Enhancing Impact Resistance of Polymer Blends via Self-Assembled

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Enhancing Impact Resistance of Polymer Blends via Self-Assembled Nanoscale Interfacial Structures Yichen Guo,*,† Xianghao Zuo,† Yuan Xue,† Yuchen Zhou,† Zhenhua Yang,† Ya-Chen Chuang,†,‡ Chung-Chueh Chang,‡ Dilip Gersappe,† and Miriam H. Rafailovich*,† †

Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794, United States ThINC Facility, Advanced Energy Center, Stony Brook, New York 11794, United States

‡

S Supporting Information *

ABSTRACT: We have designed and engineered an environmentally sustainable ternary polymer blend with the mechanical properties comparable to high impact resistant conventional polymers under the guidance of the lattice self-consistent field model. In this blend system, poly(methyl methacrylate) (PMMA) was used as the compatibilizer for the poly(lactic acid) (PLA)/ poly(butylene adipate-co-butylene terephthalate) (PBAT) blend. We characterized the compatibility of those components and found PMMA was miscible with PLA and partially compatible with PBAT, which allowed it to self-assemble to a nanoscale interfacial layer on the PLA/PBAT interface. This PMMA layer can significantly decrease the interfacial energy and strongly entangle with either PLA or PBAT, resulting in the strengthening of the interface and dramatically enhancement of the impact resistance of the ternary blend. The optimal mechanical performance was achieved when the total PMMA concentration was less than 10 wt %. Higher PMMA content embrittled the blend since the additional PMMA did not contribute to the minimization of the interfacial energy but remained in the PLA phase, increasing the glass transition temperature of the matrix.

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INTRODUCTION Since 1950, over 8.3 billion metric tons of plastics has been produced worldwide, of which approximately 80% has already become plastic waste and only 21% of this waste has been properly recycled or incinerated.1 Since nearly 99% of the discarded plastics are nonbiodegradable conventional polymers, the accumulating waste is causing serious environmental problems of increasing magnitude. Biodegradable polymers are becoming increasingly more popular since they have the greatest potential of mitigating the plastic waste pollution problem. However, the choice in biodegradable polymers is much smaller, and hence it is frequently difficult to match the thermomechanical criteria required in manufacturing when compared with conventional polymers. Of all the biodegradeable polymers, poly(lactic acid) (PLA) shows the best potential due to its good thermomechanical properties, complete renewability, and low production cost relative to other biodegradable polymers.2,3 PLA has comparable mechanical properties to some common petroleum-based polymers such as polyethylene (PE), poly(ethylene terephthalate) (PET), and © XXXX American Chemical Society

polystyrene (PS), which allows it to replace those polymers in the applications of food packaging, liquid containers, and filtration membranes.3,4 However, the brittleness of PLA restricts its application in appliance housings, medical devices, automotive parts, and protective equipment, which are usually made of conventional high-impact-resistant plastics such as high-impact polystyrene (HIPS), acrylonitrile−butadiene− styrene (ABS), and polycarbonate (PC). Although the supertoughened PLA systems with very high impact strength can be achieved via blending PLA with glycidyl methacrylate (GMA)based copolymers or other high-impact-resistant elastomers,5 the required weight fraction of those nonbiodegradable copolymers and elastomers is usually more than 20%. In order to obtain better impact resistance without compromising the biodegradability, PLA has been toughened by a highly ductile biodegradable polymer, poly(butylene adipate-coReceived: February 7, 2018 Revised: April 14, 2018

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DOI: 10.1021/acs.macromol.8b00297 Macromolecules XXXX, XXX, XXX−XXX

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interfacial modifier. Izod impact testing shows that the addition of only a few percentage of PMMA can dramatically increase the impact strength of the ternary blend, and the blend can be endued with diverse mechanical properties via tuning the component ratios. The addition of such small amount of PMMA also ensures the polymer blend has minimal nonbiodegradable component ( χdPMMA/PBAT, and the complete miscibility of PLA and PMMA implies χPLA/PMMA ≤ 0. Those parameters excellently fit the design of the SCF model, indicating that PMMA (or dPMMA) can be employed as the compatibilizer in PLA/PBAT blend, which encourages the production of PLA/ PMMA/PBAT ternary blends. Since PLA and PMMA are F

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Figure 6. (a) DSC curves of PLA/PMMA/PBAT blends. (b) Tan delta vs temperature curves of PLA/PMMA/PBAT blends obtained by DMA. (c) Impact strength of PLA/PMMA/PBAT blends and PLA/PMMA blends vs PMMA weight fraction in the PLA−PMMA cophase.

