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Preparation and characterization of antibacterial poly(lactic acid) nanocomposites with N-halamine modified silica
Journal Pre-proof Preparation and characterization of antibacterial poly(lactic acid) nanocomposites with N-halamine modified silica
Yan Zhao, Bolei Wei, Mi Wu, Huiliang Zhang, Jinrong Yao, Xin Chen, Zhengzhong Shao PII:
S0141-8130(19)37817-1
DOI:
https://doi.org/10.1016/j.ijbiomac.2019.11.125
Reference:
BIOMAC 13906
To appear in:
International Journal of Biological Macromolecules
Received date:
26 September 2019
Revised date:
13 November 2019
Accepted date:
13 November 2019
Please cite this article as: Y. Zhao, B. Wei, M. Wu, et al., Preparation and characterization of antibacterial poly(lactic acid) nanocomposites with N-halamine modified silica, International Journal of Biological Macromolecules(2019), https://doi.org/10.1016/ j.ijbiomac.2019.11.125
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© 2019 Published by Elsevier.
Journal Pre-proof
Preparation and characterization of antibacterial poly(lactic acid) nanocomposites with N-halamine modified silica Yan Zhao a, Bolei Wei a, Mi Wu a, Huiliang Zhang b, Jinrong Yao
a,
, Xin
Chen a, Zhengzhong Shao a
a
State Key Laboratory of Molecular Engineering of Polymers, Department of
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f
Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai, 200438, China.
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry,
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b
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Chinese Academy of Sciences, Changchun, 130022, China.
ABSTRACT
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In this work, silica nanoparticles modified with a new N-halamine precursor
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(EBDMH-SiO 2 NPs) were synthesized through immobilization of 3-(4'-epoxyethylbenzyl)-5,5-dimethylhydantoin (EBDMH) on the surface of amino- functionalized
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silica NPs. Then, EBDMH-SiO 2 NPs and poly(lactic acid) (PLA) were blended at 185˚C to prepare a novel environmentally friendly PLA based nanocomposite (PLA/EBDMH-SiO 2 ). The addition of EBDMH-SiO2 NPs has great influences on the thermal properties of nanocomposite. DSC results show that the grass transition temperature (Tg ), cold crystallization temperature (Tcc) and melting temperature (Tm) of PLA in nanocomposites gradually decrease with the increase of EBDMH-SiO2 NPs contents up to 5%. After that, further rise in EBDMH-SiO 2 NPs content actually
Corresponding authors E-mail address: [email protected] (J. Yao). 1
Journal Pre-proof increases Tg, Tcc, and Tm. The overall crystallization and spherulite growth rate of PLA show the similar trend. Furthermore, the introduction of EBDMH-SiO 2 NPs increases the storage modulus and viscosity of the melt of nanocomposite, providing an additional benefit for PLA blowing and injection molding. After chlorination, the N-halamine precursors on the nanocomposite surfaces are transformed into Nhalamines, which provide strong antibacterial activities against E. coli (CMCC 44103) and S. aureus (ATCC 6538), pointing to good potentials of the PLA/EBDMH-SiO 2
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nanocomposites for antibacterial applications including food packaging, filters, and a
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wide range of hygienic products.
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Keywords:
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Poly(lactic acid) (PLA)
Silica nanoparticles
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Nanocomposites
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3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH)
1. Introduction
In the last two decades, the increased production and use of plastic materials have led to an urgent need for waste disposal solutions [1,2]. To solve this problem, natural biopolymers and biodegradable synthetic polymers are investigated as alternatives to conventional petroleum plastics to reduce domestic and industrial plastic wastes [3-6]. Among biodegradable polymers, poly(lactic acid) (PLA), a degradable polymer derived from corn or sugar cane, is increasingly used to replace the traditional non-degradable polymer materials in many applications [7-9]. However, some intrinsic properties of PLA, such as slow crystallization rate and low melt 2
Journal Pre-proof strength, have caused a number of problems during PLA manufacturing [10-12]. The low crystallization rate of PLA leads to a rather long crystallization time, which is incompatible with the requirements for high efficiency of industrial processes, such as extrusion, injection molding and foaming. It has been reported that the crystallization rate of PLA can be increased by blending with polymer [13-15] and nucleating agents [16-23]. Nano-sized materials, such as montmorillonite [18], carbon nanotubes [19], calcium carbonate nanoparticles [20], nano-SiO2 [21], basalt fiber [22], and poly(D-
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lactic acid) [23-25] have been used as fillers to enhance the crystallization and melt strength of PLA.
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Due to the excellent biodegradability and biocompatibility, PLA has been used in
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biomedical materials, food packaging and hygienic products in which antibacterial
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functionality is often desired [26-28]. Some PLA materials with antibacterial properties have been developed in recent decades, such as PLA/Ag NPs transparent
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films [29], PLA/chitosan biocomposite films [30], and PLA/graphene oxide
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nanocomposites [31]. Nevertheless, all these systems suffer from certain limitations.
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For instance, although Ag NPs have been widely used in various fields, Ag ions can release into the surrounding environment, and ions can undergo chemical and biochemical conversions, which can be harmful to the environment [32,33]. Chitosan is a safe, non-toxic natural antibacterial material, but the immiscibility between PLA and chitosan has limited its further application [34]. As a kind of 2D carbon nanomaterials, GO is proved to have a good antibacterial effect, but the long-term effect of GO has been unclear so far [35]. Thus, there is a need for antimicrobial PLA which can be easily prepared, be effective within short contact times against a broad range of microorganisms, and be stable for long-term use. In this regard, N-halamine antibacterial materials are attracting continuous interest because of their unique 3
Journal Pre-proof properties, including powerful antibacterial activity, high durability, long-term stability, and regenerability [36-40]. In this work, a new N-halamine precursor with epoxy and hydantoin structures, 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH) was synthesized, and immobilized on amino- functionalized silica NPs to form EBDMH-SiO 2 NPs. Various concentrations of EBDMH-SiO 2 NPs were introduced into the PLA matrix to prepare a novel environmentally friendly PLA based nanocomposite (PLA/EBDMH-SiO 2 ).
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The effects of EBDMH-SiO 2 NPs on the crystallization behaviors, and rheological properties and antibacterial activities of PLA/EBDMH-SiO 2 nanocomposite were
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investigated. Our findings suggest that the addition of N-halamine modified silica NPs
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could provide PLA with outstanding biocidal activity, and excellent processability for
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a number of applications such as blown film extrusion, injection molding and
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2.1 Materials
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2. Experimental
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thermoforming.
Poly(lactic acid) (PLA 4032D, M w: 242 000 g/mol, PDI: 1.47, D- isomer: 2.0 wt%) was purchased from NatureWorks Co., Ltd (USA). Colloidal silica (diameter: 80 nm, density: 1.18-1.30 g/cm3 ) was from Shandong Peak-tech New Material Co., Ltd (China). 5,5-dimethylhydantoin (DMH), 1-chloromethyl-4-vinyl-benzene, 3chloroperoxybenzoic acid, and (3-aminopropyl) triethoxysilane (APTES), sodium thiosulfate solution (AR, 0.01 mol/L) were obtained from Aladdin Industrial Corporation (China). Luria Bertani agar/broth (LB) and super optimal agar/broth were provided by Sangon Biotech Co., Ltd (China). S. aureus (ATCC 6538) and E. coli (CMCC 44103) were obtained from Guangdong Huankai Microbial Sci. & Tech. Co., 4
Journal Pre-proof Ltd (China). Sodium hypochlorite solution (5%) was purchased from Adamas Reagent Co., Ltd (China). Phosphate buffered saline (PBS) was purchased from Shanghai MesGen Biotechnology Co., Ltd (China). Other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd (China).
2.2 Synthesis of 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH) The synthesis route of EBDMH is shown in Fig. 1. First, VBDMH was
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synthesized according to a method reported by Sun et. al [41], and confirmed by NMR and FT-IR spectra (Fig. S1-S2).
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20 ml of VBDMH (1.0 mmol, 2.44 g) dichloromethane (DCM) solution was
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dropwise added into 20 ml of chloroperoxybenzoic (1.3 mmol, 2.29 g) DCM solution
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at 0˚C and stirred at 0˚C for 12 hours. Then the mixture was stirred at 25˚C for another 12 hours. The DCM solution was washed in sequence by sodium sulfite
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(10%), sodium bicarbonate (5%), and water, respectively. After drying over
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anhydrous MgSO 4 , the solvent was removed by rotary evaporation to obtain the
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EGDMH as a light yellow solid ( yield 67%). Molecular weight of EBDMH: 260.29 (theoretical); 260.14 (MALDI-TOF MS). 1 H-NMR and 13 C-NMR spectra of EBDMH in DMSO-d6 are shown in Fig. 3. 1 H-NMR (400 MHz, DMSO-d6 , ppm) δ: 8.40 (s, 1H), 7.26 (d, 2H), 7.21 (d, 2H), 4.51(s, 1H), 3.90 (s, 1H), 3.10 (s, 1H), 2.82 (s, 1H), 1.29 (s, 6H). 13 C-NMR (400 MHz, DMSO-d6 , ppm) δ: 177.70, 155.56, 137.34, 137.23, 127.68, 126.42, 58.35, 51.75, 50.82, 41.12, 25.07.
2.3 Preparation of EBDMH-SiO2 NPs The preparation procedures of EBDMH-SiO 2 NPs are illustrated in Fig. 2. APTES-SiO 2 NPs was synthesized with the reported methods in literatures [42-44]. 5
Journal Pre-proof The typical process is: 15.00 g (0.06 mol) 3-aminopropyl-triethoxysilane (APTES) was dropwise added into 250 ml ethanol mixture (pH was adjusted to 9.5) of colloidal silica (100 ml) and SDS (150 mg). The mixture was stirred vigorously at room temperature for 24 hours and then refluxed for 6 hours. APTES- modified silica nanoparticles (APTES-SiO 2 NPs) were obtained by centrifugation and washing with ethanol. APTES-SiO 2 NPs (5.0 g) was dispersed in 200 ml acetonitrile by ultrasound for
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30 min, then 10 ml EBDMH (0.1 g) acetonitrile solution was dropwise added. The mixture was stirred vigorously at 60 °C for 24 hours. The resultant nanoparticles
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(EBDMH-SiO 2 NPs) were collected by centrifugation, washing with ethanol, and
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drying in vacuum.
