[Show abstract][Hide abstract] ABSTRACT: In this paper two groups supporting different views on the mechanism of light induced polymer deformation argue about the respective underlying theoretical conceptions, in order to bring this interesting debate to the attention of the scientific community. The group of Prof. Nicolae Hurduc supports the model claiming that the cyclic isomerization of azobenzenes may cause an athermal transition of the glassy azobenzene containing polymer into a fluid state, the so-called photo-fluidization concept. This concept is quite convenient for an intuitive understanding of the deformation process as an anisotropic flow of the polymer material. The group of Prof. Svetlana Santer supports the re-orientational model where the mass-transport of the polymer material accomplished during polymer deformation is stated to be generated by the light-induced re-orientation of the azobenzene side chains and as a consequence of the polymer backbone that in turn results in local mechanical stress, which is enough to irreversibly deform an azobenzene containing material even in the glassy state. For the debate we chose three polymers differing in the glass transition temperature, 32 °C, 87 °C and 95 °C, representing extreme cases of flexible and rigid materials. Polymer film deformation occurring during irradiation with different interference patterns is recorded using a homemade set-up combining an optical part for the generation of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. We also demonstrated the unique behaviour of azobenzene containing polymeric films to switch the topography in situ and reversibly by changing the irradiation conditions. We discuss the results of reversible deformation of three polymers induced by irradiation with intensity (IIP) and polarization (PIP) interference patterns, and the light of homogeneous intensity in terms of two approaches: the re-orientational and the photo-fluidization concepts. Both agree in that the formation of opto-mechanically induced stresses is a necessary prerequisite for the process of deformation. Using this argument, the deformation process can be characterized either as a flow or mass transport.
[Show abstract][Hide abstract] ABSTRACT: We report on rendering polyelectrolyte brushes photosensitive by loading them with azobenzene-containing cationic surfactants. Planar poly(methacrylic acid) (PMAA) brushes are synthesized using the “grafting from” free-radical polymerization scheme followed by exposure to a solution of photosensitive surfactants consisting of positively-charged head groups and hydrophobic tails into which azobenzene moieties are inserted. In this study the length of the hydrophobic methylene spacer connecting the azobenzene and the charged head group ranges from 4 to 10 CH2 groups. Under irradiation with UV light, the photo-isomerization of azobenzene integrated into a surfactant results in a change in size, geometry, dipole moment and free volume of the whole molecule. When the brush loaded with photosensitive surfactants is exposed to irradiation with UV interference patterns, the topography of the brush deforms following the distribution of the light intensity, exhibiting surface relief gratings (SRG). Since SRG formation is accompanied by a local rupturing of polymer chains in areas from which the polymer material is receding, most of the polymer material is removed from the surface during treatment with good solvent, leaving behind characteristic patterns of lines or dots. The azobenzene molecules still integrated within the polymer film can be removed by washing the brush with water. The remaining nano-structured brush can then be re-used for further functionalization. Although the opto-mechanically induced rupturing occurs for all surfactants, larger species do not penetrate deep enough into the brush such that after rupturing a leftover layer of polymer material remains on the substrate. This indicates that rupturing occurs predominantly in regions of high surfactant density.
[Show abstract][Hide abstract] ABSTRACT: Surface plasmons in thin metallic ﬁlms can be excited by nm-scale defects (grooves, pits) and interfere with the incident laser ﬁeld. We observe this “plasmonic hologram” with a polymer ﬁlm containing azo-benzene chromophores that is coated on the metal. It deforms into a surface relief when exposed to plasmons. The azo-benzene units isomerize and re-orient depending on local intensity and polarization. The relief period (plasmon fringes) gives access to the dispersion relation of surface plasmons in the multilayer metal-on-substrate structure.
[Show abstract][Hide abstract] ABSTRACT: Here, we report on two photosensitive amorphous polymers showing opposite behavior upon exposure to illumination. The first polymer (PAZO) consists of linear backbone to which azobenzene-containing side chains are covalently attached, while in the second polymer (azo-PEI), the azobenzene side chains are attached ionically to a polyelectrolyte backbone. When irradiated through a mask, the PAZO goes away from the intensity maxima, leaving behind topography trenches, while the direction of the mass transport of the azo-PEI polymer points towards the intensity maxima. This kind of behavior has been reported only for certain liquid crystalline polymers that exhibit in-phase reaction on illumination, that is, topography maxima coincides with the intensity maxima. Furthermore, flat nanocrystals placed on top of azo-PEI film was found to be moved together with the mass transport of the underlying polymer film as visualized using in situ atomic force microscopy (AFM) measurements. It was also demonstrated that the two polymer films respond differently on irradiation with the polarization and intensity interference patterns (IPs). To record the kinetic of the surface relief grating formation within two polymers during irradiation with different IPs, we utilized a homemade setup combining the optical part for the generation of IP and AFM. A possible mechanism explaining different responses on the irradiation of amorphous polymers is discussed in the frame of a theoretical model proposed by Saphiannikova et al. (J. Phys. Chem. B 113, 5032–5045 (2009)).
