Highly-Toughened Polylactide- (PLA-) Based Ternary Blends with

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Highly-Toughened Polylactide- (PLA‑) Based Ternary Blends with Significantly Enhanced Glass Transition and Melt Strength: Tailoring the Interfacial Interactions, Phase Morphology, and Performance Majid Mehrabi Mazidi,*,†,‡ Arman Edalat,† Reyhaneh Berahman,† and Fatemeh Sadat Hosseini‡ †

Faculty of Polymer Engineering, Institute of Polymeric Materials, Sahand University of Technology, Sahand New Town, Tabriz P.C.: 51335-1996, Iran ‡ Young Researchers and Elite Club, Darab Branch, Islamic Azad University, Darab P.C.: 74817-83143, Iran S Supporting Information *

ABSTRACT: The inherent shortcomings of polylactide (PLA) including brittleness, low glass transition temperature, and melt strength during processing were addressed through a facile melt blending of PLA with polybutadiene-g-poly(styreneco-acrylonitrile) (PB-g-SAN) core−shell impact modifier and poly(methyl methacrylate) (PMMA). Highly tough PLA-based ternary blends with drastically enhanced glass transition temperature (≈ 21 °C) and melt strength were successfully prepared. The effect of PMMA content (ranging from 0 to 30 wt %) on the phase miscibility, morphology, mechanical properties, thermal behavior, rheological properties, and toughening mechanisms of PLA/PB-g-SAN/PMMA blends with 30% PB-g-SAN was systematically investigated. It was found that PMMA can effectively tune the interfacial interactions, phase morphology and performance of incompatible PLA/PB-g-SAN blend owing to its partial miscibility with PLA matrix and miscibility with SAN shell of PB-g-SAN, as evidenced by DMTA analysis. Increase in PMMA content promoted the phase adhesion and dispersion state of PB-g-SAN terpolymer in the blends and highly toughened blends were achieved which showed incomplete break of impact specimen. The significant effect of phase morphology on imparting tremendous improvement in impact toughness was clarified. The maximum impact strength (about 500 J/m), elongation-at-break and glass transition were obtained for ternary blend with 25% PMMA. The PLA crystallinity was gradually suppressed in ternary blends upon progressive increase in PMMA content. Rheological studies showed solid-like behavior with enhanced viscosities for ternary blends. Micromechanical deformations and toughening mechanisms were studied by post-mortem fractography. Massive matrix shear yielding was found as the main source of energy dissipation triggered by suitable interfacial adhesion and microvoid formation.

1. INTRODUCTION The increasing environmental concerns over petroleum-based polymers along with the rapid depletion of the nonrenewable petroleum resources have exerted enormous research on the development of the biobased polymeric materials. Poly(lactic acid) (PLA), as a biodegradable polyester derived completely from renewable resources, has attracted a lot of attention in recent years. PLA has good biodegradability, biocompatibility, high mechanical strength, and excellent processability,1−4 which make it a promising alternative to petroleum-based plastics in widespread commercial applications.1,5 However, the inherent brittleness and low heat deflection temperature are the major bottlenecks that hinder the application of PLA in package and automotive industries.3−5 Many efforts have been made to improve the toughness and/or thermal stability of PLA through copolymerization, plasticization, and melt blending, and the last one is considered as the most economical and versatile route.5−7 In the case of physical blending, good compatibility of PLA matrix with the modifier polymer is of crucial © XXXX American Chemical Society

importance to achieve a desired mechanical performance, especially fracture toughness. This is usually obtained by functionalization of modifier polymer and/or the use of a third component as a compatibilizer in the blend system.5−8 There is a vast literature reporting the use of various biodegradable or nonbiodegradable polymers to improve the toughness of PLA materials with varied degree of success. Nonetheless, a literature survey shows that the “super-toughened” blend systems mainly included the following additives:7 glycidyl methacrylate (GMA)-functionalized copolymers and elastomers,8−12 acrylic impact modifiers and those grafted with GMA,13−17 random aliphatic copolyesters, polyurethanes, and other flexible polymers,18−23 and the blends prepared by dynamic vulcanization.24−34 Received: March 22, 2018 Revised: May 6, 2018

A

DOI: 10.1021/acs.macromol.8b00557 Macromolecules XXXX, XXX, XXX−XXX

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incompatible PLA/PB-g-SAN blend to produce a suitable interfacial bonding and uniform distribution of PB-g-SAN particles in the PLA matrix. This is because the adequate interfacial adhesion and homogeneous dispersion of rubber particles in the matrix are of crucial importance to obtain a high degree of rubber toughening. In addition to its role as adhesion promoter, PMMA with much higher glass transition and melt strength than the PLA can also address the other limitations of PLA matrix such as heat distortion temperature (HDT) and melt strength, owing to its partial miscibility with PLA matrix. A transition from brittle to fully ductile mode of failure under impact tests was observed upon the addition of as little as 5 wt % PMMA into PLA/PB-g-SAN blend containing 30 wt % PB-gSAN. Ternary blends with much higher loadings of PMMA revealed highly tough behavior under impact tests. Highly toughened ternary blends showed remarkably improved impact strength, glass transition and melt strength not only compared with unmodified PLA, but also compared with the previously reported PLA/ABS systems. The effect of PMMA content on the phase morphology, tensile properties, Izod impact strength, dynamic mechanical properties, melting and crystallinity, and rheological properties was systematically investigated. A comprehensive structure−property correlation was established for blend systems under investigation. Moreover, the failure mechanisms were studied in detail, and toughening micromechanisms were proposed.

Acrylonitrile−butadiene−styrene (ABS), composed of SAN matrix toughened by PB-g-SAN core−shell terpolymer, is a petroleum-based thermoplastic polymer which is used heavily in the electronics and automotive industries due to its tailorable properties and a relatively low cost. The toughening capability of ABS in PLA has been reported by several researchers. Li and Shimizu35 reported the reactive compatibilization of PLLA and ABS thermoplastic by SAN-GMA (a copolymer of styrene, acrylonitrile, and glycidyl methacrylate) and ETPB (ethyltriphenyl phosphonium bromide) as the catalyst. Compatibilized PLLA/ABS blends exhibited improved impact strength (by 230%) and elongation-at-break (by 590%).35 Wu and Zhang used PB-g-SAN core−shell particles to improve the toughness of PLLA.36 For PLLA blend with 6.0 wt % PB-g-SAN terpolymer particles, the elongation-at-break increased by 28 times and the notched impact strength improved by 100% comparing with those of neat PLLA. Dong et al.37 used a reactive comb polymer made from methyl methacrylate (MMA), glycidyl methacrylate (GMA), and MMA macromer to compatibilize PLA and ABS thermoplastic. They concluded that the reactive comb polymer drastically increased the interfacial adhesion between the PLA and ABS, leading to 23 times increase in fracture strain and 16 times increase in break energy. Vadori et al.38,39 adopted two additives, one acrylic copolymer and one chain extender, separately and in combination to increase mechanical properties of blend of PLA with ABS thermoplastic containing 50% PLA. A synergistic effect of both acrylic copolymer and chain extender was observed and the impact strength and tensile strain increased by almost 600% and 1000%, respectively. Zhang et al.40 investigated the effect of poly(ethylene glycol) (PEG) molecular weight (600 and 8000 g/mol) on the morphology and properties of PLA/ABS blends. They found that PEG600 was more effective in enhancing the tensile strain. The tensile strain of PLA/PEG600/ABS (45/10/45) blend was about 18%. Rigoussen et al.41 investigated the effect of cardanol in the compatibilization of PLA/ABS immiscible blends by reactive extrusion. They found that cardanol promoted the miscibility of PLA and ABS by grafting onto ABS through its phenolic ring. Compared with neat PLA, the impact strength the blend compatibilized with 3 wt % cardanol increased by 84%. The different procedures adopted by researchers to enhance the phase compatibility and interfacial adhesion between the components in PLA/ABS blends have resulted in limited improvement in impact strength of resulting PLA/ABS blends. Moreover, no considerable improvement in the other drawbacks of PLA including low glass transition temperature and melt strength has been reported in these works.15−18,35−41 In the present work, a PB-g-SAN core−shell impact modifier composed of PB core and SAN shell and poly(methyl methacrylate) (PMMA) were incorporated into PLA in order to alleviate the inherent brittleness, low glass transition temperature, and melt strength of PLA. An attempt was made to prepare PLA-based blends with far superior properties (impact toughness, glass transition, and melt strength) than those reported previously through adopting a facile and far more effective compounding. Dynamic mechanical thermal analysis (DMTA) showed that PMMA is miscible with PLA matrix at low contents while becomes partially miscible with PLA at higher loadings, consistent with the literature.42,43 It was also found that PMMA is miscible with SAN shell of PB-g-SAN terpolymer, in agreement with the literature.44−48 Therefore, PMMA functioned as a very efficient interfacial agent for

