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Effects of ionomer characteristics on reactions and properties of poly(lactic acid) ternary blends prepared by reactive blending
Polymer 53 (2012) 2476e2484
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Effects of ionomer characteristics on reactions and properties of poly(lactic acid) ternary blends prepared by reactive blending Wenjia Song, Hongzhi Liu, Feng Chen, Jinwen Zhang* Materials Science and Engineering program & Composite Materials and Engineering Center, Washington State University, Pullman, WA 99164, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2012 Received in revised form 12 March 2012 Accepted 26 March 2012 Available online 2 April 2012
Toughening of poly(lactic acid) (PLA) was studied by reactive blending PLA with ethylene/n-butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer and zinc ion-containing ionomer. The ionomer was prepared by neutralizing the ethylene/methacrylic acid copolymer (EMAA), i.e., ionomer precursor, with ZnO. The reactive interfacial compatibilization between PLA and EBA-GMA and the crosslinking of EBA-GMA during blending was studied in detail. Fractography and FT-IR analysis indicated that both the degree of neutralization (DN) of ionomer and methacrylic acid (MAA) content of ionomer precursor exhibited significant effects on interfacial compatibilization. Dynamic mechanical analysis also suggested that the crosslinking level of EBA-GMA varied with these two factors. Particle size and polydispersity of the dispersed phase were measured by image analysis of TEM micrographs of the ternary blends and correlated with the impact strength of the blends and the characteristics of the ionomer. Ionomers derived from precursor of high MAA content and/or having high DN tended to yield superior impact strength of the PLA blends. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Poly(lactic acid) Impact toughness Reactive blending
1. Introduction As one of the commercial biodegradable polymers, poly(lactic acid) (PLA), with excellent strength and stiffness, is considered to be a promising candidate to replace some petroleum-based commodity thermoplastics. However, the brittleness of PLA is a major drawback to impede its wide applications. Incorporating flexible polymers or elastomers into PLA via melt blending is a potentially cost effective way of addressing this issue. Various biodegradable polymers, including polyhydroxyalkanoate (PHA) [1,2], polycaprolactone (PCL) [3,4], poly(butylene succinate) (PBS) [5], poly(butylene adipate-coterephthalate) (PBAT) [6e9], conjugated soybean oil [10] and other biodegradable elastomers and copolymers [11e13] have been blended with PLA. Non-biodegradable polymers such as polyethylene (PE) [14e16] and many other petroleum-based polymers and elastomers [17e23] were also used in toughening PLA. Although most of the reported PLA blends showed impressively high tensile elongations (150e350%) compared with that of neat PLA, only marginal to moderate improvements in impact strength (IS) were achieved. To date, there are a few reports of PLA blends in the literature demonstrating “supertoughness”. Anderson et al. [15,16] reported
* Corresponding author. Tel.: þ1 509 335 8723; fax: þ1 509 335 5077. E-mail address: [email protected] (J. Zhang). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.03.050
supertough PLA/PE blends compatibilized by PLA-b-PE diblock copolymers. The resulting PLA/PE (80/20 w/w) blends exhibited IS as high as 760 J/m, ca. 30 times that of neat PLA. In another study reported by Oyama [22], reactive poly(ethylene-glycidyl methacrylate) (EGMA) was used to toughen PLA. After annealing at 90 C for 2.5 h, the PLA/EGMA blend achieved supertoughness with a Charpy IS of 72 kJ/m2 (ca. 50 times that of neat PLA). However, a significant reduction in tensile elongations accompanied this post-treatment of the blends. Furthermore, a direct evidence of the reactive compatibilization and thorough discussion of toughening mechanism were not yet provided. Hashima et al. [23] reported a PLA ternary blend containing 20 wt% hydrogenated styrenebutadiene-styrene block copolymer (SEBS) compatibilized by 10 wt% EGMA. A much higher Izod IS (92 kJ/m2) than that of neat PLA (3 kJ/m2) was reported. Contrary to Oyama’s annealing result, the aging at 80 C for 48 h caused a pronounced reduction in the IS of the blend to 32 kJ/m2. Recently, we introduced a novel supertough PLA ternary blend system consisting of PLA, an elastomeric ethylene/n-butyl acrylate/ glycidyl methacrylate terpolymer (EBA-GMA) and a zinc ionomer of ethylene/methacrylic acid copolymer (EMAA-Zn) [24,25]. Unlike in other supertoughened PLA blends, crosslinking of EBA-GMA induced by zinc ionomers took place in addition to the interfacial compatibilization between EBA-GMA and PLA in this ternary blend system during melt blending [24]. Previous studies demonstrated that EMAA-Zn played an important role in determining toughening
W. Song et al. / Polymer 53 (2012) 2476e2484
effect of the ternary blends and showed that both zinc ions and free carboxyl groups of methacrylic acid (MAA) in the ionomer actively participated in the dual reactions involved [24,25]. When the unneutralized EMAA-H was substituted by EMAA-Zn, interfacial wetting and consequently IS was remarkably improved [24]. With increase of the EMAA-Zn/EBA-GMA ratio in the blend composition, levels of crosslinking of EBA-GMA increased, which could lead to a higher resistance to cavitation of the dispersed particles [25]. Therefore, it was reasonably expected that the variation in the MAA content in the precursor and the extent of neutralization would exert some effects on the ultimate IS of the blends. However, an indepth understanding on how the characteristics of ionomer govern the dual reactions involved in blending and thereby affect the IS of the blend is not yet provided. In this work, we study the effects of degree of neutralization (DN) of ionomer and the MAA content in the precursor on the toughness of the PLA/EBA-GMA/EMAA-Zn (80/15/5, w/w) ternary system. Two series of ionomers with varying DN were derived by neutralizing the precursors (EMAA-H) containing 4 and 15 wt% of MAA, respectively, with ZnO in extrusion. The interfacial compatibilization was investigated using FT-IR and SEM, and the crosslinking of EBA-GMA was studied by DMA. Particle size analysis was performed to examine the effect of ionomer on the morphological aspects of the blends. 2. Experimental section 2.1. Materials The materials used in this study and some specifications are listed in Table 1. The ionomer precursors, EMAA-H containing 15 and 4 wt% MAA, were designated as EMAA15-H and EMAA4-H, respectively. Likewise, the ionomers prepared from these precursors were designated as EMAA15-Zn and EMAA4-Zn, respectively. 2.2. Preparation of ionomers EMAA-Zn ionomers were prepared by neutralizing the precursors, EMAA-H, with ZnO in a co-rotating twin screw extruder
Table 1 Characteristics of materials used in this study. Material (abbreviation)
Grade (supplier)
Specifications
Poly(lactic acid) (PLA)
PLA2002D (NatureWorks) ElvaloyÒ PTW (DuPont Co.)
MI (210 C, 2.16 kg): 5e7 g/10 min MI (190 C, 2.16 kg): 12 g/10 min; Melting point (DSC): 72 C; E/BA/GMA ¼ 66.75/28/ 5.25 (wt%)a MI (190 C, 2.16 kg): 25 g/10 min; Melting point (DSC): 92 C; Methacrylic acid content: 15.0 wt% MI (190 C, 2.16 kg): 11 g/10 min; Melting point (DSC): 109 C; Methacrylic acid content: 4.0 wt% Specific gravity: 5.67 g/cm3; Melting point: 1975 C
Ethylene/n-butylacrylate/ glycidyl methacrylate copolymer (EBA-GMA)
Ethylene/methacrylic acid copolymer (EMAA15-H)
NucrelÒ 925 (DuPont Co.)
Ethylene/methacrylic acid copolymer (EMAA4-H)
NucrelÒ 0411HS (DuPont Co.)
Zinc oxide powder (ZnO)
a
Data is cited from Ref. [26].
Baker AnalyzedÒ reagent (J.T. Baker Chemical Co.)
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(Leistritz ZSE-18) with a screw diameter of 17.8 mm and an L/D ratio of 40. Prior to extrusion, EMAA-H pellets and zinc oxide were ovendried for at least 1 day at 75 C and above 100 C, respectively. Dried ZnO powder and EMAA-H pellets in predetermined stoichiometric ratios of ZnO/COOH were manually mixed in a plastic bag. The temperature profile of the extruder barrels was 180/190/190/200/ 200/200/190/180 C from the first heating zone (next to feeding throat) to die, respectively. The screw speed of the extruder was set at 50 rpm. Vacuum venting at the seventh zone of the extruder was applied to eliminate small molecules generated during neutralization. The extrudates were pelletized and then extruded for a second time under the same condition in order to ensure a more thorough and uniform neutralization reaction. The precursors, EMAA15-H and EMAA4-H, alone were also processed under the same conditions and used as controls. The zinc ionomers obtained were designated as EMAA15-x%Zn and EMAA4-x%Zn, respectively, in which ‘x%’ indicates the value of DN. 2.3. Preparation of PLA ternary blends Prior to melt compounding, PLA pellets were dried for at least 1 day at 80 C; EMAA-Zn (or EMAA-H) pellets were dried for 1 day at 75 C; EBA-GMA pellets were dried for 1 day at 65 C. Previous study found that the blend consisting of 80 wt% PLA, 15 wt% EBAGMA and 5% EMAA-Zn and extruded at 240 C gave the superior IS [24]. In this work, therefore, the same composition and extrusion temperature were maintained for all ternary blends. Melt blending was performed in the same extruder where the above neutralization processes were carried out with a screw speed of 50 rpm. The temperature profile of the extruder was set as 210/220/230/240/ 240/240/230/220 C from the first heating zone to die, respectively. Specimens for mechanical tests were injection molded (Sumitomo SE50D) at melt temperature of 190 C and mold temperature of 35 C. Prior to injection molding, the compounds were oven-dried at 75e80 C overnight. After injection molding, all test specimens were conditioned at 23 C and 50% RH for 7 days before testing. Samples of neat PLA, PLA/EBA-GMA (80/20, w/w) and PLA/ZnO (80/0.206, w/w) binary blends were prepared under the same conditions and used as controls. In the PLA/ZnO control sample, the content of ZnO was equivalent to the molar content of zinc element in the PLA/EBA-GMA/EMAA15-60%Zn (80/15/5, w/w) ternary blend.
