poly(lactic acid) immiscible blends

Journal Pre-proof Stretch-induced crystalline structural evolution and cavitation of poly(butylene adipate-ran-butylene terephthalate)/poly(lactic acid) immiscible blends Jian Zhou, Ying Zheng, Guorong Shan, Yongzhong Bao, Wen-Jun Wang, Pengju Pan PII:

S0032-3861(19)31126-7

DOI:

https://doi.org/10.1016/j.polymer.2019.122121

Reference:

JPOL 122121

To appear in:

Polymer

Received Date: 6 November 2019 Revised Date:

19 December 2019

Accepted Date: 21 December 2019

Please cite this article as: Zhou J, Zheng Y, Shan G, Bao Y, Wang W-J, Pan P, Stretch-induced crystalline structural evolution and cavitation of poly(butylene adipate-ran-butylene terephthalate)/ poly(lactic acid) immiscible blends, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122121. 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 Jian Zhou: Investigation, Methodology, Validation, Data curation, Formal analysis, Writing - original draft Ying Zheng: Investigation Guorong Shan: Supervision Yongzhong Bao: Supervision Wen-Jun Wang: Supervision Pengju Pan: Conceptualization, Supervision, Funding acquisition, Resources, Writing - review & editing

Graphic abstract

1

Stretch-induced crystalline structural evolution and cavitation of poly(butylene adipate-ran-butylene terephthalate)/poly(lactic acid) immiscible blends Jian Zhou,a Ying Zheng,a Guorong Shan,a,b Yongzhong Bao,a,b Wen-Jun Wang,a,b Pengju Pan*,a,b

a

State Key Laboratory of Chemical Engineering, College of Chemical and Biological

Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China b

Institute of Zhejiang University-Quzhou, 78 Jiuhua Boulevard North, Quzhou 324000,

China

*Corresponding author. email: [email protected]

Abstract: Semicrystalline polymer blends show much more complicated structural and morphological evolutions during stretching, due to the interplay between phase separation and polymer crystallization. Understanding the stretch-induced multilevel structural evolutions of semicrystalline polymer blends is of fundamental importance for

their

practical

processing.

Herein,

we

choose

the

poly(butylene

adipate-ran-terephthalate) (PBAT)/poly(lactic acid) (PLA) pair as the immiscible semicrystalline blend and investigate the evolutions of crystal orientation, polymorphic crystalline structure, lamellar morphology, and the cavitation behavior of PBAT-rich PBAT/PLA blends under stretching. We find that the strength and modulus of PBAT are considerably improved after the incorporation of PLA. Crystal 1

orientation of PLA is much slower than that of PBAT crystals in their blends under stretching. PBAT undergoes the α-to-β phase transition and melt recrystallization of lamellae upon stretching, which is seldom affected by the addition of PLA. Stretch-induced cavitation is observed during stretching the PBAT/PLA blends; the cavities are mainly formed around the PBAT/PLA interface due to the interfacial debonding and in the PLA phase by the fragmentation of crystal lamellae. The formation and development of cavities in PBAT/PLA blends are significantly influenced by the blend composition, crystallization and stretching temperatures (Tc, Td). Cavitation of PBAT/PLA blends is suppressed as Tc is lowered or Td is enhanced. Keywords: PBAT; poly(lactic acid); stretch; cavitation

1. Introduction Poly(butylene adipate-ran-terephthalate) (PBAT) is a commercially- available biodegradable, semicrystalline aliphatic/aromatic copolyester that possesses good mechanical properties and processability. PBAT has been widely used to prepare the environmentally-friendly biodegradable materials such as the packaging films, compost bags, and agricultural mulching films [1]. However, the intrinsic low yield strength, low modulus, and poor stiffness of PBAT restrict its wide applications. These disadvantages can be effectively overcome by blending PBAT with the robust biodegradable polymers such as poly(lactic acid) (PLA) [2]. Due to the high strength and modulus of PLA [3], the biodegradable materials having balanced flexibility, toughness, strength, and modulus can be obtained by modifying PBAT by PLA. Several research groups have investigated the solid-state structures [4−5] and physical properties of PBAT/PLA blends [6−7]. It has been reported that PBAT and PLA are immiscible and phase-separated in their blends, due to the large difference in the 2

solubility parameters of PBAT [22.95 (cal·cm−3)0.5] and PLA [10.1 (cal·cm−3)0.5] [8-9]. Compatibility between PBAT and PLA can be improved by using the reactive compatibilizer in blending [6,10]. Stretching deformation is an inevitable process in manufacturing the polymer films. Since a major application of PBAT is used as the biodegradable films, it is of fundamental importance to understand the microstructural evolution of PBAT-based materials during stretching deformation. It is well recognized that the semicrystalline polymers undergo two major microstructural changes during stretching deformation, i.e., crystal plastic deformation and cavitation [11]. As far as the crystal plastic deformation is concerned, stretching can induce the slippage [12], fragmentation, and melt recrystallization [13] of crystalline lamellae. Generally, the block slippage of crystalline lamellae occurs first during stretching, followed by the stress-induced fragmentation/melting and recrystallization of stretched polymer chains after yielding [14−16]. We have demonstrated that PBAT obeys the melt recrystallization mechanism during stretching in the post-yield stage, accompanied by α-to-β transition of PBAT crystals [17]. Cavitation has been widely observed upon stretching the semicrystalline polymers. Cavitation can occur in the yielding [18] or post-yield stages [19] of semicrystalline

polymers

during

stretching.

