Abstract

Polarized UV light induced optical anisotropy (so called Weigert effect) of “protonated “ azobenzene-containing amino acid Schiff base copper(II) complexes and “anionic” [Ni(dmit)₂]- salts in polymethyl methacrylate (PMMA) films Which were also investigated by measuring polarized UV-Vis spectroscopy. Since the exchange of counter (long-chain alkyl pyridinium) cation of planar [Ni(dmit)₂]- complex anion is easy, it is suitable for the study of transmission of optical anisotropy via Weigert effect between cationic and anionic complexes, which was observed for the first time to our knowledge.

Keywords:Optical Anisotropy; Weigert Effect; Azobenzene; Schiff Base; Copper(II); Nickel(II); PMMA

Introduction

Measurement of viscoelastic properties in polymers, across a wide range of deformations, is commonly performed using a Rubber Process Analyzer (RPA). This strain-controlled rheometer is specifically designed to provide reliable viscoelastic data under varying strain, frequency, and temperature conditions in a simple, repeatable, and reproducible manner. The RPA operates by subjecting polymer samples, in their molten state, to cyclic sinusoidal shear deformation within a biconical, closed, and sealed die cavity. During testing, the lower die oscillates under controlled strain and frequency conditions, while the upper die remains stationary. The shear stress generated by the sample against the upper die is recorded by a torque transducer, which measures stress over time during each oscillation cycle.

To comprehensively analyse viscoelastic behaviour, the stress signal over time can be decomposed into its characteristic frequency spectrum using Fourier series analysis. This approach is particularly effective for studying uncured polymer compounds under deformation, as it provides a detailed representation of the stress response. In the linear viscoelastic regime, the relationship between strain and stress is proportional, allowing for the unambiguous determination of the elastic component, known as the storage modulus (G′), and the viscous component, known as the loss modulus (G′′). While the linear viscoelastic behaviour of polymers is well understood, significant research has also been conducted to characterize their nonlinear behaviour under large amplitude oscillatory shear (LAOS) deformation. [1] LAOS testing, often used to study filled rubber systems, has been extensively applied to analyse the nonlinear viscoelastic response of polymers. Previous studies [2–5] have primarily focused on quantifying the nonlinear response by examining the ratio of the third harmonic contribution to the first harmonic. Additionally, harmonic analysis has been applied to silica-filled rubbers, providing insights into the complex interactions between fillers and the polymer matrix [2].

This paper aims to advance the understanding of polymer behaviour under cyclic sinusoidal shear deformation, regardless of the applied strain level. By adopting a unified approach, this study ensures that the analysis remains applicable across a wide range of deformation conditions, capturing the full spectrum of viscoelastic responses. A detailed mathematical framework, based on the theory of generalized viscoelasticity [6], is presented to provide a rheological fingerprint for uncured polymer compounds. Beyond the theoretical analysis, this work demonstrates the practical application of the approach to various scenarios, including the viscoelastic behaviour of linear and branched polymers. These analyses reveal differences in molecular architecture among commercial rubber grades and enable the quantitative evaluation of filler-rubber interactions in diverse compound formulations. By bridging the gap between theoretical models and real-world applications, this study contributes valuable insights to both academic research and industrial practice. The findings presented here aim to enhance the understanding of polymer viscoelasticity, offering a robust framework for characterizing complex materials and optimizing their performance in practical applications.

Introduction

Azobenzene is known to be a molecule that undergoes cis-trans photo-isomerization by light. In particular, azo compounds present in polymer films are known to undergo molecular reori­entation upon irradiation with linearly polarized light. Many stud­ies on azopolymers have been reported, and the induction of an­isotropy has attracted great interest in the fields of liquid crystals and the like.

More than a decade, we have investigated the evaluation of molecular orientation by irradiation of linearly polarized light on polymethyl methacrylate (PMMA) films compoused of azoben­zene-containing compounds and metal complexes [1,2] (or azo-li­gand metal complexes solely included biopolymers (proteins) for other functions [3]). The molecular orientation of azobenzene in polymer films by irradiation of linearly polarized light is called Weigert effect [4]. Due to the interaction between the transition di­pole moment of azobenzene (in the same direction as the long axis of the molecule) and the electric field vector of linearly polarized light, trans azobenzene in the polymer film selectively absorbs light in the axial direction and photoisomerizes to cis isomer.

We have synthesized new metal complexes and clarified the structure-physics correlation. In recent years, we have focused on organic-inorganic composite materials that combine metal complexes with other materials (magnetic materials, catalysts, pho­to-functional materials, nanoparticles, polymers, biomolecules). We applied the Weigert effect to metal complexes, aiming to de­velop new functions and physical properties through molecular arrangement. Specifically, we prepared thin films of metal com­plexes and azobenzene in PMMA and evaluated the reorientation.

