed above in Table 2. Besides, IL (Cl) had longer alkyl chains at the N position of the imidazolium cation, which could more easily insert themselves into the cell walls of bacteria, which disturbed the peptidoglycan layer and led to cell death. To evaluate the broad-spectrum antimicrobial activity, unmodified PVDF and PVDF-g-IL nonwoven mats were tested against two “superbugs”, MRSA and VREF. The results for the unmodified PVDF, PVDF-g-IL (Cl)-0.2, and PVDF-g-IL (BF4)0.2 nonwoven mats are shown in Figure 4C. The unmodified PVDF nonwoven mats exhibited poor antimicrobial activity for all three bacteria. When IL (Cl) and IL (BF4) were immobilized on the polymer chains, the PVDF-g-IL nonwoven mats exhibited higher antimicrobial activity than before. The reduction improved from 1.6 to 5.4 for the PVDF-g-IL (Cl)-0.2 nonwoven

Figure 3. XPS C 1s core-level spectra of neat PVDF and PVDF-g-IL nonwovens with various IL loadings.

Table 2. Molar Ratios of [C]IL/[C]PVDF in the PVDF-g-IL Graft Copolymers were Calculated from Bulk Materials and the XPS C 1s Spectra Results samples

[C]IL/[C]PVDF ratioa

[C]IL/[C]PVDF ratiob

neat PVDF PVDF-g-IL(Cl)-0.05 PVDF-g-IL(BF4)-0.05 PVDF-g-IL(Cl)-0.2 PVDF-g-IL(BF4)-0.2 PVDF-g-IL(Cl)-1.0 PVDF-g-IL(BF4)-1.0

0 0.0008 0.0007 0.0031 0.0027 0.0155 0.0135

0 0.0061 0.0042 0.0190 0.0130 0.0409 0.0368

a

Calculated from the mass ratios in theory. bDetermined from the corrected XPS C 1s core-level spectral area ratios of the respective samples.

PVDF-g-IL copolymers. In the case of PVDF-g-IL (Cl)-1.0 nonwoven mats, the Cs was 0.0409, which was nearly three times the theoretical value (0.0155). We have also measured the XPS of the bulk sample (the fracture surface of as-irradiated sample), as shown in Figure S4. The real molar ratios of [C]IL/[C]PVDF is 0.015, which is almost the same to the theoretical ratios shown in Table 2.28−32 It is therefore concluded that the immobilized ionic liquids can concentrate on the surface of the nanofibers during electrospinning. The enriched ILs on the surface of the nanofibers can improve the antimicrobial activity of materials, as will be described below. The water contact angles (WCA) of the nonwoven mats were measured as a function of IL loading (Figure S5). The water contact angle of the PVDF nonwoven mats did not significantly change after electron-beam irradiation when compared with the PVDF nonwoven mats without irradiation.39 The hydrophobicity of irradiated PVDF was similar to unmodified PVDF without treatment. In the case of PVDF-g-IL nonwoven mats, the water contact angle markedly changed due to the existence 4368

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Figure 4. Antibacterial activity of PVDF-g-IL nonwovens with two kinds of ionic liquids: (A) IL (Cl) and (B) IL (BF4) of various contents; (C) the antibacterial activity of PVDF-g-IL (Cl, BF4) nonwovens for common bacteria and “superbugs”.

study the change of immobilized IL concentration on the surface of the nanofibers. As seen in Figure 6A, it is obvious that after immersion with methanol, the CH (IL) peak component intensity increased, which can be ascribed to the migration of immobilized IL from the interior to the surface of the nanofibers. The increase of IL concentration on the surface induced higher antimicrobial activity, as shown in Figure 6B. The reduction improved from 3.3 to 5.8 after immersion with methanol for 24 h. The results indicated that IL immobilized on the surface of nanofibers possessed excellent robustness and endowed the materials with durable antimicrobial activity. Mechanical Properties of PVDF-g-IL Nonwoven Mats. The mechanical properties of antimicrobial materials are also important for practical applications. In Figure 7, a tensile strength test was carried out to study the mechanical properties of PVDF-g-IL nonwoven mats. Since it had many large-scale beads, the unmodified PVDF nonwoven mats exhibited poor mechanical strength, and broke at 56.2% elongation, with a breaking strength of 4.55 MPa. However, the PVDF-g-IL (Cl)-0.05 showed improved mechanical properties, with 142.5% elongation, and a breaking strength of 5.05 MPa. The same phenomena were observed for all of the IL-grafted PVDF nonwoven mats with both types of ionic liquids, IL (Cl) and IL (BF4). The immobilization of IL on the PVDF molecular chains endowed the materials with improved mechanical performance and broadened the possibility of applications. Antimicrobial Activity of PVDF-g-IL Films. In the previous sections, the mechanical and antimicrobial properties, along with the morphology, of PVDF-g-IL nonwoven mats were studied in detail. The synthesized PVDF-g-IL can also be processed into various products. In this section, PVDF-g-IL films were prepared by solution casting, and the antimicrobial activity with one of the two types of ionic liquid were evaluated