However,

mechanism

for

the

stretch-induced cavitation of polymer is still controversial. Some researchers [20−21] suggest that the cavities are initiated inside the stretched amorphous phase; while some researchers [18,22−24] argue that the initiation of cavities around yielding is related to the crystalline phase and located in the polar parts of spherulites. The initiation of cavitation and plastic deformation of crystals can be activated concomitantly or competitively [22,25], both of which strongly depend on the initial 3

structure of materials [e.g., crystalline or amorphous state, crystallization temperature (Tc)] [26−27] and stretching conditions [e.g., stretching temperature (Td) [25, 28−29] and speed [25]]. For example, when the semicrystalline PLA is stretched at room temperature, the crystal shearing takes place around the yield point; the cavities then generate around the shear bands and rapidly lead to the brittle fracture of polymer at the rather small strain [30]. Furthermore, the cavitation of PLA is favored at lower Td with the destruction of lamellar structure; while higher Td stimulates the strain-induced crystallization of amorphous chains [31] and restrains the cavitation [32]. Stretch-induced microstructural evolution of polymer blends is more complicated than the neat polymer. Previous studies are mostly focused on the stretch-induced microstructural evolution of the rubber-toughened brittle polymers, that is, the blends composed of a brittle matrix and the dispersed elastomer phase [33−37]. It has been reported that cavitation is mainly initiated at the rubber/matrix interface upon stretching the rubber-toughened polymer, due to the poor interfacial adhesion [37−38]. Despite recent progresses, the stretch-induced microstructural evolution of semicrystalline polymer blends (especially the rigid phase-reinforced soft polymers) has been hardly investigated and its underlying mechanism is still unclear. Synergistic effects of polymer crystallization and phase separation would induce the much more complicated microstructural evolution of semicrystalline polymer blends (e.g., PBAT/PLA blend) under stretching. On one hand, crystallization of blend components would promote the microphase separation [39−40], thus influencing the deformation mechanism. On the other hand, the soft and hard domains of the phase-separated polymer blends possess different deformation abilities under stretching, resulting in the heterogeneous deformation of materials. 4

In this work, we choose the PBAT-rich PBAT/PLA blend as a representative semicrystalline polymer blend, in which the rigid, dispersed PLA phase exerts a reinforcement effect on the soft PBAT matrix. Tg of PLA (~60 °C) is between the Tc range of PBAT (−20−100 °C), which enables to build either the crystalline/crystalline or crystalline/amorphous blend by controlling the amorphous or crystalline state of PLA phase through varying the Tc of PLA. This allows us to investigate the role of crystallization of one component in the stretch-induced structural evolution of semicrystalline polymer blends. We investigate the multilevel evolution of microstructures (e.g., crystal orientation, polymorphic crystalline structure, lamellar morphology, and cavitation) in the PBAT/PLA blends during uniaxially stretching. Effects of blend composition, Tc, and Td on the multilevel structural evolution of PBAT/PLA blends were studied. Mechanism for the stretch-induced structural evolution of PBAT/PLA blends was also discussed and proposed.

2. Experimental 2.1. Materials PBAT (Mw = 71.2 kg/mol; PDI = 1.91) with the 45 mol% of BT, 55 mol% of BA units and the reactive compatibilizer (ADR-4368, BASF), a styrene-acrylic oligomer (Mw < 7000) with the multifunctional side epoxide groups, were purchased from BASF. Chemical structure of compatibilizer is illustrated in Scheme S1. PLA (Mw = 155 kg/mol, PDI = 1.62, optical purity > 99%) was supplied by Purac Co. (Gorinchem, the Netherlands). 2.2. Preparation of PBAT/PLA blend PBAT, PLA, and compatibilizer (1.0 wt% of the total weight of PBAT and PLA) were dried at 60 °C under vacuum for 24 h and then mixed in a batch intensive mixer 5

(Brabender, Germany) at 180 °C for 5 min under the rotor speed of 60 rpm. Mixing ratios of PBAT and PLA were 9/1, 8/2, and 7/3 in the blends, corresponding to the mass fraction of PLA (fPLA) of 0.1, 0.2, and 0.3, respectively. The blends are denoted as PBAT/PLA-fPLA for simplicity. After

mixing,

the

sample

was

cut

into

small

pieces

and

then

compression-molded into the films (thickness ~ 0.4 mm) after melting at 180 °C for 3 min. The molten films were transferred rapidly into the oven preset at the desired Tc (20−80 °C) and held at this Tc for 3 h for crystallization. 2.3. Characterization 2.3.1. Differential scanning calorimetry (DSC) DSC analysis was performed on a NETZSCH 214 Polyma DSC (NETZSCH, Germany) equipped with an IC70 intercooler. The sample (8−10 mg) was heated from −70 to 190 °C at 10 °C/min to measure the melting behavior. 2.3.2. Tensile tests Crystallized PBAT/PLA film was cut into a dumbbell-shaped specimen with a gauge length of 15 mm, a width of 3 mm, and a thickness of ~0.4 mm. Tensile test was conducted on a universal testing machine (Zwick/Roell Z020) equipped with a temperature-controllable unit at a crosshead speed of 10 mm/min. At least five tests were conducted for each sample and the averaged result was used. 2.3.3. WAXD and SAXS measurements In situ WAXD and SAXS measurements were performed on the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF) with an X-ray wavelength of 0.124 nm. WAXD and SAXS profiles were collected when the 6

specimen was stretched on a tensile hot stage (TST-350, Linkam Scientific Instruments, UK) with a stretching speed of 12.5 µm/s at different Td’s. 2D patterns were recorded by a Pilatus 2M detector (Dectris, Swiss) with a resolution of 1475 × 1679 pixels and a pixel size of 172 × 172 µm2. The sample-to-detector distances were 90 and 1880 mm and the exposure times of X-ray were 15 and 30 s in WAXD and SAXS measurements, respectively. SAXS data was corrected from the background and air scattering. 1D-WAXD and SAXS profiles were attained from the 2D patterns by integration within ± 45° along and perpendicular to the stretching direction via the Fit2D software. In order to evaluate the crystal orientation, the intensity distribution curves of PBAT100 and PLA110/200 reflections at various azimuthal angles were obtained from the 2D-WAXD patterns. Orientation degree of crystals is expressed by the orientation parameter (Fhkl) proposed by Hermans, Fhkl =

