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Renewable and Super-Toughened Polylactide-based Composites: Morphology, Interfacial Compatibilization and Toughening Mechanism Xiaoran Hu, Yan Li, Manqiang Li, Hailan Kang, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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Industrial & Engineering Chemistry Research

Renewable and Super-Toughened Polylactide-based Composites: Morphology, Interfacial Compatibilization and Toughening Mechanism Xiaoran Hu,a,b Yan Li,a,b Manqiang Li,a,b Hailan Kang,*,d and Liqun Zhang*,a,c a

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beijing 100029, P. R. China b

Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer

Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China c

Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology,

Beijing 100029, P. R. China d

College of Materials Science and Engineering, Shenyang University of Chemical

Technology, Shenyang, 110142, China

1 ACS Paragon Plus Environment


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KEYWORDS: toughening, compatibility, polylactide

ABSTRACT: Composites of poly(lactic acid) (PLA) and a series of renewable and biobased copolyesters (PLBSIs) were manufactured by melt blending to toughen PLA. Benefiting from reasonable macromolecular design, the introduction of lactic acid in PLBSIs not only increases the compatibility with PLA but also helps PLBSIs turn from crystalline plastic to amorphous elastomer. The increasing compatibility was proved by the decreasing particle size of PLBSIs in PLA, and the better toughening effect of elastomer than that of plastic was characterized by the thermal tensile test. Thus, the super toughened PLA composites were obtained and exhibited maximum elongation at break of 324 % and the impact strength of 35.7 kJ/m2, 50 and 15 times higher than neat PLA, respectively. Excellent performances in 3D-printed tensile tests imply the toughened PLA was ideal 3D-printing ink. The super toughened PLA will eliminate the brittleness of PLA for wide application and possess great potential for industrial and engineering fields.


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1. Introduction Bio-based polymers have drawn many interests for their advantage to lessen the utilization of petrochemical resources and eliminate environmental issues1-4. Poly (lactic acid) (PLA), a biodegradable polyester with completely renewable origins on commercial scale5,6, has been accepted as reliable materials to replace petroleum based plastics in several fields such as surgical suture and drug delivery system7. However, the inherently brittleness of PLA, evidenced by its poor impact strength and tensile toughness, significantly restricts its wide applications

8,9

. Therefore, toughening PLA had drawn a great deal of research interests over

recent years, and plenty of efforts were developed, such as plasticization, copolymerization, and physical blending10-13. Blending PLA with suitable polymers is a hopeful way to improve toughness of PLA because it is more cost-effective and facile compared to chemical modification. For the past decades, composites of PLA and low-molecular weight plasticizers, inorganic fillers and flexible polymers or elastomers have been widely reported14-18. However, most polymers in these researches presented poor compatibility with PLA, leading to unsatisfactory toughening effects. Nowadays, it is widely accepted that the compatibility between the components is key factor to determine the property of composites 19. Thus, addition of compatibilizers, reactive compatibilization and dynamical vulcanization have been developed to prepare compatible toughened PLA composites. Shibata et al. 20 use poly (butylene succinate) (PBS) as toughener for PLA toughening. By introducing lactide as compatibilizer into PBS, the poly (butylene succinate-co-L-lactate) (PBSL) helps the composites obtain enhancement in elongation at break by 1.5 times higher than PBS. Odent and colleagues

21

used ε-caprolactone and D,L-

lactide to synthesize amorphous copolyesters to toughen PLA. The presence of lactate units in the copolyesters allowed for an improved compatibility with the PLA matrix. Moreover, composites with PLA induced enhanced impact strength by 5 times higher than PLA. Zhang’s team

22

prepared compatible ternary composites of PLA, ethylenemethyl acrylate-glycidyl 3 ACS Paragon Plus Environment


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methacrylate (EMA-GMA), and poly(ether-b-amide) copolymer (PEBA) via reactive compatibilization. The ternary composites show 4 and 8 times higher in impact strength than PLA/PEBA and PLA/EMA-GMA binary composites, which both are prepared by physical melt blending, respectively. Liu et.al51 prepared unsaturated polyester elastomer (UPE)/PLA thermoplastic vulcanizate (TPV) by dynamical vulcanization to achieve highly toughened PLA. The UPE/PLA TPVs showed improved impact strength of 586.6 J/m compared to that of 16.8 J/m for neat PLA. Liu’s team

