Modern Polymer Spect..

in this 2D spectrum are all positive. Coupled with the fxt that the synchronous
correlation intensities at corresponding coordinates i Figure 1 - 14) are all negative,
one can conclude the side groups of this glassy polystyrene sample complete the
realignment induced by the dynamic strain well before the main chain.
This observation cannot be explained if the average local orientation angle of the
phenyl groups with respect to the chain axis of polystyrene is fixed. There apparently
is substantial freedom of the side groups to temporarily realign ahead of the
main chain 141 1. This finding clearly demonstrates that it is impossible to monitor
the main-chain dynamics of polystyrene by simply observing the reorientation of
side groups. Furthermore, since phenyl groups contribute significantly toward the
refractive index of polystyrene, the above observation also casts significant doubt
over the validity of directly relating birefringence results of glassy polystyrene under
a dynamic deformation to main-chain orientation dynamics.
The observation of highly localized reorientational motions of functional groups
in glassy amorphous polymers is not limited to atactic polystyrene. Similar independent
motions of different side groups are observed in many other systems, including
atactic poly(methy1 methacrylate) [42-441. Different side groups attached to
the same polymer chain, such as ester methyl and a-methyl groups of polyjmethyl
methacrylate). exhibited completely different reorientation rates (see Figure 1-8).
2D IR spectroscopy has been especially useful in elucidating submolecular-level
realignment of functional groups controlling the mechanical properties of glassy
amorphous polymers [4, 43).
The relationship between the local mobility of functional groups and glasstransition
phenomenon has also been probed successfully. New insight into the
dynamics of amorphous polymers going through glass-to-rubber transition processes
was provided. Specifically, the local mobility of individual functional groups
was found to play a very significant role in the glass-to-rubber transition of amorphous
polymers [41]. In turn, it is now possible to determine if an amorphous
polymer is in glassy or rubbery state by simply observing the characteristic local
reorientation dynamics of functional groups using 2D IR spectroscopy.
1.5.2 Semicrystalline Polymers
Semicrystalline polymers, such as polyethylene 145-471 and polypropylene (5, 481,
may also be studied by using the 2D IR technique. By taking advantage of the
enhanced spectral resolution of 2D IR, overlapped IR bands assigned to the coexisting
crystalline and amorphous phases of semicrystalline polymers can be easily
differentiated. Such differentiation has become especially useful, for example, in the
study of blends of high-density polyethylene and low-density polyethylene [47).
Here it was found that blends of polyethylenes are mixed at the molecular scale
only in the amorphous phase, while each component crystallizes separately. In this
section. an example of a 2D IR analysis applied to a film of linear low-density
polyethylene is discussed [46].
Figure 1-1 6 shows the asynchronous 2D IR spectrum of a thin film of linear lowdensity
polyethylene consisting mainly of ethylene repeat units with a small amount