O R I G I N A L P A P E R
The Properties of PLA/Oxidized Soybean Oil Polymer Blends
Baki HazerPublished online: 6 February 2014
Ó Springer Science+Business Media New York 2014
Abstract Novel polymer blends based on completely renewable polymers were reported. Polymer blends based on polylactic acid (PLA) and oxidized and hydroxylated soya bean oil polymers were prepared. Plasticization and mechanical strength effect of the soya bean oil polymers on the PLA were observed. Fracture surface analysis of the polymer blends was carried out by using scanning electron microscopy. The PLA blends showed more amorphous morphologies compared to pure PLA. The blends had better elongation at break in view of the stress–strain measure-ment. Blend of PLA with the hydroxylated polymeric soya bean oil indicated the slightly antibacterial properties.
Keywords Polylactide Hydroxylated soya oil polymer Propargyl alcohol Linoleic acid polymer Mechanical properties Antibacterial
Introduction
Environmental issue and arguably limitation of the petroleum reserves in the future dramatically push the polymer scientist to look for biodegradable plastic materials derived from renewable resources. C has been very important aliphatic biodegradable polyester derived from a renewable resource such as corn starch [1–7]. Polylactic acid (PLA) is now used worldwide as biomaterials and biodegradable packing films. However, PLA is a brittle polymer and therefore requires plasticization to obtain PLA samples with high elongation for suitable applications [8–10]. In this manner, PEG [9,10], poly(3-hydroxybutyrate) [11, 12], polyethylene [13],
acetylated micro fibrillated cellulose [14], hyperbranched polymer [15], PLA-block copolymers [16–24] were used in order to obtain new modified PLA samples. Soybean oil as a renewable material has been used as a blending partner for polylactide [25]. Isomerization of the double bonds soya oil fatty acid by RuHCl(CO)(PPh3)3 gives dienes in order to
obtain conjugated soybean oil [26]. Hillmyer et al. studied the compatibilization effect of conjugated soya oil on the PLA, in detail [25]. Inspiring from this idea, in this work, we prepared PLA blend with soya oil derivatives, autoxidized and hydroxylated soybean oil which our group have been focused on for nearly one decade [27–31]. For this purpose, soybean oil was polymerized by autoxidation under air oxygen and sunlight at room temperature via peroxidation, epoxidation and perepoxidation in order to obtain oxidized soya oil poly-mer (PSy-ox). The soya oil polypoly-mer peroxide was then con-verted to a novel wholly water soluble hydroxylated soya oil polymer (PSy-OH) from the reaction between autoxidized soya oil polymer and diethanol amine [32]. PLA blends with the autoxidized soya oil polymer and hydroxylated soya oil polymer were studied by means of mechanical properties, fracture surface analysis and antibacterial properties.
Experimental
Materials
Polylactic acid, PLA (PLLA) was a gift from RESINEX-BMY Plastik Kimya Sanayi ve Ticaret A.S¸. Bursa Turkey. GPC: Mn: 42,600, Mw: 73,600, MWD = 1.73. Soya oil was a gift from C¸ otanak/Altas¸¸ Yag˘ Su ve Tarım U¨ ru¨nleri Gıda I˙ns¸aat Otomotiv Nakliyat San. ve Tic. A. S¸. Ordu, Turkey. It contains palmitic acid (10.7 wt%), stearic acid (4.6 wt%), oleic acid (23.5 wt%), linoleic acid (52.8 wt%) B. Hazer (&)
Department of Chemistry, Bu¨lent Ecevit University, 67100 Zonguldak, Turkey
e-mail: bhazer2@yahoo.com DOI 10.1007/s10924-014-0645-z
and linolenic acid (5.7 wt%). Antioxidant adduct inside the commercial soya oil was removed by leaching with ethanol three times. Linoleic acid (cis–cis-9-12-octadeca dienoic acid) was supplied from Fluka (Steinheim, Germany), and it was used as received. Diethanol amine, propargyl alcohol and other chemicals and the solvents were supplied from Aldrich and used without further purification.
