Properties of Polymers and Gels — a.k.a. three kinds of slime!

Crosslinked Polymers: Linking things together!

This week students had a fun chemistry lesson on polymers. The students discussed the many important polymers in their lives, including starches (pasta, potatoes), cellulose (wood, plants), plastics, fabrics both natural (cotton, wool) and manufactured (rayon, nylon), and proteins. But exactly what is a polymer, you might ask? Polymers are a type of molecule made of linked chains of individual units, called monomers. Students reviewed the fact that identical monomers form chains to create polymers.  They learned that some polymers are branched and some are unbranched, that chains of polymers can be cross-linked together, and that these differences in sturcture result in materials with different properties (such as density, tensile strength, and elasticity).

Students were then able to experiment with some cross-linking polymers. We took an existing polymer (white school glue, which is polyvinyl acetate) and mixed it with a cross-link solution, tetrahydroxyborate (Borax) in water. The result was dramatic! The slimy, sticky glue changed into a rubbery, dry substance that could be stretched, formed into a ball, broken sharply apart, and bounced. We also included an unbranched polymer, that was not crosslinked by the borax, to the polymer gel in two different concentrations.  The students then had three different gels with different properties.

We then tested the elasticity of the different polymer gels by measuring the bounce height of the gel ball when dropped. Ask your students why one of the gels bounced higher than the others!

 

Polímeros: ¡entrelazando cosas!

Hoy los estudiantes tuvieron una clase de química muy entretenida sobre  polímeros y aprendieron la importancia de éstos en nuestra vida. El almidón (papas, pasta), la celulosa (madera, plantas), el plástico, las telas tanto naturales (algodón, lana) como manufacturadas (rayón y nylon) y las proteínas, son todos ejemplos de polímeros. Pero, ¿qué es un polímero? Los polímeros son un tipo de moléculas compuestas por cadenas de unidades individuales llamadas “monómeros”. En esta clase, los alumnos aprendieron que los monómeros forman cadenas para crear polímeros, los cuales a su vez, pueden unirse entre sí para crear materiales que poseen diferentes características, como densidad, resistencia y elasticidad.

Después que los estudiantes comprendieron que estamos rodeados por polímeros, tuvieron la oportunidad de experimentar con polímeros que ellos mismos crearon.

Para la actividad, tomamos un polímero ya existente (pegamento o cola fría blanca o también conocido como acetato de polivinilo) y lo mezclamos con una solución de “entrecruzamiento” (cross-linking), llamada tetrahidroxiborato (Bórax) en agua.

¡El resultado fue impresionante! El pegamento que antes era pegajoso se transformó en una sustancia seca y como de goma, la cual podía ser estirada, podía moldearse en una pelota, se podía romper y además rebotaba. Sus hijos o hijas deberían ser capaces de explicarles por qué ocurrió este fenómeno al añadir la solución de entrecruzamiento.

A continuación, probamos el efecto que tienen diferentes cantidades de polímeros sobre la elasticidad del polímero gel. Para ésto, medimos la altura de rebote de la pelota de gel. Pregúntele a sus hijos o hijas por qué una de las pelotas de gel rebotó más alto que la otra.

Author

Dr. Catherine Sukow

Dr. Sukow's interest in science education began when she was a teenager, with an extended visit to San Francisco's Exploratorium. In college, she had summer jobs in a similar, smaller, museum. She focused her Master's research at NCSU on the structure of metal silicides on silicon, and her Ph. D. work at Brandeis on the structure of crossbridged actin bundles. While volunteering in her childrens' schools, she was reminded how much fun it is to teach science, and is happy to be teaching now with Science from Scientists. In her spare time, she also enjoys yoga, choral and solo singing, and attempting a variety of international cuisines.

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