3 < co s 2 φ hkl > − 1 2

(1)

where ϕhkl is the angle between the normal direction of PBAT100 (or PLA110/200) plane and the reference axis (i.e., stretching direction). was attained from the azimuthal diffraction intensity distribution by < cos φhkl 2

∫ >=

π /2

I hkl (φ ) cos 2 φ sin φ dφ

0

∫

π /2

0

I hkl (φ ) sin φ dφ

(2)

where Ihkl(ϕ) is the diffraction intensity along the azimuthal angle ϕ. Fhkl is 0 when the crystals are randomly orientated and is −0.5 when the crystals are oriented with the normal of hkl plane perpendicular to the stretching direction. 2.3.4. Evaluation of true strain and stress

7

True (or Hencky) strain (εH) of stretched specimen was evaluated by monitoring the displacements of equally-distanced ink marks on the specimen surface [17]. The width, thickness, and marked distance of initial and stretched specimens are marked by (W0, T0, b0) and (W, T, b), respectively. W0T0b0 equals to WTb during stretching, assuming that the volume of formed cavities is neglectable. εH and σH are calculated as follows b   b0 

(3)

Fb W0T0b0

(4)

ε H = ln 

σH =

2.3.5. Scanning Electron Microscopy (SEM) Morphology of stretched samples was observed on a scanning electron microscope (SU-8010, Hitachi Co. Ltd, Japan) at an accelerating voltage of 5 kV. The stretched sample was cryo-fractured in liquid nitrogen along the stretching direction. The fractured cross-section of sample was coated with a thin layer of gold before SEM analysis.

3. Results and discussion 3.1. Thermal and mechanical properties We first investigate the thermal and mechanical properties of PBAT/PLA blends with various fPLA’s crystallized at 80 °C. Fig. 1a shows the DSC heating curves of as-prepared PBAT/PLA blends. The blends show the glass transition temperature of PBAT (Tg,PBAT) at ~−25 °C and of PLA (Tg,PLA) at ~60 °C, demonstrating that PBAT and PLA are immiscible in the blends. Neat PBAT and PBAT component in the blends show two major melting peaks at ~ 92 and ~115 °C. The low-temperature peak 8

is Tc-dependent and always locates at ~10 °C higher than Tc, which is the so-called annealing peak and caused by the melts of thin or defected crystal lamellae generated in the secondary crystallization of isothermal process [17]. PBAT/PLA blends display an additional endotherm at 155−180 °C, ascribing to the melting of PLA; the area of this peak enhances and the Tm of PLA changes little with the increase of fPLA.

Tm,PLA Tg,PBAT

Tg,PLA

Tm,PBAT

30

Tc = 80 °C, Td = 25 °C

25

fPLA 0.3 0.2

σ (MPa)

endo up

b

Heat flow

a

20 15

fPLA = 0.1

fPLA =0

10 fPLA = 0.3

0.1

5

fPLA = 0.2

0

-50

0

50

100

150

0

200

Yield strength (MPa)

c

9

60

8

50

7

40

6

30

5

0.0

0.1

0.2

fPLA

0.3

0

3

6

ε

9

12

15

Young’s modulus (MPa)

Temperature (°C)

20

Fig. 1. Thermal and mechanical properties of PBAT/PLA blends (Tc = 80 °C) with various fPLA’s: (a) DSC curves collected at 10 °C/min; (b) engineering stress-strain curves; (c) changes of yield strength and Young’ s modulus with fPLA. Fig. 1b shows the engineering stress−strain (σ−ε) curves of PBAT/PLA blends crystallized at 80 °C. Yield strength and Young’s modulus of PBAT/PLA blends are 9

summarized in Fig. 1c. All the samples exhibit the elastic stage, yield point, and strain hardening stage, similar as neat PBAT (Fig. 1b). Yield strength and Young’s modulus of PBAT are improved with the incorporation of PLA. Yield strength and Young’s modulus increase from the 6.5 MPa, 29.8 MPa of neat PBAT to 8.2 MPa, 43.7 MPa of PBAT/PLA-0.3 blend as fPLA increases from 0 to 0.3. On the contrary, the elongation-at-break of PBAT/PLA blend reduces with the increase of fPLA. However, PBAT/PLA-0.3 blend still shows a large elongation-at-break of 590%, demonstrating the good flexibility of PBAT/PLA blend. 3.2. Crystalline structural evolution during stretching In-situ WAXD measurements were conducted to explore the crystalline phase transition and crystal orientation of PBAT/PLA blends under stretching. Fig. 2a shows the selected 2D-WAXD patterns of PBAT/PLA-0.2 blend (Tc = 80 °C) during stretching at 25 °C. As shown in Fig. 2a, both the reflection rings of PBAT and PLA are observed for the PBAT/PLA-0.2 blend crystallized at Tc = 80 °C. No obvious crystal orientation is seen before the yield point (εH = 0.2), suggesting the absence of obvious structural changes of PBAT and PLA crystals in the elastic region. However, PBAT and PLA crystals have much different degrees of orientation in the blends under stretching. The reflection rings of PBAT turn into the oriented arcs and finally into spots in the equator direction (i.e., perpendicular to the stretching direction) during stretching. However, the reflection rings of PLA crystals preserve even at a large εH of 1.62, implying the little orientation of PLA crystals. As expected, stretching causes the α-to-β crystal transformation of PBAT in the blends, as demonstrated by the 1D-WAXD data (Figs. 2b, S1). As shown in Fig. 2b, expect for the (110/200) reflection of PLA α’ crystal, the (100), (104) and (105)