52

obtained ternary blend system by simultaneous

dynamic vulcanization and extrusion of PLA, an epoxy-containing elastomer, and a zinc ionomer. The ternary blends show maximum impact strength of 860 J/m, which is 35 times than that of neat PLA. These studies demonstrated the compatibility between PLA and other components dramatically contributed to excellent toughness and should be taken into consideration in toughening PLA as a dominate factor. Origin from renewable resources is the most fascinating characteristic among numerous advantages of PLA. Thus, toughening PLA without weakening its renewable origins is key issue that needs to be taken into consideration. Besides, toughening PLA through blending with elastomer is often described as an effective solution to the brittleness of PLA

23-26

.

Considering the effective toughening results of elastomers and the rapid development of biobased elastomers, a recent trend is to adopt degradable and renewable elastomers to toughen PLA. Lebarbé et.al27 achieved 420% improvement in impact strength compared to neat PLA by synthetic renewable polyesters elastomer consisting of sebacic acid, 1,10- decanediol, and hydrogenated dimer fatty acid. Kang and colleagues 28 improved impact strength by 5.5 times than PLA through blending PLA with bio-based elastomers synthesized from biomass diols and diacids. Although many researchers focused on toughening PLA with elastomers derived from renewable resources, unfortunately, most elastomers still show poor compatibility with PLA and resulted in unsatisfactory toughening effects. Thus, the challenge is to design new


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renewable and degradable elastomers with high compatibility to toughen PLA and keep the completely renewable origins. Recently, we prepared copolyesters by copolymerizing biobased monomers on commercial scale including lactic acid, sebacic acid, itaconic acid and butanediol29. Noteworthily, the introduction of lactic acid into these copolyesters provides them similar lactate structure to PLA, implying good compatibility with PLA

20,21,28

. Besides, with increasing content of

lactate into macromolecular chains, these copolyesters turn from crystalline plastic to amorphous elastomer, which ensures good toughen effect due to better toughening effect of elastomer than plastic27,30-32 . Thus, in the present study, we focused on highly toughening PLA by blending PLA with PLBSI copolyesters. The compatibility, thermal and crystalline behaviors, rheological properties, mechanical properties, morphology and toughen mechanism will be extensively investigated. Finally, the toughened PLA were used as 3D-printing ink and indicated excellent performances.

2. Experimental Section 2.1 Materials. Lactic acid (LA), sebacic acid (SeA), itaconic acid (IA), 1, 4-butanediol (BDO), and tetrabutyltitanate (TBT) were supplied by Alfa Aesar. Chloroform and methanol were procured from Beijing Chemical Works. PLA (2003D) was purchased from NatureWorks (USA). The weight-average molecular weight, polydispersity index and density of PLA is 159,000 g/mol, 1.67 and 1.24 g/cm3, respectively. 2.2 Synthesis of PLBSI. Poly(lactate/butanediol/sebacate/itaconate) bioelastomers (PLBSI) containing different content of lactic acid were prepared according our previous work 29. The molecular structure of PLBSI is presented in Figure 1. The thermal and molecular parameters, 1

H-NMR spectra and TGA trace of PLBSI were listed in supporting information. In brief,

calculated LA, BDO, SeA, IA and tri-(4-hydroxy-TEMPO) phosphate were added into a 100ml-three-neck flask. The chemicals were heating to 130℃ for 1 h and then 180℃ for 2 h 5 ACS Paragon Plus Environment


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under mechanical stir and nitrogen atmosphere. Next, the copolymerization was carried out at 220℃ for 8 hours with reduced pressure (less than 300 Pa) and TBT (0.1wt% to all reactants) as the catalyst. The PLBSIs were dissolved in chloroform and precipitated in excess methanol to remove impurities. Then, the PLBSIs were collected by filtration, extensively washed with methanol, and dried in vacuum at 60 °C. The copolyester containing 40 mol% of lactate relative to all chemical amounts was named as PLBSI-40.