2.4 Preparation of PLA/EBDMH-SiO2 nanocomposites
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PLA pellets were dried at 80˚C for 8 hours in a vacuum oven, and EBDMH-SiO 2
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NPs was ground in a planetary ball mill three times. PLA/EBDMH-SiO 2
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nanocomposites were prepared by melt blending in a mixer (Haake Rheomix 600, Germany) with a rotating speed of 60 rpm at 185˚C for 10 min. The obtained PLA nanocomposites were compression- molded (hydraulic press: 120 kg/cm2 ) at 185˚C into sheets with thicknesses of 1.0 mm for testing. The neat PLA control samples were prepared using the same method. The PLA nanocomposite sample was labelled as PLA/EBDMH-SiO 2 -x, where x was the weight percentage of EBDMH-SiO 2 NPs in nanocomposite.
2.5 Characterization and measurements 2.5.1 Fourier transform infrared (FT-IR) spectroscopy 6
Journal Pre-proof FT-IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer (ThermoFisher, USA) in the wavenumber range of 4000-500 cm-1 with a resolution of 4 cm-1 and 64 scans, respectively. The samples for FT-IR analysis were prepared by pressing with KBr.
2.5.2 Nuclear magnetic resonance (NMR) spectroscopy 1
H-NMR and
13
C-NMR spectra were obtained from a Bruker (400 MHz,
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Switzerland), using DMSO as solvent and tetramethylsilane (TMS) as interior
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chemical shift standard.
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2.5.3 Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy
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(MALDI-TOF MS)
MALDI- TOF MS measurement was performed on a MALDI-TOF MS
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spectrometer (5800, AB-SCIEX, USA). The matrix solution of dithranol (20 mg/mL),
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cationizing agent of sodium trifluoroacetate (10 mg/mL), and sample THF solution
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(10 mg/mL) were mixed in a volume ratio of matrix: cationizing agent: sample = 10: 2: 5. The mixed solution (0.5 μL) was deposited on the sample holder and dried at ambient temperature.
2.5.4 Dynamic light scattering (DLS) The particle size distribution of the silica NPs dispersed in water was measured by a DLS particle size analyzer (ZS90, Malvern, UK) with a scattering angle of 90˚.
2.5.5 Transmission electron microscope (TEM)
7
Journal Pre-proof The morphologies of the silica NPs were observed on a TEM (Tecnai G2 20 TWIN, FEI, USA).
2.5.6 Thermogravimetric analysis (TGA) TGA of the silica NPs was performed on a thermal analyzer (Perkin Elmer, USA) from 100 to 600°C at a rate of 10°C/min under air atmosphere. TGA of PLA nanocomposite samples was performed on a thermal analyzer
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(Perkin Elmer, USA) from 100 to 600 °C at a rate of 10°C/min under nitrogen
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atmosphere.
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2.5.7 Differential scanning calorimetry (DSC) measurements
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Thermal properties of PLA nanocomposite samples were performed on a DSC (Q2000, TA Instruments, USA) under nitrogen atmosphere. The sample weight varied
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between 5 and 8 mg. Samples were heated from 30 to 190 °C at a rate of 10 °C/min and
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kept in the molten state for 3 min to eliminate thermal history (1st heating), and then
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cooled to 30°C at a rate of 10°C/min. After cooling, the samples were heated back to 190°C at a rate of 10 °C/min (2nd heating). The data of glass transition temperature (Tg), cold crystallization temperature (Tcc), cold crystallization enthalpy (ΔHcc), melting temperatures (Tm) and the melting enthalpy (ΔHm) of the PLA matrix were obtained from the second heating run. The degree of crystallinity (Xc) of the samples was evaluated by the following relationship: 𝑋c (%) =
∆𝐻m
0 𝑊PLA ×∆𝐻m
× 100%
(1)
where ΔHm is the observed melting enthalpy of the PLA, ΔHm0 is the theoretical melting enthalpy of 100% crystalline PLA (93 J/g) [45] and WPLA is the weight fraction of PLA in the nanocomposites. 8
Journal Pre-proof During the isothermal melt crystallization process, the samples were heated from 30 to 190˚C at a heating rate of 30˚C/min, and held at 190˚C for 3 min, then cooled to the desired crystallization temperature (Tc) at a cooling rate of 30˚C/min and held at Tc until the isothermal crystallization was completed. The applied crystallization temperatures were 105, 110 and 115˚C, respectively.
2.5.8 Polarized optical microscope (POM)
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Spherulitic growth of neat PLA and PLA/EBDMH-SiO 2 nanocomposites was monitored by polarized optical microscope (POM, DM 2500P, Leica, Germany)
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equipped with a hot stage (Linkam THMS 600). The samples were first heated from
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room temperature to 190˚C at a rate of 30˚C/min and held at 190˚C for 5 min to erase
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thermal history, and then cooled to 130˚C at a rate of 30˚C/min for isothermal
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crystallization and held at 130˚C for 60 min.
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2.5.9 Wide angle X-ray diffraction (WAXD)
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WAXD was performed on an X-ray diffractometer (D8 ADVANCE, Bruker, Germany) using a Cu Kα radiation source (40 kV, 40 mA). The diffraction angle (2θ) was from 10˚ to 30˚. The scanning speed was 2˚/min. Before testing, all samples were first pressed into sheet with a thickness of around 1 mm on a hot press at 185°C and then transferred into another hot press at 130 °C for 3 hours.
2.5.10 Rheological properties Rheological properties of PLA nanocomposite were investigated on a rotational rheometer (HAAKE MARS III, ThermoFisher, USA) with a parallel plate diameter of 20 mm and a gap of 0.9 mm. The samples were tested at 190 °C under nitrogen to 9
Journal Pre-proof avoid thermal degradation. Dynamic frequency sweep was tested over a frequency range of 0.01-100 rad/s. The strain value was set at 1% under the linear viscoelastic region.
2.5.11 Chlorination Chlorination of EBDMH-SiO 2 NPs was carried out as follows. About 1 g of EBDMH-SiO2 NPs was ultrasonic dispersed into 100 mL of sodium hypochlorite
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solution (1%), and stirred for 1 hours at room temperature. Then chlorinated EBDMH-SiO2 NPs were collected by centrifugation, washing with distilled water,
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and drying in vacuum. The active chlorine content of the EBDMH-SiO 2 NPs was
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determined by iodometric/thiosulfate titration method [46], and calculated according
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to the following equation [47]: Cl(%) =
35.5 2
×
𝐶 ×𝑉 𝑊
× 100%
(2)
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where C and V are the concentration (mol/L) and the consumed volume (L) of the
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titrant sodium thiosulfate, respectively, and W is the weight of the chlorinated
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EBDMH-SiO2 NPs (g).
The PLA/EBDMH-SiO 2 nanocomposite sheets (3 cm × 3 cm) were chlorinated in 1 % of a commercial aqueous sodium hypochlorite solution (with 0.05 % Triton, X100) at pH 7 for 1 hours at room temperature. The sheets were washed with distilled water, and then dried at 45˚C for 1 hours to remove free chlorine. The oxidative chlorine loadings on the surfaces of PLA nanocomposite sheet was determined by a modified iodometric/thiosulfate titration method [38], and calculated with the following equation [26]: Cl (μg⁄cm2 ) = 10
35.5 2
×
𝐶×𝑉 2𝑆
(3)
Journal Pre-proof where Cl (μg/cm2 ) is the weight of oxidative chlorine per square centimeter on the surfaces of the samples, C and V are the concentration (mol/L) and the consumed volume (L) of the titrant sodium thiosulfate, respectively, and S is the surface area of the sheet (cm2 ).
2.5.12 Antibacterial assessment The antibacterial activities of the chlorinated EBDMH-SiO2 NPs and
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PLA/EBDMH-SiO 2 nanocomposite sheets were challenged by Gram-positive bacteria (S. aureus ATCC 6538) and Gram- negative bacteria (E. coli CMCC 44103). S. aureus,
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broth for E. coli) for 24 hours at 37˚C.
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and E. coli were grown in broth solutions (tryptic soy broth for S. aureus and LB
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The minimum inhibition concentration (MIC) was also determined to evaluate the antibacterial activity of the sample [48]. The harvested bacteria were diluted with
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PBS buffer (pH = 7.4) to about 105 CFU/ml. Samples with serial weights were each
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dispersed in 900 μL broth solutions, vortexed, and then sonicated for 20 min. 100 μL
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of bacteria suspension was then added into sample suspension. After incubation at 37˚C for 12 hours, 5 mL of 0.02 M sodium thiosulfate solution was added into the incubation suspension. And then resulting mixture was dispersed onto agar plates. Colonies on the plates were counted after incubation at 37˚C for 24 hours [49]. The harvested bacteria were diluted with PBS buffer (pH = 7.4) to about 104 CFU/ml. 50 μL of fresh bacteria suspension was placed on the center of a sheet (3 cm × 3 cm), and covered with another identical sheet. A gentle pressure was applied to ensure adequate contact with the bacteria. After the specified contact times (10, 30, 60, 120 and 180 min), the sheets were placed into 5 mL of 0.02 M sodium thiosulfate solution and vortexed for 2 min in order to quench the oxidative chlorine and transfer 11
Journal Pre-proof bacteria to solution. Serial dilutions were made using PBS buffer solution and each diluent was placed onto the corresponding agar plates. After incubation at 37˚C for 24 hours, bacterial colonies on the agar plates were counted [33,38]. The neat PLA sheets were tested under the same conditions to serve as control samples.
