In a study published in the journal Materials Today Communication, analysis was performed on shape memory polymer composites (SMPCs) made from industrial fabrics previously used in aerospace. The findings enabled the identification of microscopic-level repair processes that are critical for macroscopic-level shape memory capabilities.
Shape-Memory Polymers – a mainstay in the future of various industries
Shape memory polymer composites (SMPCs) belong to a class of smart material architectures that can distort and recover their original shape when stimulated.
In thermally stimulated architectures, the glass transition temperature (Tg) can be used to tune the fixation phase and the restorative properties of the original shape. Interest in these smart materials is high in a variety of industries, including biomedical and aerospace areas, where the use of propulsion is critical.
Shape Memory Behavior – Nobody Likes Polymers
Shape memory (SM) response in carbon fiber is often achieved by adding a polymer framework with SM characteristics. Shape memory polymers (SMPs) can withstand greater deformations than shape memory alloys (SMAs), even when a smaller actuation force is required to recover the undeformed baseline.
Depending on the function required, a suitable polymer framework can be used. In general, thermosetting resins have superior shape memory properties compared to thermoplastics, and epoxy resins often give the highest performance.
Epoxies have excellent thermal and mechanical properties and are widely used in all manufacturing techniques used in high impact industries such as aerospace and automotive.
Main aim of the study
The use of thermomechanical cycling to measure the SM qualities is a common approach for evaluating the shape memory behavior of the SMP.
The shape memory behavior and interactions of the different layers at the nanometric scale have yet to be studied. This research aims to thoroughly analyze these smart materials at the microscopic and macroscopic level, with special emphasis on the resulting mesostructures.
a) Scheme of SMPC structures and b) procedure for preparing SMPC samples. © Bellisario, D., et al. (2022)
This work utilized compression molding of industrially accessible materials to create two different shape memory polymeric composite structures suitable for aerospace use. A large carbon fiber cover made from commercially available thermoset “prepregs” from aerospace was first evaluated for microscale shape memory analysis.
The contribution of the shape memory interlayer due to its architecture was examined at the microscopic level. The proposed composite laminations linked the architectural properties of carbon laminates to the functionalities of SMPs. The shape memory layer sandwiched between the two shape memory polymer composite structures differed such that one contained a thin layer of shape memory epoxy resin while the second contained a shape memory epoxy foam.
The two systems are built to test the suitability of an aerospace injection molding method, such as compression molding, in producing smart architectures for aircraft. This was a significant achievement for the study, as most of the proposed novel shape memory polymeric composite materials in the existing literature could not be classified as aerospace grade materials.
Micro-CT scan and analysis of the SMPCs a) cross section, b) side view and c) analysis of SMPC with epoxy powder interlayer and d) cross section, e) side view and f) analysis of SMPC with thin layer epoxy foam. © Bellisario, D., et al. (2022)
Key findings of the study
MicroCT and SEM imaging showed strong adhesion between the carbon fiber reinforced prepreg (CFRP) layer and the epoxy interlayer, either in the form of a thin foam layer or a thin film. The homogeneity of the thin interlayer of shape memory polymer created during joint lamination was emphasized by SEM imaging, with very small porosity in the CFRP layer demonstrated by MicroCT assessment.
DMA studies showed that shape memory interlayers affected the transition region, which narrows further when shape memory resin is used. Nano-instrumented nicks and micro-notches were used to analyze shape recovery behavior at the microscale and nanoscale.
This was the first time the nano-instrumented approach was applied to this kind of shape memory polymeric composites, allowing the calculation of the temperature for SM response over a small range for both SMPC shapes.
The SM behavior of shape memory polymeric composite structures was confirmed on a macroscale using increasing pressure thermomechanical cycling and numerous thermomechanical cycling.
The SM polymer composite with the thin epoxy foam sheet showed better shape memory capabilities, as predicted, given the foam architecture. In contrast, SM polymer composite with an epoxy powder interlayer exhibited lower but impressive shape memory characteristics.
Recurring thermomechanical cycles influenced the SM behavior while preserving the architectural integrity of the generated smart materials. In addition, appropriate design of the type and amount of shape memory epoxy interlayers could significantly improve the shape memory performance.
Bellisario, D., Quadrini, F. et al† (2022). Microscopic testing of carbon fiber laminates with shape memory epoxy interlayer. Materials Today Communication† Available at: https://doi.org/10.1016/j.mtcomm.2022.103854