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graphene oxide composites toward multifunctional active packaging films
Composites Science and Technology 184 (2019) 107888
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Polyethylene/graphene oxide composites toward multifunctional active packaging films Rodrigo Silva-Leyton a, Raúl Quijada a, Roberto Bastías b, Natali Zamora b, Felipe Olate-Moya a, Humberto Palza a, * a b
Departamento de Ingeniería Química, Biotecnología y Materiales, Facultad de Ciencias Físicas y Matem� aticas, Universidad de Chile, Chile Laboratorio de Microbiología, Instituto de Biología, Facultad de Ciencias, Pontificia Universidad Cat� olica de Valparaíso, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposites Barrier properties Graphene oxide Antimicrobial materials Active materials
The addition of nanoparticles into a polymer can produce multifunctional films having not only gas barrier and antimicrobial characteristics for active packaging, but also improved mechanical and thermal behaviours. Among the different nanoparticles, graphene oxide (GO) has the potential to develope active polymers due to its layer structure and biocide property. In this study, linear low density polyethylene (LLDPE) was melt mixed with GO nanoparticles having different oxidation and exfoliation degrees. Our results shown that the elastic modulus of LLDPE composites increased with the amount of GO, and by using highly oxidated GO this reinforcement effect was observed without reducing the elongation at break. The thermal stability of the composites under either inert and oxygen atmosphere also increased as compared with the pure matrix. Regarding the barrier properties, the oxygen permeability of the composites depended on the amount and kind of filler, and while GO with low oxidation increased dramatically the permeability of the polymer, highly oxidated GO decreased this value. These results were explained through the Felske model. Water vapor permeation otherwise was not affected by the presence of the nanofillers. The LLDPE/GO nanocomposites further presented antimicrobial behaviour against Salmonella Typhi and Listeria monocytogenes strains. These results confirmed that GO can be designed as an active filler in polymer nanocomposites for packaging applications.
1. Introduction Active food packaging materials are designed not only to act as a passive barrier protecting the product from the surrounding environ ment but also to add an active function avoiding or delaying the different external processes that affect the product [1]. From the different functionalities desired for these active packaging materials, the antimicrobial behavior is one of the most interesting for products likely to present microbial growth on their surface. These antimicrobial films can inhibit microbial growth and food spoilage extending the product life. Although different approach can be found for designing antimi crobial materials, such as the use of intrinsic antimicrobial polymers [2] or the addition of organic biocide products [3], antimicrobial inorganic nanoparticles are highlighted as they can be embedded into the polymer matrix in processes that are easily implemented in the standard plastic industry further avoiding stability issues. Noteworthy, due to their high surface/volume ratio, antimicrobial nanoparticles present improved
behavior motivating the study of a broad range of biocide polymer nanocomposites, for instance using: silver, copper, titanium oxide, magnesium oxide, zinc oxide, and modified clays, among others [1,4]. From the different families of nanoparticles, those based on carbon are highlighted due to their outstanding properties, such as electrical conductivity, mechanical strength, and specific surface area, allowing the development of multifunctional polymer composites based on transition metal carbide/carbonitride (MXene), carbon nanotubes (CNT), graphite nanoplatelets, and graphene [5–7]. From all these carbon-based nanoparticles, graphene and its derivatives such as gra phene oxide (GO) has recently emerged as a new family of nanoparticles presenting outstanding properties that can be tailored by its exfoliation level and oxidation degree. For instance, graphene recently has allowed the development of different novel nanocomposites and hybrid mate rials with applications in electrodes for supercapacitors [8–10], mem branes [11], metacomposites with tunable negative permittivity [12]; lubricants without the base oil [13], anode for lithium ion batteries [14],
* Corresponding author. E-mail address: [email protected] (H. Palza). https://doi.org/10.1016/j.compscitech.2019.107888 Received 18 April 2019; Received in revised form 2 October 2019; Accepted 20 October 2019 Available online 23 October 2019 0266-3538/© 2019 Elsevier Ltd. All rights reserved.
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platinum free counter electrode for dye sensitized solar cell [15], and metal coatings [16]. Due to the high conductivity of graphene nano composites, they can be further be designed for electromagnetic inter ference shielding [17], superior electromagnetic wave absorbers [18]. Noteworthy, due to its high aspect ratio and graphene structure with hydroxyl and epoxide groups on its bulk surface, and carboxyl and carbonyl groups on its edges, GO presents several properties useful for the development of nanocomposites with different functionalities for instance in wound dressing [19]; metal coatings [20], composites with improved tribological behavior [21] and in mechanical reinforcement [22–24]. Among the different functionalities, GO present antimicrobial behaviour that can be explored for the design of biocide composite materials [25]. The strong biocide behavior of GO can be explained through several mechanisms such as those based on membrane stress, oxidative stress, and wrapping isolation [26,27]. However, although GO has been extensivelly studied in a broad range of systems for antimi crobial applications, its use as filler to produce biocide polymer nano composites toward active packaging has been barely reported [28]. Moreover, similar to clay based fillers, GO can improve the barrier property of polymer films reducing both gas and water vapor perme ability due to its high aspect ratio [29]. Therefore, GO present the po tential to be used for the development of active antimicrobial packaging materials with improved barrier behavior as GO is impermeable to most gases [30,31]. For instance, polyethylenimine and GO films prepared by a layer-by-layer assembly can exhibit significantly low oxygen and carbon dioxide transmission rates [31]. Poly(vinyl alcohol) films filled with GO nanofillers presented high barrier properties promoting the application of these composite films in the packaging industry [32]. These polymer/GO nanocomposites can further present improved me chanical behaviour and thermal stability as GO has been extensivelly studied in a broad range of polymer materials for these applications [28, 33,34]. From the different polymer matrices, linear low density poly ethylene (LLDPE) is highlighted for packaging application due to its flexibility, strength and durability [35]. In addition, LLDPE has the ability to elongate under stress, allowing them to maintain their integ rity under localized differential settlement conditions. Compared with standard low-density polyethylene, LLDPE possesses better strength, toughness, heat-resistance, cold resistance, environmental stress cracking resistance, and tearing resistance properties [36]. Despite the relevance of LLDPE in packaging and the potential of GO to produce multifunctional polymer composites toward active pack aging, LLDPE/GO nanocomposites has been barely studied in this context. Based on these antecedents, the goal of the present study is to prepare linear low density polyethylene composites using GO as filler for the development of multifunctional nanocomposites with potential ap plications in active packaging. In particular, the effect of the GO con centration and its oxidation degree on the tensile mechanical properties, thermal stability, oxygen and water vapor barrier, and the antimicrobial behaviour against two of the most relevant microorganisms present in food, was analyzed.
