Biorenewable Tough Blends of Polylactide and ... - ACS Publications

Feb 26, 2016 - Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, ...... of Houston Cullen College of Engineering Microscopy Cent...
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Biorenewable Tough Blends of Polylactide and Acrylated Epoxidized Soybean Oil Compatibilized by a Polylactide Star Polymer Sheli C. Mauck,† Shu Wang,† Wenyue Ding,† Brian J. Rohde,† C. Karen Fortune,† Guozhen Yang,† Suk-Kyun Ahn,‡,§ and Megan L. Robertson*,† †

Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-4004, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea ‡

S Supporting Information *

ABSTRACT: Polylactide (PLA), a commercially available thermoplastic derived from plant sugars, finds applications in consumer products, disposable packaging, and textiles, among others. The widespread application of this material is limited by its brittleness, as evidenced by low tensile elongation at break, impact strength, and fracture toughness. Herein, a multifunctional vegetable oil, acrylated epoxidized soybean oil (AESO), was investigated as a biodegradable, renewable additive to improve the toughness of PLA. AESO was found to be a highly reactive oil, providing a dispersed phase with tunable properties in which the acrylate groups underwent cross-linking at the elevated temperatures required for processing the blends. Additionally, the presence of hydroxyl groups on AESO provided two routes for compatibilization of PLA/AESO blends: (1) reactive compatibilization through the transesterification of AESO and PLA and (2) synthesis of a PLA star polymer with an AESO core. The morphological, thermal, and mechanical behaviors of PLA/oil blends were investigated, in which the dispersed oil phase consisted of AESO, soybean oil (SYBO), or a 50/50 mixture of AESO/SYBO. The oil additives were found to toughen the PLA matrix, with significant enhancements in the elongation at break and tensile toughness values, while maintaining the glass transition temperature of neat PLA. In particular, the blend containing PLA, AESO, SYBO, and the PLA star polymer was found to exhibit a uniform oil droplet size distribution with small average droplet size and interparticle distance, resulting in the greatest enhancements of PLA tensile properties with no observable plasticization.

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poly(p-dioxanone),21 have also been studied. A recent area of emphasis in the literature has been to identify fully or partially renewable components (which are in some cases also biodegradable), such as microbial polyesters,22,23 poly(propylene carbonate),24 and poly(tetramethylene adipate-coterephthalate),25 which can modify the mechanical properties of PLA. Vegetable oils, which are widely available, nontoxic, and biodegradable, have recently gained attention as modifiers for PLA. Numerous studies have focused on the use of epoxidized, oxidized, or hydroxylated oils which plasticize the PLA matrix, lowering the glass transition temperature (Tg), increasing the elongation at break, and in many cases also decreasing the tensile strength and modulus.26−36 Unmodified soybean oil (SYBO) has also been investigated as a toughening agent for PLA.37,38 SYBO and PLA are highly immiscible, and therefore the blend shows no plasticization of the PLA matrix; however, it was observed that the PLA/SYBO blend exhibited phase

INTRODUCTION The majority of commercial polymers produced today are sourced from products of fossil fuel refining. As there is a finite supply of fossil fuels, and their manufacturing leads to harmful environmental pollution, sustainable alternatives to fossil fuelbased polymers are currently in development.1−7 Polylactide (PLA) is a biodegradable thermoplastic that is produced commercially from plant sugars, finding applications in disposable bottles and packaging, textiles, and films, among others.2,8−10 However, PLA has limited toughness, as evidenced by low tensile elongation at break, impact strength, and fracture toughness, limiting more widespread applications of the material. Extensive research has been conducted to improve the toughness of PLA by blending PLA with a diverse array of polymers.8,11−14 Many nonbiodegradable, petroleum-sourced polymers have been investigated as blending partners for PLA, such as poly(ethylene oxide),15 polyurethanes,16 and polyethylene,17,18 which showed great enhancements in PLA mechanical properties. In addition, blends of PLA and biodegradable, petroleum-derived polymers, such as polycaprolactone,19 poly(butylene adipate-co-terephthalate),20 and © XXXX American Chemical Society

Received: December 1, 2015 Revised: January 27, 2016

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distribution characteristics: number-average molecular weight (Mn) = 57.0 kg/mol and dispersity (Đ) = 2.43 (determined with gel permeation chromatography using light scattering). Prior to use in blending, PLA was dried at 60 °C overnight and stored in a desiccator to prevent water contamination. Other blend components included consumer-grade soybean oil (SYBO, Crisco all-natural vegetable Oil) and acrylated epoxidized soybean oil (AESO, Sigma-Aldrich, inhibited with 8500 ppm monomethyl ether hydroquinone), both used as received. A PLA star polymer (referred to as STAR in this study) was synthesized (procedures and characterization described below). The blend compositions are described in Table 1.