Formation of Autoxidized Polymeric Soybean Oil (PSy-ox)
Autoxidation of soya oil was performed according to the modified procedure reported in our previous work [32]. For example, 2.7 g of soya oil spread out in a Petri dish (U = 7 cm, oil thickness: 0.7 mm) was exposed to day-light in the air at room temperature. After a given time of autoxidation (ca. 1 month), a sticky, pale yellow viscous liquid polymer layer was formed with a yield 85–90 wt%. GPC results: Mn: 6,429, Mw: 8,688, MWD: 1.351.
Hydroxylation Reaction of Soya Oil Polymer with Diethanol Amine (PSy-OH)
Hydroxylation reaction between PSy-ox and diethanol amine was performed in a round bottom flask at 90°C for 24 h according to the procedure reported in our very recent paper published [32]. As an example, a round bottom flask (250 mL) containing 5.0 g of PSy-ox and 5.0 mL of dieth-anol amine were kept in an oil bath at 90°C for a day. The crude product was dissolved in 20 mL of acetone. It was filtered and then the hydroxylated soya oil polymer was precipitated into 100 mL of petroleum ether. The dark brown viscose liquid, hydroxylated soya polymer, was dried under vacuum at room temperature for 24 h. Dark brown viscous liquid polymer layer was formed with a yield 85–90 wt%. GPC results: Mn: 5,900, Mw: 8,800, MWD: 1.48.
Hydroxylated Soya Oil Polymer with Propargyl Alcohol (PSy-pentyn)
Synthesis of hydroxylated soya oil polymer with propargyl alcohol (PSy-pentyn) was carried out using the procedure reported before [29]. In this case, propargyl alcohol instead of ethyl alcohol, was used in the hydroxylation reaction. In general, 1 g of PSy-ox, 2 mL of propargyl alcohol and 0.5 g of Fe(NO3)3.4H2O was stirred for 2 days at room
tempera-ture. The crude mixture was dissolved in 20 mL of THF. The solution was filtered and poured into 200 mL of distilled water to precipitate the hydroxy functionalized polymeric oil. The pale yellow, viscose liquid was dried under vacuum at room temperature for 24 h. GPC results: Mn: 2,622, Mw:
3,642, MWD: 1.39.1H NMR (in d ppm): 3.6: –CH2–OH, 4.0:
–CH2–O–, 2.2–2.4: H–CC– and –CH2–COO–.
Autoxidized Linoleic Acid Polymer (Plin-ox)
PLin-ox was obtained through autoxidation of linoleic acid; the same procedure mentioned above [31] was used.
1H NMR spectra of colorless viscose liquid, PLina,
con-tained characteristic peaks of the related segments: (oˆ , ppm): 5.6–6.3 ppm (the vinyl protons), 3.4–3.7 ppm (CH–O–), 2.3 ppm (–CH2–COOH). GPC results: Mn: 2,577, Mw:
4,648, MWD: 1.87.
Hydroxylated Autoxidized Linoleic Acid Polymer with Ethanol (Plin-OH)
Synthesis of hydroxylated polymeric linoleic acid (Plin-OH) was carried out using the other procedure we have reported before [29]. In general, 1 g of Plin-ox, 2 mL of ethyl alcohol and 0.5 g of Fe(NO3)3.4H2O was stirred for
2 days at room temperature. The crude mixture was dis-solved in 20 mL of THF. The solution was filtered and poured into 200 mL of distilled water to precipitate the hydroxy functionalized polymeric oil. The pale yellow viscose liquid was dried under vacuum at room tempera-ture for 24 h. Mn: 2,120, Mw: 6,488, MWD: 3.059.
Preparation of the Blends of PLA with PSy-ox, PSy-OH, Plin-ox, Plin-OH, and PSy-pentyn
Solvent-casting from chloroform solution was used to prepare polymer film samples with 0.2–0.3 mm thickness according to the procedure described in our previous publication [33]. As a typical example of PLA/PSy-ox blend, PLA was dissolved in chloroform first. To this solution was then added the oligomer in one portion. The chloroform solution of the polymer blend was stirred at room temperature for 1 h. The amount of PLA in each run was kept constant (ca. 0.70 g). The solution was filtered and poured in a Petri dish, then it was covered with a piece of paper so that the solvent evaporates slowly. A trans-parent, smooth polymer film was obtained in 1–2 days of solvent evaporation. The film was subsequently dried under vacuum for a week at room temperature.