Figure 7. Cross-sectional TEM images of ternary blends: (a) 70PLA30PBAT, (b) 66PLA4PMMA30PBAT, (c) 62PLA8PMMA30PBAT, and (d) 50PLA20PMMA30PBAT.

The impact resistance of the PLA/PMMA/PBAT ternary blends was evaluated by the Izod impact tests based on ASTM D-256. The results are plotted as impact strength vs PMMA weight fraction in the PLA−PMMA cophase in Figure 6c. From the figure, we see that without PMMA the impact strength of 70PLA30PBAT blend is 97 J/m. Replacing only 2.9 wt % of PLA with PMMA (68PLA2PMMA30PBAT sample) significantly increases the impact strength from 97 to 134 J/m. Increasing the PMMA content in the PLA−PMMA cophase causes the further improvement of the toughness. The 62PLA8PMMA30PBAT (11.4 wt % of PMMA in PLA− PMMA cophase) sample shows the highest impact strength as 168 J/m, which is 73% higher than that of 70PLA30PBAT sample. However, keep rising the PMMA content further results in the quick drop of the impact strength. To explore this phenomenon, we also plot the impact strength of PLA/PMMA binary blends vs PMMA weight fraction in Figure 6c (red line). The result shows that neat PLA has the impact strength around 34 J/m, while neat PMMA has the value only around 15 J/m.

The PLA/PMMA blends show a plateau (33−34 J/m) on the impact strength profile at low PMMA contents. Once the weight fraction of PMMA exceeds 20 wt %, the impact strength of the blend decreases significantly and quickly reaches the value of neat PMMA. This weight fraction value in binary blend corresponds to the 55PLA15PMMA30PBAT ternary blend which has 21.4 wt % PMMA weight fraction in the PLA− PMMA cophase. The ternary blend impact strength profile (black line) shows a critical point on 55PLA15PMMA30PBAT blend, where a huge impact strength decrease happens after this point. Therefore, we can conclude that the quick drop of the impact strength for the ternary blends with high PMMA contents can be attributed to the embrittlement of the PLA− PMMA cophase. The significant impact strength enhancement in PLA/ PMMA/PBAT ternary blends at low PMMA content implies the compatibilizing and binding effects of PMMA on the PLA/ PBAT interfaces. Herein, the toughening mechanism is somewhat different from rubber reinforcement which relies G

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Figure 8. Volume fraction of dPMMA vs bilayer thickness profiles for as-cast bilayer samples: (a) PBAT on 94PLA6dPMMA, (b) PBAT on 88PLA12dPMMA, (c) PBAT on 70PLA30dPMMA, and the corresponding annealed bilayer samples: (d) PBAT on 94PLA6dPMMA, (e) PBAT on 88PLA12dPMMA, and (f) PBAT on 70PLA30dPMMA. The annealing was conducted at 180 °C for 24 h. The insets show geometry of bilayer samples. The top layer surface roughness, Rq, was measured by AFM, and the fwhm interfacial width, w, and height, ϕ, were marked in each figure. Schematics of interfaces for the annealed bilayer samples: (g) PBAT on 94PLA6dPMMA, (h) PBAT on 88PLA12dPMMA, and (i) PBAT on 70PLA30dPMMA.