3. Results and discussion
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3.1 Preparation of PLA/EBDMH-SiO2 nanocomposites
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An N-halamine precursor with epoxy group (EBDMH) was synthesized from the
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oxidation of VBDMH with 3-chloroperoxybenzoic (Fig. 1). The reaction was confirmed by NMR studies. The peaks assigned to epoxy group (protons) of EBDMH
H-NMR spectrum (Fig. 3A) and at 51.75 ppm (methenyl carbon) and 50.82 ppm
(methylene carbon) in
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1
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appear at δ =3.90 ppm (methenyl proton), 2.82 and 3.10 ppm (methylene proton) in
C-NMR spectrum (Fig. 3B), respectively. Meanwhile, peaks
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corresponding to the alkene double bond of VBDMH (5.25, 5.81, 6.71 ppm in 1 H-
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NMR spectrum, and 114.83, 136.70 ppm in
13
C-NMR spectrum, Fig. S1) disappear
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completely. Furthermore, the molecular weight of EBDMH determined by MALDITOF MS (Fig. S3) is 260.14, which agrees very well with the theoretical value (260.29).
Through the reaction between epoxy group of EBDMH and amino groups on the amino- functionalized silica NPs (APTES-SiO2 NPs), EBDMH was immobilized on the surfaces of silica nanoparticles (EBDMH-SiO 2 NPs). The FT-IR spectra of SiO 2 , APTES-SiO 2 and EBDMH-SiO2 NPs were shown in Fig. 4A. In all spectra of the nanoparticles, the peaks around 800, 950, 1100, and 3500 cm-1 are attributed to symmetric stretching vibration of Si-O-Si, stretching vibration of Si-OH, antisymmetric stretching vibration of Si-O-Si, and stretching vibration of hydroxy 12
Journal Pre-proof groups, respectively. Meanwhile, the peak at 1620 cm-1 is assigned to the residual unhydrolyzed Si-O-C [40]. The peak at 1560 cm-1 in the spectrum of APTES-SiO2 NPs (curve b) is attributed to the stretching vibration of N-H groups. In curve c, a weak sharp peak at 1712 cm-1 appears, which is corresponding to the stretching vibration of C=O bond of hydantoin structure of EBDMH [50]. These findings suggest that EBDMH has been immobilized on the surface of silica nanoparticles. TGA curves of SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs are shown in Fig. 4B.
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The weight loss below 200˚C is attributed to evaporation of water and residual organic solvent [51]. In curve a, when the temperature reaches about 800˚C, the
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organic contents completely burn up and the residues are SiO 2 with a content of
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around 96.7%. In curve b, APTES-SiO 2 NPs begin to decompose at 280˚C, which
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attributed to the decomposition of aminopropyl groups [52]. After the organics completely burn up and the content of residual SiO 2 is around 93.8%. In curve c,
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EBDMH-SiO2 NPs begin to decompose at 190˚C, which is due to decomposition of
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EBDMH. From the TGA quantitative analysis, the grafting yield of EBDMH in the
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EBDMH-SiO2 NPs is calculated to be around 5.8%. In TEM studies (Fig. 5A-C), SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs show spherical shapes with smooth surface after modification, but the size of nanoparticles increases gradually. The mean particle size of SiO 2 , APTES-SiO2 , and EBDMH-SiO 2 NPs is 71.2, 85.6, and 91.8 nm, respectively. The particle size distribution from dynamic light scattering (DLS) is unimodal and narrow (Fig. 5D). The particle size of SiO2 , APTES-SiO 2 and EBDMH-SiO 2 NPs obtained from DLS is around 87.3, 146.2 and 170.7 nm, and the PDI is 0.041, 0.069 and 0.090, respectively. The increase in the size of EBDMH-SiO2 NPs further confirms that EBDMH has been immobilized onto the SiO 2 NPs surface. 13
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3.2 Effects of EBDMH-SiO2 NPs on the crystallization of PLA in the PLA/EBDMHSiO2 nanocomposites PLA/EBDMH-SiO 2 nanocomposites were prepared by melt blending in a Haake mixer at 185˚C, and compression-molded at 185˚C into sheets for testing. The crystallization behavior of PLA in the nanocomposites was investigated by the nonisothermal melt crystallization and overall isothermal crystallization kinetics of the
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nanocomposites.
Fig. 6A shows the non- isothermal crystallization thermograms of neat PLA and
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PLA/EBDMH-SiO 2 nanocomposites, and the data are summarized in Table 1. The
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addition of EBDMH-SiO 2 NPs has an apparent influence on the PLA crystallization
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kinetics in the nanocomposite. As the EBDMH-SiO 2 NPs content increases to 3%, the Tcc of PLA in nanocomposite decreases from 106.5˚C (neat PLA) to 98.4˚C. However,
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when the addition of EBDMH-SiO 2 NPs is over 5%, the Tcc of PLA increases
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gradually, even to 107.1˚C (9% of EBDMH-SiO2 NPs). The lower Tcc of PLA
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suggests that PLA in the nanocomposite may have a higher crystallization rate [53]. Meanwhile, the changes of grass transition temperature (Tg ), melting temperature (Tm ) and degree of crystallinity (Xc) of PLA in composite show the same trend with Tcc. The overall isothermal crystallization kinetics of neat PLA and PLA/EBDMHSiO2 nanocomposites was studied on DSC at different crystallization temperatures (Tc =105, 110 or 115˚C). The time-dependent relative degree of crystallinity (Xt , Fig. S4) is calculated according to the following equation [13]: 𝑋𝑡 =
𝑡 𝑑𝐻 𝑐 𝑑𝑡 𝑑𝑡 ∞ 𝑑𝐻 𝑐 ∫0 𝑑𝑡 𝑑𝑡
∫0
where dHc/dt is the heat flow at time t. 14
(4)
Journal Pre-proof The Avrami equation is used to analyze the isothermal crystallization kinetics of the polymers, and the Xt dependent t c can be expressed as [13]: 1 − 𝑋𝑡 = exp(−𝑘𝑡𝑐𝑛 )
(5)
where n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is the overall rate constant associated with both nucleation and growth contributions. The linear form of Eq. 5 can be expressed as follows: (6)
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log[−ln(1 − 𝑋𝑡 )] = log 𝑘 + 𝑛 log 𝑡𝑐
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The Avrami parameters n and k are obtained from the slopes and intercepts of the Avrami plots (Fig. S5), respectively.
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The half- time crystallization time t 1/2 is defined as the half period (50%
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crystallization) from the onset of crystallization to the end of crystallization. The value of t 1/2 can be calculated according to the following equation [54]: ln 2 𝑘
)
1⁄ 𝑛
(7)
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𝑡1/2 = (
The related crystallization kinetics parameters (n, k and t1/2 ) of the overall
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isothermal crystallization of neat PLA and PLA/EBDMH-SiO 2 nanocomposites are listed in Table 2. All value of n for neat PLA and PLA/EBDMH-SiO 2 nanocomposites are in range of 2 to 3. These findings strongly suggest that the addition of EBDMHSiO2 NPs do not change the crystallization mechanism of PLA [19]. As shown in Fig. 6B, the WAXD patterns of neat PLA exhibits three main characteristic diffraction peaks at around 14.82 ˚, 16.69˚ and 19.08˚, corresponding to diffraction of (010), (200) and (203) of PLA, respectively. These are the typical diffraction peaks of PLA crystalline in α form [55]. Meanwhile, similar diffraction peaks can be found in all WAXD patterns of PLA/EBDMH-SiO 2 nanocomposites with various EBDMH-SiO 2 NPs content, indicating that the incorporating of 15
Journal Pre-proof EBDMH-SiO2 NPs does not change the crystalline polymorphs of PLA. In addition, the change of degree of crystallinity (Wc,x ) of PLA in composite (Table S1) shows the same trend with DSC results as discussion previously. The crystallization rate can be evaluated by t 1/2 [54], and a lower t 1/2 is corresponding to a higher crystallization rate. As shown in Table 2, when the addition of EBDMH-SiO 2 NPs is below 5%, t 1/2 of PLA/EBDMH-SiO2 nanocomposites is lower than that of neat PLA under the same condition. However, if the content of
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EBDMH-SiO2 NPs is over 5%, t 1/2 of nanocomposite will increase gradually. It can be thus concluded that a small amount of EBDMH-SiO 2 NPs (< 5%) can effectively
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increase the crystallization rate.
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3.3 Effects of EBDMH-SiO2 NPs on the spherulitic morphology of PLA in nanocomposites
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The spherulitic morphologies of PLA in neat PLA and PLA/EBDMH-SiO 2
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nanocomposites during the isothermal crystallization at 130˚C were observed with
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POM. After 5 min, a few PLA spherulites with a size of around 20 μm in diameter appeared in neat PLA sample (Fig. 7A), while plenty of smaller PLA spherulites (about 10 μm) could be seen in PLA/EBDMH-SiO 2 -1 (Fig. 7B). These findings indicate that the crystallization rate of PLA has been significantly improved by EBDMH-SiO2 NPs. With the crystallization continued, the PLA spherulites in neat PLA grown up gradually, until reached around 120 μm after 60 min. On the contrast, in the nanocomposite samples, the size of PLA spherulites decreased slightly. With the combination of the results of DSC and POM analysis, it is believed that a small amount of EBDMH-SiO 2 NPs will serve as nucleating agents to enhance the crystallization rate of PLA in nanocomposites. On the other hand, at high EBDMH16
Journal Pre-proof SiO2 NPs content (> 5%), the excess EBDMH-SiO 2 NPs may restrict the movements of polymer chain segments and hinder the crystal growth process of PLA, which is similar to other inorganic nanoparticle/PLA blend systems [19,56].
3.4 Effects of EBDMH-SiO2 NPs on the rheological properties and thermal stability of PLA in the PLA/EBDMH-SiO2 nanocomposites Rheological test is an efficient tool to evaluate the melt strength of PLA upon
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processing. The frequency dependence of dynamic storage modulus (G') and complex viscosity (η*) for the neat PLA and PLA/EBDMH-SiO 2 nanocomposites are shown in
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Fig. 8. The G' of PLA/EBDMH-SiO 2 nanocomposites increases with the addition of
e-
EBDMH-SiO2 NPs (Fig. 8A). At low frequencies, the G' curve of neat PLA exhibits
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the typical terminal behavior [21], and can be expressed by the power law of G' ∝ ω2 , which is in consistent with the linear viscoelastic theory. However, with the increase
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of EBDMH-SiO 2 NPs, the terminal behavior of G' curves of nanocomposite
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disappears gradually. When the content of EBDMH-SiO 2 NPs is up to 7%, the
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particle-particle interactions among the EBDMH-SiO 2 NPs are strong enough to form the percolated network structures, which can restrain large-scale relaxations of PLA chains [57]. Therefore, the PLA/EBDMH-SiO 2 nanocomposite with high EBDMHSiO2 NPs content (> 7%) shows a solid- like flow behavior at the low-frequency region. Besides, the low- frequency η* increases with the addition of EBDMH-SiO 2 NPs, and the Newtonian plateau disappears gradually as shown in Fig. 8B. The increase of melt viscosity of PLA/EBDMH-SiO 2 nanocomposite leads to an enhancement in melt strength, which is a desirable property for PLA nanocomposite processing.
