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.

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.Access and use of this website and the material on it are subject to the Terms and Conditions set forth at Photomechanical effects in azo-polymers studied by neutron reflectometry Yager, Kevin G.; Tanchak, Oleh M.; Godbout, Chris; Fritzsche, Helmut; Barrett, Christopher J. NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=eecae0d0-f627-4c85-a847-dfacdf4964b5 http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=eecae0d0-f627-4c85-a847-dfacdf4964b5 Chemistry, McGill UniVersity, Montreal, QC, Canada, H3A 2K6, and Canadian Neutron Beam Centre, National Research Council, Chalk RiVer, ON, Canada K0J 1J0 ReceiVed July 31, 2006; ReVised Manuscript ReceiVed October 5, 2006 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.

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.

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.

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 (∼80cm2) 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.