EVOH blends: Correlation of morphology, rheology and mechanical properties

Journal Pre-proof A multiple approach in determination of interfacial tension of biodegradable meltmixed PBAT/EVOH blends: Correlation of morphology, rheology and mechanical properties Mehdi Elhamnia, Ghodratollah Hashemi Motlagh, Seyed Hassan Jafari PII:

S0142-9418(19)31934-8

DOI:

https://doi.org/10.1016/j.polymertesting.2019.106301

Reference:

POTE 106301

To appear in:

Polymer Testing

Received Date: 21 October 2019 Revised Date:

8 December 2019

Accepted Date: 15 December 2019

Please cite this article as: M. Elhamnia, G.H. Motlagh, S.H. Jafari, A multiple approach in determination of interfacial tension of biodegradable melt-mixed PBAT/EVOH blends: Correlation of morphology, rheology and mechanical properties, Polymer Testing (2020), doi: https://doi.org/10.1016/ j.polymertesting.2019.106301. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

CRediT author statement Mehdi Elhamnia: Designing and realizing the experiments, Methodology, and Writing the original draft. Ghodratollah Hashemi Motlagh: Supervision, Conceptualization and Editing. Seyed Hassan Jafari: Cosupervision, Reviewing and Editing

A multiple approach in determination of interfacial tension of biodegradable melt-mixed PBAT/EVOH blends: Correlation of morphology, rheology and mechanical properties Mehdi Elhamnia a,b, Ghodratollah Hashemi Motlagh a,b,*, Seyed Hassan Jafari a,** a

Advanced Polymer Materials and Processing Lab, School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran. b Azmoon Dana Plastic Co. (Testing Lab), 3rd Sanaat Ave, Shahre Qods, Tehran, Iran. Email: [email protected] * Correspondence to: Ghodratollah Hashemi Motlagh, E-mail: [email protected] ** Correspondence to: Seyed Hassan Jafari, E-mail: [email protected]

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ABSTRACT The interfacial tension of biodegradable melt-mixed blends of poly(butylene adipateco-terephthalate), PBAT, and poly(ethylene-co-vinyl alcohol), (EVOH), was measured by breaking thread (BT), imbedded fiber retraction (IFR), and rheological methods. The PBATrich blends were prepared under different melt mixing conditions in order to investigate the effect of mixing conditions and possibility of reactive mixing between the blend components on the blend morphology, rheology, mechanical properties and interfacial tension values. The conditions were varied based on a Taguchi design of experiment using four factors namely EVOH content (0-30 wt%), mixing time (2-15 min), rotor speed (50-90 rpm), and mixing temperature (185-200 °C), each varying at three levels. The average size of EVOH droplets in PBAT matrix was determined for each blend by a field emission-scanning electron microscopy technique. The values of interfacial tension of PBAT/EVOH were found to be 2.57±0.22 and 2.73±0.30 mN.m-1 by the BT and IFR methods. The Palierne, Gramespacher, and Bousmina models were fitted to the rheological data to verify the interfacial tension of the blends. The continuous relaxation spectrum of the blends was determined in order to obtain the relaxation time of the EVOH droplets in the PBAT matrix. The Taguchi analysis revealed that the most effective factor is the EVOH content, and other factors do not play a significant role in the ultimate properties of the blends. Finally, based on the obtained mechanical properties, the possibility of reactive mixing under the applied mixing conditions was ruled out by means of repeated differential scanning calorimetry (DSC) and rheological measurements. Keywords: Interfacial tension, PBAT, EVOH, Breaking thread method, Imbedded fiber retraction method, Rheology, Continuous relaxation spectrum, Reactive mixing, Mechanical properties

