Graphene Oxide-Driven Design of Strong and Flexible Biopolymer

Research Article pubs.acs.org/journal/ascecg

Graphene Oxide-Driven Design of Strong and Flexible Biopolymer Barrier Films: From Smart Crystallization Control to Affordable Engineering Huan Xu,†,§ Zhao-Xuan Feng,† Lan Xie,*,‡,§ and Minna Hakkarainen*,† †

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Stockholm 100 44, Sweden Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom § College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡

S Supporting Information *

ABSTRACT: Development of multifunctional, versatile biobased polymers can greatly benefit from the discovery and application of 2D sheet-like materials. For instance, the hybrid system integrating graphene oxide (GO) nanosheets with enantiomeric poly(lactic acid) (PLA) showcases several key properties that can address emerging multifunction needs such as good gas barrier and high thermal resistance. Here we revealed that large specific surface area and homogeneous dispersion of GO conferred the construction of interconnected networks in PLA even with relatively low GO contents (0.1 and 0.5 wt %). These well-extended GO nanosheets were ready to provide enormous and active platforms to nucleate preferentially the neighboring stereocomplex chains, prompting the prevailing development of stereocomplex crystals (SCs). The notable scenario associated with the GO distribution was imaged by 2D Fourier transform infrared spectroscopy, and was further elucidated by dynamic crystallization. More importantly, the nanosheets decorated with ordered PLA lamellae, in turn, contributed to the impressive enhancement in barrier and mechanical properties and chemical resistance. For example, a distinct decrease of 98.5% in oxygen permeability coefficient was observed for the composite films containing 0.5 wt % GO (6.264 × 10−17 cm3 cm cm−2 s−1 Pa−1) compared to the control sample crystallized at 150 °C (4.214 × 10−15 cm3 cm cm−2 s−1 Pa−1). The performance distinction was accompanied by the unusual combination of high tensile strength (73.5 MPa) and high elongation (13.6%), displaying an increase of 31.7% and 183.3% compared to the counterpart, respectively. This may provide a broader context for exploiting 2D nanosheets as robust cells to advance the function and property of PLA, which helps to outline the roadmap for fashioning high-performance, affordable bioplastics. KEYWORDS: Stereocomplex PLA, Graphene oxide networks, Selective stereocomplexation, Mechanical properties, Barrier films, Chemical resistance

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INTRODUCTION Several challenges remain before the full exploitation of renewable and degradable poly(L-lactic acid) (PLLA) and the ultimate goal of substituting traditional fossil-fuel plastics are reached.1−4 This is mainly associated with the inferior mechanical properties,5−9 low resistance to heat distortion,10−12 high gas/water permeation,13,14 and uncontrollable crystalline morphology of PLLA.15−19 These dwarfs, in principle, lay down paramount bottlenecks for the development of versatile PLLAbased packaging materials.20−22 Currently, the strategy pursued both by the scientific and the industrial communities involves optimizing multifunctional performances and mechanical properties of PLLA to prompt its broad commercialization in the emerging bioplastics market.23−26 It appears possible, for instance, that the incorporation of nanostructured fillers such as cellulose nanowhiskers, carbon nanotubes, and graphene oxide © XXXX American Chemical Society

(GO) nanosheets into PLLA may confer promising functionality and property enhancement.27−29 Of particular interest is two-dimensional (2D) GO owing to the high aspect ratio and excellent strength and modulus, allowing multiple properties to be simultaneously enhanced to provide desirable flexibility for designing functional polymer composites.30−33 This methodology has been well illustrated by Geng et al., opening up the possibilities to GO/biopolymer supercapacitors with a scalable method,34 GO/polyimide flexible foams with improved foaming capability,35 and devisible polymerization initiated by large GO surfaces.36 From the property-by-morphology perspective, the unique 2D GO nanosheets could construct Received: October 10, 2015 Revised: November 13, 2015

A

DOI: 10.1021/acssuschemeng.5b01273 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering

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robust “nano-barrier walls” to resist diffusing molecules, conferring the largely enhanced barrier properties of PLLA.37 As a good example, a distinct 64.6% decrease in oxygen permeability coefficient (PO2) was observed for the PLLA composites penetrated with 0.2 wt % GO compared to pure PLLA (3.76 × 10−18 m2 s−1 Pa−1).38 Another important task in expanding the applications of PLLA is to promote the intrinsically low heat deformation temperature (50−60 °C), which is far below that required for carrying daily life hot foods and beverages.39,40 Strategies to address this prominent limitation, to date, have mainly involved design and development of extraordinarily thermostable stereocomplex crystals (SCs) based on PLLA and its stereoisomer, i.e., poly(D-lactic acid) (PDLA).41−45 Compared to homocrystals (HCs) formed in PLLA or PDLA, SCs carry a set of peculiarly strong hydrogen-bonds between the stereoisomer chains, leading to distinguishably high melting temperatures (∼50 °C higher than that of HCs).46−48 The unique stereocomplex nature not only allows for the significant thermal distinction but also affords elucidation of higher resistance to hydrolytic degradation, providing the possibility to regulate the degradation kinetics of PLLA in a green and direct fashion, as reported by our group.49−51 This inspires us to conceive the design of a versatile stereocomplex system penetrated with the impermeable GO nanosheets, allowing for the appealing integration of sufficient heat shielding with the aid of thermostable SCs with enhanced gas barrier ability by the creation of robust “nano-barrier walls”. In a recent effort, we unveiled the decoration of a trace amount of GO nanosheets (0.05 wt %) by the preferred formation of SCs, not only on the enormous basal planes but also at the ultrathin edges of GO, during the melt crystallization at 165 and 180 °C.52 Consequently, the impressive combination of high heat deformation temperatures and low oxygen permeability coefficients was demonstrated, establishing the first beneficiary of utilizing GO nanosheets in exploiting the multifunctional stereocomplex poly(lactic acid) (PLA) materials. The impressive enhancement of thermal and barrier properties, in essence, can be appraised from the manipulation of crystalline morphology that exerts multiscale control over the interphase and within the matrix for PLLA composites.53−57 This methodology, moreover, has unlocked the realization of promoting the mechanical properties of PLLA, in particular for strength and stiffness.24,58 From the perspective of practical production, the desirable simplicity, versatility, and scale-up is accessible through conventional polymer processing techniques.20,59−61 The crystallization control to regulate the interfacial properties between 2D nanosheets and enantiomeric PLA could crucially advance the barrier and mechanical performances of composites.62−65 Understanding this process in the case of stereocomplex materials is challenging due to the competing nucleation and growth of HCs and SCs. Triggered by these interesting issues, we set out to reveal the role of GO nanosheets in controlling the stereocomplex crystallization of PLA, as well as the possibility to realize property enhancement and value-added bioplastics under crystallization control. This effort sheds light on the flexibility in the rotational regulation of stereocomplex crystallization by introducing highly active GO flatlands that confer the pursuit of multifunctional PLA commodities toward an optimized balance of thermal, barrier, and mechanical properties.

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EXPERIMENTAL SECTION

Materials. PLLA under the trade name of 4032D, comprising around 2% D-LA, was kindly supplied by NatureWorks (USA). It was characterized by a weight-average molecular weight of 11.9 × 104 and a number-average molecular weight of 6.6 × 104, respectively. Linear PDLA with a weight-average molecular weight of 9.6 × 104 and a number-average molecular weight of 6.2 × 104 was purchased from Polysciences, Inc. The molecular weight was determined by size exclusion chromatography. The melting points of pure PLLA and PDLA were evaluated to be 165 and 171 °C, respectively. Commercial graphite powder under the trade name of SP-1 was supplied by Bay Carbon, USA. Preparation of GO and GO Composites. Starting from the graphite powder, GO was prepared via a modified Hummer’s method.52 Widely used solution coagulation was applied to prepare the PLLA/PDLA composites loaded 0.05, 0.1, and 0.5 wt % GO (named GO0.05, GO0.1, and GO0.5, respectively), whereas pure PLLA/PDLA complex (named GO0) was prepared as a control blank. The GO contents were set at relatively low levels to obtain uniform dispersion and extension of nanosheets in the matrix, and equal-weight ratio of PLLA and PDLA (50/50) was weighted for both GO0 and GO-filled composites. Specifically, ultrasonically dispersed GO in ethyl alcohol (VWR, Germany) was dropped into the chloroform (VWR, Germany) solution containing PLLA/PDLA complex. The coagulated composites then gradually precipitated from the solution with excessive addition of ethyl alcohol. The dried coagulations were directly utilized for isothermal and nonisothermal crystallization. Polarized Optical Microscopy (POM) Observation. The dried coagulations placed on glass slides were melted at 250 °C for 5 min on a Mettler FP82HT heating stage to erase the thermal history, under the nitrogen atmosphere (80 mL/min). Then isothermal crystallization program was carried out by rapidly cooling the melts to preset crystallization temperatures, i.e., 150, 165, and 180 °C. After isothermal crystallization for 60 min, an Optiphot 2 microscope equipped with a Leica digital camera was utilized to observe the crystalline morphology. Moreover, nonisothermally crystallized samples cooled from the melts at a fixed rate of −10 °C/min were imaged. The temperature protocols are described in Figure S1, Supporting Information. Fourier Transform Infrared Spectroscopy (FTIR) Imaging. To detect GO dispersion and distribution, as well as the distribution of HCs and SCs, 2D FTIR absorbance patterns of quenched GO/PLA films and crystallized GO0 and GO0.5 were imaged using a PerkinElmer Spotlight 400 system equipped with an optical microscope (Bucks, UK). The film thickness was fixed at ∼50 μm to permit the direct comparison of absorbance intensity. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD) Measurements. To compare the crystalline morphologies developed in the composite films during isothermal and nonisothermal crystallization, 2D-WAXD measurements were carried out. For the sake of briefness, experiment details are available in our previous work.52,66 Concentrations of SCs and HCs were quantitatively analyzed based on the intensity profiles, as determined by the ratio of the area under the resolved Gaussian crystalline peaks of SCs and HCs to the total area under the unresolved diffraction curve. Apparently, the crystallinity of a specific sample should be a sum based on the concentrations of SCs and HCs. Differential Scanning Calorimetry (DSC) Characterization. The thermal behaviors of GO/PLA composites during the isothermal and nonisothermal melt crystallization were monitored by a Mettler Toledo DSC 820 under the nitrogen atmosphere (50 mL/min). Briefly, the thermal history was removed by melting the samples (5−6 mg) at 250 °C for 5 min, followed by (a) rapid cooling down to the preset temperatures (150, 165, and 180 °C) for isothermal crystallization for 60 min, (b) steady cooling at a rate of −10 °C/ min to 25 °C for nonisothermal crystallization, and (c) quenching to amorphous state prior to gradual heating up to 250 °C with a rate of 10 °C/min. The samples were directly taken from the solution B


