Content area
Full Text
Instructor Side JCE Classroom Activity: # 11
Characterizing a Cross-Linked Polymer by the Journal's Editorial Staff
About This Activity
The procedure presented here was adapted from an activity published by the Institute for Chemical Education (1). It is described in many science activity resources-some intended for young children (2). The product in the reaction is a gooey, stretchy, cross-linked polymer similar to "slime" produced by cross-linking polyvinyl alcohol (3) and to commercially available Gak and Silly Putty. The activity can be done in the chemistry laboratory or at home. If done at home, the instructor may need to provide borax powder or solution. Students may both enjoy and benefit from doing this activity outside of class with a younger student (4).
1. Making Gluep: White glue contains polyvinyl acetate. Laundry borax or sodium tetraborate decahydrate, Na2B407l0H20, forms borate ions, B(OH)4-, when dissolved in water. The borate ions form cross-links between the polyvinyl acetate molecule chains. As the polymer chains become connected, the product takes on a gel-like consistency forming the "gluep".
2. Properties of Gluep: Gluep will pick up water-soluble inks on paper and gluep balls will bounce. Students should find that the gluep is more viscous, bounces higher, and stretches less when it contains less water. Students may design additional experiments measuring solubility, elasticity, and viscosity, or other properties such as density. They might also investigate the properties of the material as it ages and dries out. Different glues will produce varying results depending on their composition. Elmers wood glue formed gluep very similar to that made from all-purpose glue. "Acid-free" or "neutral pH" glues made from polyvinyl alcohol, PVA, are available from art supply stores. Some special craft glues also contain PVA. PVA glues produce a somewhat different product, in some ways similar to the slime made with polyvinyl alcohol solution. Following the recipe in Part 1, the PVA glues tested formed a crumbly solid. Using less or no water and varying the amount of borax solution from a few drops to up to two teaspoons formed products that ranged from very slimy to stiff and rubbery. These provide an interesting comparison to both standard gluep and slime. Waterproof wood glue and washable craft glue did not form a cross-linked polymer with...