content to 11.4 wt % in PLA/PMMA cophase (62PLA8PMMA30PBAT sample, Figure 7c) results in further decreasing of PBAT domains, but further increasing the content to 28.6 wt % (50PLA20PMMA30PBAT sample, Figure 7d) shows insignificant change in domain size. It is worth to mention here that the average domain size of 62PLA8PMMA30PBAT and 50PLA20PMMA30PBAT samples is 1.5 μm, which is also the same size of PBAT domains in 100 wt % PMMA matrix shown in Figure 3b. Since the amphiphilicity of PMMA can drive it to segregate to the PLA/PBAT interfaces to lower interfacial tension, the phenomenon observed in TEM images indicates that the PMMA content on the PLA/PBAT interfaces may approach to the saturation when the overall PMMA content in PLA/PMMA cophase reaches about 11.4 wt %. The decrease of PBAT domains creates more interfaces and decreases the

only on van der Waals interactions and does not consider polymer chain structure.46 To investigate those effects, the cross-sectional structure of the ternary blends was imaged by TEM as shown in Figure 7. In the figures, we find that PBAT phase forms the dispersed domains with darker color; the PLA−PMMA cophase forms the continuous matrix with lighter color. Because of the close electron density and high miscibility, it is very hard to distinguish PLA and PMMA in the cophase. Nevertheless, we can still observe the obvious morphology changes in the samples with different PMMA contents. Figure 7a shows that 70PLA30PBAT has the PBAT domain size ranging from 2 to 3 μm, which is consistent with the SEM images above (Figure 3a). The 66PLA4PMMA30PBAT sample (Figure 7b) shows the PBAT domains with average diameter around 2 μm, indicating a certain extent of compatibilizing effect caused by the addition of PMMA. Rising the PMMA H

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component segregates to the interface between the two components with large incompatibility. This third phase serves the same purpose as the large aspect nanoparticles discussed in previous research,48 except that the tertiary polymer phase allows entanglements across the interfaces, while the large aspect ratio nanoparticles block the entanglements, weakening the interface.14 The shallow depletion grooves located between the peaks and PLA/dPMMA bottom layer region implies that the system has not but nearly reached the equilibrium. However, further annealing the samples to 30 h resulted in the dewetting of the PBAT top layer. Thus, it can be expected that the volume fraction of dPMMA at the interface of each sample shown in Figure 8d−f is slightly lower than the values at the equilibrium. The volume fraction of dPMMA segregated to the interface can be measured from the integral of the peak ϕ×w area, I = d (ϕ is peak height, w is peak fwhm, and d is the thickness of bottom layer). From Figure 8d, we see that for the PBAT on 94PLA6dPMMA sample, where the dPMMA was only 6% by volume of the PLA/dPMMA bottom layer, I = 4.5%, which means three-fourths of the dPMMA is positioned on the interface. The ϕ is 0.35, which implies that the dPMMA has not reached the saturation at the interface. Increasing the dPMMA volume fraction to 12% rises I from 4.5% to 9.3% (Figure 8e). In this case, the ϕ reaches 0.79, indicating nearly 80% of the interface has been covered by dPMMA, which drastically prevents contacts between PLA and PBAT. Once the interface is covered, any additional dPMMA does not contribute to decreasing the energy of the system; thus, the extra dPMMA remains within the PLA phase. This can be seen by the rapid rise of the dPMMA volume fraction in the PLA/ dPMMA bottom layer for the PBAT on 70PLA30dPMMA sample shown in Figure 8f. From the figure, we find that I only increases to I = 11.5%, or over 60% of the dPMMA remains uniformly dispersed in PLA phase. The ϕ shows a slightly higher value of 0.87 compared to the value (0.79) in PBAT on 88PLA12dPMMA sample. The trends of I and ϕ indicate that the segregation of dPMMA on the interface may reach a near saturation state when the dPMMA volume fraction in bottom layer is approximately 12%. This value is consistent with the TEM images shown above, where the average PBAT domain size initially decreased but then approached to a constant value when the PMMA weight fraction in the PLA/PMMA cophase was above 11.4 wt %. We can therefore assume that the equilibrium width of the interfacial layer can be estimated from the SIMS profile corresponding to 12% volume fraction of PMMA. From the figure, we can see w ∼ 131 Å ∼ 2Rg, which is in good agreement with the theoretical estimation. The SIMS profile clearly indicates that dPMMA segregates out of the miscible PLA/PMMA blend to form a monolayer across the interface. Since dPMMA is completely miscible with PLA, the layer is fully entangled. In addition, from the neutron reflectivity results, we have shown that the interfacial width between PBAT and dPMMA was 68.5 Å, which was approximately the Rg ∼ 70 Å of the dPMMA. Hence, this dPMMA interfacial layer with the width approximated to two polymer radius of gyration represents multiple entanglements crossing the interface, which can effectively enhance the interfacial adhesion of the blends.49,50 The structure of the dPMMA interface between PBAT and PLA can now be modeled as shown schematically in Figure 8g−i. In the case of PLA/PMMA/PBAT ternary blend, the PMMA is expected to form similar interfacial structure, where the entanglements of