Figure 1 Chemical structure of PLBSI 2.3 Preparation of PLBSI/PLA composites. PLA and PLBSIs were dried in vacuum at 60 ℃ for 12 h before use. According to our previous study29 and related test results in Figure S9, the PLBSI/PLA composites were fabricated through melt blending PLBSIs with PLA at 15wt% for 10 min and at 170℃ in a Haake Remix. Then, the samples were hot-pressed at 190℃to prepare 1-mm thick sheets. 2.4 Characterization and Measurements. Dynamic mechanical thermal analysis (DMTA) was employed with a V Dynamic Mechanical Thermal Analyzer (Rheometric Scientific Co.). The tension mode at 1 Hz and 3℃/min from -100 to 150 ℃ was used. Differential scanning calorimetry (DSC) test was carried out with a Mettler-Toledo DSC instrument under nitrogen. Samples were heating to 200℃ at 10℃/min and kept isothermal for 5 min. Then they were cooling to -100 ℃ at 10 ℃ /min and reheating to 200 ℃ at 10 ℃ /min.

Isothermal

crystallization of PLBSI/PLA composites were carried out with DSC by pre-melting samples at 200 ℃, then cooling to -100 ℃ and reheating to 125 ℃ at 50 ℃/min. The samples were


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fixed at 125 ℃for cold crystallization. The relative crystallinity (Xt) is calculated by Equation (1):

=

(/ )

(dH/dt)dt

(1)

Where the numerator represents heat produced at time t, and the denominator represents the total heat produced during the isothermal crystallization. Isothermal crystallization kinetics was investigated by Avrami equation 33:

1 − = exp (− )

(2)

where Xt represents relative crystallinity at time t, n is the Avrami exponent, and K is an Avrami parameter. Furthermore, taking logarithm of Equation 1twice:

log− ln(1 − ) = logK + nlogt

(3)

Where log k is the intercept and n is the slope of Avrami plot of fitted line from the experimental data. The spherulitic morphology were investigated by an optical microscope (POM) (Olympus BX51) equipped with a temperature controller (Linkam THMS 600). Rheological properties were measured by HAAKE Rheometer (RS6000) with 25-mm-plateplate arrangement. The measurements were carried out at 170 ℃ and 5% of strain rate from 0.01to100 rad/s. Stress-strain curves were obtained using a CMT 4104 Electrical Tensile Instrument (Shenzhen SANS Test Machine Co., Ltd., China) equipped with a 500-N load cell according to GBT 16421-1996. The notched Izod impact strength was obtained using Ceast, Resil Impactor machine according to GB/T 1843-2008. Morphology was investigated by a scanning electron microscope (SEM) (S4800, Hitachi Co., Japan) and transmission electron microscopy (TEM) (H-800-1 Hitachi Co., Japan).



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3. Results and Discussions 3.1 Compatibility of PLA and PLBSI/PLA composites The compatibility of components affects the morphology and interfacial interaction of composites, which may influent the mechanical property of composites. Besides, the compatibility is also the dominant factor to determine the dispersion particle size in composites. The excellent compatibility between components in a binary composites leads to uniformly dispersion of dispersed phase with small particle size and narrow particle size distribution. Finally, the compatibility determines how close two Tgs will appear in binary composites. The dispersion particle size in PLBSI/PLA composites was firstly used to evaluate the compatibility between PLBSIs and PLA. Some models were employed to calculate particle size of binary polymer composites. Among these models, the critical breakup law proposed by Wu 33 has been widely used in polymer composites system. Thus, we use the critical breakup law and our previous study34 to simulate the deformation and breakup of the PLBSI in PLBSL/PLA composites and calculated the minimum size of PLBSI dispersed phase. According to the critical breakup law, the PLBSI particles was firstly stretched to form as ellipsoid with the major semi axis L and short semi axis B (Figure S1 supporting information). Then, the PLBSI continues to breakup to reach the minimum critical diameter and deforms to be an ellipsoid

33

. The minimum diameter (αmin) of the PLBSI phase in the PLBSI/PLA

composites can be estimated. The detail calculation was presented in supporting information, and the calculated particle size is shown in Table 1. Similar to our expectation, the theoretical calculations predicted gradually decreased αmin from 250~700 nm to 50~400 nm for PLBSI-0 to PLBSI-40. The decreased αmin indicated improved compatibility between lactate-rich PLBSIs and PLA and is due to the similar lactate structure of them.


