Cellulose nanofibers reinforced biodegradable polyester blends: Ternary biocomposites with balanced mechanical properties

Journal Pre-proof Cellulose Nanofibers Reinforced Biodegradable Polyester Blends: Ternary Biocomposites with Balanced Mechanical Properties Yuankun Wang, Zeren Ying, Wenyuan Xie, Defeng Wu

PII:

S0144-8617(20)30019-9

DOI:

https://doi.org/10.1016/j.carbpol.2020.115845

Reference:

CARP 115845

To appear in:

Carbohydrate Polymers

Received Date:

26 November 2019

Revised Date:

6 January 2020

Accepted Date:

7 January 2020

Please cite this article as: Wang Y, Ying Z, Xie W, Wu D, Cellulose Nanofibers Reinforced Biodegradable Polyester Blends: Ternary Biocomposites with Balanced Mechanical Properties, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.115845

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Cellulose Nanofibers Reinforced Biodegradable Polyester Blends: Ternary Biocomposites with Balanced Mechanical Properties Yuankun Wang1

Zeren Ying1

Wenyuan Xie1,2

Defeng Wu1,3*

(1 School of Chemistry & Chemical Engineering, Yangzhou University, Jiangsu, Yangzhou, 225002, P.

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R. China) (2 Institution of Innovative Materials & Energy, Yangzhou, Jiangsu Province, 225002, P. R. China)

(3 Provincial Key Laboratories of Environmental Engineering & Materials, Jiangsu, Yangzhou, 225002,

*

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P. R. China)

Corresponding author. Tel.: +86 514 87975230; Fax: +86 514 87975244; E-mail address: [email protected].

CNFs are preferentially distributed in continuous PLA phase in PLA/PBS blend;



The presence of CNF reduces PBS droplet sizes and improves phase

The tensile strength and toughness of blend are improved in the presence of

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CNF; 

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adhesions; 

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

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Research Highlights

The overall mechanical performance can be well balanced by a two-step

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method.

ABSTRACT Blending two biodegradable aliphatic polyesters with complementary bulk properties is an easy way of tuning their final properties. In this work, the ductile poly(butylene 1

succinate) was mixed with polylactide, and as expectable, the blends show improved toughness with sharply reduced strengths. The pristine cellulose nanofibers were then used as the reinforcement for the blends. It is found that most nanofibers are dispersed in the polylactide phase because polylactide has better affinity to nanofibers, and the lower viscosity level of polylactide also favors driving nanofibers into the continuous polylactide phase during melting mixing. In this case, the strength and rigidity losses

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resulted from the presence of soft poly(butylene succinate) phase are compensated to some extent. To further improve mechanical properties, a two-step approach (reactive processing of blends, followed by the incorporation with nanofibers) was developed.

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with well-balanced mechanical performance.

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This work provides an interesting way of fabricating fully biodegradable composites

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ternary composites.

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Key words: cellulose nanofibers; aliphatic polyester blends; mechanical properties;

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1. Introduction

Blending two biodegradable aliphatic polyesters with fully different bulk properties

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is very interesting. For instance, the glassy polylactide (PLA) possesses good tensile strength, with satisfying material rigidity, while the ductile poly(butylene succinate) (PBS) or poly(ε-caprolactone) (PCL) shows better toughness, with outstanding solid plasticity. Moreover, the former has higher hydrolysis/degradation rate than the latters. Therefore, blending them together is a simple route of fabricating new biodegradable 2

materials with tailorable performance (Eastmond, 2000; Rasal, Janorkar, & Hirt, 2010; Nofar, Sacligil, Carreau, Kamal, & Heuzey, 2019). However, most biodegradable aliphatic polyester pairs are thermodynamically immiscible, and poor phase structure actually highly restricts combination of the merits of each component in their blends. Therefore, compatibilization technologies have to be employed to improve interfacial structure and phase adhesion, and the block copolymers or the random ones are the

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most commonly used compatibilizers (Vatansever, Arslan, & Nofar, 2019; Zeng, Li, & Du, 2015).

Recently, it has been reported that the addition of nanoparticles could improve phase

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structure of the immiscible polymer blends also, (De Luna & Filippone, 2016; Taguet,

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Cassagnau, & Lopez-Cuesta, 2014) because those particles act as ‘compatibilizer-like’ role in this kind of ternary systems as the minor component. The selective localization

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of particles is very important. Generally, the surface modification improves affinity of

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nanoparticles to the two matrix polymers, and as a result, those treated nanoparticles are preferentially dispersed on the polymer-polymer interfaces, thickening emulsified

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phase interfacial layers. In some cases, the pristine particles are hydrophilic and fully incompatible with two polymers, and hence could be driven onto the phase interfaces

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during mixing by the repulsive force if the two polymers were kinetically symmetrical. Both are thermodynamic compatibilization because those interface-localized particles are able to emulsify phase interfaces to a certain degree, reducing the overall mixing free-energy. In some other cases, the particles, treated or not, are selectively localized in the polymer phase with better affinity or less viscosity. This favors break-down of a 3

thread/droplet during melt mixing, or prevents coalescence of broken droplets because of physical barrier effect, which can be viewed as ‘kinetic compatibilization’. Besides the improvement of phase structure, the mechanical and thermal properties of immiscible polymer blend can be improved by the presence of nanoparticles also. Therefore, combining different roles of two kinds of matrix polymers and one type of nanoparticles together becomes a promising route to design new nanocomposites with

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tailorable properties. Many types of nanoparticles, including nanoclay (Li & Shimizu,

2004; Wang, Zhang, & Fu, 2003; Wu, Wu, Zhang, Zhou, & Zhang, 2008), carbon nanotubes (Fu et al. 2019; Göldel, Kasaliwal, & Pötschke, 2009; Krause et al., 2014;

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Wu et al., 2011b; Li & Shimizu, 2005), nano-silica (Jalali Dil & Favis, 2015a; Lee,

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Bailly, & Kontopoulou, 2012; Sangroniz et al., 2015), and cellulosic particles (Chen, Wang, Ying, Tam, & Wu, 2017; Lu, Huang, Ge, Xie, & Wu, 2018; Zhang & Zhang,

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2016), as well as other nanorods (Jalali Dil, Arjmand, Li, Sundararaj, & Favis, 2016),

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have been tried to prepare ternary composites with polymer blends. This technology has also been applied on biodegradable aliphatic polyester blend systems (Chen, Kim,

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Kim, & Yoon, 2005; Fang et al., 2019; Frackowiak, Ludwiczak, Leluk, & Kozlowski, 2016; Jalali Dil & Favis, 2015b; Jalali Dil, Virgilio, & Favis, 2016; Ojijo, Yu et al.,

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2007; Ray, & Sadiku, 2012; Wu, Zhang, Zhang, & Yu, 2009; Zhang, Xie, & Wu, 2020; Wu et al., 2011a), aiming at improving the overall performance of biodegradable blends, or exploring new kinds of biocomposites to satisfy applications. In most cases, however, the used particles are carbonaceous ones or silicates, which have negative effects on ‘green’ characteristics of the ternary nanocomposites as the biodegradable 4

materials. Accordingly, it is necessary to seek alternatives with biomass-derived and biodegradable characteristics to replace the traditional particles, and polysaccharide nanoparticles may be good options. As one of the most attractive manufactured polysaccharide nanoparticles, cellulose nanofibers (CNF) show good reinforcement to many polymers (Abdul Khalil, Bhat, & Ireana Yusra 2012; Siro & Plackett, 2010), also including the biodegradable aliphatic

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polyesters such as PLA (Frone, Berlioz, Chailan, & Panaitescu, 2013; Iwatake, Nogi,

& Yano, 2008; Jonoobi, Harun, Mathew, & Oksman, 2010; Jonoobi, Mathew, Abdi

Majid, Makinejad, & Oksman, 2012; Kowalczyk, Piorkowska, Kulpinski, & Pracella,

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2011; Vatansever, Arslan, & Nofar, 2019; Wang & Drzal, 2012; Ying et al., 2018) and