inversion during the blend preparation using melt mixing methods (when the SYBO concentration was greater than 6%) due to the significant differences in the viscosities of the PLA and SYBO at the melt mixing temperature.37 In addition, the high degree of immiscibility of PLA and SYBO, and resulting high blend interfacial tension, led to a broad size distribution of SYBO droplets in the PLA matrix.37 Furthermore, the use of conjugated SYBO has been explored, which also does not plasticize PLA and can be employed in reactive compatibilization schemes.39 Castor oil has proven to be an excellent toughening agent when blended with PLA, resulting in significant increases in the elongation at break and tensile toughness compared to neat PLA.40 In addition, castor oil does not plasticize the PLA matrix and has been inferred to produce in situ blend compatibilizers when melt mixed with PLA due to the presence of hydroxyl groups which may undergo transesterification with PLA.40 However, castor oil is not as widely available as SYBO in the United States, and processing the castor beans to produce the oil results in a waste product containing ricin, a highly toxic protein. Acrylated epoxidized soybean oil (AESO) is a multifunctional, commercially available modified soybean oil produced through the epoxidation of SYBO followed by the ring-opening of the epoxide groups with acrylic acid.41 Through this process, the carbon−carbon double bonds on the SYBO structure are functionalized with both acrylate and hydroxyl groups. AESO is a reactive oil, as the acrylate groups polymerize at elevated temperatures in the presence of air (without the addition of a radical initiator), in stark contrast to unmodified SYBO, which is a relatively nonreactive oil. Prior studies have utilized AESO and other functionalized vegetable oils as monomers for the preparation of highly cross-linked thermosets.42−46 To our knowledge, there are no reports in the literature of the use of AESO as a reactive modifier to improve the toughness of PLA. Prior studies have implemented polymerized soybean and castor oils as modifiers for PLA;38,47,48 the use of the highly reactive AESO in this study provides a route for in situ formation of the polymerized oil phase in the PLA/AESO blend. In this work, the efficacy of AESO as a modifier for improving the toughness of PLA was explored. The morphological, thermal, and mechanical behaviors of PLA/oil blends were investigated, in which the dispersed oil phase consisted of AESO, SYBO, or a 50/50 mixture of AESO/ SYBO. The cross-linking of AESO during the blend preparation process provided a route to tuning the oil phase properties. Additionally, the presence of hydroxyl groups on AESO provided two routes for compatibilization of PLA/AESO blends: (1) reactive compatibilization through the transesterification of AESO and PLA and (2) synthesis of a PLA star polymer with an AESO core. In particular, a blend containing PLA, AESO, SYBO, and the PLA star polymer exhibited significant improvements in the elongation at break and tensile toughness relative to neat PLA. This research demonstrates that the compatibilized PLA/oil blend shows great viability as a biodegradable, renewable alternative to petroleum-based engineering plastics in applications requiring enhanced toughness.

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Table 1. Blend Compositions (in wt %) blend

PLA

AESO

SYBO

STAR

PLA/SYBO PLA/AESO PLA/AESO/SYBO PLA/AESO/SYBO/STAR

95 95 95 90

0 5 2.5 2.5

5 0 2.5 2.5

0 0 0 5

The total oil content was fixed at 5%, as prior studies on PLA/ SYBO and PLA/castor oil blends showed a reduction in the tensile strength and modulus and minimal improvement in the elongation at break with higher oil contents.38,40 All polymer blends were prepared by solvent casting using the weight percentages specified for each blend (Table 1). The blend components were dissolved for 12 h in dichloromethane (VWR International LLC, BDH) with stirring (20 mL of dichloromethane per gram of blend components). The solutions were poured into aluminum pans (200 mL capacity, 11 cm i.d., 2.9 cm depth) over 12 h while allowing the solvent to evaporate in a fume hood (2 mL of solution was added to each aluminum pan every 2 h; total amounts of 0.5 g of PLA/oil and 10 mL of solution were added to each aluminum pan). The resulting films were dried in a fume hood for 12 h and then heated under vacuum at 190 °C for 10 h. For some selected samples, a drying temperature of 160 °C was employed, where specified in the paper. Specimens for tensile testing, microscopy, thermal analysis, and viscosity measurements were prepared through compression molding. Specimen geometries are discussed below for each characterization technique. Aluminum molds were coated with a polymer-release spray. Solvent-cast and vacuum-dried polymer blends were removed from the aluminum pans, cut into small pieces, and placed in the coated aluminum molds on a Carver hotpress (the press was preheated to 190 °C for 30 min). The specimens were then pressed at 190 °C three times at the following pressures and durations sequentially: 1 t for 1 min and 30 s, 1 t for another 1 min and 30 s, and 2 t for 5 min. Between each step, there was a 10 s period during which the pressure was released. Immediately following compression molding, the molds containing the specimens were removed from the press and cooled to room temperature on the benchtop. The specimens were stored in a desiccator at room temperature for at least 24 h prior to characterization. Compatibilizer Synthesis. A star compatibilizer was synthesized to improve interfacial adhesion between the immiscible PLA and oil domains in PLA/oil blends. The compatibilizer consisted of an AESO core with PLA arms. The star polymer was synthesized through the ring-opening of L-lactide, initiated from the hydroxyl groups present on AESO, following general ring-opening polymerization procedures described in ref 49. The reaction scheme is provided in Scheme 1. L-Lactide was purified through recrystallization in ethyl acetate (twice) and subsequent drying under vacuum. Oxygen and moisture free tetrahydrofuran (THF) and dichloromethane (DCM) were obtained from a Pure Process Technology solvent purification system. AESO was diluted with dry DCM and stirred with calcium hydride in vacuo, followed by filtration. DCM was removed from the AESO through distillation. The recovered AESO was stored in a glovebox freezer on molecule sieves. In the glovebox, stoichiometric amounts of AESO and L-lactide were added to a 100 mL flask and subsequently

