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Applications of Polymer, Composite, and Coating Materials

Hydrogen-bond Assembly of Polyvinyl Alcohol and Polyhexamethylene Guanidine for Non-leaching and Transparent Antimicrobial Films Jie Chen, Dafu Wei, Wuling Gong, Anna Zheng, and Yong Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14238 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Applied Materials & Interfaces

Hydrogen-bond Assembly of Polyvinyl Alcohol and Polyhexamethylene Guanidine for Non-leaching and Transparent Antimicrobial Films Jie Chen, Dafu Wei*, Wuling Gong, Anna Zheng*, and Yong Guan School of Materials Science and Engineering, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai, 200237, China.

ABSTRACT: The combination of transparency, antimicrobial activities, non-leaching of antimicrobial component and green preparation for polyvinyl alcohol (PVA) films is of importance for practical applications in industry. However, until now it remains a challenge. Herein, a facile antimicrobial PVA films containing polyhexamethylene guanidine (PHMG) is reported via a green solution casting method. Such PVA films show high transparency of 91%, above 99.99% of antimicrobial rates against Escherichia coli and Staphylococcus aureus, and non-leaching characteristic of PHMG due to the hydrogen-bond (H-bond) interaction between PHMG and PVA. The thermal stability and mechanical properties of the PVA films are further improved compared to neat PVA film. These antimicrobial films are expected to find promising applications in tissue engineering and packaging fields, which opens up a methodology to prepare non-leaching antimicrobial polymeric materials via H-bond.

KEYWORDS: PVA films, polyhexamethylene guanidine hydrochloride, hydrogen-bond, nonleaching characteristic, antimicrobial properties

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1 Introduction Recently, the synthesis, processing and application of the biocompatible and biodegradable polymers for their environmentally benign nature received more and more attention1. Among those biocompatible and biodegradable polymers, polyvinyl alcohol (PVA) holds an important position due to its excellent mechanical properties, barrier of oxygen and optical transparency, and is widely used as films,2-5 hydrogels,6-8 fibers,9-11 etc. The antimicrobial performance is high demand in many fields to prevent the disease infection and to keep a healthy and sanitary environment,12 such as antimicrobial PVA hydrogel wound dressing and antimicrobial PVA films.13 Many researches focused on the development of antimicrobial PVA products.14 Blending,15-16 graft modification of PVA17 and crosslinking of PVA18 are the common preparation methods. Polymer blending is a simple and effective physical method to obtain new materials with desired properties. PVA is suitable for blending with some polar polymers in water without organic solvents. Many antimicrobial agents, such as chitosan,4 silver,19 TiO220 and ZnO21, were used to prepare antimicrobial PVA. Among them, chitosan is widely researched because of its filmforming capacity, non-toxicity, biodegradability and biocompatibility. Hajji et al16 prepared the PVA/chitosan film with excellent antimicrobial properties. However, the mechanical properties and the transparency remarkably decreased compared to pure PVA. Moreover, chitosan of high molecular weight is soluble only in acid media, and the films lost the antimicrobial activity at higher than 6.5 pH value7. Another issue on blending method is that the blended antimicrobial agents continuously leach out from modified membrane, resulting in a decrease in the durability of antimicrobial property and even an adverse effect on the environment.