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Table 1 Critical particle size of PLBSI/PLA composites Composites Critical particle size (nm) PLBSI-0/PLA 253~700 PLBSI-20/PLA 176~659 PLBSI-40/PLA 50~399

SEM was employed to investigate the actual size of PLBSI particles. In Figure 2, where the PLBSI particles are observed as the dark phase in TEM micrographs, although a clear phaseseparated morphology was found in all composites, the uniform micron dispersion as spheres of the PLBSI particles in PLA matrix reveals good compatibility of PLBSI/PLA composites. The number-average particle diameter decreases from 1000 nm in PLBSI-0/PLA composites to 450 nm in PLBSI-40/PLA composites. The observed variation tendency of particle size of PLBSIs in PLA matrix is in accordance with the decreased αmin from PLBSI-0 to PLBSI-40 from our calculated results and also implied improved compatibility between lactate-rich PLBSIs and PLA matrix. Besides, the actual diameter for PLBSIs in the PLBSI/PLA composites is always larger than the calculated ones. The possible reason for the discrepancy may due to the coalescence of PLBSI particles after the rotational shear has stopped.



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Figure 2 SEM cryo-fractured surface micrographs (a1, b1 and c1), TEM micrographs (a2,b2 and c2) of PLBSI/PLA composites and particle diameter distribution (a3,b3 and c3) of PLBSI/PLA composites

DMTA was also used to measure the compatibility of the PLBSI/PLA composites. Figure 3 (a) presents Tanδ curves of the neat PLA and PLBSI/PLA composites. In Figure 3 (a), a sharp tanδ peak of neat PLA was found at 68℃, indicating its glass transition. For PLBSI/PLA composites, tan δ curves revealed two Tgs, the higher Tg corresponded to PLA, and the lower Tg corresponded to PLBSIs. It is widely accepted that when two polymers are miscible in the amorphous phase they will show one Tg, while the appearance of two Tgs corresponding to immiscible of each individual component. Thus, the two Tgs for all the composites indicate that PLA and PLBSIs are immiscible. However, these two Tgs shift slightly towards each other with increasing lactic acid content in PLBSIs, implying enhanced compatibility between PLA and lactate-rich PLBSIs, which is in agreement with our DSC 10 ACS Paragon Plus Environment

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data in section 3.2. This is probably caused by interfacial interaction between PLA and PLBSIs due to similar lactate structure and ester groups of them. Figure 3 (b) presents the storage modulus (E′) of neat PLA and the composites. The E′of neat PLA decreased suddenly around 65℃ for the glass transition, and increased from 95℃ for its cold crystallization. Besides, the cold crystallization temperature of composites decreased, indicating the addition of PLBSIs improved the cold-crystallization ability.

Figure 3 Dynamic viscoelastic curves of PLBSI/PLA composites: (a) tan δ versus temperature; (b) storage modulus versus temperature

Both the calculated and observed diameter of PLBSI particles and DMTA data demonstrate improved compatibility between lactate-rich PLBSIs and PLA matrix, which is due to the similar lactate structure of them. The good compatibility ensures good toughing effect of PLBSI/PLA composites, which will further discussed in the following analysis. 3.2 Thermal and crystalline behavior of PLA and PLBSI/PLA composites Figure 4 presents DSC curves of neat PLA and the composites, with relevant parameters in Table S3 (supporting information). No crystalline peaks were found in the cooling curves for all samples. The WAXD patterns (Figure S4) also confirmed neat PLA and the composites were amorphous at 25℃. Neat PLA show only one glass transition at about 61℃. However, 11 ACS Paragon Plus Environment


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PLBSI/PLA composites present two obvious Tg, demonstrating PLA and PLBSIs were phaseseparated during cooling. The two Tgs of PLBSI and PLA shift towards each other with increasing lactic acid content in PLBSIs, an indication of better compatibility between lactaterich PLBSIs and PLA, which is in accordance to DMTA data. Besides, clear enhancements in cold crystallization were found. Both the cold crystalline enthalpy (ΔHcc) and the melting enthalpy (ΔHm) are enhanced in composites, indicating the PLBSIs improved cold crystallinity of PLA. The closer Tg of PLBSIs and PLA and the enhanced cold crystallinity are due to increasing movability of PLA segments by incorporating of flexible PLBSI macromolecules.

Figure 4 DSC thermograms of PLBSI/PLA composites: (a) heating; (b) cooling.

