Blends of renewable Poly(butylene succinate) and Poly(propylene

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Blends of renewable Poly(butylene succinate) and Poly(propylene carbonate) compatibilized with Maleic Anhydride using quad screw reactive extrusion Bárbara Andrea Calderón, Matthew S McCaughey, Conor W Thompson, and Margaret J. Sobkowicz Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04757 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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Industrial & Engineering Chemistry Research

Blends of renewable Poly(butylene succinate) and Poly(propylene carbonate) compatibilized with Maleic Anhydride using quad screw reactive extrusion Bárbara A. Calderón†, Matthew S. McCaughey†, Conor W. Thompson† and Margaret J. Sobkowicz†* †Department of Plastics Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA *[email protected]

Abstract The free radical functionalization of immiscible blends of Polybutylene succinate (PBS) and Polypropylene carbonate (PPC) was successfully achieved on a novel quad screw extruder. A premade compatibilizer, PPC-grafted-Maleic anhydride (gPPC), was added in various amounts to trigger the chemical compatibilization and tailor the properties of the final blend. Titration of acid groups on the functionalized blends showed that the grafting efficiency increases with the addition of gPPC. Proton NMR corroborated the formation of new graft copolymers in the reactive blends. A change in the rheological behavior of the formulations evidenced the grafting reactions occurring at the interphase of the components. Also, gPPC suppressed droplets coalescence as seen in the SEM images. Finally, the addition of compatibilizer increased more than 100% the tensile strain at break, improved up to 50% the impact resistance and produced a small increase on flexural and elastic modulus. Keywords: reactive extrusion, free radical grafting, Polybutylene succinate, Polypropylene carbonate, Maleic anhydride Synopsis: The usage of a new biocompabitilizer for the reactive compounding of biopolymer blends improved the mechanical properties of the final compounds.

1. Introduction The conversion of renewable resources into commercial products such as plastic resins is a promising solution to reduce dependence on fossil fuels and even sequester carbon. Biobased resins come from renewable resources like plants and CO2 and their routes of synthesis are usually more ecofriendly than fossil fuel-based resins1. Many biobased polymers are also biodegradable, which means they have the potential to mitigate the mounting plastic waste the world is facing.

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Biodegradable polymers can break into naturally occurring monomers, water and CO2 via thermochemical and biologic processes in a specified time range at specific disposal conditions2. One emerging bioplastic is Polypropylene carbonate. PPC is an amorphous alternating copolymer of propylene oxide and carbon dioxide. Its synthesis route makes it CO2 negative, meaning that is consumes CO2 as part of the polymer structure instead of producing it, and is a promising candidate to mitigate climate change. PPC decomposes into CO2 and propylene carbonate which is non-toxic and can be used as a protic solvent. PPC has an elastic modulus of around 2 GPa and tensile strain at break of 150%. Its application as a bulk material is limited by its low Tg, which is close to room temperature. Moreover, PPC is very sensitive to conditions of heat and shear and has very low decomposition temperature, which complicate its melt processing3,4. Another very interesting bioplastic becoming increasingly available is Polybutylene succinate (PBS). This semi-crystalline polyester can be derived from fossil fuel or biomass fermentation by products. PBS is capable of biodegrading into CO2 and H2O at the right conditions. This polymer has a modulus of 0.26 GPa, strain at break of up to 200% and good thermal and chemical resistance5. Issues with this polymer is that since is a polyester, it experiences low melt strength, which complicates its processing and mechanical performance and its Young’s modulus is on the low end of the soft commodities. Even though bioplastics are advantageous materials from a sustainability standpoint, their disadvantages have restricted their usage in large scale applications. For this reason, there have been attempts to blend them together or with other materials to improve their performance and produce a more viable product. However, most of the blends compounded up to this point are immiscible and thus display inferior mechanical properties relative to the individual components. The primary reasons for this behavior are the unfavorable molecular interaction, large interfacial tension in the melt and coarse morphology of the final products6,7. For instance, previous studies showed that blends of PBS and PPC are immiscible under the processing conditions and ratios analyzed8–11. Yet, it is believed that if this blend is functionalized, it may exhibit competitive properties to the conventional polyolefins such as PE and PP (higher modulus and melt strength due to PPC, and higher extension at break and better thermal stability due to the presence of PBS).


