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Improvement in toughness of poly(l -lactide) (PLLA) through reactive blending with acrylonitrile–butadiene–styrene copolymer (ABS): Morphology and properties
European Polymer Journal 45 (2009) 738–746
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Improvement in toughness of poly(L-lactide) (PLLA) through reactive blending with acrylonitrile–butadiene–styrene copolymer (ABS): Morphology and properties Yongjin Li *, Hiroshi Shimizu Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 17 July 2008 Received in revised form 19 November 2008 Accepted 4 December 2008 Available online 16 December 2008
Keywords: PLLA ABS Reactive blending Impact strength Modulus
a b s t r a c t Poly(L-lactide) (PLLA) was melt-blended with acrylonitrile–butadiene–styrene copolymer (ABS) with the aim of enhancing impact strength and elongation at break of PLLA, but not sacrificing its modulus and stiffness significantly. However, PLLA and ABS were found to be thermodynamically immiscible by simply melt blending and the formed blends show deteriorated mechanical properties. The reactive styrene/acrylonitrile/glycidyl methacrylate copolymer (SAN-GMA) by incorporating with ethyltriphenyl phosphonium bromide (ETPB) as the catalyst was used as the in situ compatibilizer for PLLA/ABS blends to improve the compatibility between PLLA and ABS. The reactive process during melt blending was investigated by Fourier transformed infra-red (FTIR). It showed that the epoxide group of SAN-GMA reacted with PLLA end groups under the mixing conditions and that the addition of ETPB accelerated the reaction. Phase structure and physical properties of the compatibilized blends were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic mechanical analysis (DMA), tensile tests and impact property measurements. It was found that the size of ABS domains in PLLA matrix is significantly decreased by addition of the reactive compatibilizer. The dynamic mechanical analysis revealed markedly shifted glass transition temperatures for both PLLA and ABS, indicating the improved compatibility between PLLA and ABS. The mechanical tests showed the compatibilized PLLA/ABS blends had a very nice stiffness-toughness balance, i.e., the improved impact strength and the elongation at break with a slightly loss in the modulus. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Poly(L-lactide) (PLLA) has attracted increasing attention in recent years because it is produced from renewable recourses and is biodegradable. It has been widely used for biomedical applications such as sutures and drug delivery devices as a biodegradable and biocompatible polymer. On the other hand, PLLA has also become an alternative to traditional commodity plastics for everyday applications as an environmental friendly polymer due to its reasonable * Corresponding author. Tel.: +81 29 861 4197; fax: +81 29 861 6294. E-mail address: [email protected] (Y. Li). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.010
price and unique properties such as high strength, high stiffness, resistance to fats and oils [1,2]. Unfortunately, its toughness and heat distortion temperature are not satisfactory for the practical application. Blending PLLA with other polymers presents a versatile and economic method to obtain toughened products. Extensive studies have been carried on PLLA blends with other biodegradable soft polymers. Polycaprolactone (PCL) has been used to enhance impact strength of PLLA by many researchers [3–8]. The simply melt blending of PCL with PLLA results in an immiscible blend with little improvement of the toughness [3,4]. The addition of PLLA-b-PCL block copolymer can increase the elongation at break of the blends [5]. On the other
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hand, the reactive blending by addition of transesterification catalyst [6] or crosslinkers [7,8] in PLLA/PCL blends during melt mixing can greatly increase both the elongation at break and impact strength. Some other degradable aliphatic polyersters, such as poly(propylene carbonate) (PPC) [9], poly(butylene succinate) (PBSU) [10,11], poly(tetramethylene adipate-co-terephathalate) (PTAT) [12], poly(butylene adipate-co-terephthalate) (PBAT) [13], and polyurethane [14], have also been used to compound with PLLA for improving its impact strength by direct mechanical blending. On the other hand, blending PLLA with non-degradable polymers has also been reported. Nijenhuis et al. investigated the miscibility and mechanical properties of PLLA/poly(ethylene oxide) (PEO) blends [15]. They demonstrated that PLLA/PEO was a miscible blend and the elongation at break of the blend showed a strong increase with PEO concentration. Hillmyer et al. have succeeded in preparing PLLA/polyethylene (PE) blends with improved impact strength by adding the PLLA-b-PE block copolymer as compatibilizer [16,17]. Very recently, Piorkowska et al. reported the plasticization effects of poly(propylene glycol) (PPG) on PLLA [18]. In addition, poly(vinyl acetate) (PVAc) [19] and polyisoprene [20] have been compounded with PLLA to make toughened PLLA blends. Although these PLLA blend materials by compounding with rubbery polymers or low molecular additives have enhanced impact strength or elongation at break, almost all the blending was accompanied by a significant drop in the modulus and heat resistance. Acrylonitrile–butadiene–styrene copolymer (ABS) has a structure with a rubbery polybutadiene (PB) dispersed in a rigid styrene–acrylonitrile copolymer (SAN) matrix. It has been used as an important toughening agent for many engineering plastics. The use of ABS for impact modification of polycarbonate (PC) [21–23], nylon [24,25] and poly(butylene terephthalate) (PBT) [26–28] has been reported by a number of researchers with varied success. We considered that ABS is also a good candidate for toughening of PLLA. Although it was mentioned that ABS could improve the toughness of PLLA in a recent technical report by Nature works [29], no any detail property and morphological analysis were reported for this blend system up to now. It is expected that addition of ABS should increase the impact strength with little sacrificing of the modulus and heat resistance. In this paper, the miscibility and the properties of the PLLA/ABS blends with PLLA as the matrix by adding of the in situ compatibilizer have been investigated. It is shown that the simply blending of PLLA and ABS leads to an incompatible blend with deteriorating properties, but the addition of small amount of reactive compatibilizer in the blends results in a finer morphology and the obtained blends exhibit improved mechanical performance.
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included 1.2% of D-lactide content. The ABS was kindly provided by Grand Pacific Petrochemical Corporation. The ABS used is a high impact grade with the trade name of D-100. SAN-GMA is the copolymer of styrene, acrylonitrile, and glycidyl methacrylate (GMA). The weight ratio of [styrene]/[acrylonitrile]/[glycidyl methacrylate] in the copolymer was 65/25/10. The ethyltriphenyl phosphonium bromide (ETPB) was bought from Aldrich and was used as recieved. The blends were prepared using a Laboplasto-mill (KF70V, Toyoseiki Co. Ltd., Japan) with a twin screw at a rotation speed of 100 rpm at 200 oC for 6 min. After blending, all the samples were hot-pressed at 200 oC to a sheet with a thickness of 500 lm under the pressure of 10 MPa for 5 min, followed by quenching in ice water. 2.2. Structural characterization Morphology of the blends was observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). A Philips XL-20 SEM was used for SEM measurements at an accelerating voltage of 10 kV. All the samples were fractured after immersion in liquid nitrogen for about 10 min. The fracture surface was then coated with a thin layer of gold. TEM was carried out using a Hitachi H7000 at an acceleration voltage of 75 kV. The blend samples were ultramicrotomed at 120 oC to a section with a thickness of about 70 nm. The sections were then stained with RuO4 for 20 min. Dynamic mechanical analysis (DMA) was carried out with a Rheovibrion DDV-25FP (Orientec Corp.) in the tensile mode. All the measurements were performed in the linear region with the strain of 0.03%. Dynamic loss (tan d) was determined at a frequency of 1 Hz and a heating rate of 3 oC/min as a function of temperature from 150 oC to 175 oC. Differential scanning calorimetry (DSC) was carried out under nitrogen flow at a heating rate of 10 K/min with a Perkin-Elmer DSC-7 differential scanning calorimeter calibrated with the melting temperatures of indium and zinc. The rheological properties of PLLA/ABS blends were measured using a Rheometrics ARES strain controlled rheometer equipped with parallel plates. The geometry of the plates has a diameter of 25 mm and a gap of about 1.5 mm. Frequency sweep experiments were performed at 200 °C under nitrogen atmosphere. The frequency was ranged from 0.1 to 300 rad/s in an oscillatory shear at 10% strain. Fourier transformed infra-red (FTIR) spectra were measured with a Digilab FTS 6000ec spectrophotometer using transmission mode. The sample for FTIR measurements was prepared by direct hot press of a melt-blended sample. The thickness is about 20–40 lm.
