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Green Antibacterial Nanocomposites from Poly (lactide)/Poly (butylene adipate -co-terephthalate)/Nanocrystal Cellulose-Silver Nanohybrids Piming Ma, Long Jiang, Manman Yu, Weifu Dong, and Mingqing Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01106 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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ACS Sustainable Chemistry & Engineering

Green Antibacterial Nanocomposites from Poly (lactide)/Poly (butylene adipate -co-terephthalate)/Nanocrystal Cellulose-Silver Nanohybrids

Piming Ma*, Long Jiang, Manman Yu, Weifu Dong, Mingqing Chen*

The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China

Corresponding E-mail*: [email protected]

ACS Paragon Plus Environment


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ABSTRACT:

Silver nanoparticles (AgNPs) with a diameter of 3 - 6 nanometers

were uniformly reacted onto the surface of nanocrystal cellulose (NCC) via complexation leading to NCC-Ag nanohybrids with an AgNPs content of 8 wt%. Subsequently, antibacterial green nanocomposites containing renewable and biodegradable poly (lactide) (PLA), poly (butylene adipate-co-terephthalate) (PBAT) and NCC-Ag nanohybrids were synthesized and investigated. The PBAT as flexibilizer improved the toughness of the PLA matrix while the uniformly dispersed NCC-Ag nanohybrids enhanced the compatibility, thermal stability, crystallization and antibacterial properties of the PLA/PBAT blends. The crystallization rate and the storage modulus (E’) of the green nanocomposites were increased obviously with increasing the content of the CNC-Ag nanohybrids. Meanwhile, notably the antibacterial activity of the PLA/PBAT/NCC-Ag nanocomposites was achieved against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus cells. The antibacterial performance was mainly related to the antibacterial nature of the finely dispersed NCC-Ag nanohybrids. The study demonstrates great potential of the green nanocomposites in functional packaging and antibacterial textiles applications.

KEYWORDS: Green nanocomposite; poly (lactide); Cellulose; Nanohybrids; Antibacterial;


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INTRODUCTION In recent years, renewable and biodegradable materials have drawn considerable attention because of the environmental concerns and sustainability issues associated with petroleum-based polymers.1-2 Development of consumer products from biodegradable and renewable materials is currently of great interest and attracts many researchers.3 Poly (lactide), PLA, as one of the most promising biopolymers, has exhibited vast appeal in the past decades due to its excellent performance in renewability, biodegradability and biocompatibility.4-8 Nonetheless, some of the performances such as high brittleness and a low heat distortion temperature (HDT) due to poor crystallizability limit specific applications.9-12 Many efforts have been made to improve the toughness of PLA such as plasticization, chain extension, copolymerization and compounding with other materials.13-21 Some biobased and/or biodegradable polymers such as poly(butylene succinate),14 natural rubber,17 poly(hydroxyalkanoate)s (PHAs)22 and poly(butylene adipate-co-terephthalate) (PBAT)23-25 were used to improve the toughness of PLA. Our group recently reported that the mechanical properties of the PLA could be improved by incorporation of the PBAT and in situ compatibilization, e.g., the elongation at break was increased from 4% to 300% and the notched Izod impact toughness was increased from 28 J/m to 110 J/m.23 Signori et al developed PLA/PBAT blends for packaging applications considering their ductility. With the development of nanotechnology, PLA/PBAT-based nanocomposites were prepared, such as PLA/PBAT/clay nanocomposites.24 The thermal and morphological behaviour of PLA/PBAT blends were improved with the addition of nano-clay. Ko reported that multi-walled carbon nanotubes (MWNTs) had strong effect on thermal and rheological properties of biodegradable PLA/PBAT/MWNT nanocomposites. Therefore, both PBAT and nano filler as modifiers could enhance the mechanical and thermal properties of PLA matrix. However, PBAT in combination with nanocrystal cellulose (NCC) are seldom reported to modify PLA matrix. Moreover, functional nanocomposites such as antibacterial PLA/PBAT/NCC are even less studied although nanocrystal cellulose has been identified as important biomass fillers to enhance the



