Article pubs.acs.org/IECR
Improving Grafting Efficiency of Dicarboxylic Anhydride Monomer on Polylactic Acid by Manipulating Monomer Structure and Using Comonomer and Reducing Agent Wangcheng Liu,† Tao Liu,† Tuan Liu,† Tian Liu,† Junna Xin,† William C. Hiscox,‡ Hang Liu,†,§ Linshu Liu,*,∥ and Jinwen Zhang*,† †
School of Mechanical and Materials Engineering, Composite Materials and Engineering Center, ‡Nuclear Magnetic Resonance Center, and §Department of Apparel, Merchandizing, Design and Textiles, Washington State University, Pullman, Washington 99164, United States ∥ Eastern Regional Research Center, Agricultural Research Service, U. S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, United States S Supporting Information *
ABSTRACT: Maleic anhydride (MA) grafted polylactic acid (PLA) acting as reactive compatibilizer for PLA blends and composites has been reported. However, melt freeradical grafting of MA on PLA is often subject to steric and electron effects of the substituents in the monomer and low initiation efficiency, yielding low grafting efficiency (Eg). In this work, five dicarboxylic anhydride monomers, including MA, itaconic anhydride (IA), cis-1,2,3,6-tetrahydrophthalic anhydride (TA), 4-allyltrimellitate anhydride (ATA), and 4-methacryloxyethyl trimellitate anhydride (META), were grafted onto PLA, and the effect of steric hindrance on Eg was assessed. It is noted that Eg values followed the order of ATA > META ≫ MA ≥ TA > IA. The introduction of styrene as a comonomer selectively increased the Eg values of three electron-deficient monomers, MA, IA, and META, while Sn(Oct)2 as a reducing agent increased Eg for all monomers. Both styrene and Sn(Oct)2 exhibited a synergistic effect when used in grafting MA, IA, and META.
1. INTRODUCTION Functional polymers are used for a wide range of applications, and their preparation methods are diverse as well. Melt freeradical grafting is a preferred method for high-yield production because it is simple, fast, and economical.1 This method has been most seen in grafting maleic anhydride (MA) onto polyolefins.2,3 MA-grafted polyolefins exhibit improved adhesion ability, printability, paintability, and compatibility in places where they are needed. In recent years, the interest in renewable polymer materials has been growing continuously. Polylactic acid (PLA), a sugarbased biodegradable polyester, has received the most attention among all renewable polymers and has been frequently employed in multicomponent materials for enhanced performance. Though PLA has many polar ester groups, it still shows insufficient interfacial compatibility and reactivity in many applications. Hence, functionalized PLA is necessary for improving the compatibility in such multicomponent systems. Melt free-radical grafting is an effective way to achieve functionalized PLA. In the literature, MA-grafted PLA (PLAg-MA) is used as a reactive compatibilizer in polymer blends and composites to improve interfacial adhesion between the PLA matrix and other components such as starch,4,5 soy protein,6 natural fibers,7−9 talc,10 clay,11 microcrystalline cellulose,12 and polyester elastomer.13 Even so, unlike freeradical grafting of polyolefins, the grafting reaction of PLA itself © 2017 American Chemical Society
is seldom investigated in the literature, and until today PLA-gMA has not been commercially produced. In most cases, the grafting efficiency (Eg) of grafting MA onto PLA is low, and the conversion of MA is less than 20 wt %.14−16 Several factors account for the low Eg. First, melt freeradical grafting of polymer is subject to steric hindrance of the macromolecular chains and high viscosity of the polymer melt. The long chains impart severe steric hindrance for the macromolecular radical to attach the monomer, while the high polymer melt viscosity is not favorable for good mixing between the reagent and polymer for grafting reaction.17 Even worse, the carbonyl (CO) and methyl groups (−CH3) of PLA backbones impose more steric hindrance effect to the radical at the tertiary carbon atom, leading to low reactivity for free radical reaction. Second, MA has low reactivity in free radical reaction.1,18 Both CO groups and the cyclic ring structure exert steric effects to its CC bond. Finally, as a common obstacle for melt free-radical grafting, the primary radicals generated from the homolytic thermal decomposition of organic peroxide initiator can be trapped in the cage environment of the molten polymer matrix and then undergo Received: Revised: Accepted: Published: 3920
