Levulinic Acid as a Versatile Building Block for Plasticizer Design

Jun 19, 2019 - (8) For example, acetyl triethyl citrate and acetyl tributyl citrate, which ...... parameters, and a summarized table of ESI–MS resul...
0 downloads 0 Views 5MB Size
Download PDF
Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

pubs.acs.org/journal/ascecg

Levulinic Acid as a Versatile Building Block for Plasticizer Design Wenxiang Xuan, Minna Hakkarainen, and Karin Odelius* Department of Fibre and Polymer Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Teknikringen 56, Stockholm 100 44, Sweden

Downloaded via NOTTINGHAM TRENT UNIV on July 20, 2019 at 06:48:48 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The large potential of utilizing green platform chemicals such as levulinic acid, glycerol, and ethylene glycol as building blocks for the design of high-performance biobased plasticizers was demonstrated. From these green platform chemicals, esters with ketal or ketone functionalities and linear or branched structures were carefully designed and synthesized via a mild and solvent-free route and characterized by 1H NMR, 13C NMR, and FTIR. The effect of the structural combinations on the performance as plasticizers for polylactide (PLA), including migration resistance, was followed by a series of characterization techniques. The levulinates with ketone end-groups exhibited large capability to lower the glass transition temperature (Tg) of PLA (to 15 °C by 20 wt % plasticizer). Ketal end-groups provided additional thermal stability to the plasticizers, but their ability to lower Tg was not as good as that of ketone end-groups. Significantly improved flexibility reaching 546% elongation at break was achieved by the addition of 20 wt % ethylene glycol dilevulinate, as compared to 5% elongation at break for neat PLA. The structural differences for the plasticizers resulted in different degrees of hydrophobicity, which influenced the migration tendency of the plasticizers and also the hydrolysis rate of PLA. The branched ester structure with ketal end-groups maintained the processing window of PLA, but also lowered the hydrolysis rate of PLA in an accelerated migration test. In general, performance comparable to that of the reference plasticizer acetyl tributyl citrate (ATBC) was demonstrated, offering promise for a family of plasticizers derivable from green platform chemicals. KEYWORDS: Plasticizer, Levulinic acid, Polylactide, Migration, Acetyl tributyl citrate

■

INTRODUCTION For the design of truly sustainable plastics, full life-cycle needs to be considered including the nature and choice of additives such as plasticizers. As the potential migration from polymer matrix and the safety of the plasticizers in the environment is of importance, both during the service-life and after disposal, proper choices need to be made. Hence, effort is focused on developing sustainable plasticizers utilizing renewable sources for their synthesis, aiming at proper function during use and no hazardous effects during the entire life-cycle of the materials. These compounds are of interest, for example, to plasticize PVC1,2 and PLA.3−5 PLA is a commercial polymer synthesized from renewable resources and a promising degradable replacement for PS and PET.6 PLA is employed in several applications including food packaging.7 However, the glass transition temperature of PLA is above room temperature, making PLA a rigid and brittle material. When targeting flexible applications, adding additives such as plasticizers is considered as a well-accepted solution to enhance the properties of PLA. Given the ubiquitous calls for sustainability and environmental impact, biobased plasticizers are preferred to preserve the inherent fully biobased nature of PLA. © 2019 American Chemical Society

The plasticizing effect is indicated by the depression of glass transition temperature (Tg) and increased elongation at break for the materials. One of the general prerequisites underlying the decreased Tg and improved strain is good miscibility between the plasticizer and polymer. The miscibility can be tuned by the modification of chemical structure and molecular weight of the plasticizer, by the concentration of the plasticizer in the material, and sometimes by the processing method and temperature.8 For example, acetyl triethyl citrate and acetyl tributyl citrate, which only differ in side chain length but have the same branched structure, decrease the Tg value of PLA by 29.1 and 42.1 °C, respectively, at 20 wt % addition.9 In addition, an increased plasticizing efficiency of PLA is observed with a decrease of the molecular weight of poly(ethylene glycol) (PEG),10 and phase separation occurs when an excessive amount of PEG is present.11 The nature of the end-group is also of large importance as illustrated by methylation of PEG, which enhances the miscibility between the PLA matrix and PEG plasticizer.12 A vast variety of plasticizers have been reported for PLA including aliphatic Received: May 2, 2019 Revised: June 18, 2019 Published: June 19, 2019 12552

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering esters,13−15 phthalate,16 dicarboxylate,17 triacetine,18 citrates,9,19,20 epoxies,3 benzoates,21 glucose esters,22 cardoon seed oil derivatives,4 esters of tartaric acid,5 and polymeric substances (e.g., PEG,10,11,23,24 polyesters,25 and lactic acid oligomers26,27). In light of the enormous market volume of commodity plastics, the replacement of petroleum-based plasticizers by renewable analogues establishes need for the production of eco-friendly plasticizers, which utilize biobased building blocks and synthesis routes. The green platform chemicals are promising raw materials, offering multiple product applicability and large potential to serve as a platform for the production of biobased derivatives.28 Several green platform chemicals have already been converted into value-added green plasticizers for PLA, including derivatives of xylitol,29 succinic acid,30 glycerol,31 levulinic acid,32 and 2,5-furan dicarboxylic acid.33 The plasticizers can, due to their low molecular weight, migrate to the surface of the product during their use, thereby deteriorating the material.34 The migration behavior of a plasticizer is related to factors such as molecular weight,35,36 chemical composition,19 and structure.37 The aim of this study was therefore to design a palette of plasticizers from green platform chemicals, and to evaluate the effect of structural variations on plasticization efficiency and migration resistance in PLA blends. Here, the green platform chemicals levulinic acid and glycerol and ethylene glycol that can be attained from renewable resources were selected as the building blocks for the synthesis of ester plasticizers with linear or branched structures. The chemical composition of plasticizers was further tuned by ketalization. The results were compared to those obtained with the commercial reference plasticizer ATBC.

