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RESEARCH ARTICLE | Updated:2024-08-06
    • Synthesis, Characterization of Polyethylene Ionomers and Their Antibacterial Properties

    • Wu Jia-Jia

      a ,  

      Wang Fei

      a ,  

      Wan Peng-Qi

      b ,  

      Pan Li

      a ,  

      Xiao Chun-Sheng

      b ,  

      Ma Zhe

      a ,  

      Li Yue-Sheng

      a ,  
    • Chinese Journal of Polymer Science   Vol. 42, Issue 8, Pages: 1077-1084(2024)
    • DOI:10.1007/s10118-024-3150-4    

      CLC:
    • Published:01 August 2024

      Published Online:26 June 2024

      Received:21 March 2024

      Revised:21 April 2024

      Accepted:22 April 2024

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  • Jia-Jia Wu, Fei Wang, Peng-Qi Wan, et al. Synthesis, Characterization of Polyethylene Ionomers and Their Antibacterial Properties. [J]. Chinese Journal of Polymer Science 42(8):1077-1084(2024) DOI: 10.1007/s10118-024-3150-4.

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    Abstract

    Owing to its high production volume and wide range of applications, polyethylene has gained a great deal of attention, but its low surface energy and non-polar nature have limited its application in some important fields. In this study, ethylene/11-iodo-1-undecene copolymers were prepared and used as the intermediates to afford a series of imidazolium-based ionomers bearing methanesulfonate (CH3SO3), trifluoromethanesulfonate (CF3SO3), or bis(trifluoromethane)sulfonimide (Tf2N) counteranions. The tensile test results showed that the stress-at-break (7.8−25.6 MPa) and the elongation-at-break (445%−847%) of the ionomers could be adjusted by changing the counterion species and the ionic group contents. Most importantly, the ionomers exhibited marvelous antibacterial activities against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The ionomers bearing Tf2N exhibited antibacterial activities >99% against both S. aureus and E. coli when ionic content reached 9.1%. The imidazolium-based ionomers prepared in this work demonstrated excellent comprehensive properties, especially high-efficient and broad-spectrum antibacterial ability, exhibiting the potential for the application as the antibacterial materials in packaging, medical, and other fields.

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    Keywords

    Polyethylene; Ionomer; Polymerization catalysis; Imidazolium; Antibacterial material

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    INTRODUCTION

    Polyethylene (PE) is one of the most highly produced polyolefin materials and has a wide range of applications in daily necessities, automobiles, packaging, construction, agriculture, and other fields.[

    1,2] However, PE, which is composed of only two elements, C and H, is a typical non-polar material and suffers from the drawbacks of poor dyeability, hydrophilicity, and compatibility with polar materials.[3,4] The non-polar nature of PE limits its application in some important fields. Introducing certain polar groups, e.g., hydroxyl, ester or ionic groups, into polymer chains can effectively improve the non-polar nature of PE.[5−9] Ionomers, also known as ionic crosslinked polymers, are a class of polymers containing a small number of ionic groups (usually <10 mol%), which are distributed randomly or orderly in the backbones or branches of the polymer.[10] The introduction of ionic groups can effectively improve the performance of PE and extend its applications to high-end fields, such as ion exchange membranes,[11] high-voltage insulation materials,[12] polymer electrolytes,[13] antibacterial materials,[14] etc.[15]
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    PE ionomers are usually prepared by direct copolymerization or reactive intermediate methods. The direct copolymerization method has strict requirements for the catalyst, and only a very few catalysts are capable of directly copolymerizing ethylene with ionized monomers.[

    16−18] This approach requires a catalyst with not only a high copolymerization ability but also a good tolerance to polar ionic groups. Ye et al. used a highly tolerant α-diimide palladium to catalyze the direct copolymerization of ethylene with acrylate-based ionic monomers bearing tetraalkylammonium to obtain PE ionomers containing tetraalkylammonium ionic groups.[19] Coates et al. used the Grubbs II catalyst to directly copolymerize tetraalkylammonium-functionalized cyclooctene derivatives with cyclooctene via ROMP, and PE ionomers carrying tetraalkylammonium ionic groups in the side chains were obtained after hydrogenation.[20] Vachon et al. obtained PE ionomers by high-pressure radical copolymerization of ethylene with ion-pair comonomers comprising amine-terminated methacrylates or methacrylic acid.[12]
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    The reactive intermediate method involves the synthesis of PE containing highly reactive groups which then undergoes functional group transformations to yield functionalized PE.[

