Enhancing the Mechanical Properties of Biodegradable Polymer

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Enhancing the Mechanical Properties of Biodegradable Polymer Blends Using Tubular Nanoparticle Stitching of the Interfaces Yichen Guo, Shan He, Kai Yang, Yuan Xue, Xianghao Zuo, Yingjie Yu, Ying Liu, Chung-Chueh Chang, and Miriam H. Rafailovich ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05698 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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ACS Applied Materials & Interfaces

Enhancing the Mechanical Properties of Biodegradable Polymer Blends Using Tubular Nanoparticle Stitching of the Interfaces Yichen Guo*1, Shan He1, Kai Yang1, Yuan Xue1, Xianghao Zuo1, Yingjie Yu1, Ying Liu2, Chung-Chueh Chang2, and Miriam H. Rafailovich*1 1. Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York, 11794. USA. 2. ThINC Facility, Advanced Energy Center, Stony Brook, New York 11794. USA. ABSTRACT: “Green” polymer nanocomposites have been made by melt blending biodegradable poly (lactic acid) (PLA) and poly (butylene adipate-co-butylene terephthalate) (PBAT) with either Montmorillonite clays (Cloisite Na+), Halloysite nanotubes (HNTs), the resorcinol di (phenyl phosphate) (RDP) coated Cloisite Na+, and coated HNTs. A technique for measuring the work of adhesion ( ) between nanoparticles and their matrices was used to determine the dispersion preference of the nanoparticles in the PLA/PBAT blend system. TEM images of thin sections indicated that even though both RDP coated nanotubes and clay platelets segregated to the interfacial regions between the two immiscible polymers, only the platelets, having the larger specific surface area, were able to reduce the PBAT domain sizes. The ability of clay platelets to partially compatibilize the blend was further confirmed by the dynamic mechanical analysis (DMA) which showed that the glass transition temperatures of two polymers tend to shift closer. No shift was observed with either coated or uncoated HNTs samples. Izod impact testing demonstrated that the rubbery PBAT phase greatly increased the impact strength of the unfilled blend, but addition of only 5% of treated clay decreased the impact strength by nearly 50%. On the other hand, an increase of 9% relative to the unfilled blend sample was observed with the addition of 5% treated nanotubes. TEM cross section analysis confirmed that the RDP coated clay platelets covered most of the interfacial area, which on one hand enabled them to reduce the interfacial tension effectively, while on the other hand prevented chain entanglements across the phase boundary and increasing the overall brittleness, which has been confirmed by rheology measurements. In contrast, the RDP coated HNTs were observed to lie perpendicular to the interface, which made them less effective in reducing interfacial tension, but encouraging interfacial entanglements across the interface resulting in “stitching” the interface and increase the Izod impact of the blend. Keywords: biodegradable nanocomposites, nanoplatelets and nanotubes, work of adhesion, mechanical properties, blend compatibility, dispersing preference

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1. Introduction The white pollution caused by the lack of methods for conventional polymer product disposal urgently motivates a search for people to find an alternative polymer with biodegradability. Poly (lactic acid) (PLA) has been considered as the best candidate due to its comparable mechanical properties to other conventional plastics and relatively low production cost relative to other biodegradable polymers.1,2 Although PLA has been widely used in food packaging and the textile sector, poor impact resistance restricted its application in this new field such as in vehicle interiors and mass transportation. One of the effective and economic ways to improve the impact toughness of PLA is melt blending it with other flexible polymers. Poly (butylene adipate-co-butylene terephthalate) (PBAT), is a fully biodegradable aromatic copolyester with very good ductility which is able to effectively toughen PLA without sacrificing its biodegradability.3-5 Many studies have, therefore, focused on the PLA/PBAT blend. For example, Jiang et al.6,7 showed that even though the PLA/PBAT blend is an immiscible two phase system, the impact strength of the blend was significantly increased. Adding montmorillonite clays in the blend greatly improved its modulus, but resulted in a huge drop in elongation. Shahlari et al.8 found that the organoclay platelets tended to locate at the PLA/PBAT interface, which resulted in the reduction of the dispersed phase domain size. Ko et al.9,10 observed that the multi-walled carbon nanotubes mostly preferred to locate in the PBAT phase of PLA/PBAT blend, which correlated to the changes of rheological and crystal properties. Ludvik et al.11 used bentonite clay to uniformly disperse cellulose fiber in PLA/PBAT blends, but the weak interfacial adhesion between fibers and polymers provided limited improvement. Even though the water resistance and the modulus of the blend increased, the ductility decreased significantly. Similar findings12-14 were also reported for other blends where improvements in properties such as flame retardancy, or thermal and electrical conductivity, were achieved, at a significant cost to the impact toughness, which decreased considerably. This is a serious limitation which prevents these products from use in consumer or structural products. In order to overcome this limitation, it is important to understand the structure of the interfaces and be able to manipulate the internal morphology of the blend such that ductility is maintained. Numerous authors have shown that the dispersion of nanoparticles in an immiscible polymer blend is a delicate balance between the relative affinity of the particles with each of the polymers and the affinity of the polymers for each other, which in most cases results in segregation of the particles at the blend interfaces.15-19 We have previously20 described a simple method to measure the work of adhesion ( ) between nanoparticles and their matrix, which was shown to accurately predict the degree of intercalation of functionalized and unfunctionalized clay platelets and nanotubes. In this study, we will show that this technique can also be applied to predict the dispersion of nanoparticles


