poly (butylene adipate-co-terephthalate) blends with different viscosity ratio

Accepted Manuscript Morphology and properties of biodegradable poly (lactic acid)/poly (butylene adipateco-terephthalate) blends with different viscosity ratio Xiang Lu, Jianqing Zhao, Xiaoyun Yang, Peng Xiao PII:

S0142-9418(17)30129-0

DOI:

10.1016/j.polymertesting.2017.03.008

Reference:

POTE 4955

To appear in:

Polymer Testing

Received Date: 4 February 2017 Accepted Date: 8 March 2017

Please cite this article as: X. Lu, J. Zhao, X. Yang, P. Xiao, Morphology and properties of biodegradable poly (lactic acid)/poly (butylene adipate-co-terephthalate) blends with different viscosity ratio, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.03.008. 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.

ACCEPTED MANUSCRIPT Material Properties

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Morphology and Properties of Biodegradable Poly (lactic acid)/Poly (butylene adipate-co-terephthalate) Blends with Different Viscosity Ratio Xiang Lu1,2,3, Jianqing Zhao1,2*, Xiaoyun Yang3, Peng Xiao3* 1 College of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China 2 Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou, 510640, China 3 National-certified Enterprise Technology Center, Kingfa Scientific and Technological Co. Ltd., Guangzhou, 510663, China

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Abstract: Phase morphology exerts a tremendous influence on the properties of polymer blends. The development of the blend morphology depends not only on the intrinsic structure of the component polymers but also on extrinsic factors such as viscosity ratio, shearing force and temperature in the melt processing. In this study, various poly (butylene adipate-co-terephthalate) (PBAT) materials with different melt viscosity were prepared, and then poly (lactic acid) (PLA)/PBAT blends with different viscosity ratio were prepared in a counter-rotating twin-screw extruder under constant processing conditions. The influence of viscosity ratio on the morphology,

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mechanical, thermal and rheological properties of PLA/PBAT (70/30 w/w) blends was investigated. The experimental results showed that the morphology and properties of PLA/PBAT blends strongly depended on the viscosity ratio. Finer size PBAT phase were observed for viscosity ratio less than 1 (λ<1) compared to samples

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with λ>1. It was found that the interfacial tensions of PLA and PBAT were significantly different when the viscosity ratio was changed, the lowest interfacial tensions (0.12 mN/m) was obtained when the viscosity was 0.77. Additionally, the maximal tensile strength in PLA/PBAT blends were obtained when the viscosity ratio was 0.44, while the maximal impact properties were obtained when the viscosity ratio was 1.95. Keywords: Viscosity ratio, Morphology, Properties, PLA/PBAT blends Introduction

The blending of immiscible or miscible polymers has become an increasingly important technique for developing commercial polymer materials, which may combine the properties of several single polymers [1, 2]. It is much more cost-effective to blend two or more polymers with known properties than to synthesize new polymers with unknown properties [3]. However, since most blended polymers are immiscible, polymer blends tend to separate into two or more distinct phases [4, 5]. The final properties of such immiscible blends are strongly affected by

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the structural morphology and individual polymer components [6]. The blend morphology of multiphase polymer blends developed during processing is a complex function of blend composition, interfacial tension, viscosity ratio, shear conditions and other processing conditions [7-10]. The competing processes of droplet break-up and coalescence during melt processing determine the final morphology of these mixtures [11, 12]. It is generally agreed that, among different factors, the viscosity ratio (λ) (the ratio of the viscosity of the dispersed polymer to the viscosity of the matrix polymer) is one of the most critical variables in controlling the blend morphology [12-14]. A high viscosity ratio in many cases may result in coarse morphology, whereas matching the viscosities (λ≈1) results in much finer morphology [13]. Several theoretical and experimental studies have been performed to explain these processes, starting with the pioneering work of Taylor [15, 16]. The dimensionless parameter, known as the capillary number (Ca) is often used for describing the size of / , where R is characteristic the dispersed particles in polymer blends (Ca = size of the dispersed droplet, is the viscosity of the matrix, is shear rate, and is the interfacial tension). Droplet breakup occurs at a critical Ca value ( ). On the basis of Taylor’s study, the dependence of the morphology on the ratio of the viscosity of the dispersed phase to that of the matrix was investigated by Wu [17], who found that the relative influence of interfacial tension ( ) and viscosity ratio (λ) on phase morphology dimensions was characterized with a dimensionless Weber number:

