Modern Polymer Spect..

11 Amorphous Crystalline ,...." I
Figure 1-16. An asynchronous ZD IR
spectrum for the main chain syminetric
CH2-stretching vibration of linear
low-density polyethylene [46].
\ I...."
I I , , 1 1 -2870
2870 2855 2840
Wavenumber, VI
(ca. 3 mole'%,) of deuterium-substituted 1-octene comonomer units. The octene
comonomer is incorporated into the polyethylene chain to produce short (sixcarbon)
side branches. which reduce the melt temperature and crystallinity of the
polymer. By selectively labeling the octene units with deuterium, one can unambiguously
differentiate the dynamics of main chain and short side branches. The
spectral region shown in Figure 1-16 represents IR signals arising exclusively from
the polyethylene chain, with little contribution from the octene side branches.
The presence of asynchronous peaks in the CHZ-stretching vibration of backbone
methylene groups indicates 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-' can be assigned to the contribution from the crystalline phase of the
sample. while the band near 2863 cm-' 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 to remove the IR spectral contribution from
the crystalline component of the specimen. Because of the significant difference in
the reorientational responses of polymer chains located in the crystalline and
amorphous phase domains to a given dynamic deformation. 2D IR spectroscopy
can easily differentiate IR bands associated with the two-phase domains, even
though these bands are highly overlapped.
Figure 1-17 shows an asynchronous 2D IR spectrum correlating the backbone of
polyethylene segments located in either crystalline and amorphous domains and
deuterium-substituted branches. Asynchronous cross peaks develop between the
crystalline component of the symmetric CH2-stretching band of the polyethylene
segments and bands for octene side branches. Such asynchronicity suggests short
side branches can move independently of the polyethylene chain located in the
crystalline phase of the sample. No noticeable asynchronicity. however, is observed