keeping the bath with ice. The reaction is completed by pouring the dispersion into 0.5 L of water and adding 400 ml of H2O2 (5% weight) to remove the excess of KMnO4. The resulting graphene oxide (GO) is separated by filtration and washed with aqueous HCl and subsequently filtered with distilled water. Finally, the filtrate is subjected to two vacuum drying processes, the first at 60 � C for 12 h and the second at 110 � C for 5 h. The GO-LS sample was prepared in the same way as the GO-L but adding a sonication process after the filtration with distilled water using a Sonics brand Vibra Cell sonicator at a wave amplitude of 40% during 1 h. This solution is allowed to stand for about 4 days, until the graphene oxide is decanted, then the slurry (settled particles) is frozen for 24 h and subjected to a lyophilization process to extract the water content. GO-HS is obtained from a variation of the HummersOffeman method. After the addition of the KMnO4, the glass is removed from the ice bath and transferred to a heating plate with stir ring. The mixture is stirred for 30 min at 50 � C. After this, the mixture is slowly transferred to a beaker of 2 l with 500 ml of distilled water. The mixture is stirred for 30 min at 95 � C assisted by a heating plate. After the reaction, 400 ml of hydrogen peroxide is slowly added to the mixture to remove excess of unreacted KMnO4. The new mixture is allowed to stir by 1 h at 60 � C. At the end of the stirring, the solution is left to stand overnight and the GO settled is dried followed the method described above. Finally, the same sonication process described in the previous point is applied. Details about the characterization of the GO from the different methods used can be found elsewhere [37]. The particles were characterized by Raman spectroscopy using a Horiba HR Evolution, by FT-IR using an Agilent Cary 630 spectrometer and by scanning electro microscopy using a FEI Tecnai F20 S/TEM electronic microscope. 2.2. Nanocomposite preparation and characterization The composites were made by a batch twin-screw melt mixer (Bra bender® Plasti Corder) with a capacity of 40 cm3, at 110 RPM during 8 min using an operation temperature of 170� . Once this process is finished, the material is melt pressed at 170 � C in an HP hydraulic press (model D-50) with heating system, and water cooling system. The sample thickness were 0.1 mm for tests of permeability and 1.0 mm for tensile tests. The mechanical properties of the nanocomposites were determined by tensile-strain tests (Jinan universal tensile equipment model WDWS5), at a rate of 50 mm/min. Samples were prepared by cutting test specimens using a steel mold according to ASTM D638. Five tests were carried out for each sample and the average and the standard deviation were reported. The thermal resistance was measured with a TG 209 F1 Netzsch Libra equipment, under oxygen and nitrogen environments. Approximately 4 mg of each sample was deposited, a gas flow of 20 ml/min was applied, and the equipment applied heat from 25 [� C] to 700 [� C], with an increase of 10 � C/min by recording the mass of the samples. The oxygen permeability was measured using a gas permeability equipment (Lyssy, model L100-5000) using films of 10 � 10 � 0.01 cc with an oxygen flow of 10 [ml/min]. From the pressure difference be tween both sides of the film, the permeability values is estimated. The tests were performed in triplicate, with six measurements for each sample. The water vapor permeability was measured with the “Dry Cup Method” covering a container sealed hermetically with a film of the material under study. Into the vessel, approximately 2 g of phosphorus pentoxide (P4010) was added as a drying agent. The system was placed in a closed acrylic chamber with a relative humidity of 99.9% and a tem perature that varies between 25 � C and 30 � C. The total mass of each container was measured with an analytical balance, during a period of two weeks. The increase in mass of each container corresponds to the water vapor that has permeated through the nanocomposite and that was absorbed by the drying agent. With these measurements a graph of relative increase of the mass as a function of time is constructed and the
2. Experimental 2.1. Graphene oxide synthesis Three types of graphene oxides (GO) were elaborated having different degrees of oxidation and exfoliation: GO with low oxidation level (GO-L); GO-L sonicated after synthesis (GO-LS); and GO with high oxidation level sonicated after synthesis (GO-HS). For the synthesis of the GO-L, the Hummers-Offeman method was used consisting of an oxidation of graphite with KMnO4 and NaNO3 in concentrated sulfuric acid (37%). This oxidation is carried out using 250 ml of H2SO4 with 10 g of graphite at 23 � C. 5 g of NaNO3 are added to the solution that after 30 min of stirring is cooled to 0 [� C] using an ice bath. Then 30 g of KMnO4 are added during 4 h. When the addition is complete, the resulting dispersion is stirred at room temperature during 90 min 2
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sensor is calculated. By means of equation (1) the water vapor perme ability is calculated: � � d dm WVTR ¼ (1) ⋅ A⋅Psat ⋅HR dt where d corresponds to the thickness of the sample, A is the effective area of the sample, Psat is the saturation pressure of the water at the analysis temperature, HR is the relative humidity inside the chamber and dm/dt is the slope of the graph aforementioned. Three tests were performed per sample, reporting their average. The antimicrobial character of the samples prepared was analyzed using the international standard ISO 22196, for plastics and non-porous surfaces measuring the viable cells after 6, 8 and 24 h of incubation of Salmonella Typhi and Listeria monocytogenes strains. The tests were per formed in triplicate. 3. Results and discussions 3.1. Particle characterization Beside the characterization carried out previously to these samples showing the higher oxidation degree in GO-HS than in GO-L and GO-LS [37], Fig. 1 shows the scanning electron microscopy images of the different GO studies in order to analyze the effect of the processing on its lateral size. GO-L presented an average lateral size of around 1.3 μm that is as similar as the size of GO-LS having a value of 1.5 μm meaning that the sonication process was not able to affect the lateral size. However, the high oxidation process was able to reduce the GO average lateral size to 1.1 μm. The changes in the carbon structure arising from oxygenated groups in the GO particles were quantified by the ratio between the intensity of sp3 sites of disorder or structural defects (D band at ~ 1350 cm-1) and the intensity of sp2 sites (G band at ~ 1580 cm-1) in the carbon material surface from the Raman spectra (ID/IG) as displayed in Fig. 2 [38]. This ratio (ID/IG) is currently used as an indicator of defects in graphene structures arising from functional groups as compared with carbon array. The oxidation process was able to increase this ratio from 0.5 for a graphite sample (showed as reference) to values around 1.07 confirming the presence of functional groups in the GO samples. Despite the different oxidation process between GO-LS and GO-HS, both sample presented similar ID/IG ratio. Fig. 3 otherwise shows the FT-IR of the different samples where the change in the relative intensities of the – O (around 1710 cm 1), C ¼C (around peaks associated with the C– 1 1550 cm ), and C–O (around 1020 cm 1) stretching further confirms the higher oxidation degree in GO-HS sample.
Fig. 2. Raman spectra for the different GO samples: GO-HS; GO-LS and GO-L. The spectrum from a graphite sample is further shown for comparison.
depended on the filler concentration and its type. While at low filler concentrations the GO particles barely changed the elastic modulus of the matrix, composites with 5 wt% of filler presented increments around 30% (Fig. 4a). GO-HS filler rendered the highest elastic modulus as compared with the other two fillers at the same concentration, as tested through a two-sample t-Test assuming unequal variances (p < 0.05). In polymer nanocomposites, the elastic modulus depended on the aspect ratio and mechanical properties of the agglomerates formed by the nanoparticles rather than of the isolate particles (in this case GO single layer), as concluded analyzing micromechanical models [39]. Based on these analysis, we can conclude that GO-HS agglomerates presented the highest aspect ratio (or best dispertion level) as compared with the other two GO fillers arising from its higher oxidation and disrupted layered structure. Our results showed that GO rendered a lower reinforcement effect in LLDPE than in other polymer matrices that can be associated with the lack of a proper LLDPE/GO compatibility producing composites with agglomerates rather that well disperse individual nanoparticles [34]. Polyethylene is a non-polar polymer and it is reported that therefore the
3.2. Mechanical properties Fig. 4 shows the effect of the different GO fillers on the elastic modulus and elongation at breaks of the resulting nanocomposites allowing to conclude that the mechanical behavior of the samples
Fig. 1. Lateral size histograms from SEM analysis of: a) GO-L with an average of 1.3 μm; b) GO-LS with an average of 1.5 μm; and GO-HS with an average of 1.1 μm. 3
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break of the pure matrix. Moreover, the tendency found in composites based on GO-HS agrees with the hypothesis that this filler presents the best dispertion due to their high oxidation level facilitating the exfoli ation process and therefore increasing the aspect ration of the agglomerates. 3.3. Thermal stability After the development of polymer/clay nanocomposites, there is a growing interest in the development of layered particles as a route to increase the thermal stability of polymer matrices motivating our study of the thermal degradation of LLDPE/GO composites. Figs. 5 and 6 displays the TGA analysis for the different samples under nitrogen and air atmosphere, respectively. Under inert conditions, GO filler is able to increase the thermal stability as measured through the temperatures for 10% (T10) and 50% (T50) of weight loss (Fig. 5a and b), although the effect was highly depended on the filler concentration and type. For GO fillers produced by the Hummers’ method (GO-L and GO-LS), the largest improvements were obtained at 3 wt% with increment in T10 and T50 of 28 � C and 14 � C, respectively, as compared with the pure matrix. How ever, for GO-HS fillers the best thermal stability was at 5 wt% with values as similar as the other fillers at 3 wt%. The thermal degradation improvement in these layered particles is originated from the high aspect ratio of the filler producing a “labyrinth effect” reducing the outdiffusion of volatile decomposition products within the nanocomposites [43,44]. Similar to the mechanical behavior, the effect of filler on the thermal degradation depended on its morphology and dispertion level even showing two-step processes for complex filler morphologies [43]. In particular, a direct relationship between filler dispertion and thermal stability can be found in polymer/clay composites with a much more sensitivity as compared with the mechanical behavior [43]. Based on these results, the thermal stability in the nanocomposites displayed in Figs. 5 and 6 can be therefore explained by changes in the particle morphology and its dispertion in the polymer matrix. However, by comparing the mechanical and thermal degradation behaviours of the different composites, a different tendency is found. This mismatch can be associated to other mechanisms beside “laberynth effect” in the degradation process. The thermal degradation of polyethylene started by thermal scissions of C–C chain bonds following the formation of radical species through a molecular weight reduction by depropagation and inter/intra-transfer reactions [45]. Therefore, similar to other nanofillers, the degradation stability in polymer composites can also arise from the chemical interactions between the filler surface and the radicals rather than from the pure labyrinth effect [46,47]. For instance, graphene and GO are able to interact with radicals through the
Fig. 3. FT-IR of the different GO samples.