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1. Introduction Poly(butylene adipate-co-terephthalate) (PBAT) is a biodegradable copolymer with excellent properties particularly a balanced flexibility due to its aromatic-aliphatic structure. PBAT is mainly used for packaging and agricultural applications [1]. The disposal of plastic wastes is considered to be a serious environmental issue embossing the necessity of finding a proper alternative for common polymers. PBAT as a biodegradable copolymer is a promising alternative; however its high gas and water vapor permeability has restricted its application in packaging industry. On the other hand, poly(ethylene-co-vinyl alcohol), EVOH, has very low permeability to these permeants which accelerates most of its applications in packaging industry [2]. It would be a unique opportunity to blend PBAT and EVOH to utilize the benefits of both for achieving an improved packaging product. The properties of PBAT/EVOH blend system are not reported in the literature. Thus, studying the various properties of this system will be very useful to explore the potential advantages and applications along with the challenges and drawbacks. Interfacial tension is an essential factor in polymer blends which controls the blend morphology, compatibility and final properties. By blending two polymers, it is important to consider the properties and behavior of each polymer and the preferred morphology since the final properties of the blend are affected by the blend morphology and microstructure. In the applications where barrier properties is controlled by the dispersed phase, a higher aspect ratio of the dispersed phase and creating a three-dimensional network reduces permeability since the tortuosity and length of the permeate path increase [3-5]. PBAT and EVOH have a polar structure, thus, they potentially seem to have a good interaction and form a compatible blend. Previous reports calculated the solubility parameter by the group contribution method which was equal to 22.2 MPa0.5 [6] and 26.1 MPa0.5 [7] for PBAT and EVOH, respectively. These values are approximately close, so fairly good interaction is expected. The final properties of a polymer blends are controlled by many parameters among them, the most important are the flow history and interfacial tension [4]. The flow history is related to temperature and time of mixing, and type and intensity of the flow. In a 2009 report, Signori et al. investigated the effect of temperature and melt mixing conditions on the properties of polylactic acid (PLA)/PBAT blend. They showed that as the temperature rises, the PLA degradation increases attributed to trans-esterification reaction of the lower molecular weight PLAs with PBAT. Therefore prepared blends at higher temperature exhibited better thermo-mechanical properties and a smaller droplet size [8]. The interfacial tension between two polymers is another factor which controls the microstructure of binary polymer blends. The experimental methods for interfacial tension measurements of polymers are difficult due to the high viscosity of polymers and their complex rheological characteristics. However, there are several experimental methods to measure interfacial tension which are classified into three classes: (i) Static methods such as pendant drop, (ii) Dynamic methods such as breaking thread, and (iii) Rheological method. 3

Each method has its own advantages and limitations [9]. In the present paper, the following three methods were chosen to calculate the interfacial tension between PBAT and EVOH: 1. Breaking Thread method (BT) which is a dynamic method, 2. Imbedded Fiber Retraction method (IFR) which again is a dynamic method, and 3. Rheological method. The BT and IFR methods are extensively used in many reports. The phenomenological models of these methods are developed for Newtonian fluids which is a drawback for the methods [9]. Moreover, both polymers must be transparent.

1.1.

Breaking Thread Method (BT)

The BT method involves the observation of shape distortion of a long melt thread of polymer imbedded in another polymer. The interfacial tension between the polymers induces a pressure difference between inside and outside the thread that tends to reduce the interfacial area. So, distortions with arbitrary wavelength at the surface of the thread are generated due to Brownian motion [9]. Based on Tomokita theory [10] the interfacial tension can be calculated by measuring the evolution of these disturbances and the zero-shear viscosity of the polymers. Generally, the polymer with the higher melting point or glass transition temperature is used as the thread [11]. In 2000, Everaert et al. measured the interfacial tension of polypropylene (PP)/polystyrene (PS), polyethylene (PE)/PS, and PS/poly(2,6dimethyl-1,4-phenylene ether) (PPE) blends by the BT methods [12]. In another report, the effect of temperature on the interfacial tension was investigated by the BT method for two different polymer pairs. The values were 17.4 and 16.3 mN.m-1 at 225°C for polycarbonate (PC)/PP and polyamide (PA)/PE systems, which show high interfacial tension and so high immiscibility [13]. Furthermore, Elemans et al. measured the interfacial tension between four polymer pairs by the BT method. The measured values had a negligible difference with the reported values by other references [14] indicating good accuracy and reliability of the method. (See: [15]).

1.2.

Imbedded Fiber Retraction Method (IFR)

In the IFR method, a short polymer fiber imbedded in another polymer may retract to a sphere. The length of the fiber should be lower than a critical value, which itself depends on viscosity ratio, to retract, otherwise, the fiber will break up. It is possible to infer the interfacial tension between both polymers from the study of the evolution of the fiber and the zero-shear viscosity of the polymers. The theory of Carriere [16] was used to calculate the interfacial tension. Ellingson et al. investigated the effect of the molecular weight of PS on interfacial tension of PS/poly(methyl methacrylate), (PMMA), blend by the IFR method. The Interfacial tension was calculated from 0.48 to 1.22 mN.m-1 with a maximum error bar of 0.20 mN.m-1 [17] indicating the precision of the method. Moussaif et al. decreased the interfacial tension between PC and poly(vinylidene fluoride) from 4.5 to 0.6 mN.m-1 by adding PMMA as a compatibilizer that transform it from an immiscible blend to a compatible blend. The values of Interfacial tension were measured by the IFR method [18]. (For more examples see: [19-21]).