[Show abstract][Hide abstract] ABSTRACT: Surface Relief Gratings (SRGs) are inscribed in the Au-coated azobenzene containing photosensitive polymer films on a glass substrate. The structures consist of micrometer-period sinusoidal patterns of sub-micron amplitudes, formed by photo-isomerization and molecular reorientation processes in the polymer film during exposure to the light interference pattern that drove the formation of a SRG; the precursor is a stack sequence of Au, polymer, and glass. The SRG structures were exposed in GISAXS geometry to high-intensity X-ray radiation from a liquid Ga source (0.134 nm). Scattered
photons were registered by a 2D detector, and their intensity distribution enabled us to characterize the structures. Analysis of the 2D patterns yielded information about the pitch of the gratings as well as the thickness of the films forming the gratings. The GISAXS experiments were carried out at the Research Center Juelich.
[Show abstract][Hide abstract] ABSTRACT: We report on the experimental and theoretical investigation of a considerable increase in the rate for thermal cis → trans isomerization of azobenzene-containing molecules in the presence of gold nanoparticles. Experimentally, by means of UV–vis spectroscopy, we studied a series of azobenzene-containing surfactants and 4-nitroazobenzene. We found that in the presence of gold nanoparticles the thermal lifetime of the cis isomer of the azobenzene-containing molecules was decreased by up to 3 orders of magnitude in comparison to the lifetime in solution without nanoparticles. The electron transfer between azobenzene-containing molecules and a surface of gold nanoparticles is a possible reason to promote the thermal cis → trans switching. To investigate the effect of electron attachment to, and withdrawal from, the azobenzene-containing molecules on the isomerization rate, we performed density functional theory calculations of activation energy barriers of the reaction together with Eyring’s transition state theory calculations of the rates for azobenzene derivatives with donor and acceptor groups in para position of one of the phenyl rings, as well as for one of the azobenzene-containing surfactants. We found that activation barriers are greatly lowered for azobenzene-containing molecules, both upon electron attachment and withdrawal, which leads, in turn, to a dramatic increase in the thermal isomerization rate.
No preview · Article · Jul 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: When arranged in a proper nanoaggregate architecture, gold nanoparticles can offer controllable plasmon-related absorption/scattering, yielding distinct color effects that depend critically on the relative orientation and distance between nanoparticle constituents. Herein, we report on the implementation of novel plasmonic nanoarchitectures based on complexes between gold nanoparticles and an azobenzene-modified cationic surfactant that can exhibit a light-tunable plasmonic response. The formation of such complexes becomes possible through the use of strongly negatively charged bare gold nanoparticles (∼10-nm diameter) prepared by the method of laser ablation in deionized water. Driven by electrostatic interactions, the cationic surfactant molecules attach and form a shell around the negatively charged nanoparticles, resulting in neutralization of the particle charge or even overcompensation beyond which the nanoparticles become positively charged. At low and high surfactant concentrations, Au nanoparticles are negatively and positively charged, respectively, and are represented by single species due to electric repulsion effects having absorption peaks around 523-527 nm, whereas at intermediate concentrations, the Au nanoparticles become neutral, forming nanoscale 100-nm clusterlike aggregates and exhibiting an additional absorption peak at λ > 600 nm and a visible change in the color of the solution from red to blue. Because of the presence of the photosensitive azobenzene unit in the surfactant tail that undergoes trans-to-cis isomerization under irradiation with UV light, we then demonstrate a light-controlled nanoclustering of nanoparticles, yielding a switch in the plasmonic absorption band and a related change in the solution color. The formed hybrid architectures with a light-controlled plasmonic response could be important for a variety of tasks, including biomedical, surface-enhanced Raman spectroscopy (SERS), data transmission, and storage applications.
