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Preparation and characterization of ternary blends composed of polylactide, poly(ɛ-caprolactone) and starch
Materials Science and Engineering A 515 (2009) 207–214
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Preparation and characterization of ternary blends composed of polylactide, poly(-caprolactone) and starch Hsin-Tzu Liao ∗ , Chin-San Wu Department of Chemical and Biochemical Engineering, Kao Yuan University, Kaohsiung County, Taiwan 82151, Republic of China
a r t i c l e
i n f o
Article history: Received 30 October 2008 Received in revised form 27 February 2009 Accepted 2 March 2009 Keywords: Blends PLA PCL Starch Biodegradability
a b s t r a c t By means of a melt blending method, a ternary blend composed of polylactide (PLA), poly(-caprolactone) (PCL) and starch was prepared to conquer the major shortcomings (brittle and high price) of PLA. By the addition of PCL, PLA was tuned from rigid to ductile but its tensile strength was also reduced. To overcome the poor compatibility between PLA70 PCL30 and starch, the acrylic acid grafted PLA70 PCL30 (PLA70 PCL30 g-AA) was chosen as the alternative for the preparation of ternary blends. Owing to the formation of ester carbonyl groups, the PLA70 PCL30 -g-AA/starch blend gave a much better dispersion and homogeneity of starch in the PLA70 PCL30 -g-AA matrix, and consequently led to apparently better properties. In a soil environment, the PLA70 PCL30 /starch gave the better biodegradation than the PLA70 PCL30 -g-AA/starch but the difference was slight. Finally, the PLA70 PCL30 -g-AA/starch blend not only provided a plateau tensile strength at break up to 50 wt% starch but also provided more easily processing properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the massive use and disposal of petroleumbased plastics had been seriously caused public affairs over the environmental hazards. The main strategies to address these problems are to develop materials that are renewable, degradable, and recyclable, better known as “green materials,” [1–5] as alternatives to the petroleum-based materials. The most popular and important biodegradable polymers are aliphatic polyesters [e.g. polylactide (PLA), poly(-caprolactone) (PCL), polyethylene oxide (PEO), poly(3-hydroxybutyrate) (PHB), and polyglycolic acid] and thermoplastic proteins. The biodegradable PLA had attracted increasing attention as a candidate for us in industrial applications since it possessed excellent mechanical properties (especially in tensile strength and modulus) and was produced with corn or other starches as the starting raw material [6–10]. With these advantages, PLA was a versatile material with applications in the medical, textile, and packaging fields [11–14], but its brittleness and high price in comparison to petroleum-based thermoplastics limited its applications. So, the choice of suitable plasticizers to improve the brittle disadvantage of PLA was an important issue. Various types of chemicals (e.g. citrate esters, glucose monoesters, and partial fatty acid esters) have been tried to plasticize PLA via the polar interactions occurring between the ester groups in the plasticizer and PLA [15–20]. However, it
∗ Corresponding author. Fax: +886 7 6077788. E-mail address: [email protected] (H.-T. Liao). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.003
was observed that, upon aging of the plasticized film materials, low molecular weight plasticizers had a tendency to migrate to the film surface. In the polymer literatures, considerable efforts had been made to modify the properties of PLA by blending it with some hydrophilic polymers, such as poly(-caprolactone) (PCL), poly(vinyl alcohol) (PVA) [21,22], poly(ethylene glycol) (PEG) [23], pluronic [triblockcopolymers of PEG and poly(propylene oxide)] [24], hyaluronic acid [25], and poly(vinyl acetate) (PVAc) [26], and the results were in terms of their miscibility, morphology, physical properties, biodegradability, drug release properties, and porous material preparation. The PCL, a ductile biodegradable polymer, can be used to tune the PLA from rigid to ductile since the former has been chosen as a blending partner for the latter [27–29]. In the present study, to overcome the limitations (brittle and high price) of polylactide (PLA), it is attempted to prepare and characterize ternary blends composed of PLA, PCL, and starch. At first, we searched the optimal composition for the preparation of PLA/PCL blend and then evaluated the practicability of the addition of starch to PLA/PCL blends. The acrylic acid grafted PLA/PCL (PLA/PCL-g-AA) was used as an alternative for the preparation of ternary blends to improve the compatibility and dispersion of starch in the polymer matrix. The conclusions are based on a combination of a Fourier transform infrared (FTIR) spectroscopy, an X-ray diffractometer, a differential scanning calorimetry (DSC), a scanning electron microscopy (SEM) and an Instron mechanical tester. Additionally, the water absorption test and the soil biodegradable test were used to assess the effect of starch content on water resistance and biodegradability of ternary blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch).
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2. Experimental
The polylactide (PLA), consisting of 95% l-lactide and 5% mesolactide, and the commercial grade polycaprolactone (PCL, CAPA 6800), with a molar mass of 80,000 g/mol, were obtained from Cargill-Dow and Solvay Chemicals, respectively. Acrylic acid (AA) and benzoyl peroxide (BPO) are supplied by Aldrich Chemical Corporation (Milwaukee, WI). Before use, the acrylic acid was purified by re-crystallization from chloroform and the benzoyl peroxide, use as the initiator, was purified by dissolving it in chloroform and reprecipitating with methanol. The starch, with a granule size of 15–100 m, composed of 27% amylose and 73% amylopectine was supplied by Sigma Chemical Corporation. All the reactants were used as received. The PLA70 PCL30 -g-AA copolymer, having the grafting percentage of 6.85%, was synthesized in our laboratory as described below.
the reaction approached equilibrium after 5 h. To confirm that equilibrium reaction was established, all the reactions lasted for 6 h. The grafting product (ca. 4 g) was dissolved in 200 ml of refluxing xylene at 85 ◦ C and then the hot solution was filtered through several layers of cheesecloth. The xylene-insoluble product remaining on the cheesecloth was washed using acetone to remove the unreacted acrylic acid and was then dried in a vacuum oven at 80 ◦ C for 24 h. The xylene-soluble product in the filtrate was extracted five times, using 600 ml of cold acetone for each extraction. Using a titration method, the acrylic acid loading of the xylene-soluble polymer was calculated from the acid number and the result was expressed as the grafting percentage. About 2 g of copolymer was heated for 2 h in 200 ml of refluxing xylene. This solution was then titrated immediately with a 0.03N ethanolic KOH solution, which had been standardized against a solution of potassium hydrogen phthalate, while phenolphthalein was used as an indicator. The acid number and the grafting percentage could then be calculated using the following equations [30]:
2.2. Preparation of PLA/PCL blends
acid number (mg KOH/g) =
PLA/PCL blends were prepared by the melt blending method using a Brabender “Platograph” 200 N m W50EHT mixer (Duisburg, Germany) with a blade-type rotor. Before blending, PLA and PCL were dried in a vacuum oven at 80 ◦ C for 6 h. The mass ratios of PLA to PCL were chosen as 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 20/80 and 0/100. PLAx PCLy was used as the symbol for the PLA/PCL blend, where x and y indicated mass percentages of PLA and PCL, respectively. A determined amount of PLA and PCL was put into the Brabender instrument to melt the mixture under the conditions that the rotor speed and the blending temperature were kept at 50 rpm and 190–200 ◦ C, respectively. To ensure complete mixing of PLA and PCL, the operation lasts for another 15 min when the blending torque approaches constant (about 3 min). The hybrid products were transferred to a mold (10 cm × 10 cm × 0.1 cm) to press into thin plates by a hot press at 10 atm and 190 ◦ C for 2 min and then they were put into a dryer for cooling. Next, the cool thin plates were made into standard specimens for characterizations.