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within a polymer blend and allow us to engineer nanocomposites where all properties, including impact resistance are enhanced. In this study we focus on the biodegradable PLA/PBAT blend. Since biodegradable polymers decompose in landfills, any fillers present are then easily released into the environment. It is therefore even more critical to ensure that the additives of biodegradable composites are environmentally safe. In this case, we will use Halloysite nanotubes (HNTs) and montmorillonite clays which are natural components of soils, and have been shown to increase the degradation rate of either PLA or PBAT.21,22 But instead of functionalizing them with quaternary ammonium chloride salts, we will use resorcinol di (phenyl phosphate) (RDP). Pack et al.19 have shown that functionalization of clays with RDP is far simpler and can achieve comparable results in blend compatiblization as the more standard method of functionalization with quaternary ammonium chloride salts. In this case, the additional advantage is the fact that in contrast to quaternary ammonium chloride salts, which have high toxicity issues,23 RDP has been reported to be much safer, without significant accumulation in either soil or water.24,25 We will finally build a correlation with the nanoparticle morphology, the particle dispersing preference, the effectiveness in compatibilizing polymer blend, and the improvement in impact resistance of the nanocomposite.

2. Experimental 2.1. Materials PLA 4042D, (density: 1.24 g/cm3, molecular weight: 120,000 g/mol) and PBAT Ecoflex C1200 F, (density: 1.26 g/cm3, molecular weight: 24,000 g/mol) were purchased from Natureworks LLC and the BASF Corporation, respectively. Montmorillonite clays Cloisite Na+ (C-Na+), Cloisite 30B (C-30B) and HNTs were supplied by Southern Clay Inc. and Applied Minerals Inc., respectively. The RDP was Fyrolflex RDP provided by ICL-IP, Inc.. 2.2. RDP Coated C-Na+ (C-RDP) and HNTs (H-RDP) Preparation 20 wt% of RDP and 80 wt% of C-Na+ or HNTs were mixed into a 200ml beaker at 70 °C. The mixture was then removed from the beaker to a plastic shaker and placed in a Thinky Mixture which was set at 700 rpm for 5min. After that, the mixture was milled with a mortar and pestle. The final mixture was placed in a vacuum oven at 70 °C for 24 h in order to remove moisture. The complete absorption of RDP on the surfaces of both CNa+ and HNTs has been confirmed previously.19,20,26 2.3. Nanocomposites Preparation PLA and PBAT pellets were first poured into the chamber of a C.W. Brabender with temperature set at 180 °C and mixed at 20 rpm for 1 min. After the polymer melted, the 3 ACS Paragon Plus Environment


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nanoparticles were then added and mixed at 100 rpm for 10 min. The mixtures were molded into different shapes for the various measurements using a hot press. The ratios of the nanocomposites in this study are summarized in table 1. Table 1. Concentrations of nanocomposites used in this study PLA/PBAT/ C-Na+ 69.3/29.7/1 67.9/29.1/3 66.5/28.5/5 65.1/27.9/7 63/27/10 59.5/25.5/15