December 29, 2016 February 22, 2017 March 20, 2017 March 20, 2017 DOI: 10.1021/acs.iecr.6b05051 Ind. Eng. Chem. Res. 2017, 56, 3920−3927
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Scheme 1. Structures of the Unsaturated Dicarboxylic Anhydride Monomers and Schematic Illustration of the Grafting Reaction on PLA
reactions to form inactive matter.1,19−22 This process results in the waste of primary radicals and lower initiation efficiency. The cage effect is more critical in molten polymer medium since the viscosity is high.23 Enhancing monomer reactivity and eliminating cage effect are reasonable means to achieve high Eg. In this study, the effects of monomer structure, comonomer, and reducing agent on the melt grafting degree (Dg) and grafting efficiency (Eg) for PLA were investigated. Five unsaturated dicarboxylic anhydride monomers with different CC bond architectures (Scheme 1) were selected, and their melt free-radical grafting on PLA was compared: MA (internal CC bond in a cyclic anhydride ring), itaconic anhydride (IA) (a renewable chemical,24 terminal CC bond out of a anhydride ring), cis-1,2,3,6-tetrahydrophthalic anhydride (TA) (internal CC bond in a nonpolar cyclic alkene ring), 4allyltrimellitate anhydride (ATA) (terminal CC bond with a low steric hindrance), and 4-methacryloxyethyl trimellitate anhydride (META) (terminal CC bond, high potential for homopolymerization3,25,26). To the best of our knowledge through a literature search, to date, only MA14,16,27 and IA28,29 have been used for grafting PLA. Styrene as a comonomer is often applied in the grafting reaction to enhance the Eg of a monomer through the formation of a charge transfer complex with the anhydride monomer.6,15,30 Recently, our group first introduced the Sn(Oct)2 in a redox initiation system for grafting MA and glycidyl methacrylate (GMA) onto polyethylene, and the result demonstrated that the redox initiation system greatly improved grafting efficiency over the traditional peroxide intitator.17 However, its effect on biodegradable polyesters is unknown. This work attempts to evaluate the coeffect of both redox initiation system and comonomer on grafting efficiency of the five different anhydride monomers.
2.2. Synthesis of 4-Allyltrimellitate Anhydride (ATA) and 4-Methacryloxyethyl Trimellitate Anhydride (META). Both ATA and META were synthesized through a simple one-step reaction according to the literature.31 As an example, the procedure for synthesizing ATA is given below. Under magnetic stirring, TAC (0.01 mol) and tetrahydrofuran (50 mL) were added into a three-necked flask. After a homogeneous mixture was formed, AA (0.015 mol) with triethylamine catalyst (0.015 mol) was added dropwise at 0 °C and then continued to react for another 5 h. Upon completion of the reaction, the mixture was filtered, and THF was removed under vacuum to receive the crude product. The crude product was then purified by silica gel column chromatography using the dichloromethane and hexane (3:1) mixture as eluent. The obtained final product was a white solid with a yield of 66%. 1H NMR (ATA, 400 MHz, CDCl3, δ): 4.92−4.97 (tt, 2H, CH− CH2−O), 5.32−5.38 (dd, 1H, CH2CH−), 5.39−5.48 (dd, 1H, CH2CH−), 5.97−6,10 (m, 1H, CH2CH−), 8.07− 8.10 (dd, 1H, PhH), 8.55−8.59 (dd, 1H, PhH), 8.63−8.66 (t, 1H, PhH). The META monomer was synthesized using the same method as that for ATA synthesis. The final product was a white solid with a yield of 57%. 1H NMR (META, 400 MHz, CDCl3, δ): 1.91−1.95 (t, 3H, CH3−C−), 4.49−4.54 (m, 2H, CH2−CH2−O), 4.63−4.68 (m, 2H, O−CH2−CH2), 5.59−5.62 (t, 1H, CH2C−), 6.11−6.14 (t, 1H, CH2C−), 8.07−8.11 (dd, 1H, PhH), 8.53−8.57 (dd, 1H, PhH), 8.61−8.64 (t, 1H, PhH). The 1H NMR spectra of ATA and META are presented in the Supporting Information (Figure S1). All peaks are well assigned. 