grafting percentage (%) =
2.1. Materials
2.3. 2.3Preparation of PLA70 PCL30 -g-AA copolymer Based on the result of preliminary characterization, the PLA70 PCL30 blend was chosen as the polymer matrix to prepare the ternary blend (PLA70 PCL30 /starch). To improve the properties of PLA70 PCL30 /starch blend, the acrylic acid grafted PLA70 PCL30 (PLA70 PCL30 -g-AA) was used to replace PLA70 PCL30 to produce the ternary blend. The total amount of PLA70 PCL30 , AA, and BPO used for the manufacture of PLA70 PCL30 -g-AA are 40 g, and the weight percentages of each component are listed in Table 1. A mixture of AA and BPO was added in four equal portions at 2-min intervals to the solution of PLA70 PCL30 in 250 ml xylene to allow grafting reaction to take place. The reactions were carried out at a temperature of 85 ± 2 ◦ C and a rotor speed of 60 rpm under a nitrogen atmosphere at a flow rate of 25 cm3 /min. Preliminary experiments showed that Table 1 Grafting percentage values of acrylic acid onto PLA70 PCL30 for different BPO and AA loadings. AA loading = 10 wt%
BPO loading = 0.3 wt%
BPO (wt%)
Grafting percentage (%)
AA (wt%)
Grafting percentage (%)
0.1 0.2 0.3 0.4 0.5
2.81 4.50 6.85 7.05 7.17
5 10 15 20
5.85 6.85 7.11 7.25
± ± ± ± ±
0.08 0.14 0.21 0.21 0.22
± ± ± ±
0.18 0.21 0.21 0.22
VKOH (ml) × CKOH (N) × 56.1 polymer (g)
acid number × 72 561
(1) (2)
2.4. Preparation of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends Prior to blending, the starch was cleaned with acetone and then dried in an oven at 50 ◦ C for 24 h to remove acetone completely. To eliminate potential effects of residual AA on the PLA70 PCL30 g-AA/starch blend, it was removed from the PLA70 PCL30 -g-AA copolymer product by acetone extraction and then dried in a vacuum oven at 80 ◦ C for 6 h before preparation of blends. The mass ratios of starch to PLA70 PCL30 or to PLA70 PCL30 -g-AA were set at 10/90, 20/80, 30/70, 40/60, and 50/50. A determined amount of PLA70 PCL30 + starch or PLA70 PCL30 -g-AA + starch was put into the Brabender instrument to prepare ternary blends. The rotor speed and the blending temperature were kept at 50 rpm and 190–200 ◦ C, respectively, and the blending reaction was carried out for 18 min. After blending, blends were pressed into thin plates using a hot press and were then put into a dryer for cooling. Subsequently, the thin plates were made into standard specimens for characterization. The Brabender was also used for torque recording during the melt kneading of ternary blends. 2.5. Characterizations of blending polymers A scanning electron microscope (Hitachi microscope Model S1400, Japan) was used to study the morphology of blends and to measure the starch phase size in the polymer matrix. Before the tests, the blend was prepared in a thin film by a hydrolytic press and then the film was treated with hot water at 80 ◦ C for 24 h. Afterward, the film was coated with gold and observed by SEM. The Fourier transform infrared (FTIR) spectrophotometer (BioRad FTS-7PC type, Madison, WI), using thin films, was used to investigate the graft reaction of acrylic acid onto PLA70 PCL30 and to verify whether chemical bonding was formed in blends. The Bruker AMX400 1 H NMR spectrometer was used to obtain the spectra of PLA70 PCL30 and PLA70 PCL30 -g-AA. The sample was first dissolved in CDCl3 and sealed in a NMR tube (10 mm O.D.). Then, after being degassed, analysis was performed at conditions of 100 MHz, 30◦ pulse and 4 s cycle time. The melting temperature (Tm ) and the melting enthalpy (Hm ) of samples were determined from a differential scanning calorimeter (TA Instrument 2010 DSC system, New Castle, DE). For DSC tests, sample sizes ranged from 4 to 6 mg and the melting curves were
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recorded at a temperature range of −100 to 250 ◦ C, scanned at a heating rate of 10 ◦ C/min. The values of Tm and Hm are determined from the temperature and area of melting peaks of DSC heating thermograms of samples. Structural changes in the blends were investigated by comparison of X-ray diffraction data obtained from PLA70 PCL30 , PLA70 PCL30 -g-AA, PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch, recorded with a Rigaku D/max 3 V X-ray diffractometer, using Cu K␣ radiation at a scanning rate of 2◦ min−1 . Tensile strength and elongation at break of blends (PLA/PCL, PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch) were carried out on an Instron tensile testing machine (Model LLOYD, LR5K type) according to the ASTM D638, with a crosshead speed of 50 mm/min. A 35 mm gauge length was used during each tensile experiment. The dimensions of the dog-bone shaped specimens were prepared according to ASTM D638 type IV standard, and each sample’s width and thickness were measured before testing. Five measurements were taken for each sample and the data were averaged to obtain a mean value. Samples for measuring water absorption were in the form of 75 mm × 25 mm film strips (150 ± 5 m thickness) following the ASTM D570-81 method. The samples were dried in a vacuum oven at 50 ± 2 ◦ C for 8 h, cooled in a desiccator and then immediately weighed to the nearest 0.001 g (designated as Wc ). Thereafter, the conditioned samples were immersed in distilled water, maintained at 25 ± 2 ◦ C, for a 6-week test period. During this period, they were removed from the water at 1-week interval, gently blotted with tissue paper to remove excess water on the surface, immediately weighed to the nearest 0.001 g (designated as Ww ), and returned to the water. Each Ww was an average value obtained from three measurements. The final water uptake was calculated to the nearest 0.01% as follows. water uptake (%) =
Ww − Wc × 100% Wc
(3)
Biodegradability of the samples was studied by evaluating the weight loss of blends over time in a soil environment. Samples of 30 mm × 30 mm × 1 mm were weighed and then buried in boxes of alluvial-type soil, obtained in March 2006 from farmland topsoil before planting. The soil was sifted to remove large clumps and plant debris. Procedures used for soil burial followed the method described by Chandra and Rustgi [31]. Soil was maintained at approximately 20% moisture in weight and samples were buried at a depth of 15 cm. A control box consisted of only samples and no soil. The samples were dug out at each two weeks, during a 16-week period of soil burial, washed in distilled water, dried in a vacuum oven at 50 ± 2 ◦ C for 24 h and, before evaluation, equilibrated in a desiccator for at least a day. The samples were then weighed before returning them to the soil. The weight loss for extended burial periods, as an indication of biodegradability, was calculated as follows. weight loss (%) =
Wo − Wf × 100% Wo
(4)
where Wo is the original sample weight and Wf is the original sample weight after soil burial.
209
Fig. 1. Effect of PCL content on tensile strength and elongation values at breakpoint of PLA/PCL blends.
b value and a high εb value of 37.6 ± 1.0 MPa and 575.3 ± 59.3%, respectively. It is clear that the addition of PCL makes PLA/PCL blends a substantial reduction in b value but an increase in εb value as their PCL content increases. For example, as the PCL content of PLA/PCL blends increases from 0 to 30 wt%, the b value reduces from 53.7 ± 1.5 to 41.7 ± 1.0 MPa but the εb value increases from 3.6 ± 0.4 to 350.7 ± 30.7%. Apparently, the inherent brittle deformation behavior of the PLA specimen was successfully transformed into relatively ductile fracture behavior after blending sufficient amounts of PCL in PLA resins. In the latter study of this work, the PLA70 PCL30 blend is chosen to blend with the starch to reduce the cost of blend since it provide suitable mechanical properties applied in industrial products. 3.2. Effects of ingredients on the graft reaction The effect of BPO loading and AA loading on the grafting reaction of AA onto the PLA70 PCL30 blend had been studied and the result is given in Table 1. It can be seen that, with constant AA loading of 10 wt%, the value of grafting percentage increased steadily with an increasing of BPO loading up to 0.3 wt%, and then approached to constant. Similar to the effect of BPO loading, it can be observed that, with constant BPO loading of 0.3 wt%, the value of grafting percentage is also enhanced as the AA loading is increased, but it is increased slightly when the AA loading is beyond 15 wt%. This result may be due to the fact that at low AA loading, the diffusion of monomer molecules toward the free radical sites on the PLA70 PCL30 backbone governs the grafting extent, whereas at higher AA concentration, the grafting percentage remains almost constant as the number of free-radical sites available on the PLA70 PCL30 backbone becomes a limiting factor [32]. The grafting percentage of PLA70 PCL30 -g-AA is about 6.85% when AA loading and BPO loading are kept at 10 and 0.3 wt%, respectively, and this copolymer was used as the alternative for the preparation of ternary blends in this study.
3. Results and discussion 3.3. FTIR/NMR analysis 3.1. Mechanical properties of PLA/PCL blends The effect of PCL content on values of the tensile strength and the elongation at break for PLA/PCL blends is illustrated Fig. 1. The result shows that the neat PLA gives a high tensile strength at break ( b ) of 53.7 ± 1.5 MPa and a low elongation at break (εb ) of 3.6 ± 0.4%, exhibiting that PLA is a high modulus and brittle material. On the contrary, the neat PCL is a ductile material with a relative low
Fig. 2(A–D) shows the FTIR spectra of PLA70 PCL30 , PLA70 PCL30 g-AA, PLA70 PCL30 /starch (30 wt%), and PLA70 PCL30 -g-AA/starch (30 wt%), respectively, in the range of 500–4000 cm−1 . For the PLA70 PCL30 blend, it can be seen from Fig. 2A that there are no extra peaks can be observed except the characteristic peaks of PLA and PCL at 3300–3700, 2800–2958, 1754, 1725, and 500–1500 cm−1 [33,34], implying that the PCL is physically dispersed in the PLA
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Fig. 3.
Fig. 2. FTIR spectra of (A) PLA70 PCL30 , (B) PLA70 PCL30 -g-AA, (C) PLA70 PCL30 /starch (30 wt%), and (D) PLA70 PCL30 -g-AA/starch (30 wt%) in the wavenumber range from 500 to 4000 cm−1 .
to form a partial miscible blend. This result can be further confirmed by XRD and DSC examinations of PLA/PCL blends. It is also found that all the characteristic peaks of PLA and PCL appear in Fig. 2(B–D), revealing that the major structure of PLA and PCL is not altered during the period of grafting reaction and the blending procedure. To obtain more clear observation, the FTIR spectra in the range of 1700–1800 cm−1 were expanded. The comparison between FTIR spectra of PLA70 PCL30 and PLA70 PCL30 -g-AA [Fig. 2(A and B)] shows that an extra peak at 1709 cm−1 , assigned to –C O, as well as a broad O–H stretching absorbance at 3200–3700 cm−1 can be found in the spectrum of the later. This result demonstrates that acrylic acid had been grafted on the PLA70 PCL30 blend since the discernible shoulder near 1710 cm−1 represents the formation of free acid in the modified polymer blend. To further confirm that the AA monomer has been grafted on the PLA70 PCL30 , the 1 H NMR spectra of PLA70 PCL30 and PLA70 PCL30 -g-AA had been made and depict in Fig. 3. It can be seen that the unmodified PLA70 PCL30 shows five hydrogen peaks at 1: ı = 5.0–5.2 ppm; 2: ı = 1.4–1.5 ppm; 3: ı = 3.8–4.0 ppm; 4: ı = 1.4–1.7 ppm; and 5: ı = 2.1–2.3 ppm. This result is similar to the findings of other works [35,36]. As a comparison between Fig. 3(A and B), there are three extra hydrogen peaks at ı = 1.8–2.0, 2.2–2.4, and 5.3–5.4 ppm in the spectrum of PLA70 PCL30 -g-AA due to the grafting of AA onto the main or branch chain of the PLA70 PCL30 skeleton [37]. In addition, Fig. 2(C and D) further shows that much more intense of peaks at 3200–3700 cm−1 , assigned to O–H bond stretching vibration, can be seen in the FTIR spectra of PLA70 PCL30 /starch (30 wt%) and PLA70 PCL30 -g-AA/starch (30 wt%). This is because the –OH group of starch causes and contributes to the bond stretching vibration [31,38]. The FTIR spectrum of PLA70 PCL30 /starch (Fig. 2C) shows nearly the same absorbance of the PLA70 PCL3 blend besides
1
H NMR spectra of (A) PLA70 PCL30 and (B) PLA70 PCL30 -g-AA.
