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Reactive Compatibilization: Formation of Double-Grafted Copolymers by In situ Binary Grafting and their Compatibilization Effect Depei Chen, Hengti Wang, and Yongjin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08699 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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ACS Applied Materials & Interfaces

Reactive Compatibilization: Formation of Double-Grafted Copolymers by In situ Binary Grafting and their Compatibilization Effect

Depei Chen, Hengti Wang, Yongjin Li*

College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou 310036, People’s Republic of China

*

Corresponding author.

E-mail: [email protected] (Yongjin Li); TEL: +86-571-28867026

1

ACS Paragon Plus Environment


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ABSTRACT: Reactive compatibilizers are usually used to enhance the compatibility of

immiscible polymer blends. However, reactive linear compatibilizers containing reactive groups on the main chains form graft copolymers during reactive blending, and such graft copolymers with an asymmetric molecular structure are often “pulled in” or “pulled out” under mechanical shear. Double-grafted compatibilizers have a symmetric structure and they usually exhibit higher compatibilizing efficiency. In this work, we propose a binary grafting strategy during melt blending to form compatibilizers located at the interface of an immiscible polymer blend. Specifically, poly(methyl methacrylate) (PMMA) oligomer with carboxylic end groups (PMMA– COOH) and poly (styrene-co-glycidyl methacrylate) (SG) copolymer were simultaneously incorporated into immiscible poly(vinylidene fluoride)/poly(L-lactic acid) (PVDF/PLLA) blends. The carboxylic acid groups of both the PMMA oligomer and PLLA can react with the epoxide groups on the SG main chains. Therefore, novel compatibilizing polymers with both PMMA and PLLA chains grafted onto the SG main chains form in situ. The grafted PMMA chains can entangle with PVDF, and the grafted PLLA chains are embedded in the PLLA phase, so the double-grafted copolymers act as effective compatibilizers for the PVDF/PLLA blends. Moreover, the effects of the PMMA molecular weight and PMMA loading (number of grafted PMMA side chains) on the compatibilization efficiency were investigated. The compatibilizing efficiency increases with increasing molecular weight and number of side chains in the ranges considered in this study. This one-pot synthesis of double-grafted compatibilizers by in situ grafting provides a new and simple method to prepare double-comb compatibilizers, and it offers the possibility of high efficiency compatibilization.

KEYWORDS：Immiscible polymer blends, compatibilization, reactive blending, morphology, interface

2


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1. INTRODUCTION Blending polymers to achieve synergistic effects is an effective way to fabricate new materials.1–3 However, most polymer pairs are incompatible, which means that it is not possible to obtain high performance materials by simply melt blending.4–6 A suitable compatibilizer can improve the compatibility between the polymers and reduce the interfacial tension, improving the performance of the blended materials.7–11 Compatibilizers can be divided into reactive and non-reactive compatibilizers. Non-reactive compatibilizers are usually block or graft copolymers. They improve the compatibility between the components by non-covalent interactions.12–15 For reactive compatibilizers, a chemical reaction occurs during melt blending and the in situ formed graft/block copolymers enhance the compatibility between the polymers.16,17 Most reactive compatibilizers have a linear structure with reactive groups randomly located on the main chains.18–20 Polymer chains that can react with the reactive groups of the compatibilizers are grafted onto the linear compatibilizer, and therefore the final molecules after grafting have a one-side grafted (“single comb”) structure (Figure 1A). The grafted molecules may not be stable at the interface, and they are easily pulled into the matrix phase or dispersed phase owing to the asymmetric structure during the shearing process.12,21 Obviously, such a “pulled in” or “pulled out” process reduces the compatibilization efficiency of the compatibilizers, as shown in Figure 1A.22,23 The pregrafted molecular chains on the reactive main chains can stabilize the dynamic balance of the final compatibilizers at the interface. Therefore, high compatibilization efficiency can be achieved by using reactive graft copolymers (RGCs) as compatibilizers, as shown in Figure 1B. Recently, we fabricated the RGC poly(methyl methacrylate-co-glycidyl methacrylate)-graft-poly(methyl methacrylate) (P((MMA-co-GMA)-g-MMA)) and then used the RGC as a compatibilizer for immiscible PVDF/PLLA blends.17,24 PLLA reacted with the epoxide groups in the backbone during melt blending, so new molecules with both PMMA and PLLA chains grafted onto the main chains were achieved (Figure 1 B). The double-grafted compatibilizer could locate at the interface and showed higher compatibilization efficiency than the single-grafted compatibilizer. The double-grafted compatibilizers 3



