Hamburg workshop on the "application of synchrotron radiation in chemistry"With grazing incidence small-angle X-ray scattering (GISAXS) the limitations of conventional small-angle X-ray scattering with respect to extremely small sample volumes in the thin-film geometry are overcome. GISAXS turned out to be a powerful advanced scattering technique for the investigation of nanostructured polymer films. Similar to atomic force microscopy (AFM), a large interval of length between molecular and mesoscopic scales is detectable with a surface-sensitive scattering method. While with AFM only surface topographies are accessible, with GISAXS the buried structure is also probed. Because a larger surface area is probed, GISAXS also has a much larger statistical significance compared to AFM. Due to the high demand on collimation, GISAXS experiments are based on synchrotron radiation. Nanostructures parallel and perpendicular to the sample surface observable in thin poly(styrene- block-isoprene) diblock copolymer films are presented as an example of the possibilities of GISAXS.

Grazing incidence small-angle X-ray scattering: an advanced scattering technique for the investigation of nanostructured polymer films

The dewetting of thin polymer films and the resulting pattern formation are reviewed and previewed. Being of crucial importance for various applications, the destabilization of an initially homogeneous layer has been investigated intensively. Recently, the growing complexity of the systems under investigation introduced new questions. In this highly active research area, different complementary methods such as experiments, computer simulations and theory have yielded new insights. Particular emphasis is placed on experiments. Future perspectives are discussed in addition.

https://iopscience.iop.org/article/10.1088/0953-8984/15/36/201/pdf

Dewetting and pattern formation in thin polymer films as investigated in real and reciprocal space

In thin film geometry, the interplay between dewetting and phase separation or microphase separation controls the morphology of the polymeric structures resulting on a solid support. For the model system of polystyrene, polyparamethylstyrene and the diblock copolymer of the two homopolymers, the regime of ultrathin films is addressed experimentally. Evolving structures are probed with real and reciprocal space analysis techniques such as the optical microscopy, phase measuring interference microscopy, scanning force microscopy, neutron or x-ray reflectivity and grazing incidence small angle neutron or x-ray scattering approaches. The effective interface potential of the solid support is tuned by means of a change of the silicon substrate coating. Coating layers of silicon oxide, polyamide and polyimide are under investigation. A power law behaviour describing the most prominent in-plane length as a function of the initially prepared film thickness is observed. All reported structures have been prepared on large scale surfaces, such as typical Si wafers with 100 mm diameter.

https://iopscience.iop.org/article/10.1088/0953-8984/17/9/006/pdf

The control of thin film morphology by the interplay of dewetting, phase separation and microphase separation

Thin thermoresponsive hydrogel films of poly(N-isopropylacrylamide) end-capped with n-butyltrithiocarbonate (nbc-PNIPAM) are prepared on solid supports having silicon oxide surfaces with spin-coating. The film thickness is varied from 5 to 240 nm. As measured with optical microscopy, atomic force microscopy, and X-ray reflectivity, the films are homogeneous and smooth for films thicker than 5 nm. Microbeam grazing-incidence small-angle X-ray scattering (µGISAXS) shows that these nbc-PNIPAM films are physically cross-linked gels, where the end-group domains form the physical cross-links with a defined nearest-neighbor distance of 25 nm. Along the surface normal, with µGISAXS the presence of long-ranged correlations between substrate and film surface is detected. The thinner the nbc-PNIPAM films are, the stronger is the response to swelling in saturated water vapor atmosphere. A swelling up to a factor of 6.5 as compared to the dry film and a factor of 2.9 as compared to the collapsed film is found. The tra...

Thin Films of Poly(N-isopropylacrylamide) End-Capped with n-Butyltrithiocarbonate

Films of the semiconducting polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] with 40% fluorinated monomers, denoted PTB7-F40, are spin coated out of different solvents onto PEDOT:PSS films. The influence of the used solvents chlorobenzene, 1,2-dichlorobenzene, and 1,2,4-trichlorobenzene as well as the influence of the additive 1,8-diiodooctane (DIO) is probed with grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS). As seen with GISAXS, without DIO, the films are homogeneous and show roughness correlation with the PEDOT:PSS film surface. With DIO, an inner film structure with a size of 50-75 nm is found and the roughness correlations weaken. In addition, as seen in GIWAXS, the crystalline part of the films is influenced by the used solvent if DIO is added.

Influence of solvent and solvent additive on the morphology of PTB7 films probed via X-ray scattering.

The surface morphology of thin bilayer polymer films on top of glass substrates was investigated. The bilayer consists of a blend film of protonated and deuterated polystyrene and an underlying deuterated polystyrene film. Choosing the thickness of the top film larger than 8 times and smaller than 2 times the radius of gyration of the chains enables the determination of film thickness and confinement effects. With diffuse neutron scattering at grazing incidence in the region of total external reflection, a depth sensitivity and a contrast even at the internal polymer-polymer interface was achieved. The underlying film is conformal to the substrate, and depending on the thickness of the top film two different types of roughness correlations are observed. Thin confined films nestle to the underlying polymer films, while the stiffness of thicker bulky films provides an independent morphology. In both cases, annealing above the glass-transition temperature yields an interdiffusion at the internal polymer-polymer interface, and the polymer-air surface remains essentially unchanged with respect to roughness correlations.

Jürgen Kraus

Papers

Roughness Correlation and Interdiffusion in Thin Films of Polymer Chains