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PBS (Joy et al., 2017). The fibrous structure also favors emulsification were they used as particle emulsifier in an emulsion system (Kalashnikova, Bizot, Bertoncini, Cathala,

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& Capron, 2013; Lu et al., 2019). Therefore, using them as the minor component to be

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incorporated with biodegradable aliphatic polyester blend system may be a promising approach to obtain interesting material with improved phase structure and balanced

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performance. In this case, both matrix polymers and filler particles are biodegradable, and the obtained ternary composites are hence fully green. In this work, as-mentioned

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ternary composites were prepared using the immiscible PLA/PBS blends as matrices and CNF as the third component. The phase morphology and mechanical properties of ternary system were studied in detail, aiming at figuring out reinforcing mechanism of CNF to this kind of immiscible blends. Then, the reactive extrusion technology was applied, with the objective to develop a fully biodegradable composite with balanced 5

mechanical performance. 2. Experimental 2.1. Material preparation The commercial PLA (trade name 2003D) with density of 1.24 g cm-3 and melt flow index (MFI) of 2.9 g 10 min-1 (190 oC/2.16 kg) was purchased from NatureWorks Co. Ltd., USA. Its number average molecular weight (Mn) and weight average one (Mw)

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are 155,500 g mol-1 and 253,000 g mol-1, respectively. The commercial PBS (trade

name 3001) with density of 1.25 g cm-3 and MFI of 2.0 g 10 min-1 (190 oC/2.16 kg) was purchased from SHOWA Highpolymer Co. Ltd. (Japan). Its Mn is about 60,000 g

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mol-1 with the polydispersity index (PDI) of 2.33. All these material data are provided

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by the supplier. The CNF slurry (white aqueous suspension) with the solid contents of 5.1 wt% was purchased from Ningbo EneRol Nanotechnologies Inc., P. R. China. The

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average fiber diameter and length are 50 nm and 5 μm, respectively. The weight ratio

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of crystalline parts of fibers is 88 wt%, and their average transverse modulus is about 37.2±1.0 GPa (Ying et al., 2018). All materials were vacuum-dried at 50 oC for 24 h

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before use. The bis(1-(tert-butylperoxy)-1-methylethyl)-benzene (BIBP) was provided by Nantong Runfeng Chemical Co. Ltd., P. R. China. Its melting point and density are

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about 46 oC and 0.97 g cm-3, respectively. The purity is 95%. Preparation of PLA/PBS/CNF ternary composites followed a continuous approach

developed in the previous work, namely the solvent-assisted centrifugation method without any freeze drying step, which could guarantee good dispersion of hydrophilic polysaccharide nanocrystals in matrix polymer without any surface treatments (Xu et 6

al., 2016). By this way, CNF could be transferred from aqueous suspensions into chloroform, and then mixed with PLA/PBS solution (with predetermined blending ratios, using chloroform as the solvent) at room temperature, followed by the solution casting. The as-obtained sheet samples were then vacuum-dried for 24 h at 50 oC. To remove residual solvent, the sheet samples were then cut into pellets, and experienced further melt mixing using a Haake mini-lab (dwelling time ~3 min, Thermo Scientific

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Co., USA), with the rotation speed of 60 rpm at 180 oC. The ribbon and the dog-bone

shaped specimens for the mechanical tests were then molded by a Haake mini-jet (Thermo Scientific Co., USA). The injection and holding pressures were set as 600

C and 50 oC, respectively. Hereafter the ternary composite systems are referred as to

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o

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bar and 500 bar, respectively, and the temperatures of barrel and mold were set as 185

the BC(x/y)-s, where x/y and s are the blending ratio of two polymers (PLA/PBS w/w)

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and loadings of CNF, respectively. The compatibilized blends and their composites

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with CNF were also prepared. Firstly, the mixed PLA and PBS pellets (90/10 or 80/20, w/w) experienced reactive extrusion using the Haake mini-lab with the addition of 0.2

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wt% BIBP to obtain a compatibilized blend system (referred as to c-blend, dwelling time 6 min, 190 oC, 60 rpm using Haake mini-lab). Then, the as-obtained blends were

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dissolved in chloroform and mixed with the aqueous CNF suspension, followed by the same approach mentioned above, to prepare another kind of CNF filled PLA/PBS blend composites (referred as to c-BC-s, where s is the mass content of CNF relative to the blend matrices). Details on the sample codes and compositions are summarized in Table 1. The 0.2 wt% concentration of BIBP was an optimized results for PLA/PBS 7

blend (90/10, w/w) determined by the improvements of morphology and mechanical properties. Table 1 Abbreviations and compositions of the blend and composite samples Abbrev.

reactive processing a

PLA/PBS (w/w)

CNF (phr)

pristine PLA/PBS blends (blend x/y): 90/10

No

-

blend 80/20

80/20

No

-

blend 70/30

70/30

No

-

compatibilized PLA/PBS blends (c-blend (x/y)): c-blend(90/10)

90/10

Yes

c-blend(80/20)

80/20

Yes

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blend 90/10

-

BC(80/20)-3

80/20

BC(70/30)-3

70/30

No

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90/10

3

No

3

No

3

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BC(90/10)-3

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ternary composites based on pristine blends (BC(x/y)-s):

3

ternary composites based on compatibilized blends (c-BC-s): c-BC-3

90/10

Yes

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a: PLA/PBS blends experienced reactive extrusion with the addition of 0.2wt% BIBP.

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2.2. Characterizations

2.2.1. Morphological observations

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The dispersion and distribution of CNF in the ternary systems were detected using a

JEM-2100 transmission electron microscope (TEM, JEOL Co., Japan) performed with 120 kV accelerating voltage. The microtomed samples with the thickness of ~100 nm were used for the observation. The phase morphology was observed using a scanning electron microscope (SEM, SUPRA55, Zeiss Co. Ltd., Germany) operated with 20 kV 8

accelerating voltage. The cryo-fractured surface (smooth enough for the two-phase structure observation) and the impact-fractured surface (for the evaluation of broken mechanism) coated with gold were checked. The number average domain sizes (Dn) and volume average domain sizes of discrete PBS phase (Dv) are analyzed using SEM images, Dn   ni Di /  ni , Dv   ni Di4 /  ni Di3 i

i

i

(1)

i

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and the total numbers (n) of the domains used for analysis are about 200. 2.2.2 Drop retraction experiments

The retraction process of a PLA (or PLA/CNF composite) thread embedded in the

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PBS (or PBS/CNF composite) membrane was recorded using an optical microscope

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(Leika, DMLP, Germany). Firstly, the sandwich-like sample was put on the hot stage, and heated up to the melting point of PBS (130 oC). Then, the glass slide was adjusted

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slightly leftwards and rightwards to remove the bubbles close to the thread. Finally,

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the sample was heated up to 190 oC (higher than the melting point of PLA), and rested for 3 min to remove residual thermal and stress histories. The images were then taken

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every 10-30 s. All experiments were performed under nitrogen atmosphere. Details on deformation and retraction can be found in the previous studies (Wu, Yuan, Laredo,

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Zhang, & Zhou, 2012; Wu, Zhang, Yuan, Zhang, & Zhou, 2010b). 2.2.3. Mechanical property tests The tensile behavior of blend and composite samples was evaluated using an Instron Mechanical Tester (3367) with a crosshead speed of 10 mm min-1 and a load cell of 5 kN according to ASTM D638. The dog-bone shaped specimens (30 mm ×4 mm×2 9

mm) were used here. A cantilever beam impact testing machine (MZ-2056B, Jiangsu Mingzhu Co. Ltd., China) was used to perform impact tests according to ASTM D256. The rectangular specimens (80 mm ×10 mm×4 mm) were used. The reported moduli and strengths here are average ones of six parallel tests. All tests were performed at 25 o

C.

