Polyethylene Blends via an

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

Toughening of Cocontinuous Polylactide/Polyethylene Blends via an Interfacially Percolated Intermediate Phase Ali M. Zolali and Basil D. Favis* CREPEC, Department of Chemical Engineering, École Polytechnique de Montréal, Montréal, QC, Canada H3C 3A7 S Supporting Information *

ABSTRACT: It will be shown that an interfacially percolated rubbery phase in a cocontinuous polylactide (PLA)/linear lowdensity polyethylene (LLDPE) blend results in a significant increase in the impact strength. All blends possess a tricontinuous phase morphology in which poly(ε-caprolactone) (PCL), poly(ethylene−methyl acrylate) (EMA), and ethylene−methyl acrylate−glycidyl methacrylate (EMA-GMA) percolate at the interface of PLA/LLDPE but offer different toughening and compatibilization effects. Among these components, the addition of EMA-GMA to the binary PLA/ LLDPE blend reduces the cocontinuous PLA/LLDPE phase thickness from about 25 to 5 μm and yields a very tough material with an impact strength of about 515 J/m, which is approximately 13 times greater than the original cocontinuous PLA/LLDPE blend and more than 32 times that of PLA. The ternary blends show significant improvements in the impact strength within the tricontinuous region; however, the principal differences in the toughening effects are attributed to interfacial interactions between the phases. The interconnected network of the rubbery phase is expected to percolate the stress field throughout the tricontinuous system and reduce the detrimental dilatational stress in the bulk blend.

1. INTRODUCTION In recent years, the blending of polylactide (PLA), a biobased and biodegradable polyester which possesses high mechanical strength and processability, with other bioplastics and petroleum-based plastics has attracted much attention in both industry and academia.1,2 These studies mainly report on the toughening of PLA as its broader application is limited due to its inherent brittleness. Polyethylene (PE) is one of the most important commodity polymers that is produced both from oil and renewable resources and is used in toughening of PLA.3−5 These polymers are thermodynamically immiscible and require compatibilizers to achieve stable morphologies and superior mechanical properties.6,7 Polylactide−polyethylene block copolymers have been used to compatibilize PLA/PE blends and were found to be effective in toughening by enhancing the interfacial adhesion.3,8 Reactive compatibilizers such as copolymers of ethylene−glycidyl methacrylate (E-GMA) and ethylene−methyl acrylate−glycidyl methacrylate (EMA-GMA) have also been employed in the compatibilization of polylactide/polyolefin blends.4,9 The results show that PLA end groups react with the epoxide groups of the compatibilizers and form graft copolymers, which reinforce the interface and improve mechanical properties. Typically, in those studies the toughening of PLA was increased by a factor of 2−5. The majority of toughening studies of polymeric systems concentrate on the dispersed/matrix morphology. Ternary, quaternary, and even quinary blending is an emerging field in polymer blending which is growing and evolving at appreciable © XXXX American Chemical Society

rates. Such polymer blends form discrete phases in which the control of complex morphologies and multiple interfacial properties are the keys in obtaining materials with superior functional properties. In a recent work, we reported on the ultratoughening of PLA in a multiphase polymer material composed of percolated poly(ether-b-amide) (PEBA) at the cocontinuous interface of PLA/PA11.10 It was shown that a marked improvement in the toughening is only achieved when a tricontinuous morphology develops. Beyond this compositional region no considerable toughening is observed. Although the tricontinuous morphology was found to be essential in the toughening, ultratough behavior was only achieved when interfacial interactions were promoted by increasing PLA chain mobility at the interface through plasticization.11 Hashima et al.12 reported toughening of PLA by blending with hydrogenated styrene−butadiene−styrene block copolymer (SEBS) with the aid of E-GMA as a reactive compatibilizer. Further incorporation of polycarbonate (PC) to the ternary blend resulted in a material with ultrahigh toughness and thermal resistance aging properties. The results were attributed to the strong interfacial adhesion and negative pressure effect of SEBS at the interfaces which facilitate the plastic deformation of the PLA and PC matrices. In another work, Liu et al.13,14 systematically studied the relationship between phase morpholReceived: March 3, 2018 Revised: April 24, 2018

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DOI: 10.1021/acs.macromol.8b00464 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

speed set to 180/190/180 °C from hopper to nozzle and 100 rpm, respectively. 2.2. Rheological Properties. Rheological measurements were performed using a controlled-stress rheometer Physica MCR 501 from Anton Paar equipped with a 25 mm parallel plate disk geometry. Samples were first compression molded into disks of 25 mm diameter and 1.2 mm thickness and then conditioned at 50 °C in a vacuum oven before testing. A time sweep test was performed to determine the temperature stability of samples at 180 °C in which all samples showed acceptable thermal stability (less than 10% viscosity reduction) over more than 40 min. The frequency sweep tests were performed at 180 °C and a strain of 1%, and the complex viscosity of the neat polymers is presented in Figure 1.

ogy and interfacial adhesion with the impact behavior in PLAbased ternary polymer blends. They found that high impact toughening can be achieved through an optimum phase morphology in conjunction with suitable interfacial adhesion. Similar results have been reported in the literature on the effect of morphology and interfacial adhesion on the toughening of multicomponent polymer blends.15−18 The study of the mechanical properties of multiphase polymer blends with cocontinuous and tricontinuous phase morphology is rare despite their potential for multifunctional applications. Although considerable studies exist on multiphase blends with matrix/dispersed phase structures and the relationship between their morphology and interfacial adhesion with the mechanical properties, it is still largely unknown how the interfacial interactions can influence the mechanical properties of systems with tricontinuous morphologies. This work aims to gain an understanding of the morphology and interfacial properties in PLA/PE based ternary blends with cocontinuous and tricontinuous phase structures with a view to develop high performance biobased materials. The morphology development of PLA, PE, and three different components, i.e., poly(ε-caprolactone) (PCL), poly(ethylene−methyl acrylate) (EMA), and EMA-GMA, will be investigated. The effects of morphology and interfacial adhesion on the mechanical properties will be thoroughly examined.