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showed better balance between the pull forces from the matrix and the domain. With addition of a small amount of reactive graft compatibilizers, the interfacial tension of the incompatible blend remarkably decreased and the size of domain significantly decreased because of their better interfacial stability than the reactive linear copolymers.

Figure 1. Schematic diagrams of a contrast on compatibilizing efficiency between the incorporation of (A) reactive linear compatibilizer and (B) reactive pregraft compatibilizer in binary immiscible polymer blends.

Although RGCs exhibit better compatibilization efficiency than reactive linear copolymers, synthesis of RGCs is difficult. In particular, synthesis of oligomers with end double bonds requires harsh reaction conditions and the yield is also low.16,24 Moreover, oligomers with end double bonds have different reactivity ratios than the two monomers, which results in a low grafting ratio of the final RGCs. Considering the high reactivity of end carboxylic groups with epoxides and the reactivity dependence on the molecular weight of the oligomer/polymer, it is feasible that double-grafted copolymers can be fabricated by reactive blending with simultaneous grafting of a reactive oligomer and a polymer onto the same molecular main chain. Thus, binary grafted copolymers can be synthesized and they act as compatibilizers. In this study, we simultaneously incorporated PMMA oligomer with an end 4


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carboxylic group (PMMA–COOH) and poly(styrene-co-glycidyl methacrylate) (SG) in PVDF/PLLA blends. SG contains many epoxide groups on the main chains. The carboxylic groups of both the PMMA oligomer and PLLA readily react with the epoxide groups of SG. Because the molecular weight of PMMA–COOH oligomer is much lower than that of PLLA, the reactivity of the carboxyl group of PMMA is higher than that of PLLA.25 Therefore, grafting PMMA–COOH oligomer onto the SG main chain is faster than grafting PLLA onto the SG main chain during the melting process when the epoxide equivalent of SG is higher than the carboxylic acid equivalent of the PMMA oligomer. Formation of binary grafted SG is critical to compatibilize the PVDF/PLLA blend. Moreover, the grafting density and the chain length of grafted PMMA can be adjusted by changing the PMMA–COOH oligomer loading and the molecular weight of the PMMA–COOH oligomer. It should be noted that PVDF/PLLA blends are both scientifically and industrially important because it may be used as the new piezoelelctric materials with both high piezoelectricity and good adhesion performance.26

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis of PMMA Oligomer and SG. 2.1.1. Materials. PLLA (3001D, Nature Works, US) with Mn = 8.93 × 104 and Mw/Mn = 1.77 and PVDF (KF850, Kureha Chemicals, Japan) with Mn = 1.05 × 105 and Mw/Mn = 2.0 were used in the experiments.

2.1.2. Synthesis of Carboxyl-Terminated PMMA (PMMA–COOH). PMMA–COOH oligomer was synthesized by radical telomerization of MMA with thioglycolic acid (TAC) as a transfer agent and 4,4′-azobis-(4-cyanovaleric acid) (ACVA) as an initiator. MMA (100.14 g, 1 mol), TAC (4.6 g, 0.05 mol), and ACVA (2.80 g, 0.01 mol) were dissolved in 100 mL tetrahydrofuran (THF). The reaction was performed under nitrogen at 60 °C in a triangular flask. The reaction was stopped after 2.5 h and the product was diluted with acetone. The product was then precipitated twice in petroleum ether and once in water. The product was dried by lyophilization for 2 days. For PMMA–COOH oligomer, Mn = 2000 g/mol and Mw/Mn = 2.5. PMMA–COOH 5



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oligomer samples with Mn = 4000 and 8000 g/mol were also synthesized using the same strategy with feed molar ratios of ACVA to TAC of 1/2.25 and 1/1.23, respectively. The Mw/Mn values of the two samples are 2.4 and 2.2, respectively.