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In 2000, Xing et al. compared several methods of measuring interfacial tension including BT, IFR, retraction of deformed drop, pendant drop, and rheological method for PS/PA6 polymer pair. The advantages, limitations, and difficulties of each technique have been discussed and compared [11]. Recently, in 2017, BT, IFR, Neumann triangle, and contact angle methods were used to measure the interfacial tension of PLA/PA11 blends. They obtained 5.8 ± 0.6 and 5.4 ± 0.4 mN.m-1 values, by BT and IFR methods, respectively. Such high values are evidence of the high immiscibility. However, when the PLA/PA6 interfacial tension was measured by Neumann triangle methods, the value was significantly lower, 3.2 mN.m-1. They attributed this phenomenon to the chain mobility of stiff PLA chains. For more information see the article [22]. For the characterization of a ternary blend of PBAT/PLA/PA, Fu et al. [23] in 2018 measured the interfacial tension of the pair polymers by a dynamic method. The obtained values were: /

= 3.3 ± 0.7

.

,

/

= 5.6 ± 0.3

.

,

/

These values indicate the immiscibility of PBAT with PA and PLA.

1.3.

= 3.0 ± 0.4

/.

.

Rheological Method to estimate interfacial tension

Many investigations have been performed to understand the relationship between the morphology of polymer blends and their viscoelastic properties. For immiscible blends, high elasticity, especially in the low-frequency range, is observed. This phenomenon is due to the relaxation of the droplets of the dispersed phase [9]. Three models to predict the linear viscoelastic behavior of polymer blends have been developed by Palierne [24], Gramespacher [25], and Bousmina [26], respectively. The interfacial tension was calculated by a comparison between the modulus of the blend measured from small-amplitude oscillatory shear, SAOS, and the emulsions models of Palierne, Gramespacher, and Bousmina. Several studies have utilized this method to infer the interfacial tension between two polymers (See: Nofar et al. 2015 [27], Hassanpour et al. 2018 [28], J. Khademzadeh 2010 [29], Jalali Dil 2015 [6], and Jafari et al. 2005 [30]). PBAT/EVOH blend is expected to show unique advantages due to biodegradability of PBAT and impermeability of EVOH. However, to the best of our knowledge there are no reports regarding this blend in the literature. Interfacial tension is a key property which controls blend morphology, compatibility and therefore strongly affects the final properties of a blend. Therefore, it is highly required to measure the interfacial tension of PBAT/EVOH blend as a first step to study a polymer blend system. The breaking thread, imbedded fiber retraction, and rheological methods are widely used in many studies to measure the interfacial tension of pair polymers. Thus, the main focus of this work is to measure the interfacial tension of PBAT/EVOH blend by three methods. Moreover, the PBAT-rich blends, with the possibility of reactive mixing, are prepared under different melt mixing conditions to find a correlation between morphology, rheology, mechanical properties, and interfacial tension values.

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2. Experimental 2.1.

Materials and blending

PBAT, Ecoflex blend F C1200 from BASF Germany with melt flow rate of 2.7 g/10min and density of 1.26 g.cm-3 was used. EVOH, EVAL F101B with ethylene content of 32 mol%, melt flow rate of 1.6 g/10min and density of 1.19 g.cm-3 was supplied by Kuraray Belgium. The blends were prepared by a 60 cm-3 internal melt mixer under various mixing conditions according to Table. 1. The mixing conditions were determined based on a design of experiment using Taguchi method via Minitab 18 software. The samples are designated in accordance with Table. 1. Table 1- Sample designation, blend composition and applied mixing conditions

Code S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11

2.2.

EVOH Content (%) 0 10 10 10 20 20 20 30 30 30 100

Mixing Time (min) 15 2 5 15 2 5 15 2 5 15 15

Rotor Speed

mixing Temperature

(rpm) 90 50 70 90 70 90 50 90 50 70 90

(°C) 200 185 190 200 200 185 190 190 200 185 200

Droplet size measurement

To observe the morphology of the blends and measure the size of the dispersed phase, field emission scanning electron microscopy (FE-SEM) images were taken with a NOVA NanoSEM 450 SEM from FEI Co., America, at Oil Institute, University of Tehran, Iran. The size of the droplets was measured by ImageJ 1.44p software. For each image, at least 200 droplets were measured. The number (dn) and volume (dv) average of droplet diameter were calculated based on Eq. 1. ∑ ∑ & = ∑ ∑ Where ni is the number of droplets with diameter of di. =

6

Eq. (1)

2.1.

Breaking thread (BT) and imbedded fiber retraction (IFR) methods

For the BT method, the threads of EVOH were prepared since the EVOH had higher melting temperature than PBAT. The threads were annealed for a day at 80 ˚C in a vacuum oven for releasing residual stresses and drying. The length to diameter ratio (L/D) of the thread was about 300. The prepared thread was placed between two PBAT thin films and the obtained sandwich was placed under the lens of an optical microscope. The heat was applied until the temperature reached 190 ± 1 ˚C. Therefore, a sinusoidal disturbance was generated at the surface of the thread. Finally, the thread broke into small droplets. The generated distortion at the surface of the thread was recorded with time. These transformations were recorded by the microscope camera and the images were analyzed by ImageJ 1.44p software. The IFR method was performed at 190 ± 1 ˚C. The preparation method for EVOH fibers and the procedure were the same as the method described in the previous paragraph except, the length of fiber was lower than the critical value. The L/D was from 7 to 12 so that the fiber relaxed back to a spherical droplet.

