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Apr 17, 2018 - ABSTRACT: Low water vapor permeability is highly demanded for biodegradable packaging and agricultural film applications. However ...
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Cite This: ACS Sustainable Chem. Eng. 2018, 6, 6654−6662

Enhancement of Water Vapor Barrier Properties of Biodegradable Poly(butylene adipate-co-terephthalate) Films with Highly Oriented Organomontmorillonite Jiaxu Li,† Lei Lai,† Linbo Wu,*,‡ Steven J. Severtson,*,§ and Wen-Jun Wang*,† †

State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ‡ Institute of Polymer and Polymerization Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China § Department of Bioproducts and Biosystems Engineering, University of Minnesota, 2004 Folwell Avenue, St. Paul, Minnesota 55108, United States S Supporting Information *

ABSTRACT: Low water vapor permeability is highly demanded for biodegradable packaging and agricultural film applications. However, biodegradable poly(butylene adipate-co-terephthalate) (PBAT) films demonstrate poor water vapor barrier properties. A series of nanocomposite (NC) films composed of organically modified montmorillonite (OMMT) dispersed at levels ranging from 0 to 13 wt % in PBAT were thus generated using both film blowing and biaxial orientation. Films were characterized with wide-angle X-ray diffraction, transmission electron microscopy, thermal analysis and mechanical testing (static and dynamic), and their water vapor permeation (WVP) values were determined. The WVP of PBAT-OMMT NC films relative to that of the pure PBAT dropped and began leveling at the maximum OMMT concentrations tested. NCs for which OMMT layers better aligned with film surfaces, in this case those generated via biaxial orientation, provided for faster and more substantial decreases in WVP values relative to those produced with film blowing. The WVPs can be predicted using the Bharadwaj model, which accounts for OMMT aspect ratio, concentration, as well as orientation. The experimental results are in good agreement with the prediction values of the model. The addition of 13 wt % OMMT more than doubled the Young’s modulus, but resulting in a decrease of film tensile strength. The elongation at break was found to initially climb up to OMMT levels of about 6 wt % but declines sharply with higher concentrations. Results demonstrate the viability of reducing WVP levels of PBAT using orientated OMMT addition and provide insights on key structural parameters. KEYWORDS: Biodegradable polymer film, Poly(butylene adipate-co-terephthalate), Nanocomposite, Water vapor barrier property, Organomontmorillonite, Blown film, Biaxially oriented film, Biaxial orientation

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best suited to replace PE due to its high ductility10,11 and excellent balance between thermo-mechanical properties and biodegradability.12−15 Unfortunately, PBAT demonstrates poor gas barrier properties especially when it comes to water vapor, i.e., it possesses a high water vapor permeability (WVP).16,17 For mulching applications, water retention is a much-desired property, especially in arid regions. The WVP for PBAT under ambient conditions is reportedly around 3.3 × 10−11 g·m·m−2· s−1·Pa−1 compared with 5.5 × 10−13 g·m·m−2·s−1·Pa−1 for PE.14,18 That is, the ability of PBAT films to retain moisture is more than 2 orders of magnitude lower than that for the most commonly used PE films.

INTRODUCTION In recent years, there has been an increased research focus on identifying more sustainable alternatives to replace polymers derived from fossil fuels. An example of this is effort to find suitable substitutes for polyethylene (PE) for packaging and agricultural film applications. For the products as mulching films, collection and sorting for recycling is not practical. As a result, PE films are either landfilled or incinerated, practices that have raised serious environmental concerns.1 Although the initial cost of biodegradables plastics is higher, the elimination disposal costs make PE films a viable application area for replacement with more biodegradable alternatives.2 Currently, there are a number of polyesters being investigated for use as biodegradable packaging, agriculture films, and other applications including poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and poly(butylene adipate-coterephthalate) (PBAT).3−9 Of these, PBAT appears to be the © 2018 American Chemical Society

Received: January 27, 2018 Revised: April 17, 2018 Published: April 26, 2018 6654

DOI: 10.1021/acssuschemeng.8b00430 ACS Sustainable Chem. Eng. 2018, 6, 6654−6662

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ACS Sustainable Chemistry & Engineering

NC films as a more sustainable replacement for PE films in packaging and agricultural film applications.