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. A commercially available polylactic acid, PLA 3251D, with a melt flow index (MFI) of about 30 g/10 min (190 °C, 2.16 kg) and a density of 1.24 g/cm3, was purchased from NatureWorks LLC. Polybutadiene-g-poly(styreneco-acrylonitrile), PB-g-SAN, core−shell impact modifier with an average particle size of 430 nm was supplied by Tabriz Petrochemical Company, Iran. Other characteristics of PB-g-SAN terpolymer are given in Table S1. A commercial PMMA grade (EG920) with a glass transition temperature of 127 °C and density of 1.18 g/cm3 from LG Chemical, South Korea was chosen. All materials were dried at 80 °C under vacuum for more than 12 h to remove moisture before processing. Melt blending of the samples was performed using a lab mixer of HAAKE PolyLab OS System torque (Thermo Fisher Scientific) operating at 180 °C and 50 rpm for 6 min. The PLA and PMMA were first dry-mixed and then put into the mixing chamber. The PB-g-SAN terpolymer was incorporated into the mixing chamber after 2 min from the beginning of the mixing process. The weight fraction of PB-g-SAN phase was kept fixed at 30 wt % (corresponds to 16.8% PB rubber content) in the blends. The weight fraction of PMMA in the blends was changed from 0 to 30 wt % (Table S2). The samples were then compression molded (at 190 °C and 50 bar) into standard specimens for tensile, Izod impact, DMTA, and rheology tests. 2.2. Morphological Characterization. The phase morphology of the samples was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) instruments. For SEM experiments, the cryo-fractured surfaces, in liquid nitrogen, were gold sputtered for good conductivity of the electron beam. For some compositions, the PMMA component was selectively etched by acetone at room temperature for 20 min. The samples were dried at 80 °C overnight before microscopic imaging. For TEM analysis, ultrathin sections of about 80 nm thick were cryo-microtomed from the blends at −160 °C with a diamond knife, collected on a 200 mesh copper grid, and exposed to the vapor of ruthenium tetroxide (RuO4) for 4 h. RuO4 preferentially stained the SAN shell of PB-g-SAN rubbery phase to provide better contrast under TEM. 2.3. Contact Angle Measurements. Contact angles were measured in a sessile drop mold with KR ̈ USS DSA100 (German). B

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Macromolecules Table 1. Surface Tension Data for PLA, SAN Copolymer (Shell of PB-g-SAN) and PMMA Calculated at 180 °C surface tension (mN/m) at 23 °C

contact angle (deg)

surface tension (mN/m) at 180 °C

sample

water

diiodomethane

total (γ)

dispersion component (γd)

polar component (γp)

total (γ)

PLA SAN PMMA

67.6 ± 2 74.5 ± 2 79.0 ± 3

45.3 ± 3 52.5 ± 2 34.2 ± 3

35.50 45.30 40.50

26.44 36.80 32.40

9.06 8.50 8.10

26.00 33.10 29.94

PLA, SAN, PB-g-SAN and PMMA samples were compression-molded between clean silicon wafers at 190 °C for 5 min and then cooled to 25 °C. Contact angles were measured on 3 mL of wetting solvent at 25 °C, and the mean values of seven replicates were reported. 2.4. Mechanical Tests. Tensile tests were conducted on an Instron Universal tensile testing machine (model 3382) at a fixed crosshead speed of 5 mm/min at room temperature according to ISO 527. At least four specimens were tested for each composition and the resulting tensile properties were averaged. The impact toughness of the samples was determined by the notched Izod impact test (Zwick/ Roell B5102 pendulum impact tester) according to ASTM D256. The results of the impact strength are the average of at least six repetitions obtained at a temperature of 25 °C. 2.5. Differential Scanning Calorimetry (DSC). Thermal behavior of the samples was studied with a Netzsch-DSC 20 0F3 (Germany) instrument under nitrogen atmosphere. Each sample of 10 ± 0.2 mg was taken from a molded sheet and encapsulated in an aluminum closed pan. Samples were first heated from 30 to 200 °C at a heating rate of 20 °C/min, held at 200 °C for 5 min to erase any previous thermal history and then cooled to 30 °C at the cooling rate of 10 °C/min and heated again to 200 °C with the 10 °C/min heating rate. The results of cooling and the second heating run are presented in this work. The % of matrix (PLA) crystalline phase (X) was estimated using the following equation: Xc =

ΔHm × 100 wf ΔHm°

multiphase polymeric systems is controlled by the interfacial interactions between the blend’s components. In this work, the interfacial tension values were calculated based on the surface tension values measured by the contact angle method. The contact angles of PLA, SAN (shell of PB-g-SAN particles) and PMMA with water and diiodomethane are listed in Table 1. The surface energy (γ), dispersion (γd), and polar (γp) components of the materials can be estimated from the contact angle data by using the following two equations (eq 2 for water and eq 3 for diiodomethane) according to Wu:49 ⎛ γd γd γHp Oγ p ⎞ HO 2 ⎟ + (1 + cos θH2O)γH O = 4⎜⎜ d 2 p p⎟ d 2 γ + γ γ + γ H 2O ⎝ H 2O ⎠

(1 + cos θCH2I2)γCH I

2 2

(2)

p ⎛ γd γd γCH γp ⎞ CH 2I 2 2I 2 ⎜ ⎟ = 4⎜ d + p p⎟ d γ + γ γ + γ CH I ⎝ CH2I2 ⎠ 2 2

(3)

in which γ = γ + γ . θH2O and θCH2I2 are contact angles of the polymer with water and diiodomethane, respectively. The numerical values used were γHd 2O = 22.1 mN/m, γHp 2O = 50.7 mN/m, γdCH2I2 = 44.1 mN/m, and γpCH2I2 = 6.7 mN/m.49 The values of surface energy, dispersion and polar components of polymers are listed in Table 1. Since the melt blending was carried out at essentially higher temperatures, the use of interfacial tension values calculated from surface tension values requires to be extrapolated to the processing temperature, on the basis of data reported for variation of surface tension with temperature (−dγ/dT) and polarities (xp = γp/γ). Surface tension of SAN random copolymer was determined by means of a simple mixture rule from surface tensions of polystyrene (PS) and polyacrylonitrile (PAN).50 Surface tension values calculated at 180 °C for all the polymers are also listed in Table 1. Interfacial tension between polymers (αAB) and the thermodynamic work of adhesion (WAB) of each pairs were calculated by using the following equations:49 d

(1)

Here ΔHm is the enthalpy of melting for analyzed sample, wf is the mass fraction of PLA in the blend, and ΔHm° is the enthalpy of fusion for 100% crystalline PLA, taking the value of 93.7 J/g from the literature.38 2.6. Dynamic Mechanical Thermal Analysis (DMTA). DMTA was conducted on a DMA Q800 from TA Instruments using a dualcantilever clamp with a mode of frequency sweep/temperature ramp at the frequency of 1 Hz and oscillating amplitude of 15 μm. The samples (dimensions 12.7 × 63.5 × 3.2 mm) were heated from −140 to 150 °C at a heating rate of 3 °C min−1. To determine the heat deflection temperature (HDT), a constant load of 0.45 MPa was applied at the center of a three point bending flexural bar sample with the dimension of 55 × 12 × 2 mm3, which was heated at the rate of 2 °C min−1 from room temperature to 130 °C. The temperature at which the specimen reached a deflection of 250 μm was reported as the HDT value. 2.7. Rheological Study. The flow behavior and melt linear viscoelastic properties of different samples were investigated using a dynamic rheometer (MCR300, Anton Paar) equipped with a parallel plate geometry (diameter = 25 mm, gap = 1 mm). The frequency sweep tests were performed in the range of 0.04−625 rad/s at 190 °C under dry nitrogen atmosphere with amplitude of 1% to maintain the response of materials in the linear viscoelastic regime. The linear viscoelastic range was determined from the strain sweep test. 2.8. Fractography and Failure Mechanisms. The SEM instrument was employed to elucidate the role of phase morphology on the fracture behavior of the samples, and also to understand the micromechanisms of deformation related to the different blend systems. SEM micrographs were taken in different magnifications from the surface of impact-fractured specimens of different samples.

αAB = γA + γB −

p

4γAdγBd γAd + γBd

−

4γApγBp γAp + γBp

WAB = 2(γAdγBd)1/2 + 2(γApγBp)1/2

(4) (5)

The interfacial tension and work of adhesion values at processing temperature, calculated from the surface tension data, are listed in Table 2. The data in Table 2 demonstrate that in PLA/PB-g-SAN/PMMA ternary blend the interfacial tension values for PLA/PMMA and SAN/PMMA are lower than that of PLA/SAN. Moreover, it can be seen that the interfacial tension of PMMA/SAN couple is much smaller than that of PMMA/PLA couple, indicating that the interaction between SAN shell of PB-g-SAN particles and PMMA is stronger than interaction between PLA and PMMA. In other words, when the

3. RESULTS AND DISCUSSION 3.1. Determination of Interactions between the Components. It is well established that the morphology in C

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Macromolecules Table 2. Interfacial Tension, Spreading Coefficient, and Work of Adhesion Values for Polymer Pairs at 180 °Ca

a

polymer pairs

interfacial tension, αij (mN/m)

spreading coefficient, λij

work of adhesion, Wij (mN/m)

PLA/SAN PLA/PMMA SAN/PMMA

1.67 ± 0.3 0.94 ± 0.2 0.50 ± 0.2

λAC < 0 λBC = 0.23 > 0 λCB = −1.23 < 0

54.5 ± 3 59.3 ± 3 62.8 ± 4

Key: A, PLA; B, PMMA; C, SAN (shell of PB-g-SAN).

three polymers are melt-compounded simultaneously, PMMA has higher affinity to grafted SAN chains of PB-g-SAN terpolymer to form an interfacial layer around and/or inbetween the PB-g-SAN domains in the PLA matrix. This statement was further confirmed by using the spreading coefficient (λij) theory and calculation of λij for polymer pairs in PLA/PB-g-SAN/PMMA ternary blend. According to Hobbs et al.50 in a ternary blend of three polymers A, B and C (A is the matrix) the spreading coefficient, λCB, is defined as λCB = αBA − αCA − αBC