2. Experimental
2.3. Physical property measurements
2.1. Materials and sample preparation
Tensile tests were carried out according to the JIS K7113 test method using dumb-bell-shaped samples punched out from the molded sheets. The tests were performed using a tensile testing machine, Tensilon UMT-300 (Orientec Co.,
The PLLA sample used was commercially available with the molecular weight of Mw = 170,000. The sample
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Intensity
740
a b c d 1000 950
900
850
800
750
700
650 -1
600
550
500
450
wavenumber (cm ) Fig. 2. FTIR spectra of PLLA/SAN-GMA = 80/20 (a) after 2 min meltmixing without ETPB, (b) after 6 min melt-mixing without ETPB, (c) after 2 min with ETPB, and (d) after 6 min with ETPB.
3. Results and discussion 3.1. The reaction between PLLA with SAN-GMA
Fig. 1. SEM images of uncompatibilized (a) PLLA/ABS = 70/30, and (b) PLLA/ABS = 50/50.
Ltd.), at a crosshead speed of 10 mm/min at 20 oC and 50% relative humidity. At least five specimens were tested for each sample. Film impact tests were carried out according to JIS K7160 procedures using a standard impact tester (Toyoseiki Co. Ltd, Japan) at 20 oC and 50% relative humidity. At least five specimens were tested for each sample to get an average value.
P
C2H5 Br
Fig. 1 shows the SEM images of PLLA/ABS blends without addition of any compatibilizers. For the PLLA/ ABS = 70/30 blend, ABS forms domain dispersed in the PLLA matrix. The ABS domain size ranges from 1 to 10 lm. It is seen that the interface between the two phases is very weak, indicating a totally incompatible polymer blend. For the PLLA/ABS = 50/50 blend, ABS domains tend to connect each other and shape of the phase becomes irregular. The detail observation shows that ABS is still the domains dispersed in PLLA matrix, which can be attributed to the higher ABS viscosity than PLLA even with the same content of the two components. It is generally accepted that phase size and interface between blend components take the key role in determining
(C6H5)3P: + C2H5 Br
(1)
CH2-CH-SAN (C6H5)3P: + CH2 CH SAN O (C6H5)3P O CH2-CH-SAN + HOOC-PLLA (C6H5)3P O
(2)
(C6H5)3P: + PLLA-C-O-CH2-CH-SAN O
OH (3)
CH2-CH-SAN + HO-PLLA (C6H5)3P O
(C6H5)3P: + PLLA-O-CH2-CH-SAN OH (4)
Fig. 3. Scheme for the reaction of PLLA end groups with SAN-GMA under the catalyst of ETPB.
Y. Li, H. Shimizu / European Polymer Journal 45 (2009) 738–746
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Fig. 4. TEM images of (a) uncompatibilized PLLA/ABS = 70/30 blend, (b) PLLA/ABS = 70/30 blend with 5 wt% SAN-GMA, and (c) PLLA/ABS = 70/30 blend with 5 wt% SAN-GMA and 0.02 phr (per hundred parts of resin) ETPB.
the mechanical performance of a polymer blend. To decrease the ABS domain size and strengthen the interface between PLLA and ABS, SAN-GMA has been used as the reactive compatibilizer by incorporation of ETPB as the catalyst. It is considered that the epoxide groups can react either with –COOH or with –OH of the PLLA end groups under the catalyst to form a grafting copolymer that can function as the compatibilizer. Change et al. investigated PBT/ ABS/SAG (styrene–acrylonitrile–glycidyl methacrylate) blends in the presence of ETPB [28]. It was found that the addition of ETPB could not only promote the grafting reac-
tion but also improve the mechanical properties of the final blends. In order to verify the reaction between PLLA and SAN-GMA under the melt-mixing conditions, the blending of PLLA/SAN-GMA = 80/20 was carried out with and without ETPB. The melt-blended samples were taken out and hot-pressed into a thin film after 2 min and 6 min mixing. The films were characterized by FTIR directly. Fig. 2 shows the FTIR spectra with the wave number in the region of epoxy absorption. It is found that the intensity of the expoxy characteristic peak at 911 cm 1 decreases with prolonging the mixing time, suggesting the
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Fig. 5. TEM images of (a) uncompatibilized PLLA/ABS = 50/50 blend, (b) PLLA/ABS = 50/50 blend with 5 wt% SAN-GMA, and (c) PLLA/ABS = 50/50 blend with 5 wt% SAN-GMA and 0.02 phr ETPB.
reactions of expoxide groups. Moreover, the addition of ETPB accelerates the reaction rate as evidenced by the markedly decreased peak intensity at the same mixing time as compared with that having no ETPB. Oyama et al. have discussed the reaction mechanism of –COOH and – OH groups with epoxide groups under catalysts [30]. It is considered that ETPB dissociates easily under the mixing conditions and carries out a nucleophilic attack, which opens the epoxy ring in SAN-GMA to form an intermediate product. This product has high reactivity with –COOH or –
OH of PLLA end groups and a PLLA-g-SAN copolymer formed. It is expected that the formed copolymer can act as a good compatibilizer for PLLA/ABS blends. The reaction scheme for SAN-GMA with PLLA under the ETPB is shown in Fig. 3. 3.2. Morphology of the compatibilized PLLA/ABS blends Figs. 4 and 5 show TEM images of PLLA/ABS blends containing 50% ABS and 30% ABS, respectively. In the
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Y. Li, H. Shimizu / European Polymer Journal 45 (2009) 738–746 3.5 3.0 2.5
b
a
Heat Flow
Tan (delta)
2.0 1.5 1.0
c b
0.5 0.0
+0.2 +0 -0.2 -0.4
-0.5
-0.7
-100
-75
-50
-25
a
c d e 0
25
50
o
75
100
125
150
175
25
50
75
Temperature ( C)
100
125
150
175
200
Temperature (ºC)
Fig. 6. Dynamic loss for the PLLA/ABS blends as function of temperature. (a) Neat PLLA, (b) neat ABS, (c) PLLA/ABS = 50/50, (d) PLLA/ABS/SANGMA = 50/50/5, and (e) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/0.02 phr.