There are several potential approaches available for enhancing the gas barrier properties of a polymer including surface modifications, use of polymer blends, and introduction of fillers.19−27 Of these, the dispersion of nanoscale fillers in polymer matrices has been found to be an effective and relatively inexpensive means for lowering WVP.28 Montmorillonite (MMT) is a natural clay possessing a layered structure consisting of impermeable platelets with aspect ratios ranging from tens to thousands. When dispersed in a polymer phase, MMT provides a significant barrier to penetrating gases.16,29−40 The formation of such polymer-nanoclay composites has been attempted with various polyesters including PLA41−43 and PBAT.44 A complicating factor for these systems is the poor compatibility of the hydrophilic clay with the polymer matrix, which inhibits dispersion and polymer intercalation. This is improved with the use of organically modified MMT (OMMT). Rhim45 et al. introduced 15 w%/w OMMT into PLA and obtained a 67% decrease of WVP. Mondal46 et al. achieved a 25% decrease in the WVP of PBAT with the addition of 4 wt % OMMT. However, for higher concentrations, film barrier properties actually declined. This result indicates that while polymer−OMMT nanocomposites (NCs) can improve barrier properties, important aspects such as the dependence of performance on filler orientation is still not well understood.47,48 Nielsen49 was the first to create a model for barrier properties of NC films that takes into account the structure and morphology of fillers. This was followed by the work of Bharadwaj et al.,50 which concludes that film barrier properties of polymer matrices containing nanoscale inorganic fillers are related to filler content, structure, as well as orientation. According to the model, larger aspect ratios and better orientation (parallel to the film surface) of 2D filler structures significantly enhance performance. Thus, for fillers with high aspect ratios, a key question is how to facilitate an orientation parallel to film surfaces. Biaxial orientation is a processing technique in which polymers undergo biaxial stretching, which aligns polymer chains producing films with enhanced in-plane stiffness and strength. Such loading can also align filler particles held by polymer matrices.49 Lepot et al.51 prepared biaxially oriented polypropylene NC films with elongated and spherical ZnO particles at mass fraction of 0−7.5 wt %. However, the results indicate that increased orientation of such geometries was not beneficial to film barrier properties. Ke et al.34 prepared PET-OMMT NC films by in situ polymerization in the presence of 0−3 wt % quaternary ammonium salt modified OMMT followed by biaxial orientation. For 3 wt % OMMT, the permeation of O2 was cut in half compared to that of the pure PET film. Here, results are reviewed from a study of NC films generated using melt compositing and both laboratory film blowing and biaxial orientation to provide different levels of filler orientation. The electron microscopy, X-ray diffraction, thermal analysis, and mechanical testing were used to characterize the samples, and WVP values were measured. Although it is understood that OMMT can lower polymer WVP, this result has not be established for PBAT films nor has the additional benefits of orienting the fillers. As will be discussed, samples prepared using biaxial orientation and highly oriented filler demonstrate greatly reduced WVP values, which agree well with the Bharadwaj model predictions. The results of this study demonstrate the viability of utilizing PBAT-OMMT

