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poly-(butylene adipate-co-terephthalate) nanocomposite film with enhanced gas and water vapor barrier properties
Polymer Testing 58 (2017) 173e180
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Biodegradable graphene oxide nanosheets/poly-(butylene adipate-coterephthalate) nanocomposite film with enhanced gas and water vapor barrier properties Peng-Gang Ren a, *, Xiao-Hui Liu a
, Fang Ren a, Gan-Ji Zhong b, Xu Ji c, Ling Xu d, **
a
Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi'an University of Technology, Xi'an, Shanxi 710048, People's Republic of China College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, People's Republic of China c College of Chemical Engineering, Sichuan University, Chengdu 610065, People's Republic of China d State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, People's Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2016 Received in revised form 3 December 2016 Accepted 17 December 2016 Available online 23 December 2016
Poly-(butylene adipate-co-terephthalate) (PBAT) has captured significant interest by dint of its biodegradability, superb ductility, promising processing properties and good final properties, but the insufficient barrier performance limits its application, especially in packaging field. In the present work, improved barrier properties of PBAT films were obtained by introducing an extremely low amount of graphene oxide nanosheets (GONS). O2 and water vapor permeability coefficients were decreased by more than 70% and 36% at the GONS loading of 0.35 vol%, respectively. The enhanced barrier performance was ascribed to the outstanding impermeability and well dispersion of GONS as well as the strong interfacial adhesion between GONS and PBAT matrix. Furthermore, tensile strength and Young's modulus of GONS/PBAT nanocomposite rise up to 27.8 MPa and 72.2 MPa from 24.6 MPa to 58.5 MPa of neat PBAT, respectively, showing a prominent increase of mechanical properties compared to neat PBAT. The incorporation of GONS also endowed PBAT matrix with an excellent thermal stability. These findings provide a significant guidance for fabricating high barrier films on a large scale. © 2016 Elsevier Ltd. All rights reserved.
Keywords: PBAT Graphene oxide nanosheets Gas barrier Mechanical properties thermal stability
1. Introduction The widespread use of polymer films as food packaging is due to the advance that they are chemically and mechanically resistant, lightweight, heat-solderable [1,2]. However, overwhelming majority of polymeric packaging films (e.g., polyethylene, polypropylene, polystyrene, poly(ethylene terephthalate),etc.) are produced from non-biodegradable fossil fuels. The use of biodegradable packages propagates aggressively due to the growing need to minimize the carbon footprint in the environment [3e5]. Various biodegradable polymeric materials have been successfully developed and put into applications [6e11]. Among them, aliphatic/aromatic copolymers are thought as one of the most important series because the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P.-G. Ren), [email protected] (L. Xu). http://dx.doi.org/10.1016/j.polymertesting.2016.12.022 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
synthetic biopolymers, in general, offer greater predominance over natural ones as they can be tailored to give a wider range of properties than materials from natural resources [12e20]. For instance, copolyesters of poly(butylene adipate-co-terephthalate) (PBAT) mainly derived from 1,4-butanediol, adipic acid, and terephthalic acid have been commercialized, and a tunable balance between the biodegradation and desirable physical properties for industrial applications has been spectacularly achieved [21e23]. In addition, excellent softness and ductility of make PBAT suitable for food packaging and agricultural mulch [11,24,25]. Nevertheless, the insufficient gas barrier properties of PBAT pose one serious technical challenge for the use as oxygen-sensitive and perishable commodities. To be specific, its barrier properties including both O2 and water vapor barrier properties are seriously inferior to traditional fossil fuel films, such as polyethylene, polypropylene, polystyrene and poly(vinyl chloride) [26]. Therefore, an imperative task is to enhance the gas barrier performance of PBAT so as to make it competitive with the existing current petroleum-based polymers in
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the packaging field [27e32]. Graphene nanosheets (GNS), a monolayer of carbon atoms arranged in honeycomb networks, is found as the thinnest and strongest two dimensional material [33,34]. Due to unique graphitized planar structure, such as the extremely high specific surface area and large aspect ratios, GNS is well demonstrated to be highly effective barrier performance enhancers for carbon-based nanocomposites [35e37]. The GNS are generally recognized as “nanobarrier walls”, generating increased tortuosity for gas molecular transport, which remarkably improves barrier properties of polymer films. The tortuosity of penetration path for diffusing molecules is principally influenced by the volume fraction and morphologies (i.e., aspect ratio, exfoliation, dispersion, orientation, etc.) of GNS in the matrix, as well as the interfacial interaction between fillers and polymer matrix. Complete exfoliation and uniform dispersion of GNS is desired to construct expected torturous paths to give rise to better barrier properties of GNS/ polymer nanocomposites. Nevertheless, GNS is prone to agglomerate, which is detrimental to its uniform dispersion in polymer matrix. In contrast to GNS, graphene oxide nanosheets (GONS) is basically graphene bearing epoxide, hydroxyl, carboxyl groups and ketones, 6-membered lactol rings [38,39]. These chemical functional groups endow GONS with good interfacial interaction with polar polymers, which promotes complete exfoliation and homogeneous dispersion of GONS and improves the interfacial bonding notably. Hence, GONS are considered as forward-looking candidates for preparing high barrier polar polymer-based nanocomposite films [40e44]. For instance, Geim et al. prepared pure GONS membranes with a pronounced layered structure, revealing a complete impermeability to liquids, vapors, and gases, including helium [43]. With the addition of only 0.1 wt% GONS, the watervapor-transmission-rate of polyimide nanocomposite films was significantly reduced from 181 to 30 g mil m2 day1 [44]. In our previous work, a remarkable improvement on barrier properties of GONS/poly(viny lalcohol) nanocomposite films was successfully obtained, wherein O2 and water vapor permeability coefficients of poly(viny lalcohol) film were respectively decreased by about 98% and 68% at a GONS loading of 0.72 vol% [40]. The main objective of this study was to achieve an overall promotion of the barrier and mechanical properties of GONS/PBAT nanocomposites and to establish the relationship between microstructure and the performances of the nanocomposite. In the current study, a set of PBAT nanocomposites with low GONS loadings were fabricated through solution blending, wherein GONS were fully exfoliated and uniformly dispersed in the PBAT matrix. It was intriguing to found that O2 and H2O permeability coefficients were decreased by about 70% and 36%, respectively. Furthermore, the addition of low GONS loadings brought a prominent increase of 23% in the tensile modulus. These results could be ascribed to excellent barrier properties of GONS, its well exfoliation, uniform dispersion and strong interfacial adhesion between GONS and PBAT matrix. 2. Materials and methods 2.1. Materials The biodegradable polymer, PBAT, was purchased from Chemical Company BASF (Germany) under the trade name of Ecoflex®-F. It possesses a density of 1.31 g/cm3, a melt index of 3.5 g/10 min (190 C/2.16 kg), and a glass transition and melting temperature of about 30 C and 110e120 C (DSC analysis), respectively. The modified “Hummers” method was adopted to prepare GONS from expandable graphite, which was purchased from Qingdao Haida Graphite Co., Ltd., China with an expansion rate of 200 ml/g. Details of preparation process were reported in our previous work [45].
Anhydrous N, N-dimethyl formamide (DMF) was purchased from Chengdu Kelong Chemical Reagent Factory, Chengdu, China. Other reagents were of analytical grade and directly used without further purification.