Fig. 9. DSC heating curves of PLLA/ABS blends with a heating rate of 10 oC/min (a) PLLA/ABS = 50/50, (b) PLLA/ABS/SAN-GMA = 50/50/5, and (c) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/0.02.
1.0 0.9 0.8 0.7 0.5
Heat Flow
Tan (delta)
0.6 0.4 0.3
a
+0.2
b
+0
c
-0.2
0.2 0.1 0.0
b
-0.1 -0.2 -0.3 -100
c
-75
-50
-25
0
a 25
50
75
100
125
150
175
o
Fig. 7. Dynamic loss for the PLLA/ABS blends as function of temperature. (a) PLLA/ABS = 70/30, (b) PLLA/ABS/SAN-GMA = 70/30/5, and (c) PLLA/ ABS/SAN-GMA/ETPB = 70/30/5/0.02 phr.
3
Modulus (MPa)
10
e d c
2
10
a
b
1
10
-150 -125 -100 -75 -50 -25
0
25
50
25
50
75
100
125
150
175
200
Temperature (ºC)
Temperature ( C)
75 100 125 150 175
Temperature (ºC) Fig. 8. Storage modulus of the PLLA/ABS blends as function of temperature. (a) Neat PLLA, (b) neat ABS, (c) PLLA/ABS = 50/50, (d) PLLA/ ABS/SAN-GMA = 50/50/5, and (e) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/ 0.02 phr.
Fig. 10. DSC heating curves of PLLA/ABS blends with a heating rate of 10 oC/min (a) PLLA/ABS = 70/30, (b) PLLA/ABS/SAN-GMA = 70/30/5, and (c) PLLA/ABS/SAN-GMA/ETPB = 70/30/5/0.02.
figures, ABS was observed as the dark phase because ABS is more readily stained than PLLA by RuO4. Furthermore, ABS shows very clear core-shell structure in the TEM images and the core polybutadiene is the most easily stained due to the double bonds in the chain. It is seen that ABS was dispersed in PLLA matrix with the domain size ranging from 1 to 10 lm for the uncompatibilized blends (Figs. 4a and 5). The domain size distribution is also very large due to the immiscibility between the components. The addition of 5% SAN-GMA copolymers results in a decreased ABS domain size and a narrower size distribution, shown in Figs. 4b and 5b. Moreover, the addition of ETPB catalyst can further improve the distribution of the rubber particles as evidenced by significantly increased rubber particles in Figs. 4c and 5c. The morphological analysis indicates that SAN-GMA takes an effective role in compatibilizing PLLA/ABS blends by incorporating with ETPB.
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Table 1 Thermal properties of PLLA/ABS blends. * PLLA
Samples
Tg
PLLA ABS PLLA/ABS = 50/50 PLLA/ABS/SANMGA = 50/50/5 PLLA/ABS/SANMGA/ETPB = 50/50/5/0.02 phr PLLA/ABS = 70/30 PLLA/ABS/SANGMA = 70/30/5 PLLA/ABS/SANMGA/ETPB = 70/30/5/0.02 phr
60.5 – 60.5 63.1 64.2 61.5 63.1 66.7
*
(°C)
Tg
* SAN
(°C)
– 115.5 115.2 113.4 111.4 115.5 112.9 110.8
Tm (°C)
DHc (J/g)
DHm (J/g)
163.5 – 167.4 167.1 166.3 167.7 167.1 166.0
–24.5 – –12.0 –10.7 –9.8 –17.3 –16.7 –14.1
25.1 – 18.6 17.1 16.8 23.5 21.8 20.8
From the DMA measurement.
Table 2 Mechanical properties of PLA/PU blends. Samples
Storage modulus* (Mpa)
Static modulus (Mpa)
Tensile strength (Mpa)
Elongation at break (%)
Impact strength (KJ/m2)
PLLA ABS PLLA/ABS = 50/50 PLLA/ABS/SANMGA = 50/50/5 PLLA/ABS/SANMGA/ETPB = 50/50/ 5/0.02 phr PLLA/ABS = 70/30 PLLA/ABS/SANGMA = 70/30/5 PLLA/ABS/SANMGA/ETPB = 70/30/ 5/0.02 phr
2805 1556 2105 2110 2151
2024 807 1294 1327 1358
65.5 32.9 38.8 42.9 43.6
4.0 122 3.5 18.0 23.5
69.7 425 48.3 119.7 162.8
2548 2550 2532
1554 1405 1352
46.7 43.6 44.6
3.1 20.5 23.8
63.8 81.1 123.9
*
At 20 °C from the DMA measurement.
3.3. Thermal behavior of the compatibilized PLLA/ABS blends Figs. 6 and 7 demonstrate dynamic loss curves for PLLA, ABS, and PLLA/ABS blends with and without addition of compatibilizer. Neat PLLA shows a glass transition temperature (Tg) at 60.5 oC and ABS gives a very strong relaxation peak at 115.5 oC corresponding to the Tg of SAN phase. On the other hand, the peak at 83 oC is the Tg relaxation of PB phase in ABS. The uncompatibilized PLLA/ABS blends show almost same Tgs corresponding to the each component, indicating the thermodynamical immiscibility between PLLA and ABS. In contrast, the addition of SAN-GMA clearly changes the relaxation peak positions of both PLLA and ABS. The Tgs of PLLA and SAN shift toward each other, suggesting the improved compatibilities. The compatibilized effects of SAN-GMA are more significant by incorporation of ETPB as the catalyst, as shown by the larger Tg shift for both PLLA and SAN. The Tg of SAN decreases from 115.5 to 111.4 oC, while the Tg of PLLA increases from 60.5 to 64.2 oC. These DMA results are consistent with those by TEM. Note that the peak at about 92 oC originates from the cold crystallization of PLLA during heating where storage modulus starts to increase. Fig. 8 demonstrates the typical storage modulus curves as a function of temperature for PLLA, ABS, and PLLA/ ABS = 50/50 blends. PLLA has higher modulus than ABS at the temperature region less than 60 oC. The storage modulus of the blends is lower than that of neat PLLA, but the value is about 2100 MPa at room temperature. This means that the blends exhibit the relative high modulus and high stiffness by blending with ABS. Note that the storage modulus of all PLLA blends increases in the
temperature range of 80–100 oC due to the cold crystallization of PLLA [14]. Figs. 9 and 10 show the DSC heating curves of PLLA/ABS blends with a heating speed of 10 oC/min. All the curves show three apparent transitions upon heating, namely, a PLLA glass transition, a cold crystallization exotherm peak, and a melting endotherm peak (Table 1), while Tg of SAN phase is not clear from the DSC curves. Tg of PLLA increases with adding the compatibilizers and the catalyst enhances the shift. These results are consistent with the DMA results. However, the Tg value obtained from DSC is lower than the value obtained from DMA, which can be attributed to the different measuring mechanisms for DMA and DSC [31]. The heat of cold crystallization (DHc) and the heat of melting (DHm) decrease upon the addition of compatibilizer in the blends (Table 2), suggesting that PLLA in the compatibilized blends has lower crystallinity. The results can be attributed to the molecular interactions at the interface between PLA and amorphous ABS. Furthermore; the melting temperature of PLA in the compatibilized blends is also lower than that for the uncompatibilized one, which again indicates the improved compatibility by the reactive blending. 3.4. Melt viscosity of the compatibilized PLLA/ABS blends Fig. 11 shows the dynamic viscosity of PLLA/ABS = 50/50 blends with and without compatibilizer as a function of frequency. The viscosity decreased with an increasing shear rate, indication that the PLLA/ABS blends exhibit shear-thinning behavior. At a given shear rate, the compatibilized blends show higher viscosity in the whole
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2
4
10
Viscosity (PaS)
Impact strength (KJ/m )
160
c b
3
10
a
140 120 100 80 60
2
10
-1
0
10
1
10
40
2
10
10
PLLA
Fig. 11. Dynamic viscosity of (a) PLLA/ABS = 50/50, (b) PLLA/ABS/SANGMA = 50/50/5, and (c) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/0.02.
