- Email: [email protected]
polylactide blends
Accepted Manuscript Effect of nitrogen-doped graphene on morphology and properties of immiscible poly(butylene succinate)/polylactide blends Wei Wu, ChengKen Wu, Haiyan Peng, Qijun Sun, Li Zhou, Jiaqing Zhuang, Xianwu Cao, V.A.L. Roy, Robert K.Y. Li PII:
S1359-8368(16)32257-0
DOI:
10.1016/j.compositesb.2017.01.037
Reference:
JCOMB 4848
To appear in:
Composites Part B
Received Date: 10 October 2016 Revised Date:
23 December 2016
Accepted Date: 25 January 2017
Please cite this article as: Wu W, Wu C, Peng H, Sun Q, Zhou L, Zhuang J, Cao X, Roy VAL, Li RKY, Effect of nitrogen-doped graphene on morphology and properties of immiscible poly(butylene succinate)/ polylactide blends, Composites Part B (2017), doi: 10.1016/j.compositesb.2017.01.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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GRAPHICAL ABSTRACT:
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Effect of Nitrogen-Doped Graphene on Morphology and Properties of Immiscible Poly(butylene succinate)/Polylactide Blends Xianwu Cao,*b V. A. L. Roya and Robert K.Y. Li*a
ABSTRACT
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Wei Wu,a,b ChengKen Wu,c Haiyan Peng,a,d Qijun Sun,a Li Zhou,a Jiaqing Zhuang,a
Plastic pollution has become a serious issue to the ecosystem, and biodegradable poly(butylene
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succinate)/polylactide (PBS/PLA) blends are regarded as the promising eco-friendly alternatives to replace the non-degradable plastics based on fossil fuels. Yet, the thermodynamically immiscible nature
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of PBS and PLA hinders their extended applications. In this contribution, nitrogen-doped graphene (NG) is introduced into immiscible PBS/PLA blends by melt compounding. The incorporation of NG in PBS/PLA (70/30wt%) blends is observed to significantly improve the geometrical morphology and reduce the domain size of the dispersed PLA phase, indicating a compatibilization effect of NG on the immiscible blends. The TEM micrographs showed that the NG mainly dispersed in the PBS matrix while a small amount is located in PLA phase. When the NG concentration increases to 1.0wt%, the
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NG filled PBS/PLA nanocomposites exhibits an obvious improvement in the storage modulus and loss modulus in comparison with the pristine PBS/PLA blend. The thermal stability of the PBS/PLA/NG nanocomposites is enhanced monotonously with an increase of the NG concentration, due to the barrier
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effect of NG and good interaction between the NG and polymer matrices. Moreover, the NG is noticed to act as a nucleating agent to significantly improve the PBS crystallinity without affecting the crystal forms of PBS and PLA. The tensile strength, tensile modulus and elongation at break of the blends
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could be enhanced by the low concentration of NG.
a
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail:
[email protected]
b
National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing
Engineering of Ministry of Education, South China University of Technology, Guangzhou 510640, China. E-mail: [email protected] c
The Twenty-third Research Institute of China Electronics Technology Group Corporation, Shanghai 201900, China.
d
Key Laboratory for Material Chemistry of Energy Conversion and Storage, School of Chemistry and Chemical Engineering,
Huazhong University of Science andTechnology, Wuhan 430074, China.
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1. Introduction Plastics are indispensable materials nowadays but the plastic pollution has been
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becoming a big issue to the ecosystem. During the last two decades, great efforts have been made to circumvent the problem by developing biodegradable polymers.[1-5] To date, the applications of biodegradable polymers have spread to various areas,
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including but not limited to agriculture, packaging, biomedicine, and automotive.[6-9]
As outstanding commercial-available representatives, poly(butylene succinate) (PBS)
replace
traditional
commodity
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and polylactide (PLA) are regarded as the promising eco-friendly alternatives to thermoplastics,
due
to
their
renewability,
biodegradability and biocompatibility.[10-12]
PBS is a biodegradable thermoplastic polymer in the aliphatic polyester family,
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and synthesized by polycondensation of butanediol and succinic acid that are available from renewable resources.[13, 14] PBS holds properties that are comparable to low density polyethylene (LDPE) and polypropylene (PP) such as high flexibility
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and high impact strength. Yet, the low melt viscosity and insufficient stiffness of PBS hinder its processing and extended applications.[15-17] On the contrary, PLA is a
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biodegradable polymer that exhibits high Young’s modulus, albeit high brittleness.[18] Thus, the blending of biodegradable PBS and PLA has attracted ever-increasing interest in both academia and industry for the performance improvement purpose. A large number of studies have been done on the morphology, thermal behaviours, and mechanical
properties
of
PBS/PLA
blends
to
understand
the
materials
structure-property relation.[19-22] Nevertheless, significant phase separation is
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usually observed in PBS/PLA blends due to the thermodynamically immiscible nature of PBS and PLA, resulting in unsatisfactory blends performances.[22-24] To improve
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the compatibility of multi-component polymers, the introduction of inorganic fillers has been explored to be a simple and effective way as it is able to enhance the
interface action.[25-28] Wang et al. found that the selective localization of carbon
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black in the PBS matrix enabled enhanced processibility via increasing the melt viscosity and domain size refinement of the dispersed PLA phase, resulting in
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increased stiffness and thermal stability of the PBS/PLA blend.[25] Chen and Yoon reported that an epoxy-functionalized organo-clay (TFC) could act as a compatibilizer for the PBS/PLA blend, and they revealed that when the concentration of TFC was higher than 0.5 wt%, the clay layers were dispersed in both the PBS and PLA phases,
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and the domain size of the dispersed PBS phase became significantly smaller.[26] According to these reports,[26-28] the clay could build an interaction between the clay surface and the polymer matrix through chemical reactions, resulting in
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improvements in viscosity, thermal stability and mechanical properties of the PBS/PLA blend system.
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Graphene, a two-dimensional carbon nanomaterial, has been employed as a type
of novel filler for polymer nanocomposites.[29, 30] It has already been reported that the introduction of even a small fraction of graphene can remarkably enhance the mechanical, thermal, electrical, optical, and chemical performances of polymer matrices.[31-35] Recently, graphene and its derivatives have also been applied to improve the compatibility of immiscible blends.[36-39] Cao et al. found that the
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amphiphilicity of graphene oxide (GO) was capable of improving the compatibility of immiscible polyamide/poly(phenylene oxide) blends.[37] Ye et al. demonstrated that
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the compatibilizing effect of GO for poly(methyl methacrylate)/polystyrene blends exhibited a temperature dependence, i.e., when the processing temperature increased,
the compatibilizing effect of GO decreased.[38] Kar et al. disclosed that the
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introduction of the poly(methyl methacrylate) grafted GO (PMMA-g-GO) was able to improve the interface action between poly(vinylidene fluoride) and acrylonitrile
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butadiene styrene polymer.[39] The grafted mutually miscible polymer (i.e., PMMA) could enhance the compatibilizing effect of graphene, improving the strength and modulus of the blends simultaneously. Nevertheless, to the best of our knowledge, there is no report on the graphene filled PBS/PLA blends. Since graphene itself is
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hydrophobic, it is believed to be useful to introduce functional groups on graphene for improving the graphene dispersion and interfacial interaction with polymers.[40] Nitrogen-doped graphene (NG) is such a functionalized material with an abundant of
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polar oxygen and nitrogen groups on its surface,[41] and a large aspect ratio, thus is envisioned to be a rational candidate to offer extensive interactions between NG and
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polymers. In the present work, NG is incorporated into PBS/PLA blends by melt compounding. The phase morphology, rheological properties, thermal behaviours and mechanical properties of the PBS/PLA/NG nanocomposites are investigated in detail.