(6)

where λij is the spreading coefficient of i component over j component and αij is the interfacial tension between i and j components. For B to be encapsulated by C, λCB must be positive. In the case when both λCB and λBC are negative, B and C will tend to form separated phases in the A phase. The calculated spreading coefficient values are also given in Table 2. The spreading coefficient values in Table 2 suggest that in PLA/PB-g-SAN/PMMA ternary blend with PLA as matrix phase, PMMA tends to encapsulate the SAN shell of PB-g-SAN terpolymer by forming an interfacial layer between the PLA matrix and PB-g-SAN domains, in agreement with interfacial tension measurements. The work of adhesion data are also consistent with the interfacial tension data, in that the work of adhesion for PLA/ SAN pair is lower than that of PLA/PMMA and PMMA/SAN pairs. Moreover, the work of adhesion for PMMA/SAN is higher than that of PLA/PMMA. 3.2. Phase Miscibility of the Blends. DMTA was employed to examine the interfacial interactions and mutual miscibility of PMMA with PLA and SAN shell of PB-g-SAN components in PLA/PB-g-SAN/PMMA ternary systems. This is because the phase morphology is determined by the compatibility and extent of interactions between the components. The temperature dependence of storage modulus (G′) and damping factor (tan δ) for neat PLA, neat PMMA and PLA/PB-g-SAN/PMMA blends of different PMMA contents are shown in Figure 1. The storage modulus is directly related to the elastic response of the tested material, whereas tan δ is intimately associated with the chain relaxations that take place. From Figure 1a, it is apparent that all the curves experience a gradual decline in G′ with increase in temperature from −100 to +150 °C, as expected. Neat PLA and PMMA polymers exhibited single transitions at 65.7 and 127 °C, respectively. For PLA/PB-g-SAN binary blend and PLA/PB-g-SAN/PMMA ternary blends containing 5 and 10% PMMA, two transitions can be detected in Figure 1a, while for ternary blends containing 20−30% PMMA a third transition is also visible in the storage modulus curves. This additional transition becomes more intense as the PMMA content in the ternary blend was increased.

Figure 1. Temperature dependence of (a) storage modulus (G′), and (b) damping factor (tan δ) for neat components and PLA/PB-g-SAN/ PMMA (70 − x/30/x) blends of different PMMA contents.

The transition appeared at the temperature range of −78 to −85 °C for PLA/PB-g-SAN binary and different PLA/PB-gSAN/PMMA ternary blends is related to the PB rubbery core of PB-g-SAN core−shell particles in these blends.16,35 The transitions observed at the temperature range of 65 to 87 °C for different PLA/PB-g-SAN/PMMA blends are ascribed to the PLA-rich phase of the these blend systems.39,43 As can be seen in Figure 1a, this transition shifts forward to higher temperatures as the PMMA content in the ternary blend was increased. The third transition appeared at the temperature range of 115−126 °C is associated with the relaxation of PMMA-rich phase in the ternary blends with 20−30% PMMA.39,43 These findings indicate that a phase segregation occurs between PMMA and PLA in ternary blends composed of 20− 30% PMMA. In fact, the results demonstrate that in PLA/PB-gSAN/PMMA ternary blends, the PLA/PMMA pair is miscible up to 10% PMMA and then becomes partially miscible at higher loadings of PMMA up to 30% as PLA-rich phase transition temperature shows an increase with PMMA content. The increase in storage modulus in the temperature range of 80−100 °C observed for neat PLA, PLA/PB-g-SAN binary blend and ternary blends containing up to 10% PMMA is due to the cold crystallization of PLA.35 This increase becomes much less pronounced for ternary blend with 10% PMMA and no such cold crystallization can be detected for ternary blends containing higher than 10% PMMA. The plot of tan δ reveals more clearly the corresponding transition temperatures and the D

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complete miscibility observed for PLA/PMMA pair for ternary blends of high PMMA contents (20% and higher) prepared in this work is due to the high molecular weight (Mn = 161 000 g/ mol) of PMMA and the mixing conditions (180 °C and 50 rpm) used in this work. The high molecular weight of PMMA and relatively low mixing energy adopted for blend preparation in the present work highly restrict the mixing state and diffusion of PMMA macromolecular chains in the blends of high PMMA contents. Consequently, the PLA−PMMA pair exhibited partial miscibility. 3.3. Phase Morphology of the Blends. It is well-known that the properties of multiphase polymer blends are strongly dependent on the phase morphology. Therefore, study of the morphology of blends prepared in this work is of prime importance and useful for an attempt to establish the correlation between morphology and resulting performance. The SEM micrographs of PLA/PB-g-SAN/PMMA (70 − x/ 30/x) blends containing different amounts of PMMA are shown in Figure 2. For PLA/PB-g-SAN (70/30) blend in Figure 2a, it can be seen that the PB-g-SAN particles are highly agglomerated and formed extended structures in the PLA matrix. A clear and welldiscernible interfacial region between PB-g-SAN agglomerates and surrounding PLA matrix is apparent in Figure 2a, indicating

breadth of transition zone in different blends (Figure 1b). The glass transitions corresponding to the damping peak of different samples are summarized in Table 3. For neat PLA an intense Table 3. Glass Transition Temperatures (Tg) Corresponding to Damping (tan δ) Peaks Obtained from DMTA Test and the HDT Values of Different Samples samples (PLA/PB-g-SAN/PMMA)

Tg,PLAa (°C)

Tg,SAN (°C)

100/0/0 (neat PLA) 0/0/100 (neat PMMA) 70/30/0 65/30/5 60/30/10 50/30/20 45/30/25 40/30/30

65.7 − 65.6 64.4 68.0 74.6 86.6 81.6

− − 109.0 108.6 109.3 117.5 126.6 122.3

a

Tg,PMMAb (°C) − 127 − (miscible) (miscible) (miscible) (miscible) (miscible)

Tg,PB (°C)

HDT (°C)

− − −85.0 −84.7 −84.5 −83.5 −82.0 −87.9

54 95 52 54 56 61 63 67

PLA and PLA-rich phase. bPMMA and PMMA-rich phase.

damping peak at 65.7 °C was detected. For PLA/PB-g-SAN binary blend, the damping peak at 65 °C is related to the PLA whereas the peak at 109.3 °C is due to the SAN shell of PB-gSAN terpolymer.16 For ternary blends containing 5 and 10% PMMA, the PLA peak is located at 64.4 and 68 °C, respectively. For ternary blends containing higher loadings of PMMA up to 30%, the Tg of PLA-rich phase was initially increased and then decreased. The greatest increase in Tg was observed for ternary blend containing 25% PMMA with Tg of about 86.6 °C, which is about 21 °C higher than the Tg of neat PLA. The drop in Tg of PLA-rich phase for ternary blend with 30% PMMA could be due to the increased melt viscosity of ternary blend (see the rheology section) compared with the blends of lower PMMA content, which in turn restricts the mixing state and interdiffusion between PLA and PMMA phases. The presence of a single damping peak at the temperature range between 108.6 °C (Tg of SAN shell) and 127 °C (Tg of PMMA) indicates the miscibility of SAN shell of PB-g-SAN particles with PMMA phase. It is established that PMMA is miscible with SAN copolymers when the acrylonitrile content of the SAN is at the range of 3−35%.44−48 In this work, the grafted SAN chains of PB-g-SAN terpolymer contain 27% acrylonitrile, indicating that the PMMA and SAN shell of PB-gSAN would be miscible. At PMMA contents higher than 10%, the high temperature damping peak gradually shifted toward the damping peak of neat PMMA. The HDT data in Table 3 reveal that for PLA/PB-g-SAN binary blend the HDT is lowered compared with the neat PLA. For ternary blends of different PMMA content, it can be seen that the HDT gradually increases as the PMMA content in the blend was increased, mainly due to much greater HDT of neat PMMA than that of pure PLA and its (partial) miscibility with PLA matrix which gave rise to an increase in HDT value of the resulting blend. The HDT value of ternary blend with 30% PMMA was about 13 °C higher than that of neat PLA. The drastic enhancement of glass transition and HDT values of PLA obtained in this work upon melt blending with PMMA is of great importance as one of main drawbacks of pristine PLA for engineering applications is its low Tg and HDT values. It is well-documented that the miscibility of PLA−PMMA pair is controlled by the molecular weight of components and the processing conditions.39,43 It is worth noting that the lack of

Figure 2. SEM micrographs of cryo-fractured surfaces of PLA/PB-gSAN/PMMA (70 − x/30/x) blends containing different amounts of PMMA. Key: (a) 0 wt %; (b) 5 wt %; (c) 10 wt %; (d) 20 wt %; (e) 25 wt %; (f) 30 wt %. E