a
60 55
Stress (MPa)
50 45 40
b
35
c
d
30
50/50/5/0 50/50/5/0.02
Fig. 14. Film impact strength of neat PLLA and PLLA/ABS blends.
3.5. Mechanical properties of the compatibilized PLLA/ABS blends
e
25 20 15 10 5 0 0
10
20
30
40
Strain (%) Fig. 12. Tensile strain–stress curves for (a) neat PLLA, (b) PLLA/ABS = 50/ 50, (c) PLLA/ABS/SAN-GMA = 50/50/5, (d) PLLA/ABS/SAN-GMA/ETPB = 50/ 50/5/0.02,and (e) neat ABS.
50
a
40
Stress (MPa)
70/30/5/0 70/30/5/0.02 50/50/0/0
the compatibilized blends can be attributed to several reasons, such as the enhancing of the dispersion promoted by the compatibilizer, the increase of the total apparent volume of the dispersed phase due to the compatibilizer layer attached to the drop surfaces, the increase of interaction among the droplets and the stiffening of the interface [32].
70 65
70/30/0/0
PLLA/ABS/SAN-GMA/ETPB
Frequency (rad/s)
c
b 30
20
10
0
0
5
10
15
20
25
30
Strain (%) Fig. 13. Tensile strain–stress curves for (a) PLLA/ABS = 70/30, (b) PLLA/ABS/ SAN-GMA = 70/30/5, and (c). PLLA/ABS/SAN-GMA/ETPB=70/30/5/0.02.
frequency range investigated here as compared with the uncompatibilized blends. The increase of the viscosity of
Tensile stress–strain curves of neat PLLA, neat ABS, and PLLA/ABS blends are shown in Figs. 12 and 13. The main tensile properties such as static modulus, tensile stress, and elongation at break determined from these curves are presented in Table 2. PLLA is very rigid and shows pretty high tensile strength, but it breaks at the elongation at break of about 4%. The uncompatibilized blends are very brittle and break at even lower elongation at break due to the big domain size and very weak interface. It is obvious that the compatibilizer can improve the tensile properties significantly because the compatibilized blends undergo a clear yielding upon stretching and break at the elongation at break at 20–30%. The samples by incorporated with catalyst EBTP show almost same modulus and tensile strength as those without adding catalyst, but higher elongation at break. The film impact strength of neat PLLA and the PLLA/ABS blends is shown in Fig. 14. The uncompatibilized blends show even lower impact strength as compared with the neat PLLA. However, significant improvement can be observed with the addition of the SAN-GMA compatibilizer. The impact strength for the compatibilized PLLA/ ABS = 70/30 blend is 124 KJ/m2, which is about 100% higher than the value for neat PLLA. The modification effects of the compatibilizer are more significant for the PLLA/ ABS = 50/50 blends. The compatibilized blend gives the impact strength of 162.8 KJ/m2, as compared with the uncompatilized one of 48.3 KJ/m2. It should be noted that the modified PLLA maintains quite high tensile modulus and tensile strength with the obviously increased impact strength, which means that the obtained material having a good stiffness–toughness
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balance. Furthermore, it can be expected that the blends exhibit increased heat resistance as compared with neat PLLA due to the increased glass transition temperature, as discussed in the DMA results. 4. Conclusions Uncompatibilized blends of PLLA and ABS have a morphology with big phase size and weak interface. The blends exhibit poor mechanical properties with low elongation at break and decreased impact strength. SAN-GMA was found to be an effective reactive compatibilizer for PLLA/ABS blend by the presence of ETPB as the catalyst, as revealed by the significant improvement in the dispersion of rubber particles as well as the clearly shifted glass transition temperature of both PLLA and ABS. Compatibilized PLLA/ABS blends exhibit improved impact strength and elongation at break with a slight loss in the modulus and tensile strength. References [1] Drumright RE, Gruber RE, Henton DE. Adv Mater 2000;12:1841–6. [2] Vink ETH, Rabago R, Classner DA, Springs B, Oconnor RP, Kostad J, et al. Macromol Biosci 2004;4:551–64. [3] Tsuji H, Ikada Y. J Appl Polym Sci 1996;60:2367–75. [4] Broz ME, Vanderhart DL, Wahburn NR. Biomaterials 2003;24: 4181–90. [5] Hiljanen-vainio M, Varpomaa P, Seppala J, Tormala P. Macromol Chem Phys 1996;197:1503–23.