2. EXPERIMENTAL METHOD 2.1. Materials and Sample Preparation. Commercially available polymers of PLA and LLDPE were used as the main components. In order to modify the mechanical properties, poly(ε-caprolactone) (PCL), random poly(ethylene−methyl acrylate) (EMA) copolymer with 24 wt % methyl acrylate content, and a random terpolymer of ethylene−methyl acrylate−glycidyl methacrylate (EMA-GMA) with 24 wt % of methyl acrylate and 8 wt % of glycidyl methacrylate content were employed as the third component.19 The information on suppliers and grades of all material used in this study is summarized in Table 1. PLA and PCL were dried at 50 °C under vacuum overnight before being used in the experiments.

Figure 1. Complex viscosity of the neat polymers vs angular frequency at 180 °C. 2.3. Scanning Electron Microscopy (SEM). A cryo-microtome machine (Leica RM 2065 equipped with a cryo-chamber LN21) was employed to prepare samples for SEM observation. The Izod impact fractured samples were used as is without further modification or were cryo-fractured in a liquid nitrogen batch. PCL was extracted from the microtomed samples using acetic acid, and samples containing EMA or EMA-GMA were conditioned using cyclohexane to remove EMA or EMA-GMA and create a contrast between phases. The specimens were dried at 50 °C under vacuum in an oven and then coated with gold/ palladium by plasma deposition. The surfaces were then observed using a field emission SEM unit (JSM 7600TFE, JEOL) operated at a voltage of 2 keV. 2.4. Interfacial Tension Measurement. The interfacial tension between components was measured according to the breaking thread method. The method is based on the distortion and breakup of a molten cylindrical thread of a polymer in the matrix of the other polymer. In order to achieve this, threads (40−70 μm in diameter) were spun out of the melt and then annealed at 40 °C for 24 h under vacuum to remove the residual stress. A thread of the polymer with the higher melting temperature between two films of the other polymer was placed in a Mettler FP-82HT hot stage operating at 180 °C, and the sinusoidal distortions of the thread over time were then recorded using an optical microscope from Nikon (Optiphot-2) equipped with Streampix v.III recording software. The results of at least 5−10 measurements were analyzed using SigmaScan v.5, and the results are reported in Table 2. Further details of the technique can be found elsewhere.20 2.5. Phase Morphology Characterization. SEM micrographs were used to analyze and quantify the morphology. A digitizing table from Wacom and SigmaScan V.5 software was employed to calculate the cocontinuous domain size and the thickness of the middle phase. The measurements were done with the assumption that the

Table 1. Main Characteristics of Materials Used in the Research material

supplier

PLA LLDPE PCL EMA

NatureWorks ExxonMobil Perstorp DuPont

EMAGMA

Arkema

grade 3001D LL3402 Capa 6800 Elvaloy AC 12024s Lotader AX8900

Mwa (kg/mol) 152

b

80c

45d

density at 25 °C (g/cm3) 1.24 0.94 1.14 0.94 0.94

a c

Mw: weight-average molecular weight. bObtained from ref 20. Obtained from ref 21. dMn was obtained from ref 9.

Ternary blends of PLA/x/LLDPE, where x represents PCL, EMA, or EMA-GMA, with mass compositions of 50/x/50 were prepared through a melt blending process. All sample preparations were carried out on a corotating twin-screw extruder (TSE), Leistritz ZSE 18HP with an L/D ratio of 40, at a screw speed of 100 rpm with a temperature profile of 160/170/170/180/180/180/180 °C from hopper to die. The extrudates were quenched in a cold−water bath and pelletized and dried prior to injection molding. Dog-bone specimens of type I (ASTM D638) and impact test bars (dimensions 12.7 × 63.5 × 3.2 mm) were injection molded using a Sumimoto SE50S injection molding machine at a temperature profile and screw B


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Macromolecules Table 2. Interfacial Tension Values and Spreading Coefficients at 180 °C system

spreading coefficient (mN/m)

predicted morphology

PLA/PCL/LLDPE

γPLA/PE = 6.4 ± 0.2 γPLA/PCL = 2.0 ± 0.7 γPE/PCL = 3.3 ± 0.5

λPLA/PCL/PE = 1.1 > 0 λPCL/PLA/PE = −5.1 < 0 λPCL/PE/PLA = −7.7 < 0

complete wetting: PCL completely spreads at the interface of PLA and LLDPE

PLA/EMA/LLDPE

γPLA/PE = 6.4 ± 0.2 γPLA/EMA = 2.1 ± 1.2a γPE/EMA = 2.6 ± 0.8

λPLA/EMA/PE = 1.7 > 0 λEMA/PLA/PE = −5.9 < 0 λEMA/PE/PLA = −6.9 < 0

complete wetting: EMA completely spreads at the interface of PLA and LLDPE

γPLA/PE = 6.4 ± 0.2 γPLA/EMAGMA = 1.5 ± 0.9a γPE/EMAGMA = 2.8 ± 0.8

λPLA/EMAGMA/PE = 2.1 > 0 λEMAGMA/PLA/PE = −5.1 < 0 λEMAGMA/PE/PLA = −7.7 < 0

complete wetting: EMA-GMA completely spreads at the interface of PLA and LLDPE

PLA/EMA-GMA/LLDPE

a

interfacial tension (mN/m)

