Modern Polymer Spect..

20
I
2840
chain
-
Crystal
line
phous
t ..
Figure 1-17. An asynchronous 2D IR
spectrum of linear low-density poly-
~
ethylene comparing the strain-induced
, , , I , , , , -2870
50 2150 2050 local reorientational motions of the
Wavenumber, VI main chain and side chain [46].
between the reorientation dynamics of side branch and amorphous components of
the system.
The above result reveals that the octene side branches are excluded from the
crystalline lattice and are accumulating 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.
The high-resolution capability of 2D IR spectroscopy to differentiate highly
overlapped crystalline and amorphous IR bands has been successfully employed
for the characterization of many other semicrystalline polymers. Biodegradable
poly(hydroxyalkanoate)s, for example, were studied by this technique to show that
molecular defects created by the incorporation of comonomer units tend to accumulate
in the amorphous regions [49] in a manner similar to the case of linear lowdensity
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 polymer is
brought down to a temperature below the melt temperature.
1.5.3 Immiscible Polymer Blends
Mixtures of polymers are often immiscible and spontaneously phase separate because
of the much reduced entropy contribution to the free energy of mixing [50].