2. Experimental Section 2.1 Materials
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Nitrogen-doped graphene (NG, grade TNNRGO) with nitrogen content of 7.24 at.% and oxygen content of 7.53 at.% was supplied by Chengdu Organic Chemicals
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Co. Ltd., Chinese Academy of Sciences (Chengdu, China) (Fig. S1 and Table S1). The typical morphology of NG was shown in Fig. S2 and the thickness of NG was approximately 3 nm (Fig. S3). Poly(butylene succinate) (PBS, Bionolle 1020MD) was
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provided by Showa Polymer Co. (Tokyo, Japan), with a density of 1.26 g/cm3 and melting temperature of 115 oC. The melt flow index of PBS was determined to be 25
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g/10 min (2.16 kg, 190 oC). Polylactide (PLA, Grade 4032D) was obtained from Natureworks (Minnetonka, USA). According to the supplier, the PLA had a density of 1.24 g/cm3, and a glass transition temperature (Tg) range of 55-65 oC. The melt flow index for PLA was characterized as 7 g/10 min (2.16 kg, 210 oC).
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2.2 Nanocomposites Preparation
Prior to nanocomposites preparation, PBS and PLA pellets were dried in an oven
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at 80 oC for 5 h. Various concentration of NG was incorporated into PBS/PLA blend, with a weight ratio of PBS to PLA fixed at 70/30. The blends were prepared by melt
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compounding using an internal mixer (PLASTIC-ORDER, Brabender, Germany) with a rotation speed of 60 rpm for 5 min at a fixed temperature of 190 °C. The PBS/PLA blends with four different NG loadings, namely, 0, 0.3, 0.5 and 1 wt%, were correspondingly abbreviated as PBS/PLA0, PBS/PLA0.3, PBS/PLA0.5 and PBS/PLA1. The samples with a thickness of about 1 mm for measurement were subsequently molded into specimens by using hot compression molding (190 °C, 5
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min), followed by air quenching to room temperature.
2.3 Characterization
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The surface morphology of blends was investigated using a field emission scanning electron microscopy (FE-SEM, Quato250, FEI, USA). The samples were
cryogenically fractured in liquid nitrogen. Then the PLA phase was etched out by
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immersion in 1,4-dioxane 1 h at room temperature. Finally, the prepared surface was
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sputter-coated with gold before observation. For the tensile-fractured surfaces of the samples, they were examined directly by the FE-SEM after sputting with gold. Transmission electron microscopy (TEM, Technai 12, Philips, Netherlands) was used to characterize the dispersion of NG in PBS/PLA nanocomposites. The ultrathin slices with approximately 100 nm thick were cut using a cryo-diamond knife at -100
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o
C (EM UC6+FC6, Leica, Germany). Then the slices were transferred onto 300 mesh
copper grids and observed at an acceleration voltage of 120 kV.
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Rheological measurements were conducted on a strain-controlled dynamic rheometer (MCR302, Anton-Paar, Austria) using two parallel plates with a diameter
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of 25 mm. Dynamic frequency sweep tests were performed under a nitrogen atmosphere to evaluate the viscoelastic properties of the blends. The frequency range was set as 0.1 ~ 100 rad/s at a constant strain of 1%, which was within the linear viscoelastic regime as determined by dynamic strain sweep experiments. Before the test, all samples were heated to 190 oC and kept for 3 min to eliminate the previous thermal history.
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The thermal stability of the blends was evaluated by a thermogravimetric analyzer (TGA) (TG209F3, Netzch, Germany). Approximate 9 mg of the sample was
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heated from room temperature to 600 oC at a heating rate of 10 oC/min under nitrogen atmosphere.
The crystallization and melting behaviours of blends were characterized on a
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differential scanning calorimeter (DSC) (DSC204F3, Netzch, Germany). Approximate
5 mg of the sample was sealed in an aluminum pan. The samples were firstly heated
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from room temperature to 200 °C with the heating rate of 10 °C/min. Then the samples were kept at 200 °C for 3 min, and cooled to -50 °C at 10 °C/min subsequently. The curves of heat flow as a function of temperature were recorded. X-ray diffraction (XRD) patterns were carried out by an X-ray diffractometer
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(AXS D2, Bruker, Germany) with Cu-Kα radiation. The data were collected in the 2θ range from 5° to 30° with a step of 0.02° (2θ). The tensile properties of the blends were examined using an electronic universal
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testing machine (Model 5566, Instron, USA) with a load force of 10 kN. The dumbbell-shaped specimens with dimension of 25 × 5 × 1 mm were performed at a
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tensile speed of 5 mm/min. The reported results were average values for at least five separate specimens.