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Macromolecules a poor interfacial bonding between the components in PLA/ PB-g-SAN blend. These findings imply that the PB-g-SAN terpolymer and PLA matrix are highly incompatible, and consequently a coarse morphology is developed in PLA/PB-gSAN binary blend. Upon the addition of 5 wt % PMMA into PLA/PB-g-SAN blend, the micrograph in Figure 2b reveals that the interfacial adhesion is highly improved. An obscure interfacial region can be observed for blend containing 5 wt % PMMA, indicating increased interfacial adhesion between PB-g-SAN agglomerates and the surrounding matrix. According to DMTA results (Figure 1), PMMA is miscible with both PLA matrix and SAN shell of PB-g-SAN particles. Therefore, PMMA functions as an effective interfacial agent for PB-g-SAN terpolymer and PLA matrix by localization at the interface of PLA and PB-g-SAN components. For blend containing 10 wt % PMMA (Figure 2c) it can be seen that the compatibility of the PB-g-SAN structures and the PLA matrix is further improved. A better dispersion state (compared with blend with 5 wt % PMMA) of PB-g-SAN particles in PLA matrix is obvious in the micrograph of Figure 2c and the interfacial adhesion between the PB-g-SAN agglomerates and the PLA matrix is further enhanced. It seems that the PB-g-SAN agglomerates start to breaking down and transform into more individual particles adjacent to each other that are dispersed in the PLA matrix. In agreement with this statement, the micrograph in Figure 2d represents that a fine and uniform distribution of PB-g-SAN rubber particles in PLA matrix were achieved when the weight fraction of PMMA in the PLA/PB-g-SAN/PMMA blend reached to 20 wt %. In fact, the SEM images reveal that at 20 wt % PMMA the highly agglomerated structures of PB-g-SAN particles are collapsed and transformed into much more homogeneous dispersion of PB-g-SAN particles in PLA matrix. Further increase in PMMA content from 20 to 25 and 30 wt % led to more interfacial interactions and better wetting of PB-g-SAN particles with PLA matrix (Figure 2e). As a result, it becomes more difficult to distinguish the homogeneously distributed PB-g-SAN particles of submicron size (average size of 430 nm) from the surrounding matrix as they progressively embed into the PLA matrix with increase in PMMA content from 20 up to 30 wt % (Figure 2f). For this reason, the dispersion state of PB-g-SAN particles in ternary blends containing relatively high loading of PMMA, was examined by TEM. Figure 3 shows the TEM images of ternary blends with 25 and 30 wt % PMMA. The micrographs in Figure 3a,b reveal that the PB-g-SAN core−shell particles are homogeneously distributed in the PLA matrix for ternary blends with 25 and 30 wt % PMMA. There is no evidence of the presence of large agglomerated structures of PB-g-SAN domains in these blends. Nonetheless, the high-magnification micrographs in Figure 3a′,b′ reveals a somewhat finer dispersion of PB-g-SAN particles in PLA matrix for blend with 30 wt % PMMA compared with the blend with 25 wt % PMMA, as expected. The TEM micrographs clearly demonstrate that PMMA effectively promoted the interfacial interactions between the core−shell rubber particles and the surrounding PLA matrix in the PLA/PB-g-SAN/PMMA ternary blend which, in turn, led to a fine and uniform distribution of PB-g-SAN particles in the PLA matrix. Figure 4 shows the SEM images taken from the ternary blend containing 30 wt % PMMA in which the PMMA phase has been etched by acetone. The removal of PMMA phase around

Figure 3. TEM micrographs showing the morphology of PLA/PB-gSAN/PMMA (70 − x/30/x) ternary blends of different PMMA contents. Key: (a, a′) 25 wt %; (b, b′) 30 wt %.

Figure 4. SEM micrographs of cryo-fractured surface of PLA/PB-gSAN/PMMA (40/30/30) ternary blend. PMMA was etched by acetone.

and in-between the PB-g-SAN domains led to some accumulation of PB-g-SAN particles in these micrographs. The observation of dark cavities around and/or within the PBg-SAN aggregates indicates that the PMMA phase is mainly concentrated around and in-between the PB-g-SAN particles in the blend, promoting the wetting of PB-g-SAN particles, interfacial adhesion and improved dispersion of PB-g-SAN particles in the PLA matrix. This finding is in line with the interfacial energy and spreading coefficient measurements presented earlier (Table 2). The morphological studies performed by SEM and TEM observations from different PLA/PB-g-SAN/PMMA ternary blends as described above suggest that PMMA acts as a highly efficient interfacial agent for PLA/PB-g-SAN blend, and very good dispersion state of PB-g-SAN particles and strong interfacial adhesion between the components were obtained in this work for PLA/PB-g-SAN/PMMA ternary blends containing 20−30 wt % PMMA. The miscibility of PMMA with SAN shell of dispersed PB-g-SAN particles and its partial miscibility (at high PMMA contents as evidenced by DMTA results) with PLA matrix are responsible for the very effective F

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PLA/PB-g-SAN blend. These findings further corroborate the strong interfacial interaction between the PMMA and (SAN shell of) PB-g-SAN in PMMA/PB-g-SAN blend while poor interaction between the PLA and (SAN shell of) PB-g-SAN in PLA/PB-g-SAN blend. These findings are consistent with the interfacial tension measurements, DMTA results and SEM observations described earlier. Interestingly, the stress−strain curves in Figure 5b show that all the PLA/PB-g-SAN/PMMA ternary blends exhibited a ductile mode of failure. The transition from brittle to ductile mode of failure occurred when as little as 5 wt % PMMA was incorporated into PLA/PB-g-SAN blend. The curves in Figure 5b further demonstrate that the yield point and postyield deformation (tensile ductility) of PLA/PB-g-SAN/PMMA ternary blends are highly dependent on PMMA content in the blend. These different tensile behaviors are a direct consequence of different interfacial interactions and dispersion states of PB-g-SAN particles in the PLA matrix in these blends as described earlier. The Young’s modulus, yield stress, tensile strength and elongation at break values obtained from stress−strain curves for different samples are illustrated in Figure 6. In the case of

role of PMMA in improving the mixing state in PLA/PB-gSAN/PMMA ternary blends. 3.4. Relationship between Tensile Properties and Phase Morphology. This section elucidates how the morphology development induced by PMMA content in the PLA/PB-g-SAN/PMMA (70 − x/30/x) blends affect the macroscopic mechanical behavior of the resulting blend under uniaxial tensile test. The typical stress−strain curves of neat components, binary blends and PLA/PB-g-SAN/PMMA ternary blends of different PMMA contents are depicted in Figure 5. As can be seen in Figure 5a, neat PLA failed in a

Figure 5. Tensile stress−strain curves of different samples: (a) neat polymers and binary blends and (b) PLA/PB-g-SAN/PMMA (70 − x/ 30/x) ternary blends of various PMMA contents.

completely brittle manner with high ultimate strength (≈64 MPa) and very low (≈4.2%) strain-at-break. Neat PMMA also behaved in fully brittle mode with very low (≈3.9%) tensile strain and lower tensile strength (≈56.2 MPa) than the neat PLA. Neat PB-g-SAN terpolymer showed a rubbery-like behavior with much lower tensile strength (≈18.6 MPa) and significantly higher tensile strain (≈ 304%) compared with the neat PLA and PMMA samples. In the case of binary blends, it can be seen that PLA/PMMA and PLA/PB-g-SAN blends failed, respectively, in brittle and semibrittle modes whereas PMMA/PB-g-SAN blend fractured in ductile mode (Figure 5a). Both PLA/PB-g-SAN and PMMA/PB-g-SAN blends exhibited much lower stiffness, yield stress and tensile strength compared with their respective neat parent polymers (PLA and PMMA). However, despite the PMMA/PB-g-SAN binary blend with ductile behavior, no significant rubber toughening effect related to the dispersed PB-g-SAN terpolymer can be seen in

Figure 6. Tensile properties (Young’s modulus, yield stress, tensile strength and elongation-at-break) of reference samples and PLA/PB-gSAN/PMMA (70 − x/30/x) ternary blends of different PMMA contents. G

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bonding between the PB-g-SAN agglomerates and surrounding matrix in unmodified PLA/PB-g-SAN blend, the PB-g-SAN structures are easily debonded from the matrix at relatively low stress levels, which led to early yielding of the material. In other words, the large PB-g-SAN agglomerates act as defects in the material, and their decohesion generates large interfacial microvoids/microcracks in the blend which trigger the premature and catastrophic fracture of the material under tensile test. This is responsible for highly reduced yield stress and break stress with no improvement in tensile ductility for PLA/PB-g-SAN binary blend compared with those of pure PLA. As was observed in micrographs of Figure 2 and also outlined in Table 2, PMMA promotes the interfacial bonding between PB-g-SAN particles and PLA matrix, the degree of which is determined by PMMA content in the blend. The higher the PMMA content in the blend, the stronger the interfacial strength is. The gradual enhancement of interfacial strength between the PB-g-SAN domains and PLA matrix with increasing PMMA content in PLA/PB-g-SAN/PMMA blend, progressively raises the stress level required for debonding of the PB-g-SAN domains from the surrounding matrix during the tensile loading. Consequently, the yield stress will increase as the PMMA content in the blend was increased. The other consequence of gradual increase in interfacial adhesion between the components in PLA/PB-g-SAN/PMMA blends with PMMA content is the more efficient stress transfer from the matrix phase onto PB-g-SAN domains. As a result, the PB-g-SAN rubber particles could effectively show their toughening effect under the mechanical test. Moreover, the finer and more homogeneous distribution of PB-g-SAN particles in a larger volume of the material with PMMA content, as evidenced by morphological observations in Figure 3, triggers larger volume of the material to participate in the deformation process during the tensile test, i.e. more delocalized deformation. Therefore, the strain-at-break also increases with the degree of interfacial modification and dispersion state in PLA/PB-g-SAN/PMMA blend. However, the data in Figures 5b and 6b show that the tensile ductility of the ternary blend containing 30 wt % PMMA is significantly lower than that of ternary blends with 20 and 25 wt % PMMA, though the degree of interfacial bonding and dispersion state in the former blend is higher than that of latter ones (see Figure 2d−f and Figure 3). This finding indicates that there is an optimum level of interfacial strength above which the rubber toughening effect of PB-g-SAN particles worsen and, consequently, the tensile toughness of the blend decreases. The reduced tensile toughness for ternary blend with 30 wt % PMMA may also be due to the fact that at high loading, PMMA forms PMMA-rich domains in the PLA/PB-g-SAN/PMMA ternary blend, as evidenced by the DMTA results in Figure 1, which would adversely affect the tensile toughness of the resulting material because PMMA is a highly rigid polymer with brittle failure mode. 3.5. Relationship between the Izod Impact Strength and Phase Morphology. As stated earlier, the biobased PLA has high mechanical stiffness, strength, and excellent processability, which make it an attractive alternative to petroleumbased plastics in commercial applications. However, the inherent brittleness is one of the main drawbacks that impede the widespread applications of PLA. Therefore, in addition to tensile ductility, the improvement of PLA impact strength is also of great importance from both industry and academia points of view and has been the subject of vast research works.

