Reorientation, main-chain

In order to get a more precise description of the molecular motions occurring in BPA-PC, it is interesting to check whether, in addition to the ring motions about the 1,4 axis already described, main-chain reorientation takes place. 

Methyl dipolar coupling has been applied [39] at room temperature to investigate the occurrence of main-chain reorientation in BPA-PC. Due to the... 

The carbonyl motions of the carbonate group can be accounted for by considering jumps between two sites, corresponding to a rotation around an axis perpendicular to the C = O bond in the plane formed by the three oxygen atoms, with an angle of 40° between the two sites, over which is superimposed a rotation of 15° about the C = O bond. This latter motion requires a main-chain reorientation motion. 

At higher temperatures, around - 100 °C at 1 Hz, the ft transition involves motions of both carbonates and phenyl rings, accompanied by main-chain reorientation, which can be described in more details as follows. 

Detailed analysis of the molecular motions involved in the fi transition of BPA-PC (described in [1], Sect. 5 and summarised above) shows that, due to the chemical structure of BPA-PC, an intramolecular cooperativity intrinsically exists, above - 100 °C, between phenyl ring n-flips and carbonate conformation changes. These latter directly concern the main-chain behaviour, its reorientation, and the ease with which it undergoes trans to cis (or vice-versa) transition of the carbonate groups under an applied stress. 

H NMR parameters used depend mostly on amplitude and fioquency of reorientations of the CH3 protons due to the motion of the siloxane chain. A restricted reorientation of one monomer unit provides already an effective source for T and T2 relaxation. On the other hand, the dielectric relaxation is caused by the reorientations of the Si-O dipoles of the main chain. A few adjacent adsorbed units might be involved in the relaxation caused by adsorption-desorption. 

The E/Z-isomerization process is characterized by angular-dependent excitation and leads, therefore, to the photoselection of a preferred azobezene dye orientation. In other words, the dichroic dye units choose an orientation where the electronic transition moment is perpendicular to the light electric vector. It promotes, in turn, the cooperative reorientation of neighboring moieties, which include other fragments of the macromolecule, such as the main chain or photochemically inactive comonomer units, and low molar mass additives. Thus, a macroscopic orientation of the sample arises, and it remains long after the illumination is stopped and all the dye moieties return to the thermodynamically equilibratory -state. 

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