Calculation of the intensity and dichroism of the IR absorption bands of oriented polymers

Abstract

An algorithm for calculating the intensity and dichroism of polarized IR absorption bands of oriented polymers is proposed. The absorption curves in the IR spectrum of amorphous cis-1,4-polyisoprene and the dichroism of its absorption bands have been calculated. The polarization IR spectra of oriented amorphous cis-1,4-polyisoprene for a stretching ratio equal to 3 have been constructed. The calculated values of the absorption-band dichroism are in fairly good agreement with the experimental data.Calculation of the Band Intensity in the IR Spectrum of Amorphous cis-1,4-Polyisoprene. In the models taken for the calculation, the geometric parameters (except for the angles of rotation) of the recurring link of the chain are identical with the geometric parameters of the recurring link of the chain of crystalline cis-1,4-polyisoprene. One conformation differs from another only in the angles of internal rotation in the recurring link. The ordered polymer conformation is considered to be given if the conformation of its recurring link is given [12].In making calculations, it is assumed that the force constants, electrooptical parameters, and absorption-band half-widths are independent of the chain conformation and are the same for all conformations. The values of the force constants and electrooptical parameters are based on the data for the molecules of 3-methyl-1,5-heptadiene, 1,4-pentadiene, and 3,2,5-dimethyl-1,5-hexadiene and were taken from [13]. They were corrected so that the experimental frequencies are better reproduced. The band half-widths were taken from [1]. Because of the large volume of computations, the calculation was performed in the zeroth approximation.The vibration frequencies and the band intensities of the IR spectrum of amorphous cis-1,4-polyisoprene were calculated by the method of [1, 2]. The intensity and dichroism of the polarized absorption bands were computed by programs written on the basis of the algorithm described above. In constructing the polarization IR spectra, we used the modified program whose algorithm is given in [14].Results and Discussion. The polarization IR spectra of amorphous cis-1,4-polyisoprene constructed on the basis of the calculation are shown in Fig. 1. Table 2 gives a comparison of the calculated and the experimental values of the absorption-band dichroism. It is seen that the calculated values of the dichroism of the absorption bands are, in general, comparable to the experimental ones. However, there are absorption bands whose dichroism disagrees with the calculation even qualitatively. For example, the band at 742 cm -1 , according to the experimental results, is polarized in parallel, whereas its respective calculated band at 746 cm -1 is perpendicularly polarized. This inconsistency may be due to the fact that the chosen model of amorphous cis-1,4-polyisoprene ignores some of the conformations that can make an appreciable contribution to the intensity and dichroism of this absorption band.Conclusions. The proposed algorithm permits computing the intensity an...

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“…as reference), since it presents a unitary dichroic ratio independently of the polymer structure. While the peaks at 1367 cm − 1 (CH 3 deformation) and at 837 cm − 1 (= CH out of plane bending) were selected to calculate the dichroic ratio (R) in all the selected configurations, in order to investigate the orientation of the macromolecular chains of the cis-1,4-polyisoprene [18,19].…”

“…The acquired polarized ATR-IR spectra were analyzed considering the peak at 2960 cm −1 , related to the CH 3 asymmetric stretching, as reference band, since its dichroic ratio is unitary, independently of the polymer's structure [18,19]. In our case, for all samples the dichroic ratios of the peaks at 1367 cm −1 and 837 cm −1 were calculated, since, as previously mentioned, these two peaks are assigned to the CH 3 deformation and the CH out of plane bending of cis-1,4-polyisoprene, respectively.…”