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3.10 Wlmt Do We Learn from Cnlcailcitioizs? 129<br />

COz rock<br />

___c<br />

I<br />

GAP W S ~ W ~<br />

/<br />

Figure 3-15. Example of a dynainical situation of a po1yniel;ylene r chain -(CH2)n- which contains<br />

defects consisting of CD2 groups and of CH3 units at either ends. The motion of CD:! rocking<br />

occurs in a gap and does not couple with the host lattice, thus generating localized modes; on the<br />

contrary the frequencies of both external deformation and umbrella motions of the CH3 group<br />

occur in the frequency range spanned by the dispersion curves of the host lattice; coupling takes<br />

place and resonance modes are generated.<br />

embedded in the polymer chain is determined by the masses and the geometrical<br />

arrangement of its vibrating atoms (G matrix, Eq. (3-15)) and by the new intramolecular<br />

forces, i.e., by the new force constants (type, strength, and distance of<br />

intra-unit and inter-unit coupling (F matrix, Eqs. j3-2), (3-3) and (3-16)).<br />

Calculations carried out so far on the simplest prototypical classes of polymer<br />

describe substantially three types of phenomena (Figure 3- 15):

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