High glass transition chromophore functionalised polyimides for second-order nonlinear optical applications K. Van den Broeck a , T. Verbiest b , J. Degryse a , M. Van Beylen a , A. Persoons b , C. Samyn a, * a Laboratory of Macromolecular and Physical Organic Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium b Laboratory of Chemical and Biological Dynamics and Center for Research on Molecular Electronics and Photonics, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Received 19 September 2000; received in revised form 12 October 2000; accepted 18 October 2000 Abstract Nonlinear optical (NLO) polymers with glass temperatures (T g ) as high as 2918C were prepared, PI 2-4 by covalent bonding of a chromophore to the backbone of hydroxyl polyimides via a Mitsunobu reaction, PI 1 as a donor-embedded polyimide was synthesised by polycondensation of a diaminochromophore and 4,4 0 -(hexa¯uoroisopropylidene) diphthalic anhydride in a two-step reaction. Poled ®lms of the polymers were measured for their second-harmonic generation (SHG) effect and showed d 33 values up to 20 pm/V. Some of the ®lms show a stable nonlinear optical response even to 90% of remaining NLO-intensity at 1258C during 200 h. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polyimides; Nonlinear optics; Stability 1. Introduction Synthesis and characterisation of organic materials for the use in second-order nonlinear optical (NLO) appli- cations (e.g. second harmonic generation and electrooptic devices) are currently receiving considerable attention [1]. Particularly side-chain polymers have drawn great interest, but the relaxation of the aligned dipoles at high tempera- ture is a major drawback to be overcome. Poled order stability at high temperature is crucial and can be achieved in crosslinked systems [2,3] or by the design of high glass-transition temperature (T g ) polymers like polyimides [4±10] or maleimide-based polymers [11±14]. In this paper, we report the synthesis and nonlinear optical behaviour of a `donor embedded' side chain polyimide and three side-chain functionalised polyimides. The stability of the nonlinear response was monitored at 1258. Some of the systems investigated show excellent thermal stability. 2. Experimental part 2.1. Materials and instrumentation All reagents were purchased from Aldrich Chemical Co. and Acros Organics. Reagent grade solvents were dried when necessary and puri®ed by distillation. The glass transition and decomposition temperatures were measured with a DSC-7 apparatus from Perkin± Elmer with a heating rate of 208C/min; typically the second run was taken for measuring the T g . The decomposition temperature was estimated as the intercept of the leading edge of the thermal decomposition peak by the baseline of each DSC scan. Gel permeation chromatography (GPC) measurements were done with a Waters apparatus with a tunable absorb- ance detector and a differential refractometer, in tetrahydro- furan (THF) as eluent towards polystyrene standards. 1 H nuclear magnetic resonance (NMR) measurements were done with a Bruker 250 MHz and a Bruker 400 MHz apparatus. 2.2. Second harmonic generation measurements Thin ®lms were obtained by spincoating a solution of the chromophore functionalised poly(imide) polymers in cyclo- hexanone onto ITO substrates. The spincoated ®lms were carefully dried under vacuum for at least 48 h at a tempera- ture about 108C below the boiling point of the spincoating solvent. Subsequently they were corona-poled at a tempera- ture approximately 108C below T g . The corona-poling set-up consisted of a thin wire, positioned 1 cm above the Polymer 42 (2001) 3315±3322 0032-3861/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00761-8 www.elsevier.nl/locate/polymer * Corresponding author. Tel.: 132-16-327438; fax: 132-16-327990. E-mail address: celest.samyn@chem.kuleuven.ac.be (C. Samyn).