Because the properties of nanoparticles dependso closely on their size, size distribution and morphology, techniquesfor controlling the growth of these tiny structures is of greatinterest to materials researchers today.
A research team from the Georgia Institute of Technology andDrexel University has discovered a surprising new mechanism by whichpolymer materials used in nanocomposites control the growth ofparticles. Reported on August 28th at the 230th national meeting of theAmerican Chemical Society, the findings could provide a new tool forcontrolling the formation of nanoparticles.
Growing particles within the confinement of polymer-basedstructures is one technique commonly used for controlling nanoparticlegrowth. After formation of the particles, the polymer matrix can beremoved -- or the resulting nanocomposite used for a variety ofapplications.
In a series of experiments, the research team found a strongrelationship between the chemical reactivity of the polymer and thesize and shape of resulting nanoparticles.
"We have concentrated on the reactivity of the polymeric matrixand how that influences the growth of particles," explained RinaTannenbaum, an associate professor in Georgia Tech's School ofMaterials Science and Engineering. "We found that in the melt the keyparameter influencing particle size is actually the type of interactionwith the polymer. The molecular weight of the polymer and the synthesistemperature are almost insignificant."
In a series of experiments, Tannenbaum and her collaboratorscreated iron oxide nanoparticles within polymer films of differenttypes, including polystyrene, poly(methyl methacrylate, bisphenolpolycarbonate, poly(vinylidene di-fluouride) and polysulfone. Thepolymeric matrix was then decomposed using heat, leaving the particlesto be characterized using transmission electron microscopy.
"These polymers spanned a variety of functional groups thatdiffered in the strength and nature of their interactions with the ironoxide particles and in their position along with polymer chain,"Tannenbaum explained. "We found that the characteristic nanoparticlesize decreased with the increasing affinity -- the strength of theinteraction -- between the polymer and the iron oxide particles."
Specifically, iron oxide particles formed in stronglyinteracting polymer media tended to be small (10-20 nanometers indiameter) and pyramid-shaped, while those formed in weakly-interactingmedia tended to be larger (40 to 60 nanometers in diameter) andspherical.
The researchers also found that the length of the polymer chainwas only weakly related to the particle growth. "This means that forthe same result, we can work in the melt with lower molecular-weightmaterials and have lower glass transitions," Tannenbaum explained.
Based on the experimental results, Tannenbaum and AssociateProfessor Nily Dan of Drexel's Department of Chemical Engineeringcharted the relationship between average particle size and thereactivity of the polymer interface. That information should help otherscientists as they attempt regulate the growth of nanoparticles usingpolymer reactivity.
Tannenbaum and Dan theorize that the polymer layer surroundinga nanoparticle while it grows favors an optimal interfacial curvaturethat sets the equilibrium particle characteristics. That may be relatedto the free energy of the adsorbed polymer layer.
While the researchers focused on iron oxide in this work, theybelieve the control mechanism should be broadly applicable to otherparticles and polymeric materials.
Next, the researchers plan to explore the influence of polymersin solution -- a more complicated task involving more variables.
"In solution, the situation is much more complicated,"Tannenbaum said. "The polymer chains are on the loose, and facecompetition from the solvent. The chains will be reluctant to adsorbonto the surface of the particles, so we may end up with largerparticles than in the melt. In solution, molecular weight of thepolymer will have an impact."
The work reported at the American Chemical Society meeting ispart of a broader study of how nanoparticles interact with polymers --specifically, the interface between polymer chains and nanoparticles.
"The interface has important fundamental properties,"Tannenbaum noted. "When you look at nanocomposites, the interface is avery large component of the whole structure. You can't look at ananocomposite as having just two components -- the interface is reallya third."
Beyond their use as a means for controlling nanoparticle size,nanocomposites may also have applications of their own. Their periodicstructure, for instance, can be useful in optical and photonicapplications.
The research has been sponsored by the National Science Foundation,Petroleum Research Fund of the American Chemical Society, U.S. AirForce Research Labs and Defense Advanced Research Projects Agency(DARPA).
Tannenbaum, Dan and collaborators Melissa Zubris of GeorgiaTech, Eugene Goldberg of the University of Florida, and Shimon Reich ofthe Weizmann Institute of Science earlier reported on thepolymer-directed synthesis of nanoclusters in the journalMacromolecules.
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