interdomain distance, which enhances the craze termination ability of PBAT within the matrix. In order to trace the diffusive behaviors of PMMA within the ternary blend, a bilayer polymer thin film system was designed and profiled by SIMS. Since all three polymer components contain oxygen, carbon, and hydrogen, regular PMMA cannot be tracked in the system. Therefore, the deuterium-labeled dPMMA (same as the one used in neutron reflectivity measurements) was used to replace regular PMMA for diffusive tracking. PLA/dPMMA was first dissolved in chloroform (20 mg/mL) and then spun-cast onto the HF etched silicon wafers to form the bottom layers. Since PLA, PBAT, PMMA, and dPMMA used in this study have similar density, the volume fractions of PLA/dPMMA were selected as 94:6, 88:12, and 70:30 to correspond to the PLA−PMMA cophase component fractions of 66PLA4PMMA30PBAT, 62PLA8PMMA30PBAT, and 50PLA20PMMA30PBAT blends, respectively. It has been reported by several studies that solution-casting may cause the phase separation of PLA/PMMA blend due to the different solubility of two polymers with the solvent.30−32 In this case, we performed the AFM images in contact mode for both ascast and thermal annealed PLA/dPMMA bottom layers (Figure S2a−f), which show that the phase separation of the PLA/ dPMMA thin films caused by solution-casting can be removed by annealing the thin films at 180 °C for 30 min. The annealed PLA/dPMMA thin film bottom layers show the completely miscible morphology, which can be analogy to PLA/PMMA cophase in ternary blend. The PBAT top layer was produced as follows. The PBAT/chloroform (20 mg/mL) solution was made and spun-cast onto the UV-plasma-treated silicon wafers (1 cm × 1 cm) to form thin films. Those PBAT films were then floated off the wafers and scooped by the annealed PLA/ dPMMA bottom layers. This PBAT-top, PLA/dPMMA-bottom bilayer system can be vertically profiled by SIMS for dPMMA diffusion analysis. During the tests, a 800 μm × 800 μm area of the bilayer sample surface was rastered by a 20 kV argon + 2500 clusters gas cluster ion beam (GCIB) at an approximate rate of 300 Å /h; the sputtered negative secondary ions were collected and analyzed by a 30 kV liquid metal ion gun (LMIG) equipped with Ga+ ion source at 2−3 nA. The deuterium and silicon elements were collected to trace the diffusive position of dPMMA and determine the end of the films, respectively. The dPMMA volume fraction vs thickness profiles for both as-cast and annealed PBAT&PLA/dPMMA bilayers are plotted in Figure 8. In the figures, we can see that all the as-cast samples (Figure 8a−c) show the broad areas which can be corresponded to the interfacial areas. These unusually broad interfacial areas can be ascribed to the template effect induced by the outermost surface roughness which can transfer the roughness throughout the profiles.47 The roughness values of the top PBAT surfaces, Rq, were measured by AFM and given in each figure, where we can find that the as-cast samples have very rough surfaces with high Rq, which can be attributed to the film wrinkles generated during water floating. Those as-cast samples were annealed in a vacuum oven at 180 °C for 24 h to enable the diffusion of dPMMA within the system. The surface roughness measurements of the annealed samples show much lower Rq relative to their as-cast counterparts. The volume fraction vs thickness profiles obtained from the annealed samples were shown in Figure 8d-8f. The distribution of dPMMA can be seen in the bottom layer and also within a narrow peak at the interface. The presence of this peak is in excellent agreement with the SCF model, where the third I