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EXPERIMENTAL SECTION

Materials. Poly(butylene adipate-co-terephthalate) or PBAT was kindly provided by the Xinfu Hangzhou Co. (Hangzhou, Zhejiang, PRC). Properties reported by the manufacturer and confirmed by our characterization include density of 1.26 g/mL, aromatic content of 45 mol %, number-average molecular weight of 39.7 kDa, and polydispersity of 2.92. Sodium montmorillonite (NaMMT) with a cation exchange capacity of 119 mequiv/100g was purchased from Fenghong New Material Co. (Huzhou, Zhejiang, PRC). Methyl dihydroxyethyl hydrogenated tallow ammonium (MT(ETOH)2) was purchased from Sigma-Aldrich (St. Louis, MO). Preparation of OMMT. Organically modified MMT (OMMT) was prepared through cation exchange using MT(ETOH)2 as intercalator. In a 1000 mL beaker, NaMMT (20.0 g, exchangeable cation 23.8 mmol) was dispersed in 600 mL of deionized water at room temperature. In another 500 mL beaker, MT(ETOH)2 (12.0 g, 30 mmol) was dissolved in deionized water (200 mL) with 2.5 mL concentrated HCl. The solution was carefully poured into the clay suspension and stirred vigorously for 1 h at 80 °C. The product was then filtered and washed with a mixture of water and ethanol. This procedure was repeated several times until no Cl− was detected by AgNO3 solution. The product OMMT was then freeze-dried and stored in a desiccator until use. The specific density of OMMT was 1.80 g/cm3. Preparation of PBAT-OMMT NCs. After drying at 70 °C in a vacuum oven for 24 h, PBAT pellets and 0, 1, 3, 5, 7, 9, or 13 wt % of OMMT powder were mixed in a torque rheometer (HAAKE Polylab OS) equipped with a twin-screw extruder to prepare PBAT-OMMT NCs with different OMMT loadings. The mixing was carried out at a rotational speed of 220 rpm and a screw temperature profile of 80− 110−150−170−170−170−170−170−170−165 °C. The resulting mixture was cut into pellets and stored in a desiccator. The resulting OMMT contents were 0, 1, 3, 5, 7, 9, and 13 wt % (0, 0.7, 2.0, 3.2, 4.5, 6.3, and 9.0 vol %) and the generated NCs were designated as NC0 to NC13 based on the mass fraction of OMMT. Preparation of Films. NC pellets were dried at 70 °C in a vacuum oven for 24 h before use. Blown films (BFs) were prepared with a single screw extruder (L/D = 25:1, D = 19 mm) and a film-blowing fitting (self-made circular die with a diameter of 25 mm). The composite melt was extruded with a rotational speed of 15 rpm and a temperature profile of 80−120−170−165 °C. The take up (TUR) and blow up ratios (BUR) were controlled at 10 and 5, respectively. The BFs were air cooled and collected in rolls with 10−20 μm thicknesses. Biaxially oriented films (BOFs) were prepared using a two-step process. NC pellets were first thermo-compressed at 170 °C into 200 μm thick plates. These were stretched (simultaneously) in both horizontal directions at 80 °C using a Bruckner KARO IV tensile machine at a stretching speed of 0.1 m/s and a tensile ratio of 3.5. The thickness of the resulting BOFs ranged from 10 to 20 μm, comparable to that of BFs. Film Characterization. Fourier transform infrared spectra (FTIR) of NaMMT and OMMT were recorded with a Nicolet 5700 (ThermoFisher Co.) FTIR analyzer using slice samples. Wide angle X-ray diffraction (WAXD) spectra were recorded using a PANalytical X’Pert PRO (PANalytical Co.) with Cu Kα radiation (0.154 nm), working at 40 kV and 40 mA. The samples were scanned from 2θ = 0.8 to 15° with a scanning speed of 3°/min. The space between OMMT layers (d-spacing) was calculated by Bragg’s Law. The lamellae long period and orientation of the films were determined at 50 kV and 0.6 mA using a SAXS/WAXS system (XEUSS Co.) with a PILATUS detector and Cu Kα radiation (0.154 nm). The distance between the sample and the detector is 1365 mm. The Hermann’s orientation factors were obtained with Prasad et al. procedure,52 and the long period (Lp) was calculated by Bragg’s Law. Transmission electron microscopy (TEM) micrographs were performed using a JEM-2100F (JEOL Co.) with a 60 kV accelerating voltage. 6655

DOI: 10.1021/acssuschemeng.8b00430 ACS Sustainable Chem. Eng. 2018, 6, 6654−6662

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ACS Sustainable Chemistry & Engineering Water vapor transmission rate (WVTR) measurements were carried out following ASTM E96-16 and GB 1037-1988. Calcium chloride was encapsulated by sample films in metal cups (diameter = 7 cm) to make the relative humidity inside the cups close to zero. Test sample cups were kept in a constant temperature (38 °C) in a constant humidity (90 RH%) chamber. The total weight changes of the test sample cups over time were measured. Water vapor permeability WVP could be calculated as,

WVP =

WVTR·d P(RH1 − RH 2)

(1)

where d is the thickness of the film, P is the saturated water vapor pressure (6630 Pa at 38 °C), and RH1 and RH2 are the relative humidity of both sides of the film. Thermal transition behaviors of BOFs were record with differential scanning calorimetry (DSC, Q200, TA Instrument) under nitrogen flow. Only first heating scans were recorded from −70 to +180 °C with a heating rate of 10 °C/min. Crystallinity (χc) of all the samples can be calculated as,