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performance of polymeric materials.26,27 Consequently, the application of PLA/PBAT in the antibacterial field is still limited. As green antibacterial nanocomposites, antibacterial efficiency is strongly required in combination with some other characteristics such as environmental safety, low toxicity, cost effectiveness and ease of fabrication. Silver nanoparticles (AgNPs), copper nanoparticles (CuNPs), Zinc oxide (ZnO) and chitosan have been identified as antibacterial additives.28-33 Among these additives AgNPs are the mostly studied potential candidates in biopolymers such as PLA and PHAs to prepare antibacterial materials for functional packing application.28-30 However, the AgNPs via this method tend to aggregate leading to the loss of nano- feature and functions. As a consequence, protective agents and/or stabilizers such as starch and cellulose were applied to reduce the surface activity and prevent the agglomeration of the AgNPs.34-38 The NCC-Ag nanohybrids were prepared and utilized to reinforce PLA,27,29 PHAs30 and polyvinyl alcohol (PVA).36,37 The introduction of NCC-Ag nanohybrids improved to a certain extent the properties including mechanical and antibacterial performance of either PLA or PHAs homo-component, whereas the effect on a polymeric blends such as biodegradable PLA/PBAT blends is not clear yet. In addition, harmful reducing agents such as hydrazine and sodium borohydride (NaBH4) are generally used to prepare NCC-Ag nanohybrids in previous study, thus it would also be interesting to make NCC-Ag nanohybrids via a more environmentally friendly route.29,37,38 The primary objective of this work is to prepare antibacterial green nanocomposites containing PLA, PBAT and NCC-Ag nanohybrids with balanced properties. To achieve this objective the NCC-Ag nanohybrids were first prepared and characterized. Subsequently, PLA/PBAT/NCC-Ag nanocomposites were made by solution casting. The influence of the NCC-Ag nanohybrids on the (dynamic) mechanical, thermal, morphological, crystallization, antibacterial and hydrophilic properties of the resulting nanocomposites were investigated. The present work not only provides a simple route to fabricate antibacterial green nanocomposites with balanced properties but also reported a more environmentally friendly way to prepare fine NCC-AgNPs (3~6 nm) nanohybrids by using glucose as a reducing agent.


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EXPERIMENTAL SECTION Materials Poly (lactide) (PLA, 4032D) with Mw = 220 kDa, PDI = 1.7, and MFI = 7 g/10min (210°C, 2.16kg) was purchased from Nature Works LLC, USA. Poly(butylene adipate-co-terephthalate) (PBAT) with Mw = 170 kDa and PDI = 1.6 was supplied by Zhejiang Hangzhou XinFu Pharmaceutical Co. Ltd. Microcrystalline cellulose (MCC, purity ≥ 96%)，Concentrated sulfuric acid (purity ≥ 98%)，chloroform (purity ≥ 99.5%)，silver nitrate (AgNO3), glucose (purity ≥ 99%) were supplied by Sinopharm Group Chemical Reagent Co., Ltd., China.

Sample Preparation NCC-Ag nanohybrids: Nanocrystalline cellulose (NCC) was made from acid hydrolysis of microcrystalline cellulose (MCC) in sulfuric acid (64 wt%) following the reported procedures in literature.39,40 Nanocrystalline cellulose-silver nanohybrids (NCC-Ag) were then prepared as follows: i) 4.0 g of NCC was first dispersed in 320 mL deionized water in a three-necked flask at 70 °C, magnetically stirring at 1500 rpm for 30min; ii) AgNO3 solution (3.4 g AgNO3 in 40 mL deionized water) was transferred into the three-necked flask and mixed for 5 min; iii) glucose solution (11.9 g glucose in 40 mL deionized water) was then fed into the three-necked flask and stirred for 3 h to reduce Ag+ into AgNPs; iiii) the mixture was quenched and washed 3~4 times by successive centrifugations with deionized water to remove completely the remaining Ag species and by-products. A similar preparation route for NCC-Ag nanohybrids was reported elsewhere.41 PLA/PBAT/NCC-Ag nanocomposites: PLA/PBAT/NCC-Ag nanocomposites were prepared by solution blending. In brief, PLA and PBAT with the weight ratio of 80/20 was fist dissolved in chloroform (40 mL) and the NCC-Ag nanohybrids were ultrasonically suspended in chloroform (10 mL). The PLA/PBAT solution and NCC-Ag suspension were then mixed and stirred for 2 h at room temperature to obtain

PLA/PBAT/NCC-Ag

blends.

Subsequently,

the


PLA/PBAT/NCC-Ag


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nanocomposites were obtained by solution casting and further dried completely at 45 °C under vacuum condition. The content of NCC-Ag nanohybrids in the nanocomposites was designed from 0 to 8 wt% based on the total weight of the PLLA/PBAT component. The preparation of NCC-Ag nanohybrids and the PLA/PBAT/NCC-Ag nanocomposites are schematically shown in Scheme 1.

Scheme 1. Schematic illustration of the preparation routes of the NCC-Ag nanohybrids (left top) and the PLA/PBAT/NCC-Ag nanocomposites.

Characterizations Ultraviolet-Visible (UV–vis) Spectrophotometer: The UV-vis absorption spectra of NCC and NCC-Ag suspensions (2 mg/mL in water) were recorded by using an UV-vis spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co. Ltd.). The spectra were recorded in the wavelength range from 400 to 800 nm by using air as the blank reference. Transmission Electron Microscopy (TEM): A diluted NCC-Ag suspension in water (0.2 mg/mL) was prepared and the suspension was casted on a copper grid. Their morphology was characterized by using TEM (200 kV, JEOL-JEM-2100, Japan) after the evaporation of water. Wide Angle X-ray Diffraction (WAXD): WAXD measurements were carried out on the NCC and NCC-Ag nanohybrids by using an X-ray diffractometer (Bruker AXS


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D8, Germany) equipped with a Ni-filtered Cu Kα radiation source (1.542 Å). The measurements were operated at 40 kV and 40 mA with scan angles from 5 to 90 ° at a scanning rate of 3 °/min. Scanning

