Paper2Plastic

Over this past summer a new initiative in the Boston College chemistry department had its first year: year 0. This program, titled Paper2Plastic, is our response to the under-representation of women and minorities in the STEM fields. By introducing the students to the conditions of a research lab early in their educational careers (i.e. high school), we hope to boost their confidence and understanding of their ability to thrive in a research atmosphere.

Abstract

Paper to Plastics (P2P) is an interdisciplinary program that combines chemistry and biology in a research setting. The goal of this project is twofold: to engage students in scientific research, and to educate them about sustainability and biodegradable materials. The scientific aim of the project is to recycle unwanted office paper to the useful biodegradable polymer poly(lactic acid) (PLA). Through this program, students learn firsthand how chemistry and biology interact to form useful materials from waste. Students combine biological techniques, such as enzymatic digestion and fermentation, with chemical techniques, such as distillation and catalysis, to accomplish the conversion of waste paper into PLA. Through this summer research project, students ultimately become familiar with diverse laboratory techniques, whilst learning how their scientific interests can be used to address important social problems.

Introduction

In response to low graduation rates among American students in science, technology, engineering, and mathematics (STEM) disciplines, The National Academy of Science (NAS) recently published suggestions for modifying STEM education in grades K-12.¹ The recommendations stressed a three-dimensional model for a successful educational program: 1) Describe scientific and engineering practices, 2) Explore crosscutting concepts that have applicability across multiple scientific disciplines, and 3) Describe core ideas. These recommendations represent a dramatic paradigm shift in pedagogy away from representing science as an insurmountable collection of facts, to representing it as a creative and malleable discipline whose primary objective is to uncover the goings on of the natural world.

In line with the educational framework set forth by the NAS are educational outreach programs whose aim is to demonstrate scientific principles and thinking in the context of topics that are relevant and interesting to students. Engaging students in topics that they find interesting is particularly important, because research has shown that personal interests, experiences, and enthusiasm are a primary factor linked to later educational and career choices.^2-5 With these factors in mind, we set out to develop an outreach program designed to engage high-school-aged students in a project that best mimicked a genuine research environment. We intended to create a project that could be carried out in the summer months by colleges and universities that are equipped with standard instrumentation used in modern research environments.

In order to maximally engage the students, a socially significant and multidisciplinary topic was needed as the subject matter for the project. An ideal candidate that met these criteria is the synthesis of biodegradable polymers from waste. In the past few decades, petroleum-based plastics have fallen out of favor because they are not environmentally sustainable.⁶ Consequently, there is an increasing demand for environmentally friendly alternatives to conventional plastics. Poly(lactic acid) (PLA) is a biodegradable polymer that is suited to many of the applications in which petroleum-based polymers are typically employed, such as grocery bags and food storage containers.⁷ Currently, PLA is produced on an industrial scale from renewable starch sources, chiefly corn.^8,9 In theory, similar syntheses of PLA can be conducted on a smaller scale using any source of starch that is inexpensive and abundantly available. Used office paper is not only widely available at any college, university, or high school, but it is also an abundant and convenient source of starch. Through a series of biochemical and chemical transformations, we hypothesized that the cellulose derived from used office paper could be converted into PLA in the laboratory. Such a research project would be ideally suited to encourage high school students to participate in STEM disciplines as it addresses all three dimensions of the educational framework set forth by the NAS: 1) the students learn practices and procedures common to any scientific discipline, 2) the project uses a multidisciplinary approach to answer scientific inquiry, and 3) the project addresses many of the core ideas targeted by the NAS (e.g. matter and its interactions, the dynamic interaction of energy and ecosystems, earth and human activity, etc).¹

In this publication, we describe the “Paper to Plastics” program (P2P) that has been developed in the chemistry department at Boston College. Through a series of laboratory experiments based on existing literature and industrial techniques,^10-15 we have created a sequence of modules that harness biological catalysts as well as traditional chemistry techniques to transform used office paper into biodegradable plastic (Scheme 1). Each experiment introduces students to salient topics in biology and chemistry, such as biological and chemical catalysis, chemical reactions, and organic synthesis. Though a broad range of topics is addressed, the techniques detailed are designed to be comprehensible and fully hands-on. At every stage, there is opportunity for inquiry and adaptation. As such, the P2P program can be readily tailored to a variety of academic settings. Our aim through P2P is to cultivate excitement for scientific research in students by allowing them to work on a process that is both stimulating and socially relevant. Under the guidance of undergraduate mentors, several high-school students carried out this suite of experiments in the summer of 2013.

Scheme 1. Outline of biochemical and chemical transformations involved in the conversion of cellulose from paper into poly(lactic acid), a biodegradable plastic.

Rationale

In keeping with the interdisciplinary aim of the program, the six experimental modules shown in Table 1 encompass multiple areas within chemistry and biology. The first modules are more biology-oriented, while the latter half employs organic synthesis techniques.

Table 1. Summary of the experimental modules

The Experiments

The following modules detail the biological and chemical transformations necessary to convert office paper into poly(lactic acid) (Scheme 1). To ensure a sufficient yield for each step, each module can be supplemented with purchased reagents. A more advanced group of students may wish to attempt to carry the original paper pulp through the entire series of transformations.

Module 1: Paper pulping and floatation de-inking¹³

Students use a home blender to create a homogenous suspension of paper pulp. The process of removing ink contaminants makes use of the hydrophobic properties of ink pigments. To accomplish the de-inking step, the pulp is added to a large flask with deionized water. Upon addition of dish soap, the mixture is aerated through a tube connected to an air outlet so that bubbles form in the solution. The hydrophobic soap bubble attracts the ink particles and carries them out of the flask into a waste basin. Students then vacuum filter and oven-dry the de-inked pulp, and obtain a yield from the original paper source.

Module 2: Enzymatic digestion¹²

Cellulases are a family of enzymes produced by bacteria and fungi that catalyze the hydrolysis of cellulose.¹⁶ Cellulase is used to break down cellulose in the paper pulp into glucose monomers (Figure 1a). To achieve this, a mixture of commercially available cellulase enzymes (from Trichoderma reesei) and cellulose is incubated at 37°C, and students take aliquots at regular intervals to determine the most effective incubation time for the reaction. The glucose yield is calculated using a commercially available Glucose (GO) Assay Kit (Sigma Aldrich) that enzymatically oxidizes the glucose to gluconic acid and hydrogen peroxide. The resulting hydrogen peroxide then reacts with o-dianisidine in the presence of peroxidase to form a colored product. Product formation is monitored by absorbance at 450 nm. The percent yield can be calculated to determine the efficiency of conversion, as well as the most efficient length of incubation. To expand this module, students can opt to vary the ratio of cellulase enzyme to cellulose to determine the optimal concentration for catalysis. This module introduces students to enzyme catalysis and the effects of parameters such as temperature, time and concentration on the amount of product afforded by an enzymatic reaction. Furthermore, the students learn to use a spectrophotometer and a coupled assay to monitor glucose formation.

Figure 1. Biological transformations involved in the conversion of cellulose to lactate. (A) The general catalytic mechanism of cellulase enzymes; (B) The enzymatic transformations in Lactobacillus casei that convert glucose to pyruvate.

Module 3: Fermentation¹¹

Students utilize the microorganism Lactobacillus casei to convert glucose to lactic acid. In this module, students learn about the nutritional and environmental requirements for bacterial growth. In a similar manner to the cellulase digestion, students inoculate glucose-free growth media with bacteria, and add varying amounts of glucose. Aliquots of the fermentation broth are taken at daily intervals, and the lactic acid content of each sample is assayed with a Lactate Assay Kit (Sigma Aldrich), which utilizes a colorimetric readout to measure lactate concentrations.

This module exposes students to standard sterile techniques, by teaching them to culture L. casei in both solid agar plates as well as liquid media. They also learn about the concomitant action of a series of enzymes in a metabolic pathway that converts glucose into lactate (Figure 1b). These pathways are conserved in higher organisms and form a significant component of college level biochemistry courses, thereby providing students an early introduction to the topic of metabolism.

Module 4: Oligomerization¹⁴

The lactic acid oligomerization reaction is the first of three chemical transformations in the conversion of waste paper to PLA (step i, Scheme 2) and is the students’ first introduction to a chemical reaction, along with the prerequisite glassware and setup. During this procedure the students are introduced to the theory and design of a complex reaction setup including use of a heat source and specialized glassware such as a Dean-Stark trap. In the absence of significant quantities of lactic acid from the fermentation step, reagent-grade lactic acid can be heated in a round bottom flask to 200 °C for ~4.5 hours. During this time the water produced from the condensation reaction of lactic acid collects in a Dean-Stark trap. The volume of water collected is proportional to the progress of the reaction, and can be used to calculate the average oligomer chain length.¹⁴ By monitoring the production of water over time, the students have the opportunity to learn about chemical kinetics. Additionally, this module introduces the students to modern spectroscopic techniques such as Nuclear Magnetic Resonance (NMR) spectroscopy, which can be used to monitor the progress of the reaction and to evaluate its purity.

Scheme 2. Chemical transformations involved in the conversion of lactate to poly(lactic acid).

Module 5: Lactide Formation¹⁵

The cyclization of oligomeric PLA used to produce lactide (step ii, Scheme 2) reinforces many of the skills that the students learned during the oligomerization reaction, while adding a new level of complexity. Prior research has found that higher yields of lactide are achievable by oligomerization of lactic acid prior to cyclization rather than from direct dimerization of lactic acid. The advantage of this procedure is presumably due to entropic reasons. In order to obtain high yields of lactide, the cyclization relies on a short-path distillation under reduced pressures that removes the lactide product from the reaction mixture and drives the equilibrium towards the desired product.¹⁵ Under these conditions, the students learn how to design and plan a reaction setup to maximize control of parameters such as temperature and pressure. The students also learn how to carry out distillation under reduced pressures. After distillation, the lactide product is obtained as a mixture of diastereomers (rac and meso) and is often contaminated with small amounts of linear lactic acid dimer. In order to carry out effective lactide polymerization, the lactide monomer must be very pure. Moreover, the rac lactide diastereomer leads to polymer with more desirable mechanical properties. Therefore, the students learn how to purify the rac lactide by crystallization of the crude distillate. The progress of this reaction is monitored by the volume of the distillate and later verified through ¹H NMR spectroscopy. This module is particularly amenable for experimentation as the yield of the lactide is very sensitive to the reaction conditions. An instructive use of this module would be to have different groups carry out the lactide forming reaction under different pressures and temperatures. Yield and purity of the lactide can be used as parameters for optimization thereby illustrating to the students how systematic experimentation is used to find ideal conditions for a chemical reaction.

Figure 2. Flow chart highlighting the work flow and techniques in polymer and organometallic chemistry introduced during the polymerization step.

Module 6: Polymerization

The polymerization reaction is the final step in the conversion of used office paper to PLA (step iii, Scheme 2). New catalysts for the ring opening polymerization of lactide is currently an active area of research.^17,18 Therefore, to emphasize the research aspect of the project, we decided to carry out the polymerization of lactide using an iron catalyst (1) that was recently reported by one of us.¹⁰ The polymerization reaction also allowed for the demonstration of many common and useful techniques in polymer and organometallic chemistry (Figure 2). The catalyst precursor is air and moisture sensitive, so the students were able to learn how to handle air sensitive reagents using equipment such as inert atmosphere glove boxes, solvent purification systems, and high vacuum lines. Additionally, analysis of the reaction was carried out using gel permeation chromatography (GPC) to monitor polymer molecular weights and molecular weight distributions. For laboratories that are not equipped with such specialized equipment, there are numerous other air and moisture stable catalysts that have been developed for the polymerization of lactide.^17,18 Many of these catalysts such as tin(II) 2-ethylhexanoate¹⁹ or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)²⁰ are inexpensive and are commercially available. In the absence of gel permeation chromatography or other types of size exclusion chromatography, the progress of the polymerization reaction can easily be monitored by measuring the viscosity of the reaction mixture. Precipitation of the polymer in methanol solution also allows the students to analyze the macroscopic physical properties of the new material (e.g. T_m, T_g, etc.), which is very similar to many of the industrial plastics that they are more familiar with.

Experimental Timeline

This program was run over the course of eight weeks during the 2013 summer (June-July) meeting twice weekly. By splitting into two sections, one meeting on Mondays and Wednesdays and the other meeting on Tuesdays and Thursdays, the schedule provided adequate time for fermentations, digests, and similar procedures to progress significantly between each class period. Further streamlining these procedures is possible, which will likely allow for the program to be carried out over the course of four weeks instead of eight. Experiments range from 2-5 hours in length.

Table 2. Experimental timeline

Hazards

An advantage to this process is that these techniques have few hazards and safety issues associated with the reagents and methods used. Nevertheless, this project provides an ideal opportunity to promote safe practices in the lab. Therefore, safety glasses and appropriate attire for the laboratory is stressed. Though it is a good practice to teach students to consult Material Safety Data Sheets, the compounds used present no significant hazard.

Equipment

To carry out these experiments, the laboratory should contain basic organic glassware (round bottom flasks, condensers, etc.) and biochemical equipment (e.g. pipets, culture plates, etc.), a Dean-Stark trap, a standard vacuum manifold with a vacuum pump and air outlets, a fume hood, an analytical balance, a drying oven, a shaking incubator, and a UV-visible spectrophotometer. An inexpensive kitchen blender can be purchased from any home goods store for module 1. Modules 4 and 5 require an NMR spectrometer for optional product analysis, but product identification can also be carried out using more classical techniques such as infrared spectroscopy, viscometry, and/or melting point determinations. However, a major goal of the project is to mimic genuine research conditions; using modern analytical techniques such as NMR spectroscopy emphasizes this goal and in our experience generates excitement among the high school students.

Integration into Curriculum

The P2P program is amenable to modification to suit a range of purposes. The modular nature of the program allows for much flexibility in the duration of the program.

High School

The primary goal of the P2P program is to expose high school students to multidisciplinary research at a higher-level research institution. This program gives the students access to instrumentation and resources that are not routinely available in high-school laboratories. To highlight the benefits of this exposure, we quote some of the student feedback we received from the program: “I thought that it was really wonderful to be able to work in a real lab, instead of doing uninteresting experiments at school”, and “It was a great experience to use the high-tech equipment”. These quotes attest to the value of the opportunity that we are providing these high school students and our successes at generating interest in scientific research at an early age.

In addition to being replicated as a summer program at a research university, P2P can be extended to an entire semester project for a high-school biology or chemistry class. While some of the techniques and instruments used are outside the means of many high school facilities, many of the procedures can be adapted to the available methods. These modules could also be applied individually as an after-school club or an independent project for an advanced student. To make this a more comprehensive project, students can focus on optimizing the procedures and varying the conditions to obtain the best yield in each step or investigating the synthesis and polymerization of similar cyclic diesters.

Undergraduate

The P2P program, as designed, provides undergraduate students with an ideal setting in which to mentor younger students, whilst honing their own research skills in a variety of research arenas. We recruited two junior undergraduate students with aspirations to pursue careers in teaching and scientific research to mentor the high school students over the course of the summer. This opportunity enabled the undergraduates to independently optimize the steps of each module prior to the arrival of the high school students, thereby providing them the opportunity to perform research and troubleshoot experiments with minimal supervision. Furthermore, the mentoring of the high school students involved dissecting fundamental concepts in biology and chemistry to terms that were approachable to the high school students, thereby providing a forum to practice their teaching and mentoring skills.

Outside of the mentoring benefits provided to undergraduates interested in a career in science education, the experiments presented herein would also be suitable for an undergraduate organized laboratory setting. This project could be integrated into the curriculum of an introductory biology or chemistry lab. Similarly, this could be implemented as an independent research project for an undergraduate student.

Conclusions

The P2P program is a unique opportunity that intersects science, education, and society. It not only encourages students to think about scientific problems from a multidisciplinary perspective, but it also provides them with hands on experience in a research environment. This first-hand research experience, coupled with its social implications, provides an exciting environment for the students to learn and grow. The P2P program is ideally suited to achieve the following goals: (1) introduce high school students to a genuine scientific research environment to encourage careers in science, engineering, and technology; (2) demonstrate how scientific problems can be addressed effectively from a multidisciplinary approach; (3) provide a concrete example of how science and technology can be used to address problems with social significance; (4) provide an opportunity for high school students to interact with undergraduate students, graduate students, and professors in order to foster open discourse about pursuing higher education and the value of science degrees and careers in science and technology; and (5) develop leadership and mentoring skills among undergraduate students who are considering careers in science, engineering, and technology. We believe that the experimental modules described herein are highly versatile and easily incorporated into other outreach programs, as well as current high-school and undergraduate curriculums. By bridging the fields of chemistry and biology in a socially-relevant research endeavor, we are confident that high school students will develop an early appreciation of the scientific discipline, which will serve to increase student participation in STEM fields as outlined in the objectives of the NAS.

The whole thing: Paper to Plastics_Final