a broad O–H stretching absorbance at 3200–3700 cm−1 , implying that the hydrophilic starch is dispersed physically in the hydrophobic polymer matrix. Further comparison of the FTIR spectra of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch, the latter exhibits a new absorption peak at 1737 cm−1 , which can be assigned to the ester carbonyl stretching vibration in the copolymer. Appearance of this new absorption peak is perhaps due to the formation of an ester carbonyl functional group via the reaction between the –OH group of starch and the –COOH group of PLA70 PCL30 -g-AA [38]. 3.4. XRD analysis Fig. 4 illustrates the XRD patterns of PLA70 PCL30 , PLA70 PCL30 /starch (30 wt%), PLA70 PCL30 /starch (60 wt%), and PLA70 PCL30 -g-AA/starch (30 wt%) at 2 = 5–35◦ . As the result of Fig. 4A, it is also found that the PCL is physically dispersed in the PLA since only the characteristic diffraction peaks of PLA (2 = 16.4◦ and 18.6◦ ) and PCL (2 = 21.6◦ and 23.8◦ , reflecting the orthorhombic crystal phase of PCL) can be observed in the XRD pattern of PLA70 PCL30 (Fig. 4A). The diffraction peaks at about 2 = 16.4◦ and 18.6◦ are assigned to the reflection of ␣ crystal cell of PLA, which is orthorhombic with chains in a −10/3 helical conformation [39,40]. For the PLA70 PCL30 /starch (30 wt%) blend, it can be found from Fig. 4B that there are two extra peaks at about 15.3◦ and 17.1◦ as compared with the XRD pattern of PLA70 PCL30 (Fig. 4A). The extra peaks at 15.3◦ and 17.1◦ are assigned to amylose and amylopectine of starch, respectively, and may be due to the change in coordinate property of PLA70 PCL30 molecules when it was blended with starch [41]. The XRD spectra of the PLA70 PCL30 /starch blends, therefore, show that starch is dispersed physically in the PLA70 PCL30 matrix. Moreover, as the comparison between Fig. 4(B and C), it is clear that the higher starch content gives more intensive peaks at 17.1◦ and 15.3◦ . A comparison of the XRD patterns of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch, Fig. 4(B and D), shows a new peak in the latter at about 2 = 18.2◦ , causing by the formation of an ester carbonyl functional group [42]. In agreement with the result described in the FTIR spectroscopy analysis, the XRD pattern provides the evidence that the crystalline structure of the PLA70 PCL30 /starch blend is altered when PLA70 PCL30 -
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211
Fig. 5. Torque values versus mixing time for preparation of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends at various starch contents.
3.6. Thermal properties of blends Thermal properties (Tm and Hm ) of neat PLA, PLA/PCL blends, PLA70 PCL30 /starch blends, PLA70 PCL30 -g-AA/starch blends, and neat PCL are obtained from their DSC thermograms (not shown here). The variation of melting temperature (Tm ) and melting enthalpy (Hm ) with PCL content for PLA/PCL blends is presented in Table 2. In addition, for PLA70 PCL30 /starch and PLA70 PCL30 -gAA/starch blends, the effect of starch content on the values of Tm and Hm is illustrated in Table 3. As shown in Table 2, the melting temperatures of neat PLA and neat PCL are 161.2 and 62.5 ◦ C, respectively. For the PLA/PCL blends, there are two separate Tm s, corresponding to the PLA-rich phase and the PCL-rich phase. As compared with neat PLA and neat PCL, two separate Tm s of PLA/PCL blends shift toward each other, revealing that an amount of amorphous PCL (or amorphous PLA) is dissolved in the PLA-rich phase (or PCL-rich phase) to form a partial miscible blend. The value of melting enthalpy obtained from the integration of the melting peak indicates the crystallinity of a polymer. As compared with neat PLA and neat PCL, Table 2 shows that the percentage crystallinity of PLA and the percentage crystallinity PCL in the PLA/PCL blend are impaired and increased with the addition of PCL content, respectively. In addition, the variations in values of Tm and Hm are usually assigned to the interactions between components [45]. It can be seen from Table 2 that the Tm values of PLA and PCL in the PLA/PCL blend shift to lower and higher temperatures, respectively, as compared with those of neat PLA and neat PCL, and that the largest shift appears at the PLA70 PCL30 blend. This result reveals that the PLA70 PCL30 blend gives the strongest interaction intensity
Fig. 4. XRD pattern of (A) PLA70 PCL30 , (B) PLA70 PCL30 /starch (30 wt%), (C) PLA70 PCL30 /starch (60 wt%), and (D) PLA70 PCL30 -g-AA/starch (30 wt%).
g-AA is used to replace PLA70 PCL30 for the preparation of blend. 3.5. Torque measurements of ternary blends The curves of torque value versus mixing time for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends are presented in Fig. 5. For both blends, it can be observed that their torque value decreases with increasing starch content and blending time, and approaches a stable value when the blending time is greater than 8 min, suggesting that good mixing has occurred after 12 min. It can also be seen that the final torque decreases with increasing starch content because the viscosity of molten starch is lower than that of molten PLA70 PCL30 and molten PLA70 PCL30 -g-AA. Further, with the same starch content (e.g. 30 and 50 wt%), the torque response of PLA70 PCL30 -g-AA/starch is significantly lower than that of PLA70 PCL30 /starch. This improved rheological behavior of PLA70 PCL30 -g-AA/starch is due to the conformational change of the starch molecule [43], caused by the formation of an ester functional group as discussed under FTIR and XRD analysis. This result is in agreement with a previous study of Sagar and Merril [44]; it was shown that the melt viscosity of esterified starches also decreased with increasing molecular weight of the ester group. Table 2 Melting temperature and melting enthalpy values of PLA/PCL blends. Sample
Tm (◦ C)
Hm (J/g)
PLA Neat PLA PLA90 PCL10 PLA80 PCL20 PLA70 PCL30 PLA60 PCL40 PLA50 PCL50 PLA40 PCL60 PLA20 PCL80 Neat PCL
161.2 160.5 159.8 158.9 159.3 159.6 160.1 160.6 –
PCL ± ± ± ± ± ± ± ±
0.6 0.7 0.4 0.5 0.8 0.7 0.8 0.8
64.5 65.3 65.9 65.1 64.6 63.9 63.2 62.5
PLA ± ± ± ± ± ± ± ±
– 0.3 0.2 0.2 0.4 0.3 0.2 0.3 0.2
36.5 31.6 25.8 21.8 16.6 13.8 10.6 7.2 –
PCL ± ± ± ± ± ± ± ±
0.8 0.7 0.7 0.6 0.5 0.4 0.3 0.2
7.1 14.5 19.2 26.6 34.3 40.8 53.6 72.5
± ± ± ± ± ± ± ±
– 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
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Table 3 Melting temperature and melting enthalpy values of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends for different starch contents. Starch (wt%)
PLA70 PCL30 /starch
PLA70 PCL30 -g-AA/starch
◦
PLA 0 10 20 30 40 50
158.9 158.2 157.6 157.2 157.0 156.9
PCL ± ± ± ± ± ±
Tm (◦ C)
Hm (J/g)
Tm ( C)
0.6 0.6 0.5 0.5 0.4 0.4
65.9 65.1 64.5 64.1 63.8 63.6
PLA ± ± ± ± ± ±
0.3 0.2 0.2 0.2 0.2 0.2
21.8 18.0 16.1 14.6 12.2 10.2
PCL ± ± ± ± ± ±
0.7 0.6 0.6 0.5 0.4 0.3
19.2 17.1 15.2 13.5 11.6 9.0
among the blends. So, the PLA70 PCL30 blend was chosen to prepare the ternary blend by blending it with the starch. Table 3 gives the effect of starch content on the melting temperature (Tm ) and the melting enthalpy (Hm ) of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends. The Tm /Hm values of PLA and PCL in the PLA70 PCL30 blend are 158.9 ◦ C/21.8 J g−1 and 65.9 ◦ C/19.2 J g−1 , respectively, while those of PLA70 PCL30 -g-AA are 158.3 ◦ C/19.8 J g−1 and 65.6 ◦ C/18.5 J g−1 . The lower values of Hm and Tm for the PLA70 PCL30 -g-AA are probably due to grafted branches that disrupt the regularity of the chain structures in PLA70 PCL30 -g-AA and increase the spacing between the chains. The Tm and Hm values of ternary blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch) both decrease with an increasing of starch content due to the lower melting viscosity of starch and to the starch prohibiting movement of the polymer segments, causing polymer chain arrangement to become more difficult. With the same starch content, the PLA70 PCL30 /starch blend gives a higher Tm value but a lower Hm value (indicating lower percentage crystallinity of the blend) as compared with the PLA70 PCL30 -gAA/starch one, presumably due to the ester carbonyl functional group generated from the reaction between the –OH groups of starch and the –COOH groups of PLA70 PCL30 -g-AA. The lower per-
Hm (J/g)
PLA ± ± ± ± ± ±
0.5 0.5 0.4 0.4 0.4 0.3
158.3 157.7 157.2 156.7 156.5 156.3
PCL ± ± ± ± ± ±
0.4 0.3 0.3 0.2 0.2 0.2
65.6 64.7 64.1 63.6 63.2 62.8
PLA ± ± ± ± ± ±
0.3 0.2 0.2 0.2 0.2 0.1
19.8 18.6 18.0 17.1 16.8 16.3
PCL ± ± ± ± ± ±
0.6 0.5 0.5 0.4 0.3 0.3
18.5 17.6 17.1 16.4 16.2 15.8
± ± ± ± ± ±
0.5 0.4 0.4 0.4 0.3 0.3
centage crystallinity of the PLA70 PCL30 /starch blend is also due to the reason that the hydrophilic character of starch leads to poor adhesion with the hydrophobic PLA70 PCL30 , causing a steric effect [43,46]. The lower melting temperature is also in accordance with the lower melting viscosity of PLA70 PCL30 -g-AA/starch. Hence, the PLA70 PCL30 -g-AA/starch blend is more easily processed than the PLA70 PCL30 /starch one. 3.7. Blend morphology In this study, PLA70 PCL30 or PLA70 PCL30 -g-AA forms the polymer matrix, whereas the starch is the dispersed phase. So, good dispersion of starch, effective wetting of starch by the polymer, and strong interfacial adhesion between two phases are required to obtain satisfactory mechanical properties. To study the distribution and the phase size of starch in the polymer matrix, tensile fractured surfaces of blends were examined with a scanning electron microscope. The SEM microphotographs of PLA70 PCL30 , PLA70 PCL30 /starch, and PLA70 PCL30 -g-AA/starch are presented in Fig. 6. By examining the morphologies shown in Fig. 6(B and C), it can be seen that the PLA70 PCL30 /starch (60 wt%) blend gives the larger starch phase size (the average pore diameter) than the PLA70 PCL30 /starch (30 wt%)
Fig. 6. SEM microphotographs of (A) PLA70 PCL30 , (B) PLA70 PCL30 /starch (30 wt%), (C) PLA70 PCL30 /starch (60 wt%), and (D) PLA70 PCL30 -g-AA/starch (30 wt%).
