Pulling Single Chains out of a Collapsed Polymer Monolayer in Bad-Solvent Conditions
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Pulling Single Chains out of a Collapsed Polymer Monolayer in Bad-Solvent Conditions Vasileios Koutsos(a), Heiko Haschke(b), Mervyn J. Miles(b), Frédéric Madani(a) (a) Institute for Materials and Processes, School of Engineering and Electronics & Centre for Materials Science and Engineering, University of Edinburgh, Edinburgh, EH9 3JL, UK (b) H. H. Wills Physics Lab, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
ABSTRACT It has been theoretically predicted that a polymer chain in poor solvent conditions undergoes a phase transition from a collapsed globule to a stretched coil. In this paper, we present forcedistance curves, obtained by atomic force microscopy (AFM), showing the characteristic forceplateau associated with this transition. We show that the magnitude of the force depends on the environment (interfacial energy) that the stretched coil experiences when it is pulled-out as it is predicted by the theory. INTRODUCTION Force spectroscopy using AFM has been proven to be a unique tool to study static and dynamic properties of polymer monolayers and individual chains [1]. The force-distance (fd) curves record events associated with a variety and combination of several possible scenarios: stretching individual or several polymer coils, bond(s) rupture, desorption processes, entanglements and aggregation effects. So far most of the measurements and published work have concentrated in good solvent conditions; under these conditions polymer chains are very flexible and the entropic elasticity of single polymer chains dominates the force profiles. In poor or bad solvent conditions, although an interesting phenomenon such as the globule - stretched coil phase transition under an elongational force has been predicted [2], experimental studies and evidence of the effect are scarce. B. J. Haupt et al. in a recent paper [3] present evidence of this instability. They used polyelectrolyte chains in poor, aqueous solutions. The fd curves exhibit the force plateaus predicted by the theory. We have used PDMS chains adsorbed onto silicon wafers. The chains were extracted from their monolayer in two different bad-solvent conditions: air and water. In addition to the predicted force plateaus, the force magnitude trend associated with the two different environments compares favorably with the theory.
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EXPERIMENTAL DETAILS Polymers: We have used trimethyl terminated polydimethylsiloxane (PDMS) (chemical formula (CH3)3-Si-O(-Si-(CH3)2-O)n-Si-(CH3)3) purchased from Gelest, Inc. Two different polymer patches have been used with nominal molecular weigths (Mw) : 0.41 kg mol-1 and 62.7 kg mol-1. The polymers were filtered using PTFE membranes (hole size 0.22 µm) prior to use. Substrate: silicon wafers (111) purchased from Siltronix have been used as substrates. It is well known that the surface the silicon is oxidized and a thin silica (SiO2) layer is formed. To free the surface from contaminants and oxidize it further, the substrates were cleaned by immersing in a “piranha solution”. It consists of 3 volu
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