Introduction. Depolymerization of Poly(lactic acid) Cups to Lactic Acid. Experiment

Transcription

1 Experiment Depolymerization of Poly(lactic acid) Cups to Lactic Acid 6 Introduction Poly(lactic acid), or PLA, is a biobased polymer that is a promising alternative to fossil-fuel based plastics. PLA is derived from corn, a renewable resource, and its total production uses 25-50% less fossil energy than the production of conventional petroleum-derived plastics. i To prepare PLA, corn is fermented into lactic acid, which is then manufactured into a low molecular weight pre-polymer. This prepolymer can then be used to form the high molecular weight PLA plastic that can be made into anything from plastic cups, to clothing, to medical sutures. ii iii Pure high-molecular weight PLA is brittle. Therefore, most commercial uses of PLA incorporate other plastics into a PLA blend depending on the rigidity of the plastic needed for a specific application. iv PLA is classified as a number 7 plastic. 79

2 PLA is a polymer it consists of individual components of stable monomers chained together. In this experiment, PLA will be broken down into its lactic acid monomers, which will be quantitatively measured through a potentiometric titration. The conversion of PLA to lactic acid is called a saponification reaction. The saponification reaction is an organic hydrolysis reaction that is used to make soap. (Some green household cleaners use lactic acid as an active ingredient). Each C-O-C bond that binds two lactic acid units together is cleaved to form 2 C-O-H units. Because the reaction occurs in a basic solution, an acidic work-up is needed to convert the deprotonated lactic acid unit (present in the form of a sodium lactate salt) into the desired protonated lactic acid form: The depolymerization of PLA also demonstrates important principles of Green Chemistry: - Prevents waste: The solvent (50% ethanol) can be reused. Therefore, the E-factor, or the grams of byproduct (waste) divided by grams of product, is low. - High Atom Economy: The molecular weight of the products divided by the molecular weight of the reagents, or atom economy, is a measure of the efficiency of the reaction. What is the atom economy in this reaction? - Designing safer chemicals: lactic acid, also known as milk acid, is a naturally-occurring chemical and is therefore, biodegradable. - Safer solvents: this reaction takes place in 50% ethanol, as opposed to harmful organic solvents. - Design for Energy Efficiency: the entire hydrolysis takes place in an hour, or 15 minutes if using a microwave oven. Also, the manufacture of PLA uses less energy and fossil fuels than that of conventional fossil-fuel based plastics. - Renewable Feedstock: PLA is generally derived from corn or potato starches, which are renewable resources. - Design for degradation: the depolymerization of PLA recycles the original lactic acid used to manufacture PLA. This lactic acid can be isolated for various other uses including reincorporation into PLA. Why do the structures differ with changes in ph? Use Le Chatelier s Principle to explain the dominant species at ph 2 vs. ph 12. In this lab you will be performing a potentiometric titration of your sample of lactic acid with a known NaOH standard, using a ph meter to track changes in the ph of the acid solution as you add measured volumes of base. We usually represent such data in the form of a titration curve. 80

3 Titration of a Monoprotic Weak Acid with Strong Base 4 ph V base 1) Start- Solution is just an acid in water since no base has been added. HA + H 2 O A - + H 3 O + The ph depends on the strength of the acid and its concentration. 2) Before equivalence- A buffer exists, a mixture of HA and A - because HA is only partly neutralized by base. HA + OH - A - + H 2 O ph pk a log [A ] [HA] When exactly half of the acid has been neutralized, the ph reading gives you a very important piece of information. What is so special about the ph at ½ equivalence? Think about the proportions of A - and HA in solution. 3) At equivalence- HA and OH - react completely to give only A - in solution. The ph of this solution depends on the chemical properties of A -. Often A - is a weak base so the reaction of a weak base with the water must be considered. 4) After equivalence- The is ph dominated by the excess of the strong base added. Challenge: What would the curve look like for the titration of a weak base with a strong acid? How would you predict the ph of the buffer regions? 81