2.2.

Rheological measurement

Rheometry analysis was performed by a parallel plate MCR 301 rheometer (Physica Anton Paar, Austria) to interpret the morphology and calculate the interfacial tension of the PBAT/EVOH blends. For this purpose, tablets of 1mm thickness were prepared by hot press at 190 °C under a pressure of 100 bar. Then, they were dried at 80 ˚C for 24 h in a vacuum oven and stored in a desiccator before the rheological measurement. A strain sweep was carried out to locate the linear viscoelastic region. Then, frequency sweep runs were performed at a strain of 1% in the linear viscoelastic region under a purge of nitrogen gas at 190 °C.

2.3.

Tensile test

Tensile properties of the blends were measured by a 20kN universal tensile testing machine, JJ Test, China. The rate of tension was 20 mm/min and each reported value is an average of at least 5 specimens.

2.4.

Differential scanning calorimetry (DSC)

In order to investigate the possibility of reactive mixing, the melting and crystallization behaviors of the selected blends were analyzed by differential scanning calorimetry, ZFDSC-D2, Zufa industry Co., Shanghai, China, under dry nitrogen atmosphere. For each measurement, about 5 mg of the sample was placed in an aluminum pan and heated from room temperature to 200 °C with a constant rate of 10 °C/min. Afterward, the temperature was kept constant at 200 °C for 10 min and then cooled with a constant rate of 5 °C/min to room temperature. This cycle was repeated two or three times for each sample.

7

3. Results and Discussion 3.1.

Droplet size measurement by electron microscopy

FE-SEM micrographs of PBAT/EVOH with different compositions at various processing conditions are shown in Figure 1. A distinct disperse type morphology is seen for the blend in the range of 10 to 30% EVOH content. The number average diameter (dn) and volume average diameter (dv) of the EVOH dispersed phase were determined and the results are presented in Table 2. The droplet size will be used later to calculate interfacial tension by the rheological method.

Figure 1- The FE-SEM images of prepared blends. (S2, S3, S4 have 10% EVOH). (S5, S6, S7 have 20% EVOH). (S8, S9, S10 have 30% EVOH).

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Table 2- Number and volume average diameter of EVOH droplets into prepared blends

Sample S2 S3 S4 S5 S6 S7 S8 S9 S10

dn (µm) 1.69 ± 0.45 0.92 ± 0.26 1.18 ± 0.35 0.61 ± 0.23 1.03 ± 0.32 0.85 ± 0.19 1.74 ± 0.61 2.13 ± 0.70 1.08 ± 0.38

dv (µm) 1.95 ± 0.52 1.07 ± 0.29 1.39 ± 0.41 0.78 ± 0.28 1.23 ± 0.38 0.93 ± 0.20 2.12 ± 0.72 2.56 ± 0.82 1.34 ± 0.46

dv/dn 1.15 1.16 1.17 1.28 1.19 1.09 1.22 1.20 1.24

It is seems that the number average droplet size changes from 0.61 to 2.13 µm while the dv changes from 0.78 to 2.56 µm under the various applied mixing conditions. The polydispersity (dv/dn) of EVOH droplets varies from 1.09 to 1.28.

3.2.

Interfacial tension measurement by BT method

Due to Brownian motion, small distortions of arbitrary wavelength (λ) are generated at the surface of the thread. The amplitude of the distortions ( ) grows exponentially with time, according to Eq. 2 [9]. !−# Eq. (2) = $ exp()*+ 4 Where # and ! are the largest and the smallest diameters of the thread, respectively, as shown in Figure 2. $ is the initial amplitude of the sinusoidal distortion, t is time, and ) is the growth rate of the distortion. According to Eq. 3, ) is a function of the interfacial tension , the zero-shear viscosity of matrix ,- , the initial radius of the thread .$ , and the dimensionless growth rate Ω(/, 0+. The dimensionless growth rate is a function of the wavenumber, / (defined by Eq. 4) and the viscosity ratio 0 (defined by Eq. 5). =

Figure 2- Breaking thread geometry

)= /= 0=

Ω(/, 0+ 2,- .$

Eq. (3)

22.$ 3

Eq. (4)

,4 5,-

Eq. (5)

9

Where 3 is the wavelength of the distortions and ,4 is the zero-shear viscosity of the thread. The zero-shear viscosity of the matrix and dispersed phases was calculated by fitting the Carreau model on the small-amplitude oscillatory shear, SAOS, data will represented in Section 3.4. The values of 2362 and 9061 Pa.s, at 190 °C were obtained for ,- and ,4 , respectively. In order to obtain the interfacial tension between the polymer pair, ln ( /.$ + is plotted as a function of time, obtaining a straight line. The growth rate is calculated from the slope of the graph and the interfacial tension is obtained from Eq. 3. Tomotika et al. [10] represented the value of Ω(/, 0+ in their paper. The theory of Tomotika is based on Newtonian fluid. Figure 3 shows images taken by an optical microscope to measure interfacial tension by the BT method and Table 3 lists the values of the interfacial tension and other parameters obtained from each experiment.