2.2. Preparation of nanocomposite films Solution coagulation was employed to prepare a series of PBAT nanocomposite films containing various GONS loadings of 0, 0.1, 0.25 and 0.5 wt%. Taking the 0.25 wt% GONS/PBAT nanocomposite as an example. Adding 10 g of PBAT granules into about 200 ml of DMF solution with the aid of mild stirring for 30 min at 100 C. 25 mg of GONS was dispersed into 250 ml of DMF solution with vigorous agitation and ultrasonic treatment for 2 h at room temperature, forming a stable and uniform GONS/DMF suspension. The transparent PBAT/DMF solution then mixed with the above GONS/ DMF suspension for 15 min at 100 C under vigorous agitation. Upon completion, the homogeneous GONS/PBAT slurry was immediately added into a large amount of vigorously stirred water and the coagulated materials precipitated continuously. Thereafter, the coagulations were isolated via filtration, washed with water, left in a drying oven at 60 C to remove solvents, and further dried in a vacuum oven overnight at 60 C. Finally, the composite powders were compression molded at 160 C under a fixed pressure of 10 MPa. For comparison purposes, neat PBAT was prepared according to the same procedures. For the convenience of calculation, the weight content of GONS in the nanocomposites was converted to volume content by using the density of PBAT matrix and GONS as 1.31 and 1.80 g/cm3, respectively [24]. Thus, the volume content of GONS incorporated into the PBAT matrix can be obtained as 0, 0.07, 0.18 and 0.35 vol%, respectively.
2.3. Characterization and measurement Typical tapping-mode atomic force microscopy (AFM) measurement was performed using Nanoscope Multimode & Explore atomic force microscope (Veeco Instruments, USA) to present thickness and surface morphology of GONS. Samples for AFM images were prepared by depositing dispersion of GONS in DMF on a fresh mica substrate and allowing them to dry in air. To study the morphology of GONS in PBAT matrix, the nanocomposite samples were firstly cryo-fractured in liquid nitrogen, then the surfaces sputter-coated with gold were observed on a field emission scanning electron microscopy (SEM) (Inspect F, FEI, Finland) with an accelerated voltage of 10 kV. Two-dimensional wide angle X-ray diffraction (2D-WAXD) determination was carried out at the beamline BL15U1 of SSRF (Shanghai, China). The monochromated X-ray beam with a wavelength of 0.124 nm was focused to an area of 3 2.7 mm2 (length width), and the sample-to-detector distance was set as 173 mm. After 90 s exposure to the X-ray for the film samples (~100 mm), the 2D-WAXD images were collected with an X-ray CCD detector (Model SX165, Rayonix Co. Ltd, USA). Additionally, the WAXD intensity profiles for each 2q were obtained by integration in the azimuthal angular range of a whole circle (0360 ) from the sample patterns employing the Fit 2D package, while background scattering was subtracted from the sample patterns. Crystallization behavior was investigated by differential scanning calorimetry (DSC) on a TA Q2000 instrument. The samples (around 5e6 mg) were heated from 40 C to 200 C at a heating rate of 10 C/min under nitrogen atmosphere. Crystallinity (cc ) of all the samples can be calculated as follows:
P.-G. Ren et al. / Polymer Testing 58 (2017) 173e180 Table 1 Main characteristics of GONS/PBAT composite films. GONS loading (vol %)
Film Thickness (mm)
0 0.07 0.18 0.35
50 50 50 50
cc ¼
DН m DН 0
± ± ± ±
5 5 5 5
D1 (mm)
D2 (mm)
100 100 100 100
60 60 60 60
(1)
where DН 0 represents the melting enthalpy of 100% crystalline PBAT (114 J/g) [46], DН m is melting enthalpy measured via DSC. Thermogravimetric analysis (TGA) was carried out on a NETZSCH 209F1 instrument at a heating rate of 10 C/min under the protection of a nitrogen flow. The oxygen permeability (PO2) of neat PBAT and GONS/PBAT nanocomposites was determined at room temperature (23 ± 1 C) with 50% relative humidity on a VAC-V2 film permeability testing machine (labthink instruments, Jinan, China) according to ISO2556:1974. Oxygen gas with a purity of >99.99gcq9% was used. The gas permeation cell was separated in to two compartments by film specimens with 100 mm in diameter. Air in both compartments was continuously evacuated at least 12 h prior to testing, ensuring that the static vacuum pressure changes in the downstream compartment were smaller than the pressure changes due to the gas diffusion. Subsequently, the gases were filled in the upstream compartment at a pressure of about 1.01 105 Pa. The pressure variations in the downstream compartment were recorded as a function of time with pressure sensors. And in order to facilitate the comparisons of gas permeability coefficients that fluctuate somewhat in thickness, the time (t) was normalized to the thickness of the sample (L). As a result, the curves of the pressure and the reduced time (t/L) were obtained. Permeability coefficient of the composite films was calculated by an analysis program [40]. The water vapor transfer ratio (WVTR) of GONS/PBAT nanocomposite films was measured gravimetrically using a TSY-T1 moisture permeability testing machine (Jinan Languang Mechanical and Electrical Technology Co., Ltd., China) at room temperature and 100% relative humidity according to ISO2556:1974. A water transmission container was separated by film specimens into a dry chamber and a saturated chamber kept a constant saturated vapor pressure. With saturated water vapour entering the dry chamber through film specimens, the weight of distilled water in the water transmission container was detected by
175
microprocessor at real time to calculate the water vapor transfer ratio of film specimens. For each sample, measurements were repeated until four successive readings deviated <5% from the average value. According to ASTM standard D638: 1999, the tensile properties were measured at room temperature on an Instron universal test instrument (Model 5576, Instron Instruments, USA) with a crosshead speed of 20 mm/min and a gauge length of 20 mm. A minimum of 6 bars for each sample were tested at the same conditions, and the average values were presented with standard deviations. The main characteristics of GONS/PBAT composite films were listed in Table 1. D1 and D2 represents diameter of films for O2 and H2O permeability measurement, respectively.
3. Results and discussion 3.1. Dispersion of GONS A typical AFM image of GONS is shown in Fig. 1. The thickness of GONS is about 1 nm, which is characteristic of a fully exfoliated GONS and in good agreement with the result of Stankovich et al. [47] A single-layer GONS is expected to be thicker than pristine GNS, due to the oxygen-containing functional groups attached on both sides of graphene nanosheet and to the sp3-hybridized carbon atoms generated on the original graphene plane [38,48,49]. To access the dispersion state of GONS in the PBAT matrix, SEM and WAXD measurements were carried out. As displayed in the SEM image of 0.35 vol % GONS/PBAT film (Fig. 2a), a rough fractured surface exists and GONS marked by arrows is by and large individually exfoliated and dispersed in the PBAT matrix with no indication of aggregates, which is in contrast with the smooth fractured surface of the neat PBAT film (Fig. 2c). Fig. 2b is the high resolution magnitude of circular region in Fig. 2a. It is evident that wrinkled paper-like structure of GONS spreads out in the polymer matrix. This phenomenon can be ascribed to the strong compatibility and/or strong interfacial adhesion between GONS and PBAT matrix because of hydrogen bonds between the oxygen-containing functional groups of GONS layers and ester groups of PBAT chains, and the mechanical interlocking resulting from the special wrinkled structure of GONS [50]. Fig. 3 illustrates representative 2DWAXD patterns and the corresponding 1D-WAXD curves of graphite oxide, neat PBAT, and GONS/PBAT nanocomposites. As seen in Fig. 3a, the homogeneous diffraction rings of neat PBAT and GONS/PBAT nanocomposites suggest that PBAT crystals are randomly distributed in the nanocomposites and GONS exerts little impact on the crystalline modification of PBAT matrix. In Fig. 3b the
Fig. 1. A typical tapping-mode atomic force microscopy (AFM) image of GONS.