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Figure 1. Determination of GO dispersion and distribution. (A) SEM images of fresh film surfaces showing homogeneous dispersion and full extension of GO nanosheets, which enabled the construction of interconnected sheet networks in GO0.1 and GO0.5. The matrix was etched by flowing chloroform to expose GO nanosheets. (B) 2D-FTIR absorbance patterns showing the dispersion of GO in quenched composite films as identified by the characteristic band located at 1716 cm−1, which is assigned to CO stretching vibration of −COOH units situated at the edges. The scale bar indicates 100 μm for 2D-FTIR images. coagulation of pure complex and GO composites. The temperature protocols for the DSC thermoprograms are shown in Figure S1. Scanning Electronic Microscopy (SEM) Observation. The isothermally and nonisothermally crystallized complex and GO composites after POM observations were quenched in liquid nitrogen, followed by etching for 72 h at 20 °C in a water−methanol− dichloromethane (1:2:0.1, v:v:v) solution containing 0.025 mol/L of sodium hydroxide and 0.1 mol/L of sodium chloride. The etching process was applied to remove the amorphous phase, both for homochiral and stereocomplex chains, to expose the crystalline regions.52 An SE-4800 SEM (Hitachi, Japan), operating at a low accelerated voltage of 0.5 keV to avoid surface damages, was employed to detect the crystalline morphology for the etched samples, which were sputter-coated with a 3.5 nm-thick gold layer prior to the SEM observations. Performance Evaluation. Following ASTM standard D3958, the oxygen permeability coefficient (PO2) of quenched and annealed films with a thickness of 200 μm was determined on an Oxtran 2/21 ML instrument at 23 °C. According to ASTM standard D638, tensile properties were measured at room temperature on an Instron universal test instrument (Model 5944, Instron Instruments, USA) with a load cell of 500 N. The crosshead speed was set at 5 mm/min and the

gauge length was 20 mm. A minimum of 6 replicates for each sample were tested to obtain the average values with standard deviation.

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RESULTS AND DISCUSSION The ability for GO nanosheets to provide crystallization control and property improvement relies heavily on the filler dispersion and distribution, which were appraised from direct SEM observation and 2D-FTIR imaging identification (Figure 1). The relatively low concentrations below 1 wt % allowed the GO nanosheets to disperse uniformly and fully extend in the matrix, displaying extremely large specific surface area and numerous wrinkles (Figure 1A). From the geological point of view, these structural features are in favor of the construction of dense, impermeable “nano-barrier walls” to enhance the gas barrier properties.67 Moreover, interconnected GO networks were created in the nanocomposites containing 0.1 and 0.5 wt % GO. The observed GO content required for threedimensional network construction is much lower in comparison to the previously reported critical values (generally >1 wt %).68 This is probably ascribed to the formation of strong interactions between nanosheets and stereocomplex chains, C


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Figure 2. POM micrographs showing the spherulitic textures formed in GO/racemic PLA composites after isothermal melt crystallization. The nucleation activity of PLA was evidently enhanced in the presence of GO nanosheets, regardless of crystallization temperature. The scale bar represents 100 μm for all images.

nucleation platform for the stereocomplex crystallization of PLA. This is instructive for controlling the crystalline morphology during melt processing of enantiomeric PLLA/ PDLA blends where high temperatures were generally applied.71−73 The intrinsic X-ray diffraction reflections specifically assigned to HCs and SCs, adopting the distorted 103 and 31 helical conformations, respectively, allowed us to distinguish HCs and SCs from each other and to calculate their concentrations and distributions in GO/racemic PLA composites.71,72,74 Figure 3 compares the crystal structures for isothermally crystallized composites as evaluated by 2D-WAXD measurements. It is interesting to note that the crystallization at 150 °C yielded a mixture of HCs and SCs for all the samples, mainly arousing the diffraction reflections from the lattice planes (010), (200)/ (110), (203) and (015) of HCs,60 and (110), (300)/(030) and (220) of SCs (Figure 3Aa−d).75 Particularly, the diffraction intensity of SCs gradually increased with the addition of nanosheets, whereas opposite variations were observed for the evolution of HCs. Exclusive formation of SCs was traced for all the samples crystallized at 165 and 180 °C, as shown in Figure 3Ae−l. Furthermore, the application of higher temperature drastically weakened the diffraction intensities of GO0 but unexpectedly strengthened the diffraction intensities of GOfilled composites. Figure 3B gathers the 1D-WAXD intensity profiles extracted from the 2D-WAXD patterns, producing quantitative analyses on the evolution of HC and SC concentrations with GO contents and crystallization temperatures (Figure 3C). For direct comparison, we note that the diffraction peak positions of GO0 and GO-filled composites coincide well regardless of temperature, which is indicative for unchanged crystal form in the presence of nanosheets. Specifically, the intensity curves of

