Multiscale modelling of nano-clay filled shape memory polymer foams

Abstract

Shape memory materials (SMMs) are defined as materials that can recover their undeformed configuration after applying an external stimulus like light, temperature, magnetic field, etc. These materials are involved in many applications i.e. aerospace, biomedical and automotive. Among all SMMs, shape memory polymers (SMPs) emerged as the most promising materials. SMPs have received the attention of scientists because of many interesting properties, namely, lightweight structure, biodegradable, and cheap to manufacture. Furthermore, they can be deformed up to 200% of their original configuration keeping the ability to restore their original shape. Recently, material scientists developed SMP nanocomposites by reinforcing them with nanoinclusions (nano-clays, carbon nanotubes, etc.). This step increased the degree of interest of SMPs and expanded the range of applications in industry and medicine. Moreover, producing shape memory polymer foam nanocomposites led to a revolution in the shape memory materials field. The resulting extra lightweight materials can show the shape recovery phenomena under finite deformation with higher mechanical properties. Nano-clay filled shape memory polymer foams have heterogeneous structures at certain levels of observation. For instance, the structure of the material consists of a matrix material with randomly distributed voids of different sizes at the macroscale level. At a lower level of observation, nano-clay layers are randomly distributed through the polymer matrix. Modelling of such multifunctional materials is a big challenge according to the fact that the mechanics of the underlying atomistic nano- and continuum mechanical microstructures have considerable impacts on the material behavior at the macroscale level. Additionally, many thermomechanical phenomena arise from the lower scale levels. Within this work, a full multiscale modelling approach able to simulate the thermomechanical behaviour of the nano-clay filled epoxy shape memory polymer foam under finite strain and at atomistic, micro-, meso- and macro-scale levels is presented. In addition, the nonlinear hyperelastic and viscoelastic behaviour of the material is investigated as well as the shape recovery behaviour. Such an approach is applied by dividing the length scale of the problem into four different levels. The first, and smallest one is the atomistic scale level, in which a set of molecular dynamics models are proposed to determine the thermomechanical properties of the nanocomposite materials. Furthermore, the interfacial strength between the epoxy polymer matrix and the layers is estimated in all possible layer orientations. Moreover, the effects of nano-reinforcements on the thermal expansion coefficient as well as the transition temperature of the epoxy shape memory polymer are estimated using the MD method. The second level is the mesoscale I. A combination of the Finite Element method in conjunction with a Representative Volume Element (RVE) concept and a multiscale numerical homogenization scheme are employed to investigate nano-scale strengthening effects on the thermomechanical properties of the shape memory polymer. Moreover, analytical models are presented and used to compute the overall effective elastic constants of the RVE. On the other hand, the cellular structure of the SMPF is studied at the mesoscale II level. Real and artificial 3D geometries are generated and used in the FE method. In addition, a computational homogenization scheme is used to include the 3D RVE that is generated at the lower scale level in the analysis. Moreover, a numerical finite strain compressive test is applied to both phases (rubbery and glassy) of the SMPFs. In the next steps, all the estimated parameters of the mesoscale II level are accumulated and applied directly into the macroscopic constitutive models of both the rubbery and glassy phases. The thermomechanical performance of the nano-clay filled shape memory polymer foam is examined at temperatures lower and higher than Tg. Furthermore, two different thermomechanical loading cycles are employed to investigate the shape recovery response of the SMPF nano-composites. Last but not least, it is shown that multiscale modeling of SMPF provides a deep understanding of the material behaviours especially those which are originating from micro- and nano-scale material heterogeneous structures. Moreover, the herein applied Multiscale Materials Modelling (MMM) is considered as one of the most accurate simulation methods which is able to model available as well as future complex structures of heterogeneous nanocomposite materials.