χc =

ΔHm ΔH0· (1 − ϕw,OMMT)

Figure 1. Relative permeability as a function of ϕs for polymer− OMMT films calculated using the Bharadwaj model for L/W = 66 and 3 different filler orientations.

perpendicularly oriented relative to the film surface, they have their smallest impact on barrier properties. With this orientation, S is at its minimum value of −0.5, and the relative permeability shows only a modest decrease with ϕs. Randomly orienting the filler particles results in an order parameter of zero for which the relative permeability shows a clear decrease as a function of ϕs. The greatest impact on relative permeability is predicted to occur when the layered particles are oriented parallel to the film surface. This produces a value of 1.0 for S, its maximum, and a sharp decline in relative permeability as the filler loading is increased. For ϕs of 5 vol %, this produces a relative permeability of 0.36 compared with values of 0.61 and 0.95 corresponding to S values of 0 and −0.5, respectively. Thus, in addition to filler loading and aspect ratio, filler particle orientation plays an important role in determining the gas permeability of a polymer composite. Higher S values results in a larger decrease in permeability with increasing filler loading. Nanocomposite Microstructure. Treatment of NaMMT with methyl dihydroxyethyl hydrogenated tallow ammonium MT(ETOH)2 was used to generate the OMMT, which has an increased compatibility with the PBAT matrix. The modification of NaMMT was confirmed by FTIR spectroscopy. Figure S1 in the Supporting Information shows FTIR spectra of NaMMT and OMMT with detailed peak assignments. From the spectra, the asymmetric and symmetric stretching vibration absorptions of C−H bonds appear at 2923 and 2852 cm−1, indicating that the MT(ETOH)2 was located within the OMMT layers. Figure 2a shows the WAXD spectra of NaMMT, OMMT, PBAT, and PBAT-OMMT NC containing 5 wt % OMMT (NC5). The NaMMT exhibits a characteristic peak at 2θ = 5.98°, corresponding to a d-spacing of 1.47 nm. After organomodification, the peak shifts to 2θ = 3.70°, corresponding to a d-spacing of 2.38 nm. This shift further demonstrates that the molecular chains of the modifier MT(ETOH)2 were intercalated into clay layers successfully. When processed with PBAT using melt blending, the OMMT characteristic peak shifts to 2θ = 2.40°, corresponding to a d-spacing of 3.68 nm. This increase indicates that polymer chains have good compatibility with the OMMT layers and are able to intercalate within the OMMT layers. A TEM micrograph of NC5 (Figure 2b) shows that OMMT filler particles are evenly dispersed in the PBAT matrix. No interfacial void could be found. From the WAXD spectra of NCs with various OMMT contents (0−13 wt %) shown in Figure S2 of the Supporting Information, no obvious difference

(2)

where ϕw,OMMT represents the weight content of OMMT and ΔH0 represents the melting enthalpy of pure PBAT (114 J/g).53 Thermogravimetric analysis (TGA) measurements were carried out in nitrogen atmosphere by a thermal analyzer (TGA, Q500, TA Instrument) from 50 to 600 °C with a heating rate of 10 °C/min. The dynamic mechanical properties (E′, E″, and tan δ) were determined using a dynamic mechanical analyzer (DMA, Q800, TA Instrument) in the tensile mode. The experiments were performed at a frequency of 1 Hz and the applied strain (15 μm) from −50 to +40 °C with a heating rate of 3 °C/min. The dimensions of the specimens were 30 × 4.3 mm. Tensile properties of the films were measured with a Universal Testing Machine (Instron Instrument 3345) at a tensile speed of 50 mm/min. The films were cut into dumbbell-shape specimens. The reported tensile properties presented average values of at least 5 specimens of each sample.

