Supramolecular Networks Synthesized in Nanoparticle-Polymer Mixtures
Read full article onlineFull Article
The dispersion of solid nanoparticles within a polymeric matrix can dramatically improve the mechanical, thermal, or electrical properties of the material. One of the challenges in fabricating such composites is controlling the distribution of the particles in the polymer. However, in multicomponent mixtures that involve blends of different polymers and the nanoparticles, one can exploit the polymer–polymer and polymer–particle interactions to direct the distribution of nanoparticles within the mixture. For example, consider the case where the polymer matrix is composed of a binary blend of A and B homopolymers. Most polymer pairs are immiscible and thus these blends usually undergo phase separation. Typically, one of the polymers in the blend preferentially absorbs onto, or wets, the particles; in particular, coating the particles with A-like chains provides a mechanism for obtaining the preferential wetting interaction between the particles and the A polymers. Now the interplay between the A/B phase separation and the polymer–particle wetting interactions controls the structure of the composite. In particular, the A-like particles localize in the A domains, and depending on the composition of the AB blend, these solids can form a continuous network throughout the material or segregate into A domains that are dispersed in the B matrix. The former case is of particular importance because particles that have percolated into an interconnected network can enhance the electrical or mechanical integrity of the system.
In a previous study, we used computer simulations to determine the behavior of nanoscale rods that are immersed in a binary, phase-separating blend. The situation becomes even more complicated than the scenario described above because the rods can form liquid crystalline phases. Here the interplay among three phenomena—phase separation, wetting, and anisotropic interparticle interactions—controls the morphology of the material. We found that such cooperative effects drive the rods to form a percolating structure at very low volume fractions of the nanoparticles. In effect, the rods form a supramolecular network that extends throughout the polymer matrix. These studies revealed that the synergistic interactions among the components provide a means of manipulating the motion of the nanoscopic objects and directing their association into extensive structures.
In addition to harnessing the phase separation of polymer blends, one can exploit the microphase separation of diblock copolymers to control the dispersion of nanoparticles. Diblock copolymers are composed of an A chain that is chemically linked to a B chain. Although the A and B chains are incompatible, the diblocks cannot undergo macroscopic phase separation because of the chemical linkage between the two blocks. Instead, at a critical temperature, the diblocks microphase separate into spatially periodic structures that can exhibit spherical, cylindrical, lamellar, and other more complex geometries. Adding particles that are preferentially wetted by one of the blocks can yield spatially ordered hybrid materials. For example, adding particles to the cylindrical phase of the diblocks can result in the formation of solid “nanowires” that extend throughout the material. If these nanoparticles are metals or semiconductors, the composite can exhibit the unique electrical, optical, or magnetic properties of the inorganics and the flexibility and processibility of the polymers. In addition, the “columns” of particles provide significant reinforcement of the polymer matrix and thus improve the mechanical properties of the material.