Abbreviation BCNa1 BCNa3 BCNa5 BCNa7 BCNa10 BCNa15

PLA/PBAT/ C-RDP 69.3/29.7/1 67.9/29.1/3 66.5/28.5/5 65.1/27.9/7 63/27/10 59.5/25.5/15

Abbreviation BCRDP1 BCRDP3 BCRDP5 BCRDP7 BCRDP10 BCRDP15

PLA/PBAT/C30B 69.3/29.7/1 67.9/29.1/3 66.5/28.5/5 65.1/27.9/7 63/27/10 59.5/25.5/15

Abbreviation BC30B1 BC30B3 BC30B5 BC30B7 BC30B10 BC30B15

PLA/PBAT/ HNTs 69.3/29.7/1 67.9/29.1/3 66.5/28.5/5 65.1/27.9/7 63/27/10 59.5/25.5/15

Abbreviation BH1 BH3 BH5 BH7 BH10 BH15

PLA/PBAT/ H-RDP 69.3/29.7/1 67.9/29.1/3 66.5/28.5/5 65.1/27.9/7 63/27/10 59.5/25.5/15

Abbreviation BHRDP1 BHRDP3 BHRDP5 BHRDP7 BHRDP10 BHRDP15

2.4. Characterization Methods Transmission Electron Microscopy (TEM): Thin cross sectional films (100nm) of the PLA/PBAT blend nanocomposites were cut by a Lecia FC-7 cryomicrotome, cooled with liquid nitrogen, using a diamond knife and directly lifted onto copper grids. The morphologies of all the cross sections were imaged using a JEOL JEM1400 Transmission Electron Microscope at 80 kV. Scanning Electron Microscopy (SEM): The fracture surfaces of the broken impact samples were imaged by a JEOL JSM7600F Scanning Electron Microscope with a Schottky electron gun. Before imaging, a 10 nanometer thick film of gold was coated on the samples to increase electrical conductivity. Contact Angle Measurements: C-Na+, C-RDP and C-30B monolayers were lifted on hydrofluoric acid etched, hydrogen passivated, hydrophobic silicon wafers using the Langmuir-Blodgett (LB) technique.19,27 The monolayers were then probed by a VEECO/DI-3000 scanning force microscope (AFM) in contact mode. The layers of HNTs and H-RDP were prepared by spin casting of HNTs and H-RDP methanol dispersion on the Si wafers. The wafers with nanotubes were imaged by SEM since the surfaces were too rough to be detected by AFM. PLA and PBAT pellets were cut into little pieces (around 5 mg) and located on the Si wafers covered with nanoparticles in a vacuum oven. In order to melt the polymer pieces and let them form droplets on the nanoparticle layers, the temperature in the oven was set at 180 °C and the annealing time was 24 hours. A CAM 200 optical contact angle meter (KSV Instrument Ltd., Helsinki, Finland) was used to measure the contact angles of PLA and PBAT droplets on each nanoparticle layer. Every contact angle value represents the average measurements of 10 polymer droplets. Dynamic Mechanical Analysis (DMA): Dynamic mechanical properties of nanocomposites were measured using a TA Instruments DMA Q800 with a single cantilever bend mode at the frequency of 1 Hz. The storage modulus and tan data points were collected from -40 to 80 °C at the rate of 3°C/min.


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Rheology Measurements: Rheology performance of samples were obtained using a Bohlin Gemini HR Nano rheometer from Malverm instruments. Specimens were conducted for frequency sweep from 0.01 Hz to 10 Hz at 180 °C with the strain amplitude of 0.1% and 1%, respectively. The results were plotted as storage modulus vs. frequency. Izod Impact Test: The impact resistance of nanocomposites were evaluated by the Izod impact test according to ASTM D-256 test procedure. The Standard specimen for ASTM was 64 x 12.7 x 3.2 mm with a V-shaped notch. The result of the measurement was reported in energy lost per unit of specimen thickness (J/cm) at the notch. Each impact strength result is the average value of 10 replicates.

3. Results and Discussion 3.1. Morphology of PLA/PBAT Nanocomposites In order to determine the dispersion preference of the nanoparticles in the binary polymer blend, the relative affinity of different kinds of nanoparticles for either PLA or PBAT must be determined. Contact angle goniometry was shown to be a sensitive technique for measurement of the interfacial energy between the particles and the host polymer. A facile technique for goniometry was previously described20 where a monolayer of the inorganic component was prepared and the contact angle made by the droplet of the molten polymer was measured. In this study, monolayers were prepared on silicon wafers with either functionalized or unfunctionalized clay platelets or Halloysite nanotubes.

Figure 1. AFM images of clay monolayers: (a) C-Na+, (b) C-RDP and (c) C-30B. SEM images of nanotube layers: (d) HNTs and (e) H-RDP. The contact angle images of PLA droplets on each nanoparticle layer: (f) PLA on C-Na+, (g) PLA on C-RDP, (h) PLA on C30B (i) PLA on HNTs, (j) PLA on H-RDP, (k) PBAT on C-Na+, (l) PBAT on C-RDP, (m) PBAT on C-30B, (n) PBAT on HNTs and (o) PBAT on H-RDP.



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Table 2. Contact angles and of PLA and PBAT on different kinds of nanoparticle layers Nanoparticle layers C-Na+ C-RDP C-30B HNTs H-RDP

of PLA (˚) 71.49±3.51 39.86±2.34 35.23±2.16 47.92±2.94 36.94±2.25

with PLA (mN/m) 47.60±2.12 63.86±0.96 65.65±0.80 60.34±1.40 65.01±0.88

of PBAT (˚) 65.94±3.56 47.66±3.41 54.98±2.79 34.95±3.37 43.17±2.58

with PBAT (mN/m) 57.48±2.35 68.33±1.84 64.26±1.65 74.30±1.44 70.61±1.29

Figure 2. Surface tension of PLA and PBAT as a function of temperature.