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= / = 4λ (1) where parameter is equal to +0.84 when λ > 1, and to −0.84 when λ < 1. From eq. (1), it is concluded that the smallest particles are obtained when the viscosity ratio is near unity. The effect of viscosity ratio on the morphology and properties of immiscible polymer blends has been studied. For polyethylene/polystyrene (PE/PS) blends, Min et al. [18] found that, when the dispersed phase had a lower viscosity, it formed long fibers in the matrix, but the dispersed phase with higher viscosity was in the form of discrete droplets. Ardakani et al. [7] investigated the viscosity ratio on the phase behavior and morphology of PP/polybutene-1 blends; they found that the size of particles increased proportionally when the viscosity ratio was increased. Wu et al. [19] studied the viscosity ratio on the morphology of biocompatible ethylene-vinyl acetate copolymers/poly (caprolactone) (EVA/PCL) blends; the results indicated that the co-continuous structure and phase size depended strongly on the blending ratio and viscosity ratio. Tavanaie et al. [20] found that the viscosity ratio played an important role in controlling the size of poly (butylene terephthalate) (PBT) dispersed phase morphology in the matrix phase of PP/PBT alloy. The morphology including

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the size and size distribution of the minor phase has a profound influence on the mechanical properties of the blends [7, 12, 20]. In recent years, large amounts of petroleum-based plastic waste has been produced worldwide, and has caused serious environmental problems [21-23]. Therefore, significant effort has been put into replacing today’s fossil fuel-based polymer with the more sustainable biomaterials based on biomass [24, 25]. Poly (lactic acid) (PLA) and poly (butylene adipate-co-terephthalate) (PBAT) are two interesting biomaterials [26-29]. Unfortunately, with a glass transition temperature ranging from 55 oC to 65 o C, PLA is too stiff and brittle at room temperature, and this inherent brittleness is the major drawback that restricts its use as a common plastic material [30]. Among the flexible polymers, PBAT is considered as a good candidate for toughening PLA due to its high toughness and biodegradability [31]. The performance of PLA and PBAT is complementary. Jiang et al. [32] studied the properties of PLA/PBAT blends, and found that the tensile toughness and impact strength of the blends were greatly increased at 10 wt% or higher content of PBAT. However, PLA/PBAT blend is seen as immiscible, and phase separation usually leads to the probable deterioration in properties. As mentioned above, the morphology of blend has a profound influence on the mechanical properties. Therefore, various studies were aimed at improving the morphology of PLA/PBAT blend. The multifunctional epoxy chain extender (Joncryl ADR 4368) significantly changed the morphology and improved mechanical properties of PLA/PBAT blend [31]. Kumar et al. [33] found that glycidyl methacrylate (GMA) was an effective reactive compatibilizer to improve the interface between PLA and PBAT, and mechanical studies indicated that incorporation of 3-5 wt% GMA increased the impact strength of PLA/PBAT blend by 26.5%-51.7%, whilst retaining the tensile strength. However, apart from the compatibility between the blend components, the viscosity ratio (λ) of the blend components is another major factor in determining the blend morphology [11]. To the best of our knowledge, the effect of viscosity ratio on the morphology and properties of PLA/PBAT blend has not been investigated. This study focused on the influence of the blend viscosity ratio on the resulting blend morphology and final mechanical, thermal and rheological properties of PLA/PBAT blends. Experimental Raw Materials PLA was commercial grade (PLA 4032D) and was obtained from Natureworks, LLC (USA). PBAT (KD 1025) was supplied by Dongguan Dongruxin Polymer Technology Co., Ltd. (CHINA). Dicumyl peroxide (DCP) was obtained from

ACCEPTED MANUSCRIPT Sigma-Aldrich. All the chemicals were used as received.

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Preparation of PBAT with different viscosity Before processing, PBAT was dried at 80 oC under vacuum for 8 h. PBAT was melt blended with DCP (0, 0.1, 0.25, 0.7wt% respectively) using a Brabender Plasticorder with a chamber of 55 cm3 and roller blades at 180 °C. The rotor speed was kept at 60 rpm for 10 min. The blended PBAT materials with different viscosities were immediately pelletized. For brevity, PBAT with different DCP loading were designated as PBAT-1 (0 wt% DCP), PBAT-2 (0.1 wt% DCP), PBAT-3 (0.25 wt% DCP), and PBAT-4 (0.7 wt% DCP), respectively. Preparation of PLA/PBAT blends Before processing, PLA and PBAT were dried at 80 oC under vacuum for 4 h to

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avoid possible moisture-degradation reaction. Dried PLA and PBAT with different viscosities (PLA/PBAT: 70/30, w/w) were manually premixed by tumbling in a plastic

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zip-lock bag, and then fed into a Brabender Plasticorder with a chamber of 55 cm3 and roller blades at 190 °C. The rotor speed was kept at 60 rpm for 10 min. All the samples were molded into 1 or 4 mm sheets at 180 °C and 10 MPa for 15 min. The samples were conditioned at 50% relative humidity and 25 oC for at least 48 h before testing.

