Molecular Structural Changes of Buried Interfaces in Stretched Multilayer Polymer Films and Their Influences on Film...

Polymer multilayer films with functional tie layers are used in a variety of applications. The layer adhesion in these multilayer films is known to degrade upon stretching of the multilayer structure, which is required for certain applications. Although the loss of adhesion strength is empirically observed, the molecular basis or mechanism of this adhesion loss is unknown. In this research, we investigated buried interfaces of polymer multilayer thin films with and without stretching, in situ, to understand the reason for adhesion loss using sum frequency generation (SFG) vibrational spectroscopy. The unstretched film exhibits no SFG signals in the C═O stretching frequency region at the ethylene-vinyl alcohol copolymer/maleic anhydride (MAH) grafted polyethylene interface, indicating disordered interfacial C═O groups. This is because the C═O groups are randomly orientated at the interface due to the interfacial diffusion and interfacial chain entanglement of the two polymer materials, leading to a "diffused" interface instead of a distinct or sharp interface. For the 3× and 6× stretched films, SFG C═O stretching signals could be easily detected from the buried interfaces. Analysis of the SFG spectra collected from orthogonal directions showed that the azimuthal orientations of interfacial C═O groups were randomly distributed without a preferred alignment along the stretching direction. Further analysis of the SFG results suggests that the loss of physical entanglement at the interface, rather than interfacial chemical bond breaking, was the reason for the adhesion decrease in stretched films. This research provides important insights into future polymer thin-film material design and performance improvements.