Highly Efficient “Composite Barrier Wall” - ACS Publications

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Highly Efficient “Composite Barrier Wall” Consisting of Concentrated Graphene Oxide Nanosheets and Impermeable Crystalline Structure for Poly(lactic acid) Nanocomposite Films Hua-Dong Huang,†,‡ Sheng-Yang Zhou,† Dong Zhou,† Peng-Gang Ren,§ Jia-Zhuang Xu,† Xu Ji,*,∥ and Zhong-Ming Li*,† †

College of Polymer Science and Engineering and State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China ‡ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China § Institute of Printing and Packaging Engineering, Xi’an University of Technology, Xi’an, Shanxi 710048, P. R. China ∥ College of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China S Supporting Information *

ABSTRACT: Poly(lactic acid) (PLA), a promising sustainable packaging material, suffers from intrinsic poor gas barrier performance partly due to its innate defect of relatively low crystallization rate. In the present study, taking advantage of the excellent impermeability and heterogeneous nucleating ability of graphene oxide nanosheets (GONSs), the crystalline structure of PLA nanocomposite film was manipulated using processing techniques. We revealed that GONSs were the α-nucleating agent for PLA, inducing typical spherulite morphology. More interestingly, two-dimensional small-angle scattering characterization confirmed that GONSs were preferentially dispersed in the amorphous phase between PLA spherulites, achieving a concentrated GONS region. As a consequence, the “composite barrier wall” consisting of concentrated GONSs and impermeable PLA lamellae gave rise to O2 permeability of PLA nanocomposite film at a GONS loading of 1.0 wt % as low as 0.211 × 10−14 cm3 cm cm−2 s−1 Pa−1, reduced by ∼89.9% relative to neat amorphous PLA film. These results presented here afford new insight into the contribution of GONSs and their induced crystalline structure to the significantly enhanced barrier performance, which may also open up a promising avenue for design and fabrication of high-barrier polymer packaging materials.

1. INTRODUCTION

viewed as a stumbling block, gravely restricting its development and application in the packaging and protective industry.7,8 As a consequence, a great deal of effort is still needed to enhance the gas barrier performance of PLA film so as to fulfill the specific functional requirements in the packaging industry. Over the past decades, it has been shown to be constructive to incorporate nanoplatelets into a polymer matrix with the aim of advancing the barrier properties of polymer nanocomposite films.9−22 As representative two-dimensional nanoplatelets, graphene oxide nanosheets (GONSs) are particularly attractive for their outstanding gas barrier properties, arising from their tightly packed planar structure, extremely high specific surface area, large aspect ratio, as well as abundant oxygen-containing functional groups. These highly hydrophilic functional groups have been found to be feasible and effective at promoting full exfoliation and uniform dispersion of GONSs and significantly

The past decades have witnessed the intensely increasing interest in polymer films as popular packaging materials in the form of films, sheets, and bottles for protecting perishable food, pharmaceuticals, electrical devices, etc., because of their lightweight, versatility, low cost, and ease of manufacture.1 Nevertheless, most of the most commonly used polymer films are derived from fossil fuels and are discarded as spontaneously undegradable wastes at the end of their useful life, leading to an overdependence on limited petroleum resources and potentially serious environmental pollution. The rapidly growing awareness and worldwide concern regarding sustainable issues have therefore motivated and pushed the research community to develop more environmentally friendly alternatives for packaging applications.2,3 To this end, poly(lactic acid) (PLA), an increasingly popular polymer, has been demonstrated to be an ideal candidate to substitute for petrochemical-based traditional packaging material, originating from its desirable renewability and biodegradability, impressive mechanical performance, eminent transparency, and good processability.4−6 Unfortunately, the intrinsic poor gas barrier properties of PLA film are © 2016 American Chemical Society

Received: Revised: Accepted: Published: 9544

June 5, 2016 July 22, 2016 August 19, 2016 August 19, 2016 DOI: 10.1021/acs.iecr.6b02168 Ind. Eng. Chem. Res. 2016, 55, 9544−9554