2.3. Grafting Modification of PLA. Prior to melt extrusion, PLA pellets were dried in a vacuum oven at 70 °C for 8 h. The melt grafting reaction was performed in a corotating twin-screw microextruder (Haake MiniLab II Rheomex CTW-5). PLA was first added into extruder set at 165 °C and 100 rpm and melted for about 1.5 min, and then DCP (0.6 phr), Sn(Oct)2 (0.9 phr), styrene (2 phr), and anhydride monomer were added simultaneously. Because the major purpose of this study is to compare the grafting of different monomers, the concentrations of all reagents were not considered as variables, and the selection of concentration was based on our previous experience and other works in the literature.14,15,17,32 Because of the difference in molecular weight, each anhydride monomer was loaded by the same molar amount. Therefore, corresponding to the addition of 4
2. EXPERIMENTAL SECTION 2.1. Materials. NatureWorks PLA (3052D) was used in all grafting reactions. The following reagents were purchased from Sigma-Aldrich: maleic anhydride (MA, purity = 98%), itaconic anhydride (IA, purity = 97%), cis-1,2,3,6-tetrahydrophthalic anhydride (TA, purity = 95%); styrene (St, purity ≥ 99%), tin(II) 2-ethylhexanoate (Sn(Oct)2, purity > 92.5%), dicumyl peroxide (DCP, purity = 98%), trimellitic anhydride chloride (TAC, purity = 98%), allyl alcohol (AA, purity ≥ 98%), 2hydroxyethyl methacrylate (HM, purity = 97%); and 1naphthylmethylamine (Nap, purity = 97%). 3921
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mg sample was completely dissolved in 20 mL of chloroform and titrated by a 0.02 N KOH/methanol solution. Phenolphthalein was used as an indicator, and titration end point was gauged by both color and pH value of the sample solution. The Dg is calculated by eq 1,
phr MA, 4.6 phr IA, 6 phr TA, 9.6 phr ATA, and 12.8 phr META were loaded during grafting, respectively. Next, the temperature was raised to 205 °C (it took ∼4 min to reach the target temperature), the compound was then purged into a water bath. The selection of a low initial mixing temperature (165 °C) was to maximally avoid evaporation of some volatile anhydride monomers (Figure S2). On the basis of the differential scanning calorimetry (DSC) result in Figure S3, it took ∼1.5 min to completely melt the PLA at 165 °C. Unreacted monomers and low molecular weight oligomers in the grafted PLA were removed by dissolution and precipitation. The procedure is briefly described as follows. A 2 g portion of sample was dissolved in 40 mL of chloroform at room temperature, and then the mixture was slowly added to an excessive amount of acetone for precipitation. The precipitate was collected by filtration, washed with acetone for three times, and then dried in vacuum oven at 90 °C for 18 h. The extruded blank used as a control was prepared using the same procedure. 2.4. Postreaction with 1-Naphthylmethylamine (Nap) for Determining Grafting Degree by UV−vis. Because of the generally low grafting degree (Dg), the proton signals of the anhydride monomer were very weak in the 1H NMR spectrum of the grafted PLA and could not be used for accurate quantitative characterization. Therefore, determination of Dg mainly relies on the titration method. On the other hand, if a chromophore is introduced into the PLA molecule through reacting with the reactive anhydride groups, UV−vis spectrophotometry can be utilized to give an additional verification of the grafting reaction and a more accurate determination of Dg. In this work, the postreaction of the grafted PLA with 1-naphthylmethlamine (Nap) was performed according to the literature.33 After 0.2 g of purified sample was dissolved in 20 mL of tetrahydrofuran, Nap in a molar ratio of approximate 1.2 to 1 with respect to the anhydride group (predicted by titration, Section 2.5) was added into the solution, and the mixture was stirred at room temperature for 3 h. Under this mild reaction condition, the anhydride group was ring-opened by the primary amine of Nap in a 1:1 molar ratio.34 After reaction, the PLA samples (PLA-x-Nap, x represents one of the five anhydride monomers) were precipitated by adding 100 mL of methanol and dried in vacuum oven at 85 °C for 15 h. Neat PLA was also treated by Nap using the same procedure and used as a control. The reaction between Nap and grafted PLA (PLA-g-MA or PLA-MA-Nap as an example) is illustrated in Scheme 2. 2.5. Characterizations. a. Titration for Grafting Degree. Grafting degree (Dg) was first determined by the titration method. Titration was performed on an autotitrator (Titroline Easy 5000, SI Analytics) and the procedure is as follows. A 200
