Tensile Property Balanced and Gas Barrier Improved Poly(lactic acid

Research Article pubs.acs.org/journal/ascecg

Tensile Property Balanced and Gas Barrier Improved Poly(lactic acid) by Blending with Biobased Poly(butylene 2,5-furan dicarboxylate) Yu Long,† Ruoyu Zhang,*,†,‡ Juncheng Huang,† Jinggang Wang,† Yanhua Jiang,† Guo-hua Hu,∥ Jian Yang,§ and Jin Zhu*,† †

Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang 315201, People’s Republic of China ‡ Engineering Laboratory of Specialty Fibers and Nuclear Energy Materials (FINE), Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo, Zhejiang 315201, People’s Republic of China ∥ Laboratory of Reactions and Process Engineering (LRGP), Université de Lorraine - CNRS, 1 rue Grandville, BP 20451, 54001 Nancy, France § School of Materials and Chemical Engineering, Ningbo University of Technology, 818 Fenghua Road, Jiangbei District, Ningbo, Zhejiang 315016, People’s Republic of China ABSTRACT: A biobased poly(butylene 2,5-furan dicarboxylate) (PBF) is synthesized and blended with poly(lactic acid) (PLA). With only 5 wt % addition of PBF, the elongation at break of PLA becomes 18.5 times more than before, while the tensile modulus and tensile strength at yield keep almost the same. Morphological analysis based on scanning electron microscope pictures shows drastic shape deformation of PBF dispersed phases from sphere to fibril. The elongation ratio of PBF phase is much larger than that of PLA matrix, which is caused by a possible “plastic-rubber” transition in PBF during stretch process. On the other hand, stretch induced crystallization in PBF happened and it may increase the modulus of PBF along the tensile direction. Probably with the above two phenomena, PLA is dynamically toughened and strengthened in the elongation procedure. Moreover, the impact strength of the blends is better than those of the two pure components, and rubber-plastic transition and crystallization of PBF could improve the impact toughness too. The gas barrier property of PLA/PBF blends is also significantly enhanced by the introduction of a furan ring. It can be concluded that PLA/PBF blends are a good base material with great potential for future development in different areas. KEYWORDS: Biobased polymer, Poly(lactic acid), Poly(butylene 2,5-furan dicarboxylate), Tensile toughness, Impact strength, Heat resistance, Gas barrier

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INTRODUCTION Because of the rapidly increasing carbon dioxide (CO2) content in the atmosphere, the replacing of carbon usage from fossil based resources is necessary and urgent.1 Unfortunately, the polymer industry relies heavily on the mining of petroleum. Because polymeric materials have wide application, it is not easy to simply stop using plastics, epoxies and rubbers etc. As an alternative choice, utilizing carbon from biobased resources is an important direction for future polymers.2,3 Recently, biobased and biodegradable polymers have developed fast.4,5 Among these polymers, poly(lactic acid) (PLA) is one of the most promising candidates for wide application. Because it possesses high tensile modulus (3 GPa) and good mechanical strength (>60 MPa), it has great potential as a base polymer.6 On the other hand, its monomer can be prepared from corn, and it can be decomposed in nature.7 Therefore, PLA is a typical carbon-neutral polymer with biodegradability. In fact, it © 2017 American Chemical Society

has already found huge application possibility in different areas, such as packaging, fiber and biomedical.8−10 However, it is also widely known that before industrialized application, it must be modified chemically or physically to conquer its inherent shortness like brittleness, poor thermal properties and high gas permeability of CO2, O2 and water vapor etc.11−16 Physical modification, mainly blending, frequently with a small amount of chemical reaction, is a straightforward and efficient way in using PLA as a base polymer or matrix.17,18 Usually, one important point in physical blending is to choose materials that own obvious advantages over PLA on its weakness. For example, blending of PLA with a flexible polymer or small molecule plasticizers could achieve a lower glass transition and Received: July 3, 2017 Revised: August 22, 2017 Published: August 29, 2017 9244