Article

Industrial & Engineering Chemistry Research

collaborative contribution of GONSs and their induced crystalline structure. The obtained results are expected to help our understanding of the effect of GONSs and their induced crystalline structure on the barrier performance of polymer films and have a significant guidance for achieving high-barrier polymer nanocomposite films.

improving the interfacial adhesion with polar polymer matrix, thus giving rise to remarkably enhanced gas barrier performances of polar polymer-based nanocomposite films.12−22 For example, Tsai and co-workers prepared a polyimide nanocomposite film with a ∼83% reduction in the water vapor transmission rate at a GONS loading of 0.01 wt %.16 A multilayer thin film composed of GONSs and branched polyethylenimine exhibited an excellent gas barrier performance with an O2 permeability of 2.5 × 10−20 cm3 cm cm−2 s−1 Pa−1.15 In our previous work, gas barrier properties of PLA nanocomposite films were moderately elevated by randomly dispersed GONSs.21 Fully exfoliated, homogeneously dispersed, and highly aligned GONSs in the poly(vinyl alcohol) and cellulose matrix could maximize the tortuosity of the penetration path for diffusing molecules, thus revealing a significant enhancement against O2 diffusion at a rather low content of GONSs.17,22 On the basis of these studies, the discovery of GONSs opens up a new avenue to fabricate highbarrier polymer nanocomposite films. As a rule, enhancement efficiency of GONSs for gas barrier properties of polymer nanocomposite films strongly depends on their morphology (i.e., exfoliation, dispersion and orientation perpendicular to diffusing direction) in a polymer matrix, their inherent properties (e.g., surface area, aspect ratio, etc.), as well as the interfacial adhesion between GONSs and the polymer matrix. Additionally, for semicrystalline polymerbased nanocomposites, which consist of an impermeable crystalline phase dispersed in a permeable amorphous matrix, the barrier properties could also be optimized by tuning their supermolecular microstructure, such as crystal polymorphism, lamellar arrangement, crystallinity, etc.23−25 Recently, in addition to the excellent impermeability, GONSs have also been recognized as highly efficient heterogeneous nucleating agents to promote polymer crystallization and provide good platforms for polymer epitaxial crystallization.26−31 In this regard, it will be very interesting to harness the double effect of GONSs, namely efficient heterogeneous nucleating ability and outstanding impermeability. With the coexistence of GONSs and their induced crystalline structure, the barrier properties of polymer films are expected to be enhanced significantly. However, to the best of our knowledge, very limited effort has been devoted to investigating the crystallization−permeability relationship in GONS-based polymer nanocomposites.32 A full and generalized understanding of the effect of GONSs and their induced crystalline structure on the barrier performance of polymer films is still in its infancy. In our previous study, we found that the randomly dispersed GONSs could form a good gas barrier in the PLA matrix, thus effectively suppressing O2 and CO2 penetration through the nanocomposite films.21 Nevertheless, owing to the innate defect of PLA with relatively low crystallization rate, all the asprepared nanocomposite films were basically amorphous. The amorphous region could act as permeation paths for diffusing molecules. In this work, on the basis of our previous work on PLA crystallization behavior on the dependence of GONS loadings,30 taking advantage of the excellent impermeability and heterogeneous nucleating ability of GONSs, the crystalline structure of PLA nanocomposite films was manipulated using processing techniques. As a result, O2 permeability of the sufficiently crystallized PLA nanocomposite film at a GONS loading of 1.0 wt % was 0.211 × 10−14 cm3 cm cm−2 s−1 Pa−1, reduced by ∼89.9% relative to neat amorphous PLA film. The enhanced barrier performance was determined to be the