2.2.4. Viscoelastic characterizations

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The dynamic mechanical properties were evaluated using a Netzsch 242-C dynamic thermal mechanical analysis device (DMA) performed on the ribbon specimens (30

mm ×2 mm ×1 mm) over a temperature range -50 oC-100 oC at the rate of 5 oC min-1

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with the tensile mode (1~10 Hz). Melt rheological properties were evaluated using a

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DHR-2 rotational rheometry (TA Co., USA) equipped with the 20 mm parallel plate fixtures. The dynamic frequency sweep was performed at 180 oC with the strain level

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of 1%, which ranged within linear viscoelastic region and was determined by dynamic

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strain sweeps (1 Hz, 180 oC). Before tests, the dynamic time sweeps (1 Hz, 1% strain and 180 oC, 30 min) were performed to evaluate if the samples were thermally stable

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during following dynamic frequency sweeps. All samples were stable during dynamic frequency sweeps (about 400 s for each frequency sweeps).

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3. Results and discussion 3.1. Dispersion and distribution of CNFs in the ternary composites SEM images of the blank PLA/PBS blend shown by Figure 1(a, a') clearly disclose an immiscible phase morphology, which has been widely reported (Shibata, Inoue, & Miyoshi, 2006; Supthanyakul, Kaabbuathong, & Chirachanchai, 2016; Wang, Wang, 10

Zhang, Wan, & Ma, 2010; Wu et al., 2012; Yokohara & Yamaguchi, 2008). For the current blend system (PLA/PBS 80/20 w/w), the discrete PBS domains are dispersed in the continuous PLA phase, with the average domain size of 5-6 μm. The interfacial tension between these two polymers is evaluated by the breaking thread technique (Xing, Bousmina, & Rodrigue, 2000) modified by Bousmina and coworkers (Carriere, Cohen, & Arends, 1989). The retraction process of a PLA thread in a PBS membrane

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are shown in Figure 2(a), and the relaxation kinetics can be traced using a theoretical equation describing shape evolution of an ellipsoidal liquid drop suspended in an infinite fluid domain (Carriere, Cohen, & Arends, 1989):

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 40( p  1)   D  D0 exp   t  (2 p  3)(19 p  6) m R0 

(2)

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where D is the drop deformation parameter defined as D  ( L  B) /( L  B) (L and B

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are the major and minor axis of the ellipsoidal drop, respectively); D0 is an initial deformation parameter, p the viscosity ratio between two polymers, α the interfacial

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tension, t the relaxation time,  m the viscosity of the matrix (PLA phase), and R0 the radius of the droplet at equilibrium. The PLA-PBS phase interfacial tension is ca. 0.80

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mN m-1 for the current system (Figure 2d), slightly lower than the literature-reported

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values (1.1-3.5 mN m-1) (Wu et al., 2012; Yokohara & Yamaguchi, 2008). This is due to the differences in the bulk properties of used polymers.

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5.68 ± 0.26 μm

(a')

(b)

2.36 ± 0.10 μm

(b')

(c)

0.65 ± 0.06 μm

(c')

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(a)

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500 nm

0.32 ± 0.03 μm

(d')

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(d)

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500 nm

2 μm

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2 μm

Figure 1 SEM micrographs of the cryo-fractured surfaces of (a, a') blank blend, (b, b') BC-3, (c, c') c-blend and (d, d') c-BC-3 samples with the scale bar of 2 μm. The scale bar on the inset graphs is 500 nm. The blending ratio of PLA/PBS for all tested samples is 80/20 (w/w).

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(a)

(PLA/CNF)-in-PBS PLA-in-PBS PLA-in-(PBS/CNF)

(d)

(c)

(b)

0

30s

30s

30s

330s

660s

120s

720s

ln(D/D0)

-1

1030s

-2

180s -3

0.80 mN m-1

1020s

360s

1560s

0.50 mN m-1

-4

1.12 mN m-1 0

1530s

500 1000 1500 2000 2500 3000 3500

1080s

4180s

time (s)

Figure 2 Retraction processes of (a) a PLA thread embedded in the PBS matrix, and (b) a

PBS/CNF composite matrix recorded at 190

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PLA/CNF composite thread embedded in the PBS matrix and (c) a PLA thread embedded in the o

C. The CNF loadings in the composite

thread/membrane are 0.3 wt%. The scale bar is 50 μm. The evolution of deformation parameter

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(ln(D/D0)) of the thread against the annealing time (t) is shown in (d).

The ternary composite shows an improved phase morphology as compared with the

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blank blend, as shown by Figure 1(b, b'). The average size of discrete PBS domains

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decreases to 2-3 μm in the presence of CNF (3 wt%), which is closely related to the preferential distribution of CNFs. Figure 3 gives the TEM images of ternary systems.

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It is clear that the most CNFs are dispersed in the continuous PLA phase, instead of in the discrete PBS one (Figure 3(c, d)), highly oriented along with direction of injection

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flow. Firstly, the two polymers are kinetically asymmetric. The Newtonian viscosity of PBS is about 3638 Pa s (190 oC, @ 0.01 s-1), over twice than that of PLA (1592 Pa s, 190 oC, @ 0.01 s-1). In this case, CNFs prefer to be dispersed in the less-viscous phase, namely the continuous PLA phase, during melt mixing (Li & Shimizu, 2004; Wu et al., 2009). On the other hand, although the pristine CNF (without any treatment) 13

possesses hydrophilic surface properties, and is theoretically incompatible with PLA and PBS, its affinities to two polymers may have large differences. This is revealed by the thread retraction tests. Figure 2(b, c) show the relaxation processes of a PLA/CNF binary composite thread suspended in the neat PBS matrix and a neat PLA thread in the PBS/CNF binary composite matrix, respectively. The concentration of CNF is of low-level (merely 0.3 wt%). In this case, the binary composite thread/fiber or matrix

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membrane can be assumed as a ‘homogeneous material’ with ‘single component’ (Wu et al., 2011a), and the flow disturbance could be avoided as much as possible during drop relaxation.

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The calculated values of interfacial tensions are summarized in Figure 2d. It is clear

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that  PLA-(PBS/CNF) is higher than  (PLA/CNF)-PBS , indicating that CNFs are preferentially dispersed in the PLA phase, instead of the PBS one, because the former case shows

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lower level of mixing free-energy as the three components experience melt mixing. In

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other words, CNF has better affinity to PLA among two matrix polymers. Therefore, for current ternary systems, the dispersion of CNFs in the PLA phase is also driven by

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the thermodynamic forces. This preferential dispersion reduces viscosity differences between continuous PLA phase and discrete PBS phase, which promotes break-down

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of PBS droplets during melt mixing (Li & Shimizu, 2004). Moreover, the presence of CNFs in the continuous PLA phase acts as the physical barrier (Fu et al., 2019; Göldel et al., 2009; Li & Shimizu, 2005; Wu et al., 2011b), preventing coalescence of broken PBS droplets, which also favors decreasing the size of discrete PBS phase. Therefore, the ternary composites present improved phase morphology as compared to the blank 14

blends.

(a)

(b)

CNF

PBS

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PLA

(d)

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(c)

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Figure 3 TEM images of (a) blank blend, (b) binary PLA/CNF-3 composite sample, and (c) BC-3 and (d) BC-5 ternary composite samples. The scale bar is 1 μm (2 and 1 μm for the inset graphs in

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(a, c) and (b), respectively). The blending ratio of PLA/PBS in the blend and ternary systems is

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80/20 (w/w).