No preview · Article · Feb 2015 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: AbstractIn this paper we report on the interaction between photosensitive azobenzene-containing polymer films and on top adsorbed graphene multilayers. The photosensitive polymer film changes its topography under irradiation with light interference patterns according to their polarization distribution. The multilayer graphene follows the deformation of the polymer film and stretches accordingly. Using confocal Raman microspectroscopy we can detect the appearance of additional peaks in the Raman spectrum of the photosensitive polymer film upon irradiation indicating a molecular interaction at the interface between the graphene multilayer and the polymer matrix. Multi-component analysis of the specific Raman bands shows that the interaction involves the graphene rings and the aromatic rings of the azobenzenes causing the strong adhesion between the two materials.
[Show abstract][Hide abstract] ABSTRACT: Realization of all-optically controlled and efficient DNA compaction is the major motivation in the study of interactions between DNA and photosensitive surfactants. In this article, using recently published approach of phase diagram construction [Y. Zakrevskyy, P. Cywinski, M. Cywinska, J. Paasche, N. Lomadze, O. Reich, H.-G. Löhmannsroben, and S. Santer, J. Chem. Phys. 140, 044907 (2014)], a strategy for substantial reduction of compaction agent concentration and simultaneous maintaining the light-induced decompaction efficiency is proposed. The role of ionic strength (NaCl concentration), as a very important environmental parameter, and surfactant structure (spacer length) on the changes of positions of phase transitions is investigated. Increase of ionic strength leads to increase of the surfactant concentration needed to compact DNA molecule. However, elongation of the spacer results to substantial reduction of this concentration. DNA compaction by surfactants with longer tails starts to take place in diluted solutions at charge ratios Z < 1 and is driven by azobenzene-aggregation compaction mechanism, which is responsible for efficient decompaction. Comparison of phase diagrams for different DNA-photosensitive surfactant systems allowed explanation and proposal of a strategy to overcome previously reported limitations of the light-induced decompaction for complexes with increasing surfactant hydrophobicity.
Full-text · Article · Oct 2014 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The contraction/swelling transition of anionic PNIPAM-co-AAA particles can be manipulated by light
using interactions with cationic azobenzene-containing surfactant. In this study the influence of pHbuffers
and their concentrations, the charge density (AAA content) in microgel particles as well as the
spacer length of the surfactant on the complex formation between the microgel and surfactant is
investigated. It is shown that the presence of pH buffer can lead to complete blocking of the interactions
in such complexes and the resulting microgel contraction/swelling response. There is a clear competition
between the buffer ions and the surfactant molecules interacting with microgel particles. When working
in pure water solutions with fixed concentration (charge density) of microgel, the contraction/swelling of
the particles is controlled only by relative concentration (charge ratio) of the surfactant and AAA groups
of the microgel. Furthermore, the particle contraction is more efficient for shorter spacer length of the
surfactant. The onset point of the contraction process is not affected by the surfactant hydrophobicity.
This work provides new insight into the interaction between microgel particles and photo-sensitive
surfactants, which offers high potential in new sensor systems.
[Show abstract][Hide abstract] ABSTRACT: When azo-modified photosensitive polymer films are irradiated with light interference patterns, topographic variations in the film develop that follow the E ⃗-field vector distribution resulting in the formation of surface relief grating (SRG). The exact correspondence of the E ⃗-field vector orientation in interference pattern in relation to the presence of local topographic minima or maxima of SRG is in general difficult to determine. Here, we report on a systematic procedure to accomplish the correlation between different interference patterns and the topography of SRG. For this, we devise a new setup combining an atomic force microscope and a two-beam interferometer (IIAFM).[1–3] With this set-up, it is possible to track the topography change in-situ, while at the same time changing polarization and phase of the impinging interference pattern. To validate our results, we have compared two photosensitive materials named in short as PAZO and trimer. This is the first time that an absolute correspondence between the local distribution of E ⃗-field vectors of interference pattern and the local topography of the relief grating could be established exhaustively. In addition, using our IIAFM we found that for a certain polarization combination of two orthogonally polarized interfering beams [namely SP (↕, ↔) interference pattern] the topography forms SRG with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We have also probed for the stresses induced during the polymer mass transport by placing an ultra-thin gold film on top (5–30 nm).[5–7] During irradiation, the metal film not only deforms along with the SRG formation, but ruptures in regular and complex manner. The morphology of the cracks differs strongly depending on the E ⃗-field distribution in the interference pattern even when the magnitude and the kinetic of the strain are kept constant. The stresses developed during the polymer deformation appear to be so strong that even multi-layered graphene (thickness-10 nm) is deformed conveniently along SRG. This implies a complex local distribution of the opto-mechanical stress along the topography grating. The neutron reflectivity measurements of the metal/polymer interface indicate the penetration of metal layer within the polymer resulting in a formation of bonding layer that confirms the transduction of light induced stresses in the polymer layer to a metal film.[9, 10]