[6] Wang L, Ma W, Gross RA, McCarthy SP. Polym Degrad Stab 1998;59:161–8. [7] Harada M, Lida K, Okamoto K, Hayashi H, Hirano K. Polym Eng Sci 2008;48:1359–68. [8] Semba T, Kitagawa K, Ishiaku US, Hamada H. J Appl Polym Sci 2006;101:1825–61. [9] Ma X, Yu J, Wang N. J Polym Sci Polym Phys 2006;44:94–101. [10] Shibata M, Inoue Y, Miyoshi M. Polymer 2006;47:3557–64. [11] Chen GX, Kim HS, Kim ES, Yoon JS. Polymer 2005;46:11829–36. [12] Jiang L, Wolcott MP, Zhang J. Biomacromolecules 2006;7:199–207. [13] Liu TY, Lin WC, Yang MC, Chen SY. Polymer 2005;46:12586–94. [14] Li YJ, Shimizu H. Macromol Biosci 2007;7:921–8. [15] Nijenhuis AJ, Colstee E, Grijpma DW, Pennings AJ. Polymer 1996;37: 5849–57. [16] Anderson KS, Lim SH, Hillmer MA. J Appl Polym Sci 2003;89: 3757–68. [17] Anderson KS, Hillmer MA. Polymer 2004;45:8809–23. [18] Piorkowska E, Kulinski Z, Galeski A, Masirek R. Polymer 2006;47: 7178–88. [19] Kim KS, Chin IJ, Yoon JS, Choi HJ, Lee DC, LEE KH. J Appl Polym Sci 2001;82:3618–26. [20] Jin HJ, Chin IJ, Kim MN, Kim SH, Yoon JS. Eur Polym J 2000;36:165–9. [21] Inberg JPF, Gaymans RJ. Polymer 2002;43:4197–205. [22] Wildes G, Keskkula H, Paul DR. Polymer 1999;40:7089–107. [23] Hamada H, Tsunasawa H. J Appl Polym Sci 1996;60:353–62. [24] Kitayama N, Keskkula H, Paul DR. Polymer 2000;41:8041–52. [25] Gao G, Wang JY, Yin JH, Yu XQ, Ma RT, Tang XY, et al. J Appl Polym Sci 1999;72:683–8. [26] Hale W, Keskkula H, Paul DR. Polymer 1999;40:365–77. [27] Loyens W, Groeninckx G. Macromol Chem Phys 2002;203:1702–14. [28] Lee PC, Kuo WF, Chang FC. Polymer 1994;35:5641–50. [29] Available from: http://www.natureworkspla.com. [30] Oyama HT, Kitagawa T, Ougizawa T, Inoue T, Weber M. Polymer 2004;45:1033–43. [31] Hatakeyama T. Thermal analysis: fundamentals and applications to polymer science. New York: John Wiley & Sons; 1994. [32] Sailer C, Handge UA. Macromolecules 2007;40:2019–28.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Improvement in toughness of poly(L-lactide) (PLLA) through reactive blending with acrylonitrile–butadiene–styrene copolymer (ABS): Morphology and properties Yongjin Li *, Hiroshi Shimizu Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 17 July 2008 Received in revised form 19 November 2008 Accepted 4 December 2008 Available online 16 December 2008
Keywords: PLLA ABS Reactive blending Impact strength Modulus
a b s t r a c t Poly(L-lactide) (PLLA) was melt-blended with acrylonitrile–butadiene–styrene copolymer (ABS) with the aim of enhancing impact strength and elongation at break of PLLA, but not sacrificing its modulus and stiffness significantly. However, PLLA and ABS were found to be thermodynamically immiscible by simply melt blending and the formed blends show deteriorated mechanical properties. The reactive styrene/acrylonitrile/glycidyl methacrylate copolymer (SAN-GMA) by incorporating with ethyltriphenyl phosphonium bromide (ETPB) as the catalyst was used as the in situ compatibilizer for PLLA/ABS blends to improve the compatibility between PLLA and ABS. The reactive process during melt blending was investigated by Fourier transformed infra-red (FTIR). It showed that the epoxide group of SAN-GMA reacted with PLLA end groups under the mixing conditions and that the addition of ETPB accelerated the reaction. Phase structure and physical properties of the compatibilized blends were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic mechanical analysis (DMA), tensile tests and impact property measurements. It was found that the size of ABS domains in PLLA matrix is significantly decreased by addition of the reactive compatibilizer. The dynamic mechanical analysis revealed markedly shifted glass transition temperatures for both PLLA and ABS, indicating the improved compatibility between PLLA and ABS. The mechanical tests showed the compatibilized PLLA/ABS blends had a very nice stiffness-toughness balance, i.e., the improved impact strength and the elongation at break with a slightly loss in the modulus. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Poly(L-lactide) (PLLA) has attracted increasing attention in recent years because it is produced from renewable recourses and is biodegradable. It has been widely used for biomedical applications such as sutures and drug delivery devices as a biodegradable and biocompatible polymer. On the other hand, PLLA has also become an alternative to traditional commodity plastics for everyday applications as an environmental friendly polymer due to its reasonable * Corresponding author. Tel.: +81 29 861 4197; fax: +81 29 861 6294. E-mail address: [email protected] (Y. Li). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.12.010
price and unique properties such as high strength, high stiffness, resistance to fats and oils [1,2]. Unfortunately, its toughness and heat distortion temperature are not satisfactory for the practical application. Blending PLLA with other polymers presents a versatile and economic method to obtain toughened products. Extensive studies have been carried on PLLA blends with other biodegradable soft polymers. Polycaprolactone (PCL) has been used to enhance impact strength of PLLA by many researchers [3–8]. The simply melt blending of PCL with PLLA results in an immiscible blend with little improvement of the toughness [3,4]. The addition of PLLA-b-PCL block copolymer can increase the elongation at break of the blends [5]. On the other
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hand, the reactive blending by addition of transesterification catalyst [6] or crosslinkers [7,8] in PLLA/PCL blends during melt mixing can greatly increase both the elongation at break and impact strength. Some other degradable aliphatic polyersters, such as poly(propylene carbonate) (PPC) [9], poly(butylene succinate) (PBSU) [10,11], poly(tetramethylene adipate-co-terephathalate) (PTAT) [12], poly(butylene adipate-co-terephthalate) (PBAT) [13], and polyurethane [14], have also been used to compound with PLLA for improving its impact strength by direct mechanical blending. On the other hand, blending PLLA with non-degradable polymers has also been reported. Nijenhuis et al. investigated the miscibility and mechanical properties of PLLA/poly(ethylene oxide) (PEO) blends [15]. They demonstrated that PLLA/PEO was a miscible blend and the elongation at break of the blend showed a strong increase with PEO concentration. Hillmyer et al. have succeeded in preparing PLLA/polyethylene (PE) blends with improved impact strength by adding the PLLA-b-PE block copolymer as compatibilizer [16,17]. Very recently, Piorkowska et al. reported the plasticization effects of poly(propylene glycol) (PPG) on PLLA [18]. In addition, poly(vinyl acetate) (PVAc) [19] and polyisoprene [20] have been compounded with PLLA to make toughened PLLA blends. Although these PLLA blend materials by compounding with rubbery polymers or low molecular additives have enhanced impact strength or elongation at break, almost all the blending was accompanied by a significant drop in the modulus and heat resistance. Acrylonitrile–butadiene–styrene copolymer (ABS) has a structure with a rubbery polybutadiene (PB) dispersed in a rigid styrene–acrylonitrile copolymer (SAN) matrix. It has been used as an important toughening agent for many engineering plastics. The use of ABS for impact modification of polycarbonate (PC) [21–23], nylon [24,25] and poly(butylene terephthalate) (PBT) [26–28] has been reported by a number of researchers with varied success. We considered that ABS is also a good candidate for toughening of PLLA. Although it was mentioned that ABS could improve the toughness of PLLA in a recent technical report by Nature works [29], no any detail property and morphological analysis were reported for this blend system up to now. It is expected that addition of ABS should increase the impact strength with little sacrificing of the modulus and heat resistance. In this paper, the miscibility and the properties of the PLLA/ABS blends with PLLA as the matrix by adding of the in situ compatibilizer have been investigated. It is shown that the simply blending of PLLA and ABS leads to an incompatible blend with deteriorating properties, but the addition of small amount of reactive compatibilizer in the blends results in a finer morphology and the obtained blends exhibit improved mechanical performance.