■

ethyl acetate. The organic phase containing the products was collected and further rinsed with deionized water twice to remove impurities. The solvent was removed by rotary evaporation at reduced pressure. ED and GT were subsequently modified to form ketalized ethylene glycol dilevulinate (KED) and ketalized glycerol trilevulinate (KGT). The optimized reaction variables included time (0−72 h), temperature (25, 50, and 80 °C), and feed ratio of hydroxyl group to ketone (2:1, 3:1, and 10:1). The following conditions were subsequently used: ED (10 g, 0.04 mol) and EG (25 g, 0.40 mol) or GT (12 g, 0.03 mol) and EG (28 g, 0.45 mol) were stirred vigorously in a 100 mL three-necked round-bottomed flask at 50 °C for 48 or 72 h to reach high conversion and to obtain KED and KGT, respectively. The reaction was conducted in a nitrogen atmosphere, and the formed water was collected in a water-cooled condenser connected to a 25 mL flask. After the reaction was completed, the mixture was dispersed in deionized water and subsequently extracted using ethyl acetate. The organic phase containing KED or KGT was collected and further rinsed with deionized water three times to remove impurities. The solution was concentrated by rotary evaporation of solvent at reduced pressure. Solution Casting. Neat PLA film and plasticized PLA films were obtained by solution casting. The amount of plasticizer (ED/GT/ KED/KGT) blended with PLA was 10 or 20 wt %. The concentration of neat PLA or the combined concentration of PLA and plasticizer was 4.0 g in 100 mL of dichloromethane solution. At least 1 h of magnetic stirring at 22 °C was applied to obtain a homogeneous solution. The films were dried in Petri dishes (D = 186 mm) in a fume hood at 22 °C for 7 days, followed by at least 7 days in a drying chamber at 25 °C and reduced pressure. The average thickness of a typical film was 0.12 ± 0.02 mm. The neat PLA film was named as “PLA100”, and the plasticized films were labeled as “10P” or “20P”, in which the number 10 or 20 symbolizes the plasticizer concentration (10 or 20 wt %) and P represents the abbreviation of the plasticizer. Migration Study. According to the EU regulation on plastic packaging materials,38 the overall migration is assessed in a simulant liquid after a certain time and conditions (e.g., aging temperature and choice of simulant), and the testing method is varied depending on the application scenario. Here, three simulants (water (H2O), 10 vol % ethanol in water (EtOH), and 3 wt % acetic acid in water (AcOH)), an aging temperature of 60 °C, and the time points of 1, 5, and 10 days were selected. For each combination of conditions, triplicates were prepared by placing each PLA film (around 1 cm × 1 cm) in a 20 mL sealed vial containing 10 mL of simulant. The triplicate samples of each simulant and blend (or neat PLA) combination were then aged in a 60 °C oven. Electrospray Ionization Mass Spectrometry (ESI−MS). The migrating pattern of the low molecular weight substances from the films was determined by a Finnigan LCQ ion trap mass spectrometer in positive mode after 1, 5, and 10 days. The aged liquid sample was diluted using methanol with a volume ratio of 1:2. The flow rate of the syringe pump for the dilution was 25 μL/min, and an ion source operating at 4.5 kV was used with the capillary heater set to 200 °C. The nebulizing gas was nitrogen. Mass Loss. The mass loss was measured after 1, 5, and 10 days. Any simulant on the surface of samples was gently removed using tissue paper and cotton swabs. The weight of the samples after being stored in a drying chamber (at least 10 days, 25 °C, reduced pressure) was recorded as dry weight (w1) and was compared to the initial weight of the samples (w0). The mass loss was then determined by w − w1 mass loss (%) = 0 × 100% w0 (1)

EXPERIMENTAL SECTION

Materials. Ethylene glycol (EG, 99%, Honeywell), glycerol (GL, GPR Rectapur, VWR), levulinic acid (LeA, 97%, Sigma-Aldrich), iron(III) chloride (FeCl3, 97%, Sigma-Aldrich), sulfuric acid (H2SO4, 95−98%, Sigma-Aldrich), and p-toluene sulfonic acid monohydrate (PTSA·H2O, 98.5%, Sigma-Aldrich) were used for the synthesis of plasticizers. Sodium hydrogen carbonate (GR for analysis, Merck) and ethyl acetate (analytical reagent grade, Fisher Scientific) were utilized during the extraction to purify the plasticizers. Dichloromethane (HPLC grade, Fisher Scientific) and polylactide (5200D, NatureWorks) were utilized as the solvent and film forming polymer during solution casting of films with or without plasticizers. Tributyl 2acetylcitrate (ATBC, ≥98%, Sigma-Aldrich) was selected as reference plasticizer. Ethanol (GPR Rectapur, 96 vol %, VWR), acetic acid glacial (analytical reagent grade, Fisher Scientific), and water (hypergrade for LC−MS, Merck) were used to prepare simulants for the migration measurements. Methanol (hypergrade for LC−MS, Merck) was used for ESI−MS analysis. All chemicals were used as received. Synthesis of Plasticizers. To synthesize the plasticizers, esterification to form ethylene glycol dilevulinate (ED) and glycerol trilevulinate (GT) was performed. The type of catalyst (H2SO4, FeCl3, PTSA·H2O), catalyst concentration (0.01, 1.0, 5.0 mol % toward hydroxyl group), reaction time (0−24 h), and temperature (110 or 140 °C) were first optimized. The optimum conditions were considered to be as follows: LeA (70 g, 0.6 mol), one of the polyols (12 g, 0.2 mol for EG or 0.13 mol for GL), and catalyst PTSA (0.8 g, 0.004 mol) were stirred vigorously in a 250 mL three-necked roundbottomed flask at 110 °C for 24 h. The reaction was conducted under a nitrogen atmosphere, and the formed water was collected in a watercooled condenser connected to a 25 mL flask. After the reaction was completed, the mixture was neutralized using saturated NaHCO3 (aq) to remove the unreacted LeA. The solution was then extracted with

Size Exclusion Chromatography (SEC). The molecular weight of the neat PLA and 10-day aged PLA films was analyzed by a GPCMAX (from Malvern) equipped with autosampler, a PLgel 5 μm Guard column, and two PLgel 5 μm MIXED-D columns. The PLA was dissolved and carried by chloroform with 2 vol % toluene as an internal standard. The flow rate was 0.5 mL/min, and the temperature was at 35 °C. The calibration was created using narrow polystyrene standards with Mn ranging from 1200 to 400 000 g/mol. 12553

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Scheme of the synthesis route (a), 1H NMR spectra (b), 13C NMR spectra (c), and FTIR spectra (d) of the four synthesized potential plasticizers. 50 mL/min from 25 to 500 °C, at a heating rate of 5 °C per minute. Triplicate samples were analyzed. The onset temperature at weight loss with 5% was taken for comparison. Tensile Test. Tensile test was conducted on an INSTRON 5944 module equipped with pneumatic grips. A load cell, with 500 N as maximum load, was set at a crosshead speed of 20 mm/min. Samples were prepared by cutting the films into rectangular specimens with a constant width of 5 mm and a length of roughly 100 mm. The length between gauges was 20 mm. All samples were conditioned for 40 h at RH 50 ± 5% and 23 ± 1 °C, according to ASTM D618-13 (Standard Practice for Conditioning Plastics for Testing). At least eight specimens were measured for each sample.

Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra of the plasticizers and their PLA blends were collected on a PerkinElmer Spotlight 400, equipped with a germanium attenuated total reflection (ATR) crystal. IR absorption spectra were recorded in ATR mode from 4000 to 600 cm−1 with 16 scans at 25 °C. Nuclear Magnetic Resonance (NMR). 1H NMR and 13C NMR spectra of the plasticizers were measured on a Bruker Avance 400 spectrometer at 25 °C. 1H NMR and 13C NMR spectra were measured at 400 and 100 MHz, respectively. Chloroform-d (CDCl3) was used as a solvent, and the internal standard was CHCl3. Differential Scanning Calorimetry (DSC). Glass transition temperatures, melting temperatures, and crystallinities of the neat and plasticized PLA were measured under nitrogen atmosphere by Mettler Toledo DSC 820 Module. Triplicate samples were analyzed, and the samples were heated from 25 to 200 °C and then kept at 200 °C for 2 min. The samples then were cooled to −30 °C and kept at −30 °C for 2 min, followed by the second heating scan to 200 °C. The cooling and heating rates were 10 °C per minute. The glass transition temperatures were marked as the midpoint of glass transition in the second heating scans. The crystallinity (χc) was obtained by applying eq 2 with cold crystallization and melting enthalpies. ΔHm, ΔHcc, ΔH0m, and wPLA represent the enthalpy of melting, the enthalpy of cold crystallization, the enthalpy of melting for 100% crystalline PLA, and the weight percentage of PLA, respectively. 93.1 J/g39 was applied as the enthalpy of melting for 100% crystalline PLA. χc =

ΔHm − ΔHcc ΔHm0 × wPLA

× 100%

■

RESULTS AND DISCUSSION The green platform chemicals levulinic acid and glycerol that are already commonly acquired from biomass transformation and ethylene glycol that could potentially be attained from renewable resources, are ideal starting-points in the realization of biobased plasticizers for PLA for value-added applications such as in food packaging. Here, a variety of biobased esterplasticizers were designed from these three chemicals by utilizing the possibility to create ketone or ketal functionalities as end-groups as well as linear and branched ester structures. The effect of these structural features, influencing the hydrophobicity of the plasticizers as well as the interactions with PLA, on the blend properties and migration resistance was investigated. One of the best-functioning and most migration-resistant commercialized green plasticizer, ATBC (structure seen in Figure 2), was chosen as reference plasticizer for performance evaluation.

(2)

Thermal Gravimetric Analysis (TGA). The thermal stability of the neat plasticizers and PLA blends was evaluated using a Mettler Toledo TGA/DSC 851e module instrument under nitrogen flow of 12554

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering Synthesis and Characterization of Potential Plasticizers. Altogether four potential plasticizers with different functionalities and structures were synthesized successfully. Two esters with ketone end-groups, ethylene glycol dilevulinate (ED) and glycerol trilevulinate (GT), were obtained through condensation of levulinic acid and ethylene glycol or glycerol as presented in Figure 1a. The optimized reaction conditions ensured a conversion of alcohol of 65− 75%, yielding liquid products with slightly yellow color. To synthesize esters KED and KGT with ketal functionality, the ketone end-groups in ED and GT esters were ketalized by ethylene glycol, Figure 1a. Ketalization can be realized under the presence of a homogeneous catalyst40,41 or a heterogeneous catalyst42,43 at high temperature. These common conditions are similar to those applied during esterification. However, as observed during the optimization, the ketalization could be selectively achieved over esterification by tuning the reaction temperature. Therefore, to achieve a high degree of ketalization with preserved ester groups, a solvent- and catalyst-free process was designed and utilized. The conversion of ketones during ketalization reached 63−88%, and the obtained KED and KGT were light-yellow viscous liquids. Chemical Structure of the Plasticizers. To confirm the structures of the four plasticizer candidates, 1H NMR and 13C NMR spectra were utilized, Figure 1b,c and Figures S1−8. For the ketone-esters ED and GT, the ketone end-groups from levulinic acids were retained, which was proven by the signals from the adjacent protons at δ = 2.18 ppm (CH3−), 2.59 and 2.75 ppm (−CH2−) in the 1H NMR spectrum, and the ketone carbonyl carbon signal at δ = 206 ppm in the 13C NMR spectrum. The formation of ester-bond was verified by the signals from the ester carbonyl carbon at δ = 173 ppm in the 13 C NMR spectrum. The ketal-esters KED and KGT are ketalized products of ED and GT. Therefore, the methyl and methylene groups, previously adjacent to the ketone carbonyl carbon, shifted to δ = 1.31 ppm (CH3−), 2.01 and 2.40 ppm (−CH2−) in the 1H NMR spectrum, and also new signals of the methylene groups from 1,3-dioxolane ring were observed at δ = 3.97−3.89 ppm. In the 13C NMR spectrum, the signals at δ = 109.17 ppm and δ = 66.41 ppm correspond to the C-2 and C-4, C-5 of the 1,3-dioxalane ring, while the signals of ketone carbonyl carbon vanished. The esterification and ketalization were further confirmed by FTIR spectroscopy (Figure 1d). A strong absorption band at 1733−1736 cm−1, in the spectrum of all four esters, confirmed the existence of ester bonds, and the absorption band of the ketone groups in ED and GT was observed at 1713 cm−1. The absence of absorption at 1713 cm−1 in the spectrum of KED and KGT indicates effective ketalization of ED and GT. Furthermore, the strong C−O stretching vibration in ED and GT was indicated by the peaks at 1149−1147 cm−1, while the corresponding band of ketals exhibits characteristic triple peaks,44 which were located at 1157, 1131, 1099 cm−1 and 1164, 1130, 1097 cm−1 in the spectra of KED and KGT, respectively. In addition, the absorption at 3050−2800 cm−1 was assigned to the C−H stretching vibration. In combination with the NMR characterizations, the ketalization process was verified, and the presence of ester and ketal functionalities in the potential plasticizers was confirmed. Thermal Stability of the Plasticizers. The thermal stabilities of the four synthesized esters were determined by thermal gravimetric analysis, Figure 2. The onset temperatures of 5% weight loss (T5) and maximum weight loss temperatures

Figure 2. TGA traces of the four synthesized potential plasticizers and reference plasticizer ATBC.