Dg (wt%) =
(C KOHV − C KOHVBlank)Manhydride 2msample
100 (1)
where CKOH is the concentration of KOH (mol/mL), V is the titrant volume for grafted PLA (mL), VBlank is the titrant volume (mL) for neat PLA blank controls, Manhydride is the molecular weight of anhydride monomer (g/mol), msample is the mass of grafted sample (g). The factor 2 in the denominator accounts for the equivalence of one molar anhydride group to two molar carboxyl acid groups. b. Ultraviolet−Visible Spectroscopy (UV−vis) and Determination of Grafting Degree. UV−vis characterization of PLA/chloroform solution (0.667 mg/mL) was performed on a PerkinElmer Lambda 25 spectrometer. The calibration curve was constructed using a series of Nap solutions of different concentrations. The absorption at 283 nm was selected to construct the calibration curve and determine the concentration of the unknown (Figure S4). The grafting degree (Dg) of anhydride groups was calculated by eq 2, (C NapV ) Dg (wt%) =
(
Manhydride × 1 157.21
(0.667V )
) 100
(2)
where the CNap is obtained via equation S1 (mg/mL), V is a random volume of PLA/chloroform solution (mL), Manhydride is the molecular weight of anhydride monomer (g/mol), 157.21 is the molecular weight of Nap monomer (g/mol), 0.667 is the concentration of PLA/chloroform solution (mg/mL), and 1 is the molar number of the postreaction in section 2.4 (the anhydride group was ring-opened by the Nap monomer in 1:1 molar ratio). c. Calculation of Grafting Efficiency. The grafting efficiency (Eg) of grafted PLA samples was calculated by eq 3, Eg (%) =
weight of grafted monomer 100 weight of loaded monomer
(3)
The weight of grafted monomer (Dg × msample) is obtained via either a titration method (eq 1) or UV−vis characterization (eq 2), and the weight of loaded monomer is the feeding weight of anhydride monomers. Eg reflects the conversion rate of anhydride monomer being successfully grafted to the PLA backbone, which is more intuitive to demonstrate efficiency of monomer utilization. d. Fourier Transform Infrared Spectroscopy (FTIR). The purified sample was dissolved in chloroform (1 g/200 mL) and then casted onto the surface of a KBr crystal disk. The solvent was evaporated by heating using a heating gun prior to testing. FTIR spectra of the samples were recorded using a Thermofisher Nicolet IS 50. The sample was scanned between 500 and 4000 cm−1 with a resolution of 2 cm−1 and 64 scans. e. Proton Nuclear Magnetic Resonance Spectroscopy (1H NMR). Nuclear magnetic resonance (NMR) spectra were recorded on a Varian 600-MR spectrometer (600 MHz). A pulse angle of 90 deg, a relaxation time of 10 s, and 128 scans were applied to collect the data. Chemical shifts of 1H NMR
Scheme 2. Schematic Reaction between Nap and MAGrafted PLA
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Table 1. Results (Dg, Eg, and MW) of Grafting Five Anhydride Monomers onto PLA in the Absences of Styrene and Reducing Agent grafting results titrationb samples
Dg (wt %)
UV−visc Eg (%)
Dg (wt %)
MW (kDa)d Eg (%)
Mn
Mw
Mw/Mn
6.4 ± 0.7 9.3 ± 0.9 11.9 ± 1.0 16.7 ± 1.2 27.6 ± 2.2
65.6 62.5 65.4 56.5 44.7 65.2 36.5
147.5 141.5 133.0 133.7 95.7 117.3 86.7
2.3 2.3 2.0 2.4 2.1 1.8 2.1
a
neat-1 neat-2a PLA-g-IA PLA-g-TA PLA-g-MA PLA-g-META PLA-g-ATA
0.29 0.47 0.49 1.99 2.21
± ± ± ± ±
0.03 0.11 0.08 0.23 0.11
6.9 ± 0.7 8.8 ± 2.2 12.7 ± 2.0 17.6 ± 2.0 25.5 ± 1.5
0.27 0.50 0.46 1.88 2.39
± ± ± ± ±
0.03 0.05 0.04 0.14 0.21
a
Neat-1 and neat-2 are neat PLA before and after melt extrusion. bDg and Eg were obtained through titration. cDg an Eg were determined by measuring UV−vis adsorption of Nap-treated grafted PLA and applying eqs 1 and 2. dAverage relative MW, obtained by GPC analysis using the conventional calibration. One kDa = 1000 g/mol.
