Optical anisotropy and non-linear optical properties of azobenzene methacrylic polymers F.J. Rodrı ´guez a , C. Sa ´nchez a , B. Villacampa a, * , R. Alcala ´ a , R. Cases a , M. Millaruelo b , L. Oriol b a Departamento de Fı ´sica de la Materia Condensada, Instituto de Ciencia de Materiales de Arago ´n, Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain b Departamento de Quı ´mica Orga ´ nica, Instituto de Ciencia de Materiales de Arago ´n, Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain Received 18 August 2003; received in revised form 9 January 2004; accepted 29 January 2004 Abstract Photoinduced optical anisotropy has been studied in several methacrylic copolymers, either amorphous or liquid crystalline, with cyano azobenzene chromophores in the side chain. Illumination with linearly polarised 488 nm light gives rise to high and stable values of birefringence and dichroism, mainly in liquid crystalline films. Both dichroism and birefringence decrease with the azo content in the copolymer series while some increase can be achieved by the incorporation of biphenyl molecules to the copolymers. Photoinduced anisotropy disappears above the glass transition temperature in amorphous polymers, whereas it increases in liquid crystalline polymers (LCP) due to a thermotropic self-organisation. The non-linear optical (NLO) properties of the films have been studied by second harmonic generation (SHG) measurements. The intensity of the harmonic signal from in situ corona poled polymeric films has been measured. The effect of 488 nm light irradiation on the azo chromophores orientation and consequently on the NLO response of the films was investigated at different temperatures. The intensity and thermal stability of the second harmonic signals obtained after the two different (thermal and photoassisted) corona poling processes have been compared. q 2004 Published by Elsevier Ltd. Keywords: Azobenzene; Photoinduced optical anisotropy; Non-linear optics 1. Introduction Photoinduced optical anisotropy in azobenzene films has been extensively studied [1–16]. In most of the reported works, anisotropy has been induced by illumination with linearly polarised 488 nm light from an Ar þ laser. Under this illumination, the orientation of the azo units changes through trans–cis–trans isomerization processes. A pre- ferential orientation of the trans molecules, with its axis perpendicular to the polarisation direction of the exciting light, can be achieved. High and stable values of the optical anisotropy, which can be checked by dichroism and birefringence measure- ments, have been obtained mainly in films of liquid crystalline polymers (LCP) with the azo units in the side chain. The strong molecular interactions among the azo molecules in LCP seems to enhance the photoinduced anisotropy and to improve its stability. However, due to the presence of liquid crystalline domains with different orientations these films usually show a high light scattering that is detrimental for many applications [17]. Trying to keep strong molecular interactions while decreasing light scattering, we have prepared a series of copolymers with different contents of a non-mesogenic methacrylic unit and a promesogenic azobenzene one. This allows us to range from liquid crystalline to amorphous materials. We report in this paper the results of dichroism and birefringence measurements performed on these copolymers after illumi- nation with 488 nm light. On the other hand, azopolymer films with donor and acceptor substitutions in the azobenzene moieties can be used for second order non-linear optical (NLO) applications [18,19]. To have a second order response in a film, a polar orientation of the molecules has to be induced. This can be achieved by several procedures. In particular, a corona 0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymer.2004.01.069 Polymer 45 (2004) 2341–2348 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 34-976-761333; fax: þ 34-976-761229. E-mail address: bvillaca@unizar.es (B. Villacampa).