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1、chapter threeSuperabsorbent polymersSuperabsorbent polymers (SAPs) are materials that have the ability to absorb and retain large volumes of water and aqueous solutions. This makes them ideal for use in water absorbing applications such as baby nappies and adults incontinence pads to absorbent medic
2、al dressings and controlled release medium. 1. introductionEarly superabsorbents were made from chemically modified starch and cellulose and other polymers like poly(vinyl alcohol) PVA, poly(ethylene oxide) PEO all of which are hydrophilic and have a high affinity for water. When lightly cross -link
3、ed, chemically or physically, these polymers became water-swellable but not water-soluble. Todays superabsorbent polymers are made from partially neutralised, lightly cross- linked poly(acrylic acid), which has been proven to give the best performance versus cost ratio. The polymers are manufactured
4、 at low solids levels for both quality and economic reasons, and are dried and milled in to granular white solids. In water they swell to a rubbery gel that in some cases can be up to 99wt% water. 2. Mechanisms of swelling in superabsorbent polymers Before discussing the synthesis of superabsorbent
5、polymers it is first necessary to understand the reasons why they swell. There are several mechanisms to the process of swelling, all of which contribute to the final swelling capacity Figure 1 is a diagrammatic representation of part of the polymer network. The polymer backbone in SAP is hydrophili
6、c i.e. water loving because it contains water loving carboxylic acid groups ( COOH). When water is added to SAP there is a polymer/solvent interaction; hydration1 and the formation of hydrogen bonds are two of these interactions. a. HydrationThis is the interaction of ions of a solute with molecules
7、 of a solvent i.e. COO- and Na+ ions attract the polar water molecules b. Hydrogen Bonds Hydrogen bonds are electrostatic interactions between molecules, occurring in molecules that have hydrogen atoms attached to small electronegative atoms such as N, F and O. The hydrogen atoms are attracted to th
8、e non-bonding electron pairs (lone pairs) on other neighbouring electronegative atoms In water the electronegative atom is oxygen which pulls the hydrogens electrons towards itself setting up a dipole in the molecule. The positive hydrogen atoms are attracted to the oxygen lone pairs on other water
9、molecules. Oxygen has two lone pairs of electrons and each is capable of hydrogen bonding to two other water molecules. These effects decrease the energy and increase the entropy of the system. Due to the hydrophilic nature of SAP the polymer chains have a tendency to disperse in a given amount of w
10、ater (i.e. they are trying to dissolve in the water), which leads to a higher number of configurations for the system and also increases entropy. 3.Why doesnt SAP dissolve in water? Cross-links between polymer chains form a three-dimensional network and prevent the polymer swelling to infinity i.e.
11、dissolving. This is due to the elastic retraction forces of the network, and is accompanied by a decrease in entropy of the chains, as they become stiffer from their originally coiled state There is a balance now between the forces of retraction and the tendency for the chains to swell to infinite d
12、ilution. The degree of cross-linking has a direct effect on the level of swelling of the polymer and the strength of the network i.e. Increased cross-link density = decreased swelling capacity = increased gel strength. Cross- linking is discussed in more detail later. 4.welling in ionic polymers For
13、 ionic polymers there is another solvent/polymer interaction beyond simple mixing. The neutralised chains contain charges that repel each other (figure1). Overall electrical neutrality is maintained as the negative carboxylate groups are balanced by the positive sodium ions. Upon contact with water
14、the sodium ions are hydrated (figure 2b) which reduces their attraction to the carboxylate ions (due to the high dielectric constant of water). This allows the sodium ions to move freely within the network, which contributes to the osmotic pressure within the gel. The mobile positive sodium ions how
15、ever, cannot leave the gel because they are still weakly attracted to the negative carboxylate ions along the polymer backbone and so behave like they are trapped by a semi-permeable membrane. So the driving force for swelling is the difference between the osmotic pressure inside and outside the gel
16、. Increasing the level of sodium outside of the gel will lower the osmotic pressure and reduce the swelling capacity of the gel. The maximum swelling of the gel will occur in deionised water. 5. Osmosis and osmotic pressure What is osmosis and osmotic pressure? Osmosis can be demonstrated by a simple experiment In the above example the two solutions are se
