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Degradation rates of glycerol polyesters at acidic and basic conditions
Materials Chemistry and Physics 128 (2011) 10–11
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Degradation rates of glycerol polyesters at acidic and basic conditions Ronald A. Holser ∗ Russell Research Center, USDA-ARS, 950 College Station Road, Athens, GA, 30605 USA
a r t i c l e
i n f o
Article history: Received 27 July 2010 Received in revised form 17 February 2011 Accepted 23 February 2011 Keywords: Biomaterials Chemical synthesis Chemical techniques Oxidation
a b s t r a c t Polyesters prepared from the reaction of glycerol with mixtures of adipic and citric acids were evaluated in the laboratory to estimate degradation rates over a range of pH conditions. These renewable polymers can provide a marketable product for glycerol that is generated during biodiesel production. The polyesters were prepared without catalyst or solvent and produced water as co-product of the condensation reaction. Degradation rates for five glycerol polyester blends were determined from the amounts of the component acids released. All of the polyesters degraded rapidly to the component acids and glycerol. Polyesters exposed to acid solutions degraded 40.2% per day. Polyesters exposed to base solutions degraded 42.0% per day and polyesters exposed to neutral solutions degraded 37.5% per day. Applications for these materials exist in agricultural, environmental, and biomedical engineering products. Published by Elsevier B.V.
1. Introduction Glycerol generated during the production of biodiesel provides a nontoxic water soluble polyhydroxy compound that has numerous applications in cosmetic and pharmaceutical formulations. Glycerol also represents a versatile substrate for chemical synthesis. As biodiesel production has increased over the past several years the quantity of co-product glycerol available to industry exceeds the amount required for existing cosmetic and pharmaceutical products. However, new products have quickly developed to take advantage of this surplus glycerol. Recent research reported the synthesis and properties of a series of glycerol polyester materials prepared by condensation polymerization [1]. The reaction of glycerol with dicarboxylic acids such as adipic, azelaic, sebacic, or suberic acids proceeds without catalyst or solvent and produces polyesters with a range of physical properties. These materials provide a route to develop products with specific properties by adjusting the ratio of acids used and controlling the extent of reaction. Additional novel products were developed by combining glycerol, citric acid, and corn starch [2]. The hydrophilic character of glycerol polyesters also presents a compatible polymer matrix for the development of biocomposite materials. The hydrophilic character of the polyester facilitates bond formation to natural lignocellulosic fibers without chemical modification or surface treatments [3–5]. Such products are favored because they can be prepared from renewable substrates and follow the principles of green chemistry. These materials also show potential for a number of environmental and biomedical applications. However, the
∗ Tel.: +1 706 546 3520; fax: +1 706 546 3607. E-mail address: [email protected] 0254-0584/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.matchemphys.2011.02.071
degradation of these polymers has not been previously described. The current investigation addresses this topic and examined the breakdown of several glycerol polyesters after exposure to acid, base, and neutral solutions. 2. Experimental Glycerol, ACS grade, was obtained from Columbus Chemical Industries (Columbus, WI). Citric acid, analytical grade, was purchased from Mallinckrodt, Inc. (Paris, KY) and adipic acid, reagent grade, was obtained from the Sigma–Aldrich Co. (St. Louis, MO). Methanol, spectroscopic grade, was purchased from Fisher Scientific (Fair Lawn, NJ). Nanopure water was obtained using an Aqua Solutions system (Jasper, GA). Hydrochloric acid, ACS reagent, was obtained from J.T.Baker (Phillipsburg, NJ), sodium hydroxide was obtained from Fisher Chemicals (Pittsburgh, PA), and formic acid was obtained from Sigma-Aldrich (St. Louis, MO). Polymers were prepared in batches by mixing 25 g glycerol with 10% molar excess of adipic and citric acids. The amounts of these two acids were adjusted to obtain formulations composed of 100% adipic, 0% citric; 90% adipic, 10% citric; 80% adipic, 20% citric; 70% adipic, 30% citric; 60% adipic, 40% citric; and 50% adipic, 50% citric acids. A batch was prepared by combining the reactants in a 400 mL glass beaker placed on a temperature controlled hotplate. The reactants were heated at 70 ◦ C for 10 min with gentle stirring to obtain a homogeneous mixture. The mixture was poured into a disposable aluminum weighing pan and cured at 125 ◦ C in a vacuum oven. Samples of cured polymers, 8 mm diameter discs, were weighed and added to a 50 mL glass Erlenmeyer flasks containing 20 mL of aqueous acid, base, or neutral solution. The acid and base solutions, 0.1 N, were prepared from HCl and NaOH, respectively. The flasks were placed into a reciprocal shaking bath maintained at 37 ◦ C and set for 30 rpm oscillations. Aliquots, 0.2 mL, were taken from each flask by pipette, placed into 2 mL glass vials, and stored at 0 ◦ C pending analysis. Sampling was performed at day 1, 3, 5, 10, 24, and 45. The entire experiment was replicated. Solution samples were analyzed by high performance liquid chromatography using mass spectrometry for detection. The Thermo Scientific Accela HPLC system equipped with the Surveyor MSQ Plus MS detector and controlled by Xcalibur software was used for all analyses (Waltham, MA). Analytes were separated on a Hypersil Gold (Thermo Scientific) column that measured 50 mm × 2.1 mm packed with 1.9 micron particles. Samples were eluted using a mobile phase composition of 90% methanol and 10% water with 0.1% formic acid. A flow rate of 0.2 mL min−1 was
R.A. Holser / Materials Chemistry and Physics 128 (2011) 10–11
Fig. 1. Concentration profiles of glycerol polyesters in acid ( ), base (. . .), and neutral (—) solutions. used with injection volumes of 10 L. The detector was operated in the ESI mode with negative polarity at a probe temperature of 350 ◦ C and a cone voltage of 60 V. The detector was set to scan the range of 90–200 m/z with selected ion monitoring used to quantify the amounts of adipic and citric acid present. Calibration curves were prepared for adipic and citric acids at five levels with replicates.