2. EXPERIMENTAL SECTION 2.1. Materials. PLA (model 4032D) used in this study was provided by Nature Work (United States) with around 2% DLA. The Mw and Mn was 2.23 × 105 g mol−1 and 1.06 × 105 g mol−1, respectively. GONSs were prepared from expanded graphite by the modified “Hummers” method, and the details of the preparation process were previously reported in our work.26 Anhydrous N,N-dimethylformamide (DMF), sodium hydroxide (NaOH), and methanol were supplied by Chengdu Kelong Chemical Reagent Factory, Chengdu, China. Unless stated otherwise, all the reagents were of analytical grade and directly used without further purification. 2.2. Preparation of PLA Nanocomposite Films. As previously reported in our work,21 solution coagulation was utilized to prepare a series of PLA nanocomposite coagulates containing various GONS loadings of 0, 0.1, and 1.0 wt %. The resultant nanocomposite powders were shaped into a diameter of 100 mm and a thickness of 180 μm films for barrier measurements by compression molding at 200 °C with a pressure of 10 MPa after preheating for 5 min. To tune the crystalline structure of PLA nanocomposite films, the completely melted samples were rapidly cooled to 135 °C, isothermally crystallized for different times (15, 30, or 60 min) with a pressure of 10 MPa, and then quenched to room temperature. The detailed temperature protocol is schematically presented in Figure S1 (Supporting Information). For comparison purposes, totally amorphous films with and without GONSs were obtained by being cooled to room temperature at a fast cooling rate of ca. 40 °C/min from the melt state at 200 °C. Such a fast cooling rate markedly suppressed the ordered arrangement of PLA chains in the lattice. For the sake of briefness, the as-prepared nanocomposite films are coded as PLAX-Y. X is the weight concentration of GONSs, and Y is the isothermal crystallization time. For instance, the abbreviation of PLA0.1-30 represents PLA nanocomposite film at a GONS loading of 0.1 wt % being isothermally crystallized at 135 °C for 30 min. 2.3. Two-Dimensional Wide-Angle X-ray Diffraction Characterization. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) patterns were collected at the beamline BL15U (λ = 0.124 nm) of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) with an X-ray detector (Model Mar165). The distance between the sample and detector was 146 mm. Linear 1D-WAXD profiles were obtained from circularly integrated intensities of the 2D-WAXD patterns. The crystallinity (χc‑WAXD) of all samples obtained by a standard peak-fit procedure can be calculated by the following equation: χc‐WAXD =

∑ Acryst ∑ Acryst + ∑ A amorp

(1)

where Acryst and Aamorp are the fitted areas of the crystal and amorphous phases, respectively. 2.4. Two-Dimensional Small-Angle X-ray Scattering Measurement. Two-dimensional small-angle X-ray scattering (2D-SAXS) measurements were carried out at BL16B (λ = 9545

DOI: 10.1021/acs.iecr.6b02168 Ind. Eng. Chem. Res. 2016, 55, 9544−9554

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Industrial & Engineering Chemistry Research

3. RESULTS 3.1. Crystal Polymorphism and Crystallinity of PLA Nanocomposite Films. To examine the effect of GONSs on the crystal polymorphism and crystallinity of PLA nanocomposite films, 2D-WAXD characterization of neat PLA and its GONS nanocomposite films as a function of isothermal crystallization time was carried out. As shown in Figure 1,

0.124 nm) of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). A Mar165 CCD detector (2048 × 2048 pixels with pixel size of 80 μm) was used to collect 2D-SAXS patterns, which was placed 1895 mm from the sample. Each 2D-SAXS pattern was background corrected and then normalized using the standard procedure. One-dimensional scattering intensity distributions were obtained by circularly integrating the two-dimensional scattering patterns. To give detailed structural information on the systems, the electron density correlation function was also used to estimate the average thickness of the amorphous and crystalline regions. The one-dimensional electron density correlation function, K(z), can be derived from the inverse Fourier transformation of the experimental intensity distribution, I(q), as follows: ∞