makes IA a particular 1,1-substituted monomer and imparts great steric hindrance for the attachment of the macroradicals in reaction. IA can undergo isomerization to form citraconic anhydride (CA) when the temperature approaches the melt processing temperature,29 but CA possesses even higher steric hindrance than IA. As a result, IA displayed the lowest Eg among these five anhydride monomers. ATA and META, on the other hand, both have a terminal CC bond which have much lower steric hindrances compared to the other three monomers. ATA has a terminal allyl group and META has a methacrylate end group, both could be easily attached by the polymer macroradicals. Therefore, they exhibited much high Eg than those monomers with high steric hindrances. It is arguable if the electronegativity of the substituents of the CC bond has a significant influence on Eg. Because radical species are neutral and do not have stringent selectivity for attaching the π-bond or for the stabilization of the propagating species, free radical reaction is not as sensitive as the ionic reaction to the electronegativity of the substituent.18 For example, for these monomers, the electron-cloud density of the CC bond is probably in the order of MA < IA ≤ META < TA < ATA. The CC bond of MA should have the lowest electron density among these monomers, but it showed an intermediate Eg. For TA, the CC bond is not linked to any strong polar substituents, but its reactivity is still quite low. Therefore, the electron-cloud density of the CC bond in anhydride monomers is not considered a determining factor in grafting modification of PLA. 3.2. Structure Characterizations of the Modified PLA. Figure 1 shows the FTIR curves of neat PLA and the grafted PLA. According to the literature,17,30,35,36 the CO groups of anhydride exhibit a strong peak at ∼1780 cm−1 (symmetric stretching) and a weak peak at ∼1845 cm−1 (asymmetric stretching). Because of the general low Dg, for the grafted PLA, the peak of the anhydride at ∼1780 cm−1 was largely buried by the peak of CO group of PLA backbones at ∼1758 cm−1, and the weak peak at ∼1845 cm−1 was so small that it could only be noted by enlargement (inset in Figure 1). To verify that the low grafting degree was mainly responsible for the poor visibility of the peak associate with the symmetric stretching of CO, we prepared a blend of PLA with 5% MA and examined it using FTIR. A small shoulder at ∼1783 cm−1 (Figure S5) was noted, which was attributed to the symmetric stretching of C O groups in MA. Other grafting studies in the literature14,27,29,33 also met the same difficulty for distinguishing
peaks were reported in ppm, and CDCl3 was used as the deuterated solvent in all NMR measurements. f. Gel Permeation Chromatography (GPC). The molecular weight (MW) of PLA-g-MA was determined by a Viscotek gel permeation chromatography (GPC) system (TDA305). Tetrahydrofuran was used as the eluent with a flow rate of 1.0 mL/min. The injection volume of the sample solution was 100 μL. The temperature of the detector was set as 30 °C. All samples were fully dissolved in THF at a concentration of 1.5 mg/mL prior to characterization.