3.2. Mechanical properties of the ternary composites based on the CNF-filled pristine PLA/PBS blends

15

70

70

PLA

60

BC(90/10)-3 BC(80/20)-3 60 BC(70/30)-3

50

50

40

40

30

30

20

20

stress (MPa)

stress (MPa)

blend 90/10 blend 80/20 blend 70/30

PBS 10

10

(a)

0 0

5

10

200

(b) 0

strain (%)

5

10

200

0 250

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strain (%)

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Figure 4 Stress-strain responses for (a) blank blends (PLA/PBS: 90/10, 80/20, 70/30, w/w) and (b) ternary composites (3 wt% CNF) with various polymer blending ratios (PLA/PBS: 90/10, 80/20,

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70/30, w/w), and (c) moduli and (d) impact strengths for the blends and their composites with

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various polymer blending ratios (PLA/PBS: 95/5, 90/10, 80/20, 70/30, w/w).

The improvement of phase structure, together with the reinforcement of CNF, make

positive contribution to the mechanical properties of the blends. Figure 4(a, b) show the tensile behaviors of neat polymers and their blends, as well as ternary composites. The obtained modulus values are summarized in Figure 4c. As expectable, the blends 16

show remarkably improved elongation levels relative to the neat PLA, however, their moduli and yield strengths decrease evidently and monotonously with increased PBS concentrations. Taking the blend (90/10) sample as an example, its yield strength and Young's modulus are merely about 45 MPa and 1.8 GPa (decreased by about 30% and 20%, respectively, as compared with those of neat PLA (60 MPa and 2.3 GPa)). Thus, the improvement of elongation and toughness of the blend systems comes with a big

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sacrifice in their rigidity and strength. However, the ternary composites show higher moduli and strengths relative to the blend. For instance, the strength and modulus of

BC(90/10)-3 sample are about 50 MPa and 2.0 GPa, respectively, increased by about

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10% as compared to those of blend (90/10) sample. This improvement, as mentioned

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above, is due to the reinforcement of CNF and the increased interfacial area caused by

(a)

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the decreased PBS domain size (Chen et al., 2014; Wu et al., 2009).

PLA

(b)

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CNF

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PBS

Figure 5 (a) TEM image of BC-3 sample and (b) corresponding SEM image with the scale bars of 500 nm and 2 μm.

More importantly, the presence of CNFs has little influence on the elongations of the systems (3 wt% loading ranges in the ‘semi-dilute to semi-concentrated’ region (Ying 17

et al., 2018), as can be seen in Figure 4(a, b), and the ternary composites even have higher impact strengths than the blends (Figure 4d). This indicates that the presence of CNFs could also improve interfacial adhesion between PLA phase and PBS one, and as a result, more balanced mechanical performance is achieved. Figure 5a shows the TEM image for the BC(90/10)-3 sample. It is seen that as those CNFs dispersed in the continuous PLA phase run into a discrete PBS domain, they commonly pass around

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along with phase interface region, showing flexible fiber morphology. The effective stiffness parameter ( S eff ) can be evaluated according to (Switzer III & Klingenberg, 2003) E D 4 E / 64  64 L4 AR 4

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S eff 

(3)

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where L and D are the particle length and diameter, respectively. AR is the aspect ratio

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(AR=L/D) and  the shear stress depending on the matrix viscosity and shear rate. The S eff of the CNF used in this work is merely about 1.7×10-4 (assuming AR=100,

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and average shear rate is 66 s-1 for the melt mixing in a torque-rheometer (Marquez, Quijano, & Gaulin, 1996; Wu, Wu, Zhou, Sun, & Zhang, 2010a)). Such a small value

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indicates that the CNFs are fully flexible in the current polymer matrices during melt

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mixing, and hence cannot retain their equilibrium shape under shear flow (Wang et al., 2018; Wu, Sun, Wu, & Zhang, 2009; Wu et al., 2010a). Therefore, the CNFs close to the PLA-PBS interfaces are able to emulsify phase interfaces to a certain degree (see the arrows in Figure 5b). In other words, some CNFs play the ‘bridge-like’ role in the ternary composites, improving the phase adhesions (Wu et al., 2009). This favors load transfer and as a result, the toughening effect of PBS to PLA can be achieved much 18

better. Thus, the ternary composites show much better and more balanced mechanical performance than the blends. 3.3. Mechanical properties of the ternary composites based on the CNF-filled compatibilized PLA/PBS blends Although incorporation with CNFs is able to compensate for the strength and rigidity losses caused by the presence of ductile PBS phase, the results are not very satisfying.

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The strength and modulus of ternary composite, for instance, BC(90/10)-3 sample (50 MPa and 2.0 GPa), are still lower than those of the neat PLA (60 MPa and 2.3 GPa,

Figure 4 (a, b)). The toughened ternary composites with better strengths are expected.

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A common route is to chemically modify the surfaces of particles, thereby improving

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their affinities to the continuous matrix and to the phase interfaces of the blends (De Luna & Filippone, 2016; Taguet et al., 2014). However, this route is not that good

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because the surface modifications of nanoparticles, which consist of many procedures,

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including the repeated washing and freeze drying steps, are energy-intensive and time consuming. On the other hand, chemical treatments could amorphize polysaccharide

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nanocrystals, even destructing their rod-like structure, especially at the higher degrees of substitution (Wang et al., 2018; Xu, Wu, Lv, & Yan, 2017). Accordingly, a simple

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way is proposed here to fabricate ternary composites with good and balanced strength and toughness: preparation of the PLA/PBS blend with improved phase morphology through reactive extrusion, followed by incorporation with pristine CNFs. In this case, surface treatments of pristine CNFs may become less important.

19

G', G" (Pa)

104

103

102

blend G' G"

(a)

101 10-2

10-1

R=3000 N m-2 =1.86 N m-1

105

G', G" (Pa)

100

101

102

104

103

(b)

103

101 10-2

angular frequency (rad/s) 106

100

101

102

103

90 (d)

105

80

104

 ( o)

G', G" (Pa)

10-1

angular frequency (rad/s)

(c)

103

G'

10-1

100

101

70 60

G"

blend c-blend BC-3 c-BC-3

102

101 10-2

c-blend G' G''

102

blend c-blend BC-3 c-BC-3

50 40

102

102

103

103

ro of

105

106

R=950 N m-2 =4.95 N m-1

104

-p

106

105

106

|G*| (Pa)

angular frequency (rad/s)

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Figure 6 Dynamic storage and loss moduli for (a) neat and (b) compatibilized blend samples, as well as (c) the ternary composites (solid lines are the results predicted by Palierne model). The

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v-GP (phase angles (δ) vs. complex moduli (|G*|)) curves of the blend and composite samples are

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plotted in (d). The polymer blending ratio of all samples are 90/10 (PLA/PBS w/w) The CNF

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loadings of composites are 3 wt%.

A commonly used peroxide for the reactive processing of aliphatic polyester blends,

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BIBP (Przybysz, Zedler, Reza Saeb, & Formela, 2018), is used here. During reactive processing, its thermal decomposition generates primary radicals, attacking polymer chains and forming polymer radicals. Then, the recombination of polymer radicals occurs, leading to chain extension or cross-linking (Takamura, Nakamura, Takahashi, & Koyama, 2008; Lee, Choi, Choi, & Ha, 2019; Simmons & Kontopoulou, 2018). 20

This transesterification favors thickening of phase interface layers in an immiscible blend (Chen, & Wu, 2014; Formela, Zedler, Hejna, & Tercjak, 2018; Wang et al., 2010). Clearly, the phase morphology of PLA/PBS blend is improved after reactive processing (Figure 1 (c, c')). The average size of PBS domains reduces by more than one order of magnitude, from 5.6 μm (Figure 1 (a, a')) downshift to 0.6 μm. Palierne emulsion model (Palierne, 1990) is used here to evaluate the alteration of interfacial

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tensions after reactive compatibilization. Assuming that the droplet size distribution is narrow, the interfacial tension is hence independent of the shear and interfacial area variation (Graebling, Muller, & Palierne, 1993): 1  3 i i H i ( )

-p

Gb*  Gm*

1  2 i i H i ( )