17
Journal Pre-proof In addition, from TGA curves (Fig. S6), the onset degradation temperature of PLA is unchanged after blending with EBDMH-SiO 2 NPs. It means that the thermal stability of PLA has not been enhanced with the addition of EBDMH-SiO2 NPs.
3.5 Evaluation of antibacterial properties of EBDMH-SiO2 NPs and PLA/EBDMHSiO2 nanocomposites The antibacterial activities of EBDMH-SiO 2 NPs were tested by minimum
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inhibitory concentration (MIC). EBDMH-SiO 2 NPs with 1.07 ± 0.21% oxidative chlorine content have the MIC value of 60 mg/mL against S. aureus (ATCC 6538)
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and 60 mg/mL against E. coli (CMCC 44103).
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The antibacterial activities of the chlorinated PLA/EBDMH-SiO 2 -9 sheets with
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5.1 ± 0.3 μg/cm2 of the oxidative chlorine content were challenged by S. aureus (ATCC 6538) and E. coli (CMCC 44103) with the contact mode. As shown in Fig. 9,
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PLA/EBDMH-SiO 2 -9 inactivated 90.2% of S. aureus and 89.4% of E. coli within 10
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min. The inset photographs of agar plates in Fig. 9 shows that the bacteria on
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PLA/EBDMH-SiO 2 -9 sheets were almost totally killed after 3 hours of contact (99.97% of S. aureus and 99.91% of E. coli, respectively), while the neat PLA sheets did not provide any detectable antibacterial function.
4. Conclusion In this work, a new N-halamine precursor with epoxy and hydantoin structures (EBDMH) was synthesized and immobilized on the surface of amino- functionalized silica NPs. EBDMH-SiO2 NPs and PLA were blended at 185˚C to prepare a novel environmentally friendly PLA based nanocomposite (PLA/EBDMH-SiO 2 ). The results of DSC and POM analysis show that the addition of EBDMH-SiO 2 NPs has 18
Journal Pre-proof great influences on the thermal properties of PLA/EBDMH-SiO 2 nanocomposite. By comparing with neat PLA, the grass transition temperature (Tg ), cold crystallization temperature (Tcc) and melting temperatures (Tm ) of PLA in nanocomposites gradually decrease with the increase of EBDMH-SiO2 NPs contents, and gradually increase when the EBDMH-SiO 2 NPs is over 5%. Moreover, less than 5% of EBDMH-SiO 2 NPs improves the growth of PLA spherulites. Compared with neat PLA, the melt strength of PLA/EBDMH-SiO 2 nanocomposites is enhanced with by EBDMH-SiO 2
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NPs, which is in favor for PLA nanocomposite processing. After chlorination treatment, the N-halamine functional surface of PLA/EBDMH-SiO 2 nanocomposite
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can inactivate 99.97% of S. aureus (ATCC 6538) and 99.91% of E. coli (CMCC
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44103) in 3 hours. These unique properties make the PLA/EBDMH-SiO 2
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nanocomposites good candidates for a wide range of applications including food
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Acknowledgements
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packaging, hygienic products, filters, as well as medical textiles.
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This work was supported by the National Natural Science Foundation of China (No. 51173028). The authors further thank Prof. Yuyu Sun (University of Massachusetts at Lowell) for his critical reading and polishing of the English.
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Captions: Fig. 1. Synthesis of 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH). Fig. 2. Preparation of N-halamine precursor modified silica nanoparticles (EBDMHSiO2 NPs). Fig. 3. 1 H-NMR and
13
C-NMR spectra of EBDMH.
Fig. 4. (A) FT-IR spectra of SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs, (B) TGA
f
curves of SiO2 , SiO 2 -APTES and EBDMH-SiO2 NPs.
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Fig. 5. TEM micrographs of SiO 2 (A), APTES-SiO2 (B), EBDMH-SiO 2 NPs (C), and
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particle size distributions of SiO 2 , APTES-SiO 2 and EBDMH-SiO2 NPs determined by DLS (D). (Scale bar is 200 nm)
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Fig. 6. DSC thermograms for neat PLA and PLA/EBDMH-SiO 2 nanocomposites in
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the second heating run (A), and WAXD patterns of neat PLA and PLA/EBDMH-SiO 2 nanocomposites (B).
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Fig. 7. Spherulitic growth process of neat PLA and various nanocomposites at a
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crystallization temperature of 130˚C for different times (scale bar is 200 μm).
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Fig. 8. Frequency dependence of (A) storage modulus (G') and (B) complex viscosity (η*) for neat PLA and PLA/EBDMH-SiO 2 nanocomposites. Fig. 9. Antibacterial tests of PLA/EBDMH-SiO 2 -9 plastic sheets after chlorination against S. aureus and E. coli. Inset is photographs showing the bacterial culture plates of S. aureus and E. coli upon 180 min contact with the control and PLA/EBDMHSiO2 -9 plastic sheets. (The applied bacterial densities of S. aureus and E. coli PBS solution were 1.42 × 104 and 1.73 × 104 CFU/ml, respectively.) Table
1
Crystallization properties of neat PLA and
nanocomposites.
27
PLA/EBDMH-SiO 2
Journal Pre-proof Table 2 Isothermal crystallization kinetic parameters for neat PLA and
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Pr
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PLA/EBDMH-SiO 2 nanocomposites based on the Avrami equation.
28
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O O OH
O
O
Cl
O
NH N
NH
CH 2Cl2
N
O O
VBDMH
EBDMH
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Pr
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Fig. 1. Synthesis of 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH).
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Fig. 2. Preparation of N-halamine precursor modified silica nanoparticles (EBDMH-
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Pr
e-
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SiO2 NPs).
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C-NMR spectra of EBDMH.
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Fig. 3. 1 H-NMR and
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Fig. 4. (A) FT-IR spectra of SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs, (B) TGA
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curves of SiO2 , SiO 2 -APTES and EBDMH-SiO2 NPs.
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Fig. 5. TEM micrographs of SiO 2 (A), APTES-SiO2 (B), EBDMH-SiO 2 NPs (C), and
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particle size distributions of SiO 2 , APTES-SiO 2 and EBDMH-SiO2 NPs determined by DLS (D) (Scale bar is 200 nm).
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Fig. 6. DSC thermograms for neat PLA and PLA/EBDMH-SiO 2 nanocomposites in
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the second heating run (A), and WAXD patterns of neat PLA and PLA/EBDMH-SiO 2
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nanocomposites (B).
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Fig. 7. Spherulitic growth process of neat PLA and various nanocomposites at a
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crystallization temperature of 130˚C for different times (scale bar is 200 μm).
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Fig. 8. Frequency dependence of (A) storage modulus (G') and (B) complex viscosity
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(η*) for neat PLA and PLA/EBDMH-SiO 2 nanocomposites.
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Fig. 9. Antibacterial tests of PLA/EBDMH-SiO 2 -9 plastic sheets after chlorination against S. aureus and E. coli. Inset is photographs showing the bacterial culture plates
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of S. aureus and E. coli upon 180 min contact with the control and PLA/EBDMH-
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SiO2 -9 plastic sheets. (The applied bacterial densities of S. aureus and E. coli PBS
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solution were 1.42 × 104 and 1.73 × 104 CFU/ml, respectively.)
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Journal Pre-proof Table 1
Tg
Tcc
ΔHcc
Tm
ΔHf
Xc
SiO2 NPs (w/w)
(°C) *
(°C) *
(J/g)
(°C) *
(J/g)
(%)
100/0
60.9
106.5
30.8
169.0
35.6
38.3
99/1
57.3
103.3
28.3
167.9
38.2
41.5
97/3
56.7
98.4
20.4
166.8
41.7
46.2
95/5
57.7
100.2
22.2
167.1
39.8
45.5
93/7
60.6
104.8
26.3
168.2
38.4
44.4
91/9
61.6
107.1
28.9
169.7
36.4
43.0
pr
rn
al
Pr
e-
, obtained from the DSC curves in second heating run.
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*
f
PLA/EBDMH-
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Crystallization properties of neat PLA and PLA/EBDMH-SiO2 nanocomposites.
38
Journal Pre-proof Table 2 Isothermal crystallization kinetic parameters for neat PLA and PLA/EBDMH-SiO 2 nanocomposites based on the Avrami equation. Isothermal crystallization
Crystallization kinetic parameters
SiO2 NPs (w/w)
Temperatures (Tc , ˚C)
n
K (min-n )
t 1/2 (min)
100/0
105 110 115 105 110 115 105 110 115 105 110 115 105 110 115 105 110 115
2.49 2.65 2.76 2.72 2.75 2.76 2.79 2.87 2.88 2.26 2.84 2.89 2.81 2.66 2.89 2.68 2.72 2.74
0.33 0.07 0.03 1.11 0.19 0.03 2.21 0.27 0.05 1.97 0.25 0.04 0.14 0.08 0.03 0.08 0.05 0.02
1.34 2.35 3.91 0.84 1.59 3.07 0.56 1.38 2.41 0.63 1.43 2.71 1.79 2.29 2.73 2.28 2.72 3.96
91/9
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93/7
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95/5
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97/3
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99/1
f
PLA/EBDMH-
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Authors statements Yan Zhao a , Conceptualization, Methodology , Investigation , Writing - Original -Draft Review & Editing Bolei Wei a, Investigation (part) Mi Wu a, Writing - Original Draft (part of fig) Huiliang Zhang b , Resources, Writing - Review Jinrong Yao
a,
, Conceptualization, Methodology, Writing - Review & Editing
Xin Chen a, Writing - Review & Editing
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Zhengzhong Shao a,Writing - Review & Editing
Corresponding authors
E-mail address: [email protected] (J. Yao). 40
Yan Zhao, Bolei Wei, Mi Wu, Huiliang Zhang, Jinrong Yao, Xin Chen, Zhengzhong Shao PII:
S0141-8130(19)37817-1
DOI:
https://doi.org/10.1016/j.ijbiomac.2019.11.125
Reference:
BIOMAC 13906
To appear in:
International Journal of Biological Macromolecules
Received date:
26 September 2019
Revised date:
13 November 2019
Accepted date:
13 November 2019
Please cite this article as: Y. Zhao, B. Wei, M. Wu, et al., Preparation and characterization of antibacterial poly(lactic acid) nanocomposites with N-halamine modified silica, International Journal of Biological Macromolecules(2019), https://doi.org/10.1016/ j.ijbiomac.2019.11.125
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© 2019 Published by Elsevier.
