Chemistry
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Functionalization of Single-walled Carbon Nanotubes with Thermo-reversible Block Copolymers and Characterization by Small-angle Neutron Scattering
Chapters
Summary June 1st, 2016
A method for the functionalization of carbon nanotubes with structure-tunable polymeric encapsulation layers and structural characterization using small-angle neutron scattering is presented.
Transcript
The overall goal of this procedure is to fabricate and characterize carbon nanotube based nanoparticles containing a thermal responsive encapsulation layer in aqueous solutions. This method can help answer key questions in the nano structure engineering field such as how to fabricate stimuli responsive nano particles and investigate in situ changes in their nanoscale structure induced by external stimuli. This method can provide insight into chemical assembly based fabrication of smart nano building blocks.
It can be also applied to other systems consisting of different kinds of carbonated nanoparticles and block copolymers. To begin this procedure, add 0.175 grams of Poloxamer 407 powder in 70 grams of deuterium oxide. Then completely dissolve the polymer in solution using a magnetic stirrer for 30 minutes to one hour.
Separately add 0.01 grams of single walled carbon nanotube powder to 250 milliliter conical centrifuge tubes. Then add 31.6 milliliters of the poloxamer 407 solution to each tube. Following this, mix the suspension in each tube by vortex mixing for 5 to 10 minutes.
When finished, place tube one in a water bath and secure it using a clamp stand. Dip the tip of an ultrasonicator into the suspension of tube one. Increase the sonication power gradually from 0%until the carbon nanotubes deposited at the bottom of the tube start shattering and spreading due to the ultrasound propagated from the ultrasonicator tip.
Treat the suspension with ultrasound for 60 minutes at 20 degree Celsius while keeping the suspension temperature below 25 degree celsius using a water bath as a temperature reservoir. Next, centrifuge the crude suspensions in both tubes at 9800 times G for two hours at 20 degree celsius. After centrifugation, separately transfer 15 milliliters of each supernatant from the top of the solution to a new tube.
Dissolve 0.015 grams of 5-methylsalicylic acid or 5-MS in the supernatant from tube two and label this mixture as sample two. Label the tube containing the supernatant from tube one as sample one. Load 0.3 milliliters of each sample into an amorphous cord-span-jo cell.
Place lids on the two cells and seal them by wrapping tape securely around the lids. Following this, place one of the sealed cells between spacers from aluminum cell holder and assemble the aluminum banjo cell holder. Load the assembled cells into different sample positions of the EQ-SAN sample paddle.
Make a list of the sample positions of the paddle on the sample list form. Start the measurement by loading and executing the experiment script in the PI-DAS control window. For AFM measurements, mix 0.1 milliliters of the solution from the sample one tube with 1.9 milliliters of deionized water.
Next, place a clean silicon wafer on a spin coder. Fix the wafer position using a vacuum check. Set the rotation speed and the running time to 1500 revolutions per minute and 60 seconds respectively.
Wet the exposed surface of the wafer with the diluted sample. Then start spin coding. When finished, turn off the vacuum pump and remove the coded wafer from the spin coder.
At this point, attach the spin coded wafer on an iron disk using a double sided adhesive carbon tape. Move the disk closer to the edge of the exposed area of a scanner. Then slide the disk toward the center until the bottom surface completely covers the top of the scanner.
Following this, mount the scanning probe microscope or SPM head on the scanner and plug in the cable. Open the instrument supply control software on the computer and select the tapping mode in the system configuration window. Next, place the cantilever tip at the center of the monitor window by adjusting the coarse and fine knobs of the optical microscope and by moving the X and Y optical stages.
Align the laser by adjusting the laser alignment knobs on the SPM head. Roughly locate the red laser dot to the cantilever and move the dot to the middle of the cantilever tip by tracing the dots shown in the monitor. After laser alignment, align the detector using the photodetector knobs.
Roughly adjust the knobs to turn off four red LED lights on the SPM head. Then finely adjust the knobs until the pink reflection image is located at the center of the laser alignment window and the quadrant photo diode signal sum is greater than at least two volts. Tune the cantilever using auto tune in the cantilever tuning window.
Then run auto tune in a frequency range of zero to 1000 kilohertz. Following this, bring the wafer surface into focus of the microscope by adjusting the focus knobs. Once the wafer surface is in focus, click the engage button on the toolbar.
Select a scan size, a sampling number, and a scan rate in the popup window. Start scanning. Gradually adjust the proportional gain, the integral gain, and the vertical deflection values if the contrast between the particles and the substrate background is too low to clearly recognize particle shapes and boundaries from the scanned image.
Finally disengage the probe after measurement. By changing the temperature or by adding 5-MS additives, the SANS scattering intensities of the functionalized single walled carbon nanotube suspension show a shift to higher Q in the intermediate Q region and the development of a peak at high Q.The changes due to temperature control and 5-MS addition originate from the structural change of the poloxamer 407 encapsulation layer on the carbon nanotubes. The poloxamer carbon nanotube nanorod with the core shell cylinder structure undergoes a structural transformation from encapsulation by spherical micelles of poloxamer 407 at room temperature to encapsulation by a compact cylindrical layer of poloxamer 407 at higher temperature.
During the structural change, the spherical poloxamer 407 micelle with a radius of gyration of 45 angstroms becomes a set of single chain blobs that surround the carbon nanotube core more compactly with the radius of gyration of 30 angstroms. Although AFM images only show a dried morphology of the poloxamer carbon nanotube nanorods without water they provide evidence of debundling and dispersion of carbon nanotubes as well as the length distribution of the nanorods. This procedure can be applied to different combinations of nanoparticles in block copolymers to fabricate various stimuli responsive nano building blocks.
Following this procedure, other methods like cryo-EM or molecular dynamic simulation can be performed in order to answer questions such as the real space structure of polymer layers in solution state and detailed underlying physics related to the structure change. I hope that this video has demonstrated that a small angle neutron scattering is a very useful technique for in situ nano structure characterizations of solution samples. We also hope to see more scientists come to SNS for various neutron scattering experiment.
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