    21−23] Previously, we utilized this method to prepare ethylene/5-iodomethyl-2-norbornene/1-hexene terpolymers and then to obtain imidazolium-based PE ionomers by nucleophilic substitution reaction with N-methylimidazole.[24] Chung et al. used zirconocene to catalyze random copolymerization of ethylene and silane-protected α,ω-amino-olefin, followed by the conversion of the silane-protected amino groups to NR3+Cl groups to produce PE ionomers.[25] Cui et al. used scandium catalysts with different steric hindrance to copolymerize ethylene with 6-phenoxy-1-hexene, and then the phenoxy group was sequentially converted to Br and imidazolium cation groups, thus creating PE ionomers.[26] The ability to synthesize ionomers with well-defined structures and well-controlled ionic group distributions, as well as less restriction on the catalysts, make the reactive intermediate method a more practical approach.
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    PE materials have been widely used in biomedical applications, such as packaging, containers, implants, artificial joints, catheters, etc.[

    27−29] Therefore, there is a great demand for the development of PE materials with antibacterial properties. Currently, PE materials with antibacterial properties are usually prepared by blending or coating the antimicrobial agents directly with the polymers.[30,31] However, the non-polar nature of PE makes it incompatible with antimicrobial agents, thus limiting its performance and application in the medical field.[32] In this work, we adopted the reactive intermediate method to introduce ionic groups into the polymer side chains to address the above-mentioned issues. Ethylene and 11-iodo-1-undecene (IUD) were copolymerized using rac-ethylenebis(1-indenyl)zirconium dichloride catalyst (rac-Et(Ind)2ZrCl2) to afford ethylene/IUD (E/IUD) copolymers which acted as the intermediates. A series of imidazolium-based PE ionomers bearing methanesulfonate (CH3SO3), trifluoromethanesulfonate (CF3SO3), and bis(trifluoromethane)sufonimide (Tf2N) counteranions were synthesized via nucleophilic substitution with N-methylimidazole followed by ion-exchange reactions. These ionomers exhibited not only excellent mechanical properties but also marvelous antibacterial capabilities. The effects of ionic content and counterion type on the mechanical and antibacterial properties of PE ionomers were investigated in detail.
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    EXPERIMENTAL

    Materials

    All oxygen- and/or moisture-sensitive procedures were performed in an Etelux glove box or under nitrogen protection using standard Schlenk techniques. Anhydrous toluene was purified using the Etelux solvent purification system. The rac-Et(Ind)2ZrCl2 complex was purchased from Strem Chemicals, Inc., America. Methylaluminoxane (MAO), 1.5 mol/L in toluene was purchased from Innochem Technology, Co., Ltd., China. Commercial ethylene (99.9%) was purchased from Liufang Gas, Co., Ltd., China, and was purified by oxygen-trap columns and molecular sieves prior to use. IUD was synthesized according to the procedures described in previous literature.[

    33] N-methylimidazole (99%) and lithium bis(trifluoromethane sulphonyl)imide (LiTf2N, 98%), sodium methanesulfonate (CH3SO3Na, 98%), potassium trifluoromethanesulfonate (CF3SO3K, 98%) were purchased from Heowns Technology Co., Ltd., China.
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    Preparation of E/IUD Copolymers and Their Ionomers

    The copolymerization of ethylene and IUD was carried out in a Schlenk flask equipped with a mechanical stirrer with a total system volume of 120 mL. The flask was heated at 120 °C for 45 min under vacuum, then charged with nitrogen and evacuated to remove water and oxygen. After the flask was cooled to ambient temperature, it was placed in a water bath at a constant temperature of 25 °C. Ethylene gas was then pressurized to the flask (constant pressure of 0.1 MPa). IUD, toluene, and MAO were added sequentially. The rac-Et(Ind)2ZrCl2 complex (15 μmol) was dissolved in toluene (10 mL) and added to the flask to initiate polymerization. After stirring at 600 r/min for 20 min, the gas feed was stopped and the polymerization was terminated by air. The reaction mixture was rapidly poured into an acidic ethanol solution (5% HCl) for precipitation and the white solid was obtained after filtration. After full washing with ethanol, the resulting product was dried under vacuum at 80 °C to constant weight. The resultant copolymers were reacted with CH3SO3Na, CF3SO3K and LiTf2N, respectively, in an ion-exchange reaction, and then the PE ionomers bearing CH3SO3, CF3SO3 and Tf2N counterions, respectively were obtained.