Journal Pre-proof
Preparation and characterization of antibacterial poly(lactic acid) nanocomposites with N-halamine modified silica Yan Zhao a, Bolei Wei a, Mi Wu a, Huiliang Zhang b, Jinrong Yao
a,
, Xin
Chen a, Zhengzhong Shao a
a
State Key Laboratory of Molecular Engineering of Polymers, Department of
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f
Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai, 200438, China.
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry,
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b
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Chinese Academy of Sciences, Changchun, 130022, China.
ABSTRACT
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In this work, silica nanoparticles modified with a new N-halamine precursor
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(EBDMH-SiO 2 NPs) were synthesized through immobilization of 3-(4'-epoxyethylbenzyl)-5,5-dimethylhydantoin (EBDMH) on the surface of amino- functionalized
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silica NPs. Then, EBDMH-SiO 2 NPs and poly(lactic acid) (PLA) were blended at 185˚C to prepare a novel environmentally friendly PLA based nanocomposite (PLA/EBDMH-SiO 2 ). The addition of EBDMH-SiO2 NPs has great influences on the thermal properties of nanocomposite. DSC results show that the grass transition temperature (Tg ), cold crystallization temperature (Tcc) and melting temperature (Tm) of PLA in nanocomposites gradually decrease with the increase of EBDMH-SiO2 NPs contents up to 5%. After that, further rise in EBDMH-SiO 2 NPs content actually
Corresponding authors E-mail address: [email protected] (J. Yao). 1
Journal Pre-proof increases Tg, Tcc, and Tm. The overall crystallization and spherulite growth rate of PLA show the similar trend. Furthermore, the introduction of EBDMH-SiO 2 NPs increases the storage modulus and viscosity of the melt of nanocomposite, providing an additional benefit for PLA blowing and injection molding. After chlorination, the N-halamine precursors on the nanocomposite surfaces are transformed into Nhalamines, which provide strong antibacterial activities against E. coli (CMCC 44103) and S. aureus (ATCC 6538), pointing to good potentials of the PLA/EBDMH-SiO 2
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nanocomposites for antibacterial applications including food packaging, filters, and a
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wide range of hygienic products.
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Keywords:
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Poly(lactic acid) (PLA)
Silica nanoparticles
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Nanocomposites
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3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH)
1. Introduction
In the last two decades, the increased production and use of plastic materials have led to an urgent need for waste disposal solutions [1,2]. To solve this problem, natural biopolymers and biodegradable synthetic polymers are investigated as alternatives to conventional petroleum plastics to reduce domestic and industrial plastic wastes [3-6]. Among biodegradable polymers, poly(lactic acid) (PLA), a degradable polymer derived from corn or sugar cane, is increasingly used to replace the traditional non-degradable polymer materials in many applications [7-9]. However, some intrinsic properties of PLA, such as slow crystallization rate and low melt 2
Journal Pre-proof strength, have caused a number of problems during PLA manufacturing [10-12]. The low crystallization rate of PLA leads to a rather long crystallization time, which is incompatible with the requirements for high efficiency of industrial processes, such as extrusion, injection molding and foaming. It has been reported that the crystallization rate of PLA can be increased by blending with polymer [13-15] and nucleating agents [16-23]. Nano-sized materials, such as montmorillonite [18], carbon nanotubes [19], calcium carbonate nanoparticles [20], nano-SiO2 [21], basalt fiber [22], and poly(D-
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lactic acid) [23-25] have been used as fillers to enhance the crystallization and melt strength of PLA.
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Due to the excellent biodegradability and biocompatibility, PLA has been used in
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biomedical materials, food packaging and hygienic products in which antibacterial
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functionality is often desired [26-28]. Some PLA materials with antibacterial properties have been developed in recent decades, such as PLA/Ag NPs transparent
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films [29], PLA/chitosan biocomposite films [30], and PLA/graphene oxide
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nanocomposites [31]. Nevertheless, all these systems suffer from certain limitations.
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For instance, although Ag NPs have been widely used in various fields, Ag ions can release into the surrounding environment, and ions can undergo chemical and biochemical conversions, which can be harmful to the environment [32,33]. Chitosan is a safe, non-toxic natural antibacterial material, but the immiscibility between PLA and chitosan has limited its further application [34]. As a kind of 2D carbon nanomaterials, GO is proved to have a good antibacterial effect, but the long-term effect of GO has been unclear so far [35]. Thus, there is a need for antimicrobial PLA which can be easily prepared, be effective within short contact times against a broad range of microorganisms, and be stable for long-term use. In this regard, N-halamine antibacterial materials are attracting continuous interest because of their unique 3
Journal Pre-proof properties, including powerful antibacterial activity, high durability, long-term stability, and regenerability [36-40]. In this work, a new N-halamine precursor with epoxy and hydantoin structures, 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH) was synthesized, and immobilized on amino- functionalized silica NPs to form EBDMH-SiO 2 NPs. Various concentrations of EBDMH-SiO 2 NPs were introduced into the PLA matrix to prepare a novel environmentally friendly PLA based nanocomposite (PLA/EBDMH-SiO 2 ).
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The effects of EBDMH-SiO 2 NPs on the crystallization behaviors, and rheological properties and antibacterial activities of PLA/EBDMH-SiO 2 nanocomposite were
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investigated. Our findings suggest that the addition of N-halamine modified silica NPs
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could provide PLA with outstanding biocidal activity, and excellent processability for
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a number of applications such as blown film extrusion, injection molding and
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2.1 Materials
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2. Experimental
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thermoforming.
Poly(lactic acid) (PLA 4032D, M w: 242 000 g/mol, PDI: 1.47, D- isomer: 2.0 wt%) was purchased from NatureWorks Co., Ltd (USA). Colloidal silica (diameter: 80 nm, density: 1.18-1.30 g/cm3 ) was from Shandong Peak-tech New Material Co., Ltd (China). 5,5-dimethylhydantoin (DMH), 1-chloromethyl-4-vinyl-benzene, 3chloroperoxybenzoic acid, and (3-aminopropyl) triethoxysilane (APTES), sodium thiosulfate solution (AR, 0.01 mol/L) were obtained from Aladdin Industrial Corporation (China). Luria Bertani agar/broth (LB) and super optimal agar/broth were provided by Sangon Biotech Co., Ltd (China). S. aureus (ATCC 6538) and E. coli (CMCC 44103) were obtained from Guangdong Huankai Microbial Sci. & Tech. Co., 4
Journal Pre-proof Ltd (China). Sodium hypochlorite solution (5%) was purchased from Adamas Reagent Co., Ltd (China). Phosphate buffered saline (PBS) was purchased from Shanghai MesGen Biotechnology Co., Ltd (China). Other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd (China).
2.2 Synthesis of 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH) The synthesis route of EBDMH is shown in Fig. 1. First, VBDMH was
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synthesized according to a method reported by Sun et. al [41], and confirmed by NMR and FT-IR spectra (Fig. S1-S2).
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20 ml of VBDMH (1.0 mmol, 2.44 g) dichloromethane (DCM) solution was
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dropwise added into 20 ml of chloroperoxybenzoic (1.3 mmol, 2.29 g) DCM solution
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at 0˚C and stirred at 0˚C for 12 hours. Then the mixture was stirred at 25˚C for another 12 hours. The DCM solution was washed in sequence by sodium sulfite
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(10%), sodium bicarbonate (5%), and water, respectively. After drying over
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anhydrous MgSO 4 , the solvent was removed by rotary evaporation to obtain the
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EGDMH as a light yellow solid ( yield 67%). Molecular weight of EBDMH: 260.29 (theoretical); 260.14 (MALDI-TOF MS). 1 H-NMR and 13 C-NMR spectra of EBDMH in DMSO-d6 are shown in Fig. 3. 1 H-NMR (400 MHz, DMSO-d6 , ppm) δ: 8.40 (s, 1H), 7.26 (d, 2H), 7.21 (d, 2H), 4.51(s, 1H), 3.90 (s, 1H), 3.10 (s, 1H), 2.82 (s, 1H), 1.29 (s, 6H). 13 C-NMR (400 MHz, DMSO-d6 , ppm) δ: 177.70, 155.56, 137.34, 137.23, 127.68, 126.42, 58.35, 51.75, 50.82, 41.12, 25.07.
2.3 Preparation of EBDMH-SiO2 NPs The preparation procedures of EBDMH-SiO 2 NPs are illustrated in Fig. 2. APTES-SiO 2 NPs was synthesized with the reported methods in literatures [42-44]. 5
Journal Pre-proof The typical process is: 15.00 g (0.06 mol) 3-aminopropyl-triethoxysilane (APTES) was dropwise added into 250 ml ethanol mixture (pH was adjusted to 9.5) of colloidal silica (100 ml) and SDS (150 mg). The mixture was stirred vigorously at room temperature for 24 hours and then refluxed for 6 hours. APTES- modified silica nanoparticles (APTES-SiO 2 NPs) were obtained by centrifugation and washing with ethanol. APTES-SiO 2 NPs (5.0 g) was dispersed in 200 ml acetonitrile by ultrasound for
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30 min, then 10 ml EBDMH (0.1 g) acetonitrile solution was dropwise added. The mixture was stirred vigorously at 60 °C for 24 hours. The resultant nanoparticles
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(EBDMH-SiO 2 NPs) were collected by centrifugation, washing with ethanol, and
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drying in vacuum.