2.4. Mechanical tests Notched Izod impact tests were performed according to ASTM D256 using a Plastic Impact Tester (Tinius Olsen). Average value of five repeats was taken for each sample. Tensile tests were conducted on a universal testing machine (Instron 4466) following ASTM D638. The crosshead speed was set at 0.2 inch/min (5 mm/ min) and 2 inch/min (50 mm/min), respectively. The initial strain was measured using a 2-inch extensometer (model: 3542-0200010-ST; Epsilon technology Co., WY).
2.5. Dynamic mechanical analysis (DMA) Dynamic mechanical properties of the blends were measured using DMA Q800 (TA Instruments) under a single-cantilever straincontrolled mode with an oscillating amplitude of 15 mm and frequency of 1 Hz. The temperature was swept from 100 to 150 C at 3 C/min. The test sample size was 17.4 mm (length) 12.60 mm (width) 3 mm (thickness). For each sample, duplicate tests were conducted.
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FT-IR tests. The thicknesses of the films were measure using a screw micrometer with an accuracy of 1 mm. For each sample at least five repeat tests were conducted and the results were averaged. The DN was determined according to the following equation [27]:
a 1700 cm-1 1620 cm-1 1585 cm-1 EMAA15-H
a
EMAA15-25%Zn
1734 cm-1 1760 cm-1
EMAA15-41%Zn
Individual polymers:
1700 cm-1
EMAA15-H
EMAA15-48%Zn
EMAA15-41%Zn
EMAA15-60%Zn 1850
1800
EMAA4-H
1750
1700 1650 1600 Wavenumbers ( cm-1)
1550
EMAA4-38%Zn
1500
EBA-GMA
b
PLA
1700 cm-1 1850
1585 cm-1
1800
1750
1700
1650
1600
1550
1500
Wavenumbers ( cm-1)
EMAA4-H
b
EMAA4-24%Zn EMAA4-31%Zn
1734 cm-1 1760 cm-1
Extraction residue containing:
EMAA4-38%Zn
EMAA15-60%Zn
EMAA4-51%Zn
EMAA15-48%Zn 1850
1800
1750
1700
1650
1600
1550
1500
EMAA15-41%Zn
Wavenumbers ( cm-1)
EMAA15-25%Zn
Fig. 1. FT-IR absorption spectra of ionomer precursors and zinc ionomers in the range of 1500e1800 cm1 (a) EMAA15-H and EMAA15-Zn; (b) EMAA4-H and EMAA4-Zn.
EMAA15-H 2.6. Fourier transform-infrared spectroscopy (FT-IR)
1850 The FT-IR absorption spectra were then recorded using a Thermo Nicolet Nexus 670 spectrometer with a resolution of 4 cm1 and 32 scans.
1800
1750
1700
1650
1600
1550
1500
Wavenumbers ( cm-1)
c
2.6.1. Determination of DN of ionomers The DN of EMAA-Zn was determined by FT-IR. EMAA-H and EMAA-Zn films were prepared by hot press at 200 C and used for
1734 cm-1 1760 cm-1
Extraction residue containing: EMAA4-51%Zn
Table 2 Degree of neutralization of EMAA-Zn ionomers obtained determined by FT-IR. Ionomer type
Equivalent ratio of ZnO/MAAa
Actual degree of neutralization (DN) (%, FT-IR)
EMAA15-Zn
0:1 0.25:1 0.50:1 0.75:1 1:1 0:1 0.25:1 0.50:1 0.75:1 1:1
0 25 41 48 60 0 24 31 38 51
EMAA4-Zn
a
4 6 2 10
6 8 8 5
Equivalent stoichiometric ratio of ZnO and MAA (acid co-monomer unit in EMAA) in the reactant mixture.
EMAA4-38%Zn EMAA4-31%Zn EMAA4-24%Zn EMAA4-H 1850
1800
1750
1700 1650 1600 Wavenumbers ( cm-1)
1550
1500
Fig. 2. FT-IR spectra of individual polymers (a) and residues obtained from 1,4dioxane-extracted PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends (b and c).
W. Song et al. / Polymer 53 (2012) 2476e2484
DN ¼
1
P ni d2i dw ¼ P ni di
A1700 cm1 =DEMAAZn 100%; A1700 cm1 =DEMAAH
2479
(2)
in which A1700 cm1 was the area of peak at 1700 cm1, and DEMAA-Zn and DEMAA-H were the thicknesses of the ionomer and precursor sample, respectively.
where ni is the number of particles having the apparent particle diameter di. The polydispersity is defined as the ratio of the weight average diameter to the number average diameter, dw/dn.
2.6.2. Study of interfacial compatibilization Thin slices with thickness of w120 mm of the injection-molded ternary blend sample were prepared using a microtome. The slices were then extracted in 1,4-dioxane at room temperature under constant stirring for 10 days to dissolve any free PLA component. A very small amount of insoluble residue left after extraction was grinded with dried KBr powder and then compressed into discs for FT-IR test. All FT-IR samples were dried in a vacuum oven to eliminate the effects of residual solvent and moisture prior to testing. After the baseline correction, the deconvolution of the bands at w1760, w1734, and w1700 cm1 was made using the Lorentzian function.
3. Results and discussion
2.7. Electron microscopy 2.7.1. Observation of impact fracture surfaces The impact fracture surfaces of the specimens were sputter coated with gold and examined under a Quanta 200F field emission scanning electron microscope (FE-SEM, FEI Company) at an accelerated voltage of 15 kV. 2.7.2. Partial size analysis Ultra-thin sections (<100 nm in thickness) were sliced from the plane perpendicular to the injection flow direction using an RMC cryo ultra microtome equipped with a diamond knife at a temperature below the glass transition temperature (Tg) of the EBA-GMA phase (ca. 30 C) and then mounted on formvar-coated 200mesh nickel grids. The microstructure of the ternary blends was studied using transmission electron microscopy (TEM, Philips CM200) at an accelerated voltage of 200 kV. A semi-automated image analysis software (NIHÒ) was used to analyze the TEM images. For each sample, at least 700 particles from 4 or more TEM images were measured. The cross-sectional area (Ai) of each individual particle (i) was measured and converted into an equivalent diameter of a sphere by the equation (di¼(4Ai/p)0.5). The number (dn), and weight (dw) average diameters were determined from the following equations:
P nd dn ¼ P i i ni
(1)
3.1. Characterization of ionomers The FT-IR absorption spectra of the precursors and various ionomers in the range of 1500e1800 cm1 are shown in Fig. 1. The peak at 1700 cm1 was attributed to the carbonyl stretching vibration in the carboxylic acid dimers in the ionomer or the precursor [27,28]. The peaks at 1585 and 1620 cm1 (as appeared in the spectra of ionomers derived from EMAA15-H) were attributed to the asymmetric carboxylate stretching vibration in the zinc carboxylate in tetrahedral coordination environment and in complex acid salt structure, respectively [28]. Table 2 gives the summary of the DN of ionomers determined from FT-IR. If the added ZnO was completely reacted with the carboxyl groups, a “theoretical” DN would be obtained according to the equivalent stoichiometric ratio of ZnO/MAA in the reactant mixture. However, the “actual” DN of ionomers measured by FT-IR was lower than the theoretical one. This could be due to two main reasons. First, the ZnO used was in a form of fine powder and thus the lost during processing, by sticking on the bag, hopper and feed throat etc., was almost inevitable. Besides, the neutralization reaction was difficult to proceed to high extent of completion due to the diffusion limitation during the short residence time during extrusion. In this study, the range of DN of ionomers was from 0 to 60%. 3.2. Reactive interfacial compatibilization We previously studied the reactive interfacial compatibilization by exacting the ternary blend with 1,4-dioxane and then analyzing the residue using FT-IR. The same method was used in this study. If compatibilization reactions did occur at the interface between PLA and the crosslinked EBA-GMA elastomer, there would be a trace amount of unextractable PLA in the residue. The FT-IR absorption spectra of the each individual polymer are shown in Fig. 2a. PLA and EBA-GMA exhibited absorption peaks at 1760 cm1 and 1734 cm1, respectively, which were attributed to the stretching vibration of the ester carbonyl groups in each of the pure polymers [25]. The characteristic absorption peak at 1700 cm1 came from the
Scheme 1. Interfacial compatibilization reaction catalyzed by Zn2þ in the ionomer.
