Thermal Cis → Trans Isomerization of Covalently Attached Azobenzene Groups in Undrawn and Drawn Polyethylene Films. Characterization and Comparisons of Occupied Sites

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Abstract

The thermal cis → trans isomerization of azobenzene groups covalently attached to chains of undrawn and drawn polyethylene films (Azo-PE) has been examined in a temperature range that is well above the glass transition and well below the melting transition. Isomerizations in undrawn films at all temperatures and in a drawn film at 70 °C, the highest temperature examined, follow single-component first-order kinetics. At 20−65 °C, two first-order components whose activation energies are both smaller than that of the undrawn film are needed to describe the isomerization kinetics in the drawn state. Doped (i.e., noncovalently attached) azobenzene groups in polyethylene films (Azo/PE) also exhibit a biexponential kinetics after drawing, but the fraction of the faster component is much smaller than that in the drawn Azo-PE films. Comparisons between the rate constants for isomerization of undrawn and drawn Azo-PE and Azo/PE films at the same temperature indicate that as a consequence of probe translocations the populations of sites occupied by the probe change significantly when the doped film is drawn. The faster and slower rate components in drawn Azo-PE are assigned to cis isomers residing in interfacial and amorphous sites, respectively, on the basis of differences in polymer chain ordering, rates of relaxation, and free volumes at occupied sites in interfacial and amorphous regions. These results are discussed in terms of how the micromorphological changes in PE when it is drawn affect the dynamics of the guest molecules and how the latter provides information about the local PE matrix.

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