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Table 4 Starch phase sizes of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends for different starch contents. Starch (wt%)
Phase size (m) PLA70 PCL30 /starch
10 20 30 40 50
7.6 12.8 16.3 19.2 22.8
± ± ± ± ±
0.8 1.0 1.1 1.3 1.5
PLA70 PCL30 -g-AA/starch 2.8 3.3 3.9 4.2 4.5
± ± ± ± ±
0.5 0.6 0.7 0.8 0.8
blend. In addition, as the comparison of Fig. 6(B and D), it is clear that the PLA70 PCL30 -g-AA/starch (30 wt%) blend has the smaller starch phase size than the PLA70 PCL30 /starch (30 wt%) blend. However, it is realized that the blend has larger starch phase size, especially in the PLA70 PCL30 /starch (60 wt%), it leads to worse adhesion and compatibility between starch and polymer matrix. The bad dispersion of starch in the PLA70 PCL30 matrix is due to the wide difference in character between the hydrophobic PLA70 PCL30 and the hydrophilic starch. On the contrary, better dispersion and homogeneity of starch in the PLA70 PCL30 -g-AA matrix are due to the more similar character between the starch surface and the PLA70 PCL30 -g-AA matrix, and the formation of ester carbonyl functional groups via the reaction between the –OH groups of starch and the –COOH groups of PLA70 PCL30 -g-AA. For PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends with different starch contents were also examined by SEM but their SEM microphotographs are not shown here. The starch phase size of fractured surfaces of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends can be measured from their SEM microphotographs and the results are tabulated in Table 4. It is clear that the starch phase size of both blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch) increases with an increasing of starch content and that the latter gives much smaller phase size than the former. It is also found that fine dispersion of starch in the PLA70 PCL30 -g-AA matrix can be obtained for all PLA70 PCL30 -g-AA/starch blends, as the phase size is never greater than 4.5 ± 0.8 m. This better dispersion arises from the formation of branched and crosslinked macromolecules, since the PLA70 PCL30 -g-AA copolymer has carboxylic acid groups that can react with the hydroxyl groups of starch. So, the PLA70 PCL30 g-AA/starch blend provides the better dispersion and homogeneity of starch in the polymer matrix.
Fig. 7. Effect of starch content on tensile strength and elongation values at breakpoint of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends.
tion between the –OH groups of starch and the –COOH groups of PLA70 PCL30 -g-AA. 3.9. Water absorption
3.8. Mechanical properties of ternary blends
Water uptake versus immersed time curves for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends with different amount of starch is given in Fig. 8. It can be seen that the water resistance of PLA70 PCL30 is somewhat better than that of PLA70 PCL30 -g-AA but the PLA70 PCL30 -g-AA/starch blend exhibits better water resistance than the PLA70 PCL30 /starch blend at the same starch content. The increment of water absorption for PLA70 PCL30 /starch, as compared with the water uptake of PLA70 PCL30 -g-AA/starch, is about 0.5–2%. The lower water absorption of PLA70 PCL30 -g-AA/starch, as compared with PLA70 PCL30 /starch, is attributed to the formation of ester carbonyl functional groups. For both ternary blends, the water uptake over the 6-week test period increased with an increasing of starch content. These phenomena are similar to the findings on polyethylene/starch blends proposed by Bikiaris and Panayiotou [38] and are probably due to the increased difficulty in forming polymer chain arrangements with greater amounts of starch, and to the hydrophilic character of starch causing poor adhesion with the hydrophobic PLA70 PCL30 .
The effect of starch content on the tensile strength and the elongation at break for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends is illustrated in Fig. 7. It can be seen that the tensile strength and the elongation at break of PLA70 PCL30 are larger that those of PLA70 PCL30 -g-AA but the PLA70 PCL30 -g-AA/starch blends give better mechanical properties than the PLA70 PCL30 /starch ones. The solid circles in Fig. 7 illustrate that both the tensile strength and elongation at break of PLA70 PCL30 /starch blends decrease continuously and markedly from 41.7 ± 1.0 MPa/349.7 ± 29.9% to 18.9 ± 1.7 MPa/169.5 ± 25.4% as the starch content is increased from 0 to 50 wt%. The deterioration in mechanical properties of PLA70 PCL30 /starch blends may be due to the larger starch phase size (Table 4) and the poor wetting between starch and PLA70 PCL30 . As shown by the hollow circles in Fig. 7, the PLA70 PCL30 -g-AA/starch blends not only provide higher values of tensile strength and elongation at break than the PLA70 PCL30 /starch ones but also give a plateau mechanical property with the addition of starch up to 50 wt%. A contribution to this result may be due to the better dispersion and smaller phase size of starch in the PLA70 PCL30 -g-AA matrix, and the formation of ester carbonyl functional groups via the reac-
Fig. 8. Water uptakes of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends during water absorption.
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ester carbonyl functional groups via the reaction between –OH groups of starch and –COOH groups of PLA70 PCL30 -g-AA. The water resistance of PLA70 PCL30 -g-AA/starch was slightly higher than that of PLA70 PCL30 /starch, and in a soil environment the latter gave the better biodegradation than former. Finally, the PLA70 PCL30 g-AA/starch blend not only gave better thermal and mechanical properties than those of the PLA70 PCL30 -g-AA/starch one but also provided more easily processing properties due to its lower values of melting viscosity and blending torque. References
Fig. 9. Weight losses of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends in a soil environment.