3. RESULTS AND DISCUSSION 3.1. Comparison of Grafting Efficiency for Different Anhydride Monomers. Dg was determined by both the titration and UV−vis methods. As seen in Table 1, the Dg determined from the two methods and the corresponding calculated Eg for each anhydride monomer were very similar, suggesting the effectiveness of both determination methods. In a free radical grafting reaction, the PLA macroradicals formed by hydrogen abstraction at the tertiary carbon atoms attack the CC bond of the unsaturated anhydride monomer to form covalent linkage. The results indicate that Eg is highly depended on the type of anhydride monomers, following the order of ATA > META ≫ MA ≥ TA > IA. Each monomer exhibits unique structural characteristics (Scheme 1) and hence varies greatly in reactivity in the grafting reaction. The effect of steric hindrance on the CC bond of the anhydride monomer is the most influential factor that reduces its reactivity during the grafting procedure, but other factors may also have either independent or synergistic influence on Eg of the grafted PLA. Steric effect is considered the most influencing factor for the monomer reactivity.1,3 Owing to its low cost and simple structure, MA is the most used anhydride monomer for polymer grafting. MA has an internal (or 1,2-substituted) −CHCH- group being part of the cyclic anhydride ring which imparts serious steric hindrance for the attack of the macroradical and the continuing attack of the propagating radicals on the monomer (homopolymerization), and it often shows low grafting reactivity. Like MA, TA also has an internal −CHCH− bond in a bulky cyclic structure environment and thus exhibits a similar steric effect, though the double bond is located in a nonpolar cyclic alkene ring. It is noted that MA and TA exhibited very similar Dg and Eg suggesting the similar reactivity for the two monomers. Though IA has a terminal CH2C< group, the presence of the cyclic anhydride moiety 3923
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of methene of ATA (Figure 2c). The weak signal of the CC bond of ATA was still noticeable in PLA-g-ATA at δ 5.40 and 6.10 ppm, indicating that a trace amount of unreacted ATA monomer remained (Figure 2b). Nonetheless, the peak area for the aromatic protons was much higher than that of the residual CC bond (∼10.3:1), implying that most ATA monomers in PLA-g-ATA were grafted onto PLA backbones. The 1H NMR results of other grafted PLAs are shown in Figure S6. Because of the low Dg, some protons in the anhydride monomer could not be distinguished from that of the predominant PLA backbone and noise. Quantitative analysis of the Dg was achieved by the more sensitive UV−vis measurement (section 2.4 and Table 1) and alkali titration. Figure 3 shows the UV−vis results of the grafted Figure 1. FTIR curves of neat PLA and the grafted PLA by the five monomers.
the symmetric stretching of CO of the grafted anhydride at ∼1780 cm−1 from that of the PLA backbone at ∼1758 cm−1. The chemical structures of the grafted PLA samples were also examined by 1H NMR and UV−vis. Since the mass content of anhydride grafted onto PLA was very low, only the PLA-g-ATA sample with a relatively higher Dg (∼2.4 wt % in Table 1) displayed a clearly different 1H NMR spectrum compared with that of neat PLA (Figure 2). The signals of PLA-g-ATA that appear at δ 7.50−8.70 ppm are attributed to the protons in the benzene ring of the ATA monomer (Figure 2a), and the peaks at δ 4.85 ppm are attributed to the protons
Figure 3. UV−vis curves of neat PLA and the grafted PLA (A) before and (B) after postreaction with Nap monomer.
PLA. The absorbance of benzene rings (∼240 nm)37 of PLA-gATA and PLA-g-META are noted (Figure 3A), indicating the successful attachment of ATA and META onto PLA. Through a postreaction between the anhydride group and the UVsensitive Nap, aromatic naphthalene groups were introduced into the anhydride grafted PLA sample (Scheme 2). The absorbance of the corresponding Nap-treated grafted PLA are shown in Figure 3B. There was no change noted in the absorbance for the neat PLA after treatment because of the absence of anhydride groups. UV−vis spectrometry presents to be an sensitive characterization method for both qualitative and quantitative determinations of the grafting of monomers onto PLA.33
Figure 2. 1H NMR of ATA (orange), PLA-g-ATA (red, amplified 500 times), and neat PLA (black, amplified 500 times), and the detailed comparison of PLA-g-ATA vs neat PLA for (a) styrene region; (b) CC bond, and (c) methylene region of ATA. 3924
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Figure 4. Changes in Eg (A) and MW (B) of the samples with the addition of styrene (2 phr) as comonomer. All data are normalized with respect to the corresponding grafted PLA obtained from the reactions in the absences of styrene and Sn(Oct)2 (Table 1).