EXPERIMENTAL DETAILS

Blend Preparation. Blends were prepared containing poly(Llactide) (referred to as PLA in this work, obtained from NatureWorks LLC: Ingeo biopolymer 4043D), with the following molecular weight B

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Macromolecules Scheme 1. Synthesis of Star Polymer Containing an AESO Core and PLA Arms (2.8 Arms per Molecule on Average)

dried by the addition of molecular sieves in dry THF (with stirring for 12 h). The dried AESO and L-lactide mixture in THF was filtered through 0.45 μm nylon filters into a reaction flask containing the catalyst 1,5,7-triazabicyclo-[4.4.0]dec-5-ene (TBD, Sigma-Aldrich, 98%). After 5 min, benzoic acid (Sigma-Aldrich, ≥99.5%) was added to the flask to quench the reaction. The molar ratios used in the synthesis were 833:3.22:1.00 (L-lactide:TBD:AESO). The resulting PLA star polymer (referred to as STAR in this paper) was purified through precipitation in methanol and drying in a vacuum oven at room temperature. Characterization. Gel Permeation Chromatography (GPC). GPC was conducted on a Viscotek GPC instrument with ResiPore columns, using stabilized THF (OmniSolv, HPLC grade) as the mobile phase. The instrument operated at a temperature of 30 °C and a flow rate of 1 mL/min. The injection volume was 100 μL. Samples were prepared for GPC by dissolving in stabilized THF at a concentration of 1−2 mg/mL. The absolute number-average molecular weight, Mn, and dispersity, Đ, of the PLA homopolymer (NatureWorks LLC: Ingeo biopolymer 4043D) and PLA star polymer were determined with light scattering. A differential refractive index (dn/dc) of 0.049 mL/g was used.50,51 Nuclear Magnetic Resonance (1H NMR). 1H NMR experiments were performed on JEOL ECA-400 instrument using deuterated chloroform (Cambridge Isotope Laboratories, Inc., 99.8 atom % D) as the solvent. Chemical shifts were referenced to the solvent proton resonance (7.26 ppm). Soxhlet Extraction. Oil samples (SYBO, AESO, and a 50/50 mixture of SYBO/AESO) were vacuum-dried overnight at 190 °C. Cellulose thimbles were vacuum-dried at 60 °C for 8 h and then immediately weighed upon cooling to room temperature and removing from the vacuum oven. A known mass of dried oil was placed in the thimble. The sample-filled thimbles were placed in a Soxhlet extractor containing THF. The extraction was conducted for 24 h at 60 °C. A rotary evaporator was used to remove most of the THF from the extract. The extract was then vacuum-dried for 12 h at 60 °C. The thimble and the insoluble portion of the sample left in the thimble after extraction were also vacuum-dried at 60 °C for 12 h. The insoluble portion was weighed to determine the percentage of the sample that was insoluble. The soluble portion of the sample was subsequently analyzed with GPC. Microscopy and Image Analysis. The blend morphology was observed using a Leica DM2500 M optical microscope using a 50×

magnification lens (HCX PL Fluotar 50×/0.80 BD). A thin section (around 1 mm) of the PLA/oil blend was placed on a plain, precleaned 75 × 25 mm microslide, heated above the melting point of PLA for around 15 s, covered with a coverslip, and cooled to room temperature before imaging. This procedure produced a bubble-free specimen for imaging. ImageJ software was used to subtract dark and light background images from the sample images. The fracture surfaces of PLA/oil tensile bars were imaged using a LEO 1525 field emission scanning electron microscope at a voltage of 15 kV. The fracture surface was etched and subsequently coated with gold using a Denton Vacuum Desk V sputter coater. The gold thickness was approximately 10 nm. The micrographs were analyzed using ImageJ software to determine the area of each oil droplet, which was then converted to an equivalent diameter of a circle (Di = 2(Ai/π)1/2). No further correction of Di was made for the underestimation of Di due to the two-dimensional projection of the sphere. Additionally, droplets of a size too small to be observed at the magnification chosen have been neglected. The Sauter mean diameter (Dm) was calculated for the population of oil droplets in the micrograph: n

Dm =

∑i Di 3 n

∑i Di 2

(1)

where Di is the diameter of one oil droplet and n is the number of oil droplets in one micrograph of the blend referenced. The standard deviation was calculated based on the population of oil droplets examined within the same image. The matrix ligament thickness (average interparticle distance), T, was also calculated following literature procedures:52

⎤ ⎡⎛ ⎞1/3 π T = Dm⎢⎜ ⎟ exp(1.5 ln 2 σ ) − exp(0.5 ln 2 σ )⎥ ⎥⎦ ⎢⎣⎝ 6ϕ ⎠

(2)

where σ is a measure of the polydispersity in the particle size distribution (σ = 1 is monodisperse): N

ln σ =

∑i = 1 ni(ln di − ln Dm)2 N

∑i = 1 ni

(3)

Tensile Testing. Tensile bars were prepared according to the ASTM D 638 standard (Type V specimen). The bars had a gauge thickness of C