Isothermal cold crystallization of PLA and the composites was also measured via DSC. Isothermal crystallization thermograms and the relative crystallinity (Xt) along time (t) of cold crystallization of PLA and the composites at 125℃ were presented in Figure S5 and S6 (supporting information), respectively. The crystalline time of composites obviously shortened compared with that of neat PLA. This result demonstrates the PLBSI increase crystalline rate of PLA, in accordance to increasing Tcc obtained from the nonisothermal crystallization process. The isothermal crystalline kinetics was further investigated through 12 ACS Paragon Plus Environment

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using the classical Avrami equation. The curves of equation (3) of PLA and the composites were presented in Figure 5, where all curves exhibited good linearity, indicating the isothermal crystalline kinetics fit well with the Avrami equation

35,36

. In Table S4, the

crystalline half-time (t1/2), obviously decreases with incorporation of PLBSI, and the values of Avrami parameter K of composites were higher than that of PLA. This phenomenon implies the PLBSI particles have promoted the crystalline process of PLA, and the promotion of crystallization might be attributed to the increasing nucleation density and crystallization growth rate of PLA in these composites. Besides, the Avrami exponent n slightly varies from 2.23 to 2.31 of all the samples, implying a growth of three-dimensional-truncated spherical crystal with thermal nucleation mechanism 37.

Figure 5 Avrami plot - Effect of the on isothermal crystallization (125 °C) of PLBSI/PLA composites

Then, spherical crystal morphology of PLA and the composites was investigated by POM. In Figure 6, the images shows the spherical crystal of PLA in the composites (Figure 6(b) to (d)) is smaller than that of neat PLA (Figure 6(a)), indicating higher nucleation density of PLA in the composites than that of neat PLA. The growth rate (G) of spherical crystal of neat PLA and the composites were studied by recording the change in radii of spherical crystal 13 ACS Paragon Plus Environment


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along time and are summarized in Table S4. Neat PLA has the slowest G value of spherical crystal, and the G of spherical crystal of PLBSI/PLA composites is increasing with increasing lactic acid content in PLBSIs. Therefore, PLBSIs play both as nucleating accelerant to increase the nucleation density and as growth accelerant to increases growth rate of spherical crystal of PLA. Commonly, to an immiscible composites, an interface may generate extra nucleating sites, which may contribute to heterogeneous nucleation

38,39

. The promotion of

crystallization of matrix by immiscible elastomer for the nucleating acceleration of the matrix/elastomer interface was also reported

40-42

. As for the immiscible PLA and PLBSIs,

some interactions may occur in the interface. Thus, the interface between PLA and PLBSI may generate beneficial nucleation sites to promote crystallization, which can further accelerate growth rate of spherical crystal

Figure 6 The spherulites of (a) neat PLA, (b) PLBSI-0/PLA composites, (c) PLBSI-20/PLA composites and (d) PLBSI-40/PLA composites


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3.3 Rheological Properties of PLA and PLBSI/PLA composites Figure 7 (a) (b) (c) presented the complex viscosity (η*), storage modulus (G’), and loss modulus

(G’’)

of

neat

PLA

and

PLBSI

/PLA

composites

versus

frequency

at170℃,respectively. The η* of all materials obey near Newtonian phenomenon and shear thinning phenomenon at low and high frequency, respectively, as evidenced by decreasing viscosity with increasing frequency, which is characteristic behavior of pseudoplastic polymer composites. Besides, a more evident shear thinning phenomenon is observed for the composites than that of PLA. Such phenomenon would be related to higher viscosity of the elastic PLBSI compared to PLA. In Figure 7 (b), the G’ of all PLBSI/PLA composites are higher than that of PLA, and increased at low frequency upon increasing lactic acid content in PLBSIs. As G’ refers to elasticity of material, the higher G’ of composites may refer to reinforcement of elasticity of composites, and may be ascribed to a form relaxation process of the PLBSI with shear deformation 43. Figure 7 (c) displays the relationship between G’’ and frequency of neat PLA and PLBSI/PLA composites. Obviously, the composites have higher G’’ than that of neat PLA during the whole frequency range. Because the G’’ represents the dissipated energy the higher G’’ can be attributed to more dissipated energy absorbed by the uniform dispersed PLBSI particles in the PLA matrix.