DOI: 10.1021/acssuschemeng.7b02196 ACS Sustainable Chem. Eng. 2017, 5, 9244−9253

Research Article

ACS Sustainable Chemistry & Engineering better toughness at room temperature.19−21 However, serious decline of PLA modulus and strength inevitably accompany the improved toughness in most studies. Then, introducing inorganic and hard particles like clays or layered silicate to improve the modulus and strength of PLA becomes natural.22 The combination of the above two methods is important in modifying PLA to acquire balanced properties for potential application in daily life and industry. Except for biobased aliphatic polymers like PLA and poly(βhydroxybutyrate-co-β-hydroxyvalerate) (PHBV) etc., there is another kind of biobased polymers based on 2,5-furandicarboxylic acid (FDCA).23−25 FDCA is a biobased monomer derived from 5-(hydroxymethyl)furfural (HMF), which can be manufactured from various sources of carbohydrates, such as fructose, glucose and C6 polysaccharides etc.26−28 As an important base monomer, FDCA was identified by the US Department of Energy as one of 12 priority chemicals.25 The furan ring in FDCA is an analogue to the benzene ring in terephthalic acid, which could enhance the glass transition temperature, modulus, strength and gas permeation behavior etc. when compared to corresponding aliphatic polyesters. One typical FDCA based polyester is poly(ethylene 2,5-furan dicarboxylate) (PEF).29,30 Because PEF has an overwhelming gas barrier property to polyethylene terephthalate (PET), its potential usage as packing material has been studied for several years.31−33 At the same time, after seeing the great potential in PEF, more FDCA based polyesters are developed like poly(butylene 2,5-furan dicarboxylate) (PBF), the counterpart of polybutylene terephthalate (PBT).34−36 These FDCA based polyesters usually have comparable mechanical properties with their benzene analogues, and their other properties, like gas barrier ability, is outstanding. It is natural to find more applications for these FDCA polyesters. Blending a proper FDCA polyester with PLA is not only the aim to acquire a fully biobased polymer blends but also to endow PLA with much better properties. In this work, we blend PLA with PBF to obtain a fully biobased polymer blends. It is very interesting to find that with small amount of PBF in PLA, the tensile toughness of PLA is significantly improved while its modulus and strength are almost keep the same. It is also interesting that the impact toughness of the PLA/PBF blends is better than the two pure components. The gas barrier property of PLA/PBF, as expected, is obviously better than pure PLA. Such a biobased blend without any chemical modification is of great potential as a base material for future applications.

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Scheme 1. Synthetic Route of PBF

catalyst, triphenyl phosphate (0.12 g, 0.36 mmol) as a heat stabilizer and antioxygen 1010 (2.34 g, 1.99 mmol) were also transferred into the reaction flask. During the first stage, the reaction was carried out at 180 °C for 4 h under nitrogen to prepare intermediate oligomers (bis(hydroxybutyl)-2,5-furan dicarboxylate, BHFD). The first step of transesterification is considered to be completed after the collection of the theoretical amount of methanol. After that, the second stage of the polycondensation process began. The temperature was set to 230−245 °C for 2 h, and the reaction kettle was held at 60 Pa for 0.5 h. After the completion of reaction, molten PBF product was blown out from the flask into water by nitrogen. The product was not fractionated further by precipitation, and the cooled sample was crushed into powder for storage. Preparation of PLA/PBF Blends. Prior of blending, PLA and PBF were dried in a vacuum oven at 80 °C for at least 6 h. PLA/PBF blends with weight percentage of 100/0, 95/5, 90/10, 80/20 and 70/30 were compounded in a 20 mm corotating twin screw compounder, which was driven by a Brabender Plasti-Corder Lab-Station (Brabender GmbH & Co. KG, Germany). A processing temperature of 185 °C was chosen, and the screw rotational speed was set at 30 rpm. PBFx designates a PLA/PBF blend containing x wt % of PBF. Then, the granules of PLA/PBF blends were injected into a type of rectangular shaped molding with dimension of 80 mm (length) × 10 mm (width) × 4 mm (thickness) for impact test in a UN120SJ injection molding machine (Yizumi Precision Machinery Co., Ltd., Guangdong, China). Injection was carried out at 185 °C with a feeding pressure of 60 MPa, packing pressure of 100 MPa and mold temperature of 35 °C. All the injected samples were annealed at room temperature for at least 24 h before test. Characterizations. Nuclear Magnetic Resonance. Proton (1H) NMR spectra were measured at room temperature on a 400 MHz Bruker AVANCE III (Bruker, Switzerland) in trifluoroacetic acid (TFA-d). Gel Permeation Chromatography. GPC was carried out to determine the weight-average molecular weight (Mw) and polydispersity index (PDI) using PLgel 5 μm MIXED-C 300 × 7.5 mm column, PLgel 5 μm MIXED-C 300 × 7.5 mm column and RI detector. Chloroform was used as the eluent at a flow rate of 1 mL/min at 40 °C. The range of polystyrene standards covered ranged from 3.07 × 103 to 2.58 × 105 g/mol. A mixed solvent of chloroform/ochlorophenol (9/1, v/v) was used to dissolve PBF.