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Table 3 Peak-resolved FT-IR data of the residues from 1,4-dioxane-extracted ternary blends. Sample
PLA peak Position (cm1)
Individual polymer 1758 PLAa e EBA-GMAa Extraction residue contains EMAA15-H 1762 EMAA15-25%Zn 1761 EMAA15-41%Zn 1761 EMAA15-48%Zn 1761 EMAA15-60%Zn 1760 EMAA4-H 1762 EMAA4-24%Zn 1762 EMAA4-31%Zn 1762 EMAA4-38%Zn 1763 EMAA4-51%Zn 1762 a
EBA-GMA peak
APLA/AEBA-GMA
Area
Position (cm1)
Area
e e
e 1734
e e
e e
0.4 4.1 2.0 12.0 1.9 1.5 0.5 2.1 10.2 2.9
1734 1734 1733 1734 1734 1734 1734 1734 1734 1734
5.7 26.4 3.8 14.7 2.5 11.4 5.0 7.3 41.6 9.2
0.06 0.15 0.53 0.81 0.74 0.13 0.11 0.28 0.24 0.31
Data obtained from Ref. [29].
ionomer or the precursor both containing hydrogen-bonded COOH groups as explained in the above section. Fig. 2b and c shows the FT-IR spectra of the residues obtained after a thorough extraction of the PLA matrix from the PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends. Because the solvent (i.e., 1,4dioxane) used in the extraction only selectively dissolved the pure PLA component, the residue was believed to consist of EBAGMA, EMAA-Zn (or EMAA-H) and the in-situ formed graft copolymer resulted from interfacial reaction between EBA-GMA and PLA. It was also evidenced in the literature that GMA-functionalized copolymers could react with hydroxyl and/or carboxyl end groups of polyesters under melt processing conditions [29e31]. The resulting copolymer at the interface thus acted as a compatibilizer between the PLA matrix and the dispersed phase. The absorption of carboxylic acid dimmers (w1700 cm1) was still clearly observed in
the residues of the ternary blends containing EMAA15-Zn (or EMAA15-H) (Fig. 2b), but it was not very obvious in the residues of blends containing EMAA4-Zn (or EMAA4-H) probably due to the relatively low concentration of COOH in EMAA4-Zn (or EMAA4-H). Though Fig. 2b and c suggested the chemical reactions between PLA and EBA-GMA, these FT-IR spectra could not tell if the compatibilization reactions were through the terminal OH or the terminal COOH group of PLA with the epoxy group of EBA-GMA. Commercial PLA is prepared by bulk polymerization of lactide in the presence of catalysts especially the stannous bis-2ehtylhexanoate (tin(II) octoate) [32]. It is now generally accepted that the polymerization of lactide under the catalysis of tin(II) octoate is through a coordinationeinsertion mechanism in which tin octoate facilitates the polymerization, but the hydroxyl or other nucleophilic species are the actual initiators [33,34]. This mechanism would favor the formation of hydroxyl terminated PLA chains. Based on above evidence, the interfacial compatibilization reactions are postulated in Scheme 1. Deconvolution results of the PLA and EBA-GMA peaks in the FT-IR spectra of extraction residues from the ternary blends containing ionomer or precursor are summarized in Table 3. The extent of the compatibilization reaction can be approximately described by the amount of grafted PLA per unit of EBA-GMA used, as represented by the ratio of the areas of absorption peaks at 1760 cm1 and 1734 cm1, i.e., APLA/AEBA-GMA. The blends containing EMAA15Zn showed higher values of APLA/AEBA-GMA compared with blends containing EMAA4-Zn, suggesting that the interfacial compatibilization effect was probably more pronounced in the former cases. Within each group of blends, the value of APLA/AEBA-GMA increased with the DN of the ionomer, indicating the catalytic effect of zinc ions in the ionomer on interfacial compatibilization of the blend. The SEM images of impact fracture surfaces of the ternary blends in the vicinity of the notch are shown in Fig. 3. All PLA ternary blends exhibited similar phase structures where the domains of impact modifiers were distributed uniformly in the PLA
Fig. 3. SEM images of room temperature impact fracture surfaces (adjacent to notch) of PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) blends: (a1) EMAA15-H, (b1) EMAA1525%Zn, (c1) EMAA15-60%Zn (magnification: 4000); (a2) EMAA4-H, (b2) EMAA4-24%Zn, (c2) EMAA4-38%Zn (magnification: 5000).
W. Song et al. / Polymer 53 (2012) 2476e2484
a 2.0
1.5
all the particles in the ternary blend were very well embedded in the matrix and a higher extent of plastic deformation was observed. Similar trend was found when EMAA4-H or EMAA4-Zn were used (Fig. 3b2 and c2) except that the matrix plastic deformation seemed to be less. From the direct observation of impact fracture surfaces as a function of DN of ionomer and the total functionality level (determined by the MAA content in the precursor), it was further evidenced that reactive interfacial compatibilization effect increased with DN and MAA content in the precursor as discussed earlier in this section.
blend w/ EMAA15-60%Zn EMAA15-48%Zn
tan δ
EMAA15-41%Zn
1.0
2481
EMAA15-25%Zn EMAA15-H -60
0.5
-50
-40
-30
-20
-10
3.3. Crosslinking of EBA-GMA
0
Temperature (°C)
0.0 -120 -100 -80 -60 -40 -20
0
20
40
60
80
Temperature (°C)
b 2.0
1.5
blend w/
EMAA4-51%Zn EMAA4-38%Zn
tan δ
EMAA4-31%Zn
1.0
EMAA4-24%Zn EMAA4-H
0.5
-60
-50
-40
-30
-20
-10
0
40
60
Temperature (°C)
0.0 -120 -100 -80 -60 -40 -20
0
20
80
Temperature (°C) Fig. 4. Dependence of damping factor (tan d) on temperature for various PLA/EBAGMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends: (a) blends with EMAA15-Zn (or EMAA15-H); (b) blends with EMAA4-Zn (or EMAA4-H) (Curves were shifted vertically for clarity.).
matrix. Debonding without perceptible plastic deformation was clearly observed in both of the blends containing ionomer precursors, i.e., EMAA15-H and EMAA4-H (Fig. 3a1 and a2). When EMAA15-25%Zn (Fig. 3b1) was used, wetting of the EBA-GMA droplets by PLA was enhanced and more plastic deformation was evident at the fracture surface. As the DN of EMAA-Zn ionomer further increased to the 60% (i.e., EMAA15-60%Zn) (Fig. 3c1), almost
Damping factors (tan d) obtained from DMA measurements for various PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) blends are shown in Fig. 4. The glass transition temperature, Tg, of the PLA matrix was around 70 C, as indicated by the tan d peak at higher temperature. All ternary blends in this study exhibited a second distinct damping peak around 30 C attributed to the Tg of the EBA-GMA phase [25]. The Tg of EBA-GMA in blends with ionomer precursors appeared higher than that in the blends with ionomers. Carboxylic acid is an active curing agent for epoxy resins. Therefore, crosslinking of EBA-GMA occurred to a certain degree during compounding [24]. A schematic illustration of the crosslinking reactions between EBA-GMA and ionomer (or precursor) is given in Scheme 2. The unneutralized precursor would lead to a higher degree of crosslinking of EBA-GMA than the partially neutralized ionomer because the former had more COOH groups available for the reactions, yielding the crosslinked EBA-GMA with a higher Tg. Similarly, at the same concentration in the blends, EMAA15-H led to a higher Tg than EMAA4-H (27.6 C vs. 31 C) for the EBA-GMA phase due to the higher COOH content in the former. With increase in the DN of the EMAA15-Zn ionomer, the Tg of the EBA-GMA phase in the resulting blends decreased significantly by about 5 C (from 27.6 to 32.4 C). This decrease in the Tg of the EBA-GMA phase was likely due to the decreased degree of crosslinking of EBA-GMA which was a consequence of the declined COOH content. In contrast, for the blends containing EMAA4-Zn (or EMAA4-H), only a slight change (from 31.1 to 32.6 C) in the Tg of the EBA-GMA phase was found with variation in DN. 3.4. Morphology and mechanical properties Fig. 5 shows the phase structure of the PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends. ‘Salami’-like (i.e., domain-in-domain) structure was noted for the PLA ternary blends containing EMAA15-H and EMAA15-60%Zn. Our previous study indicated that due to the lower interfacial tension between PLA and EBA-GMA than that between PLA and EMAA-Zn (or EMAA-H), when the EBA-GMA/EMAA-Zn weight ratio was large than 1, the EMAA-Zn (or EMAA-H) formed the inclusions inside the EBA-GMA phase [24,25].
Scheme 2. Schematic crosslinking reaction of EBA-GMA with ionomer during melt compounding.
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Fig. 5. TEM images of PLA ternary blends with EMAA-Zn or EMAA-H: (a) EMAA15-H; (b) EMAA15-60%Zn; (c) EMAA4-H; (d) EMAA4-51%Zn.
Particle size analysis was performed based on the acquired TEM images. The weight average diameter (dw) and the polydispersity (dw/dn) of dispersed particles in the ternary blends are listed in Table 4. The dw decreased from 1.62 to 1.00 mm with increasing DN of EMAA15-Zn from 0 to 60%. Meanwhile, a clear decrease in the particle size polydispersity, i.e., the values of dw/dn, was also found. These results indicate that high DN favored the formation of fine and uniform dispersion for the blends containing EMAA15-Zn. This was probably in part due to the enhanced interfacial compatibilization between PLA and EBA-GMA under the catalysis of zinc ionomer, as discussed in above FT-IR and SEM results. In addition, crosslinking of EBA-GMA decreased with increasing DN due to the reduced availability of COOH, which would make it easier for the particles to break into smaller ones. For the blends containing EMAA4-Zn (or EMAA4-H), the particle size and distribution did not exhibit consistent trends with varying DN. This might be attributed the overall low contents of both zinc ions and free carboxylic acid groups which resulted in general weak crosslinking of EBA-GMA and interfacial compatibilization in the blends.