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Figure 7 Rheological properties of PLBSL/PLA composites: (a) complex viscosity (η﹡); (b) storage modulus (G’); (c) Loss modulus( G’’)

3.4 Tensile and impact Properties of PLA and PLBSI/PLA composites PLBSIs including crystalline plastic (PLBSI-0, PLBSI-20) and amorphous elastomers (PLBSI-40) were employed to toughen PLA. The toughness of PLBSI/PLA composites was investigated by tensile and impact tests. As shown in Figure 8 (a), incorporation of PLBSIs evident alters tensile behavior of PLA. The neat PLA is a brittle polymer with poor ductility evidenced by the elongation at break only around 6%, and deformed along representative brittle behavior without yielding. Compared to neat PLA, PLBSI/PLA composites present obvious yield and cold draw behavior, demonstrating a brittle-ductile transformation of PLA with addition of the PLBSI. 16 ACS Paragon Plus Environment

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Figure 8 (a) Stress-Strain curve and (b) impact strength of PLA blending with different PLBSIs

Noteworthily, the tensile strength and the elongation at break increase with increasing lactate content in PLBSIs. Indeed, increasing lactate content in PLBSIs bring them better compatibility with PLA and lead to better toughening effect. However, according to many researchers, amorphous elastomer usually achieve better toughen effect than crystalline plastic. Thus, tensile test for PLBSI-20 at 25℃ and Tm+10℃ (40℃) was used to verify if decreased crystallinity in PLBSIs also helps to achieve better toughen effect. In Figure 9, with elimination of crystallization at 40℃, rubbery PLBSI-20 helps the composites achieve 160% in elongation at break, while at 25℃, plastic PLBSI-20 only helps the composites achieve 90% in elongation at break. This obvious result illustrates better toughening effect of elastomers than that of plastics, as we expected.

Figure 9 Tensile tests for PLBSI-20/PLA at different temperature 17 ACS Paragon Plus Environment


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Measurement of the notched impact strength provides absorption of energy under highloading- fracture, which represents a more accurate and useful method. Notched impact strength tests results are presented in Figure 8 (b). Evident relationship of lactate content in PLBSIs with impact strength is found. In the case of PLBSI-0, very limited reinforcement in impact strength was found. However, increasing lactate content in PLBSIs led to a significant enhancement of impact strength. The PLBSI-40/PLA composites seems an optimal composition, inasmuch as this composites reaches the maximum impact strength of 35.3 kJ/m2, which is 15 times higher than that of neat PLA. According to the mechanical properties of PLBSI/PLA composites, it is obvious that both the increasing compatibility and turning to elastomer of PLBSIs by the introduction of lactic acid into PLBSIs succeed to achieve superior toughness.

3.5 Morphology and Toughening Mechanism The phase morphology of PLBSI/PLA composites was investigated to explore toughening mechanism between the morphology and resulting performance. The toughening efficiency of polymer composites gravely depends rest with interfacial adhesion between dispersed phase and matrix30,44. Surface tension between PLBSIs and PLA were firstly calculated and presented in Table 2. Obvious increasing surface tension between PLA and lactate-rich PLBSIs is found and indicate that high interfacial adhesion exist between PLA and lactaterich PLBSIs. Besides, as we previously mentioned, the compatibility between PLBSIs and PLA matrix increases and the crystallinity decrease with increasing lactic acid content in PLBSIs. Consequently, the higher compatibility between the lactate-rich PLBSI and PLA and elastic properties of lactate-rich PLBSI are thought to possess high toughening efficiency for their enhanced interfacial adhesion between PLA and PLBSI.


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Table 2 Contact angle and surface tension of PLA and PLBSIs γs 50.6 44.6 39.1 36.8

