Mobility and order in a supramacromolecular system

Polymer micelles are one type of the formations in which a certain nonrandomness of the macromolecular primary structure is spontaneously manifested by a supramolecular order. They are formed with a remarkable regularity e.g. in water by a spontaneous aggregation of several tens of chains of the so-called block copolymers in which a hydrophilic chain is connected with a hydrophobic one by a single chemical bond. This bond which can be regarded as a molecular interface is placed in a micelle on the interface between its spherical core, formed by aggregated hydrophobic chains, and a substantially less dense corona of extended hydrophilic chains pervaded by the water molecules. Polymer micelles are useful in a number of different ways but they also serve as a relatively simple model of a supramolecular system with a definite degree of order and a substantial mobility. Order and mobility compete only at the lower degree of order. The study of micelles contributes to the understanding of the conditions under which they start to cooperate in a kind of dynamic order.

Using a combination of nuclear magnetic resonance with light and neutron scattering, we not only elucidated the detailed structure of a model micelle but also provided the first complete analysis of the mobility of its parts. The micelle as a whole with its diameter 27 nm rotates with a frequency of about 10 kHz. At the same time, the chains of its corona make waving motions with a frequency depending on their flexibility but also on the electric charge density they carry. The main interval of a local change of the chain form is between 10-11 s on the periphery and 10-8 s in the interface region and it generally is higher in a more electrically charged corona. Small positive ions compensating the corona charge are not motionally independent of the chain motions but they cannot be considered to be distinctly ordered. The interface between the micellar core and corona is quite narrow if the corona is electrically charged. In its vicinity up to the distance of 2-3 nm the motions of the atomic groups of both core and corona are slowed down and they are relatively ordered. Deeper in the core, the chain motions are more or less statistical and substantially faster, with a correlation time between 10-10 and 10-11 s, if slower than those in the corresponding bulk polymer.

The partial dynamic order in a micelle is enforced by the molecular interface due to a limited mobility but not an immobilization of the molecules. A micelle thus apparently represents one of the foresteps of the cooperative relation between order and mobility.

BIOpolymer POstdoctoral Laboratory and educational center - BIOPOL

Otto Wichterle Centre of Polymer Materials and Technologies - CPMTOW

Centre of Biomedicinal Polymers - CBMP

Centre of Polymer Sensors - CPS

Polymers for Power Engineering - Energolab


 

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