Contrato: una vez superado el período de beca y obtenido el DEA o certificado

In this thesis, we have used experiment, theory and simulation to investigate the manipulation of single polymer chains using an AFM. These techniques have been used to study the effect of various system parameters on the detachment process of a single polymer chain from an adsorbing surface. In particular, the theoretical and simulation studies have been used to determine the effects of varying the pulling rates in AFM experiments and make predictions of features observed in the corresponding force profiles, while our experimental study has examined the effects of solvency for single chain experiments.

In our AFM experiments we have investigated the influence of solvency, partic­ ularly poor solvent conditions, on features observed in characteristic force profiles. We have shown that the extension of single PNIPAM and PEO chains in poor, aqueous solution occurs with a force that is independent of extension, i.e., a force plateau. The quantised steps associated with multiple plateaus in individual force profiles as well as the statistics of a large collection of force profiles strongly suggest that these force plateaus arc attributable to single chains. More generally, this force plateau can be understood in terms of the Rayleigh instability for a liquid column: extension of the chain results in monomer-by-monomer pull-out from the condensed globule into the poor solvent. Plateau events, can be considered in terms of a chain pulled monomer-by-monomer from an attractive potential to a zone of zero mean potential with minimal stretching. The filament of polymer between the surfaces is made up of monomers which have been pulled out of a local attractive potential. In our experiments, the potential arises from the local solvency of the chains. Our force profiles were observed to contain either exclusively Langevin events or Langevin and Plateau events, depending upon temperature and/or co-solute addition, and did not vary in character with rate of retraction of the AFM tip.

We have also used theory to study the effects of rates on AFM single chain experiments. In this study we use a simple ideal chain model, scaling analysis and activation kinetics to predict force profiles for the detachment of chains from

adsorbing surfaces by pulling a loose tether from the surface. Although we do not include detail such as finite extensibility or monomer-monomer interactions, we are able to reconstruct much of the character of experimental AFM force-profiles. Our analysis considers cases where the timescale of equilibration of the monomer-surface contacts is both shorter and longer than the timescale of the pulling experiment. When the extension rate is slow, the monomer-surface contact has ample time to exchange monomers between the pulled tether and adjacent loop and the force is constant as the chain is being ripped slowly from the surface. However, if the extension rate is made faster and commensurate with the kinetic rate of detachment, then the magnitude of the pulling force details individual detachments of monomers from the surface. We have shown that the force profile will be discontinuous marking an individual detachment and that, on average, the magnitude of the detachment force decreases with successive detachments. As the extension rate is increased, the magnitude of the detachment force increases and larger extensions are required for detachment. At very large extension rates, the applied force is sufficiently large to reduce the barrier to detachment to zero and the detachment occurs instantaneously at a yielding force, f yieid, which characterises the monomer-surface contact. At these large extension rates the force profile is ” saw-tooth" shaped with detachment forces that are equal for successive detachment events and independent of extension rate.

Unlike the experimental and theoretical investigation, our simulation study was not yet complete and will be the subject of further research. Consequently, we have only shown examples of the possible measures with our simulation. The results to date confirm our expectations on the dependence of the chain length, temperature and surface adsorption upon force profiles. We have shown examples of results obtained for pulling an end-tethered polymer chain orthogonal to the surface at a constant rate and the effect of varying the temperature and pulling rates. We also show results for varying the movement of the tip to an oscillatory motion, where the tip is cycled away and towards the surface. Finally we present representative results for applying a constant force to the end of the chain as an alternative to pulling it at a constant rate.

Our experimental study could be extended by investigating the influence, if any, of the cation in the PEO /salt studies and its correlation to the depth of the plateau event in force prohles could be determined. The PNIPAM system could be further explored by studying it in other known poor solvents, e.g., EtOH. In our theoretical study, we could extend this work by examing the effect of solvency, as well as ex­ tension rate on features observed in force profiles. A complementary experimental study could be undertaken to determine if the predictions made by our theoretical work are observed in experimental force profiles. Our simulation work will be the subject of future studies. Some of the practical studies which we intend to under-

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take include a thorough analysis of the effects of chain length, temperature, surface attraction and pulling rates on the detachment of an end-tethered chain from an adsorbing surface. Using the retraction/approach cycles to investigate the adhesion of single chains on surfaces, contrasting the two different experiment techniques of pulling the chain at a constant rate to the application of a constant force and possibly studying the effects of solvency on features observed in force profiles.