which could enable the construction of interconnected networks among the homogeneously distributed GO nanosheets even with low loadings such as 0.1 wt %. This hypothesis is supported by the examination of GO distribution by tracing the carbonyl group (CO) stretching vibration of −COOH units situated at the edges of the GO nanosheets, accounting for the excitation of characteristic band located at 1716 cm−1 in the FTIR spectrum (Figure 1B).69 The absorbance of band 1716 cm−1 was exclusively observed in GO-filled composite films, in which the intensity was in direct proportion to the GO concentration. It is apparent that the GO composites exhibited homogeneous distribution of absorbance intensity, and only some locally aggregated sheets were detected in GO0.5. The nanosheet platforms featuring large and rippled surfaces are desired to anchor the polymer chains to enhance the nucleation activity,70 which essentially governs the kinetics of both stereocomplex and homochiral crystallization. Figure 2 reveals the overall picture of GO-induced crystalline morphology in racemic PLA. During the isothermal melt crystallization, the introduction of GO nanosheets tremendously assisted the nucleation activity of PLA, showing weak relation to temperature. It is well illustrated by the increased nucleation density and decreased radius in all the composites. At 150 °C, GO0 showed a fair nucleation density. Dramatically reduced density of spherulites was observed in GO0, as the temperature was increased to 165 and 180 °C, i.e., toward the prevailing generation of SCs near or above the melting points of HCs. This contrasted with the high nucleation activity and compact spherulitic textures found in the GO composites crystallized at the higher temperatures, which were only moderately decreased in comparison to those developed at 150 °C. It is apparent that, even at ultrahigh crystallization temperatures like 165 and 180 °C, GO nanosheets were still highly active in providing a D


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Figure 3. Crystal structures of PLA in the presence of GO. (A) 2D-WAXD patterns of isothermally crystallized GO/racemic PLA composites. The diffraction rings attributed to the lattice planes (010), (200)/(110), (203), and (015) of HCs are identified in panel a, whereas those assigned to the lattice planes (110), (300)/(030), and (220) of SCs are marked in panel h. No existence of HCs was traced in the samples crystallized at 165 or 180 °C. (B) 1D-WAXD intensity profiles extracted from panel A. The peak position and corresponding assignment to the specific lattice planes are marked on the diffraction peaks. (C) Concentrations of HCs and SCs determined by fitting the intensity profiles. The sum of concentrations of HCs and SCs produces the crystallinity.

the samples crystallized at 150 °C basically show seven diffraction peaks demonstrating the coexistence of HCs and SCs, whereas only three diffraction peaks implying the exclusive formation of SCs are observed at 165 and 180 °C (Figure 3B). Figure 3C gives some intriguing results for the concentration determination of HCs and SCs. First, although high crystallinity

around 48% was achieved for all the samples after crystallization at 150 °C, the addition of GO triggered large changes for the concentrations of HCs and SCs. As the GO content increased, the concentration of SCs gradually increased to 41.6% for GO0.5 from the initial value of 9.4% for GO0, accompanied by radical decrease in the HC concentration, falling down from E


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Figure 4. Distribution of HCs, SCs, and GO nanosheets in GO0 and GO0.5 revealed by 2D-FTIR imaging. (A) Identification of HCs after crystallization at 150 °C as pointed out by the absorbance patterns at the characteristic band of 923 cm−1. GO suppressed the generation of HCs. (B) Absorbance patterns of SCs (at 910 cm−1) after crystallization at 150, 165, and 180 °C demonstrating the enhanced stereocomplex crystallization with the aid of GO. (C) Distribution of GO in crystallized GO0.5 recognized by the absorbance patterns of the characteristic band at 1716 cm−1 revealing its close relation with the distribution of SCs. The scale bar indicates 100 μm for all images.

GO0.5, supporting the assumption that homochiral crystallization was suppressed due to the preferential development of SCs in GO-filled composites. Although the development HCs was moderately suppressed, there existed a distinct enhancement of stereocomplex crystallization aided by GO nanosheets, regardless of crystallization temperature (Figure 4B). An inspection of Figure 4B,C manifests that the development of SCs in GO0.5 was closely associated with the distribution of GO sheets, to the extent that the highest concentration of SCs was found in the GO-rich region. These correlated concentration fluctuations of GO and SCs can be perceived as direct evidence of GO-assisted stereocomplex crystallization, which in turn reduces the possibility of homochiral crystallization. Figure 5 reveals the crystalline structure in GO/PLA composites, in particular, the nucleic structure and lamellar alignment. For GO0 crystallized at 150 °C, assembled SC nanospheres served as the central nuclei to trigger the directional development of lamellae,82 which may involve the packing of both stereocomplex and homochiral chains (Figure 5a). Once the nanosheets were introduced, the development of SCs was preferred. This is illustrated by the close decoration of SC nanospheres on GO nanosheets observed in the GO0.05 crystallized at 150 °C (Figure 5b,b1). Randomly distributed, Sshaped SC lamellae were templated by the GO surfaces for the GO0.05 fostered at 165 °C (Figure 5c,c1). The nanosheets were, thus, providing extensive flatlands to anchor stereocomplex crystallization in a completely random manner, in