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    Test Methods for Antibacterial Properties of Polymers

    The antibacterial properties of the copolymers and ionomers were demonstrated by the ability to suppress two of the most typical bacteria, Staphylococcus aureus (S. aureus ATCC 25923) and Escherichia coli (E. coli ATCC 25922). The sandwich method was used for examining the antibacterial properties, and the bacterial cultivation and testing procedures were detailed in published studies.[

    34] Bacterial colonies were photographed after 4 h of cocultivation at 37 °C and the number of colonies (Q) was calculated. Five sets of parallel tests were set up for each sample. Bacterial survival was calculated using the number of colonies of the experimental sample (Qsample) and positive control (Qcontrol) according to the following formula:
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    Bacterialsurvival=QsampleQcontrol×100%math 1

    RESULTS AND DISCUSSION

    Synthesis and Characterization of E/IUD Copolymers

    The introduction of halogenated groups into non-polar polymer chains not only offered polar groups and structural diversity to PE, but also provided reaction sites for subsequent conversion to various functional groups. However, the halogenated monomers, i.e., ω-halo-α-olefins, may have a poisoning effect on the active centre. According to previous study, the intensity of the poisoning effect of halogen groups on the active centre followed the order: Cl > Br > I.[

    35] And the chain transfer side reactions caused by halogen groups decrease with the increase of atom diameter (Cl < Br < I).[35] Therefore, monomers containing iodine groups were selected for the copolymerization. Moreover, halogen groups at the end of the growing chain were able to coordinate with the active center via the backbite reaction, which led to catalyst deactivation.[36] To inhibit the poisoning of the catalyst by the backbite coordination, long-chain halogenated monomers should be selected for the copolymerization. Considering all of the above, the IUD was chosen as the comonomer.
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    The rac-Et(Ind)2ZrCl2 is a typical bridged metallocene catalyst for olefin polymerization. In previous studies, this catalyst has been demonstrated to efficiently catalyze ethylene polymerization and copolymerization with various comonomers.[

    37] Moreover, the catalyst has been proven to have good tolerance towards several functional groups.[38] Thus, rac-Et(Ind)2ZrCl2/MAO was selected to synthesize the reactive intermediates.
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    Scheme 1 shows the synthesis route to E/IUD copolymers and imidazolium-based PE ionomers. Activated by MAO, rac-Et(Ind)2ZrCl2 can efficiently catalyze the copolymerization of ethylene (0.1 MPa) with IUD at 25 °C to give a series of copolymers with high molecular weight (Mw), relatively narrow molecular weight distributions (MWDs), and different IUD incorporations. As the data presented in Table 1, the incorporation of IUD increases with IUD feed. The iodine atom in IUD acted as a reactive site in the subsequent reaction process, which underwent a nucleophilic substitution reaction with N-methylimidazole.

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    Fig 1  Synthesis of E/IUD copolymers and PE ionomers.

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    Table 1  Polymerization results of ethylene and IUD. a
    EntrySampleIUD b (mL)Incorp. c (mol%)Yield (g)Activity dMw e (kDa)MWD eTm f (°C)ΔH f (J/g)XC g (%)
    1 PE / / 3.3 0.66 344 2.6 135.2 139.4 47.6
    2 E/IUD (2.9) 0.9 2.9 5.1 1.02 279 2.5 128.3 78.9 26.9
    3 E/IUD (6.1) 1.8 6.1 6.9 1.38 218 2.5 126.9 43.9 15.0
    4 E/IUD (9.1) 2.7 9.1 8.3 1.66 176 2.2 124.6 31.6 10.8
    5 E/IUD (12.2) 3.6 12.2 9.8 1.96 143 2.3 120.3 21.2 7.2
    6 E/IUD (15.7) 4.5 15.7 10.4 2.08 125 2.2 114.5 5.9 2.0
    7 E/IUD (16.6) 5.4 16.6 10.6 2.12 111 2.3 ND ND ND

    a Polymerization conditions: Cat.Zr=15 μmol, ethylene pressure=0.1 MPa, n(Al)/n(Zr)=1000, Vtotal=120 mL, copolymerization at 25 °C for 20 min; b The feed of comonomer; c Incorporation of comonomer was determined by 13C-NMR; d Catalytic activity: 106 g·molZr−1·h−1; e The weight-average molecular weight and MWD were determined by high temperature GPC; f The melting temperature and melting enthalpy were determined by DSC; g The degree of crystallinity was calculated using the melting enthalpy. ND: Not determined.