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Figure 9. SEM images taken on the fracture surfaces of the blends: (a) 100PLA, (b) 88PLA12PMMA, (c) 70PLA30PMMA, (d) 70PLA/30PBAT, (e) 62PLA8PMMA30PBAT, and (f) 50PLA20PMMA30PBAT. All the imaging areas were close to the notch tip of the impact samples. (g) Schematics of fracturing mechanisms.

that of 100PLA (62.8 °C). The fracture surface of 70PLA30PMMA shown in Figure 9c, on the other hand, has no drawn fibrils and only very limited deformations. The Tg of 70PLA30PMMA is 68.9 °C, implying that in this case the rise in temperature caused by impact force at the notch-tip region cannot reach the Tg of 70PLA30PMMA, resulting in the nonplastic deformations on the fracture surface. This can also explain the trend in the impact strength profile of PLA/PMMA blends (Figure 6c) that the impact strength values show a quick drop when the PMMA fraction reaches 30 wt %. Figure 9d shows the fracture surface of 70PLA30PBAT blend, where we can see that the PBAT domains have been delaminated from the PLA matrix under the external impact force. Most of the domains remain spherical in shape, indicating that the bonding between two phases is weak, which is in agreement with our previous interfacial analysis. The 62PLA8PMMA30PBAT blend (Figure 9e), on the other hand, shows a much different picture where the whole surface was entirely stretched and deformed, and the phase separation cannot be identified easily. This morphology is desired for polymer toughening since this extent of polymer deformation can absorb and dissipate a large

the PMMA interfacial layer with PLA and PBAT are able to bond the weak PLA/PBAT interface, thereby strengthening the whole ternary blend. To investigate the binding effect caused by the formation of nanoscale PMMA interfacial layer between PLA and PBAT, SEM images were taken on the fracture surface of the blends after the Izod impact tests. The imaging areas were selected to closely near the notch-tip region of the impact samples. Figures 9a−c show the fracture surfaces of 100PLA, 88PLA12PMMA, and 70PLA30PMMA blends to correspond the matrix phases of 70PLA30PBAT, 62PLA8PMMA30PBAT, and 50PLA20PMMA30PBAT blends, respectively. From the figures, we can see that 100PLA generated plastic deformations and elongated fibrils on the fracture surface after Izod impact tests. This impact resistant favorable phenomenon can be correlated to the heat generation by the high strain rate of Izod impact tests, which resulted in temperature rising above the Tg of PLA at the notch-tip region.51 Incorporating 12 wt % of PMMA has no effect on generating plastic deformations and drawing fibrils during Izod impact tests (Figure 9b) since the Tg of 88PLA12PMMA blend (63.5 °C) is relatively the same as J

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Figure 10. Tensile properties of PLA/PMMA/PBAT blends: (a) tensile strength, (b) elongation at break, (c) Young’s modulus, and (d) tensile stress−strain curves.

Figure 11. Mechanical properties of the ternary blends and commercialized HIPS and ABS.

amount of energy during Izod impact tests. This fracture surface morphology transition can be ascribed to the strong interfacial bonding enhanced by the PMMA interfacial layer. The neutron reflectivity measurement indicates that the PMMA can diffuse as high as one Rg length to PBAT, which allows stronger chain entanglements between PMMA and PBAT on the interface. In the case of 62PLA8PMMA30PBAT blend, during Izod impact tests, the PBAT domains were easily stretched, which simultaneously induced the stretching of the contacted PLA surfaces via interfacial chain entanglements, resulting in the large plastic deformation on the fracture surface and the best impact resistance. The 50PLA20PMMA30PBAT blend shows a significant decrease in the impact strength profile. To explore this decrease, the sample fracture surface was imaged in Figure 9f, where we find that PBAT domains still have good contact with the matrix, but only the PBAT domains stretch. One possible explanation for this morphology is that the PLA−PMMA cophase (28.6 wt % of PMMA) did not plastically deformed under impact force, which is consistent with the observation in 70PLA30PMMA sample, so that the stretching of PBAT domains failed to induce the large plastic deformation on the whole fracture surface. This plastic to nonplastic transition in PLA−PMMA cophase causes the matrix embrittlement and leads to the impact strength falling back after the point of 62PLA8PMMA30PBAT in the impact