neat components (Figure S1), the PB-g-SAN terpolymer showed the lowest tensile modulus and tensile strength. The highest values of tensile modulus and ultimate strength are related to the PMMA and PLA, respectively. The tensile strain of PB-g-SAN terpolymer was much higher than that of PLA and PMMA samples (Figure S1). The tensile properties of PLA/ PMMA binary blend were at the intermediate level between those of pure PLA and PMMA, indicating a good interfacial interaction between the components in the PLA/PMMA blend (Figure S1). Among the binary blends, the PLA/PMMA blend exhibited the highest elastic modulus and tensile strength with the lowest strain-at-break (Figure S1). Comparing the strain at break values of PLA/PB-g-SAN and PMMA/PB-g-SAN blends (Figure S1) implies poor interaction between the PLA chains and SAN shell of PB-g-SAN terpolymer in the former binary system whereas satisfactory interactions between the PMMA chains and SAN shell of PB-g-SAN particles in the latter blend system which is in accordance with DMTA and SEM results. As can be seen in Figure 6a, all the blends represent much lower tensile modulus and tensile strength values compared with neat PLA. This is due to the presence of 30 wt % PB-gSAN rubbery phase with much lower Young’s modulus (≈251 MPa) and tensile strength (≈18.6 MPa) than those for neat PLA in the prepared blends. According to Figure 6a, the increase in PMMA content from 5 to 30% in PLA/PB-g-SAN/PMMA ternary blends monotonically increased the tensile modulus and tensile strength of the resulting material. The increase in elastic modulus of the PLA/ PB-g-SAN/PMMA ternary blends with PMMA content stems from the relatively higher Young’s modulus of PMMA component (≈2080 MPa) compared with PLA component (≈1954 MPa). Although the tensile strength of PMMA is lower than that of neat PLA, the increase in the blends’ ultimate strength with PMMA content up to 25 wt % PMMA is primarily due to the interfacial bridging effect of PMMA in PLA/PB-g-SAN/PMMA ternary blends, which with increase in phase adhesion between PLA and PB-g-SAN phases, PMMA improves the load bearing capability of the resulting blends. In the case of yield stress and elongation-at-break, the data in Figure 6b demonstrate that both parameters show approximately the same trend with PMMA content. Bouzouita et al.51 have studied the structure and properties of PLA/PMMA/BS ternary blends toughened with an ethylene-acrylate impact modifier bearing epoxy moieties (BS). Compared with the PLA/BS binary blend which showed a high degree of tensile ductility, the ternary blends of different PMMA contents exhibited lower tensile ductility. For ternary PLA/PMMA/BS blends, the tensile strain decreased while the tensile modulus and ultimate strength increased upon progressive incorporation of PMMA into PLA/BS blend.51 It is well established that the factors like morphology (dispersion state) and dispersed domains/matrix interfacial strength substantially affect the yield stress of polymer blends. In fact, the yield and ultimate strength of multiphase systems is determined by the extreme values of such parameters as the interfacial adhesion, stress concentration, and defect size/spatial distribution. The gradual increase in the yield stress of PLA/ PB-g-SAN/PMMA blend with PMMA content arises from the compatibilization effect of PMMA in PLA/PB-g-SAN blend. It should be noted that the large and extended structures of PB-gSAN agglomerates in PLA/PB-g-SAN blend act as stress concentrating sites in the material. Because of poor interfacial H

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blend containing 25 wt % PMMA. The impact strength of blend with 25% PMMA (about 500 J/m) was more than 25 times greater than that of neat PLA. The impact data demonstrate that the PLA/PB-g-SAN/PMMA ternary blends with 5−15% PMMA can be classified as tough blends (regime 2), whereas the ternary blends containing 20−30% PMMA show characteristic of highly toughened blends (regime 3). The tremendous improvement in notched impact resistance achieved in this work for highly toughened blends is comparable with the impact strength results reported in the literature for PLA blends containing reactive rubbery modifiers and/or dynamically vulcanized PLA-based systems.10−30 Bouzouita et al.51 observed a synergistic effect of BS and PMMA on the impact strength of PLA/PMMA/BS ternary blends. The impact strength of ternary blend initially increased with PMMA content up to 30% and then decreased with further addition of PMMA up to 80%.51 In that work, the ternary PLA/PMMA/BS blend with 30% PMMA and 17% BS exhibited the highest impact strength, with about 13 times greater than that of neat PLA.51 It should be noted that all the PLA/PB-g-SAN/PMMA ternary blends displayed a ductile mode of failure. An intense stress whitening was developed throughout the impact fractured-surfaces of these samples under the impact loading (Figure 8a,b). Surprisingly, the impact specimen of blends

The notched Izod impact strength of pure components and binary blends are given in Figure S2. Neat PLA and PMMA underwent a catastrophic unstable fracture with low impact strength values (Figure S2). Among the different binary blends, the impact strength of PMMA/PBg-SAN blend was substantially higher than that of PLA/PMMA and PLA/PB-g-SAN blends (Figure S2). In a similar manner with the tensile test presented earlier, the impact data also demonstrate good interaction between SAN shell of PB-g-SAN terpolymer and PMMA polymer and weak interfacial bonding between the SAN shell of PB-g-SAN and PLA matrix. The notched Izod impact strength of neat PLA, binary PLA/ PB-g-SAN blend and ternary blends containing different amounts of PMMA are displayed in Figure 7.

Figure 7. Notched Izod impact strength of reference samples and PLA/PB-g-SAN/PMMA (70 − x/30/x) ternary blends of different PMMA contents. The impact data were classified into three regimes. Regime I consisted of brittle-fractured samples, while regimes II and III are related to fully ductile-fractured samples. Regimes II and III correspond to tough and highly toughened samples, respectively.

The neat PLA fractured in a completely brittle mode of failure with the Izod impact energy of about 19.7 J/m. The incorporation of 30 wt % PB-g-SAN core−shell terpolymer into PLA slightly improved the impact strength of the material, and the resulting sample also failed in completely brittle manner under the impact test. Neat PLA and binary PLA/PB-g-SAN blend which showed brittle fracture behavior were named as regime 1 samples (Figure 7). Interestingly, the presence of as little as 5 wt % PMMA in the PLA/PB-g-SAN/PMMA blend led to a remarkable improvement in impact strength with a transition in fracture mode from brittle to fully ductile (Figure 7). An intense whitening zone was developed over the entire length of crack growth plane for blend compatibilized with 5 wt % PMMA. The impact strength of this sample was more than 10 times greater than that of pure PLA. This is a large improvement in notched impact strength which was achieved at low content of PMMA. More increase in weight fraction of PMMA in the PLA/PB-gSAN/PMMA ternary blend from 5 to 30 wt % gradually enhanced the notched Izod impact strength of the resulting blend system up to the sample with 25% PMMA and then the impact toughness was reduced for sample with 30% PMMA (Figure 7). The highest impact strength value achieved was for

Figure 8. Deformed impact specimens of neat PLA and PLA/PB-gSAN/PMMA blends showing the stress-whitening zone around the fracture plane. Key: (A) neat PLA; (B) PLA/PB-g-SAN blend; and (C−H) PLA/PB-g-SAN/PMMA (70 − x/30/x) ternary blends of different PMMA contents. (C) 5 wt %; (D) 10 wt %; (E) 15 wt %; (F) 20 wt %; (G) 25 wt %; and (H) 30 wt %.

containing higher than 5% PMMA did not undergo complete separation through the crack growth fracture plane during the impact test (Figure 8a). The fall in impact toughness value observed for sample with 30% PMMA compared with the samples with 20 and 25% PMMA indicates an optimum degree of interfacial adhesion in PLA/PB-g-SAN/PMMA ternary blend, above which the material’s capability for energy absorption and dissipation under the impact loading tends to decrease. It is reported in the literature that the impact strength in multiphase polymer blends is in direct relationship with the interfacial strength.5−8 However, the impact data obtained in this work imply that too high interfacial adhesion between the I

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increases with PMMA content from 5 up to 25% and then slightly decreased for sample with 30% PMMA. These observations are consistent with the impact strength data presented in Figure 7. The toughening micromechanisms responsible for high impact toughness of blends studied in this work will be discussed in the following sections. The fall in impact strength for ternary blend with 30 wt % PMMA can be due to the fact that too high interfacial strength between the PB-g-SAN domains and surrounding PLA matrix is not beneficial for impact resistance. In addition, at progressively higher loadings of PMMA in the PLA/PB-g-SAN/PMMA ternary blend, PMMA-rich domains would develop in the ternary system, as evidenced by the DMTA results in Figure 1, which adversely affect the impact strength of the resulting material because PMMA is highly stiff with low impact strength (Figure S2). 3.6. Thermal and Crystallization Behaviors. PLA is a typical semicrystalline polymer, and its physical, mechanical, and thermal properties are highly dependent on the crystallinity and crystalline morphology in the solid state. Therefore, the study of the thermal and crystallization behavior of the PLAbased blends prepared in this work is of great importance. The DSC thermograms recorded from the second heating scan for neat PLA and different PLA/PB-g-SAN/PMMA blends are shown in Figure 9. The detail results of DSC are summarized in Table 4.