Instrumentation
Stress–Strain Measurements
Zwick/Roell Tensile Testing Machine using a 50 kg load cell with a stretch speed 100 mm/min was used for stress–
strain measurements of the rectangular shape with size 0.16 9 10 9 50 mm solvent cast film samples from CHCl3.
At least three samples from each blend composition were measured. Samples were dried at room temperature under vacuum for 10 days prior to measurement.
Scanning Electron Microscopy (SEM)
To investigate the microstructure of the polymer blends, the morphology of the fractured surfaces was elucidated by SEM. The specimens were frozen under liquid nitrogen then fractured, mounted, and coated with palladium, gold, and carbon. Scanning electron micrographs were taken on a JEOL JXA-6,335 FS scanning electron microscope (SEM) operated at 15 kV. The electron images were recorded directly from the cathode ray tube on a Polaroid film.
Gel Permeation Chromatography (GPC)
Molecular weights were determined by gel permeation chromatography instrument, Viscotek GPCmax Auto sampler system, consisting of a pump, three ViscoGEL GPC columns (G2000H HR, G3000H HR and G4000H HR), and a Viscotek differential refractive index (RI) detector with a THF flow rate of 1.0 mL/min at 30°C. The RI detector was calibrated with PS standards having narrow molecular weight distribution. Data were analyzed using Viscotek OmniSEC Omni–01 software.
Differential Scanning Calorimeter (DSC)
Thermal analysis of the obtained polymers was carried out under nitrogen using a Perkin Elmer Diamond series thermal analysis system. Dried sample was heated from -60 to 170°C under nitrogen atmosphere at a rate of 10°C/min. The elemental analyses of the samples were carried out on a LECO CHNS 90 instrument.
Determination of Antimicrobial Efficiency
The antibacterial efficiency was determined by using a method described in our previous study [33]. The bacte-rium used in this study was E. coli IFO3972 (Gram-nega-tive) and each sample was repeated five times. The casted polymer films (1.0 9 1.0 cm) were immersed into 10 mL of bacterial suspensions at McFarland 0.5 (108 cfu/mL) prepared in phosphate buffered saline (PBS). Bacteria were inoculated onto the film samples to start the proliferation. Before starting proliferation, colony forming unit was measured in 20 min to determine E. coli cfu/mL in the beginning. After 24 h of incubation at 37°C, the film samples were examined for the presence of growth inhi-bition as colony forming unit (cfu)/mL. Pour plate
technique was used to determine the number of microbes/ mL in a specimen.
Results and Discussion
Synthesis of Soya Oil Polymer Derivatives
Polylactide was blended with completely biodegradable natural modified polymers obtained from the renewable resources in order to improve packaging film properties of PLA. Five types of component; autoxidized soya oil polymer (PSy-ox), hydroxylated soya oil polymer with diethanol amine (PSy-OH), hydroxylated soya oil polymer with propargyl alcohol (PSy-pentyn), autoxidized poly-meric linoleic acid (Plin-ox) and hydroxylated polypoly-meric linoleic acid with ethanol (Plin-OH). Among these oligo-mers, PSy-ox was hydrophobic while the others-hydrox-ylated soya oil polymers were partially hydrophilic. Furthermore, PSy-pentyn is also open to tailoring modifi-cation reaction via azide-alkyn cycloaddition reaction (click chemistry) [34]. Scheme1shows the representative chemical structure of the polymeric soya oil derivatives. Each oligomer was synthesized at least three times with similar molecular weights; some PSy-ox and Plin-ox samples exhibited the higher molecular weight than 10,000 D. Oxidation of the unsaturated fatty acids is strongly related to autoxidation time [32]. However, the hydroxyl-ation process partially degrades the linkages to the oligo-mers with molecular weights lower than 10,000 D. The reason for that, some oxide linkages between triglyceride units could be cleaved by the hydroxylation reagent during the process (Scheme 1). Table1 contains the GPC results of the soya oil polymer moieties used in polymer blends.