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    Fig. 1 shows the 13C-NMR spectrum of a representative E/IUD copolymer. All resonance signals were clearly assigned according to the literature.[

    39] The signal peaks at 29.5, 29.7, 34.3 and 37.9 ppm were attributed to the ethylene units and the IUD units in the main chain. The resonance peak at 6.3 ppm was attributed to the methylene carbon adjacent to iodine atom in the IUD units. The other peaks in the range of 27.1−33.9 ppm were assigned to other carbon atoms in the side chains of the IUD units. The 13C-NMR spectrum indicated that the IUD units were isolated in the copolymer chains, for no resonance signal corresponding to the continuous insertion of IUD units was observed. These results demonstrated the random nature of E/IUD copolymers prepared by rac-Et(Ind)2ZrCl2 catalyst.
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    Fig 1  13C-NMR spectrum of the copolymer (IUD 2.9%).

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    The copolymerization results of ethylene with IUD are summarized in Table 1. All obtained copolymers displayed narrow MWDs ranging from 2.2 to 2.5 with unimodal GPC curves. The values of Mw of E/IUD copolymers decreased from 279 kDa to 111 kDa with the increase of IUD incorporation (Fig. S3 in the electronic supplementary information, ESI). This was due to the steric effect of IUD located at the end of the propagating chain slowed down the monomer coordination, thus chain transfer was accelerated. The Mw of the copolymer decreased with increasing IUD feed, because of the increased ratio of chain growth rate to chain transfer rate. Such results demonstrated the high efficiency of the rac-Et(Ind)2ZrCl2/MAO catalytic system.

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    All copolymers showed a single melting temperature (Tm) in the DSC melting curves (Fig. S4 in ESI). With the increase of IUD content, the Tms of E/IUD copolymers decreased to disappear, and the melting enthalpies (ΔHms) decreased to zero, which were determined by the position and area of the melting peaks, respectively. This was because the insertion of IUD interfered with the crystalline structure and disrupted the structural regularity of the main chains, leading to a decrease in the crystallinity (XC) of the E/IUD copolymers. The glass transition temperatures (Tgs) of E/IUD copolymers gradually shifted to lower temperatures with the increase of IUD content (Fig. S5 in ESI). This was because of the decreased crystallinity and enhanced chain mobility caused by long side chains.

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    Preparation of Imidazolium-based PE Ionomers

    To investigate the effect of the ionic groups on copolymer properties, three typical counterions (CH3SO3, CF3SO3, Tf2N) were chosen because of their easy availability and different properties.[

    40] Side chains with terminal iodine groups were randomly distributed in the molecular chains of the E/IUD copolymers. Using the carbon-iodine bond as the reactive site, the copolymers underwent nucleophilic substitution with N-methylimidazole followed by an ion-exchange reaction with CH3SO3Na, CF3SO3K or LiTf2N. In the above procedure, excess N-methylimidazole and metal salt (5 equiv. of IUD) were added to ensure that each reaction was complete. This synthetic strategy enabled the efficient synthesis of imidazolium-based PE ionomers bearing different counterions.
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    Fig. 2 displays the 1H-NMR spectra of the representative copolymer (IUD 15.7%) and the corresponding ionomers bearing different counterions (CH3SO3, CF3SO3, Tf2N). For the copolymers, the characteristic peak of CH2 protons adjacent to iodine appeared in the range of 3.0 ppm to 3.5 ppm. However, in the 1H-NMR spectra of the ionomers, no signal peaks were observed in the range from 3.0 ppm to 3.5 ppm. Meanwhile, new resonance signals appeared at 3.9−4.5 ppm, which were attributed to the CH2 (a in Fig. 2) and CH3 (b in Fig. 2) protons adjacent to the nitrogens of imidazole group. The ionomers with different counterions showed different resonance signals of imidazole groups. The chemical shifts of the CH2 proton (c in Fig. 2) between the two nitrogen atoms on the imidazole ring were quite different because of the counterions (CH3SO3: 10.0 ppm, CF3SO3: 9.3 ppm, Tf2N: 8.7 ppm). The resonance signals at 7.5 ppm were assigned to the rest CH2 protons (d and e in Fig. 2) on the imidazole ring.