dispersion of polar graphene oxide in this kind of matrix is not optimal owing to absence of positive interactions between them [40]. When compared with other composites based on GO, in this contribution we further used higher filler concentrations that can promote the agglom eration, reducing even more the reinforcement effect of the nanoparticle [34,36]. These agglomerates decrease the effective aspect ratio of the particles reducing dramatically the reinforcement effect [34,39]. Moreover, the interfacial adhesion between PE and GO is low decreasing the stress transfer and therefore the composite modulus [34]. Regarding the elongation at break (Fig. 4b), a larger effect of the type of filler is observed as while composites with GO-L presented a drastic decrease in this property, composites using GO-LS and GO-HS fillers presented values as similar as the pure polymer. In polymer nano composites, the general tendency is to decrease the elongation at break due to the presence of particles generating restrictions for the polymer movements and the failure at particle/polymer interface [41]. In particular, filler can avoid the reorientation of polymer chains leading to a premature break of the polymer chains [42]. In polymer/graphene composites, the final filler thickness, originated from the exfoliation process during melt mixer, is a relevant parameter to understand the mechanical behavior [42]. In these composites, graphene nanoparticles can inhibit the mechanisms for crack propagation in semicrystalline polymers explaining the lack of a negative effect in the elongation at break [42]. Results from Fig. 4 show therefore that GO can be designed to present a reinforcement effect without altering the elongation at
Fig. 4. Effect of the different GO fillers on the elastic modulus (a) and elongation at breaks (b) of the resulting LLDPE composites under tensile conditions. 4
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Composites Science and Technology 184 (2019) 107888
Fig. 5. Effect of the different GO fillers on the thermal degradation temperatura under inert conditions: a) temperature for 10% (T10) of weight loss and b) tem perature for 50% (T50) of weight loss.
Fig. 6. Effect of the different GO fillers on the thermal degradation temperatura under oxidative conditions: a) temperature for 10% (T10) of weight loss and b) temperature for 50% (T50) of weight loss.
sp2-carbon groups rather than oxygen-containing functional groups [46]. Therefore, GO-HS should present a lower interaction with radicals and less degradation stability. However, this filler presents the highest dispertion as concluded from the mechanical properties. Our results can therefore be explained from a competition between the two mecha nisms: a physical one based on the labyrinth effect and a chemical one based on the interactions between the filler and degradation products. The enhancement in thermal stability due to the insulator behavior of GO between the heat source and polymer surface where degradation occurs should be further considered [45]. Under oxidative conditions, a similar tendency is found with com posites presenting an improved thermal degradation as compared with the pure matrix with differences in T10 and T50 of 25 � C and 13 � C, respectively. This tendency is contrary to found in layered clays were under oxidative conditions much larger thermal stabilities are found due to the significant influence of the transport properties affecting the ox ygen diffusion. In particular, layered particles avoid the free pathway of oxygen toward the polymer reducing the radical peroxidation processes [43]. However, other characteristics of GO reducing the thermal sta bility of the resulting composites, such as its catalytic behavior under oxidative conditions [48] and low thermal stability [49], should be
further considered. To further quantify the effect of the particle on the thermal proper ties of the sample, Fig. 7 shows the values for the heat-resistance-index (THRI), defined as [50]: THRI ¼ 0:49 ⋅ ðT5% þ 0:6 ⋅ ðT30%
T5% Þ
(2)
Fig. 7 confirms the improvement in the thermal stability by adding the different particles. 3.4. Barrier properties Fig. 8a displays the effect of adding the different GO fillers on the oxygen permeability of polyethylene defined as defined as the ratio between the product of the steady-state flux of gas through the mem brane and the membrane thickness, and the transmembrane pressure difference. In general, layered particles are impermeable to molecular species improving the barrier properties when added into a polymer film through an increase in the tortuosity of molecules passing through the film. Under this condition, the permeability reduction depended on the filler aspect ratio and concentration as described elsewhere [51]. From a theoretical point of view, graphene derivatives increased the diffusion 5
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Composites Science and Technology 184 (2019) 107888
Fig. 7. Effect of the different GO and its concentration on the heat-resistance-index (THRI) for the different samples: a) under inert conditions and b) under oxidative conditions.
Fig. 8. Effect of adding the different GO fillers on the oxygen (a) and water vapor (b) permeability of polyethylene films.