Preparation of Polymer Blends
PLA/soya oil polymer blends were prepared in chloroform solution by stirring at room temperature for 1 h.
Fracture Surface Analysis of the Polymer Films SEM. The fractured surface analysis of the polymer film samples was performed by cracking the films into liquid nitrogen. Figure1
represents the SEM micrographs of the fractured surface of polymer blends. The fractured surface of PLA revealed inhomogeneity and formation of voids. The surface of pure PLA also showed layers separated into several parts.
This was an indication of its brittle structure [20]. Frac-tured surface of the polymer blends exhibited layers with surface integrity in a continuous matrix as mentioned in the literature [15]. There is not any aggregation on some local area. This can be attributed to a homogenous distribution of the soya polymer adduct into polylactide matrix. This is in
good agreement in the PLA blends with conjugated soybean oils [25]. These results also support the enhanced mechanical properties of the PLA/soya based derivatives.
Smooth film surfaces of the solvent casted films exhib-ited the different characteristic properties from their frac-ture surfaces related to slow evaporation of the solvent. Figure2exhibits SEM micrographs of the side surface of PLA blends and that of pure-PLA. Solvent casted film
surfaces of the PLA/PSy-ox, PLA/Plin-ox, PLA/Plin-OH, PLA/Pentyn2 blends contained surfaces with holes arising from the evaporated solvent, while solvent casted film surfaces of pure PLA and PLA/PSy-OH blend were very smooth without any holes. This might be the result of phase separation of the components in the blends [35].
Thermal Analysis of the PLA Blends
The temperatures observed in the DSC thermograms for the glass transition (Tg on set) and melting transition (Tm) are compiled in Table2. The PLA blends containing dif-ferent ratio of soya oligomers indicated the plasticizer effect in view of the lower Tg (17–20°C) than that of pure PLA (30°C) were essentially the same as each others. Interestingly, Tm of the polymer blends was found to be 10°C higher than that of pure PLA. DSC traces of some PLA blends have been given in Fig. 3. An exothermic peak at approximately 85°C was observed in the DSC trace of Scheme 1 Representative
chemical structure of the soybean oil based oligomers used in PLA blends:
IAutoxidized soya oil polymer, PSy-ox, II Hydroxylated soya oil polymer by using diethanol amine, PSy-OH, III
Hydroxylated soya oil polymer by using propargyl alcohol, PSy-pentyn, IV Autoxidized linoleic acid polymer, Plin-ox, and V Hydroxylated linoleic acid by using ethanol, Plin-OH
Table 1 GPC results of the soya oil polymer moieties used in PLA blends Oligomer Mn Mw MWD PSy-ox 6,359 9,136 1.44 PSy-OH 5,900 8,800 1.48 PSy-pentyn 2,622 3,642 1.39 Plin-ox 2,577 4,648 1.87 PLina-OH 2,120 6,488 3.06
pure-PLA but not in the traces of the blends. It may be argued that the additives hinder the crystallization tem-perature (Tc) in the blends.
Antibacterial Efficiency
Antibacterial efficiency of the PLA-blends with oxidized soya oil polymer and hydroxylated soya oil polymer against E. coli was evaluated. PLA-blends with hydroxyl-ated soya oil polymer exhibited ten times decrease in bacteria proliferation while PLA with oxidized soya oil polymer showed ten times bacteria growing. Table3shows the antibacterial efficacy of the some polymer blend sam-ples against E. coli.