2.1.3. Synthesis of SG. GMA (3.00 g, 0.021 mol), St (7.00 g, 0.067 mol), and AIBN (0.15 g, 9.13 × 10−4 mol) were dissolved in 15 mL of toluene. The reaction was performed under nitrogen at 90 °C in a triangular flask. The reaction was stopped after 17 h and the product was diluted with acetone. The product was then precipitated twice in petroleum ether. The product was dried by lyophilization for 2 days. For SG,

Mn = 13,000 g/mol and Mw/Mn = 2.16. SG copolymer was synthesized by addition of 30 wt % GMA monomer during copolymerization. We confirmed that all of the GMA was completely randomly copolymerized with styrene by both titration and NMR analysis (Figure S1). Based on the molecular weight of the synthesized SG and the titration results, one SG chain contains 27 epoxide groups. 2.2. Melt Blending. PVDF, PLLA, PMMA–COOH oligomer, and SG were dried at 80 °C overnight before processing. All of the blends were prepared by melt mixing at 190 °C for 10 min using a batch mixer (Haake Polylab QC) with a rotation speed of 50 rpm. All of the samples were then compression molded by continuous hot pressing (190 °C, 10 MPa) for 12 min and then cooled from 190 to 40 °C in water (10 MPa). It should be noted that the PMMA–COOH oligomer loadings were carefully calculated based on the molecular weight of PMMA–COOH oligomer with constant SG addition for all of the compatibilized samples. We designed different PMMA side chain numbers on one SG main chain by varying PMMA–COOH oligomer addition. We designate the PMMA side chain grafted SG compatibilizers as B–X–Y, where X is the molecular weight of PMMA–COOH oligomer and Y is the side chain number of the compatibilizer. For example, B-2K-1 means that the molecular weight of PMMA– COOH oligomer is 2000 and there is one PMMA side chain grafted onto the SG main chain. 2.3.

Characterization.

2.3.1.

Scanning

Electron

Microscopy

(SEM).

The

morphologies of the blends were characterized with a field emission scanning electron microscope (Hitachi S-4800). All of the films were immersed in liquid nitrogen for 15 6


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min and then fractured. Before morphology characterization, the fractured surfaces of the PVDF/PLLA blends were sputtered with gold.

2.3.2. Transmission Electron Microscopy (TEM). The samples were sliced into 70–90 nm slices with a freezing microtome and vapor stained by ruthenium tetroxide for 4 h. The phase morphologies of all of the blends were determined by TEM (Hitachi H-800).

2.3.3. Mechanical Properties. Tensile tests were performed using an Instron universal testing machine (INSTRON-5966). The tensile tests were performed at room temperature with a tension rate of 10 mm/min. Impact tests were performed according to the GB/T 16420-1996 standard.

2.3.4. Differential Scanning Calorimetry (DSC). Thermal analysis was performed by DSC using a Q2000 differential scanning calorimeter (TA Instruments). The DSC scans of all of the blends were performed at a heating/cooling rate of 10 °C/min in nitrogen atmosphere, and the testing temperature range was −50 to 200 °C.

2.3.5. Fourier Transform Infrared (FTIR) and Attenuated Total Reflection (ATR) Spectroscopy. All of the samples were compression molded before the tests. FTIR and ATR spectroscopy were performed with a Bruker VERTEX 70V spectrometer. All of the films were scanned 64 times at spectral resolution of 4 cm−1.