3. Results and discussion The concentrations of adipic acid and citric acid increased with time as the polyesters degraded in the solutions. Changes in the concentrations of adipic and citric acids were averaged and are plotted in Fig. 1 as the ratios of intermediate to final concentrations or C/Cf . Degradation rates were estimated from the slope of the plots over the first time interval. The rates were comparable for polyesters exposed to acid, base, and neutral solutions and measured 40.2%, 42.0%, and 37.5% per day, respectively. The composition of the glycerol polyester did not have a significant influence on the degradation rates. Mass spectra of the samples indicated that the polyesters degraded to adipic acid, citric acid, and glycerol without apparent side reactions at these conditions. Analysis of the solution samples showed rapid increases in adipic and citric acid concentrations as the glycerol polyesters degraded in the aqueous media. Degradation rates were estimated from the concentration changes corresponding to the release of acids from the polyester. Degradation of the polymer proceeds by hydrolysis of the ester linkages and can occur at acidic or basic con-
11
ditions although by different mechanisms. These results showed similar degradation rates for all formulations and conditions tested. It should be noted that the polyesters were formulated with an excess amount of acid so that exposure to an aqueous solution would dissolve this unreacted acid and lower the pH of the solution. As the hydrolysis reaction continues to degrade the polyester additional acidic compounds are released. In environmental product applications, the degradation of the glycerol polyester would lead to locally acidic conditions which could influence the soil chemistry. However, the biotic and abiotic components of the soil would provide buffering capacity to mitigate the influence of acids released from the degraded glycerol polyesters. The magnitude of these interactions could be predicted with detailed knowledge of the soil chemistry. Similar considerations apply to the degradation of glycerol polyesters used for biomedical products such as controlled delivery systems where the release rates may be estimated. The rapid hydrolysis and degradation of these polyesters into their components presents an advantage for the development of many new products [6–8]. The predictability of the degradation rates allows products to be designed to breakdown within a selected time period or release encapsulated bioactives at specific rates. It is anticipated that many new applications will be developed using these renewable polyester materials. 4. Conclusions Polyesters prepared from glycerol with mixtures of adipic and citric acids showed rapid degradation in the presence of weak acid, base, or neutral solutions. The degradation rates were similar for all polyester blends and solutions tested. These results demonstrated how easily the glycerol polyesters can be prepared and degraded. This is an important design consideration for agricultural, biomedical, and environmental applications. It also suggests a simple process for the separation and recovery of the polyester components to facilitate recycling. References [1] [2] [3] [4] [5] [6]
R.A. Holser, J.L. Willett, S.F. Vaughn, J. Biobased Mater. Bio. 2 (2008) 1. R.A. Holser, J. Appl. Polym. Sci. 110 (2008) 1498. A.K. Bledzki, S. Reihmane, J. Gassan, J. Appl. Polym. Sci. 59 (1996) 1329. A.K. Mohanty, S. Parija, M. Misra, J. Appl. Polym. Sci. 60 (1996) 931. E. Zini, M. Baiardo, L. Armelao, M. Scandola, Macromol. Biosci. 4 (2004) 286. N. Lucas, C. Bienaime, C. Belloy, M. Queneudec, F. Silvestre, J.E. Nava-Saucedo, Chemosphere 73 (2008) 429. [7] J.P. Eubeler, M. Bernhard, S. Zok, T.P. Knepper, Trends Anal. Chem. 28 (2009) 1057. [8] S.P. Lyu, D. Untereker, Int. J. Mol. Sci. 10 (2009) 4033.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Degradation rates of glycerol polyesters at acidic and basic conditions Ronald A. Holser ∗ Russell Research Center, USDA-ARS, 950 College Station Road, Athens, GA, 30605 USA
a r t i c l e
i n f o
Article history: Received 27 July 2010 Received in revised form 17 February 2011 Accepted 23 February 2011 Keywords: Biomaterials Chemical synthesis Chemical techniques Oxidation
a b s t r a c t Polyesters prepared from the reaction of glycerol with mixtures of adipic and citric acids were evaluated in the laboratory to estimate degradation rates over a range of pH conditions. These renewable polymers can provide a marketable product for glycerol that is generated during biodiesel production. The polyesters were prepared without catalyst or solvent and produced water as co-product of the condensation reaction. Degradation rates for five glycerol polyester blends were determined from the amounts of the component acids released. All of the polyesters degraded rapidly to the component acids and glycerol. Polyesters exposed to acid solutions degraded 40.2% per day. Polyesters exposed to base solutions degraded 42.0% per day and polyesters exposed to neutral solutions degraded 37.5% per day. Applications for these materials exist in agricultural, environmental, and biomedical engineering products. Published by Elsevier B.V.
