Membrainiacs

Introduction & Motivation

The goal of this experiment is to be a very introductory seminar on engineering with polymer materials. With that in mind, the main takeaways should be that engineering is very much based around solving problems in the real world, and different types of engineers have different materials to solve these problems with.

The polymer science and engineering concept explored in this activity is the effects of polymer architecture and morphology on membrane permeability and selectivity. Students should leave understanding that polymer engineering entails changing polymer properties such as architecture to achieve certain goals, such as separation of a mixture.

In this exercise, students will arrange pipe cleaners to create a “membrane” and flow a mixture of differently sized pom poms and smooth beads to determine the optimal configuration for selectively retaining a target.

Materials

For each group, distribute:

• 5 short Straight Pipe Cleaners (“polymer chains”)
• 3 Short Pieces of Pipe Cleaner (to create “branches” and/or “cross links”)
• Assorted Size Pom Poms (3 sizes mixed)
• Smooth objects such as different size marbles or plastic beads
• A container that can have pipe cleaners across the top
• Rubber bands, twist-ties, and/or tape to help hold the pipe cleaners in place, if necessary

Procedure

Take time to give an overview of polymer science. Polymers are used in membranes to create a mesh which will allow some molecules to pass through, while others are trapped in the membrane or stay behind. Sometimes this separation can just be on the basis of size, but the membrane can be imbued with qualities that make it “sticky” to molecules.

Throughout the exercise, try to highlight analogies to areas within polymer science. The pipe cleaners being fuzzy helps them to retain the pom poms, which are also fuzzy. This is an example of how intermolecular interactions can be leveraged to keep pom poms on one side of the membrane. However, when there are smooth beads in the mixture, the fuzzy nature of the pipe cleaners does not help to retain them. If you are hoping to make a selective membrane which captures only pom poms, this is an excellent quality!

Additionally, the gaps between pom poms and be adjusted to let more or less of the smaller pom poms through. This can be achieved a few ways, most notably by including “cross-links,” either by twisting the large pipe cleaners together in locations or connecting them with smaller pipe cleaners.

Overall, this challenge is similar to that of processing waste water. Large molecules such as bacteria need to be separated from water, a smaller molecule, before it can be returned for the city’s use!

1. Challenge students to arrange the five large pipe cleaners across the opening of the cup in order to catch as many of the larger pom poms as possible, without bending the pipe cleaners.
   
2. Once they make a “membrane,” have students drop a handful of the mixed pom poms onto the membrane. Be sure to record how many pom poms were caught, which size they were, and an illustration of the membrane arrangement. Have students experiment with different arrangements for several minutes until they can get their optimal number of pom poms caught with these pipe cleaners.
   
3. Debrief with the larger group to find out what worked well in this challenge and why they think so before proceeding.
   
4. Allow for more experimentation in the shape and arrangement of the pipe cleaners. Start by giving each group 3 small pipe cleaners.
   
   Examples include:
       ● Twisting the pipe cleaners around a pencil to mimic coiled polymer chains
       ● Using the smaller pieces of pipe cleaner to branch or connect the big pieces
       ● Changing the number of pipe cleaners allowed
       ● Twisting pipe cleaners together to mimic cross-linking
       ● Creating multiple layers of polymer networks
       ● Other configurations students come up with!
   
5. Optional: Challenge the students to come up with a membrane with a specific purpose, such as separating pom poms from beads of the same size.

Discussion

1. After the exercise, debrief with the larger group and find out what worked the best for each separation target. Ask them questions like: How would you change the shape of the pipe cleaners if you wanted more pom poms to pass by? What if you wanted to only separate out certain sized pom poms?