10

reflections of PBAT α crystals are observed for the PBAT/PLA-0.2 blend before stretching. The (104) and (105) reflections of PBAT α crystals gradually disappear and meanwhile the (104) and (106) reflections of PBAT β crystals appear with stretching beyond the yield point (εH > 0.2), in agreement with the results of neat PBAT [17]. Stretched-induced formation of PBAT β crystals is also seen in PBAT/PLA blends from the 2D-WAXD patterns (Fig. S2).

b

c

fPLA= 0.2

PLA110/200

Tc= 80 °C, Td= 25 °C

0.0

Intensity (a.u.)

Tc= 80 °C, Td= 25 °C -0.1

PBAT100 β

α

α

F

β

fPLA -0.2

εH

30

2θ (°, λ=1.24Å)

PBAT100

0.1

1.62

20

0.3 0.2

0

10

PLA110/200

-0.3 0.0

40

0.4

0.8

εH

1.2

1.6

2.0

Fig. 2. WAXD results of PBAT/PLA blends (Tc = 80 °C) stretched at 25 °C: (a) 2D-WAXD patterns of PBAT/PLA-0.2 blend stretched to different strains; (b) 1D-WAXD profiles of PBAT/PLA-0.2 blend integrated along the stretching direction; (c) Hermans’ orientation parameter, F, associated with the PBAT100 and PLA110/200 reflections.

To quantitatively analyze the crystal orientation, the orientation parameters, F, of PBAT100 and PLA110/200 reflections were evaluated from the azimuthal intensity distributions by Hermans’ equation (eq. 1). As shown in Fig. 2c, both the F100 of PBAT crystals and F110/200 of PLA crystals decrease during stretching, indicating the 11

preferential orientation of crystalline chains along the stretching direction. Compared to the rapid orientation of PBAT, the decrease of F110/200 of PLA crystals is rather slow during stretching; suggesting that the dispersed rigid PLA phase is difficult to be oriented. This is consistent with the unequal orientation degrees of polymer components in the immiscible blends [41], in which the continuous phase can be more easily oriented than the dispersed phase. In addition, crystal orientation of PBAT is not influenced by the addition of PLA; the blends with different fPLA’s show very similar F100 values at the same εH.

a

fPLA= 0.2

b

PLA110/200

fPLA= 0.2

Meridian

PLA110/200

Intensity (a.u.)

Intensity (a.u.)

Equator

εH 0

εH 0

1.62

1.62

12.8

c

0.56

13.6

12.4

14.0

12.8

13.2

13.6

14.0

2θ (°, λ=1.24 Å)

fPLA Meridian

d110/200 (nm)

13.2

2θ (°, λ=1.24 Å)

0.55

Equator

0.3 0.2 0.1 0.3 0.2 0.1

0.54

0.53 0.0

0.4

0.8

εH

1.2

1.6

Fig. 3. WAXD results corresponding to the PLA110/200 reflection (2θ = 12.6−14.2°) in PBAT/PLA blends (Tc = 80 °C) stretched at 25 °C: (a) 1D-WAXD profiles integrated along the stretching direction; (b) 1D-WAXD profiles integrated perpendicular to the stretching direction; (c) variation of d110/200 of PLA crystals along and perpendicular 12

to the stretching direction during stretching.

Fig. 3a,b show the in-situ 1D-WAXD profiles of PLA110/200 reflection integrated along and perpendicular to the stretching direction during stretching. Stretch-induced changes of the lattice spacing (d) of PLA110/200 reflection (d110/200) in these two directions are shown in Fig. 3c. Intriguingly, PLA crystals display the opposite dimensional changes in the meridional and equatorial directions. As shown in Fig. 3a,b, PLA110/200 reflection shifts gradually to the small angle in the meridional direction but to the large angle in the equatorial direction; suggesting the lateral elongation and longitudinal compression of PLA crystals under stretching. As shown in Fig. 3c, the d110/200 of PLA along equatorial direction follows the similar decreasing trends during stretching, independent of the fPLA. However, the d110/200 of PLA along meridional direction increases more significantly for the blends with high fPLA’s (e.g., 0.2, 0.3). According to the Scherrer equation [42], the increment of PLA d110/200 along stretching direction is due to i) the elongation of crystals and ii) the decrease of crystallite size induced by lamellae fragmentation. The stretch-induced fragmentation of PLA crystals is evidenced from the increase in the half-width of PLA110/200 diffraction [32]. The significant increase of PLA d110/200 in the meridional direction during stretching indicates that the lamellae fragmentation of PLA crystals is more remarkable in the blends with high fPLA (e.g., 0.3). Previous studies also demonstrate that the lamellae fragmentation can cause the cavitation of PLA [32,43] and other semicrystalline polymers [22]. We further investigate the effects of Td on the crystalline structural evolution of PBAT/PLA blend during stretching. PBAT also undergoes the α-to-β crystal transition in the PBAT/PLA-0.2 blend (Tc = 80 °C) during stretching at high Td of 80 °C, as demonstrated by the 1D-WAXD results (Fig. S3). However, the orientation 13

degrees of both PBAT and PLA crystals are influenced by Td. As shown in Fig. 4a, the orientation of PBAT crystal slows down with the increase of Td, because the high mobility of amorphous chains at high Td depresses the fragmentation of original lamellae [17]. However, the orientation of PLA crystals shows opposite variation trend with Td; it is facilitated when deformed at high Td. This might be contributed to i) the destruction or rotation of original crystal lamellae and ii) the strain-induced crystallization along stretching direction or the melt recrystallization of original crystal lamellae. As shown in Fig. 4b, the change of PLA lattice spacing slows down during stretching at high Td (80 °C), implying that the weaker stress is imposed on PLA crystals under these conditions. Therefore, the accelerated orientation of PLA crystals is mainly ascribed to the stress-induced formation of new crystals with c-axis along the stretching direction [31]. For the PBAT/PLA blend crystallized at 50 °C, PLA is not crystallized and remains in the amorphous phase. Only the orientation and polymorphic transition of PBAT crystals are observed during stretching (Fig. S4).

