Polymers in the context of "nano"

Polymers and nanotechnology

Maxim Rousseau, Polit.roo

October 10 in the framework of the project "Public lectures "Polit.<url>"" Doctor of Physical and Mathematical Sciences, Academician of the Russian Academy of Sciences, Professor of Moscow State University, Vice-Rector, Head of the Department of Innovation Policy and International Scientific Relations of Moscow State University, Chairman of the Council for Science at the Ministry of Education and Science of the Russian Federation Alexey Removich Khokhlov gave a lecture on: "Polymers in the context of "nano"".

Until the turn of the XIX – XX centuries, science studied only objects that a person can see (with the naked eye or with the help of optical devices), that is, objects whose size is larger than the wavelength of light (400 – 700 nm). Then the objects of the microcosm also came to the attention of science: first atoms, then elementary particles whose size is less than one nanometer. The range from 1 to about 500 nanometers has been ignored for a long time.

Meanwhile, this range is very important. At this level, all the molecular mechanisms that underlie life are carried out. The structure of substances at the nanoscale determines their properties. Whether we want to find out how a chemical reaction catalyst works or how a nerve impulse is transferred in a synapse, we cannot do without turning to the nanoscale.

With the development of miniaturization in the production of microcircuits, the concept of nanotechnology has also appeared, which has now spread far beyond this particular area. The special place of nanotechnology is explained by the fact that many conventional technologies are already difficult to apply to nanoobjects. However, at this level of the structure of substances, it becomes possible to use fundamentally new methods, for example, the ability of molecules to self-organize when they interact between different parts of molecules (for example, as a result of the action of van der Waals forces). or simply form ordered structures due to thermal motion. It is this self-organization of matter that allows structures to arise in the cells of living organisms. In cells, DNA and RNA molecules, various proteins, and so on are synthesized. A. R. Khokhlov gave an impressive example: if you imagine the total length of DNA molecules that are synthesized in the human body during a person's lifetime, and line up in one line, then their total length will be two light years. So far, no man-made synthesis systems can compare with a "simple" living cell.

In this regard, a biomimetic method has emerged in which the molecular structure of living systems becomes a model for those systems that people create. And these biological molecules, the functioning of which we would like to use, are polymers. Recall that polymers are substances whose molecules consist of a large number of repeating units (monomers). Here are examples of two well - known polymer compounds:

Polyethylene ...—CH ₂—CH ₂—CH ₂—CH ₂—…

Polyvinyl chloride ...— CH ₂—CHCl—CH ₂—CHCl—CH ₂—CHCl—CH ₂—CHCl—…

We see that the polyethylene monomer is the CH ₂ group, and the polyvinyl chloride is CH ₂—CHCl. Polymer molecules are called macromolecules. In synthetic polymers, the number of links in the polymer chain is usually from 10 to 10000. In natural substances, it can be much larger. A DNA molecule, for example, may consist of 10 ⁹ – 10 ¹⁰ nucleotides. The use of polymers by nature is due to a number of their properties, which, in turn, are caused by their structure.

The most important features of polymers are: long chains of monomer links, a large number of these links and the flexibility of chains. Since macromolecules are long chains, individual monomers do not have freedom of independent movement, which means that entropy is lower in polymer systems. This determines the ability of polymer systems to self-organize. Even a small energy interaction between groups of atoms leads to ordering in their arrangement.

Such self-organization occurs, for example, in materials that consist of block copolymer molecules. A block copolymer is a different polymer chain connected to each other in one by means of a covalent chemical bond. In the simplest case, a block copolymer combines two different chains, but there may be more of them, the form of their connection can be very diverse. If we take a block copolymer, the two components of which tend to deviate from each other (again due to the van der Waals interaction), then they will not be able to completely separate, because the components of the molecule are connected by a covalent bond. However, this repulsion will cause the chains of macromolecules to orient themselves in space, as a result, a structure will appear in the material. Depending on the relative length of the two components of the block copolymer, these can be spheres from one component surrounded by another, or cylinders, or layers, and so on. Therefore, by simply synthesizing block copolymer macromolecules with different length ratios, it is possible to design polymer nanostructures.

Copolymer Structures (Wikimedia Commons)

This finds practical application. One of such applications is the creation of thermoplastic elastomers. For example, a block copolymer is taken, the components of which are polyisoprene and polystyrene. Polyisoprene is a rubber, polystyrene is a polymer that hardens at room temperature. By combining them in the right proportion, we can obtain a material in which, at the nanoscale, there will be spherical inclusions of polystyrene in the rubber mass. If you raise the temperature, the polystyrene will melt, and this rubber can be given a new shape.

Alexey Khokhlov also spoke about the use of block copolymer layering in the creation of ultrathin nanostructured films. If the chains of one of the macromolecules included in the block copolymer form layers or cylinders in it, then they may have a different spatial arrangement. The cylinders can be oriented perpendicular to the surface of the film, or they can be parallel. Depending on this, the film may have different properties, for example, "allow" or "not allow" other molecules to diffuse through themselves. A. R. Khokhlov described a method proposed by him and his colleagues that allows providing the desired nanostructure in the film. It turned out that for this it is necessary that the substrate on which the film is formed is itself nanostructured. There should be areas on it that attract the components of the block copolymer in different ways.

Another example of the use of nanostructures in polymers is fuel cells. These are electrochemical devices that convert chemical energy into electrical energy. The fuel for such an element is, for example, hydrogen. Hydrogen molecules are fed to the anode, where they are split using a platinum catalyst. Protons are sent through the polymer membrane to the cathode, and the membrane does not pass electrons, so they go to the external circuit and a current arises in it. At the cathode, as a result of the combination of electrons, protons that have passed through the membrane and oxygen in the air, water is obtained. So far, such fuel cells are used in space and military technologies, but over time, when this method of energy production begins to pay off, they can replace the gasoline engines we are familiar with. An important plus of fuel cells is environmental cleanliness. Polymer nanostructures are used in a key part of a fuel cell – a membrane that conducts protons.

You can learn about other examples of the use of polymers in nanotechnology from the video recording of the lecture.

Portal "Eternal youth" http://vechnayamolodost.ru 16.10.2013