Modern Polymer Spect..

3.13 Defect Mo&s cis Structurcil Probe5 iii Poljwietliylene Cliaiiis 143
3.13.2 Conformational Defects
The study of the conformational defects in polymethylene chains has been tackled
some time ago in a systematic way by the school of Snyder by norinal inode analysis
of short-chain molecules (see, for exainplc (1041) and by our group at Milano
using NET [95. 1011. The results by both techniques are very similar. In the previous
discussion in Figure 3-14 we have pointed out that a few peaks in the calculated
g( 1’) for infinite tims polyethylene do not find corresponding infrared and/or
Ranian lines to be assigned to k = 0 motions. These additional experimental peaks
are just an evidence of the existence of extra structures which we can identify as
conformational defects using the theory we are presently discussing.
The main characteristic defect modes calculated (and observed) for tram planar
segments are the following:
1. GG defect. The wagging vibration of a CH2 group located between two adjacent
grrirclie conformations -CH-,(G)-CH?(G)-CH-,- wagging. The calculated (and
observed) frequency lies near 1355 cin-’.
2. GTG‘ defect. CH2 wagging. -CH2(G)-CH,(T)-CH-,(G’)-. This defect introduces
two wagging modes because the two CH2 groups are joined by a bond in
traizs conformation and are partly decoupled from the host lattice by the gauche
bonds. The intra-defect coupling generates out-of-phase and in-phase modes
calculated (and observed) near 1375 and 1306 c1ii-I.
3. GGTGG dc+x-t. This defect is the one which possibly exists on the surface of the
single crystal lainellae of polyeythylene if the chain re-entry forins a tight fold
along the [200] crystallographic plane [ 1051. This defect generates practically the
three lines expected for GG and GTG’ defect in the CH-, wagging region and an
additional quasi out-of band mode (near 714 cm-I) just outside the k = 0 limiting
rocking mode of the CH2 group [99].
4. Heud-to-hecicl defects of the type. -RCH-CH-,-CH-,-RCH-. The CH2 rocking
mode of a sequence of two CH-, groups between two heavy boundaries is calculated
and observed near 850 cm-’ in agreement with empirical correlations
derived from the study of ethylene-propylene copolymers [ 1061.
5. Clzaivi end modes it2 pi-alkyl clznins. When the group -CH-,-CH>-CHl-CH3 in
such chains has TTT conformation, the methyl ‘umbrella deformation’ iU) is
calculated and observed near 1375 c1n-l and the external deforniation mode 6
CH3 occurs near 890 cm-’. According to the previous discussion in this chapter
both motions should be considered end-group modes whose frequency should
remain constant when the length of the chain increases. This is indeed the case if
the chains keep the all-trams structure. However, since both motions occur in a
frequency range spanned by the dispersion cui-ves of the 1D lattice they couple
with the vibrations of the bulk and their coupling extends for a few CH2 units
along the chain. If the conformation of the chains near the ends changes, such
intramolecular coupling changes and the frequencies of the ‘end groups’ may
change. Table 3-2 reports the values of the calculated and observed frequencies
when chain ends are conformationally distorted in the TG and GT conformations.