 N. S. Yadavalli and S. Santer. J. Appl. Phys. 113, 224304 (2013)
 N. S. Yadavalli, M. Saphiannikova, N. Lomadze, L. M. Goldenberg, and S. Santer. Appl. Phys. A 113, 263 (2013)
 G. Di Florio, E. Bründermann, N. S. Yadavalli, S. Santer, and M. Havenith. Soft Matter 10, 1544 (2014)
 N. S. Yadavalli, M. Saphiannikova, and S. Santer. Appl. Phys. Lett. (2014) – accepted for publication
 N. S. Yadavalli, F. Linde, A. Kopyshev, and S. Santer ACS Appl. Mater. Interfaces 5, 7743 (2013)
 F. Linde, N. S. Yadavalli, and S. Santer. Appl. Phys. Lett. 103, 253101 (2013)
 N. S. Yadavalli and S. Santer. ACS Division of Polymeric Materials: Science & Engineering 107, 213 (2012) – ISBN: 9781622762071
 G. Di Florio, E. Bründermann, N. S. Yadavalli, S. Santer, and M. Havenith. Soft Materials (2014) – revised
 N. S. Yadavalli, J.-F. Moulin, D. Korolkov, M. Krutyeva, and S. Santer. MLZ Annual Report: Soft matter, 54 (2013) – Scientific Highlight
 N. S. Yadavalli, D. Korolkov, J-F Moulin, M. Krutyeva, and S. Santer. ACS Appl. Mater. Interfaces (2014) – revised
[Show abstract][Hide abstract] ABSTRACT: In this paper we show how graphene can be utilized as a nanoscopic probe in order to characterize local opto-mechanical forces generated within photosensitive azobenzene containing polymer films. Upon irradiation with light interference patterns, photosensitive films deform according to the spatial intensity variation, leading to the formation of periodic topographies such as surface relief gratings (SRG). The mechanical driving forces inscribing a pattern into the films are supposedly fairly large, since the deformation takes place without photo-fluidization; the polymer is in a glassy state throughout. However, until now there has been no attempt to characterise these forces by any means. The challenge here is that the forces vary locally, on a nanometer scale. Here we propose to use Raman analysis of the stretching of the graphene layer adsorbed on top of polymer film under deformation in order to probe the strength of the material transport spatially resolved. With the well-known mechanical propert
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report on differences in the response of photosensitive azobenzene containing films upon irradiation with the intensity or polarization interference patterns. Two materials are studied differing in the molecular weight: an azobenzene-containing polymer and a molecular glass formed from a much smaller molecule consisting of three connected azobenzene units. Topography changes occurring along with the changes in irradiation conditions are recorded using a homemade set-up combining an optical part for generation and shaping of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. In this way, we could reveal the unique behavior of photosensitive materials during the first few minutes of irradiation: the change in topography is initially driven by an increase in the azobenzene free volume along with the trans-cis isomerization, followed by the mass transport finally resulting in the surface relief grating. This study demonstrates the great potential of our setup to experimentally highlight puzzling processes governing the formation of surface relief gratings.
Full-text · Article · Aug 2014 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We report on the theoretical study of interaction of ionic surfactants with oppositely charged microgel particles in dilute solutions. Two approaches are proposed. Within the first approach, the micellization of the surfactants inside the microgel is taken into account while the second model focuses on the hydrophobic interactions of the surfactant tails with the hydrophobic parts of microgel subchains. It has been shown that microgels effectively absorb surfactant ions. At low surfactant concentration this absorption is realized due to an ion exchange between microgel counterions and surfactant ions. The ion exchange is significantly affected by the amount of the microgel counterions initially trapped within the microgel particles which depends on the size of the microgel, its ionization degree, cross-linking density as well as polymer concentration in the solution. Increase of the surfactant concentration causes contraction of the microgels, which can be realized as either a continuous shrinking or a jump-like collapse transition depending on the system parameters. In the collapsed state additional absorption of surfactants by microgels takes place due to an energy gain from micellization or hydrophobic interactions. This leads to microgel precipitation and successive microgel overcharging at an excess of the surfactant in the solution. The theoretical results are compared with the existing experimental data, in particular, on photosensitive surfactant/microgel complexes.