EXPERIMENTAL SECTION

Materials. The PLA (Natureworks PLA 4032D) used in this work is a production of Nature Works LLC, USA. 2,5-Dimethylfurandicarboxylate (DMFD, 99.3%) was purchased from Algal Energy and Biobased Product Group. 1,4-Butanediol (BDO, 99%), titanium butoxide (Ti(OC4H9)4, 99%) and triphenyl phosphate (98%) were all purchased from Aladdin. Antioxygen 1010 (RGANOX 1010) was bought from BASF. The synthesis of PBF was carried out in our lab. Synthesis of PBF. In a typical experiment (Scheme 1), the entire polymerization was conducted in a 1 L three-necked round-bottom flask equipped with a mechanical overhead stirrer, nitrogen inlet and air condenser. Before the transesterification period, the reaction flask was purged with nitrogen at least three times to ensure that there was no air leakage or residual oxygen inside. 2,5-Dimethylfurandicarboxylate (138 g, 0.75 mol) and 1,4-butanediol (135 g, 1.5 mol) were charged into the reaction flask at a 1:2 (DMFD/BDO) mole ratio. Immediately after, titanium butoxide (0.17 g, 0.5 mmol) as 9245


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ACS Sustainable Chemistry & Engineering Intrinsic Viscosity. A Ubbelohde viscometer (Type 541) on an AVS370 (Schott, Germany) was used to measure the intrinsic viscosity. The intrinsic viscosity of PBF was measured at 25 °C in a mixed solvent of phenol/1,1,2,2-tetrachloroethane (1/1, v/v). Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) analysis was performed by using a Mettler-Toledo METDSC (Mettler-Toledo, Switzerland). DSC measurements were performed under a nitrogen atmosphere with a flow rate of 50 mL/ min. Typical parameters for experimental procedures were as follows: first, the heating scan was recorded at a rate of 30 °C/min from −10 to +220 °C. Subsequently, samples were held at 220 °C for 3 min to erase thermal history and then it was cooled down to −10 °C for 3 min at a cooling rate of 10 °C/min. Finally, heating scans were recorded by reheating the samples at a 10 °C/min rate to 220 °C. X-ray Diffraction. The crystal in blends was determined by using a D8 ADVANCE X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a Cu Kα radiation (λ = 154 nm) target. The scanning rate was 7.86° per minute from 5° to 60°. Scanning Electron Microscopy. The surfaces of the PLA/PBF samples were first sputter-coated with gold to improve conductivity. Then, their surface morphologies were recorded by using scanning electron microscopy (SEM, EVO18 from Carl Zeiss, Germany) at an accelerating voltage of 5 kV. Thermogravimetric Analysis. Thermal degradation of the samples were studied by using a Mettler-Toledo (STARe System) machine. For this test, about 5 mg of sample was heated from 50 to 800 °C with a ramp rate of 20 °C/min under nitrogen and air atmospheres. Tensile Testing. Tensile mechanical tests on dumbbell shaped samples with dimensions of 35 mm (length) × 2 mm (neck width) × 1 mm (thickness) were carried out by using an INSTRON machine, model 5567, with elongation speed of 20 mm/min at room temperature. Ten samples per every composition were performed in this manner, and the standard deviation of each test datum was determined. Heat Deflection Temperature. Samples with the dimension 80 mm (length) × 10 mm (width) × 4 mm (thickness) were applied in HDT test, model 6911 (CEAST). The heating rate is 120 °C/h. Notched Izod Impact Test. Notched Izod impact strengths of PLA and the PLA/PBF blends were tested by a XJ-50Z (Chengde testing machine co., LTD) impact tester, according to GB/T 1843-2008. Samples were notched at least 24 h before experiment, and the impact strength was measured using a 1 J pendulum. Gas Barrier Property. PLA/PBF blends with weight percentage of 100/0, 95/5, 90/10, 80/20 and 70/30 were prepared as thin films with thickness of about 200 ± 20 μm by compression molding. Gas barrier property measurements were performed using a gas permeability tester (VAC-V2, Labthink, China) according to GB/T 1038−2000, and a water vapor permeability tester (W3/060, Labthink, China) according to GB/T 1037. The ambient temperature was set to 23 °C for gas permeability, and the settings for water vapor transmission rate were 38 °C, 90% relative humidity.

Figure 1. 1H NMR spectrum (400 MHz in TFA-d) of poly(butylene furandicarboxylate), PBF.