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    Fig 2  1H-NMR spectra of the copolymer (IUD 15.7%) and ionomers with different counterions.

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    To examine the completeness of the ion exchange, silver nitrate solution was used to test for the presence of iodide ions in the ionomers.[

    41] The detailed procedure was presented in ESI. The results demonstrated that none of the three ionomers contained iodide ions, proving that the iodine ions have been completely converted to the corresponding counterions by ion-exchange reactions. The successful preparation of imidazolium-based ionomers bearing three kinds of counterions (CH3SO3, CF3SO3, Tf2N) via the reaction intermediate method was therefore confirmed.
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    Crystallization and Thermal Properties of PE Ionomers

    The introduction of ionic groups in semi-crystalline polymers altered the values of Tm and XC. In order to explore the thermal and crystalline properties, the ionomers were carefully characterized by differential scanning calorimeter (DSC), dynamic mechanical analyzer (DMA), and wide-angle X-ray diffraction (WAXD). To clearly demonstrate the effect of different counterion types on the thermal properties, the ionomers with a relatively high ionic content of 12.2% were selected for discussion. The copolymer (IUD 12.2%) exhibited a melting peak at 120.3 °C, while the corresponding ionomers with CH3SO3, CF3SO3, and Tf2N counterion showed a melting peak at 123.9, 123.1, and 122.8 °C, respectively (Fig. 3). A similar trend was observed for other ionomers with different counterion contents.

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    Fig 3  DSC melting curves of the copolymer (IUD 12.2%) and the corresponding ionomers.

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    The above results indicated that the introduction of ionic groups led to the increase of Tm, which was attributed to the additional ionic interactions and the improved intermolecular forces. The ionomers with the same ionic content but different counterion types displayed different Tm, among which those bearing CH3SO3 counterion had the highest Tm due to their strongest ionic interaction.

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    The effect of different counterions on the crystalline behavior of the copolymers and ionomers was investigated by WAXD (Fig. S6 in ESI). Two crystalline peaks at 2θ=21.6° and 24.0° corresponding to 110 and 200 spacings and one amorphous peak at 2θ=19.6° were observed for all the copolymers and ionomers.[

    42] Although the introduction of long side chains and ionic groups disrupted the crystalline structure of the main chain, the obtained ionomers still exhibited relatively strong crystallization ability.
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    After ionization, the Tg values of the ionomers shifted from −27.8 °C of the copolymer (IUD 12.2%) to −4.9 (CH3SO3), −13.4 (CF3SO3), and −20.2 °C (Tf2N), respectively (Fig. 4). The introduced ionic groups could form ionic aggregates in polymer matrix. Such aggregation acted as physical cross-links, restricting the movement of the polymer segments, which in turn led to an increase in Tg value.[

    43,44] The binding energies between different counterions and imidazole cations were calculated using the density functional theory (DFT), which reflected the aggregation intensities in the ionomers. For example, CH3SO3 with a binding energy of −133.6 kcal/mol interacts most strongly with the imidazole cation, thus the ionomer bearing CH3SO3 displayed the highest Tg among the three imidazolium-based ionomers with the same ionic content.[41]
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    Fig 4  Typical DMA curves of the E/IUD copolymer and ionomers.

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    The effect of ionic content on Tg of the ionomers bearing the same counterion was also considered. For the ionomers bearing CF3SO3, Tg varied from −13.4 °C to −6.4 °C as the ionic content varied from 12.2% to 6.1%. The same trend was observed for the ionomers bearing CH3SO3 and Tf2N counterions. An increase in ionic content also meant an increase in side chain content, thus a greater number of side chains would lead to decreased crystallinity and enhanced chain mobility, and consequently decreased Tg.

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    Mechanical Properties of PE Ionomers

    To investigate the effect of the introduction of ionic groups on the mechanical properties, the stress-strain behaviors of the copolymers and ionomers were tested. For E/IUD copolymers, both the stress-at-break and elongation-at-break of the copolymers decreased with increasing IUD content. As presented in Fig. S7 (in ESI), the copolymer (IUD 2.9%) exhibited a stress-at-break of 20.5 MPa and an elongation-at-break of 900%. As the content of IUD reached 12.2%, the stress-at-break decreased to 2.3 MPa and the elongation-at-break decreased to 152%. On the one hand, the decrease in stress-at-break was due to the disruption of the crystalline structure of PE backbone by the incorporated IUD units. On the other hand, the decrease in elongation-at-break of the copolymers was attributed to the decreased Mw with IUD incorporation, because lower Mw weakened the chain entanglement that favored the ductility of polymers.