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Macromolecules 1.6 mm. The tensile bars were tested by an Instron tensile tester containing a 2 kN load cell, pneumatic grips (maximum force of 2 kN), and a grip pressure of 40 psig. A strain rate of 10 mm/min was employed. Thermogravimetric Analysis (TGA). The degradation temperatures of PLA and PLA/oil tensile bars were determined by TGA using a TA Instruments Q50 instrument. The samples were heated at 10 °C/min to 575 °C. The samples were tested with a balance nitrogen purge flow of 40 mL/min and a sample nitrogen purge flow of 60 mL/min. Differential Scanning Calorimetry (DSC). Thermal properties of PLA/oil blends and PLA/oil tensile bars were determined using DSC on a TA Instruments Q2000 differential scanning calorimeter with 10−15 mg samples in Tzero aluminum hermetic pans under a nitrogen purge of 50 mL/min. All temperatures scans were conducted at a rate of 10 °C/min. For the first cycle, each sample was equilibrated at 24 °C. Then, the temperature was ramped to 200 °C. For the second cycle, the sample was then cooled to 20 °C and then heated again to 200 °C. Melting temperature (Tm), crystallization temperature (Tc), glass transition temperature (Tg), and peak integrations were determined by TA Analysis software. The % crystallinity was determined using the melting peak area and the enthalpy of melting for an infinite PLA crystal, 94 J/g.53,54 Tm, Tc, and % crystallinity were determined from the first heating cycle in order to probe the properties of the tensile bar resulting from the thermal history relevant to the compression molding process. Tg was determined from the second heating cycle. The error in DSC temperature measurement is estimated to be 1−2 °C, based on analysis of repeated measurements taken on equivalent samples. Viscosity Measurement. Dynamic viscosity of the blends was measured on a TA Instruments DHR-2 rheometer. Disks (25 mm diameter and 1 mm thick) of the polymer blends were prepared through compression molding. The disks were placed between 25 mm parallel plates, and the dynamic moduli were measured using a frequency sweep from 1 to 100 rad/s (at a strain chosen to be in the linear viscoelastic region) while heating in the presence of nitrogen gas. Matrix-Assisted Laser Desorption Ionization Time-of-Flight (MALDI-TOF). MALDI-TOF experiments were performed on PLA/ oil blends using a Bruker Daltonics Autoflex II mass spectrometer equipped with an N2 laser (λ = 337 nm) operating at a frequency of 25 Hz and an accelerating voltage of 20 kV. trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) and sodium trifluoroacetate (NaTFA) were used as matrix and salt, respectively. In a typical experiment, solutions of matrix (20 mg/ mL), analyte (10 mg/mL), and salt (10 mg/mL) were prepared in THF, followed by mixing in a 20:2:1 ratio. Then, 1 μL of solution mixture was dropped on the target and allowed to dry. Mass spectra were collected in reflectron mode, and the polystyrene standard was used to calibrate the instrument.

Figure 1. Normalized GPC refractometer signal (in chloroform) obtained from neat and processed samples. (a) Neat PLA pellets (red curve) are compared to a PLA tensile bar processed through solvent casting, drying at 190 °C, and compression molding at 190 °C (black curve). (b) Data are shown for SYBO prior to (blue curve) and after (gray curve) drying at 190 °C. (c) SYBO and AESO are shown prior to drying, and AESO and a 50/50 blend of AESO/SYBO are shown after drying at 190 °C.

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used in preparation of the blends). No visible differences were noticed in the SYBO molecular weight distribution after vacuum-drying at 190 °C (Figure 1b), and the dried SYBO readily dissolved in THF. The pure AESO and 50/50 AESO/SYBO mixture were also dried under vacuum at 190 °C. Visible changes were observed in these samples after drying: the viscosity of the AESO/SYBO mixture was noticeably increased, and the pure AESO sample behaved as expected for an elastomer, consistent with the hypothesis that the AESO underwent cross-linking upon drying. Soxhlet extraction (in THF) was employed to quantify these observations: 91.8 wt % of the dried pure AESO sample was insoluble whereas 48.4 wt % of the dried 50/50 AESO/ SYBO sample was insoluble. GPC data obtained from the soluble fractions of these samples are shown in Figure 1c. Comparison of the data obtained from SYBO and AESO prior to drying with that of the dried 50/50 mixture indicates that the soluble portion of the dried 50/50 AESO/SYBO sample is almost entirely un-cross-linked SYBO, with a small amount of

RESULTS AND DISCUSSION Effect of Processing Conditions on the PLA/Oil Blend Components. Blends containing PLA and oil were prepared through solvent casting and subsequent drying at 190 °C, following the procedures described in the Experimental Details. The effect of processing on the blends components, PLA and the oils, was examined. The molecular weight distribution of the commercial PLA sample used in all blends was investigated using GPC prior to and after processing (solvent casting, drying at 190 °C, and compression molding at 190 °C) (Figure 1a). The molecular weight distribution exhibited no significant changes upon processing the PLA, and the drying protocol employed for the PLA pellets was sufficient to remove water and prevent transesterification under these conditions. The potential for cross-linking of the vegetable oils during the blend drying process was also considered. Samples of pure AESO, pure SYBO, and a 50/50 AESO/SYBO mixture were dried under vacuum at 190 °C (following the same procedure D