Mechanical Properties of the Polymer Blends
PLA was blended with five different soya oil polymer derivative: oxidized soya oil polymer, hydroxylated soya
oil polymer functionalized with diethanol amine, hydrox-ylated soya oil polymer functionalized with propargyl alcohol, oxidized linoleic acid polymer, hydroxylated lin-oleic acid polymer functionalized with ethanol. They were prepared in chloroform solution by stirring at room tem-perature for 1 h. The results of stress–strain measurement of the polymer blends were listed in Table4. Tensile strength of the polymer blends was around at 20–26 MPa while that of pure PLA was at around 34 MPa. As tensile strength of the polymer blends decreases, PLA blends with the oligomers exhibited up to ten times greater of the elongation than that of pure PLA. The elongation of the PLA blends strongly depends on the oligomer ratio of the blends. The maximum elongation values were observed as 68.98 ± 9.64 % for PLA-PSy-ox in ratio 14 wt%, 124.3 ± 9.95 % for PLA-PSy-OH in ratio 1.37 wt%, 107.53 ± 5.37 for PLA-Plinox in ratio 3.64, 96.16 ± 4.61 % for PLA-Plin-OH in ratio of 1.79 wt% and 97.65 ± 17.50 for PLA-PSy-pntyn in ratio of 6.68 wt%. In Fig. 1 SEM micrographs of the
polymer blends fracture surfaces; a Pure-PLA, b PLA/ PSy-ox (PLSy-1-82), c PLA/ PSy-OH (PL-NSy-1-51), dPLA/Plin-ox (PLinox-1-82), ePLA/Plin-OH (PLlin OH-2-86), f PLA/Pentyn2
this manner typical stress–strain curves of the PLA-blends were shown in Fig.4. In order to understand widely the blending mechanism, the stirring time dependence of the solution, temperature dependence of the mechanical
properties of the blend films, and blending in an extruder should also been studied in this manner. Propargyl deriv-ative of the soya oil polymer can also be used in click chemistry [35] in order to prepare some novel graft copolymers. Continuing research of our group on this area is on progress.
In summary, Oxidized or hydroxylated soybean oil polymers mainly affect the properties of the resulting PLA blend films from different aspects. PLA blends with the oligomers exhibited greater of the elongation than that of pure PLA depending on the oligomer ratio of the blends. As for thermal properties, the plasticizer effect of the oli-gomer addition on the PLA blends was observed. Lower Tgs and SEM images of their fracture surface can also be attributed to the compatibility of PLA blends with the additives. Moreover, the primary effect of the biodegrad-able and natural additives in the blend is to make the PLA blend completely environmental friendly. Oxidized soya Fig. 2 SEM micrographs of the
polymer blends side surfaces; aPure-PLA, b PLA/PSy-ox (PLSy-1-82), c PLA/PSy-OH (PL-NSy-1-51), d PLA/Plin-ox (PLinox-1-82), e PLA/Plin-OH (PLlin OH-2-86), f PLA/ Pentyn2
Table 2 Thermal Analysis of some polymer blends as determined by DSC
PLA blend Oligomer ratio, wt% Tg,°C Tm,°C
PLA 0 30 140 PL-Sy-ox-50-1 20.0 20 148 PL-Sy-ox-50-2 34.2 21 149 PL-NSy-1-1 11.9 20 150 PL-NSy-1-2 22.7 18 149 PL-Plin-ox-1-1 8.5 17 150 PL-Plin-ox-1-2 15.5 18 150 PL-Plin-ox-1-1 25.9 18 150
oil polymer additives also exhibit the antibacterial effect which can be promising for packaging film material.
