Surface structures of polymer solutions in confined geometries
A second-generation surface force style apparatus has been developed to probe the inter-surface and near surface structures under compressive forces. Systems of current academic interest which also have a substantial industrial impact include adsorbed and confined macromolecules, mesophases and amphiphiles in a variety of solution, melt and gel environments; the apparatus is designed to work in situ using neutron, X-ray and optical reflectivity. This section of the project is to commission the apparatus for neutron reflectivity studies and to undertake preliminary measurements on well defined systems.
The forces between macromolecular structures at surfaces, mediated by solvent, are the essence of the physics of adhesion, steric stabilisation of colloids, tack in rubbers, biofouling, lubrication and many problems in particle aggregation and coalescence. An understanding of the origin of these forces is also important in many different branches of science and engineering, ranging from fluid flow on one hand to the properties of cell engineering on the other. In recent years, experimental techniques have been developed to investigate these forces directly between two smooth surfaces bearing adsorbed flexible chains. This pioneering work began in the 1970s, in the development of a surface forces apparatus (SFA) which made it possible to bring two coated mica surfaces within a fraction of a nanometre by means of a known external force. This work has been complemented by the use of the atomic force microscope (AFM), which can provide atomic scale images and measure forces between particles and between particles and substrates with sub-nanometre resolution.
The major factor missing from these direct force measurements is that the structures of the interacting media are not known in detail, particularly as the surfaces are compressed. Our long-term aim is to use neutron, X-ray and optical reflectometry to measure directly the structural changes that occur in such confined spaces.
As shown above, we constructed a compression apparatus with which to perform the neutron reflection experiments. The apparatus was commissioned on the CRISP beamline at ISIS in November 2006. Between each set of experiments, small modifications were made to the apparatus based on our growing experience and also to allow it to be used on the different beamlines. In particular, chamfers were added to the plate that fits between the sample block and the membrane in order to make alignment easier.
A full description of the apparatus has now been published . We encourage people to work with us to apply this technique to their samples (rather than attempting to replicate the cell).
The typical procedure for the experiments performed to date has been as follows. Solutions of poly(vinyl pyrrolidone) were prepared at ~0.3 %w/w in chloroform and allowed to dissolve overnight with gentle stirring. Silicon surfaces were cleaned with a three-solvent cleaning protocol (chloroform, acetone, water) until the resulting surface was sufficiently hydrophobic as to have the water bead off them. Polymer films were prepared by spin coating yielding a ~50 nm thick layer. The films were characterised by ellipsometry.
Reflection profiles for a poly(vinyl pyrrolidone) film with both no applied pressure and 3 bar applied pressure are shown below. The compression of the film is evident in these profiles as the spacing of the fringes increases at higher pressure. More detailed analysis of these data is in progress.
We have also studied a range of other samples including polyelectrolyte multilayers .
- Measuring the structure of thin soft matter films under confinement: a surface-force type apparatus for neutron reflection, based on a flexible membrane approach. WM de Vos, LLE Mears, RM Richardson, T Cosgrove, R Dalgliesh and SW Prescott. Review of Scientific Instruments, 83, 113903, 2012.
- Nonuniform hydration and odd-even effects in polyelectrolyte multilayers under a confining pressure. WM de Vos, LLE Mears, RM Richardson, T Cosgrove, R Barker and SW Prescott. Macromolecules, 46, 1027–1034, 2013.
Last edited: Wednesday November 28, 2012
Copyright © 1996-2014 Stuart Prescott