Figure 5. Changes of Eg (A and B) and MW (C and D) of the grafted PLA with Sn(Oct)2 reducing agent (0.9 phr): (A and C) without the styrene comonomer; (B and D) with the styrene comonomer (2 phr). All data are normalized with respect to the corresponding grafted PLA obtained from the reactions in the absences of styrene and Sn(Oct)2 (Table 1).
3.3. Molecular Weight (MW) of the Grafted PLA. The MW of the grafted PLAs is shown in Table 1. All grafted PLA presented lower Mn and Mw than neat PLA. This can be attributed to the excessive backbone β-scission in the free radical reaction.14−16 β-scission commonly exists in free radical reactions, resulting in splitting of the β carbon−carbon bond next to the free radical. In comparison, PLA-g-IA and PLA-g-TA with the lowest Eg exhibited much higher MW than the other three grafted PLA samples. This result may imply that βscission occurred to a lesser extent in these two reaction systems. 3.4. Effect of Styrene as Comonomer. Figure 4 shows the effects of styrene as comonomer (2 phr) on Eg and MW. It was reported that the presence of styrene clearly enhanced the Eg values of MA, IA, and META, which increased around 145, 300, and 80%, respectively. On the contrary, the Eg values of TA and ATA were reduced by 20 and 70%, respectively. The
changes can also be explained by the variation of structural characters of these five anhydride monomers. Due to its less steric effect, styrene has a high tendency to be grafted onto the PLA backbone and form stable styryl radicals.1,17 The electron acceptor−donor interaction could activate copolymerization of the CC bond of electron-deficient monomers (MA, IA, and META) with styryl radicals,3,18,22,38 resulting in enhanced Eg. Especially for PLA-g-MA, in situ styrene-MA charge transfer complex could be formed.1,6,15,30 However, for TA and ATA which have weak electron donor−acceptor interaction with styryl radicals, their Eg values were inversely reduced. Similar phenomena were observed in the reduced Eg of grafting vinyl acetate or N-tert-butylacrylamide to polyolefins with the presence of styrene.30,39 Styryl radicals have higher resonance stability that reduces the potential for backbone β-scission.3,15,19,30,40 We also noted that PLA-g-St even exhibited very high MW (Figure S7 and 3925
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especially for ATA and META. The molecular weight (MW) of the grafted PLA exhibited a general reduction trend compared to the neat PLA, which was likely due to the βscission of the polymer backbone. Adding the electrondonating styrene comonomer greatly improved the Eg values for three electron-withdrawing anhydride monomers, namely, MA (+150%), IA (+300%), and META (+80%), which was probably achieved through the well-recognized electron donor−acceptor activation or charge transfer complex mechanism. The use of the styrene comonomer also increased the MW of the resulting grafted PLA, which was probably attributed to the formation of the styryl macroradicals that had a stabilizing effect and suppression of the backbone βscission. On the other hand, introducing Sn(Oct)2 as reducing agent in the reaction resulted in an increase in Eg for all anhydride monomers, ranging from 22.5 to 150%. This result is believed due to the redox initiation system that eliminated of cage effect during the grafting reaction. However, the addition of Sn(Oct)2 also led to serious thermal degradation of PLA backbones. The synergetic effect of styrene and Sn(Oct)2 on the enhancement of Eg was observed for PLA-g-MA (+210%), PLA-g-IA (+550%), and PLA-g-META (+100%). In comparison, although MA has a relatively low reactivity in grafting modification, it has the low-cost advantage and can achieve high Eg via applying the styrene comonomer and reducing agent in the melt free-radical grafting reaction. The findings from this study will help set a framework for future efforts of improving the Eg of the anhydride monomer for PLA and other renewable polymers and reducing monomer waste.