(Tmax) are listed for comparison (Table S1). The thermal stability of the plasticizers increased significantly after ketalization, and KGT exhibited the highest thermal stability. Comparison of ED and KED showed that, after ketalization, the T5 increased from 140 to 204 °C and the Tmax increased by 32 °C. Similarly, the T5 increased from 164 to 280 °C for KGT after ketalizing GT, and the Tmax increased by 34 to 357 °C. As compared to the reference plasticizer ATBC, KED and KGT suggested higher thermally stability due to their higher T5 and Tmax values, and ED was less stable. Although the T5 value of GT was lower than that of ATBC, the Tmax of GT was 57 °C higher than the Tmax of ATBC. Theoretical Prediction of Miscibility of PLA/Esters Blends. Miscibility is an essential criterion for the selection of plasticizers to reach, for example, high plasticizing efficiency, migration-resistance, and long service life. The miscibility for a certain polymer-plasticizer system is strongly affected by the temperature,45 chemical structure, and molecular weight of the plasticizer. Such a system can be viewed as a polymer−solvent system, in which Hansen solubility parameters have been successfully used as a tool to predict the theoretical system miscibility. The plasticizer should have solubility parameters similar to those of the polymer to ensure miscibility, and the relative energy difference (RED) number calculated from the differences in Hansen solubility parameters between plasticizer and polymer is utilized as a parameter to reflect the possible miscibility.45 The Hansen solubility parameter components and the overall solubility parameter of each plasticizer and the repeating unit of PLA were calculated from group contributions with the Hoftyzer−Van Krevelen method,46 Table S2. The calculated overall solubility parameter of PLA was calculated to be 20.6, which is within the range of the reported literature values that vary from 20.1 to 21.9 (MJ/m3)1/2.47,48 According to the obtained RED values, which were 0.4, 0.7, 0.5, and 0.7 for ED, KED, GT, and KGT, respectively, all four synthesized plasticizers and the reference plasticizer ATBC were in the same range and theoretically predicted to be miscible with PLA (RED < 1). Experimental Evaluation of the Miscibility of PLA/ Esters Blends. The neat PLA film (PLA100) and PLA blends containing 10 or 20 wt % ATBC or the synthesized esters were prepared from solution casting. The obtained transparent films were then evaluated by DSC to investigate the effect of 12555

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. DSC traces of PLA blended with 10 wt % (a) and 20 wt % (b) synthesized esters or ATBC. The Tg and Tm values were chosen from the second heating scan.

Figure 4. Thermal stabilities of PLA blends containing 10 wt % (a) and 20 wt % (b) synthesized esters or ATBC.

chemical structure on thermal properties, Figure 3. One glass transition temperature was detected for each film, and the increased depression of Tg with higher amount of added esters was widely observed, from which good miscibility of the esters in PLA was deduced. These results were in agreement with the theoretical prediction of the miscibility between the synthesized plasticizers and PLA. The blends with 10 wt % synthesized esters or ATBC all had Tg values above room temperature, ranging from 36 to 46 °C, as compared to 59 °C for PLA100, Figure 3a. The reduction in Tg was lower for the blends with ketal-esters KED and KGT. When the content of synthesized esters or ATBC was increased to 20 wt %, the Tg of the films decreased further to as low of values as 15 °C for 20ED, Figure 3b. All films portrayed Tg very close to or below 25 °C except 20KGT (Tg = 33 °C), which is beneficial for flexible applications. In comparison with previous studies on low molecular weight plasticizers for PLA, the obtained decrease in Tg is comparable. For example, for PLA (Mn of 137 000) extruded with 20 wt % triethyl citrate, tributyl citrate, acetyl triethyl citrate, or ATBC, the measured Tg values varied from 17.0 to 32.6 °C.9 In another study, a reduction in Tg to 38.9 °C from solutioncasted PLA was achieved when plasticized with 20 wt % di-2ethylhexyl-adipate, which is an approved PLA plasticizer for materials in food contact.49 Moreover, adding 20 wt % esters of tartaric acid lowered the Tg of PLA to 39.9 °C.5

A melting transition was observed for PLA100 and most of the PLA blends. Cold crystallization was also seen in 10ED, 20ED, 20ATBC, and 20GT, induced by the increased chain mobility due to the presence of the esters. The peaks of fusion and cold crystallization became broader and more significant in the thermograms as the esters content increased, although the actual crystallinity after reduction of the cold crystallization peak was generally below 0.5% and was not significantly affected by the amount of the esters (Table S3). The dual or “shoulder” fusion peak of 20ED indicated melt crystallization or the presence of two types of lamellar crystals, α-form and α′-form. The α-form is usually obtained by conventional melting crystallization conditions. The α′-form is less perfect and can be achieved by crystallization under 120 °C.50 Interestingly, there was no cold crystallization in 20KED and 20KGT as compared to the presence of cold crystallization in 20ED and 20GT. Considering the differences in ester structures and the measured Tg values, it is proposed that the more bulky ketal groups affected the mobility of the system less because of steric hindrance and increased the required free volume needed for chain mobility, thus leading to lower reduction in Tg and reduced ability for cold crystallization.51 Thermal Stability of PLA/Esters Blends. Thermal gravimetric analysis was performed on PLA and PLA blends to evaluate the effect of the different esters on the thermal stability of the blends, Figure 4 and Table S4. The blends with branched esters, GT and KGT, had higher T5 temperatures 12556

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. Mechanical properties of PLA and PLA blends. (a) Modulus. (b) Elongation at break. (c) Stress at break.

than those with linear esters, ED and KED. The highest T5 value, close to that of PLA100, was observed for the blend with KGT, that is, the ester with a branched structure and ketal endgroups. The combination of branched structure and ketal endgroups has been shown to increase the thermal stability of PVC.52 The thermal degradation of the blends with ED, KED, and the reference plasticizer ATBC consisted of two stages, the degradation or evaporation of the ester itself followed by the degradation of PLA, Figure 4. The blends with GT and KGT, independent of their content, portrayed thermal degradation patterns similar to that of PLA100. A high T5 could be an advantage during processing, as it enables processing at higher temperatures without degradation. Interestingly, although the T5 value of neat GT was quite low as compared to the other synthesized esters, its blends exhibited superior thermal stability as compared to blends with ED, KED, and ATBC. Mechanical Properties of PLA/Esters Blends. The mechanical performances of the prepared PLA films were altered, as expected, by the incorporation of low molecular weight compounds, as illustrated in Figure 5 and Figures S9 and S10. PLA100 demonstrated the highest values for both modulus and stress at break, but the lowest value of elongation at break, that is, typical properties for rigid and brittle materials with poor ductility. On the contrary, due to the enhanced chain mobility suggested by previous DSC results, the reduced Young’s modulus and improved elongation at break were observed for all of the blends, showing that the films were plasticized. For example, when 10 wt % ED or ATBC was added, the elongation at break increased from 5% to 487% and 449%, respectively; meanwhile, the Young’s modulus de-