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The functionalization of the blend interface can be carried out by the addition of the appropriate premade graft or block copolymer that acts as the compatibilizer of the blend. The reactive polymer should migrate to the interface and serve as a bridge between the two materials, hence suppressing coalesce of the minority phase and transferring loads12. The reactive polymer can be created by the addition of a low molecular weight compound that can initiate the graft/coupling reaction. Maleic anhydride (MAH) is often used as a reagent in this synthesis process as its bifunctionality makes it highly reactive. The reaction between maleic anhydride and a polymer is often trigger by a peroxide, for example, dicumyl peroxide (DCP), that produces free radical spots for the MAH to graft in the polymer backbone and form the grafted macromolecule. Wang et al.13 added MAH and DCP to a blend of starch and Polythylene (PE) found that the formation of PE-grafted-MAH enhanced the dispersion of starch and PE in the system and also improves the thermal stability of the blends. Pivsa-Art et al.14 compounded Polylactic acid (PLA) and Polypropylene (PP) with different amounts of PP-g-MA and found that the adhesion of the components was achieved by the addition of the compatibilizer. Moreover, PP-g-MA has proven to increase the mechanical properties of PP cellulosic components due to the esterification reaction between the anhydride and hydroxyl groups of the MAH and cellulose respectively15. Even though the grafted MAH polymer has been extensively used, it is known that its preparation induces large quantities of chain scission due to the free radical creation and therefore the deterioration of the properties. Moreover, its process optimization remains a challenge since it is extremely difficult to control the molecular weight reduction and the chemical structure of the resultant product16. This is likely a factor in the limited availability of compatibilizers for sustainable products. The ideal case is to synthesize a copolymer of both polymers to be added in certain quantities to the blend for compatibilization. However, these polymers have not been extensively used in industry for large scale reactive extrusion processes. This is because their synthesis has been carried out in solution, which requires large quantities of solvent, lots of energy to recover the diluent and produces significant quantities of volatile emissions. Therefore, an alternative is to form the compatibilizer in situ during compounding in continuous polymer processing equipment such as multi screw extruders, specifically, twin-screw extruders. This scalable process is referred as “reactive extrusion”. Due to the advances in extrusion technology, these versatile machines are now capable of plasticizing, pumping, mixing, blending, devolatilizing and triggering chemical reactions in a continuous, often one-step process7,17.



Co-rotating, intermeshing twin screw extruders are the preferred equipment for polymer blending and reactive extrusion. However, new advances have led to co-rotating, fully intermeshed, four parallel screw extruders capable of better mixing performance and tighter control of the compounding process. According to manufacturers, quad screw extruders (QSE) release less selfgenerated heat from the materials (which makes them ideal for heat sensitive polymers) and can extend the residence time of the run (important for reactive extrusion) in contrast to twin screw extruders. Due to their low pressure capabilities, the release of volatiles through the venting port is much more efficient18,19. To the best of our knowledge, only a few researchers have published findings using this machine for the compounding of polymer blends and composites20–22. Since quad screw extruders (QSEs) seem to be advantageous for reactive compounding of sensitive polymers, we employed this equipment to perform the reactive functionalization of PPC and PBS. As stated above, blends of this type are immiscible and there is no available data of their functionalization by means of reactive extrusion. To overcome the lack of compatibility of the system, a reactive polymer that was prepared and studied in our previous work was added to the formulations at different amounts23. The compatibilizer is Polypropylene grafted maleic anhydride (gPPC) and was prepared by free radical functionalization of Maleic anhydride using Dicumyl peroxide as the initiator. The goal of this work was to optimize the properties of PBS and PPC blends using free radical functionalization in a quad screw extruder. Specifically, we aimed to tailor the properties of the blend by the amount of compatibilizer added in the system and analyze the reaction efficiency and mixing performance by means of parallel plate rheometry, Nuclear Magnetic Resonance (NMR), Fourier Transform IR Spectroscopy FTIR, Scanning Electron Microscopy SEM and titration studies.

