Molecular ordering of PAH/MA-co-DR13 azopolymer layer-by-layer films probed by second-harmonic generation

Abstract

Molecular orientation within azopolymer thin films is important for their nonlinear optical properties and photonic applications. We have used optical second-harmonic generation (SHG) to study the molecular orientation of Layer-by-Layer (LbL) films of a cationic polyelectrolyte (poly(allylamine hydrochloride)) and an anionic polyelectrolyte containing azochromophore side groups (MA-co-DR13) on a glass substrate. The SHG measurements indicate that there is a preferential orientation of the azochromophores in the film, leading to a significant optical nonlinearity. However, both the signal strength and its anisotropy are not homogeneous throughout the sample, indicating the presence of large orientational domains. This is corroborated with Brewster angle microscopy. The average SHG signal does not increase with film thickness, in contrast to some reports in the literature, indicating an independent orientational order for successive bilayers. Analyzing the SHG signal as a function of the input and output polarizations, a few parameters of the azochromophore orientational distribution can be deduced. Fitting the SHG signal to a simple model distribution, we have concluded that the chromophores have an angular distribution with a slight in-plane anisotropy and a mean polar angle ranging from 45° to 80° with respect to substrate normal direction, with a relatively large width of about 25°. These results show that SHG is a powerful technique for a detailed investigation of the molecular orientation in azopolymer LbL films, allowing a deeper understanding of their self-assembling mechanism and nonlinear optical properties. The inhomogeneity and anisotropy of these films may have important consequences for their applications in nonlinear optical devices.

Ultrahigh Ion-Selective and Durable Nafion-NdZr Composite Layer Membranes for All-Vanadium Redox Flow Batteries

“…Positively charged PDDA is first self-assembled onto the surface of the membrane by electrostatic attraction as recast Nafion and Nafion-NdZr composite membranes exhibit −SO 3 – ; after that, negatively charged PSS is self-assembled onto the surface of the PDDA monolayer-modified metal oxide membrane. The simplicity and efficiency of this technique has led to a wide range of applications of multilayer films for sensors, nonlinear optics, photoactive films, and drug delivery . The novelty of this work is the introduction of both Nd 2 Zr 2 O 7 fillers and self-assembly of the [P-S] layer on a recast thin membrane.…”

Probing the Molecular Ordering and Thermal Stability of Azopolymer Layer-by-Layer Films by Second-Harmonic Generation

“…It is known that for samples with in-plane isotropic ordering and net polar alignment along the surface normal the SP polarization combination is allowed and the SHG signal is usually intense, while that for SS polarization combination is only nonvanishing if the sample is anisotropic along the surface plane. 27,36 Absence of signal for the SS polarization is indicative of an in-plane isotropy at a length scale smaller than the SHG wavelength (∼0.5 μm).…”

“…13 The opposite was observed for LbL films of another azopolymer, PAH/Ma-co-DR13, which were dried by nitrogen flow, and the presence of orientational domains and inhomogeneity were found by SHG and BAM (Brewster angle microscopy) measurements. 27,42 For investigating if the anisotropy either is a characteristic of that particular azopolymer or is linked to the fabrication procedure, we performed SHG measurement for two different (PAH/PS-119) 10 films fabricated at same pH 10.0, but one with nitrogen flow drying and another by spontaneous drying for 40 h. As displayed in Figure 3b, nitrogen flow drying generates an in-plane preferential ordering, but not exactly along the dipping direction (Ω = 0). On the contrary, spontaneous drying does not generate any anisotropy (and results in a weak net polar ordering; see discussion below) for the (PAH/PS-119) 10 film.…”

“…14 This drying by slow water evaporations process yields more uniform films and better molecular complexation by electrostatic interaction, 13 in contrast to the nitrogen drying process. 27 Figure 1a shows the chemical structures of the polyelectrolytes used in this work. Figure 1b represents the absorbance of films at 475 nm (near the maximum of the absorption spectrum, shown in the inset) as a function of the number of bilayers, for different pH values used in the fabrication.…”

“…This suggests that the films fabricated at this pH value have acquired a preferential orientation along the plane of the substrate upon a heating/cooling cycle, probably forming macroscopic domains (approximately hundreds of micrometers) with in-plane oriented chromophores, as we have recently reported for another azopolymer LbL film. 27 It is therefore likely that films fabricated at pH 10 have larger mobility than those produced at pH 3.5 or 7.0 due to their reduced electrostatic interaction between the polyelectrolytes, which in turn allows the rearrangement of the chains to form these anisotropic domains. Indeed, this is hypothesis is in agreement with a previous study of the thermal stability of the photoinduced birefringence of azopolymer LbL films, which has shown that films fabricated at higher pH have increased thermal stability of the birefringence.…”