Figure 3- Measurement of Interfacial tension between PBAT and EVOH by breaking thread method at 190 °C

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Table 3- Interfacial tension values of PBAT/EVOH at 190 °C by breaking thread method with varying thread diameter (d0)

Matrix

Thread

PBAT

EVOH

3.3.

$

(µm) 38 54 42 35 42

Ω (×103) 0.47 23 0.50 30 0.49 28 0.45 12 0.48 25 (89: +;<= Standard Deviation

q (×104) 9.1 8.5 8.8 4.3 8.2

X

(mN.m-1) 2.77 2.81 2.43 2.31 2.53 2.57 ± 0.22

Interfacial tension measurement by IFR method

In the IFR method, a short fiber relaxes back to a sphere. Carriere et al. [16] obtained Eq. 6 and Eq. 7 for the evolution of the imbedded fiber as a function of retracting time. . .B >? A−>? A = * .@ .@ .@ ,@

Eq. (6)

Where 3 (1 + C + C +$.H 3 .H >(C+ = D E I+ Etan 2 1−C 2

C 4 √3 C L NI − − 2+C 2 C

Eq. (7)

Where ,@ is the effective viscosity calculated from Eq. 8, .@ is the radius of a sphere having the same volume as the fiber, . is the radius of the fiber as a function of time, and .B is the initial radius of the fiber (as shown in Figure 4). The interfacial tension can be obtained from the slope of >(./.@ + − >(.B /.@ + versus retracting time. Figure 5 exhibits optical microscopic images to measure interfacial tension by the IFR method and Table 4 lists the values of the interfacial tension and the parameters obtained from each experiment. ,@ =

,- + 1.7,4 2.7

Eq. (8)

11

Figure 4- Imbedded fiber retraction geometry

Figure 5- Measurement of Interfacial tension between PBAT and EVOH by imbedded fiber recovery method at 190 °C.

12

Table 4- Interfacial tension values of PBAT/EVOH at 190 °C obtained by imbedded fiber recovery method

Matri Thread x PBAT EVOH

3.4.

.$ (µm)

(mN.m )

109 96 91 86 113 88

2.75 2.60 2.63 3.41 2.48 2.76

-1

.$ (µm) 100 92 71 143

175 (89: +;<= Standard Deviation

(mN.m-1) 2.61 2.87 3.05 2.62 2.28 2.73 ± 0.30

Rheological analysis

The rheological analysis was performed to determine the microscopic properties and the processability of the blends. The FE-SEM images can only show a very limited section of sample and may not be a good representative of the whole sample. However, rheological data can provide thorough detailed information about the microstructure of samples. Moreover, interfacial tension between the polymers can be estimated by different rheological models. Figure 6 and Figure 7 show the rheological results of the prepared PBAT/EVOH blends.

Figure 6- Storage modulus vs. frequency for the neat PBAT and EVOH and their blends at 190°C and 1% strain

13

Figure 7- Complex viscosity vs. frequency for the neat PBAT and EVOH and their blends at 190°C and 1% strain.

The storage modulus (G’) versus frequency in Figure 6 shows that the neat PBAT and EVOH exhibit Maxwellian behavior in the terminal zone. By blending these polymers at a ratio of PBAT/EVOH 90/10 (S2, S3, and S4), the storage modulus slightly increases with respect to the neat PBAT. By increasing the EVOH content to 20% (S5, S6, and S7), the slope of G’ graph decreases at low frequencies and shows a non-terminal zone. Again, by increasing the EVOH content to 30% (S8, S9, and S10), this trend continues and storage modulus of the blends enhances more than that of the neat EVOH, which can be explained by the interaction between EVOH droplets. In 2011, Li et al. also observed the same trend by studying the rheology of PLA/PBAT blends [31]. Figure 7 shows that with the addition of EVOH, the viscosity is slightly increased. For composition more than 10% (>=20%), an upturn in viscosity is seen at low frequencies which indicates a change in the blend structure. Moreover, by drawing the complex viscosity of the blends versus EVOH content (Figure 8), an upturn in viscosity can be seen above 20 wt% which shows the rheological percolation.

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Figure 8- Complex viscosity Vs. EVOH content at frequency of 0.02 and 0.055 s-1 and 1% strain

The Cole-Cole plot presented in Figure 9 can also be used to interpret the morphology of the blends. The Cole-Cole plot of EVOH is represented separately due to its high values. The appearance of two arcs is an evidence of disperse morphology for the blends where the right arc is attributed to the relaxation of the dispersed phase droplets [6].