a

b

fPLA= 0.2 Tc = 80 °C

0.55 Meridian

Td=80 °C Td=25 °C

d110/200 (nm)

0.0 PLA110/200

-0.1

F Td=25 oC

-0.2

Td=80 oC

Equator

Td=80 °C Td=25 °C

0.54

PBAT100

0.53

-0.3 0.0

0.5

1.0

εH

1.5

2.0

0.0

0.4

0.8

εH

1.2

1.6

Fig. 4. WAXD results of PBAT/PLA-0.2 blend (Tc = 80 °C) measured during stretching at different Td’s: (a) Hermans’ orientation parameter, F, associated with the PBAT100 and PLA110/200 reflections; (b) variation of d110/200 of PLA crystals along and perpendicular to the stretching direction.

14

In-situ SAXS analysis was carried out to investigate the crystalline morphological evolution of PBAT/PLA blends with various fPLA’s under stretching. Fig. 5 shows the selected 2D-SAXS patterns of PBAT/PLA blends (Tc = 80 °C) collected during stretching at 25 °C. Only the scattering peak of PBAT is observed and that of PLA is absent in these SAXS patterns, because of the similar electron densities between the crystalline and amorphous phases of PLA at 25 °C. As shown in Fig. 5, the unstretched sample shows a scattering ring, in consistent with the scattering of lamellar stacks of PBAT. Notably, some unstretched samples show slight anisotropy, due to the residual local orientation introduced during sample preparation (compression molding).

Fig. 5. Selected 2D-SAXS patterns of PBAT/PLA blends (Tc = 80 °C) with various fPLA’s measured during stretching at 25 °C: (a) fPLA = 0.1; (b) fPLA = 0.2; (c) fPLA = 0.3.

As shown in Fig. 5a, the deformation mechanism of PBAT/PLA-0.1 blend is similar to that of neat PBAT, which undergoes the melt recrystallization during stretching [17]. The scattering ring becomes ellipse during elastic deformation (εH < 0.2) and then forms an oblate shape with the boundary closing to the beam stop after yielding, indicating the slip, rotation, and damage of original PBAT lamellae. The 15

oblate ellipsoid in SAXS pattern then transforms into a highly anisotropic two-bar pattern with stretching to εH > 1.26. However, the blends with higher fPLA (0.2, 0.3) have different structural features from those of PBAT and PBAT/PLA-0.1 blend during stretching. As shown in Fig. 5b, a strong meridional scattering streak across the beam stop appears for PBAT/PLA-0.2 blend in the low q range at εH = ~0.4 (indicated by red arrow in Fig. 5b). Such a scattering feature means the formation of an elongated heterogeneous structure (e.g., fibril or cavity) perpendicular to the stretching direction [11]. This meridional scattering streak intensifies upon further stretching, accompanied by the presence of a new streak in the equatorial direction to form a crossed pattern at large strain (εH ≥ 1.26). The equatorial streak stems from the scattering of cavities with their normal perpendicular to the stretching direction. The equatorial streak spreads to the high q range at large strain, showing a diamond-like pattern (εH = ~1.63), indicating the thinning of the parallel plate cavities; this behavior evidences the reorientation of cavities along the stretching direction during further stretching. As indicated by the in-situ SAXS results, PBAT/PLA-0.3 blend has the similar structural evolution as the PBAT/PLA-0.2 sample during stretching, except for the stronger cavity scattering of the former sample. In the case of PBAT/PLA-0.3 blend, the cavities emerge much early under stretching, demonstrating that the increase of fPLA promotes the stretch-induced cavitation. Fig. 6a-c shows the 1D-SAXS profiles (along the stretching direction) of PBAT/PLA blends (Tc = 80 °C) measured during stretching at 25 °C. Except for the evolution of scattering peaks of PBAT lamellae, we find the remarkable intensifying of scattering at low q ( < 0.5) (due to the cavity scattering) for the blends with high fPLA’s (0.2, 0.3). Long spacing (LP) of PBAT crystals was calculated by Bragg 16

equation (LP = 2π/qmax), where qmax corresponds to the peak position of 1D-SAXS profile. Fig. 6c shows the εH-dependent LPs of PBAT/PLA blends measured along the stretching direction. The variation trend of LP after yielding is in accordance with the melt recrystallization deformation mechanism of neat PBAT [17]. The addition of PLA influences little on the lamellae structural evolution of PBAT in their blends during stretching at 25 °C. LPs of PBAT/PLA blends with various fPLA’s (0−0.3) are very similar at the same εH, irrespective of the degree of cavitation.

b

a

Tc = 80 °C, Td = 25 °C

Tc = 80 °C, Td = 25 °C

Intensity (a.u.)

fPLA = 0.2

Intensity (a.u.)

fPLA = 0.1

εH

εH 0

0

1.68

1.79

0.2

0.4

0.6

q (nm−1)

0.2

0.8

d

c

Long period (nm)

Intensity (a.u.)