The surfaces of the silicon wafers coated with clay platelets or nanotubes were imaged with AFM (Fig. 1a-c) and SEM (Fig. 1d, e), respectively, where we can see that C-Na+, C-RDP and C-30B platelets have successfully formed monolayers and uniformly covered the silicon wafers. The SEM images indicate that the HNTs and H-RDP, produce uniform multi-layer films, covering the Si wafers. PLA and PBAT, cut into pieces of approximately 5mg, were placed on the particle layers and allowed to melt in a vacuum oven set at 180 °C for 24 hours. Annealing allowed the droplets to reach an equilibrium conformation with well-defined Young's contact angles () which could then be imaged using the optical camera on the contact angle goniometer as shown in Fig. 1f-o. The data obtained for the different particles is summarized in Table 2. According to our previous work,20 the relative affinity between the nanoparticles and the polymer matrices can be estimated by the work of adhesion () which defines the reversible work required to separate a unit area of interface between two different materials:28

1.1 where and

are the surface tensions of two different phases, and is the interfacial tension between them. The , can be obtained from Young’s equation;

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where and are the surface tensions of the solid and liquid phases. Substituting equation 1.1 into 1.2, we can obtain the following relationship between and ; 1 cos 1.3 From equation 1.3, we find that the Young’s contact angle and surface tension of the liquid, which is the molten polymer in this case, are the only parameters determining the value of . Since, all the contact angles are formed and observed at 180 °C, we have to know the surface tension of both PLA and PBAT as a function of temperature. The surface tension measurements were conducted using the two liquid (water and diiodomethane) contact angle method.29 In order to obtain the temperature coefficient, we covered the camera stage with a hot plate and checked the exact temperature of the polymer surface using an infrared thermometer. The surface tension at different temperature points are shown in Fig. 2, where we extrapolate the data to higher temperatures and obtain values for !" = 36.13mN/m and #"$ = 40.83mN/m at 180 °C. Substituting these values and the measured contact angles into the equation (1.3), we calculate the values of summarized in Table 2. From the table, we can see that without surface coating, the of C-Na+ with PBAT (57.48 mN/m) is significantly higher than that with PLA (47.60 mN/m), which implies that C-Na+ has a higher affinity with the PBAT phase. In the case of HNTs, the of nanotubes with PBAT and PLA are 74.30 mN/m and 60.34 mN/m, respectively, which indicates that HNTs also have a higher affinity for PBAT. After surface functionalization with RDP, the of C-RDP with PLA dramatically increases from 47.60 mN/m to 63.86mN/m, and that with PBAT increases from 57.48 mN/m to 68.33mN/m, which results in the similar affinity of CRDP with both polymers. The same situation was also observed in the case of H-RDP, where the of nanotubes with either polymer approaches, the value on PLA increases from 60.34 mN/m to 65.01mN/m, while that on PBAT decreases from 74.30 mN/m to 70.61 mN/m. Since the surface functionalization is identical on the clay and nanotubes, the small differences of C-RDP and H-RDP on both polymers can be attributed to the surface roughness. The of C-30B with both PLA and PBAT on the other hand, are 65.65 mN/m and 64.26 mN/m, respectively, which are nearly equal.



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Figure 3. TEM images taken on the cross-sections of the nanocomposites: (a) PLA/PBAT, (b) BCNa5, (c) BCRDP5, (d) BC30B5, (e) BH5, (f) BHRDP5 and (g) BHRDP15.

The cross sectional images of the nanocomposites are shown in Fig. 3. From the figures we can see that nearly all the composites have similar PBAT phase domains (dark area) to the neat blend (Fig. 3a) of approximately 2 %& in extent. Two outstanding cases are the images of the BCRDP5 and BC30B5 (Fig. 3c, d), where incorporation of the C-RDP dramatically reduces the diameter of the PBAT domains to approximately 1 %&, and adding C-30B further decreases the diameter to half micron. From Fig. 3c, we can also see that the C-RDP clays are partially exfoliated and located mostly at the interfaces, with a few platelets inside the PBAT phase. From table 2 above, we can see that the values for the RDP coated clay platelets are similar for the two polymers, with only a slight preference for PBAT. On the other hand, the cross-section of BC30B5 displayed in Fig. 3d shows that the C-30B clays have higher degree of exfoliation in blend than CRDP. In addition to mostly segregate at the interface, C-30B platelets are also found equally disperse in both PLA and PBAT, which is consistent with their equal values to both polymers. Both BCRDP5 and BC30B5 systems are similar to those described by Si et al.,16 where in-situ grafts onto the clay surfaces are formed during the melt mixing process of highly immiscible blends. When the values are close, similar amounts of each polymer are adsorbed on the clays, making the platelets very effective compatibilizers. The smaller domains are then a result of the favorable energy gained in placing platelets at the interface, reducing the interfacial tension, and increasing the entropy, all balanced by the formation of additional interfacial area. This process continues until all platelets are adsorbed to the interfaces, with a few excess platelets situated on either PBAT phase or both phases depending on their preference to PLA and PBAT. It is worth to mention that the higher exfoliation degree of C-30B enables more single platelets with large specific areas to effectively compatibilize the blend, which is consistent with the fact that BC30B5 has smaller PBAT domain size than BCRDP5. The compatibility can also be detected by comparing glass transition temperature (Tg) of either PLA or PBAT in the blend. Lee et al.30 have found that the decrease of PBAT domain size always corresponds to the narrowing of the difference between two Tg. In this case, the Tg of either PLA or PBAT from each sample is measured using DMA and 8 ACS Paragon Plus Environment