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RESULTS AND DISCUSSION Predicting Permeability with Barrier Model. The Bharadwaj model provides a relationship between the permeability of a polymer composite containing a layered filler relative to the pure polymer as a function of filler content, geometry, and orientation, i.e.,50 Ps = Pp 1+

S=

1 − ϕs L 2 ϕ 2W s 3

( )(S + 12 )

1 (3cos2 θ − 1) 2

(3) (4)

where Ps and Pp are the WVP of polymer−filler systems and pure polymer, respectively, ϕs is the volume fraction of filler, L and W are the length and thickness of filler layers, and S is the order parameter of filler related to θ, which is the angle between the unit normal to the film surface (n) and filler surface (p). The model does not take into account the extent of the interfacial interactions. As discussed below, characterization of the NC samples studied indicates well-bonded interfaces. Therefore, the model is applied to this work. Figure 1 shows the predicted dependency of polymer−OMMT relative permeability (Ps/Pp) on filler loading (ϕs) for a fixed L/W of 66 and a range of filler orientations characterized by S. By choosing the same L/W value as that of OMMT used in this work, the results serve a guide in the design of experiments. It can be seen that when the layers of the filler particles are 6656

DOI: 10.1021/acssuschemeng.8b00430 ACS Sustainable Chem. Eng. 2018, 6, 6654−6662

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Figure 2. (a) WAXD spectra of NaMMT, OMMT, PBAT, and PBAT-OMMT NC5 and (b) TEM micrograph of NC5.

Figure 3. Cartoons and TEM micrographs of PBAT-OMMT NC5 generated using (a) film blowing (BF5) and (b) biaxial orientation (BOF5).

Water Vapor Barrier Properties. Values were measured for the water vapor transmission rate WVTR for films of PBAT and PBAT-OMMT NCs. The films were generated through both film blowing and biaxial orientation and contain a range of OMMT up to 13 wt %. Equation 1 was used to calculate the WVP values from these measurements. Table 1 lists the

of d-spacing is observed among these samples. However, some layer stacks appear at OMMT loading of 13 wt %, as a new peak of 2θ = 4.80° in Figure S2. TEM micrographs of BOFs of various OMMT contents (Figure S3 in the Supporting Information) also show this result. With the increase of OMMT content, layers are harder to disperse and tend to aggregate resulting in layer stacks. Film blowing and biaxial orientation are common techniques used to produce polymer films. Both BF and BOF films were transparent and turned slightly gray with the increase of OMMT loading, as shown in Figure S4. For the PBAT-OMMT system, both approaches partially align polymer chains within the film plane, which will also orient OMMT layers. However, the extent of this orientation differs. Figure 3 shows cartoons next to the actual TEM micrographs collected for PBATOMMT NC5 films produced using both film blowing (Figure 3a) and biaxial orientation (Figure 3b). Based on dimensions of the OMMT, L/W = 66 from TEM analysis, S values for the BF and BOF films estimated using the WVP data of BF0/BF5, and BOF0/BOF5, respectively, and the Bharadwaj model are 0.33 and 0.97, respectively. For films generated with the blowing process, OMMT layers are oriented along a single direction (machine direction), but the OMMT planes are only partially aligned, which provides for the lower order parameter. This produces greyish semitransparent regions around the particles due to their tilt. For films generated with biaxial orientation, less OMMT planes are visible in the micrograph. It is apparent that the OMMT layers are highly oriented parallel to the film surface as the schematic representation indicates.

Table 1. WVP values of PBAT-OMMT NC Films OMMT content

WVP (10−11 g·m·m−2·s−1· Pa−1)

Sample

(wt %)

(vol %)

BFs

BOFs

1 2 3 4 5 6 7

0 1 3 5 7 9 13

0 0.7 2.0 3.2 4.5 6.3 9.0

1.62 1.47 1.27 0.98 0.88 0.74 0.71

1.58 1.41 1.09 0.74 0.61 0.39 0.32

OMMT contents in weight and volume fraction and calculated WVP values for films produced via both blowing BFs and biaxial orientation BOFs. It can be seen there is little difference between values for the pure PBAT films generated using these two processing procedures. The addition of the OMMT lowers WVPs and these values decreases with increased filler loading. Figure 4a plots the WVP values as a function of OMMT content using data from Table 1 for films generated using both production techniques. This provides a better demonstration of the functional dependency. The values for the pure polymer are approximately the same for both BFs and BOFs, but the initial 6657

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Figure 4. (a) WVP values of BFs and BOFs with various OMMT contents and (b) comparison of results with the model proposed by Bharadwaj.