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Figure 2. Scanning electron micrographs of tensile bar fracture surfaces (left) and optical micrographs (right) of PLA/oil blends (blend compositions by weight given in parentheses).

un-cross-linked AESO in the extract. In the case of the pure AESO sample, the GPC data obtained from the soluble portion are consistent with un-cross-linked AESO (Figure 1c). We can conclude that the majority of the AESO (greater than 90%) was cross-linked in both the samples (pure AESO and the 50/50 AESO/SYBO mixture) after drying at 190 °C, while the SYBO remained unaffected. Physical Behavior of PLA/Oil Blends. Blends containing PLA and oil were prepared through solvent casting and subsequent drying at 190 °C. Specimens with dimensions following the ASTM D638 standard (Type V bar with gauge thickness of 1.6 mm) were prepared through compression molding at 190 °C. The morphological, thermal, and mechanical properties of the blends were examined. The following choices of vegetable oil were explored: SYBO, AESO, and a 50/50 mixture of SYBO/AESO.

Optical micrographs of the blends and scanning electron micrographs of tensile bar fracture surfaces are shown in Figure 2. All PLA/oil blends exhibited oil droplets dispersed throughout the PLA matrix, consistent with prior work on PLA/SYBO and PLA/castor oil blends.37−40 Image analysis was conducted on each micrograph in order to quantify the average droplet diameter (Dm) and interparticle distance (T) (Table 2) from the oil droplet size distributions (Figure S1). The oil droplets are clearly visible in the PLA/SYBO and PLA/AESO/ SYBO blend micrographs, regardless of the imaging method. The enhanced contrast in the SEM micrographs is likely is due to removal of the oil droplet during the specimen fracture, leaving voids in the PLA matrix where oil droplets previously resided.37,40 In the case of the PLA/AESO blend, SEM does not provide a clear view of the AESO droplets, likely due to cross-linking of the AESO during the blend preparation process and lack of contrast between the PLA and cross-linked AESO E

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blends, which were annealed for 4 h at 190 °C (Figures S2− S5). Castor oil was used in these model experiments (rather than AESO), as the population of molecules is significantly more homogeneous in castor oil than in AESO (87% of the fatty acids in castor oil are ricinoleic acid58). The pure PLA sample did not show any evidence of transesterification under these conditions (Figure S2), indicating the PLA pellets used in this study are relatively water free (the pellets were dried at 60 °C overnight and stored in a desiccator prior to blending). Similarly, a 50/50 PLA/castor oil blend also did not show any changes in the molecular weight distribution by MALDI after annealing at 190 °C (Figure S3). By contrast, a 95/5 PLA/ castor oil blend showed the formation of a new species (in the m/z range of 1200−2400) which may be the transesterification product between PLA and castor oil (Figure S4). The thermal properties of the PLA/oil blends were investigated with DSC (Table 3 and Figure S6). Minimal plasticization occurred in the blends with the addition of the oils as seen by the comparison of Tg values for the blends in Table 3 (53−56 °C), which were comparable to the that of neat PLA (56 °C). The % crystallinity of all blends was in the range of 1−3% and comparable to that of neat PLA (one exception is the PLA/AESO blend, in which the % crystallinity was slightly lower than that of neat PLA). The melting temperatures of the PLA/oil blends were slightly higher than that of neat PLA, and there was a significant reduction of the cold crystallization temperatures (Tc) of the blends relative to neat PLA. Reduction of Tc has been observed previously in blends of PLA and poly(butyl acrylate)59 as well as PLA and polycaprolactone60 and is attributed to an increase of nucleation rate due the presence of additional nucleation sites at the interface of the phase-separated domains. Both studies also observed an increase in degree of crystallinity of the PLA in the blend,59,60 which surprisingly was not observed in our system. The thermal degradation behavior of PLA/oil blends was examined with TGA (Figure S7 and Table S1). The onset degradation temperature for all samples was around 330 °C, and the peak in the derivative plot was observed around 355 °C. The mechanical behaviors of neat PLA and the PLA/oil blends were characterized through tensile testing. Representative tensile data are provided in Figure 3, and the average tensile properties for all samples are summarized in Table 4. Neat PLA exhibited brittle failure at around 4% strain. The addition of oil significantly increased the elongation at break and tensile toughness of the blends relative to neat PLA, regardless of the choice of oil. The moduli of all blends (2.9 GPa) were fairly independent of the oil composition and only slightly lower than that of neat PLA (3.1 GPa). Addition of SYBO, or a mixture of SYBO/AESO, resulted in a noticeable reduction in the tensile strength relative to PLA. The PLA/

Table 2. Average Droplet Diameter (Dm, μm) and Interparticle Distance (T, μm) of PLA/Oil Blendsa blend PLA/SYBO PLA/AESO PLA/AESO/SYBO PLA/AESO/SYBO/ STAR

Dm (SEM)b

Dm (OM)c

T (SEM)b

T (OM)c

6.4 ± 1.7 NAd 2.1 ± 0.5 1.5 ± 0.4

4.5 ± 1.3 2.2 ± 0.4 2.5 ± 0.5 NAe

3.4 NAd 2.0 1.2

3.6 2.3 2.5 NAe

a

Procedures for calculation of Dm and T are described in the Experimental Details. Particle size distributions analyzed from each micrograph are shown in Figure S1. bDetermined through scanning electron microscopy. cDetermined through optical microscopy. d Image has insufficient contrast for calculation of Dm and T. eImage has insufficient resolution for calculation of Dm and T.

