Research activities at the Department of Radiation Chemistry

Kinetics and mechanism of radiation induced reactions

Upon absorption of radiation energy in the matter chemical decompositions take place forming extremely reactive intermediates: ions, radicals etc. These intermediates react on a very fast time scale usually between 10-6 and 10-3 seconds. For the study of these reactive intermediates the department uses a pulse radiolysis setup. In pulse radiolysis a short pulse of accelerated electrons hits the sample resulting in an "instantaneous" production of intermediates. The formation and decay of these reactive intermediates is followed by using their light absorptions with the method of kinetic spectroscopy.

The initiation of radiation induced polymerization

The intermediates formed in monomers or monomer solutions induce a chain reaction called polymerization. Pulse radiolysis gives an excellent possibility for the study of the initiation, propagation and termination of polymerization.

The department during the last 15 years published about 30 papers on the polymerization of acrylate, methacrylate and acrylamide type monomers in solution (mainly in aqueous solution). In aqueous solution the reaction is initiated by the reactive intermediates of water radiolysis: hydroxy radical (OH), hydrated electron (eeq-) and hydrogen atom (H). They studied the reactivity of the monomers in the reaction with these intermediates and the reactivity of the radicals formed from the monomers.

Radiation degradation of organic pollutants in waters and wastewaters – Advance Oxidation Processes (AOP’s)

Polluted waters containing non-biodegradable impurities may be remediated by combining the usual treatment methods (screening, biological and chemical treatment) with one of the methods called as Advanced Oxidation Processes (AOP). Electron beam (EB) treatment which belongs to the AOP uses oxidative action of the OH radical (produced by the radiolysis of water) for the rehabilitation of polluted water and wastewater. The decomposition of most organic compounds is induced by OH radicals achieving, in some cases, their complete mineralization. EB represents a unique pre-treatment tool for the destruction of bio-resistant pollutants. Major benefits with respect to conventional treatments are: no usage of chemical additives, room temperature operation, penetration in the bulk of water even in case of turbidity, production of high concentrations of oxidizing radicals in microseconds, and simultaneous disinfection. The scientists of the department by using pulse radiolysis and end-product techniques investigate the degradation of phenols, and textile dyes in aqueous solutions.

Radiation degradation and stabilization of polymers

High-energy radiation causes chain breakage of synthetic and natural polymers. This process called as radiation degradation can in some cases be considered as useful process but in other cases it is unwanted. Radiation degradation of synthetic polymers can be a tool supporting recycling of polymer wastes. In this way e.g. additives can be produced from polytetrafluor ethylene (teflon). Radiation degradation should be avoided e.g. in case of polymer coatings of cables and wires in atomic power stations. The Department has experience in the field of radiation degradation and resistance of polymers and biopolymers.

Radiation grafting

Grafting is copolymerization of a monomer or a mixture of monomer/oligomer to a backbone polymer in order to modify its surface properties. This process is accomplished by irradiating the polymer and producing radicals and these radicals will initiate  grafting. This method is called as preirradiation grafting. In mutual grafting the polymer is covered with monomer or monomer solution and they are irradiated together. In this case the initiating radicals will be polymer-, monomer-, and solvent radicals.
By radiation grafting of monomers to the cellulose backbone some properties of cellulose e.g. crease resistance, thermo plasticity, dimensional stability pH- and thermo sensitivity, absorption capacity can be modified. The aim of grafting is often to change the compatibility of the surface by modifying the hydrophilic/hydrophobic properties of the base material. In this way polymer-cellulose composites can be produced.

Functional polymer monoliths, micro- and nanoshperes (for separation, purification, diagnostics, therapy)

Polymeric micro- and nanospheres are one of the most useful materials for many of the current technology: they are used in optical sensors, as fuel containers in nuclear fusion experiments or as antistatic coatings for spacecrafts, to mention but a few. Besides, they are increasingly utilized as functional supports in various biomedical applications ranging from purification procedures to diagnostics and therapy.
A monolith is a porous polymer rod, characterized by a system of interconnected pores with a bimodal distribution: the small pores provide the desired surface area required for the specific interactions, while the larger channels allow a high flow rate at moderate pressures. Such monoliths can be used in various separation, purification, solide state peptide synthesis, in a microfluidic device, and many others.

We are synthesizing all of them by a radiation initiated precipitation polymerization of amonomer(s) (usually diethyleneglycol dimethacrylate) solution. The characteristics and the advantages of this method are that not only could it be done free of surfactants, but also no initiator or cathalyst is necessary (usually toxic materials that could contaminate the product), the synthesis can be done at any temperature, and the particles have narrow size distribution. In case of the monoliths, they can be prepared in-situ, in a mold of suitable shape and size.
The size of the spheres as well as the pore diameter of monolith is mainly controlled by the properties of the monomer and the solvent, the use of co-monomers and their ratio, the irradiation temperature, the dose and the dose rate. For immobilization of biomolecules a desired functional group can be added either in an additional activation step, or by direct incorporation of a suitable co-monomer during the polymerization itself.

Smart nanogels (targeted drug delivery)

In parallel to macroscopic polymer gels, that are now well-established biomaterials typically used as soft contact lenses, hydrogel wound dressings and devices for controlled drug delivery, there is a growing interest in synthesis, properties and applications of microscopic polymer gels, i.e. microgels and nanogels. Nanogels are sub-micron size crosslinked polymer structures of sizes similar to a single polymer molecule in solution. Such gels have potential applications as drug and gene carriers, polymeric drugs, biomarkers, replacement of various biomolecules, but also as substrates for adsorption and separation of biomolecules, as well as for preparation of macroscopic functional polymer composites.
Nanogels are mostly obtained by emulsion polymerization. We are using a recently proposed attractive alternative method based on short pulse electron irradiation (Ulanski, P., Janik, I., Rosiak, J.M. Radiat.Phys.Chem. 1998, 52:289-294). By this technique, intramolecular crosslinking is favored. The advantage of this method is the absence of monomers and crosslinking agents, all potentially toxic materials. This simple system consists only of water and the chosen polymer, and in some case, a biomolecule to be entrapped.

Multifunctional surfaces (cell culture)

Since surface properties of a polymer determine its success or failure in most of biomedical applications, it is of utmost importance to be able to prepare tailor-made surfaces. Radiation initiated surface modification has a number of advantages over conventional methods: it is independent of temperature and no additives (initiators, catalists) that could contaminate the product are necessary; the degree of grafting and crosslinking can easily be controlled; and in some cases the modification and sterilization can be done simultaneously. Recently, plasma modification techniques are widely used, because they permit the controlled modification of the uppermost layer (about 50 nm) of polymer surfaces.
We are involved in preparation of polymer surfaces for improved protein adsorption that could result in high sensitivity in diagnostics and better cell-culture surfaces. Our results showed that that there is no universal surface treatment that will equally enhance the adsorption and retention of all proteins. Therefore, for a specific antigen or antibody or cell line an optimum surface should be developed.
We are also involved in grafting thermo-responsive hydrogels onto various polymers in order to prepare new functional surface optimized for cell growth and harvesting. The grafted surface changes the cell adhesiveness depending on the culture temperature enabling thus the harvesting of cell cultures noninvasively (without enzyme application) by simple cooling. This procedure enables the preparation of cell-sheets without scaffold.