Fig. 6 shows the stressestrain curves of the neat PLA and two representative PLA ternary blends containing EMAA4-51%Zn and EMAA15-48%Zn, respectively, at strain rate of 0.2 inch/min and 2 inch/min. Neat PLA behaved strong and stiff but brittle, as reflected by the high tensile strength, modulus and low strain-atbreak shown in its stressestrain curves. The fracture behavior of the specimens changed greatly from brittle fracture of the neat PLA to ductile fracture of the PLA ternary blends. The blends displayed stable neck growth through cold drawing during tensile testing and finally broke at a significantly higher strain-at-break (w200%) under a slow extension rate (0.2 inch/min). The tensile strength and modulus of the blends decreased by 42e48% and 32e38%, respectively, with respect to that of the neat PLA. Increasing extension rate usually results in increase in tensile strength and reduction in elongation. Fig. 6 shows that when the extension rate was increased to 2 inch/min, the strength of blends only decreased slightly while the strain-at-break drastically decreased to w20% but was still significantly higher than that of neat PLA. Almost no change in the modulus was found for both neat PLA and PLA ternary
Table 4 Mechanical properties of PLA ternary blends and particle size and distribution of the dispersed phase. Samples Controls neat PLA PLA/ZnO (80/0.206, w/w) PLA/EBA-GMA (80/20, w/w) Ternary blends EMAA15-H EMAA15-25%Zn EMAA15-41%Zn EMAA15-48%Zn EMAA15-60%Zn EMAA4-H EMAA4-24%Zn EMAA4-31%Zn EMAA4-38%Zn EMAA4-51%Zn a
IS (J/m) 25 5 20 4 102 14 97 297 411 613 680 101 126 217 189 240
10 17 79 43 56 9 13 69 41 72
Strain-at-breaka (%) 4.4 0.5 4.3 0.2 5.2 0.5 27.3 24.6 25.8 21.8 23.5 20.0 19.6 20.5 22.1 15.8
3.0 4.8 4.0 2.4 4.0 3.1 1.5 2.0 2.4 2.4
Tensile strengtha (MPa)
Modulusa (GPa)
dw (mm)
dw/dn
68.3 0.7 64.6 1.8 40.4 0.4
3.35 0.14 3.32 0.16 2.26 0.03
e e 0.30
e e 1.37
1.62 1.56 1.22 1.33 1.00 1.27 0.93 1.61 1.18 1.23
2.43 2.23 2.06 1.87 1.81 1.87 1.81 2.08 1.80 1.75
35.8 37.0 38.4 38.5 37.4 38.5 38.9 37.9 37.6 39.3
0.5 0.1 0.2 0.4 0.4 0.3 0.2 0.1 0.3 0.4
Data were obtained from tensile tests that were conducted at an extension rate of 2 inch/min.
2.06 2.12 2.19 2.17 2.13 2.24 2.28 2.19 2.18 2.24
0.08 0.07 0.02 0.05 0.07 0.04 0.03 0.02 0.06 0.04
W. Song et al. / Polymer 53 (2012) 2476e2484
70
1, 2: neat PLA 3, 4: blend w/ EMAA 4-51% Zn 5, 6: blend w/ EMAA15-48% Zn
1
Stress (MPa)
60
2
50 40 30 20
3
4
5
6
10 0 0
5
10
15
20
150
200
2483
low, ranging from 1 to 3% (data not shown). Therefore, this small difference in crystallinity was not likely a significant factor responsible for the difference in the impact strength of the PLA ternary blends. It was demonstrated that increasing DN and MAA content in the precursor contributed to PLA toughening in the ternary blends. Based on the foresaid analysis on dual reactions, such toughness improvement as a function of DN in the ternary blends could be attributed to the following factors: (1) enhanced interfacial compatibilization, as suggested by the aforesaid FT-IR and SEM results; (2) a relatively low crosslinking level of EBAGMA (as indicated by DMA results), which could lower the resistance to cavitation inside the modifier particles [35,36]. When a certain level of neutralization was attained in the EMAA4-Zn, however, the improvement in IS seemed to be independent of the variation in DN. Such limited toughening effect might be attributed to the low total functionalities (i.e. MAA content in the precursor) in the ionomer.
Strain at break (%) 4. Conclusions Fig. 6. Tensile stressestrain curves of neat PLA and PLA/EBA-GMA/EMAA-Zn (80/15/5, w/w) ternary blends under speed of extension of 2 inch/min (solid line) and 0.2 inch/ min (dash line), respectively.
blends tested under different extension rates. Table 4 listed the data of tensile strength, modulus and strain-at-break of neat PLA and its ternary blends at extension rate of 2 inch/min. In contrast to its influence on IS, manipulating DN of ionomer did not exhibit as much effect as it did on tensile toughness (or ductility) of the blends. The notched Izod IS for the ternary blends as a function of DN of EMAA-Zn and MAA content in the precursor is presented in Fig. 7 and IS data are summarized in Table 4. As compared to either of the precursors (i.e. EMAA-H), the zinc ionomers resulted in higher IS of the ternary blends. Comparatively, the ternary blends with EMAA15-Zn universally exhibited superior IS than the ones with EMAA4-Zn. The dependence of IS on DN was somewhat different between these two groups of ternary blends. The IS of the ternary blend with EMAA15-Zn continuously increased with increasing DN of ionomer, while that of blends with EMAA4-Zn initially increased then leveled off in the DN range of 30e50%. DSC study indicated the crystallinity of the PLA matrix in all the ternary blends was very
The toughness of PLA ternary blends with epoxy-containing elastomer (EBA-GMA) and zinc ionomer (EMAA-Zn) greatly depends on the characteristics of the ionomer. Interfacial compatibilization between the PLA matrix and the dispersed EBA-GMA elastomer phase during reactive blending was greatly enhanced with increases in degree of neutralization and/or functionality of EMAA-Zn. Consequently, impact strength of the resulting PLA ternary blends increased with increase in degree of neutralization of ionomers. Similarly, EMAA-Zn ionomers of high functionality exhibited much more pronounced toughening effect than the ones of low functionality. These two factors also influenced the crosslinking of EBA-GMA but to a much lesser extent compared the effects on interfacial compatibilization reactions. The change of Tg of the EBA-GMA phase in the PLA ternary blends indicates that the final extent of crosslinking of EBA-GMA largely depended on the concentration of free carboxylic acid groups in the ionomers. Combination of a relatively low extent of crosslinking of the EBAGMA phase and a strong interfacial adhesion was found to favor the achievement of supertoughness. Morphological analysis revealed that a finer and more uniform particle size and distribution accompanied the increase in the impact strength of the supertoughened blends. Acknowledgement
800 The authors are grateful for the financial support from the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant no. 2007-3550417818.
blend w/ EMAA15-Zn (or -H) blend w/ EMAA 4-Zn (or -H) Impact strength (J/m)
600
References
400
200
0 0
10
20
30
40
50
60
Degree of neutralization (%) Fig. 7. Effects of degree of neutralization and functionality of ionomers on the IS of PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) blends.
[1] Noda I, Satkowski MM, Dowrey AE, Marcott C. Macromol Biosci 2004;4(3): 269e75. [2] Schreck KM, Hillmyer MA. J Biotech 2007;132(3):287e95. [3] Semba T, Kitagawa K, Ishiaku US, Hamada H. J Appl Polym Sci 2006;101(3): 1816e25. [4] Harada M, Iida K, Okamoto K, Hayashi H, Hirano K. Polym Eng Sci 2008;48(7): 1359e68. [5] Wang R, Wang S, Zhang Y, Wan C, Ma P. Polym Eng Sci 2009;49(1):26e33. [6] Jiang L, Wolcott MP, Zhang J. Biomacromolecules 2006;7(1):199e207. [7] Zhang N, Wang Q, Ren J, Wang L. J Mater Sci 2009;44(1):250e6. [8] Coltelli MB, Maggiore ID, Bertoldo M, Signori F, Bronco S, Ciardelli F. J Appl Polym Sci 2008;110(2):1250e62. [9] Jiang L, Liu B, Zhang J. Ind Eng Chem Res 2009;48(16):7594e602. [10] Gramlich WM, Robertson ML, Hillmyer MA. Macromolecules 2010;43(5): 2313e21. [11] Li Y, Shimizu H. Macromol Biosci 2007;7(7):921e8. [12] Zhang W, Chen L, Zhang Y. Polymer 2009;50(5):1311e5.