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Grafting modification is an effective method to improve the durability of antimicrobial PVA. Si et al reported an N-halamine grafted poly(vinyl alcohol-co-ethylene) films (HAF films) with renewable antimicrobial activity by melting graft polymerization via reactive extrusion technique. The resulting HAF films exhibited integrated properties of robust mechanical strength, high transparency, rechargeable chlorination capability, and long-term durability.22 However, the residual monomer and processing complexity remained concerns. The crosslinking of PVA can be carried out via physical and chemical routes.23 Physical crosslinking, such as freezing/thawing method, induces the crystallization to form the crosslinking points.24 Generally, the freezing/thawing process always takes a long time. Chemical crosslinking of PVA needs to introduce some chemicals, such as aldehydes, dialdehydes, dicarboxylic acids, tricarboxylic acids, dianhydrides, diisocyanates and inorganic acids.25-26 Zahra et al26 prepared the crosslinked PVA films by blending PVA and quaternary ammonium modified starch (STGTMAC) using citric acid (CA) as plasticizer and glutaraldehyde (GA) as cross-linker. The results showed that the ST-GTMAC/PVA/CA/GA film has good antimicrobial property, increased tensile strength, and decreased solubility and swelling degree. However, the harmful chemical residue, the formation of by-products, increased cost and un-green procedure are main concerns. Although many approaches have been employed to develop antimicrobial PVA, a green strategy to prepare non-leaching and transparent PVA film with high mechanical properties remains a huge challenge. It is well known that Hydrogen-bonds (H-bonds) exist widely in PVA. Although Hbonds are relatively weak compared to covalent bonding, their dense clustering may extensively be formed, therefore accompanying the remarkable improvement of mechanical and thermal properties.8, 27-30 It is expected to construct transparent, non-leaching, antimicrobial PVA films with high mechanical performance via multiple H-bonds route, a green method.


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Therefore, polyhexamethylene guanidine (PHMG) was chosen as the antimicrobial agent, for its water solubility, broad-spectrum and excellent antimicrobial properties, and extensive application.31-34 A novel, transparent and non-leaching antimicrobial PVA film was prepared by a casting method of mixed aqueous solution of PVA and PHMG, a simple and green method. Due to the multiple H-bonds between PVA and PHMG, the good compatibility was observed and PVA/PHMG blend films had high optical transparency. The excellent antimicrobial properties, excellent stabilities, extremely low leaching rates and enhanced mechanical properties were also found in the PVA/PHMG blend films. It is very promising to be applied in the antimicrobial packaging and tissue engineering fields. 2 Experimental 2.1 Materials PVA (average molecular weight: 73,000-78,000; degree of hydrolysis: 98-99%) was purchased from Shanghai Titan Tech Co., Ltd. PHMG with a number-average molecular weight at 740 Da (tested by ESI-TOF-MS) was synthesized according to the reported procedure.35 2.2 Preparation of PVA/PHMG films 10 wt% PVA solution was prepared by adding 50 g of PVA in 450 g of distilled water (DI water) and stirring under a constant speed at 90 oC for 4 h. PHMG was dissolved in DI water at room temperature with magnetic stirring for 4 h. PHMG solution and PVA solution were mixed at different mass ratios, stirred for 1 h, then casted on glass petri dishes and kept at room temperature for drying. The PVA/PHMG film with 1.0 wt% PHMG was noted as PVA-1.0wt%, and so on. Finally, dried films were collected and tested. 2.3 Characterization


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2.3.1 Fourier transform infrared spectroscopy (FTIR) PVA films were scanned over the wavenumber range of 4000-400 cm-1 by Nicolet 5700 spectrometer using an attenuated total reflectance (ATR) mode to measure the functional groups on the surface. The in-situ FTIRS was used to observe the shifts of the hydroxyl absorption peak at different temperatures. 2.3.2 Mechanical properties The tensile strength and elongation at break of PVA films were measured at a speed of 50 mm/min at 23 ± 2 oC using a universal electrical testing machine (CMT-2203, MTS, America). All the samples had a gauge length of 25 mm. Each sample was tested at least five times, and the average value was calculated. 2.2.3 Morphology observations by scanning electron microscopy (SEM) The morphology of films was observed by a scanning electron microscopy (SEM, Hitachi S3400N, Japan) at 15 kV accelerating voltage. The samples were coated with gold prior to observation. 2.2.4 Antimicrobial testing The antimicrobial properties of the PVA films were tested by shaking flask method35 and ring diffusion method14. Shaking flask method is a kind of quantitative test. Escherichia coli and Staphylococcus aureus were cultured in nutrient broth at 37 oC for 24 h and further diluted to 105 CFU/ml. Then 0.10 g sample and 5 mL bacterial culture (105 CFU/ml) mixed, and shook for 1 h at 250 rpm at 37 oC. After shaking, various dilutions were prepared successively, and then 0.1 mL of this culture was seeded on LB agar in a petri dish. The dishes were incubated at 37 oC for 24 h and the number of colonies was counted. The inhibition rate of cell growth was estimated from Eq. (1):


