Published: April 14, 2011 r2011 American Chemical Society 6330 dx.doi.org/10.1021/la200422q | Langmuir 2011, 27, 6330–6337 ARTICLE pubs.acs.org/Langmuir Cellulose-Based Liquid Crystalline Photoresponsive Films with Tunable Surface Wettability L. F. V. Pinto, † S. Kundu, † P. Brogueira, ‡,§ C. Cruz, ‡,^ S. N. Fernandes, † A. Aluculesei, ‡,^ and M. H. Godinho* ,† † CENIMAT/I3N, Departamento de Ci ^ encia dos Materiais, Faculdade de Ci ^ encias e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ‡ Departamento de Física, IST-TU-Lisbon, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal § ICEMS, IST-TU-Lisbon, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal ^ CFMC-UL, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal ’ INTRODUCTION The search for molecules having the ability to perform conformational switching upon the action of specific external stimuli has been an active topic of investigation for decades. Examples of those external stimuli are mechanical stress or optical signals of particular wavelength, among others. These characteristics are especially interesting when applied to surfaces in order to create interfacial tunable systems. Along with the engineering of the surface morphology, the wetting properties are among one of the most important interfacial features. 1,2 The control of the surface wettability has been seen as an attractive technique due to its application in industry. 36 The employment of the cis-to-trans isomerization is one of the most useful methods to control surface structure and physicochemical prop- erties. Both azobenzene 7,8 and stilbene 9 derivatives can be reversibly switched between trans and cis forms using light irradiation. An important aspect of using this method is the fact that no permanent change in chemical structure is produced. Several works have been done on the wettability of low molecular weight molecules containing photoresponsive chro- mophores and polymers containing azobenzene either in back- bone or as a side chain group. 1015 Although the roughness of the surface has a great influence on the wettability, the change in dipole moment in these two isomeric states is another important factor which also causes the change in this physical property. Cellulose is the main constituent of plant cells and the mostly available polymer in nature. It is made up of β-D-glucopyranose, each of which has one primary and two secondary hydroxyl groups that can undergo esterification and etherification among other chemical reactions. Introduction of substituents can make cellulose soluble in water due to the destruction of crystalline regions, which arise from the inter- and intra-hydrogen bonding among hydroxyl groups. It is very well-known that hydroxypropylcellulose (HPC) can generate aqueous liquid crystalline solutions, and some HPC esters were found to originate thermotropic and lyotropic phases. 1618 Thermotropic cholesteric cellulose esters were referred to in the literature to undergo a cholestericnematic transition upon shearing. 19 Besides anisotropic mechanical prop- erties, as seen in other liquid crystal (LC) polymeric materials, the cholesteric cellulose esters also present some remarkable optical characteristics. 20 It has been reported in systematic studies 2123 that isotropic solutions of cellulose and cellulose derivatives were successfully electrospun into fibers using different cellulose derivatives and different electrospinning parameters, where “electrospinning” or Received: February 1, 2011 Revised: April 5, 2011 ABSTRACT: We report on a new type of liquid crystalline cellulosic films with light controllable reversible wettability. The films are prepared from a thermotropic cellulose derivative functionalized with azo-containing groups. These groups exhibit dynamic changes in interfacial properties in response to UV irradiation. The UV irradiation induces trans-to-cis isomerization in the azobenzene moiety, which causes a conformational change in the upper molecular layers of the thin films. These changes originate a hydrophobic to comparatively hydrophilic transformation of the surface. The reversible wettability of the surface results from the cis/trans photo and thermal isomerization. The UVvis absorption spectra, as well as contact angle measurements with UV irradiation, clearly support the understanding of the phenomenon. This type of surface design enables the amplification of molecular level conformational transi- tions to macroscopic changes in interface properties using the means of isomerism. This opens new opportunities in surface engineering using eco-friendly cellulose manipulation.