Mechanisms of responsive behavior in thin polymer films

Mechanisms of responsive behavior in thin polymer films 

In thin films polymer chains are highly concentrated. Assemblies of polymer chains respond cooperatively to their environment and form segregated domains. The sizes of the domains depend on interfacial tension and on the effects of constraints (for example, for chains grafted to a solid substrate, the polymer segments cannot move from the grafting point for a distance larger than the end-to-end distance of the polymer chain). Below we consider how the phase segregation mechanisms in thin polymer films can be explored for the fabrication of responsive materials. 

2 Homopolymer brush constituted of end-tethered chains stretched in good solvent (a), and collapsed in poor solvent (b). 

Although homopolymer brushes demonstrate very pronounced responsiveness in terms of switching their properties between good and poor interactions with their environment, the composition of the interface is not changed qualitatively. In other words, independent of the environment, the polymer exposed to the interface remains unchanged in its chemical nature. Thus, the switching of the thin film properties is limited by the constitution of the given polymer. 

3 Swollen thickness of a 20nm (dry thickness) poly(methacrylic acid) (PMAA) brush on a La-prism as a function of the pH of solution ( salt-free ). The brush was prepared by polymerization of methacrylic acid (bulk t= 1.5h T = 60°C). After the polymerization the PMAA brush was extracted with methanol and water for 15h. The solid line is a guide to the eye. Reprinted with permission from Ref. 17. Copyright 2002, American Institute of Physics. 

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