36.8% for GO0 to 12.3% for GO0.5. Second, the introduction of GO steadily pushed up the SC concentrations in the case of crystallization at 165 °C, climbing to 25.7% for GO0.5 from the lowest value of 10.8% for GO0. Third, it is unexpected to observe that the formation of SCs was enhanced in GO-filled composites after crystallization at 180 °C compared to the samples crystallized at 165 °C. For instance, the SC concentration of GO0.05 developed at 180 °C (20.2%) was 8.1% higher than that fostered at 165 °C (12.1%). It primarily lay in the accelerated growth of SCs at the higher temperature at the same time as the presence of GO conferred effective nucleation activity. By contrast, GO0 suffered an evident decrease of 4.0% in SC concentration. These observations emphasize the fact that GO nanosheets remarkably facilitated the generation of SCs even at ultrahigh temperatures, and the preferred formation of SCs, which has been previously perceived as nucleating agents for HCs,11,43,76−80 appeared to suppress the development of HCs to a surprising degree. HCs, SCs, and GO sheets can elicit prominent characteristic bands at 923, 910, and 1716 cm−1 in FTIR spectrum, respectively,47,52,72,81 offering the possibility to examine the evolution of HCs and SCs with the incorporation of GO by virtue of FTIR imaging technique. Figure 4 provides direct insights into GO-enabled regulation of homochiral and stereocomplex crystallization, as exemplified in GO0 and GO0.5 films. At 150 °C, Figure 4A reveals that the concentration of HCs in GO0 was moderately higher than in F


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Figure 5. Typical SEM micrographs illustrating GO-induced crystal textures. (a) GO0 crystallized at 150 °C, (b, c) GO0.05 crystallized at 150 and 165 °C, (d, e) GO0.1 crystallized at 165 and 180 °C, and (f, g) GO0.5 crystallized at 165 and 180 °C. The arrows point out the existence of GO nanosheets.

Figure 6. Competitive development of SCs and HCs examined by dynamic crystallization. (A) Cooling traces of the DSC program (−10 °C/min) from the melts for (a) GO0, (b) GO0.05, (c) GO0.1, and (d) GO0.5 (see thermoprogram in Figure S1b). Melt crystallization was significantly promoted by addition of GO. (B) Heating traces (10 °C/min) to examine the crystallization and melting behaviors of GO/PLA composites starting from the amorphous state (thermoprogram shown in Figure S1c). Greatly enlarged melting enthalpies for SCs were observed in GO-filled composites compared to GO0, indicating the preferred formation of SCs with the assistance of GO. The values of Tonset,mc and Tonset,cc are summarized in panel C.

analogy to the random epitaxy of polyethylene lamellae on the reduced GO surfaces as observed by Li.83 At the molecular level, we revealed the existence of strong interfacial interactions between the polymer chains and GO planes and edges, as

indicated by the red-shift for a set of functional groups in the FTIR spectra.52 Interestingly, the increase of GO loading gave rise to the generation of networks as indicated by the GO0.1 and GO0.5 samples (Figure 5d−g). These nanosheets triggered G


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Figure 7. Examination of GO-induced crystalline morphology during melt crystallization. (A) POM images and (B) 2D-WAXD patterns showing the enhanced nucleation activity and the preferred formation of SCs in GO-filled composites compared to GO0. The scale bar indicates 100 μm for all POM micrographs. (C) 1D-WAXD intensity curves extracted from (B). (D) Comparison of the concentrations of HCs and SCs calculated by fitting the diffraction intensity profiles. (a) GO0, (b) GO0.05, (c) GO0.1, and (d) GO0.5.

144.6 °C as observed in GO0.1. Meanwhile, the homochiral crystallization, accounting for the successive exotherm at the lower temperatures, was subjected to limited development. It was presumably derived from the involvement of homochiral chains in the preferential formation of SCs, as well as the spatial constraint originating from the extended nanosheets and dominating SC texture. Strong interactions between the chains and nanosheets were evidenced by the promotion of Tonset,cc, as clearly described in the heating program of amorphous samples (Figure 6B). Cold crystallization of GO0 was triggered at 111.5 °C, which prominently fell to ∼80 °C for the composite samples. As a response to the preferential formation of SCs in the composites during the cold crystallization, the melting enthalpies of SCs drastically climbed to around 60 J/g in direct comparison with the lowest value of 16.2 J/g for GO0. After the nonisothermal melt crystallization, POM and 2DWAXD were carried out to examine the GO-induced crystalline morphologies, especially the nucleation density, spherulitic size, and distribution of HCs and SCs (Figure 7). Figure 7A reveals large changes in the spherulitic textures of GO0 and the composites penetrated with nanosheets. Unlike the distinguishable, large spherulites developed during the isothermal crystallization (Figure 2), extremely tiny and dense crystalline entities with a radius ranging from several to ten micrometers were found in the cooled GO0. With the nucleating assistance of GO, a few large-sized spherulites, some even reaching a radius of 90 μm (GO0.1), were distinctly observed in the composites. The GO-assisted preferred nucleation of SCs

the formation of central nuclei (Figure 5d1,f1,g1) and were also engaged in the lamellar packing of SCs (Figure 5e1), which was responsible for the preferred facilitation of stereocomplex crystallization, especially at the higher temperatures (Figures 3 and 4). In particular, Figure 5g2,g3 offers a unique scenario for the formation of distorted SC lamellae, resulting primarily from the numerous lamellae originated in the selective generation of SCs anchored by the interlaced surfaces and edges of GO nanosheets. The above observations imply the GO-driven competitive development of HCs and SCs. This assumption was examined by the dynamic crystallization programs: melt crystallization during steady cooling from the melts (Figure 6A), and cold crystallization starting from the amorphous state (Figure 6B). The onset points for melt and cold crystallization (Tonset,mc and Tonset,cc, respectively) are summarized in Figure 6C. During nonisothermal melt crystallization, exothermic peaks occurring in an extremely broad temperature range (165−70 °C) were observed for all samples, as demonstrated in Figure 6A. Moreover, an inspection of Tonset,mc reveals that the enhanced nucleation activity was found in GO-filled composites with a Tonset,mc of approximately 165 °C, which was around 15 °C higher than that of GO0. Basically, the exothermic peaks occurring at the first stage, arising from the nucleation and growth of SCs, shifted to higher temperatures and exhibited larger areas after the addition of GO nanosheets. This can be exemplified by the weak exothermic peak located at 126.6 °C for GO0, which was transformed to the prominent peak at H