strength profile. The schematics of fracturing mechanisms corresponding to the blends mentioned above are shown in Figure 9g. The tensile properties of the PLA/PMMA/PBAT ternary blends were measured in order to compare with the impact strength measurements. The tensile strength, elongation at break, and Young’s modulus results are shown in Figure 10a−c, and the stress−strain curves of the ternary blends are plotted in Figure 10d. From the figures, we see that both tensile strength and Young’s modulus slightly increase as the increase of PMMA content in PLA−PMMA cophase. This is because PMMA has higher intrinsic moduli than PLA. The elongation rates, on the other hand, show insignificant changes at low PMMA content but quickly drop after the content exceeds 15%. It has been reported that in PLA/PBAT system the tensile elongation rate was mainly determined by the shear yielding of the PLA strands between PBAT particles.6 Hence, the PMMA strengthened PLA/PBAT interface has a minimal effect on the elongation reinforcement. As the increase of PMMA content, the Tg of the cophase increases, which dramatically restrict the chain mobility and cause the decrease in elongation rate. The impact toughness corresponded to the integral of the plastic regime can be seen in the stress−strain curves shown in Figure 10d, where we find that there is not significant improvement in impact toughness at low PMMA content. The difference K

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homopolymer has a similar impact as the addition of block copolymers, but it is a far more versatile and cost-effective.

between this impact toughness obtained from tensile tests and the impact strength measured by Izod impact tests can be attributed to the time scale of the measurements. In the tensile tests, the samples were stretched in a relatively slow manner, while in the case of Izod impact tests, the samples were deformed within a very short period. As mentioned above, only this high strain rate in Izod impact tests can rise the temperature at the notch-tip region, thereby allowing the plastic flow of the matrix. Therefore, the impact reinforcement of the ternary blends can only be observed in Izod impact tests. The mechanical properties of the ternary blends with different PBAT contents were also evaluated. The representative ternary blends are 71PLA9PMMA20PBAT, 62PLA8PMMA30PBAT, and 53PLA7PMMA40PBAT, since the impact resistance profiles plotted in Figure 5c and Figure S3 show that those blends have the best performance in their impact resistance profiles. It is worth to mention that the PMMA weight fractions in the PLA−PMMA cophase of 71PLA9PMMA20PBAT, 62PLA8PMMA30PBAT, and 53PLA7PMMA40PBAT are all around 11−12%, which implies that this PLA/PMMA/PBAT ternary system is highly compositional dependent. The mechanical properties of those representative ternary blends were compared with the commercialized HIPS (Polystyrol 473D) and ABS (Terluran GP-35). The results are plotted in charts and shown in Figure 11. From the figure, we can see that higher PBAT content renders blend with higher impact strength but meanwhile results in the decrease of tensile strength and Young’s modulus. The 53PLA7PMMA40PBAT blend shows the impact strength of 188 J/m, which is higher than HIPS (122 J/m) but slightly lower than ABS (215 J/m). The 71PLA9PMMA20PBAT blend shows a much lower impact strength of 117 J/m but performs the best results of tensile strength and Young’s modulus. The 62PLA8PMMA30PBAT blend has the overall mechanical performance in between 71PLA9PMMA20PBAT and 53PLA7PMMA40PBAT, which is much better than HIPS and comparable to ABS. The results indicate that we are able to produce a high impact resistant polymer blend with tunable mechanical properties and over 90% biodegradability.

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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.8b00297. SEM images on cryo-sliced cross-sectional surfaces of PLA/PMMA blends, AFM images on PLA/dPMMA blend film surfaces, impact strength of PLA/PMMA/ PBAT blends with different PBAT contents (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.G.). *E-mail: [email protected] (M.H.R.). ORCID

Yichen Guo: 0000-0002-1637-4440 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation (Inspire Award #1344267). We also acknowledge the Advanced Energy Center for access to the ThINC facility.