creased from 2053 to 1299 and 1519 MPa, respectively. In addition, almost 41% and 34% of tensile stress at break was lost in the case of 10ED and 10ATBC, respectively, as compared to PLA100. The increment of plasticizers content to 20 wt % brought further increase in the elongation at break and decreased the Young’s modulus. The increased ability for chain movements was also seen by the lower Tg values of the blends. The elongation at break for 20 wt % blends ranged from 446% to 643%, and the modulus distributed between 263 and 1416 MPa. The tensile stress was almost preserved or slightly improved for most blends. It was clear that all four synthesized esters had a plasticizing effect. It has previously been reported that an elongation at break below 150% and tensile stress of 20 MPa were determined for PLA plasticized by 20 wt % DEHA, accompanied by a high Young’s modulus close to 1500 MPa and Tg of 38.9 °C.49 For PLA plasticized with 20 wt % of glucose hexanoate,22 an elongation at break of 370%, a stress at break of 21 MPa, and Young’s modulus of 590 MPa were determined. Thus, the measured results for PLA blends prepared here were well comparable. For example, 20KGT had a relatively high Tg (33 °C) and high modulus (1416 MPa), a greater elongation (446%), and higher tensile stress at break (39 MPa), approximately equaling to 70% and over 500% of the corresponding performances of 20ATBC. Migration Study of the Plasticized PLA Films. The migrants, including plasticizers and PLA hydrolytic degradation products (PLA oligomers, LAn), dissolved in each simulant after predetermined aging periods were fingerprinted by electrospray ionization mass spectrometry (ESI−MS) to 12557

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. Positive-mode ESI−MS spectra of films stored at 60 °C for 10 days in 3 wt % acetic acid, suggesting the presence of migrated plasticizer and PLA oligomers. (a) PLA100. (b) 20KED. (c) 20GT. (d) 20KGT.

No LAn was detected by ESI−MS in any 20KGT samples irrespective of employed simulants. The main series of PLA degradation products (m/z = 200− 1100) corresponded to linear lactic acid oligomers terminated by carboxyl and hydroxyl end-groups, with the hydrogen from the carboxyl group exchanged by a sodium ion. The detected signals occurred at m/z = (72*n + 1 + 17 + 23*2 − 1), where n and 72 represent the number and molecular weight of repeating units in oligomers, respectively. The common chemical structure for the oligomers is suggested in Figures 6 and 7. Such series of double sodium adducts have been observed in a few previous studies where the degradation product patterns of PLA were recorded.53−55 One additional series of oligomers (m/z = 600−1500) with relative low intensity was detected in the spectrum of 20ED samples (Figure 7d−f). This new group of peaks appeared at m/z = (72*n + 1 + 17 + 23*3 + 18 − 2*1). Two protons were exchanged by sodium ions in this case, and similar adducts have been reported in the degradation traces of neat PLA hydrolyzed in water at 60 °C as well.53 The detection of LAn with longer chains revealed the constant cleavage of ester bonds on main chains.55 Mass Loss and Aging-Induced Changes in Thermal Properties. The mass loss of PLA blends containing 20 wt % plasticizers after 1, 5, and 10 days of aging revealed that all plasticized films had an increased mass loss with increasing aging time as expected (Figure 8a). After 10 days of aging, the PLA100 portrayed a very minor mass loss, and therefore its degraded oligomers were undetectable in the ESI−MS spectrum (Figure 6a). 20ATBC and 20ED exhibited the lowest and highest mass losses, respectively, among all tested

elucidate the effect of plasticizer and aging time on the migration patterns of plasticized PLA films, and they were compared to PLA100. The overall results are listed in Table S5. Identification of Migrated Plasticizers from Plasticized PLA Films. Migration of the plasticizers could be observed for all of the plasticized PLA films after 1 day of aging at 60 °C. The presence of migrated plasticizers was identified by their dominant characteristic peaks shown in ESI−MS spectra (Figures 6 and 7). As an example, the Mw of ATBC plasticizer is 402.5 g/mol, and therefore the peak (m/z = 425) in the ESI−MS spectra (Figure 7a−c) was assigned to the mono sodium adduct of ATBC. Similarly, the mono sodium adducts of ED (m/z = 425), KED (m/z = 369), and GT (m/z = 409) were detected by ESI−MS in Figures 6b and 7d−f, Figure 6b, and Figure 6c,d, respectively. The ketals are classically used as ketone protecting groups and are susceptible toward hydrolysis. Therefore, the relative intensity of KED in the spectra was very low, and the KGT was not even detected; instead, their hydrolyzed products of ED and GT gave strong signals in the ESI−MS spectrum of 20KED (Figure 6b) and 20KGT (Figure 6d). Identification of PLA Oligomers from Plasticized PLA Films. Within 10 days of aging, no lactic acid oligomers, the degradation products of PLA, were detected in the ESI−MS spectra of PLA100 (Figure 6a), regardless of the type of simulant and aging time. The LAn was detected in 20ATBC and 20ED after 10 days of aging in each simulant (Figure 7). However, in the case of 20KED (Figure 6b) and 20GT (Figure 6c), LAn was detected only after aging in acetic acid simulant. 12558

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

Figure 7. Positive-mode ESI−MS spectra showing migrated plasticizers and PLA oligomers leached from films that were aged at 60 °C for 10 days in three different simulants. (a) 20ATBC in water. (b) 20ATBC in 10 vol % ethanol. (c) 20ATBC in 3 wt % acetic acid. (d) 20ED in water. (e) 20ED in 10 vol % ethanol. (f) 20ED in 3 wt % acetic acid.

hydrolytic aging (Table 1). This can be compared to the significant increment of Tg for all of the plasticized films explained by the migration of the plasticizers. For 20ATBC, a lower Tg was preserved because of its lower migrating rate of the plasticizer. The aging temperature was set to 60 °C, which is higher than the Tg of PLA100, and an increase in χc was therefore observed for all of the aged films. Consequently, rising melting temperatures were obtained, and thicker crystals may be formed during hydrolysis.57 Nevertheless, the hydrolytic degradation lowered the molecular weight; that is, shorter PLA chains were formed. Thus, the value of Tm was most likely determined by the joint effect of the degree of degradation and crystallization. That effect was demonstrated by the observation that 20ED and 20ATBC had Tm values very close to each other in water simulant after aging, while 20ED had a higher degree of χc and much lower molecular weight than 20ATBC.

samples. 20ED lost more than 30 wt % of mass after 10 days; that is, at least 10 wt % of PLA was transferred into simulants. This is in accordance with the intense signals of PLA oligomers in the ESI−MS spectra (Figure 7d−f). 20KED, 20GT, and 20KGT demonstrated intermediate migration behavior, and there were no large differences in the mass loss between them. A previous study has shown that a more hydrophilic plasticizer commonly leads to faster migration.56 In agreement with this, the most hydrophilic plasticizer ED, which also had the lowest molecular weight, had the highest mass loss rate after 1 day. The changes in the thermal properties of the PLA100 and the 20 wt % plasticized PLA films in the different aging media are displayed in Figure 8b−d. After 10 days of aging, the Tg of PLA100 decreased from 59 to 57, 55, and 52 °C in water, ethanol, and acetic acid simulant, respectively, which is explained by the decreasing molecular weight induced by 12559

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

Figure 8. Mass loss during aging and thermal properties of PLA films aged 10 days. (a) Mass loss. (b) Glass transition temperatures. (c) Melting temperatures. (d) Crystallinity (PLA content counted as 100% in calculations). Tg was taken from the second heating scan; crystallinity and Tm were based on the first heating scan.