Electron

Microscopy

(SEM):

Phase

morphology

of

the

PLA/PBAT/NCC-Ag nanocomposites was characterized by SEM (3 kv, S-4800 HITACHI, Japan). The PLA/PBAT/NCC-Ag nanocomposites were cryo-fractured in liquid nitrogen and sputter-coated with a thin gold layer on the fracture surface before observation. Mechanical Properties: The tensile properties of the PLA/PBAT/NCC-Ag nanocomposites were measured with a universal tester (Instron 5967, USA) according to GBT529-2008 standard at a crosshead speed of 10 mm/min. Five specimens of each sample were tested at 25 °C and the averaged results were presented. Differential Scanning Calorimetry (DSC): The crystallization behaviour of the PLA/PBAT/NCC-Ag nanocomposites was studied by using DSC (DSC 8000, Perkin Elmer). In non-isothermal crystallization, the samples were cooled to 0 °C at 2 °C/min after melting at 190 °C for 2 min and then reheated to 190 °C at 10°C/min. For isothermal crystallization, the samples were quenched to 120 °C (50 °C/min) from 190 °C and then persisted until the crystallization was complete. The crystallinity of PLA component (Xc) is calculated via equation (1), i.e., X c = ∆H c /(ω × ∆H m0 ) , where ω is the weight fraction of PLA in the nanocomposites and ∆H m0 = 93 J/g is the melting enthalpy of 100% crystalline PLA. Polarized Optical Microscopy (POM): POM (Axio Scope 1, Carl Zeiss, Germany) equipped with a Linkam hot-stage unit (THMS600, UK) was used to investigate the crystal morphology of the PLA/PBAT/NCC-Ag nanocomposites. The samples were first melted at 190 °C for 2 min and then quenched rapidly (50 °C/min) to the designed temperatures for isothermal crystallization. Dynamic Mechanical Analysis (DMA): Dynamic mechanical properties and thermal behaviour of the PLA/PBAT/NCC-Ag specimens (15×5.3×0.2 mm3) were analyzed by using DMA-Q800 (TA Instruments) in a tensile mode. The measurement



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was performed in nitrogen atmosphere from 30 to 140 °C at 3 °C /min. A frequency of 1 Hz and amplitude of 20 µm were applied. Thermal Gravimetric Analysis (TGA): Thermal decomposition behaviour of the NCC-Ag nanohybrids and PLA/PBAT/NCC-Ag nanocomposites was investigated by TGA (1100SF, Meteler-Toledo, Switzerland) which was carried out from 20 to 600 °C at 10 °C/min in nitrogen atmosphere. Antibacterial Activity Measurement: The antibacterial activity

of the

PLA/PBAT/NCC-Ag nanocomposites was tested according to Kirby-Bauer testing protocol42 (i.e., disc diffusion test) in an agar medium. Staphylococcus aureus and Escherichia coli bacteria of approximately 106 CFU/mL were freshly grown for 4 h after taken from an overnight Mueller-Hinton Broth (MHB) culture. With this culture (1 mL), the bacterial lawn of Staphylococcus aureus and Escherichia coli were prepared on plate counter agar. The Membrane discs of PLA/PBAT/NCC-Ag nanocomposites with 15 mm in diameter were applied to determine their antibacterial activity after 24 h at 37 °C. The killing efficiency of PLA/PBAT/NCC-Ag composites against Escherichia coli was evaluated by the plate count method.43,44 The Escherichia coli was incubated at 37 °C with shaking at 200 rpm in yeast broth and nutrient broth. 100 mg of PLA/PBAT/NCC-Ag membrane was immersed in a sterile flask, in which 10 mL of bacteria culture was added. The flasks were placed in the shaker incubator for up to 24 h. At selected time intervals, 0.1 mL of bacterial culture was taken out and added into 0.9 mL of sterilized physiological saline, and then decimal serial dilutions were prepared. Properly diluted suspensions of Escherichia coli (104 CFU/mL) were spread on an agar media (containing 10 g/L peptone, 10 g/L sodium chloride, 5 g/L yeast and 20 g/L agar). The Petri dishes were sealed and incubated at 37 °C for 24 h. The bacterial cell colonies were imaged using a digital camera and counted. Hydrophilic Behaviour: Hydrophilic properties of the PLA/PBAT/NCC-Ag films were determined by measuring the water/α-bromonaphthalene contact angles which were performed at room temperature by using a Drop Shape Analysis System (OCA 40, Beijing Eastern-Dataphy Instruments, China). All the analysis was carried out at


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25 °C. Surface energy of the PLA/PBAT/NCC-Ag nanocomposite films are calculated via the Owens-Wendt-Rabel-Kaelble equation.45-47