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Fig. 2. Typical SEM images for the fractured surfaces of 0.35 vol % GONS/PBAT film with (a) low and (b) high magnification and neat PBAT film (c).
characteristic diffraction peak of graphite oxide sheets is observed at 2q ¼ 8.4 , corresponding to a layer-to-layer distance of 0.79 nm [51], much larger than that of pristine GNS (~0.34 nm), indicating adequate oxidation of GNS [52]. One can also acquire from Fig. 3b that the nanocomposites with different GONS loadings exhibit the same 1D-WAXD curves as the neat PBAT with the absence of characteristic peak of graphite oxide. It implies that GONS were fully exfoliated into individual GONS in the PBAT matrix, which is good accordance with the AFM and SEM results (Figs. 1 and 2). Moreover, the intensity of these diffraction peaks basically keeps consistent, suggesting crystallinity of the nanocomposites has hardly changed through the addition of GONS. 3.2. Barrier properties of PBAT/GONS nanocomposite films With the complete exfoliation and uniform dispersion of layered GONS throughout PBAT matrix, GONS with outstanding barrier properties are adequately reflected to enhance barrier properties of GONS/PBAT nanocomposite films by a wide margin. As shown in Fig. 4, the O2 permeability (PO2) of neat PBAT film is effectively restricted with the addition of GONS. To be specific, about 70% reduction in PO2 from 7.06 1014 to 2.15 1014 cm3 cm cm2 s1 Pa-1 is obtained by adding only 0.35 vol% GONS. The advent of impermeable GONS in PBAT matrix gives rise to the increasing path tortuosity and the decreasing cross section for O2 permeability. In addition, O2 does not always diffuse along the direction perpendicular to film and it may change its original permeability path from vertical to horizontal direction. However, the distance between GONS is much smaller than thickness of nanocomposite film, bringing about dramatic decrease of cross section for O2
permeability along the surface of film. In this work, the remarkable improvement in O2 barrier properties of PBAT films at such a low GONS loading is impressive, which demonstrates that GONS/PBAT nanocomposite films hold the key as a packaging material for protecting vulnerable to O2 degradation of perishable goods. In addition, owing to the hydrophilic groups on both GONS and PBAT molecular chains, the water vapor barrier properties of neat PBAT film are negatively interfered. Strikingly, water vapor barrier property of neat PBAT is also improved by the inclusion of GONS in this work. Direct access could be found from in Fig. 5 that the water vapor permeability of PBAT film is effectively reduced with the addition of GONS. To be specific, the addition of only 0.35 vol% GONS gives rise to about 36% reduction in H2O permeability (PH2O) from 3.98 1013 to 2.53 1013 g mm mm2 s-1 Pa-1, which further confirms that GONS are desirable nanoplatelets to enhance barrier properties of PBAT film. And the significant decline of PH2O improves the independence of barrier properties of PBAT film on environment humidity, further expands the application of PBAT film in the field of packaging and also beneficial for the agricultural film to keep moisture. According to the reported research, GONS is evidently superior to whether pristine or organo-modified clay as barrier enhancers, owing to their molecular-level dispersion and higher aspect ratio [33e35]. For GONS/PBAT nanocomposite films, moderate volume fraction (avoiding agglomeration in case of high concentration), well exfoliation and dispersion, and aspect ratio of GONS construct robust “barrier walls” to the diffusing gas molecules, conferring the highest promotion in the gas permeability resistance.
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Fig. 5. Coefficient of water vapour permeability (PH2O) for neat PBAT and its nanocomposite films as a function of GONS loadings: (a) neat PBAT, (b) 0.07 vol% GONS/ PBAT, (c) 0.18 vol% GONS/PBAT, (d) 0.35 vol% GONS/PBAT.
Fig. 6. DSC curves of neat PBAT and its GONS nanocomposite films. Fig. 3. Representative 2D-WAXD patterns (a) and the corresponding 1D-WAXD curves (b) of graphite oxide, neat PBAT and its nanocomposites with various GONS loadings.
Fig. 4. Permeability coefficient of O2 (PO2) for neat PBAT and its nanocomposite films as a function of GONS loadings: (a) neat PBAT, (b) 0.07 vol% GONS/PBAT, (c) 0.18 vol% GONS/PBAT, (d) 0.35 vol% GONS/PBAT.
3.3. Thermal behaviors of neat PBAT and its GONS nanocomposites It is well accepted that barrier properties of polymer incorporated with nanoplatelets is not only affected by the impermeable nanoplatelets, but also greatly depends on its crystallinity and crystalline morphology [36e38]. Thus, thermal behaviors of GONS/ PBAT nanocomposites are studied. The DSC heating curves of neat PBAT and its GONS nanocomposite films are shown in Fig. 6. The melting enthalpy (DHm), melting temperatures (Tm) and crystallinity (cc) are tabulated in Table 2. Tm gradually shifts to higher
Table 2 Melting enthalpy (DHm), and melting temperature (Tm), and crystallinity (cc) of neat PBAT and GONS/PBAT nanocomposite films with different GONS loadings. GONS loading (vol %)
DHm (J g1)
Tm ( C)
cc (%)
0 0.07 0.18 0.35
7.8 6.9 10.6 8.5
118.9 127.1 127.6 128.7
6.8 6.1 9.3 7.5
temperature region from 118.9 to 128.7 C, however, DHm changes in a very subtle way, ranging from 6.9 to 10.6 J g1. Meanwhile, the crystallinity (cc ) of all samples varies slightly from 6.1 to 9.3%. Therefore, it is reasonable to conjecture that the remarkable enhancement on barrier properties of GONS/PBAT nanocomposites are primarily attributed to the impermeability GONS, their complete exfoliation and strong interfacial adhesion between GONS and PBAT matrix, considering that crystallinity and crystalline structure of PBAT matrix are hardly changed. 3.4. Mechanism of the enhanced gas barrier properties by GONS According to the above analysis, the enhanced gas barrier properties of GONS/PBAT nanocomposite films are mainly assigned to the contribution of GONS. It is generally accepted that with the presence of nanoplatelets, the enhanced barrier properties of polymer-based nanocomposites are mainly due to the “torturous path effect” [36]. And there are three main factors that affect the tortuosity of penetration path for diffusing molecules: the volume fraction of the nanoplatelets; their morphology (i.e., exfoliation,
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Fig. 7. Comparison between experimental data and Bharadwaj model for Rp in terms of level of GONS loadings.
Fig. 9. Mechanical properties of neat PBAT and its nanocomposites with different GONS loadings. (A) typical strain-stress curves for (a) neat PBAT, (b) 0.07 vol% GONS/ PBAT, (c) 0.18 vol% GONS/PBAT, (d) 0.35 vol% GONS/PBAT; (B) Young's Modulus of the above samples.