3.10. Biodegradation analysis Fig. 9 shows the change in weight loss, indicating the extent of biodegradation, with time buried in soil for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends with different amount of starch. During the period of biodegradation test, the water diffused into the polymer sample, causing swelling and enhancing biodegradation. As expected, the weight loss of PLA70 PCL30 is somewhat smaller than that of PLA70 PCL30 -g-AA since the latter has better water absorption. Moreover, both the PLA70 PCL30 /starch and PLA70 PCL30 g-AA/starch blends degrade rapidly in the initial 8 weeks due to the easy degradation of starch, and a gradual decrease of weight occurs during the next 8 weeks. With the same starch content, the PLA70 PCL30 /starch blend has a higher value of weight loss (about 3–12%) than the PLA70 PCL30 -g-AA/starch one. The greater biodegradation of PLA70 PCL30 /starch may be caused by the same factors leading to its higher absorption of water. 4. Conclusions This study revealed that PLA could be tuned from rigid to ductile by incorporating PCL using the melt blending method, and that the cost of PLA/PCL blends could be further reduced by the addition of starch. The DSC tests showed that the PLA/PCL blends exhibited two separate Tm s, demonstrating that PLA and PCL formed a partial miscible blend, and that the PLA70 PCL30 blend gave the strongest interaction intensity among the blends due to the largest shift of Tm s. According to the result of thermal and mechanical examinations, the PLA70 PCL30 blend is suitable for the preparation of ternary blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch). Compatibility and mechanical properties of the PLA70 PCL30 /starch blend could be improved markedly by using PLA70 PCL30 -g-AA in place of PLA70 PCL30 . Based on the combination of FTIR, XRD, and SEM characterizations, this outcome is probable due to the better dispersion and smaller starch phase size in the PLA70 PCL30 -g-AA matrix (being always less than 4.5 ± 0.8 m for PLA70 PCL30 -g-AA/starch but larger than 7.6 ± 0.8 m for PLA70 PCL30 /starch), and the formation of
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Preparation and characterization of ternary blends composed of polylactide, poly(-caprolactone) and starch Hsin-Tzu Liao ∗ , Chin-San Wu Department of Chemical and Biochemical Engineering, Kao Yuan University, Kaohsiung County, Taiwan 82151, Republic of China
a r t i c l e
i n f o
Article history: Received 30 October 2008 Received in revised form 27 February 2009 Accepted 2 March 2009 Keywords: Blends PLA PCL Starch Biodegradability
a b s t r a c t By means of a melt blending method, a ternary blend composed of polylactide (PLA), poly(-caprolactone) (PCL) and starch was prepared to conquer the major shortcomings (brittle and high price) of PLA. By the addition of PCL, PLA was tuned from rigid to ductile but its tensile strength was also reduced. To overcome the poor compatibility between PLA70 PCL30 and starch, the acrylic acid grafted PLA70 PCL30 (PLA70 PCL30 g-AA) was chosen as the alternative for the preparation of ternary blends. Owing to the formation of ester carbonyl groups, the PLA70 PCL30 -g-AA/starch blend gave a much better dispersion and homogeneity of starch in the PLA70 PCL30 -g-AA matrix, and consequently led to apparently better properties. In a soil environment, the PLA70 PCL30 /starch gave the better biodegradation than the PLA70 PCL30 -g-AA/starch but the difference was slight. Finally, the PLA70 PCL30 -g-AA/starch blend not only provided a plateau tensile strength at break up to 50 wt% starch but also provided more easily processing properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the massive use and disposal of petroleumbased plastics had been seriously caused public affairs over the environmental hazards. The main strategies to address these problems are to develop materials that are renewable, degradable, and recyclable, better known as “green materials,” [1–5] as alternatives to the petroleum-based materials. The most popular and important biodegradable polymers are aliphatic polyesters [e.g. polylactide (PLA), poly(-caprolactone) (PCL), polyethylene oxide (PEO), poly(3-hydroxybutyrate) (PHB), and polyglycolic acid] and thermoplastic proteins. The biodegradable PLA had attracted increasing attention as a candidate for us in industrial applications since it possessed excellent mechanical properties (especially in tensile strength and modulus) and was produced with corn or other starches as the starting raw material [6–10]. With these advantages, PLA was a versatile material with applications in the medical, textile, and packaging fields [11–14], but its brittleness and high price in comparison to petroleum-based thermoplastics limited its applications. So, the choice of suitable plasticizers to improve the brittle disadvantage of PLA was an important issue. Various types of chemicals (e.g. citrate esters, glucose monoesters, and partial fatty acid esters) have been tried to plasticize PLA via the polar interactions occurring between the ester groups in the plasticizer and PLA [15–20]. However, it
∗ Corresponding author. Fax: +886 7 6077788. E-mail address: [email protected] (H.-T. Liao). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.03.003
was observed that, upon aging of the plasticized film materials, low molecular weight plasticizers had a tendency to migrate to the film surface. In the polymer literatures, considerable efforts had been made to modify the properties of PLA by blending it with some hydrophilic polymers, such as poly(-caprolactone) (PCL), poly(vinyl alcohol) (PVA) [21,22], poly(ethylene glycol) (PEG) [23], pluronic [triblockcopolymers of PEG and poly(propylene oxide)] [24], hyaluronic acid [25], and poly(vinyl acetate) (PVAc) [26], and the results were in terms of their miscibility, morphology, physical properties, biodegradability, drug release properties, and porous material preparation. The PCL, a ductile biodegradable polymer, can be used to tune the PLA from rigid to ductile since the former has been chosen as a blending partner for the latter [27–29]. In the present study, to overcome the limitations (brittle and high price) of polylactide (PLA), it is attempted to prepare and characterize ternary blends composed of PLA, PCL, and starch. At first, we searched the optimal composition for the preparation of PLA/PCL blend and then evaluated the practicability of the addition of starch to PLA/PCL blends. The acrylic acid grafted PLA/PCL (PLA/PCL-g-AA) was used as an alternative for the preparation of ternary blends to improve the compatibility and dispersion of starch in the polymer matrix. The conclusions are based on a combination of a Fourier transform infrared (FTIR) spectroscopy, an X-ray diffractometer, a differential scanning calorimetry (DSC), a scanning electron microscopy (SEM) and an Instron mechanical tester. Additionally, the water absorption test and the soil biodegradable test were used to assess the effect of starch content on water resistance and biodegradability of ternary blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch).
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2. Experimental
The polylactide (PLA), consisting of 95% l-lactide and 5% mesolactide, and the commercial grade polycaprolactone (PCL, CAPA 6800), with a molar mass of 80,000 g/mol, were obtained from Cargill-Dow and Solvay Chemicals, respectively. Acrylic acid (AA) and benzoyl peroxide (BPO) are supplied by Aldrich Chemical Corporation (Milwaukee, WI). Before use, the acrylic acid was purified by re-crystallization from chloroform and the benzoyl peroxide, use as the initiator, was purified by dissolving it in chloroform and reprecipitating with methanol. The starch, with a granule size of 15–100 m, composed of 27% amylose and 73% amylopectine was supplied by Sigma Chemical Corporation. All the reactants were used as received. The PLA70 PCL30 -g-AA copolymer, having the grafting percentage of 6.85%, was synthesized in our laboratory as described below.
the reaction approached equilibrium after 5 h. To confirm that equilibrium reaction was established, all the reactions lasted for 6 h. The grafting product (ca. 4 g) was dissolved in 200 ml of refluxing xylene at 85 ◦ C and then the hot solution was filtered through several layers of cheesecloth. The xylene-insoluble product remaining on the cheesecloth was washed using acetone to remove the unreacted acrylic acid and was then dried in a vacuum oven at 80 ◦ C for 24 h. The xylene-soluble product in the filtrate was extracted five times, using 600 ml of cold acetone for each extraction. Using a titration method, the acrylic acid loading of the xylene-soluble polymer was calculated from the acid number and the result was expressed as the grafting percentage. About 2 g of copolymer was heated for 2 h in 200 ml of refluxing xylene. This solution was then titrated immediately with a 0.03N ethanolic KOH solution, which had been standardized against a solution of potassium hydrogen phthalate, while phenolphthalein was used as an indicator. The acid number and the grafting percentage could then be calculated using the following equations [30]:
2.2. Preparation of PLA/PCL blends
acid number (mg KOH/g) =
PLA/PCL blends were prepared by the melt blending method using a Brabender “Platograph” 200 N m W50EHT mixer (Duisburg, Germany) with a blade-type rotor. Before blending, PLA and PCL were dried in a vacuum oven at 80 ◦ C for 6 h. The mass ratios of PLA to PCL were chosen as 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 20/80 and 0/100. PLAx PCLy was used as the symbol for the PLA/PCL blend, where x and y indicated mass percentages of PLA and PCL, respectively. A determined amount of PLA and PCL was put into the Brabender instrument to melt the mixture under the conditions that the rotor speed and the blending temperature were kept at 50 rpm and 190–200 ◦ C, respectively. To ensure complete mixing of PLA and PCL, the operation lasts for another 15 min when the blending torque approaches constant (about 3 min). The hybrid products were transferred to a mold (10 cm × 10 cm × 0.1 cm) to press into thin plates by a hot press at 10 atm and 190 ◦ C for 2 min and then they were put into a dryer for cooling. Next, the cool thin plates were made into standard specimens for characterizations.