RESULTS AND DISCUSSION UV-vis Absorption and Morphology of the NCC-Ag Nanohybrids Compared with the NCC. UV-vis absorption spectra of the NCC and NCC-Ag nanohybrids (suspensions, 2 mg/mL in water) are shown in Figure 1a. No peak was detected from the UV-vis absorption spectrum of NCC in the wavelength region of 350 - 600 nm. In contrast, the NCC-Ag nanohybrids showed a notable UV-vis absorption peak around 425 nm. It is known that AgNPs exhibit a surface plasmon resonance band between 350 and 500 nm.48 Therefore, the UV–vis absorption spectrum of NCC-Ag nanohybrids in comparison with that of the NCC confirmed the successful complexation of AgNPs onto the NCC surface. This can be explained by the fact that the extensive number of hydroxyl groups on the NCC surface can facilitate the complexation of silver ions to the NCC molecules and reduce Ag+ into Ag0 in the presence of glucose, and finally form nanoparticles on the surface of NCC, i.e., NCC-Ag nanohybrids.49 The structure of the NCC-Ag nanohybrids was characterized by using TEM, as shown in Figure 1b. The TEM image shows that highly dense AgNPs (i.e., black dots in Figure 1b) with diameters of 3 - 6 nm were uniformly generated along the NCC without aggregation. A TEM image of the NCC is inset in Figure 1b) showing the morphology/size of the used NCC from hydrolysis of microcrystal cellulose.



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Figure 1. (a) The UV−vis absorption spectra of NCC and NCC-Ag nanohybrids (suspensions, 2 mg/mL in water) and (b) A TEM image of the NCC-Ag nanohybrids from diluted suspension (0.2 mg/mL in water) with an inset of TEM image of the used NCC from hydrolysis of microcrystal cellulose. The blank dots in Figure 1b represent AgNPs while the short solid lines are drawn manually to indicate the alignment of AgNPs along the NCC which has low electron density.

X-ray Diffraction and Thermal Decomposition Behaviour of the NCC-Ag Nanohybrids Compared with NCC. Both NCC and AgNPs show crystalline structures. Thus the crystalline structure of the NCC-Ag nanohybrids was studied by using wide angle X-ray diffraction (WAXD) in comparison with NCC, as shown in Figure 2a. Both WAXD patterns of the NCC and the NCC-Ag nanohybrids display diffraction peaks at 2θ = 14.8, 16.5, 22.5 and 34.5°corresponding to the typical cellulose I crystalline form.50 On the other hand, the diffraction pattern of NCC-Ag nanohybrids exhibit four new peaks at 2θ = 38.2, 43.3, 64.4 and 77.2° which are assigned to the (111), (200), (220) and (311) crystal plane of AgNPs crystals, respectively. Meanwhile, two diffraction peaks of silver oxide (Ag2O) at 2θ = 32.5 and 54.5° are detected as well.36,50 The WAXD patterns confirmed the successful complexation of AgNPs on the NCC surface. Thermal gravimetric analysis (TGA) was approved as a useful method to study the thermal decomposition behaviour and to determine quantitatively the weight percentage of AgNPs in the NCC-Ag


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nanohybrids.51 The thermal decomposition behaviour of the NCC and NCC-Ag nanohybrids are shown in Figure 2b. The weight residual of NCC is around 38 wt% at 600 °C which is associate with the introduction of sulfate groups and glucose

during

the

preparation

of

NCC

and

NCC-Ag

nanohybrids,

respectively.52,53 Since the NCC underwent the same treatment as the NCC-Ag nanohybrids, the weight percentage differences at 600 °C can be determined as the complexation degree of the AgNPs which is around 8 wt% in Figure 2b. It has to be remarked that cellulose and glucose generally have comparable thermal decomposition temperatures (Td). However, the Td of NCC is lower than that of the glucose in Figure 2b because the molecular structures of cellulose were seriously damaged after sulfuric acid hydrolysis leading to breakage and defects of the cellulose chains. Such defects in combination with some residual hydrogen ions on the NCC surface may accelerate the decomposition process upon heating.54,55

Figure 2. (a) WAXD patterns and (b) TGA curves of the NCC and the NCC-Ag nanohybrids. The X-ray diffraction responses of different crystal planes and components are denoted in Figure 2a while the decomposition of NCC, AgNPs and residual glucose components are labeled on the Figure 2b.

Dynamic Mechanical Properties of the PLA/PBAT/NCC-Ag Nanocomposites. In this study, poly (lactide) and poly (butylene adipate-co-terephthalate) blend (PLA/PBAT, 80/20, wt/wt) with balanced mechanical properties was chosen to fabricate PLA/PBAT/NCC-Ag nanocomposites.



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The storage modulus (E’) and loss factor (tanδ) studied by using DMA are shown in Figure 3. It was reported that the E’ of PLA/PBAT blends was 3 GPa below their Tg (Tg-PLA ≈ 64 °C, Tg-PBAT ≈ -23 °C) while decreased to around 1 GPa between the Tg-PLA and the Tg-PBAT.23 In this study, the PLA/PBAT blend also shows moderate E’ values (approximately 1000 MPa) at 30 - 40 °C due to the softening of the PBAT components. Interestingly, the E’ of PLA/PBAT blend was enhanced by addition of the NCC-Ag nanohybrids, e.g., increased by 100 - 500 MPa at 40 °C after addition of 1 - 8 wt% of the NCC-Ag nanohybrids (Figure 3a). The reinforcing effect of NCC-Ag nanohybrids was ascribed to the high modulus and strength of NCC component and the interaction between NCC-Ag nanohybrids and polymer chains.56,57 The drop of E’ of all the PLA/PBAT/NCC-Ag nanocomposites in the neighborhood of 60 °C is due to the softening of the PLA matrix above its Tg and the lack of crystallinity.