4 Prelab: Complete the following tasks in your lab notebook. Work in small groups in lab and share your thoughts with your classmates. 1. Write a fully balanced chemical equation for the conversion of lactic acid to PLA include the acidic work-up. 2. Draw the form of the PLA polymer, the product after the depolymerization, the product after the acidic work-up, and the product after the titration. 3. What is the pka of lactic acid? 4. Of the following indicators, which one(s) would be suitable for finding the endpoint in the titration? Explain your reasoning. Phenolphthalein Bromocresol Green Bromothymol Blue 5. How many lactic acid monomer molecules do you expect in a 1g sample of PLA? (Hint: what is the molar mass of the repeating lactic acid monomer unit in PLA?) 6. How much NaOH will you need to completely react with the PLA? 7. How much HCl will you need for the acidic work-up step, assuming the reaction goes to completion? 82

5 Depolymerization of Poly(lactic acid) to Lactic Acid v Experimental The Problem Depolymerize a sample of poly(lactic acid) into lactic acid in basic solution. Quantitatively measure the amount of lactic acid in the final solution through a potentiometric titration. The Approach Work in groups of 2 depending on the availability of ph meters in your lab room. Equipment needed: PLA cups Standardized NaOH solution (~3M for the depolymerization and ~0.1M for the titration) 50mL Erlenmeyer flask stir bar watch glass hot plate 50% HCl solution ice ph meter phydrion strips buret clamps Gravity Filtration ph Meter Potentiometric Titration Apparatus Buret 24 ph Meter Clamp Hayden-McNeil Publishing, Inc. Hayden-McNeil Publishing, Inc. 83

6 1. Cut PLA cup into small pieces (the smaller the pieces, the faster the reaction why?). Measure out ~1 gram and put the pieces into a 50mL Erlenmeyer flask. Be sure to record the exact mass of the PLA in your notebook. 2. Measure out 10 ml of 3M NaOH in 50% ethanol. Add to the 50mL Erlenmeyer flask. 3. Add a stir bar to the solution and cover the top of the 50mL Erlenmeyer flask with a watch glass. 4. Heat the reaction for approximately one hour (or until all of the polymer pieces appear reacted) on a hot plate set to a medium heat. Make sure to observe any changes in the solution. The solution should be boiling, but not so much that the watch glass begins to rattle. 5. Once all the solid PLA is reacted, place the reaction mixture in a shallow ice water bath and let cool for ~5-10 minutes. 6. Take the ice bath with your reaction mixture into the fume hood. Slowly add 50% HCl until the ph is ~1-2. Test the solution with the ph paper provided. 7. Gravity filter the solution and collect the filtrate. 8. Obtain 50 ml of ~0.1M NaOH solution from the fume hood. Be sure to record the exact concentration of the solution in your notebook 9. Set up the apparatus for your titration. Obtain a buret clamp from the lab shelf. Clean your buret with soap and water, then rinse with just a few ml (~5 ml) of NaOH solution before filling the buret with the base (remember, don t fill past the 0.00 ml mark). Record your initial volume. 10. Calibrate your ph meter by following the steps in the appendix of your lab manual. Be sure to have a large beaker to collect solution from rinsing the probe with distilled water. Kim wipes can be obtained from your GSI to gently pat dry the probe. Do not use paper towels. 11. Prepare your lactic acid solution for titration. Using a pipette, remove a 1.00 ml aliquot of the filtrate and dispense into a 150mL beaker. Add 50 ml of deionized water using a graduated cylinder. Add a stir bar to your flask. 12. Clamp the probe of the ph meter to the metal rod at your station. Make sure the probe is immersed in your dilute solution past the small hole on the side of the probe, near the bulb. The stir bar must not collide with the ph meter. Your GSI will demonstrate the proper setup of the apparatus for you. 13. To perform your rough titration, start by recording the initial ph of your unknown solution. It would be helpful to have a table in your notebook where you can record the ph of the solution corresponding to the amount of base you have added. 14. With gentle stirring, add 1.00 ml of the NaOH solution to the PLA solution. Allow a few moments for the ph to stabilize, and then record the ph in your table. Repeat this step until you either reach a ph of 12 or have added 25 ml of base. 15. Draw a rough graph of the titration in your notebook to identify the equivalence point volumes. 84