[Show abstract][Hide abstract] ABSTRACT: We use a photosensitive layer containing azobenzene moieties to map near field intensity patterns in the vicinity of nano grids fabricated within a thin silver layer. It is known that azobenzene containing films deform permanently during irradiation, following the pattern of the field intensity. The photosensitive material reacts only to stationary waves whose intensity patterns do not change in time. In this paper we have found a periodic deformation above the silver film outside the nanostructure, even if the latter consists of just one groove. This is in contradiction to the widely accepted viewpoint that propagating surface plasmon modes dominate outside nano grids. We explain our observation based on an electromagnetic hologram formed by the constructive interference between a propagating surface plasmon wave and the incident light. This hologram contains a stationary intensity and polarization grating that even appears in the absence of the polymer layer.
[Show abstract][Hide abstract] ABSTRACT: Azo-modified photosensitive polymers offer the interesting possibility to reshape bulk polymers and thin films by UV-irradiation while being in the solid glassy state. The polymer undergoes considerable mass transport under irradiation with a light interference pattern resulting in the formation of surface relief grating (SRG). The forces inscribing this SRG pattern into a thin film are hard to assess experimentally directly. In the current study, we are proposing a method to probe opto-mechanical stresses within polymer films by characterizing mechanical response of thin metal films (10 nm) deposited on the photosensitive polymer. During irradiation, the metal film not only deforms along with the SRG formation, but ruptures in regular and complex manner. The morphology of the cracks differs strongly depending on the electrical field distribution in the interference pattern even when the magnitude and the kinetic of the strain are kept constant. This implies a complex local distribution of the opto-mechanical stress along the topography grating. In addition, the neutron reflectivity measurements of the metal/polymer interface indicate the penetration of metal layer within the polymer resulting in a formation of bonding layer that confirms the transduction of light induced stresses in the polymer layer to a metal film.
[Show abstract][Hide abstract] ABSTRACT: We have used polarized confocal Raman microspectroscopy and scanning near-field optical microscopy with a resolution of 60 nm to characterize photoinscribed grating structures of azobenzene doped polymer films on a glass support. Polarized Raman microscopy allowed determining the reorientation of the chromophores as a function of the grating phase and penetration depth of the inscribing laser in three dimensions. We found periodic patterns, which are not restricted to the surface alone, but appear also well below the surface in the bulk of the material. Near-field optical microscopy with nanoscale resolution revealed lateral two-dimensional optical contrast, which is not observable by atomic force and Raman microscopy.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we investigate interactions and phase transitions in polyelectrolyte-surfactant complexes formed between a cationic azobenzene-containing surfactant and two types of polyelectrolytes: natural (DNA) or synthetic (PAA: poly acrylic acid). The construction of a phase diagram allowed distancing between four major phases: extended coil conformation, colloidally stable compacted globules, colloidal instability range, and surfactant-stabilized compact state. Investigation on the complexes' properties in different phases and under irradiation with UV light provides information about the role of the surfactant's hydrophobic trans isomers both in the formation and destruction of DNA and PAA globules as well as in their colloidal stabilization. The trans isomer shows much stronger affinity to the polyelectrolytes than the hydrophilic cis counterpart. There is no need for complete compensation of the polyelectrolyte charges to reach the complete compaction. On contrary to the findings previously reported in the literature, we demonstrate - for the first time - complete polyelectrolyte compaction which occurs already at 20% of DNA (and at 50% of PAA) charge compensation. The trans isomer plays the main role in the compaction. The aggregation between azobenzene units in the photosensitive surfactant is a driving force of this process. The decompaction can be realized during UV light irradiation and is strongly influenced by the interplay between surfactant-surfactant and surfactant-DNA interactions in the compacted globules.
Full-text · Article · Jan 2014 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Recently, photosensitive surfactants have re-attracted considerable attention. It has been shown that their association with oppositely charged biologically important polyelectrolytes, such as DNA or microgels, can be efficiently manipulated simply by light exposure. In this article, we investigate the self-assembly of photosensitive surfactants as well as their interactions with DNA by calorimetric and spectroscopic methods. Critical micelle concentration (CMC), standard micellization enthalpy, entropy, and Gibbs energy were determined in different conditions (ionic strengths and temperatures) for a series of cationic surfactants with an azobenzene group in their tail. It is shown, that aggregation forces of photosensitive units play an important role in the micellization giving the major contribution to the micellization enthalpy. The onset of the aggregation can be traced from shift of the absorption peak position in the UV-visible spectrum. Titration UV-visible spectroscopy is used as an alternative, simple, and sensitive approach to estimate CMC. The titration UV-visible spectroscopy was also employed to investigate interactions (CAC: critical aggregation concentration, precipitation, and colloidal stabilization) in the DNA-surfactant complex.
Full-text · Article · Jan 2014 · The Journal of Chemical Physics