Table 1. Changes in Molecular Weight of PLA100 and Plasticized PLA after Aging for Ten Days sample

Mn (g/mol)

dispersity

sample

Mn (g/mol)

dispersity

PLA100 (before aging) PLA100, H2O PLA100, EtOH PLA100, AcOH 20ED, H2O 20ED, EtOH 20ED, AcOH 20GT, H2O 20GT, EtOH 20GT, AcOH

118000 25000 12000 8000 7000 9000 5000 12000 9000 7000

1.7 2.2 2.3 2.3 2.4 2.2 2.2 2.4 2.9 2.0

20ATBC, H2O 20ATBC, EtOH 20ATBC, AcOH 20KED, H2O 20KED, EtOH 20KED, AcOH 20KGT, H2O 20KGT, EtOH 20KGT, AcOH

34000 24000 10000 16000 14000 7000 26000 21000 8000

1.9 2.1 2.1 2.1 2.1 2.0 1.8 1.9 2.0

Molecular Weight Changes. The molecular weight of the initial PLA100 and the 10-day aged blends and PLA100 was examined by size exclusion chromatography (Table 1). Generally, the molecular weight decreased with increasing hydrolysis time for all of the tested films. The degradation of PLA was indicated by a significant decline in the molecular weight of PLA100 and the plasticized films. The initial Mn of PLA100 was 118 000 g/mol, which dropped to 25 000 g/mol after aging for 10 days in water, at the same time as the dispersity increased from 1.7 to 2.2. It has been reported that the Mn of neat PLA fell from 56 750 to 7510 g/mol, and the dispersity increased from 1.7 to 2.6 after 8 days of aging in phosphate buffer (pH 7.3) at 60 °C.58 Lower Mn values after aging were obtained when PLA was plasticized by the three synthesized plasticizers (ED, KED, and GT) as compared to

the PLA100 because the plasticized systems had higher mobility and the migrated plasticizers generated more surface area for hydrolytic attack. However, the hydrolysis rate of PLA is also governed by the balance of hydrophobicity and hydrophilicity of the plasticizers.59 Higher Mn and lower dispersity values were thereby retained in the case of 20ATBC and 20KGT, which were plasticized by more hydrophobic plasticizers with larger size. A similar inhibited hydrolysis process for PLA plasticized by ATBC has been observed before, and the hydrophobic nature of the plasticizer itself was believed as the underlying reason for suppressed water absorption and thus slower degradation rate.60 That was aligned with the fact that KGT is the most hydrophobic of the four synthesized plasticizers. Moreover, when a branched structure or ketal functionality was employed, higher Mn value 12560

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering was achieved after aging when comparing films from 20ED, 20KED, 20GT, and 20KGT.

Oil (ECSO) on Poly(Lactic Acid). Ind. Crops Prod. 2017, 104, 278− 286. (4) Turco, R.; Tesser, R.; Cucciolito, M. E.; Fagnano, M.; Ottaiano, L.; Mallardo, S.; Malinconico, M.; Santagata, G.; Di Serio, M. Cynara Cardunculus Biomass Recovery: An Eco-Sustainable, Nonedible Resource of Vegetable Oil for the Production of Poly(Lactic Acid) Bioplasticizers. ACS Sustainable Chem. Eng. 2019, 7 (4), 4069−4077. (5) Zawada, K.; Plichta, A.; Jańczewski, D.; Hajmowicz, H.; Florjańczyk, Z.; Stępień, M.; Sobiecka, A.; Synoradzki, L. Esters of Tartaric Acid, A New Class of Potential “Double Green” Plasticizers. ACS Sustainable Chem. Eng. 2017, 5 (7), 5999−6007. (6) Auras, R.; Harte, B.; Selke, S. An Overview of Polylactides as Packaging Materials. Macromol. Biosci. 2004, 4 (9), 835−864. (7) Jamshidian, M.; Tehrany, E. A.; Imran, M.; Jacquot, M.; Desobry, S. Poly-Lactic Acid: Production, Applications, Nanocomposites, and Release Studies. Compr. Rev. Food Sci. Food Saf. 2010, 9 (5), 552−571. (8) Mascia, L.; Xanthos, M. An Overview of Additives and Modifiers for Polymer Blends: Facts, Deductions, and Uncertainties. Adv. Polym. Technol. 1992, 11 (4), 237−248. (9) Labrecque, L. V.; Kumar, R. A.; Dave, V.; Gross, R. A.; McCarthy, S. P. Citrate Esters as Plasticizers for Poly(Lactic Acid). J. Appl. Polym. Sci. 1997, 66 (8), 1507−1513. (10) Ren, Z.; Dong, L.; Yang, Y. Dynamic Mechanical and Thermal Properties of Plasticized Poly(Lactic Acid). J. Appl. Polym. Sci. 2006, 101 (3), 1583−1590. (11) Hu, Y.; Rogunova, M.; Topolkaraev, V.; Hiltner, A.; Baer, E. Aging of Poly(Lactide)/Poly(Ethylene Glycol) Blends. Part 1. Poly(Lactide) with Low Stereoregularity. Polymer 2003, 44 (19), 5701−5710. (12) Lai, W.-C.; Liau, W.-B.; Lin, T.-T. The Effect of End Groups of PEG on the Crystallization Behaviors of Binary Crystalline Polymer Blends PEG/PLLA. Polymer 2004, 45 (9), 3073−3080. (13) Anakabe, J.; Zaldua Huici, A. M.; Eceiza, A.; Arbelaiz, A.; Avérous, L. Combined Effect of Nucleating Agent and Plasticizer on the Crystallization Behaviour of Polylactide. Polym. Bull. 2017, 74 (12), 4857−4886. (14) Hao, Y.; Yang, H.; Zhang, H.; Zhang, G.; Bai, Y.; Gao, G.; Dong, L. Diethylene Glycol Monobutyl Ether Adipate as a Novel Plasticizer for Biodegradable Polylactide. Polym. Bull. 2016, 73 (11), 3143−3161. (15) Yang, Y.; Xiong, Z.; Zhang, L.; Tang, Z.; Zhang, R.; Zhu, J. Isosorbide Dioctoate as a “Green” Plasticizer for Poly(Lactic Acid). Mater. Des. 2016, 91, 262−268. (16) Yang, S.-L.; Wu, Z.-H.; Meng, B.; Yang, W. The Effects of Dioctyl Phthalate Plasticization on the Morphology and Thermal, Mechanical, and Rheological Properties of Chemical Crosslinked Polylactide. J. Polym. Sci., Part B: Polym. Phys. 2009, 47 (12), 1136− 1145. (17) Wang, R.; Wan, C.; Wang, S.; Zhang, Y. Morphology, Mechanical Properties, and Durability of Poly(Lactic Acid) Plasticized with Di(Isononyl) Cyclohexane-1,2-Dicarboxylate. Polym. Eng. Sci. 2009, 49 (12), 2414−2420. (18) Ljungberg, N.; Wesslén, B. The Effects of Plasticizers on the Dynamic Mechanical and Thermal Properties of Poly(Lactic Acid). J. Appl. Polym. Sci. 2002, 86 (5), 1227−1234. (19) Ljungberg, N.; Wesslén, B. Preparation and Properties of Plasticized Poly(Lactic Acid) Films. Biomacromolecules 2005, 6 (3), 1789−1796. (20) Ljungberg, N.; Wesslén, B. Tributyl Citrate Oligomers as Plasticizers for Poly (Lactic Acid): Thermo-Mechanical Film Properties and Aging. Polymer 2003, 44 (25), 7679−7688. (21) Wan, T.; Lin, Y.; Tu, Y. Plasticizing Effect of Glyceryl Tribenzoate, Dipropylene Glycol Dibenzoate, and Glyceryl Triacetate on Poly (Lactic Acid). Polym. Eng. Sci. 2016, 56 (12), 1399−1406. (22) Yang, X.; Hakkarainen, M. Migration Resistant Glucose Esters as Bioplasticizers for Polylactide. J. Appl. Polym. Sci. 2015, 132 (18), 1−8.