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    Fig. 5 shows the effect of introducing ionic groups on mechanical properties. Compared with the copolymer (IUD 6.1%), the stress-at-break values of the ionomers increased from 9.4 MPa to 25.6 MPa (CH3SO3), 19.4 MPa (CF3SO3), and 15.8 MPa (Tf2N), respectively. However, the elongation-at-break decreased from 808% to 529% (CH3SO3), 611% (CF3SO3), and 734% (Tf2N), respectively. The same trend was observed for the copolymer and ionomers with an IUD or ionic content of 2.9% (Fig. S8 in ESI). Compared with the copolymer (IUD 12.2%), the stress-at-break values of the ionomers significantly increased from 2.3 MPa to 23.0 MPa (CH3SO3), 18.1 MPa (CF3SO3), and 10.0 MPa (Tf2N), respectively. It is noteworthy that the elongation-at-break of the ionomers with ionic content of 12.2% displays a significant increase different from the ionomers with ionic contents of 2.9% and 6.1%. The elongation-at-break of the ionomers bearing CH3SO3, CF3SO3, and Tf2N increased from 152% of the copolymer (IUD 12.2%) to 495%, 629% and 753%, respectively. The same trend was also observed for the copolymer and ionomers with an IUD or ionic content of 15.7% (Fig. S9 in ESI).

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    Fig 5  Stress-strain curves of (a) the copolymer (IUD 6.1%) and ionomers, and (b) the copolymer (IUD 12.2%) and ionomers.

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    The results discussed above indicated that the introduction of ionic groups remarkably affected the mechanical properties of the copolymers. This can be explained by the ability of ionic groups to form ionic aggregates in the polymer matrix, which act as physical cross-links to strengthen the intermolecular forces.[

    45,46] Generally, in ionomers, physical cross-links were considered as dipolar interactions between ion pairs, which formed stable quadrupoles or primary aggregates exhibiting several mutually negating dipoles.[47]
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    The effect of counterion species on the mechanical properties of the ionomers has also been considered. The stress-at-break values of the ionomers with CH3SO3 ranged from 17.9 MPa to 25.6 MPa, and the elongation-at-break values ranged from 445% to 536%. The stress-at-break values of the ionomers with CF3SO3 ranged from 14.6 MPa to 24.2 MPa, and the elongation-at-break values ranged from 549% to 716%. The stress-at-break values of the ionomers with Tf2N ranged from 7.8 MPa to 21.2 MPa, and the elongation-at-break values ranged from 681% to 847%. These results suggested that counterion type considerably impact the mechanical properties of the ionomers at the same ionic content. The stress-at-break values of the ionomers exhibited the following order: CH3SO3 > CF3SO3 > Tf2N, while the elongation-at-break values showed the opposite trend. The trend in stress-at-break was consistent with the previously reported binding intensity of the counterions with the imidazole cation.[

    41] In addition, counterions with larger ionic radius exhibited lower ionic binding intensity, thus increasing the mobility of the chain. For example, the ionomers bearing Tf2N exhibited the highest value of elongation-at-break, because Tf2N had the largest ionic radius and the lowest ionic binding intensity among the three counterions used.
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    Antibacterial Properties of PE Ionomers

    Ionomers have been reported to play an important role in biomedical applications such as antibacterial material. To verify the antibacterial properties of the prepared copolymers and ionomers, the sandwich method in which a bacterial solution was coated in the middle of two polymer membranes was employed for testing. S. aureus, a typical Gram-positive bacterium, and E. coli, a typical Gram-negative bacterium, were selected for antibacterial test.[