εH

30

0.6

0.8

q (nm−1)

35

fPLA = 0.3 Tc = 80 °C, Td = 25 °C

0.4

Tc = 80 °C, Td = 25 °C

Original lamellae

fPLA = 0.3 fPLA = 0.2 fPLA = 0.1 fPLA = 0

25 20

Newly-formed lamellae

15

0 1.63

0.2

0.4

0.6

−1

0.8

q (nm )

10

0.0

0.5

1.0

εH

1.5

2.0

Fig. 6. (a, b, c) 1D-SAXS profiles (integrated along the stretching direction) of PBAT/PLA blends (Tc = 80 °C) measured during stretching at 25 °C: (a) fPLA = 0.1; (b) fPLA = 0.2; (c) fPLA = 0.3. (d) Change of long period of PBAT during stretching.

17

We further investigate the effects of Tc and Td on the morphological evolution of PBAT/PLA blends during stretching. Fig. 7 shows the selected 2D-SAXS patterns of PBAT/PLA-0.2 blend collected during stretching at different Td’s. As indicated by the 2D-SAXS patterns of PBAT/PLA-0.2 blend crystallized at 50 °C (Fig. 7a), the scattering of cavities appears rather late after yielding and it is mainly concentrated in the equator; in the 1D-SAXS profiles (along the stretching direction) shown in Fig. S5a, the intensifying of scattering at low q is not obvious, which is different from that observed in the sample crystallized at 80 °C (Fig. 5, 6). This suggests that the amorphous state of PLA depresses the stretch-induced cavitation of PBAT/PLA-0.2 blend (Tc = 50 °C). As shown in Fig. 7b, when the PBAT/PLA blend is stretched at high Td (80 °C), the scattering rings are observed at low q for the unstretched sample, indicating an increase of LP with Td. No cavity scattering is observed during stretching at this Td (Fig. 7b, S5b). Notably, an equatorial scattering streak is observed in the blend at large strain (εH > 1.0), which is attributed to the scattering of PBAT fibril [17].

Fig. 7. Selected 2D-SAXS patterns of PBAT/PLA-0.2 blend collected during stretching: (a) Tc = 50 °C, Td = 25 °C; (b) Tc = 80 °C, Td = 80 °C . 3.3. Stretch-induced cavitation investigated by SAXS 18

In order to quantitatively evaluate the stretch-induced cavitation, we calculate the total intensity of cavities, Ic, in the whole range of scattering streak. The scattering intensity is calibrated from the sample thickness. The intensity measured before cavitation is used as the background for subtraction in the calculation of Ic. Because the electron density difference between cavity and polymer matrix is about two orders of magnitude larger than that between the crystalline and amorphous phases of polymer, the cavities govern the scattering intensity of cavitated sample in the low q region. Ic can reflect the degree of cavitation (i.e., number and size of cavities) in the stretched sample [25], the larger Ic means the stronger cavitation. Td=25 °C fPLA=0.3, Tc=80 °C

Ic (a.u.)

fPLA=0.2, Tc=80 °C fPLA=0.1, Tc=80 °C fPLA=0.2, Tc=50 °C

0.0

0.4

0.8

εH

1.2

1.6

Fig. 8. Integrated scattering intensity of cavities for PBAT/PLA blends during stretching.

Fig. 8 shows the scattering intensity of cavities of PBAT/PLA blends collected during stretching. Ic of PBAT/PLA blend (Tc = 80 °C) enhances significantly with the increase of fPLA at the same strain. Cavitation of PBAT/PLA-0.3 blend starts immediately once the external force is applied. However, the cavitation takes place late in the PBAT/PLA-0.2 and 0.1 blends, in which the critical εH’s of cavitation are 0.42 and 1.19, respectively. Additionally, the crystalline or amorphous state of PLA plays a critical role in the stretch-induced cavitation. Compared to the PBAT/PLA-0.2 19

blend crystallized at Tc = 80 °C (in which PLA is crystallized), Ic drops greatly as Tc is decreased to 50 °C (corresponding to the amorphous state of PLA). In order to compare the cavitation behavior of PBAT/PLA blends in different directions, we integrate the scattering intensities of cavities along and perpendicular to the stretching direction (IMR, IEQ); the subscripts MR and EQ denote the meridional and equatorial directions, respectively. Fig. 9a,b shows the SAXS intensities of cavity signals of PBAT/PLA blends collected in the different directions during stretching. For the PBAT/PLA-0.2 blend crystallized at Tc = 80 °C, IMR is larger than IEQ at low

εH (< 1.52) but is surpassed by IEQ at large εH (>1.52). Continuous increase of IMR during stretching indicates that the growth and reorientation of existing cavities are accompanied by the formation of new cavities with their normal parallel to the stretching direction. Similar results are also observed for the PBAT/PLA-0.1 and PBAT/PLA-0.3 blends (Fig. S6). However, IEQ is always larger than IMR for the PBAT/PLA-0.2 blend crystallized at 50 °C (Fig. 9b), implying that most of the cavities elongate along the stretching direction after formation.

a

b

fPLA= 0.2

Tc = 50 °C, Td = 25 °C

Intensity (a.u.)

Intensity (a.u.)