Rp ¼
Fig. 8. TGA curves of neat PBAT and its nanocomposites with different GONS loadings.
dispersion and orientation relative to the diffusion direction); and their aspect ratio. Herein, in order to further quantitatively evaluate the effect of GONS on barrier properties of GONS/PBAT nanocomposite films, the decrease in relative permeability (Rp ) is estimated and the comparison between experimental data and Bharadwaj model [39] is plotted in Fig. 7. Rp is defined as follows:
Rp ¼
Pc Pm
(2)
where Pc and Pm are gas permeability coefficients of GONS/PBAT nanocomposites and pristine PBAT matrix, respectively. Compared with the previous theoretical models frequently assuming that the impermeable nanoplatelets are regularly arranged in a parallel array with their main direction perpendicular to the diffusion direction [42,43], the orientational order parameter (f ) is introduced to modify the conventional model. The correspondingRp can be given by
14 1 þ 4ða=2Þð2=3Þðf þ 0:5Þ
(3)
where 4 is the volume content of nanoplatelets and a is the aspect ratio of nanoplatelets, which was roughly estimated to be about 1080 from the AFM image (Fig. 1).f ¼ 0 is indicative of randomly dispersed nanoplatelets in polymer matrix, while the nanoplatelets are well-ordered oriented array through the entire polymer matrix when f ¼ 1 and the above expression of Eq. (3) returns to that of the classic Nielson model [43]. As shown in Fig. 7, our experimental data about the reduction in Rp lie randomly between the predicted Bharadwaj values in the case f ¼ 0 and f ¼ 1, which suggest that GONS is apt to randomly disperse throughout the PBAT matrix. Theoretically, perfect orientation of nanoplatelets will maximally elevate the gas barrier properties of polymer-based nanocomposites. They are frequently obtained through special processing techniques, which may typically time- and energyconsuming. Nevertheless, in the conventional nanocomposite industry the nanoplatelets are prone to reveal randomly dispersed morphology. The new insight into the contribution of the homogeneous dispersed nanoplatelets to the gas properties of polymerbased nanocomposites provides a significant guidance to manufacture high barrier polymeric packaging materials on a large scale.
3.5. Thermal stability of neat PBAT and its GONS nanocomposites TGA was conducted to study the thermal stability of neat PBAT and its GONS nanocomposites under nitrogen atmosphere. Fig. 8 shows that the initial degradation temperature (Ti) of the neat PBAT at 5% weight loss takes place at 361.7 C when subjected to
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Table 3 Mechanical properties of neat PBAT and its nanocomposites with varying GONS loadings. GONS loading (vol %)
Yield Strength (MPa)
0 0.07 0.18 0.35
26.8 26.7 26.2 25.9
± ± ± ±
1.2 2.6 2.2 1.7
one-step degradation. As for the nanocomposites, Ti gradually shifts to higher temperature with increasing GONS loadings. Adding only 0.35 vol% GONS, Ti is increased by 5.0 C (366.7 C), retarding the degradation of the PBAT host. The distinctly enhanced thermal stability of the nanocomposites should mainly originate from the excellent barrier properties of GONS. Gas permeability through a polymer filled with a high aspect ratio, impermeable flakes can be decreased substantially via a reduced cross section for gas diffusion and a tortuous path mechanism, which makes decomposed gas difficult to escape and finally contributing to improved thermal stability [53e55]. Meanwhile, the strong interfacial adhesion between GONS and PBAT matrix can also restrict the mobility of PBAT chains in the vicinity of the GONS surface, giving rise to the remarkable thermal stability of PBAT matrix [56]. 3.6. Mechanical properties of neat PBAT and its GONS nanocomposites Fig. 9(A) is the typical strain-stress curves of neat PBAT and its GONS nanocomposites and bar chart in Fig. 9(B) showing the values of Young's Modulus. Detailed tensile results regarding tensile strength, Young's modulus, elongation at break are summarized in Table 3. Compared to the value of 24.6 MPa and 1100% for neat PBAT, the incorporation of GONS is found to give an increment of tensile strength and elongation at break and finally climb up to 27.8 MPa and 1197% for 0.35 vol% GONS/PBAT. Moreover, GONS/ PBAT nanocomposites exhibits a noteworthy enhancement in Young's Modulus, where 0.35 vol% GONS/PBAT achieves the profoundly increased modulus of 72.2 MPa in comparison with neat PBAT of 58.5 MPa, with an increment of 23.4%. The remarkable improvement of mechanical properties can be appraised from the good dispersion of GONS with high modulus and high aspect ratio and also from the strong interfacial adhesion between GONS and PBAT matrix. The incorporation of GONS can speed up PBAT performance and thus boost the attractiveness and broadens application of this biodegradable polymer. 4. Conclusions In the present work, GONS/PBAT nanocomposites with different GONS loadings ranging from 0.07 to 0.35 vol% were fabricated. SEM and 2D-WAXD techniques showed that GONS were fully exfoliated and well dispersed in the PBAT matrix. Furthermore, a low concentration of GONS had significant effect on the barrier properties, thermal stability, and mechanical properties of PBAT films. And when adding only 0.35 vol% GONS, O2 and H2O permeability coefficients of GONS/PBAT nanocomposite films decreased about 70% and 36%, respectively; Ti of the composite film was increased by 5.0 C in comparison with neat PBAT; Tensile Strength rises up to 27.8 MPa from 24.6 MPa and Young's modulus climbs up to 72.2 MPa from 58.5 MPa, showing a prominent increase of mechanical properties compared to neat PBAT. The enhanced gas barrier performances were attributed to the impermeability of GONS, their complete exfoliation and well dispersion, and strong interfacial adhesion between GONS and PBAT matrix, instead of changes in crystallinity. The work presented here suggests that the
Young's Modulus (MPa) 58.5 59.7 66.0 72.2
± ± ± ±
3.4 2.9 1.2 2.3
Elongation at Break (%) 1187 ± 11 1142 ± 9 1044 ± 12 981 ± 6
GONS/PBAT nanocomposite films with excellent barrier properties can be potentially used in food, pharmaceutical, and electronics packaging. Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (grants 51403139, 51573147, 51273131, 51473101 and 51473102), the Innovation Team Program of Science and Technology Department of Sichuan Province (grant 2014TD0002). References [1] J. Lange, Y. Wyser, Recent innovations in barrier technologies for plastic packaging - a review, Packag. Technol. Sci. 16 (2003) 149e158. [2] K. Marsh, B. Bugusu, Food packagingdroles, materials, and environmental issues, J. Food Sci. 72 (2007) R39eR55. [3] E. Chiellini, R. Solaro, Biodegradable polymeric materials, Adv. Mater 8 (1996) 305e313. rous, Nano-biocomposites: biodegradable polyester/ [4] P. Bordes, E. Pollet, L. Ave nanoclay systems, Prog. Polym. Sci. 34 (2009) 125e155. [5] S.S. Ray, M. Bousmina, Biodegradable polymers and their layered silicate nanocomposites: in greening the 21st century materials world, Prog. Mater. Sci. 50 (2005) 962e1079. [6] R.W. Lenz, Biodegradable polymers, Adv. Polym. Sci. 107 (1993) 1e40. [7] W. Amass, A. Amass, B. Tighe, A review of biodegradable polymers: uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers and recent advances in biodegradation studies, Polym. Int. 47 (1998) 89e114. [8] K. Sudesh, H. Abe, Y. Doi, Synthesis, structure and properties of polyhydroxyalkanoates: biological polyesters, Prog. Polym. Sci. 25 (2000) 1503e1555. [9] G. Scott, Green polymers, Polym. Degrad. Stab. 68 (2000) 1e7. [10] M. Okada, Chemical synthesis of biodegradable polymers, Prog. Polym. Sci. 27 (2002) 87e133. [11] Y. Doi, K. Fukuda, Biodegradable Plastics and Polymers, Elsevier Science, Amsterdam, 1994. [12] Y. Maeda, T. Maeda, K. Yamaguchi, S. Kubota, A. Nakayama, N. Kawasaki, N. Yamamoto, S. Aiba, Synthesis and characterization of novel biodegradable copolyesters by transreaction of poly(ethylene terephthalate) with copoly(succinic anhydride/ethylene oxide), J. Polym. Sci. Part A Polym. Chem. 38 (2000) 4478e4489. [13] H.J. Jin, B.Y. Lee, M.N. Kim, J.S. Yoon, Properties and biodegradation of poly(ethylene adipate) and poly(buty-lenes succinate) containing styrene glycol units, Eur. Polym. J. 36 (2000) 2693e2698. [14] E. Rantze, I. Kleeberg, U. Witt, R.J. Müller, W.D. Deckwer, Aromatic components in copolyesters: model structures help to understand biodegradability, Maromol. Symp. 130 (1998) 319e326. [15] E.S. Yoo, S.S. Im, Y. Yoo, Morphology and degradation behavior of aliphatic/ aromatic copolyesters, Macromol. Symp. 118 (1997) 739e745. [16] U. Witt, R.J. Müller, W.D. Deckwer, Studies on sequence distribution of aliphatic/aromatic copolyesters by high-resolution 13C nuclear magnetic resonance spectroscopy for evaluation of biodegradability, Maromol. Chem. Phys. 197 (1996) 1525e1535. [17] U. Witt, R.J. Müller, W.D. Deckwer, New biodegradable polyester-copolymers from commodity chemicals with favorable use properties, J. Environ. Polym. Degrad. 3 (1995) 215e223. [18] H.C. Ki, O.O.K. Park, Synthesis, characterization and biodegradability of the biodegradable aliphaticearomatic random copolyesters, Polymer 42 (2001) 1849e1861. [19] R.J. Müller, I. Kleeberg, W.D. Deckwer, Biodegradation of polyesters containing aromatic constituents, J. Biotechnol. 86 (2001) 87e95. [20] U. Witt, R.J. Müller, W.D. Deckwer, Biodegradation behavior and material properties of aliphatic/aromatic polyesters of commercial importance, J. Environ. Polym. Degrad. 5 (1997) 81e89. [21] J.C. Middleton, A.J. Tipton, Synthetic biodegradable polymers as orthopedic devices, Biomaterials 21 (2000) 2335e2346. [22] K. Fukushima, M.H. Wu, S. Bocchini, A. Rasyida, M.C. Yang, PBAT based