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Growth inhibition rate = (A-B)/A×100% (1) Where A and B are the number of the bacterial colonies observed for the control and composite film samples, respectively. Each sample was measured three times, and the average values of inhibition rates were calculated. The ring diffusion test was used to characterize the leaching characteristic. The medium for culturing bacteria was nutrient agar. Agar plates were inoculated with 0.1 ml suspensions of Escherichia coli and Staphylococcus aureus with a concentration of 108 CFU/ml. Circular pieces of films with 0.5 cm of diameter were added on the agar plates. These plates were then incubated at 37 oC for 24 h in an incubator before measuring the diameters of inhibition zone. All the tests were carried out in duplicate. 2.2.5 Leaching tests 1 g PVA film was soaked in 50 g DI water for 7 days under different temperatures. Then the soaked solution was measured on an ultraviolet (UV) spectrophotometer (Lambda 950, Perkin Elmer, America) in the range of 190-400 nm to determine the leaching rates of PHMG. The calibration Eq. (2)36 of the absorbance at 192 nm of PHMG is as follows: A192=0.0738+67.71005CPHMG (2) Where A192 represents the absorbance at 192 nm, CPHMG represents the concentration of PHMG in mg/ml. The leaching rate was calculated by the following Eq. (3): Leaching rate=CPHMGVW/ WA×100% (3) Where WA is the weight (g) of incorporated PHMG, VW is the volume (ml) of soaked solution, CPHMG represents the concentration of PHMG in mg/mL of soaked solution. 2.2.6 Differential scanning calorimeter (DSC)


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The thermal properties of PVA/PHMG films were determined by differential scanning calorimetry (DSC) (Diamond DSC, Perkin Elmer, America). About 11 mg film was placed in a sample pan of DSC equipment. An empty aluminium pan was used as reference. Samples were scanned at a heating rate of 5 oC/min between 0-300 oC. 2.2.7 Thermogravimetric Analysis The thermal stability of the PVA films was studied using thermogravimetric analysis (TGA) (STA409PC, NETZSCH, Germany). All tests were conducted under N2 atmosphere (20 ml/min) using sample weights of about 10 mg over a range of 20-600 oC at a rate of 10 oC/min. 2.2.8 Light transmission Light transmission through the films was measured on an ultraviolet-visible (UV-vis) spectrophotometer (Lambda 950, Perkin Elmer, America) in the range of 200-800 nm. 2.2.9 X-ray Diffraction The crystallization structures of PVA films were characterized by wide-angle X-ray diffraction (XRD) using a Rotating Anode X-ray Powder Diffractometer (18KW/D/max2550VB/PC, Japan) with Cu Kα radiation (λ = 1.542 Å). 3 Results and discussion 3.1 Transparency of films High optical transparency is an important characteristic of PVA films. The photos of Fig. 1-(B) intuitively confirmed the as-prepared PVA/PHMG films had almost same transparency as neat PVA film (0.1±0.02 mm thickness). Further, the transparency of films was measured by UV-vis spectrophotometer, and transmittance-wavelength curves are presented in Fig. 1-(A). The neat PVA films have a high transparency in the range of 250-800 nm, especially over 91% of transmittance in the visible light region. The similar transparency of PVA/PHMG films indicated


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that the addition of PHMG did not influence the transparency of PVA. The high transparency should attribute to the good compatibility between PVA and PHMG, which might result from their strong hydrogen-bond (H-bond) interaction.