Figure 5. (a) Comparison of WVP values of PBAT-OMMT NCs with other biodegradable polymers and with PE. (Values for the chart were obtained from references 12, 17, 18, 45−47, 49, and this work.) (b) Comparison of relative permeability values (Ps/Pp) between this work and other filler/PBAT nanocomposites.

provided in some references. It appears that BOF13 possesses the highest water vapor barrier property. Thermal and Mechanical Properties. The inorganic filler in a NC films not only lowers gas permeation by acting as a physical barrier, it can also decrease diffusion by enhancing the crystallinity of the polymer matrix.56 To examine the potential influence of this effect, the first heating thermal properties of films generated via biaxial orientation were analyzed. The first heating curves were used to preserve effects induced during film formation. Figure S5 in the Supporting Information shows the first heating thermograms. Heating thermograms for the pure PBAT subsequent to multiple heating−cooling cycles to eliminate the thermal history show a single melting transition between 110 and 115 °C. However, for the pure PBAT, films that have undergone film blowing and biaxial orientation, the first-pass heating curves show a second transition, which appears at temperatures of 46.7−50.2 °C, which is associated with poly(butylene adipate) segments.57 Table 2 lists the thermal locations of transition temperatures (Tm1 and Tm2), associated transition enthalpies (ΔHm1 and ΔHm2) and corresponding crystallinities (χc1 and χc2) as well as the total crystallinity (χc). The data collected for the PBAT-OMMT NC films (Table 2) indicate a slight increase in thermal locations of transitions with increasing OMMT levels, relative to those for the pure polymer. These shifts are small for the lower temperature transitions but appear more significant for the higher temperature ones. Along with these shifts, there is also a modest increase in the enthalpies associated with the transitions, indicating increases in the overall crystallinity.

drop in WVP is more rapid for the latter. Both curves appear to be plateauing as OMMT levels reach 13 wt %. The greatest difference in WVP values occurs at this point. The curves show that the increased orientation results in greater efficiency as well as greater effectiveness in reducing WVP. Figure 4b plots the relative permeabilities Ps/Pp calculated from the WVP data as a function of ϕs. A comparison between the relative permeability of BFs and BOFs, and the theoretical results predicted using Bharadwaj model with L/W = 66 and S values of 0.33 (film blowing) and 1 (biaxial orientation) are also shown (solid curves). The model prediction values are in good agreement with the experimental data, further supporting the conclusion that the alignment of the OMMT layers with film surfaces provides a significant enhancement of the barrier properties imparted by the OMMT. Figure 5a provides a comparison of WVP values for the films with other biodegradable polymers and PE. As discussed, most of its properties of PBAT make it an attractive (sustainable) replacement for existing polymers with the exception of its WVP, which is relatively high. Even when compared with other sustainable options, its WVP is high. As shown in the figure, the addition OMMT significantly reduces this value using either film blowing (higher end of bar) or biaxial orientation (lower end of bar). Figure 5b also shows the comparison of relative permeabilities among the BOFs and other filler/PBAT NC films, where the fillers comprise graphene oxide,17 nanoalumina,54 nanosilver,55 and different OMMTs modified with dihexylamine,39 octadecylamine,44 and cetyltrimethylammonium bromide.46 The selection of relative permeability values for comparison is because the film thicknesses were not 6658