PLA matrix and PB-g-SAN particles is not beneficial for materials’ capability for energy absorption and/or dissipation under impact loadings. The initial large and sudden raise in impact strength upon the addition of 5% PMMA into PLA/PB-g-SAN blend is undoubtedly due to improved interfacial bonding between the large PB-g-SAN agglomerates and PLA matrix, as evidenced by SEM micrographs in Figure 2a,b. According to SEM micrographs in Figure 2, the PLA/PB-g-SAN/PMMA ternary blends containing 10 and 15% PMMA showed further increase in the interfacial adhesion between the PB-g-SAN agglomerates and surrounding matrix, with little improvement in the dispersion state of PB-g-SAN structures in the PLA matrix. Therefore, it can be claimed that the corresponding increase in impact strength for samples with 5−15% PMMA is mainly related to the promotion of phase adhesion between the components rather than the change in the dispersion state of PB-g-SAN structures in PLA matrix. As stated earlier, these blend systems can be classified as tough PLA-based blends which are composed of extended PB-g-SAN agglomerated structures having good interfacial bonding with the surrounded PLA matrix (regime 2). The second jump in impact strength data was observed for blends containing 20% and higher contents of PMMA. These blends (with 20−30% PMMA) can be categorized as highly toughened blends (regime 3). The very high toughening effect of PB-g-SAN particles in these blend could be attributed to the better mixing state of PB-g-SAN terpolymer with PLA matrix and, consequently, much more homogeneous dispersion of PB-g-SAN particles in these systems as compared with the blends with lower content of PMMA (see Figure 2 and Figure 3). In other words, a change in the dispersion state of PB-g-SAN domains for ternary blends containing 20−30% PMMA compared with the blends with lower PMMA content, as observed by SEM images of Figure 2d−f, is responsible for the very high toughening effect. The breakdown of PB-g-SAN agglomerates into separated and more uniformly dispersed PB-g-SAN particles in the PLA matrix, first beginning at 20% PMMA, is responsible for a jump in impact strength by an increase in PMMA content from 15 to 20%. Further increase in impact strength at higher loadings of PMMA (25%) demonstrates that higher PMMA is required to further destroy the cohesive interaction between the PB-g-SAN particles and fully disperse the individual PB-g-SAN particles in the surrounding matrix. This statement was also confirmed by the fact that for blend with 25 and 30% PMMA the degree of wetting of PB-g-SAN particles with the surrounding matrix is highly improved, so that it is difficult to differentiate the PB-gSAN particles from the matrix phase (Figure 2d−f). Figure 7 also compares the trend of impact strength data with that of tensile strain values as a function of blend composition. It is interesting to observe that both tensile toughness (ductility) and impact toughness follow the same trend as a function of blend composition. This can further confirm the change of material toughness with blend composition achieved under either tensile or impact test in this work. The deformation behavior of impact specimens for PLA/PB-g-SAN/PMMA blends with different compositions under the notched Izod impact test is given in Figure 8. With the exception of neat PLA and binary PLA/PB-g-SAN blend, all PLA/PB-g-SAN/PMMA ternary blends exhibited a welldefined stress-whitened zone around and/or over the whole of impact-fractured surface, which is apparent in Figure 8a,b. It can be seen that the extent of stress whitening gradually

Figure 9. DSC heating flow curves for neat PLA and PLA/PB-g-SAN/ PMMA (70 − x/30/x) blend systems of different PMMA contents.

Contrary to DMTA results presented earlier, a single glass transition can be seen in the DSC curves of all PLA/PB-gSAN/PMMA ternary blends in Figure 9, indicates the complete miscibility of amorphous PMMA with semicrystalline PLA matrix in the blends regardless of PMMA content in the blend. The data in Table 4 demonstrate that the glass transition of PLA matrix in the ternary blends monotonically increased with the PMMA fraction in the blend. However, the extent of increase in glass transition with PMMA content according to DSC thermograms is much less significant than the DMTA results. As listed in Table 4, the glass transition of the PLA matrix has gradually increased with PMMA content in the blend from 59.89 °C for neat PLA to 64.53 °C for ternary blend containing 30 wt % PMMA. J

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Table 4. Thermal Properties of Neat PLA and PLA/PB-g-SAN/PMMA Blends of Different Compositions Obtained from the DSC Test samples (PLA/PB-g-SAN/PMMA)

Tg (°C)

Tcc (°C)

ΔHc (J/g)

Tm1 (°C)

Tm2 (°C)

ΔHm1 (J/g)

Xc (%)

100/0/0 (neat PLA) 70/30/0 65/30/5 60/30/10 50/30/20 45/30/25 40/30/30

59.89 60.85 61.40 62.32 62.54 63.57 64.53

98.73 103.36 137.94 140.60 − − −

35.46 24.30 10.87 5.34 − − −

150.10 151.75 − − − − −

168.20 168.23 167.34 167.88 166.40 167.07 −

49.91 30.41 12.59 5.00 0.97 0.29 −

53.26 46.35 20.67 8.90 2.07 0.69 0.00

density and viscosity of the resulting blend,52 with the result of increased cold crystallization temperature as the extent of intermolecular interactions was significantly increased. On the other hand, PMMA is a completely amorphous material and its incorporation into semicrystalline PLA matrix severely restricts the PLA macromolecular chains ability for crystallization. As a result, less PLA crystallites with highly imperfect microstructures would develop in blends with 5 and 10% PMMA compared with neat PLA sample. The limitations induced by the presence of PMMA molecular chains on the crystallization and crystalline structure of PLA component become more pronounced as the content of PMMA in the PLA/PB-g-SAN/ PMMA ternary blend was increased. For ternary blends with 20−30% PMMA no cold crystallization peak was detected in the DSC curves (Figure 9). In the case of main melting peak (Tm2), it can be seen that the Tm2 peak becomes gradually less intense as the weight fraction of PMMA in the ternary blends was increased, so that for ternary blends containing higher than 20% PMMA the Tm2 melting peak completely disappeared. In fact, the DSC data reveal that progressive introduction of PMMA at the range of 0−30% into PLA/PB-g-SAN blend gradually decreases the material’s capability for developing a crystalline structure, with the consequent of a steady reduction of degree of crystallinity of fabricated material. This is manifested by a steady decrease in ΔHc and ΔHm of PLA/ PB-g-SAN/PMMA ternary blend with PMMA weight fraction (Table 4). For ternary blend with 30% PMMA, the DSC results show the development of completely amorphous microstructure in the material. 3.7. Impact-Fractured Surface Analysis of the Blends. Study of the impact-fractured surface morphology provides useful information for understanding failure mechanisms and micromechanical deformations operating in the PLA-based samples. In this work, the representative impact-fractured surface morphologies were investigated using SEM technique. An overall view of the topography of the impact-fractured surfaces of some of PLA-base samples studied in this work is represented in Figure S3. The neat PLA shows very smooth and featureless fractured surface with no sign of much deformation, indicating a typical brittle mode of failure (Figure S3). The uncompatibilized PLA/ PB-g-SAN blend and blends compatibilized with 5 and 10% PMMA exhibited relatively rough impact fractured-surfaces (Figure S3). For samples containing 20−30% PMMA the appearance of impact fractured-surface changed into progressively smoother surface as the PMMA content in the material was increased (Figure S3). It is believed that the gradual improve in interfacial strength between the components in PLA/PB-g-SAN blend with PMMA content prevents the severe deformation and/or detachments from the bulk of the material under the impact