K (z ) =

∫0 I(q)q2 cos(qz)dq ∞

∫0 I(q)q2 dq

(2)

where z is the location measured along a trajectory normal to the lamellar surface. q is the module of scattering vector and can be expressed as q = (4π/λ)sin θ, where λ is the wavelength of the X-ray and 2θ is the scattering angle. 2.5. Differential Scanning Calorimetry Characterization. The melting behavior of PLA nanocomposite films was investigated by differential scanning calorimetry (DSC) on a TA Q2000 instrument (United States). The experiments were carried out in nitrogen atmosphere using about 5 mg samples sealed in aluminum pans. The samples were heated from 40 to 200 °C at a heating rate of 10 °C min−1. Crystallization (χc‑DSC) of all the samples can be calculated as follows: χc‐DSC

ΔHm − ΔHcc = ΔHo

Figure 1. 2D-WAXD patterns of neat PLA and its GONS nanocomposite films as a function of isothermal crystallization time.

diffuse scattering rings are found in the first pattern of PLA0, PLA0.1, and PLA1.0. This is indicative of no crystals formed in PLA nanocomposite films without isothermal crystallization, regardless of the GONS loadings. With increasing the isothermal crystallization time, several uniform diffraction rings can be seen in all three samples, and the intensities of these diffraction rings become stronger with time. The uniform diffraction rings suggest that PLA crystallites are isotropically distributed in the nanocomposite films. 1D-WAXD curves of PLA0 (a), PLA0.1 (b), and PLA1.0 (c) as a function of isothermal crystallization time, integrated in a circular manner from their corresponding 2D-WAXD patterns, are illustrated in Figure 2. When the isothermal crystallization time is beyond 30 min, it is clearly visible that two strong diffraction peaks appear at 13.4° and 15.4° in all three samples with two relatively weak diffraction peaks at 11.9° and 18.0°. These four characteristic peaks are empirically assigned to (200)/(110), (203), (010), and (015) reflections of PLA α-form crystals, respectively.26 This confirms that GONSs have no impact on crystal polymorphism of PLA isothermally crystallized at 135 °C. Additionally, WAXD measurement is also an effective way to evaluate the crystallinity of the nanocomposite films. The values of χc‑WAXD determined by eq 1 are listed in Table 1. It is taken for granted that when the crystallization time is 0 min, all three samples are totally amorphous because a rather fast cooling rate (∼40 °C min−1) in compression molding effectively inhibits the formation of PLA crystals. After isothermal crystallization at 135 °C for 15 min, there is only a tiny amount of crystals in PLA nanocomposite films with the value of χc‑WAXD less than 1%. Interestingly, as the crystallization time rises up to 30 min, PLA0.1 exhibits a significant increase in χc‑WAXD with a value of 29.9%, which is much larger than that of neat PLA film. This can be explained in terms of the highly efficient heterogeneous

(3)

where ΔHo is the enthalpy of pure of PLA crystal (93 J g−1)33 and ΔH m and ΔH cc are melting enthalpy and cold crystallization enthalpy measured via DSC, respectively. 2.6. Scanning Electron Microscopy Observation. To explore the effect of GONSs on the crystalline morphology of PLA nanocomposite films, the surface of PLA nanocomposite films isothermally crystallized at 135 °C for 30 and 60 min was etched by a water−methanol (1:2 by volume) mixture solution containing 0.025 mol L−1 of NaOH for 25 h at room temperature. 34 Subsequently, the etched samples were repeatedly cleaned by using distilled water and ultrasonication, dried in vacuum oven overnight at 50 °C, and then coated with a thin layer of gold prior to being observed. A field-emission scanning electron microscopy (SEM; Inspect F, FEI, Finland) instrument was utilized to observe the crystalline morphology of the etched samples, and the accelerated voltage was held at 5 kV. 2.7. Barrier Testing. O2 permeation analysis of PLA nanocomposite films was conducted under constant volume− variable pressure conditions by using a VAC-V 2 film permeability testing machine (Labthink instrument, Jinan, China) at room temperature with 50% relative humidity according to ISO Standard 2556:1974. The gas permeation cell was completely separated into two compartments by film samples 100 mm in diameter. Gas in both compartments was continuously evacuated at least 12 h prior to testing. Thereafter, the feed pressure of O2 was 1.01 × 105 Pa and the permeation pressure as a function of time was recorded by pressure sensors to calculate the gas permeability coefficient. 9546