Obtained from ref 30.

continuous domains in the PLA/LLDPE cocontinuous network are successions of cylinders with a diameter D. The interfacial perimeter P of the PLA/PA interface was measured and used to calculate the specific PLA/LLDPE interfacial area S using eq 121

S=

P A

3. RESULTS AND DISCUSSION 3.1. Interfacial Tensions and Spreading Coefficients. A widely used thermodynamic model of predicting the phase morphology of immiscible ternary blends is the spreading coefficient model, which employs the interfacial tensions between components to assess the final thermodynamically favorable morphology.25,26 This model defines the thermodynamic tendency of one component to engulf a second component in the matrix of a third component using eq 425,27

(1)

where A is the area of the analyzed micrograph. The average pore size was then determined with eq 222 D=

4ϕp S

λijk = γik − γij − γjk

(2)

where D represents the average pore diameter and ϕp is the volume fraction of the extracted phase. 2.6. Solvent Extraction and Gravimetry. The continuity of PLA and the middle phase component, i.e., PCL, EMA, or EMA-GMA, was measured using a gravimetric solvent extraction technique. The phase mass before and after extraction is measured by weighing samples and then using the following equation

⎛m − mfinal ⎞ ⎟⎟ × 100 %continuity = ⎜⎜ initial mx ,initial ⎝ ⎠

(4)

in which λijk are the spreading coefficients and γij are the interfacial tensions between components. The interfacial tensions between components used in this study were measured through the breaking thread method and are listed along with the associated spreading coefficients for three different ternary systems in Table 2. As predicted by the spreading theory, one positive and two negative spreading coefficients describe the tendency of one phase to segregate the two other phases from each other which is the case in all three systems. The positive λPLA/x/PE values presented in Table 2 predict a complete wetting morphology where the x phase, i.e., PCL, EMA, or EMA-GMA, completely separates PLA and LLDPE and forms a complete layer at the PLA/LLDPE interface. It is worth mentioning that three negative spreading coefficients predict a partial wetting morphology in which none of the phases engulf other phases rather all phases meet each other at a three-phase line of contact. 3.2. Morphology. Figure 2 shows the phase morphology of the PLA/LLDPE (50/50) blend and those containing 10% of a third component. The binary PLA/LLDPE 50/50 blend has a cocontinuous phase morphology where each phase is labeled in Figure 2a. The LLDPE phase is distinguishable from the PLA phase from its high roughness due to its higher crystallinity. The results of gravimetry listed in Table 3 also confirm that the binary PLA/LLDPE 50/50 blend possesses a high level of cocontinuity with the PLA phase showing about 88% continuity and the remaining LLDPE being self-supporting (see Figure S1). The PLA/LLDPE matrix remains cocontinuous in the ternary blends for all the formulations with equal PLA and LLDPE concentrations, and every third component used in the study is localized as a layer at the interface of PLA/LLDPE. Figure 2b shows the tricontinuous structure of the PLA/PCL/ LLDPE 45/10/45 ternary blend after extraction of the PCL phase where voids are clearly detectable at the interface of

(3)

in which minitial and mfinal are the mass of sample before and after extraction, respectively, and mx,initial, the mass of component x before extraction, was used to calculate the continuity of component x. Chloroform was used to extract the PLA phase, acetic acid to extract PCL, and cyclohexane to extract EMA and EMA-GMA for 2 weeks under agitation. The results are the average of four separate measurements. 2.7. Differential Scanning Calorimetry (DSC). Thermal analysis was performed using a DSC Q2000 (TA Instruments) in two different modes. First, the glass transition temperature (Tg) of the components was determined using a DSC instrument operated at modulated mode at a heating rate of 2 °C/min with an oscillation amplitude and period of ±1.27 °C and 60 s, respectively, over the temperature range of −80 to 100 °C. Second, the conventional DSC mode was used to measure the other thermal transitions. Heating and cooling runs were performed at a heating/cooling rate of 10 °C/min from 25 to 200 °C. The calculation of the crystallinity of the components was based on the specific enthalpy of fusion of the perfect crystal, ΔHf = 93.7 J/g for PLA23 and 293 J/g for LLDPE.24 2.8. Mechanical Tests. A universal tensile testing machine (Instron 4400R) was used to perform tensile tests as per the ASTM D638 standard at a crosshead speed of 50 mm/min. An impact tester unit (CS-137C-176 CSI Custom Scientific Instrument) was employed to measure impact strength according to the ASTM D256 standard. All samples were conditioned at room temperature and 50% relative humidity for at least 40 h prior to testing. An average value of five replicates was reported for each formulation. C