path (tortuosity) and consequently decreased the gas permeability of the graphene/polymer composites [52]. However, a much more complex tendency is observed in Fig. 8a as the oxygen permeability can either increase or decrease depending of the filler and its concentration. For instance, GO-HS and GO-LS fillers are able to reduce the permeability proportional to the amount of filler confirming the tendency from polymer nanocomposites having layered structures. The composite films can present a decrease in the oxygen permeability of 35% as compared with the pure polymer matrix. However, an opposite tendency is found when GO-L filler is used as the oxygen permeability increase drastically at high filler concentrations. Indeed, in Fig. 8a, for clarity just the lowest measured value for the composite having 5 wt% of GO-L filler is shown as much higher values were also obtained. The origin of this unexpected results can be traced from non-ideal contacts between the filler and the matrix producing void spaces in the interface where the permeability is significantly different from the permeability in the bulk polymer matrix [53]. These voids act therefore like fast avenues for the species passing through the composite [54]. Most of the models developed for the barrier properties of polymer composites predicted a decrease in the permeability by increasing the filler content based on ideal contact between the filler particle and matrix. However, by assuming that particles are not perfectly bonded to
the polymer matrix, the interfacial layer between the particle and the polymer can be considered as a third phase and three-phase permeation models can be used [52]. Under this condition, a modified Felske model can be used to estimate the relative permeability (Pr) accounting for the morphology and packing difficulty of particles in the polymer matrix [53]: Pr ¼
P 1 þ 2φ½ðβ γÞ=ðβ þ 2γÞ� ¼ Pm 1 φ⋅ψ ½ðβ γÞ=ðβ þ 2γÞ�
(3)
where P and Pm are the permeability of the resulting composite and the pure matrix respectively; ϕ is the volume fraction of the filler particles. In this equation, the next definitions are used:
ψ ¼1þ
ð1
φm Þ ⋅φ φ2m
� β ¼ 2 þ δ3 ⋅ λdm
2 1
γ ¼ 1 þ 2δ3
� δ3 ⋅λdI
1
(4) � δ3 ⋅λIm
(5) (6)
where δ is the ratio of outer radius of interface to pure particle radius; λdm is the permeability ratio Pd/Pm; λIm is the permeability ratio PI/Pm; 6
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λdI is the permeability ratio Pd/PI; PI is the permeability in the particle/ matrix interface; and Pd is the permeability in particle. Finally, the Felske model is able to incorporate the aspect ratio through ϕm, the maximum packing volume fraction of filler particles, that is 0.64 for random close packing of uniform spheres. By assuming that the contact between the particles and the matrix is defect free with δ ¼ 1.0; λIm ¼ 1.0; λdI ¼ 0.001; λdm ¼ 0.001; and random close packing of uniform spheres, this model account for the decrease in the permeability in the nanocomposites as displayed in Fig. 9. If we further assume that any change in the particle morphology is associated with changes in ϕm, the effect of particle aspect ratio can be studied by decreasing this parameter to 0.5 or 0.4 [55]. Fig. 9 shows that under this model, the effect of particle aspect ratio is low confirming that the changes observed in Fig. 8 between the different GO arise from an interface phenomenon rather than from changes in the particle morphology. To further confirm this hypothesis, the model was tested assuming a polymer/particle interface with δ ¼ 1.05; λIm ¼ 100; λdI ¼ 0.001; λdm ¼ 0.1; and random close packing of uniform spheres, as displayed in Fig. 9. Under these conditions the model is able to account for the increase in the permeability. Similar to the previous conditions, the particle aspect ratio is not a relevant parameter for the behavior of the composites. Therefore, this model shows that depending on the GO filler, changes in the polymer/particle interface can occur. Noteworthy, the presence of this interface in LLPE/GO-L composites can further explain the low elongation at break as compared with LLDPE/GO-HS composites. Despite the effect of both particle concentration and type of GO on the oxygen permeability of LLDPE films, the water vapor transition rate is un-changed by the presence of the nanofiller as displayed in Fig. 8b. In general, a lower effect of GO nanofillers on water vapor diffusion is expected as compared with oxygen diffusion [32]. This change is attributed to the different absorption behavior of water molecules on the polymer matrix and GO fillers, being an easier process in the latter case. This higher absorption on GO can increase the solubility coefficient of this specie compensating the lower diffusion by tortuosity meaning lower water permeability in the nanocomposite.
3.5. Antibacterial behavior Based on the previous results regarding the gas barrier and me chanical behavior, LLDPE composites with GO-LS and GO-HS fillers were selected for testing the antimicrobial behaviour against Salmonella typhi and Listeria monocytogenes bacteria, as they are commun microor ganisms found in food associated infections. The former is the most frequently isolated foodborne pathogen, and are predominantly found in poultry, eggs and dairy products [56]. The latter is a human bacterial pathogen which causes listeriosis and its sources of infection are mainly associated with raw food and working surfaces in food-processing plants [57]. Fig. 10 displays the relative bacteria reduction after contact with the composites as compared with the pure LLDPE after 6 and 24 h of exposition. These results show that the presence of GO particles can render an antimicrobial effect to the polymer matrix depending on the amount and type of filler. Although after a statistical analysis a clear effect of exposition time, filler type and its concentration, is not directly observed, some conclusions can be obtained. In particular, independent on the exposition time, composites having 5 wt% of GO-LS and 3 wt% of GO-HS present antimicrobial effect against Salmonella typhi while com posites with 1 and 3 wt% of GO-LS and 3 wt% of GO-H present antimi crobial effect against Listeria monocytogenes. The other composites present antimicrobial activity but without statistical significant. Isolate GO nanoparticles present a high antibacterial behaviour as reported elsewhere. The mechanism is based on: 1) GO adsorption ability on bacteria due to its abundant functional groups and small size; 2) membrane disruption and damage after direct contact between GO and bacteria; and 3) cells death [58]. This antimicrobial effect is asso ciated with several processes triggered by GO, such as physical damage to bacterial membranes and leakage of intracellular material, due to the sharp edges of GO [58]. The production of reactive oxygen species (ROS) produced by the oxidative stress of GO also contributes to bacterial inactivation mainly when GO penetrates the bacteria [58]. The anti microbial behavior of GO depended of several variables such as test conditions, particle lateral size, number of layers, and oxidation level, explaining the different tendencies found regarding the effect of GO [26]. For instance, regarding the effect of GO on the bacteria membrane, antimicrobial surfaces made of reduced GO show higher destruction than GO due to the high amount of edges and efficient charge transfer increasing the disruption of the membrane [27]. In GO particles as similar as our fillers, a higher antimicrobial behavior was found in GO with low amount of functional groups and less disrupted layers, vali dating the antibacterial mechanisms based on a direct contact [39]. However, when GO particles are embedded in a thermoplastic polymer matrix the direct contact between the GO and the bacteria can be ruled out. A plausible mechanism in these polymer/GO films can arise from changes on the film surface due to the presence of the carbon nano particles such as polarity and roughness, both affecting the bacteria survival [28]. Another mechanism is based on the water/oxygen diffu sion into the polymer matrix allowing the specific chemical reaction with the nanoparticles, releasing biocide active agents to the film surface and killing the bacteria. This mechanism has been applied to understand the antimicrobial behavior of polymer/copper nanocomposites pre senting ion release after immersion in water and therefore biocide ac tivity depending the amount of nanofiller [59,60]. Similar mechanism can explain the antimicrobial behavior of polymer composites with photoactive TiO2 nanoparticles [61]. In our case, water/oxygen diffu sion into the polymer composites can trigger ROS generation from GO nanofillers able to reduce bacterial survival after diffusion-out.
Fig. 9. Effect of the GO filler fraction as predicted by a modified Felske model (Equation (3)) under two polymer/particle conditions: a) defect free contact between the particles and the matrix assuming δ ¼ 1.0; λIm ¼ 1.0; λdI ¼ 0.001; λdm ¼ 0.001; and random close packing of uniform spheres (open figures); and b) presence of a polymer/particle interface assuming δ ¼ 1.05; λIm ¼ 100; λdI ¼ 0.001; λdm ¼ 0.1; and random close packing of uniform spheres (closed figures).