2.3.6. Rheological Measurements. Rheological tests were performed using a physica MCR302 (Anton Paar Instrument) at 190 °C. The plate diameter of parallel plate was 25 mm and frequency sweep was carried out from 100 to 0.01 rad s−1 at 5% strain.

3. RESULTS 3.1. Compatibilization effects of SG and PMMA–COOH oligomer on PVDF/PLLA blends. Figure 2 shows SEM images of PVDF/PLLA (50/50, w/w) blends without and with a small amount of PMMA–COOH oligomer and SG with different magnifications. For the blend without a compatibilizer, PVDF forms domains that disperse in the PLLA matrix (Figure 2 a1 and a2) owing to the much higher density and melt viscosity of PVDF than PLLA. The PVDF domain size ranges from 1 to 30 µm, and the distribution of the domain size is very wide. Moreover, there 7



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is a clear gap between the PVDF and PLLA phases (Figure 2 a2), indicating very weak interfacial adhesion. This means that the PVDF/PLLA system is a typical thermodynamic immiscible blend system. Addition of PMMA–COOH oligomer does not induce any change in the morphology of the PVDF/PLLA blend, as shown in Figure 2b1 and b2, indicating that a small amount of the PMMA–COOH oligomer has no compatibilizing effect on the blend. Incorporation of SG into the PVDF/PLLA blend leads to a drastic morphological change (Figure 2c1 and c2) compared with the raw blend without any additives. The PVDF phase stretches into long domains, and the phase structure is closer to the co-continuous structure. However, detailed analysis of the morphology indicates that the interfacial bonding is also very weak, as evidenced by the smooth fracture interface of the sample on the fracture surface and the clear gap between the phases (Figure 2 c2). This is attributed to the fact that the carboxylic acid groups of PLLA can react with the epoxide groups on the SG main chain under the blending conditions. Therefore, the viscosity of the PLLA phase drastically increases after addition of SG (see Figure S2), so the viscosity ratio of PLLA to PVDF increases and the blend morphology significantly changes. Similar results have been reported elsewhere.27,28 However, such a morphological change induced by variation of the viscosity ratio does not induce enhancement of physical properties, which will be shown in the following paragraph. This indicates that SG alone is not a real compatibilizer for PLLA and PVDF. Simultaneous addition of PMMA–COOH oligomer and SG to the blend results in totally different results. As shown in Figure 2d1 and d2, the PVDF/PLLA blend forms a co-continuous morphology. The continuous structure was also confirmed by the extracting experiments using Soxhlet extractor. The continuity of PLLA in the blend is as high as 95%. The sizes of the long and short axes of the PVDF phase are 5–20 µm and 1–3

µm, respectively, and most of PVDF domains are connected. Moreover, it is clear that the interfacial adhesion between PVDF and PLLA is drastically enhanced with binary addition of SG and PMMA–COOH oligomer (Figure 2 d2). Both the morphological change and the enhanced interface adhesion contribute to the significantly improved mechanical properties of the blends compatibilized by binary addition of SG and 8


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PMMA–COOH oligomer. It should be noted that the weight ratio between the PMMA–COOH oligomer (with Mw = 2000) and SG was carefully calculated in Figure 2d. Four PMMA side chains were grafted onto the SG main chain, assuming that all of the PMMA–COOH oligomer was grafted onto the SG main chains. In other words, we used B-2k-4 as the compatibilizer.

Figure 2. SEM images of PVDF/PLLA blends: (a) PVDF/PLLA (50/50); (b) PVDF/PLLA/PMMA–COOH (50/50/1.14); (c) PVDF/PLLA/SG (50/50/1.90); (d) PVDF/PLLA/B-2k-4(50/50/3) with different magnifications.