2. Materials and Experimental Approach 2.1 Materials The poly(butylene succinate) was supplied by VTolo (Lot number VTO1701010, extrusion grade) and had a molecular weight of 90000 g/mol, melt index of 4.0 g/10min at 190 oC/216g and density of 1.26g/cm3. The poly(propylene carbonate) used was a QPAC40 supplied by Empower Materials. The molecular weight was around 150000 and 350,000 g/mol, melt index 0.9 g/10min


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at 150 oC/216g and same density as PBS. The maleic anhydride (MAH, 99%) was supplied by Sigma-Aldrich has a molecular weight of 98.06 g/mol. The compound used to aid the chemical reaction between MAH and the polymer was Dicumyl Peroxide (DCP, 98%) supplied by Sigma Aldrich with molecular weight 270.37g/mol. 2.2 Preparation of the grafted PPC with MAH and peroxide The compatibilizer was created using a twin-screw extruder (KZW15TW-45/60MG-NH-3000, Technovel Co., Japan). The polymer was ground for improved melting using a 5 mm sieve and dry mixed with ground 2 wt% MAH and 0.1wt% DCP. The screw speed used was 155 rpm and the residence time was set to 2.1 min, which is the minimum required for completion of DCP and MAH reaction24. Since it is known that these biopolymers are shear and heat sensitive, a smooth screw profile was used for the compounding trials. See supporting information S1 for processing details and screw program. 2.3 Reactive Extrusion of PPC and PBS The reactive compatibilizer was added in three different ratios to blends of PBS and PPC by compounding in a quad screw extruder from Technovel Co., Japan (45:1 L/D) (model: WDR15QD-45MG-NH-2200). The blend composition for all the trials was 75% PBS and 25% PPC and the amount of compatibilizer (gPPC) was varied from 5 to 25%. The feeder speeds were chosen in order to obtain a residence time close to 2 min to ensure enough time for the reaction24. See processing detail and screw program in S2 under supporting information. 2.4 Characterization Techniques i)

Purification of blends and Titration

Before titration was conducted, the samples were purified to separate the grafted polymer from the unreacted MAH and DCP. The samples were dissolved in chloroform at room temperature overnight and then hydrolyzed with 2ml of 1 N hydrochloric acid. The samples were precipitated into chilled methanol and rinsed several times before drying them in a vacuum oven at 70 oC for 24 hours. For the titration analysis, 0.5 g of sample were dissolved in 20 ml of chloroform. The solutions were titrated against 0.05N of KOH solution in methanol using phenolphthalein as the indicator diluted in chloroform. The samples did not precipitate during the experiment. The MAH grafting percentage was determined using Equation 125,26:



𝐺𝐸 [%] =

𝑉𝐾𝑂𝐻 × 𝑁𝐾𝑂𝐻 × 98.06 2×𝑊

× 100

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(1)

where GE is grafting efficiency, VKOH is the difference in volume of KOH in liters used for the compatibilized formulation and the neat sample (uncompatibilized polymer or blend), NKOH KOH normality, W weight of the polymer and 98.06 the molecular weight of MAH. i)

Nuclear Magnetic Resonance

The 1H nuclear magnetic resonance (H-NMR) spectra of the samples was obtained from a Bruker & Spectrospin Advance spectroscopy of 500 MHz. To prepare the samples, 21 mg of the material were dissolved in 0.75ml of d-chloroform. The number of scans performed on each sample was 128. ii)

Fourier Transform Infrared Spectroscopy

FTIR spectra were collected on a Nicolet Spectrometer in ATR mode at 2 cm-1 resolution and 64 scans. The test specimens were small pieces of film made by compression molding at 150 oC with 5 minutes preheating/melting time and 4 minutes compression time. At least 6 specimens of each sample were analyzed in FTIR and the peak carbonyl peak locations were averaged for future analysis. iii)

Parallel Plate Rheology

A rotational rheometer (ARES-G2 from TA Instruments) set with 25 mm diameter stainless steel parallel plate fixtures was used to determine the rheological properties of the blends. The samples were 25 mm discs prepared by compression molding with 4 minutes preheating and 5 minutes compression time at 150 oC. Before molding and conducting the rheology trial the samples were dried in a vacuum oven at 50 °C for 24 hours. The tests were carried out at 150 oC. A strain sweep was used to verify that the strain used in frequency sweeps was in the linear viscoelastic region. Then frequency sweeps were conducted from 0.1 to 100 rad/s at 5% of strain and the stress relaxation response was measured by imposing a 5% step strain and recording the decay of stress with time. iv)

