For subunits 1 and 2 the required science knowledge is a basic understanding of the particulate nature of matter and particle movement based on diffusion and osmosis.
Subunit 3 applies more specific knowledge on different polymers such as superabsorbers, cyclodextrines or PVC.
Polymers belong to the group of macromolecular substances. They are compounds with the number of atoms bound by covalent bonds in a macromolecule reaching hundreds to millions. Polymers are divided into natural and synthetic. Synthetic polymers can be prepared by a set of chemical reactions of the type of polycondensation, polyaddition or polymerization. Special superabsorbing polymers (SAP) – so-called superabsorbents can be included in the group of special polymers.
What is a superabsorbing polymer?
Polymer that is able under a load to absorb liquid of at least 10 g relating to every 1 g of the dry polymer. It swells in water solutions and gel is produced.
Superabsorbents are polymers with a significant ability to absorb liquids which is manifested by increasing their volume, so-called swelling. They absorb aquatic liquid solutions and make granular gels of permanent consistency with them. The amount of absorbed liquid is significantly dependent on the content of ions and pH of the given absorbed solution.
Some superabsorbents are able to absorb as much as 2000 multiple of their weight. When using 0,9 % solution of table salt 50 multiple of its own weigt is absorbed by the polymer – it is similar in the absorption of human urine into nappies.
Superabsorbent polymers have become an integral part of hygiene products (for example in nappies, ladies hygiene, hygiene goods for incontinent patients) in the last decade. They are also used as a protection of undersea cables from moisture, textile mats under frozen food, for preserving moisture in the soil, etc.
Using polymers based on SAP, however, brings along certain problems. First of all, they are for single use. Thus they are not environmentally friendly as they are not biogradable and are difficult to recycle. Development of a new generation of superabsorbent is therefore based on renewable material (polysaccharides – pectins, starch, cellulose), which are biogradable.
Hydrogels have many important applications. For example in tissue engineering (hydrogels can contain human cells for repairing tissues), in administration of medicines, in biosensors (gels reacting with specific molecules, such as glucose or antigens), contact lenses (silicone hydrogels, poly acrylamides) and healing gels.
Cyclodextrines are polycarbohydrates α-Cyclodextrin (αCyD), which is a cyclic oligosaccharide of six α(1→4) linked α-D-glucopyranoside units, can be topologically represented as toroids (figure below). α-D-glucopyranoside units in αCyD are usually in the most stable chair conformation.
Membrane films have different polymeric composition – comparison between cellulose, parafilm, Teflon, clingfilm (polyvinylchloride), polypropylene, dialysis membranes. Need to be careful here in translation that particular household films may have local / brand names that may not be recognisable in other countries. Composition of some of the common polymers are given below.
Polyvinyl Chloride (PVC)
PVC is a flexible plastic consisting of long chains. Each chain consists of repeating units where every second carbon atom has a chlorine atom attached.
The polymerisation of vinyl chloride produces mainly atactic polymer molecules. This means that Cl are orientated randomly along the chain.
Because of the way the chlorine atoms stick out from the chain at random, and because of their large size, it is difficult for the chains to lie close together. Therefore, atactic PVC is mainly amorphous with only small areas of crystallinity.
However PVC polymer chains can also be isotactic, with Cl atoms in the same orientation along the chain. (Syndiotactic is where the Cl atoms are in opposite orientation).
When PVC plastic is bent it becomes opaque where the bend occurs. This is due to the chlorine groups being forced into the same orientation along the chain (isotactic). The polymer chains can then move closer together and become more crystalline.
Properties and uses
You normally expect amorphous polymers to be more flexible than crystalline ones because the forces of attraction between the chains tend to be weaker. However, pure PVC tends to be rather hard and rigid.
This is because of the presence of additional dipole-dipole interactions due to the polarity of the carbon-chlorine bonds. Chlorine is more electronegative than carbon, and so attracts the electrons in the bond towards itself. That makes the chlorine atoms slightly negative and the carbons slightly positive. These permanent dipoles add to the attractions due to the temporary dipoles and hence hold the chains closer together.
Plasticisers
Plasticisers are added to polymers to make the material softer, flexible and therefore more workable. PVC is the most widely plasticised polymer due to its excellent plasticiser compatibility characteristics. Molecules which contain both polar and non-polar groups act as good plasticizers, as the polar group helps retain the molecule within the polymer chains and the non-polar part separates the chains, hence increasing flexibility.
Dibutyl sebacate is a commonly used plasticiser in polymers used in food packaging and in the pharmaceutical industry in polymer coatings for tablets and granules.
The dibutyl sebacate molecule works by bonding to the polymer chain through the oxygen atom on the plasticiser. The bulky plasticiser wedges itself between the polymer chains and pushes them further apart from each other. This causes the plastic to become more flexible.
Another compound used as plasticizer is sodium lauryl sulphate.
Resourses
Polymer Website; (Macrogalleria – teaching materials);http://pslc.ws/mactest/maindir.htm - accessed 17 DEC 2010
Overview of Dialysis; Thermo Fischer Scientific Inc;http://www.piercenet.com/files/TR0020-Dialysis-overview.pdf - accessed: 17 DEC 2010
Overview of Plasticisers; AZoM.com - the A to Z of Materials online resource; http://www.azom.com/Details.asp?ArticleID=1224 - accessed 17 DEC 2010
Molekulares Sieben: Mit Einmachfolie ins Diskontinuum (Molecular Sieves experiment); CHEMKON, 2004, Vol 11,(3);http://onlinelibrary.wiley.com/doi/10.1002/ckon.200410011/pdf - accessed 17 DEC 2010
Overview of Silver nanoparticles (project report - N344); Aalborg university, Faculty of Physics and Nanotechnology,http://repetit.dk/files/projects/p3.pdf - accessed 17 DEC 2010