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Figure 3 shows the rheological behaviors of the PMMA–COOH oligomer and SG compatibilized (B-2k-4) PVDF/PLLA blends, as compared with the PVDF/PLLA blends without compatibilizers. It is clear that the compatibilized blends show much higher viscosity and storage modulus than the neat PVDF/PLLA blends, indicating the improved interface adhesion between PVDF and PLLA by the B-2k-4 compatibilizer. 5

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Figure 3. Complex viscosity and the storage modulus of PVDF/PLLA and PVDF/PLLA/B-2k-4(50/50/3) blends. Figure 4 shows the TEM images of the raw blend without compatibilizer and the B-2k-4 compatibilized blends. The compatiblized blends show thick interface layer

between PVDF and PLLA phases, while only sharp interface was observed for the blends without compatibilizer. This again means the improved interfacial adhesion between the two phases by simultaneous incorporation of PMMA–COOH oligomer and SG.

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Figure 4. TEM images of PVDF/PLLA blends: (a) PVDF/PLLA (50/50); (b) PVDF/PLLA/B-2k-4(50/50/3).

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Samples Figure 5. (a) strain-stress curves of PVDF/PLLA blends with the indicated additive; (b) elongation at break and the impact strength of PVDF/PLLA blends with the indicated additive. 11



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Figure 5a shows the strain–stress curves of PVDF/PLLA blends with and without the additives. The binary PVDF/PLLA blends are very brittle with elongation at break of about 4% and strength of about 50 MPa. The poor mechanical properties originate from the very poor compatibility between PLLA and PVDF, as evidenced by the SEM image in Figure 2a1 and a2. Single addition of PMMA–COOH or SG does not lead to any enhancement in the mechanical properties. Both samples have very low stretchability, with elongation at break of about 4.7% and 5.7%, respectively. It should be noted that the blends with only SG exhibit a much finer morphology, but there is a very limited enhancement of the mechanical properties. This clearly indicates that SG does not strengthen the interface between PLLA and PVDF, and thus it is not a compatibilizer for the PVDF/PLLA blend. The morphological change originates from chain extending effects by SG owing to the reaction between PLLA and SG, which leads an increase in the molecular weight of the PLLA phase and thus an increase in the viscosity. However, the elongation at break of the PVDF/PLLA blend significantly improves with simultaneous addition of PMMA–COOH oligomer and SG. The elongation at break of the PVDF/PLLA blend with both PMMA–COOH oligomer and SG is 230%, which is approximately 49 and 34 times higher than the values for the blends with only PMMA–COOH oligomer (4.7%) and SG (5.7%), respectively. In addition, from Figure 5a, the blends with both PMMA–COOH oligomer and SG have high tensile strength and modulus. This means that binary addition of PMMA–COOH oligomer and SG leads to high performance PVDF/PLLA blends with both excellent stretchability and high tensile strength. Figure 5 b also shows the impact strength of the PVDF/PLLA blends. The addition of only PMMA oligomer decreases the impact strength from 33.1 kJ/m2 to 25.6 kJ/m2. In contrast, the binary addition of SG and PMMA drastically enhances the impact strength of the blends from 33.1 kJ/m2 to 49.3 kJ/m2, more than 50% enhancement. The impact strength data is consistent with the tensile results. The excellent mechanical performance is attributed to grafting of PMMA–COOH oligomer and PLLA onto the SG main chains during melt blending. The double-grafted copolymer is an effective compatibilizer that is located at the interface between PLLA and PVDF. Detailed discussion will be given in Section 4. 12


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3.2. PMMA–COOH Loading Effect. We found that binary addition of PMMA– COOH oligomer and SG significantly enhances the compatibility between PLLA and PVDF. In Section 3.1, the loadings of PMMA–COOH oligomer and SG were carefully calculated, and four PMMA side chains on one SG chain was suitable for compatibilization of the PVDF/PLLA blend, assuming that PMMA–COOH oligomer is fully grafted onto the SG chains during melt blending. It is thus of great interest to investigate the PMMA–COOH oligomer loading effect (corresponding to the number of PMMA side chains) on compatibilization. Several PVDF/PLLA blend samples with various weight ratios of PMMA–COOH oligomer to SG were prepared to investigate the effect of the side chain number on compatibilization. Figure 6 shows SEM images of the PVDF/PLLA blends with constant SG loading and PMMA side chain numbers ranging from 0 to 6. The morphologies of all of the samples are very close to the co-continuous structure. With increasing PMMA–COOH oligomer loading, the PVDF phase size slightly decreases and interfacial adhesion is enhanced, indicating improved miscibility between the PVDF and PLLA phases. The samples with four PMMA side chains exhibit a fine phase structure and good interfacial adhesion.