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W. Song et al. / Polymer 53 (2012) 2476e2484 Afrifah KA, Matuana LM. Macromol Mater Eng 2010;295(9):802e11. Kim YF, Choi CN, Kim YD, Lee KY, Lee MS. Fibers Polym 2004;5(4):270e4. Anderson KS, Lim SH, Hillmyer MA. J Appl Polym Sci 2003;89(14):3757e68. Anderson KS, Hillmyer MA. Polymer 2004;45(26):8809e23. Su Z, Li Q, Liu Y, Hu GH, Wu C. Eur Polym J 2009;45(8):2428e33. Feng F, Ye L. J Appl Polym Sci 2011;119(5):2778e83. Han JJ, Huang HX. J Appl Polym Sci 2011;120(6):3217e23. Ho CH, Wang CH, Lin CI, Lee YD. Polymer 2008;49(18):3902e10. Zhang K, Ran X, Wang X, Han C, Han L, Wen X, et al. Polym Eng Sci 2011; 51(12):2370e80. Oyama HT. Polymer 2009;50(3):747e51. Hashima K, Nishitsuji S, Inoue T. Polymer 2010;51(17):3934e9. Liu H, Chen F, Liu B, Estep G, Zhang J. Macromolecules 2010;43(14):6058e66. Liu H, Song W, Chen F, Guo L, Zhang J. Macromolecules 2011;44(6):1513e22. Kaci M, Cimmino S, Silvestre C, Duraccio D, Benhamida A, Zaidi L. Macromol Mater Eng 2006;291(7):869e76.
[27] MacKnight WJ, McKenna LW, Read BE, Stein RS. J Phys Chem 1968;72(4): 1122e6. [28] Coleman MM, Lee JY, Painter PC. Macromolecules 1990;23(8):2339e45. [29] Kaci M, Benhamida A, Cimmino S, Silvestre C, Carfagna C. Macromol Mater Eng 2005;290(10):987e95. [30] Jeon HK, Kim JK. Macromolecules 1998;31(26):9273e80. [31] Benhamida A, Kaci M, Cimmino S, Silvestre C, Duraccio D. Macromol Mater Eng 2009;294(2):122e9. [32] Henton DE, Gruber P, Lunt J, Randall J. In: Mohanty A, Misra M, Drzal LT, editors. Natural fibers, biopolymers, and biocomposites. CRS Press; 2005, p.527e77. [33] Kricheldorf H, Kreiser-Saunders I, Boettcher C. Polymer 1995;36(6):1253e9. [34] Du YJ, Lemstra PJ, Nijenhuis AJ, van Aert HAM, Bastiaansent C. Macromolecules 1995;28:2124e32. [35] Dompas D, Groeninckx G. Polymer 1994;35(22):4743e9. [36] Steenbrink AC, Litvinov VM, Gaymans RJ. Polymer 1998;39(20):4817e25.
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Effects of ionomer characteristics on reactions and properties of poly(lactic acid) ternary blends prepared by reactive blending Wenjia Song, Hongzhi Liu, Feng Chen, Jinwen Zhang* Materials Science and Engineering program & Composite Materials and Engineering Center, Washington State University, Pullman, WA 99164, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 January 2012 Received in revised form 12 March 2012 Accepted 26 March 2012 Available online 2 April 2012
Toughening of poly(lactic acid) (PLA) was studied by reactive blending PLA with ethylene/n-butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer and zinc ion-containing ionomer. The ionomer was prepared by neutralizing the ethylene/methacrylic acid copolymer (EMAA), i.e., ionomer precursor, with ZnO. The reactive interfacial compatibilization between PLA and EBA-GMA and the crosslinking of EBA-GMA during blending was studied in detail. Fractography and FT-IR analysis indicated that both the degree of neutralization (DN) of ionomer and methacrylic acid (MAA) content of ionomer precursor exhibited significant effects on interfacial compatibilization. Dynamic mechanical analysis also suggested that the crosslinking level of EBA-GMA varied with these two factors. Particle size and polydispersity of the dispersed phase were measured by image analysis of TEM micrographs of the ternary blends and correlated with the impact strength of the blends and the characteristics of the ionomer. Ionomers derived from precursor of high MAA content and/or having high DN tended to yield superior impact strength of the PLA blends. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Poly(lactic acid) Impact toughness Reactive blending
1. Introduction As one of the commercial biodegradable polymers, poly(lactic acid) (PLA), with excellent strength and stiffness, is considered to be a promising candidate to replace some petroleum-based commodity thermoplastics. However, the brittleness of PLA is a major drawback to impede its wide applications. Incorporating flexible polymers or elastomers into PLA via melt blending is a potentially cost effective way of addressing this issue. Various biodegradable polymers, including polyhydroxyalkanoate (PHA) [1,2], polycaprolactone (PCL) [3,4], poly(butylene succinate) (PBS) [5], poly(butylene adipate-coterephthalate) (PBAT) [6e9], conjugated soybean oil [10] and other biodegradable elastomers and copolymers [11e13] have been blended with PLA. Non-biodegradable polymers such as polyethylene (PE) [14e16] and many other petroleum-based polymers and elastomers [17e23] were also used in toughening PLA. Although most of the reported PLA blends showed impressively high tensile elongations (150e350%) compared with that of neat PLA, only marginal to moderate improvements in impact strength (IS) were achieved. To date, there are a few reports of PLA blends in the literature demonstrating “supertoughness”. Anderson et al. [15,16] reported
* Corresponding author. Tel.: þ1 509 335 8723; fax: þ1 509 335 5077. E-mail address: [email protected] (J. Zhang). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.03.050
supertough PLA/PE blends compatibilized by PLA-b-PE diblock copolymers. The resulting PLA/PE (80/20 w/w) blends exhibited IS as high as 760 J/m, ca. 30 times that of neat PLA. In another study reported by Oyama [22], reactive poly(ethylene-glycidyl methacrylate) (EGMA) was used to toughen PLA. After annealing at 90 C for 2.5 h, the PLA/EGMA blend achieved supertoughness with a Charpy IS of 72 kJ/m2 (ca. 50 times that of neat PLA). However, a significant reduction in tensile elongations accompanied this post-treatment of the blends. Furthermore, a direct evidence of the reactive compatibilization and thorough discussion of toughening mechanism were not yet provided. Hashima et al. [23] reported a PLA ternary blend containing 20 wt% hydrogenated styrenebutadiene-styrene block copolymer (SEBS) compatibilized by 10 wt% EGMA. A much higher Izod IS (92 kJ/m2) than that of neat PLA (3 kJ/m2) was reported. Contrary to Oyama’s annealing result, the aging at 80 C for 48 h caused a pronounced reduction in the IS of the blend to 32 kJ/m2. Recently, we introduced a novel supertough PLA ternary blend system consisting of PLA, an elastomeric ethylene/n-butyl acrylate/ glycidyl methacrylate terpolymer (EBA-GMA) and a zinc ionomer of ethylene/methacrylic acid copolymer (EMAA-Zn) [24,25]. Unlike in other supertoughened PLA blends, crosslinking of EBA-GMA induced by zinc ionomers took place in addition to the interfacial compatibilization between EBA-GMA and PLA in this ternary blend system during melt blending [24]. Previous studies demonstrated that EMAA-Zn played an important role in determining toughening
W. Song et al. / Polymer 53 (2012) 2476e2484
effect of the ternary blends and showed that both zinc ions and free carboxyl groups of methacrylic acid (MAA) in the ionomer actively participated in the dual reactions involved [24,25]. When the unneutralized EMAA-H was substituted by EMAA-Zn, interfacial wetting and consequently IS was remarkably improved [24]. With increase of the EMAA-Zn/EBA-GMA ratio in the blend composition, levels of crosslinking of EBA-GMA increased, which could lead to a higher resistance to cavitation of the dispersed particles [25]. Therefore, it was reasonably expected that the variation in the MAA content in the precursor and the extent of neutralization would exert some effects on the ultimate IS of the blends. However, an indepth understanding on how the characteristics of ionomer govern the dual reactions involved in blending and thereby affect the IS of the blend is not yet provided. In this work, we study the effects of degree of neutralization (DN) of ionomer and the MAA content in the precursor on the toughness of the PLA/EBA-GMA/EMAA-Zn (80/15/5, w/w) ternary system. Two series of ionomers with varying DN were derived by neutralizing the precursors (EMAA-H) containing 4 and 15 wt% of MAA, respectively, with ZnO in extrusion. The interfacial compatibilization was investigated using FT-IR and SEM, and the crosslinking of EBA-GMA was studied by DMA. Particle size analysis was performed to examine the effect of ionomer on the morphological aspects of the blends. 2. Experimental section 2.1. Materials The materials used in this study and some specifications are listed in Table 1. The ionomer precursors, EMAA-H containing 15 and 4 wt% MAA, were designated as EMAA15-H and EMAA4-H, respectively. Likewise, the ionomers prepared from these precursors were designated as EMAA15-Zn and EMAA4-Zn, respectively. 2.2. Preparation of ionomers EMAA-Zn ionomers were prepared by neutralizing the precursors, EMAA-H, with ZnO in a co-rotating twin screw extruder
Table 1 Characteristics of materials used in this study. Material (abbreviation)
Grade (supplier)
Specifications
Poly(lactic acid) (PLA)
PLA2002D (NatureWorks) ElvaloyÒ PTW (DuPont Co.)
MI (210 C, 2.16 kg): 5e7 g/10 min MI (190 C, 2.16 kg): 12 g/10 min; Melting point (DSC): 72 C; E/BA/GMA ¼ 66.75/28/ 5.25 (wt%)a MI (190 C, 2.16 kg): 25 g/10 min; Melting point (DSC): 92 C; Methacrylic acid content: 15.0 wt% MI (190 C, 2.16 kg): 11 g/10 min; Melting point (DSC): 109 C; Methacrylic acid content: 4.0 wt% Specific gravity: 5.67 g/cm3; Melting point: 1975 C
Ethylene/n-butylacrylate/ glycidyl methacrylate copolymer (EBA-GMA)
Ethylene/methacrylic acid copolymer (EMAA15-H)
NucrelÒ 925 (DuPont Co.)
