Dielectric and Mechanical Relaxations Contribution to the Microstructure Understanding of <scp>PE</scp>/<scp>EVOH</scp>/<scp>PE</scp> Multilayer Thin Films for Stratospheric Balloons Applications

ABSTRACT To use polyethylene thin film as the envelope of pressurized stratospheric balloons with long flight duration, one has to improve its helium permeability. A multilayer film composed of outside layers of polyethylene (PE) and an inside layer of ethylene vinyl alcohol (EVOH) has been selected. The contribution of each layer to the general microstructure behavior of such material is always complex to understand. The aim of this work was to measure the improvement in helium permeability of two multilayer PE/EVOH/PE (A and B) in comparison with pure PE. The multilayer B has a higher EVOH weight percentage. In addition, a detailed analysis of the contribution of each component to the molecular mobility of the multilayer is addressed. The helium permeance of PE/EVOH/PE B has been divided by 4 thanks to EVOH compared with pure PE. Its crystalline phase as well as melting enthalpy has not changed with the addition of EVOH apart from the presence of a small melting peak associated with EVOH melting. The tensile mechanical properties of PE/EVOH/PE have been improved compared to pure PE due to the highly cohesive structure of the EVOH layer. The contribution of each component to the molecular mobility of PE/EVOH/PE has been observed and analyzed by Dynamic Mechanical Analysis (DMA) and Dielectric Dynamic Spectroscopy (DDS). γ, β and α c modes have been observed by both techniques for PE. Due to its polar nature, β and α relaxations of EVOH have been followed by DDS. Experimental correlation of molecular mobility techniques with different stimuli (mechanic or dielectric) leads to the best results regarding multicomponent materials.