Analysis of wrinkles onset and growth in multi-layered graphene-based composite films on flexible Mylar substrates

Flexible membranes are widely used in aerospace components when it is required to combine lightweight with specific optical properties and thermal resistance, in particular in thermal blankets for satellites and propulsion systems for solar sails. Their general performance can be theoretically improved by embedding graphene nanoplatelets (GnPs) in the polymer film, but their use is still severely limited by aspects related to the manufacturing. The structural and physical properties of flexible composite multi-layered films (MLFs) strongly depend on film thickness and homogeneity, which makes the interaction of the carbon nanoparticles and polymer matrices with the substrate a crucial aspect of the manufacturing process. Instability patterns, such as wrinkles, can arise on multiple scales as a consequence of thermal gradients or stresses during manufacturing, and affect the overall mechanical properties of the films. In this work, large scale flexible composite MLFs were fabricated by spray coating polymer/GnPs dispersions over Mylar substrate in multiple layers. The onset and growth of surface wrinkles due to thermal gradients during the manufacturing process was investigated by optical and electron microscopy as a function of the number of layers and GnP concentration. Experimental results were used to define a finite element model for the nonlinear static analysis of the flexible GnP-based composite layers to predict the out-of-plane deformations. In particular, wrinkles formation and how the presence of GnPs affects the overall mechanical properties of the MLFs are studied through ad hoc models.