3. Results and discussion Phase
separation
structures
determine
the
polymer
composites
performances.[42-44] Fig. 1 illustrates SEM micrographs of the etched cryofracture
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surfaces of PBS/PLA blends with various NG concentrations. The black domains indicated the position of the extracted PLA phase. It is clear that the neat PBS/PLA
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blend in Fig. 1a shows typical two-phase segregation morphology, indicating the immiscibility between PBS and PLA.[23, 24] A reduction in PLA dimension and more uniform PLA domain distribution are observed along with the incorporation of
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NG (Figs. 1b-1d). The irregular shape of the dispersed PLA domain gradually changed to a spherical shape with an increase of NG concentration in the
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nanocomposites. In addition, many tiny pores could be observed in the PBS matrix in the high resolution pictures. The changes in phase morphology can be attributed to the compatibization effect of NG on the PBS/PLA blends. The presence of polar groups on the surface of NG is speculated to interact with the polymers and thus enhance the
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adhesion between the two components.[45] It can reduce the interfacial tension, thereby, resulting into enhancement in the mechanical properties of the modified blend.[46, 47]
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To further evaluate the dispersion of NG in the nanocomposites, the TEM micrograph of PBS/PLA1 is shown in Fig. 2. The light and dark gray parts are
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corresponding to PLA and PBS phase, respectively. It can be observed that the NG disperses homogeneously in the polymer matrix. In addition, the NG is mainly dispersed in PBS phase, while a small part is located in PLA phase. The reason for selective localization of NG is that the viscosity of PBS is much lower than that of PLA.[48] In the case of the PBS/PLA/carbon black system, carbon black exhibits a similar selective localization in the continuous PBS phase.[25]
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Dynamic rheological measurement was carried out to assess the viscoelasticity performance of the PBS/PLA/NG nanocomposites in the melt state. The complex
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viscosity (η*), storage modulus (G′), loss modulus (G″) and damping factor (tanδ) of the samples as a function of frequency (ω) at 190 oC are shown in Fig. 3. It is obvious in the Fig. 3a that the incorporation of NG in the nanocomposites increases
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the complex viscosity in comparison with the pristine PBS/PLA blend in the whole
frequency range. The increased viscosity of PBS/PLA nanocomposites is mainly
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attributed to the good interaction between the NG and polymer matrices.[49] Moreover, it is noted that the non-frequency dependence of viscosity for the PBS/PLA0.3 and PBS/PLA0.5 nanocomposites is similar to that for the pristine PBS/PLA blend. However, the PBS/PLA1 nanocomposite exhibits a pronounced
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shear thinning behaviour. This phenomenon is speculated that the addition of NG hindered the chain mobility and flowability of the PBS/PLA blends. Similar trends have been observed in many other immiscible polymer systems previously.[50, 51] As
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can be seen in Figs. 3b and 3c, the G′ and G″ gradually increased with increasing the NG concentration, especially in the low-frequency region. These observations
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suggest that the NG is strongly oriented towards the flow direction and the polymer plays a prominent role in the rheological behaviour of the PBS/PLA/NG nanocomposites at higher frequencies.[52-54] Furthermore, the slopes of both moduli are lower for the PBS/PLA/NG nanocomposite than that for the pristine PBS/PLA blend, indicating that a gradual change to pseudo-solid behaviour from liquid-like state.[2, 55] The tanδ in Fig. 3d is defined as the ratio of loss modulus divided by
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storage modulus, which is used to measure the elastic response of polymers. As displayed, when increasing the NG content, the frequency at which the phase angle (δ)
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of each sample reaches its maximum shifts from 0.6 to 25.1 rad⋅s-1, and the value of the δ decreases, implying that the loading of NG enhances the pseudo-plastic behaviour of the nanocomposites.[56]
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Thermogravimetric analysis was performed to estimate the effect of
incorporating NG on the thermal stability of PBS/PLA blends. Fig. 4 exhibits the
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TGA and DTG curves of PBS/PLA blends with various NG concentrations. As shown in Fig. 4a, the incorporation of NG in the PBS/PLA matrix leads to a shift of the decomposition curves towards higher temperatures, which indicates that the NG can enhance the thermal stability of PBS/PLA blends. The decomposition temperatures
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corresponding to 5% and 50% weight loss (T5% and T50%) of the samples are summarized in Table 1. The T5% values for PBS/PLA0, PBS/PLA0.3, PBS/PLA0.5, and PBS/PLA1 are 332.0, 347.5, 347.9 and 350.5 oC, respectively, indicating the
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gradually improved thermal stability with an increase of the NG concentration. It is speculated that the NG in the blends acted as a radical scavenger which limited the
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chain cleavage and radical formation and hence delayed the onset of thermal degradation.[57, 58] From the DTG curves in Fig. 4b, two peaks (Tp1 and Tp2) are observed in the range of 360 ~ 410 oC, which correspond to the decomposition
temperatures with the maximum weight loss of PLA and PBS, respectively.[59, 60] Compared with the pristine PBS/PLA blend, the values of Tp1 and Tp2 shift upwards by 7.2 and 3.9 °C, separately, with 1 wt% NG loading. Similar results have been
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disclosed in other graphene-based nanocomposites.[61, 62] The decomposition temperature increase could be interpreted that the NG acted as the mass transfer
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barrier and protective layer. It is believed that the so-called ‘‘tortuous path’’ effect of grouping delayed the escape of volatile decomposed products. [63, 64]
Fig. 5 exhibits the heating and cooling curves of PBS/PLA/NG nanocomposites
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with various NG concentrations. The DSC data obtained from the thermographic
curves were summarized in Table 2. As shown in Fig. 5a, the addition of NG has little
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influence on glass transition temperature of PLA (Tg-PLA) in the PBS/PLA/NG nanocomposites. The peaks of cold crystallisation (Tcp) of PLA are shifted to lower temperature levels meanwhile the cold crystallization enthalpy (∆Hcc) of PLA increases with the increasing of NG concentration. Such behaviour can be associated
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with a well-known nucleating effect of NG to accelerate the cold crystallisation process of the PLA matrix, which is consistent with previous report.[65-67] In addition, no significant change in the melting peak temperature of PBS (Tp-PBS) and
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the melting peak temperature of PLA (Tp-PLA) is observed. The melting enthalpy of PBS (∆HPBS) increases from 33.2 to 51.9 J/g while the melting enthalpy of PLA
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(∆HPLA) increases from 14.3 to 18.4 J/g as the NG doping content increases from zero to 1.0 wt%. As observed in the TEM micrographs, the NG is mainly dispersed in the PBS matrix, which can provide more nucleation sites for PBS, thus resulting in the enhancement of PBS crystallinity. On the other hand, the formation of PBS crystal will restrict the PLA growth.[68] As a result, the increase of ∆HPLA is less pronounced.
From the cooling curves in Fig. 5b, the PBS/PLA/NG nanocomposites show similar
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crystallization behaviours as the pristine PBS/PLA blend does. All samples exhibit one strong exothermic peak (Tp) when cooling. The incorporation of NG leads to a
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considerable increase in the crystallization enthalpy (∆Hc) from 34.5 to 44.6 J/g, suggesting that the NG can act as a nucleation agent during the crystallization of PBS/PLA blends.
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To further understand the effects of NG on the crystal structures of PBS and PLA, the X-ray diffraction was carried out. As presented in Fig. 6, the pattern presents
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diffraction peaks at 2θ = 12.6, 15.1, 16.8 and 19.3 degrees, can be attributed to (004/103), (110), (110/200) and (203) planes of PLA α-form phase, respectively, according to the literatures.[69-71] Meanwhile, the three strong diffraction peaks located at 19.8, 22.1 and 22.9 degrees, are corresponding to the α-form of PBS crystal
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with (020), (021) and (110) planes.[72] No significant variation of the diffraction angle in the patterns is observed, indicating that the addition of NG did not affect the polymer crystal forms.
