Oleh M Tanchak

McGill University, Montréal, Quebec, Canada

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Publications (8)11.37 Total impact

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    ABSTRACT: Lasers have transformed chemistry and the everyday world. Therefore, it is not surprising that undergraduate chemistry students are frequently exposed to fairly advanced laser techniques. The usual topics studied with lasers are molecular spectroscopy and chemical kinetics. Static and dynamic fluorescence experiments seem to be particularly popular. The phenomenon of phosphorescence, on the other had, has received much less attention in the undergraduate physical chemistry laboratory. A few years ago, we developed a laser experiment that introduced students to the phosphorescence quenching of the carbazole−naphthalene system under steady-state and pulse-excitation conditions. The purpose of that experiment was to demonstrate the experimental and theoretical aspects of triplet−triplet nonradiative energy transfer between two aromatic molecules. Subsequently, we modernized the experiment by introducing an inexpensive and easy-to-use Ocean Optics data acquisition system. This not only greatly simplified the experiment, but also made it more accessible to students. This article presents the upgraded version of the experiment together with suggestions for practical implementation.Keywords (Audience): Upper-Division Undergraduate
    Journal of chemical education 08/2010; · 0.82 Impact Factor
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    ABSTRACT: Neutron reflectometry was used to determine the distribution of salt ions and water in thin poly(acrylic acid) and poly(allylamine hydrochloride) polyelectrolyte multilayers assembled with and without salt. Increasing salt concentration reverses the exclusion of water from the substrate region, eventually leading to an asymmetric segregation of water near the substrate at high salt concentration. The counterions were found to localize near the substrate in films that were either assembled with salt or were exposed to salt solutions. In addition, the capping layer of the film was found to greatly influence the counterion distribution in the multilayer.
    The Journal of Chemical Physics 09/2008; 129(8):084901. · 3.16 Impact Factor
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    ABSTRACT: The water localization in thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) was investigated with neutron reflectivity in an atmosphere of controlled humidity and with bulk water. Water was found to be distributed asymmetrically within the multilayer and to localize preferentially at the polymer surface. The diffusion of water into the multilayer did not completely penetrate to the substrate, but instead there appeared to be an exclusion zone near the Si substrate. These results help to explain previous observations of anomalous water transport kinetics in weak polyelectrolyte systems.
    Langmuir 06/2006; 22(11):5137-43. · 4.19 Impact Factor
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    ABSTRACT: We describe a novel cell design intended for the study of photoactive materials using neutron reflectometry. The cell can maintain sample temperature and control of ambient atmospheric environment. Critically, the cell is built with an optical port, enabling light irradiation or light probing of the sample, simultaneous with neutron reflectivity measurements. The ability to measure neutron reflectivity with simultaneous temperature ramping and/or light illumination presents unique opportunities for measuring photoactive materials. To validate the cell design, we present preliminary results measuring the photoexpansion of thin films of azobenzene polymer.
    Review of Scientific Instruments 04/2006; · 1.60 Impact Factor
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    ABSTRACT: Abstract: Neutron reflectometry is used to study photomechanical effects in thin films of azobenzene polymer cast onto silicon substrates. A significant photoexpansion effect, up to 17%, is observed at 25 C, due to the free volume requirement of the azobenzene chromophore photoisomerization. Above a distinct crossover temperature of ~50 C, the material response is inverted and instead photocontraction effects, of more than -15%, are observed. In this case the combined photomotion and thermal mobility enables aggregation and crystallization of the azobenzene dipoles. The photomechanical effects, which can be reversed, occur readily using a variety of irradiation powers, incident polarizations, and film thickness values. This photomechanical behavior, which appears to be general to all azo materials, is likely the origin for a wide variety of curious photomotions observed in these systems, including macroscopic bending of samples and micron-scale surface mass transport.
    01/2006;
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    Oleh M Tanchak, Christopher J Barrett