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Material Properties
Biodegradable graphene oxide nanosheets/poly-(butylene adipate-coterephthalate) nanocomposite film with enhanced gas and water vapor barrier properties Peng-Gang Ren a, *, Xiao-Hui Liu a
, Fang Ren a, Gan-Ji Zhong b, Xu Ji c, Ling Xu d, **
a
Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi'an University of Technology, Xi'an, Shanxi 710048, People's Republic of China College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, People's Republic of China c College of Chemical Engineering, Sichuan University, Chengdu 610065, People's Republic of China d State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, People's Republic of China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2016 Received in revised form 3 December 2016 Accepted 17 December 2016 Available online 23 December 2016
Poly-(butylene adipate-co-terephthalate) (PBAT) has captured significant interest by dint of its biodegradability, superb ductility, promising processing properties and good final properties, but the insufficient barrier performance limits its application, especially in packaging field. In the present work, improved barrier properties of PBAT films were obtained by introducing an extremely low amount of graphene oxide nanosheets (GONS). O2 and water vapor permeability coefficients were decreased by more than 70% and 36% at the GONS loading of 0.35 vol%, respectively. The enhanced barrier performance was ascribed to the outstanding impermeability and well dispersion of GONS as well as the strong interfacial adhesion between GONS and PBAT matrix. Furthermore, tensile strength and Young's modulus of GONS/PBAT nanocomposite rise up to 27.8 MPa and 72.2 MPa from 24.6 MPa to 58.5 MPa of neat PBAT, respectively, showing a prominent increase of mechanical properties compared to neat PBAT. The incorporation of GONS also endowed PBAT matrix with an excellent thermal stability. These findings provide a significant guidance for fabricating high barrier films on a large scale. © 2016 Elsevier Ltd. All rights reserved.
Keywords: PBAT Graphene oxide nanosheets Gas barrier Mechanical properties thermal stability
1. Introduction The widespread use of polymer films as food packaging is due to the advance that they are chemically and mechanically resistant, lightweight, heat-solderable [1,2]. However, overwhelming majority of polymeric packaging films (e.g., polyethylene, polypropylene, polystyrene, poly(ethylene terephthalate),etc.) are produced from non-biodegradable fossil fuels. The use of biodegradable packages propagates aggressively due to the growing need to minimize the carbon footprint in the environment [3e5]. Various biodegradable polymeric materials have been successfully developed and put into applications [6e11]. Among them, aliphatic/aromatic copolymers are thought as one of the most important series because the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (P.-G. Ren), [email protected] (L. Xu). http://dx.doi.org/10.1016/j.polymertesting.2016.12.022 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
synthetic biopolymers, in general, offer greater predominance over natural ones as they can be tailored to give a wider range of properties than materials from natural resources [12e20]. For instance, copolyesters of poly(butylene adipate-co-terephthalate) (PBAT) mainly derived from 1,4-butanediol, adipic acid, and terephthalic acid have been commercialized, and a tunable balance between the biodegradation and desirable physical properties for industrial applications has been spectacularly achieved [21e23]. In addition, excellent softness and ductility of make PBAT suitable for food packaging and agricultural mulch [11,24,25]. Nevertheless, the insufficient gas barrier properties of PBAT pose one serious technical challenge for the use as oxygen-sensitive and perishable commodities. To be specific, its barrier properties including both O2 and water vapor barrier properties are seriously inferior to traditional fossil fuel films, such as polyethylene, polypropylene, polystyrene and poly(vinyl chloride) [26]. Therefore, an imperative task is to enhance the gas barrier performance of PBAT so as to make it competitive with the existing current petroleum-based polymers in
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the packaging field [27e32]. Graphene nanosheets (GNS), a monolayer of carbon atoms arranged in honeycomb networks, is found as the thinnest and strongest two dimensional material [33,34]. Due to unique graphitized planar structure, such as the extremely high specific surface area and large aspect ratios, GNS is well demonstrated to be highly effective barrier performance enhancers for carbon-based nanocomposites [35e37]. The GNS are generally recognized as “nanobarrier walls”, generating increased tortuosity for gas molecular transport, which remarkably improves barrier properties of polymer films. The tortuosity of penetration path for diffusing molecules is principally influenced by the volume fraction and morphologies (i.e., aspect ratio, exfoliation, dispersion, orientation, etc.) of GNS in the matrix, as well as the interfacial interaction between fillers and polymer matrix. Complete exfoliation and uniform dispersion of GNS is desired to construct expected torturous paths to give rise to better barrier properties of GNS/ polymer nanocomposites. Nevertheless, GNS is prone to agglomerate, which is detrimental to its uniform dispersion in polymer matrix. In contrast to GNS, graphene oxide nanosheets (GONS) is basically graphene bearing epoxide, hydroxyl, carboxyl groups and ketones, 6-membered lactol rings [38,39]. These chemical functional groups endow GONS with good interfacial interaction with polar polymers, which promotes complete exfoliation and homogeneous dispersion of GONS and improves the interfacial bonding notably. Hence, GONS are considered as forward-looking candidates for preparing high barrier polar polymer-based nanocomposite films [40e44]. For instance, Geim et al. prepared pure GONS membranes with a pronounced layered structure, revealing a complete impermeability to liquids, vapors, and gases, including helium [43]. With the addition of only 0.1 wt% GONS, the watervapor-transmission-rate of polyimide nanocomposite films was significantly reduced from 181 to 30 g mil m2 day1 [44]. In our previous work, a remarkable improvement on barrier properties of GONS/poly(viny lalcohol) nanocomposite films was successfully obtained, wherein O2 and water vapor permeability coefficients of poly(viny lalcohol) film were respectively decreased by about 98% and 68% at a GONS loading of 0.72 vol% [40]. The main objective of this study was to achieve an overall promotion of the barrier and mechanical properties of GONS/PBAT nanocomposites and to establish the relationship between microstructure and the performances of the nanocomposite. In the current study, a set of PBAT nanocomposites with low GONS loadings were fabricated through solution blending, wherein GONS were fully exfoliated and uniformly dispersed in the PBAT matrix. It was intriguing to found that O2 and H2O permeability coefficients were decreased by about 70% and 36%, respectively. Furthermore, the addition of low GONS loadings brought a prominent increase of 23% in the tensile modulus. These results could be ascribed to excellent barrier properties of GONS, its well exfoliation, uniform dispersion and strong interfacial adhesion between GONS and PBAT matrix. 2. Materials and methods 2.1. Materials The biodegradable polymer, PBAT, was purchased from Chemical Company BASF (Germany) under the trade name of Ecoflex®-F. It possesses a density of 1.31 g/cm3, a melt index of 3.5 g/10 min (190 C/2.16 kg), and a glass transition and melting temperature of about 30 C and 110e120 C (DSC analysis), respectively. The modified “Hummers” method was adopted to prepare GONS from expandable graphite, which was purchased from Qingdao Haida Graphite Co., Ltd., China with an expansion rate of 200 ml/g. Details of preparation process were reported in our previous work [45].