phases. The PLA/SYBO blend exhibited a large Dm with high degree of variability in the droplet size distribution.37 The PLA/ AESO and PLA/AESO/SYBO blends exhibited a significantly more narrow droplet size distribution and lower Dm. The interparticle distance was also found to be significantly lower in the PLA/AESO and PLA/AESO/SYBO blends as compared to the PLA/SYBO blend. The reduction in Dm and T observed in blends containing AESO may potentially be explained by two phenomena which could occur during drying and processing of the blends at elevated temperatures: (1) cross-linking of AESO may inhibit coarsening of the blend, and (2) compatibilizing molecules may form through a transesterification reaction between AESO (containing hydroxyl groups) and PLA. The initial PLA/oil blend morphology was established during solvent casting, yet cross-linking of AESO occurred during the drying process, which was conducted under vacuum at 190 °C (evidence of AESO cross-linking was discussed previously). Annealing macroscopically phase separated binary blends at elevated temperatures results in coarsening of the structure; i.e., Dm increases with annealing time.55−57 In blends containing AESO, we hypothesize that cross-linking of AESO suppressed the coarsening process once the polymer reached the gel point. Additionally, a transesterification reaction between PLA and AESO may have occurred during the blend drying and processing steps, forming compatibilizing molecules. Similar behavior has been observed previously in blends of PLA and castor oil: blends containing 5 wt % castor oil exhibited a very narrow droplet size distribution with Dm = 0.70 ± 0.20 μm.40 Castor oil, like AESO, contains pendant hydroxyl groups (an average of 2.8 hydroxyl groups per molecule for both castor oil and the AESO used in this study). In order to confirm that transesterification may occur at the temperature used in this study for drying and processing the blends, we performed MALDI experiments on (1) pure PLA and (2) PLA/castor oil

Table 3. Thermal Properties of Compression-Molded PLA/Oil Tensile Bars bar PLA PLA/SYBO PLA/AESO PLA/AESO/SYBO PLA/AESO/SYBO/STAR

Tga (°C) 55.6 53 54.6 53 55.5

± ± ± ± ±

0.4 2 0.4 4 0.6

Tmb (°C) 148.1 152.0 152.0 150 150.9

± ± ± ± ±

Tcb (°C)

0.2 0.3 0.2 2 0.5

128.5 119 109 113 114

± ± ± ± ±

0.4 4 1 4 5

ΔHmb (J/g) 2.6 1.6 1.0 2.2 1.3

± ± ± ± ±

0.9 1.1 0.2 0.5 0.9

crystallinityb,c (%) 2.8 1.8 1.1 2.7 1.5

± ± ± ± ±

1.0 1.2 0.2 0.4 1.1

Characterized from second heating cycle. bCharacterized from first heating cycle. cΔH∞ m of 94 J/g was used as the enthalpy of melting for an infinite PLLA crystal.53,54 a

F

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investigated in more detail through variation of processing conditions. Our hypothesis was that the cross-linking of the AESO phase during the blending process resulted in a higher viscosity of the PLA/oil mixture, which inhibited the ability to produce defect-free tensile specimens through compression molding. In order to investigate this hypothesis further, we directly measured the dynamic viscosity of the blend through rheology (Figure 4). The PLA/AESO blend viscosity was

Figure 3. Tensile stress vs strain obtained from PLA/oil blends with varying oil type.

AESO blend had the greatest tensile strength of all the blends, which was comparable to that of neat PLA. All blends exhibited a lower stress at break as compared to neat PLA. SEM micrographs of the tensile bar fracture surfaces are shown in Figure S8. The neat PLA fracture surface is characteristic of brittle fracture. The PLA/oil blends exhibited a higher degree of deformation of the matrix, likely shear yielding,61 which can be induced by cavitation of the dispersed phase62 and has been observed previously in rubber toughened PLA blends.18,38,40 Though the PLA/AESO blend exhibited promising characteristics of high tensile strength (similar to that of neat PLA) and enhanced elongation at break and toughness relative to neat PLA, the tensile results for this blend showed wide variability, particularly in the values of the elongation at break and stress at break. Voids were observed on the surface of many tensile testing specimens prepared for the PLA/AESO mixture (the results summarized in Table 4 do not include specimens containing visible voids). In order to explore the origins of the variability of the tensile testing results obtained from the PLA/ AESO blend further, we conducted morphological and thermal analysis of PLA/AESO tensile bars which exhibited high and low elongation (73 vs 3.2% elongation at break). The two bars did not exhibit any noticeable differences in the oil droplet domain size when examined with an optical microscope. Though the bars fractured at significantly different strain values, the thermal properties were quite similar (Table S2). Therefore, the wide variability in the tensile data is not explained by the presence of morphological or thermal property differences between test specimens. This variability in the PLA/ AESO tensile behavior will be discussed in greater detail in the following section, in which discusses processing effects on the blend properties. Effect of Processing Conditions on the PLA/Oil Blends. The PLA/AESO blend exhibited wide variability in the measured tensile properties. The origin of this behavior was