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included 1.2% of D-lactide content. The ABS was kindly provided by Grand Pacific Petrochemical Corporation. The ABS used is a high impact grade with the trade name of D-100. SAN-GMA is the copolymer of styrene, acrylonitrile, and glycidyl methacrylate (GMA). The weight ratio of [styrene]/[acrylonitrile]/[glycidyl methacrylate] in the copolymer was 65/25/10. The ethyltriphenyl phosphonium bromide (ETPB) was bought from Aldrich and was used as recieved. The blends were prepared using a Laboplasto-mill (KF70V, Toyoseiki Co. Ltd., Japan) with a twin screw at a rotation speed of 100 rpm at 200 oC for 6 min. After blending, all the samples were hot-pressed at 200 oC to a sheet with a thickness of 500 lm under the pressure of 10 MPa for 5 min, followed by quenching in ice water. 2.2. Structural characterization Morphology of the blends was observed by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). A Philips XL-20 SEM was used for SEM measurements at an accelerating voltage of 10 kV. All the samples were fractured after immersion in liquid nitrogen for about 10 min. The fracture surface was then coated with a thin layer of gold. TEM was carried out using a Hitachi H7000 at an acceleration voltage of 75 kV. The blend samples were ultramicrotomed at 120 oC to a section with a thickness of about 70 nm. The sections were then stained with RuO4 for 20 min. Dynamic mechanical analysis (DMA) was carried out with a Rheovibrion DDV-25FP (Orientec Corp.) in the tensile mode. All the measurements were performed in the linear region with the strain of 0.03%. Dynamic loss (tan d) was determined at a frequency of 1 Hz and a heating rate of 3 oC/min as a function of temperature from 150 oC to 175 oC. Differential scanning calorimetry (DSC) was carried out under nitrogen flow at a heating rate of 10 K/min with a Perkin-Elmer DSC-7 differential scanning calorimeter calibrated with the melting temperatures of indium and zinc. The rheological properties of PLLA/ABS blends were measured using a Rheometrics ARES strain controlled rheometer equipped with parallel plates. The geometry of the plates has a diameter of 25 mm and a gap of about 1.5 mm. Frequency sweep experiments were performed at 200 °C under nitrogen atmosphere. The frequency was ranged from 0.1 to 300 rad/s in an oscillatory shear at 10% strain. Fourier transformed infra-red (FTIR) spectra were measured with a Digilab FTS 6000ec spectrophotometer using transmission mode. The sample for FTIR measurements was prepared by direct hot press of a melt-blended sample. The thickness is about 20–40 lm.
2. Experimental
2.3. Physical property measurements
2.1. Materials and sample preparation
Tensile tests were carried out according to the JIS K7113 test method using dumb-bell-shaped samples punched out from the molded sheets. The tests were performed using a tensile testing machine, Tensilon UMT-300 (Orientec Co.,
The PLLA sample used was commercially available with the molecular weight of Mw = 170,000. The sample
Y. Li, H. Shimizu / European Polymer Journal 45 (2009) 738–746
Intensity
740
a b c d 1000 950
900
850
800
750
700
650 -1
600
550
500
450
wavenumber (cm ) Fig. 2. FTIR spectra of PLLA/SAN-GMA = 80/20 (a) after 2 min meltmixing without ETPB, (b) after 6 min melt-mixing without ETPB, (c) after 2 min with ETPB, and (d) after 6 min with ETPB.
3. Results and discussion 3.1. The reaction between PLLA with SAN-GMA
Fig. 1. SEM images of uncompatibilized (a) PLLA/ABS = 70/30, and (b) PLLA/ABS = 50/50.
Ltd.), at a crosshead speed of 10 mm/min at 20 oC and 50% relative humidity. At least five specimens were tested for each sample. Film impact tests were carried out according to JIS K7160 procedures using a standard impact tester (Toyoseiki Co. Ltd, Japan) at 20 oC and 50% relative humidity. At least five specimens were tested for each sample to get an average value.
P
C2H5 Br
Fig. 1 shows the SEM images of PLLA/ABS blends without addition of any compatibilizers. For the PLLA/ ABS = 70/30 blend, ABS forms domain dispersed in the PLLA matrix. The ABS domain size ranges from 1 to 10 lm. It is seen that the interface between the two phases is very weak, indicating a totally incompatible polymer blend. For the PLLA/ABS = 50/50 blend, ABS domains tend to connect each other and shape of the phase becomes irregular. The detail observation shows that ABS is still the domains dispersed in PLLA matrix, which can be attributed to the higher ABS viscosity than PLLA even with the same content of the two components. It is generally accepted that phase size and interface between blend components take the key role in determining
(C6H5)3P: + C2H5 Br
(1)
CH2-CH-SAN (C6H5)3P: + CH2 CH SAN O (C6H5)3P O CH2-CH-SAN + HOOC-PLLA (C6H5)3P O
(2)
(C6H5)3P: + PLLA-C-O-CH2-CH-SAN O
OH (3)
CH2-CH-SAN + HO-PLLA (C6H5)3P O
(C6H5)3P: + PLLA-O-CH2-CH-SAN OH (4)
Fig. 3. Scheme for the reaction of PLLA end groups with SAN-GMA under the catalyst of ETPB.
Y. Li, H. Shimizu / European Polymer Journal 45 (2009) 738–746
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Fig. 4. TEM images of (a) uncompatibilized PLLA/ABS = 70/30 blend, (b) PLLA/ABS = 70/30 blend with 5 wt% SAN-GMA, and (c) PLLA/ABS = 70/30 blend with 5 wt% SAN-GMA and 0.02 phr (per hundred parts of resin) ETPB.
the mechanical performance of a polymer blend. To decrease the ABS domain size and strengthen the interface between PLLA and ABS, SAN-GMA has been used as the reactive compatibilizer by incorporation of ETPB as the catalyst. It is considered that the epoxide groups can react either with –COOH or with –OH of the PLLA end groups under the catalyst to form a grafting copolymer that can function as the compatibilizer. Change et al. investigated PBT/ ABS/SAG (styrene–acrylonitrile–glycidyl methacrylate) blends in the presence of ETPB [28]. It was found that the addition of ETPB could not only promote the grafting reac-
tion but also improve the mechanical properties of the final blends. In order to verify the reaction between PLLA and SAN-GMA under the melt-mixing conditions, the blending of PLLA/SAN-GMA = 80/20 was carried out with and without ETPB. The melt-blended samples were taken out and hot-pressed into a thin film after 2 min and 6 min mixing. The films were characterized by FTIR directly. Fig. 2 shows the FTIR spectra with the wave number in the region of epoxy absorption. It is found that the intensity of the expoxy characteristic peak at 911 cm 1 decreases with prolonging the mixing time, suggesting the
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Fig. 5. TEM images of (a) uncompatibilized PLLA/ABS = 50/50 blend, (b) PLLA/ABS = 50/50 blend with 5 wt% SAN-GMA, and (c) PLLA/ABS = 50/50 blend with 5 wt% SAN-GMA and 0.02 phr ETPB.
