Structure of Charged Polymer Chains in Confined Geometry
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Structure of Charged Polymer Chains in Confined Geometry Elliot P. Gilbert 1, Loïc Auvray 2 and Jyotsana Lal 1 1 Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, U.S.A. 2 Laboratoire Leon Brillouin, Centre d’Etudes de Saclay, Gif-sur-Yvette Cedex, France.
ABSTRACT The intra- and interchain structure of sodium poly(styrenesulphonate) when free and when confined in contrast matched porous Vycor has been investigated by SANS. When confined, a peak is observed whose intensity increases with molecular weight and the 1/q scattering region is extended compared to the bulk. We infer that the chains are sufficiently extended, under the influence of confinement, to highlight the large scale disordered structure of Vycor. The asymptotic behavior of the observed interchain structure factor is ≈ 1/q2 and ≈ 1/q for free and confined chains respectively.
INTRODUCTION An understanding of polymer conformation in reduced dimensionality has enormous technological application, such as microfabrication and miniaturization [1]. While neutron scattering has provided invaluable information of chain conformation in bulk [2], there have been relatively few scattering experiments of chains under confinement and these mainly focus on neutral chains. The significantly more complex, and biologically relevant, situation of charged chains (e.g. DNA) in confined geometry currently lacks an experimental framework with which to verify fundamental principles. We report here small-angle neutron scattering (SANS) studies of poly(styrenesulphonate) (PSSNa) when free and when confined in the nanopores of Vycor. By employing solutions composed of either pure deuterated PSSNa or suitable mixtures of protonated and deuterated PSSNa, we have separated the intramolecular and intermolecular contributions to the total scattering, and provide a theoretical understanding of their q-dependencies.
EXPERIMENTAL DETAILS All polyelectrolytes had a degree of sulphonation > 95% and were dialysed prior to solution preparation; molecular weights, MW and polydispersities, MW/MN, are shown in table 1. Since Vycor is highly susceptible to contamination, disks were cleaned by repeated boiling in H2O2 solution followed by heating to 500•C for at least three hours. The cleaned disks were opalescent but, on entering the solution, became translucent with bubbles subsequently being released, indicating the solution-displacement of air. The confined samples were studied after 2, 7 and 9 days for the low, intermediate and high MW samples respectively corresponding to the time for which no visible bubbles remained. Experiments were performed on the SAD instrument at IPNS, Argonne National Laboratory. Absolute values of intensity were obtained from calibration against a polymer standard. T1.3.1
Table I. Polyelectrolytes studied, Rg and Sharp and Bloomfield model fitting parameters for bulk ZAC solutions. Rg limits given for polystyrene, rod conformation and Benoit and Doty relation MW (Da) H/D MW/MN ZAC < MW>ZAC Rg (neutral) Rg (rod) Rg (BD) Parameter
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