Field Emission Scanning Electron Microscopy

A scanning electron microscope (Jeol JSM-6330F) was used to observe the morphology of the blends. The acceleration voltage used was 6 kV. Samples were cryogenically fractured to prevent


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any deformation on the surface to be imaged. To increase the electrical conductivity of the specimens, they were sputter coated with three thin layers of gold that were deposited on the surface over 180 seconds. v)

Mechanical Properties

The tensile properties of the neat and functionalized blends were measured on an Instron 444 machine, using a crosshead speed of 50 mm/mm in accordance with ASTM D63827. The flexural properties were obtained by means of a 3-point bend test using the same machine and a strain rate of 1.05 mm/mm in accordance with ASTM D79028. Finally, the impact properties were tested on unnotched samples by means of an Izod impact test with a 5ft-lb pendulum in accordance with ASTM D265-1029. All specimens were injection molded using a micro injection molder from DACA instruments. The tensile test samples were IV type. The samples were annealed prior the test at 50 oC for 4 hours to achieve uniform and same percent of crystallinity. The mechanical tests were conducted at room temperature. 3. Results and Discussion 3.1 Compatibilizer Characterization Figure 1 depicts sections of an overlay of the NMR spectra of compatibilizer gPPC (red) and neat PPC (blue). Additional peaks in the gPPC spectrum observed at around 7, 6, 5, 3 and 2 ppm support the belief of possible new grafted structures were formed in the system. In our previous work carried out in a batch mixer, only chemical shifts around 6 and 3 ppm were found 23. It is possible that the high shears imparted by the multi screw extruder trigger more chemical reaction which in turn produces new or more grafted structures. In our preceding publication the origin of the new chemical shifts was determined by analyzing the simulations of the NMR spectra of potential grafted structures using MestreNova software. Table S3 under supporting information, displays the chemical shifts for neat MAH, the graft reaction of MAH in the middle of PPC chain (pendant ring structure), the ring opening reaction with the terminal OH groups of PPC (ring opening structure) and the inner chain grafting structure that comes from the bonding of two PPC chains bridged by a MAH ring. From the simulations conducted the peak at 7 ppm corresponds to unreacted MAH, the peaks around 6.8 ppm probably originate from the ring opening reaction, the 6.47 and 3.97 ppm come from grafting of the MAH ring in the middle of the PPC chain and the 5.29, 2.6 and 3.07 ppm peaks from the inter chain grafting of two PPC chains. One can speculate



that the compatibilizer made on the twin screw extruder experiences more interchain grafting than the formulation made on the batch mixer. The twin screw sample produced for this work had peaks at 5 and 2 ppm that were not found on the batch samples. This could be due to the high levels of shear that cause more chain scission and free radical formation, hence leading to the prevalence of this type of reaction. The NMR results corroborate that the grafting of MAH onto PPC was carried out successfully and that there are still spots for the reaction of MAH with the blend, either by chain extension or grafting leading to branching structures.

Figure 1. Sections of the NMR spectra of (blue) neat PPC and (red) gPPC compounded in the QSE. To quantitatively determine the grafting efficiency of the compatibilizer, titration studies were performed on samples of gPPC before and after the purification. These results give an indication of the amount of acid in the samples. The samples were hydrolyzed before the analysis, meaning that the test gives a measurement of the amount of acid and anhydride moieties in the system. The grafting efficiency before and after purification was 1.5±0.1% and 0.162±0.001 respectively. The significant decrease after purification might indicate that only a small portion of MAH can react with PPC. The reactive extrusion functionalization had a residence time of 2 minutes to ensure complete reaction of DCP and MAH at the processing temperature24. However, we have not determined if the reaction continued to completion. In our previous work conducted in a batch mixer, the formulations were melt compounded for 10 minutes at 50 rpm. The GE obtained for the PPC 2% MAH and 0.1% DCP was 0.9% after purification. This value is almost 6 times higher than the one obtained on this study. It is thought that prolonged mixing during extrusion could