Figure 1. UV-vis curves (200-800 nm) (A) and pictures (B) of the neat PVA film and PVA/PHMG films. (a) PVA, (b) PVA-0.5wt%, (c) PVA-1.0wt%, (d) PVA-2.5wt%, (e) PVA-5.0wt%, (f) PVA10.0wt%. The logo of East China University of Science & Technology reproduced with permission.


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3.2 H-bonds interaction between PVA and PHMG The H-bond interaction between PVA and PHMG could be confirmed by FTIR. FTIR spectra of PVA and PVA/PHMG films were measured using an attenuated total reflectance (ATR) mode. As shown in Fig. 2-(a), pure PVA has a typically sharp and broad absorption band at 3000-3600 cm−1 centering at 3258 cm−1, arising from the stretching vibration of hydroxyl groups (O-H) due to the extensive H-bond (for that of free hydroxyls is usually only observed at 3620 cm−1). 37The neighbour strong absorption peaks at 2937 cm-1 and 2907 cm-1 are attributed to the stretching vibrations of methylene groups38. For PHMG, two absorption peaks at 3403 cm−1 and 3161 cm−1 are attributed to the stretching vibrations of N-H group12. For PVA/PHMG films, the peaks at 3403cm-1 and 3161cm-1 disappear regardless of the contents of PHMG, and the peak position of O-H group is notably shifted from 3258 cm-1 for neat PVA film to 3261 cm-1 for PVA-0.5wt%, then to 3262 cm−1 for PVA-1.0wt% and 3265 cm-1 for PVA-2.5wt%, followed by 3267 cm−1 for PVA-5.0wt% and 3270 cm−1 for PVA-10.0wt%. The shifts of higher wavenumbers of O-H peaks do not show the H-bond interaction between PVA and PHMG. However, we think the peak position of the stretching vibration of O-H group should reflect the addition of the two broad bands corresponding to the N-H group of PHMG and the O-H group of PVA. In order to remove the effect of the overlap peaks of infrared spectrum, the FTIR difference spectrum was analyzed. Fig. 2-(b) shows the FTIR difference spectra between PVA/PHMG and PHMG, and the mark 2-7 represents the difference spectrum between sample 2 and sample 7 (PHMG), and so on. The broad O-H absorption peak centered at 3258 cm−1 in pure PVA. After the H-bond interaction with PHMG, the O-H peak positions shifted to a lower wavenumber of 3230 cm-1. Fig. 2-(c) further shows the FTIR difference spectra between PVA/PHMG and PVA, and the mark 2-1 represents the difference spectrum between sample 2 and sample 1 (PVA), and


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so on. The absorption peaks of the stretching vibrations of N-H group is at 3403 cm−1 in pure PHMG. After the H-bond interaction with PVA, the peaks of N-H obviously shifted to a lower wavenumber of 3305 cm−1. The shift of N-H and O-H groups to low wavenumbers indicated the formation of new H-bonds between PVA and PHMG.39


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Figure 2. (a) FTIR spectra of PVA film, PHMG and PVA/PHMG films of 2800-3900cm-1, (b) the FTIR difference spectra between PVA/PHMG films and PHMG. The mark 2-7 represents sample 2 minus sample 7, and so on, (c) the FTIR difference spectra between PVA/PHMG films and PVA. The mark 2-1 represents sample 2 minus sample 1, and so on. (1) PVA, (2) PVA-0.5wt%, (3) PVA-1.0wt%, (4) PVA-2.5wt%, (5) PVA-5.0wt%, (6) PVA-10.0wt%, (7) PHMG. Tab. 1 shows the shifts of O-H groups with temperature obtained from in situ FTIRS. With the temperature increasing, the H-bond in neat PVA film was weakened, resulting that the peak position of the stretching vibration of O-H groups gradually shifted to high frequency, from 3331cm-1 at 40 °C to 3362 cm-1 at 100 °C. The changes of the peak position in PVA-0.5wt% and PVA-2.5wt% were similar with that of pure PVA. However, the peak position in PVA-1.0wt% almost remained unchanged from 40 °C to 70 °C, showing the better thermal stability, which might be stronger new H-bond networks between PVA and PHMG, where PHMG served as physical junctions in the system. Above 80 °C, the peak position in all the samples showed no significant distinction, which should attribute to the attenuation of H-bonds at higher temperatures.