Water Diidomethane PLA 64.25±2.21 48.26±1.62 PLBSI-0 86.22±1.9 37.96±1.56 PLBSI-20 96.32±1.6 44.25±1.22 PLBSI-40 102.3±1.5 47.96±1.28 a Polar component of surface tension b Dispersive component of surface tension c Surface tension between PLA and PLBSIs

a sp

γ 33.4 6.2 2.7 0.2

b sd

γ 17.2 38.4 36.4 36.7

c 12

γ 26.9 33.0 40.1

In order to investigate the toughening mechanisms, tensile-fractured and impact-fractured surfaces were observed using SEM. In Figure 10 (a1) to (a3), tensile-fractured surface of neat PLA exhibits mirror-like smooth surface with no obvious plastic deformation. For the PLBSI0/PLA composites, they show different behaviors and deformation steps during tensile process. Because the PLBSI particle can be as stress concentrated sites for the high elasticity over PLA, the followed concentrated stress can promote generation of a tri-axial stress in the PLBSI particle. Consequently, the interfacial debonding of the PLBSI-0 from the PLA matrix easily occurs with formation of many voids due to the poor interfacial adhesion (Table 2), which can be found from the initial step of the tensile process n in Figure 10 (b1). Then, the tri-axial stress releases near these cavities, and the yield strength decrease. Thus, yielding coms up and the cavities grow continuously. With the appearance of shear bands, these cavities are stretched along the stress direction (Figure 10 (b2)). As interfacial debonding continues, the shear yielding was observed for PLA matrix45. As a result, orientation of PLA matrix appears, which can be observed in Figure 10 (b3). For PLBSI-40/PLA composites, obvious less and smaller cavities and particles are found, indicating the PLBSI-40 particles are more bonded to the matrix and different to be debonding, which may attribute to higher interfacial adhesion and compatibility of PLBSI-40 than that of PLBSI-0 with PLA (Figure 10 (c1) to (c3)). Thus, PLBSI-40 intends to generate cavities to cause shear yielding of PLA matrix when subjected to stress. So, we suppose the internal cavitation as the main toughening



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mechanism of PLBSI-40/PLA composites. The different toughening mechanism for PLBSI0/PLA and PLBSI-40/PLA composites was further interpreted in Figure 11.

Figure 10 SEM micrographs obtained from region 1, 2 and 3 of PLBSI/PLA composites tensile samples: (a1, b1, and c1) PLA; (a2, b2, and c2) PLBSI-0/PLA; (a3, b3, and c3) PLBSI-40/PLA and (d) Schematic diagram of the measurement locations 1, 2, and 3 of the SEM micrographs of PLBSI/PLA composites

Figure 11 Schematic diagram of tensile progress for PLBSI/PLA composites 20 ACS Paragon Plus Environment

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The SEM impact-fractured surfaces of the PLBSI/PLA composites were presented in Figure 12. For PLBSI-0/PLA and PLBSI-20/PLA composites, smooth surfaces with only little fibrils and slight matrix deformation were found, indicating slight ductile fracture. The large cavities caused by the detached PLBSI particles and strong interfacial debonding with formation of cavitation appear because their poor compatibility and interfacial adhesion with PLA matrix. However, the impact-fractured surface of the PLBSI-40/PLA composites shows more evident ductile fractured characteristics. More and longer fibrils, higher deformed matrix, and blurry particles and cavities were found because of the plastic deformation and the conflation of cavities

46,47

. The dispersed PLBSI-40 particles were found to be more bonded

to the matrix, indicating strong interfacial adhesion of PLBSI-40 with PLA. This deformation process resulted in extremely enhanced impact strength.

Figure 12 SEM micrographs of the impact fractured surfaces of (a) PLA (b) PLBSI-0/PLA, (c) PLBSI-20/PLA, (d)PLBSI-40/PLA

In conclusion, during the toughening mechanism study, the interfacial adhesion affected by compatibility and crystallinity of PLBSIs were investigated in the toughening of PLA. The 21 ACS Paragon Plus Environment


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higher compatibility makes PLBSI-40 bonded to the matrix and amorphous structure allow PLBSI-40 reacted to high loading by internal cavitation. For PLBSI-0 and PLBSI-20 particles, poor compatibility allows them debonded from the matrix and high crystallinity lead to interfacial cavitation. In all composites, cavitation intends to generate cavities to cause shear yielding of PLA matrix when subjected to stress. Toughness of PLA will significantly benefit from this process. The excellent compatibility between PLBSI-40 and the PLA and amorphous structure of PLBSI-40 ensure to obtain optimal impact strength.