Ethylene/methacrylic acid copolymer (EMAA4-H)
NucrelÒ 0411HS (DuPont Co.)
Zinc oxide powder (ZnO)
a
Data is cited from Ref. [26].
Baker AnalyzedÒ reagent (J.T. Baker Chemical Co.)
2477
(Leistritz ZSE-18) with a screw diameter of 17.8 mm and an L/D ratio of 40. Prior to extrusion, EMAA-H pellets and zinc oxide were ovendried for at least 1 day at 75 C and above 100 C, respectively. Dried ZnO powder and EMAA-H pellets in predetermined stoichiometric ratios of ZnO/COOH were manually mixed in a plastic bag. The temperature profile of the extruder barrels was 180/190/190/200/ 200/200/190/180 C from the first heating zone (next to feeding throat) to die, respectively. The screw speed of the extruder was set at 50 rpm. Vacuum venting at the seventh zone of the extruder was applied to eliminate small molecules generated during neutralization. The extrudates were pelletized and then extruded for a second time under the same condition in order to ensure a more thorough and uniform neutralization reaction. The precursors, EMAA15-H and EMAA4-H, alone were also processed under the same conditions and used as controls. The zinc ionomers obtained were designated as EMAA15-x%Zn and EMAA4-x%Zn, respectively, in which ‘x%’ indicates the value of DN. 2.3. Preparation of PLA ternary blends Prior to melt compounding, PLA pellets were dried for at least 1 day at 80 C; EMAA-Zn (or EMAA-H) pellets were dried for 1 day at 75 C; EBA-GMA pellets were dried for 1 day at 65 C. Previous study found that the blend consisting of 80 wt% PLA, 15 wt% EBAGMA and 5% EMAA-Zn and extruded at 240 C gave the superior IS [24]. In this work, therefore, the same composition and extrusion temperature were maintained for all ternary blends. Melt blending was performed in the same extruder where the above neutralization processes were carried out with a screw speed of 50 rpm. The temperature profile of the extruder was set as 210/220/230/240/ 240/240/230/220 C from the first heating zone to die, respectively. Specimens for mechanical tests were injection molded (Sumitomo SE50D) at melt temperature of 190 C and mold temperature of 35 C. Prior to injection molding, the compounds were oven-dried at 75e80 C overnight. After injection molding, all test specimens were conditioned at 23 C and 50% RH for 7 days before testing. Samples of neat PLA, PLA/EBA-GMA (80/20, w/w) and PLA/ZnO (80/0.206, w/w) binary blends were prepared under the same conditions and used as controls. In the PLA/ZnO control sample, the content of ZnO was equivalent to the molar content of zinc element in the PLA/EBA-GMA/EMAA15-60%Zn (80/15/5, w/w) ternary blend.
2.4. Mechanical tests Notched Izod impact tests were performed according to ASTM D256 using a Plastic Impact Tester (Tinius Olsen). Average value of five repeats was taken for each sample. Tensile tests were conducted on a universal testing machine (Instron 4466) following ASTM D638. The crosshead speed was set at 0.2 inch/min (5 mm/ min) and 2 inch/min (50 mm/min), respectively. The initial strain was measured using a 2-inch extensometer (model: 3542-0200010-ST; Epsilon technology Co., WY).
2.5. Dynamic mechanical analysis (DMA) Dynamic mechanical properties of the blends were measured using DMA Q800 (TA Instruments) under a single-cantilever straincontrolled mode with an oscillating amplitude of 15 mm and frequency of 1 Hz. The temperature was swept from 100 to 150 C at 3 C/min. The test sample size was 17.4 mm (length) 12.60 mm (width) 3 mm (thickness). For each sample, duplicate tests were conducted.
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W. Song et al. / Polymer 53 (2012) 2476e2484
FT-IR tests. The thicknesses of the films were measure using a screw micrometer with an accuracy of 1 mm. For each sample at least five repeat tests were conducted and the results were averaged. The DN was determined according to the following equation [27]:
a 1700 cm-1 1620 cm-1 1585 cm-1 EMAA15-H
a
EMAA15-25%Zn
1734 cm-1 1760 cm-1
EMAA15-41%Zn
Individual polymers:
1700 cm-1
EMAA15-H
EMAA15-48%Zn
EMAA15-41%Zn
EMAA15-60%Zn 1850
1800
EMAA4-H
1750
1700 1650 1600 Wavenumbers ( cm-1)
1550
EMAA4-38%Zn
1500
EBA-GMA
b
PLA
1700 cm-1 1850
1585 cm-1
1800
1750
1700
1650
1600
1550
1500
Wavenumbers ( cm-1)
EMAA4-H
b
EMAA4-24%Zn EMAA4-31%Zn
1734 cm-1 1760 cm-1
Extraction residue containing:
EMAA4-38%Zn
EMAA15-60%Zn
EMAA4-51%Zn
EMAA15-48%Zn 1850
1800
1750
1700
1650
1600
1550
1500
EMAA15-41%Zn
Wavenumbers ( cm-1)
EMAA15-25%Zn
Fig. 1. FT-IR absorption spectra of ionomer precursors and zinc ionomers in the range of 1500e1800 cm1 (a) EMAA15-H and EMAA15-Zn; (b) EMAA4-H and EMAA4-Zn.
EMAA15-H 2.6. Fourier transform-infrared spectroscopy (FT-IR)
1850 The FT-IR absorption spectra were then recorded using a Thermo Nicolet Nexus 670 spectrometer with a resolution of 4 cm1 and 32 scans.
1800
1750
1700
1650
1600
1550
1500
Wavenumbers ( cm-1)
c
2.6.1. Determination of DN of ionomers The DN of EMAA-Zn was determined by FT-IR. EMAA-H and EMAA-Zn films were prepared by hot press at 200 C and used for
1734 cm-1 1760 cm-1
Extraction residue containing: EMAA4-51%Zn
Table 2 Degree of neutralization of EMAA-Zn ionomers obtained determined by FT-IR. Ionomer type
Equivalent ratio of ZnO/MAAa
Actual degree of neutralization (DN) (%, FT-IR)
EMAA15-Zn
0:1 0.25:1 0.50:1 0.75:1 1:1 0:1 0.25:1 0.50:1 0.75:1 1:1
0 25 41 48 60 0 24 31 38 51
EMAA4-Zn
a
4 6 2 10
6 8 8 5
Equivalent stoichiometric ratio of ZnO and MAA (acid co-monomer unit in EMAA) in the reactant mixture.
EMAA4-38%Zn EMAA4-31%Zn EMAA4-24%Zn EMAA4-H 1850
1800
1750
1700 1650 1600 Wavenumbers ( cm-1)
1550
1500
Fig. 2. FT-IR spectra of individual polymers (a) and residues obtained from 1,4dioxane-extracted PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends (b and c).
W. Song et al. / Polymer 53 (2012) 2476e2484
DN ¼
1
P ni d2i dw ¼ P ni di
A1700 cm1 =DEMAAZn 100%; A1700 cm1 =DEMAAH
2479
(2)
in which A1700 cm1 was the area of peak at 1700 cm1, and DEMAA-Zn and DEMAA-H were the thicknesses of the ionomer and precursor sample, respectively.
where ni is the number of particles having the apparent particle diameter di. The polydispersity is defined as the ratio of the weight average diameter to the number average diameter, dw/dn.
2.6.2. Study of interfacial compatibilization Thin slices with thickness of w120 mm of the injection-molded ternary blend sample were prepared using a microtome. The slices were then extracted in 1,4-dioxane at room temperature under constant stirring for 10 days to dissolve any free PLA component. A very small amount of insoluble residue left after extraction was grinded with dried KBr powder and then compressed into discs for FT-IR test. All FT-IR samples were dried in a vacuum oven to eliminate the effects of residual solvent and moisture prior to testing. After the baseline correction, the deconvolution of the bands at w1760, w1734, and w1700 cm1 was made using the Lorentzian function.