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be attributed to its role as a physical barrier as opposed to its influence on the matrix structure. The thermal stabilities of the NCs were gauged using thermal gravimetric analysis TGA. A pair of temperatures were used to assess this, the onset temperature of thermal degradation (Td,5), which was gauged with the temperature where 5% of the sample weight is lost, and the maximum rate of decomposition temperature (Td,max), which is the minimum in the first derivative of the weight loss curve. These are often treated as measures of the thermal stability limit and the temperature of where the maximum component is degrading, respectively. Weight loss curves and the first derivatives of the curves are shown in Figure 6. It can be seen that the curves shifts with increasing OMMT content. The weight curves are expected to level at the filler contents, keeping in mind that OMMT contains an organic component. The temperatures characterizing the degradation were extracted from the curves and are listed in Table S1 of the Supporting Information. It can be seen that both Td,5 and Td,max values of the NCs decrease with increasing OMMT content. As compared with pure PBAT, the Td,5 and Td,max of NC13 decrease by 34 °C (351 → 317 °C) and 18 °C (393 → 375 °C), respectively. The deterioration in thermal stability is ascribed to the organic intercalator, which is less thermally stable and decomposes at lower temperatures than the polymer matrix. However, it should be pointed out that the Td,5 values for the NCs greatly exceed their processing temperature (∼170 °C). Dynamic mechanical analysis DMA was performed to acquire information on chain motion of PBAT in PBATOMMT NC films generated using biaxial orientation. Figure 7 shows temperature sweeps of dynamic properties collected at 1 Hz over a range of −50 to +40 °C. From Figure 7a, it can be seen that the storage modulus (E′) decreases significantly with increasing temperature for all samples. This results from the onset of the glass transition for PBAT. As expected, E′ values for the BOFs are increased sharply by increasing OMMT levels. For example, at 25 °C, BOF13 has a value of about 520 MPa compared with 21 MPa for the pure PBAT film, an increase of nearly 25-fold. As with E′, values for the loss modulus (E″) increase with greater concentrations of OMMT (Figure 7b). The E″ values for the films pass through a maximum consistent with region of sharp decline in E′ indicating the glass transition. This also produces a maximum in the loss tangent (tan δ), which is often used to mark the glass transition temperature (Tg). From Figures 7c and S4a, it appears that the Tg values increase slightly with the addition of OMMT, ranging from −25.1 to −19.0 °C from DMA and −30.4 to −24.8 °C from

Table 2. Thermal Properties of BF0 and BOFs Sample

Tm1 (°C)

Tm2 (°C)

ΔHm1 (J/g)

ΔHm2 (J/g)

χc1 (%)

χc2 (%)

χc (%)

BF0 BOF0 BOF1 BOF3 BOF5 BOF7 BOF9 BOF13

48.1 46.7 48.1 46.9 50.2 50.0 48.1 48.2

104.5 108.1 110.3 113.1 113.2 112.4 113.4 114.3

2.58 1.34 1.78 2.08 2.56 2.70 2.89 3.16

11.63 11.72 12.74 13.62 13.13 12.12 13.54 14.41

2.5 1.2 1.6 1.9 2.3 2.5 2.8 3.2

10.2 10.3 11.3 12.3 12.1 11.4 13.1 14.5

12.7 11.5 12.9 14.2 14.4 13.9 15.9 17.7

When 13 wt % OMMT was incorporated into PBAT, the crystallinity increased from 11.5% to 17.7%. The addition of OMMT to a polymer matrix often increases its crystallinity.40 It has been suggested that the hydroxyethyl groups on OMMT provide intermolecular hydrogen bonding with PBAT chains, leading to an increase in crystallinity.17,39,42,58 According to results reported by Ren et al., such small increases of crystallinity will have little affect the barrier properties of polymer films.17 Furthermore, if these new crystalline regions are treated as nanofiller particles with L/W = 1 and S = 0, according to eq 3, a 6.2% increase of χc (ϕs = 5.6%) would produce only a 6% decrease of WVP at an OMMT content of 9 vol %. Polymer crystallite orientation and size reportedly affect WVPs.59−61 This influence was investigated using SAXS. For both BF0 and BOF0, collected patterns and plots of intensity versus both azimuthal angle and q values are shown in Figure S6 of the Supporting Information. Estimated orientation factors for BF0 and BOF0 are −0.032 and 0, respectively, while the long periods of both samples are 17.9 and 19.2 nm based on the Bragg’s Law, respectively. The measured differences of lamellae long periods and orientations in both processing processes are minor. It appears that the BOF0 has the same degree of orientation along both stretch directions,52 while the BF0 has limited lamellae orientation along the machine direction (MD) compared with its transverse direction (TD). Given that during producing of the BFs, the TUR and BUR were set as 10 and 5, respectively, the polymer viscosity and elasticity during the filmblowing process were too strong to offer the chance for PBAT chains to be aligned to a great extent along MD. Also, results indicate that the lamellae orientation and size has little effect on the WVP values for BF0 and BOF0. In other words, these results demonstrate that majority of the decrease WVP observed with the addition of the OMMT can

Figure 6. (a) Thermal gravimetric and (b) differential thermal gravimetric curves of NCs (at 10 °C/min and N2). 6659

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Figure 7. Temperature dependence of (a) storage modulus (E′), (b) loss modulus (E″), and (c) tan δ for BOFs.

