Chemistry
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Self-assembling Morphologies Obtained from Helical Polycarbodiimide Copolymers and Their Triazole Derivatives
Chapters
Summary February 7th, 2017
Here, we present a protocol to prepare and visualize secondary structures (e.g., fibers, toroidal architectures, and nano-spheres) derived from helical polycarbodiimides. The morphology characterized by both atomic force microscopy (AFM) and scanning electron microscopy (SEM) was shown to depend on molecular structure, concentration, and the solvent of choice.
Transcript
The overall goal of this protocol is to demonstrate a rational synthetic approach to helical polycarbodiimides bearing modifiable pendant groups and visualize the secondary structures assembled from them by means of atomic force microscopy. These studies are of particular interest to develop experimental procedures for the preparation of desirable architectures. These architectures may then be exploited as potential sensors, optical switches, or biomedical applications.
The main advantage of this technique is that it can be easily applied to different polycarbodiimide scaffolds to make specific assemblies, such as donuts, ribbons, fibers, superhelices, spheres, and so on. The implication of this technique extends towards using helical polycarbodiimide scaffold as potential drug carriers. Because this party-like micromolecules self assemble into unique architecture in a controllable fashion.
To begin this procedure, add one gram of ET monomer and 0.894 grams of pH monomer to a clean 20-milliliter scintillation vial containing a magnetic stir bar in a glove box. Then, add 0.018 grams of BINOL catalyst to the scintillation vial. Add approximately three to five milliliters of anhydrous chloroform and gently stir to dissolve both the monomer and the catalyst.
After capping the vial, allow the reaction mixture to stir overnight at 25 degrees Celsius. Redissolve the polymer in five to ten milliliters of chloroform and reprecipitate it in 250 milliliters of methanol to remove the residual catalyst. Then, dry the precipitant under high vacuum for 24 hours to remove the methanol.
In the glove box, add five milliliters of anhydrous THF and a magnetic stir bar to a scintillation vial containing 0.25 grams of R 50 ethynol 50 phenol composition. Then, add 0.146 grams of the desired amide to the scintillation vial. Following this, add 0.022 grams of copper iodide catalyst to the scintillation vial.
Allow the solution to stir for two minutes to form a homogeneous suspension. Now, add 0.713 grams of DBU to the homogeneous suspension and allow it to stir for two hours at 25 degrees Celsius. Cyclic reaction must be conducted for two hours.
Long reaction time should be avoided to prevent hard gel formation, such as insoluble inemulsive organic solvents. After removing the vial from the glove box, remove the magnetic stir bar and inject the greenish, gel-like solution into 250 milliliters of cold methanol, containing 0.5 milliliters of DBU. Collect the formed triazole polymer by filtration using a 15-milliliter fritted funnel, and wash it once with 250 milliliters of methanol.
After repeating the purification, dry the product of the click reaction under high vacuum for 24 hours to remove the methanol. In the glove box, mix 0.029 grams of copper chloride catalyst with 0.1 grams of the macro-initiator in a scintillation vial containing 0.101 grams of PMDETA. After adding a magnetic stir bar to the vial, add 1.51 grams of freshly distilled styrene.
Then, add approximately 12 milliliters of anhydrous toluene to dissolve the reagents. Once the vial has been sealed and removed from the glove box immerse it in an oil bath and increase the tempurature. Stir the reaction mixture at the desired tempurature for 12 hours in a fume hood.
After removing the magnetic stir bar, pour the reaction mixture into 250 milliliters of cold methanol, containing 0.5 milliliters of DBU. Then, collect the formed flakes by filtration, using a 15-milliliter fritted funnel, and wash the material once with approximately 50 milliliters of cold methanol. Filter each polymeric stock solution through a 0.45 micrometer PTFV syringe filter prior to deposition on a silicon wafer.
Immediately after depositing 200 microliters of sample on the silicon wafer, use a spin coating machine to cover the entire wafer surface with a uniform polymeric film for AFM measurements. BINAL R or S titanium catalyst-mediated coordination insertion polymerization leading to the R and S series of polycarbodiimides is shown here. The synthesis of the triazole polycarbodiimides used as macro-initiators in the ATRP reaction to produce polycarbodiimide-g-polystyrenes, or PSPCDs is displayed here.
Macromolecules can self-assemble in a thin film to form a variety of complex super-molecular architectures, such as fibers, looped fibers, superhelices, fibrous networks, ribbons, worm-like aggregates, toroidal structures, and craters. A molecular model of the triazole macro-initiator is shown here. AFM images of alkyne PCDs confirm the formation of fiber-like morphologies.
In general, diluting stock solutions resulted in diminishing the size of the aggregated morphologies formed. The morphologies formed from PSPCDs, spin coated from chloroform stock are shown here. Unlike alkyne polycarbodiimide aggregation behaviors in chloroform, examining PSPCDs revealed both crater-like assemblies and nano-sized choroidal architectures as predominate motifs.
AFM images of PSPCDs indicative of the formation of discrete nanospheres when applying a single or binary solvent system for sample deposition, with concentration-dependent particle sizes are shown here. The assembly of the individual macromolecules into spherical nanoparticles matching up closely with SEM-measured morphologies is displayed here. Remarkably, the greater micron-size aggregates may be compromised of individual nanoparticles agglomerated together.
The spin-coating method represents a convenient way to reproducibly make multiple type morphologies, including fibers, superhelices, donut-like architectures, microspheres, based on ethynol polycarbodiimides and their respective polystyrene derivatives. While preparing polycarbodiimide films on silicon wafers it should be air dried for a few hours for AMF imaging. This time we'll facilitate these macromolecules to self-organizing to specific identifiable morphologies.
And as an important practical finding of the studies is that the formation of secondary structures is strongly influenced by concentration and the solvent. Following this procedure, other methods, such as TEM, SEM, and XRD can be performed. This allows us to ask additional questions related to the structure of helical polycarbodiimides.
Our goal is to develop materials with precisely controlled kyro-architectures and tunable properties. Future applications of this method, with respect to polycarbodiimides, may include the development of kyro sensors or constructing spherical aggregates as carriers for drug delivery. In addition, we can design novel molecular scaffolds that possess defined microstructures displaying interesting kyro-optical switching properties
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