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Characterization Melt Flow Index (MFI) of PLA and PBAT blends was determined with an MTS ZRZ2452 melt flow index instrument in accordance with ISO 1133. The fracture surface of the blends was imaged by a scanning electronic microscope (SEM, HITACHI SE3400N). The cryo-fractured specimens (4mm thick) were

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obtained after being submerged in liquid nitrogen for ~ 15 min. The sample surface was sputter-coated with gold to prevent build-up of electrostatic charge during

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observation.

Contact angle tests of samples was performed with an OCA 15 PLUS apparatus

(Dataphysics Co., Ltd., Germany). The static contact angle of distilled water was measured by depositing a drop of 3−5 µL on the sample surface and the values were estimated as the tangent normal to the drop at the intersection between the sessile drop and the surface. PLA and PBAT samples for contact angle measurement were compression-molded between clean polyester films at 190 oC for 4min and then cooled to 25 oC under pressure for 1 min. To avoid solvent evaporation, images were taken within 30 s of drop deposition. All results of the contact angle were taken from the mean values of 10 replicates at different spots of the surface. Melt rheological behavior of the blends was studied using a MCR302 rheometer

ACCEPTED MANUSCRIPT (Anton Paar, Austria) in dynamic oscillation mode with a parallel-plate geometry (diameter = 25 mm, gap = 1 mm). Frequency-sweep tests were performed from 0.01 to 100 rad/s at 200 °C. To maintain the response of materials in the linear viscoelastic regime, an amplitude of 1 % was used. Differential scanning calorimetry (DSC) was performed on a Netzsch DSC

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instrument (model 204c, Germany) equipped with a liquid nitrogen-cooling accessory. Specimens were first heated from room temperature to 210 °C at a rate of 10 °C/min, kept there for 3 min to eliminate any thermal history, and then cooled to 30 °C at a rate of 10 °C/min under nitrogen atmosphere. The second scan was performed by

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reheating from 30 to 210 ° C at a rate of 10 °C/min.

Thermogravimetric analysis (TGA) was performed on a Netzsch TG209 instrument over 30 ~ 600 °C under N2 atmosphere (250 ml/min) with a 10 °C/min heating ramp.

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All thermal parameters were determined from an average of three experiments. Tensile tests to determine the yield strength, and elongation at breaking was carried out using an Instron universal machine (model 5566, USA), in accordance with ISO527, under tension mode at a single-strain rate of 20 mm/min at room temperature. An Instron POE2000 pendulum impact tester was used for impact testing.

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Results and Discussion

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Characterization of PBAT with different viscosities Torque evolution for the chain extension of PBAT initiated by DCP is shown in Fig. 1. In the initial 40 s, the torque displays a sharp and strong peak, which is attributed to melting of PBAT pellets [26]. Then, different amounts of DCP was added. With the chain extension of PBAT initiated by DCP, the dramatic increase of torque indicates the production of the molecular weight and cross-linked structure or even long-chain branching. However, the torque decreases with the mixture time after 150 s for possible thermal degradation of the polymer [34], as shown in the curves of Fig. 1. In summary, the torque of PBAT modified by DCP increases monotonously with the DCP content, compared with the torque of the original PBAT. The curves are disorganized due to the feeding process, but the final torque value is proportional to the apparent viscosity of the materials, which depends on the molecular weight and chain structure of the polymer [35]. Complex viscosity (η*) and melt flow index (MFI) are traditional parameters used to characterize the mobility of molten polymers, which depends on the molecular weight and chain structure of the polymer. Complex viscosity (η*) values of PBAT and PLA measured at 190 oC as a function of angular frequency (ω) are shown in Fig.