reactions of expoxide groups. Moreover, the addition of ETPB accelerates the reaction rate as evidenced by the markedly decreased peak intensity at the same mixing time as compared with that having no ETPB. Oyama et al. have discussed the reaction mechanism of –COOH and – OH groups with epoxide groups under catalysts [30]. It is considered that ETPB dissociates easily under the mixing conditions and carries out a nucleophilic attack, which opens the epoxy ring in SAN-GMA to form an intermediate product. This product has high reactivity with –COOH or –
OH of PLLA end groups and a PLLA-g-SAN copolymer formed. It is expected that the formed copolymer can act as a good compatibilizer for PLLA/ABS blends. The reaction scheme for SAN-GMA with PLLA under the ETPB is shown in Fig. 3. 3.2. Morphology of the compatibilized PLLA/ABS blends Figs. 4 and 5 show TEM images of PLLA/ABS blends containing 50% ABS and 30% ABS, respectively. In the
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Y. Li, H. Shimizu / European Polymer Journal 45 (2009) 738–746 3.5 3.0 2.5
b
a
Heat Flow
Tan (delta)
2.0 1.5 1.0
c b
0.5 0.0
+0.2 +0 -0.2 -0.4
-0.5
-0.7
-100
-75
-50
-25
a
c d e 0
25
50
o
75
100
125
150
175
25
50
75
Temperature ( C)
100
125
150
175
200
Temperature (ºC)
Fig. 6. Dynamic loss for the PLLA/ABS blends as function of temperature. (a) Neat PLLA, (b) neat ABS, (c) PLLA/ABS = 50/50, (d) PLLA/ABS/SANGMA = 50/50/5, and (e) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/0.02 phr.
Fig. 9. DSC heating curves of PLLA/ABS blends with a heating rate of 10 oC/min (a) PLLA/ABS = 50/50, (b) PLLA/ABS/SAN-GMA = 50/50/5, and (c) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/0.02.
1.0 0.9 0.8 0.7 0.5
Heat Flow
Tan (delta)
0.6 0.4 0.3
a
+0.2
b
+0
c
-0.2
0.2 0.1 0.0
b
-0.1 -0.2 -0.3 -100
c
-75
-50
-25
0
a 25
50
75
100
125
150
175
o
Fig. 7. Dynamic loss for the PLLA/ABS blends as function of temperature. (a) PLLA/ABS = 70/30, (b) PLLA/ABS/SAN-GMA = 70/30/5, and (c) PLLA/ ABS/SAN-GMA/ETPB = 70/30/5/0.02 phr.
3
Modulus (MPa)
10
e d c
2
10
a
b
1
10
-150 -125 -100 -75 -50 -25
0
25
50
25
50
75
100
125
150
175
200
Temperature (ºC)
Temperature ( C)
75 100 125 150 175
Temperature (ºC) Fig. 8. Storage modulus of the PLLA/ABS blends as function of temperature. (a) Neat PLLA, (b) neat ABS, (c) PLLA/ABS = 50/50, (d) PLLA/ ABS/SAN-GMA = 50/50/5, and (e) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/ 0.02 phr.
Fig. 10. DSC heating curves of PLLA/ABS blends with a heating rate of 10 oC/min (a) PLLA/ABS = 70/30, (b) PLLA/ABS/SAN-GMA = 70/30/5, and (c) PLLA/ABS/SAN-GMA/ETPB = 70/30/5/0.02.
figures, ABS was observed as the dark phase because ABS is more readily stained than PLLA by RuO4. Furthermore, ABS shows very clear core-shell structure in the TEM images and the core polybutadiene is the most easily stained due to the double bonds in the chain. It is seen that ABS was dispersed in PLLA matrix with the domain size ranging from 1 to 10 lm for the uncompatibilized blends (Figs. 4a and 5). The domain size distribution is also very large due to the immiscibility between the components. The addition of 5% SAN-GMA copolymers results in a decreased ABS domain size and a narrower size distribution, shown in Figs. 4b and 5b. Moreover, the addition of ETPB catalyst can further improve the distribution of the rubber particles as evidenced by significantly increased rubber particles in Figs. 4c and 5c. The morphological analysis indicates that SAN-GMA takes an effective role in compatibilizing PLLA/ABS blends by incorporating with ETPB.
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Table 1 Thermal properties of PLLA/ABS blends. * PLLA
Samples
Tg
PLLA ABS PLLA/ABS = 50/50 PLLA/ABS/SANMGA = 50/50/5 PLLA/ABS/SANMGA/ETPB = 50/50/5/0.02 phr PLLA/ABS = 70/30 PLLA/ABS/SANGMA = 70/30/5 PLLA/ABS/SANMGA/ETPB = 70/30/5/0.02 phr
60.5 – 60.5 63.1 64.2 61.5 63.1 66.7
*
(°C)
Tg
* SAN
(°C)
– 115.5 115.2 113.4 111.4 115.5 112.9 110.8
Tm (°C)
DHc (J/g)
DHm (J/g)
163.5 – 167.4 167.1 166.3 167.7 167.1 166.0
–24.5 – –12.0 –10.7 –9.8 –17.3 –16.7 –14.1
25.1 – 18.6 17.1 16.8 23.5 21.8 20.8
From the DMA measurement.
Table 2 Mechanical properties of PLA/PU blends. Samples
Storage modulus* (Mpa)
Static modulus (Mpa)
Tensile strength (Mpa)
Elongation at break (%)
Impact strength (KJ/m2)
PLLA ABS PLLA/ABS = 50/50 PLLA/ABS/SANMGA = 50/50/5 PLLA/ABS/SANMGA/ETPB = 50/50/ 5/0.02 phr PLLA/ABS = 70/30 PLLA/ABS/SANGMA = 70/30/5 PLLA/ABS/SANMGA/ETPB = 70/30/ 5/0.02 phr
2805 1556 2105 2110 2151
2024 807 1294 1327 1358
65.5 32.9 38.8 42.9 43.6
4.0 122 3.5 18.0 23.5
69.7 425 48.3 119.7 162.8
2548 2550 2532
1554 1405 1352
46.7 43.6 44.6
3.1 20.5 23.8
63.8 81.1 123.9
*
At 20 °C from the DMA measurement.