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increase the GE, however the extended exposure to shear and heat will not be favorable for this polymer. Therefore, a trade-off between residence time and shear must be considered. NMR results showed evidence of inner chain grafting and potential chain extension through ring opening reactions and these structures do not contain acid moieties. This means that not all the MAH consumed on reactions of this kind are captured in the titration measurement, so a corrected GE still might be higher for the compatibilizer made in the extruder. Moreover, the high value found for GE before purification proves that a large quantity of the MAH added to the PPC stays in the system and does not volatilize during processing. This could be useful since the unreacted MAH left in the compatibilizer can undergo reaction when added to the blend in second step of the blending process. 3.2 Characterization of Functionalized Blends Blends of PPC and PBS with different amounts of the confirmed gPPC were prepared and studied. Titration analysis was conducted on purified and non-purified samples to determine how much maleic acid was grafted in the blends after compounding. The results in Figure 2 show the grafting efficiency of the compatibilizer and the blends with different amounts of gPPC. The difference between non-purified and purified samples is quite large in the case of the compatibilizer because there is an excess of MAH in the system and only a small portion of MAH was grafted to the PPC. In the case of the blends, only a small fraction (up to 25%) of the formulations contain MAH, therefore the excess of the reagent in them is smaller. The results show that the grafting efficiency of the blends increases with the addition of more compatibilizer. Moreover, the compound with the highest amount of gPPC showed almost the same grafting efficiency as the compatibilizer itself. Based on the titration results for purified gPPC, the gPPC contributed 0.0072 (5% gPPC), 0.018 (15% gPPC), 0.03 (25% gPPC) wt% grafted MAH functionality to each corresponding blend sample. The fact that the blends had a higher GE value only means that the unreacted MAH present in the gPPC underwent grafting reactions during the PBS-PPC blending.



1.5

1.6 1.4 1.2 GE [%]

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1.0

non purified

purified

0.8 0.6

0.39

0.4

0.21

0.162 0.09

0.2

0.04

0.04

5%

15

0.13

0.0

er)

liz ibi

gP

PC

t pa m (co

PC

P d-g n e Bl

en Bl

P d-g

PC

% PC

en

Bl

25

%

P d-g

Figure 2. Titration results of the compatibilized blends.

Proton NMR was carried out on functionalized blends and results were compared against a neat blend without compatibilizer. Figure 3 portrays the overlay of sections of the spectra of the neat blend and the functionalized blend with 5% gPPC. The peak at 6.46 ppm was identified already in the gPPC as the hydrogen on the PPC backbone that is closest to the succinic anhydride ring (middle pendant ring), see Table S3. It was found that the 6.46 ppm peak increased with the increment of gPPC in the blend composition. The 3.99 ppm was previously identified as one of the hydrogens on the succinic anhydride coming from the same structure mentioned before. Additional signals on the 5% and 15% gPPC blends were observed around 5.7 ppm and 2.4 ppm, that do not appear on the gPPC. In order to determine the origin of these new chemical shifts, new simulations were performed on reactions between PPC and PBS (copolymer structures). Among the several potential reactions encountered in the systems that were simulated, the one presented in Scheme 1 under supporting information S4, is the only product that contains chemical shifts close to those found experimentally in the reactive blends. This structure results from linking both PPC and PBS chains together through a maleic acid or anhydride; it has a peak around 5.5 ppm (the hydrogen on the PPC closest to the maleic acid) and around 2- 3 ppm (from the hydrogens in the maleic acid). We speculate that a reaction of this kind is occurring in the system, however, it


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is also possible that the new peaks originate from new reactions between PPC and MAH or PBS with MAH as was shown on our previous work23.

Figure 3. NMR spectra of (blue) neat blend and (red) blend with 15% gPPC. Although FTIR is not as sensitive as NMR to new bonds, it is used extensively for the characterization of polymer blends to analyze their compatibility and the molecular interaction between the components30. Figure 4 depicts the FTIR spectra obtained for the neat and functionalized blends. Two new peaks were formed in all the samples that contained gPPC. The pink oval on the left shows a zoom in of the spectra around 900- 800 cm-1, which contains a new peak corresponding to the in-plane bending of the CH2 in the maleic acid structure. The pink oval on the right of the image contains a peak at 638 cm-1 which is attributed to the -COO bending bands of the grafted MAH structure26. The peaks are sharper and became more defined with gPPC which is as expected since more grated polymer with those bonds is added to the blend. The formation of copolymer during reactive extrusion can be analyzed by changes in the shape and the peak location of the FITR bands. PPC, PBS and MAH have carbonyl groups in their chemical structure which typically appear as a sharp long peak around 1700 cm-1. The location of this peak was measured in the neat blend and compared to the one of the compatibilized blends with different amounts of gPPC. All the compatibilized blends experienced a shift in the carbonyl peak location to lower wavenumber. As reported in Figure 5 (left), with the addition of more gPPC in the system, the shift in the band location is larger. Four films of each blend were tested and a standard deviation of the shift was calculated based on this. The peak locations of the neat components as well as the blends are reported in Figure 5 (right). The carbonyl peaks for PBS and the neat blend are very close to one another because the blend is mainly composed of PBS (75 wt%); the up-field shift is due to overlap with the carbonate carbonyl, which has a higher wavenumber absorption than the ester. With the addition of gPPC, the carbonyl peak moved down-



