Microhoneycomb Monoliths Prepared by the Unidirectional Freeze-drying of Cellulose Nanofiber Based Sols: Method and Extensions

This method can help answer key questions about achieving functional MHM materials for applications such as filtration, catalyst supporting, flow type batteries, sensors and bio-scaffolds. The main advantage of this technique is that it utilizes the structure directing function of cellulose nanofibers and can realize constitutional control of the targeted MHMs. This method can help answer key questions about achieving functional MHM materials for applications such as filtration, catalyst supporting, flow type batteries, sensors and bio-scaffolds.

Demonstrating the procedure with myself and Cong Wang, a grad student from our laboratory. To begin preparing the tempo-mediated oxidized cellulose nanofiber sol, first combine 66.7 grams of softwood bleached kraft pulp with 700 microliters of deionized water. Mechanically agitate the mixture for 20 minutes at 300 rpm.

Over the course of one minute, slowly add to the stirring kraft pulp suspension 20 milliliters each of a 7.5 gram per liter aqueous tempo solution and 75 gram per liter aqueous sodium bromide solution. Begin continuously monitoring the pH of the mixture with a pH meter. Adjust the mixture pH to about 10.5 with a three molar sodium hydroxide solution.

Then, over the course of several minutes, slowly pipette into the suspension 63.8 grams of aqueous sodium hypochlorite with 6 to 14%active chlorine. After addition is complete, continue adjusting the pH back to 10.5 whenever it drops to about 10. Once the pH decrease slows, time how long it takes for the pH to drop from 10.5 to 10 to determine how often to check on the mixture during the reaction.

Allow the tempo mediated oxidation to proceed normally for a bout 2.5 hours from the start of the sodium hypochlorite addition. Afterwards, filtrate the reaction mixture through a filter. And collect the oxidized cellulose nanofiber.

Then, agitate the oxidized cellulose fibers in 1.2 liters of deionized water. Rinse the fibers three times in this way. Dry a small portion of the washed fiber paste overnight in an oven at 60 degrees Celsius to determine the solid content of the paste.

Calculate the quantity of water needed to make a 1%by weight sol from the paste. Then transfer the washed fiber paste to laboratory grade blender. Add an appropriate amount of DI water to adjust the concentration of the mixture to 2%by weight.

Vigorously blend the paste to disintegrate the oxidized cellulose fibers into nanofibers. Begin with low power blending before vigorously blending the paste at high power. Add one fifth of the needed volume of deionized water to the paste, and blend thoroughly.

Repeat this process four more times to obtain the 1%by weight tempo mediated oxidized cellulose nanofiber sol. Store the sol at four degrees Celsius. We hear extracting is constant of the resulting TOCN sol.

If it is hot enough, the MHM is likely to be obtained through the newly added process. Normally, the viscosity must be greater than 20, 000 millipascal second. Prior to preparing the tempo mediated oxidized cellulose nanofiber surface oxidized carbon fiber mixed sol, reflux 1.7 grams of 300 mesh carbon fiber in 150 milliliters of concentrated nitric acid to obtain the surface oxidized carbon fibers.

Then, place in a 20 milliliter glass vial, 01 grams of the prepared surface oxidized carbon fibers, and 10 grams of the 1%by weight tempo mediated oxidized cellulose nanofiber sol. Manually shake the mixture until the surface oxidized carbon fibers are evenly distributed throughout the sol. Sonicate the mixture for five minutes to obtain an evenly dispersed TOCN SOCF mixed sol.

Store the sol at four degrees Celsius. To begin preparing a microhoneycomb monolith, fill the bottom five centimeters of the 13 by 150 millimeter or similarly sized polypropylene tube with glass beads as filler material. Slowly load at least 2.7 milliliters of the chosen sol into the tube, ensuring that the sol level is about 22 millimeters or more above the glass beads.

Avoid disturbing the sol unnecessarily to minimize the formation of bubbles in the tube. Use a narrow tipped pipette to carefully remove from the sol any bubbles that were introduced during the loading process. Store the sol sample at four degrees Celsius overnight.

Then connect the sample tube to a dipping instrument for a unidirectional freezing. Place a Dewar of liquid nitrogen under the tube. Freeze the sol at an emerging speed of either 50 centimeters per hour for the TOCN sol or 20 centimeters per hour for the mixed sols.

Keep the sol frozen once unidirectional freezing is complete. We use our cool time to constantly cool the frozen TOCN sol side hook to prevent it from sleep which will lead to the deterioration of the micromorphology of the resulting monolith. Use a saw to cut open the polypropylene tube.

Crack the frozen sol into several pieces using pliers. Freeze dry the frozen sol pieces at minus ten degrees Celsius, minus five degrees Celsius, and zero degrees Celsius for one day each sequentially to obtain the microhoneycomb monolith. Unidirectionally freezing 1%by weight TOCN sol at 50 centimeters per hour without glass beads resulted in a gradual change in orientation and channel size along the freezing direction.

The microhoneycomb morphology was retained longitudinally throughout the monolith. The bottom centimeter of the monolith was oriented toward the center of the bulk. A well-aligned honeycomb morphology was obtained two centimeters from the bottom.

The microhoneycomb channel size increased from 10 micrometers between the second and third centimeters of the monolith and then remained stable. Overall, the unidirectional freezing effect showed no influence on the morphology past three centimeters. The channel size could be tuned by altering the dipping speed and the temperature of the coolant.

Tempo oxidized cellulose nanofibers strongly tended to form the microhoneycomb structure via the unidirectional freezing process even in the presence of a second component. A mixed sol with styrene butadiene rubber yielded a composited microhoneycomb monolith with smooth walls. Tempo oxidized cellulose nanofiber microhoneycomb monoliths also supported titanium oxide nanoparticles in a well-ordered microhoneycomb monolith with nanoparticles adhering to the microhoneycomb walls.

A mixed sol with SOCF yielded a novel substructure in which the carbon fibers bridged the neighboring microhoneycomb walls. The unidirectional freeze-drying technique is a novel approach through which many regular microstructures can be obtained. We first had the idea for this method will be so well in random microhoneycomb morphology in a wood pattern.

It seems the major component of the wood is cellulose. We began to try imbuing up a similar construction with cellulose. Once mastered, this technique can be used to construct many other functional composite MHMs for targeted applications.