Jump motion polymer conformation

The detailed analysis of carbon-13 spin-lattice relaxation times of a number of polymers either in solution or in bulk at temperatures well above the glass-transition temperature has led to a general picture involving several types of motions. The segmental reorientation can be interpreted in terms of correlated conformational jumps which induce a damped orientation diffusion along the chain. It is satisfactorily described by the well-known autocorrelation functions derived from models of conformational jumps in polymer chains [4,5] which have proven to be very powerful in representing fluor-... 

The evolution of the experimental anisotropy as a function of the temperature is shown in Fig. 8. As expected, the decay rate increases as the temperature increases. For the highest temperature (t > 50 °C), it can be noticed that the anisotropy decays from a value close to the fundamental anisotropy of DMA to almost zero in the time window of the experiment (about 60 ns). This means that the initial orientation of a backbone segment is almost completely lost within this time. This possibiUty to directly check the amplitude of motions associated with the involved relaxation is a very useful advantage of FAD. In particular, it indicates that in the temperature range 50 °C 80 °C, we sample continuously and almost completely the elementary brownian motion in polymer melts. Processes too fast to be observed by this technique involve only very small angles of rotation and cannot be associated with backbone rearrangements. On the other hand, the processes too slow to be sampled concern only a very low residual orientational correlation, i.e. they are important only on a scale much larger than the size of conformational jumps. 

Just as the equilibrium conformational properties of macromolecules, the theory of which has been developed in well-known classical works by Kuhn, Flory, Volken-stein and others the kinetic properties of polymer chains can be determined by two main mechanisms of intramolecular mobility. First, it is the discrete rotational isomeric (rotameric) mechanism of mobility caused by the jump of small-chain segments (kinetic units) from certain energically stable allowed conformers into others is4-i6S) gg ond it is the continuous mechanism of motion deter-... 

The normal case with a molten or dissolved polymer is that its conformational jumps are quite rapid, compared with the tumbling and looping motions of entire chains. A typical barrier to a conformational jump in a vinyl polymer chain without external constraints is only approximately 12-20 kJ mol and is therefore readily surmounted at room temperature, typically at least 10 times per second. In this case, the local motions will contribute to the NMR relaxation parameters of the polymer, such as T, T2 and NOE. Similar motions will occur in the gel state and in the flexible solid, above Tg. 

Another important result is the similarity of the temperature variation of the correlation time r, associated with conformational jumps, and observed for all the polymers considered except polyisobutylene, to the predictions of the Williams-Landel-Ferry equation for viscoelastic relaxation, which indicates that the segmental motions observed by NMR belong to the glass-transition phenomenon. Moreover, the frequency of these intramolecular motions is mainly controlled by the monomeric friction coefficient of the polymer matrix. 

Because the configurations of the chains which predominantly contain the trans and gauche conformations of short parts formed by four C atoms (in sp hybridization) fit fairly well to the tetrahedral lattice (see chapter Conformational and Dynamic Behavior of Polymer and Polyelectrolyte Chains in Dilute Solutions, Fig. 3), to a first approximation they assumed that the basic motion of the fluorophore can be described as a jump-like rotation on the tetrahedral lattice with one characteristic time, p (which depends on the characteristic jump frequency and the conformation structure of the chain), in the form [100, 101] ... 

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