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2. The plots of the MFI of PBAT as a function of the content of DCP are illustrated in Fig. 3. It can be seen that, with increasing DCP content, the complex viscosity (η*) of the PBAT increases while the MFI decreases monotonously. According to the entanglement theory, the presence of cross-linking network structure or long-chain branching structure exacerbated the tangles of the PBAT molecular chains, so the mobility of the polymer melt was affected. All the results indicate that DCP is a good cross-linking agent for PBAT. In our experiments, the ratio of complex viscosity (η*) values at 10 rad/s between PBAT and PLA were designated as the viscosity ratio (λ) of PBAT to PLA. The MFI values, complex viscosity values at 10 rad/s of PBAT and PLA, and viscosity ratio (λ) of PBAT to PLA are listed in Table 1.

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Surface property characterizations Contact angle measurement is a traditional procedure used for the assessment of the surface energy of solids [36]. Fowkes and his co-workers [37] suggested that the

=

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surface energy (γ) of a solid or a liquid consists of the dispersive ( ( ) components: +

) and the polar

(2)

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= + (3) and the relations between contact angle (θ) and surface energy (γ) can be described by the Owens-Wendt method [38]: 1+

θ" = 2

"$⁄% + 2

"$⁄%

(4)

where γ and γ are the surface energy of the solid and liquid, respectively; θ is and

the contact angle;

and

are the dispersive and polar components of the liquid,

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respectively; and

are the dispersive and polar components of the solid,

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respectively. If the contact angles of at least two liquids with known

and

parameters, usually a polar liquid and a nonpolar liquid, are measured on a solid surface, the and , as well as the surface energy of the solid (γ ), can be calculated by combining eqs 2-4. Here, the literature values of two components for the test liquids (H2O: =50.8 dyn/cm and =22.5 dyn/cm; CH2I2: =2.3 dyn/cm and =48.5 dyn/cm) were used [39]. Fig. 4 shows digital photos of water contact angle and CH2I2 contact angle for the used PBAT and PLA. The calculated surface parameters of the tested samples are listed in Table 2. Using the parameters in Table 2, the interfacial tension values of PLA/PBAT-1, PLA/PBAT-2, PLA/PBAT-3 and PLA/PBAT-4 could be calculated according to the geometric-mean equation [40]:

%$=

$

+

%

− 2[

$ %"

$⁄%

+

$ %

"$⁄% ]

(5)

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where $% is the interfacial tension and $ and % are the surface energy of the two materials in contact. The calculated interfacial tension values of PLA/PBAT-1, PLA/PBAT-2, PLA/PBAT-3 and PLA/PBAT-4 are 22.1, 0.14, 0.12 and 0.57 mN/m, respectively. Such a high interfacial tension between PBAT-1 and PLA indicates that PBAT-1 and PLA are thermodynamically incompatible, and as a result, can merely form large-scale phase separation structure in their blend during melt blending. Generally, the smaller the interfacial tension values, the better the compatibility between the two components. Therefore, according to the interfacial tension theory, the compatibility order between PBAT and PLA is PBAT-2≈PBAT-3>PBAT-4>PBAT-1.

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Mechanical properties of PLA/PBAT blends The mechanical properties of PLA/PBAT-1, PLA/PBAT-2, PLA/PBAT-3 and PLA/PBAT-4 blends are presented in Fig. 5 (a-d). As shown in Fig. 5 (a) and (b), the tensile strength of the PBAT/PLA blends are significantly affected by the viscosity ratio between PBAT and PLA. Compared to blend of PLA/PBAT-1 (42.0 MPa), the tensile strength of PLA/PBAT-2, PLA/PBAT-3 and PLA/PBAT-4 blends increased to 53.2 MPa, 51.4 MPa, and 47.3 MPa, which increased by 26.7%, 22.4%, and 12.6%, respectively. Furthermore, as shown in Fig. 5 (c), all the PLA/PBAT blends show lower elongation at break (≤20%). However, the elongation at break of PLA/PBAT blends was also affected by the viscosity ratio, and the PLA/PBAT-3 (the viscosity ratio of PBAT to PLA is 0.77) blend shows the maximum elongation at break (20.3%). In addition to tensile properties, the impact strength of PLA/PBAT blends was also significantly affected by the viscosity ratio between PBAT and PLA. The impact strengths of PLA/PBAT-2, PLA/PBAT-3 and PLA/PBAT-4 blends were increased to 7.0 kJ/m2, 6.9 kJ/m2, and 10.8 kJ/m2, which are approximately 1.4, 1.4, and 2.1 times higher than that of PLA/PBAT-1 blend (5.1 kJ/m2), respectively. The increasing tensile strength, elongation at break and impact strength of PLA/PBAT blends are probably due to the improvement of compatibility between PLA and PBAT and the transition of phase morphology.