Tc = 80 °C, Td = 25 °C

0.4

IMR IEQ

0.8

fPLA= 0.2

1.2

εH

IMR IEQ

1.2

1.6

20

1.4

εH

1.6

1.8

c

2.0

Td = 25 °C

IMR/IEQ

1.5

1.0 fPLA=0.3, Tc=80°C fPLA=0.2, Tc=80°C

0.5

0.0

fPLA=0.1, Tc=80°C fPLA=0.2, Tc=50°C

0.0

0.4

0.8

εH

1.2

1.6

2.0

Fig. 9. SAXS intensities of cavity signals of PBAT/PLA blends collected during stretching: (a) cavity intensity of PBAT/PLA-0.2 blend (Tc = 80 °C, Td = 25 °C) in the meridian and equator directions; (b) cavity intensity of PBAT/PLA-0.2 blend (Tc = 50 °C, Td = 25 °C in the meridian and equator directions; (c) strain-dependent IMR/IEQ values of PBAT/PLA blends.

IMR/IEQ is an indicator to reflect the average orientation of cavities during stretching [32]. Fig. 9c illustrates the change of IMR/IEQ of PBAT/PLA blends during stretching. The IMR/IEQ value of PBAT/PLA-0.3 blend increases at small strain and reaches maximum at εH = ~0.5, followed by a decrease under further stretching. The peak of IMR/IEQ curve denotes the onset strain of cavity reorientation and the εH at IMR/IEQ = 1 represents the critical transition strain where most cavities have reoriented toward the stretching direction. For the PBAT/PLA blends crystallized at 80 °C, the IMR/IEQ value shows a similar variation trend with strain but εH (IMR/IEQ = 1) shifts toward smaller εH as fPLA increases, indicating the easier cavitation of the blends with high fPLA (e.g., 0.3). The maximum value of IMR/IEQ is small for the blend with low fPLA (e.g., 0.1), because the highly oriented chains at large strain retard the formation of cavities with their normal parallel to the stretching direction. Notably, the IMR/IEQ values of PBAT/PLA blend are always smaller than 1 as Tc is decreased to 50 °C, 21

indicating that the cavities are dominantly extended to the stretching direction after formation and there is no obvious reorientation.

3.4. Stretch-induced cavitation investigated by SEM Morphology of stretch-induced cavities was also analyzed by SEM in the micrometer scale. Fig. 10a shows the SEM images of PBAT/PLA-0.3 blend (Tc = 80 °C) stretched at 25 °C to different strains. Due to the different resolutions, SEM results reflect the morphology of cavities in micrometer scale, which are in two orders of magnitude larger than that detected by SAXS (in nanometer scale). Therefore, the cavity sizes detected by SEM and SAXS cannot be directly compared in our study. PLA is shown as the heterogeneously dispersed phase (size: 0.1−1.0 µm) in PBAT matrix in the SEM images of unstretched sample (image a1). Cavities are observed with stretching PBAT/PLA blend beyond the yield point (εH > 0.2). PLA domains are stress concentrators and suffer larger local strain in the PBAT/PLA blend; thus, the initiation of cavities is preferred at the PBAT/PLA phase interface (indicated by white arrow in image a2). Because of the interfacial adhesion between PBAT and PLA, some fibrils are connected around the cavities at the interface (indicated by the red arrow in image a2 of Fig. 10, Fig. S7). The cavities turn into large crazes and extend along the stretching direction with the fracture of fibrils. The formation of large crazes results in the final fracture of specimen during further stretching, accounting for the decrease of elongation-at-break for the blends with high fPLA (e.g., 0.3). Except for the cavitation, no significant orientation of PLA domains is seen and PLA domains are kept as spherical during stretching.

22

Fig. 10. SEM micrographs of PBAT/PLA blends measured before and after stretching.

Fig. 10b shows the SEM images of various PBAT/PLA blends unstretched (images b1, c1, d1) and stretched to different εH’s (0.92−1.61, images b2, c2, d2, e1, e2, f1, f2). SEM image of stretched PBAT (image b2) does not show any cavity but exhibit the microfibrils oriented along the stretching direction. Also, the formation of cavity is not obvious in the blends with low fPLA (fPLA = 0.1, images c2). Unidirectional cavities are seen in the PBAT/PLA-0.2 blend (Tc = 80 °C) along the stretching direction after stretching at Td = 25 °C (image d2), similar as the PBAT/PLA-0.3 blend. Except for fPLA, the cavitation of PBAT/PLA blend is also influenced by Tc and Td. In the case of PBAT/PLA-0.2 blend, the cavitation is absent with lowering Tc to 50 °C (image e1, e2). However, PLA domains orientate along the stretching direction in the PBAT/PLA-0.2 blend (Tc = 50 °C) after yielding; they orientate to ellipsoidal with stretching to εH = 0.92 (image e1) and further rod-like with stretching to εH = 1.61 (image e2). Similar morphological features are also seen for the PBAT/PLA-0.2 23

blend (Tc = 80 °C) stretched at high Td (i.e., 80 °C). At Td = 80 °C, the PBAT/PLA blend does not undergo cavitation and the PLA domains orientate to ellipsoidal or rod-like shape under stretching (images f1, f2). We consider that the orientation of PLA domains competes with cavitation during stretching. PLA domains are more flexible and can be easily orientated when they are amorphous (Tc < Tg,PLA) or the blend is stretched at high Td (Td > Tg,PLA). The ease of orientation of PLA domains depresses the cavitation in PBAT/PLA interfaces under stretching. 3.5. Schematic illustration of stretch-induced structural evolution According to the above-mentioned results, we explain the stretch-induced structural evolution of PBAT/PLA blend with different Tc’s and Td’s. Previous study has demonstrated that the crystals of soft PBAT matrix undergo melt recrystallization and α-to-β crystal transition during stretching [17]. Here we focus on the structural transition of PLA domains and the cavitation of PBAT/PLA blends during stretching. The stretch-induced shifting and broadening of PLA diffraction peaks (Fig. 3) verify that the cavitation occurs in PLA domains. Therefore, we conclude that two types of cavities are formed in the stretched PBAT/PLA blends. One type is generated at the PBAT/PLA interface; the other is formed in PLA domains. Cavitation of PLA domains is caused by the fragmentation of crystal lamellae, which is in the nanometer scale and beyond the resolution of SEM.