1. Introduction Glycerol generated during the production of biodiesel provides a nontoxic water soluble polyhydroxy compound that has numerous applications in cosmetic and pharmaceutical formulations. Glycerol also represents a versatile substrate for chemical synthesis. As biodiesel production has increased over the past several years the quantity of co-product glycerol available to industry exceeds the amount required for existing cosmetic and pharmaceutical products. However, new products have quickly developed to take advantage of this surplus glycerol. Recent research reported the synthesis and properties of a series of glycerol polyester materials prepared by condensation polymerization [1]. The reaction of glycerol with dicarboxylic acids such as adipic, azelaic, sebacic, or suberic acids proceeds without catalyst or solvent and produces polyesters with a range of physical properties. These materials provide a route to develop products with specific properties by adjusting the ratio of acids used and controlling the extent of reaction. Additional novel products were developed by combining glycerol, citric acid, and corn starch [2]. The hydrophilic character of glycerol polyesters also presents a compatible polymer matrix for the development of biocomposite materials. The hydrophilic character of the polyester facilitates bond formation to natural lignocellulosic fibers without chemical modification or surface treatments [3–5]. Such products are favored because they can be prepared from renewable substrates and follow the principles of green chemistry. These materials also show potential for a number of environmental and biomedical applications. However, the
∗ Tel.: +1 706 546 3520; fax: +1 706 546 3607. E-mail address: [email protected] 0254-0584/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.matchemphys.2011.02.071
degradation of these polymers has not been previously described. The current investigation addresses this topic and examined the breakdown of several glycerol polyesters after exposure to acid, base, and neutral solutions. 2. Experimental Glycerol, ACS grade, was obtained from Columbus Chemical Industries (Columbus, WI). Citric acid, analytical grade, was purchased from Mallinckrodt, Inc. (Paris, KY) and adipic acid, reagent grade, was obtained from the Sigma–Aldrich Co. (St. Louis, MO). Methanol, spectroscopic grade, was purchased from Fisher Scientific (Fair Lawn, NJ). Nanopure water was obtained using an Aqua Solutions system (Jasper, GA). Hydrochloric acid, ACS reagent, was obtained from J.T.Baker (Phillipsburg, NJ), sodium hydroxide was obtained from Fisher Chemicals (Pittsburgh, PA), and formic acid was obtained from Sigma-Aldrich (St. Louis, MO). Polymers were prepared in batches by mixing 25 g glycerol with 10% molar excess of adipic and citric acids. The amounts of these two acids were adjusted to obtain formulations composed of 100% adipic, 0% citric; 90% adipic, 10% citric; 80% adipic, 20% citric; 70% adipic, 30% citric; 60% adipic, 40% citric; and 50% adipic, 50% citric acids. A batch was prepared by combining the reactants in a 400 mL glass beaker placed on a temperature controlled hotplate. The reactants were heated at 70 ◦ C for 10 min with gentle stirring to obtain a homogeneous mixture. The mixture was poured into a disposable aluminum weighing pan and cured at 125 ◦ C in a vacuum oven. Samples of cured polymers, 8 mm diameter discs, were weighed and added to a 50 mL glass Erlenmeyer flasks containing 20 mL of aqueous acid, base, or neutral solution. The acid and base solutions, 0.1 N, were prepared from HCl and NaOH, respectively. The flasks were placed into a reciprocal shaking bath maintained at 37 ◦ C and set for 30 rpm oscillations. Aliquots, 0.2 mL, were taken from each flask by pipette, placed into 2 mL glass vials, and stored at 0 ◦ C pending analysis. Sampling was performed at day 1, 3, 5, 10, 24, and 45. The entire experiment was replicated. Solution samples were analyzed by high performance liquid chromatography using mass spectrometry for detection. The Thermo Scientific Accela HPLC system equipped with the Surveyor MSQ Plus MS detector and controlled by Xcalibur software was used for all analyses (Waltham, MA). Analytes were separated on a Hypersil Gold (Thermo Scientific) column that measured 50 mm × 2.1 mm packed with 1.9 micron particles. Samples were eluted using a mobile phase composition of 90% methanol and 10% water with 0.1% formic acid. A flow rate of 0.2 mL min−1 was
R.A. Holser / Materials Chemistry and Physics 128 (2011) 10–11
Fig. 1. Concentration profiles of glycerol polyesters in acid ( ), base (. . .), and neutral (—) solutions. used with injection volumes of 10 L. The detector was operated in the ESI mode with negative polarity at a probe temperature of 350 ◦ C and a cone voltage of 60 V. The detector was set to scan the range of 90–200 m/z with selected ion monitoring used to quantify the amounts of adipic and citric acid present. Calibration curves were prepared for adipic and citric acids at five levels with replicates.
