Handbook of Modern Coating Technologies

Self-assembled nanostructures

Self-assembly is a powerful bottom-up approach to tailor interfaces/surfaces on a nanometer scale. GISANS offers the possibility to characterize nanostructures on surfaces and interfaces and was applied to identify the self-assembly of Latex nanoparticles in water on large sur­faces [52]. In such studies the self-assembly in a liquid is directly followed by scattering from a silicon—liquid interface. Silicon has a low absorption for neutrons making such measure­ments feasible.

For an aqueous solutions of triblock copolymers, that microphase separate in water due their hydrophobic and hydrophilic parts forming micelles, the self-assembly of the micelles into crystals was studied at differently terminated solid interfaces. It turns out that a powder­like structure is formed at an interface with a larger contact angle for water, whereas epitaxial growth of a dense packed cubic structure is promoted by an interface with smaller contact angle [53]. These first experiments where done on a monochromatic instrument under a single incident beam angle. More recently a similar experiment was performed on a TOF instrument. In this case a range of wavelength impinges on the sample under the same incident angle. Some of the wavelength will be scattered at Q values larger than the critical value, some at the critical value and some below. As a result the penetration depth into the liquid is different for every wavelength and the structure can be extracted depth resolved. However, note, due to the overall low absorption of neutrons in most materials the depth resolution is significantly lim­ited [54] and often only very near surface and bulk properties can be distinguished. Fig. 4—15 depicts GISANS patterns for a micellar solution at a silicon surface treated with piranha solu­tion (top panels, hydrophilic) and OTS (lower panels, hydrophobic). The direct and the specu­larly reflected beams were masked by the beam stop. The left panels correspond to the integrated intensity from the long wavelength with a penetration depth into the polymer solu­tion of approximately 10 nm or 1 pm taking into account resolution effects. Data for the short wavelengths with a large penetration depth of about 30 pm are shown in the right panels. The

~10 nm (<1 цш)

FIGURE 4-15 GISANS data taken for a micellar solution of the polymer Pluronic F127 dissolved in D₂O in contact with a silicon surface treated with piranha solution (top panels) and OTS (lower panels). The different penetration depths, noted at top and increasing from left to right, result from the different wavelengths in the incoming beam. Crystalline ordering is clearly preferred in the vicinity of the piranha solution treated surface. GISANS, Grazing incidence small-angle scattering. Adapted from M. Wolff, J. Herbel, F. Adlmann, A.J.C. Dennison, G,

Liesche, P, Gutfreund, et al., Depth-resolved grazing-incidence time-of-flight neutron scattering from a solid-liquid interface, J. Appl. Crystallogr. 47 (1) (2014) 130-135 [55].

central column summarizes the intensities for wavelengths integrated around the critical wave­length, 0.5-0.6 nm, resulting in an intermediate penetration depth of about 10 pm. Clear qual­itative differences between the scattering from the two samples can be seen in the central panels of Fig. 4-15 where the penetration depth is nominally 10 pm. Ten well-resolved Bragg peaks, together with four weaker peaks are observed for the sample close to the piranha trea­ted surface, whereas for the OTS surface peaks of a lower intensity together with a Debye—Scherrer ring at Q = 0.4nm^_1 are observed. The insert between the left and middle panels depicts a zoom into the low Q-region and with a narrow wavelength band around 0.56 nm, which is slightly larger than the critical wavelength. On the two panels the difference in structure between both interfaces is most striking. For the largest penetration depth (right panels) both detector images become more similar indicating bulk properties.

Interestingly the surface coating does not only affect the formation of structure at the solid—liquid boundary but also the dynamics of recrystallization [56] after the cecession of shear. Which can be followed by a combination of rheology and NR, so called Rheo-NR [57], combined with time-resolved NR, GISANS [58] and OSS measurements. More recently is was
shown that the time resolution of such experiments can be considerably improved, into the sub-ms regime, if a pump probe approach is used with time stamped neutron detection and cyclable excitations [59].

• Graphene oxide

Graphene oxide (GO) has a large surface area, good mechanical strength [60] and oxygen- containing functional groups, so it has great potential for applications by modifying its sur­face structures, for example, by linking functional peptides, antibodies, and enzymes. In the past the hydration of pristine graphite oxides has been studied using neutron scattering [61,62] focusing on the dynamics of hydrated water. More recently Vorobiev et al. [63] have used isotopic contrast variation to distinguish the uptake of D₂O and ethanol into GO depos­ited on a silicon wafer. They show that NR allows to simultaneously determine the unit cell volume and its chemical composition and quantify the amounts of intercalated solvent time resolved. The results allow to quantify selectivity the permeation of solvent vapors through layered