A new class of plastics
It has long been the dream of chemists to act as creator and, with a mere lift of the finger, create well-determined molecules with even more clearly defined functions. This is what Christophe Detrembleur and his team at the Center for Education and Research on Macromolecules of the University of Liege have been attempting to do. Their objective is to create polymers with really innovative functions. They have taken a stride forward in this respect: they have perfected a method that makes it possible to control the reactivity of growing polyolefins and therefore to prepare new copolymers with brand-new properties. In addition, the process works in mild experimental conditions, at 40°C and at 10-50 bars. This is the opposite of the conventional industrial process which is not controlled and which generally takes place at much higher temperatures (several hundred degrees) and at high levels of pressure (1000 bars or more). This technology has application implications for many fields of activity such as the biomedical and energy industries and the environment.
PE, EVA, PVC (1)... These are yet more strange sets of initials that have invaded our daily lives: films used for food-packaging, pipes, glues, adhesives…, applications of this technology affect all areas whether related to food, the medical industry, the automobile industry, agriculture, or the energy industry. The techniques of industrial polymerization that makes it possible to produce these plastics have become increasingly popular in the second half of the 20th century to the point where, today, mankind is in continuous contact with one or other product that is the result of polymerization.
"Polyethylene (PE)is one of the most common industrially-produced polymers (plastics of the polyolefins family). Its chemical inertia, its transparency and its excellent mechanical properties make it an exceptional plastic. However, it is very apolar, which complicates the work when you want to mix it with other more polar polymers. When mixing them, we are looking for a synergy of properties, we combine the properties of the two polymers into one material", explains Christophe Detrembleur director of research at the FNRS in the Center for Education and Research on Macromolecules (CERM) of the University of Liege, whose last study has just been published in Nature Chemistry(2) (and which is the subject on the cover of the march 2014 volume).For example, combining a hard material with a soft material to create an elastomer, a kind of rubber which can be used as a shock-absorber. However, in most cases, an intimate mixture of the two products is an essential condition for obtaining this synergy which is often difficult with polyethylene. This can be obtained particularly by adding copolymers of ethylene and polar vinyl monomers (which are composed of strings of units of ethylene and units of polar monomers).
Apart from this application, these functional copolymers can be found in many products in our daily lives. An important example is EVA (vinyl ethyl acetate) which is found in plastic films for food-products, in smartphones, films for greenhouses and many adhesives and glues.
Controlling the growth
"These ethylene-based copolymers are mainly produced by the conventional technique known as radical polymerization which produces very interesting materials but the process is a bit chaotic", he continues. In fact, ethylene copolymers are currently industrially obtained in conditions which do not allow for precise control of the stringing of monomer units and therefore the final properties of the material.
The polymerization process takes place in three steps: initiation, propagation and termination. The first phase, initiation, consists of managing an active species, a radical (from which the technique gets its name), which will initiate the growth of chains, therefore the addition of monomers (in the case of VEA, ethylene and vinyl acetate) to each other, and enable the polymer chain to grow at a macromolecular level; this is the propagation phase. The last phase which is called termination, fixes the chains into their final configuration.
"All this is done in a chaotic and random manner all along the polymerization process in such a way that, at the end of the reaction, you have chains of different sizes which can also have different compositions and therefore different properties from each other. Here in the laboratory, we are trying to find a way of controlling the growth of these chains".
(1) PE: polyethylene, a plastic constructed by the stringing of ethylene units