Anhydrous N, N-dimethyl formamide (DMF) was purchased from Chengdu Kelong Chemical Reagent Factory, Chengdu, China. Other reagents were of analytical grade and directly used without further purification.
2.2. Preparation of nanocomposite films Solution coagulation was employed to prepare a series of PBAT nanocomposite films containing various GONS loadings of 0, 0.1, 0.25 and 0.5 wt%. Taking the 0.25 wt% GONS/PBAT nanocomposite as an example. Adding 10 g of PBAT granules into about 200 ml of DMF solution with the aid of mild stirring for 30 min at 100 C. 25 mg of GONS was dispersed into 250 ml of DMF solution with vigorous agitation and ultrasonic treatment for 2 h at room temperature, forming a stable and uniform GONS/DMF suspension. The transparent PBAT/DMF solution then mixed with the above GONS/ DMF suspension for 15 min at 100 C under vigorous agitation. Upon completion, the homogeneous GONS/PBAT slurry was immediately added into a large amount of vigorously stirred water and the coagulated materials precipitated continuously. Thereafter, the coagulations were isolated via filtration, washed with water, left in a drying oven at 60 C to remove solvents, and further dried in a vacuum oven overnight at 60 C. Finally, the composite powders were compression molded at 160 C under a fixed pressure of 10 MPa. For comparison purposes, neat PBAT was prepared according to the same procedures. For the convenience of calculation, the weight content of GONS in the nanocomposites was converted to volume content by using the density of PBAT matrix and GONS as 1.31 and 1.80 g/cm3, respectively [24]. Thus, the volume content of GONS incorporated into the PBAT matrix can be obtained as 0, 0.07, 0.18 and 0.35 vol%, respectively.
2.3. Characterization and measurement Typical tapping-mode atomic force microscopy (AFM) measurement was performed using Nanoscope Multimode & Explore atomic force microscope (Veeco Instruments, USA) to present thickness and surface morphology of GONS. Samples for AFM images were prepared by depositing dispersion of GONS in DMF on a fresh mica substrate and allowing them to dry in air. To study the morphology of GONS in PBAT matrix, the nanocomposite samples were firstly cryo-fractured in liquid nitrogen, then the surfaces sputter-coated with gold were observed on a field emission scanning electron microscopy (SEM) (Inspect F, FEI, Finland) with an accelerated voltage of 10 kV. Two-dimensional wide angle X-ray diffraction (2D-WAXD) determination was carried out at the beamline BL15U1 of SSRF (Shanghai, China). The monochromated X-ray beam with a wavelength of 0.124 nm was focused to an area of 3 2.7 mm2 (length width), and the sample-to-detector distance was set as 173 mm. After 90 s exposure to the X-ray for the film samples (~100 mm), the 2D-WAXD images were collected with an X-ray CCD detector (Model SX165, Rayonix Co. Ltd, USA). Additionally, the WAXD intensity profiles for each 2q were obtained by integration in the azimuthal angular range of a whole circle (0360 ) from the sample patterns employing the Fit 2D package, while background scattering was subtracted from the sample patterns. Crystallization behavior was investigated by differential scanning calorimetry (DSC) on a TA Q2000 instrument. The samples (around 5e6 mg) were heated from 40 C to 200 C at a heating rate of 10 C/min under nitrogen atmosphere. Crystallinity (cc ) of all the samples can be calculated as follows:
P.-G. Ren et al. / Polymer Testing 58 (2017) 173e180 Table 1 Main characteristics of GONS/PBAT composite films. GONS loading (vol %)
Film Thickness (mm)
0 0.07 0.18 0.35
50 50 50 50
cc ¼
DН m DН 0
± ± ± ±
5 5 5 5
D1 (mm)
D2 (mm)
100 100 100 100
60 60 60 60
(1)
where DН 0 represents the melting enthalpy of 100% crystalline PBAT (114 J/g) [46], DН m is melting enthalpy measured via DSC. Thermogravimetric analysis (TGA) was carried out on a NETZSCH 209F1 instrument at a heating rate of 10 C/min under the protection of a nitrogen flow. The oxygen permeability (PO2) of neat PBAT and GONS/PBAT nanocomposites was determined at room temperature (23 ± 1 C) with 50% relative humidity on a VAC-V2 film permeability testing machine (labthink instruments, Jinan, China) according to ISO2556:1974. Oxygen gas with a purity of >99.99gcq9% was used. The gas permeation cell was separated in to two compartments by film specimens with 100 mm in diameter. Air in both compartments was continuously evacuated at least 12 h prior to testing, ensuring that the static vacuum pressure changes in the downstream compartment were smaller than the pressure changes due to the gas diffusion. Subsequently, the gases were filled in the upstream compartment at a pressure of about 1.01 105 Pa. The pressure variations in the downstream compartment were recorded as a function of time with pressure sensors. And in order to facilitate the comparisons of gas permeability coefficients that fluctuate somewhat in thickness, the time (t) was normalized to the thickness of the sample (L). As a result, the curves of the pressure and the reduced time (t/L) were obtained. Permeability coefficient of the composite films was calculated by an analysis program [40]. The water vapor transfer ratio (WVTR) of GONS/PBAT nanocomposite films was measured gravimetrically using a TSY-T1 moisture permeability testing machine (Jinan Languang Mechanical and Electrical Technology Co., Ltd., China) at room temperature and 100% relative humidity according to ISO2556:1974. A water transmission container was separated by film specimens into a dry chamber and a saturated chamber kept a constant saturated vapor pressure. With saturated water vapour entering the dry chamber through film specimens, the weight of distilled water in the water transmission container was detected by
175
microprocessor at real time to calculate the water vapor transfer ratio of film specimens. For each sample, measurements were repeated until four successive readings deviated <5% from the average value. According to ASTM standard D638: 1999, the tensile properties were measured at room temperature on an Instron universal test instrument (Model 5576, Instron Instruments, USA) with a crosshead speed of 20 mm/min and a gauge length of 20 mm. A minimum of 6 bars for each sample were tested at the same conditions, and the average values were presented with standard deviations. The main characteristics of GONS/PBAT composite films were listed in Table 1. D1 and D2 represents diameter of films for O2 and H2O permeability measurement, respectively.
