Peering into polymers

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Virtually every day we benefit from products made from thin film polymers. These special layers are used for everything from time release medications to anti-reflective coatings. Neutron studies are helping us to better understand thin polymer film behaviour, further increasing their range of applications.

Thin polymer films are produced using blends of polymers with layer thickness ranging from just a few nanometres to hundreds of nanometres. Creating and characterising these systems can be difficult: like oil and water, many polymers just don’t want to mix. However, the technique of neutron reflectometry at ISIS has enabled scientists to study the interfaces between polymer layers, providing a greater understanding of these complex systems.
Michele Sferrazza, from the Université Libre de Bruxelles in Belgium and his colleague Richard Jones at the University of Sheffield have used ISIS neutrons to study the nanoscale structure in a class of polymers known as random copolymers of polyolefins. “These systems offer the possibility to control the conditions of miscibility,” Sferrazza explains.
By bringing two different polymers close together, Sferrazza and Jones were able to use neutron reflectometry to observe the fluctuations at the interface between the thin films, from immiscible systems right up to the point where the interface becomes large (quasi-miscible case). Their results showed a gradual transition from a region where long-range forces were the dominant influence on the interface fluctuations (immiscible systems) to a region where short-range forces became more important and the dependence of the width of the interface on film thickness was stronger.
“This molecular level of understanding of the interfacial structure is crucial to eventually help the design of controlled levels of adhesion, for example in coated films, products involving welded joints and in the performance of optoelectronic polymers”, says Sferrazza.
In addition, their experimental work helped to verify numerical calculations and showed that they can provide a good description of the width of the interface between two polymers. The new findings should provide information for more general theories of how interfaces behave in restricted geometries. “The fundamental physics revealed will help to underpin more applied work in polymer interfaces, of general relevance to coatings, polymer optoelectronic devices, adhesion and multiphase polymer materials”, says Sferrazza. What is more, the role of fluctuations at interfaces has a wider relevance in more complex systems, such as self-assembled surfactant phases and biological membranes. 

Michele Sferrazza (Université Libre de Bruxelles),
msferraz@ulb.ac.be. Richard Jones (University of Sheffield)

Research date: December 2007

Further Information

Interfaces and fluctuations in confined polymeric liquid mixtures: from immiscible to near critical systems, M Sferrazza and C Carelli, J Phys: Cond Matt 19 (2007) 073102

The effects of long-ranged and short-ranged forces in confined near-critical polymeric liquids, C Carelli et al, Europhysics Lett 71(2005) 763

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