Binary blend Nanoparticles with defined morphology

The word blend in linguistics means a word formed from two parts of two words. In polymer science polymer blends means polymer mixtures, a class of materials analogues to the metal alloys. Blending of polymers is a simple and economic way to create new materials meeting specific desired properties. The other alternative is to synthesize such materials eventually facing the organic chemistry design difficulties. The low entropy of mixing polymers makes the process thermodynamically unfavorable, unless there are some specific interactions between the mixed polymers. As a result, in thermal equilibrium typically a phase separation between the blend components takes place. The main challenge facing the blending of polymers is the control of the length scale of the phase separation. One of the most important applications, where the control of the phase separation is crucial for the performance is the organic solar cells. In organic solar cells a blend of an electron donating polymer and electron accepting one is formed. The dimensions of the phase separation between the two polymers should be in the range of the exciton diffusion length [1-3] (in semiconductors, exciton diffusion length is the average distance traveled by the electron-hole pair before recombination). Only under this condition the charge transfer at the interface between the two polymer layers can take place and the solar cell performs efficiently. The thin polymer blend layers for such applications are commonly deposited by spin coating from solution containing both polymers. The morphology of the thin layer prepared in this way is highly influenced by the preparation conditions such as the surface properties of the substrate, the solvent from which the blend was deposited, the temperature, and the annealing temperature [4-9]. Therefore controlling the length scale of phase separation in layers casted or spin coated from solutions is difficult and is a matter of trials and errors. Recently a novel nanoparticle approach relying on the mini emulsion process was presented, by which the length scale of phase separation of polymer blends is controllable down to few tens of nanometers [1-3, 10-12]. The method is based on forming a mini emulsion of the polymer solution in water and subsequently evaporating the solvent to obtain the polymer nanospheres dispersed in water. The process enables the control of the polymer particle size in the range of 50-500 nm [13-15]. The blending is done through two different approaches: either by mixing nanoparticles of pure polymers (nanoparticle blends), or by fabricating blend nanoparticles (composite particles) by starting with a mutual solution of the two polymers. Solar cells based on the mini emulsion approaches have been fabricated and their efficiency was studied [1-3], nevertheless the morphology of the polymer blends used for the fabrication was difficult to study. The difficulty is hidden in finding polymer pairs that have electronic contrast to enable the morphology study by electron microscopy