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listed in table 3. The table shows that the Tg of PLA and PBAT in the neat blend are 68.1°C and -21.9 °C, respectively, and the difference is 90 °C. The two Tg stay relatively the same in the cases of BCNa5, BH5 and BHRDP5, which is consistent with the fact that they have comparable PBAT domain size with the neat blend. On the other hand, the BCRDP5 sample that has much smaller PBAT domains shows the difference of the two Tg narrows to 86.1 °C, and that of BC30B5 sample narrows further to 85.1 °C, which again, confirms the compatibilizer function of C-RDP and C-30B platelets. Table 3. Glass transition temperature (Tg) of PLA/PBAT blend and nanocomposites from tan δ curves. Sample PLA/PBAT BCNa5 BCRDP5 BC30B5 BH5 BHRDP5

Tg of PBAT (°C) -21.9 -21.3 -20.0 -20.7 -22.0 -21.8

Tg of PLA (°C) 68.1 68.1 66.1 64.4 68.2 67.9

Difference between two Tg (°C) 90.0 89.4 (-0.6) 86.1 (-3.9) 85.1 (-4.9) 90.2 (+0.2) 89.7 (-0.3)

The compatibilization observed above disappears in BCNa5 sample. From Fig. 3b we find that, without surface functionalizing, nearly all the C-Na+ platelets stay in the PBAT phase as tactoids, which is consistent with their relatively low value with PBAT. On the other hand, the of C-Na+ with PLA is significantly lower, driving all the particles into the PBAT phase. A similar behavior is observed for the BH5 sample in Fig. 3e, where the HNTs are seen to aggregate only in the PBAT phase. In this case, the values of HNTs are somewhat better than those of C-Na+, explaining the better dispersion of the HNTs. Yet, the values between the polymers are too dissimilar to permit segregation to the interfaces. It is interesting to contrast the interfacial behavior of the BHRDP5 sample (Fig. 3f), with that of the BCRDP5, since C-RDP and H-RDP have identical surface properties and hence their values with both PLA and PBAT are similar. In Fig. 3f, we find that, similar to the behavior of the C-RDP, the H-RDP are localized at the interfaces between the polymer domains. Yet, since the surface area of the nanotubes is much smaller than that of the clay platelets, they are less effective at forming in-situ grafts, and hence not efficient at reducing the interfacial tension or compatibilizing the blend.31 This is clearly evidenced by the fact that the domain size is not reduced despite the large concentration of particles at the interface. It is interesting to note that much H-RDP at the interface are observed arraying perpendicular to the interface. This particular morphology can be ascribed to the free energy change of the nanocomposite system.16,32-35 In a typical binary polymer/nanoparticles blend, the total free energy includes the interfacial energy between both polymers, the interfacial energy between nanoparticle surfaces and each polymer, and the bending energy of nanoparticles,36 which can be expressed by the following equation;16 ' ( &) * &) &'+,- 1.4