Figure 4. Dynamic viscosity of PLA and PLA/oil blends at (a) 160 °C and (b) 190 °C. The blends were solvent cast and dried at 190 °C, followed by compression molding at 190 °C. The dynamic viscosity was measured at the specified temperatures.

comparable to that of neat PLA and higher than that of the PLA/SYBO blend, regardless of the frequency used in the measurement. This trend was observed regardless of the measurement temperature (190 and 160 °C). Interestingly, the blend containing the 50/50 AESO/SYBO mixture showed behavior similar to that the PLA/AESO blend at high frequencies and similar behavior to that of the PLA/SYBO blend at low frequencies. With the hypothesis that lowering the drying temperature would limit the extent of cross-linking of the AESO droplets, thus improving the processability of this blend, we also prepared tensile testing specimens through drying at 160 °C. Lowering the drying temperature did not

Table 4. Tensile Properties of PLA/Oil Blendsa,b blend PLA PLA/SYBO PLA/AESO PLA/AESO/SYBO PLA/AESO/SYBO/STAR

tensile strength (MPa) 64 44 62 44 44

± ± ± ± ±

2 3 1 4 1

elongation at break (%) 4.1 24 31 20 39

± ± ± ± ±

0.9 14 39 9 6

toughness (MPa) 1.9 8 10 6 11

± ± ± ± ±

0.5 4 12 3 2

stress at break (MPa) 58 25 33 29 30

± ± ± ± ±

2 6 11 3 1

modulus (GPa) 3.1 2.9 2.9 2.9 2.90

± ± ± ± ±

0.1 0.1 0.1 0.1 0.02

a

The error is quantified through measurement of the standard deviation of results obtained. Bars with visible defects were excluded. bBlends were vacuum-dried at 190 °C for 12 h, followed by compression molding at 190 °C. G

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highest elongation at break and tensile toughness values of all the PLA/oil blends (with a high degree of reproducibility), which was a considerable improvement over the properties of neat PLA: the elongation at break and tensile toughness of the PLA/AESO/SYBO/STAR blend were 10 and 6 times greater than that of neat PLA, respectively (Table 4 and Figure 3).

improve the repeatability of the tensile testing measurement of the PLA/AESO blend (an example of a bar with low elongation at break is shown in Table S2). Synthesis of PLA Star Polymer Compatibilizer. A PLA star polymer (STAR) was synthesized through the ring-opening polymerization of L-lactide, initiated from the hydroxyl groups of AESO (Scheme 1). Through 1H NMR analysis (Figure S9), it was determined that the AESO employed in this study contained an average of 2.8 acrylate groups per molecule (and therefore 2.8 hydroxyl groups per molecule, on average; refer to Scheme 1). The presence of epoxide groups was not detected in the AESO sample (characteristic peaks located in the vicinity of 3 ppm were not observed). However, the molecules present in the AESO sample in theory may contain anywhere from 0 to 9 acrylate groups (and therefore hydroxyl groups), as the number of double bonds per triglyceride ranges from 0 to 9 in soybean oil7 (i.e., compare the various types of fatty acids which are found in soybean oil, such as stearic and palmitic acid, which are fully saturated, to linolenic acid, which contains three double bonds). On the basis of this characterization of the AESO sample, we assume that the PLA star polymer contains an average of 2.8 arms per molecule (with molecules present that contain a range of 0−9 arms per molecule). 1 H NMR analysis (Figure S10) was employed to confirm the presence of characteristic peaks associated with PLA and the AESO core in the PLA star polymer. GPC data obtained from the PLA star polymer are shown in Figure S11. The characteristics of the PLA star polymer molecular weight distribution were determined to be Mn = 45.6 kg/mol and Đ = 3.55 (using GPC with light scattering). The high dispersity can be explained by the wide variation in the AESO chemical structure and resulting variation in the numbers of arms per molecule in the PLA star polymer, as is discussed above. In addition, the conversion of L-lactide was quite high (>95%), and some degree of transesterification is likely, further broadening the distribution. Physical Properties of PLA/Oil Blends Compatibilized by a PLA Star Polymer. Prior studies have demonstrated the influence of architecture (linear, branched, star) on the properties of PLA63,64 as well as the ability of PLA star polymer additives to modify the properties of linear PLA,65,66 including studies in which the PLA star polymer was synthesized using a vegetable oil (i.e., castor oil or epoxidized soybean oil) as a macroinitiator for the lactide monomer.67−69 In this study, the PLA star polymer was employed as a compatibilizer for PLA/oil blends. Based on the results of the mechanical testing of PLA/oil blends, the PLA/AESO/SYBO blend was chosen for investigating the effect of addition of the star polymer. While all PLA/oil blends exhibited a significant improvement in the elongation at break and tensile toughness relative to neat PLA (Table 4), the PLA/AESO/SYBO blend was chosen for compatibilization studies as the average droplet size Dm was significantly less than that of the PLA/SYBO blend (Table 2), and it also exhibited greater reproducibility and lower error in the measured tensile parameters as compared to the PLA/AESO blend (Table 4). The addition of the PLA star polymer compatibilizer had a dramatic effect on the average droplet size and interparticle distance of the PLA/AESO/SYBO blend, which were reduced to 1.5 and 1.2 μm, respectively (Table 2 and Figure 2). The Tg and percent crystallinity of the compatibilized blend were indistinguishable from that of neat PLA (Table 3). The compatibilized PLA/AESO/SYBO blend also exhibited the