2.4 Preparation of PLA/EBDMH-SiO2 nanocomposites
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PLA pellets were dried at 80˚C for 8 hours in a vacuum oven, and EBDMH-SiO 2
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NPs was ground in a planetary ball mill three times. PLA/EBDMH-SiO 2
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nanocomposites were prepared by melt blending in a mixer (Haake Rheomix 600, Germany) with a rotating speed of 60 rpm at 185˚C for 10 min. The obtained PLA nanocomposites were compression- molded (hydraulic press: 120 kg/cm2 ) at 185˚C into sheets with thicknesses of 1.0 mm for testing. The neat PLA control samples were prepared using the same method. The PLA nanocomposite sample was labelled as PLA/EBDMH-SiO 2 -x, where x was the weight percentage of EBDMH-SiO 2 NPs in nanocomposite.
2.5 Characterization and measurements 2.5.1 Fourier transform infrared (FT-IR) spectroscopy 6
Journal Pre-proof FT-IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer (ThermoFisher, USA) in the wavenumber range of 4000-500 cm-1 with a resolution of 4 cm-1 and 64 scans, respectively. The samples for FT-IR analysis were prepared by pressing with KBr.
2.5.2 Nuclear magnetic resonance (NMR) spectroscopy 1
H-NMR and
13
C-NMR spectra were obtained from a Bruker (400 MHz,
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Switzerland), using DMSO as solvent and tetramethylsilane (TMS) as interior
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chemical shift standard.
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2.5.3 Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy
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(MALDI-TOF MS)
MALDI- TOF MS measurement was performed on a MALDI-TOF MS
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spectrometer (5800, AB-SCIEX, USA). The matrix solution of dithranol (20 mg/mL),
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cationizing agent of sodium trifluoroacetate (10 mg/mL), and sample THF solution
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(10 mg/mL) were mixed in a volume ratio of matrix: cationizing agent: sample = 10: 2: 5. The mixed solution (0.5 μL) was deposited on the sample holder and dried at ambient temperature.
2.5.4 Dynamic light scattering (DLS) The particle size distribution of the silica NPs dispersed in water was measured by a DLS particle size analyzer (ZS90, Malvern, UK) with a scattering angle of 90˚.
2.5.5 Transmission electron microscope (TEM)
7
Journal Pre-proof The morphologies of the silica NPs were observed on a TEM (Tecnai G2 20 TWIN, FEI, USA).
2.5.6 Thermogravimetric analysis (TGA) TGA of the silica NPs was performed on a thermal analyzer (Perkin Elmer, USA) from 100 to 600°C at a rate of 10°C/min under air atmosphere. TGA of PLA nanocomposite samples was performed on a thermal analyzer
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(Perkin Elmer, USA) from 100 to 600 °C at a rate of 10°C/min under nitrogen
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atmosphere.
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2.5.7 Differential scanning calorimetry (DSC) measurements
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Thermal properties of PLA nanocomposite samples were performed on a DSC (Q2000, TA Instruments, USA) under nitrogen atmosphere. The sample weight varied
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between 5 and 8 mg. Samples were heated from 30 to 190 °C at a rate of 10 °C/min and
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kept in the molten state for 3 min to eliminate thermal history (1st heating), and then
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cooled to 30°C at a rate of 10°C/min. After cooling, the samples were heated back to 190°C at a rate of 10 °C/min (2nd heating). The data of glass transition temperature (Tg), cold crystallization temperature (Tcc), cold crystallization enthalpy (ΔHcc), melting temperatures (Tm) and the melting enthalpy (ΔHm) of the PLA matrix were obtained from the second heating run. The degree of crystallinity (Xc) of the samples was evaluated by the following relationship: 𝑋c (%) =
∆𝐻m
0 𝑊PLA ×∆𝐻m
× 100%
(1)
where ΔHm is the observed melting enthalpy of the PLA, ΔHm0 is the theoretical melting enthalpy of 100% crystalline PLA (93 J/g) [45] and WPLA is the weight fraction of PLA in the nanocomposites. 8
Journal Pre-proof During the isothermal melt crystallization process, the samples were heated from 30 to 190˚C at a heating rate of 30˚C/min, and held at 190˚C for 3 min, then cooled to the desired crystallization temperature (Tc) at a cooling rate of 30˚C/min and held at Tc until the isothermal crystallization was completed. The applied crystallization temperatures were 105, 110 and 115˚C, respectively.
2.5.8 Polarized optical microscope (POM)
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Spherulitic growth of neat PLA and PLA/EBDMH-SiO 2 nanocomposites was monitored by polarized optical microscope (POM, DM 2500P, Leica, Germany)
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equipped with a hot stage (Linkam THMS 600). The samples were first heated from
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room temperature to 190˚C at a rate of 30˚C/min and held at 190˚C for 5 min to erase
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thermal history, and then cooled to 130˚C at a rate of 30˚C/min for isothermal
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crystallization and held at 130˚C for 60 min.
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2.5.9 Wide angle X-ray diffraction (WAXD)
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WAXD was performed on an X-ray diffractometer (D8 ADVANCE, Bruker, Germany) using a Cu Kα radiation source (40 kV, 40 mA). The diffraction angle (2θ) was from 10˚ to 30˚. The scanning speed was 2˚/min. Before testing, all samples were first pressed into sheet with a thickness of around 1 mm on a hot press at 185°C and then transferred into another hot press at 130 °C for 3 hours.
2.5.10 Rheological properties Rheological properties of PLA nanocomposite were investigated on a rotational rheometer (HAAKE MARS III, ThermoFisher, USA) with a parallel plate diameter of 20 mm and a gap of 0.9 mm. The samples were tested at 190 °C under nitrogen to 9
Journal Pre-proof avoid thermal degradation. Dynamic frequency sweep was tested over a frequency range of 0.01-100 rad/s. The strain value was set at 1% under the linear viscoelastic region.
2.5.11 Chlorination Chlorination of EBDMH-SiO 2 NPs was carried out as follows. About 1 g of EBDMH-SiO2 NPs was ultrasonic dispersed into 100 mL of sodium hypochlorite
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solution (1%), and stirred for 1 hours at room temperature. Then chlorinated EBDMH-SiO2 NPs were collected by centrifugation, washing with distilled water,
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and drying in vacuum. The active chlorine content of the EBDMH-SiO 2 NPs was
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determined by iodometric/thiosulfate titration method [46], and calculated according
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to the following equation [47]: Cl(%) =
35.5 2
×
𝐶 ×𝑉 𝑊
× 100%
(2)
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where C and V are the concentration (mol/L) and the consumed volume (L) of the
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titrant sodium thiosulfate, respectively, and W is the weight of the chlorinated
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EBDMH-SiO2 NPs (g).
The PLA/EBDMH-SiO 2 nanocomposite sheets (3 cm × 3 cm) were chlorinated in 1 % of a commercial aqueous sodium hypochlorite solution (with 0.05 % Triton, X100) at pH 7 for 1 hours at room temperature. The sheets were washed with distilled water, and then dried at 45˚C for 1 hours to remove free chlorine. The oxidative chlorine loadings on the surfaces of PLA nanocomposite sheet was determined by a modified iodometric/thiosulfate titration method [38], and calculated with the following equation [26]: Cl (μg⁄cm2 ) = 10
35.5 2
×
𝐶×𝑉 2𝑆
(3)
Journal Pre-proof where Cl (μg/cm2 ) is the weight of oxidative chlorine per square centimeter on the surfaces of the samples, C and V are the concentration (mol/L) and the consumed volume (L) of the titrant sodium thiosulfate, respectively, and S is the surface area of the sheet (cm2 ).
2.5.12 Antibacterial assessment The antibacterial activities of the chlorinated EBDMH-SiO2 NPs and
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PLA/EBDMH-SiO 2 nanocomposite sheets were challenged by Gram-positive bacteria (S. aureus ATCC 6538) and Gram- negative bacteria (E. coli CMCC 44103). S. aureus,
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broth for E. coli) for 24 hours at 37˚C.
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and E. coli were grown in broth solutions (tryptic soy broth for S. aureus and LB
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The minimum inhibition concentration (MIC) was also determined to evaluate the antibacterial activity of the sample [48]. The harvested bacteria were diluted with
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PBS buffer (pH = 7.4) to about 105 CFU/ml. Samples with serial weights were each
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dispersed in 900 μL broth solutions, vortexed, and then sonicated for 20 min. 100 μL
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of bacteria suspension was then added into sample suspension. After incubation at 37˚C for 12 hours, 5 mL of 0.02 M sodium thiosulfate solution was added into the incubation suspension. And then resulting mixture was dispersed onto agar plates. Colonies on the plates were counted after incubation at 37˚C for 24 hours [49]. The harvested bacteria were diluted with PBS buffer (pH = 7.4) to about 104 CFU/ml. 50 μL of fresh bacteria suspension was placed on the center of a sheet (3 cm × 3 cm), and covered with another identical sheet. A gentle pressure was applied to ensure adequate contact with the bacteria. After the specified contact times (10, 30, 60, 120 and 180 min), the sheets were placed into 5 mL of 0.02 M sodium thiosulfate solution and vortexed for 2 min in order to quench the oxidative chlorine and transfer 11
Journal Pre-proof bacteria to solution. Serial dilutions were made using PBS buffer solution and each diluent was placed onto the corresponding agar plates. After incubation at 37˚C for 24 hours, bacterial colonies on the agar plates were counted [33,38]. The neat PLA sheets were tested under the same conditions to serve as control samples.
3. Results and discussion
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3.1 Preparation of PLA/EBDMH-SiO2 nanocomposites
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An N-halamine precursor with epoxy group (EBDMH) was synthesized from the
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oxidation of VBDMH with 3-chloroperoxybenzoic (Fig. 1). The reaction was confirmed by NMR studies. The peaks assigned to epoxy group (protons) of EBDMH
H-NMR spectrum (Fig. 3A) and at 51.75 ppm (methenyl carbon) and 50.82 ppm
(methylene carbon) in
13
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1
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appear at δ =3.90 ppm (methenyl proton), 2.82 and 3.10 ppm (methylene proton) in
C-NMR spectrum (Fig. 3B), respectively. Meanwhile, peaks
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corresponding to the alkene double bond of VBDMH (5.25, 5.81, 6.71 ppm in 1 H-
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NMR spectrum, and 114.83, 136.70 ppm in
13
C-NMR spectrum, Fig. S1) disappear
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completely. Furthermore, the molecular weight of EBDMH determined by MALDITOF MS (Fig. S3) is 260.14, which agrees very well with the theoretical value (260.29).