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Tensile tests were conducted to investigate the effects of NG on the mechanical properties of PBS/PLA blends. The tensile-fractured surfaces of the PBS/PLA blends
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with various concentrations of NG are shown in Fig. 7. Clearly, the neat PBS/PLA blend (Fig. 7a) exhibits a relatively flat and smooth fracture surface as compared with those of PBS/PLA/NG nanocomposites (Fig. 7b-7d), indicating weak resistance to crack propagation.[73] Furthermore, the fractured surface becomes rougher with the increasing of NG concentration, suggesting the deformation process would certainly consume more energy. It is noted that some embedded NG can be found on the
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fractured surface, as highlighted by the red circles in Fig. 7. These results demonstrate the good interfacial interaction between the NG and the polymer matrix.[74] The
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tensile stress-strain curves are shown in Fig. 8, and the results are summarized in Table 4. From Fig. 8, it can be seen that the pure PBS/PLA blend exhibits similar tensile performance as the published paper.[24] In addition, the PBS/PLA/NG
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nanocomposites show an increasing tendency with NG concentration in both tensile
strength, Young’s modulus and elongation at break, which is in good agreement with
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the tensile fractrued surfaces. When the NG concentration increased to 1.0 wt%, the tensile strength and Young’s modulus were dramatically enhanced to 27.95MPa and 381.03 MPa, respectively, which were higher than those of the pristine PBS/PLA blend by 38% and 20%. The elongation at break was also increased from 7.90% to
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10.17% with the increasing concentration of NG. The main reason for such improvements is the strong interfacial interactions between the NG and the polymer matrices.[75-77] The abundant nitrogen-containing and oxygen-containing functional
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groups in the NG can help to transfer the applied load from the main matrix to the to the dispersed phase.[46] Moreover, it is possible that the increase of crystallinity as
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revealed by DSC analyses also contributes to the improvement of the strength and modulus. On the other hand, the better miscibility of PBS and PLA phases in the presence of NG allows for better load transfer across the system and hence better modulus and elongation at break of the blends. Similar tensile behaviors have been reported for other graphene reinforced immiscible blends.[78, 79] In the case of PBS/PLA0.5, the slight decrease in elongation at break, compared to the value for the
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PBS/PLA0.3, may be ascribed to the defects in the samples.
4. Conclusions
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The NG filled PBS/PLA nanocomposites were prepared using melt compounding method. The morphology observation revealed that the NG mainly dispersed in the
PBS matrix while a small amount is located in PLA phase. The NG had a good
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compatibilizing effect on the immiscible PBS/PLA (70/30 by weight) blends,
improving the phase separation and resulting in more uniform and smaller dispersed
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PLA domains. The dynamical rheology measurements showed that the storage modulus, loss modulus and complex viscosity of PBS/PLA/NG nanocomposites increased with an increase of the NG concentration. TGA results exhibited that the NG could significantly improve the thermal stability of the PBS/PLA blend due to the
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barrier effect of NG and the enhanced interaction between the NG and polymer matrices. The NG could act as a nucleating agent to enhance the crystallinity of PBS without affecting the crystal forms of PBS and PLA. As expected, a small addition of
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NG apparently increased the tensile strength, Young’s modulus and elongation at
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break of PBS/PLA blends.
5. Acknowledgements We thank the funds from Guangdong Province (2014A030310218) and Shenzhen City (JCYJ20140617143643478) for basic scientific research.
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Table Captions Table 1 TGA data of PBS/PLA blends with various NG concentrations.
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Table 2 Thermographic parameters of PBS/PLA blends with various NG concentrations.
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Table 3 Tensile properties of PBS/PLA blends with various NG concentrations.
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Figure Captions
Fig. 1 SEM micrographs of the etched cryofracture surfaces of PBS/PLA blends with various NG concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1. The inserts are
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high magnification micrographs.
Fig. 2 TEM micrograph for the sample of the PBS/PLA1.
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Fig. 3 Dynamic rheological properties of PBS/PLA blends with various NG concentrations: (a)
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complex viscosity, (b) storage modulus, (c) loss modulus, and (d) phase angle.
Fig. 4 TGA thermograms of PBS/PLA blends with various NG concentrations: (a) TGA, and (b) DTG.
Fig. 5 DSC thermograms of PBS/PLA blends with various NG concentrations: (a) heating, and (b) cooling.
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Fig. 6 XRD patterns of PBS/PLA blends with various NG concentrations.
Fig. 7 SEM micrographs of the tensile-fractured surfaces of PBS/PLA blends with various NG
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concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1.
Fig. 8 Tensile stress-strain curves of PBS/PLA blends with various NG concentrations. The insert is the
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optical image of the tensile samples.
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Table 1 TGA data of PBS/PLA blends with various NG concentrations. Tp1 (oC)
Tp2 (oC)
T 5% (°C)
T50% (°C)
PBS/PLA0
366.3
398.6
332.2
382.9
PBS/PLA0.3
370.5
398.8
347.5
385.5
PBS/PLA0.5
370.8
399.5
347.9
385.9
PBS/PLA1
374.1
402.5
350.5
390.3
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Samples
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Table 2 Thermographic parameters of PBS/PLA blends with various NG concentrations. First heating
First cooling
Tg-PLA
Tcp
∆Hcc
Tp-PBS
∆HPBS
Tp-PLA
∆HPLA
(oC)
(oC)
(J/g)
(oC)
(J/g)
(oC)
(J/g)
(oC)
(J/g)
PBS/PLA0
58.5
99.4
2.6
114.8
33.2
166.3
14.3
76.9
34.5
PBS/PLA0.3
58.5
97.3
3.4
115.6
50.4
166.8
17.2
76.7
43.8
PBS/PLA0.5
58.6
96.4
4.9
114.7
50.5
166.4
18.2
77.4
44.1
PBS/PLA1
58.5
95.8
5.5
114.9
51.9
166.7
18.4
78.3
44.6
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Tp
∆Hc
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Samples
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Table 3 Tensile properties of PBS/PLA blends with various NG concentrations. NG concentration
Tensile strength
Young’s modulus
Elongation at break
(wt%)
(MPa)
(Mpa)
(%)
PBS/PLA0
0
20.19±2.02
317.16±37.83
7.90±0.99
PBS/PLA0.3
0.3
22.79±1.23
332.73±28.54
9.81±0.51
PBS/PLA0.5
0.5
24.38±1.05
345.79±14.82
9.45±0.67
PBS/PLA1
1.0
27.95±1.32
381.03±17.59
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Samples
10.17±0.64
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Fig. 1 SEM micrographs of the etched cryofracture surfaces of PBS/PLA blends with various NG concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1. The inserts are
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high magnification micrographs.
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Fig. 2 TEM micrograph for the sample of the PBS/PLA1.
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Fig. 3 Dynamic rheological properties of PBS/PLA blends with various NG concentrations: (a)
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complex viscosity, (b) storage modulus, (c) loss modulus, and (d) phase angle.
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Fig. 4 TGA thermograms of PBS/PLA blends with various NG concentrations: (a) TGA, and (b) DTG.
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Fig. 5 DSC thermograms of PBS/PLA blends with various NG concentrations: (a) heating, and (b) cooling.
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Fig. 6 XRD patterns of PBS/PLA blends with various NG concentrations.
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Fig. 7 SEM micrographs of the tensile-fractured surfaces of PBS/PLA blends with various NG
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concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1.
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Fig. 7 Tensile stress-strain curves of PBS/PLA blends with various NG concentrations. The insert is the
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optical image of the tensile samples.
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HIGHLIGHTS:
performance of the immiscible PBS/PLA blends.
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1. Nitrogen doped graphene was used as a compatibilizer to improve the
2. Nitrogen doped graphene was mainly dispersed in the PBS matrix
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while a small amount is located in PLA phase.