MPa at the yielding point with a strain of 2.7%. When the strain reaches ∼7.4%, the PLA sample directly broke. On the other hand, PBF shows much better tensile toughness.35 After an conspicuous yielding at strain of 3.3%, the stress gradually increases with strain before its break at ∼259.5%. The large elongation at break of PBF is probably related to its low glass transition temperature, as will be shown and discussed below. The tensile toughness of PLA/PBF blends benefits from the tensile behavior of PBF. As shown in Figure 2 and Table 1, the elongation at break of PLA/PBF blends increases with PBF content from 5 wt % to 20 wt % and it reaches ∼223% for PBF20, which is ∼30 times larger than that of pure PLA. In most cases, toughening PLA by polyesters will drastically reduce its modulus and tensile strength at yield.38,39 However, a prominent and interesting phenomenon in this work is that only a mild decrease of the modulus in PLA/PBF blends is found. For PBF5, its Young’s modulus and tensile strength at yield show very little decrease when compared to pure PLA in Table 1. But its elongation at break increases to 183.5%, which is about 24.8 times larger than that of PLA. The tensile property of PLA is thus well balanced by the addition of only 5% PBF. The increase of PBF content will cause gradual increase of elongation at break but it will also mildly reduce the modulus and strength at the same time. For example, PBF20 has the lowest Young’s modulus among the three blends of 2661 MPa, which is 18.5% lower than pure PLA of 3265 MPa. Similarly, the tensile strength at yield of PBF20 is 64.1 MPa, which is 11.7 MPa less than that of pure PLA. In order to understand such interesting behavior, we investigated the internal morphology before and after the stretch. Morphological Study. Cryo-fractured surfaces of PBF5, PBF10 and PBF20 before stretch are illustrated in Figure 3 with a magnification of 5000×. Sea−island morphology can be clearly seen from these samples, which means a poor compatibility between PLA and PBF. The average diameters of PBF phase, Ds, in these blends are statistically analyzed and listed in Table 2. As the PBF content increases, the size of the dispersed phase increases too. The dependence of phase dimension on PBF content could be the result of the coalesence of PBF droplets in the melt state.40 In Figure 4, at the broken region of the samples, we show the cryo-fractured surfaces of the three stretched blends in both Y− Z and X−Y planes. The Y−Z plane is perpendicular to the tensile stretch, and the X−Y plane is parallel to the stretching direction. From the Y−Z plane, the existence of the PBF fibrillar structure (in yellow circle) is obvious in the PLA matrix. In the X−Y plane, we could see the PBF fiber phase along the stretching direction as indicated by the yellow line. The shape change of the PBF phase from sphere to fiber is induced by the elongation process, and the average diameter of

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RESULTS AND DISCUSSION Characterization of the Synthesized PBF Polyester. PBF (Scheme 1) was synthesized by using a two-step melt polycondensation method was described in detail in experimental part and previous work.37 The intrinsic viscosity of PBF was 1.06 at ambient temperature. Figure 1 shows the 1H NMR spectrum of the PBF sample. All peaks concided well with previous publication on PBF NMR spectrum.35,36 The weightaverage molecular weight (Mw) and the polydispersity index (PDI) of PBF were determined by GPC as 27 kg/mol and 1.7, respectively. Mechanical Properties. Tensile properties of all the samples were measured at room temperature, and the representative tensile curves of each sample are shown in Figure 2. In Figure 2a, the brittleness of neat PLA is obvious. At first, tensile stress increases with strain rapidly and reaches ∼65 9246


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ACS Sustainable Chemistry & Engineering

Figure 2. (a) Typical tensile curves of pure PLA and PLA/PBF blends at room temperature. (b) Tensile curve of pure PBF.

droplet, and are 2.7, 8.8 and 2.6 times larger than the elongation at break of the corresponding PLA blends. Especially, the elongtaion ratio of the dispersed phase in PBF10 is the largest among the three samples, which reachs 17.5. The reason remains unknown and it may be related with the interdistance of PBF droplets.41 In some elastomer toughened PLA systems, we could observe the shape change of the dispersed phase; however, such drastic shape deformation has rarely been observed.42 The above mophological observation imply that the large shape deformation of PBF phase could undertake lots of external stress and strain, and PLA is efficiently toughened.43 Except for the shape change of PBF phase, the content of PBF also affect the toughening too, as PBF20 has the largest elongation at break. Nevertheless, the shape change of the dispersed droplets take the major role in tougheing PLA. Because the PBF content has little improvement in elongation at break. The deformation of the PBF domain is very interesting and extraordinary. The gap, as shown in Figure 3, between the PLA matrix and PBF spherical phases, which would prevent the transfer of stress from matrix to dispersed phase, disappears in Figure 4 between the fibrillar PBF phase and the PLA matrix. In thermodynamically incompatible blends, stretch induced debonding and cavitation can be frequently found.44−46 However, such a phenomenon is not observed in PLA/PBF blends. It is very unusual that the stress can still be transferred so well in such a highly incompatible blend. In order to find more information in this special system, we investigated the morphology in different necking regions of the stretched sample. The stretched PBF10 sample is taken as an example and it was fractured along the tensile direction in liquid