Through the reaction between epoxy group of EBDMH and amino groups on the amino- functionalized silica NPs (APTES-SiO2 NPs), EBDMH was immobilized on the surfaces of silica nanoparticles (EBDMH-SiO 2 NPs). The FT-IR spectra of SiO 2 , APTES-SiO 2 and EBDMH-SiO2 NPs were shown in Fig. 4A. In all spectra of the nanoparticles, the peaks around 800, 950, 1100, and 3500 cm-1 are attributed to symmetric stretching vibration of Si-O-Si, stretching vibration of Si-OH, antisymmetric stretching vibration of Si-O-Si, and stretching vibration of hydroxy 12
Journal Pre-proof groups, respectively. Meanwhile, the peak at 1620 cm-1 is assigned to the residual unhydrolyzed Si-O-C [40]. The peak at 1560 cm-1 in the spectrum of APTES-SiO2 NPs (curve b) is attributed to the stretching vibration of N-H groups. In curve c, a weak sharp peak at 1712 cm-1 appears, which is corresponding to the stretching vibration of C=O bond of hydantoin structure of EBDMH [50]. These findings suggest that EBDMH has been immobilized on the surface of silica nanoparticles. TGA curves of SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs are shown in Fig. 4B.
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The weight loss below 200˚C is attributed to evaporation of water and residual organic solvent [51]. In curve a, when the temperature reaches about 800˚C, the
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organic contents completely burn up and the residues are SiO 2 with a content of
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around 96.7%. In curve b, APTES-SiO 2 NPs begin to decompose at 280˚C, which
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attributed to the decomposition of aminopropyl groups [52]. After the organics completely burn up and the content of residual SiO 2 is around 93.8%. In curve c,
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EBDMH-SiO2 NPs begin to decompose at 190˚C, which is due to decomposition of
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EBDMH. From the TGA quantitative analysis, the grafting yield of EBDMH in the
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EBDMH-SiO2 NPs is calculated to be around 5.8%. In TEM studies (Fig. 5A-C), SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs show spherical shapes with smooth surface after modification, but the size of nanoparticles increases gradually. The mean particle size of SiO 2 , APTES-SiO2 , and EBDMH-SiO 2 NPs is 71.2, 85.6, and 91.8 nm, respectively. The particle size distribution from dynamic light scattering (DLS) is unimodal and narrow (Fig. 5D). The particle size of SiO2 , APTES-SiO 2 and EBDMH-SiO 2 NPs obtained from DLS is around 87.3, 146.2 and 170.7 nm, and the PDI is 0.041, 0.069 and 0.090, respectively. The increase in the size of EBDMH-SiO2 NPs further confirms that EBDMH has been immobilized onto the SiO 2 NPs surface. 13
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3.2 Effects of EBDMH-SiO2 NPs on the crystallization of PLA in the PLA/EBDMHSiO2 nanocomposites PLA/EBDMH-SiO 2 nanocomposites were prepared by melt blending in a Haake mixer at 185˚C, and compression-molded at 185˚C into sheets for testing. The crystallization behavior of PLA in the nanocomposites was investigated by the nonisothermal melt crystallization and overall isothermal crystallization kinetics of the
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nanocomposites.
Fig. 6A shows the non- isothermal crystallization thermograms of neat PLA and
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PLA/EBDMH-SiO 2 nanocomposites, and the data are summarized in Table 1. The
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addition of EBDMH-SiO 2 NPs has an apparent influence on the PLA crystallization
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kinetics in the nanocomposite. As the EBDMH-SiO 2 NPs content increases to 3%, the Tcc of PLA in nanocomposite decreases from 106.5˚C (neat PLA) to 98.4˚C. However,
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when the addition of EBDMH-SiO 2 NPs is over 5%, the Tcc of PLA increases
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gradually, even to 107.1˚C (9% of EBDMH-SiO2 NPs). The lower Tcc of PLA
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suggests that PLA in the nanocomposite may have a higher crystallization rate [53]. Meanwhile, the changes of grass transition temperature (Tg ), melting temperature (Tm ) and degree of crystallinity (Xc) of PLA in composite show the same trend with Tcc. The overall isothermal crystallization kinetics of neat PLA and PLA/EBDMHSiO2 nanocomposites was studied on DSC at different crystallization temperatures (Tc =105, 110 or 115˚C). The time-dependent relative degree of crystallinity (Xt , Fig. S4) is calculated according to the following equation [13]: 𝑋𝑡 =
𝑡 𝑑𝐻 𝑐 𝑑𝑡 𝑑𝑡 ∞ 𝑑𝐻 𝑐 ∫0 𝑑𝑡 𝑑𝑡
∫0
where dHc/dt is the heat flow at time t. 14
(4)
Journal Pre-proof The Avrami equation is used to analyze the isothermal crystallization kinetics of the polymers, and the Xt dependent t c can be expressed as [13]: 1 − 𝑋𝑡 = exp(−𝑘𝑡𝑐𝑛 )
(5)
where n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is the overall rate constant associated with both nucleation and growth contributions. The linear form of Eq. 5 can be expressed as follows: (6)
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log[−ln(1 − 𝑋𝑡 )] = log 𝑘 + 𝑛 log 𝑡𝑐
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The Avrami parameters n and k are obtained from the slopes and intercepts of the Avrami plots (Fig. S5), respectively.
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The half- time crystallization time t 1/2 is defined as the half period (50%
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crystallization) from the onset of crystallization to the end of crystallization. The value of t 1/2 can be calculated according to the following equation [54]: ln 2 𝑘
)
1⁄ 𝑛
(7)
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al
𝑡1/2 = (
The related crystallization kinetics parameters (n, k and t1/2 ) of the overall
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isothermal crystallization of neat PLA and PLA/EBDMH-SiO 2 nanocomposites are listed in Table 2. All value of n for neat PLA and PLA/EBDMH-SiO 2 nanocomposites are in range of 2 to 3. These findings strongly suggest that the addition of EBDMHSiO2 NPs do not change the crystallization mechanism of PLA [19]. As shown in Fig. 6B, the WAXD patterns of neat PLA exhibits three main characteristic diffraction peaks at around 14.82 ˚, 16.69˚ and 19.08˚, corresponding to diffraction of (010), (200) and (203) of PLA, respectively. These are the typical diffraction peaks of PLA crystalline in α form [55]. Meanwhile, similar diffraction peaks can be found in all WAXD patterns of PLA/EBDMH-SiO 2 nanocomposites with various EBDMH-SiO 2 NPs content, indicating that the incorporating of 15
Journal Pre-proof EBDMH-SiO2 NPs does not change the crystalline polymorphs of PLA. In addition, the change of degree of crystallinity (Wc,x ) of PLA in composite (Table S1) shows the same trend with DSC results as discussion previously. The crystallization rate can be evaluated by t 1/2 [54], and a lower t 1/2 is corresponding to a higher crystallization rate. As shown in Table 2, when the addition of EBDMH-SiO 2 NPs is below 5%, t 1/2 of PLA/EBDMH-SiO2 nanocomposites is lower than that of neat PLA under the same condition. However, if the content of
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EBDMH-SiO2 NPs is over 5%, t 1/2 of nanocomposite will increase gradually. It can be thus concluded that a small amount of EBDMH-SiO 2 NPs (< 5%) can effectively
e-
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increase the crystallization rate.
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3.3 Effects of EBDMH-SiO2 NPs on the spherulitic morphology of PLA in nanocomposites
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The spherulitic morphologies of PLA in neat PLA and PLA/EBDMH-SiO 2
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nanocomposites during the isothermal crystallization at 130˚C were observed with
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POM. After 5 min, a few PLA spherulites with a size of around 20 μm in diameter appeared in neat PLA sample (Fig. 7A), while plenty of smaller PLA spherulites (about 10 μm) could be seen in PLA/EBDMH-SiO 2 -1 (Fig. 7B). These findings indicate that the crystallization rate of PLA has been significantly improved by EBDMH-SiO2 NPs. With the crystallization continued, the PLA spherulites in neat PLA grown up gradually, until reached around 120 μm after 60 min. On the contrast, in the nanocomposite samples, the size of PLA spherulites decreased slightly. With the combination of the results of DSC and POM analysis, it is believed that a small amount of EBDMH-SiO 2 NPs will serve as nucleating agents to enhance the crystallization rate of PLA in nanocomposites. On the other hand, at high EBDMH16
Journal Pre-proof SiO2 NPs content (> 5%), the excess EBDMH-SiO 2 NPs may restrict the movements of polymer chain segments and hinder the crystal growth process of PLA, which is similar to other inorganic nanoparticle/PLA blend systems [19,56].
3.4 Effects of EBDMH-SiO2 NPs on the rheological properties and thermal stability of PLA in the PLA/EBDMH-SiO2 nanocomposites Rheological test is an efficient tool to evaluate the melt strength of PLA upon
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processing. The frequency dependence of dynamic storage modulus (G') and complex viscosity (η*) for the neat PLA and PLA/EBDMH-SiO 2 nanocomposites are shown in
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Fig. 8. The G' of PLA/EBDMH-SiO 2 nanocomposites increases with the addition of
e-
EBDMH-SiO2 NPs (Fig. 8A). At low frequencies, the G' curve of neat PLA exhibits
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the typical terminal behavior [21], and can be expressed by the power law of G' ∝ ω2 , which is in consistent with the linear viscoelastic theory. However, with the increase
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of EBDMH-SiO 2 NPs, the terminal behavior of G' curves of nanocomposite
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disappears gradually. When the content of EBDMH-SiO 2 NPs is up to 7%, the
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particle-particle interactions among the EBDMH-SiO 2 NPs are strong enough to form the percolated network structures, which can restrain large-scale relaxations of PLA chains [57]. Therefore, the PLA/EBDMH-SiO 2 nanocomposite with high EBDMHSiO2 NPs content (> 7%) shows a solid- like flow behavior at the low-frequency region. Besides, the low- frequency η* increases with the addition of EBDMH-SiO 2 NPs, and the Newtonian plateau disappears gradually as shown in Fig. 8B. The increase of melt viscosity of PLA/EBDMH-SiO 2 nanocomposite leads to an enhancement in melt strength, which is a desirable property for PLA nanocomposite processing.