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to over 95% for EMA and EMA-GMA. The continuity of the PLA phase is also found to be at 88−99% which, considering the remaining self-supporting porous LLDPE, suggests a tricontinuous morphology for the all ternary blends with the PLA/x/LLDPE 45/10/45 concentration. However, the third components used in these blends result in different compatibilization effects. The PLA (DPLA) and LLDPE (DPE) phase sizes of the blends are summarized in Table 3. The binary PLA/LLDPE 50/50 blend shows a phase size in the range of 22−26 μm, which remains at the same size scale with the addition of 10% PCL to the PLA/LLDPE (1:1) system. In contrast, the addition of EMA and EMA-GMA significantly reduces the PLA/LLDPE phase to about 5 μm, which is almost 5 times smaller than the original binary blend. The marked decrease in the phase size of the cocontinuous PLA/LLDPE is important since it shows that an interfacially percolated phase can act as a compatibilizer at the interface and reduce the phase size of a cocontinuous system. Figure 3 displays a schematic of phase ordering and possible interfacial interactions in these systems. In the PLA/LLDPE

Figure 2. SEM micrographs of (a) 50PLA/50LLDPE, (b) 45PLA/ 10PCL/45LLDPE, (c, c′) 45PLA/10EMA/45LLDPE, and (d, d′) 45PLA/10EMA-GMA/45LLDPE. Note that acetic acid is used to extract PCL and cyclohexane is employed to extract EMA and EMAGMA.

PLA/LLDPE. Also, some very tiny voids of about 300 nm, extracted from the PLA phase, are identified as PCL dispersed particles trapped within the PLA phase due to the high viscosity of polymeric systems. EMA forms a completely wet layer at the interface of PLA/LLDPE with a thickness of 0.6 μm which is significantly lower than the 1.9 μm thickness of the system with 10% (see Figure 2c). It will be shown that this can be attributed to the greater interfacial interactions of EMA with both the PLA and LLDPE phases compared with PCL, which only interacts with PLA as reported in the literature.28 Similar to EMA, EMA-GMA also develop into a 0.5 μm completely wet layer at the PLA/LLDPE interface (see Figure 2d). These are typical phase morphologies observed in a ternary blend with complete wetting behavior.26,29 The results of morphology quantification and continuity measurements for the blends are presented in Table 3. As the SEM images indicate, the gravimetry analysis also proves the existence of a tricontinuous phase morphology in all ternary blends with equal PLA and LLDPE concentrations. The continuity of the middle phase is in the range of 82% for PCL

Figure 3. Schematic illustrating the interfacial interactions in the PLA/ x/LLDPE systems.

50/50 blend, no interfacial interaction is envisaged and the cocontinuous phase size is the largest. Although the PCL phase thermodynamically favors to the formation of a completely wet layer at the interface of the PLA and LLDPE phases, no appreciable reduction in the phase size is observed. This can be attributed to the very limited interfacial interaction of PCL with PLA and no interaction with LLDPE. However, EMA and

Table 3. Morphological Characteristics of the Blends sample PLA/LLDPE 50/50 PLA/PCL/LLDPE 45/10/45 PLA/PCL/LLDPE 60/10/30 PLA/EMA/LLDPE 45/10/45 PLA/EMA/LLDPE 60/10/30 PLA/EMA-GMA/LLDPE 45/10/45 PLA/EMA-GMA/LLDPE 60/10/30 a

layer thickness (μm)

DPLA (μm)

DPE (μm)

PLA phase continuitya (%)

1.9 ± 0.7

22.6 ± 8.2 23.6 ± 8.7

26.6 ± 7.3 22.1 ± 8.7

88 ± 11 90 ± 9

(LDPE + x) phase continuitya (%)

middle phase continuitya (%) 82 ± 5

39 ± 5 0.6 ± 0.2

5.8 ± 2.5

8.5 ± 5

98 ± 6

97 ± 6 43 ± 6

0.5 ± 0.2

4.6 ± 1.6

5.3 ± 3.1

99 ± 5

95 ± 6 41 ± 5

Obtained from the gravimetry analysis. D


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Macromolecules particularly EMA-GMA are copolymers of ethylene and methyl acrylate and are expected to interact with both the PLA and LLDPE phases (c, d). It is worth mentioning that a commonly employed method to compatibilize cocontinuous blends is to add premade block copolymers or to use in situ generation of graft block copolymers through a reactive process at interface.30−32 Block copolymers with suitable design of chemistry, molecular weight, and chain structure can localize at the interface and stabilize the morphology; however, micelle formation often occurs and can reduce the effectiveness of additives by lowering the interfacial concentration of block copolymer. In contrast to block copolymers, no such a problem as micellation occurs in tricontinuous immiscible blends since the morphology of phase-separated multicomponent polymer blends is thermodynamically driven and is related to the interfacial energy between components. 25,27 Thus, the compatibilization effect observed here can be attributed to the thermodynamically favored phase morphology and interfacial interactions at the interfaces. These interfacial interactions are evaluated in section 3.4. 3.3. Mechanical Properties. Table 4 compares the impact strength, elastic modulus, tensile yield strength, and elongation

strength significantly increases to about 360 J/m and the elongation at break to about 40%. A substantial enhancement in the impact strength and elongation at break at 515 J/m and 70%, respectively, is achieved by the addition of 10% EMAGMA to the cocontinuous PLA/LLDPE 50/50. Although the impact strength and elongation at break improve through the addition of the third component to the cocontinuous PLA/ LLDPE system, only modest improvements in the elastic modulus and tensile strength are observed which are typically observed in the toughening of glassy polymer using rubbery components.33 Figure 4 shows the photographs of impact