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Table 1． The shifts of wavenumber (cm-1) of -OH groups in different temperatures. T (oC)

Wavenumbers of O-H groups (cm-1) PVA

PVA-0.5wt%

PVA-1.0wt%

PVA-2.5wt%

40

3331

3334

3334

3334

50

3335

3339

3340

3340

60

3346

3346

3340

3343

70

3352

3353

3340

3347

80

3354

3357

3354

3353

90

3357

3354

3354

3353

100

3362

3362

3364

3361

3.3 X-ray Diffraction The H-bond interaction between PVA and PHMG also could be analysed by XRD. Xray diffraction patterns of neat PHMG, PVA, and PVA/PHMG composite films with various PHMG contents are presented in Fig. 3. The sharp diffraction peaks centred at 2θ = 19.6° of neat PVA correspond to the (101) plane of PVA crystals.40 For PHMG, no evident diffraction peak can be observed, which means PHMG has an amorphous structure. For the PVA/PHMG composite films, the peak position remained unchanged, suggesting the good compatibility of PVA and PHMG and no transformation in crystal compared with PVA. However, the areas of diffraction peaks at 2θ = 19.6° gradually decreased with the content of PHMG increasing until 2.5 wt% of PHMG, then reversely increased with the content of PHMG increasing. The decrease of areas of the diffraction peaks suggests the crystallinity of PVA decreased to a minimum value at 2.5 wt% PHMG due to the constraint of H-bond between PVA


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and PHMG, and the reverse increases of peak areas should be attributed to the easier motion of PVA chains to crystallize due to more PHMG.

Figure 3. X-ray diffraction patterns of PHMG, PVA, and PVA/PHMG composite films with various PHMG contents. 3.4 Thermal properties of PVA and PVA/PHMG films DSC and TGA measurements were carried out to determine the effects of PHMG and H-bond on the thermal properties. The DSC data in Table 2 showed that pure PVA exhibited a melting point (Tm) of 224.3 °C and a degree of crystallinity (c) of 37.6% based on 138.6 J/g of melting enthalpy (ΔHm0) of 100% crystalline PVA.41 The Tm monotonously decreased with increasing the concentration of PHMG. For instance, the PVA-10.0wt% film displays a Tm of 218.7 °C, 5.6 °C lower than that of pure PVA. The decrease of Tm was perhaps due to the relatively low molecular weight of PHMG and H-bond interaction between PVA and PHMG.


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However, with the increase of PHMG concentration, the χc of PVA/PHMG films first decreased to a minimum value at 2.5 wt% PHMG, then increased to higher value at 10.0 wt% PHMG than that of pure PVA film. The decrease of χc should be related with the restricted PVA network due to the H-bond interaction between PVA and PHMG. However, when the content of PHMG was above 2.5 wt%, it might be difficult to form the restricted PVA network due to the excessive PHMG. On the contrary, the motion of PVA chain would be easier due to the low molecular weight PHMG, which was beneficial to crystallize. Therefore, the χc increased. The results are consistent with those of XRD. Table 2. Detailed data obtained from DSC curves of PVA film and PVA/PHMG films. Samples

c

PVA

PHMG

Tm

(wt%)

(wt%)

(oC)