However, the interaction between metal complexes and azo­benzene in polymer films remains unclear because it is difficult to investigate the molecular orientation of the complexes alone by spectroscopic measurements. Therefore, for the first time, we conducted this study to introduce cationic and anionic complexes, which are expected to have stronger Coulombic attraction, into PMMA films, and to confirm whether the results of the direct pho­to-orientation of azo complexes are transmitted to other indirect complexes, as in the case of neutral molecules.

A 5×10_-3 mmol solution of [Ni(dmit)₂]- anion [5] and proton­ated azobenzene+arginine Schiff base Cu(II) complex or protonat­ed azobenzene+lysine Schiff base Cu(II) complex [6] (Figure 1) in acetone was mixed with a 10 wt% PMMA solution in acetone. The mixture was then dropped onto a glass slide and heated at 323 K for 30 min in a hot stirrer.

The optical state of the film was measured using polarized UV-Vis spectroscopy, and the results were shown in a circular dia­gram to confirm the presence or absence of optical anisotropy of the film (Figures 2 and 3). The UV light source equipped with a po­larizing filter was used. The measurement results were evaluated based on the following R and S values:

Here, A₀ and A₉₀ are the absorbance parallel to the polariza­tion plane of the irradiated light and the absorbance perpendicu­lar to it, respectively. The further the R parameter is from 1, and the further the S parameter is from 0, the greater the anisotropy (Table 1). For comparison, similar results for PMMA film contain­ing sigle component of azobenzene+arginine/lysine Schiff base Cu(II) complex (without [Ni(dmit)₂]- salt) have been reported elsewhere [6].

(Figure 4-6) depict DFT optimized structure and electrostat­ic potential surface of anion of [Ni(dmit)₂]- (Figure 4) and cat­ions of trans- (left) and cis- (right) forms of protonated azoben­zene+arginine or lysine Schiff base Cu(II) complexes (Figures 5 & 6), respectively. The planar anions have a centrosymmetric charge distribution. On the other hand, the two chiral cations have a pos­itive charge localized on the protonated ammonium group and a relatively negative charge on the azo group. The dipole moment along the long molecular axis is small for the cis-form azo group with a short molecular length. The calculation results show that optical anisotropy occurs, in which the electric vector of polarized ultraviolet light is aligned perpendicular to the long molecular axis, and that the planar anions are also oriented parallel, which is rational in terms of both shape and charge.

We have also attempted DFT calculations for excited states, namely simulated UV-vis spectra with transition dipole moment for both components of ionic metal complexes. Straight forwardly, predominant peaks (mainly in the UV region) should be attribut­ed to π-π* transitions in p-conjugated organic ligand moieties for each one. Overall optical anisotropy observed experimentally was caused by these strong absorption (resulting from selection rule and oscillator strength of electric dipole transitions) and changing molecular orientation of planar moieties of molecule (anion) or azo-group (cations) due to Weigert effect. It should have shown that there was an intermolecular interaction in the PMMA film, which leads to an increasingly higher optical anisotropy (of the absorption bands reproduced in the calculations) of both compo­nents.

However, reliable simulation and deep argument such as the specific features of this interaction in ion pairs (like a donor-ac­ceptor in charge transfer complexes), the appearance of new ab­sorption bands (like a Kasha’s rule for J- or H-aggregates) due to the interaction (like a transition dipole-dipole interaction of exci­tons depending on mutual distances and angles) in PMMA matrix, and the information that helps in the separation of the spectra (intensity ratios and transition dipole moments of the substituent units) were beyond the limits of the method of conventional DFT computation. It is also interesting to know how the cationic and anionic complexes are dispersed within the PMMA chains while interacting with each other electrostatically.

Conclusion

In this way, we could indicate transmission of optical anisotropy via Weigert effect between cationic and anionic complexes under the condition that Coulomb force is acting. Which was observed for the first time to our knowledge. The π-π* transition of [Ni(dmit)₂]- had a peak at 390 nm. By incorporating rotonated azobenzene+arginine/lysine Schiff base Cu(II) complexes, which have optical anisotropy, into a PMMA film, an increase in optical anisotropy was confirmed at the peak of the π-π* transition of [Ni(dmit)₂]-. Specifically, for Ni(dmit)₂]- and protonated azobenzene+arginine Schiff base Cu (II) complex, the R values at 388 nm were ini: 0.9374, UV: 0.9203, and Vis: 0.9278, and the optical anisotropy increased after UV irradiation, and decreased after visible light irradiation. For [Ni(dmit)₂]- and protonated azobenzene+lysine Schiff base Cu(II) complex, on the other hand, the R values at 382 nm were ini: 0.9377, UV: 0.9121, and Vis: 0.9311, and the optical anisotropy increased after UV irradiation, similar to the Arg complex, and decreased after visible light irradiation.

References

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