24

Fig. 11. Schematic representation of multilevel structural evolutions of PBAT/PLA blends with different Tc’s during stretching at various Td’s: (a) PBAT/PLA-0.1 blend, Tc = 80 °C, Td = 25 °C; (b) PBAT/PLA-0.2 blend, Tc = 80 °C, Td = 25 °C; (c) PBAT/PLA-0.2 blend, Tc = 50 °C, Td = 25 °C; (d) PBAT/PLA-0.2 blend, Tc = 80 °C, Td = 80 °C.

Fig. 11 illustrates the multilevel structural evolution of PBAT/PLA blends with various Tc’s during stretching at different Td’s. As illustrated in Fig. 11a, in the case of PBAT/PLA blends with low content of crystallized PLA domains (fPLA = 0.1, Tc = 80 25

°C), the dispersed PLA domains are rigid and have low deformation ability during stretching at Td = 25 °C. Due to the low content of dispersed PLA phase, the soft PBAT phase is preferentially deformed and the local stress concentrated at PLA domains is lower in PBAT/PLA-0.1 blend during stretching. Therefore, no micrometer-scaled cavity is generated around the PLA domains during stretching, as shown in the SEM results. However, SAXS and WAXD results indicate that the nanometer-scaled cavities are still formed in the PLA lamellar stacks during stretching, similar as the cavitation of semicrystalline homopolymers (image a2) [11]. As illustrated in Fig. 11b (image b1, b2), for the blends with high fPLA (e.g., PBAT/PLA-0.2, 0.3 blends), the coupling between PLA domains is strengthened and thus forms the stress-bearing network in the blend, leading to the more intensified stress concentration around the PBAT/PLA interfaces. Therefore, the cavities are initiated and propagated by interfacial debonding during stretching. Additionally, the cavitation also takes place in the PLA domains. As illustrated in Fig. 11c (image c1, c2), for the blends with amorphous PLA domains (Tc = 50 °C), the better deformation ability of amorphous PLA domains facilitates the dissipation of energy under stretching. Besides, the microphase separation of polymer blends can be promoted by the crystallization of polymer, in which the crystallization of one polymer expels the amorphous chains of another polymer to the other domains [39-40]. Therefore, PBAT and PLA would have better compatibility in the blends crystallized at Tc = 50 °C than that crystallized at Tc = 80 °C, due to the crystallization of PLA in the latter case. For the blends with amorphous PLA domains, the absence of PLA crystallization would improve the interfacial adhesion between PLA and PBAT phases. Therefore, the synergistic effects of enhanced deformation ability of PLA domains and the better interfacial adhesion 26

would prohibit the cavitation in the PBAT/PLA interphase and PLA domains. However, the cavities can be initiated in the elongated amorphous PLA domains and orientates along the stretching direction at large strain (Fig. 12, c2). As illustrated in Fig. 11d (image d1, d2), for the blends with crystallized PLA domains (Tc = 80 °C) and stretched above Tg,PLA (e.g., Td = 80 °C), PLA amorphous phase is in the rubbery state and possesses high chain mobility. PLA has better deformation ability to accommodate the stress in both the domains and crystal lamellae, restraining the cavitation of PBAT/PLA blend at high Td. Moreover, the strain-induced crystallization and the rotation of original lamellae of PLA take place during stretching at high Td. The newly-formed PLA crystals (indicated by the red line in image d2) with their c-axis along the stretching direction can induce the accelerated orientation.

4. Conclusions In summary, we have elucidated the changes of polymorphic crystalline structure, crystal orientation, lamellar morphology, and the cavitation behavior of PBAT-rich PBAT/PLA blends under stretching deformation. Effects of fPLA, Tc, and Td on the multilevel structural evolutions of PBAT-rich PBAT/PLA blends are clarified. PBAT/PLA blends are immiscible and phase-separated; PLA is shown as the dispersed phase in the PBAT-rich PBAT/PLA blends. The blends have much improved strength and modulus compared to neat PBAT. PBAT lamellae experience the melt-recrystallization evolution and the α-to-β crystal transition during stretching in the PBAT/PLA blends, regardless of the incorporation of PLA. However, the orientation of PLA crystals is much slower than that of PBAT crystals during stretching. Cavities are generated in the PBAT/PLA blends upon stretching, which are mainly initiated from the PBAT/PLA interface (because of the interfacial debonding) 27

and the PLA phase (because of the crystalline fragmentation). Stretch-induced cavitation of PBAT/PLA blends is governed by fPLA, Tc, and Td, which is preferred in the PBAT/PLA blends with high fPLA but is depressed with the decrease of Tc or the increase of Td. This study would advance our understanding on the multilevel structural transitions of polymer blends under the stretching deformation and is helpful for tailoring the microstructures and physical properties of polymer blends in industrial processing.

Acknowledgements This work was financially supported by the National Key Research and Development Program of China (2016YFB0302400). WAXD and SAXS were measured on the beamline BL16B1 of SSRF, China.

Appendix A. Supplementary data Supplementary data associated with this article can be found in the on-line version.

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Highlights PBAT/PLA blend has higher strength and modulus than neat PBAT. Stretch-induced multilevel structural evolutions of PBAT/PLA blend are influenced by the blend composition, crystallization, and stretching temperatures. PLA undergoes crystal orientation and fragmentation in the PBAT/PLA blend during stretching. Cavities are mainly formed at the PBAT/PLA interface upon stretching the PBAT/PLA blend.

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:

None