DOI: 10.1021/acs.iecr.6b02168 Ind. Eng. Chem. Res. 2016, 55, 9544−9554

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Industrial & Engineering Chemistry Research

Figure 2. 1D-WAXD curves of PLA0 (a), PLA0.1 (b), and PLA1.0 (c) as a function of isothermal crystallization time, integrated in a circular manner from their corresponding 2D-WAXD patterns shown in Figure 1.

relatively high loading of GONSs (1.0 wt %) restricts, rather than promotes, the formation of PLA crystals, displaying the lowest χc‑WAXD of 6.6% in all three samples. The GONSloading-induced transition of PLA crystallization kinetics from promotion to restriction is quite consistent with our previous work.30 Finally, after a time long enough for PLA to crystallize completely, the χc‑WAXD of all three samples is almost identical, varying slightly in the range of 43.2−44.7% (Table 1). The results about crystallinity will be further confirmed by DSC measurement below. 3.2. Melting Behavior of PLA Nanocomposite Films. Figure 3 delineates melting traces of neat PLA and its GONS nanocomposite films as a function of isothermal crystallization time. It can be seen from Figure 3a that PLA0-0 displays an apparent endothermic peak at around 62 °C overlapping the routine shoulder peak assigned to the glass transition. This endothermic peak is probably related to the physical aging effect of the compression molded films owing to short-range ordered structure relaxation.35,36 In the compression molding, the rapid cooling rate can notably suppress the movement of PLA chains, ultimately giving rise to a handful of short-range ordered structures, i.e., so-called mesomorphic phase. PLA chains are subsequently rearranged for a more favorable energetic configuration during heating in DSC measurement. This result is in line with our previous work.21 As the temperature increases, there is an exothermic peak for cold crystallization and two endothermic peaks for crystal melting in the PLA0-0. The mechanism of the double melting peaks have been demonstrated to be based on melting of the crystals formed in the cold crystallization stage during heating and followed by recrystallization and further melting processes at

Table 1. DSC Results Summarized from Melting Traces Shown in Figure 3 and Crystallinity Obtained by WAXD Characterization of Neat PLA and its GONS Nanocomposite Films as a Function of Isothermal Crystallization Time

nucleating effect of GONSs at a relatively low loading so that the crystallization rate of PLA is enhanced dramatically and more crystals are formed in the same time.26,30,31 In contrast, a 9547

DOI: 10.1021/acs.iecr.6b02168 Ind. Eng. Chem. Res. 2016, 55, 9544−9554

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Industrial & Engineering Chemistry Research

Figure 3. Melting traces of PLA0 (a), PLA0.1 (b), and PLA1.0 (c) as a function of isothermal crystallization time.

