The second order nonlinear susceptibility χ(2) of a novel class of poled polymer films was investigated by means of the attenuated total reflection (ATR) method. The polymers are composed of an aromatic polyester backbone to which NLO active stilbene chromophores and flexible alkyl side chains are alternately attached. The optical properties and the dynamics of relaxation of the poled state were strongly depending on the length of the aliphatic side chains. The relaxation determined at different temperatures above and below Tg showed a temperature dependence that differs from an Arrhenius type behaviour. An empirical equation which relates the temperature dependence of the relaxation times to the glass transition temperature is presented. The polyester main chains could be macroscopically aligned in strong magnetic fields. Electric poling of the samples with aligned main chains results in ratios of χ 333(2) χ 113(2) which deviate significantly from the ratio of 3 one obtains for samples with isotropically distributed main chains. This behaviour is explained by the solid state structure of this kind of polymer, which is best described by a sequence of alternating layers of polyester backbones and flexible side chains.
Novel rigid rod polymers substituted with NLO-active chromophores have been developed towards application in electrooptical signal modulation. The materials are based on rigid polyesters and polyesteramids, in which the chromophores are either covalently linked to the backbone by short flexible spacers or directly incorporated into the main chain. In the bulk these systems form macroscopically ordered structures with layers of rigid rod backbones separated by the side chain segments. The properties and stability of the induced polar order can thus be adjusted by morphological parameters like the layer distance or the orientation of the main chains relative to the substrate. The relaxation of the NLO activity with time is described by a multiexponential decay and shows enhanced time-temperature stability even above 100 0 C. The temperature dependence of the relaxation times exhibits unusual features that distinguishes these systems from conventional NLO side chain polymers. BASIC CONCEPTSThe development of optical communication systems has triggered intense research in second order nonlinear optical (NLO) polymers [1]. In the widely accepted poled polymer approach, NLO-active chromophores are either dissolved in a polymer matrix or linked to a polymer chain, and then oriented in a strong electric poling field. The polar orientational order is then frozen in by cooling the sample into the glassy state of the polymer. However, most of the current materials are based on polymers with flexible main chains. The stability of the polar order in these materials decreases tremendously when the temperature is raised near to the glass transition temperature.Recently a new type of molecular design was introduced in which NLO-active chromophores are linked to rigid rod polyesters and polyesteramids [2][3][4][5]. Rigid polymers containing aliphatic side chains have been widely studied [6][7][8][9]. When films are cast from solution, layered structures have generally been observed, in which layers of backbone chains are well separated by the flexible side chains. Structural parameters like the layer spacing depend strongly on the substitution pattern and on the length of the side chains. X-ray diffraction experiments parallel and perpendicular to the film surface suggest that the films are laterally isotropic, with the layers oriented parallel to the substrate plane, as shown in Figure la. If now some of the sidechains are replaced by NLO-active chromophores, this anistropic structure should force the chromphores to an upright orientation (Figure lb). Consequently the nonlinear optical properties should differ strongly from those observed in conventional side chain polymers based on a flexible polymer backbone. This is especially true when the chromophores are linked to the main chain by a rigid connection. In the latter case the chomophores will be only allowed to rotate around the main chain axis.
The widely accepted concept of introducing a fictive temperature (Tf) to describe the structural relaxation below the glass transition temperature is transposed to explain the orientational dynamics of nonlinear optical (NLO) active chromophores in poled polymer systems. The influence of different thermal histories on the results of differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) gives evidence for a direct connection between the structural relaxation as measured by DSC and the relaxation of polar order as measured by TSDC. With the fictive temperature being determined from the DSC experiments it is possible to relate the equilibrium dynamics of the NLO chromophores above the glass transition temperature as determined by dielectric spectroscopy to the nonequilibrium relaxation times of polar order relaxation below Tg over 12 decades in time. The obtained time‐temperature dependence for the relaxation times, based on Scherer's extension of the Adam‐Gibbs theory (AGV), shows a Vogel‐Fulcher behavior above Tg and an Arrhenius‐like behavior far below Tg. Below Tg the temperature dependence of the relaxation times of polar order is in excellent agreement with an empirically found modified Vogel‐Fulcher equation, which modeled the relaxation of polar order of different poled polymer systems in an universal manner. The AGV theory further offers a direct approach to introduce the influence of the thermal history on the time‐temperature characteristics of polar order relaxation.
Polymers composed of an aromatic polyester backbone to which NLO‐active chromophores and flexible alkyl side chains are attached have been developed as a class of second order nonlinear optical materials. Thin films suitable to be used as waveguides could be produced by spin coating technique and were investigated for their NLO‐properties. The polymers were investigated by optical microscopy, DSC, X‐ray diffraction, 2H‐NMR, dielectric relaxation spectroscopy and — for their optical properties — by the method of attenuated total reflection of visible light. The solid state structure is best described by an arrangement of alternating layers of polyester backbones and flexible side chains with good layer correlation, especially for derivatives with long side chains. The low nonlinear optical response of these derivatives seems to be due to dipole pair formation of the chromophores. Polyesters with short side chains show a similar main chain packing but substantially different optical properties. In addition, they could be macroscopically aligned in strong magnetic fields. Anisotropic samples prepared in this way indicated a restricted orientational freedom of the chromophores in the space available between the backbone layers. Consequently the ratio of the second order susceptibilities χ zzz(2) versus χ zxx(2) deviated significantly from the expected isotropic value of three.
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