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Here, is the interfacial energy between the two polymers, * is the interfacial energy when nanoparticles are presented at the interface, ( is the total number of nanoparticles in the system, & is the number of nanoparticles segregated at the interface, and ) is the surface area of the single nanoparticle. In this equation, the third term stands for the unfavorable energy penalty induced by bending large aspect ratio nanoparticles to conform to the shape of the interface. The nanotubes, being much longer than the clays, are more sensitive to interfacial curvature, and more likely to bend when they are confined parallel to the interface. Hence at lower concentration, they prefer to lie vertically relative to the domain interfaces, where the bending penalty is reduced, as shown in Fig. 3f. It is interesting to note that even though they are pinned to the interface, most of the tube length is situated in the PBAT phase, where the value is larger, and the adsorption energy more favorable. Increasing the concentration of the H-RDP results in agglomeration of the particles at the domain interface (Fig. 3g), which produces stiffer bundles, and increases their resistance to bending. At the same time, the surface area of the bundles is larger than that of the individual nanotubes, making them more efficient at reducing the interfacial tension, which compensates the gain of bending penalty. Hence, a transition is observed where the nanotubes become oriented tangentially to the interfacial curvature. The particular nanoparticle dispersion and cross section morphology described above have strong correlation with the impact resistance of the corresponding nanocomposite, which will be discussed in detail in the following sections. 3.2. Impact Resistance of PLA/PBAT Nanocomposites The impact strength results of PLA/PBAT blend and the nanocomposites was measured and the results are plotted in Fig. 4 as a function of nanoparticle concentration. From the figure we find that adding C-Na+ and HNTs in the blend shows similar influence on impact strength, the impact values gradually decrease with the increasing filler concentration and the value of BH is always higher than that of BCNa at each concentration. In the case of BCRDP, addition of as little as 5% nanoparticles significantly reduces the impact strength from 80 J/m of the neat blend to 38 J/m. BC30B samples show the worst results that 5% of C-30B decreases the impact strength to only 27 J/m. The striking feature is the change for the BHRDP samples, where an increase from 80 J/m to 87 J/m of impact strength was observed at 5% concentration. The structural integrity of a blend against impact or fracture is determined by the ability of the two polymers to achieve some degree of inter-diffusion and entanglements across the domain interfaces.37-39 This is possible either in the case of the neat blend, or in the blends where the particles are sequestered in one phase such as the cases of BCNa and BH. From the figure we therefore see that the decrease in impact strength of BCNa and BH samples with increasing nanoparticle loading is small. On the other hand, when the particles are at the interface, such as the case of C-RDP or C-30B, a very effective, hard barrier is formed preventing chain interpenetration across the domain interfaces, which 10 ACS Paragon Plus Environment

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mechanically weakens the interface as illustrated in Fig. 5a. This is clearly seen by the steep decrease in impact strength with increasing C-RDP or C-30B concentration. The presence of the H-RDP on the other hand, does not present a barrier to chain penetration. In fact, the perpendicular orientation of the nanotubes increases the interfacial adhesion by “sewing” or reinforcing the interface, with minimal disturbance of the chain entanglement across the interfacial regions (Fig. 5b). Hence in this case, an increase in impact strength is observed with increasing concentration reaching a maximum at a value of 5%. As the concentration increases over this value, the impact strength begins to decrease, which can be ascribed to the transition of the nanotube orientation from vertical to tangential at the interface. In the tangential mode, the nanotubes are now interfering with the chains crossing the interface. The interference, even in this situation is not as severe as for the C-RDP or C-30B platelets, but nonetheless, it is clear that it disturbs the entanglements and contributes to a decrease in the impact strength.

Figure 4. Effects of nanofillers loading on the impact strength of neat blend and nanocomposites.



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Figure 5. Schematic illustration of (a) C-RDP or C-30B blocking the chain entanglements on the interface, and (b) H-RDP pulling out of the interface of PLA/PBAT blend during izod impact test.

Figure 6. SEM images taken on the fracture surfaces of the nanocomposites: (a) PLA/PBAT, (b) BCNa5, (c) BCRDP5, (d) BC30B5, (e) BH5, (f) BHRDP5 and (g) BHRDP15.

In order to investigate both toughening and embrittlement mechanisms in the PLA/PBAT matrix, the fracture surfaces of the samples after impact testing were imaged using SEM. 12 ACS Paragon Plus Environment