Figure 3. (a) Storage modulus (E’) and (b) loss factors (tan δ) of the PLA/PBAT/NCC-Ag nanocomposites as functions of temperature and composition. The arrow in Figure 3b shows the increase in Tan δ peak temperatures with increasing the NCC-Ag nanohybrids content.

The loss factors (tan δ) of the PLA/PBAT/NCC-Ag nanocomposites are shown in Figure 3b where the tan δ peak temperature is referred to as α-relaxation temperature (Tα) of PLA component in this study.58 As indicated by the arrow in Figure 3b that the Tα-PLA of the PLA/PBAT/NCC-Ag nanocomposites increased obviously with increasing the NCC-Ag nanohybrids content. Compared with PLA/PBAT blend the Tα


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of PLA in the nanocomposites with 8 wt% of NCC-Ag nanohybrids was 10 °C higher. The NCC-Ag nanohybrids with large specific area would immobilize the polymeric chains/segments at the interface resulting in higher Tα-PLA values which explains the reinforcing effect of the NCC-Ag nanohybrids on the nanocomposites as well.

Morphology and Tensile Properties of the PLA/PBAT/NCC-Ag Nanocomposites. Micro-morphology is important to the properties of nanocomposites, hence the morphology of the PLA/PBAT/NCC-Ag nanocomposites was studied by using SEM, as shown in Figure 4a-d. All samples exhibit a typical sea-island type structure. The oval spherical domains with the size of 2 - 5 µm represent the PBAT phase distributed in the PLA matrix. The relatively large domain size and notable interfacial debonding indicate a poor compatibility between the PLA and PBAT components which is in agreement with literature.23 Whereas, the compatibility was improved after addition of the NCC-Ag nanohybrids resulting in stronger interfacial adhesion (Figure 4a/d). It was also reported in the literature that nanoparticles (SiO2) reduced the domain size of poly(styrene) (PS) by a factor of about three in the PP/PS/SiO2 system because the nanoparticles could lead to a formation of barrier preventing the coalescence of the second phase polymer.59,60 It can be concluded from the SEM images that the NCC-Ag nanohybrids are well embedded in the polymeric phases (PLA and/or PBAT) because seldom NCC-Ag nanohybrids are visible on the fracture surfaces. Even the loading of NCC-Ag nanohybrids is as high as 8 wt%, only a small amount of NCC-Ag nanohybrids are exposed on the fractured surfaces (white dots in Figure 4d). The morphology of this sample is further characterized by using TEM, as shown in Figure 4d’. The AgNPs with diameters of 10-40 nm are clearly seen in both light PLA and dark PBAT phases. The binding interaction between the NCC and AgNPs is relatively weak and it could break during subsequent processing.49 Consequently, the AgNPs may agglomerate to a certain extent leading to larger particle size in the nanocomposites than that in the original NCC-AgNPs nanohybrids (Figure 1b). PLA is stiff showing a tensile strength (Ts) of 55 MPa and elongation at break (Eb)



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of 3-5%.61 Toughness of PLA was improved by the incorporation of PBAT while compromised to a certain extent by the addition of rigid NCC-Ag nanohybrids, as shown in Figure 4e. In principle, transesterification between the polyesters (PLA and PBAT) may exist, which can improve the compatibility between the PLA and the PBAT phases, whereas the extent of transesterification should be very small due to the low temperature processing (RT) and the absence of any catalyst. The balanced mechanical properties (Ts = 13 - 20 MPa, Eb = 8 - 25%) of the PLA/PBAT/NCC-Ag nanocomposites in combination with superior antibacterial performances (see discussion below) still deserve niche applications in practice, e.g., food and hygienic functional packaging, aseptic environment in hospital and/or laboratory.

Figure 4. (a-d) SEM images (scale bar = 10 µm) of the PLA/PBAT/NCC-Ag nanocomposites with the NCC-Ag content of (a) 0 wt%, (b) 1 wt%, (c) 4 wt% and (d) 8 wt%, respectively. (d’) A TEM image of the PLA/PBAT/NCC-Ag nanocomposite with 8 wt% of the NCC-Ag nanohybrids. The PLA, PBAT and NCC-Ag nanohybrids are denoted in the images. Tensile strength and elongation at break of PLA/PBAT/NCC-Ag nanocomposites as a function of NCC-Ag content loading. (e) The tensile strength (Ts) and elongation at break (Eb) of the PLA/PBAT/NCC-Ag nanocomposites.

