Electric-Field-Induced Layer-by-Layer Fabrication of Second-Order Nonlinear Optical Films with High Thermal Stability

Abstract

A promising methodology for fabricating highly thermally stable organic second-order nonlinear optical (NLO) thin films containing low molecular weight chromophoric (LMWC) molecules was sustained by electric-field-induced layer-by-layer assembly. The new LMWC molecule but-2-enedioic acid mono- [2-({4-[4-(2-carboxy-2-cyano-vinyl)-phenylazo]-phenyl}-methyl-amino)-ethyl] ester (BCPE) was first designed and synthesized successfully, which possesses two negative groups at both ends and still retains the molecular polarity after ionization. Under a dc electric field, the density of the oriented chromophores BCPE deposited on the substrate is increased largely, and upon UV irradiation, accompanied with the transition of the ionic bonds between the layers to covalent bonds, the resulting NLO films possess excellent environmental stability and high thermal stability (the resulting films can retain 92% of the polar order up to 200°C), which may rival the properties of the latest poled polymer films for electro-optic devices.Recently, various novel and high-performance secondorder nonlinear optical (NLO) organic materials, owing to their wide application in electro-optical (E-O) modulated devices and advantages over inorganic materials, have been prepared and investigated by numerous research groups. 1 Compared with poled polymer materials which have been developed widely and remarkably in the past 20 years, 2 organic NLO film materials fabricated using low molecular weight chromophoric (LMWC) molecules have attracted less attention. Using LMWC molecules, we can design and prepare more kinds of NLO films purposely and further explore the relationship between the structures and the properties. 3 As we know, highly ordered arrangements of LMWC molecules can be obtained by Langmuir-Blodgett (LB) techniques and LMWC molecules can be ordered by self-assembly, through selective ionic interaction, hydrogen bonds, complexation, and so forth, with the layer-by-layer (LbL) technique. 4 However, LB film materials are restricted in the application of nonlinear optics because of their poor mechanical stability. In turn, when the LbL technique is used for LMWC molecule organization, the self-ordered degree, density, and stability of LMWC molecules in LbL films still need to be improved effectively due to the weak driving force for assembly and instable molecular interaction between layers. 5 Accordingly, it is hitherto still considered to be a challenge for researchers to find a constructive combination of LMWC molecules and polyelectrolytes that could be used to improve the deposition density of LMWC molecules and construct stable NLO films with high performance for E-O devices. 6 In this study, the authors clarify a new strategy that first introduces the electric-field-induced LbL assembly technique to the preparation of stable NLO multilayer films. Using this method, a new LMWC molecule (see the Supporting Information) and the polyelectrolyte diazoresin Koetse, M.; Laschewsky, A.; Wischerhoff, E.; Jullien, L.; Persoons, A.; Verbiest, ...

Thermal effect of polymer layers deposition on polypropylene nonwoven fabric

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