3. The thermal stability of the PBS/PLA/NG nanocomposites is
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enhanced monotonously with an increase of the NG concentration. 4. The tensile strength, tensile modulus and elongation at break of the
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blends could be enhanced by the low concentration of NG.
S1359-8368(16)32257-0
DOI:
10.1016/j.compositesb.2017.01.037
Reference:
JCOMB 4848
To appear in:
Composites Part B
Received Date: 10 October 2016 Revised Date:
23 December 2016
Accepted Date: 25 January 2017
Please cite this article as: Wu W, Wu C, Peng H, Sun Q, Zhou L, Zhuang J, Cao X, Roy VAL, Li RKY, Effect of nitrogen-doped graphene on morphology and properties of immiscible poly(butylene succinate)/ polylactide blends, Composites Part B (2017), doi: 10.1016/j.compositesb.2017.01.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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GRAPHICAL ABSTRACT:
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Effect of Nitrogen-Doped Graphene on Morphology and Properties of Immiscible Poly(butylene succinate)/Polylactide Blends Xianwu Cao,*b V. A. L. Roya and Robert K.Y. Li*a
ABSTRACT
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Wei Wu,a,b ChengKen Wu,c Haiyan Peng,a,d Qijun Sun,a Li Zhou,a Jiaqing Zhuang,a
Plastic pollution has become a serious issue to the ecosystem, and biodegradable poly(butylene
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succinate)/polylactide (PBS/PLA) blends are regarded as the promising eco-friendly alternatives to replace the non-degradable plastics based on fossil fuels. Yet, the thermodynamically immiscible nature
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of PBS and PLA hinders their extended applications. In this contribution, nitrogen-doped graphene (NG) is introduced into immiscible PBS/PLA blends by melt compounding. The incorporation of NG in PBS/PLA (70/30wt%) blends is observed to significantly improve the geometrical morphology and reduce the domain size of the dispersed PLA phase, indicating a compatibilization effect of NG on the immiscible blends. The TEM micrographs showed that the NG mainly dispersed in the PBS matrix while a small amount is located in PLA phase. When the NG concentration increases to 1.0wt%, the
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NG filled PBS/PLA nanocomposites exhibits an obvious improvement in the storage modulus and loss modulus in comparison with the pristine PBS/PLA blend. The thermal stability of the PBS/PLA/NG nanocomposites is enhanced monotonously with an increase of the NG concentration, due to the barrier
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effect of NG and good interaction between the NG and polymer matrices. Moreover, the NG is noticed to act as a nucleating agent to significantly improve the PBS crystallinity without affecting the crystal forms of PBS and PLA. The tensile strength, tensile modulus and elongation at break of the blends
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could be enhanced by the low concentration of NG.
a
Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail:
[email protected]
b
National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing
Engineering of Ministry of Education, South China University of Technology, Guangzhou 510640, China. E-mail: [email protected] c
The Twenty-third Research Institute of China Electronics Technology Group Corporation, Shanghai 201900, China.
d
Key Laboratory for Material Chemistry of Energy Conversion and Storage, School of Chemistry and Chemical Engineering,
Huazhong University of Science andTechnology, Wuhan 430074, China.
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1. Introduction Plastics are indispensable materials nowadays but the plastic pollution has been
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becoming a big issue to the ecosystem. During the last two decades, great efforts have been made to circumvent the problem by developing biodegradable polymers.[1-5] To date, the applications of biodegradable polymers have spread to various areas,
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including but not limited to agriculture, packaging, biomedicine, and automotive.[6-9]
As outstanding commercial-available representatives, poly(butylene succinate) (PBS)
replace
traditional
commodity
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and polylactide (PLA) are regarded as the promising eco-friendly alternatives to thermoplastics,
due
to
their
renewability,
biodegradability and biocompatibility.[10-12]
PBS is a biodegradable thermoplastic polymer in the aliphatic polyester family,
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and synthesized by polycondensation of butanediol and succinic acid that are available from renewable resources.[13, 14] PBS holds properties that are comparable to low density polyethylene (LDPE) and polypropylene (PP) such as high flexibility
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and high impact strength. Yet, the low melt viscosity and insufficient stiffness of PBS hinder its processing and extended applications.[15-17] On the contrary, PLA is a
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biodegradable polymer that exhibits high Young’s modulus, albeit high brittleness.[18] Thus, the blending of biodegradable PBS and PLA has attracted ever-increasing interest in both academia and industry for the performance improvement purpose. A large number of studies have been done on the morphology, thermal behaviours, and mechanical
properties
of
PBS/PLA
blends
to
understand
the
materials
structure-property relation.[19-22] Nevertheless, significant phase separation is
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usually observed in PBS/PLA blends due to the thermodynamically immiscible nature of PBS and PLA, resulting in unsatisfactory blends performances.[22-24] To improve
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the compatibility of multi-component polymers, the introduction of inorganic fillers has been explored to be a simple and effective way as it is able to enhance the
interface action.[25-28] Wang et al. found that the selective localization of carbon
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black in the PBS matrix enabled enhanced processibility via increasing the melt viscosity and domain size refinement of the dispersed PLA phase, resulting in
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increased stiffness and thermal stability of the PBS/PLA blend.[25] Chen and Yoon reported that an epoxy-functionalized organo-clay (TFC) could act as a compatibilizer for the PBS/PLA blend, and they revealed that when the concentration of TFC was higher than 0.5 wt%, the clay layers were dispersed in both the PBS and PLA phases,
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and the domain size of the dispersed PBS phase became significantly smaller.[26] According to these reports,[26-28] the clay could build an interaction between the clay surface and the polymer matrix through chemical reactions, resulting in
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improvements in viscosity, thermal stability and mechanical properties of the PBS/PLA blend system.
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Graphene, a two-dimensional carbon nanomaterial, has been employed as a type
of novel filler for polymer nanocomposites.[29, 30] It has already been reported that the introduction of even a small fraction of graphene can remarkably enhance the mechanical, thermal, electrical, optical, and chemical performances of polymer matrices.[31-35] Recently, graphene and its derivatives have also been applied to improve the compatibility of immiscible blends.[36-39] Cao et al. found that the
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amphiphilicity of graphene oxide (GO) was capable of improving the compatibility of immiscible polyamide/poly(phenylene oxide) blends.[37] Ye et al. demonstrated that
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the compatibilizing effect of GO for poly(methyl methacrylate)/polystyrene blends exhibited a temperature dependence, i.e., when the processing temperature increased,
the compatibilizing effect of GO decreased.[38] Kar et al. disclosed that the
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introduction of the poly(methyl methacrylate) grafted GO (PMMA-g-GO) was able to improve the interface action between poly(vinylidene fluoride) and acrylonitrile
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butadiene styrene polymer.[39] The grafted mutually miscible polymer (i.e., PMMA) could enhance the compatibilizing effect of graphene, improving the strength and modulus of the blends simultaneously. Nevertheless, to the best of our knowledge, there is no report on the graphene filled PBS/PLA blends. Since graphene itself is
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hydrophobic, it is believed to be useful to introduce functional groups on graphene for improving the graphene dispersion and interfacial interaction with polymers.[40] Nitrogen-doped graphene (NG) is such a functionalized material with an abundant of
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polar oxygen and nitrogen groups on its surface,[41] and a large aspect ratio, thus is envisioned to be a rational candidate to offer extensive interactions between NG and
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polymers. In the present work, NG is incorporated into PBS/PLA blends by melt compounding. The phase morphology, rheological properties, thermal behaviours and mechanical properties of the PBS/PLA/NG nanocomposites are investigated in detail.
