Stimuli-responsive polymer brushes into nanoconfined geometry

The conformational behavior of end-grafted diblock copolymer brushes inside a nanoporous alumina membrane in poor and good solvent is experimentally studied. For this purpose an apparatus has been specially designed which enables the monitoring of the volumetric flow rate vs. differential pressure generated by fluid flowing through the membrane. INTRODUCTION: Polymer brushes are formed when polymer chains are tethered at one end to a surface at sufficient density to overlap and stretch away from the surface. 1 Stimulusresponsive polymer brushes are a category of polymer brushes that exhibit a change in their conformation, surface energy, or charge state, triggered by an external stimulus such as a change of solvent, temperature, pH, ionic strength, light, mechanical stress, electric or magnetic field, e.t.c. 2 In many processes, particles or macroscopic surfaces bearing adsorbed polymer molecules pass from a good solvent for the polymer to a poor solvent. This external stimulus, by change of the solvent, has been utilized in regulating the switching properties of the polymer brush. This paper describes a method that allows the layer thickness of a polymer (polystyrene), adsorbed within the pores of an alumina membrane, to be determined as the solvency is changed. For this purpose, an ensemble of nanotubes embedded in an alumina membrane, bearing Polystyrenepoly(ethylene oxide) (PS-PEO) diblock copolymer brushes, undergoes permeation measurements in a suitable custom made apparatus. The switching process can be controlled by treating the brushes with selective solvents. When these brushes are treated with toluene, PS chains swell and stretch away from the surface to preferentially occupy the outer-most layer, reducing the hydrodynamic pore diameter and increasing the resistance in flow. Conversely, when the brushes are treated with poor solvent such as isopropanol, chains partially collapse increasing the hydrodynamic pore diameter and decreasing the resistance in flow. Intermediate states in the stretching behavior of the polymer brush, are possible to appear by gradually switching the quality of the solvent. The latter is achieved by mixing poor and good solvent in various volume fractions. EXPERIMENTAL METHODS: A nanoporous membrane (Whatman) with nominal pore diameter of 200nm and 60μm thickness is fitted to a permeable cell made from Teflon. The cell is tightly fixed to a glass cylindrical tube, 150cm in height and 6.82mm in diameter. The glass tube is appropriately scaled with engraved marks at the outer surface. Sealing is achieved by placing o-rings between the different parts, (membrane-cell-glass tube) as shown in Fig. 1. The glass tube is filled with the appropriate solvent and the fluid height level, h, as a function of the time, t, is recorded. The hydrostatic pressure difference imposed by the fluid column upon the membrane is at the order of 10kPa and functions as the driving force for the flow establishment. This apparatus, simple and versatile, offers superior convenience over other proposed experimental setups in the literature. More specifically: a) there is no need of any pump adequately calibrated for the flow establishment. b) there is no need of differential manometers for measuring the pressure drop imposed by the presence of the porous membrane. c) there is no need of any flow meter with careful and laborious calibration.