3.3. Thermal behavior of the compatibilized PLLA/ABS blends Figs. 6 and 7 demonstrate dynamic loss curves for PLLA, ABS, and PLLA/ABS blends with and without addition of compatibilizer. Neat PLLA shows a glass transition temperature (Tg) at 60.5 oC and ABS gives a very strong relaxation peak at 115.5 oC corresponding to the Tg of SAN phase. On the other hand, the peak at 83 oC is the Tg relaxation of PB phase in ABS. The uncompatibilized PLLA/ABS blends show almost same Tgs corresponding to the each component, indicating the thermodynamical immiscibility between PLLA and ABS. In contrast, the addition of SAN-GMA clearly changes the relaxation peak positions of both PLLA and ABS. The Tgs of PLLA and SAN shift toward each other, suggesting the improved compatibilities. The compatibilized effects of SAN-GMA are more significant by incorporation of ETPB as the catalyst, as shown by the larger Tg shift for both PLLA and SAN. The Tg of SAN decreases from 115.5 to 111.4 oC, while the Tg of PLLA increases from 60.5 to 64.2 oC. These DMA results are consistent with those by TEM. Note that the peak at about 92 oC originates from the cold crystallization of PLLA during heating where storage modulus starts to increase. Fig. 8 demonstrates the typical storage modulus curves as a function of temperature for PLLA, ABS, and PLLA/ ABS = 50/50 blends. PLLA has higher modulus than ABS at the temperature region less than 60 oC. The storage modulus of the blends is lower than that of neat PLLA, but the value is about 2100 MPa at room temperature. This means that the blends exhibit the relative high modulus and high stiffness by blending with ABS. Note that the storage modulus of all PLLA blends increases in the
temperature range of 80–100 oC due to the cold crystallization of PLLA [14]. Figs. 9 and 10 show the DSC heating curves of PLLA/ABS blends with a heating speed of 10 oC/min. All the curves show three apparent transitions upon heating, namely, a PLLA glass transition, a cold crystallization exotherm peak, and a melting endotherm peak (Table 1), while Tg of SAN phase is not clear from the DSC curves. Tg of PLLA increases with adding the compatibilizers and the catalyst enhances the shift. These results are consistent with the DMA results. However, the Tg value obtained from DSC is lower than the value obtained from DMA, which can be attributed to the different measuring mechanisms for DMA and DSC [31]. The heat of cold crystallization (DHc) and the heat of melting (DHm) decrease upon the addition of compatibilizer in the blends (Table 2), suggesting that PLLA in the compatibilized blends has lower crystallinity. The results can be attributed to the molecular interactions at the interface between PLA and amorphous ABS. Furthermore; the melting temperature of PLA in the compatibilized blends is also lower than that for the uncompatibilized one, which again indicates the improved compatibility by the reactive blending. 3.4. Melt viscosity of the compatibilized PLLA/ABS blends Fig. 11 shows the dynamic viscosity of PLLA/ABS = 50/50 blends with and without compatibilizer as a function of frequency. The viscosity decreased with an increasing shear rate, indication that the PLLA/ABS blends exhibit shear-thinning behavior. At a given shear rate, the compatibilized blends show higher viscosity in the whole
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Y. Li, H. Shimizu / European Polymer Journal 45 (2009) 738–746
2
4
10
Viscosity (PaS)
Impact strength (KJ/m )
160
c b
3
10
a
140 120 100 80 60
2
10
-1
0
10
1
10
40
2
10
10
PLLA
Fig. 11. Dynamic viscosity of (a) PLLA/ABS = 50/50, (b) PLLA/ABS/SANGMA = 50/50/5, and (c) PLLA/ABS/SAN-GMA/ETPB = 50/50/5/0.02.
a
60 55
Stress (MPa)
50 45 40
b
35
c
d
30
50/50/5/0 50/50/5/0.02
Fig. 14. Film impact strength of neat PLLA and PLLA/ABS blends.
3.5. Mechanical properties of the compatibilized PLLA/ABS blends
e
25 20 15 10 5 0 0
10
20
30
40
Strain (%) Fig. 12. Tensile strain–stress curves for (a) neat PLLA, (b) PLLA/ABS = 50/ 50, (c) PLLA/ABS/SAN-GMA = 50/50/5, (d) PLLA/ABS/SAN-GMA/ETPB = 50/ 50/5/0.02,and (e) neat ABS.
50
a
40
Stress (MPa)
70/30/5/0 70/30/5/0.02 50/50/0/0
the compatibilized blends can be attributed to several reasons, such as the enhancing of the dispersion promoted by the compatibilizer, the increase of the total apparent volume of the dispersed phase due to the compatibilizer layer attached to the drop surfaces, the increase of interaction among the droplets and the stiffening of the interface [32].
70 65
70/30/0/0
PLLA/ABS/SAN-GMA/ETPB
Frequency (rad/s)
c
b 30
20
10
0
0
5
10
15
20
25
30
Strain (%) Fig. 13. Tensile strain–stress curves for (a) PLLA/ABS = 70/30, (b) PLLA/ABS/ SAN-GMA = 70/30/5, and (c). PLLA/ABS/SAN-GMA/ETPB=70/30/5/0.02.
frequency range investigated here as compared with the uncompatibilized blends. The increase of the viscosity of
Tensile stress–strain curves of neat PLLA, neat ABS, and PLLA/ABS blends are shown in Figs. 12 and 13. The main tensile properties such as static modulus, tensile stress, and elongation at break determined from these curves are presented in Table 2. PLLA is very rigid and shows pretty high tensile strength, but it breaks at the elongation at break of about 4%. The uncompatibilized blends are very brittle and break at even lower elongation at break due to the big domain size and very weak interface. It is obvious that the compatibilizer can improve the tensile properties significantly because the compatibilized blends undergo a clear yielding upon stretching and break at the elongation at break at 20–30%. The samples by incorporated with catalyst EBTP show almost same modulus and tensile strength as those without adding catalyst, but higher elongation at break. The film impact strength of neat PLLA and the PLLA/ABS blends is shown in Fig. 14. The uncompatibilized blends show even lower impact strength as compared with the neat PLLA. However, significant improvement can be observed with the addition of the SAN-GMA compatibilizer. The impact strength for the compatibilized PLLA/ ABS = 70/30 blend is 124 KJ/m2, which is about 100% higher than the value for neat PLLA. The modification effects of the compatibilizer are more significant for the PLLA/ ABS = 50/50 blends. The compatibilized blend gives the impact strength of 162.8 KJ/m2, as compared with the uncompatilized one of 48.3 KJ/m2. It should be noted that the modified PLLA maintains quite high tensile modulus and tensile strength with the obviously increased impact strength, which means that the obtained material having a good stiffness–toughness
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balance. Furthermore, it can be expected that the blends exhibit increased heat resistance as compared with neat PLLA due to the increased glass transition temperature, as discussed in the DMA results. 4. Conclusions Uncompatibilized blends of PLLA and ABS have a morphology with big phase size and weak interface. The blends exhibit poor mechanical properties with low elongation at break and decreased impact strength. SAN-GMA was found to be an effective reactive compatibilizer for PLLA/ABS blend by the presence of ETPB as the catalyst, as revealed by the significant improvement in the dispersion of rubber particles as well as the clearly shifted glass transition temperature of both PLLA and ABS. Compatibilized PLLA/ABS blends exhibit improved impact strength and elongation at break with a slight loss in the modulus and tensile strength. References [1] Drumright RE, Gruber RE, Henton DE. Adv Mater 2000;12:1841–6. [2] Vink ETH, Rabago R, Classner DA, Springs B, Oconnor RP, Kostad J, et al. Macromol Biosci 2004;4:551–64. [3] Tsuji H, Ikada Y. J Appl Polym Sci 1996;60:2367–75. [4] Broz ME, Vanderhart DL, Wahburn NR. Biomaterials 2003;24: 4181–90. [5] Hiljanen-vainio M, Varpomaa P, Seppala J, Tormala P. Macromol Chem Phys 1996;197:1503–23.
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