3. Results and discussion 3.1. Dispersion of GONS A typical AFM image of GONS is shown in Fig. 1. The thickness of GONS is about 1 nm, which is characteristic of a fully exfoliated GONS and in good agreement with the result of Stankovich et al. [47] A single-layer GONS is expected to be thicker than pristine GNS, due to the oxygen-containing functional groups attached on both sides of graphene nanosheet and to the sp3-hybridized carbon atoms generated on the original graphene plane [38,48,49]. To access the dispersion state of GONS in the PBAT matrix, SEM and WAXD measurements were carried out. As displayed in the SEM image of 0.35 vol % GONS/PBAT film (Fig. 2a), a rough fractured surface exists and GONS marked by arrows is by and large individually exfoliated and dispersed in the PBAT matrix with no indication of aggregates, which is in contrast with the smooth fractured surface of the neat PBAT film (Fig. 2c). Fig. 2b is the high resolution magnitude of circular region in Fig. 2a. It is evident that wrinkled paper-like structure of GONS spreads out in the polymer matrix. This phenomenon can be ascribed to the strong compatibility and/or strong interfacial adhesion between GONS and PBAT matrix because of hydrogen bonds between the oxygen-containing functional groups of GONS layers and ester groups of PBAT chains, and the mechanical interlocking resulting from the special wrinkled structure of GONS [50]. Fig. 3 illustrates representative 2DWAXD patterns and the corresponding 1D-WAXD curves of graphite oxide, neat PBAT, and GONS/PBAT nanocomposites. As seen in Fig. 3a, the homogeneous diffraction rings of neat PBAT and GONS/PBAT nanocomposites suggest that PBAT crystals are randomly distributed in the nanocomposites and GONS exerts little impact on the crystalline modification of PBAT matrix. In Fig. 3b the
Fig. 1. A typical tapping-mode atomic force microscopy (AFM) image of GONS.
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Fig. 2. Typical SEM images for the fractured surfaces of 0.35 vol % GONS/PBAT film with (a) low and (b) high magnification and neat PBAT film (c).
characteristic diffraction peak of graphite oxide sheets is observed at 2q ¼ 8.4 , corresponding to a layer-to-layer distance of 0.79 nm [51], much larger than that of pristine GNS (~0.34 nm), indicating adequate oxidation of GNS [52]. One can also acquire from Fig. 3b that the nanocomposites with different GONS loadings exhibit the same 1D-WAXD curves as the neat PBAT with the absence of characteristic peak of graphite oxide. It implies that GONS were fully exfoliated into individual GONS in the PBAT matrix, which is good accordance with the AFM and SEM results (Figs. 1 and 2). Moreover, the intensity of these diffraction peaks basically keeps consistent, suggesting crystallinity of the nanocomposites has hardly changed through the addition of GONS. 3.2. Barrier properties of PBAT/GONS nanocomposite films With the complete exfoliation and uniform dispersion of layered GONS throughout PBAT matrix, GONS with outstanding barrier properties are adequately reflected to enhance barrier properties of GONS/PBAT nanocomposite films by a wide margin. As shown in Fig. 4, the O2 permeability (PO2) of neat PBAT film is effectively restricted with the addition of GONS. To be specific, about 70% reduction in PO2 from 7.06 1014 to 2.15 1014 cm3 cm cm2 s1 Pa-1 is obtained by adding only 0.35 vol% GONS. The advent of impermeable GONS in PBAT matrix gives rise to the increasing path tortuosity and the decreasing cross section for O2 permeability. In addition, O2 does not always diffuse along the direction perpendicular to film and it may change its original permeability path from vertical to horizontal direction. However, the distance between GONS is much smaller than thickness of nanocomposite film, bringing about dramatic decrease of cross section for O2
permeability along the surface of film. In this work, the remarkable improvement in O2 barrier properties of PBAT films at such a low GONS loading is impressive, which demonstrates that GONS/PBAT nanocomposite films hold the key as a packaging material for protecting vulnerable to O2 degradation of perishable goods. In addition, owing to the hydrophilic groups on both GONS and PBAT molecular chains, the water vapor barrier properties of neat PBAT film are negatively interfered. Strikingly, water vapor barrier property of neat PBAT is also improved by the inclusion of GONS in this work. Direct access could be found from in Fig. 5 that the water vapor permeability of PBAT film is effectively reduced with the addition of GONS. To be specific, the addition of only 0.35 vol% GONS gives rise to about 36% reduction in H2O permeability (PH2O) from 3.98 1013 to 2.53 1013 g mm mm2 s-1 Pa-1, which further confirms that GONS are desirable nanoplatelets to enhance barrier properties of PBAT film. And the significant decline of PH2O improves the independence of barrier properties of PBAT film on environment humidity, further expands the application of PBAT film in the field of packaging and also beneficial for the agricultural film to keep moisture. According to the reported research, GONS is evidently superior to whether pristine or organo-modified clay as barrier enhancers, owing to their molecular-level dispersion and higher aspect ratio [33e35]. For GONS/PBAT nanocomposite films, moderate volume fraction (avoiding agglomeration in case of high concentration), well exfoliation and dispersion, and aspect ratio of GONS construct robust “barrier walls” to the diffusing gas molecules, conferring the highest promotion in the gas permeability resistance.
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Fig. 5. Coefficient of water vapour permeability (PH2O) for neat PBAT and its nanocomposite films as a function of GONS loadings: (a) neat PBAT, (b) 0.07 vol% GONS/ PBAT, (c) 0.18 vol% GONS/PBAT, (d) 0.35 vol% GONS/PBAT.
Fig. 6. DSC curves of neat PBAT and its GONS nanocomposite films. Fig. 3. Representative 2D-WAXD patterns (a) and the corresponding 1D-WAXD curves (b) of graphite oxide, neat PBAT and its nanocomposites with various GONS loadings.
Fig. 4. Permeability coefficient of O2 (PO2) for neat PBAT and its nanocomposite films as a function of GONS loadings: (a) neat PBAT, (b) 0.07 vol% GONS/PBAT, (c) 0.18 vol% GONS/PBAT, (d) 0.35 vol% GONS/PBAT.