Table 4. Mechanical Properties of the Pure Components and Various Melt Blended Systems

Figure 4. Photographs of impact fracture testing bar of (a) PLA/PCL/ LLDPE 45/10/45, (b) PLA/EMA/LLDPE 45/10/45, and (c) PLA/ EMA-GMA/LLDPE 45/10/45.

sample PLA LLDPE PLA/LLDPE 50/50 PLA/LLDPE 70/30 PLA/PCL/LLDPE 60/10/30 PLA/PCL/LLDPE 45/10/45 PLA/EMA/ LLDPE 60/10/30 PLA/EMA/ LLDPE 45/10/45 PLA/EMA-GMA/ LLDPE 60/10/30 PLA/EMA-GMA/ LLDPE 45/10/45

Izod impact strength (J/m)

Young’s modulus (GPa)

tensile strength (MPa)

elongation at break (%)

15.9 ± 0.7 470 ± 22 39.5 ± 2.3

4.3 ± 0.3 0.7 ± 0.2 1.5 ± 0.2

65.6 ± 1.2 18.2 ± 2 21.3 ± 0.4

3.6 >500 20 ± 2

29.6 ± 1.2

2.2 ± 0.1

34.3 ± 0.2

15 ± 4

51.4 ± 6.5

2.1 ± 0.1

26.7 ± 0.6

22 ± 5

100.0 ± 6.8

1.7 ± 0.2

22.8 ± 0.5

35 ± 5

89.8 ± 8.5

1.9 ± 0.1

29.2 ± 0.1

25 ± 6

358.8 ± 30.7

1.6 ± 0.1

22.2 ± 1.2

39 ± 5

138.1 ± 18.4

1.8 ± 0.1

28.1 ± 0.9

36 ± 4

514.6 ± 18.4

1.6 ± 0.1

24.4 ± 0.9

70 ± 12

fracture testing bars of the three different ternary blends. A partial break is observed in these samples in which the sample containing EMA-GMA shows the toughest fracture behavior with a clear whitening at the fracture area. As discussed in the Morphology section, the ternary blends with the PLA/LLDPE ratio maintained at 1 possess a tricontinuous morphology. Increasing the concentration of PLA in the samples containing a fixed 10% of middle component, i.e., changing the composition ratio of PLA/ LLDPE, converts the tricontinuous morphology to a matrix/ dispersed where a core−shell morphology possibly forms, in which LLDPE is the core phase and the middle component is the shell phase. The mechanical properties listed in Table 4 indicate that the impact strength of the ternary blends deteriorates upon the change in the PLA/x/LLDPE composition from 45/10/45 to 60/10/30. The gravimetry analysis results also show that the continuity of the LLDPE phase is around 40%, implying low continuity, i.e., approaching a dispersed phase morphology. These results suggest that the cocontinuous morphology plays a critical role in the toughening of the PLA/x/LLDPE systems. Figure 5 compares the notched Izod impact strength of the ternary PLA/x/LLDPE blends as a function of PLA content. The PLA content is given as a function of PLA in (PLA plus LLDPE), which is equal to 90% in the ternary systems containing a fixed third component at 10%. Thus, 50% PLA in Figure 5 refers to 45% PLA/10% x/ 45% LLDPE blend. Figure 5 shows that an important increase in impact strength is obtained at 50% PLA with an impact strength up to 515 J/m for the PLA/EMA-GMA/LLDPE blend while limited improvements are achieved above 60% PLA. Similar results have been reported for the ternary PLA/ polyether-b-amide/polyamide-11 blends in the literature in which the highest toughness has been achieved when all phases are fully continuous constructing a tricontinuous morphology.10 It should be noted that the tricontinuous region is estimated from the gravimetry and morphological analyses results.

at break of the neat polymers and blends. PLA is a glassy semicrystalline polymer demonstrating high modulus and tensile strength but poor elongation at break and notched Izod impact strength. Although it has been reported that the addition of linear low-density polyethylene to PLA can result in a significant improvement over the impact toughness of PLA,3 the binary blend of PLA/LLDPE 50/50 and 70/30 only show a slightly improved impact and tensile toughness of 39.5 and 29.6 J/m, respectively (see Table 4). This suggests that interfacial modification is necessary for obtaining enhanced mechanical properties as reported in the literature.5,8,19 The addition of 10% PCL to the binary PLA/LLDPE 50/50 increases the notched Izod impact strength to 100 J/m with a limited improvement in the elongation at break up to 35%. When 10% EMA is added to the binary PLA/LLDPE 50/50, the impact E