2. Experimental Section 2.1 Materials
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Nitrogen-doped graphene (NG, grade TNNRGO) with nitrogen content of 7.24 at.% and oxygen content of 7.53 at.% was supplied by Chengdu Organic Chemicals
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Co. Ltd., Chinese Academy of Sciences (Chengdu, China) (Fig. S1 and Table S1). The typical morphology of NG was shown in Fig. S2 and the thickness of NG was approximately 3 nm (Fig. S3). Poly(butylene succinate) (PBS, Bionolle 1020MD) was
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provided by Showa Polymer Co. (Tokyo, Japan), with a density of 1.26 g/cm3 and melting temperature of 115 oC. The melt flow index of PBS was determined to be 25
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g/10 min (2.16 kg, 190 oC). Polylactide (PLA, Grade 4032D) was obtained from Natureworks (Minnetonka, USA). According to the supplier, the PLA had a density of 1.24 g/cm3, and a glass transition temperature (Tg) range of 55-65 oC. The melt flow index for PLA was characterized as 7 g/10 min (2.16 kg, 210 oC).
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2.2 Nanocomposites Preparation
Prior to nanocomposites preparation, PBS and PLA pellets were dried in an oven
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at 80 oC for 5 h. Various concentration of NG was incorporated into PBS/PLA blend, with a weight ratio of PBS to PLA fixed at 70/30. The blends were prepared by melt
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compounding using an internal mixer (PLASTIC-ORDER, Brabender, Germany) with a rotation speed of 60 rpm for 5 min at a fixed temperature of 190 °C. The PBS/PLA blends with four different NG loadings, namely, 0, 0.3, 0.5 and 1 wt%, were correspondingly abbreviated as PBS/PLA0, PBS/PLA0.3, PBS/PLA0.5 and PBS/PLA1. The samples with a thickness of about 1 mm for measurement were subsequently molded into specimens by using hot compression molding (190 °C, 5
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min), followed by air quenching to room temperature.
2.3 Characterization
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The surface morphology of blends was investigated using a field emission scanning electron microscopy (FE-SEM, Quato250, FEI, USA). The samples were
cryogenically fractured in liquid nitrogen. Then the PLA phase was etched out by
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immersion in 1,4-dioxane 1 h at room temperature. Finally, the prepared surface was
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sputter-coated with gold before observation. For the tensile-fractured surfaces of the samples, they were examined directly by the FE-SEM after sputting with gold. Transmission electron microscopy (TEM, Technai 12, Philips, Netherlands) was used to characterize the dispersion of NG in PBS/PLA nanocomposites. The ultrathin slices with approximately 100 nm thick were cut using a cryo-diamond knife at -100
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o
C (EM UC6+FC6, Leica, Germany). Then the slices were transferred onto 300 mesh
copper grids and observed at an acceleration voltage of 120 kV.
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Rheological measurements were conducted on a strain-controlled dynamic rheometer (MCR302, Anton-Paar, Austria) using two parallel plates with a diameter
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of 25 mm. Dynamic frequency sweep tests were performed under a nitrogen atmosphere to evaluate the viscoelastic properties of the blends. The frequency range was set as 0.1 ~ 100 rad/s at a constant strain of 1%, which was within the linear viscoelastic regime as determined by dynamic strain sweep experiments. Before the test, all samples were heated to 190 oC and kept for 3 min to eliminate the previous thermal history.
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The thermal stability of the blends was evaluated by a thermogravimetric analyzer (TGA) (TG209F3, Netzch, Germany). Approximate 9 mg of the sample was
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heated from room temperature to 600 oC at a heating rate of 10 oC/min under nitrogen atmosphere.
The crystallization and melting behaviours of blends were characterized on a
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differential scanning calorimeter (DSC) (DSC204F3, Netzch, Germany). Approximate
5 mg of the sample was sealed in an aluminum pan. The samples were firstly heated
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from room temperature to 200 °C with the heating rate of 10 °C/min. Then the samples were kept at 200 °C for 3 min, and cooled to -50 °C at 10 °C/min subsequently. The curves of heat flow as a function of temperature were recorded. X-ray diffraction (XRD) patterns were carried out by an X-ray diffractometer
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(AXS D2, Bruker, Germany) with Cu-Kα radiation. The data were collected in the 2θ range from 5° to 30° with a step of 0.02° (2θ). The tensile properties of the blends were examined using an electronic universal
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testing machine (Model 5566, Instron, USA) with a load force of 10 kN. The dumbbell-shaped specimens with dimension of 25 × 5 × 1 mm were performed at a
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tensile speed of 5 mm/min. The reported results were average values for at least five separate specimens.