Table 1. Tensile Property of PLA/PBF Blends, PLA and PBF sample PLA PBF5 PBF10 PBF20 PBF

Young’s modulus (E, MPa) 3265 3200 3147 2661 1412

± ± ± ± ±

29 45 23 46 83

tensile strength at yield (σ, MPa) 75.8 74.8 71.1 64.1 32.0

± ± ± ± ±

2.0 1.2 3.1 1.9 2.7

elongation at break (ε, %) 7.4 183.5 202.0 223.0 259.5

± ± ± ± ±

1.3 17.5 21.6 6.3 24.6

Figure 3. SEM micrographs of freeze fractured surface of different PLA blends: (a) 5% PBF, (b) 10% PBF and (c) 20% PBF.

fibers, Df, is counted in Table 2. The elongation ratio of the PBF phase after stretch near the broken region, Lf/Ds, is calculated as dividing the fiber length by the diameter of Table 2. Dimensions of PBF Droplet and PBF Fiber Samples droplet (before stretch) fibril (after stretch)

a

Ds (μm) Vb (μm3) Dfc (μm) Sfd (μm2) Lfe (μm) εPBF (Lf/Ds) εPBF/εPLA blends

PBF5 0.79 0.27 0.30 0.07 3.86 4.9 2.7

± ± ± ±

0.13 0.1 0.02 0.008

PBF10 0.80 0.28 0.18 0.02 14.0 17.5 8.8

± ± ± ±

PBF20

0.07 0.07 0.01 0.003

0.99 0.51 0.33 0.09 5.67 5.7 2.6

± ± ± ±

0.07 0.1 0.01 0.006

Ds is the diameter of PBF droplets. bV is the volume of PBF droplets. cDf is the diameter of PBF fiber after stretch. dSf is the cross-sectional area of PBF fiber after stretch. eLf is the length of PBF fiber after stretch.

a

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Figure 4. SEM micrographs of freeze fractured surface of different PLA blends after tensile break, Y−Z plane: (a) Right, three-dimensional coordinate in sample; Left, broken region in the PLA/PBF sample. (b) 5% PBF, (c) 10% PBF and (d) 20% PBF; X−Y plane: (e) 5% PBF, (f) 10% PBF and (g) 20% PBF. The yellow dashed arrow indicates the stretching direction.

nitrogen. The X−Y plane of the sample is shown in Figure 5. Regions B, C and D in Figure 5a are three positions in the

Figure 6. Thermal transition temperatures of pure polymers and blends are summarized in Table 3. Neat PLA shows typical

Figure 6. DSC curves of PLA, PBF and PLA/PBF blends. (a) Second heating curves with 10 °C/min between 25 and 200 °C. (b) Cooling curves with 10 °C/min between 200 and 250 °C.

Figure 5. Morphology of PBF10 in different necked down regions after tensile experiment. (a) Right, three-dimensional coordinate in sample; Left, different necked down regions in the PLA/PBF sample; (b) morphology in region B; (c) morphology in region C and (d) morphology in region D. The arrow indicates the stretching direction.

thermal transitions with glass transition temperature (Tg) at 60.4 °C, cold crystallization temperature (Tcc) at 111.7 °C and melting temperature (Tm) at 164.6 and 169.4 °C, respectively.9,47,48 The glass transition temperature of pure PBF is observed at 37.8 °C, and it is only 12.8 °C higher than the room temperature. During mechanical stretching, the sample temperature will increase due to the molecular friction.49,50 Since the tensile tests were carried out at room temperature, it is a high possibility that the internal temperature of PBF sample or PBF domain exceeds its glass transition temperature during elongation. Above the glass transition temperature, PBF will behave like an elastomer and the large elongation at break in Figure 2b can be understood. In PLA/PBF blends, the deformation of PBF domain could undergo a similar process. After heating up the PBF domain to higher than its glass transition temperature, the drastic deformation from sphere to fibril happens, and it efficiently toughens PLA in the tensile experiment. Combination of DSC and XRD data would show a more interesting fact in stretched blends. XRD results are illustrated in Figure 7a,b,c for static and stretched samples. In Figure 7a, diffraction peaks of pure PLA and PLA blends are shown. The