3. Results and discussion
2.7. Electron microscopy 2.7.1. Observation of impact fracture surfaces The impact fracture surfaces of the specimens were sputter coated with gold and examined under a Quanta 200F field emission scanning electron microscope (FE-SEM, FEI Company) at an accelerated voltage of 15 kV. 2.7.2. Partial size analysis Ultra-thin sections (<100 nm in thickness) were sliced from the plane perpendicular to the injection flow direction using an RMC cryo ultra microtome equipped with a diamond knife at a temperature below the glass transition temperature (Tg) of the EBA-GMA phase (ca. 30 C) and then mounted on formvar-coated 200mesh nickel grids. The microstructure of the ternary blends was studied using transmission electron microscopy (TEM, Philips CM200) at an accelerated voltage of 200 kV. A semi-automated image analysis software (NIHÒ) was used to analyze the TEM images. For each sample, at least 700 particles from 4 or more TEM images were measured. The cross-sectional area (Ai) of each individual particle (i) was measured and converted into an equivalent diameter of a sphere by the equation (di¼(4Ai/p)0.5). The number (dn), and weight (dw) average diameters were determined from the following equations:
P nd dn ¼ P i i ni
(1)
3.1. Characterization of ionomers The FT-IR absorption spectra of the precursors and various ionomers in the range of 1500e1800 cm1 are shown in Fig. 1. The peak at 1700 cm1 was attributed to the carbonyl stretching vibration in the carboxylic acid dimers in the ionomer or the precursor [27,28]. The peaks at 1585 and 1620 cm1 (as appeared in the spectra of ionomers derived from EMAA15-H) were attributed to the asymmetric carboxylate stretching vibration in the zinc carboxylate in tetrahedral coordination environment and in complex acid salt structure, respectively [28]. Table 2 gives the summary of the DN of ionomers determined from FT-IR. If the added ZnO was completely reacted with the carboxyl groups, a “theoretical” DN would be obtained according to the equivalent stoichiometric ratio of ZnO/MAA in the reactant mixture. However, the “actual” DN of ionomers measured by FT-IR was lower than the theoretical one. This could be due to two main reasons. First, the ZnO used was in a form of fine powder and thus the lost during processing, by sticking on the bag, hopper and feed throat etc., was almost inevitable. Besides, the neutralization reaction was difficult to proceed to high extent of completion due to the diffusion limitation during the short residence time during extrusion. In this study, the range of DN of ionomers was from 0 to 60%. 3.2. Reactive interfacial compatibilization We previously studied the reactive interfacial compatibilization by exacting the ternary blend with 1,4-dioxane and then analyzing the residue using FT-IR. The same method was used in this study. If compatibilization reactions did occur at the interface between PLA and the crosslinked EBA-GMA elastomer, there would be a trace amount of unextractable PLA in the residue. The FT-IR absorption spectra of the each individual polymer are shown in Fig. 2a. PLA and EBA-GMA exhibited absorption peaks at 1760 cm1 and 1734 cm1, respectively, which were attributed to the stretching vibration of the ester carbonyl groups in each of the pure polymers [25]. The characteristic absorption peak at 1700 cm1 came from the
Scheme 1. Interfacial compatibilization reaction catalyzed by Zn2þ in the ionomer.
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Table 3 Peak-resolved FT-IR data of the residues from 1,4-dioxane-extracted ternary blends. Sample
PLA peak Position (cm1)
Individual polymer 1758 PLAa e EBA-GMAa Extraction residue contains EMAA15-H 1762 EMAA15-25%Zn 1761 EMAA15-41%Zn 1761 EMAA15-48%Zn 1761 EMAA15-60%Zn 1760 EMAA4-H 1762 EMAA4-24%Zn 1762 EMAA4-31%Zn 1762 EMAA4-38%Zn 1763 EMAA4-51%Zn 1762 a
EBA-GMA peak
APLA/AEBA-GMA
Area
Position (cm1)
Area
e e
e 1734
e e
e e
0.4 4.1 2.0 12.0 1.9 1.5 0.5 2.1 10.2 2.9
1734 1734 1733 1734 1734 1734 1734 1734 1734 1734
5.7 26.4 3.8 14.7 2.5 11.4 5.0 7.3 41.6 9.2
0.06 0.15 0.53 0.81 0.74 0.13 0.11 0.28 0.24 0.31
Data obtained from Ref. [29].
ionomer or the precursor both containing hydrogen-bonded COOH groups as explained in the above section. Fig. 2b and c shows the FT-IR spectra of the residues obtained after a thorough extraction of the PLA matrix from the PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends. Because the solvent (i.e., 1,4dioxane) used in the extraction only selectively dissolved the pure PLA component, the residue was believed to consist of EBAGMA, EMAA-Zn (or EMAA-H) and the in-situ formed graft copolymer resulted from interfacial reaction between EBA-GMA and PLA. It was also evidenced in the literature that GMA-functionalized copolymers could react with hydroxyl and/or carboxyl end groups of polyesters under melt processing conditions [29e31]. The resulting copolymer at the interface thus acted as a compatibilizer between the PLA matrix and the dispersed phase. The absorption of carboxylic acid dimmers (w1700 cm1) was still clearly observed in
the residues of the ternary blends containing EMAA15-Zn (or EMAA15-H) (Fig. 2b), but it was not very obvious in the residues of blends containing EMAA4-Zn (or EMAA4-H) probably due to the relatively low concentration of COOH in EMAA4-Zn (or EMAA4-H). Though Fig. 2b and c suggested the chemical reactions between PLA and EBA-GMA, these FT-IR spectra could not tell if the compatibilization reactions were through the terminal OH or the terminal COOH group of PLA with the epoxy group of EBA-GMA. Commercial PLA is prepared by bulk polymerization of lactide in the presence of catalysts especially the stannous bis-2ehtylhexanoate (tin(II) octoate) [32]. It is now generally accepted that the polymerization of lactide under the catalysis of tin(II) octoate is through a coordinationeinsertion mechanism in which tin octoate facilitates the polymerization, but the hydroxyl or other nucleophilic species are the actual initiators [33,34]. This mechanism would favor the formation of hydroxyl terminated PLA chains. Based on above evidence, the interfacial compatibilization reactions are postulated in Scheme 1. Deconvolution results of the PLA and EBA-GMA peaks in the FT-IR spectra of extraction residues from the ternary blends containing ionomer or precursor are summarized in Table 3. The extent of the compatibilization reaction can be approximately described by the amount of grafted PLA per unit of EBA-GMA used, as represented by the ratio of the areas of absorption peaks at 1760 cm1 and 1734 cm1, i.e., APLA/AEBA-GMA. The blends containing EMAA15Zn showed higher values of APLA/AEBA-GMA compared with blends containing EMAA4-Zn, suggesting that the interfacial compatibilization effect was probably more pronounced in the former cases. Within each group of blends, the value of APLA/AEBA-GMA increased with the DN of the ionomer, indicating the catalytic effect of zinc ions in the ionomer on interfacial compatibilization of the blend. The SEM images of impact fracture surfaces of the ternary blends in the vicinity of the notch are shown in Fig. 3. All PLA ternary blends exhibited similar phase structures where the domains of impact modifiers were distributed uniformly in the PLA
Fig. 3. SEM images of room temperature impact fracture surfaces (adjacent to notch) of PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) blends: (a1) EMAA15-H, (b1) EMAA1525%Zn, (c1) EMAA15-60%Zn (magnification: 4000); (a2) EMAA4-H, (b2) EMAA4-24%Zn, (c2) EMAA4-38%Zn (magnification: 5000).
W. Song et al. / Polymer 53 (2012) 2476e2484
a 2.0
1.5
all the particles in the ternary blend were very well embedded in the matrix and a higher extent of plastic deformation was observed. Similar trend was found when EMAA4-H or EMAA4-Zn were used (Fig. 3b2 and c2) except that the matrix plastic deformation seemed to be less. From the direct observation of impact fracture surfaces as a function of DN of ionomer and the total functionality level (determined by the MAA content in the precursor), it was further evidenced that reactive interfacial compatibilization effect increased with DN and MAA content in the precursor as discussed earlier in this section.
blend w/ EMAA15-60%Zn EMAA15-48%Zn
tan δ
EMAA15-41%Zn
1.0
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EMAA15-25%Zn EMAA15-H -60
0.5
-50
-40
-30
-20
-10
3.3. Crosslinking of EBA-GMA
0
Temperature (°C)
0.0 -120 -100 -80 -60 -40 -20
0
20
40
60
80
Temperature (°C)
b 2.0
1.5
blend w/
EMAA4-51%Zn EMAA4-38%Zn
tan δ
EMAA4-31%Zn
1.0
EMAA4-24%Zn EMAA4-H
0.5
-60
-50
-40
-30
-20
-10
0
40
60
Temperature (°C)
0.0 -120 -100 -80 -60 -40 -20
0
20
80
Temperature (°C) Fig. 4. Dependence of damping factor (tan d) on temperature for various PLA/EBAGMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends: (a) blends with EMAA15-Zn (or EMAA15-H); (b) blends with EMAA4-Zn (or EMAA4-H) (Curves were shifted vertically for clarity.).
matrix. Debonding without perceptible plastic deformation was clearly observed in both of the blends containing ionomer precursors, i.e., EMAA15-H and EMAA4-H (Fig. 3a1 and a2). When EMAA15-25%Zn (Fig. 3b1) was used, wetting of the EBA-GMA droplets by PLA was enhanced and more plastic deformation was evident at the fracture surface. As the DN of EMAA-Zn ionomer further increased to the 60% (i.e., EMAA15-60%Zn) (Fig. 3c1), almost
Damping factors (tan d) obtained from DMA measurements for various PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) blends are shown in Fig. 4. The glass transition temperature, Tg, of the PLA matrix was around 70 C, as indicated by the tan d peak at higher temperature. All ternary blends in this study exhibited a second distinct damping peak around 30 C attributed to the Tg of the EBA-GMA phase [25]. The Tg of EBA-GMA in blends with ionomer precursors appeared higher than that in the blends with ionomers. Carboxylic acid is an active curing agent for epoxy resins. Therefore, crosslinking of EBA-GMA occurred to a certain degree during compounding [24]. A schematic illustration of the crosslinking reactions between EBA-GMA and ionomer (or precursor) is given in Scheme 2. The unneutralized precursor would lead to a higher degree of crosslinking of EBA-GMA than the partially neutralized ionomer because the former had more COOH groups available for the reactions, yielding the crosslinked EBA-GMA with a higher Tg. Similarly, at the same concentration in the blends, EMAA15-H led to a higher Tg than EMAA4-H (27.6 C vs. 31 C) for the EBA-GMA phase due to the higher COOH content in the former. With increase in the DN of the EMAA15-Zn ionomer, the Tg of the EBA-GMA phase in the resulting blends decreased significantly by about 5 C (from 27.6 to 32.4 C). This decrease in the Tg of the EBA-GMA phase was likely due to the decreased degree of crosslinking of EBA-GMA which was a consequence of the declined COOH content. In contrast, for the blends containing EMAA4-Zn (or EMAA4-H), only a slight change (from 31.1 to 32.6 C) in the Tg of the EBA-GMA phase was found with variation in DN. 3.4. Morphology and mechanical properties Fig. 5 shows the phase structure of the PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) ternary blends. ‘Salami’-like (i.e., domain-in-domain) structure was noted for the PLA ternary blends containing EMAA15-H and EMAA15-60%Zn. Our previous study indicated that due to the lower interfacial tension between PLA and EBA-GMA than that between PLA and EMAA-Zn (or EMAA-H), when the EBA-GMA/EMAA-Zn weight ratio was large than 1, the EMAA-Zn (or EMAA-H) formed the inclusions inside the EBA-GMA phase [24,25].