Conclusion
A new method has been evaluated to prepare PLA films enhanced elasticity by blending soya oil polymer deriva-tives up to 20 wt%. The oligomers obtained from soya oil derivatives were all miscible with PLA. The increase in strain of the polymer blends were observed up to ten times. Hydroxylated soya oil polymer most increased the elasticity among the oligomers while the tensile strength of the blends
was slightly lower than that of PLA. The PLA blends with alcohol oligomer also slightly acquired the antibacterial property. The novel environmentally friendly PLA-blends with soya oil polymers can be promising film packaging material obtained from completely renewable resources. Fig. 3 DSC thermograms of some PLA blends; a PL-PSy-ox-50-1,
b PL-PSy-OH-1-1, c PL-Plin-ox-1-1, d Pure-PLA
Table 3 Antibacterial efficacy of the polymer samples against E. coli: PLA, PLA/PSy-ox and PLA/PSy-OH
Sample E. coli proliferation (cfu/mL)
In the beginning After 24 h PL-NSy-1-51 1.99 9 105 1.53 9 104 PL-PSy-ox-50-3 4.78 9 105 5.68 9 106 PLA (Cargill) 8.18 9 105 2.23 9 105 E. coli (control) 2.18 9 105 3.45 9 105
Table 4 The results of stress–strain measurement of the polymer blends
Code PSyox (wt%) Stress (MPa) Elongation (%) PLA-Cargill 0.00 34.69 ± 3.55 13.61 ± 4.17 PSy-1-15 2.05 26.20 ± 1.33 44.04 ± 24.36 PSy-1-25 3.33 27.89 ± 1.40 44.11 ± 6.09 PSy-1-50 6.50 24.77 ± 1.33 47.21 ± 12.93 PLSyox-50-3 12.4 24.39 ± 2.79 60.26 ± 6.60 PLSyox-50-1 14.0 20.97 ± 0.54 68.98 ± 9.64 PSy-1-252 26.2 17.48 ± 0.74 23.86 ± 3.42 Code NSy-1 (wt%) Stress (MPa) Elongation (%) PL-NSy-1-10 1.37 19.85 ± 1.14 124.3 ± 9.95 PL-NSy-1-20 2.66 14.98 ± 2.26 107.96 ± 11.72 PL-NSy-1-51 6.68 14.49 ± 2.60 94.12 ± 26.75 PL-NSy-1-1 11.9 19.45 ± 2.37 89.23 ± 12.51 PL-NSy-1-2 22.7 16.96 ± 3.69 50.38 ± 22.89 Code PLinox-1 (wt%) Stress (MPa) Elongation (%) PLinox-1-15 2.06 21.07 ± 2.06 65.77 ± 34.50 PLinox-1-27 3.64 19.57 ± 1.78 107.53 ± 5.37 PLinox-1-42 5.61 24.28 ± 1.38 97.64 ± 9.74 PLinox-1-1 8.54 26.23 ± 0.24 98.40 ± 3.03 PLinox-1-2 15.5 21.49 ± 2.07 79.14 ± 15.61 PLinox-1-3 25.9 25.26 ± 2.40 71.09 ± 14.46 Code PLinaOH-2 (wt%)
Stress (MPa) Elongation (%)
PL-PLinaOH-2-13 1.79 26.27 ± 1.45 96.16 ± 4.61 PL-PLinaOH-2-29 3.85 26.67 ± 3.12 29.18 ± 9.54 PL-PLinaOH-2-55 7.06 25.88 ± 1.75 35.55 ± 6.81 PL-PLinaOH-2-86 10.6 23.51 ± 2.24 49.09 ± 24.26 PL-PLinaOH-2-113 13.7 21.91 ± 0.87 44.65 ± 20.34 PL-PLinaOH-2-220 23.7 17.69 ± 1.85 33.05 ± 10.19 Code Pntyn-1 (wt%) Stress (MPa) Elongation (%) PLpntyn-1-14 1.88 22.77 ± 0.69 63.22 ± 5.73 PLpntyn-1-30 3.95 23.41 ± 1.90 58.73 ± 23.32 PLpntyn-1-52 6.68 21.79 ± 1.39 97.65 ± 17.50 PLpntyn-2 11.5 24.00 ± 0.52 67.04 ± 7.29 PLpntyn-1-112 13.6 21.56 ± 1.73 25.99 ± 3.60 PLpntyn-1 22.1 19.61 ± 1.72 31.72 ± 3.73
Acknowledgments This work was supported by; both the Bu¨lent Ecevit University Research Fund (#BEU-2012-10-03-13) and TUBI-TAK (Grant # 211T016). The Author thanks to Cem Berk for taking SEM micrographs, thanks to O¨ zcan Cura (Cilas Kauc¸uk-Devrek), Ceyda Pembeci Kodolbas¸ (TU¨ BI˙TAK-MAM) for antibacterial ana-lysis, Elvan Sulu and Timur S¸anal for molecular weight measurements.
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