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    ABSTRACT: A light-induced expansion of thin films of polymers containing azobenzene chromophores was characterized in real time by single wavelength ellipsometry. An initial expansion of the azo polymer films was found to be irreversible with an extent of relative expansion observed of 1.5-4% in films of thickness ranging from 250 to 1400 Å. A subsequent and reversible expansion was observed with repeated irradiation cycles, achieving a relative extent of expansion of 0.6-1.6%. Control over the extent of this reversible expansion was achieved by varying the pump beam power or irradiation duration, allowing these polymers to function as reversible photomechanical materials. Introduction Thin films of azobenzene (azo)-containing polymers have many potential applications as photoactive materi-als for micropatterning, 1 for reversible optical storage, 2,3 and as sensors, 4-7 based on the light-activated inter-conversion between the two geometrical isomers. The photochemical trans-to-cis isomerization and the ther-mal or photochemical back-conversion to the energeti-cally favored trans isomer are well-studied phenom-ena. 8,9 The photoinduced trans-cis isomerization gen-erally takes picoseconds to milliseconds to complete in solution, 10,11 while the thermal relaxation from cis to trans is a slower process that requires tens of seconds to many hours depending on the chromophore 12 and the nature of the polymer backbone. 13,14 Although the photochemical trans-to-cis isomerization of azobenzene has been well studied for nearly 50 years, 15 many interesting questions concerning the behavior of poly-meric azo systems remain unanswered. For example, spin-cast films of azo polymers have been found to produce surface relief gratings (SRGs) when exposed to interfering laser irradiation, 16-18 whereby the polymer moves across a surface in response to light of certain polarizations. The mechanism of the mass transport well below T g is still unresolved, but it has been suggested that the formation of SRGs is possibly due to a photoexpansion effect. 19,20 Recently, similar interest has been given to the photodeformation properties of other azo thin films, such as studies with free films of azo-polyester which have shown macroscopic deforma-tion upon irradiation with low power circularly polarized light for both amorphous or liquid crystalline poly-mers. 21 Last, Ikeda and co-workers have also demon-strated a light-induced bending of free-standing liquid-crystal network azo films, whereby light impinging on the surface causes the material to bend and curl along the polarization director, relaxing back to flat when the cis-to-trans back-isomerization is activated at a different wavelength. 22,23 Despite some similarities to previous observations of photochemically induced free volume changes in azo films measured by ellipsometry 24 or by total attenuated reflection, 25,26 no current explanation for a mechanical force at the molecular level exists for the phenomena of both SRG photoinscription and LC film curling. In addition to any fundamental insight to be gained from a detailed study of this effect, an understanding of the photoexpansion behavior of these azo polymer systems might provide development of some interesting applications based on this piezoelectric-like response, such as photoactuation or artificial muscles. In this paper, we use null ellipsometry to confirm a photomechanical response in azo polymers directly for the first time and then characterize this photomechani-cal effect for thin films poly-Disperse Red 1 acrylate (PDR1A) and copolymers of acrylic acid (AA) with varying dye content, under various irradiation condi-tions and film thicknesses. In addition to providing a thorough characterization of the photomechanical effect for development of future photomechanical materials, this work also allows insight into the previously unre-solved mechanisms of light-activated material response, such as LC film bending and the mass transport effect of SRG formation.
    Macromolecules. 01/2005;
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    ABSTRACT: We describe a variable temperature, relative humidity 0%–100% RH, and bulk liquid neutron reflectometry sample cell suitable for the study of polymeric and biomimetic materials e.g., lipid bilayers. Compared to previous reflectometry cells, one of the advantages of the present sample environment is that it can accommodate ovens capable of handling either vapor or bulk liquid hydration media. Moreover, the design of the sample cell is such that temperature gradients are minimal over a large area 80 cm 2 allowing for the nontrivial 100% RH condition to be attained. This permits the study, by neutron reflectometry, of samples that are intrinsically unstable in bulk water conditions, and is demonstrated by the lamellar repeat spacing of lipid bilayers at 100% RH being indistinguishable from those same bilayers hydrated in liquid water. © 2005 American Institute of Physics.
    Review of Scientific Instruments 01/2005; 76. · 1.60 Impact Factor
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    Oleh M Tanchak, Christopher J Barrett
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    ABSTRACT: The swelling behavior of thin polyelectrolyte multilayers assembled from poly(acrylic acid) and poly(allylamine hydrochloride) under various pH conditions was characterized in real time by in situ single wavelength ellipsometry. Both the rates and extent of swelling were found to be independent of the solution environment, yet they depended strongly on the pH conditions under which the layers were fabricated. The transport mechanism could be determined by analyzing the rate of swelling and was also observed to vary significantly with the initial assembly conditions of the multilayer films. The swelling behavior was non-Fickian for films constructed with a deposition pH of 3.5, 5.0, and 6.5 for both polyelectrolytes. The time scales to reach maximum swelling depended strongly on pH assembly conditions and varied over at least 3 orders of magnitude from seconds to tens of minutes. Swelling of the films was also observed to depend strongly on the ambient humidity to which the films were exposed before measurement; for example, when equilibrated at 45% relative humidity prior to swelling, films assembled at pH) 3.5 required ∼1800 s to reach equilibrium, while the same films reached maximum swelling in ∼3 s when pre-exposed to an environment of 20% relative humidity.
    Chemistry of Materials - CHEM MATER. 01/2004; 16(14).