higher temperature.21,37 One can also observe in Figure 3a that with increasing isothermal crystallization time, the signals of short-range ordered structure relaxation and cold crystallization gradually subside, or even disappear in the PLA0-60, wherein the typical shoulder peak corresponding to glass transition and only one melting peak at high temperature are observed. This implies that 60 min is long enough for PLA to crystallize completely. When GONSs are incorporated, PLA nanocomposite films isothermally crystallized for different times exhibit melting behavior that is substantially similar to that of their neat PLA counterparts. The detailed results are summarized in Table 1. It is worth noting that the value of crystallinity obtained by eq 3 (χc‑DSC) is a little higher than that of χc‑WAXD, arising from the difference of detection principle for the two techniques. The collection of WAXD patterns is a nondestructive test, while the acquisition of DSC signals strongly depends on the thermal-induced transition of tested samples, such as endothermic or exothermic processes, or changes in heat capacity. Thus, it is more convincing to elucidate the inherent structure of PLA nanocomposite films and calculate their crystallinity by WAXD characterization. Even so, the variation tendency of χc‑DSC as a function of GONS loading and isothermal crystallization time are basically in accordance with that of χc‑WAXD, indirectly consolidating the WAXD results. 3.3. Crystalline Morphology of PLA Nanocomposite Films. As depicted in Figure 4, SEM observation for the etched surfaces of neat PLA and its GONS nanocomposite films is carried out to visually examine the impact of GONSs on the crystalline morphology of PLA nanocomposite films. At first sight, only typical PLA spherulites are exhibited in all three cases and the inclusion of GONSs has no significant effect on the geometric framework of PLA crystals. Nevertheless, it can

be vividly seen from Figure 4a,b that the addition of only 0.1 wt % GONSs results in a remarkable increase in the number of PLA spherulites with smaller size compared with neat PLA film isothermally crystallized for 30 min. This obviously originates from the excellent heterogeneous nucleating ability of GONSs, effectively increasing the nucleating density for PLA crystallization.26,30 Surprisingly, as the GONS loading rises up to 1.0 wt % (Figure 4c), fewer and smaller PLA spherulites are achieved. As previously reported,30 the critical gelation concentration of GONSs to form a network structure in the PLA matrix was estimated to be about 1.0 wt % and the GONS network slowed the motion of PLA chains tremendously and restricted their relaxation. As a result, a confined crystallization behavior of PLA was induced by the high concentration of GONSs. Therefore, the unpredictable crystalline morphology of PLA nanocomposite films at a GONS loading of 1.0 wt % (Figure 4c) mainly accounts for the formed GONS network, which not only reduces the nucleation activity of GONSs but also limits the ordered arrangement of PLA chains to form initial stable nuclei. Finally, as displayed in Figure 4a′−c′, after a time long enough for isothermal crystallization, PLA spherulites almost fill up the entire visual field. All three samples reach a thermodynamically stable state with nearly identical crystallinity. 3.4. Barrier Properties of PLA Nanocomposite Films. Figure 5 illustrates O2 permeability coefficient (PO2) of neat PLA and its GONS nanocomposite films as a function of isothermal crystallization time. As previously reported, the addition of GONSs could effectively block the O2 penetration in PLA nanocomposite films. A more than 35.4% reduction in PO2 from 2.087 × 10−14 to 1.348 × 10−14 cm3 cm cm−2 s−1 Pa−1 was achieved by adding 1.0 wt % GONSs.21 As for neat PLA 9548

DOI: 10.1021/acs.iecr.6b02168 Ind. Eng. Chem. Res. 2016, 55, 9544−9554

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corresponding counterparts. At the time point of 30 min, PLA0.1 displays the lowest PO2 with the value of 0.803 × 10−14 cm3 cm cm−2 s−1 Pa−1 in all three samples because of its highest crystallinity of 29.9%. More significantly, when the isothermal crystallization time increases to 60 min, the PO2 of PLA nanocomposite films at a GONS loading of 1.0 wt % is as low as 0.211 × 10−14 cm3 cm cm−2 s−1 Pa−1, about 84.3% and 89.9% reduction relative to PLA0-60 and PLA0-0, respectively. Such substantial improvement on gas barrier properties of PLA films is much higher than that obtained in our previous work,21 making PLA films more competitive than petroleum-based polymers in the packaging industry. The methodology proposed here opens up an avenue toward designing and fabricating high-barrier polymer nanocomposite films for packaging applications.