Table S1), suggesting the stabilization effect of styrene for PLA macroradicals. When styrene was used as a comonomer, except for PLA-g-META, all the other grafted PLA samples exhibited significantly higher MW (typically Mw) than their counterparts prepared without using styrene. Because methacrylate monomers have high tendency for homopolymerization, the aforementioned charge transfer complex or stabilizing effect of styrene for the propagating macroradicals has little influence on grafting of META, typically for the backbone β-scission of PLA-g-META. 3.5. Effect of Sn(Oct)2 as Reducing Agent. Recently, our group first introduced a DCP/Sn(Oct)2 redox initiation system for grafting modification of polyethylene by MA and GMA.17 The oxidation of Sn(II) to Sn(IV) induces one DCP molecule to generate one primary radical, preventing the formation of primary radical pairs in the macromolecular cage.17,41 Figure 5 shows the effect of Sn(Oct)2 as reducing agent (0.9 phr) on Eg of the anhydride monomer and MW for the grafted PLA. Without applying styrene as comonomer (Figure 5A,C), Sn(Oct)2 enhanced Eg for all five anhydride monomers, indicating that the redox initiation system successfully eliminated the cage effect and improved the initiation efficiency. Except for PLA-g-ATA, the increases in Eg for other grafted PLA were all over 50%. This result suggests that the redox system is more effective for the monomer with a low Eg. Meanwhile, using Sn(Oct)2 resulted in a noticeable decrease in the MW, irrespective of the monomers used. Typically, the MW of the grafted PLA prepared in the presence of Sn(Oct)2 was only 30.6% that of neat PLA (Figure S7 and Table S1). Sn(Oct)2 can act as a catalyst for an intermolecular transesterification reaction, which might contribute to the reduction of the MW of PLA.42 As we mentioned above, Eg can be improved by two independent mechanisms: (1) redox initiation system to eliminate cage effect, and (2) copolymerization of styrene with electron-deficient anhydride monomers. Therefore, applying the styrene comonomer and Sn(Oct)2 reducing agent together can result in the changes of Eg and MW (Figure 5B,D). In particular, a synergetic effect on Eg was noted in PLAg-MA, PLA-g-IA, and PLA-g-META. Taking PLA-g-IA as an example, Eg was more than 6 times that of the original formulation without styrene and Sn(Oct)2. However, since styrene cannot strongly interact with TA and ATA during the chain propagation, the overall Eg of PLA-g-TA and PLA-g-ATA were lower than that of the other three. In addition, because the intermolecular transesterification reaction catalyzed by Sn(Oct)2 is irrelevant to backbone β-scission from a free radical reaction, styrene as a radical stabilizer cannot effectively prevent MW degradation. Similarly, the redox system-initiated PLA-g-St also produced severe degradations (Figure S7 and Table S1).
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b05051. Reaction schemes of ATA and META; 1H-NMR spectra; TGA, DSC, UV-vis, FTIR, and GPC curves (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (J. Zhang). *Email:
[email protected] (L. Liu). ORCID
Jinwen Zhang: 0000-0001-8828-114X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was partially supported by Grant No. 807241000-098-06S from the Agricultural Research Service, U.S. Department of Agriculture for the project of Development of Composite Materials with More Cellulosic Fibers and Less Conventional Thermoplastics for Light Weight-Bearing Applications.
4. CONCLUSIONS The results from this study indicate that melt free-radical grafting efficiency (Eg) of unsaturated dicarboxylic anhydride monomers onto PLA is greatly influenced by the chemical nature of the carbon−carbon double bond (steric hindrance and/or electronegativity of the substituents) of the monomer and use of comonomer and redox initiation system. The Eg values of the five anhydride monomers used followed the order of ATA > META ≫ MA ≥ TA > IA, which generally corresponded to the order of steric hindrance for these monomers. The results suggest that the steric hindrance of the anhydride monomer was the determining factor for Eg,
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DOI: 10.1021/acs.iecr.6b05051 Ind. Eng. Chem. Res. 2017, 56, 3920−3927