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Morphology of PLA/PBAT blends SEM was employed to identify the phase structures of the PLA/PBAT blends. Fig. 6 gives the SEM images for PLA/PBAT blends with various viscosity ratios. Clearly, all samples present typical two-phase island-sea morphology, where discrete droplets of the minor phase (PBAT) are dispersed in the matrix (PLA). Finer size PBAT phase is observed for viscosity ratio less than 1 (λ<1) compared to samples with λ>1. The average domain size of PLA/PBAT-1 blend is about 0.5-1µm. With increasing viscosity ratio (from 0.23 to 0.77), the increase of average domain size of PBAT is not obvious for blends of PLA/PBAT-2 and PLA/PBAT-3. However, for blend of PLA/PBAT-4 (viscosity ratio is 1.95), the average domain size increased to 2-3µm, which is approximately 3 times larger than that of PLA/PBAT-1 blend. Moreover, all the cryo-fractured surfaces of the blends are smooth, indicating that PLA and PBAT

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are immiscible thermo-dynamically. In addition to the cryo-fracture surfaces of PLA/PBAT blends, the impact fracture surface of the specimens was characterized to further investigate the toughening effect of the materials. For rubber-toughened plastic system, two kinds of cavitation can be induced during impact testing[26]:internal cavitation in the rubber domains and debonding cavitation between rubber components and plastic components. Internal cavitation occurs when there is a strong interfacial adhesion between the components, and debonding cavitation exists between two components when impact stress is higher than bonding strength at the interface [26, 41]. The PBAT phase can act as stress concentrators in the PLA/PBAT blends because the elasticity of PBAT is different from that of PLA. The PLA matrix can deform more easily during debonding to achieve shear yielding. In the PLA/PBAT-1 blend (Fig. 7 a), interfacial debonding between PLA and PBAT-1 after impact was clearly observed and it exhibited distinct island-sea-type morphology, which is in accordance with the poor interfacial adhesion between PLA and PBAT-1. Additionally, internal cavitation in PBAT domains after impact was clearly observed in PLA/PBAT-2, PLA/PBAT-3 and PLA/PBAT-4 blends (Fig. 7 b, c, and d). However, for rubber-toughened plastic systems, the toughening effect is not only related to compatibility, but also to the rubber particle size [42, 43]. In this study, with increasing viscosity ratio, the interfacial tension between PBAT and PLA decreased and the size of PBAT phase increased. The maximal tensile strength in PLA/PBAT blends was obtained when the viscosity ratio was 0.44, while the maximal impact properties were obtained when the viscosity ratio was 1.95. Good interfacial adhesion and moderate dispersed phase particle size could contribute to the improvement mechanical properties of PLA/PBAT blends.

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Thermal properties of PLA/PBAT blends PLA and PBAT are semicrystalline polymers, and the mechanical properties of PLA/PBAT blends strongly depend on their crystallization behavior. The crystallization and melting behavior of PLA/PBAT blends was examined to investigate whether the crystallization of PLA and PBAT influenced the mechanical properties of the blend system. Only the first heat scan of the DSC data is presented because the mechanical properties of the blends can only be affected by the crystalline state of PLA and PBAT in the molded samples. Fig. 8 shows the melting curves of the various PLA/PBAT blends and PBAT, and Table 3 displays a summary of the DSC results. The relative crystallinity (+ ) of PLA component of all samples is calculated as follows: + where ∆:

,-."

,-."

=

∆01 234" 5∆066 234" 7 ∆01 234" 89

(7)

is the measured melting enthalpy by DSC, ∆:

,-."

is the cold

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<

,-."

is the enthalpy of pure PLA crystal

is the weight fraction of PLA component in the blend.