3. Results and discussion The concentrations of adipic acid and citric acid increased with time as the polyesters degraded in the solutions. Changes in the concentrations of adipic and citric acids were averaged and are plotted in Fig. 1 as the ratios of intermediate to final concentrations or C/Cf . Degradation rates were estimated from the slope of the plots over the first time interval. The rates were comparable for polyesters exposed to acid, base, and neutral solutions and measured 40.2%, 42.0%, and 37.5% per day, respectively. The composition of the glycerol polyester did not have a significant influence on the degradation rates. Mass spectra of the samples indicated that the polyesters degraded to adipic acid, citric acid, and glycerol without apparent side reactions at these conditions. Analysis of the solution samples showed rapid increases in adipic and citric acid concentrations as the glycerol polyesters degraded in the aqueous media. Degradation rates were estimated from the concentration changes corresponding to the release of acids from the polyester. Degradation of the polymer proceeds by hydrolysis of the ester linkages and can occur at acidic or basic con-
11
ditions although by different mechanisms. These results showed similar degradation rates for all formulations and conditions tested. It should be noted that the polyesters were formulated with an excess amount of acid so that exposure to an aqueous solution would dissolve this unreacted acid and lower the pH of the solution. As the hydrolysis reaction continues to degrade the polyester additional acidic compounds are released. In environmental product applications, the degradation of the glycerol polyester would lead to locally acidic conditions which could influence the soil chemistry. However, the biotic and abiotic components of the soil would provide buffering capacity to mitigate the influence of acids released from the degraded glycerol polyesters. The magnitude of these interactions could be predicted with detailed knowledge of the soil chemistry. Similar considerations apply to the degradation of glycerol polyesters used for biomedical products such as controlled delivery systems where the release rates may be estimated. The rapid hydrolysis and degradation of these polyesters into their components presents an advantage for the development of many new products [6–8]. The predictability of the degradation rates allows products to be designed to breakdown within a selected time period or release encapsulated bioactives at specific rates. It is anticipated that many new applications will be developed using these renewable polyester materials. 4. Conclusions Polyesters prepared from glycerol with mixtures of adipic and citric acids showed rapid degradation in the presence of weak acid, base, or neutral solutions. The degradation rates were similar for all polyester blends and solutions tested. These results demonstrated how easily the glycerol polyesters can be prepared and degraded. This is an important design consideration for agricultural, biomedical, and environmental applications. It also suggests a simple process for the separation and recovery of the polyester components to facilitate recycling. References [1] [2] [3] [4] [5] [6]
R.A. Holser, J.L. Willett, S.F. Vaughn, J. Biobased Mater. Bio. 2 (2008) 1. R.A. Holser, J. Appl. Polym. Sci. 110 (2008) 1498. A.K. Bledzki, S. Reihmane, J. Gassan, J. Appl. Polym. Sci. 59 (1996) 1329. A.K. Mohanty, S. Parija, M. Misra, J. Appl. Polym. Sci. 60 (1996) 931. E. Zini, M. Baiardo, L. Armelao, M. Scandola, Macromol. Biosci. 4 (2004) 286. N. Lucas, C. Bienaime, C. Belloy, M. Queneudec, F. Silvestre, J.E. Nava-Saucedo, Chemosphere 73 (2008) 429. [7] J.P. Eubeler, M. Bernhard, S. Zok, T.P. Knepper, Trends Anal. Chem. 28 (2009) 1057. [8] S.P. Lyu, D. Untereker, Int. J. Mol. Sci. 10 (2009) 4033.