Evidence for the applicability of a novel procedure (swelling–poling–deswelling) to produce a stable alignment of second order NLO-chromophores covalently attached to a cross-linked PMMA or polystyrene polymeric network Daniele Marinotto a , Séverine Proutière b , Claudia Dragonetti a, ⁎, Alessia Colombo a , Paolo Ferruti b , Danilo Pedron c , Maria Chiara Ubaldi d , Silvia Pietralunga d a Dip. di Chimica Inorganica Metallorganica e Analitica “Lamberto Malatesta” and Centro di Eccellenza CIMAINA dell'Università degli Studi di Milano and UdR dell'INSTM, V. Venezian 21, 20133 Milano, Italy b Dip. di Chimica Organica e Industriale and Centro di Eccellenza CIMAINA dell'Università degli Studi di Milano, via Venezian 21, 20133 Milano, Italy c Dip. di Scienze Chimiche dell'Università di Padova and UdR INSTM di Padova, Via Marzolo 1, 35131 Padova, Italy d Politecnico di Milano, Dip. Elettronica e Informazione, Lab. Policom, via G. Colombo, 81, 20133 Milano, Italy abstract article info Article history: Received 21 September 2010 Received in revised form 8 February 2011 Available online 8 March 2011 Keywords: Electrical poling; In situ-second harmonic generation; Nonlinear optics; Polymethylmethacrylate; Polystyrene The “swelling–poling–deswelling” technique is a new procedure of poling crosslinked polymeric network carrying covalently attached NLO (nonlinear optical) chromophores. It is based upon a solvent-swollen crosslinked polymeric network before submission to poling. Under electrical poling, the matrix is deswelled without heating above the polymer Tg (glass transition temperature) obtaining a significant improvement of the stabilization of the alignment of the chromophores and therefore of the SHG (second harmonic generation). We determined the d 33 values of DR1 chromophore linked in different manner to PMMA (polymethylmethacrylate) and polystyrene. Crosslinked PMMA gives very good results, in fact the 40% of the d 33 value remains after 4 months respect to d 33 evaluated a couple of hours after poling. Interestingly the same stability in the time of the d 33 is observed with the crosslinked polystyrene matrix. In terms of d 33 after poling, the two systems carrying DR1 (Disperse Red 1) moieties covalently attached to the polystyrene matrix (side-chain and crosslinked) behave in a similar manner, but in terms of stability, the linear polymeric system is the best (75%), higher than all systems investigated. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In the last two decades much attention has been focused on the development of organic and organometallic second order nonlinear optical (NLO) hybrid materials with significant and long lasting Second Harmonic Generation (SHG), in which the chromophore is stably aligned [1–5]. So far, the interest of researchers has concen- trated upon the study of the NLO chromophore/polymer material in order to achieve large electro-optic coefficients, high time stability and a good optical quality, especially for applications. The most common methods for generating noncentrosymmetry of dipolar molecules include electric field poling of polymers, utilization of liquid crystalline order, self-assembly, or Langmuir–Blodgett film formation [6,7]. Among these many methods, by far the most convenient and one of the most successful, is that of electric field poling [8,9]. Electric field poling is performed in electrode contact poling or corona-poling [8,9]. Electrode contact poling requires high quality polymers films without any defects to avoid dielectric breakdown. Corona poling, the other commonly used poling method, has the advantage that extremely high corona fields can be applied compared to that for contact electrode poling and the quality of the thin film is not a critical issue. Several efficient methodologies were suggested to minimize the reorientation process: the chromophore is usually physically dispersed in a host polymer, or covalently bonded to the polymer as a side chain, or incorporated into the main chain of the polymer [10–13]. To collect a SHG signal I(2ω), the traditional technique is to apply an electrical poling to induce chromophores' orientation above the glass transition temperature, Tg. However, this poling technique suffers from various disadvantages [14,15]. For example, the mobility of the NLO chromo- phores is not completely inhibited even much below the Tg of the polymer, leading to an easy loss of alignment of NLO chromophores after removal of the poling [16,17]. Moreover, physically dispersed NLO chromophores may undergo phase separation, aggregation, crystalliza- tion, or leaching. More recently, in order to improve the stability of the SHG response, crosslinked polymers have been considered by performing the cross- linking under poling [9]. In the crosslinked system the chromophores Journal of Non-Crystalline Solids 357 (2011) 2075–2080 ⁎ Corresponding author. Tel.: +39 02 50314358; fax: +39 02 50314405. E-mail address: claudia.dragonetti@unimi.it (C. Dragonetti). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.02.031 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol