Orientation change of polyelectrolytes in linearly elongated polyelectrolyte multilayer measured by polarized UV spectroscopy

Supported polyelectrolyte multilayer (PEM) thin films produced by layer-by-layer self assembly, show interesting phenomena upon elongation and release like lithography-free surface structuring, controlled wettability, and stimuli-responsive nanovalves. To understand these effects, the structure and orientation change of PEM, when exposed to linear elongation were investigated by means of pyrene labeled polystyrenesulphonate and polarized UV spectroscopy. The samples were prepared either in solution containing 0.5 M NaCl or in salt free solutions, deposited on sheets of PDMS rubber and elongated laterally up to 10%. Our results show, that if the PEM is produced in the presence of salt the PEM contains a local polarity, depending on the orientation of molecules within the PEM. Upon elongation, the differences in oriented local polarity disappear. In the case of PEM prepared in salt free condition, the polarity within the PEM is independent of the molecular orientation. Upon elongation the orientation of the transition dipole moment changes from isotropic to anisotropic. This effect is strongly enhanced by the presence of NaCl in the solutions from which the PEM were formed.