Polymers and Hydrogels

Hydrogels are polymers which can absorb large quantities of water.  They are alsol known as “smart” polymers, because they can change shape quickly in response to changes in their environment.  Examples include hair gel, water-retaining gel, eg in disposable nappies or plant pots, and soft contact lenses. Alison demonstrated hair gel’s excellent qualities on Tiny.  One source explains:  

“Hydrogels are networks of long polymer chains that attract and store large volumes of water. It is not uncommon for 90% of a hydrogel to be water. As water molecules are attracted and stick to the polymer chain network, this causes it to swell forming a gel.”

Hair Gel Collapse Experiment

We added salt to hair gel and found that the gel shrank and just a little liquid was left.

Salt replaces water in the polymer structure of the gel, because it’s more strongly attracted to the polymer than water is.  This is why you should choose your styling products carefully if you want to look cool in the sea.

RSC : Experiments with hydrogels 

University of Warwick : Hair Gel collapse experiment

Polymers : Disposable Nappies

We looked at a type of hydrogel, found in disposable nappies, and tested which of several brands of nappy was best value for money in terms of absorbency.

Method: We had Boots own brand, which cost 18.5p per nappy, Pampers, which cost 26p each, and Naty “natural” nappies, which were 28.2p.  We pulled the nappies apart and extracted the polymer granules, then added distilled water and measured how much water each collection of granules could absorb.

Results: Boots own brand absorbed 650ml ( so the cost of nappy divided by the liquid absorbed gives us 0.028p per ml), Pampers absorbed 800ml (0.0325p per ml) and Naty absorbed 680ml (0.041p per ml), although one group had an unexpected result of 900ml.

Conclusion: If all you’re concerned about is absorbency, the Boots own-brand nappies were best value for money.  However, there might be other factors which would influence your decision, such as whether the nappy fitted well, or environmental factors.

  http://lavenderpondnotes.tumblr.com/post/118965025616/polymers-disposable-nappies

Things to look at:

Catalyst -a magazine for teenage chemistry students - has a nice article about hydrogels which discusses medical applications.

 RSC experiment on hydrogels - discusses the chemistry behind the hydrogel.

Identifying Polymers

We used density to identify polymers:

Last month we synthesised a polymer, nylon, and this time we investigated some other types of polymer. We talked about what polymers are - long molecules formed when lots of smaller molecules join together, end-to-end.  They occur in nature, eg rubber, DNA, hair and nails are polymers.  We often think first of man-made polymers though, and these are known as “synthetic polymers”, such as nylon, PVC etc.. These can present challenges in waste disposal because they don’t break down, hence the concerns about the environmental hazards of plastics. 

We tried to identify some common polymers by comparing their relative densities.  Each group had six liquids of known densities in test tubes, and a selection of small pieces of polymer - most of which were raided from the recycling bin. We placed a sample of each polymer into each liquid and noted whether it floated or sank.  If it floated, it was less dense than the liquid, and if it sank, it was more dense.  In this way we were able to identify a density range for each sample, in grams per cubic cm (g/cm3).  We looked at a table of common polymers and their densities, and tried to work out which each was.  Finally, we looked at the recycling symbol which had been secretly kept back from each sample, to see if we were right.  

The RSC provided the following table::

Polymer Density range/g cm -3

EPS - expanded polystryrene 0.02 - 0.06

PP - polypropylene 0.89 - 0.91

LDPE - low density polyethylene 0.91 - 0.93

HDPE - high density polyethylene 0.94 - 0.96

PS - polystryrene 1.04 - 1.11

PVC - polyvinyl chloride (PVC) 1.20 - 1.55

PET - polyethylene terephthalate 1.38 - 1.40

When we checked our results, we’d correctly identified many of the polymers, but some were a little outside the ranges stated in the table.  The RSC say: “But note that materials made of polymers may also contain other substances as fillers, plasticisers, stabilisers, etc., which may make the density of a particular sample fall outside the ranges indicated; note the wide range of PVC densities in the table above.”

 It’s important to be able to correctly identify polymers in order to recycle them, and in fact industrial mixed recycling facilities use a process a little like this to sort polymers.  This BBC podcast about carbon, polymers and recycling includes a visit to a recycling plant to find out how it’s done.

Some other links to recap and take you further on this topic:

RSC Identifying Polymers - Classic Chemistry Experiments

The Polymer Party - short video about polymers, good fun.

From DNA to Silly Putty: the diverse world of polymers.   - TED Ed video , including environmental concerns.

BBC Bitesize on Polymers - recap and activities

Fantastic Plastic - from Catalyst, a science magazine for teenagers. “ This Catalyst article looks at the use of polymers in the manufacture of household items. The exciting thing about polymers is that it is possible to make polymers behave in so many different ways by organising their long chain molecules in different ways – polymers are the ultimate designer material. The article looks at their molecular chain and also the future for polymers in manufacturing.”

Robert Krampf : Polymers and Slime - video from the popular science teacher with fun activities.

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