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Figure 9- Cole-Cole plot of imaginary viscosity versus real viscosity for (a) PBAT and prepared blends. (b) EVOH

3.5.

Interfacial tension measurement by rheological method

As noted in introduction, another technique for obtaining the interfacial tension between two polymers is to employ rheological data. This method is accurate and reliable for polymer emulsions, but it is also acceptable for melt polymer blends. In this method, represented models are fitted to small-amplitude oscillatory shear (SAOS) data and the interfacial tension can be calculated. Three well-known models are used: (1) Gramespacher & Meissner [25], (2) Palierne [24], and (3) Bousmina [26]. The models are presented in Table 5. Table 5- Rheological models for calculation of Interfacial tension

Model

Gramespacher & Meissner

[25]

Equation , 3 S 3 ′ ′ ′ OPQ@ ?1 − A 4 = R4 O4 (S+ + (1 − R4 +O- (S+ + 3 3 1+S 3 3 S3 , " " " OPQ@ ?1 − A 4 = R4 O4 (S+ + (1 − R4 +O- (S+ + 3 3 1+S 3 “m”: matrix. “d”: dispersed phase. R4 : volume fraction of dispersed phase. , = ,- U1 +

3 = 3$ U1 +

(HVW +

R + + 4

H(HVW +X R4 Z ; 3$ Y(VW +X

(VW H( aVW b+ R Z c(VW +( VW + 4

;

16

=

3 = 3$ U1

\]^ ( aVW b+( VW + c$(VW + _`X ( aVW b+ + c(VW +( VW + R4 Z ; d

\

= \e

f

∗ OP∗ (S+ = O(S+

Palierne [24]

1 + 3R4 h(S+ 1 − 2R4 h(S+

h (S+ = 4 ∗ (S+ ∗ (S++(16O ∗ (S+ i j k2O+ 5O4∗ (S+l + (O4∗ (S+ − O+ 19O4∗ (S++ . 40 ∗ (S+ + O ∗ (S+j + (2O ∗ (S+ + 3O ∗ (S++(16O ∗ (S+ + 19O ∗ (S++ i j iO4 4 4 . OP∗ (S+

Bousmina [26]

∗ (S+ ∗ (S+Ns p2 iO4∗ (S+ + j + 3O+ 3R4 LO4∗ (S+ + − O. . o r ∗ (S+ = Oo r ∗ (S+ − 2R LO ∗ (S+ + ∗ (S+Nr o2 iO4∗ (S+ + j + 3O − O 4 4 . . n q

In the Palierne model, the dispersed phase should be spherical droplets and it is applicable in the linear viscoelastic region only. Also, this model assumes two viscoelastic phases with uniform particle size and constant interfacial tension. (See: Nofar et al. 2015 [27], Hassanpour et al. 2018 [28], Xing 2000 [11], Khademzadeh 2010 [29], Jalali Dil 2015 [6], and Jafari et al. 2005 [30]). The Bousmina model is the modified Kerner model. The Kerner model takes into account zero interfacial tension [26]. Bousmina model has been applied in some papers (e.g. [28]) to calculate interfacial tension. The obtained rheological data were fitted by these three models and calculated interfacial tension values for PBAT/EVOH blends are given in Table 6. Table 6- Obtained interfacial tension values for some PBAT/EVOH blends based on rheological data at 190 °C

Sample

Palierne

89: (mN.m-1) Bousmina

Gramespacher

S2 S3

1.4 UF

UF UF

UF 1.05

S4

1.91

3.62

5.1

S5 S6 S9 S10

1.32 UF 1.25 2.10

1.70 UF 1.48 2.99

1.73 1.46 2.52 3.46

Nominal average

1.60

2.45

2.55

UF: unexpected fitting parameter was obtained. NF: no fit was obtained. As mentioned, since these theories have been developed for polymer emulsions, in some cases, reliable values for interfacial tension cannot be obtained. This is because of the much higher modulus of polymer melts as compared to polymer emulsions. Therefore the 17

terms of the equation which have the expression

_`X ]^

can be neglected in comparison with

other terms and have no significant effect on the results. However, the obtained nominal averages for Palierne, Bousmina and Grameschaper models are in good agreement with the experimental results obtained by BT and IFR methods in Section 3.2 and Section 3.3. On the other hand, according to the models [25], [26] the relaxation time of droplets could be estimated by using the models and interfacial tension. The equations employed to predict the relaxation time are given in Table 7. Table 7- Relaxation time calculated from different theories

Model

Equation

Gramespacher & Meissner

34 = 3$ E1 +

Bousmina

34 =

Palierne

34 =

5(19d + 16+ R I 4(d + 1+(2d + 3+ 4

. ,- d + 3/2 − R4 (d − 1+ E I (1 − R4 +

. ,- (19d + 16+(2d + 3 − R4 (d − 1++ E I 10(d + 1+ − 2R4 (5d + 2+

Ref. [25]

[26] [26]

* The parameters 3$ , d, R4 was introduced in Table 5.

