Working Group of Optical Phenomena

Head: Jaroslav Holoubek, PhD (phone +420-296 809 321, e-mail holou@imc.cas.cz)

Research

The research activities in this group are focused on physical and physico-chemical studies of various supramolecular systems consisting of homopolymers, block and random copolymers, gels, either in the bulk or in the presence of large or small amount of solvents or solvent mixtures, using a large variety of optical (mainly light scattering) methods. Particular attention is paid to (a) kinetics and dynamics of polymer systems during phase separation, to association, complexation and micelle formation, (b) ordering processes, on microscopic, mesoscopic and macroscopic scales, and (c) electrically conducting polymers.

The experimental techniques used include static and dynamic light scattering with fully automatic instruments (ALV), time-resolved light scattering, laser speckle analysis, birefringence analysis and image analysis. The group has facilities for polymer synthesis.

(a) Phase separation on microscopic and macroscopic level determines many applications of polymer systems. We study the kinetics of these systems which describes the pathway of the system to equilibrium, and the dynamics of the systems, which analyze microscopic motion of system components determining macroscopic, application-related properties.

We investigate also the formation and evolution of organized structure in mixtures of copolymers especially in micro- and macrophase separated regions. The systems under consideration include mixtures of block copolymers with chemically identical and/or different blocks (symmetric, asymmetric), systems with crystallizable and/or stereoregular blocks and systems of triblock copolymers.

We investigate these polymer systems in the melt and solid state and also as dilute or concentrated solutions in various solvent systems. The effort should lead to gathering of substantial new information about the formation of organized structures in copolymer systems which are ingredients of advanced nanotechnology.

(b) Chemical reactivity of surfaces and physical interactions at surfaces control both chemical and physical sorption processes and determine the degree of order of adsorbed or chemically anchored molecules. Some strategies in modern materials science such as biomimetic chemistry and surface engineering focus on the additional control of chemical and physical interactions. The research includes both the preparation of chemically patterned surfaces through surface chemical reactions and self-assembly processes and the structur investigation of the surface.

The basic idea of self-organization of polymers by a variety of interactions can be demonstrated on many polymer systems (environment-responsive polymer systems) such as:

• Thermosensitive micelles from block copolymers of poly(ethylene glycol) with thermosensitive polypeptides.
• Polymer-polymer complexes stabilized by hydrogen bonding in solutions of polycarboxylic acides.
• Polymer-polymer complexes between DNA and weak polycarboxylic acids stabilized by hydrogen bonding.
• pH-sensitive polymer-polyelectrolyte complexes between zwitterionic copolymers and polyelectrolytes (including DNA) or surfactants and phospholipids.

c) The research of conducting polymers is oriented on polyaniline and polypyrrole. The optimization of synthetic conditions on the electric and utility properties is a frequent goal. Surface polymerization of aniline yields thin conducting polymer films. These can be used for the coating of various substrates (inorganic oxides, ferrites, wood mass, polymer membranes) with conducting overlayer. The preparation of polyaniline in the presence of steric stabilizers yields colloidal forms.

Temperature dependence of the distribution A(?) of relaxation times ? obtained by dynamic light scattering from the same ternary system as in the neighboring Figure. Three dynamic processes have been identified (from left to right): cooperative diffusion of the transient polymer network, self-diffusion of a single block copolymer molecule through the mixture and a large-scale structure relaxation the dynamics of which is slowed down by five orders of magnitude on cooling to 50-40 °C. In this temperature range, the two solvents become fully incompatible.

Send your comments to:
holou@imc.cas.cz
Last update: 22.04.2004