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specific interactions between PLA and LLDPE as no change in the glass transition temperature (Tg) and melting temperature (Tm) of the components is observed. The crystallinity of PLA, though, is increased from 2 to about 17%, which can be attributed to the enhanced nucleation of PLA by the molten LLDPE at the interface between the two phases.34 It has been reported that compatibilization with a suitable block copolymer is necessary to obtain a high performance PLA/LLDPE blend.3,6 The addition of PCL replaces the PLA/ LLDPE interface with the two new interfaces of PLA/PCL and LLDPE/PCL that possess significantly lower interfacial tensions than that of PLA/LLDPE at 6.4 mN/m (see Table 2). PLA and PCL have some interfacial interactions28 resulting in partial miscibility which is reflected in their low interfacial tension at 2.0 mN/m. LLDPE/PCL also shows a low interfacial tension at 3.3 mN/m. Although the addition of PCL is expected to improve the interfacial adhesion between the components, almost no change in the size of the phase morphology of the PLA/PCL/LLDPE system is observed when it is compared to that of the binary PLA/LLDPE 50/50 blend (see Figure 2b and Table 3). This suggests limited interfacial interactions exist between phases during morphology development, in particular between PCL and LLDPE. EMA, which is a copolymer of ethylene and methyl acrylate, however, has interactions with both the PLA and LLDPE phases. It has been shown that EMA is partially miscible with PLA due to the interactions between the methyl acrylate segments of EMA and PLA.35 Additionally, there is a natural affinity between the ethylene blocks of EMA and LLDPE which promotes the interfacial adhesion between the LLDPE and EMA phases. Accordingly, the significant reduction observed in the size of the cocontinuous PLA/LLDPE phase morphology can be attributed to the improved interfacial interaction of EMA with both PLA and LLDPE phases (see Figure 2c and Table 3). The thermal properties of the binary PLA/EMA and LLDPE/EMA blends are listed in Table 5 to further examine the possible interfacial interactions between components (see Figure S2). The Tg of EMA is increased from −48 to −30 °C in the blend of PLA/EMA 80/20. The other second-order transitions of LLDPE/EMA are difficult to characterize through DSC analysis due to segmental similarity between the ethylene blocks of EMA and LLDPE; however, the natural affinity between ethylene segments can potentially enhance the interfacial interactions. The interfacial interactions between EMA-GMA and PLA/ LLDPE are supposed to be further enhanced due to the presence of the reactive epoxide groups (glycidyl group) of GMA segments. In addition to the interactions of methyl acrylate groups of EMA-GMA with PLA chains,35,36 it has been shown that the reactive groups can react with the terminal groups of PLA (hydroxyl and carboxyl groups)12,37 and form graft copolymers at interface.13,14,38 The reduced interfacial tension of PLA/EMA-GMA at 1.5 mN/m suggests a much more compatible interface compared to the PLA/EMA interface due to the interfacial interactions and reactions at the interface. The LLDPE/EMA-GMA interface also possesses a reduced interfacial tension at 2.8 mN/m in comparison to that of the PLA/LLDPE interface at 6.4 mN/m The thermal properties of the PLA/EMA-GMA/LLDPE system also indicate good interactions between EMA-GMA and PLA/LLDPE. The Tg of EMA-GMA increases from −35 to −30 °C, and the Tg of PLA shows a 2 °C reduction in the

Figure 5. Impact strength and phase continuity of ternary blend formulations. The weight fraction of PLA is based on (PLA + LLDPE). The third, intermediate phase is held at 10% of the total weight. Hence, the compositions follow PLA/10/LLDPE where PLA + LLDPE = 90. Note that 0% and 100% represent pure LLDPE and PLA, respectively. The dashed lines are a guide to the eye.

To evaluate the influence of the middle phase content on impact strength, the concentration of this phase was varied from 0 to 15% in the ternary PLA/x/LLDPE blends, and the results are presented in Figure 6. For all three different middle

Figure 6. Impact strength as a function of the middle phase content in PLA/x/LLDPE where the PLA:LLDPE ratio is maintained at 1:1.

phase components, the impact strength increases with middle phase content and levels off at 10% at values of about 100, 370, and 530 J/m for PCL, EMA, and EMA-GMA, respectively. These results can be attributed to the interfacial coverage of the middle phase at the PLA/LLDPE interface which has been shown to correlate with impact strength of ternary systems with interfacially percolated rubbery phase.10 3.4. Interfacial Interactions. A range of interfacial interactions are possible between the components in the ternary PLA/x/LLDPE blends studied here. PLA and LLDPE are thermodynamically immiscible as is reflected in the high interfacial tension of PLA/LLDPE at 6.4 mN/m, which is presented in Table 2. The thermal properties of the binary PLA/LLDPE 50/50 blend also confirm that there are no F


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Macromolecules Table 5. Thermal Properties of the Pure Polymers and Blendsa Tg (°C)

samples PLAb LLDPEc PCLd EMAe EMA-GMAf PLA/PCL 80/20 LLDPE/PCL 80/20 PLA/EMA 80/20 LLDPE/EMA 80/20 PLA/EMA-GMA 80/20 LLDPE/EMA-GMA 80/20 PLA/LLDPE 50/50 PLA/PCL/LLDPE 45/10/45 PLA/EMA/LLDPE 45/10/45 PLA/EMA-GMA/LLDPE 45/10/45

Tm,PLA (°C)

61 −50 −48, −10 −35

Tm,LLDPE (°C)

170

Xc,PLA (%)

125.1 56 61e 50f 168.6

19.7

15 124.5

59b/−30e −46e 59b/−30f, −8f −35f 61b

166

17.2 4

124.3 167 169.3 167.9 168.4 168.5

Xc,LLDPE (%)

2

17.5 7

124.6 124.9 124.2 124.1 124.2

17.3 13 18.7 24.5

12.5 14.5 12.5 15.6 17.6

a f

Tg: glass transition temperature; Tcc: cold crystallization temperature; Tm: melting temperature; Xc: crystallinity (%). bPLA. cLLDPE. dPCL. eEMA. EMA-GMA.