4( / Ri )[2Gm* ( )  5Gd* ( )]  [Gd* ( )  Gm* ( )][16Gm* ( )  19Gd* ( )] 40( / Ri )[Gm* ( )  Gd* ( )]  [2Gd* ( )  3Gm* ( )][16Gm* ( )  19Gd* ( )]

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H i ( ) 

re

where

(4)

(5)

in which, Ri, i , ω and α are the ith particle fraction radius, the ith volume fraction of

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dispersed phase, the angular frequency, and the interfacial tension. Gb* ( ) , Gm* ( ) ,

ur

and Gd* ( ) are the complex modulus of the blend, matrix, and the dispersed phase, respectively. The best predictions are shown in Figure 6 (a, b). Details on the model

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fitting can be found elsewhere (Wu et al., 2009; Wu et al., 2010b; Wu et al., 2012). It is clear that after reactive compatibilization, the interfacial tension level between PLA and PBS lowers remarkably (for the pristine blend, the tension value divergence from that obtained by the fiber retractions is due to the different principles of testing). It should be addressed also that the Palierne model is commonly used for the description 21

on the immiscible blend systems, which may be not very valid for the systems with partial miscibility such as the compatibilized blends. Therefore, the reported value for the c-blend sample here may be inaccurate, but the decreased trend could be viewed as a criterion for the improved phase adhesion after reactive compatibilization.

-40

o

-28.2 C

-30

-20

-10

0

10

Tg(PBS)

(1)

-29.8 oC -28.8 oC -30.5 oC

(4) (5)

-29.2 oC

(6)

-50

-25

0

25

56.6 oC 53.3 oC 53.9 oC 52.2 oC 50

temperature (oC)

75

100

125

re

-75

58.3 oC

(2) (3)

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tan 

-50

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Tg(PLA)

(1) (2) (3) (4) (5) (6) -60

(1) PBS (2) PLA (3) blend (4) c-blend (5) BC-3 (6) c-BC-3

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Figure 7 Dynamic loss factors for (1) PBS, (2) PLA, (3) blend, (4) c-blend, (5) BC-3 and (6) c-BC-3. The polymer blending ratio of the blend and ternary composites is 90/10 (PLA/PBS, w/w).

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The summarized data are the values of Tg of PLA (right) and PBS (left).

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Relative to the two kinds of blends, their ternary systems show increased moduli at the low-frequency region. This is due to the flow repression caused by the presence of

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fibrous particles (Chen, Xu, Huang, Wu, & Lv, 2017; Wu, Wang, Zhang, & Zhou, 2012; Wang et al., 2015; Wu, Wu, Wu, & Zhang, 2006; Wu, Wu, Zhang, & Wu, 2007; Yuan, Wu, Zhang, & Zhou, 2011). The two ternary composite systems still show the viscous flow-dominated responses at current CNF loadings because their loss moduli are higher than storage ones at the low-frequency region (Figure 6c). In other words, 22

the percolated network of CNFs does not form in these two systems because 3 wt% loadings range in the semi-dilute region of CNF in the current systems, as mentioned above. Therefore, the low-frequency phase angles of two ternary composite samples are still higher than 45o, as can be seen in Figure 6d. It is notable that the CNF filled compatibilized blend (c-BC-3) present higher middle-frequency modulus and lower middle-frequency phase angle than the CNF filled pristine blend (BC-3), as shown by

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the arrow in Figure 6 (c, d). In general, the fluctuation of middle-frequency modulus for a polymer blend (Figure 6 a) is caused by the shape relaxations of discrete phase

in the matrix (Wu et al., 2009; Wu et al, 2012). This is because the total area of phase

-p

interfaces and the interfacial energy are changing periodically during oscillatory shear

re

(Yu, Bousmina, Grmela, & Zhou, 2002). The increased shape relaxation responses of c-BC-3 indicates that more phase interfaces form in this sample, which is confirmed

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by the SEM images shown in Figure 1 (d, d'). It is seen that its size of discrete PBS

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domains is merely about 0.3 μm, far lower than that of BC-3, and also reduces by half as compared to that of c-blend, indicating that the presence of CNFs is able to further

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improve phase morphology of compatibilized PLA/PBS blend. Besides, both the compatibilized blend (c-blend) and its ternary composite (c-BC-3)

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still retain two glass transition temperatures (Tg) of each polymer component, as can be seen in Figure 7. This is very important because it indicates that two matrix phases, strong PLA and tough PBS, in these two systems, can show their own bulk properties (Wang et al., 2010; Wu et al., 2010b), respectively. As compared to the pristine blend, the compatibilized one (c-blend) shows a slight decrease of the Tg (PLA phase), which 23

is attributed to improved two-phase miscibility (Wu, et al., 2011b; Wu, Zhang, Yuan, Zhang, & Zhou, 2010b). The Tg of PBS is not that sensitive, which is indicative of its lower reactivity than that of PLA (Yang, Clenet, Xu, Odelius, & Hakkarainen, 2015) because the two polymers are thermally asymmetric. The presence of CNF nearly has no evident influence on the Tgs of two polymer matrices. Figure 8 reveals the tensile and impact behaviors of two kinds of blends and their ternary composites. Due to the

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improved phase adhesion, c-blend sample shows better tensile strength and modulus

(51 MPa and 1.93 GPa) than the pristine blend (45 MPa and 1.83 GPa), with the same

level of elongation at break. The toughness also increases markedly, from 22 kJ m-2 to

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35 kJ m-2. The mechanical performance of c-BC-3 sample is within the expectations:

re

its strength and modulus are about 58 MPa and 2.35 GPa, respectively, very close to, or even higher than those of the neat PLA. Clearly, the reinforcement of CNFs in the

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compatibilized blend is much better than in the pristine one. What is more attractive is

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that c-BC-3 retains the same good level of elongation and impact strength (160% and 31 kJ m-2) with those of c-blend, far higher than those of neat PLA (5% and 13 kJ m-2).

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Therefore, balanced mechanical performance is achieved on the PLA based materials in this case.

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This balanced performance is synergistically resulted from improved phase structure

and presence of rigid particles. Reactive compatibilization improves phase adhesion between PLA and PBS, and in this case, the reinforcement role of CNFs could be better played due to decreased PLA-PBS phase interfacial defects. On the other hand, the improved phase adhesion favors reducing yield stress or increasing crack initiation 24

energy (Nagarajan, Mohanty, & Misra, 2016; Odent, Raquez, & Dubois, 2015), and as a result, c-BC-3 sample shows a combination of shear yielding and multiple crazing phenomenon on its broken surface (Figure 9c), while the pristine blend sample merely shows the former (Figure 9b). The addition of CNFs results in an evident increase of material rigidity, which is against the occurrence of cavitation in polymer bulks (Kim & Michler, 1998; Odent et al., 2015). Therefore, c-BC-3 sample shows the thickened

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shear belts, accompanied by a weakened multiple crazing trend (Figure 9d). However, as discussed in the morphological section, the preferential dispersion of CNFs in the

continuous PLA phase highly restrains coalescence of broken PBS droplets during

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melt mixing, reducing discrete PBS phase size (Figure 1d'). For a rubber-toughened

re

blend with brittle matrix, the optimum rubbery particle size, d0, correlates with the

(Wu, 1992):

log d0  1.19  14.1 e

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intrinsic brittleness of matrix (Wu, 1990), which follows a least-squares regression

(6)

na

where  e is the entanglement density of matrix polymer, applicable to the brittle

ur

matrix ( e <0.15 mmol cm-3). The  e value of PLA ranges in 0.12-0.14 mmol cm-3 (Joziasse, Topp, Veenstra, Grijpma, & Pennings, 1994; Schindler & Harper, 1979),

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and the optimum rubbery domain size is ca. 0.15-0.30 μm. The size of PBS phase in c-BC-3 sample is about 0.32 μm, very close to this optimum size region. In this case, c-BC-3 still retains good toughness.