4. Conclusion The addition of GO into a LLDPE matrix can be used as a proper route to design novel multifunctional polymer nanocomposites presenting improved mechanical, thermal, barrier and antimicrobial behaviours. The effect of the filler highly depended on the characteristics of the GO, 7
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Fig. 10. Relative bacteria reduction of the composite surfaces as compared with the pure LLDPE sample: a) 6 h of contact against Salmonella typhi strain; b) 6 h of contact against Listeria monocytogenes strain; c) 24 h of contact against Salmonella typhi strain; and d) 24 h of contact against Listeria monocytogenes strain. *: Sta tistically significant difference between experiment groups with pure LLDPE at the same time (p < 0.05).
in particular the oxidation level and sonication process. In particular, PE/GO-HS composites presented the highest Young’s modulus as compared with the pure polymer matrix without altering the elongation at break. The presence of GO further increased the thermal degradation temperatures of the polymer under both nitrogen and oxygen atmo spheres. Noteworthy, depending of the filler content and kind of GO, the permeability of oxygen can either decrease or increase as explained by a modified Felske model. Additionally, the material presented antimi crobial activity against two of the most relevant bacteria strains in food processing. All these results open up the possibility to use GO nanofillers for the development of active films for packaging applications.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Polyethylene/graphene oxide composites toward multifunctional active packaging films Rodrigo Silva-Leyton a, Raúl Quijada a, Roberto Bastías b, Natali Zamora b, Felipe Olate-Moya a, Humberto Palza a, * a b
Departamento de Ingeniería Química, Biotecnología y Materiales, Facultad de Ciencias Físicas y Matem� aticas, Universidad de Chile, Chile Laboratorio de Microbiología, Instituto de Biología, Facultad de Ciencias, Pontificia Universidad Cat� olica de Valparaíso, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposites Barrier properties Graphene oxide Antimicrobial materials Active materials
The addition of nanoparticles into a polymer can produce multifunctional films having not only gas barrier and antimicrobial characteristics for active packaging, but also improved mechanical and thermal behaviours. Among the different nanoparticles, graphene oxide (GO) has the potential to develope active polymers due to its layer structure and biocide property. In this study, linear low density polyethylene (LLDPE) was melt mixed with GO nanoparticles having different oxidation and exfoliation degrees. Our results shown that the elastic modulus of LLDPE composites increased with the amount of GO, and by using highly oxidated GO this reinforcement effect was observed without reducing the elongation at break. The thermal stability of the composites under either inert and oxygen atmosphere also increased as compared with the pure matrix. Regarding the barrier properties, the oxygen permeability of the composites depended on the amount and kind of filler, and while GO with low oxidation increased dramatically the permeability of the polymer, highly oxidated GO decreased this value. These results were explained through the Felske model. Water vapor permeation otherwise was not affected by the presence of the nanofillers. The LLDPE/GO nanocomposites further presented antimicrobial behaviour against Salmonella Typhi and Listeria monocytogenes strains. These results confirmed that GO can be designed as an active filler in polymer nanocomposites for packaging applications.
1. Introduction Active food packaging materials are designed not only to act as a passive barrier protecting the product from the surrounding environ ment but also to add an active function avoiding or delaying the different external processes that affect the product [1]. From the different functionalities desired for these active packaging materials, the antimicrobial behavior is one of the most interesting for products likely to present microbial growth on their surface. These antimicrobial films can inhibit microbial growth and food spoilage extending the product life. Although different approach can be found for designing antimi crobial materials, such as the use of intrinsic antimicrobial polymers [2] or the addition of organic biocide products [3], antimicrobial inorganic nanoparticles are highlighted as they can be embedded into the polymer matrix in processes that are easily implemented in the standard plastic industry further avoiding stability issues. Noteworthy, due to their high surface/volume ratio, antimicrobial nanoparticles present improved
behavior motivating the study of a broad range of biocide polymer nanocomposites, for instance using: silver, copper, titanium oxide, magnesium oxide, zinc oxide, and modified clays, among others [1,4]. From the different families of nanoparticles, those based on carbon are highlighted due to their outstanding properties, such as electrical conductivity, mechanical strength, and specific surface area, allowing the development of multifunctional polymer composites based on transition metal carbide/carbonitride (MXene), carbon nanotubes (CNT), graphite nanoplatelets, and graphene [5–7]. From all these carbon-based nanoparticles, graphene and its derivatives such as gra phene oxide (GO) has recently emerged as a new family of nanoparticles presenting outstanding properties that can be tailored by its exfoliation level and oxidation degree. For instance, graphene recently has allowed the development of different novel nanocomposites and hybrid mate rials with applications in electrodes for supercapacitors [8–10], mem branes [11], metacomposites with tunable negative permittivity [12]; lubricants without the base oil [13], anode for lithium ion batteries [14],
* Corresponding author. E-mail address: [email protected] (H. Palza). https://doi.org/10.1016/j.compscitech.2019.107888 Received 18 April 2019; Received in revised form 2 October 2019; Accepted 20 October 2019 Available online 23 October 2019 0266-3538/© 2019 Elsevier Ltd. All rights reserved.
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platinum free counter electrode for dye sensitized solar cell [15], and metal coatings [16]. Due to the high conductivity of graphene nano composites, they can be further be designed for electromagnetic inter ference shielding [17], superior electromagnetic wave absorbers [18]. Noteworthy, due to its high aspect ratio and graphene structure with hydroxyl and epoxide groups on its bulk surface, and carboxyl and carbonyl groups on its edges, GO presents several properties useful for the development of nanocomposites with different functionalities for instance in wound dressing [19]; metal coatings [20], composites with improved tribological behavior [21] and in mechanical reinforcement [22–24]. Among the different functionalities, GO present antimicrobial behaviour that can be explored for the design of biocide composite materials [25]. The strong biocide behavior of GO can be explained through several mechanisms such as those based on membrane stress, oxidative stress, and wrapping isolation [26,27]. However, although GO has been extensivelly studied in a broad range of systems for antimi crobial applications, its use as filler to produce biocide polymer nano composites toward active packaging has been barely reported [28]. Moreover, similar to clay based fillers, GO can improve the barrier property of polymer films reducing both gas and water vapor perme ability due to its high aspect ratio [29]. Therefore, GO present the po tential to be used for the development of active antimicrobial packaging materials with improved barrier behavior as GO is impermeable to most gases [30,31]. For instance, polyethylenimine and GO films prepared by a layer-by-layer assembly can exhibit significantly low oxygen and carbon dioxide transmission rates [31]. Poly(vinyl alcohol) films filled with GO nanofillers presented high barrier properties promoting the application of these composite films in the packaging industry [32]. These polymer/GO nanocomposites can further present improved me chanical behaviour and thermal stability as GO has been extensivelly studied in a broad range of polymer materials for these applications [28, 33,34]. From the different polymer matrices, linear low density poly ethylene (LLDPE) is highlighted for packaging application due to its flexibility, strength and durability [35]. In addition, LLDPE has the ability to elongate under stress, allowing them to maintain their integ rity under localized differential settlement conditions. Compared with standard low-density polyethylene, LLDPE possesses better strength, toughness, heat-resistance, cold resistance, environmental stress cracking resistance, and tearing resistance properties [36]. Despite the relevance of LLDPE in packaging and the potential of GO to produce multifunctional polymer composites toward active pack aging, LLDPE/GO nanocomposites has been barely studied in this context. Based on these antecedents, the goal of the present study is to prepare linear low density polyethylene composites using GO as filler for the development of multifunctional nanocomposites with potential ap plications in active packaging. In particular, the effect of the GO con centration and its oxidation degree on the tensile mechanical properties, thermal stability, oxygen and water vapor barrier, and the antimicrobial behaviour against two of the most relevant microorganisms present in food, was analyzed.
