In the Laboratory
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Polymer–Plastics Experiments for the Chemistry Curriculum
Eugene S. Stevens,* Kyle Baumstein, James-Michael Leahy, and David C. Doetschman Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902; *
[email protected] Laboratory experiments involving polymers1 are not represented in undergraduate curricula in proportion to the prevalence of polymer materials in modern society or the number of polymer chemists (1). New experiments are finding their way into the chemistry curriculum (2–9), but some aspects of polymer chemistry that demonstrate the importance of chemistry in producing polymeric materials are particularly absent. For example, mechanical properties are closely related to chemical structure and critically important for applications. Also, polymers are widely used as matrices in drug delivery applications. It is timely to introduce chemistry students to these topics. At this university polymer–plastics experiments were developed for upper-level physical chemistry and biophysical chemistry laboratories. The laboratory experiments integrate three components: a chemistry component for improved learning of chemical structure–property relationships; a materials component for learning materials formulation and mechanical properties; and, through the use of degradable polymers, an environmental component for learning the relevance of science to environmental conservation and sustainable technologies (10, 11). A Web site was developed for these experiments (12). Experiment 1: Mechanical Properties of Polymers–Plastics
into a Teflon-coated mold; a commercial baking pan 25-cm × 15-cm suffices. Thorough air-drying takes several days; alternatively, oven drying can be used. When the film is dry, 0.25-in. × 3.5-in. specimens are cut using a metal template. Specimen thickness is measured with a micrometer. In tensile tests (13, 14), specimens are placed between two grips and drawn. The testing instrument measures the increasing stress, σ, σ = F兾A where F is the force being applied (load) in Newtons, and A is the original cross-sectional area (width × thickness) of the specimen in m2. Tensile strength at break is the stress at break in Pa. The instrument simultaneously measures the tensile strain, ε, which is the increase in the length, l, (extension, or elongation) of the specimen divided by the original length of that portion of the specimen being measured, l0, (called the gage length): ε = (l − l0 )兾l0. The percent elongation at break is the strain multiplied by 100. The combined results are displayed as a stress–strain curve. Young’s (elastic) modulus, E, is calculated as the initial slope of the stress–strain curve, which is often observed to be linear with plastic films: σ = Eε
Experimental Procedure In this experiment, glycerol is used as a plasticizer1 (but in these formulations, water also acts as a plasticizer). A glycerol stock solution is provided that is 2% v兾v of glycerol in water; the density of pure glycerol is 1.26 g兾mL. Students cast three films of glycerol-plasticized agar, with agar:glycerol weight percent compositions of 50:50, 60:40, and 80:20. A fourth film is prepared with 30% agar, 30% starch, and 40% glycerol. In each case the total weight of polymer plus glycerol is 4 g. Water is added to the polymer and glycerol solution to bring the total volume to 120 mL. The beaker contents are heated with stirring to 85–95 C and poured
Modulus is a measure of the “stiffness” of the polymer or plastic. Additional details are given in the Supplemental Material.W
Hazards The chemicals are nonhazardous. The sharp instrument used to cut specimens must be handled with care. Results Measurements were made on an Instron Model 5543 testing system with a 100 N load cell using a 2-in. gage length and a speed of 2 in.兾min. Results from two classes are shown in Table 1.
Table 1. Student Tensile Data Sample (Wt %) Agar
Starch
Glycerol
Tensile Strength (SD, SDm )/MPa
Laboratory
E (SD, SDm )/MPa
ε (SD, SDm ) (%)
50
00
50
Physical
12.9 (5.2, 1.7)
0115 (47, 26)
57 (28, 10)
50
00
50
Biophysical
15.5 (7.8, 2.4)
0209 (155, 54)
31 (21, 8)
60
00
40
Physical
25.4 (7.4, 2.5)
0433 (200, 95)
34 (11, 5)
60
00
40
Biophysical
25.2 (9.1, 2.7)
0818 (400, 135)
12 (6, 4)
80
00
20
Physical
61.7 (8.0 3.8)
2500 (520, 180)
06.7 (3.8, 1.3)
80
00
20
Biophysical
46.3 (9.3, 3.9)
2160 (820, 250)
03.1 (1.0, 0.7)
30
30
40
Physical
13.9 (4.9, 1.6)
0186 (108, 56)
39 (16, 7)
30
30
40
Biophysical
12.6 (4.1, 1.4)
0404 (163, 67)
12 (6, 5)
NOTE: The number of measurements was 9 or 10.
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In the Laboratory
Discussion Despite the variance, students are able to draw several conclusions: (i) both tensile strength and modulus increase as the quantity of plasticizer is decreased; (ii) elongation increases with increased plasticizer; and (iii) replacing part of the agar with starch weakens and softens the films. These are significant observations that teach (a) the dependence of mechanical properties on materials formulation, (b) the dependence of mechanical properties on the structure of the polymer, and (c) the role of blends and composites in manipulating properties. Students observe that biopolymers can be processed so as to produce useful biodegradable plastic materials (bioplastics). Biopolymers are usually presented in a biochemical context and synthetic polymers in a materials context. The experiment illustrates that this need not be so. Part of the variance comes from student inexperience with sample preparation and instrument operation and from film thickness inhomogeneities. The average standard deviations of the mean for tensile strength (±11%), modulus (±19%), and elongation (±24%) are not much different from between-laboratory reproducibilities for polyethylene (13). Temperature and humidity variations also play a role. For example, on average the relative humidity during the physical chemistry laboratory (fall semester) was 5% higher than during the biophysical chemistry laboratory (spring semester). The apparent systematic difference in elongation between the two laboratories may be the result of humidity differences. For comparison, low-density polyethylene has a tensile strength at break of 23.6 MPa, an elastic modulus of 371 MPa, and an elongation at break of 205% (13). Experiment 2: Drug Release from a Polymer Matrix
Experimental Procedure Chloroform, 15.0 mL, and 100.0 mg of poly(lactic acid–co-glycolic acid) (PLGA) are stirred, but not heated, until the polymer is completely dissolved. Salicylic acid (SA), 50.0 mg, is then added with further stirring. The solution is poured into an evaporation dish and set in the hood to dry. Distilled water, 40 mL, is thermally equilibrated in a 25 C water bath. The dried PLGA–SA film, 36.0 ± 1.0 mg, is placed in a beaker supported in the water bath. To begin the sampling, the 25 C water is added to the film sample, the contents stirred once, and the first aliquot of 1.00 mL is immediately removed (nominal zero time) with a pipetor and placed in a test tube. Additional aliquots are removed after 3, 5, 15, 30, 60, 90, and 120 min. The entire procedure is repeated at 37 C. The released SA is complexed with Fe3+ by adding 1.00 mL of 2.0% (w兾v) Fe(NO3)37H2O in 0.10 M HNO3 to each of the samples and mixed. The complex is purple–pink and has an absorption maximum near 535 nm. An increase in absorbance is observed at increasing times as the drug is released. One mL of the Fe3+ reagent in 1.00 mL of distilled water provides a reference for the absorption measurements. With excess iron ion, the concentration of the complex is proportional to SA concentration. Beer’s law applies, so the absorbance of the solution is proportional to SA concentration. 1532
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As a control, 12.0 mg SA is used instead of the film sample. Two-mL aliquots are filtered through a syringe after they are removed from the beaker. Samples only need to be taken for approximately 30 min. 1.00 mL of each aliquot is transferred to another test tube and 1.00 mL of ferric ion reagent is added. A control experiment can also be done at 37 C.
Data Analysis On the time scale of the experiment, release by degradation of the polymer is not significant. The release kinetics are determined by water diffusion into the film, salicylic acid dissolution, and salicylic acid diffusion out of the film, resulting in a complex release profile. An empirical equation sometimes used to describe drug release (15, 16) is fraction released = kt n at time t where k is a release constant and n is a constant less than 2. Assuming that the measured absorbance is proportional to the fraction released gives A (t ) = k ′ t n ln A(t ) = ln k ′ + n ln t
where k´ is the apparent release constant. A plot of ln A(t) versus lnt in the present case is not linear, which demonstrates the complexity of the release kinetics. But the limiting value of n as t approaches zero is found to be close to 0.5 indicating “pseudo” Fickian diffusion. The apparent release constant for SA release from the polymer matrix can be compared with the control, at both temperatures. Assuming Arrhenius behavior, students can estimate an apparent enthalpy of activation for the release and compare release from the polymer matrix with the control.
Hazards Chloroform causes cancer in laboratory animals and is listed as a probable human carcinogen. Inhalation and ingestion are harmful and may be fatal. Salicylic acid and the polymer are nontoxic.
Results Typical limiting slopes of plots of ln A versus lnt range from 0.42 to 0.67, consistent with Fickian diffusion playing an initial role in the release. Values of the apparent release constant, k´, are shown in Table 2.
Table 2. Typical Values of the Apparent Release Constant
Experiment
k´
Control (25 °C)
0.37
Release from polymer (25 °C)
0.13
Control (37 °C)
0.41
Release from polymer (37 °C)
0.23
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Discussion The values of k´ indicate slower release from the polymer matrix at both temperatures. Assuming Arrhenius behavior, the data in Table 2 give an apparent enthalpy of activation for release from the polymer matrix of 36 kJ mol1, significantly higher than the control value, 6.6 kJ mol1. Evaluation of Student Interest and Learning Several evaluation tools indicated student interest and learning. Fourteen out of 22 students in the biophysical chemistry laboratory, when asked to name the “best” experiment in the course, selected one or both of the polymer– plastics experiments. Students liked being introduced to the practical applications of polymers. On the pre- and post-experiment administration of a content-based questionnaire, students demonstrated increased knowledge of plastics formulations, mechanical properties, and environmental aspects of plastics and polymer degradation. Increased learning of polymer chemical structures was marginal. Additional details are provided in the Supplemental Material.W Acknowledgments Partial support for this work was provided by the National Science Foundation Course, Curriculum, and Laboratory Improvement (CCLI) program through grant DUE 0310454. Support was also received from the Camille and Henry Dreyfus Foundation Special Grant Program in the Chemical Sciences (SG-04-014). WSupplemental
Material
Instructions for students, notes for the instructor, and the content-based questionnaire are available in this issue of JCE Online. Note 1. Polymers are long-chain molecules with molecular masses commonly exceeding 25,000 g兾mol. Common natural polymers include polysaccharides such as starch, cellulose, and agar; proteins,
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such as gelatin; and others. Plastics are polymeric materials that have been processed to form three-dimensional shapes or films. Plastics typically contain additive chemicals to improve processing or properties of the final product. For example, plasticizers are softening agents that improve flexibility.
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