• Dr. Anton Kiriy
• Vera Bocharova
• Prashant Sinha
• Constantin Demedinok

Virtually all knowledge of physical, chemical or engineering processes has been deduced from bulk measurements. This has led scientists in many cases to think of molecular processes in the context of ensemble averages. However, if one wants to know molecular properties, one should look at individual molecules themselves, one at a time, and compare them.

Most of conventional methods available for polymer science provide information referring to the ensemble average over many polymer chains. On the other hand, the observation and manipulation of molecules would be the method to directly probe and modify local properties of polymer individual molecules. This approach opens a fascinating area of research - single molecule experiment. At the same time, the direct observation of individual molecules also permitting the investigation of ensemble properties through the ensemble of individual measurements.

• study of the conformations of single flexible polyelectrolyte molecules (PE) in controlled environment
• investigation of phase transitions of PE at the level of single molecule events and at the level molecular assembles
• study of PE adsorption at the level of single molecule events
• study of single molecule conformations for PE of different molecular architecture
• interpolyelectrolyte complexes: a single molecule insight
• polymer analytics: direct determination of the molecular weight

fig 1. P2VP, PMB, and P2VP₇-PS₇ star-like copolymer adsorbed onto the mica from acidic water

Among the methods which can be used for visualization of polymer molecules, atomic force microscopy (AFM) certainly occupies a leading place. Before our works the experiments were performed mainly with relatively "thick" polymeric molecules if the diameter of the chain was well larger then 1 nm: dendronized polymers, "molecular brushes", and some naturally occurring polymers: viruses, proteins and duplex DNA. However, most of the synthetic polymers and many natural polymers have much thinner chains.

We visualized "normal sized" polymers like polyvinylpyridine (PVP) with the aid of AFM. This finding allowed us to study largely discussed problem, a coil-to-globule transition of polyelectrolytes (PE), and visualized a theoretically predicted, so-called, pearl-necklace conformation (left figure).

Block copolymers of more complicated architectures were also visualized and studied their reconformation. Diverse conformational transition of single molecules of star-shaped polystyrene-poly(2-vinylpyridine) block copolymer (PS7-P2VP7) induced by different solvents was studied by AFM. At low concentrations in selective solvents PS7-P2VP7 exists in molecularly dissolved state and form unimolecular micelles. The number of P2VP arms of the unimers as well as the aggregation number of the multimolecular micelles and morphological details of the structures were directly analyzed. We confirmed the star-shaped nature of PS7-PVP7 star block copolymer and directly counted the amount of PVP arms (right figure).

To improve the AFM visualization of single positively charged polymer chains deposited on the substrates of a relatively high roughness we developed the simple contrasting procedure via counter ion exchange between Cl^- anions and bulky HCF anions or negatively charged nanoclusters of Prussian Blue.

Thus, polymer single molecules can be now considered not only as representative of the ensemble molecules, but also as individual nanoscale objects which can be used for future nanotechnology for the fabrication of single molecule electronic devices. Our findings are also important from fundamental point of view, since developed approach can be successfully applied for investigation of various "classical" problems in polymer science, such as polymer reconformation, interpolyelectrolyte complex formation, polymer diffusion, adsorption, etc.

One of the most important property of polyelectrolytes (PEs) is their ability to form stable complexes (PECs) with oppositely charged spices due to cooperative Coulombic attraction and release of small counterions. Interaction of oppositely charged polymers plays an important role in nature for various biological processes (such as compaction of DNA in cells). PECs of naturally occurred and synthetic PEs were extensively studied for many medicinal, pharmaceutical, and large-scale industrial applications such as flocculation, coating, separation, and purification of biopolymers. PECs are also useful for microencapsulation and controlled release of drugs and biological objects (enzymes, cells, microorganisms, proteins, DNA). On the other hand, formation of PECs on the surface via layer-by-layer deposition is a simple and versatile technique for assembling multilayered membranes and thin film optoelectronic devices. Despite of considerable interest, understanding of PEs and their complexes constitutes still a challenging task Here we report on the AFM observation of complexes between polycation poly(methacryloyloxyethyl dimethylbenzyl ammonium chloride) (PMB) and polyanion polysterene sulfonic acid (PSA) at different mixing ratio.

PECs between long polycation and short polyanion adsorbed onto mica were studied by atomic force microscopy. If one component is taken in excess, a rapid coupling of the oppositely charged polyions firstly leads to the formation of non-equilibrium structures when collapsed PEC-particles co-exist with unreacted PEs molecules. The equilibrium PEC-particles possess micelle-like core-shell morphology if short polyion is taken in excess. When long PE is given in excess, equilibrium PECs are stabilized by wrapping of long polyion around hydrophobic segments of the PEC. We propose that transformations of initially formed non-equilibrium aggregates proceed through slow reactions (addition or/and substitution) of primary complexes with unreacted PEs chains that finally leads to equilibrium PECs with optimized morphology. As expected, mixing of oppositely charged PEs at near stoichiometric ratio leads to highly aggregated water-insoluble PECs.

fig. 4 Excess of the longer PE

fig. 5 Excess of the shorter PE

The standard methods of molecular weight determination are hardly applicable to polyelectrolytes (PEs) and macromolecules with complex architecture, such as molecular brushes.
The direct visualization of the adsorbed polymer chains and subsequent measuring of their contour length by tapping mode (TM) atomic force microscopy (AFM) could, in principle, allow for the determination of the molecular weight. In general, the degree of polymerization, DP, can be determined by dividing the contour length, LN, onto a length of a monomer unit for certain local conformation, lmon (for vinyl polymers lmon can vary from about 0.15 to 0.25 nm, depending on the conformation). It is clear, that for semi-stretched chains a proper resolution of the molecular details is not guarantied (some of molecular fragments can be hidden in unresolved loops) and a complete uniformity of the local conformations for all repeat units can not be achieved. In contrast, in the case of the ultimately stretched chain the molecular contour can be easily resolved and the maximal possible value of the lmon equal to 0.25 nm for “the longest” all-trans local conformation can be applied for DP determination.
We developed simple procedure to ultimately stretch PE chains upon adsorbtion onto mica, hydrophobized by octylamine.

fig. 6 Simple Method for Stretching and Alignment of Single Adsorbed Synthetic Polycation molecules

• Polyelectrolyte Conformations and Phase Transitions:
  Atomic Force Microscopy, Neutron, Light Scattering and Computer Simulation Studies
  • Funds: DFG

• Prof. Dr. Klaus Huber,
  Universität Paderborn
• Prof. Dr. Hartmut Löwen,
  Institut für Theoretische Physik II, Heinrich-Heine-Universität
• Dr. Rene Messina,
  Institut für Theoretische Physik II, Heinrich-Heine-Universität
• Prof. Alexei R. Khohlov,
  Moscow State University
• Dr. Petr Stephanek,
  Institute for Macromolecular Chemistry, Prague, Czech Republic
• Prof. Constantinos Tsitsilianis,
  University of Patras, Greece
• Dr. Christophe Detrembleur,
  Universite der Liege, Belgium
• Prof. Dr. Chris Toprakcioglu,
  University of Patras, Greece
• Prof. Sergiy Minko
  Clarkson University