grafting percentage (%) =
2.1. Materials
2.3. 2.3Preparation of PLA70 PCL30 -g-AA copolymer Based on the result of preliminary characterization, the PLA70 PCL30 blend was chosen as the polymer matrix to prepare the ternary blend (PLA70 PCL30 /starch). To improve the properties of PLA70 PCL30 /starch blend, the acrylic acid grafted PLA70 PCL30 (PLA70 PCL30 -g-AA) was used to replace PLA70 PCL30 to produce the ternary blend. The total amount of PLA70 PCL30 , AA, and BPO used for the manufacture of PLA70 PCL30 -g-AA are 40 g, and the weight percentages of each component are listed in Table 1. A mixture of AA and BPO was added in four equal portions at 2-min intervals to the solution of PLA70 PCL30 in 250 ml xylene to allow grafting reaction to take place. The reactions were carried out at a temperature of 85 ± 2 ◦ C and a rotor speed of 60 rpm under a nitrogen atmosphere at a flow rate of 25 cm3 /min. Preliminary experiments showed that Table 1 Grafting percentage values of acrylic acid onto PLA70 PCL30 for different BPO and AA loadings. AA loading = 10 wt%
BPO loading = 0.3 wt%
BPO (wt%)
Grafting percentage (%)
AA (wt%)
Grafting percentage (%)
0.1 0.2 0.3 0.4 0.5
2.81 4.50 6.85 7.05 7.17
5 10 15 20
5.85 6.85 7.11 7.25
± ± ± ± ±
0.08 0.14 0.21 0.21 0.22
± ± ± ±
0.18 0.21 0.21 0.22
VKOH (ml) × CKOH (N) × 56.1 polymer (g)
acid number × 72 561
(1) (2)
2.4. Preparation of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends Prior to blending, the starch was cleaned with acetone and then dried in an oven at 50 ◦ C for 24 h to remove acetone completely. To eliminate potential effects of residual AA on the PLA70 PCL30 g-AA/starch blend, it was removed from the PLA70 PCL30 -g-AA copolymer product by acetone extraction and then dried in a vacuum oven at 80 ◦ C for 6 h before preparation of blends. The mass ratios of starch to PLA70 PCL30 or to PLA70 PCL30 -g-AA were set at 10/90, 20/80, 30/70, 40/60, and 50/50. A determined amount of PLA70 PCL30 + starch or PLA70 PCL30 -g-AA + starch was put into the Brabender instrument to prepare ternary blends. The rotor speed and the blending temperature were kept at 50 rpm and 190–200 ◦ C, respectively, and the blending reaction was carried out for 18 min. After blending, blends were pressed into thin plates using a hot press and were then put into a dryer for cooling. Subsequently, the thin plates were made into standard specimens for characterization. The Brabender was also used for torque recording during the melt kneading of ternary blends. 2.5. Characterizations of blending polymers A scanning electron microscope (Hitachi microscope Model S1400, Japan) was used to study the morphology of blends and to measure the starch phase size in the polymer matrix. Before the tests, the blend was prepared in a thin film by a hydrolytic press and then the film was treated with hot water at 80 ◦ C for 24 h. Afterward, the film was coated with gold and observed by SEM. The Fourier transform infrared (FTIR) spectrophotometer (BioRad FTS-7PC type, Madison, WI), using thin films, was used to investigate the graft reaction of acrylic acid onto PLA70 PCL30 and to verify whether chemical bonding was formed in blends. The Bruker AMX400 1 H NMR spectrometer was used to obtain the spectra of PLA70 PCL30 and PLA70 PCL30 -g-AA. The sample was first dissolved in CDCl3 and sealed in a NMR tube (10 mm O.D.). Then, after being degassed, analysis was performed at conditions of 100 MHz, 30◦ pulse and 4 s cycle time. The melting temperature (Tm ) and the melting enthalpy (Hm ) of samples were determined from a differential scanning calorimeter (TA Instrument 2010 DSC system, New Castle, DE). For DSC tests, sample sizes ranged from 4 to 6 mg and the melting curves were
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recorded at a temperature range of −100 to 250 ◦ C, scanned at a heating rate of 10 ◦ C/min. The values of Tm and Hm are determined from the temperature and area of melting peaks of DSC heating thermograms of samples. Structural changes in the blends were investigated by comparison of X-ray diffraction data obtained from PLA70 PCL30 , PLA70 PCL30 -g-AA, PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch, recorded with a Rigaku D/max 3 V X-ray diffractometer, using Cu K␣ radiation at a scanning rate of 2◦ min−1 . Tensile strength and elongation at break of blends (PLA/PCL, PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch) were carried out on an Instron tensile testing machine (Model LLOYD, LR5K type) according to the ASTM D638, with a crosshead speed of 50 mm/min. A 35 mm gauge length was used during each tensile experiment. The dimensions of the dog-bone shaped specimens were prepared according to ASTM D638 type IV standard, and each sample’s width and thickness were measured before testing. Five measurements were taken for each sample and the data were averaged to obtain a mean value. Samples for measuring water absorption were in the form of 75 mm × 25 mm film strips (150 ± 5 m thickness) following the ASTM D570-81 method. The samples were dried in a vacuum oven at 50 ± 2 ◦ C for 8 h, cooled in a desiccator and then immediately weighed to the nearest 0.001 g (designated as Wc ). Thereafter, the conditioned samples were immersed in distilled water, maintained at 25 ± 2 ◦ C, for a 6-week test period. During this period, they were removed from the water at 1-week interval, gently blotted with tissue paper to remove excess water on the surface, immediately weighed to the nearest 0.001 g (designated as Ww ), and returned to the water. Each Ww was an average value obtained from three measurements. The final water uptake was calculated to the nearest 0.01% as follows. water uptake (%) =
Ww − Wc × 100% Wc
(3)
Biodegradability of the samples was studied by evaluating the weight loss of blends over time in a soil environment. Samples of 30 mm × 30 mm × 1 mm were weighed and then buried in boxes of alluvial-type soil, obtained in March 2006 from farmland topsoil before planting. The soil was sifted to remove large clumps and plant debris. Procedures used for soil burial followed the method described by Chandra and Rustgi [31]. Soil was maintained at approximately 20% moisture in weight and samples were buried at a depth of 15 cm. A control box consisted of only samples and no soil. The samples were dug out at each two weeks, during a 16-week period of soil burial, washed in distilled water, dried in a vacuum oven at 50 ± 2 ◦ C for 24 h and, before evaluation, equilibrated in a desiccator for at least a day. The samples were then weighed before returning them to the soil. The weight loss for extended burial periods, as an indication of biodegradability, was calculated as follows. weight loss (%) =
Wo − Wf × 100% Wo
(4)
where Wo is the original sample weight and Wf is the original sample weight after soil burial.
209
Fig. 1. Effect of PCL content on tensile strength and elongation values at breakpoint of PLA/PCL blends.
b value and a high εb value of 37.6 ± 1.0 MPa and 575.3 ± 59.3%, respectively. It is clear that the addition of PCL makes PLA/PCL blends a substantial reduction in b value but an increase in εb value as their PCL content increases. For example, as the PCL content of PLA/PCL blends increases from 0 to 30 wt%, the b value reduces from 53.7 ± 1.5 to 41.7 ± 1.0 MPa but the εb value increases from 3.6 ± 0.4 to 350.7 ± 30.7%. Apparently, the inherent brittle deformation behavior of the PLA specimen was successfully transformed into relatively ductile fracture behavior after blending sufficient amounts of PCL in PLA resins. In the latter study of this work, the PLA70 PCL30 blend is chosen to blend with the starch to reduce the cost of blend since it provide suitable mechanical properties applied in industrial products. 3.2. Effects of ingredients on the graft reaction The effect of BPO loading and AA loading on the grafting reaction of AA onto the PLA70 PCL30 blend had been studied and the result is given in Table 1. It can be seen that, with constant AA loading of 10 wt%, the value of grafting percentage increased steadily with an increasing of BPO loading up to 0.3 wt%, and then approached to constant. Similar to the effect of BPO loading, it can be observed that, with constant BPO loading of 0.3 wt%, the value of grafting percentage is also enhanced as the AA loading is increased, but it is increased slightly when the AA loading is beyond 15 wt%. This result may be due to the fact that at low AA loading, the diffusion of monomer molecules toward the free radical sites on the PLA70 PCL30 backbone governs the grafting extent, whereas at higher AA concentration, the grafting percentage remains almost constant as the number of free-radical sites available on the PLA70 PCL30 backbone becomes a limiting factor [32]. The grafting percentage of PLA70 PCL30 -g-AA is about 6.85% when AA loading and BPO loading are kept at 10 and 0.3 wt%, respectively, and this copolymer was used as the alternative for the preparation of ternary blends in this study.
3. Results and discussion 3.3. FTIR/NMR analysis 3.1. Mechanical properties of PLA/PCL blends The effect of PCL content on values of the tensile strength and the elongation at break for PLA/PCL blends is illustrated Fig. 1. The result shows that the neat PLA gives a high tensile strength at break ( b ) of 53.7 ± 1.5 MPa and a low elongation at break (εb ) of 3.6 ± 0.4%, exhibiting that PLA is a high modulus and brittle material. On the contrary, the neat PCL is a ductile material with a relative low
Fig. 2(A–D) shows the FTIR spectra of PLA70 PCL30 , PLA70 PCL30 g-AA, PLA70 PCL30 /starch (30 wt%), and PLA70 PCL30 -g-AA/starch (30 wt%), respectively, in the range of 500–4000 cm−1 . For the PLA70 PCL30 blend, it can be seen from Fig. 2A that there are no extra peaks can be observed except the characteristic peaks of PLA and PCL at 3300–3700, 2800–2958, 1754, 1725, and 500–1500 cm−1 [33,34], implying that the PCL is physically dispersed in the PLA
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Fig. 3.
Fig. 2. FTIR spectra of (A) PLA70 PCL30 , (B) PLA70 PCL30 -g-AA, (C) PLA70 PCL30 /starch (30 wt%), and (D) PLA70 PCL30 -g-AA/starch (30 wt%) in the wavenumber range from 500 to 4000 cm−1 .
to form a partial miscible blend. This result can be further confirmed by XRD and DSC examinations of PLA/PCL blends. It is also found that all the characteristic peaks of PLA and PCL appear in Fig. 2(B–D), revealing that the major structure of PLA and PCL is not altered during the period of grafting reaction and the blending procedure. To obtain more clear observation, the FTIR spectra in the range of 1700–1800 cm−1 were expanded. The comparison between FTIR spectra of PLA70 PCL30 and PLA70 PCL30 -g-AA [Fig. 2(A and B)] shows that an extra peak at 1709 cm−1 , assigned to –C O, as well as a broad O–H stretching absorbance at 3200–3700 cm−1 can be found in the spectrum of the later. This result demonstrates that acrylic acid had been grafted on the PLA70 PCL30 blend since the discernible shoulder near 1710 cm−1 represents the formation of free acid in the modified polymer blend. To further confirm that the AA monomer has been grafted on the PLA70 PCL30 , the 1 H NMR spectra of PLA70 PCL30 and PLA70 PCL30 -g-AA had been made and depict in Fig. 3. It can be seen that the unmodified PLA70 PCL30 shows five hydrogen peaks at 1: ı = 5.0–5.2 ppm; 2: ı = 1.4–1.5 ppm; 3: ı = 3.8–4.0 ppm; 4: ı = 1.4–1.7 ppm; and 5: ı = 2.1–2.3 ppm. This result is similar to the findings of other works [35,36]. As a comparison between Fig. 3(A and B), there are three extra hydrogen peaks at ı = 1.8–2.0, 2.2–2.4, and 5.3–5.4 ppm in the spectrum of PLA70 PCL30 -g-AA due to the grafting of AA onto the main or branch chain of the PLA70 PCL30 skeleton [37]. In addition, Fig. 2(C and D) further shows that much more intense of peaks at 3200–3700 cm−1 , assigned to O–H bond stretching vibration, can be seen in the FTIR spectra of PLA70 PCL30 /starch (30 wt%) and PLA70 PCL30 -g-AA/starch (30 wt%). This is because the –OH group of starch causes and contributes to the bond stretching vibration [31,38]. The FTIR spectrum of PLA70 PCL30 /starch (Fig. 2C) shows nearly the same absorbance of the PLA70 PCL3 blend besides
1
H NMR spectra of (A) PLA70 PCL30 and (B) PLA70 PCL30 -g-AA.