Figure 6. SEM micrographs of PVDF/PLLA (50/50) blends with PLLA-g-SG-g-PMMA of various PMMA chains: (a) PVDF/PLLA/SG; (b) PVDF/PLLA/B-2k-1; (c) PVDF/PLLA/B-2k-2; (d) PVDF/PLLA/B-2k-4; (e) PVDF/PLLA/B-2k-6.

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The PMMA–COOH oligomer loading has a great influence on the mechanical properties of the PVDF/PLLA blends, as shown in Figure 7a. The compatibilized blend with only one PMMA side chain has elongation at break of about 11%, which is higher than that of the blend without a PMMA side chain. With increasing number of PMMA side chains, the elongation at break increases and the maximum elongation at break occurs for the sample with four PMMA side chains. The elongation at break is as high as 218% (Figure 7b). Overloading of PMMA–COOH oligomer results in a decrease in the elongation at break. It is considered that too much PMMA–COOH oligomer leads to either overgrafting or some ungrafted PMMA–COOH oligomer under the processing conditions.

(a)

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250 200

gn s in rea c n i

ro be um

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Samples Figure 7. (a) strain-stress curves of PVDF/PLLA blends with PLLA-g-SG-g-PMMA of various PMMA chain numbers; (b) elongation at break of PVDF/PLLA blends with PLLA-g-SG-g-PMMA of various PMMA chain numbers. 14


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3.3. PMMA–COOH Molecular Weight Effect. Based on the characteristic symmetrical structure of the in situ formed compatibilizers, the molecular weight (chain length) of the PMMA chain should have a significant effect on the compatibilization efficiency of the PVDF/PLLA blend. Therefore, PMMA–COOH oligomers with various molecular weights were fabricated and they were then used as compatibilizers accompanied with SG. In this section, the side chain number was fixed at two for all of the blends. Figure 8 shows SEM images of blends containing PMMA side chains with different molecular weights. It is clear that all of the blends maintain an almost co-continuous structure, but the phase size decreases with increasing molecular weight of the PMMA–COOH oligomer. It is considered that the length of the side chain should be sufficiently long to make the PMMA chains fully penetrate into the PVDF phase. Clearly, better compatibilization can be achieved with longer PMMA side chains.

Figure 8. SEM micrographs of PVDF/PLLA (50/50) blends with PLLA-g-SG-g-PMMA of various PMMA molecular weights:(a) PVDF/PLLA/SG;(b) PVDF/PLLA/B-2k-2;(c) PVDF/PLLA/B-4k-2;(d) PVDF/PLLA/B-8k-2. Figure 9 shows the mechanical properties of the blends with different PMMA 15



side chain molecular weights. The stretchability of the compatibilized blend increases with increasing molecular weight of the PMMA side chain. The blend compatibilized by SG and PMMA with molecular weight of 8000 exhibits elongation at break of about 270%, whereas the blend with PMMA with molecular weight of 2000 shows elongation at break of 50%. It is clear that the longer the side chain of PMMA, the stronger the molecular entanglement of PMMA with PVDF, leading to higher compatibilization efficiency.

