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The structural evolution in poly(styrene-b-butadiene) (P(S-b-B)) diblock copolymer thin films during treatment with cyclohexane vapor is investigated in-situ and in real-time using grazing-incidence small-angle X-ray scattering (GISAXS) [1]. Both the swelling and the drying process are investigated. The lamellae are initially perpendicular to the film surface, i.e. the film is laterally nanostructured. Cyclohexane is a good solvent for PB and a theta solvent for PS, i.e., it is slightly selective. Using incident angles above and below the polymer critical angle, structural changes near the film surface and in the entire film are distinguished. We find that, during swelling, the initially perpendicular lamellae tilt within the film. Our computer simulations [2] show that this is due to the tendency of the copolymers to assume less stretched chain conformations, i.e. the lamellae shrink upon solvent uptake. Since long-range mass transport is not easily possible, tilting allows satisfying the space-filling condition when the lamellae are shrinking. Surface-sensitive GISAXS experiments show that, at the film surface, the lamellae eventually vanish at the expense of a thin PB wetting layer. During the subsequent drying, the perpendicular lamellae reappear at the surface, and finally, PS blocks protrude because of the solvent selectivity. By modeling the 2D GISAXS images, the time-dependent height of the protrusions can be quantitatively extracted. Figure 1. (a) 2D GISAXS images during swelling (top row) and drying (bottom row). The times after the beginning of the vapor treatment are indicated. (b) Snapshots from computer simulations of perpendicular lamellae which tilt during the film swelling. From left to right, the degree of swelling increases.
abstracts
Thermo-responsive polymers can exhibit a demixing transition which is of the lower critical solution temperature (LCST) type. The collapse transition of polymers with such LCST behavior is of great interest for applications where a strong change of volume is desired even for small changes of an external stimulus such as temperature. Examples of applications are valves in micro-fluidics, the release of drugs in the body or sensors. A polymer which is a promising candidate in this context is poly(N-isopropylacrylamide), PNIPAM. It exhibits a LCST of about 32 0C that is attributed to alterations in the hydrogen-bonding interactions of the amide group. A polymer with a higher transition temperature as compared to PNIPAM is the thermosensitive acrylate poly(methoxydiethylene glycol acrylate) or PMDEGA. In thin film geometry, such thermoresponsive polymers are of particular interest for use as thermosensitive surfaces, artificial pumps and muscles, light modulation systems and optical switches. The strong volume change translates into a change of the film thickness due to the thin film geometry. We investigate the kinetics of chain collapse of thin thermosensitive films as a function of quench depth. Homopolymer films and amphiphilic block copolymer films are compared. With time resolved neutron reflectivity (NR) we follow the kinetics of the transition from a swollen to a collapsed thermoresponsive film. Within 15 seconds a full NR curve is probed, which allows to determine the evolution of film thickness and of the water content in real time. Thus, we investigate in-operando the switching behavior caused by a thermal trigger. The observed complex three-step switching of the films is discussed.