a broad O–H stretching absorbance at 3200–3700 cm−1 , implying that the hydrophilic starch is dispersed physically in the hydrophobic polymer matrix. Further comparison of the FTIR spectra of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch, the latter exhibits a new absorption peak at 1737 cm−1 , which can be assigned to the ester carbonyl stretching vibration in the copolymer. Appearance of this new absorption peak is perhaps due to the formation of an ester carbonyl functional group via the reaction between the –OH group of starch and the –COOH group of PLA70 PCL30 -g-AA [38]. 3.4. XRD analysis Fig. 4 illustrates the XRD patterns of PLA70 PCL30 , PLA70 PCL30 /starch (30 wt%), PLA70 PCL30 /starch (60 wt%), and PLA70 PCL30 -g-AA/starch (30 wt%) at 2 = 5–35◦ . As the result of Fig. 4A, it is also found that the PCL is physically dispersed in the PLA since only the characteristic diffraction peaks of PLA (2 = 16.4◦ and 18.6◦ ) and PCL (2 = 21.6◦ and 23.8◦ , reflecting the orthorhombic crystal phase of PCL) can be observed in the XRD pattern of PLA70 PCL30 (Fig. 4A). The diffraction peaks at about 2 = 16.4◦ and 18.6◦ are assigned to the reflection of ␣ crystal cell of PLA, which is orthorhombic with chains in a −10/3 helical conformation [39,40]. For the PLA70 PCL30 /starch (30 wt%) blend, it can be found from Fig. 4B that there are two extra peaks at about 15.3◦ and 17.1◦ as compared with the XRD pattern of PLA70 PCL30 (Fig. 4A). The extra peaks at 15.3◦ and 17.1◦ are assigned to amylose and amylopectine of starch, respectively, and may be due to the change in coordinate property of PLA70 PCL30 molecules when it was blended with starch [41]. The XRD spectra of the PLA70 PCL30 /starch blends, therefore, show that starch is dispersed physically in the PLA70 PCL30 matrix. Moreover, as the comparison between Fig. 4(B and C), it is clear that the higher starch content gives more intensive peaks at 17.1◦ and 15.3◦ . A comparison of the XRD patterns of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch, Fig. 4(B and D), shows a new peak in the latter at about 2 = 18.2◦ , causing by the formation of an ester carbonyl functional group [42]. In agreement with the result described in the FTIR spectroscopy analysis, the XRD pattern provides the evidence that the crystalline structure of the PLA70 PCL30 /starch blend is altered when PLA70 PCL30 -
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Fig. 5. Torque values versus mixing time for preparation of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends at various starch contents.
3.6. Thermal properties of blends Thermal properties (Tm and Hm ) of neat PLA, PLA/PCL blends, PLA70 PCL30 /starch blends, PLA70 PCL30 -g-AA/starch blends, and neat PCL are obtained from their DSC thermograms (not shown here). The variation of melting temperature (Tm ) and melting enthalpy (Hm ) with PCL content for PLA/PCL blends is presented in Table 2. In addition, for PLA70 PCL30 /starch and PLA70 PCL30 -gAA/starch blends, the effect of starch content on the values of Tm and Hm is illustrated in Table 3. As shown in Table 2, the melting temperatures of neat PLA and neat PCL are 161.2 and 62.5 ◦ C, respectively. For the PLA/PCL blends, there are two separate Tm s, corresponding to the PLA-rich phase and the PCL-rich phase. As compared with neat PLA and neat PCL, two separate Tm s of PLA/PCL blends shift toward each other, revealing that an amount of amorphous PCL (or amorphous PLA) is dissolved in the PLA-rich phase (or PCL-rich phase) to form a partial miscible blend. The value of melting enthalpy obtained from the integration of the melting peak indicates the crystallinity of a polymer. As compared with neat PLA and neat PCL, Table 2 shows that the percentage crystallinity of PLA and the percentage crystallinity PCL in the PLA/PCL blend are impaired and increased with the addition of PCL content, respectively. In addition, the variations in values of Tm and Hm are usually assigned to the interactions between components [45]. It can be seen from Table 2 that the Tm values of PLA and PCL in the PLA/PCL blend shift to lower and higher temperatures, respectively, as compared with those of neat PLA and neat PCL, and that the largest shift appears at the PLA70 PCL30 blend. This result reveals that the PLA70 PCL30 blend gives the strongest interaction intensity
Fig. 4. XRD pattern of (A) PLA70 PCL30 , (B) PLA70 PCL30 /starch (30 wt%), (C) PLA70 PCL30 /starch (60 wt%), and (D) PLA70 PCL30 -g-AA/starch (30 wt%).
g-AA is used to replace PLA70 PCL30 for the preparation of blend. 3.5. Torque measurements of ternary blends The curves of torque value versus mixing time for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends are presented in Fig. 5. For both blends, it can be observed that their torque value decreases with increasing starch content and blending time, and approaches a stable value when the blending time is greater than 8 min, suggesting that good mixing has occurred after 12 min. It can also be seen that the final torque decreases with increasing starch content because the viscosity of molten starch is lower than that of molten PLA70 PCL30 and molten PLA70 PCL30 -g-AA. Further, with the same starch content (e.g. 30 and 50 wt%), the torque response of PLA70 PCL30 -g-AA/starch is significantly lower than that of PLA70 PCL30 /starch. This improved rheological behavior of PLA70 PCL30 -g-AA/starch is due to the conformational change of the starch molecule [43], caused by the formation of an ester functional group as discussed under FTIR and XRD analysis. This result is in agreement with a previous study of Sagar and Merril [44]; it was shown that the melt viscosity of esterified starches also decreased with increasing molecular weight of the ester group. Table 2 Melting temperature and melting enthalpy values of PLA/PCL blends. Sample
Tm (◦ C)
Hm (J/g)
PLA Neat PLA PLA90 PCL10 PLA80 PCL20 PLA70 PCL30 PLA60 PCL40 PLA50 PCL50 PLA40 PCL60 PLA20 PCL80 Neat PCL
161.2 160.5 159.8 158.9 159.3 159.6 160.1 160.6 –
PCL ± ± ± ± ± ± ± ±
0.6 0.7 0.4 0.5 0.8 0.7 0.8 0.8
64.5 65.3 65.9 65.1 64.6 63.9 63.2 62.5
PLA ± ± ± ± ± ± ± ±
– 0.3 0.2 0.2 0.4 0.3 0.2 0.3 0.2
36.5 31.6 25.8 21.8 16.6 13.8 10.6 7.2 –
PCL ± ± ± ± ± ± ± ±
0.8 0.7 0.7 0.6 0.5 0.4 0.3 0.2
7.1 14.5 19.2 26.6 34.3 40.8 53.6 72.5
± ± ± ± ± ± ± ±
– 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
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Table 3 Melting temperature and melting enthalpy values of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends for different starch contents. Starch (wt%)
PLA70 PCL30 /starch
PLA70 PCL30 -g-AA/starch
◦
PLA 0 10 20 30 40 50
158.9 158.2 157.6 157.2 157.0 156.9
PCL ± ± ± ± ± ±
Tm (◦ C)
Hm (J/g)
Tm ( C)
0.6 0.6 0.5 0.5 0.4 0.4
65.9 65.1 64.5 64.1 63.8 63.6
PLA ± ± ± ± ± ±
0.3 0.2 0.2 0.2 0.2 0.2
21.8 18.0 16.1 14.6 12.2 10.2
PCL ± ± ± ± ± ±
0.7 0.6 0.6 0.5 0.4 0.3
19.2 17.1 15.2 13.5 11.6 9.0
among the blends. So, the PLA70 PCL30 blend was chosen to prepare the ternary blend by blending it with the starch. Table 3 gives the effect of starch content on the melting temperature (Tm ) and the melting enthalpy (Hm ) of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends. The Tm /Hm values of PLA and PCL in the PLA70 PCL30 blend are 158.9 ◦ C/21.8 J g−1 and 65.9 ◦ C/19.2 J g−1 , respectively, while those of PLA70 PCL30 -g-AA are 158.3 ◦ C/19.8 J g−1 and 65.6 ◦ C/18.5 J g−1 . The lower values of Hm and Tm for the PLA70 PCL30 -g-AA are probably due to grafted branches that disrupt the regularity of the chain structures in PLA70 PCL30 -g-AA and increase the spacing between the chains. The Tm and Hm values of ternary blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch) both decrease with an increasing of starch content due to the lower melting viscosity of starch and to the starch prohibiting movement of the polymer segments, causing polymer chain arrangement to become more difficult. With the same starch content, the PLA70 PCL30 /starch blend gives a higher Tm value but a lower Hm value (indicating lower percentage crystallinity of the blend) as compared with the PLA70 PCL30 -gAA/starch one, presumably due to the ester carbonyl functional group generated from the reaction between the –OH groups of starch and the –COOH groups of PLA70 PCL30 -g-AA. The lower per-
Hm (J/g)
PLA ± ± ± ± ± ±
0.5 0.5 0.4 0.4 0.4 0.3
158.3 157.7 157.2 156.7 156.5 156.3
PCL ± ± ± ± ± ±
0.4 0.3 0.3 0.2 0.2 0.2
65.6 64.7 64.1 63.6 63.2 62.8
PLA ± ± ± ± ± ±
0.3 0.2 0.2 0.2 0.2 0.1
19.8 18.6 18.0 17.1 16.8 16.3
PCL ± ± ± ± ± ±
0.6 0.5 0.5 0.4 0.3 0.3
18.5 17.6 17.1 16.4 16.2 15.8
± ± ± ± ± ±
0.5 0.4 0.4 0.4 0.3 0.3
centage crystallinity of the PLA70 PCL30 /starch blend is also due to the reason that the hydrophilic character of starch leads to poor adhesion with the hydrophobic PLA70 PCL30 , causing a steric effect [43,46]. The lower melting temperature is also in accordance with the lower melting viscosity of PLA70 PCL30 -g-AA/starch. Hence, the PLA70 PCL30 -g-AA/starch blend is more easily processed than the PLA70 PCL30 /starch one. 3.7. Blend morphology In this study, PLA70 PCL30 or PLA70 PCL30 -g-AA forms the polymer matrix, whereas the starch is the dispersed phase. So, good dispersion of starch, effective wetting of starch by the polymer, and strong interfacial adhesion between two phases are required to obtain satisfactory mechanical properties. To study the distribution and the phase size of starch in the polymer matrix, tensile fractured surfaces of blends were examined with a scanning electron microscope. The SEM microphotographs of PLA70 PCL30 , PLA70 PCL30 /starch, and PLA70 PCL30 -g-AA/starch are presented in Fig. 6. By examining the morphologies shown in Fig. 6(B and C), it can be seen that the PLA70 PCL30 /starch (60 wt%) blend gives the larger starch phase size (the average pore diameter) than the PLA70 PCL30 /starch (30 wt%)
Fig. 6. SEM microphotographs of (A) PLA70 PCL30 , (B) PLA70 PCL30 /starch (30 wt%), (C) PLA70 PCL30 /starch (60 wt%), and (D) PLA70 PCL30 -g-AA/starch (30 wt%).