The relaxation time obtained according to the equations presented in Table 7 can be compared with the values obtained from the SAOS data. To calculate the relaxation time of droplets from the rheological data, it is needed to obtain the continuous spectrum of the relaxation time. Maxwellian model (Eq. 9 and Eq. 10) was non-linearly fitted to the storage and loss modulus. Thus, the discrete spectrum of the relaxation was obtained [32–34]. Subsequently, using piecewise cubic Hermite spline described in [32], the continuous relaxation spectrum was obtained and the results are plotted in Figure 10. According to the figure, the relaxation peak of EVOH droplets is distinctly visible for the EVOH content above 20%. v

O (S+ = t Eu (3 + ′

w

v

O " (S+ = t Eu (3 + w

S 3 I 1+S 3

Eq. (9)

S3 I 1+S 3

Eq. (10)

Where u (3 + [Pa] are the modes of the discrete spectrum; N is the number of modes; and 3 are relaxation times.

18

Figure 10- Continuous relaxation spectrum calculated from frequency sweep data for the PBAT/EVOH blends

In Table 8, the relaxation times of the droplets obtained from the models are compared with the ones obtained based on the relaxation spectrum. The calculated values are in the same order of magnitude which suggests a relative agreement between both sets of the results. Table 8- Calculated interfacial tension from three various models

Sample

Volume Fraction

Relaxation Time from Rheological data (sec)

S5

0.210

S6 S7 S8 S9 S10

0210 0210 0.312 0.312 0.312

3.6.

Effect of mixing conditions on mechanical properties

Relaxation Time from models (sec) Palierne

Bousmina

Gramespacher

18.1

8.6

2.3

5.4

18.1 19.9 18.9 18.9 18.3

16.3 10.3 27.6 39.5 15.7

4.3 2.7 7.2 10.3 4.1

7.0 6.4 26.0 32.4 14.8

The effect of mixing condition on the mechanical properties of the blends was investigated by tensile test. Figure 11 (a) shows the yield stress and tensile strength of the blends without significant effect of the mixing condition and EVOH content. But stress at break has decreased below those of both PBAT and EVOH. Figure 11 (b) shows the strain at yield and strain at break of the samples. Similarly no significant change is observed for strain 19

at yield but strain at break has slightly decreased with a large drop at 30% EVOH. These data were imported to Minitab 18 software as the responses. Figure 12 shows the results of Taguchi analysis that reveals that the EVOH content is the more effective factor but not the mixing condition. The effect of mixing condition on the morphology of PP/PC was also investigated by Favis [35]. It was seen that rotor speed and mixing time after 2 minutes do not have a significant effect on the final morphology.

Figure 11- Tensile properties of the PBAT/EVOH blends and the neat components: (a) yield stress (solid symbols) and tensile strength (open symbols), and (b) strain at yield (solid symbols) and strain at break (open symbols).

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Figure 12- Response plots from Taguchi method- The effect of mixing condition on (a) yield stress, (b) tensile strength, (c) strain at yield, and (d) strain at break.

Evaluating the tensile test results suggests that both the tensile strength and strain at break for the blends are smaller than the single polymers i.e. EVOH and PBAT. On the other hand the yield stress of the blends is higher than that for PBAT but with negative deviation from mixture law. Therefore it turns out that the tensile properties of the blends are lower than what expected from mixture law. Poor mechanical properties of a blend could be usually due to poor compatibility and high interface tension. This was unexpected since good compatibility for EVOH and PBAT was assumed due to hydrogen bonding between oxygen of PBAT and OH of EVOH. Moreover reactive mixing was a possible phenomenon to take place between PBAT and EVOH through exchange and trans-esterification reactions to improve. The low tensile strength and strain at break of the blends can be explained by the fact that stiff EVOH droplets hinder the elongation of PBAT chains by defects developed at the interface so that strain at break decreases and PBAT continuous phase cannot undergo strain hardening and consequently tensile strength at break decreases. This is more pronounced as the amount of EVOH increases so that for 30% EVOH blend the strain at break decreases by 50% of the PBAT. The negative deviation from mixture law testifies that the interfacial adhesion is not as good as expected e.g. the internal hydrogen bonding in EVOH over rules the external possible hydrogen bonding between EVOH and PBAT. I. Akiba and S. Akiyama et al [36]

21

reported similar situation for EVOH/PA where internal hydrogen bonding of EVOH chains was stronger than interaction between amide group of PA and OH group of EVOH. Furthermore it is unlikely for reactive mixing to take place between EVOH and PBAT which will be investigated in the next section.