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As shown in Fig. 8 (a), all PLA/PBAT blends exhibit similar thermograms, and all the thermograms show cold crystallization behavior of PLA and subsequent melting of PLA crystals. The presence of an endothermic peak near the glass transition temperature (=> ) was attributed to the enthalpy relaxation effects that reflect the

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thermal history of the samples. Surprisingly, the endothermic melting peak for PBAT component in PLA/PBAT blends did not appear. This may be attributed to the fact that, compared to PLA component, the related thermal signal for PBAT component is very weak. With increasing viscosity ratio between PBAT and PLA, the cold crystallization peak of the PLA component shifts to lower temperature after the addition of PBAT. However, only a slight change in = and + of the PLA component was observed with increasing viscosity ratio. It should be pointed out that, with increasing viscosity ratio, the => of PLA component shift from 56.2 oC to 53.7 o C, which is closer to the => of PBAT component (-30 oC [45]). This indicates that compatibilization between PLA and PBAT was a significant improvement, which was in agreement with the results of surface property measurements. Therefore, on the basis of the above results, the significantly increased mechanical properties of the PLA/PBAT-2, PLA/PBAT-3, and PLA/PBAT-4 blends can be attributed to the

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improved interfacial compatibilization between PLA and PBAT, rather than the change in crystallinity of PLA.

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Thermal stability of PLA/PBAT blends In order to investigate the influence of viscosity ratio on the thermal stability of blends, TGA analysis was performed on the PLA/PBAT blends. The TGA curves and the first derivative TGA (DTG) curves of the pure PLA, PBAT and PLA/PBAT

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blends are shown in Fig. 9 and Fig. 10, respectively. Finally, Table 4 gives a summary of the TGA and DTG data: (1) onset degradation temperature (=? ", which was taken arbitrarily as the temperature at 5 wt% degradation occurred; (2) maximum degradation temperature ( = @A ), the temperature at which maximum rate of degradation occurred; and (3) the percentage char formation at 550 oC. All results were reported as averages of three independent tests. As shown in Fig. 9, the TGA and DTG curves indicate that PBAT has better thermal stability than PLA. With introduction of DCP to PBAT, the thermal stability of PBAT first decreased, and then increased with increasing DCP content. Fig. 10 shows that the thermal degradation of the PLA/PBAT blends consists of two weight losses between 250 oC and 450 oC, corresponding to the PLA and PBAT, respectively. All the blends presented similar behavior, and the corresponding results are tabulated

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C, 368.7 oC, 367.4 oC, and 368.3 oC for PLA/PBAT-1, PLA/PBAT-2, PLA/PBAT-3, and PLA/PBAT-4 blends, which were close to that of pure PLA (367.4 oC). These

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results indicated that the incorporation of PBAT improved the thermal stability of the polymer materials.

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Dynamic rheological analysis Complex viscosity (η*), storage modulus (G’), and loss modulus (G’’) of PLA/PBAT blends at 200 oC versus angular frequency (ω) are shown in Fig. 11 (a), (b) and (c), respectively. The shear melt viscosity of the samples shows pseudoplastic

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flow behavior, regardless of their viscosity ratio (Fig. 10 a). The dependence of the complex viscosity on viscosity ratio of PLA/PBAT blends at low, moderate and high

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constant shear rates (0.1, 10, and 100 rad/s-1, respectively) is shown in Fig. 12. The complex viscosity at low shear rate (0.1 rad/s-1) shows both positive and negative deviations from a monotonic change from the lowest (PLA/PBAT-1) to the highest (PLA/PBAT-4) viscosity ratio. However, at moderate (10 rad/s-1) and high (100 rad/s-1) levels of shear rate, the melt viscosity became closer to a monotonic change. Interestingly, the lowest differences among measured complex viscosities from low and high levels of shear rates are observed in the PLA/PBAT-2 blends. It seems that a good relationship exists between these rheological properties and the mechanical properties of the PLA/PBAT-2 blend because the best tensile properties are also observed for the PLA/PBAT-2 blend. According to Fig. 12, higher differences of complex viscosities of the samples (PLA/PBAT-1, PLA/PBAT-3 and PLA/PBAT-4) compared to the sample (PLA/PBAT-2) over all ranges of shear rates (0.1-100 rad/s-1)

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are observed. Consequently, it can be concluded that the effect of viscosity variation of the dispersed phase (PBAT) on the complex viscosity of the PLA/PBAT blends is stronger. In other words, the rheological properties (η*, G’, G’’) of the PLA/PBAT blends are more affected by viscosity ratio. Conclusions

The viscosity ratio plays an important role in the morphology of PBAT dispersed phase in the matrix phase of PLA/PBAT blends. SEM images and contact angle measurement indicate that an increase in the viscosity ratio results in an improvement of compatibility between PLA and PBAT. Due to the improvement of compatibility and the transition of phase morphology, the best tensile properties were obtained for the PLA/PBAT-2 (viscosity ratio is 0.4) and PLA/PBAT-3 (viscosity ratio is 0.77)

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Corresponding Author Jianqing Zhao* E-mail: [email protected] Peng Xiao* E-mail: [email protected] Notes The authors declare no competing financial interest.