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Figure 8. SEM images describing the crystal architecture in nonisothermally cooled GO/PLA composites from the melts. (a) GO0, (b) GO0.05, (c) GO0.1, and (d) GO0.5. Tiny spherulitic SCs were involved in the assembly of spherulites for all samples, and the strong interactions between the GO nanosheets and PLA chains were clearly discernible. GO nanosheets were ready to provide enormous platforms for nucleating SC nanospheres, and (b2), (c2), and (d2) illustrate that the single- or few-layered nanosheets were closely connected to the SC nanospheres.

should be responsible for the observation of dramatically increased size, in contrast to the sparse self-nucleation in GO0. The prevailing generation of SC spherulites in the composites appeared to spatially restrict the growth of neighboring HCs, as further evidenced by the decreased diffraction intensity of HCs in 2D-WAXD measurements (Figure 7B). In the cooled GO0 and composites, we noticed the coexistence of HCs and SCs in the diffraction patterns, mainly ascribing to the lattice planes (200)/(110) and (203) of the HCs and lattice planes (110), (300)/(030), and (220) of the SCs. Figure 7C manifests that the diffraction peaks of SCs shifted to higher intensity as the GO content rose, accompanied by the reverse variations for the diffraction intensities of HCs. The concentrations of HCs and SCs were compared for GO0 and GO-filled composites in Figure 7D. The overall crystallinity steadily grew with the addition of nanosheets, climbing to 34.8% for GO0.5 from the lowest value of 17.2% for GO0. The substantially increased concentrations of SCs, in essence, should be responsible for it. An over 4-fold increase in the SC concentration was achieved in GO0.5 (32.4%) compared to that of GO0 (5.9%). Figure 8 offers insights to interpret how the SCs met GO nanosheets during cooling by examining the crystal architecture. Figure 8a reveals that huge quantities of nanosized SC

spheres, sharing the similar structural characters with the stereocomplex microspheres observed by Tsuji82 and Biela et al.,84 were involved in the directional growth of lamellae that assembled into regular spherulites. Evidently, the SC nanospheres not only launched the primary nucleation but were also engaged in the growth proceeding, indicating the key role of SCs in governing the crystalline morphology (Figure 8a1,a2). The density of SC nanospheres was notably increased by application of nanosheets, leading to the generation of incredibly compact nanospheres existing in GO-filled composites (Figure 8b−d). This explains the significant decline in the diameter of SC spheres, falling from approximately 1 μm for GO0 to around 650 nm for GO0.05 and further to about 320 nm for GO0.1 and GO0.5 (Figure 8a2−d2). Figure 8b1−d1 provides some interesting scenarios with respect to the interactions between the nanospheres and nanosheets. Specifically, acting as extremely large platforms, the well extended, mono- or few-layered nanosheets were closely wrapped by dense SC nanospheres, which nucleated the spherulitic development (Figure 8b1−d1). Moreover, the nanosheet edges were ready to induce the formation of SC nanospheres, creating tenacious filaments with the matrix, as clearly pointed out in Figure 8d2. The high nucleation activity I


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Figure 9. Schematic representation comparing the evolution of homochiral and stereocomplex crystallization in (a) GO0 and (b) GO reinforced composites. Preferential nucleation of SCs assisted by GO, both on the basal planes and at the edges, leads to the dominating development of SCs and spatially hindered HCs, whereas limited SCs are generated during the simultaneous growth of HCs for GO0.

Figure 10. Performance evaluation to demonstrate GO-enabled multifunction by crystallization control. (A) Comparison of PO2 for quenched and annealed composite films suggesting that the decoration of GO nanosheets improved the resistance to oxygen permeation. (B) Tensile strength, (C) Young’s modulus, and (D) elongation at break of quenched and annealed composite films revealing the crystallization-driven combination of high strength and ductility in GO-filled PLA films.

of nanosheets for SCs, in principle, elicited the evolution of the distributions of HCs and SCs in pure PLA complex and GOfilled composites, i.e., GO-driven steady increase in SC concentration and suppressed generation of HCs.