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CONCLUSIONS The efficacy of vegetable oils as additives for improving the toughness of PLA was explored. Specifically, the use of acrylated epoxidized soybean oil (AESO), soybean oil (SYBO), and a 50/50 mixture of the oils was investigated. AESO is a reactive oil that can undergo cross-linking by heating in the presence of air whereas SYBO is a nonreactive oil. Greater than 90% of the AESO was cross-linked in all PLA/oil blends, whereas the molecular weight distribution of SYBO was unaffected when exposed to elevated temperatures (190 °C) at which the blends were processed. The morphology of all PLA/ oil blends consisted of oil droplets dispersed throughout the PLA matrix. The PLA/SYBO blend exhibited the largest average oil droplet diameter and greatest breadth of the droplet size distribution. The use of AESO or a 50/50 mixture of AESO/SYBO in the PLA/oil blend resulted in a smaller average droplet diameter and narrower droplet size distribution. We hypothesized that the reduction in the droplet diameter was due to either suppression of blend coarsening through AESO cross-linking or formation of compatibilizing molecules through transesterification of PLA and AESO during the blend preparation process. Minimal plasticization was observed in the PLA/oil blends, with Tg values comparable of that to neat PLA. The percent crystallinity of the blends was also comparable, and the melting temperature was slightly higher than that of neat PLA. The presence of the oil droplet phase resulted in rubber toughening of the PLA matrix, in which the elongation at break and tensile toughness of all PLA/oil blends was higher than that of neat PLA. The PLA/AESO blend also exhibited a higher tensile strength than all other PLA/oil blends, which was comparable to neat PLA. However, the PLA/ AESO tensile properties were highly variable, with large standard deviations in the elongation at break and tensile toughness values. This variability in the PLA/AESO blend properties was attributed to the cross-linking of the oil during the blend preparation process, which hindered the preparation of defect-free specimens for tensile testing. Such behavior was not observed in the PLA/SYBO or PLA/AESO/SYBO blends. The PLA/AESO/SYBO blend, with desirable attributes of highly reproducible tensile properties and a relatively narrow oil droplet size distribution and low average droplet diameter, was chosen for investigation of the effect of compatibilization on the PLA/oil blend properties. A PLA star polymer was synthesized through the ring-opening polymerization of L-lactide, initiated by the hydroxyl groups present on AESO. Addition of the PLA star polymer to the PLA/AESO/SYBO blend resulted in a reduction in the average droplet diameter and increased reproducibility of the measured tensile parameters. The compatibilized blend exhibited similar Tg and percent crystallinity to neat PLA. Addition of the PLA star polymer to the PLA/AESO/SYBO blend also resulted in the highest elongation at break and tensile toughness values of all the PLA/ oil blends (a factor of 10 and 6 times improvement over neat PLA, for the elongation at break and tensile toughness, respectively). This research demonstrates that the quaternary blend of PLA, AESO, SYBO, and the PLA star polymers shows H

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great viability as a biodegradable, renewable alternative to petroleum-based engineering plastics in applications requiring enhanced toughness over the performance of neat PLA.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02613. Particle size distributions of SEM and optical micrographs obtained on PLA/oil blends (Figure S1); MALDI-TOF spectra and GPC data obtained on PLA and PLA/castor oil blends (Figures S2−S5); raw DSC data obtained from PLA and PLA/oil blends (Figure S6); TGA data obtained from PLA and PLA/oil blends (Figure S7 and Table S1); SEM micrographs of the fracture surfaces of PLA and PLA/oil tensile bars (Figure S8); thermal properties of PLA/AESO blends prepared at a lower processing temperature (160 °C) and contrasting bars of differing strain at break (Table S2); NMR data obtained from AESO (Figure S9); and NMR and GPC data obtained from the star polymer (Figures S10 and S11) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel 713-743-2748 (M.L.R.). Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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Article

ACKNOWLEDGMENTS

We thank Jeffrey D. Rimer and Jihae Chung for access to and training on the optical microscope and Haleh Ardebili and Mejdi Kammoun for access to and training on the thermogravimetric analysis instrument. We thank John C. Wolfe, Rebecca Kusko, and Apeksha Awale for access to and training on the scanning electron microscope in the University of Houston Cullen College of Engineering Microscopy Center, and Debora F. Rodrigues and Hang N. Nguyen for access to and training on the gold sputterer. We appreciate the assistance of Charles Anderson for access and training in the University of Houston Department of Chemistry Nuclear Magnetic Resonance Facility. We also appreciate the assistance of Kunlun Hong and Vivek Yadav for gel permeation chromatography data analysis. We thank Gila E. Stein, Leonard P. Trombetta, and Kim Mai Le for helpful discussions. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This material is based upon work supported by the National Science Foundation under Grant DMR-1351788 and Grant CMMI1334838. This work is funded in part by the Norman Hackerman Advanced Research Program of the Texas Higher Education Coordinating Board (003652-0022-2013). C.K.F. acknowledges support from the Research Experiences for Teachers program through the Engineering and Computer Science Site at the University of Houston: Innovations in Nanotechnology, sponsored by the National Science Foundation (NSF-EEC 1130006). I

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