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Table 4 Starch phase sizes of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends for different starch contents. Starch (wt%)
Phase size (m) PLA70 PCL30 /starch
10 20 30 40 50
7.6 12.8 16.3 19.2 22.8
± ± ± ± ±
0.8 1.0 1.1 1.3 1.5
PLA70 PCL30 -g-AA/starch 2.8 3.3 3.9 4.2 4.5
± ± ± ± ±
0.5 0.6 0.7 0.8 0.8
blend. In addition, as the comparison of Fig. 6(B and D), it is clear that the PLA70 PCL30 -g-AA/starch (30 wt%) blend has the smaller starch phase size than the PLA70 PCL30 /starch (30 wt%) blend. However, it is realized that the blend has larger starch phase size, especially in the PLA70 PCL30 /starch (60 wt%), it leads to worse adhesion and compatibility between starch and polymer matrix. The bad dispersion of starch in the PLA70 PCL30 matrix is due to the wide difference in character between the hydrophobic PLA70 PCL30 and the hydrophilic starch. On the contrary, better dispersion and homogeneity of starch in the PLA70 PCL30 -g-AA matrix are due to the more similar character between the starch surface and the PLA70 PCL30 -g-AA matrix, and the formation of ester carbonyl functional groups via the reaction between the –OH groups of starch and the –COOH groups of PLA70 PCL30 -g-AA. For PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends with different starch contents were also examined by SEM but their SEM microphotographs are not shown here. The starch phase size of fractured surfaces of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends can be measured from their SEM microphotographs and the results are tabulated in Table 4. It is clear that the starch phase size of both blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch) increases with an increasing of starch content and that the latter gives much smaller phase size than the former. It is also found that fine dispersion of starch in the PLA70 PCL30 -g-AA matrix can be obtained for all PLA70 PCL30 -g-AA/starch blends, as the phase size is never greater than 4.5 ± 0.8 m. This better dispersion arises from the formation of branched and crosslinked macromolecules, since the PLA70 PCL30 -g-AA copolymer has carboxylic acid groups that can react with the hydroxyl groups of starch. So, the PLA70 PCL30 g-AA/starch blend provides the better dispersion and homogeneity of starch in the polymer matrix.
Fig. 7. Effect of starch content on tensile strength and elongation values at breakpoint of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends.
tion between the –OH groups of starch and the –COOH groups of PLA70 PCL30 -g-AA. 3.9. Water absorption
3.8. Mechanical properties of ternary blends
Water uptake versus immersed time curves for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends with different amount of starch is given in Fig. 8. It can be seen that the water resistance of PLA70 PCL30 is somewhat better than that of PLA70 PCL30 -g-AA but the PLA70 PCL30 -g-AA/starch blend exhibits better water resistance than the PLA70 PCL30 /starch blend at the same starch content. The increment of water absorption for PLA70 PCL30 /starch, as compared with the water uptake of PLA70 PCL30 -g-AA/starch, is about 0.5–2%. The lower water absorption of PLA70 PCL30 -g-AA/starch, as compared with PLA70 PCL30 /starch, is attributed to the formation of ester carbonyl functional groups. For both ternary blends, the water uptake over the 6-week test period increased with an increasing of starch content. These phenomena are similar to the findings on polyethylene/starch blends proposed by Bikiaris and Panayiotou [38] and are probably due to the increased difficulty in forming polymer chain arrangements with greater amounts of starch, and to the hydrophilic character of starch causing poor adhesion with the hydrophobic PLA70 PCL30 .
The effect of starch content on the tensile strength and the elongation at break for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends is illustrated in Fig. 7. It can be seen that the tensile strength and the elongation at break of PLA70 PCL30 are larger that those of PLA70 PCL30 -g-AA but the PLA70 PCL30 -g-AA/starch blends give better mechanical properties than the PLA70 PCL30 /starch ones. The solid circles in Fig. 7 illustrate that both the tensile strength and elongation at break of PLA70 PCL30 /starch blends decrease continuously and markedly from 41.7 ± 1.0 MPa/349.7 ± 29.9% to 18.9 ± 1.7 MPa/169.5 ± 25.4% as the starch content is increased from 0 to 50 wt%. The deterioration in mechanical properties of PLA70 PCL30 /starch blends may be due to the larger starch phase size (Table 4) and the poor wetting between starch and PLA70 PCL30 . As shown by the hollow circles in Fig. 7, the PLA70 PCL30 -g-AA/starch blends not only provide higher values of tensile strength and elongation at break than the PLA70 PCL30 /starch ones but also give a plateau mechanical property with the addition of starch up to 50 wt%. A contribution to this result may be due to the better dispersion and smaller phase size of starch in the PLA70 PCL30 -g-AA matrix, and the formation of ester carbonyl functional groups via the reac-
Fig. 8. Water uptakes of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends during water absorption.
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ester carbonyl functional groups via the reaction between –OH groups of starch and –COOH groups of PLA70 PCL30 -g-AA. The water resistance of PLA70 PCL30 -g-AA/starch was slightly higher than that of PLA70 PCL30 /starch, and in a soil environment the latter gave the better biodegradation than former. Finally, the PLA70 PCL30 g-AA/starch blend not only gave better thermal and mechanical properties than those of the PLA70 PCL30 -g-AA/starch one but also provided more easily processing properties due to its lower values of melting viscosity and blending torque. References
Fig. 9. Weight losses of PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends in a soil environment.
3.10. Biodegradation analysis Fig. 9 shows the change in weight loss, indicating the extent of biodegradation, with time buried in soil for PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch blends with different amount of starch. During the period of biodegradation test, the water diffused into the polymer sample, causing swelling and enhancing biodegradation. As expected, the weight loss of PLA70 PCL30 is somewhat smaller than that of PLA70 PCL30 -g-AA since the latter has better water absorption. Moreover, both the PLA70 PCL30 /starch and PLA70 PCL30 g-AA/starch blends degrade rapidly in the initial 8 weeks due to the easy degradation of starch, and a gradual decrease of weight occurs during the next 8 weeks. With the same starch content, the PLA70 PCL30 /starch blend has a higher value of weight loss (about 3–12%) than the PLA70 PCL30 -g-AA/starch one. The greater biodegradation of PLA70 PCL30 /starch may be caused by the same factors leading to its higher absorption of water. 4. Conclusions This study revealed that PLA could be tuned from rigid to ductile by incorporating PCL using the melt blending method, and that the cost of PLA/PCL blends could be further reduced by the addition of starch. The DSC tests showed that the PLA/PCL blends exhibited two separate Tm s, demonstrating that PLA and PCL formed a partial miscible blend, and that the PLA70 PCL30 blend gave the strongest interaction intensity among the blends due to the largest shift of Tm s. According to the result of thermal and mechanical examinations, the PLA70 PCL30 blend is suitable for the preparation of ternary blends (PLA70 PCL30 /starch and PLA70 PCL30 -g-AA/starch). Compatibility and mechanical properties of the PLA70 PCL30 /starch blend could be improved markedly by using PLA70 PCL30 -g-AA in place of PLA70 PCL30 . Based on the combination of FTIR, XRD, and SEM characterizations, this outcome is probable due to the better dispersion and smaller starch phase size in the PLA70 PCL30 -g-AA matrix (being always less than 4.5 ± 0.8 m for PLA70 PCL30 -g-AA/starch but larger than 7.6 ± 0.8 m for PLA70 PCL30 /starch), and the formation of
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