Video Science - Mix it Up! – Cross Linking Polymers

Of all the iterations we see of science lessons preparing “Silly Putty” from borax and glue, we’d like to see students looking carefully at each aspect of this classic chemical reaction, including the solution chemistry and the simple math involved preparing each reactant.

Sodium tetraborate (Borax is also termed Granubar^md), the common laundry detergent additive, lends itself well to this experiment because it is quite easy and safe to handle in the laboratory. It is a white crystalline solid with a chemical formula of Na₂B₄O₇*5H₂O (the *5H₂O indicates that the compound is in the pentahydrate form, as were our copper sulfate crystals in our Zinc/Copper redox segment). It is sometimes used in farming to reach the correct boron concentration in soil, yet more commonly used as a laundry detergent supplement. We use a 4% solution in our segment that is “scaled up” to produce a 2 liter sample, yet in a classroom situation, you may elect to have students add 4 grams of the solute to 96 ml of distilled water to the same effect. You may also lower the concentration of borax to 3% (3 grams solute to 97 ml solvent) if you find (as we did) that the solution saturates at 4%.

Alternatively, you may elect to warm the distilled water to drive the full 4 gram sample of solute into solution, or perhaps introduce a variable into the experiment by asking six teams to prepare samples at increasing concentrations (Team 1 – 1%, Team 2- 2%, etc.) and then gather the class to evaluate the six product samples to judge which concentration was optimal based on the stability of the final product. In dissolving the glue by 80%, we model adding the glue to the distilled water in a wide mouth Nalgene^tm graduate or pitcher. In preparing a classroom (student) version of the experiment, we would scale the sample down to 50 ml of glue diluted with 40 ml of distilled water. As the glue is nontoxic and water soluble, we find it manageable to work with in a science lab, even with smaller children. In lieu of a glass or Pyrex stirring rod, we can recommend a wood splint or popsicle stick if teachers are concerned with breakage as students stir the reacted polymers (quite thick and viscous, potentially causing the glass rod to “snap” if overeager students stir the reaction vigorously. Student will note the good solubility of the glue in the distilled water, and you may take a moment to comment on the pigmentation of the glue (titanium dioxide is in fact the same material that is added to skim milk to make it appear “whiter”, richer and more appetizing).

Rinse in the Water Clear…: Rinsing the reacted product in tap or distilled water will wash away any unreacted sodium tetraborate. Without rinsing, you may find your sample is tacky and leaves the skin of your hands dry by sapponifying the fats and oils in your skin (aka “dishpan hands). Yet the rinsed sample, with some stretching and kneading, can be formed into a clean, pliable sample of a white amorphous solid that kids will enjoy handling because of its “nonNewtonian” properties (it behaves like a liquid under some conditions, as a solid under others). Students normally ask to keep the reacted samples. If you do send them home, send them sealed in a plastic bag and keep them in your lab until the end of the day to avoid student temptation to handle the material in other classes. Caution the students to avoid letting the reacted polymer come into contact with carpet, upholstered surfaces or fabrics (it can stick to fibers). If a sample becomes dry and brittle, it can be rehydrated with a few drops of distilled water. You may choose to tint the solution or the dissolved glue with vegetable dye, yet we normally do not to avoid staining our hands when handling it.

Bonus! Adding your extra sodium tetraborate solution to your dissolved polyvinyl alcohol bags (see our segment titled “Slime” will cross link the polymers in that experiment, producing an amorphous yet clear final product that can be trained to climb down a windowpane through the effects of gravity and surface tension. Stirring your solution with a highlighter will allow your final sample to fluoresce under a black light.