Crystallization Behaviour of the PLA/PBAT/NCC-Ag Nanocomposites. The


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crystallization and melting behaviour of polymer nanocomposites are not only important to the crystal structure and crystallinity but also crucial to the ultimate properties such as mechanical behaviour, heat resistance and processability. Therefore, the effect of the NCC-Ag nanohybrids on the crystallization and melting behaviour of the PLA/PBAT/NCC-Ag nanocomposites were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The cooling and subsequent heating DSC curves are shown in Figure 5. The corresponding parameters such as crystallization peak temperature (Tc), crystallinity (Xc), cold crystallization temperature (Tcc) and melting peak temperature (Tm) are labeled as well in Figure 5. The PLA/PBAT blend without NCC-Ag nanohybrids shows poor crystalline ability. As a consequence, the Tg and Tcc responses are significant in the subsequent heating process (Figure 5b). Interestingly, the crystallization rate of the PLA/PBAT/NCC-Ag nanocomposites was speeded up leading to a gradual increase in Tc and Xc, i.e., Tc from 96 to 108 °C and Xc from 24 to 49% with increasing the NCC-Ag nanohybrids content up to 8 wt% (Figure 5a). In the meantime, the PLA/PBAT/NCC-Ag nanocomposites with higher NCC-Ag nanohybrids content show weaker cold crystallization behaviour (Figure 5b). Double endothermic peaks are detected for PLA/PBAT/NCC-Ag samples which are assigned to a melting/re-crystallization/re-melting mechanism. Some imperfect and small crystals upon melting re-crystallized into more stable crystals which has higher melting temperatures.62-64 Apparently, the NCC-Ag nanohybrids behave as nucleating agents promoted the crystallization of the nanocomposites which, however, had less effect on the melting temperatures. It has to be remarked that the crystallization of PBAT component was suppressed in the nanocomposites probably due to the size confinement effect.65



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Figure 5. (a) Cooling and (b) subsequent heating DSC curves of the PLA/PBAT/NCC-Ag nanocomposites with various NCC-Ag nanohybrids content. The Tc, Xc, Tg, Tcc and Tm represent the crystallization peak temperature, crystallinity, glass transition temperature, cold crystallization temperature and melting peak temperature, respectively.

Isothermal crystallization behaviour of the PLA/PBAT/NCC-Ag nanocomposites was investigated at 120 °C and the DSC curves (heat flow versus crystallization time) are shown in the Figure 6a. Clearly, the crystallization peaks of the nanocomposites shift to left side accompanied by an increase in sharpness and intensity when the NCC-Ag nanohybrids content increases. Half-life crystallization time (t1/2) defined as the time required to reach 50% of the final crystallinity is a widely used parameter to evaluate the overall crystallization rate of a (semi-)crystalline material. The smaller t1/2 value the higher crystallization rate. The t1/2 values of the PLA/PBAT/NCC-Ag nanocomposites obtained from the isothermal crystallization process were plotted in Figure 6b, which decreased by 45% with increasing the NCC-Ag nanohybrids content up to 8 wt%. These results demonstrate a faster crystallization of the nanocomposites at higher NCC-Ag nanohybrids content which is consistent with the non-isothermal crystallization results.


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Figure 6. (a) DSC isothermal crystallization curves of the PLA/PBAT/NCC-Ag nanocomposites carried out at 120 °C and (b) the corresponding half-life crystallization time (t1/2) of the nanocomposites as a function of NCC-Ag nanohybrids content. The arrow shows the decrease in peak time (Figure 6a) and t1/2 with increasing the NCC-Ag nanohybrids content.

The crystal morphology of the PLA/PBAT/NCC-Ag nanocomposites was observed by using POM at 120 °C, as shown in Figure 7. Spherulites with a diameter of around 20 µm were observed in the PLA/PBAT blend without any NCC-Ag nanohybrids after an induction period of 10 min (Figure 7a). The spherulites with classical maltese cross pattern correspond to only the PLA component since the selected temperature is higher than the Tm of the PBAT component. The PLA spherulitic sizes did not change that much after addition of the NCC-Ag nanohybrids, whereas the spherulite density (i.e., the number of the spherulites per unit area) was increased significantly. At the NCC-Ag nanohybrids content as high as 4 and 8 wt%, it is even difficult to distinguish an individual spherulite due to the high spherulite density which vividly displays a large(r) number of nuclei sites. Apparent, the increased nuclei sites were resulted from the nucleation effect of the NCC-Ag nanohybrids. Therefore, the POM images offer a direct insight into the nucleation efficiency of the NCC-Ag in the nanocomposites, which interpreted the enhanced overall crystallization rate of PLA component in the presence of a sufficient amount of NCC-Ag nanohybrids.



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Figure 7. Polarized optical microscopy images (in the same magnification) of the PLA/PBAT/NCC-Ag nanocomposites that isothermally crystallized at 120 °C for 10 min with the NCC-Ag nanohybrids content of (a) 0 wt%, (b) 1 wt%, (c) 4 wt% and (d) 8 wt%, respectively. The PLA spherulitic morphology with higher magnification was inset in the top right of image (a) while the NCC-Ag aggregates were indicated by arrows in the images (c) and (d).