■

CONCLUSIONS A family of biobased plasticizers, with ketone or ketal endgroups and linear or branched ester structures, was successfully synthesized by utilizing ethylene glycol and the green platform chemicals levulinic acid and glycerol. The structures of the synthesized plasticizes were confirmed by 1H NMR, 13C NMR, and FTIR. All synthesized plasticizers significantly decreased the Tg of PLA, where ketal and branched plasticizer structures resulted in less reduced Tg. The thermal stability of the plasticizer itself was increased by ketal end-groups, and the thermal stability of plasticized PLA was retained when a branched structure was employed together with ketal endgroups. The four synthesized plasticizers ensured comparable enhancement in ductility and a somewhat higher Young’s modulus, as compared to PLA with reference plasticizer ATBC. The ESI−MS analysis suggested fast migration of plasticizers and PLA hydrolysis during the migration study. The hydrolysis rate of PLA was decreased by the addition of branched plasticizer with ketal end-groups. Thus, the plasticizers designed from levulinic acid and especially the one with branched structure and ketal functionality demonstrated good potential to be introduced as a biobased highperformance plasticizer for PLA.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02439. NMR spectra of the four synthesized plasticizers, tensile curves of PLA film and PLA blends with plasticizers, thermal stability data related to plasticizers and blends, calculation of solubility parameters, and a summarized table of ESI−MS results (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Minna Hakkarainen: 0000-0002-7790-8987 Karin Odelius: 0000-0002-5850-8873 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge The China Scholarship Council (CSC) for the financial support of this work. Many thanks to Yunsheng Xu and Geng Hua for the assistance with NMR analysis.

■

REFERENCES

(1) Howell, B. A.; Sun, W. Biobased Plasticizers from Tartaric Acid, an Abundantly Available, Renewable Material. Ind. Eng. Chem. Res. 2018, 57 (45), 1. (2) Howell, B. A.; Lazar, S. T. Biobased Plasticizers from Carbohydrate-Derived 2,5-Bis(Hydroxymethyl)Furan. Ind. Eng. Chem. Res. 2019, 58 (3), 1222−1228. (3) Carbonell-Verdu, A.; Samper, M. D.; Garcia-Garcia, D.; SanchezNacher, L.; Balart, R. Plasticization Effect of Epoxidized Cottonseed 12561

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562

Research Article

ACS Sustainable Chemistry & Engineering

(44) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; John Wiley & Sons Ltd.: Chichester, 2001. (45) Hansen, C. M. Polymer Additives and Solubility Parameters. Prog. Org. Coat. 2004, 51 (2), 109−112. (46) Van Krevelen, D. W.; Te Nijenhuis, K. Cohesive Properties and Solubility. Properties of Polymers; Elsevier: New York, 2009; pp 189− 227. (47) Murariu, M.; Da Silva Ferreira, A.; Alexandre, M.; Dubois, P. Polylactide (PLA) Designed with Desired End-Use Properties: 1. PLA Compositions with Low Molecular Weight Ester-like Plasticizers and Related Performances. Polym. Adv. Technol. 2008, 19 (6), 636−646. (48) Abbott, S. CHEMICAL COMPATIBILITY OF POLY(LACTIC ACID): A PRACTICAL FRAMEWORK USING HANSEN SOLUBILITY PARAMETERS. Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications 2010, 83−95. (49) Martino, V. P.; Ruseckaite, R. A.; Jiménez, A. Thermal and Mechanical Characterization of Plasticized Poly (L-Lactide-Co-D,LLactide) Films for Food Packaging. J. Therm. Anal. Calorim. 2006, 86 (3), 707−712. (50) Saeidlou, S.; Huneault, M. A.; Li, H.; Park, C. B. Poly(Lactic Acid) Crystallization. Prog. Polym. Sci. 2012, 37 (12), 1657−1677. (51) Ruellan, A.; Ducruet, V.; Domenek, S. Plasticization of Poly (Lactide). Poly(lactic acid) Science and Technology: Processing, Properties, Additives and Applications 2014, 124−170. (52) Wang, Y.; Nie, X.; Li, X. Synthesis and Characterization of Novel Pentaerythritol Ester as PVC Plasticizer. J. Appl. Polym. Sci. 2016, 133 (47), 44227. (53) Yang, X.; Xu, H.; Odelius, K.; Hakkarainen, M. Poly(Lactide)g-Poly(Butylene Succinate-Co-Adipate) with High Crystallization Capacity and Migration Resistance. Materials 2016, 9 (5), 313. (54) Yang, X.; Clénet, J.; Xu, H.; Odelius, K.; Hakkarainen, M. Two Step Extrusion Process: From Thermal Recycling of PHB to Plasticized PLA by Reactive Extrusion Grafting of PHB Degradation Products onto PLA Chains. Macromolecules 2015, 48 (8), 2509− 2518. (55) Höglund, A.; Hakkarainen, M.; Edlund, U.; Albertsson, A.-C. Surface Modification Changes the Degradation Process and Degradation Product Pattern of Polylactide. Langmuir 2010, 26 (1), 378−383. (56) Andersson, S. R.; Hakkarainen, M.; Albertsson, A.-C. Tuning the Polylactide Hydrolysis Rate by Plasticizer Architecture and Hydrophilicity without Introducing New Migrants. Biomacromolecules 2010, 11 (12), 3617−3623. (57) Tsuji, H.; Ikada, Y. Blends of Aliphatic Polyesters. II. Hydrolysis of Solution-cast Blends from Poly(L-lactide) and Poly(E-caprolactone) in Phosphate-buffered Solution. J. Appl. Polym. Sci. 1998, 67 (3), 405−415. (58) Hakkarainen, M.; Albertsson, A.-C.; Karlsson, S. Weight Losses and Molecular Weight Changes Correlated with the Evolution of Hydroxyacids in Simulated in Vivo Degradation of Homo- and Copolymers of PLA and PGA. Polym. Degrad. Stab. 1996, 52 (3), 283−291. (59) Reed, A. M.; Gilding, D. K. Biodegradable Polymers for Use in Surgery - Poly(Glycolic)/Poly(Iactic Acid) Homo and Copolymers: 2. In Vitro Degradation. Polymer 1981, 22 (4), 494−498. (60) Höglund, A.; Hakkarainen, M.; Albertsson, A.-C. Migration and Hydrolysis of Hydrophobic Polylactide Plasticizer. Biomacromolecules 2010, 11 (1), 277−283.