keeping the bath with ice. The reaction is completed by pouring the dispersion into 0.5 L of water and adding 400 ml of H2O2 (5% weight) to remove the excess of KMnO4. The resulting graphene oxide (GO) is separated by filtration and washed with aqueous HCl and subsequently filtered with distilled water. Finally, the filtrate is subjected to two vacuum drying processes, the first at 60 � C for 12 h and the second at 110 � C for 5 h. The GO-LS sample was prepared in the same way as the GO-L but adding a sonication process after the filtration with distilled water using a Sonics brand Vibra Cell sonicator at a wave amplitude of 40% during 1 h. This solution is allowed to stand for about 4 days, until the graphene oxide is decanted, then the slurry (settled particles) is frozen for 24 h and subjected to a lyophilization process to extract the water content. GO-HS is obtained from a variation of the HummersOffeman method. After the addition of the KMnO4, the glass is removed from the ice bath and transferred to a heating plate with stir ring. The mixture is stirred for 30 min at 50 � C. After this, the mixture is slowly transferred to a beaker of 2 l with 500 ml of distilled water. The mixture is stirred for 30 min at 95 � C assisted by a heating plate. After the reaction, 400 ml of hydrogen peroxide is slowly added to the mixture to remove excess of unreacted KMnO4. The new mixture is allowed to stir by 1 h at 60 � C. At the end of the stirring, the solution is left to stand overnight and the GO settled is dried followed the method described above. Finally, the same sonication process described in the previous point is applied. Details about the characterization of the GO from the different methods used can be found elsewhere [37]. The particles were characterized by Raman spectroscopy using a Horiba HR Evolution, by FT-IR using an Agilent Cary 630 spectrometer and by scanning electro microscopy using a FEI Tecnai F20 S/TEM electronic microscope. 2.2. Nanocomposite preparation and characterization The composites were made by a batch twin-screw melt mixer (Bra bender® Plasti Corder) with a capacity of 40 cm3, at 110 RPM during 8 min using an operation temperature of 170� . Once this process is finished, the material is melt pressed at 170 � C in an HP hydraulic press (model D-50) with heating system, and water cooling system. The sample thickness were 0.1 mm for tests of permeability and 1.0 mm for tensile tests. The mechanical properties of the nanocomposites were determined by tensile-strain tests (Jinan universal tensile equipment model WDWS5), at a rate of 50 mm/min. Samples were prepared by cutting test specimens using a steel mold according to ASTM D638. Five tests were carried out for each sample and the average and the standard deviation were reported. The thermal resistance was measured with a TG 209 F1 Netzsch Libra equipment, under oxygen and nitrogen environments. Approximately 4 mg of each sample was deposited, a gas flow of 20 ml/min was applied, and the equipment applied heat from 25 [� C] to 700 [� C], with an increase of 10 � C/min by recording the mass of the samples. The oxygen permeability was measured using a gas permeability equipment (Lyssy, model L100-5000) using films of 10 � 10 � 0.01 cc with an oxygen flow of 10 [ml/min]. From the pressure difference be tween both sides of the film, the permeability values is estimated. The tests were performed in triplicate, with six measurements for each sample. The water vapor permeability was measured with the “Dry Cup Method” covering a container sealed hermetically with a film of the material under study. Into the vessel, approximately 2 g of phosphorus pentoxide (P4010) was added as a drying agent. The system was placed in a closed acrylic chamber with a relative humidity of 99.9% and a tem perature that varies between 25 � C and 30 � C. The total mass of each container was measured with an analytical balance, during a period of two weeks. The increase in mass of each container corresponds to the water vapor that has permeated through the nanocomposite and that was absorbed by the drying agent. With these measurements a graph of relative increase of the mass as a function of time is constructed and the
2. Experimental 2.1. Graphene oxide synthesis Three types of graphene oxides (GO) were elaborated having different degrees of oxidation and exfoliation: GO with low oxidation level (GO-L); GO-L sonicated after synthesis (GO-LS); and GO with high oxidation level sonicated after synthesis (GO-HS). For the synthesis of the GO-L, the Hummers-Offeman method was used consisting of an oxidation of graphite with KMnO4 and NaNO3 in concentrated sulfuric acid (37%). This oxidation is carried out using 250 ml of H2SO4 with 10 g of graphite at 23 � C. 5 g of NaNO3 are added to the solution that after 30 min of stirring is cooled to 0 [� C] using an ice bath. Then 30 g of KMnO4 are added during 4 h. When the addition is complete, the resulting dispersion is stirred at room temperature during 90 min 2
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sensor is calculated. By means of equation (1) the water vapor perme ability is calculated: � � d dm WVTR ¼ (1) ⋅ A⋅Psat ⋅HR dt where d corresponds to the thickness of the sample, A is the effective area of the sample, Psat is the saturation pressure of the water at the analysis temperature, HR is the relative humidity inside the chamber and dm/dt is the slope of the graph aforementioned. Three tests were performed per sample, reporting their average. The antimicrobial character of the samples prepared was analyzed using the international standard ISO 22196, for plastics and non-porous surfaces measuring the viable cells after 6, 8 and 24 h of incubation of Salmonella Typhi and Listeria monocytogenes strains. The tests were per formed in triplicate. 3. Results and discussions 3.1. Particle characterization Beside the characterization carried out previously to these samples showing the higher oxidation degree in GO-HS than in GO-L and GO-LS [37], Fig. 1 shows the scanning electron microscopy images of the different GO studies in order to analyze the effect of the processing on its lateral size. GO-L presented an average lateral size of around 1.3 μm that is as similar as the size of GO-LS having a value of 1.5 μm meaning that the sonication process was not able to affect the lateral size. However, the high oxidation process was able to reduce the GO average lateral size to 1.1 μm. The changes in the carbon structure arising from oxygenated groups in the GO particles were quantified by the ratio between the intensity of sp3 sites of disorder or structural defects (D band at ~ 1350 cm-1) and the intensity of sp2 sites (G band at ~ 1580 cm-1) in the carbon material surface from the Raman spectra (ID/IG) as displayed in Fig. 2 [38]. This ratio (ID/IG) is currently used as an indicator of defects in graphene structures arising from functional groups as compared with carbon array. The oxidation process was able to increase this ratio from 0.5 for a graphite sample (showed as reference) to values around 1.07 confirming the presence of functional groups in the GO samples. Despite the different oxidation process between GO-LS and GO-HS, both sample presented similar ID/IG ratio. Fig. 3 otherwise shows the FT-IR of the different samples where the change in the relative intensities of the – O (around 1710 cm 1), C ¼C (around peaks associated with the C– 1 1550 cm ), and C–O (around 1020 cm 1) stretching further confirms the higher oxidation degree in GO-HS sample.
Fig. 2. Raman spectra for the different GO samples: GO-HS; GO-LS and GO-L. The spectrum from a graphite sample is further shown for comparison.
depended on the filler concentration and its type. While at low filler concentrations the GO particles barely changed the elastic modulus of the matrix, composites with 5 wt% of filler presented increments around 30% (Fig. 4a). GO-HS filler rendered the highest elastic modulus as compared with the other two fillers at the same concentration, as tested through a two-sample t-Test assuming unequal variances (p < 0.05). In polymer nanocomposites, the elastic modulus depended on the aspect ratio and mechanical properties of the agglomerates formed by the nanoparticles rather than of the isolate particles (in this case GO single layer), as concluded analyzing micromechanical models [39]. Based on these analysis, we can conclude that GO-HS agglomerates presented the highest aspect ratio (or best dispertion level) as compared with the other two GO fillers arising from its higher oxidation and disrupted layered structure. Our results showed that GO rendered a lower reinforcement effect in LLDPE than in other polymer matrices that can be associated with the lack of a proper LLDPE/GO compatibility producing composites with agglomerates rather that well disperse individual nanoparticles [34]. Polyethylene is a non-polar polymer and it is reported that therefore the
3.2. Mechanical properties Fig. 4 shows the effect of the different GO fillers on the elastic modulus and elongation at breaks of the resulting nanocomposites allowing to conclude that the mechanical behavior of the samples
Fig. 1. Lateral size histograms from SEM analysis of: a) GO-L with an average of 1.3 μm; b) GO-LS with an average of 1.5 μm; and GO-HS with an average of 1.1 μm. 3
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break of the pure matrix. Moreover, the tendency found in composites based on GO-HS agrees with the hypothesis that this filler presents the best dispertion due to their high oxidation level facilitating the exfoli ation process and therefore increasing the aspect ration of the agglomerates. 3.3. Thermal stability After the development of polymer/clay nanocomposites, there is a growing interest in the development of layered particles as a route to increase the thermal stability of polymer matrices motivating our study of the thermal degradation of LLDPE/GO composites. Figs. 5 and 6 displays the TGA analysis for the different samples under nitrogen and air atmosphere, respectively. Under inert conditions, GO filler is able to increase the thermal stability as measured through the temperatures for 10% (T10) and 50% (T50) of weight loss (Fig. 5a and b), although the effect was highly depended on the filler concentration and type. For GO fillers produced by the Hummers’ method (GO-L and GO-LS), the largest improvements were obtained at 3 wt% with increment in T10 and T50 of 28 � C and 14 � C, respectively, as compared with the pure matrix. How ever, for GO-HS fillers the best thermal stability was at 5 wt% with values as similar as the other fillers at 3 wt%. The thermal degradation improvement in these layered particles is originated from the high aspect ratio of the filler producing a “labyrinth effect” reducing the outdiffusion of volatile decomposition products within the nanocomposites [43,44]. Similar to the mechanical behavior, the effect of filler on the thermal degradation depended on its morphology and dispertion level even showing two-step processes for complex filler morphologies [43]. In particular, a direct relationship between filler dispertion and thermal stability can be found in polymer/clay composites with a much more sensitivity as compared with the mechanical behavior [43]. Based on these results, the thermal stability in the nanocomposites displayed in Figs. 5 and 6 can be therefore explained by changes in the particle morphology and its dispertion in the polymer matrix. However, by comparing the mechanical and thermal degradation behaviours of the different composites, a different tendency is found. This mismatch can be associated to other mechanisms beside “laberynth effect” in the degradation process. The thermal degradation of polyethylene started by thermal scissions of C–C chain bonds following the formation of radical species through a molecular weight reduction by depropagation and inter/intra-transfer reactions [45]. Therefore, similar to other nanofillers, the degradation stability in polymer composites can also arise from the chemical interactions between the filler surface and the radicals rather than from the pure labyrinth effect [46,47]. For instance, graphene and GO are able to interact with radicals through the
Fig. 3. FT-IR of the different GO samples.