3.7.

Investigation of reactive mixing

The hydroxyl group of EVOH can act as catalyst for ester exchange reactions or take part in trans-esterification reactions. Gui et al. [37] studied PLA/EVOH properties where related the observed improved properties of the blends to reactive mixing between the two polymers but did not provide any evidence or detailed description. On the other hand Signori et al. [8] showed that PBAT is more heat stable than PLA therefore ester exchange or transesterification reactions of PBAT/EVOH blends are of less possibility to take place as compared to PLA/EVOH. To investigate the possible reactive mixing in PBAT/EVOH blends in this work, dynamic melt rheology and DSC analysis were employed for a high EVOH content blend i.e. S8 blend containing 30 wt% EVOH and the single polymers. In DSC analysis the PBAT/EVOH(30%) blend and PBAT were heated at 10 °C/min from room to 200 C and were kept for 10 min at this high temperature to allow for possible reactions and then slowly cooled down to room temperature. This cycle was repeated two times to see the last heating cycle for evaluating any change in the melting behavior of PBAT component due to ester exchange or trans-esterification reactions. The results are shown in Figure 13 with a broad melting peak from 95 to 145 °C for PBAT [38], [39]. The coldcrystallization-temperature (Tcc) of PBAT has increased due to the nucleation effect of EVOH solid droplets. The Tcc of EVOH has also increases a little probably due to the plasticization effect of PBAT chains. Similar behavior was reported by As’habi et al. [40] for PLA/LDPE blends. More importantly there is no significant difference between the first and last melting behavior of PBAT/EVOH blend i.e. the melting point and melting enthalpy of PBAT are very similar in the first and last heating cycle. Therefore it can be deduced that ester exchange or trans-esterification reactions have not occurred at all or have been negligible to be detected.

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Figure 13- DSC graphs. Three cycles of heating and cooling of PBAT, PBAT/EVOH(30%), and EVOH.

The dynamic rheometry results are shown in Figure 14 where the samples were characterized through a frequency cycle at 190 °C for a total time of about 20 min which was longer than mixing time of the blends. As seen the complex viscosities are near the same for the increasing or decreasing frequency sweep saying that reactions or molecular weight reduction have not significantly occurred.

23

Figure 14- Complex viscosity of PBAT, PBAT/EVOH(30%), and EVOH versus frequency from 0.05 to 600 s-1 and return to 0.05 s-1 at 190 °C.

4. Conclusion The interfacial tension, rheological properties, morphology, tensile properties and absence of reactive blending for PBAT/EVOH blends are reported for the first time here. The interfacial tension between PBAT and EVOH were found to be 2.57 ± 0.22 and 2.73 ± 0.30 mN.m-1 by breaking thread (BT) and imbedded fiber retraction (IFR) methods, respectively. In another approach, rheological models of Palierne, Gramespacher, and Bousmina were applied to SAOS rheological data to estimate the interfacial tension of the blends. The obtained values for interfacial tension from these models at varying EVOH contents were in good agreement with the BT and IFR results. A disperse morphology was observed for the melt blended PBAT/EVOH blends by FESEM images in which the droplet size of the EVOH dispersed phase was between 0.6 to 3 µm for various blend compositions. The rheological measurements and obtained Cole-Cole plots showed the features of disperse morphology for the PBAT/EVOH blends. A rheological percolation at 20% EVOH was assigned as a result of an upturn of viscosity versus the EVOH content. The continuous relaxation spectrum extracted from the SAOS data exhibited that the relaxation time of the dispersed droplets in PBAT matrix is about 18 sec.

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The results of tensile tests for studying the effect of mixing condition revealed that the most effective factor with regard to the mechanical properties is the EVOH content. Furthermore, mixing time, rotor speed, and mixing temperature showed no significant effects on the tensile properties of the blends. With increasing the EVOH content the yield stress of PBAT remained fairly the same but its strain at break and tensile strength decreased likely due to the low strain at break of the EVOH phase. Finally, the rheological measurements and DSC results of virgin PBAT, PBAT/EVOH(30%), and virgin EVOH ruled out the possibility of reactive mixing between PBAT and EVOH under the applied mixing conditions. No significant effect of EVOH droplets on the crystallinity amount and melting point of PBAT was observed.

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Highlights: • Measured the interfacial tension of PBAT/EVOH blend by breaking thread (2.57 ± 0.22 mN.m-1) and imbedded fiber retraction (2.73 ± 0.30 mN.m-1) methods • Employed the SAOS rheological data using various models to validate the obtained interfacial tension values • Calculated the continuous relaxation spectrum and the relaxation time of droplets using the SAOS data • Established a correlation between the measured interfacial tension and mechanical, rheological and morphology of the blends • Ruled out the possibility of reactive mixing between PBAT and EVOH under the applied mixing conditions

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: