In the Laboratory
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An Introduction to the Scientific Process: Preparation of Poly(vinyl acetate) Glue Robert G. Gilbert, Christopher M. Fellows,* James McDonald, and Stuart W. Prescott Key Centre for Polymer Colloids, University of Sydney, Sydney, NSW 2006, Australia; *
[email protected] Introducing high school and university students to the scientific method has been the subject of a significant amount of research (1–9). It is important for potential scientists to learn early that (i) the scientific process requires creative thought and personal interaction with one’s colleagues, (ii) there may be no simple answer to questions that have been posed, (iii) there may be several equally valid “correct” answers based on available knowledge, and (iv) the teacher is not omniscient. Some knowledge of the process as well as the content of science is also useful to help students who may never encounter science again to make informed judgments in a world full of arguments based on poor or false science. We have developed a practical exercise that has been successful in giving students a feel for the scientific process while introducing them to synthetic polymer colloids in an everyday context: glue. A series of polymer latexes (suspension of polymer particles in water) is prepared, then tested to see how effectively different formulations behave as adhesives. The exercise was designed in such a way that the students: 1. Gain practical experience in free-radical emulsion polymerization; 2. Realize the importance of good experimental design; 3. Collaborate with other students in the research; 4. Practice real science on a small scale, developing and testing their own hypotheses; and 5. Realize the limitations of certain methodologies, e.g., the variability of certain testing processes.
The outcomes of the experiment suggested that the practical had been a more successful introduction to the process of “doing science” than the more typical experience of doing an isolated exercise with a single correct result known in advance. Synthetic polymer colloids are important industrial materials (10) (water-based paints, adhesives, rubber tires, Teflon, etc.) and the preparation of a simple dispersion of polymer in water can be a valuable aid to the study of surfactants (11–15) and free-radical polymerization (16–24 ). The latex used here is poly(vinyl acetate) containing varying amounts of poly(vinyl alcohol). This material is well known as a common wood glue (“PVA”) and is a good real-world example. The many functional roles of poly(vinyl alcohol) provide an ideal way to adjust the scope of the investigation to any level of complexity desired, making the exercise suitable for students from the later years of high school to advanced undergraduates. Concept This practical links polymers and colloids to everyday phenomena and helps students relate chemical structures to macroscopic properties. Directions for synthesis of the latex
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using free-radical emulsion polymerization were provided. The overall reaction is shown in Scheme I. CH2
CH
K2S2O8, surfactant,
O n
C
O
H C C H2 O
∆
CH3
C
O
CH3 n
Scheme I
By adding small amounts of poly(vinyl alcohol) to the reaction mixture, the students obtain latexes with different adhesive properties. They are then required to investigate the bonding between the latexes and substrate materials, both qualitatively and quantitatively. Development of the procedure for testing the latex is left in the hands of the students, who are provided with a limited “toy box” of tools and materials. A Typical Implementation The practical was run in two sessions of two hours each. Six groups of students did the practical during a two-week period. Before the students commenced practical work, an introductory lecture on polymer chemistry was presented, and after all students had completed the experimental work, a review session was held to analyze the results. In the first laboratory session, “recipe sheets” were handed out and students prepared one of three poly(vinyl acetate) latexes with differing amounts of poly(vinyl alcohol) in a glass bottle. The reaction bottles were placed in an agitated, thermostatted water bath at 60 °C and heated and shaken overnight. In the morning they were removed by a demonstrator. For the rest of the first laboratory session, students gathered in groups of 12, each with a facilitator–demonstrator, to plan their tests. Groups were encouraged to make use of results obtained by previous groups and were given the choice of optimizing previous tests or exploring new testing options. The demonstrators answered any questions on theory or previous work and gently steered the planning process by suggesting that the students list the variables that need to be controlled in an adhesion test (such as contact surface area, glue thickness) and helping them analyze their ideas critically. To explore various possibilities of bond failure, demonstrators encouraged students to think about various adhesive tests, including shear, peel, and tensile tests. Students from different sessions were encouraged to collaborate, to develop increasingly sophisticated tests. In the second laboratory session, students carried out the tests they had devised and tried to interpret the results obtained for different samples in light of the chemistry of each
Journal of Chemical Education • Vol. 78 No. 10 October 2001 • JChemEd.chem.wisc.edu
In the Laboratory
sample. Demonstrators helped them gain maximum information from the results of their tests by observing the “failures” critically—was the failure adhesive, cohesive, or structural? The overall conclusion drawn by the students in this practical was that addition of poly(vinyl alcohol) to the reaction mixture makes a better glue. The mechanism they proposed was that it gives stronger bonding owing to increased hydrogen bonding with the cellulose substrate. The mechanism is, of course, significantly more complex, allowing this practical to be used at many different levels up to experienced polymer chemists. In summary, poly(vinyl alcohol): 1. Acts as a steric stabilizer, enhancing the colloidal stability of the polymer particles; 2. Lowers the glass transition temperature, giving better film formation; 3. Can impart non-Newtonian fluid properties to the glue; and 4. Can increase water sensitivity of the glue.
The increase in water sensitivity has a negative impact on glue properties—students may enjoy dunking the pieces of wood they could not separate in hot water and watching them fall apart. Equipment With the exception of the agitated water bath, equipment is available at supermarkets and hardware stores and chemicals are readily available. The practical has been carried out with a variety of heating–shaking apparatus including a “bottle polymerizer” (six sealable glass bottles, which may be placed in supports in a thermostatted water bath and agitated by rotating the setup) and a more widely available thermostatted agitator (e.g., Haake SWB 20). Gentle agitation of a horizontal reaction bottle was found to be most efficient. The “toy box” contained the following items: high-density fiberboard (e.g., Masonite hardboard), cardboard, loose-weave fabric (e.g., cheesecloth), denim, paper, scissors, permanent markers, pens, string, wire, pliers, rulers, fishing scales, and 4-L bottles. Retort stands and clamps, plentiful water, balances for weighing ingredients, an electric drill, and an oven to dry glued samples at 60 °C were useful.
held in a metal cage while heating. It is important that the reaction itself be carried out in a well-ventilated area, preferably a fumehood, to avoid exposure to monomer fumes should a bottle leak or break. Evaluation The practical received positive reviews from the highschool students involved. From a sample size of 127 students, 68% agreed “the lecture and practical developed my interest in the subject”; 66% agreed “the lab work relied on my own initiative to develop the experiments”; 80% agreed “overall as a learning experience I would rate these experiments as worthwhile”. Top responses when asked to nominate two things they found most helpful in the exercise: 47% liked the introductory lecture; 34% appreciated the freedom to make their own experiment in the lab. Each group of students developed good ideas that the staff members had not thought of previously. Though passing information from one practical group to the next was done informally, there was strong support from the students for a more structured format for reporting to their successors. One possible procedure would be for each group after the first to carry out one test suggested by the previous group, and one of their own. For undergraduates, one can reduce the time allotted, although we gave undergraduate students the same amount of time and expected a higher standard of data and analysis. One can also use a redox initiation system, which allows the reaction to be performed in a much shorter time or without heating (25, 26 ). Overall, both organizers and students regarded the practical as a successful introduction to polymer chemistry and the experience of scientific research. W
Supplemental Material
Supplemental material in this issue of JCE Online provides detailed guidelines, information, and equipment and materials lists for the instructor, and instructions for the students, including an adhesive test report sheet with questions. Acknowledgments
Hazards The chemicals used in the experiment are vinyl acetate, poly(vinyl alcohol), Aerosol OT and Aerosol MA (commercial surfactants), potassium persulfate, and sodium hydrogencarbonate. Care must be taken to ensure that students take every possible precaution to avoid contact with vinyl acetate; this monomer is a suspected carcinogen and should be kept in a bottle with a calibrated pump-top dispenser in a fume hood. Aerosol OT and Aerosol MA are irritants and should be handled with caution. At the reaction temperature and concentrations used, there should be no explosive hazard from the potassium persulfate initiator, but care should be taken that students do not accidentally or intentionally add much larger quantities of initiator, as breakage of the reaction bottles with danger of broken glass may occur. For this reason it is preferable that the reaction bottles be wrapped in cloth or
The assistance of the students and organizers of the 30th Professor Harry Messel International Science School in developing and testing this practical is gratefully acknowledged. The Key Centre for Polymer Colloids is established and supported under the Australian Research Council’s Research Centres Program. Literature Cited 1. 2. 3. 4. 5. 6. 7.
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In the Laboratory 8. Young, J. A. J. Chem. Educ. 1957, 34. 9. Kieffer, W. F. J. Chem. Educ. 1951, 28, 300. 10. Gilbert, R. G. Emulsion Polymerization: A Mechanistic Approach; Academic: London, 1995. 11. Furton, K. G.; Norelus, A. J. Chem. Educ. 1993, 70, 254. 12. Dominguez, A.; Fernandez, A.; Gonzalez, N.; Iglesias, E.; Montenegro, L. J. Chem. Educ. 1997, 74, 1227. 13. Soran, P. D.; Neal, E. E.; Smith, B.; Mullen, K. I. J. Chem. Educ. 1996, 73, 819. 14. Crisp, P. T.; Eckert, J. M.; Gibson, N. A. J. Chem. Educ. 1983, 60, 236. 15. Wood, J. A. J. Chem. Educ. 1972, 49, 161. 16. Ceska, G. W. J. Chem. Educ. 1973, 50, 757. 17. Mendicuti, F.; Martin, O.; Tarazona, M. P. J. Chem. Educ.
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Journal of Chemical Education • Vol. 78 No. 10 October 2001 • JChemEd.chem.wisc.edu