Thermal

Decomposition

Behaviour

of

the

PLA/PBAT/NCC-Ag

Nanocomposites. The thermal decomposition cures of the PLA/PBAT/NCC-Ag nanocomposites with different NCC-Ag nanohybrids contents are shown in Figure 8. All the PLA/PBAT/NCC-Ag nanocomposites lose approximately 5% in weight between 100 and 200 °C corresponding to the combined water and solvent during fabrication. A related platform was followed between 200 and 300 °C. However, a transesterification-induced degradation mechanism at this temperature range (200-300 °C) was proposed in literature.62,66 Interestingly, the decomposition of both PLA (Region A around 350 °C) and PBAT (Region B around 400 °C) components shifted to higher temperatures with increasing the NCC-Ag nanohybrids loading up to 4 wt% and then leveled off. To further study this behaviour, decomposition activation


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energy (Et) of PLA component was analyzed from the TGA data by the integral method, i.e., equation (2).67, 68 2 ln  ln (1/1 − α )  = Etθ / RTmax

（2）

where α is the decomposed fraction of the sample, Tmax is the temperature at the maximum rate of weight loss, θ is defined as (T-Tmax) and R is the gas constant. Et values of PLA component were calculated from the slopes of the straight lines of ln[ln(1/(1-α)] versus θ and inset in Figure 8b. Obviously, the activation energy of the PLA was significantly increased after addition of the NCC-Ag nanohybrids, e.g., from 55 kJ/mol to73 kJ/mol at 4 wt% of the NCC-Ag nanohybrids. The higher Et values, the more energy required to decompose the materials. Thus the improved thermal stability of the polymer matrix is resulted from the higher decomposition activation energy being associated with the AgNPs in the NCC-Ag nanohybrids since NCC component decomposed prior to the matrix. It has to be remarked that the influence of PBAT on the thermal stability of PLA in all the samples should be equal since all the samples contain the same content of PLA and PBAT components (a fixed ratio, 80/20) and thus it is not taken into account in the discussion.

Figure 8. (a) TGA curves of the PLA/PBAT/NCC-Ag nanocomposites and (b) the drawing of partial enlargement of the TGA curves from Figure 8a. The decomposition activation energy (Et) of PLA component in the PLA/PBAT/NCC-Ag nanocomposites are inset in Figure 8b as a function of NCC-Ag content.



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Antibacterial Activity Determination. Packaging materials with antibacterial functions have received considerable attention notably in the field of food, medical and health. In this study，the NCC-Ag nanohybrids were introduced into PLA/PBAT blends

to

prepare

antibacterial

films.

The

antibacterial

activity

of

the

PLA/PBAT/NCC-Ag nanocomposite films was studied by using a testing protocol similar to Kirby-Bauer disc diffusion test,40 and the results are shown in Figure 9. According to Kirby-Bauer theory, bacteria would adhere to the surface of the films or grow at the interface if the antibacterial activity of the films is poor, otherwise, inhibition rings around the films would be observed. Impressively, the PLA/PBAT/NCC-Ag nanocomposite films (B1 and B2) exhibit notable inhibition rings upon the bacteria growth. The thickness of the inhibition rings in the growth of Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus is around 2.6 and 1.8 mm, respectively. Therefore, the Kirby-Bauer disc diffusion test demonstrates a high antibacterial activity of the PLA/PBAT/NCC-Ag nanocomposites in killing both Gram-negative E. coli and Gram-positive S. aureus. In contrast, hardly any inhibition zone is observed around the PLA/PBAT films (A1 and A2). Therefore, the bacterial inactivation of the nanocomposites was ascribed to the presence of the well dispersed AgNPs (Figure 4d’) and thus large antibacterial surfaces. Since the selected E. coli and S. aureus represent the typical Gram-negative and Gram-positive cells, respectively, the material should be effective in inhibiting most of bacteria based on the current results. On the other hand, we want to emphasize that bacteria cannot be seen on the surface of the composite with NCC-Ag (B1 and B2 in Fig. 9). This phenomenon in combination with the inhibition zones clearly indicate that bacteria cannot survive in these areas, i.e., they would be killed effectively by the composites. A plate count 43,44 was applied to evaluate the killing efficiency of these composites against bacteria (E. coli) and the corresponding digital images are shown in Figure S1. The killing efficiency (ke) of the composites against Escherichia coli is finally determined to be 99.7% calculated by using equation (3), i.e.,


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(3) where CBlank and CA represent the number of bacterial cell colonies from PLA/PBAT/NCC-Ag and blank samples after 24 h of incubation, respectively.

Figure 9. Digital images of the PLA/PBAT and the PLA/PBAT/NCC-Ag nanocomposite films against (a) Gram-negative Escherichia coli and (b) Gram-positive Staphylococcus aureus demonstrating inhibition rings around the films. The A1 and A2 represent the PLA/PBAT films as a reference while the B1 and B2 correspond to the PLA/PBAT/NCC-Ag nanocomposite films with 8 wt% of the NCC-Ag nanohybrids.