(23) Pillin, I.; Montrelay, N.; Grohens, Y. Thermo-Mechanical Characterization of Plasticized PLA: Is the Miscibility the Only Significant Factor? Polymer 2006, 47 (13), 4676−4682. (24) Younes, H.; Cohn, D. Phase Separation in Poly(Ethylene Glycol)/Poly(Lactic Acid) Blends. Eur. Polym. J. 1988, 24 (8), 765− 773. (25) Coativy, G.; Misra, M.; Mohanty, A. K. Microwave Synthesis and Melt Blending of Glycerol Based Toughening Agent with Poly(Lactic Acid). ACS Sustainable Chem. Eng. 2016, 4 (4), 2142− 2149. (26) Martin, O.; Averous, L. Plasticization and Properties of Biodegradable Multiphase Systems. Polymer 2001, 42, 6209−6219. (27) Burgos, N.; Tolaguera, D.; Fiori, S.; Jiménez, A. Synthesis and Characterization of Lactic Acid Oligomers: Evaluation of Performance as Poly(Lactic Acid) Plasticizers. J. Polym. Environ. 2014, 22 (2), 227−235. (28) Bozell, J. J.; Petersen, G. R. Technology Development for the Production of Biobased Products from Biorefinery Carbohydrates The US Department of Energy’s “Top 10” Revisited. Green Chem. 2010, 12 (4), 539−554. (29) Buchanan, C. M.; Buchanan, N. L.; Edgar, K. J.; Lambert, J. L. Novel Carbohydrate Esters and Polyol Esters as Plasticizers for Polymers, Compositions and Articles Including Such Plasticizers and Methods of Using the Same. WO 03/062314 A1, 2003. (30) Stefanisin, K. L.; Shah, U. B.; O’rourke, S.; Wentworth, G. Biopolymer Compositions Having Improved Flexibility. WO 2011/ 100101 A1, 2011. (31) Wan, T.; Lin, Y.; Tu, Y. Plasticizing Effect of Glyceryl Tribenzoate, Dipropylene Glycol Dibenzoate, and Glyceryl Triacetate on Poly(Lactic Acid). Polym. Eng. Sci. 2016, 56 (12), 1399−1406. (32) Leibig, C.; Mullen, B.; Mullen, T.; Rieth, L.; Badarinarayana, V. Cellulosic-Derived Levulinic Ketal Esters: A New Building Block. ACS Symp. Ser. 2011, 1063, 111−116. (33) Bhattacharjee, D.; Sehanobish, K. FDCA-Based Polyesters Made with Isosorbide. WO 2014/100257 A2, 2014. (34) Ljungberg, N.; Andersson, T.; Wesslen, B. Film Extrusion and Film Weldability of Poly (Lactic Acid) Plasticized with Triacetine and Tributyl Citrate. J. Appl. Polym. Sci. 2003, 88, 3239−3247. (35) Model, P. I. Mathematical Modelling in Application To Plasticizers. Handb. Plast. 2012, 573−580. (36) Ljungberg, N.; Wesslén, B. Thermomechanical Film Properties and Aging of Blends of Poly(Lactic Acid) and Malonate Oligomers. J. Appl. Polym. Sci. 2004, 94 (5), 2140−2149. (37) Wypych, G. Effect of Plasticizers on Properties of Plasticized Materials. Handbook of Plasticizers; ChemTec Publishing: Toronto, 2012; pp 209−306. (38) Plastic Materials and Articles Intended to Come into Contact with Food. Commission Regulation (EU) No. 10/2011, 2011; pp 1− 132. (39) Fischer, E. W.; Sterzel, H. J.; G, W. Investigation of the Structure of Solution Grown Crystals of Lactide Copolymers by Means of Chemical Reactions. Kolloid-Z. u. Z. Polym. 1973, 251, 980− 990. (40) Leibig, C.; Mullen, B.; Mullen, T.; Rieth, L.; Badarinarayana, V. Cellulosic-Derived Levulinic Ketal Esters: A New Building Block. Renewable and Sustainable Polymers 2011, 1063, 111−116. (41) Gonzalez-Arellano, C.; Arancon, R. A. D.; Luque, R. Al-SBA-15 Catalysed Cross-Esterification and Acetalisation of Biomass-Derived Platform Chemicals. Green Chem. 2014, 16, 4985−4993. (42) Gonzalez-Arellano, C.; De, S.; Luque, R. Selective Glycerol Transformations to High Value-Added Products Catalysed by Aluminosilicate-Supported Iron Oxide Nanoparticles. Catal. Sci. Technol. 2014, 4, 4242−4249. (43) Amarasekara, A. S.; Animashaun, M. A. Acid Catalyzed Competitive Esterification and Ketalization of Levulinic Acid with 1, 2 and 1, 3-Diols: The Effect of Heterogeneous and Homogeneous Catalysts. Catal. Lett. 2016, 146, 1819−1824. 12562

DOI: 10.1021/acssuschemeng.9b02439 ACS Sustainable Chem. Eng. 2019, 7, 12552−12562