Self-aggregation, H-bonding, and photoresponse in film and solution states of azobenzene containing polyurea
We critically understand the hydrogen bonding interactions and electronic transitions occurring in a thin film as well as in solution of a photo-responsive polymer, azo-polyurea (azo-PU). We synthesize azo-PU by covalent attachment of the azobenzene chromophore to the main chain of polyurea. Azo-PU shows reversible photoisomerization between trans and cis states upon light exposure, the occurrence of which is typically analysed using the π-π* and n-π* electronic transition peaks in the UV-visible absorption spectrum. We find that the π-π* and n-π* bands undergo a redshift and blueshift respectively on dissolving azo-PU in DMF solvent, resulting in a single overlapped peak in the spectrum. However, upon UV irradiation, these bands split into two independent transitions that are characteristic of azo-PU solid films. These observations are explained based on the changes in polymer-polymer and polymer-solvent interactions through hydrogen bonding and self-aggregation tendency. The experimental findings are corroborated using DFT simulations which provide useful insights into electronic orbital transitions, electron distribution, and hydrogen bonding interaction through IR vibrational modes.
Red-shifted optical absorption in films of azo-polyurea - polystyrene blends: Structural correlations and implications
We report the optical absorption characteristics in thin film coatings of a blend of two polymers: azo-polyurea (azo-PU) and polystyrene (PS). It is shown using UV–visible spectroscopy that the absorption spectrum of the resultant blend undergoes a distinct and decisive redshift towards the visible wavelength regime, accompanied by broadening and overlapping of the characteristic absorption peaks. Light exposure studies reveal that the red-shifted optical absorption induces trans to cis isomerization of the blended polymer films upon exposure to visible wavelengths, which would otherwise occur typically only by stimulus from ultraviolet wavelengths. The extent of redshift in the absorption maxima and broadening in the absorption spectrum of the polymer blend films are found to be sensitive to the amounts of the constituent polymers. The changes in spectral configuration are understood in terms of lattice structural changes that occur in azo-PU chains due to the incorporation of PS, in particular the disarrangement in hydrogen bonding. It is suggested that the modified optical absorption characteristics would endow significant advantages, such as the possibility of replacing UV with visible light in applications requiring cis → trans and trans → cis isomerization, as well as photoswitchability due to trans-cis-trans cycling.
Photochemically assisted patterning: An interfacial hydrodynamic model perspective
Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.
Light-assisted self-organization and pattern formation in thin films of azobenzene-containing polyurea
In this work, ordered patterns having nanoscale features and resolutions in the range of 5–75 μm are produced from thin films of azo-polyurea (azo-PU) using photoinduced dewetting. Azo-PU is synthesized through polyaddition reactions, and reversible isomerization between cis – trans states is demonstrated by exposure to light of wavelengths 365 and 450 nm. This photochemical response is exploited to produce patterns by selective exposure using photomasks followed by subsequent annealing to induce polymer flow and dewetting. Marangoni flow is found to be a dominant mechanism that controls morphological evolution of patterns. The patterns can be switched between pillar-like and hole-like morphologies by stimulating the forward or backward isomerization reaction, and pattern resolution can be controlled using photomasks of different feature sizes. Preferential adsorption of proteins on these patterns is proposed as a potential application.