field in contrast to the neat uncompatibilized blend, see Figure 5 (right). The gPPC peak is 0.16 cm-1 higher than the neat PPC. This shift from the neat polymer is not only quite small but is upfield (opposite direction to the one of the compatibilized blends). Moreover, the carbonyl peak band of MAH appears at a higher wavenumber (around 1770 cm-1), meaning that the unreacted MAH that is in the blends is not causing the shift either.

Figure 4. FTIR spectra of uncompatibilized and compatibilized blends. It is believed that the formation of a new graft copolymer (due to either chain extension or grafting) may cause the red shift in the absorption bands, perhaps due to more intimate mixing of the components and slightly more hydrogen bonding character. Jo et al.31 attributed the carbonyl shift of the styrene-acrylic acid copolymer and PMMA blends to specific interactions involving hydrogen bonding between the two components. According to previous research conducted on PET/PC blends, the shift in peak location of this nature is due to new copolymers created by ester exchange reaction32,33. This is another evidence of grafting reaction and better interaction between the components in the PPC/PBS functionalized system.


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Carbonyl Peak Shift

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0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

0.66

0.23 0.14

gPPC 5%

gPPC 15%

gPPC 25%

Sample Neat PBS Neat PPC gPPC MAH Neat Blend 25% gPPC Blend

Carbonyl peak 1712.3±0.02 1736.11±0.07 1736.26±0.07 1774.00 1712.82±0.04 1712.15±0.04

Figure 5. Carbonyl peak shift of gPPC blends (left) and carbonyl peak location of the blends and blend components (right)

The rheological behavior of blends at low frequencies or strains provides valuable information about the extent of reaction and the structures formed in a reactive system. In this work, parallel plate measurements of complex viscosity and modulus vs. frequency were used to characterize the mixing behavior of the functionalized blends. Figure 6 depicts the complex viscosity for the neat blend and the blend with different amounts of compatibilizer. The neat, uncompatibilized blend has a lower viscosity than the 5 and 15% gPPC blends whereas the 25% gPPC blend has the lowest viscosity plateau of all. The viscosity increase indicates the occurrence of interfacial chemical reactions in the 5 and 15% system34. It is known that the rheological behavior of a polymer blend is tightly related to the morphology changes with the addition of a compatibilizer. The intent of adding gPPC is to functionalize the interface between the two polymers, suppress the droplet coalescence and reduce or eliminate interfacial slip. Van Puyvelde et al.35 discovered that blends of PA-6/EPR experienced a significant viscosity increase when the blend was compatibilized. These results support the belief that the interfacial slip is due to loss of entanglements at the interface between the polymer components and will trigger a change in the viscosity plateau of the blend36. The zero-shear viscosity of the blends was estimated by fitting the Cross Model37 to the viscosity curve; results are depicted in Table 1. The value cannot be presumed proportional to molecular weight; however, it can be taken as a relative measurement of the molecular weight of the matrix or entanglement growth at the interface area if compared to blends of the same composition. All the functionalized blends have a higher zero shear viscosity than the neat blend, even the gPPC 25% blend that appeared to have decreased viscosity in the shear thinning region. This means that the gPPC blends experience an increase in reinforcement of the matrix around the



