3D-C/shape memory polymer composite for space deployable applications

• Main Author: Shivakumar, Ranjana
• Other Authors: Teo Hang Tong Edwin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Functional materials; Engineering::Materials::Composite materials
• Online Access: https://hdl.handle.net/10356/143891
• Institution: Nanyang Technological University

Large components such as solar panels, solar sails and antenna in the satellite are generally designed as deployable structures, that are stowed during launch and deployed in orbit using electromechanical deployment systems. The structural integrity, reliability, weight and stowage volume of the deployment system are always critical factors to be accounted for satellite design. The traditional deployment mechanism has certain drawbacks such as increased mass, stowage volume, high cost and failure modes. Smart deployable structures using shape memory materials are being researched extensively due to their lightweight, low cost, ability to be packed easily and controllable deployment. In this thesis, I have developed and tested a new smart material using three- dimensional graphene (3D-C) and shape memory polyimide (SMPI) which can be used to build a simple, reliable, and low-cost self-deployable component for space. Any material developed for space should be resilient to harsh environmental conditions such as atomic oxygen (AO), ultraviolet and ionising radiation, extreme thermal cycles and vacuum. Polyimides (PIs) are a suitable candidate for space application due to their high thermal stability, excellent radiation shielding capacity, and high glass transition temperature. Recently, several shape memory polyimides (SMPIs) were developed by combining the shape memory effect with the excellent mechanical and thermal properties of PI to extend their application to deployable satellite structures. However, they have poor thermal conductivity leading to non-uniform temperature distribution and increased time to conduct heat throughout the SMPI. Since the majority of the SMP are heat activated, the poor thermal conductivity leads to a large thermal gradient across the material. The large temperature gradient results in non-uniform shape recovery leading to internal stress and eventually crack formation. Their poor thermal conductivity hinders their application and reliability as large area actuator. Hence in this thesis, 3D-C with high electrical and thermal conductivity is combined with SMPI’s good mechanical strength and environmental stability. First, the effect of loading fraction of 3D-C interconnected filler on the properties of epoxy-based shape memory polymer (SMP) is studied. A series of 3D-C/SMP composite with different volume fractions of 3D-C (0.1 to 1.5 vol.%) are prepared, and their thermal, mechanical and shape memory properties are studied and compared. It is observed that, as the 3D-C content increases, the thermal conductivity of the composite increases and as well as the shape transformation performances. However, interestingly, in conflict with typical nanofillers, the mechanical strength decreases at higher loading fraction due to the obstruction of polymer chains by interconnected branches. Hence, to leverage on the extraordinary improvement of thermal conductivity by 3D-C, a careful design based on the amount of loading is essential to obtain a balance between thermal conductivity, mechanical strength and shape memory performance. Proofs of concept experiments are also conducted to demonstrate the applicability of 3D-C/SMP composites for large area actuator as well as tape spring hinge design. 3D-C/SMPI composite is synthesised by infusing 60 ppi 3D-C (foam with large pores) with a viscous PAA followed by thermal imidization. The developed 3D-C/SMPI composite exhibits greatly enhanced thermal conductivity (500%), high glass transition temperature (~175°C) and thermal decomposition temperature (5% weight loss at ~500°C), showing excellent thermal stability required for space application. 3D-C/SMPI also demonstrates good shape memory behaviour with high shape fixity (97.6%) and shape recovery (90.27%) in the third thermomechanical cycle with stable electrical and shape memory behaviour after ground-stimulated space environmental tests. 3D-C/SMPI composite exhibits good compatibility with the GEO environment as well. To increase the lifetime of the composite at LEO altitudes, AO exposure study of 3D-C/PI (Polyimide similar to Kapton), is conducted. Polyhedral Oligomeric Silsesquioxane (POSS) is added to 3D-C or PI or both and the resulting composite’s AO durability, electrical and mechanical properties are compared. It is revealed that adding POSS to PI, reduces the erosion yield to 4.67 × 10-25 cm3/O-atom (one order of magnitude lower than that of Kapton), extending its durability beyond 10 years in Low earth orbit. Adding POSS to 3D-C alone does not seem to have any significant effect on the AO durability since the majority of the film mass is PI. Thus, the films with unprotected PI have an erosion yield similar to pure PI films. Lastly, the utilisation of 3D-C/SMPI in deployment mechanism is also investigated by building two shapes of 3D-C/SMPI samples. A box-shaped 3D-C/SMPI is fabricated with an infused heater to demonstrate the high complexity sequential deployment of a multi-joint structure. The seamless shape recovery process of the box into a flat structure is seen, demonstrating the feasibility of large area actuator with 3D-C infused SMPI. The sample is scheduled to be launched and tested in space (in collaboration with SaRC, NTU). It is being subjected to a series of pre-flight qualification tests including launch vibration test and thermal vacuum cycling test. The model is intact after being subjected to launch vibrational loads (acceleration levels of 7g in longitudinal and 2.5g in the lateral axis). In addition, a tape spring shaped 3D-C/SMPI sample is fabricated, demonstrating the ability of the SMPI to be manufactured as a hinge structural material with the addition of 3D-C. The developed 3D-C/SMPI material can be used in small deployable mechanisms (hinges or release devices) as well as large structural members.