Overlapping the evolution of crystalline morphologies observed during the isothermal and nonisothermal crystallization clearly indicates that GO nanosheets promote the preferential launch of stereocomplex nucleation and assist the prevailing growth of SCs by spatially suppressing the J


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Figure 11. Fracture mechanism responsible for the exceptional promotion of ductility in crystallized GO composites. (A) Typical SEM images showing fracture surface morphology after tensile failure. Obviously crystallized GO0 and amorphous GO0.5 were characterized by brittle fracture behavior, whereas rich plastic deformation was found in crystallized GO0.5. (B, C) Schematic scenarios comparing the fracture mechanisms in crystallized GO0 and GO0.5, respectively. Semirigid chain backbone led to brittle fracture of GO0, whereas crystal-decorated nanosheets in GO composites may serve as tenacious units to permit lamella slip on the surfaces and transfer neighboring stress, finally rendering the distinct transition to ductile fracture behavior.

homochiral crystallization. Figure 9 schematically depicts the major morphological features during the competitive stereocomplex and homochiral crystallization, in which the SCs at the same time facilitate the nucleation of HCs. In pure enantiomeric PLA (Figure 9a), the nuclei of SCs are created in advance due to the relatively high crystallization ability, which in turn anchor and aid the neighboring homochiral chains to pack into the homochiral lamellae. Given the SCassisted homochiral crystallization, there are fewer chances to form stereocomplex nuclei after the involvement of adjacent homochiral chains into the HC lamellae. The possibility to develop SCs will be further reduced with the compact formation of HCs, which imposes a strong spatial hindrance

onto the growth of SCs. As a result, low concentrations of SCs restricted by the surrounding HCs are generated during both the isothermal melt crystallization at 150 °C (Figures 3 and 4) and the nonisothermal crystallization (Figures 6 and 7). The formation of limited SCs may unfavorably lead to inadequate contribution to thermal resistance enhancement. Figure 9b describes the overarching nucleation of SCs by virtue of GO nanosheets. The high nucleation activity of nanosheets mainly lies in the strong absorption and anchoring of stereocomplex chains, in which both the basal planes and ultrathin edges provide the landing platforms.52 Provided with the dense creation of SC nuclei prior to the launch of HCs, stereocomplex chains are involved in the growth of SCs at the early stage K


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Figure 12. Digital photos showing the improved chloroform resistance for crystallized GO0.5. GO0-150, and GO0.5-150 (GO0 and GO0.5 crystallized at 150 °C, respectively), and GO0.5-Quenched (GO0.5 quenched from the melts) film sheets (∼0.3 g) were immersed in chloroform (30 mL) for up to 10 min. The GO0 film, albeit crystallized at 150 °C, showed the lowest resistance to chloroform, being swollen after only 20 s. The quenched GO0.5 was completely swollen after 3 min, whereas the crystallized GO0.5 was not corroded even after 10 min (finally being swollen from 12 min).

for GO0.1, and over 12% for GO0.5, showing weak relation to crystallization temperature. These observations are unprecedented in comparison with the normal hardening of composite materials after introducing stiff nanofilllers. Stimulated by the unusual combination of strength, stiffness, and ductility, we further revealed the structural features existing in crystallized GO composites giving rise to the brittle-toductile transition. The fracture behaviors of crystallized GO0 and quenched and crystallized GO0.5 were appraised from the fractured surfaces after tensile failure (Figure 11A). Notably, some significant changes distinguished the fractured surfaces of the crystallized GO0.5 from those of crystallized GO0 and quenched GO0.5. In clear contrast to the flat and smooth surface morphology observed in crystallized GO0 and quenched GO0.5 (Figure 11Aa,b), the fractured surfaces of crystallized GO0.5 were extremely bumpy and coarse with a scale-like appearance (Figure 11Ac,d). Such a creation of numerous plastic deformation, which is in great need for dissipating a large amount of energy, is an unexpected phenomenon that is rarely reported in plasticizer-free composite systems, especially after addition of stiff nanofillers.58 Figure 11B,C extracts the key morphological features of crystallized GO0 and GO composites that are responsible for the tensile behavior distinctions. Presenting semirigid chain backbone, PLA chains with defects are prone to be directly fractured without sufficient chain segment regularization or plastic deformation (Figure 11B).58 This scenario explains the poor ductility of enantiomeric PLA, which is slightly deteriorated with the creation of stiff regions during crystallization.61 It is fairly convincing that the existence of GO nanosheets decorated with rich lamellae serve as strong and resilient ligaments or filaments, rendering a multifunctional reinforcement of PLA composites in terms of strength, modulus, and ductility (Figure 11C). This scenario arises from the three main morphological features governing the resistance to stress deformation: (1) a higher volume percentage of crystalline regions was generated due to GOtriggered high nucleation activity for PLA, decreasing the amount of relatively defective amorphous chains; (2) a large amount of stress can be transferred and dissipated at the GO/ PLA interfaces through epitaxial lamellae closely connecting nanosheets and neighboring chains, conferring the tenacious extension of chains and lamellae along the deformation