25

2600

70

PLA blend c-blend c-BC-3

50 2400

modulus (MPa)

stress (MPa)

50

40

30

20

40

2200

30

20 2000

impact strength (KJ/m2)

60

60

10

10

(a)

0 0

5

10

150

200

250

PL A

blend c-blend

0

c-BC-3

ro of

strain (%)

(b)

1800

Figure 8 Mechanical properties of neat PLA, pristine and compatibilized blend samples (blend,

c-blend), as well as ternary compatibilized composite (c-BC-3): (a) stress-strain responses and (b)

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tensile moduli and impact strengths. The polymer blending ratio of the blends and ternary

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composite is 90/10 (w/w, PLA/PBS).

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Although the ternary composites developed in this work are not that outstanding in terms of each mechanical property (for instance, their tensile strengths are lower than

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those of the binary PLA composites reinforced with nanoparticles, and the toughness is also not as good as that of the PLA based blends mixed with elastomers), they show

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acceptable and balanced overall mechanical performance, possessing tensile strengths

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and moduli very close to those of the neat PLA, with highly improved toughness. It is also attractive that both the matrix polymers, PLA and PBS, and the reinforcement, CNF, are biodegradable. The two-step route reported in this work, namely the reactive processing of biodegradable aliphatic polyester blends, followed by the incorporation with pristine CNFs, is hence an efficient way to achieve good combination of strength and toughness, fabricating green and interesting polymeric composites with balanced 26

(b)

(c)

(d)

-p

(a)

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performance.

2 μm

re

2 μm

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Figure 9 SEM images of the impact-fractured surfaces of (a) PLA, (b) pristine blend (blend), (c) compatibilized blend (c-blend), and (d) ternary compatibilized composite (c-BC-3). The polymer

is 2 μm.

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4. Conclusions

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blending ratio of the blend and ternary composite samples is 90/10 (w/w, PLA/PBS). The scale bar

Pristine CNFs were used as the third component to tune the phase morphology and

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mechanical properties of immiscible PLA/PBS blends with the objective to prepare ternary biocomposites with balanced mechanical performance and fully biodegradable characteristics. Both the tensile strengths and impact ones are improved remarkably because of the reinforcement of CNFs and the decreased PBS phase size resulted from the preferential dispersion of most CNFs in the continuous PLA phase. However, the 27

improvement is not very satisfying because the tensile strengths and Young's moduli of as-obtained ternary composites are still less than those of neat PLA, despite highly improved toughness. More balanced performance is expected. A two-step approach consisting of reactive extrusion of PLA/PBS blends, and further incorporation with pristine CNFs, was developed here to prepare ternary composites. As-obtained ternary systems present good and balanced mechanic performance, with doubled toughness

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and the same strength level with those of neat PLA. The merit of each component,

such as rigidity of PLA, ductility of PBS, and reinforcement of CNFs, can be well combined in this case. An interesting ternary composite with balanced strengths and

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toughness, and fully green and ecofriendly characteristics, is finally obtained based on

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biodegradable polyester blends and biomass-derived polysaccharide nanoparticles. Acknowledgement

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The authors gratefully thank the National Natural Science Foundation of China

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ur

na

(51573156) for the financial support.

28

References Abdul Khalil, H. P. S., Bhat, A. H., & Ireana Yusra A. F. (2012). Green composites from sustainable cellulose nanofibrils: A review. Carbohydrate Polymers, 87, 963-979. Carriere, C. J., Cohen, A., & Arends, C. B. (1989). Estimation of interfacial tension using shape evolution of short fibers. Journal of Rheology, 33, 681-689. Chen, G. X., Kim, H. S., Kim, E. S., & Yoon, J. S. (2005). Compatibilization-like effect of reactive

ro of

organoclay on the poly(L-lactide)/poly(butylene succinate) blends. Polymer, 46, 11829-11836.

Chen, J. X., Lu, L. L., Wu, D. F., Yuan, L. J., Zhang, M., Hua, J. J., et al. (2014). Green

poly(ε-caprolactone) composites reinforced with the electrospun polylactide/poly(ε-caprolactone)

-p

blend fiber mats. ACS Sustainable Chemistry and Engineering, 2, 2102-2110.

re

Chen, J. X., Wang, Y. K., Ying, Z. R., Tam, K. C., & Wu, D. F. (2017). Morphology and mechanical

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properties of poly(β-hydroxybutyrate)/poly(ε-caprolactone) blends controlled with cellulosic particles. Carbohydrate Polymers, 174, 217-225.

na

Chen, J. X., & Wu, D. F. (2014). Poly(trimethylene terephthalate)/poly(butylene succinate) blend: Phase behavior and mechanical property control using transesterification system as the

ur

compatibilizer. Materials Chemistry and Physics, 148, 554-561. Chen, Y., Xu, C. J., Huang, J., Wu, D. F., & Lv, Q. L. (2017). Rheological properties of

Jo

nanocrystalline cellulose suspensions. Carbohydrate Polymers, 157, 303-310.

De Luna, M. S., & Filippone, G. (2016). Effects of nanoparticles on the morphology of immiscible polymer blends - Challenges and opportunities. European Polymer Journal, 79, 198-218. Eastmond, G. C. (2000). Poly(ε-caprolactone) blends. Advance in Polymer Science, 149, 59-223. Fang, H. G., Wang, S. L., Ye, W. J., Chen, X., Wang, X. H., Xu, P., et al. (2019). Simultaneous 29

improvement of mechanical properties and electromagnetic interference shielding performance in eco-friendly polylactide composites via reactive blending and MWCNTs induced morphological optimization. Composites Part B-Engineering, 178, 107452. Frackowiak, S., Ludwiczak, J., Leluk, K., & Kozlowski, M. (2016). New class of shear oriented, biodegradable packaging material. Composites Part B-Engineering, 92, 1-8. Formela, K., Zedler, Ł., Hejna, A., & Tercjak. A. (2018). Reactive extrusion of bio-based polymer

ro of

blends and composites - Current trends and future developments. eXPRESS Polymer Letters, 12, 24-57.

Frone, A. N., Berlioz, S., Chailan, J. F., & Panaitescu, D. M. (2013). Morphology and thermal

-p

properties of PLA-cellulose nanofibers composites. Carbohydrate Polymers, 91, 377-384.

re

Fu, Z. A., Wang, H. T., Zhao, X. W., Li, X., Gu, X. Y., & Li, Y. J. (2019). Flame-retarding

lP

nanoparticles as the compatibilizers for immiscible polymer blends: simultaneously enhanced mechanical performance and flame retardancy. Journal of Materials Chemistry A, 7, 4903-4912.

na

Göldel, A., Kasaliwal, G., & Pötschke, P. (2009). Selective localization and migration of multiwalled carbon nanotubes in blends of polycarbonate and poly(styrene-acrylonitrile). Macromolecular

ur

Rapid Communications, 30, 423-429.

Graebling, D., Muller, R., & Palierne, J. F. (1993). Linear viscoelastic behavior of some incompatible

Jo

polymer blends in the melt: Interpretation of data with a model of emulsion of viscoelastic liquids. Macromolecules, 26, 320-329.

Iwatake, A., Nogi, M., & Yano, H. (2008). Cellulose nanofiber-reinforced polylactic acid. Composites Science and Technology, 68, 2103-2106. Jalali Dil, E., & Favis, B. D. (2015a). Localization of micro and nano- silica particles in a high 30

interfacial tension poly(lactic acid)/low density polyethylene system. Polymer, 77, 156-166. Jalali Dil, E., Arjmand, M., Li, Y., Sundararaj, U., & Favis, B. D. (2016). Assembling copper nanowires at the interface and in discrete phases in PLA-based polymer blends. European Polymer Journal, 85, 187-197. Jalali Dil, E., & Favis, B. D. (2015b). Localization of micro- and nano-silica particles in heterophase poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends. Polymer, 76, 295-306.

ro of

Jalali Dil, E., Virgilio, N., & Favis, B. D. (2016). The effect of the interfacial assembly of nano-silica in poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends on morphology, rheology and mechanical properties. European Polymer Journal, 85, 635-646.