3. Results and discussion Phase
separation
structures
determine
the
polymer
composites
performances.[42-44] Fig. 1 illustrates SEM micrographs of the etched cryofracture
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surfaces of PBS/PLA blends with various NG concentrations. The black domains indicated the position of the extracted PLA phase. It is clear that the neat PBS/PLA
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blend in Fig. 1a shows typical two-phase segregation morphology, indicating the immiscibility between PBS and PLA.[23, 24] A reduction in PLA dimension and more uniform PLA domain distribution are observed along with the incorporation of
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NG (Figs. 1b-1d). The irregular shape of the dispersed PLA domain gradually changed to a spherical shape with an increase of NG concentration in the
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nanocomposites. In addition, many tiny pores could be observed in the PBS matrix in the high resolution pictures. The changes in phase morphology can be attributed to the compatibization effect of NG on the PBS/PLA blends. The presence of polar groups on the surface of NG is speculated to interact with the polymers and thus enhance the
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adhesion between the two components.[45] It can reduce the interfacial tension, thereby, resulting into enhancement in the mechanical properties of the modified blend.[46, 47]
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To further evaluate the dispersion of NG in the nanocomposites, the TEM micrograph of PBS/PLA1 is shown in Fig. 2. The light and dark gray parts are
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corresponding to PLA and PBS phase, respectively. It can be observed that the NG disperses homogeneously in the polymer matrix. In addition, the NG is mainly dispersed in PBS phase, while a small part is located in PLA phase. The reason for selective localization of NG is that the viscosity of PBS is much lower than that of PLA.[48] In the case of the PBS/PLA/carbon black system, carbon black exhibits a similar selective localization in the continuous PBS phase.[25]
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Dynamic rheological measurement was carried out to assess the viscoelasticity performance of the PBS/PLA/NG nanocomposites in the melt state. The complex
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viscosity (η*), storage modulus (G′), loss modulus (G″) and damping factor (tanδ) of the samples as a function of frequency (ω) at 190 oC are shown in Fig. 3. It is obvious in the Fig. 3a that the incorporation of NG in the nanocomposites increases
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the complex viscosity in comparison with the pristine PBS/PLA blend in the whole
frequency range. The increased viscosity of PBS/PLA nanocomposites is mainly
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attributed to the good interaction between the NG and polymer matrices.[49] Moreover, it is noted that the non-frequency dependence of viscosity for the PBS/PLA0.3 and PBS/PLA0.5 nanocomposites is similar to that for the pristine PBS/PLA blend. However, the PBS/PLA1 nanocomposite exhibits a pronounced
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shear thinning behaviour. This phenomenon is speculated that the addition of NG hindered the chain mobility and flowability of the PBS/PLA blends. Similar trends have been observed in many other immiscible polymer systems previously.[50, 51] As
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can be seen in Figs. 3b and 3c, the G′ and G″ gradually increased with increasing the NG concentration, especially in the low-frequency region. These observations
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suggest that the NG is strongly oriented towards the flow direction and the polymer plays a prominent role in the rheological behaviour of the PBS/PLA/NG nanocomposites at higher frequencies.[52-54] Furthermore, the slopes of both moduli are lower for the PBS/PLA/NG nanocomposite than that for the pristine PBS/PLA blend, indicating that a gradual change to pseudo-solid behaviour from liquid-like state.[2, 55] The tanδ in Fig. 3d is defined as the ratio of loss modulus divided by
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storage modulus, which is used to measure the elastic response of polymers. As displayed, when increasing the NG content, the frequency at which the phase angle (δ)
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of each sample reaches its maximum shifts from 0.6 to 25.1 rad⋅s-1, and the value of the δ decreases, implying that the loading of NG enhances the pseudo-plastic behaviour of the nanocomposites.[56]
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Thermogravimetric analysis was performed to estimate the effect of
incorporating NG on the thermal stability of PBS/PLA blends. Fig. 4 exhibits the
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TGA and DTG curves of PBS/PLA blends with various NG concentrations. As shown in Fig. 4a, the incorporation of NG in the PBS/PLA matrix leads to a shift of the decomposition curves towards higher temperatures, which indicates that the NG can enhance the thermal stability of PBS/PLA blends. The decomposition temperatures
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corresponding to 5% and 50% weight loss (T5% and T50%) of the samples are summarized in Table 1. The T5% values for PBS/PLA0, PBS/PLA0.3, PBS/PLA0.5, and PBS/PLA1 are 332.0, 347.5, 347.9 and 350.5 oC, respectively, indicating the
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gradually improved thermal stability with an increase of the NG concentration. It is speculated that the NG in the blends acted as a radical scavenger which limited the
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chain cleavage and radical formation and hence delayed the onset of thermal degradation.[57, 58] From the DTG curves in Fig. 4b, two peaks (Tp1 and Tp2) are observed in the range of 360 ~ 410 oC, which correspond to the decomposition
temperatures with the maximum weight loss of PLA and PBS, respectively.[59, 60] Compared with the pristine PBS/PLA blend, the values of Tp1 and Tp2 shift upwards by 7.2 and 3.9 °C, separately, with 1 wt% NG loading. Similar results have been
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disclosed in other graphene-based nanocomposites.[61, 62] The decomposition temperature increase could be interpreted that the NG acted as the mass transfer
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barrier and protective layer. It is believed that the so-called ‘‘tortuous path’’ effect of grouping delayed the escape of volatile decomposed products. [63, 64]
Fig. 5 exhibits the heating and cooling curves of PBS/PLA/NG nanocomposites
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with various NG concentrations. The DSC data obtained from the thermographic
curves were summarized in Table 2. As shown in Fig. 5a, the addition of NG has little
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influence on glass transition temperature of PLA (Tg-PLA) in the PBS/PLA/NG nanocomposites. The peaks of cold crystallisation (Tcp) of PLA are shifted to lower temperature levels meanwhile the cold crystallization enthalpy (∆Hcc) of PLA increases with the increasing of NG concentration. Such behaviour can be associated
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with a well-known nucleating effect of NG to accelerate the cold crystallisation process of the PLA matrix, which is consistent with previous report.[65-67] In addition, no significant change in the melting peak temperature of PBS (Tp-PBS) and
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the melting peak temperature of PLA (Tp-PLA) is observed. The melting enthalpy of PBS (∆HPBS) increases from 33.2 to 51.9 J/g while the melting enthalpy of PLA
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(∆HPLA) increases from 14.3 to 18.4 J/g as the NG doping content increases from zero to 1.0 wt%. As observed in the TEM micrographs, the NG is mainly dispersed in the PBS matrix, which can provide more nucleation sites for PBS, thus resulting in the enhancement of PBS crystallinity. On the other hand, the formation of PBS crystal will restrict the PLA growth.[68] As a result, the increase of ∆HPLA is less pronounced.
From the cooling curves in Fig. 5b, the PBS/PLA/NG nanocomposites show similar
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crystallization behaviours as the pristine PBS/PLA blend does. All samples exhibit one strong exothermic peak (Tp) when cooling. The incorporation of NG leads to a
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considerable increase in the crystallization enthalpy (∆Hc) from 34.5 to 44.6 J/g, suggesting that the NG can act as a nucleation agent during the crystallization of PBS/PLA blends.
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To further understand the effects of NG on the crystal structures of PBS and PLA, the X-ray diffraction was carried out. As presented in Fig. 6, the pattern presents
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diffraction peaks at 2θ = 12.6, 15.1, 16.8 and 19.3 degrees, can be attributed to (004/103), (110), (110/200) and (203) planes of PLA α-form phase, respectively, according to the literatures.[69-71] Meanwhile, the three strong diffraction peaks located at 19.8, 22.1 and 22.9 degrees, are corresponding to the α-form of PBS crystal
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with (020), (021) and (110) planes.[72] No significant variation of the diffraction angle in the patterns is observed, indicating that the addition of NG did not affect the polymer crystal forms.
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Tensile tests were conducted to investigate the effects of NG on the mechanical properties of PBS/PLA blends. The tensile-fractured surfaces of the PBS/PLA blends
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with various concentrations of NG are shown in Fig. 7. Clearly, the neat PBS/PLA blend (Fig. 7a) exhibits a relatively flat and smooth fracture surface as compared with those of PBS/PLA/NG nanocomposites (Fig. 7b-7d), indicating weak resistance to crack propagation.[73] Furthermore, the fractured surface becomes rougher with the increasing of NG concentration, suggesting the deformation process would certainly consume more energy. It is noted that some embedded NG can be found on the
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fractured surface, as highlighted by the red circles in Fig. 7. These results demonstrate the good interfacial interaction between the NG and the polymer matrix.[74] The
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tensile stress-strain curves are shown in Fig. 8, and the results are summarized in Table 4. From Fig. 8, it can be seen that the pure PBS/PLA blend exhibits similar tensile performance as the published paper.[24] In addition, the PBS/PLA/NG
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nanocomposites show an increasing tendency with NG concentration in both tensile
strength, Young’s modulus and elongation at break, which is in good agreement with
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the tensile fractrued surfaces. When the NG concentration increased to 1.0 wt%, the tensile strength and Young’s modulus were dramatically enhanced to 27.95MPa and 381.03 MPa, respectively, which were higher than those of the pristine PBS/PLA blend by 38% and 20%. The elongation at break was also increased from 7.90% to
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10.17% with the increasing concentration of NG. The main reason for such improvements is the strong interfacial interactions between the NG and the polymer matrices.[75-77] The abundant nitrogen-containing and oxygen-containing functional
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groups in the NG can help to transfer the applied load from the main matrix to the to the dispersed phase.[46] Moreover, it is possible that the increase of crystallinity as
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revealed by DSC analyses also contributes to the improvement of the strength and modulus. On the other hand, the better miscibility of PBS and PLA phases in the presence of NG allows for better load transfer across the system and hence better modulus and elongation at break of the blends. Similar tensile behaviors have been reported for other graphene reinforced immiscible blends.[78, 79] In the case of PBS/PLA0.5, the slight decrease in elongation at break, compared to the value for the
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PBS/PLA0.3, may be ascribed to the defects in the samples.
