SHG and Orientation Phenomena in Polymer Films and Organic-Inorganic Hybrid Materials
The observation of even-order NLO properties in centrosymmetric materials by application of an external electromagnetic field is due to the induction of an internal field, Eint. In inorganic glasses the formation of Eint is governed by ion migration, while in polymers and organic-inorganic hybrid materials this internal field is controlled mainly by dipole reorientation and depends on the microscopic hyperpolarizability of the material and the number of permanent dipoles per unit volume. Chromophores with large conjugated ð-systems, such as the azo-benzene disperse red 1 (DR1), are known to exhibit high hyperpolarizability.
Chromophore-containing polymers are of special interest for a variety of photonic applications because they may exhibit very efficient second-order NLO response after corona poling. However, the long-term stability of this NLO response is often low due to the limited stability inherent in such polymeric materials. For this reason, the development and exploitation of more stable polymers with NLO properties is of special significance. One solution may arise frïm organic-inorganic hybrids which combine useful properties of organic and inorganic materials like high mechanical and dimensional stability, low optical loss and good thermal stability. Experiments include the preparation of copolymer-doped silica glasses by the sol-gel technique or composites based on layered structures of differently charged ionic polymers and metallic nanoparticles (work in progress).
The investigation of structural configurations in polymers exhibiting a high corona poling-induced SHG intensity is essential for understanding the involved mechanisms. The identification of structural changes connected with the chromophore and/or the polymer matrix during poling helps to specify critical factors that govern the generation of a high NLO response, as evaluated by the strength of the SHG signal. Several methacrylate-type polymers containing disperse red 1 (DR1) were synthesized and studied. The matrix polymers differed primarily in polarity and bulkiness of their side chains (R).
Due to its conjugated ð-system, DR1 induces a strong NLO response in many polymeric and organic-inorganic hybrid matrices. This large and planar molecule may form aggregates depending on the matrix polarity, as resulted from 1H-NMR and optical spectroscopy studies.
The structure of the chromophore and of the co-polymer matrix is investigated by various techniques including 1H-NMR and vibrational spectroscopy. Thermal parameters are examined together with structural changes occurring within co-polymer films by increasing temperatures. Polarized Raman, infrared spectra and SHG measurements allow establishing the degree of order in thin films before and after thermal poling. In situ measurements of the SHG response emphasize the necessity of a tight temperature regime during poling. The observed NLO response is strong and exhibits substantial long term stability. The relative intensities of SHG signals measured in sp and pp polarized è-scans indicate a high degree of poling-induced order in the poled co-polymer films. Although poling-induced NLO responses arise mainly from the alignment of the chromophore molecules, the arrangement of the polymers side chains (R) contributes likewise to the overall order. Orientation phenomena in the polymer may be assessed by considering the tacticity of the side chains, as derived from H-NMR and infrared spectroscopy.
Solvatochromism and Aggregation of the Chromophore
The optical spectra of DR1-methacrylate type co-polymers (P1, P2, P3) revealed solvatochromism in the form of a bathochromic red shift of the maximum of the absorption band upon increasing polarity of the co-polymer. On the other hand, aggregation of chromophores is reflected in a blue shift of the maximal absorption with increasing chromophore loading. Although this effect was observed for all three concentration series of the chromophore containing polymers, it is strongest in P1 the most polar of the three polymers.
1H-NMR spectra of the aromatic protons of the chromophore provide also information on aggregation effects. The spectrum of P3 shows instead of one broad central line two separate resonances for the protons on the nitro-substituted ring. In addition, the resonance of the "d" protons next to the amino group was found strongly up-shifted in the P3 co-polymer. These effects can be explained by the interaction of the low polarity and bulky side chain of P3 with the chromophore, as this chain coils around the DR1 molecule (a). Because of the much shorter side chains this option was not observed for the more polar co-polymers P1 and P2. Nevertheless, the optical spectra suggest that local environments of lower polarity can be created in co-polymers Pi and P2 by aggregation of the chromophore molecules (b).
Tacticity of Polymer Side Chains by Infrared Spectroscopy
Infrared spectra of an unpoled co-polymer film heated to increasing temperatures. Bands providing information on tacticity are indicated in blue, while bands connected with the chromophore are marked in red. The intensity of the methacrylate-skeleton band at 1060 cm-1 is a measure of the syndiotactic side chain segments. The splitting of the asymmetric ester (C-(C=)-O) stretch between 1200 and 1300 cm-1 depends strongly on the tacticity of the side chains (see next figure for the quantitative evaluation). Decomposition of the chromophore is reflected in the disappearance of the corresponding bands in spectra measure after heating above 250 oC.
Increasing temperature leads to a significant increase in the relative content of atactic side chain segments in the polymeric film. The 1215-1315 cm-1 band envelope can be deconvoluted in three pairs of bands with maximum intensities at 1245 + 1273 cm-1 for atactic, 1257 + 1266 cm-1 for isotactic, and 1234 + 1284 cm-1 for syndiotactic side chain segments. The intensity of the infrared shoulder at 1060 cm-1 (yellow triangles, in arbitrary units) is also a measure for syndiotactic entities and it was found to decrease in parallel with the bands characteristic of syndiotactic order. The lines in the figure are drawn as guide for the eye.
SHG Signals of Corona-poled Co-polymer Films
In situ measurements during corona poling of MDR1-containing polymer films reveal an increase of the SHG signal with increasing poling time. For the sample above, the SHG intensity increases also as the poling temperature is lowered from 125 to 100 oC (these temperatures are well below Tg of the polymer). However, the SHG signal drops by more than an order in magnitude when the external electric field is switched off. Lasting SHG signals were obtained for this polymer sample at higher poling temperatures and voltages.
Normalized SHG intensity recorded as a function of the incident angle between the probe beam and the sample (?-scans). The square root of the ratio of the SHG intensities measured for pp and sp polarization allows assessing the degree of the poling-induced alignment. Any value larger than 3 indicates a high degree of alignment not only for the chromophore but also for the polymeric matrix.