PVA

100.0

0

224.3

52.1

37.6

PVA-0.5wt%

99.5

0.5

223.2

47.0

34.1

PVA-1.0wt%

99.0

1.0

222.1

46.9

34.1

PVA-2.5wt%

97.5

2.5

221.4

38.4

28.4

PVA-5.0wt%

95.0

5.0

219.3

46.0

34.9

PVA-10.0wt%

90.0

10.0

218.7

49.4

39.6

ΔHm (J/g)

(%)

Fig. 4 shows the TGA and DTG curves of neat PVA and PVA/PHMG films. The slight decrease of mass above 100 °C should attribute to the removal of moisture in all the samples.42 The DTG curve in Fig 4-(a2) showed that the process of weight loss of PVA film had two degradation stages, corresponding to two peaks at 328 °C (Tmax1) and 433 °C (Tmax2), respectively. The first degradation stage attributed to the dehydration of the side chain hydroxyl group, which further resulted in the formation of unstable enol structure


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having keto-enol tautomerism equilibrium and the decreased thermal stability of main chain. The second stage of the weight loss was related with the degradation of main chain. 43

For the PVA/PHMG films, Tmax1 and Tmax2 gradually increased with PHMG content increasing until 2.5 wt% PHMG. Above 2.5 wt% PHMG, Tmax1 decreased while Tmax2 still increased with the PHMG content increasing. Compared with pure PVA, all the peak shapes of DTG curves of PVA/PHMG films remained unchanged except PVA-2.5wt%, implying the degradation mechanism of PVA/PHMG samples were the same as that of pure PVA. The increases of Tmax1 and Tmax2 of PVA/PHMG samples perhaps might be related with the physical crosslinking between PVA and PHMG within 2.5 wt% PHMG. Above 2.5 wt% PHMG, the numbers of physical crosslinking decreased, Tmax1 decreased. However, Tmax2 increased, such as 452 °C of Tmax2 of PVA-10.0wt% film (19 °C higher than that of PVA), which should attribute to the strong capacity of PHMG of capturing and eliminating free radicals generated by the degradation of main chains at high temperature. Furthermore, it is assumed that the thermal degradation of PVA/PHMG films obeys the linear mixing law based on the respective weight fraction, then the thermal degradation curves of PVA/PHMG films could be calculated by the thermal degradation curves of PVA and PHMG. The calculated curves were named as the theoretical degradation curves, as shown in Fig. 4-(c1) and Fig. 4-(c2). The theoretical TGA curve and DTG curve of PVA1.0wt% were the almost same as those of pure PVA. That was, when the interaction between PVA and PHMG was not considered, the addition of 1.0 wt% PHMG would not affect the thermal stability of PVA. But in fact, the observed thermal weight loss curves of PVA-1.0wt% were remarkably different from the theoretical curves, implying the


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interaction between PVA and PHMG had significant effect on the thermal stability. Overall, the thermal stability of PVA/PHMG films was better than that of neat PVA when the PHMG contents were lower than 1.0 wt%.


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Figure 4. TGA (a1) and DTG curves (a2) of PVA, PVA-0.5wt%, PVA-1.0wt%, PVA-2.5wt% and PHMG. TGA (b1) and DTG curves (b2) of PVA, PVA-5.0wt%, PVA-10.0wt% and PHMG. TGA (c1) and DTG curves (c2) of PVA, PHMG, PVA-1.0wt% and theoretical PVA-1.0wt%. Table 3. Degradation data of PVA and PVA/PHMG films obtained from TGA and DTG curves. Samples

Ti/℃

Tmax1/°C

Tmax2/°C

Final residual weight/%

PVA

246

328

433

2.4

PVA-0.5wt%

267

337

444

3.5

PVA-1.0wt%

274

355

447

4.1

PVA-2.5wt%

258

327

397 and 431

7.2

PVA-5.0wt%

256

312

439

8.4

PVA-10.0wt%

254

309

452

14.0

PHMG

340

370

482

11.0

3.5 Surface and cross section morphologies of PVA and PVA/PHMG films Fig. 5 shows the scanning electron microscope (SEM) images of surfaces and cross sections of the PVA/PHMG films. Pure PVA film (Fig. 5-(a2)) had a dense and smooth surface, and its cross section (Fig. 5-(a1)) was compact and continuous.44 All the PVA/PHMG films also exhibited smooth surface and homogeneous cross sections with no obvious phase separation, crack or pores, further indicating the good compatibility of the two polymers due to the H-bond between PVA and PHMG.