necking region, and their distances to the broken region decreased sequentially. In region B in Figure 5b, which is the farthest position to the broken region, the spherical PBF phase is deformed into ellipsoid, and the gap seems to disappear. Nevertheless, the miscibility between PLA and PBF is still poor, as evidenced by the dispersed PBF phases on the surface of PLA matrix. Such a particle−matrix interface is caused by the insufficient interfacial adhesion. In region C in Figure 5c, columnar PBF phases start to form, and in region D the laminar PBF phases are very obvious in Figure 5d. Again, in these fractured surfaces, the PBF phase could be peeled off from PLA matrix. Although, in these regions, there is no obvious gap between PBF and PLA, the compatibility between the two phases remains poor. Possible mechanism analysis will the presented after with DSC and XRD results. Calorimetry and XRD Study. Calorimetric study helps one to discover more information about PLA/PBF blends, and their second heating and cooling DSC curves are displayed in 9248


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ACS Sustainable Chemistry & Engineering Table 3. Thermal Transition Data of PLA, PLA/PBF Blends and PBF Samplesa Tg (°C)

Tcc (°C)

Tm (°C)

PLA

60.4

111.7

PBF5 PBF10 PBF20 PBF

60.0 60.4 60.3 37.8

99.8 99.7 99.0 102.0

164.6 169.4 168.0 168.0 167.9 166.3

Tc (°C)

120.5 104.0

ΔHcc (J/g)

ΔHm (J/g)

17.4

31.1

ΔHc (J/g)

13.7

25.9 23.5 21.3 30.3

30.2 33.6 36.4 42.0

4.3 10.1 15.1 11.7

3.3 2.5

ΔHm−ΔHcc (J/g)

a Scanning rate: 10°C/min. Abbreviations in Table 3: glass transition temperature (Tg), cooling crystallization temperature (Tcc), melting temperature (Tm), crystallization temperature (Tc), enthalpy of cold crystallization (ΔHcc), enthalpy of melting (ΔHm) and enthalpy of crystallization (ΔHc).

reference, it is difficult to know the crystallinity change. But the increase of the melting enthalpy from 32 to 42 J/g is clear. Such a phenomenon means that the mechanical energy is transformed into thermal enthalpy in polymer. The stretch induced crystallization could enhance the PLA and PBF modulus in tensile direction, and such phenomenon may partially explain the mild decrease of modulus and tensile strength at yield in PLA/PBF blends. Based on the data and discussion all above, we can propose a possible picture in Scheme 2 for the structural evolution during Scheme 2. Morphologies of PBF Phases in Different Positions of Tensile Sample before and after Stretch

Figure 7. Effect of stretch on the crystals of PLA, PLA/PBF blends and PBF. (a) PLA and PLA/PBF blends before stretch. (b) PLA and PLA/PBF blends after stretch. (c) PBF before stretch and after stretch. (d) DSC curves of PBF before stretch and after stretch at 30 °C/min by first heating.

the elongation of PLA blends. Considering the morphologies in Figure 5, it can be said that the deformation ratio or local strain increases as the distance to the broken point decreases. Therefore, different shapes of PBF phases in Figures 4 and 5 could reflect the shape changing process of the PBF phases in broken region. The spherical domain experiences in sequence the ellipsoidal, columnar, laminar and fibrillar shapes as the local strain gradually increases. The detailed study on the above mechanism requires multiscale and in situ experiments, but we can propose a possible one here.53 Once the stress is applied, the shape deformation happens and internal friction will heat up the sample, especially around the stress concentration region of PBF domain. Because the glass transition temperature of PBF is close to room temperature, the plastic-rubber transition happens in the PBF domain. As a result, the mobility of polymer chain is largely enhanced and its flow is relatively easy. Microscopically, the shape of PBF droplets is deformed from sphere to fibrillar. In consequence, the tensile tougheness of PLA is efficiently improved by the extrem shape deformation of PBF phase. Nanoscopically, on the other hand, the stretch induced recrystallization and crystallization could happen and the modulus of PLA and PBF increases. Spherullites of PLA and PBF are destroyed and new types of crystals formed in the elongation procedure. In turn, the oriented crystals could enhance the modulus and strenght of PLA/PBF along the

