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2870 2855 2840
Figure 8.25 Asynchronous 2D IR spectrum of LLDPE in the CH2 symmetric stretching region. Crystalline and amorphous bands around 2855 and 2863 cm-1 are split. (Reprinted from Polymer Solutions, Blends, and Interfaces, R. S. Stein, M. M. Satkowski, and I. Noda, pp. 109-131, Copyright (1992), with permission from Elsevier.)
short (six-carbon) side branches, which reduce the melt temperature and crys-tallinity of the polymer. By selectively labeling the octene units with deuterium, one can unambiguously differentiate the dynamics of the main chain and short side branches. The spectral region shown in Figure 8.25 represents IR signals arising exclusively from the polyethylene chain, with little contribution from the octene side branches.
The presence of asynchronous peaks in the CH2 stretching vibration of backbone methylene groups indicates that there are two distinct bands in this region of the IR spectrum of the linear low-density polyethylene sample. The band located near 2855 cm-1 is assignable to the contribution from the crystalline phase, while the band near 2863 cm-1 is due to the amorphous component of this semicrystalline polymer. The above assignment can be easily verified by heating the sample above its melt temperature; the IR spectral contribution from the crystalline component of the specimen disappears. Because of the significant difference in the reorientational responses of polymer chains located in the crystalline and amorphous phase domains, 2D IR dichroism spectroscopy can separate
Dynamic 2D IR Dichroism Spectra of Polymers
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Figure 8.26 Asynchronous 2D IR spectrum of side-chain-deuterated LLDPE. Cross peaks develop between the side-chain bands and the backbone crystalline band. (Reprinted from Polymer Solutions, Blends, and Interfaces, R. S. Stein, M. M. Satkowski, and I. Noda, pp. 109-131, Copyright (1992), with permission from Elsevier.)
IR bands associated with the two-phase domains, even though these bands are highly overlapped. Asynchronous cross peaks (Figure 8.26) develop between the bands for the polyethylene backbone located in either the crystalline or the amorphous domains and bands for deuterium-substituted octene side branches. Such asynchronicity indicates that short side branches can move independently of the polyethylene main chain located in the crystalline phase of the sample. No noticeable asynchronicity, however, is observed between the reorientation dynamics of side-branch and amorphous components of the system.
The above observation suggests that the octene side branches are excluded from the crystalline lattice and accumulate preferentially in the noncrystalline region of the polyethylene. This result agrees with the view that the depression of the melt temperature and crystallinity of linear low-density polyethylene is caused by the inability of the crystalline lattice to incorporate chain branches above a certain length. It is not possible, however, to determine conclusively from this 2D IR data if the short branches of linear low-density polyethylene are uniformly distributed within the amorphous phase domain, or preferentially accumulate near the interface between the amorphous and crystalline regions.
Dynamic 2D Correlation Spectroscopy Based on Periodic Perturbations
The high resolution capability of 2D IR spectroscopy in differentiating overlapped crystalline and amorphous IR bands has been successfully used for characterizing many other semicrystalline polymers. Biodegradable poly(hydroxyalkanoate)s, for example, have been studied by this technique.23 It was shown that molecular defects created by the incorporation of branched comonomer units tend to accumulate in the amorphous regions in a manner similar to the case of linear low-density polyethylene. Additives, such as plasticizers which are miscible in the molten state of semicrystalline polymers, also tend to be excluded from the crystal lattice and preferentially accumulate in the amorphous phase, once the system is brought to a temperature below the melt temperature.
Figures 8.27 and 8.28 show 2D IR correlation spectra for the CH stretching region of poly(3-hydroxybutyrate), orPHB. PHB is a biodegradable semicrystalline aliphatic polyester produced by microorganisms.24 PHB films were cast from
Synchronous 2D IR Dichroism Spectrum
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Figure 8.27 Synchronous 2D IR spectrum of PHB. (Reprinted from Analytica Chim-ica Acta, 250, C. Marcott, I. Noda, and A. E. Dowrey, p. 131, Copyright (1991), with permission from Elsevier.)
Dynamic 2D IR Dichroism Spectra of Polymers
Figure 8.28 Asynchronous 2D IR spectrum of PHB. (Reprinted from Analytica Chim-ica Acta, 250, C. Marcott, I. Noda, and A. E. Dowrey, p. 131, Copyright (1991), with permission from Elsevier.)
chloroform solution, vacuum dried, and annealed at 60 °C overnight. The samples were analyzed at room temperature with a dynamic IR spectrometer with a low-amplitude (0.1 %) oscillatory (23 Hz) strain. In this spectral region, well-defined correlation peaks for symmetric and asymmetric CH stretching vibrations of the side-group methyl and backbone methylene groups are observed in the synchronous 2D IR spectrum (Figure 8.27). The negative cross peaks at 2978 and 2997 cm-1 indicate the two dipole transition moments for the doubly degenerate asymmetric methyl stretching bands are reorienting in the directions perpendicular to each other. Peaks at 2936 cm-1 indicate the dipole transition moment for the 2997 cm-1 band reorients along with the methylene transition moments in the direction perpendicular to the axis of the applied dynamic strain.