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Figure 5. SEM images for cross-sections (a1, b1, c1, d1) and surfaces (a2, b2, c2, d2) of PVA films. a) pure PVA film, b) PVA-1.0wt%, c) PVA-5.0wt%, d) PVA-10.0wt%. 3.6 Antimicrobial properties of PVA/PHMG films Fig. 6-(1)a1 and Fig. 6-(1)b1 indicated that pure PVA film has no capability of deactivation against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). However, the survival colonies sharply reduced for PVA/PHMG composite films. Even after 10 cycles of water-rinsed, the antimicrobial properties were still excellent, as shown in Fig. 6-(1)a2 and Fig. 6-(1)b2. Table 4 lists the antimicrobial rates of PVA/PHMG films after washed with DI water for different times.


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The inhibition rates of all the composite films samples against E. coli and S. aureus were higher than 99.99%. The increasing of washing number did not affect the antimicrobial properties of PVA/PHMG films, indicating the excellent and durable capabilities of deactivation against bacteria due to the H-bond between PHMG and PVA. Moreover, the ring diffusion test (Fig. 6-(2)) confirmed the non-leaching characteristic of PHMG in PVA/PHMG films. No inhibition rings were observed even when the concentration of PHMG reached 10.0 wt%. It was concluded that the obtained antimicrobial PVA/PHMG films were stable without PHMG leakage. Meanwhile, no colonies were observed in the interface between PVA/PHMG films and agar, revealing the excellent contact antimicrobial properties. In order to accurately determine the trace leaching, the quantitative analysis of leaching PHMG was carried out by a more sensitive ultraviolet (UV) absorption spectrometry analysis. The results are shown in Fig. 7-(a). A sharp peak at 192 nm belonged to the UV absorption of C=N group of PHMG. Based on the calibration equation, the calculated leaching rates of PHMG at 20 °C, 30 °C and 40 °C, were only 0.04 %, 0.12 % and 0.14 %, respectively (Fig. 7-(b)). This should attribute to the tight bond between PVA and PHMG via H-bond to prevent the release of PHMG from PVA/PHMG films. With the temperature increasing the slight rise of leaching rates was due to the weakness of H-bond although this weakened phenomenon was not significantly observed in the FTIR results of PVA-1.0wt% film.


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Figure 6. (1) Antimicrobial photos of pure PVA film against E. coli (a1) and against S. aureus (b1). Antimicrobial photos of PVA-0.5wt% film against E. coli (a2) and against S. aureus (b2) after 10 cycles of DI water washing. (2) Photos of inhibition zone against E. coli (left) and against S. aureus (right). a: PVA, b: PVA-0.5wt%, c: PVA-1.0wt%, d: PVA2.5wt%, e: PVA-5.0wt%, f: PVA-10.0wt%.