It is worth noting that the temperature span of the glass transition of the ternary blends is much broader than that of the neat PLA, which is suggested to arise from the local heterogeneity and concentration fluctuation in the segmental length scale.52 For neat PLA and PLA/PB-g-SAN binary blend an exothermic peak, attributed to the cold crystallization during heating can be observed in Figure 9. Compared with neat PLA, the cold crystallization temperature (Tcc) of PLA/PB-g-SAN blend occurred at higher temperature, indicating the heterogeneous nucleating effect of PB-g-SAN domains in the PLA matrix. Compared with the DSC data, the DMTA results (Table 3) revealed no change in the glass transition of PLA/PB-g-SAN binary blend (65.6 °C) compared with that of neat PLA (65.7 °C) as expected from the SEM images and interfacial tension measurements. Therefore, the increase in the Tcc of PLA/PB-gSAN blend compared with that of neat PLA is mainly due to the heterogeneous nucleating effect of PB-g-SAN domains in the PLA matrix. The double endothermic melting peaks ranging from 150 to 170 °C observed for neat PLA and binary PLA/PB-g-SAN blend are associated with the melting of PLA crystals,11,30,53 with the lower temperature melting peak (Tm1) relating to melting of less perfect PLA lamellar crystals and the higher temperature melting peak (Tm2) to melting of more perfect PLA crystals.11,30,53 The incorporation of PMMA into PLA/PBg-SAN blend caused a dramatic change in the melting and crystallization of resulting samples (Figure 9). For ternary blends containing 5 and 10% PMMA, the cold crystallization temperature was dramatically increased compared to PLA/PBg-SAN binary blend (Table 4), the extent of which is much greater than the amount of increase in the glass transition for theses blends. Therefore, one can conclude that this drastic increase in Tcc of these blends is mainly due to the enhanced phase adhesion between the submicron (average size of 430 nm) dispersed PB-g-SAN particles and surrounding PLA matrix, which prevents the interfacial slippage and, thereby, significantly promotes the heterogeneous nucleating effect of PB-g-SAN domains in the PLA matrix. The cold crystallization peak of these blends became very broad and much less intense compared with neat PLA and PLA/PB-g-SAN samples (Figure 9). Furthermore, both cold crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) underwent a significant drop upon the incorporation of 5 and 10% PMMA into PLA/PB-g-SAN blend (Table 4), reflecting the reduced crystallization of PLA component in these samples. The DMTA results showed that PMMA is partially miscible with PLA matrix. Moreover, owing to much higher molecular weight and melt viscosity of PMMA compared to PLA (see Dynamic Rheology), the addition of PMMA to PLA/PB-g-SAN blend enhances the entanglement K

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Macromolecules test. Therefore, the impact-fractured surface becomes smoother as the degree of interfacial bonding in the blend and, thereby, the yield strength of the material was increased (Figure S3). 3.8. Micromechanical Deformations and Toughening Mechanisms. This section investigates the micromechanical deformations accompanying the impact fracture process for different PLA/PB-g-SAN/PMMA blend systems. An attempt was also made to propose the toughening micromechanisms operating in the toughened blends. The SEM images taken from the impact-fractured surfaces of different PLA-based samples are represented in Figures 10 and 11. These images are

Figure 11. SEM images of the impact-fractured surfaces of PLA/PB-gSAN/PMMA (70 − x/30/x) blend systems containing different amounts of PMMA. Key: (a, a′) 20 wt %; (b, b′) 25 wt %; and (c, c′) 30 wt %.

surface of ternary PLA/PB-g-SAN/PMMA blends is quite different from that of PLA/PB-g-SAN binary blend. The high magnification micrograph in Figure 10b′ shows the shear yielding and plastic deformation of PLA matrix for ternary blend containing 5% PMMA. The lack of coarse interfacial debonding between the PB-g-SAN agglomerates and PLA matrix for this sample implies a good interfacial adhesion between the components in the blend, which is clearly visible in the micrographs. The matrix material around the PB-g-SAN agglomerates has been plastically deformed, and a number of fibrils have been developed during the fracture process. It can be seen that the incorporation of 5% PMMA into PLA/PB-gSAN blend activated the yielding and plastic flow of the matrix material through promotion of interfacial adhesion between the PB-g-SAN agglomerates and PLA matrix, which led to good stress transfer from the matrix onto PB-g-SAN particles under the impact loading. Moreover, close examination of the micrograph in Figure 10b′ reveals the formation of a number of tiny voids either within the PB-g-SAN agglomerates or at the interface between the PB-g-SAN agglomerates and PLA matrix. It is believe that these tiny voids are formed due to debonding and/or internal cavitation of PB-g-SAN rubber particles during the impact loading. The same micromechanical deformation processes can be observed for ternary blend containing 10% PMMA (Figure 10c,c′). Matrix plastic drawing is visible in the micrographs, and it seems that the extent of fibril formation under the impact-

Figure 10. SEM images of the impact-fractured surfaces of PLA/PB-gSAN/PMMA (70 − x/30/x) blend systems containing different amounts of PMMA. Key: (a, a′) 0 wt %; (b, b′) 5 wt %; and (c, c′) 10 wt %.

corresponding to the back of crack initiation region of impactfractured surface. For PLA/PB-g-SAN binary blend in Figure 10a,a′, the micrographs clearly reveal vast debonding of PB-gSAN agglomerated structures from the surrounding PLA matrix followed by the development of interfacial voids throughout the fractured surface. There is also evidence of detachment of PB-gSAN domains from the matrix during the impact fracture, leaving holes composed of tiny spots on the surface. These findings indicate very poor interfacial interaction between PB-gSAN domains and PLA matrix, resulting in inefficient stress transfer from the matrix onto rubber domains during the impact test. Therefore, unsatisfactory improvement in impact strength for PLA/PB-g-SAN blend is obtained compared with neat PLA (Figure 7). The morphology of impact-fractured L

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the resultant material. Both these factors adversely affect the material’s capability for energy absorbing shear yielding and plastic deformation processes under the impact loading.54−56 According to SEM micrographs described above, it is concluded that the matrix shear yielding and plastic deformation is the main source (mechanism) of energy absorption and/or dissipation under the notched Izod impact tests for both tough and highly toughened PLA-based blends prepared in this work. It is worth noting that the microvoid formation via interfacial debonding and/or internal cavitation of PB-g-SAN rubber particles, as detected in the SEM micrographs of Figures 10 and 11, also contributes to the activation of matrix shear yielding process. In fact, it is well documented that these two processes are interconnected with each other.54−56 Therefore, the deformation sequences in PLA/ PB-g-SAN/PMMA ternary blends studied in this work are probably consisted of the following stages: (1) stress concentration around the agglomerated structures or individual particles of PB-g-SAN terpolymer, (2) microvoid formation through partial debonding and/or internal cavitation of PB-gSAN rubber particles, and (3) relieving the concentrated triaxial stress fields followed by the activation of shear yielding process in the matrix ligament around and in-between the dispersed PB-g-SAN domains. 3.9. Dynamic Rheology. It is well-known that the dynamic rheological properties are highly sensitive to interfacial interactions and phase morphology in multiphase polymeric systems. The dynamic rheological properties as a function of angular frequency can provide reliable information about the microstructure of polymer blends. The change in interfacial interactions and dispersion state of the components upon compatibilization is usually manifested at low frequency regions, while the intermediate and high frequency regions are affected by the change in the bulk of blend’s components such as phase miscibility, branching, molecular weight, entanglement density, etc.57,58 Therefore, the dynamic rheology test was further employed in the present wok to substantiate our understanding of the phase miscibility, interfacial interactions and change in bulk properties of the PLA/PB-gSAN/PMMA ternary systems as a function of PMMA content in the blend. The frequency dependence of complex viscosity (η*), storage modulus (G′), and damping factor (tan δ) for neat polymers and different PLA/PB-g-SAN/PMMA blends are displayed in Figure 12. According to complex viscosity data in Figure 12a, the neat PLA showed a Newtonian behavior over the entire range of angular frequency, whereas neat PMMA and PB-g-SAN with much higher viscosities compared with pure PLA exhibited a strong shear thinning effect as a function of frequency. The difference between the viscosities of neat components becomes more pronounced as the angular frequency was reduced, and it is visible that the PB-g-SAN terpolymer had the highest viscosity among the neat components at low frequency region. The PLA/PB-g-SAN blend revealed much higher melt viscosity than the neat PLA over the entire range of frequency. In fact, the incorporation of 30 wt % PB-g-SAN into PLA completely disappeared the Newtonian behavior of PLA so that a steady shear thinning response was observed with angular frequency. The same viscosity behavior is also obvious for PLA/PB-gSAN/PMMA ternary blends containing different amounts of PMMA. According to Figure 12a, the gradual increase in PMMA content from 5 to 30 wt % in PLA/PB-g-SAN/PMMA ternary

fracture test has increased in this sample compared with the sample containing 5% PMMA. It can be seen that the shear yielding is mostly due to the deformation of matrix material located at the close proximity of the PB-g-SAN agglomerates. According to SEM micrographs of Figure 10c,c′ the highly stretched fibrils are formed at the interface between the PB-gSAN agglomerates and PLA matrix and/or as a bridging lines connecting the separated agglomerates to each other. It should be noted that the formation and development of these highly stretched fibrils is a viscoelastic process which absorbs and dissipates a considerable amount of the fracture energy. These fibrils not only can act as bridging lines to better distribute the applied load on the larger volume of the material, but also they can function as crack bridging lines which restrict the formation and propagation of microcracks through the material. The latter function of fibrils is especially important when they are formed at the interfacial region between the PBg-SAN agglomerates and matrix, as these sites are under high stress concentration and, therefore, are highly susceptible for microcrack initiation and/or propagation. The formation of some microvoids inside the PB-g-SAN agglomerates and/or at their interfacial region with the matrix is also visible in the micrographs (Figure 10c,c′), similar to what observed for sample with 5% PMMA. A smoother fractured surface was observed for PLA/PB-gSAN/PMMA ternary blends containing 20−30% PMMA as compared with the blends of lower PMMA content, as depicted in Figure 11. As stated earlier, this is probably due to higher degree of phase adhesion between the PB-g-SAN particles and PLA matrix in these ternary blends which increases the yield and ultimate strength of the material. For blend with 20% PMMA, the SEM images in Figure 11a,a′ show the yielding and plastic flow of the material under the impact test. Some tiny voids are also visible on the fractured surface which can be ascribed to debonding and/or internal cavitation of PB-g-SAN particles. Exactly the same deformation micromechanisms are also operating in the ternary blend composed of 25% PMMA as can be seen in Figure 11b,b′. In the case of blend with 30% PMMA, the SEM images in Figure 11c,c′ illustrate somewhat less shear yielding and plastic flow of the matrix material compared to the blends with 25% PMMA. Moreover, close examination of the micrographs reveal that the extent of microvoid formation on the fractured surface inside or around the PB-g-SAN rubber particles is highly reduced for blend with 30 wt % PMMA compared with the blend with 25 wt % PMMA. In fact, the micrograph of Figure 11c′ reveals that the PB-gSAN rubber particles are strongly adhered to the PLA matrix with no sign of the formation of tiny microvoids on the fractured surface. These observations clearly indicate much higher interfacial strength between PB-g-SAN particles and PLA matrix in ternary blend with 30% PMMA compared with the ternary blends with lower PMMA content. The lower capability of this sample for shear yielding and plastic deformation is responsible for its lower impact strength value compared with the sample with 25% PMMA (Figure 7). The less intense plastic flow of the sample with 30% PMMA is due to very strong interfacial bonding between the PB-g-SAN particles and PLA matrix in this sample. The very high degree of interfacial strength not only prevents the PB-g-SAN particles from partial debonding and/or internal cavitation under the impact load, but also significantly increases the yield stress of M