-p

Jonoobi, M., Harun, J., Mathew, A. P., & Oksman, K. (2010). Mechanical properties of cellulose

re

nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion.

lP

Composites Science and Technology, 70, 1742-1747.

Jonoobi, M., Mathew, A. P., Abdi Majid, M. M., Makinejad, D., & Oksman, K. (2012). A comparison

na

of modified and unmodified cellulose nanofiber reinforced polylactic acid (PLA) prepared by twin screw extrusion. Journal of Polymers and the Environment, 20, 991-997.

ur

Joy, J., Jose, C., Yu, X. Y., Mathew, L., Thomas, S., & Pilla, S. (2017). The influence of nanocellulosic fiber, extracted from Helicteres isora, on thermal, wetting and viscoelastic properties of

Jo

poly(butylene succinate) composites. Cellulose, 24, 4313-4323.

Joziasse, C. A. P., Topp, M. D. C., Veenstra, H., Grijpma, D. W., & Pennings, A. J. (1994). Supertough poly(lactide)s. Polymer Bulletin, 33, 599-605. Kalashnikova, I., Bizot, H., Bertoncini, P., Cathala, B., & Capron, I. (2013). Cellulosic nanorods of various aspect ratios for oil in water Pickering emulsions. Soft Matter, 9, 952-959. 31

Kim, G. M., & Michler, G. H. (1998). Micromechanical deformation processes in toughened and particle-filled semicrystalline polymers: Part 1. Characterization of deformation processes in dependence on phase morphology. Polymer, 39, 5689-5697.

Kowalczyk, M., Piorkowska, E., Kulpinski, P., & Pracella, M. (2011). Mechanical and thermal properties of PLA composites with cellulose nanofibers and standard size fibers. Composites Part A-Applied Science and manufacturing, 42, 1509-1514.

ro of

Krause, B., Schneider, C., Boldt, R., Weber, M., Park, H. J., & Pötschke, P. (2014). Localization of carbon nanotubes in polyamide 6 blends with non-reactive and reactive rubber. Polymer, 55, 3062-3067.

-p

Lee, J. C., Choi, M. C., Choi, D. H., & Ha, C. S. (2019). Toughness enhancement of poly(lactic acid)

Lee, S. H., Bailly, M.,

lP

Degradation and Stability, 160, 195-202.

re

through hybridization with epoxide-functionalised silane via reactive extrusion. Polymer

&

Kontopoulou,

M.

(2012).

Morphology and properties of

na

poly(propylene)/ethylene-octene copolymer blends containing nanosilica. Macromolecular Materials and Engineering, 297, 95-103.

ur

Li, Y. J., & Shimizu, H. (2004). Novel morphologies of poly(phenylene oxide) (PPO)/polyamide 6 (PA6) blend nanocomposites. Polymer, 45, 7381-7388.

Jo

Li, Y. J., & Shimizu, H. (2005). Co-continuous polyamide 6 (PA6)/acrylonitrile-butadiene-styrene (ABS) nanocomposites. Macromolecular Rapid Communications, 26, 710-715.

Lu, Y., Huang, J., Ge, L. L., Xie, W. Y., & Wu, D. F. (2018). Selective localization of cellulose nanocrystals in the biodegradable poly(vinyl alcohol)/poly(ε-caprolactone) blend composites prepared by Pickering emulsions. Polymer, 156, 136-147. 32

Lu, Y., Qian, X. L., Xie, W. Y., Huang, J., Zhang, W. T., & Wu, D. F. (2019). Rheology of the sesame oil-in-water emulsions stabilized by cellulose nanofibers. Food Hydrocolloids, 94, 114-127. Marquez, A., Quijano, J., & Gaulin, M. (1996). A calibration technique to evaluate the power-law parameters of polymer melts using a torque-rheometer. Polymer Engineering and Science, 36, 2556-2563. Nagarajan, V., Mohanty, A. K., & Misra, M. (2016). Perspective on polylactic acid (PLA) based

ro of

sustainable materials for durable applications: Focus on toughness and heat resistance. ACS Sustainable Chemistry & Engineering, 4, 2899-2916.

Nofar, M., Sacligil, D., Carreau, P. J., Kamal, M. R., & Heuzey, M. C. (2019). Poly (lactic acid) blends:

-p

Processing, properties and applications. International Journal of Biological Macromolecules, 125,

re

307-360.

lP

Odent, J., Raquez, J., & Dubois, P. (2015). Highly toughened polylactide-based materials through melt-blending techniques. In S. Fakirov (Eds.), Biodegradable Polyesters. Berlin: Wiley-VCH

na

Verlag GmbH & Co. KGaA.

Ojijo, V., Ray, S. S., & Sadiku, R. (2012).Effect of nanoclay loading on the thermal and mechanical

ur

properties of biodegradable polylactide/poly[(butylene succinate)-co-adipate] blend composites. ACS Applied Materials & Interfaces, 4, 2395-2405.

Jo

Palierne, J. F. (1990). Linear rheology of viscoelastic emulsions with interfacial tension. Rheologica Acta, 29, 204-214.

Przybysz, M., Zedler, Ł., Reza Saeb, M., & Formela, K. (2018). Structure-property relationships in peroxide-assisted blends of poly(ε-caprolactone) and poly(3-hydroxybutyrate). Reactive and Functional Polymers, 127, 113-122. 33

Rasal, R. M., Janorkar, A. V., & Hirt, D. E. (2010). Poly(lactic acid) modifications. Progress in Polymer Science, 35, 338-356. Sangroniz, L., Antonio Moncerrate, M., De Amicis, V. A., Palacios, J. K., Fernández, M., Santamaria, A., et al. (2015). The outstanding ability of nanosilica to stabilize dispersions of Nylon 6 droplets in a polypropylene matrix. Journal of Polymer Science Part B: Polymer Physics, 53, 1567-1579. Schindler, A., & Harper, D. (1979). Polylactide. II. Viscosity - molecular weight relationships and

ro of

unperturbed chain dimensions. Journal of Polymer Science: Polymer Chemistry Edition, 17, 2593-2599.

Shibata, M., Inoue, Y., & Miyoshi, M. (2006). Mechanical properties, morphology, and crystallization

-p

behavior of blends of poly(L-lactide) with poly(butylene succinate-co-L-lactate) and poly(butylene

re

succinate). Polymer, 47, 3557-3564.

lP

Simmons, H., & Kontopoulou, M. (2018). Hydrolytic degradation of branched PLA produced by reactive extrusion. Polymer Degradation and Stability, 158, 228-237.

na

Siro, I., & Plackett, D. (2010). Microfibrillated cellulose and new nanocomposite materials: A review. Cellulose, 17, 459-494.

ur

Supthanyakul, R., Kaabbuathong, N., & Chirachanchai, S. (2016). Random poly(butylene succinate-co-lactic acid) as a multi-functional additive for miscibility, toughness, and clarity of

Jo

PLA/PBS blends. Polymer, 105, 1-9.

Switzer III, L. H., & Klingenberg, D. J. (2003). Rheology of sheared flexible fiber suspensions via fiber-level simulations, Journal of Rheology, 47, 759-778. Taguet, A., Cassagnau, P., & Lopez-Cuesta, J. M. (2014). Structuration, selective dispersion and compatibilizing effect of (nano)fillers in polymer blends. Progress in Polymer Science, 39, 34

1526-1563. Takamura, M., Nakamura, T., Takahashi, T., & Koyama, K. (2008). Effect of type of peroxide on cross-linking of poly(l-lactide). Polymer Degradation and Stability, 93, 1909-1916. Vatansever, E., Arslan, D., & Nofar, M. (2019). Polylactide cellulose-based nanocomposites. International Journal of Biological Macromolecules, 137, 912-938. Wang, R. Y., Wang, S. F., Zhang, Y., Wan, C. Y., & Ma, P. M. (2010). Toughening modification of

ro of

PLLA/PBS blends via in situ compatibilization. Polymer Engineering and Science, 49, 26-33.