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Fig. 6a shows the PLA/PBAT neat blend, where we can identify the PBAT as the phase forming the spherical domains, with diameters of approximately 2%&, similar to the features observed in the TEM images above. From Fig. 6a we can also see that the PBAT phase is well dispersed within the PLA phase. Since this phase is rubbery, it is easily deformed and acts as a stress concentrator which absorbs much of the impact energy during the Izod impact tests. Observation of the SEM image shows the formation of long fibers, originating on the PBAT domains; deformations on the delaminated PLA surfaces where the PBAT phase was removed, were produced during fracture to dissipate the impact energy. Therefore, the PLA/PBAT blend (80J/m) performs much better than PLA (30J/m) in the Izod impact test. The fracture surfaces of the blends with unfunctionalized clays and nanotubes (BCNa5 and BH5) are displayed in Fig. 6b and e, where we can see that the surface morphology is very similar to the neat blend. The PBAT domain sizes are comparable and multiple stress fibers and deformations are also apparent on the interfaces. A few very large PBAT domains are observed, which appear to have fractured, revealing tactoids of either C-Na+ or HNTs. These large domains containing particle agglomeration are able to disturb the ability of the distributed PBAT phase to absorb the impact energy since particle agglomeration can create weak particle-particle interfacial areas. The number of these large domains increase with increasing particle concentration, which is consistent with the fact that the impact strength of either BCNa or BH sample decreases gradually with increasing particle concentrations. Since the of HNTs with PBAT (74.30 mN/m) are much higher than that of C-Na+ with PBAT (57.48 mN/m), BH samples always show better impact strength than BCNa samples at each particle concentration. In the case of BCRDP5 sample (Fig. 6c), we can see that the diameter of PBAT domains has been decreased from 2 %& to 1 %& which is consistent with the TEM Fig. 3c. Furthermore, very few fibrils are observed on the PBAT phase and the delaminated surfaces of the PLA were smooth, indicating that no chain entanglements occurred during fracture. These very clean interfaces observed are consistent with the embrittlement mechanism illustrated in Fig. 5a that the C-RDP platelets have formed very effective barriers to prevent chain entanglements. Similar and even worse situation can be seen in BC30B5 sample (Fig. 6d) that the fibrils around the PBAT phase are totally absent. Furthermore, the smaller PBAT domain (500 (& ) generates larger interface areas weakened by impenetrable C-30B platelets, which corresponds to the fact that BC30B samples have the worst impact strength results. Since the specific surface areas of single platelets are huge, the large reduction of the impact strength can be observed even at low clay concentrations for both BCRDP and BC30B samples. Fig. 6f and 6g show the SEM images of the fracture surfaces at low and high concentrations of the H-RDP. From the figures we find that the domain sizes in both cases are similar, and not reduced relative to the neat blend. Close examination of the 13 ACS Paragon Plus Environment


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BHRDP5 image (Fig. 6f) clearly shows that deformations can be observed on the delaminated surfaces of the PLA. In addition, many nanotube tips and stretched polymer tips are found on the PBAT phase surface and delaminated PLA surface (Fig. 6f insert), respectively, implying that the vertically orientated nanotubes on the interface have been pulled out from the PLA phase under the external impact force. These morphologies are consistent with the toughening mechanism illustrated in Fig. 5b where the H-RDP can reinforce the interface without blocking chain entanglements. It is worth mentioning that we have measured the Young’s modulus and tensile strength of neat PLA/PBAT blend and BHRDP5 samples using a Instron 5542 (Instron Co., Grove City, PA) facility according to the ASTM D-638, type M standard with the extension rate set at 5 mm/min. The results listed in table 4 show that in addition to the better impact strength, BHRDP5 also has higher Young’s modulus and tensile strength than the neat blend, which can be attributed to the high and interaction between rigid nanotubes and polymer matrices. Fig. 6g shows the fracture surface of BHRDP15, where we see that the PBAT phases are fully covered by tangentially oriented H-RDP and the deformation and fibrils at the interface cannot be found, which indicates the interference of nanotube agglomerations on the chain entanglements crossing the interface. This corresponds to the drop of impact strength for the BHRDP samples with higher H-RDP concentrations. Table 4. Mechanical properties of PLA/PBAT blend and BHRDP5. Sample PLA/PBAT BHRDP5

Impact Strength (J/m) 79.9±2.5 86.7±4.9

Young’s Modulus (MPa) 2164±118 2521±143

Tensile Strength (MPa) 48.1±3.6 51.6±6.6

3.3. Rheological Properties of PLA/PBAT Nanocomposites The influence of the particle morphology on the rheological response was also measured. The storage modulus (G’) plotted as a function of frequency for strain amplitudes of 0.1 and 1% are shown in Fig. 7a and 7b, respectively. From the figures we can see that the rheological response of the neat blend is modified to the largest extent by the addition of either of the functionalized clays, C-30B or C-RDP, where the G’ is increased by more than three orders of magnitude. This gel-like response at low frequencies is attributed to the strong interactions between the clay platelets and the both polymer components. It is interesting to note that in Fig. 7b, where the strain amplitude is higher, we can identify a G’ peak (pointed by arrows) on both BCRDP5 and BC30B5 curves. This peak is shown more clearly in the linear expanded plot of this region (insert). It has been proposed by several authors that stick slip causes anomalies in elongation, shear rate and shear stress40-42 due to extreme elongation of partially confined chains. Since the dynamic modulus is also a function of these parameters, we propose that the peak corresponds to the slip at the blend interfaces with higher frequencies. The observed slippage is consistent with the decrease in entanglements due to the clay platelet morphology. Despite the similarity in , this peak is absent in the BHRDP5 curve. This rheological response is also consistent with the TEM image shown in Fig. 3f, where the H-RDP stay 14 ACS Paragon Plus Environment