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blends gradually increased the melt viscosity of the resulting ternary blend at intermediate and high frequencies. At low frequency region, the melt viscosity of blends increased at first with PMMA content up to 20 wt % and then was not further affected by PMMA content for blends with higher PMMA content. The storage modulus data in Figure 12b demonstrate that PLA has much lower melt elasticity than the PB-g-SAN and PMMA. All the PLA-based blends showed much higher melt elasticity compared to the neat PLA. Moreover, the binary PLA/PB-g-SAN blend and different ternary blends represented a plateau response at low frequencies, indicating a solid-like behavior with nonterminal relaxation. The solid-like behavior arises from the presence of agglomerates and/or network-like structures of PB-g-SAN domains dispersed in the different blends.57,58 Similar to melt viscosity discussed above, the gradual incorporation of PMMA into PLA/PB-g-SAN blend from 5 to 30 wt % steadily raised the storage modulus of the blend at intermediate and high frequencies. However, at low frequency region, the storage modulus increased at first with PMMA content up to the ternary blend with 20% PMMA and then remained unchanged with further loading of PMMA into blend, probably due to saturation of interfacial region with PMMA (Figure 12b). Since neat PMMA has much higher melt viscosity and elasticity compared with the neat PLA, the steady increase in the viscosity and elasticity of the PLA/PB-g-SAN/PMMA ternary blends with PMMA content at intermediate and high frequency regions indicate the partial miscibility of PMMA component with the PLA matrix in the ternary blends. This statement is further confirmed by the fact that the dynamic viscoelastic properties at intermediate and high frequencies are dominated by the bulk properties of the blend’s components rather than the interfacial region between the phases.57,58 In the case of damping factor, the data in Figure 12c,d show that the PB-g-SAN and PMMA have much lower loss factor compared with PLA. Therefore, the PLA-based blends also have lower loss factors than the neat PLA. The data further demonstrate that the damping capacity of the ternary blends at high frequencies gradually reduced with PMMA content in the blend while a reverse trend can be observed at low frequencies as a function of PMMA content (Figure 12d). The former is due to the lower damping of the PMMA homopolymer compared with neat PLA, while the latter is ascribed to the change in the interfacial interactions and phase morphology of the PLA/PB-g-SAN blend with PMMA content. It is well documented that the Cole−Cole plots provide useful information about the structural changes in multiphase polymer systems.52 For ternary blends studied in this work the Cole−Cole plots (Figure S4) showed linear curves (contrary to neat PLA and PMMA which showed semicircular arcs) and an increase in PMMA content in the blends gave rise to a change in the plots at both high- and low-frequency regions. In fact, it was found that, with a gradual increase in PMMA content, the curves shifted toward the neat PMMA component, indicating the miscibility of PLA/PMMA pair in the ternary system. The plot of the phase angle δ as a function of the absolute value of the complex modulus |G*| (van Gurp−Palmen (vGP) plot),59 was used additionally to evaluate the miscibility of the components in the PLA/PB-g-SAN/PMMA ternary blends (Figure S5). For neat PLA and PMMA, the δ values increased with the reduction of |G*|. Because of much higher melt

Figure 12. Frequency dependence of dynamic viscoelastic properties of neat components and PLA/PB-g-SAN/PMMA (70 − x/30/x) blend systems of different compositions. Key: (a) complex viscosity; (b) storage modulus; (c, d) damping factor. The effect of PMMA on the damping factor of ternary blends is better depicted in part d. N

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improved the dispersion state of PB-g-SAN terpolymer in PLA matrix, which in turn led to very high toughening effect. At the same time, PMMA significantly enhanced the glass transition and melt strength of PLA matrix as evidenced by DMTA and dynamic rheological studies. Miscibility and thermal properties of the PLA/PB-g-SAN/PMMA ternary blends were investigated by DMTA and DSC techniques, respectively. Highly tough ternary blends exhibited negligible degree of crystallinity with significantly higher glass transition temperatures (ranging from 10 to 21 °C) than the neat PLA, depending on the PMMA content. It was found that too high interfacial strength is not necessarily beneficial to achieve a higher degree of impact toughness as the stress-relieving deformation micromechanisms become more restricted at very high levels of interfacial bonding. The fractography analysis of the impact-fractured surfaces of the toughened blends revealed that the massive shear yielding and plastic flow of the matrix material was the main source of energy absorption and/or dissipation during the impact test, which was triggered by suitable interfacial adhesion and microvoid formation around and/or inside the PB-g-SAN rubber particles. The present work provides a facile method to prepare highly toughened PLA/PB-g-SAN/PMMA ternary blends with far more impact toughness and significantly higher glass transition temperature and melt strength values than those for the previously reported PLA/PB-g-SAN blends.

viscosity and elasticity of the neat PMMA compared with the neat PLA (Figure 12), the PMMA showed much lower δ range with greater |G*| value. The lowest range of δ and the greatest range of |G*| were related to neat PB-g-SAN, as expected (Figure S5). The PLA/PB-g-SAN/PMMA blends of different PMMA content exhibited predominant elastic response (Figure S5). The results demonstrated that the maximum δ decreased, the vGP plots shifted toward lower δ values, while the |G*| gradually increased as the PMMA fraction in the ternary blends was increased (Figure S5). The decrease in δ together with the shift of |G*| toward higher values indicated the enhanced elasticity of the ternary blends with PMMA content, which originated from the increased entanglement density52 of the PLA/PMMA pair in the blend with PMMA content as a result of their mutual partial miscibility. These findings are in line with those observed in Figure 12 for the effect of PMMA content in these ternary blends on the frequency dependence behavior of η*, G′, and tan δ. In summary, the dynamic viscoelastic properties discussed above reveal that PMMA affects the rheological properties of the PLA/PB-g-SAN/PMMA ternary blends at both low and high frequency regions. The effect of PMMA content on the low-frequency behavior of PLA/PB-g-SAN/PMMA blends (observed for blends up to 20 wt % PMMA, probably due to interfacial saturation) stems from its localization around and/or in-between the PB-g-SAN particles in the blend, which improves the interfacial interactions between the PB-g-SAN and surrounding PLA and, thereby, promotes the dispersion state of PB-g-SAN particles in the PLA matrix. The impact of PMMA on the rheological properties at intermediate and high frequencies (observed for all blends, up to 30 wt % PMMA) originates from its partial miscibility with PLA matrix and also the formation of PMMA-rich domains in the ternary blends with 20−30 wt % PMMA as evidenced by DMTA results. By considering that PMMA has much higher melt viscosity and elasticity compared to neat PLA, its incorporation into PLA/ PB-g-SAN blend followed by its molecular-scale mixing (as a result of its partial miscibility) with PLA macromolecular chains increases the entanglement density of the blend matrix.52 The degree of improvement in the entanglement density of PLA is in direct relationship with PMMA content in the blend for compositions studied in the present work.52 The increase in entanglement density of the matrix causes an increase in melt viscosity and elasticity followed by a decrease in damping factor of the resulting blend at intermediate and high frequency regions. This is because the increase in intermolecular (physical) interactions via chain entanglements generates more restrictions for macromolecular flow, movements and/ or slippage under shear deformations. In addition, according to the rheological results one can infer that the incorporation of PMMA into PLA/PB-g-SAN blend would significantly increase the melt strength PLA matrix and thereby the resulting system. This is because the macromolecular chain relaxations are severely retarded as the entanglement density and physical interactions in the resulting blends were increased.52,57,58 This enhanced melt strength is of great importance, as one of drawbacks of PLA is its low melt strength.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00557. Characteristics of the PB-g-SAN terpolymer, notation and composition of different samples studied in this work, tensile properties, notched Izod impact strengths, SEM micrographs, Cole−Cole plots, and Van Gurp− Palmen plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.M.M.). ORCID

Majid Mehrabi Mazidi: 0000-0003-1889-6258 Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS Owing to its partial miscibility with PLA matrix and miscibility with SAN shell of PB-g-SAN terpolymer, PMMA promoted the interfacial interactions between the components in PLA/PB-gSAN/PMMA (70 − x/30/x) ternary blends and significantly O

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DOI: 10.1021/acs.macromol.8b00557 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00557 Macromolecules XXXX, XXX, XXX−XXX