Figure 7. SEM micrographs of the impact fracture surface at the notch root of (a, a′) PLA/LLDPE 50/50, (b, b′) PLA/PCL/LLDPE 45/10/45, (c, c′) PLA/EMA/LLDPE 45/10/45, and (d, d′) PLA/EMA-GMA/LLDPE 45/10/45. The white bars denote 10 μm.

binary PLA/EMA-GMA 80/20 blend, implying interfacial interactions between PLA and EMA-GMA. Figure 2d′ shows nanometer size fibrils, which bridge the gap between the PLA and LLDPE phases (the extracted EMAGMA phase); however, the gap in Figure 2c′ is completely extracted (the EMA phase). These fibrils are likely graft copolymers of PLA and EMA-GMA, PLA-g-(EMA-GMA), which extend from the PLA phase to the LLDPE phase and are not affected by cyclohexane due to the insolubility of PLA segments with the solvent. These are covalently bound to the PLA chains on the PLA side and physically interact on the LLDPE side through the ethylene segments of EMA-GMA.39 These interactions can reduce the interfacial energy and alter the dynamics of interface and the pressure difference across the interface which can consequently result in the formation of smaller interconnected domains.40−42 3.5. Toughening Mechanism. The mechanical properties can be explained based on the interfacial interactions between components. PLA and LLDPE are immiscible polymers with a very weak interfacial adhesion.6 This results in a poor stress transfer at the interface and failure of the blend under mechanical testing. The poor impact strength of the binary PLA/LLDPE 50/50 blend at about 39 J/m can be attributed to the coarse morphology and weak interfacial adhesion of PLA/

LLDPE. The addition of 10% PCL to the system replaces the weak PLA/LLDPE interface with two new interfaces which are stronger, at least the PLA/PCL interface, than the PLA/LLDPE interface. The higher interfacial adhesion along with the interconnected rubbery-like PCL network result in a considerable improvement in the impact strength to 100 J/m. The addition of 10% EMA as a middle phase to PLA/LLDPE, however, creates two new highly interactive interfaces which significantly enhances the mechanical performances. The high impact strength at about 360 J/m can be directly related to the higher interfacial adhesion and finer PLA/LLDPE domain size in the PLA/EMA/LLDPE 45/10/45 system. The highly reactive EMA-GMA demonstrates significantly more improved impact strength at about 515 J/m owing to the enhanced interfacial adhesion along with its very fine phase morphology. Multiple interfaces in multicomponent blends can affect the crystallinity of components through mainly nucleation enhancement at the interface.43 The crystallinity of PLA is increased through melt blending in the binary and ternary blends (see Table 5). In particular, the very low crystallinity of PLA at 2% is increased to above 13% (13−24.5%) in the ternary PLA/x/ LLDPE 45/10/45 blends. The nucleation at the rubber/matrix interface produces crystallographically oriented material at the interface region.3,44 These oriented layers possess a reduced G


Article

Macromolecules

Figure 8. SEM micrographs of the cryo-fractured surface of a cross section underneath the Izod impact fracture surface of (a, a′) PLA/LLDPE 50/ 50, (b, b′) PLA/PCL/LLDPE 45/10/45, (c, c′) PLA/EMA/LLDPE 45/10/45, and (d, d′) PLA/EMA-GMA/LLDPE 45/10/45. The white bars denote 10 μm.

resistance to plastic deformation45 which percolates at the interconnected interface of the cocontinuous phase morphology, resulting in tough behavior. Thus, in addition to the effect of the tricontinuous phase morphology and interfacial adhesion, the increased toughness in the ternary blends with tricontinuous phase morphology can also be attributed to the presence of overlapping crystalline layers at the interface. SEM micrographs of the impact fracture surface of the binary and ternary PLA/LLDPE based blends are presented in Figure 7. The binary PLA/LLDPE 50/50 blend shows a very rough fracture surface but a closer look at the fracture area reveals a brittle fracture behavior of the PLA domains as marked by arrows in Figure 7a′. The cryo-fractured surface under the impact fracture area in Figure 8a,a′ clearly shows interfacial debonding and crazing at the PLA/LLDPE interface. This suggests that the low impact strength of this sample at 39 J/m can be attributed to the brittle fracture of the PLA domains mainly due to the very poor interfacial adhesion between PLA and LLDPE at the interface. The ternary PLA/PCL/LLDPE 45/10/45 blend also demonstrates a rough fracture surface, but with some plastic deformation (Figure 7). Despite the rougher surface which is manifested in the improved impact strength at 100 J/m, the brittle fracture of the PLA domains as indicated by arrows in Figure 7b′ still limits the impact toughness of the system. Although the interfacial adhesion between the PLA and PCL phases is good, the relatively low impact strength obtained in this blend is attributed to the brittle fracture of the PLA domains mainly due to the poor stress transfer and crazing at the PCL/LLDPE interface with poor interfacial adhesion. The brittle fracture of the PLA domains and the interfacial debonding are indicated by arrows in Figures 7b′ and 8b′, respectively. Replacing PCL with EMA in the PLA/PCL/LLDPE 45/10/ 45 blend results in a much tougher system with a notched impact strength at about 360 J/m. This is reflected in the topography of the impact fracture surface in Figure 7c, showing a uniform rough surface with significant plastic deformation. Although the roughness of the surface seems to be reduced in these blends, the fine and uniform roughness implies much more energy dissipation as compared with the PCL middle phase blend due to a higher fracture surface formed during the fracture process. No domains demonstrating brittle fracture mode is found upon closer observation of the fracture surface of PLA/EMA/LLDPE 45/10/45, implying effective stress