crystallization, resulting ultimately in the prevailing development of SCs. Moreover, the spatial confinement generated by the neighboring large-sized stereocomplex crystalline entities and the well-extended nanosheets should be responsible for the suppressed development of HCs.85 It is apparent from Figure 10 that the combination of GO addition and crystallization control greatly benefit the barrier and mechanical properties of PLA. We see from Figure 10A that PO2 of quenched PLA films was much lowered after introduction of GO nanosheets that may serve as impermeable “nano-barrier walls” to resist oxygen permeation, showing a gradual fall from 4.869 × 10−15 cm3 cm cm−2 s−1 Pa−1 for GO0 to 2.553 × 10−16 cm3 cm cm−2 s−1 Pa−1 for GO0.5. This barrier regulation can be further promoted by decorating the GO nanosheets with the aid of controlled crystallization, as exemplified by the lowest PO2 achieved in GO0.05 crystallized at 150 °C (6.264 × 10−17 cm3 cm cm−2 s−1 Pa−1), displaying a distinct decrease of 75.7% compared to the quenched GO0.5. The prominent control arises from the four main morphological features retarding the gas permeation process, namely, enlarged active territories of impermeable “nano-barrier walls” after decoration of GO edges with compact lamellae,52 enhanced interfacial interactions between PLA matrix and crystal-decorated nanosheets, improved resistance to diffusing gas molecules due to the existence of GO-induced rich crystals especially the SCs, as well as reduced solubility of oxygen given the decreased amorphous regions.20 Figure 10B−D manifests the obvious distinctions in the tensile properties for quenched GO-filled composites and those submitted to crystallization control. With the addition of strong and stiff GO nanosheets, tensile strength and Young’s modulus were increased gradually with weak relation to the crystallization temperature, climbing to approximately 70 and 2200 MPa for GO0.5 from around 52 and 1600 MPa for GO0, respectively (Figure 10B,C). Of more significance is the unusual enhancement of ductility achieved by crystallization control, as shown in Figure 10D. It is obvious that GO0 was characterized by the typical low elongation of ∼5%, regardless of crystalline morphology. The enhancement of ductility by virtue of tailoring the crystalline morphology in GO-filled composites was observed. Although quenched GO composites presented poor ductility in the same level with that of GO0, elongation was increased to over 8% for GO0.05, around 12% L


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the crystallization temperature. When combined with high tensile strength (∼70 MPa), modulus (∼2200 MPa), and elongation (∼13%), our structured GO0.5 composite films are conferred with great potential for a diverse range of applications. These interesting findings facilitate the rational design by nanosheet decoration to reach diverse functionalities from high thermostability to excellent gas barriers and to improved chemical resistance, all of interest for wider application of PLA-based materials.

direction; and (3) lamellae slip between GO nanosheets and epitaxial lamellae allows for the increase of ductility and thus the strong retardation of crack propagation. The unusual combination of strength and ductility in PLA composites containing GO nanosheets decorated with numerous lamellae suggests interesting generalizations concerning the role of morphology control in creating evolutionary innovations and adaptive radiation for the industrial manufacture of highperformance PLA. Equally desirable, from both academic and commercial points of view, is the opening of potential for diverse applications ranging from packaging to aviation. In addition to the notable improvement in barrier and mechanical properties, it is interesting to discover that the composite films containing GO networks decorated with SCs were characterized by much higher resistance to chloroform. As Ma et al. reported, the resistance to chloroform corrosion was improved after the cross-linked stereocomplexation between enantiomeric PLA chains, enabling the observation of undissolved gel.86 Figure 12 suggests that the GO0 film was preferentially inflected in chloroform after only 10 s, followed by the fast swelling in 30 s. The GO0 was dominated by HCs (36.8%) rather than SCs (9.4%, Figure 3), which caused the unfavorable erosion of the amorphous and homocrystallized regions. No existence of gel-like entities, however, was found in the solution dissolving GO0 (Figure 12d). This was presumably due to the extremely low concentration of SCs in the solution. With the incorporation of chemically stable GO networks, the quenched GO0.5 showed moderately higher resistance to chloroform swelling, displaying substantial inflection after 60 s. Once the GO frameworks were decorated by prevailing SCs (41.6%, Figure 3) instead of HCs (12.3%), the crystallized GO0.5 presented the highest stability in chloroform, showing the undamaged shape and good condition even after 10 min. This unexpected observation is probably due to the synergetic effects of GO networks and high concentration of SCs. Furthermore, the construction of interconnected large GO sheets, which were closely decorated by SCs, may provide effective shielding from chloroform corrosion. This finding may help our composite films find wider applications under chemically corroding conditions.

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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.5b01273. Temperature protocols for crystallization programs, examination of thermally induced chain degradation and oxidization in GO0 melts by SEC and FTIR, SEM images examining the GO quality during crystallization, DSC heating traces of isothermally crystallized GO/PLA composites, SEM observation of nucleic and lamellar textures for GO0 annealed at 165 and 180 °C, SEM and TEM images showing the morphology of individual GO nanosheets (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*L. Xie. Email: [email protected]. *M. Hakkarainen. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are deeply indebted to Dr. Patrick Baker from the Department of Molecular Biology and Biotechnology, University of Sheffield for his kind help during the X-ray measurements. H.X., Z.-X.F., and L.X. are grateful to the financial support from the China Scholarship Council (CSC) for studying abroad.

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CONCLUSIONS Mechanically strong and thermally stable PLA barrier films were realized by integrating SCs-decorated GO nanosheets in enantiomeric PLA. In the PLA matrix, GO nanosheets were homogeneously dispersed and fully extended, principally due to the relatively low GO loadings and adequate interactions between GO and PLA. These morphological features conferred the construction of interconnected networks of GO nanosheets. During the examination of isothermal and nonisothermal crystallization, the selectively accelerated stereocomplex crystallization in the presence of GO rendered the prevailing development of SCs in the direct proportion to the GO loadings. At the same time, the generation of HCs was suppressed, primarily due to the limited availability of homochiral chains and spatial hindrance caused by the surrounding nanosheets and SCs. More importantly, the decoration of GO with ordered PLA lamellae served as strong and resilient ligaments, accounting for the promising combination of low PO2, high strength, high ductility, and improved chemical resistance for GO/PLA composites. This was exemplified by the distinct fall of 98.5% in PO2 of annealed GO0.5 in comparison to that of GO0, showing weak relation to

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