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perpendicular to the interface, and hence do not interfere with entanglements across the interface. The rheological response of BCNa5 and BH5 is very similar, and is consistent with the fact that both C-Na+ and HNTs are segregating in the PBAT phase, and are not interfacially active. The higher G’ value of BH5 can be ascribed to the higher value of HNTs to both polymers. A similar discontinuity was previously observed for PLA/PBAT blends when a reactive compatibilizer, Joncryl, was added.43-45 This method is somewhat similar to the addition of the clays with the large values. Joncryl was shown to form in-situ chain extension, or blocks, thereby lowering the interfacial tension, while Si et al.16 showed that clay surface can form in-situ grafts at the interface which are also effective at lowering the interfacial tension. The main difference between these two cases though is the ability to form cross interfacial entanglements as the interfacial regions are increased and the interfacial tension is reduced. In the case of the block copolymer, entanglements form freely, and the interfaces have an elasticity similar to the component polymers. In the case of the clay/graft platelets, Si et al. argued that the rigidity of the clays adds an additional term to the interface which resists deformation.16 It is possible that this term may also be responsible for the abrupt slippage observed here. These effects are of course not present in when the H-RDP lies perpendicular to the interface. This conformation may not be efficient at compatibilization, but is clearly effective in reinforcement and reducing slippage at the interfaces.

Figure 7. Storage modulus (G’) vs frequency curves of neat blend and nanocomposites at 180 /: (a) Strain amplitude is 0.1% and (b) Strain amplitude is 1%.

4. Conclusion We have shown that the dispersion of nanoparticles within a polymer blend can also be predicted by the . Using the as a guide, we have produced blends of the biodegradable PLA and PBAT, and MMT-clay or HNTs functionalized with RDP which is an environmentally benign compound. The results show that addition of PBAT greatly 15 ACS Paragon Plus Environment


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increases the Izod impact strength of PLA. Addition of unfunctionalized clays or nanotubes gradually decreases the impact strength relative to the neat blend. Adding functionalized C-RDP or C-30B though had a dramatic effect on the blend, rapidly decreasing the impact strength even at very low concentrations. SEM images of the fracture surfaces reveal that C-RDP and C-30B were powerful compatibilizers of the blend, effectively reducing the domain sizes. DMA measurements confirmed this and indicated a change in Tg, consistent with partial compatibilization. TEM images also confirmed the reduction of the domain size and revealed the localization of the C-RDP and C-30B platelets at the domain interfaces. C-RDP and C-30B platelets were partially exfoliated and formed an effective barrier to block the chain interpenetrations across the interface. This was further supported by rheology measurements, and the lack of stress fibrils and smooth fracture surfaces imaged with the SEM. The functionalized H-RDP on the other hand, further increased the impact strength with increasing concentration up to about 5%, after which they also decreased the impact strength. TEM images revealed that at low concentrations the H-RDP were situated perpendicular to the interface, and the SEM images indicated that the nanotubes were able to reinforce the interfaces via a “stitching” mechanism. With increasing concentration, the tubes agglomerated into bundles that were positioned parallel to the interface, blocking the chain entanglements and contributing to a weakening of the interfacial adhesion. Author Information Corresponding Authors *Phone: +01-631-632-2843. E-mail: [email protected] (Y.G.) *Phone: +01-631-632-2843. E-mail: [email protected]. (M.H.R.) Acknowledgments The authors acknowledge for the support from ICL Industrial Products. We would also like to acknowledge the Advanced Energy Center for access to the ThINC facility. References (1) Lunt, J. Large-Scale Production, Properties and Commercial Applications of Polylactic Acid Polymers. Polym. Degrad. Stab. 1998, 59, 145-152. (2) Vink, E.T.H.; Rabago, K.R.; Glassner, D.A.; Gruber, P.R. Applications of Life Cycle Assessment to NatureWorksTM Polylactide (PLA) Production. Polym. Degrad. Stab. 2003, 80, 403-419. (3) Wu, C. Antibacterial and Static Dissipating Composites of Poly(butylene adipate-co-terephthalate) and Multi-Walled Carbon Nanotubes. Carbon 2009, 47, 3091-3098. (4) Witt, U.; Einig, T.; Yamamoto, M.; Kleeberg, I.; Deckwer, W.D.; Muller, R.J. Biodegradation of Aliphatic-Aromatic Copolyesters: Evaluation of the Final Biodegradability and Ecotoxicological Impact of Degradation Intermediates. Chemosphere 2001, 44, 289-299.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6a 30x24mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6b 30x24mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6c 30x24mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6d 30x24mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6e 30x24mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6f 30x24mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6g 30x24mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7a 17x13mm (600 x 600 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7b 17x13mm (600 x 600 DPI)


Enhancing the Mechanical Properties of Biodegradable Polymer

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