characteristic diffraction peak of PLA at 15.8° is clearly shown in these samples and its intensity decreases with the decreasing content of PLA. The indistinctive peak at 24.1° is attributed to PBF crystals, which can be found in Figure 7c of unstretched PBF data too.34 In Figure 7b, the diffraction curves of the stretched PLA and its blends are shown. It is interesting to notice that the intensity of PLA peak at 15.8° is stronger in blends than in neat PLA. It seems that stretch induced crystallization happens in PLA matrix.51,52 However, the strongest stretch induced crystallization effect happens in the PBF phase. The newly risen diffraction peak at 24.1° in Figure 7b,c should be attributed to the stretch induced crystals of PBF. As the content of PBF increases, the intensity of this characteristic peak increases rapidly. In Figure 7c, we can see more clearly the changes of PBF crystals induced by elongation. Unlike PLA, the crystals of PBF before and after stretch are completely different. The original diffraction peak at 21° disappears after elongation, and two new peaks at 16.9° and 24.5° form. It means that the crystal lattice and crystal form of PBF is completely altered by the stretch. We cut small pieces of PBF from the samples before and after stretch, and examine their melting behavior with temperature ramping rate of 30 °C/ min in Figure 7d. It is clear that the melting point and enthalpy before and after elongation are different in pure PBF. Since the PBF crystal form changed and no basic data can be found in 9249


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ACS Sustainable Chemistry & Engineering tensile direction. Therefore, the modulus and tensile strength at yield of PLA/PBF blends is dynamically improved during the elongation procedure and the observed mild decrease of the two parameters can be explained. Notch Impact Property. The notch impact strength of PLA is also improved by the addition of PBF, and it has a maximum in the PBF10 sample as shown in Figure 8a. When

Figure 9. TGA curves of PLA, the blends and PBF. (a) Samples in nitrogen atmosphere, inner is the first derivative. (b) Samples in air atmosphere, inner is the first derivative.

interesting to see that PBF has Td5% of 350 °C, higher than that in N2. Unfortunately, the underlying mechanism is unknown and more investigation is needed in the future. In this case, PLA has Td5% of 324 °C.54 But the degradation behavior of the PLA blends shows a similar behavior with PLA. The above data is also collected in Table 4. Table 4. Thermostability of PLA, PLA/PBF Blends and PBF nitrogen

air

samples

Td5% (°C)

Tdmax (°C)

Td5% (°C)

Tdmax (°C)

PLA PBF5 PBF10 PBF20 PBF

334 316 312 316 316

365 365 368 368 388

324 320 315 313 350

375 368 363 360 355

The heat deflection temperature (HDT) of a PLA blend is an important parameter for its application in some areas.55 The HDT of neat PLA is not high, about 51.3 °C for 0.45 MPa load and 46.3 °C for 1.8 MPa load, as shown in Table 5. With the

Figure 8. (a) Notched Izod impact of PLA, PLA/PBF blends and PBF. SEM micrographs of the samples with impact fracture: (b) PBF5, (c) PBF10, (d) PBF20, (e) PLA and (f) PBF.

Table 5. Heat Resisting Property of PLA and PLA Blends compared to pure PLA, the percentage increases of impact strength are 39.6%, 44.0% and 24.5% for PBF5, PBF10 and PBF20, respectively. An interesting and important fact is that the impact strength of both PLA and PBF is lower than those of PLA/PBF blends. The improved impact strength in PLA/PBF blends may be related with plastic-rubber transition and stress induced crystallization in PBF domains too. However, there is also an apparent difference between them. In impact experiment, the strain is small even microscopically, and the above two transition is probably weak. Consequently, the enhancement of impact toughness by adding PBF is not that strong. Therefore, the motion of polymer chain is completely different between tensile and impact experiments, and we are unable to observe the details from SEM pictures in Figure 8. Nevertheless, the impact fracture surface of PLA and PBF reveal more friability than those of PLA/PBF blends. Thermal Stability and Heat Resistance. Thermogravimetric analysis was performed for assessing the thermal stability of the blends. The remaining weight percentage is displayed as a function of temperature in Figure 9. In nitrogen atmosphere in Figure 9a, PLA has Td5% (thermal decomposition temperature at 5% weight lost) of 334 °C, whereas PBF has Td5% of 316 °C. At the same time, the blends show degradation behavior between PLA and PBF, which means that PLA and PBF have very limited interference during thermal degradation. When the degradation occurs in air in Figure 9b, it is very

HDT (°C)