dispersion of polar graphene oxide in this kind of matrix is not optimal owing to absence of positive interactions between them [40]. When compared with other composites based on GO, in this contribution we further used higher filler concentrations that can promote the agglom eration, reducing even more the reinforcement effect of the nanoparticle [34,36]. These agglomerates decrease the effective aspect ratio of the particles reducing dramatically the reinforcement effect [34,39]. Moreover, the interfacial adhesion between PE and GO is low decreasing the stress transfer and therefore the composite modulus [34]. Regarding the elongation at break (Fig. 4b), a larger effect of the type of filler is observed as while composites with GO-L presented a drastic decrease in this property, composites using GO-LS and GO-HS fillers presented values as similar as the pure polymer. In polymer nano composites, the general tendency is to decrease the elongation at break due to the presence of particles generating restrictions for the polymer movements and the failure at particle/polymer interface [41]. In particular, filler can avoid the reorientation of polymer chains leading to a premature break of the polymer chains [42]. In polymer/graphene composites, the final filler thickness, originated from the exfoliation process during melt mixer, is a relevant parameter to understand the mechanical behavior [42]. In these composites, graphene nanoparticles can inhibit the mechanisms for crack propagation in semicrystalline polymers explaining the lack of a negative effect in the elongation at break [42]. Results from Fig. 4 show therefore that GO can be designed to present a reinforcement effect without altering the elongation at
Fig. 4. Effect of the different GO fillers on the elastic modulus (a) and elongation at breaks (b) of the resulting LLDPE composites under tensile conditions. 4
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Fig. 5. Effect of the different GO fillers on the thermal degradation temperatura under inert conditions: a) temperature for 10% (T10) of weight loss and b) tem perature for 50% (T50) of weight loss.
Fig. 6. Effect of the different GO fillers on the thermal degradation temperatura under oxidative conditions: a) temperature for 10% (T10) of weight loss and b) temperature for 50% (T50) of weight loss.
sp2-carbon groups rather than oxygen-containing functional groups [46]. Therefore, GO-HS should present a lower interaction with radicals and less degradation stability. However, this filler presents the highest dispertion as concluded from the mechanical properties. Our results can therefore be explained from a competition between the two mecha nisms: a physical one based on the labyrinth effect and a chemical one based on the interactions between the filler and degradation products. The enhancement in thermal stability due to the insulator behavior of GO between the heat source and polymer surface where degradation occurs should be further considered [45]. Under oxidative conditions, a similar tendency is found with com posites presenting an improved thermal degradation as compared with the pure matrix with differences in T10 and T50 of 25 � C and 13 � C, respectively. This tendency is contrary to found in layered clays were under oxidative conditions much larger thermal stabilities are found due to the significant influence of the transport properties affecting the ox ygen diffusion. In particular, layered particles avoid the free pathway of oxygen toward the polymer reducing the radical peroxidation processes [43]. However, other characteristics of GO reducing the thermal sta bility of the resulting composites, such as its catalytic behavior under oxidative conditions [48] and low thermal stability [49], should be
further considered. To further quantify the effect of the particle on the thermal proper ties of the sample, Fig. 7 shows the values for the heat-resistance-index (THRI), defined as [50]: THRI ¼ 0:49 ⋅ ðT5% þ 0:6 ⋅ ðT30%
T5% Þ
(2)
Fig. 7 confirms the improvement in the thermal stability by adding the different particles. 3.4. Barrier properties Fig. 8a displays the effect of adding the different GO fillers on the oxygen permeability of polyethylene defined as defined as the ratio between the product of the steady-state flux of gas through the mem brane and the membrane thickness, and the transmembrane pressure difference. In general, layered particles are impermeable to molecular species improving the barrier properties when added into a polymer film through an increase in the tortuosity of molecules passing through the film. Under this condition, the permeability reduction depended on the filler aspect ratio and concentration as described elsewhere [51]. From a theoretical point of view, graphene derivatives increased the diffusion 5
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Fig. 7. Effect of the different GO and its concentration on the heat-resistance-index (THRI) for the different samples: a) under inert conditions and b) under oxidative conditions.
Fig. 8. Effect of adding the different GO fillers on the oxygen (a) and water vapor (b) permeability of polyethylene films.