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(b)

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300

lec mo

250 200

re inc

ng asi

t igh we r ula

s ain ch e PVDF/PLLA/ sid

PVDF/PLLA/ B-8k-2

B-4k-2

150 100

PVDF/PLLA/ B-2k-2

50 PVDF/PLLA/ SG 0

Samples Figure 9. (a) strain-stress curves of PVDF/PLLA blends with PLLA-g-SG-g-PMMA of various PMMA molecular weight; (b) elongation at break of PVDF/PLLA blends with PLLA-g-SG-g-PMMA of various PMMA molecular weight. 16


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4. DISCUSSION We have shown that binary addition of PMMA–COOH oligomer and SG leads to enhanced compatibility between PVDF and PLLA. Moreover, both the PMMA– COOH oligomer loading and the molecular weight of the PMMA–COOH oligomer are critically important for the compatibility. Such a one-pot compatibilization strategy is convenient and technically simple. At the same time, the reactive PMMA– COOH oligomer can be replaced by other polymers with reactive groups to compatibilize other polymer blends, using this "building blocks" assembly strategy. It is considered that the technique shows generality for many immiscible polymer blends. Therefore, it is important to elucidate the compatibilization mechanism of the combination of PMMA–COOH oligomer with SG. We first confirmed the binary grafting reaction of both the PMMA–COOH oligomer and PLLA onto the SG main chain under the melt processing conditions. The ternary PMMA–COOH oligomer, PLLA, and SG blends were melt processed, and the unreacted PMMA–COOH oligomer and SG were extracted using THF as the solvent. Note that PLLA does not dissolve in THF under the experimental conditions. The residue after extraction was then characterized. As shown in Figure 10a, numerous nanodomains with sizes of 10–30 nm are dispersed in the matrix of the residue. The white matrix is PLLA and dark nanodomains are SG grafted PMMA because SG-g-PMMA is ready to be stained by RuO4 and PLLA cannot be stained by RuO4. It is considered that the domains formed because of the microphase separation of binary grafted PLLA-g-SG-g-PMMA from the PLLA matrix. Both PMMA and SG are immiscible with PLLA, so the domain size would be in the micrometer scale if only PMMA was grafted onto the SG main chains. The fact that nanodomains with very small phase sizes are observed after extracting PMMA and SG indicates that both PMMA and PLLA are grafted onto the SG main chains during melt blending. PLLA is immiscible with both PMMA and PS, so microphase separation was observed in PLLA-g-SG-g-PMMA.

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Heat Flow(W/g)

(b)

pure PLLA SG PMMA-COOH PLLA-g-SG-g-PMMA

2nd heating

Exo up

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-50

0

50

100

150

200

Temperature(°C) Figure 10. (a) TEM images of the extracted residue of ternary blending of PLLA/ SG/PMMA; (b) DSC curves of pure PLLA, PMMA–COOH oligomer, SG and PLLA-g-SG-g-PMMA.

Binary grafting of PMMA and PLLA onto the SG main chains is confirmed by the DSC curves in Figure 10b for pure PLLA, PMMA–COOH oligomer, SG, and the extracted residue of ternary blending of PLLA/SG/PMMA during heating. The double-grafted copolymers have two Tg values. The lower Tg value corresponds to the segment motion of grafted PLLA while the higher value corresponds to grafted PMMA. Moreover, the characteristic absorption bands at 1759 and 1213 cm−1 for 18


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PLLA and 1723 and 1187 cm−1 for PMMA (Figure S3) again indicate binary grafting of PMMA and PLLA under the melt-mixing conditions. We also investigated the reaction processes of PMMA–COOH oligomer and PLLA with SG at different reaction times. In order to increase the reaction degree of epoxy group in SG, we increased the loadings of PMMA–COOH oligomer, PLLA. In Figure 11, the ratio of the epoxy group absorption bands at 910 cm−1 (A910) to the benzene ring skeleton vibration absorption bands at 1600 cm-1 can indicate the reaction degree of the epoxy group. The absorbance ratio Ra = A910/A1600 of PLLA-g-SG linearly decreases with increasing reaction time, indicating that there is a gradual reaction between PLLA and SG. Ra decreases much faster as a function of the reaction time for the PMMA–SG system. This means that PMMA has a higher reaction rate with SG than PLLA. This is rational because PMMA–COOH oligomer has a much lower molecular weight than PLLA under the mixing conditions, so the PMMA–COOH oligomer would graft onto SG first. Note that SG contains 30 wt % GMA monomer. One SG molecular chain contains 27 epoxide groups. Therefore, there are sufficient epoxide groups for the following grafting of PLLA after grafting of the PMMA–COOH oligomer. 1.0 0.8 Ra(A910/A1600)