Figure 8. (a) Strain−stress curves for BFs and BOFs and (b) mechanical properties comparison of BOFs with various OMMT contents.

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CONCLUSIONS The results of this study demonstrate the ability of OMMT to significantly lower WVP values when added to PBAT-OMMT NCs up to levels of 13 wt %. It is clear that treatment of NaMMT with MT(EtOH)2 facilitates both filler dispersion and polymer intercalation. Thermal analysis indicates that the reduced permeability is primarily a consequence of added fillers acting as a physical barrier rather than inducing increase of crystallinity. While the initial reduction in WVP is significant, plateauing occurred for higher filler loadings. In addition to OMMT modification and concentration, the influence of layer orientation was examined here. Two techniques, film blowing and biaxial orientation, were used to generate NC films. Both techniques generate films with enhanced in-plane polymer chain alignment, which aligns OMMT particles. However, OMMT layers were better oriented parallel to film surfaces with biaxial orientation. This was determined from TEM micrographs and with resulting order parameters S from the Bharadwaj model. The enhanced orientation is credited for the greater efficiency and effectiveness of OMMT at reducing the WVP in BOFs compared with BFs. The relationship between relative permeability and OMMT vol % was found to be in agreement with the prediction values using Bharadwaj model for both films. As expected, the added OMMT increased the BOF stiffnesses, nearly doubling E from pure PBAT to BOF at 13 wt % OMMT loading while the tensile strength declined over this entire region. The elongation at break increased up to OMMT levels of 5 wt %, but values dropped sharply beyond this. The degradation in both strength and ductility were attributed to the presence of aggregated filler particles. A comparison of WVP values with existing and potential products indicates that the biodegradable PBAT-OMMT NCs may serve as a viable substitute for packaging and agricultural film applications.

DSC. The differences are due to the different scanning rates used. Mechanical properties of the films were characterized using tensile tests. Figure 8a provides examples of strain−stress curves for BFs and BOFs. It is obvious that, owing to the highly oriented PBAT chains, BOFs show remarkable high strength than BFs. On the contrary, after being stretched, BOFs show lower elongation at break than BFs. The tensile strength at break, Young’s modulus, and elongation at break of BOF5 are 69 MPa, 133 MPa, and 185% against to them of BF5 31 MPa, 120 MPa, and 600%. Figure 8b plots the mechanical properties extracted from the strain−stress curves of BOFs as a function of added OMMT levels. The clay nanoparticles increase the BOF stiffness, nearly doubling the Young’s modulus (E) to 156 MPa from its initial value for pure PBAT of 85 MPa. At higher OMMT levels, it appears that E values begin to plateau. The increase is attributed largely to the reinforcing influence of the inorganic filler and to a lesser extent the slight increase in polymer crystallinity. The plateauing behavior is inconsistent with that predicted by micromechanical models such as Halpin−Tsai,62 and may be attributed to greater aggregation of particles at higher concentrations, which limits bonding with particle surfaces. As the BOFs become stiffer, they also lose strength. The tensile strength for the pure PBAT is 109 MPa, which drops and plateaus to a value of 56 MPa at 13 wt %. It is possible that increased aggregation of the OMMT would produce an increased concentration of defects, providing the decline in tensile strength. It is interesting that the elongation at break initially increases from 160% for pure PBAT to 185% for the BOF5 sample. It then drops linearly to 112% for the film containing the highest OMMT level. 6660

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ACS Sustainable Chemistry & Engineering

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00430. FTIR spectra of NaMMT and OMMT, WAXD spectra of NCs, TEM micrographs, DSC thermograms for BOFs, SAXS patterns and intensities as a function of azimuthal angle and q values for BF0 and BOF0, characteristic decomposition temperatures of NCs, and photographs of films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] (W.-J. Wang) *[email protected] (L. Wu) *[email protected] (S. J. Severtson) ORCID

Linbo Wu: 0000-0001-9964-6140 Wen-Jun Wang: 0000-0002-9740-2924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the Nation Key Research and Development Program of China (2016YFB0302400) and the National Natural Science Foundation of China (Grant 21420102008).

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