transfer along the PLA/EMA and EMA/LLDPE interfaces due to good interfacial adhesion (see Figure 7c′). Figure 8c,c′ shows the cryo-fractured surface of the PLA/EMA/LLDPE 45/ 10/45 blend in which no debonding is observed, confirming substantial interfacial adhesion between components. A similar fracture behavior with much more intense matrix plastic deformation is observed in the fracture surface of the PLA/EMA-GMA/LLDPE 45/10/45 blend which is shown in Figure 7d. Furthermore, the cryo-fractured surface under the fractured area reveals no interfacial debonding in Figure 8d,d′. The fracture surface result and high notched Izod impact strength of 515 J/m obtained from this blend suggest that the very fine tricontinuous morphology and reinforcement of the PLA/EMA-GMA interface due to the interfacial interactions are very effective in the toughening of this ternary polymer blend. These results suggest a shear yielding mechanism in the toughening of the PLA/EMA/LLDPE and PLA/EMA-GMA/ LLDPE 45/10/45 blends which are in line with the previous studies on the toughening of multicomponent polymer blends.10,46 The results suggest that the interfacial adhesion is the main controlling factor in the toughening of the PLA/x/LLDPE systems with tricontinuous morphology. While some improvement in the interfacial adhesion is likely to occur due to entanglements at the interface,47 a significant enhancement is expected when specific interactions exist between components.48,49 Upon the addition of PCL to the binary 50/50 blend, the weak PLA/LLDPE interface is replaced by the PLA/ PCL and PCL/LLDPE interfaces. The PLA/PCL interface possesses a good interfacial adhesion while the PCL/LLDPE interface is still a weak one (Figure 3b). However, the interconnected rubbery network of PCL facilitates the process of stress transfer throughout the system and a marginal improvement in the impact strength is obtained. In contrast to PCL, EMA is a rubbery copolymer of ethylene and methyl acrylate which has good interactions with both the PLA and LLDPE phases (Figure 3c). It is well-known that rubber cavitation is the main dilatational deformation process and an important part of the toughening mechanism in rubbertoughened polymers.50 The cocontinuous structure of the PLA/LLDPE matrices provides a continuous interface in which the interfacially percolated rubbery network can cavitate throughout the sample. Thus, the percolated EMA at the PLA/LLDPE interface markedly improves the impact strength H


Article

Macromolecules of the system as it significantly lowers the dilatational stress by percolating stress throughout the system and effectively transferring stress along the phases. This significantly facilitates the dilatational deformation of the PLA domains and yielding process. In contrast to the tricontinuous PLA/EMA/LLDPE 45/10/45 system, the interconnected interfacial area, which extends over the entire sample, is significantly reduced in the systems beyond the tricontinuous composition region. This means that only discrete interfacial areas within the PLA matrix exist, and the stress fields around rubbery domains cannot effectively percolate throughout the sample. EMA-GMA is even more effective than EMA in the toughening of the PLA/ LLDPE blend due to the possibility of enhanced interfacial reactions, i.e., formation of graft copolymers at the PLA/EMAGMA interface13,14,38 and interpenetration of LLDPE segments of EMA-GMA into the amorphous phase of the LLDPE phase.39 Knowing that the only difference between the systems with EMA and EMA-GMA is the possibility of a graft reaction between the glycidyl groups of EMA-GMA and PLA, it shows to what extent strong interfacial adhesion can influence the toughening of a system with a tricontinuous morphology.

Basil D. Favis: 0000-0002-7980-3740 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the NSERC Network for Innovative Plastic Materials and Manufacturing Processes (NIPMMP) for supporting this work.

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4. CONCLUSION Three different systems based on the cocontinuous PLA/ LLDPE system are shown to form thermodynamically driven tricontinuous structures in which PCL, EMA, or EMA-GMA is percolated at the PLA/LLDPE interface as the middle phase. These components possess different levels of interfacial interactions with the PLA/LLDPE matrices which influence the morphology and toughness of the systems. The middle phase is found to compatibilize the cocontinuous PLA/LLDPE phase morphology where EMA-GMA results in the highest compatibilization effect by reducing the domain size from about 25 μm to about 5 μm while only a slight decrease is obtained with PCL at 22 μm. EMA is also effective in compatibilizing the morphology by reducing the cocontinuous domain size to about 7 μm. The toughness of the binary PLA/LLDPE 50/50 blend increases from about 40 to 100 J/m upon addition of PCL and further increases to about 360 J/m with EMA. However, an impact strength of about 515 J/m, a 13-fold improvement over the neat PLA/LLDPE blend and more than 32-fold improvement over the neat PLA, is achieved with EMAGMA. The results clearly indicate the importance of both a tricontinuous morphology and strong interfacial interactions in obtaining materials of high toughness. Since it is also possible to obtain polyethylene from renewable sources, these results provide a route to achieve high performance materials at high biobased content.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00464. Figures S1 and S2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; tel +1 514 340-4711 ext 4527; fax +1 514 340-4159 (B.D.F.). ORCID

Ali M. Zolali: 0000-0002-8459-068X I


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J


Polyethylene Blends via an

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