compatibilized of the droplets after processing in contrast to the neat blend, even though the reactive blends contain gPPC which is exposed twice to shear and heat stress. It appears that the gPPC 25% blend experienced some chain architecture changes along with potential degradation. The neat blend seems to have a slightly longer viscosity plateau and its onset of shear thinning occurs at a higher frequency. The functionalized blends have a more power-law-like behavior and seem to shear thin at lower frequencies. The effect becomes more pronounced with the addition of more gPPC in the system. In homopolymers, this trend is often related to increasing amount of branching in the resin while keeping the Mw the same. Higher content of side branches shifts the viscosity plateau to lower frequency or shear rate due to the lubrication effect of the side group chains37. A similar effect could be encountered in this blend type. The grafting reaction could lead to long or short chain branches depending on where in the chain grafting occurs. The MAH serves as a bridge between polymer chains creating branches that could potentially form a network structure, if fully reacted. This same behavior but with a more significant impact was found in blends of PLA/PBAT using an epoxy resin as compatibilizer by Al-Itry et al.38 the interpretation was that some epoxy functions were consumed to react in the interface during compounding. Figure 6 also depicts the evolution of the dynamic storage modulus of the blends. The samples experienced different behavior with the addition of compatibilizer. Higher storage moduli of all the compatibilized blends can be observed in comparison to the neat blend. Especially the moduli of the 5 and 15% gPPC samples plateaued at very low frequencies. This is also known as the “relaxation shoulder” and it is thought to appear due to the formation of copolymer at the interface, i.e., the blends become more elastic with the addition of the compatibilizer. This effect was also encountered by Al-Itry et al.38 in the compounding of PLAT and PBAT, by Sailer et al.39 during the blending of PA and SAN with MAH and by Riemann et al.40 when compounding PS and PMMA with different amount of diblock copolymer.


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Figure 6. Complex viscosity and Storage modulus vs. Frequency of blends. Table 1. Zero shear viscosity of neat components and blends. Sample Zero Shear Viscosity Neat Blend 1192±28 Blend 5% gPPC 2155±47 Blend 15% gPPC 2698±160 Blend 25% gPPC 1656±140 Stress relaxation measurements were also carried out to analyze if interfacial viscoelasticity is experienced in the systems. In theory, compatibilized blends show a longer relaxation event in step-strain tests than their uncompatibilized counterparts41. Figure 7 portrays the relaxation modulus vs time for all the samples. In agreement with the other rheology results, moduli of the 5 and 15% gPPC blends are slightly higher than that of the neat blend and the 25% gPPC one. The shoulder that occurs at around 0.08s is related to the relaxation time of neat PBS. It seems to be more pronounced in the 25% blend, but it became less evident for the 15 and 5% ones. The relaxation of these blends happens almost instantly due to their fluid like behavior, however, it is evident that the three functionalized blends experience the same trend in their curves, where the neat blend seems to relax slightly faster. The delay in the functionalized blends could be related to the relaxation of the copolymer in the interface of the two polymer components. This was found by Al-Itry et al.38, during the stress relaxation analysis; not only was the relaxation time of the compatibilized blends prolonged but also the modulus increased, meaning that the contribution of



the interface was significant. Silva et al.36 discovered the same trend and found that beyond a certain amount of compatibilizer the relaxation time was shorter than the neat blend, indicating that just small amounts are sufficient to modify the morphology of the blend but not to avoid the slip between the two phases.

Figure 7. Stress Relaxation results of the neat and compatibilized blends. Some of the factors that affect the morphology of an immiscible blend are the composition, processing conditions, viscosity ratio of the components and interfacial tension. Immiscible blends typically demonstrate a coarse morphology that can coalesce in molding or other post-blending processes. Therefore, the addition of the right compatibilizer can improve the miscibility of the system and this usually translates to a finer structure unable to coalesce42. Figure 8 depicts the SEM images of the freeze fracture surface of the blends compounded on the QSE. All the images showed a matrix-droplets morphology, evidencing the immiscibility of this blend. However, the addition of the compatibilizer significantly changed the morphology of the systems. The particle size distribution seems to be quite broad for all the compatibilized blends, though the overall tendency is to a smaller droplet size in comparison to neat blend. Also, the shape of the PPC phase is not spherical but more of an elongated form. In the case of the neat blend, the particle size is around 5 um and they seem to have very sharp boundaries, evidencing the lack of interfacial agent in between the components. In the case of the functionalized blends, the particle size is around 1 to 2 um and with smoother edges, which is evidence of the interfacial reaction between polymers.


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Moreover, the minority phase seems to be much more finely dispersed in the compatibilized blends. These findings were also discovered by Zahedi et al.43, in blends of PET/PC with Lanthanum Acetyl Acetonate, and by Huneault et al.44 when mixing PLA with thermoplastic starch and MAH.