4. Conclusions
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The NG filled PBS/PLA nanocomposites were prepared using melt compounding method. The morphology observation revealed that the NG mainly dispersed in the
PBS matrix while a small amount is located in PLA phase. The NG had a good
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compatibilizing effect on the immiscible PBS/PLA (70/30 by weight) blends,
improving the phase separation and resulting in more uniform and smaller dispersed
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PLA domains. The dynamical rheology measurements showed that the storage modulus, loss modulus and complex viscosity of PBS/PLA/NG nanocomposites increased with an increase of the NG concentration. TGA results exhibited that the NG could significantly improve the thermal stability of the PBS/PLA blend due to the
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barrier effect of NG and the enhanced interaction between the NG and polymer matrices. The NG could act as a nucleating agent to enhance the crystallinity of PBS without affecting the crystal forms of PBS and PLA. As expected, a small addition of
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NG apparently increased the tensile strength, Young’s modulus and elongation at
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break of PBS/PLA blends.
5. Acknowledgements We thank the funds from Guangdong Province (2014A030310218) and Shenzhen City (JCYJ20140617143643478) for basic scientific research.
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Table Captions Table 1 TGA data of PBS/PLA blends with various NG concentrations.
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Table 2 Thermographic parameters of PBS/PLA blends with various NG concentrations.
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Table 3 Tensile properties of PBS/PLA blends with various NG concentrations.
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Figure Captions
Fig. 1 SEM micrographs of the etched cryofracture surfaces of PBS/PLA blends with various NG concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1. The inserts are
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high magnification micrographs.
Fig. 2 TEM micrograph for the sample of the PBS/PLA1.
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Fig. 3 Dynamic rheological properties of PBS/PLA blends with various NG concentrations: (a)
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complex viscosity, (b) storage modulus, (c) loss modulus, and (d) phase angle.
Fig. 4 TGA thermograms of PBS/PLA blends with various NG concentrations: (a) TGA, and (b) DTG.
Fig. 5 DSC thermograms of PBS/PLA blends with various NG concentrations: (a) heating, and (b) cooling.
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Fig. 6 XRD patterns of PBS/PLA blends with various NG concentrations.
Fig. 7 SEM micrographs of the tensile-fractured surfaces of PBS/PLA blends with various NG
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concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1.
Fig. 8 Tensile stress-strain curves of PBS/PLA blends with various NG concentrations. The insert is the
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optical image of the tensile samples.
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Table 1 TGA data of PBS/PLA blends with various NG concentrations. Tp1 (oC)
Tp2 (oC)
T 5% (°C)
T50% (°C)
PBS/PLA0
366.3
398.6
332.2
382.9
PBS/PLA0.3
370.5
398.8
347.5
385.5
PBS/PLA0.5
370.8
399.5
347.9
385.9
PBS/PLA1
374.1
402.5
350.5
390.3
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Samples
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Table 2 Thermographic parameters of PBS/PLA blends with various NG concentrations. First heating
First cooling
Tg-PLA
Tcp
∆Hcc
Tp-PBS
∆HPBS
Tp-PLA
∆HPLA
(oC)
(oC)
(J/g)
(oC)
(J/g)
(oC)
(J/g)
(oC)
(J/g)
PBS/PLA0
58.5
99.4
2.6
114.8
33.2
166.3
14.3
76.9
34.5
PBS/PLA0.3
58.5
97.3
3.4
115.6
50.4
166.8
17.2
76.7
43.8
PBS/PLA0.5
58.6
96.4
4.9
114.7
50.5
166.4
18.2
77.4
44.1
PBS/PLA1
58.5
95.8
5.5
114.9
51.9
166.7
18.4
78.3
44.6
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Tp
∆Hc
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Samples
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Table 3 Tensile properties of PBS/PLA blends with various NG concentrations. NG concentration
Tensile strength
Young’s modulus
Elongation at break
(wt%)
(MPa)
(Mpa)
(%)
PBS/PLA0
0
20.19±2.02
317.16±37.83
7.90±0.99
PBS/PLA0.3
0.3
22.79±1.23
332.73±28.54
9.81±0.51
PBS/PLA0.5
0.5
24.38±1.05
345.79±14.82
9.45±0.67
PBS/PLA1
1.0
27.95±1.32
381.03±17.59
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Samples
10.17±0.64
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Fig. 1 SEM micrographs of the etched cryofracture surfaces of PBS/PLA blends with various NG concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1. The inserts are
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high magnification micrographs.
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Fig. 2 TEM micrograph for the sample of the PBS/PLA1.
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Fig. 3 Dynamic rheological properties of PBS/PLA blends with various NG concentrations: (a)
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complex viscosity, (b) storage modulus, (c) loss modulus, and (d) phase angle.
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Fig. 4 TGA thermograms of PBS/PLA blends with various NG concentrations: (a) TGA, and (b) DTG.
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Fig. 5 DSC thermograms of PBS/PLA blends with various NG concentrations: (a) heating, and (b) cooling.
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Fig. 6 XRD patterns of PBS/PLA blends with various NG concentrations.
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Fig. 7 SEM micrographs of the tensile-fractured surfaces of PBS/PLA blends with various NG
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concentrations: (a) PBS/PLA0, (b) PBS/PLA0.3, (c) PBS/PLA0.5 and (d) PBS/PLA1.
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Fig. 7 Tensile stress-strain curves of PBS/PLA blends with various NG concentrations. The insert is the
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optical image of the tensile samples.
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HIGHLIGHTS:
performance of the immiscible PBS/PLA blends.
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1. Nitrogen doped graphene was used as a compatibilizer to improve the
2. Nitrogen doped graphene was mainly dispersed in the PBS matrix
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while a small amount is located in PLA phase.
3. The thermal stability of the PBS/PLA/NG nanocomposites is
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enhanced monotonously with an increase of the NG concentration. 4. The tensile strength, tensile modulus and elongation at break of the
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blends could be enhanced by the low concentration of NG.