4. DISCUSSION It is well-accepted that the gas barrier properties of semicrystalline polymer incorporated nanoplatelets are determined to a large degree by the impermeable nanoplatelets, as well as its crystalline structure.23,24,38 In our case, the results reported in the preceding sections depicted that O2 barrier property of PLA nanocomposite films was elevated by a large margin. Undoubtedly, the efficiently enhanced barrier performance is closely related to the crystalline structure of PLA and the morphology of GONSs in the PLA matrix. The new insight into the contribution of crystalline structure and GONSs to the gas barrier property of PLA nanocomposite films is expected to help us further understand the mechanism of GONS advanced barrier performances and to provide guidance for designing and manufacturing high-barrier polymeric packaging materials. As for a semicrystalline polymer, the lamellar structure is frequently analogous to impermeable nanoplatelet and all the parameters pertaining to crystalline structure play an important role in determining the eventual barrier properties. First, the crystallinity is directly related to the content of the impermeable phase in the semicrystalline polymer. The increase of crystallinity not only increases the tortuosity of penetration path but also decreases the amount of amorphous phase through which the diffusing molecules can permeate.23,38 Second, crystalline morphology mainly refers to the dispersion and orientation of the crystalline phase in the semicrystalline polymer, where it is widely recognized that polymer lamellae are uniformly dispersed in the continuous amorphous phase and the high alignment of polymer lamellae perpendicular to diffusing direction can maximize the tortuosity of the penetration path for diffusing molecules.24,39 Lastly, crystal polymorphism and geometric size determine the intrinsic barrier performance of polymer lamellae.25 In our case, as shown in Figure 2, all the samples displayed the same characteristic diffraction peaks of PLA α-form crystals, regardless of the concentrations of GONSs and isothermal crystallization time. The uniform diffraction rings (Figure 1) and spherulite structure (Figure 4) revealed that PLA lamellae were evenly dispersed in all cases. The crystallinity of PLA nanocomposite films was calculated by DSC and WAXD techniques, as listed in Table 1. Nevertheless, the geometric size of PLA lamellae is still absent. SAXS characterization is a powerful tool for studying the microstructure of polymer materials, which is employed to quantitatively evaluate the average thickness of crystalline regions, in an effort to shed light

Figure 4. Typical SEM images of PLA spherulites for PLA0 (a, a′), PLA0.1 (b, b′), and PLA1.0 (c, c′) with isothermal crystallization times of 30 min (a, b, c) and 60 min (a′, b′, c′).

Figure 5. O2 permeability coefficient (PO2) of neat PLA and its GONS nanocomposite films as a function of isothermal crystallization time.

films, it is anticipated that the longer the isothermal crystallization time, the lower the PO2, owing to the gradual increase in crystallinity.23,38 Specifically, PLA0-60 with the crystallinity of 44.3% shows a P O 2 of 1.109 × 10 −14 cm3 cm cm−2 s−1 Pa−1, which is reduced by 46.9% relative to PLA0-0. These results indicate that GONSs are evidently superior to PLA crystals in enhancing gas barrier performances of PLA films, arising from their uniform dispersion and high aspect ratio. With the coexistence of GONSs and crystalline structure, barrier properties of PLA nanocomposite films are expected to be improved more significantly compared to their 9549

DOI: 10.1021/acs.iecr.6b02168 Ind. Eng. Chem. Res. 2016, 55, 9544−9554

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Industrial & Engineering Chemistry Research

with increasing isothermal crystallization time. Combined with the WAXD results shown in Figure 1, it can be speculated that the occurrence of scattering signals may be attributed to PLA crystals causing electron density contrast. The time evolution of 1D-SAXS profiles of PLA0, PLA0.1, and PLA1.0 is then illustrated in Figure 7. The 1D-SAXS profiles of PLA0-0, PLA0.1-0, and PLA1.0-0 show the typical features of an amorphous polymer. Unexpectedly, a very weak scattering maximum about q is observed in the vicinity of 0.31 nm−1, corresponding to a long spacing of ∼20.3 nm in the PLA0.1-15, while the scattering peak is absent in the PLA0-15 and PLA1.015. Taking into account the rather low crystallinity of the three samples (