7 16. Set up your precise titration following the same procedure as above. Dispose of your solution in the proper container and be sure to wash and dry the beaker thoroughly before adding a new diluted lactic acid solution. 17. Perform the precise titration. Add 1 ml increments of base as you did above (recording ph readings as you go), but when you come within 1 ml of an equivalence point, slow down the titration and add smaller amounts of base (dropwise, around 0.2 ml). You ll need to make an Excel graph of your precise titration, so be sure to collect as much data around the equivalence points as you can. 18. Place excess acid and base solutions in the appropriately labeled containers near the fume hood. A five gallon bucket will be provided for collection of all the solutions from this experiment. Calculating Equivalence Points from Titration Data The solid line above shows how ph changes during a titration of nitric acid, HNO 3, with the strong base sodium hydroxide, NaOH. The ph changes dramatically around 16 ml, indicating the equivalence point has been reached in the titration. To accurately determine equivalence point it is best to compute the first derivative of the titration curve. The first derivative (dotted line) shows a maximum when the slope is the steepest, which should coincide with the equivalence point of the titration. Using this data the equivalence volume is actually ml. Accurate determination of the equivalence point depends on the quality of the data. For this method to work well, it is important to add base is very small increments around the equivalence point. In the data set below the student added 0.05 ml of base at a time. Adding even smaller amounts, fractions of a drop, will produce even greater accuracy. Vbase ph dph/dv

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9 Experiment 6- Depolymerization of Poly(lactic acid) Cups to Lactic Acid Report Sheet GSI Name Partners Generate a good graph of ph vs. Volume Titrant for your sample and attach it to this report. Also graph the first derivative of the ph graph versus volume titrant. (Hint: If you use Excel, the SLOPE function will be useful). Data: 1. Attach a copy of your titration curve to this report. 2. Label important data points and regions on the curve. Results and Discussion: 1) Determine the amount of lactic acid titrated in the solution. 2) What evidence suggests that lactic acid is actually in the final solution? (Hint: Calculate the pka of the acid.) Are any assumptions being made about the final solution that would affect the data from the titration? 3) What assumptions were made when calculating the amount of NaOH that would be needed to react the entire PLA sample? How do you think using a real-world reagent versus a pure chemical affects the data? 4) Draw the molecular structure of PLA before and after the depolymerization step. 5) How does the molecule change after the acidic-work-up? 87

10 6) What different species are in your solution immediately after the depolymerization? 7) What different species are in your solution after the acidic work-up? Green Chemistry Connections Read through the green chemistry principles provided in the beginning of the lab manual. 8) What principles are illustrated through this depolymerization of PLA? 9) Another way to depolymerize the PLA is to heat the plastic with store bought white vinegar for 65 hours. Which method is greener, the NaOH/HCl or the vinegar? i Vink, Erwin T.H., et. al. Applications of Life Cycle Assessment to NatureWorks polylactide (PLA) production. Science Direct [Online] Nov 11, 2002, p (accessed July 31, 2011). ii NatureWorks LLC. Products and Applications. Applications.aspx (accessed Aug 3, 2011). iii Gruber, Patrick and Michael O Brien. Polylactides NatureWorks PLA. Wiley-VCH [online], p (accessed July 31, 2011). iv Gruber, Patrick. Commodity Polymers from Renewable Resources: Polylactic Acid. National Center for Biotechnology Information [Online]. (accessed Aug 1, 2011). v Boice, Jennifer N. et. al. Molecular Recycling: Application of the Twelve Principles of Green Chemistry in the Diversion of Post-consumer Poly(lactic acid) Waste. The Journal of Materials Education [Online] May 5, (accessed Aug 3, 2011). 88