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AUTHOR INFORMATION

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blends, and the best impact properties were obtained for the blend of PLA/PBAT-4 (viscosity ratio is 1.95). With the incorporation of various PBAT with different viscosity, the thermal stability of the polymer materials was improved. Dynamic rheological analysis indicated the rheological properties (η*, G’, G’’) of the PLA/PBAT blends were more affected by the morphology and compatibility of the blends.

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Table 1 Melt flow index, complex viscosity and viscosity ratio of the used PBAT and PLA

PBAT-1 PBAT-2 PBAT-3 PBAT-4 PLA

90.8 49.3 27.4 7.68 28.8

η* (10 rad/s, Pa*s) 112.6 194.3 373.5 947.7 485.9

viscosity ratio (λ) of PBAT to PLA 0.23 0.40 0.77 1.95 -

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Table 2 Measured contact angles (25 oC) and calculated values of surface energy of the used PBAT and PLA EFG H (o) EIFG JG (o) KL (mJ/m2) Samples γ (mJ/m2) KM (mJ/m2) PBAT-1 73.7 19.7 2.60 4.81 7.41 PBAT-2 76.0 20.9 43.7 3.90 47.7 PBAT-3 81.2 18.5 44.4 4.91 49.3 PBAT-4 78.8 15.8 46.3 2.61 48.9 PLA 75.8 30.3 39.9 4.82 44.7

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Table 3. Glass transition temperatures (=> ), cold crystallization temperatures (= ), melting temperatures (= ), cold crystallization enthalpies (∆: ), melting enthalpies (∆: ), and relative crystallinities (+ ) of PLA component in PLA/PBAT blends Samples NPP NQ ∆FPP ∆FQ RP NO (oC) o o ( C) ( C) (J/g) (J/g) (%) PLA/PBAT-1 56.2 93.5 168.3 19.1 28.4 16.7 PLA/PBAT-2 55.1 92.1 168.7 18.2 28.0 17.5 PLA/PBAT-3 53.7 89.2 169.1 18.5 28.2 17.4 PLA/PBAT-4 54.2 85.7 168.9 18.7 28.5 17.6

Table 4. Onset degradation temperature at 5% weight loss (=? ), temperature at maximum degradation rate (= @A ), and % charred residues at 550 °C for PLA/PBAT

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NQTU 367.4 368.5 368.7 367.4 368.3

Charred residues at 550 oC 1.7 1.5 1.8 1.9 1.8

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Samples PLA PLA/PBAT-1 PLA/PBAT-2 PLA/PBAT-3 PLA/PBAT-4

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Fig. 1 Torque evolutions for reactions between PBAT and DCP

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Fig. 2 Complex viscosity (η*) versus angular frequency (ω) for PBAT and PLA

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Fig. 3 Melt flow index of PBAT as a function of the content of DCP

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Fig. 4. Digital photos of water contact angle (a-e) and CH2I2 contact angle (a’-e’)

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for the used PBAT and PLA

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Fig. 5 Mechanical properties of PLA/PBAT blends,

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(a) stress-strain curves, (b) tensile strength, (c) elongation at break, (d) impact strength

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Fig. 6 SEM images of the cryo-fracture surfaces of PLA/PBAT blends

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(a) PLA/PBAT-1, (b) PLA/PBAT-2, (c) PLA/PBAT-3, (d) PLA/PBAT-4

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Fig. 7 SEM images of the impact fractured surfaces of PLA/PBAT blends

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(a) PLA/PBAT-1, (b) PLA/PBAT-2, (c) PLA/PBAT-3, (d) PLA/PBAT-4

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Fig. 8 Melting curves of various PLA/PBAT blends and PBAT

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Fig. 9 TGA and DTG curves of various PLA and PBAT

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Fig. 10 TGA and DTG curves of PLA/PBAT blends

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Fig. 11 Rheological properties for the PLA/PBAT blends with different viscosity ratio

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(a) Complex viscosities, (b) storage modulus,(c) loss modulus

Fig. 12 Complex viscosities of PLA/PBAT blends versus viscosity ratio

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at different constant shear rates