Figure 8. SEM images of (a) the neat blend, (b) blend gPPC 5%, (c) blend gPPC 15% and (d) blend gPPC 25%. The mechanical properties of polymer blends depend on the molecular weight of the components, the morphological structure and the interfacial adhesion between the phases. Polymers of high molecular weight have higher entanglement degree and fewer end groups, which in turn gives them better mechanical properties than polymers of lower molecular weight. This is why degradation effects negatively impact the mechanical performance of polymers; especially, in biopolymers as they are sensitive to heat and shear exposure. The morphology of the blend also plays an important role on the mechanical properties; smaller domains and good adhesion between the phases aids load transfer from matrix to droplets and improves the toughness of the material. The tensile properties of the blends with and without the compatibilizer are depicted in Figure 9



(a). The stress at break of the formulations slightly decreased with the addition of gPPC. This correlates with the results found by Yao et al.45 when blended PLA with PPC and MAH. The decay in strength is attributed to the unreacted MAH that acts as a plasticizer, improving the flexibility of the blend but reducing its resistance to the tensile load. According to our titration results, there is some MAH left in the formulations after compounding, hence, plasticization could be a potential reason for this result. Nevertheless, the decrease in stress at break is very small and some of the results are within the error, so it is concluded that this property is not significantly affected by the addition of gPPC. Moreover, the elastic modulus experiences a small increase when adding 5 and 15% gPPC; these results are also within the error of the measurement, meaning that the stiffness of the blend does not improve nor decrease with more gPPC in the system. On the other hand, the results for the strain at break are more promising, there is a significant increase of more than 100% for all the samples and the difference becomes greater with more compatibilizer in the blend. A blend between PPC and PBS is expected to be flexible since both components have high strain at break. However, the results of the neat blend evidence the poor interface of this formulation. It is remarkable that with only a 2 minute residence time in the QSE the gPPC compatibilized blends attained such a large improvement to strain at break and toughness; many similar blend studies are conducted at low speeds in batch mixers with a much longer residence time. Finally, Figure 9 (b) shows a small increase on the flexural stiffness of the samples with addition of gPPC, the trend is more pronounced than that found for the Young’s modulus. The improvement in impact strength is more noteworthy. The increase is more than double for the 5% and 25% gPPC and is 1.7 times greater for the 15% one. Semba et al.46 also found a significant increase in impact strength when blending PLA with PCL and DCP. They attribute this effect to the better interfacial adhesion of the components. These results proved that the compatibilizer is acting as a bridge for the load transfer between the matrix and the PPC droplets. Also, the smaller particle size gives rise to higher surface area and prevents the crack propagation during impact, hence, increasing its toughness.


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Figure 9. Tensile properties (a), impact and flexural properties (b) of neat and reactive blends.

4. Conclusions The preparation of functionalized blends of sustainable polyesters and polycarbonates was successfully achieved, and titration and spectroscopy were conducted to identify the degree of functionalization and the types of chemical structure formed in the system. From the titration results, the addition of more gPPC in the blend increased the grafting efficiency of the MAH in the polymer. From spectroscopy techniques it was found that the compatibilizer formed bonds between both phase components (Particularly, the 5% and 15% gPPC blends, according to NMR). Parallel plate rheology showed that the compatibilizer functionalized the interface of the system to increase resistance to flow, especially in the case of 5 and 15% gPPC blends. Microscopy results demonstrated the emulsification effect created by the gPPC in the formulations, preventing coalescence and enhancing the dispersion of the PPC droplets in the matrix. Finally, the strain at break and the impact resistance improved significantly with the incorporation of more gPPC in the system, evidencing the performance of the compatibilizer at the interface. Functionalized blends of this kind are expected to be easier to handle than neat PPC due to the physical stability offered by the PBS (crystalline), have a thermal stability close to that of neat PBS, higher viscosity during processing due to the highly viscous PPC phase and the good mechanical properties from both materials.



5. Acknowledgments The authors wish to acknowledge the National Science Foundation (NSF) (grant number CMM1350445) for funding this study.

6. Reference (1) (2) (3)

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PPC

no PPC-g-MAH

with PPC-g-MAH

+

PBS

w i t h P P C


+

PPC-g-MAH

Blends of renewable Poly(butylene succinate) and Poly(propylene

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