3.6 3D-printed performances of PLBSI/PLA composites 3D-printing technology can fabricate materials to various shapes and shows great potential in many fields. Recently, PLA was applied in 3D-printing fields though the printed products are short of toughness. In this study, we employed the highly toughened PLA as a 3D-printing ink. Significant improvement is achieved in 3D-printed PLBSI-40/PLA composites compared to 3D-printed neat PLA evidenced in Figure 13. Then, tensile test was carried out to evaluate the practicability of 3D-printed toughened PLA by comparing the mechanical performances of 3D-printed samples to that of the normal compressive samples and some PLA-based composites reported by references48-50 including polyethylene glycols (PEG)/PLA, hydroxyapatite (HA)/PLA and triallyisocyanurate (TAIC)/PLA composites (Figure 14(a)). The tensile strength of 3D-printed samples are litter lower than that of compressive samples due to the layer by layer structure of 3D-printed samples. However, their elongation at break are nearly the same, indicate the excellent toughness will remain after 3D-printing process. Besides, the 3D-printed PLBSI/PLA composites show obvious higher elongation at break than that of other 3D-printed PLA-based composites, indicating an outstanding toughness for 3Dprinted applications. In Figure 14 (b), cryo-fractured surface of 3D-printed PLBSI/PLA tensile samples was presented to study the microstructure. The high-porosity microstructure was found and all pores were open and interconnected. 22 ACS Paragon Plus Environment

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Besides, no defects in layer and frame are observed. The satisfied mechanical properties and reliable microstructure of 3D-printed samples indicate the highly toughened PLBSI/PLA composite is good 3D-printing ink and have great potential to be applied in industrial and engineering applications.

Figure 13 Evidence of significant improvement in toughness of (b) 3D-printed PLBSI40/PLA composites (a) compared to 3D-printed neat PLA

Figure 14 (a) Comparison on mechanical properties of compressive PLBSI/PLA, 3Dprinted PLBSI/PLA and 3D-printed PLA-based composites (b) cryo-fractured surface of 3Dprinted PLBSI/PLA tensile samples obtained by SEM.

4. Conclusion 23 ACS Paragon Plus Environment


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In this study, super toughened PLA was obtained by blending PLA with PLBSIs, which are completely synthesized from natural renewable resource. The compatibility between PLBSIs and PLA were evaluated by theoretical calculation, SEM, DMTA and DSC. demonstrating that compatibility between PLBSIs and PLA increase with increasing lactate content in PLBSIs. DSC and POM study show the addition of PLBSIs improved cold crystallization of PLA, and promoted both nucleation density and growth rate of spherical crystal. Investigation on rheology indicates the complex viscosity, storage modulus and loss modulus of the composites were all higher than those of neat PLA at low frequencies. Thermal tensile test indicates rubbery PLBSI achieve better toughening effect than plastic PLBSI. Importantly, super toughened PLBSI-40/PLA composites exhibiting maximum elongation at break of 324 % compared to 3.8% for neat PLA and the impact strength of 35.7 KJ/m2 compared to 2.4 KJ/m2 for neat PLA. Furthermore, morphological observed by SEM indicated that the significant toughening effects is ascribed to highly deformed matrix caused by cavitation of PLBSI particles with suitable interfacial adhesion affected by excellent compatibility and amorphous structure between PLBSI and PLA. Consequently, the introduction of lactic acid in PLBSIs not only increases the compatibility with PLA but also helps PLBSIs turn from crystalline plastic to amorphous elastomer. Both of the increasing compatibility and turning to elastomer succeed to achieve superior toughening effect. Finally, the similar mechanical properties of 3D-printed samples compared to that of normal compressive samples and other PLA-based composites indicate the highly toughened PLBSI/PLA composite is good 3D-printing ink. The PLBSI/PLA composites with high toughness can be applied in both industrial and engineering applications.

ASSOCIATED CONTENT Supporting Information


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Calculation of critical diameter of PLBSI particles in PLBSI/PLA composites, hydrolysis of PLA and PLBSI-40/PLA composites, DSC exotherms of isothermal cold-crystallization of PLBSI/PLA composites and WAXD patterns of PLBSI/PLA composites are listed in supporting Information. Supporting Information is available from the http://pubs.acs.org. or from the authors. AUTHOR INFORMATION Corresponding Author Correspondences should be addressed to Prof L. Q. Zhang at [email protected] and Dr. H. L. Kang at [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Natural Science Foundation of China (Grant No. 50933001 and 51221002).

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