PLA

PBF5

PBF10

PBF20

0.45 MPa 1.8 MPa

51.3 °C 46.3 °C

50.1 °C 45.3 °C

50.3 °C 45.3 °C

50.3 °C 45.6 °C

addition of 5% PBF, the HDT of PLA blend slightly decreases. Interestingly, the increase of PBF content does not cause a continuous drop of the HDT in PLA blends. It may be related with little increase of crystallinity in the PLA matrix with the addition of PBF. The well maintained HDT of PLA blends allows further improvement by physical or chemical modification. Gas Barrier Property. PLA based composites are thought to be safe as food packaging,56 and their gas barrier property is crucial for food preservation. Hereby, we evaluated the gas barrier property of PLA/PBF blends, and Figure 10a shows the data for oxygen transmission rate (OTR), carbon dioxide transmission rate (CDTR) and water vapor transmission rate (WVTR) in PLA, PBF and PLA blends. It is clear that pure PBF has a much better gas barrier property than pure PLA, and the gas barrier property of PLA blends increased with the increase of PBF content. When PBF content was 30%, CDTR, OTR and WVTR were 7.4, 6.3 and 2 times better than those of pure PLA, respectively. For comparison, we also measured the gas barrier property of PEs and PET in Figure 10b. By adding PBF into the PLA 9250



Research Article

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APPENDIX First, we assume that the volume of a PBF dispersed domain keeps the same during stretch. Second, we statistically analyzed the radius (Rs) of PBF droplets before stretch and the radius (Rf) of PBF fiber after stretch with the help of ImageJ (free software released by NIH). It is worth to mention that we approximately take fiber as cylinder in calculation for simplicity. Finally, the average length of PBF fiber can be calculated as the following eqs 1−3: Sf = πR f 2

Figure 10. (a) Gas barrier property of PLA, PBF and PLA blends films. (b) Comparison of gas barrier property in films of PLA, PLA/ PBF blends, HDPE, LDPE and PET.

(1)

Where Sf the cross-sectional area of the elongted PBF fiber and Rf the radius of the fiber.

V=

matrix, the gas barrier property of PLA blends improved. The OTR and CDTR of PBF30 become close to those of PET. Further, OTR and CDTR of PBF were better than those of PET. But the WVTR of PBF30 was 3−6 times lower than that of HDPE and LDPE. This is maybe because the oxygen atom in the furan ring affected the polarity of PBF, which make the polymer chain easier to combine with water. Polyolefin, on the other hand, has inherent hydrophobicity and possesses a good water barrier property.57

4 πR s 3 = Sf × Lf 3

(2)

Where V the volume of PBF dispersed phase, Rs the radius of PBF droplet before elongation and Lf the length of PBF fiber. Lf

4 πR s 3) ( 3 =

Sf

(3)

In this way, we can acquire Lf from the above calculations. The elongation ratior of PBF phase can be defined as Lf/Ds, and Ds is the diameter of PBF droplet before tensile experiment.

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CONCLUSIONS In this work, a 2,5-furan dicarboxylate based polyester PBF is synthesized through transesterification and polycondensation. Then, it is blended with PLA to prepare fully biobased polymer blends, and their basic properties are evaluated. By adding only 5% PBF, the elongation at break of PLA increases to ∼183.5%, whereas the modulus and tensile strength at yield drop less than 2%. With the increase of PBF content in the blends, the elongation at break gradually increases, and the modulus and the tensile strength at yield decrease mildly. The SEM micrographs in different necking regions show the deformation process of PBF domains. Classical debonding and caviation phenomena do not occur in the immiscible PLA/PBF blends. The gap between PLA and PBF phases seems disappear during the tensile procedure. This interesting and extraordinary balance between modulus and tensile toughness is possibly due to the glass-amorphous transition and stretch induced crystallization in PBF phase. During the elongation, internal molecular friction heats up PBF to an elastic state, at which extreme deformation from sphere to fibril happens. Simultaneously, stretch induced crystallization happens in both the PLA matrix and PBF domains, which strengthen the stiffness along the tensile direction. In this way, the blend is dynamically toughened and strengthened during the elongation process. However, more experiments are needed to uncover the whole process through in situ and multiscale investigation. Another interesting fact is that the impact strengths of the blends are better than those of both PLA and PBF. It is possible that a similar mechanism in tensile test may be applied in impact experiments too. Thermal stability and HDT of PLA/PBF blends do not change too much when compared with PLA, which imply the possibility of further modification. Since PBF owns an overwhelming gas barrier property than PLA, the addition of PBF improves the gas barrier ability of PLA matrix. In conclusion, PLA/PBF blends, as base material, show considerable potential for further development and commercial application like eco-packaging. More work, including reactive blending, is worth to do in this fully biobased polymer blends.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Ruoyu Zhang). *E-mail: [email protected] (Jin Zhu). ORCID

Ruoyu Zhang: 0000-0002-3502-8738 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Ningbo Science and Technology Bureau Project (Grant No. 2013B6005), The National Key Research and Development Program of China (Grant No. 2017YFB0303000) and National Natural Science Foundation of China (Grant No. 51603107). Authors also thank Prof. Jiawei Zhang and Mr. Yukun Jian for their assistance in scheme drawing.

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