Superabsorbent Polymers: Properties, Uses, and Absorbency

Polymer Chains and Hydration

Introductions:

Superabsorbent polymers are three-dimensional cross-linked polymers, with the ability to absorb several hundreds times their mass in water without dissolving. The polymer usually contains ionized functional groups. The commercial uses of sodium polyacrylate ([CH2–CH–(COONa)–]n), which is currently the most widely used superabsorbent polymer, include nappies and feminine hygiene products to ensure dryness. It also has a role in disaster management as it is used in inflatable anti-flood sandbags, which expand to form a barrier as they come into contact with water. 

In its powdered state (before the addition of water) the polymer chains in sodium polyacrylate are arranged in a random coil, lined with carboxyl groups (- COOH). When hydrated with water, the carboxyl groups present in the polymer dissociate into anionic carboxylate ions which have a negative charge (

Cross-linking due to hydrogen bonding, the strongest form of intermolecular attraction, prevents the polymer from dissolving when the bond has been stretched to its maximum point. A hydrogen bond is an electrostatic interaction that occurs in molecules that contain H+ ions bonded to electronegative atoms such as N, F and O (elements with the highest electronegativities). In water, the electronegative atom is O. The oxygen present in water attracts the electron away from the covalent bond and towards itself, setting up a permanent dipole in the molecule.

Conclusion
My results agreed with my hypothesis which was that the higher the temperature of the water, the higher the mass of water absorbed by the sodium polyacrylate crystals. This is showed by my graph above as there is a straight line and a positive corelation. This is because the entropy of the system increases since more heat energy is put into the system. Entropy is a thermodynamic quantity used to measure the degree of randomness amongst atoms in each system. As heat transfers from the aliquot to the sodium polyacrylate crystals, the temperature of the system increases. The atoms have a great velocity, and - as shown the equation below - collide with one another with more force, since momentum is directly proportional to velocity. p = mv. Accordingly, the kinetic energy of the system also increases it is directly proportional to

As a result, the movement of water into the sodium polyacrylate crystals occurs faster, so more water is absorbed into the polymer per unit of time. In this sense, temperature does not affect the amount of water absorbed by sodium polyacrylate crystals per se, but rather, the mass of water that can be absorbed in each amount of time. In other words, at 5°C the crystals may eventually absorb the same amount of water that it does at 65°C, but this will occur over a longer period. 

Evaluation Limitations:
The crystals of sodium polyacrylate needed to be submerged in the aliquots for a longer period of time, in order to determine experimentally whether or not a lower temperature simply prolongs the polymer from absorbing the same amount of water that it would at a higher temperature. Clearly, two minutes was not a sufficient amount of time to make such an observation. Furthermore, despite being obtained from the same brand of nappy, the polymer crystals are likely to have varied in size. This may have influenced the results since a crystal with a larger diameter will be able to absorb more water than a crystal with a smaller diameter. In addition, the difference in the values of mass of calcium ions was far less than those of temperature. It is, therefore, difficult to make a fair comparison of the results and the effect of calcium ion content was not as fully explored as it could have been.

Extensions:
In light of its limitations, a possible extension of this experiment could be to measure the diameter of the polymer crystals and use only those closest in size when obtaining results. The crystals should also be submerged in the test sample for a longer period of time, and a greater difference in calcium ions should be used in order to meaningfully investigate its effect on the absorbency of sodium polyacrylate. A variety of superabsorbent polymers should also be used, as opposed to just one, as was done here.