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Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Antimicrobial rates of PVA/PHMG films after washed by DI water for different cycles. Antimicrobial rates against

Antimicrobial rates against

Escherichia coli (%)

Staphylococcus aureus（%）

Samples

Before washing

Cycles of water washing 1

5

10

0

0

0

Before washing 0

Cycles of water washing 1

5

10

0

0

0

Neat PVA

0

PVA-0.5wt%

99.999

99.999 99.999 99.998

99.999

99.999 99.999 99.998

PVA-1.0wt%

99.999

99.999 99.999 99.999

99.999

99.999 99.999 99.999

PVA-2.5wt%

99.999

99.999 99.999 99.999

99.999

99.999 99.999 99.999

PVA-5.0wt%

99.999

99.999 99.999 99.999

99.999

99.999 99.999 99.999

PVA-10.0wt%

99.999

99.999 99.999 99.999

99.999

99.999 99.999 99.999


21


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Figure 7. (a) UV absorption spectra of leachate solution of PVA-1.0wt% in DI water for 7 days at different temperatures. (b) The leaching rates of PHMG in the PVA-1.0wt% at different temperatures. 3.7 Mechanical Properties PVA displays a tensile strength of 47.4 MPa and elongation at break of 1143.3% (Fig. 8). Within 1.0 wt% of PHMG, the tensile strength of PVA/PHMG increased with the amount of PHMG increasing. Compared with the pure PVA film, the tensile strength of PVA-1.0wt% increased by approximately 10%, changing from 47.4 MPa to 51.9 MPa (Fig. 8-(a)). On the contrary, above 1.0 wt% of PHMG, the tensile strength of PVA/PHMG films decreased with the amount of PHMG increasing. The tensile strength of PVA-5.0wt% reduced to 43.7 MPa. It seemed that PVA-1.0wt% had a largest tensile strength, which was consistent with the stronger H-bond networks confirmed by above FTIR. This further indicated that the H-bond network and crosslinking points at 1.0 wt% PHMG might be at an optimized state. The elongation at breaks are shown in Fig. 8-(b). The elongation at break of PVA/PHMG films gradually reduced with the amount of PHMG increasing, which should be due to two factors. One was the low elongation at break of PHMG oligomer itself. The other was the formation of crosslinking network via H-bond to limit the stretch.


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Figure 8. Mechanical properties of PVA and PVA/PHMG films. (a) tensile strength, (b) elongation at break. 4 Conclusions In this work, a green aqueous solution casting method was developed to obtain the transparent, non-leaching and antimicrobial PVA films by blending PVA and PHMG. The antimicrobial films have broad-spectrum and excellent antimicrobial properties (higher than 99.99% of antimicrobial rates against Escherichia coli and Staphylococcus aureus). The formation of hydrogen-bond crosslinking network between PVA and PHMG ensures the stability, transparency, and non-leaching characteristic, which was confirmed by FTIR, thermogravimetric analysis, UV-vis spectra, water washing test, inhibition zone test and leaching


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rate test. Furthermore, this physical interaction also contributes to the uniform and smooth morphology, increased thermal stability, and improved mechanical properties. This work provides a green and promising approach for large-scale industrial application to produce transparent, non-leaching and antimicrobial polymeric products by creating hydrogen-bond network.

AUTHOR INFORMATION Corresponding Author Dafu Wei: [email protected], Anna Zheng: [email protected] ACKNOWLEDGMENT The authors thank The Key Laboratory of Advanced Polymer Materials of Shanghai (Grant No. ZD20170203) and Shanghai Leading Academic Discipline Project (B502) for funding this work. REFERENCES (1) Hunt, A. J.; Budarin, V. L.; Breeden, S. W.; Matharu, A. S.; Clark, J. H. Expanding the Potential for Waste Polyvinyl-alcohol. Green Chem. 2009, 11 (9), 1332-1336. (2) Monjazeb Marvdashti, L.; Koocheki, A.; Yavarmanesh, M. Alyssum Homolocarpum Seed Gum-Polyvinyl Alcohol Biodegradable Composite Film: Physicochemical, Mechanical, Thermal and Barrier Properties. Carbohyd. Polym. 2017, 155, 280-293.


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TOC

Transparent and non-leaching antimicrobial PVA films was obtained by hydrogen-bond interaction with PHMG via a green casting method.


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Hydrogen-bond Assembly of Polyvinyl Alcohol and ... - ACS Publications

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