Wang, T., & Drzal, L. T. (2012). Cellulose-nanofiber-reinforced poly(lactic acid) composites prepared by a water-based approach. ACS Applied Materials & Interfaces, 4, 5079-5085.

-p

Wang, Y., Cheng, Y. X., Chen, J. X., Wu, D. F., Qiu, Y. X., Yao, X., et al. (2015). Percolation

lP

various thicknesses. Polymer, 67, 216-226.

re

networks and transient rheology of polylactide composites containing graphite nanosheets with

Wang, Y., Zhang, Q., & Fu, Q. (2003). Compatibilization of immiscible poly(propylene)/polystyrene

na

blends using clay. Macromolecular Rapid Communications, 24, 231-235. Wang, Y. K., Xu, C. J., Wu, D. F., Xie, W. Y., Wang, K., Xia, Q. R., et al. (2018). Rheology of the

ur

cellulose nanocrystal filled poly(ε-caprolactone) biocomposites. Polymer, 140, 167-178. Wu, D. F., Lin, D. P., Zhang, J., Zhou, W. D., Zhang, M., Zhang, Y. S., et al. (2011a). Selective

Jo

localization of nanofillers: Effect on morphology and crystallization of PLA/PCL blends. Macromolecular Chemistry and Physics, 212, 613-626.

Wu, D. F., Sun, Y. R., Lin, D. P., Zhou, W. D., Zhang, M., & Yuan, L. J. (2011b). Selective localization behavior of carbon nanotubes: Effect on transesterification of immiscible polyester blends. Macromolecular Chemistry and Physics, 212, 1700-1709. 35

Wu, D. F., Sun, Y. R., Wu, L., & Zhang, M. (2009). Kinetic study on the melt compounding of polypropylene/multi-walled carbon nanotube composites. Journal of Polymer Science Part B: Polymer Physics, 47, 608-618. Wu, D. F., Wang, J. H., Zhang, M., & Zhou, W. D. (2012). Rheological properties of carbon nanotubes filled poly(vinylidene fluoride) composites. Industrial and Engineering Chemistry Research, 51, 6705-6713.

ro of

Wu, D. F., Wu, L., Wu, L. F., & Zhang, M. (2006). Rheology and thermal stability of polylactide/clay nanocomposites. Polymer Degradation and Stability, 91, 3149-3155.

Wu, D. F., Wu, L., Zhou, W. D., Sun, Y. R., & Zhang, M. (2010a). Relations between the aspect ratio

-p

of carbon nanotubes and the formation of percolation networks of biodegradable

re

polylactide/carbon nanotube composites. Journal of Polymer Science Part B: Polymer Physics, 48,

lP

479-489.

Wu, D. F., Wu, L. F., Zhang, M., & Wu, L. (2007). Effect of epoxy resin on rheology of

na

polycarbonate/clay nanocomposites. European Polymer Journal, 43, 1635-1644. Wu, D. F., Wu, L. F., Zhang, M., Zhou, W. D., & Zhang, Y. S. (2008). Morphology evolution of

ur

nanocomposites based on poly(phenylene sulfide)/poly(butylene terephthalate) blend. Journal of Polymer Science Part B: Polymer Physics, 46, 1265-1279.

Jo

Wu, D. F., Yuan, L. J., Laredo, E., Zhang, M., & Zhou, W. D. (2012). Interfacial properties, viscoelasticity, and thermal behaviors of poly(butylene succinate)/polylactide blend. Industrial and Engineering Chemistry Research, 51, 2290-2298. Wu, D. F., Zhang, Y. S., Yuan, L. J., Zhang, M., & Zhou, W. D. (2010b). Viscoelastic interfacial properties of compatibilized polylactide/poly(ε-caprolactone) blend. Journal of Polymer Science 36

Part B: Polymer Physics, 48, 756-765. Wu, D. F., Zhang, Y. S., Zhang, M., & Yu, W. (2009). Selective localization of multi-walled carbon nanotube in polylactide/poly(ε-caprolactone) blend. Biomacromolecules, 10, 417-424. Wu, S. (1990). Chain structure, phase morphology, and toughness relationships in polymers and blends. Polymer Engineering and Science, 30, 753-761.

structure: a review. Polymer International, 29, 229-247.

ro of

Wu, S. (1992). Control of intrinsic brittleness and toughness of polymers and blends by chemical

Xing, P. X., Bousmina, M., & Rodrigue, D. (2000). Critical experimental comparison between five techniques for the determination of interfacial tension in polymer blends: Model system of

-p

polystyrene/polyamide-6. Macromolecules, 33, 8020-8034.

re

Xu, C. J., Chen, J. X., Wu, D. F., Chen, Y., Lv, Q. L., & Wang, M. Q. (2016). Polylactide/acetylated

lP

nanocrystalline cellulose composites prepared by a continuous route: A phase interface-property study. Carbohydrate Polymers, 146, 58-66.

na

Xu, C. J., Wu, D. F., Lv, Q. L., & Yan, L. L. (2017). Crystallization temperature as the probe to detect polymer-filler compatibility in the poly(ε-caprolactone) composites with acetylated cellulose

ur

nanocrystals. Journal of Physical Chemistry C, 121, 18615-18624. Yang, X., Clenet, J., Xu, H., Odelius, K., & Hakkarainen, M. (2015). Two step extrusion process: From

Jo

thermal recycling of PHB to plasticized PLA by reactive extrusion grafting of PHB degradation products onto PLA chains. Macromolecules, 48, 2509-2518.

Ying, Z. R., Wu, D. F., Xie, W. Y., Wang, Z. F., Qiu, Y. X., & Wei, X. J. (2018). Rheological and mechanical properties of polylactide nanocomposites reinforced with the cellulose nanofibers with various surface treatments. Cellulose, 25, 3955-3971. 37

Yokohara, T., & Yamaguchi, M. (2008). Structure and properties for biomass-based polyester blends of PLA and PBS. European Polymer Journal, 44, 677-685. Yu, W., Bousmina, M., Grmela, M., & Zhou, C. X. (2002). Modeling of oscillatory shear flow of emulsions under small and large deformation fields. Journal of Rheology, 46, 1401-1418. Yu, Z. Y., Yin, J. B., Yan, S. F., Xie, Y. T., Ma, J., & Chen, X. S. (2007). Biodegradable poly(L-lattice)/poly(ε-caprolactone)-modified montmorillonite nanocomposites: Preparation and

ro of

characterization. Polymer, 48, 6439-6447.

Yuan, L. J., Wu, D. F., Zhang, M., & Zhou, W. D. (2011). Rheological percolation behavior and

Engineering Chemistry Research, 50, 14186-14192.

-p

isothermal crystallization of poly(butylene succinate)/carbon nanotube composites. Industrial and

lP

blends. RSC Advances, 5, 32546-32565.

re

Zeng, J. B., Li, K. A., & Du, A. K. (2015). Compatibilization strategies in poly(lactic acid)-based

Zhang, G. R., Xie, W. Y., & Wu, D. F. (2020). Selective localization of starch nanocrystals in the

227, 115341.

na

biodegradable nanocomposites probed by crystallization temperatures. Carbohydrate Polymers,

ur

Zhang, X., & Zhang, Y. (2016). Reinforcement effect of poly(butylene succinate) (PBS)-grafted cellulose nanocrystal on toughened PBS/polylactic acid blends. Carbohydrate Polymers, 140,

Jo

374-382.

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