Contact

Angle

and

Surface

Energy

of

the

PLA/PBAT/NCC-Ag

Nanocomposites. Hydrophilic property and surface energy are important to the application of polymeric films. The hydrophilic property and the surface energy of the PLA/PBAT/NCC-Ag nanocomposite films were characterized by measuring the water/α-bromonaphthalene contact angles using a dynamic drop shape analysis system. The contact angle and surface energies of the PLA/PBAT/NCC-Ag nanocomposite films are shown in Table 1. Clearly, the PLA/PBAT blends gave the highest contact angle (W-CA = 70.1°, B-CA = 40.3°). The W-CA and B-CA of the PLA/PBAT film is decreased by 6.7° and 8.9°, respectively, after addition of even as low as 1 wt% of the NCC-Ag nanohybrids which brought plenty hydrophilic hydroxyl



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groups to the PLA/PBAT matrix. As expected, the CA values decrease monotonically with increasing the NCC-Ag nanohybrids content and the nanocomposites exhibit hydrophilic feature already at 8 wt% of the NCC-Ag nanohybrids (W-CA = 46.1°, decreased by 24.0°; B-CA = 11.4°, decreased by 28.9° ). On the other hand, the surface energy of the PLA/PBAT was increased gradually by increasing the NCC-Ag content. In general, pollution occurs on a hydrophilic rather than a hydrophobic substrate

due

to

an

interfacial

adhesion

reason.

As

a

consequence,

bacteria/microorganism in pollutant prefers to adhere to a humid substrate. This feature may make the bacteria/microorganism to be killed more efficiently by the antibacterial agents (NCC-Ag) and facilitate the sterilization process.

Table1. Contact angle (CA) and surface energies of the PLA/PBAT/NCC-Ag samples. Samples (PLA/PBAT/NCC-Ag) NCC-Ag-0wt% NCC-Ag-1wt% NCC-Ag-4wt% NCC-Ag-8wt%

W-CA (°) 70.1 63.4 56.3 46.1

B-CA (°) 40.3 31.4 22.5 11.4

surface energy (mJ·m-2) 43.38 49.21 54.90 61.98

Notes: W-CA: water contact angle; B-CA: α-bromonaphthalene contact angle.

CONCLUSIONS Nanocrystalline cellulose complexed silver nanoparticles (NCC-Ag) were synthesized by liquid phase chemical reduction method using glucose as a reducing agent. The silver nanoparticles (AgNPs) with a diameter of 3-6 nm and a content of 8 wt% were successfully and uniformly complexed onto the NCC surfaces as confirmed by the UV–vis, WAXD, TEM and TGA. Subsequently, poly (lactic acid)/poly (butylene adipate-co-terephthalate)/NCC-Ag (PLA/PBAT/NCC-Ag) nanocomposites were prepared by solution compounding the three components. Both of the room temperature storage modulus (E’) and thermal stability of the PLA/PBAT/NCC-Ag nanocomposites were enhanced by the addition of NCC-Ag nanohybrids whereas the tensile properties were compromised to a certain extent. The NCC-Ag nanohybrids


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also increased the glass transition temperature (Tg) of the PLA component indicating indirectly the interaction between the NCC-Ag nanohybrids and the PLA chains. The micro-morphology analysis showed poor compatibility between the PLA and PBAT components which however was improved after addition of the NCC-Ag nanohybrids. In both non-isothermal and isothermal conditions, the DSC and POM results showed efficient nucleation effect of the NCC-Ag nanohybrids that accelerated the crystallization of the PLA matrix leading to high(er) crystallization temperature (Tc), short(er) half-life crystallization time (t0.5) and small(er) spherulitic size. The Kirby-Bauer disc diffusion test demonstrated high antibacterial activity of the PLA/PBAT/NCC-Ag nanocomposites against both of the Gram-negative Escherichia coli and the Gram-positive Staphylococcus aureus. The obvious antibacterial performance is mainly resulted from the presence of NCC-Ag nanohybrids in the nanocomposites. Therefore, the PLA/PBAT/NCC-Ag green nanocomposites with balanced properties and excellent antibacterial performance have great potential applications in the fields of functional packaging, antibacterial textiles and so on.

SUPPORTING INFORMATION The digital images of CFUs of Escherichia coli in agar plates after 24 h of incubation. This material is available at free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Tel.: +86 510 85917019

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS



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This work is supported by the Natural Science Foundation of Jiangsu Province (BK20130147), National Natural Science Foundation of China (51573074, 51303067) and the Fundamental Research Funds for the Central Universities (JUSRP51624A).

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For Table of Contents Use Only

Green Antibacterial Nanocomposites from Poly (lactide)/Poly (butylene adipate -co-terephthalate)/Nanocrystal Cellulose-Silver Nanohybrids

Piming Ma, Long Jiang, Manman Yu, Weifu Dong, Mingqing Chen

Green PLA/PBAT/NCC-Ag nanocomposites were prepared with balanced properties and high antibacterial activity which is sustainable for the development of polymeric materials.


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Green Antibacterial Nanocomposites from Poly ... - ACS Publications

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