Scheme 2. Schematic crosslinking reaction of EBA-GMA with ionomer during melt compounding.
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Fig. 5. TEM images of PLA ternary blends with EMAA-Zn or EMAA-H: (a) EMAA15-H; (b) EMAA15-60%Zn; (c) EMAA4-H; (d) EMAA4-51%Zn.
Particle size analysis was performed based on the acquired TEM images. The weight average diameter (dw) and the polydispersity (dw/dn) of dispersed particles in the ternary blends are listed in Table 4. The dw decreased from 1.62 to 1.00 mm with increasing DN of EMAA15-Zn from 0 to 60%. Meanwhile, a clear decrease in the particle size polydispersity, i.e., the values of dw/dn, was also found. These results indicate that high DN favored the formation of fine and uniform dispersion for the blends containing EMAA15-Zn. This was probably in part due to the enhanced interfacial compatibilization between PLA and EBA-GMA under the catalysis of zinc ionomer, as discussed in above FT-IR and SEM results. In addition, crosslinking of EBA-GMA decreased with increasing DN due to the reduced availability of COOH, which would make it easier for the particles to break into smaller ones. For the blends containing EMAA4-Zn (or EMAA4-H), the particle size and distribution did not exhibit consistent trends with varying DN. This might be attributed the overall low contents of both zinc ions and free carboxylic acid groups which resulted in general weak crosslinking of EBA-GMA and interfacial compatibilization in the blends.
Fig. 6 shows the stressestrain curves of the neat PLA and two representative PLA ternary blends containing EMAA4-51%Zn and EMAA15-48%Zn, respectively, at strain rate of 0.2 inch/min and 2 inch/min. Neat PLA behaved strong and stiff but brittle, as reflected by the high tensile strength, modulus and low strain-atbreak shown in its stressestrain curves. The fracture behavior of the specimens changed greatly from brittle fracture of the neat PLA to ductile fracture of the PLA ternary blends. The blends displayed stable neck growth through cold drawing during tensile testing and finally broke at a significantly higher strain-at-break (w200%) under a slow extension rate (0.2 inch/min). The tensile strength and modulus of the blends decreased by 42e48% and 32e38%, respectively, with respect to that of the neat PLA. Increasing extension rate usually results in increase in tensile strength and reduction in elongation. Fig. 6 shows that when the extension rate was increased to 2 inch/min, the strength of blends only decreased slightly while the strain-at-break drastically decreased to w20% but was still significantly higher than that of neat PLA. Almost no change in the modulus was found for both neat PLA and PLA ternary
Table 4 Mechanical properties of PLA ternary blends and particle size and distribution of the dispersed phase. Samples Controls neat PLA PLA/ZnO (80/0.206, w/w) PLA/EBA-GMA (80/20, w/w) Ternary blends EMAA15-H EMAA15-25%Zn EMAA15-41%Zn EMAA15-48%Zn EMAA15-60%Zn EMAA4-H EMAA4-24%Zn EMAA4-31%Zn EMAA4-38%Zn EMAA4-51%Zn a
IS (J/m) 25 5 20 4 102 14 97 297 411 613 680 101 126 217 189 240
10 17 79 43 56 9 13 69 41 72
Strain-at-breaka (%) 4.4 0.5 4.3 0.2 5.2 0.5 27.3 24.6 25.8 21.8 23.5 20.0 19.6 20.5 22.1 15.8
3.0 4.8 4.0 2.4 4.0 3.1 1.5 2.0 2.4 2.4
Tensile strengtha (MPa)
Modulusa (GPa)
dw (mm)
dw/dn
68.3 0.7 64.6 1.8 40.4 0.4
3.35 0.14 3.32 0.16 2.26 0.03
e e 0.30
e e 1.37
1.62 1.56 1.22 1.33 1.00 1.27 0.93 1.61 1.18 1.23
2.43 2.23 2.06 1.87 1.81 1.87 1.81 2.08 1.80 1.75
35.8 37.0 38.4 38.5 37.4 38.5 38.9 37.9 37.6 39.3
0.5 0.1 0.2 0.4 0.4 0.3 0.2 0.1 0.3 0.4
Data were obtained from tensile tests that were conducted at an extension rate of 2 inch/min.
2.06 2.12 2.19 2.17 2.13 2.24 2.28 2.19 2.18 2.24
0.08 0.07 0.02 0.05 0.07 0.04 0.03 0.02 0.06 0.04
W. Song et al. / Polymer 53 (2012) 2476e2484
70
1, 2: neat PLA 3, 4: blend w/ EMAA 4-51% Zn 5, 6: blend w/ EMAA15-48% Zn
1
Stress (MPa)
60
2
50 40 30 20
3
4
5
6
10 0 0
5
10
15
20
150
200
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low, ranging from 1 to 3% (data not shown). Therefore, this small difference in crystallinity was not likely a significant factor responsible for the difference in the impact strength of the PLA ternary blends. It was demonstrated that increasing DN and MAA content in the precursor contributed to PLA toughening in the ternary blends. Based on the foresaid analysis on dual reactions, such toughness improvement as a function of DN in the ternary blends could be attributed to the following factors: (1) enhanced interfacial compatibilization, as suggested by the aforesaid FT-IR and SEM results; (2) a relatively low crosslinking level of EBAGMA (as indicated by DMA results), which could lower the resistance to cavitation inside the modifier particles [35,36]. When a certain level of neutralization was attained in the EMAA4-Zn, however, the improvement in IS seemed to be independent of the variation in DN. Such limited toughening effect might be attributed to the low total functionalities (i.e. MAA content in the precursor) in the ionomer.
Strain at break (%) 4. Conclusions Fig. 6. Tensile stressestrain curves of neat PLA and PLA/EBA-GMA/EMAA-Zn (80/15/5, w/w) ternary blends under speed of extension of 2 inch/min (solid line) and 0.2 inch/ min (dash line), respectively.
blends tested under different extension rates. Table 4 listed the data of tensile strength, modulus and strain-at-break of neat PLA and its ternary blends at extension rate of 2 inch/min. In contrast to its influence on IS, manipulating DN of ionomer did not exhibit as much effect as it did on tensile toughness (or ductility) of the blends. The notched Izod IS for the ternary blends as a function of DN of EMAA-Zn and MAA content in the precursor is presented in Fig. 7 and IS data are summarized in Table 4. As compared to either of the precursors (i.e. EMAA-H), the zinc ionomers resulted in higher IS of the ternary blends. Comparatively, the ternary blends with EMAA15-Zn universally exhibited superior IS than the ones with EMAA4-Zn. The dependence of IS on DN was somewhat different between these two groups of ternary blends. The IS of the ternary blend with EMAA15-Zn continuously increased with increasing DN of ionomer, while that of blends with EMAA4-Zn initially increased then leveled off in the DN range of 30e50%. DSC study indicated the crystallinity of the PLA matrix in all the ternary blends was very
The toughness of PLA ternary blends with epoxy-containing elastomer (EBA-GMA) and zinc ionomer (EMAA-Zn) greatly depends on the characteristics of the ionomer. Interfacial compatibilization between the PLA matrix and the dispersed EBA-GMA elastomer phase during reactive blending was greatly enhanced with increases in degree of neutralization and/or functionality of EMAA-Zn. Consequently, impact strength of the resulting PLA ternary blends increased with increase in degree of neutralization of ionomers. Similarly, EMAA-Zn ionomers of high functionality exhibited much more pronounced toughening effect than the ones of low functionality. These two factors also influenced the crosslinking of EBA-GMA but to a much lesser extent compared the effects on interfacial compatibilization reactions. The change of Tg of the EBA-GMA phase in the PLA ternary blends indicates that the final extent of crosslinking of EBA-GMA largely depended on the concentration of free carboxylic acid groups in the ionomers. Combination of a relatively low extent of crosslinking of the EBAGMA phase and a strong interfacial adhesion was found to favor the achievement of supertoughness. Morphological analysis revealed that a finer and more uniform particle size and distribution accompanied the increase in the impact strength of the supertoughened blends. Acknowledgement
800 The authors are grateful for the financial support from the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant no. 2007-3550417818.
blend w/ EMAA15-Zn (or -H) blend w/ EMAA 4-Zn (or -H) Impact strength (J/m)
600
References
400
200
0 0
10
20
30
40
50
60
Degree of neutralization (%) Fig. 7. Effects of degree of neutralization and functionality of ionomers on the IS of PLA/EBA-GMA/EMAA-Zn (or EMAA-H) (80/15/5, w/w) blends.
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