17
Journal Pre-proof In addition, from TGA curves (Fig. S6), the onset degradation temperature of PLA is unchanged after blending with EBDMH-SiO 2 NPs. It means that the thermal stability of PLA has not been enhanced with the addition of EBDMH-SiO2 NPs.
3.5 Evaluation of antibacterial properties of EBDMH-SiO2 NPs and PLA/EBDMHSiO2 nanocomposites The antibacterial activities of EBDMH-SiO 2 NPs were tested by minimum
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inhibitory concentration (MIC). EBDMH-SiO 2 NPs with 1.07 ± 0.21% oxidative chlorine content have the MIC value of 60 mg/mL against S. aureus (ATCC 6538)
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and 60 mg/mL against E. coli (CMCC 44103).
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The antibacterial activities of the chlorinated PLA/EBDMH-SiO 2 -9 sheets with
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5.1 ± 0.3 μg/cm2 of the oxidative chlorine content were challenged by S. aureus (ATCC 6538) and E. coli (CMCC 44103) with the contact mode. As shown in Fig. 9,
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PLA/EBDMH-SiO 2 -9 inactivated 90.2% of S. aureus and 89.4% of E. coli within 10
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min. The inset photographs of agar plates in Fig. 9 shows that the bacteria on
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PLA/EBDMH-SiO 2 -9 sheets were almost totally killed after 3 hours of contact (99.97% of S. aureus and 99.91% of E. coli, respectively), while the neat PLA sheets did not provide any detectable antibacterial function.
4. Conclusion In this work, a new N-halamine precursor with epoxy and hydantoin structures (EBDMH) was synthesized and immobilized on the surface of amino- functionalized silica NPs. EBDMH-SiO2 NPs and PLA were blended at 185˚C to prepare a novel environmentally friendly PLA based nanocomposite (PLA/EBDMH-SiO 2 ). The results of DSC and POM analysis show that the addition of EBDMH-SiO 2 NPs has 18
Journal Pre-proof great influences on the thermal properties of PLA/EBDMH-SiO 2 nanocomposite. By comparing with neat PLA, the grass transition temperature (Tg ), cold crystallization temperature (Tcc) and melting temperatures (Tm ) of PLA in nanocomposites gradually decrease with the increase of EBDMH-SiO2 NPs contents, and gradually increase when the EBDMH-SiO 2 NPs is over 5%. Moreover, less than 5% of EBDMH-SiO 2 NPs improves the growth of PLA spherulites. Compared with neat PLA, the melt strength of PLA/EBDMH-SiO 2 nanocomposites is enhanced with by EBDMH-SiO 2
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NPs, which is in favor for PLA nanocomposite processing. After chlorination treatment, the N-halamine functional surface of PLA/EBDMH-SiO 2 nanocomposite
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can inactivate 99.97% of S. aureus (ATCC 6538) and 99.91% of E. coli (CMCC
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44103) in 3 hours. These unique properties make the PLA/EBDMH-SiO 2
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nanocomposites good candidates for a wide range of applications including food
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Acknowledgements
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packaging, hygienic products, filters, as well as medical textiles.
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This work was supported by the National Natural Science Foundation of China (No. 51173028). The authors further thank Prof. Yuyu Sun (University of Massachusetts at Lowell) for his critical reading and polishing of the English.
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Captions: Fig. 1. Synthesis of 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH). Fig. 2. Preparation of N-halamine precursor modified silica nanoparticles (EBDMHSiO2 NPs). Fig. 3. 1 H-NMR and
13
C-NMR spectra of EBDMH.
Fig. 4. (A) FT-IR spectra of SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs, (B) TGA
f
curves of SiO2 , SiO 2 -APTES and EBDMH-SiO2 NPs.
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Fig. 5. TEM micrographs of SiO 2 (A), APTES-SiO2 (B), EBDMH-SiO 2 NPs (C), and
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particle size distributions of SiO 2 , APTES-SiO 2 and EBDMH-SiO2 NPs determined by DLS (D). (Scale bar is 200 nm)
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Fig. 6. DSC thermograms for neat PLA and PLA/EBDMH-SiO 2 nanocomposites in
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the second heating run (A), and WAXD patterns of neat PLA and PLA/EBDMH-SiO 2 nanocomposites (B).
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Fig. 7. Spherulitic growth process of neat PLA and various nanocomposites at a
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crystallization temperature of 130˚C for different times (scale bar is 200 μm).
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Fig. 8. Frequency dependence of (A) storage modulus (G') and (B) complex viscosity (η*) for neat PLA and PLA/EBDMH-SiO 2 nanocomposites. Fig. 9. Antibacterial tests of PLA/EBDMH-SiO 2 -9 plastic sheets after chlorination against S. aureus and E. coli. Inset is photographs showing the bacterial culture plates of S. aureus and E. coli upon 180 min contact with the control and PLA/EBDMHSiO2 -9 plastic sheets. (The applied bacterial densities of S. aureus and E. coli PBS solution were 1.42 × 104 and 1.73 × 104 CFU/ml, respectively.) Table
1
Crystallization properties of neat PLA and
nanocomposites.
27
PLA/EBDMH-SiO 2
Journal Pre-proof Table 2 Isothermal crystallization kinetic parameters for neat PLA and
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Pr
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PLA/EBDMH-SiO 2 nanocomposites based on the Avrami equation.
28
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O O OH
O
O
Cl
O
NH N
NH
CH 2Cl2
N
O O
VBDMH
EBDMH
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Pr
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Fig. 1. Synthesis of 3-(4'-epoxyethyl-benzyl)-5,5-dimethylhydantoin (EBDMH).
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Fig. 2. Preparation of N-halamine precursor modified silica nanoparticles (EBDMH-
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Pr
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SiO2 NPs).
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C-NMR spectra of EBDMH.
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Fig. 3. 1 H-NMR and
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Fig. 4. (A) FT-IR spectra of SiO 2 , APTES-SiO 2 and EBDMH-SiO 2 NPs, (B) TGA
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curves of SiO2 , SiO 2 -APTES and EBDMH-SiO2 NPs.
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Fig. 5. TEM micrographs of SiO 2 (A), APTES-SiO2 (B), EBDMH-SiO 2 NPs (C), and
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particle size distributions of SiO 2 , APTES-SiO 2 and EBDMH-SiO2 NPs determined by DLS (D) (Scale bar is 200 nm).
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Fig. 6. DSC thermograms for neat PLA and PLA/EBDMH-SiO 2 nanocomposites in
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the second heating run (A), and WAXD patterns of neat PLA and PLA/EBDMH-SiO 2
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nanocomposites (B).
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Fig. 7. Spherulitic growth process of neat PLA and various nanocomposites at a
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crystallization temperature of 130˚C for different times (scale bar is 200 μm).
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Fig. 8. Frequency dependence of (A) storage modulus (G') and (B) complex viscosity
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(η*) for neat PLA and PLA/EBDMH-SiO 2 nanocomposites.
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Fig. 9. Antibacterial tests of PLA/EBDMH-SiO 2 -9 plastic sheets after chlorination against S. aureus and E. coli. Inset is photographs showing the bacterial culture plates
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of S. aureus and E. coli upon 180 min contact with the control and PLA/EBDMH-
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SiO2 -9 plastic sheets. (The applied bacterial densities of S. aureus and E. coli PBS
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solution were 1.42 × 104 and 1.73 × 104 CFU/ml, respectively.)
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Journal Pre-proof Table 1
Tg
Tcc
ΔHcc
Tm
ΔHf
Xc
SiO2 NPs (w/w)
(°C) *
(°C) *
(J/g)
(°C) *
(J/g)
(%)
100/0
60.9
106.5
30.8
169.0
35.6
38.3
99/1
57.3
103.3
28.3
167.9
38.2
41.5
97/3
56.7
98.4
20.4
166.8
41.7
46.2
95/5
57.7
100.2
22.2
167.1
39.8
45.5
93/7
60.6
104.8
26.3
168.2
38.4
44.4
91/9
61.6
107.1
28.9
169.7
36.4
43.0
pr
rn
al
Pr
e-
, obtained from the DSC curves in second heating run.
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*
f
PLA/EBDMH-
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Crystallization properties of neat PLA and PLA/EBDMH-SiO2 nanocomposites.
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Journal Pre-proof Table 2 Isothermal crystallization kinetic parameters for neat PLA and PLA/EBDMH-SiO 2 nanocomposites based on the Avrami equation. Isothermal crystallization
Crystallization kinetic parameters
SiO2 NPs (w/w)
Temperatures (Tc , ˚C)
n
K (min-n )
t 1/2 (min)
100/0
105 110 115 105 110 115 105 110 115 105 110 115 105 110 115 105 110 115
2.49 2.65 2.76 2.72 2.75 2.76 2.79 2.87 2.88 2.26 2.84 2.89 2.81 2.66 2.89 2.68 2.72 2.74
0.33 0.07 0.03 1.11 0.19 0.03 2.21 0.27 0.05 1.97 0.25 0.04 0.14 0.08 0.03 0.08 0.05 0.02
1.34 2.35 3.91 0.84 1.59 3.07 0.56 1.38 2.41 0.63 1.43 2.71 1.79 2.29 2.73 2.28 2.72 3.96
91/9
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93/7
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95/5
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97/3
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99/1
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PLA/EBDMH-
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Authors statements Yan Zhao a , Conceptualization, Methodology , Investigation , Writing - Original -Draft Review & Editing Bolei Wei a, Investigation (part) Mi Wu a, Writing - Original Draft (part of fig) Huiliang Zhang b , Resources, Writing - Review Jinrong Yao
a,
, Conceptualization, Methodology, Writing - Review & Editing
Xin Chen a, Writing - Review & Editing
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Zhengzhong Shao a,Writing - Review & Editing
Corresponding authors
E-mail address: [email protected] (J. Yao). 40