This protocol allows the novel physical transformation of chitin, a high volume waste material, that is notoriously difficult to work with. The main advantage of this technique is the simplicity of the processing methodology. As it does not require specialized materials.
Only the basic equipment commonly found in chemistry laboratories. We demonstrated this method with chitin, but this technique can be modified to create expanded versions of other polymers and bio polymers that form gels. In the simplest version of this technique, performed as demonstrated.
Patience is important. The jelling rate will change with daily and seasonal changes to humidity. One day before preparing the expanded chitin, dry at least 1.2 grams of chitin flakes for 24 hours in an 80 degrees Celsius oven.
The next morning in a fume hood and wearing chemical resistant gloves and goggles, add 15 grams of lithium chloride to 285 grams of D-MAc, In a 500 milliliter Erlenmeyer flask, containing a 50 milliliter PTFE line magnetic stir bar. Cap the flask with a rubber septum and place it on a heating stir plate. Place, a temperature probe into the mixture through the septum and stir the mixture at 400 rotations per minute at 80 degrees Celsius, for about 4 hours.
When all of the lithium chloride has dissolved, add 1 gram of the oven dried chitin flakes to the 5%lithium chloride D-MAc solution. And transfer the resulting solution into a 500 milliliter round bottom flask, containing a 50 milliliter PTFE lined magnetic stir bar. Place the flask cap with the rubber septum on a stirring heat block.
Pierce to septum with a needle to allow the flask to vent and heat the solution at 80 degrees Celsius, with stirring at 400 rotations per minute for 24 to 48 hours. When all of the chitin has dissolved, allow the resultant chitin sol-gel to cool to room temperature for about one hour, with stirring. Once the solution reaches room temperature, place the flask in an ice bath, with continued stirring for approximately 20 minutes, until the temperature stabilizes.
To prepare a 100 milliliter slurry of sodium hydride and D-MAc. In a fume hood, wearing chemical resistant gloves and goggles, wash approximately 1 gram of sodium hydride removed from mineral oil storage. 3 times with 10 milliliters of hexane per wash.
Add 0.82 grams of the wash sodium hydride to a fresh 250 milliliter Erlenmeyer flask, containing 100 milliliters of D-MAc at a PTFE line magnetic stir bar, and swirl the mixture, to produce a sodium hydride D-MAc slurry. To form the chitin gel, add the entire volume of sodium hydride slurry to the cooled sol-gel, while vigorously stirring the gel solution. Then replace the cap and continue to stir the mixture at 400 rotations per minute, until a gel forms.
After the gel is formed, add 100 milliliters of deionized water to the flask in a fume hood, and remove the expanded chitin foam from the flask, breaking the foam into pieces, if necessary. Place the foam in a crystallization dish sufficiently large enough to hold the foam, along with 1000 milliliters of deionized water. Rinse the isolated gel 3 times, with 500 milliliters of deionized water per wash, before soaking the gel in 1000 milliliters of deionized water, 500 milliliters of methanol, and 1000 milliliters of fresh deionized water for 24 per immersion.
After the last deionized water wash, allow the gel to air dry for 24 to 48 hours. The gel can then be dried in the oven for 48 hours at 85 degrees Celsius under ambient air or an lyophilizer, at minus 43 degrees Celsius and 0.024 millibars of pressure, for 48 hours. When a solid chitin foam is formed, use a mortar and pestle to grind the foam into a fine powder.
Before drying, the knead chitin flakes, exhibit a core sand appearance. After drying, the expanded chitin morphology resembles a kernel of popcorn, regardless of the method. Scanning electron micrographs revealed that knead chitin is a compact dense structure, while the expanded chitin, resembles crinkled paper or wrinkled sheets.
In x-ray diffraction studies, knead chitin displays a strong peak at 19.3 degrees corresponding to its crystal plane. Which decreases in intensity after baking, or lyophilizing. Suggesting, that drying changes the crystallinity index of the chitin.
Measurement of the specific surface area obtained from nitrogen physisorption isotherms, shows the greatest uptake volume, for the expanded foams. Confirming the more open and porous structure of these samples. Despite these changes in morphology, the expansion process does not appear to affect the chemical structure of the chitin.
As observed in these representative IR spectrograms. Similar observations are noted, after thermogravimetric analysis. With the onset of thermal decomposition of all three samples occurring at 260 degrees Celsius, and the maximum decomposition rate occurring at a higher temperature for chitin flakes, due to its more compact morphology.
The increase in specific surface area is accompanied by an expected increase in the maximum copper uptake by chitin. However, these differences in uptake disappear, when the copper uptake is normalized by the surface area. It's important to remember that D-MAc and NAH are dangerous chemicals, that must be handled carefully.
Always work in a fume hood, and wear the appropriate personal protective equipment. The limited nitrogen adsorption isotherms we collected, can only provide specific surface area. Full nitrogen absorption isotherms, can give porosity information that is critical to determining the usefulness in size-selective applications.