Scheme 2. Schematic Illustration of the Antimicrobial Mechanism of PVDF-g-IL Nonwovens with Bacteria

mats. The results indicated that the PVDF-g-IL nonwoven mats possessed broad-spectrum antimicrobial activity. The antimicrobial activity of PVDF-g-IL (Cl)-0.05 was also examined using a plate-counting assay. S. aureus (ATCC 6538) was selected as the model bacteria. Figure 5 shows images of the culture plates. The control plate had dense colonies after 0 and 24 h. The same colony density was observed on the plates of unmodified PVDF nonwoven mats. In contrast, no surviving bacterial colonies were found on the PVDF-g-IL (Cl)-0.05 nonwoven mats. The results suggest that the antimicrobial activity originated with the IL (Cl) on the surface of the nanofibers, rather than from the PVDF matrix. As antimicrobial materials, the durability of activity is essential for practical applications. In Figure 6, the durability of antimicrobial activity was investigated with immersion of the PVDF-g-IL nonwoven mats in methanol before and after antimicrobial testing. Methanol is a good solvent for IL (Cl) and IL (BF4). XPS C 1s core-level spectra were obtained to 4369

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Figure 5. Photographs of the antimicrobial assay against S. aureus (ATCC 6538): (A) control sample after 0 h, (B) control sample after 24 h, (C) PVDF nonwovens after 24 h, (D) PVDF-g-IL (Cl)-0.05 nonwovens after 24 h.

Figure 6. (A) XPS C 1s core-level spectra of PVDF-g-IL nonwovens before and after methanol immersion, (B) antibacterial activities of nonwovens against S. aureus.

against Gram-positive and Gram-negative bacteria. The results are shown in Figure 8. Without the addition of ionic liquid, the unmodified PVDF films displayed poor antimicrobial activity against E. coli and S. aureus bacteria. The difference of antimicrobial activity between PVDF nonwoven mats and films may be ascribed to the change in surface area. The PVDF nonwoven mats had larger surface areas than the films and could interact better with the bacteria. The stronger interaction between PVDF and the bacteria induced the deformation and death of the bacteria. When 0.05 wt % IL (Cl) was immobilized onto the polymer chains, the films exhibited excellent antimicrobial activity against E. coli bacteria, and the reduction improved from 0.1 to 4.1. This reduction improved gradually with increasing IL (Cl) loading. The same phenomenon was observed with the PVDF-g-IL (BF4) films.

Figure 7. Tensile test of PVDF-g-IL nonwovens with various contents of ILs. 4370

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Figure 8. Antibacterial properties of PVDF-g-IL films with two types of ionic liquids: (A) IL (Cl) and (B) IL (BF4) of various IL loadings.

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CONCLUSIONS

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ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.biomac.7b01416. Additional data, including FTIR analysis of PVDF-g-IL materials, the grafting efficiency, and the TGA results, XPS of as-irradiated sample, and the water contact angle of nonwovens (PDF).

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REFERENCES

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Novel PVDF-g-IL graft copolymers were designed and synthesized via an irradiation-induced graft reaction. Because of the good miscibility between imidazolium-type ionic liquids and PVDF chains, the ILs can interact with the PVDF and become immobilized efficiently onto the polymer chains. PVDF-g-IL exhibited excellent mechanical properties along with good processability, very similar to unmodified PVDF. The nonwoven mats and films made from PVDF-g-IL graft copolymers displayed broad spectrum antimicrobial activity against common bacteria (E. coli, S. aureus) and “superbugs” (VREF, MRSA) and high antimicrobial efficiency at extremely low IL loading (0.05 wt %). Additionally, when covalently tethering ILs onto PVDF polymer chains, the materials possessed durable antibacterial activity. The present work provides a simple and universal strategy for immobilizing functional ionic monomers into polymer substrates, which may have significant potential applications in healthcare and other industries.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 57128867899. Tel.: +86 7128867026. ORCID

Yongjin Li: 0000-0001-6666-1336 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21674033, 21374027) and National Key R&D Program of China (2017YFB0307704). 4371

DOI: 10.1021/acs.biomac.7b01416 Biomacromolecules 2017, 18, 4364−4372

Article

Biomacromolecules

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DOI: 10.1021/acs.biomac.7b01416 Biomacromolecules 2017, 18, 4364−4372