    48,49] As shown in Fig. S10 (in ESI), it is evident that the copolymers have certain antibacterial activities before the introduction of ionic groups. After co-cultivating with the copolymers for 4 h, the survival rate of S. aureus ranged from 65.1% to 73.3%, and that of E. coli ranged from 65.4% to 76.7% (Fig. S11 in ESI). The antibacterial activities of the copolymers were slightly improved with the increase of the IUD content. This was because the surface environment of the copolymer membrane was not suitable for bacterial propagation and the iodine atoms were toxic to bacteria.
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    The antibacterial activities of the ionomers were also considered. For imidazolium-based ionomers with the same ionic content, the ionomers bearing Tf2N displayed the best antibacterial activities against both S. aureus and E. coli (Fig. S12 in ESI). The ionomers bearing Tf2N were chosen to discuss the effect of ionic content on the antibacterial properties of the ionomers. The ionomers with different ionic contents exhibited different antibacterial activities (Fig. 6a). As shown in Fig. 6(b), after 4 h of co-cultivation with the ionomers containing 6.1%, 9.1%, 12.2%, and 15.7% Tf2N counterion, the corresponding survival rates for S. aureus were 3.8%, <0.1%, <0.1%, <0.01%, and for E. coli were 3.9%, 0.2%, 0.1%, <0.1%. The above results indicated that the antibacterial activities of the ionomers improved with the increase of ionic content. The imidazole cation group on the side chain can interact electrostatically with the bacterial surface carrying negative charges. The electrostatic interaction can cause cell rupture or change the permeability of cell membranes, resulting in the leakage of intracellular components of the bacterial cell which in turn led to the death of the bacterium.[

    50]
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    Fig 6  (a) Photographs of bacterial colonies and (b) survival rates of S. aureus and E. coli after co-cultivation with the ionomers bearing Tf2N for 4 h.

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    The effect of counterion type on the antibacterial activities of the ionomers was further investigated. The ionomers containing 9.1% ionic group but different counterions were chosen for discussion. The ionomers with different counterion types exhibited different antibacterial activities (Fig. 7a). As presented in Fig. 7(b), the survival rate of S. aureus was 70.0% and that of E. coli was 71.6% after 4 h of co-cultivation with the copolymer (IUD 9.1%). After co-cultivation with ionomers bearing CH3SO3, CF3SO3, and Tf2N for 4 h, the corresponding survival rates for S. aureus are 6.7%, 4.6% and <0.1%, and for E. coli are 4.8%, 3.4% and 0.2%. The ionomers bearing Tf2N exhibited excellent antibacterial activities of more than 99% against both bacteria when the ionic content reached 9.1%. The above results indicated that the antibacterial properties of ionomers with the same ionic content but different counterions displayed the following order: CH3SO3 < CF3SO3 < Tf2N.

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    Fig 7  (a) Photographs of bacterial colonies and (b) survival rates of S. aureus and E. coli after co-cultivation with the copolymer (IUD 9.1%) and the corresponding ionomers for 4 h.

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    Among three kinds of imidazolium-based ionomers, the ionomers bearing Tf2N exhibited the best antibacterial activities against S. aureus and E. coli. The Tf2N counterion had the weakest ionic interaction with the imidazole cation, thus the imidazole cation was more likely to interact electrostatically with negatively charged cell membranes rather than Tf2N. In contrast, the interaction between CH3SO3 and imidazole cation was too strong, preventing the imidazole cation group from effectively interacting with the cell membrane. The above results demonstrated the advantages of imidazolium-based ionomers in efficient and broad-spectrum antibacterial activity and their potential application as antibacterial materials.

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    CONCLUSIONS

    In conclusion, a series of imidazolium-based PE ionomers bearing different types of counterions were efficiently prepared. The ionomers were obtained from E/IUD copolymers with iodine groups in the side chains via nucleophilic substitution and ion exchange reactions. The ionomers exhibited elevated Tm and Tg compared with the E/IUD copolymers, which was attributed to ionic aggregation. The stress-at-break (7.8−25.6 MPa) and the elongation-at-break (445%−847%) of the ionomers could be adjusted over a wide range by changing the counterion species and the ionic group contents. The stress-at-break values of ionomers exhibited the following order: CH3SO3 > CF3SO3 > Tf2N, while the elongation-at-break values showed the opposite trend. The ionomers showed excellent antibacterial activities against S. aureus and E. coli. The antibacterial ability of ionomers with the same ionic content but different counterions displayed the following order: CH3SO3 < CF3SO3 < Tf2N. The ionomers bearing Tf2N exhibited antibacterial activities >99% against both S. aureus and E. coli when ionic content reached 9.1%.

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    This work not only provided a practicable approach for the synthesis of PE ionomers, but also demonstrated the great potential of PE ionomers in biomedical applications. Furthermore, the PE ionomers also showed potential applications in the field of thermoplastic elastomers due to their reversible interactions including crystallization and ionic cross-linking. Related work is in progress.

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