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0.6 PLLA-SG

0.4 0.2 PMMA-SG

0.0 0

1

2 3 4 Processing time (min)

5

Figure 11. The absorbance ratio Ra(A910/A1600) of PLLA-SG and PMMA –SG.

For the immiscible PVDF/PLLA blend, the compatibilization mechanism for binary addition of PMMA–COOH oligomer and SG is shown in Figure 12. During 19



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melt blending, the carboxylic groups of both PMMA and PLLA are readily react with the epoxide groups of SG (Figure 12 I). PMMA–COOH oligomer has higher mobility and higher reactivity with the epoxides, so the PMMA–COOH oligomer will be grafted onto the SG main chains (Figure 12 II). If the epoxide equivalent of SG is higher than the carboxylic acid equivalent of the PMMA oligomer, PLLA will also be grafted onto the SG main chain to form binary grafting (Figure 12 III). Such a binary grafted copolymer can be located at the interface between PLLA and PVDF because of the specific interaction of PVDF with PMMA and entanglement of grafted PLLA with the PLLA phase. Thus, a strong interface can be achieved. It should be mentioned that both the length and the grafted number of the PMMA–COOH oligomer are important for compatibilization in the binary addition system. The molecular weight of PMMA should be higher than the critical entanglement molecular weight with PVDF, which has been reported to be 5000.26 The compatibilization efficiency increases with increasing molecular weight of the side chain (Figure 8). The grafting number is another critical parameter for the compatibilization efficiency by this in situ grafting strategy. SG with four PMMA side chains exhibits the best compatibilization effect when using PMMA–COOH oligomer with molecular weight of 2000. Obviously, a small number of PMMA side chains may not balance the grafted PLLA chains with a large molecular weight. However, the grafted side chains lead to steric hindrance for further grafting, so too much PMMA– COOH oligomer results in a decrease in the compatibilization effect, as shown in Figures 6 and 7.

Figure 12. Schematic diagram of the in-situ formation of double grafted copolymers (I) before grafting; (II) PMMA grafted; (III) PMMA and PLLA double grafted. 20


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5. CONCLUSION Binary addition of PMMA–COOH oligomer and SG significantly enhances the compatibility of PVDF/PLLA blends. Both PMMA and PLLA can be grafted onto the SG main chains by the reaction between COOH and epoxide groups under the melt processing conditions. The in situ formed double-grafted compatibilizer acts as a compatibilizer for PVDF/PLLA blends. The graft density and molecular weight of the PMMA–COOH oligomer significantly affect the compatibilizing efficiency. Such a one-pot strategy for synthesizing double-grafted compatibilizers is very simple and industrially feasible. It provides a new method to compatibilize immiscible polymer blends by reactive blending.

Supporting Information The 1H-NMR spectrum of SG and PLLA-g-SG-g-PMMA, the complex viscosity ߟ∗ of PLLA, PVDF and PLLA-g-SG. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: (Y.L.) [email protected]. Fax: +86 571 28867899. Telephone: +86 57128867026. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21674033, 21374027).

REFFERENCES (1) Li, Y. J.; Shimizu, H. Compatibilization by Homopolymer: Significant 21



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Table of Contents

Reactive Compatibilization: Formation of Double-Grafted Copolymers by In situ Binary Grafting and their Compatibilization Effect Depei Chen, Hengti Wang, Yongjin Li*

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Reactive Compatibilization: Formation of Double ... - ACS Publications

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