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REFERENCES

(1) Geyer, R.; Jambeck, J. R.; Law, K. L. Production, use, and fate of all plastics ever made. Science Advances 2017, 3, e1700782. (2) Gross, R. A. Biodegradable Polymers for the Environment. Science 2002, 297, 803−807. (3) Lunt, J. Large-Scale Production, Properties and Commercial Applications of Polylactic Acid Polymers. Polym. Degrad. Stab. 1998, 59, 145−152. (4) Williams, C.; Hillmyer, M. Polymers from Renewable Resources: A Perspective for a Special Issue of Polymer Reviews. Polym. Rev. 2008, 48, 1−10. (5) Nagarajan, V.; Mohanty, A. K.; Misra, M. Perspective on Polylactic Acid (PLA) Based Sustainable Materials for Durable Applications: Focus on Toughness and Heat Resistance. ACS Sustainable Chem. Eng. 2016, 4, 2899−2916. (6) Jiang, L.; Wolcott, M. P.; Zhang, J. Study of Biodegradable Polylactide/Poly(Butylene adipate-Co-Terephthalate) Blends. Biomacromolecules 2006, 7, 199−207. (7) Gu, S.-Y.; Zhang, K.; Ren, J.; Zhan, H. Melt rheology of polylactide/Poly(Butylene adipate-Co-Terephthalate) blends. Carbohydr. Polym. 2008, 74, 79−85. (8) Mohanty, A. K.; Parulekar, Y.; Chhidambarakuemar, M.; Kositruangchai, N.; Harte, B. R. Biodegardable Polymeric Nanocomposites Particularly for Packaging. U.S. Patent US7619025 B2, 2009. (9) Kumar, M.; Mohanty, S.; Nayak, S.; Parvaiz, M. R. Effect of glycidyl methacrylate (GMA) on the thermal, mechanical and morphological property of biodegradable PLA/PBAT blend and its nanocomposites. Bioresour. Technol. 2010, 101, 8406−8415. (10) Schott, H. Solubility parameter, specific molar cohesion, and the solubility of ethylene oxide in polymers. Biomaterials 1982, 3, 195− 198. (11) Siemann, U. The solubility parameter of poly(Dl-Lactic acid). Eur. Polym. J. 1992, 28, 293−297. (12) Jiang, L.; Liu, B.; Zhang, J. Properties of Poly(Lactic acid)/ Poly(Butylene adipate-Co-Terephthalate)/Nanoparticle Ternary Composites. Ind. Eng. Chem. Res. 2009, 48, 7594−7602.

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CONCLUSIONS We have shown that the SCF model can be used to guide the design of ternary polymer blends. If polymer A had the χ parameter with polymer B lower than the χ parameter between polymers B and C, and polymers A and C had the χ parameter less than or equal to zero, the polymer A can become a compatibilizer for the blend of B and C by forming a nanoscale layer on the interface. For the PLA/PBAT blend, PMMA was selected since it had low χ value with PBAT and high miscibility with PLA. SIMS analysis indicated that PMMA was able to form an interfacial layer of approximately 2Rg,PMMA thick at the PLA/PBAT interface, which is in good agreement with the SCF model. Izod impact tests showed that the ternary PLA/PMMA/ PBAT blend had the best performance when the PMMA weight fractions in the PLA−PMMA cophase was around 11− 12%, where a nearly complete PMMA monolayer was formed at the interface. Once the interface was saturated, incorporation of additional PMMA occurred mostly in the PLA phase, resulting in increased brittleness. At the optimal concentration of PMMA, the mechanical properties of the blend were higher than HIPS and comparable to ABS, while still maintaining a high degree of biodegradability. Hence, we were able to demonstrate that the use of small amount of a tertiary L

DOI: 10.1021/acs.macromol.8b00297 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00297 Macromolecules XXXX, XXX, XXX−XXX