3.3. Thermal behaviors of neat PBAT and its GONS nanocomposites It is well accepted that barrier properties of polymer incorporated with nanoplatelets is not only affected by the impermeable nanoplatelets, but also greatly depends on its crystallinity and crystalline morphology [36e38]. Thus, thermal behaviors of GONS/ PBAT nanocomposites are studied. The DSC heating curves of neat PBAT and its GONS nanocomposite films are shown in Fig. 6. The melting enthalpy (DHm), melting temperatures (Tm) and crystallinity (cc) are tabulated in Table 2. Tm gradually shifts to higher
Table 2 Melting enthalpy (DHm), and melting temperature (Tm), and crystallinity (cc) of neat PBAT and GONS/PBAT nanocomposite films with different GONS loadings. GONS loading (vol %)
DHm (J g1)
Tm ( C)
cc (%)
0 0.07 0.18 0.35
7.8 6.9 10.6 8.5
118.9 127.1 127.6 128.7
6.8 6.1 9.3 7.5
temperature region from 118.9 to 128.7 C, however, DHm changes in a very subtle way, ranging from 6.9 to 10.6 J g1. Meanwhile, the crystallinity (cc ) of all samples varies slightly from 6.1 to 9.3%. Therefore, it is reasonable to conjecture that the remarkable enhancement on barrier properties of GONS/PBAT nanocomposites are primarily attributed to the impermeability GONS, their complete exfoliation and strong interfacial adhesion between GONS and PBAT matrix, considering that crystallinity and crystalline structure of PBAT matrix are hardly changed. 3.4. Mechanism of the enhanced gas barrier properties by GONS According to the above analysis, the enhanced gas barrier properties of GONS/PBAT nanocomposite films are mainly assigned to the contribution of GONS. It is generally accepted that with the presence of nanoplatelets, the enhanced barrier properties of polymer-based nanocomposites are mainly due to the “torturous path effect” [36]. And there are three main factors that affect the tortuosity of penetration path for diffusing molecules: the volume fraction of the nanoplatelets; their morphology (i.e., exfoliation,
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Fig. 7. Comparison between experimental data and Bharadwaj model for Rp in terms of level of GONS loadings.
Fig. 9. Mechanical properties of neat PBAT and its nanocomposites with different GONS loadings. (A) typical strain-stress curves for (a) neat PBAT, (b) 0.07 vol% GONS/ PBAT, (c) 0.18 vol% GONS/PBAT, (d) 0.35 vol% GONS/PBAT; (B) Young's Modulus of the above samples.
Rp ¼
Fig. 8. TGA curves of neat PBAT and its nanocomposites with different GONS loadings.
dispersion and orientation relative to the diffusion direction); and their aspect ratio. Herein, in order to further quantitatively evaluate the effect of GONS on barrier properties of GONS/PBAT nanocomposite films, the decrease in relative permeability (Rp ) is estimated and the comparison between experimental data and Bharadwaj model [39] is plotted in Fig. 7. Rp is defined as follows:
Rp ¼
Pc Pm
(2)
where Pc and Pm are gas permeability coefficients of GONS/PBAT nanocomposites and pristine PBAT matrix, respectively. Compared with the previous theoretical models frequently assuming that the impermeable nanoplatelets are regularly arranged in a parallel array with their main direction perpendicular to the diffusion direction [42,43], the orientational order parameter (f ) is introduced to modify the conventional model. The correspondingRp can be given by
14 1 þ 4ða=2Þð2=3Þðf þ 0:5Þ
(3)
where 4 is the volume content of nanoplatelets and a is the aspect ratio of nanoplatelets, which was roughly estimated to be about 1080 from the AFM image (Fig. 1).f ¼ 0 is indicative of randomly dispersed nanoplatelets in polymer matrix, while the nanoplatelets are well-ordered oriented array through the entire polymer matrix when f ¼ 1 and the above expression of Eq. (3) returns to that of the classic Nielson model [43]. As shown in Fig. 7, our experimental data about the reduction in Rp lie randomly between the predicted Bharadwaj values in the case f ¼ 0 and f ¼ 1, which suggest that GONS is apt to randomly disperse throughout the PBAT matrix. Theoretically, perfect orientation of nanoplatelets will maximally elevate the gas barrier properties of polymer-based nanocomposites. They are frequently obtained through special processing techniques, which may typically time- and energyconsuming. Nevertheless, in the conventional nanocomposite industry the nanoplatelets are prone to reveal randomly dispersed morphology. The new insight into the contribution of the homogeneous dispersed nanoplatelets to the gas properties of polymerbased nanocomposites provides a significant guidance to manufacture high barrier polymeric packaging materials on a large scale.
3.5. Thermal stability of neat PBAT and its GONS nanocomposites TGA was conducted to study the thermal stability of neat PBAT and its GONS nanocomposites under nitrogen atmosphere. Fig. 8 shows that the initial degradation temperature (Ti) of the neat PBAT at 5% weight loss takes place at 361.7 C when subjected to
P.-G. Ren et al. / Polymer Testing 58 (2017) 173e180
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Table 3 Mechanical properties of neat PBAT and its nanocomposites with varying GONS loadings. GONS loading (vol %)
Yield Strength (MPa)
0 0.07 0.18 0.35
26.8 26.7 26.2 25.9
± ± ± ±
1.2 2.6 2.2 1.7
one-step degradation. As for the nanocomposites, Ti gradually shifts to higher temperature with increasing GONS loadings. Adding only 0.35 vol% GONS, Ti is increased by 5.0 C (366.7 C), retarding the degradation of the PBAT host. The distinctly enhanced thermal stability of the nanocomposites should mainly originate from the excellent barrier properties of GONS. Gas permeability through a polymer filled with a high aspect ratio, impermeable flakes can be decreased substantially via a reduced cross section for gas diffusion and a tortuous path mechanism, which makes decomposed gas difficult to escape and finally contributing to improved thermal stability [53e55]. Meanwhile, the strong interfacial adhesion between GONS and PBAT matrix can also restrict the mobility of PBAT chains in the vicinity of the GONS surface, giving rise to the remarkable thermal stability of PBAT matrix [56]. 3.6. Mechanical properties of neat PBAT and its GONS nanocomposites Fig. 9(A) is the typical strain-stress curves of neat PBAT and its GONS nanocomposites and bar chart in Fig. 9(B) showing the values of Young's Modulus. Detailed tensile results regarding tensile strength, Young's modulus, elongation at break are summarized in Table 3. Compared to the value of 24.6 MPa and 1100% for neat PBAT, the incorporation of GONS is found to give an increment of tensile strength and elongation at break and finally climb up to 27.8 MPa and 1197% for 0.35 vol% GONS/PBAT. Moreover, GONS/ PBAT nanocomposites exhibits a noteworthy enhancement in Young's Modulus, where 0.35 vol% GONS/PBAT achieves the profoundly increased modulus of 72.2 MPa in comparison with neat PBAT of 58.5 MPa, with an increment of 23.4%. The remarkable improvement of mechanical properties can be appraised from the good dispersion of GONS with high modulus and high aspect ratio and also from the strong interfacial adhesion between GONS and PBAT matrix. The incorporation of GONS can speed up PBAT performance and thus boost the attractiveness and broadens application of this biodegradable polymer. 4. Conclusions In the present work, GONS/PBAT nanocomposites with different GONS loadings ranging from 0.07 to 0.35 vol% were fabricated. SEM and 2D-WAXD techniques showed that GONS were fully exfoliated and well dispersed in the PBAT matrix. Furthermore, a low concentration of GONS had significant effect on the barrier properties, thermal stability, and mechanical properties of PBAT films. And when adding only 0.35 vol% GONS, O2 and H2O permeability coefficients of GONS/PBAT nanocomposite films decreased about 70% and 36%, respectively; Ti of the composite film was increased by 5.0 C in comparison with neat PBAT; Tensile Strength rises up to 27.8 MPa from 24.6 MPa and Young's modulus climbs up to 72.2 MPa from 58.5 MPa, showing a prominent increase of mechanical properties compared to neat PBAT. The enhanced gas barrier performances were attributed to the impermeability of GONS, their complete exfoliation and well dispersion, and strong interfacial adhesion between GONS and PBAT matrix, instead of changes in crystallinity. The work presented here suggests that the
Young's Modulus (MPa) 58.5 59.7 66.0 72.2
± ± ± ±
3.4 2.9 1.2 2.3
Elongation at Break (%) 1187 ± 11 1142 ± 9 1044 ± 12 981 ± 6
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