path (tortuosity) and consequently decreased the gas permeability of the graphene/polymer composites [52]. However, a much more complex tendency is observed in Fig. 8a as the oxygen permeability can either increase or decrease depending of the filler and its concentration. For instance, GO-HS and GO-LS fillers are able to reduce the permeability proportional to the amount of filler confirming the tendency from polymer nanocomposites having layered structures. The composite films can present a decrease in the oxygen permeability of 35% as compared with the pure polymer matrix. However, an opposite tendency is found when GO-L filler is used as the oxygen permeability increase drastically at high filler concentrations. Indeed, in Fig. 8a, for clarity just the lowest measured value for the composite having 5 wt% of GO-L filler is shown as much higher values were also obtained. The origin of this unexpected results can be traced from non-ideal contacts between the filler and the matrix producing void spaces in the interface where the permeability is significantly different from the permeability in the bulk polymer matrix [53]. These voids act therefore like fast avenues for the species passing through the composite [54]. Most of the models developed for the barrier properties of polymer composites predicted a decrease in the permeability by increasing the filler content based on ideal contact between the filler particle and matrix. However, by assuming that particles are not perfectly bonded to
the polymer matrix, the interfacial layer between the particle and the polymer can be considered as a third phase and three-phase permeation models can be used [52]. Under this condition, a modified Felske model can be used to estimate the relative permeability (Pr) accounting for the morphology and packing difficulty of particles in the polymer matrix [53]: Pr ¼
P 1 þ 2φ½ðβ γÞ=ðβ þ 2γÞ� ¼ Pm 1 φ⋅ψ ½ðβ γÞ=ðβ þ 2γÞ�
(3)
where P and Pm are the permeability of the resulting composite and the pure matrix respectively; ϕ is the volume fraction of the filler particles. In this equation, the next definitions are used:
ψ ¼1þ
ð1
φm Þ ⋅φ φ2m
� β ¼ 2 þ δ3 ⋅ λdm
2 1
γ ¼ 1 þ 2δ3
� δ3 ⋅λdI
1
(4) � δ3 ⋅λIm
(5) (6)
where δ is the ratio of outer radius of interface to pure particle radius; λdm is the permeability ratio Pd/Pm; λIm is the permeability ratio PI/Pm; 6
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λdI is the permeability ratio Pd/PI; PI is the permeability in the particle/ matrix interface; and Pd is the permeability in particle. Finally, the Felske model is able to incorporate the aspect ratio through ϕm, the maximum packing volume fraction of filler particles, that is 0.64 for random close packing of uniform spheres. By assuming that the contact between the particles and the matrix is defect free with δ ¼ 1.0; λIm ¼ 1.0; λdI ¼ 0.001; λdm ¼ 0.001; and random close packing of uniform spheres, this model account for the decrease in the permeability in the nanocomposites as displayed in Fig. 9. If we further assume that any change in the particle morphology is associated with changes in ϕm, the effect of particle aspect ratio can be studied by decreasing this parameter to 0.5 or 0.4 [55]. Fig. 9 shows that under this model, the effect of particle aspect ratio is low confirming that the changes observed in Fig. 8 between the different GO arise from an interface phenomenon rather than from changes in the particle morphology. To further confirm this hypothesis, the model was tested assuming a polymer/particle interface with δ ¼ 1.05; λIm ¼ 100; λdI ¼ 0.001; λdm ¼ 0.1; and random close packing of uniform spheres, as displayed in Fig. 9. Under these conditions the model is able to account for the increase in the permeability. Similar to the previous conditions, the particle aspect ratio is not a relevant parameter for the behavior of the composites. Therefore, this model shows that depending on the GO filler, changes in the polymer/particle interface can occur. Noteworthy, the presence of this interface in LLPE/GO-L composites can further explain the low elongation at break as compared with LLDPE/GO-HS composites. Despite the effect of both particle concentration and type of GO on the oxygen permeability of LLDPE films, the water vapor transition rate is un-changed by the presence of the nanofiller as displayed in Fig. 8b. In general, a lower effect of GO nanofillers on water vapor diffusion is expected as compared with oxygen diffusion [32]. This change is attributed to the different absorption behavior of water molecules on the polymer matrix and GO fillers, being an easier process in the latter case. This higher absorption on GO can increase the solubility coefficient of this specie compensating the lower diffusion by tortuosity meaning lower water permeability in the nanocomposite.
3.5. Antibacterial behavior Based on the previous results regarding the gas barrier and me chanical behavior, LLDPE composites with GO-LS and GO-HS fillers were selected for testing the antimicrobial behaviour against Salmonella typhi and Listeria monocytogenes bacteria, as they are commun microor ganisms found in food associated infections. The former is the most frequently isolated foodborne pathogen, and are predominantly found in poultry, eggs and dairy products [56]. The latter is a human bacterial pathogen which causes listeriosis and its sources of infection are mainly associated with raw food and working surfaces in food-processing plants [57]. Fig. 10 displays the relative bacteria reduction after contact with the composites as compared with the pure LLDPE after 6 and 24 h of exposition. These results show that the presence of GO particles can render an antimicrobial effect to the polymer matrix depending on the amount and type of filler. Although after a statistical analysis a clear effect of exposition time, filler type and its concentration, is not directly observed, some conclusions can be obtained. In particular, independent on the exposition time, composites having 5 wt% of GO-LS and 3 wt% of GO-HS present antimicrobial effect against Salmonella typhi while com posites with 1 and 3 wt% of GO-LS and 3 wt% of GO-H present antimi crobial effect against Listeria monocytogenes. The other composites present antimicrobial activity but without statistical significant. Isolate GO nanoparticles present a high antibacterial behaviour as reported elsewhere. The mechanism is based on: 1) GO adsorption ability on bacteria due to its abundant functional groups and small size; 2) membrane disruption and damage after direct contact between GO and bacteria; and 3) cells death [58]. This antimicrobial effect is asso ciated with several processes triggered by GO, such as physical damage to bacterial membranes and leakage of intracellular material, due to the sharp edges of GO [58]. The production of reactive oxygen species (ROS) produced by the oxidative stress of GO also contributes to bacterial inactivation mainly when GO penetrates the bacteria [58]. The anti microbial behavior of GO depended of several variables such as test conditions, particle lateral size, number of layers, and oxidation level, explaining the different tendencies found regarding the effect of GO [26]. For instance, regarding the effect of GO on the bacteria membrane, antimicrobial surfaces made of reduced GO show higher destruction than GO due to the high amount of edges and efficient charge transfer increasing the disruption of the membrane [27]. In GO particles as similar as our fillers, a higher antimicrobial behavior was found in GO with low amount of functional groups and less disrupted layers, vali dating the antibacterial mechanisms based on a direct contact [39]. However, when GO particles are embedded in a thermoplastic polymer matrix the direct contact between the GO and the bacteria can be ruled out. A plausible mechanism in these polymer/GO films can arise from changes on the film surface due to the presence of the carbon nano particles such as polarity and roughness, both affecting the bacteria survival [28]. Another mechanism is based on the water/oxygen diffu sion into the polymer matrix allowing the specific chemical reaction with the nanoparticles, releasing biocide active agents to the film surface and killing the bacteria. This mechanism has been applied to understand the antimicrobial behavior of polymer/copper nanocomposites pre senting ion release after immersion in water and therefore biocide ac tivity depending the amount of nanofiller [59,60]. Similar mechanism can explain the antimicrobial behavior of polymer composites with photoactive TiO2 nanoparticles [61]. In our case, water/oxygen diffu sion into the polymer composites can trigger ROS generation from GO nanofillers able to reduce bacterial survival after diffusion-out.
Fig. 9. Effect of the GO filler fraction as predicted by a modified Felske model (Equation (3)) under two polymer/particle conditions: a) defect free contact between the particles and the matrix assuming δ ¼ 1.0; λIm ¼ 1.0; λdI ¼ 0.001; λdm ¼ 0.001; and random close packing of uniform spheres (open figures); and b) presence of a polymer/particle interface assuming δ ¼ 1.05; λIm ¼ 100; λdI ¼ 0.001; λdm ¼ 0.1; and random close packing of uniform spheres (closed figures).
4. Conclusion The addition of GO into a LLDPE matrix can be used as a proper route to design novel multifunctional polymer nanocomposites presenting improved mechanical, thermal, barrier and antimicrobial behaviours. The effect of the filler highly depended on the characteristics of the GO, 7
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Fig. 10. Relative bacteria reduction of the composite surfaces as compared with the pure LLDPE sample: a) 6 h of contact against Salmonella typhi strain; b) 6 h of contact against Listeria monocytogenes strain; c) 24 h of contact against Salmonella typhi strain; and d) 24 h of contact against Listeria monocytogenes strain. *: Sta tistically significant difference between experiment groups with pure LLDPE at the same time (p < 0.05).
in particular the oxidation level and sonication process. In particular, PE/GO-HS composites presented the highest Young’s modulus as compared with the pure polymer matrix without altering the elongation at break. The presence of GO further increased the thermal degradation temperatures of the polymer under both nitrogen and oxygen atmo spheres. Noteworthy, depending of the filler content and kind of GO, the permeability of oxygen can either decrease or increase as explained by a modified Felske model. Additionally, the material presented antimi crobial activity against two of the most relevant bacteria strains in food processing. All these results open up the possibility to use GO nanofillers for the development of active films for packaging applications.
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